A typical prior art workpiece processor, as illustrated in FIG. 1, includes vacuum plasma processing chamber assembly 10, a first circuit 12 for driving a planar excitation antenna 48 consisting of a coil for exciting ionizable gas in chamber assembly to a plasma state, a second circuit 14 for applying RF bias to a workpiece holder in chamber assembly 10, and a controller arrangement 16 responsive to sensors for various parameters associated with chamber assembly 10 for deriving control signals for devices affecting the plasma in chamber assembly 10. Controller 16 includes microprocessor 20 which responds to various sensors associated with chamber assembly 10, as well as circuits 12 and 14, and signals from operator input 22, which can be in the form, for example, of a keyboard. Microprocessor 20 is coupled with memory system 24 including hard disk 26, random access memory (RAM) 28 and read only memory (ROM) 30. Microprocessor 20 responds to the various signals supplied to it to drive display 32, which can be a typical computer monitor.
Hard disk 26 and ROM 30 store programs for controlling the operation of microprocessor 20 and preset data associated with different recipes for the processes performed in chamber assembly 10. The different recipes concern gas species and flow rates applied to chamber assembly 10 during different processes, the output power of AC sources included in circuits 12 and 14, the vacuum applied to the interior of chamber assembly 10, and initial values of variable reactances included in matching networks of circuits 12 and 14.
Plasma chamber assembly 10 includes chamber 40 having non-magnetic cylindrical side wall 42 and non-magnetic base 44, both of which are frequently metal and electrically grounded. Dielectric, typically quartz, window 46 is fixedly positioned on the top edge of wall 42.
Wall 42, base 44 and window 46 are rigidly connected to each other by suitable gaskets to enable a vacuum to be established within the interior of chamber 40. Plasma excitation antenna 48 includes coil 49, that is planar or dome shaped, and can be configured as disclosed in Ogle, U.S. Pat. No. 4,948,458 or Holland et al., U.S. Pat. No. 5,759,280 or Holland et al, U.S. Pat. No. 5,800,619 sits on or in very close proximity to the upper face of window 46. Antenna 48 reactively supplies magnetic and electric RF fields to the interior of chamber 40, to excite ionizable gas in the chamber to a plasma, schematically illustrated in FIG. 1 by reference numeral 50.
The upper face of base 44 carries holder (i.e. chuck) 52 for workpiece 54, which is typically a circular semiconductor wafer, a rectangular dielectric plate such as used in flat panel displays or a metal plate. Workpiece holder 52 typically includes metal plate electrode 56 which carries dielectric layer 58 and sits on dielectric layer 60, which is carried by the upper face of base 44. A workpiece handling mechanism (not shown) places workpiece 54 on the upper face of dielectric layer 58. Workpiece 54 is cooled by supplying helium from a suitable source 62 to the underside of dielectric layer 58 via conduit 64 and grooves (not shown) in electrode 56. With workpiece 54 in place on dielectric layer 58, d.c. source 66 supplies a suitable voltage through a switch (not shown) to electrode 56 to clamp, i.e., chuck, workpiece 54 to holder 52.
With workpiece 54 secured in place on chuck 52, one or more ionizable gases from one or more sources 68 flow into the interior of chamber 40 through conduit 70 and port 72 in sidewall 42. For convenience, only one gas source 68 is shown in FIG. 1. The interior of conduit 70 includes valve 74 and flow rate gauge 76 for respectively controlling the flow rate of gas flowing through port 72 into chamber 40 and measuring the gas flow rate through port 72. Valve 74 responds to a signal microprocessor 20 derives, while gauge 76 supplies the microprocessor with an electric signal indicative of the gas flow rate in conduit 70. Memory system 24 stores for each recipe of each workpiece 54 processed in chamber 40 a signal indicative of desired gas flow rate in conduit 70. Microprocessor 20 responds to the signal memory system 24 stores for desired flow rate and the monitored flow rate signal gauge 76 derives to control valve 74 accordingly.
Vacuum pump 80, connected to port 82 in base 44 of chamber 40 by conduit 84, evacuates the interior of the chamber to a suitable pressure, typically in the range of one to one hundred millitorr. Pressure gauge 86, in the interior of chamber 40, supplies microprocessor 20 with a signal indicative of the vacuum pressure in chamber 40.
Memory system 24 stores for each recipe a signal indicative of desired vacuum pressure for the interior of chamber 40. Microprocessor 20 responds to the stored desired pressure signal memory system 24 derives for each recipe and an electric signal from pressure gauge 86 to supply an electric signal to vacuum pump 80 to maintain the pressure in chamber 40 at the set point or predetermined value for each recipe.
Optical spectrometer 90 monitors the optical emission of plasma 50 by responding to optical energy emitted by the plasma and coupled to the spectrometer via window 92 in side wall 42. Spectrometer 90 responds to the optical energy emitted by plasma 50 to supply an electric signal to microprocessor 20. Microprocessor 20 responds to the signal spectrometer 90 derives to detect an end point of the process (either etching or deposition) that plasma 50 is performing on workpiece 54. Microprocessor 20 responds to the signal spectrometer 90 derives and a signal memory system 24 stores indicative of a characteristic of the output of the spectrometer associated with an end point to supply the memory with an appropriate signal to indicate the recipe has been completed. Microprocessor 20 then responds to signals from memory system 24 to stop certain activities associated with the completed recipe and initiate a new recipe on the workpiece previously processed in chamber 40 or commands release of workpiece 54 from chuck 52 and transfer of a new workpiece to the chuck, followed by instigation of another series of processing recipes.
Excitation circuit 12 for driving coil 49 of antenna 48 includes constant or variable frequency RF source 100 (see Barnes et al U.S. Pat. No. 5,892,198), typically having a frequency of 4.0±10% MHz or 13.56±10% MHz. Source 100 drives variable gain power amplifier 102, typically having an output power in the range between 100 and 3000 watts. Amplifier 102 typically has a 50 ohm output impedance all of which is resistive and none of which is reactive. Hence, the impedance seen looking back into the output terminals of amplifier 102 is typically represented by (50+j0) ohms, and cable 106 is chosen to have a characteristic impedance of 50 ohms.
For any particular recipe, memory system 24 stores a signal for desired output power of amplifier 112. Memory system 24 supplies the desired output power of amplifier 102 to the amplifier by way of microprocessor 20. The output power of amplifier 102 can be controlled in an open loop manner in response to the signals stored in memory system 24 or control of the output power of amplifier 102 can be on a closed loop feedback basis, as known in the art.
The output power of amplifier 102 drives coil 49 via cable 106 and matching network 108. Matching network 108, configured as a “T,” includes two series legs including variable capacitors 112 and 116, as well as a shunt leg including fixed capacitor 114. The antenna 48 includes excitation terminals 122 and 124, respectively connected to (1) a first end of coil 49 and one electrode of capacitor 112 and (2) a second end of coil 49 and a first electrode of series capacitor 126, having a grounded second electrode; or terminal 124 can be connected directly to ground. The value of capacitor 126 is preferably selected as described in the commonly assigned, previously mentioned, Holland et al. '200 patent.
Electric motors 118 and 120, preferably of the step type, respond to signals from microprocessor 20 to control the values of capacitors 112 and 116 in relatively small increments to maintain an impedance match between the impedance seen by looking from the output terminals of amplifier 102 into cable 106 and by looking from cable 106 into the output terminals of amplifier 102. Hence, for the previously described (50+j0) ohm output impedance of amplifier 102 and 50 ohm characteristic impedance of cable 106, microprocessor 20 controls motors 118 and 120 so the impedance seen looking from cable 106 into matching network 108 is as close as possible to a matched impedance of (50+j0) ohm. Alternatively, microprocessor 20 controls the frequency of source 100 and the capacitance of capacitor 116 to achieve a matched impedance between the source and the load it drives. As a result of a matched impedance being attained, the current flowing through capacitors 112 and 126 and the leads connecting the capacitors to terminals 122 and 124, is typically within a couple of percent of its very high maximum value. The very high current in these leads has an adverse effect on the uniformity of the density of plasma 50.
To control motors 118 and 120 or the frequency of source 100 and motor 120 to maintain matched conditions between the impedance seen looking into the output terminals of amplifier 102 and the impedance amplifier 102 drives, microprocessor 20 responds to signals from conventional sensor arrangement 104. The signals are indicative of the impedance seen looking from cable 106 into matching network 108; usually the signals represent the absolute values of the current and voltage reflected toward the sensor from capacitor 118, and the phase angle between the reflected current and voltage. Alternatively, sensors are provided for deriving signals indicative of the power that amplifier 102 supplies to its output terminals and the power reflected by cable 106 back to the output of amplifier 102. Microprocessor 20 responds, in one of several known manners, to the sensed signals sensor arrangement 104 derives to control motors 118 and 120 or the frequency of source 100 and motor 120 to attain the matched condition.
Because of variations in conditions in the interior of chamber 40 which affect plasma 50, the plasma has a variable impedance. The conditions are aberrations in the flow rate and species of the gas flowing through port 72, aberrations in the pressure in chamber 40 and other factors. In addition, noise is sometimes supplied to motors 118 and 120 causing the motors to change the values of capacitors 112 and 116. All of these factors affect the impedance reflected by the load including plasma 50 back to the output terminals of amplifier 102. Microprocessor 20 responds to the output signals of sensor 104, to vary the values of capacitors 112 and 116 or the frequency of source 100, to maintain the impedance driven by the output terminals of amplifier 102 matched to the output impedance of the amplifier.
Circuit 14 for supplying RF bias to workpiece 54 via electrode 56 has a construction somewhat similar to circuit 12. Circuit 14 includes constant frequency RF source 130, typically having a frequency such as 400 kHz, 2.0 MHz or 13.56 MHz. The constant frequency output of source 130 drives variable gain power amplifier 132, which in turn drives a cascaded arrangement including directional coupler 134, cable 136 and matching network 138. Matching network 138 includes a series leg comprising the series combination of fixed inductor 140 and variable capacitor 142, as well as a shunt leg including fixed inductor 144 and variable capacitor 146. Motors 148 and 150, which are preferably step motors, vary the values of capacitors 142 and 146, respectively, in response to signals from microprocessor 20.
Output terminal 152 of matching network 138 supplies an RF bias voltage to electrode 56 by way of series coupling capacitor 154 which isolates matching network 138 from the chucking voltage of d.c. source 66. The RF energy circuit 14 applies to electrode 56 is capacitively coupled via dielectric layer 48, workpiece 54 and a plasma sheath between the workpiece and plasma to a portion of plasma 50 in close proximity with chuck 52. The RF energy chuck 52 couples to plasma 50 establishes a d.c. bias in the plasma; the d.c. bias typically has values between 50 and 1000 volts. The d.c. bias resulting from the RF energy circuit 14 applies to chuck 52 accelerates ions in the plasma 50 to workpiece 54.
Microprocessor 20 responds to signals indicative of the impedance seen looking from cable 136 into matching network 138, as derived by a known sensor arrangement 139, to control motors 148 and 150 and the values of capacitors 142 and 146 in a manner similar to that described supra with regard to control of capacitors 112 and 116 of matching network 108.
For each process recipe, memory system 24 stores a set point signal for the net power flowing from directional coupler 134 into cable 136. The net power flowing from directional coupler 134 into cable 136 equals the output power of amplifier 132 minus the power reflected from the load and matching network 138 back through cable 136 to the terminals of directional coupler 134 connected to cable 136. Memory system 28 supplies the net power set point signal associated with circuit 14 to microprocessor 20. Microprocessor 20 responds to the net power set point signal associated with circuit 14 and the output signals that directional coupler 134 supply to power sensor arrangement 141. Power sensor arrangement 141 derives signals indicative of output power of amplifier 132 and power reflected by cable 136 back toward the output terminals of amplifier 132.
FIG. 2 is a perspective view of an antenna consisting of a planar coil of the type schematically illustrated in FIG. 6 of the previously mentioned '619 patent and which has been incorporated as the coil of antenna 48 in processors of the type illustrated in FIG. 1. The coil illustrated in FIG. 2 includes a single winding 160 including inner and outer concentric metal turns 162 and 164, each of which has a square cross-section and is shaped as a sector of a circle extending through an angle of approximately 340 degrees. Opposite ends of turns 162 and 164 respectively include excitation terminals 166 and 168, respectively connected by metal posts (i.e. current feeds) 170 and 172 to one electrode of capacitor 112 of matching network 108 and to one electrode of capacitor 126; alternatively, post 172 connects excitation terminal 168 to ground directly. Consequently, the RF (i.e. AC) current which flows in posts 170 and 172 is approximately equal to the RF current which flows in turns 162 and 164. The ends of turns 162 and 164 remote from terminals 166 and 168 are connected to each other by straight metal strut 174 that extends generally radially between turns 162 and 164 and has the same cross-sectional configuration as the turns.
The two turn coil of FIG. 2 differs from an ideal two turn coil which consists of two coaxial circular loops having constant, equal amplitude RF currents flowing therein throughout the length of each loop. Such an ideal two turn coil would provide, to the plasma 50 in chamber 40, electric and magnetic fields having complete cylindrical symmetry. The coil of FIG. 2, as well as all practical coils that can be used as the coil of antenna 48, has connections (such as strut 174 that connects turns 162 and 164) between any loops or windings included in the coil, and current feed points, such as excitation terminals 166 and 168 that connect posts 170 and 172 to turns 162 and 164. These connections prevent all practical coils from having the complete cylindrical symmetry of the idealized coil.
The currents in the practical coil of FIG. 2 can be expressed as the sum of the current in the ideal portions of the coil, i.e., turns 162 and 164, plus the current in a hypothetical perturbation coil that includes terminals 166 and 168, posts 170 and 172, and strut 174. The hypothetical perturbation coil thus includes the effects of the current feeds formed by posts 170 and 172, the “missing” sections of the loops formed by turns 162 and 164, as well as strut 174 which forms a connection between the loops formed by turns 162 and 164. The current flowing in the hypothetical perturbation coil, including the high current flowing in the current feeds formed by posts 170 and 172, has a tendency to cause azimuthal asymmetry in the magnetic field coupled by the coil to the plasma, resulting in azimuthal asymmetry in the plasma density processing the workpiece.
One object of the present invention is to provide a new and improved plasma processor including a plasma having a density with reduced azimuthal asymmetry.
Another object of the present invention is to provide a new and improved antenna arrangement for a plasma processor.
An added object is to provide a new and improved plasma processor antenna arrangement for enabling the plasma of the processor to have density with relatively low asymmetry.
An additional object of the present invention is to provide a new and improved antenna arrangement for a plasma processor, wherein the antenna arrangement is arranged so that the perturbing effects of RF feeds that supply current to the antenna arrangement are reduced compared to a typical prior art arrangement.
A further object of the present invention is to provide a new and improved plasma processor wherein a relatively high current flows in a plasma excitation coil of an antenna while a substantially lower amplitude current flows in the leads connecting the antenna to circuitry which drives the antenna.