JP2597099B2 - Multi-source ellipsometry - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial application field" This invention applies linearly polarized light to a sample, measures the ellipticity of elliptically polarized light reflected by the optical state of the surface of the sample, and measures the film thickness of the sample. Ellipsometry, which measures the ellipsometry.

[Prior Art] FIG. 2 shows a conventional automatic ellipsometry (ellipsometry). Light from the light source 1 is linearly polarized by the polarizer 2, and the linearly polarized light is incident on the thin film sample 3. The incident light is reflected as elliptically polarized light having a specific ellipticity and direction depending on the optical state of the surface of the sample 3. The reflected light is changed in its polarization state into a change in the amount of light by the rotation analyzer 4 and is incident on the photodetector 5, where it is converted into an electric signal.

The light quantity I converted by the rotary analyzer 4 is represented by I (A) = I ₀ (1 + αcosA + βsin ² A), where A is the azimuthal angle of the analyzer 4 at that time. I ₀ is the amount of incident light, and α and β are Fourier coefficients. Therefore, α and β can be determined by measuring I (A). It is known that α and β have the following relationship with the polarization state of incident light and the surface state of the sample 3.

α = −cosΨ ′, β = sin2Ψ′cosΔ tanΨ ′ = cosP tanΨ P is the azimuthal angle of the polarizer 2, and therefore the direction Ψ and the ellipticity Δ of the elliptically polarized light polarized on the sample 3 are Becomes From this relationship,, and Δ can be obtained by knowing P, α, and β.

Complex amplitude reflectance _P on the surface of the sample 3, _S is _{P = (r 1P + r 2P} e -2iδ) / (1 + r 1P r 2P e -2iδ) S = (r 1S + r 2S e -2iδ) / (1 + r 1S r _2S e ^−2iδ ) r _1P = (n ₁ ^COSφ ₀ −n ₀ COSφ ₁ ) / (n ₁ COSφ ₀ + n ₀ COSφ
₁ ) r _1S = (n ₀ COSφ ₀ −n ₁ COSφ ₁ ) / (n ₀ COSφ ₀ + n ₁ COSφ
₁ ) r _2P = (n ₂ COSφ ₁ −n ₁ COSφ ₂ ) / (n ₂ COSφ ₁ + n ₁ COSφ
₂ ) It is known that n ₀ , n ₁ , and n ₂ are the air (or vacuum), the refractive index of each of Sample 3 and the substrate of Sample 3,
d is the film thickness of sample 3, φ ₀ is the incident angle of light, and φ ₁ is sample 3.
Is the refraction angle of φ, φ ₂ is the refraction angle of the substrate of sample 3,
P indicates the direction of polarization, and λ is the wavelength of light used for measurement.

Here, if the complex amplitude reflectances _P and _S are displayed separately by the real number amplitude reflectances R _P and R _S and the phase changes δ _P and δ _S , which can be measured, _P = R _P e ^iδP and _S = R _S e ^iδS , [psi, delta is the relationship _{tanΨ = R P / R S,} Δ = δ P -δ S. Therefore, it usually corresponds to the values of Ψ and Δ
Calculate n, d to create a chart (Ψ, Δ chart) and find n, d corresponding to the actual measurement value, or use a successive approximation calculation method
Find n and d.

"Problems to be solved by the invention" In the conventional ellipsometry (ellipsometry),
When the reflected light approaches linearly polarized light, the measurement accuracy deteriorates, and the error in the obtained film thickness d increases.

"Means for solving the problem" According to the present invention, a plurality of light sources having different wavelengths are prepared,
When the reflected light from the sample with respect to the light from one light source is close to linearly polarized light, the light from another light source is used for measurement. As described above, in the present invention, the measurement accuracy of the film thickness of the sample is improved by always using the fact that the optical distance of the sample is different with respect to different wavelengths to measure the polarization ratio with high accuracy.

"Embodiment" FIG. 1 shows an embodiment of the present invention, in which a thin film sample 12 is disposed in a vacuum vessel 11. In this example, two light sources 13 and 14 having different wavelengths are provided, and lasers are used as the light sources 13 and 14. Choppers 15 and 16 are provided to turn on and off each of the light emitted from the light sources 13 and 14, so that light from both light sources 13 and 14 is not simultaneously incident on the measurement system. Light sources 17 and 18 for detecting the operating positions of the choppers 15 and 16 are provided, and the optical fiber 2 is transmitted through the light sources 17 and 18 and the choppers 15 and 16, respectively.
Each end face of 1,22 is opposed. Photodetectors 23 and 24 are arranged to face the other end surfaces of the optical fibers 21 and 22. This photo detector 2
The chopper position signal is input to the computer 25 from 3 and 24.

Light from the light sources 13 and 14 is propagated on the same optical axis by the wedge mirror 26. The light from the wedge mirror 26 is converted into linearly polarized light by the polarizer 27, and is incident on the thin film sample 12 in the vacuum vessel 11 through the window 28 of the vacuum vessel 11. The reflected light of the sample 12 is incident on the rotary analyzer 31 outside the vacuum vessel 11 from the window 29 of the vacuum vessel 11. In order to detect the angular position of the rotation analyzer 31,
The light source 33 and one end of the optical fiber 34 are arranged with the rotating plate 32 of the rotating analyzer 31 interposed therebetween. A photodetector 35 is provided at the other end of the optical fiber 34, and a pulse signal corresponding to the rotation angle of the rotation analyzer 31 obtained from the photodetector 35 is input to the computer 25. The emitted light from the rotating analyzer 31 is converted into an electric signal by the photodetector 36 and input to the electronic computer 25.

The measuring method of the refractive index and the film thickness of the sample 12 using each light from the light sources 13 and 14 is the same as the conventional method. Measurement is performed using light from one light source 13, and when reflected light from the sample 12 approaches elliptically polarized light to linearly polarized light, measurement is performed using light from the other light source 14. This utilizes the fact that the above-mentioned phase difference δ differs depending on the wavelength and the refractive index, so that when measured at two wavelengths, it becomes linearly polarized light for different Ψ. That is, light from the two light sources 13 and 14 is alternately temporally incident on the sample 13, and when the light exceeds a certain ellipticity, the refraction calculated from the data based on one wavelength light and the data based on the other wavelength light. By obtaining the ratio n and the film thickness d, the film thickness of the sample 12 is always measured with high accuracy.

The number of light sources for measurement may be increased and these may be switched for measurement.

[Effects of the Invention] As described above, according to the present invention, if the reflected light of the sample approaches linearly polarized light, the polarization ratio can always be measured with high accuracy by changing the wavelength of the light to be measured. Can be measured with high accuracy.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing a conventional method.

Claims (1)

(57) [Claims] In a polarization analysis method, linearly polarized light is incident on a sample, the ellipticity of elliptically polarized light reflected by the optical state of the surface of the sample is measured, and the film thickness of the sample is measured. When light reflected from the sample with respect to light from one light source is close to linearly polarized light, light from another light source is incident on the sample and measurement is performed. Multi-source ellipsometry.