Over the last 25 years driven by both the development of new technologies such as fast computers and solid state devices and the necessity to precisely inspect these increasingly tiny engineering surfaces, the field of surface metrology has exploded in both its technological sophistication and its range of application. Advances in illumination sources, such as lasers, and in solid state detectors and optoelectronic devices in general have fueled the development of a wide range of instruments that can not only map surface topography but also determine other features such as displacement or dispersion. Innovative techniques and technologies have greatly increased the range of measurable objects, so now even difficult surfaces with high slopes or steps and narrow, deep trenches can be measured. Many of these surface profiling techniques were developed from distance measuring or focus detection techniques, and they often require scanning to obtain the surface profile. This chapter describes instruments such as the stylus profiler, scanning probe microscope, confocal microscope and the interferometric optical profiler that are most often used to determine surface topographies of not only very small, typically engineering surfaces, but also smooth and large surfaces such as aspheres and glass plates.

Surface Profilers, Multiple Wavelength, and White Light Intereferometry

The conventional signal model of confocal sensors is well established and has proven to be exceptionally robust especially when measuring rough surfaces. Its physical derivation however is explicitly based on plane surfaces or point like objects, respectively. Here we show experimental results of a confocal point sensor measurement of a surface standard. The results illustrate the rise of severe artifacts when measuring curved surfaces. On this basis, we present a systematic extension of the conventional signal model that is proven to be capable of qualitatively explaining these artifacts.

Improved signal model for confocal sensors accounting for object depending artifacts.

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The confocal-detection principle is open especially for use in medical applications. For inspection systems applications for technical objects in reflection confocal setups are of growing importance. For such applications the confocal measurements need to have a very short measuring time. A fast detection system is needed and to satisfy this requirement only a small number of height levels are measured and a fast-evaluation algorithm is used. Drawbacks of the reduction of height levels are a greater influence of noise and additional systematic errors on the measured heights. Study the effects of the reduction are calculated, different evaluation algorithms are analyzed, and the optimization of the parameters is discussed.

Signal evaluation for high-speed confocal measurements

Optical aberrations of microscope lenses are known as a source of systematic errors in confocal surface metrology, which has become one of the most popular methods to measure the surface topography of microstructures. We demonstrate that these errors are not constant over the entire field of view but also depend on the local slope angle of the microstructure and lead to significant deviations between the measured and the actual surface. It is shown by means of a full vectorial high NA numerical model that a change in the slope angle alters the shape of the intensity depth response of the microscope and leads to a shift of the intensity peak of up to several hundred nanometers. Comparative experimental data are presented which support the theoretical results. Our studies allow for correction of optical aberrations and, thus, increase the accuracy in profilometric measurements.

Systematic errors on curved microstructures caused by aberrations in confocal surface metrology.

We present a numerical study of the performance of a one-dimensional Confocal Scanning Optical Microscope (CSOM) when used as a profiler of highly reflecting surfaces. The study is conducted by means of a computer simulation that describes the interaction between the incident beam and the sample in a fairly rigorous manner. The degradation of the depth discrimination as a function of the local surface slope is illustrated with examples, and a criterion for the maximum allowable slope is proposed. We also show that the instrument has difficulties in profiling surfaces that have steps or sharp edges and it is argued that, in general, problems are encountered when the surface profile cannot be considered locally flat in the scale of the Point Spread Function (PSF) of the optical system. This implies that the lateral resolution of the instrument is far worse as a profilometer than when used as an imaging system, and that the unresolved features introduce spurious details on the estimated profile.

J.F. Aguilar

Papers

On the limitations of the confocal scanning optical microscope as a profilometer