  Future far-infrared astrophysics observatories will require focal plane
arrays containing thousands of ultra-sensitive, superconducting detectors, each
of which needs to be optically coupled to the telescope. At longer wavelengths,
many approaches have been developed including feedhorn arrays and macroscopic
arrays of lenslets. However, with wavelengths as short as 25 microns, optical
coupling in the far-infrared remains challenging. In this paper, we present a
novel approach for fabricating far-infrared monolithic silicon microlens arrays
using grayscale lithography and deep reactive ion etching. The design,
fabrication, and characterization of the microlens arrays are discussed. We
compare the designed and fabricated lens profile, and calculate that the
fabricated lenses will achieve 84% encircled power for the designed detector,
which is only 3% less than the designed performance. We also present methods
developed for anti-reflection coating microlens arrays and for a
silicon-to-silicon die bonding process to hybridize microlens arrays with
detector arrays.
