Fabrication and Performance of Diffractive Optics for Quantum Well Infrared Photodetectors

Diffractive optical elements (microlenses) for quantum well infrared photodetectors (QWIPs) were fabricated by two techniques: 1) standard lithography of a binary optical structure and 2) PMMA pattern transfer for an analog diffractive optic structure. The binary lenses were fabricated by sequential contact lithography and etching using two binary masks. The analog diffractive lenses were fabricated in PMMA by direct-write e-beam lithography followed by acetone development. The resulting PMMA surface relief profiie was transferred into the GaAs by dry etching. Both types of lenses were etching into GaAs using an electron cyclotron resonance (ECR) microwave plasma etching system. Although the lenses were fabricated accurately, the performance of the QWIPs was not improved as much as expected due to the angle-of-incidence sensitivity of the QWIP light-coupling grating. The lenses would have likely improved the performance of d tectors capable of absorbing normally incident light.