<title>High-speed binary optic microlens array in GaAs</title>

Abstract

We report the development of binary optic microlens arrays in GaAs. The application of this microlens array is for (gamma) -hardening of HgCdTe focal plane arrays. We intend to reduce the effective spot size of the picture elements and provide significant nuclear hardening for the focal plane array by considerable volume reduction of the detector elements. The microlens design is an eight-phase level approximation to an ideal kinoform lens. The lenses are designed to focus into the GaAs or air with a focal length of 480 micrometers or 148 micrometers respectively, at (lambda) equals 9 micrometers . Arrays of square lenses and individual circular lenses were fabricated. The square lens dimensions and f-numbers are 120 micrometers X 120 micrometers , f/1.23; 240 micrometers X 240 micrometers , f/0.62; and 480 micrometers X 480 micrometers , f/0.31, respectively. Designs include correction for spherical aberration. A set of four 10X projection masks was designed using graphic language (GPL) interfaced to computer-generated binary optics elements. The binary optic pattern was etched into the 3'' diameter GaAs substrate by reactive ion etching. Highly anisotropic etch profiles were obtained with feature heights in excess of 2 micrometers . Measured microlens efficiency for f/1.23 microlenses was as high as 60. The average measured value for a whole array was 55. Measurement of the point spread function at (lambda) equals 10.6 micrometers demonstrates optical concentration. This demonstration of binary optic microlenses in GaAs is of considerable importance to the future integration of purely optical and optoelectronic functions on a single wafer.