The present invention relates to a composite wafer and a method for producing the same.
A method of sticking a wafer of lithium tantalate (Lithium Tantalate: may be abbreviated to LT) into which a hydrogen ion is implanted in advance and a wafer of lithium tantalate via a metal film, and performing a thermal treatment, thereby causing delamination with heat while avoiding a problem due to a difference in thermal expansion coefficients, has been known (for example, see Non-Patent Document 1).
When lithium tantalate or lithium niobate (Lithium Niobate: may be abbreviated to LN) is used for a supporting wafer, a charge is generated also in a thin film LT or LN which is an active layer in accordance with polarization possessed by LT or LN as the supporting wafer, and thus characteristics are negatively affected.
Hereinafter, the present invention will be described through embodiments of the invention, but the following embodiments do not limit the invention according to claims. In addition, not all of the combinations of features described in the embodiments are essential to the solution of the invention.
The LT layer 110 is polarized. For example, the LT layer 110 is a single crystal, and is electrically polarized in a Z-axis direction of the crystal even without an external electric field. In this manner, the LT layer 110 is formed as an active layer which exerts a function such as a piezoelectric effect.
The LT layer 110 has a thickness of several hundred nm, for example. An LN layer may be used instead of the LT layer 110.
On the other hand, the LT substrate 400 is not substantially polarized. A state of not being substantially polarized herein is weaker than at least the polarization of the LT layer 110, and it includes not only a state in which polarization is not caused at all when there is no external electric field, but also a state in which polarization is not intentionally caused but was originally caused, a state in which polarization is remained even after going through a process of eliminating the polarization, a state in which polarization is caused in a level that does not affect exertion of the function of the LT layer 110, and the like. Furthermore, for example, the polarization of the LT substrate 400 is preferably 0.5 pC/N or less in an absolute value of a d33 meter.
The LT substrate 400 has a thickness of several hundred μm, for example, and it gives a mechanical strength when handling the composite wafer 10. Instead of the LT substrate 400, another substrate having a little difference in expansion coefficients from an active layer, for example, an LN substrate may be used.
The interposed layer 200 is disposed between the LT layer 110 and the LT substrate 400 in a thickness direction. The interposed layer 200 preferably has insulation properties, and is preferably easy to process, for example, easy to make a mirror surface with polishing. The interposed layer 200 may be at least one of SiO2, SiON, or SiN. The interposed layer 200 is not polarized in a state where external voltage has not been applied.
500 in
501 in
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503 in
Note that, a treatment of causing non-polarization may be positively performed on the LT substrate 400. For example, by raising a temperature of the LT substrate 400 to a temperature of a Curie point (phase transition point) or higher, the polarization caused in the LT substrate 400 is destructed. Note that, the Curie point of LT is around 607° C., and the Curie point of LN is around 1160° C.
504 in
In the above-described sticking surfaces, the LT substrate 100 and the LT substrate 400 are stuck. In the present embodiment, the LT substrate 100 and the LT substrate 400 are stuck via the interposed layer 200. When at least either of the sticking surfaces has been subjected to the activation treatment, the sticking may be performed at ordinary temperature. Note that, instead of the activation treatment, in the sticking, the sticking may be performed with a high temperature of several hundred degrees (and optionally also with a high pressure).
505 in
According to the present embodiment as above, by using substrates having thermal expansion coefficients that are equal or close to each other for the LT substrate 100 which becomes the active layer and the LT substrate 400 which becomes the supporting substrate, a warpage is less likely to be generated at the time of the thermal treatment, and the temperature can be raised to a temperature that enables delamination. Furthermore, since the LT substrate 400 which becomes the supporting substrate is not substantially polarized, a negative effect to the LT layer 110 which is the active layer can be avoided.
A SiO2 film was formed for 700 nm by a PVD (sputtering) method on a 42° Y-cut LT 100 mmφ wafer (with polarization) having a thickness of 0.35 mm, and polishing was performed to 500 nm. This wafer was stuck to various supporting substrates after being subjected to a surface treatment by a plasma activation method, and the temperature was raised.
A SiO2 film was formed for 700 nm by the PVD (sputtering) method on a 160° Y-cut LN 100 mmφ wafer (with polarization) having a thickness of 0.35 mm, and polishing was performed to 500 nm. This wafer was stuck to various supporting substrates after being subjected to a surface treatment by a plasma activation method, and the temperature was raised. The results were the same as Example 1.
H+ ions were implanted with 100 keV in a dose amount of 7.5e16 atoms/cm 2 into a 42° Y-cut LT 100 mmφ wafer (with polarization) having a thickness of 0.35 mm which becomes the active layer. Then, a film of SiO2 was formed by the PVD (sputtering) method, and polishing was performed. This wafer was stuck to various supporting substrates after being subjected to a surface treatment by the plasma activation method, and the temperature was raised to 180° C. Then, delamination was performed along an implantation interface with a SiGen method (mechanical delamination method), and polishing was performed on the surface to make the thickness of LT to 500 nm, followed by a thermal treatment of 550° C. to obtain a composite wafer.
A resonator was created for these composite wafers, and a Qmax value was measured near 2 GHz. A Q value is a sharpness of a signal peak, and a value thereof is an index for measuring performance of a device. The results are shown in
The same experiment as Example 3 was conducted by using a 160° Y-cut LN 100 mmφ wafer (with polarization) having a thickness of 0.35 mm which becomes an active layer. The temperature was raised to 450° C. before delamination. The results showed a tendency similar to Example 3.
The results were almost the same even when the film of the interposed layer was formed by a CVD (chemical vapor deposition) method, or the material of the interposed layer was changed to SiON or SiN, in Example 1. It became clear that the present invention is not dependent on the film formation method or the material of the interposed layer.
While the present invention has been described by way of the embodiments, the technical scope of the present invention is not limited to the scope described in the above-described embodiments. It is apparent to persons skilled in the art that various alterations or improvements can be made to the above-described embodiments. It is also apparent from the description of the claims that embodiments added with such alterations or improvements can be included in the technical scope of the present invention.
The operations, procedures, steps, stages, or the like of each process performed by a device, system, program, and method shown in the claims, embodiments, or drawings can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or drawings, it does not necessarily mean that the process must be performed in this order.
Number | Date | Country | Kind |
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2021-025398 | Feb 2021 | JP | national |
The contents of the following patent application(s) are incorporated herein by reference: NO. 2021-025398 filed in JP on Feb. 19, 2021NO. PCT/JP2022/004806 filed in WO on Feb. 8, 2022
Number | Date | Country | |
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Parent | PCT/JP2022/004806 | Feb 2022 | US |
Child | 18450431 | US |