Spot grid array imaging system

A high data-rate spot-grid array imaging system is provided that compensates for stage vibrations and overcomes the severe linearity requirements of prior art systems. Embodiments include an imaging system with a two-dimensional and periodic array of lenses, each lens imaging a spot in an object plane, such as a semiconductor substrate to be inspected, upon an image plane to image a two-dimensional and periodic array of spots. A sensor is provided in a conjugate image plane with a two-dimensional and periodic array of readout elements, each collecting the signal from one of the spots. A mechanical system moves the substrate in a direction which is nearly parallel to an axis of the array of spots such that as the substrate is moved across the spot array in the scan direction (the y-direction) the spots trace a path which leaves no gaps in the mechanical cross-scan direction (the x-direction). A compensator, such as a servo or a movable mirror, compensates for mechanical inaccuracies in the moving stage, thereby increasing imaging accuracy. In other embodiments, the motion of the mechanical system provides a small overlap between coverage areas of lenses of the lens array in consecutive columns, thereby overcoming the severe linearity requirements of prior art systems and allowing the utilization of cost effective microlens arrays.

FIELD OF THE INVENTION

The present invention relates to an imaging system. The present invention has particular applicability in optical imaging systems optimized for automated defect inspection.

BACKGROUND ART

Optical imaging involves the reproduction or imaging of a scaled image in an object plane upon an image plane. High-resolution imaging is termed microscopy. Such imaging is refered to as electronic imaging when an optoelectronic device such as an array of charged coupled devices (called a CCD ) is used to sample the optical signal in the image plane and translate it into an electrical signal.

Automated optical inspection is a technique for measuring the integrity of an object by collecting an image of it and comparing that image to a reference (e.g., comparing a die to a data-base for photolithographic masks), to another part of that object (such as die-to-die inspection for semiconductor wafers), or to a reference image (die-to- golden image ). Disadvantageously, when conducting high-resolution inspection of large semiconductor substrates, the FOV of the imaging system cannot cover the entire substrate to be inspected, so the substrate must be moved or stepped across the FOV, thereby increasing inspection time. To increase throughput, some conventional automated inspection tools continuously scan the substrate in one direction while imaging an orthogonal one-dimensional optical FOV. Once the substrate is traversed in the scanning direction, it is typically moved in the other (cross-scan) direction by a distance of one FOV, and then its path is retraced, creating a serpentine motion path.

Other optical imaging systems for inspecting semiconductor substrates utilize a spot grid array to achieve high throughput. In these systems, an imager typically includes a two-dimensional and periodic array of lenses, each lens imaging a spot in an object plane, such as a substrate to be inspected, upon an image plane to image a two-dimensional and periodic array of spots from the object plane upon the image plane. A sensor, such as a CCD, is provided in a conjugate image plane with a two-dimensional and periodic array of readout elements, each collecting the signal from a spot in the object plane. A mechanical system moves the substrate such that as the substrate is moved across the spot array in the scan direction (the y-direction) the spots trace a path which leaves no gaps in the mechanical cross-scan direction (the x-direction). Thus, imaging of very large FOVs is accomplished by employing an array of optical elements each having a minimal FOV, rather than complex large-FOV optics. Optical imaging devices utilizing a spot grid array are described in U.S. Pat. No. 6,248,988 to Krantz, U.S. Pat. No. 6,133,986 to Johnson, U.S. Pat. No. 5,659,420 to Wakai, and U.S. Pat. No. 6,043,932 to Kusnose.

These and other previous implementations of spot-grid array concepts suffer from several limitations. To achieve the very high data-rates required for high-end inspection with all-mechanical stage scanning, a large array is required. For example, a data-rate of 10 Gpix/sec with a 100 nm pixel and a 32 32 lens array requires a stage velocity of 100 nm (10 10 9 )/(32 32) m/sec, which is impractical due to the stage-turn around time, the motion accuracy requirement and the stage complexity and cost. To reduce the required speed to a more reasonable stage speed, a larger array is needed. A 320 320 array, for example, requires a stage speed of 10 mm/sec, which is a very reasonable rate. Moreover, the frame-rate would be reduced to 100 KHz, vs. 10 MHz for a 32 32 array. The lower data rate is compatible with the pulse rate of Q-switched lasers (i.e., several tens of KHz), which thus enables using high-efficiency frequency conversion for short wavelength and hence high-resolution imaging. By using a somewhat larger array (for example 1000 1000), the frame-rate (pulse-rate) requirement is further reduced (to 10 KHz), enabling the use of Excimer lasers (such as a 157 nm F2 laser) and thus even finer resolution.

However, some major problems prevent the use of prior art technologies for large arrays, such as stage vibrations, relatively limited focus capabilities, imaging linearity, dielectric layer interference, and limited fault detection and classification capabilities. Each of these problems will now be discussed in turn.

The magnitude of stage mechanical vibrations increases with the time passed between adjacent pixels. This time is equal to the reciprocal of the frame-rate multiplied by the number of rows in the array. For the 10 GPS and 320 320 array scenario discussed above, this is 3 millisecond, vs. 3 microseconds for a 32 32 array. Image processing cannot be used to compensate for these vibrations, because parts of the image can be missing, thereby reducing accuracy. It is noted that electron imaging systems are more susceptible to stage mechanical vibrations as the mechanical stage move in vacuum.

A further limitation of prior art spot grid array implementations arises from the fact that inspecting with confocal imaging requires very tight focus control, which is very difficult to achieve at high scan rates with large NA short-wavelength optics. To overcome this problem, simultaneous multi-height confocal imaging is necessary. However, while taking several height-slice images sequentially, as described in the prior art, is compatible with a one frame review mode, it is not compatible with the continuous motion requirements of inspection systems.

Another limitation to large arrays in the prior art is the linearity requirement on the lens array, imaging optics and detector arrays. To obtain good results from a spot grid array system, close tolerances on the linearity of the optics is important both for the microlens array and for the de-magnification optical elements. The optical spots must be located on an exactly rectilinear grid with very exact distances between the spots. Such extreme linearity is difficult and expensive to achieve.

Another limitation of prior art technology is the need to employ a coherent laser source to achieve sufficient power density for high-speed inspection. Many inspected substrates are covered by transparent or semi-transparent dielectric layers, which cause interference phenomena between the surfaces of the dielectric layers. As the thickness of these layers varies across the wafer, the phase of the reflections of the coherent light from the top and bottom of the dielectric layer varies. Moreover, the interference can be either constructive or destructive. These interference phenomena cause a change in the reflected power despite the absence of defects or irregularities, limiting the accuracy of defect detection and thereby limiting the capability of the system to identify true defects.

A further limitation of prior art spot grid array techniques arises from the limited fault detection and classification ability resulting from the collection of light signals from a single angular section of an object. As a result, fault detection and analysis may require more than a single inspection, thus dramatically increasing the amount of data that needs to be processed and collected for reliable detection and classification of faults.

There exists a need for a low-cost, accurate, high-speed imaging system with a large FOV for reducing manufacturing costs and increasing production throughput.

SUMMARY OF THE INVENTION

The present invention provides a high data rate spot grid array imaging system that compensates for stage vibrations.

The present invention further provides a high data rate spot grid array imaging system having a small overlap between coverage areas of lenses of a lens array in consecutive columns, thereby overcoming the severe linearity requirements of prior art systems and allowing the utilization of cost effective microlens arrays.

The present invention further provides for the employment of broadband illumination and broad illumination spots to overcome dielectric layers interference without reducing the throughput of the imaging system.

The present invention further provides for the collection of reflected light from the spots formed on the substrate from several directions simultaneously, thereby improving the fault classification and detection capabilities of the imaging system.

The present invention further provides for the simultaneous collection of data from more than one distance from the inspected substrate, thus allowing the selection of one or more relevant data sets out of a plurality of data sets, instead of mechanically moving the substrate upwards or downwards.

According to the present invention, the foregoing and other features are achieved in part by an imaging system with a two-dimensional and periodic array of lenses, each lens imaging a spot in an object plane, such as a substrate to be inspected, upon an image plane to image a two-dimensional and periodic array of spots from the object plane upon the image plane. A sensor is provided in a conjugate image plane with a two-dimensional and periodic array of readout elements, each collecting the signal from a spot in the object plane. A mechanical system moves the substrate in a direction nearly parallel to an axis of the array of spots such that as the substrate is moved across the spot array in the scan direction, the spots trace a path which leaves no gaps in the mechanical cross-scan direction. A compensator is provided for compensating for mechanical inaccuracies in the moving stage.

DESCRIPTION OF THE INVENTION

An embodiment of the present invention will now be described with reference to FIGS. 1 a and 2 . As shown in FIG. 1 a , a radiation source such as a light source 100 ; e.g. at least one laser, diode or lamp, provides a light beam. Illumination optics 110 , such as a conventional collimator, collimates the beam and brings it to the desired width. Collimating optics 110 may include a polarizing element to ensure that the light reaches polarizing beam-splitter 120 with the polarization that will reflect the beam into the imaging path. A quarter-wave plate 130 is used to rotate the polarization of the illumination light by 90 degrees. The collimated light impinges upon a lens array 140 where it is focused by the individual elements into an array of separate spots 150 upon an object to be imaged 160 , such as a semiconductor substrate. Each element in lens array 140 can be an individual lens, such as a micro-lens, or a multiple lens element. Reflected light from substrate 160 is redirected through lens array 140 and quarter wave-plate 130 , and reaches polarizing beam-splitter 120 with a polarization rotated by 90 degrees or half a wavelength with respect to the illumination beam. It therefore passes through polarizing beam-splitter 120 . An optical telescope 170 is used to image the back pupil plane of the lens array 140 (which can be the same plane as lens array 140 ) upon a two-dimensional detector array 180 , such as a CCD array corresponding to lens array 140 , such that each CCD pixel sees one spot on substrate 160 . Signals from detector array 180 are read out by a data acquisition section 190 , which can be used to transfer them to an image processing unit 191 and/or an image display unit 192 . Telescope 170 can be placed in an intermediate image plane where light reflected from spot array 150 forms an intermediate image substantially equal in size to lens array 140 , to demagnify the intermediate image before it reaches detector array 180 .

Substrate 160 is carried on a mechanical stage 165 which is moved in the y direction in a direction which is nearly parallel to one of the axes y of the array of spots 150 . The deviation from parallelicity is such that as substrate 160 is moved a distance substantially equal to the length L of the spot array in the scan direction y, the spots trace a path which leaves no gaps in the mechanical cross-scan direction (the x direction).

Several types of lenses can be used in lens array 140 , such as standard lenses or microlenses of either the refractive or diffractive type. For relatively low NA's and large FOVs, plastic diffractive elements can be used. This allows for FOVs of many tens of centimeters across or even more. For high NA applications microlens arrays (typically tens of microns across) can be used. If diffractive lens elements are used, the lens array can further contain an aperture array (i.e., a pinhole array) to truncate the higher scattering orders created by the diffractive elements. Diffractive lenses are particularly suitable when used in conduction with short wavelengths of light, such as extreme ultraviolet (EUV) of about 13 nm, to practice the invention.

Moreover, the micro-lens arrays referred to herein, such as lens array 140 , can be a single array of lenses, or multiple arrays arranged in series, as per coventional optical techniques, so the optical paths of the individual lens elements from the separate arrays form a compound lens. Such an arrangement results in arrays of compound lenses having a higher numerical aperture than can be obtained with arrays of single lenses. Such compound micro-lens arrays can be mechanically assembled by stacking individual lens arrays, or manufactured by, for example, well-known MEMS (micro-electro mechanical systems) manufacturing techniques.

FIG. 2 schematically depicts spot array 150 in the substrate (object) plane. For simplicity, FIG. 2 shows an 8 wide (a-h) by 6 deep (1-6) array of spots. When practicing the present invention, the array typically comprises at least several hundred lens elements, resulting in a corresponding number of spots. The shift in the mechanical cross-scan x direction between the lens centers of lenses in consecutive lines determines the pixel size in the x direction (i.e., the projection p x on the x-axis of the distance between the e'th lens in the first line e1 and the e'th lens in the 2nd line e2). The pixel size reflects how densely substrate 160 is sampled. To obtain continuous coverage of substrate 160 , the last lens in column d6 must trace a path no more than one pixel away in the cross-scan x direction from the tangent of the first lens in an adjacent column (c1). The pixel size in the mechanical scan y direction p y (not shown) is determined by the distance traversed between the spot center of a given spot between two consecutive samplings of the detector; that is, the distance between the center of a spot f4 at time 0 ( f4t0 ) and the same spot one sampling interval later ( f4t1 ). This distance is determined by multiplying the stage velocity and sampling interval.

Substrate motion can be achieved by any means ensuring accurate and linear motion, such as can be obtained from a conventional interferometer-controlled stage with linear motors and air-bearings, commercially available from Anorad Corporation of New York. To correct the residual inaccuracy such as that created by mechanical vibrations of the stage, a servo 175 can be included to control an optical element for moving the spot array and compensating for the substrate mis-location. In the embodiment of FIG. 1 a , the movable optical element may be the lens array 140 itself. In another embodiment of the present invention, the angle of incidence upon the back pupil of lens array 140 may be changed by means of a movable mirror, an electro-optic or an acousto-optic element in the optical illumination path and/or the collection path.

To ensure the spots 150 are focused on substrate 160 , any focus error which needs to be corrected is measured using conventional techniques, as disclosed, for example, in U.S. Pat. No. 6,124,924, the entire disclosure of which is incorporated herein by reference. Correction is then implemented either by moving substrate 160 in the z direction (i.e., up or down in relation to lens array 140 ), by moving lens array 140 , or by another optical element (not shown) which is moved to compensate. If substrate 160 is not planar, lens array 140 or another optical element can be tilted to compensate for the substrate's local (i.e., within FOV) slope.

In the embodiment of the present invention shown in FIG. 1 a , either a collimated, partially collimated, or non-collimated illumination source 100 can be used. In an embodiment of the present invention where illumination source 100 is collimated, lens array 140 is placed one focal distance away from substrate 160 to create an array of focused spots 150 . If the lens elements of lens array 140 have negligible on-axis aberrations, an array of diffraction-limited spots 150 is obtained. In that case, the role of the collection optics (ref. nos. 120 , 130 , 140 , 170 ) is to image lens array 140 's back pupil on detector array 180 with a resolution requirement determined by the size of the individual lenses in array 140 . Since the size of the lenses is typically in the tens of microns to several millimeters range if microlenses are used to make up lens array 140 , and the size of the spots 150 is in the tenths of a micron to tens of microns range, the requirement for imaging the back pupil of lens array 140 is much simpler than a requirement to image the entire FOV. Since resolution in this case is obtained by the illumination, illumination source 100 needs to be a laser light source to provide sufficient brightness. This embodiment of the present invention is essentially a laser-scanning microscope where the only scanning element is mechanical stage 165 . If a pin-hole 171 is inserted in the focal point of the telescope 170 and aligned with illumination source 100 , telescope 170 becomes a confocal microscope.

In embodiments of the present invention where a partially-collimated or non-collimated illumination source 100 is employed, the spots in array 150 are not diffraction limited. In this case, placing pin-hole 171 at the focal point of telescope 170 ensures imaging of diffraction limited spots. This embodiment of the present invention is essentially an array of imaging microscopes each imaging one spot, with the motion of stage 165 used to create continuous coverage of substrate 160 .

In another embodiment of the present invention, a standard beam splitter is used in place of polarizing beam splitter 120 . Consequently, there is no need for quarter wave plate 130 .

In a further embodiment of the present invention illustrated in FIG. 1 d, the illumination path does not pass through lens array 140 but reaches substrate 160 via a different path. That path may be either illumination of all the area of the FOV or include a lens array 140 a or an equivalent diffractive optical element as shown to illuminate only the array of spots 150 .

In a further embodiment of the present invention illustrated in FIGS. 1 j-l, the back pupils of the lenses in array 140 are not identical for the illumination path and the collection path. This difference is achieved by placing an aperture either in the illumination path or in the collection path or in both. For example, by blocking the pupil centers in the illumination path and allowing only those centers through in the collection path we get a darkfield microscope. This blocking of the center of each lens allows finer resolution. The cost is stronger side-lobes, but these can be accommodated if a large lens array is used (e.g., 320 320 vs. 32 32).

Referring now to FIG. 1 j, a darkfield microscope according to this embodiment of the present invention is obtained by placing a plane 115 in the illumination path between illumination optics 110 and beam splitter 120 of FIG. 1 a . As shown in FIG. 1 k, plane 115 has dark circles 115 b blocking the light in the center of individual circular apertures 115 a corresponding to lens array 140 , thereby resulting in a ring aperture. Another plane 125 , as shown in FIGS. 1 j and 1 l, is placed between beam splitter 120 and telescope 170 , and is the reverse of plane 115 ; i.e., the center 125 b of each element 125 a is transparent, and the remainder opaque, resulting in a central circular aperture. Alternatively, planes 115 and 125 can be switched (i.e., central circular apertures in the illumination path and ring apertures in the collection path), and the darkfield microscope of this embodiment of the present invention will nevertheless result.

For high resolution imaging, it is desirable to create spot array 150 using lenses with a large numerical aperture; e.g., about 0.8. However, micro-lenses typically have a numerical aperture of about 0.4 or less. In a still further embodiment of the present invention illustrated in FIG. 1 b, micro-lens array 140 a employing inexpensive and readily available micro-lenses with low numerical apertures (e.g., about 0.1) is used to create a relatively large array of spots in an intermediate plane IP, which array is then de-magnified to the desired spot array size and projected by conventional optics 145 (called a relay optical system or relay optics ) upon substrate 160 . This embodiment allows the use of inexpensive micro-lenses, thereby lowering the cost of the imaging system.

The use of microlens technology results in a relatively flat optical surface that is in close proximity (typically several tens of microns or even less) with the substrate. Referring again to FIG. 1 a, in one embodiment of the present invention, the gap between lens array 140 and substrate 160 is filled with a liquid having an optimized refractive index; e.g., an index of refraction larger than that of air (n>1) which leads to improved resolution. Whereas a large NA and large FOV lens would have a large curvature requiring a substantial liquid media, the use of an array of lenses allows for a much lower volume of liquid. This embodiment of the present invention provides the advantage of immersion microscopy of effectively shortening of the optical wavelength, hence obtaining a finer resolution limit. A further advantage of large FOV immersion microscopy is the ability to do inspection of substrates processed in a liquid environment before their drying, such as in chemical mechanical polishing (CMP).

In an alternative embodiment of the present invention, illustrated in FIG. 1 c, an array of lasers 100 a, each laser individually controlled, is used as the light source to create a spot array on substrate 160 . Laser array 100 a can comprise an array of vertical cavity surface emitting lasers (VCSELs), available from Band Gap Engineering of Colorado. VCSELs are semiconductor lasers that emit light from the top of the chip, straight up. Light from laser array 100 a is passed through a lens 120 a to illuminate substrate 160 . Beam splitter 120 a is placed in a conjugate plane of light reflected from substrate 160 , so light from laser array 100 a passes through the conjugate plane, and reflected light from substrate 160 is directed to detector array 180 , as shown in FIG. 1 c. Thus, a lens array is not needed in this embodiment of the present invention.

The present methodology is compatible with photo-electron emission microscopy (PEEM). In a PEEM implementation, the system illuminates spots on a substrate (e.g., spots 150 on substrate 160 ) and collects emitted electrons to perform electron rather than photon (optical) imaging. Thus, detector array 180 comprises conventional sensors for detecting photo-electron emissions, such as a multi-channel plate (MCP) coupled to a CCD detector array, or a scintillator coupled to a CCD or MCP. Use of the present invention's discrete spot illumination with well separated spots enables high resolution PEEM with low resolution requirements for the electron imaging system, which only needs to provide sufficient resolution to prevent cross-talk between the separate spots.

The present invention also enables fast and efficient confocal imaging with continuous stage motion. In a further embodiment of the present invention, an array of pin-holes of the desired size and with separation corresponding to the micro-lens array is placed in a conjugate image plane and adjusted so the pinholes are concentric with the individual spot elements. Referring now to FIG. 1 e, microlens array 141 is used as the focusing optics to generate the conjugate image plane 141 a , and pinhole array 142 is placed in conjugate image plane 141 a concentric with the microlenses of lens array 141 . This technique is advantageous over prior art confocal imaging systems because there are no pin-holes in the illumination path and all of the source brightness is used. Moreover, this technique is compatible with laser as well as white-light illumination. Furthermore, the elements of microlens array 141 can be inexpensive diffractive microlenses, and the pinholes of pinhole array 142 can be advantageously sized to allow only the center spot generated by the diffractive microlenses pass to detector array 180 , while blocking out undesirable side bands.

In a further embodiment of the present invention, illustrated in FIG. 1 f, one or more conventional beam-splitting elements 210 are inserted in the collection path to split the conjugate plane of lens array 140 back pupil. Focusing optics 220 a-c, such as a microlens array similar to lens array 140 , is inserted for each conjugate pupil plane to form multiple conjugate image planes 221 a-c. For each conjugate image plane 221 a-c, a pin-hole array 230 a-c is placed with a different lateral shift relative to the best focus plane; that is, the distance d1 between lens array 220 a and pin hole array 230 a is different than the distance d2 between lens array 220 b and pin hole array 230 b, and the distance d3 between lens array 220 c and pin hole array 230 c is different than distance d1 or d2. By placing an imaging array (CCD) 180 a-c after each pin-hole array 230 a-c, multiple images are simultaneously generated, each of a different height slice on substrate 160 . In this way, the multiple imaging arrays 180 a-c can be used to simultaneously inspect the same spot on substrate 160 from different heights. The data from the multiple arrays 180 a-c can then be resampled to generate the image of the best focus plane. For example, gray level information for a given pixel on the surface of substrate 160 from each of the arrays 180 a-c can be processed by signal processor 240 to compensate for imperfect focus tracking.

In yet another embodiment of the present invention, reflected light from the spots formed on the substrate is collected from several directions simultaneously. This multi-perspective imaging technique enables defect detection and classification to be conducted with greater accuracy, since certain types of defects reflect light in characteristic known directions. Thus, the presence or absence of reflected light at a particular angle in relation to the substrate can be used to determine the presence of a particular type of defect.

The multi-perspective imaging of this embodiment of the present invention can be achieved by placing several optical systems, such as microlens arrays 340 a, 340 b, and associated detector arrays 380 a, 380 b at different angles with relation to substrate 160 , as depicted in FIG. 1 g. Instead of lens arrays 340 a, 340 b, any conventional optical systems can be employed that are capable of imaging the entire field of view of substrate 160 with the resolution of the separation of spots 150 . Alternatively, as illustrated in FIG. 1 h, a single lens array 1040 is provided comprising diffractive elements 1010 , such as in the spaces between lenses 1020 . Diffractive elements 1010 divert light scattered at different angles from substrate 160 to either particular regions of detector array 180 or to several detector arrays 180 , 1080 a, 1080 b, as shown in FIG. 1 i.

In the embodiment of FIGS. 1 a and 2 , the shift in the mechanical cross-scan x direction between the lens centers of lenses in consecutive lines determines the pixel size in the x direction (e.g. the projection p x on the x-axis of the distance between the e'th lens in the first line e1 and the e'th lens in the 2nd line e2). Moreover, the last spot in one column (d6) passes a distance of one cross-scan pixel (p x ) away from the path of the spot created by the first lens in an adjacent column (c1). Therefore, the distance between the lens columns or the lens pitch determines the number of lens rows in the array (n r ).

In an alternative embodiment of the present invention, a larger number of rows (n r ) are used, and the array is tilted such that the x-axis separation between the paths of lens in consecutive rows is a fraction (f) of the pixel-size (p x /f). The substrate velocity is chosen such that it transverses a distance in the y-axis a factor f larger than a single pixel (p y /f). Referring now to FIG. 3 a , wherein a simple scan pattern is shown, for a given pixel created by lens b1 1 , the subscript stands for the writing period, the y neighbor on top is b1 2 , and the x neighbor on the left is b3 n where n s/p y (to create a rectangular array, the value of s/ py needs to be an integer). In FIG. 3 b , however, an interlace scanning pattern is created (it shows f 2 for simplicity). In this case, b1 1 and b1 2 will be separated by a distance of 2p y , where the adjacent pixel to b1 1 will be b2 n and n s/2p y . b1 2 will be shifted relative to b1 1 in a diagonal with a slant of 1/f. Therefore, for a large f the separation is mainly in the y direction. The result is a continuous coverage of the substrate achieved by an interleaving of f periodic structures offset in both axes.

An advantage of the interleaving of this embodiment of the present invention is a larger number of individual spots in a given FOV. Therefore for an identical pixel-rate requirement the array read rate ( frame-rate ) can be lower since there are more elements in the array. When practicing this embodiment, close tolerances on the linearity of the motion of the mechanical stage and on the inter-lens spacing are necessary. Furthermore, the light source must be in the form of short pulses, rather than continuous waves (CW).

To obtain good results when practicing the spot array concept of the present invention, close tolerances on the linearity of the optics is important both for the microlens array and for the de-magnification optical elements. The optical spots must be located on an exactly rectilinear grid with very exact distances between the spots. For example, if we have a grid 1000 rows deep, the thousandth row spot of column n must pass accurately near the location which was viewed by the first row's spot of column n 1. Assuming a desired accuracy of {fraction (1/10)} th of a pixel, this implies linearity of one tenth of a pixel over the length of the FOV. Where the lens pitch is equal to 100 pixels, the linearity requirement is therefore 1:10 6 (1000 rows * 100 pixels pitch/0.1 pixel tolerance 10 6 ). This requirement for extreme accuracy is problematic if mechanical vibrations are present.

In a further embodiment of the present invention, this severe linearity requirement is removed by creating a small overlap between the coverage areas of the lenses in consecutive columns, thereby reducing the deleterious effects of mechanical vibration on the system. This is achieved by providing additional rows of lenses; e.g., adding rows 7 and 8 in the spot array of FIG. 2 . Furthermore, in most automated inspection systems, such as Applied Materials' WF-736, the image comparison is done between two locations along the substrate scanning direction. The additional rows of pixels of this embodiment enable pixels generated by individual columns to be compared to pixels generated by the same column. Moreover, image processing algorithms typically require operations on a given pixel's neighbor. The overlap between columns (i.e., the additional rows of pixels) is preferably sufficient to provide spare pixels (typically 1 to 5 pixels) to ensure that neighboring pixels used for purposes of an algorithm are all from the same column. In this way, spot d6 does not have to be compared with a remote spot such as c1. This embodiment essentially makes the lenses of each column into an individual data-path. It is also compatible with the use of a modularized image processing approach; for example, each column feeding into a separate image processing module. Such a modularized approach simplifies and speeds processing.

In this embodiment of the present invention, the linearity requirement is reduced to the distance between rows of an individual column which pass in the vicinity of each other. In the non-interlaced basic approach this distance is one lens pitch. For the case described above this is a linearity requirement of 1:1000 (100 pixels pitch/0.1 pixel tolerance). If interlacing is used (see FIG. 3 b ) the linearity requirement is multiplied by the interlace factor and thus becomes 1:10,000 for an interlace factor of 10.

As discussed above, a limitation of the prior art is the need to work with a coherent laser source to achieve sufficient power density for high-speed inspection. Many inspected substrates are covered by transparent or semi-transparent dielectric layers which cause interference phenomena between the surfaces of the dielectric layers. As the thickness of these layers varies across the wafer, the phase of the reflections from the top and bottom of each dielectric layer varies, and the resulting interference can be either constructive or destructive. This causes a change in the reflected power despite the absence of defects or irregularities, thereby limiting the capability of the system to identify true defects. To overcome this limitation of laser sources, some prior art inspection systems use broadband lamp illumination, which causes the effects of constructive and destructive interference to average out, rendering reflection intensity less dependent on dielectric layer thickness fluctuations.

However, lamp sources do not have the brightness of lasers, and this fact results in problems when the light from the lamp source is collimated prior to reaching the lens array. The more collimated the light at the lens array, the lower the available power. Low power requires a relatively long integration time to achieve a reasonable signal-to-noise ratio, and thereby limits the system's throughput. On the other hand, if the light is not collimated, the lens will not focus it to a diffraction-limited spot. A large illumination spot will either degrade the system's resolution or require a means in the collection optics, such as a pinhole array, to block out a portion of the large light spot and again create a weak signal requiring longer integration time and hence reduced throughput. Thus, prior art broadband lamp illumination schemes do not deliver adequate performance for spot grid array inspection systems.

FIG. 4 illustrates an embodiment of the present invention that overcomes the above-discussed limitations of prior art broadband illumination systems. In this embodiment, partially collimated broadband light from a lamp source enabling sufficient illumination of the substrate is used, creating illumination spots S which are larger than the diffraction limit. The imaging CCD array 500 shown in FIG. 4 is designed with pixel sizes corresponding to the system's required resolution. Each illumination spot S is thus imaged on more than one pixel 510 at a time.

As the substrate moves beneath the lens array 500 of this embodiment (e.g., along the y-axis), the same substrate location is imaged by corresponding pixels 510 of detector array 500 as the substrate location is illuminated by different parts of illumination spot S. The signal from the rows of pixels 510 is added in sync with the motion of the substrate, thus adding together the signals generated by different parts of illumination spot S. This embodiment of the present invention can be implemented either by charge transfer on a conventional CCD array, or by any well-known analog or digital technique, either on or off the detector array chip.

Furthermore, the signal corresponding to the same location on the substrate generated by the following lens and collected on other pixels 510 in detector array 500 can also be added to the signal from the previous lens. The example in FIG. 4 shows an integration of pixels from 10 consecutive rows for one lens' spot and an integration of 10 lenses overall an integration of 100 pixels. This 100-fold improvement in the effective brightness of the illumination source enables the use of a lamp source rather than a laser for overcoming the interference issues discussed above, while providing adequate throughput.

The detector array of this embodiment can be a uniform grid. In this case, only some portions of it would be utilized. Alternatively, it can be composed of a dense array for each lens, separated by an area that can be used for supporting electronics. For the example illustrated in FIG. 4 , we have a 10 by 10 pixel sub-array 520 for each lens, with a pitch equal to 100 by 100 pixels. Detector array 500 is tilted at the same angle as in the previously described embodiments to ensure full substrate coverage as well as integration of the signal from the sequential lenses illuminating a given area. Use of broadband illumination, as in this embodiment of the present invention, requires the use of refractive and not diffractive lens elements, as the latter's focal length is linearly dependent on wavelength.

In a further embodiment of the present invention illustrated in FIG. 5 , two corresponding substrates 640 a, 640 b, such as two identical dies from the same wafer, are placed on a movable stage 650 , and one die is used as a reference for inspection of the other die. A radiation source 600 , which can be any one of the illumination sources described above, provides light that impinges upon lens arrays 630 a, 630 b, as through illumination optics 610 and beam splitters 620 a, 620 b as necessary, to irradiate identical arrays of spots on substrates 640 a and 640 b. Lens arrays 630 a, 630 b can be any of the arrays discussed above.

Signals from substrates 640 a and 640 b are collected by detector arrays 660 a, 660 b, and the resulting images compared by processor 670 to determine if defects exist on one of the substrates 640 a, 640 b. For example, the gray levels of corresponding pixels of the two images are compared, and if they differ by more than a predetermined threshold amount, processor 670 determines that a defect exists at that pixel location. As in previous embodiments of the present invention, movable stage 650 moves such that substantially the entire surface of each substrate 640 a, 640 b is irradiated and imaged. However, an advantage of this embodiment of the present invention is that since both substrates 640 a, 640 b undergo the same vibrations of stage 650 , the unwanted effects of that vibration are not relevant, and do not need to be compensated for, as they do in the other embodiments described herein.

The following examples illustrate the calculation of various parameters relevant to the practice of the present invention:

Definitions

D Pitch between spots on substrate in microns

p Pixel size on substrate in microns

n y and n x number of rows and columns in array respectively

N total number of lens in array

V stage velocity in y direction in microns/sec

Since FOV D * n x , n y D/p. Thus, the total number of lenses N is calculated by:

For a given data-rate requirement (DR) the frame rate (FR) and hence stage velocity required are:

For a given pixel size, increasing the FOV is key to obtaining a larger number of pixels in the array, and hence to reduced frame-rates and stage velocity requirements (when using interleaving as shown in FIG. 3 b , the number of rows and hence array elements increases and the frame-rate goes down, but the stage velocity requirement remains unchanged). In the embodiments of the present invention employing direct lens array to substrate imaging, the FOV is not a limitation. However, when using conventional optics to re-image the microlens array upon the substrate, FOV becomes an issue.

If the pixel size is reduced to 10 nm and the FOV is increased to 10 mm, the total number of array spots is N 10,000/0.01 10 6 . Keeping the frame-rate (FR) at 10 6 frames/second, the data rate (DR) of the present invention becomes 10 12 pixels/second or one Tera-pixels/second. The stage velocity (V) at this DR is 10 mm/sec. This system according to the present invention is three orders of magnitude faster than any prior art system. Of course, such a system requires conventional image acquisition and image processing systems capable of handling a high data-rate. For example, the resolution of the system according to this embodiment of the present invention can be obtained with EUV (Extended UV with a wavelength 13-14 nm) optics.