GB2058395A

GB2058395A - Directional High-cut Spatial Frequency Filter

Info

Publication number: GB2058395A
Application number: GB8027602A
Authority: GB
Original assignee: Agency of Industrial Science and Technology; Dai Nippon Printing Co Ltd
Current assignee: Dai Nippon Printing Co Ltd; National Institute of Advanced Industrial Science and Technology AIST
Priority date: 1979-08-28
Filing date: 1980-08-26
Publication date: 1981-04-08
Also published as: JPS6027964B2; DE3031816C2; FR2464495A1; FR2464495B1; DE3031816A1; JPS5633621A; GB2058395B

Abstract

A directional high-cut spatial frequency filter obtained by combining a spatial frequency filter made by photographically recording the optical Fourier transform spectrum intensity distribution of a regular pattern with a directional high-cut filter 20 of which the high-pass characteristic is increased, according to the directional component proportion of the optical Fourier transform spectrum intensity distribution, in directions in which spectral components are more than in other directions. The directional high- cut filter is generally made of a light absorbing material, such as a thick sheet cut in a predetermined configuration, or by blackening the spatial frequency filter in such configuration. <IMAGE>

Description

SPECIFICATION Directional High-Cut Spatial Frequency Filter This invention relates to a laser beam diffraction pattern spatial frequency filtering system (hereinafter referred to as " a spatial filtering system", when applicable) in which the defects of a regular pattern are detected by utilizing optical diffraction phenomena, and more particular to a spatial frequency filter (hereinafter referred to as "a spatial filter", when applicable) prepared by photographically recording the optical Fourier transform spectrum intensity distribution of a regular, normal pattern, in the system.

Recently, a spatial filtering system utilizing laser beams has been put in practical use in order to detect defects included in patterns which are arranged regularly for instance as in various metal filters and IC masks.

A system in which a two-dimensional image is scanned with a thin light beam to provide output signals, which are processed with an electronic computer is known as one of the two-dimensional image processing systems. While this conventional system is disadvantageous in that the processing is intricate, the processing time is relatively long and accordingly it is not economical, the above-described spatial filtering system is advantageous in that the spatial parallel processing of a two-dimensional image can be achieved readily with an optical system relatively low in manufacturing cost at a higher speed.

Therefor, the spatial filtering system has been realized as a defect inspecting device for inspecting industrial products having regular twodimensional patterns such as metal filters, IC masks, fabrics, etc.

The fundamental arrangement of the optical system in the defect inspecting device utilizing the spatial filtering system will be described with reference to Fig. 1.

A laser beam 2 emitted by a laser oscillator 1 is applied to a collimator 3, where it is converted into an enlarged, parallel beam 4, which is applied to an object 5 to be inspected. The object 5 is positioned on the front focal plane of a Fourier transform lens 7, on the rear focal plane of which the Fourier transform spectrum of the object 5 is provided by means of a light beam 6 which is obtained through the diffraction of the parallel beam 4 which is caused when passing through the object 5. A spatial filter 8 is disposed on the rear focal plane of the Fourier transform lens 7.

The spatial filter 8 is a negative picture film on which is recorded the Fourier transform spectrum intensity distribution of the normal pattern of the object 5 to be inspected (by the Fourier transform lens 7). Accordingly, only a spectrum corresponding to the normal pattern is absorbed by the spatial filter 8, and a spectrum corresponding to a defective pattern is allowed to pass through the spatial filter 8.

In this connection, it should be noted that the position of the spatial filter 8 is on the front focal plane of an inverse Fourier transform lens 10.

Therefore, a light beam 9 passed through a spatial filter 8 is subjected to inverse Fourier transformation, so that it appears on the rear focal plane of the lens 10 as an inverse Fourier transformation image obtained through spatial filtering, i.e. an image indicating only the defective parts of the object to be inspected. The image is detected by an optical detector 11. The provision of a screen instead of the optical detector 11 allows the defective parts to appear as bright dots. Accordingly, in the latter case, the defects of the object can be visually inspected.

One example of a conventional spatial filter 8 will be described with reference to Figs. 2(a) through 2(c). Fig. 2(a) shows two-dimensional cross stripes. Fig. 2(b) shows a spatial filter 8 obtained by photographically recording the optical Fourier transform spectrum intensity distribution of the two-dimensional cross-stripes, the spatial filter 8 having an opaque portion 1 2 and a transparent portion 13 which are made respectively black and non-black according to the spectral intensities. Fig. 2(c) shows relationships between the geometrical dimensions of the twodimensional cross stripes and the optical Fourier transform spectrum intensity distribution in the x direction.In Fig. 2(c), spatial frequencies X=x/(R f) (where A is the wavelength, and f is the focal length) are plotted on the horizontal axis, and the intensity distribution l(X) which is defined as follows, on the vertical axis;

The intensity distribution is in such a form that interference fringes

attributing to plural slits are modified with the diffraction image

(a dot in the figure) of a slit. In the abovedescribed equation (1), A, B and C are the length of one side of a square lattice, the distance between adjacent lattices, and the overall length, respectively, as shown in Fig. 2(a).

In general, in the inspection of industrial articles or products having a regular pattern for defects according to the spatial filtering system, a spatial filter which is obtained by photographically recording on a 35-mm film or the like the optical Fourier transform spectrum intensity distribution of the normal pattern is employed. This spatial filter is advantageous in that it can be readily manufactured and accordingly it is low in manufacture cost; however, it is still disadvantageous in the following points: Its optical system is considerably severe in mechanical accuracy so that the spatial filter may not be displaced from the optical axis or may not be turned around the optical axis.Especially, in the case where an ordinary convex lens is employed as the Fourier transform lens, a curved focal plane is provided, as a result of which a higher-order diffraction light beam becomes out of focus.

Furthermore, as the higher-order diffraction light beam is excessively small in spectral intensity, it is impossible to achieve the recording in the dynamic range of 102--103 (lug. &commat; sec) of a photographic film. Thus, the spatial filter cannot filter the higher-order diffraction light beam, thus being unable to provide a sufficiently high S/N ratio.

In order to eliminate these difficulties, a method of using a directional filter, or a method of employing a special Fourier transform lens providing a flat Fourier transform plane has been proposed in the art.

One example of a spatial filter called "a directional filter" is as shown in Fig. 3. The spatial filter is provided for a pattern in which, as in twodimensional cross stripes, the main components of its Fourier transform spectrum are distributed in the directions of 0 and 900. The spatial filter has an opaque portion (hatched) 14 and the remaining transparent portion 1 5 to allow the optical system to be lower in mechanical accuracy; that is, this spatial filter is more practical in use. With the spatial filter, the components in the directions of 00 and 900, i.e.

the inner linear components of the pattern, being regarded as the normal components, are filtered out, and the remaining components, being regarded as the defective components, are allowed to pass through the spatial filter.

Accordingly, it is impossible for the spatial filter to filter a non-directional false defect spectrum as in a corner radius (cf. R in Fig. 6), and accordingly the S/N ratio of the spatial filter is not sufficiently high.

On the other hand, the latter method is not practical because the special high resolution Fourier transform lens providing a flat Fourier transformation plane is considerably expensive, several millions of yen. Even if the lens is used, the problem on the dynamic range of a photographic film is still left to be solved.

Therefore, the higher-order diffraction light beam filtering effect cannot be expected as required.

Accordingly, an object of this invention is to eliminate all of the above-described difficulties accompanying a conventional spatial filter.

More specifically, an object of the invention is to provide a directional high-cut spatial frequency filter with which industrial products having regular two dimensional patterns can be inspected for defects, with high S/N ratios, high defect outputs and excellent image quality with relatively low mechanical accuracy and without harming the feature of the conventional spatial filtering system that the spatial parallel processing of two-dimensional images can be achieved readily with an optical system relatively low in manufacturing cost.

The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.

In the accompanying drawings: Fig. 1 is an explanatory diagram showing the fundamental arrangement of a defect inspecting device utilizing a spatial filtering system; Fig. 2(a) is an explanatory diagram showing two-dimensional cross stripes; Fig. 2(b) is a plan view of a spatial filter obtained by photographically recording the optical Fourier transform spectrum intensity distribution of the two dimensional cross stripes in Fig. 2(a); Fig. 2(c) is a graphical representation indicating the relationship between the geometrical dimensions of the two-dimensional cross stripes and the optical Fourier transform spectrum intensity distribution; Fig. 3 is a plan view of a directional filter; Fig. 4(a) is a plan view of a spatial filter which is obtained by photographically recording an industrial product having two-dimensional cross stripes;; Fig. 4(b) is an enlarged view of the essential part of the spatial filter in Fig. 4(a); Fig. 5(a) is a plan view showing one example of a directional high-cut spatial filter according to this invention, which is obtained by combining the spatial filter in Fig. 4(a) with a directional high-cut filter; Fig. 5(b) is an enlarged view of the essential part of the filter in Fig. 5(a); Fig. 6 is an explanatory diagram showing openings or holes, and a defect in an industrial product having two-dimensional cross stripes; Fig. 7(a) is an explanatory diagram showing openings or holes, and a defect in a metal filter; Fig. 7(b) is an enlarged view showing the essential part of the metal filter; Fig. 8 is an enlarged view showing a directional high cut spatial filter for the metal filter;; Fig. 9 is a graphical representation indicating the S/N ratios of the directional high-cut spatial filter and a circular low-pass spatial filter; Fig. 10 is a graphical representation indicating the optical intensities of a defect in the use of the directional high-cut spatial filter and in the use of the circular low-pass spatial filter; Fig. 11 is a graphical representation indicating optical axis displacement with S/N ratio in the use of the directional high-cut spatial filter and in the use of the conventional spatial filter; and Fig. 12 is a graphical representation indicating S/N ratio with rotation around the optical axis in the use of the directional high-cut spatial filter and in the use of the conventional spatial filter.

As conductive to a full understanding of this invention, first a conventional spatial filterwill be described with reference to Figs. 4(a) and 4(b).

The conventional spatial filter has portions 16.

which are blackened as a result of the recording of the optical Fourier transform spectrum intensity distribution of an industrial product such as a metal filter having two-dimensional cross stripes, nameiy, opaque portions 16, and transparent portions 1 7-1 and 1 7-2 which are not blackened because the spectral intensity is weak.

As shown in Fig. 4(a), the main components of the two-dimensional cross stripes spectrum are distributed in the directions of 0 and 900, and the component distribution is a considerably little in a direction 0 between the directions of 0 and 900. Accordingly, a black distribution reflecting the spectrum intensity distribution as it is, is provided on a photographic film. Thus, in the directions of 0 and 900, the blackening is effected for the higher-order diffraction light region, while in the direction of O only the lowerorder diffraction light region is blackened.The diameter of a black spot is different from the spot diameter (for instance, several micrometers (cm)) in diffraction limit which is defined according to the size of the light source. That is, in a diffraction light region high in intensity, the order of which is close to the O-th order, the black spot has a large diameter of the order of 100 to 1 50 Mm because of 6ver-exposure and halation in the recording operation; however, as the order of a diffraction light region is increased, the spot diameter is decreased.

In the case where an object 5 to be inspected having a defect 1 8 as shown in Fig. 6 is subjected to spatial filtering according to the system described with reference to Fig. 1 by using the spatial filter as shown in Figs. 4(a) and 4(b), among the spectral components of the defect 1 8, the higher-order spectral components and the lower-order spectral components pass through the transparent portions 17-1 and 1 7-2, respectively, and appear as bright spots on the inverse Fourier transformation plane. Industrial products such as metal filters having openings or windows generally have corner radii (R) 19 as shown in Fig. 6.Therefore, in the above-described spatial filtering, the higher-order spectral components, in the direction (),of the corner radius (R) 1 9 pass through the transparent portions 1 7-1, and are also allowed to appear as bright spots on the inverse Fourier transformation plane; that is, socalled false defect outputs are provided, thus decreasing the S/N ratio.

The noise components of the conventional spatial filtering system can be classified into noise components in the optical system (caused by optical scattering due to the uneven surface of the lens and dust on the lens surface) and noise components attributing to high-order diffraction light beams which pass through the higher-order spectrum region where the photographic film is not sufficiently blackened because of insufficient spectral intensity and defocusing. A larger part of each of the two different noise components can pass through the transparent portions 1 7-1. The transparent portions 1 7-1 occupy a larger part of the spatial filter.Therefore, although the noise components of the optical system and of the higher-order diffraction light are considerably low in intensity, they cannot be negligible in the amount of transmitted light when surfaceintegrated. The invention is based on this new findings.

The higher-order diffraction light beams passing through the spatial filter include those from the linear peripheral portions of the square lattice openings. This can be explained on the theory of marginal waves that higher-order diffraction light beams are those from the periphery of an opening (cf. "Wave Optics" by Hiroshi Kubota, p. 257, published by Iwanami Shoten, 1971). These higher-order diffraction light beams pass through portions of the spatial filter which are not blackened, in the directions of 0 and 900. As is clear from the above description, the higher-order diffraction light beams, forming a part of the spectral components of a defect, are important; however, they are nothing but the main components of noise, when considered as a whole.Accordingly, an improvement of the S/N ratio of the spatial filter requires the provision of a spatial filter which passes the diffraction light components of a defect as much as possible, and filters out the higher-order diffraction light beams as much as possible which cause the noise components.

Figs. 5(a) and 5(b) shows one example of a spatial filter according to this invention. The spatial filter can be obtained by adding a directional high-cut filter 20 having portions (hatched in Figs. 5(a) and 5(b)) to the spatial filter in Fig. 4 which is obtained through photographical recording. As is apparent from the configuration of the hatched portions, the high-cut filter 20 masks up to the lower-order diffraction light region the spectrum of which is recorded, in the direction 0 in which the higher-order diffraction light region is not recorded; while in the spectral main component directions, i.e. in the directions of 0 and 900, the spectrum is recorded up to the higher-order diffraction light region, thus providing a filtering function.Thus, the high-cut filter 20 opens in a high-pass tendency, and has a configuration according to the directional component proportion of the optical Fourier transform spectrum intensity distribution of twodimensional cross stripes.

Almost all the higher order diffraction light beams around the direction ocean be cut off by adding the directional high-cut filter 20 having such a configuration to the conventional spatial filter obtained through photographical recording.

Therefore, almost all the effects of noise components of the optical system, of corner radii, and of marginal diffraction light can be eliminated. On the other hand, of the spectral components of the defect 18, only lower order diffraction light beams are passed through the filter in the direction 0, and in the directions of 0 and 900 diffraction light beams up to higher order diffraction light beams are passed through the filter as much as possible according to the filtering capability of the spatial filter obtained through photographical recording, so that reduction of the optical output of the defect 18 is prevented as much as possible, thus remarkably improving the S/N ratio.

The effects of the directional high-cut filter resides not only in an increase of the optical output of the defect and in an improvement of the S/N ratio. This will be explained with respect to its image quality. In the combination of a circular low-pass filter and a spatial filter obtained through photographical recording, the S/N ratio is improved; however, the resultant image is considerably poor in quality because of circular hole diffraction. On the other hand, the directional high-cut filter, having a high-pass tendency in the spectral main component direction, is scarcely affected by diffraction, thus providing an excellent inverse Fourier transformation image.

Furthermore, in the lower order diffraction light region which is mainly used, the black spot diameter of the recorded spectrum is ten to twenty times as large as the spot diameter in the diffraction limit which is defined by the size of the light source. Therefore, the allowances for the rotation around the optical axis and the displacement from the optical axis can be relatively large, and accordingly the mechanical accuracy of the spatial filter may be greatly decreased.

The directional high-cut filter may be made of any material if it can absorb light Therefore, it may be prepared by blackening in a predetermined configuration the spatial filter obtained through photographical recording, or may be prepared by sticking a black paper cut in a predetermined configuration onto the spatial filter. Thus, the directional high-cut filter can be manufactured readily and at low cost. The black paper cut in the predetermined configuration may be positioned several millimeters away from the spatial filter in the direction of the optical axis.

As is apparent from the above description, the employment of the directional high-cut spatial filter which is, according to the invention, obtained by combining the spatial filter prepared by photographically recording the Fourier transform spectrum intensity distribution of a regular pattern with the directional high-cut filter having the high-pass tendency in directions in which more components are provided according to the directional component proportion of the spectrum, makes it possible to realize a device which is relative low in mechanical accuracy but inspects industrial products having regular twodimensional patterns for defects with high accuracy, without harming the feature of the spatial filtering system that the spatial parallel process of two-dimensional images can be readily achieved with an optical system relatively low in cost.

The effects of this invention will be described.

Figs. 7(a) and 7(b) are an explanatory diagram showing a metal filter, and an enlarged view of the essential part of the metal filter, respectively.

The metal filter is 0.2 mm in thickness and has a regular two-dimensional pattern. The metal filter has been used to confirm the effects of the invention. In Figs. 7(a) and 7(b), reference numeral 21 designates an opening or hole in the metal filter, and reference numeral 22 designates a defect.

Fig. 8 is an enlarged view of one example of the directional high-cut spatial filter according to the invention which is obtained by combining a directional high-cut filter 24 with a spatial filter 23 which is obtained by photographically recording the optical Fourier transform spectrum intensity distribution of the metal filter shown in Fig. 7, in the case where the light source is a He Ne laser, and the Fourier transformation lens is a convex lens with a focal distance (f) of 250 mm and a diameter of 100 mm. The main components of the Fourier transformation spectrum lie in the directions of 0 and 900.

However, as most spectral components lie in the direction of 00, the high-pass characteristic of the directional high-cut filter 24 is increased in the direction of 0 so as to provide (00 direction)/(900 direction) = F/E=2.

Fig. 9 is a graphical representation indicating the ratio of the optical intensity of the defect 22 to the optical intensity of the corner noise which is measured on the inverse Fourier transformation plane, i.e. S/N ratio, with variations of the value Df in Fig. 8, in the case where the directional high-cut spatial filter shown in Fig. 8 is used (Curve A), and in the case where a circular lowpass spatial filter made by combining a circular (D0) low-pass filter with the spatial filter 23 obtained through photographical recording (Curve B). The S/N ratio of the conventional spatial filter is five (S/N=5).

Fig. 10 is a graphical representation indicating the optical intensity of the defect 22 measured in Fig. 9, in comparision of the directional high-cut spatial filter (Curve A) and the circular low-pass spatial filter (Curve B). The optical intensity of the conventional spatial filter is 2.3.

As is apparent from Figs. 9 and 10, the S/N ratio of the directional high-cut spatial filter according to the invention is about five times as high as that of the conventional spatial filter. The S/N ratio in the use of the directional high-cut spatial filter is better than that in the use of the circular low-pass spatial filter, and optical intensity of the defect 22 is higher. The optical intensity of the defect 22 with D=30 in which the S/N ratio reaches its peak in the case of the directional high-cut spatial filter is lower only by 20% than the optical intensity in the case of the conventional spatial filter; while the optical intensity in the case of the circular low-pass spatial filter is lower by 80%.

Fig. 11 shows effects of optical axis displacement to the S/N ratio. It is assumed that the limit of S/N ratio effective for satisfactory inspection is four (S/N=4). Then, the conventional spatial filter (Curve B) obtained by photographical recording cannot be used when the optical axis is.

shifted at least 10 ,um; while the directional highcut spatial filter (Curve A) can be used even when the optical axis is shifted 35 ,um.

Fig. 12 indicates effects of rotation around the optical axis to S/N ratio. If it is assumed that the effective limit is S/N =4, then the conventional spatial filter (Curve B) cannot be used when it turns around the optical axis through at least 0.2 degree; while the degree of rotation about the optical axis allowable to the directional high-cut spatial filter (Curve A) of the invention is 1.0 degree.

As is clear from the above description, according to the spatial filtering system employing the directional high-cut spatial filter of the invention, industrial products having regular two-dimensional patterns can be inspected for defects with high S/N ratios, high defect outputs, and excellent image quality, and with relatively low mechanical accuracy. Thus, the practical effects of the invention should be highly appreciated.

Claims

1. A directional high-cut spatial frequency filter comprising in combination: a spatial frequency filter which is prepared by photographically recording the optical fourier transform spectrum intensity distribution of a regular pattern; and a directional high-cut filter of which the highpass characteristic is increased, according to the directional component proportion of said optical Fourier transform spectrum intensity distribution, in directions in which spectral components are more than in other directions.

2. A filter as claimed in claim 1, in which said directional high-cut filter is made of material absorbing light.

3. A filter as claimed in claim 2, in which said material is a black sheet which is cut in a predetermined configuration.

4. A filter as claimed in claim 2, in which said directional high-cut filter is prepared by blackening said spatial frequency filter in a predetermined configuration.

5. A filter as claimed in claim 2, in which said directional high-cut filter made of material absorbing light is positioned apart from said spatial frequency filter.

6. A directional high-cut spatial frequency filter substantially as hereinbefore described with reference to the accompanying drawings.