Integrated Passive Components A Brief Overview of LTCC Surface

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					         Integrated Passive Components: A Brief Overview of LTCC
                    Surface Mount and Integral Options

                                            A.J. Piloto
                                         Kyocera America
                                       San Diego, CA 92122

                                            ABSTRACT

         Emerging personal electronic devices and Radio Frequency (RF) systems are demanding
reductions in both size and cost. A significant portion of passive component (i.e. resistors, capacitors,
and inductors) integration is being driven by these reductions in size and cost. The integration
technology must address stringent reliability and maintainability requirements, reducing life cycle cost
and achieving improved performance over a wide range of sizes and functional requirements. The
applications for this technology, from standard electronics modules to single MMIC packages to laptops
and cell phones, are both desktop and handheld. This paper will outline some of the activity in this
exciting new area and include specific examples of material sets for Integrated Passive Components
(IPC’s) and their impact on monolithic microwave multichip packaging.

INTRODUCTION

        Passive components such as resistors,
capacitors and inductors, once viewed as
ancillary, are emerging into an exciting new area
of integration.     Upon disassembly of any
computer, cell phone or PDA, you would
discover that hundreds if not thousands of
passive       components      surround     active
semiconductors. As described by Kapadia, et. al.,
many wireless products have passive to active        Figure 1. Many Passive Components
                                    1,2            Typically Surround Active Semiconductors
component ratios of 15:1 to 30:1.          A less
extreme example, shown in figure 1, shows an
ASIC controller and 4 regulators surrounded by 12 capacitors and 5 resistors.

        Integrated Passive Components, IPC’s, generally fall into three (3) categories:

        •    Resistors
        •    Capacitors
        •    Inductors

Combinations of these devices provide for timing loops around analog devices, tuning
structures around RF circuitry, filters, and resistive loads on differential pairs among other
applications. The NEMI Roadmap foresees the movement from virtually 100% discrete
passive component usage in 1996 to 70% integration by 2004.3 The electrical
functionality of each of these components typically dictates that disparate materials are
used to construct these devices:

                                                                                                       1
          •    Resistors: Generally comprised of ruthenium oxide (RuO2) or tantalum oxide
               (Ta2O5).4
          •    Capacitors: Generally comprised of silicates or titanates
          •    Inductors: Generally metallic conductors in some spiral shape to provide
               inductance

The             composite
requirements of IPC’s                                                                                         Increasing
                                                                                                            Circuit Density

are typically not met by                           Silicon   Semiconductors
                                                   GaAs      ε = 3.8-11
conventional packaging                                               r



technologies such as                         BCB
                                                                   Multilayer Thin Film
organic packaging, on-                                             ε = 2.1-3.5
                                                                          r



chip solutions, high                      Polyimide
                                                                          ε    r
                                                                            Multilayer Ceramics/LTCC
temperature         cofired                                                 ε = 3.8-16,000
                                                                                   r
                                     Low Temperature                        Resistors
ceramic,                            Cofired Ceramic (LTCC)
                                                                              ε Multilayer Ceramics/HTCC
single/multilayer      thin          High Temperature                             ε = 6.0-7000
                                                                                       r
                                                                                           r

                                   Cofired Ceramic (HTCC)
film (MCM-D) or thick
film hybrid technology.                                                                 Multilayer Organics
                                                                                        ε = 2.5-3.5
                                  Cyanate Ester, Epoxy Glass                                   r

As shown in figure 1,
low temperature cofired
ceramic            (LTCC)         Figure 1. Low Temperature Ceramic is one of the emerging
technology is one of                         Technologies Capable of IPC development
several          emerging
technologies, along with advanced organic and on-chip solutions which can encompasses
the attributes of each while minimizing the detriments. Additionally, as shown in figure 1,
LTCC displays the widest range of dielectric properties available for any packaging
technology, enabling the incorporation of passive components.5 Furthermore, LTCC
technology can incorporate resistive material thus enabling the integration of both
resistors and capacitors within the substrate. The three (3) examples described herein give
a brief overview of the IPC’s and the LTCC materials from which they are fabricated.

WAVEGUIDE FILTERS

       Cofired ceramics offer many advantages for achieving higher packaging densities
in RF/microwave and digital integrated circuits. The integration of high speed digital and
DC power circuits along with RF/microwave structures using cofired ceramics, for
example, is becoming common practice for many wireless and microwave module
applications. Although rectangular waveguide technology has been demonstrated before,
a waveguide filter recently demonstrated illustrates the versatility of this technology. 6,7,8,9

         One of the significant advantages of a waveguide structure over printed
transmission lines is the lower insertion loss characteristic it provides. Desirable for low
loss filter applications, the waveguide structures can be embedded into multilayer cofired

                                                                                                                         2
ceramic assemblies without significant
crosstalk. Additionally, these structures can be
efficiently transitioned to other transmission
line structures such as microstrip or stripline.

        The first LTCC waveguide filter
described in this section was introduced and
patented using inductive windows.10 This
structure is shown in Figure 2. The sidewalls of
the waveguide may be constructed by using Figure 2 - Inductive Window Filter can be
                                                   Integrated into a Ceramic Substrate or
metal filled vias. The initial design is based on                 Package.
the synthesis procedures of Young and
Mathaei.11 The final design is completed by using full wave analysis with a combination
of mode matching methods and optimization techniques by Zaki.12,13 The measured
results compared to the modeled results are shown in figure 3.




          SIMULATED RESPONSE                               MEASURED RESPONSE

             Figure 3. Measure vs. Simulated Response for Waveguide Filter in LTCC

These filters, shown in figure 4., were fabricated using the Ferro A6, calcium borosilicate
LTCC. The inductive window filter measures 2.40
x 0.36 x 0.011 inches and is embedded in a 0.076
inch thick LTCC substrate.

ENCODER CIRCUIT14

        One trend in the development of IPC’s is to
integrate layers of multiple dielectric constant
materials within one substrate as shown in figure 5.    Figure 5. Trend: Multiple Dielectric
                                                                Constant Substrates
In this case high dielectric constant materials are
embedded within lower dielectric constant, insulator materials. Developed by the Kyocera
Central Research Laboratory, this substrate/packaging technology integrates three (3)
dielectric constant materials, BaTiO 3, CaTiO3, and magnesium silicate (Mg2SiO4).


                                                                                          3
        Furthermore, surface thick film
resistors are applied in a postprocessing
step. A cross section of this substrate and
its properties are shown in figure 6 and 7
respectively.

        In order to make cofiring possible,
constituent ceramic materials must be
sintered at the same temperature. Figure 4. The Inductive Window Filter Measures
                                              2.40 x 0.36 x 0.011 inches and is Integrated into
Additionally,    shrinkage     match    and           0.076 inch thick LTCC substrate.
moderate interdiffusion (for adhesion) must
be considered in the development of a multiple dielectric constant substrate.
                                                                                              Barium     Calcium
                                                                                  Fosterite              Titanite
                                                                                              Titanate
                                                       Dielectric Constant          7.1        2000        110

                                                                  -7
                                                         CTE (x 10-7 /oC)           120         120         X

                                                                          Ω
                                                   Insulation Resistance (Ω cm)     1014       >1012      >1013
                                                     Thermal Coefficient of         N/A        +/- 15    +/- 120
                                                    Capacitance (%, ppm/oC)
                                                      Dissipation Factor (%)        2.0        0.75       0.01




Figure 6. Multiple Dielectric Constant Substrate   Figure 7. Properties of the Multiple Dielectric
 Integrates Barium Titanate, Calcium Titanate                 Constant Constituents
                 and Forsterite

        The capacitor types in this substrate
                                                      After               Before
technology are rated at UJ and X7R. An example
encoder circuit showing both a piece part count
reduction and size reduction before and after the
application of a multiple dielectric constant
substrate is shown in figure 8. The capacitor
characteristics are shown in figure 9. Alternative
capacitor compositions that densify at less than
1100oC have also been explored to provide not Figure 8. “Before and After” Encoder
                                                   Substrate with IPC is Significantly
only X7R characteristics but also C0G Smaller than with SMD Discrete Passives
characteristics.15




                                                                                                                    4
SURFACE MOUNT RC NETWORK
                                                                                          X7R       UJ                                                    X7R                                UJ
        In some cases, the desired use of
                                                    Electrode Size (Sq. mm.)              2.5      3.0
LTCC is for IPC’s that integrate several
discrete functions; that is, integrated             Dielectric Thickness (µm)
                                                                            (µ             22       45

resistors, capacitors and inductors in a                   No. Layers                      10       2
surface mount device.              While the            Capacitance (nF)                   50       3
dominant usage of thick film resistor                Dissipation Factor (%)               2.0      0.75
materials is for surface-fired devices,
                                                     Breakdown Voltage (V)               >500     >1000
there are emerging applications for
cofired buried resistor systems, including
LTCC packages with buried passives                Figure 9. Example of Demonstrated Capacitor
                                                              Values and Configuration.
discussed above, and more recently, the
AVX patented |Z|Chip™ integrated series RC components.16,17 This application places
different constraints on the buried resistor system compared to surface firing resistors.
An obvious difference is the local environment during firing, but there are others as well.
In producing LTCC integrated passives, special attention is given to the buried resistor
because of its sensitivity to process
conditions In many applications, the
buried resistors cannot be trimmed,                                Print Thickness vs. Resistance
                                                                       800

therefore they must be very predictable                                700


in their electrical properties after firing.
                                                   Resistance (Ohms)




                                                                       600

                                                                       500
This means the resistors must have very                                400

uniform,        reproducible          printing                         300

characteristics, as it is at this step that the                        200


virtual tolerance of the device is set.                                100

                                                                        0
Also, given that the resistors, dielectric
                                                                                                         10.00

                                                                                                                 10.50

                                                                                                                          11.00

                                                                                                                                  11.50

                                                                                                                                          12.00

                                                                                                                                                  12.50

                                                                                                                                                          13.00

                                                                                                                                                                  13.50

                                                                                                                                                                          14.00

                                                                                                                                                                                  14.50

                                                                                                                                                                                          15.00

                                                                                                                                                                                                  15.50

                                                                                                                                                                                                          16.00
                                                                             8.00

                                                                                    8.50

                                                                                           9.00

                                                                                                  9.50




and conductors are cofired, densification                                                                                Print Thickness (microns)


upon sintering must be well matched to
minimize distortion during firing.              Figure 10. Sheet Resistance vs. Print Thickness for a
                                                                                    Commercial Cofired Buried Resistor18
        The     AVX        |Z|Chip™      is
component that has pushed material and process development for cofired electronic
materials. These devices are made in the structure of multilayer capacitors, and can be
fabricated with RuO2-based resistive inks in place of traditional, conductive electrodes
embedded in the low K, DuPont 951 dielectric system.18 With the added internal
resistance of each layer, the devices perform as integrated series resistor-capacitor (RC)
networks. Other than the mechanically robust cofired structure, the principal benefit of
these type of devices is that the components can be made very small, currently as 0603
and 0402 case sizes for a "discrete" device forming a single RC network, and as 0612 four-
pair arrays. Whereas traditional use for both surface-fired and buried resistors is for single
layer components, the |Z|Chip™ typically has 10-30 internal resistor layers with 10-20
micron fired dielectric thickness. Given this structure, it is necessary to minimize resistor
print thickness to reduce cost and increase volumetric efficiency. However, as resistor
print thickness is reduced, the sensitivity of sheet resistance to the absolute tolerance of
                                                                                                                                                                                                                  5
print height becomes an issue. For example, given a printing process with an absolute
print thickness control of ± 0.5 micron, one could nominally produce untrimmed resistors
with a tolerance of ± 6.5% at 15 micron print thickness, but only ± 10% if print thickness
is 10 microns (Fig. 10 ).

        Additionally, the resistor sintering shrinkage must be well matched to the dielectric
to avoid formation of internal defects from differential densification, especially when
closely-spaced multiple resistor layers are used. However, devices with high layer counts
can be made economically to at least ± 10% tolerance ranges without trimming when
three conditions are met:

       •   The cofired material suite is designed
           to have similar densification rates,
       •   There is minimal interaction between
           the glass component of the dielectric
           and the resistor, and
       •   There is uniform printing of internal
           inks.

A cross-section of this type of device is shown in
Figure 11.
                                                       Figure 11. Cross-Section of a Cofired RC
CONCLUSIONS
                                                                   IPC, |Z|Chip™

         Integrated Passive Components (IPC’s) are in demand from the marketplace. It is
the availability of cost effective materials with stable, predictable properties that enables
thick film cofired IPD’s. The material properties need to be tailored to match sintering
temperature, shrinkage rate, coefficient of thermal expansion as well as providing reaction
bonding, all the while meeting electrical performance. The few examples presented in this
paper show that as the material performance and capabilities are disseminated throughout
the industry, the engineering creativity to meet this challenge is unleashed. This sparks
previously unseen solutions in passive component integration. The focus that drove the
original material development is usually eclipsed by the myriad of uses that exploit the
full range of the new materials characteristics. Returning to IPC’s, the growth and
miniaturization of both digital and RF electronics will continue to provide demand and
focus for future developments addressed with cofired materials.

ACKNOWLEDGMENTS

      The author wishes to acknowledge the contributions and encouragement of Jeff
Howell, Andy Ritter and Elizabeth Rose to this article.

REFERENCES


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