JP2875746B2 - Interference film thickness meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference film for measuring the thickness of a transparent or translucent thin film applied to the surface of an object by tilting a wavefront reflected from the upper surface and the lower surface of the film and causing them to interfere with each other. It concerns the thickness gauge.

[0002]

2. Description of the Related Art An interference film thickness meter known as a prior art separates reflected waves from the upper and lower surfaces into P-polarized light and S-polarized light, and tilts each of them by a Wollaston prism to form an interval between interference fringes. Was measured by measuring the distance between the peak positions of the light intensity distribution on one line of the video camera (FIG. 4A).

[0003]

On the other hand, at present, 1 μm
It is required to measure the following thin films with a resolution of about 0.01 μm. However, the conventional interference film thickness meter is used to measure a relatively thick film having a film thickness of about 1 μm to 30 μm, and the resolution is limited to 0.1 μm. This is because when measuring the film thickness of the thin film, the interference fringes of the reflected waves of the upper surface and the lower surface overlap the reference interference fringes of the upper surface reflected waves and the lower surface reflected waves (FIG. 4B). The reason is that it is difficult to measure the interval between the reference interference fringe and the thickness interference fringe (the interference fringes of the vertically reflected wavefront) in the state.

On the other hand, in the interference film thickness meter, since interference fringes are formed in the Wollaston prism, the measurement becomes impossible due to scratches on the prism, adhesion of dust, and the like. There is also a problem that it takes time for processes and cleaning, and the cost rises.

The present invention has been made in view of the above-mentioned problems of the prior art, and its main purpose is to form a thin film having a thickness of 1 μm or less such that interference fringes do not overlap each other on an image sensor.
An object of the present invention is to provide an interference film thickness meter which can measure at a resolution of about 1 μm and is easy to handle and maintain.

[0006]

According to the present invention, a film formed directly on a substrate or through another film is irradiated with illumination light and reflected from the upper and lower surfaces of the film. The light is divided into a P-polarized wave and an S-polarized wave by a birefringent prism, tilted in different directions, and the film thickness is measured by measuring the interval of the film thickness interference fringe generated at the intersection of the wavefronts. In the film thickness meter , the surface without the target film
At the intersection of the P-polarized wavefront and the S-polarized wavefront reflected from
Means for storing the reference interference fringe only waveform that, to form an image including the thickness fringe by the light receiving unit, the interference fringe image obtained by imaging the waveform of the reference interference fringe in the light receiving means Means for subtracting from the waveform, and means for obtaining the film thickness of the film from the interval between the peak positions of the pair of interference fringe waveforms after subtracting the waveform of the reference interference fringe. Achieved by providing.

[0007]

As described above, the waveform of the reference interference fringe is stored in advance, and the waveform of the reference interference fringe is obtained from the measured interference fringe waveform.
And only the film thickness interference fringes of the reflected wavefront from the upper surface and the reflected wavefront from the lower surface are left on both sides of the position where the reference interference fringes existed, and the peak position interval of these interference fringes is measured to be １ ／.
Accordingly, the thickness of a thin film of 1 μm or less can be easily and accurately measured. At this time, for example, when a Wollaston prism is used, the apex angle is reduced and the reference interference fringes between the reflected wavefronts from the upper surface and between the reflected wavefronts from the lower surface, and the thickness interference fringes of the reflected wavefronts from the upper and lower surfaces are reduced. It is even better to increase the distance between them. Further, even if the birefringent prism has a flaw, dust or the like, the influence thereof is reduced or completely removed by the subtraction processing.

[0008]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a preferred embodiment of the present invention. FIG. 1 is a diagram showing a relationship between an optical element and a processing block of an interference film thickness meter to which the present invention is applied. Illumination light guided from the light emitting device via the optical fiber is reflected from the light projecting device 1 to the half mirror 2 and illuminates the object to be measured via the objective lens 3.
The light reflected on the object to be measured passes through the half mirror 2 and enters the Wollaston prism 6 as a birefringent prism via the imaging lens 4 and the polarizing filter 5.
The light is split into a polarized wave and an S-polarized wave, tilted, and enters the CCD camera 9 via the polarizing filter 7 and the relay lens 8.

When the light enters the CCD camera 9, the light is photoelectrically converted, amplified by an amplifier / filter block 10, filtered, and displayed as an image on a video monitor 11.
(FIG. 2) and the image capturing block 12
Is subjected to image capture and A / D conversion. Here, in the present embodiment, the CCD camera 9 has an element of 768 dots × 494 lines, of which an image of 512 dots × 128 lines at the center is defined as a measurement area.

The A / D-converted image of 512 dots × 128 lines is input to the computer 13, and is averaged as described later, the subtraction of the reference interference fringe waveform, the detection of the peak of the film thickness interference fringe, the calculation of the distance between peaks and the film. The thickness is calculated, and the result is output from an output device (not shown).

Hereinafter, the processing after light including interference fringes is incident on the CCD camera 9 will be described in detail. First, a raw output waveform from the CCD camera 9 as shown in FIG. 3A is amplified by an amplifier / filter block 10, filtered, and a signal waveform from each line is converted into a waveform as shown in FIG. To be shaped. Then, the image capturing block 12 cuts data of 128 dots at both ends from the necessary signal waveforms of the central 128 lines, and trims the waveform of each line into a waveform as shown in FIG.

Next, the signal waveforms from the 128 lines are averaged in the computer 13, and the signal waveform obtained as a result of performing the above measurement on the object to be measured on which no film is formed in advance, that is, only the reference interference fringes is obtained. Is reduced (FIG. 3 (d)) . Later, it obtains each waveform at the peak of the film thickness fringe waveform A, B in FIG. 3 (d), CCD peak value interval
Is calculated from the number of elements. The film thickness d is given by the following equation, where y is the peak interval, n is the refractive index of the film, and α is the inclination angle of the P-polarized wavefront and the S-polarized wavefront by the Wollaston prism.

[0013]

## EQU1 ## d = y · sin α / 2n

In this way, even with a film thickness of 1 μm or less, which is impossible with a conventional interference film thickness meter, high-precision measurement with a resolution of about 0.01 μm and a reproducibility of about ± 0.02 μm is possible. Becomes Here, even if the Wollaston prism and other optical systems have scratches, dust and the like, the signal waveforms of many lines are averaged, and the same optical system including the same Wollaston prism has no film in advance. The effect is reduced by subtracting the signal waveform having only the reference interference fringes obtained by the film thickness measurement performed on the measurement object from the signal waveform having the film thickness interference fringes obtained by the actual film thickness measurement. Or completely removed.

In this embodiment, a Wollaston prism is used as the birefringent prism.
Gran Thomson prism, lotion prism, etc.
It goes without saying that it is only necessary to be able to separate and incline the P-polarized light and the S-polarized light. In addition, it is difficult to remove an error due to a temperature drift in an image pickup tube such as a pidicon or a new vidone, and this also causes a problem in that the resolution cannot be improved. It was resolved.

As another embodiment of the present invention, a line sensor having a small element pitch can be used instead of a CCD camera. In this case, the resolution can be improved by increasing the magnification at which the interference fringes are combined on the line sensor.

When a line sensor is used, the position on the interference fringe to be scanned is constant, so that the Wollaston prism and the Wollaston prism on the optical system are liable to be affected (noise) such as scratches and dust adhesion. In the same manner as in the above embodiment, a signal waveform having only a reference interference fringe obtained by a film thickness measurement performed in advance on a film-free measurement object using the same optical system including the same Wollaston prism is measured by actual film thickness measurement. The influence is reduced or completely eliminated by subtracting from the signal waveform having the film thickness interference fringe obtained by the above. Further, similarly to the averaging process by the CCD camera, the above influence can be reduced by arranging a plurality of line sensors and measuring at the same time, and averaging each output waveform. Needless to say, these effects can be further eliminated by combining them. In addition, if scanning of the line sensor is performed a plurality of times at staggered times, random noise and fixed noise can be simultaneously removed.

[0018]

As is apparent from the above description, according to the interference film thickness meter according to the present invention, the intersection of the P-polarized wave front and the S-polarized wave front reflected from the surface in the absence of the target film in advance. The waveform of the reference interference fringe generated in is stored, and the waveform of the reference interference fringe is subtracted from the measured interference fringe waveform .
Only the film thickness interference fringes of the reflected wavefront from the upper surface and the reflected wavefront from the lower surface are left on both sides of the position where the reference interference fringes existed, and when the peak position interval of these interference fringes is measured, the thin film is 1 μm or less. However, the thickness can be easily and accurately measured. At this time, for example, when a Wollaston prism is used, the apex angle is reduced and the reference interference fringes between the reflected wavefronts from the upper surface and between the reflected wavefronts from the lower surface, and the thickness interference fringes of the reflected wavefronts from the upper and lower surfaces are reduced. It is even better to increase the distance between them. Further, even if the birefringent prism has a flaw, dust or the like, the influence thereof is reduced or completely removed by the subtraction processing. Accordingly, handling and maintenance are facilitated, and maintenance costs are reduced. Furthermore, if the fringe intervals are measured on a large number of lines and the average value is taken, σ becomes 1/10 statistically when 100 lines are measured. This can be expected, that is, the resolution can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between an optical element and a processing block of an interference film thickness meter to which the present invention is applied.

FIG. 2 is a diagram showing an image image of a CCD camera.

FIGS. 3A to 3D are diagrams illustrating a procedure for processing a signal waveform when an image including interference fringes generated by a Wollaston prism is received by a CCD camera.

FIG. 4 (a) is a diagram for explaining the principle of film thickness measurement using a conventional interference film thickness meter, and FIG. 4 (b) shows a problem when a thin film thickness is measured using a conventional interference film thickness meter. The figure similar to (a) shown.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Projector 2 Half mirror 3 Objective lens 4 Imaging lens 5 Polarizing filter 6 Wollaston prism 7 Polarizing filter 8 Relay lens 9 CCD camera 10 Amplifier / filter block 11 Video monitor 12 Image capturing block 13 Computer

Claims (1)

(57) [Claims] An illumination light is applied to a film formed directly on a substrate or through another film, and reflected light from the upper and lower surfaces of the film is converted into a P-polarized wave and an S-polarized wave by a birefringent prism. In the interference film thickness meter for measuring the film thickness by measuring the interval of the film thickness interference fringes generated at the intersection of their wavefronts by tilting them in different directions, the target film is not included in advance. P reflected from the surface in the state
Means for storing only the waveform of the reference interference fringes generated at the intersection of the polarization wavefront and the S-polarization wavefront; and forming an image including the film thickness interference fringes by a light receiving means, and changing the waveform of the reference interference fringes to Means for subtracting from the interference fringe waveform of the image formed by the light receiving means, and means for determining the film thickness of the film from the interval between the peak positions of the pair of interference fringe waveforms after subtracting the waveform of the reference interference fringe. An interference film thickness meter comprising: