FPC failure mode and detection

[Warren Straszheim]

This is out of left field, but let me try anyway.

We are running an Oxford Instruments Wave WDS system on our FEI Quanta. It is basically the current version of the old Microspec spectrometer, in case that means anything to anyone.

The FPC recently failed and we are trying to get it going again. We have looked at the preamp and the main control box. They don’t appear to be the problem. The current thought is that the detector itself has failed. So, for those who have much more experience with such things than I do, I have some questions.

How common is it for an FPC to fail?
Is there a good way to determine that the FPC has gone bad?
What fails? Does the wire break or get contaminated?
Any other advice?

Thanks in advance.

Warren Straszheim
Materials Analysis and Research Lab
Iowa State University

[sem-geologist]

I have experience only with Cameca WDS and its FPC. While I have no idea how old FEI WDS are made, maybe some of failure modes known to my experience could hint something.

I guess it is 1 atm FPC? Those then use thin grid supported film for isolation of P10 gas from escaping into the vacuum.
Is there a bubbler at the end of P10 gas exhaust from the spectrometer? In our WDS case we close the gas inlet, wait for pressure in inlet to drop and if we see then backward bubbles being sucked into the spectrometer - it is clear then - the window has a hole (or critical number of holes). Also wrong gas, back flow of atmospheric gas into spectrometer (i.e. gas closed for long term) can lead to suppression of townsend avalanche (ions recombine with electrons after ionisation instantly suppressing the avalanche) or grow into continuous glow discharge.

The next thing which could fail is HV generator and powering and voltage buffer line of the anode wire. One of failure modes of capacitors is "shorted" (normally capacitor is "open circuit" for DC) and thus would short high voltage with ground. The resistors sometimes can fail to making it "open". As wire in FPC then in both cases would never get high enough voltage - no Townsend avalanches would be produced -> no counts.

Finally, preamplifier is just 1st step. The shapping amplifier follows it. If there would be a way to see if shapping amplifier or preamplifier generates pulses (or cascades in case of pre-amplifier) it would be then clear that counting electronics malfunction. Signal from shapping amplifier is easy to get, with not much influencing the signal. probing preamplifier directly with oscilloscope howere is much harder thing. 

When troubleshooting FPC there are  two things to be aware: deadly high voltage!, and very sensitive signal path in preamplifier. For the first then rather resistance is checked of components after completely physically powering down and disconnecting supply to the HV generator and taking all precaucious  that the capacitors are discharged. Albeit, measuring resistance of capacitor at low voltage with multimeter could miss the shorting seen at the high voltage.  For the second point - connecting probe wires to preamplifier can mess with preamplifier circuit as it is very much succeptible to the capacitance added by probe wires.

Finally FPC can age (I am not sure how much  that is possible on the SEM) especially when running at very high count rate continuously. It is well known process from CERN and seems P10 gas byproducts from ionisation can get deposited on anode wire and the cathode walls, and in both cases it makes the operation of GFPC worse and worse (It affects PHA mostly and is recognizable by growth of broad distribution at higher energy in PHA). However despite contamination the FPC should still function - it is long term deterioration of counter rather than instantaneous failure.