JPH10311708A - Film thickness meter of interference type - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a film thickness of interference type capable of accurately measuring simultaneously the film thicknesses of a thin film formed on the substrate of a wafer, etc., and capable of measuring the film thickness distribution, even where these film thicknesses exist plurally within ranges of micron and submicron order. SOLUTION: Light is emitted from a light source 10 on a thin film having a plurality of film thicknesses formed on the silicon substrate of a wafer 50, the interfering light reflected on the front and rear of the film thickness is separated into the light of each wavelength by a spectral prism 16. The intensity of light separated into each wavelength thereof is made photoelectric transfer into an electric signal by a detector 18, and the light intensity distribution of the interfering light is detected. This light intensity distribution of the interfering light is made Fourier transform into a frequency component by a fast Fourier transform(FFT) 20, and the frequency band of light intensity distribution of the interfering light corresponding to each film thickness is elected by a filter 22, on the basis of the distribution of frequency component thereof. The frequency component is made inverse Fourier transform into the light intensity distribution by an inverse FFT 24 per elected frequency band, and the film thickness is found from the light intensity distribution by a fitting 26.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to interferometric film thickness meter, in particular oxide film formed on a silicon substrate (SiO ₂₎ thin film thickness and a metal film such as (W, Al, Cu, etc.) The present invention relates to an interference-type film thickness meter that optically measures a step.

[0002]

2. Description of the Related Art In recent years, with the demand for higher integration of semiconductor devices, the density and the number of wafers have been increased. As a result, the depth of focus of an exposure apparatus for forming a circuit wiring pattern on a wafer becomes shallower, and a high degree of flatness of the wafer is required. A chemical mechanical polishing (CMP) apparatus is known as an apparatus for realizing this high-precision planarization.
The chemical mechanical polishing apparatus polishes the wafer surface by pressing the surface of the wafer against a polishing cloth while supplying a chemical abrasive to the polishing surface.

As a method of controlling the polishing amount, for example, when an insulating film (SiO ₂ ) is formed on a wafer substrate, the thickness of the insulating film is measured by an interference type film thickness meter or the like. There are known ways to do this. The interference type film thickness meter irradiates the insulating film on the wafer surface with light, reflects the light on the front surface and the back surface of the insulating film, and detects the light intensity of each wavelength of the interference light. Then, the film thickness is determined from the light intensity (for example, peak wavelength) of each wavelength.

[0004]

However, an aluminum wiring pattern is formed on a substrate of a wafer by a thin film forming apparatus (such as a CVD apparatus), and an insulating film (SiO 2) is formed on the wiring.
_{In the case where 2} ) is formed, a wiring pattern exists in the range of microns and submicrons due to recent high integration, and a plurality of film thicknesses may exist in this range. In this case, when trying to measure the film thickness of the insulating film by the above-mentioned conventional interference type film thickness meter, in the spot of the light irradiated on the insulating film, various steps of the wiring pattern are included, and as a result, a plurality of film thicknesses are obtained. Only the average film thickness could be measured. For this reason, there was a problem that it was not possible to detect whether or not the surface was flattened at the portion where the wiring pattern was formed.

The present invention has been made in view of such circumstances, and even when a plurality of thin films formed on a substrate such as a wafer have a thickness in the order of microns or submicrons, An object of the present invention is to provide an interference-type film thickness meter capable of simultaneously measuring these film thicknesses with high accuracy and measuring a film thickness distribution.

[0006]

According to the present invention, there is provided an interference type film thickness meter for measuring a plurality of film thicknesses of a thin film formed on a substrate. Light irradiating means for irradiating light, light reflected on the front surface of the thin film of the wafer, and spectral means for separating interference light of light reflected on the back surface of the thin film into each wavelength, and each wavelength separated by the spectral means. Photoelectric conversion of the light intensity into an electrical signal, photoelectric conversion means for detecting a light intensity distribution with respect to the wavelength of the interference light, Fourier transform means for Fourier transforming the light intensity distribution detected by the photoelectric means into frequency components, A filter for selecting a frequency band of the light intensity distribution of the interference light with each film thickness based on the distribution of the frequency components obtained by the Fourier transform means, and extracting the frequency component for each of the selected frequency bands. Inverse Fourier transform means for inversely Fourier transforming the frequency components extracted for each frequency band by the filter means into a light intensity distribution for each frequency band, and for each frequency band obtained by the inverse Fourier transform means And a film thickness calculating means for obtaining a plurality of film thicknesses of the thin film of the wafer based on the light intensity distribution of each frequency band based on the light intensity distribution.

According to the present invention, a thin film having a plurality of film thicknesses formed on a substrate is irradiated with light, and the interference light reflected on the front and rear surfaces of the thin film is split into light of each wavelength. Then, the separated light intensities of the respective wavelengths are photoelectrically converted into electric signals, and the light intensity distribution of the interference light is detected. Next, the light intensity distribution of the interference light is Fourier-transformed into frequency components, and a frequency band of the light intensity distribution of the interference light with each film thickness is selected based on the distribution of the frequency components. Then, the frequency components are inversely Fourier-transformed into the light intensity distribution for each of the selected frequency bands, and the film thickness is obtained from the light intensity distribution.

Thus, even when there are a plurality of film thicknesses of the thin film formed on the substrate, it is possible to simultaneously measure each of these film thicknesses with high accuracy.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a configuration diagram showing one embodiment of an interference type film thickness meter according to the present invention. Hereinafter, a case where the thickness of the insulating film (SiO ₂ ) formed on the substrate of the wafer is measured will be described.

As shown in FIG. 1, light is emitted from a light source 10 such as a halogen lamp or a xenon lamp. The light emitted from the light source 10 is guided to a lens 14 via a beam splitter 12, condensed to an optimum spot by the lens 14, and irradiated on the surface of the wafer 50. As shown in FIG. 2, on a surface of a silicon substrate 51 of a wafer 50, an aluminum wiring pattern 52 is formed on the order of microns or submicrons, and an insulating film (Si) is formed thereon.
O ₂ ) 54 are formed. The light applied to the wafer 50 is applied to, for example, the area of the spot S including the wiring pattern 52 as shown in FIG. The portion advances into the insulating film and is reflected on the substrate surface 54 or the wiring pattern surface 54C. Accordingly, the light applied to the insulating film is converted into the insulating film 54 having a plurality of film thicknesses shown in FIG.
Are reflected by the front surface 54A and the back surfaces 54B and 54C of the first and second surfaces.

The light reflected by the insulating film 54 interferes with a phase difference caused by an optical path difference corresponding to the thickness of the insulating film 54, and the interfering light (interfering light) as shown in FIG. The light is reflected by the beam splitter 12 and enters the spectral prism 16. The interference light incident on the spectral prism 16 is split into light of each wavelength, and the light is detected by the detector 18.
Is incident on.

The detector 18 is composed of a light receiving element for converting light into an electric signal, and the interference light incident on the detector 18 is photoelectrically converted for each wavelength. Thus, a light intensity distribution indicating the light intensity of each wavelength of the interference light is obtained. FIG. 3 is a diagram showing an example of the light intensity distribution obtained by the detector 18. In the figure, the horizontal axis indicates wavelength, and the vertical axis indicates light intensity. As shown in FIG.
The light intensity distribution obtained by 8 forms peaks and valleys of intensity by interference. Here, assuming that the refractive index of the insulating film is n and the thickness of the insulating film is d, the wavelength λ at which the light intensity of the interference light is maximized
Is nd = (2m + 1) λ / 4, and the condition of the minimum wavelength λ is nd = (2m) λ / 4. still,
m is the order of interference and is an integer.

Here, the interference light reflected by the insulating film 54 includes the film thickness and the interference light interfering with the film thickness as shown in FIG. Accordingly, the light intensity distribution observed by the interference between the film thicknesses is superimposed on the light intensity distribution shown in FIG. 3 obtained by the detector 18. That is, the light intensity distribution of the interference light interfering with the film thickness shown in FIG. 2 is assumed to be the distribution shown in FIG. 4A, and the light intensity distribution of the interference light interfering with the film thickness shown in FIG. 3, the light intensity distribution shown in FIG. 3 obtained by the detector 18 is a distribution obtained by adding the light intensity distributions shown in FIGS. 4 (1) and 4 (2).

Accordingly, the light intensity distribution obtained by the detector 18 is then decomposed into light intensity distributions for each film thickness, and each film thickness is determined from the decomposed light intensity distribution. Therefore, as shown in FIG. 1, the waveform of the light intensity distribution obtained by the detector 18 is first subjected to Fourier transform by an FFT (Fast Fourier Transform: Fast Fourier Transform) 20, and the waveform of the light intensity distribution is decomposed into frequency components.

A characteristic frequency band is extracted by the filter 22 from the distribution (frequency distribution) of the decomposed frequency components. For example, a frequency band near a peak is extracted from the frequency distribution. As shown in FIG.
In the case of the light intensity distribution of the interference light with two types of film thicknesses, peaks are generated at two places in the frequency distribution, and the frequency bands near the peaks at these two places are respectively extracted. If the two types of film thickness are clearly different,
Frequency components may be separated into a wide band and a low band.
When there are many different film thicknesses, the same number of peaks appear in the frequency distribution. In this case as well, frequency bands near the peaks are extracted.

Thus, the light intensity distribution of the interference light generated by the plurality of film thicknesses is divided into the frequency components of the light intensity distribution for each film thickness. Then, based on the frequency components of each frequency band extracted by the filter in this way, the inverse FF
The inverse Fourier transform is performed by T24, and the light intensity distribution indicated by the frequency component in each frequency band is obtained. Thus, the light intensity distribution of the interference light according to each film thickness is obtained. In the case of the light intensity distribution shown in FIG. 3, the light of the interference light according to the film thickness and the film thickness shown in FIGS. An intensity distribution is obtained respectively.

After obtaining the light intensity distribution of the interference light according to each film thickness as described above, fitting 2
Each film thickness is calculated according to 6. The fitting 26 calculates a film thickness at which the difference between the light intensity distribution obtained by the inverse FFT 24 and the light intensity distribution calculated by the theoretical formula is minimized. That is, the film thickness of the theoretical formula is determined such that the light intensity distribution of the theoretical formula using the film thickness as a parameter optimally matches the light intensity distribution obtained by the measurement. Thereby, each film thickness is obtained from the light intensity distribution of the interference light by each film thickness.
The film thickness and the film thickness are obtained from the light intensity distribution of the interference light according to the film thickness and the film thickness shown in FIGS.

The above-mentioned interference type film thickness meter is, for example, a CMP type.
Mounted on the device. As shown in FIG. 2, when a wiring pattern 52 is formed on a silicon substrate 51 of a wafer, and an insulating film 54 is formed thereon, the insulating film surface is polished. Measure thickness and thickness. If the height of the wiring pattern is known, if the sum of the height and the film thickness matches the film thickness, it can be determined that the surface of the insulating film has been polished flat.

The method of obtaining each film thickness from the light intensity distribution of the interference light with each film thickness obtained by the inverse FFT 24 is not limited to the above-described fitting, but may be performed, for example, from the peak wavelength of the light intensity distribution. Is also good. In the above embodiment, the Si formed on the substrate of the wafer
The case where the thickness of the insulating film such as O ₂ is measured has been described. However, the present invention is not limited to this, and it is possible to transmit light even with a metal film (for example, tungsten W, aluminum Al, copper Cu, etc.) formed on the substrate. Therefore, the thickness of these metal films can also be measured. Therefore, for example, when there is a step on the surface of the metal film, this step can be measured.

[0020]

As described above, according to the interference type thickness meter according to the present invention, a thin film having a plurality of film thicknesses formed on a substrate is irradiated with light and reflected on the front and rear surfaces of the thin film. The interference light is split into light of each wavelength. Then, the separated light intensities of the respective wavelengths are photoelectrically converted into electric signals, and the light intensity distribution of the interference light is detected. Next, the light intensity distribution of the interference light is Fourier-transformed into frequency components, and a frequency band of the light intensity distribution of the interference light with each film thickness is selected based on the distribution of the frequency components. Then, the frequency components are inversely Fourier-transformed into the light intensity distribution for each of the selected frequency bands, and the film thickness is obtained from the light intensity distribution. Thus, even when there are a plurality of thin film thicknesses formed on the substrate, these film thicknesses can be simultaneously measured with high accuracy.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an interference type film thickness meter according to the present invention.

FIG. 2 is a cross-sectional view illustrating a configuration of a surface of a wafer as an example of a workpiece to be measured.

FIG. 3 is a diagram illustrating an example of a light intensity distribution obtained by a detector.

4 (1) and 4 (2) are diagrams showing the film thickness and the light intensity distribution observed with the film thickness shown in FIG. 2, respectively.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Light source 12 ... Beam splitter 14 ... Lens 16 ... Spectral prism 18 ... Detector 20 ... FFT (fast Fourier transform) 22 ... Filter 24 ... Inverse FFT 26 ... Fitting

Claims (2)

[Claims] 1. An interference type film thickness meter for measuring a plurality of film thicknesses of a thin film formed on a substrate, comprising: a light irradiating means for irradiating light to the thin film of the wafer; Reflected light and spectral means for separating the interference light of the light reflected on the back surface of the thin film into respective wavelengths; and photoelectrically converting the light intensity of each wavelength spectrally separated by the spectral means into an electric signal; A photoelectric conversion unit that detects a light intensity distribution with respect to a wavelength, a Fourier transform unit that Fourier-transforms the light intensity distribution detected by the photoelectric unit into a frequency component, and a frequency component distribution obtained by the Fourier transform unit. Filter means for selecting a frequency band of the light intensity distribution of the interference light according to the film thickness, and extracting the frequency component for each of the selected frequency bands; Inverse Fourier transform means for inverse Fourier transforming the extracted frequency components into light intensity distributions for each of the frequency bands, and each frequency band based on the light intensity distribution for each frequency band obtained by the inverse Fourier transform means A film thickness calculating means for obtaining a film thickness indicated by each light intensity distribution and obtaining a plurality of film thicknesses of the thin film of the wafer. 2. The method according to claim 1, wherein the film thickness calculating unit performs fitting by using a light intensity distribution for each frequency band obtained by the inverse Fourier transform unit and a light intensity distribution theoretically obtained for a predetermined film thickness. The interference type film thickness meter according to claim 1, wherein a predetermined film thickness is calculated.