Principal Investigator Karl Berggren
Patterning of superconducting thin films at the nanoscale has enabled numerous technologies used in signal detection and digital circuits. For instance, super- conducting nanowire single photon detectors (SNSPDs) and, more recently, the nanocryotron (nTron) both make use of the ability to pattern niobium nitride films at dimensions < 100 nm. Electron beam lithography of these devices often employs the negative tone resist hydrogen silsesquioxane (HSQ) due to its high resolution and superior line edge roughness. Development of HSQ and adhesion promotion of HSQ to the substrate surface are both facilitated by tetramethylammonium hydroxide (TMAH), making it an integral chemical in the fabrication process. However, despite the prevalent use of TMAH in patterning superconducting films, its influence on the film itself has yet to be fully studied.
Here we have investigated the effects of exposing NbN thin films to 25% TMAH. We show that TMAH modifies the surface chemistry of the film by reacting with the NbN to form niobium-based clusters, which are visible by scanning electron micrograph inspection In addition to thinning the overall NbN film and reducing its critical temperature, the formation of niobium clusters creates a barrier to reactive ion etching in CF4, threatening the lithographic pattern transfer. While characterization such as FTIR has been employed to identify the compounds created by this reaction, future work is needed to study the mechanism through which the hexaniobate species interfere with the reactive ion etch chemistry.