Abstract:
A fast scan axis for a high speed flat bed scanner has an elongated reciprocating table for securing the material to be scanned. The table is narrower than the material and portions of the material overhang the table. The overhanging portions are supported by stationary support surfaces with a plurality of air ports in the surface thereof for providing an air bearing support for the overhanging portions. The moving mass of the scanner is thus kept small enabling fast scanning of the material.

Description:
RELATED APPLICATIONS 
     This application claims benefit of the filing date of U.S. application Ser. No. 60/381091 filed on May 17, 2002. 
    
    
     TECHNICAL FIELD 
     The invention relates to the field of optical scanning and more particularly to scanning flat materials having at least one relatively flat surface. 
     BACKGROUND 
     Industrial scanners are commonly used to scan surface(s) of a material for the purpose of altering or inspecting the surface. Such alteration may involve patterning of the surface using selectively applied laser radiation to deposit material from a donor sheet and/or to change or remove a layer coated on the surface. In many applications, there is a requirement to scan the surface of a material in a rectilinear fashion and when the material is a relatively flat sheet, a flat bed scanner is commonly used. Specific example applications of flat bed scanning include fabrication of flat panel displays, printed circuit boards and printing plates. The material having the surface to be scanned is placed on a table, and the table is moved rapidly back and forth underneath an optical scanning head to scan the surface along a first axis. To complete the scan of the surface, motion along a second axis orthogonal to the first axis may also be necessary depending on the configuration of the optical scanning head. This may be achieved by moving the table or by moving the optical scanning head in a direction aligned with the second orthogonal axis. 
     When the surface being scanned is large, the mass of such a table and the inertial forces involved in creating the relative motion between the table and the optical scanning head may become prohibitive. Prior art scanners, such as that disclosed in PCT application WO 00/49563, have sought to address this problem by supporting the material having the surface to be scanned on rollers. The scanning speed of such systems is limited by the inertia of the rollers, since the rollers still have to change direction each time that the scan direction is reversed. Additionally, the use of moving parts that can wear is undesirable in circumstances where the material must be scanned or otherwise processed in extremely clean environments. Other disadvantages of rollers include deformation caused by contact between the material and the plurality of rollers, which contact the material at a plurality of points. Continuous support is important in cases where downward pressure is applied to the material by, for example, a donor material sheet in intimate contact with the material surface. 
     SUMMARY OF THE INVENTION 
     In a first aspect of the present invention, a fast axis scan apparatus used in a high speed scanner for scanning a flat material is provided. The scan apparatus has an elongate table that is adapted to be driven in a reciprocating manner in a direction aligned with the fast axis. The table has a chuck for securing the material to the table such that at least a portion of the material laterally overhangs the table. There is a stationary support surface for each overhanging portion of the material. The stationary support surface is disposed substantially parallel to the overhanging material and has a plurality of air ports for providing air bearing flotation to the overhanging material portions. 
     In another aspect of the present invention, a method for scanning a flat material in a fast scan direction is provided. The method has the following steps:
         (a) clamping the material over only a portion of its surface area so that there is at least one overhanging portion;   (b) supporting the at least one overhanging portion using an air bearing surface;   (c) scanning the material in the fast scan direction by moving it back and forth in a reciprocating manner.
 
For an understanding of the invention, reference will now be made, by way of example, to the following detailed description in conjunction with the accompanying drawings.
       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate by way of example only preferred embodiments of the invention: 
         FIG. 1  is a perspective view of a flatbed scanner according to the present invention; 
         FIG. 2  is an exploded view of the vacuum chuck, moving table and a portion of the main beam of the scanner of  FIG. 1 ; 
         FIG. 3  is a partial cross-sectional view of the air bearing and main beam of the scanner of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of the scanner of  FIG. 1 . 
         FIG. 5  is an enlarged partial cross-sectional view of the scanner of  FIG. 1 ; and 
         FIG. 6  is a partial cross-sectional view of an alternative embodiment of the scanner according to the present invention. 
     
    
    
     DESCRIPTION 
       FIG. 1  shows a flatbed scanner in accordance with a particular embodiment of the present invention which is suited for patterning a glass substrate used in the fabrication of flat panel displays (FPD). A main beam  1  is mounted at its ends on a pair of supports  2  (only one of which is visible in the drawing of  FIG. 1 ). As described in more detail below, table  3  is supported for sliding motion on beam  1  using air bearings. Table  3  comprises a vacuum chuck  4  on its upper surface. Vacuum chuck  4  provides vacuum suction which holds a material  5  in a fixed orientation on the surface of vacuum chuck  4 . In the illustrated embodiment, material  5  is a sheet of glass. Vacuum chuck  4  supports and holds a central portion of material  5  while laterally overhanging portions of material  5  are supported by a pair of stationary air bearing support surfaces  6 . 
     Air bearing support surfaces  6  comprise a plurality of air ports  32  which expel pressurized air (or other gas) to provide an air bearing (i.e. an air gap) for contactless flotation of the laterally overhanging portions of material  5 . In this manner, air bearing support surfaces  6  help to support and maintain the substantially flat orientation of material  5  during scanning, while avoiding direct contact with material  5 . The air channelled between material  5  and air bearing surfaces  6  also provides a retaining force which tends to maintain the alignment of material  5  above air bearing surfaces  6 . 
     One or more endless belts  7  extend between and are entrained over pulleys  10  and  31 . Motor  8  is driveably coupled to pulley  31  such that rotational motion of a shaft of motor  8  causes corresponding linear motion of belt  7 . Belt  7  is attached to table  3 , and under actuation from motor  8 , imparts longitudinal reciprocating motion thereto. In this description the direction parallel with the longitudinal reciprocating motion of belt  7  is referred to as the “fast scan axis” and the orthogonal direction is referred to as the “cross scan axis”. The drive mechanism incorporating motor  8 , belt  7  and pulleys  10 ,  31  is advantageous because it is simple and cost effective, it keeps the heat generated by motor  8  outside the high accuracy scanning areas and it involves a small number of moving parts whose collective mass is relatively small when compared to the mass of the moving parts in alternative drive mechanisms. 
     The components driven by motor  8  comprise mainly table  3 , vacuum chuck  4 , material  5 , belt  7  and pulleys  10 ,  31 . These components have a collective mass which is small in comparison to the mass of moving parts in alternative drive mechanisms. The attachment (not shown) of belt  7  to table  3  is preferably positioned as close as possible to the center of gravity of table  3 , which reduces torque imparted on table  3 , thus helping to prevent rotational errors. 
     The position of table  3  (and material  5 ) along the fast scan axis of the scanner is indicated by a linear encoder  9 . Linear encoder  9  preferably comprises an adhesive tape scale, with a corresponding read-head (not shown). Examples of such linear encoders include Thox™, manufactured by Renishaw of Gloucestershire, U.K. Using encoder  9  as a reference, the reciprocating motion of table  3  (and materials) along the fast scan axis need not be absolutely uniformly applied. Encoder  9  may provide synchronization to the writing or reading operation. This synchronization removes constraints from the belt drive system which allows, for example, some stretching under load. Drive electronics and motors are known to those skilled in the art and are commercially available from suppliers such as Anorad (USA), Fanuc (Japan), Siemens (Germany) and others. 
     Crosswise scanning (i.e. along the cross scan axis) is provided by sleeve  13  which is slideably mounted on air bearing beam  11 . Air bearing beam  11  comprises a plurality of apertures which provide an air bearing that supports sleeve  13  above the surface of beam  11 . Air bearing beam  11  spans the cross scan width of material  5  and is supported on either side of material  5  by supports  12 . Supports  12  maybe coupled to beam  1  via a machine base (not shown). 
     An optical scanning head  14  is attached to sleeve  13 . In one embodiment of the invention, optical scanning head  14  comprises a multi channel thermal laser imaging head along with a CCD camera which may be used, for example, for locating registration indicia on material  5 . An example of a suitable optical scanning head  14  is the Squarespot™ thermal imaging head manufactured by Creo Inc. of Burnaby, British Columbia Canada. 
     Unlike the fast scan axis which incorporates linear encoder  9  to provide synchronization and to ease the associated actuation accuracy requirements, cross scan motion of sleeve  13  and optical scanning head  14  may require more precise position control. In the illustrated embodiment, the cross scan motion of sleeve  13  and optical scanning head  14  is controllably actuated by linear motor  15  in a closed-loop fashion, using linear encoder  16  for position feedback. Controllable linear motion systems, such as the one used to controllably actuate sleeve  13  and optical scanning head  14 , complete with encoder, motor and drive electronics are widely available and well known to those skilled in the art. 
     Details of the coupling of table  3  to beam  1  and vacuum chuck  4  are shown in the exploded view of  FIG. 2 . Table  3  is preferably made from material having high specific stiffness, such as carbon-fiber based composite material for example. Carbon-fiber based materials have the advantage of allowing the tailoring of the material thermal expansion coefficient to thermally match the other parts of the system and the material being scanned. Such balancing of thermal coefficients of expansion is well known in optical systems where a degree of thermal invariance is desired. Carbon-fiber based materials may be fabricated to have a target co-efficient of thermal expansion that may very small or even be negative. 
     Table  3  is equipped at each end with a plurality of air bearing pads  17  which surround beam  1 . In a preferred embodiment, twelve air bearing pads  17  are used, six at each end of table  3 . Only three air bearing pads are visible in  FIG. 2 . 
     Air bearing pads  17 , shown in more detail in  FIG. 3 , are preferably made of aluminium. Each pad  17  comprises a flat annular shaped portion  33 , a shallow recess  25  and a flow restrictor aperture  26 . Apertures  26  of pads  17  are fed by an air (or other gas) distribution system  18 . Pads  17  are sized to cover almost the full width of beam  1  on each of its faces. In the illustrated embodiment, pads  17  are sized such that a single pad  17  covers the top face of beam  1 , a single pad  17  covers the bottom face of beam  1  and pairs of pads  17  cover the side faces of beam  1 . 
     As an example, for a 100 mm×200 mm cross-section beam the diameter of each pad may be about 80 mm, the thickness about 8 mm, and shallow recess  25  may be a slight taper with an outside diameter of about 40 mm and a central depth of about 0.025 mm. Flat portion  33  of each pad  17  is preferably lapped to correspond to the flatness of beam  1 . Restrictor aperture  26  of each pad  17  is about 0.5 mm in diameter and about 2 mm long. A secondary taper or countersink (not shown) with an outside diameter of about 3 mm may be provided at the periphery of aperture  26 , as a transition between recess  25  and aperture  26 . The outer peripheral edges of each pad  17  at the edges of flat portion  33  may be chamfered. In this example embodiment, the thickness of pads  17  is about 1 mm less than the desired operating gap between table  3  and beam  1 . 
     For each pad  17 , a spring washer  27  may be inserted in this gap to press pad  17  towards beam  1  during assembly of table  3 . Low shrinkage epoxy resin  28  may then be injected via aperture  29  to fill the gap and secure the pads  17  in correct alignment with beam  1 . 
     Vacuum chuck  4  is preferably mounted to table  3  in a manner which allows a small range of rotational adjustment. Such rotational adjustment permits alignment of the reciprocating scanning motion along the fast scan axis with registration marks on material  5 . The small rotational adjustment, typically below 5 mrad, is preferably provided by one or more flexure joints located between chuck  4  and table  3 . Flexure and rolling motions minimize the generation of particulate matter and, consequently, are preferred over contact sliding in applications requiring a high degree of cleanliness such as FPD fabrication. 
     In the illustrated embodiment of  FIG. 2 , one end of chuck  4  is mounted on a rotational flexure pivot  20  on table  3  using fastener  24 . The other end of chuck  4  rests on or is secured by fasteners  24  to a band  21 . Band  21 , which may be made of steel for example, extends between a pair of rollers  40 , one of which is actuated by a gear motor  22 . In this manner, gear motor  22  may be activated to move one end of chuck  4 , such that chuck  4  may be rotated slightly about flexure pivot  20 . 
     Referring now to  FIG. 4 , air bearing support surfaces  6  preferably comprise a honeycomb or hollow construction, allowing air (or other gas) to be supplied via inlet port  30  and to escape via outlet ports  32 . The interior structure of air bearing support surfaces  6  may be divided into chambers  31  and inter-connected via apertures  42 . Such a construction helps to prevent the internal pressure of air (or other gas) distorting the upper air bearing support surfaces. 
     The more detailed view of  FIG. 5  shows grooves  36  which allow the air (or other gas) to escape without lifting material  5  any more than necessary. Typically, air pressure inside chamber  31  may be in a range between 2–5 psi and nozzles  32  may be between 2–4 mm in diameter. 
     Vacuum chuck  4  is preferably fabricated from three plates of carbon fiber composite, as shown in  FIG. 5 . Top plate  4 A comprises a plurality of small apertures  35  which allow the vacuum to securely grip material  5 . Bottom plate  4 C comprises mounting holes through which fasteners  24  mount chuck  4  onto table  3 , as described above. Bottom plate  4 C also comprises a vacuum connection  34 . Middle plate  4 B has openings formed therethrough to distribute the vacuum to apertures  35 . The three plates  4 A,  4 B, and  4 C are bonded together by an adhesive such as epoxy resin, to form a stiff, lightweight vacuum chuck  4 . 
     In one particular embodiment, vacuum chuck  4  has dimensions of 150×1000×9 mm, and is made of three plates  4 A,  4 B  4 C of carbon fiber composite, each of which is 3 mm thick. As may be seen in  FIG. 2 , vacuum chuck  4  extends longitudinally (i.e. in a direction parallel with the fast scan axis) past table  3 . This allows clearance between table  3  and supports  2  and allows table  3  to be moved along the entire fast scan axis. 
     The operation of the scanner is briefly explained with reference to  FIG. 1 . The description provided herein relates to the use of the scanner in the fabrication of a FPD. Those skilled in the art will appreciate that other applications may involve slightly different operation of the scanner. 
     A glass sheet of material  5  is positioned on vacuum chuck  4 . The initial alignment of material  5  on chuck  4  may be aided by mechanical stops or electronic means, such as a CCD camera. Once positioned and aligned, vacuum is applied to chuck  4  and is transmitted to material  5  through apertures  35  to secure material  5  on the surface of chuck  4 . Pressurized air (or other gas) is then applied to air bearing support surfaces  6  to provide flotation to the transversely overhanging portions of material  5 . Preferably, material  5  floats about 0.05 mm above air bearing support surfaces  6 . For this reason, the top surface of vacuum chuck  4  is preferably positioned about 0.05 mm higher than the height of air bearing support surfaces  6 , so that material  5  is maintained in a flat condition. 
     Air (or other gas) pressure of 40 to 80 psi is applied to the air bearing pads  17  of table  3 . This pressure provides a slight outward stretching of the dimensions of table  3  and creates a corresponding air gap of about 0.005 mm between pads  17  and beam  1 . This air gap permits table  3  to run freely on beam  1 . Under these conditions, motor  8  reciprocably drives table  3  using belt  7  and pulleys  10 ,  31 , to move material  5  forward and backward along the fast scan axis. Similarly, air (or other gas pressure) is also applied to the apertures of air bearing beam  11  to provide a small air gap for sleeve  13 . Linear motor  15  moves sleeve  13  and optical scanning head  14  to the desired position along the cross scan axis. 
     Prior to performing any operation (i.e. such as imaging, for example), a second alignment procedure may be performed using reference indicia (not shown) provided on material  5 . Such indicia may be targets such as engraved cross marks or may be one or more edges of material  5 . As a part of this second alignment procedure, material  5  and optical scanning head  14  are moved over two or more indicia separated in the fast scanning axis and the rotation of vacuum chuck  4  is adjusted as described above about flexure pivot  20  to bring material  5  into accurate alignment with the fast scanning axis. 
     Typically, in FPD fabrication, material  5  has been sensitized by applying a coating layer to the surface. In such embodiments, the flatbed scanner may be used to apply patterning radiation. However, this process may vary considerably depending on the application and materials in use. 
     In order to scan the patterning radiation over the surface of material  5 , table  3  is reciprocated back and forth along the fast scanning axis, while optical head  14  emits a beam, or multiple beams, of patterning radiation. The beams are switched on and off in response to imaging data which defines the pattern to be imparted to material  5 . 
     Following each movement along the fast axis, optical scanning head  14  is indexed along the cross scan axis so that during the next motion along the fast scan axis, a new area of material  5  is patterned. Additionally or alternatively, optical scanning head  14  may be indexed to allow interleaving of the patterning radiation according to the particular configuration of the radiation sources on optical scanning head  14 . In this manner, the entire surface of material  5  is patterned. The vacuum supply to chuck  4  may then be interrupted and material  5  may be removed from the scanner for further processing. 
     In an alternative embodiment, it may be required to accommodate a cover sheet in intimate contact with material  5  (for example, when using a donor sheet for the purpose of thermal transfer patterning). In  FIG. 6 , a carrier plate  37  is attached to vacuum chuck  4 . Carrier plate  37  is larger than the sheet of material  5 . Carrier plate  37  comprises vacuum holes  35  which penetrate carrier plate  37  and are connected to a plurality of vacuum grooves  38 . Vacuum grooves  38  run along the periphery of carrier sheet  37  and serve to draw down a donor sheet  39  by evacuating the space between donor sheet  39  and carrier plate  37 . 
     Additional grooves (not shown) may also run under material  5  to secure material  5  to carrier plate  37 . Such additional grooves may be connected to the same vacuum supply as the peripheral vacuum grooves  38  or to a different vacuum supply. Advantageously, separating the vacuum supply allows material  5  to be positioned and secured before applying donor sheet  39  and also allows convenient application of subsequent donor sheets should more than one donor be necessary in a particular process. The patterning then takes place in a similar manner to that previously described except that the patterning radiation is operative to transfer material from the donor sheet to the surface of material  5 . 
     In another embodiment, carrier sheet  37  is used for material  5  that is not sufficiently rigid or sufficiently flat for successful scanning without additional support. Carrier sheet  37  may then be used to impart the required stiffness or flatness to material  5 . Such a need may arise in the scanning of printed circuit boards or other materials that are not sufficiently stiff to be supported by air flotation alone. 
     Another alternative involves the provision of additional vacuum holes on the underside of carrier sheet  37 . In operation, the vacuum applied via apertures  35  may also be applied to the holes on the underside of carrier sheet  37 , thus providing additional pull down force between air bearing surfaces  6  and carrier sheet  37 . Similarly, if a carrier sheet  37  is always used on a particular system, the air (or other gas) supply to air bearing surfaces  6  may be dispensed with in favour of an air supply to carrier sheet  37  via a second set of holes for supplying air (or other gas), instead of supplying vacuum. Another method of increasing the stiffness of carrier sheet  37  may involve the use of magnets which pull down to air bearing surface  6 . 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention. 
     In particular, multiple optical heads  14  may be used on the same cross scan beam  11 . Optical head(s)  14  may also be provided with a means for vertical motion if required for purposes of focusing or other operations. Table  3  may be powered by a brushless linear motor or other actuation mechanism instead of the belt drive system described. Table  3  as described is made from a carbon-fiber based composite material, but may also be made from other materials having relatively high specific stiffness such as fiber reinforced aluminium, aluminium foam or magnesium, for example. 
     Optical scanning head  14  may be a read head or a write head or both. While air bearings are shown in the described embodiments, other forms of a non-contacting bearing may be used such as fluidic or magnetic bearings. Air bearing pads  17  may also be made from carbon fiber composite. In the embodiment depicted in  FIG. 6 , grooves  36  may be connected to a vacuum system to pull down material  5  in order to load the air bearings created by nozzles  32 . 
     Alternatives for air flotation systems are also known in the art. While the air bearing surfaces described above are configured for atmospheric release of injected air (or other gas), it is also possible to provide a second plurality of ports, connected to a vacuum, to provide a stiffer support for material  5 .