Thickness, Shape and Flatness Measurement of Semiconductor Wafers - MTI Instruments - #1

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THICKNESS MEASUREMENT THICKNESS ASTM F657: The distance through a wafer between corresponding points on the front and back surface. Thickness is expressed in microns or mils (thousandths of an inch). TOTAL THICKNESS VARIATION (TTV) ASTM F657: The difference between the maximum and minimum values of thickness encountered during a scan pattern or series of point measurements. TTV is expressed in microns or mils (thousandths of an inch). B Tw WAFER Gtotal MEASUREMENT AXIS PROBE A PROBE B A Figure 1 Figure 1 above shows a wafer placed between two non-contact measurement probes. By monitoring changes between the upper probe face and the upper wafer surface (A) and the bottom probe face and the bottom wafer surface (B), thickness can be calculated. First the system must be calibrated with a wafer on known thickness (Tw). The area of known thickness is placed between the probes and an upper probe to wafer gap (A) and a lower probe to wafer gap (B) is acquired. The total gap (Gtotal) between the upper and lower probes is then calculated as follows: Gtotal = A + B + Tw With the system calibrated, wafers of unknown thickness can now be measured. When the wafer is placed between the probes, a new value of A and B is acquired. Thickness is calculated as follows: Tw = Gtotal - (A + B) During an automated scanning of the wafer, a series of point measurements is taken and stored. Following completion of the scan, TTV is calculated as follows: TTV = Tmax – Tmin