# Calibration of a faraday cup?

1. Jun 15, 2012

### d.sonali20

what does calibrating a faraday cup mean??

2. Jun 15, 2012

### Bob S

A Faraday cup is a re-entrant cavity designed to measure charge from ion or electron sources in vacuum. Calibrating means verifying the accuracy of charge measurement. Sometimes charge can leak out, so there may be grids to prevent back-scattered electrons from leaking out the back. Sometimes the Faraday cup is too thin, thus allowing charge from MeV-energy beams to leak out through the cup walls.

3. Jun 15, 2012

### yungman

I don't know either? It's the amplifier you might need to calibrate!!!:rofl:

It is not calibrate that matter, when you design with certain gain, they work!!!! Those are the easy part. Those are just simple transimpedance amps like an inverted op-amp type. For ultra low current like pA type, the feedback resistor can be 10GΩ.

For low current transimpedance amp, leakage is the biggest problem. If you are trying to measure 10pA but the input leakage from the faraday cup, cable to the input of the amp is 5pA, then you have big error.

So what I usually do to make sure nothing is leaking is to use a signal generator.....or even a DC power supply; driving through a large resistor to simulate a current source. Use this to drive into the the input of the amp with the faraday cup and all connected. By V=IR, you know how much current you are driving. Read the result and compare. If you don't get the correct reading, you troubleshoot.

4. Jun 15, 2012

### Bob S

My experience has been that it is easier to build an inverting charge integrator using a good op-amp with 1 pA or lower input bias current and a high quality integrating capacitor with measured capacitance value. The output voltage V is the input (Faraday cup) charge Q divided by the feedback capacitance C (V=Q/C).

If your Faraday cup is in vacuum, with a high voltage beam of milliamps or more average current, it will get hot. Then you will need a de-ionized water cooling loop (another possible current leakage path).

5. Jun 15, 2012

### yungman

When getting down to the mud, this can be quite a big topic. Noise, settling time, vibration noise all come into play.

6. Jun 15, 2012

### nsaspook

Yes it can, in semiconductor fabs using ion implanters there are dose correction factors in each machine that can be used to match the actual dose vs the dose measurement faradays. A set of special test wafers are dosed with a series of levels/dopants and checked by a certified lab using X-Ray Emission Spectrometry/SIMS. The results of the tests are then used to provide the correction factors. After any work on a machine that might effect the dosing system a quick check is made by looking at the degree of surface damage from the ion beam with Therma-wave (TW) and sheet resistance (Rs).

http://www.electroiq.com/articles/sst/print/volume-45/issue-8/features/metrology/in-fab-techniques-for-baselining-implant-dose-contamination.html [Broken]

Last edited by a moderator: May 6, 2017
7. Jun 15, 2012

### yungman

I spent almost half my career designing electronic system for all different types of SIMs, TOF, Dynamic etc. Also add on's for CAMECA.