Method and apparatus for testing refractive index

Abstract

The present application provides a method and device for measuring refractive index, the method comprising: (1) installing a sample holder on an Abbe refractometer, fixing a holder reflective layer on the sample holder, the sample holder comprising a fixed flat plate and a fixing device for fixing the holder reflective layer; (2) fixing a test sample between the holder reflective layer and the fixed flat plate, so that the part of the test sample as the object of refractive index measurement is close to the holder reflective layer; (3) turning on the light source of the Abbe refractometer, so that the light finally reaches the ocular lens or infrared viewing mirror of the Abbe refractometer; (4) rotating the rotating wheel shaft of the Abbe refractometer, and recording the reading of the Abbe refractometer when the critical emergent light is located at the center of the crosshair of the ocular lens or infrared viewing mirror as the refractive index of the test sample.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of refractive index measurement, and particularly relates to a testing method and a testing device for refractive index. BACKGROUND

[0002] Polyimide film is widely used as "golden film" in the market, including being used as adhesives, separation membranes, photoresists, dielectric buffer layers, liquid crystal alignment agents, electro-optical materials, etc. as high-temperature thermal insulation materials, and being used as motor slot insulation and cable enameled wire materials as high-heat insulation materials. In addition, due to the softness, good dimensional stability and excellent dielectric properties of the film, polyimide film is suitable for use as a substrate or cover layer of a ribbon cable or a soft printed circuit, etc.

[0003] The mechanical properties, optical properties and thermal properties of the oriented polyimide film in different directions have great differences. For example, in terms of mechanical properties, the tensile strength and fatigue strength are significantly increased in the orientation direction, but decreased in the direction perpendicular to the orientation direction. Therefore, it is quite critical to use a non-destructive in-situ detection technology to characterize the molecular orientation of the film in each process of polyimide film production.

[0004] The optical anisotropy or birefringence of polyimide is a convenient and sensitive index for characterizing the molecular orientation of polyimide at each stage and for various applications. The birefringence of polyimide depends on two factors: monomer structure and molecular orientation degree. Therefore, for polyimide with consistent monomer structure, the molecular orientation can be judged from the birefringence. The greater the molecular orientation degree, the more regular the arrangement of the polymer chain along the in-plane direction, and the greater the difference between the in-plane and out-of-plane refractive indices. Therefore, the size of the birefringence can be used to measure the orientation of the polymer.

[0005] The determination of the refractive index of thin film material is mainly based on the principles of geometric optics or light interference and diffraction, and the refractive index of the sample is determined by comparing the propagation direction and phase difference of the incident light and the reflected and refracted light. At present, the main methods for measuring the refractive index of thin film are the ellipsometry method, the prism coupling method, the interference method and the traditional Abbe refractometer method.

[0006] Ellipsometry is a common method for measuring the refractive index and thickness of optical films. It projects a polarized light beam non-perpendicularly to the surface of the sample to be measured, and determines the optical properties of the sample, such as the thickness of the film, the complex refractive index of the material, etc., by the change in the polarization state of the reflected or transmitted light. This method is suitable for measuring transparent or weakly absorbing, thin film samples with a thickness less than one film thickness period, and a known substrate material, but it has different measurement accuracies at different incident angles and wavelengths. Moreover, the ellipsometry measurement process is complex and expensive. Because ellipsometry requires monitoring the optical properties of the refracted light, the polyimide sample has a serious absorption of the refracted light, and the optical properties of the refracted light cannot be determined, thereby affecting the test results. On the other hand, ellipsometry cannot test the refractive index of a sample of a non-independent support film, such as a polyimide film that has been bonded.

[0007] The working principle of the prism coupling method is that when light is incident on a prism, the incident angle changes with the rotation of the rotating table. At a certain incident angle, photons will enter the film through the air slit and be transmitted, so that the energy of the photons detected by the detector will decrease, forming a concave shape, which is a leaky mode. During the test, the test sample serves as the waveguide layer, which is between the cover layer (air layer) and the substrate layer. By changing the polarization direction of the incident light, the refractive index of the film in different directions can be tested. Therefore, if the sample has absorption to the light during the full reflection transmission of the light in the waveguide layer, the critical angle in the prism will be affected, thereby affecting the accuracy of the test.

[0008] The measurement process of the interference method for measuring the birefringence of the film is complex and easy to wear the film surface. The optical interference method requires a very high precision of the instrument and is complex to operate.

[0009] The traditional Abbe refractometer measures the refractive index of a substance by the grazing incidence method based on the principle of total reflection. When light is incident on a test sample from a less dense substance (air), the incident angle and the refractive angle meet the Snell formula. When the incident light is perpendicular to the interface normal, the refractive angle reaches the maximum, and the incident light at this time is called grazing light, and the corresponding refractive angle is called the critical angle of refraction or the total reflection angle. The refracted light is refracted by the prism, reflected by the swing mirror, and finally enters the eyepiece. The refractive index of the substance to be tested satisfies the formula: where A is the apex angle of the prism, N is the refractive index of the prism, i0 is the exit angle, and the signs in the formula depend on the difference between the refractive index of the sample to be tested and the refractive index of the prism. The schematic diagram of the traditional Abbe refractometer is shown in Figure 1 When measuring the birefringence of an anisotropic sample to be tested, the Figure 1The eyepiece in the Abbe refractometer is replaced by a polarizing eyepiece, and the ordinary light and the extraordinary light are observed by adjusting the direction of the polarizing eyepiece. When the refractive index and the birefringence of the polyimide are tested by the Abbe refractometer, there are two problems: one is that the polyimide has a strong absorption to the light, and it is difficult to monitor the exit light, and the bright-dark boundary line cannot be observed in the eyepiece; the other is that the polyimide and the carrier cannot be separated, and the measurement cannot be carried out, because the light cannot exit from the lower surface after entering the film from the upper surface, and the exit light is collected by the traditional Abbe refractometer method.

[0010] In the industry requiring quality control by measuring the refractive index, it is required to analyze the refractive index measurement results of different samples under the same conditions, including a clear and accurate wavelength. In order to generate a clear wavelength, the most commonly used by the refractometer is the sodium D line, which is equivalent to 589.3 nm. Since the sodium D line is a widely used, reliable and stable light source, it has been used for a long time in the study of refractive index.

[0011] In summary, the ellipsometry method, the prism coupling method, the interference method and the traditional Abbe refractometer method have two problems in detecting the refractive index of the polyimide film: one is that the polyimide has a low transmittance in the visible light range due to its high aromatic ring density, and it is difficult to detect the refractive index of the polyimide film, especially the refractive index under high absorption rate wavelength (such as sodium light 589 nm). In addition, the light is absorbed by the film during its propagation in the polyimide film, so it is also difficult to detect the refractive index of the polyimide film by analyzing the phase difference or the fringe (ellipsometry method and interference method) or the waveguide characteristics (prism coupler) of the reflected and refracted light; the second is that the above methods require the film as an independent film or have clear requirements for the substrate material, and when the polyimide film and the substrate (such as glass, copper foil) cannot be separated, these methods cannot test the refractive index of the polyimide film.

[0012] There is another Abbe refractometer which uses an internal light source to transmit light through the prism, and the light is emitted from the optically dense material (prism) to the optically sparse material (sample). When the incident angle is greater than or equal to the critical angle, total reflection occurs, and when the incident angle is less than the critical angle, refraction and reflection occur, so there is a boundary line, which is the curve of the total reflection light with a refractive angle of 90°. This Abbe refractometer has three problems: one is that the test wavelength is limited by the instrument because the light source is built-in; the second is that the boundary line is not obvious, and errors are easily generated when observed with the eyepiece, especially when the test has interference fringes, which easily interferes with the judgment of the test results; the third is that there is no report using this method to test the birefringence. SUMMARY

[0013] In order to overcome the deficiencies of the prior art, the present application provides a method for measuring the refractive index and birefringence of a material, particularly a thin film or other solid material, which can be colored and semi-transparent. The present application is particularly suitable for measuring the refractive index of micron to millimeter scale thin films, particularly polyimide thin films. The present application can non-destructively and quickly measure the refractive index of a thin film at a specific wavelength.

[0014] In particular, the present application provides a method for measuring the refractive index, the method comprising the following steps:

[0015] (1) mounting a sample holder on an Abbe refractometer, fixing a holder reflecting layer on the sample holder, the sample holder comprising a fixed flat plate and a fixing device for fixing the holder reflecting layer;

[0016] (2) fixing a test sample between the holder reflecting layer and the fixed flat plate, so that the part of the test sample as the object of refractive index measurement is in close contact with the holder reflecting layer;

[0017] (3) turning on the light source of the Abbe refractometer, so that the light passes through the filter and enters the holder reflecting layer, critical total reflection occurs at the interface between the holder reflecting layer and the test sample, returning to the holder reflecting layer, then passing through the prism of the Abbe refractometer, and finally reaching the ocular lens or infrared viewing lens of the Abbe refractometer;

[0018] (4) rotating the rotating wheel shaft of the Abbe refractometer, and recording the reading of the Abbe refractometer when the critical outgoing light is located at the center of the crosshair of the ocular lens or infrared viewing lens as the refractive index of the test sample.

[0019] In one or more embodiments, the refractive index of the test sample is anisotropic, the ocular lens is a polarizing ocular lens, and the step (4) comprises: setting the polarization direction of the polarizing ocular lens, recording the reading of the Abbe refractometer when the critical outgoing light is located at the center of the crosshair of the ocular lens or infrared viewing lens as the refractive index of the test sample corresponding to the polarization direction; preferably, the polarization direction comprises the north-south direction and / or the east-west direction, the refractive index when the polarization direction is the north-south direction is the refractive index of the extraordinary light, and the refractive index when the polarization direction is the east-west direction is the refractive index of the ordinary light.

[0020] In one or more embodiments, the step (4) further comprises: fixing a silicon photodiode at the center of the crosshair of the ocular lens or infrared viewing lens, measuring the current passing through the silicon photodiode with a microammeter, rotating the rotating wheel shaft of the Abbe refractometer, recording the correspondence between the reading of the Abbe refractometer and the photocurrent reading of the microammeter, and recording the reading of the Abbe refractometer when the photocurrent reading changes as the refractive index of the test sample.

[0021] In one or more embodiments, the step (4) further comprises adjusting the brightness of the light source so that the reading of the microammeter is ≤ 25 μA, preferably ≤ 20 μA.

[0022] In one or more embodiments, the sample holder further comprises a distance adjusting device for adjusting the distance between the fixing plate and the holder reflecting layer so that the test sample is fixed between the holder reflecting layer and the fixing plate.

[0023] In one or more embodiments, the distance adjusting device comprises one or more selected from a magnetic attraction device, a lock, a spring and a lifting screw.

[0024] In one or more embodiments, the distance adjusting device comprises a lifting screw for displacing the fixing plate in the direction away from the holder reflecting layer.

[0025] In one or more embodiments, the holder reflecting layer has an isotropic refractive index which is greater than the refractive index of the test sample.

[0026] In one or more embodiments, the holder reflecting layer has a refractive index of n1, and the test sample has a refractive index of n o , (n1-n o ) / n o ≤ 20%, preferably (n1-n o ) / n o ≤ 10%.

[0027] In one or more embodiments, the material of the part of the test sample which is the object of the refractive index measurement comprises polyimide.

[0028] The present application also provides a sample holder for an Abbe refractometer, which comprises a fixing plate and a fixing device for fixing a holder reflecting layer.

[0029] In one or more embodiments, the sample holder further comprises a distance adjusting device for adjusting the distance between the fixing plate and the holder reflecting layer so that the test sample is fixed between the holder reflecting layer and the fixing plate.

[0030] In one or more embodiments, the distance adjusting device comprises one or more selected from a magnetic attraction device, a lock, a spring and a lifting screw.

[0031] In one or more embodiments, the distance adjusting device comprises a lifting screw for displacing the fixing plate in the direction away from the holder reflecting layer.

[0032] The present invention also provides an apparatus for testing refractive index, the apparatus comprising an Abbe refractometer and a sample holder as described in any embodiment herein; preferably, the apparatus further comprises a silicon photodiode and a microammeter. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of a traditional Abbe refractometer.

[0034] Figure 2 This is a schematic diagram of a sample holder in some embodiments of the present invention.

[0035] Figure 3 This is a schematic diagram of a refractive index measuring device in some embodiments of the present invention.

[0036] Figure 4 This is a schematic diagram of the photocurrent testing device in some embodiments of the present invention.

[0037] Figure 5 This is a schematic diagram of a silicon photodiode in some embodiments of the present invention.

[0038] Figure 6 This is a typical photocurrent-Abbe refractometer reading relationship curve in this invention.

[0039] Figure 7 The graph shows the relationship between the photocurrent and the Abbe refractometer reading when the Abbe refractometer reading changes by 0.005 during the test of the out-of-plane refractive index of the thin film in Example 1.

[0040] Figure 8 The graph shows the relationship between the photocurrent and the Abbe refractometer reading when the Abbe refractometer reading changes by 0.0001 during the test of the out-of-plane refractive index of the thin film in Example 1.

[0041] Figure 9 The graph shows the relationship between the photocurrent and the Abbe refractometer reading when the Abbe refractometer reading changes by 0.005 during the in-plane refractive index test of the thin film in Example 1.

[0042] Figure 10 The graph shows the relationship between the photocurrent and the Abbe refractometer reading when the Abbe refractometer reading changes by 0.0001 during the in-plane refractive index test of the thin film in Example 1.

[0043] Figure 11 The graph shows the relationship between the photocurrent and the Abbe refractometer reading when the Abbe refractometer reading changes by 0.005 during the test of the out-of-plane refractive index of the thin film in Example 2.

[0044] Figure 12 The graph shows the relationship between the photocurrent and the Abbe refractometer reading when the Abbe refractometer reading changes by 0.0001 during the test of the out-of-plane refractive index of the thin film in Example 2.

[0045] Figure 13 A plot of photocurrent versus Abbe reading for a change in Abbe reading of 0.005 in the in-plane refractive index of the test film in Example 2.

[0046] Figure 14 A plot of photocurrent versus Abbe reading for a change in Abbe reading of 0.0001 in the in-plane refractive index of the test film in Example 2.

[0047] Figure 15 A plot of photocurrent versus Abbe reading for a change in Abbe reading of 0.005 in the out-of-plane refractive index of the test film in Example 3.

[0048] Figure 16 A plot of photocurrent versus Abbe reading for a change in Abbe reading of 0.0001 in the out-of-plane refractive index of the test film in Example 3.

[0049] Figure 17 A plot of photocurrent versus Abbe reading for a change in Abbe reading of 0.005 in the in-plane refractive index of the test film in Example 3.

[0050] Figure 18 A plot of photocurrent versus Abbe reading for a change in Abbe reading of 0.0001 in the in-plane refractive index of the test film in Example 3. DETAILED DESCRIPTION

[0051] To enable persons skilled in the art to understand the features and effects of the present application, the following is a general description and definition of the terms and phrases mentioned in the specification and claims. Unless otherwise specified, all technical and scientific words used herein are used in the ordinary meaning understood by those skilled in the art of the present application, and in the event of a conflict, the definition in the present specification shall prevail.

[0052] Theories or mechanisms described and disclosed herein, whether correct or not, should not be considered limiting of the scope of the present application, which is limited only by the claims. The present application can be implemented in ways other than those specifically described herein.

[0053] Herein, "comprising", "including", "containing", and like terms are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. For example, if a process is described as comprising A and B, it is understood that A and B are essential elements of the process, and that the process can also include additional elements, e.g., C, unless otherwise specified.

[0054] Herein, all the features defined in the form of numerical range or percentage range such as numerical value, quantity, content and concentration are only for the sake of brevity and convenience. Accordingly, the description of numerical range or percentage range should be considered to have covered and specifically disclosed all possible sub-ranges and individual numerical values (including integers and fractions) within the range.

[0055] Herein, when describing the embodiments or examples, it should be understood that they are not intended to limit the present application to these embodiments or examples. On the contrary, all alternatives, modifications, and equivalents of the methods and materials described herein that are within the scope of the claims are to be included.

[0056] Herein, for the sake of brevity, all possible combinations of the technical features in each embodiment or example are not described. Therefore, as long as the combinations of the technical features do not contradict each other, the technical features in each embodiment or example can be combined arbitrarily, and all possible combinations should be considered to be within the scope of the present specification.

[0057] The present application provides a method for testing the refractive index of a material, such as a thin film, particularly a polyimide thin film, by using the critical total reflection method. The method of the present application overcomes the problems of the prior art that it is difficult to measure the birefringence of polyimide thin films and the carrier cannot be peeled off, and overcomes the problem that the birefringence of polyimide cannot be tested in the waveband with high absorption, because when the absorption is high, the light transmission in the thin film is hindered, and the prior art all uses light transmission in the thin film to achieve measurement, while the present application can achieve full-waveband testing of the birefringence of polyimide; on the other hand, it also has the advantages of convenient measurement and high precision.

[0058] The present application also designs a sample holder for testing the refractive index of a material, which can be simply and conveniently connected with an Abbe refractometer, without the need for excessive adjustment of the entire device, so as to simply and accurately achieve the purpose of testing the refractive index of a material.

[0059] The method for measuring the refractive index of the present application comprises the following steps:

[0060] (1) installing the sample holder on the Abbe refractometer, fixing the holder reflection layer on the sample holder, wherein the sample holder comprises a fixed flat plate and a fixing device for fixing the holder reflection layer;

[0061] (2) fixing the test sample between the holder reflection layer and the fixed flat plate, so that the part of the test sample as the object of refractive index measurement is in close contact with the holder reflection layer;

[0062] (3) Turn on the light source of the Abbe refractometer so that the light passes through the filter and enters the support reflective layer. At the interface between the support reflective layer and the test sample, critical total internal reflection occurs and the light returns to the support reflective layer. Then, it passes through the prism of the Abbe refractometer and finally reaches the eyepiece or infrared observation mirror of the Abbe refractometer.

[0063] (4) Rotate the rotating wheel of the Abbe refractometer and record the reading of the Abbe refractometer when the critical outgoing ray is located at the center of the cross line of the eyepiece or infrared observation mirror as the refractive index of the test sample.

[0064] Furthermore, when the refractive index of the test sample is anisotropic, a polarizing eyepiece is used. Step (4) includes: setting the polarization direction of the polarizing eyepiece, testing the refractive index when the polarization direction is north-south (i.e., the out-of-plane refractive index in the film thickness direction), and testing the refractive index when the polarization direction is east-west (i.e., the in-plane refractive index in the film in the in-plane direction). In this invention, isotropy and anisotropy refer to the refractive index.

[0065] In the Abbe refractometer used in this invention, the critical outgoing ray is measured by rotating the rotating wheel of the Abbe refractometer, thereby adjusting the angle of the oscillating mirror (a mirror inside the Abbe refractometer that captures the critical outgoing ray). The reading of the Abbe refractometer at the center of the eyepiece is taken as the refractive index of the test sample. In the Abbe refractometer, the oscillating mirror angle, the outgoing ray angle, and the refractive index reading are all calibrated by the manufacturer. The Abbe refractometer used in this invention requires its refractive index range to cover the refractive index of the test sample.

[0066] In some embodiments of the present invention, a sample holder for testing the refractive index of a material is used, such as Figure 2 As shown. The sample holder of the present invention includes a fixing plate and a fixing device for fixing the reflective layer of the holder. The structure of the fixing device is not particularly limited, as long as it can fix the reflective layer of the holder; for example, it can be a clamping device, a magnetic suction device, etc. In some embodiments, the sample holder further includes a distance adjustment device for adjusting the distance between the fixing plate and the reflective layer of the holder, so that the test sample can be fixed between the reflective layer of the holder and the fixing plate. The distance adjustment device is not particularly limited, as long as it can fix the test sample between the reflective layer of the holder and the fixing plate. The distance adjustment device can include one or more selected from magnetic suction devices, latches, springs, lifting screws, etc. In some embodiments, the distance adjustment device includes a lifting screw for displacing the fixing plate in the direction of distance from the reflective layer of the holder, thereby fixing the test sample between the reflective layer of the holder and the fixing plate.

[0067] In the present application, the sample holder can be flexibly mounted and dismounted for testing sample, the holder reflection layer is used as a light tight layer, the light source is incident on the test sample through the holder reflection layer, and then the light is reflected and partially refracted. The fixed flat is used to fix the test sample. The fixed flat is light-tight to avoid the influence of ambient light on the test. The light finally forms an upper dark and lower bright field after being conducted through the optical path.

[0068] In the present application, the holder reflection layer needs to be parallel on the upper and lower surfaces. Therefore, the holder reflection layer is in the form of a flat plate with parallel upper and lower surfaces. The refractive index of the holder reflection layer needs to be higher than that of the test sample, and the holder reflection layer is isotropic. The thickness of the holder reflection layer is not particularly required, and is preferably between 1 cm and 3 cm. The material of the holder reflection layer can be high refractive index glass, artificial sapphire, etc.

[0069] In the present application, after the sample holder and the holder reflection layer are mounted, the holder reflection layer is tightly attached to the prism of the Abbe refractometer.

[0070] Figure 3 The present application is a schematic diagram of the refractive index measuring device and an optical path schematic diagram. The refractive index measuring device of the present application includes the sample holder of the present application, an Abbe refractometer, a silicon photodiode, and a microammeter. The Abbe refractometer includes a light source, a filter, a prism, an eyepiece, a rotating wheel shaft, and a swing mirror (not shown). The silicon photodiode is fixed at the center of the crosshair of the eyepiece of the Abbe refractometer. The microammeter is connected to the silicon photodiode. Figure 3

[0071] In the present application, when the test sample is an isotropic sample, Figure 3 The light shown in the figure is the light when critical reflection occurs, at which time the incident angle a satisfies formula (1): sin a · n1 = sin 90° · n o , where n1 is the refractive index of the holder reflection layer, n o is the refractive index of the test sample. When the critical reflection light is transmitted into the main prism through the holder reflection layer, the incident angle is also a because the upper and lower edges of the holder reflection layer are parallel, and the refractive angle β of the main prism satisfies formula (2): sin β · N = sin a · n1, where N is the refractive index of the main prism. Formula (3) can be obtained by combining formula (1) and formula (2): sin β · N = n o When a conventional Abbe refractometer is used, the refractive angle β1 of the light incident on the main prism satisfies formula (4): sin 90° · n o = sin β1 · N, i.e. n o = sin β1 · N. It can be known from formula (3) and formula (4) that β = β1. Therefore, in the present application, when the critical reflection light is located at the center of the eyepiece, formula ​Wherein A is the prism vertex angle, N is the prism refractive index, i0 is the exit angle, the positive and negative signs in the formula depend on the refractive index difference between the sample to be tested and the prism.

[0072] The refractive index of the test sample is independent of the refractive index of the support reflection layer, so the present application can use the test system of a conventional Abbe refractometer to complete the reading of the refractive index of the test sample.

[0073] In the embodiment in which the test sample is an anisotropic sample such as polyimide, the test sample has birefringence, and the rotating polarizing eyepiece is used, when the polarization direction is the north-south direction, at this time the critical reflection angle a satisfies formula (5): sin a·n1=sin 90°·n e , wherein n e is the refractive index of the extraordinary light, that is, the refractive index in the thickness direction of the film, and when the polarization direction is the east-west direction, at this time the critical reflection angle a satisfies formula (6): sin a·n1=sin 90°·n o , wherein n o is the refractive index of the ordinary light, that is, the refractive index in the in-plane direction of the film. In the present application, the ordinary light refers to light that obeys the law of refraction, and the extraordinary light refers to light that does not obey the law of refraction.

[0074] In the conventional Abbe refractometer test method for testing the refractive index, the bright-dark boundary is observed by the naked eye, and the bright-dark boundary may be inaccurate due to environmental and individual differences.

[0075] In the present application, the identification of the bright-dark boundary can be improved by adjusting the refractive index of the support reflection layer.

[0076] Regarding the selection of the refractive index of the support reflection layer, in theory, when the refractive index of the support reflection layer is less than the refractive index of the main prism, when the incident angle is greater than the critical angle (total reflection angle) calculated according to Snell's law, the test light is totally reflected and does not enter the main prism, and cannot enter the test system. However, in the present application, the boundary in the eyepiece field of view is the light at the critical reflection angle when the incident angle a is at the critical reflection angle, and the light in the eyepiece field of view is located near this critical reflection angle, regardless of the difference between the refractive index of the support reflection layer and the refractive index of the Abbe refractometer main prism, according to formula (3), at this time, β=arcsin(n o / N), which is less than 90°, and the test light enters the test system. Therefore, in the present application, the support reflection layer serves as an external sample holder, and the material of the support reflection layer can be selected according to requirements, as long as it has a refractive index greater than that of the test sample and is an isotropic substance, which is an important advantage of the present application.

[0077] When light is emitted from a light source, passes through an interference filter, and enters the reflective layer of the support, refraction and reflection occur between the reflective layer and the test sample. According to Fresnel's law, the intensity of the reflected light is related to the refractive index n1 of the reflective layer and the refractive index n of the test sample. o The ratio (i.e., n1 / n) o Closely related to n1 / n. When the test direction is in-plane, the brightness of the light above the eyepiece boundary increases with n1 / n. o As n increases, the light intensity above the eyepiece boundary increases with n1 / n. When the test direction is the film thickness direction, the light intensity increases with n1 / n. o The refractive index is increased first and then decreased. Because the refractive index of the test sample itself remains constant, in order to minimize the brightness of the light above the boundary line and make its contrast with the boundary line more obvious, this invention selects a refractive index n1 that is relatively small (slightly larger than the sample's refractive index n) when testing the in-plane direction of the thin film. o This can be achieved by using a reflective layer with a support structure. When testing the film thickness direction, a particularly high refractive index n1 or a value similar to the sample's refractive index n can be selected. o A suitable reflective support layer is needed. Polyimide films have a very high refractive index, making it difficult to find a reflective support layer with a refractive index approximately twice that of the film. Therefore, when testing the refractive index of polyimide films, selecting a reflective support layer with a refractive index close to that of the film will make the light-dark boundary more distinct. Without a micro-galvanometer to monitor the photocurrent, choosing a suitable reflective support layer can help clearly determine the light-dark boundary, a function that existing reflective methods for testing refractive index cannot achieve.

[0078] Furthermore, the present invention preferably uses photocurrent to quantify illuminance. In the present invention, a silicon photodiode can be fixed on an eyepiece or an infrared observation mirror (when measuring the refractive index under infrared light), and a micro-ammeter can be connected to the leads of the silicon photodiode, thereby quantifying the illuminance into photocurrent, effectively reducing the testing error caused by visual judgment.

[0079] Figure 4 This is a schematic diagram of the photocurrent testing device that can be used in this invention, showing the connection between the eyepiece of the Abbe refractometer and the micro-galvanometer. Figure 5 This is a schematic diagram of a silicon photodiode that can be used in this invention. Figure 5 In the diagram, the upper part of the diode is circular, with the gray portion representing its incident light window, which can be 1.1mm x 1.1mm in size. The silicon photodiode is positioned such that the incident light window is flush against the eyepiece of the Abbe refractometer, with the lead perpendicular to the eyepiece and pointing outwards. The micro-galvanometer and the silicon photodiode are connected by wires.

[0080] Preferably, step (4) of the method of the present application comprises: fixing the silicon photodiode at the center of the crosshair of the eyepiece or infrared viewing scope, measuring the current through the silicon photodiode with a microammeter, rotating the rotating wheel shaft of the Abbe refractometer, recording the correspondence between the readings of the Abbe refractometer and the photocurrent readings of the microammeter, and taking the reading of the Abbe refractometer when the photocurrent reading changes as the refractive index of the test sample.

[0081] In the present application, using a microammeter to quantify the degree of light intensity can not only avoid the error in the judgment of the boundary line by the naked eye, but also effectively prevent the influence of interference fringes and the like on the result, that is, whether the absorption rate of the test sample for the wavelength is large or small, using a microammeter is beneficial to accurate measurement of the refractive index. In the absence of interference fringes, a typical plot of the photocurrent as a function of the Abbe refractometer reading is as shown in Figure 6

[0082] The present application has the following beneficial effects:

[0083] (1) The present application can enhance the contrast of the eyepiece boundary line by selecting the material of the support reflection layer in the sample support.

[0084] (2) The present application can use photocurrent to quantify the illumination.

[0085] (3) The present application can test polyimide films attached to other substrates, such as single-sided copper-clad plate films.

[0086] (4) The present application has a simple sample preparation and testing process, high precision, and wide testing range.

[0087] (5) The present application can be used as a non-destructive, in-situ detection technique for polyimide.

[0088] (6) The present application uses a reflection method for testing, which can test the refractive index at a wavelength with high absorption, which is not achievable by the prior art.

[0089] The present application will be described below in the form of specific examples. It should be understood that these examples are merely illustrative and are not intended to limit the scope of the present application. The methods, reagents and materials used in the examples are conventional in the art unless otherwise specified. The raw material compounds in the examples can be purchased through commercial channels.

[0090] Device Example: Sample Support and Refractive Index Measuring Device

[0091] The sample support of the device example is as shown in Figure 2 The sample support comprises a lifting screw, a fixed plate and a fixing device for mounting the support reflection layer, and the lifting screw is used to adjust the distance between the fixed plate and the support reflection layer mounted on the fixing device.​

[0092] The refractive index measuring device of the present equipment example is shown in Figure 3 The refractive index measuring device comprises a sample holder, an Abbe refractometer, a silicon photodiode and a microammeter as shown in Figure 2 The Abbe refractometer comprises a light source, a filter, a prism, an eyepiece and a rotating wheel shaft. The filter is an interference filter. The eyepiece is a polarizing eyepiece. The silicon photodiode is fixed at the center of the crosshair of the eyepiece of the Abbe refractometer. The microammeter is connected to the silicon photodiode. After the sample holder and the holder reflecting layer are installed into the Abbe refractometer, the holder reflecting layer is tightly attached to the prism of the Abbe refractometer.

[0093] Example 1: Test the birefringence of polyimide at 633 nm wavelength

[0094] Using the sample holder and the refractive index measuring device of the present equipment example, selecting an interference filter with 633 nm wavelength, placing a 25 μm thick polyimide film on the sample holder, adjusting the lifting screw to make the fixed flat plate completely cover the upper surface of the polyimide film, selecting an optical glass with refractive index of 1.9 as the holder reflecting layer, setting the direction of the polarizing eyepiece as the north-south direction, testing the refractive index of the film thickness direction, adjusting the brightness of the light source to make the final reading of the microammeter within the range of 20 microamperes (at this time the photoelectric current is linearly related to the illumination), rotating the wheel shaft of the Abbe refractometer, recording the relationship between the Abbe refractometer reading and the photoelectric current, judging the mutation point according to the Abbe refractometer reading-photoelectric current curve, and the Abbe refractometer reading at the mutation point is the refractive index of the film thickness direction (out-of-plane refractive index); rotating the direction of the polarizing eyepiece as the east-west direction, and testing the refractive index of the film in-plane direction (in-plane refractive index) by the same method, the results are shown in Figures 7-10 Figure 7 and Figure 8 are the corresponding relationship diagrams of the photoelectric current and the Abbe refractometer reading when testing the out-of-plane refractive index of the film, Figure 7 is the corresponding relationship diagram tested by changing the Abbe refractometer reading by 0.005, Figure 8 is the corresponding relationship diagram tested by changing the Abbe refractometer reading by 0.0001 after determining the approximate range. Figure 9 and Figure 10 are the corresponding relationship diagrams of the photoelectric current and the Abbe refractometer reading when testing the in-plane refractive index of the film, Figure 9 is the corresponding relationship diagram tested by changing the Abbe refractometer reading by 0.005, Figure 10 is the corresponding relationship diagram tested by changing the Abbe refractometer reading by 0.0001 after determining the approximate range. Figures 7-10 It can be concluded that the out-of-plane refractive index and the in-plane refractive index are 1.6350 and 1.7360 respectively.

[0095] ​Comparative Example 1: Test birefringence of polyimide at 633 nm wavelength

[0096] The refractive index of the polyimide film of Example 1 was tested by prism coupling instrument, and the result was basically consistent with that of Example 1.

[0097] It was found during the test that although the measurement accuracy of the prism coupling instrument was high, the measurement accuracy was easily affected by factors such as the adjustment state of the system, the quality of the optical elements, and the environmental noise. In particular, when the sample to be tested has absorption at the test wavelength, it is difficult to obtain accurate results.

[0098] Example 2: Test birefringence of polyimide at 633 nm wavelength

[0099] The other conditions were the same as those of Example 1, only the support reflective layer was replaced with optical glass with a refractive index of 1.8, and the corresponding relationship graph of the photoelectric current and the Abbe refractometer reading obtained by the test is shown in Figures 11-14 .

[0100] Example 3: Test birefringence of polyimide at 633 nm wavelength

[0101] The other conditions were the same as those of Example 1, only the support reflective layer was replaced with optical glass with a refractive index of 2.2, and the corresponding relationship graph of the photoelectric current and the Abbe refractometer reading obtained by the test is shown in Figures 15-18 .

[0102] The Abbe refractometer reading near the mutation point of the Abbe refractometer reading-photoelectric current curve in Examples 1-3 and the corresponding photoelectric current values are shown in Table 1.

[0103] Table 1

[0104]

[0105]

[0106] As can be seen from Table 1, the same results were obtained by the tests of Examples 1-3, and the Abbe refractometer readings at the mutation points of the Abbe refractometer reading-photoelectric current curve when testing the in-plane refractive index and the out-of-plane refractive index were 1.635 and 1.736, respectively, i.e., the in-plane refractive index was 1.635 and the out-of-plane refractive index was 1.736. However, compared with Examples 1 and 3, the curve mutation point of Example 2 was more obvious, especially when testing the out-of-plane refractive index, it was easier to judge the mutation point. The reason is as described above, the refractive index of the support reflective layer and the polyimide in Example 2 is the closest, and thus a more obvious mutation is obtained.

[0107] Example 4: Test birefringence of polyimide on single-layer copper-clad plate at 633 nm wavelength

[0108] A polyimide film with a thickness of 12.5 μm is tested for birefringence by a prism coupling instrument, and then the polyimide film is combined with a copper foil by a hot-pressing method to prepare a single-layer copper-clad plate.

[0109] A sample holder and a refractive index measuring device are used, an interference filter with a wavelength of 633 nm is selected, a 12.5 μm thick polyimide film of the single-layer copper-clad plate is placed on the sample holder, the lifting screw is adjusted so that the fixed plate completely covers the upper surface of the sample to be tested, optical glass with a refractive index of 1.8 is selected as the holder reflecting layer, the direction of the polarizing eyepiece is set to the north-south direction, the refractive index in the thickness direction of the film is tested, the brightness of the light source is adjusted so that the reading of the final microammeter is within the range of 20 μA (at this time, the photocurrent is linearly related to the illumination), the wheel shaft of the Abbe refractometer is rotated, and the relationship between the Abbe refractometer reading and the photocurrent is recorded, the mutation point is determined according to the Abbe refractometer reading-photocurrent curve, and the Abbe refractometer reading at the mutation point is the refractive index in the thickness direction of the polyimide film (out-of-plane refractive index); the direction of the polarizing eyepiece is rotated to the east-west direction, and the refractive index in the in-plane direction of the film (in-plane refractive index) is tested by the same method, the out-of-plane refractive index is measured to be 1.6202, and the in-plane refractive index is measured to be 1.7468. Compared with the data tested by the prism coupling instrument, the results are basically consistent.

[0110] Example 5: Test the birefringence of polyimide at a wavelength of 1550 nm

[0111] A sample holder and a refractive index measuring device are used, an interference filter with a wavelength of 1550 nm is selected, an infrared viewing mirror is installed on the Abbe refractometer (the light path enters the infrared viewing mirror after passing through the polarizing eyepiece), a photodiode is installed at the cross center of the infrared viewing mirror, a 12.5 μm thick polyimide film is placed on the sample holder, the lifting screw is adjusted so that the fixed plate completely covers the upper surface of the sample to be tested, optical glass with a refractive index of 1.8 is selected as the holder reflecting layer, the direction of the polarizing eyepiece is set to the north-south direction, the refractive index in the thickness direction of the film is tested, the brightness of the light source is adjusted so that the reading of the final microammeter is within the range of 20 μA (at this time, the photocurrent is linearly related to the illumination), the wheel shaft of the Abbe refractometer is rotated, and the relationship between the Abbe refractometer reading and the photocurrent is recorded, the mutation point is determined according to the Abbe refractometer reading-photocurrent curve, and the Abbe refractometer reading at the mutation point is the refractive index in the thickness direction of the polyimide film (out-of-plane refractive index); the direction of the polarizing eyepiece is rotated to the east-west direction, and the refractive index in the in-plane direction of the film (in-plane refractive index) is tested by the same method, the out-of-plane refractive index is measured to be 1.5868, and the in-plane refractive index is measured to be 1.7192. Compared with the data tested by the prism coupling instrument, the results are basically consistent.

[0112] Example 6: Test the birefringence of polyimide at a wavelength of 589 nm

[0113] The sample holder and the refractive index measuring device of the equipment example are used, the interference filter with a wavelength of 589 nm is selected, the 25 μm thick polyimide film is placed on the sample holder, the lifting screw is adjusted so that the fixed plate completely covers the upper surface of the sample to be measured, the optical glass with a refractive index of 1.8 is selected as the support reflection layer, the direction of the polarizing eyepiece is set to the north-south direction, the refractive index in the thickness direction of the film is tested, the brightness of the light source is adjusted so that the reading of the final microammeter is in the range of 20 microamperes (at this time, the photocurrent is linearly related to the illumination), the wheel shaft of the Abbe refractometer is rotated, and the relationship between the Abbe refractometer reading and the photocurrent is recorded, the mutation point is judged according to the Abbe refractometer reading-photocurrent curve, and the refractometer reading at the mutation point is the refractive index in the thickness direction of the film (out-of-plane refractive index); the direction of the polarizing eyepiece is rotated to the east-west direction, and the refractive index in the in-plane direction of the film (in-plane refractive index) is tested by the same method, and the out-of-plane refractive index and the in-plane refractive index are measured to be 1.6178 and 1.7602, respectively.

[0114] Because the absorption rate of polyimide reaches more than 80% at the test wavelength, it is difficult to accurately test the refractive index at 589 nm by using the existing equipment.

[0115] Verification example 1: calculation of the birefringence of polyimide at a wavelength of 589 nm

[0116] The refractive indices of the sample in example 6 at 638 nm, 720 nm and 1540 nm are tested by using the prism coupling instrument, the results are shown in table 2, and the out-of-plane refractive index and the in-plane refractive index at 589 nm are calculated to be 1.6179 and 1.7601, respectively, by using the Cauchy dispersion formula. It can be seen that the refractive indices tested in example 6 and calculated by using the Cauchy dispersion formula are highly consistent.

[0117] Obviously, although the refractive index at a specific wavelength (such as 589 nm) can be calculated by using the Cauchy dispersion formula, three or more sets of refractive index data at other wavelengths are required, and the calculation process is relatively complicated, while the method of the present application is more direct and more convenient.

[0118] Table 2

[0119]

Claims

1. A method of measuring the refractive index of a thin film, characterized by, The method comprises the following steps: (1) mounting a sample holder on an Abbe refractometer, fixing a holder reflecting layer on the sample holder, the sample holder comprising a fixing flat plate and a fixing device for fixing the holder reflecting layer, the holder reflecting layer being isotropic, the refractive index of the holder reflecting layer being isotropic and greater than the refractive index of the test sample, the holder reflecting layer being in the shape of a flat plate with its upper and lower surfaces parallel; (2) fixing the test sample between the holder reflecting layer and the fixing flat plate, so that the part of the test sample as the refractive index measurement object is in close contact with the holder reflecting layer; (3) turning on the light source of the Abbe refractometer, so that the light passes through the filter and enters the holder reflecting layer, critical total reflection occurs at the interface between the holder reflecting layer and the test sample, the light returns to the holder reflecting layer, and then passes through the prism of the Abbe refractometer and finally reaches the ocular lens or infrared viewing lens of the Abbe refractometer; (4) rotating the rotating wheel shaft of the Abbe refractometer, and recording the reading of the Abbe refractometer when the critical outgoing light is located at the center of the crosshair of the ocular lens or infrared viewing lens as the refractive index of the test sample.

2. The method of claim 1, wherein, The refractive index of the test sample is anisotropic, the ocular lens is a polarizing ocular lens, and the step (4) comprises: setting the polarization direction of the polarizing ocular lens, and recording the reading of the Abbe refractometer when the critical outgoing light is located at the center of the crosshair of the ocular lens or infrared viewing lens as the refractive index of the test sample corresponding to the polarization direction.

3. The method of claim 2, wherein, The polarization direction comprises the north-south direction and / or the east-west direction, the refractive index when the polarization direction is the north-south direction is the refractive index of the extraordinary light, and the refractive index when the polarization direction is the east-west direction is the refractive index of the ordinary light.

4. The method of claim 1, wherein, The step (4) further comprises: fixing a silicon photodiode at the center of the crosshair of the ocular lens or infrared viewing lens, measuring the current passing through the silicon photodiode with a microammeter, rotating the rotating wheel shaft of the Abbe refractometer, recording the corresponding relationship between the reading of the Abbe refractometer and the photocurrent reading of the microammeter, and taking the reading of the Abbe refractometer when the photocurrent reading changes as the refractive index of the test sample.

5. The method of claim 4, wherein, The step (4) further comprises: adjusting the brightness of the light source so that the reading of the microammeter is ≤25 μA.

6. The method of claim 4, wherein, The step (4) further comprises: adjusting the brightness of the light source so that the reading of the microammeter is ≤20 μA.

7. The method of claim 1, wherein, The sample holder further comprises a distance adjusting device for adjusting the distance between the fixing flat plate and the holder reflecting layer, so that the test sample is fixed between the holder reflecting layer and the fixing flat plate.

8. The method of claim 7, wherein, The distance adjusting device comprises one or more selected from a magnetic attraction device, a lock, a spring and a lifting screw.

9. The method of claim 7, wherein, The distance adjusting device comprises a lifting screw for causing the fixing flat plate to displace in the direction away from the holder reflecting layer.

10. The method of claim 1, wherein, The refractive index of the support reflective layer is n1 and the refractive index of the test sample is n o , (n1-n o ) / n o ≤ 20%.

11. The method of claim 1, wherein, The refractive index of the support reflective layer is n1 and the refractive index of the test sample is n o , (n1-n o ) / n o ≤ 10%.

12. The method of claim 1, wherein, The material of the part of the test sample as the refractive index measurement object comprises polyimide.

13. A sample holder for an Abbe refractometer for use in the method of any one of claims 1 to 12, characterized in that The sample holder comprises a fixing flat plate and a fixing device for fixing the holder reflecting layer; the holder reflecting layer is isotropic, and the holder reflecting layer is in the shape of a flat plate with its upper and lower surfaces parallel.

14. The sample holder of claim 13, wherein, The sample holder further comprises a distance adjusting device for adjusting the distance between the fixing flat plate and the holder reflecting layer, so that the test sample is fixed between the holder reflecting layer and the fixing flat plate.

15. The sample holder of claim 14, wherein, The distance adjustment device comprises one or more selected from a magnetic attraction device, a lock, a spring, and a lifting screw.

16. The sample holder of claim 14, wherein, The distance adjustment device comprises a lifting screw for displacing the fixed flat plate in the direction of the reflective layer of the distance support.

17. An apparatus for testing the refractive index of a thin film, characterized by The device comprises an Abbe refractometer and the sample support of claim 13.

18. The apparatus of claim 17, wherein, The device further comprises a silicon photodiode and a microammeter.

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