Impedance matched coatings are used for absorber applications to couple mid-to far-infrared (IR) radiation to an ultrasensitive bolometric detector suspended on an ultrathin dielectric membrane. In order to provide adequate responsivity, the bolometric detector must have a low heat capacity. Furthermore, to achieve an optimal signal-to-noise ratio, spectral filtering of the incident radiation is desirable.
Conventional approaches for absorber coatings are susceptible to aging, which hinders the transient optical efficiency of the instrument. Additionally, conventional absorber coatings have a high heat capacity that impacts the performance of low-background cryogenic detectors. Conventional absorber coatings also are reactive in the short wavelength limit, which correspondingly reduces their coupling efficiency. Furthermore, some thin film absorber coatings have extremely high intrinsic stress, which can cause the dielectric membranes to break or bend.
Niobium Titanium Nitride (NbTiN) Thin Film Coating, developed at NASA Goddard Space Flight Center, can optically couple light to a bolometric detector that is suspended on an ultra-thin dielectric membrane. The coating can also filter out low-frequency spectral components that would increase the photon-limited noise of the detector.
NbTiN thin film coatings are fabricated on dielectric substrates using a specialized reactive sputtering co-deposition process. Two different sputtering sources are used in which one source contains a niobium sputtering target and the other contains a titanium sputtering target. The niobium and titanium are deposited in a nitrogenrich environment.
The NbTiN coating can be used by depositing it on one side of an ultra-thin silicon membrane and have a well-defined optical impedance requirement for a specific application — NbTiN coating can be deposited on non-silicon membranes as well. The NbTiN coating has low intrinsic stress, which makes it mechanically compatible with integration on ultra-thin dielectric membranes. The coating possesses the optical impedance required for high optical efficiency absorption. Furthermore, the coating has a very low superconducting transition temperature, which enables it to filter out radiation at certain frequencies.
The NbTiN coating is especially useful for ultrasensitive cryogenic bolometric detector applications and can be fabricated in a reproducible manner while not complicating the fabrication process of detector architectures.