CN116148216A

CN116148216A - Refractive index testing method and device

Info

Publication number: CN116148216A
Application number: CN202211080807.1A
Authority: CN
Inventors: 顾玲玲; 郑天成
Original assignee: Shanghai Zhonghua Technology Co ltd
Current assignee: Shanghai Zhonghua Technology Co ltd
Priority date: 2022-09-05
Filing date: 2022-09-05
Publication date: 2023-05-23

Abstract

The present invention provides a method and apparatus for measuring refractive index, the method comprising: (1) Installing a sample support on the Abbe refractometer, and fixing a support reflecting layer on the sample support, wherein the sample support comprises a fixing flat plate and a fixing device for fixing the support reflecting layer; (2) Fixing the test sample between the support reflecting layer and the fixing plate to make the part of the test sample as the refractive index measuring object cling to the support reflecting layer; (3) Turning on a light source of the Abbe refractometer to enable light to finally reach an ocular or an infrared observation mirror of the Abbe refractometer; (4) And rotating a rotating wheel shaft of the Abbe refractometer, and recording the reading of the Abbe refractometer when the critical emergent ray is positioned at the center of the cross line of the ocular or the infrared observation mirror as the refractive index of the test sample.

Description

Refractive index testing method and device

Technical Field

The invention belongs to the technical field of refractive index measurement, and particularly relates to a refractive index testing method and a refractive index testing device.

Background

Polyimide films are widely used in the market as "gold films" including as high temperature insulation materials for adhesives, separation films, photoresists, dielectric buffer layers, liquid crystal aligning agents, electro-optic materials, etc., as a high thermal insulation material for motor slot insulation and as a cable enamel wire material. In addition, polyimide films are suitable for use as substrates or coverlays for ribbon cables or flexible printed circuits, etc., due to their softness, good dimensional stability, and superior dielectric properties.

The oriented polyimide film has great differences in mechanical properties, optical properties, thermal properties and the like in different directions, such as the tensile strength and fatigue strength are remarkably increased in the orientation direction and are reduced in the direction perpendicular to the orientation direction. Therefore, in various technological processes of polyimide film production, it is quite critical to characterize the molecular orientation inside the film by using a nondestructive in-situ detection technique.

The optical anisotropy or birefringence of polyimide is a convenient and sensitive indicator of the molecular orientation of polyimide characterizing various stages and applications. The birefringence of polyimide depends on two factors: monomer structure and degree of molecular orientation. Therefore, the molecular orientation of a polyimide having a uniform monomer structure can be determined from the birefringence. The larger the degree of orientation of the polymer, the more ordered the alignment of the polymer chains in the in-plane direction, the larger the difference in refractive index between the in-plane and out-of-plane directions, so the magnitude of birefringence can be used to measure the orientation of the polymer.

The refractive index measurement of the film material is mainly based on the geometrical optics principle or the light interference and diffraction principle, and the refractive index of the sample is measured by comparing the propagation direction and phase difference of incident light and reflected and refracted light. At present, the current time of the process, ellipsometry, prism coupling, interferometry, and conventional Abbe refractometry are the primary methods of measuring the refractive index of thin films.

Ellipsometry is a common method for measuring refractive index and thickness of optical thin films, which is to project a beam of polarized light onto the surface of a sample to be measured non-perpendicularly, and to infer the optical properties of the sample, such as the thickness of the thin film, the complex refractive index of the material, etc., by the change of the polarization state of the reflected light or transmitted light, and is suitable for measuring transparent or weakly absorbing thin film samples having a thickness of less than one film thickness period and known as a base material, but it has different measurement accuracy due to different incident angles and wavelengths. And ellipsometry is a complex and expensive measurement process. Because ellipsometry requires monitoring the optical properties of refracted light, polyimide samples absorb the refracted light severely, and the optical properties of the refracted light cannot be known, thus affecting the test results. Ellipsometry, on the other hand, fails to test the refractive index of samples of non-freestanding support films, such as polyimide films that have been bonded.

The prism coupling method has the working principle that when light is incident on the prism, the incident angle changes along with the rotation of the rotary table, and at a certain incident angle, photons enter the film through the air slit to be transmitted, so that photon energy obtained by the detector is reduced, a recess is formed, and the leakage mode is formed. During the test, the test sample acts as a guided wave layer 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, in the process of total reflection propagation of the light rays in the waveguide layer, if the light rays are absorbed by the sample, the judgment of the critical angle in the prism can be affected, so that the accuracy of the test cannot be ensured.

The measurement process of measuring the birefringence of the film by interferometry is complex to adjust and is easy to abrade the surface of the film. The optical interferometry has high precision requirement and complex operation.

The conventional Abbe refractometer measures the refractive index of a substance by a grazing incidence method based on the principle of total reflection. When light is injected into a test sample by an optical-hydrophobic substance (air), the incident angle and the refraction angle accord with the Snell formula, and when the incident light is perpendicular to the normal line of an interface, the refraction angle reaches the maximum, at this time, the incident ray is called glancing ray, and the corresponding refraction angle is called refraction critical angle or total reflection angle. The refracted light rays are refracted by the prism, reflected by the swinging reflector and the like, and finally enter the ocular lens. The refractive index of the substance to be measured satisfies the formula:

wherein A is the apex angle of the prism, N is the refractive index of the prism, i ₀ For the exit angle, the sign in the formula depends on the difference in refractive index between the sample to be measured and the prism. A schematic diagram of a conventional abbe refractometer is shown in fig. 1, when testing the birefringence of an anisotropic test object, the eyepiece in fig. 1 is changed to a polarizing eyepiece, and the ordinary ray and the extraordinary ray are observed by adjusting the direction of the polarizing eyepiece. When polyimide refractive index and birefringence are measured with an abbe refractometer, there are two problems: first, polyimideThe light has strong absorption effect, so that emergent light is difficult to monitor, and the cut-off can not be observed in the ocular; secondly, when polyimide and the carrier cannot be separated, measurement cannot be performed because light rays cannot exit from the lower surface after entering the film from the upper surface, and the emergent light is collected by the traditional Abbe refractometer method.

In industries where quality control by measuring refractive index is required, it is required to analyze refractive index measurements of different samples under the same conditions (including a well-defined, precise wavelength). To generate a well-defined wavelength, the refractometer most commonly uses the sodium D line, which corresponds to 589.3nm. Since the sodium D-line is a widely used, reliable and stable light source, it has long been used in the study of refractive index.

In summary, ellipsometry, prism coupling, interferometry and conventional abbe refractometer have two problems in detecting the refractive index of polyimide films: first, polyimide exhibits brown or yellow color due to its high aromatic ring density, and therefore has low light transmittance in the visible light range, and it is difficult to detect the refractive index of a polyimide film, particularly at a high absorption wavelength (for example, 589nm for sodium light). In addition, light is absorbed by the film during the propagation inside the polyimide film, so it is also difficult to detect the refractive index of the polyimide film by analyzing the phase difference or fringes (ellipsometry and interferometry) or waveguide characteristics (prism coupler) of the reflected and refracted rays thereof; secondly, the above methods require the film as a separate film or have clear requirements for the substrate material, and when the polyimide film and the substrate (e.g., glass, copper foil) cannot be separated, none of these methods can test the refractive index of the polyimide film.

There is another abbe refractometer, which uses a built-in light source to transmit light through a prism, then the light is emitted to an optical-hydrophobic substance (sample) by an optical dense substance (prism), when the incident angle is larger than or equal to a critical angle, total reflection occurs, when the incident angle is smaller than the critical angle, refraction and reflection occur, so that a boundary exists, and the boundary is a curve of total reflection light with a refraction angle of 90 degrees conducted by an optical path. There are three problems with this Abbe refractometer: firstly, because the light source is built in, the selectivity of the test wavelength is limited by the instrument; secondly, the boundary is not obvious, errors are easy to generate when the eyepiece is used for observation, and particularly when interference fringes exist in the test, the judgment of the test result is easy to be interfered; and thirdly, the third step of, in the case of the vehicle, the method for testing the birefringence is not reported at present.

Disclosure of Invention

To overcome the deficiencies of the prior art, the present invention provides a method of measuring the refractive index and birefringence of a material, particularly a film or other solid material, which may be colored, translucent. The invention is particularly useful for measuring refractive index of micrometer to millimeter scale films, particularly polyimide films. The invention can nondestructively and rapidly measure the refractive index of the film under specific wavelength.

Specifically, the present invention provides a method of measuring refractive index, the method comprising the steps of:

(1) Installing a sample support on the Abbe refractometer, and fixing a support reflecting layer on the sample support, wherein the sample support comprises a fixing flat plate and a fixing device for fixing the support reflecting layer;

(2) Fixing the test sample between the support reflecting layer and the fixing plate to make the part of the test sample as the refractive index measuring object cling to the support reflecting layer;

(3) Turning on a light source of the Abbe refractometer, enabling light to enter a support reflecting layer after passing through the optical filter, enabling critical total reflection to occur at the interface of the support reflecting layer and the test sample, enabling the light to return to the support reflecting layer, enabling the light to finally reach an ocular or an infrared observation mirror of the Abbe refractometer through a prism of the Abbe refractometer;

(4) And rotating a rotating wheel shaft of the Abbe refractometer, and recording the reading of the Abbe refractometer when the critical emergent ray is positioned at the center of the cross line of the ocular or the infrared observation mirror as the refractive index of the test sample.

In one or more embodiments, the refractive index of the test sample is anisotropic, the eyepiece is a polarizing eyepiece, and step (4) comprises: setting the polarization direction of a polarized ocular, and recording the reading of an Abbe refractometer when the critical emergent ray is positioned at the center of a cross line of the ocular or an infrared observation mirror as the refractive index of a test sample corresponding to the polarization direction; preferably, the polarization direction includes a north-south direction and/or an east-west direction, the refractive index when the polarization direction is the north-south direction is the refractive index of the extraordinary ray, and the refractive index when the polarization direction is the east-west direction is the refractive index of the ordinary ray.

In one or more embodiments, the step (4) further comprises: the method comprises the steps of fixing a silicon photodiode in the center of a cross line of an ocular or an infrared observation mirror, measuring current passing through the silicon photodiode by using a microcurrent meter, rotating a rotating wheel shaft of an Abbe refractometer, recording the corresponding relation between the reading of the Abbe refractometer and the photocurrent reading of the microcurrent meter, and taking the reading of the Abbe refractometer when the photocurrent reading is suddenly changed as the refractive index of a test sample.

In one or more embodiments, the step (4) further comprises: the brightness of the light source is adjusted, the reading of the microammeter is made to be 25. Mu.A or less, preferably 20. Mu.A or less.

In one or more embodiments, the sample holder further comprises a distance adjustment device for adjusting the distance between the holding plate and the holder reflective layer such that the test sample is held between the holder reflective layer and the holding plate.

In one or more embodiments, the distance adjusting means includes one or more selected from a magnetic attraction means, a latch, a spring, and a lifting screw.

In one or more embodiments, the distance adjustment means includes a lifting screw for displacing the fixing plate in a direction away from the support reflective layer.

In one or more embodiments, the refractive index of the scaffold reflective layer is isotropic and greater than the refractive index of the test sample.

In one or more embodiments, the refractive index of the stent reflective layer is n ₁ The refractive index of the test sample is n _o ，(n ₁ -n _o )/n _o Less than or equal to 20%, preferably (n) ₁ -n _o )/n _o ≤10％。

In one or more embodiments, the material of the portion of the test sample that is the subject of refractive index measurement comprises polyimide.

The invention also provides a sample support for the Abbe refractometer, which is used for fixing the flat plate and a fixing device for fixing the reflecting layer of the support.

The present invention also provides a device for testing refractive index comprising an abbe refractometer and a sample holder according to any of the embodiments herein; preferably, the device further comprises a silicon photodiode and a micro-ammeter.

Drawings

Fig. 1 is a schematic diagram of a conventional abbe refractometer.

Fig. 2 is a schematic view of a sample holder in some embodiments of the invention.

Fig. 3 is a schematic diagram of a refractive index measurement device in some embodiments of the invention.

Fig. 4 is a schematic structural diagram of a photocurrent testing apparatus in some embodiments of the present invention.

Fig. 5 is a schematic diagram of a silicon photodiode in some embodiments of the present invention.

FIG. 6 is a graph showing a typical photocurrent-Abbe refractometer reading relationship in the present invention.

FIG. 7 is a plot of photocurrent versus Abbe refractometer reading for the film out-of-plane refractive index of example 1 as a function of 0.005.

FIG. 8 is a plot of photocurrent versus Abbe refractometer reading for the change in the out-of-plane refractive index of the film of example 1 at 0.0001.

FIG. 9 is a plot of photocurrent versus Abbe refractometer reading for the 0.005 variation in Abbe refractometer reading for the in-plane refractive index of the film of example 1.

FIG. 10 is a plot of photocurrent versus Abbe refractometer reading for the 0.0001 variation in Abbe refractometer reading for the in-plane refractive index of the film of example 1.

FIG. 11 is a plot of photocurrent versus Abbe refractometer reading for the film out-of-plane refractive index of example 2 as a function of 0.005.

FIG. 12 is a plot of photocurrent versus Abbe refractometer reading for the change in the out-of-plane refractive index of the film of example 2 at 0.0001.

FIG. 13 is a plot of photocurrent versus Abbe refractometer reading for the 0.005 variation in Abbe refractometer reading for the in-plane refractive index of the film of example 2.

FIG. 14 is a plot of photocurrent versus Abbe refractometer reading for the 0.0001 variation in Abbe refractometer reading for the in-plane refractive index of the film of example 2.

FIG. 15 is a plot of photocurrent versus Abbe refractometer reading for the film out-of-plane refractive index of example 3 as a function of 0.005.

FIG. 16 is a plot of photocurrent versus Abbe refractometer reading for the 0.0001 variation in Abbe refractometer reading for the out-of-plane refractive index of the film tested in example 3.

FIG. 17 is a plot of photocurrent versus Abbe refractometer reading for the 0.005 variation in Abbe refractometer reading for the in-plane refractive index of the film of example 3.

FIG. 18 is a test film of example 3 Abbe refractometer with in-plane refractive index readings are plotted as photocurrent versus abbe refractometer readings for a change of 0.0001.

Detailed Description

So that those skilled in the art can appreciate the features and effects of the present invention, a general description and definition of the terms and expressions set forth in the specification and claims follows. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs, and in the event of a conflict, the present specification shall control.

The theory or mechanism described and disclosed herein, whether right or wrong, is not meant to limit the scope of the invention in any way, i.e., the present disclosure may be practiced without limitation to any particular theory or mechanism.

Herein, "comprising," "including," "containing," and similar terms are intended to cover the meaning of "consisting essentially of … …" and "consisting of … …," e.g., "a consisting essentially of B and C" and "a consisting of B and C" should be considered to have been disclosed herein when "a comprises B and C" is disclosed herein.

All features such as values, amounts, and concentrations that are defined herein in the numerical or percent ranges are for brevity and convenience only. Accordingly, the description of a numerical range or percentage range should be considered to cover and specifically disclose all possible sub-ranges and individual values (including integers and fractions) within the range.

Herein, when embodiments or examples are described, it should be understood that they are not intended to limit the invention to these embodiments or examples. On the contrary, all alternatives, modifications, and equivalents of the methods and materials described herein are intended to be included within the scope of the invention as defined by the appended claims.

In this context, not all possible combinations of the individual technical features in the individual embodiments or examples are described in order to simplify the description. Accordingly, as long as there is no contradiction between the combinations of these technical features, any combination of the technical features in the respective embodiments or examples is possible, and all possible combinations should be considered as being within the scope of the present specification.

The invention provides a method for testing the refractive index of materials, such as films, particularly polyimide films, by using a critical total reflection method. On one hand, the method of the invention solves the problems that the prior art is difficult to measure the birefringence of the polyimide film which cannot be peeled off by the carrier, and solves the problem that the polyimide birefringence cannot be tested in a waveband with high absorptivity, because light is blocked when the absorptivity is high and light is transmitted in the film, and the prior art realizes measurement by utilizing the light transmitted in the film, and the invention can realize the full waveband test of the birefringence of the polyimide; on the other hand, the device has the advantages of convenient measurement and high precision.

The invention also designs a sample support for testing the refractive index of a material, which can be simply and conveniently connected with an Abbe refractometer, the refractive index of the test material can be simply, conveniently and accurately realized without excessively adjusting the whole device.

The refractive index measuring method of the present invention comprises the steps of:

(1) The sample support is arranged on an Abbe refractometer, the support reflecting layer is fixed on the sample support, the sample support comprises a fixing plate and a fixing device for fixing the reflecting layer of the support;

Further, when the refractive index of the test sample is anisotropic, using a polarizing eyepiece, step (4) includes: setting polarization direction of polarized ocular, testing refractive index when the polarization direction is north-south direction, namely out-of-plane refractive index in film thickness direction, the refractive index when the polarization direction is the east-west direction is tested, namely the in-plane refractive index of the in-plane direction of the film. In the present invention, isotropy and anisotropy are with respect to refractive index.

In the Abbe refractometer used in the invention, critical emergent light is obtained when the emergent light is positioned in the center of the ocular lens by rotating the rotating wheel axle of the Abbe refractometer, so that the angle of the swinging reflector (a mirror for capturing the critical emergent light in the Abbe refractometer) is adjusted, and the reading of the Abbe refractometer is the refractive index of the test sample. In the Abbe refractometer, the angle of the swinging reflector, the emergence angle and the refractive index reading are all calibrated by manufacturers. Abbe refractometers suitable for use in the present invention require that their refractive index range cover the refractive index of the test sample.

A sample holder for testing the refractive index of a material in some embodiments of the invention is shown in fig. 2. The sample support of the present invention comprises a fixation plate and a fixation device for fixing the support reflective layer. The structure of the fixing device is not particularly limited as long as the support reflection layer can be fixed, and for example, a clamping device, a magnetic attraction device, and the like can be used. In some embodiments, the sample holder further comprises a distance adjustment device for adjusting the distance between the holding plate and the holder reflective layer to enable the test sample to be held between the holder reflective layer and the holding plate. The distance adjusting means is not particularly limited as long as the test sample can be fixed between the support reflection layer and the fixing plate. The distance adjusting means may comprise one or more selected from the group consisting of magnetic means, catches, springs, lifting screws, etc. In some embodiments, the distance adjustment device includes a lift screw for displacing the fixation plate in a direction away from the support reflective layer, thereby securing the test sample between the support reflective layer and the fixation plate.

In the invention, the sample support can flexibly mount and dismount the test sample, the support reflecting layer is used as the light-tight layer, and the light source reflects and partially refracts after entering the test sample through the support reflecting layer. The fixation plate is used to fix the test sample. The stationary plate is opaque to avoid the effects of ambient light on the test. After the light is finally conducted through the light path, a dark and bright visual field is formed.

In the invention, the upper and lower surfaces of the support reflecting layer are required to be parallel. Therefore, the support reflecting layer is in a flat plate shape with parallel upper and lower side surfaces. The refractive index of the standoff reflective layer needs to be higher than that of the test sample and the standoff reflective layer is isotropic. The thickness of the support reflective layer is not particularly limited and is preferably between 1cm and 3 cm. The material of the support reflecting layer can be high refractive index glass, artificial sapphire and the like.

In the invention, after the sample support and the support reflecting layer are installed, the support reflecting layer is tightly attached to the prism of the Abbe refractometer.

FIG. 3 is a graph of refraction in some embodiments of the invention schematic diagram of the rate measurement device and schematic diagram of the optical path. The refractive index measuring device comprises the sample support, the Abbe refractometer, the silicon photodiode and the micro-ammeter. Wherein the abbe refractometer comprises a light source, a filter, a prism, an eyepiece, a rotating hub and a swinging mirror (not shown in fig. 3). The silicon photodiode is fixed in the center of the crisscross line of the eyepiece of the Abbe refractometer. The micro-ammeter is connected with the silicon photodiode.

In the present invention, when the test sample is an isotropic sample, the light shown in fig. 3 is the light at which critical reflection occurs, and the incident angle α satisfies the formula (1): sin alpha n ₁ ＝sin90°·n _o Wherein n is ₁ N is the refractive index of the support reflective layer _o To test the refractive index of the sample. When critical reflected light is transmitted into the main prism in the support reflecting layer, the upper and lower sides of the support reflecting layer are parallel, the incident angle is alpha, and the refraction angle beta of the main prism meets the formula (2): sin β·n=sin α·n ₁ Where N is the refractive index of the primary prism. Formula (3) can be obtained by combining formula (1) and formula (2): sin beta.n=n _o . When a conventional Abbe refractometer is used, the angle of refraction beta incident on the primary prism ₁ Satisfy formula (4): sin90 DEG n _o ＝sinβ ₁ N, i.e. N _o ＝sinβ ₁ N. From the formula (3) andequation (4) shows that β=β ₁ . Therefore, in the invention, when the critical reflection light is positioned at the center of the ocular lens, the formula is also satisfied

Wherein A is the apex angle of the prism, N is the refractive index of the prism, i ₀ For the exit angle, the sign in the formula depends on the difference in refractive index between the sample to be measured and the prism.

The refractive index of the test sample is irrelevant to the refractive index of the support reflecting layer, so the invention can use the test system of the traditional Abbe refractometer to complete the reading of the refractive index of the test sample.

In an embodiment in which the test sample is an anisotropic sample such as polyimide, the test sample has a birefringence phenomenon, rotates a polarizing eyepiece, and when the polarization direction is the north-south direction, the critical reflection angle α satisfies the formula (5): sin alpha n ₁ ＝sin90°·n _e Wherein n is _e When the polarization direction is the east-west direction, the critical reflection angle α satisfies the formula (6): sin alpha n ₁ ＝sin90°·n _o Wherein n is _o The refractive index of ordinary light, that is, the refractive index in the in-plane direction of the film. In the present invention, the ordinary ray means a ray which obeys the law of refraction, and the extraordinary ray means a ray which does not obey the law of refraction.

In the conventional method for testing refractive index by using an Abbe refractometer, naked eyes are used for observing the cut-off, and the cut-off can be judged inaccurately due to differences of environment, individuals and the like.

In the invention, the identification of the cut-off can be improved by adjusting the refractive index of the support reflective layer.

Regarding the selection of the refractive index of the support reflection layer, theoretically, when the refractive index of the support reflection layer is smaller than the refractive index of the main prism, and when the incident angle is larger than the critical angle (total reflection angle) calculated according to the snell principle, the test light is totally reflected, and does not enter the main prism, but cannot enter the test system. However, in the present invention, the boundary line in the eyepiece view is a light ray when the incident angle α is at the critical reflection angle, and the eyepiece viewLight in the field is located near the critical reflection angle, and beta=arcsin (n) is calculated according to formula (3) no matter how much the refractive index of the support reflective layer differs from the refractive index of the Abbe refractometer main prism _o and/N) is smaller than 90 degrees, and test light enters the test system. Therefore, the support reflecting layer is used as an external sample support, and the material of the support reflecting layer can be selected according to the requirements, so that the support reflecting layer only needs to meet the condition that the refractive index is larger than that of a test sample and is an isotropic substance, which is an important advantage of the invention.

When light is emitted by the light source and enters the support reflecting layer through the interference filter, refraction and reflection occur between the support reflecting layer and the test sample, and according to the Fresnel theorem, the light intensity of the reflected light and the refractive index n of the support reflecting layer ₁ Refractive index n of test sample _o Ratio (i.e. n) ₁ /n _o ) Closely related. When the testing direction is the in-plane direction of the film, the brightness of the light above the ocular boundary is along with n ₁ /n _o The brightness of light above the boundary of the ocular lens increases with n when the test direction is the film thickness direction ₁ /n _o The increase increases and then decreases. Since the refractive index of the test specimen itself is unchanged, the refractive index n can be selected when testing the in-plane direction of the film in order to make the brightness of the light above the boundary as low as possible and make the contrast between the light and the boundary more obvious ₁ Smaller (slightly larger than the refractive index n of the sample) _o And then) a support reflective layer. While the refractive index n can be selected when testing the film thickness direction ₁ Particularly large or with the refractive index n of the sample _o A relatively close support reflector. The polyimide film itself has a large refractive index, and it is difficult to find a support reflection layer about twice as large as the refractive index. Therefore, when the refractive index of the polyimide film is tested, the light-dark cut-off can be made more obvious by selecting the support reflective layer which is relatively close to the refractive index of the polyimide film. Under the condition that a microcurrent meter is not arranged for monitoring photocurrent, a proper support reflecting layer is selected, so that the bright-dark cut-off can be clearly judged, and the function which cannot be realized by the existing reflection method refractive index testing equipment can be realized.

Further, the present invention preferably uses photo-current to quantify illuminance. In the invention, the silicon photodiode can be fixed on the ocular or the infrared observation mirror (when the refractive index under infrared light is measured), and the micro-ammeter is connected on the lead wire of the silicon photodiode, so that the brightness is quantized into photocurrent, and the test error caused by naked eye judgment is effectively reduced.

Fig. 4 is a schematic structural diagram of a photocurrent testing device usable in the present invention, showing a connection manner of an eyepiece of an abbe refractometer and a microcurrent meter. Fig. 5 is a schematic diagram of a silicon photodiode that may be used in the present invention. In fig. 5, the upper part of the diode is circular, wherein the gray part represents its incident light window, which may be 1.1mm by 1.1mm in size. The placement direction of the silicon photodiode is: the incident light window is tightly attached to the ocular of the Abbe refractometer, and the lead wire is outwards perpendicular to the ocular direction. The micro-ammeter is connected with the silicon photodiode through a wire.

Preferably, step (4) of the method of the invention comprises: the method comprises the steps of fixing a silicon photodiode in the center of a cross line of an ocular or an infrared observation mirror, measuring current passing through the silicon photodiode by using a microcurrent meter, rotating a rotating wheel shaft of an Abbe refractometer, recording the corresponding relation between the reading of the Abbe refractometer and the photocurrent reading of the microcurrent meter, and taking the reading of the Abbe refractometer when the photocurrent reading is suddenly changed as the refractive index of a test sample.

In the invention, the micro-ammeter is used for quantifying the light intensity degree, so that not only can the error of judgment of a boundary line by naked eyes be avoided, but also the influence of interference fringes and the like on the result can be effectively prevented, that is, no matter whether the absorption rate of a test sample for wavelength is large or small, the micro-ammeter is used for accurately measuring the refractive index. A plot of typical photocurrent as a function of abbe refractometer reading without interference fringes is shown in fig. 6.

The invention has the following beneficial effects:

(1) The invention can enhance the boundary contrast of the ocular by selecting the material of the support reflecting layer in the sample support.

(2) The present invention can use photo-current to quantify illuminance.

(3) The invention can test polyimide films attached to other substrates, such as single-sided copper-clad plate films.

(4) The invention has simple sample preparation and test process, high precision and wide test range.

(5) The invention can be used as a nondestructive and in-situ detection technical means of polyimide.

(6) The invention uses reflection method to test the refractive index under the wavelength of high absorptivity, which can not be realized by the prior art.

The invention will be illustrated by way of specific examples. It should be understood that these examples are illustrative only and are not intended to limit the scope of the invention. The methods, reagents and materials used in the examples are those conventional in the art unless otherwise indicated. The starting compounds in the examples are all commercially available.

Equipment example: sample holder and refractive index measuring device

The sample holder of this apparatus example is shown in fig. 2. The sample support comprises a lifting screw, a fixing flat plate and a fixing device, wherein the fixing device is used for installing a support reflecting layer, and the lifting screw is used for adjusting the distance between the fixing flat plate and the support reflecting layer installed on the fixing device.

The refractive index measuring device of this embodiment is shown in fig. 3. The refractive index measuring device includes a sample holder, an Abbe refractometer, a silicon photodiode, and a micro-ammeter shown in FIG. 2. The Abbe refractometer comprises a light source, an optical filter, a prism, an ocular and a rotating wheel shaft. The optical filter is an interference filter. The ocular is polarized ocular. The silicon photodiode is fixed in the center of the crisscross line of the eyepiece of the Abbe refractometer. The micro-ammeter is connected with the silicon photodiode. After the sample support and the support reflecting layer are arranged in the Abbe refractometer, the support reflecting layer is tightly attached to a prism of the Abbe refractometer.

Example 1: testing the Birefringence index of polyimide at 633nm wavelength

Using a sample support and a refractive index measuring device of equipment, selecting an interference filter with a wavelength of 633nm, placing a polyimide film with a thickness of 25 mu m on the sample support, adjusting lifting screws to enable a fixed flat plate to completely cover the upper surface of the polyimide film, selecting optical glass with a refractive index of 1.9 as a support reflecting layer, setting the direction of a polarized eyepiece to be in the north-south direction, testing the refractive index in the thickness direction of the film, adjusting the brightness of a light source, enabling the reading of a final micro-ammeter to be in a 20 microampere range (at the moment, the photocurrent and illuminance form a linear relationship), rotating an axle of the Abbe refractometer, recording the relationship between the reading of the Abbe refractometer and the photocurrent, judging a mutation point according to the reading-photocurrent curve of the Abbe refractometer, wherein the reading at the mutation point is the refractive index (out-of-plane refractive index) in the thickness direction of the film; the refractive index (in-plane refractive index) of the film in the in-plane direction was measured in the same manner with the rotating polarizing eyepiece in the east-west direction, and the results are shown in fig. 7 to 10. Fig. 7 and 8 are graphs showing correspondence between photocurrent and abbe refractometer reading when the out-of-plane refractive index of the film is measured, fig. 7 is a graph showing correspondence between abbe refractometer reading measured with a change of 0.005, and fig. 8 is a graph showing correspondence between abbe refractometer reading measured with a change of 0.0001 after the approximate range is determined. Fig. 9 and 10 are graphs showing correspondence between photocurrent and abbe refractometer reading when the in-plane refractive index of the film is measured, fig. 9 is a graph showing correspondence between abbe refractometer reading measured with a change of 0.005, and fig. 10 is a graph showing correspondence between abbe refractometer reading measured with a change of 0.0001 after the approximate range is determined. As can be derived from fig. 7-10, the out-of-plane and in-plane refractive indices are 1.6350 and 1.7360, respectively.

Comparative example 1: testing the Birefringence index of polyimide at 633nm wavelength

The same polyimide film as in example 1 was tested for refractive index using a prism coupler, and the results were substantially identical to those of example 1.

In the test process, although the measurement accuracy of the prism coupler is higher, the measurement accuracy is easily affected by factors such as the adjustment state of the system, the quality of an optical element, environmental noise and the like, and particularly when a sample to be tested is absorbed under the test wavelength, accurate results are difficult to obtain.

Example 2: testing the Birefringence index of polyimide at 633nm wavelength

Other conditions were the same as in example 1, except that the support reflection layer was replaced with optical glass having a refractive index of 1.8, and the correspondence between photocurrent and abbe refractometer reading obtained by the test was as shown in fig. 11 to 14.

Example 3: testing the Birefringence index of polyimide at 633nm wavelength

Other conditions were the same as in example 1, except that the support reflection layer was replaced with an optical glass having a refractive index of 2.2, and the correspondence between the photocurrent obtained by the test and the abbe refractometer reading was as shown in fig. 15 to 18.

Abbe refractometer readings and corresponding photoelectric values in the vicinity of the mutation points of the Abbe refractometer readings-photocurrent curves in examples 1 to 3 are shown in Table 1.

TABLE 1

As can be seen from table 1, the tests of examples 1-3 gave the same results, abbe refractometer readings at the abbe refractometer reading-photocurrent curve mutation points at the in-plane refractive index test and the out-of-plane refractive index test are 1.635 and 1.736, respectively, i.e., the in-plane refractive index is 1.635, and the out-of-plane refractive index is 1.736. However, the curve mutation points of example 2 are more apparent than those of examples 1 and 3, and particularly, the mutation points are easier to judge when the out-of-plane refractive index is measured. The reason is that, as described above, the refractive indices of the support reflective layer and polyimide in example 2 are closest, and thus a more pronounced abrupt change is obtained.

Example 4: testing the double refractive index of polyimide on single-layer copper-clad plate at 633nm wavelength

And testing the birefringence of a polyimide film with the thickness of 12.5 mu m through a prism coupler, and then combining the polyimide film with a copper foil through a hot pressing method to prepare the single-layer copper-clad plate.

Using a sample support and a refractive index measuring device of equipment, selecting an interference filter with 633nm wavelength, placing a polyimide film surface with the thickness of 12.5 mu m of a single-layer copper-clad plate on the sample support, adjusting a lifting screw to enable a fixed flat plate to completely cover the upper surface of a sample to be measured, selecting optical glass with the refractive index of 1.8 as a support reflecting layer, setting the direction of a polarized eyepiece to be in the north-south direction, testing the refractive index of the film in the thickness direction, adjusting the brightness of a light source, enabling the reading of a final micro-ammeter to be in a 20 microampere range (at the moment, the linear relation between photocurrent and illuminance), rotating an axle of the Abbe refractometer, recording the relation between the reading of the Abbe refractometer and the photocurrent, judging a mutation point according to the reading-photocurrent curve of the Abbe refractometer at the mutation point, and obtaining the refractive index (out-of-plane refractive index) of the thickness direction of the polyimide film; the refractive index (in-plane refractive index) of the film in the in-plane direction was measured by the same method with the rotating polarizing eyepiece in the east-west direction, and the out-of-plane refractive index 1.6202 was measured to be 1.7468. And comparing the data tested by using the prism coupler, wherein the two results are basically consistent.

Example 5: testing the Birefringence index of polyimide at 1550nm wavelength

Using a sample support and a refractive index measuring device of equipment, selecting an interference filter with 1550nm wavelength, installing an infrared observation mirror on an Abbe refractometer (an optical path enters the infrared observation mirror after passing through a polarized eyepiece), installing a photodiode in the cross center of the infrared observation mirror, placing a polyimide film with the thickness of 12.5 mu m on the sample support, adjusting a lifting screw to enable a fixed flat plate to completely cover the upper surface of a sample to be measured, selecting optical glass with the refractive index of 1.8 as a support reflecting layer, setting the polarized eyepiece direction as the north-south direction, testing the refractive index of the film thickness direction, adjusting the brightness of a light source, enabling the reading of a final microcurrent meter to be in a 20 microampere range (at the moment, the light current and the light illuminance are in a linear relation), rotating the wheel axle of the Abbe refractometer, and recording the relation between the Abbe refractometer reading and the light current curve, and judging a mutation point according to the Abbe refractometer reading-light current curve, wherein the reading of the refractometer at the mutation point is the refractive index (out-of plane refractive index) in the film thickness direction; the refractive index (in-plane refractive index) of the film in the in-plane direction was measured by the same method with the rotating polarizing eyepiece in the east-west direction, and the out-of-plane refractive index 1.5868 was measured to be 1.7192. And comparing the data tested by using the prism coupler, wherein the two results are basically consistent.

Example 6: testing the Birefringence index of polyimide at 589nm wavelength

Using a sample support and a refractive index measuring device of equipment, selecting an interference filter with a wavelength of 589nm, placing a polyimide film with a thickness of 25 mu m on the sample support, adjusting a lifting screw to enable a fixed flat plate to completely cover the upper surface of a sample to be measured, selecting optical glass with a refractive index of 1.8 as a support reflecting layer, setting the direction of a polarized eyepiece to be in the north-south direction, testing the refractive index of the film in the thickness direction, adjusting the brightness of a light source, enabling the reading of a final micro-ammeter to be in a 20 microampere range (at the moment, the photocurrent and the illuminance form a linear relation), rotating an axle of the Abbe refractometer, recording the relation between the reading of the Abbe refractometer and the photocurrent, judging a mutation point according to the reading-photocurrent curve of the Abbe refractometer, and obtaining the reading of the refractometer at the mutation point as the refractive index (out-of-plane refractive index) in the thickness direction of the film; the refractive index (in-plane refractive index) of the film in the in-plane direction was measured by the same method with the rotating polarizing eyepiece in the east-west direction, and the measured out-of-plane refractive index and in-plane refractive index were 1.6178 and 1.7602, respectively.

Since polyimide has an absorptivity of 80% or more at the test wavelength, it is difficult to precisely test the refractive index at 589nm using the existing equipment.

Verification example 1: calculation of the Birefringence index of polyimide at 589nm wavelength

The sample of example 6 was tested for refractive index at 638nm, 720nm, 1540nm using a prism coupler, and the results are shown in Table 2, and the out-of-plane refractive index and in-plane refractive index at 589nm were 1.6179 and 1.7601, respectively, using the Cauchy dispersion formula. It can be seen that the refractive index obtained by the test of example 6 is highly consistent with the refractive index calculated using the Cauchy dispersion formula.

It is obvious that, although refractive index of specific wavelength (such as 589 nm) can be calculated by using the Cauchy dispersion formula, three or more sets of refractive index data of other wavelengths are needed, and the calculation process is complex, but the method of the invention is more direct and more convenient.

TABLE 2

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Claims

1. A method of measuring refractive index, the method comprising the steps of:

2. The method of claim 1, wherein the refractive index of the test sample is anisotropic, the eyepiece is a polarizing eyepiece, and step (4) comprises: setting the polarization direction of a polarized ocular, and recording the reading of an Abbe refractometer when the critical emergent ray is positioned at the center of a cross line of the ocular or an infrared observation mirror as the refractive index of a test sample corresponding to the polarization direction; preferably, the polarization direction includes a north-south direction and/or an east-west direction, the refractive index when the polarization direction is the north-south direction is the refractive index of the extraordinary ray, and the refractive index when the polarization direction is the east-west direction is the refractive index of the ordinary ray.

3. The method of claim 1, wherein the step (4) further comprises: the method comprises the steps of fixing a silicon photodiode in the center of a cross line of an ocular or an infrared observation mirror, measuring current passing through the silicon photodiode by using a microcurrent meter, rotating a rotating wheel shaft of an Abbe refractometer, recording the corresponding relation between the reading of the Abbe refractometer and the photocurrent reading of the microcurrent meter, and taking the reading of the Abbe refractometer when the photocurrent reading is suddenly changed as the refractive index of a test sample.

4. The method of claim 3, wherein step (4) further comprises: the brightness of the light source is adjusted so that the micro-ammeter reading is less than or equal to 25 muA, preferably less than or equal to 20 muA.

5. The method of claim 1, wherein the sample holder further comprises a distance adjustment device for adjusting a distance between the holding plate and the holder reflective layer such that the test sample is held between the holder reflective layer and the holding plate;

preferably, the distance adjusting device comprises one or more selected from a magnetic attraction device, a lock catch, a spring and a lifting screw;

preferably, the distance adjusting means includes a lifting screw for displacing the fixing plate in a direction away from the support reflection layer.

6. The method of claim 1, wherein the refractive index of the scaffold reflective layer is isotropic and greater than the refractive index of the test sample.

7. The method of claim 1, wherein the refractive index of the standoff reflective layer is n ₁ The refractive index of the test sample is n _o ，(n ₁ -n _o )/n _o Less than or equal to 20 percent, good qualitySelecting (n) ₁ -n _o )/n _o ≤10％。

8. The method of claim 1, wherein the material of the portion of the test sample that is the subject of refractive index measurement comprises polyimide.

9. A sample holder for an abbe refractometer, characterized in that the sample holder comprises a fixing plate and a fixing device for fixing a reflecting layer of the holder;

preferably, the sample support further comprises a distance adjusting device, wherein the distance adjusting device is used for adjusting the distance between the fixing plate and the support reflecting layer so that the test sample is fixed between the support reflecting layer and the fixing plate;

10. A device for testing refractive index, comprising an abbe refractometer and the sample holder of claim 9;

preferably, the device further comprises a silicon photodiode and a micro-ammeter.