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

 Four Point Probing is a method for measuring the resistivity of a substance.  Impurity concentrations can be estimated from the resistivity

 Bulk or volume resistivity ( r ) is measured in ohms- cm  Independent of sample size or shape  Sheet resistance ( r s ) is measured in ohms-per-square  Can be used to measure the value a resistor in a IC

 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.

Contact Lever Probe head electrical connection Probe Head Mounting Chuck (Aluminum base with Teflon surface

 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 ( r s ) = (k)(V/I)= ohms/square

 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=r s 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)

 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  r s = (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

 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

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.

 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