Precision measurement of two-axis positions and tilt motions using a surface encoder

A prototype surface encoder consisting of a sinusoidal angle grid and a two-dimensional slope sensor is developed for two-axis position measurement. The angle grid has a three dimensional micro-structured surface, which is a superposition of periodic sinusoidal waves in the X- and Y-directions with spatial wavelengths of 150 μm and amplitudes of 100 nm over an area of up to 150 mm in diameter. The laser-based slope sensor is used to read local slope profiles of the grid surface. The X- and Y-positions together with tilt motions about the X-, Y- and Z-axes can be detected with resolutions of 0.1 μm and 1 arc-second through scanning a multi-spot laser beam across the grid surface at a constant speed.