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IEEE Transactions on Applied Superconductivity

Fabrication and Characterization of Niobium Diffusion-Cooled Hot-Electron Bolometers on Silicon Nitride Membranes

A. Datesman, J.C. Schultz, Arthur W. Lichtenberger, Dathon R. Golish, C. K. Walker, J. Kooi

Published Jun 1, 2005• 6 citations
Publisher

Abstract

We have successfully fabricated niobium diffusion-cooled hot-electron bolometer (HEB) mixers on membranes of silicon nitride less than one micron thick. This advance has allowed us to construct a 1 /spl times/ 5 HEB receiver array intended for operation at 1.45 THz. This article provides an overview of the integration of the HEB array chip with silicon micromachined backshorts and feedhorns, discusses materials issues surrounding the device fabrication, reports resistance and I-V measurements, and compares HEBs fabricated on silicon nitride to similar devices on quartz substrates.

DOI
10.1109/tasc.2005.850124

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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, 2001•2 citations
We have successfully fabricated niobium diffusion-cooled hot-electron bolometer (HEB) mixers on membranes of silicon nitride less than one micron thick. This advance has allowed us to construct a 1 /spl times/ 5 HEB receiver array intended for operation at 1.45 THz. This article provides an overview of the integration of the HEB array chip with silicon micromachined backshorts and feedhorns, discusses materials issues surrounding the device fabrication, reports resistance and I-V measurements, and compares HEBs fabricated on silicon nitride to similar devices on quartz substrates.
We report on the development of a multipixel hot electron bolometer (HEB) receiver fabricated using silicon membrane technology. The receiver comprises a 2 × 2 array of four HEB mixers, fabricated on a single chip. The HEB mixer chip is based on a superconducting NbN thin-film deposited on top of the silicon-on-insulator (SOI) substrate. The thicknesses of the device layer and handling layer of the SOI substrate are 20 and 300 μm, respectively. The thickness of the device layer is chosen such that it corresponds to a quarter-wave in silicon at 1.35 THz. The HEB mixer is integrated with a bow-tie antenna structure, in turn designed for coupling to a circular waveguide, fed by a monolithic drilled smooth-walled horn array.
We report in this paper a new concept for 2.7 THz superconducting Niobium nitride (NbN) Hot-Electron Bolometer mixer (HEB). The membrane process was developped for space telecommnunication applications a few years ago and the HEB mixer concept is now considered as the best choice for low-noise submillimeter-wave frequency heterodyne receivers. The idea is then to join these two technologies. The novel fabrication scheme is to fabricate a NbN HEB mixer on a 1 μm thick stress-less Si3N4/SiO2 membrane. This seems to present numerous improvements concerning : use at higher RF frequencies, power coupling efficiency, HEB mixer sensitivity, noise temperature, and space applications. This work is to be continued within the framework of an ESA TRP project, a 2.7 THz heterodyne camera with numerous applications including a SOFIA airborne receiver. This paper presents the whole fabrication process, the validation tests and preliminary results. Membrane-based HEB mixer theory is currently being investigated and further tests such as heterodyne and Fourier transform spectrometry measurement are planed shortly.
We present a characterization of a 1.5 THz waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixer which can be pumped by a commercial solid state tunable local oscillator (LO) source. The NbTiN HEB mixer is made from a 12 nm thick NbTiN thin film deposited on a quartz substrate at room temperature. A gold electrode is formed in situ on the NbTiN thin film without breaking vacuum to ensure good contact. The uncorrected DSB receiver noise temperature is measured to be 1700 K at 1.5 THz, whereas the mixer noise temperature is derived to be 1000 K after corrections for losses of the input optics and the intermediate frequency (IF) amplifier chain. The required LO power absorbed in the HEB mixer is evaluated to be 340 nW by using an isothermal technique. The IF gain bandwidth is supposed to be about 1.3 GHz or higher. The present results show that good performance can be obtained at 1.5 THz even with a relatively thick NbTiN film (12 nm), as in the case of 0.8 THz. In order to investigate the cooling mechanism of our HEB mixers, we have conducted performance measurements for a few HEB mixers with different microbridge sizes both at 1.5 and 0.8 THz. The noise performance of the NbTiN HEB mixers is found to depend on the length of the NbTiN microbridge. The shorter the microbridge is, the lower the receiver noise temperature is. This may imply a contribution of the diffusion cooling in addition to the phonon cooling.
Hot electron bolometer mixers (HEBM) are expected to be used as low-noise heterodyne mixers in the terahertz frequency range. We are developing quasi-optical HEBMs for applications such as atmospheric remote sensing and radio astronomy in the terahertz frequency range. Therefore, we developed a fabrication method using niobium nitride (NbN) ultrathin films deposited on a silicone (Si) substrate for terahertz applications. First, a magnesium oxide buffer layer 50 nm thick was deposited on a Si substrate at approximately 525 °C. After deposition of this layer, NbN ultrathin films were deposited at ambient temperature. Using the buffer layers, we found the transition temperature of the 3.5-nm-thick NbN thin films to be approximately 10 K. The same method was applied to fabricate NbN-HEBMs on a Si substrate. The fabricated quasi-optical mixers were designed to operate at 3.1 THz. The receiver noise temperature was found to be approximately 1930 K (DSB) at 3.1 THz.
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,
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