Multiple frequency pulsing of multiple coil source to control plasma ion density radial distribution

Abstract

A method is provided for processing a workpiece supported on a support surface in a chamber of a plasma reactor. A process gas is introduced into the chamber and a plasma is generated with pulse-modulated RF power. The method comprises successively repeating the following cycle: (a) concentrating the plasma in the chamber in a center-high plasma ion distribution for a first on-time duration; (b) permitting plasma to drift during a first off-time duration away from the center-high plasma ion distribution; (c) concentrating the plasma in the chamber in an edge-high plasma ion distribution for a second on-time duration; and (d) permitting plasma to drift during a second off-time duration away from the edge-high plasma ion distribution. The method further comprises adjusting a plasma process rate near a center of the workpiece by adjusting a duty cycle of the first on-time and first off-time. The method also comprises adjusting a plasma process rate near a periphery of the workpiece by adjusting a duty cycle of the second on-time and second off-time.

Description

TECHNICAL FIELD

The disclosure concerns the processing of a workpiece or semiconductor wafer in a plasma reactor having plural overhead coils for applying RF plasma source power, and in particular a method for controlling and improving uniformity of the radial distribution of plasma ion density.

BACKGROUND

A plasma reactor that generates an inductively coupled plasma is capable of etching thin films on a workpiece such as a semiconductor wafer at a relatively high etch rate. Such a reactor has an inductively coupled plasma source power applicator, typically a coil antenna, coupled to an RF power generator. As the wafer diameter has increased in recent years, the chamber size has increased accordingly, requiring larger coil antennas, which greater inductance and more concentrated power deposition profiles. Power deposition tends to peak in narrow annular regions underlying the coil antenna or underlying inner and outer coil antennas. Such concentrated profiles cause large peaks in the plasma ion density distribution that are difficult to compensate, leading to reduced process uniformity across the wafer. Some improvement in process uniformity can be achieve using two (or more) concentric coil antennas over the reactor ceiling, one antenna overlying the wafer periphery and the other being closer to the wafer center. Even though such a configuration can improve process uniformity, the concentrated peaks in the power deposition profiles of the inner and outer coil antennas lead to process non-uniformities that are difficult to reduce.

SUMMARY

A method is provided for processing a workpiece supported on a support surface in a chamber of a plasma reactor. A process gas is introduced into the chamber and a plasma is generated with pulse-modulated RF power. The method comprises successively repeating the following cycle: (a) concentrating the plasma in the chamber in a center-high plasma ion distribution for a first on-time duration; (b) permitting plasma to drift during a first off-time duration away from the center-high plasma ion distribution; (c) concentrating the plasma in the chamber in an edge-high plasma ion distribution for a second on-time duration; and (d) permitting plasma to drift during a second off-time duration away from the edge-high plasma ion distribution. The method further comprises adjusting a plasma process rate near a center of the workpiece by adjusting a duty cycle of the first on-time and first off-time. The method also comprises adjusting a plasma process rate near a periphery of the workpiece by adjusting a duty cycle of the second on-time and second off-time.

In one embodiment, the adjustment of the plasma process rate near a center of the workpiece comprises reducing the plasma process rate near the center of the workpiece and the adjusting the first duty cycle comprises reducing the first duty cycle. In one embodiment, the adjustment of a plasma process rate near a periphery of the workpiece comprises reducing the plasma process rate near the periphery of the workpiece and the adjusting the second duty cycle comprises reducing the second duty cycle.

In one embodiment, the reduction in plasma process rate near the center of the workpiece and the reduction in plasma process rate near the periphery of the workpiece reduces non-uniformity in distribution of process rate across the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 illustrates a plasma reactor adapted to carry out processes disclosed herein.

FIGS. 2A, 2B and 2C illustrate a chronological sequence depicting how plasma ion distribution spreads out over time following a trailing edge of pulsed RF power applied to the inner coil only in the reactor of FIG. 1.

FIGS. 3A, 3B and 3C illustrate a chronological sequence depicting how plasma ion distribution spreads out over time following a trailing edge of pulsed RF power applied to the outer coil only in the reactor of FIG. 1.

FIGS. 4A and 4B are contemporaneous timing diagrams of pulse waveforms that pulse-modulate RF power applied to the inner and outer coils, respectively, of the reactor of FIG. 1.

FIGS. 5A, 5B, 5C, 5D, 5E and 5F illustrate a chronological sequence depicting how plasma ion distribution alternately (a) concentrates during pulse on times below one or the other of the inner and outer coils of FIG. 1, and (b) spreads out during off times between pulses.

FIGS. 6A, 6B, 6C, 6D, 6E and 6F depict radial distributions of etch rate corresponding to FIGS. 5A, 5B, 5C, 5D, 5E and 5F, respectively.

FIG. 7 illustrates a process in accordance with one embodiment.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings in the figures are all schematic and not to scale.

DETAILED DESCRIPTION

FIG. 1 illustrates a plasma reactor adapted to carry but certain processes in accordance with embodiments disclosed herein. The reactor includes a vacuum chamber 100 enclosed by a cylindrical side wall 105 and a disk-shaped ceiling 110. A wafer support pedestal 120 within the chamber 100 has a top insulating layer 121, a conductive base 122, a cylindrical side wall 123 and a mesh electrode 124 within the insulating layer 121. The top of the insulating layer 121 defines a wafer support surface 125 that is separated from the mesh electrode 124 by a very thin portion of the insulating layer 121. A semiconductor wafer 126 can be supported on the wafer support surface 125. A process gas supply 130 furnishes process gases to the chamber 100 through gas injection apparatus 135 of any suitable type, such as individual gas injectors 135a, 135b. Chamber pressure is controlled by a vacuum pump 140 and by the flow rate of process gases from the injection apparatus 135. The reactor of FIG. 1 further includes an inductively coupled plasma source power applicator consisting of an inner coil antenna 150 and an outer coil antenna 155 concentric with the inner coil antenna 150. Each coil antenna 150, 155 may consist of an individual conductor or wire helically wound in a torus (as shown in FIG. 1), or may be a flat winding helical winding. In order to enhance inductive coupling of RF power from the antennas 150, 155 into the chamber 100, the ceiling 110 may be formed of an insulating material. The side wall 105 may be metal that is coupled to RF ground. The inner coil antenna 150 receives RF power from a first RF power generator 160 through a first pulse gate 162 and through a first RF impedance match circuit or element 164. The outer coil antenna 155 receives RF power from a second RF power generator 165 through a second pulse gate 167 and through a second RF impedance match circuit 169. A programmable controller 170 or other suitable device controls the pulse gates 162, 167. RF plasma bias power from an RF generator 180 is coupled to the wafer 126 by applying it to the mesh electrode 124 (or alternatively to the conductive base 122) through an RF impedance match element 182. Optionally, a second power generator 184 may be coupled to the mesh electrode 124 (or alternatively to the conductive base 122) through another impedance match element 186. The RF bias generator 180 may be a low frequency generator while the RF generator 184 may be a high frequency or very high frequency generator. The two source power generators 160, 165 may be of similar frequencies (e.g., LF or HF) that are sufficiently offset from one another to avoid coupling between them, for example.

It is our discovery that pulse-modulating the RF power applied to the coil antennas 150, 155 can be performed or controlled in such a way as to solve the problem non-uniformity caused by the concentrated power deposition profile of the coil antennas 150, 155. In one embodiment, this is accomplished by causing the plasma to alternate between different ion density distributions, so that the plasma processing results reflect a time average of the different distributions. During each pulse cycle, RF power is turned off at the trailing edge of the pulse, which permits the plasma to drift away from a concentrated profile to a more diffuse profile. This movement in plasma distribution provides an time-averaged plasma distribution that has better uniformity.

In one embodiment, FIGS. 2A, 2B and 2C depict plasma distribution across the process region overlying the wafer 126 in chronological sequence. The regions labeled 190 and 191 in each of FIGS. 2A, 2B and 2C correspond to zones of concentrated plasma ion density. For example, the region 190 may have on the average of about 10¹¹ ions/cc, while the region 191 may have on the average of about 1.5·10¹⁰ ions/cc. The remainder of the chamber 100 outside of both regions 190, 191 has a much lower plasma ion density, on the average less than 10¹⁰ ions/cc. FIG. 2A represents the distribution during the time that RF power is applied to the inner coil antenna 150 only. FIG. 2B illustrates the distribution shortly after power has been turned off and FIG. 2C illustrates the distribution after power has been turned off for a somewhat longer time. The time differences between FIGS. 2A, 2B and 2C may be on the order of 0.01-100 milliseconds. In FIG. 2A, plasma ions concentrate over the center of the wafer. In FIG. 2B, removal of RF power causes the plasma to drift out of the concentrated profile of FIG. 2A and begin to distribute outwardly away from the center. In FIG. 2C, the continue drift of the plasma results in any even greater radial spreading of plasma ion distribution away from the center and toward the periphery of the process region.

In one mode, the inner coil RF power is pulse modulated by the gate 162 with a desired repetition rate and duty cycle in which the plasma ion distribution corresponds to FIG. 2A during the “ON” time and during the time between pulses from the distribution of FIG. 2A to that of FIG. 2B and later to that of FIG. 2C. The spreading of the distribution depicted in FIGS. 2B and 2C is halted at the beginning of the next cycle when RF power is again applied to the inner coil 150.

In another embodiment, FIGS. 3A, 3B and 3C depict plasma distribution across the process region overlying the wafer 126 in another chronological sequence involving the outer coil 155. The regions labeled 193 and 194 in each of FIGS. 3A, 3B and 3C correspond to zones of concentrated plasma ion density. For example, the region 193 may have on the average of about 10¹¹ ions/cc, while the region 194 may have on the average of about 1.5.10¹⁰ ions/cc. The remainder of the chamber 100 outside of both regions 193, 194 has a much lower plasma ion density, on the average less than 10¹⁰ ions/cc. FIG. 3A represents the distribution during the time that RF power is applied to the outer coil antenna 155 only. FIG. 3B illustrates the distribution shortly after power has been turned off and FIG. 3C illustrates the distribution after power has been turned off for a somewhat longer time. The time differences between FIGS. 3A, 3B and 3C may be on the order of 0.01-100 milliseconds. In FIG. 3A, plasma ions concentrate over or near the periphery of the wafer. In FIG. 3B, removal of RF power causes the plasma to drift out of the concentrated profile of FIG. 3A and begin to distribute inwardly toward the center and away from the periphery. In FIG. 3C, the continue drift of the plasma results in any even greater radial spreading of plasma ion distribution away from the periphery and toward the center of the process region.

In one mode, the outer coil RF power is pulse modulated by the gate 167 with a desired repetition rate and duty cycle in which the plasma ion distribution corresponds to FIG. 3A during the “ON” time and during the time between pulses from the distribution of FIG. 3A to that of FIG. 3B and later to that of FIG. 3C. The spreading of the distribution depicted in FIGS. 3B and 3C is halted at the beginning of the next cycle when RF power is again applied to the outer coil 155.

In one mode, pulses of RF power are applied alternately to the inner and outer coils 150, 155. FIGS. 4A and 4B are contemporaneous time domain diagrams of enabling signals applied to the pulse gates 162, 167 by the controller 170 in such a mode. FIGS. 5A through 5F depict a chronological sequence of changing plasma ion distributions in the process zone over one cycle corresponding to FIGS. 4A and 4B. Reference is now made to FIGS. 4A, 4B and 5A through 5F. From time T1 to time T2, RF power is applied to the inner coil 150 only, so that plasma distribution is concentrated over the center of the process region (FIG. 5A). At time T2, power is turned off, and the plasma begins to drift or progressively spread outward and away from the center to become less concentrated. This trend (FIGS. 5B and 5C) continues until time T3, when RF power is applied to the outer coil 155. The causes the plasma to concentrate near the edge of the process zone (FIG. 5D). At time T4, power is turned off, and the plasma begins to drift toward the center of the process region, spreading more over time as depicted in FIGS. 5E and 5F. This continues until time T5, when a new cycle is begun and power is again applied to the inner coil 150, at which point the distribution returns to that depicted in FIG. 5A.

The resulting change in etch rate distribution is depicted in the chronological sequence of FIGS. 6A through 6F. In FIG. 6A, etch rate peaks at the wafer center (time T1 through time T2) during application of RF power to the inner coil 150. In FIGS. 6B and 6C, etch rate begins to decrease over the center and increase away from the center (time T2 through T3). In FIG. 6D, progression of etch rate away from the center and toward the edge results in concentration of the maximum etch rate at the periphery (time T3 through T4) while RF power is applied to the outer coil 155. In FIGS. 6E and 6F, RF power is turned off and etch rate distribution drifts back away from the periphery and toward the center. The foregoing cycle repeats itself at time T5.

Etch results on the wafer at the end of the etch process are the time-average of all of the etch rate distributions (samples of which are depicted in FIGS. 6A through 6F) over the entire etch process. The progression of etch rate distribution actually consists of a continuum of distributions, not merely the six discrete distributions of FIGS. 6A through 6F. The time average spans this continuum of distributions. This time average of etch rate (or ion density) distribution is far more uniform than can be achieved by the conventional methods in which RF power is continuously applied to one or both of the inner and outer coils 150, 155. The plasma drift resulting in the progression of etch rate distribution as depicted in FIGS. 6A-6F provides a far more continuous movement of etch rate across the wafer, which tends to reduce or eliminate areas of lower etch rate or areas of peak etch rate.

In an exemplary process, a polysilicon film overlying a thin gate oxide layer is to be etched to form polysilicon gates. A silicon etch gas, such a fluorine-containing species, is introduced into the chamber 100 of FIG. 1 with the wafer 126 supported on the pedestal 120. RF bias power is applied to the electrode 124 from the RF generator 180. The controller 170 causes RF power to be applied alternately to the inner and outer coils 150, 155 in accordance with the waveforms of FIGS. 4A and 4B. The duty cycles of the pulses applied to the inner coil gate 162 (FIG. 4A) and of the pulses applied to the outer coil gate 167 (FIG. 4B) are adjusted to maximize uniformity of the time average of the progression of etch rate distributions realized over many cycles of the waveforms of FIGS. 4A and 4B.

For example, if the etch rate distribution is too concentrated at the center during the inner coil on-time (times T1-T2) and moreover is too concentrated at the edge during the outer coil on-time (times T3-T4), then this excessive concentration is compensated by reducing the duty cycles of both the pulses applied to the inner coil gate 162 (FIG. 4A) and the pulses applied to the outer coil gate 167 (FIG. 4B). This allows greater time during which no RF power is applied to either coil 150, 155 and the plasma drifts away from its more concentrated distribution states, and provides a more uniform time-averaged etch rate distribution. The duty cycles of the control pulses governing the pulsed RF on the two coils 150, 155 (FIGS. 4A and 4B) may be the same or may be different depending upon the differences in design or performance of the two coils 150, 155. In the illustrated example of FIGS. 4A and 4B, the duty cycles of the pulse waveform of FIG. 4A and the pulse waveform of FIG. 4B are approximately the same and are on the order of about ⅙. However, other choices of duty cycle may be made depending upon a particular reactor design and process recipe. In another embodiment, the duty cycle may be increased, so as to decrease the power off interval (e.g., from time T2 to time T3) to a lesser time period. In the example of FIGS. 4A and 4B, the pulse widths of the two control signals are depicted as being the same. However, the pulse widths (duty cycles) of the pulse signals of FIGS. 4A and 4B may be chosen independently and differ significantly from one another.

As one example involving different duty cycles applied to the inner and outer coils 150, 155, if etch rate is higher over the center and weaker at the periphery, then the duty cycle of the pulse waveform of FIG. 4A applied to the inner coil gate 162 may be decreased and/or the duty cycle of the pulse waveform of FIG. 4B applied to the outer coil gate 167 may be increased. Conversely, if etch rate is predominant over the periphery and weak at the center, then the duty cycle of the pulse waveform of FIG. 4A applied to the inner coil gate 162 may be increased and/or the duty cycle of the pulse waveform of FIG. 4B applied to the outer coil gate 167 may be decreased.

In some applications, the duty cycles of the pulsing of the gates 162, 167 may be set to relatively high values. For example, both duty cycles may exceed 50%, in which case the “on” time periods of the two coils 150, 155 will be partially contemporaneous or overlapping. In other words, each coil will be turned off after the other coil has been turned on. In this case, RF coupling between the two coils can be minimized by offsetting the frequencies of the two RF generators 160, 165. As one possible example of this, the two RF generators may have respective frequencies of 2.75 MHz and 2.25 MHz.

The period or length of one cycle in the pulse waveforms of FIGS. 4A and 4B (i.e., the period from time T1 to time T5) is in one embodiment relatively short, for example on the order of 0.01-100 milliseconds. Shorter values of this time period provide the best continuity of etch performance and minimize fluctuations within a single etch process or step.

A process in accordance with one embodiment is depicted in FIG. 7. The wafer 126 is placed in the chamber 100 and a process gas is introduced (block 200 of FIG. 7). RF power is applied to the inner and outer coils 150, 155 through the respective pulse gates 162, 167 (block 210). The controller 170 enables power flow through the respective gates 162, 167 during alternate time windows defined by applying alternating pulses waveforms to the gates 162, 167 (block 215). The duty cycles of the respective pulse waveforms are adjusted to optimize uniformity of radial ion distribution over the wafer (block 220). If the inner coil 150 produces an excessively concentrated or peak ion distribution or etch rate over the wafer center, then the duty cycle of the pulse waveform applied to the inner coil 150 is reduced (block 221). This decrease in inner coil duty cycle allows more time for plasma drift following temporary RF power removal to spread or even out the ion distribution or counter the non-uniform distribution created during the preceding pulse of RF power. Alternatively (or in addition) to decreasing the inner coil duty cycle, the outer coil duty cycle may be increased, provided that this increase does not result in excessive ion concentration by the outer coil 155. If the outer coil 155 produces an excessively concentrated or peak ion distribution or etch rate over the wafer periphery, then the duty cycle of the pulse waveform applied to the outer coil 155 is reduced (block 222). This decrease in outer coil duty cycle allows more time for plasma drift following temporary RF power removal to spread or even out the ion distribution or counter the non-uniform distribution created during the preceding pulse of RF power. Alternatively (or in addition) to decreasing the outer coil duty cycle, the inner coil duty cycle may be increased provided that this increase does not result in excessive ion concentration by the inner coil 150. In one embodiment, if a higher overall plasma ion density (or higher process rate or higher etch rate) is desired, then the duty cycle of one or both waveforms may be increased up to a point at which uniformity may be compromised (block 223).

While foregoing embodiments have been described with reference to RF generators 160, 165 with pulsed gates 162, 167 for pulse modulating the RF outputs of the generators 160, 165, the generator 160 and corresponding gate 162 may be combined in one unit as a commercially available pulse-modulated RF generator. Likewise, the generator 165 and corresponding gate 167 may be combined in another similar unit.

While the foregoing description of embodiments having at least two coils (e.g., the inner and outer coils 150, 155) have been described with reference to operational modes in which the RF power to both coils is pulse-modulated, in another embodiment both coils 150, 155 are driven with RF power but only one of the two coils is driven with pulse-modulated RF power. In such an embodiment, for example, RF power to the inner coil 150 would be pulse modulated in accordance with the sequence of FIG. 4A, while RF power to the outer coil would be applied continuously. Alternatively, RF power to the outer coil 155 would be pulsed modulated in accordance with the sequence of FIG. 4B, while RF power to the inner coil would be applied continuously. The controller 170 may be configured to implement either of these embodiments by pulsing one of the two gates 162, 167 while continuously enabling (holding “on”) the other of the two gates. This corresponds to an operational mode in which the pulse duty cycle to one of the coils is 100%.

While the foregoing description of embodiments having at least two coils (e.g., the inner and outer coils 150, 155) have been described with reference to separate independent RF power generators for each coil (e.g., the RF power generators 160, 165), in another embodiment only a single RF generator is employed and has its RF output power apportioned among the different coils. For example, as indicated in dashed line in FIG. 1, the RF generator 160 may have its output coupled to both gates 162, 167. In this case, the individual gates 162, 167 perform the pulse-modulation functions governed by the controller 170 as described above, and in addition each includes conventional RF circuitry that enables the controller 170 to control the amount of RF power admitted through each of the gates 162, 167. With this latter feature, the controller 170 in this embodiment apportions the RF power from the generator 160 to the two coils 150, 155.

While foregoing embodiments have been described with reference to an inductively coupled RF power applicator consisting of two coils, an inner coil 150 and an outer coil 155, in another embodiment there may be only a single coil (e.g., either the inner coil 150 or the outer coil 155 or a single coil at an intermediate location). Alternatively, more than one coil may be present, but only a single coil is driven by RF power, the remaining coil (or coils) being inactive. The single coil would be driven by a single RF power generator (e.g., the generator 160) with pulse modulation of the RF power being performed by a pulsed gate (e.g., the gate 162) pulsed by the controller 170 in accordance with a chosen pulsing sequence.

While embodiments having more than one coil have been described above with reference to two coils (i.e., the inner and outer coils 160, 165), such embodiments may have more than two coils, e.g., three or four or more coils. Generally at least some or all of such coils may be concentric as in the embodiment of FIG. 1.

While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1. A method of processing a workpiece supported on a support surface in a chamber of a plasma reactor, comprising:

supplying a process gas into said chamber, said chamber comprising a concentric inner and outer coil antennas over the chamber and facing the support surface and said chamber comprises a first and second pulse-modulated RF power sources of first and second RF frequencies for respective ones of said inner and outer coil antennas;

applying pulse-modulated RF power to said inner and outer coil antennas by successively repeating the following cycle: (a) applying RF power from said first source to said inner coil antenna for a first on-time duration corresponding to a first duty cycle, and at the end of said first on-time duration refraining from applying RF power to said inner coil antenna; (b) applying RF power from said second source to said outer coil antenna for a second on-time duration corresponding to a second duty cycle, and at the end of said second on-time duration refraining from applying RF power to said outer coil antenna;

adjusting a plasma process rate near a center of said workpiece by adjusting said first duty cycle; and

adjusting a plasma process rate near a periphery of said workpiece by adjusting said second duty cycle.

2. The method of claim 1 wherein adjusting the plasma process rate near the center of said workpiece comprises reducing the plasma process rate near the center of the workpiece and said adjusting said first duty cycle comprises reducing said first duty cycle.

3. The method of claim 2 wherein adjusting the plasma process rate near the periphery of said workpiece comprises reducing the plasma process rate near the periphery of the workpiece and said adjusting said second duty cycle comprises reducing said second duty cycle.

4. The method of claim 3 wherein said reducing the plasma process rate near said center of said workpiece and said reducing of the plasma process rate near said periphery of said workpiece reduces non-uniformity in distribution of process rate across said workpiece.

5. The method of claim 1 wherein said first and second frequencies are offset from one another.

6. The method of claim 1 further comprising coupling RF bias power to said workpiece.

7. The method of claim 6 wherein said RF bias power has a frequency different from said first and second frequencies.

8. The method of claim 1 wherein said process gas comprises an etchant precursor and the plasma process rate an etch rate.

9. The method of claim 1 wherein said cycle has a period of about 0.01-100 milliseconds.

10. The method of claim 1 wherein the durations of said first and second on-time durations are one the order to 0.01-100 milliseconds.

11. A method of processing a workpiece supported on a support surface in a chamber of a plasma reactor, comprising:

introducing a process gas into the chamber and generating a plasma in said chamber with pulse-modulated RF power;

successively repeating the following cycle: (a) concentrating the plasma in said chamber in a center-high plasma ion distribution for a first on-time duration; (b) permitting plasma to drift during a first off-time duration away from said center-high plasma ion distribution; (c) concentrating the plasma in said chamber in an edge-high plasma ion distribution for a second on-time duration; (d) permitting plasma to drift during a second off-time duration away from said edge-high plasma ion distribution;

adjusting a plasma process rate near a center of said workpiece by adjusting a duty cycle of said first on-time and first off-time; and

adjusting a plasma process rate near a periphery of said workpiece by adjusting a duty cycle of said second on-time and second off-time.

12. The method of claim 11 wherein adjusting the plasma process rate near the center of said workpiece comprises reducing the plasma process rate near the center of the workpiece and said adjusting said first duty cycle comprises reducing said first duty cycle.

13. The method of claim 12 wherein adjusting the plasma process rate near the periphery of said workpiece comprises reducing the plasma process rate near the periphery of the workpiece and said adjusting said second duty cycle comprises reducing said second duty cycle.

14. The method of claim 13 wherein said reducing the plasma process rate near said center of said workpiece and said reducing of the plasma process rate near said periphery of said workpiece reduces non-uniformity in distribution of process rate across said workpiece.

15. The method of claim 11 wherein said first and second frequencies are offset from one another.

16. The method of claim 11 further comprising coupling RF bias power to said workpiece.

17. The method of claim 16 wherein said RF bias power has a frequency different from said first and second frequencies.

18. The method of claim 11 wherein said process gas comprises an etchant precursor and the plasma process rate an etch rate.

19. The method of claim 11 wherein said cycle has a period of about 0.01-100 milliseconds.

20. The method of claim 11 wherein the durations of said first and second on-time durations are one the order to 0.01-100 milliseconds.

21. The method of claim 1 or 11 further comprising holding one of said first and second duty cycles at 100%.

22. A method of processing a workpiece supported on a support surface in a chamber of a plasma reactor, comprising:

supplying a process gas into said chamber, said chamber comprising concentric inner and outer coil antennas over the chamber and facing the support surface;

continuously applying RF power to one of said inner and outer coil antennas;

applying pulse-modulated RF power to the other one of said inner and outer coil antennas by successively repeating the following cycle: (a) applying RF power to said other coil antenna for an on-time duration corresponding to a duty cycle, (b) at the end of said on-time duration refraining from applying RF power to said other coil antenna; and

adjusting radial distribution of a plasma process rate over said workpiece by adjusting said duty cycle.

23. A method of processing a workpiece supported on a support surface in a chamber of a plasma reactor, comprising:

supplying a process gas into said chamber, said chamber comprising concentric inner and outer coil antennas over the chamber and facing the support surface, and said chamber configured to receive RF power provided from a common RF power source to said inner and outer coil antennas;

apportioning RF power from said common RF power source to said inner and outer coil antennas;

pulse-modulating the RF power applied to said inner and outer coil antennas by successively repeating the following cycle of (a) followed by (b): (a) applying RF power from said common source to said inner coil antenna for a first on-time duration corresponding to a first duty cycle, and at the end of said first on-time duration refraining from applying RF power to said inner coil antenna; (b) applying RF power from said common source to said outer coil antenna for a second on-time duration corresponding to a second duty cycle, and at the end of said second on-time duration refraining from applying RF power to said outer coil;

adjusting a plasma process rate near a center of said workpiece by adjusting said first duty cycle; and

adjusting a plasma process rate near a periphery of said workpiece by adjusting said second duty cycle.

24. A method of processing a workpiece supported on a support surface in a chamber of a plasma reactor, comprising:

supplying a process gas into said chamber, said chamber comprising a coil antenna;

applying pulse-modulated RF power to said coil antenna by successively repeating the following cycle: (a) applying RF power to said coil antenna for a first on-time duration corresponding to a duty cycle, (b) at the end of said on-time duration refraining from applying RF power to said coil antenna; and

adjusting radial distribution of a plasma process rate by adjusting said duty cycle.