JP2008020318A - Device and method for measuring film thickness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new film thickness measuring device. <P>SOLUTION: This film thickness measuring device consists of a light source 4, polarizing plates 3, 6, a beam splitter 7, a 1/4-wavelength plate 9, an objective lens 8, an image focusing lens 2, and a CCD sensor 1. This film thickness measuring device acquires an interference image between a glass plate 11 and a sample 10, which are in contact state with the white polarization interference method. A chromium thin film 12 and a silica thin film 13 are formed on the glass plate 11. Its color information is converted into an HSV color space to obtain a hue value, and based on the correction result of the clearance of thickness between two faces, a true contact part equivalent to the clearance thickness or zero film thickness is visualized. The resolution of clearance thickness measurement is above ± 3nm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a novel film thickness measuring apparatus.
Moreover, this invention relates to the novel film thickness measuring method used for the said film thickness measuring apparatus.

  When the surface of the sample is a rough surface or a low reflectance surface, measurement and observation of the true contact area under lubrication (corresponding to an oil film thickness of zero) has traditionally used total reflection of light at the contact surface with the prism. This method is widely used, and there is an application example to a wet paper material (see Patent Document 1). However, this is an image viewed from an angle with respect to the surface (the aspect ratio does not become 1), so post-processing is required, and blurring occurs when the magnification is set high. There was a drawback that the measurement accuracy was lower than that.

  In the case of non-lubricated conditions, a device using white polarization interferometry has been reported as a solution to the above-described drawbacks (see Patent Document 2). However, this has the disadvantage that it is difficult to obtain a contrast interference image because the difference in refractive index between the glass plate and the lubricating oil is small under lubrication.

  As a solution to the above-mentioned drawbacks, in the case where the contact pressure is high (Hertz contact) against the surface of a smooth, high-reflectance surface such as a steel ball, normal multiple reflection interference in which only the chrome reflective film is coated on the glass surface In contrast to this method, a method of measuring a thickness on the order of nm (spacer method) in which a silica spacer film is further provided is known (see Non-Patent Document 1). This method has also been reported for the case where roughness is added to the steel ball surface (see Non-Patent Document 2).

  Furthermore, in EHL film thickness measurement for steel balls (with high reflectivity and smooth surface), interference fringes are obtained by multiple reflection interferometry using only a normal chromium reflection film, and HSV color space is used as color information. An example of film thickness measurement by calibration between hue and clearance has been reported (see Non-Patent Document 3).

  The inventor has disclosed the technical contents related to the present invention (see Non-Patent Document 4). This Non-Patent Document 4 is considered to be applicable to Patent Act Article 30 (1).

JP-A-8-247747 JP 2005-55413 A G.J.JOHNSTON, R.WAYTE and H.A.SPIKES: The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts, Tribology Transactions, 34,2 (1991) 187-194 H.A.Spikes and P.M.Cann: The development and application of the spacer layer imaging method for measuring lubricant film thickness, PIME part J, 215 (2001) 261-277 L. Gustafsson, E. Hoglund and O. Marklund: Measuring lubricant film thickness with image analysis, ImechE J. Enginerring Tribology, 208 (1994) 199-205 Masao Eguchi and Takashi Yamamoto: Clearance measurement and visualization of true contact using white spacer interferometry and HSV color space, Tribology Conference Proceedings, Tokyo 2006-5, 229-230

As described above, in the case where the contact surface pressure is high with respect to the surface of a smooth and high-reflectance surface such as a steel ball, a silica spacer film is further added to the normal multiple reflection interference method in which only the chrome reflection film is coated on the glass surface. A film thickness measuring method (spacer method) of nm order to which is given is known. This method has also been reported in the case of adding roughness to the steel ball surface.
However, since the image processing work is performed using luminance information of the RGB color space, it is necessary to pay attention to the three elements of RGB, and there is a problem that the prospect of the calibration work is deteriorated.

As described above, in EHL film thickness measurement for steel balls, interference fringes are obtained by multiple reflection interferometry using only ordinary chromium reflective films, and the color information is determined by calibration between hues and gaps using the HSV color space. An example of film thickness measurement has been reported.
However, due to the occurrence of an optical phase delay due to the chromium reflecting film coating, there is a problem that the hue cannot be defined in a gap of 95 nm or less, that is, the film thickness cannot be measured.

  Therefore, development of a novel film thickness measuring apparatus and film thickness measuring method that solves such problems is desired.

The present invention has been made in view of such problems, and an object thereof is to provide a novel film thickness measuring apparatus.
Another object of the present invention is to provide a novel film thickness measuring method used for the film thickness measuring apparatus.

  In order to solve the above problems and achieve the object of the present invention, a film thickness measuring apparatus of the present invention comprises a light-transmitting substrate in which a reflective film and a spacer film are sequentially formed, and the spacer film is in contact with a sample, and the spacer film From the opposite side, there is provided irradiation means for irradiating the light transmissive substrate with white light, light receiving means for receiving reflected light of the white light, and color information acquisition means for acquiring color information from the reflected light.

  In the thickness measurement method of the present invention, a reflective film and a spacer film are sequentially formed on a light-transmitting substrate, the spacer film is brought into contact with a sample, and white light is applied to the light-transmitting substrate from the side opposite to the spacer film. Irradiate, receive the reflected light of the white light, and acquire color information from the reflected light.

  The present invention has the following effects.

  In the film thickness measuring apparatus of the present invention, a reflective film and a spacer film are sequentially formed, and a light-transmitting substrate that contacts the sample with the spacer film, and white light is applied to the light-transmitting substrate from the side opposite to the spacer film. Since it has irradiation means for irradiating, light receiving means for receiving the reflected light of the white light, and color information acquisition means for acquiring color information from the reflected light, a novel film thickness measuring device can be provided.

  In the thickness measurement method of the present invention, a reflective film and a spacer film are sequentially formed on a light-transmitting substrate, the spacer film is brought into contact with a sample, and white light is applied to the light-transmitting substrate from the side opposite to the spacer film. Irradiating, receiving the reflected light of the white light, and acquiring color information from the reflected light, it is possible to provide a novel film thickness measurement method.

  Hereinafter, the best mode for carrying out the invention relating to a film thickness measuring apparatus and a film thickness measuring method will be described.

  The film thickness measuring apparatus of the present invention will be described. In the film thickness measuring apparatus of the present invention, a reflective film and a spacer film are sequentially formed, and a light-transmitting substrate that contacts the sample with the spacer film, and white light is applied to the light-transmitting substrate from the side opposite to the spacer film. Irradiation means for irradiating, light receiving means for receiving reflected light of the white light, and color information acquisition means for acquiring color information from the reflected light.

  FIG. 1 shows a schematic diagram of a film thickness measuring apparatus using interference fringes according to the present invention. The basic optical system is a previously reported white polarization interferometry method (Masao Eguchi, Takashi Yamamoto: Measurement of true contact area with low reflectance rough surface under non-lubrication, tribologist, 50, 6 ( 2005) Same as 471.). The outline of the measurement principle is as follows. The optical system is configured using a stereo microscope, CCD camera, epi-axial illumination device and white polarized light. The CCD camera 1 is built in the CCD camera. A light transmissive substrate (glass plate 11) having a spacer film (silica thin film 13, film thickness: 420 nm) as a top layer and a reflective film (chrome thin film 12, light transmittance: 64%) coated thereon is pressed against the sample 10. ing. As the sample 10, an object having a low reflectance rough surface is employed. The true contact portion clearance can be visualized by the interference fringes by the white interference method. Hereinafter, this method is referred to as “white spacer interferometry”.

  At this time, a polarizing plate composed of a polarizer 6 and an analyzer 3 and a quarter-wave plate 9 are inserted into the optical system so that the sample 10 is an object having a rough surface and a low reflectance and is under lubrication. An interference image having a clear hue distribution with contrast can be obtained.

The light transmissive substrate is made of glass, sapphire, or polycarbonate.
The reflective film contains chromium or silver.
The thickness of the reflective film is preferably several nm.
The spacer film contains silica or a DLC film (diamond-like carbon film).

  The thickness of the spacer film is preferably in the range of 130 nm to 900 nm. When the film thickness is 130 nm or more, there is an advantage that the interference fringes can be made chromatic. When the film thickness is 900 nm or less, there is an advantage that a decrease in luminance of the interference fringes can be suppressed.

  As the sample, steel balls, rubber tires, friction materials for brakes, friction materials for wet clutches, and the like can be used.

  The film thickness measuring method of the present invention will be described. In the thickness measurement method of the present invention, a reflective film and a spacer film are sequentially formed on a light-transmitting substrate, the spacer film is brought into contact with a sample, and white light is applied to the light-transmitting substrate from the side opposite to the spacer film. Irradiate, receive the reflected light of the white light, and acquire color information from the reflected light.

  An example of the calibration method is shown in FIG. In this method, a ball bearing steel ball 14 having a known high-precision surface shape is used to contact the glass plate 11 and a clearance generated between the two surfaces is used. This clearance can be analyzed in detail by Hertz's contact theory and can generate clearances on the order of nm.

  As can be seen from FIG. 2, a chromium thin film 12 is formed on the glass plate 11. This is to increase the reflectance. By simply forming the chrome thin film 12, the phase delay of the reflected light occurs due to the property of light as an electromagnetic wave, and the optical path difference corresponding to the delay (point of zero optical clearance) becomes achromatic, and the zero-order dark interference fringes It does not lead to formation. Therefore, a silica thin film 13 is further formed on the chromium thin film 12 for the purpose of extending the optical path difference and improving the wear resistance and peel resistance of the chromium thin film. The silica thin film 13 changes the RGB interference luminance of the true contact portion corresponding to the zero gap portion, and as a result, chromatic interference fringes appear.

  Thus, if the spacer method is combined with the white polarization interference method, the film thickness can be measured. At this time, if interference light is dispersed and the peak of its spectral distribution is used, the relationship of the following equation (1) exists between the brightness and darkness of the interference fringes. It is assumed that air exists in the clearance.

Here, n _air : air refractive index, h _air : air film thickness, n _sp : spacer film refractive index, h _sp : spacer film thickness, φ: phase lag, λ: wavelength of dispersed light, N: integer, but white light is incident vertically. However, it is difficult to measure the two-dimensional film thickness distribution by this method. Therefore, use of interference light intensity (luminance) can be considered. The interference light intensity IR in the case of two-beam interference is expressed by the following equation (2).

Here, I ₁ and I ₂ are the light intensities of the two light beams, and h _{opt is the} optical clearance (= n _air h _air + n _sp h _sp ). Also, considering the low coherence of white light, and assuming the attenuation coefficient of the luminance amplitude as K, it can be approximated by the following equation (3) (P. Sandoz and G. Tribillon: Profilometry by Zero-order Interference Fringe Identification, J. of Modern Opt., 40 (1993) 1691.).

  Due to the low coherence of the white light, high-order interference fringes that hinder the analysis are attenuated, and the analysis becomes easy. Although it is considered that the sample to be used in the present invention and the low reflectance sample surface exhibit an intermediate behavior between the two-beam interference and the multiple interference, the approximation by the formula (3) having a good view is sufficient within the scope of the present invention. Thought.

FIG. 3 shows the distribution of interference fringe light intensity (luminance) caused by white interference using the spacer method, which is plotted against the three primary color wavelengths of RGB. The letters “Cr” and “SiO ₂ ” indicate the contribution of each thin film to h _opt (optical clearance). The film thickness of the silica thin film represented by SiO ₂ can be arbitrarily adjusted, and as a result, the color (RGB value) at the time of true contact can be adjusted. The interference light intensity shown in FIG. 3 is used for the two-dimensional distribution measurement of the film thickness.

  In general, image processing work is performed using luminance information in the RGB color space. However, it is necessary to pay attention to the three elements of RGB, and the prospect of the calibration work is poor. Therefore, we focused on the HSV color space (hue H, saturation S, brightness V). If the hue H is used, it is possible to have information equivalent to the luminance information of the RGB three primary colors with a single value (normalized to 0 to 1.0). The hexagonal pyramid model shown in FIG. 4 was used for conversion of the color space from RGB to HSV. The conversion formula from RGB to HSV is shown in the following formula (4).

  Hue H has the advantage of being less susceptible to illumination brightness because it simultaneously evaluates the RGB light intensity.

  The color space conversion method from RGB to HSV is not limited to the method using the above-described hexagonal pyramid model and equation (4). In addition, as a conversion method of the color space from RGB to HSV, a bihexagonal pyramid model, conversion based on the definition of Haydn, conversion based on the definition of Raines, and the like can be adopted.

FIG. 5 shows an example of a simulation of the HSV value-clearance relationship based on real data approximation of a white interference image. However, the RGB light intensity (luminance) shown in FIG. 5 (a) is expressed by Equation (3) and Table 1. The representative wavelength of RGB filter of digital camera is R = 595nm, G = 540nm, B = 505nm, φ = 0.27, _hsp = 420nm, _nsp = 1.46, luminance amplitude attenuation coefficient K = 10 ^-6 nm ^-2 Assumed. As shown in FIG. 5B, the hue H changes with a period of about 265 nm, and the inclination thereof also changes. It can be seen that the clearance in one cycle can be calculated by using this hue. On the other hand, the saturation S and the lightness V change irregularly and have a small change width, and are not suitable for the measurement of the clearance.

Since the hue at the point of zero clearance varies depending on the spacer film thickness and the like, it is possible to vary the detection sensitivity (change in hue) of the true contact portion by shifting this point. FIG. 6 is an example showing this from the simulation results for hue H in FIG. When the hue is between 0.78 and 0.85 (equivalent to h _opt = 604 to 597 nm), the absolute value of the inclination | ΔH / Δh _opt | is 0.01 nm ⁻¹ or more, and a sensitive detection of a true contact portion can be expected. Here we examine the resolution of clearance measurement (true contact extraction). Considering the analysis of an acquired image with 256 gradations using RGB, the luminance change is about 100 gradations. Accordingly, it is possible to expect Δh _opt = 1 nm by setting the hue resolution to ΔH = 0.01 and applying the detection sensitivity of 0.01 nm ⁻¹ described above. Even if this value is considered as a safe side and the value is three times the deviation, the resolution of ± 3 nm is sufficiently obtained.

  Next, for the purpose of calibrating the relationship between hue and gap, the order and coordinate position on the plane of the interference fringes of each RGB representative wavelength were examined. Fig. 7 shows the relationship between the coordinates of the center of the interference fringe (G and B wavelengths) and the clearance read from the order under Hertz contact using a steel ball (steel ball diameter: 4.763 mm, maximum Hertz pressure: 245 MPa) Indicates. The broken line is a regression polynomial that evaluates G and B measurement points simultaneously. By interpolation with this polynomial, a smooth relationship was obtained up to the clearance of the interference fringe generation position of the lowest order (G bright part 3rd order, clearance about 119 nm). In addition, the figure also shows the deformed outline calculated theoretically between the glass plate and the steel ball when the load is zero (rigid body) and the load (0.134N) is applied. From this figure, the experimental points and extrapolation lines almost coincided with the theoretical outline, and the validity of extrapolation was confirmed. From this result, this regression polynomial was used for the clearance smaller than about 119 nm.

  FIG. 8 is an example of the relationship between the hue H measured in the vicinity of the center line of the Hertz contact portion and the above-described clearance. In the figure, Y = 223 and Y = 232 represent the pixel position in the vertical direction of the horizontal 640 × vertical 480 pixel image in which the hue value was measured. Almost on the diameter of the Hertzian contact circle. The hue at the zero gap is 0.82 and the period is about 260 nm.

  In addition, the result of the hue by the above-described simulation is also shown in the figure. In the region where the clearance exceeds 30 nm, the hue values of the two are almost the same, and the effectiveness of Hue H and the effectiveness of Equation (3) are confirmed. The cause of the inconsistency is considered to be a problem in the vicinity of the Hertz contact portion and the degree of approximation in the interference light intensity simulation. From this figure, if the luminance change of RGB is about 100 gradations and the resolution of hue H is also ΔH = 0.01, from the gradient of ΔH / Δh = -1.0 / 300nm = -0.01 / 3nm shown in the figure, It can be said that this method has a resolution of about ± 3 nm.

  FIG. 9 shows the result of plotting the hue value at the center of the Hertz contact against the maximum Hertz contact pressure, and a hue plot corresponding to the pressure distribution under a set of contact conditions. The circle mark indicates the hue value at the center of the Hertz contact, and the square mark indicates the hue value corresponding to the pressure distribution from the center to the peripheral edge inside the Hertz contact. Clearly, the hue increases with increasing surface pressure, indicating the dependency of hue on surface pressure. Furthermore, the hue with zero contact pressure is estimated to be about 0.82, and this value can be regarded as the starting point of true contact. This dependence is considered to be because the silica spacer film was compressed and thinned by Hertz pressure.

A report on the thinning phenomenon of silica spacer layers (G. Guangteng, PM Cann, AV Olver, HA Spikes: Lubricant Film Thickness in Rough Surface, Mixed Elasthydrodynamic Contact, J. Tribology, 122, 1 (2000) 65. ), A value of Δh _sp / Δp _max = −1 nm / 100 MPa is obtained under substantially the same conditions (h _sp = 400 nm). From this value and the above-described detection sensitivity ΔH / Δh = −0.01 nm ⁻¹ , ΔH / Δp _max = 0.01 / 100 MPa is obtained. This inclination is shown in the figure. Considering the nonlinearity of the detection sensitivity, the main cause of the surface pressure dependence of the hue is considered to be the thinning of the silica spacer film thickness.

  In extracting the true contact portion, it is necessary to grasp the dependence of the hue on the surface pressure in advance and to give a width. Further, this dependence indicates the possibility of simultaneously measuring the contact pressure at the true contact portion, which has been impossible in the past when the true contact such as mixed lubrication and the oil film layer coexist.

  The application of the film thickness measuring apparatus and the film thickness measuring method of the present invention will be described. The present invention can be applied to the visual measurement of the oil film thickness and the true contact portion for a contact surface having a low reflectance rough surface. Specifically, measurement and contact of oil film thickness (film thickness) between machine sliding parts where liquids such as lubricating oil exist, especially friction drive surfaces (wet clutch surfaces, road surfaces and tire surfaces), and seal surfaces Applicable for condition monitoring.

From the above, according to the best mode for carrying out the present invention, the following advantages can be obtained.
(1) By adopting polarization white light interferometry, it is possible to obtain a contact image seen from the front with respect to the surface, and the true contact portion between an object having a rough surface and a low reflectance surface and a transparent (glass) plate The film thickness distribution can be visualized.
(2) By using HSV color information (hue value) based on CCD image information of three RGB colors and using a (mechanical) calibration device, resolution exceeding ± 3 nm is not affected by the usage environment such as being resistant to uneven illumination. Can measure the oil film thickness and extract the true contact area.
(3) An optimum calibration curve can be constructed by adjusting the thickness of the silica spacer and the RGB three-color light intensity.
(4) Even the contact surface (mixed lubrication surface) where the oil film portion and the true contact portion coexist can be identified, and the contact pressure of the true contact portion can also be estimated.

  The present invention is not limited to the best mode for carrying out the above-described invention, and various other configurations can be adopted without departing from the gist of the present invention.

It is a figure which shows an example of the apparatus used for this invention. It is a figure which shows the glass plate used for this invention. It is a figure which shows the white interference fringe light intensity distribution using the spacer method. It is a figure used for description of HSV color space (HSV hexagonal pyramid model). It is a figure which shows the simulation result about the relationship between an HSV value and a clearance gap. It is a figure which shows the change of the detection sensitivity in simulation. It is a figure which shows the relationship between a radius and a clearance gap. It is a figure which shows the relationship between a hue value and a clearance gap (film thickness). It is a figure which shows the surface pressure dependence of a hue.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... CCD sensor, 2 ... Imaging lens, 3 ... Analyzer, 4 ... Halogen lamp, 5 ... Light guide, 6 ... Polarizer, 7 ... Beam splitter, 8 ... Objective lens, 9 ··· 1/4 wavelength plate, 10 ··· Sample, 11 ··· Glass plate, 12 ··· Chrome thin film, 13 ··· Silica thin film, 14 · · · Steel ball

Claims (12)

A reflective film and a spacer film are sequentially formed, and a light-transmitting substrate that contacts the sample with the spacer film;
Irradiation means for irradiating the light transmissive substrate with white light from the side opposite to the spacer film,
A light receiving means for receiving the reflected light of the white light;
A film thickness measurement device comprising color information acquisition means for acquiring color information from the reflected light. The film thickness measuring apparatus according to claim 1, wherein the reflective film contains chromium or silver. The film thickness measuring apparatus according to claim 1, wherein the spacer film contains silica or a DLC film (diamond-like carbon film). The film thickness measuring apparatus according to claim 1, wherein the thickness of the spacer film is in a range of 130 nm to 900 nm. The film thickness measuring apparatus according to claim 1, wherein the light transmissive substrate is made of glass, sapphire, or polycarbonate. The film thickness measuring apparatus according to claim 1, wherein the color information is HSV color information. A reflective film and a spacer film are sequentially formed on the light transmissive substrate,
Contacting the spacer film with the sample;
Irradiate the light transmissive substrate with white light from the side opposite to the spacer film,
Receiving the reflected light of the white light,
A film thickness measurement method for obtaining color information from the reflected light. The film thickness measuring method according to claim 7, wherein the reflective film contains chromium or silver. The film thickness measuring method according to claim 7, wherein the spacer film contains silica or a DLC film (diamond-like carbon film). The film thickness measuring method according to claim 7, wherein the thickness of the spacer film is in a range of 130 nm to 900 nm. The film thickness measuring method according to claim 7, wherein the light-transmitting substrate is made of glass, sapphire, or polycarbonate. The film thickness measurement method according to claim 7, wherein the color information is HSV color information.

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