The IV Ramp Circuit for Langmuir Probe

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The IV Ramp Circuit forLangmuir Probe

• Yee Yue Fei
• Techpos 04

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Plasma

• 4th state of matter.
• Consist of Electrons ,ions and neutrals.
• Temperature range from 0.1 eV several keV

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Important Plasma Parameter

• Electron, ion and neutral temperature
• Charge density
• Plasma potential

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Plasma Diagnostic Techniques

• Interferometer (Optical Microwave)
• Spectroscopy
• Spectral Intensity
• Langmuir Probe

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A Simple Langmuir Probe Setup

• Ramp Voltage is applied to the plasma and the V/I
  characteristic is recorded using an oscilloscope.
• Data from the oscilloscope is then sent to a PC
  for further analysis.
• Ion as well as electron temp. and density can be
  found from V/I.
• These characteristics are important for setting
  the plasma process parameters.

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Probe Working Principle
I
Electron saturation
Ion saturation
Vp
Vf
V
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Objective

• To design a high speed ramp voltage circuit for
  Langmuir probe supply.
• To diagnose a continuous microwave plasma using
  the developed Langmuir probe system.

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Building Block of Langmuir Probe

• Probe
• Probe Ramp Voltage Supply
• Data Acquisition Device

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The Probe

• Essentially a metal, normally tungsten, that is
  inserted in a Plasma for diagnostics purposes.
• Normally insulated by mica or quartz, exposing
  only small section of tips to the Plasma.

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Circuit Block Diagram

• Consists of 3 parts.
• Generator
• Amplifier
• Sensing conditioning

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Circuit diagram
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Circuit diagram

• Current sensing using a resistor.
• Buffer invert by opamp.
• Fed to DAQ together with respective probe voltage
  for analysis.
• Offset is nulled by resistor.

• Ground referance, output can be feed directly to
  DAQ.

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DAQ Design

• NI DAQ card is used.
• Voltage signal is attenuated using NI SCC-A10
• Current signal is fed directly.
• Analysis and result displayed using LabView.

• Ramp Gen. is float by current sensing resistor.

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Result Discussion
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Circuit Performance
Circuit tested with 300W resistor.
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Specification of Circuit
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LabView screenshot
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Result for 50mTorr Argon plasma
Te 11eV
ne 5e10 /cm³
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Conclusion

• A fully functional and highly customizable
  Langmuir Probe driving circuit is developed.
• Various control and interfacing options built-in
  allow the circuit to suit wider application.

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Acknowledgement

• The work at Uniten is financially supported by
  IRPA,
• Grand No 09-99-03-0014-EA-001.