Grid plate encoder based positioning system and method

A method of positioning an element, comprising:

• providing a grid plate with a grid plate surface;
• providing an encoder unit with one or more optical sensors for sensing a grid plate surface pattern of the grid plate surface;
• receiving setpoint coordinates specifying a desired position of the element;
• computing compensated coordinate data corresponding to estimated position data expected from the encoder unit when the element is positioned at the desired position;
• using feedback control to position the element where the estimated position data correspond to the compensated coordinate data,
• wherein the method further comprises:
• providing a wafer stage for carrying a wafer having optically detectable marks at a wafer surface facing the grid plate surface;
• providing a calibration bead movable between the wafer stage and the grid plate, the calibration head including the encoder unit with the one or more optical sensors for sensing the grid plate surface pattern of the grid plate surface and a mark sensor for sensing the optically detectable marks,
• wherein the calibration head is configured for being positioned at a plurality of mutually different lateral positions, and wherein the compensated coordinate data is computed from measurement data obtained from the encoder unit and from the mark sensor at the plurality of mutually different lateral positions.

A method of computing compensation parameters for use with a grid plate in a grid plate encoder based positioning system, the grid plate having a grid plate surface with a surface pattern, the grid plate encoder based positioning system comprising an encoder unit with one or more optical sensors for optically sensing the surface pattern at the grid plate surface and a signal processor for computing estimated position data from sense signals obtained from said one or more optical sensors;

• the method comprising:
• providing a reference wafer having optically detectable marks at a wafer surface facing the grid plate surface;
• providing a calibration head between the grid plate and the reference wafer, the calibration head having a mark sensor facing said wafer surface and having at least the one or more optical sensors of the encoder unit facing the grid plate surface;
• positioning the calibration head at a plurality of laterally different positions between the grid plate and the reference wafer to obtain respective position indications from the mark sensor indicative for its sensed position with respect to the optically detectable marks, and to obtain a respective encoder position indication from the encoder unit indicative for its position with respect to the grid plate surface;
• and
• computing the compensation parameters based on the position indications.