A microprocessor controller for an accurate positioning system

This paper describes a microprocessor-based (Zilog Z80) position control system for an X-Y table which is a precision specimen stage in an electron-beam lithographic machine used to define patterns on semiconductor wafers. The controller receives commands which specify the coordinates, on a user specified Cartesian grid, to which the workpiece is to be moved. After checking that the requested position is within the table´s movement range the microprocessor rotates and translates these user coordinates into the representation used by the position sensor, a two-axis laser interferometer. This interferometer measures the positions of mirrors, which are mounted on the specimen table, to an accuracy of 79 nm. Coarse positioning of the table is achieved by means of stepper motors which drive a pair of crossed lead screws whilst fine adjustment of the specimen´s effective position is provided by applying a small offset to the electron-beam deflection drivers via digital-to-analogue converters. Thus although the table is only near to the required position, the specimen is nevertheless effectively aligned with the working point of the electron-beam. When the table has been placed in the desired position and come to rest, a signal is given to indicate that processing may resume. An operator panel provides for push-button manual control of the drive motors and a liquid crystal display shows the table´s position.