1. Procedure for Pro4 using Keithley
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 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

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
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.

6. Resistivity Probe Stand
Contact Lever
Probe head electrical connection
Probe Head
Mounting Chuck (Aluminum base with Teflon surface

7. 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

8. 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)

9. To measure the thickness of a wafer Use a caliper or the micrometer stage shown below
To turn on, press and hold the right button
To turn off, press and hold both buttons
The reading on the screen is in mm – a reading of .400mm = 400 microns (um)

10. Procedure for using the Pro-4
Enter thickness in microns (um) as measured

11. Procedure for using the Pro-4
Choose either single or multiple readings per wafer. Load the wafer under the 4 point probe head and lower the head using the handle

12. Procedure for using the Pro-4
Record resistivity in ohm-cm as shown on the screen
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

13. 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

14. Summary
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

15. Sample Wafer Calculations Using voltage and current measurements
A current of 1.0 mA is passed through the wafer and a voltage reading of 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) = ohms/square
The wafer is measured as 0.40 mm = .04 cm
r = (135.9)(.04cm) = 5.43 ohm-cm