METHOD FOR POLISHING A SILICON WAFER

Abstract

Method for polishing a silicon wafer comprising a double-sided polishing step (S2) as a lapping step followed by a final polishing step (S4, S5) which includes the following: a preliminary polishing step (S4) using a preliminary polishing unit including a first plate provided with a first polishing pad on its surface and a first polishing head for polishing a surface of a silicon wafer by rotating the first plate and the silicon wafer held by the first polishing head while the silicon wafer is brought into contact with the first polishing pad and a first polishing medium is supplied to the first polishing pad; and a subsequent final polishing step (S5) using a final polishing unit including a second plate provided with a second polishing pad on its surface and a second polishing head for further polishing the surface of the silicon wafer by rotating the second plate and the silicon wafer held by the second polishing head while the silicon wafer is brought into contact with the second polishing pad and a second polishing agent is supplied to the second polishing pad, wherein in the upstream polishing step (S4) the first polishing agent is a first alkaline aqueous solution containing abrasive grains with a density of 1 x 10 14 / cm 3or more, is provided to perform polishing (S4A) at a polishing rate of 100 nm / min to 300 nm / min, and the feed is then switched to a feed of a second alkaline aqueous solution containing a water-soluble polymer and abrasive grains with a density of 5 x 10 13 / cm 3 or contains less to perform polishing (S4B) at a polishing rate of 5 nm / min to 20 nm / min, the silicon wafer is separated from the first polishing head after the upstream polishing step (S4) and transferred to the final polishing unit, while water is supplied to the surface of the silicon wafer, and the silicon wafer is attached to the second polishing head, and in the final polishing step (S5) a third alkaline aqueous solution containing a water-soluble polymer and abrasive grains with a density of 5 x 10 13 / cm 3or contains less than the second polishing compound is provided to perform polishing at a polishing rate of 5nm / min to 20nm / min.

Description

Technical field

[0001] This disclosure relates to a method for polishing a silicon wafer, which has significance in a final polishing step of mirror polishing on one side of a silicon wafer that has been subjected to double-sided polishing. background

[0002] A process for manufacturing a silicon wafer primarily involves a single-crystal pulling step to form a single-crystal ingot and a processing step for the formed single-crystal ingot. This processing step typically includes a cutting step, a lapping step, a chamfering step, an etching step, a mirror polishing step, a cleaning step, and so on. These steps produce a silicon wafer with a mirror-finished surface. The mirror polishing step is performed in several stages, for example, a double-sided polishing step (lapping step) in which both sides of a silicon wafer are polished simultaneously, followed by a final polishing step to achieve a mirror finish on one side of the silicon wafer.The final polishing is typically performed using a polishing unit, which includes a plate with a polishing pad on its surface and a head for holding a silicon wafer. One side of the silicon wafer, held on the head, is pressed against the polishing pad, and the head and plate are rotated together while a polishing solution (polishing slurry), an alkaline aqueous solution containing abrasive particles, is delivered to the polishing pad. Accordingly, one side of the silicon wafer is polished using mechanochemical polishing (i.e., chemical-mechanical polishing: CMP) to a mirror-like surface with excellent smoothness, combining the mechanical polishing action of the abrasive particles with the chemical polishing action of the alkaline aqueous solution.

[0003] Here, the final polishing step is carried out in two or more phases, including one or more upstream polishing steps performed by one or more upstream polishing units, and a final polishing step subsequently performed by a final polishing unit. WO 2010 / 140671A1 (PTL 1) describes that in a final polishing step, an alkaline aqueous solution containing a water-soluble polymer and abrasive grains with a density of 5 x 10⁻⁶ is used. 13 / cm 3 or less than the amount of polishing solution used, resulting in a silicon wafer with reduced PIDs (process-induced defects).

[0004] Furthermore, JP 2016 – 51 763 A (PTL 2) discloses a method for polishing a silicon wafer, comprising: a first polishing step of polishing a surface of the silicon wafer while a first polishing fluid is supplied to a polishing cloth, comprising: a primary agent in the form of an alkaline aqueous solution; and abrasive grains, wherein the first polishing fluid does not comprise a water-soluble polymer; following the first polishing step, a protective film formation step of supplying a protective film formation solution, comprising a water-soluble polymer, to the polishing cloth after use in the first polishing step and bringing the protective film formation solution into contact with the polished surface of the silicon wafer that underwent the first polishing step in order to form a protective film on the polished surface;and a second polishing step of polishing the surface of the silicon wafer where the protective film is formed by the protective film formation step, while a second polishing fluid is supplied to a polishing cloth different from the polishing cloth used in the first polishing step. This fluid comprises a primary agent in the form of an alkaline aqueous solution; abrasive grains; and a water-soluble polymer. The polishing process is a technique for suppressing the formation of water spots, which are detrimental in the process from the first polishing step to the second polishing step, where a silicon wafer is provided in air, by forming the protective film described above.

[0005] The JP H11 - 140 427 A, the JP 2014 – 4 658 A, the JP H09 - 102 475 A and the WO 2016 / 129 215 A1 reveal various polishing processes, some of which use several polishing steps with the addition of different polishing agents. CITATION LIST Patent literature PTL 1: WO 2010 / 140 671 A1 PTL 2: JP 2016 – 51 763 A SUMMARY (Technical Problem)

[0006] The PIDs evaluated in patent literature 1 are linear bulge defects, as in Fig. Figure 1A from PTL 1 illustrates the formation of these PIDs, and the mechanism is assumed to be as follows. Abrasive grains and other various types of foreign material in the polishing solution during the final polishing process cause linear scratches on the silicon wafer surface with a certain probability. These linear scratches are process-damaged areas, and the etch rate of these damaged areas is lower than that of other parts of the wafer surface in subsequent etching steps, including the cleaning step immediately after final polishing and the final cleaning step after inspection; consequently, linear protrusions are formed. In PTL 1, the wafer surface is measured using a laser particle counter (SP2, manufactured by KLA Tencor). The defects, identified as LPD-N with a size of 35 nm or larger, are identified as PIDs, and the number of PIDs is evaluated.

[0007] According to the procedure for polishing a wafer in PTL 1, in the final polishing step, the formation of PIDs can indeed be suppressed by reducing the number of abrasive grains to 5 x 10 13 / cm 3 or less suppressed. However, according to the studies of the inventors of this disclosure, it was found that the formation of relatively shallow scratches cannot be sufficiently suppressed by the wafer polishing process in PTL 1. The shallow scratches are revealed by subjecting the wafer surface to a measurement using a laser particle counter (SP3, manufactured by KLA Tencor) in normal mode, and the defects are different from the PIDs evaluated in PTL 1.

[0008] Furthermore, the method for polishing a wafer in PTL2 does not take into account the reduction of scratches with relatively shallow depth, as described above; according to the studies of the inventors of this disclosure, it was found that the formation of scratches with relatively shallow depth cannot be sufficiently suppressed by this polishing method either.

[0009] It could therefore be helpful to provide a method for polishing a silicon wafer that can suppress the formation of not only PIDs but also shallow scratches. (Solution to the problem)

[0010] We conducted thorough studies with a view to solving the above problem and made the following discoveries. In the conventional final polishing step, the upstream polishing step is performed using a polishing compound with a relatively high alkaline etch rate and numerous abrasive grains, while the polishing to achieve higher flatness in the final polishing step is performed using a polishing compound with a low alkaline etch rate and fewer abrasive grains. Immediately after the upstream polishing step, the wafer is transferred to a final polishing unit, to which water is supplied.Here, the surface of the wafer was rendered water-repellent immediately after the upstream polishing step due to the aforementioned characteristics of the polishing compound used in that step. The inventors considered that if the wafer, with its water-repellent surface preserved, were transferred and subjected to final polishing, the abrasive grains would aggregate on the wafer surface during the final polishing phase. Accordingly, they provided a solution to prevent this abrasive grain aggregation by switching the polishing compound supplied to the wafer surface from the upstream polishing compound to a final polishing compound used during the upstream polishing step, thereby rendering the wafer surface hydrophilic immediately after the upstream polishing step.It was found that the wafer surface, which underwent the upstream polishing step and the final polishing step, exhibited reduced shallow scratches.

[0011] A method according to claim 1 is provided. The dependent claims define further embodiments. The disclosure is based on the findings described above and has the following primary features. (1) A method for polishing a silicon wafer comprising a double-sided polishing step as a lapping step followed by a final polishing step comprising: a preliminary polishing step using a preliminary polishing unit including a first plate provided with a first polishing pad on its surface and a first polishing head for polishing a surface of a silicon wafer by rotating the first plate and the silicon wafer held by the first polishing head while the silicon wafer is brought into contact with the first polishing pad and a first polishing compound is supplied to the first polishing pad; and a subsequent final polishing step using a final polishing unit including a second plate provided with a second polishing pad on its surface and a second polishing head for further polishing the surface of a silicon wafer by rotating the second plate and the silicon wafer held by the second polishing head while the silicon wafer is brought into contact with the second polishing pad and a second polishing agent is supplied to the second polishing pad, wherein in the upstream polishing step a first alkaline aqueous solution, the abrasive grains with a density of 1 x 10 14 / cm 3or contains more than the first polishing agent is provided to perform polishing at a polishing rate of 100 nm / min to 300 nm / min, and the feed is then switched to a feed of a second alkaline aqueous solution containing a water-soluble polymer and abrasive grains with a density of 5 x 10 13 / cm 3 or contains less, in order to perform polishing at a polishing rate of 5nm / min to 20nm / min, The silicon wafer is separated from the first polishing head after the upstream polishing step and transferred to the final polishing unit, while water is supplied to the surface of the silicon wafer, and the silicon wafer is attached to the second polishing head, and In the final polishing step, a third alkaline aqueous solution containing a water-soluble polymer and abrasive grains with a density of 5 x 10 13 / cm 3or contains less than the second polishing compound is provided to perform polishing at a polishing rate of 5nm / min to 20nm / min. (2) The method for polishing a silicon wafer according to (1) above, wherein in the upstream polishing step the first polishing agent is changed from the first alkaline aqueous solution to the second alkaline aqueous solution after a polishing time has elapsed in which a target amount of polishing is achieved in the step. (3) The method for polishing a silicon wafer according to (1) or (2) above, wherein in the upstream polishing step the second alkaline aqueous solution is provided for a period of 10 seconds or more. (4) The method for polishing a silicon wafer according to a process described in (1) to (3) above, wherein in the final polishing step an undiluted alkaline solution containing the water-soluble polymer and the abrasive grains is mixed with pure water in a dilution tank, thereby preparing the third alkaline aqueous solution, The prepared third alkaline aqueous solution is supplied to the final polishing unit using a piping system that communicates with the dilution tank, and Before the third alkaline aqueous solution runs out in the dilution tank, the undiluted alkaline solution and the purified water are added back into the dilution tank to prepare a new portion of the third alkaline aqueous solution. (5) The method for polishing a silicon wafer according to (4) above, the undiluted alkaline solution and the purified water are newly added to the dilution tank, while the third alkaline aqueous solution continues to occupy up to 10% or more of the volume of the dilution tank. (6) The method for polishing a silicon wafer according to one of (1) to (5) above, where in the first to third alkaline aqueous solutions the mean primary particle diameter of the abrasive grains is in the range of 10 nm to 70 nm. (7) The method for polishing a silicon wafer according to one of (1) to (6) above, wherein in the first to third alkaline aqueous solutions the abrasive grains contain SiO2 particles. (8) The method for polishing a silicon wafer according to one of (1) to (7) above, wherein the first alkaline aqueous solution contains one or more alkalis selected from potassium hydroxide, sodium hydroxide, tetramethylammonium hydroxide and tetraethylammonium hydroxide, and The second and third alkaline aqueous solutions contain ammonia. (9) The method for polishing a silicon wafer according to one of (1) to (8) above, wherein in the second and third alkaline solutions the water-soluble polymer is one or more selected from hydroxyethylcellulose, polyethylene glycol and polypropylene glycol. (Beneficial effect)

[0012] The method for polishing a silicon wafer according to this disclosure can suppress the formation of not only PIDs but also shallow scratches. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The following applies to the accompanying drawings: Fig. Figure 1 illustrates a flowchart illustrating the steps for manufacturing a silicon wafer, including a method for polishing a silicon wafer according to an embodiment of this disclosure; Fig. Figure 2 is a schematic view illustrating a feeding mechanism for a polishing agent used in a final polishing step in a method for polishing a silicon wafer according to an embodiment of this disclosure, and Fig. 3A, Fig. 3B, Fig. 3C and Fig. 3D are LPD maps obtained in comparison example 1, comparison example 2, example 1 and example 2 respectively. Detailed description

[0014] A method for polishing a silicon wafer according to an embodiment of this disclosure is described with reference to Fig. 1 described. A silicon wafer is produced according to the flow diagram from Fig. 1. In the preliminary process of step S1, a cutting step, a lapping step, a chamfering step, an etching step, etc., are performed, and the shape of the wafer is formed by double-sided polishing (DSP step) in step S2. The silicon wafer, which has undergone double-sided polishing, is cleaned in step S3, followed by a final polishing step, including a pre-polishing step in step S4 and a final polishing step in step S5. The silicon wafer, which has undergone the final polishing step, is cleaned in step S6 and subsequently examined for wafer planarity and the presence or absence of visible defects and blemishes in step S7, followed by a final cleaning step in step S8, and then undergoes a surface inspection in step S9 before being shipped.

[0015] A method for polishing a silicon wafer according to this embodiment relates to the final polishing step in the above process. The final polishing step is a two-phase polishing process, including the upstream polishing step S4 using the upstream polishing unit and a subsequent final polishing step S5 using a final polishing unit.

[0016] In the upstream polishing step S4, using the upstream polishing unit including a first plate which is provided with a first polishing pad on its surface and a first polishing head, a surface of the silicon wafer is polished by rotating the first plate and the silicon wafer which is held by the first polishing head, while the silicon wafer is brought into contact with the first polishing pad and a first polishing agent is supplied to the first polishing pad.

[0017] For the first polishing agent used in the upstream polishing step, a polishing agent with a relatively high alkaline etching rate and containing numerous abrasive grains is preferably used. In this embodiment, such a polishing agent is a first alkaline aqueous solution containing abrasive grains with a density of 1 x 10 14 / cm 3or more. The polishing rate of silicon using the first alkaline aqueous solution is preferably 100 nm / min to 300 nm / min. A polishing rate of 100 nm / min or more would not impair productivity, and a polishing rate of 300 nm / min or less would not roughen the wafer surface; accordingly, the wafer surface can be polished uniformly. To achieve such a polishing rate, the first alkaline aqueous solution does not contain water-soluble polymers and preferably contains one or more alkalis selected from potassium hydroxide (KOH), sodium hydroxide (NaOH), tetramethylammonium hydroxide (TMAH), and tetraethylammonium hydroxide (TEAH). The density of the abrasive grains is not specifically limited, as long as they are 1 x 10 14 / cm 3 or more; however, the density with regard to preventing the aggregation of the abrasive grains is preferably 1 x 10 15 / cm 3or less. In this embodiment, after polishing, while the first alkaline aqueous solution mentioned above is being provided, the solution to be provided is changed to a second alkaline aqueous solution with specifications equivalent to those of a third alkaline aqueous solution as described below.

[0018] Immediately after the upstream polishing step, the wafer is separated from the first polishing head and transferred to the final polishing unit, while water is supplied to the wafer surface to prevent it from drying out. This transfer can also be referred to simply as "underwater transfer." Specifically, the wafer is picked up in a transfer container and transferred to the final polishing unit, with water being supplied to the wafer surface. Alternatively, the wafer transfer channel can be filled with water, and the wafer can be transferred through this channel. Furthermore, the wafer can be picked up in a water-filled transfer container, and the container can be used for the transfer.

[0019] In the final polishing step S5, using the final polishing unit including a second plate equipped with a second polishing pad on its surface and a second polishing head, the surface of the silicon wafer is further polished by rotating the second plate and the silicon wafer, which is held by the second polishing head, while the silicon wafer is brought into contact with the second polishing pad and a second polishing agent is supplied to the second polishing pad.

[0020] For the second polishing agent used in the final polishing step, a polishing agent with a low alkaline etch rate and fewer abrasive grains is preferably used. In this embodiment, a third alkaline aqueous solution containing a water-soluble polymer and abrasive grains with a density of 5 x 10⁻⁵ is used. 13 / cm 3or contains less than such a polishing agent is used. The polishing rate of silicon using the third alkaline aqueous solution is preferably 5 nm / min to 20 nm / min. A polishing rate of 5 nm / min or more does not result in a long polishing time to achieve a desired amount of polishing; accordingly, productivity would not be impaired, and it can have a sufficient effect on removing defects formed on the wafer surface in the upstream polishing step. A polishing time of 20 nm / min or less does not result in an excessive etching effect of an alkali and would accordingly not worsen the roughness of the wafer surface. To obtain such a polishing rate, the third alkaline aqueous solution contains a water-soluble polymer and preferably contains ammonia.Preferably, one or more water-soluble polymers selected from hydroxyethylcellulose (HEC), polyethylene glycol (PEG), and polypropylene glycol (PPG) are used. The density of the abrasive grains is not specifically limited, as long as they are 5 x 10⁻⁶. 13 / cm 3 or less; however, the density is preferably 2 x 10 to ensure minimum polishability. 13 / cm 3This reliably improves the smoothness of the wafer surface. It is noted that the third alkaline aqueous solution contains the water-soluble polymer, has a higher viscosity than the first alkaline aqueous solution, and preferably has a viscosity of 1.5 mPa·s to 5.0 mPa·s at a service temperature (18 °C to 25 °C). If the viscosity is less than 1.5 mPa·s, the polishing compound would flow easily and the desired etch rate would not be achieved; conversely, if the viscosity is 5.0 mPa·s or higher, the polishing compound would remain and adhere to the wafer surface, even after cleaning following final polishing.

[0021] In this embodiment, as in Fig. Figure 1 illustrates, importantly, the changeover of a slurry provided in the upstream polishing step from an upstream polishing slurry (step S4A) to a final polishing slurry (step S4B). Specifically, in the upstream polishing step, the first alkaline aqueous solution described above is provided as the first polishing agent, and the feed is changed to the second alkaline aqueous solution with specifications equivalent to those of the third alkaline aqueous solution described above. The second alkaline aqueous solution has specifications equivalent to those of the third alkaline aqueous solution used in the final polishing and must contain a water-soluble polymer and abrasive grains with a density of 5 x 10⁻⁶. 13 / cm 3or less. Other requirements for the second alkaline aqueous solution are the same as those for the previously mentioned third alkaline aqueous solution.

[0022] In this embodiment, the formation of not only PIDs but also shallow scratches can be suppressed by such a change of polishing agents, and the inventors consider their process to be as follows. (1) The second alkaline aqueous solution contains fewer abrasive grains than the first alkaline aqueous solution; accordingly, the abrasive grains left on the wafer surface are less likely to be aggregated during underwater transfer from the upstream polishing unit to the final polishing unit. (2) The water-soluble polymer contained in the second alkaline aqueous solution protects the wafer surface undergoing underwater transfer; accordingly, abrasive grains left on the wafer surface are less likely to aggregate during underwater transfer. (3) When final polishing is started, the composition of the agent (the second alkaline aqueous solution) left on the wafer surface is similar to the second polishing agent to be provided (the third alkaline aqueous solution), so that the pH of the polishing agent on the wafer surface hardly changes, and accordingly, the abrasive grains are less likely to aggregate. (4) It is important that the second alkaline aqueous solution contains abrasive particles, although fewer than those contained in the first alkaline aqueous solution. Since the concentration of abrasive particles in the first alkaline aqueous solution used in the upstream polishing is high, the abrasive particles simply aggregate and adhere to the wafer. If the second alkaline aqueous solution does not contain abrasive particles and contains only a water-soluble polymer, such as the solution described in PTL 2 for forming a protective film, a protective film of the water-soluble polymer will form on the water surface; however, this will not be sufficient to completely remove the abrasive particles adhering to the wafer.In this embodiment, the abrasive grains in the second alkaline aqueous solution, which aggregate and adhere to the wafer surface during the upstream polishing, are removed. This results in fewer abrasive grains adhering to the wafer surface in the subsequent transfer phases and the beginning of the final polishing. As described above, the density of the abrasive grains in the second alkaline solution is preferably 2 x 10⁻⁶. 13 / cm 3 or more.

[0023] It is noted that processes (1) and (2) above are achieved provided that the transfer from the upstream polishing unit to the final polishing unit is carried out by underwater transfer. Accordingly, in this embodiment, underwater transfer is also one of the important steps for suppressing the formation of shallow scratches. Furthermore, underwater transfer can also prevent the formation of water stains on the wafer surface caused by drying and thickening of the initial alkaline aqueous solution used in the upstream polishing. Even if the abrasive particles in the initial alkaline aqueous solution used in the upstream polishing adhere to the side surfaces or beveled portions of the wafer, these abrasive particles can be removed by underwater transfer.

[0024] The timing of the change of the polishing agents is not specifically restricted; however, in the preceding polishing step, after a polishing time has elapsed in which a target amount of polishing in the step is achieved, the agent to be provided is preferably changed from the first alkaline aqueous solution to the second alkaline aqueous solution.

[0025] Furthermore, to fully achieve the processes and effects of changing the polishing agents described above, the second alkaline aqueous solution is preferably provided in the upstream polishing step for a period of 10 s or more. Although the upper limit of the period is not specifically restricted, it is preferably 300 s or less for productivity reasons.

[0026] In the first to third alkaline aqueous solutions, the mean primary particle diameter of the abrasive grains preferably lies in the range of 10 nm to 70 nm. If the particle diameter is less than 10 nm, the abrasive grains aggregate, forming coarse particles with a large diameter, which would cause PIDs; on the other hand, if the particle diameter exceeds 70 nm, the roughness of the wafer surface would be worsened after polishing because the particle diameter is excessively large.It is noted that “mean primary particle diameter” here refers to a value of the specific surface area of ​​spherical particles calculated by a BET method (a method for calculating the specific surface area of ​​a sample from the amount of molecules whose adsorption cross-sectional area has been determined and which are adsorbed on the surfaces of the powder particles at a temperature of liquid nitrogen), converted into the diameter of the spherical particles.

[0027] In the first to third alkaline aqueous solutions, the abrasive grains used can include: ceramics, such as silicon dioxide or aluminum oxide; diamond or silicon carbide alone or a compound thereof; a high molecular weight polymer, such as polyethylene or polypropylene; etc. However, for reasons of low cost, dispersibility in the polishing compounds, ease of controlling the abrasive grain diameter, etc., SiO2 particles are preferred. Furthermore, SiO2 particles can be prepared, for example, by either a dry process (combustion process, arc process) or a wet process (sedimentation process / sol-gel process). The abrasive grains to be used can have a spherical shape, a cocoon-like shape, etc.

[0028] The first to third alkaline aqueous solutions preferably do not contain an oxidizing agent (such as hydrogen peroxide (H₂O₂), sodium persulfate, etc.). If an oxidizing agent is included, not only is the wafer surface oxidized and the etch rate reduced, but the abrasive grains also adhere to the surface during oxidation, causing PIDs.

[0029] For the second polishing pad, a material such as non-woven cloth, suede, polyurethane foam, polyethylene foam, porous fluororesin, etc., can be used.

[0030] Next, in this embodiment, the formation of shallow scratches can be more completely suppressed by developing a method for providing the third alkaline aqueous solution (second polishing agent) in the final polishing step as follows. This point is made with reference to Fig. 2 described. Fig. Figure 2 is a schematic view illustrating a system for supplying the polishing compound used in the final polishing step. In the final polishing step, an undiluted alkaline solution containing a water-soluble polymer and abrasive grains is mixed with purified water in a dilution tank 10 to dilute the undiluted alkaline solution by a factor of several tens, thereby preparing the third alkaline aqueous solution. Specifically, purified water is added to the dilution tank 10 through a water tube 16, then a desired quantity of the undiluted alkaline solution is added to the dilution tank 10 through an undiluted solution tube 14, and then purified water is added again.After the undiluted alkaline solution is first introduced into the dilution tank 10, the subsequent addition of purified water causes bubble formation due to the influence of the water-soluble polymer in the undiluted alkaline solution, and the aggregation of abrasive grains in the polishing compound occurs readily. To address this problem, purified water is first supplied so that its liquid level is higher than an outlet port of the undiluted solution tube 14 in the tank. The undiluted alkaline solution is then supplied, followed by another supply of purified water. The mixture of purified water and undiluted alkaline solution is then stirred.The method of stirring the liquid mixture is not specifically restricted; for example, stirring can be carried out using a circulation pump, or can be carried out by placing a stirring device, which includes a stirring rod of a given shape and its drive unit, in the tank.

[0031] The prepared third alkaline aqueous solution is supplied to a polishing unit (final polishing unit) via a piping system that communicates with the dilution tank 10. Specifically, the third alkaline aqueous solution is transported to the final polishing unit via a feed pipe 18 that communicates with the bottom of the dilution tank 10. If the second alkaline aqueous solution used in the upstream polishing step has specifications equivalent to those of the third alkaline aqueous solution, the third alkaline aqueous solution is transported to the upstream polishing unit via the feed pipe 18 that communicates with the bottom of the dilution tank 10 and can be used as the second alkaline aqueous solution.

[0032] If the prepared third alkaline aqueous solution is completely transported from the dilution tank 10 to the final polishing unit, and after the dilution tank 10 has been emptied, the undiluted alkaline solution and purified water are added back into the dilution tank and stirred to prepare a new (fresh) third alkaline aqueous solution; the pH of the liquid changes significantly, and consequently, abrasive particles aggregate, which has been found to lead to the formation of shallow scratches. Therefore, in this embodiment, before the third alkaline aqueous solution in the dilution tank 10 runs out, the undiluted alkaline solution and purified water are added back into the dilution tank 10 to prepare a new third alkaline aqueous solution.Accordingly, the change in the pH value of the liquid can be reduced; consequently, the abrasive grains hardly aggregate and the formation of shallow scratches can be more completely suppressed.

[0033] To achieve this process and effect, the undiluted alkaline solution and purified water are preferably added to the dilution tank 10, while the third alkaline aqueous solution continues to occupy up to 10% or more of the volume of the dilution tank 10. Furthermore, with regard to productivity, the undiluted alkaline solution and purified water are added to the dilution tank 10, while the third alkaline aqueous solution continues to occupy up to 50% or less of the volume of the dilution tank 10.

[0034] The silicon wafer, which has undergone the final polishing step described above, is rinsed and submerged within 24 hours of the end of the final polishing step before being cleaned (typically with sulfuric acid and ozone) in step S6. Performing submerged storage and cleaning before polishing compound components and abrasive grains adhere to the wafer surface can reduce PIDs.

[0035] The above embodiment describes an example of a two-phase polishing process, including an upstream polishing step S4 using an upstream polishing unit and a subsequent final polishing step S5 using the final polishing unit, as shown in Fig. Figure 1 illustrates this. However, this disclosure is not limited to the embodiment above, and the upstream polishing step can be performed using two or more upstream polishing units. In this case, several upstream polishing steps can be performed using several upstream polishing units, with the last step of the upstream polishing steps S4 in this case. Fig. can correspond to 1. EXAMPLES (Comparison example 1)

[0036] Following a conventional procedure, 25 silicon wafers with a diameter of 300 mm, which had undergone cleaning after double-sided polishing, were subjected to a final polishing step under the following conditions. In a preliminary polishing step, a first alkaline aqueous solution containing no water-soluble polymer, TMAH as an alkali, and abrasive grains (SiO2 particles with a mean primary particle diameter of 35 nm) with a density of 2.5 x 10 14 / cm 3 It contains a first polishing agent. The polishing rate of silicon using the first alkaline aqueous solution was 200 nm / min. The polishing time was 300 s.

[0037] In a final polishing step, a third alkaline aqueous solution containing HEC as a water-soluble polymer, ammonia as an alkali, and abrasive grains with a density of 5 x 10⁻⁵ was used. 13 / cm 3(SiO2 particles with a mean primary particle diameter of 35 nm) was provided as a second polishing agent. The polishing rate of silicon using the third alkaline aqueous solution was 10 nm / min, and the viscosity of the solution at 25 °C was 3 mPa·s. The polishing time was 300 s. For the procedure to provide the second polishing agent in the final polishing step, undiluted alkaline solution and purified water were added to a dilution tank, which was emptied, and mixed to prepare a new polishing agent.

[0038] The silicon wafer, which underwent the final polishing step, was subjected to cleaning, inspection, and a final cleaning step according to conventional procedures. Furthermore, the evaluation of PIDs and shallow scratches on the silicon wafer was performed by surface inspection as follows. (Example 1)

[0039] Polishing was performed under the same conditions as in Comparison Example 1 above, with the exception of the following points, and the evaluation of PIDs and shallow scratches was carried out. Specifically, in Example 1, in the upstream polishing step, after polishing using the first alkaline aqueous solution above for 300 s, the polishing compound was changed to a second alkaline aqueous solution with specifications equivalent to those of the third alkaline solution above, and polishing was continued for another 30 s. (Comparative example 2)

[0040] Polishing was performed under the same conditions as in Example 1 above, with the exception of the following points, and the evaluation of PIDs and shallow scratches was carried out. Specifically, in Comparative Example 2, an alkaline aqueous solution with the same specifications as the third alkaline aqueous solution above was used, except that it did not contain abrasive particles. (Example 2)

[0041] Polishing was performed under the same conditions as in Example 1 above, with the exception of the following points, and the evaluation of PIDs and shallow scratches was carried out. Specifically, in Example 2, for the procedure to provide the second polishing compound in the final polishing step, undiluted alkaline solution and purified water were added to the dilution tank before the third alkaline aqueous solution in the dilution tank ran out (particularly at the time when the third alkaline aqueous solution continued to occupy up to 20% of the volume of the dilution tank) and were mixed to prepare a new polishing compound. <Auswertung von PIDs>

[0042] The surface of each wafer was measured using a laser particle counter (SP2, manufactured by KLA Tencor), and defects 35 nm or larger, classified as LPD-N, were identified as PIDs, and their number was counted. The average number of defects across the 25 silicon wafers was three for comparison example 1, three for comparison example 2, two for example 1, and one for example 2. Accordingly, the formation of PIDs was completely suppressed in all comparison examples 1, 2 and 1, 2. < Evaluation of shallow scratches>

[0043] The surface of each wafer was measured using a laser particle counter (SP3, manufactured by KLA Tencor) in normal mode, and a map of the wafer surface was generated using LPDs detected as defects of 36 nm or larger, with rows of detected defects being plotted in Fig. Three long lines were observed, which were recognized as shallow scratches. It is noted that Fig. Figures 3A to 3D each illustrate an LPD map of the 25 wafers that were stacked. In comparison example 1, scratches were formed in 15 of the 25 wafers and, as shown in the diagram, had... Fig.As can be seen in Figure 3A, long scratches had a length of approximately 140 mm, which was close to the radius of the wafer. In comparison example 2, scratches formed in 15 out of 25 of the wafers. In contrast, in example 1, scratches formed in 10 out of 25 of the wafers; accordingly, the number of scratches was considerably reduced compared to comparison examples 1 and 2, and the length of the detected scratches was significantly shorter. In example 2, no scratches formed in the 25 wafers. INDUSTRIAL APPLICABILITY

[0044] According to the disclosed method for polishing a silicon wafer, the formation of not only PIDs but also shallow scratches can be suppressed. REFERENCE MARK LIST 10 Dilution tank 14 Undiluted solution tube 16 Pure water pipe 18 Feed pipe

Claims

[1] Method for polishing a silicon wafer comprising a double-sided polishing step (S2) as a lapping step followed by a final polishing step (S4, S5) comprising: a preliminary polishing step (S4) using a preliminary polishing unit including a first plate provided with a first polishing pad on its surface and a first polishing head for polishing a surface of a silicon wafer by rotating the first plate and the silicon wafer held by the first polishing head while the silicon wafer is brought into contact with the first polishing pad and a first polishing medium is supplied to the first polishing pad; and a subsequent final polishing step (S5) using a final polishing unit including a second plate provided with a second polishing pad on its surface and a second polishing head for further polishing the surface of the silicon wafer by rotating the second plate and the silicon wafer held by the second polishing head while the silicon wafer is brought into contact with the second polishing pad and a second polishing agent is supplied to the second polishing pad, wherein in the upstream polishing step (S4) the first polishing agent is a first alkaline aqueous solution containing abrasive grains with a density of 1 x 10 14 / cm 3or more, is provided to perform polishing (S4A) at a polishing rate of 100 nm / min to 300 nm / min, and the feed is then switched to a feed of a second alkaline aqueous solution containing a water-soluble polymer and abrasive grains with a density of 5 x 10 13 / cm 3 or contains less to perform polishing (S4B) at a polishing rate of 5 nm / min to 20 nm / min, the silicon wafer is separated from the first polishing head after the upstream polishing step (S4) and transferred to the final polishing unit, while water is supplied to the surface of the silicon wafer, and the silicon wafer is attached to the second polishing head, and in the final polishing step (S5) a third alkaline aqueous solution containing a water-soluble polymer and abrasive grains with a density of 5 x 10 13 / cm 3or contains less than the second polishing compound is provided to perform polishing at a polishing rate of 5nm / min to 20nm / min. [2] Method for polishing a silicon wafer according to claim 1, wherein in the upstream polishing step (S4) the first polishing agent is changed from the first alkaline aqueous solution to the second alkaline aqueous solution after a polishing time has elapsed in which a target amount of polishing is achieved in the step. [3] Method for polishing a silicon wafer according to claim 1 or 2, wherein in the upstream polishing step (S4) the second alkaline aqueous solution is provided for a period of 10 seconds or more. [4] Method for polishing a silicon wafer according to any one of claims 1 to 3, wherein in the final polishing step (S5) an undiluted alkaline solution containing the water-soluble polymer and the abrasive grains is mixed with pure water in a dilution tank (10), thereby preparing the third alkaline aqueous solution, the prepared third alkaline aqueous solution is supplied to the final polishing unit using a piping system that communicates with the dilution tank (10), and Before the third alkaline aqueous solution in the dilution tank (10) runs out, the undiluted alkaline solution and the purified water are added to the dilution tank (10) to prepare a new portion of the third alkaline aqueous solution. [5] Method for polishing a silicon wafer according to claim 4, wherein the undiluted alkaline solution and the purified water are newly added to the dilution tank (10), while the third alkaline aqueous solution continues to occupy up to 10% or more of a volume of the dilution tank (10). [6] Method for polishing a silicon wafer according to any one of claims 1 to 5, wherein in the first to third alkaline aqueous solution the mean primary particle diameter of the abrasive grains is in the range of 10 nm to 70 nm. [7] Method for polishing a silicon wafer according to any one of claims 1 to 6, wherein the abrasive grains in the first to third alkaline aqueous solution contain SiO2 particles. [8] Method for polishing a silicon wafer according to any one of claims 1 to 7, wherein the first alkaline aqueous solution contains one or more alkalis selected from potassium hydroxide, sodium hydroxide, tetramethylammonium hydroxide and tetraethylammonium hydroxide, and The second and third alkaline aqueous solutions contain ammonia. [9] Method for polishing a silicon wafer according to any one of claims 1 to 8, wherein in the second and third alkaline solution the water-soluble polymer is one or more selected from hydroxyethylcellulose, polyethylene glycol and polypropylene glycol.

Images