JPH09304291A - Foreign matter inspecting apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect foreign matter to relax the modulation caused by the shape of a circuit pattern by photoelectrically converting lights from the inspection regions of two surfaces of a photomask and adding both square root value signals. SOLUTION: The reflected light 2 of light 1 goes toward an object lens L5 to illuminate the circuit pattern drawing screen of a photomask 5 as reflected HM. The light 3 (10) going toward a lens L5 (L6 ) in the scuttered light from the surface OB is formed into an image on an image surface IM1 (IM2 ) and an image element DR (Dr ) outputs an image signal IRO (IMO). This is signal voltage iR (ir ) of one scanning line and a signal 27 (28) containing this is inputted to a square root calculator 21 (22). Signals 29, 30 have the voltage value corresponding to the square root of the signals 27, 28 and are respectively multiplied by k', l' to become signals 31, 32 which are, in turn, added to become a signal 33. The part reflecting the foreign matter in a waveform iR of the signal 33 becomes a part d' and the signal 33 is inputted to be binarized by threshold values TH, TL and the part d' exceeds the threshold value TH by width δand, therefore, a signal 34 having a rectangular waveform DET of width δ is outputted to detect foreign matter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a foreign matter inspection apparatus, and more particularly to an apparatus for optically detecting a foreign matter on a photomask on which a circuit pattern of a semiconductor device or the like is drawn.

[0002]

2. Description of the Related Art Conventionally, an apparatus of this type erases an image of a defect-free circuit pattern by using a bright-field transmission image and a bright-field reflection image obtained by beam scanning to detect a foreign substance. Referring to FIG. 1 schematically showing an optical system that also employs a conventional foreign matter inspection apparatus, a light ray 1 emitted from a light source LN passes through lens groups L1 and L2 and is reflected by a mirror M1 to become a light ray 2. . Ray 2 is the lens group L
3, L4 is transmitted, is reflected by the half mirror HM, is transmitted through the objective lens L5 to become illumination light, and illuminates the circuit pattern drawing surface 7 on the photomask 5. A circuit pattern 6 is formed on the photomask 5. Of the reflected light / scattered light generated from the photomask 5, the light ray 3 traveling in the reflection direction enters the objective lens L5. The one-dimensional photoelectric conversion element, that is, the one-dimensional image pickup element D _R is arranged so that the detection surface 11 is located on the imaging plane conjugate with the circuit pattern drawing surface 7 of the photomask 5 on the object plane OB. . The light ray 3 forms an image on the photoelectric conversion element D _R to form a reflected illumination visual field image. The photoelectric conversion element D _R outputs the image signal I _R0 . This image signal I _R0 is shown in FIG. Note that FIG. 2A is an enlarged view of a dense portion of the circuit pattern 6 on the photomask 5 and the foreign matter DE.

Similarly, of the light rays emitted from the photomask 5, the light ray 10 directed in the transmission direction is the objective lens L6.
Incident on. The one-dimensional image pickup device D _T is arranged so that the detection surface 12 is located on an image formation surface which is conjugate with the circuit pattern drawing surface 7 of the photomask 5 on the object surface. Ray 1
0 forms an image on the image sensor D _T to form a transmitted illumination field image. The image sensor D _T outputs the image signal I _T0 .
This image signal I _T0 is shown in FIG.

A signal processing unit (not shown) to which the two image signals I _R0 and I _T0 after photoelectric conversion are input adjusts their gains and adds them to generate an electric signal I _DET . When no foreign matter is present on the photomask 5, the gains k and l are adjusted so that the I _DET signal has a constant value m, as shown in the following expression (1), regardless of the movement of the photomask 5. To be done. _{I DET = kI R0 + lI T0} = m = const (1) gain k, l reflects the reflectance and transmittance of the mask blank photomask 5 of the circuit pattern 6, respectively.

Incidentally, the amplitude reflectance α _DE and the amplitude transmittance β _DE of the foreign matter DE generally do not match the amplitude transmittance β and the amplitude reflectance α of the circuit pattern. Therefore, a part of the image signals I _R0 and I _T0 are modulated as D _r and D _t as shown in FIGS. 2B and 2C, so that the above equation (1) does not hold. . Therefore, as shown in FIG. 2D, an appropriate threshold value T _L above and below the average level which is a constant value m,
By setting T _H , it becomes possible to detect the signal D _ES of the foreign matter DE.

[0006]

However, FIG.
The waveform of (d) is obtained by the light receiving objective lens L5.
This is when the numerical aperture of L6 is infinite, and the finite numerical aperture does not work as ideal. For example, the L / S that can be resolved by a 100 × microscope is about 1 μm pitch, and at such a pitch, the image contrast becomes poor, and even if there is no foreign matter on the photomask 5, a transmission image and a reflection image can be obtained. Modulation occurs in the signal obtained by adding and, which hinders the detection of foreign matter.

For example, a Foucault pattern (grating with a duty of 50%) close to the resolution limit of the objective lens will be described as an example. As shown in FIG. 3A, the pitch P = 2.
It is ideal if the amplitude reflectance distribution αf ₁ (x) indicated by the broken line is approximately μm and can be reproduced on the image plane. Also, the amplitude transmittance distribution βf indicated by the broken line
_It would be ideal if all ₂ (x) could be reproduced on the image plane. However, the numerical aperture of the objective lens is finite, and FIG.
As shown in (c) and FIG. 3D, the spectrum within the area of | y | <y ₀ on the Fourier transform plane can be imaged, but the other spectra are not imaged. Figure 3 (c)
In addition, in FIG. 3D, | y |
Only the spectra αF ₁₀ (y) and βF ₂₀ (y) in the area of ₀ are drawn. These spectra include only three rays of 0th order light and ± 1st order light. Therefore, the amplitude distribution of the light wave that can be reproduced is only the sinusoidal amplitude distributions αf ₁₀ (x) and βf ₂₀ (x) drawn by the solid lines in FIGS. ing. In this case, since the intensity signal photoelectrically converted is proportional to the square of the amplitude distribution, as shown in FIGS. 3 (e) and 3 (f), it has a point that is asymmetric in the vertical direction. It becomes a waveform, and the above equation (1) cannot be satisfied any longer.

The present invention has been made in view of the above-mentioned conventional problems, and provides a foreign matter inspection apparatus which can alleviate the modulation caused by the shape of the circuit pattern without depending on the advanced image processing technique. The purpose is to do.

[0009]

To achieve the above object, according to the present invention, there is provided a foreign matter inspection device for optically detecting a foreign matter on a photomask having a predetermined pattern drawn on only one surface. There is an illuminating means for illuminating the inside of the inspection area on the one surface of the photomask, and a first photoelectric conversion means facing the one surface of the photomask, and a light beam generated from the inside of the inspection area on the photomask. A first photoelectric conversion unit that receives and photoelectrically converts light; and a second photoelectric conversion unit that faces the other surface of the photomask and that receives and photoelectrically converts light rays generated from the inspection region of the photomask. A first square root value calculating unit that outputs a first square root value signal that is proportional to a square root of a first photoelectric conversion signal value that is output from the photoelectric conversion unit, and a second square root value output unit that outputs the first square root value signal from the second photoelectric conversion unit. Proportional to the square root of photoelectric conversion signal value Second square root value calculating means for outputting a second square root value signal, additional value calculating means for adding the first square root value signal and the second square root value signal, and outputting an additional value signal, and an additional value There is provided a foreign matter inspection device including a signal processing unit that detects a foreign matter based on a signal.

In this case, preferably, the signal processing means has an optical or electrical gain adjusting means for adjusting the relative gains of the first and second square root value signals, and for the added value signal. A foreign substance is detected by setting a predetermined threshold value. Further, it is preferable that the first and second objective lenses, whose fields of view are respectively located in the inspection area, are provided between the first and second photoelectric conversion means and the inspection area. Further, it is preferable that the first and second photoelectric conversion means are one-dimensional or two-dimensional image pickup elements arranged on an image plane conjugate with the inspection region.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a signal processing system capable of detecting the absolute value of the amplitude of a reflected image and the absolute value of the amplitude of a transmitted image is used. The foreign matter is inspected by using the relationship of the following expression (2) in which the sum of the values becomes constant. k'α | f ₀ (x) f ₁ (x) | + l'β | f ₀ (x) f ₂ (x) | = f0 (x) = const (2) where f ₀ (x) is Amplitude distribution of illumination light, αf ₁ (x) represents the reflectance distribution of the circuit pattern, βf ₂ (x) represents the transmittance distribution of the circuit pattern, and k′α = 1′β = 1 Then, k'and l'are determined. By the way, what is observed in the photoelectric conversion element, which is a square law detector, is an amount proportional to the intensity distribution of the wavefront. Since this is intensity = | amplitude | ² , ｜
The value of the amplitude | can be reproduced. Therefore, in the present invention, when detecting the foreign matter on the photomask,
The signal proportional to the absolute value of the amplitude is obtained by taking the square root of the signal value of the image signal proportional to each light intensity of the reflection image obtained by illuminating the inspection position and the transmission image obtained by illuminating the same position. Generate two amplitude image signals with values. Then, by adding these two amplitude image signals, the image information of the circuit pattern having the known reflectance distribution and transmittance distribution is removed, and only the image information of the foreign matter whose reflectance and transmittance are random. Are visualized by simple signal processing, for example, threshold processing.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, an optical system satisfying the above equation (2) is used, and the signal processing section has a function of calculating a square root. The optical system applicable to the present invention is of a type in which a photoelectric conversion element, that is, an image pickup element is arranged on the image plane. Referring to FIG. 1 showing an optical system used in a first embodiment of a foreign matter detecting apparatus according to the present invention, a light source LN
A light ray 1 emitted from is magnified by the lens groups L1 and L2 and is reflected by the mirror M1 to become a light ray 2. Ray 2
Is focused by the lens groups L3 and L4 near the rear focal plane of the objective lens L5 at a predetermined aperture angle, and the half mirror HM
Is reflected toward the objective lens L5. Ray 2
Is refracted by the objective lens L5 and becomes a substantially parallel light beam, and illuminates the circuit pattern drawing surface 7 of the photomask 5. The photomask 5 is held by the holding unit 8 so that the circuit pattern drawing surface 7 of the photomask 5 is located on the object plane OB of the objective lens L5. The photomask 5 can be moved in parallel to the object plane OB by the horizontal moving mechanism 9, so that any point of the photomask 5 can be positioned within the field of view of the objective lenses L5, L6.

Of the scattered light generated from the object plane OB, a light ray traveling toward the objective lens L5, for example, a light ray 3 is refracted by the objective lens L5, and the object plane OB.
The image is formed on the image plane IM1 that is conjugate with. The image sensor D _R having the light detection surface 11 positioned on the image plane IM1 outputs an image signal I _R0 . Objective lenses L5 and L6 have an optical axis A
It is arranged along X, and the object plane of the objective lens L6 is matched with the object plane OB of the objective lens L5. Of the scattered light generated from the object plane OB, a light ray that is directed toward the objective lens L6, for example, a light ray 10 is refracted by the objective lens L6 and is conjugate to the object plane OB.
Image on 2. The image sensor D _T whose light detection surface 12 is located on the image plane IM2 outputs an image signal I _T0 .

FIG. 4 is a block diagram showing a signal processing unit according to the present invention. In the case where the image pickup devices D _R and D _T are one-dimensional linear sensors or two-dimensional image pickup devices, one scanning line of the output signals I _R0 and I _T0 will be described as an example. The output signals I _R0 and I _T0 are the signal voltages i _R and i _T of one scanning line, respectively, and are represented by the following formulas (3) and (4). i _R = α ² | f ₀ (x) f ₁ (x) | ² (3) i _T = β ² | f ₀ (x) f ₂ (x) | ² (4) schematically shown in FIG. The vertical displacement of the signal waveform indicates the relative voltage value, and the horizontal direction indicates the relative position of the x coordinate.

The signal 27 containing the luminance signal i _R is input to the square root calculator 21, while the signal 28 containing the luminance signal i _T
Is input to the square root calculator 22. Signals 29 and 30 are
Each of them has a voltage value corresponding to the square root of the luminance signals 27 and 28, and their waveforms are as shown. The signals 29, 30 are then respectively sent to the amplifiers 23, 24 by k '.
The signals 31 and 32 having the waveforms shown in the figure are multiplied by 1 '. These signals 31 and 32 are added up by the adder 25 to become a signal 33 having the waveform shown in the figure. According to the equation (2), the signal 33 has a constant direct-current waveform regardless of the x coordinate in the portion having no defect, that is, the portion having no foreign matter, but the portion in the waveform i _R that reflects the foreign matter on the circuit pattern. The portion corresponding to d remains in the signal 33 and becomes the portion d ′.

The signal 33 is input to the window comparator 26 and becomes a binarized signal D ₀ by two predetermined threshold values T _H and T _L. In the example shown in FIG. 4, signal 3
A part of the part d ′ of the waveform of 3 has a threshold value T _H of x
Since it exceeds the width δ in the direction, the window comparator 26 outputs a signal 34 having a rectangular waveform D _ET whose width is δ as shown in the figure, so that the foreign matter is detected. Since the signal 33 fluctuates up and down according to the foreign substance information included in the signals I _R0 and I _T0 , it is necessary to binarize it with two threshold values.

FIG. 5 is a block diagram showing an example of an analog type square root calculating circuit. The input signal voltage e _i becomes a signal 44 of lne _i by the logarithmic amplifier 41 and a signal e _i ′ of 1/2 lne _i by the amplifier 42. The signal e _i 'is
The output signal voltage e ₀ = expe _i '= by the antilogarithmic amplifier 43
It becomes _ei ^1/2 . FIG. 6 is a block diagram showing an example in which the circuit configuration shown in FIG. 4 is configured by a digital circuit,
This digital circuit has an analog / digital converter 5
1, 52 are added. Then, the signal processing executed by the signal processing unit 53 surrounded by a broken line is a numerical calculation by a digital circuit, and the function of each unit in the signal processing unit 53 is a unit having the same reference numeral in FIG. Corresponding to that.

FIG. 7 shows a second foreign matter detection device according to the present invention.
The optical system used for an Example is shown. In the optical system shown in FIG. 7, the light beam 1 emitted from the light source LN is magnified by the lens groups L1 and L2 and reflected by the vibrating mirror Mm. The lens groups L3 and L4 include the objective lens L
5 plays a role of relaying on the pupil conjugate plane AP which is conjugate to the pupil plane. In the relay lens groups L3 and L4, the optical axis AX ′ is generated by folding the optical axis AX by the half mirror HM.
Are arranged along. The oscillating mirror Mm has a rotation center Mp, which is perpendicular to the paper surface, is included on the pupil conjugate plane AP, and intersects the optical axis AX ′. The objective lenses L5 and L6 are arranged along the optical axis AX. The objective lens L5 has its object surface OB positioned on the pattern drawing surface 7 of the photomask 5, and the objective lens L6 has its object surface positioned on the object surface OB. The image sensor D is provided on the image plane IM1 of the objective lens L5.
The light detection surface 11 of _R is positioned, and the light detection surface 12 of the image sensor D _T is attached to the image plane IM2 of the objective lens L6.
Is located.

Of the scattered light generated from the circuit pattern drawing surface 7 of the photomask 5, a light ray traveling toward the objective lens L6, for example, a light ray 10 is imaged on the image plane IM1. The illumination light beam 2 is reflected by the vibrating mirror Mm, passes through the relay lens groups L3 and L4, and then the half mirror H.
It is reflected by M and refracted by the objective lens L5 to form a beam spot on the object plane. By vibrating the vibrating mirror Mm about the rotation center Mp as an axis,
The beam spot is one-dimensionally optically scanned in the x direction on the object plane OB. An image is formed on each of the image planes IM1 and IM2 at a point corresponding to an illumination point that continuously moves as a result of optical scanning.

The image pickup devices D _R and D _T are one-dimensional line sensors, and output the light quantity of the image points corresponding to the respective optically scanned object points as a one-dimensional image signal. Thus, the second book
Also in the embodiment, one-dimensional image output can be obtained as in the first embodiment. In the second embodiment, the illumination luminous flux is condensed to improve the illumination brightness, so that the electrical S / N ratio is increased.
It is possible to improve the ratio. The signal processing method in the second embodiment is no different from that in the first embodiment.

[0021]

As described above, according to the present invention, it is possible to provide a foreign matter inspection apparatus which can alleviate the modulation caused by the shape of the circuit pattern without depending on the advanced image processing technique.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an optical system used in a first embodiment of a foreign matter inspection apparatus according to the present invention.

FIG. 2 is a diagram illustrating an inspection principle in a conventional foreign matter inspection apparatus.

FIG. 3 is a diagram illustrating an inspection principle in a conventional foreign matter inspection apparatus.

FIG. 4 is a block diagram showing a signal processing unit in the foreign matter inspection apparatus according to the present invention.

FIG. 5 is a block diagram showing a square root calculation circuit.

FIG. 6 is a block diagram showing another signal processing unit in the foreign matter inspection apparatus according to the present invention.

FIG. 7 is a schematic diagram showing an optical system used in a second embodiment of the foreign matter inspection apparatus according to the present invention.

[Explanation of symbols]

5 Photomask 6 Circuit pattern 7 Circuit pattern drawing surface 8 Holding part 11, 12 Light detection surface 21,22 Square root calculator 23,24 Amplifier 25 Adder 26 Wind comparator 41 Logarithmic amplifier 42 Amplifier 43 Inverse logarithmic amplifier 51,52 Analog / Digital converter 53 Signal processing unit AP Pupil conjugate plane AX Optical axis DE Foreign matter D _R , D _T Image sensor (photoelectric conversion element) IM Image plane Mm Vibration mirror Mp Rotation center OB Object plane

Claims (4)

[Claims] 1. A foreign matter inspection apparatus for optically detecting a foreign matter on a photomask in which a predetermined pattern is drawn only on one surface, wherein an inspection region within the one surface of the photomask is formed. An illuminating means for illuminating; a first photoelectric conversion means facing the one surface of the photomask, for receiving and photoelectrically converting a light beam generated from the inspection region of the photomask; Second photoelectric conversion means facing the other surface of the mask, which receives and photoelectrically converts a light beam generated in the inspection area of the photomask, and is output from the first photoelectric conversion means. A first square root value calculating means for outputting a first square root value signal proportional to a square root of the first photoelectric conversion signal value; and a square root of a second photoelectric conversion signal value output from the second photoelectric conversion means. The second which is proportional to A second square root value calculating means for outputting a root value signal; an addition value calculating means for adding the first square root value signal and the second square root value signal and outputting an addition value signal; A signal processing unit for detecting the foreign matter based on a value signal, the foreign matter inspection apparatus. 2. The signal processing means comprises the first and second signal processing means.
2. The foreign matter is detected by having an optical or electrical gain adjusting means for adjusting the relative gain of the square root value signal of the above, and by setting a predetermined threshold value for the added value signal. The foreign matter inspection device described. 3. A first objective lens and a second objective lens, the fields of view of which are respectively located in the inspection area, between the first and second photoelectric conversion means and the inspection area. The foreign matter inspection device according to 1 or 2. 4. The foreign matter according to claim 3, wherein the first and second photoelectric conversion means are one-dimensional or two-dimensional image pickup devices arranged on an image plane conjugate with the inspection region. Inspection device.