A large variety of cryogenic detectors need to be fabricated on thin dielectric membranes in order to have high signal-to-noise attributes. Unfortunately, many of the etching processes used to define the detectors can roughen or even completely dissolve the membranes. These types of membrane damage degrade the detector performance and limit fabrication yield.
The Microspec project relies on the use of ultra-low-noise superconducting resonators in order to achieve unprecedented resolving power in a compact and lightweight far-infrared spectrometer. The resonator material of choice is titanium nitride, which is a difficult material to etch, and is deposited on a hot (>200 °C) substrate.
Prior techniques of etching titanium nitride involve the use of fluorinated and chlorinated plasmas that readily etch Si, Si3N4, and SiO2 membranes. Thus, ultra-low-noise resonators cannot be fabricated on these substrates. An alternative is to use sapphire substrates, which are not attacked by the etchants used to pattern titanium nitride. Unfortunately, sapphire is a much more expensive membrane option than Si, Si3N4, or SiO2.
Ge liftoff masks can be used to lift off thick TiN films. Unfortunately, the acid used to dissolve the Ge during liftoff etches the TiN quite readily (100A/min). Furthermore, the lifted films have unusual sidewall profiles, which make them difficult to integrate with other metallization layers.
This innovation entails the fabrication of a thin metallic film liftoff mask that consists of a metallic film bilayer, which comprises a Cu lower layer and Nb upper layer. The Nb layer is reactive ion etched and the Cu lower layer is etched in a dilute nitric acid etchant. The degree of undercut of the mask can be controlled by immersion time in nitric acid. This type of mask allows for the deposition of metallic thin films on hot substrates and patterning of the metallic films without roughening the substrate. An alternate embodiment of this innovation would be to use silver instead of copper as the under layer. This would be done if the deposition were conducted on a very hot (500 to 600 °C) substrate, as the Ag and Si are immiscible at those temperatures.