METHOD FOR PRINTING A LASER MARKER AND METHOD FOR PRODUCING LASER-MARKED SILICON WAFERS

Abstract

Method for producing a laser-labeled silicon wafer, comprising: a laser marking printing step of printing a laser mark having a multitude of points on a silicon wafer obtained by cutting a single crystal silicon ingot grown using a predetermined process and subjecting it to beveling, an etching step of performing an etching at least on a laser-marked area of ​​the silicon wafer, and a polishing step of performing a polishing on both surfaces of the silicon wafer that has undergone the etching step, wherein each of the plurality of points is formed using laser light having a wavelength in an ultraviolet range, wherein each of the plurality of points is formed by irradiating with laser light a plurality of times, wherein a number of pulses in the plurality of irradiations with laser light is 15000 or more per second, wherein an amount removed by etching in the etching step is 1.5 µm or more and 12.5 µm or less per side, and where processing deformation is removed around all points.

Description

TECHNICAL AREA

[0001] This disclosure relates to a method for printing laser markings and a method for producing a laser-marked silicon wafer. BACKGROUND

[0002] Silicon wafers are conventionally used as substrates for semiconductor devices. These silicon wafers are manufactured as follows: First, a single-crystal silicon ingot, grown using, for example, the Czochralski process (CZ process), is cut into blocks. The perimeter of each block is ground, and the block is then sliced ​​into wafers.

[0003] Next, each silicon wafer obtained by slicing undergoes beveling and, if necessary, one or more processes, such as lapping, surface grinding, and double-disc grinding. The perimeter of the front or back surface of the silicon wafer that has undergone the aforementioned processes may be printed with an identification code (identification mark) by laser irradiation for wafer management or identification purposes. The laser-printed mark (hereinafter referred to as the "laser mark") is composed of characters and symbols, each depicted with a set of multiple indentations (dots), and is sized to be identifiable visually or by means of a camera or similar device.Traditionally, the laser light used has a wavelength in the infrared range (see, for example, JP H11-000772 (PTL 1)).

[0004] Irradiation with the aforementioned laser light forms a ring-shaped raised area around each point. Accordingly, at least one laser-marked area of ​​the laser-marked silicon wafer (hereinafter also referred to as the "laser-marked area") is etched to remove the raised area, followed by polishing of the silicon wafer surface (see, for example, JP 2011-029355 A (PTL 2)). The polished silicon wafer undergoes final cleaning. Furthermore, the silicon wafers are subjected to a variety of inspections, and those that meet specified quality standards are shipped as products. US 2008 / 0011852A1 also discloses a method for printing a laser mark. LIST OF COUNTERPOINTS Patent Literature PTL 1: JP H11- 000 772 A PTL 2: JP 2011-029355A PTL 3: US 2008 / 0 011 852 A1 SUMMARY (Technical Problem)

[0005] As semiconductor devices have become increasingly miniaturized and integrated in recent years, it is necessary for the silicon wafers to be very flat. Furthermore, the device formation areas are expanding radially outwards from the wafers year after year, necessitating that the wafer edge areas also exhibit high flatness.

[0006] As described above, the raised area formed around each point is removed by etching. However, an investigation carried out by the inventor of this disclosure has shown that a machining deformation around the point, caused by the thermal effect of the laser irradiation, remains without being completely removed, even though the raised area is removed as described above. If a silicon wafer with such a residual machining deformation is subjected to polishing, the wafer surface would not be polished uniformly, and the flatness of the wafer edge area would be reduced.

[0007] In view of the above problem, it might be helpful to provide a laser marking printing process and a process for producing a laser-marked silicon wafer that can reduce the machining deformation left around points forming a laser mark. (Solution to the problem)

[0008] The following features are proposed for solving the above problem. [1] Method for printing a laser mark having a plurality of points on a silicon wafer, comprising forming each of the plurality of points using laser light having a wavelength in an ultraviolet range. [2] Method for printing a laser marking according to [1] above, wherein each of the plurality of points is formed by irradiating with laser light a plurality of times. [3] Method for printing a laser marking according to [2] above, wherein the number of pulses in the plurality of irradiations with laser light is 15000 or more per second. [4] Method for producing a laser-labeled silicon wafer, comprising: a laser marking printing step of printing a laser mark using the method for printing a laser mark, according to one of the preceding [1] to [3], on a silicon wafer obtained by cutting a single crystal silicon ingot grown using a specified method and subjecting it to beveling; an etching step of performing an etching at least on a laser-marked area of ​​the silicon wafer, and a polishing step of performing a polishing on both surfaces of the silicon wafer that has undergone the etching step. [5] Method for producing a laser-labeled silicon wafer according to [4] above, wherein the amount removed by etching in the etching step is 1.5 µm or more per side. [6] Method for producing a laser-labeled silicon wafer according to [4] or [5] above, wherein an etchant used in the etching step is one of an acidic etchant and an alkaline etchant. (Beneficial effect)

[0009] This revelation can reduce the processing deformation that remains around points forming a laser mark. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a flowchart of a process for producing a laser-labeled silicon wafer according to this disclosure; Fig. 2 is a diagram showing the relationship between the wavelength of laser light and the amount removed by etching and the processing deformation remaining around points; and Fig. Figure 3 is a diagram showing the relationship between the number of pulses per second in a laser irradiation and the processing deformation left around points. DETAILED DESCRIPTION (Laser marking printing process)

[0010] Embodiments of this disclosure will now be described with reference to the drawings. The laser marking printing method according to this disclosure is a method for printing a laser marking having a plurality of dots onto a silicon wafer. The plurality of dots is formed using laser light having a wavelength in the ultraviolet range.

[0011] As described above, an investigation carried out by the inventor has shown that although the raised area formed by laser irradiation is removed by etching, the processing deformation around points remains without being completely removed, and the flatness of the wafer edge area is reduced by subsequent polishing.

[0012] The inventor has extensively studied ways to reduce the aforementioned machining deformation. As will be described in the examples below, a substantial increase in the amount removed by etching (for example, 50 µm) can completely eliminate machining deformation around points. However, the amount removed by etching in the current wafer manufacturing process is approximately a few micrometers, making the above measure undesirable from a manufacturing cost perspective.

[0013] Therefore, the inventor thoroughly investigated the possibilities of reducing the processing deformation around points without significantly increasing the amount removed by etching, as described above. As a result, the inventor arrived at the idea of ​​forming each of the multitude of points that constitute a laser marking using laser light with a wavelength in the ultraviolet range.

[0014] In particular, conventional methods, such as that described in PTL 1, typically use laser light with an infrared wavelength to form points that constitute a laser marking. The inventor investigated the relationship between machining deformation around points and the wavelength of laser light. As a result, as described in examples below, it was found that machining deformation is reduced by reducing the wavelength of the laser light. In particular, it was found that machining deformation around points is substantially reduced when using laser light with an ultraviolet wavelength, and this finding led to this disclosure. The following description discusses each feature of this disclosure.

[0015] A silicon wafer to be printed with a laser marking may use one obtained by performing a wafer edge grinding, cutting and chamfering known per se and optionally one or more processes, such as lapping, surface grinding and double-disc grinding, on a single-crystal silicon ingot grown using the CZ process or the Floating Zone process (FZ process).

[0016] During the growth of a single-crystal silicon ingot, the oxygen concentration, carbon concentration, nitrogen concentration, and the like can be appropriately adjusted so that the silicon wafer cut from the grown silicon ingot exhibits the desired characteristics. Furthermore, suitable dopants can be added to obtain a wafer with n-type or p-type conductivity.

[0017] The diameter of the silicon wafer can be, for example, 200 mm, or it can be 300 mm or more (for example, 300 mm or 450 mm). Furthermore, the thickness of the silicon wafer can be a suitable thickness, depending on the specifications (for example, 700 µm to 1 mm).

[0018] As described above, it is important that each of the multiple points forming a laser mark in this disclosure is formed using laser light having a wavelength in the ultraviolet range. This can reduce machining deformation around points by minimizing the thermal effect resulting from laser irradiation. It should be noted that here, "a wavelength in the ultraviolet range" means a wavelength of 355 nm or less.

[0019] A laser light source used can be, for example, an argon laser, an excimer laser, or a YAG laser. Of these, a YAG laser, which is typically used as a light source for laser marking, is preferred.

[0020] The depth of each spot formed by a single laser irradiation does not depend on the laser's output power and is, for example, approximately 2 µm to 10 µm, although it depends on the device used. If a spot with a desired depth cannot be formed using a single laser irradiation, it can be formed by multiple irradiations.

[0021] In this case, the number of pulses in the majority of laser irradiations is preferably 10,000 or more per second, more preferably 15,000 or more per second. Setting the number of pulses in the majority of laser irradiations to 10,000 or more per second reduces the energy per pulse irradiation, thus further reducing the machining deformation around the point. Furthermore, setting the number of pulses in the laser irradiation to 15,000 or more per second almost completely eliminates the machining deformation around the point.

[0022] The silicon wafer printed with a laser marking, as described above, then undergoes an etching step and a polishing step. In the etching step, the raised areas formed around the laser marking are removed. Since the residual processing deformation around the points is reduced compared to the case where a conventional method is used, the silicon wafer edge area can exhibit improved flatness after the polishing step. (Method for producing a laser-labeled silicon wafer)

[0023] A method for producing a laser-labeled silicon wafer according to this disclosure is now described. Fig. Figure 1 presents a flowchart of a process for producing a laser-marked silicon wafer according to this disclosure. As shown in this diagram, this disclosure comprises a laser-marking printing step of printing a laser mark using the laser-marking printing method described above, according to this disclosure, on a silicon wafer obtained by cutting and chamfering a single-crystal silicon ingot grown using a specified process, such as the CZ process or the FZ process; an etching step of performing an etching at least on a laser-marked area of ​​the silicon wafer (Step 2); and a polishing step of performing a polishing on both surfaces of the silicon wafer that has undergone the etching step (Step 3).

[0024] First, the laser marking printing step of step S1 is identical to the step performed by the aforementioned laser marking printing method according to this disclosure, so the description is not repeated. With regard to this step, the residual machining deformation around points can be reduced by forming each of the plurality of points using laser light having a wavelength in the ultraviolet range.

[0025] Following the preceding laser marking printing step, etching is performed on at least one laser-marked area of ​​the silicon wafer in step S2 (etching step). This etching step removes the raised area formed around each dot by the laser irradiation in step S1 and can also eliminate any warping of the silicon wafer caused by lapping.

[0026] The preceding etching step can be carried out, for example, by immersing the silicon wafer printed with the laser marking into an etching medium that fills an etching container and holding it there while the wafer is rotated.

[0027] The etchant used can be acidic or alkaline, with an alkaline etchant being preferred. A nitric acid or hydrofluoric acid etchant can be used as the acidic etchant. However, an aqueous solution of sodium hydroxide or potassium hydroxide is preferred as the alkaline etchant. This etching step removes the raised area around each laser-marked dot and can also eliminate any warping of the silicon wafer caused by lapping.

[0028] The amount removed by the above etching process is preferably 0.5 µm or more per side, more preferably 1.5 µm or more per side. As will be described in examples below, machining deformation can be almost completely removed if the amount removed by the etching process is 1.5 µm per side.

[0029] On the other hand, the upper limit of the amount removed by etching with regard to removing the aforementioned machining deformation around the points and removing the curvature caused by lapping is not specified, but with regard to manufacturing costs it is preferably 15 µm or less.

[0030] Following the preceding etching step, polishing is performed on the surfaces of the silicon wafer that underwent the etching step in step S3 (polishing step). In this polishing step, both surfaces of the etched wafer are polished using a polishing suspension containing abrasive particles.

[0031] In this polishing step, mirror polishing is performed on both surfaces of the silicon wafer by the following steps: placing the silicon wafer in a support; holding the wafer between an upper and a lower plate, each with a polishing cloth attached; flowing a suspension, for example, colloidal silica, into the space between the upper and lower plates and the wafer; and rotating the upper and lower plates relative to the support plate. This reduces irregularities on the wafer surfaces, resulting in a very flat wafer.

[0032] In particular, an alkaline suspension containing colloidal silica as abrasive grains can be used as the polishing suspension.

[0033] Following the preceding polishing step, a final polishing step is performed on one side, in which at least one of the silicon wafer's surfaces is polished sequentially. This final polishing step includes polishing only one surface as well as polishing both surfaces. If both surfaces are polished, one surface is polished first, followed by the other.

[0034] After final polishing, the silicon wafer that has undergone polishing is subsequently cleaned. In particular, particles, organic material, metal, etc., are removed from the wafer surfaces, for example, using an SC-1 cleaning solution, which is a mixture of aqueous ammonia, a hydrogen peroxide solution, and water, or an SC-2 cleaning solution, which is a mixture of hydrochloric acid, a hydrogen peroxide solution, and water.

[0035] Finally, the flatness of the cleaned silicon wafer, the number of LPDs on the wafer surface, damage, contamination of the wafer surface, etc., are examined. Only silicon wafers that meet the specified quality requirements in these examinations are shipped as products.

[0036] Therefore, the processing deformation around the points forming the laser marking can be reduced, thus a laser-marked silicon wafer with a high flatness of the edge area can be produced. EXAMPLES

[0037] Examples of this revelation will now be described; however, this revelation is not limited to these examples. <Relationship between processing deformation around points and wavelength of laser light> (Examples 1 to 3)

[0038] A laser-marked silicon wafer according to this disclosure was produced according to the method described in Fig. The process was prepared according to the flowchart shown in Figure 1. First, a laser mark was printed onto an edge section of a silicon wafer (laser marking printing step). Specifically, a 300 mm diameter single-crystal silicon ingot grown using the CZ process was cut into blocks, and one of the blocks underwent wafer edge grinding and was then sliced. The resulting silicon wafer was chamfered and lapped, and the laser mark, formed by a multitude of dots with a depth of 55 µm, was then printed onto the edge region of the back surface of the silicon wafer. For this laser marking printing, a laser light with a wavelength of 355 nm in the ultraviolet range was used. Furthermore, the dots were formed by irradiation with laser light 18 times, and the number of laser pulses per second was 15,000.

[0039] The silicon wafer, which was printed with the laser marking on the edge region of the back surface as described above, was subjected to an etching step. In particular, an aqueous potassium hydroxide solution was used as an etchant, and the amount removed by etching was set to 1.5 µm (Example 1), 12.5 µm (Example 2), and 50 µm (Example 3) per side.

[0040] The etched silicon wafer then underwent double-sided polishing. Specifically, the etched silicon wafer was first placed in a support and held between an upper and a lower plate, each with a polishing cloth attached. Mirror polishing was then performed on both surfaces of the silicon wafer by pouring an alkaline polishing suspension containing colloidal silica into the space between the upper and lower plates and the wafer, rotating the upper and lower plates and the support plate in opposite directions. The amount of material removed by the double-sided polishing was approximately 5.0 µm per side.

[0041] After performing a final polish on the silicon wafer that had undergone the aforementioned polishing, a cleaning process was subsequently carried out, thereby obtaining a laser-marked silicon wafer according to this disclosure. The laser-marked area of ​​the obtained laser-marked silicon wafer was subjected to machining deformation measurements around the points using an XRT system manufactured by Rigaku Corporation. A obtained XRT image is shown in Fig. 2 shown. (Comparative example 1)

[0042] As in Example 1, a laser-labeled silicon wafer was prepared. However, it should be noted that the laser light used to print the laser label had a wavelength in the infrared range (wavelength 1064 nm). All other conditions were the same as in Example 1. A resulting XRT image is shown in Fig. 2 shown. (Comparative example 2)

[0043] As in Example 2, a laser-labeled silicon wafer was prepared. However, it should be noted that the laser light used to print the laser label had a wavelength in the infrared range (wavelength 1064 nm). All other conditions were the same as in Example 2. A resulting XRT image is shown in Fig. 2 shown. (Comparative example 3)

[0044] As in Example 3, a laser-labeled silicon wafer was prepared. However, it should be noted that the laser light used to print the laser label had a wavelength in the infrared range (wavelength 1064 nm). All other conditions were the same as in Example 3. A resulting XRT image is shown in Fig. 2 shown. (Comparative example 4)

[0045] As in Example 1, a laser-labeled silicon wafer was prepared. However, it should be noted that the laser light used in printing the laser label had a wavelength in the visible range (wavelength 532 nm). All other conditions were the same as in Example 1. A resulting XRT image is shown in Fig. 2 shown. (Comparative example 5)

[0046] As in Example 2, a laser-labeled silicon wafer was prepared. However, it should be noted that the laser light used in printing the laser label had a wavelength in the visible range (wavelength 532 nm). All other conditions were the same as in Example 2. A resulting XRT image is shown in Fig. 2 shown. (Comparative example 6)

[0047] As in Example 3, a laser-labeled silicon wafer was prepared. However, it should be noted that the laser light used in printing the laser label had a wavelength in the visible range (wavelength 532 nm). All other conditions were the same as in Example 3. A resulting XRT image is shown in Fig. 2 shown.

[0048] Fig. Figure 2 shows the relationship between the wavelength of laser light and the amount removed by etching, and the residual processing deformation around the points; the points around which processing deformation remained are shown, while the points around which no processing deformation remained are not shown. The ratio of the number of points around which processing deformation remained, relative to the total number of points, is also given under each image.

[0049] Fig. Figure 2 shows that the processing deformation remained by many points when the amount removed by etching was 1.5 µm and 12.5 µm per side and the wavelength of the laser light was 1064 nm and 532 nm (compare examples 1, 2, 4 and 5). On the other hand, it shows Fig. 2. that the processing deformation around all points was removed when the wavelength of the laser light was 355 nm. Although the processing deformation around all points was removed with all wavelengths of laser light when the etching level was 50 µm per side, it should be noted that the amount to be removed of 50 µm is not desirable in terms of cost, since the amount removed by etching in the current wafer fabrication process is approximately a few micrometers. <Relationship between number of pulses in laser irradiation and residual processing deformation around points> (Example 4)

[0050] As in Example 1, a laser-labeled silicon wafer was prepared. However, it should be noted that the number of pulses in the laser irradiation for forming the multitude of points was set to 10,000. All other conditions were the same as in Example 1. A resulting XRT image is shown in Fig. 3 shown. (Example 5)

[0051] As in Example 2, a laser-labeled silicon wafer was prepared. However, it should be noted that the number of pulses in the laser irradiation for forming the multitude of points was set to 10,000. All other conditions were the same as in Example 2. A resulting XRT image is shown in Fig. 3 shown. (Example 6)

[0052] As in Example 1, a laser-labeled silicon wafer was prepared. However, it should be noted that the number of pulses in the laser irradiation for forming the multitude of points was set to 20,000. All other conditions were the same as in Example 1. A resulting XRT image is shown in Fig. 3 shown. (Example 7)

[0053] As in Example 2, a laser-labeled silicon wafer was prepared. However, it should be noted that the number of pulses in the laser irradiation for forming the multitude of points was set to 20,000. All other conditions were the same as in Example 2. A resulting XRT image is shown in Fig. 3 shown. (Example 8)

[0054] As in Example 1, a laser-labeled silicon wafer was prepared. However, it should be noted that the number of pulses in the laser irradiation for forming the multitude of points was set to 25,000. All other conditions were the same as in Example 1. A resulting XRT image is shown in Fig. 3 shown. (Example 9)

[0055] As in Example 2, a laser-labeled silicon wafer was prepared. However, it should be noted that the number of pulses in the laser irradiation for forming the multitude of points was set to 25,000. All other conditions were the same as in Example 2. A resulting XRT image is shown in Fig. 3 shown.

[0056] Fig. Figure 3 shows the relationship between the number of pulses per second in the laser irradiation and the processing deformation remaining around the points. Fig. 3 also includes the XRT images of Example 1 and Example 2, which are in Fig. 2 are specified. Fig. Figure 3 shows that when the number of pulses per second was 10,000, the processing deformation was removed around many points, but some processing deformation remained around a few points. Conversely, it was found that the processing deformation was removed around all points when the number of pulses per second in the laser irradiation was 15,000, 20,000, and 25,000. INDUSTRIAL APPLICABILITY

[0057] This revelation makes it possible to reduce the machining deformation that remains around points forming a laser marking, and is therefore useful in the semiconductor wafer manufacturing industry.

Claims

[1] Method for producing a laser-labeled silicon wafer, comprising: a laser marking printing step of printing a laser mark having a multitude of points on a silicon wafer obtained by cutting a single crystal silicon ingot grown using a predetermined process and subjecting it to beveling, an etching step of performing an etching at least on a laser-marked area of ​​the silicon wafer, and a polishing step of performing a polishing on both surfaces of the silicon wafer that has undergone the etching step, wherein each of the plurality of points is formed using laser light having a wavelength in an ultraviolet range, wherein each of the plurality of points is formed by irradiating with laser light a plurality of times, wherein a number of pulses in the plurality of irradiations with laser light is 15000 or more per second, wherein an amount removed by etching in the etching step is 1.5 µm or more and 12.5 µm or less per side, and where processing deformation is removed around all points. [2] Method for producing a laser-labeled silicon wafer according to claim 1, wherein an etchant used in the etching step is one of an acidic etchant and an alkaline etchant.

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