Showing papers by "Joanna Schmit published in 2006"

Surface Profilers, Multiple Wavelength, and White Light Intereferometry

Abstract: 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.

Spectrally resolved phase-shifting interferometry of transparent thin films: sensitivity of thickness measurements.

Abstract: Spectrally resolved white-light phase-shifting interference microscopy can be used for rapid and accurate measurements of the thickness profile of transparent thin-film layers deposited upon patterned structures exhibiting steps and discontinuities. We examine the sensitivity of this technique and show that it depends on the thickness of the thin-film layer as well as its refractive index. The results of this analysis are also valid for any other method based on measurements of the spectral phase such as wavelength scanning or white-light interferometry.

Observation of nanoscale dynamics in cantilever sensor arrays

Abstract: Silicon micro cantilevers are used as transducers for a wide range of physical, chemical and biochemical stimuli, where they exhibit exquisite sensitivity (10?18?g, 10?15?J, 10?9?M, etc) over a wide range of temperatures (100?mK?1300?K). This is accomplished by inducing static bending in the cantilever structure or by changing the cantilever's resonant behaviour, both easily measurable responses. There is increasing interest in using higher-order resonant modes to achieve extra sensitivity; however, this raises the question of exactly which modes are excited in the cantilever. Using strobed interferometric microscopy we are able to probe the dynamic behaviour of individual (100 ? 500 ? 1??m3) cantilevers in an eight cantilever array over frequencies from 0 to 1?MHz. We present data that enable spatial visualization of 16 cantilever modes with nanometre-scale amplitudes; combined with finite element analysis calculations we directly assign mode indices. We discuss the reversal of specific modes between experiment and theory, the uniformity of individual cantilevers in the array and the clear relationship between boundary conditions and resonant behaviour. Our conclusion is that the assignment of a resonant frequency spectrum is fairly complex and does not necessarily follow simple intuition.

Two-wavelength interferometric profilometry with a phase-step error-compensating algorithm

Abstract: We show how an eight-step algorithm with a high tolerance for phase-shift miscalibration can be used with a conventional Mirau interferometer, with only minor modifications to the software, for two-wavelength interferometric profilometry of surfaces exhibiting steps and discontinuities.

Testing of Aspheric Wavefronts and Surfaces

Abstract: Aspheric wavefronts with spherical aberration are produced by optical systems using spherical as well as aspherical surfaces. Aspheric surfaces are used in optical systems in order to improve aberration correction and, frequently, to decrease the number of optical elements needed to make this correction satisfactorily. However, if these surfaces are tested while being isolated from the rest of the optical system to which they belong, they frequently produce aspherical wavefronts. The interferometric testing and measurement of aspherical wavefronts are not as simple as in the case of spherical or flat wavefronts. To test aspherics, often a null test is issued. The usual definition of a null test is that which produces a fringe-free field when the desired wavefront is obtained. Then, if a tilt between the wavefront under test and the reference wavefront is added and the paraxial curvature of them are equal, straight and parallel fringes are obtained. Under these conditions, any deviation from straightness of the fringes is a graphical representation of the wavefront deformation. This is the ideal testing procedure because the desired wavefront is easily identified and measured with high accuracy. There are several methods to obtain this null test, but sometimes this is not simple and may even be a source of possible errors. Typically, if a quantitative retrieval of the wavefront is desired, the interferogram is imaged onto a CCD detector. Then, the straightness of the fringes for a perfect wavefront is useful but not absolutely necessary. However, the minimum fringe spacing should be larger than twice the pixel size in the detector. This is the wellknown Nyquist condition, which may be impossible to satisfy if the wavefront has a strong asphericity. In a Fizeau or Twyman–Green interferogram, a strong rotationally symmetric aspheric wavefront has many fringes when taken at the paraxial focus setting as shown in Figure 12.1(a). By adding a small curvature to the wavefront, that is, by adding defocusing, the minimum fringe spacing can be slightly reduced. For

Applications of imaging interferometry

Abstract: Here we report application of imaging interferometry to the study of nanomechanical motion in biosensors and living biological systems. Using strobed interferometric microscopy we are able to probe the dynamic behavior of individual (100 x 500 x 1 micron) cantilevers in an eight cantilever array over frequencies from 0 – 1 MHz. In a related approach, we have developed an interferometric method to measure cell-specific mechanical signals in real time. This yields real-time diagnostic information about cell structure, metabolism and movement, along with response to chemical and physical stimuli. Our new approach makes use of “nanomirrors” fixed to the cell membrane. These mirrors act as nanoscopic displacement probes and can be interrogated, rapidly, by optical profiling metrology. Keywords: Optical profiler, microcantilever, live cell imaging 1. MICRO CANTILEVER DYNAMICS Silicon micro cantilevers are used as transducers for a wide range of physical, chemical and biochemical stimuli, where the exhibit exquisite sensitivity (10

Thickness-profile measurement of transparent thin-film layers by spectrally resolved phase-shifting interferometry

Abstract: Spectrally resolved white-light phase-shifting interferometry has been used for accurate measurements of the spectral phase of the wave reflected from a micromachined surface. The phase is linearly related to the wave number, and the slope of the graph of the phase vs. the wave number, for any point on the test surface, gives the absolute value of the optical path difference at this point. These values can be used to generate a line profile of the test surface. However, if the test surface is coated with a transparent thin film, multiple reflections affect the phase of the reflected wave. The values obtained for the phase then depend on the thickness and the refractive index of the film and exhibit an additional nonlinear variation with the wave number, which can be modeled using thin-film theory. We show that this additional nonlinear phase can be measured directly using spectrally resolved white-light interferometry. The thickness profile of the film can then be obtained by a least-squares fit to the experimental phase data.