Composite substrate manufacturing process

Abstract

Composite substrate manufacturing process with: (a) a step in which a substrate stack having a diameter of 4 inches or more is highly polished, the substrate stack comprising a piezoelectric substrate and a support substrate bonded together, and the high-gloss polishing is carried out on the side of the piezoelectric substrate until the thickness of the piezoelectric substrate reaches 3 µm or less; (b) a step in which data on the distribution of the thickness of the highly polished piezoelectric substrate are generated; and (c) a step in which, based on the thickness distribution data, machining is carried out using an ion beam machine equipped with a direct current excited Ar beam source to produce a composite substrate, wherein step (c) is carried out in such a way that beam output powers of the ion beam machine are determined for respective points on the surface of the piezoelectric substrate and the piezoelectric substrate is machined in accordance with the beam output powers, wherein the irradiation time is constant over the entire surface of the piezoelectric substrate and thicker areas are irradiated with higher beam output powers, and wherein the piezoelectric substrate has a thickness of 3 µm or less,the difference between the greatest and smallest thickness of the piezoelectric substrate across the entire plane is 60 nm or less, and the piezoelectric substrate has such crystallinity that the half-width of an X-ray diffraction rocking curve is 100 arcseconds or less.

Description

[0001] The invention relates to a composite substrate manufacturing method.

[0002] The use of an ultra-thin piezoelectric layer is expected to realize an acoustic wave device capable of operating at a higher frequency than previously possible. Chemical vapor deposition (CVD) and "smart cut" technology are two exemplary processes capable of forming such thin layers. These two processes are well-known techniques. The "smart cut" technology, for example, is described in paragraph

[0004] of JP 2010-109949 A.

[0003] While the CVD process and Smart Cut technology can produce thin layers with a highly uniform thickness, these processes each have the following problems. 1) CVD - The crystallinity is very poor. - The directions of the crystal axes are limited. 2) “Smart Cut” technology - Crystal defects remain due to insufficient recovery from ion implantation damage.

[0004] In addition to the two methods above, polishing piezoelectric layers to a thin thickness has been attempted. For example, JP H09-208399 A discloses a manufacturing method for a composite substrate in which a piezoelectric LiNbO3 substrate bonded to a 2-inch support substrate is first ground until the thickness reaches approximately 10 µm and then further polished with a CMP machine until the thickness reaches approximately 3 µm. If necessary, further planarization follows using PACE (Plasma Assisted Chemical Polishing). However, this approach has resulted in cracks occurring during polishing or has failed to achieve a uniform thickness.

[0005] Furthermore, JP 2004-221816 A and JP 2007-214215 A disclose a planarization method in which the thickness distribution of a substrate is measured and machining is performed with an ion beam machine. The irradiation time with the ion beam is determined for respective points on the substrate surface, and the substrate is machined according to the respective irradiation time. JP 2007-214215 A discloses an ion beam machine with a direct current-excited Ar beam source, but proposes a plasma, an electron beam, or reactive plasma etching as an alternative to the ion beam machine. As a further alternative, it is proposed to keep the ion beam irradiation time constant and to control the output power of the ion beam machine for each area on the substrate surface.

[0006] It is an object of the invention to provide a thin piezoelectric single crystal layer with high crystallinity, desired crystal axis, uniform thickness and thin damage layer.

[0007] The above object is achieved by a composite substrate manufacturing method comprising the features of claim 1. Short description of the drawings Fig. 1 is a sectional view of a piezoelectric substrate in Example 1, which is a comparative example. Fig. 2 is a sectional view of a piezoelectric substrate in Example 2, which is an example of the present invention. Description of implementation examples

[0008] A composite substrate obtainable by the composite substrate manufacturing method of the present invention is described below. The composite substrate comprises a piezoelectric substrate and a support substrate bonded together, and it has a diameter of 4 inches or more. The piezoelectric substrate has a thickness of 3 μm or less, and the difference between the maximum thickness and the minimum thickness of the piezoelectric substrate is 60 nm or less across the entire plane surface of the piezoelectric substrate. Furthermore, the piezoelectric substrate has such crystallinity that the half-width of an X-ray diffraction rocking curve is 100 arc seconds or less.

[0009] Examples of the materials of the piezoelectric substrates include lithium tantalate, lithium niobate, lithium niobate-lithium tantalate solid solution single crystal, lithium borate, langasite and rock crystal.

[0010] Examples of support substrate materials include silicon, sapphire, aluminum nitride, alumina, alkali-free glass, borosilicate glass, quartz glass, lithium tantalate, lithium niobate, lithium niobate-lithium tantalate solid solution single crystal, lithium borate, langasite, and rock crystal. The size of the support substrate is such that the diameter is the same as that of the piezoelectric substrate and the thickness is 100 to 1000 µm, preferably 150 to 500 µm.

[0011] An electrode pattern may be formed on the surface of the piezoelectric substrate. Thus, the composite substrate of this embodiment can be used as an acoustic wave device.

[0012] Next, a process for manufacturing the composite substrate is described below. Step (a):

[0013] A substrate stack (a composite substrate to be polished) is provided, which has a diameter of 4 inches or more and comprises a piezoelectric substrate with a thickness of 100 to 1000 µm and a support substrate with a thickness of 100 to 1000 µm, which are bonded together. The side of the piezoelectric substrate is mirror-polished until the thickness of the piezoelectric substrate reaches 3 µm or less. The substrate stack is a stack of a piezoelectric substrate and a support substrate, which are bonded to each other via an organic adhesive layer or directly. Exemplary materials of the organic adhesive layers include epoxy resins and acrylic resins. Direct bonding can be achieved by activating the respective bonding surfaces of the piezoelectric substrate and the support substrate and pressing the substrates against each other while the bonding surfaces are facing each other.For example, the bonding surfaces can be activated by irradiating the bonding surfaces with an ion beam of an inert gas (such as argon) or by irradiation with plasma or a neutral atom beam.

[0014] For example, step (a) can be performed by first grinding the piezoelectric substrate side of the substrate stack with a grinder and then lapping it with a lapping machine. The piezoelectric substrate side is further polished with a CMP machine until the thickness of the piezoelectric substrate reaches 3 µm or less. In this way, the thickness of the piezoelectric substrate can be efficiently reduced to 3 µm or less. The term "CMP" is short for chemical mechanical polishing. Step (b):

[0015] Thickness distribution data of the highly polished piezoelectric substrate is generated. For example, the thickness distribution data can be generated by measuring the thickness of the highly polished piezoelectric substrate with an optical thickness gauge that utilizes laser interferometry. In this way, the thickness distribution data can be generated with high accuracy. Step (c):

[0016] The piezoelectric substrate is machined with an ion beam machine based on the thickness distribution data. This step results in a composite substrate in which the piezoelectric substrate has a thickness of 3 µm or less, the difference between the maximum and minimum thickness of the piezoelectric substrate across the entire plane is 60 nm or less, and the piezoelectric substrate has such crystallinity that the half-width of an X-ray diffraction rocking curve is 100 arcseconds or less.

[0017] Step (c) is performed by inputting the thickness distribution data into the ion beam machine to determine the beam outputs for the respective points on the surface of the piezoelectric substrate, and machining the piezoelectric substrate in accordance with the beam outputs. This allows for precise machining. In this case, the irradiation time is constant, and thicker areas are irradiated with higher beam outputs.

[0018] The machining in step (c) involves an ion beam machine equipped with a direct current excited Ar beam source. Although an ion beam machine equipped with a plasma excited Ar beam source is usable, the use of an ion beam machine with a direct current excited Ar beam source is advantageous in that it further reduces damage to the surface of the piezoelectric substrate.

[0019] According to the above-described method for manufacturing the composite substrate of this embodiment, it is possible to eliminate the problems associated with the CVD method and "smart cut" technology, and to produce thin piezoelectric single-crystal layers with high crystallinity, a desired crystal axis, and a uniform thickness. The composite substrates manufactured by this method can be used in acoustic wave devices. EXAMPLESExample 1 (comparison example):

[0020] A silicon substrate (a support substrate) and a LiTaO3 substrate (a piezoelectric substrate) were provided, each polished on both sides and each having a thickness of 230 µm and a diameter of 4 inches. These substrates were placed in a vacuum chamber with a vacuum degree on the order of 10 -6Pa was introduced and held so that the bonding surfaces faced each other. The bonding surfaces of both substrates were irradiated with an Ar beam for 80 seconds to remove inactive layers and activate the surfaces. Subsequently, the substrates were brought into contact with each other and bonded together under a load of 1200 kgf. The resulting substrate stack was recovered, and the side of the piezoelectric substrate was ground with a grinder until the thickness of the piezoelectric substrate reached 10 µm. Next, the substrate stack was placed on a lapping machine, and the piezoelectric substrate was polished using diamond slurry until its thickness reached 3 µm. Furthermore, the surface of the piezoelectric substrate was mirror-polished with a CMP machine until its thickness reached 0.8 µm. Colloidal silica was used as an abrasive during this process.The thickness of the piezoelectric substrate was measured with an optical thickness gauge using laser interferometry. The thickness across the entire surface of the piezoelectric substrate was 0.8 µm ± 0.1 µm. The measurement was performed at a total of 80 points across the entire flat surface of the piezoelectric substrate, excluding the beveled edges.

[0021] The substrate stack obtained in the above manner was placed on an ion beam machine equipped with a plasma-excited Ar beam source. Next, the thickness data of the piezoelectric substrate obtained by the above-mentioned measurement with the optical thickness gauge were imported into the ion beam machine, and the machining amounts, or more precisely, the lengths of the Ar beam irradiation time, were determined for the respective measurement points on the piezoelectric substrate. The length of the beam irradiation time was adjusted by controlling the feed rate of the substrate stack. While changing the feed rate of the substrate stack, the entire surface of the piezoelectric substrate was irradiated with an Ar beam of constant output. The beam spot was 6 mm in diameter.RF plasma was excited under conditions where the ion acceleration voltage and ion current were constant at 1300 eV and 30 mA, respectively. The actual machining time was approximately 5 minutes.

[0022] The thickness of the piezoelectric substrate in the machined substrate stack (in the composite substrate of this example) was measured again. Consequently, the median thickness was 0.76 µm, and the difference between the maximum and minimum thicknesses was 24 nm across the entire surface. A rocking curve was recorded using an X-ray diffraction meter. The full width at half maximum (FWHM) was 80 arcseconds, the same as that of the bulk single crystal. This result confirmed that there was no deterioration in crystallinity.

[0023] This composite substrate was used in surface acoustic wave filters to realize devices that had small frequency deviations and showed excellent filtering properties. Example 2 (Example according to the invention):

[0024] A substrate stack was prepared in the same manner as in Example 1. The thickness of the piezoelectric substrate was measured with an optical thickness gauge using laser interferometry. As a result, the thickness across the entire surface of the piezoelectric substrate was 1.0 µm ± 0.12 µm. Similar to Example 1, the total number of measurement points was 80.

[0025] The substrate stack obtained in the above manner was placed on an ion beam machine equipped with a direct current-excited Ar beam source. Next, the thickness data of the piezoelectric substrate obtained by the above-mentioned measurement with the optical thickness gauge were imported into the ion beam machine, and the machining amounts, or more precisely, the Ar beam output powers, were determined for the respective measurement points on the piezoelectric substrate. While the substrate stack was advanced at a (constant) feed rate of 0.5 mm / s, the entire surface of the piezoelectric substrate was irradiated with the Ar beam, with the Ar beam output powers varying between 20 and 100 W. The beam spot was 6 mm in diameter.Since the feed rate was constant, the length of the beam irradiation time was the same over the entire surface of the piezoelectric substrate.

[0026] The thickness of the piezoelectric substrate in the machined substrate stack (in the composite substrate of this example) was measured again. Consequently, the median thickness was 0.92 µm, and the difference between the maximum and minimum thicknesses was 50 nm across the entire surface. A rocking curve was recorded using an X-ray diffraction meter. The full width at half maximum (FWHM) was 65 arcseconds, which was the same as that of the bulk single crystal. This result confirmed that there was no deterioration in crystallinity.

[0027] A TEM was used to examine sections near the surface of the piezoelectric substrates in Example 1 and Example 2. The examination revealed the presence of damage layers on the surface of both substrates. Fig. 1 is a cross-sectional image obtained in Example 1, and Fig. Figure 2 is a cross-sectional image obtained in Example 2. The thickness of the damage layer (the black surface layer) in Example 1 was 10 nm, and the damage layer in Example 2 had a thickness of 3 nm. This result demonstrated that an ion beam machine equipped with a DC-excited Ar beam source causes less damage to the surface than an ion beam machine equipped with a plasma-excited Ar beam source.

[0028] To evaluate the influence of the damage layer on the device properties, electrodes of SAW (surface acoustic wave) resonators were formed on the piezoelectric substrates in the composite substrates of Examples 1 and 2. The electrode spacing was 4 µm. Similar resonator properties with a center frequency of approximately 930 MHz were achieved between the resonators of the present invention and resonators fabricated on conventional piezoelectric substrates. Although it has been shown that the presence of damage layers with a thickness of approximately 10 nm does not affect the properties, a damage layer with a thickness of only approximately 3 nm is preferred.

[0029] The invention can be used in acoustic wave devices such as SAW filters.

Claims

[1] Composite substrate manufacturing process with: (a) a step of mirror-polishing a substrate stack having a diameter of 4 inches or more, the substrate stack comprising a piezoelectric substrate and a support substrate bonded to each other, and mirror-polishing is performed on the piezoelectric substrate side until the thickness of the piezoelectric substrate reaches 3 µm or less; (b) a step in which data of the distribution of the thickness of the highly polished piezoelectric substrate are generated; and (c) a step in which, based on the thickness distribution data, machining is carried out with an ion beam machine equipped with a direct current-excited Ar beam source to produce a composite substrate, wherein step (c) is carried out in such a manner that beam outputs of the ion beam machine are determined for respective points on the surface of the piezoelectric substrate and the piezoelectric substrate is machined in accordance with the beam outputs, wherein the irradiation time is constant over the entire surface of the piezoelectric substrate and thicker areas are irradiated with higher beam outputs, and wherein the piezoelectric substrate has a thickness of 3 µm or less,the difference between the maximum thickness and the minimum thickness of the piezoelectric substrate over the entire plane surface is 60 nm or less, and the piezoelectric substrate has a crystallinity such that the half-width of an X-ray diffraction rocking curve is 100 arcseconds or less. [2] The composite substrate manufacturing method according to claim 1, wherein a damage layer of the machined piezoelectric substrate has a thickness of 3 nm. [3] The composite substrate manufacturing method according to claim 1 or 2, wherein the step (a) is performed in such a manner that the piezoelectric substrate side of the substrate stack is first ground with a grinder and then lapped with a lapping machine, and the piezoelectric substrate side is further mirror-polished with a CMP machine until the thickness of the piezoelectric substrate reaches 3 µm or less. [4] The composite substrate manufacturing method according to any one of claims 1 to 3, wherein the step (b) is performed in such a manner that the thickness distribution data is generated by measuring the thickness of the mirror-polished piezoelectric substrate with an optical thickness gauge using laser interferometry.

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