An Automated RF Tuner for Silicon Carbide Electronic Device Processing for Power Devices and I.C.’s Melissa Spencer, Dept of ECE EMRL

RF energy strikes a plasma which etches the SiC material Reactive Ion Etching Plasma Chemistry Etch: SF6+He, H2, O2 Ash: O2 RF Power 10-100 W typical Pressure 100-600 mT RF energy strikes a plasma which etches the SiC material

Etch rate is a sensitive function of plasma power Problem* 10 Percent power change  20% etch rate variation SF6 :O2 (5:10 sccm) Pressure = 100 mT Electrode to sample spacing = 25.4 cm Power (W) vs Etch rate (A/min) Etch rate (Angstroms/minute) Power (W) 1600 1400 1200 1000 800 600 400 200 5 10 15 20 25 30 35 40 Etch rate is a sensitive function of plasma power *J. Bonds, G. E. Carter, J.B. Casady and J.D. Scofield, Spring MRS Meeting, April 2000

Impedance Matching The load impedance is matched to the source impedance by a “T” network: X1 X2 PForward X3 RF Source ZP =ZS PReflected X1, X2 variable capacitors and X3 variable inductance Control algorithm to minimize PR vs.  ZP

Design Constraints Maintain a maximum VSWR of 1.1 to 1 source to load match Dedicated on-board micro-controller PC for monitoring the tuning Manual and Automatic Mode Operates with a 60 Hz, 120V AC power source Conforms to FCC emission limits for RF devices Packaged in a 19-in. aluminum rack mount enclosure Cost not to exceed $1000 dollars

Implementation Hardware Software MFJ Roller Inductor Tuner Detection Circuitry for PForward and PReflected Basic Stamp II Controller Servo Motors (impedance device tuning) AC to DC Converter A/D Converters Software Impedance Tuning Algorithm

Detection Circuitry for Forward and Reflected Power System Block Diagram Detection Circuitry for Forward and Reflected Power MFJ-962D RF in Monitoring PC Servo Motor/Driver Micro-Controller Servo Motor/Driver Servo Motor/Driver RF out Goal: Maintain constant power transfer

Schematic A/D Driver A/D Stamp

Control PC Board Control PC Board double-sided to reduce board size -- PC board designed to fit in RF shielded enclosure inside chassis Board in connected to external sources by connectors RF Filter Feedthroughs used to eliminate RF noise at Stamp and other control electronics

Automated RF Tuner RF Filter Feedthroughs Power Supply Detection Circuitry Inductor Servos Control Board Capacitors Servo RF Shielding

Detect Reflected Power Matching Algorithm Algorithm utilizes MFJ tuning procedure of varying -inductor, load capacitor, inductor, source capacitor Step Procedure 1 Operator Tunes to a Set Point 2 Auto Mode is set 3 Measures Pforward and Preflected 4 Checks Threshold 5 Varies Tuning if Necessary 6 Repeat as Necessary to maintain VSWR < 1.1 to 1 Vary Impedance Vary Impedance? Detect Reflected Power

Functional Tests

RIE Test Results Includes: RIE Etching Chamber, Automated RF Tuner, and PC Results: O2 Plasma Minimum VSWR Attained 10 W 25 W 50 W 400 mT 1.02 to 1 1.04 to 1 1.05 to 1 300 mT 1.03 to 1 1.05 to 1 1.05 to 1 200 mT 1.02 to 1 1.03 to 1 1.04 to 1 RIE Chamber Monitoring PC Automated RF Tuner (All below a VSWR of 1.1 to 1)

Design Constraints Maintain a maximum VSWR of 1.1 to 1 source to load match Tuning Algorithm continuously maintains at least a 1.1 to 1 VSWR Dedicated on-board micro-controller, which maintains primary control over the tuning network PC only has a display function over the tuning PC Interface for monitoring the tuning RF source provide accurate means for measuring the forward and reflected power Manual and Automatic Mode Tuning algorithm includes both manual and automatic modes of operation

Design Constraints cont.. Operates with a 60 Hz, 120V AC power source On-board power supplies 5 VDC from 120 VAC source Conforms to FCC emission limits for RF devices FCC allows unlimited emission at scientific frequencies such as 13.56 MHz Packaged in a 19-in. aluminum rack mount enclosure Components were chosen which fit easily in the existing rack mount Cost not to exceed $1000 dollars Parts total about $850 dollars

Future Work Modular Device Communication with computer Feedback circuitry can be replace with other detection devices Communication with computer Reflected Power could be compensated with RF source Automate other power related parameters Electrode spacing could be varied automatically to utilize optimal spacing

An Automated RF Tuner for Silicon Carbide Electronic Device Processing for Power Devices and I.C.’s Melissa Spencer, Dept of ECE EMRL