Polyimide Buffer Layer for STJ Photon Detector

We have proposed and been developing ultra low noise photon detector using superconducting tunnel junctions (STJs). The STJs are fabricated on polyimide (PI) layer which is prepared on sapphire substrate. The PI is coated on the substrate before making the STJ. It acts as buffer layer to suppress a great deal of phonon events which come from the substrate. For low energy application such as soft X-ray and extreme ultraviolet, the STJs with PI buffer layer is a very attractive detector because the spectrum of low energy photons can measured clearly without noise that depends on phonon events in the substrate. We fabricated the STJs with PI buffer layer which are different thickness and measured X-ray spectrum of 5.9 keV. We found that polyimide of 100 nm thickness is enough to suppress phonon events in low energy range.