TWI598565B - Method of measuring thickness of thin film - Google Patents

Description

Method of measuring film thickness

The present invention relates to a method of measuring film thickness, and more particularly to a method of measuring film thickness, which allows for convenient and rapid measurement of film thickness.

With the rapid development of technology in all recent industrial fields, microfabrication has been required for semiconductors, microelectromechanical systems (MEMS), flat panel displays, optical components, and the like, and currently requires ultra-precision fabrication on a nanometer scale. In addition, the required manufacturing patterns have become complicated, and therefore, it is increasingly important to measure the thickness of the fine film.

Devices that have been widely used to measure film thickness and reflectivity are based on reflectometry. Reflectors, broadly referred to as thin-film layer measurement systems, are non-contact and non-destructive measuring devices that measure multilayer film properties and directly measure target objects without specific preparation or processing.

A conventional reflectometer performs scanning of as many wavelengths as possible using light from a white light source at predetermined intervals, and obtains light intensity signals at respective wavelengths via a black and white camera. Then, the plurality of light intensity signals obtained at the respective wavelengths are synthesized to plot a reflectance versus the overall spectrum, and thereby the film thickness can be measured based on the reflectance chart.

In the conventional method of measuring the thickness, the reflectance of the film to be tested is measured, and then the thickness is read from a table in which the thickness corresponding to the reflectance measured using the standard sample is first included. In other words, the reflectance of the film to be tested is measured, and then the thickness is read from a look-up table based on a standard sample.

However, there is a problem with the conventional method of measuring the thickness, that is, the error generated when collecting the standard sample data is applied to all the substrates actually produced, and whenever the process is changed, the reflectance of the thickness and the new standard sample must be established immediately. database.

Accordingly, the present invention is directed to solving the aforementioned problems, and an aspect of the present invention will provide a method of measuring film thickness in which the extinction coefficient of a film is determined and the phase is directly calculated from the reflectance without constructing a standard sample. A database of thickness versus reflectivity, which facilitates and quickly measures film thickness.

According to an embodiment of the present invention, a method for measuring a film thickness is provided, the method comprising: obtaining a light intensity signal of the film and obtaining a reflectance of the film based on the light intensity signal; assuming that the refractive index of the air and the refractive index of the film are a real number, and assuming that the refractive index of the substrate formed by the film is a complex number; converting the reflectance into a normal reflectance; determining whether the error is within a preset error range, regardless of the maximum value of the converted normal reflectance and the first reference value The first difference of the difference is also a second difference indicating a difference between the minimum value of the converted normal reflectance and the second reference value a value; if the first difference or the second difference is within the error range, the phase is captured; the phase is recovered; and the film thickness is calculated based on the recovered phase.

After determining the error, if it is determined that the first difference or the second difference is out of the error range, the method may further include compensating the normal reflectance such that the first difference or the second difference may be within the error range.

The first reference value may be 1 and the second reference value may be -1.

1‧‧‧air

2‧‧‧film

3‧‧‧Substrate

4‧‧‧ objects

100‧‧‧Reflometer

110‧‧‧White light source

120‧‧‧beam splitter

130‧‧‧ camera

140‧‧‧Image Processor

The above and/or other aspects of the present invention will be apparent from the following description of the exemplary embodiments in conjunction with the accompanying drawings in which: FIG. A reflectometer for implementing the method of measuring film thickness according to the present invention; and FIG. 2 is a flow chart sequentially illustrating a method for measuring film thickness according to an embodiment of the present invention; and FIG. 3 is a view, The figure shows a film measured by the method of Fig. 2; Fig. 4 is a view showing the operation of obtaining and converting the reflectance in the method of Fig. 2; Fig. 5 is a view, which is a view The operation of compensating for the normal reflectance in the method of Fig. 2; and Fig. 6 is a view illustrating the operation of drawing the phase in Fig. 2.

Hereinafter, an embodiment of a method of measuring a film thickness according to the present invention will be described in detail by referring to the accompanying drawings.

1 is a view illustrating a reflectometer for implementing a method of measuring a film thickness according to the present invention; and FIG. 2 is a flow chart sequentially illustrating a measurement for use according to an embodiment of the present invention. Method of film thickness; FIG. 3 is a view showing a film measured by the method of FIG. 2; and FIG. 4 is a view showing obtaining and converting reflectance in the method of FIG. Fig. 5 is a view illustrating an operation of compensating for the regular reflectance in the method of Fig. 2; and Fig. 6 is a view illustrating the operation of capturing the phase in Fig. 2.

Referring to FIG. 1, a general reflectometer for implementing the method of measuring film thickness includes a white light source 110, a beam splitter 120, a camera 130, and an image processor 140. In this exemplary embodiment, description will be made by way of example: The article 4 to be measured by the reflectometer 100 in this exemplary embodiment includes a substrate 3 and a film 2 coated onto the substrate 3. Further, the top of the film 2 is exposed to the air 1.

The white light source 110 is a light source for emitting white light, and may include a halogen lamp, a light emitting diode (LED), or the like. On the back side of the white light source 110, a collimating lens or the like for collimating white light emitted from the white light source 110 may be disposed.

The beam splitter 120 emits light from the white light source 110 to the object. Further, light reflected from the object 4 enters the camera 130 via the beam splitter 120, and will be described below.

The camera 130 is arranged above the object 4 and obtains a light intensity signal of the light reflected from the object 4.

In general, camera 130 includes a charge coupled device (CCD) camera having a suitable number of pixels for the area to be measured. In front of the camera 130, a concentrating lens can be arranged to converge the incident light from the beam splitter 120.

Image processor 140 calculates a reflectance chart relative to the overall spectrum.

Then, if the light intensity signal, which is compared with the overall spectrum of the light reflected from the object 4, is divided by the light intensity signal as compared with the overall spectrum of the light incident on the object 4, the reflectance can be plotted as compared with the overall spectrum. chart.

After the aforementioned reflectance chart is generated at each position on the film 2, the thickness of the film 2 can be measured at a corresponding position based on the reflectance chart. Since the method of measuring the thickness of the film 2 based on the reflectance chart is well known to those of ordinary skill in the art, a detailed description thereof will be omitted.

Hereinafter, a method of measuring the film thickness via the reflectometer 100 will be described in accordance with an embodiment of the present invention.

Referring to FIGS. 2 to 5, a method of measuring film thickness according to an embodiment of the present invention includes the following operations: obtaining reflectance, assuming refraction, performing conversion, determining error, compensating for regular reflectance, drawing phase, and restoring Phase, and calculate thickness.

In the operation step of obtaining the reflectance, the light intensity signal of the film 2 is obtained, and then the reflectance R of the film 2 is determined based on the light intensity signal.

Here, the light intensity signal, which is compared with the overall spectrum of the light incident on the object 4, by dividing the light intensity signal relative to the overall spectrum of the light reflected from the object 4, can be determined relative to the overall spectrum. Reflectivity R.

The obtained reflectance R is represented by the following equation [1].

Where R is the reflectance, r ₁₂ is the Fresnel reflection coefficient of the interface between the air 1 and the film 2, r ₂₃ is the Fresnel reflection coefficient of the interface between the film 2 and the substrate 3, and β is light The phase variation when passing through the film 2.

β is represented by the following equation [2].

Where d is the thickness of the film 2, λ is the wavelength of light, N ₂ is the degree of refraction of the film 2, and θ ₂ is the angle of refraction of the interface between the air 1 and the film 2.

In the operation of assuming the degree of refraction, it is assumed that the degree of refraction N _{1 of the} air 1 and the degree of refraction N ₂ of the film ₂ are real numbers, and it is assumed that the degree of refraction N ₃ of the substrate 3 formed by the film 2 is a complex number.

Each degree of refraction can be represented by N = n - ik (where n is the index of refraction and k is the extinction coefficient). Assuming that the degree of refraction of the air 1 and the degree of refraction of the film 2 are real numbers, this means that the extinction coefficient (k) in the refractive index N ₂ of the air 1 and the refractive index N ₂ of the film 2 is 0, and the refraction of the substrate 3 The extinction coefficient (k) in degree N ₃ is not zero.

In the operation of performing the conversion, the reflectance R is converted into the normal reflectance R'.

In the equation [1], the Fresnel reflection coefficient r _{12 at the} interface between the air 1 and the film 2 and the Fresnel reflection coefficient r _{23 at the} interface between the film 2 and the substrate 3 are respectively obtained by the following equations [3] and [4] ] indicated.

Wherein N ₁ is the degree of refraction of air, N ₂ is the degree of refraction of the film, N ₃ is the degree of refraction of the substrate, θ ₁ is the incident angle of the interface between the air 1 and the film 2, and θ ₂ is the air 1 and the film 2 The angle of refraction of the interface, and θ ₃ is the angle of refraction of the interface between the film 2 and the substrate 3.

Since it is assumed that the degree of refraction of air N ₁ and the degree of refraction N ₂ of the film are real numbers, r ₁₂ is a real number. Also, since it is assumed that the refractive index N ₃ of the substrate is a complex number, r ₂₃ is a complex number. Therefore, r _{23 is} represented by the following equation [5].

r ₂₃ = A + iB ...[5]

If equation [5] and equation [6] below are applied to equation [1], the reflectivity R is expressed by the following equation [7]: exp(-i2β)=cos(2β)-i ̇sin(2β)... [6]

If "Acos(2β)+Bsin(2β)=X" is substituted into equation [7], then X is represented by the following equation [8]:

Then, the normal reflectance R' is obtained by the following expression [9], and the expression [9] uses A and B in the equation [5] and X in the equation [8].

In the operation of determining the error, determining whether the first difference is within a preset error range, the first difference indicating between the maximum value 11 of the normal reflectance R' converted in the converting operation step and the first reference value difference.

Referring to Fig. 4, the normal reflectance R' obtained by the equation [8] is represented in the form of a graph. In the foregoing operation of assuming the degree of refraction, if it is assumed that the refractive index N ₂ of the film is a real number, the maximum value 11 of the normal reflectance R' is approximately the first reference value, for example, approximately 1.

If the first difference indicating the difference between the maximum value 11 of the normal reflectance and the first reference value is within the preset error range, it is assumed that the refractive index N ₂ of the film is a real number.

At the same time, the first difference indicating the difference between the maximum value 11 of the normal reflectance and the first reference value is not determined, but the minimum value 12 and the second indicating the normal reflectance R' converted in the conversion operation step can be determined. Whether the second difference of the difference between the reference values is within a preset error range. If the second difference is within the preset error range, it is assumed that the refractive index N ₂ of the film is a real number. In this case, the second reference value is -1.

In the operation of compensating for the normal reflectance, if it is determined that the first difference or the second difference exceeds the error range, the normal reflectance is compensated so that the first difference or the second difference is within the error range.

Fig. 5 illustrates the normal reflectance 21 when the first difference or the second difference exceeds the error range, and the normal reflectance R' when the first difference or the second difference is compensated to be within the error range.

If the first difference or the second difference exceeds the error range, the normal reflectance R' is compensated by Fourier transform, wavelet transform or the like such that the first difference or the second difference can be within the error range. In other words, the normal reflectance 21 when the first difference or the second difference exceeds the error range is corrected to the normal reflectance R' when the first difference or the second difference is within the error range, so that the regular reflection The maximum value 11 of the rate may approximate the first reference value, or the minimum value 12 of the normal reflectance may approximate the second reference value.

In the operation step of capturing the phase, if it is not necessary to compensate the normal reflectance or after the compensation of the normal reflectance, the first difference or the second difference is within the error range, the phase β is taken.

The phase β extracted from the equation [8] using the trigonometric function is expressed by the following equation [9].

In the operation of restoring the phase, the phase captured in the phase capture operation is restored.

Due to the phase ambiguity, the phase 31 captured in the phase capture operation differs from the actual phase 32 by an integer multiple of π. This phase is ambiguously counted and the phase 31 is recovered to the actual phase 32.

In the thickness calculation operation, the thickness d of the film 2 is calculated based on the phase 32 restored in the phase recovery operation. Here, if the actual phase 32 and the theoretical phase of the phase blur are considered to be substituted into the equation and then solved by the linear equation, it is possible to calculate the film thickness d.

Since the operation of restoring the phase and calculating the thickness is generally used by a person of ordinary skill in the art using a reflectometer for measuring film thickness, a detailed description of such operations will be omitted.

As described above, according to the embodiment of the present invention, the method of measuring the film thickness can determine the film extinction coefficient without constructing a database on the relationship between the thickness and the reflectance of the standard sample, and directly calculate the phase based on the reflectance, thereby facilitating and Film thickness is measured quickly.

According to an embodiment of the present invention, the method of measuring the thickness of a film can measure the film thickness conveniently and quickly.

Although a few exemplary embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that The embodiments are subject to change on the basis of the principles and spirit of the invention, and the scope of the invention is defined in the scope of the appended claims and their equivalents.

Claims (2)

A method for measuring the thickness of a film, the method comprising the steps of: obtaining a light intensity signal of a film, and obtaining a reflectance of the film based on the light intensity signal; assuming the degree of refraction of the air and the degree of refraction of the film a real number, and the refractive index of one of the substrates formed by the film is a complex number; the reflectance is converted into a normal reflectance; and whether an error is within a preset error range, the error is a first difference or a second difference, the first difference indicating a difference between a maximum value of the converted normal reflectance and a first reference value, the second difference indicating a minimum value of the converted normal reflectance and a a difference between the second reference values; if the first difference or the second difference is determined to be outside the error range, the normal reflectance is compensated such that the first difference or the second difference can be in the Within the error range; if the first difference or the second difference is within the error range, a phase is retrieved; the captured phase is restored; and the thickness of the film is calculated based on the restored phase. The method of claim 1, wherein the first reference value is 1 and the second reference value is -1.