BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite substrate and a method of producing the same.

2. Description of the Related Art

Acoustic wave devices include interdigital transducer (IDT) electrodes formed on one surface of a piezoelectric substrate and pick up a signal in a specific frequency band. In recent years, composite substrates obtaining by bonding a thin piezoelectric substrate to a support substrate with a small thermal expansion coefficient have been used to improve thermal characteristics of the acoustic wave devices. As an example of these composite substrates, a composite substrate is known in which lithium tantalate or lithium niobate is used for the piezoelectric substrate and silicon or quartz is used for the support substrate (see PTL 1).

CITATION LIST

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2006-319679

SUMMARY OF THE INVENTION

In general, in a composite substrate, IDT electrodes are formed on a positively-polarized surface of a piezoelectric substrate, and a negatively-polarized surface of the piezoelectric substrate is bonded to a support substrate. To produce the composite substrate, the piezoelectric substrate and the support substrate are first bonded by a direct bonding method, and a surface of the piezoelectric substrate is then ground and polished to thin the piezoelectric substrate. When the bonding is performed by the direct bonding method, the bonding surface of the piezoelectric substrate and the bonding surface of the support substrate are irradiated with an argon beam. At this time, when the negatively-polarized surface of the piezoelectric substrate is irradiated with the argon beam, the arithmetic mean roughness Ra after the irradiation when an area 10 μm square is measured with an atomic force microscope (AFM) is about 0.5 nm.

When the bonding is performed by the direct bonding method, bonding strength is increased as the arithmetic mean roughness Ra of the bonding surface is decreased in addition to adhering substances such as an oxide film and a liquid on the surface being completely removed and the surface being activated. It is accordingly desirable that the arithmetic mean roughness Ra of the bonding surface be decreased as much as possible.

The present invention has been made to address the problem, and a primary object of the present invention is to increase the bonding strength in the composite substrate when the piezoelectric substrate and the support substrate are bonded by the direct bonding method.

Solution to Problem

The composite substrate according to the present invention comprises

a piezoelectric substrate, one surface of which is a negatively-polarized surface and another surface of which is a positively-polarized surface; and

a support substrate bonded to the positively-polarized surface of the piezoelectric substrate by direct bonding.

In the composite substrate, the positively-polarized surface of the piezoelectric substrate is directly bonded to one surface of the support substrate. Now, comparing the positively-polarized surface of the piezoelectric substrate subjected to the direct bonding process with the negatively-polarized surface subjected to the direct bonding process, the surface roughness of the former is better than the surface roughness of the latter when the degree of removal from the surface of the piezoelectric substrates is identical. Accordingly, the bonding strength is higher in the former case than in the latter case when the bonding is performed by the direct bonding method. Examples of the direct bonding process include ion beam irradiation such as an inert gas (argon etc.) and plasma or neutral atomic beam irradiation.

In the composite substrate according to the present invention, the negatively-polarized surface of the piezoelectric substrate may be etched with the strong acid, the etching rate at which the negatively-polarized surface is etched with the strong acid may be higher than the etching rate at which the positively-polarized surface is etched with the strong acid, and the etching rate at which the support substrate is etched with the strong acid may be higher than the etching rate at which the negatively-polarized surface is etched with the strong acid. In the composite substrate, the positively-polarized surface of the piezoelectric substrate is bonded to one surface of the support substrate, and hence the surface of the piezoelectric substrate is the negatively-polarized surface that is etched with the strong acid at a higher rate. Accordingly, the duration required for etching the surface of the piezoelectric substrate with the strong acid by a predetermined thickness, that is, the duration for which the entire composite substrate is immersed into the strong acid during etching is shorter than when the surface of the piezoelectric substrate is the positively-polarized surface. Since the etching rate for the support substrate is higher than for the negatively-polarized surface of the piezoelectric substrate, the support substrate is also etched while the entire composite substrate is immersed into the strong acid. However, since the duration of the immersion into the strong acid is shorter than when the surface of the piezoelectric substrate is the positively-polarized surface as described above, progress in the etching of the support substrate can be inhibited to such a degree that the bonding strength is not affected. Accordingly, in the composite substrate, the bonding strength between the support substrate and the piezoelectric substrate can be sufficiently ensured even after the piezoelectric substrate surface is etched with the strong acid.

In the composite substrate according to the present invention, the strong acid is preferably hydrofluoric nitric acid or hydrofluoric acid. Use of hydrofluoric nitric acid or hydrofluoric acid enables the surface of the piezoelectric substrate (negatively-polarized surface) to be etched at a relatively high rate.

In the composite substrate according to the present invention, the support substrate preferably has a thermal expansion coefficient smaller than the thermal expansion coefficient of the piezoelectric substrate. In this manner, when the composite substrate is used to fabricate an acoustic wave device, the dimensional variation of the piezoelectric substrate due to a variation in temperature can be reduced and frequency characteristics can be suppressed from varying due to a variation in temperature of the acoustic wave device.

In the composite substrate according to the present invention, the piezoelectric substrate is preferably a lithium tantalate (LT) substrate or a lithium niobate (LN) substrate, and the support substrate is preferably a silicon substrate or a glass substrate. Because the LT substrate and the LN substrate have a large polarization vector, the etching rate at which the positively-polarized surface is etched with the strong acid is likely to differ to a large extent from the etching rate at which the negatively-polarized surface is etched with the strong acid, and application of the present invention is significant. In addition, application of the present invention is significant because the silicon substrate and the glass substrate are etched with the strong acid at a rate higher than in the case of the LT substrate and the LN substrate, and the degree of progress in the etching needs to be suppressed by decreasing the duration for which each substrate is immersed into the strong acid.

A method of producing a composite substrate according to the present invention includes a bonding step of bonding a positively-polarized surface of a piezoelectric substrate, one surface of which is a negatively-polarized surface and another surface of which is a positively-polarized surface, to one surface of a support substrate by a direct bonding method to produce the composite substrate.

In the method of producing a composite substrate, the positively-polarized surface of the piezoelectric substrate is directly bonded to one surface of the support substrate. Now, comparing the positively-polarized surface of the piezoelectric substrate subjected to the direct bonding process with the negatively-polarized surface subjected to the direct bonding process, the surface roughness of the former is better than the surface roughness of the latter. Accordingly, the bonding strength is higher in the former case than in the latter case when the bonding is performed by the direct bonding method.

The method of producing a composite substrate according to the present invention further include a substrate thinning step of grinding and polishing the negatively-polarized surface of the piezoelectric substrate of the composite substrate obtained in the bonding step, and an etching step of etching, with a strong acid, an affected layer generated in the negatively-polarized surface in the substrate thinning step. In the piezoelectric substrate and support substrate, an etching rate at which the negatively-polarized surface is etched with the strong acid may be higher than an etching rate at which the positively-polarized surface is etched with the strong acid and an etching rate at which the support substrate is etched with the strong acid may be higher than the etching rate at which the negatively-polarized surface is etched with the strong acid. In the substrate thinning step, the affected layer (layer in which the quality of the material has been changed due to grinding and polishing) is generated in the negatively-polarized surface. In the etching step, the affected layer is etched and removed with the strong acid. At this time, the duration required for etching the affected layer generated in the surface of the piezoelectric substrate with the strong acid, that is, the duration for which the entire composite substrate is immersed into the strong acid during etching is shorter than when the surface of the piezoelectric substrate is the positively-polarized surface. Since the etching rate for the support substrate is higher than for the negatively-polarized surface of the piezoelectric substrate, the support substrate is also etched while the entire composite substrate is immersed into the strong acid. However, since the duration of the immersion into the strong acid is shorter than when the surface of the piezoelectric substrate is the positively-polarized surface as described above, progress in the etching of the support substrate can be inhibited to such a degree that the bonding strength is not affected. Accordingly, in the composite substrate, the bonding strength between the support substrate and the piezoelectric substrate can be sufficiently ensured even after the piezoelectric substrate surface is etched with the strong acid.

In the method of producing a composite substrate according to the present invention, hydrofluoric nitric acid or hydrofluoric acid is preferably used as the strong acid. Use of hydrofluoric nitric acid or hydrofluoric acid enables the surface of the piezoelectric substrate to be etched at a relatively high rate.

In the method of producing a composite substrate according to the present invention, a substrate having a thermal expansion coefficient smaller than a thermal expansion coefficient of the piezoelectric substrate is preferably prepared as the support substrate. In this manner, when the composite substrate is used to fabricate an acoustic wave device, the dimensional variation of the piezoelectric substrate due to a variation in temperature can be reduced and frequency characteristics can be suppressed from varying due to a variation in temperature of the acoustic wave device.

In the method of producing a composite substrate according to the present invention, an LT substrate or an LN substrate is preferably prepared as the piezoelectric substrate, and a silicon substrate or a glass substrate is preferably prepared as the support substrate. Because the LT substrate and the LN substrate have a large polarization vector, the etching rate at which the positively-polarized surface is etched with the strong acid is likely to differ to a large extent from the etching rate at which the negatively-polarized surface is etched with the strong acid, and application of the present invention is significant. In addition, because the silicon substrate and the glass substrate are etched with the strong acid at a higher rate compared with the LT substrate and the LN substrate and highly need to be immersed into the strong acid for a shorter duration so that the degree of progress in the etching is suppressed, application of the present invention is significant.

