Performance of cryogenic microbolometers and calorimeters with on-chip coolers

Abstract

Astronomical observations of cosmic sources in the far-infrared and X-ray bands require extreme sensitivity. The most sensitive detectors are cryogenic bolometers and calorimeters operating typically at about 100 mK. The last stage of cooling (from 300 mK to 100 mK) often poses significant difficulties in space-borne experiments, both in system complexity and reliability. We address the possibility of using refrigeration based on normal metal/insulator/superconductor (NIS) tunnel junctions as the last stage cooler for cryogenic thermal detectors. We compare two possible schemes: the direct cooling of the electron gas of the detector with the aid of NIS tunnel junctions and the indirect cooling method, when the detector lattice is cooled by the refrigerating system, while the electron gas temperature is decreased by electron-phonon interaction. The latter method is found to allow at least an order of magnitude improvement in detector noise equivalent power, when compared to the direct electron cooling.A thermal detector system, such as a bolometer or a calorimeter, consists of a thermal sensing element (TSE) which is connected to a heat sink. The TSE typically consists of an absorber and a thermometer. The thermometer can be a transition-edge sensor [1-3], or a normal metal/insulator/superconductor NIS tunnel junction thermometer [4,5]. In this paper we address the fundamental question of whether one should cool the electrons of the detector directly, i.e. cool the TSE below the heat sink temperature, or alternatively cool the heat bath of the TSE. The principle of the NIS cooler has been introduced in Refs. [6] and [7].The sensitivity of thermal detectors is strongly influenced by the detector temperature. The figure of merit for bolometric detectors is the noise equivalent power (NEP). To calculate this quantity for thermal detectors coupled to on-chip coolers, we have to evaluate the fluctuations of the electron gas temperature due to the power exchange with the lattice, with the superconductor of the cooler (in the case of direct cooling), and due to the bias of the thermometer (see Fig. 1). We can then define the NEP as the optical input power (the power to be detected) needed to produce a change in the temperature of the electron gas equal to the square root of the mean square of these fluctuations. In the following calculations the noise introduced by the bias powerQ b is supposed to be very small.

Electron–phonon heat exchange in layered nano-systems

“…Because of this, if the normal metal island absorbs radiation, the device turns into a very sensitive radiation detector 5,8 . When it works as a detector, the normal metal island can be kept at the nominal working temperature either by cooling it directly through NIS junctions (eventually through the thermometer junctions), or indirectly through electron-phonon coupling to a cold substrate [9][10][11] . Therefore in any situation, i.e.…”

Electron–phonon heat transport and electronic thermal conductivity in heavily doped silicon-on-insulator film

“…Investigation of the heat transport processes in heavily doped silicon at low temperature is important both for developing various low temperature devices such as hot electron bolometers 1 or microcoolers 2 and also for fundamental physics. Electron-phonon interaction is very weak at low temperature and this leads to a large temperature difference between electron and phonon systems even when a relatively small power is introduced into the system.…”

Heat Exchange Between Electrons and Phonons in Nanosystems at Sub-Kelvin Temperatures

“…The supporting membrane is dielectric and provides the thermal insulation of the TSE from the bulk material [1][2][3][4]. To ensure the level of sensitivity required by the astrophysical observations, the thickness of the TSE should be of the order of 10 nm, the thickness of the supporting membrane is of the order of 100 nm, whereas the working temperature of the device may be about 100 mK [5,6]. The layers of materials that form the detector are shown schematically in Fig.…”

“…The TSE may consist of a normal metal strip coupled to a superconducting antenna [2,3,[5][6][7][8][9]. When the incident radiation is absorbed, its energy is dissipated into the normal metal strip.…”