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

3. 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

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 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

7. 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

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

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

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 to to to 1
300 mT to to to 1
200 mT to to 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 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 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

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