AbstractâA desire to incorporate multiple functions into a single component continues to drive product size reduction in the wireless electronics industry.
2004 IEEE Ultrasonics Symposium
Modular Integration of RF SAW Filters Martin P. Goetz, senior member, IEEE Clarisay, Inc. Dallas, TX 75204 USA
Chris E. Jones, member, IEEE Clarisay, Inc. Dallas, TX 75204 USA
Abstract—A desire to incorporate multiple functions into a single component continues to drive product size reduction in the wireless electronics industry. In the radio portion of wireless handsets, many of the functions have been modularized. Because surface acoustic wave (SAW) devices are built using a piezoelectric substrate, they require unique packaging in order to ensure their functionality. Some form of a hermetic cavity over the active area of the SAW die is necessary to avoid damage or mass loading to the unpassivated surface. This paper will review the nature of the RF SAW filters and current packaging approaches. A novel encapsulation process for these filters will be discussed. The active area of the filter is protected at the wafer level by a low profile hermetic lid. It can thereafter be tested, singulated and integrated along with other devices into a module. Examples of modules used in wireless products with integrated SAW filters will be given.
pass band and good out-of-band rejection. However, because SAW filters must be fabricated on piezoelectric substrates, they are difficult to monolithically integrate on semiconductor chips. Furthermore, a SAW filter requires specialized packaging with a protective cavity over the active area of the filter circuitry in order to avoid mechanical clamping and mass loading from moisture or other contamination. Traditionally, this cavity is provided by ceramic or metal packages. More recently, chip-scale type packages for SAW filters have provided progress in the attempt to make the filters smaller [2]. Wafer-scale packaging techniques, in which protective caps are fabricated over the active surface area of the filter prior to singulation from the wafer, have provided further size reduction [3]. RF modules integrate SAW filters using a variety of assembly techniques. These include mounting bare SAW filters, low noise amplifiers (LNA) and mixers on a silicon substrate, and then mounting the subsystem into a ceramic package with a cavity. Others integrate bare SAW filters, switches, and a direct conversion receiver on a ceramic substrate, covering the module with a metal lid [4]-[5].
Keywords-surface acoustic wave, SAW, filter, module, packaging, wafer-level.
I.
INTRODUCTION
There are various forms of integration strategies for microelectronics. One strategy is the system-on-chip (SoC) approach, which requires integrating various functions, such as logic, memory, analog and RF into a single device using a monolithic substrate. Silicon is a common platform for this technology, yet other substrates such as silicon germanium and silicon-on-insulator (SOI) are also used.
An advanced wafer-level encapsulation of SAW devices has been realized that provides the size advantage of a bare die and the environmental protection of a packaged die. The encapsulation process is performed at the wafer level, leveraging an in-line batch process flow. Because of this, both device size and product cost can be reduced compared to existing packaging technologies.
Another approach to integration, termed system-inpackage (SiP), provides a combination of discrete and integrated semiconductor devices, plus passives, into a standardized package form factor. SiP is a derivative of other forms of module level integration, termed hybrids and multichip modules (MCM). There is virtually no real difference in the three module terms as far as integration strategy is concerned. They simply eliminate one level of packaging before being connected to each other.
II.
WAFER-LEVEL ENCAPSULATION PROCESS
RF SAW filters are fabricated on substrates that are both piezoelectric and pyroelectric in nature. This places severe limits on the allowable process temperatures and applied pressures by which the wafers can be exposed [6]-[8]. The substrate builds up an electric charge as the wafer is exposed to large temperature gradients, which can cause irreparable damage to the SAW device.
The wireless industry is moving quite aggressively to introduce products that offer more functionality in one device. Cell phones are organizers and web browsers (smart phones). Personal digital assistants (PDA) are web browsers and telephones. Web pads have become personal communicators, including integrated cameras to capture still pictures or live video. The ubiquitous ‘all in one’ communicator is here [1].
The wafer level encapsulation process involves bonding the SAW filter wafer to a mating wafer using a photodefinable adhesive. Silicon is chosen as the mating wafer for its mechanical strength and low cost processing via standard semiconductor techniques. The inherent piezoelectric properties as well as the ceramic nature of the SAW substrate required extensive review of process pressure and temperature applied during bonding in order to ensure a stable mechanical bond without causing breakage or voiding. The ability to etch the silicon without damaging the patterned
RF SAW filters are frequently used in wireless devices to meet the requirements of low insertion loss across a wide This work was sponsored by Clarisay, Inc., Dallas, TX USA
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2004 IEEE Ultrasonics Symposium metalization on the SAW filter was a key component for this process. A photo-definable adhesive for bonding the two wafers was required so that the portion of the material which corresponds to the active surface area of the SAW device was removable, thus eliminating the need to etch a cavity into the mating wafer. A B-stage benzocyclobutene (BCB) adhesive resin was selected, which is noted for its low moisture absorption, low ionic content and excellent adhesion for wafer bonding [9].
impedance of a multi-band low noise amplifier (LNA), a filter plus matching components would be required for each band. If a filter module was designed to accommodate multiple bands, and assembled with discrete passive elements, specifically inductors and capacitors, on an organic or ceramic substrate, this could make the total component count as high as 16 for a quad-band filter module. Therefore, based on these considerations, these goals for the filter are determined by the following factors:
Fig. 3 illustrates the process flow for creating the wafer level encapsulated SAW filter. In order to accurately align and bond the two wafers together, alignment marks were placed on both the SAW device wafer and the mating wafer. Since the mating wafer is opaque, it required alignment marks on both sides to utilize a double-sided alignment technique. Registration was performed through a directional camera system, which was built into the alignment system of the bonding machine. The alignment accuracy was designed for ±2µm.
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smallest device profile (l, w, h)
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cost of assembly
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cost of encapsulated SAW filter
At some point, the goals will reach a low enough value to create a compelling reason to integrate the filters. A few examples are given that illustrate the kinds of applications that would benefit from an integratable filter. A. Front-End Modules RF front-end modules (FEM) integrate the functions of the antenna switch module, with that of SAW bandpass filters. The antenna switch module consists of a diplexer, Tx/Rx switch, and low pass / high pass filter functions. The diplexer and LC filters are typically embedded into a low temperature co-fired ceramic (LTCC) substrate and the discrete switch and filters are assembled onto the substrate. There may be a digital controller IC integrated as well. A dual-band FEM using encapsulated RF SAW filters is shown in Fig. 2. The filters and a GaAs switch are assembled using a wirebond assembly process. A surface mount component is also added during the assembly process.
2-SE-BAL SAW Filters Module size: 5.4mm x 4.0mm
Figure 1. Wafer level encapsulation process for SAW filters.
Once the wafers were bonded, the process resembles a more traditional semiconductor process flow. The mating wafer is patterned and processed with deep reactive ion etching etching (DRIE) to remove silicon from the edges of the SAW filter, thereby exposing the outside of the adhesive. After the etching is complete, the wafer is passivated with SiO2 to provide a hermetic seal around the edge of the adhesive. Fig. 4 shows the post-etch results of the process, before the passivation is applied. III.
RF MODULE APPLICATIONS
SAW filters are key components in modern mobile phone applications. The possibility to realize impedance conversion and balun functionality for balanced outputs are key elements in the receiver section of the radio. With multi-band phones, the component count goes up based on the number of bands supported. For example, to match the complex input
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Figure 2. Dual-band RF front-end module with two wafer level encapsulated SAW filters assembled onto ceramic substrate.
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2004 IEEE Ultrasonics Symposium B. Integrated CDMA Duplexer With the availability of low profile wafer level encapsulation for SAW filters, the possibility to integrate duplexers into modules can be realized. An example of a CDMA SAW duplexer integrated onto a printed circuit board with associated phase matching is shown in Fig. 5. The duplexer solution consists of a single die for both Tx and Rx filtering. An external phasing network is designed as part of the module. In one configuration, a π-matching network with two SMT capacitors and an inductor can be used to provide the match. If the module were to use an LTCC substrate, the network could be embedded. Alternatively, a λ/4 delay line could be designed either as a separate component, or into the substrate. Fig. 6 shows the response of the duplexer as shown.
Figure 3. Quad-band RF front-end module with two dual-band wafer level encapsulated SAW filters assembled onto ceramic substrate.
An expansion of an FEM is shown in Fig. 3, which is designed as a quad-band. The low band filters (AMPS/EGSM) are fabricated on one chip and the high band filters (DCS/PCS) are combined on another chip. Combining the low bands together and the high bands together provided another level of integration, thereby reducing customer assembly costs. The AMPS and EGSM filters were designed with the same metal thickness requiring no extra processing steps. Similarly, the DCS and PCS filters were designed with the same metal thickness. The response of the complex impedance matching filters are shown in Fig. 4.
One of the key challenges in duplexer design is controlling the parasitics in general and the ground inductance in particular. It is necessary to tune the resonance nulls created by the parasitics to achieve isolation and rejection performance in a duplexer. Coupling between ground currents leads to increased ground inductance. These currents at the die level had to be carefully designed to meet the isolation and rejection performance. Various software tools and techniques are now commonly used to address these issues [10]-[14].
Figure 5. Encapsulated CDMA duplexer assembled onto printed circuit board with discrete matching elements.
Figure 6. Measured bassband response of CDMA duplexer, including matching network and channel isolation.
Figure 4. Response of dual band wafer level encapsulated filters. Top: AMPS/EGSM, Bottom: DCS/PCS.
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2004 IEEE Ultrasonics Symposium C. Transceiver Module A stacked die integration of a wafer level encapsulated SAW filter mounted onto a thin film integrated passive device (IPD), mounted onto a transceiver IC and assembled onto a leadframe QFN package. The IPD was used as an interposer and provided interconnect redistribution, a matching network between the SAW filter and the transceiver, and a thermal insulator. A dual band, direct conversion transceiver integrated dual balanced input LNAs, dual quadrature mixers, baseband filtering, and receive and transmit VCO circuitry on a SiGe:C BiCMOS process platform with 10mm thick Cu inductors and 1.6 fF/mm2 metal-insulator-metal (MIM) capacitors [15]. The module used single-ended input, balanced output filters designed for EGSM and DCS bands. The IPD interposer was designed to provide two matched networks: one each for EGSM and DCS bands. The matching networks employ MIM capacitors and integrated inductors in a process flow similar to that of the transceiver but without transistors. The IPD also includes an option to provide direct conversion between filter and LNA without matching network. The IPD design used existing scalable library models for the inductors and capacitors [16].
commercial, environmental, and reliability conditions, and provides a low cost alternative to conventional packaging approaches. Numerous applications were illustrated to emphasize the opportunity of integrating SAW filters into modules that may not otherwise be available. ACKNOWLEDGMENT
The authors wish to express their appreciation to the following for their contributions: Kushal Bhattacharjee, Oleg Lusev, Jagan Rao, Paul Lindars, Al Hatcher, Marc-Andre and Sarah Schwab from Clarisay, and Cliff Vaughn and Robert Jones from Motorola. REFERENCES [1] [2]
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[13] Figure 7. Integrated transceiver module with SAW filter, integrated passive device, and transceiver packaged in plastic QFN.
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IX. SUMMARY AND CONCLUSION
[15]
Wafer level encapsulation of SAW devices has been developed for wireless modular integration. The wafer-level packaging processes and configuration differ from conventional SAW chip scale packaging, since it is designed as a hermetically sealed bare die. This type of encapsulation addresses the need for smaller, more compact SAW filters and lends itself to design flexibility within the module. This technology provides a device that is robust enough to handle
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[16]
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2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control Joint 50th Anniversary Conference
