Observing the universe in the submillimeter spectral region

The Herschel Space Observatory, launched in May 2009, has opened a new window to space, a view of the cold universe and the earliest phases of star formation and galactic evolution. So far, the models available to researchers about formation and evolution processes have not equaled actual observations. In addition to the atmosphere’s opacity, the main reason for this is that such phenomena cannot be followed in visible light, but only in the far infrared and beyond. These wavelength bands are affected by heavy radiation absorption in the Earth’s atmosphere. Herschel is overcoming these limits by providing temperature and matter-density measurements, ascertaining the chemical species involved and by yielding information about the dynamics of the objects investigated.

With its 3.5-meter diameter mirror, Herschel is the largest space telescope ever placed in orbit. Designed to benefit the world scientific community, the satellite carries a massive cryostat, holding 2,500 liters of helium, a superfluid liquid serving to cool instruments to temperatures lower than –271 °C. For astrophysicists, the aim is to minimize the infrared, and sub-millimeter emission from the new observatory and enhance contrast for the sources being observed.

Meeting the demands of the Herschel Space Observatory made a strong contribution to the development of bolometers for wavelengths shorter than 1 millimeter and to spurring research in this area.

CEA, with strong support from CNES, the French space research agency, contributed to the development, construction, and qualification of two of the three instruments carried by the Herschel satellite, two medium-resolution bolometric imaging spectrometers. One is a Photodetector Array Camera and Spectrometer (PACS), effective in the 60–210 mm spectral range, and the other is a Spectral and Photometric Imaging Receiver (SPIRE), which is effective in the 200–650 mm range.


A series of technological challenges
CEA’s concrete contribution proved particularly significant with regard to the PACS imaging spectrometer through the involvement of a number of divisions and institutes, including the Nanosciences and Cryogenics Institute (INAC: Institut Nanosciences et cryogénie), and the Institute for Research on the Fundamental Laws of the Universe (IRFU). Second, it proved significant in that this cooperation enabled construction of a complete camera, including the detectors, the 0.3-K cryocooler, the mechanical and electronic systems required for control, and data acquisition purposes, along with, obviously, the bolometer arrays specifically developed for the mission.

CEA teams have mastered fabrication of focal planes involving several thousand pixels, by way of collective fabrication processes, derived from the silicon microtechnology developed by Leti, CEA’s Laboratory for Electronics & Information Technology. This involves a technological leap forward compared to a few tens, or possibly hundreds, of detectors available from other laboratories. The limitations faced by these laboratories stem from the devices used to collect radiation – feed-horns positioned in front of each detector, integrating spheres – with each pixel requiring manual assembly, etc. The breakthroughs achieved at Leti benefited from the exceptional context of the 1990s, marked by the rise of microelectronics, with the advent, in particular, of chips affording the ability to hold a variety of sensors, obtained by deep, collective etching of the silicon (micro-accelerometers for the airbags fitted to automobiles, methane detectors for domestic use). This technological spurt was complemented by a more relaxed circulation of information than had prevailed in earlier years: older ideas, concerning the absorption of electromagnetic waves, were coming back in favor. Leti researchers tweaked these to meet the issue of detection to be compatible with collective fabrication processes. That was the last obstacle to the fabrication of large detector arrays.