Transcript Four Point Probe - WordPress for the College of

What is Four Point Probing
 How the system works
 Pro 4 Set Up
 Simple Calculations behind Four Point
Probing
 Procedure for using Pro4


Four Point Probing is a method for
measuring the resistivity of a substance.

Impurity concentrations can be
estimated from the resistivity


Bulk or volume
resistivity (r) is
measured in ohmscm
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
The 4 point probing setup consists of 3 key components

Pro-4 probing station from LUCAS
Source
Meter
LABS with 4 point probe head

KEITHLEY 2400 power/source meter

Computer with Pro4 software and
interface
The 4 point probing setup can measure resistivity or the
thickness of a film. But, either one has to be known.
Pro-4
Software
Probing
Station
Contact Lever
Probe head
electrical connection
Probe Head
Mounting Chuck
(Aluminum base with
Teflon surface

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

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)
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
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.

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