US20050178654A1 - High deposition rate sputtering - Google Patents

Abstract

Methods and apparatus for high-deposition sputtering are described. A sputtering source includes an anode and a cathode assembly that is positioned adjacent to the anode. The cathode assembly includes a sputtering target. An ionization source generates a weakly-ionized plasma proximate to the anode and the cathode assembly. A power supply produces an electric field between the anode and the cathode assembly that creates a strongly-ionized plasma from the weakly-ionized plasma. The strongly-ionized plasma includes a first plurality of ions that impact the sputtering target to generate sufficient thermal energy in the sputtering target to cause a sputtering yield of the sputtering target to be non-linearly related to a temperature of the sputtering target.

Description

BACKGROUND OF INVENTION
• Sputtering is a well-known technique for depositing films on substrates. Sputtering is the physical ejection of atoms from a target surface and is sometimes referred to as physical vapor deposition (PVD). Ions, such as argon ions, are generated and then directed to a target surface where the ions physically sputter target material atoms. The target material atoms ballistically flow to a substrate where they deposit as a film of target material.
• Diode sputtering systems include a target and an anode. Sputtering is achieved in a diode sputtering system by establishing an electrical discharge in a gas between two parallel-plate electrodes inside a chamber. A potential of several kilovolts is typically applied between planar electrodes in an inert gas atmosphere (e.g., argon) at pressures that are between about 10⁻¹and 10⁻²Torr. A plasma discharge is then formed. The plasma discharge is separated from each electrode by what is referred to as the dark space.
• The plasma discharge has a relatively constant positive potential with respect to the target. Ions are drawn out of the plasma, and are accelerated across the cathode dark space. The target has a lower potential than the region in which the plasma is formed. Therefore, the target attracts positive ions. Positive ions move towards the target with a high velocity. Positive ions then impact the target and cause atoms to physically dislodge or sputter from the target. The sputtered atoms then propagate to a substrate where they deposit a film of sputtered target material. The plasma is replenished by electron-ion pairs formed by the collision of neutral molecules with secondary electrons generated at the target surface.
• Magnetron sputtering systems use magnetic fields that are shaped to trap and to concentrate secondary electrons, which are produced by ion bombardment of the target surface. The plasma discharge generated by a magnetron sputtering system is located proximate to the surface of the target and has a high density of electrons. The high density of electrons causes ionization of the sputtering gas in a region that is close to the target surface.
• One type of magnetron sputtering system is a planar magnetron sputtering system. Planar magnetron sputtering systems are similar in configuration to diode sputtering systems. However, the magnets (permanent or electromagnets) in planar magnetron sputtering systems are placed behind the cathode. The magnetic field lines generated by the magnets enter and leave the target cathode substantially normal to the cathode surface. Electrons are trapped in the-electric and magnetic fields. The trapped electrons enhance the efficiency of the discharge and reduce the energy dissipated by electrons arriving at the substrate.
• Conventional magnetron sputtering systems deposit films that have relatively low uniformity. The film uniformity can be increased by mechanically moving the substrate and/or the magnetron. However, such systems are relatively complex and expensive to implement. Conventional magnetron sputtering systems also have relatively poor target utilization. The term “target utilization” is defied herein to be a metric of how uniform the target material erodes during sputtering. For example, high target utilization would indicate that the target material erodes in a highly uniform manner.
• In addition, conventional magnetron sputtering systems have a relatively low deposition rate. The term “deposition rate” is defined herein to mean the amount of material deposited on the substrate per unit of time. In general, the deposition rate is proportional to the sputtering yield. The term “sputtering yield” is defined herein to mean the number of target atoms ejected from the target per incident particle. Thus, increasing the sputtering yield will increase the deposition rate.
BRIEF DESCRIPTION OF DRAWINGS
• This invention is described with particularity in the detailed description. The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals Indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
• FIG. 1 illustrates a cross-sectional view of a known magnetron sputtering apparatus having a pulsed power source.
• FIG. 2 illustrates a cross-sectional view of a prior art cathode assembly having a cathode cooling system.
• FIG. 3 illustrates a known process for sputtering material from a target.
• FIG. 4 illustrates a cross-sectional view of an embodiment of a magnetron sputtering apparatus according to the present invention.
• FIG. 5A throughFIG. 5D illustrate cross-sectional views of the magnetron sputtering apparatus ofFIG. 4.
• FIG. 6 illustrates graphical representations of the applied voltage, current, and power as a function of time for periodic pulses applied to the plasma in the magnetron sputtering apparatus ofFIG. 4.
• FIG. 7A throughFIG. 7D illustrate various simulated magnetic field distributions proximate to the cathode assembly for various electron ExB drift currents in a magnetically enhanced plasma sputtering apparatus according to the invention.
• FIG. 8 illustrates a graphical representation of sputtering yield as a function of temperature of the sputtering target.
• FIG. 9 illustrates a process for sputtering material from a target according one embodiment of the present invention.
• FIG. 10 illustrates a cross-sectional view of a cathode assembly according to one embodiment of the invention.
• FIG. 11 is a flowchart of an illustrative process of enhancing a sputtering yield of a sputtering target according to the present invention.
DETAILED DESCRIPTION
• The sputtering process can be quantified in terms of the sputtering yield. The term “sputtering yield” is defined herein to mean the number of target atoms ejected from the target per incident particle. The sputtering yield depends on several factors, such as the target species, bombarding species, energy of the bombarding ions, and the angle of incidence of the bombarding ions. In typical known sputtering processes, the sputtering yield is generally insensitive to target temperature.
• The deposition rate of a sputtering process is generally proportional to the sputtering yield. Thus, increasing the sputtering yield typically will increase the deposition rate. One way to increase the sputtering yield is to increase the ion density of the plasma so that a larger ion flux impacts the surface of the target. The density of the plasma is generally proportional to the number of ionizing collisions in the plasma.
• Magnetic fields can be used to confine electrons in the plasma to increase the number of ionizing collisions between electrons and neutral atoms in the plasma. The magnetic and electric fields in magnetron sputtering systems are concentrated in narrow regions close to the surface of the target. These narrow regions are located between the poles of the magnets used for producing the magnetic field. Most of the ionization of the sputtering gas occurs in these localized regions. The location of the ionization regions causes non-uniform erosion or wear of the target that results in poor target utilization.
• Increasing the power applied between the target and the anode can increase the production of ionized gas and, therefore, increase the target utilization and the sputtering yield. However, increasing the applied power can lead to undesirable target heating and target damage. Furthermore, increasing the voltage applied between the target and the anode increases the probability of establishing an undesirable electrical discharge (an electrical arc) in the process chamber. An undesirable electrical discharge can corrupt the sputtering process.
• Pulsing the power applied to the plasma can be advantageous since the average discharge power can remain low while relatively large power pulses are periodically applied. Additionally, the duration of these large voltage pulses can be preset so as to reduce the probability of establishing an electrical breakdown condition leading to an undesirable electrical discharge. However, very large power pulses can still result in undesirable electrical discharges and undesirable target heating regardless of their duration.
• FIG. 1 illustrates a cross-sectional view of a knownmagnetron sputtering apparatus100 having apulsed power source102. The knownmagnetron sputtering apparatus100 includes avacuum chamber104 where the sputtering process is performed. Thevacuum chamber104 is positioned in fluid communication with avacuum pump106 via aconduit108. Thevacuum pump106 is adapted to evacuate thevacuum chamber104 to high vacuum. The pressure inside thevacuum chamber104 is generally less than 100 Pa during operation. Afeed gas source109, such as an argon gas source, is coupled to thevacuum chamber104 by agas inlet110. Avalve112 controls the gas flow from thefeed gas source109.
• Themagnetron sputtering apparatus100 also includes acathode assembly114 having atarget116. Thecathode assembly114 is generally in the shape of a circular disk. Thecathode assembly114 is electrically connected to afirst output118 of thepulsed power supply102 with anelectrical transmission line120. Thecathode assembly114 is typically coupled to the negative potential of thepulsed power supply102. In order to isolate thecathode assembly114 from thevacuum chamber104, aninsulator122 can be used to pass theelectrical transmission line120 through a wall of thevacuum chamber104. A groundedshield124 can be positioned behind thecathode assembly114 to protect amagnet126 from bombarding ions. Themagnet126 shown inFIG. 1 is generally shaped in the form of a ring that has itssouth pole127 on the inside of the ring and itsnorth pole128 on the outside of the ring. Many other magnet configurations can also be used.
• Ananode130 is positioned in thevacuum chamber104 proximate to thecathode assembly114. Theanode130 is typically coupled to ground. Asecond output132 of thepulsed power supply102 is also typically coupled to ground. Asubstrate134 is positioned in thevacuum chamber104 on asubstrate support135 to receive the sputtered target material from thetarget116. Thesubstrate134 can be electrically connected to a biasvoltage power supply136 with atransmission line138. In order to isolate the biasvoltage power supply136 from thevacuum chamber104, aninsulator140 can be used to pass theelectrical transmission line138 through a wall of thevacuum chamber104.
• In operation, thepulsed power supply102 applies a voltage pulse between thecathode assembly114 and theanode130 that has a sufficient amplitude to ionize the argon feed gas in thevacuum chamber104. The typical ionization process is referred to as direct ionization or atomic ionization by electron impact and can be described as follows:
  Ar+e⁻→Ar⁺+2e⁻
  where Ar represents a neutral argon atom in the feed gas and e⁻ represents an ionizing electron generated in response to the voltage pulse applied between thecathode assembly114 and theanode130. The collision between the neutral argon atom and the ionizing electron results in an argon ion (Ar⁺) and two electrons.
• The negativelybiased cathode assembly114 attracts positively charged ions with sufficient acceleration so that the ions sputter the target material from thetarget116. A portion of the sputtered target material is deposited on thesubstrate134.
• The electrons, which cause the ionization, are generally confined by the magnetic fields produced by themagnet126. The magnetic confinement is strongest in aconfinement region142 where there is relatively low magnetic field intensity. Theconfinement region142 is substantially in the shape of a ring that is located proximate to the target material. Generally, a higher concentration of positively charged ions in the plasma is present in theconfinement region142 than elsewhere in thechamber104. Consequently, thetarget116 is eroded more rapidly in areas directly adjacent to the higher concentration of positively charged ions. The rapid erosion in these areas results in undesirable non-uniform erosion of thetarget116 and, thus relatively poor target utilization.
• Dramatically increasing the power applied to the plasma can result in more uniform erosion of thetarget116 and higher sputtering yield. However, the amount of applied power necessary to achieve this increased uniformity can increase the probability of generating an electrical breakdown condition that leads to an undesirable electrical discharge between thecathode assembly114 and theanode130 regardless of the duration of the pulses. An undesirable electrical discharge will corrupt the sputtering process and cause contamination in thevacuum chamber104. Additionally, the increased power can overheat the target and cause target damage.
• Sputtering yields are generally determined experimentally. The yield dependence on the bombarding ion energy approximately exhibits a threshold that is between about 10-30 eV, followed by a nearly linear range that extends to several hundred eV. At higher energies, the dependence is less than linear. Sputtering processes are generally most energy efficient when the ion energies are within the linear range.
• Sputtering systems are generally calibrated to determine the deposition rate under certain operating conditions. The erosion rate of thetarget116 can be expressed by the following equation:$R=k\frac{\mathrm{JYM}}{\rho }\stackrel{.}{A}/\min$
  where k is a constant, J is the ion current density in mA/cm², Y is the sputtering yield in atoms/ion, and M is the atomic weight in grams, and ρ is the density in gm/cm³of the target material. The deposition rate is generally proportional to the sputtering yield Y.
• FIG. 2 illustrates a cross-sectional view of a priorart cathode assembly114′ having a cathode cooling system. Thecathode assembly114′ includestarget116′. The cathode cooling system also includes aconduit150 that contains afluid152 for conducting heat away from thecathode assembly114′. The fluid152 can be a liquid coolant or a gas, for example.
• In operation,ions154 in a plasma impact asurface156 of thetarget116′. The impact of theions154 generatesheat158 at thesurface156. Additionally, the impact of theions154 eventually dislodgesatoms160 from thesurface156 of thetarget116′ causing sputtering. Theheat158 that is generated by the ion impact radiates through thecathode assembly114′. Thecathode assembly114′ is in thermal communication with theconduit150. The fluid152 absorbs theheat158 and transfers it away from thecathode assembly114′.
• FIG. 3 illustrates a known process for sputtering material from atarget116′. Anion154 having a mass M_iand a velocity v_iimpacts atarget particle162 having a mass M_twhich is initially at rest on thesurface156 of thetarget116′. Theion154 impacts thesurface156 at normal incidence. The momentum from theion154 transfers to thetarget particle162 driving thetarget particle162 into thetarget116′.
• Thus, the ejection of a sputteredparticle164 from thetarget114′ generally requires a sequence of collisions for a component of the initial momentum vector to change by more than ninety degrees. Typically, anincident ion154 experiences a cascade of collisions and its energy is partitioned over a region of thetarget surface156. However, the sputtering momentum exchange occurs primarily within a region extending only about ten angstroms below thesurface156. Theincident ion154 generally strikes twolattice atoms166,168 almost simultaneously. This low energy knock-on receives a side component of momentum and initiates sputtering of one or more of its neighbors. The primary knock-on is driven into thetarget114′, where it can be reflected and sometimes returned to thesurface156 to produce sputtering by impacting the rear of asurface atom170.
• A fraction of the kinetic energy of theincident ion154 is transferred to thetarget particle162. This kinetic energy transfer function can be expressed as follows:$\varepsilon =\frac{4M_i{M}_t}{{\left(M_i+M_t\right)}^2}$
• The sputtering yield Y can be expressed as follows, assuming perpendicular ion incidence onto a substantially planar surface156:$Y=k\times \varepsilon \times \frac{E}{U}\times \alpha \left(M_t/M_i\right)$
  where k is a constant, ε is the energy transfer function, α is a near-linear function of the ratio of the mass of thetarget atom162 to the mass of theincident ion154, E is the kinetic energy of theincident ion154, and U is the surface binding energy for the target material. For most sputtering materials, the mass dependence of ε α does not vary greatly from one material to another. The primary material-sensitive factor is the surface binding energy, and this has only a first power dependence.
• At energies above 20-30 eV, heavy particles can sputter atoms from a surface of a target. The sputtering yield increases rapidly with energy up to a few hundred eV, with 500-1000 eV argon ions being typically used for physical sputtering. Above a few hundred eV, there is a possibility thations154 will be implanted in thetarget116′. This can especially occur at energies over 1 keV.
• FIG. 4 illustrates a cross-sectional view of an embodiment of amagnetron sputtering apparatus200 according to the present invention. Themagnetron sputtering apparatus200 includes achamber202, such as a vacuum chamber. Thechamber202 is coupled in fluid communication to avacuum system204 through avacuum valve206. In one embodiment, thechamber202 is electrically coupled to ground potential.
• Thechamber202 is coupled to afeed gas source208 by one ormore gas lines207. In one embodiment, thegas lines207 are isolated from the chamber and other components byinsulators209. Additionally, thegas lines207 can be isolated from the feed gas source using in-line insulating couplers (not shown). A gasflow control system210 controls the gas flow to thechamber202. Thegas source208 can contain any feed gas. For example, the feed gas can be a noble gas or a mixture of noble gases. The feed gas can also be a reactive gas, a non-reactive gas, or a mixture of both reactive and non-reactive gases.
• Asubstrate211 to be sputter coated is supported in thechamber202 by asubstrate support212. Thesubstrate211 can be any type of work piece such as a semiconductor wafer. In one embodiment, thesubstrate support212 is electrically coupled to anoutput213 of abias voltage source214. Aninsulator215 isolates thebias voltage source214 from a wall of thechamber202. In one embodiment, thebias voltage source214 is an alternating current (AC) power source, such as a radio frequency (RF) power source. In other embodiments (not shown), thesubstrate support212 is coupled to ground potential or is electrically floating.
• Themagnetron sputtering apparatus200 also includes acathode assembly216. In one embodiment, thecathode assembly216 includes acathode218 and asputtering target220 composed of target material. In one embodiment, thecathode218 is formed of a metal. In one embodiment, thecathode218 is formed of a chemically inert material, such as stainless steel. Thesputtering target220 is in physical contact with thecathode218. In one embodiment, thesputtering target220 is positioned inside thecathode218 as shown inFIG. 4. The distance from thesputtering target220 to thesubstrate211 can vary from a few centimeters to about one hundred centimeters.
• The target material can be any material suitable for sputtering. For example, the target material can be a metallic material, polymer material, superconductive material, magnetic material including ferromagnetic material, non-magnetic material, conductive material, non-conductive material, composite material, reactive material, or a refractory material.
• Thecathode assembly216 is coupled to anoutput222 of amatching unit224. Aninsulator226 isolates thecathode assembly216 from a grounded wall of thechamber202. Aninput230 of thematching unit224 is coupled to afirst output232 of apulsed power supply234. Asecond output236 of thepulsed power supply234 is coupled to ananode238. Aninsulator240 isolates theanode238 from a grounded wall of thechamber202. Anotherinsulator242 isolates theanode238 from thecathode assembly216.
• In one embodiment (not shown), thefirst output232 of thepulsed power supply234 is directly coupled to thecathode assembly216. In one embodiment (not shown), thesecond output236 of thepulsed power supply234 and theanode238 are both coupled to ground. In one embodiment (not shown), thefirst output232 of thepulsed power supply234 couples a negative voltage impulse to thecathode assembly216. In another embodiment (not shown), thesecond output236 of thepulsed power supply234 couples a positive voltage impulse to theanode238.
• In one embodiment, thepulsed power supply234 generates peak voltage levels of between about 5 kV and about 30 kV. In one embodiment, operating voltages are generally between about 50V and 1 kV. In one embodiment, thepulsed power supply234 sustains discharge current levels that are on order of about 1 A to 5,000 A depending on the volume of the plasma. Typical operating currents varying from less than about one hundred amperes to more than a few thousand amperes depending on the volume of the plasma. In one embodiment, the power pulses have a repetition rate that is below 1 kHz. In one embodiment, the pulse width of the pulses generated by thepulsed power supply234 is substantially between about one microsecond and several seconds.
• Theanode238 is positioned so as to form agap244 between theanode238 and thecathode assembly216 that is sufficient to allow current to flow through aregion245 between theanode238 and thecathode assembly216. In one embodiment, thegap244 is between approximately 0.3 cm and 10 cm. The surface area of thecathode assembly216 determines the volume of theregion245. Thegap244 and the total volume of theregion245 are parameters in the ionization process as described herein.
• Ananode shield248 is positioned adjacent to theanode238 and functions as an electric shield to electrically isolate theanode238 from the plasma. In one embodiment, theanode shield248 is coupled to ground potential. Aninsulator250 is positioned to isolate theanode shield248 from theanode238.
• Themagnetron sputtering apparatus200 also includes amagnet assembly252. In one embodiment, themagnet assembly252 is adapted to create amagnetic field254 proximate to thecathode assembly216. Themagnet assembly252 can includepermanent magnets256, or alternatively, electro-magnets (not shown). The configuration of themagnet assembly252 can be varied depending on the desired shape and strength of themagnetic field254. The magnet assembly can have either a balanced or unbalanced configuration.
• In one embodiment, themagnet assembly252 includes switching electro-magnets, which generate a pulsed magnetic field proximate to thecathode assembly216. In some embodiments, additional magnet assemblies (not shown) can be placed at various locations around and throughout thechamber202 to direct different types of sputtered target materials to thesubstrate212.
• In one embodiment, themagnetron sputtering apparatus200 is operated by generating themagnetic field254 proximate to thecathode assembly216. In the embodiment shown inFIG. 2, thepermanent magnets256 continuously generate themagnetic field254. In other embodiments, electro-magnets (not shown) generate themagnetic field254 by energizing a current source that is coupled to the electro-magnets. In one embodiment, the strength of themagnetic field254 is between about fifty gauss and two thousand gauss. After themagnetic field254 is generated, the feed gas from thegas source208 is supplied to thechamber202 by the gasflow control system210.
• In one embodiment, the feed gas is supplied to thechamber202 directly between thecathode assembly216 and theanode238. Directly injecting the feed gas between thecathode assembly216 and theanode238 can increase the flow rate of the gas between thecathode assembly216 and theanode238. Increasing the flow rate of the gas allows longer duration impulses and thus, can result in the formation higher density plasmas. The flow of the feed gas is further discussed herein.
• In one embodiment, thepulsed power supply234 is a component of an ionization source that generates the weakly-ionized plasma. The pulsed power supply applies a voltage pulse between thecathode assembly216 and theanode238. In one embodiment, thepulsed power supply234 applies a negative voltage pulse to thecathode assembly216. The amplitude and shape of the voltage pulse are such that a weakly-ionized plasma is generated in theregion246 between theanode238 and thecathode assembly216.
• The weakly-ionized plasma is also referred to as a pre-ionized plasma. In one embodiment, the peak plasma density of the pre-ionized plasma is between about 10⁶and 10¹²cm⁻³for argon feed gas. In one embodiment, the pressure in the chamber varies from about 10⁻³to 10 Torr. The peak plasma density of the pre-ionized plasma depends on the properties of the specific plasma processing system.
• In one embodiment, thepulsed power supply234 generates a low power pulse having an initial voltage that is between about 100V and 5 kV with a discharge current that is between about 0.1 A and 100 A in order to generate the weakly-ionized plasma. In some embodiments the width of the pulse can be on the order of 0.1 microseconds to one hundred seconds. Specific parameters of the pulse are discussed herein in more detail.
• In one embodiment, thepulsed power supply234 applies a voltage between thecathode assembly216 and theanode238 before the feed gas is supplied between thecathode assembly216 and theanode238. In another embodiment, thepulsed power supply234 applies a voltage between thecathode assembly216 and theanode238 after the feed gas is supplied between thecathode assembly216 and theanode238.
• In one embodiment, a direct current (DC) power supply (not shown) is used to generate and maintain the weakly-ionized or pre-ionized plasma. In this embodiment, the DC power supply is adapted to generate a voltage that is large enough to ignite the pre-ionized plasma. In one embodiment, the DC power supply generates an initial voltage of several kilovolts between thecathode assembly216 and theanode238 in order to generate and maintain the pre-ionized plasma. The initial voltage between thecathode assembly216 and theanode238 creates a plasma discharge voltage that is on the order of 100V to 1000V with a discharge current that is on the order of 0.1 A to 100 A.
• The direct current required to generate and maintain the pre-ionized plasma is a function of the volume of the plasma. In addition, the current required to generate and maintain the pre-ionized plasma is a function of the strength of the magnetic field in theregion245. For example, in one embodiment, the DC power supply generates a current that is on order of 1 mA to 100 A depending on the volume of the plasma and the strength of the magnetic field in theregion245. The DC power supply can be adapted to generate and maintain an initial peak voltage between thecathode assembly216 and theanode238 before the introduction of the feed gas.
• In another embodiment, an alternating current (AC) power supply (not shown) is used to generate and maintain the weakly-ionized or pre-ionized plasma. For example, the weakly-ionized or pre-ionized plasma can be generated and maintained using electron cyclotron resonance (ECR), capacitively coupled plasma discharge (CCP), or inductively coupled plasma (ICP) discharge. AC power supplies can require less power to generate and maintain a weakly-ionized plasma than a DC power supply. In addition, the pre-ionized or weakly-ionized plasma can be generated by numerous other techniques, such as UV radiation techniques, X-ray techniques, electron beam techniques, ion beam techniques, or ionizing filament techniques. In some embodiments, the weakly-ionized plasma is formed outside of theregion245 and then diffuses into theregion245.
• Forming a weakly-ionized or pre-ionized plasma substantially eliminates the probability of establishing a breakdown condition in thechamber202 when high-power pulses are applied between thecathode assembly216 and theanode238. Uniformly distributing the weakly-ionized or pre-ionized plasma over the cathode area results in a more uniform strongly ionized plasma when a high power pulse is applied. The probability of establishing a breakdown condition is substantially eliminated because the weakly-ionized plasma has a low-level of ionization that provides electrical conductivity through the plasma. This conductivity greatly reduces or prevents the possibility of a breakdown condition when high power is applied to the plasma.
• Once the weakly-ionized plasma is formed, high-power pulses are then generated between thecathode assembly216 and theanode238. In one embodiment, thepulsed power supply234 generates the high-power pulses. The desired power level of the high-power pulse depends on several factors including the desired deposition rate, the density of the pre-ionized plasma, and the volume of the plasma, for example. In one embodiment, the power level of the high-power pulse is in the range of about 1 kW to about 10 MW. In one embodiment, the high-power pulses are rapidly applied across the weakly-ionized plasma. In one embodiment, the high-power pulses are substantially instantly applied across the weakly-ionized plasma in a substantially explosive manner. This rapid application of the high-power pulses can result in a surface layer of thetarget220 being almost instantly evaporated.
• Each of the high-power pulses are maintained for a predetermined time that, in one embodiment, is in the range of about one microsecond to about ten seconds. In one embodiment, the repetition frequency or repetition rate of the high-power pulses is in the range of between about 0.1 Hz to 1 kHz. In order to minimize undesirable substrate heating, the average power generated by thepulsed power supply234 can be less than one megawatt depending on the volume of the plasma.
• In one embodiment, the high-power pulse is applied so rapidly that a surface layer of thetarget220 is substantially vaporized and only a small quantity of heat is conducted through thecathode assembly216. In one embodiment, the thermal energy in at least one of thecathode assembly216, theanode238, and thesubstrate support212 is conducted away or dissipated by liquid or gas cooling such as helium cooling (not shown).
• The high-power pulses generate a strong electric field between thecathode assembly216 and theanode238. This strong electric field is substantially located in theregion245 across thegap244 between thecathode assembly216 and theanode238. In one embodiment, the electric field is a pulsed electric field. In another embodiment, the electric field is a quasi-static electric field. The term “quasi-static electric field” is defined herein to mean an electric field that has a characteristic time of electric field variation that is much greater than the collision time for electrons with neutral gas particles. Such a time of electric field variation can be on the order of ten seconds. The strength and the position of the strong electric field will be discussed in more detail herein.
• The high-power pulses generate a highly-ionized or a strongly-ionized plasma from the weakly-ionized plasma. For example, the discharge current that is formed from this strongly-ionized plasma can be on the order of 5 kA for a pressure that is on the order of about 100 mTorr and 10 Torr.
• Since thesputtering target220 is typically negatively biased, the positively charged ions in the strongly-ionized plasma accelerate at high velocity towards the sputteringtarget220. The accelerated ions impact the surface of thesputtering target220, causing the target material to be sputtered. The strongly-ionized plasma of the present invention results in a very high sputtering rate of the target material.
• In addition, the strongly-ionized plasma tends to diffuse homogenously in theregion246 and, therefore tends to create a more homogeneous plasma volume. The homogenous diffusion results in accelerated ions impacting the surface of thesputtering target220 in a more uniform manner than with conventional magnetron sputtering. Consequently, the surface of thesputtering target220 is eroded more evenly and, thus higher target utilization is achieved. Furthermore, since the target material is sputtered more uniformly across the surface of thesputtering target220, the uniformity and homogeneity of the material deposited on thesubstrate211 is also increased without the necessity of rotating thesubstrate211 and/or themagnet assembly252.
• In one embodiment, the high-power pulsedmagnetron sputtering system200 of the present invention generates a relatively high electron temperature plasma and a relatively high density plasma. One application for the high-power pulsedmagnetron sputtering system200 of the present invention is ionized physical vapor deposition (IPVD), which is a technique that converts neutral sputtered atoms into positive ions in order to enhance the sputtering process.
• FIG. 5A throughFIG. 5D illustrate cross-sectional views of thesputtering apparatus200 having thepulsed power supply234. For example,FIG. 5A illustrates a cross-sectional view of thesputtering apparatus200 having thepulsed power supply234 at a time before thepulsed power supply234 is activated.FIG. 5A illustrates thecathode assembly216 including thesputtering target220. Thecathode assembly216 is coupled to theoutput222 of thematching unit224. Theinput230 of thematching unit224 is coupled to thefirst output232 of thepulsed power supply234. Thesecond output236 of thepulsed power supply234 is coupled to theanode238.
• Theanode238 is positioned so as to form agap244 between theanode238 and thecathode assembly216 that is sufficient to allow current to flow through theregion245 between theanode238 and thecathode assembly216. In one embodiment, the width of thegap244 is between approximately 0.3 cm and 10 cm. The surface area of thecathode assembly216 determines the volume of theregion245. Thegap244 and the total volume of theregion245 are parameters in the ionization process as described herein. Thegas lines207 providefeed gas256 from the feed gas source208 (FIG. 4) proximate to theanode238 and thecathode assembly216.
• In operation, thefeed gas256 from thegas source208 is supplied by the gas flow control system210 (FIG. 4). Preferably, thefeed gas256 is supplied between thecathode assembly216 and theanode238. Directly injecting thefeed gas256 between thecathode assembly216 and theanode238 can increase the flow rate of thefeed gas256. This causes a rapid volume exchange in theregion245 between thecathode assembly216 and theanode238, which permits a high power pulse having a longer duration to be applied across thegap244. The longer duration high power pulse results in the formation of more dense plasma. This volume exchange is described herein in more detail.
• FIG. 5B illustrates thecathode assembly216 after thefeed gas256 is supplied between thecathode assembly216 and theanode238. Thepulsed power supply234 applies a voltage pulse between thecathode assembly216 and theanode238. In one embodiment, thepulsed power supply234 applies a negative voltage pulse to thecathode assembly216. The characteristics of the voltage pulse are chosen such that anelectric field260 develops between thecathode assembly216 and theanode238 that creates a weakly-ionizedplasma262 in theregion245 between theanode238 and thecathode assembly216. The weakly-ionizedplasma262 is also referred to as a pre-ionized plasma.
• In one embodiment, thepulsed power supply234 generates the weakly-ionizedplasma262 by generating a low power pulse having an initial voltage that is in the range of 100V to 5 kV with a discharge current that is in the range of 0.1 A to 100 A. In some embodiments, the width of the pulse can be in the range of 0.1 microseconds to one hundred seconds. Specific parameters of the pulse are discussed herein in more detail.
• In another embodiment, an alternating current (AC) power supply (not shown) is used to generate and maintain the weakly-ionized orpre-ionized plasma262. The weakly-ionized orpre-ionized plasma262 can be generated and maintained using electron cyclotron resonance (ECR), capacitively coupled plasma discharge (CCP), or inductively coupled plasma (ICP) discharge. Generating the pre-ionized plasma using an AC power supply can be more energy efficient than generating the pre-ionized plasma using a DC power supply.
• In addition, the pre-ionized or weakly-ionizedplasma262 can be generated by numerous other techniques, such as UV radiation techniques, X-ray techniques, electron beam techniques, ion beam techniques, or ionizing filament techniques. These techniques include components used in ionization sources according to the invention. In some embodiments, the weakly-ionized plasma is formed outside of theregion245 and then diffuses into theregion245.
• In one embodiment, as thefeed gas256 is pushed through theregion245, the weakly-ionizedplasma262 diffuses generally in a homogeneous manner through theregion264. The homogeneous diffusion tends to facilitate the creation of a highly uniform strongly-ionized plasma in theregion264. In one embodiment (not shown), the weakly-ionizedplasma262 is trapped proximate to thecathode assembly216 by a magnetic field. Specifically, electrons in the weakly-ionizedplasma262 are trapped by a magnetic field generated proximate to thecathode assembly216. In one embodiment, the strength of the magnetic field is in the range of fifty to two thousand gauss.
• Thepulsed power supply234 generates high-power pulses between thecathode assembly216 and the anode238 (FIG. 5C) after the weakly-ionizedplasma262 is formed. The desired power level of the high-power pulses depends on several factors including the density of the weakly-ionizedplasma262 and the volume of the plasma. In one embodiment, the power level of the high-power pulse is in the range of about 1 kW to about 10 MW or higher.
• The high-power pulses generate a strongelectric field266 between thecathode assembly216 and theanode238. The strongelectric field266 is substantially located in theregion245 between thecathode assembly216 and theanode238. In one embodiment, theelectric field266 is a pulsed electric field. In another embodiment, theelectric field266 is a quasi-static electric field. The strength and the position of the strongelectric field266 will be discussed in more detail herein.
• FIG. 5D illustrates the high-power pulses generating a highly-ionized or a strongly-ionizedplasma268 from the weakly-ionizedplasma262. The strongly-ionizedplasma268 is also referred to as a high-density plasma. The discharge current that is formed from the strongly-ionizedplasma268 can be on the order of about 1,000 A or more with a discharge voltage in the range of 50V to 1,000V for a pressure that is in the range of 5 mTorr and 10 Torr. In one embodiment, the strongly-ionizedplasma268 tends to diffuse homogenously in theregion264. The homogenous diffusion creates a more homogeneous plasma volume.
• The homogenous diffusion results in acceleratedions272 in the strongly-ionizedplasma268 impacting the surface of thesputtering target220 in a more uniform manner than with conventional magnetron sputtering. Consequently, the surface of thesputtering target220 is eroded more evenly and, thus higher target utilization is achieved. Furthermore, since the target material is sputtered more uniformly across the surface of thesputtering target220, the target material is deposited more uniformly on the substrate211 (FIG. 4) without the necessity of rotating the substrate and/or the magnetron.
• The strongelectric field266 facilitates a multi-step ionization process of thefeed gas256 that substantially increases the rate at which the strongly-ionizedplasma268 is formed. Referring again toFIG. 2D, the multi-step or stepwise ionization process is described as follows. A pre-ionizing voltage is applied between thecathode assembly216 and theanode238 across thefeed gas256, which forms the weakly-ionizedplasma262. The weakly-ionizedplasma262 is generally formed in theregion245 and diffuses to theregion264 as thefeed gas256 continues to flow.
• In one embodiment, a magnetic field254 (FIG. 4) is generated in theregion245 and extends to the center of thecathode assembly216. This magnetic field tends to assist in diffusing electrons from theregion245 to theregion264. The electrons in the weakly-ionizedplasma262 are substantially trapped in theregion264 by themagnetic field245. In one embodiment, the volume of weakly-ionizedplasma262 in theregion245 is rapidly exchanged with a fresh volume offeed gas256.
• Thepulsed power supply234 applies a high-power pulse between thecathode assembly216 and theanode238 after the formation of the weakly-ionized plasma262 (FIG. 2C). This high-power pulse generates the strongelectric field266 in theregion245 between thecathode assembly216 and theanode238. The strongelectric field266 results in collisions occurring betweenneutral atoms270, electrons (not shown), andions272 in the weakly-ionizedplasma262. These collisions generate numerousexcited atoms274 in the weakly-ionizedplasma262.
• The accumulation ofexcited atoms274 in the weakly-ionizedplasma262 alters the ionization process. In one embodiment, the strongelectric field266 facilitates a multi-step ionization process of an atomic feed gas that significantly increases the rate at which the strongly-ionizedplasma268 is formed. The multi-step ionization process has an efficiency that increases as the density ofexcited atoms274 in the weakly-ionizedplasma262 increases. In one embodiment the strongelectric field266 enhances the formation of ions of a molecular or atomic feed gas.
• In one embodiment, the distance orgap244 between thecathode assembly216 and theanode238 is chosen so as to maximize the rate of excitation of the atoms. The value of theelectric field266 in theregion245 depends on the voltage level applied by thepulsed power supply234 and the dimensions of thegap244 between theanode238 and thecathode assembly216. In some embodiments, the strength of theelectric field266 is in range of 10V/cm to 10⁵V/cm depending on various system parameters and operating conditions of the plasma system.
• In some embodiments, thegap244 is in the range of 0.30 cm to 10 cm depending on various parameters of the desired plasma. In one embodiment, theelectric field266 in theregion245 is rapidly applied to the pre-ionized or weakly-ionizedplasma262. In some embodiments, the rapidly appliedelectric field266 is generated by applying a voltage pulse having a rise time that is in the range of 0.1 microsecond to ten seconds.
• In one embodiment, the dimensions of thegap244 and the parameters of the appliedelectric field266 are varied to determine the optimum condition for a relatively high rate of excitation of theatoms270 in theregion245. For example, an argon atom requires an energy of about 11.55 eV to become excited. Thus, as thefeed gas256 flows through theregion245, the weakly-ionizedplasma262 is formed and theatoms270 in the weakly-ionizedplasma262 experience a stepwise ionization process.
• Theexcited atoms274 in the weakly-ionizedplasma262 then encounter the electrons (not shown) that are in theregion264. Theexcited atoms274 only require about 4 eV of energy to ionize whileneutral atoms270 require about 15.76 eV of energy to ionize. Therefore, theexcited atoms274 will ionize at a much higher rate than theneutral atoms270. In one embodiment,ions272 in the strongly-ionizedplasma268 strike thecathode assembly216 causing secondary electron emission. These secondary electrons interact with neutral270 orexcited atoms274 in the strongly-ionizedplasma268. This process further increases the density ofions272 in the strongly-ionizedplasma268 as thefeed gas256 is replenished.
• The multi-step ionization process corresponding to the rapid application of theelectric field266 can be described as follows:
  Ar+e⁻→Ar*+e⁻
  Ar^*+e⁻→Ar⁺+2e⁻
  where Ar represents aneutral argon atom270 in thefeed gas256 and e⁻ represents an ionizing electron generated in response to apre-ionized plasma262, when sufficient voltage is applied between thecathode assembly216 and theanode238. Additionally, Ar* represents anexcited argon atom274 in the weakly-ionizedplasma262. The collision between theexcited argon atom274 and the ionizing electron results in an argon ion (Ar⁺) and two electrons.
• Theexcited argon atoms274 generally require less energy to become ionized thanneutral argon atoms270. Thus, theexcited atoms274 tend to more rapidly ionize near the surface of thecathode assembly216 than theneutral argon atoms270. As the density of theexcited atoms274 in the plasma increases, the efficiency of the ionization process rapidly increases. This increased efficiency eventually results in an avalanche-like increase in the density of the strongly-ionizedplasma268. Under appropriate excitation conditions, the proportion of the energy applied to the weakly-ionizedplasma262 that is transformed to theexcited atoms274 is very high for a pulsed discharge in thefeed gas256.
• Thus, in one aspect of the invention, high power pulses are applied to a weakly-ionizedplasma262 across thegap244 to generate the strongelectric field266 between theanode238 and thecathode assembly216. This strongelectric field266 generatesexcited atoms274 in the weakly-ionizedplasma262. Theexcited atoms274 are rapidly ionized by secondary electrons emitted by thecathode assembly216. This rapid ionization results in a strongly-ionizedplasma268 having a large ion density being formed in thearea264 proximate to thecathode assembly216. The strongly-ionizedplasma268 is also referred to as a high-density plasma.
• In one embodiment of the invention, a higher density plasma is generated by controlling the flow of thefeed gas256 in theregion245. In this embodiment, a first volume offeed gas256 is supplied to theregion245. The first volume offeed gas256 is then ionized to form a weakly-ionizedplasma262 in theregion245. Next, thepulsed power supply234 applies a high power electrical pulse across the weakly-ionizedplasma262. The high power electrical pulse generates a strongly-ionizedplasma268 from the weakly-ionizedplasma262.
• The level and duration of the high power electrical pulse is limited by the level and duration of the power that the strongly-ionizedplasma268 can absorb before the high power discharge contracts and terminates. In one embodiment, the strength and the duration of the high-power electrical pulse are increased and thus the density of the strongly-ionizedplasma268 is increased by increasing the flow rate of thefeed gas256.
• In one embodiment, the strongly-ionizedplasma268 is transported through theregion245 by a rapid volume exchange offeed gas256. As thefeed gas256 moves through theregion245, it interacts with the moving strongly-ionizedplasma268 and also becomes strongly-ionized from the applied high-power electrical pulse. The ionization process can be a combination of direct ionization and/or stepwise ionization as described herein. Transporting the strongly-ionizedplasma268 through theregion245 by a rapid volume exchange of thefeed gas256 increases the level and the duration of the power that can be applied to the strongly-ionizedplasma268 and, thus, generates a higher density strongly-ionized plasma in theregion264.
• As previously discussed, to increase the efficiency of the ionization process, a magnetic field (not shown) can be generated proximate to thecathode assembly216. The magnetic field substantially traps electrons in the weakly-ionizedplasma262 and secondary electrons from thecathode assembly216 proximate to thecathode assembly216. The trapped electrons ionize theexcited atoms274 generating the strongly-ionizedplasma268. In one embodiment, the magnetic field is generated in theregion245 to substantially trap electrons in the area where the weakly-ionizedplasma262 is ignited.
• In one embodiment, a strongly-ionizedplasma268 according to the present invention is used to sputter magnetic materials. Conventional magnetron sputtering generally is not suitable for sputtering magnetic materials, since the magnetic field generated by the magnetron can be absorbed by the magnetic target. Traditional RF diode sputtering can be used to sputter magnetic materials. However, this method generally results in very poor uniformity of the sputtered film, relatively low plasma density, and relatively low deposition rate.
• According to the present invention, magnetic materials can be sputtering by driving a target assembly including a magnetic target material with a RF power supply (not shown). For example, the RF power supply can provide a power of up to about 10 kW. A substantially uniform weakly-ionized plasma can be generated by applying RF power across a feed gas that is located proximate to the target assembly. The strongly-ionized plasma is generated by applying a strong electric field across the weakly-ionized plasma as described herein. Since the RF power supply applies a negative voltage bias to the target assembly, ions in the strongly-ionized plasma bombard the target material causing sputtering.
• In one embodiment, a strongly-ionizedplasma268 according to the present invention is used to sputter dielectric materials. Dielectric materials are sputtered according to the present invention by driving a target assembly including a dielectric target material with a RF power supply (not shown). For example, the RF power supply can provide a power of up to about 10 kW. A substantially uniform weakly-ionized plasma can be generated by applying RF power across a feed gas that is located proximate to the target assembly.
• In one embodiment, a magnetic field can be generated proximate to the target assembly in order to trap electrons in the weakly-ionized plasma. The strongly-ionized plasma is generated by applying a strong electric field across the weakly-ionized plasma as described herein. Since the RF power supply applies a negative voltage bias to the target assembly, ions in the strongly-ionized plasma bombard the target material causing sputtering.
• In one embodiment, a DC power supply (not shown) is used to create a weakly-ionizedplasma232 for sputtering a dielectric target material according to the present invention. In this embodiment, the dielectric target material is positioned relative to thecathode218 such that an area of thecathode218 can conduct a direct current between theanode238 and thecathode218.
• FIG. 6 illustrates graphical representations320,322, and324 of the absolute value of applied voltage, current, and power, respectively, as a function of time for periodic pulses applied to the plasma in thesputtering apparatus200 ofFIG. 4. In one embodiment, at time t₀(not shown), thefeed gas256 flows proximate to thecathode assembly216 before thepulsed power supply234 is activated. The time required for a sufficient quantity offeed gas256 to flow proximate to thecathode assembly216 depends on several factors including the flow rate of thefeed gas256 and the desired pressure in theregion245.
• In the embodiment shown inFIG. 6, thepower supply234 generates a constant power. At time t₁, thepulsed power supply234 generates a voltage326 across theanode238 and thecathode assembly216. In one embodiment, the voltage326 is approximately between 100V and 5 kV. The period between time t₀and time t₁(not shown) can be on the order of several microseconds up to several milliseconds. At time t₁, the current328 and the power330 have constant values.
• Between time t₁and time t₂, the voltage326, the current328, and the power330 remain constant as the weakly-ionized plasma262 (FIG. 5B) is generated. The voltage332 at time t₂is in the range of 100V to 5 kV. The current334 at time t₂is in the range of 0.1 A to 100 A. The power336 delivered at time t₂is in the range of 0.01 kW to 100 kW.
• The power336 generated by thepulsed power supply234 partially ionizes thegas256 that is located in theregion245 between thecathode assembly216 and theanode238. The partially ionized gas is also referred to as a weakly-ionized plasma or apre-ionized plasma262. As described herein, the formation of the weakly-ionizedplasma262 substantially eliminates the possibility of creating a breakdown condition when high-power pulses are applied to the weakly-ionizedplasma262. The suppression of this breakdown condition substantially eliminates the occurrence of undesirable arcing between theanode238 and thecathode assembly216.
• In one embodiment, the period between time t₁and time t₂is in the range of one microsecond to one hundred seconds to allow thepre-ionized plasma262 to form and be maintained at a sufficient plasma density. In one embodiment, the power336 from thepulsed power supply234 is continuously applied in order to maintain the weakly-ionizedplasma262. Thepulsed power supply234 can be designed so as to output a continuous nominal power in order to sustain the weakly-ionizedplasma262.
• Between time t₂and time t₃, thepulsed power supply234 delivers a large voltage pulse338 across the weakly-ionizedplasma262. In some embodiments, the large voltage pulse338 has a voltage that is in the range of 200V to 30 kV. In some embodiments, the period between time t₂and time t₃is in the range of 0.1 microsecond to ten seconds. Between time t₃and time t₄, the large voltage pulse338 is applied before the current across the weakly-ionizedplasma262 begins to increase. In one embodiment, the period between time t₃and time t₄can be between about ten nanoseconds and one microsecond.
• Between time t₄and time t₅, the voltage340 drops as the current342 increases. The power344 also increases between time t₄and time t₅, until a quasi-stationary state exists between the voltage346 and the current348. The period between time t₄and time t₅is on order of several hundreds nanoseconds.
• In one embodiment, at time t₅, the voltage346 is in the range of 50V to 30 kV, the current348 is in the range of 10 A to 5 kA and the power350 is in the range of 1 kW to 10 MW. The power350 is continuously applied to the plasma until time t₆In one embodiment, the period between time t₅and time t₆is in the range of one microsecond to ten seconds.
• Thepulsed power supply234 delivers a high power pulse having a maximum power350 and a pulse width that is sufficient to transform the weakly-ionizedplasma262 to a strongly-ionized plasma268 (seeFIG. 2D). At time t₆, the maximum power350 is terminated. In one embodiment, thepulsed power supply234 continues to supply a background power that is sufficient to maintain the plasma after time t₆.
• In one embodiment, thepower supply234 maintains the plasma after the delivery of the high-power pulse by continuing to apply a power352 to the plasma that is in the range of 0.01 kW to 100 kW. The continuously generated power maintains the pre-ionization condition in the plasma, while thepulsed power supply234 prepares to deliver the next high-power pulse.
• At time t₇, thepulsed power supply234 delivers the next high-power pulse (not shown). In one embodiment, the repetition rate between the high-power pulses is in the range of 0.1 Hz and 1 kHz. The particular size, shape, width, and frequency of the high-power pulses depend on various factors including process parameters, the design of thepulsed power supply234, the geometry of thesputtering system200, the volume of plasma, the density of the strongly-ionizedplasma268, and the pressure in theregion245. The shape and duration of the leading edge356 and the trailing edge358 of the high-power pulse354 is chosen so as to sustain the weakly-ionizedplasma262 while controlling the rate of ionization of the strongly-ionizedplasma268.
• In another embodiment (not shown), thepower supply234 generates a constant voltage. In this embodiment, the applied voltage320 is continuously applied from time t₂until time t₆. The current322 and the power324 rise until time t₆where they maintain a constant voltage level, and then the voltage320 is terminated. In one embodiment, the values for the current, power and voltage are optimized for generating exited atoms.
• In one embodiment of the invention, the efficiency of the ionization process is increased by generating a magnetic field proximate to thecathode assembly216. The magnetic field substantially traps electrons in the weakly-ionizedplasma262 proximate to thecathode assembly216. The trapped electrons ionize theexcited atoms274 thereby generating the strongly-ionizedplasma268. In this embodiment, the magnetically enhanced plasma has strong diamagnetic properties. The term “strong diamagnetic properties” is defined herein to mean that the magnetically enhanced high-density plasma discharge tends to exclude external magnetic fields from the plasma volume.
• FIG. 7A throughFIG. 7D illustrate various simulatedmagnetic field distributions400,402,404, and406 proximate to thecathode assembly216 for various electron ExB drift currents in a magnetically enhanced plasma sputtering apparatus according to the invention. The magnetically enhanced plasma generating apparatus includes amagnet assembly252 that is positioned proximate to thecathode assembly216. Themagnet assembly252 generates a magnetic field proximate to thecathode assembly216. In one embodiment, the strength of the magnetic field is in the range of fifty to two thousand gauss. The simulatedmagnetic fields distributions400,402,404, and406 indicate that high-power plasmas having high current density tend to diffuse homogeneously in anarea246 of the magnetically enhanced plasma sputtering apparatus.
• The high-power pulses applied between thecathode assembly216 and theanode238 generate secondary electrons from thecathode assembly216 that move in a substantially circular motion proximate to thecathode assembly216 according to crossed electric and magnetic fields. The substantially circular motion of the electrons generates an electron ExB drift current. The magnitude of the electron ExB drift current is proportional to the magnitude of the discharge current in the plasma and, in one embodiment, is approximately in the range of three to ten times the magnitude of the discharge current.
• In one embodiment, the substantially circular electron ExB drift current generates a magnetic field that interacts with the magnetic field generated by themagnet assembly252. In one embodiment, the magnetic field generated by the electron ExB drift current has a direction that is substantially opposite to the magnetic field generated by themagnet assembly252. The magnitude of the magnetic field generated by the electron ExB drift current increases with increased electron ExB drift current. The homogeneous diffusion of the strongly-ionized plasma in theregion246 is caused, at least in part, by the interaction of the magnetic field generated by themagnet assembly252 and the magnetic field generated by the electron ExB drift current.
• In one embodiment, the electron ExB drift current defines a substantially circular shape for a low current density plasma. However, as the current density of the plasma increases, the substantially circular electron ExB drift current tends to describe a more complex shape as the interaction of the magnetic field generated by themagnet assembly252, the electric field generated by the high-power pulse, and the magnetic field generated by the electron ExB drift current become more acute. For example, in one embodiment, the electron ExB drift current has a substantially cycloidal shape. Thus, the exact shape of the electron ExB drift current can be complex and depends on various factors.
• For example,FIG. 7A illustrates themagnetic field lines408 produced from the interaction of the magnetic field generated by themagnet assembly252 and the magnetic field generated by an electron ExB drift current410. The electron ExB drift current410 is generated proximate to thecathode assembly216 and is illustrated by a substantially circularly shaped ring. In the example shown inFIG. 7A, the electron ExB drift current410 is approximately 100 A.
• In one embodiment of the invention, the electron ExB drift current410 is in the range of three to ten times as great as the discharge current. Thus, in the example shown inFIG. 7A, the discharge current is in the range of 10 A to 30 A. Themagnetic field lines408 shown inFIG. 7A indicate that the magnetic field generated by themagnet assembly252 is substantially undisturbed by the relatively small magnetic field that is generated by the relatively small electron ExB drift current410.
• FIG. 7B illustrates themagnetic field lines412 produced from the interaction of the magnetic field generated by themagnet assembly252 and the magnetic field generated by an electron ExB drift current414. The electron ExB drift current414 is generated proximate to thecathode assembly216. In the example shown inFIG. 7B, the electron ExB drift current414 is approximately 300 A. Since the electron ExB drift current414 is typically between about three and ten times as great as the discharge current, the discharge current in this example is in the range of 30 A and 100 A.
• Themagnetic field lines412 that are generated by themagnet assembly252 are substantially undisturbed by the relatively small magnetic field generated by the relatively small electron ExB drift current414. However, themagnetic field lines416 that are closest to the electron ExB drift current414 are somewhat distorted by the magnetic field generated by the electron ExB drift current414. The distortion suggests that a larger electron ExB drift current should generate a stronger magnetic field that will interact more strongly with the magnetic field generated by themagnet assembly252.
• FIG. 7C illustrates themagnetic field lines418 that are produced from the interaction of the magnetic field generated by themagnet assembly252 and the magnetic field generated by an electron ExB drift current420. The electron ExB drift current420 is generated proximate to thecathode assembly216. In the example shown inFIG. 7C, the electron ExB drift current420 is approximately 1,000 A. Since the electron ExB drift current420 is typically between about three and ten times as great as the discharge current, the discharge current in this example is approximately between 100 A and 300 A.
• Themagnetic field lines418 that are generated by themagnet assembly252 exhibit substantial distortion that is caused by the relatively strong magnetic field generated by the relatively large electron ExB drift current420. Thus, the larger electron ExB drift current420 generates a stronger magnetic field that strongly interacts with and can begin to dominate the magnetic field generated by themagnet assembly252.
• The interaction of the magnetic field generated by themagnet assembly252 and the magnetic field generated by the electron ExB drift current420 substantially generatesmagnetic field lines422 that are somewhat more parallel to the surface of thecathode assembly216 than themagnetic field lines408,412, and416 inFIG. 7A andFIG. 7B. Themagnetic field lines422 allow the strongly-ionizedplasma268 to more uniformly distribute itself in thearea246. Thus, the strongly-ionizedplasma268 is uniformly diffused in thearea246.
• FIG. 7D illustrates themagnetic field lines424 produced from the interaction of the magnetic field generated by themagnet assembly252 and the magnetic field generated by an electron ExB drift current426. The electron ExB drift current426 is generated proximate to thecathode assembly216. In the example shown inFIG. 7D, the electron ExB drift current426 is approximately 5 kA. The discharge current in this example is approximately between 500 A and 1,700 A.
• Themagnetic field lines424 generated by themagnet assembly252 exhibit relatively high distortion due to their interaction with the relatively strong magnetic field generated by the relatively large electron ExB drift current426. Thus, in this embodiment, the relatively large electron ExB drift current426 generates a very strong magnetic field that is substantially stronger than the magnetic field generated by themagnet assembly252.
• A large electron ExB drift current can enhance the rate of ionization of the strongly-ionized plasma by trapping secondary electrons from thecathode assembly216. The secondary electrons ionize neutral and excited atoms and molecules. A strongly-ionized plasma having a high density of ions can increase the deposition rate in a plasma sputtering process according to the invention.
• The deposition rate of a sputtering process can be expressed as follows:
  R_D=K×Y×I
  where K is a geometrical factor, Y is the sputtering yield, and I is the discharge current. Thus, the deposition rate is proportional to the sputtering yield Y. The sputtering yield Y is defined as the number of atoms sputtered per incident ion and depends on the type of ions that bombard the target surface, the energy of the ions, the incident angle of the bombarding ions, the binding energy of the target material, and the target temperature. In a typical sputtering process involving a so-called “cold cathode,” the temperature of the target is gradually heated and that heat is dissipated using liquid cooling as described with reference toFIG. 2. This typical sputtering process uses momentum and energy exchange of bombarding ions to dislodge target atoms.
• To substantially increase the deposition rate, the present invention generates a very high discharge current and a very high sputtering yield. The high-power pulse is applied almost instantaneously, causing an explosive reaction at the surface of the target. The explosive reaction rapidly vaporizes a surface layer of the target.
• FIG. 8 illustrates agraphical representation500 of sputtering yield Y as a function of temperature T of thesputtering target220 ofFIG. 4. A higher deposition rate can be achieved according to the present invention by using a thermal sputtering process. In one embodiment, the deposition rate is increased by increasing the sputtering yield. The sputtering yield of thetarget220 is increased as the temperature of thetarget220 is increased. As the temperature of thetarget220 is increased, the sputtering yield slowly increases in a substantially linear manner as shown inregion502. In theregion502, the sputtering yield generally does not depend on the temperature of thetarget220.
• When the temperature of the target reachestemperature T₀504, the sputtering yield increases at a non-linear rate. In one embodiment, the sputtering yield increases at an exponential rate. The temperature T₀is approximately equal to 0.7 T_m, where T_mis the melting point of the target material. In another embodiment, the temperature T₀is approximately equal to U/40 k, where U is the binding energy for a surface atom and k is Boltzman's constant.
• The sputtering yield at or above the target temperature T₀can be expressed as follows:$Y_T=c\frac{\tau }{\sqrt{T_o+\Delta T_M}}\exp \left(-\frac{\mathrm{<figure-callout\; label="U\; T"\; filenames="US20050178654A1-20050818-D00011.png"\; state="\{\{state\}\}">U</figure-callout>}}{T_{}}\right),\tau =\frac{{\mathrm{<figure-callout\; label="R"\; filenames="US20050178654A1-20050818-D00011.png"\; state="\{\{state\}\}">R</figure-callout>}}^2}{\kappa }{\left(\frac{{\mathrm{<figure-callout\; label="T"\; filenames="US20050178654A1-20050818-D00011.png"\; state="\{\{state\}\}">T</figure-callout>}}_0+\Delta T_M}{U}\right)}^2$
  where ΔT_Mis the maximum difference of the target temperature from the temperature T₀, R is the initial radius of the high temperature area on the target, τ is the time period for the high temperature in the high temperature area, κ is the coefficient for the temperature conductivity, and U is the binding energy.
• When high power pulses having the appropriate power level and duration are applied to the plasma according to the present invention, the sputtering yield increases non-linearly. In one embodiment, the sputtering yield increases substantially in an exponential manner. According to one embodiment of the invention, a high power pulse is applied to a weakly-ionized plasma262 (FIG. 5C) for a relatively short duration. This high power pulse creates a strongly-ionized plasma268 (FIG. 5D) that contains a high density ofenergetic ions272.
• Thus, in one embodiment, a very large quantity of explosive energy at the target surface results in a sputtering yield that increases exponentially. The explosive energy causes the temperature at the target surface to increase rapidly. This rapid increase in temperature results in a surface layer of the target being substantially evaporated and sputtered at a very high rate compared with known sputtering techniques. In one embodiment, the deposition rate of a target material is greater than one micron per minute.
• FIG. 9 illustrates aprocess510 for sputteringatoms512 from atarget220 according one embodiment of the invention. In one embodiment, thetarget220 is negatively biased. The negative bias causes theenergetic ions272 in the strongly-ionizedplasma268 to vigorously impact thesurface514 of thetarget220, thereby causing the temperature of the target to rapidly increase. When the temperature of the target reaches the temperature T₀, the sputtering yield increases nonlinearly. In one embodiment, the sputtering yield increases almost exponentially. Consequently, the deposition rate rapidly increases. The sputtering yield can depend on the properties of the target material, such as the crystal structure, the binding energy of the surface atoms, and/or the melting point of the target material.
• FIG. 10 illustrates a cross-sectional view of acathode assembly216 according to one embodiment of the invention. When the temperature of thetarget220 reaches a certain level, the target material is evaporated in an avalanche-like manner. In one embodiment, the high-power pulse generatesthermal energy516 into only a shallow depth of thetarget220 so as to not substantially increase an average temperature of thetarget220. The target material is almost instantly evaporated. However damage to thetarget220 itself is minimal because thermal energy only penetrates into the shallow skin depth.
• FIG. 11 is a flowchart600 of an illustrative process of enhancing a sputtering yield of a sputtering target according to the present invention. The process is initiated (step602) by pumping thechamber202 down to a specific pressure (step604). Next, the pressure in the chamber is evaluated (step606). In one embodiment, a feed gas is then pumped into the chamber (step608). The gas pressure is evaluated (step610). If the gas pressure is correct, the pressure in the chamber is again evaluated (step612).
• If the pressure in the chamber is correct, an appropriate magnetic field254 (FIG. 4) can be generated proximate to the feed gas256 (FIG. 5B) (step614). In one embodiment, amagnet assembly252 can include at least one permanent magnet, and thus the magnetic field is generated constantly, even before the process is initiated. In another embodiment, amagnetic assembly252 includes at least one electromagnet, and thus the magnetic field is generated only when the electromagnet is operating. In one embodiment, themagnetic field254 is then evaluated (step616).
• Thefeed gas256 is ionized to generate a weakly-ionized plasma262 (step618). In one embodiment, the weakly-ionizedplasma262 is generated by creating a relatively low current discharge in thegap244 between thecathode assembly216 and theanode238. In another embodiment (not shown), the weakly-ionizedplasma262 can be generated by creating a relatively low current discharge between an ionizing electrode and thecathode assembly216. In yet another embodiment (not shown), an electrode is heated to emit electrons proximate to thecathode assembly216. In this embodiment, a relatively low current discharge is created between theanode238 and the electrode in order to generate the weakly-ionizedplasma262. In the embodiment shown inFIG. 4, for example, the weakly-ionizedplasma262 is generated by applying a potential across thegap244 between thecathode assembly216 and theanode238 after the introduction of thefeed gas256.
• When the gas is weakly-ionized (step620), A strongly-ionized plasma268 (FIG. 5D) is generated from the weakly-ionized plasma262 (step622). In one embodiment, the strongly-ionizedplasma268 is generated by applying a high-power pulse between thecathode assembly216 and theanode238. As described herein, the high-power pulse causes a strongelectric field266 to be generated in thegap244 between theanode238 and thecathode assembly216.
• The strongelectric field266 causes the feed gas to experience stepwise ionization. In one embodiment, the feed gas includes a molecular gases and the strongelectric field266 increases the formation of ions that enhance the strongly-ionizedplasma268. In one embodiment, the strongly-ionizedplasma268 is substantially homogeneous in thearea264 ofFIG. 5D. This homogeneity results in more uniform erosion of thesputtering target220 and, therefore, relatively high target utilization. Since thecathode assembly216 is negatively biased relative to theanode238, thecathode assembly216 attracts ions from the strongly-ionized substantially uniform plasma. This causes the ions to bombard thecathode assembly216, thereby resulting in sputtering of the target material.
• In one embodiment, the strongly-ionized plasma is enhanced through the rapid exchange of the strongly-ionized plasma with a fresh volume of feed gas256 (step624). This rapid exchange occurs in theregion245. In one embodiment, the rapid exchange of the strongly-ionized plasma occurs during the duration of the high-power pulse.
• After the strongly-ionizedplasma268 is formed (step626), the sputtering yield is monitored (step628) by known monitoring techniques. If the sputtering yield is insufficient (step630), the power delivered to the plasma is increased (step632). In one embodiment, increasing the magnitude of the high-power pulse applied between thecathode assembly216 and theanode238 increases the power delivered to the plasma. In one embodiment, the power delivered to the plasma is sufficient to vaporize a surface layer of the target. This increases the sputtering yield in a substantially nonlinear fashion.
• The sputter yield is again evaluated (step628). This process continues until the sputter yield is sufficient (step630), and sputtering continues (step634). Once the sputter deposition is completed (step636), the sputter process is terminated (step638).
• Equivalents
• While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined herein.

Claims (33)

1-30. (canceled)

31. A sputtering source comprising:

a) a cathode assembly that is positioned adjacent to an anode, the cathode assembly including a sputtering target;

b) an ionization source that generates a weakly-ionized plasma from a feed gas proximate to the anode and the cathode assembly; and

c) a power supply that generates a voltage pulse between the anode and the cathode assembly that creates a strongly-ionized plasma from the weakly-ionized plasma, at least one of an amplitude, a rise time, and a duration of the voltage pulse being chosen to increase a density of ions in the strongly-ionized plasma enough to generate sufficient thermal energy from ion bombardment of a surface layer of the sputtering target to cause a sputtering yield to be related to a temperature of the sputtering target.

32. The sputtering source ofclaim 31 wherein the sputtering yield is related to a temperature of a surface of the sputtering target.

33. The sputtering source ofclaim 31 wherein the sputtering yield is linearly related to the temperature of the sputtering target.

34. The sputtering source ofclaim 31 further comprising a gas flow controller that controls a flow of the feed gas so that the feed gas diffuses the strongly-ionized plasma.

35. The sputtering source ofclaim 34 wherein the controller controls the flow of the feed gas to allow additional power to be absorbed by the strongly ionized plasma, thereby generating additional thermal energy in the sputtering target.

36. The sputtering source ofclaim 31 wherein the thermal energy generated in the sputtering target from ion bombardment does not substantially increase an average temperature of the sputtering target.

37. The sputtering source ofclaim 31 further comprising a magnet that is positioned to generate a magnetic field proximate to the weakly-ionized plasma, the magnetic field substantially trapping electrons in the weakly-ionized plasma proximate to the sputtering target.

38. The sputtering source ofclaim 31 wherein the voltage pulse generated between the anode and the cathode assembly excites atoms in the weakly-ionized plasma and generates secondary electrons from the cathode assembly, the secondary electrons ionizing a portion of the excited atoms, thereby creating the strongly-ionized plasma.

39. The sputtering source ofclaim 31 wherein the power supply generates a constant power.

40. The sputtering source ofclaim 31 wherein the power supply generates a constant voltage.

41. The sputtering source ofclaim 31 wherein the ionization source is chosen from the group comprising an electrode coupled to a DC power supply, an electrode coupled to an AC power supply, a UV source, an X-ray source, an electron beam source, an ion beam source, an inductively coupled plasma source, a capacitively coupled plasma source, and a microwave plasma source.

42. The sputtering source ofclaim 31 wherein a rise time of the voltage pulse is chosen to increase an ionization rate of the strongly-ionized plasma.

43. The sputtering source ofclaim 31 wherein a presence of weakly-ionized plasma reduces the probability of developing an electrical breakdown condition when the power supply generates the voltage pulse between the anode and the cathode assembly.

44. The sputtering source ofclaim 31 wherein the strongly-ionized plasma is substantially non-uniform proximate to the cathode assembly.

45. The sputtering source ofclaim 31 wherein a distance between the anode and the cathode assembly is chosen to increase an ionization rate of strongly-ionized plasma.

46. The sputtering source ofclaim 31 wherein the rise time of the voltage pulse is in the range of approximately 0.1V/μsec to 100V/μsec.

47. The sputtering source ofclaim 31 wherein the discharge voltage for the weakly ionized plasma is in the range of approximately 100V to 1,000V.

48. The sputtering source ofclaim 31 wherein the amplitude of the voltage pulse is in the range of approximately 200V to 30,000V.

49. The sputtering source ofclaim 31 wherein a pulse width of the voltage pulse is in the range of approximately 0.1 μsec to 100 sec.

50. A method for high deposition rate sputtering, the method comprising:

a) ionizing a feed gas to generate a weakly-ionized plasma proximate to a cathode assembly that comprises a sputtering target; and

b) applying a voltage pulse to the cathode assembly to generate a strongly-ionized plasma from the weakly-ionized plasma, at least one of an amplitude, a rise time, and a duration of the voltage pulse being chosen so that ions in the strongly-ionized plasma bombard a surface layer of the sputtering target to generate sufficient thermal energy in the surface layer of the sputtering target to cause a sputtering yield to be related to a temperature of the sputtering target.

51. The method ofclaim 50 wherein the sputtering yield is linearly related to the temperature of the sputtering target.

52. The method ofclaim 50 wherein the sputtering yield is non-linearly related to the temperature of the sputtering target.

53. The method ofclaim 50 wherein the rise time of the voltage pulse is in the range of approximately 0.1V/μsec to 100V/μsec.

54. The method ofclaim 50 wherein the amplitude of the voltage pulse is in the range of approximately 200V to 10,000V.

55. The method ofclaim 50 wherein a pulse width of the voltage pulse is in the range of approximately 0.1 μsec to 100 sec.

56. The method ofclaim 50 further comprising generating a magnetic field proximate to the sputtering target, the magnetic field trapping electrons proximate to the sputtering target.

57. The method ofclaim 50 wherein the applying the voltage pulse to the cathode assembly generates excited atoms in the weakly-ionized plasma and generates secondary electrons from the sputtering target, the secondary electrons ionizing a portion of excited atoms, thereby creating the strongly-ionized plasma.

58. The method ofclaim 50 further comprising diffusing the weakly-ionized plasma with a volume of the feed gas while ionizing the volume of the feed gas to create additional weakly-ionized plasma.

59. The method ofclaim 50 further comprising exchanging a volume of feed gas to diffuse the strongly-ionized plasma while applying the voltage pulse to the cathode assembly to generate additional strongly-ionized plasma from the volume of the feed gas.

60. The method ofclaim 50 wherein a presence of weakly-ionized plasma reduces the probability of developing an electrical breakdown condition when the power supply generates a voltage pulse between cathode and anode assembly.

61. The method ofclaim 50 wherein the ionizing the feed gas comprises exposing the feed gas to one of a static electric field, an AC electric field, a quasi-static electric field, a pulsed electric field, UV radiation, X-ray radiation, an electron beam, and an ion beam.

62. The method ofclaim 50 wherein the ions in the strongly-ionized plasma cause a surface layer of the sputtering target to evaporate during the applied voltage pulse.

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