Silicon wafers are ordinarily highly conductive and easy to measure with standard capacitive displacement sensors (See MTI’s Proforma 300i). Measuring the thickness of GaAs wafers that have high bulk resistivity (>10k Ohm/cm) is a little more difficult because the wafers act as non-conductive insulators in a capacitive sensors measuring field. Fortunately, MTI has a solution to this problem.

It’s possible to measure the thickness and TTV of high resistivity semiconducting wafers (like GaAs) using non-contact capacitive sensor technology. Accuracy demands the dielectric constant or permittivity of the wafer being measured remain constant. Manufacturing wafer material requires control and measurement to remain consistent; thickness ranges need to be less than several millimeters.
Some examples of semi conductive materials that can be measured by MTI’s New Digital Accumeasure TM are Sapphire, Gallium Arsenide, Indium Phosphide, and Silicon Nitride semiconductor wafers.
The Digital Accumeasure TM has several unique advantages over previous sensors: 
  • â–¶ The measurement is non contact so is ideal for surfaces that shouldn’t be touched
  • â–¶ Highly accurate thickness measurements typically less than a micron (.00004 inch) can be performed
  • â–¶ A special dielectric mode of operation for semi-conducting wafer measurement provides highly accurate thickness measurements
  • â–¶ User adjustable digital filtering/smoothing is selectable and a 20,000 samples/second rate will meet almost all process measurement applications
  • â–¶ Easily calibrated to measure thickness of insulating material, without knowing the dielectric constant of the materials being measured, if you have a sample of known thicknesses
  • â–¶ Directly reads the thickness and sends the results to a standard PC running MTI’s Digital Accumeasure TM display program

The same program assists in setting up the initial calibration needed to measure thickness with a few simple steps that are easy to follow.

GaAs wafer measurement

Placing a GaAs wafer between a capacitance probe and a ground plane creates a capacitive circuit that can be analyzed to determine total thickness

Set up is quick and easy.
Simply determine the range limit of the probe (by adjusting the height until the indicator on the amplifier just turns blue and before it turns red) and entering zero in the first sample thickness. Press “calibrate” and place a wafer of known thickness between the probe and ground plane. Now enter the known thickness into the second sample thickness and calibrate the second sample. This has stored the dielectric constant which will be used to measure all remaining wafers. Digital Accumeasure TM Capacitance Amplifier is also perfect for measuring other non-conductive commodities such as Plastic, Glass, Optical Disc’s, Paper, and Rubber.

MTI Instruments, Inc. offers several styles and types of non-contact sensors that can be customized to your exact requirements. If you have a difficult application to solve, contact our experienced team of application engineers who will thoroughly analyze your requirements and guide you to a practical, cost-effective solution.

Would you like to contact MTI Instruments directly to learn more about the products used in this application note?