Ellipsometer micro-spot calibration method

Abstract

The present application relates to a kind of ellipsometer micro light spot effect calibration method, comprising: S1, using ellipsometer measurement to obtain the periodic light intensity signal of selected standard measurement sample;S2, select a certain section as analysis waveband in full waveband, based on ellipsometer system model, adjust micro light spot phase difference, obtain the curve of film thickness and incident angle variance with micro light spot phase difference, with quadratic function fitting curve and find the lowest point of curve;S3, the micro light spot phase difference in ellipsometer system model is fixed at the micro light spot phase difference corresponding to the lowest point of curve, calibrate all system parameters and standard measurement sample film thickness in analysis waveband wavelength by wavelength, calculate the mean of film thickness and incident angle in analysis waveband;S4, the film thickness and incident angle in ellipsometer system model are fixed at the mean of film thickness and incident angle in analysis waveband calculated, calibrate all system parameters in full waveband wavelength by wavelength.The ellipsometer can be accurately calibrated by the method.

Description

Technical Field

[0001] This invention relates to the field of optical scattering measurement, and specifically to a method for calibrating a micro-spot of an ellipsometer. Background Technology

[0002] Ellipsometry, compared to microscopic morphology measurement methods such as scanning electron microscopy and atomic force microscopy, offers advantages such as high speed, low cost, non-contact, and non-destructive operation, leading to its widespread application in advanced process online monitoring. The ellipsometer measurement process can be summarized as follows: light emitted from the light source is phase-modulated by polarizers and waveplates in the polarizing arm, projected onto the surface of the sample, and reflected. The reflected light, carrying information about the sample, is phase-demodulated by waveplates and polarizers in the analyzer arm and then received by a light intensity detector. It is easy to understand that the final light intensity received by the detector is related not only to the sample itself but also to the ellipsometer system parameters (including the incident angle, the characteristic parameters of the polarizing devices such as the polarizers and waveplates, and their placement azimuth). Therefore, to accurately obtain sample information from the received light intensity, precise system calibration of the ellipsometer is essential.

[0003] As semiconductor technology nodes continue to shrink, semiconductor devices are also constantly evolving towards miniaturization, with the size of the chip processing area on a wafer reduced to tens of micrometers. To meet the requirements of micro-area measurement, ellipsometers must be equipped with micro-spot assemblies composed of multiple lenses to reduce the probe spot size from several millimeters to tens of micrometers. However, the lenses in the micro-spot assembly typically exhibit stress birefringence, resulting in a certain phase delay, which affects the final received light intensity and consequently the measurement of the sample under test. Therefore, for accurate measurement of the sample under test, the micro-spot effect must be precisely calibrated during system calibration. Summary of the Invention

[0004] This invention addresses the technical problems existing in the prior art by providing a method for calibrating the micro-spot effect of an ellipsometer, comprising the following steps:

[0005] S1, select the periodic light intensity signal of the selected standard measurement sample by using an ellipsometer;

[0006] S2. Select a segment within the entire band as the analysis band. Based on the ellipsometer system model, adjust the phase difference of the micro-spot to obtain the curves of film thickness and incident angle variance as a function of the phase difference of the micro-spot. Fit the curve with a quadratic function and find the lowest point of the curve.

[0007] S3, fix the phase difference of the micro-spot in the ellipsometer system model to the phase difference of the micro-spot corresponding to the lowest point of the curve, and perform wavelength-by-wave calibration on all system parameters and the film thickness of the standard measurement sample within the analysis band, and calculate the mean values ​​of film thickness and incident angle within the analysis band;

[0008] S4. Fix the film thickness and incident angle in the ellipsometer system model to the average values ​​of film thickness and incident angle within the calculated analysis band, and calibrate all system parameters wavelength by wavelength across the entire band.

[0009] Further, step S2 includes:

[0010] S201: Select a segment within the entire band as the analysis band, fix the phase difference of the micro-spot at a certain value, perform wavelength-by-wave calibration on all system parameters and standard film thickness within the analysis band, and calculate the variance of the film thickness and incident angle calibration values ​​within the analysis band.

[0011] S202, adjust the phase difference of the micro-spot and repeat step S201 to obtain the curves of film thickness and incident angle variance as a function of the phase difference of the micro-spot. Fit the curve with a quadratic function and find the lowest point of the curve.

[0012] Furthermore, the analysis band is selected within the visible light range.

[0013] Furthermore, the calibration method for the system parameters includes:

[0014] The periodic light intensity obtained from the actual measurement by the ellipsometer is converted into Fourier coefficients.

[0015] Construct a system model function whose input is the system parameters and whose output is the Fourier coefficients of the simulated light intensity of the system model;

[0016] Adjust the input parameter values ​​of the system model function so that the output simulated Fourier coefficients match the measured Fourier coefficients.

[0017] The method of this invention enables precise system calibration of an ellipsometer. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a typical double-rotating-waveplate ellipsometer.

[0019] The attached diagram lists the components represented by each number as follows:

[0020] 1. Light source, 2. Polarizer, 3. First rotating waveplate, 4. First micro-spot assembly, 5. Sample stage, 6. Sample, 7. Second micro-spot assembly, 8. Second rotating waveplate, 9. Analyzer, 10. Detector. Detailed Implementation

[0021] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0022] This invention provides a method for calibrating the micro-spot effect of an ellipsometer, comprising the following steps:

[0023] S1, select the periodic light intensity signal of the selected standard measurement sample by using an ellipsometer.

[0024] The principle of a typical double-rotating-waveplate ellipsometer is as follows: Figure 1 As shown, its core components include a light source 1, a polarizing arm (mainly composed of a polarizer 2, a first rotating waveplate 3, and a first micro-spot assembly 4), a sample stage 5, an analyzer arm (mainly composed of an analyzer 9, a second rotating waveplate 8, and a second micro-spot assembly 7), and a detector 10. Specifically, the system model of the ellipsometer can be represented by the following formula:

[0025]

[0026] Where t represents time, S in Let I be the Stokes vector of the light emitted by the light source. out The system output light intensity is represented by P, A, C1, and C2, which are the azimuth angles of the polarizer arm, analyzer arm, first rotating waveplate, and second rotating waveplate, respectively. δ1 and δ2 represent the phase delays of the first and second rotating waveplates, respectively. μ1 and μ2 represent the phase delays of the first and second micro-spot components, respectively. offset For the phase difference of the micro-spot; M A and M P M is the characteristic Mueller matrix of the polarizer. C M is the characteristic Mueller matrix of the phase delayer, R is the rotation matrix; s The Mueller matrix is ​​the feature matrix of the sample. and Δ s These are the ellipticity parameters of the sample, which depend on the sample film thickness THK and the incident angle AOI, i.e. Δ S =Δ S (AOI, THK), ω1 and ω2 are the angular frequencies of the first and second rotating waveplates, respectively, and their ratio is a constant value. Therefore, the light intensity received by the detector is a one-cycle signal.

[0027] By placing the standard sample on the ellipsometer sample stage, the detector can receive the periodic light intensity signal reflected by the sample for subsequent analysis.

[0028] S2. Select a segment within the entire band as the analysis band. Based on the ellipsometer system model, adjust the phase difference of the micro-spot to obtain the curves of film thickness and incident angle variance as a function of the micro-spot phase difference. Fit the curve with a quadratic function and find the lowest point of the curve.

[0029] Specifically, step S2 includes the following sub-steps:

[0030] S201: Select a segment within the entire band as the analysis band, fix the phase difference of the micro-spot at a certain value, perform wavelength-by-wave calibration on all system parameters and standard film thickness within the analysis band, and calculate the variance of the film thickness and incident angle calibration values ​​within the analysis band.

[0031] Sample elliptic parameters and Δ s The phase difference Δ of the micro-spot depends on the film thickness and the incident angle, so it is easy to find from the system model formula. offset It exhibits a strong coupling with film thickness and incident angle. If the incident angle, sample film thickness, and micro-spot phase difference Δ are simultaneously calibrated... offset If this is not done correctly, it is usually difficult to obtain the correct results. To achieve decoupling between parameters, the phase difference Δ of the micro-spot can be... offset The parameters in the system model are calibrated wavelength by wavelength within the selected analysis band, and the variances of the film thickness and incident angle within the analysis band are calculated.

[0032] Preferably, the analysis band can be selected within the visible light range, because the light intensity is generally stronger and the noise is weaker in the visible light range, making the analysis results more accurate.

[0033] Preferably, system parameter calibration can be achieved as follows: First, the periodic light intensity actually measured by the ellipsometer is converted into Fourier coefficients; then, a system model function is written, whose input is the system parameters and whose output is the Fourier coefficients of the simulated output light intensity; finally, the input parameter values ​​of the system model function are adjusted through algorithms such as nonlinear fitting so that the output simulated Fourier coefficients match the measured Fourier coefficients. There is a wealth of information available on calibration methods, which will not be elaborated upon here.

[0034] S202, adjust the phase difference of the micro-spot and repeat step S201 to obtain the curves of film thickness and incident angle variance as a function of the phase difference of the micro-spot. Fit the curve with a quadratic function and find the lowest point of the curve.

[0035] According to the analysis in S201, the phase difference Δ of the micro-spot is... offset It exhibits a strong coupling with film thickness and incident angle. Therefore, the phase difference Δ of the micro-spot is... offset Significant differences exist in the film thickness and incident angle obtained from different numerical calibrations. In principle, when the phase difference Δ of the micro-spot... offset When the value is near the true value, the film thickness and incident angle within the analysis band are also closer to the true value, and the corresponding variance should be smaller. Therefore, this invention proposes that the micro-spot phase difference can be traversed and step S201 can be repeated to obtain the curves of the film thickness and incident angle variance as a function of the micro-spot phase difference; furthermore, a quadratic function can be used to fit the curves, and the micro-spot phase difference corresponding to the lowest point of the curve can be found as the true value of the micro-spot phase difference within the analysis band.

[0036] S3. Fix the phase difference of the micro-spot in the ellipsometer system model to the phase difference of the micro-spot corresponding to the lowest point of the curve. Perform wavelength-by-wavelength calibration on all system parameters and standard measurement sample film thickness within the analysis band, and calculate the mean values ​​of film thickness and incident angle within the analysis band.

[0037] Within the analysis band, the phase difference of the micro-spot is fixed to the true value obtained in step S202, and all other parameters in the system model are calibrated wavelength by wavelength, and the mean values ​​of film thickness and incident angle are calculated.

[0038] S4. Fix the film thickness and incident angle in the ellipsometer system model to the average values ​​of film thickness and incident angle within the calculated analysis band, and calibrate all system parameters wavelength by wavelength across the entire band.

[0039] In step S3, the phase difference of the micro-spot is fixed at its true value, thus the calibrated film thickness and incident angle results are relatively accurate. To further eliminate the influence of factors such as noise, the film thickness and incident angle can be fixed at the average value within the selected analysis band, and all system parameters except for these two can be calibrated wavelength by wavelength across the entire band.

[0040] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0041] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for calibrating a micro-spot on an ellipsometer, characterized in that, include: S1, select the periodic light intensity signal of the selected standard measurement sample by using an ellipsometer; S2. Select a segment within the entire band as the analysis band. Based on the ellipsometer system model, adjust the phase difference of the micro-spot to obtain the curves of film thickness and incident angle variance as a function of the phase difference of the micro-spot. Fit the curve with a quadratic function and find the lowest point of the curve. S3, fix the phase difference of the micro-spot in the ellipsometer system model to the phase difference of the micro-spot corresponding to the lowest point of the curve, and perform wavelength-by-wave calibration on all system parameters and the film thickness of the standard measurement sample within the analysis band, and calculate the mean values ​​of film thickness and incident angle within the analysis band; S4. Fix the film thickness and incident angle in the ellipsometer system model to the average values ​​of film thickness and incident angle within the calculated analysis band, and calibrate all system parameters wavelength by wavelength across the entire band.

2. The method according to claim 1, characterized in that, Step S2 includes: S201: Select a segment within the entire band as the analysis band, fix the phase difference of the micro-spot at a certain value, perform wavelength-by-wave calibration on all system parameters and standard film thickness within the analysis band, and calculate the variance of the film thickness and incident angle calibration values ​​within the analysis band. S202, adjust the phase difference of the micro-spot and repeat step S201 to obtain the curves of film thickness and incident angle variance as a function of the phase difference of the micro-spot. Fit the curve with a quadratic function and find the lowest point of the curve.

3. The method according to claim 1 or 2, characterized in that, The analysis band is selected within the visible light range.

4. The method according to claim 1 or 2, characterized in that, The calibration method for the system parameters includes: The periodic light intensity obtained from the actual measurement by the ellipsometer is converted into Fourier coefficients. Construct a system model function whose input is the system parameters and whose output is the Fourier coefficients of the simulated light intensity of the system model; Adjust the input parameter values ​​of the system model function so that the output simulated Fourier coefficients match the measured Fourier coefficients.

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