US20050133361A1 - Compensation of spacing between magnetron and sputter target - Google Patents

Abstract

A lift mechanism for and a corresponding use of a magnetron in a plasma sputter reactor. A magnetron rotating about the target axis is controllably lifted away from the back of the target to compensate for sputter erosion, thereby maintaining a constant magnetic field and resultant plasma density at the sputtered surface, which is particularly important for stable operation with a small magnetron, for example, one executing circular or planetary motion about the target axis. The lift mechanism can include a lead screw axially fixed to the magnetron support shaft and a lead nut engaged therewith to raise the magnetron as the lead nut is turned. Alternatively, the support shaft is axially fixed to a vertically moving slider. The amount of lift may be controlled according a recipe based on accumulated power applied to the target or by monitoring electrical characteristics of the target.

Description

RELATED APPLICATIONS
• This application claims benefit of U.S. provisional application Ser. No. 60/529,209, filed Dec. 12, 2003. It also is related to concurrently filed U.S. patent application entitled MECHANISM FOR VARYING THE SPACING BETWEEN SPUTTER MAGNETRON AND TARGET.
FIELD OF THE INVENTION
• The invention relates generally to sputter deposition of materials. In particular, the invention relates to a movable magnetron that creates a magnetic field to enhance sputtering.
BACKGROUND ART
• Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of integrated circuits. The more conventional type of sputtering, as originally applied to integrated circuits as well as to other applications, deposits upon a workpiece a planar layer of the material of the target. However, the emphasis has recently changed in the use of sputtering for the fabrication of integrated circuits because vertical interconnects through inter-level dielectrics having the high aspect ratios now being used present a much greater challenge than the horizontal interconnects. Furthermore, the horizontal interconnects are being increasingly implemented by electrochemically plating copper into horizontally extending trenches while sputtering is being reserved for liner layers deposited onto the sidewalls in the holes in which the vertical interconnects are formed or also deposited onto the walls of the horizontal trenches.
• It has long been known that sputtering rates can be increased by the use of amagnetron10, illustrated in the schematic cross-sectional view ofFIG. 1, positioned in back of a sputteringtarget12. The magnetron projects amagnetic field14 across the face of thetarget12 to trap electrons and hence increase the plasma density. Themagnetron10 typically includes at least twomagnets16,18 of anti-parallel magnetic polarities perpendicular to the face of thetarget12. Amagnetic yoke20 supports and magnetically couples the twomagnets16,18. The resultant increased plasma density is very effective at increasing the sputtering rate adjacent the parallel components of themagnetic field14. However, as illustrated in the cross-sectional view ofFIG. 2, anerosion region22 develops adjacent the magnetic field, which brings afront surface24 of thetarget12 closer to themagnetron10, whichfront surface24 is the surface being currently sputtered. The erosion illustrated inFIG. 2 emphasizes an erosion pit adjacent themagnetron10. In typical operation, themagnetron10 is scanned over the back of thetarget12 to produce a more uniform erosion pattern. Nonetheless, even if a target eroded to a planar surface, the fact remains that after erosion the surface of the target being sputtered is closer to themagnetron10 than before erosion.
• The target erosion presents several problems if the lifetime of thetarget12 is to be maximized. First, the erosion pattern should be made as uniform as possible. In conventional planar sputtering, uniformity is improved by forming themagnets16,18 in a balanced, relatively large closed kidney-shaped ring and rotating the magnetron about the central axis of the target. Secondly, the erosion depth can be compensated by adjusting the spacing between the target and the wafer being sputter deposited, as disclosed by Tepman in U.S. Pat. No. 5,540,821. Futagawa et al. disclose a variant in U.S. Pat. No. 6,309,525. These schemes have primarily addressed the dependence of deposition rate on the separation between the wafer and the effective front face of thetarget12. These approaches do not address how the erosion affects the magnetic enhancement of sputtering.
• The erosion problem has been complicated by the need to produce a highly ionized sputter flux so that the ionized sputter atoms can be electrostatically attracted deep within high aspect-ratio holes and be magnetically guided, as has been explained for an SIP reactor by Fu et al. in U.S. Pat. No. 6,306,265, incorporated herein by reference in its entirety. The apparatus described therein uses a small triangularly shaped magnetron to effect self-ionized sputtering, taking into account three factors. First, it is advantageous to reduce the size of the magnetron in order to concentrate the instantaneous sputtering to a small area of the target, thereby increasing the effective target power density. Secondly, the concentrated magnetic field of the small magnetron increases the plasma density adjacent the portion of the target being sputtered, thereby increasing the ionization fraction of the target atoms being sputtered. The ionized sputter flux is effective at being attracted deep within high aspect-ratio holes in the wafer. However, the target erosion affects the effective magnetic field at the target face being sputtered, thereby changing the sputtering rate and the ionization fraction. Thirdly, the small magnetron makes uniform target sputtering that much more difficult. Various magnetron shapes, e.g., triangular have been used to increase the uniformity of sputtering, but their uniformity is not complete. Instead, annular troughs are eroded into the target even in the case of rotary magnetrons.
• Two major operational effects are readily evident in the use of conventional rotary magnetrons, particularly small magnetrons. First, as illustrated in theplot26 of the graph ofFIG. 3, the deposition rate falls from its initial rate with target usage, here measured in target kilowatt-hours of cumulative power applied to the target since it was fresh. The target usage corresponds to both the amount of target that has been eroded since the target was put into service with a substantially planar and uneroded surface and to the number of wafers that have been deposited in a repetitive process. We believe that the decrease is believed arises at least indirectly from the target erosion in which the target surface being sputtered is no longer optimized for the magnetic field since its separation from the magnetron is varying. The sputtering degradation can be compensated by either increasing the length or sputtering or increasing the target power. Secondly, the non-uniformity of sputtering reduces the lifetime of the target to a number N₁at which the erosion trough maximum approaches the target backing plate or, in the case of an integral target, a minimum thickness of the target. At this point, to prevent either sputter deposition of the backing material or breakthrough of the target, the target must be discarded even though substantial target material survives away from the erosion troughs. Costs would be saved for target purchase, operator time, and production throughput if the target lifetime is increased.
• Hong et al. have presented a planetary magnetron as a solution to the uniformity problem for a high-density plasma reactor in U.S. patent application Ser. No. 10/152,494, filed May 21, 2002, now published as Application Publication 2003-0217913, and incorporated herein by reference in its entirety. As illustrated in the cross-sectional view ofFIG. 4, aplasma reactor30 has a fairly conventional lower reactor including areactor wall32 which supports asputtering target34 through anadapter36 andisolator38 in opposition to apedestal electrode40 supporting thewafer42 to be sputter deposited with the material of thetarget34. Avacuum pump system44 pumps the vacuum chamber to a level of a few milliTorr or less while agas source46 supplies a working gas such as argon through amass flow controller48. Aclamp ring50 holds thewafer42 to thepedestal electrode40 although an electrostatic chuck may alternatively be used. An electrically groundedshield52 protects thereactor walls32 and further acts as an anode in opposition to thetarget34 while aDC power supply54 negatively biases thetarget34 to a few hundred volts to excite the argon working gas into a plasma. The positively charged argon ions are accelerated to the negativelybiased target34, which they strike and dislodge or sputter atoms of the target material. The sputtered atoms are ejected from thetarget34 with fairly high energy in a wide beam pattern and thereafter strike and stick to thewafer42. With sufficiently high target power and high plasma density, a substantial fraction of the sputtered atoms are ionized. Preferably, anRF power supply58, for instance oscillating at 13.56 MHz, biases thepedestal electrode40 through acapacitive coupling circuit60 such that a negative DC self-bias develops on thewafer42, which accelerates the positively charged sputter ions deep within high-aspect ratio holes being sputter coated.
• According to the invention, amagnetron70 positioned in back of thetarget34 projects its magnetic field in front of thetarget34 to create a high-density plasma region72, which greatly increases the sputtering rate of thetarget34. If the plasma density is high enough, a substantial fraction of sputtered atoms are ionized, which allows additional control over the sputter deposition. Ionization effects are particularly pronounced in sputtering copper, which has a high self-sputtering yield, as copper ions are attracted back to the copper target and sputter further copper. The self-sputtering allows the argon pressure to be reduced, thereby reducing wafer heating by argon ions and reducing argon scattering of copper atoms, whether ionized or neutral, as they travel from thetarget34 to thewafer42.
• In the described embodiment, themagnetron70 is substantially circular and includes an innermagnetic pole74 of one magnetic polarization with respect to and extending along acentral axis76 of thechamber32 as well as thetarget34 andpedestal electrode40. It further includes an annularouter pole78 surrounding theinner pole74 and of the opposed magnetic polarity along thecentral axis76. Amagnetic yoke80 magnetically couples the twopoles74,78 and is supported on acarrier81. The total magnetic intensity of theouter pole78 is substantially greater than that of theinner pole74, for example by a factor of greater than 1.5 or 2.0, to produce an unbalanced magnetron which projects its unbalanced magnetic portion towards thewafer42 to thereby confine the plasma and also guide sputtered ions towards thewafer42. Typically, theouter pole78 is composed of plural cylindrical magnets arranged in a circle and having a common annular pole piece on the side facing thetarget34. Theinner pole74 may be composed of one or more magnets, preferably with a common pole piece. Other forms of magnetrons are encompassed by the invention.
• The high plasma densities achieved by this configuration as well as that of Fu et al. are achieved in part by minimizing the area of themagnetron70. The encompassing area of themagnetron70 is typically less than 10% of the area of thetarget34 being scanned by themagnetron70. The magnetron/target area ratio may be less than 5% or even less than 2% if uniform sputtering is otherwise maintained. As a result, only a small area of thetarget34 is subject to an increased target power density and resultant intensive sputtering. That is, the sputtering at any instant of time is highly non-uniform. To compensate for the non-uniformity, arotary drive shaft82 rotated by adrive source84 and supporting themagnetron70 circumferentially scans themagnetron70 about thechamber axis76. However, as has been described with respect to the reactor of Fu et al., the resultant annular troughs in the target may produce significant radial non-uniformity in the sputtering.
• Hong et al. significantly reduce the sputtering non-uniformity by the use of aplanetary scanning mechanism90 to cause themagnetron70 to move along a planetary or other epicyclic path over the back of thetarget34 with respect to thecentral axis76. Their preferredplanetary gear mechanism90 for achieving planetary motion includes, as additionally and more completely illustrated inFIG. 5, a fixedgear92 fixed to ahousing94 and adrive plate96 fixed to therotary shaft82. In the reactor of Hong et al., thehousing94 is stationary. Thedrive plate96 rotatably supports anidler gear98 which engages the fixedgear92. Thedrive plate96 also rotatably supports afollower gear100 engaged with theidler gear98. Ashaft102 of thefollower gear100 is fixed also to thecarrier81 so that themagnetron70 supported on thecarrier81 away from thefollower shaft102 rotates with thefollower gear100 as it rotates about the fixedgear92 to execute the planetary motion.Counterweights110,112 are fixed to the non-operative ends of thedrive plate96 and thecarrier81 to reduce bending and shimmy on therotary drive shaft82 and thefollower shaft102. Particularly in copper sputtering which achieves a high ionization ratio Cu⁺/Cu⁰of sputtered copper ions, thesputter reactor30 ofFIG. 4 advantageously includes a magnetic coil ormagnet ring114 annular about thecentral axis76 to guide the copper ions to thewafer42.
• Because theDC power supply54 delivers a significant amount of power to thetarget34 and a high flux of energetic ions bombard thetarget34 thereby heating thetarget34, it is conventional to immerse themagnetron70 as well as theplanetary mechanism90 in a coolingwater bath116 enclosed in atank118 sealed to thetarget34 and the fixed drive-shaft housing94. Unillustrated fluid lines connect thebath116 with a chiller to recirculate chilled deionized water or other cooling fluid to thebath118.
• The planetary magnetron scanning, because of its convolute path across thetarget34, greatly improves the uniformity of target erosion so that thetarget34 is more uniformly eroded and results in a nearly planar sputtering surface even as the target is eroded. As a result, the target utilization is greatly improved. Nonetheless, as thetarget34 erodes generally uniformly, the magnetic field at its sputtering face is changing and apparently on average decreasing. The change affects the sputtering rate, which as described above has been observed to decrease. The plots presented inFIG. 3 are speculative. Actual experimental data are presented inFIG. 6. Plot120 presents the measured deposition rate for copper in the planetary magnetron chamber ofFIG. 4 having an axially fixed magnetron and with 28 kW of DC target power and 600W of RF bias power as a function of target usage in kilowatt-hours. Plot121 presents the deposition rate for a small axially fixed magnetron executing simple rotary motion, as described by Fu et al., with 56 kW of DC target power. Although the fall off in the simple rotary chamber is not as great in the planetary chamber, it is still significant. It is pointed out, however, that it may be advantageous to more heavily sputter the outer regions of thetarget34, particularly when the target/wafer spacing is relatively small in order to compensate for the geometric effect of greater deposition at the wafer center. Such intended non-uniformity can be achieved by adjusting the length of the rotation arms in a planetary chamber or by changing the shape or radial position of the magnetron in a simple rotary chamber. Even in this case, the deposition rate decreases with target usage.
• A second set of non-uniformity problems is not immediately addressed by the planetary scanning mechanism. The small area of themagnetron70 advantageously produces a high target power density and high plasma density and hence increases sputtering rate and increases the fraction of ionized sputter atoms which are drawn deep within high aspect-ratio holes to coat the sides and bottom of via holes. However, the magnetic field and hence the plasma density depend upon the distance between the target sputtering surface and the magnetron. As a result, as thetarget34 is being sputtered, even if uniformly, the plasma density is changing and hence the sputtering rate and the ionization rate upon which the via sidewall coverage depends are changing. The effect is exacerbated for a small magnetron because the gradient of the magnet field is greater. As a result, the changing magnetic field and plasma density destabilizes the process causing variation in bottom and sidewall coverages across the lifetime of the target. It has generally been accepted that the high-performance sputtering is different at the end of the lifetime of the target than at the beginning. Plot122 inFIG. 7 shows the measured target voltage andplot123 inFIG. 8 shows the measured mean bias voltage with respect to target usage for the axially fixed planetary magnetron with the aforementioned values of target and bias power. There is a significant rise in the target voltage and the magnitude of the bias voltage with increased sputtering. However, the bias voltage is subject to fluctuations of about +20V with the maximum magnitude greatly increasing to about 150V at maximum usage. The instability is readily apparent from theplot122 ofFIG. 7 for target voltage and theplot123 ofFIG. 8 for bias power. The change of sputtering rate can be compensated by increasing the sputtering duration, but this does not address the sidewall coverage. In any case, the increased sputtering period decreases throughput and introduces another variable into the queuing plan. The variation in plasma density because of reduced magnetic field can be partially compensated by increasing the target power. Such power compensation however involves an ad hoc relationship which needs to be determined for each set of conditions and also reduces the ability to maximize plasma densities and sputtering rates with limited power supplies.
• Halsey et al. in U.S. Pat. No. 5,855,744 show an apparatus for deforming a linear magnetron as it scans across a rectangular target. In one embodiment, multiple actuators moving shafts along multiple respective axes deform the magnetron. Mizouchi et al. in U.S. Pat. No. 6,461,485 discloses a single vertical actuator for compensating for end effects in linear scanning.
• Demaray et al. in U.S. Pat. No. 5,252,194 discloses a slider mechanism for vertically moving a large magnetron to adjust the magnetic field at the front of the target.
• Schultheiss et al. in U.S. Pat. No. 4,927,513 discloses a magnetron lift mechanism to control magnetic properties of sputtered layers.
SUMMARY OF THE INVENTION
• The invention includes the method and apparatus for compensating erosion of a plasma sputtering target by moving the magnetron away from the back of the target as the front of the target is eroded. The compensation provides a more constant magnetic field and plasma density at the surface of the target being sputtered and results in a more stable sputtering process.
• The lift mechanism may include a lead screw mechanism including a lead screw and lead nut. The lead screw may be axially fixed to the magnetron and a lead nut threadably engaged with the lead screw. Rotation of the lead nut vertically moves the magnetron. The lead screw may be azimuthally fixed while the lead nut is axially fixed. The lead nut may be manually moved or moved under the control of a motor or other actuator coupled to the lead nut by a gear or a linear lead screw mechanism or linear actuator.
• The amount of lift my be dictated by a predetermined recipe or by a measured cumulative power applied to the target. Alternatively, the target resistance or power characteristic or the physical erosion depth may be monitored to determine when additional lift is required.
• The magnetron lift mechanism may also be used to control the magnetic field at the face of the sputtering target for control of the sputtering process other than simple compensation of target erosion.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIGS. 1 and 2 are cross-sectional views functionally illustrating the effect of magnetron sputtering as the target is being eroded.
• FIG. 3 is a graph illustrating the dependencies in the prior art and according to the invention of the sputtering deposition rate as a function of the target usage.
• FIG. 4 is a schematic cross-sectional view of a plasma sputter reactor with a planetary magnetron.
• FIG. 5 is an isometric view of a planetary magnetron.
• FIG. 6 is a graph illustrating the experimentally determined dependencies in the prior art of the sputtering deposition rate as a function of target usage.
• FIG. 7 is a graph illustrating the experimentally determined dependencies in the prior art and in the practice of the invention of target voltage as a function of target usage.
• FIG. 8 is a graph illustrating the experimentally determined dependencies in the prior art and in the practice of the invention of bias voltage as a function of target usage.
• FIG. 9 is a graph illustrating as a function of target usage both a sequence of target spacings used in an experiment verifying the invention and the resultant sputtering deposition rate.
• FIG. 10 is a cross-sectional view of a lead screw lift mechanism for raising the magnetron.
• FIG. 11 is an orthographic view of the exterior portions of the lead screw lift mechanism ofFIG. 10 and a spur gear mechanism for driving the spacing between target and magnetron.
• FIG. 12 is an orthographic view of a first embodiment of a lead screw mechanism for driving the spacing compensation.
• FIG. 13 is an orthographic view of a dual slider mechanism for driving the spacing compensation to be used in the lead screw mechanism ofFIG. 10.
• FIG. 14 is an orthographic view of a collar, slider and its case to be used in the slider mechanism ofFIG. 13.
• FIG. 15 is an orthographic view of a bracket used with the slider mechanism ofFIG. 13.
• FIG. 16 is an orthographic view of the slider mechanism ofFIG. 13 additionally including a magnetron rotation motor.
• FIG. 17 is a schematic representation of a computerized lift control system.
• FIGS. 18 and 19 are schematic cross-sectional view of two hollow cathode magnetrons incorporated the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• The erosion of the front of the target in magnetron sputtering can be compensated by moving the magnetron away from the back of the target. As illustrated inFIG. 4, alift mechanism124 controllably raises themagnetron70 with respect to the back of thetarget34, preferably in an amount commensurate to the amount of the front surface of thetarget34 that has been eroded since thetarget34 was installed with a flesh planar front surface. The compensation should focus on the areas of thetarget34 being more heavily eroded since they contribute a higher fraction of sputtered atoms. While the conventional design criterion minimizes the distance between themagnetron70 and the back of thetarget34 and maintains the separation at this initial spacing, one preferred criterion of the invention maintains an approximately constant spacing between themagnetron70 and the front of thetarget34 facing thewafer42. The nearly constant spacing maintains a substantially constant magnetic field at the surface of thetarget34 being sputtered. The nearly constant magnetic field removes one variable from the process conditions determining sputtering performance, not just sputtering rate but also ionization fraction among other effects. Thereby, sputtering time or target voltage do not need to be adjusted for target usage. Movement of the magnetron to maintain a substantially constant magnetic field at the front face of thetarget34 stabilizes the sputtering process over the life of the target and enables a substantially constant deposition rate despite target usage, as schematically illustrated inplot126 ofFIG. 3. Also, the sidewall and bottom coverages may be maintained substantially constant over the life of the target. Furthermore, the lifetime of the target considerably increases to a value N₂as the target is nearly uniformly eroded almost to its backing plate.
• Although other implementations are possible, thelift mechanism124 can be easily incorporated into the conventional design by allowing thehousing94 to be axially movable by the lift mechanism while still maintaining its fluid seal to thetank118.
• A first embodiment of the invention used to verify the effects of compensating the magnetron-to-target spacing uses a series of shims of varying thickness placed between themagnetron70 ofFIG. 5 and thecarrier81. As the target erodes, the previous shim is replaced by a thinner shim. As a result, themagnetron70 is being moved away from the back of thetarget34 along a single axis at the center of themagnetron70 although that axis is moving as themagnetron70 is moved along a planetary path. As a result, a nearly constant magnetic field can be maintained at the sputtering surface of thetarget34 over the target life thereby stabilizing the sputtering process. The manual shimming process may be alternatively effected by shims placed between the otherwisestationary housing94 and the roof of thetank118.
• Actual experimental data using a copper target and a planetary magnetron in the reactor ofFIG. 4 are presented inFIG. 9. Plot125 shows the magnet-to-target (MTS) spacing, specifically to the back of the target as the shim thickness was occasionally increased whileplot126 in shows the measured deposition rate. Even in these preliminary experiments, the deposition rate is maintained nearly constant. With the use of the invention, a copper target may be used to deposit thin copper seed layers on up to 20,000 wafers. Plot121 inFIG. 6 also shows actual data for the moderated change of deposition rate as a function of target usage as the shims were being replaced. Plot127 inFIG. 7 shows the dependence of measured target voltage as a function of target usage. Plot128 inFIG. 8 shows the dependence of the mean of the measured bias voltage as a function of target usage. Further, although not illustrated, the deviations of the maximum and mean bias voltage from the mean do not significantly change with target usage.
• These results could be improved particularly for target and bias power by more frequently moving the magnetron with a finer resolution. These results also show that target and bias voltages are sensitive indicators of the amount of erosion and hence the need for spacing compensation. These voltages are easy to monitor during production. Current is another sensitive measurement for electrical supplies generating constant power. Alternatively, if the power supplies are set to generate constant voltage or current, the complementary quantity or power may be measured. These electrical measurements typically amount to monitoring the resistance of the plasma under some set electrical condition. Therefore, the compensation can be dynamically controlled by measuring one or both of these voltages (or other quantities) during production and comparing them to baseline values. When the deviation exceeds a threshold, the compensation may be performed to bring the measured value closer to the baseline value. It is also possible to optically or otherwise measure the physical depth of erosion of the target and use the depth measurement to initiate the compensation. Nonetheless, it has proven satisfactory to keep track of cumulative target power and move the magnetron at values experimentally determined for a given sputter recipe.
• Although the first embodiment relying on shims is effective, it clearly presents operational difficulties as the sputter reactor needs to be shut down and the magnetron removed from the water bath to allow manual replacement of its shims. It is greatly desired to perform the spacing compensation from outside the water bath and preferably under computerized electrical control.
• One set of embodiments is based on converting thestationary housing94 to a vertically movable but in large part azimuthally fixedhousing94 driven by alead screw mechanism130, as illustrated in the cross-sectional view ofFIG. 10. Therotary drive shaft82 includes acentral bore132 for flowing chilled cooling water to the center of thetank118 ofFIG. 4 near the back of thetarget34. The cooling water flows out of the water bath through an unillustrated outlet in aroof142 or other wall of thetank118. The top of thedrive shaft82 is coupled by yet further unillustrated belts or other means to themotor84. Thedrive plate96 of the planetary mechanism is fixed to the bottom of thedrive shaft82 and rotates with it. Tworing bearings134,136 rotatably support aboss138 of thedrive shaft82 within thehousing94. An annulardynamic seal139 the seals the fluid within thebath116 from thebearings134,136 and the exterior.
• Atail140 of thehousing94 axially passes through an aperture in thetank roof142 but is azimuthally fixed by other means. The fixedgear92 of the planetary mechanism is fixed to the end of thehousing tail140. As a result, when thehousing94 is vertically moved, the fixedgear92, thedrive plate96, and the rest of theplanetary mechanism90 andmagnetron70 are also vertically moved along thecentral axis76.
• Asupport collar146 is fixed to thetank roof142 and sealed to it with an O-ring placed in an O-ring groove147. An annular bellows148 surrounding the upper portion of thehousing tail140 is sealed on opposed ends to thehousing94 and to the inner portion of thesupport collar146 to slidably seal the fluid in thebath116 from the exterior as well as from most of the mechanical parts of thelift mechanism130 while allowing axial movement between thehousing94 and driveshaft82 on one hand and thetank roof142 on the other. Thebellows148 should accommodate a movement of about ¾″ (2 cm) corresponding to the usable thickness of thetarget34. Other types of slidable fluid seals are possible. The fixedcollar146 rotatably supports an internally threadedlead nut150 through tworing bearings152,154. Aninner retainer ring156 fixed to thelead nut150 and anouter retainer ring158 fixed to thecollar146 trap theupper bearing152 against thelead nut150 and thecollar146. Another similar retainer ring configuration beneath thelead nut150 traps thelower bearing154. Thelead nut150 can thus rotate about thecentral axis76 but is axially fixed to thetank top142.
• The external threads of a azimuthally fixed but vertically movablelead screw164 engage the internal threads of thelead nut150. Thelead screw164 supports thehousing94 on its upper surface. Thehousing94 may be fixed to thelead screw164 or guide pins may couple them to prevent relative rotational movement. A plurality ofscrews166 hold thelead screw164 to thetank top142 through compression springs168. As a result, thelead screw164 is rotationally fixed as it engages therotatable lead nut150 but the compression springs168 accommodate limited vertical motion of thelead screw164. The axial fixing of thelead nut150 to thetank top142 provides a wide mechanical base for the heavy rotating magnetron, thereby reducing shimmy and allowing the reduction of the clearance between themagnetron70 and the back of thetarget34.
• In operation, if thelead nut150 is rotated clockwise, the azimuthally fixedlead screw164 rises and lifts thehousing94 and the attachedrotary shaft82 andmagnetron70 away from thetarget34. Counter-clockwise rotation of thelead nut150 produces the opposite axial movement of lowering themagnetron70 toward thetarget34. The lift drive mechanism for rotating thelead nut150 is easily formed outside of the coolingbath116. Two types of lift drive mechanisms will be described.
• A first embodiment of a rotational lift drive includes a spur gear drive170 illustrated in the orthographic view ofFIG. 11. The outer rim of theinner retainer ring156 is partially formed with atoothed gear172 in agear ledge173 extending from only part of theinner retainer ring156. Thetoothed gear172 engages with alift drive gear174 controllably driven by alift motor176, which maybe mounted on thetank roof142 with a vertically oriented drive shaft to which thetoothed gear172 is fixed. Thelift motor176 is preferably a stepper motor rotating a fixed angle for each motor pulse with a separate control signal controlling the direction of rotation. Thereby, thelead nut150 ofFIG. 10 is controllably rotated to raise or lower thelead screw164 and hence thehousing94 and attacheddrive shaft82 andmagnetron70.
• Anoptical position sensor175 includes twoarms175a,175bspaced to accommodate thegear ledge173 as it rotates in lifting the magnetron. Onearm175acontains an optical emitter, such as an light emitting diode, while theother arm175bcontains a light detector, such as a photodiode. Theposition sensor175 is used to calibrate the rotation of thegear172 using thegear ledge173 as a flag. Thelift motor176 rotates thegear172 toward theposition sensor175 until thegear ledge173 enters between thearms175a,175bof theposition sensor175 and blocks the emitted light from the optical detector. The controller notes that position as a home position. Thestepper motor176 is then stepped in the opposite direction by a controlled number of pulses to a desired rotation location of thegear172 and hence vertical position of the magnetron. Other position sensors may be used.
• Thedrive shaft motor84 may be vertically mounted on thetank roof142 through amotor mount180. Thedrive shaft motor84 drives amotor drive gear182 through optional unillustrated gearing to reduce the rotation rate. Ashaft drive gear186 is formed in acapstan188 fixed to thedrive shaft82. Aribbed belt190 is wrapped over both themotor drive gear182 and theshaft drive gear186 so that themotor84 rotates thedrive shaft82 in executing the planetary motion of themagnetron70. Because thedrive shaft82 and the attachedshaft drive gear186 are raised and lowered in operation relative to themotor mount180 and attachedmotor drive gear182, the teeth of at least theshaft drive gear186 must be wide enough to accommodate the slip or axial movement of thebelt190 relative to teeth of thatgear186 and themotor drive gear182 may be formed with two rims to limit the axial movement of thebelt190 on thatgear182. Arotary fluid coupling194 is mounted on the top of thedrive shaft82 to allow cooling water lines to be connected to thecentral bore132 of therotating drive shaft82.
• A second embodiment of a rotational lift drive includes alead screw mechanism200 illustrated in the orthographic, partially sectioned view ofFIG. 12. Asupport collar202 is fixed to thetank roof142 and rotatably supports thelead nut150 through a ring bearing204 trapped by anupper retainer ring206. Alead nut lever208 extends radially outwardly from thelead nut150 and has twoparallel arms210 formed at its end. A pivot connection including twoarms212 at the back of thelift motor176, an unillustrated pivot pin through them, and a mount for the pin, pivotally mounts thelift motor176 to thetank roof142 in a horizontal orientation. The horizontally orientedlift motor176 rotates ashaft216 having a lead screw formed on its distal end. Anut box220 threadably captures the lead screw of thedrive shaft216 and is pivotally supported by apin218 fixed to thearms210 of thelead nut lever208. Thereby, rotation of thelift motor176 rotates thelead nut150 to raise or lower along thecentral axis72 thelead screw164 and hence thehousing tail140 and attacheddrive shaft82 andmagnetron70.
• The second embodiment ofFIG. 12 can be easily modified to replace thelift motor176 with a hydraulic or pneumatic linear actuator driving a shaft pivotally coupled at its end to thearms210 of thelead nut lever208. Yet further, the second embodiment could be manually controlled by the operator manually rotating thelead nut lever208. Other combinations of gears, levers, and actuators or motors can be used to implement the lead nut lift mechanism.
• The lead nut lift mechanism offers several advantages. It is concentric about the lift axis and the support shaft for the magnetron. The magnetron is supported on an azimuthally fixed lead screw threaded into a larger lead nut that is axially fixed to a yet larger structure. Hence, the lead nut lift mechanism offers low vibration and flexing of the relatively heavy rotating magnetron. The design is mechanically simple, thereby increasing reliability and reducing cost.
• A second type of lift mechanism is adouble slider mechanism230 illustrated orthographically inFIG. 13. It includes anelongated collar232, also illustrated orthographically inFIG. 14, adapted to be fixed and sealed to thetank roof142. A vertically orientedslider case234 fixed to thecollar232 includes two vertically extending and horizontally stacked tracks, one of which is formed on one side byrails235. The two tracks respectively trap twosliders236, which are fixed together and can together vertically move along therails235. Only theexterior slider236 is illustrated. Two sliders fixed to each other provide greater stiffness in supporting the relatively heavy load. A vertically extending back238 is fixed to thecollar232 to provide a rigid mount for theslider case234 and other parts. A vertically orienteddrive shaft240 is rotatably supported in the top end of theslider case234. The lower end of thedrive shaft240 is threaded as a lead screw and engages corresponding threads formed in an unillustrated lead box axially supporting bothsliders236. As a result, when thedrive shaft240 rotates, thesliders236 move up or down within theslider case234.
• Abracket250 illustrated orthographically inFIG. 15 has a base252 sized to fit onto theexterior slider234. A plurality of throughholes254 drilled through thebracket base252 pass screws256 threaded into theexterior slider234 to snugly hold thebracket base252. Locatingpins258 may be inserted into theexterior slider234 to engage corresponding holes formed in the bottom of thebracket base252. Thebracket250 further includes atubular collar260 having anaperture262 sized to closely hold thehousing94 ofFIG. 10, with suitable modifications of thathousing94, and an upperaxial end264 to support thehousing94. Thehousing94 may be fixed to thetubular collar260 or engage it through pins so that thehousing94 does not rotate.
• Returning toFIG. 13, ashaft gear270 is fixed to themagnetron drive shaft82 ofFIG. 10, which is rotatable within but vertically fixed in thehousing94. Thehousing94 itself is not rotatable but can move vertically with respect to thecollar232 and thetank roof142. The verticallymovable housing94 maybe sealed to thetank roof142 by an assembly including thebellows148 ofFIG. 10 to be axially movable with respect to thetank roof142 over a limited throw. Theshaft gear270 is similar to theshaft gear186 ofFIG. 11 and may be driven by thebelt190 driven by the vertically orientedmotor84. Thebelt190 as applied toFIG. 13 should be able to vertically slide along the teeth of theshaft gear270.
• A slider drive mechanism includes aplate276 fixed to end of theslider case234 which passes the end of theslider shaft240 to be fixed to aslider gear278. Theplate276 also supports below a vertically orientedslider motor280 having a drive shaft fixed to amotor gear282. Aribbed belt284 is wrapped around the slider and motor gears278,282 so that theslider motor280 can move theslider234 up and down within theslider case250 to thereby vertically move thehousing94 and attachedmagnetron70 relative to thetank roof142 and the back of thetarget34.
• A modifieddouble slider mechanism290 illustrated orthographically inFIG. 16 includes a modifiedbracket292 having ashelf294 extending outward from the top of thetubular collar260. Ashaft drive motor296 andgear box298 are supported on the bottom of theshelf294 to drive amotor gear300. Aribbed belt302 engages both themotor gear300 and theshaft gear270 to thereby rotate themagnetron shaft82 and cause the attachedmagnetron70 to execute planetary motion. Because themotor296 andmotor gear300 are axially fixed to thehousing94 and hence moves with themagnetron shaft82, thebelt270 does not need to slip along the teeth of thegears270,300.
• The described compensation mechanisms may be used in a number of ways for compensating target erosion. It is possible to perform the lifting and compensating during the plasma excitation and sputter deposition, but it is preferable instead to perform it after one wafer is processed and before the next one is processed. Even though motor controlled, the mechanisms may be essentially manually controlled by on occasion instructing the lift motor to move a set amount corresponding to a desired lift of the magnetron. However, the lift compensation algorithm is advantageously incorporated into the recipe for which a machine is being used and a computerized controller performs the compensation as well as controls the other chamber elements according to the recipe. In view of the limited axial throw of about 2 cm and the large number of wafers which maybe deposited with a single target over many weeks of even continuous processing, it is reasonable to compensate the spacing on only an occasional basis, for example, once an hour or once a day or more specifically after a large number of wafers have been processed.
• In a control procedure emphasizing the optimized process in which the reactor is being used, the amount of displacement may be determined empirically for a given combination of target, magnetron, initial target/magnetron spacing, and general operating conditions developed for a step in the fabrication of a chip design A convenient unit of target usage is total kilowatt-hours of use since the target was fresh so that the process recipe keeps a running total of kilo-watt hours and adjusts the spacing as a function of the total kilowatt-hours according to a compensation algorithm incorporated into the process recipe and set during development of the recipe. The compensation may be controlled once a set period for this unit has passed. For a given process, wafer count is nearly as good a usage unit.
• A dynamic control algorithm may also be effective. As is evident from theplots122,127 ofFIG. 7, the measured target voltage can be tracked and correlated with deviations from a predetermined value set by the recipe. when the measured voltage deviates by a set voltage increment, the magnetron maybe moved upwardly by a set spacing increment experimentally determined beforehand to largely compensate the voltage increment. In fact, the empirical algorithm may be obtained in corresponding fashion during development of the process in which the development engineer tracks changes in the target voltage as a function of target usage and experimentally determines what spacing compensation is necessary to bring the target voltage back to a set value.
• It is also possible to directly measure the position of the sputtering surface of the target by optical or other means or to measure the thickness of the target by separate electrical means, both approaches providing a measurement of target erosion.
• It is desirable that the compensation be directly measurable by a feedback measurement, for example, the angular position of the set nut or of the linear position of the slider or an angular displacement of one of the rotary parts, all measured from a known position. For example, theposition sensor175 ofFIG. 11 acts as a limit indicator useful for resetting after a power outage or computer glitch.
• It is noted that the baseline magnetron-to-target spacing may vary from one recipe to another and the described lift mechanisms may be used to initially obtain the baseline spacing for a fresh target as well as to maintain it during extended target usage.
• FIG. 17 schematically illustrates an example of a control system for adjusting the spacing S between the front face of themagnetron70 and the back surface of thetarget34 as well as controlling other parts of the sputter reactor. Thelift motor176 is preferably implemented in a stepper motor that is connected through a schematically illustrated mechanical drive310 (for example that ofFIGS. 10 and 11) which can selectively raise or lower themagnetron70. Aflag312 attached to themechanical drive310, and aposition sensor314 detects the position of theflag312, for example, at the extreme of the travel of themechanical drive310 in which themagnetron70 is farthest from thetarget34.
• Acomputerized controller316 is conventionally used to control the sputtering operation according to a process recipe stored within thecontroller316 on arecordable medium318, such as a recordable disk. Thecontroller316 conventionally controls thetarget power supply54 as well as otherconventional reactor elements44,48,58,84, and114. Additionally according to the invention, thecontroller316 controls thestepper motor176 with a controlled series of pulses and a directional signal to drive the magnetron70 a controlled distance in either direction. Thecontroller316 stores the current position of themagnetron70 and, if additional movement is desired, can incrementally move themagnetron70. However, on startup or after some unforseen interrupt, thecontroller316 raises themagnetron70 away from thetarget34 until theposition sensor314 detects theflag312. The setting of thestepper motor176 at this flagged position determines a home position. Thereafter, thecontroller316 lowers themagnetron70 to a desired position or spacing S from thetarget34. This limit detection maybe implemented by theposition sensor175 ofFIG. 11.
• The recipe stored within thecontroller316 may contain the desired compensation rate, for example, as a function of kilowatt hours of power applied to thetarget34 from thepower supply54 or alternatively as a compensation for variation in target voltage. Thecontroller316 can monitor the applied power through awatt meter320 connected between thepower supply54 and the target. However, thepower supply54 is often designed to deliver a selectable constant amount of power. In this case, the total power consumption can be monitored by software within thecontroller316 with no direct power measurement. Thecontroller316 may also monitor the target voltage with avoltmeter322 connected to the power supply line to thetarget34. As mentioned previously, target voltage is a sensitive indicator of the need to compensate the spacing between magnetron and target.
• The spacing compensation may be advantageously applied to the roof magnetron used with a target having an annular vault formed in its surface, as has been described by Gopalraja et al. in U.S. Pat. No. 6,451,177, incorporated herein by reference in its entirety. The invention can also be applied to a sputter reactor having ahollow cathode magnetron330 schematically illustrated inFIG. 18, such as disclosed by Lai et al. in U.S. Pat. No. 6,193,854, incorporated herein by reference in its entirety. Thehollow cathode magnetron330 includes atarget332 formed with a single right circular cylindrical vault extending about acentral axis334 and facing an unillustrated pedestal supporting the wafer. Unillustrated biasing means applied to thetarget332 relative to an anode excites the sputtering working gas into a plasma to sputter the portions of thetarget332 inside the vault to thereby coat a layer onto the wafer of the material of thetarget332.
• Permanent magnets336, usually axially aligned, are placed around the exterior of acircumferential sidewall338 of thetarget332 to serve several functions including intensifying the plasma adjacent thesidewall338. However, in some implementations, the magnets are horizontally aligned to create a bucking field within the vault adjacent thesidewall338. According to the invention, motors or other types ofactuators340 selectively move themagnets336 radially with respect to thecentral axis334 to compensate for sputtering erosion of thetarget sidewalls338. Thehollow cathode magnetron330 may additionally include aroof magnetron342 positioned in back of a disk-shapedroof342 of thetarget332. Theroof magnetron342 may be stationary or be rotated about thecentral axis334. According to the invention, a motor orother actuator346 maybe used to axially move theroof magnetron342 along thecentral axis334 to compensate for erosion of thetarget roof344. However, as has been previously discussed, the various magnet movements may be used alternatively to tune the sputtering process to an initial state as well as to maintain it there.
• An alternativehollow cathode magnetron350 schematically illustrated inFIG. 19 uses asidewall coil352 wrapped around thetarget sidewall338 to produce an axial magnetic field inside the target vault. According to the invention, anadjustable power supply354 supplying the coil current is adjusted, for among other reasons, to compensate for target erosion such that a more constant magnetic field is produced adjacent the interior surface of the eroding target.
• The compensation mechanism is not limited to those which have been described. For example, especially in the case that the magnetron executes only simple rotary motion, the rotary shaft supporting the magnetron can be directly lifted if an additional dynamic or slidable seal allows leak-free axial movement of the rotary shaft. Other types of lift mechanisms and lift drives may be used in achieving the control or compensation of the target/magnetron spacing However, the lead-screw lift mechanism130 ofFIG. 10 has effectively been used for compensating an SIP magnetron of the Fu patent which executes simple rotary motion.
• Although the above described lift mechanisms have been described for raising a magnetron away from the target backside, they may be used as well to lower the magnetron. Also, the apparatus maybe used for purposes other than compensating for target erosion.
• Although the invention has been developed for copper sputtering, it may be used for sputtering other materials dependent on the target material and whether a reactive gas is admitted to the chamber. Such materials include nearly all metals and metal alloys and their reactive compounds used in sputter deposition, including but not limited to Cu, Ta, Al, Ti, W, Co, Ni, NiV, TiN, WN, TaN, Al—Cu alloys, Cu—Al, Cu—Mg, etc.
• The invention may be also applied to other magnetrons such as the more conventional large kidney-shaped magnetrons and to other magnetrons not intended to ionize the sputtered atoms. Nested magnetrons are not required. Long-throw sputter reactors can benefit from the invention. Inductive RF power may be coupled into the magnetron sputter reactor to increase the source power. Although the invention is particularly useful with scanned magnetrons, it may also be applied to stationary magnetrons. It may also be applied to magnets used more for confining the plasma and guiding ions rather than strictly for increasing the plasma density.
• Accordingly, the invention greatly stabilizes a sputtering process over the lifetime of the target with relatively minor additions to the sputter apparatus.
• The above described embodiments do not encompass all possible implementations and uses of the invention. The coverage of the invention should be determined primarily by the specific language of the claims.

Claims (22)

1. In a magnetron sputter reactor including a chamber sealable to a sputtering target, a support in said reactor for holding a substrate to be processed, and a magnetron placeable on a backside of said target opposite said support, a magnetron control system comprising:

a lift mechanism affixed to a support of said magnetron capable of varying a distance of said magnetron from said backside of said target; and

a controller for controlling a degree of said varying said distance during processing of a sequence of substrates.

2. The lift mechanism ofclaim 1, including a motor driving said lift mechanism in response to said controller.

3. A sputter reactor, comprising:

a target affixed to vacuum chamber;

a pedestal within said chamber for support a substrate in opposition to said target;

a magnetron positioned on a back side of said target;

a power supply selectively applying power to said target to excite a plasma within said chamber to thereby sputter material from a front side of said target onto said substrate;

a mechanism for varying a spacing between said magnetron and said target; and

a controller controlling said mechanism to adjust said spacing during a predetermined sequence of depositing said material onto a plurality of said substrates.

4. The reactor ofclaim 3, wherein said controller adjusts said spacing to compensate for an erosion of said front side of said target by said plasma.

5. A method of sputtering onto a substrate support on a support in a sputtering reactor chamber including a target fixed to said chamber and a magnetron disposed on a back of said target opposite said support, comprising:

plural first steps of exciting a plasma in said chamber and depositing material of said target onto sequential ones of a plurality of substrates; and

plural second steps of lifting said magnetron away from said backside of said target performed during or after different ones of said first steps.

6. The method ofclaim 5, further comprising rotating said magnetron about a central axis of said target.

7. The method ofclaim 5, wherein said rotating step causes said magnetron to execute planetary motion about said central axis.

8. The method of anyclaim 5, wherein said second steps lift said magnetron to compensate for an erosion depth of said target after said target enters service.

9. A magnetron sputtering method, comprising adjusting a magnetic field, which is assisting generation of a plasma in sputtering a target, in compensation for erosion of a front of said target by particles from said plasma.

10. The method ofclaim 9, wherein said adjusting comprises moving a magnetron away from a back of said target.

11. The method ofclaim 9, wherein said adjusting reduces variations in said magnetic field at said front of said target caused by said erosion.

12. The method ofclaim 9, wherein said adjusting reduces variations in said plasma at said front of said target caused by said erosion.

13. The method ofclaim 9, wherein a degree of said adjusting is determined according to a recipe for said sputtering.

14. A method for use with a plasma sputter reactor chamber mounting a target facing an interior of the chamber and with a magnetron positioned on the backside of said target exteriorly of the chamber, comprising the steps of:

exciting a plasma in said chamber so as to deposit material of said target onto sequential ones of a plurality of substrates;

lifting said magnetron away from the backside during or after said exciting of said plasma or deposition of material; and

a controller controlling a degree of said lifting at an intermediate time of processing of said plurality of substrates.

15. The method ofclaim 14, wherein said controller controls said degree of said lifting according to an amount of erosion of a front side of said target during processing of said plurality of substrates.

16. The method ofclaim 14, wherein there is no substantial effective movement of said magnetron towards said target during a sequence of depositions of said material onto different substrates.

17. The method ofclaim 16, in which said lifting of said magnetron is performed during or after any one or more of said exciting of said plasma or said depositing of material.

18. The method ofclaim 16, in which said lifting is at performed plural different times.

19. The method of claims14, further comprising rotating said magnetron about a central axis of said target to execute circular movement about said central axis.

20. The method ofclaim 14, further comprising rotating said magnetron to execute planetary motion about said central axis.

21. A hollow cathode sputter reactor, comprising:

a vacuum chamber arranged about a central axis;

a support within said chamber for holding a substrate to be processed;

a sputter target sealable to said chamber and having a right cylindrical vault arranged about said central axis in opposition to said support formed by a generally disk-shaped roof and a generally tubular sidewall;

a magnet assembly disposed adjacent a back surface of either said roof or said sidewall; and

a lift mechanism supporting said magnet assembly and controllably varying a distance between said magnet assembly and said back surface.

22. The reactor ofclaim 21, wherein said lift mechanism includes an actuator controllably moving a support member of said magnet assembly.

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