Integrated Micro Coaxial Air-Lines with Perforations

Abstract

Three-dimensional micro coaxial transmission lines have been fabricated and measured on CMOS grade silicon wafers. One of the unique features of the structure is the perforated, coral-like outer shield and signal line which provides a means to control characteristic impedance and facilitates sacrificial layer removal after multi-layer deposition. These lines can thus be used to realize multilayer vertical interconnects with embedded MEMS devices for RF to mm-wave frequencies. Measured S-parameters through 40 GHz for lines of varying impedance and perforation density are presented

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Integrated Micro Coaxial Air-Lines with Perforations
Saravana P. Natarajan, Thomas M. Weller and Andrew M. Hoff
Department of Electrical Engineering, University of South Florida, Tampa, FL, USA
Abstract-Three-dimensional micro coaxial transmission lines
have been fabricated and measured on CMOS grade silicon
wafers. One of the unique features of the structure is the
perforated, coral-like outer shield and signal line which provides
a means to control characteristic impedance and facilitates
sacrificial layer removal after multi-layer deposition. These lines
can thus be used to realize multilayer vertical interconnects with
embedded MEM devicesfor RF
Measured S-parameters through 40 GHz for lines of varying
impedance and perforation density are presented.
to mm-wave
frequencies.
Index
transmission lines, suspended transmission lines
Terms-micro
coaxial, interconnects,
MEMS
I. INTRODUCTION
The most common type ofmicrowave transmission lines are
microstrip and CPW, which can be broadly classified as
printed transmission lines. But with the increasing complexity
in MMIC circuitry and miniaturization of chips it becomes
necessary to place circuits in close proximity. Microstrip and
CPW are not ideally suited for high-density applications due
to their radiating nature at mm-wave frequencies. So, there
arises
structures such as micro coaxial lines (herein referred to as
'tcoax') which exhibit minimal radiation and offer good
performance across RF to mm-wave frequencies.
Thin
microstrip
platforms [1] and suspended lines on dielectric posts [2] were
proposed for similar reasons. Though the suspended lines
provided
an
advantage
due
substrate/dielectric losses, radiation and coupling still remain
unsolved issues. Realizing a coaxial line in the micro scale on
a wafer,
circuitry, is a formidable task owing to the 3-D structure ofthe
coaxial line. On-wafer micro coaxial transmission lines were
first realized by Bishop et al in 1991
frequencies up to 200 GHz. The space between the center
conductor and the outer shielding was filled with probimide,
which led to dielectric losses at high frequencies. This work
was followed by the development of air-filled outer-shield
cavities where the signal lines of the micro coaxial lines were
suspended on metallic posts, designed as quarter wave short
circuit stubs [4], [5]. These designs overcome the dielectric
loss problem with the air-filled cavities, but the use ofmetallic
posts causes degradation in the RF performance and also
restricts the suspension height ofthe signal line to the quarter
a needfor
shielded,
on-wafer transmissionline
film
lines
suspended
on
polyimide
to
theelimination of
to co-exist with other conventional microwave
[3]
for operating
wavelength size. Moreover the fabrication of these designs
involves
complex
Nevertheless,
researchers have
couplers [6], [7] based on a micromachined coax structure
using this technique.
This paper introduces a 3-D micro coaxial transmission line
with an air dielectric and a center conductor suspended on
polyimide pedestals. The rectangular cross-sectional lines are
completely shielded with electroplated metallic walls on the
sides, top and bottom. The air cavity and the inherent fixed-
fixed beam topology can be utilized to realize MEM structures
within the ptcoax. The release of embedded MEM structures,
and control of the characteristic impedance, is enabled via
coral-like perforations
Measuredperformance of these
(gCORAL) lines, measuring 20
wide center conductors, indicates loss of 2 dB/cm at 20 GHz.
For the results presented herein the lines were fabricated
without the top shield layer.
relatively
stereo
demonstrated
lithography
steps.
andfilters
in the shielding and signal
micro-coaxial
tm in height with -60 tm
line.
air-lines
II. DESIGN AND FABRICATION
A. Design
The most challenging part of the gCORAL design is to
retain compatibility with basic MEMS fabrication techniques
so that the lines can co-exist with RF MEMS components and
MMIC circuits on-wafer. The width of the signal lines,
pedestals, and total width of the structure are designed to
provide minimum dielectric loss and dispersion by keeping the
relative permittivity inside the cavity close to that of air. This
is achieved by the use of polyimide
combined volume is not more than 10-12% ofthe total cavity
stand-offs whose
GROUND
PROBEPAD
POLYIMIDE PEDETALS
(WVDTH H0 TO 140 pmr
HEi
51 |- 20pm)
OUND PANE
HOLES
-pm,HEII HT-20pm )
L E
LAD
SIDEWLL(WIDTH =BO
I.
GOLD ELECTROPLAED
VIATO GROUND
SIGNAL LINE
WITHHOLES
POLY
PATFORM
(HEIGHT-10pm
IDE
Figure 1 -Top view ofthe perforated ,ICORAL structure.
0-7803-9542-5/06/$20.00 ©2006 IEEE

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volume. Typical dimensions used in this work are indicated in
Figure
configuration is shown in Figure 2.
lines are performed using Ansoft's HFSS.
1, and a conceptual view of a 3-D multi-layer
The simulations of the
Figure 2. 3-D rendering /cross sectional view of a multilayer
,ICORAL structure.
The characteristic impedance values for the transmission
lines were estimated using equations governing a microstrip
line. The jaCORAL structure can be considered as a suspended
microstrip line shielded by metallic walls. Considering a low
volume ratio (10%) of polyimide in the air cavity, the relative
permittivity can be approximated to about 1.3. The effective
dielectric constant, which is given by (1), is a function of the
W/H ratio ofthe structure, where W is the width of the signal
line (in jam) and H is the height ofthe pedestal (in jam).
Zo :=
£r+1
2
£r- 1
Ceff=
+
h
2 l1+12-
zI
120c
2
3
w
w
h
w
h
9cff-+ 1.393+ -In - + 1.444
(1)
(2)
The knowledge of Feffand the fixed pedestal height of 10 jam
can be used to calculate the widths of lines needed to achieve
a particular value of Zo using (1) and (2). It was arbitrarily
chosen to have conductor dimensions of 40, 60, 80 and 100
jam for this initial study in order to achieve Zo values of
around 50, 40, 35 and 25Q, respectively. These values are
compared to the Zo values extracted from the measurement
data in Section III below.
One ofthe salient features ofthe design is the periodic hole
pattern in the signal line, top and bottom metallic shields.
These holes
essentially as lumped tuning elements to modify the effective
characteristic impedance. The effects of variations in the hole
density are also explored in Section III.
Many of the geometrical parameters were held constant
across the designs. The polyimide pedestals are 80 to 140 jam
are electrically small and can thus be used
squares, depending on signal line width, and 10 pm in height.
They are placed 350 ptm apart to support the signal line like a
fixed-fixed beam. The total width and height ofthe structure is
470 ptm and 20 ptm respectively (see Figure 1). The length of
the structures was held constant at 2000 ptm for all the
designs. The ends ofthe ptcoax line are terminated in Ground-
Signal-Ground pads
microwave probes. The ground pads are connected to the
bottom ground plane using electroplated vias. The side walls
are 80 ptm thick, made of electroplated gold. The signal line,
top and bottom metallic shield are made of 1
electroplated/thermally evaporated gold metallization.
for probing with conventional GSG
ptm thick
B. Fabrication
The ptcoax structures were fabricated on low resistivity
(7-13 Q.cm) silicon wafers. The fabrication steps are given
below-
1.
Ground holes are formed by thermal evaporation and
lift-off of 1 ptm thick Cr/Au metallization on a clean,
bare, low resistivity silicon wafer
Polyimide (HD 4010) is spun and patterned to form the
pedestals and the platforms for the probe pads
Gold is electroplated in a AZ P4620 resist mould to
form the side walls of 10 ptm height
1 jtm thick layer of Cr/Au is thermally evaporated and
patterned to form the signal line suspended on the
pedestals
The sidewalls are further electroplated to a total height
of 20 ptm and the resulting upper cavity is filled with
sacrificial material (AZ P4620)
1 ptm thick layer of Cr/Au is thermally evaporated and
patterned to form the top lid with the holes (Note that
this step was not performed for the structures described
in this paper).
The sacrificial layer is removed using AZ 300T resist
stripper and transferred to DI water followed by two
stages of anhydrous methanol. The sample is then
transferred to a convection oven at 900 C and held for
15 minutes to evaporate the residual methanol.
2.
3.
4.
5.
6.
7.
Figure 3 - Micro photograph of a ItCORAL structure.
MEM structures such as these are generallyreleasedusing a
criticalpoint dry (CPD) methodusing liquid carbon-dioxide to

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426
avoid damage to the beams due to surface tension ofthe liquid
releasing agents. The suspension height of 10 ptm and the span
of350 ptm for the fixed-fixed beams result in very low surface
tension and capillary effects, thus eliminating the need for a
CPD. A completed line is shown in Figure 3.
III. MEASUREMENT AND RESULTS
The fabricated structures were measured in the frequency
range up to 40 GHz. A probe-tip Through-Reflect-Line
calibration was performed on a GGB CS-5 substrate prior to
measurements to establish a 50-Ohm reference impedance.
co
a
I..
-5
-10
-15
-20
-25
-30
-35
-40
-45
0
5
1015
Frequency (GHz)
2025 3035
amount of loss caused by the radiation that may occur.
0
5
1015
Frequency (GHz)
20253035 40
Figure 5 - Loss factor ofthe ,ICORAL structures.
Finally, the measured S21 referenced to 50Q is shown in
Figure 6.
40
Figure 4 - Sit ofthe gCORAL structures.
A.
Effect ofCharacteristic Impedance Variations
Measured results are presented for ptCORAL structures of
23, 32, and 38Q characteristic impedances in Figures 4 - 6.
The characteristic impedances ofthe lines are calculated from
the measured results using (3).
Zo=(Zsc*ZOc)'2
(3)
Where Zo - characteristic impedance of the line, Zoc- open
circuited input impedance, and Zsc - short circuited input
impedance. The calculations were performed by terminating
the measured 2-port S-parameter data sets with ideal open-
and short-circuit impedances using Advanced Design System
software. Table 1 presents a comparison between the design
estimates and the measured values of Zo. The discrepancy
between the calculated and measured values can be attributed
to imperfections in fabrication process, leading to a change in
the width/height ratio, in turn affecting the Zo ofthe line.
Figure 5 shows the loss factor ofthe structures referenced to
a 50Q system. The losses can be further improved by using a
thicker signal line and ground plane metallization, to avoid
losses due to skin depth and substrate interaction effects,
respectively.
The absence of the top lid also adds a small
C,)
0
-0.2
-0.4
-0.6
-0.8
-1
-1.2
-1.4
-1.6
-1.8
-2
0
5
10 152025
Frequency (GHz)
3035 40
Figure 6 - S21 ofthe gCORAL structures.
Table 1
Comparison ofCalculated and Measured Zo
Line Width
40
(~
Calculated
52
37 lo 40
Measured
52
37to40
(Q
60
80
100
tm
3931 26
2
23
Zo(Q)
27l_1
27to31
B.
Effect ofHoles Pattern Variations
For comparison purposes, the measured characteristics of
lines with a nominal 32Q characteristic impedance (when no
holes are present) and varying degrees of hole density are
shown in Figures 8 and 9.
The hole density is essentially
equal in the ground plane and lower shield for these tests. The
return loss data (Figure 9) shows that the characteristic

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427
impedance is increased toward 50Q (increasing return loss) as
the hole density increases; this
capacitance ofthe line is being reduced. The data in Figures 7
and 8 shows that only a minimal change in loss occurs due to
the opening ofholes, which has some significance because of
the presence of the low resistivity substrate beneath the
ground shield.
is as expected since the
0.16
0.14
0.12
o
0.1
u- 0.08
Ue
U,
°9 0.06
0.04
0.024
0
0
5
1015
Frequency (GHz)
202530
Figure 7 - Loss factor for 32Q lines with different hole
densities.
a
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
05 10 15
Frequency (GHz)
202530
IV. CONLCUSION
3-D micro coaxial transmission lines have potential as a
building block for complex MMIC circuitry. This research is
being extended to produce MEM based capacitors, switches,
filtersand phase
within
structure. The air filled cavity can be utilized to realize MEM
devices thus creating an integrated and space conservative RF
subsystem within the transmission
ptcoax structures can be configured as vertical interconnects in
multi layer circuitry such as antenna array-feed network.
shifters
the
ptcoax waveguide
line. Furthermore, the
ACKNOWLEDGEMENT
The authors would like to acknowledge HD Microsystems for
providing us with free HD 4010 polyimide samples and
related chemicals for use in this work and Harris Corporation
-GCSD, Palm Bay, FL for the financial support.
3540
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H. S. Lee et al, "Design and Characterisation ofmicromachined
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Jennifer A. Bishop et al, "Monolithic Coaxial Transmission
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al, "Characterization of Thin film
[2]
line withdielectric post
for millimetre-wave
[3]
[4]
[5]
et
al,
of Microfabricated
[6]
[7]
35 40
Figure 8 - S21 for 32Q lines with different hole densities.
-10
-15
-20
a
-
-25
Cl,
-30
-35
-40
0
5
10 15
Frequency (GHz)
2025 30 3540
Figure 9-Sit for 32Q lines with different hole densities.