characterization of electrical properties of glass and
TRANSCRIPT
Characterization of Electrical Properties of Glass and Transmission Lines on
Thin Glass up to 50 GHz
Wasif Tanveer Khan*#, Jialing Tong#, Srikrishna Sitaraman#, Venky Sundaram#, Rao Tummala#, and John
Papapolymerou#
#School of Electrical and Computer Engineering
Georgia Institute of Technology, Atlanta GA 30332-0250 *Department of Electrical Engineering
Lahore University of Management Sciences, Lahore, Pakistan
Abstract
This paper presents, for the first time, the
characterization of electrical properties of Glass/ZIF stack-up
and transmission lines on glass/ZIF up to 50 GHz. Ring
resonators, co-planar wave guide (CPW), CPWs with Thru-
Package-Vias (TPVs) and microstrip to CPW transitions are
designed, fabricated and measured on a 300/33 µm glass/ZIF
substrate. The Short-Open-Load-Through (SOLT) calibration
technique was used to measure the fabricated structures.
Measurements show promising RF performance of glass and
T.L on glass up to 50 GHz. An insertion loss of 0.05 dB/mm
at 20 GHz and 0.12 dB/mm at 50 GHz for a CPW line has
been measured. The microstrip to CPW transition exhibited
0.24 dB/mm of loss and a thru-package-via exhibited a loss
of 0.34 dB at 50 GHz. A dielectric constant of 4.95 and loss
tangent of 0.012 at 50 GHz is also reported.
.
I. Introduction
In addition to the excellent electrical properties,
packaging substrates/interposers should be cost effective and
compatible with different package integration technologies.
One property, whose importance cannot be ignored, is the I/O
density that can be achieved on a particular interposer
technology. To enable the packaging of next generation
systems, many new materials have been explored for
advanced package design. Interposer technology has evolved
over the years from ceramics to organics to silicon. Some of
the limitations of ceramic substrate are their higher cost
(because of smaller tile size) and high co-firing temperature.
Organic substrates suffer from poor dimensional stability
requiring large via-capture pads, which make them
unsuitable for very high IOs with fine pitch interconnections.
Therefore, researchers explored the option to develop Si-
based interposers. Si-based interposer has two limitations; (a)
because of the requirement to have an insulation wall around
via-walls, the process is expensive and (b) the size of Si-
based interposers is limited by the size of the Si wafer.
Recently, thin glass that can be processed in large panel size
has been proposed as a superior alternative interposer
technology to address the aforementioned problems of
organic and silicon based interposers [1]. Some of the
challenges, such as low cost formation of vias and low
thermal conductivity, associated with glass based interposers
have been recently addressed. The other advantages, which
glass-based interposer offer over the other available
interposer technologies include: low-cost alternative to 2.5 D
MCM package, packaged 3D ICs directly without requiring
organic BGAs, 10x higher IO density than organic packages
at 2-10x lower cost per mm2 than wafer Si interposers, 5 µm
redistribution layer (RDL) on both sides and 10x lower signal
loss than oxide-line TSV interposers. Because of the tremendous amount of media streaming,
video calling and high definition TV and gaming, the biggest
challenge for the industry is the increasing demand of high
data rates. Utilization of mm-wave frequencies is an
attractive option to meet this high demand. To investigate
the viability of glass at mm-wave frequencies,
characterization of electrical parameters of glass and
transmission lines on glass is very important. In this work,
for the first time, we report the characterization of glass and
T.Ls on glass from DC to 50 GHz.
II. Characterization of Glass/ZIF stack-up Substrate
To characterize a microwave substrate, various
methods/techniques can be utilized such as free space [2]
method, open resonator method [3], filled waveguide method
[3], cavity resonator method [4], and ring resonator method
[4]. Broad band techniques are suitable for high-loss and low
Q materials, whereas resonant methods are used to extract
electrical properties of a low loss material. Each method has
its own range of applicability and limitations. The parallel
plate method [5] is used only for lower frequencies whereas
the free-space method requires extra components (horn
antennas and a suitable substrate mounting setup). Resonant
cavities based techniques are accurate but not broadband;
they allow to extract the loss tangent and dielectric constant
at only one frequency.
Figure 1. Glass-ZIF Stack-up
978-1-4799-8609-5/15/$31.00 ©2015 IEEE 2138 2015 Electronic Components & Technology Conference
Using the ring resonator method, we can extract electrical
parameters at multiple frequencies. Therefore, in this work
we have used the ring resonator method to extract the
dielectric parameters of glass-ZIF stack-up.
The glass-ZIF stack-up, illustrated in Fig. 1, was used in this
work. The thickness of glass and ZIF was 300 µm and 33
µm, respectively. The thickness of the metallization on both
top and bottom sides was 10 µm.
A. Design of a Ring Resonator
The microstrip ring resonator (MRR), shown in Fig. 2, was
designed on the stack-up displayed in Fig. 1.
Figure 2. Ring Resonator
The MRR produces periodic resonant peaks in its S21
response. The extraction of the dielectric constant (Ɛr) of the
substrate depends on the location of the resonant frequency of
a resonator for a given radius. The desired resonant
frequency and corresponding radius of the ring was derived
using equation (1)
where fo corresponds to the nth resonant frequency of a ring
with mean radius rm, effective dielectric constant Ɛeff and c
being the speed of light in vacuum [6]. Ring resonators with
fundamental resonant frequencies of 3 GHz, 6 GHz, and 9
GHz were designed to extract the electrical parameters at
multiple frequencies. The design parameters of MRR are
tabulated in Table 1. The simulated S21 response of the
designed MRR is displayed in Fig. 3.
Figure 3. Simulated S21 response of Ring Resonator
Table I
Ring Resonator Design Parameters
Dimensions (mm)
mrr_w 0.684 dia(9GHz) 17.764
dia(3GHz) 53.374 c_g 0.04
dia(6GHz) 26.668
B. Extraction of Dielectric Electrical Parameters
The effective permittivity can be calculated using equation
(1). The extraction of relative permittivity and loss tangent
was performed using the dispersive permittivity model of
effective permittivity from [7]. The relative permittivity at
nth resonant frequency is extracted by using microstrip line
equations (2-4).
In equation (2), Mt represents the term that accounts for the
effect of metallization and is defined in equation (3).
where t, W, and h represent metallization thickness,
microstrip width and height of the substrate, respectively.
F(W/h) is defined in equation (4).
The ring resonator method gives the total loss at the
frequency locations of each resonant peak [8] and subtracting
theoretical values for conductor loss gives us the dielectric
loss.
Then the dielectric loss is inserted into the following
equation to get the loss tangent.
III. Characterization of Transmission Lines on Glass/ZIF
stack-up Substrate
Microstrips and CPWs are the most commonly used
transmission lines in planar microwave circuit design. For
the measurement and packaging of microstrip
designs/circuits, microstrip to coplanar wave guide
transitions are commonly utilized. When designing RF
circuits, electromagnetic simulators are used with the
assumption that metals are perfect electric conductors and
have no metal loss. Even when a conductivity value is added,
simulated transmission lines do not take the metal and the
dielectric surface roughness into account. Dielectric and
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conductor surface roughness might create significant
conductor losses which may not be detected in simulations.
That’s why transmission line structures have been designed
and measured to investigate the performance of T.Ls on
glass. Modeling, design, fabrication and characterization of
low loss and high aspect ratio 55 μm diameter through
package vias (TPVs) in 300 μm thick glass interposers was
reported in [9].To investigate the effect of Thru-Package-vias
(TPVs) on multi-layer interconnected transmission lines, we
have also designed CPW structures with Thru-Package-vias
(TPVs), as shown in Fig. 4 (c).
Figure 4. Top and Cross-sectional view of transmission lines
on glass.
The designed structures are illustrated in Fig. 4, and design
parameters of transmission lines are tabulated in Table II.
Table II
Transmission Line Design Parameters
Dimensions (mm)
l 6.2,11.2 g 0.026
w 0.17
The 3-D EM simulations were performed in HFSS. The
simulated S-parameters of 6.2 mm and 11.2 mm long CPW
lines are illustrated in Fig. 5-6. It can be seen from Fig. 5 that
the reflection coefficient is below than -10 dB from DC to 50
GHz and the insertion loss of 11.2 mm long line is less than
2 dB across the whole frequency range.
Figure 5. Simulated S11 of simple CPW lines without TPVs
Figure 6. Simulated S21 of CPW lines
IV. Fabrication
A two-metal-layer substrate can be fabricated with one
metallization layer on each side of the 300 um thick glass.
Figure 7. Fabricated Test Vehicle (a) Complete Wafer, (b)
CPWs with TPV structures, (c) CPW line, (d) CPW to
microstrip transition.
In the following, a short description of the fabrication process
is presented. A thin dry-film polymer ZEONIFTM (ZiF) ZS
of thickness 33µm, was laminated on both sides of the glass
substrate prior to the other processes. Such an approach was
shown to help in handling and metallization of the glass [10].
For the top and bottom metallization layers, electro-less
copper deposition process was used to form seed layers on the
surfaces of the polymer-laminated glass. Dry-film photo-
resist was then laminated on the seeded samples and UV
exposure was performed using designed masks to pattern the
photo-resist. Subsequently, the electrolytic copper plating was
employed to achieve desired copper pattern. The thickness of
copper on both metallization levels was 10µm. Finally, the
photo-resist was stripped and the seed layers were etched
away, which concluded the typical semi-additive process
(SAP). The fabricated test vehicle is displayed in Fig. 7 and
magnified view of three ring resonator structures is displayed
in Fig. 8.
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.
Figure 8. Fabricated Ring Resonators
V. Measurements and Results
A 250 µm Cascade MicroTech Infinity probe was used to
probe the fabricated structures. All S-parameter
measurements were performed using an Agilent/Keysight
E8361 network analyzer. The measurements are in close
agreement with simulated results. The slight deviation in
measurements is attributed to the slight process variation
(different dielectric height) during the fabrication process. In
the following sections, we will discuss the measured results
of ring resonators, and different transmission lines.
A. Extraction of Dielectric Electrical Parameters of Glass
substrate
The ring resonators designed for different resonant
frequencies were measured. Since the resonator designed at 3
GHz covers all the frequency points, to avoid clutter in the
measured results, measured S21 response of only the 3 GHz
resonator is displayed in Fig. 9.
Figure 9. Measured S21 Response of 3 GHz Ring Resonator
The extracted relative permittivity of glass-ZIF stack-up is
4.938 at 20.46 GHz. The extracted loss tangent of the stack-
up is shown in Fig. 10. The loss tangent remains below
0.0085 up to 40 GHz. The loss tangent value increases as the
frequency increases. It varies from 0.0075 to 0.012 across the
whole frequency range.
0 10 20 30 40 500.005
0.01
0.015
Frequency (GHz)
Loss
Tan
gen
t (t
an
)
Figure 10. Measured/extracted Loss Tangent.
B. Characterization of Transmission lines on Glass
substrate
The CPW lines of two different lengths (6.2 mm and 11.2
mm) were fabricated and measured. The measured results are
displayed in Fig. 11-12. Reflection coefficient in Fig. 11 is
below -10 dB and shows that the T.Ls are well matched. It
can be seen from Fig. 12 that the CPW lines on glass
substrate exhibits a loss of 0.12 dB/mm at 50 GHz. This is a
promising performance and is comparable to the performance
of transmission lines on other low-loss substrates such as RO
3003, LCP and LTCC.
Figure 11. Measured S11 of CPW lines on Glass
The measured S-parameters of CPW lines which were
patterned on both sides of Glass-ZIF stack-up and
interconnected with Thru-Package-vias (TPVs) are shown in
Figs. 13-14. The diameter of TPVs was 55 µm. It can be seen
that the transmission line structures are well matched and an
insertion loss of less than 2.5 dB for a T.L of 3 mm long
across the complete frequency range (DC- 50 GHz) is
achieved.
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Figure 12. Measured and Simulated S21 of CPW line on
Glass
This 3mm long (CPW with TPVs) structure shows 0.8
dB/mm of loss. As discussed earlier, the CPW line exhibits a
0.12 dB/mm loss, which means that two TPVs exhibit a loss
of 0.68 dB. This translates into a loss of 0.34 dB per thru-
package via (TPV).
Figure 13. Measured S11 of CPW with TPV on Glass
The measured S-parameters of a 14 mm long 50 ohms
microstrip to CPW transition are illustrated in Fig. 15-16,
which shows an insertion loss of 0.24 dB/mm
Figure 14. Measured S21 of CPW with TPV on Glass
Figure 15. Measured S11 of microstrip to CPW transition on
glass substrate
Figure 16. Measured S21 of microstrip to CPW transition on
glass substrate
The table 5 summarizes the performance of different
transmission lines in dB/mm.
Table III
Performance Summary of T.Ls
Loss (dB/mm)
T.L Loss T.L Loss
CPW 0.12 microstrip 0.24
CPW with
TPV
0.8 *Loss per
TPV
0.34
*Loss per TPV is not expressed in dB/mm. This loss is expressed as “loss
per thru-package-via”.
Conclusions
In this paper, for the first time, the characterization of glass-
ZIF stack-up and transmission lines on glass was presented.
Microstrip ring resonator method was used to extract the
dielectric properties of glass-ZIF stack-up. The extracted
relative permittivity was reported to be 4.938 at 20.46 GHz.
The extracted loss tangent varies from 0.0075 to 0.012 across
the whole frequency range. Because of the low-loss tangent of
the substrate at mm-wave frequencies, glass substrate can be
used to develop some low-cost high-performance mm-wave
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modules. We also designed and reported the loss of CPW
lines, CPW with TPVs and CPW to microstrip transition on
glass substrate. An insertion loss of 0.05 dB/mm at 20 GHz
and 0.12 dB/mm at 50 GHz for a CPW line was measured.
The microstrip line exhibited 0.24 dB/mm of loss and a thru-
package-via exhibited a loss of 0.34 dB at 50 GHz. This
promising performance of transmission lines and TPVs
shows that mm-wave circuits/components can be designed
with very good performance on a glass substrate up to 50
GHz. This work paves the way for System-on-Package
solutions on low-cost thin glass substrate.
.
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