JPS63186130A - Ellipsometer - Google Patents

Abstract

PURPOSE:To simplify the constitution of an ellipsometer and to shorten a measurement time by setting the optical axis of irradiation light almost at right angles to a film to be measured and measuring the light intensity of a specific part of the output light of an analyzer by a photodetector. CONSTITUTION:The output light of a laser light source 3 is converted by a lens 20 into parallel light, which is polarized linearly by a polarizer 21 and projected on the film 2 to be measured almost at right angles through a 1/4 wavelength plate 22 and a half-mirror 23. Light reflected by the film 2 to be measured is converted by a lens 24 into parallel light, which is reflected by the mirror 23. Only the light of the part is extracted by a pupil 26 and the photodetector 27 measures the intensity of the light to find the film thickness and refractive index of the film 2 to be measured. Thus, the constitution is simplified and the measurement time is shortened.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an ellipsometer that uses light to measure the thickness and refractive index of a thin film formed on a semiconductor surface, etc.
In particular, it relates to an ellipsometer that can simultaneously irradiate light with multiple conditions.

<Prior art> Ellipsometer uses the polarization characteristics of light to
This method measures the refractive index related to the thickness of a thin film with a thickness of 100 mL and the composition of this 7a film, and is particularly used in the manufacture of 6n chips such as LSIs.

Such ellipsometers are classified into photometric type and extinction type depending on their measurement method.

FIG. 4 shows the configuration of a photometric ellipsometer.

In FIG. 4, 1 is a substrate, 2 is formed on the substrate 1,
This is a film to be measured whose flA thickness and refractive index are measured.

3 is a laser light source, and its output light is polarized by a light leaker 4. This polarized light is irradiated onto the film to be measured 2 via a lens 5 and a quarter wavelength plate 6. 7 is the optical path of the incident light. The light reflected from the upper and lower surfaces of the film to be measured 2 passes through an analyzer 8, and after the light other than the laser light source is removed by a filter 9, its light intensity is measured by a semiconductor photodetector 10.

11 is an optical path of reflected light.

In such a configuration, the polarizer 4 is fixed at the 45'' direction.The light from the laser light source 3 is linearly polarized by the polarizer 4 and irradiated onto the film to be measured 2. A specific polarization component of the reflected light is selected by an analyzer 8, and its light intensity is measured by a semiconductor photodetector 10. This light intensity changes depending on the rotation angle of the analyzer 8.

If one rotation of the analyzer 8 is divided into n equal parts and the m-th light intensity is r current, then 1m=Io (1+αcos (4πm/n)+β
sin (4πm/n)), and from this the Fourier coefficient α-=a/I.

β=b/I.

However, a-2 (Σ shoulder, cos (4πm/n))/nb=2
(Σ Itasin (4πm/n))/nFL-
1 is (q). From these Fourier coefficients α and β, the reflected light S
The phase shift Δa3, which is the change in the phase difference between the wave and the p-wave, and the change in the principal axis direction A are Δ−cos'(β/(1−α2)π/2)v=cos−
'(-α)/2. Since the phase shift Δ and principal axis direction change A are functions of the film thickness and refractive index of the film 2 to be measured, the film thickness and refractive index can be determined from these values. Note that the quarter-wave plate 6 provides a phase difference between the S wave and the p wave,
It has nothing to do with basic operations. The photometric type has the characteristic of being able to perform measurements quickly.

Next, the operation of the extinction type will be explained. In the extinction type, the orientation of the quarter-wave plate 6 is ±45° in FIG. 4, and both the polarizer 4 and the analyzer 8 are rotated to find the point where the output of the semiconductor photodetector 10 becomes minimum. In such a state, the polarizer 4
The light entering the film to be measured 2 becomes elliptical polarized light by the 1/4 wavelength plate 6 and the 1/4 wavelength plate 6, and this elliptical polarized light is reflected by the film to be measured 2 as linearly polarized light. Since the output of the semiconductor photodetector 10 is minimal, the analyzer 8 is located at a position orthogonal to the linearly polarized light.
The angles of the polarizer 4 and analyzer 8 at this time are P and A, respectively.
Then, the phase shift Δ of the reflected light and the principal axis direction change force are Δ−(π/2>−2P A=A. This measurement is characterized by high accuracy.

<Problems to be Solved by the Invention> However, such an ellipsometer has the following drawbacks. FIG. 5 shows the relationship between the thickness of the silica layer on silicon, the phase shift Δ, and the change in principal axis orientation. In FIG. 5, the horizontal axis represents the change in principal axis orientation, the vertical axis represents the phase shift Δ, and the numbers with arrows represent the refractive index of the thin film. In the figure, the refractive index is 1.362, 1.450, 1.4
75 3 types of itl! These are the measurement results of the membrane. Further, the number next to the curve is the phase difference δ caused by one round trip of light in the thin film, and for a thin film with a refractive index of 1.362, 20' corresponds to 308 people. As can be seen from this figure, there are two problems with measurements using an ellipsometer. 1
One is that the measured value has periodicity with a period of δ = 180°,
A plurality of film thicknesses correspond to the same Δ and village. Second, in the vicinity of δ=O0, the curve representing the measured value hardly changes depending on the refractive index of the thin film, and therefore the refractive index cannot be determined from Δ and A.

In order to solve these two problems, attempts have been made to change the angle of incidence of the incident light that enters the thin film, but it is necessary to change the angle of incidence so that the optical axes on the incident side and the reflective side do not shift. Therefore, large-scale equipment is required.

In addition, since the light is roughly divided diagonally with respect to WIM'A, it is necessary to adjust the optical axes on the incident side and reflection side, which makes the operation complicated, and also requires an adjustment groove, which makes the device difficult. It also has the disadvantage of being large.

<Object of the Invention> An object of the invention is to provide an ellipsometer that has a simple configuration and can perform measurements at a plurality of angles of incidence without moving the optical system.

<Means for solving the problem> In order to solve the above problem, in the present invention, light from a light source such as a laser is polarized by a polarizer, and this polarized light is focused by a lens system and directed onto the film to be measured. The reflected light from the film to be measured is separated from the incident light using an optical system such as a half mirror, a specific polarized component is extracted using an analyzer, and the intensity of that light is measured using a photodetector. It is something. Furthermore, the optical axis of the irradiation light applied to the film to be measured is perpendicular to the film to be measured, and the light intensity of a predetermined portion of the output light from the analyzer is measured by the photodetector.

Effect> Since the optical axis of the incident light is perpendicular to the film to be measured,
In particular, it is not necessary to align the optical axes on the incident side and the reflective side. Furthermore, since the light from the light source is converged by the lens system and irradiated onto the film to be measured, measurement of a plurality of incident lights having different incident angles can be performed without changing the arrangement of the optical system.

<Embodiment> FIG. 1 shows an embodiment of an ellipsometer according to the present invention. Note that the same elements as in FIG. 4 are given the same reference numerals, and their explanations will be omitted. In FIG. 1, 20 is a lens into which the output light from the laser light source 3 is input and converts it into parallel light. 21
is a polarizer, into which light parallelized by the lens 10 is incident and polarized. Reference numeral 22 denotes a quarter-wave plate, into which light polarized by the polarizer 21 is input, giving a phase difference between the p-wave and the s-wave. 23 is a half mirror, which transmits the light transmitted through the quarter-wave plate 22 and reflects the light reflected from the film to be measured 2. 24 is a lens, half mirror 2
3 is converged to be measured I! It irradiates M2 and converts the reflected light into parallel light. 25 is an analyzer which decomposes the light reflected by the half mirror 23 into specific polarization components. Reference numeral 26 denotes a pupil that extracts a portion of the transmitted light from the analyzer 25, and the light intensity of the light passing through the pupil 26 is converted into an electrical signal by a photodetector 27 such as a photodiode array. The photodiode arrays are arranged at the same radius from the optical axis, as will be described later.

36 is a shadow that restricts light near the optical axis.

Next, the operation of this embodiment will be explained for the photometric type. Second
Figures <A) and (B) are A-A= and B-B in Figure 1, respectively.
A cross-sectional view of the optical path seen from the direction of the arrow at point ' is shown.

The output light from the laser light source 3 is linearly polarized in the direction of an arrow 28 by a polarizer 21 . This light passes through the half mirror 23 and is irradiated onto the film 2 to be measured. The main azimuth axis of the quarter-wave plate 22 is straight! 9A-A', the quarter-wave plate 22 does not substantially affect the transmitted light, so the light passing through points 29, 30, and 31 in FIG. ,
Only the S-wave contains equal parts of the p-wave component and the S-wave component. That is, in the case of a photometric type, the quarter wavelength plate 22 may not be provided. The light reflected by the film to be measured 2 is converted into parallel light by the lens 24 and reflected by the half mirror 23, but the point 29
The light passing through ~31 respectively points to point 32~ in Figure 2 (B).
Pass through 34. Points 29-31 and 32-34 are points equidistant from the optical axis 0.0', and each photodiode of the photodiode array of the photodetector 27 is located at a portion corresponding to these points 32-34. It is arranged so that Only that portion of this light is extracted by the III 26, and the light intensity is measured by the photodetector 27.

Since the reflectance of the p-wave and the S-wave differs depending on the thickness and refractive index of the film 2 to be measured, the light intensity of the light passing through the point 32, that is, the p-wave, is P p N, its reflectance is Rp, and the light passing through the point 33 is If the light intensity of the light, that is, the S wave, is Ps, and its reflectance is Rs, the light intensity of the incident light at the two points is the same, so the measured object II! The principal axis direction change ψ due to 2 is '4f-tan
-' ((Pp /Po) / (Ps /Po
) )=tan'I(Rp/Rs). In other words, the light intensity at points 32 and 33 is detected by the photodetector 27.
By measuring with , it is possible to determine the principal axis orientation change type. Furthermore, since the light passing through point 31 contains equal parts of p-wave and S-wave components, and their phases are shifted by π/2, the light at point 34 through which the reflected light passes is as shown in Figure 3. It is elliptically polarized as shown in the figure. From the rotation angle of the analyzer 25 at which the light intensity is maximum or minimum at the point 34 shown in FIG. 2 (B) and the light intensity at that time, the length of the long axis of this elliptical light a1 the length of the short axis The bl ellipse azimuth angle Ω is determined, and the reflectance of the p-wave and the S-wave is determined from the measured values at points 32 and 33. The phase shift Δ can be calculated from these values. Since this phase shift Δ and principal axis orientation change flux are expressed as functions of the film thickness and refractive index of the film to be measured 2, the film thickness and refractive index of the film to be measured can be determined from these values. In addition, point 34
The light passing through is measured n times by changing the angle of the analyzer 25,
As in the conventional example, the Fourier coefficients may be obtained to obtain the phase shift Δ and the principal axis orientation change flux. Furthermore, since the light transmitted through the half mirror 23 is converged by the lens 24 and irradiated onto the film to be measured 2, if the distance from the center is changed in FIG. 2(B), the film to be measured II! It is possible to change the angle of incidence of irradiation. That is, the points at distances rl and r2 respectively correspond to the incident angle ψ+s 9'2.

Next, the operation of the extinction type will be explained. The output light from the laser light source 3 is converted into linearly polarized light in the direction of arrow 28 in FIG. 2<A) by a polarizer 21. Next, the principal axis direction of the quarter-wave plate 22 is set to a 45° direction as indicated by the dotted line 35.

Considering the light passing through two points, the light that hits the film to be measured 2 becomes circularly polarized light (the p-wave component and the S-wave component are equal in magnitude and their phase difference is π/2), Since the reflectivity of the S-wave and the P-wave of the film 2 is different, the light passing through the point 32 becomes elliptical polarized light. Polarizer 21 and analysis here? ! 25 is rotated so that the intensity of light passing through point 32 becomes minimum, and if the rotation angles of polarizer 21 and analyzer 25 at that time are P and Δ, phase shift Δ=π/2−2P The principal axis direction change east=A, and the film thickness and refractive index of the film 2 to be measured can be determined from these values.

In the embodiment shown in FIG. 1, the photodetector 27 uses a photodiode array arranged one-dimensionally along the circumferential direction of the optical path, but it is also possible to use a photodiode array arranged two-dimensionally. It's okay. In this way, measurements at different angles of incidence can be performed simultaneously. Alternatively, a single detector may be used and the detector may be moved for measurement.

In addition, a Faraday rotation element is used as a polarizer, and 1/4
An element capable of changing the phase, such as Babinet Soleil, may be used as the wave plate.

Furthermore, in this embodiment, measurement was explained in which only the analyzer 25 was rotated (photometric type) and the polarizer 21 and the analyzer 25 were rotated (extinction type). /4 wavelength plate 22
to be fixed.

■ Rotate the polarizer 21 and 1/4 wavelength plate 22, and
to be fixed.

■Rotate the 1/4 wavelength plate 22 and analyzer 25, and
to be fixed.

■ Rotate the polarizer 21, 1/4 wavelength plate 22 and analyzer 25
to be fixed.

■ Rotate the analyzer 25, polarizer 21 and 1/4 wavelength plate 22
to be fixed.

■ Rotate the polarizer 21, 1/4 wavelength plate 22 and analyzer 25
to be fixed.

■■ Change the azimuth to the quarter wavelength plate 22.

It can also be measured by In this case, ■～■ is 1
The phase difference of the /4 wavelength plate 22 is fixed, and the phase difference can be varied by using Babinet Soleil, BSC, etc. or by using birefringence.

<Effects of the Invention> As specifically explained based on the embodiments above, in this invention, light from a light source is polarized by a polarizer, and this polarized light is converged by a lens system to be almost perpendicular to the film to be measured. irradiate the film to be measured, separate the light reflected from the film to be measured from the incident light using an optical system, and measure the light intensity with a photodetector via an analyzer to determine the film thickness and refractive index of the film to be measured. I made it. Therefore, by changing the radius of the position to be measured with the photodetector, the angle of incidence on the film to be measured can be changed, and there is no need to change the arrangement of the optical system to change the angle of incidence, so the configuration is simple and easy to handle. The measurement time can be shortened.

In addition, since the light is made to enter the film to be measured almost perpendicularly, the incident light and the optical axis of the splitting target coincide, eliminating the need for a mechanism and work for aligning the optical axis, making it possible to downsize. And it is easy to handle.

Furthermore, since lights with different incident angles and lights with different S-wave and P-wave ratios can be detected at the same time, photometry, extinction, and other measurement methods can be used without changing the basic configuration.

[Brief explanation of the drawing]

Figure 1 is a diagram showing one embodiment of the ellipsometer according to the present invention, Figure 20 is a diagram showing the correspondence of measurement points, Figure 3 is a diagram showing the state of elliptical polarization, and Figure 4 is a diagram of a conventional ellipsometer. FIG. 5 is a characteristic curve diagram showing the measurement results. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Thin film to be measured, 3... Laser light source, 20.24... Lens, 21... Polarizer, 22
...1/4 wavelength plate, 23...Half mirror-125.
...analyzer, 2G...pupil, 27...photodetector. Figure 1 DE6REES

Claims (1)

[Scope of Claims] An ellipsometer that irradiates a film to be measured with polarized light and determines the film thickness and refractive index of the film to be measured from the amount of change in polarization of reflected light reflected by the film to be measured, comprising: a light source; a polarizer into which the output light of this light source is incident, a lens system that converges the output light of this polarizer and irradiates it onto the film to be measured, and a light reflected by the film to be measured and the film to be measured. an optical system that separates the irradiation light that is irradiated onto the object, an analyzer into which the reflected light separated by the optical system is input, and a photodetector into which the output light of the analyzer is input,
An ellipsometer characterized in that the optical axis of the irradiated light is made substantially perpendicular to the film to be measured, and the light intensity of a predetermined portion of the output light of the analyzer is measured by the photodetector.