Silicon wafer polishing method

Abstract

To provide a polishing method of a silicon wafer, capable of reducing a minute defect and a DIC defect of a front surface and an end surface in the silicon wafer after a polishing while sufficiently securing a stock allowance of a silicon wafer front surface.SOLUTION: A polishing method of a silicon wafer includes at least three or more polishing steps of: an initial polishing step; a middle polishing step; and a finishing polishing step. The initial polishing step executes: a first polishing step (S1-1) of performing a polishing of the silicon wafer by dropping a polishing agent of an alkali base containing at least a water-soluble polymer and a loose abrasive; a second polishing step (S1-2) of performing a rough polishing of the silicon wafer by dropping the polishing agent of the alkali base including the loose abrasive to polishing cloth excluding the water-soluble polymer after that; and a third polishing step (S1-3) of performing a polishing of the silicon wafer by dropping the polishing agent of the alkali base containing the water-soluble polymer and the loose abrasive to the polishing cloth after that.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for polishing a silicon wafer. [Background technology]

[0002] Silicon wafer polishing is usually performed in multiple stages using different types of polishing cloth and abrasives (multi-stage polishing). The most common method for polishing silicon wafers is chemical mechanical polishing (CMP), which combines chemical and mechanical polishing. By using an abrasive (slurry) in which minute silica particles are dispersed in an alkaline-based aqueous solution, excellent smoothness and a mirror-like finish can be obtained. In particular, in the polishing process of silicon wafer manufacturing, silicon wafers with a defect-free surface and high smoothness are required, and efforts are being made to optimize the combination of polishing consumable materials such as slurries and polishing cloths, as well as the polishing conditions. In general multi-stage polishing, the first polishing is called primary polishing, the polishing performed after the primary polishing is called secondary polishing, and the polishing performed after the secondary polishing is called finish polishing.

[0003] For example, Patent Document 1 discloses, as an example of the above-mentioned multi-stage polishing, a polishing step in which a silicon wafer is subjected to primary polishing, secondary polishing, and finish polishing in that order. Specifically, the example (paragraph 0053) states that the primary polishing "uses an alkaline-based polishing agent that contains abrasive grains but does not contain a water-soluble polymer." This primary polishing removes the native oxide film on the surface of the silicon wafer and repairs any damage caused in previous processing steps. It also states that the secondary polishing consists of polishing with an alkaline-based polishing agent followed by rinse polishing (paragraph 0032).

[0004] Furthermore, Patent Document 2 discloses, as an example of multistage polishing, a multistage polishing process including a primary polishing step, an intermediate polishing step, and a finish polishing step in this order. In the multistage polishing of Patent Document 2, silicon wafers are polished with polishing slurry A containing water-soluble polymer A and abrasive grains in a primary polishing step, then polished with polishing slurry B containing water-soluble polymer B and abrasive grains in an intermediate polishing step, and further polished with polishing slurry C containing water-soluble polymer C and abrasive grains in a finish polishing step. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-21719 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-185672 Summary of the Invention [Problem to be solved by the invention]

[0006] In the polishing method described in Patent Document 1, in the first stage (primary polishing) of multistage polishing, polishing is performed using an alkali-based polishing agent that contains abrasive grains but does not contain a water-soluble polymer. However, when a silicon wafer surface having foreign matter such as dust adhering thereto is polished with an alkali-based polishing agent containing abrasive grains but not containing a water-soluble polymer, surface roughness (haze unevenness), DIC (Differential Interference Contrast) defects, and the like may occur, and there is also a risk of problems such as deterioration of the bevel surface roughness (roughness of the bevel surface).

[0007] Furthermore, in the primary polishing step of the polishing method described in Patent Document 2, the silicon wafer is polished with a polishing slurry containing a water-soluble polymer and abrasive grains, which may result in insufficient removal of the native oxide film on the surface of the silicon wafer and in insufficient repair of damage caused in the pre-processing step, and may also result in insufficient removal of the silicon wafer.

[0008] In order to solve the above problems, the present inventors have investigated a polishing method for the first stage (primary polishing) of multistage polishing. The first stage (primary polishing) of multi-stage polishing is carried out by performing at least the first to third polishing steps, and the first polishing step uses an alkaline-based polishing agent containing a water-soluble polymer and free abrasive grains, the second polishing step uses an alkaline-based polishing agent containing free abrasive grains but no water-soluble polymer, and the third polishing step uses an alkaline-based polishing agent containing a water-soluble polymer and free abrasive grains.The inventors have found that by doing so, it is possible to suppress surface roughness (haze unevenness) and DIC (Differential Interference Contrast) defects, and to suppress deterioration of bevel surface roughness (roughness of the bevel surface). Furthermore, it was discovered that in the second polishing step, which is sandwiched between the first and third polishing steps using an alkali-based abrasive containing a water-soluble polymer and free abrasive grains, an alkali-based abrasive containing free abrasive grains but no water-soluble polymer is used, which makes it possible to remove the native oxide film on the surface of the silicon wafer and repair damage that occurred in the previous processing step, while also ensuring the removal stock of the silicon wafer. Based on these findings, the present inventors have arrived at the present invention.

[0009] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for polishing silicon wafers that can reduce microdefects and DIC defects on the surface or edge face of the polished silicon wafer while ensuring a sufficient removal stock on the surface of the silicon wafer. [Means for solving the problem]

[0010] The silicon wafer polishing method according to the present invention, which has been made to achieve the above-mentioned object, is a method for polishing silicon wafers by pressing a silicon wafer held by a polishing head against a polishing cloth attached to a polishing platen, dropping an alkali-based abrasive onto the polishing cloth, and sliding the silicon wafer across the polishing cloth, and the method comprises at least three polishing steps including an initial polishing step, an intermediate polishing step, and a finish polishing step, and the initial polishing step comprises at least a first polishing step in which an alkali-based abrasive containing a water-soluble polymer and free abrasive grains is dropped onto the polishing cloth to polish the silicon wafer, followed by a second polishing step in which an alkali-based abrasive containing free abrasive grains but no water-soluble polymer is dropped onto the polishing cloth to roughly polish the silicon wafer, and then a third polishing step in which the alkali-based abrasive containing a water-soluble polymer and free abrasive grains is dropped onto the polishing cloth to polish the silicon wafer, and then the intermediate polishing step and the finish polishing step are carried out using the alkali-based abrasive containing a water-soluble polymer and free abrasive grains.

[0011] According to the present invention, by performing the first polishing step using an abrasive containing a water-soluble polymer, the surface and edge face of the silicon wafer are sufficiently protected by the water-soluble polymer before polishing, and therefore dust adhering to the surface of the silicon wafer can be effectively removed. By protecting the surface and edge face of the silicon wafer with the water-soluble polymer, it is possible to suppress an increase in the roughness of the surface and edge face of the silicon wafer in the second polishing step that is subsequently performed using an abrasive that contains abrasive grains but does not contain the water-soluble polymer.

[0012] Furthermore, in the second polishing step, which is sandwiched between the first and third polishing steps that use an alkali-based abrasive containing a water-soluble polymer and free abrasive grains, an alkali-based abrasive containing free abrasive grains but no water-soluble polymer is used, which makes it possible to remove the native oxide film on the surface of the silicon wafer and repair damage that has occurred in the previous processing step, while also ensuring a sufficient amount of silicon wafer stock removal. By carrying out such an initial polishing step, it is possible to reduce micro-defects and DIC defects on the surface and end face of the polished silicon wafer. In the intermediate polishing step and the finish polishing step, an alkali-based polishing agent containing a water-soluble polymer and free abrasive grains, as disclosed in Patent Document 2, can be used.

[0013] Here, it is desirable that the polishing time in the initial polishing step is shorter than the polishing times in the intermediate polishing step and the finish polishing step, and that the polishing times in the first to third polishing steps are in the following order: "polishing time in the second polishing step > polishing time in the third polishing step > polishing time in the first polishing step." By controlling the polishing time of the first stage polishing in this way, it is possible to suppress a decrease in the removal amount of the silicon wafer due to rough polishing without reducing productivity.

[0014] The polishing time of the first polishing step is preferably set to a time required for the abrasive to spread over the entire silicon wafer, and the polishing time of the second polishing step is preferably set to a time required to ensure a removal allowance for the silicon wafer.

[0015] The polishing time in the first polishing step is set to a short time, which is the time it takes for the abrasive to spread over the entire silicon wafer, and therefore, sufficient time can be ensured for rough polishing in the second polishing step. Furthermore, since the polishing time of the second polishing step is set to a time that allows a sufficient removal amount of the silicon wafer to be secured, it is possible to suppress a decrease in the removal amount of the silicon wafer, and it is possible to effectively improve the surface roughness after intermediate polishing and finish polishing.

[0016] It is also preferable that the polishing time in the initial polishing step be in the range of 90% or more and less than 100% of the polishing time in the intermediate polishing step and the finish polishing step. In this way, by making the polishing time of the initial polishing shorter than the polishing time of the intermediate polishing and the finish polishing, it is possible to suppress a decrease in productivity.

[0017] In order to adequately protect the surface and edge of the silicon wafer, it is desirable that the average molecular weight of the water-soluble polymer contained in the abrasive used in the first polishing step be 5,000 or more and 50,000 or less. The surface and edge of the silicon wafer can be protected by the water-soluble polymer by using a water-soluble polymer with an average molecular weight of 5000 or more. On the other hand, water-soluble polymers with an average molecular weight of more than 50,000 are not preferred because they interfere with the subsequent intermediate polishing and finish polishing. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a method for polishing a silicon wafer that can reduce microdefects and DIC defects on the surface or end face of the polished silicon wafer while ensuring a sufficient removal stock on the surface of the silicon wafer. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a flowchart showing an example of a method for polishing a silicon wafer according to the present invention. [Figure 2] FIG. 2 is a diagram showing an image of the method for polishing a silicon wafer according to the present invention. [Figure 3] FIG. 3 is a diagram showing the results of comparison between the number of defects and LPD due to polishing in Example 1 and the number of defects and LPD due to polishing in the comparative example. [Figure 4] FIG. 4 is a schematic diagram of a single-side polishing machine for polishing silicon wafers. [Figure 5] FIG. 5 is a schematic diagram showing the general configuration of a multistage polishing apparatus equipped with a plurality of single-side polishing apparatuses. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of a method for polishing silicon wafers according to the present invention will be described with reference to Figs. 1 and 2. Fig. 1 is a flowchart showing the method for polishing silicon wafers according to this embodiment, and Fig. 2 is a diagram showing an image of the method for polishing silicon wafers according to this embodiment. The present invention is not limited to this embodiment. In addition, in the specification and drawings of this application, elements that can be similarly described may be assigned the same reference numerals, and duplicate explanations may be omitted.

[0021] First, the polishing apparatus used in the method for polishing silicon wafers according to the present invention can be a commonly used polishing apparatus, and for example, either a single-sided polishing apparatus or a multi-stage polishing apparatus equipped with a plurality of single-sided polishing apparatuses can be used. The configuration of the apparatus used in this embodiment will be briefly described below.

[0022] <Device configuration> FIG. 4 is a schematic diagram of a single-side polishing apparatus for polishing silicon wafers, and FIG. 5 is a schematic diagram showing the general configuration of a multistage polishing apparatus equipped with a plurality of single-side polishing apparatuses. In FIG. 4, a single-side polishing apparatus 100 includes a polishing head 1 having a retainer ring 2 and a membrane 3 as a silicon wafer holder, a polishing platen 4, and a polishing cloth 5 attached to the polishing platen 4. The polishing head 1 holds one silicon wafer W by means of an annular retainer ring 2 provided so as to surround the silicon wafer W, and a membrane 3 that abuts against the upper surface of the silicon wafer W and applies a pressing force. In the single-side polishing apparatus 100, the silicon wafer W held by the polishing head 1 is pressed against a polishing cloth 5 attached to a polishing platen 4, a predetermined alkali-based abrasive is dropped onto the polishing cloth 5, and the polishing platen 4 and the polishing head 1 are rotated relative to each other to slide the silicon wafer W, thereby polishing the silicon wafer W. Then, this single-sided polishing device is used to carry out the primary polishing step (initial polishing), and then another single-sided polishing device is used to carry out the secondary polishing step (intermediate polishing) and the tertiary polishing step (finish polishing).

[0023] 5 includes three polishing plates 4 (4a, 4b, 4c), polishing cloths 5a, 5b, 5c individually attached to the polishing plates 4a, 4b, 4c, a polishing head 1 that holds a silicon wafer W, and a loader 6 that moves the polishing head 1. The multistage polishing apparatus 200 performs the final polishing process of a silicon wafer W in multiple stages, for example, in a clockwise direction, including a primary polishing process (initial polishing) using the polishing cloth 5a and an abrasive, a secondary polishing process (intermediate polishing) using the polishing cloth 5b and an abrasive, and a tertiary polishing process (finish polishing) using the polishing cloth 5c and an abrasive. In this case, the multistage polishing apparatus 200 uses polishing cloths and polishing agents prepared according to the purpose of each polishing step, and the loader 6 moves the polishing head 1 for each polishing step to perform multistage polishing.

[0024] The method for polishing silicon wafers of this embodiment is carried out using the polishing apparatus described above, and as shown in FIG. 1, at least polishing steps including an initial polishing step (primary polishing step) S1, an intermediate polishing step (secondary polishing step) S3, and a finish polishing step (tertiary polishing step) S5 are carried out.

[0025] In particular, as shown in FIG. 1, the method for polishing silicon wafers according to the present invention is characterized in that, in an initial polishing process (primary polishing process) S1, it comprises at least a first polishing step S1-1 in which an alkali-based abrasive containing a water-soluble polymer and free abrasive grains is dropped onto the polishing cloth to polish the silicon wafer, a second polishing step S1-2 in which an alkali-based abrasive containing free abrasive grains but no water-soluble polymer is dropped onto the same polishing cloth to roughly polish the silicon wafer, and a third polishing step S1-3 in which an alkali-based abrasive containing a water-soluble polymer and free abrasive grains is dropped onto the same polishing cloth again to polish the silicon wafer. The intermediate polishing step (secondary polishing step) S3 and the finish polishing step (tertiary polishing step) S5 are polishing steps that use a conventional alkali-based polishing agent containing a water-soluble polymer and free abrasive grains.

[0026] (Initial polishing process (primary polishing process) S1) As described above, the initial polishing (step S1 in FIG. 3) is composed of the first polishing step S1-1, the second polishing step (rough polishing) S1-2, and the third polishing step S1-3. In the first polishing step S1-1 and the third polishing step S1-3, the silicon wafer is polished using an alkali-based abrasive containing a water-soluble polymer and free abrasive grains, and in the second polishing (rough polishing) step S1-2, the silicon wafer is polished using an alkali-based abrasive containing free abrasive grains but not a water-soluble polymer.

[0027] The reason why an alkali-based polishing agent is used here is that if the pH of the polishing agent is lower than 9, colloidal silica tends to aggregate, while if the pH is higher than 12, colloidal silica tends to dissolve in the solution. Therefore, an alkali-based polishing agent with a pH of 9 to 12 is preferred. This pH adjustment is achieved by adding a KOH solution.

[0028] Furthermore, as the water-soluble polymer contained in the abrasive used in the first stage polishing, hydroxyethyl cellulose (HEC) is used in order to efficiently protect the silicon wafer and improve the surface roughness. Specifically, for example, hydroxyethyl cellulose having an average molecular weight of 5,000 or more and 50,000 or less is used. If the average molecular weight of the hydroxyethyl cellulose is less than 5,000, it is not preferable because it cannot sufficiently protect the surface and edge face of the silicon wafer W. Also, if the average molecular weight of the hydroxyethyl cellulose exceeds 50,000, it is not preferable because it will hinder subsequent polishing.

[0029] The water-soluble polymer used in the first stage polishing is not limited to the above, but may also be a cellulose derivative such as hydroxypropyl cellulose or methyl cellulose, or a polymer containing an N-vinyl type monomer unit such as an N-vinyl chain amide.

[0030] The free abrasive grains contained in the polishing agent used in the first stage polishing are colloidal silica, specifically, colloidal silica having an average grain size of 30 nm to 150 nm. The content of free abrasive grains contained in the polishing agent used in the first polishing stage and the particle size of the abrasive grains are preferably changed depending on the purpose of the first to third polishing steps. The free abrasive grains contained in the polishing agent are not limited to these, and may be fumed silica, alumina particles, chromium oxide particles, etc.

[0031] The polishing cloth used in the first stage polishing is not particularly limited, but can be appropriately determined (hardness, softness, etc.) from the viewpoints of protecting the silicon wafer and the removal amount of the silicon wafer W. As the polishing cloth, general nonwoven fabric type, polyurethane type, suede type, etc. can be used without any particular limitation.

[0032] Furthermore, the load (polishing load) applied to the silicon wafer during the first stage polishing can be determined on a case-by-case basis depending on the desired quality. The load applied to the silicon wafer W is mainly due to the pressure applied by the polishing head, and the pressure at this time is preferably in the range of 3 kPa to 20 kPa. With such a pressure, sufficient removal can be obtained, good flatness quality can be achieved, and the introduction of defects can be minimized.

[0033] (First polishing step S1-1) In the first polishing step S1-1, the silicon wafer W held by the polishing head is pressed against the polishing cloth, an alkali-based polishing agent containing a water-soluble polymer and free abrasive grains is dropped onto the polishing cloth, and the polishing platen and the polishing head are rotated relative to each other to slide the silicon wafer W.

[0034] In the first polishing step S1-1, polishing is performed for a polishing time (also referred to as a first time), for example, within a time range of 1 second to 15 seconds. The purpose of the first polishing step S1-1 is to spread (permeate) the hydroxyethyl cellulose over the entire silicon wafer W rather than to polish the surface of the silicon wafer W, and it is desirable to set the time for this step to a relatively short time in order to ensure sufficient time for rough polishing, for example, about 1 to 15% of the polishing time allocated for the first-stage polishing.

[0035] By performing the first polishing step S1-1, the surface and edge of the silicon wafer can be adequately protected using an abrasive containing a water-soluble polymer immediately after the start of the first-stage polishing, and dust adhering to the silicon wafer can be effectively removed, thereby suppressing haze and DIC defects caused by dust.

[0036] It is desirable that the abrasive used in the first polishing step S1-1 contains fewer free abrasive grains than those used in other polishing processes, from the viewpoint of protecting the surface and edge face of the silicon wafer.

[0037] (Second polishing step S1-2) In the second polishing step (rough polishing) S1-2, the silicon wafer W held by the polishing head is pressed against the same polishing cloth as in the first polishing step S1-1, an alkali-based polishing agent containing free abrasive grains but not containing a water-soluble polymer is dropped onto the polishing cloth, and the polishing platen and the polishing head are rotated relative to each other to slide the silicon wafer W. Specifically, for example, an alkali-based abrasive containing colloidal silica with an average particle size of 30 nm or more and 150 nm or less and not containing hydroxyethyl cellulose is dropped onto a polishing cloth attached to a polishing platen to perform the second polishing step (rough polishing).

[0038] If the average particle size of the colloidal silica is less than 30 nm, it is not preferable because there is a possibility that the removal amount on the surface of the silicon wafer W may not be sufficiently ensured. In addition, the average particle size of the colloidal silica is preferably larger than that of the abrasive grains used in the first polishing step, so as to efficiently perform rough polishing, or the content of the abrasive grains is preferably larger than that of the abrasive used in the first polishing step.

[0039] In this second polishing step (rough polishing) S1-2, rough polishing is performed for a second time using an abrasive that does not contain hydroxyethyl cellulose in order to efficiently polish the surface of the silicon wafer W, ensure a sufficient removal amount of the silicon wafer W, and ensure flatness quality. The polishing time of the second polishing step (also referred to as the second time) is preferably longer than the first time described above and the polishing time of the third polishing step (also referred to as the third time) described below, and is, for example, approximately 50 to 90% of the polishing time allocated to the initial polishing.

[0040] (Third polishing step S1-3) In the third polishing step S1-3, the silicon wafer held by the polishing head is pressed against the same polishing cloth as in the second polishing step S1-2, an alkali-based polishing agent containing a water-soluble polymer and free abrasive grains is dropped onto the polishing cloth, and the polishing platen and the polishing head are rotated relative to each other to slide the silicon wafer.

[0041] Specifically, polishing is performed by dropping an alkali-based abrasive containing hydroxyethyl cellulose with an average molecular weight of 5,000 to 50,000 and colloidal silica with an average particle size of 30 nm to 150 nm onto a polishing cloth attached to a polishing platen. In the third polishing step S1-3, in order to reduce damage to the silicon wafer after the second polishing (rough polishing) step S1-2 using an abrasive that does not contain hydroxyethyl cellulose, polishing is again carried out for a third time using an abrasive that contains hydroxyethyl cellulose. Therefore, from the viewpoint of reducing damage to the silicon wafer, it is preferable that the abrasive used in the third polishing step contains fewer free abrasive grains than the abrasive used in the second polishing step, or that abrasive grains with a smaller grain size be used.

[0042] The third time period is intended to reduce damage to the silicon wafer W, and is desirably longer than the first time period, which is intended to allow the hydroxyethyl cellulose to spread (permeate) over the entire silicon wafer W. On the other hand, from the viewpoint of ensuring a sufficient polishing rate, it is desirable that the time period be shorter than the second time period. Therefore, the polishing time for each polishing step is preferably "second time>third time>first time", and is preferably about 5 to 35% of the polishing time allocated to the first-stage polishing, for example.

[0043] After that, when the third polishing is completed, the polishing head is replaced with another polishing platen (abrasive cloth) by, for example, a loader of the multistage polishing device, and the process moves to the second polishing process (intermediate polishing process) (steps S2 and S3 in Figure 3).

[0044] In the first stage polishing of this embodiment, it is preferable that the rotation speed of the polishing platen is "rotation speed when performing the first polishing step > rotation speed when performing the 22nd polishing step > rotation speed when performing the third polishing step." When performing the first polishing step, it is desirable to quickly cover the entire silicon wafer with the water-soluble polymer (to spread the abrasive over the entire silicon wafer W as quickly as possible) and to use the fastest rotation speed in the initial polishing in order to protect the surface and edge surface of the silicon wafer. In addition, from the viewpoint of suppressing the occurrence of surface defects, it is preferable to gradually reduce the rotation speed of the polishing platen in the order of the second polishing step and the third polishing step.

[0045] Furthermore, from the viewpoint of productivity, the polishing time for the initial polishing should be shorter than the polishing times for the intermediate polishing and finish polishing described below, and the difference between the polishing times for the intermediate polishing and finish polishing should be as small as possible. For example, it is desirable that the polishing time be in the range of 90% or more but less than 100% of the polishing time for the intermediate polishing and finish polishing, and it is even more desirable that the polishing time be in the range of 95% or more but less than 100% of the polishing time for the intermediate polishing and finish polishing. The polishing time for intermediate polishing and finish polishing can be determined appropriately depending on the desired quality.

[0046] (Secondary polishing process (intermediate polishing)) The secondary polishing step (intermediate polishing) can be a conventional polishing step. Specifically, the silicon wafer W is polished using an abrasive containing a water-soluble polymer and free abrasive grains. In this intermediate polishing, the particle size of the colloidal silica contained in the polishing agent is reduced during the polishing process in order to further reduce the roughness of the silicon wafer surface. That is, as shown in Figure 2, intermediate polishing (1) and intermediate polishing (2) are performed by changing the particle size of colloidal silica. The particle size of colloidal silica in the first half of intermediate polishing is set larger than the particle size of colloidal silica in the second half of intermediate polishing.

[0047] After that, when the secondary polishing process (intermediate polishing) is completed, the polishing head is replaced with another polishing platen (abrasive cloth) by, for example, a loader of the multi-stage polishing device, and the process moves to the tertiary polishing process (finish polishing process) (steps S4 and S5 in Figure 1).

[0048] (Tertiary polishing process (finish polishing)) In the final polishing, a conventional polishing process can be used. Specifically, the silicon wafer W is polished using an abrasive containing a water-soluble polymer and free abrasive grains. The abrasive used in the final polishing is the same as that used in the latter half of the intermediate polishing (the abrasive contains small particle sizes of colloidal silica). In addition, it is desirable to use a soft abrasive cloth, such as a suede type, in the final polishing.

[0049] As described above, in the first polishing step of the characteristic initial-stage polishing according to the present invention, an abrasive containing a water-soluble polymer is used for a time period ranging from 1 second to 15 seconds, for example, to sufficiently protect the surface and edge face of the silicon wafer with the water-soluble polymer and polish it, thereby making it possible to effectively remove dust adhering to the surface of the silicon wafer. This makes it possible to suppress surface roughness and DIC defects caused by dust on the surface and edge of the silicon wafer, as well as deterioration of bevel surface roughness, during subsequent rough polishing using an abrasive that does not contain water-soluble polymers.

[0050] Furthermore, in the second polishing step, which is sandwiched between the first and third polishing steps using an alkali-based abrasive containing a water-soluble polymer and free abrasive grains, polishing is performed using an alkali-based abrasive containing free abrasive grains but not a water-soluble polymer. This makes it possible to remove the native oxide film on the surface of the silicon wafer and repair damage that has occurred in the previous processing step, while also ensuring a sufficient amount of silicon wafer stock removal. Furthermore, even if intermediate polishing and finish polishing are performed using conventional general techniques after the first-stage polishing of this embodiment, the surface roughness after intermediate polishing and finish polishing can be effectively improved. [Example]

[0051] An example of the method for polishing a silicon wafer according to the present invention will now be described.

[0052] Example 1 In Example 1, a polishing process consisting of a primary polishing process (first-stage polishing), a secondary polishing process, and a tertiary polishing process shown in FIG. 2 was carried out on a silicon wafer having a diameter of 300 mm.

[0053] In Example 1, the initial polishing (first polishing, second polishing, third polishing) was carried out for 100 seconds while reducing the rotation speed of the polishing platen 4a. Various conditions for each polishing step are as shown in Table 1. (First polishing) Polishing was performed using an alkali-based polishing agent containing hydroxyethyl cellulose with an average molecular weight of 30,000 and colloidal silica with an average particle size of 35 nm under the conditions shown in Table 1. After polishing, a second polishing (coarse polishing) was performed on the same polishing cloth 5a under the conditions shown in Table 1.

[0054] (Second polishing) Polishing was performed using an alkali-based polishing agent containing colloidal silica with an average particle size of 40 nm under the conditions shown in Table 1. After polishing, a third polishing was performed on the same polishing cloth 5a under the conditions shown in Table 1.

[0055] (Third Polishing) Polishing was carried out under the conditions shown in Table 1 using an alkali-based polishing agent containing hydroxyethyl cellulose with an average molecular weight of 30,000 and colloidal silica with an average particle size of 35 nm.

[0056] (Secondary polishing process, tertiary polishing process) After the primary polishing, the polishing head was moved onto the polishing platen 4b (polishing cloth 5b) in 10 seconds, and the secondary polishing step was carried out. The polishing conditions for the secondary polishing step are as shown in Table 1. After the secondary polishing step was completed, the tertiary polishing step was carried out. The polishing conditions for the tertiary polishing step are as shown in Table 1.

[0057] (Comparative Example) In the comparative example, a silicon wafer having a diameter of 300 mm was polished in the primary polishing step without using a water-soluble polymer, by the same rough polishing as in the second polishing of Example 1, under the conditions shown in Table 1. As shown in Table 1, the secondary polishing step and the tertiary polishing step were the same as those in Example 1.

[0058] [Table 1]

[0059] Then, for the silicon wafers after the tertiary polishing process of Example 1 and the comparative example, the number of LPDs of 26 nm or more on the silicon wafer surface and the number of edge defects were measured using a laser scattering particle counter (SurfScan SP-3 manufactured by KLA-Tencor Corporation). The results are shown in Figure 3. FIG. 3 is a diagram showing the results of a comparison between the number of defects caused by polishing in Example 1 and the number of defects caused by polishing in the Comparative Example, and shows the percentage of defects in Example 1 when the number of LPDs of 26 nm or more and the number of edge defects on the silicon wafer surface in the Comparative Example are set to 100%. A comparison of Example 1 and the Comparative Example confirmed that application of the polishing method of the present invention improved both the number of LPDs of 26 nm or larger and the number of edge defects on the silicon wafer surface. More specifically, it was confirmed that the number of LPDs of 26 nm or larger was reduced by approximately 30% in the polishing of Example 1. Similarly, it was confirmed that the number of edge defects was reduced by approximately 70%. It was also confirmed that the polishing of Example 1 ensured sufficient removal of the silicon wafer. More specifically, it was confirmed that the polishing rate of the primary polishing step in Example 1 was reduced by only approximately 5% compared to the primary polishing step in the Comparative Example. [Explanation of symbols]

[0060] 1 polishing head 2 retainer ring 3. Membrane 4 (4a, 4b, 4c) Polishing plate 5(5a, 5b, 5c) Polishing cloth 100 Single side polishing equipment 200 Multi-stage polishing equipment S1 Initial polishing process (primary polishing process) S1 S1-1 First polishing step S1-2 Second polishing step S1-3 Third polishing step

Claims

1. A method for polishing a silicon wafer, comprising: pressing a silicon wafer held by a polishing head against a polishing cloth attached to a polishing platen; dropping an alkali-based abrasive onto the polishing cloth; and sliding the silicon wafer across the polishing cloth, the method comprising: The method has at least three polishing steps including a first polishing step, an intermediate polishing step, and a finish polishing step, In the first polishing step, at least a first polishing step in which an alkali-based polishing agent containing a water-soluble polymer and free abrasive grains is dropped onto the polishing cloth to polish the silicon wafer; Thereafter, a second polishing step is performed by dropping an alkali-based polishing agent containing free abrasive grains and no water-soluble polymer onto a polishing cloth to roughly polish the silicon wafer; a third polishing step in which an alkali-based polishing agent containing a water-soluble polymer and free abrasive grains is dropped onto a polishing cloth to polish the silicon wafer; and Thereafter, the intermediate polishing step and the finish polishing step are carried out using an alkali-based polishing agent containing a water-soluble polymer and free abrasive grains.

1. A method for polishing a silicon wafer, comprising:

2. a polishing time in the initial polishing step is shorter than a polishing time in the intermediate polishing step and a polishing time in the finish polishing step; and the polishing times of the first to third polishing steps are set in the following order: "polishing time of the second polishing step>polishing time of the third polishing step>polishing time of the first polishing step"; 2. The method for polishing a silicon wafer according to claim 1, wherein the polishing step comprises the steps of:

3. The polishing time of the first polishing step is set to the time until the abrasive spreads over the entire silicon wafer.

3. The method for polishing a silicon wafer according to claim 2, wherein the polishing step comprises the steps of:

4. The polishing time of the second polishing step is set to a time that allows a removal allowance of the silicon wafer to be secured.

3. The method for polishing a silicon wafer according to claim 2, wherein the polishing step comprises the steps of:

5. The polishing time in the initial polishing step is set to be in the range of 90% or more and less than 100% of the polishing time in the intermediate polishing step and the finish polishing step.

3. The method for polishing a silicon wafer according to claim 2, wherein the polishing step comprises the steps of:

6. the average molecular weight of the water-soluble polymer contained in the polishing agent used in the first polishing step is set to 5,000 or more and 50,000 or less; 2. The method for polishing a silicon wafer according to claim 1, wherein the polishing step comprises the steps of:

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