Four Point Probe

1
Four Point Probe

• Procedure for Pro4 using Keithley

2
Overview

• What is Four Point Probing
• How the system works
• Pro 4 Set Up
• Simple Calculations behind Four Point Probing
• Procedure for using Pro4

3
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

4
Resistivity vs Resistance

• Bulk or volume resistivity is measured in ohms-cm
• Independent of sample size or shape

• Sheet resistance is measured in ohms-per-square
• Used for measuring resistivity of thin films

5
Pro-4 Set Up
The 4 point probing setup consists of 3 key
components

• 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 can measure resistivity
or the thickness of a film. But, either one has
to be known.
6
Resistivity Probe Stand
7
How the system works

• 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

8
Calculations

• 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

9
Resistivity measurements

• 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

10
Procedure for using the Pro-4

• Click on the Auto Test tab for multiple
  measurements or Single Test tab for a single
  reading.
• Select size and shape of the sample using the
  tabs at the bottom of the page. Type in the
  required information.
• Place the sample on the mounting table and then
  move the sample to position it at the required
  location
• Turn down the lever so that all the needles on
  the probe head are in contact with the wafer.

11
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