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 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 waferUse a caliper or the micrometer stage shown below

To turn on, press and hold the right buttonTo turn off, press and hold both buttonsThe 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

Procedure for using the Pro-4

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

Record resistivity in ohm-cm as shown on the screen

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

Summary

Sample Wafer CalculationsUsing 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