KR100992839B1 - Spectroscopic Ellipsometer with a Microspot Module - Google Patents

Abstract

The present invention provides a spectroscopic ellipsometer comprising a light source, a polarizer, a focusing optical system, an analyzer, and a spectroscope, wherein the focusing optical system has a concave mirror in which a reflection surface is a concave mirror and a convex mirror in which a through hole is formed in the center thereof. As an inverse Cassegrain type optical system composed of a main diameter having an effective diameter smaller than the minor diameter is used as.

According to the present invention as described above, it can be driven in a wide wavelength band from the ultraviolet region to the near infrared region and can implement a small beam diameter of about 10 ㎛ size at the sample position, and the polarization state is not changed because the optical axis is not twisted while using the reflector Since it does not change, there is an effect that can significantly improve the measurement accuracy.

Spectroscopic ellipsometer, microspot, resolution, reflective, mirror, mirror, focusing optical system, vacuum ultraviolet light.

Description

Spectroscopic Ellipsometer with a Microspot Module

The present invention relates to a spectroscopic ellipsometer, and more particularly, by implementing a focused optical system using an inverse casee grain structure optical system, a small beam spot having a size of about 10 μm can be used over a wide wavelength band from vacuum ultraviolet to near infrared region. It relates to a micro spot spectroscopic ellipsometer having.

Ellipsometry is a method that obtains the optical thickness and complex regulation ratio of the sample by injecting light having a specific polarization state into the sample and analyzing the changed polarization state of the reflected light. It is very useful for the measurement.

An ellipsometer is an apparatus that controls, measures, and analyzes the polarization state of light using elliptic analysis technology. A general ellipsometer includes a light source, a polarization control module, a sample, an analyzer module, and a light detector. An ellipsometer driven over a wide wavelength band is called a spectroscopic ellipsometer.

On the other hand, the semiconductor industry is currently progressing from a process having a line width of 60 nm to a process having a line width of 45 nm. Accordingly, the thickness of the thin film is also reduced, and the demand for improving the spatial resolution by one step is increasing in the spectroscopic ellipsometer, which is essentially used for monitoring the thickness of the thin film in the semiconductor field.

In a spectroscopic ellipsometer, a focusing optical system for focusing light onto a sample is used. There are many cases of implementing a focusing optical system using one or more reflectors. In the case of using a reflector, the polarization state is changed while reflecting light, which may cause a measurement error. A polarization control module is usually disposed between the reflector and a sample, and an example thereof is disclosed in Japanese Patent Laid-Open No. 2005-3666. have. 1 shows a spectroscopic ellipsometer according to Japanese Patent Laid-Open No. 2005-3666. Referring to FIG. 1, the light emitted from the light source 31 is focused on the sample after being reflected by the reflector 333. It can be seen that the polarizer 321 is disposed between the reflector 333 and the sample. However, in the non-axis two-reflection boundary configuration using two concave mirrors, there is no rotational symmetry with respect to the optical axis, so a change in polarization and non-axis aberration occur. In order to reduce this, the incident angle to the reflector should be made as small as possible, so that the distance between the major and minor diameters, the minor diameter, and the sample surface becomes longer, and thus, the polarization change is not sufficiently suppressed. In addition, the spectroscopic ellipsometer has a structural problem that the polarizer should be installed between the secondary diameter and the sample, so that the distance between the reflector and the sample is further increased, which causes the spot size to increase in proportion.

This is called a microspot ellipsometer when a focused optical system is included that makes the beam size very small at the sample location. As described above, it is structurally complicated to use a non-axis 2 reflector as the focused optical system. Since there is a limit to the reduction, the conventional microspot spectroscopic ellipsometer mainly reduces the spot size by using a focusing lens. Among the methods using the focusing lens, a method using a lens having a high numerical aperture as in US Patent No. 5,596,411, a method of having a small projection angle as in US Patent Application No. 10 / 319,189, US Patent No. 6,829,049 As in the arc, there is a method using a synthetic lens. However, U. S. Patent No. 5,596, 411 and U. S. Patent Application No. 10/319, 189 are not applicable to conventional spectroscopic ellipsometers that incline light at an angle of incidence around 75 degrees. U. S. Patent No. 5,596, 411 The disadvantage is that the structure is very complicated.

In particular, the focusing lens has a relatively easy focusing point when using monochromatic light. Therefore, it is not difficult to design and manufacture a microspot ellipsometer. However, in a spectroscopic ellipsometer using a wide band of wavelengths, chromatic aberration generated by the change of wavelength Therefore, even when the optimized focusing optical system is applied, it is not easy to make the beam spot at the sample position very small. For example, the beam diameter of a commercial spectroscopic ellipsometer operating in the wavelength range of 200-800 nm is about 25 um.

In a focused optical system using a lens, two or more lenses may be used to correct chromatic aberration for use in a wide wavelength band. When the lens is used, light passing through the central axis can be used to realize a coaxial focused optical system, and it is easy to manufacture and align the optical system, but it uses chromatic aberration and spherical lens due to wavelength dispersion of lens material. There are spherical aberrations that follow, and aberrations such as astigmatism and coma. In addition, there is no material having a high transmittance from the vacuum ultraviolet region to the near infrared region, and a material having a considerable transmittance from the ultraviolet to the near infrared region is limited to several kinds of special optical crystals. As a result, it is very difficult to produce a beam spot having a diameter of several tens of micrometers at a sample position over a wide wavelength band from a vacuum ultraviolet region to a near infrared region.

On the other hand, reducing the beam size at the sample position in the spectroscopic ellipsometer is significantly different from reducing the beam size in a general optical instrument such as an optical microscope. In the case of general optical measuring devices, only the amount of light is measured and light is incident perpendicularly to the sample plane. Therefore, the transmitted or reflected light measured vertically proceeds perpendicularly to the sample plane to design an optical system that conforms to the diffraction limit even when using a focused optical system having a relatively simple structure. I can make it. In addition, since these optical systems are mostly used for imaging in the visible light region, vacuum ultraviolet rays are not used.

However, in the case of an ellipsometer, the distance between the optical system and the sample should be sufficiently secured because the light is obliquely incident on the sample surface. That is, when obliquely incident at an angle of incidence of about 75 degrees, which is the angle most used in an ellipsometer, the numerical aperture (NA) of the focusing optical system must be smaller than 0.2 so that the sample surface is not interfered by the optical system. In this sense, US Patent No. 5,596,411 and US Patent Application No. 10 / 319,189 are not appropriate.

In addition, since ellipsometers control and measure polarization, in addition to reducing the beam size, the effect of the focused optical system on the polarization state should be minimized. In the spectroscopic ellipsometer, it is advantageous to use light of a wide wavelength from vacuum ultraviolet to near infrared. In summary, the spectroscopic ellipsometer required for the next-generation semiconductor process is driven at an angle of incidence of about 75 degrees and has a beam diameter of about 10 µm at a sample position and a numerical aperture of 0.2 or less over a wavelength range of 150-800 nm. It is necessary to have a focused optical system that does not change the

The present invention has been made to solve the above problems, an object of the present invention is to operate in a wide wavelength band from the ultraviolet region to the near infrared region by using a focused optical system of the reverse casee grain structure and the size of about 10 ㎛ in the sample position It is to provide a reflective focusing optical system that realizes a small beam diameter of and a spectroscopic ellipsometer using the same.

Another object of the present invention is to provide a reflection type focusing optical system and a spectroscopic ellipsometer using the reflector so that the polarization state is not changed because the optical axis is not twisted while using the reflector.

According to an aspect of the present invention for achieving the above object, in the spectroscopic ellipsometer comprising a light source, a polarizer, a focusing optical system, an analyzer and a spectroscope, the focusing optical system is a concave mirror form the center of the reflecting surface There is provided a micro spot spectroscopic ellipsometer, wherein an inverse cassgrain type optical system is used, which consists of a main diameter having a through hole formed therein and a reflecting surface in the form of a convex mirror, the main diameter having an effective diameter smaller than the sub diameter.

Here, the radius of curvature of the major diameter is 35 to 45 mm, the radius of curvature of the minor diameter is 100 to 120 mm, the distance between the main diameter and the minor diameter is 60 to 80 mm, the distance between the minor diameter and the focal point is preferably 130 to 150 mm.

In addition, the effective diameter of the main diameter is more preferably 6 ~ 10 mm, the effective diameter of the minor diameter is 35 ~ 45 mm.

Moreover, it is more preferable that the diameter of the through hole of the said secondary diameter is 8-12 mm.

Using the focusing optical system according to the present invention as described above, has the following advantages.

First, since it is composed of a reflector only, chromatic aberration does not occur, it is possible to provide a high resolution optical system without being limited to the wavelength band, and it is applicable to a wide wavelength band covering vacuum ultraviolet rays and near infrared rays because there is no absorption of light by transmission.

Second, since it has a rotationally symmetrical structure around the optical axis, it can be utilized in the same structure as a conventional refractive type optical system using a lens, and it is possible to configure a coaxial optical system so that the optical alignment is simple and does not affect the polarization state.

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

2 is a schematic view for explaining the structure of the focusing optical system according to the present invention, Figure 3 shows the path of light in the focusing optical system according to the actual design specifications of the present invention.

As shown in FIG. 2, in the focusing optical system according to the present invention, the reflection surface is a concave mirror, and the sub-diameter 20 having a through hole H in the center thereof, and the reflection surface is a convex mirror, are formed in the convex mirror. Inverse Cassegrain type optical system composed of the main diameter 10 having a small effective diameter is used.

The reverse casee grain optical system is a two-reflection optical system composed of a small convex major diameter 10 and a large concave minor diameter 20 as shown in FIG. 2. Although the optical system has the best aberration characteristics among the two reflection systems composed only of spherical surfaces, it has a disadvantage in that the minor diameter is larger than the main diameter, so it is difficult to use as a large diameter optical system, but is suitable for a small diameter optical system in which the diameter of the light beam is small.

As shown in FIG. 3, the light generated from the light source passes through the through hole H formed in the secondary diameter 20, and then is primarily reflected in the convex surface of the primary diameter 10, and then, at the concave surface of the secondary diameter 20. It is a structure that is focused on a sample after being differentially reflected. Here, if the curvature radius, separation distance, effective diameter, etc. of the main diameter 10 and the minor diameter 20 are precisely controlled, the reflected light 2 may be focused at a point on the sample.

The third order aberrations of the inverse casee grain optics are detailed in S. Rosin's paper [1] in 1968, and the conditions under which the third order spherical aberration and the third order coma are corrected when the object is at infinity in a spherical second reflection optical system. silver

일 때가 실용성 있는 해이며, 이 때 a₁= 4.236이다. 수학식 1의 조건은 3차 수차를 근사한 Seidel 수차를 보정하여 얻은 결과이므로, 실제 광학계에서 최적화된 값은 이와 약간 다른 값을 가지게 된다. Given by In the above formula, m ₂ is the horizontal magnification of the minor diameter, and a ₁ is the ratio of the height h _{1 at} which the ambient light enters the main diameter and the height h ₂ at which the minor diameter is incident. Out of 2 years

Is a practical solution, where a ₁ = 4.236. Since the condition of Equation 1 is obtained by correcting Seidel aberration approximating the third order aberration, the optimized value in the actual optical system has a slightly different value.

In the present invention, an optimal embodiment was derived using a numerical analysis technique. Table 1 shows the design specifications of the optimum embodiment having an effective focal length of 33 mm and an entrance pupil diameter of 8 mm. The shape of the optical system according to this design is shown in FIG.

Table 1 shows that the numerical aperture is 0.115 for the infinite object point, and the diameter of the beam considering diffraction is 9.5 μm at wavelength 800 nm and 2.1 μm at wavelength 200 nm. In the focused optical system of the present invention, a ₁ = 4.387, m ₂ = -1.597, and it can be seen that the result is almost close compared with the approximation obtained through Equation 1 considering only the third order aberration.

The aberration of the focused optical system of the present invention is very well corrected since the aberration is about 1/50 of the wavelength even at 200 nm, which is 0.02λ (rms, λ = 200 nm) within a field of view (FOV) of 2 °. (Average is judged to be excellent if aberration is 1/10 or less of wavelength).

Among the light incident through the through hole H of the secondary diameter 20, the light near the center of the optical axis is first reflected by the principal diameter 10 and then the second reflected light from the secondary diameter 20 passes again through the principal diameter and is shielded. However, there is a disadvantage in that the central luminous flux cannot be used. However, in the elliptical spectrometer, the incident light intensity is increased by the reduced luminous flux.

Incident angles of the light incident on the main mirror 10 and the light incident on the secondary mirror 20 to each mirror surface are 11 degrees or less, and both the reflective surface of the major mirror 10 and the reflective surface of the secondary mirror 20 have high reflectances. Therefore, the difference between the reflection coefficients of p-wave and s-wave according to the reflection of incident light is very small. In particular, the present optical system has rotational symmetry, and thus the difference in the reflection coefficients of the p wave and the s wave cancels each other within the first approximation, thereby making it possible to ignore a change in polarization state due to reflection.

Figure 4 shows the overall structure of a spectroscopic ellipsometer to which the focusing optical system according to the present invention is applied.

As shown in FIG. 4, the spectroscopic ellipsometer according to the present invention is completely identical to the structure of a conventional spectroscopic ellipsometer except for the structure of the focusing optical system 400.

That is, light emitted from the light source 100 is transmitted through the optical fiber, and the light emitted from the optical fiber is converted into parallel light using a focusing lens (not shown) and then passes through the polarization control module 200. Light incident in parallel through the polarization control module 200 creates a micro spot within 10 μm on the sample surface through the focusing optical system 400 of the present invention.

The light reflected and emitted from the sample surface is applied to the reflector module 500 by applying the reflective focusing optical system 400 of the present invention symmetrically once again to make parallel light. The light passing through the analyzer module 500 is incident to the optical fiber by using a focusing lens (not shown), and the other end of the optical fiber is incident to the spectrometer 700 which divides the light by wavelength and measures the intensity of light for each wavelength. Done.

 The focusing optical system 400 according to the present invention does not change the polarization state and is the same as the existing spectroscopic ellipsometer except for the focusing optical system, and thus the principle and driving method of the spectroscopic ellipsometer equipped with the focusing optical system are the same as the conventional method.

5 is a graph showing the shape and wavefront aberration of the incident pupil, and FIG. 6 shows the jumper function for the wavelength 546.8 nm.

In FIG. 5, as described above, the center portion of the incident pupil may generate an area where light does not reach, and the shape and wavefront aberration of the incident pupil of FIG. 5 and the point spread function of FIG. 6 may have diffraction limits. It shows the imaging characteristics of. The light has wave characteristics, and due to the diffraction phenomenon, the spot cannot be made smaller than the diffraction limit in the optical system.

Although the present invention has been described in connection with the above-mentioned preferred embodiments, it is possible to make various modifications or variations without departing from the spirit and scope of the invention. Accordingly, the appended claims are intended to cover such modifications or changes as fall within the scope of the invention.

1 is a view showing the structure of a spectroscopic ellipsometer using a conventional reflector.

2 is a schematic view for explaining the structure of a focusing optical system according to the present invention.

Figure 3 illustrates the shape and path of light of the optimized optical system of the present invention.

Figure 4 shows the overall structure of a spectroscopic ellipsometer to which the focusing optical system according to the present invention is applied.

5 is a graph showing the shape and wavefront aberration of the incident pupil.

<Description of main drawing code>

10: principal diameter 20: minor diameter

100: light source 200: polarization control module

300: motor and encoder 400: focusing optical system

500: light detector 600: motor and encoder

700: Spectroscope

Claims (4)

A light source for generating light; A polarization control module for polarizing the light generated by the light source; A first focusing optical system for focusing the light in the polarization state to a sample; A sample which receives a focused light and converts and reflects a polarization state; A second focusing optical system positioned symmetrically with the first focusing optical system and converting light reflected and emitted from the surface of the sample into parallel light; An analyzer module for detecting parallel light converted by the second focusing optical system; And After spectroscopy the detected light for each wavelength, and includes a spectrometer for measuring the amount of spectroscopic light, The first and second converging optical systems have an inverse casing grain type consisting of a sub-mirror having a reflecting surface in the form of a concave mirror and a through-hole formed in the center thereof, and a reflecting surface of a convex mirror in a main diameter having an effective diameter smaller than the sub-caliber. A micro spot spectroscopic ellipsometer, wherein an optical system is used. The method of claim 1, The radius of curvature of the major diameter is 35 ~ 45mm, the radius of curvature of the minor diameter is 100 ~ 120mm, the distance between the major diameter and the minor diameter is 60 ~ 80mm, the distance between the minor diameter and the focal point is a micro-spot spectroscopic ellipsometer. The method of claim 2, The effective diameter of the major diameter is 6 ~ 10mm, the effective diameter of the minor diameter is a micro-spot spectroscopic ellipsometer, characterized in that. The method of claim 2, Micro-spot spectroscopic ellipsometer, characterized in that the aperture of the through hole of the secondary diameter is 8 ~ 12mm.

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