GB2357158A - Inspecting objects with multi-wavelength ultraviolet radiation - Google Patents

Abstract

A method of inspecting objects, such as wafers and photomasks, comprises illuminating the object with ultraviolet radiation at multiple wavelengths. A multi-wavelength UV image is formed with a single broadband collector and the image is detected, and preferably analysed to identify and classify defects. Preferably the UV wavelengths are less than 400nm and separated by at least 10nm. A plurality of fluorescent images may be formed.

Description

2357158 Broad Spectrum Ultraviolet Catadioptric Imaging System The present

invention relates to optical systems adapted for imaging in the ultraviolet (UV) portion of the spectrum, and in particular to broadband UV catadioptric imaging optics, i.e. systems employing a combination of one or more lens elements and one or more reflecting (mirror) elements in series. The invention is addressed especially to systems that have been designed to correct for imaging and color aberrations.

Catadioptric imaging systems for the deep ultraviolet spectral region (about 0.15 to 0.30 pm wavelength) are known. U.S. Pat. No. 5,031,976 to Shafer and U.S. Pat. No. 5,488,229 to Elliott and Shafer disclose two such systems. These systems employ lens elements made from only a single refractive material, namely fused silica, since it is practically the only material that combines good transmission of deep UV light with desirable physical properties. For example, fluoride glasses (based on CaF 2. LiF, etc.), while trans missive of deep UV light, are generally considered too soft, making lens formation difficult. Thus, fluoride glass materials are normally avoided whenever possible.

In the above-noted 1976 Shafer patent, an optical system is disclosed, which is based on the Schupmann achromatic lens principle producing an achromatic virtual image, and which combines it with a reflective relay to produce a real image. The system, reproduced here as Fig. 7, includes an aberration corrector group of lenses 101 for providing correction of image aberrations and chromatic variation of image aberrations, a focusing lens 103 receiving light from the K:KLA-001.APL -2 Oducing an intermediate image 105, a group 101 for pr ame material as the other lenses field lens 107 of the s 1 05, a tl-lick lens 109 placed at the intermediate image 11, whose power and with a plane mirror back coating itudinal position is selected to correct the primary long color of the system in conjunction with t.kie focusing lens 103, and a spherical mirror 113 located between the - age and the thick lens 109' for producing a intermediate Im 140st of the focusing p(-)wer of the final image 115. herical mirror 113- It has a system is due to the sP er-mediate image 105 to small central hole near the int 105 to pass light from the intermediate image C, mirror coating allow log. Th - therethrough to the thick lens log a:.so has a small Ill on the back of the thick lens low light focused by the spherical central hole 119 to al ugh to the f inal image 115. while mirror 113 to pass thro Olor is corrected by the primary longitudinal (axial) c e field lens 3.07 placed at thick lens 109r the offner-tYP it'L,ve power to the intermediate image IC)5 has a POS placing the field correct secondary longitudinal color. rmediate image 105 lens slightly to one side of the inte, axial corrects tertiary longitudinal color. Thus chromatic aberrations are completely c,orrected over a broad spectral range. The system incidentlY also for narrow band lateral C0107r but fails to corrects plete correction of residual (secondary and proVide COM ver a broad UV spectrum higher order) lateral color 0 patent to Elliott and The above-noted 1229 tical system ides a modified version of the Op Shafer proV which has been optimized for use in of the 1976 patent, - ir laser applica 0.193 Am wavelength high power exclln- tions, such as ablation of a surface 121', as seen in e aberration corrector group Fig. S. This system has th S 105,, field - termediate fOcu 101,' focusing lens 103', 1 in mirror surfaces 1111 and 113' ens 109', thick 1 lens 107" therein and a 117, and 119' with small central openings 1976 patent, but here the f ocus 115' of the prior final at the inter 107, has been repositioned SO th field lens

K-"-ool -AP, - '. t.

mediate image or focus 1051 lies outside of the field lens 1071 to avoid thermal damage from the high power densities produced by focusing the excimer laser light.

Further, both mirror surfaces 1111 and 113' are formed on lens elements 1081 and 1091. The combination of all light passes through both lens elements 1081 and 1091 provides the same primary longitudinal color correction of the single thick lens 109 in Fig. 7, but with a reduction in total glass thickness. Since even fused silica begins to have absorption problems at the very short 0.193 pm wavelength, the thickness reduction is advantageous at this wavelength for high power levels.

Though the excimer laser source used for this optical system has a relatively narrow spectral line width, the dispersion of silica near the 0.193 4m wavelength is great enough that some color correction still needs to be provided. Both prior. systems have a numerical aperture of about 0.6.

Longitudinal chromatic aberration (axial color) is an axial shift in the focus position with wavelength.

The prior system seen in Fig. 7 completely corrects for primary, secondary and tertiary axial color over a broad wavelength band in the near and deep ultraviolet (0.2 pm to 0. 4 pm). Lateral color is a change in magnification or image size with wavelength, and is not related to axial color. The prior system of Fig. 7 completely corrects for primary. lateral color, but not for residual lateral color. This is the limiting aberration in the system when a broad spectral range is covered.

The invention can provide a catadioptric imaging system with correction of image aberrations, chromatic variation of image aberrations, longitudinal (axial) color and lateral color, including residual (secondary and higher order) lateral color correction over a broad spectral range in the near and deep ultraviolet spectral band (0. 2 to 0. 4 gm).

In addition to color correction, it is also desired to provide a UV imaging system useful as a K:XLA-001.APL 4 microscope objective or as microlithography optics with a large numerical aperture for the final image and with a field of view of at leasi 0.5 mm. The system is preferably telecentric.

In such a catadioptric imaging system an achromatic multi-element field lens is used. made from two or more different refractive materials, such as fused silica and fluoride glass. The field lens may be a doublet or preferably a triplet, which may be cemented together or alternatively spaced slightly apart. Because fused silica and fluoride glass do riot differ substantially in dispersiori in the deep ultraviolet, th(, individual powers of the several component elements of the field lens need to be o- high maglUtude. Use of such an achromatic field lens allows riot only axial color, but also lateral color to be completely corrected over a broad spectral range. Oniy on field tens component need be of a different refractive material than the other lenses of the system.

An optical system includes a focusing lens 2roup with plural lens element; , preferably all formed from a single type of material, with refractive surfaces havirg curvatures and positions selected to focus light to an intermediate image with hi;h levels of correction in the final image of both image aberrations and chromaiic variation of aberrations over a UV wavelength band of at least 0.20 to 0. 29 J).m, aiid preferably extending over 0.20 to 0.40 Am. Systerns adapted for a UV band that includes the 0. 193 um wavelength are also possible, The system also includes the aforementioned field lens group positioned near the intermediate image to prov,de correction of chromacic aberrations including residual axial and lateral color. 71'he intermediate image plane may be located either inside or outside the field lens group depending on the optimization. A catadioptric group includes a concave spherical reflector, which may either be a mirror or a reflectively coated lens element, and a planar or near planar reflector near the final image, which is a reflectively coated lens element. Both reflective elements have central optical apertures therein where reflective material is absent, allowing light from the intermediate image to pass through the concave reflector, be reflected by the planar (or near planar) reflector onto the concave reflector, and pass through the planar (or near planar) reflector, traversing the associated lens element or elements on the way.

The imaging system provides a numerical aper- ture of at least 0.7, a large field size of about. 0.5 mm and substantially flat field imaging over a broad wave length band extending into the deep UV portion of the spectrum. The system is useful in a number of optical configurations, including bright field illumination, directional and ring (nondirectional) dark field illu mination, fluorescence imaging, full sky scatterometer and confocal microscope con f ig-urat ions. UV imaging systems provide not only better optical resolution, but also better material identification due to strong varia tions in the reflectivity and absorption of UV light by materials, strong scattering (proportional to X'4), higher orders of diffraction, and fluorescence in the UV spectrua. Broad band UV imaging systems can have UV lamps as light sources, which provide incoherent light for no speckle imaging, and which enable other special imaging techniques, such as Nipkow disk type confocal microscopy, to be used in the UV spectrum. Possible applications for the broad band, deep UV objective lens include wafer and photomask inspection, material masking and cutting applications, UV lithography, biological microscopy, metallurgical microscopy, spectroscopic analysis of specimen materials, and others.

K-KLA-001.APL The invention is diagrammatically illustrated by way of example in the accompanying drawings in which:- Fig. I is a schematic side view of a catadioptric imaging system in accord with the present invention.

Fig. 2 is an enlarged portion of the imaging system of Fig. 1 in the vicinity of the inter-mediate image 13 showing elements of an achromatic field lens group for the system.

Fig. 3 is an enlarged portion, comparable to Fig. 2, of a catadioptric imaging syste-r-a in accord with the present invention showing elements of an alternative achromatic field lens group for the system.

Fig. 4 is a schematic side view of a catadioptric tube lens designed to accompany the imaging system of Fig. 1 when used as an infinity-corrected microscope objective.

Fig. 5 is a schematic side view of a portion of a catadioptric imaging system in accord with the present invention used for a dark-field light scatter imaging wafer inspection device, showing an ob.Lique laser beam illumination source.

Fig. 6 is a schematic side view of a wafer inspection apparatus employing the cat-adioptric imaging system of the present invention as a UV objective of the inspection apparatus.

Figs. 7 and 8 are schematic side views of catadioptric imaging systems of the pz-ior art.

With reference to Fig. 1, a catadioptric imaging system of the present invention, which is especially suited for use in broad-band deep-ultraviolet applications, is made up of a focusing lens group 11 forming an intermediate image 13, a field lens group 15 disposed proximate to the intermediat'e image 13 for providing correction of chromatic aberrations, and a catadioptric group 17 focusing the light from the intermediate image 13 to a final image 19. The imaging r,.KLA-001.APL system is optimized to correct both monochromatic (Seidel) aberrations and chromatic aberrations (longi- tudinal and lateral), as well as chromatic variations of the monochromatic aberrations, over a wavelength band that extends into the deep ultraviolet (UV) portion of the spectrum, including at least 0.20 to 0.29 Am UV light and preferably extending over a broad band covering 0.20 to 0.40 Am UV light. Both ranges include the 0.248 Am KrF excimer laser line and the 0.248 Am and 0.254 Am mercury emission lines. The broader spectral range also includes the 0.365 Am mercury emission line (commonly known as the i-line), the 0.351 Am XeF excimer laser line, and the 0.325 Am He-Cd laser line. A wide assortment of other laser and arc lamp emission wavelengths in the ultraviolet are also available. The system could also be adapted to provide chromatic aberration-corrected imaging over other UV wavelength ranges. For example, a 0.19 to 0.40 Am wavelength band that includes the 0.193 Am ArF excimer laser line is also possible. Narrower bands might also be used. The catadioptric system of the present invention can be adapted for a number of UV imaging applications, including as a L77 microscope objective, a collector of surface scattered UV light in a wafer inspection apparatus, or as mask projection optics for a UV photolithography system.

The focusing lens group 11 in Fig. I consists of seven lens elements 21-27, with two of the lenses 21 and 22 separated by a substantial distance from the remaining five lens elements 23-27. In particular, the separation of the pair of lenses 21 and 22 from the remaining five lens elements 23-27 in this focusing lens group is typically on the order of at least one-half the total combined thickness of the five lens elements 23-27 forming the main focusing subgroup. For example, lens elements 23-27 may span a distance of about 60 mm and lens element 22 may be 30 to 60 mm, from lens element 23.

The actual dimensions depend on the scale chosen for the K:KLA-001 APL design. The two lerises 21 and 22 form a nearly zero-power doublet for the correction of chromatic variation of monochromatic image aberrations, such as chromatic variation of coma and astigmatism. By having this doublet 21 and 22 relatively far from the rest of the system components, the shift of the light beam on these two lenses with field angle is maximized. That, in turn, helps greatly in achieving the best correction of chromatic variation of aberrations. The five lenses 23 27 of the main focusing subgroup in Fig. 1 consist of a thick strong negative meniscus lens 23, an opposite facing strongly-curved negative meniscus lens 24, a strong biconvex lens 25, a strong positive meniscus lens 26, and an opposite-facing strong ly-curv ed, but very weak, meniscus lens 27 of either positive or negative power. Variations of this subgroup of lens 23-27 are possible. The subgroup focuses the light to an intermediate image 13. The curvature and positions of the lens surfaces are selected to minimize monochromatic aberrations and also to cooperate with. the doublet 21-22 to minimize chromatic variations of those aberrations.

The field lens group 15, seen in an expanded view in Fig. 2, typically comprises an achromatic triplet, although a doublet might also be used. Both fused silica and CaF2 glass materials are used. Other possible deep UV transparent refractive materials can include MgF2, SrF2, LaF3 and LiF glasses, or mixtures thereof. Note, however, that some of' these materials car, be birefringent if they are not completely amorphous and contain microcrystals. Because the dispersions between the two UV transmitting materials, CLF2 glass and fused silica, are not very different in the deep ultraviolet, the individual components of the group 15 are quite strong. The triplet 15 may comprise a fused silica negative meniscus lens 31, a CaF, bic-onvex (positive) lens 33 and a fused silica biconcave (negative) lens 35, all cemented together. The optimized design for this configuration places the intermediate image 13 inside the -9-.

triplet group 15. Alternatively, as seen in Fig. 3, the achromatic f ield lens group may comprise two fused silica, opposite facing negative meniscus lenses 51 and 53, spaced slightly apart (typ. about 1.0 mm), followed by a CaF2 biconvex (positive) lens 55 nearly abutting the second of the meniscus lenses 53. The optimized design for this second configuration allows the intermediate image 13 to be formed outside the field lens group 15 beyond the CaF, lens. Either embodiment of the field lens group 15 has surface curvatures and positions selected to correct residual (secondary and tertiary) axial and lateral color. Primary color aberrations are corrected mainly by the lens elements in the catadioptric group 17- in combination with the focusing lens group 11. Use of two or more different refractive material types in the field lens groups, such as both fused silica and CaF2 glass, allows residual lateral color to be completely corrected, in addition to the axial color corrections provided by prior single-material field lenses.

As seen in Figs. 2 and 3, the intermediate focus 13 may be located either inside or outside of the field lens group 15. If the intermediate image 13 is inside the group 15, maximum aberration correction is achieved. Alternatively, it may be desirable to have the image 13 outside the field lens group 15 in cases where there is danger that high optical power densities may cause damage to the glass material of one or more of the field lens elements. Further-more, small imaging errors due to glass inhomogeneities are less of a factor when the field lens group 15 is placed somewhat away from the intermediate image 13.

The catadioptric group 17 seen in Fig. 1 includes a first optical element consisting of a fused silica meniscus lens 39 with a concave reflective surface coating 41 on a back surface of the lens 39, and also includes a second optical element consisting of a first silica lens 43 with a reflective surface coating 45 on a back surface of the lens 43. (The front surfaces of the K:KLA-001.APL two lens elements 39 and 43 of the catadioptric group 17 face each other.) The reflective surface, coatings 41 and are typically composed of aluminum, possibly with a MgF, overcoat to prevent oxidation. Alum.inum has a nearly uniform reflectivity of at least 92% over the entire near and deep UV wavelength range. Other metals commonly used as reflective coatings in the visible portion of the spectrum have reflectivities that vary considerably with wavelength or even become opaque in the deep UV. For example, silver drops to only 4% reflectivity at 0.32 im.

Possible alternatives to aluminum, but with somewhat lower reflectivities near 60%, include molybdenum, tungsten and chromium. These may be favored in ce " rtain high power applications, such as laser ablation.

Specialized coatings, including long-wave pass, short wave pass and band pass dichroic reflective materials, partially transmissive and reflective material coatings, and fluorescent coatings, could all be used for a variety of specialized applications.

The first lens 39 has a hole 37 centrally formed therein along the optical axis of the system. The reflective coating 41 on the lens surface likewise ends at the central hole 37 leaving a central optical aperture through which light can pass unobstructed by either the lens 39 or its reflective coating 41. The optical aperture defined by the hole 37 is in the vicinity of the intermediate image 13 so that there is minimum optical loss. The achromatic field lens group 15 is positioned in or near the hole 37. The second lens 43 does not normally have a hole, but there is a centrally located opening or window 47 on the surface reflective coating 45 where the coating is absent, leaving another optical aperture at the central window location 47. The optical aperture in lens 39 with its reflect.".ve coating 41 need not be defined by a hole 37 in the lens 39, but rather could simply be defined by a window in the coating 41 where reflective coating material is absent, just as with lens 43 and coating 45. In that case, light would pass K:KLA-001.AP1 one additional time through the refractive surfaces of lens 39.

The coated mirror 45 may be either flat or preferably slightly curved. The slight curvature will provide some centering tolerance for that element, Moreover, if the reflective element 45 is slightly curved, it will make contact with a wafer surface or other object to be imaged by the catadioptric system less likely and avoid the damage to both mirror coating 45 and the object which would result from any such contact.

Light from the intermediate image 13 passes through the optical aperture 37 in the first lens 39 then through the body of the second lens 43 where it is reflected back through the body of the second lens 43 by the planar or near planar mirror coating 45 on the back surface of the lens 43. The light then passes through the first lens 39, is reflected by the -mirror surface 41 and passes back through the body of the f irst lens 39.

Finally the light, now strongly convergent passes through the body of the second lens 43 for a third time, through the optical aperture 47 to the final image 47. The curvatures and positions of the first and second lens surfaces are selected to correct primary axial and lateral color in conjunction with the focal lens group The optical elements may be assembled with or without cemented interfaces. Cementing lens elements simplifies assembly, resulting in a less expensive objective. It also results in a more robust device in which the cemented elements are far less likely to go out of alignment. Moreover, the cementing process can be used to seal environmental sensitive materials, such as the CaF, field lens element between other elements. On the other hand, since the polymeric materials used as cement in lens systems can be damaged by deep UV light possibly leading to degradation of the optical system and K:XLA-001 APL providing uncertain lifetime in some high power UV appli cations, non-cemented systems will be preferred in those high power deep UV system applications where long-term reliability is a significant issue. This is an important consideration in the selection of a cemented or non cemented design for the field lens groul..) elements located near the inter-mediate image where UV radiation is most concentrated.

Specific values for two examples of optimized broad-band system designs, one for the field lens group of Fig. 2 and the other for the alternate field lens group of Fig. 3, follow. The lens surface data are based on refractive index values (relative to air) for the wavelengths 0.200, 0.220, 0.245, 0.290 and 0.400 gm. The resulting designs have a numerical ape:,fture of about o.9 and a field size of about 0.5 mm diameter. Variations of the design can be done for a slightly lower numerical aperture, for example about 0.7, by simply reoptimizing the surface curvatures for the desired parameters. such a variation would be suitable for reticle inspection, where longer working distances are preferred. Likewise, with slight adjustments to the surface curvatures, and allowing for a narrower wavelength band over which longitudinal and lateral are corrected, the system can be optimized to include 0.193 um ArF excimer laser light over a wide band of 0.19 to 0.40 pm or over a narrower band.

K:KLA-001.APL Leng 12ata (úMbOdiment 4 1) Radius of Surface Curvature (mm) Spacing (mm) Material 1 1105.7 4.000 fused silica 2 53.051 2.500 air 3 284.061 5.000 fused silica 4 - 57.181 60.000 air 39.782 15.000 fused silica 6 13.379 7.977 air 7 - 12.955 5.169 fused silica 8 - 17.192 1.000 air 9 42.964 8.000 fused silica - 55.269 1.000 air 11 18.436 8.000 fused silica is 12 91.574 6.253 air 13 - 20.802 4.000 fused silica 14 - 21.768 17.120 air is 7.217 5.494 fused silica 16 2.259 3.000 CaF2 glass 17 11.760 1.500 fused silica 18 373.721 39.730 air 19 flat 7.000 fused silica reflector/ flat - 7.000 fused silica 21 flat 36.000 air 22 50.470 - 9.500 fused silica reflector/ 23 64.290 9.500 fused silica 24 50.470 36.000 air 25 flat 7.000 fused silica 26 flat 1.500 air CKLA-001.APL J&ns Data (Embodiment III Radius of Surface Curvature (mm) Spacing (mm) Material 1 67.007 4.000 fused silica 2 50.308 2.000 air 3 120.297 6.000 fused silica 4 - 37.494 30.636 air 24.138 10.000 fused silica 6 13.441 9.532 air 7 13.518 7.546 fused silica 8 17.997 1.000 air 9 34.465 6.000 fused silica 517.022 1.000 air 11 18.268 10.000 fused silica 12 965.352 4. 181 air 13 30.177 9.746 fused silica 14 28.138 7.892 air 19.346 2.500 fused silica 16 36.530 1.0010 air 17 6.687 5.026 fused silica 18 2.044 0. 017 air 19 2.044 2.000 CaF2 glass 90.635 36.108 air 21 908.968 7.000 fused silica reflector/ 22 1000.0 - 7.000 fused silica 23 908.968 36.000 air 24 48.244 - 9.500 fused silica reflector/ 25 63.204 9.500 fused silica 26 48.244 36.000 air 27 908.968 7.000 fused silica 28 1000.0 1.500 air K:KLA-001.01 With reference to Fig. 4, a tube design for using the imaging system of Fig. 1 as a microscope objective is shown. Illumination of a sample surface being imaged by the objective of Fig. 1 may be made through 'the objective itself, by means of an ultraviolet light source 61, such as a mercury vapor lamp or excimer laser, together with conventional illumination optics 63, 65, 67, leading to a beamsplitter 69 in the objective's optical path. The imaging path for light received from the objective of Fig. I is via transmission through the beamsplitter 69 to a microscope tube, whose design may also be catadioptric. The tube elements include a pair of opposite facing negative meniscus lenses 71 and_ 73 closely spaced to one another, and two spherical mirrors 75 and 77 spaced from each other and from the pair of lenses 71 and 73 by at least 400 mm. The curvature of mirror 75 is concave toward the lenses 71 and 73 and the mirror 77, while the curvature of mirror 77 is convex toward the mirror 75, both curvatures being at least 1000 mm radius, i.e. nearly flat. The mirrors 73 and 75 fold the optical path off-axis so that the system length is under 500 mm. One example optimized for the particular objective seen in Fig. I has the following characteristic refractive and reflective surfaces for optical elements 71, 73, 75 and 77:

Radius of Surface Curvature (mm) Spacing (mm) Material 1 92.965 4.989 fused silica 2 96.420 1.000 air 3 89.440 4.616 fused silica 4 87.394 413.186 air 1219.4 413.186 reflector 6 1069.7 463.186 reflector K:KLA-001.APL Referring now to Fig. 5, yet al,,Iother UsS for - I is for wafex" inspection, the imaging system of Fig namely as a directional dark field, scattered light collector. A uV laser illumination source si directs a beam 85 through holes 83 and 87 formed in lenses 39" and 43" and reflective coatings 41" and 45" of the catadiop tric group onto a surface 89 to be inspected. Alterna tively, only the reflective coatings 41" and 45" might be absent or only partially reflective to form transparent or at least partially transmissive windows for the light beam 85. The beam 85 might also enter the system from below the hemispherical reflector 41". The angle of incidence is oblique, i.e. at least 60 from verti.cal due to the high numerical aperture (about 0.90) of the imaging system. Illumination may be done from more than one direction and angle of incidence. 'The specularly reflected light 93 passes through holes 91 and 95 formed in lenses 39" and 4111 and reflective coatings 41" and 45" of the catadioptric group (or in the coatings 41" and 45" only). UV light scattered by features on the sample surface 89 are imaged by the catadiopt:i.-ic imaging system of Fig. 1, beginning with the catadiopt-ric group, then through the achromatic field lens group, and focusing lens group, to the tube elements 71, 73, 75 and 77 of the tube system (absent the illumination group 61-69).

Ring dark field illumination can be used instead of the directional dark field illumination of

Fig. 5. in that case, a ring illumination source, such as a ring shaped f lash lamp, shines a ring or partial ring of light through a matching hole or partially reflective area of the coating in the hemispherical reflector. This can be done for more than one angle of incidence of light on the object to be observed.

in yet another alternate embodiment, the objective can be used as a dome scatterometer. The reflective components in such a system may be coated witft long pass, short pass or bandpass, diffuse, fluorescent K:KLA-001.APL coatings. The optical components themselves are then imaged onto a set of detectors placed around the hemi spheric reflector to measure the full sky scattering pattern from the fluorescent emission of the coatings.

Alternatively, a dichroic or partially reflective and partially transmissive hemispheric mirror coating would allow direct measurement of scattered light transmitted through the coating. Fig. 6 shows a wafer inspection apparatus that can use the catadioptric

imaging system as a UV objective 86 for the apparatus. The apparatus may be constructed according to one or more of U.S. Patents 4,247,203; 4,556,317; 4,618,938; and 4,845,558 of the assignee of - the present invention. A semiconductor wafer 82 with a plurality of integrated circuit die 84 at some stage of formation on the wafer 82 is shown lying in a carrier or stage 80. The stage 80 is capable of providing movement of the wafer 80 with translational X and Y and rotational 0 motion components relative to a UV microscope objective 86, such as the catadioptric imaging system seen in Fig.

Light 83 collected from a die 84 or a portion of a die and formed into a magnified image of that die or portion by the objective 86 is transferred through a relay lens or lens system 90, such as the tube lens system seen in Fig. 4, into the aperture of a video or CCD array camera 92 sensitive to deep UV light. The output 94 of the camera 92 is fed into a data processor 96, which compares pixel data relating to the UV image of the die or die portion either to data corresponding to other portions of the image or to stored data from previous images relating to other die or other die portions. The results of this comparison are fed as data 98 to an output device, such as a printer or a CRT display, or to a data storage unit.

one advantage of the broad band UV objective lens of the present invention with lateral color correc tion is its large field size of about 0.5 mm diameter, compared to prior narrow band UV lenses that have a field

K:KLA-001.APL size on the order of 0.1 mm. or less. This provides a field with at least 25 times greater area, allowing for high speed inspection of a wafer surface, reticle or similar object. Inspections that previously took 20 to 30 minutes to complete can now be done in about one minute. The new lens also has a significantly flattened field, which is a must for surface viewing and inspection. Note that no broad band UV objective previously exists. The current Zeiss Ultrafluor 10ox objective needs tuning a ring and refocus in order to be used at a specific wavelength.

However, the most important advantage is the objective's multi--wavelength capability. Prior UV_objec- tives are relatively narrow band designs in which good performance is limited to single wavelength sources, because of significant chromatic aberrations over wave length bands as small as 10 nm in the deep UV (e.g., near 248 nm). In many applications, multi -wavelength sources, such as Xenon flash lamps and arc lamps, are the preferred light source, due to their low cost and absence of coherent artifacts. Such sources demand primary and residual longitudinal and lateral color correction over a broader wavelength band of at least 20 nm, and preferably over 100-200 nm wide bands. In other cases, multiple light sources at widely different wavelengths may be used in a single system, again demanding broad band color correction in the UV spectrum.

For a wafer fab facility with both i-line (365 nm) and deep UV 248 nm-based steppers, the broad band Uv lens of the present invention enables a reticle inspec tion system to have selectable i-line or 248 nm wave length illumination to match the exposure wavelength for which a reticle or mask has been constructed. Such wave length matching is important, for example, for inspecting advanced phase shifting masks. Likewise, the broad band UV lens of the present invention allows for construction of a system with selectable wavelength for improved ins pection of photoresist on wafers. Photoresist is a K:KLA-001 ALPL material that is transparent to visible light, providing low contrast for inspection at those wavelengths.

However, photoresist becomes opaque at the shorter UV wavelengths with different resists becoming opaque at different wavelengths. Thus, wafers with i-line photo resist can be inspected with high sensitivity at a wave length of about 313 nm, where it is opaque. Wafers with deep UV (248 nm) photoresist can be inspected at a different wavelength around 220 nm. The lens system of the present invention allows the same inspection appa ratus to inspect both kinds of photoresist.

In a similar fashion, multiple wavelength imaging of UV light can help in understanding the observed image. For example, different materials vary in their reflectivities at different UV wavelengths. That is, they have what by analogy to color in visible light might be termed "UV color". Most metals other than aluminum become opaque, while silicon becomes more reflective in deep UV light. If combined with a UV camera having a UV photodetector imaging array and a combination of wavelength selective UV transmission filters, the broad band UV objective lens of the present invention can be used to provide a 11UV color" image of the object being inspected. This would be useful in defect and feature classification on a wafer. The UV imaging array can be made, for example, with CsZTe, Cs 3Sb, K2CsSb or GaAsP detector elements. Silicon-based back thinned or microlens array CCDs have also been developed for UV imaging.

Likewise, a system can be built that analyzes materials based on fluorescence. Many materials, including most organic materials, such as photoresists, fluoresce, but they respond to different excitation wavelengths and they emit at different fluorescence wavelengths. With the broad band UV imaging lens of the present invention, a fluorescence detection system can be built with adjustable wavelength from about 0.2 to 0.4 14M. By analyzing the fluorescence wavelength, the K-KLA-001 APL compositions of the materials being observed can be determined. The UV reflective components of the catadioptric system can be coated with long pass, short pass, or bandpass dichroic coatings to image the fluorescence signals while rejecting the reflected or scattered excitation light.

The depth of focus of an optical system (proportional to wavelength and inversely proportional to the square of the system's numerical aperture) is intrin sically very short in the ultraviolet spectrum (typically on the order of 0. 1 to 0. 5 Am). This can create a problem in imaging patterned wafers and other similar surfaces with nonplanar profiles. With the broadband UV optics of the present invention, we can use multiple uv wavelength imaging at different depths and computer soft ware integration of the resulting images to extend the depth of focus to about 1 Am. For exa-raple, we can scan the surface of a wafer or other object at three different UV colors with about a 10 to 50 nm wavelength separation (e.g., at 0.20, 0.22 and 0.24);m) using three different focal planes for the different wavelengths to image dif ferent slices of the surface. A confocal microscope configuration with the UV objective olf. I the present invention and with three detectors having corresponding bandpass filters could be used for this purpose. The three images can then be integrated by a computer to produce a composite with the increased depth of focus.

The small depth of focus of the high N.A. lens system can also be used to advantage to produce high resolution image slices at various depths that can be integrated to form a 3-D image.

The UV objective lens of the present invention is useful in many different microscopy techniques, including the previously mentioned bright field, dark field and fluorescence techniques, as well as phase contrast, polarization contrast, differential inter ference contrast and other techniques. For example, the system may be used in a confocal microscope geometry using a UV lamp and full field imaging instead of a scanning laser device. Some or all of these techniques can be used simultaneously or in sequence within the same objective lens.

K:KLA-001,.APL -------- 22

Claims (1)

1. A method of inspecting objects for defects, compnsr-ig:

   illuminating an object with ultraviolet radiation at mtiltiple wavelengths, forming a multi-wave length ultraviolet image with a single broadband collector o.- ultraviolet radiation, and detecting said multi-wave length ultraviolet image, The method of claim 1, wherein said object being inspected comprises a waf(:r surface comprising photoresist.

   The method of claim 1, wherein said object being inspected comprises a phas- shift photomask.

   4. The method of any preceding claim, further cornpnsing analyzing said mu.ti I wavelength ultraviolet image so as to identify and classify defects on said object.

   5. The method of any of the above claims, wherein illuminating the object comprises illuminating the object with ultraviolet radiation having a wavelength of less t;aan approximately 400 nanometers.

   6. The method of any of the above claims, wherein illuminating the object comprises illuminating the object with ultraviolet radiation at multiple wavelengths, wherein each of the multiple wavelengths is separated by at least approximately 10 nm.

   7. The method of any of the above claims, further compnsing forming a plurality of fluorescent images with the single broadband collector of ultraviolet radiation, wh--rein each of the plurality of fluorescent images is detected c-it a wavelength band separatcd by at least approximately 50 nanometers.

   --- 233 The method of any of the above claims, further comprising forming a plurality of fluorescent images with the single broadband collector of ultraviolet radiation, wherein each of the plurality of fluorescent images is detected at a wavelength band separated by at least approximately 50 nanometers. and further comprising converting the plurality of images to visible light for visual inspection of defects.

   9. The method of any of the above claims, further comprising, for each ultraviolet illumination wavelength, forming a corresponding ultraviolet image slice of the object at a different focal plane from image slices corresponding to the other illumination wavelengths and forming a composite image by means of computer integration of the image slice, the composite image characterized by a greater depth of focus than any of its constituent 1maQe slices.

   10. The method of any of the above claims, wherein said multi -wavelength image formed by said broadband collector has a field size of at least about 0.5 mm diameter.

   11. The method of any of the above claims, wherein forming said multiwavelength image comprises moving a relative position of said object to said collector such that different portions of said object are successively imaged.

   12. The method of any of the above claims, wherein said object is illuminated by an incoherent multi-wavelength ultraviolet source.

   13. The method of any of the above claims, wherein detecting said multiwavelength image comprises passing said image through one or more wavelength- selective ultraviolet transmission filters.

   14. The method of any of the above claims, further comprising converting the detected multi-wavelength image to a visible light image for visual observation.

   24 15. The method of any of the above claims, wherein said object being observed is selected from the group comprising semiconductor,vafers, photomasks, reticles, materials undergoing masking or cutting, biological samples. metallurgical samples, an(I samples undergoing spectroscopic analysis.

   16. The method of any of the above claims, wherein said broadband collector comprises a catadioptric imaging system.

   17. A method of inspecting objects for defects, substantially as herein described w',th reference to Figs. I to 6 of the accompanying drawings.

   18. A method of inspecting objects for defects comprising:

   illuminating an object with ultraviolet radiation having a wavelength which is less than 400 nanometers; forming a plurality of fluorescent ultraviolet images with a single broadband collector of ultraviolet radiation, each image detected at a wavelength band separated from other wavelength bands of images of the same object by at least 50 nanometers; converting said plurality of images to visible light for visual inspection of defects.

   19. A method of inspecting objects for defects comprising:

   illuminating an object with ultraviolet radiation at multiple wavelengths separated by at least nm, for each ultraviolet illumination wavelength, forming a corresponding ultraviolet image slice of the object at a different focal plane from image slices corresponding to the other illumination wavelengths, and forming a composite image by means of computer integration of the image slices, the composite image characterized by a greater depth of focus than any of its constituent image slices.

   26 20. A broad-band deep-ultraviolet achromatic catadioptric imaging system, comprising:

   a focusing lens group including a plurality of lens elements, all formed from a single refractive material ty-pe, with refractive surfaces thereof disposed at first predetermined positions along an optical path o f the system and having curvatures and said positions selected to focus ultraviolet light at an intermediate image within the system, and simultaneously to also provide in combination with the rest of the system, high levels oE Correccaan Ln --he final image or- 'c)or_'n -mage abrrations and chromatic variation of aberrations over a wavelength band including at least 0.20-0.29;;m, a field lens group with a net positive power disposed along said optical path proximate to said inter mediate image, the field lens group including a plurality of lens elements formed from at least z_wo different refractive materials with different dispersions, with refractive surfaces of the lens elements of the field lens group disposed at second predetermined positions and having curvatures selected to provide correction of chromatic aberrations including at least secondary lateral color of the system over said wavelength band, a catadioptric group including a first optical element having at least a concave reflective surface with a central optical aperture therein disposed along said optical path proximate to said intermediate image so that ultraviolet light from the intermediate image can pass therethrough, the catadioptric group also including a second optical element which is a lens with a reflective mirror coating on a rear surface of said lens except for a central area on said rear surface where said mirror coating is absent, the optical elements of said catadioptric group being arranged such that ultraviolet light from the intermediate image transmitted through said central optical aperture 'of said first optical :% 27 element of said catadioptric group passes through the lens portion of said second Optical element of said catadioptric group, reflects from said reflective mirror coating on said lens rear surface, passes back through said lens portion towards said concave reflective surface of said first optical element, is reflected thereby and passes a third time through said lens portion of said second optical element and through said central area of said lens rear surface to form a final image beyond said catadioptric group.

   ?J. The imaging system of claim 20 wherein said wavelength band includes 0.20-0.40 pm.

   22. The imaging system of claim 20 wherein said wavelength band includes 0.193 pm.

   23). The imaging system of claim 20 wherein said single refractive material type of said focusing lens group is fused silica.

   24. The imaging system of claim20 wherein said field lens group is formed from lens elements made of fused silica and a fluoride glass.

   25. The imaging system of claim20 wherein said first optical element of said catadioptric group comprises a concave mirror with a central hole therein forming said central optical aperture.

   29 26. The imaging system of claim2O wherein said first optical element of said catadioptric group comprises a meniscus lens with a concave reflective surface coating thereon.

   27, The imaging system of claim'26 wherein said central optical aperture is formed by a central hole in said meniscus lens.

   29- The imaging system of claim 26 wherein said central optical aperture is formed by a central area on said meniscus lens where said concave reflective surface coating is absent.

   29. The imaging system of claim20 wherein said catadioptric group is characterized by reflective surface curvatures selected to provide at least, a 0.8 numerical aperture and a 0.5 mm field of view for said final image of the imaging system.

   30. A catadioptric imaging system, comprising- first negative lens, second positive biconvex lens closely spaced to the first lens to form a substantially zero-power corrector group for chromatic variations of image aberra tions, a third negative meniscus lens spaced from the second lens, a fourth negative meniscus lens with concave surfaces of the third and fourth lenses facing each other, a fifth positive biconvex lens, a sixth positive meniscus lens, 29 a seventh meniscus lens of near zero-power with concave surfaces of the sixth and seventh lenses facing each other, the third through seventh lenses being closely spaced together to form a focusing lens group with minimal image aberrations, said focusing lens group providing an intermediate image, eighth, ninth and tenth lenses forming an achromatic field lens group positioned proximate to said intermediate image, said field lens group including at least one positive convex lens of a different refractive material type than all other lenses in the system and at least one negative meniscus lens, the field lens group having a net positive power, an eleventh negative meniscus lens with a convex surface facing said first lens having a reflective coating thereon and with a first central optical aperture therein proximate to said intermediate image, and a twelfth near zero-power, substantially flat lens with a reflective coating'on a surface facing away from the first lens with a second central optical aperture therein, said twelfth lens spaced apart from the eleventh lens, the eleventh and twelfth lenses with their respective reflective coatings forming a catadioptric group providing a light focusing relay for the intermediate image to provide a final image proximate to the second optical aperture.

   The imaging system of claim 30 wherein the third lens is spaced at least 30 mm, from the second lens, and said twelfth lens is spaced at least 30 mm from the eleventh lens.

   J2. The imaging system of claim 30 wherein the focusing lens group, field lens group and catadioptric group have refractive and reflective surfaces that are characterized by the following dimensional values:

   Radius of Surface 1 Curvature (mm) Spacin g (mm) Material 1 1105.7 4.000 fused silica 2 53.051 2.500 air 3 284.061 5.000 fused silica 4 57.181 60.000 air 39.782 15.000 fused silica 6 13.379 7.977 air 7 12.955 5.169 fused silica 17.192 1.000 air 9 42.964 8.000 fused silica 55.269 1.000 air 11 18.436 8.000 fused silica 12 91.574 6.253 air 13 20.802 4.000 fused silica 14 21.768 17.120 air 7.217 5.494 fused silica 16 2.259 3.000 CaF, glass 17 11.760 1.500 fused silica 18 373.721 39.730 air 19 flat 7.000 fused silica reflector/ flat 7. 000 fused silica 21 flat 36.000 air 22 50.470 9.500 fused silica reflector/ 23 64.290 9.500 fused silica 24 50.470 36.000 air flat 7.000 fused silica 26 flat 1.500 air 31 3 The imaging system of claim 30 wherein the focusing 33.

   lens group, field lens group and catadioptric group have refractive and reflective surfaces that are characterized by the following dimensional values.

   Radius of Surface 1 Curvature (mm) Spacing (mm) Material 1 - 67.007 4.000 fused silica 2 50.308 2.000 air 3 120.297 6.000 fused silica 4 37.494 30.636 air 24.138 10.000 fused silica 6 13.441 9.532 air 7 13.518 7.546 fused silica 8 17.997 1.000 air 9 34.465 6.000 fused silica 517.022 1.000 air 11 18.268 10.000 fused silica 12 965.352 4.181 air 13 30.177 9.746 fused silica 14 28.138 7.892 air 19.346 2.500 fused silica 16 36.530 1.000 air 17 6.687 5.026 fused silica 18 2.044 0.017 air 19 2.044 2.000 CaF, glass - 90.635 36.108 air 21 - 908.968 7.000 fused silica reflector/ 22 1000.0 - 7.000 fused silica 23 - 908.968 36.000 air 24 48.244 - 9.500 fused silica reflector/ 63.204 9.500 fused silica 26 48.244 36.000 air 27 908.968 7.000 fused silica 28 1000.0 1.500 air