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