JPS63196805A - Surface internal measuring method - Google Patents

Abstract

PURPOSE:To enhance measuring accuracy, by a method wherein beam is allowed to irradiate two surfaces having an interval therebetween and the luminous intensity distribution of the interference fringe formed on the beam receiving surface of the reflected beams from said surfaces is subjected to Fourier transformation and the frequency corresponding to the frequency length of the interference fringe is detected from the Fourier transformation data. CONSTITUTION:Laser beam is allowed to obliquely irradiate two surfaces overlapping so as to have an interval therebetween and the beams reflected from both of two surfaces are received to detect the luminous intensity distribution of the interference fringe formed on the beam receiving surface of said reflected beams and, from this detection signal, luminous intensity is converted to data so as to show luminous intensity (g) as frequency-f distribution. This frequency (f) is the one corresponding to the frequency length of X-direction distribution. Since the obtained data is dispersed in a noise component and frequency f0 to be calculated protrudes clearly as compared with other frequency, said frequency f0 can be easily detected. The interval between two surfaces can be calculated from the detected frequency f0 according to a predetermined formula. By using high speed Fourier transformation as Fourier transformation, all of a series of processings on and after the interference fringe is converted to data can be performed by a computer.

Description

[Detailed Description of the Invention] [Summary] Laser light is irradiated onto two overlapping surfaces with an interval, and the interval between the two surfaces is calculated from the frequency corresponding to the periodic length of interference fringes formed by the light reflected from both surfaces. In surface spacing measurement, the light intensity distribution of interference fringes is Fourier-transformed and the frequency is detected from that data, making it easier to find accurate frequencies and improving measurement accuracy.

[Industrial application field]

The present invention provides a method for measuring the distance between two surfaces in which the distance between the two surfaces is calculated from the frequency corresponding to the periodic length of interference fringes formed by the light reflected from both surfaces by irradiating a laser beam onto two surfaces that overlap with a distance. about.

The above surface spacing measurement has the feature that it can be performed from one side without contact. For example, in the manufacture of semiconductor devices, it is used to measure the spacing between a mask and a wafer during close exposure in photolithography technology, or to measure the spacing between a mask and a wafer during close exposure in photolithography technology. It can be used to measure the thickness of deposited films.

[Conventional technology]

The formation of interference fringes in the above surface spacing measurement is performed as shown in the explanatory diagram of FIG.

References: ^Precision Wide-rang
e Optical Gap Measurement T
echnique (precision optical measurement over a wide range of distances)” D, C, Flanders & T, M, Lysz
czarz.

J, Vac, Sci, ↑echno1. Bl (1983
) PP, 1196-1199.

In the figure, 1 and 2 are two overlapping surfaces with an interval d, and a single wavelength laser beam 3 is directed obliquely to these two surfaces.
The light is irradiated so as to be focused on surface 1.

The light receiving surface 6 is arranged so as to receive both the reflected light 4 from the surface 1 and the reflected light 5 from the surface 2.

Then, due to the interference between the reflected lights 4 and 5, interference fringes having periodicity in the X direction shown in the figure are formed on the light receiving surface 6.

The light intensity g of this interference fringe is such that the wavelength of the laser beam 3 is λ, the angle of incidence of the laser beam 3 is 45 degrees, the reflectance of surface 1 is RI, the reflectance of surface 2 is R2, and the transmittance of surface 1 is TI. , the light-receiving surface 6 is perpendicular to the line passing through the center of the reflected light 4, the distance from the reflection point of surface 1 to the light-receiving surface 6 is l, and the distance in the X direction on the light-receiving surface 6 is xl
If d/It and x/l are extremely small, then g=RI +R2(TI)2+2(R,R2)''2T.

X cos(2ff (2”d (1+ (x/j
')(x2/212)))/λ) is given by the formula, and the light intensity distribution of the interference fringe, that is, the light intensity
The directional distribution (the distribution of the light intensity g with respect to X) has a constant period length X6 as shown in FIG.

Therefore, if f6-1/Xo, the interval d is d-(λ
i! /(2”)f.

is given in a form proportional to fo.

Therefore, the interval d can be calculated by finding fo or the period length p0.

Here, fo is a frequency corresponding to period length XQ with (1/length) as a dimension, and period length xo corresponds to its wavelength.

From the above, the distance d can be measured by using a photodetector such as a COD line sensor that can detect the light intensity distribution of interference fringes on the light receiving surface 6, and by processing its output signal. .

The conventional method is as shown in the flowchart of FIG. That is, (1) interference fringes are formed by the method described above and detected by the photodetector 6;

(2) Convert the detection signal into data such that the light intensity distribution of interference fringes can be expressed as shown in FIG.

■ Find the apex of the protrusion from the data and detect the period length x□.

■ Calculate the interval d from the detected period length xO and complete the measurement.

[Problem that the invention seeks to solve]

However, with this method, it is easy to accurately detect the period length XQ if the data obtained when the detection signal of the photodetector 6 is converted into data as shown in FIG. Due to the contamination of
, ) may become inaccurate, or worse, it may become difficult to detect the period length X6, making it difficult to accurately determine the frequency f0.

This reduces the accuracy of measuring the distance d or makes it impossible to measure it.

[Means for solving problems]

The above problem is solved by irradiating a laser beam of a single wavelength obliquely onto two overlapping surfaces with a gap between them, and receiving the light reflected from both surfaces, resulting in the formation of interference fringes on the receiving surface. The surface spacing measuring method of the present invention detects an intensity distribution, performs Fourier transform on the light intensity distribution, detects a frequency corresponding to the periodic length of the interference fringes from the data, and calculates the spacing between the two surfaces from the frequency. solved by.

[Effect]

In this method, the detection of the light intensity distribution is the same as the conventional method, but since the light intensity distribution is Fourier transformed,
The distribution of light intensity with respect to the frequency explained above with respect to the frequency fo is obtained, and from that data, the frequency f. fc is being detected.

On the other hand, the noise that makes it difficult to accurately detect the period length x0 in the conventional method generally does not have a constant frequency and has a level lower than the level of interference fringes.

Therefore, in the data on the distribution of light intensity with respect to the above-mentioned frequencies, the frequency r0, which is determined by the dispersion of noise components, stands out more clearly than the others, so by detecting this protrusion, it is easy to detect the accurate frequency fo. I can do it.

Thus, according to the present method, it is possible to improve the measurement accuracy of the distance between the two surfaces (d mentioned above) compared to the conventional method.

〔Example〕

The following is a first example of surface distance measurement using the method of the present invention.
This will be explained with reference to FIG.

FIG. 1 is a flowchart of an embodiment of the method of the present invention, and FIG. 2 is a frequency distribution diagram of the Fourier-transformed light intensity.

The flow of the embodiment shown in FIG. 1 is as follows. That is, (1) similar to the conventional method, interference fringes are formed and detected by the photodetector 6; The contents are as explained using FIG.

(2) Similar to the conventional method, convert the detection signal into data so that the light intensity of the interference fringes can be expressed as an X-direction distribution.

As explained earlier, the data is expected to be as shown in Figure 4 when expressed in a diagram, but due to the inclusion of noise,
It will look like the figure.

■ Fourier transform the above data to create data that can represent the light intensity as a frequency distribution. This frequency is the frequency previously explained as frequency f0. When the obtained data is expressed in a diagram with the frequency f as shown in FIG. 2, for example, the frequency fo clearly stands out from the others for the reason explained above.

(2) Detect the frequency fo from the data obtained in (2). This detection is extremely easy as it is the detection of the apex of the protrusion in FIG. 2, and the frequency f obtained by this detection is
o is much more accurate than the period length xO in the conventional method.

■ Calculate the interval d from the detected frequency fo and complete the measurement.

Thus, according to the present method, the accurate frequency fo at
Since it is possible to obtain the distance d, it is possible to improve the measurement accuracy of the distance d compared to the conventional method.

In the case of this method, as can be seen from the above-mentioned flow, by using fast Fourier transform (FFT) for the Fourier transform in step (2), it is possible to perform all of the series of processing from step (1) onward by a computer.

〔Effect of the invention〕

As explained above, according to the configuration of the present invention, a laser beam is irradiated onto two overlapping surfaces with an interval, and the interval between the two surfaces is adjusted from a frequency corresponding to the periodic length of interference fringes formed by the light reflected from both surfaces. In surface spacing measurement for calculating , it is possible to easily obtain an accurate frequency, which has the effect of making it possible to improve measurement accuracy.

[Brief explanation of the drawing]

Figure 1 is a flowchart of an embodiment of the method of the present invention, Figure 2 is a frequency distribution diagram of Fourier-transformed light intensity, Figure 3 is an explanatory diagram of interference fringe formation, and Figure 4 is the X-direction distribution of light intensity according to the formula. 5 is a flowchart of the conventional method, and FIG. 6 is an actual light intensity distribution diagram in the X direction. In the figure, 1.2 is the two overlapping surfaces, 3 is the laser beam, 4.5 is the reflected light, 6 is the photodetector (light receiving surface), d is the distance between 1 and 2, X is the direction of the period of the interference fringe, x (1 is the periodic length of the interference fringe, fo is the frequency corresponding to po, g is the light intensity. , 277

Claims (1)

[Claims] A laser beam of a single wavelength is irradiated diagonally onto two overlapping surfaces with a gap between them, and the light reflected from both surfaces is received, and the light intensity distribution of interference fringes formed on the receiving surface is detected. A surface spacing measuring method comprising: Fourier transforming the light intensity distribution, detecting a frequency corresponding to the periodic length of the interference fringes from the data, and calculating the spacing between the two surfaces from the frequency.