Method for polishing a silicon wafer

Abstract

Method for polishing a silicon wafer, comprising: a first polishing step for polishing one side of a surface of the silicon wafer by supplying a first polishing fluid comprising a primary agent in the form of an aqueous alkali and abrasive grains to a polishing cloth, wherein the first polishing fluid does not comprise a water-soluble polymer; Following the first polishing step, a step to form a protective film by supplying a protective film-forming solution comprising a water-soluble polymer to the polishing cloth after its use in the first polishing step and to establish contact between the protective film-forming solution and the polished surface of the silicon wafer subjected to the first polishing step, in order to form a protective film on the polished surface; and a second polishing step for polishing one side of the surface of the silicon wafer, wherein the protective film is formed into a polishing cloth by supplying a second polishing fluid comprising a primary agent in the form of an aqueous alkali, abrasive grains and a water-soluble polymer, which differs from the polishing cloth used in the first polishing step.

Description

TECHNICAL AREA

[0001] The present invention relates to a method for polishing a silicon wafer. TECHNICAL BACKGROUND

[0002] In recent years, chemical-mechanical polishing (CMP), which involves the relative rotation of a silicon wafer and a polishing cloth while a polishing fluid containing abrasive particles (e.g., silicon dioxide particles) is introduced into an aqueous solution, has become a widely used method for polishing the surface of silicon wafers. CMP is a hybrid technique combining mechanical polishing by the abrasive particles with chemical polishing by the aqueous solution. It is well known that combining these two polishing processes results in a high degree of flatness on the silicon wafer surface. CMP of the silicon wafer is typically performed in several steps, progressing from a double-sided polishing step to a single-sided polishing step.

[0003] The double-sided polishing step is performed to polish the silicon wafer to a desired thickness. More precisely, the front and back sides of the silicon wafer are simultaneously polished at a relatively high speed using a hard polishing cloth made of polyurethane or similar materials. This double-sided polishing flattens the silicon wafer, reducing variations in thickness and removing waviness components (e.g., nanotopography).

[0004] Single-sided polishing is performed to improve the roughness of one of the silicon wafer surfaces after double-sided polishing. Specifically, single-sided polishing is carried out using a soft polishing cloth, such as a suede cloth, in combination with abrasive micro-grains to reduce the micro-roughness (e.g., nanotopography and haze) of one of the silicon wafer surfaces. Single-sided polishing is performed in several stages, varying the polishing cloth type, the abrasive grain size, and the alkali concentration in the polishing fluid.

[0005] A technique relating to the one-sided polishing step for improving the opacity of a silicon wafer surface has been disclosed, comprising: an abrasive used for high-gloss polishing of the silicon wafer, wherein the abrasive contains alkaline silicon dioxide, a water-soluble polymer, and an organic ring compound; and a method for polishing the silicon wafer using the abrasive (see patent reference 1). Further polishing methods for semiconductors are known in the prior art, including a two-sided polishing method from DE 10 2013 218 880 A1, a method in which end polishing or mirror polishing is performed from DE 10 2009 031 356 A1, and a method for simultaneously polishing the front and back sides of a semiconductor wafer from US 2014 / 0141613 A1. LIST OF COUNTER-POINTS PATENT LITERATURE

[0006] Patent literature 1: JP S53 - 21 430 A SUMMARY OF THE INVENTION PROBLEM(S) TO BE SOLVED BY THE INVENTION

[0007] As also reported in patent literature 1, the water-soluble polymer is generally added to the polishing fluid in the one-sided polishing step to reduce the degree of turbidity of the surface of the silicon wafer.

[0008] On the uppermost surface layer of the silicon wafer, processing defects (stresses) induced by the mechanical action of double-sided polishing remain after the double-sided polishing step. Accordingly, the subsequent single-sided polishing step is necessary to polish the silicon wafer in order to remove these processing defects. However, since the polishing speed (polishing rate) is significantly reduced when the water-soluble polymer is added to the polishing fluid, removing the processing defects requires a considerable amount of time and a large number of steps.

[0009] Regarding the aforementioned problem, the silicon wafer undergoes an initial single-sided polishing treatment (hereinafter also referred to as rough polishing) with the polishing fluid to which no water-soluble polymer has been added, following the double-sided polishing step. This allows for the removal of processing defects without reducing the polishing speed. In the second and subsequent single-sided polishing treatments (hereinafter also referred to as finish polishing), the silicon wafer is polished with the polishing fluid to which the water-soluble polymer has been added, enabling the silicon wafer to exhibit a desired surface roughness.

[0010] However, experiments conducted by the inventors have shown that numerous micro-LPDs (LPD = light spot defects) with a size of 45 nm or less form on the surface of the fabricated epitaxial silicon wafer when an epitaxial growth treatment is applied to the surface of the silicon wafer produced by the aforementioned rough and finish polishing. Furthermore, as described below, the inventors have also found that water spot defects form on the surface of the silicon wafer after it has undergone rough polishing, and that these water spot defects cause the formation of the micro-LPDs on the epitaxial silicon wafer.

[0011] One object of the invention is to provide a method for polishing a surface of a silicon wafer, wherein the method is suitable for reducing the formation of water spot defects on the surface of the silicon wafer in a one-sided polishing step. MEANS TO SOLVENT THE PROBLEM(S)

[0012] The inventors have carefully studied the cause of the formation of micro-LPD on the surface of an epitaxial silicon wafer.

[0013] To investigate the cause of micro-LPDs on the surface of the epitaxial silicon wafer, an image of the LPDs was examined, and a substantial examination of the LPDs was performed using an atomic force microscope (AFM). Furthermore, the LPDs were examined using a wafer defect inspection system (MAGICS, manufactured by Lasertec Corporation) and an AFM, allowing for the classification of the detected LPDs.

[0014] Fig. Figure 1 shows the LPD observed on the surface of the epitaxial silicon wafer, which shows that many micro-LPDs are distributed on the surface of the epitaxial silicon wafer.

[0015] Fig. Figure 2 is a graph showing the results of the defect classification of the LPDs observed on the surface of the epitaxial silicon wafer. The graph shows that the number of concave microdefects is greater than that of typical epitaxial defects such as SF (stacking defects) and Hillock.

[0016] Fig. Figure 3 shows an RKM image of the concave microdefects observed on the surface of the epitaxial silicon wafer. Fig. Figure 4 shows a height difference of an upper surface between A and B according to Fig. 3. As can be seen from the Fig. 3 and Fig.As can be clearly seen in Figure 4, the concave microdefects actually observed on the surface of the epitaxial silicon wafer are crater-shaped concave microdefects with a diameter of approximately 26 µm and a height of approximately 15 nm and a ring-shaped protruding circumference.

[0017] The results suggest that the concave microdefects determine the level of the number of micro-LPDs.

[0018] Next, the cause of the concave microdefects on the surface of the epitaxial silicon wafer was investigated. Specifically, the surface of the silicon wafer was examined in DIC mode with respect to coordinate positions where defects had a concave shape and a height difference of 3 nm or more, and coordinate positions where concave microdefects were observed on the surface of the epitaxial silicon wafer before it underwent epitaxial growth treatment, using a surface defect inspection device (Surfscan SP-2: manufactured by KLA-Tencor Corporation). A comparison of the two coordinate positions confirmed that the coordinate positions where the defects had a concave shape with a height difference of 3 nm or more essentially coincided with the coordinate positions where the concave microdefects were observed.

[0019] Accordingly, the use of an RKM on a section where the defect was detected on the surface of the silicon wafer yielded a significant observation. The results are presented in Fig. 5 shown. Fig. Figure 6 shows a height difference of the upper surface between C and D according to Fig. 5. As can be seen from the Fig. 5 and Fig. As can be clearly seen in Figure 6, it was confirmed – similar to the concave microdefects observed on the surface of the epitaxial silicon wafer – that the concave microdefects actually observed on the surface of the silicon wafer were a crater-shaped defect with a diameter of approximately 20 µm and a height of approximately 22 nm and a ring-shaped protruding perimeter (hereinafter referred to as the water spot defect).

[0020] Since the annular water stain defect observed on the surface of the silicon wafer has a central part that is thinner than its circumference, it was concluded that the reaction forming the water stain defect involves etching. More precisely, it is concluded that the water stain defect forming on the surface of the silicon wafer is caused by a phenomenon in which an aqueous solution containing an alkaline component and similar substances is present on the surface of the silicon wafer in the form of water droplets. The surface of the silicon wafer is etched by the alkaline component in the aqueous solution, and simultaneously, oxygen from the air enters the water droplets and reacts with the alkaline component, leading to the formation of a reactant salt.

[0021] It has been confirmed that one step in which this phenomenon may occur is the one-sided polishing step, where the water spot defect arose when the silicon wafer was conveyed in air during the transition from rough polishing to finish polishing.

[0022] It is assumed that the water spot defect may be formed because water droplets containing a polishing suspension component remain on the surface of the silicon wafer after it has undergone the coarse polishing step in the one-sided polishing step.

[0023] Fig. Figure 7 shows a suspected mechanism for the formation of the water stain defect. A lower section in Fig. Figure 7 corresponds to an upper section and shows a partially enlarged view of the top surface layer of the silicon wafer.

[0024] After completion of the coarse polishing step in the one-sided polishing step, the polishing suspension used for coarse polishing is present as a residue on the polished surface of the silicon wafer. The polished surface is hydrophobic. Accordingly, as in the section above... Fig. Figure 7(A) shows water droplets containing polishing suspension forming on the polished surface. As shown in the lower section of Fig. 7(A) is shown, the (in the figure by K + and OH -The alkaline component (designated Si) originating from the polishing suspension is present in the water droplets. Furthermore, silicon (designated Si) from the top surface layer of the silicon wafer, which comes into contact with the water droplets, is washed out and trapped in the water droplets. During the transition from the completion of the coarse polishing to the subsequent finish polishing, oxygen present in the air (designated O2) is trapped in the water droplets while the polishing head is positioned in an elevated position.

[0025] Next, after completion of the rough polishing, the silicon wafer is transported into the air in a dry state while attached to the polishing head to proceed with the next step of finishing the polishing.

[0026] Although the silicon wafer is transported in air, KOH in the water droplets etchs the silicon on parts of the polished surface to which the water droplets adhere. This etching reaction forms K₂SiO₃ and H₂SiO₃. The water droplets evaporate during the transport of the silicon wafer in air. Consequently, the K₂SiO₃ and H₂SiO₃ that have formed remain in such a way that they accumulate around the area where the water droplets were previously present (see the lower section of Fig. 7(B)).

[0027] During final polishing, the polished surface of the silicon wafer undergoes final polishing (see the section above). Fig.7(C)). Since the polishing component in the final polishing process is small, the water spot defects that occurred during the transport of the silicon wafer in air are not completely removed, so that a crater-shaped water spot defect remains on the surface of the silicon wafer after it has undergone final polishing (see the lower section in Fig. 7(C)).

[0028] It should be noted that the water spot defect is defined as an annular defect with a height of at least approximately 10 nm and a diameter of at least approximately 10 µm when a comprehensive inspection is carried out in DIC mode using a surface defect inspection device (Surfscan SP-2, manufactured by KLA-Tencor Corporation) for defects that each have a concave shape with a height difference of 3 nm or more.

[0029] As described above, it was found that the formation of micro-LPD on the surface of the epitaxial silicon wafer is caused by the water spot defect formed on the silicon wafer during air transport during the transition from coarse polishing to final polishing in the one-sided polishing step.

[0030] The investigation of the invention was based on the following findings.

[0031] According to one aspect of the invention, a method for polishing a silicon wafer comprises: a first polishing step for polishing one side of a surface of the silicon wafer by supplying a first polishing fluid containing a primary agent in the form of an aqueous alkali and abrasive grains to a polishing cloth, wherein the first polishing fluid does not contain a water-soluble polymer; following the first polishing step, a step for forming a protective film by supplying a solution containing a water-soluble polymer to the polishing cloth after its use in the first polishing step and for bringing the solution into contact with the polished surface of the silicon wafer subjected to the first polishing step to form a protective film on the polished surface;and a second polishing step for polishing one side of the surface of the silicon wafer, wherein the protective film is formed by supplying a second polishing fluid containing a primary agent in the form of an aqueous alkali, abrasive grains, and a water-soluble polymer to a polishing cloth that differs from the polishing cloth used in the first polishing step. According to a further aspect of the invention, a method for polishing a silicon wafer according to claim 2 is provided.

[0032] According to the preceding aspect of the invention, in the step of producing the protective film, which is carried out following the first polishing step, the solution containing the water-soluble polymer is brought into contact with the polished surface of the silicon wafer to form a protective film. This process forms the protective film on the polished surface of the silicon wafer after it has undergone the first polishing step.

[0033] The protective film shields the polished surface from atmospheric oxygen during air transport of the silicon wafer, specifically during the transition from the first polishing step (rough polishing) to the second polishing step (finish polishing). Although the polishing fluid remains on the polished surface after rough polishing is complete, the protective film acts as an etch inhibitor, preventing etching caused by the alkaline component of the polishing fluid.

[0034] Since the protective film prevents the polished surface from being exposed to air, a reaction between the polished surface and atmospheric oxygen is prevented. Furthermore, the polished surface remains free from corrosion caused by the alkaline component remaining in the polishing fluid. Consequently, the formation of waterspot defects in the one-sided polishing step during air transport of the silicon wafer from coarse to final polishing can be reduced. This, in turn, reduces the concave microdefects on the manufactured epitaxial silicon wafer caused by waterspot defects.

[0035] In general, a polishing cloth used in a single-sided polishing step is softer than one used in a double-sided polishing step. Since the silicon wafer is polished while submerged in the polishing cloth, a correspondingly large rebound effect occurs at the wafer's periphery due to a reaction force of the polishing cloth, resulting in peripheral shear drop of the wafer, which is referred to as "edge rolloff".

[0036] In the initial phase of coarse polishing, the temperature of the heat generated by the polishing process is high in the central part of the wafer and low at the periphery. Consequently, any initial oxide film present on the wafer's surface prior to coarse polishing, located in the central part where the generated heat is high, tends to be removed first, followed by the removal of the initial oxide film present at the periphery.

[0037] Since the protective film is formed on the polished surface during the protective film formation step, while the solution containing the water-soluble polymer is applied to the polishing surface of the coarse polishing cloth, water-soluble polymer remains in the coarse polishing cloth. Consequently, the coarse polishing in the first polishing step is performed in the next cycle with the same coarse polishing cloth containing the water-soluble polymer. In this case, the water-soluble polymer reduces the removability of the original oxide film from the silicon wafer surface.

[0038] Since the removability of the original oxide film is reduced during coarse polishing using the coarse polishing cloth in which the water-soluble polymer remains, the polishing fraction on the wafer's circumference is correspondingly reduced if the removal of the original oxide film from the wafer's surface is delayed. In other words, by applying the polishing method according to the preceding aspect of the invention, the degree of roll-off at the edges of the silicon wafer during coarse polishing can be reduced in the next and subsequent cycles.

[0039] According to the foregoing aspect of the invention, the water-soluble polymer in the solution for forming a protective film preferably has a concentration in the range of 10 ppm to 30 ppm.

[0040] As long as the concentration of the water-soluble polymer in the solution for forming a protective film remains within the aforementioned range during the protective film formation step of this embodiment, the protective film on the polished surface of the silicon wafer can be formed with sufficient thickness to prevent the formation of the water spot defect. Furthermore, as long as the concentration of the water-soluble polymer remains within the aforementioned concentration range, the amount of water-soluble polymer remaining in the coarse polishing cloth is low enough not to impair the coarse polishing process. Accordingly, the polishing rate is not drastically reduced during the coarse polishing stage in the next cycle.Furthermore, as long as the concentration of the water-soluble polymer is within the aforementioned concentration range, the thickness of the protective film formed on the polished surface of the silicon wafer during the protective film formation step is approximately several hundred Å (several tens of nm), which allows for the removal of the protective film formed on the surface of the wafer in a short time during the subsequent final polishing.

[0041] According to the preceding aspect of the invention, the water-soluble polymer used in the solution to form a protective film is preferably a polymer compound with a cellulose structure or a non-ionic polymer compound. Specifically, hydroxyethylcellulose is mentioned as an example of a polymer compound with a cellulose structure. Examples of non-ionic polymer compounds include polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone, polyethylene glycol, polypropylene glycol, and polyethylene oxide, one or more of which may contain the polymer compound. By using such a water-soluble polymer, the protective film can be easily formed on the polished surface of the silicon wafer. BRIEF DESCRIPTION OF THE DRAWING(S) Fig. Figure 1 is an image of LPD formed on the surface of an epitaxial silicon wafer. Fig.Figure 2 shows the result of a defect classification of the micro-LPD present on the surface of the epitaxial silicon wafer. Fig. Figure 3 shows an RKM image of a concave microdefect with a diameter of 26 µm and a height of 15 nm, where the defect is present on the surface of the epitaxial silicon wafer. Fig. Figure 4 shows a height difference of an upper surface between A and B according to Fig. 3. Fig. Figure 5 shows an RKM image of a water spot defect with a diameter of 20 µm and a height of 22 nm, where the defect is present on the surface of the epitaxial silicon wafer. Fig. Figure 6 shows a height difference of the upper surface between C and D according to Fig. 5. Fig. Figure 7 shows a suspected mechanism for the formation of the water stain defect. Fig.Figure 8 is a top view showing an assembly of a polishing device according to an exemplary embodiment of the invention. Fig. Figure 9 is a partially cut-off side view of the polishing device according to the exemplary embodiment. Fig. Figure 10 shows a protective film formation process in the exemplary embodiment. Fig. Figure 11 shows the number of micro-LPDs formed on the surface of the epitaxial silicon wafer in Example 1, Example 2 and Comparison Example 1. Fig. Figure 12 shows the respective surface roughness Ra of the silicon wafer in Example 1, Example 2 and Comparative Example 1. Fig. Figure 13 is an illustration of the LPD formed on the surface of the epitaxial silicon wafer according to Example 1. Fig. Figure 14 is an illustration of the LPD formed on the surface of the epitaxial silicon wafer according to Example 2. Fig. Figure 15 is an illustration of the LPD formed on the surface of the epitaxial silicon wafer according to comparative example 1. Fig. Figure 16 shows a relationship between a concentration of the water-soluble polymer and the number of water stain defects according to Example 3. DESCRIPTION OF THE FORM(S)

[0042] Below, with reference to the accompanying drawings, an exemplary embodiment(s) of the invention will be described. Design of the polishing device for wafers

[0043] Fig. Figure 8 is a top view showing an assembly of a polishing device according to an exemplary embodiment of the invention. Fig. Figure 9 is a partially cut-off side view of the polishing device according to the exemplary embodiment.

[0044] As in Fig.As shown in Figure 8, a polishing device 1 is designed for polishing the surface of a wafer W in several stages using a polishing suspension. In particular, the polishing device 1 is configured to perform a rough polishing treatment of the wafer W, a first finishing polishing treatment to polish the wafer W to a finer surface roughness than that achieved in the rough polishing treatment, and a second finishing polishing treatment to polish the wafer W to a finer surface roughness than that achieved in the first finishing polishing treatment. Although the finishing polishing treatment in the exemplary embodiment has two stages, comprising the first finishing polishing treatment and the second finishing polishing treatment, the finishing polishing treatment can be performed in a single stage.

[0045] As in Fig.As shown in Figure 8, the polishing device 1 comprises: a polishing unit 2 provided in a box-shaped housing 20, a rotatable wafer holding unit 3 provided on a top side of the polishing unit 2 and a polishing head 4 provided on the rotatable wafer holding unit 3.

[0046] The polishing unit 2 is configured to, for example, clean the polishing head 4 and polish and rinse the wafer W as required. The polishing unit 2 comprises a cleaning unit 21, a coarse polishing unit 22, a first finishing polishing unit 23, and a second finishing polishing unit 24, arranged along a circumference of an upper surface of the housing 20. It should be noted that the coarse polishing unit 22 is described in detail, while the descriptions of the first finishing polishing unit 23 and the second finishing polishing unit 24 are simplified, since the coarse polishing unit 22, the first finishing polishing unit 23, and the second finishing polishing unit 24 have the same basic structure. Cleaning unit 21

[0047] The wafer W is mounted on the cleaning unit 21 as required. The wafer W is polished by the coarse polishing unit 22 and the like, while held by the polishing head 4. The polished wafer W is remounted on the cleaning unit 21 by the polishing head 4. The wafer W is conveyed outwards as required. Furthermore, the cleaning unit 21 is configured to clean the polishing head 4, which is located on its upper surface. Coarse polishing unit 22

[0048] As in the Fig. 8 and Fig.As shown in Figure 9, the coarse polishing unit 22 comprises: a rotary drive unit 221 for the surface plate provided in the housing 20; a disc-shaped coarse polishing surface plate 222 provided on a rotary shaft of the rotary drive unit 221 for the surface plate; a unit (not shown) for supplying coarse polishing fluid and a unit (not shown) for supplying a solution to form a protective film.

[0049] A coarse polishing cloth 223 is provided on an upper surface of the coarse polishing surface plate 222. The coarse polishing fluid supply unit applies the coarse polishing fluid to a polishing surface of the coarse polishing cloth 223 as required. The protective film forming solution supply unit applies the protective film forming solution to the polishing surface of the coarse polishing cloth 223 as required. First finishing polishing unit 23

[0050] The first finishing polishing unit 23 comprises: a rotary drive unit 231 for the surface plate; a first finishing polishing surface plate 232; a unit for supplying a first finishing polishing fluid, configured to supply a first finishing polishing fluid to a polishing surface of a first finishing polishing cloth 233, and a unit for supplying a rinsing fluid. Second finishing polishing unit 24

[0051] The second finishing polishing unit 24 comprises: a rotary drive unit 241 for the surface plate, a second finishing polishing surface plate 242c, a unit for supplying a second finishing polishing fluid configured to supply a second finishing polishing fluid to a polishing surface of a second finishing polishing cloth 243, and a unit for supplying a rinsing fluid.

[0052] The rotatable wafer holding unit 3 is configured to hold and rotate the wafer W in order to convey the wafer W in the specified sequence in air to the cleaning unit 21, the rough polishing unit 22, the first finishing polishing unit 23, and the second finishing polishing unit 24. The rotatable wafer holding unit 3 comprises: a holding spindle 31 arranged in the housing 20 and a bracket 32 ​​in the form of a substantially cruciform box in plan view, which is provided on a rotating shaft of the holding spindle 31. Two polishing heads 4 are provided at each end of the cruciform bracket 32. A head rotation drive unit 33 is configured to rotate each of the polishing heads 4 provided in the bracket 32.

[0053] In the rough polishing unit 22, the rotary drive unit 221 for the surface plate and the head rotary drive unit 33 define a rotary drive unit. In the first finishing polishing unit 23, the rotary drive unit 231 for the surface plate and the head rotary drive unit 33 together define a rotary drive unit. In the second finishing polishing unit 24, the rotary drive unit 241 for the surface plate and the head rotary drive unit 33 together define a rotary drive unit. Function of the wafer polishing device

[0054] Next, a method for polishing the wafer W is described in connection with a function of the polishing device described above.

[0055] First, the holder 32 is lowered, causing the wafer W, located on the cleaning unit 21, to be drawn in and held by a wafer suction device (not shown) of the polishing head 4. Next, the holder 32 is moved upwards and then rotated 90° to move the polishing head 4, holding the wafer W, into a position above the coarse polishing unit 22. First polishing step

[0056] The holder 32 is lowered while the head rotation drive unit 33 rotates the polishing head 4. As the coarse polishing fluid 224 is supplied to the coarse polishing cloth 223, the wafer W, held by the polishing head 4, is brought into contact with the polishing surface of the rotating coarse polishing cloth 223. During this process, the coarse polishing is carried out as described in Fig. 10(A) shown, on the wafer W, while the wafer W is pressed onto the coarse polishing cloth 223 in the presence of the coarse polishing fluid 224.

[0057] The coarse polishing cloth 223 can be a velour or suede cloth. It is advisable to choose a relatively stiff cloth to increase the polishing rate.

[0058] The first polishing step corresponds to the coarse polishing step in the single-sided polishing step and is performed to remove an initial oxide film formed on one surface of wafer W, resulting in the removal of processing defects on the surface of wafer W. The coarse polishing fluid 224 used in the first polishing step contains abrasive particles to increase the polishing rate. Examples of abrasive particles include colloidal silicon dioxide, cerium oxide, diamond, and aluminum oxide.

[0059] Coarse polishing fluid 224 does not contain a water-soluble polymer. Since the water-soluble polymer significantly reduces the polishing rate, it is not practical for coarse polishing fluid 224, which requires a high polishing rate, to contain the water-soluble polymer. However, as long as the water-soluble polymer is in a concentration range where no protective film W1 is formed on a polished surface of the wafer W after coarse polishing, and the polishing rate is not affected, a small amount of the water-soluble polymer may be included to prevent the abrasive particles from accumulating.

[0060] It is desirable to use an aqueous alkali as the coarse polishing fluid 224, containing a main alkaline substance and having a regulated pH value in the range of 8 to 13. Representative examples of alkaline agents include potassium hydroxide, sodium hydroxide, lithium hydroxide, tetramethylammonium hydroxide, and piperazine. Additionally, an aqueous ammonium carbonate solution and an aqueous alkali to which a specific amine has been added are exemplary examples of aqueous alkalis.

[0061] The other polishing conditions in the first polishing step include a polishing time, a rotational speed of the polishing head 4, a contact pressure of the wafer W and the like, and can be adjusted appropriately depending on the desired polishing percentage.

[0062] After completion of the first polishing step, the supply of the coarse polishing fluid 224 is stopped. Step in the formation of the protective film

[0063] Following the first polishing step, a step to form a protective film is performed. This protective film formation step is carried out to prevent the formation of waterspot defects on the polished surface of the wafer W, typically during the transition from rough polishing to finish polishing.

[0064] Again according to Fig. 8 In the protective film formation step after the first polishing step, a solution containing a water-soluble polymer is applied to the polishing surface of the coarse polishing cloth 223 while the polishing surface of the coarse polishing cloth 223 is in contact with the polished surface of the wafer W. During this process, the protective film formation solution 225 is applied as described in Fig. Figure 10(B) shows the wafer W being brought into contact with the polished surface of the wafer. This contact forms the protective film W1 on the polished surface of the wafer W.

[0065] To perform the protective film formation step, the polished surface of wafer W is exposed to air when the holder 32 is moved upwards to separate the polished surface of wafer W from the coarse polishing cloth 223. Since the defect of unsaturated bonding sites is present on the polished surface when wafer W is conveyed in air from the coarse polishing unit 22 to a polishing stage at another unit (e.g., the first finishing polishing unit 23), a water spot defect forms on the surface of wafer W due to contamination by airborne particles and / or a reaction with oxygen. The water spot defect formed in this stage proves to be a concave microdefect after the wafer W has undergone epitaxial growth and is ultimately detected as a micro-LPD.

[0066] Accordingly, to minimize the exposure of the polished surface of the wafer W to air, the step of forming the protective film, which corresponds to the polishing stage in which the first polishing step is carried out, is preferably performed on the coarse polishing cloth 223.

[0067] When the protective film-forming solution is brought into contact with the polished surface of wafer W, the wafer W and the polishing stage can be rotated or stopped. To reliably form the protective film W1 by applying the protective film-forming solution to a position corresponding to the center of wafer W, it is desirable that the protective film-forming solution be brought into contact with the polished surface of wafer W while the wafer W and the polishing stage (i.e., the coarse polishing cloth 223) are rotated in opposite directions.When both the wafer W and the polishing stage are rotated while the wafer W is pressed against the coarse polishing cloth 223, the polished surface of the wafer W undergoes a process of etching by an alkaline component in the solution to form a protective film, followed by a process of removing any etch residue by the coarse polishing cloth 223 after the initial coarse polishing. In other words, in the first polishing step (i.e., the coarse polishing), processing damage caused by abrasive grains on the polished surface of the wafer W can be removed, and a predetermined protective film W1 can be formed on the polished surface of the wafer W, since the polishing of the surface of the wafer W in the protective film formation step occurs in the absence of abrasive grains. Solution for forming a protective film 225

[0068] The water-soluble polymer contained in the solution for forming a protective film 225 is preferably a polymer compound with a cellulose structure or a non-ionic polymer compound. An exemplary example of the polymer compound with a cellulose structure is hydroxyethylcellulose. Examples of the non-ionic polymer compound include polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone, polyethylene glycol, polypropylene glycol, and polyethylene oxide. The water-soluble polymer preferably has the form of a linear micelle with a hydrophobic group at one end and a hydrophilic group at the other end.

[0069] The water-soluble polymer preferably has a concentration in the range of 10 ppm to 30 ppm. At concentrations below 10 ppm, a protective film W1 sufficient to provide the desired effects may not form on the polished surface of the wafer W, even if the protective film formation solution 225 is brought into contact with the polished surface of the wafer W. In this case, the water stain defect may form on the polished surface of the wafer W.

[0070] On the other hand, if the concentration exceeds 30 ppm, the thickness of the protective film W1 formed on the polished surface of the wafer W becomes too great, so that the polishing time for removing the protective film W1 in the subsequent second polishing step can be increased.

[0071] The solution for forming a protective film 225 preferably contains the alkaline liquid as the main substance.

[0072] Immediately after coarse polishing, the coarse polishing fluid 224 is present in a space between the coarse polishing cloth 223 and the polished surface of the wafer W. Accordingly, the protective film formation solution 225 is introduced into this space where the coarse polishing fluid 224 remains. If the protective film formation solution 225 does not contain an alkaline component, a large pH difference arises between the coarse polishing fluid 224 and the protective film formation solution 225. Due to such a large difference, the components present in the space between the coarse polishing cloth 223 and the polished surface of the wafer W can accumulate. These accumulated components can cause damage to the polished surface of the wafer W. Therefore, the pH of the protective film formation solution 225 is preferably adjusted by adding a small amount of an alkaline component (e.g., an amine).In this configuration, the type of alkaline component is not specifically limited, as long as the pH of the solution for forming a protective film 225 is in a range of 10 to 11.

[0073] As described above, the solution for forming a protective film 225 preferably does not contain abrasive grains in the protective film formation step, since the processing damage caused by abrasive grains in the first polishing step can be removed by continuing to polish the polished surface of the wafer W without abrasive grains. Protective film W1

[0074] The protective film W1 is an organic film formed by the attachment of the hydrophobic group of the water-soluble polymer to the polished surface of the wafer W.

[0075] The protective film W1 preferably has a thickness in the range of 10 nm to 100 nm. With a thickness within this range, even when the wafer W is transported in air from the coarse polishing unit 22 to the first finishing polishing unit 23, it is possible to prevent the polished surface from coming into contact with the alkaline component caused by the coarse polishing fluid 224 and the oxygen in the air, thus preventing the polished surface of the wafer W from being etched by these components.

[0076] By performing the protective film formation step as described above, the protective film W1 is formed on the polished surface of the wafer W. As in Fig.As shown in Figure 10(C), the typical formation of the water spot defect during the transport of the silicon wafer in air during the transition from rough polishing to final polishing can be reduced because the protective film W1 is present on the polished surface of the wafer W.

[0077] After completion of the protective film formation step, the supply of the protective film formation solution 225 is stopped, and the rotation of the polishing head 4 is terminated. The polishing head 4 is moved upwards while holding the wafer W. Next, the holder 32 is moved upwards and then rotated 90 degrees to position the polishing head 4, which is holding the wafer W, above the first finishing polishing unit 23. Second polishing step

[0078] The second polishing step corresponds to the finishing polishing step in the one-sided polishing step and is performed to reduce surface roughness.

[0079] In the exemplary embodiment, the second polishing step is carried out in two stages, comprising the first finishing polishing by the first finishing polishing unit 23 and the second finishing polishing by the second finishing polishing unit 24.

[0080] Again according to Fig. 8 The holder 32 is lowered while the head rotation drive unit 33 rotates the polishing head 4. The finishing polishing fluid is supplied to the first finishing polishing cloth 233. The wafer W, held by the polishing head 4, is brought into contact with the polishing surface of the rotating first finishing polishing cloth 233. This process applies the finishing polish to the wafer W while the wafer W is pressed onto the first finishing polishing cloth 233. The first finishing polishing cloth 233 can be a velour cloth or a suede cloth.

[0081] The finishing polishing fluid contains abrasive particles. Specifically, the finishing polishing fluid may contain a mixture of abrasive particles such as colloidal silicon dioxide, diamond, and aluminum oxide. The average diameter of each abrasive particle must be selected within a range that prevents particle clumping and thus avoids processing defects such as micro-scratches. The average diameter is preferably in the range of 10 nm to 50 nm.

[0082] It is desirable to use an aqueous lye containing an alkaline principal substance and having a regulated pH in the range of 8 to 13 as the ready-to-use polishing fluid. Examples of aqueous lye include: an aqueous lye or an aqueous alkaline carbonate solution to which either a basic ammonium salt, a basic potassium salt, or a basic sodium salt has been added as the alkaline agent, and an aqueous lye to which an amine has been added. Furthermore, a water-soluble polymer is added to the ready-to-use polishing fluid. The water-soluble polymer used in the ready-to-use polishing fluid is preferably a polymer compound with a cellulose structure or a non-ionic polymer compound. In particular, hydroxyethylcellulose is an exemplary example of a polymer compound with a cellulose structure.Examples of non-ionic polymer compounds include polyvinyl alcohol, polyacrylamide, polyvinylpyrrolidone, polyethylene glycol, polypropylene glycol, and polyethylene oxide.

[0083] The concentration of the water-soluble polymer in the solution used to form a protective film in the initial step is preferably lower than the concentration of the water-soluble polymer in the finishing polishing fluid used in the second polishing step. If the concentration of the water-soluble polymer in the solution used to form the protective film is higher than the concentration of the water-soluble polymer in the finishing polishing fluid, the amount of water-soluble polymer derived from the solution used to form the protective film and introduced into the second polishing step is increased, causing the polishing rate in the second polishing step to fall below a predetermined value, which can impair the formation of a surface haze.

[0084] After the first finishing polishing step by the first finishing polishing unit 23 is completed, the rotation of the polishing head 4 is stopped, and it is moved upwards while holding the wafer W. The holder 32 is moved upwards and then rotated 90 degrees to move the polishing head 4, which holds the wafer W, into a position above the second finishing polishing unit 24.

[0085] The second finishing polishing step by the second finishing polishing unit 24 is performed to achieve a finer surface roughness than that achieved in the first finishing polishing step. Since the second finishing polishing step is performed in the same manner as the first, with the exception of the treatment described above, a description of the second finishing polishing step is omitted.

[0086] Performing the second finishing polish as described above creates a micro-roughness with a degree of turbidity.

[0087] Finally, after the second finishing polishing step is completed by the second finishing polishing unit 24, the holder 32 is moved upwards and then rotated 90 degrees to move the polishing head 4, which holds the wafer W, into a position above the cleaning unit 21. The holder 32 is lowered, releasing the wafer W, which has been sucked in and held by the wafer suction device of the polishing head 4, to be moved back onto the cleaning unit 21. Advantage(s) of the embodiment(s)

[0088] As described above, the exemplary embodiment described above offers the following advantages. (1) In the step of forming the protective film following the first polishing step, the solution containing the water-soluble polymer is brought into contact with the polished surface of the wafer W to form the protective film W1 on the polished surface.

[0089] Since the protective film W1 prevents the polished surface from being exposed to air, a reaction between the polished surface and atmospheric oxygen is prevented. Furthermore, the polished surface is free from corrosion caused by the alkaline component remaining in the polishing fluid. Consequently, in the one-sided polishing step, the formation of water spot defects during air transport of the silicon wafer, as it transitions from rough polishing to final polishing, is reduced. This, in turn, reduces the concave microdefects on the manufactured epitaxial silicon wafer caused by water spot defects.

[0090] (2) The water-soluble polymer of the solution 225 supplied during the protective film formation step remains in the coarse polishing cloth 223 used in the previous cycle. Accordingly, in the first polishing step of the next cycle, the coarse polishing is performed with the coarse polishing cloth 223 in which the water-soluble polymer remains. In this case, the water-soluble polymer impairs the removability of the original oxide film, so that the removal of the original oxide film at the periphery of the wafer is delayed compared to the center of the wafer. Consequently, the degree of rolloff at the edges W of the wafer can be reduced in the next and subsequent cycles.

[0091] (3) The water-soluble polymer in the solution for forming a protective film 225 has a concentration in the range of 10 ppm to 30 ppm. Accordingly, the protective film W1 is formed on the polished surface of the wafer W to such an extent that it is thick enough to prevent the formation of the water spot defect. Furthermore, as long as the concentration of the water-soluble polymer remains within the aforementioned concentration range, the amount of water-soluble polymer remaining in the coarse polishing cloth 223 is low enough that the coarse polishing process is not affected. Consequently, the polishing rate is not drastically reduced during the coarse polishing process in the next cycle. Further embodiment(s)

[0092] It goes without saying that the scope of the invention is not limited to the above exemplary embodiment, but includes various improvements and modifications of the design, as long as these improvements and modifications are compatible with a problem of the invention.

[0093] In particular, the wafer W can be polished while the unit for supplying a protective film solution supplies the protective film solution 225, although in the foregoing exemplary embodiment the protective film W1 is formed by bringing the protective film solution 225 into contact with the polished surface of the wafer W in the protective film formation step.

[0094] Furthermore, cleaning by rinsing with clean water can be performed between the first polishing step and the protective film formation step, or between the protective film formation step and the second polishing step. The coarse polishing unit 22 of the polishing device 1 includes a liquid supply unit for rinsing with clean water. Cleaning by rinsing with clean water is carried out by a suitable supply of the rinsing liquid to rinse the polished surface of the wafer W onto the polishing surface of the coarse polishing cloth 223.

[0095] Although the second polishing step is performed in two stages, comprising the first finishing polish by the first finishing polishing unit 23 and the second finishing polish by the second finishing polishing unit 24, the second polishing step can also be performed in a single stage. Examples

[0096] The invention is described in more detail below with reference to examples. However, it should be noted that the scope of the invention is not limited by these examples. Example 1

[0097] First, a 300 mm diameter silicon wafer was produced by successively performing the steps of cutting, chamfering, lapping, etching, double-sided polishing, and washing. Next, the washed silicon wafer was processed using the methods described in Fig. The polishing device 1 shown in Figure 8 is subjected to the following one-sided polishing step.

[0098] In the one-sided polishing step according to Example 1, the first polishing step (coarse polishing), the protective film formation step, and the second polishing step (finish polishing) were performed sequentially. The coarse polishing fluid used and the solution for forming a protective film are shown in Table 1. Example 2

[0099] The treatments in the steps according to Example 2 were carried out under the same conditions as according to Example 1, except that the alkaline agent of the solution used in the protective film formation step was replaced by an amine and PEG (polyethylene glycol) was added as a water-soluble polymer in addition to HEC (hydroxyethylcellulose), as shown in Table 1. Comparative example 1

[0100] The treatments in the steps according to Comparative Example 1 were carried out under the same conditions as according to Example 1, except that the step of forming the protective film according to Example 1 was not carried out and cleaning by rinsing was carried out after the first polishing step. Table 1 Coarse polishing fluid Solution for forming a protective film Example 1 Example 2 Comparative example 1 Alkaline agent KOH ammonia Amin - Polymer (HEC)[ppm] 0 10 10 - Polymer (HEC)[ppm] 0 0 20 -

[0101] By means of a CVD process, an epitaxial silicon film with a thickness of 4 µm was formed on one surface of each of the silicon wafers subjected to the one-sided polishing step under the above conditions, thereby producing an epitaxial silicon wafer.

[0102] Evaluation criteria for the wafer included the number of LPDs of the epitaxial silicon wafer, the examination of an LPD image of the epitaxial silicon wafer, and the surface roughness of the silicon wafer before it was subjected to epitaxial growth.

[0103] The number of LPDs and the examination of the LPD image of the epitaxial silicon wafer were obtained by examining the surface of the wafer in DIC mode using a surface defect inspection device (Surfscan SP-2; manufactured by KLA-Tencor Corporation).

[0104] The surface roughness Ra of the silicon wafer was measured using a surface roughness measuring device (manufactured by Chapman).

[0105] Fig. Figure 11 shows the number of micro-LPDs formed on the surface of the epitaxial silicon wafer in each case for Example 1, Example 2 and Comparative Example 1. Fig. Figure 12 shows the surface roughness Ra of the silicon wafer for Example 1, Example 2, and Comparative Example 1. Fig. 13, Fig. 14 to Fig.Figure 15 shows illustrations of the LPD formed on the surface of the epitaxial silicon wafer in each of Example 1, Example 2 and Comparative Example 1.

[0106] As from the Fig. 11 and Fig. As can be clearly seen in Figure 12, both the number of micro-LPDs and the surface roughness Ra in examples 1 and 2, according to which the protective film was formed, tend to be lower compared to the comparison example 1, in which no protective film was formed after the first polishing step.

[0107] In a comparison between Example 1 and Example 2, both the number of micro-LPDs and the surface roughness Ra are lower in Example 2. Therefore, it is concluded that using the solution with a high polymer concentration to form a protective film leads to the formation of a protective film with a higher protective potential.

[0108] The Fig. 13, Fig. 14 to Fig.15 confirm, as a result of the decisive analysis of the LPDs formed on the surface of the epitaxial silicon wafer, that the number of micro-LPDs in each of examples 1 and 2 is lower than in the comparison example 1. This is assumed to be caused by a decrease in the concave microdefects originating from the water stain defect.

[0109] Furthermore, when comparing Example 1 and Example 2, the number of LPDs is low in Example 2. Therefore, it is assumed that the water stain defect was sufficiently prevented.

[0110] Since the solution used to form a protective film according to Example 2 contains PEG as a water-soluble polymer, it can be concluded from the results that the protective film formed from PEG has a high passivation effect and a high etch-stopping effect. Example 3

[0111] Silicon wafers were prepared in the same manner as in Example 2, except that the epitaxial growth treatment was omitted, the protective film formation step used a solution containing only PEG as the water-soluble polymer, and the PEG concentration in the protective film formation solution was adjusted. The polymer concentration was set to 0 ppm, 5 ppm, 10 ppm, 20 ppm, 30 ppm, 50 ppm, and 100 ppm.

[0112] Regarding the obtained silicon wafer, the number of water spots, the polishing rate, and an edge profile and a surface profile of the wafer near the edges (the rolloff at the edges: ERO) were measured.

[0113] The results are in Fig. 16 shown. It is pointed out that one in Fig.The index value shown as the ordinate in Figure 16 is a value relative to the result if the solution used had a PEG concentration of 0 ppm, in other words, a solution containing no PEG.

[0114] Through Fig. 16 confirms that the solution for forming a protective film with a higher polymer concentration shows a higher protective effect through the protective film, which can prevent the formation of water spots, since the number of water spots formed decreases when the polymer concentration of the solution for forming a protective film is increased.

[0115] On the other hand, the results show that the polishing rate decreases when the polymer concentration of the solution used to form a protective film is increased. It is concluded that this is because the polymer remains in the polishing cloth when the polymer concentration is high, leading to a reduction in the polishing rate during the coarse polishing stage in the next cycle.

[0116] The results show that the most suitable range of polymer concentration in the solution for forming a protective film, in which the number of water spots is low and the polishing rate is high, is 10 ppm to 30 ppm. Furthermore, the results show that edge roll-off is reduced when the polymer concentration of the protective film formation solution is increased. This is thought to be because the polymer remaining in the polishing cloth acts on the silicon wafer during the coarse polishing in the next cycle, delaying the removal of the original oxide film and thus preventing edge roll-off. It was confirmed that edge roll-off was also prevented within the most suitable polymer concentration range and that edge roll-off was further prevented by performing the protective film formation step following the initial polishing step to form the protective film on the polished surface. EXPLANATION OF THE REFERENCE SYMBOLS

[0117] 1...Polishing device, 2...Polishing unit, 3...Rotating wafer holding unit, 4...Polishing head, 20...Housing, 21...Cleaning unit, 22...Coarse polishing unit, 23...First finishing polishing unit, 24...Second finishing polishing unit, 31...Holding spindle, 32...Holder, 33...Head rotary drive unit, 221...Rotary drive unit for the surface plate, 222...Coarse polishing surface plate, 223...Coarse polishing cloth, 224...Coarse polishing fluid, 225...Protective film solution, 231...Rotary drive unit for the surface plate, 232...First finishing polishing surface plate, 233...First finishing polishing cloth, 241...Rotary drive unit for the surface plate, 242...Second finishing polishing surface plate, 243...Second polishing cloth, W... Wafer, W1... Protective film

Claims

[1] Method for polishing a silicon wafer, comprising: a first polishing step for polishing one side of a surface of the silicon wafer by supplying a first polishing fluid comprising a primary agent in the form of an aqueous alkali and abrasive grains to a polishing cloth, wherein the first polishing fluid does not comprise a water-soluble polymer; Following the first polishing step, a step to form a protective film by supplying a protective film-forming solution comprising a water-soluble polymer to the polishing cloth after its use in the first polishing step and to establish contact between the protective film-forming solution and the polished surface of the silicon wafer subjected to the first polishing step, in order to form a protective film on the polished surface; and a second polishing step for polishing one side of the surface of the silicon wafer, wherein the protective film is formed into a polishing cloth by supplying a second polishing fluid comprising a primary agent in the form of an aqueous alkali, abrasive grains and a water-soluble polymer, which differs from the polishing cloth used in the first polishing step. [2] Method for polishing a silicon wafer, comprising: a first polishing step for polishing a surface of the silicon wafer by supplying a first polishing fluid comprising a primary agent in the form of an aqueous alkali and abrasive grains to a polishing cloth, wherein the first polishing fluid does not comprise a water-soluble polymer; Following the first polishing step, a step to form a protective film by supplying a protective film-forming solution comprising a water-soluble polymer and not containing abrasive grains to the polishing cloth after its use in the first polishing step and to establish contact between the protective film-forming solution and the polished surface of the silicon wafer subjected to the first polishing step, in order to form a protective film on the polished surface; and a second polishing step for polishing the surface of the silicon wafer, wherein the protective film is formed into a polishing cloth by supplying a second polishing fluid comprising a primary agent in the form of an aqueous alkali, abrasive grains and a water-soluble polymer, which differs from the polishing cloth used in the first polishing step. [3] Method for polishing the silicon wafer according to claim 1 or 2, wherein the water-soluble polymer in the solution for forming a protective film has a concentration in the range of 10 ppm to 30 ppm. [4] Method for polishing the silicon wafer according to any one of claims 1 to 3, wherein the water-soluble polymer used in the solution to form a protective film is a polymer compound having a cellulose structure or a non-ionic polymer compound.

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