Surface processing method

Abstract

A surface processing method for planarizing a surface of an object to be processed (W), i.e., a surface to be processed (W1), includes a step, a treatment, or a step in which a current flows through the object to be processed as an anode in a state in which a grindstone layer (32) and the surface to be processed are disposed facing each other with an alkaline electrolyte (S) interposed therebetween, and the surface to be processed (W1) is planarized by a planarizing step in which the surface to be processed (W1) is planarized by a planarizing step in which the surface to be processed (W1) is planarized. Thus, an oxide is generated on the surface to be processed due to anodic oxidation. The oxide generated on the surface to be processed due to anodic oxidation is removed by the grindstone layer.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application is based on Japanese Patent Application No. 2022-93027 filed on June 8, 2022, and the contents thereof are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to a surface processing method for flattening the surface of a workpiece, that is, a processed surface. Background Art

[0004] Patent document 1 provides a polishing method using anodization, which can efficiently process difficult-to-process materials such as single-crystal SiC into a target shape with a high-quality surface without scratches and damage. Specifically, the polishing method disclosed in Patent document 1 includes an anodizing step and a polishing step. The anodizing step is a step in which a voltage is applied to the workpiece as an anode in the presence of an electrolyte to form an oxide film on the surface of the workpiece by causing a current of a specified current density to flow through the workpiece. The polishing step is a step in which an abrasive material having a Mohs hardness intermediate between the workpiece and the oxide film is used to selectively polish and remove the oxide film. Moreover, the polishing method disclosed in Patent document 1 performs the anodizing step and the polishing step simultaneously to flatten the surface of the workpiece. It should be noted that the polishing method disclosed in Patent document 1 is included in the technology called ECMP. ECMP is the abbreviation of Electro-Chemical Mechanical Polishing.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2017-92497 Summary of the invention

[0008] The inventors conducted in-depth research and found that in the previous ECMP, if the processing speed is increased, many scratches will be generated on the processed surface, thus there is a problem of increased surface roughness. In addition, after investigating the main reasons, the inventors found the following points of interest. For example, the condensed sludge adheres to the workpiece and the grindstone and bites into the processing interface, thereby generating scratches. Or, for example, since the hardness of the oxide film generated by anodization is high, a higher pressure is required during processing, thereby introducing strain to the processed surface and generating scratches.

[0009] The present disclosure has been made in view of the above-mentioned circumstances, etc. That is, the present disclosure provides a technique capable of achieving both high processing speed and good surface roughness in a surface processing method (eg, grinding method, rough polishing method, or polishing method) using anodizing, for example.

[0010] According to one aspect of the present disclosure, a surface processing method is a method for flattening the surface of a workpiece, that is, a processed surface, and includes the following steps, treatments, or processes:

[0011] The workpiece is used as an anode and current is passed through the abrasive stone layer and the workpiece surface in a state where the abrasive stone layer and the workpiece surface are arranged opposite to each other with an alkaline electrolyte interposed therebetween, thereby generating oxides on the workpiece surface by anodic oxidation.

[0012] The oxide generated on the processed surface due to anodic oxidation is removed by the grindstone layer.

[0013] It should be noted that in each column in the application document, each element is sometimes marked with a reference symbol in brackets. In this case, the reference symbol is only an example of the correspondence between the element and the specific structure described in the embodiment described later. Therefore, the present disclosure is not limited by the description of the reference symbol. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic structural diagram of a surface processing device for implementing a surface processing method according to an embodiment of the present disclosure.

[0015] Figure 2 It is a plan view showing the surface roughness measurement position of the processed surface after grinding in the example and the comparative example.

[0016] Figure 3 It is a figure which shows the surface roughness measurement result of the processed surface after grinding|polishing in Example and comparative example.

[0017] Figure 4 It is a graph showing the measurement results of the hardness of the oxide film produced on the processed surface in the examples and comparative examples.

[0018] Figure 5 This is a graph showing the ECMP polishing rate of SiC wafers as workpieces in Examples and Comparative Examples.

[0019] Figure 6 Graph showing the ECMP polishing rate of SiO 2 optical glass as a workpiece in Examples and Comparative Examples.

[0020] Figure 7 This is a graph showing changes in polishing rate when the pH of the electrolyte solution is changed.

[0021] Figure 8 This is a graph showing changes in the ability to remove silicon dioxide particles as foreign matter when the pH of the electrolyte solution is changed.

[0022] Fig. 9 This is a graph showing changes in the ability to remove ceria particles as foreign matter when the pH of the electrolyte solution is changed. DETAILED DESCRIPTION

[0023] (Implementation Method)

[0024] The following describes the embodiments of the present disclosure based on the accompanying drawings. It should be noted that if various modified examples that can be used for one embodiment are inserted in the middle of a series of descriptions related to the embodiment, it may hinder the understanding of the embodiment. Therefore, the modified examples are not inserted in the middle of a series of descriptions related to the embodiment, but are collectively described in the following text.

[0025] (Surface processing equipment)

[0026] Reference Figure 1 , the surface processing device 1 is configured in a manner that can flatten a processed surface W1 that is a main surface of the workpiece W. "Flattening" is, for example, grinding or polishing, specifically, for example, rough grinding, fine grinding, rough polishing, or fine polishing. The "main surface" is a surface in a layered or plate-like object that is orthogonal to the thickness direction, and is also referred to as a "plate surface". Specifically, in the present embodiment, the surface processing device 1 is configured in a manner that can perform a flattening process that refers to anodizing on the processed surface W1 of the workpiece W. That is, the surface processing device 1 has an ECMP device or is configured as an ECMG device. ECMG is the abbreviation of Electro-Chemical Mechanical Grinding. The workpiece W is, for example, a silicon-based material, typically a semiconductor material such as a silicon carbide semiconductor.

[0027] The surface processing device 1 includes a container 2, a surface processing pad 3, a driving device 4, and a power supply device 5. As described later, Figure 1 The following description using this figure is only simplified for the purpose of briefly explaining the surface processing method of this embodiment, and is not necessarily consistent with the specific device structure actually manufactured and sold. In addition, in order to simplify the description, a right-handed XYZ coordinate system is set as shown in the figure. In this embodiment, the positive direction of the Z axis represents vertically upward, that is, the direction opposite to the direction of gravity. In addition, the mutually orthogonal X-axis and Y-axis directions both represent horizontal directions. In the following, for the sake of convenience of description, the positive direction side of the Z axis corresponding to vertically upward is sometimes referred to as "upward", and the opposite side is referred to as "downward".

[0028] The container 2 is formed in the shape of a bathtub with an opening at the top. The container 2 is constructed in such a manner that the workpiece W is held at the bottom and immersed in an electrolyte S that does not contain an etching agent component. The etching agent component is a component that constitutes a dissolving solution that has the ability to dissolve the oxide film (i.e., the oxide generated in a film shape) generated on the workpiece W1 due to anodization, for example, hydrofluoric acid. The electrolyte S that does not contain an etching agent component used in this embodiment is an alkaline electrolyte with a pH of 9 to 13. As an alkaline compound for making the pH of the electrolyte S 9 to 13, for example, potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, etc. can be used. The surface processing pad 3 and the driving device 4 are arranged above the container 2. The container 2, the surface processing pad 3 and the driving device 4 are arranged so that they can be relatively moved in the up and down directions by a lifting mechanism not shown in the figure. In addition, the container 2, the surface processing pad 3 and the driving device 4 are arranged so that they can be relatively moved in the in-plane direction (i.e., horizontal direction) along the workpiece W1 by a sliding mechanism not shown in the figure.

[0029] The surface processing pad 3 has an electrode 31 and a grindstone layer 32. The electrode 31 is a plate-shaped component made of a good conductor such as a metal, for example, formed of a copper plate, etc. The grindstone layer 32 is joined to the electrode 31. That is, the surface processing pad 3 has a structure in which the electrode 31 and the grindstone layer 32 are joined in the thickness direction of the surface processing pad 3. The grindstone layer 32 is formed by a grindstone (for example, a cerium dioxide grindstone, etc.) as a grinding material or a grinding material. The surface processing pad 3 is arranged so that the grindstone layer 32 and the processing surface W1 of the workpiece W are arranged relative to each other with the electrolyte S sandwiched therebetween. Moreover, the surface processing pad 3 is arranged so that: by being rotationally driven by the driving device 4 in a state in which the grindstone layer 32 is arranged relative to the processing surface W1 with the electrolyte S sandwiched therebetween, the oxide film generated on the processing surface W1 due to anodization can be selectively polished or ground away.

[0030] The driving device 4 is configured to drive the surface processing pad 3 to rotate around a rotation axis in the vertical direction. The power supply device 5 is provided in such a manner that a voltage is applied in the presence of an electrolyte S with the workpiece W as an anode and the electrode 31 in the surface processing pad 3 as a cathode, thereby flowing a current, i.e., an anodic oxidation current, for anodic oxidation of the workpiece W, i.e., the workpiece W1, which is the workpiece to be processed of the grinding stone layer 32.

[0031] (Overview of Surface Processing Method of Embodiment)

[0032] The surface processing method (i.e., grinding method or polishing method) of the workpiece W of the present embodiment is a method for flattening a processing surface W1 as the surface of the workpiece W, using Figure 1 Specifically, the surface processing method of this embodiment includes the following steps, treatments or processes:

[0033] (1) When current is passed through the workpiece W as an anode in a state where the grinding stone layer 32 and the workpiece surface W1 are arranged to face each other with the alkaline electrolyte S interposed therebetween, an oxide film is formed on the workpiece surface W1 by anodic oxidation.

[0034] (2) The oxide film formed on the processed surface W1 by anodic oxidation is removed by the grindstone layer 32 .

[0035] According to the surface processing method of the workpiece W of the present embodiment, the hardness of the oxide film generated in the form of a film on the workpiece W1 due to anodic oxidation is lower than that of the non-oxidized area. Therefore, the oxide film is selectively removed from the workpiece W1 by wiping the workpiece W1 with the grindstone layer 32. As a result, the workpiece W1 is well flattened with a desired surface roughness. It should be noted that the generation of the oxide film based on anodic oxidation and the removal of the oxide film based on the grindstone layer 32 can be performed simultaneously or with a time difference.

[0036] The surface processing method of this embodiment can be well applied to the surface flattening processing of semiconductor materials such as SiC wafers that require nanometer-level surface roughness. Here, the current status of this surface flattening processing is that it limits the lead time in semiconductor wafer manufacturing. Therefore, in this surface flattening processing, increasing the processing speed as much as possible (that is, the processing quality cannot be reduced) will lead to improved manufacturing efficiency in semiconductor wafer manufacturing. In this aspect, the inventors conducted in-depth research and found the following issues: In the previous ECMP and ECMG, if the processing speed is to be increased, many scratches will be generated on the processed surface W1, thereby increasing the surface roughness. In addition, after examining the main reasons, the inventors found the following focus.

[0037] (Point 1) The coagulated sludge adheres to the workpiece W and the grindstone layer 32 and bites into the machining interface, thereby generating scratches.

[0038] (Point 2) Since the oxide film produced by anodization has high hardness, a high pressure is required during processing, thereby introducing strain into the processed surface W1 and causing scratches.

[0039] Therefore, the inventors have proposed a method for obtaining good surface roughness of the processed surface W1 while increasing the processing speed based on the following mechanism in order to solve the above-mentioned problems from the above-mentioned viewpoints.

[0040] (Mechanism 1) The potential between the work surface W1 and the sludge is controlled to suppress the aggregation of the sludge and to suppress the adhesion of the sludge to the work surface W1.

[0041] (Mechanism 2) By reducing the hardness of the oxide film generated by anodization, the pressure during processing is reduced, thereby suppressing the generation of strain and scratches on the processed surface W1.

[0042] Moreover, in order to realize the above-mentioned mechanism, the inventor has found a method of using an alkaline aqueous solution as the electrolyte S in anodizing. By using an alkaline electrolyte in anodizing, the zeta potential of the workpiece W and the sludge can be unified to be negative. Thus, the aggregation of the sludge and the abrasive grains released from the grinding stone layer 32 can be suppressed, and the adhesion of the sludge to the workpiece surface W1 can be suppressed. In addition, the hardness of the oxide film generated during anodizing can be reduced, thereby making it easy to remove the oxide film based on the grinding stone layer 32. In addition, by utilizing the chemical reaction between the oxide film and the alkaline aqueous solution, the oxide film can be removed at low pressure. In addition, the wear rate of the grinding stone layer 32 is reduced. Therefore, according to this embodiment, it is possible to have both high processing speed and good surface roughness.

[0043] (Example 1: Surface roughness)

[0044] First, in ECMP using SiC wafers as workpieces W, the finish of the workpiece W1 was evaluated when pH 7 salt water and pH 12 alkaline electrolyzed water (specifically, potassium hydroxide aqueous solution) were used as electrolytes S. Specifically, the workpiece W1 was subjected to a DC current of 10 mA / cm 2 The current density was 0.500 for 30 minutes, and the scanning white light interferometer New View8300 manufactured by ZYGO was used to measure the Figure 2 The surface roughness Ra of the total of 9 points P1 to P9 on the processed surface W1 shown in FIG. Figure 3 . Figure 3 The bar graph shows the average value of the 9 points, and the vertical line segment shows the variance σ of the statistical value corresponding to the crop deviation. Figure 3 As shown, it was confirmed that the average value and deviation of the surface roughness were improved by using the alkaline electrolyte. In addition, the appearance was observed, and as a result, when the comparative example was salt water, a large number of scratches were generated on almost the entire surface of the processed surface W1, but when the embodiment was alkaline water, almost no scratches were generated.

[0045] (Example 2: Oxide film hardness)

[0046] Figure 4 The results of measuring the hardness of the oxide film produced on the processed surface W1 are shown. The electrolyte S and current conditions are the same as above. The hardness of the oxide film was measured using a TI980 TriboIndenter manufactured by Bruker Company through a micro Vickers hardness test in accordance with JIS Z2244. The indentation depth was 200 nm. Figure 3 Similarly, the vertical line segment represents the variance σ as a statistical value corresponding to the deviation of the measured values ​​of multiple points. Figure 4 As shown, the hardness of the oxide film is reduced by using an alkaline electrolyte.

[0047] (Example 3: Processing speed)

[0048] Figure 5 Evaluation results of the polishing rate in ECMP using a SiC wafer as the workpiece W are shown. Figure 6 The evaluation results of the polishing rate in ECMP using SiO2 optical glass as the workpiece W are shown. Figure 5 As shown in FIG. 1 , the polishing speed of SiC wafers can be increased by about 2 times by using an alkaline electrolyte. Figure 6 As shown, the polishing speed of SiO2 optical glass is increased by about 3 times by using alkaline electrolyte. According to these results, it can be understood that the polishing speed is increased by the chemical reaction of oxide film and alkaline electrolyte.

[0049] Figure 7 The results of evaluating the pH dependence of the polishing rate when the pH of the electrolyte S is changed in ECMP using SiC wafers as the workpiece W are shown. pH 12 and pH 12.5 are assumed to be the state of a "fresh" alkaline electrolyte with no usage history. On the other hand, pH values ​​lower than these are assumed to be the time-dependent changes in pH due to the elapsed processing time. Figure 7 As shown, the polishing speed is almost constant between pH 9 and 12.5, and the polishing speed is doubled compared with the case of pH about 7, i.e., salt water. Thus, it is confirmed that the reduction in processing speed and processing quality caused by the change in the properties of the alkaline electrolyte over time can be well suppressed.

[0050] (Example 4: Removal capability of silicon dioxide particles)

[0051] Figure 8 The results of evaluating the removal rate of silicon dioxide particles from the processed surface W1 in ECMP using a SiC wafer as the processed object W are shown. Fig. 9 The results of evaluating the removal rate of ceria particles from the processed surface W1 in ECMP using a SiC wafer as the processed object W are shown. Fig. 9 In the figure, the circle indicates a particle size of 0.2 μm, the triangle icon indicates a particle size of 0.3 μm, and the × icon indicates a particle size of 0.5 μm. The removal rate was evaluated by measuring the wafer surface before and after processing using a surface foreign matter inspection device manufactured by YGK. Figure 8 and Fig. 9As shown, it is confirmed that the use of alkaline electrolyte can exert a good foreign matter removal ability from the processed surface W1. That is, in the case of salt water with a pH of 7, the zeta potential of the processed surface W1 is positive, while the zeta potential of foreign particles such as silicon dioxide and cerium dioxide is negative, thereby generating an electrostatic attraction between the two. This helps foreign particles to adhere to the processed surface W1. In contrast, by using an alkaline electrolyte, the zeta potential of both the processed surface W1 and the foreign particles is negative, generating an electrostatic repulsion between the two. This effectively suppresses foreign particles from adhering to the processed surface W1.

[0052] (Summary of Examples)

[0053] Thus, according to this example, it was confirmed that high processing speed and good processing quality can be achieved by anodic oxidation using an alkaline electrolyte. Specifically, the effects of improving processing speed, processing efficiency and processing quality were observed in the pH range of 9 to 13 of the alkaline electrolyte.

[0054] (Variation Example)

[0055] The present disclosure is not limited to the above-mentioned embodiments. Therefore, the above-mentioned embodiments may be appropriately modified. Representative modified examples are described below. In the description of the following modified examples, the differences from the above-mentioned embodiments are mainly described. In addition, in the above-mentioned embodiments and modified examples, the same or equal parts are marked with the same symbols. Therefore, in the description of the following modified examples, regarding the constituent elements with the same symbols as the above-mentioned embodiments, as long as there is no technical contradiction or special additional description, the description in the above-mentioned embodiment can be appropriately cited.

[0056] The present disclosure is not limited to the specific device configuration shown in the above-mentioned embodiments. Figure 1 and use Figure 1 The above description is simplified only for the purpose of explaining the general structure and function of the surface processing device 1 of the present embodiment, and is not necessarily consistent with the specific device structure actually manufactured and sold. Specifically, for example, in the above embodiment, there is a structure in which the workpiece W is held on the container 2 side, and the surface processing pad 3 is rotatably held above the workpiece W on the other hand. However, the present disclosure is not limited to the above manner. That is, it may also be a structure in which, for example, the workpiece W is held on the chuck side that is arranged above the container 2 and rotationally driven by the drive device 4, and the surface processing pad 3 is held on the container 2 side on the other hand.

[0057] The electrolyte S may contain an etchant component. That is, the surface processing method of the present disclosure can grind or grind the processing surface W1 by selectively removing the oxide film generated by anodic oxidation using both the etchant and the surface processing pad 3 .

[0058] The type and size of the workpiece W are not particularly limited. That is, the present disclosure can be well applied to any workpiece W that can be processed by anodic oxidation for surface flattening. It should be noted that "using anodic oxidation" can also be expressed as "using anodic oxidation". In addition, the diameter of the workpiece W when it is a semiconductor ingot or wafer is not particularly limited. In addition to the typical 6-inch size, the present disclosure can also be well applied to sizes of 1 to 8 inches.

[0059] The structure of the surface processing pad 3, such as the type and material of the grinding stone layer 32, is not particularly limited. That is, within the scope of the effect of the present disclosure, the details of the structure of the surface processing pad 3 (such as the type and material of the grinding stone layer 32) can be appropriately selected from the available details.

[0060] The elements constituting the above-mentioned embodiments are not necessarily required, except for the cases where they are explicitly stated as being particularly necessary or where they are clearly required in principle. In addition, when referring to the numerical values ​​such as the number, amount, and range of the constituent elements, the present disclosure is not limited to the specific numerical values, except for the cases where they are explicitly stated as being particularly necessary or where they are clearly limited to specific numerical values ​​in principle. Similarly, when referring to the shapes, directions, and positional relationships of the constituent elements, the present disclosure is not limited to the shapes, directions, and positional relationships, except for the cases where they are explicitly stated as being particularly necessary or where they are limited to specific shapes, directions, and positional relationships in principle.

[0061] The modified examples are not limited to the above examples. That is, for example, in addition to the above examples, as long as multiple embodiments are not technically contradictory, they can be combined with each other. Similarly, as long as multiple modified examples are not technically contradictory, they can be combined with each other.

[0062] (Public opinion)

[0063] As is apparent from the above description of the embodiment and the modified examples, this specification discloses at least the following viewpoints.

[0064] [Viewpoint 1]

[0065] A surface processing method for flattening a surface of a workpiece (W), i.e., a processed surface (W1), comprises the following treatments, procedures or steps:

[0066] The grinding stone layer (32) is arranged opposite to the processed surface with an alkaline electrolyte (S) sandwiched therebetween, and current is passed through the processed object as an anode, thereby generating oxides on the processed surface by anodic oxidation.

[0067] The oxide generated on the processed surface due to anodic oxidation is removed by the grindstone layer.

[0068] [Viewpoint 2]

[0069] In aspect 1, the pH of the alkaline electrolyte is 9-13.

[0070] [Viewpoint 3]

[0071] In aspects 1 and 2, the workpiece is a silicon-based material.

[0072] [Viewpoint 4]

[0073] In viewpoint 3, the workpiece is a silicon carbide semiconductor.

[0074] [Viewpoint 5]

[0075] In aspects 1 to 4, by controlling the potential between the processed surface and the sludge by anodic oxidation using the alkaline electrolyte, aggregation of the sludge and abrasive grains released from the grindstone layer is suppressed, and adhesion of the sludge to the processed surface is suppressed.

[0076] [Viewpoint 6]

[0077] In aspects 1 to 5, the hardness of the oxide is reduced by anodic oxidation using the alkaline electrolyte, thereby facilitating removal of the oxide by the grindstone layer.

[0078] [Viewpoint 7]

[0079] In aspects 1 to 6, the surface processing speed is increased by anodization using the alkaline electrolyte.

[0080] [Viewpoint 8]

[0081] In aspects 1 to 7, the wear rate of the grinding stone layer is reduced by anodic oxidation using the alkaline electrolyte.

[0082] [Viewpoint 9]

[0083] In aspects 1 to 8, the removal of the oxide by the grindstone layer is grinding or polishing (ie, for example, rough grinding, fine grinding, rough polishing, or fine polishing).

Claims

1. A surface processing method for flattening a surface of a workpiece (W), i.e., a processed surface (W1), The grinding stone layer (32) and the processed surface are arranged opposite to each other with an alkaline electrolyte (S) interposed therebetween, and the processed object is used as an anode to pass a current, thereby generating oxides on the processed surface by anodic oxidation. The oxide generated on the processed surface by anodic oxidation is removed by the grindstone layer.

2. The surface processing method according to claim 1, wherein: The pH of the alkaline electrolyte is 9-13.

3. The surface processing method according to claim 1 or 2, wherein: The workpiece is a silicon-based material.

4. The surface processing method according to claim 3, wherein: The workpiece is a silicon carbide semiconductor.

5. The surface processing method according to claim 1, wherein: By controlling the potential of the processed surface and the sludge by anodic oxidation using the alkaline electrolyte, aggregation of the sludge and abrasive grains released from the grindstone layer is suppressed, and adhesion of the sludge to the processed surface is suppressed.

6. The surface processing method according to claim 1, wherein: By reducing the hardness of the oxide through anodic oxidation using the alkaline electrolyte, the oxide can be easily removed by the grindstone layer.

7. The surface processing method according to claim 1, wherein: The surface processing speed is increased by anodizing using the alkaline electrolyte.

8. The surface processing method according to claim 1, wherein: The wear rate of the grinding stone layer is reduced by anodizing using the alkaline electrolyte.

9. The surface processing method according to claim 1, wherein: The removal of the oxide by the grindstone layer is grinding or lapping.

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