Lumped and distributed element design for LTCC radio filters

Lumped and distributed element design for LTCC radio filters
Jens Müller
Micro Systems Engineering GmbH & Co.
Schlegelweg 17
D-95180 Berg/Germany
Phone: +49 9293 78 64
E-Mail: jmueller@mse.biotronik-erlangen.de
Claude Guichaoua
Solectron Brittany/France
Route de Quimper
F-29590 Pont-de-Buis/France
Phone : +33 2 98 81 33 04
E-Mail : ClaudeGuichaoua@bty.slr.com
Abstract
In the perspective of the 3 rd generation of mobile systems (3G) and in the current evolution of 2G,
multi-mode wireless terminals are developing to provide more services for users. Modes of interest are
GSM 900, GSM 1800, IS95, TETRAPOL, TETRA, WB-CDMA, APCO 25, DIIS and GPS.
Very tight technical requirements (TETRAPOL, APCO 25), along with integration constraints (multimode
capability in a user-friendly cabinet) and price pressure (-10% a year), make the new platform a
challenge. In that context, smart implementation of wide-band VCOs and tuneable filters are
compulsory to meet market requirements.
LTCC is a convenient technology for the manufacturing of band pass filters for a bandwidth of 380
MHz to 2400 MHz as required by radio telephony systems [1,2]. For lumped element filters, the best
choice turns out to be a hybrid solution with a mix of soldered components on top of the package and
embedded components. Components which would require a large area or too many layers to be
integrated in LTCC can be selected as SMDs.
Due to the relatively low frequency and therefore “long” wavelength, the distributed filters require
considerable space to be realised in LTCC having a permittivity of about 6-10. A combline filter design
was found to be the best in terms of size and electrical performance.
Demonstrators were built up and the results of the S-Parameter measurement will be discussed.
Key words: LTCC, RF-filters, passive integration, miniaturisation
Introduction
LTCC technology provides three major
benefits for filter designs:
first, the ceramic nature of the substrate
allows the implementation of resonators of
significant quality factor;
second, the hybrid assembly process
allows the mounting of a variable capacitor
and varactor diodes to provide the tuning
of this type of filters as well as a BGA
connection to the mother board;
third, the implementation of embedded
passive components like resistors,
capacitors and coils, along with. the fact
that this technology allows a multilayer
design which is nearly unlimited (20 layer
substrates have been already successfully
manufactured), allows an outstanding level
of integration.
The paper demonstrates two
approaches for LTCC filter modules – lumped
and distributed element designs. The
development of the lumped BGA-filter module
started with the electrical filter design. After
electrical optimisation on the schematic level
(including feasibility for integrated
components) the elements of the circuit were
analysed for size and layers required to make
them in LTCC. Optimisation was focussed on
the size of the filter package. The maximum
layer count defined was 8. The embedded
components (capacitors and inductors) were
designed based on equations developed
earlier [3]. Fine tuning of the component
performance including interference and
coupling effects was made with electrical field
simulation.

Distributed filters were designed and
optimised using a 3D electromagnetic field
solver. These filters were not limited to 8 layers
to allow a more flexible design.
The demonstrators were realised with
DuPont 951 material using silver pastes.
Solder paste stencil printing was used for
bumping. After SMD mounting, the filters were
singulated and assembled on a test board for
electrical characterization.
Passive Integration
Passive integration is defined as a
combination of circuit carrier (substrate, board)
with passive components like resistors,
capacitors and inductors in one technology.
Passive elements may be integrated on the
surface or embedded in a multilayer structure
of the substrate. The goal is to achieve:
Reduced module sizes
Lower costs (component reduction)
Improved electrical and thermal
performance
Higher reliability (reduced number of I/O)
LTCC-resistors are achieved by
printing a pattern with a specific resistor paste.
These pastes are available from 1 Ohm/sq. to
> 10 MOhm/sq. Due to printing and other
process and material tolerances, these
resistors need to be trimmed. Therefore,
precise resistors are placed on the surface of
the substrate for accessing them by laser. For
lower requirements on the tolerance (e.g.
> 25%) resistors can be embedded in the
LTCC substrate. This can be useful for buried
absorbers or internal heaters [4]. Embedded
resistors may be trimmed to a desired value by
high-voltage-pulse-trimming [5]. This method is
very paste and design dependent.
Fig. 1: Inductor designs for LTCC
Inductors in LTCC are made of line
elements which form a spiral or a helix. The
inductance is a function of numerous
parameters like number of windings, tape
thickness, position to the ground plane, line
width etc. Fig. 1 shows typical designs for
LTCC inductors. Useful inductance values for
printed RF-coils are in the range of 1…100nH.
Capacitors can be designed as
interdigital (planar comb structure) or as plate
capacitors (Fig. 2). They are simple to
integrate since they are printed together with
the conductor line pattern. The capacitance is
a function of permittivity, plate area, distance
between plates and number of plates (for a
multilayer capacitor). Using the tape as
dielectrics between the plates limits the useful
range from 0.2 ….10pF. These capacitors are
compatible with RF-requirements in terms of
self resonance frequency and quality. Higher
capacitance densities can only be achieved by
inserting a special high-k-material between the
plates [6]. Due to printing accuracy and lot to
lot variation, a tolerance of about > 15% is
achievable. These materials show also a poor
dielectric loss behaviour which makes filter
designs with high selectivity impossible. These
materials were mainly developed for
decoupling capacitors or similar applications.
Fig. 2: Multilayer plate capacitor
For microwave frequencies however,
passive integration is not limited to the lumped
passive components mentioned above.
Structures made of line elements are used to
create filters, resonators, couplers etc.
Depending on the line type chosen, these
structures are on the surface (microstrip line)
or embedded between ground planes
(stripline). The latter provides reduced
interactions with neighbouring components
due to the complete shielding. The size of
these components is related to the signal
wavelength. For frequencies above 20 GHz
such filters become small. In the frequency
band of interest here, they are rather large and
therefore not suited for high integration. The
advantages of line or distributed filters are the
low insertion loss and the good selectivity.
General filter design
The LTCC filters specifications were
defined from a complete radio architecture
study based on a multi-mode architecture

covering both professional and public market.
According to standard constraints (UMTS,
TETRAPOL,…) the global radio architecture
was defined including 1dB compression point
requirements, Mixer image rejection, Mixer
harmonics, Local Oscillators leakage filtering,
noise dimensioning, inter-modulation requirements
and filtering budget.
Lumped element filter design
The general flow for the filter design is
shown in Fig. 3. Designing a filter starts with
the system requirements mentioned above. In
our case the filter design and it’s optimisation
was performed by GENESYS 1 .
SMD selection
(RF-properties
from vendor)
System
Specification
Electrical filter
synthesis, design
& optimisation
Critical component
analysis/feasibility
(sensitivity)
Component
partitioning
SMT/embedded
Single component
design
Single component
verification by
3D-simulation
Placement in
module
Module verification
by mixed 3D- and
"Black Box"
simulation
Finalise Layout
Filter type,
order
estimated
properties
(e.g. quality)
layer count,
thickness,
estim. area
Component
modification
n
correct
behavior?
y
Placement
modification
correct
behavior?
Fig. 3: Design flow for filter development
1 Courtesy of EAGLEWARE Inc
n
y
Ideal components are used for the first
approximation (Fig. 4). In the second step,
typical element properties like losses, self
resonance frequency etc. are added and
optimisation is repeated. Critical components
can be traced by a tolerance sensitivity
analysis showing the highest impact to the
overall behaviour, which is also a measure for
manufacturability.
Fig. 4: Example of filter schematic
Next, the schematic is divided into
integrated components and SMDs (e.g. actives
for tunable filters or large capacitors). With a
coarse design of the embedded components
based on library elements or semi-empirical
equations a first placement study and size
estimation can be derived. If the area of the
module is much larger than the area for the
SMDs on top, some of the large integrated
components should be changed to a SMD-type
to get an optimum in module size (Fig. 5).
Large values as SMD
Fig. 5: Component partitioning
After partitioning the “fine” design of
embedded components is done. There are
many parameters which can be adversely
varied like plate area versus number of layers
for a capacitor or number of turns versus coil
radius for inductors. Optimisation of these
designs can be either driven by size
constraints or costs (e.g. number of layers).
The physical dimensions of each component
are obtained from semi-empirical equations or
known similar elements which are already
available in a data base. A library with
measured or simulated electrical parameters
helps to reduce the design time and increases
the confidence in the design. New components
should be optimised or verified by a 3dimensional
electrical simulation until they
show the correct behaviour (Fig. 6).

Fig. 6: Simulation structure of a 3D coil
S11
Fig. 7: Coil simulation results (S11, S21)
up to 6 GHz
The designed LTCC-components are
then arranged according to their interconnections.
Symmetries in the schematic
should also lead to symmetries in the placement
to obtain equal conditions.
Finally, a module simulation including
electrical models of the SMDs, the embedded
components, all connections and the module
interface (e.g. solder bumps) helps to find out
possible problems due to parasitic effects like
cross talk or mutual inductivity. Process or
material tolerances are used to assess
repeatability and manufacturability. However,
complex modules may be still too much for
simulation programs despite the increasing
computing power.
Fig. 8: 3D-structure of the module
S21
S11, S21[dB]
0
-20
-40
-60
- 7µm
-80
0 400 800 1200 1600
Frequency [Mhz]
+ 7µm
Fig. 9: Influence of tape thickness
variation on the frequency
response
Distributed filter design
Based on the specifications from the
general filter design, the synthesis of the
distributed filters was performed using
GENESYS software 1 (M/FILTER module).
Combline and hairpin topologies were selected
due to their easy implementation and
integration in LTCC technology. The number of
poles, the transformation functions (Tchebychev
or Elliptic) were defined in order to meet
the specifications.
The initial layout was implemented in
IE3D 2 software for electromagnetic simulations
and optimisation in terms of space and
performance. Additionally, manufacturing
constraints and tolerances as material thickness
were taken into account in the design
optimisation, in order to improve manufacturing
yield.
Fig. 10: Combline filter structure (top and
bottom ground planes not
shown)
2 ZELAND Software Inc

S11, S21 [dB]
0,000
-10,000
-20,000
-30,000
-40,000
1,9 1,95 2 2,05 2,1 2,15 2,2 2,25 2,3
frequency [GHz]
Fig. 11: Simulation results for the combline
filter
LTCC filter realisation
Both introduced filters were
manufactured using DuPont Greentape 951.
Though, the lumped element harmonic filter
uses only 6 tape layers, it was made of 8
layers to allow other filter solutions on the
same test coupon. The two additional layers
were provided with feed-through vias for
connecting the solder bumps of the BGA-type
module. Due to the required thickness of the
combline structure, this filter was made of 14
layers. The conductor material applied was Ag
to maintain very low conductive losses.
After LTCC-processing, all modules
were solder bumped using a stencil printing
technique and reflow. The bump pitch is
800 µm with a bump diameter of about
300 µm. Additional 0402-SMT components
were assembled on the harmonic filter
modules prior to singulation from the panel.
Fig. 12: Combline filter module
A test PCB (Fig. 14) with 50calibration
structures and SMA-connectors
was used to measure the scattering
parameters with a network analyzer (HP
8753C).
Fig. 13: Completed harmonic filter
Fig. 14: RF-Test-PCB with mounted
filters
Measurement results are shown in
Figs. 15/16. The harmonic filter behaviour
differs from the predicted performance
according to the simulations. The main
changes are the decreased stop frequency
and the attenuation in the stop band. The
former leads also to a larger attenuation in the
pass band. A first analysis revealed a
dominating influence of the inductors self
resonance frequency and additional parasitic
elements associated with the mounted SMDs.
A detailed investigation is to follow.
A slightly higher attenuation was
obtained in the pass band of the combline
filter. This may be attributed to the AgPd-top
and –bottom ground metallisation (higher Rsq.).
However, the overall prediction of the filter
behaviour is in good agreement with the test
results (Fig. 16). For a first estimation of
production tolerances, several filters have
been measured and plotted in the same
diagram. Fig. 17 shows data from 4 different
filters from the same manufacturing lot. The
filter to filter dispersion is very low, verifying
the constant layer thickness (material

tolerances) and the precise stacking alignment
(manufacturing tolerances).
Fig. 15: Measured S-parameters of the
harmonic filter
dB
0,000
-10,000
-20,000
-30,000
-40,000
1,9 1,95 2 2,05 2,1 2,15 2,2 2,25 2,3
frequency [GHz]
S11[dB]
S21 [dB]
S11 [dB] sim
S21 [dB] sim
Fig. 16: Measured vs. simulated Sparameters
for the combline filter
Fig. 17: Measured S-Parameters of 4
combline filters
Summary and conclusions
Band pass filters for RF-applications in
the frequency range from 380 MHz to 2.4 GHz
have been designed and manufactured using
LTCC technology. Distributed filters (line
coupled) have the disadvantage of size in the
frequency range considered. The combline
design offers the smallest footprint. Due to the
well defined structures and boundary
conditions of line filters, simulation results were
in very good agreement with the measured
data.
Lumped element filters using
embedded inductors and capacitors allow
highly integrated designs. Mixing external
SMDs with buried components permits very
small module dimensions. However, mixed
component designs are difficult to simulate
accurately. Interactions between embedded
elements and external components may
become a dominating factor in the electrical
function. The frequency behaviour of the
harmonic filter realised showed large
deviations from the simulated one. Reasons
for this will be investigated by sub-structure
measurements. Based on these data, a new,
more realistic model will be established.
Acknowledgements
The authors would like to thank the
colleagues from the Technical University of
Ilmenau for the support in measuring the
structures. The work carried out in the project
Multi-Modules is funded by the EU within the
program IST under the project number 1999-
20260.
References
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Microwave Engineering Europe, June 2002, pp. 15-
18.
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Proceedings 38 th IMAPS Nordic Conference,
Oslo/Norway, 2001.
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Passive RF-Components in LTCC”, Proceedings of
the Pan Pacific Conference, Maui/Hawaii, Febr.
1997.
[4] DuPont Thick Film News: “Highlyintegrated
mm-wave modules use LTCC and
metallised plastic covers,” DuPont Horizons, No. 18,
Dec. 2001.
[5] W. Ehrhardt, H. Thust.: “Trimming of buried
RuO2-based Thick Film Resistors in Multilayer
Technology (LTCC) by Energy of High Voltage
Pulses”, Proceedings of the 38th IMAPS Nordic
Conference, Oslo, Sept. 2001.
[6] J. Müller, D. Josip: “Integrated Capacitors
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