Fig. 1: Production scheme for High-Tc Bolometer

Fig. 2: Production of silicon-on-nitride using commercial SOI wafers

Fig. 3: Schemetical cross-section of bolometer

Fig. 4: SEM picture of bolometer multilayer structure

Introduction

Because of the central importance of the OH radical in the stratospheric ozone balance, ESA is currently coordinating investigations on different techniques to measure the global climatology of stratospheric OH. 

Within this context, a new project has been started recently in collaboration with SRON (in Utrecht, The Netherlands), IHT (University of Hannover, Germany) and IPHT (Jena, Germany), involving the development of a high-Tc superconductor transition-edge bolometer . The bolometer is required as the detector for the so called OHIO instrument. 

The OHIO instrument is a Fabry-Perot based satellite instrument designed for remote sensing of atmospheric OH. The Fabry-Perot together with a reflection grating is used to select the 85 micrometer emission line of OH. To put minimial demands on the satellite resources a mechanical cryo-cooler is used for cooling. The minimum operating temperature of the detector is therefore 35 K. Further requirements for the detector are a time constant <0.3 s, a diameter around 1.1 mm, and a noise equivalent power (NEP) smaller than 4.0E-12 W/Hz^1/2 at 85 micron. None of the presently available detectors can meet these requirements. The most promising candidates to realise these specifications are high-Tc superconductor (HTS) transition edge bolometers. 

Bolometer design

The investigated bolometer essentially consists of a 1 micrometer Si₃N₄ membrane with a high-Tc superconducting GdBaCuO film on top of it. The operating temperature (equal to T_c) is about 88 K. A thin (100 to 300 nm) monocrystalline silicon layer on top of the Si₃N₄ membrane enables the epitaxial growth of the superconductor. This is necessary for a low 1/f noise level of the high-Tc film. 
The bolometer production scheme is shown in fig. 1. The first and last step in this scheme are done at MESA, the second and third at IHT Hannover, and the fourth step at IPHT Jena. 

Work done at MESA

The thin monocrystalline silicon layer on top of the Si₃N₄ membrane is made at MESA using a bond-and-etchback technology, shown in fig.2. A key step is the Chemical Mechanical Polishing CMP treatment on the wafers with Si₃N₄, in order to make these wafers bondable. After the epitaxial deposition of the complete multilayer structure, a membrane is etched, to obtain thermal isolation of the sensing element. A schematic cross-section is shown in fig. 3. A SEM picture of the obtained multilayer (still on a full substrate) is shown in fig. 4. 

Bolometer Performance

Bolometers have been produced on 2x2 mm² and 3x3 mm² membranes. The T_c of these bolometers is typically around 90 K, and show a very low intrinsic noise level, indicating a high quality superconductor. These bolometers are performing very close to the stated requirements, with a best measured electrical NEP (3x3 mm² membrane) of about 1.5 E-12 W/Hz^1/2. Combined with the best measured optical absorption efficiency this would lead to an optical NEP of 3.5 E-12 W/Hz^1/2. These values make this bolometer comparable with the best High-T_c superconductor bolometers currently available.

The performance of the bolometer can be further improved by reducing the thermal conductance between the sensing element and the silicon substrate. This can be done by making a smaller membrane which is supported by several long, narrow beams from a 4x4 mm² membrane. The first measurements on such samples indicate that a reduction of the thermal conductance with a factor 10 should be possible. This work has been continued by SRON, IHT and IPHT.

S. Sánchez, C. Gui, M. Elwenspoek, 'Spontaneous direct bonding of thick silicon nitride', Journal of Micromechanics and Microengineering Vol. 7, No. 3, pp 111-113, September 1997

S. Sánchez, M. Elwenspoek, C. Gui, M.J.M.E. de Nivelle, R. de Vries, P.A.J. de Korte , M.P. Bruijn, J.J. Wijnbergen, W. Michalke, E. Steinbeiß, T. Heidenblut, B. Schwierzi, "A High-Tc Superconductor Bolometer on a Silicon Nitride Membrane", Journal of Microelectromechanical Systems, in press (1997)

M.J.M.E. de Nivelle, M.P. Bruijn, R. de Vries, J.J. Wijnbergen, P.A.J. de Korte, S. Sánchez, M. Elwenspoek, T. Heidenblut, B. Schwierzi, W. Michalke, E. Steinbeiß, "Low noise high-Tc superconducting bolometers on silicon nitride membranes for far-infrared detection", Journal of Applied Physics Vol. 82, No. 10, pp. 4719-4726 (1997)
 

MESA Research Institute The Netherlands Prof. M. Elwenspoek Ir. S. Sánchez Space Research Organization Netherlands Dr. P. de Korte Dr. M. de Nivelle Dr. R. de Vries Dr. M. Bruijn Dr. J.J. Wijnbergen Institut fur Halbleitertechnologie und Werkstoffe der Elektrotechnik Hannover Germany Dr. rer. nat. B. Schwierzi Dr.-Ing. R. Ferretti Dipl.-Phys. T. Heidenblut Institut fur Physikalische Hochtechnologie Jena Germany Dr. E. Steinbeiß Dr. W. Michalke

1. Space Research Organization Netherlands  Welcome High-Tc bolometer development 2. Institut fur Physikalische Hochtechnologie