JP2005184340A - Surface acoustic wave chip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave chip having good frequency temperature characteristics in which chip size is reduced. <P>SOLUTION: The surface acoustic wave chip 10 comprises an interdigital electrode 16 and a reflector 18 formed on a crystal substrate 14 wherein the interdigital electrode 16 is formed of a metal material and the reflector 18 is formed of a heavier metal material. When an ST cut crystal substrate or an in-plane rotation ST cut crystal substrate is employed, a surface wave propagating on the crystal substrate 14 can be a Rayleigh wave. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a surface acoustic wave chip, and more particularly to a surface acoustic wave chip suitable for reducing the chip size.

  A surface acoustic wave (SAW) chip is mainly configured by forming a pair of interdigital electrodes (IDT) and a reflector on the surface of a piezoelectric substrate made of a material that generates a piezoelectric effect. For example, a quartz substrate or the like is used as the piezoelectric substrate. Further, as an electrode material for the IDT or reflector, for example, aluminum or an aluminum alloy is used. When an electrical signal is applied to the IDT of such a SAW chip, a periodic mechanical strain is generated between the electrode fingers due to the piezoelectric effect generated in the piezoelectric substrate, and surface acoustic waves are excited, along the meshing direction of the electrode fingers. Is propagated. Since the propagated surface acoustic wave is reflected by the reflector and returns to the IDT side, the surface acoustic wave propagates on the surface of the piezoelectric substrate provided with the IDT and the reflector. Such a SAW chip is mounted on a package to form a SAW device such as a SAW resonator or a SAW filter, and is mounted on a package together with an oscillation circuit to form a SAW oscillator.

As an example of a SAW filter, aluminum is used on an ST-cut 90 ° X-direction propagation crystal substrate whose Euler angles (φ, θ, ψ) are represented by (0 °, 120 ° to 145 °, 90 °). And a reflector made of a metal material having a density higher than that of aluminum and using an SH wave as a surface wave (see Patent Document 1). In Patent Document 1, when an IDT composed of electrodes having a high density is formed on a quartz substrate having a small electromechanical coupling coefficient, and the reflection coefficient of the electrode is large, reflection between the electrodes of the IDT becomes large and elasticity is increased. Although it is disclosed that the surface wave energy is confined between the IDTs, and the surface acoustic wave is difficult to propagate from the IDT, it is disclosed that it is desirable to form the electrode with aluminum having a small reflection coefficient. Not disclosed.
JP 2002-152003 A

  As a piezoelectric substrate of a SAW chip, an ST cut quartz substrate whose Euler angles (φ, θ, ψ) are represented by (0 °, 113 ° to 135 °, 0 °), (0 °, 113 ° to 135 °, An in-plane rotated ST-cut quartz substrate represented by ± (40 ° to 49 °) is used. In the case of an ST cut quartz substrate in which Euler angles (φ, θ, ψ) are represented by (0 °, 113 ° to 135 °, 0 °), an IDT and a reflector are formed using aluminum on the quartz substrate. When a surface acoustic wave is excited, the reflection coefficient per short-circuit electrode constituting the reflector is about 0.02. For this reason, in order to sufficiently reflect the surface acoustic wave, 100 or more short-circuit electrodes are required, which increases the size of the SAW chip. In addition, the distance between the short-circuit electrodes of the reflector is provided corresponding to the wavelength of the surface acoustic wave excited on the piezoelectric substrate, so when a surface acoustic wave in the low frequency range is excited on the piezoelectric substrate, In the case of exciting a surface acoustic wave, there is a problem that the size of the SAW chip becomes large.

  In addition, SAW resonators and SAW oscillators are required to resonate at a constant frequency with high accuracy. That is, the frequency accuracy required for the SAW resonator and SAW oscillator is required to be within ± 100 to ± tens of ppm or less in combination with various factors such as frequency temperature characteristics. For this reason, SAW chips used in SAW resonators and SAW oscillators are required to resonate at a very constant frequency, and therefore the deterioration of frequency temperature characteristics cannot be ignored. By the way, in the case of a SAW filter, since a certain pass band width is required, it is not necessary to resonate a certain frequency, and the idea that a certain frequency must be resonated with high accuracy like a SAW resonator or a SAW oscillator arises. Absent. That is, in the SAW filter, there is little need to consider the factor of frequency temperature characteristics.

  The present invention has been made to solve the problems of the prior art, and an object of the present invention is to provide a surface acoustic wave chip having a good frequency temperature characteristic while reducing the size of the SAW chip.

  In order to achieve the above object, a surface acoustic wave chip according to the present invention is a surface acoustic wave chip having a comb electrode and a reflector on a quartz substrate, wherein the comb electrode is formed of a metal material. The reflector is made of a metal material having a specific gravity heavier than that of the metal material, and the surface wave propagating on the quartz substrate is a Rayleigh wave. As a result, the reflection coefficient per short-circuit electrode constituting the reflector is increased, so that the reflection coefficient of the reflector can be sufficiently obtained even if the number of short-circuit electrodes is reduced as compared with the case where a light metal material is used. be able to. The length of the surface acoustic wave (SAW) chip can be reduced by reducing the number of short-circuit electrodes. Further, since a light metal material is used for the interdigital electrode (IDT), the frequency temperature characteristics of the SAW chip are good.

  The quartz substrate is characterized in that the Euler angle is (0 °, 113 ° to 135 °, 0 °) or (0 °, 113 ° to 135 °, ± (40 ° to 49 °)). The quartz substrate whose Euler angle is represented by (0 °, 113 ° to 135 °, 0 °) is an ST cut quartz substrate, and is (0 °, 113 ° to 135 °, ± (40 ° to 49 °)). The represented quartz substrate is an in-plane rotated ST-cut quartz substrate. The type of surface acoustic wave excited by the SAW chip differs depending on the cut angle of the quartz substrate, but if an ST cut quartz substrate or an in-plane rotated ST cut quartz substrate is used, Rayleigh waves can be excited.

  Further, the interdigital electrode is made of aluminum or an aluminum alloy, and the reflector is made of gold. Thereby, IDT can be formed with a light metal material, and a reflector can be formed with a heavy metal material. Therefore, the SAW chip can be miniaturized and the frequency temperature characteristic is good.

  Further, when the SAW chip described above is mounted on a package to form a SAW resonator, and an SAW oscillator is formed together with the SAW chip to form a SAW oscillator, the SAW resonator and the SAW oscillator can be reduced in size. In addition, a SAW resonator or a SAW oscillator that can resonate at a constant frequency can be provided.

  Hereinafter, preferred embodiments of the surface acoustic wave chip according to the present invention will be described. FIG. 1 is an explanatory diagram of the cut angle of the quartz substrate. The crystal axis of quartz is defined by the X axis (electric axis), the Y axis (mechanical axis), and the Z axis (optical axis). In the surface acoustic wave (SAW) chip 10 according to the present embodiment, an ST cut quartz substrate whose Euler angles (φ, θ, ψ) are represented by (0 °, 113 ° to 135 °, 0 °), Alternatively, an in-plane rotated ST-cut quartz substrate represented by (0 °, 113 ° to 135 °, ± (40 to 49 °)) is used. The ST-cut quartz substrate is obtained by rotating a quartz crystal Z plate 12 whose Euler angles (φ, θ, ψ) are (0 °, 0 °, 0 °) around the X axis by θ = 113 ° to 135 °. It is a quartz substrate cut out along the surface to be cut. The axes obtained by rotating the Y axis and the Z axis by θ around the X axis are defined as the Y ′ axis and the Z ′ axis, respectively. The in-plane rotated ST-cut substrate is a quartz substrate obtained by rotating the ST-cut quartz substrate in the XY ′ plane, that is, rotating ψ = ± (40 ° to 49 °) around the Z ′ axis.

  FIG. 2 is an explanatory diagram of the SAW chip. FIG. 2A is a plan view of the SAW chip, and FIG. 2B is a cross-sectional view in which the SAW chip is partially enlarged. FIG. 2 shows only main components of the SAW chip. The SAW chip 10 according to the present embodiment is provided with a pair of interdigital electrodes 16 (IDTs 16) by alternately engaging a plurality of electrode fingers on a quartz substrate 14 subjected to ST cut or in-plane rotation ST cut. In this configuration, reflectors 18 are provided at both ends of the IDT 16 along the meshing direction of the electrode fingers. The reflector 18 is formed by paralleling a plurality of electrode fingers and short-circuiting both ends of the electrode fingers. The electrode fingers constituting the reflector 18 are short-circuit electrodes. The IDT 16 is made of a metal material having a low specific gravity, such as aluminum or an aluminum alloy. The reflector 18 is made of a metal material having a specific gravity heavier than that of the metal material used for the IDT 16, such as gold.

  When an electric signal is applied to the IDT 16 of the SAW chip 10 as described above, a periodic mechanical strain is generated due to the piezoelectric effect of the quartz substrate 14 to excite the surface acoustic wave and propagate along the direction in which the electrode fingers are engaged. . The type of surface acoustic wave excited at this time varies depending on the cut angle of the quartz substrate 14, and Rayleigh waves are excited in the SAW chip 10 using the ST cut quartz substrate or the in-plane rotated ST cut quartz substrate. Since the surface acoustic wave propagating from the IDT 16 is reflected by the reflector 18 and returns to the IDT 16 side, the surface acoustic wave propagates between the reflectors 18 via the IDT 16.

  At this time, since the reflector 18 is made of a metal material having a high specific gravity, that is, gold, the reflection coefficient per one short-circuit electrode is increased. FIG. 3 shows the relationship between η and the reflection coefficient per short-circuit electrode when an ST-cut quartz substrate is used. FIG. 4 shows the relationship between η and the reflection coefficient per short-circuit electrode when an in-plane rotated ST-cut quartz substrate is used. Here, η is the ratio of the electrode pitch P to the electrode width L (η = L / P), and H / λ is a value obtained by normalizing the electrode height H with the wavelength λ (λ = 2P). In both FIG. 3 and FIG. 4, a plurality of values with different values of H / λ are described. In FIGS. 3 and 4, what is indicated by a white symbol is a reflector 18 formed of gold. For comparison, a black symbol indicates that a reflector is formed using aluminum. In both cases where the value of H / λ is 0.03 and 0.04, if the reflector 18 provided on the ST-cut quartz substrate and the in-plane rotated ST-cut quartz substrate is made of gold, per short-circuit electrode It can be seen that the reflection coefficient increases.

  FIG. 5 shows the relationship between the reflection coefficient of the reflector 18 and the normalized frequency when the reflector 18 provided on the ST cut quartz substrate is made of gold. Note that η = 0.6 and H / λ = 0.03. The respective reflection coefficients when the reflector 18 is formed with the number of short-circuit electrodes being 30, 40, or 50 are shown. From FIG. 5, when 30 short-circuit electrodes are used, the reflection coefficient of the reflector 18 is 0.97, and when 40 short-circuit electrodes are used, the reflection coefficient of the reflector 18 is 0.99. It can be seen that the reflection coefficient of the reflector 18 when 50 is used is 1. FIG. 6 shows the relationship between the reflection coefficient of the reflector and the normalized frequency when the reflector provided on the ST cut quartz substrate is made of aluminum. Note that η = 0.6 and H / λ = 0.03. In FIG. 6, 105 short-circuit electrodes are used, and it can be seen that the reflection coefficient of the reflector at this time is 0.98.

  FIG. 7 shows the relationship between the reflection coefficient of the reflector and the normalized frequency when the reflector provided on the in-plane rotated ST-cut quartz substrate is made of gold. Note that η = 0.5 and H / λ = 0.03. The respective reflection coefficients when the reflector 18 is formed with the number of short-circuit electrodes being 15, 20, or 30 are shown. From FIG. 7, the reflection coefficient of the reflector 18 when 15 short-circuit electrodes are used is 0.95, and the reflection coefficient of the reflector 18 when 20 short-circuit electrodes are used is 0.99. It can be seen that the reflector 18 has a reflection coefficient of 1 when 30 are used. FIG. 8 shows the relationship between the reflection coefficient of the reflector and the normalized frequency when the reflector provided on the in-plane rotated ST-cut quartz substrate is formed of aluminum. Note that η = 0.5 and H / λ = 0.03. In FIG. 8, 250 short-circuit electrodes are used, and it can be seen that the reflection coefficient of the reflector at this time is 0.93.

  Therefore, when using either an ST-cut quartz substrate or an in-plane rotating ST-cut substrate, if a metal material having a heavy specific gravity, i.e., gold, is used for the reflector 18, the reflection coefficient per short-circuit electrode increases, and the reflector It can be seen that the number of short-circuit electrodes used for 18 can be reduced. Any of the cases shown in FIGS. 5 to 8 can be applied to the SAW chip.

  Further, since a light metal material, that is, aluminum is used for the IDT 16, the frequency temperature characteristic is good. FIG. 9 shows frequency-temperature characteristics when an IDT is formed of aluminum on an ST cut quartz substrate. H / λ is 0.028, and η is 0.5. FIG. 10 shows frequency-temperature characteristics when an IDT is formed of gold on an ST-cut quartz substrate. H / λ is 0.03 and 0.04, and η is 0.5. From FIG. 10, it is understood that when H / λ is 0.03 and the temperature range is −40 ° C. to 85 ° C., the frequency deviation when IDT is formed using gold is +1700 ppm to −2000 ppm. On the other hand, from FIG. 9, when H / λ is 0.028 and the temperature range is −40 ° C. to 85 ° C., the frequency deviation when IDT 16 is formed using aluminum is 0 ppm to −140 ppm. I understand that.

  FIG. 11 shows frequency-temperature characteristics when an IDT is formed of aluminum on an in-plane rotated ST-cut quartz substrate. H / λ is 0.0368, and η is 0.5. FIG. 12 shows frequency-temperature characteristics when an IDT is formed of gold on an in-plane rotated ST-cut quartz substrate. H / λ is 0.03 and 0.04, and η is 0.5. From FIG. 12, it is found that when H / λ is 0.03 and the temperature range is −40 ° C. to 85 ° C., the frequency deviation when gold is used to form IDT is +180 ppm to −250 ppm. On the other hand, as shown in FIG. 11, when H / λ is 0.0368 and the temperature range is −40 ° C. to 85 ° C., the frequency deviation when IDT 16 is formed using aluminum is 0 ppm to −55 ppm. I understand that.

  When the IDT is formed of a metal having a high specific gravity, that is, gold, the frequency temperature characteristics are deteriorated due to the weight of the gold and the difference in the coefficient of linear thermal expansion between the gold and the crystal. On the other hand, when IDT is formed using a metal having a low specific gravity, that is, aluminum or an aluminum alloy, frequency temperature characteristics may be good regardless of whether an ST-cut quartz substrate or an in-plane rotated ST-cut substrate is used. Understand.

  The SAW chip is adjusted so that the frequency temperature characteristic is minimized in the operating temperature range. For this reason, when the frequency temperature characteristic is represented by a quadratic function or a cubic function, the amount of frequency fluctuation is minimized by adjusting the apex of these functions to be located at the center of the operating temperature range. A SAW chip formed using an ST-cut quartz substrate exhibits a frequency temperature characteristic represented by a quadratic function, and a SAW chip formed using an in-plane rotated ST-cut quartz substrate is a frequency temperature represented by a cubic function. Show properties. Therefore, in the SAW chip using the ST cut quartz substrate, the apex of the frequency temperature characteristic moves by changing the value of η or H / λ. In addition, the SAW chip using the in-plane rotation ST-cut quartz substrate moves the apex of the frequency temperature characteristic by changing the value of the in-plane rotation angle ψ, η, or H / λ. Therefore, by forming the IDT using aluminum on the quartz substrate and adjusting the values of the in-plane rotation angles ψ, η, or H / λ, the frequency variation can be minimized.

  Thus, since the reflector 18 is formed of a heavy metal material, that is, gold, on the ST-cut quartz substrate or the in-plane rotated ST-cut quartz substrate, the reflection coefficient per one short-circuit electrode is increased, and the number of short-circuit electrodes is reduced. The SAW chip 10 can be formed with fewer. When the SAW chip 10 that excites surface acoustic waves having a resonance frequency of 106.25 MHz is manufactured by forming 60 pairs of IDT logarithms with an electrode pitch P of 14.28 μm and forming a reflector using aluminum, the short-circuit electrode is 105 This is necessary, and the length of the SAW chip is 4890.6 μm. On the other hand, when the reflector 18 is formed using gold, 40 short-circuit electrodes are required, and the length of the SAW chip 10 is 2964 μm. Therefore, by changing the metal forming the reflector from aluminum to gold, the length of the SAW chip can be shortened from 5 mm to 3 mm, and the area ratio can be reduced by 40%. Therefore, the number of SAW chips 10 taken from one quartz wafer can be increased, and the yield can be increased by 40%.

  In addition, since the IDT 16 is formed using a metal material having a light specific gravity such as aluminum or an aluminum alloy, even if a temperature change occurs in the operating temperature range of the SAW chip, a change in frequency can be minimized. Therefore, the frequency temperature characteristics of the SAW chip can be improved, and a constant frequency can be obtained.

  When the SAW chip 10 according to the present embodiment is mounted on a package, or an oscillation circuit is mounted on the package together with the SAW chip, a SAW resonator or SAW oscillator with a small size and high frequency accuracy can be obtained.

It is explanatory drawing of the cut angle of a quartz substrate. It is explanatory drawing of the surface acoustic wave chip concerning this Embodiment. It is a figure which shows the relationship between the reflection coefficient per short circuit electrode, and (eta) when using an ST cut quartz substrate. It is a figure which shows the relationship between the reflection coefficient per short circuit electrode, and (eta) when an in-plane rotation ST cut quartz substrate is used. It is a figure which shows the relationship between the reflection coefficient of a reflector, and a normalized frequency when the reflector provided on the ST cut quartz substrate is formed with gold. It is a figure which shows the relationship between the reflection coefficient of a reflector, and a normalized frequency when the reflector provided on the ST cut quartz substrate is formed with aluminum. It is a figure which shows the relationship between the reflection coefficient of a reflector, and the normalized frequency when the reflector provided on the in-plane rotation ST cut quartz substrate is formed with gold. It is a figure which shows the relationship between the reflection coefficient of a reflector, and the normalized frequency when the reflector provided on the in-plane rotation ST cut quartz substrate is formed with aluminum. It is a figure which shows the frequency temperature characteristic when the interdigital electrode is formed with aluminum on the ST cut quartz substrate. It is a figure which shows the frequency temperature characteristic when forming the interdigital electrode with gold | metal | money on a ST cut quartz crystal substrate. It is a figure which shows a frequency temperature characteristic when IDT is formed with aluminum on the in-plane rotation ST cut quartz substrate. It is a figure which shows a frequency temperature characteristic when forming IDT with gold | metal | money on the in-plane rotation ST cut quartz substrate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ...... SAW chip | tip, 14 ...... Quartz substrate, 16 ... Interdigital electrode (IDT), 18 ... Reflector.

Claims (3)

  A surface acoustic wave chip having a comb electrode and a reflector on a quartz substrate, wherein the comb electrode is formed of a metal material, and the reflector is formed of a metal material having a specific gravity heavier than the metal material. The surface acoustic wave chip, wherein the surface wave propagating on the quartz substrate is a Rayleigh wave.   The Euler angle of the quartz substrate is (0 °, 113 ° to 135 °, 0 °) or (0 °, 113 ° to 135 °, ± (40 ° to 49 °)). 2. The surface acoustic wave chip according to 1. 3. The surface acoustic wave chip according to claim 1, wherein the interdigital electrode is made of aluminum or an aluminum alloy, and the reflector is made of gold.

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