W. Michalke

LOW NOISE HIGH-TC SUPERCONDUCTING BOLOMETERS ON SILICON NITRIDE MEMBRANES FOR FAR-INFRARED DETECTION

High-Tc GdBa2Cu3O7 – delta superconductor bolometers with operation temperatures near 89 K, large receiving areas of 0.95 mm2 and very high detectivity have been made. The bolometers are supported by 0.62 µm thick silicon nitride membranes. A specially developed silicon-on-nitride layer was used to enable the epitaxial growth of the high-Tc superconductor. Using a gold black absorption layer an absorption efficiency for wavelengths between 70 and 200 µm of about 83% has been established. The noise of the best devices is fully dominated by the intrinsic phonon noise of the thermal conductance G, and not by the 1/f noise of the superconducting film. The temperature dependence of the noise and the resulting optimum bias temperature have been investigated. In the analysis the often neglected effect of electrothermal feedback has been taken into account. The minimum electrical noise equivalent power (NEP) of a bolometer with a time constant tau of 95 ms is 2.9 pW/Hz1/2 which corresponds with an electrical detectivity D* of 3.4 × 1010 cm Hz1/2/W. Similar bolometers with tau = 27 ms and NEP = 3.8 pW/Hz1/2 were also made. No degradation of the bolometers could be observed after vibration tests, thermal cycling and half a year storage. Measurements of the noise of a Pr doped YBa2Cu3O7 – delta film with Tc = 40 K show that with such films the performance of air bridge type high-Tc bolometers could be improved.

A high-T/sub c/ superconductor bolometer on a silicon nitride membrane

In this paper, we describe the design, fabrication, and performance of a high-T/sub c/ GdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// superconductor bolometer positioned on a 2/spl times/ 2-mm/sup 2/ 1-/spl mu/m-thick silicon nitride membrane. The bolometer structure has an effective area of 0.64 mm/sup 2/ and was grown on a specially developed silicon-on-nitride (SON) layer. This layer was made by direct bonding of silicon nitride to silicon after chemical mechanical polishing. The operation temperature of the bolometer is 85 K. A thermal conductance G=3.3/spl middot/10/sup -5/ W/K with a time constant of 27 ms has been achieved. The electrical noise equivalent power (NEP) at 5 Hz is 3.7/spl middot/10/sup -2/ WHz/sup -1/2/, which is very close to the theoretical phonon noise limit of 3.6/spl middot/10/sup -12/ WHz/sup -1/2/, meaning that the excess noise of the superconducting film is very low. This bolometer is comparable to other bolometers with respect to high electrical performance. Our investigations are now aimed at decreasing the NEP for 84-/spl mu/m radiation by further reduction of G and adding an absorption layer to the detector. This bolometer is intended to be used as a detector in a Fabry-Perot (FP)-based satellite instrument designed for remote sensing of atmospheric hydroxyl.

High-T/sub c/ bolometers with silicon-nitride spiderweb suspension for far-infrared detection

High-T/sub c/ GdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//(GBCO) superconducting transition edge bolometers with operating temperatures near 90 K have been made with both closed silicon-nitride membranes and patterned silicon-nitride (SiN) spiderweb-like suspension structures. As a substrate silicon-on-nitride (SON) wafers are used which are made by fusion bonding of a silicon wafer to a silicon wafer with a silicon-nitride top layer. The resulting monocrystalline silicon top layer on the silicon-nitride membranes enables the epitaxial growth of GBCO. By patterning the silicon-nitride the thermal conductance G is reduced from about 20 to 3 /spl mu/W/K. The noise of both types of bolometers is dominated by the intrinsic noise from phonon fluctuations in the thermal conductance G. The optical efficiency in the far infrared is about 75% due to a goldblack absorption layer. The noise equivalent power NEP for FIR detection is 1.8 pW//spl radic/Hz, and the detectivity D* is 5.4/spl times/10/sup 10/ cm /spl radic/Hz/W. Time constants are 0.1 and 0.6 s, for the closed membrane and the spiderweb like bolometers respectively. The effective time constant can be reduced with about a factor 3 by using voltage bias. Further reduction necessarily results in an increase of the NEP due to the 1/f noise of the superconductor.

HTS BOLOMETERS FOR FIR APPLICATIONS

For the visible spectral range different types of sensitive radiation detectors are available especially high sensitive and fast quantum detectors based on semiconducting materials. However, for the IR range there is a lack of detectors, especially for the FIR range between 10 µm and 1000 em wavelength. This wavelength region is of actual importance for spectroscopic purposes as well as for communication systems. For spectroscopic investigations two competing techniques are under developement, the optical and the rf-technique.

IR detector system based on high-Tc superconducting bolometer on Si membrane

An infrared detector system based on high-Tc superconducting (HTS) membrane bolometer is reported. Superconducting transition-edge bolometer has been manufactured by silicon micromachining using an epitaxial GdBa2Cu3O7-x film on an epitaxial yttria- stabilized zirconia buffer layer on silicon. The active area of the element is 0.85 X 0.85 mm2. The membrane thickness is 1 micrometers , those of the buffer layer and HTS films are 50 nm. The detectivity of bolometer, D*, is 3.8 X 109 cm Hz1/2 W-1 at 84.5 K and within the frequency regime 100 < f < 300 Hz. The optical response is 580 V/W at time constant 0.4 ms. This is one of the fastest composite type HTS-bolometer ever reported. The bolometer is mounted on a metal N2-liquid cryostat, which fits the preamplifier. With the volume of N2-reservoir being 0.1 liter, the cryostat holds nitrogen for about 8 hours. Using only wire heater with constant current, the temperature stability of about 0.03 K/h is achieved. The detector system can be used in IR- Fourier spectroscopy at wavelengths longer than the typical operating range of semiconductor detectors (wavelength greater than about 20 micrometers ).