US006672739B1 (12) United States Patent (10) Patent N0.: US 6,672,739 B1 Argyle et al. (45) Date of Patent: Jan. 6, 2004

(54) BEAM HOMOGENIZER IBM Technical Disclosure Bulletin vol. 37 (12), 469—471 (1994). (75) Inventors: Bernell Edwin Argyle, Hopewell H. J. Gerritsen, W. J. Hannon, and E. G. Ramberg; Elimi Junction, NY (US); Je?'ery Gregory nation of speckle noise in holograms With redundancy McCord, San Jose, CA (US) Applied Optics vol 7 (11), 2301—2311, (1968). Gordon W. Ellis; Fiber—optic phase randomiZer fro micro (73) Assignee: International Business Machines scope illumination by laser J. Cell. Biology vol 83, p303 , Corp., Armonk, NY (US) (1979). ( * ) Notice: Subject to any disclaimer, the term of this Robert Hard, Robert Zeh, and Robert D. Allen; Phase patent is extended or adjusted under 35 randomized laser illumination for microscopy , J. Cell Sci. U.S.C. 154(b) by 0 days. 23, 335—343 (1977). S. K. Dey, M. J. BoWman, and A. D. Booth; AneW technique (21) Appl. No.: 09/386,017 for improving domain pictures in Kerr—effect microscopy using a laser source; J. Scienti?c Instr. (J. Physics E) vol 2, (22) Filed: Aug. 30, 1999 162—164 (1969). Malcolm J. BoWman; TWo neW methods f improving optical (51) Int. Cl.7 ...... F21K 27/06 image quality; Appl. Optics vol. 7 2280—2284 (1968). * cited by examiner (52) US. Cl...... 362/259; 362/553; 372/9; 385/28; 385/901 Primary Examiner—Sandra O’Shea Assistant Examiner—John Anthony Ward (58) Field of Search ...... 385/15, 27, 28, (74) Attorney, Agent, or Firm—Rodney T. Hodgson 385/29, 31, 32, 39, 133, 901, 147; 362/553, (57) ABSTRACT 259; 372/9, 12—15 An apparatus, system, and method for illuminating a litho (56) References Cited graphic mask or an object in a microscope is presented, Whereby the output of a laser beam homogeniZer is imaged U.S. PATENT DOCUMENTS on to a ?eld such as the object plane for lithographic

4,744,615 A * 5/1988 Fan et a1...... 219/121.61 application or on to the rear focal plane of an epi illuminat

5,634,920 A * 6/1997 Hohla ...... 606/12 ing objective lens as a source for Wide ?eld illumination at 5,889,278 A * 3/1999 Richard ...... 250/214 R an object plane in a microscope (for application to magni?ed OTHER PUBLICATIONS imaging of Weak phase objects)., and the image of the output is dithered With respect to the ?eld B. E. Argyle, D. A. Herman, and B. Petek; Reduction of coherence—related noise in laser based imaging systems 40 Claims, 7 Drawing Sheets

12 U.S. Patent Jan. 6, 2004 Sheet 1 0f 7 US 6,672,739 B1

F.i9‘ 1 10 j Prlor Art

101 Fig. 2 Prior Art

10] Fig. 3 19 Prior Art 30

Fig. 4 Prior Art U.S. Patent Jan. 6, 2004 Sheet 2 0f 7 US 6,672,739 B1

62

Fig. 14 U.S. Patent Jan. 6, 2004 Sheet 3 0f 7 US 6,672,739 B1 U.S. Patent Jan. 6, 2004 Sheet 4 0f 7 US 6,672,739 B1 U.S. Patent Jan. 6, 2004 Sheet 5 0f 7 US 6,672,739 B1 U S Patent Jan. 6, 2004 Sheet 6 0f 7 US 6,672,739 B1 U S Patent Jan. 6, 2004 Sheet 7 0f 7 US 6,672,739 B1 US 6,672,739 B1 1 2 LASER BEAM HOMOGENIZER SUMMARY OF THE INVENTION The present invention is a system, apparatus and method FIELD OF THE INVENTION for uniformly illuminating a ?eld, (such as an object in an object plane or the rear focal plane of an epi illuminating The ?eld of the invention is the ?eld of apparatus for objective lens) by imaging the output aperture of a laser or homogenizing laser light for illumination of objects in other light beam homogeniZer on the ?eld using an appa microscopes and in photolithographic applications. ratus Which dithers the image of the output of the beam BACKGROUND OF THE INVENTION homogeniZer With respect to the ?eld perpendicularly to the optical axis or dithers the output of the beam homogeniZer 10 may be used as illumination sources for micro so that the image of the beam homogeniZer dithers With scopes and for photolithographic systems. Laser beams are respect to the object or the rear focal plane. The dither typically single mode beams or multimode beams, and the means, in a preferred embodiment, is synchroniZed With beam homogeneity and coherence properties of such beams dynamic effects in the object or With a video recording may not be sufficient for the application. For single mode device to provide enhanced resolution. laser beams, the light distribution is Gaussian in a line taken 15 perpendicular to the beam. For uniform illumination, the BRIEF DESCRIPTION OF THE DRAWINGS “top” of the beam may be used, but much of the light intensity is then throWn aWay. Multimode beams may have FIG. 1 shoWs a prior art system of a laser beam homog a “top hat” distribution, but unless all modes have nearly eniZer. equal population, the intensity may vary over the “top hat”. FIG. 2 shoWs a prior art system for homogeniZing a laser In applications Where intense laser light exposes highly beam. non-linear photoresists, for example, to produce highest FIG. 3 shoWs a prior art system. resolution exposure in photoresist, a feW percent variation in FIG. 4 shoWs a prior art system. light intensity may be suf?cient to degrade the system capability. For microscopic investigations of Weak phase 25 FIG. 5 shoWs a sketch of the system of the invention. objects, such variation may also lead to masking of the small FIGS. 6(a—a) shoW images of the ?ber face recorded for difference in re?ected light from neighboring objects, and to various different dithering means producing motions up to introduction of artifacts related to the coherence of light several mode spacings. re?ected from different parts of the objects in the ?eld of FIGS. 7(a—a) shoW object images corresponding to the vieW. various modal ?ber patterns and dithering means recorded in An excellent overvieW of microscope illuminators is FIG. 6. included in S. Inoue, Video Microscopy, Plenum Press, NeW FIG. 8(a) shoWs an image of the ?ber face recorded as a York, NY, 1986. doughnut-shaped time average of circularly dithered motions larger than a face. Prior art systems have used systems of diffusers in the 35 laser beam path to homogeniZe the beam. Prior art systems FIG. 8(b) shoWs an object image When a dithering-means such as outlined in an abstract by G. W. Ellis, “A Fiber-Optic axis of rotation is Misaligned With the optic axis by 30 Phase-RandomiZer for Microscope Illumination by Laser”. degrees. J. Cell Biol. 83, 303a (1979), have used optical ?bers as FIG. 9 shoWs an object image When a dithering-means beam homogeniZers to convert near single mode laser beams axis of rotation is aligned Parallel to the optic axis. to multimode beams for illumination systems in micro FIGS. 10(a—b) shoW object images comparing hg-arc and scopes. Prior art systems have used light tunnels, as in US. dithered laser Illumination. Pat. No. 4,744,615, and systems of lenses, as in US. Pat. No. FIGS. 11(a—b) shoW an image of the dithered ?ber face 4,475,027, and holographs, as in US. Pat. No. 5,610,733, to and the associated object image, respectively. homogeniZe laser beams. The output of such beam homog 45 eniZers is often insufficient for exacting lithographic and FIG. 12 shoWs an an image of the difference betWeen tWo microscopic Work, especially on a small scale. Prior art object images taken sequentially in time. systems R. D. Allen, “Phase-Randomized Laser Illumination FIG. 13 shoWs an object image presenting magneto-optic for Microscopy”, J. Cell Sci. 23, 335 (1977), have used polar Kerr contrast. rotating Wedges in combination With a diffuser in a laser FIG. 14 is a sketch of a holloW pieZoelectric tube holding beam to move the laser beam around on the object being an optical ?ber. illuminated. Such rotating Wedges average out small scale inhomegenuities in the laser beam, but leave large scale DETAILED DESCRIPTION OF THE inhomogenuities in place. INVENTION OBJECTS OF THE INVENTION 55 FIG. 1 shoWs a prior art system such as described in detail in US. Pat. No. 4,744,615, Whereby a laser 10 produces a It is an object of the invention to produce an apparatus, laser beam 12 Which passes through an optical system 13 system, and method for illuminating an object or mask (here denoted as a simple lens) into the entrance aperture 14 uniformly over a certain area. of a laser beam homogeniZer 15. The laser beam homog It is an object of the invention to produce an apparatus, eniZer may be constructed from four re?ecting plates (only system, and method for illuminating Weak phase object in a tWo shoWn in the elevation vieW of FIG. 1) arranged in the microscope With high ef?ciency. form of a square or rectangular tube. Anumber of light rays It is an object of the invention to produce an apparatus, 16 are shoWn as a guide to the eye. The exit aperture 17 of system, and method for illuminating an object or mask the beam homogeniZer 15 is imaged by an optical system 18 uniformly over a certain area With high efficiency, and to 65 (here denoted by a symbol for a lens)on to an object 19. In record dynamic effects With improved spacial and temporal a prior art case, object 19 may be a mask Which has apertures resolution and contrast. for some of the light rays 16 to pass Which may be further US 6,672,739 B1 3 4 focused by another optical system (not shown) on to a Work of the microscope, and then conducted by the optical system piece (not shown). The laser beam homogeniZer system as of the microscope to uniformly illuminate the object plane of shoWn passes light from the laser 10 Which has not re?ected the microscope, or may be used to image the ?ber face on from the plates of the laser beam homogeniZer 15 and and illuminate a mask or other object directly Without produces an inverted image of the eXit aperture 16 on the passing through an intermediate image forming stage. object 19. Light Which has bounced multiple times from the FIG. 14 is a sketch of a holloW pieZoelectric tube 60 top, bottom, and side plates of beam homogeniZer plates 15 holding an optical ?ber 50. One end of the pieZo tube 60 is also form such images on object 19. The light from each held rigidly in a holder 62. The pieZoelectric tube 60 may multiple bounce comes from a different region of laser beam 12, and hence non uniformities on laser beam tWelve are have one grounded electrode (not shoWn) inside the tube, averaged out by the multiple overlapping images. The angle and four electrode (not shoWn) in stripes longitudinally of the light rays 16 striking the object may be controlled by along the sides of the tube. Application of voltages to the the focusing action of the optical system 13 and the number various electrodes Will cause the holloW tube to bend and move the optical ?ber eXit face 52 in horiZontal and vertical of bounces the light rays 16 make on the plates of the beam homogeniZer 15. The uniform illumination of the object 19, directions as shoWn by the arroWs in FIG. 14. 15 and the controlled angle Which the light rays Which eXit a Microscopic objects can often be categoriZed as ampli mask are important in use of the mask and laser system in tude objects or phase objects in so far as the interaction of lithography and in of material. Residual inho light With the object produces changes in light amplitude or mogenuities in the irradiation of the object 19 are, hoWever, phase, respectively. To render an image of a phase object on still a problem. photographic ?lm or video screen requires conversion from FIG. 2 shoWs a prior art system for homogeniZing a laser a phase distribution to an amplitude distribution. This is beam. A rotating diffuser plate 20 is rotated by a motor 22 typically accomplished using polariZed light for illumination to homogeniZe the output beam 24 so that the image of the and by introducing optical phase-analyZing elements in the diffuser plate 20 may be formed on an object 19. The setup vieWing path such as an optical analyZer and compensator. When these elements are adjusted to produce phase-to sketched in FIG. 2 is advantageously used in a microscope 25 illumination system. Unfortunately, a laser such as an argon amplitude conversion, most of the light incident on the used in a setup such as shoWn in FIG. 2 produces object is discarded. The residual light that remains in the a beam having coherent properties Which lead to a residual vieWing path carries the spatial information of interest for speckle pattern When the object 19 is an object illuminated studies of the phase object. When conventional illumination in the microscope system. The spot of light produced on the is used, the images formed in a high poWer microscope are object 19 has the time averaged intensity of the spot of light typically so Weak as to be undetectable With even the formed on the diffuser plate 20, Which is typically a Gaus sensitive dark-adapted eye. We say that such a Weak-contrast sian beam, and the illumination is non-uniform over the ?eld image comes from a “Weak phase object”, or WPO. of illumination. To render images of WPOs places great demands on the microscope, the photometric sensing, and signal processing. FIG. 3 shoWs a prior art system Where a rotating Wedge 35 30 is used to rotate a spot of light from the laser beam on the Special digital signal processing methods have been devel object 19. Depending on the Wedge angle of the Wedge 30, oped to enhance image contrast for video microscopy. The the image of the laser beam rotates about a point With greater Hamamatsu C1966 image processor that Was developed by or lesser amplitude. In essence, each point of the image of microbiologists at the Woods Hole Oceanographic Institute the beam on the object 19 rotates about the point Where it is one eXample. Imaging cards installed in the personal Would be imaged if there Were no Wedge. computer can also process Weak video signals. Processing FIG. 4 shoWs a prior art system Whereby a laser beam consumes time. The problem to be solved is to overcome processing limitations and loW throughput. passes through a beam homogeniZer, in this case an optical ?ber 40 , and the optical ?ber is vibrated by a pieZo electric The approach of the invention sketched in FIG. 5 is to greatly enhance the strength of the microscope’s incident crystal 42 to change the mode propagation patterns in the 45 ?ber, and hence change the speckle pattern formed by the Wide ?eld illumination to Well above What is normally light from the eXit face 44 of the ?ber When the eXit face 44 available from conventional sources. Speci?c means are of the ?ber is imaged on the object 19. Such a system still described to inject high intensity laser illumination into the has a residual effect due to incomplete homogeniZation and microscope to replace the less intense, unsteady, incoherent incomplete averaging of the light over the eXit face 44 of the arc illumination and overcome the problem of image noise ?ber. associated With laser coherence. This method thereby FIG. 5 shoWs a sketch of the system of the invention used greatly enhances the contrast and the effective optical reso for a microscope illuminator. An optical ?ber 50 With other lution of processed images; it enhances the optical limit of components described beloW is used as a beam homogeniZer perception. to homogeniZe a laser beam 12. The eXit face 52 of the ?ber 55 Earlier methods using laser light to enhance Wide-?eld provides illumination to the object using an optical system illumination intensities have been found to introduce image comprising a collimating lens means 53, a rotating Wedge artifacts unrelated to the nature of the WPO: image noise and 30, and the optical system of a microscope 55. A camera 56 non-uniformity. and a recording device 57 record images of the illuminated The method proposed and demonstrated here uses a laser object in a preferred embodiment. A means 58 for dithering light stream in combination With a multi-mode ?ber or other the eXit face 52 perpendicular to the optical aXis is shoWn. laser beam homogeniZer together With a means to homog Such a means 58 may be a simple mechanical means, or the eniZe the spatial variations of illumination intensity at the means may be an acousto optic, magneto optic, electrooptic, object caused by localiZed ray bundles emanating from or other means as knoWn in the art for modifying the different ?ber modes. propagation of light. The invention may be used as shoWn 65 The reduction-to-practice of the present invention is dem With a microscope, Where the light from the eXit face of the onstrated by forming high magni?cation Wide-?eld images ?ber is ?rst imaged in a ?eld on the rear focal plane (RFP) of laser-illuminated magnetic recording head devices and US 6,672,739 B1 5 6 comparing them quantitatively With images formed using a Different means are available to render amplitude images conventional 100 W mercury arc-lamp supplied Within the of WPOs in a polariZing microscope: one using Wide-?eld same microscope. The 100 W Hg-arc is considered the most illumination is based on video detection and image process intense incoherent source available for visible light micros ing; another uses a spot focused illumination With spot copy. We shoW that (1) image artifacts due to laser coher photometric detection and mechanical scanning to render the ence and ?ber modes are minimal, (2) dynamic range is images. The compatibility of Wide-?eld illumination and increased, (3) image S/N ratio is increased, (4) ‘effective’ video imaging makes this method more time-efficient optical resolution is improved, and (5) ef?ciency of though because electron beams in the video camera scan much put is improved With diminished time consumed by multiple faster than mechanical methods scanning a sample under focused spot illumination. Wide-?eld illumination is also steps of image processing (image accumulation, averaging 1O more consistent With procedures conventionally used in and subtraction). Also, synchroniZing the mode scrambler optical microscopy. Laser-spot focused imaging has until and dithering means to an integer of the video frame rate noW produced images With the best signal-to-noise ratios by permits the advantage (6) that WPO motions may be means of lock-in ampli?cation and synchronous detection. recorded in real time (at video rates). This bene?ts video HoWever, Wide-?eld video detection and processing can be recording not only device responses (for example, motions 15 used to capture both synchronous- and asynchronous time of magnetiZation inside recording heads being stimulated varying phenomenon for time rates of change sloWer than With applied electrical currents), but also micro biological video frame rates. Responses at shorter times have been cell motions, for example in a squid neural axons responding imaged With stroboscopic averaging using Wide-?eld 10 ns to the ?ring of a synapse. pulsed laser illumination and sub 10th nanosecond focused laser illumination. The later is used currently for character Potential Application and Implementation iZing spatial and temporal responses inside DASD recording The signi?cance to overcoming inadequacies of prior-art heads. Light intensity requirements for focused illumination microscope sources (insuf?cient arc intensity and stability, are less severe than for Wide-?eld illumination. HoWever, and artifacts of laser coherence), is the improvement in lasers (CW or pulsed) available commercially can supply capability to image a large class of WPOs that are of current 25 adequate poWer (1—4 W) for both. Microscopes such as the technological or biological importance. Zeiss Axiomat are capable of accepting these poWer levels Examples of important WPOs are: Without suffering damage to the optical elements, especially (1) micro biological cells suspended in an optically iso for Wide-?eld illumination Where the laser focus spot is in air tropic liquid (visible due to the cell’s optical behind the objective, not inside objective elements. birefringence); Another bene?t of the convenience and information con tent made possible by Wide-?eld image recording, is to (2) defects producing hot spots in integrated circuits facilitate pulsed-laser strobing of magnetic responses inside carrying electrical current (visible due to thermal DASD recording heads. Strobing With sub-tenth stress-induced birefringence in a polymer ?lm nanosecond time resolution can enable studies of magnetic overcoat13); 35 instability knoWn to be problem in making magneto (3) magnetic responses and/or defects in magnetic devices resistive (MR) heads. Motions of domain Walls inside such as recording heads (visible due to Kerr- or Faraday MR-head shields can be captured stroboscopically While magneto-optic effects intrinsic to the constituent mag recordings of the instability (changes in the MR transfer netic materials), and function) are recorded electronically. Knowledge of Wall (4) exposed pattern in photolithgraphic ?lm (made visible motion behavior in magnetic shields and hoW they correlate as due to small index of refraction difference from With instability in MR response should improve understand non-exposed region). ing the fundamental nature of the problem. The instability These are just four of many WPOs capable of being problem permeates a large segment of the magnetic storage magni?ed and vieWed With polariZed light microscopy. industry, one that depends on stable MR read heads and Magnetic materials subdivide into 3 optical groups: highly 45 inductive Write heads. This problem is expected to Worsen as re?ecting, transparent, and absorbing (non-re?ective). Iron DASD data rates increase. based ferrites for example, span a range from being trans parent through opaque (black) due mostly to interaction of Sensitivity and Degree of Dif?culty iron-oxide With visible light. WPOs of importance to frontier studies in micro biology An example of a strong phase object (SPO) Would be or technology exhibit phase shifts as small as a feW ten materials used in liquid crystal personal computer displays thousandths of the Wave length of light. Its origin in the case and ?at-panel monitors. Certain types of liquid crystals of magnetic phase images is magnetic circular- or elliptical exhibit large circular birefringence, hence large angles of birefringence. Circular birefringence causing the polar Kerr rotation for polariZed incident light. effect produces a rotation of the plane of polariZation upon PolariZed light microscopy of WPOs and SPOs can be of 55 re?ection from a material magnetiZed perpendicular to its great bene?t to diagnostics in manufacturing, defect surface. Components of magnetiZation parallel to the plane monitoring, device developmental studies, studies on neW of the ?lm produce the Longitudinal Kerr effect (elliptical thin ?lm magnetoresistive (MR) materials for MR read birefringence). These effects can also be imaged With kerr heads, ferrite materials 3 for recording heads, and for microscopy8. The amount of rotation for NiFe alloys is very fundamental investigations for better scienti?c understand small-only of the order of ten milli-degrees1 in Permalloy ing of properties of magnetic ?lms/ materials/liquids, or ?lms thicker than the optical skin depth, 6. Films thinner stress-induced birefringent ?lms/liquids/materials. Other than 6 yield smaller Kerr-signals. Thus, investigations of means for their imaging, such as electron microscopy, thin ?lm permalloy deposited on glass or on transparent atomic force microscopy and magnetic force microscopy, antiferromagnetic ?lms used for biasing the MR element, are more limited in their application. In practice 65 Will also bene?t from the greater illumination intensity. unfortunately, these methods typically offer only a slightly Non-magnetic rotations due to the microscope itself, can improved spatial resolution at great sacri?ce to throughput. overWhelm small Kerr rotations. The ordinary Snell’s laW of US 6,672,739 B1 7 8 refraction at spherical lens surfaces can cause rotation of the objective’s rear focal plane (RFP) and the “point source” is plane of polarized light. The effect is as large as 5 degrees positioned in one arm of the polariZation extinction cross in a 100X/ 1.3 NA objective lens. To remove this effect, the seen When the polariZers are crossed and the Bertrend lens illuminating ray bundle must be injected into the lens along is focused on the RFP. This position minimiZes the disper a plane de?ned by one of the great arcs on the lens surface sion in out-put polariZations of the objective due to the and the light polariZation must be parallel- or perpendicular snell’s laW refraction effect1>2 . to that plane2. The “point source” in this invention is an image of the Background light may be uniform or non-uniform, time output end of a 200 pm core diameter optical ?ber being dependent or constant. Uniform constant background can be illuminated at its input end by focusing a laser beam. (Note: A diffuser plate called a phase randomiZer that is present in eliminated by sequential digital image processing steps 10 including analog-to-digital conversion, image summation, the illuminator section of some microscopes uniformly and background image subtraction. The time consumed by distributes the arc- or lamp source image over the objective this process may eXceed time constants of disturbances in rear-focal-plane area. This plate must be removed When the microscope system causing deterioration of the image. imaging magnetic WPOs.) This bright end-face of the ?ber is imaged at the RFP and thereby produces an outgoing These disturbances may be caused by the illumination 15 system producing changes in light amplitude or polariZation, parallel ray bundle, providing nearly uniform illumination on the object planel. Image distortion due to residual non or by motions in a mechanical system too Weak to prevent uniform background illumination may be diminished using relative motions of microscope optics and object stage. A-to-D conversion, digital averaging1'3, and subtraction As mentioned above, the process of conversion from a from a reference image that contains only the backgroundl. phase object to amplitude image using nearly crossed polar It is often necessary to integrate many video images to iZers (and, in some cases, an optical compensator1'4), Will average noise and obtain suitable background subtraction. strongly reduce the light intensity at the camera. The inten For this reason, systems for image processing should utiliZe sity of an arc source reaching the video camera attached to frame memories having large piXel depth. Depths larger than the microscope is generally smaller than the threshold of the 6—8 bits of human eye gray-level response, are required. camera response. This effect can severely limit the dynamic 25 This large piXel depth enables summing single-frame images range and the signal-to-noise ratio. of minute WPO phase variations into image amplitudes and Expansion of the dynamic range calls for increasing the variations ?lling up higher level bits. Non-Zero bits remain light intensity at the detector. Light intensity at the camera ing after subtracting the higher level background image, are is proportional to object illumination and to sin2(6+/—A), residuals in a loWer bit-slice range. The bit-slice selection Where 6 is the angle the analyZer is offset from 90 degrees for the difference image may then be adjusted Within the to the polariZer, and Where A (put simply) is due to the image processor so as to not address empty higher-level bits object’s phase distribution. Detector intensity may be and to render an image from the loW level bits and produce increased by increasing 6 HoWever, the resulting increase in gray level contrast suitable to the vieWer’s eye. This image background intensity causes increased detector noise 4. too can be stored in a digital frame memory. Issues as these become particularly acute When the micro It is desirable for increased efficiency to reduce the time 35 typically consumed in completing the sequence of events for scope’s optical magni?cation is increased, because intensity background subtraction and enhancement. Raising the inten at the detector falls off inversely as the square of magni? sity of the illumination Will shorten this duration. Increasing cation for objects illuminated With transmitted light intensity also produces images With greater effective reso (Faraday contrast) and to the fourth poWer of magni?cation lution and helps reduce image deterioration during process in re?ection (Kerr contrast)4. This drop off severely limits ing due to thermal/mechanical drifts in relative positions of both the magni?cation and the ‘effective image resolution’. microscope optics and stage. Mechanical drift as little as 0.1 To remove image distortion due to a non constant back micron during frame grabbing and image processing can ground intensity, digital image processing is used. It is used reduce ‘effective resolution’ beloW the fundamental limit set also to subtract background that arises from non-magnetic by the Wavelength of the light and the objective numerical re?ections or refractions in the microscope lenses, beam 45 aperture. splitters, or other objects in the light stream, including the In order to decrease image processing time, the illumina sample under inspection, its protective overcoat ?lms, cover tion source must not only be intense but steady. Video slips, etc. Non-uniform background is especially prominent detectors With higher photo sensitivity Would also help, but When vieWing WPOs in a re?ection microscope Where the these tend to produce larger shot noise. A source of inco illumination passes to the object through the objective lens. herent light that is considered to be the brightest, is the 100 Phase changes at the beam splitter and lens surfaces produce W Hg-arc lamp. Arc illumination is unsteady hoWever, in its non-uniform background affects due to optical interference amplitude, phase (polarization) and spatial distribution. and refraction, respectively. The refraction effect is particu Unsteadiness causes noise, requiring an additional time larly strong. It is due to Snell’s laW of refraction Which budget for averaging. Lasers can provide suitable steadiness and brightness. Laser use leads hoWever, to image artifacts dictates that a ray passing through a spherical refracting 55 (noise and distortions) because of the coherence of the light surface does not retain its incident phase unless both the that is detrimental to image quality and its interpretation. For incident and emergent rays lie in a plane common to an arc eXample, images produced With laser illumination contain of the great circle2. (*SpecialiZed optics With a meniscus eXtraneous concentric rings due to interference of the inci lens designed to correct for this effect have been designed dent Wave front With spherical Waves generated When plane but are not available commercially. Each type of objective Waves diffract off dust particles. Also, in the case of illumi requires a different type of meniscus lens.) Non-uniform nation through the objective lens (epi-illumination), bands illumination may also arise due to dispersion in light polar of light and dark intensity are introduced into the image due iZations Within the ray bundle passing through the objective. to optical interference in the beam splitter. This effect Will distort the conversion from a phase distri We describe here a laser illuminator source that is intense, bution to an amplitude distribution. Uniformity may be 65 steady, and diminishes the image artifacts due to laser nearly established hoWever, When Koehler optics is utiliZed coherence; it has advantages over method’s reported so as to sharply focus an image of the source onto the previously5'7. US 6,672,739 B1 9 10 Preferred Embodiment distribution from the ?ber than occurs When the Wedge 30 is not present or not spinning. FIG. 6(b) shoWs the ?ber mode A suitable light intensity for the microscope is the CW distribution averaged over 64 video frames With Method #1 Argon-ion laser, one capable of producing one or a feW being applied. The corresponding illumination pattern on the Watts of poWer. Apulsed laser such as an may object recorded in FIG. 7b, is improved over FIG. 7a, but is also be used. AWater supply for cooling the laser and poWer obviously yet unsatisfactory. supply, one that is stabiliZed against ?uctuations in Water pressure, may be useful to improve steadiness of the beam. Method #2: Fiber modes can also be scrambled by Control of laser intensity With photo diode signals being fed mechanically vibrating or dithering some length of the ?ber back to the poWer supply (not shoWn), can also be bene?cial. isolated from its ends. The dithering or homogeniZation Was The feedback photo-sensor is best positioned doWnstream obtained by fastening several loops 54 of the ?ber to a from the ?ber output so as to stabiliZe the light intensity for speaker cone 56 and applying an ac current. Results from illumination from Within the microscope. applying this method Which are shoWn in FIGS. 6c and 7c, Were taken after optimiZing frequency and amplitude of the System & Procedure speaker vibrations. This produced a someWhat smoother 15 time-averaged ?ber output (FIG. 6c) than Method #1 but The illumination system in FIG. 5 conducts a 1—4 Watt only minor improvement in illumination uniformity at the light beam from a CW or pulsed laser 10 (short Wavelength recording head object (FIG. 7c). Method #3: An optical for best optical resolution) through a focusing lens 13 onto Wedge 30 driven by a motor 32 is positioned in the beam the input face 14 of a multi-mode ?ber 50 Which is held and after the ?ber output coupling lens ahead of the microscope. manipulated With the aid of a ?ber positioner (not shoWn). Wedge 30 is constructed by starting With a ?at circular disc (A single-mode ?ber made of quartZ glass for blue light having parallel faces, then angle-lapping one face to provide propagation and having a 3 micron core diameter for single a continuous reduction in thickness, and ?nally polishing to mode propagation, produces a smaller optic ?eld and has a an optical ?nish. Images shoWn in FIGS. 6d and 7d, Were loWer threshold of damage than the multimode quartZ ?ber. obtained using a disc made into a Wedge having continuous The damage threshold for a 3 micron quartZ ?ber, is only 80 25 reduction in thickness by 1 mm over its 10 cm disc diameter. Light emanating from the preferred multimode ?ber Acounter balancing Weight 59 is applied at the thinnest edge 50 (FIG. 5) is collected and directed into the microscope 55 to prevent vibration during spinning. The transmitted illu by a Wide-angle lens (eg a 35 mm camera lens) 53. Inside mination beam is centered at an outer region near the disc the microscope 55 the beam re?ects off the beam splitter of perimeter. an epi-illuminator (not shoWn) and forms an image of the ?ber face 52 at the rear focal plane (RFP) of the objective Experimentally, the Wedge 30 Was ?rst rotated about an lens. An image of the ?ber face in the RFP may be vieWed axis at a small angle to the axis of the illumination, and then in the binocular of the microscope after inserting the Ber an axis parallel to the axis of microscope. Changes in Wedge trand lens (a small adjustable telescope objective that ?ips 30 position along the path betWeen collector lens and microscope Were also investigated. The latter has the greater into the body tube in front of the ocular and provides a 35 means for inspecting the microscope’s internal optics). Light in?uence on the RFP image of the ?ber face, the object rays from the ?ber source pass through the objective and image and the modal pattern. illuminate the object. This bright ?ber output face thereby In the parallel alignment case, Wedge rotation causes the replaces the conventional source in the conventional micro rear-focal-plane image of the ?ber (the source) to move in a scope illumination scheme including conventional Koehler circular motion. When the Wedge 30 is located close to the optics (the optical arrangement-not shoWn- for imaging the coupling lens, the time averaged motion is the doughnut light source at the objective’s rear focal plane). FIGS. shape recorded in FIG. 8(a). In the most preferred 6(a—d), 8a and 11a are images of the ?ber face recorded for embodiment, the Wedge 30 is located farther doWnstream various different dithering means that scramble the ?ber from the collector lens. This position produces circular motions of smaller radius still visible in the objective RFP. modes and homogeniZe the output light. The illuminated 45 object (visible With Bertrand lens removed) produces micro Aradius of motion only a feW times the ?ber’s mode spacing scopic images [FIGS. 7(a—a), 8(b), 9, 10(a—b), 11(b), 12 and instead of a feW times the ?ber diameter, is suitable for 13]. These patterns (images) are convolutions of the incident producing uniform illumination. Smaller motions position illumination pattern from the ?ber and the object function. the instantaneous location of the ?ber image closer to its The ?ber output image in FIG. 6(a), taken Without applying average location. The speci?c location of the ?ber in the any means of mode dithering, is a speckled pattern indicat RFP is important to the speci?c interpretation of Kerr signals ing that many modes are supported by the ?ber. Approxi in terms of the vector orientation of the magnetiZation in the mately 104 modes are theoretically possible in a 100 micron magnetic sample or device diameter ?ber. The object image in FIG. 7a corresponding to In the case of Wedge 30 rotation about an axis oblique to the modal ?ber pattern FIG. 6a, is too mottled to be of use 55 the optic axis (FIGS. 6(a) and 7(a'), there occurs a larger because the pattern contrast is much larger than that from amount of motion in- and out-of-focus in addition to rotation many types of WPOs. of the ?ber face image. Experiments using a 30 degree tilt Method #1: The diffuser 20 is a spinning glass disk, Which for the rotation axis of the Wedge 30, shoWed the additional has a randomly undulated surface (unpolished Pyrex glass defocusing motion tends to diminish image focus (and Works Well, for example). This surface produces a spatial resolution). This is exempli?ed by the fuZZy horiZontal phase variation across the Width of the incident laser beam. edges in FIG. 8b, Whereas FIG. 9, Which Was recorded When Rotating the Wedge 30 modulates the laser input to the ?ber. the Wedge 30 Was rotated about an axis parallel to the optic A ?ber coupling lens 13 conducts the modulated laser beam axis, produces the sharpest focus and edge-de?nition. 12 into a spectrum of modes of the multi-mode ?ber 50. The Comparison With Conventional Hg-arc Illumination spinning Wedge 30 acting on the laser light, scatters it into 65 a larger number of ?ber modes varying in time. This A comparison of conventional microscope illumination achieves a more uniform time-averaged output intensity With laser/?ber illumination Was made using mercury-arc US 6,672,739 B1 11 12 illumination. FIGS. 10(a—b) Were made using the 100 W dithering to the video frame rate. The synchroniZation Hg-arc, the most intense conventional source for optical desired occurs When the frequencies of each method for microscopy. The single-frame image in FIG. 10a, represent dithering (disc rotation frequencies, speaker vibration ing the Hg-arc source, Was obtained using the same micro frequency) are tuned to an integral number multiplied by the scope settings (magni?cation, polariZer offset angle, etc.) as video frame rate (30 HZ). Indeed, residual contrast recorded for the laser illuminated images FIGS. 7(a—a) Striations in in FIG. 12 did occur under slightly detuned conditions. The FIG. 10a are due to the discrete nature of the camera contrast residue is therefore due to the dithering method photocathode and the variability in pixel photosensitivity. itself, as produced for example by spatially non-uniform or This type of image nonuniformity occurs naturally When temporally non-periodic Wedge 30 rotation or speaker vibra operating the camera at an illumination intensity near its 10 tion or pieZo 50 vibration. This result means the mechanical threshold of photoresponse. In order to capture FIG. 10a in quality of motors and pieZos and optical properties of discs a single frame it Was necessary to set camera at its are important. The motors used here Were supplied by maximum,—16 times higher gain than Was required for the manufacturers of lock-in ampli?ers and are normally used to laser/?ber illumination 1—4 images of FIGS. 7 ((a—a) Stria rotate light chopper blades so as to provide synchroniZed tions shoWn in FIG. 10a are overcome in FIG. 10b taken 15 reference signals for phase sensitive lock-in techniques11 When the gain Was reduced slightly and a greater number of applied to detection of Weak electrical signals. images accumulated—tWice as many frames compared With Tests on background effects over longer periods of time the laser/?ber case of FIG. 7. It is clear that conventional reveal difference images that may Worsen With time. Such light sources require greater image processing times to deterioration can occur due to thermal/mechanical drifts in capture images of microscopic Weak phase objects being the system. System drifts may include drifts in the laser magni?ed With polariZed light microscopes than When more (beam intensity and pointing), or relative movements in ?ber intense laser sources are used. positioner and laser, or in microscope and object stage. Combination Method Sequential difference images like the one exempli?ed in FIGS. 7(a—a) are stable When system components of high The results of applying all three methods of homogeni 25 quality are used. We use the Zeiss Axiomat polariZing Zation are exempli?ed in FIGS. 11a and 11b shoWing the microscope set up in the inverted mode because it affords ?ber face image and the object image, respectively. The ?ber rigidity and long term stability. Its mechanical axis of image shoWn for slight oblique-axis rotation includes in- and symmetry is colinear With its optic axis12; its inverted form out of focus motion as Well as slight rotation of the ?ber offers four rigid posts for direct attachment of the object source. The object image (FIG. 11b Was taken for the best stage. The original stage supplied With the axiomat Was case of parallel-axis rotation. This nearly uniform illumina replaced With a custom designed microscope stage attached tion result (except for the dust particles and linear marks rigidly to the four posts.) The laser component is a Coherent caused by surface lapping of the device) occurs by combin Innova 90 argon laser. We baffle the Water supply against ing all three ?ber illumination methods for mode homog supply ?uctuations in the Water ?oW used to cool the laser eniZation. Slight residual non uniform illumination is still 35 and its poWer supply. present, hoWever and can be caused by motions of any of the The nearly complete suppression of background effects three means for dithering that are not synchroniZed With the (nonmagnetic, or non WPO related) We accomplish With this video raster. This Was investigated by applying tWo image invention, enables magnetic response in magnetic devices to processing steps sequentially called Mottle FreeZe and be imaged in real time (even devices composed.of magnetic Mottle Subtract. Mottle FreeZe averages 64 video frames materials such as permalloy that exhibit Weak magneto-optic and places the result in a frame memory. This is the method Kerr effects, and even mono-atomic layers of magnetic ?lms used to obtain the previous images and includes setting the of similar 3d atomic origin such as cobalt). Background bit-slice to 6—13. The Mottle Subtract function subtracts this suppression Will also improve video imaging of other Weak image from all subsequent sequential images. Each differ phase objects such as biological cells or latent images in ence image that folloWs sequentially in time, Will contain 45 lithographically exposed photo resist. Zero bits if there is no change in the object and its illumi nation conditions. So, changes in background only that may Polar Kerr-effect Contrast Image of Magnetic occur can be imaged When no magnetic change is being Device applied to the sample. Recording them as images on a It isn’t possible to demonstrate real-time imaging of Weak vieWable (printable) scale of gray levels, requires applying phase objects in a paper draft. HoWever, the overall quality a DC offset to each difference image. After the Mottle of the invention is exempli?ed in its application to rendering FreeZe/Mottle Subtract functions are applied and stopped an image of a magneto-optic feature as shoWn in FIG. 13. and the DC Offset is applied, the images as shoWn in FIG. (This is taken on a recording head sample of the same design 12 appear. The high brightness is due to an applied DC as imaged Without magnetic contrast in FIGS. 6—12.) FIG. offsetlo. The residual contrast Was so Weak that it had to be 55 13 shoWs polar Kerr contrast measuring distribution and ampli?ed signi?cantly. Ampli?cation for FIG. 12 Was pro strength of the magnetiZation produced When a dc current is duced by decreasing the bit-slice to the loWest level (0—5) applied to the head’s integrated coil. Polar Kerr contrast Which is Well beloW the bit-slice 6—13 used in non subtracted Which is generally produced When the focused source is images, FIGS. 6—11. The residual structure of FIG. 12, centrally positioned in the objective RFP generally measures having barely any contrast at all, shoWs nearly ideal back the perpendicular component of magnetiZation. Accumula ground suppression. This has resulted from combining all tions of 64 frames Were obtained and stored in frame three Methods 1—3. This residual background from applying memory, one accumulation each for equal plus- and minus Methods 1—3 is Weaker than occurs (but not shoWn here) 80 mA dc currents applied to the recording head’s integrated With other forms of light source including 100 W Hg-arc and coil. Alternating the sign of the current alternates polarity of laser/?ber Methods 1—3 taken one- or tWo at a time. 65 north and south poles at the pole tips. So, a double Kerr The background effect exempli?ed in FIG. 12 can be contrast image is produced When the tWo stored images are extinguished completely by synchroniZing all three forms of subtracted from each other. A DC offset is applied to the US 6,672,739 B1 13 14 difference image to avoid negative bits and to match image Wavelength illumination for providing a higher optical intensity to a vieWer’s optimal eye response. The dark- and resolution and depth of ?eld. Refractive lensing using light levels of intensity in FIG. 13 shoW polarity and strength quartZ and quartZ-?uorite objectives may be used to respectively, of the perpendicular magnetiZationcomponent. produce parfocal observation in the ultraviolet and A mid-gray level, arbitrarily set When the DC off-set level is visible Wavelength regions. set, represents Zero perpendicular magnetiZation. This gray level permeates regions on non magnetic material lying Alternative Methods outside the pole tips. Alternate methods to scramble the image of the output The optical resolution in this pole-tip imaged in FIG. 13 face of the ?ber include vibrating or rotating a or lens is remarkable. Compare the detail and sharpness at the edge (one being used to form the image), or vibrating a ?ber face of the non-magnetic gap against a scaling reference given by located physically (instead of optically) in the objective the square shaped pole tip 5x5 microns in area, and the RFP. These methods differ in three respects from the case of nonmagnetic gap betWeen the square tip and ?rst Wide tip the rotating Wedge-shaped disc. The optical path length is knoWn from the manufacturing process to be only 0.75 not modulated. The intensity of illumination at the object is microns. The sharpness at the edges at the gap indicates the 15 increased. HoWever, the ef?ciency and adaptability of such system and method of this invention are capable to provide methods to selective changes of a microscope’s objective an ‘effective magneto-optical resolution’ of 0.1 micron (100 lenses are poor, because ?ber or mirror positions Would need nm) or beloW. (At least the “optical limit of perception” is to be reestablished folloWing eXchanges of objectives. This that small.) A similar conclusion is made by inspection of Would be necessary in order for the ?ber image to be normal images taken in re?ection Without magnetic contrast, compatible With the different objective lens’s properties, FIGS. 7d and 9, for example. The edge roughness (scallops) e.g., location of each objective’s RFP. is also real in the object; it too is resolved to this same level of resolution (or perception). SUMMARY A neW illumination method for optical microscopy of Further Embodiments 25 Weak phase objects (WPOs) is disclosed. It utiliZes a laser This disclosure also claims other conditions producing beam coupled to a polariZing microscope through a multi similar quality. These include use of: mode optical ?ber. Single-mode optical ?bers provide (1) a prism illuminator in place of the half-silvered beam steady illumination, but their smaller diameter limits their splitter for the Epi illuminator in the microscope; laser poWer handling capability (<100 The multi (2) different types of lasers as light sources including: cW mode ?ber can carry the 1—5 Watts needed for Wide-?eld and pulsed diode lasers such as GaAs and GaN other illumination in the polariZing microscope Without damaging III—V lasers; solid state lasers such as Nd-YAG and its optical elements. In order to homogeniZe nonuniform AleXandrite lasers and frequency doubled, tripled, and time-varying illumination caused by ?uctuating ?ber modes, frequency converted solid state lasers, and pulsed gas a unique method is applied. Without this method, the uneven lasers such as eXcimer lasers, especially eXcimer lasers pattern of illumination from multi-mode ?bers destroys the having short Wavelength such as XeCl, XeF, and ArF Weak contrast images of WPOs such as magnetic devices lasers, and ?uorine lasers. and biological cells that are developed When their phase (3) translational motion of the RFP ?ber image instead of distributions are converted to amplitude images using a rotational motion (retains pure linear polariZation polariZing optical microscope. The unique method to modu needed for more precise quantitative M-vector late and time-average image distortions caused by multi analysis); mode ?ber?s modal illumination results in illumination that (4) dithering of the end of the ?ber perpendicular to the is steadier (effectively) and brighter than conventional high ?ber aXis instead of dithering the image of the ?ber; intensity sources (for eXample, the 100 W Hg-arc lamp). (5) other types of Wide-?eld microscopes including 45 Three dithering methods provided for homogeniZing the transmission- in place of re?ection-microscopes light, are periodic in time. Therefore, the illumination vari (negates use of epi-illuminator With beam splitter or ability caused by any or all three can be removed by simply prism) microscopes With achromatic mirror objectives synchroniZing the dithering rate to an integer multiple of the and cartadioptric lenses (extension to uv or ir Wave video frame rate. This synchroniZation makes it possible to length regions); and apply image processing With subtraction to remove essen (6) other types of video cameras including CCD. Note: tially all background light, including that produced Within Both oil-immersion and non-immersion microscopy the microscope itself. Thus, images and videos of WPO systems are options claimed for inclusion. Fiber optic motions are attained Which have high S/N ratios and Wide cables may be selected for their maXimum transmission dynamic range. Detection of these images may accom plished With non-intensi?ed video cameras like the at blue, green or red Wavelength regions. Prism epi 55 illumination may be selected over half-silvered epi Chalnicon, the NeWvicon, etc. illumination on the basis that the microscope Would Advantages then produce four times greater illumination intensity at the sample object The prism may be replaced by a Light intensity noW becomes a variable for use in adjust sliver-shaped front surface mirror partially blocking the ing the microscope system, thereby placing less of the lens aperture. Both prism and FS-mirror methods burden to match the response of the video Camera on the diminish hoWever, the objective’s numerical aperture image processing functions. This variable enables optimiZ and make the optical resolution asymmetric. The lens ing Kerr contrast better than When polariZer/analyZer/ system the microscope may be re?ective instead of compensator settings alone, are adjusted. refractive. Re?ective (cartadioptric) lensing overcomes 65 Images can be made at higher magni?cations and better the optical opacity of glass (refractive) optics at deep ‘effective resolution’ than is possible using conventional ultraviolet Wavelengths, and permits using shorter sources. US 6,672,739 B1 15 16 Intensity being constant in time and in amplitude (across 167—176. (*published by Microscope Publications Ltd., 2 the optic ?eld) permits quantitative experiments measuring McCrone MeWs, BelsiZe lane, London, NW3 5BG, magnetization distributions. MagnetiZation-vector mapping England.) is then obtainable in principle from combining tWo or three 13.—IBM invention ?led by B.Argyle, A. Halperin, M. types of Kerr contrast, (polar, longitudinal and transverse). Scaman and E. Yarmchuk as part of IBM Docket F19 Strengthened photometric signals resulting from 98-012. Original disclosure F18-97-0620. increased intensity improve imaging and recording of local 14. Cameras having a loWer threshold of response, such as iZed magnetic responses, eg domain Wall movements, the silicon-intensi?ed-target (SIT) camera, can also be Barkhausen jumps, domain structure instability, etc. in mag used for loW light level imaging. HoWever, SIT cameras have a higher level of shot noise because they have loWer netic devices or materials. 10 Strengthened photometric signals also enable localiZed quantum efficiency—only 10% compared With 99% for Kerr-effect hysteresis loops to be recorded With greater the Chalnicon camera. Shot noise Will therefore occur in speed, dynamic range, and precision. the images taken With a SIT camera unless additional processing time is consumed for increased image accu Similar advantages are incurred imaging other types of mulation and averaging. Weak phase objects including the tWo described brie?y 15 Obviously, many modi?cations and variations of the above. present invention are possible in light of the above teach REFERENCES ings. It is therefore to be understood that, Within the scope of the appended claims, the invention may be practiced 1. B. E. Argyle, B. Petek and D. Herman, “Optical Imaging otherWise then as speci?cally described. of Domains in Motion”, JAP, Vol. 61, 4303 (1987). 20 2. B. E. Argyle, “A Magneto-optic Microscope System for We claim: 1. Apparatus for illuminating a ?eld With a laser beam, Magnetic Domain Studies”, Proc. Symposium on Mag comprising; netic Materials, Processes and Devices, 85—109, Vol 90—8, (1990), The Electrochemical Society, Inc. a beam homogeniZer, the beam homogeniZer having an entrance aperture for receiving the laser beam and an Pennington, N.J., Edited by L. T. RomankiW and D. A. 25 Herman, Jr. eXit aperture for emitting the homogeniZed laser beam; 3. R. Schaefer, B. E. Argyle and P. L. Trouilloud, “Domain a lens means for imaging the eXit aperture on to the ?eld, Studies in Single-Crystal Ferrite MiG Heads With Image and; Enhanced, Wide-Field Microscopy”, IEEE Trans. a dither means for dithering the image of the eXit aperture Magnetics, Vol.28, No.5, (September 1992). in a direction having a component parallel to the ?eld. 4. S. Inoue, Video Microscopy, Plenum Press, NeW York, 2. The apparatus of claim 1, Where the beam homogeniZer NY, 1986. is a light tunnel. 5. G. W. Ellis, “A Fiber-Optic Phase-RandomiZer for Micro 3. The apparatus of claim 1, Where the beam homogeniZer scope Illumination by Laser”, J. Cell Biol. 83, 303a is a light pipe. (1979). 4. The apparatus of claim 1, Where the beam homogeniZer 6. R. Hard, R. Zeh, and R. D. Allen, “Phase-Randomized is an optical ?ber. Laser Illumination for Microscopy”, J. Cell Sci. 23, 335 5. The apparatus of claim 4, Where the optical ?ber suffers (1977). a time varying strain. 7. M. J. BoWman, “TWo NeW Methods for Improving 6. The apparatus of claim 4, Where the eXit aperture of the Optical Image Quality”, Appl. Opt. 7, 2280 (1968). optical ?ber is physically moved in a direction perpendicular 8. An adjustable source location is required for establishing to the optical aXis of the optical ?ber. speci?c magneto-optic sensitivities: polar Kerr-, trans 7. The apparatus of claim 1, Where the dither means is a verse Kerr-, or longitudinal Kerr-effect sensitivityl’2 . The rotatable Wedge placed betWeen the eXit aperture and the longitudinal and transverse Kerr effect develops contrast ?eld. among magnetic domains lying parallel to the object 45 8. The apparatus of claim 1, Where the dither means is a plane, While the polar Kerr effect contrasts perpendicular electro-optic device. magnetic components. To produce the oblique and per 9. The apparatus of claim 1, Where the dither means is a pendicular illuminations appropriate for longitudinal and magneto-optic device. polar contrasts respectively, the output face of the ?ber is 10. The apparatus of claim 1, Where the dither means is an simply translated Within the objective RFP using an X,Y acousto-optic device. adjustable positioner. Placing the image of the ?ber onto 11. The apparatus of claim 1, Where the dither means is a the optic aXis yields polar-Kerr contrast, a sensitivity to rotatable lens section. only the component of magnetiZation Which is perpen 12. The apparatus of claim 1, Where ?eld is the rear focal dicular to the object surface. AnaXial positions produce plane of an optical microscope. longitudinal or transverse contrast depending on the ori 13. The apparatus of claim 1, Where the lens means entation of the light polariZation With respect to the images the eXit aperture on a lithographic mask in the ?eld. illumination plane-of-incidencel’z. 14. The apparatus of claim 12, further comprising an 9. B. Petek, B. E. Argyle and D. A. Herman,“Reduction of optical recording system for recording images of an object coherence related noise in laser based imaging systems”, in the object plane of an optical microscope, and Wherein the IBM Technical Disclosure Bulletin, Vol. 37, No.12, 60 dither means is synchroniZed With the optical recording December (1994), pp. 469—471. system. 10. (An offset supplied Within the Hamamatsu C 1966 15. A system for illuminating an object in an ?eld, Processor as default value, Was used) comprising; 11. Floyd M. Gardner, Phaselock Techniques, 2nd edition, a laser for producing a laser beam; John Wiley & Sons, 1979. 65 a beam homogeniZer, the beam homogeniZer having an 12. W. C. McCrone, “The NeW Zeiss Axiomat Microscope”, entrance aperture for receiving the laser beam and an The Microscope*, 21, No.3 July—October (1973), pp. eXit aperture for emitting the homogeniZed laser beam; US 6,672,739 B1 17 18 a lens means for imaging the exit aperture on to the ?eld, 28. A method for illuminating a ?eld, comprising; and; directing a laser beam into the entrance aperture of a beam a means for dithering the image of the eXit aperture in a homogeniZer; direction having a component parallel to the ?eld. imaging the eXit aperture of the beam homogeniZer on to 16. The system of claim 15, Where the beam homogeniZer the ?eld, and; is a light tunnel. dithering the image of the eXit aperture in a direction 17. The system of claim 15, Where the beam homogeniZer having a component parallel to the ?eld. is a light pipe. 29. The method of claim 28, Where the beam homogeniZer is a light tunnel. 18. The system of claim 15, Where the beam homogeniZer 10 30. The method of claim 28, Where the beam homogeniZer is an optical ?ber. is a light pipe. 19. The system of claim 18, Where the optical ?ber suffers 31. The method of claim 28, Where the beam homogeniZer a time varying strain. is an optical ?ber. 20. The system of claim 15, Where the dither means is a 32. The method of claim 31, Where the optical ?ber suffers 15 rotatable Wedge. a time varying strain. 21. The system of claim 15, Where the dither means is a 33. The method of claim 28, Where the dither means is a electro-optic device. rotatable Wedge. 22. The system of claim 15, Where the dither means is a 34. The method of claim 28, Where the dither means is a magneto-optic device. electro-optic device. 23. The system of claim 15, Where the dither means is an 35. The method of claim 28, Where the dither means is a acousto-optic device. magneto-optic device. 36. The method of claim 28, Where the dither means is an 24. The system of claim 15, Where the dither means is a acousto-optic device. rotatable lens section. 37. The method of claim 28, Where the dither means is a 25 25. The system of claim 15, Where the ?eld is the rear rotatable lens section. focal plane (RFP) of an optical microscope, further com 38. The method of claim 28, Where the ?eld is the rear prising an optical microscope. focal plane of an optical microscope. 26. The system of claim 15, Where the lens means images 39. The method of claim 28, Where the lens means images the eXit aperture on a lithographic mask. the eXit aperture on a lithographic mask. 27. The system of claim 25 further comprising an optical 40. The method of claim 38, further comprising recording recording system for recording images of an object in the images of an object in the object plane of an optical object plane of the optical microscope, and Wherein the microscope in synchronism With the dither of the image. dither means is synchroniZed With the optical recording system. * * * * *