Specific Process Knowledge/Characterization/Four-Point Probe
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Four-Point Probe
The set-up of the four-point probe is based on four tungsten carbide electrodes/probes. The power source (DC) sends a current through the two outer electrodes. A digital voltage meter (DVM) measures the potential drop between the two inner electrodes.
The sheet resistance can then be calculated using the equation below:
This equation is only valid if: i) the material being tested is no thicker than 40% of the spacing between the probes, and ii) the lateral size of the sample is sufficiently large. If this is not the case, then geometric correction factors are necessary to account for the sample's size, shape, and thickness.
Bulk resistivity for wafers and films = sheet resistance x wafer thickness in cm (Unit: ohm.cm)
Bulk resistivity for a semi-infinite volume = 2 x π x S x V/I, where S is the probe spacing in cm (Unit: ohm.cm)
A sample is usually defined as a “semi-infinite volume” if it extends to infinity in all directions below a plane on which four probes are located.
Note:
Volume Resistivity = Volume Resistance = Bulk Resistivity = Bulk Resistance
Sheet Resistivity = Sheet Resistance = Surface Resistivity = Surface Resistance
Limits of the Measurement
1. The material must be capable of being probed, i.e. the probes must be able to make ohmic contact with the material e.g. Germanium, Silicon and metals. Materials such as Gallium Arsenide cannot normally be probed unless it is doped and measured with special measuring techniques, such as those in the Four Dimensions Inc. GaAs probe.
2. Very low resistivity material, e.g., aluminum, gold, platinum, may require the maximum current from the current source to achieve a reading on the digital voltage display. Only very thin films from 10’s nm up to 1 micron thickness can be measured. Samples with expected sheet resistance less than 1 ohm/square may not be possible to measure.
3. High resistivity material, e.g., ion-implanted Silicon wafer, silicon on sapphire, can be measured using very low current (about 1uA or less) and trying to avoid a greater voltage indication than 200mV. Sheet resistance of up to 10^7 ohms/square can be measured.
4. Low measurement result could be from electrical noise due to poor contact condition, thermally induced voltages, actinic effects, offset voltages produced by the devices in the current source etc.
There are two four-point probes at DTU Nanolab. Four-point probe from Jandel and Four-Point Probe from Veeco.
Four-Point Probe from Jandel
The Four point probe is manufactured by Jandel Engineering Limited and using Jandel probe head, a cylindrical probe type B: 1.00 mm spacing, 100 micron tip radius, 100g load linear is being use at Nanolab and the RM3000+ Test Unit which can supply constant current between 10nA and 99.99mA, and measure voltage from 0.01mV to 1250mV. The unit supplies a constant current and can display the resultant voltage, sheet resistance, or volume resistivity, depending on the chosen function. For calculating volume resistivity (for wafers or bulk samples), it is possible to input wafer thickness or probe spacing as required.
There is an Autorange feature: it will begin at 10 nA and increase the current by a factor of 10 until a suitable level of measuring voltage is reached, then it will stop and display the voltage or ohms/square on the screen.
For sheet resistance measurements, the quoted range is 1 milliohm/square to 5 × 10^8 ohms/square. Measurements outside this range are possible but with possible reduced accuracy. For volume (bulk) resistivity measurements, the quoted range is 1 milliohm.cm to 1 × 10^6 ohm.cm. Measurements outside of this range may be possible, but will depend on sample type, e.g., whether the sample is a thin layer.
The multiposition probe stand can measure on wafers up to 200mm in diameter. The Θ movement clicks in four positions at 90 degrees, and the linear movement can be adjusted in up to 10 positions, providing repeat placement accuracy of ±1 mm.
The user manual, technical information, and contact information can be found in LabManager:
Four-Point Probe from Veeco
The Four-Point Probe is a Veeco FPP-5000. The main purpose is to measure film resistance and resistivity on a 4" or 6" wafer. And it can also be used to find film thickness by knowing film resistivity.
Only 4inch or 6inch wafers can be fitted in the system because a special holder is needed.
The user manual, technical information and contact information can be found in LabManager:
Overview of the performance of the Four-Point Probe at DTU Nanolab
| Four point probe - Jandel | Four point probe - Veeco | |
| Purpose | Measure voltage, sheet resistance or volume resistivity at varied current | Measure resistance, resistivity or film thickness(*) at 100uA |
| Unit | mV, ohm/square, ohm.cm(wafer), ohm.cm(volume) | ohm/square, ohm.cm, |
| Substrate | Small pieces up to 200mm wafer | Only 4inch or 6inch wafer |
(*) Knowing the resistivity