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IEEE Transactions on Applied Superconductivity

Speed measurements of diffusion-cooled tantalum bolometers

Anders J. Skalare, William R. McGrath, B. Bumble, Henry George LeDuc

Published Jun 1, 2003• 3 citations
Publisher

Abstract

Thin tantalum films were deposited on silicon wafers using a niobium seed layer to promote alpha-phase growth. These films were patterned into submicrometer size diffusion-cooled bolometers with superconducting transition temperatures of up to 2.35 K and a transition width of about 200 mK. The thermal relaxation times of the devices were determined by measuring the device impedance as a function of frequency and by fitting a theoretical model to the data. Measured relaxation times at low bias voltages range from 0.75 GHz for a 400-nm long device to 6 GHz for a 100-nm device, excluding electrothermal feedback. This should allow sufficiently high instantaneous bandwidths for most low-noise mixer applications in astrophysics.

DOI
10.1109/tasc.2003.813670

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Applied Physics Letters•Sep 16, 1996•200 citations
Thin tantalum films were deposited on silicon wafers using a niobium seed layer to promote alpha-phase growth. These films were patterned into submicrometer size diffusion-cooled bolometers with superconducting transition temperatures of up to 2.35 K and a transition width of about 200 mK. The thermal relaxation times of the devices were determined by measuring the device impedance as a function of frequency and by fitting a theoretical model to the data. Measured relaxation times at low bias voltages range from 0.75 GHz for a 400-nm long device to 6 GHz for a 100-nm device, excluding electrothermal feedback. This should allow sufficiently high instantaneous bandwidths for most low-noise mixer applications in astrophysics.
We describe superconducting diffusion-cooled hot-electron bolometers that were fabricated fromtantalum films grown on a thin niobium seed layer. The seed layer promotes single-phase growth of the Ta films, resulting in high-quality bolometers with transition temperatures up to 2.35 K and transition widths of less than 0.2 K. An S-parameter measurement set-up in a He-3 cryostat was used to measure device impedance versus frequency of a 400 nm long device at a temperature of 400 mK. It is shown that a 3 dB roll-off frequency of about 1 GHz can be achieved when the device resistance matches the impedance of the embedding network (no electrothermal feedback). This would lead to a prediction of 16 GHz for a 100 nm device, and indicates that a heterodyne mixer using a Ta HEB should be able to operate at several GHz even with a significant amount of electrothermal feedback.
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
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 report a systematic, comprehensive set of measurements on the dynamics and noise processes in diffusion and phonon-cooled superconducting hot-electron bolometer mixers which will serve as ultralow noise detectors in THz heterodyne receivers. The conversion efficiency and output noise of devices of varying lengths were measured with radio frequency between 8 and 40 GHz. The devices studied consist of 100-Å-thin film Nb bridges connected to thick (1000 Å), high conductivity normal metal (Au) leads. The lengths of the devices studied range from 0.08 to 3 μm. For devices longer than the electron–phonon interaction length Le–ph≡Dτe–ph, with D the diffusion constant and τe–ph−1 the electron–phonon interaction rate, the hot electrons are cooled dominantly by the electron–phonon interaction, which in Nb is too slow for practical applications. If the device length is less than πLe–ph(≈1 μm at 4.2 K), then out diffusion of heat into the high conductivity leads dominates the cooling process. In this limit, the intermediate frequency (IF) bandwidth is found to vary as L−2, with L the bridge length, as expected for diffusion cooling. The shortest device has an IF bandwidth greater than 6 GHz, the largest reported for a low-Tc superconducting bolometric mixer. The dominant component of the output noise decreases with frequency in the same manner as the conversion efficiency, consistent with a model based on thermal fluctuations. The noise bandwidth is larger than the gain bandwidth, and the mixer noise is low, ranging from 100 to 530 K (double sideband). The crossover from phonon dominated to diffusion dominated behavior is also demonstrated using noise thermometry measurements in the normal state. Scalar measurements of the device differential impedance in the intermediate state agree with a theoretical model which takes into account the thermal and electrical dynamics. We also present detailed comparisons with theoretical predictions of the output noise and conversion efficiency.
We present a theoretical analysis and experimental evaluation of a transition-edge superconducting bolometer for detecting infrared and millimeter waves. The superconducting film is voltage biased and the current is read by a superconducting quantum interference device ammeter. Strong electrothermal feedback maintains the sensor temperature within the transition, gives a current responsivity that is simply the inverse of the bias voltage, and reduces the response time by several orders of magnitude below the intrinsic time constant C/G. We evaluated a voltage-biased bolometer that operates on the Tc∼95 mK transition of a tungsten film with a thermal conductance of G∼1.2×10−9 W/K. As expected, the electrical noise equivalent power of 3.3×10−17/W√Hz is close to the thermal fluctuation noise limit and is lower than that of other technologies for these values of G and temperature. The measured time constant of 10 μs is ∼100 times faster than the intrinsic time constant.