Egan 448
The Kostas
Research Center
Four Point Probe/ Sheet Resistance Measuring
System for Thin Films:User Manual
Jeff Cox and Rich DeVito
Equipment List for Sheet Resistance Measurements: 4 Point Probe.
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Agilent 344010 6.5 digit DMM
Lucas Labs SP 4 point probe head (tungsten carbide probe tips)
100 ohm Precision Resistor ( Ohmite , Dale…)
HP 6177C DC Current Source.
Custom DPDT switch box ( required when 1 DMM is available for both I
and V measurement)
 Jandel Scientific ITO film Calibation Standard
 Various Banana Connectors…
I.
Introduction
The four point probe measuring system is primarily used to measure the sheet resistivity
of thin films and semiconductors wafers. The corresponding sheet resistivity values help
engineers determine the quality of thin films deposited on a 3” or 4” silicon wafer.
In this setup the sheet resistance is measured by sending a DC current through the outer
two probes by raising the stage until slight contact has been made by the wafer and probe
tips. This connection completes an electrical circuit which then induces a voltage in the
inner two probes. This voltage is read through a digital multimeter and entered into the
“Sheet Resistivity Test” spreadsheet.
There are two formulas used to calculate resistivity of thin films. The first is a very
simple formula which relies on a geometric factor k (4.532) and the second is a more
complicated formula which relies on hyperbolic sine functions. Both are shown below in
their respective order:
Equ. 1 4.532
ln 2
·
Equ. 2 Here t is tf (film thickness). From these equations we can calculate the film sheet
resistance which is:
Equ. 3 Rs ( / sq ) 

V 
 4.532     Fc
tf
I
Normally Fc=1 if the following approximations are true:
1. ( d>> s , probe spacing: The diameter of the sample is much larger than the
probe spacing ( this may not be true for small chips, see below)
2. (tf<< s probe spacing) The film thickness is much smaller than the probe
spacing this is usually true since the probe spacing here is on the order of
0.050” .
3. Exception: For a square 10 x10 mm chip which is often used here in the lab
the correction factor of Fc=0.9 must be applied to the above equations.
II.
Equipment Hookup
The hookup of the various components is shown in the figure 1 below. V1 is used to
measure the voltage across the Resistor R1. This is used to calculate the current
through the sample. It is usually set to 4.532 mA. The Voltage on V2 is the voltage
drop across the sample due to the current passing through. This is used in equation 3
to calculate the sheet resistance. If only one Voltmeter is available the leads will have
to be switched out for both measurements. A switch box is proved for this in the
present setup.
1. To measure current place the switch in the current position and adjust current
reading on voltmeter (divide by 100 to get reading in amps).
2. To measure sheet resistance place the switch in sheet resistance position and
read the voltmeter, then use equation 3 above to calculate sheet resistance
I Figure 1. Dual meter setup. IF only one meter is available a switch box will switch
lead pairs A and B between the one meter.
III.
Procedure
1. Insert silicon wafer securely into vacuum chuck as in figure 2
Figure 2: Inserting wafer in chuck
Note: Occasionally the 3” wafers are made slightly large and do not fit into the
chuck. When this occurs, place a 4” silicon wafer into the chuck and perform the
test on top of it.
2. Turn on the HP 6177C DC current source and the Agilent 34401A digital
multimeter figures 3 and 4 respectively. ( wait several minutes for the current
source to stabilize)
Figure 3: Current Source ‐ Turn knob to "AMPS" setting
Figure 4: Multimeter ‐ Push "Power" button in
3. Place the switch box in the current senting to set and measure the current flow
through the tungsten carbide probe tips
Below, Figure 5 displays the proper way to setup a current measurement using the
Agilent multimeter. In order to display the voltage reading on the multimeter,
press the “DC V” button
Figure 5: Standard probes in multimeter
The reasing is in volts and the resistor in the resitor box is 100 ohms so just dived
the voltage by 100 to get the current in amps.
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For example if the current supply is adjusted so that the voltage displays
0.453V then the current is 0.452V/100ohms = 4.52 mA
Figure 6 demonstrates how to change the amount of current traveling through the
system. Many technicians set their current source to 4.532 mA which will
theoretically cancel out the geometric constant k, in equations 1 and 3.
Figure 6: Adjusting the amount of DC current
4. Adjusting the multimeter to display the voltage of the wafer
In order to display the voltage reading on the multimeter, press the “DC V”
button. Figure 7 shows the proper way of connecting the four point probe’s
banana plugs into the Agilent multimeter to measure the induced voltage.
Figure 7: Multimeter set up to measure voltage
5. Modifying the x-y coordinates of the stage to the desired location
Figure 1: Adjusting coordinates of stage
6. Raising the stage to make contact with the tungsten carbide probe tips
Twisting the knob towards you will cause the stage to lower, twisting the knob
away from you will cause the stage to rise. Figure 9 demonstrates how to adjust
the height of the stage.
Figure 9: Adjusting stage height
Note: Only raise the stage until slight contact has been made with all four probe
tips. The tips can easily be damaged if too much force is applied on the system.
7. Lower the stage and repeat test as many times as deemed necessary
Note: Once testing is complete, lower stage until its initial bottom position has
been reached.
8. Turn off the hp6177C DC current source and the Agilent 34401A digital
multimeter
9. Remove silicon wafer from vacuum chuck