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Four Point Probe

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.

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Four Point Probe

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  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 vsSheet 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 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.

  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 Keithleysource 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 to measure the wafer thickness • Convert caliper reading in mm to cm • Resistivity of wafer r=rs 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)

  9. Sample Wafer Calculations • 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

  10. 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

  11. 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.

  12. 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.

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