Several conventional methods for absolute sphericity testing of optical surfaces are known. One method has been used previously for real time interferometry, but a detailed investigation has not been carried out so far. In this work the known conventional methods are reviewed and assessed for suitability in real time interferometry. One method has been applied to phase stepping interferometry and examined. Adjustment peculiarities, experimental results of deviation measurements on normal surfaces, and application of normals are reported. Measuring errors, especially coherent noise, are analyzed and a special subsequent digital spatial filtering technique for diminution of noise is described.

Absolute sphericity measurement

A microlithographic projection lens is expected to give constant results from one lens to another, anywhere within the field, over the full Rayleigh depth of focus, and with a finite variation of exposure dose. In particular, the requirements for image dimension and position control are an order of magnitude smaller than the features being printed. These demand that the paper design have wavefront correction significantly better than the Marechal criterion, and almost zero distortion. Furthermore, impressive theoretical performance is of no use if the manufactured lens cannot always approach the ideal as closely as possible. Lens designers traditionally bridge the gap between customer requirements and production realities. Compensators -- parameters that are adjusted to compensate for construction imperfections -- are a well-known way of relaxing tolerances. For a lens on the edge of possibility, they come to the aid of the designer caught between two vociferous groups of people with contradictory needs -- the users and the manufacturers. A lens will be shown that is used in a Wafer Stepper capable of printing features in the 0.7- to 0.6-micron range, and the rationale given for an extensive scheme of compensation, both before and after assembly. A selected number of elements are adjustable after assembly, while the lens is tested both interferometrically and lithographically, after which they are fixed in place.

Compensator Selection In The Tolerancing Of A Microlithographic Lens

A lens mount including a lens cell wherein a lens is mounted through flexures. The lens seat in the lens cell has a radius of curvature match-mated to the lens surface which rests upon it. The rotational symmetry axis of the lens seat is coincident with the axis of rotation of the lens cell. The optical axis of the second lens surface is coincident with the axis of rotation of the lens cell.

Mounting for high resolution projection lenses

We describe a relatively simple, but highly effective, approach to the system design and alignment of an all-refractive Offner null corrector and phase-measuring Shack cube interferometer. In addition we outline procedures for fabricating and testing the optical components. Allowable errors for all parameters are determined by a tolerance analysis that separates axisymmetric and residual figure errors. An open construction optics frame provides a high degree of metering flexibility by incorporating simple kinematic mounts that provide adjustment of each lens while also allowing the lens to be removed and replaced with < 2-μm absolute repeatability. Nonaxisymmetric alignment errors are removed by rotating the optics on a high-precision bearing. Axial spacings are measured with contact transducers attached to both ends of an Invar metering rod. Two completed systems have guided the stressed-lap polishing of 1.8-mf/1.0 and 3.5-m f/1.5 aspheric mirrors.

Optical metrology for two large highly aspheric telescope mirrors

A high-quality optical system has been developed for the optical processing of remote sensing imagery. Picture formats in excess of 107 pixels can be accommodated. Examples are given which involve satellite photography, sonar and airborne radar images.

Spatial Filtering Applied to Remote Sensing Imagery

Centring was done by interferometric measurement of surface height and the lens cells were assembled in plain diamond turned barrels; improvements in symmetry of about an order of magnitude over results previously reported were found.

A.J. Overill

Papers

Some Experiments on Precision Lens Centring and Mounting