MONOLITHIC SILICON BOLOMETERS COUPLED TO WAVEGUIDE
Peter T. Timbie1, Christine A. Allen2, Tina C. Chen3,
Sean Cordone1, Khurram Farooqui4,
We have measured
the performance of ion-implanted silicon bolometers developed for observations
of faint astrophysical sources at millimeter wavelengths.
These devices are mounted in waveguide to couple to a single mode
of electromagnetic radiation. We
measure optical coupling efficiencies of ~ 90% across a full waveguide
band. When cooled to 0.1 K and operated under low background conditions,
the devices have a time constant of 1.5 – 4 ms and NEP ~
We have developed monolithic silicon bolometers for
use in an experiment to measure the spatial structure (anisotropy) in the
Cosmic Microwave Background (CMB). Since the CMB is an unperturbed relic of
the hot Big Bang, studies of its structure can yield a wealth of information
about the early Universe. Measurements of anisotropy in the CMB are difficult
because the level of anisotropy is five orders of magnitude below the 2.7
Kelvin CMB. Our approach is to
measure the anisotropy in the CMB using a balloon-borne telescope called the
Medium Scale Anisotropy Measurement (MSAM2) . The low background from the
atmosphere at an altitude higher than 30 km allows the use of ultra-sensitive
The MSAM2 radiometer has five channels spanning the W and D bands: 65-80 GHz, 80-95 GHz, 95-110 GHz, 130-150 GHz and 150-170 GHz. The bands were chosen to take advantage of windows in the atmospheric opacity at millimeter wavelengths and to allow discrimination between astrophysical foregrounds and the CMB. We have coupled our detectors directly to single-mode waveguide, thereby utilizing the advantages of single-mode technology such as low-sidelobe antennas, high quality filters, and diffraction-limited angular resolution. In addition, by coupling the detector directly to waveguide, the absorber can be made much smaller than a wavelength. This approach greatly reduces the time-constant of a thermal detector and allows rapid scanning of the sky without loss of sensitivity. Moreover, the cross-section for cosmic ray hits is small.
Monolithic silicon bolometers, introduced by Downey et al. , were been fabricated at the NASA Goddard Space Flight Center . The device consists of a micromachined thin silicon substrate suspended from a silicon frame by silicon legs which also function as the thermal link to the heat bath. The thermistor is ion-implanted in the substrate. The thermistor, the absorber and the thermal link can be optimized separately. Our waveguide-to-bolometer coupling scheme is similar to that introduced by Peterson and Goldman  for composite bolometers. However, in our design ( Figure 1) the absorber of the bolometer consists of a thin resistive bismuth film deposited on the narrow silicon substrate oriented along the E-plane. An adjustable backs in the waveguide behind the absorber is used to match the impedance of the absorber to the waveguide. The thermistor is located outside the waveguide. The silicon substrate and legs pass through a small slot in the broad wall of the waveguide. The reflectance has been measured to be better than –10 dB across an entire waveguide band.
Design and Fabrication
our bolometers the thermistors are produced by implanting silicon wafers
with phosphorus and 50% boron compensation to a concentration near the
metal-insulator transition. At this concentration, the phonon-assisted
hopping conduction mechanism has a strong dependence on temperature.
The behavior of resistance with temperature of the thermistor
is described by:
Load curves of the bolometers are measured at a variety of cold plate temperatures from
~ 100 mK to 200 mK in the dark.
From these we determine that a typical device has
T0 = 13 K, R
0 = 380W, and G =
2.2 X 10-11 W/K at 0.1 K.
We have fabricated and tested in the lab and in a balloon flight monolithic silicon bolometers coupled to waveguide. These devices are particularly promising for sensitive measurements at millimeter wavelengths