US20040190215A1 - Electrostatic chuck having dielectric member with stacked layers and manufacture - Google Patents

Abstract

An electrostatic chuck comprises a dielectric member comprising (i) a first layer comprising a semiconductive material, and (ii) a second layer over the first layer, the second layer comprising an insulative material. The insulative material has a higher electrical resistance than the semiconductive material. An electrode in the dielectric member is chargeable to generate an electrostatic force. The chuck is useful to hold substrates, such as semiconductor wafers, during their processing in plasma processes.

Description

CROSS-REFERENCE
• This application is a continuation of U.S. patent application Ser. No. 10/095,914, filed on Mar. 12, 2002, entitled, “ELECTROSTATIC CHUCK HAVING COMPOSITE DIELECTRIC LAYER AND METHOD OF MANUFACTURE”, which is a divisional of U.S. patent application Ser. No. 09/596,108, filed on Jun. 16, 2000, entitled, “DIELECTRIC COVERED ELECTROSTATIC CHUCK”, which is a divisional of U.S. Pat. No. 6,108,189, issued on Aug. 22, 2000, entitled “ELECTROSTATIC CHUCK HAVING IMPROVED GAS CONDUITS”, which is a continuation-in-part of U.S. Pat. No. 5,720,818, issued Feb. 24, 1998, entitled, “Conduits for Flow of Heat Transfer Fluid to the Surface of an Electrostatic Chuck,” by Weldon et al., all of which are incorporated herein by reference.[0001]
BACKGROUND
• The present invention relates to an electrostatic chuck for holding substrates in a process chamber.[0002]
• Electrostatic chucks are used to hold substrates in various applications, including for example, holding a silicon wafer in a process chamber during semiconductor fabrication. A typical electrostatic chuck comprises an electrode covered by an insulator or dielectric layer. When the electrode of the chuck is electrically biased with respect to the substrate by a voltage, an attractive electrostatic force is generated that holds the substrate to the chuck. In monopolar electrode chucks, an electrically charged plasma above the substrate induces electrostatic charge in the substrate that electrostatically holds the substrate to the chuck. A bipolar electrode chuck comprises bipolar electrodes that are electrically biased relative to one another to provide the electrostatic attractive force.[0003]
• With reference to FIGS. 1[0004]aand1b, the electrostatic attractive force generated byelectrostatic chucks10a,10bcan be of different types. As schematically illustrated in FIG. 1a, adielectric layer11 with a high electrical resistance results in the generation of coulombic electrostatic forces from the accumulation of electrostatic charge in thesubstrate12 and in theelectrode13 of the$F=-{\varepsilon }_0{\varepsilon }_r\frac{1}{2}{\left(\begin{array}{c}V\\ t\end{array}\right)}^2$

  
• chuck[0005]10a. The coulombic electrostatic force is described by the equation: where ∈₀and ∈_rare the dielectric constant of vacuum and relative dielectric constant of thedielectric layer11, respectively, V is the voltage applied to theelectrode13, and t is the thickness of the dielectric layer. The electrostatic force increases with increased relative dielectric constant ∈_rof thedielectric layer11.
• With reference to FIG. 1[0006]b, Johnsen-Rahbek electrostatic attraction forces occur when aninterface14 of a low resistancedielectric layer15 and thesubstrate12 comprise an interfacial contact resistance much greater than the resistance of thedielectric layer15, i.e., when the resistance of thedielectric layer15 from about 10¹¹to about 10¹⁴Ωcm. In these chucks, free electrostatic charge drifts through thedielectric layer15 under the influence of the electric field and accumulates at the interface of thedielectric layer15 and thesubstrate12, as schematically illustrated in FIG. 1b. The charge accumulated at the interface generates a potential drop represented by the equation:$F=-{\varepsilon }_0\frac{1}{2}{\left(\begin{array}{c}V\\ \delta \end{array}\right)}^2$

  
• where σ denotes the contact resistance of the[0007]air gap14 between thesubstrate12 and the low resistancedielectric layer15. The Johnsen-Rahbek electrostatic attractive force is much larger for an applied voltage than that provided by coulombic forces because (i) polarization in thedielectric layer15, and (ii) free charges at the interface14 (which have a small separation distance from the accumulated charges in the substrate) combine to enhance electrostatic force. A strong electrostatic force securely clamps thesubstrate12 onto the chuck and improves thermal transfer rates. Also, it is desirable to operate the chuck using lower voltages to reduce charge-up damage to active devices on thesubstrate12.
• It is known to use ceramic layers to fabricate the low conductivity Johnsen-Rahbek electrostatic chucks. For example, various formulations of Al[0008]₂O₃doped with low levels of 1-3 wt % TiO₂to form low resistance ceramic layers are disclosed in Watanabe et al., “Relationship between Electrical Resistivity and Electrostatic Force of Alumina Electrostatic Chuck,” Jpn. J. Appl. Phys., Vol. 32,Part 1, No. 2, 1993; and “Resistivity and Microstructure of Alumina Ceramics Added with TiO₂Fired in Reducing Atmosphere,” J. of the Am. Cer. Soc. of Japan Intl. Edition, Vol. 101, No. 10, pp. 1107-1114 (July 1993). In another example, U.S. Pat. No. 4,480,284 discloses a chuck having a ceramic dielectric layer made by flame spraying Al₂O₃, TiO₂, or BaTiO₃over an electrode and impregnating the pores of the ceramic layer with a polymer. Whereas pure Al₂O₃ceramic has a resistivity on the order of 1×10¹⁴ohm cm, the alumina/(1-3 wt % titania) ceramic formulations typically have lower resistivities on the order of 1×10¹¹to 1×10¹³, and consequently are more suitable for fabricating Johnsen-Rahbek electrostatic chucks. However, one problem with such ceramic layers is that the volume resistivity of the ceramic decreases to low values with increasing temperature, which results in large current leakages that exceed the capacity of the chuck power supply.
• Another problem with low resistance ceramic formulations is their low charge accumulation and dissipation response time, i.e., the speed at which electrostatic charge accumulates or dissipates in the chuck. The charge accumulation time is the time to reach electrostatic charge saturation and depends on the resistivity of the dielectric layer. Typical resistivities of conventional ceramics of greater than 1×10[0009]¹²Ωcm result in relatively slow charging times, often as high as 5 to 10 seconds. The high resistance also results in a slow dechucking time, which is the time it takes for the electrostatic charge accumulated in the chuck to dissipate after the voltage applied to the electrode is turned off. It is desirable for the chuck to provide rapid chucking and dechucking to provide high process throughput.
• Yet another problem with conventional electrostatic chucks occurs during their use in semiconductor processes that use plasma environments and, in particular, high density plasma environments. A plasma is an electrically conductive gaseous medium formed by inductively or capacitively coupling RF energy into the process chamber. High density plasmas which are generated using a combined inductive and capacitive coupling source typically comprise a thin plasma sheath having a large number per unit volume of energetic plasma ions. The high density plasma species permeate into the interfacial gap between the substrate and the chuck, or the potential differences at the backside of the substrate cause formation of glow discharges and electrical arcing at the backside of the substrate. It is desirable to have an interfacial region that is more resistant to plasma permeation and that can reduce plasma formation even when charged plasma species penetrate into the gap.[0010]
• The formation of glow discharges and arcing at the interfacial gap below the substrate causes additional problems when the substrate is cooled or heated by a heat transfer gas, such as helium, supplied to the interface between the chuck and the substrate via channels in the body of the chuck. The heat transfer gas serves to enhance thermal heat transfer rates. However, the pressure of the heat transfer gas below the substrate counteracts and reduces the electrostatic clamping force exerted on the substrate. Because the semiconductor plasma processing is typically carried out at low pressures, the helium gas pressure increases the size of the interfacial gap below the substrate, causing increased permeation and penetration of the high density plasma into the gap. Additional problems occur when the heat transfer gas passes through gas holes in the chuck that are surrounded by the electrode of the chuck which is supplied by a high power AC voltage. Instantaneous changes in potentials can ionize the heat transfer gas adjacent to the gas distribution holes, particularly when the diameter of the gas hole is relatively large and provides a long mean free path which allows avalanche breakdown of gas molecules in the gas holes. Ceramic chucks typically have large diameter gas holes because it is difficult to drill small holes having diameters less than 0.5 mm because the ceramic at the edges of the holes shatters or chips off during drilling. Arcing and glow discharges within these large diameter gas holes in ceramic chucks cause deterioration of the regions adjacent to the gas distribution holes, including the adjacent dielectric layer and overlying substrate.[0011]
• Thus, there is a need for an electrostatic chuck that reduces plasma glow discharges and arcing in the interfacial gap between a substrate and chuck, particularly when heat transfer gas is provided to the interface. There is also a need for an electrostatic chuck that deactivates or prevents plasma formation at the gas feeding apertures in the chuck. There is a further need for a chuck having a low conductivity dielectric covering or enclosing the electrode which provides higher electrostatic clamping forces, rapid chucking and dechucking response times, and controlled leakage of current from the electrode.[0012]
SUMMARY
• An electrostatic chuck comprises a dielectric member comprising (i) a first layer comprising a semiconductive material, and (ii) a second layer over the first layer, the second layer comprising an insulative material. An electrode is in the dielectric member. The electrode is chargeable to generate an electrostatic force that is useful to hold a substrate in plasma processes.[0013]
• In another version, the electrostatic chuck comprises a dielectric member comprising (i) a first layer comprising a resistivity of from about 5×10[0014]⁹Ωcm to about 8×10¹⁰Ωcm, and (ii) a second layer over the first layer, the second layer comprising a resistivity of from about 1×10¹¹to about 1×10²⁰Ωcm. An electrode is in the dielectric member.
• In a further version, the electrostatic chuck comprises a dielectric member comprising (i) a first semiconductive layer having a resistivity that is sufficiently low to provide (i) a charging time of less than about 3 seconds, and (ii) allow accumulated electrostatic charge to substantially dissipate in less than about 1 second; and (ii) a second insulative layer over the first semiconductive layer, the second insulative layer having a resistivity higher than the first semiconductive layer. An electrode is in the dielectric member.[0015]
DRAWINGS
• These features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings which illustrate versions of the invention, where:[0016]
• FIG. 1[0017]a(Prior Art) is a schematic view of an electrostatic chuck that operates by coulombic electrostatic forces;
• FIG. 1[0018]b(Prior Art) is a schematic view of an electrostatic chuck that operates by Johnsen-Rahbek electrostatic forces;
• FIG. 2 is a schematic side view of a process chamber comprising an embodiment of the electrostatic chuck of the present invention;[0019]
• FIG. 3[0020]ais a schematic side view of an electrostatic chuck comprising a unitary body of dielectric material enclosing an electrode and having gas flow conduits extending therethrough;
• FIG. 3[0021]bis a schematic top view of the chuck of FIG. 3ashowing the outlet of the conduits;
• FIG. 4[0022]ais a schematic top view of an electrostatic chuck comprising gas flow conduits in grooves on the surface of the chuck;
• FIG. 4[0023]bis a schematic sectional side view of another version of the electrostatic chuck showing the dielectric member with gas flow conduits and electrical isolators;
• FIG. 5[0024]ais a perspective partial sectional view of an annular ring which can be used to form the gas flow channel and gas flow conduits in the chuck;
• FIG. 5[0025]bis a sectional side schematic view showing an electrical isolator comprising dielectric coatings on a gas flow conduit in the annular ring of FIG. 5a;
• FIG. 5[0026]cis a sectional side schematic view showing an electrical isolator comprising a dielectric insert in a gas flow conduit in the annular ring of FIG. 5a;
• FIG. 5[0027]dis a sectional side schematic view showing an electrical isolator comprising a porous plug of dielectric material in a gas flow conduit in the annular ring of FIG. 5a;
• FIGS. 6 and 7 are schematic side views showing fabrication of electrical isolators that comprise dielectric inserts fitted in the gas flow conduits;[0028]
• FIG. 8[0029]ais a schematic side view showing a dielectric insert having multiple openings in a gas flow conduit;
• FIG. 8[0030]bis a schematic side view showing a dielectric insert made of a porous material in a gas flow conduit;
• FIG. 8[0031]cillustrates three hole patterns that can be used in the dielectric inserts of FIGS. 8aand8b;
• FIG. 8[0032]dis a schematic side view of a composite dielectric insert comprising a non-porous dielectric sleeve surrounding a porous dielectric center;
• FIG. 8[0033]eis a top view of the composite dielectric insert of FIG. 8d;
• FIG. 8[0034]fis a schematic side view of a porous dielectric insert positioned in a gas flow conduit;
• FIG. 9[0035]ais a schematic side view of a composite electrical isolator comprising a non-porous dielectric sleeve surrounding a porous dielectric center, with an annular gas flow opening therebetween;
• FIG. 9[0036]bis a schematic side view of a composite electrical isolator comprising a ceramic core and a polymer sleeve;
• FIG. 9[0037]cis a schematic side view of a composite electrical isolator comprising a tubular insert and an outwardly extending spacer ledge;
• FIG. 10[0038]ais a schematic side view of a composite electrical isolator comprising a tapered non-porous dielectric sleeve surrounding a porous dielectric center;
• FIG. 10[0039]bis a schematic side view of another version of a composite electric isolator;
• FIG. 11 is a schematic side view of an electrical isolator comprising an embedded electrical lead;[0040]
• FIGS. 12[0041]athrough12fillustrate a method of forming an electrical isolator in a gas flow conduit;
• FIGS. 13[0042]athrough13care schematic sectional side views showing fabrication of an electrical isolator comprising a tapered porous plug in a gas flow conduit;
• FIG. 13[0043]dis a partial sectional perspective view of an electrostatic chuck fabricated according to FIGS. 13athrough13c;
• FIG. 14 is a schematic side view of an electrical isolator comprising porous material covered by dielectric;[0044]
• FIG. 15 is a schematic side view of the electrostatic chuck comprising a composite dielectric layer having a first dielectric layer (such as a semiconducting dielectric) covering a central portion of the electrode, and a second dielectric layer (such as an insulator or dielectric) covering a peripheral portion of the electrode;[0045]
• FIG. 16 is a schematic view of a plasma glow discharge spraying apparatus;[0046]
• FIG. 17 is a schematic view of a detonation gun flame spray apparatus;[0047]
• FIG. 18 is a schematic view of an electrode arc spraying apparatus; and[0048]
• FIG. 19 is a schematic sectional view of a preferred grain structure of a semiconducting dielectric layer formed on an electrode using the apparatus of FIG. 18.[0049]
DESCRIPTION
• The present invention relates to an electrostatic chuck that exhibits reduced plasma glow discharges and electrical arcing at the interface of the substrate and chuck and provides fast chucking and dechucking response times. The electrostatic chuck is described in the context of holding substrates in a process chamber, as illustrated in FIG. 2. The[0050]process chamber50 schematically represents an “HDP” decoupled plasma chamber commercially available from Applied Materials Inc., Santa Clara, Calif., and described in commonly assigned patent application Ser. No. 07/941,507, filed on Sep. 8, 1992, which is incorporated herein by reference. The particular embodiment of theprocess chamber50 is suitable for plasma processing ofsemiconductor substrates55; however, the present invention can also be used with other process chambers or in other processes without deviating from the scope of the invention.
• The[0051]process chamber50 includes aprocess gas source60 that feeds agas distributor62 for introducing process gas into thechamber50 and a throttledexhaust65 for exhausting gaseous byproducts. A plasma is formed from the process gas using a plasma generator that couples RF energy into the chamber, such as aninductor coil70 adjacent to theprocess chamber50 powered by acoil power supply75. The chamber also includes cathode andanode electrodes80,85 that capacitively couple energy into thechamber50. The frequency of the RF voltage applied to the cathode andanode80,85 and/or theinductor coil70 is typically from about 50 Khz to about 60 MHZ, and more typically about 13.56 MHZ; and the power level of the RF voltage/current applied to the coil or process electrodes is typically from about 100 to about 5000 Watts.
• An[0052]electrostatic chuck100 is used to hold asubstrate55 for plasma processing in theprocess chamber50. In one version, theelectrostatic chuck100 comprises anelectrode110 covered by, and more preferably embedded in, adielectric member115 that electrically isolates the electrode from the substrate. Theelectrode110 embedded in thedielectric member115 provides increased electrical isolation from the plasma environment. Optionally, abase105 supports the chuck, and a heattransfer fluid circulator88 circulates heat transfer fluid through thechannels90 in the base to transfer heat to or from thechuck100. In another version, shown for example in FIG. 3a, thechuck100 is formed by adielectric member115 comprising a layer of dielectric material covering a metal plate that serves as theelectrode110. By “dielectric member”115 it is meant both the dielectric layer covering theelectrode110 and the unitary dielectric member having theelectrode110 embedded therein.
• Referring to FIG. 2, to operate the[0053]chuck100, theprocess chamber50 is evacuated to a low pressure, and a robot arm (not shown) transports asubstrate55 from a load-lock transfer chamber through a slit valve into thechamber50. A lift finger assembly (not shown) has lift fingers that are elevated through thechuck100 by a pneumatic lift mechanism. The robot arm places thesubstrate55 on the tips of the lift fingers, and the pneumatic lift mechanism, under the control of a computer system, lowers the substrate onto thechuck100. After the substrate is placed on thechuck100, theelectrode110 of the chuck is electrically biased with respect to thesubstrate55 by achuck voltage supply120 to electrostatically hold the substrate. After process completion, the pneumatic lift mechanism raises the lift pins to raise thesubstrate55 off thechuck100, allowing the substrate to be removed by the robotic arm. Before raising the lift pins, thesubstrate55 can be electrically decoupled or dechucked by dissipating the residual electrical charges holding the substrate to the chuck (after the voltage to the electrode is turned off) by grounding theelectrode110 and/orsubstrate55.
• In the embodiment shown in FIGS. 3[0054]aand3b, the chuck comprises amonopolar electrode110 embedded in, or covered by, thedielectric member115. Theelectrode110 comprises a metal layer composed of copper, nickel, chromium, aluminum, molybdenum, or combinations thereof; that typically has a thickness of from about 1 μm to about 100 μm, and more typically from 1 μm to 50 μm. For asubstrate55 having a diameter of 200 to 300 mm (6 to 8 inches), theelectrode110 typically comprises an area of about 50 to about 250 sq. cm. In operation, a voltage applied to themonopolar electrode110 causes electrostatic charge to accumulate in the electrode (or in thedielectric member115 covering theelectrode110 for Johnsen-Rahbek chucks). The plasma in thechamber50 provides electrically charged species of opposing polarity which accumulate in thesubstrate55. The accumulated opposing electrostatic charges result in an attractive electrostatic force that electrostatically holds thesubstrate55 to thechuck100.
• Alternatively, the embedded[0055]electrode110 can also comprisebipolar electrodes110a,110b, as shown in FIGS. 4aand4b, that comprises at least two substantially coplanar electrodes that generate substantially equivalent electrostatic clamping forces. A differential electrical voltage is applied to each of thebipolar electrodes110a,110bto maintain the electrodes at differential electric potential to induce electrostatic charge in thesubstrate55 and electrodes. Thebipolar electrodes110a,110bcan comprise two opposingsemicircular electrodes110a,110bwith an electrical isolation void therebetween that is covered by thedielectric member115 as shown in FIG. 4a.Alternative electrode configurations110a,110binclude inner and outer rings of electrodes, polyhedra patterned electrodes, or other segmented electrode forms embedded in the dielectric member as shown in FIG. 4b.
• In the arrangement shown in FIG. 3[0056]b, the electrode comprises an electricallyconductive plate110 that is covered by adielectric member115 comprising a layer or coating of dielectric material. Themetal plate electrode110 is shaped and sized to correspond to the shape and size of thesubstrate55 to maximize heat transfer and provide a large electrostatic holding surface for the chuck. For example, if thesubstrate55 is disk shaped, a right cylindrically shaped plate is preferred. Typically, the metal plate comprises an aluminum cylinder having a diameter of about 100 mm to 225 mm, and a thickness of about 1.5 cm to 2 cm. By “electrode”110 it is meant any of the aforementioned versions of the electrode, including both the embedded electrode layer and the metal plate electrode.
• Particular aspects of the[0057]electrostatic chuck100 of the present invention and illustrative methods of fabricating the chuck will now be described. However, the present invention should not be limited to the illustrative examples and methods of fabrication described herein. Also, it should be understood that each of the individual components, layers, and structures described herein, for example, a semiconducting dielectric layer or electrical isolator structures, can be used independently of one another and for applications other than electrostatic chucking, as would be apparent to those of ordinary skill.
• Electrical Isolator in Conduit[0058]
• One feature of the[0059]electrostatic chuck100 of the present invention relates to a plurality of heat transfergas flow conduits150 that extend through one or more of thebase105,electrode110, anddielectric member115, as shown in FIGS. 2 through 4b. Agas supply channel155 supplies heat transfer gas to theconduits150 via agas supply tube160 connected to a heattransfer gas source165. A typicalgas flow conduit150 comprises (1) aninlet202 for receiving gas from agas channel155, and (2) anoutlet204 for delivering the gas to atop surface170 of thedielectric member115 on thechuck100. The gas at thetop surface170 of thechuck100 regulates the temperature of thesubstrate55 by transferring heat to or from thesubstrate55. Thesubstrate55 held on thechuck100 covers and seals the edges of thedielectric member115 to reduce leakage of heat transfer gas from the peripheral edge of thechuck100. Thedielectric member115 can also comprisegrooves162 that are sized and distributed to hold heat transfer gas such that substantially the entire surface of thesubstrate55 is uniformly heated or cooled, such for example a pattern of intersecting channels that cut through thedielectric member115. Preferably, at least oneconduit150 terminates in agroove162, and more preferably, theconduits150 terminate at one or more intersections of thegrooves162. Alternative groove patterns are described in, for example, U.S. patent application Ser. No. 08/189,562, entitled “Electrostatic Chuck” by Shamouilian, et al., filed on Jan. 31, 1994, which is incorporated herein by reference. Thegas flow conduits100,gas supply channel155, andgrooves162 are formed by conventional techniques, such as drilling, boring, or milling. Typically, the heat transfer gas comprises helium or argon at a pressure of about 5 to about 30 Torr.
• Referring to FIGS. 3[0060]aand4b,electrical isolators200 are located in theoutlet204 of thegas flow conduits150 to reduce or prevent plasma formation from the gas provided by theconduits100. This version of the electrostatic chuck is useful for holding substrates in high density plasma environments, for example, where the electromagnetic energy coupled to the chamber is on the order of 5 to 25 watts per cm2 at frequencies of 1 to 20 MHZ. High density plasmas typically contain a higher ion density of charged plasma species in thin plasma sheaths and/or plasma ions having ion energies in excess of 1,000 eV. During operation of thechuck100, the pressure of heat transfer gas below thesubstrate55 counteracts and reduces the electrostatic clamping force on thesubstrate55 to form spaces or gaps at the interface. In high density plasma environments, the thin plasma sheath formed above thesubstrate55 penetrates into these spaces forming an arc or glow discharge at the back of thesubstrate55 which can burn holes in thesubstrate55 orchuck100. Theelectrical isolator structures200 reduce or altogether prevent formation of a plasma in the spaces adjacent to theconduit150 to significantly improve the lifetime of thechuck100.
• The[0061]electrical isolators200 are fabricated from any dielectric material, including ceramics and thermoplastic or thermoset polymers. Suitable polymers include polyimide, polyketone, polyetherketone, polysulfone, polycarbonate, polystyrene, nylon, polyvinylchloride, polypropylene, polyethersulfone, polyethylene terephthalate, fluoroethylene propylene copolymers, and silicone. Engineering thermoplastics and thermoset resins loaded with about 35% to about 45% by volume glass or mineral fillers can be injection molded to form theelectrical isolator200. Suitable ceramic materials include Al₂O₃, AlN, SiO₂, Si₃N₄; of which aluminum oxide, aluminum nitride, silicon nitride, and mixtures thereof, are preferred. More preferably, the dielectric material comprises aluminum oxides which provide a degree of chemical compatibility with the aluminum of the electrode and base or a mixture of aluminum oxides and silicon oxides, as described below. The dielectric breakdown strength of the dielectric material is preferably from about 4 to 40 volts/micron, and the electrical resistance is preferably from about 10¹¹to 10²⁰Ωcm.
• Preferably, the[0062]electrical isolator200 comprises a plasma-deactivating material that is capable of deactivating, and consequently altogether preventing formation of a plasma adjacent to thegas conduits150 below thesubstrate55. The plasma-deactivating material comprises a porous, high surface area material lining the internal surfaces of theconduit150 that prevents plasma formation by limiting the kinetic energy and/or dissipating the electrical charge of ionized gaseous species. Although the plasma deactivation mechanism is not precisely known, it is believed that the high surface area provides active recombination sites that strip the electrical charge from plasma species incident on the surface. Also, tortuous small diameter pores in the plasma deactivating material control the kinetic energy of charged plasma species in the pores by providing a small mean free path that limits acceleration, and resultant avalanche breakdown of the charged species, that is necessary to ignite a plasma. The small mean free path also results in fewer energy transferring collisions between charged gas species which further reduces plasma formation. In this manner, the porous and/or high surface area plasma-deactivating material prevents formation of a plasma in the regions below the substrate that are adjacent to theconduits100.
• The[0063]electrical isolators200 preferably comprise continuous passages therethrough that have small linear dimensions (i.e., diameter or length) which prevent avalanche breakdown and plasma formation in the holes. Preferably, the diameter of the conduits is less than about 0.5 mm, and more preferably less than about 0.25 mm. At these dimensions, the operating pressure and power of thechamber50 are too low to permit ionization of the heat transfer gas, thereby preventing formation of a plasma in the regions adjacent to the outlets of thegas flow conduits50 and electrically isolating the surroundingelectrode110. The shape and distribution of the pores, volume percent porosity, pore size and distribution, and surface area of the plasma deactivating material all affect its plasma deactivating properties. Preferably, the plasma deactivating material comprises small diameter, randomly oriented, tortuous pores which, in conjunction with the spaces between the separated grains, form continuous pathways or pore passageways having small diameters extending through the material. The randomly orientated pores are desirable to produce tortuous passageways that avoid straight line pathways while providing continuous passageways that allow heat transfer gas to flow therethrough. The diameters of these pathways are typically of the same order of magnitude as the ceramic particles used to form the porous material. The tortuous pathways increase the number of effective collisions between the charged gaseous species and between the charged species and the pore wall surfaces. Preferably, the porous material comprises pore passageways that are typically sized from about 250 to about 375 μm in length, and having diameters ranging from about 1 to about 100 μm. Preferably, the volume percent porosity of the plasma deactivating material is from about 10 to about 60 volume %, and more preferably from about 30 to about 40 volume %. Most preferably, the plasma-deactivating material typically comprises a surface area from about 20 cm²/g to about 300 cm²/g.
• The plasma-deactivating material can be formed in the[0064]conduit150 using conventional ceramic fabrication, thermal spraying. In one preferred embodiment, the plasma-deactivating material comprises a mixture of aluminum oxides and silicon oxides. The aluminum oxide grains are held together with intermixed silicon oxide glassy phase, and the resultant structure comprises continuous pathways that are formed between the ceramic grains and through its pores. The porous material can be formed by mixing the desired composition of alumina and silica, pouring the formulation in a mold shaped as the insert, and sintering the mixture at 1400° C. to melt the silica glass around the alumina. The resultant structure has a high porosity of about 5 to 50%, and tortuous pores having diameters typically ranging from 1 to about 25 microns.
• In another method of fabrication, a flame spraying method is used to form the plasma-deactivating material. In this method, a high temperature flame of a combustible mixture of gases, for example, acetylene and oxygen, is formed and a ceramic powder formulation corresponding to the desired composition of the plasma deactivating material is sprayed through the hot flame. The flame spraying method provides a relatively low heat or kinetic energy input to the sprayed ceramic particles, allowing them to move relatively slowly and cool off during travel to the incident surface. The cooling and low kinetic energy impact on the conduit walls results in solidified plasma-deactivating material that comprises spherical ceramic particles which retain their shape and have extensive tortuous pathways between the particles and have high surface areas.[0065]
• Electrical Isolator Structures[0066]
• The[0067]electrical isolator200 in thegas flow conduit150 can have many different shapes and forms. In one embodiment that is easy to fabricate, thegas supply channel155 withgas flow conduits150 is machined in anannular metal ring180 that is inserted in, and forms a portion of theelectrode110, as illustrated in FIGS. 5ato5d. Referring to FIG. 5a, theannular ring180 comprises agas supply channel155 machined in its underside in close proximity to its upper surface with athin metal layer185 therebetween.Conduits150 are machined through themetal layer185 in an annular configuration. Theinlets202 of theconduits150 are bored through the thin layer ofmetal185 prior to the application of the overlying dielectric member115 (not shown), or the conduits can be formed after the application of dielectric member by simultaneously boring outlet holes204 through thedielectric member115 and thinmetallic layer185. Theannular ring180 is sized to fit along the periphery of theelectrostatic chuck100, and is sealed at edges and surfaces190 adjacent to the central portion of theelectrode110 to reduce leakage of heat transfer gas. Preferably, as shown in FIG. 5b, theannular ring180 andelectrode110 form a first annulargas flow channel155aand an overlying and concentric secondannular channel155bhaving a larger width. Theannular ring180, including a plurality ofpredrilled conduits150 spaced apart around the length of the annular ring, is fitted into this combination of channels. The dielectric member115 (not shown) is applied over the surface of thering180 and processed to the desired thickness. Thereafter, an opening is drilled through thedielectric member115 to connect to theconduit150 to allow heat transfer gas to flow fromchannel155 to the surface of the electrostatic chuck.
• In the version shown in FIG. 5[0068]b, theelectrical isolator200ain thegas flow conduit150 comprises a plurality ofdielectric coatings205,210 covering the sidewalls of the conduit. Thedielectric coatings205,210 can be deposited directly on sidewalls of the conduits formed in a annular ring180 (as shown) or can be deposited on top of another “sidewall” dielectric coating that is initially deposited on the sidewalls of theconduit150. For example, the first or inner sidewalldielectric coating210 can comprise a highly electrically insulative layer, and theouter coating205 can comprise a coating of porous plasma deactivating material.
• FIG. 5[0069]cillustrates another embodiment in which the electrical isolator comprises a preformed dielectric insert200binserted in theconduit150 and having at least one continuous hole or passageway206 that allows heat transfer or other gas to flow through theconduit150. The dimensions of the hole are selected to reduce plasma formation, and preferably comprise a diameter equal to or less than about 0.4 mm. The preformed dielectric insert200bcan be fabricated from electrical insulator or dielectric material, plasma-deactivating material, or mixtures thereof. In general, the dielectric inserts200bare fabricated by positioning the insert into theconduit150 drilled in anannular ring180 with an apex of the insert extending from theelectrode110. A layer of dielectric (not shown) is formed over thesurface208 of theannular ring180 andelectrode110 and processed to the desired thickness to expose the hole206 of the insert200b. Thereafter, the apex is removed, for example, by grinding or ablating; or adielectric member115 is formed around the apex to hold the dielectric insert200bin position. The dielectric insert200belectrically isolates theannular ring180 from process plasma which may penetrate the outlet ofgas flow conduit150 and reduces arcing between thesubstrate55 supported upon the surface of theelectrostatic chuck100 and the electrically conductiveannular ring180.
• FIG. 5[0070]dillustrates yet another embodiment of the present invention where the electrical isolator comprises aplug200cof dielectric material that substantially fills up the outlet of thegas flow conduit150. Theplug200ccomprises continuous pathways such as interconnected pore passageways, microcracks, and separated grain boundary regions that extend through the entire plug.Suitable plugs200chave porosities ranging from about 10 to about 60 volume %. In this embodiment, an overlyingdielectric member115 covers theplug200cto hold the plug in place, and an opening is drilled through the dielectric member and stopped on the top surface of theporous plug200c. The continuous pore pathways formed by the intersection of one or more pores, microcracks, and separated grain boundary regions in theporous plug200callow heat transfer gas to flow therethrough, while reducing or preventing limiting plasma formation in theconduit150.
• Another version of the electrical isolator, as shown in FIG. 6, comprises a[0071]cylindrical dielectric insert300 having aboss301 around its circumference and a vertically extendingcavity308 extending from the bottom and having a closed offapex306. Thedielectric insert300 is conically shaped with taperingsides314 at an angle of about 26°. The central portion is cylindrical with a diameter of about 1.5 mm (60 mils), and the entire insert has a diameter of about 3.2 mm (127 mils). Asocket hole313 is bored in theelectrode110 to connect to the underlyinggas supply channel155 in theelectrode110. Thedielectric insert300 is fitted into thesocket hole313 with the bottom of itsboss301 resting on the side portions of thesocket hole313, leaving a clearance between the bottom ofdielectric insert300 and thegas supply channel155. Heat transfer gas flows from thegas supply channel155 into the vertically extendingcavity308 formed withindielectric insert300. After thedielectric insert300 is fit into thesocket hole313 of theelectrode110, adielectric member302 is formed over the surfaces of both thedielectric insert300 andelectrode110. Thereafter, thedielectric member302 is ground back toline304 which is below the closed end of thecavity308 of theinsert300, to expose thecavity308 at itsapex306 allowing heat transfer gas to flow therethrough. Preferably, a plurality ofdielectric inserts300 are inserted into respective socket holes313 spaced apart along theelectrode110, or into socket holes formed in theannular ring180 which is thereafter joined to theelectrode110.
• FIG. 7 shows another embodiment of the electrical isolator comprising a[0072]tubular sleeve320 shaped as a right circular cylinder with anaxial opening328 therethrough. Theaxial opening328 passes through the entire sleeve320 (or has an upper closed end, not shown). Thetubular sleeve320 is inserted in corresponding socket holes334 in theelectrode110 that connects to the underlyinggas supply channel155 of thechuck100. Asecond socket hole335 is drilled partially throughelectrode110 to form anannular ledge336 at the bottom of thesocket hole335 that supports thetubular sleeve320. Optionally, atubular sleeve320 is held in theelectrode110 by an annular weld or brazed joint326 extending around thesleeve320 at the top of theelectrode110 or by an interference fit. After thetubular sleeve320 is fitted intosocket hole335, adielectric member322 is formed over the surface ofinsert sleeve320, and thereafter ground back toline324 to expose theopening332 of thesleeve320. Instead of weldingsleeve320 in place,layer322 can be processed so that it leavesdielectric insert320 unexposed.Openings332 are then drilled through asemiconducting dielectric member322 to connect with theopening328 in thedielectric insert sleeve320. Preferably, a plurality of suchtubular sleeves320 are positioned around theelectrode110.
• FIGS. 8[0073]ato8fshow additional embodiments of theelectrical isolators200 of the present invention. The overlyingdielectric member115 which forms the upper surface of theelectrostatic chuck100 is not shown so that the underlying structures can be shown with more clarity. Thedielectric insert510 illustrated in FIG. 8acomprises a plurality ofopenings516 leading togas flow channel155.Dielectric insert510 is shaped to fit intoannular ring180 and comprises a dome-shaped upper surface that, after application of an overlying dielectric member (not shown), can be ground or ablated to expose theopenings516 of thedielectric insert510 while leaving a portion of the upper surface of theelectrode110 and the insert covered by the overlying dielectric member.
• The electrical isolator of FIG. 8[0074]balso comprises adielectric insert520 that uses an overlying dielectric layer (not shown) to hold it in place. The overlying dielectric layer (which serves as the dielectric member115) is applied over the surface of theinsert520,annular ring180, andelectrode110; and thereafter, ground or ablated to the desired thickness. The conduits through the overlying dielectric layer anddielectric insert520 are drilled through the overlying dielectric layer and insert520 to connect togas flow channel155. FIG. 8cshows typical hole patterns which can be drilled through the dielectric inserts510,520 of FIGS. 8aand8b, respectively. Alternatively, the dielectric inserts510,520 can be fabricated from porous material without drilling holes therethrough, allowing the continuous pores and passageways of the insert to allow heat transfer gas to flow therethrough.
• In the embodiment shown in FIG. 8[0075]c, the conduits or grooves are formed in theelectrical isolator200 by laser micro-machining, a grinding wheel, or diamond/cubic boron nitride drilling. A preferred laser is an excimer UV laser having a short wavelength and high energy that is operated at a relatively low power level to reduce redeposition of drilled aluminum particles onto the walls of the openings and onto the dielectric member. Such aluminum contamination can cause arcing of thedielectric member115. The number ofoutlet openings204 for the conduit depends on the heat transfer load and the gas flow rate required to handle this load. For anelectrostatic chuck100 used with an 200 mm (8 inch) silicon wafer, a suitable number ofoutlets204 or openings for the gas flow conduits range from about 12 to about 24, and the openings are positioned in a ring-shaped configuration around the perimeter of theelectrostatic chuck100. Preferably, the diameters of theoutlets204 are less than or equal to about 0.20 mm, and more preferably about 0.175 mm.
• Another series of dielectric insert designs, shown in FIGS. 8[0076]dthrough8f, are positioned in theannular ring180 fitted in anelectrode110 having twoannular trenches602,604 therein. In FIG. 8d, thedielectric insert610 comprises a tubular non-porousdielectric sleeve616 surrounding aporous dielectric insert618. The dome-shaped upper portion ofdielectric insert610 allows the dielectric member115 (not shown) to hold it in place. The overlyingdielectric member115 is ground or ablated to expose porous adielectric insert618, as shown in the top view of FIG. 8e. This allows heat transfer gas to flow through thechannel155 and theporous dielectric insert618 to the surface of the dielectric member. The non-porousdielectric sleeve616 is shaped to form a small angle with theadjacent surface612 of theannular ring180, allowing deposition of a contiguous coating without voids or cavities at the interface of thesleeve616 andring180. The upper surface ofdielectric insert616 is roughened to provide a strong bond with thedielectric member115. Preferably, thedielectric sleeve616 has greater tensile strength and modulus than theinsert618 to provide a more reliable joint between thesleeve616 andannular ring180. This also reduces formation of voids betweendielectric sleeve616 andring180 which can cause flaws in the overlying dielectric coating (not shown). FIG. 8fillustrates anotherdielectric insert620 that entirely comprises a porous dielectric material, such as the plasma-deactivating material having continuous pore passageways therein. The porosity and pore size distribution of the porous material is selected to reduce formation of plasma in and adjacent to thedielectric insert620.
• FIG. 9[0077]ashows yet another preferred configuration of adielectric insert630 comprising adielectric sleeve636 and adielectric center plug638. An annular ring shapedopening640 is betweensleeve636 andcenter plug638.Center plug638 is held in place by an adhesive or ceramic bonding material such asfusible glass ceramic642, which anchors plug638 tosleeve636. By adjusting the size ofdielectric center plug638, the gas flow rate throughdielectric insert630 is adjusted. Again, an overlying dielectric member115 (not shown) is applied over the surfaces of theelectrode110,annular ring180, anddielectric insert630. Subsequently the overlyingdielectric member115 is processed to expose theopening640 indielectric insert630 while leaving at least a portion ofsleeve636 entrapped below the overlying dielectric member.
• FIG. 9[0078]bshows a preformedelectrical isolator200 comprising aporous plug820 in apolymer sleeve832, the sleeve sized to hold theporous plug820 in theconduit150 in thedielectric member115 orelectrode110. Preferably, thesleeve832 is made of a ductile, lubricative, and slippery surfaced polymeric material, such as Teflon® (trademark of DuPont Company), or a silicone containing material. Because of its ductility and lubricative surface, thesleeve832 facilitates insertion of the hard, brittle, and fracture-prone ceramicporous plug820 into theconduits150 of the chuck. Also, the ductile and flexible polymer conforms its shape to fit snugly into the conduit, to eliminate the need for machining the conduit and/or the porous plug to precise tolerances. In the fabrication process, theporous plug820 is first press fitted into thepolymer sleeve832, and the assembledelectrical isolator200 is then press fitted into theoutlet204 of theconduit150. The preformed insert in thesleeve832 defines at least one continuous passageway that allows gas to flow through the insert. While the ceramic insert can be fabricated from aluminum oxide, aluminum nitride, silicon dioxide, zirconium oxide, silicon carbide, silicon nitride, or mixtures thereof; of which aluminum oxide, aluminum nitride, or silicon nitride, are preferred.
• In the embodiment shown in FIG. 9[0079]c, theelectrical isolator200 comprises an outwardly extendingspacer835 that is sized to hold aninsert830 inconduits840,850. Preferably, thespacer835 is made of a ceramic or plastic material, such as Teflon® (trademark of DuPont Company). Thespacer835 has a toptubular portion860 and a bottomtubular portion865 separated by acentral ledge870 having a cross-sectional area greater than the inner diameter ofconduits840,850. Thespacer835 aligns and holds in place thetubular insert830 during assembly of the chuck, and prevents ingress ofbonding material855, such as molten solder, for example indium, into theconduits840,850 during bonding of thechuck100 to abase105. Prior to joining thechuck100 to the base105 the toptubular portion860 is inserted into theconduit840 on the lower surface of the chuck and the bottomtubular portion865 is then aligned with and inserted into theconduit850 in the base. The base/chuck assembly is placed into a mold, which is then evacuated by a vacuum pump and into which molten solder is injected. The thickness of thecentral ledge870 of a plurality ofspacers835 interposed between thechuck100 and the base105 hold the chuck at a predetermined distance from the metal base to provide a uniform bond line of predetermined thickness. A uniform bond line provides uniform thermal resistance which in turn promotes good heat transfer between the base105 and thechuck100. This is particularly advantageous in the embodiment in which thebase105 advantageously comprises heattransfer fluid channels90 that are used to circulate heat transfer fluid to heat or cool thechuck100 to regulate the temperature of thesubstrate55. As shown in FIG. 2, thebase105 compriseschannels90 through which heat transfer fluid can be circulated byfluid circulator88 to heat or cool thechuck100 as needed to maintain substrate temperature.
• Yet another embodiment of a composite dielectric insert is shown in FIG. 10[0080]a. In this embodiment, thedielectric insert650 comprises a porous dielectric material shaped in the form of an inverted T-shape structure, and having aboss652 around its circumference, the boss comprising avertical cylinder654 with a closedupper end656 centered on a disc portion658. Thevertical cylinder654 of theboss652 typically has a diameter of about 1 to about 3 mm and the disc portion658 a diameter of about 3 to about 5 mm. Anon-porous sleeve660 is shaped to fit and surround thevertical cylinder654. The tapered upper surface of thenon-porous sleeve660 is roughened to allow strong adherence to the overlyingdielectric member115. To fabricate the chuck, a socket hole662 is bored in theelectrode110 to connect to the underlyinggas supply channel155 in theelectrode110.Dielectric insert650 is fitted into the socket hole662 with the bottom of itsboss652 resting on the bottom portions of socket hole662, exposing the relatively wide area of the disc658 to allow heat transfer gas to ingress into theinsert650 from thegas supply channel155, and thereafter flow into the vertically extendingcylinder654. After theinsert650 is fit into socket hole662 ofelectrode110, adielectric member115 is formed over the surfaces of thesleeve650, the tapered roughened surface of thesleeve660, and adjacent surfaces of theelectrode110. Thereafter, thedielectric member115 is ground back to expose the closed end of thecylinder654, and the porous pathways therein allow heat transfer gas to flow through.
• In the embodiment shown in FIG. 10[0081]b, thedielectric insert670 comprises aboss672 having conically shaped tapering sides at674 at an angle of about 26°. In this version, anon-porous sleeve676 comprising a tubular shape with an inwardly extendingcap678 is shaped and sized to fit over thedielectric insert670. Theupper surfaces680 of thenonporous sleeve676 are roughened to form a surface having a strong mechanical adherence. Glass or ceramic cement can be used to bond thecap678 of the nonporous insert onto theboss672 of the porous insert. Thereafter, the composite insert is positioned in acorresponding hole682 in theelectrode110, and thedielectric member115 is formed over the insert and thereafter ablated or ground to expose the surface of the porous insert, as described above.
• In the embodiment shown in FIG. 11, the[0082]electrical isolator200 comprises a dielectric material shaped in the form of a column or pin836 having an embeddedelectrical conductor lead838 that is electrically connected to the groundedbase105 of the chuck. Thelead838 is electrically connected to the base that is typically maintained at an electrical ground, to bring the ground potential applied to the base closer to the substrate to suppress the formation of plasma and electrical arcing in theconduit150. Eachgas supply channel155 of the chuck contains a centrally positioneddielectric pin836 having a diameter sized smaller than the conduit to provide an annular or circumferential opening that allows gas to flow from thechannel155 past thedielectric pin836 and below the substrate. Thedielectric pin836 is held in place inchannel155 by an adhesive orbonding material720 applied to the base of thepin836.
• FIGS. 12[0083]athrough12fillustrate a preferred embodiment of the present invention which provides ease in fabrication. Referring to FIG. 12e, the final structure comprises anelectrode110 including at least onegas supply channel155 which containsdielectric insert718.Dielectric insert718 is sized to provide anannular opening716 that allows gas to flow from thechannel155 and past thedielectric insert718, as shown in FIG. 12f. Thedielectric member115 overlying theelectrode110 also includes at least one opening directly overchannel155, the opening sized to allow insertion ofdielectric insert718 with theannular opening716 around theinsert718. Thus, heat transfer gas can flow fromchannel155 to the surface of thedielectric member115 via theannular opening716.Dielectric insert718 is held in place inchannel155 by an adhesive orbonding material720. It is not critical thatdielectric insert718 be centered in theopening710 through thedielectric member115, as long as the heat transfer gas can flow through theannular opening716.
• Fabrication of this embodiment is shown in FIGS. 12[0084]athrough12f. FIG. 12ashows agas supply channel155 formed in theelectrode110, and at least one hole oropening710 is drilled through thesurface706 of theelectrode110 to connect with heat transfergas flow channel155, as shown in FIG. 12b. The diameter ofopening710 is generally, but not by way of limitation, about 2 mm (0.080 inches) or larger. Although this diameter is not critical, the tolerance of the selected diameter should be held within about ±0.13 mm (±0.005 inches). As shown in FIG. 12c, a space-holdingmasking pin712 is then held inopening710 andchannel155 so that overlyingdielectric member115 can be formed without excessive dielectric material entering intoopening710. This is the reason the tolerance of the diameter of opening710 should be carefully controlled. Maskingpin712 is preferably constructed from a material which does not adhere to thedielectric member115, such as a Teflon® (trademark of DuPont Company) maskingpin712. Space-holdingmasking pin712 is generally 3 to 6 diameters high; being sufficiently tall to allow pulling out thepin712 after forming thedielectric member115, and sufficiently small to reduce shadowing of thedielectric member115 around maskingpin712.
• The[0085]dielectric member115 is typically applied to a thickness which is from about 250 to about 600 microns (10 to 20 mils) greater than the desired final thickness; and after application of thedielectric member115 and removal of themasking pin712, as shown in FIG. 12d, thedielectric member115 is ground to final thickness, and cleaned of grinding residue. This provides a smooth,flush surface722 to thedielectric member115, which is flat to at least 25 microns, i.e., all points on the surface lie within two parallel planes spaced 25 microns apart.Annular opening716 typically has a diameter of about 2 mm (0.08 inches) or more to permit removal of surface residue, such as the grinding residue. This is an advantage over other embodiments of this invention which have smaller diameter openings and are more difficult to clean.
• A measured quantity of adhesive or bonding ceramic[0086]720 is then deposited at the base ofchannel155, directly beneathopening710. The thickness ofadhesive layer720 is sufficient to compensate in variations in the length ofdielectric pin718 while maintaining the smoothness of the chuck surface across thedielectric member115 anddielectric pin718.Dielectric pins718 are typically fabricated from ground ceramic, such as alumina, and have a diameter ranging from about 0.76 mm to about 0.102 mm (0.003 to 0.005 inches) less than the bore diameter ofopening710. Typically, thedielectric pins718 are cut at least ¼ mm (0.010 inch) shorter than the bore depth throughdielectric member115 andelectrode110 to the bottom726 ofchannel155.Dielectric pins718 may be cut as much as 1 mm (0.040 inch) undersized in length.
• Dielectric pins[0087]718 are inserted throughopening710 and into adhesive720 resting on the bottom726 ofchannel155. It is important that thepins718 are positioned to provide a flush top surface724, and this is accomplished using the depth of penetration ofpins718 into the thickness of adhesive720 to make up any differences in length ofpins718. It is not critical thatdielectric pins718 be centered within thebore openings716 and710, and variation is allowable, as apparent from FIG. 12f. The heat transfer gas flows out of theannular opening716 betweendielectric pin718 andelectrode110. In an alternative method, the dielectric pin is held in place by an opening machined into the bottom surface726 into whichdielectric pin718 is interference fitted or staked.
• For[0088]electrostatic chucks100 used to hold 8-inch diameter semiconductor wafers, approximately 12 to 24 conduits havingdielectric inserts718 are positioned in a ring around the periphery of thechuck100. Thecircular opening710 in electrode110 (or annular ring180) typically ranges from about 1 to about 10 mm (0.040 to about 0.400 inches) in diameter, and the dielectric insert has an outer diameter of approximately 0.123 mm (0.005 inches) smaller than the diameter of the cavity. These dimensions are adjusted depending on the kind of heat transfer gas used, the pressure in the process chamber, and the desired gas flow rate to the surface of theelectrostatic chuck100.
• Where the[0089]electrical isolator200 is in close contact with an opening in theelectrode110 orannular ring180, a close contact can be achieved using an interference fit or press fit. During press fitting of the dielectric insert200bin the opening, a uniform pressure should be applied to the surface of the dielectric insert to prevent fracture of the brittle insert using a tool designed to fit and apply even pressure to the surface of the dielectric insert during press fitting. In general,electrical isolators200 comprising ceramic dielectric inserts should be small in size, about 0.5 mm (0.020 inches) to about 10 mm (0.400 inches) in diameter, to avoid mechanical failure from compressive loads applied as a result of temperature cycling during substrate processing. The incompatibility of the thermal coefficient of expansion between the ceramic dielectric and theelectrode110 of theelectrostatic chuck100 creates these compressive loads during temperature cycles. The small size of the dielectric insert also makes it possible to compression fit the insert into theelectrostatic chuck100 in an interference fit. Also, the insert should be tapered toward its bottom edge to permit easier insertion into the receiving cavity. Since alumina containing dielectric inserts have relatively hard and sharp edges, the insert can be pressed into the underlying aluminum cavity with sufficient pressure to cut into the aluminum and provide a close press fit. A close press fit is also obtained by deforming the conductive material in contact with the dielectric insert (staking). The dielectric insert can also be closely fitted using a machined interference fit of about at least about 0.025 mm (0.001 inch). Also, a large interference fit between the insert and surrounding base increases the strength of the bond of the overlying dielectric coating applied on the base and insert, reducing thermal expansion microcracking of the overlying dielectric member, which can lead to penetration of high density plasmas, and rapid breakdown of the dielectric member overlying the insert and base.
• Another structure suitable for forming the[0090]electrical isolator200 comprises aporous plug800, the manufacture of which is shown in FIGS. 13ato13d. Theporous plug800 has substantially continuous pores, and/or interconnected microcracks and pores that forms continuous pathways that allow heat transfer gas to flow through theplug800, while simultaneously deactivating or limiting plasma formation in theconduit150. Referring to FIG. 13a, a hole having a straightwalled inlet802 and atapered outlet804 is bored through theelectrode110 to form agas flow conduit150. Thereafter, an underlyingdielectric layer806 is deposited on the sidewalls of theconduit150, and thesurface810 of thedielectric layer806 at theoutlet804 is roughened, for example by grit blasting, to form a rough serrated surface that provides strong mechanical adherence of the porous plasma deactivating material, and of thedielectric member115 overlying the conduit. Preferably, the dielectric layer is deposited by plasma spraying to provide rough surfaces yielding better adhesion. A tapered conically shapedporous plug800 is formed over the roughed surface to fill the outlet of theconduit150, by for example, thermal spraying. Alternatively, as shown in FIG. 13d, one ormore conduits150 terminating in circular grooves805 are formed in the surface of thechuck100, a layer ofdielectric material806 is deposited in the groove805 and roughened, and a porous covering803 is filled in the groove805. Preferably, at least one groove805 is formed in the peripheral edge of theelectrode110 to cool the peripheral edge of the substrate. This configuration has the further advantage of facilitating manufacture by allowing theunderlying dielectric layer806 and the porous covering803 to be deposited by rotating thechuck100 under the applicator nozzle of the spraying apparatus. Preferably, plasma or flame spraying is used to form theporous plug structure800 to obtain a high porosity plug. Thereafter, the overlyingdielectric member115 is deposited on theplug800 to hold the plug in place, as shown in FIG. 13b. Either an opening is drilled only through the dielectric member115 (not shown) and stopped on the top surface of theporous plug800; or the surface of the dielectric member is ground or ablated until the apex812 of the plug is exposed, as shown in FIG. 13c, allowing heat transfer gas to flow through the porous pathways in the plug.
• In the embodiments shown in FIGS. 13[0091]ato13c, the taperedoutlet804 comprises a nonvertical surface which allows firm adherence and uniform deposition of a thermally sprayeddielectric member115. It has been discovered that when a thermally sprayed coating is applied to a perpendicular surface, i.e., in the same plane as the spraying direction of the spraying process, the solidified spray has low adhesion on the vertical surfaces and forms loose grains near the corners and edges of the vertical surfaces resulting in spalling and flaking off of thedielectric member115. Thus, preferably, theoutlet804 of theconduit150 has non-vertical surfaces that define a tapered region therebetween. The tapered region is also configured to reduce the penetration of plasma into theconduits100, the sloped non-vertical walls forming an apex at the surface of the chuck. Theporous plug800 is deposited in the tapered region to substantially entirely fill the tapered region of the outlet. Typically, theoutlet804 has tapered sides that form an angle of about 26° with a vertical axis through the conduit, and comprises a first smaller diameter of at least about 1 mm, and a second large diameter of less than about 5 mm.
• In yet another method of fabrication, the[0092]electrical isolators200 are formed by filling theoutlet204 of theconduit150 with porous material. Theoutlet204 of theconduit150 forms an annular ring that extends continuously along, and adjacent to, the circumferential perimeter of the chuck, as shown in FIG. 13d, to provide heat transfer gas below the entire perimeter of thesubstrate55. Preferably, granules of dielectric material mixed with organic binder are packed in theoutlet204 of theconduit150, and sintered to form granular material bonded to the inner surfaces of theconduit150 having convoluted passageways and interconnected pores. Because the dimensions of the resultant interconnecting pores tend to be roughly equal to the size of the granules, it is also preferred to use ceramic granules having an average mean diameter of less than or equal to about 0.4 mm, and more preferably less than or equal to 0.25 mm. Preferably, the granules consist of the same material as thedielectric member115 to increase their adhesion to the inner surface of thegas conduits155 and reduce thermal stresses. Thereafter, a layer of ceramic material is deposited over theelectrical isolator200, and a gas flow or gas pressure is maintained in theelectrical isolator200 during deposition of the overlying dielectric layer to prevent plugging of pre-drilled holes or pores of the porous materials. After thedielectric member115 is formed on the surface of thechuck100, a thickness of about 200 to 250 μm of the top surface of the dielectric member is ground or ablated to expose the underlyingelectrical isolator200. The grinding process is performed using a diamond grit-coated grinding wheel that is registered accurately relative to thechuck100 to grind-off the correct thickness of the dielectric layer, and deionized water grinding fluid is used to reduce contamination.
• Another embodiment of the[0093]porous plug configuration820 is shown in FIG. 14. In this version, a hole having a straightwalled inlet822 and atapered outlet824 having a continuously varying multi-radius sidewalls, is bored through anannular ring180 to form the gas flow conduit. The annular ring is mounted in a cavity in theelectrode110 so that thering180 rests uponledges826 in the base. Thereafter, anunderlying dielectric member828 is deposited on theoutlet824 of the conduit and adjacent surfaces of theelectrode110. A tapered conically shapedporous plug800 is formed over anunderlying dielectric member828 to substantially fill theoutlet824 by a suitable deposition method, such as for example thermal spraying, and more preferably by plasma or flame spraying. Thereafter, an overlyingdielectric member115 is deposited on theplug800 to hold the plug in place. Either an opening is drilled only through the dielectric member115 (not shown) and stopped on the top surface of theporous plug820 without drilling through the plug; or the surface of the dielectric member is ground or ablated until the apex of the plug is exposed (not shown) allowing heat transfer gas to flow through the porous pathways in the plug.
• Semiconducting Dielectric Member[0094]
• In another aspect, the present invention is directed to an[0095]electrostatic chuck100 comprising one ormore electrodes110 covered by, and more preferably embedded in, a dielectric member having semiconducting properties that provides fast charging and discharging response time and rapid chucking and dechucking ofsubstrates55 held on thechuck100. Thesemiconducting dielectric member115 can be used in conjunction with theelectrical isolators200 or separately without using theelectrical isolators200. Thedielectric member115 comprises a unitary body of semiconducting dielectric material covering or enclosing theelectrode110 therein, as shown in FIGS. 2 and 4b; or one or more layers of semiconducting dielectric material covering an electricallyconductive electrode110 that serves as theelectrode110, as shown in FIG. 3a. In both versions, thesemiconducting dielectric member115 comprises atop surface170 configured to support a substrate. Upon application of a voltage to theelectrode110, the semiconducting properties of thedielectric member115 allow rapid accumulation of electrostatic charge in the dielectric member, particularly at the interface between the dielectric member and thesubstrate55. For electrostatic charge to accumulate in thedielectric member115 the semiconducting material has to be sufficiently leaky to allow a small leakage current to flow from theelectrode110 through thedielectric member115. If the leakage current is too small, chucking speed is slow, and substrate processing throughput is reduced. Conversely, an excessively high leakage current can damage the active devices formed on thesubstrate55.
• The amperage of the leakage current that can be tolerated in the[0096]chuck100 also depends upon the voltage applied to theelectrode110. The higher the applied voltage, the larger the leakage current that can be tolerated without completely losing the electrostatic clamping force from excessive current leakage through thesemiconducting dielectric member115. However, the maximum operating voltages that can be used to electrostatically hold semiconductor substrates are limited to about 2000 volts, and if exceeded, can cause charge-up damage of the active devices in thesubstrate55. Thus, the leakage current provided by the semiconducting material should be sufficiently low to retain electrostatic charge in thedielectric member115, during operation the chuck at voltage levels of about 100 to about 1500 volts. It has been discovered that optimal leakage currents from thedielectric member115, that provide quick charging response times, without damaging the devices on the substrate, are at least about 0.001 mAmps/cm², and more preferably from about 0.002 mAmp/cm²to about 0.004 mAmp/cm². A suitable leakage current is achieved by controlling the resistivity of thesemiconducting dielectric member115. Thus, preferably, the resistivity of thesemiconducting dielectric member115 is sufficiently low to allow conductance of a low amperage leakage current that provides a quick charging time of less than about 3 seconds, and more preferably less than about 1 second. Thesemiconducting dielectric member115 also has a resistivity sufficiently low to provide rapid dissipation of accumulated electrostatic charge when the voltage applied to theelectrode110 is turned off. Preferably, the resistivity of thesemiconducting dielectric member115 is sufficiently low to allow accumulated electrostatic charge to substantially entirely discharge or dissipate in less than about 1 second, and more preferably in less than about 0.5 second. Conventional dielectric members typically have dechucking times of 5 to 10 seconds, which is about five to ten times longer than that provided by semiconducting dielectric member of the present invention.
• While a low resistance semiconducting[0097]dielectric member115 is desirable for rapid chucking and dechucking, a chuck having an excessively low resistance dielectric member will allow excessive charge to leak out. The resistance of thesemiconducting dielectric member115 needs to be sufficiently high to maintain a supply of electrostatic charge at the interface of thechuck100 andsubstrate55, even though a portion of the electrostatic charge leaks or dissipates through themember115. Any leakage current allows electrostatic charge to continually dissipate from thedielectric member115. Thus, electrostatic charge must accumulate at the dielectric/substrate interface at a rate equal to or greater than the rate of charge dissipation to provide an equilibrium mode in which a supply of accumulated electrostatic charge is maintained at the dielectric/substrate interface.
• In a preferred version, the[0098]semiconducting dielectric member115 comprises a resistance in a preferred range of resistivity Δρ that provides such a combination of opposing properties. The resistivity range Δρ of thesemiconducting dielectric member115 is defined by (i) a first lower resistivity ρL that is sufficiently low to allow a leakage current to flow from the electrode when the operating voltage is applied to the electrode to form accumulated electrostatic charge at the interface of thesubstrate55 and thesemiconducting dielectric member115; and (ii) a second higher resistivity ρH that is sufficiently high to maintain accumulated electrostatic charge at the interface during operation of the chuck without use of excessively high operating voltages that damage the substrate. The optimal range Δρ of resistivity of thesemiconducting dielectric member115 is preferably from about 5×10⁹to about 8×10¹⁰Ωcm, and more preferably from about 1×10¹⁰to about 5×¹⁰Ωcm. This range of resistivity is substantially lower than conventional dielectric members which have resistivities exceeding 1×10¹¹Ωcm, and more often exceeding 1×10¹³Ωcm.
• The[0099]semiconducting dielectric member115 having the described properties can be fabricated from ceramic materials, polymers, and mixtures thereof. Suitable ceramic materials include (i) oxides such as Al₂O₃, BeO, SiO₂, Ta₂O₅, ZrO₂, CaO, MgO, TiO₂, BaTiO₃, (ii) nitrides such as AlN, TiN, BN, Si₃N₄), (iii) borides such as ZrB₂, TiB₂, VB₂, W₂B₃, LaB₆, (iv) silicides such as MoSi₂, WSi_x, or TiSi_x. Preferably, thesemiconducting dielectric member115 having a resistivity in the preferred range of resistivities of Δρ comprises a composition of aluminum oxide doped with (i) transition metals or metal oxides, such as for example, Ti, Cr, Mn, Co, Cu, TiO₂, Cr₂O₃, MnO₂, CoO, CuO, and mixtures thereof; (ii) alkaline earth metals or oxides, such as for example, Ca, Mg, Sr, Ba, CaO, MgO, SrO, or BaO; or (iii) a combined oxide formulation, such as for example, CaTiO₃, MgTiO₃, SrTiO₃, and BaTiO₃. The dopant material is added in a sufficient quantity to provide semiconducting properties to the aluminum oxide dielectric material. By semiconducting it is meant a material having a conductivity in between that of a metal and an insulator.
• Preferably, the[0100]dielectric member115 comprises a unitary body of multiple layers of semiconductor and/or insulating material enclosing the electrodes, each layer typically having a thickness of from about 10 μm to about 500 μm. Thedielectric member115 comprises a cover layer that electrically isolates thesubstrate55 from theelectrode110, and a support layer which supports the electrode and electrically isolates theelectrode110 from aconductive electrode110. The material and thickness of the cover layer are selected to allow the DC voltage applied to the electrode to electrostatically hold the substrate by means of Coulombic or Johnsen-Rahbek electrostatic attractive forces. The thickness of the layer covering the electrode is typically from about 100 μm to about 300 μm. Preferably, material of the cover layer comprises a dielectric constant of at least about 2. Additionally, a protective coating (not shown) can be applied on the exposed surface of the dielectric member to protect the semiconductor layer from erosive processing environments
• A preferred composition of the semiconducting dielectric member comprises aluminum oxide doped with titanium oxide in a weight percent content of at least about 8 wt %, and preferably at least about 12 wt %. Whereas, pure aluminum oxide has a resistivity of 10[0101]¹⁴Ωcm and a characteristic charging response time of about 103 seconds; the highly doped aluminum oxide has a resistivity typically ranging from about 5×10⁹Ωcm to about 8×10¹⁰Ωcm. It is believed that the low resistivity results from titanium-metal rich grains or grain boundaries that are formed in the aluminum oxide material, titanium-metal rich regions comprising Ti³⁺ in solid solution in the aluminum oxide structure. However, the resistivity can also be dependent upon microstructural factors other than Ti³⁺ formation, for example, formation of highly conductive Al_xTi_yO_zphases within the alumina grains or at grain boundary regions. Formation of highly conductive titanium-metal rich alumina grains is particularly prevalent when the TiO₂—Al₂O₃mixture is exposed to an oxygen-deficient or reducing environment, such as an inert gas environment during fabrication.
• The[0102]semiconducting dielectric member115 operates by Johnsen-Rahbek forces providing a higher electrostatic clamping force for relatively low chuck voltages. The low chuck voltages reduce the potential for damage to active regions in thesubstrate55. Also, the lower chuck voltages reduce the risk of plasma generation at the dielectric/substrate interface. The semiconductor dielectric is sufficiently leaky that upon application of a voltage to the electrode, the semiconducting dielectric member allows rapid accumulation of electrostatic charge at the dielectric/substrate interface. Furthermore, the lowresistance semiconducting layer115 provides electrostatic charge dissipation response times of less than about 1 second, and more typically less than about 0.5 seconds, with little or no residual charge or sticking forces. The extremely low charging and charge dissipation response time provides rapid chucking and dechucking with the electrostatic holding force rising almost instantaneously with applied voltage, and decreasing almost instantaneously to zero when the applied voltage is turned off. Also, unlike conventional ceramic formulations, the resistivity of the highly doped alumina coatings did not appear to change during use at temperatures ranging from −10° C. to 100° C. These novel and unexpected advantages of thesemiconducting dielectric member115 provide significant benefits for electrostatic chucks.
• In yet another aspect of the invention, as shown in FIG. 15, a[0103]composite dielectric member115 comprising a firstdielectric material172 having first electrical properties; and a second dielectric material174 having second electrical properties, is used to cover the electrode110 (which is illustrated as a base105 that serves as the electrode, but also includes the embedded electrode version). In a preferred configuration, the firstdielectric material172 is disposed over a central portion of the electrode110 (which is substantially entirely covered by thesubstrate55 during operation of the chuck100); and the second dielectric material174 is disposed over a peripheral portion of theelectrode110 and comprises an annular rim extending around the first dielectric member. This configuration allows tailoring of the properties of the composite dielectric layers across the radial surface of the chuck. This is desirable to provide different electrical properties at the edge of the chuck which is closer to the plasma sheath than the center which is covered by the substrate.
• The properties of the first and second[0104]dielectric members172,174 are tailored to achieve different electrical properties at different portions of thechuck100. For example, thefirst dielectric member172 can comprise a semiconducting material as described above. During operation of thechuck100, the first dielectric member is substantially entirely covered by the substrate which serves as a dielectric material that electrically insulates the semiconducting layer and reduces shorting between the semiconducting layer and the plasma. In this version, the second dielectric member174 comprises an insulator that has a higher resistivity than the semiconducting dielectric member to prevent plasma discharge at the exposed peripheral portions of the chuck. The resistivity of the insulating second dielectric member174 is sufficiently high to prevent electrical discharge or arcing between the surrounding plasma environment and the peripheral portions of the chuck electrode. Preferably, the second dielectric member174 has a resistance of at least about 1×10¹¹Ωcm, and more preferably from about 10¹³Ωcm to about 1×10²⁰Ωcm. This configuration prevents shorting and arcing between the leaky semiconducting dielectric member and the plasma and the resultant pinholes in the dielectric member that cause failure of the chuck. In another example, the compositedielectric coating115 can comprise afirst dielectric member172 having a first dielectric breakdown strength, and a second dielectric member174 having a second dielectric breakdown strength. Preferably, the second dielectric breakdown strength is higher than the first dielectric breakdown strength to prevent plasma discharge or electrostatic charge neutralization at the peripheral edge of the chuck.
• The[0105]composite dielectric member115 can also be made from multiple vertically stacked layers. For example, a multilayercomposite dielectric member115 can comprise (i) an Al₂O₃—TiO₂layer providing semiconducting electrical properties; and (ii) a more insulative second layer, such as polyimide, Teflon®, SiO₂, or ZrO₂. For example, the multilayer structure can be tailored to provide increased electrostatic charge retention at thetop surface170 of the chuck, and/or faster electrostatic charge accumulation and dissipation response times through the body of the dielectric member. This can be accomplished by forming a thin second dielectric member having a high resistivity over a first dielectric member having a lower resistivity. Because the electrostatic force is largely attributable to the charge concentrated near the surface of thedielectric member115 such multilayer coatings can provide excellent surface charge retention characteristics, without affecting charge dissipation from the underlying layer. The multiple dielectric members preferably comprise a combination of semiconducting and insulator dielectric members.
• The semiconducting or[0106]composite dielectric member115 can be formed by a variety of conventional methods, as apparent to those skilled in the art, including for example, isostatic pressing, thermal spraying, sputtering, CVD, PVD, solution coating, or sintering a ceramic block with theelectrode110 embedded therein; as would be apparent to those skilled in the art. In the methods described below, thesemiconducting dielectric member115 is used to cover at least a portion of the electrically conductive base that serves as theelectrode110, or is used to cover or entirely enclose anelectrode110 to form an electrostatic member that can be supported by the base.
• A preferred method of forming a[0107]unitary dielectric member115 with an embedded electrode uses a pressure forming apparatus, such as an autoclave, platen press or isostatic press. Isostatic presses are preferred because they apply a more uniform pressure on the dielectric member and electrode assembly. A typical isostatic press comprises a pressure resistant steel chamber having a diameter ranging from about 1 to 10 feet. A pressurized fluid is used to apply pressure on an isostatic molding bag. A powdered precursor is prepared comprising a suitable ceramic compound as described above is mixed with an organic binder selected to burn off during sintering. The precursor is placed along with the electrode structure in the isostatic molding bag and the bag is inserted in the isostatic press. The fluid in the pressure chamber is pressurized to apply an isostatic pressure on the dielectric assembly. It is desirable to simultaneously remove air trapped in the isostatic molding bag using a vacuum pump to increase the cohesion of the powdered precursor. The unitary dielectric member/electrode assembly is removed from the molding bag and sintered to form a unitary dielectric with the electrode embedded. Thegas flow conduits150 are formed in the dielectric member by conventional techniques, such as drilling, boring, or milling. Preferably, at least some of theconduits150 terminate at the periphery of thechuck100, to provide heat transfer gas to the peripheral edge of thesubstrate55.
• After deposition, the surface of the[0108]dielectric member115 is fine ground to obtain a highly flat surface to efficiently electrically and thermally couple thesubstrate55 on thechuck100. In a typically high density plasma, the driving point RF bias impedance presented by the plasma is very low. To achieve uniform ion flux energy to thesubstrate55 it is necessary to uniformly couple RF energy from the plasma through thesubstrate55 to provide a constant plasma sheath voltage across the surface of thesubstrate55. Nonuniform plasma sheath voltages result in different processing rates or attributes across the substrate surface. The uniformity of the plasma sheath voltage is a function of the impedance/area of the plasma sheath, thesubstrate55, the gap between thesubstrate55 and thechuck100, and thechuck100. A nonuniform impedance or rough surface on thechuck100 creates uneven impedances between the chuck and the substrate, resulting in nonuniform plasma sheath voltage. Thus, it is desirable for thechuck100 to have a substantially flat and planardielectric member115 to provide uniform impedance in the gap between thedielectric member115 and the substrate. Besides providing strong electrical coupling, a flat and smooth dielectric member also provides strong thermal coupling and good heat transfer properties from thesubstrate55 to thechuck100. Thus conventional diamond grinding wheels are used to grind thesemiconducting dielectric member115 to a surface roughness of about 0.007±0.001 mm, which is typically less than about 30 rms.
• Alternatively, the[0109]dielectric member115 can comprise a layer of dielectric material formed directly on theelectrode110 orelectrode110 using thermal spraying methods, such as for example, plasma glow discharge spraying, flame spraying, electric wire melting, electric-arc melting, and detonation gun techniques, as described below. Prior to use of the thermal spraying methods, the upper surface of theelectrode110 or base105 (which to avoid repetition are collectively referred to herein as electrode110), that is typically made of a conductive metal such as aluminum or copper, is abraded by grit blasting to provide a roughened surface that enhances adhesion of thedielectric member115. In the grit blasting process, the surface of theelectrode110 is blasted at a predetermined grit spray incidence angle. Furthermore, by rotating the base during blasting, microscopic grooves are formed which undercut the aluminum surface to provide mechanical interlocking of a dielectric member that is subsequently formed on the grooved and undercut surface. In this process, theelectrode110 is fixed to a rotating turntable that rotates theelectrode110 around a centerline. The grit is blasted onto the surface of theelectrode110 using a nozzle oriented at an angle to the surface of the base. The nozzle travels from the outer edge to the center of the base at a variable speed to maintain the depth and the pitch of the grit blasted grooves. Typically, the rate of nozzle travel increases as the nozzle moves from the outer edge toward the center. For example, analuminum electrode110 was fixed to a turntable which rotated at about 20 to 30 revolutions per minute (rpm), and the angle of incidence of the nozzle relative to surface of thealuminum electrode110 was about 70°. A grit of particle size of about 60 to 80 mesh, was grit blasted using a paint removal type nozzle, onto the base. The height of the grit blasted grooves was about 0.025 mm (0.001 inch), and their pitch was about 0.073 mm (0.003 inch).
• After preparation of the surface of the[0110]electrode110, a coating of semiconducting material is formed on theelectrode110. Preferably, a thermal spraying process is used to apply the selected ceramic formulation. For example, an alumina-titania composition is sintered to form a homogeneous frit, and ground to form a fine particle sized ceramic powder having an average particle size ranging from about 10 to about 100 μm. The spraying process partially melts and energetically impacts the ceramic powder onto theelectrode110. Typically, theelectrode110 is maintained at a temperature of about 60° C. to about 80° C., and the ceramic powder is thermally sprayed at an angle of about 80° to 90° (nearly perpendicular) to the surface of the base. The thermally sprayed coatings can bounce-off the surface, so it is important to apply the coating at a proper angle to the base to reduce microcracking and provide dense layers. The high kinetic energy of the molten fine ceramic particles provide a dense, low porosity, dielectric member having the desired semiconducting properties and low resistivity. Thesemiconducting dielectric member115 should be sufficiently dense to completely electrically insulate theelectrode110 of thechuck100. A low or zero electrical resistance at any point in thesemiconducting dielectric member115 can result in an electrical short. Low electrical resistance can occur when thesemiconducting dielectric member115 is damaged during spraying, i.e., by large macroscopic cracks; or if the dielectric coating is too porous and allows plasma to permeate through the pores and electrically short thedielectric member115. After cooling, the peel strength of the thermally sprayed alumina/titania was tested using ASTM methods and found to have improved by about 20% over that obtained using other coating methods.
• Different thermal spray methods of forming the[0111]semiconducting layer115 will now be described. Referring to FIG. 16, a plasma glow discharge spraying process uses aplasma gun240 consists of a cone-shapedcathode242 inside acylindrical anode244 which forms a nozzle. An ionizable inert gas, typically argon, argon/hydrogen, or argon/helium, is flowed through the plasma zone between the electrically biased anode and cathode where it is ionized to form a plasma. Ceramic powders injected into the plasma zone are accelerated and melted by the high temperature plasma. Molten droplets are propelled onto theelectrode110, where they solidify and accumulate to form a thick, well-bonded, and dense semiconductingdielectric member115. The process has sufficient thermal energy to completely melt high temperature ceramic materials, such as alumina and/or titania.
• In the flame spraying method, a highly combustible mixture of acetylene and oxygen is used to melt a sprayed ceramic powder sprayed through the flame. In this method, a high temperature flame is produced using a combustible mixture of gases, for example acetylene and oxygen, as shown in FIG. 17. A typical[0112]flame spraying gun250 comprises afuel supply252 and anoxygen supply254. The oxygen enriched fuel mixture is ignited by a sparking means, such as aspark plug256. The resultant high velocity ignited gas melts the ceramic particles injected through thenozzle258 and the molten particles impinge on theelectrode110. The flame spraying method provides a relatively low heat or energy input to the ceramic powder. The low kinetic energy ceramic particles travel relatively slowly from the flame to the surface of theelectrode110 allowing the particle to cool during travel. As a result of the cooling, and the relatively low kinetic energy impact on theelectrode110 the solidified plasma-deactivating material comprises spherical ceramic particles that retain their shape, providing pores and tortuous pathways between the particles that provide a high surface area.
• Another method comprises a detonation gun technique (not shown). In this method, a rapidly expanding mixture of ignited gases imparts a high kinetic energy to powdered ceramic material that provides a dense coating on impact with the[0113]electrode110. In the detonation gun, a series of detonation explosions are used to provide extremely high energy molten ceramic particles that impact theelectrode110 to form a very dense ceramic material having novel electrical properties. The high velocity detonation melts and expels the ceramic particles from a gun like nozzle directed toward theelectrode110. Typically, the hot expanded gases comprise a velocity of about 600 m/sec (2000 ft/sec) to about 900 m/sec (3000 ft/sec), and a succession of such detonations provide the resultant coating thickness on the substrate.
• Preferably, the[0114]dielectric member115 is formed by an electric arc melting method, as shown in FIGS. 18 and 19. A typical electric arc melter comprises a circular ring-shapedcathode262 with ahole264 therethrough, and a needle-shaped anode266 centered within the cathode (as shown in FIG. 18) or adjacent to the cathode (not shown). The fine ceramic powder from asource268 is sprayed around the anode using carrier gas from acarrier gas supply270, at a feeding rate of about 2 to about 10 gm/min. The powdered ceramic material is transported by a carrier gas through thechannels272 on either side of the needle-shaped anode266 and is directed through theopening264 having a diameter of about 1 to 10 mm. Anelectric arc274 is formed by applying a voltage V sufficiently high to substantially entirely melt the ceramic powder being sprayed into the arc. The ceramic powder melts in the high temperatureelectric arc274 and highly energetically impinges on theelectrode110. Also, important in the electric arc melting process is the distance d between the ring-shapedcathode262, the anode nozzle266, and thesubstrate55, commonly referred to as the spray distance. The distance d between the arcing electrodes and the chuck electrode is selected so that the ceramic powder impinges on the chuck electrode in a substantially molten state.
• The carrier gas that is used to transport the ceramic powder can be an inert gas, a reducing gas, or an oxidizing gas. A reducing gas can increase formation of non-stoichiometric transition metal compounds in the alumina to reduce the resistivity of the ceramic while retaining its mechanical properties. Also, oxidizing gasses are generally undesirable because they cause excessive oxidation of the alumina resulting in high resistivity[0115]dielectric members115. Preferably, the carrier gas comprises a non-reactive gas, such as an inert gas, for example, argon, helium, or xenon. Most preferably, argon gas is used to transport the ceramic particles at a flow rate of about 20 to 100 l/min.
• The ceramic powder sprayed into the[0116]electric arc274 melts while passing through the highly energetic and extremely hotelectric arc274 to form molten droplets that impinge on theelectrode110. The energized molten grains impinge on the base and rapidly solidify due to conduction and convection cooling at the incident surface. The in-flight convection cooling of the molten droplets is minimized by the high kinetic energy imparted to the molten droplets by the electric arc. This restricts grain growth and improves homogeneity by reducing segregation of impurities. Although the mechanism is not understood, it was discovered that the electric arc melting process provided flattened ceramic grains (schematically illustrated in FIG. 19), small grain sizes, and grain boundary compositions that give rise to entirely different electrical and thermal properties, such as the controlled electrical resistivity desired in the semiconducting layer. Of primary importance is the droplet velocity and temperature, which are controlled by the ratio of the kinetic energy to heat input provided by the electric arc melting process to the ceramic powder traveling through the arc. The high kinetic energy and heat input provided to the ceramic particles by the electric arc melting process results in a high speed “splatting” of molten particles on the surface of theelectrode110 causing spreading of the particles, rapid cooling from 500-600° C. to room temperature, and solidification in about 15-20 microseconds. This provides a dense coating with the required distribution of conductive titania species in the alumina composition. The electric arc melting methods provided highly dense Al₂O_{3 /TiO2}compositions having resistivities of from 1 to 5×10¹¹Ω-cm. Scanning electron microscope (SEM) photomicrographs showed dense coatings with homogeneously dispersed porosity of less than about 10%, and often less than about 5%.
• The thermally sprayed ceramic coatings form[0117]submicron microcracks276 upon cooling and solidification that permit thedielectric member115 to expand or stretch to conform with the differential thermal expansion between thedielectric member115,electrode110, and/orelectrical isolators200, without forming large-sized cracks or delaminating from theunderlying electrode110. Large cracks allow plasma to enter through themicrocracks276 thereby damaging theelectrode110 anddielectric member115. However, small microcracking is desirable as long as the cracks are submicron sized, relatively uniformly distributed, and formed along inhomogeneous grains and grain boundaries without propagating through the entire thickness of thedielectric member115. Such controlled microcracking prevents delamination and cracking-off of thedielectric member115 from the thermal expansion stresses at high process temperatures. For example, microcracking prevents aluminum oxide containing dielectric member115 (which has a thermal expansion close to that of pure alumina of about 4.3×10⁻⁶in/in/° F.) from delaminating and separating from the underlying aluminum electrode110 (which has a much higher thermal expansion of about 13×10⁻⁶in/in/° F.).
• Although the present invention has been described in considerable detail with regard to the preferred version thereof, other versions are possible. For example, the[0118]semiconducting dielectric member115 can be used in other applications, and can be fabricated from equivalent compositions that provide quick chucking and dechucking response times. Also, theelectrical isolator200 can be fabricated in many other shapes and forms that are equivalent in function to the illustrative structures herein. Therefore, the appended claims should not be limited to the description of the preferred versions contained herein.

Claims (25)

What is claimed is:

1. An electrostatic chuck comprising:

(a) a dielectric member comprising:

(i) a first layer comprising a semiconductive material; and

(ii) a second layer over the first layer, the second layer comprising an insulative material; and

(b) an electrode in the dielectric member.

2. An electrostatic chuck according toclaim 1 wherein the first layer comprises a resistivity of from about 5×10⁹Ωcm to about 8×10¹⁰Ωcm.

3. An electrostatic chuck according toclaim 1 wherein the second layer comprises a resistivity of at least about 1×10¹¹Ωcm.

4. An electrostatic chuck according toclaim 1 wherein the second layer comprises a resistivity of from about 1×10¹¹to about 1×10²⁰Ωcm.

5. An electrostatic chuck according toclaim 1 wherein the first layer comprises Al₂O₃.

6. An electrostatic chuck according toclaim 1 wherein the first layer comprises TiO₂.

7. An electrostatic chuck according toclaim 1 wherein the first layer comprises AlN.

8. An electrostatic chuck according toclaim 1 wherein the electrode is embedded in the first layer of the dielectric member.

9. An electrostatic chuck according toclaim 1 wherein the second layer comprises AlN.

10. An electrostatic chuck according toclaim 1 wherein the second layer comprises SiO₂or ZrO₂.

11. An electrostatic chuck according toclaim 1 wherein the second layer comprises polyimide or Teflon®.

12. An electrostatic chuck according toclaim 1 wherein the dielectric member is fabricated by sintering ceramic powders.

13. An electrostatic chuck comprising:

(a) a dielectric member comprising:

(i) a first layer comprising a resistivity of from about 5×10⁹Ωcm to about 8×10¹⁰Ωcm; and

(ii) a second layer over the first layer, the second layer comprising a resistivity of from about 1×10¹¹to about 1×10²⁰Ωcm; and

(b) an electrode in the dielectric member.

14. An electrostatic chuck according toclaim 13 wherein the first layer comprises Al₂O₃.

15. An electrostatic chuck according toclaim 13 wherein the first layer comprises TiO₂.

16. An electrostatic chuck according toclaim 13 wherein the electrode is embedded in the first layer of the dielectric member.

17. An electrostatic chuck according toclaim 13 wherein the second layer comprises SiO₂.

18. An electrostatic chuck according toclaim 13 wherein the second layer comprises ZrO₂.

19. An electrostatic chuck comprising:

(a) a dielectric member comprising:

(i) a first semiconductive layer having a resistivity that is sufficiently low to provide (i) a charging time of less than about 3 seconds, and (ii) allow accumulated electrostatic charge to substantially dissipate in less than about 1 second; and

(ii) a second insulative layer over the first semiconductive layer, the second insulative layer having a resistivity higher than the first semiconductive layer; and

(b) an electrode in the dielectric member.

20. An electrostatic chuck according toclaim 19 wherein the first semiconductive layer comprises a resistivity of from about 5×10⁹Ωcm to about 8×10¹⁰Ωcm.

21. An electrostatic chuck according toclaim 19 wherein the second insulative layer comprises a resistivity of from about 1×10¹¹to about 1×10²⁰Ωcm.

22. An electrostatic chuck according toclaim 19 wherein the first semiconductive layer comprises Al₂O₃.

23. An electrostatic chuck according toclaim 19 wherein the electrode is embedded in the first semiconductive layer of the dielectric member.

24. An electrostatic chuck according toclaim 19 wherein the second insulative layer comprises SiO₂.

25. An electrostatic chuck according toclaim 19 wherein the second insulative layer comprises ZrO₂.

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