Method of manufacturing bonded substrate

Abstract

A method of manufacturing a bonded substrate, which has a quartz substrate and a piezoelectric substrate bonded, includes irradiating a bonding surface of the quartz substrate and a bonding surface of the piezoelectric substrate with ultraviolet light under a pressure lower than atmosphere pressure. After the irradiation, the bonding surface of the quartz substrate and the bonding surface of the piezoelectric substrate are brought into contact. And the quartz substrate and the piezoelectric substrate are pressurized in a thickness direction to bond the bonding surfaces.

Claims

1. A method of manufacturing a bonded substrate having a quartz substrate and a piezoelectric substrate bonded, the method comprising: irradiating a bonding surface of the quartz substrate and a bonding surface of the piezoelectric substrate set within a processing apparatus with ultraviolet light under a pressure lower than atmosphere pressure to activate the bonding surface of the quartz substrate and the bonding surface of the piezoelectric substrate; after the irradiation, bringing the bonding surface of the quartz substrate and the bonding surface of the piezoelectric substrate into contact; and pressurizing the quartz substrate and the piezoelectric substrate in a thickness direction to bond the bonding surfaces.

2. The method according to claim 1, wherein heating at a predetermined temperature is performed in the pressurization step.

3. The method according to claim 1, wherein the quartz substrate is obtained by growing a crystal by a hydrothermal synthesis method and cutting it out in an arbitrary direction.

4. The method according to claim 1, wherein an amorphous layer is interposed on one or both of the bonding surfaces of the quartz substrate and the piezoelectric substrate.

5. The method according to claim 4, wherein the amorphous layer is attached by a thin film forming method.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, advantages and features of the present invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein:

(2) FIG. 1 is a schematic diagram illustrating a bonded substrate and a surface acoustic wave element in an embodiment of the present invention;

(3) FIG. 2 is a schematic diagram illustrating a bonded substrate and a surface acoustic wave element in another embodiment;

(4) FIG. 3 is a schematic diagram illustrating a bonding processing apparatus used for manufacturing the bonded substrate in an embodiment of the present invention;

(5) FIG. 4 is a diagram for explaining a bonding mode of the quartz substrate and the piezoelectric substrate in the same embodiment;

(6) FIG. 5 is a schematic diagram illustrating a surface acoustic wave device in the same embodiment;

(7) FIG. 6 is a diagram showing calculation results of a phase velocity of the bonded substrate in the same embodiment; and

(8) FIG. 7 is a diagram illustrating calculation results of an electromechanical coupling factor of the bonded substrate in the same embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) Hereafter, a bonded substrate and a surface acoustic wave element of an embodiment of the present invention are described.

(10) As shown in FIG. 1, a bonded substrate 5 has a quartz substrate 2 and a piezoelectric substrate 3 bonded via a bonding interface 4. The quartz substrate 2 and the piezoelectric substrate 3 are bonded at the bonding interface 4 through covalent bonding.

(11) The quartz substrate 2 preferably has 150 to 500 μm of thickness. The piezoelectric substrate 3 preferably has a thickness corresponding to 0.05 to 10 wavelengths with respect to the wavelengths of a surface acoustic wave.

(12) The quartz substrate 2 can be used, for example, which is obtained by growing a crystal by a hydrothermal synthesis method and cutting it out in an arbitrary direction. The piezoelectric substrate 3 can employ a proper material and be composed, for example, of lithium tantalate or lithium niobate. In particular, it can employ lithium tantalate that is Y-cut at 36° in plane orientation and X-propagating or lithium niobate that is Y-cut at 128° and X-propagating.

(13) Moreover, as shown in FIG. 2, an amorphous layer 6 can be interposed between the quartz substrate 2 and the piezoelectric substrate 3 to afford a surface acoustic wave element 1A. Notably, the same configurations as those in the aforementioned embodiment are given the same signs and their description is omitted.

(14) In this embodiment, when the amorphous layer 6 is interposed, a bonding interface exists between the amorphous layer 6 and the quartz substrate 2, and on the other side of the amorphous layer 6, a bonding interface exists between the amorphous layer 6 and the piezoelectric substrate 3. The material of the amorphous layer 6 is not specially limited in the present invention but SiO.sub.2, Al.sub.2O.sub.3 or the like can be used. Moreover, the thickness of the amorphous layer is desirably 100 nm or less.

(15) Notably, in forming the amorphous layer 6, the amorphous layer 6 is formed by forming a thin film on the surface of the quartz substrate 2 or the piezoelectric substrate 3. Moreover, amorphous layers can be formed on both the surface of the quartz substrate 2 and the surface of the piezoelectric substrate 3.

(16) The amorphous layer can be formed by a known method, and chemical vapor deposition or physical vapor deposition such as sputtering can be used.

(17) Next, manufacturing of the bonded substrate and the surface acoustic wave element is described with reference to FIG. 3.

(18) A quartz substrate and a piezoelectric substrate of predetermined materials are prepared. Notably, when an amorphous layer is formed on the bonding surface, with respect to one or both of the quartz substrate and the piezoelectric substrate targeted for the formation, deposition processing is performed on the bonding surface side. A method of the deposition processing is not specially limited but a thin film forming technique such as a vacuum vapor deposition method or a sputtering method can be used. For example, an amorphous layer which has 100 nm or the less of the thickness can be formed on the bonding surface by electron cyclotron resonance plasma deposition. This amorphous film can be formed to have exceedingly high film density, and hence, the degree of activation of the bonding surface is high, which results in generation of more OH groups.

(19) The prepared quartz substrate 2 and piezoelectric substrate 3 are set in a processing apparatus 20 with a tightly-sealed structure. The figure presents only the quartz substrate 2.

(20) A vacuum pump 21 is connected to the processing apparatus 20, and the processing apparatus 20 is evacuated, for example, at a pressure not more than 10 Pa. Discharge gas is introduced into the processing apparatus 20 and discharge is performed by a discharge apparatus 22 in the processing apparatus 20 to generate ultraviolet light. The discharge can be performed by using a method of applying high frequency voltage or the similar method.

(21) The quartz substrate 2 and the piezoelectric substrate 3 are set in the state where they can be irradiated with ultraviolet light, and the bonding surfaces of these are irradiated with ultraviolet light to be activated. Notably, in the case where an amorphous layer is formed on one or both of the quartz substrate 2 and the piezoelectric substrate 3, the irradiation with ultraviolet light is performed with the surface of the amorphous layer being as the bonding surface.

(22) On the quartz substrate 2 and the piezoelectric substrate 3 that have undergone the irradiation with ultraviolet light, the bonding surfaces of them are brought into contact with each other and heated at ambient temperature or a temperature not more than 200° C., and pressure is applied across both of them to perform bonding. The applied pressure can be set at 10 Pa and the processing time can be set to be approximately from 5 minutes to 4 hours. It should be noted that neither the pressure value nor the processing time is specially limited in the present invention.

(23) By the aforementioned processing, the quartz substrate 2 and the piezoelectric substrate 3 are securely bonded at the bonding interface through covalent bonding.

(24) FIG. 4 shows states of the bonding surfaces of the quartz substrate 2 and the piezoelectric substrate 3.

(25) Portion A of the figure shows a state where the bonding surfaces are activated by irradiation with ultraviolet light and OH groups are formed on the surfaces. Portion B of the figure shows a state where the substrates are brought into contact with each other, and pressurized and heated to perform bonding. In the bonding, the OH groups react with one another to make covalent bonding between the substrates. Extra H.sub.2O is removed outside in heating.

(26) The aforementioned steps afford the bonded substrate. With respect to the bonded substrate, as shown in FIG. 1, patterns of interdigital electrodes 10 are formed on the principal surface of the piezoelectric substrate 3. A method of forming the interdigital electrodes 10 is not specially limited but a proper method can be used. Moreover, for the shape of the interdigital electrode 10, a proper shape can be employed. The aforementioned steps afford the surface acoustic wave element 1.

(27) As shown in FIG. 5, the surface acoustic wave element 1 can be set in a packaging 31, connected to not-shown electrodes, and sealed with a lid 32 to be provided as a surface acoustic wave device 30.

Example 1

(28) Hereafter, an example of the present invention is described.

(29) A bonded substrate was obtained based on the aforementioned embodiment. A SAW resonator was provided on the principal surface of the piezoelectric substrate such that the propagation direction of an LLSAW was the X-direction.

(30) In this example, as the piezoelectric substrate, lithium tantalate was used in which the thickness was 100 μm and the plane orientation was X-cut and 31° Y-propagating. Moreover, as the quartz substrate, a crystal that was grown by a hydrothermal synthesis method and cut out in the ST-cut direction to have 250 μm of thickness was used.

(31) The bonded sample was polished on the lithium tantalate side to be thin with approximately 10 μm of thickness.

(32) For the sample material obtained by making the piezoelectric substrate thin after bonding the quartz substrate and the piezoelectric substrate, a phase velocity and an electromechanical coupling factor of an LLSAW were calculated, the results of which are presented in FIGS. 6 and 7.

(33) The electromechanical coupling factor K.sup.2 was obtained from phase velocities Vf and Vm in the case where the electric conditions on the piezoelectric substrate were set to be the free surface (Free) and the shorted surface (Metallized), based on K.sup.2=2 (Vf−Vm)/Vf [%].

(34) By using the longitudinal-type LSAW, the sample material gave the phase velocity approximately 1.5 times faster than a phase velocity (approximately 4100 m/s) of an LSAW on 36 to 45° Y-cut and X-propagating lithium tantalate which is currently widely used as a SAW filter substrate. Moreover, while K.sup.2 of the longitudinal-type LSAW solely with X-cut and 31° Y-propagating lithium tantalate is 2.3%, approximately 3 times K.sup.2 was obtained when the thickness of the piezoelectric substrate was 0.15 to 0.35 wavelengths. This value is larger than K.sup.2 (approximately 5%) of an LSAW on 36 to 45° Y-cut and X-propagating lithium tantalate.

(35) For the obtained bonded substrate, its bonding strength was measured by a method of tensile testing (application of tension perpendicular to the wafer plane). As a result, it was found that 4 MPa or more (converted into the value per unit area) of bonding strength was obtained, which was excellent bonding strength, further leading to the bulk destruction.

(36) As above, the present invention has been described based on the aforementioned embodiment and example. The scope of the present invention is not limited to the contents of the aforementioned description, but proper modifications of the aforementioned embodiment and example can occur without departing from the scope of the present invention.

INDUSTRIAL APPLICABILITY

(37) The present invention can be used for a SAW resonator, a SAW filter, a highly-functional piezoelectric sensor, a BAW device and the like.