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\begin{document}

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

\large

\noindent
{\bf PI Walker:} 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.
He will spend $\sim$2 months per year working on the project.
However, due to the PI's salary being maintained by his 
academic appointment, only graduate student support is being
requested for this project.

\smallskip

\noindent
{\bf Co-I Lichtenberger} leads the team at the University of 
Virginia and will combine the HEB device expertise from 
the UMass group with their on processing expertise to 
make high quality arrays of HEBs.  He requests 5\% support per year
for these efforts. 

\smallskip

\noindent
{\bf Co-I Yngvesson} has a great deal of expertise in the area
of phonon-cooled HEBs and will participate in the modeling,
fabrication, and testing of devices.  He requests 5\% support per year
for these efforts. 

\smallskip 

\noindent
{\bf Co-I Kulesa} brings experience with the bias electronics, control
systems, and software used for submillimeter and THz frequency
heterodyne arrays and has partaken in the integration of such an
instrument (PoleSTAR) at the AST/RO telescope.  He will participate in kind
with the development of the proposed THz array.  His salary is being
provided by Steward Observatory.


\smallskip
\normalsize
\clearpage

\medskip
\centerline {\Large \bf Development of Integrated Array Building Blocks} 
\centerline {\Large \bf for Future Large Format Heterodyne Systems} 
  
\section{Introduction}
           
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, digital signal processing, and telescope design made the 
construction of such an instrument tractable. Here we propose
to implement these technologies to develop the first integrated, far-infrared
heterodyne array.  The array will utilize NbN and NbTiN
phonon-cooled 
Hot Electron Bolometers (HEB) in an efficient, micromachined, waveguide
mount.  HEB devices are inherently broadband, require very little local 
oscillator (LO) power and do not require a magnetic field 
for operation.  These properties make
HEBs ideal for array applications. The noise performance of HEBs starts
to become competitive with SIS mixing devices at $\sim 1$~THz and HEBs are
the most sensitive mixer technology above 1.4~THz (Yngvesson 2003). It
is in this frequency range where laser micromachining can be used to fabricate
efficient, compact arrays of feedhorns and waveguide structures
\citep{wal98}. The goal of the proposed effort is to
demonstrate the viability of large format arrays in the FIR by developing
a sensitive, single pixel 1.5 THz receiver that can serve as a
building block for arrays and, once successfully tested, use this
design to build a prototype 
1x10 mixer array. Ultimately, the array would be used on the AST/RO telescope
at the South Pole to perform [N~II] observations of southern molecular clouds. 
Once such an array has been demonstrated, a technological roadmap to 
achieving the goal of large format, far-infrared (FIR) heterodyne arrays will 
become clear.

We have assembled a team with experience in each aspect of the proposed
investigation. Co-I Yngvesson (UMass) is an expert in the field of
phonon-cooled 
HEBs and will work closely with Co-I's Lichtenberger and Kooi in developing
the optimum mixer design. Co-I Kooi (Caltech) will perform HFSS
simulations of the 
mixers. Co-I Lichtenberger's team at UVa will be responsible for fabricating
the HEBs from NbN and NbTiN films. The micromachined 
waveguide mounts, IF/bias electronics, cryogenics, optics, and IF processor 
will be the responsibility of the PI's group, including co-I Kulesa,
at the University
of Arizona. In addition, Karl Jacobs at the University of Cologne has agreed 
to provide high quality NbTiN films to the UVa team for patterning into HEBs.
Members of this team have worked together in the past on a number of
successful projects, including a 1.5 THz single pixel receiver (TREND)
for AST/RO, a 4 pixel, 810 GHz array SIS receiver (PoleSTAR) for
AST/RO, and a 7 pixel, 345 GHz array (DesertSTAR) for the Heinrich
Hertz Telescope. In each case, this was the very first instrument of
its kind.  The experience gained in building these instruments will be
fully utilized in the development of the proposed integrated array. 


\section {Scientific Motivation}

\subsection {High Resolution, Spectroscopic Imaging Arrays in the Far-Infrared}
 
In the wavelength regime between 300 and 60 microns there are a number
of 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.  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{3.5in}{\includegraphics[angle=0,scale=0.32]{ISM.eps}}
\parbox{2.8in}{\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 wavelengths 
    play pivotal roles in shaping the relevant physical processes.}}
\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, 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. 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 1x10 array for AST/RO
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 SARA program guidelines, the 
proposed work will 1) increase array format size, 2) reduce detector noise,
3) provide new fabrication and formatting techniques, and 
4) extend wavelength coverage. 


\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.8in}{\includegraphics[angle=0,scale=0.9]{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}


\section {Technical Description}

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

\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 efficiently pump more than
   one or two detectors at any given time.

\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}

\noindent However, through the arduous efforts of many researchers in the
field, most of these hurdles have now been overcome. In the following
sections, we will describe plans for an instrument that combines a number 
of leading edge technologies to make a compact, sensitive 
submillimeter heterodyne array. 

\subsection {\it State-of-the-Art of Phonon-Cooled HEBs (PHEB) THz Heterodyne Receivers}

Phonon-Cooled Hot Electron Bolometer (PHEB) heterodyne detectors for
 the frequency range 1 THz to 2.5 THz have recently developed into a
 well established technology for designing low-noise receiver systems
 for that frequency range. The first  such receiver system on a radio
 telescope  detected an astronomical spectral line from the
 Insterstellar Medium (CO,   in 1998. Recently (October 2002 to
 February 2003), a team of researchers based at the University of
 Massachusetts, Amherst, with collaborators at UMass/Lowell,
 University of Arizona, the Harvard/Smithsonian Center for
 Astrophysics, and CalTech, installed TREND (Terahertz REceiver with
 NbN Device), an HEB receiver system for 1.25 THz to 1.5 THz at the
 AST/RO submillimeter wave telescope at the US South Pole Station. The
 TREND team included several of the investigators behind the present
 proposal. This receiver is ready to start observations now that
 periods of  ultra-low precipitable water vapor weather will occur
 during the austral winter season  
 UMass was one of the first groups to start developing the NbN HEB
 technology (1994), and thus has extensive experience in this
 field. NbN HEB devices in particular have a number of significant
 advantages: 

\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.5in}{\includegraphics[angle=0,scale=0.41]{perf.eps}}
\parbox{3.5in}{\includegraphics[angle=0,scale=0.43]{S12.eps}}
\end{minipage}
\caption{\small \label{perf} [LEFT]: Receiver noise
    performance of the HEB device for TREND. [RIGHT]: PHEB
    sensitivity to LO power and bias voltage. }
\end{figure}


\begin{enumerate}

\item A low receiver noise temperature over a wide tunable frequency
range. Measured data with the twin-slot antenna are shown in
Figure~\ref{perf} [LEFT]. 
Similar bandwidth can be expected in the waveguide version. A
waveguide mounted NbN HEB device was recently tested by 
Kawamura et al (2003) and yielded comparable receiver noise
temperatures over a similar frequency range. 

\item NbN PHEB devices are very insensitive to changes in applied LO
  power (= bias current) or bias voltage, as shown in Figure~\ref{perf}
  [RIGHT].  

\item The device size is not as small as that of diffusion-cooled
  devices. A typical optimum size is 0.4 ?m (length) x 4 ?m
  (width). The devices are thus relatively easy to fabricate with high
  yield. E-beam lithography is preferred.  

\item The LO power required for a device of the size in 1) is of the
  order of a few hundred nW, which is sufficiently low to be supplied
  by either multiplier or laser sources. 

\item The (THz) impedance is easily adjusted to the required value
  which matches an antenna or waveguide circuit. In particular, the 40
  ohm impedance of the waveguide structure presented in this proposal
  can be met by a slight widening of the device.  

\item The IF bandwidth at which the noise temperature has doubled
  compared with low IF frequencies (the "Receiver Noise Temperature
  Bandwidth") is about 5.5 to 6 GHz for device as has been shown by
  recent measurements at UMass.  

\item NbN films are available from an experienced source (Moscow State
  Pedagogical University), and are under development at the Univ. of
  Virginia. NbTiN films are another, more recent alternative, which
  have shown roughly similar performance. UVa is also developing the
  capability to deposit these films. In the interim, Karl Jacobs at
  the University of Cologne (see commitment letter) has agreed to
  provide them.  

\item The UMass group is developing a series of diagnostic and
  modeling techniques funded by a contract from NASA Langley Research
  Center, which can be used in the proposed effort. 

\item Silicon is a preferred substrate, and is compatible with the
  technology proposed here. The silicon substrate need only be 1 to
  2~$\mu$m thick to guarantee normal functioning of the HEB.  

\item UMass/Amherst is using a flexible laser system and the group is
  experienced with  testing of all aspects of HEB devices over a wide
  range of THz frequencies.   

\end{enumerate}


\subsection{\it Beamlead HEBs in Waveguide Mounts}

%        ********* Jacob's Section ***************

\subsubsection{\it Necessity and Design Concept}
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 though, the use of waveguide
components offers certain advantages over quasi-optical techniques as
they are based on well-established microwave practices. Some examples
include: Focal plane array receivers with feedhorns that produce
well-defined on axis Gaussian beam patterns, balanced receivers that
efficiently use all of the available LO power (no need for beam
splitter or narrowband diplexer/etalon), the construction of sideband
separation receivers, and the possibility of continuous comparison
(correlation) receivers.  


\begin{figure}[htb]
\centering
\begin{minipage}{7.5in}
\parbox{4in}{\includegraphics[angle=0,scale=0.32]{beamleadchip.eps}}
\parbox{2.4in}{\caption{\small \label{beamleadchip} Rendering of the
    beamlead chip seated in a silicon carrier block. For the 1800 GHz
    center design, the chip width is 50$\mu$m and has a nominal thickness
    of 4-6~$\mu$m. The IF/Bias lines use part of the RF choke as a ground
    layer and are separated from the ground plane by a 0.2$\mu$m SiO
    insulating layer. }}
\end{minipage}
\end{figure}

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 terahertz
frequencies very difficult if not impossible. 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, Schottkey 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 terahertz 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 been all single pixel, and their designs are not necessarily
scalable to the kind of imaging arrays we are proposing to build.  

We believe that to move forward into the teraherz regime a much more
integrated approach is required. To do so however one has to consider
the requirements of the actual mixing device, in this case a phonon
cooled hot electron bolometer (HEB). First of all, for the response
time to be fast (IF) there has to be a thermal bath for the phonon's
to escape into. This excludes the use of micron sized silicon-nitride
membranes \citep{kooi01}. Secondly, HEB mixers exhibit extremely large
instantaneous bandwidth. 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}[htb]
\centering
\begin{minipage}{7in}
\parbox{4in}{\includegraphics[angle=0,scale=0.33]{E-field.eps}}
\parbox{2.3in}{\caption{\small \label{Efield} Electric Field
    distribution in the plane of the silicon beamlead chip. Note that
    even though there are strong fields in the space surrounding the
    chip, there is very little leakage, e.g. the coupling efficiency
    is better than 90\% over the entire TE10 mode waveguide band. }}
\end{minipage}
\end{figure}

Fortunately, development in several key areas make it now practical to
consider integrated imaging arrays for the teraherz regime. For one,
laser micromachining techniques allow successful waveguide designs to
be scaled to terahertz frequencies \citep{wal98}. Secondly, GaAs beamlead
techniques in high frequency multipliers have been used with great
success \citep{schle01,chatto02}. 
There is no reason to believe the same the same won't
hold for silicon as GaAs and silicon have very similar physical
properties. 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.
And 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 the design is equally well
applicable to SIS arrays for frequencies below a terahertz. 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).  

\begin{figure}[htb]
\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}

\subsubsection{Model Results}


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}



\subsection{\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 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{SEM}. 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
structures in order to fabricate ultra-thin chip beam-lead HEB 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.  

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 laser based
micromaching tool 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=0.39]{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 {Micromachined Waveguide Mounts}

HEBs have been successfully used in both quasi-optical and waveguide
mixer mounts \citep{bur99, kaw01}. When waveguide and quasi-
optical mixers are made at the same frequency with comparable mixing devices, 
laboratory measurements show waveguide mixers outperform quasi-optical 
mixers both electrically and optically \citep{koo98,koo94,wal92}. 
This occurs for two
reasons: 1) because
waveguide has an intrinsically lower loss than the microstrip
structures used in
planar designs, and 2) waveguide feedhorns can couple
to the fundamental Gaussian mode with a ~98\% efficiency, compared
to the $\leq$ 89\% efficiency of dielectric lens-planar antennas
\citep{gol98}. 
 
At wavelengths shorter than 400~$\mu$m (800~GHz), rectangular waveguide
structures, feedhorns, and backshorts become extremely difficult to
fabricate using standard machining techniques. We have used a new laser
milling technique to fabricate high quality, THz waveguide components and
feedhorns. Once metallized, the structures have the properties of standard
waveguide components \cite{wal98}. With this process, waveguide
components of varying height and width can be machined quickly
and at low cost to an accuracy of $\sim 1$~$\mu$m.  
{\it By combining this new laser micromachining process with HEBs, 
the construction of high performance, large format, waveguide 
imaging arrays at THz frequencies becomes possible}.

As a demonstration of the impact of laser micromachining technology, 
single proof-of-concept 1.5~THz corrugated feedhorns and 1x10 arrays
of 1.5~THz corrugated horns were fabricated 
using the laser micromachining system at the University of Arizona. Figure~\ref{lasermm} is an electron microscope image of a 1.5 THz corrugated feedhorn,
with $\lambda$/4 waveguide matching to full-height rectangular
waveguide.  Figure~\ref{lasermmfig} demonstrates the repeatability and
consistency of the laser micromachining process.  The entire array of
feedhorns was etched in less than 2 hours. 
 
\begin{figure}[!htb]
\begin{minipage}{7.0in}
\parbox{3in}{\includegraphics[scale=0.9,angle=0]{1500ghz_corrugated.ps}}
\parbox{3.4in}{\caption{\label{lasermm} \small SEM of portion of
a prototype 1.5 THz feedhorn and waveguide structure. The feedhorn and matching
waveguide are together 2~mm long.  The structure was etched using 4.3
Watts of laser power focused into a 6 $\mu$m spot in 200 Torr of
chlorine gas.  The laser beam was scanned at 5~cm/s and incrementally
moved 2~$\mu$m  between line scans.  Under these conditions,
nominally 1~$\mu$m shavings are removed per pass of the laser over the
surface.  The total etch time is one hour, not including the overhead
time for pattern generation and stage motion.}}
\end{minipage}
\end{figure}

As a test of our fabrication and assembly process, we performed beam
pattern measurements on a laser micromachined 2~THz corrugated
feedhorn \citep{wal98}. The differences between the theoretical and
measured beam profiles were within the resolution limit and amplitude
sensitivity of the test set-up.

Waveguide surface roughness values measured with atomic force
microscopy are typically on the order of 200 nm RMS. This surface
quality is already sufficient to provide low-loss waveguide
performance to $\ge 10$~THz.  The RMS surface roughness can be reduced
even further, to under 25~nm, using standard polishing etches based on
HF-HNO$_3$-HC$_2$H$_3$O$_2$ solutions.

Our initial investigations into the application of laser micromachining
were conducted under a Collaborative Research and Development Agreement (CRDA) 
between the PI (at the University of Arizona) and Lincoln Laboratory.
One purpose of the CRDA is to transfer technology, such as the 
micromachining process described here, out of national laboratories into
the private sector. Based upon the success of this collaboration,
the Engineering Division of the National Science Foundation, NASA
SARA program, and, most recently, Army Research Office have awarded
funds to construct a laser micromachining system
optimized for the fabrication of THz waveguide and optical components
at Steward Observatory. This system is now fully operational and we are
ready to use it for the development of imaging arrays.

\subsection{ \it Focal Plane Array Concept}

All waveguide components
for the arrays will be micromachined out of silicon and 
assembled into an integrated package. The array design and 
fabrication can be readily scaled for use at shorter wavelengths.
To illustrate how 2D arrays can be fabricated with this technology,
a conceptual design for a $4\times 4$ array of micromachined
fixed-backshort, waveguide mixers is shown in Figure~\ref{focalplane}. The
focal plane array consists of a ``Horn Block'', ``Bolometer Block'',
and ``Backshort Block''.

\begin{figure}[!htb]
\begin{minipage}{7in}
\parbox{4.1in}{\includegraphics[scale=0.65,angle=0]{corrugated.eps}}
\parbox{2.4in}{\caption{\label{lasermmfig} \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}

\vspace{-0.2in}

\subsubsection{Horn Block}

The Horn Block to be demonstrated in this proposal 
is assembled from a single $1\times 10$ corrugated feedhorn
subarray, formed by bonding two gold plated, silicon
wafers in which mirror images of the corrugated feedhorns have been
laser micromachined. Alignment between the two halves of a horn
subarray is insured by posts and corresponding holes wet-etched in
each half before laser machining. Assembly of the package
will be done with the aid of an infrared microscope and micromanipulator
system designed originally for flip-chip bonding. 
Each horn is micromachined with a circular-to-rectangular waveguide
transition to match the input impedance of the HEB.


\subsubsection{Bolometer Block}

The Bolometer Block serves as the carrier for the beamlead devices
and the `PC board' on which DC bias is applied and IF power conveyed from
the HEB. It too is made from silicon. The window and slots across which
the HEB is mounted is formed using the laser micromachining system.
If during assembly there is a small gap between the Bolometer, Horn, or 
Backshort Blocks, significant signal could
be lost due to the generation of unwanted substrate modes. In the 
array design, we have reduced this effect dramatically by using a
suspended stripline RF choke design similar to that of Blundell \& Tong
and micromachining a novel RF choke structure \citep{hes01} at the
Horn and Junction Block interface. With these chokes in place, more
than 1~mil (milli-inch) separation can be easily tolerated between 
the Horn, Bolometer, and Backshort Blocks. The choke and waveguide
probe designs were optimized at both 1.5 and 5~THz by collaborator
Kooi using Hewlett-Packard's High Frequency  Structure Simulator (HFSS). 

\begin{figure}[!htb]
\centering
\begin{minipage}{7in}
\parbox{4.4in}{\includegraphics[scale=0.4,angle=0]{Fig7-color.eps}}
\parbox{2in}{\caption{\label{focalplane} \small Assembly Drawing of a
    1.5~THz Array Mixer Block}}
\end{minipage}
\end{figure}


\begin{figure}[htb]
\centering
\includegraphics[angle=0,scale=0.45]{Assembly2.eps}
\caption{\small \label{assembly} Exploded view of an individual array mixer assembly.}
\end{figure}

An exploded view of an individual mixer block is shown in Figure~\ref{assembly}. 
The feedhorns and waveguide backshorts will be micromachined and gold plated 
by the Arizona group using existing facilities. The fabrication of the 
array Bolometer Block, including the HEB, waveguide probe, and IF output lines,
will be a joint effort between the UofA and the UVa.

For heterodyne arrays efficient telescope illumination can be achieved with a
$2f\lambda$ separation ($\sim 3\omega$) between feedhorns in the
telescope focal 
plane. At the Nasmyth focus of AST/RO this separation is $\sim 3.6$~mm.
Since all the mixers share a common 
Bolometer Block, crosstalk between array pixels could be a concern. 
Using HFSS, we have carefully constructed a 2x2 simulation of an array
with only $\sim$1~mm separation 
and found the crosstalk to be down below the -35~dB level across 
the band. 
The separation between array elements in the proposed
array is three times as great, suggesting crosstalk in the Bolometer
Block will not be a significant problem.

The IF output of each mixer is ultimately conveyed to wire bonding pads
on the periphery of the bolometer block by CPW lines. 
The IF impedance of HEBs provides a good, broadband match to 50~$\Omega$ 
transmission line, so no sophisticated IF impedance transformers are required. 


\subsubsection{Backshort Block}

The fixed rectangular backshort is laser micromachined on a pyramidal
structure designed to fit the cavity behind the membrane. The
pyramidal structures can be readily made by wet-etching silicon
through an SiN mask evaporated on the wafer \citep{reb87}. Once
etched, the Backshort Wafer is gold plated. With this design
the performance of the array can be optimized in the lab by trying
different backshort lengths.

\subsection{IF/Bias Distribution system}

\subsubsection{IF Processing and Backend Spectrometers}

Unlike the situation often encountered in conventional single pixel
and array receivers, in the proposed system it is the choice of the IF 
frequency and bandwidth that determines the size and cost of the array camera. 
In the proposed design the waveguide mixers are micromachined out of silicon
using computer controlled laser micromachining and wet etching techniques.
With this approach it costs the same to make $\sim$4 mixers as to
make just one. In both SIS and HEBs fabrication runs, typically 100's of 
devices are made at once on a single wafer. Since HEBs require
$\sim$30 times less LO power than SIS devices, a commercially available
FIR laser system can easily drive the entire array.

The additional cost per pixel is driven by the cost of the 
IF components and the spectrometer needed to process the signal.
Fortunately these costs are also dropping. For the [\nii], H$_2$D+,
and CO 11-10 transitions, laser lines 
yielding IF frequencies of 1.7 GHz (Yngvesson, private communication), 
7.64 (Borieko \& Betz 1993), and 5.2 GHz (Chin 1980) are available. 
IF frequencies up to 7~GHz have been demonstrated to yield low noise
response in NbN phonon-cooled HEBs (Yngvesson et al., 2002). 
Wideband, low-noise amplifiers are now available with sufficient
bandwidth to cover all three IF frequencies (Weinreb 2003).  

In an earlier effort, the PI and his collaborators designed and built
a 4 pixel, 810 GHz array receiver. This system has a complete IF
processing system, including a 4 x 1 GHz array AOS provided by the
University of Cologne. We plan to utilize this system for
field testing the proposed array.  Funds for a $10 \times 2$~GHz correlator
system to service the full array are being requested in a separate
grant. 

\medskip

\noindent {\bfseries With the proposed approach, there is only a small cost
increase in going from a single to multipixel system.}

\medskip

\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 (Pleiades)
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 10 channel
version of this system for the proposed array.

\subsection{Local Oscillator}

The laser LO system for the array will be supplied by NASA/GSFC as a result
of an SBIR development awarded to DeMaria
ElectroOptics Systems (DEOS, now part of Coherent), of Bloomfield, CT, for 
the design and construction of a waveguide CO$_2$-pumped submillimeter local
oscillator suitable for use on SOFIA (see supporting letter from
G. Chen).  The high output power of the SIFIR
laser will provide ample LO power to pump HEB arrays of $>$100 pixels.


\subsection{Array Cryostat}

Before building the 1x10 array, we will conduct performance
tests on single channel versions of phonon cooled NbN and
NbTiN HEBs at UMass using a conventional wet dewar.

A new, larger cryostat will be needed for 
housing the 1x10 array. Our ultimate goal is to use the array on
the Nasmyth focus of AST/RO (see Figure~\ref{integration} below, and
accompanying letter from A. Stark, AST/RO PI). For this
we will need a closed-cycle cryostat capable of 4~K operation
with very low microphonics. Commercially available (CryoMech Inc.)
are now available that meet these requirements. These referigerators
have mounting rings similar to that of a standard CTI 350
coldhead. For this project, We will modify an existing Precision 
Cryogenics dewar to accomodate the pulse tube referigerator.
The load capacity of the pulse tube at 4K is {\it more than adequate}
to cool both the mixers and the first stage IF amplifiers.

\begin{figure}[!htb]
\centering
\begin{minipage}{7in}
\parbox{3.8in}{\includegraphics[scale=1,angle=0]{Fig11.eps}}
\parbox{2.3in}{\caption[Integration of the Array on
    AST/RO]{\label{integration} \small
AST/RO optical path to the proposed hetrodyne receiver array at the
Nasmyth focus}} 
\end{minipage}
\end{figure}


\subsection{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. From the warm receiver room (located just below
the telescope) optical constraints limit the size of
the array to 3x3. Although the proposed 1x10 array has only one
more pixel, it has the advantage of being extensible to 100 or
more elements. The instrument will be completely enclosed in a 
housing that will keep it at a suitable operating temperature.

The telescope beam first reflects off a secondary and tertiary 
mirror before it passes through a silicon beam splitter (Walker et
al., 2001). The beam
splitter injects the local oscillator beam from the FIR laser
(located in the warm receiver room) into the focal plane array. 
The horns are already spaced to provide adequate coverage of the
focal plane. Only a dielectric lens is required to make the 
final match between the telescope beams and the array feedhorns.


\section {Educational Impact of Proposed Research}

\subsection{University of Arizona}

Almost all the instrumentation designed and fabricated in 
the Steward Observatory Radio Astronomy Lab (SORAL) was done 
by students under the supervision of the PI. Past Ph.D. grads
from the lab include Jason Glenn (now a faculty member at
U.C. Boulder) and Gopal Narayanan (faculty member at U. Mass.).
Currently, there are 4 grad students and 2 undergrads 
working in the lab on a wide range of instrumentation projects. 
When appropriate, the students use the instruments they 
have built toward their undergraduate or graduate degrees.
Even though the salary of only one Arizona graduate student is 
requested, all the students will benefit from the project.

The PI is now in the process of organizing an `Astro-Technology'
initiative at the University of Arizona. The goal is to build
a cross-disciplinary program that facilitates
the exchange of faculty members, postdocs, students, ideas,
and physical resources between departments (e.g.
Astronomy, Planetary Sciences, Optical Sciences, and Electrical
Engineering) interested in applying leading-edge technology to the
solution of astrophysical problems. The proposed instrument will
serve as a model for this initiative, illustrating how diverse 
technologies can be fused to provide a new, powerful instrument 
for observational astronomy.

\subsection{University of Massachusetts}

The subcontract to the University of Massachusetts will contain funds
to support a graduate student to work with Prof. Yngvesson in designing
and characterizing prototype HEB beamlead mixers. The student will
work closely with fellow graduate students both at the University
of Arizona and Virginia. It is likely that this student will also
participate in field testing the array on AST/RO.


\subsection{University of Virginia}

The subcontract to the University of Virginia will be used to
partly fund a graduate research assistant. The student will
work with Dr. Lichtenberger and other team members to
develop the optimum strategy for fabricating and assembling
the micromachined components into a working mixer.


\section{Plan of Action}


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

\hline

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

$\bullet$  Fabrication of first beamlead HEBs & & & \bigX & \\

$\bullet$  Test single pixel performance & & \bigX & & \\

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

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

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

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

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

$\bullet$  Help test performance of array at AZ & \bigX & \bigX & & \\

\hline
{\large \bf YEAR 3}: Field Testing at AST/RO & & & &\\
\hline 

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

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

$\bullet$  Install array at AST/RO & \bigX & \bigX & & \bigX \\

\hline
\end{tabular}
\end{table}



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\end{thebibliography}

\clearpage

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

\large 
\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.

\eject

\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 Microwave and Electronics Laboratory (MEL) within the Department of
Electrical and Computer Engineering at the University of Massachusetts, has an
internationally recognized research and teaching program in the area
of microwave  
engineering, and solid state devices. Most of the proposed effort will
be performed 
within LAMMDA (Laboratory for Millimeter Wave Devices and
Applications). A brief 
description of available resources, relevant to this proposal, follows.
Microwave equipment, including several HP 8510 automatic network analyzers,
one of which is usable to 60 GHz, is routinely used by graduate
students, as well as a Cascade wafer probe, with probes to 40 GHz. We
also have sources, and waveguide 
equipment, for frequencies up to 110 GHz. Microwave integrated
circuits can be quickly 
fabricated in-house..
Measurements at low temperatures can be performed either in LN2 or LHe
dewars, or in 
a CTI Model 350 mechanical refrigerator. One LHe dewar incorporates a
superconducting magnet for fields to 5 T.
Micro-fabrication facilities include a submicron mask aligner (Karl Suss
MJB3UV), a multi-target sputtering system (Sputtered Film, Inc., S-Gun
Turbosystem, 
Series II), and E-beam evaporator (CHA Industries, SE-600), a plasma
reactive ion etcher 
(Microscience, Inc.), and a scanning electron microscope (AMRAY 1810D).
A CO2 laser-pumped sub-mm gas-laser is available, with associated
instrumentation, such as Spectrometer, power meters, detectors,
interferometers, etc. A Fourier-Transform Spectrometer can be used for
measurements from the Near IR to THz. 
\vfill
\eject


\end{document}




