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\begin{document}

\noindent {\bf \Large SUMMARY OF PERSONNEL AND WORK EFFORTS}

\vspace{2cm}


\begin{table}[!hbt]
\renewcommand{\arraystretch}{1.25}
\renewcommand{\thefootnote}{\alph{footnote}}
\large
\centering
\label{workeffort} 
\begin{tabular}{|l|c|c|c|c|c|}
\hline
 & & & \multicolumn{3}{|c|}{\bf Work Commitment} \\ \cline{4-6} 
{\bf Name} & {\bf Organization} & {\bf Role} & {\bf Year 1} & {\bf Year 2} & {\bf Year 3} \\ \hline \hline
Dr. Christopher K. Walker & Univ. of Arizona & PI & 10\% & 10\% & 10\% \\ \hline
Dr. Gordon Chin & NASA/GSFC & Co-I & 10\% & 10\% & 10\% \\ \hline
Dr. Jonathan Kawamura & JPL & Co-I & 10\% & 10\% & 10\% \\ \hline
Mr. Jacob Kooi & Caltech & Co-I & \dots \footnotemark & \dots & \dots \\ \hline
Dr. Craig Kulesa & Univ. of Arizona & Co-I & 25\% & 25\% & 25\% \\ \hline
Dr. Gopal Narayanan & Univ. of Massachusetts & Co-I & 10\% & 10\% & 10\%
 \\ \hline
Dr. Art Lichtenberger & Univ. of Virginia & Co-I & 10\% & 10\% & 10\%
 \\ \hline 
Dr. Sander Weinreb & Caltech & Co-I & \dots & \dots & \dots \\ \hline
\end{tabular}
\end{table}
\footnotetext{Dots (\dots) represent contributed effort. }


\clearpage

\medskip
\centerline {\Large \bf Development of Space Terahertz Array Receivers
  (STARs)}

\section{Objectives and Significance of the Proposed Research}
           
The advent of large format ($\sim$100 pixel) spectroscopic imaging cameras
in the far-infrared (FIR) will fundamentally change the way astronomy is
performed in this important wavelength regime. While the possibility of
such instruments has been discussed for more than two decades
\citep{gil79}, only recently have advances in mixer technology, device
fabrication, micromachining, low-noise amplifiers, and digital signal
processing made the construction of such an instrument tractable. Here we
propose to implement these technologies to develop the first integrated,
far-infrared heterodyne arrays.  The arrays will utilize state-of-the-art
SIS and phonon-cooled Hot Electron Bolometers (HEB) mixing devices in
efficient, micromachined, waveguide mounts.  The goal of the proposed
effort is to demonstrate the viability of large format arrays in the FIR
by developing proof-of-concept compact, integrated, 1x8 SIS and HEB array
receivers at 810~GHz and 1.5~THz respectively. The arrays will serve as
building blocks for much larger 2 dimensional arrays.  As a technological
demonstration, the prototype arrays will be incorporated into a dual
polarization receiver system for use on the AST/RO telescope at the South
Pole.  We have developed a work plan that will place this technology at a
TRL of 4+ at by the end of the investigation. Once such arrays have been
demonstrated, a technological roadmap to achieving the goal of large
format, far-infrared (FIR) heterodyne arrays will become clear.  The
STARs team will submit an E/PO proposal to develop hands-on activities
for grades K-12 based upon the proposed research activities. 


\section {Scientific Motivation}

\subsection*{High Resolution, Spectroscopic Imaging Arrays in the Far-Infrared}


\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.3in}{\includegraphics[angle=0,scale=0.28]{ISM.eps}}
\parbox{2.9in}{\caption{\small \label{ISM} The life cycle of
    the interstellar medium and the stars that form
    from it; this relationship constitutes the basis for the evolution
    of galaxies, including our own Milky Way.
    Throughout this cycle, different (labeled) ionic and molecular
    species with strong emission lines at far-infrared and submillimeter 
    wavelengths 
    play pivotal roles in shaping the relevant physical processes.}}
\end{minipage}
\end{figure}
 
The wavelength regime between 300 and 60 microns hosts numerous atomic and
molecular emission lines that are key diagnostic probes of the
interstellar medium. These include transitions of [\cii], [\nii], [\oi],
HD, H$_2$D$^+$, OH, CO, and H$_2$O.  In Giant Molecular Clouds (GMCs),
evolved star envelopes, and planetary nebulae, these emission lines can be
extended over many arc minutes and possess complicated line profiles that
can only be disentangled using high resolution (${\rm R} = \lambda /
\Delta \lambda > 10^6$) spectroscopy. Observations of these lines are
crucial to understanding the delicate interplay between the interstellar
medium and the stars that form from it (see Figure~\ref{overview}). 
This feedback is central to all
theories of galactic evolution, and it must be understood at high angular
resolution in the Milky Way before it can be properly interpreted in
external galaxies near and far.  These spectral lines are pivotal probes
of important physical processes because they relate directly to the
chemistry and energy balance of the ISM; it is the cooling provided by
these lines that partially regulates the collapse of molecular cloud cores
into stars and traces the formation of molecules in evolved stars that
ultimately replenish the ISM.  Due to the hot (T $\ge$ 300K) and dense
($n_{\rm H} > 10^{10}\ {\rm cm}^{-3}$) conditions expected in many
protoplanetary disks, emission lines in the FIR will also play a pivotal
role in cooling and molding young planetary systems.  Both high spectral
resolution and imaging capabilities are necessary to probe velocity fields
and disentangle the emission of protoplanetary disks from that of
ubiquitous molecular outflows and ambient envelope and cloud material.


\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.7in}{\includegraphics[angle=0,scale=0.8]{overview-white.eps}}
\parbox{2.6in}{\caption{\small \label{overview} Overview of SOFIA/Herschel 
Terahertz-frequency spectroscopic instruments in development.  The
proposed instrument will provide a technological roadmap to making
heterodyne arrays with 100+ pixels.}}
\end{minipage}
\end{figure}

The only instruments capable of realistically providing the required 
spectral resolution and spatial coverage for these studies are heterodyne 
array receivers. Unfortunately, above 1~THz none exist. Figure~\ref{overview} 
compares the capabilities of SOFIA \& Herschel's 
first light instruments in terms of number of pixels versus spectral 
resolution. When viewed in this way, there is a clear, urgent need for large 
format heterodyne instruments (Young et al. 2003). 
Fortunately, recent breakthroughs in detector 
technology, micromachining, local oscillators, amplifier technology, and
backend spectrometers now make the construction of large arrays of
heterodyne receivers possible. The proposed array development
will not only provide a technological roadmap for large format
spectroscopic imaging cameras, it will also serve to demonstrate how
such arrays can maximize the scientific return from single aperture
telescopes such as AST/RO, the South Pole Submillimeter Telescope 
(SPST), SOFIA, and SAFIR. In terms of relevancy to the OAT program, the 
proposed work will help fulfill Goal 10, Mission and Science Measurement 
Technology Theme, RFA 10.2(b). 

We have assembled a team with experience in each aspect of the proposed
investigation. Members of this team have worked together in the past on a
number of successful projects, including:

\begin{itemize}
\setlength{\topsep}{-1ex}
\setlength{\partopsep}{-1ex}
\setlength{\itemsep}{-0.4ex}
\setlength{\parsep}{-0.4ex}
\item a 4 pixel, 810~GHz SIS array receiver (PoleSTAR) for AST/RO  
\item a 1 pixel, 1.1~THz HEB receiver for the Heinrich Hertz Telescope (HHT)
\item a dual polarization 492/810~GHz receiver for AST/RO  
\item a 7 pixel, 345~GHz SIS array receiver (DesertSTAR) for the HHT
\end{itemize}

The HEB and SIS array receivers are the first astronomical receivers 
of their kind.  As in the proposed work, waveguide technology was used
to optimize mixer performance in each of these instruments. 
Representative spectra taken with these instruments are shown in 
Figures~\ref{orion} and \ref{ngc6334}.    
The experience gained in building these instruments will be fully utilized
in the development of the proposed integrated arrays.

\begin{figure}[!hbt]
\centering
\begin{minipage}{6.5in}
\parbox{4.3in}{\includegraphics[scale=0.35,angle=0]{orion.eps}}
\parbox{2.0in}{\caption{\label{orion} {\small Integrated intensity of the 
J~=~$7 \rightarrow 6$ line of CO from the Orion molecular cloud,
observed at the Heinrich Hertz Telescope on Mt. Graham, Arizona
\citep{wils01}.  The intensity scale is shown as a bar on the 
right side of the map. Selected positions of the map have been
measured in the 1.037 THz line of CO~J~=~$9 \rightarrow 8$ from the
HHT \citep{kawa02}, and are displayed on the far right.  This
demonstrates the feasibility of near-THz submillimeter astronomy from
ground-based observatories. }}}
\end{minipage}
\end{figure}


\section {Technical Approach}

Unlike the situation with bolometric detectors, heterodyne 
receiver systems are coherent, retaining information about both the amplitude 
and phase of the incident photon stream. High resolution spectroscopy can, in 
principle, be performed in this same wavelength regime using incoherent 
detectors together with frequency dispersive quasi-optical devices such as 
gratings and Fabry-Perot interferometers. However, the size requirement of 
quasi-optical devices and/or the need to scan in order to construct a 
spectrum make them too cumbersome or insensitive for many applications. 

In the past the fabrication of integrated arrays of heterodyne mixers
at submillimeter wavelength was prohibited by several factors:

\vspace{-0.2cm}
\begin{itemize}
\setlength{\topsep}{-1ex}
\setlength{\partopsep}{-1ex}
\setlength{\itemsep}{-0.4ex}
\setlength{\parsep}{-0.4ex}

\item Sensitive mixing devices either did not exist or were difficult to
   fabricate.

\item When mixers were available, their performance would often vary
   significantly from device to device.

\item There was insufficient LO power to simultaneously pump more than
   one or two detectors.

\item Stacking more than a few mixer blocks together in the focal plane
   with their associated backshorts (if necessary), IF amps, magnets,
   bias lines, etc. was mechanically complex and could overload the
   cryogenic system.

\item The cost of the frontend components and the required backend
   spectrometer were prohibitive.

\end{itemize}

\vspace{-0.2cm}
\noindent However, through the arduous efforts of many researchers, most of these hurdles have now been overcome. 


\begin{figure}[!t]
\centering
\begin{minipage}{7in}
\parbox{4.2in}{\includegraphics[scale=0.4,angle=0]{ngc6334-polestar.eps}}
\parbox{2.2in}{\caption{\label{ngc6334} {\small Integrated intensity map of the 
J=4$\rightarrow$3 line of CO from the NGC~6334 molecular cloud.  
The first heterodyne array spectra at 810~GHz are
shown at right, taken with 3 of the 4 channels of the PoleSTAR array 
at AST/RO, built by the PI's group in collaboration with teams at the
University of Cologne and Caltech.  The footprint of the proposed 1x8
array at 1.5~THz is shown for comparison. }}}
\end{minipage}
\end{figure}

\subsection{\it Mixer Design}

     At frequencies above ~200 GHz no amplifiers exist and incoming
signals must first be down converted to frequencies where low noise
amplifiers are available. At these high frequencies three types of mixing
devices are currently in use;  Schottky diodes, superconductor insulator
superconductor (SIS) junctions, and hot electron bolometer (HEB) mixers.
Schottky diode mixers have been in use on ground, airborne, and
space-based platforms for many years. They are reliable and provide modest
sensitivity, which improves as they are cooled.  However, they require
prodigious amounts (of order mW)  of local oscillator (LO) power to work
efficiently. At submillimeter wavelengths gas lasers are often used to
provide this level of LO power.  Both SIS and HEB mixers utilize
superconductors to realize significant increases in sensitivity over that
of Schottky devices and require orders of magnitude less LO power.  
State-of-the-art SIS mixers have 30 times the sensitivity of the Schottky
mixers used on SWAS and the two Microwave Limb Sounder (MLS) instruments.
SIS mixers are used to ~1.2 THz. Above this frequency the bandgap
structure of SIS devices severely limits their performance and HEB mixers
are employed. To operate, these mixers must be cooled to $\sim 4$~K. We
propose to develop prototype 1x8 arrays of SIS and HEB mixers. The SIS
array will be optimized for observing the $\sim 810$~GHz lines of
[$^{12}$CI], [$^{13}$CI], and CO ($J=7\rightarrow6$). The HEB array will
provide low noise performance between 1.4 to 2.2~THz. Over this frequency
range several astrophysically important lines occur, including [NII] and
[CII].

Most receivers currently in operation at millimeter and submillimeter-wave
observatories utilize simple, single-ended, DSB mixers. For our array
development work we will utilize a balanced mixer architecture. Balanced
mixers have two principal advantages: (1) sideband noise from the LO is
suppressed (the sideband noise can get significant with the large
multiplications required for THz LOs), and (2) all the available LO power
is coupled into the mixer, eliminating the need for cumbersome, lossy,
band-limiting, quasi-optical diplexers (e.g. Martin-Puplett or Fabry-Perot
interferometers). The main disadvantage of balanced mixers compared to
traditional single-ended mixers is that they require two mixing devices
operating in quadrature. However, with modern micromachining and
photolithographic processing techniques it is now possible to fabricate
entire balanced mixer receivers in a single mixer block and achieve the
device uniformity required for proper operation.

\begin{figure}[!hbt]
\centering
\includegraphics[angle=0,scale=0.7]{balancedmixer.eps}
\includegraphics[angle=0,scale=0.25]{Bias-IF-Block2.10.1.03-1.eps}
\begin{minipage}{7in}
\parbox{3.9in}{a)}
\parbox{2.5in}{b)}
\end{minipage}
\caption{\small \label{balancedmixer} a) Overview of
    balanced mixer topology: the sky and LO signals are coupled to the
    individual mixers through a quadrature hybrid.   IF output is
    combined either by opposite biasing of the two mixers, or with a
    180$^o$ IF hybrid. b) Pictoral representation of how the balanced
    mixer topology will be realized in an array mixer.}
\end{figure}

One common topology of balanced mixers is one in which the signal and
the LO are coupled  
to the individual mixers through a quadrature hybrid (see
Figure~\ref{balancedmixer}a). A 180 degree IF  
hybrid then combines the IF output of the two mixers, so that all the
down-converted 
signal appears in one port and all the LO noise appears at the other
port. It is possible  
to omit the IF hybrid and simply connect the two IF ports in parallel,
if the two mixers are biased with opposite polarity \citep{kerr03}. 

Figure~\ref{balancedmixer}b is a pictoral representation of how the balanced
mixer topology will be realized in an array mixer. A scalar feedhorn
residing in the telescope's focal plane efficiently couples energy into
full height rectangular waveguide. A branchline coupler \citep{clau00}
functions as the 90$^\circ$ hybrid. The waveguide output of the feedhorn
goes into one input port of the coupler and LO power into the other. Each
output port of the hybrid contains both signal and local oscillator power.
These are mixed by either an SIS or HEB device (depending on the design
frequency) fabricated on a thin ($\le 2$~$\mu$m) SOI beamlead substrate
that partially extends across each output waveguide. LO power enters the
mixer block from the back and is distributed to each mixer using waveguide
power dividers. Since the `rear' of the mixer block contains the LO power
distribution system, the IF output of each mixing device is conveyed via
coplanar waveguide on silicon (in the 1.5~THz block) or a microstrip
PC board (in the 810~GHz block) to bonding pads on the `front' (horn side) 
of the mixer. An IF distribution board is bump-bonded to the front of
the mixer block (see Figure~\ref{subassembly}). Clearance holes are
drilled in the board
at the location of each horn aperture. The mixer block/IF distribution board
assembly is mounted onto an array motherboard containing mixer 
bias and IF power combining circuitry. The power-combined IF outputs are  
amplified by low-noise, MMIC devices also mounted on the
motherboard. The mixer 
and IF amplifier operate at 4~K. A novel microstrip ribbon cable carries 
the IF's from all 8 mixers in the array to a break-out board located on the 
80~K radiation shield of the array cryostat. A 3-D CAD rendition of the 
array subassemblies is shown in Figure~\ref{subassembly}. Below we discuss 
how the subassemblies will be fabricated.


\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{4.3in}{\includegraphics[angle=0,scale=0.35]{STARs3D3.9.30.03-4.eps}}
\parbox{1.9in}{\caption{\small \label{subassembly}  CAD representation of an array subassembly.}}
\end{minipage}
\end{figure}

\subsection{Waveguide Mount}   

Excellent progress in the development of submillimeter-wave SIS and
HEB mixers has been demonstrated in recent years. At frequencies below
800~GHz these mixers are typically implemented using waveguide
techniques, while above 800~GHz quasi-optical (open structure) methods
are often used. In many instances the use of waveguide
components offers advantages over quasi-optical techniques. The principal
advantages of waveguide are:

\begin{enumerate}

\item Due to its larger cross sectional area, waveguide has a lower loss 
per unit length than any other transmission line. At any frequency
they can be built, waveguide mixers are capable of providing the
lowest noise performance. 

\item High efficiency, $\sim 99$\%, feedhorns can be used to couple energy
in and out of waveguide circuits. In comparison, the dielectric
lens-planar antennas used with quasioptical mixers typically have an
efficiency $\leq$ 89\% \citep{gol98}.  

\item As discussed above, balanced mixers offer several significant
advantages for array applications. When implemented in waveguide, the
mixing devices can be configured so they can be reversed bias relative to
each other. With this arrangement the IF output of each mixing device can
be combined in a simple microstrip Wilkinson power combiner, obviating the
need for a bulky IF hybrid \citep{kerr03}. In quasioptical balanced mixers
the mixing devices are biased in series. In this instance, an IF
hybrid is required, making it difficult to realize compact, integrated
arrays.
 
\item With waveguide, over half a century of technological developments
can be brought to bear on a problem. Recent advances in micromachining
techniques, together with the availability of sophisticated E\&M modelling
software permit the scaling of successful waveguide well into the
far-infrared \citep{wal98}.

\end{enumerate}


\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.8in}{\includegraphics[angle=0,scale=0.3]{branch_coupler.eps}}
\parbox{2.8in}{\caption{\small \label{coupler} SEM imaging of a 1.5~THz branchline coupler based on the ALMA design, fabricated in 2003 using the laser micromachining system at the University of Arizona.}}
\end{minipage}
\end{figure}


At THz frequencies, the hardest part of realizing a balanced, waveguide
mixer design is the input quadrature hybrid. A 3 dB, 90 degree hybrid can
be realized in waveguide using a branch-line coupler. The branch-line
coupler consists of two or more quarter wavelength sections (branches)
that bridge across a pair of waveguides, between the broad walls.  A
six-branch, branch-line hybrid with a percentage bandwidth of $\sim 30$\%
has been designed and tested at WR-10 for ALMA receivers [4]. The width of
the branch-lines at 100 GHz was 15 mils. Scaling these dimensions to 1.5
THz, the width of the required branch lines is $\sim$1.0 mils. Using the
laser micromachining system at the University of Arizona, we have
succeeded in fabricating a 1.5 THz branchline coupler based on the ALMA
design. A photograph of a laser micromachined 1.5~THz split-block, 
quasioptical power combiner that utilizes a branchline coupler is shown 
in Figure~\ref{coupler}. An array
of 1.5~THz corrugated feedhorns made with the same system is shown in
Figure~\ref{feedhorns}. With the laser micromachining system these
structures are etched in silicon and then plated with several skin depths
of gold to make them behave as traditional hollow waveguide. At the
University of Massachusetts a complementary micro-milling system has been
developed which is also capable of machining similar waveguide structures
directly in copper \citep{nara99}. The two 
micromachining approaches are quite complementary. The laser micromachining
system can make small waveguide structures to micron accuracy, but has 
relatvely low volumetric removal rates. It is best suited for making
waveguide structures above 1~THz. The UMass system is not optimized
for making structures as small, but has higher volumetric removal rates. 
In the proposed development effort, the University of Arizona will 
laser machine the 1.5~THz mixer array. The University of Massachusetts will 
use their micromilling system to fabricate the 810~GHz array.

\begin{figure}[!htb]
\begin{minipage}{7in}
\parbox{4.1in}{\includegraphics[scale=0.5,angle=0]{corrugated.eps}}
\parbox{2.4in}{\caption{\label{feedhorns} \small SEM of three
laser-machined 1.5~Thz corrugated horns in a $1\times10$ array,
demonstrating the accuracy and consistency of the Arizona
micromachining system.}}
\end{minipage}
\end{figure}

\subsection{Mixing Devices}

\subsubsection{Probe Design}

Over the years a number of waveguide to microstrip transitions have been
proposed; most of which are implemented in reduced height waveguide with
$<$35\% RF bandwidth \citep{Tong96}.  Unfortunately reducing the waveguide
height makes machining of mixer components at THz frequencies more
difficult. It also increases RF loss as the current density in the guide
is increased. An additional disadvantage of existing high frequency
waveguide mixers is the way the active device (SIS, HEB, Schottky diode)
is mounted in the waveguide. Traditionally the junction, and its
supporting substrate, is positioned in a narrow channel across the
waveguide. This structure forms a partially filled dielectric waveguide,
whose dimensions must kept small to prevent energy from leaking out the
channel. At frequencies exceeding a THz this mounting scheme becomes
impractical. Because of the fabrication and mounting challenges,
quasi-optical mixers are typically used at the shorter wavelength.
However, published results from quasi-optical THz mixers have all been
single pixel, and their designs are not readily scalable to the kind of
imaging arrays we are proposing to build.

We believe that to move forward into the THz regime a much more integrated
approach is required. To do so however one has to consider the
requirements of the actual mixing device. For frequencies $>$1~THz
phonon-cooled, hot electron bolometer (HEB) mixers have demonstrated the
best performance \citep{khos03,yngv03}.

The IF bandwidth of phonon-cooled HEBs is directly
related to how rapidly accumulated heat can be dumped to the thermal bath.
Therefore the device mounting must provide an efficient heat conduction
path. Secondly, HEB mixers exhibit extremely large instantaneous bandwidth
at signal frequencies. Though advantageous in many respects, one has to be
careful to avoid saturating the device with incident thermal radiation. A
waveguide structure naturally does this.  Thirdly, HEB devices produced to
date have not proven to be very uniform across a wafer. Thus basing an
entire focal plane array on the uniformity of single wafer is risky. A
better approach is to have a design of many densely packed HEB mixer chips
that (after processing) can be RF pre-selected. The added advantage of
such a design is that the chips can be used in any type of mixer
configuration, e.g. array, balanced or sideband separating.

\begin{figure}[!hbt]
\centering
\begin{minipage}{7in}
\parbox{3in}{\includegraphics[angle=0,scale=0.3]{beamlead-topview.eps}}
\parbox{3in}{\caption{\small \label{topview} Top view of the silicon
    beamlead chip. Note how the photonic crystals (just 4~$\mu$m in
    height) help disperse the E-field leakage outside the
    waveguide. The beamlead chip is suspended in 6~$\mu$m of ``air''. }}
\end{minipage}
\end{figure}

Fortunately, development in several key areas make it now practical to
consider integrated imaging arrays for the THz regime. For one, laser
micromachining techniques allow successful waveguide designs to be scaled
to THz frequencies \citep{wal98}. Secondly, GaAs beamlead techniques in
high frequency multipliers have been used with great success
\citep{schle01,chatto02}. We propose to use a similar beamlead approach
for mounting HEB (and SIS) devices in our micromachined mixer blocks.
Because of its excellent thermal properties and high device yields, we
will make our beamlead device substrates out of silicon. The silicon
beamlead substrates will provide an excellent cooling path for both HEB
and SIS devices.  Thirdly, a very broad bandwidth full-height waveguide to
thin film microstrip transition has recently been reported on by
\citet{kooi03}. The combination of a 45\% RF bandwidth and full-height
waveguide turns out to be ideal for HEB THz mixer designs. Fourthly,
extensive membrane experience \citep{koo98} has shown the importance to
suspend the substrate. Fifthly, photonic crystals junctions
\citep{hesler01} can be used to suppress stray (or scatter) fields.
Finally, progress in electromagnetic field simulation tools such as HFSS
\citep{ansoft02} has given us an important tool in not only optimizing,
but also analyzing the designs against misalignment in any special
dimension. It should be noted that our design approach is equally well
applicable to SIS arrays for frequencies below a THz. In the latter case
the magnetic field required to suppress the Josephson currents will have
to be applied uniformly across the entire wafer (HEB mixers do not require
a magnetic field making them attractive mixing devices for larger scale
spectroscopic imaging arrays). The UMass team will use Ansoft 
Maxwell 3D modelling software to design efficient magnetic field 
concentrators for the SIS array mixer block. 

The results of an HFSS simulation of a 1.8~THz HEB silicon beamlead device
suspended across full height waveguide is shown in Figure~\ref{topview}. 

The silicon chip (described in Section~\ref{art}) vertical alignment is
set by the beamleads.  Extensive HFSS computer simulations show the chip
height variation to be very tolerant (Figure~\ref{losses}), and in any
event to be much larger that the specified ``SOI''
(Si~``device-layer''/insulator~``thin''~/Si~``handle'') substrate
variation of 0.5~$\mu$m.  Sideways alignment of the beamlead chip is set
lithographically, and is not an issue (small fraction of a wavelength).
For the 1400--2200~GHz design, the silicon chip is suspended by 6~$\mu$m
of space on both the top and bottom giving it an effective dielectric
constant of 3.2. We anticipate using a combination of wet etching,
reactive ion etching (RIE), and laser silicon micromachining techniques.
All dimensions are well within the capabilities of these techniques.  
Test chips with less than 10~$\mu$m of clearance have been fabricated and
yield no handling problems.

\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{4.2in}{\includegraphics[angle=0,scale=0.29]{losses.eps}}
\parbox{2.5in}{\includegraphics[angle=0,scale=0.29]{impedance.eps}}
\end{minipage}
\caption{\small \label{losses} {\bfseries [LEFT]:} Input return loss for
    the silicon beamlead chip as the substrate thickness is increased
    from 3.5~$\mu$m to 6~$\mu$m. In each case the backshort position was
    adjusted to optimize the results. It is evident that the silicon
    substrate height variation is very non-critical as long as the
    backshort depth is compensated accordingly. {\bfseries [RIGHT]:}  
    Beamlead chip input
    impedance. The impedance locus is centered around 40~$\Omega$, which is
    determined by the structure. Varying the substrate height has
    little effect on the probe impedance, e.g. performance.}
\end{figure}



\subsubsection{\it Beamlead Device Fabrication}
\label{art}
%        ********* Art's Section ****************       

At UVA we have developed a high yield, precise quartz beam lead
process for SIS millimeter array applications \citep{bass03,bass02}. We are
currently working toward a Si based thin-chip beam lead process
suitable for the micromachined HEB and SIS receiver applications discussed in
this proposal. While our existing quartz based beam lead process uses
a dicing saw and lapping to define the finished chip dimensions, the
requirements of our proposed THz elements require much greater
dimensional control than these techniques could afford.   


\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.2in}{\includegraphics[angle=0,scale=0.3]{art-layers.eps}}
\parbox{3in}{\caption{\small \label{art-layers} UVA SOI process for
    fabricating beam lead ultra thin Si mixers: (a) the beam leads and
    mixer circuitry are first fabricated atop an SOI wafer,  (b) the
    SOI wafer is then mounted device side down on a glass carrier
    wafer and (c, d) the handle silicon removed,  (e) the oxide layer
    is patterned and etched and (f) then used as a mask for an
    anisotropic Si reactive ion etch to precisely define the chip
    perimeter and then removed, and (g)  the chips are complete
    following separation from the carrier wafer. }}
\end{minipage}
\end{figure}

Our Si based approach begins by first fabricating the desired
superconducting circuits atop the device layer of a ``SOI''
(Si'device-layer'/insulator'thin'/Si'handle') substrate.  We have access
to SOI wafers with Si device layer thickness between 0.5 and 10~$\mu$m
with a tolerance of better than 0.5$\mu$m.  The remaining beam-lead
process is shown in Figure~\ref{art-layers}. The beam leads are next
fabricated using our established thick resist and plating process.  The
SOI chip is then mounted, device side down, atop a quartz carrier wafer
using a clear mounting wax that has a melting point of $\sim$100C.  
Mounting the wafer with a transparent wax and carrier wafer allows for a
subsequent backside lithographic alignment process to properly orient the
wafer with respect to the circuit and beam lead structures for the
subsequent precise chip perimeter lithographic and RIE steps. A
combination of mechanical lapping and chemical etching is next used to
remove the thick bulk ``handle'' silicon layer. The revealed oxide layer
is next patterned using backside photolithographic alignment and RIE
etching.  The patterned oxide then serves as an etch mask for a reactive
ion etch of the exposed device silicon.  The device silicon is etched
through to the quartz carrier and gold beam leads, thereby precisely
defining the individual chips.  The chips are then removed from the
carrier wafer by dissolving the mounting wax.

To date we have developed an SOI wafer process \citep{bass03b}
that is compatible
thermally and chemically with the processing of the HEBs and mixer
circuitry on the SOI wafer. {\bfseries Our initial efforts have
  demonstrated that we are able to fabricate chips with this process and
handle Si chips with a single thickness (no frame) as thin as 1~micron as
shown in Figure~\ref{SEM}}.  We have gone to pains to develop this process
so that it will be compatible with our existing beam lead technology.
Likewise, this process should also be compatible with our established HEB
processes.

We propose to integrate this new SOI process with our existing beam-lead
technology and our proven ability to fabricate HEB and SIS structures in
order to fabricate ultra-thin chip beam-lead mixer elements suitable for
the proposed integrated focal plane arrays. We have previously
demonstrated the ability to form high yield beam lead structures on
quartz. We have also an established HEB fabrication process using a
focused ion beam (FIB) tool to carve the HEB structure
\citep{date99,date01}. We have recently also developed a more traditional
Ebeam resist process for fabricating HEB structures \citep{bass03c}. It
should be noted that these processes are suitable for defining both phonon
cooled and diffusion cooled HEB geometries. Our UHV magnetron sputtering
system is also equipped for sputtering Nb and NbTiN films for these
devices. In support of our ongoing array development efforts, our
colleagues at Moscow State Pedagogical University have recently deposited
high quality NbN films on SOI wafers for device fabrication at UVa.
Devices made using Moscow State films are in wide use and have excellent
performance.

With this SOI process we have excellent control of both final chip
thickness and lateral dimensions to better than 0.5$\mu$m. Final chip
thickness can also be measured in our laboratory to better than 100nm
accuracy. Coupled with the capabilities of Arizona's and UMass'
micromaching tools in forming the remaining waveguide blocks and
'accepting' fixturing for the beam lead arms, along with the use of
UVA's FIB system for local trimming of the critical waveguide block
features, we should be able to use this beam lead thin-chip technology
with integrated receiver arrays to great advantage. 

\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.25in}{\includegraphics[angle=0,scale=0.28]{SEM-HEB.eps}}
\parbox{3in}{\includegraphics[angle=0,scale=0.57]{prototype.eps}}
\end{minipage}
\caption{\small \label{SEM} {\bfseries [LEFT]:}  SEM image of an  HEB
    structure formed with our focused ion Beam (FIB) process.  It
    should be noted  that these processes are suitable for defining
    both phonon cooled and diffusion cooled HEB geometries. 
    {\bfseries [RIGHT]:} The chip is a
    prototype for future THz chip designs that will feature non-linear
    superconducting mixing elements, microwave circuitry, as well as
    thick gold beam leads. The gold beam leads will provide electrical
    and thermal contact for the chip. The beam leads will also serve
    as a mechanical support, allowing the chip to be suspended in the
    middle of a waveguide. }
\end{figure}

\subsection{Local Oscillators}

\subsubsection{LO Power Distribution}

With an array receiver LO power must be efficiently distributed between
pixels. The balanced mixer architecture being proposed here is ideal for
this purpose, allowing efficient, broad-band, waveguide power splitters to
be utilized. A schematic representation of the proposed LO power
distribution system is shown in Figure~\ref{LOsplitter}. LO power is
coupled into the array via a scalar feedhorn. The power is then subdivided
into 8 equal parts by a `tree' network of 7, E-plane, Y-junctions 
\citep{nara02, kerr01}.

All the mixer and LO waveguide components are machined together in a
single split block structure. This is true whether the waveguide is
laser machined in silicon or micromilled in copper. Guide pins are
used to ensure alignment between the two halves of the block. 


\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{4.2in}{\includegraphics[angle=0,scale=0.2]{STARs3D3.9.30.03-1.eps}}
\parbox{2.2in}{\caption{\small \label{LOsplitter} CAD representation of
the LO power distribution system.  LO power is coupled into the array via
a scalar feedhorn, then subdivided into 8 equal parts by a
`tree' network of 7, E-plane, Y-junctions. }}
\end{minipage}
\end{figure}


\subsubsection{SIS -- 810~GHz Array}

The PI's team has recently completed construction of a 4-pixel, 810~GHz
array receiver (PoleSTAR) for the AST/RO telescope at the South Pole
\citep{wal01}. A 
photograph of PoleSTAR mounted on the focal plane of AST/RO is provided in
Figure~\ref{polestar}.  A solid-state, JPL 810~GHz LO chain, a by-product
of the {\it Herschel} program, is used with the array. The LO produces so
much output power ($\sim 0.25$~mW) that even after being split 4 ways a 
simple, Mylar beam splitter can be used to inject the LO into the signal 
beams. With the beamsplitter, only $\sim 3$\% of the LO power ($\sim 2\,\mu$W)
actually makes it into each mixer. In contrast, the balanced mixer design 
being proposed here can theoretically couple LO power into the mixer with 
an efficiency approaching 100\%. There could be as much as $\sim 12$~dB
of loss in the waveguide power splitter and there would still be enough 
LO power to efficiently drive all 8 mixers. LO power levels can be 
controlled by adjusting bias voltages on the multiplier or slightly detuning 
the Gunn oscillator driving the chain.



\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.5in}{\includegraphics[angle=0,scale=0.35]{polestar.eps}}
\parbox{2.9in}{\caption{\small \label{polestar} PoleSTAR, the PI's
four-pixel, 810~GHz array receiver mounted at the AST/RO telescope at the
South Pole.  Optical path through the focal plane reimaging optics is
highlighted in yellow.  The gold anodized solid state JPL 810~GHz LO chain
is seen in the foreground.  }}
\end{minipage}
\end{figure}

 
\subsubsection{HEB - 1.4 - 2.2~THz Array}

The laser LO system for the 1.4 - 2.2 THz array has already been supplied
by NASA/GSFC and is in operation in the PI's lab (Figure~\ref{laser}; see
supporting letter from Co-I G.~Chin). The FIR laser system was designed
and built by DeMaria ElectroOptics Systems (DEOS, now part of Coherent),
of Bloomfield, Conn. specifically for use as a THz LO source on {\it
SOFIA}. It is a self-contained, computer controlled, turn-key system
capable of providing 5-10~mW of LO power at the frequencies of interest
\citep{muel03}, orders of magnitude more than needed to pump the proposed
array. The output power of the DEOS/Coherent laser can be set under
control computer.  

\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.7in}{\includegraphics[angle=0,scale=0.65]{DEOSLaser.eps}}
\parbox{2.7in}{\caption{\small \label{laser} DEOS laser system for the 
1.4--2.2~THz array, supplied by NASA/GSFC and in operation in the PI's 
lab. }}
\end{minipage}
\end{figure}


\subsection{IF/Bias Distribution system}

\subsubsection{IF Amplifiers}

As discussed above, IF outputs from each balanced mixer are power combined 
on the array motherboard and then amplified
by indium phosphide (InP) MMIC amplifier modules. These amplifier modules have been designed
and fabricated by Sander Weinreb's group at Caltech. 

During the past  4 years wideband, very low noise, cryogenic monolithic
microwave integrated circuit (MMIC) amplifiers have been developed with
design and testing at JPL and Caltech and foundry fabrication at TRW (now
Northrop Grumman Space Systems,  NGST) and HRL.  These LNA's utilize 
0.1~$\mu$m gate length, high electron-mobility transistors 
(HEMT's) and match the requirements needed for densely packed focal plane 
arrays in terms of noise temperature,  chip size,  DC power dissipation, 
yield, and bandwidth.  

Prototype chips tested at 12~K and higher temperatures are available with 
frequency ranges of 1 to 12~GHz,  1 to 60~GHz, and 1 to 110~GHz with noise 
temperature increasing with frequency. The 1 to 12~GHz chip is 
most appropriate for this application because of its match to the
IF bandwidth of SIS and HEB devices and its very low noise and power 
consumption.   A typical 1-12~GHz  amplifier is described in 
Figure~\ref{IFamps}.  The chip
achieves noise temperature of $<$5~K consuming 20~mW  of power at 12~K when 
driven from a 50 ohm generator impedance.   The input match of the LNA is 
close to 50 ohms over the 4 to 12~GHz range;  this is often required for 
stable operation with an SIS mixer (but not with an HEB mixer where a lower 
frequency IF is advantageous).   We propose to optimize the LNA using 
external microstrip
input circuit design and chip revisions if needed,  for the proposed mixers 
in terms of frequency range, power consumption at 4~K, input match, and noise 
temperature.

In addition to the MMIC design an important issue for this program is compact 
packaging  compatible with large focal plane arrays. A custom designed 
ribbon cable with alloy conductors 
for low thermal conductivity and ground-signal-ground coplanar waveguide 
output transmission lines will be designed and tested for cross-talk and 
impedance match.  The "ground" conductors can be at RF ground but also serve 
as DC conductors for transistor and mixer bias. Miniature strip connectors 
with 0.5~mm conductor spacing are available.  We believe this  strip MMIC IF 
amplifier approach can be realized with pixel spacing in the 2 to 5~mm range. 
An important parameter is the isolation of the amplifier input and output.  
We will design appropriate shields and absorbers for this purpose.

It is entirely feasible to integrate a line array of IF amplifiers  in a 
larger chip with 4, 8, or 16 amplifiers.  We do not propose implementing 
this in our prototype program but will investigate this possibility.



\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.8in}{\includegraphics[angle=0,scale=0.4]{IF-performance.eps}}
\parbox{2.5in}{\caption{\small \label{IFamps} Performance of the 
4--12~GHz IF amplifiers produced by S. Weinreb's group at Caltech.  Noise 
temperatures are $\le $5~K, with gains of $\sim$35~dB when cooled to 12~K.}}
\end{minipage}
\end{figure}


\subsubsection{Array Bias Control System}

A computer controlled array bias system has already been developed by the
PI's group at Arizona and is currently being used with their 4-channel,
SIS 810~GHz array (PoleSTAR) on the AST/RO telescope at the South Pole and
a 7-channel, 345~GHz SIS array (DesertSTAR) soon to be installed on the HHT.
The system allows manual tuning of each mixer and amplifier, plus
automated bias point selection.  All voltages (and currents) are
controlled and monitored through a flexible graphical user interface (GUI)
written in C and controlled by a single Linux-based PC.  We will build a
16 channel version of this system to allow simultaneous operation of the
810~GHz SIS and ~1.5~THz HEB arrays.

\subsection{Implementation on AST/RO}

Once the arrays have been fully characterized in the laboratory, we plan
to implement them on the AST/RO telescope at the South Pole (see
support letter from AST/RO PI Stark). The 
atmospheric transparency of the site, together with its mechanical and
optical design, make AST/RO an excellent testbed for new THz technologies.
AST/RO is the home of the first 810 GHz SIS array receiver (PoleSTAR) and
the first 1.5 THz HEB receiver (TREND). The PI's team played a key role in
the design and construction of both prototype systems. The proposed effort
is a natural extension of these earlier efforts.

\subsubsection{Array Spectrometer}

AST/RO has 6 Cologne AOSs, each of which can provide 1 GHz of bandwidth.
We will build an IF processor that will take the 16 IF outputs from the
array and downconvert them so that three of them will share a single AOS.
The resulting bandwidth of each pixel is 330 MHz at 1 MHz resolution. This
translates into 123 km/s of velocity coverage at a resolution of 0.4 km/s
for the 810~GHz array and 67~km/s of velocity coverage at 0.2~km/s
resolution for the 1.5~THz array. These bandwidths and resolutions are
suitable for a wide variety of Galactic astronomy projects; from large
scale mapping of molecular clouds to searching for emission from
protoplanetary disks. 

\subsubsection{Array Cryostat}

Before building the prototype arrays, we will conduct performance
tests on single channel versions of the SIS and HEB array mixers 
using a conventional wet dewar.
Our ultimate goal is to implement the arrays on the Nasmyth focus of
AST/RO (see Figure~\ref{integration}). 
For this we will need a closed-cycle cryostat
capable of 4~K operation with very low microphonics. Commercially
available (CryoMech Inc.) pulse-tube refrigerators are now available that
meet these requirements.  These refrigerators have mounting rings similar
to that of a standard CTI-350 coldhead. For this project, we will modify
an existing Precision Cryogenics dewar to accommodate the pulse tube
refrigerator. The load capacity of the pulse tube at 4K is more than
adequate to cool both array mixers and the first stage IF amplifiers.
If necessary, a small Helium 4 reservoir can be added to the system
to ensure thermal stability. 


\subsubsection{Optics}

The array will be bolted on the Nasmyth focus of AST/RO as shown in
Figure~\ref{integration}. This position was chosen because of the greater
field of view it provides.  The converging {\it f}/6.5 telescope beam
passes through an AR coated, crystalline quartz vacuum window and IR
blocking filters before being split by a polarizing wire grid mounted on
the 4~K coldplate. One polarization is reflected into the 810~GHz array.
The orthogonal polarization passes through the grid and is incident on the
1.5~THz array.  By employing polarization diplexing it is possible to
observe at both frequencies simultaneously. Given the variability of the
1.5 THz atmospheric window, this is a significant advantage. Not only will
it be possible to observe in two lines at once, but the arrangement
permits the radio pointing obtained in the much more transparent 810 GHz
atmospheric window to be readily applied to the 1.5~THz array. With the
proposed balanced mixer approach, LO power is coupled into the mixer via
waveguide, significantly reducing the complexity of the fore-optics.


\begin{figure}[!htb]
\centering
\begin{minipage}{7in}
\parbox{4.1in}{\includegraphics[scale=0.2,angle=0]{ASTRO-3D-3.eps}}
\parbox{2.3in}{\caption[Integration of the Array on
    AST/RO]{\label{integration} \small
AST/RO optical path to the proposed heterodyne receiver array at the
Nasmyth focus}} 
\end{minipage}
\end{figure}

\section {Project Management}

\subsection{Organization}

The STARs team brings together the instrument design, development,
and management experience of Co-Investigors from 4 universities and two NASA
centers. Our team of Co-Is has worked together on a number of successful
projects in the past and is looking forward to continuing our collaboration
through the proposed effort. The organizational structure of the STARs
project is shown in Figure~\ref{orgchart}. A description of the role
each team member will play is provided below. 

\smallskip

\noindent
{\bf PI Walker (Univ. of Arizona):} The proposed effort will be a major 
component 
of the PI's research over the next three years. He will
have overall responsibility for the project and coordinate
activities between the various groups. 

\smallskip
\noindent
{\bf Co-I Chin (NASA/GSFC)} comes to this project with extensive
experience as a project scientist for SWAS.  He 
will provide the DEOS laser LO for the
1.5~THz array. 

\smallskip
\noindent
{\bf Co-I Kawamura (JPL)} has a great deal of expertise in the area
of phonon-cooled HEBs and will participate in the modeling,
fabrication, and testing of devices. 

\smallskip
\noindent
{\bf Co-I Kooi (Caltech)} is a world leader in the design of submillimeter 
mixers and
receiver systems. He will perform extensive simulations to optimize the
performance of the arrays and will participate in their performance
characterization and integration on the AST/RO telescope. 

\smallskip
\noindent
{\bf Co-I Kulesa (Univ. of Arizona)} brings experience with the bias 
electronics and software used for submillimeter heterodyne arrays and has
taken part in the integration of such instruments at AST/RO and the HHT.
He will apply this experience to the development of similar systems 
for the proposed arrays.

\smallskip

\noindent
{\bf Co-I Lichtenberger (Univ. of Virginia)} leads the team at the 
University of 
Virginia which has considerable experience in the fabrication of SIS and,
more recently, HEB devices. His team will be responsible for making the high
quality SIS and HEB beamlead devices on thin SOI membranes. 

\smallskip 

\noindent
{\bf Co-I Narayanan (Univ. of Mass.)} has played a key role in the 
development of the
micromilling machine at the University of Massachusetts and has
performed and extensive study of HEB and SIS balanced mixers. He will
lead the effort to machine the 810~GHz SIS array mixer. 


\smallskip

\noindent
{\bf Co-I Weinreb (Caltech)} will lead the effort in developing 
the intermediate
frequency amplifiers and distribution system for the proposed arrays. His team
at Caltech has already developed IF amplifier modules that meet many of
our design requirements.



\begin{figure}[!htb]
\centering
\begin{minipage}{7in}
\parbox{4.7in}{\includegraphics[angle=0,scale=0.4]{orgchart.eps}}
\parbox{1.8in}{\caption{\small \label{orgchart} The STARs organizational chart
    shows clear lines of accountability.}}
\end{minipage}
\end{figure}



\subsection{Plan of Action}

A chart showing project milestones and responsibilities is provided below.


\begin{table}[!hbt]
\centering
\renewcommand{\arraystretch}{1.04}
\begin{tabular}{|l|c|c|c|c|c|}
\hline 
 & {\bf \large Arizona} & {\bf \large UMass} & {\bf \large UVa} & {\bf \large Caltech} & {\bf \large JPL} \\
\hline 
{\large \bf YEAR 1}:  Optimize Single Pixel & & & & & \\

\hline

$\bullet$  Complete single-pixel mixer models in HFSS & \bigX & \bigX & & \bigX & \bigX \\

$\bullet$ Design IF motherboard using ADS  & \bigX & & & \bigX & \\

$\bullet$  Fabrication of first beamlead HEBs \& SIS's & & & \bigX & & \\

$\bullet$  Laser-micromachine 1.5 THz mixer blocks & \bigX & & & & \\

$\bullet$  Machine 810~GHz mixer blocks & & \bigX & & & \\ 

$\bullet$  Fabricate single channel IF  & \bigX & & & \bigX & \\

$\bullet$ Assess single pixel performance at 810~GHz \& 1.5~Thz& \bigX & \bigX & & \bigX &  \bigX \\
 \ in test dewar & & & & & \\

\hline 
{\large \bf YEAR 2}: Array Implementation & & & & & \\
\hline 

$\bullet$ Construct array bias control system  & \bigX & & & & \\

$\bullet$  Complete array mixer models in HFSS & \bigX & \bigX & & \bigX & \bigX \\

$\bullet$  Optimization of HEB \& SIS beamlead devices & & & \bigX & \bigX & \\

$\bullet$ Optimize design and fabricate array motherboard & \bigX & & & \bigX & \\

$\bullet$  Build array cryostat & \bigX & & & & \\


\hline
{\large \bf YEAR 3}: Array Integration and Testing & & & & & \\
\hline 

$\bullet$  Mount 810~GHz \& 1.5~THz arrays in cryostat   & \bigX & & & &  \\

$\bullet$  Fabrication of spare HEB \& SIS devices & & & \bigX & & \\

$\bullet$  Build optics and mount for AST/RO & \bigX & & & & \\

$\bullet$  Characterize performance of array at Arizona & \bigX & \bigX & &  \bigX &  \bigX \\

$\bullet$  Prepare instrument for shipment to AST/RO& \bigX & \bigX & & \bigX &  \bigX \\
 \ in following year  & & & & & \\
\hline
\end{tabular}
\end{table}


\clearpage

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\end{thebibliography}

\clearpage

\noindent{\large \bf FACILITIES, EQUIPMENT \& OTHER RESOURCES}

\subsection*{ \it University of Arizona, Steward Observatory}

In 1992 the PI established a laboratory
(the Steward Observatory Radio Astronomy Laboratory, SORAL) for
the development of state-of-the-art submillimeter-wave receiver systems.
The laboratory contains (or has ready access to) all the equipment
(spectrum analyzers, network analyzers, vacuum pumps, cryogenic
support facilities, etc.) needed for the development of receivers.

The NSF Division of Astronomical Sciences awarded
the PI funds to construct a 7 element array receiver at 345 GHz.
This array will begin full operation on the HHT in October 2003.
In the past, SORAL has produced facility 230 and 492 GHz receivers
for the HHT and, recently, was the lead group in the design and
construction of a dual channel 492/810 GHz receiver and 4 pixel
810 GHz array receiver (PoleSTAR) for AST/RO at the South Pole.
The 492/810 GHz receiver has been in operation for 5 years and is the primary
facility receiver for the telescope. The 810 GHz array was commissioned
in November 2000 and is the first array to go into operation in this
important atmospheric window.  In Fall 2002, SORAL worked
very closely with Sigfrid Yngvesson at UMass in preparing a
single pixel, 1.5 THz HEB receiver for integration onto AST/RO.  This
instrument, TREND, is now in operation.  The experience
SORAL personnel have gained in the design and deployment of these
receiver systems will be fully utilized in the construction of the
proposed prototype systems.

As a result of its successful collaboration with Lincoln Laboratory
in applying laser micromachining technology to the fabrication
of THz components, the Physical Foundations of Enabling Technologies
Program in the Engineering Division of the NSF awarded the PI
a grant to construct a laser micromachining facility at Steward Observatory.
This system is now fully operational.
The design of this facility is based on the successful Lincoln
Laboratory system and optimized for fabricating the mixer arrays described
in this proposal. The availability of a state-of-the-art laser micromachining
system at Steward Observatory provides the community with a {\it powerful,
new} ability to develop a wide variety of needed THz components.
Photolithographic facilities are also available at Steward Observatory
and the Electrical Engineering Department for fabricating
the wet-etched portions of the focal plane arrays.

Through a grant through the Defense University Research Instrumentation
Program (DURIP), SORAL has recently purchased a new laser for the
micromachining system, an e-beam evaporator for metallizing micromachined
components, and a thin film deposition system for depositing nitride
and other films.

SORAL also has licenses for
both Ansoft's High Frequency Structure Simulator (HFSS) and
Agilent Advanced Design Systems (ADS) software packages. These programs
are used to accurately model and optimize mixers and other
crucial receiver components.

\subsection*{\it University of Virginia}

The Superconductor Device Laboratory (SDL) operates within the Applied 
Electrophysics Laboratory (AEpL) in the Department of Electrical 
Engineering at the University of Virginia. The primary facility 
consists of a 6,000 square foot laboratory, of which roughly two-thirds 
is clean room space. This facility is indicative of the
institutional support that the SDL enjoys as a result of our long term
commitment to excellent research in solid state technology, and also to
education. The AEpL program involves graduate and undergraduate
students, approximately thirty-five of which are currently involved
in ongoing research. 

The laboratory is fully equipped for the investigation and fabrication 
of complex Nb superconductive circuits. These processing capabilities include: 
UHV loadlocked 3" DC magnetron sputtering of aluminum, 
niobium and gold in a dedicated trilayer system, high vacuum loadlocked 
3" magnetron sputtering of insulators and metals, UV and deep UV contact 
photolithography with sub-micron resolution, a  Karl Zeiss 982 field 
emission scanning electron microscope with 1 nm resolution, two fluorine 
and a chlorine based reactive ion etchers, plasma etching,  a multisource 
Temescal E-beam vacuum evaporation, dedicated ion milling station,
and electrochemical plating and etching.  The laboratory also has two 
high speed dicing saws, a computer controlled Tencor surface profiler, 
an FSM laser based thin film stress measurement system, and a Westbond 
wirebonder. 

The SDL also benefits greatly from cooperative research efforts with
other departments. In particular the Materials Science and Electrical 
Engineering Departments share an FEI-200 focused ion beam system (\$500K). 
Also, a cooperative research program with the Department of Physics has 
led to the establishment of our Far Infrared Receiver Laboratory. This 
facility is fully equipped to design, assemble and evaluate millimeter 
and submillimeter wavelength mixers and multipliers. Sources include 
two submillimeter wavelength gas laser systems (300~GHz - 4.5~THz) and a 
variety of millimeter wavelength sources, multipliers and amplifiers from 
Millitech, J.E. Carlstrom and Radiometer Physics GmbH. A Bruker IFS 66V 
Fourier Transform Infrared Spectrometer (120~GHz - 225~THz) is available 
for materials and component evaluation, as well as HP network analyzers 
to 100~GHz (8510 and 8720s), spectrum analyzers to 110~GHz, 
and a variety of power meters, microscopes and probe stations.
Computer systems include two HP 735 Apollo Workstations each running
Hewlett-Packard's high frequency structure simulator and microwave design 
software.

\subsection*{\it University of Massachusetts}

The University of Massachusetts (UMass) operates a 14~m millimeter
telescope facility called the Five College Radio Astronomy Observatory
(FCRAO). In collaboration with the Instituto Nacional de Astrofisica,
Optica y Electronica (INAOE), Mexico, UMass is also building a 50
meter telescope called Large Millimeter Telescope/Gran Telescopio
Milimetrico (LMT/GTM) in Puebla, Mexico, to operate between 1 and 3~mm
wavelength bands.  In support of the operation of the FCRAO 14~m, and
to develop receiver systems for the LMT/GTM. we have three radio
astronomy laboratories spread over the functionalities of heterodyne
receivers (Millimeter Wave Instrumentation Laboratory), continuum
receivers (Cryogenic Device Laboratory), and a digital technology
Laboratory (involved in High-speed autocorrelator spectrometer
work). The Millimeter Wave Lab has state-of-the-art facilities for the
development of millimeter, submillimeter and terahertz receivers and
local oscillator systems and backend spectrometers. The laboratory
contains all the equipment (spectrum analyzers, network analyzers,
vacuum pumps, cryogenic support facilities, etc.) needed for the
development of receivers.  The NSF Division of Astronomical Sciences
funds the operation and instrument development program on the FCRAO as
a university radio astronomy facility.  
%Historically, the FCRAO has
%been a leader in low noise receiver systems using Schottky diode
%mixers for frequencies from 80 to 600 GHz. 
The list of instrumental
accomplishments of the radio group include QUARRY (QUabbin ARRaY), the
first fully integrated mm-wave array receiver, SEQUOIA (SEcond QUabbin
Observatory Imaging Array), currently the world's fastest 3~mm array
receiver that uses MMIC preamplifiers, and development of receivers
for SWAS (Submillimeter Wave Astronomy Satellite). Current projects
also include receiver development for SOFIA, the NASA/DARA Airborne
Observatory, and the HIFI system on the FIRST spacecraft.
% Sorry Gopal!  I had to nuke these words to make page requirements...

%, primarily in
%work on submillimeter local oscillators. The laboratory has also been
%involved in the design and fabrication of mixer-block elements for a
%seven element 345 GHz focal plane array receiver (Desert STAR) for the
%Heinrich Hertz Telescope (HHT), on Mount Graham, Arizona. Some of the
%recent research activities in the Millimeter Wave Lab include the
%development of a novel dual-polarized, sideband-separating, balanced
%230 GHz SIS receiver system for the LMT, and in the development of THz
%HEB mixer technology for arrays.  
The experience of FCRAO personnel,
gained in the design and deployment of these receiver systems will be
fully utilized in the construction of the proposed prototype systems.

UMass also has licenses for both Agilent's and Ansoft's High Frequency
Structure Simulator (HFSS), Ansoft's Maxwell 3D, CST Microwave Studio
(a time-domain structure simulator), Advanced Design System (ADS), and
Microwave Design System (MDS) software packages. These programs are
used to accurately model and optimize mixers, multipliers, and other
crucial receiver components.

Micro-assembly facilities include three long working distance high
resolution stereo microscopes (Nikon SMZ-10 and SMZ-U), two very high
resolution metallurgical scopes.

UMass radio astronomy has a full machine shop with a Bridgeport 2 axis
CNC milling machine, a Haas Mini Mill 3-axis CNC milling machine, jig
bores, jeweler's lathe and conventional mills, lathes and
grinders. UMass radio astronomy also has a special purpose high-speed
micro-NC machine that is custom-built for the fabrication of high
frequency waveguide components. This custom-built micromilling machine
employs high-speed pneumatic spindles, precision positioners, custom
milling software, a multi-tool holding capability, and has been used
for fabricating successful waveguide blocks for frequency multipliers
and mixers up to 2.5 THz.

\vfill
\eject


\end{document}
