KR100870132B1 - Spectroscopic ellipsometer using acoustic-optic tunable filter and ellipsometry using thereof - Google Patents

Abstract

A spectroscopic ellipsometer using an acoustic-optic tunable filter and an ellipsometric method using the same are provided to implement a spectroscopic ellipsometer including a spectral imaging ellipsometer more conveniently by using an acoustic-optic tunable filter(40) as both spectral and polarization devices. A spectroscopic ellipsometer includes a light source(2) emitting homogeneous light; a specimen part(3) in which the homogeneous light emitted from the light source is reflected; a polarization and modulation part(4) which includes an acoustic-optic tunable filter(40) having a wavelength bandwidth determined according to the homogeneous light reflected from the specimen part and an RF signal(63) from an acoustic-optic tunable filter driver(61), and generates two diffracted lights whose wave fronts are perpendicular to each other; a detection unit(5) measuring the two diffracted lights from the acoustic-optic tunable filter(40) to a two-dimensional imaging device at the same time and compensating for the polarized light confusion error generated in the acoustic-optic tunable filter; and a signal processing control part(6) synchronizing and controlling the two-dimensional imaging device to compensate for the polarized light confusion error and calculating the refractive index and thickness of specimen.

Description

Spectroscopic Ellipsometer Using Acousto-optic Modulation Filter and Analysis Method of Ellipse Using It {Spectroscopic Ellipsometer using Acoustic-Optic Tunable Filter and Ellipsometry using Technical}

1 is a schematic diagram showing the operation of the conventional ellipsometer.

Figure 2 is a block diagram showing the configuration of a spectroscopic ellipsometer according to the present invention.

Figure 3 is a block diagram showing the configuration of an acoustic optical modulation filter according to the present invention.

Figure 4 is a schematic diagram showing the diffraction polarization generated in the acoustic optical modulation filter according to an embodiment of the present invention.

Figure 5 is a block diagram showing the configuration of an acoustic optical modulation filter according to an embodiment of the present invention.

Figure 6 is a procedure showing an elliptic analysis method for measuring the thickness and refractive index of the specimen according to the present invention.

<Description of Signs of Major Parts of Drawings>

1: spectroscopic ellipsometer,

2: light source,

3: Psalm,

4: spectral and polarization modulator,

5: detection unit,

6: signal processing and control unit,

7: Incident light,

9: measuring light,

9a: e- diffracted light,

9b: o- diffracted light,

40: acoustic optical modulation filter,

55: measurement data,

61: acoustic optical modulation filter driver,

62: acoustic optical modulation filter control signal,

63: RF signal,

64: light source polarization control signal.

The present invention relates to a spectroscopic ellipsometer using an acoustic optical modulation filter and an elliptic analysis method using the same, and more particularly, to a spectroscopic ellipse that can improve the accuracy of thin film measurement including a multilayer thin film using polarization characteristics at various wavelengths. The present invention relates to an analyzer and an elliptic analysis method thereof.

In general, an elliptic analysis method is an analysis method for determining optical or structural characteristics of a sample by measuring a change in polarization state of light reflected or transmitted from the sample. Initially mainly used for the thickness of thin oxide films, but with the development of personal computers, automated and spectroscopic ellipsometers were developed, and the so-called modeling process was established to quantitatively quantify surface and multilayer thin films. Phosphorus analysis was possible.

Such a spectroscopic ellipsometer (SE) is described in detail in US Pat. Nos. 5,625,455 and 5,373,359.

The spectroscopic ellipsometer is an optical measuring instrument that has the advantages of nondestructiveness, non-interference, structure analysis ability of multi-layer thin film, and high thickness resolution (units of several units). Has contributed greatly to many research and industrial applications. Since the late 1980s, interest in real-time measurement of thin film growth has been greatly increased, and since the short-wave ellipsometer or spectroscopic ellipsometer was installed in the thin film growth equipment or the etching equipment, the gap growth refractive index during the initial growth of the thin film was observed. Research to measure the change in the film thickness or composition ratio of the thin film in real time is being actively conducted.

The acoustooptic modulation filter generates acoustic waves using piezos on both ends of the anisotropic crystal. The standing wave of the acoustic wave is formed in the crystal and acts as a diffraction grating, so that it can be used as a band pass filter for the wavelength. An advantage of the acousto-optic modulation filter is that changing the frequency of the RF signal applied to the piezo changes the period of the standing wave in the crystal, thereby changing the center wavelength of the band pass filter. That is, it can be used as a filter that can pass only an arbitrary wavelength band by changing the RF signal.

Another characteristic of the acoustooptic modulation filter is that anisotropy produces diffracted light of different polarization states in a direction perpendicular to each other. That is, when light having an arbitrary wavelength bandwidth and an arbitrary polarization state is incident on the acoustooptic modulation filter, diffracted light having two different polarization states determined by the direction of the crystal is generated at different angles. The wavelength band is determined by the RF signal applied to the crystal.

When the polarization characteristic of the acoustooptic modulation filter is used, crosstalk errors occur between diffracted light of each polarization because anisotropic crystals or standing waves are not perfect.

The ellipsometer analyzes the light incident on the thin film to a predefined polarization state and then analyzes the polarization state of the light reflected from the thin film. That is, the refractive index and the thickness of the thin film are calculated by extracting the change of the polarization state by the thin film by the previously considered theoretical model. Elliptical analyzers come in many forms, but the most basic device is a linear polarizer located in a monochromatic light source. Thus, light is in a linearly polarized state. The linearly polarized light is reflected from the specimen, and the intensity of the measured light changes as the polarization changes, and the change is calculated to calculate the thickness and refractive index of the thin film. The polarizer of the light source unit and the rotating polarizer of the detector may be interchanged with each other. In addition, various elements for changing the polarization state may be used.

The measuring principle of the elliptical analyzer is as follows.

1 is a schematic diagram showing the principle of measuring the thickness and refractive index of the specimen using the elliptic analysis method. That is, as shown in FIG. 1, when light is reflected from the surface of the specimen, its polarization state is changed. The fact that the polarization state of the reflected light varies with the optical properties of the material and the thickness of the layer was discovered by Drude in 1889.However, starting from the field of electrochemistry, the advantages of this method are known and are increasingly being studied in other fields. It required decades of use before it could be used.

It is well known that the elliptical analyzer 1 can be classified into a polarizer (not shown) for linearly polarizing a light source, a specimen (not shown) to be measured, and an analyzer and a detector for measuring the polarization state deformed by the specimen (not shown). It is a fact.

In this elliptic analyzer 1, the refractive index n and the extinction coefficient k, which are imaginary parts, can be measured at the same time. In the reflectance measurement, a very difficult process is required for accurate measurement, but in the case of the ellipsometer (1), since one polarization state becomes the reference of the other polarization state, there is no need to move the test equipment again, and it is measured according to the light source or the temperature. The error is very small

The dielectric constant becomes a dielectric function, not a constant, due to the dispersion phenomenon in which the optical or dielectric properties of a material vary depending on the wavelength of light.

The dielectric function is expressed as a complex function consisting of real and imaginary parts with ε (w) = ε ₁ + i ε ₂ , and has a relation with the complex refractive index as follows.

N = n + ik

ε = N ²

In practice, the measured value of the ellipsometer 1 is (φ, Δ), which is called an ellipsometer value or an ellipsometer angle. The values of these two variables are used to determine the desired information, such as the optical or microstructural properties of the material.

Regardless of the surface of a pure tube or covered with a thin film, the reflection coefficient can be calculated by the following equation by knowing the structure of the sample, the optical thickness, refractive index, and angle of incidence of its components.

Where | r _{p (s)} | is the ratio of the magnitude of the electric field strength (E _{ip (s)} ) of the incident wave and the electric field strength (E _{rp (s)} ) of the reflected wave. And δ _{p (s)} is the phase change after reflection. Using this, the complex reflection coefficient ratio ρ can be defined.

From this two angles are defined. That is, the elliptic analyzer (1) equation

The input formulas of Δ and φ are the same as in [Equation 7] or [Equation 8].

Δ = δ _p -δ _s

The phase difference between the p-wave and the s-wave incident on the same phase as Δ is obtained after reflection, and tanφ is the ratio of the magnitude of the reflection coefficient. When the angle of incidence is 0 °, the two angles are meaningless. Therefore, the angle between two ellipsometers (1) can be measured only if the material has an incidence angle at all times unless the material is anisotropic. In addition, since the phase difference is 0 ° to 180 °, the elliptic solver angle of the bulk material has a certain range, but there is no limitation on the (φ, Δ) value in the case of the thin film. In general, the reflected light becomes elliptical polarization as shown in FIG.

In particular, in the case of a multi-layered thin film, the scattering matrix S can be expressed by the interfacial matrix I and the layer matrix L, which are expressed as follows.

S = I ₁₂ L ₂ I ₂₃ L ₃ ...... I _{jj +1} L _{j +1 ......} I _n L _{n +1} I _{n + 1n + 2}

Where I _{jj +1} is the interface matrix representing the phenomenon at the interface between the jth and (j + 1) th layers, and L _{j +1} is the layer matrix representing the phenomenon when passing through the (j + 1) th layer. These matrices are represented by Fresnel's reflection coefficient, transmission coefficient, and the like.

Details thereof are easy to those skilled in the art, and thus detailed descriptions thereof will be omitted. After all, the scattering matrix has a factor of 2 × 2. After calculating the scattering matrices for the p-wave and the s-wave, respectively, the reflection coefficient can be obtained as follows.

This complex reflection coefficient ratio is used to find the ellipsometer angle φ, Δ.

Since the ellipsometer is used to obtain the refractive index and thickness by fitting to the theoretical model using the measurement results, various measurement results are required for measuring the multilayer thin film or for more accurate measurement. For this purpose, a spectroscopic ellipsometer has been proposed. Spectroscopic ellipsometer is an improved elliptical analyzer to measure at various wavelengths, and monochromator or spectrometer which can scan wavelength is used.

Another drawback of elliptical analyzers is obtaining a single measurement point. Therefore, if the measuring area is large, it should be measured while moving the specimen. Therefore, in view of the technical level, the most advanced ellipsometer is a spectroscopic elliptical analyzer, and the present invention can also be implemented as a spectroscopic ellipsoidal analyzer because the present invention can apply an imaging elliptic analysis method. The spectroscopic ellipsometer is complicated by the increase in the number of optical elements used, the alignment error or interference and polarization error between each accumulate cumulative mechanical and software alignment of the device is difficult and difficult to maintain.

The polarization measuring apparatus using the conventional acousto-optic modulation filter 40 has no part for adjusting the polarization state incident on the measurement specimen. In addition, the imaging elliptic analyzer using the conventional acoustooptic modulation filter simply uses the acoustooptic modulation filter as a spectroscopic element, and uses two acoustooptic modulation filters or additional polarizers. There was a disadvantage that the measurement time becomes longer.

In order to solve this problem, the present invention is to measure the polarization change generated by the incident light of a predetermined polarization state reflected from the specimen and simultaneously measure two diffracted light of different polarization states generated by the acoustic optical modulation filter. In addition, the polarization state can be measured without a separate polarizer or an optical element for controlling the polarization state in front of the detector.

Another object of the present invention is to measure both diffracted light generated by the acoustooptic modulation filter to compensate for crosstalk errors due to polarization generated by the acoustooptic modulation filter to enable more accurate analysis.

Other objects, specific advantages and novel features of the present invention will become more apparent from the preferred embodiments of the following detailed description which is described in connection with the accompanying drawings.

The elliptic analyzer 1 using the acoustic optical modulation filter according to the present invention comprises a light source 21 for emitting single light; A specimen portion 3 on which monochromatic light emitted from the light source 21 is reflected; A wavelength and polarization modulator 4 including an acoustic optical modulation filter 40; A detection section 5 for measuring the two diffracted lights 9a, 9b from the acoustic optical modulation filter 40 with the two-dimensional imaging elements 51a, 51b; And a signal processing controller 6 for compensating for the polarization crosstalk error and calculating the refractive index and the thickness of the specimen 31.

Next, the spectroscopic ellipsometer 1 using the acoustooptic modulation filter 40 and the elliptic analysis method using the same will be described with reference to the accompanying drawings.

2 is a block diagram of a spectroscopic ellipsometer using an acoustic optical filter according to the present invention. Looking at the configuration diagram of the spectroscopic ellipsometer using the acoustooptic filter, the light from the light source 2 is a light having a polarization state through the polarizer 22 to pass through the specimen 31, the specimen 31 The light passing through becomes elliptical polarization because of the anisotropy of the specimen (31). As such, the light reflected by the specimen 31 is incident on the acoustooptic modulation filter 40 and is deformed according to polarization and spectral characteristics. At this time, the diffracted light 9a and 9b of two different polarization states are generated, and they also have a specific wavelength bandwidth determined by the RF signal 63 emitted from the acoustic optical modulation filter driver 61. . Two diffracted lights 9a, 9b from the acoustooptic modulation filter 40 are measured by two two-dimensional rotating imaging elements 51a, 51b. The measured results 55a and 55b are transmitted to the signal processing and control unit 6 to compensate for the polarization crosstalk error and calculate the refractive index and thickness of the specimen.

At this time. By measuring both the diffracted light 9a and 9b generated by the acoustooptic modulation filter 40, it is possible to compensate for crosstalk due to polarization generated by the acoustooptic modulation filter 40 to perform a more accurate analysis. It becomes possible. In particular, in the spectroscopic ellipsometer 1, the crosstalk error amount of polarized light varies depending on the wavelength, and thus the crosstalk error amount of each diffracted light according to the wavelength is measured through measurement in advance. After that, the values obtained in the actual measurement are calibrated using the result of the measurement in advance.

The directions of polarization states perpendicular to each other are expressed in the e-direction and the o-direction as follows.

δ _e (λ): ratio of the e-polarized state measured from the o-direction diffraction light measurement unit at the wavelength λ

δ _o (λ): The ratio of the o-polarized state measured from the e-direction diffracted light measuring unit at the wavelength λ

I _e (λ): Intensity of light measured from the e-direction diffracted light measurement unit at wavelength λ

I _o (λ): Intensity of light measured from the o-directional diffracted light measurement unit at wavelength λ

i _e (λ): the intensity of the e-direction component of the incident light at the wavelength λ

i _o (λ): the intensity of the o-direction component of the incident light at the wavelength λ

The intensity of light measured in each diffracted light can be expressed as follows.

Therefore, by measuring the two diffracted light (9a, 9b) and using the above equation, it is possible to accurately obtain the polarization state of each incident light by compensating the polarization crosstalk error irrespective of the change in the intensity of the incident light.

This will be described in detail below. Passing through the acoustic optical modulation filter 40, as shown in FIG. 2, the +1 and -1 orders of light are simultaneously acquired. This is called o-diffraction light 9b and e-diffraction light 9a. However, these two diffracted lights 9a and 9b are mutually perpendicular to each other, which has the same effect as measuring s-wave and p-wave which are perpendicular to each other in the elliptical analyzer 1 described above. Therefore, once the intensity of the o-diffraction light and the e-diffraction light are known, (φ, Δ) can be calculated by the following equations (7) and (8). By comparing the measured (φ, Δ) and the theoretical model (φ, Δ), the minimum square error (Square Error) is minimized and the thickness and refractive index of the specimen are measured. In order to find out the intensity of the incident light 7, the intensity of the diffracted light 9a, 9b is required. When the intensity of the diffracted light is known, the wavelength? Of the light to be scanned is fixed as described above. 12] and [Equation 13] to calculate the intensity of the incident light.

That is, at this time, the e-diffracted light 9a and the o-diffracted light 9b respectively enter two imaging elements positioned in the polarization direction, and the e-diffracted light 9a is due to the characteristics of the acoustooptic modulation filter 40. And o-diffracted light (9b) are not completely spectroscopic, but included in some e-diffracted light (9a) and o-diffracted light (9b) (similarly, some o-diffracted light is included in e-diffracted light And enters an imaging device (not shown).

In order to accurately measure the thickness and refractive index of the thin film, it is necessary to know the information of the e-diffraction light and the o-diffraction light which are completely separated from the two imaging devices (not shown). In this case, since there is a limit that does not satisfy this condition, the intensity information (I _o (λ), I of incomplete e-diffraction light 9a and o-diffraction light 9b incident on two imaging devices (not shown)). _{From e} (λ)), the intensity information i _e (λ) and i _o (λ) of the complete e-incident light 7 and o-incident light 7 of the actual incident light 8 must be extracted.

3 is a cross-sectional view showing the configuration of the acoustooptic modulation filter 40.

The acoustooptic modulation filter 40 has piezoelectric pads 42a and 42b attached to both ends of the anisotropic crystal 41, and a standing wave of the acoustic wave by the RF signal 63 coming from the acoustooptic modulation filter driver 61. (44) occurs. The incident light 8 causes diffraction within the anisotropic structure of the crystal 41, resulting in diffracted light 9a, 9b having two polarization states perpendicular to each other. Further, the diffracted light 9a, 9b has a specific wavelength bandwidth corresponding to the RF signal 63 due to the diffraction phenomenon. The wavelength λ of the diffracted light 9a, 9b is determined as follows by the luminous flux c, the refractive index difference Δn in each direction of the anisotropic crystal, and the frequency F of the RF signal.

4 is a detailed view showing that the diffracted light is separated, and in order to accurately calculate the separation of the diffracted light, an e-diffraction light detector 51a is provided with an o-diffraction light blocking filter 10a and an o-diffracted light The detection unit 51b is provided with an o- diffraction light blocking filter 10b.

That is, as shown in FIG. 4, if the blocking filter 10b capable of blocking only the o-diffracted light 9b is placed in the direction in which the o-diffracted light 9b passes, no signal is originally detected by the o-direction detector 51b. Is not.

However, as described above, since the e-diffracted light 9a and the o-diffracted light 9b are not completely separated due to the characteristics of the acoustooptic modulation filter 40, they should not be included in the o-direction detector 51b. The e-diffraction light 9a is measured.

In the e-direction detector 51a, only the e-diffracted light 9a is detected by filtering the o-diffracted light 9b. The ratio of the measured e-diffraction light 9a is expressed as δ _e (λ). In FIG. 4, the e-diffraction light blocking filter 10a is provided in place of the o-diffraction light blocking filter 10b. The ratio of the o-diffracted light 9b detected at (51a, 51b) is expressed by δ _o (λ).

If δ _e (λ) and δ _o (λ) are measured in advance through these settings, the intensity information measured by the e-direction measuring unit and the o-direction measuring unit is used to determine the complete e-diffracted light and o-diffracted light of the actual incident light. The intensity information i _e (λ), i _o (λ) can be extracted. That is, I _e (λ) measured by the e-direction measuring unit is (1-δ _e (λ), which is a portion except for δ _e (λ), which is a ratio flowing in the o direction among the actual e-diffraction light intensities i _e (λ). ) The sum of the intensities of _o -diffracted light, δ _o (λ) i _o (λ), that should not have been originally incident on the intensities of i _e (λ). The same interpretation is also possible for I _o (λ) measured at the o-bar measurement unit.

5 relates to an embodiment of a spectroscopic ellipsometer 1 using an acoustooptic modulation filter 40.

The light source unit 2 includes a light source 21 and a polarizer 22, and an auxiliary detection device 23 may be attached to compensate for a change in light source intensity over time. The polarizer 22 may use a general polarizer and may be rotated to change the direction of linear polarization. In addition, an optical element that can change the polarization state such as a phase retarder or a woolen prism to the desired state of the device may be used. In particular, devices such as the Liquid Crystal Variable Retarder (LCVR) can use electrical signals to control the amount of delayed phase. The signal 64 for changing the state of the polarizer 22 is received from the signal processing and control section 6.

This signal may be referred to as the intensity data of the diffracted polarized light described above, and is measured in such a manner as to compensate for the intensity of the e-direction diffraction light 9a and the o-direction diffraction light 9b of the incident light. By correcting the data on the refractive index and the thickness and repeating the data it is possible to calculate the correct thickness and refractive index of the specimen (31).

6 is a procedure showing the measurement procedure of the thickness and refractive index of the spectroscopic ellipsometer using the above-described acoustic optical modulation filter 40. After the light is first emitted from the light source unit 2 (S10), the light becomes linearly polarized light in the polarizer 22 (S20). The linearly polarized light is incident on the specimen part 3 on which the specimen 31 is mounted. In this way, light incident on the specimen part 3 becomes an elliptically polarized light according to the thickness and the refractive index of the specimen (S30).

Here, separate lens units 24a and 24b may be mounted according to the structure of the elliptical analyzer or the degree of condensing of the light source. In the above manner, the light emitted from the light source is linearly polarized in a state in which the polarization state is controlled and is incident on the specimen 31.

 The reflected and polarized light is incident on the spectral and polarization modulator 4 where the acoustic optical modulation filter 40 is located (S40). The acoustic optical modulation filter 40 receives the RF signal 63 from the acoustic optical modulation filter driver 61, wherein the lens 43 may be mounted on the specimen part 3 side according to the type of the elliptic analyzer 1. have. The above RF signal 63 makes the diffracted light 9a, 9b generated from the acoustooptic modulation filter 40 to have a band width of a specific wavelength (S50).

Two diffracted lights 9a and 9b having a specific wavelength band width are then incident on the detector 5. The detection unit is equipped with lens units 50a and 50b for imaging or collecting light, and the two diffracted lights 9a and 9b passing through the lens units 50a and 50b are transferred to the two-dimensional imaging elements 51a and 51b. Measurement is made (S60). The two-dimensional imaging elements 51a and 51b are synchronized and controlled by the signal processing and control unit 6, and the measured intensity 55a and 55b of the diffracted light 9a and 9b are transmitted to the signal processing and control unit 6. The polarization crosstalk error is compensated for in the above-described manner, and the refractive index and the thickness of the specimen 31 are calculated (S70 and S80).

At this time, the signal processing and control unit 6 not only adjusts the linear polarization state of the incident light 7 by adjusting the linear polarizer 22 generated from the light source 21, but also calculates a signal 62 for adjusting the acoustooptic modulation filter. Play a role at the same time.

The spectroscopic ellipsometer using the acoustooptic modulator according to the present invention uses the acoustooptic modulating filter 40 as a spectroscopic and polarizing element at the same time to more easily implement a spectroscopic ellipsometer including a spectroscopic image ellipsometer, thereby including multiple thin films. The thickness and refractive index of the specimen can be measured accurately at high speed.

In order to more easily implement such a spectroscopic ellipsometer, the optical optical modulation filter is provided, and the signal processing and control unit for controlling the linearly polarized state of the emitted signal and the incident light is provided.

Claims (8)

A light source 2 for emitting single light; A specimen part 3 on which monochromatic light emitted from the light source is reflected; The monochromatic light reflected from the specimen 3 and the acoustic optical modulation filter 40 having a predetermined wavelength bandwidth determined by receiving the RF signal 63 from the acoustic optical modulation filter driver 61 are perpendicular to each other. A wavelength and polarization modulator 4 for generating two diffracted lights; Polarization crosstalk generated by the acoustooptic modulation filter 40 by simultaneously measuring the diffracted light from the acoustooptic modulation filter 40 with the two-dimensional imaging elements 51a and 51b in two orthogonal directions. A detector 5 for compensating for the error; Signal processing control unit 6 for compensating for the polarization crosstalk error by synchronizing and controlling the two-dimensional imaging elements (51a, 51b), and calculates the refractive index and thickness of the specimen; Spectroscopic Ellipsometer. delete delete delete delete delete Emitting light from the light source unit (S10); Linearly polarizing the emitted light with a polarizer (s20); Reflecting the linearly polarized light from the specimen (31); Injecting the light reflected from the specimen (31) into the acoustooptic modulation filter (s40); Generating two diffracted polarizations in the acoustooptic modulation filter (s50); Detecting the diffracted polarized light by the detector 5 (s60); Measuring intensity of incident light through a method of measuring intensity of diffracted polarized light (s70); Compensating for a polarization crosstalk error of linearly polarized light with data obtained from the intensity of the incident light 7 (s80); Measuring the thickness and the refractive index of the specimen (31) using the polarization crosstalk error (s90); Elliptical analysis method using an acoustic optical modulation filter comprising a. The method of claim 7, wherein And transmitting the signal to the polarizer (22) in the signal processing and control unit (6) to compensate for the linearly polarized light crosstalk error.

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