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Defense Technical Information Center 
Compilation Part Notice 

ADPO 11774 

TITLE: Laser Micromachining of THz Components 
DISTRIBUTION: Approved for public release, distribution unlimited 

This paper is part of the following report: 

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

To order the complete compilation report, use: ADA398789 

The component part is provided here to allow users access to individually authored sections 
of proceedings, annals, symposia, etc. However, the component should be considered within 
the context of the overall compilation report and not as a stand-alone technical report. 

The following component part numbers comprise the compilation report: 

ADP01 1730 thru ADP01 1799 


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. 


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, 

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 

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). 


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 

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, 

Fig. 6: Beam combiner and LO injection element 

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


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. 


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: 


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 


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.