DTIC ADP011774: Laser Micromachining of THz Components

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ADPO 11774

TITLE: Laser Micromachining of THz Components
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TITLE: International Conference on Terahertz Electronics [8th], Held in
Darmstadt, Germany on 28-29 September 2000

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Laser Micromachining of THz Components

Christian Drouet d'Aubigny, Christopher Walker, Bryan Jones, Christopher Groppi, John Papapolymerou

Abstract - Laser micromachining techniques can be used to
fabricate high-quality waveguide and quasi-optical
components to micrometer accuracies. Successful GHz
waveguide designs can be directly scaled to THz
frequencies. We expect this promising technology to allow'
the construction of the first fully integrated THz heterodyne
imaging arrays. At the University of Arizona, construction
of the first laser micromachining system designed for THz
waveguide components fabrication has been completed.
Once we have tested and characterized our system we will
use it to construct prototype THz 1x4 focal plane mixer
arrays, AR coated silicon lenses, THz LO sources, phase
gratings and more. The system can micromachine
structures down to a few microns accuracy and up to 6
inches across in a short time. This paper discusses the design
and performance of our laser micromachining system, and
illustrates the type and range of components this exciting
new technology will make accessible to the THz community.

Introduction

Laser processing offers many advantages over
conventional machining of micrometer sized components
[i]. Thanks to the ability to finely focus laser light,
smaller features can be achieved with improved
tolerances. Because chemically activated laser etching is
a non contact process, there is no mechanically induced
material damage, no hard to remove particulate residues,
and no tool wear or machine vibration. Laser fabrication
therefore yields finer finishes, improved accuracy, and
lower process overheads. The chemical activation on
which this process is based minimizes the etching energy
requirement and therefore reduces the potential for
cracking [1].

Laser Micromachining Principles

An Argon-Ion laser is used to heat a microscopic portion
of the silicon substrate in a chlorine ambient. At the onset
of melting, volatile silicon chlorides are formed. The
highly non-linear activation energy of the process
confines etching to a molten zone only a few microns
across. Crystalline materials have the benefit that un-
etched portions of the molten zone grow back epitaxially,
allowing controlled shavings to be removed plane by
plane. Structures can thus be built by limiting the etch
depth at each scan plane, to typically 1 jam, (see Fig. 1).

C. Drouet d’Aubigny, C. Walker, B. Jones and C. Groppi are with
Steward Observatory, The University of Arizona, Tucson, AZ 85721,
USA

J. Papapolymerou is with The Department of Electrical and Computer
Engineering, The University of Arizona, Tucson, AZ 85721, USA

direction of scan

Fig. 1: Schematic representation of laser etching of

silicon in chlorine ambient. Using high
numerical aperture (NA) optics the reaction can
be confined to a region only a few micrometer
in size. The obvious trade off of high NA is a
tapering of the beam that can be significant for
some applications eg. Vertical walls [1].

At Steward Observatory we have built a laser
micromachining system that follows the successful
Lincoln Laboratory design and is optimized for THz
applications.

Fig. 2: Schematic of Steward Observatory k laser

micromachining system

In our design the 18W Argon-Ion laser beam is expanded
to 16mm, then deflected, using a commercial X-Y
galvomirror scanner, onto achromatic scanning lens. The
focused beam is then introduced through a fused silica
window into a stainless steel reaction chamber containing
the sample (see fig. 2).

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The wafer surface is typically biased to 100°C using an
IR illumination source shining through a second window
on the back side of the reaction chamber (sec Fig 3).

Fig. 3: Detailed view of the galvomirror scanner,

reaction chamber, 1R heater and X-Y-Z motion
stages

The process is monitored through the focusing optics
using a CCD with a plate scale of 7 pm per pixel. The
scanning system is driven directly from computer
generated patterns which can be constructed using
Autodesk's AutoCAD. The ensemble is mounted on
computer controlled X-Y-Z precision motion stages (sec
Fig. 3) allowing the stitching of large structures.

Before operation the cell is evacuated, then filled with
99.9 % pure chlorine gas to 100 Torr. After 2 hours of
machining the remaining chlorine gas and silicon
chlorides arc vented through a scrubbing bubbler before
release in the atmosphere. Figure 4 shows the system in
our laboratory. The chlorine and nitrogen gas cylinders
arc stored in the gas cabinet on the right. The central
hood houses the laser, reaction chamber, and optics. The
vacuum pump and chlorine scrubbers arc contained in the
small gas cabinet on the left.

Fig. 4: Ensemble view of the Steward Observatory laser

micromachining system

Figure 5 shows the electronic shutter and beam expander
portion of the optical system with the laser turned on.

Fig. 5: Laser, shutter and beam expander assembly

Results and Possible Applications

The laser micromachining system will permit the direct
scaling of a wide variety of waveguide and optical
structures to THz frequencies. One such device is a
“Magic-T”. Figure 6 is a conceptual design for a
0.85 THz mixer that coherently combines the signals
from two independent telescopes using a Magic T before
downconversion. The local oscillator is injected using a
microm a chined directional coupler. An array of such
mixers is shown in Figure 6. We plan to propose to build
such an instrument [3] for use on the Large Binocular
Telescope now being constructed on Mount Graham,
Arizona.

Fig. 6: Beam combiner and LO injection element

design (top). Proposed beam combiner array for
the LBT (bottom).

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Figure 7 is a conceptual drawing of an integrated,
niicromachincd, 2 THz array receiver being developed
for SOFIA, the Stratospheric Observatory for Far Infrared
Astronomy [3]. Test feedhorns for the array (Fig. 8) were
fabricated using the parent laser microtnachining system
at Lincoln Laboratory and successfully tested at Steward
Observatory [4].

STAR Mixer Assembly Diagram

Feedhorn Block

Bolometer Array Block

Backshort Block

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Fig. 7: SOFIA 2 THz, 4x4 array design

Fig. 8: Feedhorn produced at Lincoln Laboratories.

Conclusion

Laser etching not only makes the construction of THz
waveguide arrays tractable but it is also ideally suited to
make submillimeter phase gratings, high efficiency
feedhorns to replace Winston cones in large bolometer
arrays, AR grooving in silicon lenses and more. Optical
diagrams, waveguide pictures and conceptual diagrams
are available at:

http://soral.as.arizona.edu/micromachining.html

Acknowledgements

This project made possible by a Collaborative Research
Agreement (CRDA) between the University of Arizona
and MIT Lincoln Laboratory, was supported through
NSF grant #9800260

References

1. Theodore M. Bloomstein, 1996, Ph-D Thesis, MIT

2. C. Y. Drouet d’Aubigny et al, 2000, SPIE Proceeding, “Radio
Telescopes”, Vol. 4015, p268

3. C. K. Walker et al, 2000, SPIE Proceeding, “Airborne Telescope
Systems”, Vol. 4014, pi 25

4. C. K. Walker et al, 1997, 8 th Int. Symp. on Space Terahertz
Technology, p358.

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