Automated Impedance Matcher

Overview

The CMU Hacker Fab develops low-cost, open-source alternatives to democratize nanofabrication and replace expensive semiconductor manufacturing equipment. One of its core tools is a DIY RF sputtering chamber, where plasma conditions make the chamber impedance shift throughout a deposition run.

Traditionally, an operator keeps tuning variable capacitors by hand to reduce reflected power, measured as VSWR (Voltage Standing Wave Ratio). Commercial automated matching networks typically cost $3,000–$8,000+, making them a poor fit for a student-run fab. This project built a custom automated RF matching network to remove that manual tuning step at a much lower cost.

System Overview

The matcher replaces manual knob-turning with high-precision digital control. A Teensy 4.1 and custom T-network read forward and reverse power signals from the RF line, then drive 0.9° stepper motors at 1/64 microstepping connected to air-variable capacitors.

The closed-loop controller uses coordinate descent with finite-difference gradient estimation to search for better capacitor positions, stabilizing VSWR around 1.2 while maintaining 95W forward power for more than an hour without human intervention.

PCB Design

I designed a custom two-layer PCB around the Teensy 4.1, motor drivers, and active cooling. The board needed careful shielding and grounding to suppress RF interference in the feedback loop, keeping the microcontroller and drivers reliable in a high-noise environment.

Sensor Integration

To measure VSWR, I reverse-engineered a COTS Surecom SW-112 VSWR meter and tapped into its internal toroidal sensing lines. I then built an analog sensing path into the Teensy’s ADCs using TLV2462CP buffers so the measurement circuit would not cause voltage sag on the ~1 MΩ high-impedance lines or alter RF behavior.

Firmware

The firmware processes digitized forward and reverse power, runs the coordinate descent matcher, and drives the actuators in real time. I also built a local interface with an I2C OLED and rotary encoder so operators can switch between automated and manual tuning while monitoring VSWR and power telemetry.

Results

The final system removed a tedious manual tuning step and lowered the operational barrier for students running thin-film depositions. The build cost was $636, an 80%+ reduction compared with $3,000+ entry-level commercial alternatives. The hardware and firmware were developed under open-source licensing so other Hacker Fabs can replicate the design.