Four Point Probe
Procedure for Pro4 using Keithley
Overview
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What is Four Point Probing
How the system works
Pro 4 Set Up
Simple Calculations behind Four Point Probing
Procedure for using Pro4
What is Four Point Probing
• Four Point Probing is a method for measuring
the resistivity of a substance.
• Impurity concentrations can be estimated from
the resistivity
Resistivity vs Sheet Resistance
• Bulk or volume
resistivity (r) is
measured in ohms-cm
• Sheet resistance (rs) is
measured in ohms-persquare
• Independent of sample
size or shape
• Can be used to
measure the value a
resistor in a IC
Pro-4 Set Up
The 4 point probing setup consists of 3 key components

Pro-4 probing station from LUCAS LABS
Source
Meter
with 4 point probe head

KEITHLEY 2400 power/source meter

Computer with Pro4 software and
interface
Pro-4
Software
Probing
Station
The 4 point probing setup can measure resistivity or the thickness of a film.
But, either one has to be known.
Resistivity Probe Stand
Contact Lever
Probe head electrical
connection
Probe Head
Mounting Chuck
(Aluminum base with
Teflon surface
How the system works
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Current is passed through the two outer probes
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Voltage is measured between the two inner probes
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Read and record both current and voltage values from the
Keithley source meter
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Sheet Resistance is measured using (V/I) and k
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V = volts, I = Amps (convert current reading to amps)
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k=constant factor = to 4.53 when the wafer diameter is
much greater than the probe spacing – typical for wafers
•
Sheet resistance (rs) = (k)(V/I)= ohms/square
For the bulk resistivity of a wafer
• The thickness of the wafer/film
must be known – use calipers or
micrometer block to measure the
wafer thickness
• Convert caliper reading in mm to
um (microns)
• Resistivity of wafer will be shown
on the computer screen
• There is a second k factor but for
our work this k factor is not a
factor and can be ignored
(typically >.995)
To measure the thickness of a wafer
Use a caliper or the micrometer stage shown below
To turn on, press
and hold the right
button
To turn off, press
and hold both
buttons
The reading on the
screen is in mm –
a reading of
.400mm = 400
microns (um)
Procedure for using the Pro-4
Enter thickness in microns (um)
as measured
Choose either single or
multiple readings per wafer.
Load the wafer under the 4
point probe head and lower
the head using the handle
Procedure for using the Pro-4
Record resistivity in ohm-cm as
shown on the screen
Lower the probe heading
using the contact lever until
the computer screen shows a
measurement taking place.
Once completed, the screen
will show the resistivity and
V/I
Saving the data
• After the measurement is
completed, the resistivity at each
location will be displayed on the left
hand side of the screen.
• When all the points are tested, the
data can be saved and read using
excel
• The Pro-4 can be used to measure resistivity or the thickness. But, either
one has to be known.
• Typically the thickness of the wafer can be measured
• The # of points to be tested and the shape of the sample can be selected.
• A single point or multiple points on the sample can be tested to obtain
the average resistivity.
• The resistivity is automatically displayed when the thickness is entered.
• Additional information is also displayed including V/I
Sample Wafer Calculations
Using voltage and current measurements
• A current of 1.0 mA is passed through the
wafer and a voltage reading of 0.030 v is
noted. I = 1.0 MA = .001 amp
• V/I = .030 v/.001A = 30 ohm
• rs = (V/I) k = (.030/.001)(4.53) = (30)(4.53) =
135.9 ohms/square
• The wafer is measured as 0.40 mm = .04 cm
• r = (135.9)(.04cm) = 5.43 ohm-cm