Method for evaluating the hydrophilicity level of silicon wafers

Abstract

To provide a silicon wafer contact angle measurement method which can detect a severe difference in a hydrophilicity level of a silicon wafer surface that cannot be detected by contact angle measurement using pure water.SOLUTION: A silicon wafer contact angle measurement method according to the present invention comprises the steps of: dropping droplets onto a surface of a silicon wafer; and measuring a contact angle of the surface of the silicon wafer from an image of the droplets. The droplets are made of the aqueous solution having the larger surface tension than the surface tension of pure water.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for measuring a contact angle of a silicon wafer and a method for evaluating a surface state of a silicon wafer. [Background technology]

[0002] Conventionally, one method for evaluating the surface condition of a silicon wafer has been to drop a droplet of pure water on the surface of the silicon wafer and measure the contact angle of the surface of the silicon wafer from an image of the dropped droplet.

[0003] For example, in Patent Document 1 (see Example 7), a wafer is cleaned at 70°C for 10 minutes using SC-1 containing chelating agent TTHA, and is rinsed with pure water containing 100 ppm HF heated to 50°C, and the change in water droplet contact angle with respect to the rinsing time is investigated. Here, when the rinsing time is 30 minutes or less, the water droplet contact angle is 5°, which is thought to be due to a natural oxide film remaining on the wafer surface, and when the rinsing time is 120 minutes, the water droplet contact angle is 60°, which is thought to be due to the natural oxide film on the wafer surface being removed and the bare silicon surface being exposed. In this way, it has been conventional to determine whether a wafer surface is hydrophilic or hydrophobic by measuring the contact angle of the wafer surface with a water droplet. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-216098 Summary of the Invention [Problem to be solved by the invention]

[0005] As is clear from Patent Document 1, when a native oxide film is formed on the wafer surface, the wafer surface is basically hydrophilic, and the contact angle of the wafer surface is generally 5° or less when measured by dropping pure water on the wafer surface. However, the present inventors focused on a new problem of wanting to detect a severe difference in the hydrophilicity level of the wafer surface, which is not different from the contact angle value of the wafer surface measured with pure water. However, no technology capable of solving such a problem existed in the past.

[0006] In view of the above problems, an object of the present invention is to provide a method for measuring a contact angle of a silicon wafer, which is capable of detecting a severe difference in hydrophilicity level on a silicon wafer surface that cannot be detected by contact angle measurement using pure water. [Means for solving the problem]

[0007] In order to solve the above problem, the present inventors came up with the idea of ​​measuring the contact angle of the silicon wafer surface with a droplet of an aqueous solution having a surface tension greater than that of pure water. This is because they thought that if the contact angle of the wafer surface is measured with an aqueous solution having a surface tension greater than that of pure water, a contact angle measurement value greater than that of pure water would be obtained, and therefore it would be possible to detect a severe difference in hydrophilicity level of the wafer surface that cannot be detected by contact angle measurement with pure water. As a result of the inventors' experiments, it was confirmed that it is possible to detect a severe difference in hydrophilicity level by measuring the contact angle of the silicon wafer surface with a droplet of an aqueous solution having a surface tension greater than that of pure water.

[0008] The gist and configuration of the present invention are as follows. [1] Dropping droplets onto a surface of a silicon wafer; measuring a contact angle on the surface of the silicon wafer from the image of the droplet; Including, The method for measuring a contact angle of a silicon wafer, wherein the droplet is made of an aqueous solution having a surface tension greater than that of pure water.

[0009] [2] The method for measuring a contact angle of a silicon wafer according to the above [1], wherein the aqueous solution is at least one selected from the group consisting of an aqueous sodium chloride solution, an aqueous potassium chloride solution, and an aqueous magnesium chloride solution.

[0010] [3] The method for measuring a contact angle of a silicon wafer according to the above [1] or [2], wherein the concentration of the aqueous solution is 10 mass % or more.

[0011] [4] The method for measuring a contact angle of a silicon wafer according to any one of the above [1] to [3], wherein the amount of the droplet is within a range of 0.3 to 3.0 μL.

[0012] [5] The method for measuring a contact angle of a silicon wafer according to any one of the above [1] to [4], wherein the humidity of the environment in which the contact angle is measured is within a range of 30 to 70% RH.

[0013] [6] A method for measuring a contact angle of a silicon wafer according to any one of the above [1] to [5], comprising the steps of measuring a contact angle on a surface of the silicon wafer under a plurality of conditions in which the amount of a droplet dropped on the surface is different from one another, and determining a relationship between the amount of the droplet and the measured value of the contact angle under the plurality of conditions.

[0014] [7] The method for measuring a contact angle of a silicon wafer according to [6] above, further comprising measuring an amount of the droplet from an image of the droplet.

[0015] [8] The method for measuring a contact angle of a silicon wafer according to any one of the above [1] to [7], wherein a surface layer portion of the silicon wafer is an oxide film, and the oxide film forms the surface.

[0016] [9] The contact angle measuring method for a silicon wafer according to the above [8], wherein the oxide film is a native oxide film.

[0017]

[10] A method for measuring a contact angle of a silicon wafer according to any one of the above [1] to [9]; evaluating a surface state of the silicon wafer based on the measured value of the contact angle; A method for evaluating the surface state of a silicon wafer having the above structure. Effect of the Invention

[0018] According to the method for measuring a contact angle of a silicon wafer of the present invention, it is possible to detect a severe difference in the hydrophilicity level of the silicon wafer surface which cannot be detected by contact angle measurement using pure water. [Brief description of the drawings]

[0019] [Figure 1] FIG. 2 is a diagram illustrating Young's equation for a contact angle. [Diagram 2] 1 is a graph showing the measurement results of contact angles according to an example of the invention and a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] (Method for measuring contact angle of silicon wafer) A method for measuring a contact angle of a silicon wafer according to an embodiment of the present invention includes a step of dropping a droplet on a surface of a silicon wafer and a step of measuring a contact angle of the surface of the silicon wafer from an image of the droplet, and is characterized in that the droplet is made of an aqueous solution having a surface tension greater than that of pure water. According to this embodiment, it is possible to detect a severe difference in hydrophilicity level of the silicon wafer surface that cannot be detected by contact angle measurement using pure water.

[0021] The silicon wafer subjected to the contact angle measurement of this embodiment is preferably a single crystal silicon wafer. In addition, it is preferable that the surface layer of the silicon wafer is an oxide film, and the oxide film forms the surface of the silicon wafer. In particular, the oxide film is not particularly limited as long as it is a SiO2 film, and examples of the oxide film include a thermal oxide film and a natural oxide film, but it is particularly preferable that the oxide film is a natural oxide film.

[0022] In the silicon wafer manufacturing process, a suitable timing for applying the contact angle measuring method according to the present embodiment is immediately before single wafer spin cleaning. In general, the process immediately before single wafer spin cleaning is a pre-cleaning process or an inspection process performed following the pre-cleaning process, and at the end of the pre-cleaning process, a natural oxide film is formed on the wafer surface. Specifically, in the pre-cleaning process, the wafer is cleaned using a combination of an SC1 cleaning tank, an HF tank, an ozone tank, etc., and then rinsed with pure water and dried. When the inspection process is performed, inspection of particles and scratches on the wafer surface, inspection of the wafer shape (flatness), etc. are performed. Thus, a natural oxide film is formed on the surface of the silicon wafer immediately before the single wafer spin cleaning, and the wafer surface is basically hydrophilic. Specifically, the contact angle of the wafer surface is approximately 5° or less when measured by dropping pure water on the wafer.

[0023] However, in reality, the level of hydrophilicity of the wafer surface varies depending on the storage conditions of the wafer before being subjected to single wafer spin cleaning, but the contact angle of the wafer surface measured with pure water does not vary. For example, after the above pre-cleaning process and optional inspection process, the wafer is stored in a container called a FOUP (Front-Opening Unified Pod), and as the storage period increases, slight accumulation of organic matter may occur on the wafer surface. In addition, if the wafer is not dried sufficiently after the above pre-cleaning process, water vapor may be generated in the FOUP and adsorbed on the wafer surface, causing polarization of water molecules on the wafer surface. With wafers that have such a severely inferior hydrophilicity level, the cleaning solution does not spread all over the wafer surface during the first step of single-wafer spin cleaning (for example, spin cleaning with ozone water), and the continuity of the cleaning solution film on the wafer surface is not maintained, resulting in localized areas on the wafer surface where the cleaning solution does not reach. As a result, particles remain even after single-wafer spin cleaning, or uneven etching occurs after single-wafer spin cleaning, resulting in an increase in LPD.

[0024] For silicon wafers that are found to have a severely inferior hydrophilicity level by the contact angle measurement method of this embodiment, a countermeasure can be taken such as performing a pretreatment to enhance the hydrophilicity of the wafer surface prior to single-wafer spin cleaning. In other words, the contact angle measurement method of this embodiment can be said to be an effective method for reliably reducing LPD after single-wafer spin cleaning.

[0025] Referring to FIG. 1, when a drop of liquid is deposited on a solid surface, the following Young's equation holds true: Gamma S =γ L cosθ+γ SL Where: Gamma S : Surface tension of a solid Gamma SL : Interfacial tension between solid and liquid Gamma L : Surface tension of the liquid θ: contact angle γ S is the force that pulls the end point in Figure 1 to the left in an attempt to reduce the surface area of ​​the solid, i.e., the gas / solid interface. γ SL is the force that pulls the end point to the right, trying to reduce the area of ​​the solid / liquid interface. γ L acts in the tangential direction of the liquid contour in an attempt to reduce the surface area of ​​the liquid, i.e., the gas / liquid interface, and its horizontal component γ L cosθ pulls the end point to the right. When the droplet is at rest, these three forces balance each other and Young's equation holds true.

[0026] In this embodiment, the surface tension γ L1 Surface tension γ is greater than L2 It is essential to measure the contact angle by dropping a droplet of an aqueous solution having the surface tension γ L1 Surface tension γ is greater than L2If the contact angle of the wafer surface is measured with an aqueous solution having the above formula, a contact angle θ2 larger than the contact angle θ1 measured with pure water can be obtained. This makes it possible to detect a severe difference in the hydrophilicity level of the wafer surface that cannot be detected by contact angle measurement with pure water. Specifically, an image of a droplet dropped on the surface of the silicon wafer is obtained, and the contact angle is measured from this image. The contact angle can be measured by a standard method, for example, the θ / 2 method, the tangent method, or the curve fitting method.

[0027] The aqueous solution used in this embodiment has an interfacial tension γ SL2 The interfacial tension between the silicon wafer surface (SiO2) and pure water γ SL1 It is preferable that γ is equal to or larger than γ. This makes it possible to reliably obtain a contact angle θ2 that is larger than the contact angle θ1 measured with pure water. SL1 and γ SL2 However, it is difficult to measure the surface tension of pure water, γ L1 and the surface tension γ of the aqueous solution used in this embodiment. L2 It is possible to measure the contact angles θ1 and θ2. Here, the tension γ S Since is constant, γ SL1 and γ SL2 Here, it is possible to grasp the relationship between the surface tension of the liquid γ L can be measured by the hanging drop method.

[0028] The aqueous solution used in this embodiment is preferably at least one selected from the group consisting of an aqueous sodium chloride solution, an aqueous potassium chloride solution, and an aqueous magnesium chloride solution. These aqueous solutions are easy to prepare and have an appropriate surface tension. The concentration of these aqueous solutions is not particularly limited, but is preferably 10% by mass or more from the viewpoint of exerting an appropriate surface tension, and the upper limit is allowed up to the solubility.

[0029] The amount of droplets used in contact angle measurement is preferably set within the range of 0.3 to 3.0 μL. If the amount of droplets is 0.3 μL or more, the effects of evaporation and volatilization of the droplets are small, and the error in the contact angle measurement is not large, and if the amount of droplets is 3.0 μm or less, the droplets are not easily crushed by their own weight, and the error in the contact angle measurement is not large.

[0030] The humidity of the environment in which the contact angle is measured is preferably within the range of 30 to 70% RH. If the humidity is 30% RH or higher, the effect of evaporation and volatilization of the droplets is small, and the error in the contact angle measurement does not become large, and if the humidity is 70% RH or lower, the number of water molecules adsorbed on the silicon wafer surface due to condensation does not increase too much, so the error in the contact angle measurement does not become large.

[0031] Details will be described with reference to FIG. 2 in the examples, but in this embodiment, it is preferable to measure the contact angle of the surface of a silicon wafer under a plurality of conditions in which the amount of droplets dropped on the surface is different from each other, and to grasp the relationship between the amount of droplets and the measured value of the contact angle under the plurality of conditions. The inventors have found that the difference in the severe hydrophilicity level can also be detected as the difference in the droplet amount dependency of the contact angle. That is, it was found that the ratio of the change in the contact angle to the change in the droplet amount is large in a wafer with a poor hydrophilicity level, and the ratio of the change in the contact angle to the change in the droplet amount is small in a wafer with a good hydrophilicity level. Therefore, as shown in FIG. 2, the measurement data can be plotted on a plane with the droplet amount on the horizontal axis and the contact angle on the vertical axis, and the difference in the hydrophilicity level can be detected based on the droplet amount dependency of the contact angle.

[0032] At this time, it is preferable to measure (calculate) the amount (volume) of the droplet that is actually dropped from the image of the droplet. The amount of droplet can be set by the contact angle meter used, but there may be a certain degree of error between the droplet amount set by the device and the amount of droplet that is actually dropped. Therefore, by plotting the actually measured droplet amount instead of the device set value, the droplet amount dependency of the contact angle can be grasped more accurately.

[0033] From the viewpoint of more accurately grasping the droplet amount dependency of the contact angle, the contact angle is preferably measured under three or more conditions with different droplet amounts, more preferably under five or more conditions. Although there is no particular upper limit on the number of conditions, the number of conditions can be eight or less since the accuracy saturates.

[0034] (Method for evaluating the surface condition of silicon wafers) A method for evaluating a surface state of a silicon wafer according to an embodiment of the present invention includes the above-described method for measuring a contact angle of a silicon wafer according to an embodiment of the present invention, and a step of evaluating a surface state of the silicon wafer based on the measured value of the contact angle.

[0035] For example, based on the difference in the measured contact angle, it is possible to detect severe differences in the hydrophilicity level of the silicon wafer surface that cannot be detected by contact angle measurements using pure water.

[0036] Furthermore, as described above, based on the droplet volume dependency of the contact angle, it is possible to detect severe differences in the hydrophilicity level of the silicon wafer surface that cannot be detected by contact angle measurement using pure water. EXAMPLES

[0037] Two single crystal silicon wafers (diameter 300 mm) were prepared by performing a pre-cleaning process in which the wafers were cleaned in a combination of SC1 cleaning tank, HF tank, and ozone tank after mirror polishing, then rinsed with pure water and then dried. The two silicon wafers were not dried sufficiently after the pre-cleaning process, which is thought to have caused water vapor to be generated in the FOUP and adsorbed onto the wafer surface, resulting in polarization of water molecules on the wafer surface. In addition, a natural oxide film was formed on the surface layer of both silicon wafers.

[0038] [Level 1] One of the two silicon wafers was subjected to contact angle measurement according to the following invention examples and comparative examples immediately after being removed from the FOUP. [Level 2] The other of the two silicon wafers was subjected to a pretreatment in which the surface of the silicon wafer was exposed to the downflow in the clean room, and then subjected to the contact angle measurement according to the following invention example and comparative example. In the pretreatment, the fan rotation speed was 1300 rpm, and the treatment time was 300 seconds.

[0039] Both the level 1 and level 2 silicon wafers have a natural oxide film on the surface, and the wafer surface is basically hydrophilic. However, the level of hydrophilicity of the level 1 silicon wafer is slightly lower due to the influence of the polarization of water molecules, whereas the level 2 silicon wafer appears to have a high level of hydrophilicity because the polarization of water molecules is eliminated by the pretreatment.

[0040] (Example of the invention) The contact angle on the surface of each silicon wafer was measured by the θ / 2 method under the following conditions. The set liquid amount was set under the following three conditions, and the actual amount of the dropped liquid was measured from the image of the dropped liquid. Equipment: Portable contact angle meter PCA-11 manufactured by Kyowa Interface Science Co., Ltd. Dropped liquid type: 20% by mass NaCl aqueous solution Set droplet volume: 3 conditions: 0.5μL, 1.0μL, 2.0μL Measurement points: 5 points on the wafer (1-2 cm intervals from the center to the edge) Environmental humidity: 40%RH

[0041] (Comparative Example) The contact angle on the surface of each silicon wafer was measured by the θ / 2 method under the following conditions. The set liquid amount was set under the following two conditions, and the actual amount of the dropped liquid was measured from the image of the dropped liquid. Equipment: Portable contact angle meter PCA-11 manufactured by Kyowa Interface Science Co., Ltd. Dripping liquid type: Pure water Set droplet volume: 2 conditions: 1.0 μL and 2.0 μL Measurement points: 5 points on the wafer (1-2 cm intervals from the center to the edge) Environmental humidity: 40%RH

[0042] [Measurement results] In the examples and comparative examples, the average value of the contact angle measurements (average value of five points) and the average value of the droplet volume measurements (average value of five points) were calculated for each set droplet volume. Figure 2 shows a graph in which the measurement data is plotted with the droplet volume measurements (average value of five points) on the horizontal axis and the contact angle measurements (average value of five points) on the vertical axis.

[0043] In the contact angle measurements according to the comparative example, the average contact angle was 5° or less for both level 1 and level 2, regardless of the droplet volume. Contact angles below 5° are unreliable, so are shown as 5° in FIG. 2. In contrast, in the contact angle measurements according to the example of the invention, when the set droplet volume was 0.5 μL, the average contact angle was 21.9° for level 1 and 19.8° for level 2. In this way, the example of the invention was able to detect severe differences in hydrophilicity levels on the silicon wafer surface that could not be detected by the contact angle measurements according to the comparative example.

[0044] 2, in the examples of the invention, the ratio of the change in contact angle to the change in droplet volume was large at level 1, which had a poor level of hydrophilicity, whereas the ratio of the change in contact angle to the change in droplet volume was small at level 2, which had a high level of hydrophilicity. This shows that in the examples of the invention, differences in hydrophilicity levels can also be detected based on the dependency of the contact angle on the droplet volume.

[0045] [Additional Experiments] Thereafter, each of the silicon wafers of levels 1 and 2 was first subjected to spin cleaning with ozone water, then to single-wafer spin cleaning in which three sets of a combination of a spin cleaning with hydrofluoric acid and then a spin cleaning with ozone water were performed, and finally, spin drying was performed at a wafer rotation speed of 1500 rpm. -Conditions for spin cleaning with ozone water Concentration: 25mg / L Flow rate: 1.0L / min Processing time per session: 200 seconds Wafer rotation speed: 500 rpm -Conditions for single wafer spin cleaning using hydrofluoric acid Concentration: 1% by mass Flow rate: 1.0L / min Processing time per session: 50 seconds Wafer rotation speed: 500 rpm

[0046] The surface of each silicon wafer was then measured using a laser particle counter (KLA-Tencor, Surfscan SP7) in HS (High Sensitivity) mode to determine the number of LPDs with a size of 15 nm or more. The level 1 silicon wafer had 200 LPDs, while the level 2 silicon wafer had 5 LPDs.

[0047] This indicates that the severe difference in hydrophilicity level of the silicon wafer surface, which cannot be detected by contact angle measurement with pure water, causes the difference in the number of LPDs after single-wafer spin cleaning. - For wafers with poor hydrophilicity, in the first step of single wafer spin cleaning (e.g., spin cleaning with ozone water), the cleaning solution does not spread all over the wafer surface, and the continuity of the cleaning solution film on the wafer surface is not maintained, resulting in localized areas on the wafer surface where the cleaning solution does not reach. -As a result, particles remain even after single-wafer spin cleaning, and etching unevenness occurs after single-wafer spin cleaning, resulting in an increase in LPD. It is considered that.

[0048] In this regard, according to the examples of the present invention, the difference in the severe hydrophilicity level of the silicon wafer surface that leads to the difference in the number of LPDs after single-wafer spin cleaning can be detected before single-wafer spin cleaning. Therefore, for silicon wafers that are found to have a poor severe hydrophilicity level as a result of the contact angle measurement according to the examples of the present invention, a measure can be taken such as performing a pretreatment to increase hydrophilicity before performing single-wafer spin cleaning. In other words, the present invention can be said to be an effective method for reliably reducing LPDs after single-wafer spin cleaning. [Industrial Applicability]

[0049] According to the method for measuring a contact angle of a silicon wafer of the present invention, it is possible to detect a severe difference in the hydrophilicity level of the silicon wafer surface which cannot be detected by contact angle measurement using pure water.

Claims

1. Dropping droplets of an aqueous solution having a surface tension greater than that of pure water onto a surface of the silicon wafer; measuring a contact angle on the surface of the silicon wafer from the image of the droplet; is performed under a plurality of conditions in which the amount of droplets dropped on the surface is different from each other, The contact angle measurements under the plurality of conditions are carried out for a plurality of silicon wafers; A method for evaluating hydrophilicity levels of silicon wafers, comprising evaluating a difference in hydrophilicity levels of the surfaces of the plurality of silicon wafers based on a ratio of a change in contact angle to a change in droplet volume obtained by the measurement.

2. 2. The method for evaluating a hydrophilicity level of a silicon wafer according to claim 1, wherein the aqueous solution is at least one selected from the group consisting of an aqueous sodium chloride solution, an aqueous potassium chloride solution, and an aqueous magnesium chloride solution.

3. 3. The method for evaluating a hydrophilicity level of a silicon wafer according to claim 1, wherein a concentration of the aqueous solution is 10 mass % or more.

4. The method for evaluating a hydrophilicity level of a silicon wafer according to any one of claims 1 to 3, wherein the amount of the droplet is within a range of 0.3 to 3.0 μL.

5. The method for evaluating a hydrophilicity level of a silicon wafer according to any one of claims 1 to 4, wherein the humidity of an environment in which the contact angle is measured is within a range of 30 to 70% RH.

6. The method for evaluating a hydrophilicity level of a silicon wafer according to any one of claims 1 to 5, further comprising measuring an amount of the droplet from an image of the droplet.

7. 7. The method for evaluating a hydrophilicity level of a silicon wafer according to claim 1, wherein a surface layer portion of the silicon wafer is an oxide film, and the oxide film forms the surface.

8. The method for evaluating a hydrophilicity level of a silicon wafer according to claim 7 , wherein the oxide film is a native oxide film.

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