Topography Measurements of High Gradient and Reflective Micro-structures by Digital Holography

Introduction

Due to both, 100% fill factor and the high shape gradient, the silicon molds are difficult to fabricate and are also demanding in characterization by optical methods. In the paper we present a method that overcomes some of measurement problems and can be used for recovering high numerical aperture (NA) shape of reflective microstructures, such as silicon molds. To achieve this practical goal we use the digital holography in microscope configuration with afocal imaging system [1] working in reflection mode. The standard method for topography reconstruction in optical full field metrology uses thin element approximation (TEA). In this paper we deal with the high NA optical field generated by an object. TEA would produce significant errors and cannot be applied. There are two algorithms that allow shape reconstruction with smaller error: the extended depth of focus (EDOF) [2] and the local ray approximation (LRA) [3,4]. The first one computes the shape from unwrapped measured phase. The phase is used in a refocusing algorithm to obtain the local object height from the optical field. The phase in this plane is reconstructed by TEA algorithm. Second algorithm is based on analysis of a local ray’s optical path differences in object. Both algorithms can be applied under condition that entire optical field is transferred by the imaging system. Also the algorithms require knowledge of the precise location of the plane from which the phase originates [5].