- #1
d.sonali20
- 24
- 0
what does calibrating a faraday cup mean??
What is the purpose of calibrating a faraday cup in electronic systems?
- Thread starter d.sonali20
- Start date
-
- Tags
- Calibration Faraday
Click For Summary
Discussion Overview
The discussion revolves around the calibration of Faraday cups in electronic systems, particularly in the context of measuring charge from ion or electron sources in vacuum. Participants explore various aspects of calibration, including the implications of charge leakage, the design of amplifiers, and the challenges associated with low current measurements.
Discussion Character
- Technical explanation
- Debate/contested
- Experimental/applied
Main Points Raised
- Some participants describe a Faraday cup as a re-entrant cavity for measuring charge, emphasizing the need for calibration to ensure accuracy, particularly in preventing charge leakage.
- One participant suggests that the calibration of the amplifier may be more critical than the Faraday cup itself, highlighting issues with input leakage that can affect measurements.
- Another participant shares their experience with designing inverting charge integrators, noting the importance of using low input bias current op-amps and high-quality integrating capacitors for accurate charge measurement.
- Concerns about noise, settling time, and vibration noise are raised as significant factors that can complicate the calibration process.
- A participant mentions the use of dose correction factors in semiconductor fabrication, indicating that calibration can involve complex testing and verification processes to ensure accurate dose measurements.
- One participant shares their extensive experience in designing electronic systems for various types of mass spectrometry, suggesting a depth of practical knowledge in the field.
Areas of Agreement / Disagreement
Participants express a range of views on the importance of different aspects of calibration, with no consensus on a singular approach or understanding of the calibration process for Faraday cups. Multiple competing perspectives on the significance of various factors involved in calibration remain evident.
Contextual Notes
Participants mention various challenges such as charge leakage, noise, and the need for specialized equipment, indicating that the discussion is limited by the complexity of the calibration process and the specific applications being considered.
- #2
Bob S
- 4,662
- 8
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
yungman
- 5,741
- 291
I don't know either? It's the amplifier you might need to calibrate!
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.
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
Bob S
- 4,662
- 8
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).
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
yungman
- 5,741
- 291
When getting down to the mud, this can be quite a big topic. Noise, settling time, vibration noise all come into play.
- #6
nsaspook
Science Advisor
- 1,560
- 5,162
yungman said:
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
Last edited by a moderator:
- #7
yungman
- 5,741
- 291
I spent almost half my career designing electronic system for all different types of SIMs, TOF, Dynamic etc. Also add on's for CAMECA.
Similar threads
- · Replies 3 ·
- · Replies 5 ·
- · Replies 1 ·
- · Replies 19 ·
- · Replies 84 ·
- · Replies 2 ·
Graduate
An electron inside a Faraday cage
- · Replies 13 ·
- · Replies 5 ·
- · Replies 6 ·