A previously reported interferometer without intermediate optics is used to perform measurements on an aspherical extreme ultraviolet lithography mirror substrate. Acousto-optic modulation based phase shifting is used together with a novel phase retrieval algorithm to retrieve the phase distribution from our interferograms. The phase distribution is then processed by a previously reported inverse propagation algorithm to give the shape of the mirror under test. Our results are compared with measurements performed with conventional Fizeau interferometry and the discrepancies are discussed with reference to systematic error sources inherent in the classical and novel interferometers.
This paper will illustrate several approaches to retrieving the shape of aspherical reflective surfaces as used in EUV Lithography, from measurements from a previously reported angstrom-accuracy interferometer. First, the working principles of the interferometer will be reviewed, and typical measurement data expected from the instrument will be presented. Several methods will then be introduced for retrieving the reflector shape from such measurements. These methods will include approaches based on ray tracing, approximate diffraction calculations, and linearization of rigorous diffraction calculations which use a novel numerical scheme to reduce calculation time of the diffraction integral. The methods will be compared on the basis of accuracy, calculation time and extendibility.
Standard heterodyne interferometer can be used as phase modulation subsystem in a novel interferometer designed to measure the figure of projecting mirrors with 0.1 nm accuracy. This article discusses possible operational principles of the sensor and presents experimental results for fast sampling type sensor prototype.
We report on the progress of the development of a three-wavelength light source for an earlier published absolute, two point source interferometer to measure absolute optical path differences (OPD) with angstrom accuracy over the range of millimeters. The light source system should produce three different wavelengths between 630nm and 640nm simultaneously, providing two synthetic wavelengths that enable the absolute OPD measurement. Due to requirements of the detection system, the frequencies of two of these lasers have to be stabilized with an accuracy of the order of 10-7, while the third laser is stabilized to better than 10-8. For the former, tuneable external cavity diode lasers are used, whereas the latter is a commercial frequency stabilized HeNe laser. Two different locking schemes and their relative merits are evaluated: Molecular absorption locking, guaranteeing long-term stability, versus Fabry Perot locking, with the flexibility of choice of the desired frequencies. Recent measurement results for both locking schemes are presented.
An external cavity diode laser is used as source for a frequency modulated continuous wave (FMCW) interferometer intended to determine absolute optical path differences (OPD) of up to 3mm with a target accuracy of 0.3 microns. This interferometer will eventually be paired with a heterodyne interferometer to extend the accuracy of the system to 0.1nm. Rather than using injection current modulation for the FMCW subsystem, the frequency sweep is provided by moving a mirror of the external cavity with a piezo element. A noise source analysis for this particular setup is provided, and estimates of the maximum attainable accuracy are made.
The stringent figure requirements for optics used in Extreme UV (EUV) projection systems pose a significant challenge to metrologists. The large asphericities combined with low RMS figure error tolerances require novel measurement techniques. Such a method to measure absolute optical path differences (OPD) with 0.1 nm accuracy over ranges of several millimeters is presented. The method combines two different measurement techniques: frequency modulation to determine the absolute OPD with an accuracy of half a wavelength and heterodyning to determine the OPD modulo one wavelength with an accuracy of 0.1 nm. By combining the results of the two methods, the absolute OPD is determined with the large range of the frequency modulation and the high accuracy of the heterodyning. Measurement results for the heterodyning method are presented. In addition, a method to dynamically characterize the frequency behavior of a sweeping tunable external cavity diode laser is shown.
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