Stefan Sanchez

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.

Spontaneous direct bonding of thick silicon nitride

Wafers with LPCVD silicon-rich nitride layers have been successfully direct bonded to silicon-rich nitride and boron-doped silicon surfaces. A chemical - mechanical polishing treatment was necessary to reduce the surface roughness of the nitride before bonding. The measured surface energies of the room-temperature bond were comparable to values found for Si - Si hydrophilic bonding. A mechanism similar to this bonding is suggested for silicon nitride bonding.

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.