JPS6271804A - Film thickness measuring device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a film thickness measuring device for transparent films, and particularly to a film thickness measuring device suitable for measuring relatively thick films such as photoresists in semiconductor manufacturing.

[Background of the invention]

Conventionally, to measure the thickness of transparent thin films such as anti-reflection films,
Light interference, polarization diffraction, etc. are used. However, when measuring a thick film such as photoresist using these methods, the film thickness is too large for the wavelength of the light used, so the order of interference becomes unknown and measurement becomes impossible. Ta. In order to determine the order of interference, for vapor deposited films, etc.
By continuously monitoring the film thickness during the deposition,
It is also possible to measure the film thickness by counting the number of arrows, but when a film is formed instantaneously, such as with photoresist, the above method of monitoring film thickness cannot be adopted. Therefore, conventionally, when measuring the film thickness of photoresist,
A part of the film was peeled off from the substrate and the difference in level between the photoresist adhesion surface and the photoresist surface was measured using an interference method or TallyStep. However, with this method, the work process is troublesome and there is a risk of destruction or damage to the object to be measured.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to solve the drawbacks of the conventional measuring methods described above, and to provide a film thickness measuring device that can measure the thickness of a relatively thick transparent film such as a photoresist without contact and with high precision. .

[Summary of the invention]

In order to achieve the above object, the present invention provides an illumination system that illuminates a transparent film to be measured with light including a plurality of wavelengths, such as white light, and a dispersion element that spectrally reflects interference light from both the front and back surfaces of the transparent film. and a detection element that detects the light separated by the dispersive element.The detection signal output by the detection element uses the channeled spectrum, which is a phenomenon caused by interference of light, that is, the phenomenon in which the intensity of reflected light differs depending on the wavelength. The technical point is to be able to measure the thickness of the transparent film from the period of .

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of an optical system showing an embodiment of the present invention, where a transparent photoresist 2 coated on a wafer W is
The beam is irradiated via the beam splinter 3 by collimated white light.

The light reflected by the respective surfaces of the photoresist 2 and the wafer W passes through the beam splitter 3, passes through a spatial filter system consisting of a pair of lenses 4.6 and an aperture 5, is split into spectra by a spectroscopic prism 7, and then passes through a lens 8. The light is focused onto a photoelectric detection element 9, such as a -dimensional CCD.

At this time, the reflected light from the surfaces of the photoresist 2 and the wafer W interfere to form bright and dark stripes on the photoelectric detection element 9.

This stripe has a thickness of the photoresist 2 of d, a refractive index of n,
If the wavelength of light is λ, 'l n d = mλ (m = 1.2.3 ---
-----) becomes brighter at a wavelength that satisfies 2 n d
= (m+44) It becomes dark at a wavelength that satisfies λ. Therefore, a signal as shown in FIG. 2 is obtained from the photoelectric detection element (-dimensional CCD) 9. This signal is aperiodic with respect to wavelength λ, but! /λ, a periodic signal is obtained as shown in FIG. In order to obtain the period from the signal shown in FIG. 3, the signal shown in FIG. 3 may be Fourier transformed. If the period of this signal is X(μm-')'/x as the spatial frequency, the thickness d of the photoresist is given by d=1/(2n x)---(1).

Therefore, the bright and dark interference fringes caused by the light split by the spectroscopic prism 7 are detected photoelectrically, and the film thickness d can be determined from the period of the detection signal using equation (1).

On the other hand, since a fine pattern of unevenness is generally formed on a wafer, when light is projected onto this surface, a large amount of scattered light is included in the light flux reflected from that surface. The scattered light becomes noise and adversely affects measurement accuracy, so in order to remove this, an aperture 5 (spatial filter) is provided at the pupil position between the pair of lenses 4 and 5. As the photoelectric detection element 9, a one-dimensional CCD is used in the embodiment shown in FIG. 1, but it may also be an array sensor such as a PDA (photodiode array), an image pickup tube, a solid-state image sensor, an image dissector tube, etc. It may be an image sensor such as. Furthermore, as shown in FIG. 4, by moving the spectroscopic prism (dispersive element) Ta to scan the wavelength, a single detector 9a such as a phototransistor or photodiode can be used to receive the light instead of the above-mentioned image sensor. It's okay.

Furthermore, instead of moving the dispersion element 7a, the wavelength of the light source may be scanned, and the photoresist 2 may be irradiated with illumination light of the scanned wavelength. Further, as the dispersion element, a diffraction grating may be used instead of the spectroscopic prism, or an acousto-optic (AO) element having an acousto-optic effect may be used.

Now, in order to find the period from the signal in Fig. 3, it is sufficient to Fourier transform the signal, but in reality, the optical system including the wafer W and the photoresist 2 has wavelength characteristics (spectral characteristics). Therefore, it is desirable to measure and calculate the wavelength characteristics of the optical system in advance, obtain their values, and correct the characteristics so that the signal shown in FIG. 3 approaches the ideal sine wave shape as much as possible. (It is best to perform this correction using a computer.) Also, since the signal in Figure 3 contains unnecessary DC components, the frequency (period) may not be determined, especially if the photoresist is thin. Since the error increases, the average value of the maximum value (average) and minimum value (average) of the data is subtracted from the data and the fifth value is calculated.
The signal should have a neat shape as shown in the figure.

Furthermore, the data area is expanded to an area where there is no data (however, the data in the expanded area is 0 [zero]), and FFT (
When a fast Fourier transform is performed, a continuous signal as shown by the solid line in FIG. 6 is obtained. The values indicated by black dots in FIG. 6 are the values obtained when FFT is performed without enlarging the data area. This value is determined by the reciprocal of the region of 1/λ in FIG. 5, and is discontinuous. Therefore, if you want to know the smallest possible value, it is necessary to expand the l/λ region and perform FFT. The film thickness d can be determined with high accuracy from the coordinates giving the maximum value of this FFTO value, that is, the frequency of the signal shown in FIG. 5, using the above-mentioned equation (1). At this time, if we do not convert from Fig. 3 to Fig. 5, that is, remove the DC component, F 1? The maximum value of T is the origin (frequency 0), so the frequency to be found is the maximum value excluding the origin (local maximum value).If the film is thick, this frequency will be a large value, so the maximum value at the origin It is clearly distinguished from the value and can be easily determined. However, as the film becomes thinner, it may become impossible to separate it from the maximum value at the origin, or even if it can be separated, the error will increase. Converting the signal to is an essential condition.

Now, assuming that the wavelength range of white light is λ = 0.4 to 0.8 μm and the refractive index of the photoresist is n = 1.5, the minimum film thickness d win is (from equation 11, the maximum film thickness d = is the sample If the number of points (number of light receiving elements in the case of CCD) is N, then 110.4-110.8 2x1.5 2=0.
133N (μm) For example, if the number of sample points N=64, d m-=o,
133N=8.5 (#m). Furthermore, the resolution when the data area is expanded ten times is 110.4-110.8 2x1.5 10.

The above explanation is for the case where a transparent photoresist is coated on an opaque wafer W as shown in FIG. 1, but in general, a film such as Sing, A1, etc. is formed on the wafer. A photoresist is applied over it. If the film under the photoresist is opaque, the above II theory holds true. However, if the film is transparent, there will be interference of multiple beams of light, and the reflected light will be complicated. The method of measurement in that case will be described below. FIG. 7 shows the state of reflected light when the transparent film 10 is formed on the wafer W, and FIG. 8 shows the state of reflected light when the photoresist 2 is further coated on the transparent film 10 of FIG. It is a cross-sectional explanatory view. In addition, Fig. 9 shows the results of Fourier transform (FFT) of the data based on the reflected light shown in Fig. 7, and Fig. 1O shows the lines showing the results of Fourier transform (FFT) based on the reflected light shown in Fig. 7. It is a diagram. In the state shown in FIG. 7, the reflected light flux a from the wafer W and the reflected light flux from the surface of the transparent film 10 interfere with each other. Since this is the same as the conventional two-beam interference, a curve having a peak at only one location as shown in FIG. 9 is obtained. However, in the case of three-beam interference caused by three reflected light beams a, b, and c reflected from the surfaces of the wafer W, the transparent film 10, and the photoresist 3, as shown in FIG. Peaks appear in certain places. One of these places is
Reflected light a from the surface of the wafer W and the transparent film 10 on the wafer
-b, and appears in the same position as in FIG.

Among the other two peaks, the one with higher frequency is due to the reflected light a-c from the surfaces of the wafer W and the photoresist 2, and the remaining one peak is due to the transparent film 10 on the wafer W ( That is, this is due to reflected light b-c from the back surface of the photoresist 2 and the front surface of the photoresist 2.

Therefore, in order to determine the film thickness of photoresist 2, before applying the first photoresist, data is collected using the apparatus shown in FIG. Find the peak position and store it in the memory device. Next, apply photoresist 2, take the data in the same way, and perform Fourier transformation.
It is sufficient to obtain peaks as shown in FIG. 10, determine each peak from the stored data, and obtain the thickness.

However, depending on the thickness of the photoresist 2, the sought peak may not be distinguishable from the initially stored peak. At that time, the peak position to be obtained can be determined from the difference between the high frequency peak position and the stored peak position.

〔Effect of the invention〕

As described above, according to the present invention, the reflected light from the film to be measured is separated, the light is detected by the detection element, and the film thickness is measured from the period of the detection signal, so there is no need for troublesome operations. The thickness of the film can be measured without contact and with very high accuracy.

[Brief explanation of drawings]

FIG. 1 is an optical system layout diagram showing one embodiment of the present invention, FIG. 2 is an output signal curve diagram output from the detection element in the embodiment of FIG. 1, and FIG. A signal curve diagram obtained by converting the coordinates of the output signal, FIG. 4 is an optical system diagram showing a second embodiment of the present invention that is different from the embodiment shown in FIG. 1, and FIG. The signal curve diagram, Figure 6, after removing
Fig. 5 is a curve diagram obtained by expanding the data area and performing fast Fourier transform. Fig. 7 is an illustration of reflected light when a transparent film is formed on a wafer. Fig. 8 is a curve diagram obtained by expanding the data area of Fig. 5 and performing fast Fourier transform. Furthermore, FIG. 9 is an explanatory diagram of reflected light when photoresist is applied, and FIG. 9 is a curve diagram obtained by detecting the reflected light of FIG. Figure 10 is the 8th
A curve diagram obtained by detecting the reflected light shown in the figure using the embodiment apparatus shown in FIG. 1 and fast Fourier transforming the data is shown. [Explanation of symbols of main parts] 1---Illumination system 2--Photoresist (transparent film)
3.-beam splitter

Claims (4)

[Claims] (1) An illumination system that illuminates the transparent film to be measured with light containing multiple wavelengths, a dispersion element that separates reflected interference light from both the front and back surfaces of the transparent film, and a detection of the light that has been separated by the dispersion element. What is claimed is: 1. A film thickness measuring device comprising: a detection element, and configured to be capable of measuring the film thickness of the transparent film from the period of a detection signal outputted by the detection element. (2) The dispersion element is a spectroscopic prism or a diffraction grating, and the reflected interference light incident on the dispersion element is transmitted through a pair of lenses (4, 6) and an aperture (5) provided at the pupil position.
2. The film thickness measuring device according to claim 1, wherein scattered light is removed by a spatial filter comprising: (3) The detection element is an array sensor such as a C, C, D, or PDA, or an imaging element such as an image pickup tube, a solid-state image sensor, or an image dissector tube, and the detection element is an image pickup device such as an image pickup tube, a solid-state image sensor, or an image dissector tube, and the detection element is an array sensor such as a C, C, D, or PDA, or an imaging device such as an image pickup tube, a solid-state image sensor, or an image dissector tube. 3. The film thickness measuring device according to claim 1, wherein the device is configured to detect the light intensity and interval of interference fringes. (4) The detection element is a photomultiply, a phototransistor,
Or one detector such as a photodiode (9a)
3. A film thickness measuring device according to claim 1, wherein said dispersive element is a rotating spectroscopic prism (7a) that scans wavelengths.