Plasma Potential Determination in RF Capacitively Coupled Plasma by Measuring Electrode Voltage. Nagoya university Hironao Shimoeda

Transcription

1 Plasma Potential Determination in RF Capacitively Coupled Plasma by Measuring Electrode Voltage. Nagoya university Hironao Shimoeda

2 Introduction The University of Texas at Dallas International Center for Advanced Materials Processing (ICAMP) Prof. Matthew John Goeckner Major: Physics URL: Prof. Lawrence J. Overzet Major: Electrical engineering URL: 2 / 13

3 Introduction Dealey Plaza in the historic West End district of downtown Dallas, Texas The assassination site; the mark on the road where John F. Kennedy was hit. The Texas School Book Depository; at the rooftop of that building, the assassin shot at JFK. 3 / 13

4 Background Plasma potential V p Decision of electron temperature, Ion accelerations between bulk plasma and sheaths Measurement of V p Probe diagnostics Advantage Simplicity Disadvantage Plasma perturbations Noncontact measurement of V p Measurement of electrode voltage Electrode voltage = DC self bias voltage (V dc ) + RF voltage (V rf ) (V dc Decision of ion energy in the sheaths) 4 / 13

5 Relation between V dc and V p Plasma Equivalent Circuits V 1 = V p, V 2 = V p -V dc V 1 0 V V p V dc A 1 V 2 A 2 (a)dc or low frequency plasma, (b)high frequency plasma, and (c) Simplification of (b) by considering R p << R S1, R S2, and most ions moving to electrodes V 1 V 2 = A 2 A 1 Substituting V 1 = V p, V 2 = V p -V dc in the equation (1) and arranging, we obtain V p = [1] ttp://timedomaincvd.com/cvd_fundamentals/plasmas/capacitive_plasma.html [2] Principles of Plasma Discharges and Materials Processing Potential diagram of V p and V dc [1] a A2 A1 A2 A1 (a 2.5) [2] (1) a a 1 V dc (2) 5 / 13

6 V 1 (V) Relation between electrode voltage and V p Experimental [3] R=A 2 /A 1 We can find more accurate V p by comparing one calculated from V dc with one obtained from electrode voltage waveforms. [3] J. W. Coburn and K. Eric, J. Appl. Phys (1972). V 2 (V) V 1 =V p, V 2 =V p -V t a V 1 V 2 = A 2 A 1 (0.98 < a < 1.4 when 0.09 < A 2 /A 1 < 0.29.) 6 / 13

7 Objective V dc measurements Construction of a circuit to measure DC component. Dependence of V dc as a function of the electrode position. V p calculation from outputs V rf measurements Construction of a voltage divider circuit Dependence of V p as a function of the electrode position. 7 / 13

8 Experimental h Liquid injection plasma system Ar gas flow rate : 20 sccm RF power: 1-20 W Gas pressure : 16 Pa h (the electrode position) : 0, 2.5, 5.0 cm A 2 /A 1 : (h = 0 cm), (h = 2.5, 5.0 cm) A 1 : Area of the grounded chamber A 2 : Area of the energized electrode 8 / 13

9 Dependence of V dc on the electrode position -V dc (V) 600 h = 0 cm h = 2.5 cm h = 5.0 cm 400 Circuit diagram to measure DC component RF power (W) Dependence of V dc on h as a function of RF power Increase in V dc Increase in discharge voltage Decrease slightly in V dc with changing h 9 / 13

10 V p (V) V p calculation with V dc V 1 V 2 = C sh2 C sh1 = A 2s 1 A 1 s 2 C sh1, C sh2 : sheath capacitances s 1, s 2 : sheath thicknesses In RF plasma, ion sheaths can t follow the RF voltage. s 1 /s 2 ~ 1 (also as reported [3] ) In the equation (2), it is assumed that a = 1. V p = A2 A1 A2 A1 1 V dc (2) 10 h = 0 cm h = 2.5 cm h = 5.0 cm RF power (W) Dependence of V p on h as a function of RF power Decreases in averaged V p and V dc with changing h Decrease in the electron temperature [3] J. W. Coburn and K. Eric, J. Appl. Phys (1972). 10 / 13

11 Measurement of electrode voltage Re(Z) ( ) Im(Z) ( ) Methods of measuring electrode voltage (V rf + V dc ) High voltage probe Voltage divider RF high voltage have to be divided into measurable voltages. Voltage divider circuit Equivalent circuit of general electric element for RF voltage ex. R=1 MΩ, L=0.1 μh, C=0.1 pf f=10 Hz jωl << R, 1/jωC >> R Z~R f=10 MHz jωl << R, 1/jωC << R Z~1/jωC Dependence of 5.4 MΩ resistor on frequency For RF voltage, influences of inductance and capacitance are critical Frequency (MHz) / 13

12 V p (V) Evaluation of V p by measuring V rf h = 0 cm h = 2.5 cm h = 5.0 cm 0-10 RF voltage divider circuit Theoretical V rf 1114 V O V rf : amplitude of RF discharge voltage V O : amplitude of measured RF voltage RF power (W) Dependence of V p on h as a function of RF power Actual measurement V rf 71 V O Incorrect values on capacitors and inductors. The behaviors of V p were obviously incorrect. Induced voltages or RF noises in MN Inaccurate V rf 12 / 13

13 Summary V p in the RF capacitively coupled plasma was determined by measuring V dc with changing the electrode position. Decreases in averaged V p and V dc with changing h Decrease in the electron temperature V p was determined by measuring V rf with changing the electrode position by using a voltage divider circuit. Correct V p was hardly obtained because of induced voltages or RF noises. Future works More accurate measurements of V rf by getting circuit lines as short as possible. Identification of frequency dependencies of electric elements. Comparisons of electron density, electron temperature, and V p with other techniques. 13 / 13

14 Thank you for your kind help and support! 14 / 13