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Optical Wafer Warpage Measurement System

 

The measurement station consists of a unique linear air bearing stage with an integral air bearing rotary table. This has originally been developed specifically for wafer warpage measurement for a German wafer manufacturer. It can be used for wafers upto 300 mm diameter. These stages are an integral part of a single granite block. The unit consists of a massive granite block and features a moving stage made from aluminum with a hard anodised surface, 400x400x50 mm, with integrated air channels, vacuum preload, and can accommodate a vacuum chuck. The side rails are granite with a linearity of 1 µm.

The whole unit sits on passive anti vibration dampers and on a metal frame. The linear stage is driven by a steel band with DC motor/gear to minimise vibrations. Inside the linear stage rests a rotational air bearing table, diameter 300 mm, running coplanar on the granite base surface with vacuum preload and DC motor with gear and encoder.

The profile (warpage) measurement is made such that one takes a profile line across the wafer and then rotates the wafer by a certain angle and again makes a profile, etc. So all profile lines add up to a star-like graph which is evaluated. (in the picture you see a tactile pick up but naturally we will provide an optical sensor according to the customer’s requirements, e.g. an optical displacement sensor working according to the confocal principle. It has small enough a spot size to resolve details of the surface and lends itself well for surface profiling.

The controller of the sensor is a PC card. The sensor is attached to the granite bridge of the granite base unit via a manually adjustable Z displacement stage.

Windows XP / 7 data controlling and data acquisition software. After setting the required parameters (data point density, travel length, etc.) the measurement can be started on a mouse click and the results will be displayed and can be saved or sent to another PC.

Further details on request.