Future Trends OF SAW Filters
Tarak HDIJI, Hassene MNIF, Mourad LOULOU, LETI, Electronic and Information Technology Laboratory, National Engineering School of Sfax B.P.W, 3038 Sfax, Tunisia,
Phone: (+216) 98 438 956 Fax: (+216) 74 275 595. E-mail: firstname.lastname@example.org; email@example.com; Mourad.firstname.lastname@example.org ;
AbstractThe key issues for the new generation of SAW filters will be enhanced functionality, miniaturization and integration. The possibilities of realization of miniature RF-SAW device with high frequencies on CMOS substrate for new generation of mobile and wireless communication system is described in this paper. The optimal design and performance of these filters can be reached by using new smart materials and profiting of advances in MEMS (Micro-Electro-Mechanical System) technology. Simulation results are presented using new thin films as piezoelectric materials and transducers.
Key words MEMS, Smart materials, SAW, CMOS, thin films. .
I. INTRODUCTION The trade-offs between noise, frequency of operation, tolerance of interferers, power dissipation, and cost constitute the principal challenge in todays wireless systems. In recent years, progress has been made in efforts to shift to high frequencies in information communication fields such as mobile and optical communications, leading to rapid growth in the gigahertz (GHz) band communications market.
Consequently there has been an increase in demand for filters that function in the GHz band, and filters employing dielectric and piezoelectric materials are being actively developed. Compared with dielectric filters, surface acoustic wave (SAW) filters have good characteristics in the attenuation range, and can be formed in a compact package. Even if further steps in monolithic integration of RF-functions into standard CMOS ICs are anticipated there are several types of RF-components which cannot be replaced until this moment. Amongst these components are the RF-filters. All mobile phones need RF-filters to protect the sensitive receive (Rx) path from interference by transmit (Tx) signals from other users and noise from various RF sources.
As a consequence new phone designs rely on RF-modules which include RF-filters. Recently SAW filters have been integrated in RF modules in various forms using a variety of techniques. Advanced packaging of SAW devices has been developed for modular integration such as low- temperature co-fired ceramic (LTCC) which offers high-quality passive integration capability in the substrate . But adding passives requires additional LTCC layers that increase the thickness of the module and limit in the future, the possibility of miniaturisation of mobile communication products which need smaller and thinner size.
SAWs which are manufactured in a very cost competitive way using standard IC manufacturing, will play an important role in future mobile communication as they offer significant advantages as compared to others types of filters in condition of:
Profiting of advances in micromachining technologies which have successfully achieved miniaturized and integral versions of low loss, highly selective band pass filters covering a wide range of frequencies .
Manufacturing of piezoelectric material with innovative compound to be the structural layer on the CMOS substrate (Silicon Wafer).
Better transducer design, using standard CMOS process fabrication of IDT and profiting of the advances in MEMS technology (e.g. electron beam photolithography).
In the first part of this paper, the operating principle of SAW filters is described. The great success of the SAW technology is also attributable to the potential for ultra precision fabrication of these devices by the latest photolithographic techniques and film thin deposited in substrate development. The last, is focused in the second part. Then, the performance of state-of-the-art MEMS (Micro-Electro-Mechanical System) technology that SAW filters can benefit will be shown. Finally, Simulation results are presented for different design of SAW filter and different substrates and the role and future trend of SAW filters in future mobile communication devices will be reviewed.
II. SAW FILTER In general, a SAW filter is composed of a piezoelectric
material, and comb electrodes called Inter-Digital Transducers (IDT) formed on top of that. As shown in Fig. 1, when a signal is applied to the input electrode, a SAW is generated. This wave propagates to the output electrode where it is converted to an electrical signal. In this mode the element functions as a filter, allowing passage only of frequencies that satisfy f =v/ , where v is the propagation velocity of the SAW, and is the wavelength, four times the electrode line width. In order to operate a SAW filter at high frequency, it is necessary to use a material with a high propagation velocity, or use a fine electrode line width.
Bandwidth (BW) is represented by BW = 2xf/N where N is the number of finger-pair. Propagation velocity is material specific, so when using conventional materials, the problem has been handled by using a finer electrode line width.
However, process constraints put limits on electrode line widths, so with conventional materials practical use has not been achieved at frequencies above 2.5 GHz.
Figure 1. SAW IDT structure on piezoelectric substrate.
In order to prevent spurious signals, the portions of the surface wave traveling to the edges of the substrate must be attenuated by means of acoustic absorbers.
The shape of the pass-band curve is a function of the envelope of the finger overlap W in the transducer. These rectangular functions produces an equally rectangular acoustic impulse response in the time domain and via Fourier transform a sinx/x shaped amplitude response in the frequency domain with a linear phase response. The product of the transfer functions of the two transducers yields the transfer function of the SAW filter as a whole. Therefore, in order to produce a desired frequency response in SAW filters, it is generally necessary to vary the finger overlap length W of the transducers.
To achieve the best SAW performance , it is necessary to:
Adopt a suitable technological concept for the design of SAW devices.
Select a perfect substrate material which affect the piezoelectric coupling constant k2, the temperature coefficient of delay (TCD) etc.
Use piezoelectric films which should have a smooth surface morphology, sharp interface, and perfect c-axis texture.
Consider a SAW filter which reveals high phase velocity and large electromechanical coupling coefficient as the thickness of piezoelectric film increases.
Use highly polished substrates of single crystal material and the photolithographic patterning process employed in the semiconductor industry.
Include in simulation, second order effects such as reflections, triple transit echoes, and temperature effects.
III. SAW SUBSTRATES Multilayer structure like SiO2/IDT/ZnO/diamond  has a
high propagation velocity, so it is more advantageous than
conventional materials such as quartz, Lithium Niobate (LiNbO3) and Lithium Tantalite (LiTaO3), for operation at high frequencies. Surface-acoustic-wave (SAW) technology based on conventional materials has been successful at frequencies up to 1 GHz but a combination of intrinsic propagation loss, surface degradation effects and fabrication difficulties make the operation of SAW devices at higher frequencies more difficult.
Although SAW devices using LiTaO3 and quartz have proven highly successful, manufacturability would be considerably eased if SAW devices could be readily integrated with CMOS devices. In a recent paper, Lee and Jeong  proposed Mg-doped epitaxial GaN as suitable for SAW devices integrated with other materials. To achieve cost and size reductions,  developed a low cost manufacturing technology for RF substrates and a high performance passive process technology for RF integrated passive devices (IPDs).
IV. ADVANCES OF MEMS TECHNOLOGY MEMS technology has recently been employed to
fabricate several radio components, such as filters, oscillators, switches and voltage-controlled capacitors. However, the implementation of highly integrated, single-chip radio-transceiver front end module (FEM) based on MEMS has not yet been possible, since the different components have required incompatible fabrication processes.
The very recent advances in the field , provide a promising approach to realize highly integrated radios transceiver. It seems feasible that a single manufacturing process can be used to fabricate the components, such as electro-acoustic transmission lines, filters, resonators, oscillators, mixers and others. Furthermore, CMOS electronics can be integrated into the same chip with MEMS components. However, it is also evident that a multitude of challenges still remains in order to realize such transceivers.
MEMS is a powerful technology capable of enabling devices to overcome all limitations. Fig.2 shows the arsenal of high quality components enabled by MEMS.
Figure 2. MEMS-enabled RF components.
The potential of micro-machining technology to implement passive components suitable for RF has been recognized several years ago . In IDT fabrication techniques, the new surface micromachining techniques like in photolithography and highlights the fact that advances in MEMS technology can improve the frequency range of SAW devices more than achieved until now. This can be done by reducing the IDT finger width and spacing. The metal transducer electrodes are now multilayer composites  to handle the power.
V. SUMILATION RESULTS The SAWFAT software is used to simulate bidirectional
surface acoustic wave filter. The layout generated by the software consists of two IDTs (unapodized + apodized) as shown in Fig. 3. The input and output finger pairs are respectively: Nin=1262 and Nout=1178.
0 500 1000 1500 2000 2500-1
Output (N2=1178)Input (N1=1262) Output (N2=1178)
Figure 3. IDT Layout.
Two multilayer substrates: ZnO/SiO2/Si , AlN/GaN/Sapphire and a conventional piezoelectric material: LiTaO3 (112) are chosen for the SAW filter simulation. Table. I summarize the characteristics of these piezoelectric substrates used in this work.
TABLE I. SUBSTRARES CHARACTERISTICS
Electromechanical coupling coefficient
LiTaO3 (112) 3300 0.64 ZnO/SiO2/Si 4000 1.2
AlN/GaN/Sapphire 5280 3.1
Both the load and input resistance are assumed to be 50. Fig. 4, Fig. 5 and Fig. 6 shows the simulation results of the frequency response of the SAW filters for the previous cases with a wavelength =15 m.
The experimental results can be summarized as follows: The AlN/GaN/sapphire structure SAW filter exhibited excellent SAW characteristics with less insertion loss, higher center frequency than that of the SAW filter with
ZnO/SiO2/Si structure and at a lower level the SAW Filter with LiTaO3(112) substrate .
205 206 207 208 209 210 211 212 213 214 215-100
W1/W2 =100.5 / 100.5 IL = -11.4 dB
Figure 4. Frequency response of the SAW filtre with LiTaO3(112).
260 262 264 266 268 270 272 274 276 278 280-100
W1/W2 =100.5 / 100.5 IL = -3.4 dB
Figure 5. Frequency response of the SAW filtre with ZnO/SiO2/Si .
340 342 344 346 348 350 352 354 356 358 360-100
W1/W2 =100.5 / 100.5 IL = -1.1 dB
Figure 6. Frequency response of the SAW filtre with AlN/GaN/Sapphire.
Fig. 7 shows that we can reach the center frequency of 1 GHz like in the mobile transceiver (GSM) if the wavelength reaches the value of 5 m.
1000 1010 1020 1030 1040 1050 1060 1070 1080 1090 1100-100
W1/W2 =300.0 / 300.0 IL = -1.2 dB
Figure 7. Frequency response of the SAW filtre with AlN/GaN/Sapphire and = 5 m.
So, with the advances of MEMS and NEMS technology, we can fabricate a SAW filters with center frequencies of few GHz and more depending in the limits of IDTs fabrication and test.
VI. FUTURE TREND OF SAW FILTER Enhanced functionality and space reduction are basic
driving forces for further integration of SAW filters into whole front end modules (FEMs). High performance SAW filters can be mounted together with active components on (LTCC) substrate and other structures. Mature filter technologies gradually improve due to evolutionary advances in materials science, computer modeling, and mass manufacturing experience. Recently, a miniature SAW device is designed and fabricated at 1 GHz for wireless communication system using MEMS Technology .
The realization of miniature RF SAW device on CMOS substrate with electronics for mobile and wireless communication system has become very important in recent years. To be able to produce RF SAW, we will realize what follows:
Design based on developed advanced modeling techniques such as analytical and finite element modeling.
Use specified MEMS CAD tools.
Use hetero-structure, which is piezoelectric film on non-piezoelectric substrate combination which has higher, SAW propagation loss, lower temperature-
coefficient of delay (TCD), and best coupling coefficient K2 than those of typical piezoelectric substrates.
Design and fabrication of Inter-Digital Transducer (IDT) using standard BiCMOS or CMOS process taking into consideration the critical structures of the arrays of narrow parallel metal strips of the IDT to reach the Gigahertz range .
The requirements on SAW filters for future generation systems extend from optimum electrical performance to extensive miniaturization and integration. Enhanced functionality and space reduction are basic driving forces for further integration of SAW filters into whole front end SAW modules. This let us concluded that advances in MEMS make it possible to reduce the electrode width to submicron and fabricate a multilayer piezoelectric structure. So, SAW technology should enjoy a good future
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