Four Point Probe
The Magnetron model 800 resistivity/conductivity test system measures the resistivity of silicon slices using a collinear four point probe array. These measurements can be taken either in units of Ohms (resistance) or Ohms per square (sheet resistance) - assuming and infinite thin layer geometry. It also has a typing mode that incorporates both thermoelectric and rectification capabilities for determining if the sample is P- or N-type. The resistance range for our probe is about 10 - 106 Ohms between the two inner probe tips. Lower Resistances have large amounts of error and higher resistances create voltages too great to be measured even at the lowest current settings.
Average percent error for different amounts of total resistance between the inner two probes
- A current source built into the base.
- A probe stand adjustable to heights of over two inches.
- The Digital Panel Meter (DPM) mounted above (includes a voltmeter and circuitry that calculates the resistance).
- Digital Panel Meter (DPM) (mounted on top)
- Power switch: Switch left to turn on the instrument
- Votlage range buttons: Select the range of the voltmeter: 10mV, 100mV, 1V, 10V
- TEST toggle button: Activiates/deactivates the calibration testing mode.
- THERM button: Momentarily activates thermoelectric pulsing (used in typing mode). Do not hold this button down, as it will cause incorrect readings.
- Out Reg. LED: Indicates that the measurement could be unreliable. This can be caused if the compliance voltage is exceeded (try reducing the current level), a probe tip has broken, the head is not making proper contact (make sure it's in the full down position), or the current source has malfunctioned.
- Actuator Arm (The lever immediately below the Digital Panel Meter): Raises and lowers the probe head.Note that the probe head must be in the full down position to activate the probes and take measurements. The height can be adjusted using the probe head height knob.
- Function Knob (on the left side of the base): Switches between measuring resistivity (RES.) and typing mode (used to determine if the sample is P- or N-type). Typing mode should only be used for a few seconds at a time, then the knob should be returned to the RES position.
- Range Knob (on the right side of the base): Controls the system's current range in either ohms or ohms/square. Ohms: 1uA, 10uA, 100uA, 1mA, 10mA, 100mA. Ohms/square: .453uA, 4.53uA, 45.3uA, .453mA, 4.53mA, 45.3mA.
- Polarity Button (on the back of the probe stand): Reverses the polarity of the current source when pressed and held. This typically should change the sign of the measured value, but not the magnitude. If the magnitude changes, verify that the probe tips are making proper contact with the sample.
- Probe head height knob (can be reached through a hole in the bottom of the base): Adjusts the height of the probe head. The probe head should be close to, but not touching the sample (The probe tips should make contact and be pressed up into the probe head. They are weighted to apply the correct amount of pressure).
- Out of Regulation LED: Indicates that the readings could be unreliable.
- Commpliance voltage may be exceeded. Reduce current level.
- A probe tip may have broken off. Replace probe head.
- Probe head may not be contacting the sample properly. Make sure proper contact is made with the probe head in the full down position. A micro-switch cuts off the current if the head is not all the way down.
- Current source has malfunctioned.
- Push in the 100mV button on the Digital Panel Meter (DPM).
- Make sure the TEST button on the DPM is in the IN position (for calibration mode).
- Make sure the Function knob (on the left side of the base) is set to RES (for resistance measurements).
- Turn the current range knob (on the right side of the base) to either 1mA or .453mA.
- Turn on the power switch.
- Allow the system approximately 15 minutes to stabilize. (The manual says to do this, but it probably is not necessary.)
- The DPM should read "CAL 100.0 Ohms" or "CAL 4.53 Ohms/SQ" ± 1 digit. If not, it may need calibration.
- Turn off calibration mode by toggling the TEST button to the OUT position on the DPM.
- The system is now ready to make measurements.
Loading the sample
- Make sure the probe head is in the full up position by moving the actuator arm all the way to the right.
- Place your sample on the platform and slide it under the probe head. Be careful not to knock it against the probe tips. If it does not fit, then remove the sample, tip the four point probe up on its back, and adjust the probe height knob by reaching through the hole in the bottom of the base.
- The probe tips sticking out of the bottom of the probe head are weighted or spring loaded to exert the right amount of pressure on your sample. When the probe head is fully lowered, the tips should be pressed up into the head but the head itself should not make contact with your sample (it could exert too much pressure and break it).
- Slowly lower the probe head by pulling the actuator arm toward you and to the left. When the probe head is in the full down position, it should make proper contact with your sample as described in #3. If the head is to high or too low, remove your sample and adjust the probe head height knob by reaching through the hole in the bottom of the base.
- The probe tips must make proper contact and the head must be in the full down position before measurements can be taken. (A micro-switch deactivates the current source if the head is not in the full down position.)
- Basic Resistance / Sheet Resistance measurement
- To measure total resistance (Ohms) between the two inner probes, use one of the 10n Amp current settings (where n is an integer). To measure sheet resistance (Ohms/SQ), use one of the 4.53x10n Amp current settings. Note that these sheet resistance measurements assume an infinite thin layer geometry and will not be accurate if your geometry is significantly different (see geometric considerations under the Theory section).
- Set the current range knob to the lowest setting in either Ohms or Ohms/SQ, depending on the type of measurement you need to make. (1uA for Ohms or .453uA for Ohms/SQ)
- Set the voltmeter (DPM) to its lowest setting: 10mV.
- Increase the current until the DPM is overloaded (it will blank out all digits except for the leftmost "1").
- Increase the voltmeter setting on the DPM until a value can be read again. Note: Measurements on the four point probe are generally more accurate when higher currents are used. However, the 10V setting on our four point probe does not work very well. It often can't take a reading, or the reading jumps all over the place. When it does occasionally give a stable reading, the reading is accurate. I recommend using the highest current that can still be measured using the 1V setting, if possible.
- Repeat the above two steps until either the current supply or the voltmeter is maxed out.
- Record your measurement.
- Typing Operation (determines if the sample is P- or N-type)
- The function knob should not be left in TYPE position for more than 5-6 seconds. Read over this section so you know what to do before beginning. Typing mode induces current into the wafer and can damage probe heads with plastic bodies if left on too long.
- Turn the function knob from RES to TYPE.
- If the system indicates N or P, return the knob to the RES position and you are done.
- If the N and P symbols flash on and off, then thermoelectric pulsing is necessary. Momentarily press in on the THRM button (not longer than 1-2 seconds) to get the reading. Holding it down too long could cause an incorrect reading. If N and P still flash, then it is undeterminable. Return the function knob to RES and you are done.
- If the N and P symbols light up but do not flash, then the probe is not sensitive enough to determine the sample's type.
- Basic Resistance / Sheet Resistance measurement
- Raise the probe head to its full up position and remove your sample.
- Make sure the function knob has been returned to the RES position.
- Turn off the power switch.
Why 4 probes instead of 2?
When the resistance of the probe/sample contact becomes comparable to the resistance of the sample itself, it is necessary to separate the probes that measure the voltage from the probes that conduct the current. Otherwise, The contact resistance would be measured in addition to the sample resistance. By placing the voltmeter probes between the current source probes, only the voltage drop caused by that part of the sample itself is measured. This is standard practice for measuring the resistivity of semiconducting materials.
For infinitely large layers with thickness T less than half the probe spacing s,
1. Ohms: Resistance
2. Ohms per square (Ohms/SQ): Sheet resistance
- For a two-dimensional material, this is the resistance seen from one side of a square to the other, regardless of the size of the square. This is because for a square, L=W. Thus, L/W=1 and R=RsL/W becomes R=Rs. Note that the sheet resistance does not take into account the layer thickness, T.
3. Ohm-cm: Resistivity.
4. Converting sheet resistance (Ohms/SQ) to resistivity (Ohm-cm)