Method for measuring contact angle of silicon wafer and method for evaluating surface state of silicon wafer

Abstract

Provided is a contact angle measurement method for a silicon wafer capable of detecting a severe difference in hydrophilicity level of the surface of the silicon wafer that cannot be detected in a pure water-based contact angle measurement. The contact angle measurement method for a silicon wafer of the present invention includes: a step of dropping a droplet onto the surface of a silicon wafer; and a step of measuring the contact angle of the surface of the silicon wafer from an image of the aforementioned droplet, the aforementioned droplet being formed from an aqueous solution having a surface tension greater than that of pure water.

Description

Technical Field

[0001] This invention relates to a method for measuring the contact angle of silicon wafers and a method for evaluating the surface condition of silicon wafers. Background Technology

[0002] Previously, one method for evaluating the surface condition of silicon wafers involved the following operation: dropping a drop of pure water onto the surface of the silicon wafer and measuring the contact angle of the silicon wafer surface based on the image of the dropped drop.

[0003] For example, in Patent Document 1 (refer to Example 7), a wafer cleaned at 70°C for 10 minutes using SC-1 with added chelating agent TTHA was rinsed with pure water containing 100 ppm HF heated to 50°C, and the change in water droplet contact angle relative to rinsing time was investigated. Here, when the rinsing time was less than 30 minutes, the water droplet contact angle was 5°, which could be considered as the natural oxide film remaining on the wafer surface. When the rinsing time was 120 minutes, the water droplet contact angle was 60°, which could be considered as the natural oxide film on the wafer surface being removed, exposing the bare silicon surface. In this way, conventional methods have been used to determine whether a wafer surface is hydrophilic or hydrophobic by measuring the wafer surface contact angle based on water droplets.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 6-216098 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] As clearly stated in Patent Document 1, when a natural oxide film is formed on the wafer surface, the wafer surface is essentially hydrophilic, and when measured with pure water, the contact angle of the wafer surface is approximately 5° or less. However, the inventors have focused on a novel problem: detecting a stringent difference in the level of hydrophilicity of the wafer surface that is not significantly different when measured with pure water. However, no prior art has been available that can solve this problem.

[0009] In view of the above-mentioned problems, the object of the present invention is to provide a contact angle measurement method for silicon wafers that can detect severe differences in hydrophilicity levels on the surface of silicon wafers that cannot be detected in contact angle measurements based on pure water.

[0010] Methods for solving problems

[0011] To address the aforementioned issues, the inventors conceived of using droplets formed from an aqueous solution with a surface tension greater than that of pure water to measure the contact angle of a silicon wafer surface. This is because if an aqueous solution with a surface tension greater than that of pure water is used to measure the contact angle of the wafer surface, a contact angle measurement value larger than that obtained using pure water can be obtained. Therefore, it is possible to detect severe differences in hydrophilicity levels on the wafer surface that are undetectable in contact angle measurements based on pure water. Furthermore, experimental results from the inventors confirm that by using droplets formed from an aqueous solution with a surface tension greater than that of pure water to measure the contact angle of the silicon wafer surface, severe differences in hydrophilicity levels can be detected.

[0012] The main structure of this invention is as follows.

[0013] [1] A method for measuring the contact angle of a silicon wafer, comprising:

[0014] The process of adding liquid droplets to the surface of a silicon wafer; and

[0015] The process of determining the contact angle of the silicon wafer surface based on the aforementioned droplet image.

[0016] The aforementioned droplets are formed from an aqueous solution with a surface tension greater than that of pure water.

[0017] [2] According to the contact angle measurement method of silicon wafer described in [1] above, the aforementioned aqueous solution is at least one selected from sodium chloride aqueous solution, potassium chloride aqueous solution and magnesium chloride aqueous solution.

[0018] [3] According to the contact angle measurement method of silicon wafer described in [1] or [2] above, wherein the concentration of the aforementioned aqueous solution is 10% by mass or more.

[0019] [4] The method for measuring the contact angle of a silicon wafer according to any one of [1] to [3] above, wherein the amount of the aforementioned droplet is in the range of 0.3 to 3.0 μL.

[0020] [5] The method for measuring the contact angle of a silicon wafer according to any one of [1] to [4] above, wherein the humidity of the environment in which the contact angle is measured is in the range of 30 to 70% RH.

[0021] [6] The contact angle measurement method for silicon wafers according to any one of [1] to [5] above has the following steps: measuring the contact angle of the surface of the silicon wafer under multiple conditions where the amount of droplets added to the surface is different from each other, and determining the relationship between the amount of droplets under the multiple conditions and the measured value of the contact angle.

[0022] [7] According to the contact angle measurement method of silicon wafer described in [6] above, the amount of the droplet is measured based on the image of the droplet.

[0023] [8] The contact angle measurement method of the silicon wafer according to any one of [1] to [7] above, wherein the surface portion of the aforementioned silicon wafer is an oxide film, and the oxide film forms the aforementioned surface.

[0024] [9] According to the contact angle measurement method of silicon wafer described in [8] above, wherein the aforementioned oxide film is a natural oxide film.

[0025]

[10] A method for evaluating the surface state of a silicon wafer, which has the following characteristics:

[0026] The method for measuring the contact angle of a silicon wafer as described in any one of [1] to [9] above; and

[0027] The process of evaluating the surface condition of the aforementioned silicon wafer based on the measured contact angle value.

[0028] Invention Effects

[0029] The contact angle measurement method for silicon wafers according to the present invention can detect severe differences in hydrophilicity levels on the surface of silicon wafers that cannot be detected in contact angle measurements based on pure water. Attached Figure Description

[0030] Figure 1 This is a diagram illustrating Young's formula related to the contact angle.

[0031] Figure 2 This is a graph showing the measurement results of the contact angle described in the invention example and the comparative example. Detailed Implementation

[0032] (Method for measuring the contact angle of silicon wafers)

[0033] A method for measuring the contact angle of a silicon wafer according to one embodiment of the present invention is characterized by comprising: a step of dropping a droplet onto the surface of a silicon wafer; and a step of measuring the contact angle of the surface of the silicon wafer based on an image of the droplet, wherein the droplet is formed from an aqueous solution having a surface tension greater than that of pure water. According to this embodiment, it is possible to detect severe differences in the hydrophilicity level of the silicon wafer surface that are undetectable in contact angle measurements based on pure water.

[0034] The silicon wafer used for contact angle measurement in this embodiment is preferably a single-crystal silicon wafer. Furthermore, it is preferable that the surface layer of the silicon wafer is an oxide film, and that this oxide film forms the surface of the silicon wafer. In particular, the oxide film is not particularly limited to any SiO2 film; examples include thermally oxidized films and naturally oxidized films, with naturally oxidized films being particularly preferred.

[0035] In the silicon wafer manufacturing process, the preferred time to apply the contact angle measurement method described in this embodiment is before the single-wafer spin cleaning. Generally, the process before the single-wafer spin cleaning is a pre-cleaning process or an inspection process immediately following the pre-cleaning process. 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 by combining an SC1 cleaning tank, an HF tank, an ozone tank, etc., then rinsed with pure water and dried. In the case of the inspection process, the wafer surface is inspected for particles, damage, etc., and the wafer shape (flatness), etc. As such, a natural oxide film is formed on the surface of the silicon wafer before it is supplied for single-wafer spin cleaning, and the wafer surface is basically hydrophilic. Specifically, when measuring with pure water, the contact angle of the wafer surface is approximately 5° or less.

[0036] In practice, depending on the storage conditions of the wafer up to the point of spin cleaning, the level of hydrophilicity on the wafer surface can vary to the extent that the contact angle value of the wafer surface measured using pure water does not differ. For example, after the aforementioned pre-cleaning process and optional inspection process, the wafer is stored in a container called a FOUP (Front-Opening Unified Pod). As the storage time increases, slight accumulation of organic matter sometimes occurs on the wafer surface. In addition, if the drying after the aforementioned pre-cleaning process is insufficient, water vapor may be generated in the FOUP and adsorbed onto the wafer surface, causing polarization of water molecules on the wafer surface. For wafers with such poor hydrophilicity, in the initial step of spin cleaning (e.g., spin cleaning based on ozone water), the cleaning solution does not spread throughout the wafer surface, the continuity of the cleaning solution film cannot be maintained on the wafer surface, and local areas on the wafer surface where the cleaning solution cannot reach are created. As a result, particles remain after spin cleaning, or uneven etching occurs after spin cleaning, leading to increased LPD (Liquidity Limiting Defect).

[0037] Therefore, the contact angle measurement method described in this embodiment can be performed immediately before the single-wafer spin cleaning, that is, immediately after the aforementioned pre-cleaning process or the inspection process immediately following the pre-cleaning process. For silicon wafers where the contact angle measurement method of this embodiment determines a severe hydrophilicity level difference, a pretreatment to improve the hydrophilicity of the wafer surface can be performed before single-wafer spin cleaning. In other words, the contact angle measurement method described in this embodiment reliably reduces LPD after single-wafer spin cleaning, and therefore can be considered an effective method.

[0038] Reference Figure 1 If a liquid is added to a solid surface, then Young's formula holds true.

[0039] γ S =γ L ·cosθ+γ SL

[0040] Here,

[0041] γ S Surface tension of solids

[0042] γ SL Interfacial tension between solids and liquids

[0043] γ L Surface tension of liquids

[0044] θ: Contact angle

[0045] γ S The goal is to reduce the surface area of ​​the solid, i.e., the interfacial area between the gas and the solid. Figure 1 The force pulling the endpoint of the γ to the left. SL It is the force that pulls the endpoint to the right in an attempt to reduce the interfacial area between the solid and the liquid. γ L It aims to reduce the surface area of ​​the liquid, i.e., the interfacial area between the gas and liquid, and works along the tangential direction of the liquid profile; its horizontal vector γ L • cosθ pulls the endpoint to the right. When the droplet is at rest, these three forces are in equilibrium, and Young's formula holds.

[0046] In this embodiment, a substance with a surface tension γ greater than that of pure water is dropped onto the wafer surface. L1 Large surface tension γ L2 It is important to determine the contact angle of droplets formed from aqueous solutions. This is particularly important if the surface tension γ of the solution, which is greater than that of pure water, is utilized. L1 Large surface tension γ L2 By using an aqueous solution to measure the contact angle of the wafer surface, a contact angle θ2 larger than that measured using pure water can be obtained. This allows for the detection of stringent differences in hydrophilicity levels on the wafer surface that are undetectable in water-based contact angle measurements. Specifically, an image of a droplet being dropped onto the surface of a silicon wafer is acquired, and the contact angle is measured based on this image. The contact angle can be measured using conventional methods, such as the θ / 2 method, the tangent method, or curve fitting.

[0047] In this embodiment, the aqueous solution is preferably set to a surface tension γ between the silicon wafer surface (SiO2) and the aqueous solution. SL2 The interfacial tension γ between the silicon wafer surface (SiO2) and pure water is greater than or equal to that of pure water. SL1An aqueous solution. Therefore, a contact angle θ2 that is larger than the contact angle θ1 measured using pure water can be reliably obtained. It should be noted that it is difficult to practically measure γ. SL1 and γ SL2 However, it is possible to measure the surface tension γ of pure water. L1 and the surface tension γ of the aqueous solution used in this embodiment L2 And contact angles θ1 and θ2. Here, due to the surface tension γ of the silicon wafer (SiO2) S It is constant, therefore, it is possible to control γ. SL1 and γ SL2 The magnitude relationship. Here, the surface tension γ of the liquid. L The pendant drop method can be used for determination.

[0048] The aqueous solution used in this embodiment is preferably selected from at least one of sodium chloride aqueous solution, potassium chloride aqueous solution, and magnesium chloride aqueous solution. This is because these aqueous solutions are easy to prepare and have moderate surface tension. The concentration of these aqueous solutions is not particularly limited, but from the viewpoint of achieving moderate surface tension, it is preferably 10% by mass or more, and the upper limit is acceptable up to the solubility.

[0049] The droplet volume for contact angle measurement is preferably set within the range of 0.3 to 3.0 μL. This is because: if the droplet volume is above 0.3 μL, the influence of droplet evaporation and volatilization is small, and the error in contact angle measurement will not increase; if the droplet volume is below 3.0 μL, the droplet is unlikely to be destroyed by its own weight, and the error in contact angle measurement will naturally not increase.

[0050] The optimal humidity range for measuring the contact angle is 30%–70% RH. This is because: if the humidity is above 30% RH, the effect of droplet evaporation and volatilization is small, and the error in contact angle measurement will not increase; if the humidity is below 70% RH, the amount of water molecules adsorbed on the silicon wafer surface due to condensation will not increase excessively, and therefore, the error in contact angle measurement will naturally not increase.

[0051] Details are referred to in the embodiments. Figure 2 While this will be explained, in this embodiment, it is preferable to measure the contact angle of the silicon wafer surface under multiple conditions where the amount of droplets added to the aforementioned surface varies, thereby determining the relationship between the amount of droplets and the measured contact angle under these multiple conditions. The inventors have discovered that even severe differences in hydrophilicity levels can be detected as differences in the droplet-amount dependence of the contact angle. It is understood that in wafers with severely poor hydrophilicity levels, the rate of change in contact angle relative to the rate of change in droplet amount is large, while in wafers with severely excellent hydrophilicity levels, the rate of change in contact angle relative to the rate of change in droplet amount is small. Therefore, it is possible to... Figure 2In this way, the measurement data are plotted on a plane with the horizontal axis set to droplet volume and the vertical axis set to contact angle, and the difference in hydrophilicity level is detected based on the droplet volume dependence of the contact angle.

[0052] At this point, it is preferable to determine (calculate) the actual amount (volume) of droplets added based on the droplet image. The droplet amount can be set using the contact angle meter used, but there is sometimes some degree of error between the device setting and the actual amount of droplets added. Therefore, by plotting the measured droplet amount instead of the device setting, the droplet amount dependence of the contact angle can be more accurately determined.

[0053] From the viewpoint of more accurately understanding the droplet quantity dependence of the contact angle, it is preferable to measure the contact angle under three or more conditions where the droplet quantities are different, and more preferably under five or more conditions. There is no particular upper limit to the number of conditions; since accuracy will saturate, the number of conditions can be set to eight or less.

[0054] (Methods for evaluating the surface condition of silicon wafers)

[0055] An embodiment of the present invention provides a method for evaluating the surface condition of a silicon wafer, comprising: the contact angle measurement method for a silicon wafer described in the above embodiment of the present invention; and a step of evaluating the surface condition of the silicon wafer based on the measured contact angle value.

[0056] For example, based on the difference in the measured contact angle, it is possible to detect severe differences in the hydrophilicity level of a silicon wafer surface that cannot be detected in contact angle measurements based on pure water.

[0057] Furthermore, as mentioned above, based on the droplet amount dependence of the contact angle, it is possible to detect severe differences in hydrophilicity levels on silicon wafer surfaces that are undetectable in contact angle measurements based on pure water.

[0058] Example

[0059] Two single-crystal silicon wafers (300mm in diameter) were prepared and subjected to the following pre-cleaning process after mirror polishing: an SC1 cleaning tank, an HF tank, and an ozone tank were combined to clean the wafers, followed by rinsing with pure water and drying. It can be assumed that the drying of the two silicon wafers after the pre-cleaning process was insufficient. Therefore, water vapor was generated in the FOUP and adsorbed onto the wafer surface, causing water molecule polarization on the wafer surface. Additionally, a natural oxide film formed on the surface of both silicon wafers.

[0060] [Level 1]

[0061] The contact angle of one of the two silicon wafers, immediately after being removed from the FOUP, is used for the contact angle measurement described in the following inventive and comparative examples.

[0062] [Level 2]

[0063] For the other of the two silicon wafers, a pretreatment process is performed to expose the surface of the silicon wafer to a downflow in a cleanroom, followed by contact angle measurement as described in the following inventive and comparative examples. In the pretreatment, the blade rotation speed is set to 1300 rpm and the treatment time is set to 300 seconds.

[0064] It should be noted that both Level 1 and Level 2 silicon wafers have a natural oxide film on the surface, and the wafer surface is basically hydrophilic. It can be considered that the level 1 silicon wafer has a slightly lower level of hydrophilicity due to the influence of water molecule polarization, while the level 2 silicon wafer achieves a high level of hydrophilicity because the water molecule polarization is eliminated due to the pretreatment.

[0065] (Example of an invention)

[0066] The contact angle of each silicon wafer surface was determined using the θ / 2 method under the following conditions. It should be noted that the liquid volume was set to the following three conditions, and the actual amount of liquid added was determined based on the image of the added droplets.

[0067] Apparatus: Portable contact angle meter PCA-11 manufactured by Kyowa Interface Science Co., Ltd.

[0068] Type of solution added: 20% (w / w) NaCl aqueous solution

[0069] Set the droplet volume to 0.5 μL, 1.0 μL, or 2.0 μL.

[0070] Measurement points: 5 points within the wafer surface (from the center towards the edge, spaced 1-2 cm apart).

[0071] Ambient humidity: 40% RH

[0072] (Comparative Example)

[0073] The contact angle of each silicon wafer surface was determined using the θ / 2 method under the following conditions. It should be noted that the liquid volume was set to the following two conditions, and the actual amount of liquid added was determined based on the image of the added droplets.

[0074] Apparatus: Portable contact angle meter PCA-11 manufactured by Kyowa Interface Science Co., Ltd.

[0075] Type of solution added: pure water

[0076] Set the droplet volume to either 1.0 μL or 2.0 μL.

[0077] Measurement points: 5 points within the wafer surface (from the center towards the edge, spaced 1-2 cm apart).

[0078] Ambient humidity: 40% RH

[0079] [Measurement Results]

[0080] In the inventive and comparative examples, for each set droplet volume, the average value of the contact angle (average of 5 points) and the average value of the droplet volume (average of 5 points) are calculated. The horizontal axis is set to the droplet volume measurement value (average of 5 points), and the vertical axis is set to the contact angle measurement value (average of 5 points). The measurement data are plotted, and the resulting graph is displayed on... Figure 2 .

[0081] In the contact angle measurements described in the comparative example, both level 1 and level 2 were independent of droplet volume, and the average contact angle was less than 5°. Since contact angles less than 5° have low reliability, therefore... Figure 2 The value is expressed as 5°. In contrast, in the contact angle measurement described in the inventive example, with a droplet volume of 0.5 μL, the average contact angle in level 1 is 21.9°, and the average contact angle in level 2 is 19.8°. Thus, in the inventive example, it is possible to detect severe differences in the hydrophilicity level of the silicon wafer surface that cannot be detected in the contact angle measurement described in the comparative example.

[0082] Furthermore, according to Figure 2 It is clear that in the invention example, at level 1 with poor hydrophilicity, the ratio of the change in contact angle to the change in droplet volume is large; conversely, at level 2 with high hydrophilicity, the ratio of the change in contact angle to the change in droplet volume is small. Therefore, in the invention example, the difference in hydrophilicity level can also be detected based on the droplet volume dependence of the contact angle.

[0083] [Additional Experiment]

[0084] Subsequently, for each silicon wafer in Level 1 and Level 2, the following single-wafer spin cleaning was performed: first, spin cleaning was performed using ozone water; then, a combination of spin cleaning based on hydrofluoric acid and subsequent spin cleaning based on ozone water was performed in 3 sets; finally, spin drying was performed at a wafer speed of 1500 rpm.

[0085] - Conditions for ozone water-based rotary cleaning

[0086] Concentration: 25 mg / L

[0087] Flow rate: 1.0L / minute

[0088] Processing time per session: 200 seconds

[0089] Chip rotation speed: 500 rpm

[0090] - Conditions for single-piece rotary cleaning based on hydrofluoric acid

[0091] Concentration: 1% by mass

[0092] Flow rate: 1.0L / minute

[0093] Processing time per cycle: 50 seconds

[0094] Chip rotation speed: 500 rpm

[0095] Subsequently, a laser particle counter (KLA-Tencor Surfscan SP7) was used to measure the surface of each silicon wafer in HS (High Sensitivity) mode to determine the number of LPDs with a size greater than 15 nm. In the silicon wafer of Level 1, there were 200 LPDs, while in the silicon wafer of Level 2, there were 5 LPDs.

[0096] This indicates that even minute differences in hydrophilicity levels on the silicon wafer surface, undetectable in water-based contact angle measurements, can lead to variations in the number of LPDs after spin cleaning of a single wafer. Therefore, it can be assumed that even slight differences in hydrophilicity levels, undetectable in water-based contact angle measurements,

[0097] - In the initial steps of monolithic spin cleaning (e.g., ozone-water based spin cleaning), wafers with poor hydrophilicity will not allow the cleaning solution to spread throughout the wafer surface. This results in the cleaning solution film failing to maintain continuity on the wafer surface, creating localized areas where the cleaning solution cannot reach.

[0098] As a result, particles remain after single-wafer spin cleaning, or uneven etching occurs after single-wafer spin cleaning, leading to increased LPD.

[0099] Regarding this, according to the inventive example, it is possible to detect severe differences in the hydrophilicity level of the silicon wafer surface, which are related to the difference in the number of LPDs after spin cleaning, before spin cleaning. Therefore, based on the contact angle measurement results described in the inventive example, for silicon wafers determined to have severe hydrophilicity differences, a countermeasure can be taken to perform a pretreatment to improve hydrophilicity before spin cleaning. That is, it can be said that the present invention is an effective method for reliably reducing LPD after spin cleaning.

[0100] Industrial utilization

[0101] The contact angle measurement method for silicon wafers according to the present invention can detect severe differences in hydrophilicity levels on the surface of silicon wafers that cannot be detected in contact angle measurements based on pure water.

Claims

1. A method for evaluating the hydrophilicity level of silicon wafers, wherein, The experiment was conducted under several conditions where the amount of liquid droplets added to the surface of a silicon wafer varied: The process of adding droplets to the surface of a silicon wafer, wherein the droplets are formed from an aqueous solution having a surface tension greater than that of pure water; as well as The process of determining the contact angle of the silicon wafer surface based on the image of the droplets. Contact angles of multiple silicon wafers were measured based on the aforementioned conditions. The differences in the hydrophilicity levels of the surfaces of the plurality of silicon wafers are evaluated based on the ratio of the change in contact angle to the change in droplet volume obtained by the measurement.

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

3. The method for evaluating the hydrophilicity level of a silicon wafer according to claim 1 or 2, wherein, The concentration of the aqueous solution is 10% by mass or more.

4. The method for evaluating the hydrophilicity level of a silicon wafer according to claim 1 or 2, wherein, The amount of the droplets is in the range of 0.3 to 3.0 μL.

5. The method for evaluating the hydrophilicity level of a silicon wafer according to claim 1 or 2, wherein, The humidity of the environment in which the contact angle was measured was in the range of 30-70%RH.

6. The method for evaluating the hydrophilicity level of a silicon wafer according to claim 1 or 2, wherein, The amount of the droplet is determined based on the image of the droplet.

7. The method for evaluating the hydrophilicity level of a silicon wafer according to claim 1 or 2, wherein, The surface of the silicon wafer is an oxide film, which forms the surface.

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

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