Fabrication and Receiver Measurements of a Diffusion-cooled Hot-electron Bolometer at 800 GHz

Abstract

We present a fabrication process for diffusion-cooled niobium hot-electron bolometers using a self-aligned process with gold as an initial protection layer and aluminum as an etch mask that is removed by a wet etch. This process proves to be reproducible and has a yield of ~70 % per batch. Great care has been taken to avoid processing temperatures above 120 °C to minimize degradation of the Nb film. We have performed heterodyne measurements of an HEB in a fixed tuned waveguide mixer at 804 GHz and at an IF-range from 1-2 GHz. The approximate dimensions of the bridge are 240 x 200 x 14 nm3. The Niobium bridge has a critical temperature of 6.0 K and a normal state resistance of 16.5 Ω. A standard Y-factor measurement leads to a minimum noise temperature of 1200 K at 1 GHz IF. The coupled LO-power is estimated by the isothermal method to be 21 nW. Introduction Superconducting Hot-Electron-Bolometers (HEB) utilizing thin superconducting films have become established as sensitive mixers in the THz region. The thermal response time τ can be very small if the superconductor is thin enough that only the electron gas is heated and the cooling is fast enough. The bolometers described in this paper are cooled by out-diffusion of the hot electrons in normal conducting heat sinks (diffusion-cooled HEB) rather than interactions with phonons of the lattice (phononcooled HEB) [1]. To meet the requirements for astronomical applications such as SOFIA, thermal response times down to several picoseconds are needed to get intermediate frequencies up to 10 GHz. Because of τ ~ L [1] the dimensions of the bolometer bridge have to be very small. We have fabricated diffusion-cooled HEBs with short (L<300 nm) and narrow (<200 nm) bridges consisting of thin (10-14 nm) Niobium (Nb) on z-cut quartz substrates. The normal conducting heat sinks consist of 70 nm Gold (Au). In the following paragraph 1 e-mail: stodolka@ph1.uni-koeln.de we discuss problems and merits of several approaches for the fabrication of diffusioncooled HEBs. Device Fabrication Aside from a good reproducibility, the essential requirement for the fabrication of a diffusion-cooled HEB is a good transparency of the interface between the heat sinks and the material of the bolometer bridge. A thermal resistance results in a slower out-diffusion of the hot electrons and thus increases the effective bridge length and thermal response time. Defining the bolometer bridge by lift-off with subsequent deposition of the heat sinks (Fig. 1) [2] has the major disadvantage of high requirements for the overlap accuracy of the Electron-Beam-Lithography (EBL) system to position the heat sinks on the contact areas of the bridge. A solution is offered by the self-aligned process (Fig. 2). Here, the substrate is covered with a blanket layer of Nb. The heat sinks and a metal strip of the width of the bolometer bridge are deposited in succession. The bridge is now etched out of the blanket layer by using reactive ion etching (RIE) where the heat sinks and the metal strip serve as etch masks: the spacing between the heat sinks determines the bridge length while the metal strip defines the width. The overlap between Nb and Au heat sinks is large. Fig. 1: Outline of HEB fabrication by lift off (top view) a) Bolometer bridge defined by liftoff. b) Deposition of the heat sinks. The contact area (shaded) to the heat sinks is small. bolometer bridge