Vertical scanning interferometry with a mixed-coherence light source

Abstract

ABSTRACT Micro interferometers are power ful optical instruments for 3D -surface metrology that usually adopt one of two different concepts for the data acquisition: the Phase Shifting Interferometry (PSI) and the Vertical Scanning Interferometry (VSI). In our approach we generate an illumination with mi xed coherence characteristics by superposition of light beams from a broadband incandescent lamp and from a laser source. With a novel data evaluation technique we are able to obtain with a single vertical scan a su rface profile that has the resolution and the accuracy of PSI and the measurement range of VSI. As an example, we present a surface that ha s a step of about 1,3µm and a shallow hole of about 0.1µm depth that could be entirely surveyed in a single vertical scan. Keywords: Interference microscopy, micro interferometer, interferometry, three-dimensional microscopy, shape measurement 1. INTRODUCTION Components produced with microtechnological techniques are not only in an advanced development stage, but some of them can already be found in our usual live, as for example in the medical technology, in the telecommunication and in the automobile industry. The industrial production of these components needs not only special technologies, but also a robust process control and a reliable quality management. To satisfy these requirements, it is necessary to provide an adequate and powerful technique for 3D measurement. A high measurement speed and a high resolution in all three directions in the space are the most important aspects for a measurement technique to be employed in the microtechnological production. In this field we can find diverse solutions, which are based on different measurement principles. An elementary method is based on a stylus that is tracked across the surface to detect the shape. During a measurement, this sensor is continuously kept in contact with the object surf ace and the method is therefore not suitable for the control of sensitive products. For this reason, non-contact optical measurement devices have been developed. There exist two main sampling principles: one of them, analog ously to the technique based on the stylus, makes use of a point-measurement, while the second one has a full-field capability. According to our basic requests (object size, measurement speed, resolution), we prefer to apply optical interferometric techniques based on microscopes. This solution can provide a non-contact surface survey with a vertical resolution in the nano-scale and a measurement range of some hundred micrometers. The typical field of view (FOV) for this full-field measurement is between 2000x2000 micron and 200x200 micron, while the lateral resolution depends on the applied optical configuration (illumination, optics, and CCD Sensor resolution). 1.1 Basics of micro interferometry Most microinterferometers are built by adding a beamsplitter and a reference mirror to a stan dard microscope setup. The application of the classical Michelson arrangement is only possible with an objective that has a low magnification and a long working distance. For objectives with higher magnification and shorter working distance it was necessary to develop some new and more suitable types of interferometric configuration, as the Mirau or Linnik setups. The basis for a full-field interferometric measurement is a series of interferograms taken with a digital camera, given that each pixel can register an independent interferometric signal. The shape of the registered bursts depends on the coherence property of the applied light source. Depending on the characteristics of the light source there are indeed two basic measurement concepts: the Phase Shifting Interferometry (PSI) and the Ver tical Scanning Interferometry (VSI) [1][2]. The processing routines for the computation of the object topology are thus different and specific for the signal characteristics. A short description of the mentioned methods are hereafter presented: In VSI a broadband light source is employed and the distan ce between the object and the interferometer is sampled by equidistant steps with the motion of eith er the object or the objective lens. At each step position an image of the surface