JP2010054312A - Method of measuring contact angle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of measuring a contact angle capable of obtaining the contact angle of an area conveniently and precisely even in an area with a micrometer order or less, which is impossible to optically measure a contact angle. <P>SOLUTION: The method of measuring the contact angle includes the steps of measuring adhesion, with the cantilever of a scanning probe microscope, from a force curve acting between the distal end of the cantilever and a sample in an area referred on a sample, observing a droplet dropped on the area referred and measuring a contact angle referred; determining, from the adhesion of the area referred to, the contact angle of the area referred to, and Young's equation, the relational expression of the adhesion and the contact angle previously, measuring the adhesion from the force curve in the area where the contact angle on the sample to be measured, and determining the contact angle of the area where the angle to be measured by inserting the value of the adhesion of the area to be measured in the relational equation. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for measuring a contact angle used as a measure for evaluating the wettability of a solid surface with a liquid, and more particularly, to a method for measuring a contact angle using a scanning probe microscope in a minute part in the micron to submicron range. .

  In general, the hydrophilicity and water repellency of a sample are evaluated by the contact angle of a droplet on the sample surface. This contact angle refers to the angle on the side containing the liquid among the angles formed between the tangent line drawn to the liquid and the solid surface at the contact point of the solid, liquid and gas phases. Conventionally, the contact angle can be measured by using an optical microscope with a goniometer that can read the angle, directly measuring the contact angle by observing the droplet from the side, or photographing the contour of the droplet and analyzing the image. This was done by measuring the angle.

  For example, FIG. 7 shows a contact angle measuring device using a conventional contact angle measuring method known as a sessile drop method (see Patent Document 1). The contact angle measuring device comprises a sample stage 71, a syringe 74 for dropping a droplet on the sample 73, an injection needle 75, a light source 76, a measuring camera 77, and a measurement result calculation / display device (personal computer) 78. The droplets 72 and the sample 73 thus formed and the measurement camera 77 are arranged in a line at the same height. An angle formed by the dropped droplet 72 and the horizontally placed sample 73 is measured from an image obtained by the measurement camera 77 as a contact angle.

In recent years, by using an ultra-fine injection needle with an inner diameter of about 5 μm, a droplet with a droplet volume of 10 to 1000 picoliters and a minimum diameter of several tens of μm is produced, and the contact angle on a fine surface within 100 μm is It can be measured and used for evaluation of wettability related to a fine pattern provided on a semiconductor wafer, a glass substrate or the like.
JP 2007-108007 A

  However, the miniaturization of patterns used in semiconductors, semiconductor photomasks, nanoimprints, micromachines, etc. is progressing more and more, and as a method for evaluating the material surface in a finer area, a micrometer order or smaller area is used. The measured value of the contact angle at is obtained. However, due to the vapor pressure, the droplets cannot be made smaller, for example, water cannot form water droplets on the order of micrometer order or less, and as a result, the contact angle in a minute local region on the order of micrometer order or less. There was a problem that measurement was not possible. As described above, the conventional contact angle measurement method in which a droplet is formed on a sample and the contact angle of the droplet is optically measured has a limit in the measurable region.

  Therefore, the present invention has been made in view of the above problems. That is, the object of the present invention is to provide a simple and accurate measurement even in a region where the contact angle is measured in the micrometer order or less and the contact angle cannot be optically measured by forming a droplet. It is an object to provide a contact angle measuring method capable of obtaining the contact angle of the region well.

  In order to solve the above-mentioned problems, the present inventor obtained a relationship with the contact angle by measuring the adhesion force acting between the cantilever probe and the sample using a scanning probe microscope as a result of various studies. The contact angle measurement method of the present invention has been completed.

  The contact angle measurement method according to the invention of claim 1 is a contact angle measurement method of a droplet on a sample, and refers to a region where the contact angle of a droplet on the sample can be optically measured. Using the cantilever of the scanning probe microscope, the adhesion force is measured from the force curve acting between the tip of the cantilever probe and the sample in the reference region on the sample, and the optical microscope is used. The contact angle of the reference region is measured by observing a droplet dropped on the reference region from the side, and the adhesion force of the reference region, the contact angle of the reference region, and Young's Based on the equation, a relational expression between the adhesion force and the contact angle is obtained in advance, and then the force acting between the tip of the cantilever probe and the sample in the region where the contact angle on the sample is to be measured. Measure the adhesive force from the curve, It is characterized in that the value of the adhesion of Teisu should region was inserted into the equation determining the contact angle of the area to be the measurement.

  A contact angle measuring method according to a second aspect of the present invention is the contact angle measuring method according to the first aspect, wherein an adhesion force acting between the probe of the cantilever and the sample is a van der Waals force and a meniscus force. It is characterized by being expressed in the sum.

A contact angle measurement method according to a third aspect of the present invention is the contact angle measurement method according to the first or second aspect, wherein the adhesion force is F, the radius of curvature of the tip of the probe of the cantilever is R, and the probe is measured. Γ ^d _C is the dispersion force component of the surface energy of the needle, γ _{L is} the surface energy of the liquid in which the meniscus is formed between the cantilever probe and the sample, and γ ^d _L is the dispersion force component of the surface energy of the liquid. The contact angle θ of the region to be measured is given by the following equation, where A is the motion coefficient and A is the contact angle of the droplet.

  The contact angle measuring method according to a fourth aspect of the invention is the contact angle measuring method according to any one of the first to third aspects, wherein the region to be measured on the sample is in the horizontal direction of the sample surface. The width is at least one direction and is in the micrometer to submicrometer range.

  The contact angle measurement method according to the invention of claim 5 is the contact angle measurement method according to any one of claims 1 to 4, wherein the adhesion force acting between the probe of the cantilever and the sample is measured. Is performed in a constant humidity environment.

  According to the contact angle measuring method of the present invention, in addition to the contact angle measurement in a wide area on the sample to which the conventional contact angle measuring method can be applied, the micron order on the sample that could not be measured conventionally is submicron. It is possible to measure a contact angle even in a small local region of the order, and it is possible to evaluate the wettability of the micro region in a non-destructive manner. Furthermore, it is possible to predict the contact angle in an ultra-fine region of several tens of nanometers where the adhesion force can be measured with the AFM cantilever probe.

  Hereinafter, a contact angle measurement method according to the best embodiment of the present invention will be described in detail with reference to the drawings.

In the present invention, using the cantilever of the scanning probe microscope, the adhesion force acting between the cantilever and the sample is measured in the reference region on the sample. In the present invention, the region to be referred to is a wide region to which a conventional contact angle measuring method capable of obtaining a contact angle by directly observing a droplet on a sample with an optical microscope, for example, several tens of μm square or more. This area is shown. The contact angle obtained by measuring in the reference area is the contact angle of the reference area. In the measurement, it is desirable that the reference region on the sample has the same surface state as the minute region to be measured.
In the present invention, the order of measurement of the adhesion force measurement and the contact angle measurement in the reference region on the sample is not particularly limited, and either may be first.

  In the present invention, as a scanning probe microscope used for measuring the adhesion force, the surface of the sample is scanned with a probe called a cantilever provided with a probe at one end of the cantilever, and the size and shape of the pattern on the sample surface are measured. A microscope is used, and an atomic force microscope (hereinafter referred to as “AFM”) is most preferable. It can also be used with other microscopes collectively called Microscope (SPM). In the following description, a case where the contact angle measuring method of the present invention is used for AFM will be described as an example.

  FIG. 1 shows a measurement principle diagram as an example of an AFM used for measuring the adhesion force of the present invention. As shown in FIG. 1, when a small probe 12 having a cantilever structure is gradually brought closer to the surface of the sample 11, an attractive force or a repulsive force acts between them at a certain distance, and the cantilever 13 provided with the probe 12 Distortion occurs. The distortion is detected by the optical sensor 15 due to the change in the reflection angle of the laser beam 14 applied to the back surface of the cantilever 13, and the amount of distortion of the cantilever 13 is constant when the probe 12 is scanned on the surface of the sample 11. The distance between the sample 11 and the probe 12 is controlled. The scanning of the surface of the sample 11 (X direction, Y direction) and the control of the distance between the sample 11 and the probe 12 (Z direction) are performed by the piezo scanner 16, and the voltage applied to each scanner is analyzed. An image of the surface of the sample 11 is obtained. When the probe is moved to a predetermined position in the region to be measured, the force curve is obtained and the adhesion force is measured.

  The force curve (also called force distance curve) is the upper and lower limits of the distance between the AFM probe and the sample, and the fixed point is moved up and down. The distance between the probe and the sample and the force acting on the cantilever (cantilever It is a curve plotting the relationship with the amount of deflection). From the amount of deflection read from the force curve and the spring constant of the cantilever, the adhesion force can be obtained according to Hooke's law.

  As described above, the AFM is provided with a sharp pointed probe at the free end of a cantilever having a length of 100 μm to 2000 μm made of an elastic body having a predetermined spring constant. This is a device that measures the local shape and physical properties of a sample from the displacement of the cantilever caused by an attractive force such as van der Waals force acting between atoms at the tip and the sample surface.

  As the material and shape of the cantilever probe used in the present invention, conventionally used materials and shapes can be applied. That is, as the probe material, silicon, silicon nitride, silicon oxide, silicon carbide or the like is used, and further, a metal such as gold or titanium, or carbon is coated to impart conductivity to the material surface. Things are used. As the shape of the probe, a shape having a sharpened tip such as a pyramid-shaped quadrangular pyramid or cone is used. For example, in the case of a quadrangular pyramid, the apex angle of the tip is about 70 degrees and the radius of curvature is about several tens of nm. It is. The probe may be manufactured integrally with the cantilever, or a probe manufactured separately from the cantilever may be attached to the cantilever.

Next, the adhesion force acting between the cantilever probe and the sample will be further described.
FIG. 2 is a schematic diagram for explaining the adhesion force acting between the probe of the cantilever and the sample. As shown in FIG. 2, when the cantilever probe 22 is brought close to the surface of the sample 21, a surface force acts between the sample 21 and the probe 22 to generate an adhesive force. Assuming that this adhesion force is F, the adhesion force (F) is the van der Waals force (f ₁ ) attracted by the inter-solid atoms of the sample 21 and the probe 22 as shown in FIG. And the meniscus force (f ₂ ) formed between the sample 21 and the probe 22 by condensation of moisture in the atmosphere. Since there is moisture condensation in the atmosphere, it is not necessary to form droplets (water droplets) on the sample 21 in advance in measuring the adhesion force.
F = f ₁ + f ₂ = 4πR (γ ^d _C γ ^d _Sample ) ^1/2 + 4πRγ _L cos θ (1)
Here, R: radius of curvature of tip of cantilever probe, γ ^d _C : dispersive force component of surface energy of cantilever probe, γ ^d _Sample : dispersive force component of surface energy of sample, γ _L : liquid forming meniscus ( Surface energy of water), θ: contact angle of liquid (water).

On the other hand, the phenomenon between the liquid and the solid surface is simply expressed by the degree of wetting of the liquid onto the solid surface. FIG. 3 is a schematic diagram illustrating the force acting on the liquid on the sample surface. As shown in FIG. 3, when the droplet 33 is formed on the surface of the sample 31, the surface energy of the sample is expressed as γ _Sample , between the sample and the liquid with respect to the contact angle representing the degree of wettability, that is, hydrophilicity and water repellency. When the interface energy is γ _SL , the liquid surface energy is γ _L , and the contact angle is θ, the force relationship indicated by the three arrows is balanced, and the following Young's formula (2) holds.
γ _Sample = Γ _L cosθ + γ _SL (2)

The adhesion work W _SL when the sample is wetted with liquid at the interface between the two solid and liquid media in contact is expressed by the following equation (3).
W _SL = γ _Sample + γ _L −γ _SL (3)

Here, when only the dispersion force mainly contributes to the interaction of the two media, the dispersion force component of the surface energy of the water that formed the meniscus is γ ^d _L ,
W _SL = 2 (γ ^d _Sample γ ^d _L ) ^1/2
From the equation (3), the interfacial energy γ _SL between the sample and the liquid is expressed by the equation (4).
γ _SL = γ _Sample + γ _L −2 (γ ^d _Sample γ ^d _L ) ^1/2 (4)

From the above formula (2) and formula (4),
(Γ ^d _Sample ) ^1/2 = γ _L (1 + cos θ) / 2 (γ ^d _L ) ^1/2 (5)
Get.

From the above formulas (1) and (5), the relationship between the adhesion force F and the contact angle θ is expressed by formula (6). Here, it is assumed that the sample has an interaction mainly due to dispersion force.
F = 4πR (γ ^d _C ) ^1/2 γ _L (1 + cos θ) / 2 (γ ^d _L ) ^1/2 + 4πRγ _L cos θ (6)

ここで、上記の接触角θの数式に用いている各符号の意味を改めて以下に記す。
θ：液体（水）の接触角、F：試料とカンチレバー探針との間に働く付着力、Ｒ：カンチレバー探針先端の曲率半径、γ^d _C：カンチレバー探針の表面エネルギーの分散力成分、γ_L：液体（水）の表面エネルギー、γ^d _L：液体（水）の表面エネルギーの分散力成分、Ａ：キャリブレーション係数 In the actual measurement of the adhesive force, calibration due to a measuring instrument such as the curvature radius R of the tip of the probe and the spring constant of the cantilever is required. In addition, factors such as the surface roughness of the sample and the cantilever are included. Therefore, if the measured value of adhesive force is F _exp and the calibration coefficient is A,
F _exp = AF = A [4πR (γ ^d _C ) ^1/2 γ _L (1 + cos θ) / 2 (γ ^d _L ) ^1/2 + 4πRγ ^d _L cos θ]
Thus, the contact angle θ shown in the following equation is obtained.

Here, the meaning of each symbol used in the mathematical formula of the contact angle θ will be described again below.
θ: contact angle of liquid (water), F: adhesion force acting between sample and cantilever probe, R: radius of curvature of cantilever probe tip, γ ^d _C : dispersion force component of surface energy of cantilever probe, γ _L : surface energy of liquid (water), γ ^d _L : dispersion force component of surface energy of liquid (water), A: calibration coefficient

In the contact angle measuring method of the present invention, the radius of curvature r _k of the meniscus formed between the sample and the cantilever probe is expressed as the Kelvin equation as shown in the following equation (7). Related to.
r _k = γV / RTlog (p / p _s ) (7)
Where p / p _s : relative humidity, γ: surface energy, V: molar volume, R: gas constant, T: temperature, r _k : radius of curvature (Kelvin radius)

When the meniscus is a spherical concave water meniscus, the measurement temperature is 20 ° C., and γV / RT is 0.54 nm, the radius of curvature changes with respect to the relative humidity. FIG. 4 is a diagram showing the relationship between the radius of curvature of the meniscus formed between the sample and the cantilever probe and the relative humidity of the measurement environment.
As shown in FIG. 4, the radius of curvature increases as the humidity increases. As a result, the area where the meniscus force works increases, and the rate of change of the radius of curvature relative to the change in humidity increases as the humidity increases. When the humidity is 60% or more, the radius of curvature rapidly increases. A change in the humidity of the environment in which the adhesion force is measured causes a change in the radius of curvature of the meniscus, which fluctuates the value of the adhesion force. Therefore, the measurement of the adhesive force in the present invention is preferably performed in an environment where the humidity is constant (constant humidity) at a relative humidity of 50% or less, more preferably at a low humidity of 40% or less.

As described above, the value of the contact angle θ on the sample surface can be obtained from the adhesive force F obtained from the force curve measured by the AFM, using the mathematical formula [Equation 1] of the contact angle θ. Since the tip of the AFM cantilever probe is extremely small, it is possible to predict the contact angle not only in the micrometer to submicrometer region of the sample but also in the ultrafine region of several tens of nanometers.
Hereinafter, the present invention will be described in detail by way of examples.

  As a sample, a nanoimprint template made of quartz glass in which a fine line / space uneven pattern was formed on one main surface was used. Method of evaluating surface water wettability when a mold release agent (OPTOOL DSX: manufactured by Daikin Industries, Ltd.) is formed on the entire pattern side of the template and the template surface is surface treated with the mold release agent. The contact angle measurement method of the present invention was applied to the above. The contact angle measurement method including the following measurement of adhesion force was performed in a constant temperature and humidity environment with a temperature of 20 ° C. and a humidity of 40%.

  First, an AFM (L-trace: manufactured by SII Nano Technology) is used as a scanning probe microscope, a probe made of silicon (Si) is used as a cantilever, and a cantilever is used in a reference region on a sample. The adhesion force acting between the probe and the sample was measured. The region to be referred to was a wide region having a pattern width of several tens of μm or more.

  Next, using a contact angle meter (DM500: manufactured by Kyowa Interface Science Co., Ltd.), distilled water was dropped as a water droplet from the tip of the injection needle of the contact angle meter to the region to be referred to, and formed on the sample surface. The contact angle of the area to be referred to was measured by observing the water droplet from the side using an optical microscope. Measurements were taken at several locations in the reference area.

Next, a relational expression between the adhesion force and the contact angle was obtained based on the adhesion force F of the reference region obtained above, the contact angle of the reference region, and the Young's formula. Here, although Si is used for the probe, it is assumed that a natural oxide film is formed on the surface, and the surface energy of the probe is expressed as γ _C = γ _{SiO 2} .
F = 4πR (γ _SiO2 γ _Sample ) ^1/2 + 4πRγ _L cosθ
R is the radius of curvature of the tip of the cantilever probe, γ is the surface energy, γ _Sample is the surface energy of the sample, γ _L is the surface energy of the water forming the meniscus, and θ is the contact angle of water.

In the above embodiment, Si, which is often used as a probe material, is used as an example, and a natural oxide film is formed on the surface thereof. Surface energy γ _SiO2 is used, but other materials are used for the probe. When used, the surface energy value corresponding to the material is used.

Further, the following Young's formula is established for water droplets formed on the sample surface. γ _SL is the interfacial energy between the sample and the water droplet.
γ _Sample = Γ _L cosθ + γ _SL

On the other hand, when the sample is wetted with water at the interface between the two solid and liquid media that are in contact, the following relationship is established from the formula relating to the adhesion work as described above.
γ _SL = γ _Sample + γ _L −2 (γ ^d _Sample γ ^d _L ) ^1/2

Therefore, the following equation is obtained.
(Γ ^d _Sample ) ^1/2 = γ _L (1 + cos θ) / 2 (γ ^d _L ) ^1/2

From the above formula, when the contact angle θ is obtained by setting the calibration coefficient due to the measuring instrument to A, the contact angle θ is expressed by the following formula.

  FIG. 5 shows the calculated value of the contact angle θ, the adhesion force acting between the cantilever probe in the reference area and the sample, and the contact angle of the water droplet in the reference area on the sample surface. The actual measured values are illustrated.

  In FIG. 5, the vertical axis represents the adhesion force (nN), the horizontal axis represents the contact angle (degree), the black circle represents the measured value, and the solid line represents the calculated value of the contact angle θ. The two dotted lines are individual calculated values of van der Waals force and meniscus force, respectively. As shown in FIG. 5, the actual measured value and the calculated value almost coincide with each other, indicating the high reliability of the value based on the calculated value of the contact angle θ formula.

  FIG. 6 is a diagram for calculating the contact angle of the liquid on the sample surface from the adhesion force acting between the cantilever probe and the sample. The vertical axis represents the contact angle (degrees), the horizontal axis represents the adhesion force (nN), and the black circle represents The calculated value and the white circle are measured values. As shown in FIG. 6, the actual measurement value and the calculated value almost coincide with each other.

  Therefore, in the region where the contact angle on the sample is to be measured, the adhesion force acting between the cantilever probe and the sample is measured, and the value of the adhesion force is inserted into the formula of the contact angle θ to be measured. A contact angle measurement method for obtaining the contact angle of the contact is possible.

  According to the contact angle measuring method of the present invention, if the relational expression between the contact force θ and the adhesion force F acting between the cantilever probe and the sample shown above is obtained, Even at the measurement location measured by the optical contact angle measurement method, the adhesion force is measured from the focus curve of the AFM without using the optical contact angle measurement method, and inserted into the above relational expression to obtain the contact angle. Can be obtained.

  Furthermore, according to the contact angle measurement method of the present invention, not only can a contact angle be measured from a micron order to a submicron order, but also a cantilever of an AFM, which cannot be measured by the conventional contact angle measurement method. It is also possible to predict the contact angle in the ultra-fine region of several tens of nanometers where the adhesive force can be measured with this probe.

It is a measurement principle figure as an example of AFM used for measuring the adhesive force of the present invention. It is a schematic diagram explaining the adhesive force which acts between a cantilever probe and a sample. It is a schematic diagram explaining the force which acts on the liquid of a sample surface. It is a figure which shows the relationship between the curvature radius of the meniscus formed between a sample and a cantilever probe, and relative humidity. It is a figure which shows the correlation with the contact force of the liquid force of the adhesion force which acts between a cantilever probe and a sample, and a sample surface. It is a figure which calculates the contact angle of the liquid of a sample surface from the adhesive force which acts between a cantilever probe and a sample. It is a figure which shows an example of the contact angle measuring apparatus by the conventional contact angle measuring method.

Explanation of symbols

11, 21 Sample 12, 22 Cantilever probe 13 Cantilever 14 Laser light 15 Optical sensor 16 Piezo scanner 17a X, Y axis scanning system 17b Z axis servo system 18 Computer 19 Preamplifier 20 Monitor 23 Water f ₁ Van der Waals force f ₂ Meniscus force 31 Sample 33 Droplet 71 Sample stage 72 Droplet 73 Sample 74 Syringe 75 Injection needle 76 Light source 77 Measuring camera 78 Measurement result calculation / display device (PC)

Claims (5)

A method for measuring the contact angle of a droplet on a sample, comprising:
The region where the contact angle of the droplet on the sample can be optically measured is referred to as a region,
Using the cantilever of the scanning probe microscope, the adhesion force is measured from the force curve acting between the tip of the cantilever probe and the sample in the reference region on the sample,
Using an optical microscope, measure the contact angle of the reference area by observing from the side the droplet dropped on the reference area,
Based on the adhesion force of the region to be referred to, the contact angle of the region to be referred to, and the Young's formula, a relational expression between the adhesion force and the contact angle is obtained in advance.
Next, in the region where the contact angle on the sample is to be measured, the adhesion force is measured from the force curve acting between the tip of the cantilever probe and the sample,
A contact angle measurement method, wherein the contact angle value of the region to be measured is obtained by inserting an adhesion force value of the region to be measured into the relational expression.   The contact angle measuring method according to claim 1, wherein an adhesion force acting between the cantilever probe and the sample is indicated by a sum of van der Waals force and meniscus force. A liquid in which the adhesion force is F, the radius of curvature of the tip of the cantilever probe is R, the dispersion force component of the surface energy of the probe is γ ^d _C , and a meniscus is formed between the cantilever probe and the sample. Where γ _{L is} the surface energy of the liquid, γ ^d _L is the dispersion force component of the surface energy of the liquid, A is the calibration coefficient, and θ is the contact angle of the droplet. The contact angle measuring method according to claim 1, wherein the contact angle is measured.

  The region to be measured on the sample is a region from a micrometer to a submicrometer in at least one horizontal width of the sample surface. The contact angle measuring method as described.   The contact angle measurement method according to any one of claims 1 to 4, wherein the adhesion force acting between the probe of the cantilever and the sample is measured in a constant humidity environment.

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