K. A. Ingersoll

Generation of periodic surface corrugations.

The generation of periodic surface corrugations by ion-beam milling and chemical etching of grating relief patterns in photoresist is analyzed. A general treatment is developed for gratings of any desired period on substrates of arbitrary reflectivity, but particular emphasis is given to the generation of gratings with deep grooves and fine periods (Lambda < 3000 A) on GaAs. Analysis of the intensity distribution in photoresist for both p- and s-polarized incident beams reveals that the standing waves generated by reflection from the substrate are diminished for p-polarized beams, but the existence of a displaced grating for certain ranges of substrate reflectivity and angle of incidence severely limits groove depth in resist. The requirements are given for the establishment of an intensity maximum at the photoresist-substrate interface, a condition desired for subsequent chemical etching. It is shown further that the alternative use of a quarterwave intermediate oxide layer to achieve this condition on GaAs results in a lower limit being imposed on grating period. Constantintensity contours approximating the groove profiles in resist demonstrate that an imbalance in incident beam intensity may lead to severing of the resist stripes, and the dependence of this phenomenon on substrate reflectivity is determined. For beams of equal intensity, a similar phenomenon occurs with increasing reflectivity of the substrate. The transfer of a grating relief pattern to the substrate by ion-beam milling is treated by considering the erosion profiles produced by ion bombardment. This analysis is used to examine the influence of milling geometry on the depth and shape of the groove. Although the ion-beam milling rate of GaAs is several times greater than AZ-1350 photoresist, it is shown that the groove aspect ratio (depth/period) in GaAs can be no more than about 1.2, a figure that is obtained, surprisingly, by milling at the angle of maximum removal rate of photoresist. For a metal substrate, the groove aspect ratio decreases with increasing grating period. For gratings produced by chemical etch, the problem of the weakly exposed layer of resist adjacent to the substrate is solved by using a combination of ion-beam milling and chemical etching. Using a preferential chemical etch with a sufficiently slow etch rate, gratings with well-defined planar features, a period ~2500 A, and a groove aspect ratio >0.6 have been produced on GaAs.

Ion polishing with the aid of a planarizing film.

An ion polishing technique employing a planarizing film in conjunction with ion-beam erosion at the planarizing angle was used to remove surface irregularities associated with conventional polishing of optical surfaces. By maintaining a planar surface throughout the erosion process, nonuniform erosion resulting from faceting, redeposition, and ion reflection is eliminated. Smooth surfaces on fused quartz are obtained by erosion of a planarizing film of photoresist at an angle of 60 degrees. The method is applicable to a wide variety of materials and may be useful for removing the surface roughness limitation on the laser-induced damage threshold of optical surfaces.

Planarization of patterned surfaces by ion beam erosion

The angular dependence of ion beam erosion has been utilized to achieve planarization of patterned surfaces. Deviation from planarity of <500 A has been demonstrated on planarized SiO2 covered patterned Si surfaces with 1.5‐μm‐deep recesses. The technique should be applicable to a wide variety of materials with different physical, chemical, optical, and electrical properties, and should be particularly useful in very high speed integrated circuit development.

Liquid Filters for the Visible and Near Infrared

Many experiments in spectroscopy require the use of optical filters. The type most commonly used are glass filters such as those produced by Corning Glass Company. In most applications, glass filters are convenient to use for they come in a standard 5 cm × 5 cm size and are readily available. However, for some experiments this type of filter cannot be used for a variety of reasons. Excessive heating from a source of high intensity radiation may fracture the filter, and effective cooling is difficult. A geometry other than the standard 5 cm × 5 cm square is sometimes required, but fabrication of a number of glass filters of

Liquid Filters for the Ultraviolet, Visible, and Near Infrared

This paper describes some liquids that have been found to be useful as sharp cutoff optical filters in the uv, visible, and near ir. The range of useful transmission extends to approximately 6 micro in the ir.

Interference gratings blazed by ion‐beam erosion

The use of ion‐beam erosion to transform rounded symmetric grating relief patterns into asymmetric profiles with planar facets and sharp angular features is described. Calculated profiles are confirmed by experimental profiles produced in GaAs but with significant modifications caused by redeposition. Higher efficiency interference (holographic) gratings for spectroscopic and integrated optics applications are suggested by these results.

An oblique shadow deposition technique for altering the profile of grating relief patterns on surfaces

An oblique shadow deposition technique for altering the profile of grating relief patterns on surfaces is described. The technique essentially represents a method whereby a series of grating masks may be superimposed in precise registration with each other. By appropriate choice of deposition angle, deep grooves may be generated from shallow profiles, rounded or sinusoidal profiles may be transformed to structures with planar vertical walls, and symmetric profiles may be made asymmetric. In conjunction with ion‐beam milling, the technique has been used to increase the depth of fine period gratings on GaAs and to produce grooves ∼4000 A deep on the surface of YAG, 10 times deeper than could be obtained by ion‐beam milling of photoresist.

Generation of surface gratings with periods < 1000 Å

Simple techniques for doubling the spatial frequency of surface‐relief gratings are demonstrated. Grating periods as small as 750 A have been produced and further subdivision appears possible.

Asymmetric triangular grating profiles with 90° groove angles produced by ion-beam erosion

A simple phenomenological model has been developed to account for the effects of redeposition during transverse ion-beam erosion of grating relief patterns on surfaces. The model predicts the evolution with time of an inclined facet in the substrate, and the dependence of the facet angle theta(r) on angle of incidence of the ion beam, erosion rates of mask and substrate, and the geometrical parameters of the mask. The results are illustrated by calculation of the angular dependence of theta(r) for AZ-1350 photoresist masks on GaAs and SiO(2) substrates and for Ti on SiO(2). A strong dependence of theta(r) on mask intercept angle alpha is found, except for a limited range in angle of incidence of the ion beam. Combined with results derived previously, the facet angles theta(S) and theta(r) and the groove angle theta(g) of the asymmetric triangular groove profiles produced in these systems have been determined. A groove angle of 90 degrees cannot be obtained for AZ-1350 on GaAs and SiO(2), but in appropriate conditions a 90 degrees groove angle may be obtained in SiO(2) with a Ti mask.