CN109141224B - Interference reflection type optical thin film microscopic measurement method based on structured light - Google Patents

Interference reflection type optical thin film microscopic measurement method based on structured light Download PDF

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CN109141224B
CN109141224B CN201811166494.5A CN201811166494A CN109141224B CN 109141224 B CN109141224 B CN 109141224B CN 201811166494 A CN201811166494 A CN 201811166494A CN 109141224 B CN109141224 B CN 109141224B
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interference pattern
structured light
optical film
light
image
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CN109141224A (en
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岳慧敏
周政
宋一平
方宇耀
黄易杨
刘永
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University of Electronic Science and Technology of China
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/04—Measuring microscopes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
    • G01B11/2441—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures using interferometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02015—Interferometers characterised by the beam path configuration
    • G01B9/02025—Interference between three or more discrete surfaces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02041—Interferometers characterised by particular imaging or detection techniques
    • G01B9/02043—Imaging of the Fourier or pupil or back focal plane, i.e. angle resolved imaging
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02041—Interferometers characterised by particular imaging or detection techniques
    • G01B9/02044—Imaging in the frequency domain, e.g. by using a spectrometer
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/0209—Low-coherence interferometers
    • G01B9/02091—Tomographic interferometers, e.g. based on optical coherence
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
    • G06V20/00—Scenes; Scene-specific elements
    • G06V20/60—Type of objects
    • G06V20/69—Microscopic objects, e.g. biological cells or cellular parts

Abstract

An interference reflection type optical thin film microscopic measurement method based on structured light belongs to the technical field of optical imaging. The method comprises the following steps: the method comprises the following steps that firstly, structured light is used as a light source and is irradiated on the surface of a sample to be detected after passing through a microscope, the sample to be detected comprises a transparent substrate and an optical film, the optical film comprises a plurality of optical film layers, the structured light is reflected on the multilayer optical film layers of the optical film after passing through the transparent substrate of the sample to be detected, reflected light of all the optical film layers forms multi-beam interference and forms an interference pattern on the surface of the transparent substrate, and a clear interference pattern is formed on the surface of the transparent substrate by adjusting the distance between the microscope objective and the sample to be detected; and secondly, shooting a deformed interference pattern obtained by amplifying the interference pattern on the surface of the transparent substrate through a microscope objective, and determining the film quantity or the surface appearance of the corresponding position of the optical film according to the light intensity information of each part in the collected deformed interference pattern. The method is accurate, simple, rapid and wide in application range.

Description

Interference reflection type optical thin film microscopic measurement method based on structured light
Technical Field
The invention belongs to the technical field of optical imaging, and particularly relates to an interference reflection type optical thin film microscopic measurement method and system adopting structured light illumination.
Background
Graphene (Gr) is a new material with the characteristics of high conductivity, high toughness, high strength, ultra-large specific surface area and the like, and is widely applied to the fields of electronics, aerospace industry, new energy, new materials and the like, but a graphene film IA rapid, efficient and accurate detection method for the layer number and the micro-morphology is lacked. The following four methods are generally used to detect the number of layers of the graphene-like optical thin film: optical microscope, atomic force microscope, transmission electron microscope, raman spectroscopy. The optical microscope detection method is simple and rapid, does not damage the sample, but is limited to the substrate with obvious contrast difference, such as Si/SiO2，Si3N4PMMA, etc.; the atomic force microscope detection method is directly effective, but has small observation range, low efficiency and the result accuracy is influenced by various factors; the transmission electron microscope detection method is simple and visual, but the result accuracy is limited, and the sample can be damaged in the sample preparation process; the Raman spectrum detection method is quick and effective, has the characteristics of non-destructiveness and high resolution, and is only suitable for Gr in an AB stacking mode. The above four detection methods are often limited by various reasons, and the detection process is complex and the accuracy of the detection result is not high.
An Interference Reflection Microscope (IRM) is an optical method for detecting the number of layers and the microscopic morphology of an optical thin film of graphene-like, the method can measure the number of layers and the morphology of the optical thin film on a substrate by only one transparent substrate, incident light penetrates through the transparent substrate to generate multi-beam Interference in the optical thin film layer, reflected light of different interfaces forms Interference patterns on the transparent substrate, the number of layers and the surface morphology of the optical thin film can be well represented by Interference intensity images, an existing IRM microscope light source usually adopts a halogen lamp or a lamp box of halogen gas to carry out illumination, and the measured image based on the IRM method is reported to be much higher than the signal-to-noise ratio and the contrast ratio of the measured images of an atomic force microscope and a scanning electron microscope.
Disclosure of Invention
Aiming at the problems of higher complexity, higher detection cost, small application range, accuracy, limitation and the like of the traditional detection mode of the number of layers of the optical film on the transparent substrate, the invention provides the interference reflection type optical film micro-measurement method based on the structured light.
In order to achieve the above purpose, the invention provides the following technical scheme:
an interference reflection type optical thin film microscopic measurement method based on structured light is characterized by comprising the following steps:
the method comprises the following steps that firstly, structured light is used as a light source and is irradiated on the surface of a sample to be detected after passing through a microscope, the sample to be detected comprises a transparent substrate and an optical film, the optical film comprises a plurality of optical film layers, the structured light is reflected on the multilayer optical film layers of the optical film after passing through the transparent substrate of the sample to be detected, reflected light of all the optical film layers forms multi-beam interference and forms an interference pattern on the surface of the transparent substrate, and a clear interference pattern is formed on the surface of the transparent substrate by adjusting the distance between the microscope objective and the sample to be detected;
and secondly, shooting a deformation interference pattern of the interference pattern on the surface of the transparent substrate after the interference pattern is amplified by the microscope objective, and determining the film quantity and the surface appearance of the corresponding position of the optical film according to the acquired light intensity information of each position in the deformation interference pattern.
And further, the structured light is coded structured light with phase shift, an image of modulation degree information is formed by extracting modulation degree information of light intensity in the deformed interference pattern in the second step, a relation between the modulation degree information and the focusing degree when the interference pattern is shot is established, the modulation degree information which is not clear in focusing in the modulation degree information image is removed to obtain a final modulation degree information image, and the film quantity and the surface appearance of the corresponding position of the optical film are determined according to the gray scale of the final modulation degree information image.
The encoding structured light with the phase shift adopts stripe structured light with N-step phase shift, in the first step, the stripe structured light is generated by using a micro OLED display, the micro OLED display respectively generates the stripe structured light with different phases for N times to form the stripe structured light with the N-step phase shift, wherein N is a positive integer greater than 3;
in the second step, a deformed interference pattern formed after the fringe structure light generated by the micro OLED display passes through the optical path in the first step is shot to obtain a corresponding N-width deformed interference pattern;
obtaining light intensity modulation degree information from the N amplitude deformation interference patterns
Figure BDA0001821192710000021
Wherein N is an element of [0, N-1 ]]，InThe intensity of the distorted interference pattern representing the phase shift of the nth step,
Figure BDA0001821192710000022
indicating the phase shift amount of the fringe structure light generated when the fringe structure light generated by the nth phase shift is equal to 0, based on the modulation degree information IBDetermining the number of film layers and the surface topography of the corresponding position of the optical film according to the gray scale information in the characterization image.
Furthermore, the structured light is coded structured light without phase shift, a deformed interference pattern is shot in the second step, a two-dimensional orthogonal coordinate system is established in the deformed interference pattern, and phase information of light intensity with coordinates (x, y) of any point in the two-dimensional orthogonal coordinate system is extracted
Figure BDA0001821192710000023
Determining the film quantity and the surface morphology of the corresponding position of the optical film, and the specific steps are as follows:
a. performing two-dimensional Fourier transform on the deformed interference pattern to obtain a Fourier spectrum phi (u, v) of the deformed interference pattern;
b. performing frequency shift on the Fourier spectrum phi (u, v) of the deformed interference pattern to enable the frequency spectrum of the deformed interference pattern to generate a magnitude f in two coordinate axes, namely x and y directions, of the two-dimensional orthogonal coordinate systemxAnd fyPerforming inverse Fourier transform on the frequency spectrum of the image after frequency shift to obtain an image I after frequency shifts(x,y)；
c. Obtaining an image I by inverse Fourier transformsReal part information I { I of (x, y)s(x, y) } and imaginary information R { I }s(x,y)}Performing phase angle calculation to obtain phase angle (x, y) containing height and surface reflectivity information of the corresponding position of the optical film
Figure BDA0001821192710000031
Further, the contrast of the stripe-structured light is changed by changing the frequency of the micro OLED display.
Further, the transparent substrate is arranged on the upper layer of the optical film, and the modulation degree information I of the light intensity in the deformed interference pattern is changed by adjusting the refractive indexes of the medium under the transparent substrate and the optical film according to the refractive index of the optical filmBCharacterizes the contrast of the image.
Further, a detection system is set up before measurement, the detection system comprises a structural light illumination module 1, a beam splitter 2, a microscope objective 3 and an image acquisition module 6, the beam splitter 2 is a semi-transparent semi-reflecting mirror, the image acquisition module 6, the microscope objective 3 and a sample to be detected are coaxially arranged, the beam splitter 2 is arranged between the image acquisition module 6 and the microscope objective 3, the structural light illumination module 1 is used for generating collimated structural light, the collimated structural light is reflected by the beam splitter 2 and then penetrates through the microscope objective 3 to be imaged on the surface of the sample to be detected, and a deformation interference pattern amplified by the microscope objective 3 is acquired by the image acquisition module 6 after being transmitted by the beam splitter 2.
The working principle of the invention is as follows:
the interference reflection type microscope IRM is a technology which utilizes linearly polarized light with a single wavelength to illuminate, incident light beams are reflected through different interfaces and form interference intensity images on the surface of a glass substrate, so that the appearance of an optical film on the glass substrate is measured microscopically, finally received intensity signals of the interference images are related to the distance from the surface of the optical film to the glass substrate, and the traditional IRM microscope is not required to be calculated and directly used for imaging and collecting.
The invention is based on an IRM microscope, replaces the traditional IRM microscope light source which uses a halogen lamp or a lamp box of halogen gas for illumination with a coding light source which can generate high-contrast structured light, the traditional IRM needs linear polarized light with single wavelength, because the structure design of an optical film substrate is special, incident light can generate multi-beam interference on the optical film layer, and interference patterns with alternate light and shade are formed on the surface of a transparent substrate, and the light source in the invention generates unpolarized light with non-single wavelength, but can also generate interference. The introduction of the structured light illumination can obtain an image of modulation information by calculating the fringe modulation degree of the acquired deformation interference pattern on the basis of not changing the main body structure of the IRM microscope, and then the film quantity and the surface morphology of the corresponding position of the optical film are determined according to the gray scale of the modulation degree image.
Corresponding information analysis and processing are carried out based on different used structured light, for example, when the fringe structured light with N-step phase shift is adopted, the deformation interference patterns corresponding to the N-step phase shift are processed together, and by establishing the relation between the focusing (separating) degree and the modulation degree, the modulation degree image only keeps the part with clear focusing in the microscopic measurement image, but the part with unclear focusing is removed, so that the defocusing blur in the IRM image is reduced, and the imaging contrast is improved; the structured light can increase the high-frequency information of an image, the structured light illuminates the high-frequency information of a sample to be coded in a low-frequency region, then the high-frequency information passes through a frequency domain limited passband of the optical microscope and is decoded in a high-frequency region, the spectral range of the optical microscope is expanded, and the imaging resolution of the system is improved.
When the non-phase-shift method is adopted to finish measurement, only one deformation interference pattern is processed, the Fourier transform frequency shift is utilized to realize filtering operation, the information of the fringe part is filtered, the phase information related to the surface height and reflection of the optical film is obtained, and the number of layers and the defects of the optical film can be well represented through the intensity of the phase image.
The invention has the beneficial effects that: the method improves the resolution of the interference reflection type microscope system of the traditional light source and the accuracy of detecting the number of layers of the traditional optical film, reduces the defocusing influence in the measurement image of the traditional microscope system, has the advantages of accuracy, simplicity, quickness and wide application range, and is particularly suitable for detecting the number of layers and the micro-morphology of the graphene optical film.
Drawings
Fig. 1 is a schematic structural diagram of a measurement system constructed based on the structured light-based interference reflection type optical thin film micro-measurement method for detecting the number of layers and the shape of an optical thin film on a glass substrate.
FIG. 2 is a schematic diagram of the principle of detecting the number of layers and the shape of an optical thin film on a glass substrate by using the interference reflection type optical thin film microscopic measurement method based on structured light provided by the invention.
Fig. 3 is a schematic flow chart of the detection of the fringe structure light using N-step phase shift in the embodiment.
Reference numerals: the device comprises a structured light illumination module 1, a beam splitter 2, a microscope objective 3, an optical film substrate 4, a camera lens 5 and an image acquisition module 6.
Detailed Description
The principles and features of the present invention are described in detail below with reference to specific embodiments and the accompanying drawings of the specification:
the invention introduces the structured light into an interference reflection type microscope to replace a traditional light source, based on the measurement method provided by the invention, a detection system of the interference reflection type microscope is built in some embodiments before measurement, the detection system comprises a structured light illumination module 1, a beam splitter 2, a microscope objective 3 and an image acquisition module 6, an optical film and a transparent substrate form an optical film substrate 4, the beam splitter 2 is a semi-transparent and semi-reflective mirror, the image acquisition module 6, the microscope objective 3 and a sample to be measured, namely the optical film substrate 4 are coaxially arranged, the beam splitter 2 is arranged between the image acquisition module 6 and the microscope objective 3, and the spatial positions of the structured light illumination module 1, the surface of the optical film and the image acquisition module 6 form a positive projection and positive shooting mode, so that light is imaged on the surface of the optical film. Structured light illumination module 1 can adopt the lighting apparatus that can produce the hi-lite, high contrast structured light produces structured light, lighting apparatus can also link to each other with control system and form structured light illumination module 1 that can produce multiple mode structured light, structured light that structured light illumination module 1 produced can pass through the beam splitting meter reflection after the collimation earlier, structured light after the reflection passes through micro objective 3 and images on the sample surface that awaits measuring, the deformation interference pattern after the micro objective enlargies is gathered by image acquisition module 6 after passing through beam splitting meter 2 transmission, structured light illumination module 1 needs the encoding structured light that output can satisfy micro little visual field requirement of micro objective 3. The image acquisition module 6 at least comprises an image acquisition device such as a camera and the like, the deformation coding structured light which is modulated by the surface of the optical film and amplified by the microscope objective 3 is acquired through a camera lens 5, the image acquired by the image acquisition module 6 is transmitted to an image data processing module at the rear end, the image data processing module carries out the structureless light processing on a group of collected phase shift images containing the structured light, the super-resolution and high-contrast optical film microscopic image is obtained by calculating the modulation degree image of the deformation coding structured light image, and the image data processing module can adopt a computer or other intelligent terminal devices with image data processing function software.
As shown in fig. 1, the shooting mode in this embodiment is: controlling the structured light illumination module 1 to generate coded structured light, adjusting the coded structured light generated by the structured light illumination module 1 to enable the contrast of a deformed coded stripe on the surface of a transparent substrate of the optical film to be higher, enabling the coded structured light to horizontally enter from one side of an axis formed by the image acquisition module 6, the microscope objective 3 and a sample to be detected in the detection system, reflecting by the beam splitter 2 at 45 degrees, imaging to the surface of the optical film by the microscope objective 3, and adjusting the positions of the structured light illumination module 1, the surface of the optical film and the image acquisition module 6 in the space to enable the coded structured light to meet the law of reflection; structured light passes through the microscope objective 3 and then enters the surface of the optical thin film through the transparent substrate, and is reflected in the optical thin film, the thickness and the refractive index of the optical thin film meet the condition of forming multi-beam interference, the incident light is reflected for multiple times in the optical thin film to form multi-beam interference, and a multi-beam interference pattern with alternate light and shade can be imaged on the surface of the transparent substrate. When an image is collected, a focal plane is required to be ensured to be within the range of the depth of field of the image collection module 6, so that the displacement table carrying the optical film substrate moves up and down, an interference pattern formed on the surface of the optical film is focused, the interference pattern enters the image collection module 6 after being amplified by the microscope objective 3 and is imaged on an imaging target surface of the image collection module, the image collection module 6 hands the collected image to a subsequent image data processing module for processing, and the microscopic information of the surface of the optical film can be clearly represented by calculating the image of the modulation degree information of the deformed interference image generated by the coded structured light, so that an interference reflection intensity image of the surface of the optical film is obtained, the interference reflection intensity image reflects the microscopic information of the surface of the optical film, and compared with an IRM microscope with a traditional light source, the transverse resolution and the image contrast of the optical microscopic image are.
The invention codes the structured light to have contrast structured light, define stripe contrast to be the ratio of the difference and sum of maximum and minimum of the light intensity of the stripe, namely the ratio of the average gray scale of the black part of the stripe to the average gray scale of the white part, the contrast of the structured light in the invention is preferably above 0.5, including: any one of the coded structured light with the phase shift and the coded structured light without the phase shift; the encoding structure light without phase shift in the art includes any one of linearly polarized structure light and non-linearly polarized structure light. The working process and working principle of the present invention are described in detail below by taking coded structured light with phase shift and coded structured light without phase shift as the light source of the IRM microscope, respectively.
The first embodiment is as follows: the encoding structure light with phase shift adopts standard N-step phase-shift sine stripe light as an example, and as shown in fig. 3, is a schematic flow chart for detecting the surface morphology and the number of film layers of an optical thin film by adopting standard N-step phase-shift sine stripe light, and the specific method is as follows:
the structured light illumination module adopts a 12.78mm multiplied by 9mm high-brightness micro OLED display, the structured light illumination module 1 also comprises a collimation module connected with a collimation micro display, and as the micro OLED illumination is adopted, the imaging effect needs to be ensured, so that a 35mm lens is adopted for collimation, and the structured light illumination module 1 can generate convergent structured light; the micro OLED display changes the pattern of the projected stripe structure light along with a computer signal, the micro OLED display is controlled by the computer to change the structure light with different phases at a certain frequency, the stripe structure light is output in N steps, only the phase of the stripe structure light generated in each step is different, wherein N is a positive integer greater than 3; in this embodiment, the image capturing module 6 is a CCD camera, the CCD camera is an Allied Vision Technologies MG-1060C, and the camera lens 5 is a telecentric lens (computer TEC-M55) with a focal length of 55mm and a minimum line field of view of 2 mm.
The structured light with N-step phase shift generated by the structured light illumination module 1 is controlled to be reflected by the beam splitter 2 and then imaged on the surface of the optical film substrate 4 through the microscope objective 3, the image acquisition module 6 acquires the deformation coding structured light which is amplified and output by the microscope objective 3 after being modulated by the optical film substrate 4 step by step, and the acquired N deformation interference patterns are processed by the rear-end image data processing module. It should be noted that, since the snr of the collected intensity image is related to the types of the media above and below the optical film substrate, in order to optimize the contrast between different film regions of the optical film, the present invention needs to select the upper and lower layer media with proper refractive indexes, and change the modulation information I of the light intensity in the deformed interference pattern by adjusting the refractive indexes of the transparent substrate, the optical film and the media below the optical filmBThe contrast of the characterization image is relatively clear in focus of the microscopic image acquired by the camera of the image acquisition module 6 by adjusting the distance between the objective table and the microscope objective 3, and in order to make the interference image of the optical thin film contain the most information, the fringe frequency of the microdisplay OLED in the structured light illumination module 1 needs to be further adjusted, and the fringe contrast in the microscopic image is made as high as possible.
The intensity I of the sinusoidal fringe image generated by the structured light illumination module 1 in this embodimentinExpressed as:
Figure BDA0001821192710000061
in the formula (1), A is the maximum light intensity which can be generated by the structured light illumination module 1,
Figure BDA0001821192710000062
representing the spatial frequency of sinusoidal fringes with direction, N ∈ [0, N-1 ]]When n is 0The generated fringe structure light passes through a deformed interference pattern obtained by the optical path of the first step to be used as an initial pattern,
Figure BDA0001821192710000063
a and b are constants describing the contrast of the sine stripe, and satisfy a + b of 1(a + b) by representing the phase shift amount of the phase in the N-th step sine image in the N-step phase shift with respect to the phase of the sine image obtained when N is 0>0 and b>0) The conditions of (1).
The light intensity of the deformed interference pattern of the nth phase shift (i.e. the (n + 1) th structured light in the generated structured light) collected and received by the image collection module 6 is expressed as:
Figure BDA0001821192710000071
in the formula (2), R is a reflectivity parameter of the IRM microsystem, and R and the phase difference caused by different film layer numbers on the surface of the optical film
Figure BDA0001821192710000072
Associated with the wavelength λ of the light wave, I0The background light intensity when the structured light is illuminated, m represents the modulation degree of the stripes,
Figure BDA0001821192710000073
spatial frequency, δ, of sinusoidal fringesnThe phase shift amount between the stripe-structured light phase-shifted at the nth step and the initial stripe-structured light when n is 0.
The modulation degree m of the stripes in the formula (2) is a quantity related to the focal depth of the micro-measurement system and the micro-morphology of the object to be measured, and the expression is as follows:
Figure BDA0001821192710000074
m in formula (3)maxThe value of (a) is the maximum modulation degree in the focal depth range of the microscope system, z represents the depth corresponding to the object plane to be measured in the depth range of the camera depth of field relative to the focusing plane, mmaxDepth z in the focal depth rangeaIs the FWHM a system constant, which is largeThe minimums depend on the magnification and numerical aperture of the microscope system.
As can be seen from the formula (3), the fringe modulation degree m contains the characteristic of the microscopic morphology of the object surface, and on the premise of not considering the contribution of the reflectivity R, the modulation degree information of the deformed coding structure light microscopic image with N-step phase shift is solved, so that the morphological characteristic of the optical film can be well represented through the gray scale of the image of the modulation degree information.
In the interference reflection type structured light microscope system of the embodiment, because the incident light generates multi-beam interference in the thin film layer, the reflectivity of the interference pattern imaged on the surface of the transparent substrate enhances the characterization capability of the modulation degree m on the number of layers of the optical thin film, and finally obtained modulation degree information IBIs represented by formula (4):
IB＝R(d,λ)·m (4)
the reflectivity R of the interferometric reflection microscope system in the formula (4) is a quantity related to the thickness d and the wavelength λ of the optical thin film, and changes with the thickness of the optical thin film under the premise that the wavelength is not changed.
Establishing a relation between modulation degree information and focusing degree when the interference pattern is shot in a measuring process, removing the modulation degree information which is not clear when the interference pattern is shot in an image of the modulation degree information, determining the film layer quantity and the surface appearance of the corresponding position of the optical film according to the gray scale of the finally obtained image of the modulation degree information, and solving the modulation degree information I through an equation (5)B：
Figure BDA0001821192710000075
N is an element of [0, N-1 ] in the formula (5)]N is the number of phase shift steps, InThe deformed sine stripe image information which represents the phase shift of the nth frame collected by the image collecting module,
Figure BDA0001821192710000081
the phase shift amount of the stripe-structured light generated when n is 0 compared to the stripe-structured light generated by the n-th phase shift is shown. The structured light pattern is projected by the lighting module, and the structured light pattern can be distorted according to the appearance of the optical filmThe influence of structured light illumination is removed by applying the calculation method of the formula (5) to the multi-frame phase-shifted deformed sine stripes acquired by the image acquisition module to obtain modulation degree information I of surface defects and the number of layers of the optical film with high contrast and high resolutionBIs used to characterize the image.
To illustrate the modulation degree information I in the systemBThe mechanism of realizing super resolution of the intensity image is that the nth step phase shift deformation coding stripe image information I obtained from an image acquisition systemnExpression (2) of (a) starts the analysis.
Another expression of formula (2) is:
Figure BDA0001821192710000082
wherein F represents the fourier transform of the signal,
Figure BDA0001821192710000083
is the spatial frequency of the sinusoidal fringes,
Figure BDA0001821192710000084
for the spatial frequency extending along the phase shift direction of the structured light after the structured light is added,
Figure BDA0001821192710000085
for the frequency domain impulse response function, exp (ix) + exp (-ix) is another mathematical expression of cos (x)/2.
In this form IBCan be expressed as:
Figure BDA0001821192710000086
generally, the image shot by people is an intermediate frequency signal in a frequency domain, high-frequency information of a sample is encoded to a low-frequency region by using structured light to illuminate, then the high-frequency information passes through a frequency-domain limited passband of an optical microscope and is decoded to a high-frequency region, the spectral range of the optical microscope is expanded, and information of higher frequency components is obtained. And the high-frequency components correspond to the fine structure of the sample, and the acquisition of the higher-frequency components means that the finer sample structure can be observed, namely, the imaging resolution is improved, so that the super-resolution of breaking the Abbe diffraction limit can be realized by the method.
Modulation degree information IBThe areas with different gray scales in the image correspond to the areas with different numbers of optical thin film layers on the glass substrate, the gray scale of the area in the modulation degree image gradually becomes deeper along with the increase of the optical thin film layer of a certain area on the glass substrate, and the micro-defect of the optical thin film also presents different gray scales from the surrounding, wherein the formula (8) represents the modulation degree information I under the IRM microscope calculated by the multi-beam interference formulaBExpression (c):
Figure BDA0001821192710000091
in the formula IinAs to the intensity of the incident light,
Figure BDA0001821192710000092
for the phase change of the light after passing through the optical film, n2Complex refractive index of the optical film, reflection coefficient: r is12＝(n1-n2)/(n1+n2)，r23＝(n2-n3)/(n2+n3) The refractive index of the covering medium of the upper and lower layers of the optical film is n1And n3，d2It is shown that the thickness of the optical film is related only to the number of layers and the surface topography of the optical film.
In order to describe the gray scale variation between different film layers, the relationship between the contrast ratio C and the number m of film layers can be expressed by the following formula (9):
Figure BDA0001821192710000093
in the formula (2) ImAnd Im-1Representing an image IBThe m-th layer and the m-1 layer area.
The contrast of the determined graphene substrate can reach 30-35%, and the detection of the number of layers and the detection of the micro-morphology of the optical thin film can be clearly carried out.
Example two: the coding structure light without phase shift takes non-phase shift stripe junction light as an example, and the method for obtaining the surface appearance and the film quantity of the optical film by utilizing the non-phase shift stripe junction light comprises the following steps:
the micro OLED display is also used to generate the stripe structure light, but in this embodiment the micro OLED display screen is made to project only one frame of sinusoidal stripe image and this image is captured in the image capture module 6. Establishing two-dimensional coordinate axes in the xy direction in the deformed interference pattern, wherein light intensity information I (x, y) at coordinates (x, y) in the deformed interference pattern comprises phase information related to the height of the object and the surface reflectivity
Figure BDA0001821192710000094
As shown in equation (10).
Figure BDA0001821192710000095
In the formula (10)
Figure BDA0001821192710000096
The amount of true phase change, f, due to the height of the optical film and the surface reflectivityxAnd fyW is a relation between light intensity at the image (x, y) and a true phase variation amount, and the relation is another expression of expression (1).
To obtain information about the surface of an object, we extract the true phase using the direction of the Fourier transform
Figure BDA0001821192710000097
Such as (11)
Figure BDA0001821192710000101
The real phase is extracted by using a Fourier transform method in the formula (11)
Figure BDA0001821192710000102
The method comprises the following three steps: firstly, performing two-dimensional Fourier transform on an original image to obtain a Fourier spectrum phi (u, v) of the original image; second, a frequency shift is performed on the Fourier spectrum phi (u, v) of the original image such that the image spectrum occurs in the x and y directions with a respective magnitude fxAnd fyThe frequency shift of the frequency domain can be completed by a mathematical method, and the inverse Fourier transform is carried out on the frequency spectrum of the image after the frequency shift to obtain an image I after the frequency shifts(x, y); thirdly, obtaining an image I by inverse Fourier transformsReal part information I { I of (x, y)s(x, y) } and imaginary information R { I }s(x, y) calculating the phase angle, wherein the obtained phase angle is the real phase comprising the height and surface reflectivity information of the optical film
Figure BDA0001821192710000103
In this embodiment, the method for processing the non-phase-shifted stripe junction light is to use fourier transform frequency shift to implement filtering operation, and filter out information of a stripe portion to obtain phase information related to the height and reflection of the surface of the optical film. In the method, the phase image
Figure BDA0001821192710000104
The image intensity of (2) can well represent the number of layers and defects of the optical film.
In summary, the invention provides a non-contact and nondestructive detection method and system for the number of layers and the shape of an optical film on a transparent substrate, and the invention introduces structured light illumination in a traditional interference reflection type microscope system, and realizes the purpose of measuring the number of layers and the surface micro-shape of the optical film on the transparent substrate in the optical microscope system based on the principle that the structured light illumination realizes super-resolution and micro-chromatography. The detection system has simple structure, and solves the problem that the traditional detection method for the number of layers and the micro morphology of the optical film is complex; the resolution of the interference reflection type microscope system is improved; the defocusing influence in the measurement image of the traditional microscope system is reduced; therefore, the invention has the advantages of accuracy, simplicity, quickness and practicality. The detection system and the detection method provided by the invention are suitable for detecting the micro-morphology and the structure of the optical film on the transparent substrate, are particularly suitable for detecting the number of layers and the micro-morphology of the graphene optical film, and have wide prospects in the preparation, transfer and detection processes of the optical film.
The foregoing is a detailed description of the invention with reference to specific embodiments, which are intended to be illustrative only, and not to be construed as limiting the scope of the invention, which is intended to be covered by the following claims.

Claims (5)

1. An interference reflection type optical thin film microscopic measurement method based on structured light is characterized by comprising the following steps:
the method comprises the following steps that firstly, structured light is used as a light source and is irradiated on the surface of a sample to be detected after passing through a microscope, the sample to be detected comprises a transparent substrate and an optical film, the optical film comprises a plurality of optical film layers, the structured light is reflected on the multilayer optical film layers of the optical film after passing through the transparent substrate of the sample to be detected, reflected light of all the optical film layers forms multi-beam interference and forms an interference pattern on the surface of the transparent substrate, and a clear interference pattern is formed on the surface of the transparent substrate by adjusting the distance between the microscope objective and the sample to be detected;
and secondly, shooting a deformed interference pattern of the interference pattern on the surface of the transparent substrate after the interference pattern is amplified by the microscope objective, forming an image of modulation information by extracting modulation degree information of light intensity in the deformed interference pattern, establishing a relation between the modulation degree information and the focusing degree when the interference pattern is shot, removing the modulation degree information which is not focused clearly in the modulation degree information image to obtain a final modulation degree information image, and determining the film quantity and the surface appearance of the corresponding position of the optical film according to the gray scale of the final modulation degree information image.
2. The structured-light based interferometric reflective optical thin film microscopy measurement method according to claim 1,
in the first step, a micro OLED display is used for generating stripe structure light, the micro OLED display respectively generates the stripe structure light with different phases for N times to form the stripe structure light with N-step phase shift, wherein N is a positive integer greater than 3;
in the second step, a deformation interference pattern formed after the fringe structure light generated by the micro OLED display passes through the optical path in the first step is shot, so that a corresponding N-width deformation interference pattern is obtained;
obtaining light intensity modulation degree information from the N amplitude deformation interference patterns
Figure FDA0002408379360000011
Wherein N is an element of [0, N-1 ]]，InThe intensity of the distorted interference pattern representing the phase shift of the nth step,
Figure FDA0002408379360000012
indicating the phase shift amount of the fringe structure light generated when the fringe structure light generated by the nth phase shift is equal to 0, based on the modulation degree information IBDetermining the number of film layers and the surface topography of the corresponding position of the optical film according to the gray scale information in the characterization image.
3. The method of claim 2, wherein the contrast of the structured light fringes is changed by changing the frequency of the micro OLED display.
4. The interferometric reflective optical thin film microscopic measurement method based on structured light according to claim 1, wherein in the second step, a deformation interference pattern is photographed, a two-dimensional orthogonal coordinate system is established in the deformation interference pattern, phase information of light intensity at any point in the two-dimensional orthogonal coordinate system is extracted to determine the number of the film layers and the surface topography of the corresponding position of the optical thin film corresponding to the point, and the specific steps are as follows:
a. performing two-dimensional Fourier transform on the deformed interference pattern to obtain a Fourier spectrum of the deformed interference pattern;
b. performing frequency shift on a Fourier frequency spectrum of the deformed interference pattern, so that the frequency spectrum of the deformed interference pattern is subjected to respective frequency shift in the directions of two coordinate axes of the two-dimensional orthogonal coordinate system, and performing inverse Fourier transform on the frequency spectrum of the image subjected to frequency shift to obtain an image subjected to frequency shift;
c. and performing phase angle calculation on the real part information and the imaginary part information of the image after the frequency shift obtained by inverse Fourier transform to obtain phase angle, wherein the obtained phase angle is the phase information which contains the height and the surface reflectivity information of the corresponding position of the optical film at any point selected in the two-dimensional orthogonal coordinate system.
5. The interferometric reflection type optical thin film micro-measurement method based on the structured light according to claim 1, characterized in that a detection system is set up before measurement, the detection system comprises a structured light illumination module (1), a beam splitter (2), a microscope objective (3) and an image acquisition module (6), the beam splitter (2) is a half-mirror, the image acquisition module (6), the microscope objective (3) and the sample to be measured are coaxially arranged, the beam splitter (2) is arranged between the image acquisition module (6) and the microscope objective (3), the structured light illumination module (1) is used for generating the collimated structured light, the collimated structured light is reflected by the beam splitter (2), then passes through the microscope objective (3) and is imaged on the surface of the sample to be measured, and the deformed interference pattern amplified by the microscope objective (3) is transmitted by the beam splitter (2) and then is acquired by the image acquisition module (6).
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