MTI Instruments Inc. has developed a thickness measurement device that eliminates the effect of varying target conductivity. Called the Push-Pull probe, it’s a unique version of the AccumeasureTM amplifier series. This special design provides accurate surface information for wafer bow and warp.
Bow is the deviation of the center point of the median surface of a free, un-clamped wafer from the median surface to the reference plane. Where, the reference plane is defined by three corners of equilateral triangle. This definition is based on now obsolete ASTM F534.
Warp is the difference between the maximum and the minimum distances of the median surface of a free, un-clamped wafer from the reference plane defined above. This definition follows ASTM F657, and ASTM F1390.
In this design, each probe consists of two capacitance sensors built into one probe body. Each sensor is driven at the same AC voltage with a 180 degree phase shift between signals. The shift allows the current to travel across the target surface rather than through the target to ground, eliminating inaccuracies created by poorly grounded targets. The individual output signals are summed by the amplifier producing a single 0 to 10 VDC output that is proportional to the probe-to-wafer gap.
Measuring wafers of differing resistivity (below 10k ohm/cm) has no effect on device accuracy. Tests that measure materials with significantly different bulk resistivity (Computer Hard Disks, Si Wafers, Ge Wafers, and Optical Disks) demonstrate no need to recalibrate the probes for changes in target material. Electrically grounding the measurement stage or leaving it un-grounded has no effect on measurement accuracy. Thickness measurements are taken using every portion of the voltage output range with no decrease in device accuracy.
These probes are standard in the ProformaTM Series Wafer Measurement Tools. See Digital Accumeasure for multi-points measurement and AS-562-PA push pull probe amplifier series for high volume requirement.