Joined body of piezoelectric material substrate and support substrate, and acoustic wave element

Abstract

A bonded body includes a supporting substrate, a piezoelectric material substrate of a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate, and a bonding layer bonding the supporting substrate and piezoelectric material substrate. A material of the bonding layer is silicon oxide. Provided that the bonding layer is divided into a piezoelectric material substrate-side bonding part and a supporting substrate-side bonding part, the piezoelectric material substrate-side bonding part has a nitrogen concentration higher than a nitrogen concentration of the supporting substrate-side bonding part.

Claims

1. A bonded body comprising: a supporting substrate; a piezoelectric material substrate comprising a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and a bonding layer for bonding said supporting substrate and said piezoelectric material substrate, wherein said bonding layer consists essentially of silicon oxide; and wherein an intermediate part has a nitrogen concentration of 1×10.sup.19 atoms/cm.sup.3 or higher and 5×10.sup.20 atoms/cm.sup.3 or lower and a piezoelectric material substrate-side bonding part and a supporting substrate-side bonding part have nitrogen concentrations of 4×10.sup.17 atoms/cm.sup.3 or higher and 9×10.sup.18 atoms/cm.sup.3 or lower, provided that said bonding layer is divided into said piezoelectric material substrate-side bonding part, said supporting substrate-side bonding part and said intermediate part between said piezoelectric material substrate-side bonding part and said supporting substrate-side bonding part, and wherein said intermediate part has a carbon concentration of 1×10.sup.19 atoms/cm.sup.3 or higher and 5×10.sup.20 atoms/cm.sup.3 or lower and said piezoelectric material substrate-side bonding part and said supporting substrate-side bonding part have carbon concentrations of 1×10.sup.17 atoms/cm.sup.3 or higher and 3×10.sup.18 atoms/cm.sup.3 or lower.

2. The bonded body of claim 1, wherein a fluorine concentration of said intermediate part is higher than a fluorine concentration of said piezoelectric material substrate-side bonding part and a fluorine concentration of said supporting substrate-side bonding part.

3. The bonded body of claim 1, wherein said piezoelectric material substrate has a thickness of 4.0 μm or smaller.

4. A bonded body comprising: a supporting substrate; a piezoelectric material substrate comprising a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and a bonding layer for bonding said supporting substrate and said piezoelectric material substrate, wherein said bonding layer consists essentially of silicon oxide; and wherein an intermediate part has a nitrogen concentration of 1×10.sup.19 atoms/cm.sup.3 or higher and 5×10.sup.20 atoms/cm.sup.3 or lower and a piezoelectric material substrate-side bonding part and a supporting substrate-side bonding part have nitrogen concentrations of 4×10.sup.17 atoms/cm.sup.3 or higher and 9×10.sup.18 atoms/cm.sup.3 or lower, provided that said bonding layer is divided into said piezoelectric material substrate-side bonding part, said supporting substrate-side bonding part and said intermediate part between said piezoelectric material substrate-side bonding part and said supporting substrate-side bonding part, and wherein said intermediate part has a fluorine concentration of 3×10.sup.18 atoms/cm.sup.3 or higher and 8×10.sup.19 atoms/cm.sup.3 or lower, and said piezoelectric material substrate-side bonding part and said supporting substrate-side bonding part have fluorine concentrations of 1×10.sup.18 atoms/cm.sup.3 or higher and 2×10.sup.18 atoms/cm.sup.3 or lower.

5. The bonded body of claim 4, wherein said piezoelectric material substrate has a thickness of 4.0 μm or smaller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1(a) shows a piezoelectric material substrate 1, and FIG. 1(b) shows the state that plasma is irradiated onto a surface of the piezoelectric material substrate to generate an activated surface.

(2) FIG. 2(a) shows the state that a silicon oxide film is provided on a supporting substrate, and FIG. 2(b) shows the state that plasma is irradiated onto a bonding face of the silicon oxide film.

(3) FIG. 3(a) shows a bonded body of the piezoelectric material substrate and supporting substrate, FIG. 3(b) shows the state that the piezoelectric material substrate of a bonded body is thinned by processing, and FIG. 3(c) shows an acoustic wave device.

(4) FIG. 4(a) shows the piezoelectric material substrate, FIG. 4(b) shows the state that a silicon oxide film is provided on the piezoelectric material substrate, and FIG. 4(c) shows the state that plasma is irradiated onto a bonding face of the silicon oxide film to activate it.

(5) FIG. 5(a) shows a bonded body of the piezoelectric material substrate and supporting substrate, FIG. 5(b) shows the state that the piezoelectric material substrate of a bonded body is thinned by processing, and FIG. 5(c) shows an acoustic wave device.

(6) FIG. 6 is a graph showing typical resonance characteristics of an acoustic wave device.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

(7) The present invention will be described in detail below, appropriately referring to the drawings.

(8) FIGS. 1 to 3 relate to a first embodiment of the present invention.

(9) First, as shown in FIG. 1(a), a piezoelectric material substrate 1 having a pair of main faces 1a and 1b is prepared. Then, as shown in FIG. 1(b), plasma is irradiated onto the bonding face 1a of the piezoelectric material substrate 1 as arrows A to obtain a surface activated bonding face 5.

(10) Further, as shown in FIG. 2(a), a silicon nitride film 4 on a surface 3a of a supporting substrate 3 is formed. Then, as shown in FIG. 2(b), plasma is irradiated onto a surface 4a of the silicon nitride layer 4 as arrows B to perform surface activation to form an activated bonding face 6.

(11) The activated bonding face 5 on the piezoelectric material substrate 1 and activated bonding face 6 of the silicon oxide film 4 on the supporting substrate 3 are contacted and directly bonded with each other to obtain a bonded body 8 shown in FIG. 3(a). Here, a bonding layer 7 made of a material of silicon oxide, between the supporting substrate 3 and piezoelectric material substrate 1 is generated. Provided that the bonding layer 7 is divided into a piezoelectric material substrate-side bonding part 7a and supporting substrate-side bonding part 7b, a nitrogen concentration of the piezoelectric material substrate-side bonding part 7a is made higher than a nitrogen concentration of the supporting substrate-side bonding part 7b. The piezoelectric material substrate-side bonding part 7a contacts the piezoelectric material substrate 1, and the supporting substrate-side bonding part 7b contacts the supporting substrate 3.

(12) An electrode may be provided on the piezoelectric material substrate 1 at this stage. Further, preferably, as shown in FIG. 3(b), the main face 1b of the piezoelectric material substrate 1 is processed to make the substrate 1 thinner, to obtain a thinned piezoelectric material substrate 1A. 9 represents a processed surface. Then, as shown in FIG. 3(c), an electrode 10 may be formed on the processed surface 9 of the piezoelectric material substrate 1A of the bonded body 8A to obtain a piezoelectric material device 11.

(13) According to the present embodiment, the bonding layer 7 is made of silicon oxide, and provided that the bonding layer 7 is divided into the piezoelectric material substrate-side bonding part 7a and the supporting substrate-side bonding part 7b, the nitrogen concentration N7a of the piezoelectric material substrate-side bonding part 7a is higher than the nitrogen concentration N7b of the supporting substrate-side bonding part. A ratio (N7a/N7b) of the nitrogen concentration N7a of the piezoelectric material substrate-side bonding part 7a with respect to the nitrogen concentration N7b of the supporting substrate-side bonding part may preferably be 10 or higher and may more preferably be 100 or higher. Further, from a practical viewpoint, the ratio (N7a/N7b) may preferably be 1000 or lower.

(14) The nitrogen concentration N7a of the piezoelectric material substrate-side bonding part 7a may preferably be 1E19 atoms/cm.sup.3 or higher and may more preferably be 5E19 atoms/cm.sup.3 or higher. Further, the nitrogen concentration N7a may be 8E20 atoms/cm.sup.3 or lower. Further, the nitrogen concentration N7b of the supporting substrate-side bonding part 7b of the bonding layer may preferably be 4E17 to 7E18 atoms/cm.sup.3. Besides, “1E19” indicates “1×10.sup.19” and the other numerical values are interpreted in the same manner.

(15) Additionally, the bonding layer 7 is divided into the two parts in the direction of thickness to provide the piezoelectric material substrate-side bonding part 7a and supporting substrate-side bonding part 7b, provided that the thicknesses of the respective bonding parts are the same. Further, the piezoelectric material substrate-side bonding part 7a contacts the piezoelectric material substrate 1, and the supporting substrate-side bonding part 7b contacts the supporting substrate 3.

(16) According to a preferred embodiment, a carbon concentration C7a of the piezoelectric material substrate-side bonding part 7a is higher than a carbon concentration C7b of the supporting substrate-side bonding part 7b. A ratio (C7a/C7b) of the carbon concentration C7a of the piezoelectric material substrate-side bonding part 7a with respect to the carbon concentration C7b of the supporting substrate-side bonding part may preferably be 10 or higher and more preferably be 100 or higher. Further, from a practical viewpoint, the ratio (C7a/C7b) may preferably be 1000 or lower.

(17) For the present embodiment, the carbon concentration C7a of the piezoelectric material substrate-side bonding part 7a may preferably be 1E19 atoms/cm.sup.3 or higher and may more preferably be 5E19 atoms/cm.sup.3 or higher. Further, the carbon concentration C7a may be 1E21 atoms/cm.sup.3 or lower. Further, the carbon concentration C7b of the supporting substrate-side bonding part 7b of the bonding layer 7 may preferably be 1E17 to 3E18 atoms/cm.sup.3.

(18) According to a preferred embodiment, a fluorine concentration F7a of the piezoelectric material substrate-side bonding part 7a is higher than a fluorine concentration F7b of the supporting substrate-side bonding part 7b. A ratio (F7a/F7b) of the fluorine concentration F7a of the piezoelectric material substrate-side bonding part 7a with respect to the fluorine concentration F7b of the supporting substrate-side bonding part 7b may preferably be 5 or higher and may more preferably be 50 or higher. Further, from a practical viewpoint, the ratio (F7a/F7b) may preferably be 500 or lower.

(19) For the present embodiment, the fluorine concentration F7a of the piezoelectric material substrate-side bonding part 7a may preferably be 3E18 atoms/cm.sup.3 or higher and may more preferably be 6E18 atoms/cm.sup.3 or higher. Further, the fluorine concentration Fla may be 2E20 atoms/cm.sup.3 or lower. Further, the fluorine concentration F7b of the supporting substrate-side bonding part 7b of the bonding layer 7 may preferably be 1E17 to 2E18 atoms/cm.sup.3.

(20) FIGS. 2, 4 and 5 relate to a second embodiment of the present invention.

(21) First, as shown in FIG. 4(a), a piezoelectric material substrate 1 having a pair of main faces 1a and 1b is prepared. Then, as shown in FIG. 4(b), a silicon oxide film 12 is film-formed on the first main face 1a of the piezoelectric material substrate 1. Then, as shown in FIG. 4(c), plasma is irradiated onto a bonding face 12a of the silicon oxide film 12 as arrows A to obtain a surface-activated bonding face 13.

(22) Further, as shown in FIG. 2(a), a silicon oxide film 4 is formed on a surface 3a of a supporting substrate 3. Then, as shown in FIG. 2(b), plasma is irradiated onto a surface 4a of the silicon oxide film 4 as arrows B to perform surface activation, to provide an activated bonding face 6.

(23) Then, an activated bonding face 13 of the silicon oxide layer 12 on the piezoelectric material substrate and the activated bonding face 6 of the silicon oxide film 4 on the supporting substrate are contacted and directly bonded with each other to obtain a bonded body 18 shown in FIG. 5(a). Here, the silicon oxide film 12 and silicon oxide film 4 are integrated through the direct bonding to generate a silicon oxide layer 17.

(24) Provided that the bonding layer 17 is divided into a piezoelectric material substrate-side bonding part 17a, supporting substrate-side bonding part 17c, and intermediate part 17b, a nitrogen concentration of the intermediate part 17b is higher than a nitrogen concentration of the piezoelectric material substrate-side bonding part 17a and a nitrogen concentration of the supporting substrate-side bonding part 17c. The piezoelectric material substrate-side bonding part 17a contacts the piezoelectric material substrate 1, and the supporting substrate-side bonding part 17c contacts the supporting substrate 3.

(25) An electrode may be provided on the piezoelectric material substrate 1 at this stage. Further, preferably, as shown in FIG. 5(b), the main face 1b of the piezoelectric material substrate 1 is processed to make the substrate 1 thinner, to obtain a thinned piezoelectric material substrate 1A. 9 represents a processed surface. Then, as shown in FIG. 5(c), a predetermined electrode 10 may be formed on the processed surface 9 of the piezoelectric material substrate 1A of the bonded body 18A to obtain a piezoelectric material device 21.

(26) According to the present embodiment, the bonding layer 17 is made of a material of silicon oxide, and provided that the bonding layer 17 is divided into the piezoelectric material substrate-side bonding part 17a, intermediate part 17b, and supporting substrate-side bonding part 17c, a nitrogen concentration N17b of the intermediate part 17b is higher than a nitrogen concentration N17a of the piezoelectric material substrate-side bonding part 17a and a nitrogen concentration N17c of the supporting substrate-side bonding part 17c. Ratios (N17b/N17a) and (N17b/N17c) of the nitrogen concentration N17b of the intermediate part 17b with respect to the nitrogen concentration N17a of the piezoelectric material substrate-side bonding part 17a and the nitrogen concentration N17c of the supporting substrate-side bonding part 17c may preferably be 10 or higher and may more preferably be 100 or higher. Further, the ratio (N17b/N17a) and ratio (N17b/N17c) may preferably be 1000 or lower from a practical viewpoint.

(27) Further, the bonding layer 17 is divided, in the direction of thickness, into three parts which are the piezoelectric material substrate-side bonding part 17a, intermediate part 17b, and supporting substrate-side bonding part 17c, and the thicknesses of the respective bonding parts 17a, 17b, and 17c are made the same. Further, the piezoelectric material substrate-side bonding part 17a contacts the piezoelectric material substrate 1A, and the supporting substrate-side bonding part 17c contacts the supporting substrate 3.

(28) The nitrogen concentration N17b of the intermediate part 17b may preferably be 1E19 atoms/cm.sup.3 or higher and may more preferably be 5E19 atoms/cm.sup.3 or higher. Further, the nitrogen concentration N17b may preferably be 5E20 atoms/cm.sup.3 or lower. The nitrogen concentration N17a of the piezoelectric material substrate-side bonding part 17a and the nitrogen concentration N17c of the supporting substrate-side bonding part 17c may preferably be 9E18 atoms/cm.sup.3 or lower. Further, the nitrogen concentrations N17a and N17c may preferably be 4E17 atoms/cm.sup.3 or higher.

(29) According to the present embodiment, a carbon concentration C17b of the intermediate part 17b is higher than a carbon concentration C17a of the piezoelectric material substrate-side bonding part 17a and higher than a carbon concentration C17c of the supporting substrate-side bonding part 17c. Ratios (C17b/C17a) and (C17b/C17c) of the carbon concentration C17b of the intermediate part with respect to the carbon concentration C17a of the piezoelectric material substrate-side bonding part 17a and the carbon concentration C17c of the of the supporting substrate-side bonding part 17a may preferably be 10 or higher and may more preferably be 100 or higher. Further, the ratio (C17b/C17a) and ratio (C17b/C17c) may preferably be 1000 or lower on a practical viewpoint.

(30) The carbon concentration C17b of the intermediate part 17b may preferably be 1E19 atoms/cm.sup.3 or higher and may more preferably be 5E19 atoms/cm.sup.3 or higher. Further, the carbon concentration C17b may preferably be 5E20 atoms/cm.sup.3 or lower. The carbon concentration C17a of the piezoelectric material substrate-side bonding part 17a and the carbon concentration C17c of the supporting substrate-side bonding part 17c may preferably be 3E18 atoms/cm.sup.3 or lower. Further, the carbon concentrations C17a and C17c may be 1E17 atoms/cm.sup.3 or higher.

(31) According to the present embodiment, a fluorine concentration F17b of the intermediate part 17b is higher than a fluorine concentration F17a of the piezoelectric material substrate-side bonding part 17a and higher than a fluorine concentration F17c of the supporting substrate-side bonding part 17c. Ratios (F17b/F17a) and (F17b/F17c) of the fluorine concentration F17b of the intermediate part 17b with respect to the fluorine concentration F17a of the piezoelectric material substrate-side bonding part 17a and the fluorine concentration F17c of the supporting substrate-side bonding part 17c may preferably be 5 or higher, and may more preferably 50 or higher. Further, the ratio (F17b/F17a) and ratio (F17b/F17c) may more preferably be 500 or lower from a practical viewpoint.

(32) The fluorine concentration F17b of the intermediate part 17b may preferably be 3E18 atoms/cm.sup.3 or higher and may more preferably be 6E18 atoms/cm.sup.3 or higher. Further, the fluorine concentration F17b may preferably be 8E19 atoms/cm.sup.3 or lower. The fluorine concentration F17a of the piezoelectric material substrate-side bonding part 17a and the fluorine concentration F17c of the supporting substrate-side bonding part 17c may preferably be 2E18 atoms/cm.sup.3 or lower. Further, the fluorine concentrations F17a and F17c may be 1E17 atoms/cm.sup.3 or higher.

(33) The respective constituents of the present invention will be described further in detail below.

(34) Although the material of the supporting substrate 3 is not particularly limited, preferably the material is selected from the group consisting of silicon, quartz, sialon, mullite, sapphire and translucent alumina. It is thus possible to further improve the temperature characteristics of frequency of acoustic wave device 11 or 21.

(35) The silicon oxide layers 4 and 12 are formed on the supporting substrate 3 or optionally on the piezoelectric material substrate 1. Although the method of film-forming the silicon oxide layers 4 and 12 is not particularly limited, sputtering, chemical vapor deposition (CVD) and vapor deposition may be used. Preferably, the supporting substrate 3 is a silicon substrate, and in this case, the silicon oxide film 4 can be formed by sputtering of oxygen or ion injection onto the surface of the silicon substrate, or by heating under oxidizing atmosphere.

(36) The thickness of the silicon oxide film 4 or 12 may preferably be 0.05 μm or larger, more preferably be 0.1 μm or larger and particularly preferably be 0.2 μm or larger. Further, the thickness of the silicon oxide film may preferably be 3 μm or smaller, preferably 2.5 μm or smaller and may more preferably be 2.0 μm or smaller.

(37) The piezoelectric material substrate 1 used in the present invention is made of single crystals of a lithium tantalate (LT), lithium niobate (LN) or lithium niobate-lithium tantalate solid solution. As the materials have high propagation speeds of a surface acoustic wave and large electro-mechanical coupling factors, it is preferred for use in a surface acoustic wave device for high frequency and wide-band frequency applications.

(38) Further, the normal direction of the main surface of the piezoelectric single crystal substrate 1 is not limited. For example, in the case that the piezoelectric material substrate 1 is made of LT, it is preferred to use the substrate rotated from Y-axis toward Z-axis by 32 to 50° (180°, 58° to 35°, 180° on Eulerian angle representation) around X-axis, which is a direction of propagation of a surface acoustic wave, because of a low propagation loss. In the case that the piezoelectric material substrate 1 is made of LN, (i) it is preferred to use the substrate rotated from Z-axis toward −Y-axis by 37.8° (0°, 37.8°, 0° on Eulerian angle representation) around X-axis, which is a direction of propagation of a surface acoustic wave, because of a large electro-mechanical coupling factor. Alternatively, (ii) it is preferred to use the substrate rotated from Y-axis toward Z-axis by 40 to 65° (180°, 50 to 25°, 180° on Eulerian angle representation) around X-axis, which is a direction of propagation of a surface acoustic wave, because a high acoustic speed can be obtained. Further, although the size of the piezoelectric material substrate 1 is not particularly limited, for example the diameter may be 100 to 200 mm and thickness may be 0.15 to 1 μm.

(39) Plasma is then irradiated onto the bonding face 1a of the piezoelectric material substrate 1, the bonding face 12a of the silicon oxide film 12 on the piezoelectric material substrate 1 or bonding face 4a of the silicon oxide film 4 on the supporting substrate 3 at a temperature of 150° C. or lower to activate the bonding face 1a, 4a or 12a. Although it is preferred to irradiate nitrogen plasma, the bonded bodies 8A and 18A of the present invention can be obtained even when oxygen plasma is irradiated.

(40) The pressure during the surface activation may preferably be 100 Pa or lower and may more preferably be 80 Pa or lower. Further, the atmosphere may be composed of nitrogen only, oxygen only, or a mixture of nitrogen and oxygen.

(41) The temperature during the irradiation of the plasma is preferably 150° C. or lower. It is thereby possible to obtain the bonded bodies 8 and 18A, each having a high bonding strength and no deterioration of the crystallinity. The temperature during the plasma irradiation is preferably 150° C. or lower and may more preferably be 100° C. or lower.

(42) Further, the energy of the irradiated plasma is preferably 30 to 150 W. Further, a product of the energy of the irradiated plasma and irradiation time period is preferably 0.12 to 1.0 Wh.

(43) The bonding face 5 and bonding face 6 of the substrates after the plasma treatment or the bonding face 13a on bonding face 6 are contacted with each other at a room temperature. Although these may be treated under vacuum, the bonding faces are preferably contacted under atmosphere. It is thus possible to realize a composite substrate maintaining the distribution of nitrogen, carbon and fluorine incorporated into silicon oxide by plasma treatment.

(44) According to a preferred embodiment, the bonding face 1a of the piezoelectric material substrate 1 and the bonding faces 4a and 12a of the silicon oxide layers 4 and 12 are subjected to a flattening process before the plasma treatment. The method of flattening the respective bonding faces 1a, 4a and 12a may include lapping, chemical mechanical polishing (CMP), and the like. Further, the flattened face may preferably have an Ra of 1 nm or lower and may more preferably have an Ra of 0.3 nm or lower.

(45) The bonding face 5 of the piezoelectric material substrate 1 and the bonding face 6 of the silicon oxide film 4 on the supporting substrate 3, or the bonding face 13 of the silicon oxide film 12 on the piezoelectric material substrate 1 and bonding face 6 of the silicon oxide film 4 on the supporting substrate 3 are contacted and bonded with each other. Thereafter, an annealing treatment is preferably performed to improve the bonding strength. The temperature during the annealing temperature may preferably be 100° C. or higher and 300° C. or lower.

(46) The bonded bodies 8A and 18A of the present invention may preferably be applied as acoustic wave devices 11 and 21.

(47) As acoustic wave devices 11 and 21, a surface acoustic wave device, Lamb wave-type device, thin film resonator (FBAR) or the like are known. For example, the surface acoustic wave device is produced by providing input side IDT (Interdigital transducer) electrodes (also referred to as comb electrodes or interdigitated electrodes) for oscillating surface acoustic waves and IDT electrodes on the output side for receiving the surface acoustic wave on the surface of the piezoelectric material substrate. By applying a high frequency signal on the IDT electrodes on the input side, an electric field is generated between the electrodes, so that the surface acoustic wave is oscillated and propagated on the piezoelectric material substrate. Then, the propagated surface acoustic wave is drawn as an electrical signal from the IDT electrodes on the output side provided in the direction of the propagation.

(48) A material forming the electrode 10 of the piezoelectric material substrate 1A may preferably be aluminum, an aluminum alloy, copper or gold, and more preferably be aluminum or an aluminum alloy. The aluminum alloy may preferably be Al with 0.3 to 5 weight % of Cu mixed therein. Ti, Mg, Ni, Mo or Ta may be used instead of Cu.

EXAMPLES

Inventive Example A1

(49) An acoustic wave device 11 shown in FIG. 3(c), according to the method described referring to FIGS. 1 to 3, was produced.

(50) Specifically, a 42Y-cut X-propagation LiTaO.sub.3 substrate (piezoelectric material substrate) 1 having a thickness of 200 μm and both main faces polished into mirror surfaces and a high-resistance (>2 kΩ.Math.cm) Si (100) substrate (supporting substrate) 3 having a thickness of 675 μm were proposed. Both substrates have sizes of 150 mm. A silicon oxide layer 4 was film-formed by sputtering into a thickness of 500 nm on the supporting substrate 4. As the silicon oxide layer 4 after the film-formation had a surface roughness Ra of 0.6 nm, the surface was slightly polished by CMP (Chemical mechanical polishing) so that the Ra was improved to 0.3 nm.

(51) The bonding face 1a of the piezoelectric material substrate 1 and the bonding face 4a of the silicon oxide layer 4 on the supporting substrate 3 were both subjected to cleaning and surface activation. Specifically, ultrasonic cleaning using pure water was performed, and the substrate surfaces were dried by spin dry. The supporting substrate 3 after the cleaning was then introduced into a plasma activation chamber, and the bonding face 4a was activated by nitrogen gas plasma at 30° C. Further, the piezoelectric material substrate 1 was similarly introduced into the plasma activation chamber, and the bonding face 1a was subjected to surface activation by nitrogen gas plasma at 30° C. The time period of the surface activation was 40 seconds and the energy was 100 W. The ultrasonic cleaning and spin dry as described above were performed again for removing particles adhered during the surface activation.

(52) Then, the positioning of the respective substrates was performed, and the bonding faces 5 and 6 of the substrates were contacted with each other at room temperature. The substrates were contacted with the piezoelectric material substrate 1 positioned as the upper side. As a result, it was observed that by a so-called bonding wave that the adhesion of the substrates was spreading, indicating that good preliminary bonding was completed. Then, the bonded body was inserted into an oven filled with nitrogen atmosphere and held at 120° C. for 10 hours, for improving the bonding strength.

(53) The surface 1b of the piezoelectric material substrate 1 of the bonded body 8 after the heating was subjected to grinding, lapping and CMP polishing so that the thickness of the piezoelectric material substrate 1A was 16, 8, 4, 2 or 1 μm. The nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts were measured using SIMS method (secondary ion mass spectroscopy) for the thus obtained bonded bodies 8A. As the SIMS measuring system, the “CAMECA IMS-7f” was used, Cs+ was used as primary ion specie, and the primary acceleration voltage was 15.0 kV. The detection region was 30 μmφ.

(54) The thickness of the piezoelectric material substrate 1A was measured by an optical measurement system (“F20” supplied by Filmetrix Corporation) applying optical interference. An IDT electrode 10 made of aluminum metal was provided on the surface 9 of the polished piezoelectric material substrate 1A by a photolithography process. The period A of the electrode was 4 μm, so the oscillation frequency was about 1000 MHz. The thickness of the IDT electrode 10 was 200 nm and 80 pairs of reflectors on both sides of the 200 pairs of IDT electrodes 8, respectively, were provided to produce an acoustic wave device 11 (SAW resonator) of 1 port. The impedance characteristics of the thus produced acoustic wave device (SAW resonator) 11 was measured by a network analyzer “E5071C” supplied by Agilent Corporation. FIG. 6 shows typical resonance characteristics. A resonance peak at about 1000 MHz (fs) and an anti-resonance peak at about 1050 MHz (fr) were observed. The ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was measured. The results are shown in Table 3.

Inventive Example A2

(55) In the inventive example A1, plasma of mixed gases of 80 percent nitrogen gas and 20 percent oxygen gas was used instead of nitrogen plasma. When the gas composition was changed, the matching was appropriately changed so that the reflection electric power took the minimum value. The other conditions were the same as those in inventive example A1.

(56) According to the same procedures as the inventive example A1, nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts in the thus obtained bonded body 8 were measured and the measurement results are shown in Table 1. Further, as with inventive example A1, the piezoelectric material substrate was processed so that the thickness of the piezoelectric material substrate 1A was 16, 8, 4, 2 or 1 μm. The ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was measured. The results are shown in Table 3.

Comparative Example A1

(57) Each bonded body and SAW device were produced and subjected to the same measurements as inventive example A1. However, the silicon oxide film was film-formed on the piezoelectric material substrate 1 and not film-formed on the side of the supporting substrate 3. Further, the surface activation of the silicon oxide film and supporting substrate 3 was performed, and the activated surfaces of the silicon oxide film and supporting substrate 3 were directly bonded by irradiating nitrogen plasma.

(58) The nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts were measured for the thus obtained bonded bodies, and the measurement results are shown in Table 2. The piezoelectric material substrate 1 was processed so that the thickness of the piezoelectric material substrate was 16, 8, 4, 2 or 1 μm according to the procedure of inventive example A1, and the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fs was measured. The results are shown in Table 3.

(59) TABLE-US-00001 TABLE 1 Nitrogen Carbon Fluorine concen- concen- concen- tration tration tration Measurement points atoms/cm.sup.3 atoms/cm.sup.3 atoms/cm.sup.3 Inventive Average value of 7.1E19 2.6E20 4.4E19 Example piezoelectric A1 material substrate- side bonding part Average value of 4.6E18 3.2E17 9.7E17 supporting substrate side bonding part Inventive Average value of 3.2E19 2.1E20 9.9E19 Example piezoelectric A2 material suhstrate- side bonding part Average value of 1.8E18 1.6E17 3.4E17 supporting substrate- side bonding part

(60) TABLE-US-00002 TABLE 2 Nitrogen Carbon Fluorine concen- concen- concen- tration tration tration Measurement points atoms/cm.sup.3 atoms/cm.sup.3 atoms/cm.sup.3 Comparative Average value of 1.6E18 8.1E17 2.3E17 Example piezoelectric A1 material substrate- side bonding part Average value of 8.3E19 1.2E20 8.8E18 supporting substrate- side bonding part

(61) TABLE-US-00003 TABLE 3 Ratio of admittance value (dB) Thickness of piezoelectric material substrate after Inventive Inventive Comparative processing (μm) Example A1 Example A2 Example A1 16.0 42.5 44.0 43.3 8.0 40.7 42.5 42.8 4.0 43.3 42.2 40.0 2.0 44.3 45.1 39.3 1.0 46.5 44.9 35.8

(62) As shown in Table 3, even in the case in which the piezoelectric material substrate 1A is extremely thinned by processing to a thickness of 2.0 to 1.0 μm in an acoustic wave device (SAW device) 11 using the bonded body 8A of the present invention, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was not deteriorated and was good. On the other hand, according to the bonded body of the comparative example A1, as the thickness of the piezoelectric material substrate decreases, the ratio of the admittance valuer at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr deteriorated. In particular, according to the bonded body of the comparative example A1, when the piezoelectric material substrate was processed to a thickness of 4.0 μm or smaller, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was considerably deteriorated.

Inventive Example B

(63) In the inventive example A1, the material of the piezoelectric material substrate was changed to a 128Y-cut X-propagation lithium niobate. As a result, the same results as those of the inventive example A1 were obtained.

Inventive Example C1

(64) An acoustic wave device 21 shown in FIG. 5(c), according to the method illustrated referring to FIGS. 2, 4 and 5, was produced.

(65) Specifically, a 42Y-cut X-propagation LiTaO.sub.3 substrate (piezoelectric material substrate) 1 having a thickness of 200 μm and both main faces polished into mirror surfaces and a high-resistance (>2 kΩ.Math.cm) Si (100) substrate (supporting substrate) 3 having a thickness of 675 μm was prepared. Both substrates have sizes of 150 mm. Silicon oxide films 12 and 4 were film-formed by sputtering in thicknesses of 250 nm on the piezoelectric material substrate 1 and supporting substrate 3, respectively. The respective silicon oxide films 12 and 4 after the film-formation each had a surface roughness Ra of 0.6 nm measured by AFM, and the surface was slightly polished by CMP (Chemical mechanical polishing) so that Ra was improved to 0.3 nm.

(66) The bonding face 12a of the silicon oxide film 12 on the piezoelectric material substrate 1 and the bonding face 4a of the silicon oxide layer 4 on the supporting substrate 3 were then subjected to washing and surface activation. Specifically, ultrasonic cleaning using pure water was performed and the substrate surfaces were dried by spin drying.

(67) The supporting substrate 3, after the cleaning, was then introduced into a plasma activation chamber to activate the bonding face 4a of the silicon oxide film 4 with nitrogen gas plasma at 30° C. Further, the piezoelectric material substrate 1 was similarly introduced into the plasma activation chamber so that the bonding face 12a of the silicon oxide film 12 was subjected to surface activation with nitrogen gas plasma at 30° C. The time period of the surface activation was 40 seconds and the energy was 100 W. The ultrasonic cleaning and spin drying as described above were performed again for removing particles adhered during the surface activation.

(68) Then, the positioning of the respective substrates was performed, and the activated bonding faces 13 and 6 of the substrates were contacted with each other at room temperature. The substrates were contacted with the piezoelectric material substrate 1 positioned as the upper side. As a result, it was observed with the so-called bonding wave that the adhesion of the substrates was spreading, indicating that good preliminary bonding was completed. Then, the bonded body was inserted into an oven filled with nitrogen atmosphere and held at 120° C. for 10 hours, for improving the bonding strength.

(69) The surface of the piezoelectric material substrate 1 of the bonded body 18, after the heating, was subjected to grinding, lapping and CMP processing so that the thickness of the piezoelectric material substrate 1A was 16, 8, 4, 2 or lea m. The nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts were measured using SIMS method (secondary ion mass spectroscopy) for the thus obtained bonded bodies 18A. As the SIMS measuring system, “CAMECA IMS-7f” was used, Cs+ was used as primary ion specie, and the primary acceleration voltage was 15.0 kV. The detection region was 30 μmφ. The results are shown in Table 4.

(70) An IDT electrode 10 made of aluminum metal was provided on the processed surface 9 of the polished piezoelectric material substrate 1A, and an acoustic wave device 21 (SAW device) of 1 port by photography process. The period A of the electrode was 4 μm, so that the oscillation frequency was about 1000 MHz. The impedance characteristics of the thus produced acoustic wave device (SAW resonator) 21 was measured by a network analyzer “E5071C” supplied by Agilent corporation. The ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was measured. The results are shown in Table 6.

Inventive Example C2

(71) In inventive example C1, a plasma of mixed gases of 80 percent of nitrogen gas and 20 percent of oxygen gas, instead of nitrogen plasma, was used. The experiment was otherwise performed according to the same procedure as that for inventive example C1.

(72) The nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts were measured for the thus obtained bonded body 18A according to the same procedure as inventive example C1, and the measurement results are shown in Table 4. Further, the piezoelectric material substrate 1A was processed to thicknesses of 16, 8, 4, 2 and 1 μm according to the same procedure as that of inventive example C1, and the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fs was measured. The results are shown in Table 6.

Comparative Example C1

(73) The respective bonded bodies and acoustic wave devices (SAW devices) were produced according the same procedure as inventive example C1 and subjected to the measurements as those for inventive example C1. However, the silicon oxide film was film-formed on the piezoelectric material substrate and not film-formed on the side of the supporting substrate. Further, the silicon oxide film and supporting substrate were subjected to the surface activation, and the activated surfaces of the silicon oxide film and supporting substrate were directly bonded with each other by irradiation of nitrogen gas plasma.

(74) The nitrogen concentration, carbon concentration and fluorine concentration at the respective parts were measured for the thus obtained bonded body and the measurement results are shown in Table 5. Further, the piezoelectric material substrate 1A was processed to thicknesses of 16, 8, 4, 2, and 1 μm using the same procedure as for inventive example A1, and the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fs was measured. The results are shown in Table 6.

(75) TABLE-US-00004 TABLE 4 Nitrogen Carbon Fluorine concen- concen- concen- tration tration tration Measurement points atoms/cm.sup.3 atoms/cm atoms/cm.sup.3 Inventive Average value of 1.8E18 5.3E17 6.3E17 Example piezoelectric C1 material substrate- side bonding part Average value of 6.1E19 9.9E19 3.2E19 intermediate part Average value of 2.6E18 4.1E17 9.7E17 supporting substrate- side bonding part Inventive Average value of 8.8E17 4.9E17 2.8E17 Example piezoelectric C2 material substrate- side bonding part Average value of 2.9E19 1.7E20 9.5E18 intermediate part Average value of 7.6E17 2.5E17 4.4E17 supporting substrate- side bonding part

(76) TABLE-US-00005 TABLE 5 Nitrogen Carbon Fluorine concen- concen- concen- tration tration tration Measurement points atoms/cm.sup.2 atoms/cm.sup.2 atoms/cm.sup.2 Comparative Average value of 1.6E18 8.1E17 4.3E17 Example piezoelectric C1 material substrate- side bonding part Average value of 1.6E18 8.1E17 6.6E17 intermediate part Average value of 8.3E19 1.2E20 1.2E19 supporting substrate- side bonding part

(77) TABLE-US-00006 TABLE 6 Ratio of admittance values (dB) Piezoelectric material substrate Inventive Inventive Comparative after processing Example C1 Example C2 Example C1 16.0 43.9 41.4 43.3 8.0 42.2 42.5 42.8 4.0 44.8 42.1 40.0 2.0 46.6 43.3 39.3 1.0 47.3 46.1 35.8

(78) As shown in Table 6, according to the acoustic wave device (SAW device) 21 using the bonded body 18A of the present invention, even in the case when the thickness of the piezoelectric material substrate 1A was made extremely small such as 2.0 to 1.0 μm by processing, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was not deteriorated and was good. On the other hand, according to the bonded body of comparative example C1, as the thickness of the piezoelectric material substrate was thinner, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fs deteriorated. In particular, according to the bonded body of comparative example C1, when the thickness of the piezoelectric material substrate was 4.0 μm or smaller by processing, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was considerably deteriorated.

Inventive Example D

(79) The material of the piezoelectric material substrate 1 was changed to lithium niobate in inventive example C1. As a result, similar results as those of the inventive example C1 were obtained.