ABSTRACT
Digital holography is well suited for the investigation of microcomponents. As an interferometric method, it offers both a field-wise and noncontact approach to very fragile components and a high sensitivity with respect to geometric quantities such as coordinates and displacements. The smallness of these objects diminishes the practical consequences of the limited space–bandwidth product (SBP) of the existing sensors, such as the restricted angle resolution of digital holograms. Nevertheless, several boundary conditions have to be observed carefully. This topic will be discussed in detail. We start with a description of the basic principles of DH, including the analysis of the recording process and the discussion of the main reconstruction techniques. Afterward, the interferometric methods for shape and displacement measurement are presented. Because of the importance of unwrapping of mod-2π phase distributions for the quantitative evaluation process, a section is dedicated to it. We then discuss representative applications of DH for the investigation of microcomponents. Because the surface of the object under test plays a crucial role, we distinguish between microcomponents having technical (rough) surfaces and those having optical (smooth) surfaces. For the first type of objects, both the shape and displacement of various components subjected to thermal and mechanical load are measured. The measurement of material parameters is demonstrated on test samples taken from a silicon wafer. In an example of a gas sensor fabricated by the deposition of different material layers, the presence of residual stress is investigated. Microlens arrays made of different materials and enabling refractive and diffractive functions represent the object class having a smooth surface. Special attention will be given to the problems that the inspector has to face while using DH in lensless and/or microscope configuration for the practical evaluation and testing of microcomponents. Approaches and solutions that have been discovered and successfully applied to overcome those problems will be described and discussed, furnishing examples of the real-world applications. As an extension of the first edition, we have added a chapter that covers the calibration of interferometric setups with respect to the determination of quantitative three-dimensional shape and displacement data.
