JPH0331705A - Method for measuring film thickness in optical system - Google Patents

Abstract

PURPOSE:To improve the accuracy of measurement by using parallel beams when the film thickness of a thin film is measured by means of an optical system film thickness measuring instrument utilizing interference stripes and using concentrated beams obtd. from high magnification of an objective lens to the thin film for the measurement of references. CONSTITUTION:A photosensitive thin film is formed by applying organic high conductor to an aluminium element tube and the film thickness is measured from the interference stripes of spectral reflection spectra by the optical system film thickness measuring instrument using parallel beams. In the measurement of references, the photosensitive thin film itself is measured by an objective lens of 50-magnification, and in the measurement of the film thickness, the wavelength area is divided into three areas and the film thickness values of respective areas are measured by the objective lens of 5-magnification. Consequently, a spectral reflection spectrum generated at the time of using the reference as the base of the thin film can be prevented from generating a peak shifting phenomenon, the difference between the film thickness values due to the difference of the wavelengths and that of measuring positions of the references can be reduced, a more stable value is obtained and the measuring accuracy can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for measuring film thickness in an optical system, and more particularly to a method for measuring film thickness in an optical system that uses reflection interference to measure the thickness of a thin film.

[Conventional technology]

Conventionally, when measuring the thickness of a thin film using an optical film thickness measurement device that uses reflective interference, the reference measurement is performed using parallel light at low magnification, using the substrate under the thin film as a reference. . Therefore, although the interference fringes of the spectral reflection spectrum undergo a peak shift, the effect of this peak shift is ignored in calculating the film thickness value.

FIG. 3 is a spectral reflection spectrum diagram for thin film thickness measurement to explain an example of such a conventional method.

As shown in Figure 3, in conventional film thickness measurement, the aluminum substrate under the photoreceptor thin film, which is the object to be measured, is used as a reference and measured with a 5x objective lens. The film thickness was measured using a 5x objective lens. The measurement results are shown in Table 1, and (
a) and (b) are two film thickness measurement points, and ■ and ■ are two positions on the aluminum substrate at the time of reference measurement. Furthermore, in Table 1, 1■-■1 indicates the difference in the film thickness value due to the difference in the reference measurement position, and dmax-
dmin indicates the difference between the highest and lowest film thickness values in three wavelength ranges.

In Table 1, the difference in film thickness values 1■-■1 due to differences in reference measurement positions and the difference dmax-dmin between the maximum and minimum film thickness values in the three wavelength ranges each have large variations. The absolute value is also large. Further, there is a large variation in the film thickness value due to the difference in wavelength, and the film thickness value changes greatly no matter where the reference is set.

The reason for the above is that a peak shift occurs in FIG. 3, and conventionally, the film thickness value is measured while ignoring the influence of this peak shift.

FIG. 4 is a spectral reflection spectrum diagram for thin film thickness measurement to explain another conventional example.

As shown in FIG. 4, this film thickness measurement is a thin film thickness measurement using a different object to be measured than the above-mentioned object, and is carried out under the same conditions as the above-mentioned measurement conditions. Table 2 shows the measurement results for the case in which an aluminum substrate was used. In other words, it is understood that the change in film thickness value is large due to a difference in wavelength, and that the change in film thickness value is large depending on where in the thin film itself is used as a reference.

The above is because a peak shift similar to that in FIG. 3 occurs in FIG. 4.

Table 1 [Problems to be Solved by the Invention] In the conventional thin film thickness measurement method described above, the substrate under the thin film is measured with parallel light for reference measurement, so interference fringes in film thickness measurement peak. Since this has caused a shift, the difference in film thickness values depending on the wavelength range becomes large.
This method has the disadvantage of increasing measurement errors.

An object of the present invention is to provide a method for measuring film thickness in an optical system that can prevent such a peak shift of interference fringes, reduce differences in film thickness values depending on wavelength range, and improve measurement accuracy. .

[Means to solve the problem]

The film thickness measurement method in the optical system of the present invention uses an optical system film thickness measurement device to measure the film thickness of a thin film using interference fringes, and the thin film itself is used as a reference measurement with a high magnification of the objective lens. While the measurement is performed using concentrated light, the film thickness is also configured to be measured using parallel light.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a spectral reflection spectrum diagram for thin film thickness measurement for explaining the first embodiment of the present invention.

As shown in FIG. 1, this example uses a photoreceptor thin film coated with an organic photoconductor on an aluminum tube, similar to the object to be measured explained in FIG. 3 and Table 1, and an optical film thickness measuring device. uses a well-known measuring device.

First, the film thickness is calculated using the following equation (1) from the interference fringes of the spectral reflection spectrum obtained by an optical film thickness measuring device.

Here, d is the film thickness value, N is the number of bright and dark stripes in the spectral reflection spectrum, n is the refractive index of the thin film, and λ is the wavelength.

Next, as measurement conditions, the reference measurement is performed by measuring the photoreceptor thin film itself of the object to be measured using a 50x objective lens, and the film thickness is measured using a 5x objective lens. On the other hand, there are three wavelength ranges (λmin to λnax), namely 640 (nm) to 720 (nm).

650 (nm) ~ 730 Cntn), 660 (nm)
) to 740 (nm), and the film thickness value is determined in each wavelength region. There were two film thickness measurement points, which correspond to (a) and (b) in Table 3 showing the measurement results. There are also two reference measurement points, which correspond to ■ and ■ in Table 3.

As a result of such thin film thickness measurement, Table 1 shows the difference in film thickness value 1■-■1 due to the difference in the reference measurement position and the difference dm between the highest and lowest film thickness values in three types of wavelength ranges.
ax-dmin is shown.

Comparing the results in Table 3 with the results in Table 1 above, we find that in this example, the difference in film thickness values due to the difference in the reference measurement position is 1■-■1, and the difference in film thickness values in the three types of wavelength regions. Both the difference dmax-dmin between the highest value and the lowest value can be reduced. In addition, variations in film thickness values due to differences in wavelength are small, and changes in film thickness values can be made small no matter where the reference is set, so that -layer stability can be achieved and measurement accuracy can be improved.

The reason for the above is that no peak shift appears in FIG.

FIG. 2 is a spectral reflection spectrum diagram of thin film thickness measurement for explaining a second embodiment of the present invention.

As shown in FIG. 2, in this example, the object to be measured was a photoreceptor thin film obtained by coating an aluminum tube with an organic photoconductor that was different from the object to be measured in the first example. The film thickness is measured using the photoreceptor thin film itself of the object to be measured as a reference under the same conditions as in the first embodiment. The present example corresponds to the above-mentioned conventional FIG. 4 and Table 2, and the measurement results are shown in Table 4. Furthermore, (a),
(b), ■, and ■ are the same as in the first embodiment described above.

Comparing the results in Table 4 with the results in Table 2 above, it is found that, similar to the first example, this example also shows that the change in film thickness due to the difference in wavelength is small. It is possible to measure the thickness of a thin film with high accuracy and stability, without causing any changes in the thickness value.

The above can also be understood from the fact that no peak shift appears in FIG.

〔Effect of the invention〕

As explained above, in the method for measuring film thickness in an optical system of the present invention, when measuring the film thickness of a thin film using an optical film thickness measuring device, the reference is the thin film itself of the object to be measured, and the high magnification of the objective lens is used. By measuring with , it is possible to prevent the peak shift of interference fringes in the spectral reflection spectrum that occurs when the reference is taken from the substrate under the thin film, and to prevent differences in film thickness values due to differences in wavelength range and reference measurement position. This has the effect of making it smaller, obtaining more stable values, and increasing measurement accuracy.

[Brief explanation of drawings]

Fig. 1 is a spectral reflection spectrum diagram for thin film thickness measurement to explain the first embodiment of the present invention, and Fig. 2 is a spectral reflection spectrum diagram for thin film thickness measurement to explain the second embodiment of the present invention. Reflection spectrum diagrams, FIGS. 3 and 4 are spectral reflection spectrum diagrams of thin film thickness measurement for explaining one conventional example and another example, respectively.

Claims (1)

[Claims] When measuring the thickness of a thin film using interference fringes using an optical film thickness measurement device, the reference measurement uses focused light with a high magnification of the objective lens to measure the thin film itself. A method for measuring film thickness in an optical system, characterized in that measurement is performed using parallel light.