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\subsection{Bolometer Arrays for Submillimeter Cosmology}\label{kreysa} 
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{\bf{
{Dr. Ernst Kreysa}\\
{Max-Planck-Institut f\"ur Radioastronomie}\\
{Auf dem H\"ugel 69}\\
{D-53121 BONN}\\
{Telefon: 0228/525269}\\
{Fax: 0228/525435}\\
{Email: ekreysa@mpifr-bonn.mpg.de}\\}}


\subsubsection{Abstract}

Large bolometer arrays of ever increasing size and sensitivity are the
detectors of choice for observations of redshifted thermal emission from
young galaxies and fluctuations of the cosmic microwave background
temperature at millimeter- and submillimeter wavelengths.  Based on
extensive experience in the development and construction of bolometer
arrays, we propose to push research and development toward a next
generation of these arrays.


\subsubsection{Current status of research}

Presently, several developments are taking place that will allow the
manufacture of very large arrays of bolometers.  Foremost among these is
the microlithographic fabrication of electronic and mechanical components.
It is not the shrinking of the device size that is of interest here,
because the dimension of the absorbing structure in a bolometer is tied to
the operating wavelength.  It is rather the reliability combined with the
ease of increasing the array size at a small incremental cost that make
microlithographic techniques attractive for bolometer technology.  If one
compares the mapping speed of two bolometer arrays of different size
(i.e., number of elements $n$), the mapping speed -- defined as the time to
reach a given signal-to-noise ratio for a given area -- scales with $n$,
provided the individual bolometers have the same sensitivity.  This is
strictly true only for the case when the area of sky to be mapped is much
larger than the array, which applies for much of the current
front-line research, e.g., the search for cosmological point sources. 

At the extremes, bolometer arrays can be classified as fully efficient
(sparsely sampled, with channel separations of 2 beams) or as fully sampled
(half beam separation).  A recent study for SPIRE (the bolometer camera of
FIRST) indicates that the speed advantage of a fully sampled array over a
fully efficient one is less than a factor of two.  However, the fully
sampled array has 16 times as many elements, which implies a large increase
in system and data complexity.  For the same number of elements (and wires) 
the fully efficient array is much more sensitive for large scale mapping. 
For both array types, Nyquist sampling will be achieved naturally during a 
continuous scan.

The sensitivity of (sub)mm bolometers can be limited by two effects.  The
first is the fundamental photon background noise, i.e.,  the statistical
fluctuations of the thermal photon field.  The second is the time variation
of large-scale inhomogeneous emission in the atmosphere, mostly from water
vapour.  Current semiconducting bolometers are already close to being
limited by photon noise.  If the sensitivity cannot be improved
significantly, it is more sensible to push the limits of array size
instead.  The skynoise is generally much higher than the photon noise, and
it is of non-Gaussian character, so that the sensitivity does not integrate
down with the square root of integration time. Highest sensitivities can 
only
be achieved with low, stable backgrounds, i.e., by moving to the best
possible site.

There have been various recent proposals for new detector devices.
Of those, we consider superconducting transition edge bolometers to be the 
most likely to reach technical maturity in the near future.

Existing state-of-the art instrumentation on ground-based facilities
include
\begin{itemize}
\item a series of Max-Planck Millimeter Bolometer (MAMBO) arrays
built at the MPIfR and installed at the IRAM 30 m telescope on Pico
Veleta, Spain for the 1-mm window -- a 117-element array 
has just replaced the previous 37-element array; 
\item a 19-element MPIfR array for the 870 $\mu$m window at the 10 m 
Heinrich-Herz-Telescope (HHT) on Mount Graham, Arizona;
\item a new 37-element array (SIMBA) for the 1-mm window at the 
Swedish-ESO Submillimetre Telescope (SEST) on La Silla, Chile, that the 
MPIfR is currently deploying;
\item the SCUBA arrays for the 870 $\mu$m and 450 $\mu$m windows at the 15 
m James-Clerk-Maxwell Telescope on Mauna Kea, Hawaii; 
\item BOLOCAM for the 1-mm window at the 10.4 m Caltech Submillimeter 
Telescope, on Mauna Kea.  
\end{itemize}


\subsubsection{Own experience}

The proposer has extensive experience in the theory, design, and building
of detectors for millimeter- and submillimeter-wavelength astronomy.
Since the 1970s, he has concentrated on bolometer systems, which have
been very successfully used at various facilities.  His group has built
all of the MPIfR bolometers, namely several generations of MAMBO, 
the facility bolometer instruments for the SEST
and its next generation instrument SIMBA, a multi-color bolometer and a
bolometer array for the HHT, and various other systems.

Dr.  Kreysa is very much interested in the astronomical questions that can
be addressed by his bolometer arrays, in particular research on dust
emission from the high redshift universe and studies of the cosmic
microwave background radiation.

The present generation of bolometers built at the MPIfR are based on the 
very reliable Neutron-Transmutation-Doped Germanium (NTD-GE) thermistors. 
We were involved in the development of these thermistors by the group of 
E.E. Haller at LBNL in Berkeley. Unfortunately the production of NTD-Ge 
chips is not very compatible with microfabricaton of bolometers. The chips 
have to be attached manually to the bolometer structures.

Bolometers require a weak thermal link between the absorber of the 
radiation and the cold surface. In our case this a free-standing membrane 
of Silicon-Nitride deposited by chemical vapor deposition. The complete 
array structures, including wiring and absorber, are micromachined on a 
Silicon wafer in the microlab of UC Berkeley. The wafer is then matched to 
an electroformed array of horn antennas. This proven design can be refined 
and scaled to 1000 elements. At the wafer level, a transition to Indium 
bump bonding will required in order to increase the reliability of 
thermistor attachment.



\subsubsection{Goals of the project}

The goal of the project is to design, develop, and operate state-of-the-art
continuum detector arrays for unique frontline telescopes like the AST and
the IRAM 30m telescope.

The first major project will be the development of a 1000-element bolometer
array for the AST, operating in the 870 $\mu$m band with a total field of
view of $\approx 20$ arcminutes.  To search for high-redshift dust
emission, the 870 $\mu$m window is best because of a high
source flux and good atmosheric transparency.  In addition, a
37-element bolometer array covering the 350 $\mu$m window 
will be built for 
the beginning of the AST operation. It will be essential for determining
the spectral index of sources found, e.g., through 870$\mu$m surveys.

In the later phases of the project, the unique quality of the AST site
should allow us to take advantage of the extreme submm windows at 450
$\mu$m and 350 $\mu$m with very large arrays, of the order of several
thousand elements.  This will require new technology.  At present
superconducting bolometers with multiplexed SQUID readout look
promising, but detector development is a rapidly evolving
field which must be monitored carefully.  

Because of the short time left until the begin of the AST operation 
(early 2003)
we will initially rely mostly on proven technology.
Still, significant research is necessary on the several issues which
are critical for the success of our new developments:
\begin{itemize} 
\item silicon micromachining of the bolometer structure,
\item bump bonding of the thermistors,
\item cold multiplexers,
\item highly effective thermal rejection filters,
\item closed-cycle cryogenics,
\item powerful, low interference, data acquisition.
\end{itemize}

The size of arrays might eventually be limited by the number of wires
and/or the electrical dissipation of the pre-amplifiers.  There are several
promising multiplexing technologies under development which would reduce
the problem considerably, most of which are being developed for
superconducting devices.  In a relatively high background environment, the
multiplexing capability, and not the sensitivity advantage, is the
major attraction of the new superconducting bolometer technology.  For
Arrays with NTD-Ge thermistors the multiplexing problem remains to be
solved, but some of promising ideas are already being investigated.

In a ground-based environment, bolometers are working under a high thermal
background.  Filters must be employed on all intermediate temperature
shields to restrict the background to a passband in the Rayleigh-Jeans tail
of the 300 K peak.  It does not help to increase the cooling power of the
cryogenic system, because any out-of-band power would overwhelm the
bolometers.  Scaling the filter size with array dimensions is problematic
too.  Given the limited thermal conductivity of some filter components, the
out-of-band power they absorb could heat them and thus increase the thermal
load on the bolometers.  It is likely that filter technology will limit the
maximum array size.

At an altitude of 5000m, all observations must be
performed with a minimum of physical intervention and effort by the
staff.  Even at this exeptional site excellent submm condition can arrive
with little warning, and the equipment has to be on standby for long
periods.  Cryogenics are a particular challenge in that respect.  For the
arrays we have to provide temperatures of 100mK, ideally without the need
for liquid helium.  Pulse tube coolers reach 4K with low vibration levels
and can be combined with adiabatic demagnetisation refridgerators for
100mK.  Both need further development for our purposes.

Also needed is a substantial effort in data acquistion hard- and
software.  The cold multiplexers need special backends which have to take
into account the intrinsic sensitivity of bolometers to interference of
any kind.  Mapping wide areas with large arrays will involve high data
rates and the data acquisition has to be optimized for that observing
mode. 

\subsubsection{Estimate of resources expected from DFG}
To be discussed.

\subsubsection{Members of research group relevant for the project}
Dr. Ernst Kreysa, Dr. Hans-Peter Gem\"und, Dr. Lothar Reichertz,
Andrea Raccanelli, Giorgio Siringo

\subsubsection{Connections to other projects of current SFB proposal}
The project is closely related to the project lead by Dr. F. Bertoldi,
which utilizes new bolometer technology for cosmological studies.

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