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An Automated RF Tuner for Silicon Carbide Electronic Device Processing

An Automated RF Tuner for Silicon Carbide Electronic Device Processing for Power Devices and I.C.’s. Melissa Spencer, Dept of ECE EMRL. Reactive Ion Etching. Plasma Chemistry Etch: SF 6 +He, H 2 , O 2 Ash: O 2 RF Power 10-100 W typical Pressure 100-600 mT.

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An Automated RF Tuner for Silicon Carbide Electronic Device Processing

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

  2. 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

  3. Problem* 1600 1400 Power (W) vs Etch rate (A/min) 1200 1000 Etch rate (Angstroms/minute) 10 Percent power change  20% etch rate variation 800 600 400 SF6 :O2 (5:10 sccm) Pressure = 100 mT Electrode to sample spacing = 25.4 cm 200 0 0 40 5 10 30 35 15 20 25 Power (W) 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

  4. 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

  5. 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

  6. Implementation • Hardware • 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

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

  8. Schematic A/D Driver A/D Stamp

  9. 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

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

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

  12. Functional Tests

  13. 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)

  14. 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

  15. 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

  16. Future Work • Modular Device • 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

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

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