⚡ FET Design Spotlight: Electron Beam Circuit Protection with PAD Diodes

Aug 12

2 min read

High-voltage electron beam systems demand precision—and protecting sensitive measurement circuitry is critical to maintaining accuracy and minimizing costly downtime.

In this latest FET Design Spotlight, we look at how Linear Systems PAD Diodes have provided reliable circuit protection in electron beam applications for more than 35 years.

Electron Beam Circuit Protection by PAD Diodes

"High-voltage applications happen to be a difficult place to involve sensitive measurement and control electronics. Often, because of the high impedances involved, certain control nodes need protection from transients, like high-voltage arcs and parasitic oscillations within the elements themselves. High-voltage sensing resistors will invariably have distributed capacitances acting as low-reactance impedances with the fast rise times of an arc. Because op-amps used in these circuits typically have FET inputs for the least loading, protection diodes are needed to prevent transient voltage surges from damaging circuitry. Diodes used here must have the same or lower leakage as the amplifier nodes they protect to maintain measurement accuracy. For these applications, LS PAD diodes have been used with consistent results on all of the Electron Beam circuits I've designed over the past 35 years.

This application uses PAD-1 diodes to protect nA current sensing and high-voltage monitoring of a semiconductor wafer bias supply floating on a supply of 10–30 kV. Noise in charged beam columns is minimized when all supplies are referenced to a common point in parallel, as shown here. Without Picoamp Diode circuit protection, even 1 pF distributed circuit capacitances could, and would, render resident instrumentation inoperative after an arc and cost, sometimes, days of downtime. This circuit measures the electron current yield on the wafer, whether it was negative or positive, regardless of the wafer bias voltage value. The graph shows there to be minimal bias voltage excursion effect on the wafer current measurement by using complementary summing."

— Kirkwood Rough