An Automated RF Tuner forSilicon Carbide Electronic Device Processing

for Power Devices and I.C.’sMelissa Spencer, Dept of ECE

EMRL

Reactive Ion Etching

RF energy strikes a plasma which etches the SiC material

Plasma ChemistryPlasma Chemistry

Etch: SF6+He, H2, O2

Ash: O2

RF PowerRF Power

10-100 W typical

PressurePressure

100-600 mT

Problem*

Etch rate is a sensitive function of plasma power

10 Percent power change

20% etch rate variation

*J. Bonds, G. E. Carter, J.B. Casady and J.D. Scofield, Spring MRS Meeting, April 2000

SF6 :O2 (5:10 sccm)Pressure = 100 mTElectrode to sample spacing = 25.4 cm

Power (W) vs Etch rate (A/min)

Etc

h r

ate

(An

gstr

oms/

min

ute

)

Power (W)

1600

1400

1200

1000

800

600

400

200

00 5 10 15 20 25 30 35 40

Impedance MatchingThe load impedance is matched to the source

impedance by a “T” network:

X1 X2

X3

X1, X2 variable capacitors and X3 variable inductanceControl algorithm to minimize PR vs. ZP

PForward

PReflected

RF Source =ZS

ZP

• 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

Design Constraints

• 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

Implementation

System Block Diagram

Monitoring PC

Monitoring PC

Micro-Controller

Micro-Controller

ServoMotor/Driver

ServoMotor/Driver

ServoMotor/Driver

ServoMotor/Driver

ServoMotor/Driver

ServoMotor/Driver

Detection Circuitry for Forward and

Reflected Power

Detection Circuitry for Forward and

Reflected Power

RF inRF in

RF outRF out

Goal: Maintain constant power transfer

MFJ-962D

vcc -cs20 1

7 Vin-

6 Vin+

9 Vref/ 2

ADC0804

7 Vin-

6 Vin+

9 Vref/ 2

ADC0804

vcc20

D3 15

D0 18

D2 16D1 17

D6 12D5 13D4 14

-WR 3-RD 2

D7 11

-intr 5-cs

1

-

+

For Voltageinput

inputRefl Voltage

5V

5V

2k

10k 1uF

1uF

5V

24PWRGND 23RES 22+5V 21P15 20P14 19P13 18P12 17P11 16P10 15P9 14P8 13

56789101112

234

1

P5

P7P6

P4P3P2P1P0

TX

GNDATNRX

Serial

Moto

r 3

Moto

r 2

Moto

r 1

Au

to O

ut p

ut

Au

to I

np

ut

Cable 5V SwitchReset

LM358

LT1029

-intr 5

D6 12

D1 17D0 18

D4 14D3 15D2 16

D5 13

-RD 2

D7 11-WR 3

A/D Driver A/D Stamp

Schematic

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

Servos

Automated RF Tuner

RF Filter Feedthroughs

Capacitors

Power Supply

RF Shielding

Inductor

Detection Circuitry

Servo

Con

trol

Boa

rd

Matching Algorithm

Algorithm utilizes MFJ tuning procedure of varying -inductor, load capacitor, inductor, source capacitor

Vary Impedance

Vary Impedance

?

Detect Reflected

Power

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

Functional Tests

RequirementMotor

Control TestSuite

ControlSignalTest

AlgorithmTest

PhysicalPackaging

RIE Run

MatchingAbility

On-boardController

PC Interface

Dual ModeOperation

Power

EMC1 N/A N/A N/A N/A N/A

Packaging

Cost 1 EMC testing does not have to be tested due to this qualification as a scientific instrument .

RIE Test Results

RIE Chamber

Monitoring

PC

Au

tom

ated

RF

T

une

r

Includes: RIE Etching Chamber, Automated RF Tuner, and PC

Results: O2 Plasma

Minimum VSWR Attained

10 W 25 W 50 W400 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 (All below a VSWR of 1.1 to 1)

• 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

• 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

Design Constraints cont..

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

An Automated RF Tuner forSilicon Carbide Electronic Device Processing

for Power Devices and I.C.’sMelissa Spencer, Dept of ECE

EMRL