Program Number: 4EE08, Paper Identification Number: 279274 1 Abstract—We are developing a proof-of-concept for a diffu-sion cooled hot electron bolometer (HEB) array submillimeter camera. The ultimate objective is to create a working 64 pixel array with the University of Arizona for use on the Heinrich Hertz Telescope. We have fabricated Nb HEBs using a novel angle deposition process. We have characterized these devices using heterodyne mixing at 20 GHz. We also report on optimiza-tions in the fabrication process that improve device performance and a dc screening test for device quality. Using this process, HEBs with a sharp resistive transition (<300 mK) can consis-tently be produced. These devices have good suppression of su-perconductivity in the contact pads, with a pad transition ~1 K below the main bridge transition. Furthermore, the bandwidth was investigated above and below the pad transition and found to be ~15% larger for 400 nm long bridges with non-superconducting contact pads. Index Terms—Hot Electron Bolometers, superconductivity proximity effect, electron-beam fabrication I. INTRODUCTION superconducting microbridge connected to thick, normal metal contacts forms the active element of the supercon-ducting Hot Electron Bolometer (HEB), which has dem-onstrated promising performance as a terahertz detector [1-5]. HEBs have several desirable characteristics: unlike SIS tunnel junctions, they are not limited by the gap frequency [6]; they require very small local oscillator power; their simple geome-try, with low stray impedance, facilitates integration in multi-pixel arrays; and they have demonstrated IF bandwidth as large as 9 GHz [1-5]. Recently, we have fabricated diffusion-cooled niobium HEBs. These devices are being developed for a large format array camera for use in the 810 GHz atmospheric window on the Heinrich Hertz Telescope operated by the University of Arizona. This can serve as a model for future THz camera designs. We require a reasonably sharp resistive transition of the superconductor, a critical temperature (Tc) of 4-5 K to op-erate in a pumped 4He cryostat, and contact pads that either do not superconduct or have a much lower Tc than the bridge. Here we present our new fabrication process along with initial characterization results. This process can consistently produce Manuscript received August 29, 2006. This work was supported by NSF-AST, NASA-JPL, and a NASA Graduate Student Research Fellowship for M. O. Reese. All authors are in the Department of Applied Physics of Yale University, New Haven, CT 06520-8284 USA. All correspondence should be directed to Daniel Prober (phone: 203-432-4280; fax: 203-432-4283; e-mail: daniel.prober@ yale.edu). high quality devices, having produced hundreds. Three are discussed. II. FABRICATION METHOD The essential goal has been to produce Nb HEBs using a single lithographic patterning and no cleaning step between the deposition of the Nb and the normal metal of the contact pad. (Here we use Al, because Tbath>Tc,Al). Such a process has numerous advantages. First, it is relatively simple. Previously, diffusion-cooled Nb HEBs have been made in complicated multi-step processes requiring as many as five separate metal vacuum deposition steps, two to three photolithography steps and two to three electron beam lithography steps, as well as a reactive ion etching step [7,8]. Our process requires only one electron beam lithography step and one metal vacuum deposi-tion step. Furthermore, the previous methods required process-ing steps that could result in degradation of the Nb film after its deposition, whereas ours does not. Last, device to device performance across even a single wafer proved to be variable in the original process for making Nb HEBs. One possibility for this variability was that the argon plasma step used to re-Niobium Hot Electron Bolometer Development for a Submillimeter Heterodyne Array Camera Matthew O. Reese, Daniel F. Santavicca, Luigi Frunzio, Daniel E. Prober A Fig. 1. Deposition process: a) e-beam pattern PMMA, top view [Fig. 1b and 1c are side views of slice 1, Fig. 1d is a side view of slice 2], b) sputter Nb, c) angle evaporate Al, Al sticks on side of resist in bridge region, d) final result after liftoff. The Nb in the bridge center is slightly thinner than the contact region (see text).Program Number: 4EE08, Paper Identification Number: 279274 2move the Au capping layer affected the Nb bridge underneath in an inconsistent fashion, since there was no etch selectivity. The argon plasma etched Nb at a similar rate as Au. By avoid-ing an etch step, device to device variability may be mini-mized. Our structure is patterned as shown in Fig. 1. We use a con-verted Scanning Electron Microscope (FEI Sirion XL40) to expose a pattern in a 380 nm thick monolayer of 950K poly-methyl methacrylate (PMMA) spun on a substrate of silicon or fused silica. [When using fused silica, we evaporate 10-15 nm of Al on top of our PMMA before e-beam writing, to reduce charging effects. Before developing our resist, we remove the Al with a 5 min. etch in MF-312 developer (4.9% by volume tetra methyl ammonium hydroxide in water), then arrest the process with 20 s in isopropyl alcohol (IPA).] Developing is done in a solution at 25oC of 1:3 methyl isobutyl ketone:IPA for 20 s in an ultrasonic bath, arrested by 20 s in IPA in ultra-sound. The substrate is then blown dry with N2. The sample is loaded into a Kurt J. Lesker Supersystem III Series multi-sputtering and evaporation system with a base pressure of ~10-6 Pa. First the 2” diameter Nb target is pre-sputtered for 2 min in an Ar plasma with a power of 350 W and a pressure of 0.17 Pa and a flow rate of ~82 sccm. The sample is then ion beam cleaned by Ar with current of 4.7 mA for 15 s using a 3 cm Kaufman-type gun to help adhesion to the substrate. Then, a Nb film is sputtered for 9 s, using the same parameters as stated above. And then, with as little delay as possible (typi-cally ~3 min.), we angle evaporate 200 nm of Al at 1 nm/s [9]. The system has a rotating “J-arm” on which the sample is held during the deposition. By rotating the J-arm to different posi-tions in the chamber different sources may be selected. To achieve angle evaporation in this system, the J-arm is rotated to a position so that it is not directly above the evaporation source. Using geometry, the angle may be precisely calculated (the plane of the J-arm is vertically separated