CN116642412A - Optical measurement method and device for film thickness - Google Patents

Abstract

The invention discloses a film thickness optical measurement method, which comprises the following steps: s1, determining a light source for measurement and emitting measurement light; s2, focusing measuring light to the surface of a measuring object through an optical fiber and a condensing objective lens to form a converging light beam; s3, reflecting the converging light beam by the measuring object to form interference light; s4, coupling interference light into a spectrum analyzer through a condensing objective lens and an optical fiber, and carrying out light splitting to obtain wavelength distribution corresponding to the interference light; s5, calculating the characteristics of the wavelength distribution corresponding to the interference light through the data processing part to obtain the thickness of the film layer, the absolute/relative reflectivity of the surface and the inside of the measurement structure and the refractive index of the measurement film material, and compared with the prior art, the method adopts a spectral analysis algorithm of LSP periodic chart and EMD modal decomposition to obtain the film thickness, and effectively eliminates the interference peak value formed by the low-frequency component in the light source spectrum in period estimation; the influence of noise on the period estimation can be better overcome.

Description

Optical measurement method and device for film thickness

technical field

The invention relates to the field of thin film measurement, in particular to an optical measurement method and device for thin film thickness.

Background technique

Optical thin film is an important method to improve optical properties such as reflectivity and absorptivity of optical system. Thin film technology and thin film materials are widely used in many fields. Film thickness is an important parameter affecting film performance, so measuring film thickness has significant significance and broad application scenarios.

At present, there are probe method, wavelength extreme value method, quartz crystal oscillator method, wide spectrum scanning method and ellipsometry, etc., among which the non-contact non-destructive thickness measurement methods are mainly wide spectrum scanning method and ellipsometry two kinds. The ellipsometry uses the principle that the polarization state of the light reflected by the film changes, and can achieve a measurement accuracy of 0.01nm. However, the measurement equipment ellipsometer is complex and expensive, and is not suitable for applications that require low precision and fast measurement. Scenes. Both the wavelength extreme value method and the wide spectrum scanning method belong to the spectral analysis method, and are film thickness detection methods based on the interference effect of light and the reflection and transmission theory of light. The traditional wavelength extremum method is often used to monitor the growth of thin films. When the optical thickness nh (refractive index multiplied by thickness) of the thin film reaches an integral multiple of 1/4 of the wavelength of the monitored light, the transmittance of the thin film appears extreme. Due to the low change rate of transmittance near the extreme point, the measurement accuracy of the wavelength extreme value method is limited.

The traditional algorithm interpolates the data and resamples at equal intervals, and then uses the discrete Fourier transform to estimate the period. However, because the interpolation changes the statistical characteristics of the data, additional errors are introduced, and the discrete Fourier transform is easily affected by signal noise, and its ability to estimate weak periods is poor.

Contents of the invention

The purpose of the present invention is to perform interpolation and resampling of data at equal intervals for traditional algorithms, because the interpolation changes the statistical characteristics of the data and introduces additional errors, and the discrete Fourier transform is easily affected by signal noise. Aiming at the poor estimation ability of , an optical measurement method and device for film thickness are proposed.

In order to achieve the above object, the present invention adopts the following technical solutions:

A kind of film thickness optical measurement method, comprises the following steps:

S1. Determine the light source for measurement and emit the measurement light;

S2. Focus the measurement light to the surface of the measurement object through the optical fiber and the condensing objective lens to form a converging beam;

S3. The measuring object reflects the converging light beam to form interference light;

S4. Coupling the interference light into the spectrum analyzer through the condenser objective lens and the optical fiber, performing light splitting, and obtaining the wavelength distribution corresponding to the interference light;

S5. The data processing unit calculates the characteristics of the wavelength distribution corresponding to the interference light, and calculates the thickness of the film layer, the measurement structure surface, the absolute/relative reflectivity inside, and the refractive index of the measurement film material.

As a further preferred embodiment of the present invention, the measuring object reflecting the converging light beam to form interference light comprises that the front surface and the rear surface of the measuring object respectively reflect the converging light beam.

As a further preference of the present invention, the spectral analysis algorithm of LSP periodogram and EMD mode decomposition is used in the S5.

An optical measurement device for film thickness, characterized in that it includes a light source, the optical fiber, and the optical fiber couples the measurement light emitted by the light source;

Two condensing objective lenses, one of which focuses the measuring light to the surface of the measuring object to form a converging light beam, and the other condensing objective lens focuses the interference light reflected from the front and back of the measuring object to the end face of the optical fiber;

A spectrum analyzer, which analyzes the interference light;

A data processing unit, the data processing unit performs calculations.

As a further preference of the present invention, the light sources include LED light sources and SLD superluminescent light emitting diode light sources.

As a further preference of the present invention, the condensing objective lens is set as a 10x microscope objective lens to improve the utilization rate of light energy.

As a further preference of the present invention, the optical fiber is configured as a Y-shaped multi-mode optical fiber or a single-mode optical fiber to improve light energy utilization.

As a further preference of the present invention, the angle between the converging light beam and the measuring object is 20°-90°.

Beneficial effect:

A method and device for optical measurement of film thickness proposed by the present invention, compared with the prior art, has the following beneficial effects:

1. The low-cost and fast measurement of film thickness can be realized by adopting the method and device of the present invention;

2. Using LSP periodogram and EMD modal decomposition spectral analysis algorithm to obtain the film thickness, effectively eliminate the interference peak formed by the low frequency component in the light source spectrum in the period estimation; it can better overcome the influence of noise on the period estimation;

3. With the device of the present invention, the optical path is simple, the cost is low, and the installation is convenient.

Description of drawings

Fig. 1 is a kind of film thickness measuring device schematic diagram of the present invention;

Fig. 2 is a schematic diagram of another film thickness measuring device according to the present invention;

Figure 3 shows the results of spectrum and film thickness calculation before and after EMD mode decomposition of the sample;

Fig. 4 is that the FFT of the embodiment of the present invention is more susceptible to the impact figure of signal noise than LSP;

FIG. 5 is a diagram of the relative error of the film thickness caused by the incident angle error of 1° according to the embodiment of the present invention.

The meanings of reference signs in the figure: 1. light source, 2. optical fiber, 3. condenser objective lens, 4. thin film, 5. spectrum analyzer, 6. data processing unit.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

An optical measurement method and device for film thickness can measure the film layer thickness on the surface of a measured object with a single layer or laminated structure, and the invention can be applied to the measurement of the film thickness of plastic films and semiconductor films.

Example 1:

A kind of film thickness optical measurement method, comprises the following steps:

S1. Determine the light source for measurement, and emit measurement light.

The light source for measurement is selected, and the light source emits a spectrum in a specific band range.

S2. Focus the measurement light to the surface of the measurement object through the optical fiber and the condensing objective lens to form a converging beam.

The measurement light emitted by the light source is coupled to a condenser objective lens with an optical fiber, and the measurement light is focused to the surface of the measured object through a condenser objective lens.

S3. The measuring object reflects the converging light beam to form interference light.

The measuring object reflecting the converging light beam to form interference light includes that the front surface and the rear surface of the measuring object respectively reflect the converging light beam.

S4. Coupling the interference light into the spectrum analyzer through the light-condensing objective lens and the optical fiber, performing light splitting, and obtaining the wavelength distribution corresponding to the interference light.

The interference light is collected by another condensing objective lens and focused to the end face of the optical fiber, and the optical fiber couples the interference light to the spectrum analyzer.

S5. The data processing unit calculates the characteristics of the wavelength distribution corresponding to the interference light, and calculates the thickness of the film layer, the measurement structure surface, the absolute/relative reflectivity inside, and the refractive index of the measurement film material.

The periodicity, reflectivity and transmittance of the spectroscopic reflection light interference spectrum of the front and back surfaces of the film can be calculated through the characteristics of the light wavelength distribution.

The data processing department first adopts the full-spectrum fitting method to find the cycle or frequency of the transformed signal, and multiply the obtained frequency by Π to obtain the film thickness. The full-spectrum fitting method utilizes all spectral information for period estimation, does not depend on the specific value of a certain extreme point, and is not easily affected by signal noise.

Because the pixel-wavelength correspondence of the spectrum analyzer is nonlinear, the periodic signal obtained from the reflection spectrum or transmission spectrum of the film is a non-uniform sampling signal, and the density of sampling points is inversely proportional to the wavelength. The Lomb-ScarglePeriodogram (LSP) algorithm is used for period estimation, which is less affected by noise and sequence finiteness. It has higher reliability than FFT when the signal-to-noise ratio is low. It does not need to interpolate and resample the data, and does not change the statistical characteristics of the original data. .

Because the spectrum of the light source itself is not flat, the transmission spectrum has the same low-frequency intensity variation as the light source, which affects the period estimation. In order to eliminate this effect, high-frequency component extraction is performed on the spectral data Empirical Mode Decomposition (EMD). The first eigenmode is used for subsequent calculations. The first eigenmode filters out the low-frequency changes caused by the characteristics of the light source, and can better describe the spectrum formed by thin-film interference. There is no need to design the cut-off frequency according to the wavelength, refractive index and film thickness. , the highest frequency eigenmode can be adaptively extracted.

The spectral analysis algorithm using LSP periodogram and EMD mode decomposition can quickly and accurately calculate the film thickness under the condition of narrow-band light source and low signal-to-noise ratio, and the algorithm has high reliability.

Example 2:

An optical measuring device for film thickness comprises a light source 1 and an optical fiber 2 that couples the measurement light emitted by the light source 1 .

The light source 1 includes an LED light source and a SLD superluminescent light emitting diode light source, and the light source emits a wide spectrum and highly reliable spectrum with a specific wavelength range to the surface of the measured object.

The optical fiber 2 is configured as a Y-shaped multimode optical fiber or a single-mode optical fiber.

Two condensing objective lenses 3, one of which focuses the measuring light to the surface of the measuring object to form a converging light beam, and the other condensing objective lens 3 focuses the interference light reflected from the front and back of the measuring object to the end face of the optical fiber 2.

The condensing objective lens 3 converges the light emitted by the superradiant light source to the surface of the measurement object, and focuses the light reflected by the surface of the measurement object into the spectrometer, and the condensing objective lens 3 is set as a 10 times microscopic objective lens.

The angle between the converging light beam and the measuring object is 20°-90°.

Spectrum analyzer 5, which analyzes the interference light, disperses the interference light (broad spectrum) reflected by the front and rear surfaces of the measurement object through the grating, and uses the detector to detect the distribution characteristics of different light wavelengths.

A data processing unit 6, which performs calculations.

According to the optical wavelength distribution characteristics collected by the spectrum analyzer 5, the transformation and period estimation methods are improved and optimized, and the period estimation is performed using the Lomb-Scargle Periodogram (LSP) algorithm, which overcomes the problem of non-uniform sampling of data and is affected by noise. Less affected.

Test light source 1 is not an ideal light source, that is, the intensity of the light source at different wavelengths is different, that is, the light source itself is not flat, and there are low-frequency components, which will seriously affect the period estimation, so the Empirical Mode Decomposition (EMD) mode is used Decompose and extract the high-frequency components in the spectral signal, and filter out the low-frequency components.

The first eigenmode is used for subsequent calculations. The first eigenmode filters out the low-frequency changes caused by the characteristics of the light source, and can better describe the spectrum formed by thin-film interference. There is no need to design the cut-off frequency according to the wavelength, refractive index and film thickness. , which can adaptively extract the highest frequency eigenmode

The spectral analysis algorithm using LSP periodogram and EMD mode decomposition can quickly and accurately calculate the film thickness under the condition of narrow-band light source and low signal-to-noise ratio, and the algorithm has high reliability.

The periodicity, reflectivity and transmittance of the spectroscopic reflection light interference spectrum of the front and back surfaces of the film can be calculated through the characteristics of the light wavelength distribution.

Example 3:

Sample A is a test object with a film thickness of 10-700um. The test object uses at least one layer of acrylic plastic film. The schematic diagram of the device is shown in Figure 1.

Choose a full-spectrum white LED light source to emit a spectrum in the range of 440nm to 670nm;

Y-type multimode fiber and 10x microscope objective lens are used to improve the utilization rate of light energy. Among them, Y-type multimode fiber is formed by paralleling two multimode fibers with a core diameter of 50um in the direction of the optical axis of the object under test. .

The measurement light emitted by the light source is coupled to the condenser objective lens with an optical fiber, and then focused onto the surface of the measured object through the condenser objective lens to form a converging beam. interfere with light.

Then the interference light is collected by the condenser objective lens and focused to the end face of the optical fiber, and the optical fiber couples the interference light into the spectrum analyzer.

The spectrum analyzer splits the interference light, and finally the detector obtains the wavelength distribution characteristics of the reflected light. From the spectral wavelength distribution characteristics, calculate the thickness of the film layer and measure the absolute/relative reflectivity of the surface and interior of the structure, and also measure the refractive index of the thin film material.

The reflection of the converging light beam on the interface on both sides of the film, the thickness of the film is h, the refractive index of the upper medium is n ₀ , the refractive index of the film is n, the refractive index of the substrate is n _G , the incident angle is θ ₀ , the refraction angle is θ, and the exit angle is θ _G , and the reflection coefficient is r.

Transmittance T, reflectivity R:

Among them, λ is the wavelength of the incident light, and n1 is the refractive index of the measured object;

Define the wavenumber K ₁ :

According to the phase difference δ=4πnhcosθ/λ, δ=2K ₁ h.

The reflectance R and transmittance T are transformed as follows:

Both T _K1 and R _K1 can be expressed as a cosine function about K ₁ , and the frequency defining the wave number K ₁ is related to the film thickness h. The thickness of the film is only related to the changing rules of the reflection and transmission spectra, and has nothing to do with the actual reflectance and transmittance values. Therefore, it is not necessary to accurately measure the reflectance and transmittance during measurement, and the film thickness can also be calculated.

According to the above formula, based on the discrete Fourier transform, the periodic component can be effectively extracted from the non-uniform time series. The core idea is to construct a periodic function: F=acos(ωt)+bsin(ωt), and do nonlinear fitting with the data sequence. The parameters a and b are solved for the regression model by the least square method, and the residual sum of squares between the observed value and the estimated value is the smallest, and the objective function E(ω)=[X(t _i )-F(t _i )] ² min. The processing of LSP periodogram for non-uniform data is to introduce a redundant parameter τ for each t to ensure time-shift invariance. After adding redundant parameters, the periodic function constructed by LSP is F=acos(ω(t-τ))+ bsin(ω(t-τ)).

Lomb deduces the LSP periodogram, and obtains

In this embodiment, the spectrum analyzer has a measurement range of 505nm-638nm, the detector has 2048 pixels in total, and the resolution of the spectrometer is 0.07nm. Since the spectrum of the light source itself is not straight, that is, the intensity of the light source is different at different wavelengths, the collected reflection spectrum still has the same intensity change as the light source. Whether Fourier transform or LSP periodogram algorithm is used, this low-frequency change Both will affect the results. In order to eliminate this effect, the spectral data is extracted by EMD mode decomposition for high-frequency components.

Use the device of the present invention to take the reflectance spectrum of sample A as an example, as shown in Figure 3, (a) is a part of the original reflectance spectrum of the sample, that is, the spectrum that has not been modally decomposed. It can be seen that due to the spectral characteristics of the light source itself, the Its intensity shows an increasing trend from 0 to 600 pixels, and decreases slightly after 600 pixels. Using the original data for calculation, the result of (b) is obtained, and (b) is the solution result of the original spectrum; due to the existence of low frequency components, Make it appear a peak near 0. When the film thickness is small, the peak formed by the low frequency component is closer to the peak sought; when the waveform of the reflection spectrum is not obvious, that is, when the signal-to-noise ratio is low, the height of the peak caused by the low frequency component may be It is several times of the desired peak, and even submerges the desired peak; (c) is the high-frequency component selected after the original data is subjected to EMD mode decomposition, that is, the first mode separated from the spectrum, which effectively suppresses the low-frequency intensity of the light source. change; (d) is the solution result of the data after modal decomposition.

Use the device of the present invention to take the reflectance spectrum of a sample B with a thickness of about 395um as an example, as shown in Figure 4, (a) is the original spectrum of sample B, it can be seen that the waveform produced by thin film interference is not obvious, and the signal-to-noise ratio is low, (b) and (c) are the results of calculation using Fourier transform FFT and LSP periodogram algorithm respectively. Although both images have correct peaks at 395um, other false peaks in (b) affect the reading, while (c) using the LSP periodogram for period estimation has good identifiability, which further proves that the LSP algorithm is more reliable than FFT for film thickness calculation.

The device of the present invention is used to measure 6 film samples, and the data processing and calculation are carried out by the method described above. The calculation results are obtained in the following table:

The absolute errors of measurement are all less than 3um, only the relative error of No. 1 sample reaches 2.16%, and the rest relative errors are all controlled within 1%. Due to the small thickness of No. 1 sample, the film material is soft and prone to deformation. It is not ruled out that the deformation caused by the thickness change during clamping, disassembling the sample or transferring and transporting. The experiment proves the feasibility of this method for measuring film thickness and the reliability of EMD mode decomposition and LSP periodogram for spectral data processing and film thickness calculation.

Example 4:

The schematic diagram of the device is shown in Figure 2. The SLD superluminescent light-emitting diode light source is used to emit a wide spectrum, high reliability, and low coherence. The same photons continuously generate stimulated radiation during the propagation process in the gain medium, and the stimulated emitted photons increase exponentially, from the initial spontaneous emission to the amplified spontaneous emission. Compared with the first embodiment The white light LED used, in the process of SLD gain, the photons near the center gain wavelength get greater amplification, and the photons far away from the center gain wavelength get smaller amplification, thus narrowing the spectrum. A semiconductor optoelectronic device between a laser diode (Laser Diode, LD) and a light emitting diode (Light Emitting Diode, LED), which can realize the high-power characteristics of the laser and the wide-spectrum characteristics of the LED, and reduce the time coherence. The light source used in this embodiment generates measurement light with a wavelength of 815-865 nm, and the spectral width is 1/4-1/5 of that of LED.

The optical fiber adopts a single-mode optical fiber with a core diameter of 9um. Because the coherence of the SLD light source is at the bending of the optical fiber and the connection end face of the spectrometer, other similar interference phenomena are produced except for the spectral interference of the reflected light on the surface of the film under test. . Therefore, single-mode optical fiber is used to reduce the influence of additional interference fringes. In the measurement device, it is necessary to fix the optical fiber with a hard plastic tube to reduce the suspected interference fringes caused by bending and jitter. At the same time, the fiber end face is coated to reduce end face reflection.

The incidence mode of the optical fiber is oblique incidence, and the emitted light and the received light are converged by two 10x microscope objective lenses respectively. When measuring, first place a flat mirror at the sample, adjust the optical path to maximize the energy of the reflected light, and then place a thin film sample to measure the transmission spectrum T. EMD is used to preprocess the transmission spectrum T, and then transformed. LSP is used to estimate the period of the transformed data, and the abscissa corresponding to the maximum peak of the periodogram is extracted, that is, the film thickness. The data processing method is similar to the first embodiment.

The film thickness h is proportional to the frequency factor K ₁ of T _K1 and R _K1 , and the partial derivative of K ₁ with respect to the incident angle θ ₀ is: where _θ0 is the angle of incidence, θ is the angle of refraction, λ is the wavelength of the incident light, and _K1 is the defined wavenumber. For every λ=λ _i , the incident angle error Δθ ₀ causes K ₁ to increase by K′ ₁ Δθ ₀ . Correspondingly, the film thickness h increases by K' ₁ Δθ ₀ /K ₁ times.

When the normal incidence method is used, θ ₀ is near 0, and the partial derivative is the infinitesimal quantity of the same order as θ ₀ , and the error of the incident angle has little influence on the calculation of the film thickness. Will Take the derivative of θ ₀ , let it be 0, and get θ ₀ ≈ 48.9°, at this time /> The absolute value of is the maximum value. That is, the influence of the incident angle error on the film thickness error increases gradually from 0 to 48.9°, and then decreases gradually.

Taking θ ₀ ≈45°, n ₀ =0, n=1.5 as an example, the incident angle error of ±1° causes a relative error of 0.497% in film thickness. Figure 5 shows the film thickness error caused by the incident angle error of ±1° when the incident angle is from 0 to 80°.

In this embodiment, the spectrum analyzer has a measuring range of 793-865nm, the detector has 2048 pixels in total, and the resolution of the spectrometer is 0.04nm. SLD was selected as the test light source, the measuring light range was 815-865nm, and the 6 film samples were measured by oblique incidence. The measurement results are as follows:

In the measurement experiment of 6 samples, the maximum error of oblique incidence is less than 16um. The relative error is within 5%. The errors of LED light vertical incidence and SLD light oblique incidence measurement are close, which verifies the feasibility of SLD light oblique incidence measurement method.

The basic principles, main features and advantages of the present invention have been shown and described above. Those skilled in the industry should understand that the above-mentioned embodiments do not limit the present invention in any form, and all technical solutions obtained by means of equivalent replacement or equivalent transformation fall within the protection scope of the present invention.

Claims (8)

1. A film thickness optical measurement method, is characterized in that, comprises the following steps: S1. Determine the light source for measurement and emit the measurement light; S2. Focus the measurement light to the surface of the measurement object through the optical fiber and the condensing objective lens to form a converging beam; S3. The measuring object reflects the converging light beam to form interference light; S4. Coupling the interference light into the spectrum analyzer through the condenser objective lens and the optical fiber, performing light splitting, and obtaining the wavelength distribution corresponding to the interference light; S5. The data processing unit calculates the characteristics of the wavelength distribution corresponding to the interference light, and calculates the thickness of the film layer, the measurement structure surface, the absolute/relative reflectivity inside, and the refractive index of the measurement film material. 2 . The method for optical measurement of film thickness according to claim 1 , wherein the measuring object reflects the converging light beam to form interference light comprising: the front surface and the rear surface of the measuring object respectively reflect the converging light beam. 3 . 3. A kind of film thickness optical measuring method according to claim 1, is characterized in that, adopts the spectral analysis algorithm of LSP periodogram and EMD modal decomposition in the described S5. 4. An optical measuring device for film thickness, characterized in that it includes a light source, the optical fiber, and the optical fiber couples the measurement light emitted by the light source; Two condensing objective lenses, one of which focuses the measuring light to the surface of the measuring object to form a converging light beam, and the other condensing objective lens focuses the interference light reflected from the front and back of the measuring object to the end face of the optical fiber; A spectrum analyzer, which analyzes the interference light; A data processing unit, the data processing unit performs calculations. 5. A kind of film thickness optical measuring device according to claim 4, is characterized in that, described light source comprises LED light source and SLD superluminescent light-emitting diode light source. 6. A kind of thin film thickness optical measurement device according to claim 4, is characterized in that, described condensing objective lens is set as 10 times microscope objective lens. 7 . The optical measuring device for film thickness according to claim 4 , wherein the optical fiber is configured as a Y-shaped multi-mode optical fiber or a single-mode optical fiber. 8 . 8 . The optical film thickness measuring device according to claim 4 , wherein the angle between the converging light beam and the measuring object is 20°-90°.