Abstract:
A precision printing system uses multiple scan beams that an acousto-optic modulator (AOM) separately modulates. An array of optical elements such as dove prisms separately rotates each of the beams about a central ray of the beam to eliminate blurred edges, skew, and variations in line thickness caused by the direction of propagation of acoustic waves in the AOM being at an angle to a scan direction. In particular, the amount of rotation is selected so that in the final image the direction in which illumination progresses across a cross-section of a beam is in a direction opposite the scan direction. A method of making the array includes attaching rods to a flat, grinding or polishing the combination of the rods and flat to form three planar regions that correspond to facets on prisms. Removing the rods/prisms from the flat. Using photolithography and etching to form grooves in a substrate, and a mounting the rods/prisms in the grooves on the substrate. Integrated circuit processing techniques that control the spacing, widths, and depths of the grooves provide precise control of critical alignment of the prisms.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This patent document is related to and incorporates by reference in its entirety, co-filed U.S. patent application Ser. No. 09/273,115, entitled “Laser Pattern Generator”, filed Mar. 19, 1999. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention relates to printing systems using multiple scan beams and particularly to optical systems associated with acousto-optic modulators in such systems. 
     2. Description of Related Art 
     Printing systems including scanners are suitable for a variety of applications including printing text on paper, patterning photoresist during integrated circuit manufacture, and creating masks or reticules for projection-type photolithography systems. For integrated circuit applications, the printing systems typically require submicron precision. FIG. 1A illustrates the basic architecture of a precision printing systems  100  that employs scanning. System  100  includes: a light source  110  such as a laser; an acousto-optic modulator  120  that controls intensity of one or more input beams  135 ; prescan optics  130  that control the position, shape, and collimation of input beams  135 ; a scanning element  140  such as a polygon mirror that sweeps scan beams  145  along a scan direction; and post-scan optics  150  that focus scan beams  145  on an image plane  160 . Scanning of scan beams  145  forms scan lines that expose a pattern in an image area of plane  160 . Acousto-optic modulator  120  modulates the intensity of input beams  135  to select the pattern that scan beams  145  expose in image plane  160 . 
     A conventional acousto-optic modulator includes a block of material such as fused silica through which input beams propagate. To turn on, turn off, or change the intensity of an input beam, a transducer generates an acoustic wave that crosses the path of the input beam in the block. The acoustic wave locally changes the optical properties of the block and deflects part of the input beam. A beam stop later in the optical train blocks the undeflected part of the beam. A concern for a precision scanner having a conventional acousto-optic modulator is the orientation of the scanning direction relative to propagation of the acoustic waves that modulate the input beams. If the propagation direction and the scanning direction are not collinear, turning beams on or off can reduce sharpness of edges or create undesired skew or directional bias in a pattern being illuminated. FIG. 1B illustrates an illuminated region  170  of a scan line formed when an acoustic wave deflects an input beam in a direction  178  (after convolution through the system optics  130  and  150 ) that is perpendicular to a scan direction  172 . Deflection direction  178  typically corresponds to the direction of propagation of the acoustic wave. As acousto-optic modulator  120  turns on input beam  135 , a cross-section  174  of the beam expands in direction  178 . Accordingly, the initially illuminated part of region  170  is narrow and toward one edge until the input beam has a fully illuminated cross-section such as cross-section  175 . Similarly, when acousto-optic modulator  120  turns off input beam  135 , one edge of the input beam darkens first, and a shrinking cross-section  176  of the beam causes illuminated region  170  to recede toward the opposite edge. This reduces sharpness at the edges of illuminated regions formed by multiple scan lines, skews rectangular illuminated areas, and causes pattern lines at 45° to the scan direction to differ in thickness from pattern lines at 135° to the scan direction. 
     Acoustic waves in an acousto-optic modulator propagating opposite the scan direction (after convolution through scanner optics) eliminates skew and 45°/135° bias and sharpens edges of illuminated regions. However, in scanning systems using multiple beams, projections of the scan beams along the scan direction typically overlap. For example, as shown in Fig. 1C, beams  132 ,  134 ,  136 , and  138  overlap when viewed along scan direction  172 . This creates a brush that illuminates a strip in the image plane without gaps between adjacent beams. With this configuration, an acoustic wave propagating along or opposite scan direction  172  would affect multiple beams. Generally, the separation  133  between beams inside acousto-optic modulator  120  must be more than a beam diameter to permit acoustic waves  122 ,  124 ,  126 , and  128  to independently modulate respective beams  132 ,  134 ,  136 , and  138 . Accordingly, to provide independent control of the intensities of beams  132 ,  134 ,  136 , and  138 , acoustic waves  122 ,  124 ,  126 , and  128  in acousto-optic modulator  120  must propagate at an angle relative to scan direction  172 . 
     Systems and methods are sought that use simultaneous scan beams for faster scanning but avoid the skew, blurred edges, and directional bias associated with acousto-optic modulators having acoustic waves propagating at an angle to the scan direction. 
     SUMMARY 
     In accordance with the invention, a multi-beam scanner includes an array of optical elements such as dove prisms. Each optical element effectively rotates the direction in which illumination progress across the cross-section of an associated beam when an acousto-optic modulator turns on the beam. The amount of rotation is selected so that at the image plane where scan lines form, the beams cross-sections expand or brighten along a direction opposite the scan direction. 
     One embodiment of the invention is a scanning system that includes: a source of multiple beams; a modulator positioned to separately control intensities of the beams, an array of optical elements associated with the beams; a scanning element that sweep the beams along a scanning direction; and post-scan optics that direct the beams to form scan lines in an image plane. When the modulator turns on one of the beams, illuminated areas in a cross-section of the beam progress in a brightening direction. Each optical element acts on the associated beam to change brightening direction so that in the image plane the brightening direction is along or opposite the scanning direction. In one specific embodiment, the array of optical elements is an array of dove prisms. 
     To avoid varying in the relative positions of beams, dove prisms in an array have uniform geometry and are uniformly positioned relative to central axes of the beams. A process for making dove prisms with uniform geometry attaches multiple rods or pieces of fiber optic material to an optical flat with sides of the rods in contact to keep the rods parallel to each other. The combined assembly including the rods and the flat are then ground or polished to form planar surfaces at opposite ends of the rods. Forming these planar surface forms front and back facets of the dove prisms. The lengths of the dove prisms (i.e., the distances between front and back facets) are uniform since all rods are polished at once. A top surface of the assembly is ground or polished to form side facets where total internal reflections occur in the dove prisms. Once polishing of the assembly forms the dove prisms, the dove prisms are removed from the optical flat for mounting on a substrate to form the array. To prepare the substrate, known integrated circuit processing techniques such as photolithography and etching form parallel grooves with precise spacing, shape, and size in the substrate. The dove prisms are placed in the grooves with the side facets of the dove prisms oriented as required to rotate each beam by the desired amount. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a prior art printing system. 
     FIG. 1B illustrates the relation between the cross-section of a scan beam that is being turned on and then off and the shape of a resulting illuminated region. 
     FIG. 1C shows the orientation of the scan direction, multiple source beams, and multiple acoustic waves that separately modulate the scan beams in an acousto-optic modulator. 
     FIG. 2 shows a precision printing system incorporating a dove prism array in accordance with the invention. 
     FIGS. 3A and 3B respectively illustrate rotation of multiple scan beams by a single dove prism and by a combination of a large dove prism and an array of dove prisms in accordance with the invention. 
     FIG. 4 shows an orientation of an acousto-optic modulator and an array of dove prisms in accordance with the invention. 
     FIG. 5 illustrates the operation of a dove prism on a modulated beam. 
     FIGS. 6 and 7 show an assembly including rods and a flat before and after polishing to convert the rods into dove prisms during a process in accordance with the invention. 
     FIG. 8 shows a dove prism removed from the assembly of FIGS. 6 and 7. 
     FIG. 9 shows a dove prism array where in accordance with an aspect of the invention, dove prisms are mounted in grooves formed in a substrate. 
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with an aspect of the invention, a precision printing system employing a scanner and multiple scan beams includes an array of dove prisms. An acousto-optic modulator or deflector in the printing system controls the intensities of the scan beams. As the acousto-optic modulator turns a beam on or off, successive portions of a scan beam cross-section brighten or darken along the direction of propagation of acoustic waves in the acousto-optic modulator. Each dove prism in the array independently rotates the direction of brightening of an associated scan beam so that in the image plane of the printing system, the scan beams brighten in a direction along or opposite the scan direction. To create a dove prism array with suitable accuracy for precision printing, a rigid or crystalline substrate, such as silicon, is etched with a series of parallel grooves for mounting of the dove prisms. Placing dove prisms formed from fiber-optic material or fine silica rods in the grooves precisely aligns the prisms in the array. 
     FIG. 2 shows a precision printing system  200  that employs scanning in accordance with an embodiment of the invention. A prescan portion of system  200  includes a beam source  210 , an acousto-optic modulator (AOM) 220, and prescan optics  230 . Beam source  210  forms multiple input beams  219  having intensities which AOM 220 modulates. Prescan optics  230  direct modulated input beams  229  onto a scanning element  240 . In accordance with an aspect of the invention, an optical array  222  and brush rotation optics  228  between AOM 220 and scanning element  240  change the direction in which portions of modulated beams  229  successively brighten or darken as AOM 220 turns on or shuts off the beams. Array  222  is an array of prisms or other optical elements that change the brightening directions for individual beams while preserving the orientation of a brush formed by the beams. Brush rotation optics  228 , which can be a K mirror or a single dove prism, rotates the brush to the appropriate orientation for scanning and can also change the brightening directions of the individual beams. 
     Scanning element  240  directs multiple scan beams  249  into post-scan optics  250 . Scanning element  240  is preferably a rotating polygon mirror but alternatively an oscillating mirror or a rotating holographic element could be employed. Post-scan optics  250  focuses scan beams  249  as scan beams  249  sweep along scan lines on a surface of a workpiece. Post-scan optics  250  include a scan lens  252  and a reduction lens  258 . In an exemplary embodiment of system  200 , scan lens  252  is an f-θ lens because such lenses are known to provide a uniform scanning rates. Scan lens  252  can alternatively be another type of lens such as a f-sin θ lens, which reduces scan line bow for extended scan brushes but causes the scan rate to be non-uniform. Reduction lens  258  reduces the scan line and resulting image size as required for the image to be formed on the workpiece. For the exemplary embodiment, the workpiece is a mask, a reticule, an unprocessed wafer, or a partially processed wafer that is coated with a layer of photoresist. A precision stage system  260  which is connected to an interferometer  262  and an alignment system  264  positions and moves the workpiece as required for alignment and indexing of scan lines. Alignment system  264  identifies the positions of alignment marks on the workpiece as viewed through reduction lens  258  and accordingly determines the position of the alignment relative to the scan lines. Interferometer  262  monitors the movement of the workpiece for indexing. 
     Beam source  210  includes a light source  211 , beam-shaping elements  212  and  214 , a beam steering system  213 , a beam splitter  215 , and brush optics (or telescope)  216 . Light source  211  is preferably a laser that generates a coherent beam of suitable power and wavelength for the printing performed. In the exemplary embodiment of the invention, light source  211  is a laser that produces a monochromatic beam of deep ultraviolet light having power greater than about 0.1 W. For example, a “Sabre Fred” system available from Coherent, Inc. delivers 0.5 W beam at 244 nm or 257 nm. Such deep UV lasers have a resonant cavity free of oxygen and moisture to prevent formation of ozone and degradation of the BBO crystal. These lasers with appropriate components in the rest of system  200  can achieve a minimum feature size of 360 nm with a uniformity of ±20 nm and a placement accuracy of less than 20 nm at exposure doses up to 200 mJ/cm 2  using multiple pass printing. The environment for the optical system and workpiece is a clean environment that is kept at a temperature controlled to ±0.05° C. High purity nitrogen is a suitable purge gas where required. 
     Beam preparation optics  212  and beam steering system  213  direct the beam from light source  211  to beam splitter  215  which splits the beam into multiple input beams  219  for AOM 220. Beams  219  form a “brush” for simultaneous illumination of multiple scan lines. In the exemplary embodiment, beam splitter  215  and telescope  216  form thirty-two input beams  219  which are along a line with a  404.8- μm on center spacing. The center two beams are separated by an extra 202.4 μm (one half the normal spacing) for a total of 607.2 μm. The separation provides gaps between input beams  219  sufficient for AOM 220 to modulate the intensity of each beam independently. Rotation optics  228  rotates the brush so that, in the image plane of system  200 , the projections of beams  219  along the scan direction overlap so that the scan brush illuminates a continuous strip with some overlap of the separate scan lines. Additionally, after scan across a set of 32 scan lines, the entire brush is indexed (moved in the image perpendicular to the scan direction) by half the width of the brush. The larger spacing between the center beams of the brush causes beams in the top half of the brush in a subsequent scan to be interleaved with beams the lower half of the brush in the previous scan. This provides a more uniform illumination for smoother imaging. 
     AOM 220 is a block of material such as fused silica having a patterned layer of lithium niobate bonded to one surface. Electric signals applied to the contacts lithographically defined in a conductive layer over the lithium niobate layer create multiple acoustic waves. Each acoustic wave propagates through the path of an associated input beam in the block, deflects the associated input beam, and controls the intensity reaching an aperture that selects only the diffracted beam. A rasterizer  224  generates the signals that create the acoustic waves and as a result controls the intensity of modulated beams  229 . In particular, rasterizer  224  divides each scan line into pixels and generates the signals as required for each pixel to have a desired intensity. A timing generator  226  generates a pixel clock signal that is synchronized with the movement of the scan beams. A facet detection system  242  detects the orientation of scanning element  240  to identify the beginnings of scan lines. While a scan beam sweeps across a scan line, timing generator  226  generates the pixel clock for the scan line. If the motion of scanning element  240  and the characteristics of post-scan optics  250  cause the scan beams to scan at a uniform rate, the pixel clock signal is uniform. Otherwise, the pixel clock period can vary according to variations in the scan rate. Co-filed patent App. Ser. No. UNKNOWN describes a timing generator suitable for systems having a non-uniform scan rate. 
     To provide the maximum space for separate acoustic waves that separately control individual beam intensities, the acoustic wave propagate along a direction perpendicular to the line of input beams  229 . FIG. 3A illustrates input beams  219  that are in a line  310  with the direction  312  of propagation of the acoustic waves perpendicular to line  310 . Direction  312  is the direction in which successive portions of beam  229  become illuminated as AOM 220 turns on the beam and is sometimes referred to herein as a brightening direction. If array  222  were not present, a brush rotation optics  228 , which rotates line  310  to a line  320  relative to scan direction  330 , would rotate brightening direction  312  by the same amount. Accordingly, a direction  322  in which beam cross-sections brighten would be perpendicular to line  320  and at angle to scan direction  330  as shown in FIG.  3 A. This would blur edges, skew rectangles, and create a 45°/135° bias in the scanned image as described above. In system  200 , array  222  separately rotates the brightening direction  312  of each modulated beam  229 . For example, as illustrated in FIG. 3B, array  222  rotates the brightening direction  312  of modulated beams  229  in line  310  to a brightening direction  314  while keeping modulated beams  229  in line  310 . Direction  314  is such that when brush rotation optics  228  rotate line  310  to line  320 , the beam cross-sections brighten along a direction  324  that is opposite scan direction  330 . 
     Array  222  should be positioned in the paths of modulated beams  229  where beams  229  are tightly collimated or focused. As shown in FIG. 2, array  222  is immediately adjacent or part of AOM 220. Alternatively, array  222  can be at or near a focus in prescan optics  230 . FIG. 4 shows a possible orientation of AOM 220 and array  222 . As shown, each modulated beam  229  from AOM 220 has in array  222  an associated dove prism  410  that rotates the brightening direction for that beam. In the exemplary embodiment, beams  229  and dove prisms  410  are 404.8 μm apart on center to match the separation of beams in the brush. Each prism  410  is formed as described below from a cylindrical rod approximately 400 μm in diameter. Beams  229  have a cross-section preferably less than about 150 μm in diameter. 
     FIG. 5 illustrates the operation of a dove prism  500  on a beam  229 . Prism  500  has a three flat facets  510 ,  520 , and  530  that affect the transmission of modulated beam  229 . Base facet  520  is parallel to an x-z plane where the z axis is parallel to the incident direction of beam  229 . Front and back facets  510  and  520  are an angle φ with the x-z plane and at an angle (90°-φ) with an x-y plane. Beam  229  has a brightening direction at an angle θ with the base facet  520 , and in the cross-section of beam  229 , brightening illuminates a ray B before a central ray A and ray A before a ray C. As shown in FIG. 5, rays A, B, and C are refracted at facet  510 , experience total internal reflection at facet  520 , and are refracted at surface  530 . Because of the total internal reflection, an exit beam  529  has a brightening direction with a y component that is the negative of the y component of the brightening direction of beam  229 . Thus, dove prism  500  effectively rotates the brightening direction of beam  229  by an angle of 2θ. Ideally, exit beam  529  is otherwise the same as incident beam  229 , i.e., exit beam  529  propagates along the same line as incident beam  229 . However, if the angle (nominally φ) facet  510  makes with base facet  520  differs from the angle (also nominally φ) facet  530  makes with base facet  520 , beams  229  and  559  propagate in slightly different directions. Additionally, dove prism  500  shifts exit beam  529  in the y direction unless the incident position of beam  229  and the length of dove prism  500  cause reflection of central ray A from the exact mid-point of prism  500 . 
     Changes in position and direction of beams  229  in array  222  are not critical if each beam experiences the same change. To achieve only uniform changes, dove prisms  410  of FIG. 4 are identical, as nearly as possible, and have common alignment with beams  229 . FIGS. 6,  7 ,  8 , and  9  illustrate a method for making a dove prism array that provides only uniform changes in beam position and direction across a multi-beam scanning brush. 
     FIG. 6 illustrates an initial step for forming multiple dove prisms having uniform geometry. The initial step bonds rods  610  to a precision optical flat  620  using an adhesive or optical pitch. Rods  610  may be fiber optic material, glass, fused silica, quartz, or any sufficiently transparent material capable of withstanding the required power and wavelength of light in beams  229 . Rods  610  are in contact with each other to keeps rods  610  parallel to each other. The rods have a diameter that is controlled to submicron accuracy and on the order of 300 μm to 400 μm. A second step of the process grinds or polishes the combined assembly of rods  610  and flat  620  to create flat surfaces  710 ,  720 , and  730  as shown in FIG.  7 . Surface  720  is on the top of rods  610  and parallel to flat  620 . Surfaces  710  and  730  are at the ends of rods  610  and at an angel with surface  720  sufficient to refract an incident beam to surface  720  for total internal reflection. After grinding, rods  610  are removed from optical flat  620  to provide multiple matched dove prisms such as dove prism  800  of FIG.  8 . Dove prism  800  has facets  810 ,  820 , and  830  that correspond to the facets  510 ,  520 , and  530  of FIG. 5 described above. Dove prism  800  also has a rounded (portion of a cylinder) surface  840  that is not involved in the optical operation of a dove prism. The dove prisms formed in a batch as described here are nearly identical with any geometric errors likely to be common to all of the prisms. 
     FIG. 9 shows a cross-section of a completed dove prism array  900 . To complete the array, photolithography techniques known from integrated circuit manufacturing processes mask a polished silicon wafer  920  for etching of precisely spaced grooves  922  of uniform size. In an exemplary, embodiment grooves  922  are v grooves formed by etching with potassium hydroxide (KOH). Each previously formed rod  800  has its rounded surface  840  in one of grooves  922 . Rods  800  are rotated so that the normals to flat surfaces  820  are at an angle θ with the brightening directions of associated beams  229 . The resulting rotation of the brightening directions is 2θ. Grooves  922  precisely fix the locations of the central axes of rods  800  so that the direction and position of the central axes are well controlled. The rotations and placement along the length of grooves  922  can vary from rod to rod. The effects of such variations are not critical. In particular, placement error along the channel only has a second order effect on the positions of the beams that is dependent on angular error in the orientation of the front and back facets. Variation in the rotations of facets  820  affect how close the brightening directions will be to opposite the scanning direction, but an error of a degree or less is a relatively small effect when compared to uncorrected case where the brightening direction can be nearly 90° from the scanning direction. 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.