JP7294282B2 - measuring device - Google Patents

Description

The present invention relates to a measuring device for measuring film thickness.

2. Description of the Related Art As an example of conventional technology of a measuring device for measuring a film thickness, for example, a film thickness measuring device disclosed in Patent Document 1 is known. The film thickness measuring apparatus according to Patent Document 1 obtains an interference image between the glass plate and the sample in contact with each other by white polarized light interferometry. A chromium thin film and a silica thin film are formed on the glass plate. The color information is converted to the HSV color space to obtain the hue value, and based on the result of calibration with the thickness of the gap between the two surfaces, the real contact portion corresponding to the thickness of the gap or film thickness of zero is visualized.

An oil film thickness measuring device disclosed in Patent Document 2 is also known. The measuring device according to Patent Document 2 includes a transparent plate, a light source, a microscope, a camera, and an arithmetic device. A light source and a camera are arranged on the top side of the transparent plate. The light source instantaneously irradiates the contact portion between the transparent plate and the rolling element with light in a plurality of wavelength ranges. The camera captures an image of the contact area magnified by a microscope when the interference light is incident thereon to generate an interference image. The computing device acquires the intensity for each wavelength band from the interference image and converts it into an oil film thickness. When the light emission time of the light source is L [μs], the rolling element rotation speed is V [m/s], the microscope magnification is M, the image sensor size and the number of pixels of the camera is S [μm], and NP , 0<L×V+(S/NP)/M≦5.

Furthermore, a sliding device disclosed in Patent Document 3 is also known. In the sliding device according to Patent Document 3, the transparent sliding material and the reflective sliding material move relative to each other while receiving a load, and the liquid film exists between the transparent sliding material and the reflective sliding material. The sliding material and the liquid film are made of a material that transmits light, and the reflective sliding material is made of a material that reflects light. The light from the white light source is made into light composed of monochromatic light of three wavelengths using a band-pass filter, and the light is transmitted through the transparent sliding material and the liquid film to irradiate the reflective sliding material, thereby causing light interference. , the camera measures the brightness of each of the two or more different wavelengths of light in the resulting light interference. A computing device calculates the film thickness of the liquid film based on the brightness of each of the two or more lights measured by the camera.

Japanese Patent Application Laid-Open No. 2008-020318 JP 2017-207316 A JP 2017-053690 A

However, the conventional measuring device has the following problems. In relation to Patent Document 1, since the white light source contains various wavelength components, the optical film thickness and hue calibration curves are unique to the observation system (camera, lens, light source), and calibration work is performed separately. (calibration curve specific correction problem). In addition, since the hue turns back in the range of 0 to 1, there is a problem that the possible range of film thickness conversion is limited to one period (approximately 265 nm) (calibration curve period problem).

In addition, related to Patent Documents 2 and 3, there are also the following problems. That is, if the sample has a portion with a large surface roughness due to abrasion or the like, the intensity of the reflected light at that portion is attenuated due to irregular reflection, and thus it deviates from the calibration curve, making film thickness conversion difficult. Fig. 9 shows interferograms of a sample for explaining this problem, Fig. 9(a) is an interferogram of a sample without wear, and Fig. 9(b) is an interferogram of a sample including partial wear. each shown. Thus, specimens with wear have a significantly reduced intensity of reflected light in the interference image. This means that the occurrence of wear cannot be tolerated when observing the lubrication phenomenon over time, and it is a factor that significantly reduces the degree of freedom in measurement, such as only completely non-contact lubrication phenomena can be handled (diffuse reflection problem).

Also, when oil exists between the glass and the sample, light absorption occurs due to the oil, and the reflected light is attenuated according to the absorption coefficient that depends on the wavelength and the type of oil. In order to accurately estimate the film thickness, it is necessary to obtain this absorption coefficient for each wavelength and type of oil (absorption problem).

Furthermore, when the interference image is collectively converted into a film thickness, if the RGB brightness becomes locally abnormal due to the reasons described above or for other reasons, only that portion will have a film thickness that is significantly different from the surroundings. Reliability as a film thickness distribution is lowered (peculiar pixel problem).

Theoretically, no calibration work is required, but even if the above problem does not exist, it is rare for the RGB luminance to perfectly match the calibration curve. It is necessary to manually enter the relationship between RGB luminance and film thickness values to correct the calibration curve (calibration curve manual correction problem).

SUMMARY OF THE INVENTION It is an object of the present invention to provide a measuring apparatus in which the influence of the surface state of a sample is suppressed and the film thickness can be measured more easily.

In order to achieve the above object, the measuring apparatus according to claim 1 comprises: a light source that emits monochromatic light having three or more wavelengths; a first sliding portion that transmits the monochromatic light; a semi-permeable film that reflects a portion of the monochromatic light that has passed through the portion and transmits the other portion; a second sliding portion that reflects the monochromatic light that has passed through the semi-permeable film; a load applying portion that causes relative motion between the first sliding portion and the second sliding portion; and interference between the monochromatic light reflected by the semi-transparent film and the monochromatic light reflected by the second sliding portion. an imaging unit that acquires an interference image obtained by making the interference image, and a brightness distribution image for each wavelength region of the monochromatic light obtained by the interference image, is converted into a hue distribution image, and based on a predetermined correspondence relationship between the optical film thickness and the hue value obtaining an optical film thickness distribution image by converting the hue value of each pixel of the hue distribution image into an optical film thickness; and a calculation unit for calculating the film thickness distribution, wherein the calculation unit separates the hue value of each pixel of the hue distribution image into a sine component and a cosine component, and calculates the hue for each of the sine component and the cosine component. After performing the filtering process, a further hue filtering process is performed to remove the singular pixels in the hue distribution image by taking the arctangent and restoring it.

Further, according to the invention of claim 2 , in the invention of claim 1 , the filter used in the hue filtering process is a Gaussian filter.

The invention according to claim 3 is the invention according to claim 1 or claim 2 , wherein the calculation unit selects a hue A further process is performed to restore the sharpness of the hue distribution image by selecting the optical thickness value that is closest to the optical thickness obtained from the filtered hue .

Advantageous Effects of Invention According to the measuring apparatus of the present invention, it is possible to suppress the influence of the surface state of the sample and to provide a measuring apparatus capable of measuring the film thickness more easily.

It is a block diagram showing an example of a measuring device concerning an embodiment. FIG. 4A is a diagram showing the relationship between optical film thickness and RGB luminance, and FIG. 4B is a diagram showing the relationship between optical film thickness and hue value, according to the embodiment. FIG. 8A is a diagram for explaining an optical film thickness conversion process, and FIG. 8B is a diagram for explaining a reference film thickness determination process, according to an embodiment; (a) is an image obtained by converting an interference image including partial wear into a hue distribution, and (b) is an image obtained by subjecting the image of (a) to the optical film thickness conversion processing according to the present embodiment. there is (a) is a diagram showing the relationship between an interference image in the presence of a unique pixel and an abnormal distribution of optical film thickness that occurs when the interference image is converted into an optical film thickness distribution, and (b) shows a large number of structures. The hue distribution image before performing the hue filtering process and the hue distribution image after performing the hue filtering process on the interference image including the defect are shown. (a) is a diagram for explaining the principle of the hue filtering process according to the present embodiment, and (b) is an image of the optical film thickness distribution when the hue filtering process according to the present embodiment is applied. FIG. 10 is a diagram for explaining a film thickness defect complementing process according to the embodiment; 4 is a flow chart showing the flow of processing of a measurement processing program according to the embodiment; (a) shows an interference image of a sample without wear, (b) shows an interference image of a sample with partial wear.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. It should be noted that in the following description, characters in italics expressed in mathematical expressions are written in characters that are not in italics.

<Configuration of measuring device>
An example of the configuration of a measuring device 10 according to the present embodiment will be described with reference to FIG. The measuring device 10 is a device for measuring the thickness of a transparent film (optical film thickness) on a sliding surface, such as the thickness of an oil film, and is sometimes called a sliding device. The basic principle of optical film thickness measurement by the measuring device 10 is optical interferometry. The measuring device 10 is roughly divided into an optical system 30 , a sliding system 40 , and an arithmetic device 21 .

(Optical system)
Optical system 30 includes light source 15 , bandpass filter 16 , passageway 17 , microscope 18 , half mirror 19 and camera 20 . Light source 15 is a part that generates measurement light used in measurement apparatus 10, and in the present embodiment, a white light source is used as an example. The bandpass filter 16 is an element that extracts monochromatic measurement light from the light of the light source 15, which is white light. In this embodiment, as will be described later, light of three wavelengths is used as measurement light, so the band-pass filter 16 transmits monochromatic light of these three wavelengths. The measurement light transmitted through the band-pass filter 16 passes through the passage 17, is reflected by the half mirror 19, and enters the sliding system as the illumination light Pi. The microscope 18 includes a lens (not shown) and the like, and has a function of enlarging a contact portion (described later).

Reflected light Pr reflected by the sliding system 40 passes through the half mirror 19 and enters the camera 20 . The camera 20 measures the brightness of each of the three wavelengths of light in the optical interference generated by the sliding system 40 . As will be described later, in this embodiment, three wavelengths of 600 nm, 560 nm and 470 nm are selected as monochromatic light. This is because a general color camera is used as a photodetector in consideration of the RGB spectral sensitivities of a general color camera. The full width at half maximum (FWMH) of each monochromatic light is approximately 10 nm. In this embodiment, the smaller the full width at half maximum, the better, but even when monochromatic light with a full width at half maximum of 10 nm is used, reasonable results are obtained. In the present embodiment, a band-pass filter is used to configure the irradiation light Pi only from monochromatic light, but it may be realized by combining a light source that emits monochromatic light, such as an LED or a laser.

In this embodiment, the case of using visible light as the wavelength of the light to be irradiated will be described as an example. This principle of measurement also holds true for light (electromagnetic waves). In this embodiment, three monochromatic lights are used, but this is the minimum number of wavelengths, and the number of wavelengths may be four or more.

(sliding system)
A sliding system 40 of the measuring device 10 includes a transparent member 11 , a translucent film 12 , a reflecting member 13 and a load mechanism 14 . The measurement target of the measuring device 10 is the thickness of a transparent film, for example, an oil film (not shown) formed between the translucent film 12 and the reflecting member 13 . The oil film is, for example, a lubricant that is supplied to the contact portion (“contact portion”) between the reflecting member 13 and the translucent film 12 . The oil film exists between the semi-transparent film 12 and the reflecting member 13, and the semi-transparent film 12 provided on the side opposite to the optical system 30 side of the transparent member 11 and the oil film are arranged so as to be in contact with each other. 13 and the oil film are in contact.

The transparent member 11 and the oil film transmit light of three wavelengths, and the reflecting member 13 reflects light of three wavelengths. Irradiation light Pi incident from the optical system is transmitted through the transparent member 11 and the oil film and is irradiated onto the reflecting member 13 to cause optical interference.

The load mechanism 14 rotates the transparent member 11 and applies a load to the transparent member 11 and the reflecting member 13 to cause them to move relative to each other.

Arithmetic device 21 is a device for centrally controlling the entire measurement device 10, and is composed of, for example, a PC (Personal Computer) or the like. The computing device 21 includes a CPU, ROM, RAM, etc., not shown, and executes a measurement processing program, which will be described later. That is, the brightness of the interference light is obtained for each wavelength region from the interference image, the brightness is converted into a hue, and the hue is converted into an optical film thickness.

(Operation of measuring device)
First, the transparent member 11 is rotated by the load mechanism 14, and white light from the light source 15 is incident on the bandpass filter 16 while the transparent member 11 and the reflective member 13 are moving relative to each other while receiving a load. Irradiation light Pi composed of monochromatic light of three wavelengths transmitted through the band-pass filter 16 is reflected by the half mirror 19 and enters the sliding system 40 . Part of the irradiation light Pi composed of monochromatic light of three wavelengths is reflected at the interface between the semi-transparent film 12 and the oil film, and another part of the irradiation light Pi composed of monochromatic light of three wavelengths (other part) is reflected. The light is reflected at the interface between the member 13 and the oil film. When light interference occurs due to both reflected lights, the camera 20 captures an image of the light interference distribution, and each time the camera 20 captures an image, the image is input to the arithmetic unit 21 . Then, a measurement processing program, which will be described later, is executed by the computing device 21 for each of the input images.

<Background technology of the present embodiment>
(Definition of transparent film)
In the present embodiment, a measuring device capable of measuring the thickness distribution of a transparent film existing between sliding materials will be described, but before that, the definition of a transparent film will be described. In the present embodiment, the transparent film does not mean only a film such as a film of water that hardly causes attenuation of light due to transmission, but a film that causes attenuation of light due to transmission such as a film of engine oil. That is, it also means a colored transparent film. Furthermore, in the sense of the light-transmitting region existing between the sliding surfaces, it also means the gap between the sliding surfaces when no liquid or solid is contained. A specific example is the troughs of surface roughness when surfaces having surface roughness slide against each other in the air or in a vacuum. In this embodiment, unless otherwise specified, the transparent film means the above.

(Importance of transparent film thickness distribution measurement)
In sliding parts of mechanical elements, transparent films are present between the sliding surfaces in many cases. For example, many sliding parts are lubricated with liquids such as oil, and those liquids are often transparent, so there is a transparent film between the sliding surfaces. In addition, a transparent film may be formed on the surface of the sliding material due to the chemical reaction between the sliding material and surrounding substances. Specific examples include a reaction film of ZnDTP when steel is rubbed in engine oil and an oxide film when steel is rubbed in air. All of these are transparent. Furthermore, there may be a region between the sliding surfaces that does not have a transparent film as an object but that transmits light. A specific example is the troughs of surface roughness when surfaces having surface roughness slide against each other in air or vacuum. Even in such cases, it is useful to measure the thickness distribution in that region. This is because the zero point distribution of film thickness means the real contact point distribution.

Since the thickness distribution of these transparent films affects the frictional properties, it is important to measure the thickness distribution of the transparent films in the design of sliding parts.

(optical interferometry)
"Light interferometry" is widely used as a method for measuring transparent film thickness distribution, especially oil film thickness distribution. This is a method of measuring transparent film thickness using light interference.

The principle of optical interferometry is as follows. That is, one of the sliding members is made of a material that transmits light, and the material of the other sliding member is made of a material that reflects light. These sliding materials are sometimes called "transparent sliding material" and "reflective sliding material", respectively. Light from a light source is transmitted through the transparent sliding member and irradiated to the other sliding member through the transparent film. At this time, the refractive index difference between the transparent sliding member and the transparent film is appropriately set so that the light is partly reflected and partly transmitted at the interface therebetween. If the refractive index difference is small and reflection is not sufficiently generated, a partially reflective film may be formed on the surface of the transparent sliding member that is in contact with the transparent film.

The light that has passed through the transparent film is reflected by the reflective sliding member, passes through the transparent film and the transparent sliding member again, and returns toward the light source. In this embodiment, the light reflected at the interface between the transparent sliding material and the transparent film is referred to as "surface reflected light", and the light reflected at the reflecting sliding material is referred to as "back surface reflected light". Optical path difference occurs between front reflected light and back reflected light, causing optical interference. When the surface of the reflective sliding material is observed through the transparent film from the transparent sliding material side with a photodetector (camera, etc.), optical interference is observed. be. Since the color (wavelength and luminance) of this light interference is affected by the optical path difference, that is, the transparent film thickness, information on the transparent film thickness can be obtained from the interference color. This is the principle of optical interferometry.

<Measurement principle of the present embodiment>
The measurement apparatus 10 according to the present embodiment is intended to solve the above-described problems with respect to the above-described film thickness measurement method using optical interference. The measurement principle of the measurement device 10 according to this embodiment will be described in more detail below. In the present embodiment, an example in which the number of monochromatic light wavelengths used for measurement is three will be described.

Reflected light luminance I _j of each monochromatic light (j = 1, 2, 3) is defined by I _1j and I _2j as the intensity of reflected light on the front surface (front surface reflected light) and the intensity of reflected light on the back surface (back surface reflected light). is the intensity, λ _j is the wavelength, and t is the optical film thickness to be measured. FIG. 2(a) is a diagram plotting Equation 1 when each of the three wavelengths is λ ₁ =600 nm, λ ₂ =560 nm, and λ ₃ =470 nm. In addition, in the following description, the case where the measuring object of the measuring device 10 is an oil film will be described as an example.

Although the luminance is often treated as an integer value of 0 to 255 in data processing, it is defined by a numerical value in the range of 0 to 1 in this embodiment. Also, the reflected light luminance _Ij of each monochromatic light corresponds to the RGB luminance of the photographed image, and a wavelength combination is selected within a range in which the crosstalk phenomenon does not occur with respect to the spectral sensitivity of the camera. In the following description, each of the three wavelengths is λ ₁ =600 nm, λ ₂ =560 nm, and λ ₃ =470 nm.

ここで、Ｒは裏面における乱反射による減衰率、ｋは測定対象である油膜の吸収係数である。 The above (Equation 1) is an equation in the case where there is no decrease in brightness due to the diffuse reflection problem and the absorption problem among the problems in the conventional technology described above. (Formula 2) below.

Here, R is the attenuation rate due to irregular reflection on the back surface, and k is the absorption coefficient of the oil film to be measured.

ただし、（０≦Ｈ≦２π） On the other hand, hue H is defined by the following (Formula 3) using RGB luminance ( _Ir , _Ig , _Ib ).

However, (0≤H≤2π)

In the prior art, there are cases where the hue is classified according to the magnitude relationship of each luminance, but in the present embodiment, the hue is defined by an inverse trigonometric function and radians in consideration of ease of handling. Substituting the above reflected light from monochromatic light into the equation of hue shown in (Equation 3) as I _r =I ₁ , I _g =I ₂ , and I _b =I ₃ , (Equation 3) is shown below ( Equation 4) is obtained.

Here, it is assumed that R and k are equal regardless of the wavelength, and furthermore, the light amount and the light receiving sensitivity are I ₁₁ =I ₁₂ =I ₁₃ for the front reflected light intensity and I ₂₁ =I ₂₂ =I ₂₃ for the back reflected light intensity. is established, the following (Equation 5) is obtained by substituting (Equation 2) into (Equation 4) and rearranging.

That is, by adjusting (calibrating) the amount of light and the photosensitivity in advance, the hue H depends only on the wavelength of each monochromatic light and the optical film thickness, as shown in (Equation 5). Therefore, according to the measuring device 10 according to the present embodiment, it is possible to suppress the influence of the diffuse reflection problem and the absorption problem described above. This is determined by I _1j , I _2j even though the amplitude (i.e., RGB luminance magnitude) in the calibration curve is reduced by R and k, since hue H is independent of RGB luminance magnitude. This is because if the amplitude center and amplitude attenuation rate (R, k) are the same for each monochromatic light, the hue H does not change.

FIG. 2(b) shows a calibration curve of the optical film thickness t and the hue H, which was created based on the (Equation 5) in the optical film thickness range of 0 nm to 1500 nm. Here, as described above, in this embodiment, the wavelengths are λ ₁ =600 nm, λ ₂ =560 nm, and λ ₃ =470 nm. This calibration curve is analytically calculated using (Equation 5) from the selected wavelength combination of monochromatic light regardless of the combination of the camera 20 and the lens (microscope 18). Therefore, pre-calibration becomes unnecessary, and the above-mentioned calibration curve-specific correction problem that the optical film thickness and hue calibration curves are unique to the observation system (camera, lens, light source) can be solved.

<Batch Conversion from Hue to Optical Film Thickness: Optical Film Thickness Conversion Processing>
From the above, a calibration curve for optical film thickness and hue is analytically derived, and hue can be converted to optical film thickness using (Equation 5). However, as described above, since the hue folds back at a period of about 250 nm (periods P1, P2, and P3 are shown in FIG. 2(b)), there are multiple film thickness candidate values for one hue. , a problem similar to the calibration curve period problem discussed above is still unsolved. In other words, there remains the problem that the convertible range is limited within the turn-around period even if (Equation 5) is used. Therefore, in the present embodiment, the hue monotonously decreases in the range from the optical film thickness of 0 nm to 1040 nm indicated by the dotted line R in FIG. , and adopted a new method to uniquely determine the film thickness. Here, as for the monotonic decrease of the hue, as shown in FIG. Although it takes a value in the range of ˜2π, it monotonously decreases within each period, so when the curve of each period is made continuous, the entire optical film thickness monotonically decreases. However, the range in which the hue monotonously decreases and the cycle of the repeated optical film thickness change depending on the combination of the three wavelengths. Hereinafter, a conversion method from hue to optical film thickness according to this idea (hereinafter sometimes referred to as "optical film thickness conversion processing") will be described in detail.

The optical film thickness distribution is determined by the unevenness and slope of the surface of the sample. It is considered that the change remains in the range of several nanometers to ten-odd nanometers at most. In that case, if the optical film thickness of a certain reference pixel (hereinafter referred to as “reference pixel”) and the hue of its adjacent pixels are known, the optical film thickness of the adjacent pixel is , which is closest to the optical film thickness of the reference pixel, can be uniquely determined. However, this determination method is based on the premise that the hue monotonously changes in the vicinity of the optical film thickness of the reference pixel (hereinafter referred to as "reference film thickness"). That is, as long as the optical film thickness of an arbitrary reference pixel is determined by a predetermined method, the optical film thickness can be sequentially determined from the reference pixel under the above assumption. That is, finally, all the hues around the reference pixel can be uniquely converted into the optical film thickness. The optical film thickness conversion processing will be described in more detail below.

[Procedure 1]: An arbitrary reference pixel is selected, and an optical film thickness (hereinafter sometimes referred to as "reference film thickness") corresponding to the hue of the reference pixel is determined. Here, determination of the reference film thickness means determination of one optical film thickness from among a plurality of optical film thickness value candidates based on a predetermined procedure. One of the predetermined procedures is manual, for example, a human observes the hue of the reference pixel, or an empirical rule is used to determine which hue cycle to enter by comparing with the hue curve of FIG. 2(b). , the optical film thickness of that period is adopted as the optical film thickness of the reference pixel. As for the method of determining the reference film thickness, there is a method of automatically determining the reference film thickness in addition to the manual method. The method of automatically determining the reference film thickness (reference film thickness determination processing) will be described later.

[Procedure 2]: Move to an adjacent pixel, and select an optical film thickness close to the movement source (reference pixel) from the optical film thickness value candidates corresponding to the hue. Of course, this selection can be made automatically.
[Procedure 3]: [Procedure 2] is linearly executed and repeated until the edge of the interference image is reached.
[Procedure 4]: Returning to the reference pixel, [Procedure 2] and [Procedure 3] are repeated for all directions, and the entire interference image is converted into an optical film thickness distribution.

With reference to FIG. 3A, the procedure of the optical film thickness conversion process will be specifically described. In FIG. 3A, four pixels PX1, PX2, PX3, and PX4 are arranged in the horizontal direction.

First, in [Procedure 1], the pixel PX1 is selected as the reference pixel, and the hue value of the reference pixel is determined.
The example of FIG. 3A illustrates a case where the hue value of the reference pixel is determined to be 3.14. When the hue value is determined, the corresponding optical film thickness value candidate is determined. Therefore, for example, a person visually determines one of the optical film thickness value candidates, and the hue value is shown in FIG. 2(b). Determine which period of the hue curve you are in. As a result, the optical film thickness corresponding to the hue value of the reference pixel is determined to be 511 nm among the candidates for the optical film thickness (767 nm, 511 nm, and 256 nm).

Following [Procedure 2], the hue value of the pixel PX2 adjacent to the reference pixel PX1 is automatically determined to be 2.09, and the optical film thickness is automatically determined to be 527 nm within the range of the period P2. be done. The same applies to pixels PX3 and PX4 below.

The above method is close to the conventionally known phase shift method for a white light source, but the phase shift method requires unwrapping of hues, which complicates the filtering and filtering removal processes described later. Therefore, in the present embodiment, the hue is handled as it is without converting the hue.

<Example>
Effects of the measuring device 10 according to the present embodiment will be described with reference to FIG. FIG. 4(a) shows the hue distribution obtained by converting the interference image including partial wear shown in FIG. 9(b), and FIG. The optical film thickness distribution obtained by performing the optical film thickness conversion process which concerns is shown. In addition, in FIG. 4B, the parts other than the sliding part are masked for easy viewing. As shown in FIG. 9(b), although the RGB brightness in the original interference image is greatly reduced due to abrasion, the use of the hue gives an optical film thickness at a level that does not pose any problem as data. I know there is.

<Reference film thickness determination processing>
Here, reference film thickness determination processing for selecting a reference pixel and automatically executing processing for determining a reference film thickness will be described. FIG. 2(a) is a diagram showing calibration curves of the optical film thickness and RGB luminance when each of the three measurement wavelengths is λ ₁ =600 nm, λ ₂ =560 nm, and λ ₃ =470 nm. rice field. If there is no reduction in RGB brightness due to diffuse reflection or absorption problems and the optical film thickness is within this range, the RGB brightness of each pixel in the interference image will match any combination of RGB brightness on this calibration curve. should be. Based on this, _{the error} norm v (t) is defined by the following (Equation 6).

Now, among the plurality of error norms obtained by substituting the plurality of film thickness candidate values obtained from the hue value of each pixel into (Equation 6), the minimum error norm of the pixel is defined as the minimum error norm of the pixel. A thickness candidate value is defined as a provisional film thickness value. This minimum error norm is 0 for pixels in which the RGB luminance has not decreased at all, and the provisional film thickness value that gives this minimum error norm should be the actual optical film thickness value.

However, in reality, the minimum error norm does not become 0 because most of the pixels have reduced RGB luminance due to irregular reflection problems and absorption problems. Furthermore, if the decrease in brightness is large, there is a possibility that an incorrect provisional film thickness value has been selected, and in that case the minimum error norm is also considered to be large. Conversely, it can be said that a pixel with a small minimum error norm has a small decrease in brightness and high reliability, so such a pixel should be selected as a reference pixel. Specifically, the reference pixel having the highest reliability is automatically selected and the reference film thickness is automatically determined by the following procedure.
[Procedure 1]: A plurality of film thickness candidate values are calculated from the hue value of each pixel. Among a plurality of error norms obtained by substituting these into (Equation 6), the minimum error norm of the pixel is stored as the minimum error norm of the pixel, and the film thickness candidate value that gives the minimum error norm is stored as the provisional film thickness value.
[Procedure 2] The processing of [Procedure 1] is performed for all pixels, the pixel with the smallest minimum error norm is set as a reference pixel, and its provisional film thickness value is set as a reference film thickness value.

By the reference film thickness determination process described above, it is possible to automatically determine a highly reliable reference pixel and a reference film thickness value. However, there is a possibility of erroneously detecting a specific pixel on rare occasions. In such a case, a pixel whose brightness is not lowered may be manually selected as a reference pixel, and the above [Procedure 1] may be performed on that pixel to automatically determine the optical film thickness value.

The reference film thickness determination process will be described in more detail with reference to FIG. 3(b). In the example shown in FIG. 3B, the interference color (RGB luminance: R=0.356, G=0.727, B=0.728) of the pixel PX1 is converted into a hue, and the hue value of the pixel PX1 is 3. .14. In this case, candidates for the optical film thickness are 256 nm, 511 nm, and 767 nm, respectively, corresponding to the periods P1, P2, and P3 shown in FIG. 2(b). At this time, the error norms for these optical film thicknesses are 0.521, 0.001 and 0.497, respectively. Therefore, the minimum error norm of pixel PX1 is 0.001 ([procedure 1]). The provisional film thickness value in this case is 511 nm. As a result of calculating the minimum error norm for all pixels, if the pixel PX1 has the smallest minimum error norm, the pixel PX1 is selected as the reference pixel and the reference film thickness is determined to be 511 nm.

<Hue filtering processing>
By the above-described procedure, it is possible to accurately convert the interference image into the optical film thickness while avoiding the problems in the conventional technology. Here, the above procedure is based on the premise that the change in the optical film thickness between pixels remains several nanometers to ten-odd nanometers without extreme steps. However, in reality, there may be pixels where the hue is greatly disturbed due to camera noise, local scratches, foreign matter, etc. (hereinafter referred to as "peculiar pixels"). For example, in the interference image of FIG. 9A, although there is no abrasion, there are many portions where the optical film thickness changes suddenly due to structural defects. In such a case, since the information of adjacent pixels is dragged by the above procedure, if a certain point is a singular pixel, not only the optical film thickness at that pixel but also the optical film thickness conversion of the subsequent pixels will be inaccurate. is obtained.

The influence of the peculiar pixel will be described with reference to FIG. FIG. 5(a) <1> is an example of an interference image including a specific pixel, showing the specific pixel together with a reference pixel. FIG. 5(a) <2> shows the conversion process from the interference image shown in FIG. 5(a) <1> to the optical film thickness distribution. As shown in FIG. 5(a) <2>. As the conversion to the optical film thickness is performed linearly from the position B of the reference pixel to the position S of the specific pixel, the abnormal distribution of the optical film thickness spreads from the position S to the opposite side of the position B. Therefore, it is practically necessary to remove these peculiar pixels.

In general image processing, filtering processing is often performed to remove peculiar pixels. Gaussian filters and median filters are typical filters, but these are performed individually for each of the RGB luminances of the interference image. If it is normal image processing, there is no problem as it is, but with regard to the hues handled here, normal filtering may not work well. This is because hue can be said to be obtained by excluding saturation and lightness information from RGB luminance and extracting the relative relationship between R, G, and B. include and filter. Therefore, the hue in the interference image after filtering may be unsuitable for use in optical film thickness conversion, such as the occurrence of unnatural folding of colors due to the influence of these influences. This is particularly noticeable when the brightness is greatly reduced due to irregular reflection problems, absorption problems, or the like.

Ｈｓ、Ｈｃは、値が－１～１で定義され、かつ折り返しではないため、Ｈｓ、Ｈｃに色相フィルタリング処理を行っても不連続は発生しない。そして、色相フィルタリング処理後の値をＨｓ’、Ｈｃ’とすると、Ｈｓ’、Ｈｃ’を以下に示す（式８）で戻すことにより不連続を回避した色相フィルタリング処理後の色相Ｈ’が得られる。

Therefore, in this embodiment, a new method of applying a Gaussian filter to hues instead of interference images is adopted. Since the hue H is defined as 0≤H≤2π, if the Gaussian filter is applied directly to the hue, the folded portions of 0 and 2π are separated even though they are originally continuous. This results in an appropriate hue distribution. In order to avoid this problem, in the present embodiment, the hue H is separated into a sine component Hs and a cosine component Hc as shown in (Equation 7) below.

Hs and Hc are defined with values from -1 to 1 and are not folded, so even if Hs and Hc are subjected to hue filtering, discontinuity does not occur. Assuming that the values after the hue filtering process are Hs' and Hc', the hue H' after the hue filtering process that avoids discontinuity can be obtained by returning Hs' and Hc' according to (Equation 8) shown below. .

FIG. 5(b) <1> shows the hue distribution before the hue filtering process (i.e., the hue distribution according to (Equation 5)) obtained by converting the interference image including many structural defects shown in FIG. 9(a). FIG. 5B <2> shows the hue distribution obtained by subjecting the hue distribution of FIG. Note that a Gaussian filter is used as the filter in this example. As shown in FIG. 5(b) <1>, many color jumps occur in the hue distribution before the hue filtering process. This color jumping itself indicates the optical film thickness of the defective portion, so it is significant information on one side, but it becomes a factor of conversion error. On the other hand, referring to FIG. 5(b) <2>, the hue filtering process can remove these color jumps to a level at which there is no problem in the hue distribution (grains are no longer conspicuous). I understand.

<Hue Filtering Restoration Processing: Sharpness Restoration Processing>
The hue filtering process enables automatic film thickness conversion from the hue even if the hue image contains a specific pixel. However, the image of the optical film thickness distribution after the hue filtering process is reduced in sharpness due to the filtering. Therefore, it is preferable to restore the original sharpness. The sharpness restoring process will be specifically described below with reference to FIG.

First, the optical film thickness is determined from the hue after hue filtering. This determination is made according to [Procedure 1] to [Procedure 4] described above. In the example shown in FIG. 6A, 520 nm is determined for the pixel PX1, 530 nm for the pixel PX2, and 550 nm for the pixel PX3. Next, for each pixel, the optical film thickness value closest to the optical film thickness obtained from the hue after the hue filtering process is selected from among the optical film thickness value candidates corresponding to the hue distribution before the hue filtering process. In the example shown in FIG. 6A, a value of 511 nm is selected for the pixel PX1, 527 nm for the pixel PX2, and 579 nm for the pixel PX3. As described above, it is possible to acquire the distribution image of the optical film thickness from which the peculiar pixels are removed while maintaining the sharpness before applying the hue filtering process.

FIG. 6(b) shows an image of the optical film thickness distribution obtained by applying the hue filtering process to the interference image shown in FIG. 9(a). In FIG. 6(b), portions not directly related to the measurement of the non-sliding surface are removed by masking for ease of viewing. As is clear from FIG. 6(b), even when a large number of defects are included, it is possible to obtain an accurate optical film thickness distribution by applying the hue filtering process.

<Complementary treatment for film thickness defects>
By using the hue filtering process, most sliding surfaces can be converted into optical film thickness, but there are surface conditions that are difficult to deal with even with the hue filtering process. For example, there are defects such as extreme steps and scratches due to laser textures.

With reference to FIG. 7, a method of optical film thickness conversion useful in the above case, that is, a film thickness missing portion complementing process will be described. First, as shown in FIG. 7 <1>, an arbitrary reference pixel 1 is selected, and optical film thickness conversion is performed along a straight line. The optical film thickness conversion at this time is performed according to [Procedure 1] to [Procedure 4] of the optical film thickness conversion process. Then, when reaching a pixel where the optical film thickness is discontinuous, the conversion is terminated at that portion. Through the above processing, an optical film thickness distribution including a linear data defect starting from a specific pixel is obtained.

Next, as shown in FIG. 7 <2>, a reference pixel 2 other than the reference pixel 1, which can reach data loss while avoiding the peculiar pixels, is selected, and the conversion is performed along a straight line from the reference pixel 2 again. At this time, if a point whose optical film thickness value is known by optical film thickness conversion for the reference pixel 1 is selected as the reference point, there is no need to manually select the optical film thickness value. With this processing, the optical film thickness value can be obtained even in the data missing portion in the optical film thickness conversion for the reference pixel 1 . Instead, a new data missing portion is generated, and the optical film thickness value determined by converting the optical film thickness of the first reference pixel is input as it is to the new data missing portion. By repeating until there are no missing data portions other than the above, it is possible to acquire the optical film thickness distribution of the region excluding the specific pixels, as shown in FIG. 7 <3>.

<Measurement processing>
With reference to FIG. 8, the measurement process performed in measurement apparatus 10 according to the present embodiment will be described. FIG. 8 is a flow chart showing the processing flow of the measurement processing program executed in the measurement device 10. As shown in FIG. This measurement processing program is stored in a storage means such as a ROM (not shown) provided in the arithmetic unit 21, for example. Deploy and run.

Referring to FIG. 8, an interference image is acquired from camera 20 in step S10.

In step S11, the interference image acquired in step S10 is converted into a hue image.

In step S12, the hue image converted in step S11 is subjected to hue filtering processing. In this hue filtering process, a Gaussian filter, for example, is used as a filter after separating the hue into a sine component and a cosine component.

In step S13, a reference pixel is selected and a reference film thickness is determined by the reference film thickness determination process described above.

In step S14, the above-described optical film thickness conversion processing ([Procedure 1] to [Procedure 4]) is executed to convert the hue into the optical film thickness.

In step S15, it is determined whether or not there is a film thickness defect caused by the peculiar pixel, and if the determination is negative, the process proceeds to step S16. On the other hand, if the determination is affirmative and a defective portion due to a specific pixel occurs, the film thickness defective portion complementing process is performed, and the optical film thickness conversion is repeated by changing the reference pixel until the defective portion is filled. . It should be noted that the film thickness defect compensation process is a process that may be performed as necessary, and is not necessarily an essential process.

In step S16, hue filtering restoration processing is performed to restore the sharpness of the image of the optical film thickness, and this measurement processing program ends. Note that the hue filtering restoration process is a process that may be performed as needed, and is not necessarily an essential process.

As described in detail above, according to the measuring apparatus according to the present embodiment, even when the hue of the interference image is used, the optical film thickness conversion of the entire interference image can be performed at high speed. In addition, the range of optical film thickness that can be converted in the measuring apparatus 10 is a range in which the hue monotonically decreases with respect to the optical film thickness, for example, the combination of measurement wavelengths is (λ ₁ =600 nm, λ ₂ =560 nm, λ ₃ =470 nm ), it becomes 0 nm to 1040 nm, which solves the calibration curve period problem that the optical film thickness range that can be converted into the optical film thickness is limited to one period (about 265 nm) of the optical film thickness. is planned.

Here, referring to FIG. 2A, in the range of the optical film thickness from 0 nm to 200 nm, there is a section where the monotonically decreasing gradient (inclination) is small, and there is a possibility that the conversion accuracy deteriorates in this section. In that case, a transparent film called a spacer layer may be arranged on the translucent film 12 on the surface of the transparent member 11 with a thickness of, for example, 150 nm to 200 nm. As a result, the optical film thickness becomes larger than the actual film thickness by the product of the thickness of the spacer layer and the refractive index.

In addition, in the present embodiment, the information of the adjacent pixels is referred to, and furthermore, the missing film thickness part is supplemented as necessary, so that the above-mentioned peculiar pixel problem of reliability deterioration due to the occurrence of local optical film thickness deviation can be avoided. occurrence is also suppressed. In addition, in the measurement apparatus 10 according to the present embodiment, if the reference film thickness determination process is adopted, the whole process is automated, so that manual calibration work is actually required. Problems have also been improved. As described above, the present invention solves the problems of the conventional technology described above, and makes it possible to improve the applicable range, convenience, and accuracy of real-time observation of the optical film thickness on the sliding surface. .

10 measuring device 11 transparent member 12 translucent film 13 reflecting member 14 load mechanism 15 light source 16 bandpass filter 17 passage 18 microscope 19 half mirror 20 camera 21 arithmetic device 30 optical system 40 sliding system P1, P2, P3 period PX1, PX2 , PX3, PX4 Pixel Pi Irradiation light Pr Reflected light

Claims (3)

a light source that emits monochromatic light of three or more wavelengths;
a first sliding portion that transmits the monochromatic light;
a semi-transparent film that reflects part of the monochromatic light transmitted through the first sliding portion and transmits the other portion;
a second sliding portion that reflects the monochromatic light transmitted through the semi-permeable film;
a load applying unit that causes the first sliding portion and the second sliding portion to move relative to each other while applying a load;
an imaging unit that acquires an interference image in which the monochromatic light reflected by the semi-transparent film and the monochromatic light reflected by the second sliding portion are caused to interfere with each other;
converting the luminance distribution image for each wavelength region of the monochromatic light from the interference image into a hue distribution image, and determining the hue value for each pixel of the hue distribution image based on a predetermined correspondence relationship between the optical film thickness and the hue value; a computing unit that acquires an optical film thickness distribution image by converting it into an optical film thickness, and computes the optical film thickness distribution of the transparent film formed between the semipermeable film and the second sliding portion; including
The calculation unit separates the hue value of each pixel of the hue distribution image into a sine component and a cosine component, performs hue filtering processing on each of the sine component and the cosine component, and then obtains an arctangent to obtain the original value. measuring device, further performing a hue filtering process for removing singular pixels in the hue distribution image by returning to The measuring device according to claim 1, wherein the filter used in the hue filtering process is a Gaussian filter. The calculation unit selects the value of the optical film thickness closest to the optical film thickness obtained from the hue after the hue filtering process from among the optical film thickness value candidates corresponding to the hue distribution before the hue filtering process. 3. The measuring device according to claim 1, further comprising a process of restoring sharpness of the hue distribution image.

Images