Characterization of a geometrically desensitized interferometer for flatness testing Xavier Colonna de Lega, Jim Biegen, Dave Stephenson, Peter de Groot Zygo Corporation, Laurel Brook Road, Middlefield, CT 06455, USA ABSTRACT We describe the detailed design of a geometrically desensitized interferometer using two transmission diffraction gratings. A number of models of the instrument are used to eliminate object ghosts and stray light contributions. We then investigate analytically the influence of object slope variations on the instrument precision. We show that the part can be located at a measurement location where the metrology is optimized. Analytical and raytracing models demonstrate excellent agreement with experiment. Keywords: desensitized interferometry, flatness testing, diffraction gratings 1. INTRODUCTION Geometrically desensitized interferometers (GDI) constitute an interesting class of instruments in that they take advantage of both interferometric and geometric optical metrology techniques. They make efficient tools for measuring engineering surfaces since they provide a large dynamic range and work with both rough and smooth surfaces. The basic principle of a GDI consists of sending two beams of light with different angles on the object surface before recombining them. The choice of angles adjusts the sensitivity of the setup. It is then possible to desensitize the interferometer to accommodate large surface departure and roughness. GDI are different from grazing-incidence interferometers in which only one beam reflects at a very high angle of incidence from the surface. In this paper we characterize a GDI in which the two measurement beams are produced and recombined by a pair of diffraction gratings1'2. We will first recall the basic properties of this instrument. In a second step we will discuss a number of design considerations related to the multiple diffracted orders and stray beams created by the dual grating assembly. Finally, we will study the effect of the combination of object slope and part defocus. In particular we will show that locating the part within an optimum measurement volume minimizes geometric errors. 2. MAIN PROPERTIES OF THE INTERFEROMETER A schematic of the interferometer is shown in Figure 1 . A collimated beam comes at an angle to a first diffraction grating, which we call the coarse grating. Diffraction in the +1 and —1 orders creates two beams that propagate a distance h before reaching the second grating, called the fine grating. A second diffraction in the —1 and +1 orders deviates these beams so that the pair of rays crosses in a plane located at a distance h ' from the second grating. In practice h = h ' when the fine grating frequency is twice that of the coarse grating, which is the case for our instrument. This geometry provides two main benefits. First, the working distance of the instrument is determined by the grating separation h, which is on the order of 50 mm in our setup. This is a considerable advantage for industrial applications when compared to a prism-based grazing-incidence interferometer where the part has to be brought as close as possible (at most a few hundred microns away) to the optical reference surface . The second benefit is the fact that the grating pair forms an achromatic interferometer4. Hence there is no requirement on the temporal coherence of the source. The possible use of white light has already been demonstrated5. In practice the coherence length of the source is determined by its spatial extent in the plane of incidence (the plane defined by Figure 1). Part of the SPIE Conference on Three-Dimensional Imapino, Ontical Metroloqy. and lnsiection IV Boston. Massachusetts • November 1998 284 SPIE Vol. 3520 • 0277-786X/99/$10.00 Copyright 1998 Society of Photo-Optical Instrumentation Engineers. This paper was published in Proceedings of SPIE and is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.