Surface acoustic wave device

Abstract: A surface acoustic wave device comprises a surface-acoustic-wave element 1 with interdigital electrodes 15 on a piezoelectric substrate 10 packaged in a package 20,25. A suitable part of the package, e.g. the lid 20, is provided with a metal pattern 10, and corresponding metal bumps 11a, 11b, 11c are formed on a bonding pad section of the surface-wave-device element. The metal bumps 11 are thus contact-connected with the metal pattern 21 of the package, reducing its size and making it more reliable compared to the conventional wire-bonded package.

Description: The present invention relates to a surface-acoustic-wave device adapted for high frequencies, and particularly to a surface-acoustic-wave device that has its preferred performance adapted for use in the field of land mobile radio telephone systems or portable radio telephone systems and can be manufactured by simplified bonding processes to improve reliability and is suitable for mass production. A surface-acoustic-wave device, such as a surface-acoustic-wave filter, is a three-terminal or four-terminal- type element that has an electrode for input and output formed for instance of Al on a piezoelectric substrate having a large electromechanical coupling coefficient and a relatively small temperature/frequency coefficient, such as a 36° Y-X LiTaO 3 monocrystalline substrate. Several interdigital electrodes (also called comb electrodes) are provided, the teeth of which are fashioned so that the width (L) of the electrode strip, the spacing (S) between adjacent electrode strips and the pitch (P) of the electrode strips normally satisfy L=S=λ/4, P=λ/2, where λ is the wavelength of the surface wave. For example, in order to obtain a center frequency of 836 MHz, λ=4.8 µm is derived from the acoustic velocity 4090 m/s of a surface wave propagating under the interdigital electrode on the substrate. Thus the pitch of the electrodes should be 2.4 µm and the width and spacing of the electrodes are set to 1.2 µm. Typical devices have one pair of comb electrodes for input and output opposing each other. However, in certain fields of use, such as in the field of mobile radio telephones, portable radio telephones and so forth, surface-acoustic-wave filters need a low amplitude attenuation (for example, less than or equal to 3 to 5 dB of insertion loss) and high suppression (for example, 24 to 25 dB of attenuation magnitude out of a pass band) in a wide range (for example, wider than or equal to 25 MHz of pass band at a center frequency higher than or equal to 836 MHz). Various methods have been proposed in order to satisfy the foregoing demands. As one of the typical methods, there is the surface-acoustic-wave filter having a multi-electrode construction (see M. Lewis, 1982 Ultrasonic Symposium proceedings, P.12, for example). Fig. 1 shows the electrode pattern of a conventional surface-acoustic-wave device, which illustrates the surface-acoustic-wave filter with multi-electrode construction (in this example 5 inputs and 4 outputs). In the drawing, a device element 1' has comb electrodes 15 constituting input and output electrodes arranged alternately on a 36° Y-X LiTaO 3 monocrystalline piezoelectric substrate. The device element is thus of a multi-electrode construction having five stages at the input side and four stages at the output side. The signal side electrode terminals are connected respectively to an input terminal pad 11'a and output terminal pad 11'b which are, in turn, connected to external circuits. Although not illustrated, reflectors may be provided at both sides for improving the characteristics. The reference numeral 11'c denotes grounding electrode strip pads formed electrically independent of each other. The reference numeral 111' denotes a metal pattern for shielding formed surrounding the periphery of the element except for the input and output terminal pads 11'a and 11'b. These comb electrodes, terminal pad and metal pattern and so forth, are typically formed simultaneously in photo-lithographic technology by deposition of the same metal, such as Al or Al-Cu alloy. It should be noted that the illustrated comb electrode is of a so-called normal type normal construction that has an equal overlapping length of comb electrode strips. Width and number of the electrode strips on the drawing should not be considered accurate and are schematic for simplification of the drawing. Fig. 2 shows an example of the practical packaging of the conventional surface acoustic wave device. In the drawing, the reference numeral 1' denotes the above-mentioned device element in multi-pole construction. The reference numeral 2' denotes a package that comprises a ceramic-box-type container formed with a metallic wiring pattern 21' on an intermediate step portion of the inner wall, for example (normally this metallic wiring pattern is formed in a pattern corresponding to that of the terminal pads of the device element 1', and is connected to a not shown external terminal of the package). In the practical installation of the element, after die-bonding the device element 1' onto the bottom of the package 2', connection is established between the input terminal pad 11'a and the output terminal pad 11'b (including the grounding metal pattern 111' for shielding if necessary) of the device element 1' and the metallic wiring pattern 21' by means of wires 7. Finally, the metallic lid plate 20' is sealed and fitted to complete the surface acoustic wave device, such as the multi-electrode construction of the surface acoustic wave filter. However, in the above-mentioned conventional surface-acoustic-wave device, a space for wire bonding is required in the package. Particularly, in the multi-electrode construction of the device, there are many terminal pads and accordingly an increased number of wire bondings is required, which in turn requires more space. Furthermore, to provide an enhancement of the working frequency range of the device to be used, the size of the comb electrodes and the terminal pads becomes smaller, making connection by wire bonding more difficult. It is also difficult to check the quality of bonding. Furthermore, problems in miniaturization of the surface-acoustic-wave device are encountered owing to the large size of the package in proportion to the size of the device element. It is desirable to solve such problems. Flip-chip methods of mounting SAW devices are known, for instance, from US 4737742 (Alps Electric) and 4864470 (Pioneer), which afford a space-saving alternative to wire bonding. The latter document describes a SAW chip bonded to a ceramic base plate and encapsulated by a shrink-setting substance, the base plate having metal tracks and bumps for the flip-chip bonding. Reference may also be made to EP-A2-0472856, published on 4 March 1992, which also shows a flip-chip packaging for SAW devices. The invention provides a surface-acoustic-wave device in which a surface-acoustic-wave device element comprising interdigital electrodes on a piezoelectric substrate is packaged in a package provided with a metal pattern, the package including a frame surrounding the piezoelectric substrate and a lid aligned with the frame, the metal pattern being provided on the lid of the package, at least one metal bump being formed on a bonding pad section of the surface-acoustic-wave device element, wherein the lid is arranged to seal the substrate and to press the or each metal bump of the device element in such a way as to establish a contact connection between the bump and the said metal pattern of the lid. A plurality of metal bumps for grounding may be contact-connected with the metal pattern. Also, a continuous raised grounding pattern may be formed along the periphery of the surface of the surface-acoustic-wave device element, on which the electrode is arranged. Alternatively, the surface-acoustic-wave device element formed of the grounding bump pattern may be contact-connected with the metal pattern, and the whole element may be coated with a resin. Furthermore, a recess or projection for positioning of the surface-acoustic-wave device element may be provided on the metal pattern. Methods of making such a device are also described, including lift-off methods where a resist pattern complementary to the bumps is formed on the piezoelectric substrate, deposition of one or more metal layers takes place and the resist is then lifted off, leaving the desired pattern of bumps. With devices of the kind on which the present invention is based, since the metal bump is provided on the terminal pad of the surface-acoustic-wave device element, it is not necessary to perform wire bonding at packaging, thus facilitating installation of the element. This is particularly effective for the multi-electrode construction type and can provide a higher reliability. Also, since it is not necessary to provide a space for wire bonding in the package 2, the device can be made compact. Furthermore, it is simple to perform testing of the connection of the signal lines and grounding lines by means of a tester, and isolation between an input and an output can be improved by providing the grounding metal pattern. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view illustrating an electrode pattern in a conventional surface-acoustic-wave device; Fig. 2 is a plan view (Fig. 2(A)) and a cross-sectional view in the Y-Y direction (Fig. 2(B)) of the conventional surface acoustic wave device element; Figs. 3(A) to (F) illustrate views of a surface-acoustic-wave device in accordance with the present invention, that is, a plan view (Fig. 3(A)), an X-X' direction cross-sectional view (Fig. 3(B)), a Y-Y' direction cross-sectional view (Fig. 3(C)), a back view of a package lid plate (Fig. 3(D)), a Y-Y' direction cross-sectional view in Fig. 3(D) (Fig. 3(E)), and a cross-sectional view showing the sealed state of the package (Fig. 3(F)); Fig. 4 shows a pass band characteristics of the embodiment of Fig. 3; Fig. 5 shows pass band characteristics of a filter in the conventional surface-acoustic-wave device; Fig. 6 is an illustration showing a second SAW device relating to the invention; Fig. 7 is an illustration showing a third SAW device relating to the invention; Fig. 8 is an illustration showing a fourth SAW device not within the scope of the invention; Fig. 9 is an illustration showing a fifth SAW device relating to the invention; Fig. 10 is an illustration showing a sixth SAW device relating to the invention; Fig. 11 is an illustration of a seventh SAW device; Fig. 12 is an illustration showing an example of a method of forming a bump; Fig. 13 is a Af vs. temperature characteristic curve in the cases of contact connection and fixed connection to a LiTaO 3 monocrystalline substrate; Fig. 14 is a curve of insertion loss vs. time in an ageing test in an 800 MHz band pass filter; and Fig. 15 is an outlined cross-sectional view showing contact resistance of a SAW device. Fig. 3 shows the first embodiment of the present invention; in (A) to (F) of this figure, a surface-acoustic-wave device chip, a package and packaging are schematically shown in order. In the plan view of Fig. 3(A), the reference numerals lla and llb denote metal bumps provided on an input and output terminal pad. The reference numerals llc and lld denote metal bumps provided on a grounding terminal pad and a grounding pattern. The metal bumps are respectively formed of Au, for example. Figs. 3 (B) and 3 (C) are X-X' and Y-Y' and are cross-sections of the element shown in Fig. 3 (A). The reference numeral 15 denotes a comb electrode forming the propagation path of a surface acoustic wave. Fig. 3 (D) shows an example of back side view of a lid plate. The reference numeral 20 denotes a ceramic plate formed with input and output metal patterns 21a and 21b and a grounding metal pattern 21c formed of an Au layer, for example. The Y - Y' cross section of the lid plate is shown in Fig. 3 (E). Fig. 3 (F) is a cross-section of the package in the enclosed condition, in which the surface wave device 1 is die-bonded on the bottom of a ceramic box-shaped container 25. Each of the metal bumps 11 on the surface wave device 1 is made in contact with each metal pattern 21 of the lid plate 20 and interconnected therewith by depressing. Simultaneously, the periphery of the lid plate 20 and the upper edge of the package 2 is bonded with a metal or resin (not shown) for completing the packaging of the element (not shown in the figure). In this case, the upper part of the comb (propagation area) 15 of the surface acoustic wave is spaced from the lid plate 20 by a distance equal to the height of a bump, thus producing no deterioration of performance. It should be noted that, for simplification of the drawing, external connection terminals are omitted from the illustration. As can be appreciated from the figures, the connection of the element can be significantly simplified and is made compact. Also, because of no possibility of wire breakage, it may hold increased yield and reliability. It should be noted that although the foregoing discussion is given for the process in which the lid plate 20 is connected with the metal bumps 11 after die-bonding the surface acoustic wave device 1 onto the bottom of the container 25, it is, of course, a possible alternative that the element and the lid plate via the metal bump 11 are initially connected, and subsequently the container 25 is sealed and enclosed. Fig. 4 shows the pass band characteristics of an embodiment of the invention, and Fig. 5 shows the pass band characteristics of the conventional surface acoustic wave filter, which are shown for comparison with respect to the case where the identical design of the elements of 7 inputs - 6 outputs are used. There can be seen no noticeable difference. This demonstrates a remarkable advantage to be obtained by implementation of the element and the package according to the present invention, without causing any drawbacks. When the intervals (B) between the metal patterns 21a, 21b of the signal line and the metal pattern 21c are made smaller than the width (A) of the metal bumps 11a, 11b, precise positioning of the metal bumps and the metal patterns of the package by checking conduction between the metal patterns 21a, 21b for signal lines and the grounding metal pattern 21c is possible. Fig. 6 shows a second device relating to the present invention, which is shown clearly with (partially sectioned) an assembling perspective view of the device. In Fig. 6, reference numeral 20' denotes a lid plate made of a metal, 23 a frame made of ceramic, 21' a metal pattern formed on an upper end face of the ceramic frame 23, 22 a bottom plate made of ceramic, and 24 a connection terminal to an external circuit. Parts equivalent to those discussed with respect to the preceding figures will be represented by the same reference numerals and a discussion of the equivalent parts will be omitted. The shown device is an example of how to connect the surface-acoustic-wave device element 1 to the bottom plate 22 via the metal bump 11, the principle and effect of which is similar to those of the foregoing first embodiment. Fig. 7 shows a third device relating to the present invention, which is illustrated clearly by the (partially sectioned) assembling perspective view similar to the former figure. In the figure, the reference numeral 20" denotes a cap made of a metal, for example. The reference numeral 222 denotes a bonding material of metal or resin for sealing and bonding the cap 20" and the bottom plate 22. This device is essentially similar in construction to the foregoing second device and can be selectively used therewith depending upon the way of use. Fig. 8 shows a version of an SAW package not being an embodiment of the present invention. In this figure Fig. 8 (A) shows a plan view of the element, and 8 (B) shows a cross section of the device. In the drawing, the portion enclosed by the broken line represents a propagation path region of the surface acoustic wave formed by the comb electrode 15. The reference numeral 111 denotes a grounding bump pattern, formed with line-shaped bulged portions of Au and so forth and extending along the peripheral portion of the element except for the portion of the metal bumps 11a and 11b. By providing the grounding bump pattern 111, the shielding ability of the element can be enhanced. Accordingly, when the surface-wave device 1 is connected to the bottom plate 22 provided with the metal pattern 21 (similarly for the lid plate) through the metal bump 11 and the grounding bump pattern 111, resin will not enter the inside of the element even when the outer surrounding is coated by an outer resin layer 3. Also, the characteristics will not be adversely affected even when the cap of metal and so forth is not provided. A metal layer may be formed on the surface-acoustic-wave device at the side opposite the operational surface for providing a further enhanced shielding effect. Fig. 9 shows a fifth device relating to the present invention. In this figure, 9 (A) and 9 (B) show modifications of the metal pattern 21c on the lid plate (or bottom plate, though this is not within the scope of the attached claims) of the package shown in Fig. 3. In either case, the grounding metal patterns 21c are separated into input side and output side. This is effective to use when better isolation between the input and output is desired. Fig. 10 shows a sixth device relating to the present invention. In the figure, 200 denotes a recess of 10 to 20 µm in depth, formed in the metal pattern 21 of the bottom plate (or lid plate 20). This recess 200 may be formed solely in the metal pattern 21, or as an alternative may be formed in the ceramic substrate per se located therebelow. Figs. 10 (A and B) show the recess 200 formed corresponding to the metal bumps 11 of the surface-wave element 1. On the other hand, Figs. 10 (C and D) show an elongated recess 200 used for the grounding metal bump. In either case, since the metal bumps engage with the recess 200, it facilitates the positioning of the element and thus make it stable. Fig. 11 (A) shows a seventh device relating to the present invention. Contrary to the foregoing sixth device, projections 201 of a height of about 20 µm, for example, are provided on the metal pattern 21. Similar to the recess 200, the projection 201 can be formed solely with the metal pattern 21. It is of course possible to form the projection with the ceramic substrate per se located below the metal pattern. The effect is essentially equivalent to that of the case where the recess 200 of the sixth device is formed. Fig. 11(B) shows a modification. A practical example of the formation of the metal bump 11 as set forth above will be discussed below with reference to the drawings. Fig. 12 shows an example of a process for forming a bump which can be used with the invention, in which the principal steps are illustrated in order. Step (1): On the piezoelectric substrate 10 made of 36° Y-X LiTaO 3 monocrystal, a layer, such as a 200 to 300nm-thick Al-Cu layer, is formed thereon by deposition. With known photo-lithographic technology, the surface acoustic wave propagation path of multi-electrode construction with the alternately arranged input and output electrodes forming comb electrodes 15, such as 5-stage input side and 4-stage output side, and the input terminal pad ll'a and the output terminal pad 11'b connected to respective signal side electrode strips are formed. Step (2): For the substrate processed as set forth above, a thick resist pattern layer 4 of 30 to 40 µm thick is formed for the area excluding the portion to form the metal bump (for example 100 to 200 µm□). (See Japanese Patent Application No. 62-244271 and Japanese Patent Application No. 62-248059). Step (3): For the substrate process as set forth above, an under coat layer 5 for promoting bonding, such as about 25 nm thick of TaMo layer, is deposited, and a metal layer 6, such as about 50 nm of Au, is deposited as a plated undercoat layer thereon. Step (4): On the metal layer 6 of the substrate processed as set forth above, 30 to 40 µm thick of Au layer is formed by electroplating, for example. Step (5): By processing the substrate formed as set forth above with a solvent such as acetone to remove the thick resist pattern layer 4, the metal bump 11 as shown and for use in the present invention can be formed. The foregoing embodiments show mere examples; it is needless to say that as long as it is consistent with the principle of the invention, any appropriate material, construction and production process or the combination thereof can be used for implementing the invention. As set forth, according to the present invention, since the metal bump 11 is formed on the terminal pad of the surface device element 1, it becomes unnecessary to perform wire bonding upon packaging, which facilitates installation. Particularly, this is effective for the multi-electrode construction type and can provide higher reliability. Also, since it is not necessary to provide a space for wire bonding in the package 2, the device can be made compact. Furthermore, it is simple to perform testing of the connection of the signal lines and grounding lines by means of a tester, and can improve isolation between input and output by providing the grounding metal pattern 21c. Therefore, the invention contributes to making the surface acoustic wave device compact and improving quality and reliability. Figure 13 is a frequency shift vs. temperature characteristic curve. A frequency shift Δf = f T -f 30°C is shown in the case of a contact connection and fixed connection caused by a connection of LiTaO 3 monocrystalline substrate with a bump. Figure 14 is an insertion loss vs. time characteristic curve of an aging test in an 800 MHz band-pass filter. Figure 15 is an outlined cross-sectional view showing a means for reducing contact resistance of surface-acoustic-wave devices in accordance with the present invention. In Fig. 15, a lid 34 is mounted on a housing in which nitrogen gas is sealed into the surface-acoustic-wave device 30 via a molding material 32 by means of an Au-Sn sealed member 36 and a bump 38 at the bottom end of the package is contact-connected with an electrode 40. In this case, the contact resistance between the bump 38 and the electrode 40 is nearly equal to zero for high frequencies. Because contact between the electrode and the bump is maintained by virtue of an appropriate pressure of the molding member 32 or the lid 34, even if a thin insulating layer of a depth of up to 0.1 µm at the contacting surface 44 is formed, the electrostatic capacitance is more than 10 pF, so in the case of a high frequency of 1GHz, impedance is no longer affected. A suitable load might be 200 to 500 gram weight. From experiment, it has been determined that a force of less than 200 gr. weight provides a low reliability and a contact of more than 500 gr. weight leads to destruction of a bump. Durability per bump amounts to several grams weight.

Claims: A surface-acoustic-wave device in which a surface-acoustic-wave device element (1) comprising interdigital electrodes (15) on a piezoelectric substrate (10) is packaged in a package (2) provided with a metal pattern (21), the package including a frame (25, 23) surrounding the piezoelectric substrate (10) and a lid (20, 20', 34) aligned with the frame, the metal pattern (21) being provided on the lid of the package (2), at least one metal bump (11) being formed on a bonding pad section of the surface-acoustic-wave device element, wherein the lid is arranged to seal the substrate and to press the or each metal bump (11) of the device element in such a way as to establish a contact connection between the bump and the said metal pattern (21) of the lid. A surface-acoustic-wave device according to claim 1, in which the lid (20, 20', 34) is sealed to the frame (25, 23) by bonding with a metal. A surface-acoustic-wave device according to claim 1 or 2, wherein a plurality of metal bumps for grounding (11c) are contact-connected with the metal pattern (21). A device according to claim 1 or 2, wherein a bump pattern (111) for grounding is formed along the periphery of the surface on which the electrodes of the surface-acoustic-wave device element are arranged. A device according to any preceding claim, wherein a recess (200) or a projection (201) for positioning the surface-acoustic-wave device element is provided on the metal pattern (21) for engaging the metal bumps in order to position the SAW device element. A device according to any preceding claim, wherein the spacing B between the metal patterns (21a, 21b) for signal lines and the metal pattern (21c) for grounding is smaller than the widths A of the metal bumps (11a, 11b) for the signal lines of the surface-acoustic-wave device element. A device according to any preceding claim, in which the metal bumps (11) are provided at locations different from the propagation path (15) of a surface acoustic wave emitted by the surface-acoustic-wave device. A device according to any preceding claim, wherein the piezoelectric substrate (10) is pressed by a force of 200 to 500 gram weight (2 to 5 N) against the package.

Inventors: IKATA OSAMU, MATSUDA TAKASHI, MIYASHITA TSUTOMU, SATOH YOSHIO, TAKAMATSU MITSUO, IKATA, OSAMU, MATSUDA, TAKASHI, MIYASHITA, TSUTOMU, SATOH, YOSHIO, TAKAMATSU, MITSUO, IKATA, OSAMU,C/O FUJITSU LIMITED, MATSUDA, TAKASHI,C/O FUJITSU LIMITED, MIYASHITA, TSUTOMU,C/O FUJITSU LIMITED, SATOH, YOSHIO,C/O FUJITSU LIMITED, TAKAMATSU, MITSUO,C/O FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: H03H9
IPC codes: H03H9