Inspection method and apparatus for inspecting a particle, if any, on a substrate having a pattern

What is claimed is:

1. An inspection apparatus for inspecting a particle, if any, on a substrate having a pattern, said apparatus comprising:

light producing means for producing (i) first light having a first state of polarization and a first wavelength, and (ii) second light having a second state of polarization, different from the first state of polarization, and a second wavelength, different from the first wavelength;

light projecting means for projecting at least the first light to a position of inspection upon the substrate;

detecting means for detecting heterodyne interference light produced on the basis of the second light and light scattered at the inspection position and having its state of polarization changed, by the scattering, from the first state of polarization; and

inspecting means for inspecting a particle, if any, on the substrate on the basis of an output of said detecting means.

2. An inspection apparatus according to claim 1, wherein said light projecting means comprises scanning means for deflecting at least the first light to optically scan a surface to be inspected.

3. An apparatus according to claim 2, further comprising a lens system for imaging a scanning area of the surface upon a detection surface of said detecting means.

4. An apparatus according to claim 1, wherein the second light is projected at the same position irradiated with the first light.

5. An apparatus according to claim 1, wherein the second light is not projected on the surface to be inspected and wherein the second light and scattered light resulting from the first light are caused to interfere with each other.

6. An apparatus according to claim 1, wherein said light producing means produces the first light and second light in a combined flux.

7. An apparatus according to claim 1, wherein said light producing means produces the first light and the second light separately.

8. An apparatus according to claim 1, wherein the first light comprises linearly polarized light of a predetermined direction while the second light comprises linearly polarized light of another direction different from the predetermined direction.

9. An apparatus according to claim 1, wherein at least one of the first light and second light comprises circularly polarized light.

10. An apparatus according to claim 1, wherein said detecting means detects interference light produced on the basis of the second light and light advancing in a particular direction sideways to the direction of incidence of the first light.

11. An apparatus according to claim 10, wherein the particular direction has an angle in a range of 90-180 deg.

12. An apparatus according to claim 1, further comprising a setting mechanism for setting the state of polarization of the first light.

13. An apparatus according to claim 1, further comprising a setting mechanism for setting the state of polarization of the second light.

14. An apparatus according to claim 1, further comprising inspecting means comprising signal processing means including a phase synchronization loop circuit, wherein said inspecting means inspects the state of the inspection position on the basis of processing, through said signal processing means, a beat signal of the detected interference light.

15. An apparatus according to claim 1, further comprising (i) oscillating means for producing a high frequency signal of a frequency different from a beat signal of the detected interference light, (ii) multiplying means for multiplying the beat signal and the high frequency signal, and (iii) inspecting means for inspecting the state of the inspection position on the basis of the result of multiplication.

16. A transfer system, comprising:

an inspection apparatus as recited in claim 1, for inspecting an original having a pattern; and

a transfer apparatus for transferring onto a substrate the pattern of the original inspected by said inspection apparatus.

17. An original cleaning and inspecting system, comprising:

a cleaning apparatus for cleaning an original; and

an inspection apparatus as recited in claim 1, for inspecting the original cleaned by said cleaning apparatus.

18. An apparatus according to claim 1, wherein the inspection position lies on a mask having a pattern to be transferred, and wherein a particle if any on the mask can be inspected while being distinguished from the pattern of the mask.

19. An inspection apparatus, comprising:

first light producing means for producing first light having a first wavelength, and second light having a second wavelength, different from the first wavelength;

second light producing means for producing third light having a third wavelength, and fourth light having a fourth wavelength, different from the third wavelength;

light projecting means for projecting at least the first light and the third light to a position of inspection;

first detecting means for detecting heterodyne interference light produced on the basis of the second light and light scattered at the inspection position as a result of irradiation by the first light; and

second detecting means for detecting heterodyne interference light produced on the basis of the fourth light and light scattered at the inspection position as a result of irradiation by the third light.

20. An apparatus according to claim 19, wherein the first light and the second light have different states of polarization, and wherein the third light and the fourth light have different states of polarization.

21. An apparatus according to claim 19, wherein the inspection position lies on a mask having a pattern to be transferred, and wherein a particle if any on the mask can be inspected while being distinguished from the pattern of the mask.

22. An inspection apparatus for inspecting a particle, if any, on a substrate having a pattern, said apparatus comprising:

light producing means for producing (i) first light having a first state of polarization and a first wavelength, and (ii) second light having a second state of polarization, different from the first state of polarization and a second wavelength, different from the first wavelength;

light projecting means for projecting at least the first light to a position of inspection upon the substrate;

first detecting means for detecting heterodyne interference light produced on the basis of the second light and light scattered in a first direction from the inspection position as a result of irradiation with the first light;

second detecting means for detecting heterodyne interference light produced on the basis of the second light and light scattered in a second direction, different from the first direction, from the inspection position as a result of irradiation with the first light; and

inspecting means for inspecting a particle, if any, on the substrate on the basis of outputs of said first and second detecting means.

23. An apparatus according to claim 22, wherein the inspection position lies on a mask having a pattern to be transferred, and wherein a particle if any on the mask can be inspected while being distinguished from the pattern of the mask.

24. An inspection method for inspecting a particle, if any, on a substrate having a pattern, said method comprising the steps of:

producing (i) first light having a first state of polarization and a first wavelength, and (ii) second light having a second state of polarization, different from the first state of polarization, and a second wavelength, different from the first wavelength;

projecting at least the first light to a position of inspection upon the substrate;

detecting heterodyne interference light produced on the basis of the second light and light scattered at the inspection position and having its state of polarization changed, by the scattering, from the first state of polarization; and

inspecting a particle, if any, on the substrate on the basis of the detection in said detecting step.

25. An inspection method according to claim 24, wherein said projecting step comprises deflecting at least the first light to optically scan a surface to be inspected.

26. A device manufacturing method, comprising the steps of:

inspecting an original having a pattern in accordance with an inspection method as recited in claim 24; and

transferring onto a substrate the pattern of the inspected original.

27. A device manufactured by a device manufacturing method which comprises the steps of:

inspecting an original having a pattern in accordance with an inspection method as recited in claim 24; and

transferring onto a substrate the pattern of the inspected original.

28. A method according to claim 24, wherein the inspection position lies on a mask having a pattern to be transferred, and wherein a particle if any on the mask can be inspected while being distinguished from the pattern of the mask.

Description Submit all comments and votes

FIELD OF THE INVENTION AND RELATED ART

This invention relates to a method and an apparatus usable, as an example, in the manufacture of microdevices such as semiconductor devices, for inspecting the surface of an article. More particularly, the invention is concerned with a method and an apparatus for optically inspecting the presence/absence of minute particles or defects of the surface of an article. In another aspect, the invention is concerned with a method and an apparatus for manufacturing microdevices such as semiconductor devices by using such an inspection method.

For manufacture of semiconductor devices such as ICs or liquid crystal displays, for example, a circuit pattern formed on an original (called a "reticle" or "photomask") is transferred to the surface of a workpiece or wafer having a resist coating by using a semiconductor printing apparatus (called an "exposure apparatus"). If in this transfer process there are minute particles (foreign particles) on the surface of the original, such particles are also transferred (printed) on the wafer. This causes decreased yield of IC manufacture. Particularly, in a case where the same circuit pattern is printed on different zones of a wafer sequentially in accordance with the step-and-repeat method, only one particle on the original is printed on every zone of the wafer. This results in a possibility that all the chips produced from this wafer are defective, leading to a substantial decrease in the yield of IC manufacture.

In the IC manufacturing process, it is therefore desired to inspect the presence/absence of minute particles on an original, and many proposals have been made in this respect. FIG. 66 shows an example of an inspection apparatus. In this example, the presence/absence of any foreign particle is examined by detecting scattered light from the particle.

More particularly, in FIG. 66, a laser beam from a laser light source 151 is transformed into a laser beam best suited to inspection, by means of a polarizer 152, a filter 153, a collimating system 154 and so on. Mirror 155 directs the laser beam to a scanning optical system comprising a scanning mirror 157 and an f-.theta. lens 158. The scanning laser beam from the f-.theta. lens 158 is converged on the surface 160, to be inspected, of a reticle or the like having a circuit pattern formed thereon, and thus a scanning light spot 159 is formed thereon. Scanning stage system 166 serves to relatively move the scanning spot 159 and the surface 160 in a direction perpendicular to the direction of the scan by the scanning spot 159, whereby a two-dimensional scan of the entire surface 160 is assured.

A detection system comprising a lens system 161, a polarizer 162, an aperture 163 and a photoelectric detector 164 is disposed to receive backward or sideward scattered light. As regards the disposition of this detection system, since there is scattered light from the circuit pattern or the like on the surface 160 which light has a particular direction of diffraction, the detection system has to be disposed off such a direction so as not to receive the unwanted diffraction light.

If in this structure there is no particle within the range of the scanning spot 159, no scattered light is detected by the photoelectric detector 164. If there is any particle, it produces scattered light isotropically and, therefore, the photoelectric detector 164 detects any scattered light. Thus, by processing an output signal of the detector in a signal processing system 165, the presence/absence of any foreign particle on the surface can be inspected.

However, this type of inspection apparatus involves such inconveniences as follows:

(1) Where a very small particle of a size of about 0.3 micron or less is to be detected, the produced scattered light has a very low intensity. It is therefore not easy to detect the particle-scattered light with good sensitivity.

(2) There is a case wherein, depending on the circuit pattern used, scattered light goes from the pattern toward the detector. In the detection system like this example which is based only on the intensity information of the scattered light, it is not easy to discriminate the particle-scattered light from the pattern-scattered light. This leads to a decreased signal-to-noise (S/N) ratio.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide an inspection method or apparatus by which even very small particles or defects on a surface can be detected with a good S/N ratio.

It is another object of the present invention to provide a method or apparatus for manufacture of microdevices such as semiconductor devices, using such an inspection method.

In accordance with an aspect of the present invention, there is provided an inspection apparatus comprising: light producing means for producing (i) a first light beam having a first state of polarization and a first wavelength, and (ii) a second light beam having a second state of polarization, different from the first state of polarization, and a second wavelength, different from the first wavelength; light projecting means for projecting at least the first light beam to a position of inspection; and detecting means for detecting heterodyne interference light produced on the basis of the second light beam and the light scattered at said position and having its state of polarization changed by the scattering from the first state of polarization.

These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a first embodiment of the present invention.

FIG. 2 is a schematic view for explaining the generation of scattered light and detection of a beat signal.

FIG. 3 is a schematic view for explaining a beat signal detected.

FIG. 4 is a schematic view for explaining the time width of a beat signal detected.

FIG. 5 is a diagrammatic view of a beat signal detecting system.

FIG. 6 is a schematic view of a detection optical system of this embodiment.

FIG. 7 is a schematic view of a modified form of the detection optical system.

FIG. 8 is a schematic view of a second embodiment of the present invention.

FIG. 9 is a schematic view of a third embodiment of the present invention.

FIG. 10 is a schematic view for explaining reference light and scattered light.

FIG. 11 is a schematic view for explaining the difference in wave front of scattered lights from a circuit pattern and from a particle.

FIG. 12 is a schematic view of a fourth embodiment of the present invention.

FIGS. 13A and 13B are graphs each showing the relationship between the relative intensity of scattering and the angle of scattering in particulate size.

FIG. 14 is a schematic view of a detection optical system of this embodiment.

FIG. 15 is a schematic view of a modified form of the detection system of the fourth embodiment.

FIG. 16 is a schematic view of a fifth embodiment of the present invention.

FIG. 17 is a schematic view of a sixth embodiment of the present invention.

FIGS. 18A, 18B and 18C are graphs each showing the relationship between the relative intensity of scattering and the angle of scattering in a P-polarized component and an S-polarized component.

FIG. 19 is a schematic view of a seventh embodiment of the present invention.

FIGS. 20A and 20B are graphs each showing an example of a waveform of a signal detectable in the seventh embodiment.

FIG. 21 is a schematic view of an eighth embodiment of the present invention.

FIG. 22 is a schematic view of a ninth embodiment of the present invention.

FIG. 23 is a schematic view of a tenth embodiment of the present invention.

FIG. 24 is a schematic view of a modified form of a detection system.

FIG. 25 is a schematic view of another modified form of a detection system.

FIG. 26 is a schematic view of an eleventh embodiment of the present invention.

FIG. 27 is a perspective view of a portion of the FIG. 26 embodiment.

FIG. 28 is a schematic view, showing the relationship between a circuit pattern and a light spot.

FIG. 29 is a schematic view, showing the manner of a scan of a surface, to be inspected, with a light spot.

FIG. 30 is a schematic view, showing an example of the waveform of a detection signal which is obtainable with the apparatus of the eleventh embodiment.

FIG. 31 is a diagrammatic view of a signal processing circuit.

FIG. 32 is a schematic view, showing the waveforms of signals at respective portions of the signal processing circuit of FIG. 31.

FIG. 33 is a schematic view of a twelfth embodiment of the present invention.

FIG. 34 is a schematic view, showing the waveform of a detection signal obtainable with the apparatus of the twelfth embodiment.

FIG. 35 is a diagrammatic view of a signal processing circuit.

FIG. 36 is a schematic view, showing the waveforms of signals at respective portions of the signal processing circuit of FIG. 35.

FIG. 37 is a schematic view of a thirteenth embodiment of the present invention.

FIG. 38 is a schematic view of a fourteenth embodiment of the present invention.

FIG. 39 is a schematic view of a fifteenth embodiment of the present invention.

FIG. 40 is a schematic view of a sixteenth embodiment of the present invention.

FIG. 41 is a schematic view of a seventeenth embodiment of the present invention.

FIG. 42 is a schematic view of an eighteenth embodiment of the present invention.

FIG. 43 is a schematic side view of a nineteenth embodiment of the present invention.

FIG. 44 is a schematic plan view of the nineteenth embodiment of the present invention.

FIG. 45 is a schematic view for explaining the generation of scattered light from the surface being inspected.

FIG. 46 is a schematic perspective view of a twentieth embodiment of the present invention.

FIG. 47 is a schematic front view of the twentieth embodiment of the present invention.

FIG. 48 is a schematic side view of the twentieth embodiment of the present invention.

FIG. 49 is a schematic plan view of the twentieth embodiment of the present invention.

FIG. 50 is a schematic view for explaining the function of an aperture of the twentieth embodiment.

FIG. 51 is a schematic view for explaining the function of the aperture of the twentieth embodiment.

FIG. 52 is a schematic perspective view of a twenty-first embodiment of the present invention.

FIG. 53 is a schematic perspective view of a twenty-second embodiment of the present invention.

FIG. 54 is a schematic perspective view of a twenty-third embodiment of the present invention.

FIG. 55 is an enlarged view of a light scattering structure used in the twenty-third embodiment.

FIG. 56 is a schematic perspective view of a twenty-fourth embodiment of the present invention.

FIG. 57 is a schematic view of a twenty-fifth embodiment of the present invention.

FIGS. 58A and 58B are schematic views each for explaining a preferable direction of detection for scattered light.

FIG. 59 is a diagrammatic view of a twenty-sixth embodiment of the present invention.

FIGS. 60A and 60B are schematic views each for explaining a beat signal detected.

FIG. 61 is a schematic and diagrammatic view of a twenty-seventh embodiment of the present invention.

FIG. 62 is a schematic and diagrammatic view of an example of the light source system of the twenty-seventh embodiment.

FIG. 63 is a schematic and diagrammatic view of a twenty-eighth embodiment of the present invention.

FIG. 64 is a schematic and diagrammatic view of a twenty-ninth embodiment of the present invention, which is applied to a semiconductor device manufacturing system.

FIG. 65 is a schematic and diagrammatic view of a thirtieth/embodiment of the present invention, which is applied to an original cleaning and inspection system.

FIG. 66 is a schematic view of a known type inspection apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, description will be made of some preferred embodiments of the present invention which is applied to an apparatus for inspecting a surface of an article such as an original (reticle or photomask) or a workpiece (wafer) used the in manufacture of semiconductor devices, more particularly, to an apparatus for inspecting foreign particles such as dust adhered to the surface to be examined or any defects such as scratches on that surface. As a matter of course, the invention is applicable not only to the field of semiconductor device manufacture but also to any other fields wherein the surface inspection is required.

Embodiment 1

FIG. 1 shows a first embodiment of the present invention. Denoted in the drawing at 1 is a laser light source; at 2 is a polarizing plate: at 3 is a filter system; at 4 is a collimator optical system; at 5 and 12 are polarization beam splitters each for separating a laser beam into two mutually orthogonal polarization light components or for combining them; at 6 and 7 are mirrors; at 8 and 9 (10 and 11) are a set of an acousto-optic device and a driver therefor, for modulating the laser beam at a suitable shift frequency; at 13 is a scanning mirror such as a polygonal mirror or a galvano mirror; at 14 is an f-.theta. lens system; at 15 is a polarization beam splitter having an elongated shape corresponding to the region to be scanned; at 16 and 17 are intensity attenuating filter systems; at 18 and 19 are mirrors; at 20 is a scanning spot; at 21 is the surface of a reticle or the like which is to be inspected; at 22 is a scanning stage system; at 26 is a lens system for directing scattered light from the scanning spot 20 to a photoelectric detector 29; at 27 is a polarization filter; at 28 is a slit-like aperture; at 29 is the aforesaid photoelectric detector; and at 30 is a beat signal processing system.

The laser beam produced by the laser light source 1 is transformed by the polarizing plate 2 and the filter system 3 into a laser beam of suitable intensity having mutually orthogonal linearly polarized light components, which is then collimated by the collimator optical system. This laser beam is then separated by the polarization beam splitter 5 into a P-polarized laser beam and an S-polarized laser beam. Of these laser beams, the P-polarized laser beam is reflected by the mirror 6 and is modulated at a shift frequency .omega. by the acousto-optic device 9 which is driven by the driver 8. On the other hand, the S-polarized laser beam is modulated at a shift frequency .omega.+.DELTA..omega. by the acousto-optic device 11 which is driven by the driver 10, and it is reflected by the mirror 7. These two linearly polarized and frequency modulated laser beams are combined by the polarization beam splitter 12, whereby a single laser beam 12a having two mutually orthogonal, linearly polarized light components with a difference attributable to the relative shift frequency difference .DELTA..omega., is produced.

As regards the arrangement for producing such a laser beam 12a as having the above-described property, it is not limited to the above-described example. An alternative is that only one acousto-optic device is used to modulate only one of the two laser beams at a shift frequency .DELTA..omega.. As a further alternative, a Zeeman laser light source may be used or an injection current to a semiconductor laser light source may be modulated.

The laser beam 12a is directed to an optical scanning system which comprises the scanning mirror 13 and the f-.theta. lens system 14, and the laser beam emanating therefrom is separated by the polarization beam splitter 15 into a P-polarized laser beam 15a (shift frequency .omega.) and an S-polarized laser beam 15b (shift frequency .omega.+.DELTA..omega.).

The separated laser beam 15a is received by the filter system 16 whereby an intensity suitable to the particle inspection is set. Then, it is converged through cooperation of the mirror 18 upon the surface 21, to be inspected, at an angle of incidence (.theta.), whereby a spot 20 is formed. On the other hand, the S-polarized laser beam 15b is received by the filter system 17 whereby an intensity suitable to the particle inspection is set. Then, it is converged through cooperation of the mirror 19 at an angle of incidence (.phi.), into a spot 20 on the surface 21. Namely, the laser beams 15a and 15b are converged into the same spot 20 at different incidence angles. The intensity ratio of the laser beams 15a and 15b impinging on the spot 20 may be 1:100, for example. Also, the spot 20 may have a size of about 10 microns. Here, as regards the optical length from the polarization beam splitter 15 to the scanning spot 20, the same length is set to the P-polarized laser beam 15a and the S-polarized laser beam 15b. This assures "interference" even when the spatial coherent length of the laser beam is not very long. Also, while the photoelectric detector 29 is disposed in the direction of zero-th order diffraction light (angle of emission of .theta.) of the P-polarized laser beam 15a (angle of incidence of .theta.), this angle may be so selected as to minimize impingement upon the detector 29 of that scattered light which is produced from anything other than a foreign particle or a fault on the surface 21 (for example, light diffractively scattered by a circuit pattern of a reticle).

With the rotation of the scanning mirror 13, the scanning spot 20 moves along a direction perpendicular to the sheet of the drawing, to optically scan the surface 21. Also, the scanning stage 22 relatively moves the surface 21 in a direction (depicted by an arrow in the drawing) perpendicular to the optical scan direction with the spot 20, whereby the surface 21 as a whole can be scanned two-dimensionally.

In the present embodiment, particular notice is taken of three kinds of light, among those produced from the scanning spot 20 toward the photoelectric detector 29, that is: (1) zero-th order diffraction light 23 (P-polarized light) of the P-polarized laser beam 15a; (2) back scattered light 24 (P-polarized light plus S-polarized light) of the S-polarized laser beam 15b, depolarized by a foreign particle or fault; and (3) back scattered light 25 (S-polarized light) of the S-polarized laser beam 15b, produced by the circuit pattern formed on the surface 21. Here, the cause of depolarization attributable to a foreign particle or fault on the surface is that: since generally the surface irregularity on such a foreign particle or fault is large, when the light is irregularly reflected and scattered, the state of polarization is disturbed to generate a polarized component different from the plane of polarization of the input light. If, on the other hand, the surface is relatively uniform and smooth, such as the surface on a circuit pattern, the depolarization of scattered light is small.

The zero-th order diffraction light 23 of P-polarized light (shift frequency .omega.) produced in the direction toward the photoelectric detector 29 and the P-polarized light component of the back scattered light 24 (shift frequency .omega.+.DELTA..omega.) attributable to any particle or fault, have the same or coinciding plane of polarization. Therefore, they cause optical heterodyne interference. By photoelectrically converting this interference light, a beat signal is obtained. Namely, in the optical heterodyne method, since the zero-th order diffraction light 23 is a reference light and it comprises P-polarized light, the light that can interfere with this light to provide a beat signal, is only the scattered light 24 (among those back scattered) which has a P-polarized light component as a result of depolarization. This means that: if there is scattered light from a circuit pattern, it does not cause a beat signal; or alternatively, if a beat signal is produced, it is of very low level. Thus, the present embodiment assures inspection of any foreign particle or fault, with a very high sensitivity and a very high S/N ratio.

The above-described scattered light as received by the lens system 26 is then received by the polarization filter 27 having an optical property effective to pass only a P-polarized light component, whereby unwanted light components such as an S-polarized light component can be blocked. This effectively reduces beat signal noises attributable to unwanted mixture of noise polarized light. After this, the light passes through the slit-like aperture 28 and reaches the photoelectric detector 29. The detection signal obtained by the detector 29 is processed by the beat signal processing system 30, and the presence/absence of any foreign particle or fault is discriminated on the basis of the state of the beat signal.

In the present embodiment, the zero-th order diffraction light 23 comprising P-polarized light is used as a reference light and it is caused to interfere (heterodyne interference) with the P-polarized light resulting from depolarization, to thereby obtain a beat signal. However, even if the relationship of the P-polarization and the S-polarization is interchanged totally, the detection can be done in a similar way. An example of doing this may be that: the characteristics of the polarization beam splitter 15 are changed so as to provide a laser beam 15a of S-polarized component and a laser beam 15b of P-polarized component while, on the other hand, a polarization filter 27 having a property for passing only S-polarized light components is used.

Next, generation of scattered light as well as detection of a beat signal in the present embodiment will be explained in greater detail. FIG. 2 illustrates generation of scattered light on an occasion when a foreign particle and a circuit pattern are present in the neighborhood of the position of a scanning spot. Denoted in the drawing at 201 is a foreign particle of a size of about 0.3 micron, adhered to the surface 21 to be inspected; at 202 is a circuit pattern; and at 203 is a beat signal detecting system including a polarization filter, as an example.

As described with reference to FIG. 1, the P-polarized laser beam 15a having been modulated at a shift frequency .omega. and the S-polarized laser beam 15b having been modulated at a shift frequency .omega.+.DELTA..omega., are incident on the same spot position with respective angles of incidence of .theta. and .phi.. The beat signal detecting system 203 is disposed in the direction in which the zero-th order diffraction light is produced from the laser beam 15a by the surface 21 to be inspected and, with regard to the S-polarized laser beam 15b, the beat signal detecting system 203 is disposed in the direction of back scattering.

Here, since as compared with the size (about 0.3 micron) of particles to be inspected the scanning spot has a sufficiently large diameter of about 10 microns, irrespective of the presence or absence of such a particle the zero-th order diffraction light 23 can reach the beat signal detecting system 203 while the state of polarization of the input light is retained substantially unchanged. This can be explained from the fact that, in the phenomenon of diffraction of light, the higher the order of diffraction of light is, the more it depends on the high frequency component (spatial frequency) of the reflection surface while the zero-th order diffraction light depends on the low frequency component of the reflection surface. Namely, the zero-th order diffraction light is less affected by a minute structure within the spot.

Assuming now that the P-polarized laser beam 15a and the S-polarized laser beam 15b impinging on the surface 21 have respective electric fields E.sub.1 and E.sub.2, then they can be expressed as follows:

E.sub.1 =Ep.multidot.exp{j(.omega.t+.theta..sub.1)} (1)

E.sub.2 =Es.multidot.exp[j{(.omega.+.DELTA..omega.)t+.theta..sub.2 }](2)

Now, if the zero-th order diffraction light 23 from the scanning spot 20, the back scattered light 24 by the foreign particle and the back scattered light 25 by the circuit pattern are denoted by F.sub.1, R.sub.1 and R.sub.2, respectively, then they can be expressed as follows:

F.sub.1 =.alpha.Ep.multidot.exp{j(.omega.t+.theta.'.sub.1)}(3)

(where .alpha. is the efficiency of zero-th order diffraction)

R.sub.1 =.DELTA.E.sub.1 s.multidot.exp[j{(.omega.+.DELTA..omega.)t+.theta.'.sub.2 }]+.DELTA.E.sub.1 p.multidot.exp[j{(.omega.+.DELTA..omega.)t+.theta.'.sub.2 }](4)

R.sub.2 =.DELTA.E.sub.2 s.multidot.exp[j{(.omega.+.DELTA..omega.)t+.theta.'.sub.3 }](5)

Here, since only those having the same or coinciding plane of polarization cause interference and since any S-polarized component is blocked by a polarization filter included in the beat detecting system 203, the intensity I of a combined beat signal to be detected by the beat signal detecting system 203 can be expressed as follows: ##EQU1##

The amplitude .DELTA.E.sub.1 p of the P-polarized component, produced as a result of depolarization by the particle defined by equation (4), is very small. Since, however, from the third term of equation (6), Ep is significantly larger than .DELTA.E.sub.1 p, the output voltage of the beat signal detected by the beat signal detecting system 203 has a good sensitivity as compared with a case where the back scattered light 24 by the particle is detected directly.

Also, where the DC component and the AC component (frequency .DELTA..omega.) of the beat signal obtained in equation (6) are extracted selectively in accordance with an appropriate method, it is possible to avoid noise components such as stray light to thereby further enhance the S/N ratio.

Now, the beat signal to be detected will be explained. FIG. 3 shows an example of the waveform of such a beat signal. Denoted in the drawing at 301 is the axis which represents time t; at 302 is the axis which represents the intensity I of the signal outputted; at 306 is the beat signal to be detected; at 303 is the DC component of the beat signal; at 304 is the AC component of the beat signal; at 305 is the time width or period (.DELTA.t) in which the beat signal is detected.

As described hereinbefore, if there is no particle or fault within the scanning light spot, then no beat signal is detected. However, if there is any particle or fault, a beat signal of a frequency .DELTA..omega. as depicted at 306 is produced in the time width .DELTA.t. This time width .DELTA.t (305) in which the beat signal is produced is determined by the size of the scanning spot 20 and the scan speed of the spot 20 over the surface to be inspected. Namely, in FIG. 4, the time period from a moment at which an end of the scanning spot 20 moving at a speed V (402) just reaches the particle 201 to a moment at which the scanning spot 20 comes just to the position 20', corresponds to the beat signal time width .DELTA.t (305) in FIG. 3.

Referring now to FIG. 5, details of an example of the structure of the beat signal processing system 30, for processing a beat signal detected, will be explained. Denoted in the drawing at 801 is a preamplifier for amplifying a beat signal detected by the photoelectric detector 29; at 802 is a signal processing system for detecting individually the DC component 808 and the AC component 807 of the amplified beat signal; at 803 is a zero-th order diffraction light monitor system for monitoring any change in intensity of the zero-th order diffraction light 23, through detection of the DC component 808; at 809 is a correction signal for correcting a change in intensity of the zero-th order diffraction light 23 as detected by the monitoring; at 804 is a frequency filter for extracting a signal of frequency .DELTA..omega. of the detected AC component, for correction with the correction signal 809; at 805 is a counter for counting the number of particles or faults on the basis of comparison of the output of the frequency filter with a certain threshold for discrimination of a foreign particle or fault; and at 806 is a computer for memorizing and/or displaying the number of particles or faults or the positions of them on the surface 21.

From equation (6), the AC component 807 and the DC component 808 of the beat signal detected by the detector 29 can be expressed as follows:

AC comp.=2.alpha.Ep.DELTA.E.sub.1 p.multidot.cos (.DELTA..omega.t+.theta.'.sub.2 -.theta.'.sub.1) (7)

DC comp.=(.alpha.Ep).sup.2 +.DELTA.E.sub.1 p.sup.2 .perspectiveto.(.DELTA.Ep).sup.2 (8)

From equation (7), it is seen that the amplitude of the AC component of the beat signal