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Four Point Probe

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Four Point Probe

Procedure 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 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

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

Source Meter

Pro-4 Software

Probing Station

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
- Resistance is measured using ohms law
- The thickness of the film must be known

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

- 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

- 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.

Saving the data

- When all the points are tested, the data can be saved and read using excel

- After the measurement is completed, the resistivity at each location will be displayed on the left hand side of the screen.