Chin Oi Hoong
Oi Hoong Chin is an Associate Professor at the Department of Physics, University of Malaya and heads the Plasma Technology Research Centre. Her research interests include basic gas discharge physics, plasma diagnostics (electric probes and optical emission spectroscopy) and applications of plasma in biology and surface modification.
Address: Department of Physics, Faculty of Science Building, University of Malaya, 50603 Kuala Lumpur, MALAYSIA
Address: Department of Physics, Faculty of Science Building, University of Malaya, 50603 Kuala Lumpur, MALAYSIA
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of the Langmuir probe. The system is ideal for extensive probe data collection in the study of steadystate
plasmas.
The Langmuir probe is a simple but effective tool for the diagnostic of fairly large volume,
relatively cold and low density plasmas.F rom the I-V .characteristico f the probe, plasmap arameters
such ag the electron temperature, electron density, space potential and electron energy distribution
can be deduced. For a time-varying plasma, the pulsed Langmuir probera must be used. In the case
of a steady-state plasma, it is usually sufficient to use the point-by-point technique, although the
pulsed method can also be employed. However, the process of obtaining the Langmuir probe characteristic
by the point-by-point method and its analysis is tedious and time-consuming if it is done
manually. This is particularly true in casesw here the Langmuir probe is employeda s a basicd iagnostic
and extensived ata are required to be collectedc oncurrently with other diagnosticso f the plasma.
In this note, a simple microcomputer-basedd ata acquisition systemf or the Langmuir probe is
described. The system employs an Apple-II microcomputer to control the variation of the probe's
bias potential as well as to monitor the probe curent automatically.
The data acquisition system consists of a 64k RAM Apple-II micrommputer which is supported
by a high-resolution CRT display unit; two mini floppy diskdrives and a dot matrix printer with highresolution
graphic capability (Fig. l). Data acquisition is done via a multi-channel A/D + D/A interface
card (Mountain Computer Inc.) which can be plugged directly into the expansion slot of the Apple-II.
This interface card provides 16 cannelse ach of analog-todigital and digital-to-analogc onverters.T he
rangeo f its analogi nput/output level is -5V to +5V, with a digital equivalenceo f 0 to 255.
The experimentasl et-upo f the miqocomputer-basedo uble Langmuirp robe measuremenotf
a plasma is as shown in Fig. 2. A I kO resistor is connected in place of the usual microammeter for
measuringth e probe curent, Ip. The potentiald rop acrosst he I kS2r esisteri s expectedt o be of the
order of mV only and hence it must be amplified before it is fed into an A/D channel of the computer.
This is done by using a HP2470A data amplifier.
The probe potential Vp is supplied by using the HP457A power amplifier acting as a variable
power supply. The output of the power amplifier can be varied within the range of -3OV to +30V
by applying a voltage of -3V to +3V to its input. Thus when used in conjunction with a digitaltoanalog
converter, the HP467A power amplifier functions as a low voltage digitally controlled power
supply. This iurangementis good enoughf or the presentp urpose.I f -higherv oltagesa re required,
the digitally controlled power supply described by Fanelli and Merangelli" may be employed.
radio frequency (RF) inductively coupled Argon plasma. A passive compensation is
incorporated into the Langmuir probe to provide the high impedance to the probe tip in the
fundamental drive frequency of 13.56 MHz. This will enable the probe tip to follow the RF
fluctuation in the plasma and allows the “dc” probe characteristic to be measured. The
properties of the RF inductively coupled Argon plasma are measured in the pressure range
of 0.01 mbar–0.4 mbar and the RF incident power from 20 W–280 W. The radial and axial
distributions of the RF Argon plasma are also studied. Comparison of the results from the
Langmuir probes with compensation and without compensation demonstrates the effect of
the compensating component.
of the Langmuir probe. The system is ideal for extensive probe data collection in the study of steadystate
plasmas.
The Langmuir probe is a simple but effective tool for the diagnostic of fairly large volume,
relatively cold and low density plasmas.F rom the I-V .characteristico f the probe, plasmap arameters
such ag the electron temperature, electron density, space potential and electron energy distribution
can be deduced. For a time-varying plasma, the pulsed Langmuir probera must be used. In the case
of a steady-state plasma, it is usually sufficient to use the point-by-point technique, although the
pulsed method can also be employed. However, the process of obtaining the Langmuir probe characteristic
by the point-by-point method and its analysis is tedious and time-consuming if it is done
manually. This is particularly true in casesw here the Langmuir probe is employeda s a basicd iagnostic
and extensived ata are required to be collectedc oncurrently with other diagnosticso f the plasma.
In this note, a simple microcomputer-basedd ata acquisition systemf or the Langmuir probe is
described. The system employs an Apple-II microcomputer to control the variation of the probe's
bias potential as well as to monitor the probe curent automatically.
The data acquisition system consists of a 64k RAM Apple-II micrommputer which is supported
by a high-resolution CRT display unit; two mini floppy diskdrives and a dot matrix printer with highresolution
graphic capability (Fig. l). Data acquisition is done via a multi-channel A/D + D/A interface
card (Mountain Computer Inc.) which can be plugged directly into the expansion slot of the Apple-II.
This interface card provides 16 cannelse ach of analog-todigital and digital-to-analogc onverters.T he
rangeo f its analogi nput/output level is -5V to +5V, with a digital equivalenceo f 0 to 255.
The experimentasl et-upo f the miqocomputer-basedo uble Langmuirp robe measuremenotf
a plasma is as shown in Fig. 2. A I kO resistor is connected in place of the usual microammeter for
measuringth e probe curent, Ip. The potentiald rop acrosst he I kS2r esisteri s expectedt o be of the
order of mV only and hence it must be amplified before it is fed into an A/D channel of the computer.
This is done by using a HP2470A data amplifier.
The probe potential Vp is supplied by using the HP457A power amplifier acting as a variable
power supply. The output of the power amplifier can be varied within the range of -3OV to +30V
by applying a voltage of -3V to +3V to its input. Thus when used in conjunction with a digitaltoanalog
converter, the HP467A power amplifier functions as a low voltage digitally controlled power
supply. This iurangementis good enoughf or the presentp urpose.I f -higherv oltagesa re required,
the digitally controlled power supply described by Fanelli and Merangelli" may be employed.
radio frequency (RF) inductively coupled Argon plasma. A passive compensation is
incorporated into the Langmuir probe to provide the high impedance to the probe tip in the
fundamental drive frequency of 13.56 MHz. This will enable the probe tip to follow the RF
fluctuation in the plasma and allows the “dc” probe characteristic to be measured. The
properties of the RF inductively coupled Argon plasma are measured in the pressure range
of 0.01 mbar–0.4 mbar and the RF incident power from 20 W–280 W. The radial and axial
distributions of the RF Argon plasma are also studied. Comparison of the results from the
Langmuir probes with compensation and without compensation demonstrates the effect of
the compensating component.