Semiconductor wafer manufacturing method

Abstract

To provide a semiconductor wafer manufacturing method and an etching solution that can suppress an increase in the moisture content of the etching solution without performing a dressing operation, even when the etching amount per wafer is small, for example, less than 20 μm, and can suppress changes in the glossiness of the silicon wafer that change depending on the moisture content of the etching solution, thereby stabilizing quality. [Solution] A method for manufacturing a semiconductor wafer made of single crystal silicon, comprising an etching step in which etching is performed using an etching solution containing a mixed acid of at least hydrofluoric acid and nitric acid, wherein in the etching step, a portion of the used etching solution is discharged and new mixed acid is injected into the etching solution to adjust the composition of the mixed acid in the etching solution, and the mixed acid has a water (H2O) content of 32 mass% or less.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a semiconductor wafer made of single crystal silicon. [Background technology]

[0002] Semiconductor wafers made of single crystal silicon (hereinafter referred to as silicon wafers) are manufactured by slicing a single crystal silicon ingot, lapping the sliced ​​surfaces to make them flat, and then immersing the sliced ​​surfaces in an etching solution to remove distortions from the lapping and impurities, followed by polishing, cleaning, etc. In the etching step, for example, Patent Document 1 discloses a technique of performing etching using an etching solution containing hydrofluoric acid at a concentration of 50%, nitric acid, and water, the etching solution containing water at a mass ratio of 50% or more. Furthermore, JISH0609:1994 describes that hydrofluoric acid with a concentration of 49 to 50% should be used as an etching solution for silicon wafers, and also describes that the mass ratio of water in the etching solution should be 33% or more. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-157616 Summary of the Invention [Problem to be solved by the invention]

[0004] In an etching process, silicon wafers are generally etched to a target etching amount (the thickness of the silicon wafers to be dissolved in the etching process), so that a batch of multiple silicon wafers is immersed in an etching solution for a predetermined time corresponding to the etching amount, and after the predetermined time has elapsed, the etched silicon wafers are removed from the etching solution, and then new silicon wafers (a batch of multiple silicon wafers) are similarly immersed in the etching solution, repeating this process to continuously perform the etching process. When etching is performed continuously in this manner, the silicon wafers immersed in the etching solution react with the hydrofluoric acid and nitric acid contained in the etching solution, increasing the amount of reactants and decreasing the amounts of hydrofluoric acid and nitric acid in the etching solution. Therefore, for each batch, a process is performed in which a portion of the etching solution is discharged and a new mixed acid containing hydrofluoric acid and nitric acid is injected depending on the amount of silicon wafers dissolved.

[0005] Furthermore, in the etching process, the hydrofluoric acid and nitric acid contained in the etching solution react with the silicon wafer to generate water, and in addition, water adhering to the surfaces of the silicon wafer and the etching jig used to hold the silicon wafer during etching during cleaning in the pre-etching process can be mixed into the etching solution, causing the moisture content of the etching solution to increase with each increase in the number of etching processes (batch count).In particular, it is known that the glossiness of silicon wafers after etching changes depending on the moisture content of the etching solution, and if the moisture content of the etching solution increases with each increase in the number of etching batch counts, the glossiness of the silicon wafers after etching also changes accordingly, resulting in a problem of reduced quality of the silicon wafers.

[0006] This problem is particularly likely to occur when the etching amount per silicon wafer is small. This is because, when the etching amount per wafer is small, the amount of silicon wafer dissolved in the etching solution is smaller than when the etching amount per wafer is large. Consequently, the amount of etching solution used and the amount of mixed acid injected into the etching solution are also reduced. On the other hand, the amount of water adhering to and contaminating the silicon wafers and etching jigs remains almost constant even when the etching amount changes, assuming the same amount of silicon wafers per batch (number and size of silicon wafers). Therefore, the moisture content of the etching solution tends to increase overall. Furthermore, as the moisture content of the etching solution increases, the glossiness of the silicon wafers decreases, resulting in a deterioration in the quality of the silicon wafers.

[0007] Therefore, in the past, when the etching amount per wafer was small, before the moisture content of the etching solution became high (after performing the etching process a predetermined number of batches), most of the etching solution was replaced with a newly prepared mixed acid, and a dressing operation was performed to dissolve a predetermined amount of Si without introducing moisture from the silicon wafer or etching tool, thereby preventing the change in moisture content of the etching solution from exceeding a certain value and suppressing the change in gloss, thereby preventing a decrease in the quality of the silicon wafer. However, since performing the dressing operation periodically results in production loss and leads to increased costs, there has been a need for a method of manufacturing silicon wafers that suppresses changes in gloss after etching even when the etching amount per wafer is small, even without performing the dressing operation.

[0008] The present invention aims to provide a semiconductor wafer manufacturing method and etching solution that can suppress an increase in the moisture content of the etching solution without dressing, suppress changes in the glossiness of the silicon wafer that changes depending on the moisture content of the etching solution, and stabilize quality, even when the etching amount per wafer is small, for example, less than 20 μm. [Means for solving the problem]

[0009] The present invention is summarized as a method for manufacturing a semiconductor wafer according to any one of the following (1) to (9). (1) A method for manufacturing a semiconductor wafer made of single crystal silicon, comprising an etching step of performing etching using an etching solution containing a mixed acid obtained by mixing at least hydrofluoric acid and nitric acid, wherein in the etching step, a portion of the used etching solution is discharged and new mixed acid is injected into the etching solution to adjust the composition of the mixed acid in the etching solution, and the mixed acid has a water (H2O) content of 32 mass% or less. In the etching step, the amount of the etching solution used and the amount of the mixed acid added are determined based on the amount of silicon dissolved by etching, and in the etching step, the semiconductor wafer is continuously etched by less than 20 μm. A method for manufacturing semiconductor wafers. (2) A method for manufacturing a semiconductor wafer made of single crystal silicon, comprising an etching step of performing etching using an etching solution containing a mixed acid obtained by mixing at least hydrofluoric acid and nitric acid, wherein in the etching step, a part of the used etching solution is discharged and the mixed acid is newly injected into the etching solution to adjust the composition of the mixed acid in the etching solution, and the mixed acid contains water (H 2 0) content is 32 mass% or less, and in the etching step, the discharge amount of the etching solution used and the amount of the mixed acid to be added are determined based on the amount of silicon dissolved by etching, and a process of etching the semiconductor wafer to 20 μm or more and a process of etching the semiconductor wafer to less than 20 μm are performed using the etching solution. ( 3 ) In the mixed acid, the content of hydrogen fluoride (HF) is 13 mass% or more, or (2) 10. A method for manufacturing a semiconductor wafer according to claim 9. ( 4 ) The mixed acid is prepared by mixing hydrofluoric acid having a concentration of 55% by weight or more and nitric acid having a concentration of 70% by weight or more. or (2) 10. A method for manufacturing a semiconductor wafer according to claim 9. ( 5 ) The mixed acid has a mass ratio of hydrogen fluoride (HF) to nitric acid (HNO3) of 1:2 to 1:5. or (2) 10. A method for manufacturing a semiconductor wafer according to claim 9. ( 6 ) The mixed acid further contains acetic acid (CH3COOH), and the mass ratio of hydrogen fluoride (HF) and nitric acid (HNO3) to acetic acid (CH3COOH) is 3:1 to 7:1, or (2) 10. A method for manufacturing a semiconductor wafer according to claim 9. ( 7 (1) The amount of the etching solution used and the amount of the mixed acid injected are determined so that the amount of silicon wafer dissolved in the etching solution is 10 to 20 g / L. or (2) 10. A method for manufacturing a semiconductor wafer according to claim 9. ( 8) The water content in the etching solution is maintained at 27 mol / L or less. or (2) 10. A method for manufacturing a semiconductor wafer according to claim 9. [Effects of the Invention]

[0010] According to the present invention, by using a mixed acid to be injected into the etching solution containing 32 mass % or less of water (H2O), even when the etching amount per wafer is small, for example, less than 20 μm, it is possible to suppress an increase in the moisture content of the etching solution without performing a dressing operation, by simply discharging a portion of the used etching solution and injecting new mixed acid into the etching solution in accordance with the amount of silicon dissolved in the etching process, thereby adjusting the concentration of the mixed acid in the etching solution, and as a result, it is possible to suppress changes in the glossiness of the silicon wafer, which changes depending on the moisture content of the etching solution, and stabilize the quality. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a graph showing the results of a simulation of the transition of the moisture content of an etching solution when etching is performed using a conventional mixed acid. [Figure 2] 1 is a graph for explaining the relationship between the moisture content of an etching solution and the glossiness of a silicon wafer after etching. [Figure 3] 1 is a graph showing the results of a simulation of the transition of the moisture content of an etching solution when etching is performed using the mixed acid of an example. [Figure 4] 1 is a flowchart showing an etching process according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of a semiconductor wafer manufacturing method according to the present invention will be described below with reference to the drawings. Silicon wafers are generally manufactured through the following steps: (1) a slicing process in which a single-crystal silicon ingot is sliced ​​into silicon wafers; (2) a chamfering process in which the side periphery of the silicon wafer is ground to adjust the diameter, chamfer shape, and orientation flat length; (3) a lapping process in which the main surface of the silicon wafer is polished to make the silicon wafer uniform; (4) an etching process in which the silicon wafer is immersed in an etching solution to remove processing strain caused by the lapping process; (5) a heat treatment process in which the silicon wafer is heated to eliminate oxygen donors present in the silicon wafer and stabilize the resistivity of the silicon wafer; and (6) a polishing process in which the main surface of the silicon wafer is polished with high precision to increase the flatness of the main surface of the silicon wafer. The semiconductor wafer manufacturing method according to this embodiment is characterized by the etching process (3) among the above silicon wafer manufacturing processes, more specifically, the wet etching process in which acid etching is performed using an etching solution.

[0013] Specifically, in a conventional wet etching process, a mixed acid containing hydrofluoric acid and nitric acid is prepared, and a certain amount of silicon is dissolved in this mixed acid to prepare an etching solution, and silicon wafers are immersed in this etching solution to etch the silicon wafers. Furthermore, for each batch, a portion of the used etching solution is discharged and new mixed acid is added to the etching solution, and the etching process is performed while controlling so that the amount of silicon dissolved in the etching solution is constant and the concentrations of hydrogen fluoride and nitric acid in the etching solution are constant.

[0014] In this embodiment, the mixed acid added to the etching solution is characterized by having a lower water (H2O) content than conventional mixed acids. Specifically, as shown in Table 4 below, the mixed acid according to this embodiment is characterized by having a water (H2O) ratio of 32 mass% or less. An example of a conventional mixed acid and an example of a mixed acid according to this embodiment will be described below. Table 1 below shows an example of the composition of a conventional mixed acid, and Table 2 below shows an example of the composition of a mixed acid according to this embodiment. Table 3 below shows the mass ratio of each compound in the mixed acid shown in Table 1 below, and Table 4 below shows the mass ratio of each compound in the mixed acid shown in Table 2 below. [Table 1] [Table 2] [Table 3] [Table 4]

[0015] As shown in Tables 1 and 2, the conventional mixed acid contains 29 volume percent hydrofluoric acid with a concentration of 50 mass percent, 56 volume percent nitric acid with a concentration of 70.5 mass percent, 13 volume percent acetic acid with a concentration of 100% (almost 100%), and 2.93 volume percent water. Furthermore, 50 mass percent hydrofluoric acid is an aqueous solution containing 50 mass percent hydrogen fluoride (HF), and 70.5 mass percent nitric acid is an aqueous solution containing 70.5 mass percent nitric acid (HNO). In other words, half of the mass of 50 mass percent hydrofluoric acid is water (H2O), and similarly, 29.5% of the mass of 70.5 mass percent nitric acid is water (H2O). Therefore, in conventional mixed acids, the proportions of hydrogen fluoride (HF), nitric acid (HNO3), acetic acid (CH3COOH), and water (H2O) that make up the mixed acid are 12.9 mass% hydrogen fluoride (HF), 43.2 mass% nitric acid (HNO3), 10.5 mass% acetic acid (CH3COOH), and 33.3 mass% water (H2O). Note that in Table 1, the volume ratios of components other than water are rounded to the nearest whole number.

[0016] In contrast, as shown in Tables 3 and 4, an example (Example) of mixed acid according to this embodiment can be configured to contain 26% by volume of hydrofluoric acid with a concentration of 55% by mass, 58% by volume of nitric acid with a concentration of 70.5% by mass, 14% by volume of acetic acid with a concentration of 100% (almost 100%), and 0.01% by volume of water. In this case, the mixed acid of the Example contains 13.4% by mass of hydrogen fluoride (HF), 44.9% by mass of nitric acid (HNO), 10.9% by mass of acetic acid (CHCOOH), and 30.8% by mass of water (HO).

[0017] Tables 3 and 4 above are examples of mixed acids according to this embodiment, and the present invention is not limited to these compositions. For example, the mixed acid according to this embodiment can be prepared so that the mass ratio of hydrogen fluoride (HF) to nitric acid (HNO3) is 1:2 to 1:5. The mixed acid according to this embodiment also contains acetic acid (CH3COOH), and can be prepared so that the mass ratio of hydrogen fluoride (HF) and nitric acid (HNO3) to acetic acid (CH3COOH) is 3:1 to 7:1. Furthermore, as shown in Table 3 above, a buffer such as acetic acid can be included, but the buffer is not limited to acetic acid, and any agent or chemical having buffering properties similar to acetic acid can be used.

[0018] As shown in Table 2, the water (H2O) content in the conventional mixed acid is 33 mass% or more, whereas the water (H2O) content in the mixed acid according to this embodiment is 32 mass% or less, as shown in Table 4. As a result, in this embodiment, as will be described below, the moisture content of the etching solution can be stabilized, and the gloss rate of the silicon wafer after etching can be stabilized according to the moisture content.

[0019] In the mixed acid according to this embodiment, the water (H2O) content may be 32% by mass or less, for example, 31% by mass or less, 30% by mass or less, or 29% by mass or less. On the other hand, the lower limit of the water (H2O) content is not particularly limited, but is preferably 28% by mass or more, and may be 29% by mass or more, or even 30% by mass or more. If the water (H2O) content in the mixed acid is less than 28% by mass, it becomes difficult to maintain a constant moisture content in the etching solution, which can lead to changes in the gloss, roughness, flatness, and other properties of the silicon wafer after etching. Such changes in the gloss, roughness, and flatness of the silicon wafer can lead to problems such as wafer detection failure due to changes in rear surface reflectance, abnormal heat transfer from the chuck due to changes in roughness, and pattern defects due to poor flatness. Furthermore, when the water (H2O) content in the mixed acid is low, less than 28% by mass, maintaining a constant moisture content in the etching solution requires a correspondingly increased amount of mixed acid replacement per batch, which increases the number of dressing operations and reduces the efficiency of silicon wafer production. However, even when the water (H2O) content in the mixed acid is less than 28% by mass, this problem can be suppressed by adding water to the etching solution in an amount equivalent to the amount of water in the mixed acid such that the water (H2O) content in the mixed acid is 28% by mass or more.

[0020] Here, factors that cause the moisture content of the etching solution to fluctuate will be explained. Factors that cause the moisture content of the etching solution to increase include the following factors (A) to (C). (A) Water generation by chemical reaction between silicon wafer and mixed acid (B) Water that adheres to and gets mixed into silicon wafers and etching jigs during cleaning in the previous process (C) Injection of mixed acid containing water into etching solution Furthermore, factors that reduce the moisture content of the etching solution include the following factor (D). (D) Discharge of etching solution containing water The moisture content of the etching solution may decrease due to evaporation of water contained in the etching solution, but this will not be discussed in this embodiment because the change in moisture content due to evaporation is slight. The above factors (A) to (D) will be explained below.

[0021] (A) Water generation by chemical reaction between silicon wafer and mixed acid In the etching process, the silicon wafer is immersed in an etching solution containing a mixed acid containing hydrofluoric acid and nitric acid. When the silicon wafer is immersed in the etching solution, the silicon wafer (Si) reacts with the nitric acid (HNO3) contained in the etching solution, as shown in the following formula (1), and is converted into silicon dioxide (SiO2), and water (2H2O) is generated. Si+2HNO3→SiO2+NO2+NO+H2O…(1) In addition, silicon dioxide (SiO2) is converted into hexafluorosilicic acid (H2SiF6) by hydrogen fluoride (HF) contained in the etching solution, as shown in the following formula (2), and water (2H2O) is also produced during this process. SiO2+6HF→H2SiF6+2H2O…(2) Thus, in the etching process of a silicon wafer, 3 moles of water (H2O) are produced per mole of silicon (Si).

[0022] (B) Water adhering to and contaminating silicon wafers and their jigs during cleaning in the previous process In this embodiment, a silicon wafer lapping process is performed as a pre-etching process. In the lapping process, silicon wafers are lapped using a lapping device, and then multiple lapped silicon wafers are placed in a designated jig and cleaned all at once. In the etching process of this embodiment, the cleaned silicon wafers are transferred to an etching jig and immersed in an etching solution, thereby etching the silicon wafers. Therefore, when immersed in the etching solution, water droplets adhere to the surfaces of the silicon wafers and the etching jig. When the silicon wafers and the etching jig are immersed in the etching solution, the water droplets adhering to the silicon wafers and the etching jig become mixed into the etching solution. Note that if the number and size of silicon wafers per batch are constant, the amount of water adhering to and mixed into the silicon wafers and the etching jig will be approximately constant. Note that if the silicon wafers are completely dried before being immersed in the etching solution, "burns," "uneven etching," and "a coating that is insoluble in hydrofluoric acid" may occur on the main surfaces of the silicon wafers. Therefore, the etching process is generally performed while the silicon wafers are wet.

[0023] (C) Injection of mixed acid containing water into etching solution During etching, reactants such as silicon dioxide and hexafluorosilicic acid increase in the etching solution, while the mixed acid is consumed and reduced. Therefore, in this embodiment, a process is performed in which a portion of the etching solution is discharged and new mixed acid according to this embodiment is injected for each batch process, depending on the amount of silicon dissolved by etching (which may be the amount of reactants generated). As described above, the mixed acid injected into the etching solution contains water, and injecting the mixed acid into the etching solution increases the moisture content of the etching solution by the amount of water contained in the mixed acid. Here, the amount of mixed acid (amount of water) injected into the etching solution varies depending on the etching amount. The greater the etching amount, the greater the amount of mixed acid used in the etching solution, and therefore the greater the amount of mixed acid injected into the etching solution.

[0024] The mixed acid injected into the etching solution contains water, and reducing the amount of water in the mixed acid can reduce the moisture content of the etching solution. Therefore, while conventional mixed acids have a water content of 33% by mass or more, the mixed acid according to this embodiment has a water content of 32% by mass or less. The water contained in the mixed acid can be divided into (a) water already contained in the chemicals, such as hydrofluoric acid and nitric acid, and (b) water added during the preparation of the mixed acid (water not contained in the chemicals). In this embodiment, the amount of water in (a) is reduced by using hydrofluoric acid and nitric acid with higher concentrations than conventional ones, and / or the amount of water in (b) is reduced during the preparation of the mixed acid, thereby reducing the water content in (a) to 32% by mass or less. Specifically, while conventionally, hydrofluoric acid with a concentration of 50% by mass and nitric acid with a concentration of 70.5% by mass are generally used, in this embodiment, the amount of water in (a) can be reduced by using hydrofluoric acid with a concentration of 55% by mass or more and nitric acid with a concentration of 75% by mass or more. Furthermore, in the past, the amount of water (b) added to the mixed acid was 3% by volume or more of the total mixed acid, but in this embodiment, the amount of water (b) added to the mixed acid is set to 2% by volume or less, thereby adjusting the water content in the total mixed acid to 32% by mass or less.

[0025] (D) Discharge of etching solution containing water As described above, in the etching process, for each batch, a portion of the etching solution is discharged and the mixed acid according to the present embodiment is injected, depending on the amount of silicon dissolved by etching. The discharged etching solution contains not only the water originally contained in the mixed acid (C) above, but also water generated by the reaction between the silicon wafer and the mixed acid (A) above, and water adhering to the silicon wafer or the etching jig from the previous process (B) above and mixed in. Therefore, when a portion of the etching solution is discharged, a portion of the water from (A) to (C) is also discharged, and the moisture content of the etching solution decreases accordingly.

[0026] The moisture content of the etching solution is maintained constant and stabilized when the increase in the moisture content of the etching solution due to the above (A) to (C) and the decrease in the moisture content of the etching solution due to the above (D) are the same. Figure 1 is a graph showing the results of a simulation of the change in the moisture content of the etching solution when a silicon wafer is etched multiple times using a conventional mixed acid. Figure 1 also shows the moisture content (mol / L) of the etching solution when the etching depth of the silicon wafer is X1 (X1≧20 μm) μm and when the etching depth of the silicon wafer is X2 μm (X1>X2, X2<20 μm). In the example shown in FIG. 1, the simulation was performed for the cases where the etching amount of the silicon wafers was X1 μm and X2 μm, with the number of silicon wafers per batch, the size (diameter) of the silicon wafers, the amount of water mixed into the etching solution by (B), and the amount of silicon dissolved in the etching solution before the start of etching being the same, while the discharge amount of the etching solution and the injection amount of mixed acid per batch were set to amounts proportional to the etching amount.

[0027] As a result of the simulation, as shown in FIG. 1, whether the etching depth is X1 μm or X2 μm, the moisture content of the etching solution initially increases and then converges to a constant moisture content as the number of batches increases. This is because, before the start of etching, the moisture content of the etching solution is the same as the moisture content of the mixed acid being injected. Therefore, the amount of water lost due to (D) above and the amount of water increased due to (C) above are approximately the same. As a result, the amount of water in the etching solution increases by the amount of water increased due to (A) and (B) above. On the other hand, as the number of batches increases, the moisture content of the etching solution increases (the moisture content of the etching solution becomes higher than the moisture content of the mixed acid being injected). As a result, even with the same discharge amount as at the beginning of etching, the amount of water lost as the etching solution is discharged increases. Gradually, the difference between the amount of water lost in the etching solution due to (D) above and the amount of water increased in the etching solution due to (A) to (C) above becomes smaller, and finally, the moisture content converges to a predetermined value.

[0028] In the example shown in FIG. 1 , comparing the change in moisture content of the etching solution when the silicon wafer etching depth is X1 μm with the change in moisture content of the etching solution when the silicon wafer etching depth is X2 μm, when the etching depth is less than 20 μm (X2 μm), the moisture content of the etching solution converges higher than when the etching depth is large (X1 μm), and the time required for the moisture content of the etching solution to stabilize is also longer (the number of batches is also larger). This is because when the etching depth is small, the amount of silicon dissolved in the etching solution is smaller than when the etching depth is large, so the amount of etching solution used is less, and the amount of water lost due to (D) above is also smaller. As a result, in order to balance the increase in the moisture content of the etching solution due to (A) to (C) above and the decrease in the moisture content of the etching solution due to (D) above, the moisture content of the etching solution needs to be higher, and the moisture content converges accordingly. Furthermore, the higher the moisture content of the etching solution that converges, the more batches are required to reach that moisture content, and the longer the change in moisture content of the etching solution continues. Thus, when the amount of silicon wafer etched is small, the moisture content of the etching solution that converges deviates significantly from the moisture content of the initial etching solution, and a long time is required for the moisture content to converge (a large number of batches), resulting in the following problems: the change in gloss of the silicon wafer that depends on the moisture content of the etching solution becomes large, and it also takes a long time for the gloss to stabilize.

[0029] Here, referring to FIG. 2, the relationship between the moisture content of the etching solution and the glossiness after etching will be described. FIG. 2 is a graph showing the relationship between the moisture content of the etching solution and the glossiness of the silicon wafer after etching. Furthermore, the example shown in FIG. 2(A) shows the change in moisture content and the change in glossiness in the etching solution when etching is performed using the conventional mixed acid shown in Tables 1 and 2 above, with a small etching amount of less than 20 μm, X2 μm. Furthermore, the example shown in FIG. 2(B) shows the change in moisture content and the change in glossiness in the etching solution when etching is performed using the mixed acid according to the present embodiment shown in Tables 3 and 4 above, with a small etching amount of less than 2 μm, X2 μm. Furthermore, in the example shown in FIG. 2(A), since the moisture content of the etching solution increases with the number of batches, a dressing operation is performed every 40 batches, in which most or part of the etching solution is replaced with a new etching solution.

[0030] As shown in Figure 2(A), when etching is performed using a conventional mixed acid at a small etching amount of X2 μm, the moisture content of the etching solution increases as the number of batch processing of silicon wafers increases, and the gloss of the silicon wafer after etching decreases accordingly. In contrast, as shown in Figure 2(B), when the mixed acid according to this embodiment is used, even when etching is performed at a small etching amount of X2 μm, the moisture content of the etching solution remains almost constant, and the gloss of the silicon wafer also remains almost constant, thereby stabilizing the quality of the silicon wafer after etching.

[0031] As described above, the gloss level after etching varies depending on the moisture content of the etching solution, and therefore, when a conventional mixed acid is used, if the etching amount is small, the moisture content of the etching solution changes significantly as etching is repeated, resulting in unstable silicon wafer quality. Furthermore, conventionally, in order to maintain the gloss level of the silicon wafer within a certain range, a dressing operation has been performed in which most or part of the etching solution is replaced with a newly prepared mixed acid, but performing the dressing operation periodically has the problem of causing production losses and increasing costs.

[0032] In contrast, in this embodiment, by setting the moisture content in the mixed acid to 32 mass % or less, the etching solution can converge at a low moisture content even when the etching amount is small, and the time until the moisture content of the etching solution converges can be shortened (the number of batches can be reduced), as shown in Fig. 3. Fig. 3 is a graph showing the results of a simulation of the transition of the moisture content (mol / L) of the etching solution when a silicon wafer is etched using the mixed acid having the composition according to this embodiment.

[0033] 3, the mixed acid according to this embodiment was a mixed acid having the composition shown in Table 3 and Table 4. Also in the example shown in Fig. 3, the simulation was performed under the same conditions as in the example shown in Fig. 1, including the number of silicon wafers to be etched per batch, the size of the silicon wafers, the amount of water adhering to and mixed in the silicon wafers and the etching jig from the previous process, the amount of silicon dissolved in the etching solution before the start of etching, the amount of etching solution discharged, and the amount of mixed acid injected (the amount injected corresponding to the etching amounts X1 and X2).

[0034] As shown in FIG. 3, when etching was performed using the mixed acid according to this embodiment with a small etching amount of X2 μm (less than 20 μm), the moisture content of the etching solution converged at a lower level than when etching was performed using a conventional mixed acid with the same etching amount of X2 μm. Specifically, when etching was performed using a conventional mixed acid with an etching amount of X2 μm, the moisture content of the etching solution converged at a higher level than when etching was performed with a large etching amount of X1 μm (20 μm or more). However, when etching was performed using the mixed acid according to this embodiment with an etching amount of X2 μm, the moisture content of the etching solution converged and remained low, similar to when etching was performed with an etching amount of X1 μm. This is because, in this embodiment, the mixed acid contains a low content of water (HO), so the increase in water content of the etching solution due to (C) above is small. As a result, when the moisture content of the etching solution is low, the increase in water content due to (A) to (C) above and the decrease in water content due to (D) above are balanced. Furthermore, when etching was performed using the mixed acid according to this embodiment at a small etching amount of X2 μm, the moisture content of the etching solution converged in fewer batches than when etching was performed using a conventional mixed acid at the same etching amount of X2 μm. Furthermore, in this embodiment, the moisture content is maintained low even when the etching amount is small compared to conventional methods, eliminating the need for a dressing operation and improving the efficiency of the etching process. Note that when the mixed acid according to this embodiment is used at a large etching amount of X1 μm, there is a possibility that moisture may be insufficient, so water may be added to the etching solution in advance.

[0035] Thus, it was found that by using the mixed acid according to this embodiment, the moisture content of the etching solution can be converged to a low value even when the etching amount is small, that is, less than 20 μm. In this embodiment, the change in the moisture content of the etching solution is small, and the time until the moisture content of the etching solution converges is short, so that the change in the gloss of the silicon wafer according to the moisture content of the etching solution is also small, and the gloss can be stabilized in a short time.

[0036] In this embodiment, the etching process can be performed by a known method except for changing the composition of the mixed acid. For example, the etching time, etching temperature, etc., depending on the target etching amount, can be the same as known conditions.

[0037] Next, the etching process according to this embodiment will be described in detail. Fig. 4 is a flowchart showing the etching process according to this embodiment. Before the etching process according to this embodiment is performed, the silicon wafer to be etched is cleaned and then transferred to a jig dedicated to etching.

[0038] As shown in FIG. 4, in step S101, first, a mixed acid to be used in the etching process according to this embodiment is prepared. In this embodiment, as shown in Tables 3 and 4 above, a mixed acid containing 32% by mass or less of water (H2O) is prepared using hydrofluoric acid with a concentration of 55% by mass or more and / or nitric acid with a concentration of 75% by mass or more. Note that the nitric acid used in this embodiment preferably has a lower concentration than fuming nitric acid with a concentration of 80% by mass or more (nitric acid with a concentration of less than 80% by mass). This is because fuming nitric acid with a concentration of 80% by mass or more may cause the container to break due to internal pressure caused by an increase in temperature, is highly corrosive, is difficult to handle, and may also reduce the surface roughness of the silicon wafer in micro-regions.

[0039] Furthermore, in step S102, an etching solution is prepared. Specifically, in this embodiment, before etching a silicon wafer, a certain amount of silicon is dissolved in the mixed acid prepared in step S101 to prepare an etching solution to be used in the etching process. The amount of silicon dissolved in the etching solution is not particularly limited and can be set appropriately based on the target quality, but in this embodiment, the etching solution can be prepared so that the amount of silicon dissolved in the etching solution is, for example, 10 to 20 g / L.

[0040] In step S103, a silicon wafer to be etched is immersed in the etching solution prepared in step S102. The silicon wafer to be etched is lapped as a pre-etching step, then placed in an etching jig and washed with water along with the etching jig. After washing, the silicon wafer is immersed in the etching solution along with the etching jig. Therefore, the silicon wafer and the etching jig are immersed in the etching solution with water droplets adhering to their surfaces. This causes the water adhering to the silicon wafer and the etching jig to mix with the etching solution, increasing the moisture content of the etching solution. In step S103, the silicon wafer is immersed in the etching solution for a predetermined time corresponding to the target etching amount (thickness of the silicon wafer) so that the target etching amount (thickness of the silicon wafer) is etched. After the predetermined time has elapsed, the silicon wafer is removed from the etching solution and washed.

[0041] In step S104, a portion of the used etching solution is discharged. In this embodiment, the amount of etching solution to be discharged is determined based on the amount of silicon dissolved in the etching solution in step S103. Specifically, the amount of etching solution to be discharged is determined as shown in the following formula (1). Etching solution discharge volume = weight of silicon dissolved by etching (g) / target initial silicon concentration in etching solution (g / L) (1) For example, if the target initial concentration of silicon in the etching solution is 20 g / L and the amount of silicon dissolved by the etching process in step S103 is 40 g, 2 L of etching solution will be discharged. The amount of silicon dissolved by the etching process can be calculated from the etching time in step S103 (the time the silicon wafers are immersed in the etching solution), the number of silicon wafers per batch, etc.

[0042] In step S105, the mixed acid prepared in step S101 is injected into the etching solution. The amount of mixed acid injected in step S105 can be the same as the amount of etching solution discharged in step S104. This makes it possible to maintain the amount of silicon dissolved in the etching solution at a constant value, and also makes it possible to keep the water content (mol / L) in the etching solution at or below a certain value.

[0043] In step S106, it is determined whether the etching process has been completed. For example, if there are still silicon wafers remaining that have not been etched, the process returns to step S103, and the etching process is performed on the unetched silicon wafers. On the other hand, if etching of all silicon wafers to be etched has been completed, the etching process according to this embodiment ends. Note that when all etching has been completed, the etching liquid that has been used up until now may be replaced with a new etching liquid, or may be used as is for the next etching process.

[0044] Furthermore, if necessary, the etching process can be evaluated. For example, it can be configured to evaluate whether the silicon wafer has been etched by a target amount and whether the target glossiness has been achieved. If the etching evaluation shows that the amount of etching is insufficient, the process can be returned to step S103, and the silicon wafer can be immersed in the etching solution again to etch it.

[0045] As described above, the method for producing a silicon wafer according to this embodiment includes an etching step in which etching is performed using an etching solution containing a mixed acid of at least hydrofluoric acid and nitric acid, and in this etching step, a portion of the used etching solution is discharged and new mixed acid is added to the etching solution to adjust the concentration of the mixed acid in the etching solution, and the mixed acid contains 33 mass% or less of water (H2O). This allows the etching process to be performed without performing a dressing operation not only when etching a silicon wafer to a depth of 20 μm or more, but also when etching a silicon wafer to a depth of less than 20 μm, and also prevents a decrease in the gloss of the etched silicon wafer, making it possible to produce high-quality silicon wafers.

[0046] Although the amount of change in the moisture content of the etching solution can be suppressed by increasing the moisture content of the etching solution in advance, if the moisture content of the etching solution is high, the glossiness decreases, and the desired quality may not be obtained. In this embodiment, the etching solution can be maintained at a low moisture content, so that silicon wafers with high glossiness can be continuously manufactured.

[0047] Although the preferred embodiments of the present invention have been described above, the technical scope of the present invention is not limited to the above-described embodiments. Various modifications and improvements can be made to the above-described embodiments, and such modifications and improvements are also included in the technical scope of the present invention.

[0048] For example, in the above-described embodiment, a configuration was illustrated in which hydrofluoric acid with a concentration of 55% by mass or more and / or nitric acid with a concentration of 75% by mass or more was used, and the water (H2O) content in the mixed acid was set to 32% by mass or less, but the hydrofluoric acid used may have a concentration of 60% by mass or more, or even 65% by mass or more. Similarly, the nitric acid used may have a concentration of 80% by mass or more, or even 90% by mass or more.

Claims

1. A method for manufacturing a semiconductor wafer made of silicon single crystal, comprising: an etching step of performing etching using an etching solution containing a mixed acid of at least hydrofluoric acid and nitric acid; In the etching step, a part of the used etching solution is discharged and the mixed acid is newly injected into the etching solution, thereby adjusting the composition of the mixed acid in the etching solution; In the mixed acid, water (H ２ 0) content is 32% by mass or less, In the etching step, the amount of the etching solution to be discharged and the amount of the mixed acid to be added are determined based on the amount of silicon dissolved by etching; In the etching step, the semiconductor wafer is continuously etched by less than 20 μm.

2. A method for manufacturing a semiconductor wafer made of silicon single crystal, comprising: an etching step of performing etching using an etching solution containing a mixed acid of at least hydrofluoric acid and nitric acid; In the etching step, a part of the used etching solution is discharged and the mixed acid is newly injected into the etching solution, thereby adjusting the composition of the mixed acid in the etching solution; In the mixed acid, the content of water (H 2 O) is 32 mass% or less, In the etching step, the amount of the etching solution to be discharged and the amount of the mixed acid to be added are determined based on the amount of silicon dissolved by etching; A method for manufacturing a semiconductor wafer, comprising: using the etching solution to etch the semiconductor wafer by 20 μm or more; and using the etching solution to etch the semiconductor wafer by less than 20 μm.

3. 3. The method for producing a semiconductor wafer according to claim 1, wherein the mixed acid contains hydrogen fluoride (HF) at a content of 13 mass % or more.

4. 3. The method for manufacturing a semiconductor wafer according to claim 1, wherein the mixed acid is prepared using hydrofluoric acid having a concentration of 55% by weight or more and / or nitric acid having a concentration of 75% by weight or more.

5. The hydrogen fluoride (HF) and nitric acid (HNO ３ 3. The method for producing a semiconductor wafer according to claim 1, wherein the mass ratio of the compound (II) to the compound (II) is 1:2 to 1:

5.

6. The mixed acid is acetic acid (CH ３ COOH), hydrogen fluoride (HF) and nitric acid (HNO ３ ) and acetic acid (CH ３ 3. The method for producing a semiconductor wafer according to claim 1, wherein the mass ratio of the hydroxybenzoate to the hydroxybenzoate (COOH) is 3:1 to 7:

1.

7. 3. The method for manufacturing a semiconductor wafer according to claim 1, wherein the discharge amount of the used etching solution and the injection amount of the mixed acid are determined so that the amount of silicon dissolved in the etching solution is 10 to 20 g / L.

8. 3. The method for producing a semiconductor wafer according to claim 1, wherein the moisture content in the etching solution is maintained at 27 mol / L or less.

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