The majority of radio receivers, transmitters, and components operating at millimeter and submillimeter wavelengths utilize rectangular waveguides in some form. However, conventional machining techniques for waveguides operating above a few hundred GHz are complicated and costly. This paper reports on the development of silicon micromachining techniques to create silicon-based waveguide circuits which can operate at millimeter and submillimeter wavelengths. As a first step, rectangular WR-10 waveguide structures have been fabricated from (110) silicon wafers using micromachining techniques. The waveguide is split along the broad wall. Each half is formed by first etching a channel completely through a wafer. Potassium hydroxide is used to etch smooth mirror-like vertical walls and LPCVD silicon nitride is used as a masking layer. This wafer is then bonded to another flat wafer using a polyimide bonding technique and diced into the U-shaped half wavelengths. Finally, a gold layer is applied to the waveguide walls. Insertion loss measurements show losses comparable to those of standard metal waveguides. It is suggested that active devices and planar circuits can be integrated with the waveguides, solving the traditional mounting problems. Potential applications in terahertz instrumentation technology are further discussed

Mcgrath, William R.

Tai, Yu-Chong

Walker, Christopher

Yap, Markus

English

NASA Technical Reports Server

Page 316 Third International Symposium on Space Terahertz Technology
Z 5 4N 9 3 -
SILICON MICROMACfflNED WAVEGUIDES FOR
MILLIMETER AND SUBMILLIMETER WAVELENGTHS
MarkusYap,1 Yu-Chong Tai,1 William R. McGrath,2 Christopher Walker 3
1. Department of Electrical Engineering, California Institute of Technology,
Pasadena, CA91125
2. Center for Space Microelectronics Technology, Jet Propulsion Laboratory,
California Institute of Technology, Pasadena, CA91109
3. Department of Astronomy, University of Arizona, Tucson, AZ 85726
Abstract ~ The majority of radio receivers, transmitters, and components operating
at millimeter and submillimeter wavelengths utilize rectangular waveguides in some
form. However, conventional machining techniques for waveguides operating above a
few hundred GHz are complicated and costly. This paper reports on the development
of silicon micromachining techniques to create silicon-based waveguide circuits
which can operate at millimeter and submillimeter wavelengths. As a first step,
rectangular WR-10 waveguide structures have been fabricated from (110) silicon
wafers using micromachining techniques. The waveguide is split along the broad wall.
Each half is formed by first etching a channel completely through a wafer. Potassium
hydroxide is used to etch smooth mirror-like vertical walls and LPCVD silicon nitride
is used as a masking layer. This wafer is then bonded to another flat wafer using a
polyimide bonding technique and diced into the U-shaped half waveguides. Finally a
gold layer is applied to the waveguide walls. Insertion loss measurements show
losses comparable to those of standard metal waveguides. It is suggested that
active devices and planar circuits can be integrated with the waveguides, solving the
traditional mounting problems. Potential applications in Terahertz instrumentation
technology are further discussed.
https://ntrs.nasa.gov/search.jsp?R=19930018565 2020-03-17T05:44:16+00:00Z
Third International Symposium on Space Terahertz Technology Page 317
I. Introduction
Rectangular waveguide is a well characterized transmission medium which is
used in a variety of complex rf components and circuits. Many sophisticated
applications including radar, communications systems, test instruments, and
heterodyne radiometers use waveguide components up to millimeter wave
frequencies. The long history of development of waveguide components provides a
broad base of knowledge to synthesize and evaluate new designs for higher
frequencies.
Waveguide is typically fabricated from metals such as brass and copper using
conventional machining techniques. However, at frequencies above a few hundred
GHz, waveguide becomes so small (less than 0.3 mm x 0.15 mm for 500 GHz -
1000 GHz waveguide) that fabrication utilizing these conventional techniques is time
consuming, costly and difficult In addition, mounting active and passive devices such
as mixer diodes, filters and planar probes on these waveguides is difficult.
A substantial research effort in recent years has been devoted to fabricating
micromechanical structures in silicon using micromachining techniques. Moveable
structures such as slider, gears, and spiral springs in the dimensional scale of 50-200
jim have been fabricated [1, 2]. We have taken a new approach in developing and
adapting silicon micromachining techniques to create silicon-based waveguide
circuits which can operate up to millimeter and submillimeter wavelengths.
As a first step we have started fabricating rectangular waveguides for frequencies
between 100 GHz and 1000 GHz. Here we only emphasize WR-10 waveguides
(operating at 75 GHz - 115 GHz) because it is compatible with our existing
measurement equipment. Conventional WR-10 waveguide is a rectangular channel
with inner wall dimensions of 0.1 x 0.05 inches. Our waveguide, however, is made of
two half sections split along the broadwall as shown in Fig. 1. The reason for
splitting the waveguide is to simplify the fabrication process and to facilitate
integration of planar circuits and devices, which is further discussed in Section IV.
Waveguide Channel
Top Si Wafer
B
onding Layer
ottom Si Wafer
(b)
Fig. 1. a) The waveguide is split into 2 half sections,
b) One half section of a waveguide.
Page 318 Third International Symposium on Space Terahertz Technology
n. Fabrication Process
The fabrication process for the half sections with emphasis on the cross
section is shown in Fig. 2. A thick (0.05 inches) double-side polished silicon wafer
with (110) surface orientation is used. The major flat has a normal in the [111]
direction within 0.4°. After a standard piranha-bath cleaning, a 1000 A Low
Pressure Chemical Vapor Deposited (LPCVD) silicon nitride layer is deposited on
both sides of the wafer. Photolithographic techniques are then utilized to pattern
the waveguide etching windows. Photoresist is used as a masking layer for etching
the silicon nitride windows with an SFg plasma. The silicon nitride is used as an
etching mask to define b, the waveguide height, shown in Fig. Ib. After removal of
the photoresist using acetone solvent as shown in Fig. 2a, the wafer is put in a
reflux system and etched in a water based solution of 40 % KOH at 80 °C. Figure
2b shows the wafer after it has been etched completely through to form half of the
waveguide. The etching rate of (110) silicon in this KOH solution is 2 jim/min and
the etching ratio of (110):(111) planes is 170:1. At this rate 0.05 inches (1270 urn)
of silicon is etched thru in -11 hours. Following removal of the nitride mask using
hot hydrophosporic acid at 150 °C, a polyimide bonding technique is used to glue
these etched grooves to a smooth silicon wafer with an identical thickness (0.05
inches) as shown in Fig. 2c. The wafer is then diced into pieces of half waveguides.
Such a half waveguide is shown in Fig. 2d. Metalization is done by first depositing a
thin (200 A) chrome layer followed by a thicker (5000 A) gold layer on the waveguide
walls using vacuum evaporation. Further metalization is done by electroplating gold
to a thickness of ~3 Jim to reduce rf conduction losses.
HI. Experimental Results
In order to perform insertion loss measurement, we designed a pair of brass
mounting blocks. The two waveguide. half-sections are put on the brass mounting
blocks and mated together. This allows the silicon waveguide to be connected to
microwave test equipment using conventional waveguide flanges. The silicon
waveguides are rugged and can be firmly clamped to metallic flanges. The insertion
loss of the WR-10 waveguide is measured over a frequency range of 75 GHz to 110
GHz. The measurement system is shown in Fig. 3. The source is a BWO which
produces several milliwatts. A reference sweep is first taken without the waveguide.
This is compared to a sweep with the waveguide inserted between the source and
detector. The insertion loss for a 2.5 cm long section of waveguide is shown in Fig. 4
(the small wiggles in these curves are noise and do not reflect any resonances in the
waveguide components). The measured loss is about 0.05 dB per wavelength (at 100
GHz) across most of the band. This is very good performance and is comparable to
the result for commercially available waveguide which shows a loss of about 0.024
dB per wavelength. The small difference of 0.026 dB per wavelength is most probably
due to differences in the quality of the gold plated surfaces. Our evaporated gold
showed small pits which were still present in the plated layer. Also there was no
gold on the ends of the silicon waveguide where contact was made to the metallic
flanges of the test equipment. We expect improvements in the gold surface to be
directly reflected in improvements in the rf losses.
Third International Symposium on Space Terahertz Technology Page 319
(110) silicon (0.05 inches)
polyimide bonding layer
KOH etch
silicon nitride (1000 A)
(a)
(100) silicon (0.05 inches)
(c)
^*>chro:
f' gold
ime(250A)
 (5000 A)
(d)
Fig. 2. A cross section view of the fabrication process.
BWO
75 GHz to 110 GHz WAVEGUIDE UNDERTEST
\
DETECTOR
1J < » " < ^10 dB 10 dB
SWEEPER
SCALAR
NETWORK
ANALYZER
Fig. 3. Block diagram of millimeter wave insertion loss test system.
Page 320 Third International Symposium on Space Terahertz Technology
0 dB Reference
I I
75 80 85 90 95
FREQUENCY [GHz)
100 105 110
)- (b)
0 dB Reference
I
75 80 85 90 95
FREQUENCY [GHz]
100 105 110
Fig. 4. (a) Measured loss of a 2.5 cm long section of Si-based WR-10 waveguide.
The surface of the silicon was metallized with approximately 3 ujn of gold to
reduce rf losses, (b) Measured loss of a 2.5 cm long section of conventional
metallic waveguide.
IV. Discussion
Waveguide circuits are preferable at frequencies above 100 GHz since
waveguide has the advantage of adjustable rf tuning. This solves the difficulties of
accurately designing fixed-tuned planar microwave integrated circuits.
Unfortunately, millimeter and submillimeter waveguide components are hard to
manufacture by conventional machining techniques. We have shown here the
feasibility of making silicon waveguides. It is also possible to use silicon
micromachining techniques to fabricate other components such as: directional
couplers, waveguide transformers, waveguide-to-planar circuit transitions,
low-loss filters, rectangular and conical feedhorns, and dichroic plates. This wide
variety of waveguide components will become the building blocks for complicated
circuits. For example, complex mixer and frequency multiplier embedding circuits
can be built. These are important for ground-based and space-based radar,
communications, and remote-sensing applications.
Third International Symposium on Space Terahertz Technology Page 321
Silicon micromachined waveguide components have several important
advantages: 1) These structures are produced by projecting the desired
pattern onto silicon with photolithographic techniques. Therefore waveguides with
dimensions suitable for use above 100 GHz can be easily fabricated. 2)
Dimensional accuracy is in the order of a few microns, which is essential for the
fabrication of high-Q components. 3) The waveguide walls would be
atomically smooth, thereby minimizing rf losses [3]. 4) Several versions of a
single component (with variations of a critical parameter) can be produced at the
same time on a single wafer. This would allow for rapid optimization and reduced
cost compared to conventional machining techniques where only one variation at a
time is produced. 5) Most importantly, active and passive devices can be
integrated with the waveguide. For example, a thin (~ l^im) rf transparent silicon
nitride membrane can be fabricated across the end of the waveguide or parallel to
its length in the E-field direction. Active devices such as Schottky diodes and SIS
tunnel junctions as well as micromechanical rf tuning elements[l, 4] can then be
fabricated directly on the membrane as shown schematically in Fig. 5. This would
eliminate the long-standing problem of mounting the devices and would represent a
significant advance for waveguide technology.
Active Device
Silicon wafer with several
etched waveguides and
devices
RFin
Etched
Waveguide
RF Filter
Fig.5 A schematic view of an integrated waveguide circuit. Several waveguide
components can be produced on a single wafer. Active devices and planar
circuits can be integrated directly on thin membranes spanning the
waveguide. Micromechanical rf tuning elements can also be included in the
waveguide.
Page 322 Third International Symposium on Space Terahertz Technology
Currently, we are fabricating WR-10 waveguides with SiN membranes in
between the two half pieces of the waveguide. Metalization of the half sections of
these waveguides will require selective plating of the silicon walls without plating
the silicon nitride membranes. Tungsten substitution of silicon in an LPCVD
environment [5] is proposed to meet this need. In this process, tungsten hexafluoride
(WFg) gas attacks silicon surface and a thin layer of tungsten atoms substitute for
silicon atoms on the surface. Further metalization can be done by electroplating the
tungsten surface with gold.
V . Summary
We have demonstrated a new approach in fabricating waveguide' circuits using
silicon micromachining technology. In particular, we have fabricated a 100 GHz silicon
rectangular waveguide. The insertion loss of 0.05 dB/X is comparable to a
commercially available metal waveguide. As we improve our plated gold quality, we
expect to improve the insertion loss. We have also proposed a new approach of
integrating active/passive devices and micromechanical rf tuning elements with
waveguide.
This work is supported in part by California Institute of Technology President's Fund
under grant PF-347 and the Jet Propulsion Laboratory, California Institute of
Technology, under contract with the National Aeronautics and Space Administration
and the Innovative Science and Technology Office of the Strategic Defense Initiative
Organization.
Third International Symposium on Space Terahertz Technology Page 323
References
[1]. L.-S. Fan, Y.-C. Tai, and R. S. Muller, "Integrated Movable MicromechanicaJ
Structures for Sensors and Actuators," IEEE Trans, on Electron Devices, vol.
35, pp. 724-730, June 1988.
[2]. M. Mehregany, K. J. Gabriel, and W. S. N. Trimmer, "Integrated Fabrication of
Polysilicon Mechanisms," IEEE Trans, on Electron Devices, vol. 35, pp.
719-730, June 1988.
[3]. F. J. Tischer, "Experimental Attenuation of Rectangular Waveguides at
Millimeter Wavelengths," IEEE Trans. Microwave Theory and Tech., vol.
MTT-27, pp. 31-37, January 1979.
[4]. V. M. Lubecke, W. R. McGrath, and D. B. Rutledge, " Sliding Backshorts for
Planar Circuits," International Journal of Infrared and Millimeter Waves, vol.
12, pp. 1387-1397, December 1991.
[5]. N. Kobayashi, M. Suzuki, and M. Saitou, "Tungsten Plug Technology Using
Substitution of W for Si," IEEE Trans, on Electron Devices, vol. 37, pp.
577-582, March 1990.


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