JP2019117397A - Substrate processing method - Google Patents

Abstract

To effectively prevent cracking in a substrate on which a function element is formed when processing is performed for thinning the substrate.SOLUTION: A substrate processing method for thinning a substrate 200 that is anisotropic with respect to linear expansion coefficient includes the steps of: bonding the substrate 200 to a reinforcement member 204 that is isotropic with respect to the linear expansion coefficient; processing the substrate 200 bonded to the reinforcement member 204 for thinning the substrate 200; and peeling the substrate 200 off the reinforcement member 204. The bonding step includes heating a thermally fusible material interposed between the reinforcement member 204 and the substrate 200 to a temperature of 100°C or more and 200°C or less to fuse and then cool it for solidification, thereby bonding the reinforcement member 204 and the substrate 200.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method of processing a substrate using a reinforcing member, and a functional element comprising a substrate reinforced by the reinforcing member, for example, a lithium niobate crystal substrate on which an optical waveguide element is formed is thinly processed using a reinforcing member. And an optical waveguide device comprising the substrate reinforced by the reinforcing member. In the present specification, the term "functional element" refers to any type of element formed on a substrate so as to exert some electrical and / or optical function. For example, an optical waveguide element, an electric circuit An element etc. shall be included.

  In the fields of optical communication and light measurement, many light control devices such as light modulators using an optical waveguide element in which an optical waveguide is formed on a substrate having an electro-optical effect are used.

  In the optical waveguide device, an electrode for applying an electric field to the optical waveguide is further provided in proximity to the optical waveguide formed on the substrate surface, and light modulation etc. is performed by controlling the refractive index of the optical waveguide by the applied electric field. The light control function of

As a substrate of such an optical waveguide device, for example, lithium niobate (LiNbO ₃ ) (also referred to as “LN”) which is a ferroelectric crystal is widely used. When LN is used as the substrate of the optical waveguide device, it is necessary to reduce the dielectric loss of the substrate in order to reduce the driving voltage required for light control or to improve the electrical characteristics required for driving by a high frequency signal.

  As a technique for reducing the drive voltage of the optical waveguide device and improving the electrical characteristics, in the case of a Z-cut LN substrate, processing in the vicinity of the optical waveguide to form a ridge structure or in the case of an X-cut LN substrate the substrate thickness It is known to process it thinly to 10 μm or less.

  The thinning process of the LN substrate is performed, for example, in the procedure shown in FIG. First, as shown in FIG. 4A, an optical waveguide is formed on the front surface (upper surface in the drawing) of the LN substrate 400. The formation of the optical waveguide can be performed by a known method such as a Ti thermal diffusion method. Next, the front surface of the LN substrate 400 (the surface on which the optical waveguide is formed) is attached, for example, with a resin 404, to a polishing jig 406 made of metal (FIG. 4B). Subsequently, the back surface of the LN substrate 400 is polished using a suitable polishing agent until the thickness of the LN substrate 400 becomes a desired thickness (FIG. 4C).

  However, in the manufacturing process including such a process of processing the LN substrate thinly, the LN substrate is easily cracked during the processing, which can be one factor to reduce the manufacturing yield of the optical waveguide device. Generally, in the thinning process, as described above, the front surface (front side) of the LN substrate is bonded and fixed to a polishing jig made of metal or the like, and the rear surface of the LN substrate is polished. Here, in general, a resin or the like that melts at a high temperature is often used to bond the jig and the LN substrate, and when the resin is cured by returning to normal temperature, thermal expansion / contraction of the jig accompanying temperature fluctuation The difference between the thermal expansion and contraction of the LN substrate generates a stress in the LN substrate, which makes it easy to cause the LN substrate to crack in the subsequent steps.

  The stress generated in the LN substrate can be prevented in principle by matching the linear expansion coefficient of the jig with the linear expansion coefficient of the LN substrate, but the LN substrate is in the direction perpendicular to the direction parallel to the crystal axis. In fact, it is difficult to match the linear expansion coefficients because they have linear expansion coefficients different from each other (have anisotropy).

  Conventionally, as a technique for preventing the generation of substrate cracks in a device, a piezoelectric substrate made of anisotropic crystal such as lithium tantalate is a member such as sapphire which does not have anisotropy and has a linear expansion coefficient close to that of the piezoelectric substrate. It is known that the support layer is bonded to a support layer comprising the support layer, and the support layer thickness to the entire thickness after bonding is a predetermined ratio or less (Patent Document 1).

  However, the above-mentioned prior art is intended to be applied to a piezoelectric device, and in an optical waveguide element requiring a chip size 10 times or more larger than that of the piezoelectric device, the generation of stress is sufficiently reduced by the above configuration. It is difficult to do in the first place. Generally, when manufacturing an optical waveguide device, a plurality of optical waveguide devices are formed in a larger circular substrate, and the substrate is thinned by polishing the back surface of the circular substrate (batch processing) . Therefore, even if the above-mentioned prior art is applied in the process of thinning a circular substrate for batch processing, it is extremely difficult to prevent the stress generated in the substrate.

  Furthermore, even from the viewpoint of the configuration of the optical waveguide device, the above-described conventional techniques differ in the thickness of the support layer on the premise of an anisotropic crystal substrate having a thickness of several tens of μm (50 to 70 μm). In an optical waveguide device which is required to be processed to a thickness of about 10 μm or less, which is less than half the thickness of the anisotropic crystal substrate, it is difficult to secure mechanical strength that can withstand practical use by the configuration. .

WO2014 / 010696A1

  From the above background, in a substrate on which a functional element such as an optical waveguide element is formed, when processing such as reducing the thickness of the substrate, it is possible to effectively prevent the occurrence of cracks in the substrate. The realization of the processing method is required.

One aspect of the present invention is a method of processing a substrate having a functional element formed on one surface, and bonding any surface of the substrate to a mechanical structure through a reinforcing member. The material constituting the reinforcing member is stressed at the interface between the substrate and the reinforcing member due to the temperature fluctuation within the range of the temperature fluctuation of the substrate which may occur in any step for performing the processing. The strain on the surface of the substrate, which is generated by the occurrence of the stress, is selected to be a compressive strain or a tensile strain equal to or less than a predetermined strain amount.
According to another aspect of the present invention, the substrate is made of a material having an anisotropy of linear expansion coefficient, and the material constituting the reinforcing member has an isotropic linear expansion coefficient. is there.
According to another aspect of the present invention, the substrate comprises lithium niobate, and the predetermined amount of strain is 110 ppm.
According to another aspect of the present invention, the type of strain generated on the surface of the substrate is specified by the difference between the linear expansion coefficient of the substrate and the linear expansion coefficient of the material constituting the reinforcing member.
According to another aspect of the present invention, the processing is processing for reducing the thickness of the substrate.
Another aspect of the present invention is a functional element formed on a substrate, comprising a reinforcing member bonded to one surface of the substrate, wherein the material of the reinforcing member is within the operating temperature range of the functional element. In the temperature fluctuation, the strain on the surface of the substrate, which is generated when a stress is generated at the interface between the substrate and the reinforcing member due to the temperature fluctuation, becomes a compressive strain or a tensile strain smaller than a predetermined strain amount. To be chosen.
Another aspect of the present invention is an optical waveguide device constituted by an optical waveguide formed on the surface of a lithium niobate substrate, comprising: a reinforcing member bonded to one surface of the substrate; The material is a strain on the surface of the substrate, which is generated when a stress is generated at the interface between the substrate and the reinforcing member due to the temperature variation in the temperature variation within the operating temperature range of the optical waveguide device. The tensile strain is selected so as to be equal to or less than a predetermined strain amount.

It is a flowchart which shows the outline of the manufacturing process of an optical waveguide element based on one Embodiment of this invention. It is a figure which shows the state of LN board | substrate stuck on the grinding jig in the manufacturing process shown in FIG. FIG. 7 schematically shows distortion that may occur in the LN substrate when the signs of the linear expansion coefficient difference between the LN substrate and the reinforcing member are different in the directions parallel and perpendicular to the crystal axis of the LN crystal. It is a figure for demonstrating the procedure of the conventional thinning process of LN board | substrate. It is the figure which showed typically a mode that a crack generate | occur | produces by the tensile strain produced in LN board | substrate.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present embodiment relates to a method of manufacturing an optical waveguide device manufactured by thinning a substrate to a thickness of about 10 μm. FIG. 1 is a manufacturing process of an optical waveguide device according to an embodiment of the present invention. It is a flow figure showing an outline.

  First, an optical waveguide is formed on the surface of the LN substrate (S100). The LN substrate is, for example, an X-cut LN substrate, and the formation of the optical waveguide on the substrate surface can be performed by any known method such as a Ti diffusion method. Further, in this step, an electrode for controlling a light wave propagating in the optical waveguide may be further formed.

  Next, the LN substrate is attached to the polishing jig via the reinforcing member (S102). FIG. 2 is a view showing the state of the LN substrate attached to the polishing jig, FIG. 2 (a) is a top view thereof, and FIG. 2 (b) is a side view thereof. The front surface of the LN substrate 200 (the surface on which the optical waveguide is formed (the surface on the lower side of the LN substrate 200 in FIG. 2B)) is, for example, a polishing jig made of metal via the reinforcing member 204 It is attached to 206.

  This attachment can be performed by using a resin or the like that melts at a high temperature. For example, after the resin is sandwiched between the LN substrate 200 and the reinforcing member 204, the entire LN substrate 200, the reinforcing member 204, and the polishing jig 206 are heated to a temperature of 100 to 200 ° C. to melt the resin. Thereafter, the LN substrate 200 and the reinforcing member 204 can be bonded to each other by cooling to room temperature and curing the resin. The reinforcing member 204 and the polishing jig 206 can be bonded to each other using the above-described resin or an appropriate bonding resin that can withstand the above-described temperature.

  Subsequently, the back surface of the LN substrate 200 is polished and thinned until the LN substrate 200 has a desired thickness (for example, about 10 μm) (S 104), and then the LN substrate 200 is peeled off from the reinforcing member 204 ( S106) The LN substrate 200 subjected to the thinning process is diced (S108), and the manufacturing process of the optical waveguide device is completed.

  FIG. 1 shows an outline of the manufacturing process of the optical waveguide device, and the application of other processes performed in relation to manufacturing, such as inspection in each process, lamination of other members to the LN substrate, etc. It is not something to exclude.

  In the manufacturing process shown in FIG. 1, the material selection of the reinforcing member 204 used in step S102 is important in order to prevent the occurrence of a crack in the LN substrate 200 in the polishing process (thinning process) in step S104. The magnitude of the stress generated in LN substrate 200 and the type and magnitude of the strain generated on the surface of LN substrate 200 due to the stress depend on the linear expansion coefficient of reinforcing member 204.

  The inventor of the present invention configures the reinforcing member 204 using various materials, and reveals the causal relationship between the physical characteristics of the material of the reinforcing member 204 and the occurrence of a crack in the LN substrate 200, (1) LN The substrate is unlikely to crack with respect to the stress in the compression direction generated with the reinforcing member due to thermal expansion / contraction, and tends to cause cracking with respect to the stress in the extension direction (tensile direction), (2) When stress is applied to the LN substrate in the tensile direction, the generation of cracks can be effectively prevented by limiting the maximum stress to a value such that the amount of strain of the LN substrate resulting from the stress is 110 ppm or less. The findings of the

  FIG. 5 is a view schematically showing how a crack is generated due to a tensile strain generated in the LN substrate. As shown, cracks are less likely to occur with respect to compressive strain (eg, generated in a perpendicular direction) with respect to the crystal axis of LN substrate 500 (eg, generated in a parallel direction). ) Cracks are likely to be generated for tensile strain (arrows 504 and 506), and various shapes as shown by reference numerals 512a to d, for example, due to the growth of a slight crack caused due to the tensile strain Cracks can run in various directions.

  The amount of strain is defined by the ratio of the amount of change in the length caused by the occurrence of the stress to the length measured on the LN substrate along the direction of the occurrence of stress.

  The present invention is based on the above findings, and in selecting the material of the reinforcing member 204, as in the prior art, using the selection criterion that “the linear expansion coefficient of the material is closer to the linear expansion coefficient of the LN substrate” is used. Instead, the above distortion amount of the LN substrate 200 generated with respect to the temperature fluctuation during processing is used as a reference.

  That is, the reinforcing member 204 is configured by "a material which causes a compressive strain or a tensile strain of 110 ppm or less, a strain generated in one direction in the substrate surface of the LN substrate against a temperature fluctuation generated in a processing step". I assume. The type of strain of the LN substrate 200 generated with respect to temperature change can be estimated by the difference in linear expansion coefficient (or coefficient of linear expansion) between the LN substrate 200 and the material of the reinforcing member 204.

The following table is a table showing coefficients of linear expansion of various materials a to d that can be selected for the reinforcing member 204, which are commercially available at the time of filing of the present application. For reference, the following table also shows the linear expansion coefficient of LN crystal.

  From Table 1, when the material a or c is used as the reinforcing member 204, only tensile strain occurs in the LN substrate 200, and when the material b or d is used as the reinforcing member 204, the LN substrate 200 is used. It can be seen that compressive strain occurs in the direction parallel to the crystal axis and tensile strain occurs in the direction perpendicular to the crystal axis. FIG. 3 is a view schematically showing a strain generated in the LN substrate 200 when the material b or d is used as the reinforcing member 204. As shown in FIG. As described above, in the LN substrate 200, compressive strain (arrows 300 and 302) occurs in the direction parallel to the crystal axis, and tensile strain (arrows 304 and 306) occurs in the direction perpendicular to the crystal axis.

  According to experiments conducted by the inventor of the present application, the LN substrate 200, the reinforcing member 204, and the polishing jig 206 are heated to a temperature of 100 to 200 ° C. to melt the resin for bonding, and then cooled to room temperature. When the resin is cured, cracks occur in the LN substrate 200 when the materials a to c are used as the reinforcing member 204, and no cracks occur in the LN substrate 200 when the material d is used as the reinforcing member 204. The occurrence of cracks was effectively prevented. The value obtained by multiplying the linear expansion coefficient difference and the temperature change width shown in Table 1 is different from the above-described allowable 110 ppm of tensile strain. This is because fixation of the LN substrate 200 to the reinforcing member 204 in the process of cooling the heated substrate to normal temperature and generation of stress in the LN substrate 200 due to the fixation start at a temperature in the middle of the cooling process. It is thought that it depends on the matter.

  As described above, in the method of manufacturing an optical waveguide device according to the present embodiment, when the LN substrate is polished, the LN substrate is a polishing jig that is another mechanical structure via the reinforcing member. The material of the reinforcing member is selected such that the strain of the LN substrate generated due to temperature fluctuation in the manufacturing process is compressive strain or tensile strain of 110 ppm or less. Thereby, in the manufacturing method according to the present embodiment, the occurrence of cracks in the LN substrate can be effectively prevented.

  In the embodiment described above, the optical waveguide device using the LN substrate is described. However, the present invention is not limited to this, and the substrate may be another substrate when any processing is performed on any substrate constituting any functional device. When bonding to a mechanical structure (for example, a jig suitable for the processing), the substrate is bonded to the mechanical structure via a reinforcing member corresponding to the reinforcing member 204 in the process. Cracks on the surface of the substrate due to temperature fluctuations can be effectively prevented. In this case, in the material of the reinforcing member, the strain generated in the substrate due to the stress generated at the interface between the substrate and the reinforcing member due to the temperature fluctuation during the process is a compressive strain or a tensile force of a predetermined strain or less. It is selected to be distortion.

  Further, in the embodiment described above, in order to process thin the thickness of the LN substrate on which the optical waveguide is formed, the substrate surface on which the optical waveguide is not formed is attached to the polishing jig through the reinforcing member. However, the present invention is not limited to this, in the case of a substrate on which a functional element such as an optical waveguide is not formed, or in the case of processing that can be performed without damaging the functional element by providing a protective film on the functional element. Any arbitrary surface of the substrate (for example, any surface suitable for the processing) may be bonded to another mechanical structure via a reinforcing member.

  Further, in the present embodiment, the reinforcing member 204 is used when thinning the LN substrate 200. However, the present invention is not limited to this. A reinforcing member is attached to one surface of the optical waveguide device formed on the LN substrate. In addition, the mechanical strength of the optical waveguide device is to be improved, and the material of the reinforcing member is used as the interface between the substrate and the reinforcing member due to the temperature fluctuation in the temperature fluctuation within the operating temperature range of the optical waveguide device. Strain on the surface of the substrate generated by the generation of stress may be selected to be compressive strain or tensile strain less than a predetermined strain (for example, 110 ppm mentioned above) .

  Similarly, not only the optical waveguide element but also a reinforcing member is bonded to one surface of the substrate of the functional device formed on the substrate to improve the mechanical strength of the functional device, and the material of the reinforcing member is In the temperature fluctuation within the operating temperature range of the functional element, the strain on the surface of the substrate generated by the stress generated at the boundary surface between the substrate and the reinforcing member due to the temperature fluctuation is a compressive strain or a predetermined predetermined strain amount It may be selected to have the following tensile strain.

  200, 400, 500 ... LN substrate, 204 ... reinforcement member, 206, 406 ... polishing jig, 404 ... resin.

Claims (3)

A substrate processing method for thinning a substrate having anisotropy with respect to a linear expansion coefficient,
Bonding the substrate to a reinforcing member having isotropy with respect to a linear expansion coefficient;
Processing the substrate bonded to the reinforcing member to reduce the thickness of the substrate;
Peeling the substrate from the reinforcing member;
Including
In the bonding step, the heat fusible material interposed between the reinforcing member and the substrate is heated and melted to a temperature of 100 ° C. or more and 200 ° C. or less, cooled and solidified by cooling. Bonding the member and the substrate,
Substrate processing method. The reinforcing member is made of a SUS-based material,
The substrate processing method according to claim 1. The substrate comprises lithium niobate
The substrate processing method according to claim 1.

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