JPH06147837A - Method and equipment for measuring film thickness optically - Google Patents

Abstract

PURPOSE:To eliminate measurement error of film thickness due to incident angle by storing a table of phase difference and amplitude reflectance ratio calculated from specific complex indexes of thin film and substrate for combinations of arbitrary film thickness and incident angles. CONSTITUTION:Light emitted from a semiconductor laser light source 1 transmits through a polarizing plate 2 and impinges, as a linearly polarized light, on a thin film formed on a substrate 3. Light reflected on the thin film transmits through a polarizing plate 4 and impinges on a light receiving element 5 where the intensity of light is converted into an electric signal. Rotational angle of the polarizing plate 4 is read out by means of an encoder 4a and converted into an electric angle. Output waveform being obtained through independent rotation of the polarizing plate 4 is delivered, in the form of an optical intensity signal from the light receiving element 5 and a rotational angle signal from an encoder 4a, to a signal processor 6 where the signals are subjected to Fourier transform thus determining phase difference and amplitude reflectance ratio. A retrieval unit 8 then retrieves a film thickness corresponding to the phase difference and amplitude reflectance ratio obtained from a table stored in a memory 7 and displays the film thickness thus retrieved at a display section 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical film thickness measuring device for optically measuring the film thickness of a thin film formed on the surface of a substrate.

[0002]

2. Description of the Related Art Semiconductor devices such as LSI have been highly integrated, and film thickness control in each process of the manufacturing process is very important for improving the yield.
Further, the gate oxide film tends to be further thinned as the integration becomes higher. Recently, it is required to accurately measure the film thickness of an extremely thin film as compared with a wavelength on the order of 1 to 100 nm. Ellipsometry is a method used to measure the thickness of such a thin film.

When light of wavelength λ is incident on the thin film formed on the surface of the substrate, reflection is repeated at the upper and lower boundary surfaces of the thin film.
As shown in FIG. 1, when the thin film M is formed on the substrate 3, the p component (parallel to the incident surface) and the s component (parallel to the incident surface) at the boundary surface G1 between the air and the thin film and the boundary surface G2 between the thin film and the substrate ( Amplitude reflectances (perpendicular to the plane of incidence) are r _1p , r _1s , r _2p , r _2s
Assuming that the phase difference when light makes one round trip in the thin film M is δ, the amplitude reflectances R _p and R _s of the p component and the s component of the entire film are given by the equation (1), as is well known. The phase difference δ is given by the equation (2). The relationship between the incident angle and the refraction angle is given by equation (3). However, n and d are the refractive index and film thickness of the thin film, φ,
φ'is the angle of incidence and the angle of refraction, respectively. Where Δ _p ,
Δ _s is the amount of phase jump due to reflection.

When polarized light having a complex amplitude of E _p ⁱ and E _s ⁱ respectively enters the surface of the thin film M, the p component and the s component E _p ^r , E _s ^r of the reflected light are If E _p ^r = R _p · E _p ⁱ E _s ^r = R _s · E _s ⁱ and the complex amplitude ratios of the p component and s component of the incident light and the reflected light are X _i and X _r , respectively, then X _r = [R _p / R _s ] .X _i .

In the polarization analysis method, the amplitude reflectance ratio R _p / R _s is obtained by knowing the incident polarized light state X _i and the reflected polarized light state X _r, and the refractive index n and the film thickness d of the thin film M are numerically analyzed. Is a method of asking for. From the formula (1), the amplitude reflectance ratio is R _p / R _s = r _p / r _s · exp [i ( _Δp −Δ _s )] = tan Ψ · exp (iΔ) ・ ・ ・ (4) (tan Ψ = R _p / r _s , Δ = Δ _p −Δ _s )). The refractive index n and the film thickness d of the thin film M have been obtained by measuring Ψ and Δ by various conventionally known methods.

Although it is clear that δ in the equation (2) depends on the refractive index of the thin film M, the amplitude reflectance of the boundary surface used in the equation (1) also depends on the refractive index and the absorption coefficient of the film. is doing. Generally, the refractive index of a film is unknown as well as the film thickness, so the refractive index must be measured. However, in polarization analysis, two measured values, Ψ and Δ, can be obtained, so the film thickness and refractive index are obtained. be able to.

[0007]

Here, a film structure in a semiconductor manufacturing process, for example, SiO ₂ on a Si substrate,
For Si ₃ N ₄ ... On a Si substrate, the refractive index and absorption coefficient of the Si substrate and the film are generally known. Therefore, at a given wavelength, if the film structure is known, the amplitude reflectance is uniquely determined, and the parameter that determines the amplitude reflectances R _p and R _s is only the film thickness. It is apparent from the equation (2) that the phase difference δ is determined by the refraction angle calculated from the refractive index and the incident angle by the equation (3).

In the conventional film thickness measurement, the incident angle is set to a constant angle, and the film thickness and the refractive index are determined without changing. For this reason, there has been a problem that the measurement value largely changes due to an error in the setting of the incident angle and a fluctuation occurring during the measurement. Especially, in the film thickness measurement of a film extremely thin as compared with a wavelength of 1 to 100 nm, this variation causes a great decrease in measurement accuracy.

In view of the above problems, it is an object of the present invention to eliminate a film thickness measurement error caused by an incident angle.

[0010]

According to the present invention, the p-component of reflected light and the s component are defined by assuming that the complex refractive index of the thin film and the substrate is known for any combination of the arbitrary thickness of the thin film and the arbitrary incident angle.
The phase difference due to the reflection of the component and the amplitude reflectance ratio of the p component and the s component are calculated to create a list, which is stored in the storage means. After that, the phase difference and the amplitude reflectance ratio are measured by a known polarization analysis method, and the film thickness and the incident angle corresponding to the measured values are searched and obtained in the list stored in the storage means. did. Regarding light of a predetermined wavelength that is obliquely incident on and reflected by a thin film formed on the surface of a substrate having a predetermined complex refractive index, the phase difference and the amplitude reflectance ratio are further determined by measuring the rotation angle and the light intensity of the analyzer. The required film thickness measuring device was constructed as a preferable one. Further, the film thickness measuring device for obtaining the phase difference and the amplitude reflectance ratio by measuring the rotation angle of the compensator and the light intensity is preferably constructed.

[0011]

[Function] The film thickness and the incident angle are obtained by searching based on the measured values by the ellipsometry method in the list calculated in advance. According to a preferred apparatus, the film thickness can be obtained by obtaining the phase difference and the amplitude reflectance ratio by measuring the rotation angle and light intensity of the analyzer or compensator.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention is shown in FIGS. 1, 2 and 3
Will be described. FIG. 1 is a diagram for explaining the situation of a light beam incident on a thin film on a substrate, FIG. 2 is a configuration diagram of this embodiment, and FIG. 3 is a block diagram of this embodiment. Light emitted from a semiconductor laser light source 1 having a wavelength of 670 nm passes through the polarizing plate 2 to become linearly polarized light, and is incident on the thin film M on the substrate 3 at an incident angle φ. The incident angle φ does not need to be strictly adjusted, and may be set within the angle range described in the table below. The reflected light passes through the rotating polarizing plate 4 and enters the light receiving element 5, and the intensity of the light is converted into an electric signal. The rotation angle of the polarizing plate 4 is read by the encoder 4a provided on the support ring of the polarizing plate 4 and converted into an electric signal. An output waveform obtained by rotating the polarizing plate 4 independently is input to the signal processing device 6 as a light intensity signal from the light receiving element 5 and a rotation angle signal from the encoder 4a, and Fourier transform is performed here to perform phase difference and amplitude. Calculate the reflectance ratio. After that, the retrieval device 8 retrieves the film thickness d corresponding to the phase difference Δ and the amplitude reflectance ratio Ψ obtained from the list stored in the storage device 7, and displays the film thickness on the display unit 9.

The table gives the refractive index and the absorption coefficient of the substrate and the thin film as predetermined numerical values, the film thickness d is in the range of 0 to 400 nm in steps of 10 nm, and the incident angle φ is in the range of 10 to 45 °. Ψ and Δ are calculated in advance from the above equation in steps of 10 ″ and prepared in advance. Taking a SiO ₂ film on a Si substrate as an example, the refractive index and the absorption coefficient are Then, Ψ and Δ are calculated, and then a table showing a part of them is created.

[0013]

[Table 1]

D (nm) θ (degree) Ψ (degree) Δ (degree) ············ 0 15 ° 30′00 ″ 10 15 ° 30′00 ″ 20 15 ° 30′00 ″ 30 15 ° 30′00 ″ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ 100 15 ° 30'00 ″ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・・ ・ 0 15 ° 30'10 ″ 10 15 ° 30′10 ″ 20 15 ° 30′10 ″ 30 15 ° 30′10 ″ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ 100 15 ° 30 '10 "·························· For the other combinations of the substrate and the thin film, a table can be similarly created.

A second embodiment of the present invention will be described with reference to FIG. FIG. 4 is a block diagram of the second embodiment. The light emitted from the light source 1 passes through the polarizing plate 2 to become linearly polarized light, and then passes through the quarter-wave plate 2a as a compensator and then enters the thin film M. The reflected light passes through the rotating polarizing plate 4,
Upon entering the light receiving element 5, the intensity of light is converted into an electric signal. The extinction direction is obtained by rotating the quarter-wave plate 2a. In the present embodiment, instead of rotating the polarizing plate 4, the quarter wavelength plate 2a is rotated to obtain the phase difference and the amplitude reflectance ratio, and detailed description thereof will be omitted.

According to the embodiment, since the film thickness can be measured without depending on the incident angle, the device can be easily adjusted and an unstable sample stage can be used. Therefore, the film thickness measuring device can be manufactured at low cost. it can.

It is also possible to measure in a situation where the normal direction of the thin film is not fixed like oil on the water surface.

In the embodiment, a table is prepared for a combination of an arbitrary film thickness of the thin film and an arbitrary incident angle. However, if the arbitrary combination cannot be stored due to the limitation of the capacity of the storage means, the substrate and It may be stored in a limited range for each combination of thin films, or a part of the combination of the thickness of the thin film and the incident angle may be stored. The unstored portion may be obtained by interpolation or linear or nonlinear programming. When the constraint is large, it may be obtained by an analytical method.

[0019]

As described above, according to the present invention, the film thickness and the incident angle are preliminarily calculated, and the film thickness and the incident angle are obtained by searching based on the measurement value by the ellipsometry method. There is no measurement error.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a situation of a light beam incident on a thin film on a substrate.

FIG. 2 is a configuration diagram of a first embodiment.

FIG. 3 is a block diagram of a first embodiment.

FIG. 4 is a configuration diagram of a second embodiment.

[Explanation of symbols]

 M thin film 1 light source 2 polarizing plate 2a quarter wave plate 3 substrate 4 polarizing plate 4a encoder 5 light receiving element 6 signal processing device 7 storage device 8 search device 9 display unit

Claims (3)

[Claims] 1. A method for measuring the film thickness of a thin film based on light having a predetermined wavelength that is obliquely incident on and reflected by a thin film formed on a surface of a substrate having a predetermined complex refractive index. Phase difference due to reflection of p component and s component of reflected light and amplitude reflectance of p component and s component calculated by a predetermined complex refractive index of the thin film and the substrate with respect to a combination of film thickness and arbitrary incident angle. A table consisting of calculated values of the ratio is stored, and the film thickness and the incident angle corresponding to the phase difference and the amplitude reflectance ratio calculated using the information obtained by the ellipsometry are searched from the table. And then measuring the film thickness of the thin film. 2. A polarizer inserted in an optical path of an incident light beam of a predetermined wavelength obliquely incident on a thin film formed on a substrate surface having a predetermined complex refractive index, and a polarizer inserted in an optical path of a reflected light beam from the thin film. In a film thickness measuring device having a rotatable analyzer and a light receiving element that receives a light beam that has passed through the analyzer and outputs a light intensity signal, an arbitrary film thickness of the thin film and an arbitrary incident angle A list of the phase difference due to the reflection of the p component and s component of the reflected light and the calculated value of the amplitude reflectance ratio of the p component and the s component calculated by the predetermined complex refractive index of the thin film and the substrate for the combination. A storage unit that stores a table, a calculation unit that calculates the phase difference and the amplitude reflectance ratio by using the rotation angle information of the analyzer and the value of the light intensity signal, and a film thickness corresponding to the calculation value. Search for the incident angle from the list above A film thickness measuring device comprising means and display means for displaying the retrieved value. 3. A polarizer inserted in the optical path of an incident light beam of a predetermined wavelength obliquely incident on a thin film formed on a substrate surface having a predetermined complex refractive index, and a polarizer inserted in the optical path of a reflected light beam from the thin film. Rotatable analyzer, a light receiving element that receives a light beam that has passed through the analyzer and outputs a light intensity signal, and a rotatable quadrant that is inserted in the optical path of either the incident light beam or the reflected light beam. In a film thickness measuring device having a one-wave plate, a p component of reflected light calculated by a predetermined complex refractive index of the thin film and the substrate for a combination of an arbitrary film thickness of the thin film and an arbitrary incident angle. And a storage means for storing a list consisting of a phase difference due to reflection of the s component and a calculated value of the amplitude reflectance ratio of the p component and the s component, rotation angle information of the quarter wavelength plate, and the light intensity signal. Value of the phase difference and the amplitude reflectance ratio A film thickness measuring apparatus, comprising: a calculating unit that calculates a value, a searching unit that searches the list for a film thickness and an incident angle corresponding to the calculated value, and a display unit that displays the searched value.