Abstract: A wafer prober for testing uncut die of a semiconductor wafer has an alignment accuracy in the X-Y plane within the sub-micron range. The wafer, held by a chuck, is positioned in the X-Y plane by using a pair of linear servo motors each driven by a servo drive unit providing drive control, monitoring and diagnostic functions. Positional information is obtained from a pair of linear encoders, one along each drive axis. Positional information is also obtained from a pair of resolvers, one for the vertical axis and the other for rotational position about the vertical axis. Ways and runner blocks are used to provide substantial downforce capability and high speed translational movement. In addition, the present invention uses a general purpose computer providing for an on-board controller card interfacing with the servo drives via a fiber optic coupler.