Laser-induced dry etching of integrated InP microlenses

Maskless laser dry etching of monolithically integrated InP microlenses for infrared-optical coupling between optoelectronic devices and a glass fiber at 1.53 mu m wavelength is reported. The process is based on the projection of a conventional chromium-on-quartz reticle on to the InP wafer surface using a pulsed 248 nm KrF excimer laser. Etching proceeds with a fluence-dependent rate in a chlorine atmosphere as described in detail before. The lens shape is defined by a rotationally symmetric fluence distribution with smooth radial variation in the image plane of the reticle, The demagnified image only represents the mean local transmission of the reticle since the dimensions of chromium structures on the reticle are chosen so small that they are not optically resolved on the wafer surface. A typical lens has 50 mu m diameter, 5-10 mu m maximum etch depth at the edge, and needs 3000 laser pulses within 150 s for fabrication. Upscaling is possible by etching an array of lenses simultaneously in an expanded field of view and by increasing the pulse repetition rate. The lens surfaces are smooth and deviations from the nominal design are below 50 nm (lambda/10). The coupling losses to single-mode fibers were measured with a laser diode on a separate chip to be 4+/-1 dB.