JPH06147838A - Film thickness measuring method - Google Patents

Abstract

PURPOSE:To allow measurement of microarea by storing a noise output signal to be generated from a light receiving element when a stage for mounting a thin film is blackened and then irradiated with white light thereby canceling optical noise in a film thickness measuring apparatus. CONSTITUTION:A stage is covered with a black cloth and a white light source 1 is lighted. Light emitted from the light source 1 passes through a lens 2 and an objective lens 6 and impinges on the black cloth which does not reflect the light toward a photoelectric converting element 14. But flare caused by scattering or reflection at respective parts of an optical system constituting a film thickness measuring apparatus passes through a lens 8, a polarizer, and the like and impinges on the element 14 to produce noise output. The noise output signal is measured and stored in a storing/operating unit 15. A thin film is then mounted at a predetermined position on the stage and the light source 1 is lighted. Subsequently, output signal from the element 14 is measured and fed to the unit 15 where the quantity of light is corrected according to measurements previously stored therein thus producing a corrected quantity of light for each wavelength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a film thickness measuring method,
In particular, the present invention relates to a film thickness measuring method for measuring the film thickness by spectrophotometrically measuring interference light of a thin film such as an oxide film formed on a semiconductor substrate.

[0002]

2. Description of the Related Art In recent years, semiconductor devices such as LSI have been highly integrated. The degree of integration of this semiconductor device depends on the film thickness of a thin film such as an oxide film formed in each manufacturing process, and it is necessary to control the film thickness of the thin film with high accuracy in order to achieve high integration. It is getting higher and higher.
Conventionally, as a film thickness measuring method for performing this kind of optical film thickness measurement, the spectral reflectance of the thin film to be measured is obtained, and the extreme value (maximum value or minimum value) is calculated from the obtained spectral reflectance curve. However, the method of calculating the film thickness value from the wavelength and the refractive index to obtain the extreme value is used.

In this type of film thickness measuring method, white light is applied to a sample through a diaphragm at a position conjugate with the sample surface, and reflected light is guided to a spectroscope through a diaphragm at a position conjugate with the sample surface.
When measuring a fine area, the aperture on the light receiving side is narrowed down.

Before passing through these optical systems, some of the incident light rays are reflected by various optical surfaces and reach the detector, which causes noise. Conventionally, in order to reduce noise, the diaphragm on the irradiation side is made equal to or slightly larger than the diaphragm on the light receiving side to block light not related to measurement, thereby reducing the generation of noise.

[0005]

However, although the above-mentioned method can reduce noise, there is a problem that a correct spectral intensity curve cannot be obtained because the noise cannot be completely erased. Furthermore, there is a problem in that it is impossible to measure while observing the entire surface of the sample because the irradiation area is narrow.

In view of the above problems, the present invention eliminates the noise (flare) of light in the film thickness measuring apparatus and enables the measurement of a minute area while observing the entire surface of the sample. For the purpose of providing.

[0007]

According to the present invention, a thin film to be measured is irradiated with white light, the reflected interference light of the thin film is spectroscopically measured, and its spectral reflectance is measured in a predetermined wavelength range. In the film thickness measuring method for calculating the thickness, the noise output signal generated by the light receiving element is stored in the storage means in advance when white light is radiated with the portion on the stage on which the thin film is placed darkened. Every other time, in the spectrophotometry, the noise output signal is subtracted from the measurement output signal generated by the light receiving element to calculate a predetermined spectrophotometric curve.

A film thickness measuring method for irradiating a thin film to be measured with white light, spectroscopically measuring reflected interference light of the thin film and measuring the spectral reflectance within a predetermined wavelength range to calculate the film thickness of the thin film. In advance, when a white light is radiated in advance in a dark state on the stage where the thin film is placed, the noise output signal generated by the light receiving element and the light amount monitor on the stage where the thin film is placed are monitored. When the white light is radiated in the installed state, the ratio of the monitor output signal generated by the light receiving element is stored in the storage means, and at the time of spectrophotometry, the ratio and the measurement output signal generated by the light receiving element are stored. A film thickness measuring method characterized in that a predetermined spectrophotometric curve is calculated by subtracting a value obtained by multiplying a monitor output signal generated by a light receiving element with a light quantity monitor installed at a position is also desirable.

[0009]

According to the present invention, the noise output signal generated by the light receiving element can be measured and corrected when white light is emitted while the portion on the stage on which the thin film is placed is darkened. .

Further, when white light is radiated with the portion on the stage on which the thin film is placed darkened, the light output monitor is used without directly reducing the noise output signal generated by the light receiving element. It is also possible to measure the noise output signal generated by the light receiving element when the white light is emitted at, and use the ratio of the two to make a correction.

Regarding the correction according to the present invention and the measurement of each output signal value used for the correction, the film thickness measuring device having the function of monitoring the light amount of the light source inside the film thickness measuring device and the film thickness measuring device not having the function. The case will be described separately. If there is no light quantity monitor, first, with the part on the stage on which this thin film is placed dark, turn on the light source, rotate the diffraction grating, and output the noise output signal generated by the light receiving element over all wavelengths. Measure and measure the measured value in this state as I
_flare.i . And remember this. Next, the thin film is set at a predetermined position, the measurement output signal received by the light receiving element is measured, and the measured value in this state is defined as I _measure.i . The light quantity I _i after noise removal and correction is expressed by the following equation. I _i = I _measure.i −I _flare.i . ． ． ． (Equation 1)

In the case of having a light quantity monitor, the noise output signal generated by the light receiving element is measured in a state where the portion on the stage on which the thin film is placed is darkened in advance as in the above, and the measured value in this state is measured. I _flare.i . Then, a light quantity monitor is set at a position on the stage on which the thin film is placed, the measurement output signal received by the light receiving element is measured, and the measurement value in this state is defined as I _monitor0.i . Then, the ratio I _flare.i / I _monitor0.i of both is calculated and stored. When actually measuring the film thickness, set the light intensity monitor to the position on the stage where the thin film is placed immediately before or immediately after the measurement, measure the measurement output signal received by the light receiving element, and measure in this state. Measured value is I _monitor1.i
And Next, the thin film is set at a predetermined position, the measurement output signal received by the light receiving element is measured, and the measured value in this state is defined as I _measure.i . The light quantity I _i after noise removal and correction is expressed by the following equation. I _i = I _measure.i- [I _flare.i / I _monitor0.i ] I _monitor1.i . ． ． ． (Formula 2)

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the film thickness measuring method according to the present invention will be described with reference to FIGS. FIG. 1 is a diagram showing a spectral intensity curve. (A) is a spectral intensity curve when noise is mixed in reflected light from a thin film, (b) is a spectral intensity curve of only noise, and (c) is noise correction. It is a spectrum intensity curve of the reflected light from the thin film removed by. FIG. 2 is a system configuration diagram of an embodiment of a film thickness measuring device to which the film thickness measuring method according to the present invention is applied. A sample S such as a substrate is usually placed on the stage T, and the measurement site is positioned at a predetermined position. The light emitted from the white light source 1 is reflected by the half mirror 7 arranged on the optical axis through the lens 2, the diaphragm 3, the lens 4 and the diaphragm 5, and the sample S mounted on the stage T is used as an objective lens. Illuminate via 6. Then, the light reflected from the sample surface enters the objective lens 6 again,
The light passes through the half mirror 7 and is converged on the entrance slit 11 formed in the light receiving portion of the spectroscope through the lens 8, the diaphragm 9 and the lens 10. The light that has passed through the entrance slit 11 is reflected by the mirror M2, is incident on the diffraction grating 13, is spectrally separated, and is incident on the photoelectric conversion element 14. This photoelectric conversion element 14
Is a CCD, which performs photoelectric conversion and outputs an electric signal corresponding to a predetermined light intensity distribution. After the electric signal is calculated by the storage / calculation device 15, the film thickness value is displayed on the display device 16.

Next, the operation of correcting the measurement in the case where the light quantity monitor is not provided in such a film thickness measuring apparatus will be described. The stage T is covered with a black cloth that absorbs light and does not reflect light, and the white light source 1 is turned on. The light emitted from the white light source 1 enters the lens 2 and illuminates the black cloth through the objective lens 6, but is absorbed by the black cloth and does not reflect, so it again enters the objective lens 6 and enters the lens 8, spectroscope, etc. It does not enter the photoelectric conversion element 14 via the light. However, there is flare light due to scattering and reflection in each part of the optical system that constitutes the film thickness measuring device, and this flare light enters the lens 8 and enters the photoelectric conversion element 14 via a spectroscope or the like to generate noise output. .
The diffraction grating 13 of the spectroscope is rotated to measure the noise output signal generated by the photoelectric conversion element 14 over all wavelengths, and this measured value is input to the storage / calculation device 15 as I _flare.i as a correction value and stored. .

Then, the thin film S is placed at a predetermined position on the stage T, the white light source 1 is turned on, and similarly, the output signal generated by the photoelectric conversion element 14 is measured over all wavelengths, and the measured value I _measure.i Is input to the memory / arithmetic unit 15. The light quantity I _i is stored and calculated by the measured value I _measure.i first.
_Corrected by the measured value I _flare.i stored in
Is calculated for each wavelength. I _i = I _measure.i −I _flare.i . ． ． ． (Equation 1) The film thickness value is calculated by substituting the light quantity I _{i of} each wavelength according to a known calculation formula for calculating the film thickness from the spectral intensity curve stored in the storage / calculation device 15 in advance, and the display device 16 Displayed above. The measured value I _measure.i is shown in Fig. 1.
The measured value I _flare.i corresponds to the spectral intensity curve when noise is mixed in the reflected light from the thin film shown in (a), and the measured value I _flare.i is shown in FIG.
It corresponds to the spectral intensity curve of only the noise shown in, and corresponds to the spectral intensity curve of the reflected light from the thin film in which the noise is corrected and removed.

The magnitude of the noise output changes with temperature, humidity, etc., and the temperature, humidity, etc. of the environment in which the device is placed also changes with time. Therefore, since the correction value changes with time, it is desirable to measure I _flare.i for each film thickness measurement.

Next, the operation of correcting the measurement in the case of having the light quantity monitor in such a film thickness measuring apparatus will be described. The light quantity monitor unit 17 shown in FIG. 2 is compatible with the objective lens 6 and can be attached to the body of the film thickness measuring device.
It comprises a lens 17a and a mirror 17b below it. First, as in the case without the light quantity monitor, the stage T is covered with a black cloth that absorbs light and does not reflect, and the noise output signal generated by the photoelectric conversion element 14 is measured over all wavelengths.
This measured value is stored in I _flare.i by _{inputting it} into the memory / calculation device 15.

Next, the objective lens 6 is removed and replaced with the light quantity monitor unit 17, and the white light source 1 is turned on. The light emitted from the white light source 1 enters the lens 2 and the lens 17
The light is reflected by the mirror 17b via a, enters the lens 17a again, and enters the photoelectric conversion element 14 via the lens 8 and the spectroscope. The output signal generated by the photoelectric conversion element 14 is measured over all wavelengths, and the measured value I _monitor0.i is input to the storage / calculation device 15. And ratio of both I _flare.i / I
_{The monitor0.i} is calculated and stored in the memory / arithmetic unit 15.

When actually measuring the film thickness of the thin film S, the output signal is measured using the light quantity monitor unit 17 prior to the measurement, and the measured value I _monitor1.i is input to the memory / arithmetic unit 15. Remember.

Then, the thin film S is placed at a predetermined position on the stage T, the white light source 1 is turned on, and similarly, the output signal generated by the photoelectric conversion element 14 is measured over all wavelengths, and the measured value I _measure.i Is input to the memory / arithmetic unit 15. The light amount I _i is the measured value I _measure.i and the measured value I measured using the light amount monitor unit 17 prior to the measurement.
Using the _Monitor1.I, corrected by the ratio _I flare.i / I monitor0.i which is stored in the storage-arithmetic unit 15 calculated in advance, is calculated for each wavelength according to Equation 2. I _i = I _measure.i- [I _flare.i / I _monitor0.i ] I _monitor1.i . ． ． ． (Formula 2) The film thickness value is calculated by substituting the light amount I _{i of} each wavelength according to a known calculation formula for calculating the film thickness from the spectral intensity curve stored in the storage / calculation device 15 in advance, and the display device 16 Displayed above.

The magnitude of the noise output changes with temperature, humidity, etc., and the temperature, humidity, etc. of the environment in which the device is placed also changes with time. But the ratio I _flare.i / I
_{Since monitor0.i} is the ratio of the measured values at the same time, the change due to the influence of the environment is small. Also, it is _necessary to cover the stage T with a black cloth that absorbs light and does not reflect it, and prepare a black cloth each _time to measure I _flare.i. However, the light quantity monitor unit 17 that can be easily compatible with the objective lens 6 is used. It's not annoying to use.

According to the present embodiment, it is not necessary to make the iris 3 and 5 on the irradiation side equal to or slightly larger than the iris 9 on the light receiving side in order to reduce noise so as to shield light not involved in the measurement. The size of each diaphragm can be set according to the object of measurement. Even when measuring a fine region, it is not necessary to narrow the diaphragm on the light receiving side.

[0023]

According to the present invention, it is possible to eliminate the noise (flare) of light in the film thickness measuring device and to measure a minute area while observing the entire surface of the sample.

[Brief description of drawings]

FIG. 1A is a diagram showing a spectral intensity curve when noise is mixed in reflected light from a thin film. (B) is a diagram showing a spectral intensity curve of only noise. (C) is a diagram showing a spectral intensity curve of reflected light from a thin film in which noise is corrected and removed.

FIG. 2 is a system diagram of a film thickness measuring device to which the present invention is applied.

[Explanation of symbols for main parts]

 1 ... White light source 2 ... Lens 3 ... Aperture 4 ... Lens 5 ... Aperture 6 ... Objective lens 7 ... Half mirror 8 ... Lens 9 ... Aperture 10. ..Lens 11 ... Aperture 12 ... Reflecting mirror 13 ... Spectroscope 14 ... Detector 15 ... Memory / arithmetic device 16 ... Display device

Claims (2)

[Claims] 1. A film thickness measuring method for measuring a film thickness by spectrophotometrically measuring interference reflected light of white light applied to a thin film, wherein a portion on a stage on which the thin film is mounted is previously darkened. When the white light is emitted in this state, the noise output signal generated by the light receiving element is stored in the storage means, and during spectrophotometry, the noise output signal is subtracted from the measurement output signal generated by the light receiving element to obtain the spectrum. A method for measuring film thickness, which comprises calculating reflectance and measuring film thickness. 2. A film thickness measuring method for measuring a film thickness by spectrophotometrically measuring interference reflected light of white light applied to a thin film, wherein a portion on a stage on which the thin film is placed is previously darkened. When the white light is radiated in the above state, the noise output signal generated by the light receiving element and the white light radiated in the state where the light amount monitor is installed in the portion on the stage where the thin film is placed, the light receiving element The ratio of the monitor output signal generated by the above is stored in the storage means, and the measurement output signal generated by the light receiving element during spectrophotometry is used to measure the ratio and the light receiving monitor with the light quantity monitor installed at the position. A film thickness measuring method, wherein a film thickness is measured by calculating a spectral reflectance by subtracting a value obtained by multiplying a monitor output signal generated by an element.