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Four Point ProbeProcedure for Pro4 using Keithley
Overview
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
Independent of sample size or shape
Sheet resistance (rs) is measured in ohms-per-square
Can be used to measure the value a resistor in a IC
Pro-4 Set Up
Pro-4 probing station from LUCAS
LABS with 4 point probe head
KEITHLEY 2400 power/source meter
Computer with Pro4 software and
interface
The 4 point probing setup consists of 3 key components
Probing Station
Source Meter
Pro-4 Software
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 Current is passed through the two outer probes
Voltage is measured between the two inner
probes
Read and record both current and voltage
values from the Keithley source meter
Sheet Resistance is measured using (V/I) and k
V = volts, I = Amps (convert current reading to
amps)
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 to measure
the wafer thickness
Convert caliper reading in
mm to cm
Resistivity of wafer r=rs x
thickness in cm
There is a second k factor
but for our work this k
factor is not a factor and
can be ignored (typically
>.995)
Sample Wafer Calculations
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
The Pro-4 can be used to measure resistivity or
the thickness. But, either one has to be known.
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
Resistivity measurements
1. Click on the Auto Test tab for multiple
measurements or Single Test tab for
a single reading.
2. Select size and shape of the sample
using the tabs at the bottom of the
page. Type in the required information.
3. Place the sample on the mounting
table and then move the sample to
position it at the required location
4. Turn down the lever so that all the
needles on the probe head are in
contact with the wafer.
Procedure for using the Pro-4
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
Saving the data