The Proforma 300i from MTI Instruments is a system used to measure conductive wafers, silicon wafers, or gallium wafers. Other types can also be measured in the indium class if the doping is low and the resistance is long enough for the system to measure.
The Proforma 300i from MTI Instruments can measure thickness of wafers and total thickness variation (TTV). As an option with the Proforma 300i, it can measure bow and it has an extended range of 1.7 millimeters, where standard equipment generally measures a maximum gap of 1.2 millimeters. This could be a good modification or add-on if measuring thicker wafers.
The Proforma 300i consists of a numeric display, keyboard, and a Teflon coated deck to allow the wafer and wafer ring to slide on the deck without scratching the wafer. There are dials on the side of the machine to adjust and level the feet to ensure the most accurate readings. The keyboard allows for manual entry and setting up the instrument changing modes of operations calibrations.
When turning on the Proforma 300i, allow the amplifiers to thermally stabilize, or warm up, for 30 minutes. The cooling fan can always remain on but should also be turned on during the warmup time.
The Proforma 300i should be calibrated with a wafer of known calibrated thickness or by using a gauge block, which are a certified thickness. To calibrate the Proforma 300i, center the calibrated wafer between the probes. By using the keyboard and arrow menu functions, change the mode to calibrate. Then set the machine measurements to CAL STD to “teach” the machine the thickness of the wafer being used as the standard, known, measurement.
The Proforma 300i also contains a USB port to save the data collected on a thumb drive. Insert the thumb drive into the USB and press the Save button on the keyboard to save the data after each measurement is taken. Another computer can control the Proforma 300i through ethernet connectivity. The connections/ports for this feature are located on the back of the machine.
To measure TTV, using the menu, change the mode to five-point, and move the wafer to the points to be measured. After each point, press select to record the measurement. This will measure the total thickness variation and deviation among the points.
Bow can also be measured by changing the mode on the main menu to BOW. Using a wafer ring, measure one side, then flip the wafer, and measure the second side. The measurements may also be saved to the thumb drive by selecting save from the keyboard.
To measure more exotic materials such as gallium arsenide, or indium phosphide, there are a few modifications that must be made to the Proforma 300i. Unscrew the nosepiece and by using an Allen key, rotate the probe 180 degrees. The probe must be gaffed with a gauge block. Reinstall the screw and tighten the nosepiece. The machine will then require re-calibration using the techniques discussed above.
As a note, due to the dielectric constant of the air, there will always be a measurement reading even when there is no wafer under the probe.