Jpn. J. Appl. Phys. Vol. 40 (2001) pp. 3718–3721 Part 1, No. 5B, May 2001 c 2001 The Japan Society of Applied Physics
Very Small-Sized Resonator Filter Using Shear Horizontal Wave on Quartz Michio K ADOTA∗ , Toshimaro YONEDA, Koji F UJIMOTO, Takeshi NAKAO and Eiichi TAKATA Murata Mfg. Co., Ltd., Nagaokakyo-shi, Kyoto 617-8555, Japan (Received November 23, 2000; accepted for publication January 29, 2001)
By composing interdigital transducers (IDTs) and reflectors consisting of films made of a heavy metal such as Au, Ta or W on an ST cut 90◦ X propagation (direction perpendicular to X-axis) quartz substrate, the authors realized a new type of shear horizontal (SH) wave. This wave has an excellent temperature characteristic, a large electromechanical coupling factor (k), and a large reflection coefficient at reflector electrodes. The square of this electromechanical coupling factor (k 2 = 0.28 ∼ 0.34%) and the reflection coefficient at reflector electrodes are 2.2 ∼ 2.6 times and 30 ∼ 35 times, respectively, as large as those of a Rayleigh wave on an ST cut X propagation quartz substrate. The authors applied this technology to filters for the first intermediate frequency (first IF) stage of a global system for mobile communications (GSM) in the nominal center frequency from 200 to 400 MHz. As a result, we succeeded in developing the first IF filter having a low insertion loss, an excellent temperature characteristic (frequency shift: 1 ppm/◦ C) and a small package size (3 × 3 mm2 ), which is as small as a radio frequency (RF) surface acoustic wave (SAW) filter, for the first time. KEYWORDS: shear horizontal wave, quartz, resonator filter, intermediate frequency filter, heavy metal film, longitudinally coupled, small size
1. Introduction The surface acoustic wave (SAW) filter has been playing an important role as a key device in mobile phone systems. The mobile phone is urgently required to be small and the SAW filter is also required to be smaller. In particular, the requirement for small SAW filters for the first intermediate frequency (IF) stage, which is larger than a radio frequency (RF) SAW filter, is very urgent. Because most first IF SAW filters are required to have a low center frequency, a narrow bandwidth, a high selectivity, a high stop band rejection and good temperature stability, it is considered that it is very difficult to make them smaller. The smallest among our first IF SAW filters of 200 ∼ 300 MHz for global system for mobile communications (GSM) use which has a large market has been 5 × 7 mm2 .1) Some of these require an external coil to extend the pass bandwidth. One of the authors reported a very small IF SAW filter that was realized by using a reflection of the Bleustein-Gulyaev-Shimize (BGS) or shear horizontal (SH) wave at the edges of substrates.2–4) However, these substrates are not suitable for SAW filters requiring good temperature stability and a narrow pass bandwidth. For this reason, a SAW transversal filter or a SAW resonator filter using the Rayleigh wave on an ST cut X propagation (ST-X) quartz substrate is applicable. In both cases, the sizes are large because the former one requires two interdigital transducers (IDTs) with many electrode fingers, a propagation path and two absorbers, and the latter one requires about 300 reflector grating fingers on each side of an IDT.5) An acoustic wave propagating in the direction perpendicular to the X-axis on an ST cut (ST-90◦ X) quartz substrate also has a good temperature coefficient of frequency (TCF). This substrate is well known for SH-type waves with a high SAW velocity such as surface transverse wave (STW), surface skimming bulk wave (SSBW) or shallow bulk acoustic wave (SBW),6) but their electromechanical coupling factor (k) is small. By utilizing this high velocity, Nishikawa et al. fabricated a 1.6 GHz SAW filter which has Al-IDT electrodes on an ST-90◦ X quartz substrate.7) However, the electromechanical coupling factor is ∗ E-mail
small (k 2 = 0.09%) at their used Al normalized thickness, H/λ = 0.01 (H is thickness and λ is the SAW wavelength). This filter’s package size is large because both of the two sets of IDTs consist of 150 pairs for this SAW filter. This package consists of hermetic sealed can case of diameter 12 mm.7) The authors have realized an SH wave which had an excellent TCF, a large electromechanical coupling factor, and a large reflection coefficient, by combining IDTs and reflectors consisting of films made of heavy metal film such as Au, Ta or W on an ST-90◦ X quartz substrate. We have realized a low loss and small longitudinally coupled resonator filter for first IF of GSM in the 200 to 400 MHz range with very few reflector fingers by using this SH wave (one thirtieth of the number of grating fingers of a conventional resonator filter). However, the size of our newly developed first IF filter is as small as that of an RF SAW filter, although a first IF SAW filter in general is much larger than an RF SAW filter. 2. SH Wave on the ST-90◦ X Quartz Substrate One of us obtained an SH wave which had a high SAW velocity and a large electromechanical coupling factor (k 2 = 0.25%) by combining this ST-90◦ X quartz substrate and a ZnO thin film.8) We considered that the SH wave with a large electromechanical coupling factor could be generated by means of combining an ST-90◦ X quartz substrate and a heavy metal electrode instead of a ZnO thin film. We analyzed a leaky SAW (LSAW) in the IDT/ST-90◦ X quartz substrate structure by the method of Campbell.9) Figure 1 shows the SAW velocities as a function of the thickness of various metal materials. In the case of heavy metals such as Au, Ta and W compared with Al, the shifts of the SAW velocities with the change of metal thickness are large. Figure 2 shows the dependency of the electromechanical coupling factor on the metal thickness. The heavy metals such as Au, Ta or W on the ST-90◦ X quartz substrate have large electromechanical coupling factors (k 2 0.25%) at H/λ 0.005. These values are 2.2 to 2.6 times as large as that of the Rayleigh wave on the ST-X quartz substrate. This wave has only an SH component (U2 displacement), without propagation attenua-
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Fig. 1. Dependence of SAW velocities on thicknesses of various metals.
Fig. 2. Dependence of electromechanical coupling factor on thicknesses of various metal films.
tion. Figures 3(a) and 3(b) show distributions of the U2 displacement at various Al thicknesses and various thicknesses of Au, Ta, and W (H/λ 0.025), respectively. In proportion to the metal thickness, the displacement is concentrated in the surface layer of the substrate. The displacement of heavy metal film on the ST-90◦ X quartz substrate is concentrated more than that of Al metal film. For example, at metal thickness H/λ = 0.025, almost all of the displacement in the Al-film/quartz structure is concentrated in the layer within 4λ from the surface, but that of the heavy metal film is concentrated in the layer within 1λ. Table I lists the reflection coefficients and various properties of the SH wave on (Au, Ta, W, and Al)-fingers/ST-90◦ X quartz substrates and the Rayleigh wave on Al-fingers/ST-X quartz substrates. The reflection coefficients were calculated by an admittance ratio (Ym/Yo) obtained by using the finite element method (FEM) at the metallization ratio of 0.5 and normalized metal thickness H/λ = 0.02, where the reflection coefficient is |2 × (1 − Ym/Yo)/(1 + Ym/Yo)|. It has been clarified that the reflection coefficients of the (Au, Ta, or W)-fingers/ST-90◦ X quartz substrate show almost the same values, and they are 30 ∼ 35 times larger than that of a Rayleigh wave on the Al-fingers/ST-X quartz substrate and 20 ∼ 23 times larger than the reflection coefficient on the Al-fingers/ST-90◦ X quartz substrate. The propriety of these values was confirmed by comparing the calculated fitting results with the measured filter characteristics. The SH wave on heavy metal film on the ST-90◦ X quartz substrate has many
Fig. 3. (a) Distribution of displacement U2 at normalized Al thickness H/λ = 0 ∼ 0.025. (b) Distribution of displacement U2 at normalized thickness of heavy metal H/λ = 0 ∼ 0.025.
Table I. SAW properties of SH waves of various metal films on ST-90◦ X quartz substrate and Rayleigh wave of Al film on ST-X quartz substrate. Au, Ta, W film on ST-90◦ X
Al film on ST-90◦ X
Metal thickness (H/λ)
0.01 ∼ 0.025
0.017)
—
k 2 (%)∗
0.28 ∼ 0.34
0.09
0.13
Velocity(m/s)
3500 ∼ 4500
4500
3100
1
1
1
f/f/◦ C(ppm/◦ C)∗∗
not
large7)
Al film on ST-X
not large1) 35 ∼ 40
Stop band rejection(dB)
large 75
Package size
small 3 × 3 mm2
large7) 12 mmφ
large1) 5 × 7 or 4.8 × 9.1 mm2
Reflection co.∗∗∗
0.36 ∼ 0.42
0.018
0.012
20
∗ k 2 : Electromechanical coupling factor ∗∗ f/f/◦ C: Frequency shift/◦ C ∗∗∗ Reflection coefficient at H/λ = 0.02 and metalization ratio of 0.5.
advantages as mentioned above. Most of the values for AlIDT/ST-90◦ X quartz were obtained from ref. 7. 3. Application to Longitudinally Coupled Resonator Filter While a resonator filter using the Rayleigh SAW on the ST-X quartz substrate requires about 300 Al grating reflec-
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tor fingers on each side of the Al-IDT,5) our newly developed resonator filter utilizing the above-mentioned structure requires only about 10 grating fingers for the reflector at most because of its large reflection coefficient. Therefore, we investigated the new structure, not using the edge reflection but using small reflectors. We have applied a longitudinally coupled resonator filter utilizing this structure to a first IF filter for GSM. The nominal center frequencies (fc) of such IF filters for GSM currently range from 200 to 400 MHz. They require a narrow bandwidth of about 0.1% and an excellent TCF. For example, Figs. 4 and 5 show the characteristics of a 241 MHz filter with W electrodes and those of a 360 MHz filter with Ta electrodes, respectively. Figure 6 shows the spurious characteristics of a 225 MHz filter with Ta electrodes. Figure 7 shows Smith charts of the 360 MHz filter. The insertion losses of the 241, 360, and 225 MHz filters are the 3.5, 3.9, and 3.5 dB, and the 3 dB bandwidths are 245, 270, and 240 kHz, respectively. The stop band rejection, which is the attenuation at fc−50 MHz to fc−3 MHz and fc+3 MHz to fc+50 MHz, is larger than 75 dB. These filters consist of 28 pairs of IDTs, each reflector having 10 fingers, with 2 stages of the longitudinally coupled type. The distance between an IDT and a finger of a reflector is 0.46λ. The metal thickness H/λ of W and Ta is about 0.02. The aperture is 25λ. The terminal impedances are 800 // − 0.4 pF, 920 // − 0.6 pF, and 1 k// − 0.4 pF, respectively. Figure 8 shows a test cir-
M. K ADOTA et al.
Fig. 6. Spurious characteristics of 225 MHz first IF filter for GSM system.
Fig. 7. Smith chart of 360 MHz first IF filter for GSM system.
Fig. 8. Test circuit of first IF filter for GSM system.
Fig. 4. tem.
Frequency characteristics of 241 MHz first IF filter for GSM sys-
Fig. 5. tem.
Frequency characteristics of 360 MHz first IF filter for GSM sys-
cuit consisting of a balanced circuit using baluns at the input and output terminals. The inductances and capacitances are tuned to impedance matching. A 0 dB level is determined when A and B in Fig. 8 are directly connected electrically. A filter consisting of Au showed a similar characteristic, although this study demonstrates ones consisting of W or Ta. As shown in Fig. 9, although our previous filter package size is 4.8 × 9.1 mm or 5 × 7 mm2 ,1) our newly developed filter chip size is 1.1 × 1.7 mm2 and package size is 3 × 3 × 1.15 (height) mm3 . The length of 1.1 mm of the chip is the width of the chip in the direction of SAW propagation, and that of 1.7 mm is the length of the chip in the perpendicular direction. If this filter utilizes the edge reflection, the width of the chip must be less than 0.8 mm. However, the other length of the chip is not changed, thus the total package size is not changed. A first IF SAW filter in general is much larger than an RF SAW filter because the former frequency is lower than the later one. A wire-bonding type of SAW filter is larger than a face-down type because the former requires an area for wire bonding. Although our newly developed IF filter is a low-
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4. Conclusions
Fig. 9. Comparison of newly developed filter chips and packages and previous ones.
The authors obtained an SH wave with a large coupling factor (k 2 = 0.28 ∼ 0.34%), a large reflection coefficient, and an excellent temperature property by combining IDTs and reflectors consisting films made of heavy metal films on the ST-90◦ X quartz substrate. This SH wave was applied to the first IF filter for GSM. The authors were able to realize longitudinally coupled resonator filters for the first IF for GSM ranging from 200 to 400 MHz and consisting of 2 stages, each small reflector with only 10 grating fingers, 28 total pairs of IDTs on one stage for the first time. Their filters have many advantages such as their small size (3 × 3 mm2 ), low loss (3.5 ∼ 3.9 dB), excellent temperature characteristic (frequency shift per 1◦ C = 1 ppm/◦ C), a large stop band rejection (75 dB), and no need for an expanding coil to extend the bandwidth. The size of our newly developed IF filter is the same as that of a wire-bonding type of the RF SAW filter, although the first IF SAW filter in general is much larger than the RF SAW filter. Acknowledgement The authors thank Mr. S. Arai, Director of Murata Mfg. Co., Ltd. for his support, and many co-researchers for their practical assistance.
Fig. 10. Frequency shift characteristics of newly developed filter in the temperature range from −25 to 80◦ C.
frequency and wire-bonding type, the size is as small as that of an RF SAW filter. The previous filters often required an external coil to extend the pass bandwidth, but our newly developed filter does not require an external coil because it has a large electromechanical coupling factor compared with that of a Rayleigh SAW on the ST-X quartz substrate. Figure 10 shows a frequency shift at the temperature change from −25 to 80◦ C. The frequency shift per 1◦ C (total frequency shift from −25 to 80◦ C/measured temperature range) is 1 ppm/◦ C. As shown in Fig. 10, our newly developed filter has good temperature characteristics, the same as a filter utilizing Rayleigh wave on the ST-X quartz substrate.
1) Murata Catalog: Cat. No. p35-2 (1998) p. 25 [in Japanese]. 2) M. Kadota, J. Ago, H. Horiuchi and H. Morii: 28th Electro-Mechanical Symp. Tokyo (1999) p. 103. 3) M. Kadota, J. Ago, H. Horiuchi and H. Morii: IEEE Ultrason. Symp. (1999) p. 55. 4) M. Kadota, J. Ago, H. Horiuchi and H. Morii: Proc. 20th Symp. Ultrason. Electronics, Tokyo, 1999, Jpn. J. Appl. Phys. 39 (2000) 3045. 5) M. Tanaka, T. Morita, K. Ono and Y. Nakazawa: 38th Annual Freq. Cont. Symp. (1984) p. 286. 6) I. V. Avramov: IEEE Trans. Ultrason. Ferrolectric. & Freq. Cont. 40 (1993) 459. 7) T. Nishikawa, A. Tani, C. Takeuchi and J. Minowa: Proc. 3rd Meet. Ferroelectric Materials and Their Apprications, Kyoto, 1981, Jpn. J. Appl. Phys. 20 (1982) Suppl. 20-4, p. 29. 8) M. Kadota: 29th Electro-Mechanical Symp. Chiba (2000) p. 187 [in Japanese]. 9) J. J. Campbell and W. R. Jone: IEEE Trans. Sonic. & Ultrason. SU-15 (1968) 209.
