# Modeling and Measurements of Novel Monolithic Filters

Authors: Aly H. Aly; Badawy El-Sharawy; Adalbert Beyer
Journal: Active and Passive Electronic Components
 (2008)

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## Abstract

This paper presents novel multilayer tuneable highQ-filters based on hairpin resonators including ferroelectric materials. This configuration allows the miniaturization of these filters to a size that makes them suitable for chip and package integration and narrow-band applications. The main focus was miniaturizing filters with coupled loops using multilayer dielectric substrates. A further goal was to increase the quality factor of these distributed filters by embedding high dielectric materials in a multilayer high- and low-k (dielectric constant) substrate that is supported by LTCC technology. An improved W-shape bandpass filter was proposed with a wide stopband and approximately  5%  bandwidth.


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## Body

## 1. Introduction

At microwave frequencies, the parallel-coupled-resonator
filters [1] and
the hairpin resonator filters [2] are the most widely used filters because of
their design, ease of fabrication, and low cost. However, these filters have
some disadvantages such as: large size, high loss, especially when the number
of coupled resonators is increased, low stopband rejection, and spurious
response at the filter’s harmonics.Many filter topologies were explored using coupled-line theory in strip
line and microstrip to improve the performance. A cross-coupled topology was
proposed [3] to create controlled attenuation poles or transmission zeros by
allowing different coupling paths with different phases for the input signal.
These transmission zeros are created in the stopband either above, below, or
between resonances to enhance filter rejection [4, 5]. A drawback of this
technique is the high loss of the
filter, especially when zeros are placed near the resonant frequency. Another drawback
is the bandwidth asymmetry created from concentration of rejection on one
stopband (above or below resonance) and relaxed on the other band. Different
coupling schemes for resonator filters were introduced in [6].Stepped
impedance hairpin resonators were introduced in [7] to reduce the resonator
size and enhance the performance. In [8], the hairpin resonator’s length is
further decreased by capacitive loading at the end of the resonator. The main
disadvantage of capacitive loaded resonators is the high loss due to the
increased resonator capacitance. This explains why all of the applications of
this filter use high-temperature superconductors (HTSs) [9] which have a
limited usage in wireless applications. Slow-wave open-loop resonators were
presented in [10]. These allow for various elliptic and quasielliptic function
responses to be realized at the expense of high insertion loss. This loss
limits this filter to HTS applications. Another coupled-line filter using
defected ground structure (DGS) was presented in [11] in which a defect in the
ground plane below the coupled lines added more transmission zeros to the
resonance characteristics of the filter. Utilizing this DGS may not be
applicable in terms of filter integration and fabrication. Zigzag hairpin comb
resonators have been proposed in [12], to further reduce the size, but the
filter is suffering from a very high capacitance from winding the hairpin
resonators. This winding increases the losses, which is why this filter was
only utilized in HTS technology where conductor loss is of no concern.Besides the improvement in the resonator structure, feed structures also
play an important role in optimizing the performance the filters. Different
feed topologies have been used such as: parallel-coupled-feed structure [1],
tapped-line [13, 14], and the end-coupled feed structures. Most of the feed
structures are capacitively coupled feeds or electric field coupling, which can
increase the overall capacitance of the filter and reduce Q. Optimization of
the feed location and length controls the strength of the coupling, which
controls bandwidth and Q.The progress and feasibility of filter
integration are usually coupled to filter tuning. Electronic tuning is used to compensate
for manufacturing and process variations. Ferroelectric materials have been
suggested for both filter tuning and size reduction [15–17]. Most ferroelectric
materials have relative dielectric constant greater than 200. There is a
tradeoff between material Q and tunableness. Materials like KTaO3 have high Q, greater than 10 000 but have poor tunableness of less that 1% per
V/μm. The preferred
choice has been BaSrTiO3 which has a relative
dielectric constant equal or greater than 5000 and tunableness of 20% per V/μm
but has a poor Q of less than 100. The most common design is the lumped element
filter with ferroelectric loaded capacitors [17]. This approach yields a low Q
of less than 30 due to the low Q of the lumped elements.This paper
describes the properties of a multilayer dielectric mode filter coupled through
hairpins to increase coupling and reduce size. Tight coupling is required to achieve
low insertion loss. Mediumk and high Q material, such as KTaO3, is inserted between two
layers of high k material such as BaSrTiO3. The high k material
pulls the fields inside the low k material to enhance both tunableness and Q.
The present modes are EM dielectric modes in the ferroelectric material for the
design of band-reject filter and oscillator applications.Now, the paper is organized as follows. Section2 describes some
properties of the dielectric mode hairpin filters. In Section 3, the topic ‘‘multilayer
filters” is treated. Section 4, an overview of the LTCC technology and the
LTCC substrate configuration is shown. Section 5 presents the properties of
hair-pin resonator filter. Section 6 presents the fabrication and measurements
of different coupled-line filters, and the paper is concluded in Section 7.


## 2. Dielectric Mode Hairpin Filters

Simple band-reject filters (BRFs) can be
constructed using a dielectric resonator (DR) placed next to a microstrip line or
by coupling ferroelectric bars made of KTaO3 (with a relative dielectric
constant of 240 and Q of about 10 000) to a 50 ohms microstrip line as shown in
Figure 1. The present dimensions of the microstrip and ferroelectric bar are
required for on-chip integration.Figure 1
FBAR coupled to a microstrip line.In order to increase the coupling and to
enhance the filter characteristics, a hairpin is wrapped around the
ferroelectric bar as shown in Figure2.Figure 2
FBAR coupled to a microstrip hairpin with side coupling.The performance of the filter depicted
in Figure2 is calculated versus frequency using the HFSS software package
from Ansoft, Pa, USA,
and is shown in Figure 3.Figure 3
Frequency response of an FBAR coupled to a microstrip hairpin
with side coupling.A significant improvement in coupling andQ, as compared to the simple
bar, is observed.Next, the hairpin is coupled to the 50 ohms line at an edge of the
hairpin as shown in Figure4. Also the resonant frequency is reduced by a
factor of 2 for the same bar dimension. When the field distribution is studied,
it has been found that the loop of the hairpin produces a maximum magnetic
field in the ferroelectric bar resulting in effectively a virtual ground at the
loop. A maximum electric field occurs at the open end. Thus, the dimension of
the resonant bar is reduced to λ/4 instead of λ/2 as compared to case
of a conventional dielectric resonator. To verify the type of mode, the hairpin
is coupled to the 50 ohms line at an edge of the hairpin as shown in Figure 4.
The resonant frequency did not change significantly, as shown in Figure 5.Figure 4
FBAR coupled to a hairpin with
edge coupling.Figure 5
Frequency response of an FBAR coupled to a hairpin with edge
coupling.


## 3. Multilayer Filters

The material, which is used in the above-described devices as ferroelectric
bars, is made of KTaO3 (with a relative dielectric constant of 240 and Q of about 10 000), and it has
a very poor tuning, but fairly high Q. In order
to increase the tunableness, another
material, namely, a BaSrTiO3 layer can be used in conjunction with KTaO3 as shown in
Figure 6. Thus, multilayer filter structures may be obtained.Figure 6
Geometry of a multilayer BRF.In all of following simulations, a
silicon dioxide substrate was used with an effective dielectric constant of 4
and a thickness of 15μm which gives a 50-ohms microstrip line of 23 μm width. The conductor thickness is set to 10 μm. Ferroelectric
bars’ width and thickness are optimized to give a high Q.Figure7 shows the response of a multilayer
filter depicted in Figure 6. Here, a hairpin with 2 μm thick BaSrTiO3 is used.Figure 7
Frequency response of a multilayer structure.This picture also shows the new response when a 3- micron KTaO3 is inserted in the middle of the BaSrTiO3 layer. The
figure indicates
an improvement in Q of up to 20%. The resonant frequency shifted downward as
expected.Subsequently, the tunableness of the
multilayer structure is tested by applying a DC voltage between the hairpin and
the ground plane of the microstrip line.A decent tunableness in the order of 5% is predicted for a 20 volt
variation in the DC voltage as shown in Figure8.Figure 8
Frequency response (S21 (dB))—of tunable multilayer structure.


## 4. LTCC Substrate Configuration

LTCC
is a multilayer ceramic substrate technology. The multilayer architecture can
be produced using stacked ceramic tapes that are used to apply conductive,
dielectric, and/or resistive parts. These single sheets have to be laminated
together and fired in one step at a relatively low temperature. This saves
time, money, and reduces circuit dimensions. LTCC technology offers a high
level of integration, buried components, low loss, and system robustness. LTCC
technology has been established in mobile communication techniques in the
frequency range of a few GHz. LTCC has been investigated and has developed a
good reputation for the high-frequency applications. Examples include: WLAN at
5 GHZ [18], radar sensors at 24 GHz and 77 GHz [19], and digital radio networks
operating from 20 to 60 GHz [20] have also been reported.Advances in new materials and fabrication technologies opened a new
window in improving filter performance. One of these technologies is LTCC. Due
to its thick conductor, losses of the LTCC filter can be reduced. The filter
size can be also reduced by using embedded high dielectric-constant substrates.
Also, one of the major advantages of using LTCC is its low cost and short
fabrication cycle. Filters presented in this paper are formed between a highk material (k=65) followed by a low
k material (k=6).
Figure 9 shows the cross-sectional area of the mixed
k substrate.Figure 9
Cross-section of substrate
layers using LTCC technology.Table1 shows the material properties of the mixed
k substrate
measured at 2.5 GHz.Table 1
LTCC material
properties.
Kind of tapek @ 2.5 GHzLoss Factor @ 2.5 GHzHertape CT  7076.374.56∗10E−3Hertape 76568.71.73∗10E−3


## 5. Properties of Hair-Pin Resonator Filter

Figure10 shows the concept of using a hair-pin line as a resonator to
design a simple bandpass filter (BPF).Figure 10
A single-resonator hair-pin filter with capacitance coupling at the input
and output.A
single capacitively coupled hair-pin resonator is used. The folded resonator
length is 8800 microns on the LTCC multilayer substrates that are described above.
Dimensions of the filter were set to comply with the LTCC design rules. The
spacing between the resonator and the capacitive feeds,S, is set to 125
microns, which is the minimum spacing between conductors. The thickness of the
conductor is 10 microns. The filter was simulated using the HFSS package. The
effective dielectric constant is εeff=40 and gives a resonance around
2.66 GHz.Since
the opposite sides of the hairpin resonator have opposite potentials, there is
a virtual ground created at the center of the resonator. This makes the filter
similar to the comb line filter [9] and makes this filter appropriate for
planar applications that do not require vias connected to the ground plane.Simulation results of the filter are shown in Figure11.Figure 11
Simulated response of
single hair-pin resonator filter with capacitive coupling.It can be seen from the response of the filter that the insertion loss
is very high (≅10 dB). This is due to the weak coupling between the feeds and the
resonator. Weak coupling is desirable for narrow-bandwidth filter, but it comes
at the expense of increasing filter loss.To
further illustrate the effect of changing the spacingS,
Table 2 shows
performance of the filter for different values of S.Table 2
Effect of changingS
on the filter response.
S(μm)fr (GHz)I.L (dB)R.L. (dB)3 dB-B.W. (MHz)1002.66103.3173502.656.95.1780252.616.255.9191It
can be recognized from the table that there is a tradeoff between insertion
loss and bandwidth.The
above discussion explains the basic operation of the hair-pin resonators. It is
shown that narrow bandwidth can be obtained by using weak coupling at the
expense of the insertion loss. This may render the filter useless in some
applications. To increase coupling, parallel feed lines can be used to increase
the coupling area. Figure12 shows a single hair-pin filter with parallel feed
lines.Figure 12
Single hair-pin resonator
filter having two orientations with parallel coupled-feed lines.While Figure12 shows two different orientations of the resonator, with
respect to the feed lines, in Figure 13 the responses of the filter are given.Figure 13
Simulated magnitude response of hair-pin resonator filter having two
orientations with parallel coupled-feed lines.It can be seen from the figure that if a resonator is in the same
orientation as the feed line, then it has higher losses. This is because the crowding
currents at coupling edges can increase loss [17]. A comparison between the two
cases is shown in Table 3 in which the unloaded Q is higher for the opposite
orientation.Table 3
Properties of the hair-pin resonator filter having two
orientations
with parallel coupled-feed lines.
fr
(GHz)I.L
(dB)R.L.
(dB)3 dB-B.W. (MHz)QlQulOpposite
orientation2.471.3117.64006.244.2Same
orientation2.534.188.3922011.530.1


## 6. Fabrication and Measurements of Coupled-Line Filters Using LTCC

The
proposed coupled-line filter substrates will use embedded high dielectric
material to reduce the size, and improveQ. Two filter configurations
were considered in this layout. The first configuration is the strip-line
configuration. A ground ring was formed around the filter to shield it and to
connect the top and bottom metallization as shown in
Figure 14.Figure 14
Configuration (1): metal layers.In this layout, a minimum conductor width of 100 microns and a minimum
metal spacing of 125 microns are considered. The second configuration is the
microstrip configuration which was the same as configuration (1) without the
top ground plane. The metallization of all layers for the
second configuration is shown in Figure15.Figure 15
Configuration (2): metal layers.With a patch area of 76 mm by 76 mm, several filter structures using strip-line and microstrip, along
with a calibration structure, were included in the layout. The latter is done
for on-chip calibration and to exclude any parasitic effects in the
measurement.In the following, the
measurement results of several coupled-line filters implemented using the
mixed-k topology are reported.


### 6.1.
Strip-Line Versus
Microstrip

Following the discussion
in Section4 about the parasitic effects on the filter performance, a single
hair-pin  resonator filter with line feed was implemented in both the strip-line
and the microstrip configurations. Figure 16 shows the filter layout.Figure 16
Layout of
single hair-pin filter with parallel coupled-feed lines.The line feed was about a
quarter the length of the hair pin(≈λ/8). As
predicted previously, the microstrip configuration gave lower losses and a
better quality factor than the strip-line configuration due to lower loss in
the ground plane. Figure 17 shows the S21  for both configurations.Figure 17
S21 for
single hair-pin filter with (a) strip line, and with (b) microstrip line
configuration.


### 6.2. Loop Feeds Versus Coupled-Line Feeds

To
investigate the feasibility of using a loop feed instead of a coupled-line
feed, a hair-pin resonator filter was  fabricated
using coupled line and loop feeds. Figure18 shows the filter layout.Single hair-pin resonator
filter with (a) coupled-line feed and (b) loop.
(a)(b)The filter responses are shown in Figure19.Figure 19
S21 for hair-pin resonator filter with (a)
coupled-line feed and (b) loop feeds.


### 6.3.W-Shape Resonator

To reduce the size and increaseQ, a new
resonator configuration using the W-shape is presented and compared to the
existing folded resonator shape as shown in Figure 20.(a) Folded resonator, (b)W-shape resonator.
(a)(b)Results show (see Figure21) that the W-shape resonator filter
gives a quality factor of 79 compared to the folded resonator that gave a
quality factor of about 58. The
higher resonance frequency of the W-shape as compared to the folded resonator was
an indication of less capacitance.S21 (dB) for (a) capacitive (folded resonator) and
(b)  W-shape resonator.
(a)(b)


## 6.1.
Strip-Line Versus
Microstrip

Following the discussion
in Section4 about the parasitic effects on the filter performance, a single
hair-pin  resonator filter with line feed was implemented in both the strip-line
and the microstrip configurations. Figure 16 shows the filter layout.Figure 16
Layout of
single hair-pin filter with parallel coupled-feed lines.The line feed was about a
quarter the length of the hair pin(≈λ/8). As
predicted previously, the microstrip configuration gave lower losses and a
better quality factor than the strip-line configuration due to lower loss in
the ground plane. Figure 17 shows the S21  for both configurations.Figure 17
S21 for
single hair-pin filter with (a) strip line, and with (b) microstrip line
configuration.


## 6.2. Loop Feeds Versus Coupled-Line Feeds

To
investigate the feasibility of using a loop feed instead of a coupled-line
feed, a hair-pin resonator filter was  fabricated
using coupled line and loop feeds. Figure18 shows the filter layout.Single hair-pin resonator
filter with (a) coupled-line feed and (b) loop.
(a)(b)The filter responses are shown in Figure19.Figure 19
S21 for hair-pin resonator filter with (a)
coupled-line feed and (b) loop feeds.


## 6.3.W-Shape Resonator

To reduce the size and increaseQ, a new
resonator configuration using the W-shape is presented and compared to the
existing folded resonator shape as shown in Figure 20.(a) Folded resonator, (b)W-shape resonator.
(a)(b)Results show (see Figure21) that the W-shape resonator filter
gives a quality factor of 79 compared to the folded resonator that gave a
quality factor of about 58. The
higher resonance frequency of the W-shape as compared to the folded resonator was
an indication of less capacitance.S21 (dB) for (a) capacitive (folded resonator) and
(b)  W-shape resonator.
(a)(b)


## 7. Conclusion

In
this paper, integration of passive filters on silicon substrates is
investigated. Simple hair-pin band reject filters (BRFs), that support
dielectric mode, are presented. Ferroelectric bars were used as resonant elements, compared to
transmission line hair-pin resonators; an improvement of about 10 inQ is
achieved. Multilayer dielectric ferroelectric material is discussed and gives
about 5% tunableness. Furthermore,
the miniaturization of filters introduced was achieved by using an embedded
high-k substrate configuration that is feasible using an LTCC process.
An improved W-shape bandpass filter was proposed with a wide stopband and
approximately 5% bandwidth.



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*Source: 537069-2008-11-11.xml*