MULTI-FREQUENCY OPERATION OF RIE AND ICP SOURCES *

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1 45th International Symposium of the American Vacuum Society Baltimore, Maryland November 2-6, MULTI-FREQUENCY OPERATION OF RIE AND ICP SOURCES * Shahid Rauf and Mark J. Kushner Department of Electrical & Computer Engineering, Urbana-Champaign *Work supported by AFOSR/DARPA, SRC and NSF. AVS98_P1_F1

2 AGENDA Introduction Computational Model Asymmetric Capacitively Coupled Discharge Electrode Currents and Voltages Effect of Frequency, Voltage Waveform and Source Interaction Inductively Coupled Plasma Electrode Currents and Voltages Effect of Frequency and Source Interaction Conclusions AVS98_P1_F2

3 INTRODUCTION Source frequency has a strong influence on plasma characteristics in radio frequency (rf) discharges. Multiple sources at different frequencies are often simultaneously used to separately optimize the magnitude and energy of ion fluxes to the substrate. The sources can, however, nonlinearly interact if the frequencies are sufficiently close, and resulting plasma and electrical characteristics can be different than due to individual sources. A plasma equipment model has been used to investigate the interaction of multiple frequency sources in both capacitively and inductively coupled discharges. In this talk, we discuss the effect of frequency on plasma and electrical characteristics, and describe the consequences of source interactions. AVS98_P1_F3

4 THE COMPUTATIONAL MODEL Our computational platform consists of the coupled Hybrid Plasma Equipment Model (HPEM) and a circuit model. The circuit model uses intermediate results from the HPEM to compute voltages (dc, fundamental and harmonics) at electrodes. CIRCUIT MODULE I,V (coils) Eθ ELECTRO- MAGNETICS MODULE Eθ(r,z,φ) B(r,z,φ) ELECTRON MONTE CARLO SIMULATION ELECTRON ENERGY EQN./ BOLTZMANN MODULE The Hybrid Plasma Equipment Model S(r,z), Te(r,z), µ(r,z) FLUID- KINETICS SIMULATION HYDRO- DYNAMICS MODULE ne, Te, vi Vrf, Vdc CIRCUIT MODEL NON- COLLISIONAL HEATING s(r,z), I (coils), J(r,z,φ) N(r,z) Es(r,z,φ) AVS98_P1_F4

5 SHEATH-CIRCUIT MODEL The reactor and circuitry are replaced by the following equivalent circuit. Sheath 1 Sheath 2 Sheath N Reactor RA 1 RA 2 RA N CA 1 CA 2 CA N LA 1 RC 1 LC 1 LA 2 RC N-1 LC N-1 LA N RB 1 CC 1 RB 2 CC N-1 RB N CB 1 CB 2 CB N LB 1 LB 2 LB N VS 1 VS 2 VS N AVS98_P1_F4a

6 GEC REFERENCE CELL We first explore the effects of frequency and source interactions in the capacitively coupled GEC reference cell with Ar at 1 mtorr. Sources and blocking capacitors have been connected to both electrodes. 1.9 [e] (Max = 1.18 x 1 1 cm-3) Electrode 2 C2 E2 Showerhead V2 E1 G. Electrode 1 Dark Space Shield Pump Port. 1.2 Radius (cm) C1 V1 Ar, 1 mtorr, V1 = 1 V, V2 = V. AVS98_P1_F5

7 SHEATH VOLTAGES AND CURRENTS A negative dc voltage appears across the capacitor C1 (dc bias) to balance currents through the powered and grounded surfaces. The sheath currents are fairly nonlinear with large higher harmonics. 5 V G V E1 +V C1.15 I E V C1. I E2 I G -15 V E Time (µs) Time (µs) 59. Ar, 1 mtorr, V1 = 1 V, V2 = V, MHz. AVS98_P1_F6

8 EFFECT OF SOURCE FREQUENCY Total current through electrodes and walls increases with frequency because of enhancement of displacement current. Larger current leads to more electron heating and larger electron densities. Electron temperature decreases with frequency, resulting in better electron confinement between electrodes and smaller dc biases. AVS98_P1_F IE1 IG IE Frequency (MHz) Ar, 1 mtorr, V1 = 1 V, V2 = V Above E1 Centerx.1 Sheath (E1) Bias (C1) Frequency (MHz)

9 INTERACTION OF MULTIPLE FREQUENCY SOURCES - I In these results, a MHz source (V1=1 V) is connected to E1 while a MHz source is connected to E2. Electron density increases with V2 (27.12 MHz) due to the enhancement of displacement currents. The dc bias magnitude on E1 decreases with increasing V2 (27.12 MHz) due to source interactions Electrode E1 Sheath Bias (C1) 1 Centerx.1 5 Above E V2(27.12 MHz) (V) Ar, 1 mtorr, V1 (13.56 MHz) = 1 V. AVS98_P1_F Sheath Electrode E2 Bias (C2) V2(27.12 MHz) (V)

10 INTERACTION OF MULTIPLE FREQUENCY SOURCES - II In these results, we show the sheath voltages and currents for only the MHz source, MHz source and their combination. Sheath voltages at E1 and E2 are primarily governed by the sources connected to them. The sheath voltage at the grounded wall is, however, in the linear regime and the two sources interact increasing the sheath voltage drop. DC bias at E1, which is the difference between the dc sheath voltage at E1 and wall, therefore decreases in magnitude. AVS98_P1_F Both Both Both Both (a) (b) (c) (d) (e) Both Time (1/13.56 µs)

11 ARBITRARY VOLTAGE WAVEFORMS! By varying the rf bias voltage waveform, one can control the dc bias, sheath voltage, plasma characteristics and the ion energy distribution at the substrate (a) -9. (b) (c) (d) AVS98_P1_F π ωt (rad) π ωt (rad) 2π π ωt (rad) 2π Optical &Discharge Physics

12 EFFECT OF WAVEFORM ON PLASMA PARAMETERS Waveforms which have higher first harmonic lead to larger dc biases. Higher first harmonics also lead to enhanced power deposition in the plasma and higher electron densities. Since displacement current increases with frequency, waveforms with larger amplitudes at higher harmonics result in larger plasma densities. Triangular Wave Sinosoidal Wave Square Wave [e] (11 cm-3) [e] (11 cm-3) [e] (11 cm-3) Radius (cm) Radius (cm) Radius (cm) AVS98_P1_F18

13 INDUCTIVELY COUPLED PLASMA (ICP) SOURCE We next consider the effects of rf bias frequency and rf source interaction in an ICP reactor. For circuit simulation, the dielectric window is replaced by effective capacitors. [e] (Max = 5.4 x 111 cm-3) Coils Showerhead CW4 CW Electrode (S1) S2 S3 C1 V1 2 AVS98_P1_F1. Pump Port. 17. Radius (cm) Ar, 2 mtorr, 5 W, V1 (13.56 MHz) = 1 V.

14 TYPICAL SHEATH VOLTAGES AND CURRENTS Most of the rf current flows out through the grounded surfaces. Sheath voltage is larger near the grounded walls at 1 MHz because of low plasma density (i.e., high impedance) adjacent to them V4 V1 V5-1 V2, MHz -14 2π 4π ωt (radians) I2 I3. I5 I π 4π ωt (radians) I1 1 MHz Ar, 2 mtorr, 5 W, V1 (1 MHz) = 1 V. AVS98_P1_F11

15 EFFECT OF RF BIAS SOURCE FREQUENCY Since plasma is generated by the inductive source, rf bias frequency does not significantly affect the electron density. Displacement current through the sheaths increases with bias frequency, enhancing the total sheath current Frequency (MHz) 5 3 Bias (C1) -3 Sheath Ar, 2 mtorr, 5 W, V1 = 1 V. AVS98_P1_F Frequency (MHz)

16 AVS98_P1_F13 WHY DOES RF BIAS FREQUENCY STRONGLY EFFECT DC BIAS? Most of the current through the substrate is conduction while that at grounded surfaces is displacement. An increase in rf bias frequency, therefore, decreases the sheath impedance more strongly at grounded surfaces than at the substrate. The resulting disproportionate change in sheath voltage at different surfaces modifies the dc bias at the substrate V2,3-1 V MHz -14 2π 4π ωt (radians) Ar, 2 mtorr, 5 W, V1 (3 MHz) = 1 V I2 I3 3 MHz -2 2π 4π ωt (radians) I1

17 DEPENDENCE OF DC BIAS ON INDUCTIVE POWER DEPOSITION Dc bias was positive at low frequencies (in the previous results) because electron density is much larger near the powered electrode than the grounded surfaces. By powering only the outer two coils, electron density profile can be shifted towards the grounded wall (S3). This changes the dc bias at the powered substrate (S1) from 1.8 V to V. 15. [e] (Max = 3.2 x 111 cm-3) 1 7 S1.. Radius (cm) S3 Ar, 2 mtorr, 5 W, V1 (13.56 MHz) = 1 V. AVS98_P1_F14

18 RF SOURCE INTERACTION IN ICP REACTOR In these results, a MHz source (V1 = 1 V) is connected to S1 while a AVS98_P1_F MHz source is connected to S3. Sheath voltage at S1 is mainly governed by the rf bias source. The two rf sources, however, interact at the grounded surface S2 and change the sheath voltage there. Dc bias at S1 is therefore modified VS3(27.12 MHz) (V) VSheath Bias (C1) Electrode S VS3(27.12 MHz) (V) Ar, 2 mtorr, 5 W, V1 (13.56 MHz) = 1 V.

19 CONCLUSIONS The effect of rf bias source frequency and source interactions have been discussed in both capacitively and inductively coupled sources. In the capacitively coupled GEC reference cell, frequency had a significant effect on currents and electron density, but not dc bias. On the other hand, rf bias source frequency appreciably modified the dc bias in the inductively coupled plasma reactor, but not plasma density. Multiple rf sources at different frequencies were found to interact with each other in both inductively and capacitively coupled reactors. This interaction was strong near surfaces where sources were not attached and very weak in sheaths connected to the sources. Due to this inhomogeneous and nonlinear response of different sheaths to multiple sources, the electrical characteristics of the discharge were significantly modified. AVS98_P1_F16

DYNAMICS OF NONLINEAR PLASMA-CIRCUIT INTERACTION *

Seminar in Plasma Aided Manufacturing University of Wisconsin, Madison, Wisconsin September 18, 1998. DYNAMICS OF NONLINEAR PLASMA-CIRCUIT INTERACTION * SHAHID RAUF Department of Electrical & Computer