Silicon nitride micromesh bolometer array for submillimeter astrophysics

Abstract

We present the design and performance of a feedhorn-coupled bolometer array intended for a sensitive 350-m photometer camera. Silicon nitride micromesh absorbers minimize the suspended mass and heat capacity of the bolometers. The temperature transducers, neutron-transmutation-doped Ge thermistors, are attached to the absorber with In bump bonds. Vapor-deposited electrical leads address the thermistors and determine the thermal conductance of the bolometers. The bolometer array demonstrates a dark noise-equivalent power of 2.9 ϫ 10 Ϫ17 W͌͞Hz and a mean heat capacity of 1.3 pJ͞K at 390 mK. We measure the optical efficiency of the bolometer and feedhorn to be 0.45-0.65 by comparing the response to blackbody calibration sources. The bolometer array demonstrates theoretical noise performance arising from the photon and the phonon and Johnson noise, with photon noise dominant under the design background conditions. We measure the ratio of total noise to photon noise to be 1.21 under an absorbed optical power of 2.4 pW. Excess noise is negligible for audio frequencies as low as 30 mHz. We summarize the trade-offs between bare and feedhorn-coupled detectors and discuss the estimated performance limits of micromesh bolometers. The bolometer array demonstrates the sensitivity required for photon noise-limited performance from a spaceborne, passively cooled telescope.

Observing extended sources with theHerschelSPIRE Fourier Transform Spectrometer

“…The SPIRE FTS detector arrays use feedhorns to couple radiation to the bolometric sensors (Turner et al 2001). Each feedhorn consists of a conical concentrator in front of a circular section waveguide designed to act as a spatial filter, allowing only certain electromagnetic modes to propagate along the waveguide.…”

“…It contains an imaging photometric camera and an imaging Fourier Transform Spectrometer (FTS). Both subinstruments use arrays of bolometric detectors operating at ∼300 mK (Turner et al 2001) with feedhorn focal plane optics giving sparse spatial sampling over an extended field of view . The FTS is composed of two bolometer arrays with overlapping bands.…”

Antenna-coupled TES bolometers for the SPIDER experiment

“…14 Another significant advantage of microstrip-coupled bolometer geometry is that the thermalization time constant is much shorter than the external (bolometer) time constant, providing excellent thermal efficiency. 3,15 While in theory small-volume bolometers might someday provide lower noise equivalent power (NEP) at a higher physical temperature, 16, 17 a micro-machined mechanically-isolated bolometer allows the characteristics of the termination, the thermistor, and the thermal conductance to be optimized independently. State-of-the-art photolithography can now reliably produce silicon nitride legs with extremely high aspect ratio.…”

“…1 Bolometers can provide photon noise-limited sensitivity over a wide frequency range. However, the existing feedhorn-coupled micromesh bolometers [2][3][4] face difficulties in extending the frequency coverage and scaling to kilo-pixel instruments. The micromesh bolometers with low thermal conductance (both the "spiderweb" 2 and the "PSB" 4 type) have currently been demonstrated with architectures suitable for frequencies ν >60 GHz.…”

In-flight calibration of theHerschel-SPIRE instrument

“…For the purposes of discussion of the calibration and performance estimation we treat SPIRE as two separate instruments that are spatially separated at the focal plane of the Herschel telescope: a three band imaging photometric camera with bands centred nominally on 250 μm (PSW), 350 μm (PMW), and 500 μm (PLW) with detector arrays of 139, 88 and 43 respectively feedhorn coupled NTDbolometers (Turner et al 2001), and a two band imaging Fouriertransform spectrometer (FTS) covering 194−313 μm (SSW) and…”