Semiconductor wafer manufacturing method

Abstract

The present invention provides a semiconductor wafer manufacturing method and etching solution that can suppress the increase in the moisture content of the etching solution without dressing, even when the etching amount per wafer is small, for example, less than 20 μm, thereby suppressing changes in the glossiness of the silicon wafer which vary depending on the moisture content of the etching solution, and stabilizing the quality. [Solution] A method for manufacturing a semiconductor wafer made of a silicon single crystal, comprising an etching step in which etching is performed using an etching solution containing a mixed acid which is a mixture 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 water (H2O) content of the mixed acid is 32% by mass or less, the method for manufacturing a semiconductor wafer.

Description

Technical Field

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

Background Art

[0002] A semiconductor wafer made of a silicon single crystal (hereinafter referred to as a silicon wafer) is manufactured by slicing an ingot of single crystal silicon, flattening the sliced surface by lapping, and then performing an etching process of immersing it in an etching solution to remove lap distortion and impurities, and then performing polishing, cleaning, etc. In the above etching process, for example, as described in Patent Document 1, an etching solution containing 50% by mass of hydrofluoric acid, nitric acid, and water, and containing 50% or more of water by mass ratio is used for etching. In addition, JISH0609:1994 describes using hydrofluoric acid with a concentration of 49 to 50% as an etching solution for a silicon wafer, and also describes that the mass ratio of water in the etching solution is 33% or more.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the etching process, generally, to etch a silicon wafer to a target etching amount (the thickness of the silicon wafer to be dissolved in the etching process), multiple silicon wafers are treated as one batch and immersed in the etching solution for a predetermined time corresponding to the etching amount. After the predetermined time has elapsed, the etched silicon wafer is removed from the etching solution, and then the etching process is carried out continuously by repeating the process of immersing new silicon wafers (multiple silicon wafers for one batch) in the etching solution. When etching is carried out continuously in this way, the silicon wafer immersed in the etching solution reacts with the hydrofluoric acid and nitric acid contained in the etching solution, increasing the amount of reactants and decreasing the amount of hydrofluoric acid and nitric acid in the etching solution. Therefore, for each batch, a portion of the etching solution is drained according to the amount of silicon wafer dissolved, and a new mixed acid containing hydrofluoric acid and nitric acid is injected.

[0005] Furthermore, during the etching process, in addition to the reaction of hydrofluoric acid and nitric acid contained in the etching solution with the silicon wafer to produce water, water adhering to the surface of the silicon wafer and the etching jig used to hold the silicon wafer during etching due to cleaning in the pre-etching process can mix with the etching solution. As a result, the water content of the etching solution can increase with each etching cycle (batch cycle). In particular, it is known that the glossiness of the silicon wafer after etching changes depending on the water content of the etching solution. Therefore, if the water content of the etching solution increases with each etching batch cycle, the glossiness of the silicon wafer after etching will also change, leading to a decrease in the quality of the silicon wafer.

[0006] In particular, the above problem tends 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 compared to when the etching amount per wafer is large, and consequently, the amount of etching solution discharged and the amount of mixed acid injected into the etching solution are also smaller. On the other hand, the amount of water adhering to and mixed into the silicon wafer and etching jig remains almost the same even if the etching amount changes, provided that the amount of silicon wafers per batch (number and size of silicon wafers) is the same. Therefore, the overall moisture content of the etching solution tends to increase. And as the moisture content of the etching solution increases, the glossiness of the silicon wafer decreases, and the quality of the silicon wafer deteriorates.

[0007] Therefore, conventionally, when the etching amount per wafer is small, before the moisture content of the etching solution becomes high (after performing the etching process a predetermined number of times), a dressing process is performed in which a predetermined amount of Si is dissolved in the etching solution without the introduction of moisture from the silicon wafer or etching-specific jigs. This prevents the change in the moisture content of the etching solution from exceeding a certain value, thereby suppressing the change in gloss and preventing a deterioration in the quality of the silicon wafer. However, performing the dressing process regularly results in production losses and increases costs. Therefore, there has been a demand for a silicon wafer manufacturing method that suppresses changes in gloss after etching without the need for dressing when the etching amount per wafer is small.

[0008] The present invention aims to provide a method for manufacturing semiconductor wafers and an etching solution that can stabilize quality by suppressing the increase in the moisture content of the etching solution and suppressing the change in glossiness of the silicon wafer, which varies depending on the moisture content of the etching solution, without performing a dressing procedure, 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 essentially a method for manufacturing a semiconductor wafer as described in any of the following (1) to (9). (1) A method for manufacturing a semiconductor wafer made of a silicon single crystal, comprising an etching step in which etching is performed using an etching solution containing a mixed acid which is a mixture 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 water (H2O) content of the mixed acid is 32% by mass or less, the method for manufacturing a semiconductor wafer. (2) The method for manufacturing a semiconductor wafer as described in (1) above, wherein the mixed acid contains 13% by mass or more of hydrogen fluoride (HF). (3) The method for manufacturing a semiconductor wafer as described in (1) above, wherein the mixed acid is prepared using hydrofluoric acid with a concentration of 55% by weight or more and nitric acid with a concentration of 70% or more. (4) The method for manufacturing a semiconductor wafer according to (1) above, wherein the ratio of hydrogen fluoride (HF) and nitric acid (HNO3) by mass in the mixed acid is 1:2 to 1:5. (5) The method for manufacturing a semiconductor wafer according to (1) above, wherein in the etching step, the amount of etching solution used and the amount of 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 to a thickness of less than 20 μm. (6) The method for manufacturing a semiconductor wafer as described in (1) above, wherein in the etching step, the amount of etching solution used and the amount of mixed acid added are determined based on the amount of silicon dissolved by etching, and the semiconductor wafer is etched to a thickness of 20 μm or more and the semiconductor wafer is etched to a thickness of less than 20 μm using the etching solution. (7) The method for manufacturing a semiconductor wafer according to (1) above, wherein the mixed acid further contains acetic acid (CH3COOH), and the ratio of hydrogen fluoride (HF) and nitric acid (HNO3) to acetic acid (CH3COOH) by mass is 3:1 to 7:1. (8) The method for manufacturing a semiconductor wafer according to (1) above, wherein the amount of etching solution used and the amount of mixed acid injected are determined so that the amount of silicon wafer dissolved in the etching solution is 10 to 20 g / L. (9) The method for manufacturing a semiconductor wafer as described in (1) above, wherein the moisture content of the etching solution is maintained at 27 mol / L or less. [Effects of the Invention]

[0010] According to the present invention, by making the mixed acid injected into the etching solution contain 32% by 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 the increase in the water content of the etching solution by simply draining a portion of the used etching solution and injecting new mixed acid into the etching solution according to the amount of silicon dissolved in the etching process, without performing a dressing procedure, thereby adjusting the concentration of the mixed acid in the etching solution. As a result, it is possible to suppress changes in the glossiness of the silicon wafer, which varies depending on the water content of the etching solution, and stabilize the quality. [Brief explanation of the drawing]

[0011] [Figure 1] This graph shows the simulation results of the change in the water content of the etching solution when etching is performed using conventional mixed acids. [Figure 2] This graph illustrates the relationship between the water content of the etching solution and the glossiness of the silicon wafer after etching. [Figure 3] This graph shows the simulation results of the change in the water content of the etching solution when etching is performed using the mixed acid described in the example. [Figure 4] This flowchart shows the etching process according to this embodiment. [Modes for carrying out the invention]

[0012] The following describes an embodiment of the semiconductor wafer manufacturing method according to the present invention with reference to the figures. A silicon wafer is generally manufactured through the following steps: (1) a slicing step in which a single-crystal silicon ingot is sliced ​​to cut out a silicon wafer; (2) a chamfering step in which the edges of the silicon wafer are ground to adjust the diameter, chamfer shape, orientation flat length, etc.; (3) a lapping step in which the main surface of the silicon wafer is polished to make the thickness of the silicon wafer uniform; (4) an etching step in which the silicon wafer is immersed in an etching solution to remove processing distortion caused by the lapping step; (5) a heat treatment step 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 step in which the main surface of the silicon wafer is polished with high precision to improve the flatness of the main surface of the silicon wafer. The semiconductor wafer manufacturing method according to this embodiment is characterized by the etching step in (3) above, more specifically, the wet etching step in which acid etching is performed using an etching solution, among the above silicon wafer manufacturing steps.

[0013] Specifically, in the 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. The silicon wafer is then immersed in this etching solution to etch it. In addition, for each batch, a portion of the used etching solution is drained, and fresh mixed acid is added to the etching solution. The etching process is carried out while controlling the amount of silicon dissolved in the etching solution to be constant, and the concentrations of hydrofluoric acid and nitric acid in the etching solution to be constant.

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

[0015] As shown in Tables 1 and 2, the conventional mixed acid contains 29% by volume of hydrofluoric acid with a concentration of 50% by mass, 56% by volume of nitric acid with a concentration of 70.5% by mass, 13% by volume of acetic acid with a concentration of 100% (almost 100%), and 2.93% by volume of water. Further, hydrofluoric acid with a concentration of 50% by mass is an aqueous solution containing 50% by mass of hydrogen fluoride (HF), and nitric acid with a concentration of 70.5% by mass is an aqueous solution containing 70.5% by mass of nitric acid (HNO3). That is, half of the mass of hydrofluoric acid with a concentration of 50% by mass is water (H2O), and similarly, 29.5% of the mass of nitric acid with a concentration of 70.5% by mass is water (H2O). Therefore, in the conventional mixed acid, the ratios of hydrogen fluoride (HF), nitric acid (HNO3), acetic acid (CH3COOH), and water (H2O) constituting the mixed acid are 12.9% by mass for hydrogen fluoride (HF), 43.2% by mass for nitric acid (HNO3), 10.5% by mass for acetic acid (CH3COOH), and 33.3% by mass for water (H2O). In Table 1, the volume ratios other than water are shown by rounding off the decimal places.

[0016] In contrast, as shown in Tables 3 and 4, an example of the mixed acid according to this embodiment (Example) 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 proportions of hydrogen fluoride (HF), nitric acid (HNO3), acetic acid (CH3COOH), and water (H2O) in the mixed acid of the Example are 13.4% by mass of hydrogen fluoride (HF), 44.9% by mass of nitric acid (HNO3), 10.9% by mass of acetic acid (CH3COOH), and 30.8% by mass of water (H2O).

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

[0018] As shown in Table 2 above, conventional mixed acids have a water (H2O) content of 33% by mass or more, whereas, as shown in Table 4 above, the mixed acid according to this embodiment has a water (H2O) content of 32% by mass or less. As a result, in this embodiment, as will be explained below, the water content of the etching solution can be stabilized, and the glossiness of the silicon wafer after etching can be stabilized according to the water 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 it is preferably 28% by mass or more, can be 29% by mass or more, and can even be 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 water content in the etching solution, and consequently, the glossiness, roughness, flatness, etc. of the silicon wafer after etching tend to change. If the glossiness, roughness, and flatness of the silicon wafer tend to change, there is a risk of wafer detection failure due to changes in back surface reflectivity, abnormal heat transfer from the chuck due to changes in roughness, and pattern defects due to deterioration of 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 larger amount of mixed acid replacement per batch, increasing the number of dressing operations and thus reducing the manufacturing efficiency of silicon wafers. However, even when the water (H2O) content in the mixed acid is less than 28% by mass, this problem can be mitigated by adding water to the etching solution equivalent to the amount of water that would result in a water (H2O) content of 28% or more in the mixed acid.

[0020] Here, we will explain the factors that cause the moisture content of the etching solution to fluctuate. Factors that increase the moisture content of the etching solution include the following (A) to (C). (A) Production of water by chemical reaction between silicon wafer and mixed acid (B) Water that adheres to and mixes with silicon wafers and etching jigs due to cleaning in the previous process. (C) Injection of mixed acid containing water into etching solution Furthermore, the following factors (D) can be cited as causes for a decrease in the water content of the etching solution. (D) Discharge of etching solution containing water Although factors such as the evaporation of water contained in the etching solution can also reduce the moisture content of the etching solution, the change in moisture content due to evaporation is negligible and therefore will be omitted in this embodiment. The elements (A) to (D) described above will be explained below.

[0021] (A) Production of water by chemical reaction between silicon wafer and mixed acid In the etching process, the silicon wafer is etched by immersing it in an etching solution containing a mixed acid including hydrofluoric acid and nitric acid. When the silicon wafer is immersed in the etching solution, as shown in equation (1) below, the silicon wafer (Si) reacts with the nitric acid (HNO3) contained in the etching solution to change into silicon dioxide (SiO2), and water (2H2O) is produced in the process. Si+2HNO3→SiO2+NO2+NO+H2O…(1) Furthermore, as shown in formula (2) below, silicon dioxide (SiO2) is converted to hexafluorosilicic acid (H2SiF6) by hydrogen fluoride (HF) contained in the etching solution, and water (2H2O) is also produced in this process. SiO2 + 6HF → H2SiF6 + 2H2O…(2) Thus, in the silicon wafer etching process, 3 moles of water (H2O) are generated for every 1 mole of silicon (Si).

[0022] (B) Water that adheres to the silicon wafer or its fixtures and becomes mixed in due to cleaning in the previous process. In this embodiment, a silicon wafer wrapping process is performed as a pre-etching step. In the wrapping process, silicon wafers are wrapped using a wrapping device, and then multiple wrapped silicon wafers are placed in a predetermined jig and washed all at once. In the etching process of this embodiment, the washed silicon wafers are transferred to an etching jig and immersed in an etching solution to etch the silicon wafers. Therefore, when immersed in the etching solution, water droplets adhere to the surfaces of the silicon wafers and the etching jig, and 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 mix into the etching solution. If the number and size of silicon wafers per batch are constant, the amount of water adhering to and mixing with the silicon wafers and the etching jig will be approximately constant. If the silicon wafers are completely dried before immersing them in the etching solution, "burning," "etching unevenness," and "films that do not dissolve with hydrofluoric acid" may occur on the main surface of the silicon wafers. Therefore, generally, the etching process is performed with the silicon wafers in a wet state.

[0023] (C) Injection of mixed acid containing water into etching solution When etching is performed, the amount of reactants such as silicon dioxide and hexafluorosilicic acid in the etching solution increases, while the amount of mixed acid consumed decreases. Therefore, in this embodiment, for each batch process, a portion of the etching solution is discharged and new mixed acid according to this embodiment is injected, depending on the amount of silicon dissolved by etching (or the amount of reactants generated). As mentioned above, the mixed acid injected into the etching solution contains water, so when mixed acid is injected into the etching solution, the water content of the etching solution increases by the amount of water contained in the mixed acid. Here, the amount of mixed acid (amount of water) injected into the etching solution changes according to the amount of etching, and the more etching is done, the more mixed acid is used in the etching solution, and therefore the more mixed acid is injected into the etching solution.

[0024] Here, the mixed acid injected into the etching solution contains water, and by reducing the amount of water contained in the mixed acid, the water content of the etching solution can be reduced. For this reason, in conventional mixed acids, the water content in the entire mixed acid was 33% by mass or more, but in the mixed acid according to this embodiment, the water content in the entire mixed acid is 32% by mass or less. The water contained in the mixed acid can be divided into (a) water that is already contained in chemicals such as hydrofluoric acid and nitric acid, and (b) water added when preparing the mixed acid (water not contained in the chemicals). In this embodiment, the amount of water in (a) above is reduced by using hydrofluoric acid or nitric acid at a higher concentration than conventional methods, and / or by reducing the amount of water in (b) above when preparing the mixed acid, thereby reducing the water content in the entire mixed acid to 32% by mass or less. Specifically, conventionally, hydrofluoric acid with a concentration of 50% by mass or nitric acid with a concentration of 70.5% by mass is generally used, but in this embodiment, the amount of water in (a) above can be reduced by using hydrofluoric acid with a concentration of 55% by mass or more, or nitric acid with a concentration of 75% by mass or more. Furthermore, while conventionally the amount of water (b) added to the mixed acid was 3% by volume or more of the total mixed acid, in this embodiment, the amount of water (b) added to the mixed acid is 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 in proportion to the amount of silicon dissolved by etching, and the mixed acid according to this embodiment is injected. The discharged etching solution contains not only the water originally contained in the mixed acid in (C) above, but also water generated by the reaction between the silicon wafer and the mixed acid in (A) above, and water that has been adhering to the silicon wafer and etching jig from the preceding process in (B) above and has been mixed in. Therefore, by discharging a portion of the etching solution, some of the water from (A) to (C) is also discharged, and the water content of the etching solution decreases accordingly.

[0026] The moisture content in the etching solution is maintained and stabilized when the increase in moisture content due to (A) to (C) above equals the decrease in moisture content due to (D) above. Here, Figure 1 is a graph showing the simulation results of the change in 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 amount of the silicon wafer is X1 (X1 ≥ 20 μm) μm and when the etching amount of the silicon wafer is X2 μm (X1 > X2, X2 < 20 μm). In the example shown in Figure 1, simulations were performed for two cases: one where the etching amount of the silicon wafer was X1 μm, and another where it was X2 μm. The number of silicon wafers per batch, the size (diameter) of the silicon wafer, the amount of water mixed into the etching solution according to (B), and the amount of silicon dissolved in the etching solution before etching began were kept the same under these conditions. However, the amount of etching solution discharged and the amount of mixed acid injected per batch were set to be proportional to the etching amount.

[0027] The simulation results, as shown in Figure 1, show that in both cases where the etching amount is X1 μm and X2 μm, the water content of the etching solution initially increases, and then converges to a constant water content as the number of batches increases. This is because, before etching begins, the water content of the etching solution is the same as the water content of the injected mixed acid, so the decrease in water due to (D) above and the increase in water due to (C) above are almost the same, and as a result, the amount of water in the etching solution increases by the amount of water increased by (A) and (B) above. On the other hand, as the number of batches increases, the water content of the etching solution increases (the water content of the etching solution becomes higher than the water content of the injected mixed acid), and as a result, even with the same discharge rate as at the beginning of etching, the amount of water that decreases with the discharge of the etching solution increases, and gradually the difference between the decrease in water in the etching solution due to (D) above and the increase in water in the etching solution due to (A) to (C) above becomes smaller, and finally converges to a predetermined water content.

[0028] Furthermore, in the example shown in Figure 1, comparing the change in the moisture content of the etching solution when the etching amount of the silicon wafer is X1 μm with the change in the moisture content of the etching solution when the etching amount of the silicon wafer is X2 μm, in the case of X2 μm, which is a small etching amount of less than 20 μm, the moisture content of the etching solution that converges is higher compared to the case of X1 μm, which is a large etching amount, and the time required for the moisture content of the etching solution to stabilize is also longer (the number of batches also increases). This is because, when the etching amount is small, the amount of silicon dissolved in the etching solution is small compared to when the etching amount is large, so the amount of etching solution used is reduced, and therefore the amount of water reduction due to (D) above is also small. As a result, in order for 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 to balance out, the moisture content of the etching solution needs to be higher, and therefore the converged moisture content is higher. Furthermore, the higher the moisture content of the etching solution as it converges, the more batches are required to reach that moisture content, and consequently, the longer the change in the moisture content of the etching solution persists. Thus, when the amount of silicon wafer etched is small, the moisture content of the etching solution as it converges deviates significantly from the initial moisture content of the etching solution, and the time required for the moisture content to converge is long (many batches are required). As a result, there is a problem in that the change in the glossiness of the silicon wafer in response to the moisture content of the etching solution becomes large, and the time required for the glossiness to stabilize also becomes long.

[0029] Here, referring to Figure 2, the relationship between the moisture content of the etching solution and the glossiness after etching will be explained. Figure 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 Figure 2(A) shows the change in moisture content of the etching solution and the change in glossiness when etching is performed at a small etching depth of less than 20 μm (X2 μm) using the conventional mixed acid shown in Tables 1 and 2 above. Furthermore, the example shown in Figure 2(B) shows the change in moisture content of the etching solution and the change in glossiness when etching is performed at a small etching depth of X2 μm using the mixed acid according to this embodiment shown in Tables 3 and 4 above. In addition, in the example shown in Figure 2(A), since the moisture content of the etching solution increases with each batch, 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 with a small etching amount of X2μm using a conventional mixed acid, the water content of the etching solution increases as the number of batch processing cycles of silicon wafers increases, and consequently, the glossiness of the silicon wafer after etching decreases. In contrast, as shown in Figure 2(B), when using the mixed acid according to this embodiment, the water content of the etching solution remains almost constant even when etching with a small etching amount of X2μm, so the glossiness of the silicon wafer also remains almost constant, and the quality of the silicon wafer after etching can be stabilized.

[0031] Thus, because the glossiness after etching changes depending on the water content of the etching solution, when using conventional mixed acids, if the etching amount is small, the water content of the etching solution changes significantly when etching is repeated, resulting in the problem of unstable silicon wafer quality. In addition, conventionally, in order to maintain a certain range of glossiness on silicon wafers, a dressing process was performed in which most or part of the etching solution was replaced with a newly prepared mixed acid. However, performing this dressing process regularly resulted in production losses and increased costs.

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

[0033] In the example shown in Figure 3, the mixed acid used according to this embodiment had the compositions shown in Tables 3 and 4. Furthermore, in the example shown in Figure 3, the simulation was performed under the same conditions as in the example shown in Figure 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 into the silicon wafers and etching jig from the previous process, the amount of silicon dissolved in the etching solution before etching began, the amount of etching solution discharged, and the amount of mixed acid injected (injection amount corresponding to etching amounts X1 and X2).

[0034] As shown in Figure 3, when etching was performed with a small etching amount of less than 20 μm (X2 μm) using the mixed acid according to this embodiment, the etching solution converged to a lower moisture content compared to when etching was performed with the same etching amount of X2 μm using the conventional mixed acid. Specifically, when etching was performed with a conventional mixed acid with an etching amount of X2 μm, the moisture content of the etching solution converged to a higher moisture content than when etching was performed with a larger etching amount of 20 μm or more (X1 μm), whereas when etching was performed with the mixed acid according to this embodiment with an etching amount of X2 μm, the moisture content of the etching solution converged to and was maintained at a low level, similar to when etching was performed with an etching amount of X1 μm. This is because, in this embodiment, the water (H2O) content in the mixed acid is low, so the increase in water in the etching solution due to (C) above is small, and as a result, the increase in water due to (A) to (C) above and the decrease in water due to (D) above are balanced when the moisture content of the etching solution is low. Furthermore, when etching was performed with a small etching amount of X2 μm using the mixed acid according to this embodiment, the moisture content of the etching solution converged in fewer batches than when etching was performed with the same etching amount of X2 μm using the conventional mixed acid. Moreover, in this embodiment, compared to the conventional method, the moisture content is maintained at a low level even when the etching amount is small, eliminating the need for dressing and improving the efficiency of the etching process. Note that in the mixed acid according to this embodiment, if the etching amount is large at X1 μm, there is a possibility of insufficient moisture, so the etching solution can be configured to be pre-watered.

[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, less than 20 μm. Furthermore, 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 the change in glossiness of the silicon wafer in response to the moisture content of the etching solution is also small, and the glossiness can be stabilized in a short time.

[0036] In this embodiment, etching can be performed using known methods, except for changing the composition of the mixed acid. For example, etching time and etching temperature, depending on the target etching amount, can be performed under the same conditions as known.

[0037] Next, the etching process according to this embodiment will be described in detail. Figure 4 is a flowchart of 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 an etching jig.

[0038] As shown in Figure 4, in step S101, first, the 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. It is preferable that the nitric acid used in this embodiment has a lower concentration than fuming nitric acid with a concentration of 80% by mass or more (i.e., 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 when the temperature rises, is highly corrosive and difficult to handle, and may also reduce the surface roughness in the micro region of the silicon wafer.

[0039] Furthermore, in step S102, the etching solution is prepared. Specifically, in this embodiment, before etching the silicon wafer, a certain amount of silicon is dissolved in the mixed acid prepared in step S101 to prepare the etching solution to be used for etching. 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, for example, the etching solution can be prepared so that the amount of silicon dissolved in the etching solution is 10 to 20 g / L.

[0040] In step S103, the silicon wafer to be etched is immersed in the etching solution prepared in step S102. The silicon wafer to be etched is wrapped in 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. As a result, the silicon wafer and etching jig are immersed in the etching solution with water droplets adhering to their surfaces, and this water mixes into 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 amount of etching required, so that the target amount of etching (the thickness of the silicon wafer) is achieved. 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 etching solution used 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 amount = Weight of silicon dissolved by etching (g) / Target initial concentration of silicon 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, then 2 L of etching solution will be discharged. The amount of silicon dissolved by the etching process can be determined from the etching time in step S103 (the time the silicon wafer is immersed in the etching solution) and the number of silicon wafers per batch.

[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 allows the amount of silicon dissolved in the etching solution to be maintained at a constant value, and the water content (mol / L) in the etching solution to be kept below a certain value.

[0043] In step S106, a determination is made as to whether the etching process is complete or not. For example, if there are still silicon wafers 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 the etching of all silicon wafers to be etched is complete, the etching process according to this embodiment is completed. Once all etching is complete, the etching solution that has been used may be replaced with a new etching solution, or it may be used as is for the next etching process.

[0044] Furthermore, the etching process can be evaluated as needed. For example, the system can be configured to evaluate whether the silicon wafer has been etched to the target amount or whether it has achieved the target gloss level. If the etching evaluation reveals that the amount of etching is insufficient, the system can return to step S103 and immerse the silicon wafer in the etching solution again to perform the etching process.

[0045] As described above, the silicon wafer manufacturing method according to this embodiment includes an etching step in which etching is performed using an etching solution containing a mixed acid, which is a mixture of at least hydrofluoric acid and nitric acid. In this etching step, a portion of the used etching solution is discharged, and new mixed acid is injected into the etching solution to adjust the concentration of the mixed acid in the etching solution. The mixed acid is characterized by containing water (H2O) at a concentration of 33% by mass or less. This makes it possible to perform etching without performing a dressing procedure in between, not only when etching silicon wafers to a thickness of 20 μm or more, but also when etching silicon wafers to a thickness of less than 20 μm, and to suppress the decrease in glossiness of the etched silicon wafer, thereby producing high-quality silicon wafers.

[0046] While it is possible to suppress changes in the moisture content of the etching solution by increasing its moisture content beforehand, a high moisture content in the etching solution can reduce glossiness, potentially resulting in undesirable quality. In this embodiment, the etching solution can be maintained at a low moisture content, enabling the continuous production of high-gloss silicon wafers.

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

[0048] For example, in the embodiment described above, a configuration was exemplified 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 to set the water (H2O) content in the mixed acid to 32% by mass or less. However, for example, the hydrofluoric acid used can have a concentration of 60% by mass or more, or even 65% by mass or more. Similarly, the nitric acid used can 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 a silicon single crystal, The process includes an etching step in which etching is performed using an etching solution containing at least a mixed acid, which is a mixture of hydrofluoric acid and nitric acid. In the etching process, a portion of the used etching solution is discharged, and the composition of the mixed acid in the etching solution is adjusted by injecting the mixed acid again into the etching solution. In the mixed acid, water (H ２ A method for manufacturing a semiconductor wafer, wherein the content of 0) is 32% by mass or less.

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

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

4. Hydrogen fluoride (HF) and nitric acid (HNO) in the aforementioned mixed acid ３ A method for manufacturing a semiconductor wafer according to claim 1, wherein the content ratio of ) by mass is 1:2 to 1:

5.

5. In the etching process, the amount of etching solution used and the amount of mixed acid added are determined based on the amount of silicon dissolved by etching. The method for manufacturing a semiconductor wafer according to claim 1, wherein in the etching step, the semiconductor wafer is continuously etched to a thickness of less than 20 μm.

6. In the etching process, the amount of etching solution used and the amount of mixed acid added are determined based on the amount of silicon dissolved by etching. A method for manufacturing a semiconductor wafer according to claim 1, comprising the steps of etching the semiconductor wafer to a thickness of 20 μm or more using the etching solution, and etching the semiconductor wafer to a thickness of less than 20 μm.

7. The aforementioned mixed acid is acetic acid (CH ３ It further contains COOH, hydrogen fluoride (HF), and nitric acid (HNO). ３ ) and acetic acid (CH ３ A method for manufacturing a semiconductor wafer according to claim 1, wherein the content ratio of COOH by mass is 3:1 to 7:

1.

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

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

Images