# Four Point Probe ```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 is measured
in ohms-cm

Sheet resistance is
measured in ohmsper-square

Independent of
sample size or shape

Used for measuring
resistivity of thin films
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
electrical connection
Mounting Chuck
(Aluminum base with
Teflon surface

Current is passed through
the two outer probes

Voltage is measured
between the two inner
probes

Resistance is measured
using ohms law

The thickness of the film
must be known

If the spacing between the probe
points is constant, and the conducting
film thickness is less than 40%, and the
edges of the film are more than 4 times
the spacing distance from the
measurement point, the average
resistance of the substance is given by


› Rs= k x V/I

For bulk resistivity, the sample is much
larger than the spacing of the probes,
the sheet resistance is defined as the
resistance equal to the bulk resistivity
divided by the sample thickness. The
equation is given by…
› Rs= P/t


Rs = Resistivity of the
substance
K = constant for the
average resistivity
that is equal to 4.53
P = Resistance
t = thickness of the
substance

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
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