Interferometer system and method for recording an interferogram using weighted averaging over multiple frequencies, and method for providing and manufacturing an object having a target surface
An interferometer system comprises a reference surface, a support for an object providing an object surface, a radiation source for emitting radiation of an adjustable frequency onto the reference surface and the object surface, a position-sensitive radiation detector, a controller for adjusting a plurality of different frequencies of the radiation emitted by the radiation source, and an integrator for averaging the interference patterns superposed on the radiation detector at different frequencies. Moreover, there is provided a method for recording an interferogram, a method for providing an object with a target surface as well as a method for manufacturing an object with a target surface.
This application is a continuation of International Application No. PCT/EP02/07080 filed on Jun. 26, 2002, which International Application was published by the International Bureau on Jan. 9, 2003, and which was not published in English, the entire contents of which are incorporated herein by reference. This application also claims the benefit of DE 101 30 902.3 filed on Jun. 27, 2001, the entire contents of which are incorporated herein by reference.
The tilted relationship between the reference and test mirrors of a Fizeau interferometer is used to spatially separate the reflections from the two surfaces. The separate beams are filtered through a spatial polarization element that provides different states of polarization to the beams. The beams are subsequently recombined to form a substantially collinear beam that is processed using a spatial-phase-shift interferometer that permits quantitative phase measurement in a single video frame. Alternatively, two beams with orthogonal polarization are injected into the Fizeau cavity at different angles, such that after reflection from the reference and test optics they are substantially collinear. Unwanted reflections are blocked at the focal plane through the use of a circular aperture. Short coherence length light and a delay line may be used to mitigate stray reflections, reduce measurement integration times, and implement temporal phase averaging.
An interferometer for measuring the flatness, variation of thickness and parallelism of large, thin transparent wafers held vertically and undistorted by gravitational forces. A sodium lamp provides monochromatic light that is diffusely reflected off a background screen towards the wafer. A reference flat surface is positioned sufficiently closely to the wafer that an interference fringe pattern is formed that is a measure of the air gap between the reference flat surface and the surface of the wafer being measured. The background screen is angled so that the fringe pattern, which is localized at the air gap can be viewed at an angle to a normal to the wafer. The background screen has trapezoidal markings that, when viewed at the angle required to see the fringes, appear as a grid. This allows counting of the number of fringes visible within a fixed distance to characterize the surface flatness.
