Superconducting hot electron bolometers for terahertz sensing

Matthew O. Reese

Published Jan 1, 2006• 7 citations

Abstract

Superconducting Hot Electron Bolometers (HEBs) are good candidates for detecting weak signals in the submillimeter or terahertz range. In this thesis work, a novel fabrication method was developed to make two types of niobium HEBs for different applications. HEBs were designed, fabricated, and then characterized at dc, microwave, and THz frequencies. The first type is a diffusion-cooled HEB, made with a short bridge that determines its cooling time. In this thesis, bridges were typically 400 nm long with bandwidths of about 1 GHz. These diffusion-cooled HEBs were developed as part of a collaboration with the University of Arizona (UA), to develop a proof-of-concept heterodyne array submillimeter camera. Devices were fabricated on thin fused quartz and silica substrates for waveguide coupling in the UA system for the astrophysically interesting 345 and 810 GHz atmospheric windows. The goal of this collaboration is to provide a basis of comparison between Nb diffusion-cooled HEB mixers and superconductorinsulator-superconductor mixers at these frequencies. The second type is a phonon-cooled HEB, made with a ∼3 µm long bridge. Its thermal response is dictated by the electron-phonon relaxation time. These devices were developed in collaboration with Prof. C. Schmuttenmaer’s lab in the Yale Chemistry department, Prof. G. Blake at Caltech,

Related papers

Niobium Hot Electron Bolometer Development for a Submillimeter Heterodyne Array Camera

Matthew O. Reese, Daniel F. Santavicca, Luigi Frunzio +1 more

We are developing a proof-of-concept for a diffusion 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 optimizations 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 consistently be produced. These devices have good suppression of superconductivity 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.

arXiv:cond-mat/0606494IEEE Transactions on Applied SuperconductivityJun 1, 200711 citations

Low-noise and wideband hot-electron superconductive mixer for terahertz frequencies

Boris S. Karasik, Anders J. Skalare, William R. McGrath +7 more

Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range from 1.1 THz to 2.5 THz. The mixer device is a 0.15 - 0.3 micrometer microbridge made from a 10 nm thick Nb film. This device employs diffusion as a cooling mechanism for hot electrons. The double sideband noise temperature was measured to be less than or equal to 3000 K at 2.5 THz and the mixer IF bandwidth is expected to be at least 10 GHz for a 0.1 micrometer long device. The local oscillator (LO) power dissipated in the HEB microbridge was 20 - 100 nW. Further improvement of the mixer characteristics can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated. The HEB mixer is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies. HEB receivers are planned for use on the NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) and the ESA Far Infrared and Submillimeter Space Telescope (FIRST). The prospects of a submicron-size YBa2Cu3O7-(delta ) (YBCO) HEB are discussed. The expected LO power of 1 - 10 (mu) W and SSB noise temperature of approximately equals 2000 K may make this mixer attractive for various remote sensing applications.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIENov 13, 19981 citation

Development of phonon cooled hot-electron-bolometers (HEB) for applications in THz radioastronomy

Theo Andreas Scherer, Michael Frommberger, Matthias Schicke +1 more

Heterodyne detection of weak THz signals can be obtained with ultrathin superconducting microbridge bolometers where frequency mixing is achieved by rapid relaxation of hot electrons in resistive hot spots. These devices do not suffer from the superconducting band gap limit of conventional SIS mixers. Phonon cooled HEBs from NbN allow mixing with IF bandwidths up to several GHz. We discuss NbN thin film optimization and present characterization methods which allow to investigate the phonon cooling path.

International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)Oct 4, 20060 citations

Terahertz (THz) direct detectors based on superconducting HEBs with thermal, THz and microwave biasing methods (Conference Presentation)

Jian Chen

Superconducting hot electron bolometers (HEBs) are widely used as terahertz (THz) heterodyne detectors (mixers) and becoming the most advantageous candidate for low noise receiver at frequencies above 1.2 THz. However, there is still lack of accurate theoretical modeling which explains the complex mechanism of different aspects of superconducting HEBs. These difficulties do not prevent researchers around the world from making experimental progress. In the last few years, superconducting HEBs are also used as THz direct detectors to develop the array for the applications in biology, medicine and security systems. Recently, a noise equivalent power (NEP) of 0.2 pW/√Hz at 2.5 THz is reported for the superconducting HEBs with thermal biasing. Instead of thermal one, microwave (MW) which was first used to stabilize the HEB mixer is also applied to bias the device as a direct detector. Furthermore, THz can be used to bias the HEBs, if THz sources can be provided easy. Although MW biasing can improve sensitivity of the superconducting HEB direct detectors, little work has been done to compare the responsivity and NEP among the THz direct detectors with thermal, THz and MW biasing. To find a best biasing method, we have studied the similarities and differences of the direct detectors based on the superconducting HEBs with three different biasing methods. The fabricated NbN HEB detectors consist of a complementary logarithmic-spiral antenna made of gold and an NbN film (bridge) connecting across the antenna’s inner terminals. We have fabricated the niobium nitride (NbN) superconducting HEB mixers with the system double sideband (DSB) noise temperature better than 8 times of the quantum limit: hf/kB (where h is the Planck constant, kB is the Boltzmann constant and f is the operating frequency) at the frequencies higher than 1 THz. Here, the mixer chips with the system DSB noise temperature of about 400 K and the intermediate frequency (IF) gain bandwidth (GBW) of larger than 5 GHz at 4.2 K and 0.65 THz have been used. As the direct detectors, the current responsivity of 290 A/W and 105 A/W are obtained for MW and THz biasing, which about one order higher than that of thermal biasing. The NEP of 2.7 pW/√Hz is obtained with MW biasing, which can be expected to be improved in the future.

Next-Generation Spectroscopic TechnologiesMay 14, 20190 citations

Low-noise and wideband hot-electron superconductive mixers for THz frequencies

Boris S. Karasik, Anders J. Skalare, Rolf A. Wyss +5 more

Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range 1.1-2.5 THz. The mixer device is a 0.15-03 /spl mu/m microbridge made from a 10 nm thick Nb film. This device employs diffusion as a cooling mechanism for hot electrons. The double sideband noise temperature was measured to be /spl les/3000 K at 2.5 THz and the mixer IF bandwidth is /spl ap/9-10 GHz for a 0.1 /spl mu/m long device. The local oscillator (LO) power dissipated in the HEB microbridge was 20-100 nW. Further improvement of the mixer characteristics can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated The HEB mixer Is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies. HEB receivers are planned for use on the NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) and the ESA Far Infrared and Submillimeter Space Telescope (FIRST). The prospects of a submicron-size YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) HEB are also discussed. The expected LO power of 1-10 /spl mu/W and SSB noise temperature of /spl ap/2000 K may make this mixer attractive for various remote sensing applications.

IEEE International Conference on Terahertz Electronics (THz)Nov 27, 20025 citations

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

J. Stodolka, Kristof J. P. Jacobs

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

Dec 1, 20012 citations