CMOS-compatible mid-IR metamaterial absorbers for out-of-band suppression in optical MEMS

The design and fabrication of wideband mid-infrared metamaterial absorbers are presented. The emphasis is put on the shape-tolerant design for using masked UV (i-line) lithography and CMOS-compatible fabrication to enable on-chip co-integration with detector and readout circuits in a MEMS foundry while maintaining wafer throughput. The CMOScompatibility implies the use of aluminum rather than the commonly used high conductivity metals. The use of masked lithography rather than e-beam lithography in the fabrication of metamaterial absorbers for the mid-infrared range between 3 and 4 μm introduces the challenge of the shape-tolerant design of the unit cell. Moreover, the sensitivity of the fabricated metamaterials to the surface roughness and exposure dose were investigated in this paper. The throughput advantage of masked lithography has been exploited in the fabrication of mid-infrared absorbers over an area of several mm². The measurements confirm the theoretical spectral response and a 98% peak absorption at an angle close to perpendicular incidence. Measurements at different angles show that the absorption spectrum only deviates marginally from normal incidence for angles up to 30°. The combined CMOS-compatibility and masked lithography enable batch fabrication and the on-chip integration of the metamaterial absorbers with MEMS devices and sensors.