Fabrication of waveguide tapers by semitransparent mask photolithography

An improved process for semitransparent mask lithography is described. The uniformity of mask transmittance is increased by adapting the electron beam exposure field size to a binary multiple of the pattern period. In addition surface roughness of resist patterns is reduced by exposure of the resist-coated sample to an acetone atmosphere which results in negligible micro roughness. Using this process waveguide tapers employing a symmetric layer structure were fabricated. The tapers exhibit fiber-chip coupling losses of 1.6 dB after Fresnel correction and lateral alignment tolerances of +or-2.2 mu m for 1 dB excess loss. These values are comparable to results obtained for waveguide tapers fabricated using direct write electron beam lithography. This proves the described process to be suitable for the fabrication of relief type structures for optoelectronic integrated circuits.