The objective of this work was to develop a thin film metal liftoff process that would allow one to accurately pattern two-micron-wide (or wider) features. The goal of this innovation was to pattern thin metal films on silicon substrates. The thin metal films can be deposited using physical vapor deposition techniques. The metallic films to be lifted off were deposited via DC magnetron sputtering, in which the mean free path of the metal atoms to be deposited is on the order of one micron. Thus, the deposited metal could conformally coat structures to fill in gaps that were greater than approximately one micron tall.
The process uses a liftoff mask comprising a metallic germanium and a polymer layer. The thickness and undercut of both layers can be well controlled, which permits a wide variety of thin film geometries to be accurately lifted off.
The steps involved include fabricating the liftoff mask, depositing the metal, and lifting off the metal in acetone. Fabrication of the liftoff mask consists of spinning on the polymer layer, depositing the germanium layer, patterning the germanium layer using a polymeric photoresist, etching the germanium layer using a reactive ion etcher (RIE), and etching the polymer layer using an oxygen plasma.
Thinned 1811 photoresist and poly (methyl methacrylate) (PMMA) were used as the polymer liftoff layer. Some of the Si substrates consisted of ultra-thin layers (0.5 micron) under which a metallic layer was deposited. This metallic layer acted as a mirror that resulted in optical interference during the photolithographic exposure process (used to pattern the film). This interference could occasionally result in distortion of the patterned feature.
The novel features of this innovation are the high degree of control over the thickness of each liftoff mask layer as well as the amount of undercut in the polymer layer. The undercut is precisely and reproducibly controlled inside the RIE by setting the amount of oxygen gas flow, power, and ash time.
The advantages of having precise control over the liftoff mask geometry are that the feature size can be controlled, the feature sidewall morphology can be controlled, and a much larger number of metallic thin films can be lifted off (than by using conventional techniques).
Furthermore, the germanium outer layer provides good thermal contact between the substrate and metallic substrate holder. This results in prevention of temperature spikes during deposition, which can lead to unwanted outgassing of the polymer layer or even prevent liftoff from occurring (due to polymer crosslinking).
Alternate embodiments of this innovation involve using different polymer layer materials and different germanium thicknesses.
This work was done by Ari Brown and Amil Patel of Goddard Space Flight Center. GSC-16664-1