Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70383—Direct write, i.e. pattern is written directly without the use of a mask by one or multiple beams
  • G03F7/704—Scanned exposure beam, e.g. raster-, rotary- and vector scanning
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
  • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
  • G02B26/10—Scanning systems
  • G02B26/12—Scanning systems using multifaceted mirrors
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
  • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
  • G02B26/10—Scanning systems
  • G02B26/12—Scanning systems using multifaceted mirrors
  • G02B26/125—Details of the optical system between the polygonal mirror and the image plane

Definitions

• This invention relates to laser scanners and to optical systems for sweeping an image along a scan line, and more specifically, to flying spot (raster) scanners used for precise electronic imaging applications.
• Photolithography is commonly employed to produce repeatable patterns on devices such as integrated circuits, flat panel displays, and printed circuit boards.
• a conventional photolithography process begins with coating a device with a layer of photoresist.
• An image projection system for example, using an object reticle or a sequential scanning, illuminates selected regions of the photoresist with light that changes the properties of the illuminated regions. Using the changed properties, the photoresist is developed by removing the illuminated or not-illuminated regions (depending on the type of photoresist) to create a patterned mask for processing of the device.
• a variety or different photolithography devices have been developed for image projection.
• a laser raster scanner (also known as a raster output scanner, flying spot scanner, or flat-bed scanner) is a photolithography device which scans one or more focused and spatially modulated laser beams in a series of scan lines covering a surface being patterned.
• the laser raster scanning systems can be used as a reticle making tool or as a direct-imaging device, eliminating steps associated with manufacture and use of reticles. Whether a laser raster scanner illuminates a region depends on the laser beam's intensity as the beam passes the region.
• Such laser raster scanners use imaging systems adapted for light having wavelengths at which a photoresist has high sensitivity. This generally occurs in the ultraviolet region of the spectrum.
• a basic architecture for a laser raster scanner includes the f- ⁇ lens system that may or may not include a rotating polygon mirror to sweep the beam and/or prepolygon optical system. Distinguishing features of scanner architectures are described below.
• a first distinguishing feature is spectral performance, in particular the spectral center line and spectral bandwidth.
• Most laser scanners are designed for monochromatic light, but a few scanners are color corrected for 3 -color visible applications.
• Achromatizing a refractive system for a raster scanner is complicated because such systems generally use high-index glasses to aid in aberration control. These glasses tend to limit the spectral range of the scanner to visible and near infra-red wavelengths.
• a second distinguishing feature of scanner optics is use of passive motion compensation (PMC).
• PMC passive motion compensation
• a scan lens has an anamorphic architecture to re-image the polygon facet in the cross-scan (sagittal) direction.
• Most scanners for xerographic laser printers use PMC to remove facet wobble of low-cost ballbearing polygon mirrors.
• scan lenses alternatively use rotationally symmetric optics, and the polygon mirror must be taller to accommodate the height of a four-fold symmetric input beam clear aperture (e.g., round or square). The polygon mirror is therefore more massive and requires more drive power for rotation.
• a third distinguishing feature is the method used to inject a beam onto a polygon mirror and into the scan lens.
• the predominant method is tangential injection in which an input beam is in the plane of the swept scan line.
• Figs. 1 A and IB respectively illustrate top and side views of a scan lens system 100 using tangential injection.
• an input beam 105 reflects from a folding mirror 110 so that input beam 105 and a reflected beam 115 are in a plane that is perpendicular to the rotation axis of a polygon mirror 120 and includes the optical axis of post-polygon lens elements 130 and 140.
• Non-PMC scan lenses use tangential injection unless specialized architectures are used (e.g., U.S. Pat. Ser. No. 4,682,842) since sagittal input places the scan line above or below the tangential meridian of the polygon mirror, and introduces scan line bow with rotationally-symmetric optics due to the distortion present in the lens for f- ⁇ linearity correction.
• Figs. 2A and 2B respectively illustrate top and side views of a scan lens system 200 using sagittal injection.
• an input beam 205 reflects from a folding mirror 210 so that input beam 205 and a reflected beam 215 are in a plane containing the rotation axis of a polygon mirror 220 and the optical axis of post-polygon lens elements 230 and 240.
• a sagittal injection makes scan line "bow" or deviation from a straight scan line difficult to control.
• the classical optical aberration referred to as distortion introduces bow in the scan line at an image plane 250.
• Distortion is introduced to scan lenses to provide the f- ⁇ correction, and is fundamentally non-zero for any scan line that does not lie on the tangential meridian of the image plane. This bow is inherent to sagittal injection in which the image plane is off-axis.
• Figs. 3A and 3B illustrate a fourth distinguishing feature, telecentricity.
• a telecentric scanner 300 has the chief ray of a swept beam 305 substantially perpendicular in both meridians to an image plane 310 across the length of a scan line.
• Both anamorphic and rotationally symmetric telecentric designs exist are known and described, for example, in U.S. patents Ser. No. 4,056,307 and 4,527,858, respectively.
• Telecentricity is extremely important in high-precision, high-resolution scanning systems.
• a system may be considered "telecentric" if the chief ray is perpendicular within a third the subtended cone angle of the focused ray bundle to the final image plane in both meridians across all fields and scan positions.
• a non-telecentric scan lens 350 as shown in Fig. 3B has a scan beam 355 with a chief ray that meets an image plane 360 at a substantial angle to perpendicular.
• the variations in the chief ray angle across the scan field for a non- telecentric scanner causes two problems. First, the spot size on image plane 360 grows at the edges of a scan line, due to oblique projection of the focused spot onto the image plane. Second, small shifts in focal plane location cause absolute pixel placement errors. For a chief ray angle in the cross-scan direction, focal plane shifts result in pixel placement errors that mimic magnification errors. If the chief ray angle is in the cross scan direction, out-of-focus scan lines may appear bowed.
• a fifth distinguishing feature is performance with multiple beam (data channel) input to the scan lens system. Multiple beams allow faster writing speeds with reasonable electronic data rates and polygon mirror rotational velocities. With a single beam system, distortion can be added to the design to provide f- ⁇ linearization of the fast-scan beam position. With a multiple beam system, f- ⁇ linearization is not necessarily sufficient to control the fixed channel-to-channel spacing across the scan line. Localized separation between first and last channels (i.e. fixed magnification in both slow and fast axes) must be maintained across the scan line to prevent pixel placement errors within the multiple beam field of view.
• the variation in beam magnification in the fast-scan direction is referred to as differential distortion. Variation in magnification in the slow scan direction is referred to as differential bow.
• a sixth distinguishing feature of laser scanner architecture is the number of resolvable spots in the scan line.
• Precision applications typically require spot diameters from 25 microns down to 2 microns, with absolute pixel placement accuracy down to a tenth of the spot diameter.
• a precision, refractive telecentric lens system may achieve up to 20,000 resolvable spots per scan line.
• a typical non-telecentric xerographic scanners may have about 9,000 resolvable spots in a scan line, although more spots are achievable if significant spot size variation across the scan line is allowed.
• a precision scan lens is sought which provides the features and performance desirable for a high resolution, radiometrically efficient scanner.
• an embodiment of the invention provides an improved scanning system that incorporates the best of the above features within a performance range suitable for photolithographic applications.
• an optical system in accordance with an exemplary embodiment of the invention includes a telecentric scan lens having a sagittal injection and passive motion compensation (PMC) and achieves high radiometric efficiency for ultraviolet laser light, low differential distortion for multichannel beams, and up to 15,000 resolvable spots per scan line. Radiometric efficiency is important because ultraviolet laser power is expensive, and the speed of the scanning system is related to the power delivered to the image plane.
• the exemplary embodiment utilizes a catadioptric architecture that maximizes transmission efficiency by using a combination of reflective optics in conjunction with refractive elements that have high transmission of UV light.
• the exemplary system corrects aberrations over multiple UV wavelengths, thereby optimizing the use of available laser power.
• the optical system for a scanner incorporates anamorphic passive motion compensation.
• PMC is useful because PMC reduces the heat load on the system since a polygon mirror for the system can be thinner to require less power for rotation at high speeds.
• PMC also reduces system cost because PMC permits use of less precise motor-polygon assemblies. The projected size of the spot on the polygon mirror in the tangential
• sagittal input combined with PMC creates a bi-laterally symmetric optical system that allows aberrations to be corrected for greater numerical apertures and scan angles than tangential input systems, yielding more resolvable spots in a scan line.
• the invention provides for sagittal input in a unique manner that fundamentally minimizes cross-scan distortion.
• the telecentricity (perpendicularity of the chief ray in both meridians to the image plane) of the scan lens removes variation in spot placement as a function of image defocus. This eases the requirement on work piece flatness and focal plane alignment with the exposed media.
• an optical path from a rotating polygon mirror of a scan lens encounters a spherical lens, a cylindrical lens element; a first sphero-cylindrical lens element; a concave spherical mirror; a convex cylindrical mirror; and a second sphero-cylindrical lens element.
• the scan lens also includes injection optics for a beam to the polygon mirror.
• the injection optics like the post-polygon optics, can be anamo ⁇ hic.
• the injection optics include a concave cylindrical mirror positioned to receive a beam of collimated light at a non-zero angle with a radius of curvature of the concave cylindrical mirror; a cylindrical lens, and a folding mirror.
• the optical materials and coatings in the scanner are matched to the spectral sensitivity of the photo-sensitive media and for photoresist exposure, are suitable for ultraviolet light having wavelengths of about 340 to 390 nm.
• One embodiment of an optical system in accordance with the invention includes: a cross-scan cylinder mirror, a cross-scan cylinder lens, a folding mirror that provides sagittal input of the beam to a rotating polygon mirror, a spherical meniscus lens, a piano-cylinder lens, a first sphero-cylinder lens, a primary spherical mirror, a secondary cylindrical mirror, and a second sphero-cylinder lens.
• Figs. 1 A and IB show a scan lens with tangential injection of a beam to a polygon mirror.
• Figs. 2A and 2B show a scan lens with sagittal injection of a beam to a polygon mirror.
• Figs. 3 A and 3B respectively show telecentric and non-telecentric scan lenses.
• Figs. 4A and 4B show a top view and a side view of a laser scanner in accordance with an embodiment of the invention.
• Figs. 5 A and 5B respectively show a side view and a top view of scan optics in accordance with an embodiment of the invention.
• Fig. 6 shows a schematic representation of sagittal input for an embodiment of the invention.
• Figs. 7A, 7B, 7C, and 7D shows performance curves for an exemplary embodiment of the invention.
• a scan lens system in accordance with an exemplary embodiment of the invention is an unobscurred catadioptric optical system incorporating anamorphic elements to implement passive motion compensation.
• all of the refractive optical elements have high transmission of UV light, and may be modified to use other materials such as calcium fluoride for deep UV light or high- index glasses for improved performance with the visible light.
• the system implements sagittal input in a manner that is consistent with but not limited to an unobscurred catadioptric design.
• the system is telecentric at the image plane, and is color-corrected for multiple ultraviolet wavelengths.
• the system implements f- ⁇ correction.
• the system is capable of imaging up to 12 independent channels while maintaining stated performance criteria, and is capable of resolving over 15,000 pixels per line with pixel spacing equal to the full width at half maximum (fwhm) spot diameter.
• a raster scanner 400 in accordance with an embodiment of the invention shown in Figs. 4A and 4B includes a laser 410 with required beam shaping optics, a multi-channel modulator 420, scan optics 430, and a precision stage 490 for holding a workpiece.
• Laser 410 generates a collimated light beam 415 which modulator 420 converts into a modulated beam 425 containing separate collimated sub-beams.
• laser 410 is a UV argon ion laser
• beam 425 contains ultraviolet light of wavelengths 363.8 nm, 351.4 nm, and 351.1 nm and is split into two or more separate sub-beams.
• Modulation of beam 425 changes the intensities of the individual sub-beams typically turning sub-beams on and off, but gray scale intensity control can also be employed to provide an optimum irradiance profile to the beam eventually written to the photosensitive media.
• a co-filed U.S. provisional patent application entitled “ACOUSTO- OPTIC MODULATOR ARRAY WITH REDUCED RF CROSSTALK", Atty. Docket No. P-4296-US, describes a modulator for the exemplary embodiment of the invention.
• Beam 425 from modulator 420 has a diameter that defines a stop size for scan optics 430.
• Scan optics 430 forms an image of beam 425 and sweeps that image across a scan line in an image plane.
• An optional optical relay 480 reforms the image from scan optics 430 on a workpiece held by stage 490 so that a final image of the modulated beam sweeps along a scan direction at the surface of the workpiece.
• Precision stage 490 moves the workpiece perpendicular to the scan line direction. Movement of the workpiece can be continuous during scanning or may only occur each time scan optics 430 completes a scan line.
• Scan optic 430 includes receiving optics 440, folding mirror 450, a polygon mirror 460, and post-polygon optics 470.
• Receiving optics 440 performs initial shaping of beam 425 to generate a converging beam 445 which folding mirror 450 directs to polygon mirror 460.
• Receiving optics 440 and folding mirror 450 are sometimes referred to as injection optics since they inject the modulated beam onto polygon mirror 460.
• Scan lens 470 focuses beam 465 to reduce the separation between separate sub-beams and focus each sub-beam.
• Scan lens 470 has anamorphic focusing which reduces or eliminates perpendicular offsets of an image from a desired scan line due to facet signature or wobble in rotating polygon mirror 460.
• Figs. 5 A and 5B respectively show a side view and a top view of scan optics 430 in accordance with an embodiment of the invention.
• a collimated multi-channel light beam bundle enters scan lens 430 and is reflected by a folding mirror 510 into the pre-polygon optics consisting of a cylindrical mirror 520 and a bi-cylindrical refractive element 530.
• the purpose of the pre-polygon cylindrical optics provides motion compensation through use of a multi-element focusing system to accommodate the relatively large numerical aperture.
• the focused beam bundle 535 then impinges a second folding mirror 450 sometimes referred to as the injection mirror, sending a beam bundle 455 into polygon mirror 460 at a sagittal angle.
• system 430 uses an optical system with a polygon rotation axis 462 perpendicular to the scan lens' optical axis, centers the focused beam bundle on the optical axis through a polygon facet, and re-images to the scan line such that the scan line is also centered on the optical axis.
• a sagittal input system 600 of the class used in exemplary embodiment is schematically illustrated in Fig. 6.
• pre-polygon optics 610 focuses an input beam 605 which a folding mirror 620 directs onto a facet 630 of a polygon mirror.
• Post-polygon optics 640 re-images polygon facet 630 at the focal plane of the scanner as with tangential input systems.
• system 600 accomplishes injection and re-imaging using an off-axis section of the corrected clear aperture of the system rather than using laser beams that are symmetrically centered about the optical axis. Let the focused light from the polygon mirror have a subtended angle of ⁇ .
• the aberration-corrected acceptance cone of the cross-scan optical system is designed to be 2 ⁇ + 2 ⁇ , where ⁇ is a displacement angle as required for beam 635 to clear folding mirror 620.
• Beam 625 can be injected into the polygon facet 630 using injection mirror 620 at an angle below the centerline by ⁇ /2 + ⁇ .
• the converging beam is substantially focused on the optical centerline of post-polygon optics 640 at the polygon facet 630, and reflects at an angle ⁇ /2 + ⁇ above the optical centerline, such that reflected beam 635 clears the top of injection mirror 620 and enters post-polygon scan optics 640.
• Distortion-induced bow is introduced in a scan line when the scan line fails to intercept the tangential meridian of the optics 640. Since the polygon facet is re- imaged at the scan line in the cross-scan plane to intercept the optical axis, and the polygon axis is perpendicular to said optical axis, there is no bow in the scan line due to distortion. For multi-channel systems, this is the minimum-bow configuration since the channels cannot be brought closer to the optical axis and distortion-induced bow is minimized. Mention should be made at this point that the off-axis nature of the optimized aperture is critical in implementing a centered, catadioptric architecture.
• beam bundle 455 reflects off of rotating multi-facet polygon mirror 460.
• Beam bundle 455 in the tangential direction underfills a facet of polygon mirror 460. Since sagittal offset is used, the projected beam bundle size on the polygon face in the tangential direction is minimized, and the diameter of polygon mirror 460 can be reduced accordingly.
• the polygon diameter is 5.33 inches in diameter, yielding a scan efficiency of 85% with a 12-facet polygon mirror.
• the sagittal input method of this invention can be used with active facet-tracking schemes as well, allowing further reduction in polygon diameter.
• Active facet tracking shifts the beam bundle to maintain the position of the beam bundle at the center of a polygon facet while the polygon mirror rotates.
• polygon mirror 460 has twelve facets which rotate about a rotation axis 462 at about 7500 rpm.
• Fig. 5B illustrates a facet 461 of polygon mirror 460 and a resulting direction for respective reflected beam bundle 465.
• facet 461 is in the position show in Fig. 5B, an image is formed in image plane near a first end 596 of the a scan line.
• polygon mirror 460 rotates so that beam bundle is reflected of 15 an opposite end of facet (i.e. rotates slightly less than 30° in the exemplary embodiment), the final image forms on an opposite end 597 of the scan line.
• Polygon mirror 460 may be mounted on precision air bearings to minimize wobble during rotation.
• the passive motion compensation reduces the effects from wobble and keeps an image from forming off the desired scan line. Accordingly, polygon mirror 460 can use roller bearings or other less expensive bearings and still achieve high performance. In addition, the passive motion compensation reduces the required facet height thus reducing air resistance and allowing use of a lower thermal load motor to drive the polygon mirror 460.
• the first post-polygon optical elements, spherical meniscus lens 540 and piano-cylinder lens 550, which form a doublet, and a sphero-cylinder lens 560 are refractive elements of fused silica or BK7, which both effectively transmit light having wavelengths down to 350 nm.
• BK7 fused silica
• System 400 can also work effectively at shorter wavelengths (at least down to 190 nm) if calcium fluoride is substituted for BK7.
• Beam bundle 564 from lens 560 reflects off of a primary spherical mirror 570, while passing over a secondary cylindrical mirror 570.
• Beam bundle 575 from mirror 570 reflects off of cylinder mirror 570 so that a beam bundle 585 passes through a sphero-cylinder lens 590, which is designed such that its clear aperture does not encroach into beam bundle 575.
• Lens 590 focuses a beam bundle 595 at the scan lens' focal plane. Since an off-axis section of the centered lens 590 is used, the chief ray of the incident bundle 595 is not perpendicular to the optical axis of the post-polygon lens elements. However, by redefining the focal plane to be normal to the chief ray of the bundle 595, the telecentricity requirement for the architecture is now met.
• Figs. 7A, 7B, 7C, and 7D show performance curves for the exemplary embodiment.
• Fig. 7A is a plot of the diameter at which the intensity of a spot falls to 1/e over a range of scan angles corresponding to a scan line.
• the exemplary embodiment provides spots with a variation less than one tenth of the spot diameter.
• Fig. 7B indicates the ratio of the spots' major and minor axes for sub-beams 2, 3, 4, and 5 respectively in upper- left, upper-right, lower-right, and lower-left corners of a beam bundle. For each sub-beam, the spot is nearly circular across the range of polygon angles.
• Fig. 7C indicates the differential distortion between sub-beams in the upper-left and lower- left and the differential distortion between sub-beams in the upper-right and lower- right of the beam bundle. As indicated, differential is less than about 0.5%.
• Fig. 7D indicates the cross-scan position of sub-beams 2, 3, 4, and 5 across the range of polygon angles corresponding to a scan line. The position of diagonally located sub-beams 2 and 4 or 3 and 5 track each other to provide uniform spacing between scan lines formed by sub-beams if the sub-beams are oriented along a diagonal running from top-left to bottom-right (or from top-right to bottom left) of a square cross-section (i.e., aperture) for a beam bundle.
• This appendix contains an optical listing of the exemplary embodiment of the invention. The listing formatted and defines parameters as in the "Code V" optical design software available from Optical Research Associates.
• ADC 100 BDC: 100 CDC: 100
• ADC 100 BDC: 100 CDC: 100
• ADC S19 100 100 100 100 100 100 100 100 100 100
• ADC S21 100 100 100 100 100 100 100 100 100 100 100

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• 1998
  • 1998-06-19 KR KR1019997012346A patent/KR20010014242A/ko not_active Application Discontinuation
  • 1998-06-19 CA CA002296595A patent/CA2296595A1/en not_active Abandoned
  • 1998-06-19 JP JP2000502439A patent/JP2001509613A/ja active Pending
  • 1998-06-19 IL IL13375198A patent/IL133751A0/xx unknown
  • 1998-06-19 WO PCT/US1998/012464 patent/WO1999003012A1/en not_active Application Discontinuation
  • 1998-06-19 EP EP98930259A patent/EP0995144A1/en not_active Withdrawn
  • 1998-07-07 TW TW087111000A patent/TW394853B/zh not_active IP Right Cessation

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