A technique has been developed for patterning thin metallic films that are, in turn, used to fabricate microelectronics circuitry and thin-film sensors. The technique uses germanium thin films as lift-off masks. This requires development of a technique to strip or undercut the germanium chemically without affecting the deposited metal. Unlike in the case of conventional polymeric lift-off masks, the substrate can be exposed to very high temperatures during processing (sputter deposition). The reason why polymeric liftoff masks cannot be exposed to very high temperatures (>100 °C) is because (a) they can become cross linked, making lift-off very difficult if not impossible, and (b) they can outgas nitrogen and oxygen, which then can react with the metal being deposited. Consequently, this innovation is expected to find use in the fabrication of transition edge sensors and microwave kinetic inductance detectors, which use thin superconducting films deposited at high temperature as their sensing elements.
Transition edge sensors, microwave kinetic inductance detectors, and their circuitry are comprised of superconducting thin films, for example Nb and TiN. Reactive ion etching can be used to pattern these films; however, reactive ion etching also damages the underlying substrate, which is unwanted in many instances. Polymeric lift-off techniques permit thin-film patterning without any substrate damage, but they are difficult to remove and the polymer can outgas during thin-film deposition. The outgassed material can then react with the film with the consequence of altered and non-reproducible materials properties, which, in turn, is deleterious for sensors and their circuitry.
The purpose of this innovation was to fabricate a germanium lift-off mask to be used for patterning thin metal films. The germanium can either be thermally or electron-beam evaporated onto Si(001) wafers. The evaporation rates and deposited thicknesses are 0.2 nm/s and 0.5 nm/s, and 620 nm and 500 nm for thermal and electron beam evaporation, respectively. The germanium can be patterned either via polymeric lift-off, using 1 micron of LOR- 5a (Microchem) and 1.3 microns of S- 1811 (Shipley) photoresists, or with lithographic patterning using 1.3 microns of S-1811 photoresist. In both cases, the photoresist is exposed to UV light using a mask aligner (MA-6, SUSS) and developed in a commercially available developer. In the case of lift-off, the germanium is removed in 1165 (Microchem); in the case of lithographic patterning, the germanium is removed in a dilute hydrochloric acid solution. The photoresist can be stripped in acetone. The desired metal thin film (Nb, TiN, NbN, Au) is deposited and is lifted-off in dilute hydrochloric acid. The reliability of the lift-off process is dependent upon the amount of undercut in the germanium mask during the germanium patterning process.
This work was done by Ari Brown of Goddard Space Flight Center. GSC-16147-1