Abstract:
In accordance with the present invention a scanning method for flat objects obtains reciprocating ballistic motion without any physical contact between the mutually moving parts, as contact can create particulate matter causing defects. The method ensures that the peak forces attained during scanning are minimized to prevent dislodgement of particulate material and excessive vibration. The method furthermore ensures that both surfaces of flat plate objects be scanned simultaneously.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     None 
     STATEMENT REGARDING FEDERALLY SPONSORED R&amp;D 
     Not applicable 
     REFERENCE TO MICROFICHE APPENDIX 
     Not applicable 
     FIELD OF THE INVENTION 
     The invention pertains to the general field of scanning and in particular to two-dimensional scanning of flat objects. 
     BACKGROUND OF THE INVENTION 
     Scanning is a central aspect of many industrial, commercial and technological processes. This is done either to collect information by numerous sensing means, such as in optical scanning where optical data is gathered over a relatively large area, or with and actuator head where the scanning is undertaken with a view to effect an influence on a point by point basis over the area being scanned. The present invention concerns itself with 2-dimensional scanning where there is no physical contact between the scanning device and the object being scanned. 
     Prior art in the field of has established the practice of ensuring correct two-dimensional positioning of scanning devices by driving the scanning system to predetermined positions based on incremented coordinates in both the X and Y directions. In some cases the object being scanned is placed on a platen and the latter is moved along a 2-dimensional raster path, while in others the scanning head is moved relative to the object to be scanned which is affixed to a stationary platen. Both situations have in common the fact that the control system assumes the coordinates of the position that is scanned or illuminated to be those to which the scanning head or platen has been directed by the control system. 
     U.S. Pat. No. 4,245,259 teaches an X-Y scanning device in which the coordinates of the scanned point are not predetermined, but instead either the scanning head or the scanned platen moves with respect to the other member in at least one direction in a free running oscillatory shuttle motion, the initial and make-up power being provided by a low inertial motor and the return stroke provided by energy conserving springs of suitable stiffness at the end of the travel of the scanning head. In this case the instantaneous position of the moving member is determined by measurement and is not predetermined through some programmed means. This particular invention seeks to attain a smooth and uniform scanning velocity utilizing a minimum of drive power. The invention incorporates a conventional roller drive. 
     U.S. Pat. No. 2,476,496 teaches the use of linear motors in scanning devices, the invention addressing itself in particular to a multipurpose facsimile machine. The driven member moves along a guide rail and terminates against shock absorbing springs. The linear motor in this invention comprises coils in the moving member and alternating magnetic and non-magnetic materials in the guide rail along which the moving member glides with the coils concentric to the guide rails. 
     U.S. Pat. No. 4,348,697 describes a facsimile apparatus a similar machine which generates signals representing information for more than one scan line during each traversal of the scanning head or carriage. 
     U.S. Pat. No. 4,764,815 discloses yet another shuttling scanner. This particular invention addresses itself, amongst other aims, to the reduction of the mechanical forces involved in larger and heavier high resolution scanning devices. It seeks to do so by means of springs, either mechanical or air, at either end of the shuttle traverse. In this invention the springs are supported independently from either the scanning head or the platen being scanned. 
     U.S. Pat. No. 4,750,721 seeks to address the forces that destabilize scanning devices by using a counter weighting approach that seeks to maintain the center of mass of the system as a whole in a fixed position. It does so by means of an intermediate stage operating much like a recoilless system. It also employs a linear motor as drive system. 
     The above prior art therefore clearly discloses the concept of a scanning device in which at least one of either the platen or the scanning head moves in at least one dimension by a ballistic shuttling motion, the position of the moving part being determined by measurement rather than by driving it to a predetermined position. Means of reducing the mechanical forces on the scanner as a whole, with a view to attaining positioning accuracy, are also disclosed in this prior art. 
     Within the semiconductor integrated circuit manufacturing industry there are applications requiring extremely accurate positioning of scanning heads and platens. Often these systems are comparatively large, representing significant mass and momentum in their moving parts. Simultaneously this industry has great concern regarding the generation of particulate matter that go directly to the quality of the product being produced. This results in the equipment being placed in Class 10 clean facilities with extreme precautions regarding the generation and disturbance of particulate matter. Any scanning devices generating significant forces whilst executing shuttling actions will therefore run the risk of freeing up such particulate matter and exacerbating the quality concerns. Any equipment containing rigid surfaces that come into contact periodically is undesirable, as such contact generates particulate matter. All prior art exhibits this problem. 
     There is therefore considerable merit in devising a means of reducing the peak forces required to drive the scanning system. There is also merit in ensuring that neither the traverse nor the execution of the return stroke of shuttling scanners requires physical contact between any bulky parts in mutual motion as such contact would similarly be a source of forces leading to loosened particulate matter and consequent contamination. 
     Within the printed circuit board industry there is an added consideration that the boards should be scanned on both sides simultaneously, thereby eliminating a costly additional processing step. There is therefore considerable merit in an approach that provides this additional functionality. 
     In the specific area of the reciprocation of the shuttling part of scanners, the prior art described in U.S. Pat. No. 2,476,496 teaches the use of springs while U.S. Pat. No. 4,764,815 describes the use of either springs or air springs. Both of these devices have shortcomings that lead to excessive forces being generated during the reciprocation process of the shuttling part. In the case of a spring the force in the opposite direction to the motion of the shuttle increases linearly with the compression while in the case of air springs it increases even more rapidly with compression. The result is quite a precipitous stop to the shuttle with resulting high peak forces. There is therefore much merit in devising an alternative means to effect the slowing and turnaround reciprocation of the shuttle carriage of the scanner if this means will lead to lower peak forces during the stroke of the shuttle carriage. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention a scanning method for flat plate objects obtains reciprocating ballistic motion without any physical contact between the mutually moving parts. The method ensures that the peak forces attained during scanning are minimized to prevent dislodgement of particulate material. The method furthermore ensures that both surfaces of flat plate objects may be scanned simultaneously. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a two-dimensional electromechanically driven shuttling translation stage that embodies the invention 
     FIG. 2 shows a cross-sectional view of the shuttle mounting system perpendicular to the direction of ballistic motion. Air circuit ducts are depicted schematically. 
     FIGS.  3 ( a ) and ( b ) show alternative implementations of the reciprocating mechanism for the ballistic shuttle, relying on pressurized air in the case of the former and on springs and air cushions in the case of the latter 
     FIG. 4 shows a shuttling translation system that allows printing two sides of a flexible surface simultaneously 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates the essence of the preferred embodiment of the method disclosed in this invention. 
     Using a stationary base  1  and air bearing tracks  2 ,  3  and  4  a shuttle carriage  5  carrying the flat plate object or planar substrate  6  that is being scanned is ballistically shuttled along one axis by a linear motor comprising of permanent magnet armatures  7  and  8  and wound stators  9  and  10 . By virtue of the air bearings, depicted in greater detailed in later FIG. 2, the motion of the shuttle carriage is subject to very low frictional losses. By suitable electrical signals supplied to the stators  9  and  10  the shuttle carriage may be started, stopped, slowed, reciprocated, interrupted or otherwise affected in its motion. 
     The enduring merit of employing a linear motor in this way is that its retarding force can be made largely constant with distance of the armatures  6  and  7  within the coils of the stators  8  and  9 . In this respect it differs markedly from the springs and air springs taught by the prior art. This creates far lesser peak forces during the slowing and turnaround reciprocation of the shuttling carriage than is the case with springs or air springs. This then also forms the basis for the invention leading to lesser contamination in that any resident particulate matter in the scanner system will be much less likely to be dislodged in the method described here. 
     The cross-scanning fixture  11  allows the scanner head  12  to address a swatch  13  of multiple tracks in the shuttling direction on the planar substrate being scanned  6 . The scanner head is moved in the direction perpendicular to the shuttle direction by the action of a lead screw  14  driven by a motor  15 , or any other conventional means such linear motors, friction drives, metal belts and timing belts etc. 
     In order to secure the substrate  6  being written to the shuttle carriage  5 , a means  16  of attaching the substrate is provided. In this preferred embodiment the preferred means is vacuum, but mechanical means may also be employed. 
     The pressurized air that is required for various functions depicted in FIG. 2 is fed to the shuttle via a coiled air hose  17  while the lateral motion of the shuttle is constrained in contact-less low friction means which, in the case of this preferred embodiment, is a single vertical air bearing  18 . This air bearing may also be viewed in a different aspect in FIG. 2 
     The position of the shuttle is not preprogrammed and driven, but rather the system uses the shuttling motion to its greatest benefit and simply reads the precise position of the shuttle very accurately from the linear encoder  19  using the encoder readout station  20 . This encoder may be either conventional or interferometric as required by the precision specifications of the application. Once linear motors  9  or  10  place the shuttle in motion, the motor is no longer used (in the case of a linear motor the armature may be disengaged) and the shuttle carriage motion is ballistic with the shuttle carriage moving at constant speed. 
     Referring now to FIG. 2, the shuttle carriage and base  1  are shown in cross-section. A venturi generator  21  is employed to avoid an additional vacuum connection to the moving shuttle. The presence of such a connecting hose will constitute an avoidable encumbrance to the shuttle that is intended to have an little energy loss as possible. 
     All vacuum and pressure lines  22  and  23  are shown schematically. Pressure circuit duct  23  supplies the pressurized air for horizontal air bearing  24  and vertical air bearing  25 . Vacuum pull-down pocket  26  provides the counter-force for balancing the horizontal air bearing  24 , the vacuum for this function being routed via duct  22 . Vacuum grooves  27 , supplied with vacuum by duct  22 , hold the substrate  6  being scanned to the shuttle carriage  5 . Other methods of substrate hold-down and air bearing counter force can be substituted. Conventional double-side air bearings similar to  18  can also be used instead of single-sided bearings  24  to eliminate the need for counter-force generators  26 . 
     Also depicted in cross-section in this drawing are the linear motor armature  7  and stator  9 , the linear encoder  19 , and air bearing tracks  2 , 3  and  4 . 
     A unique aspect of the method disclosed here is that it nowhere requires intermittent physical contact between the shuttle carriage and the base, thereby reducing the transmission of peak forces through the system. The only contact is hose  17 , however, this is a permanent connection while particulate material is generated from intermittent contact. Connection  17  can be eliminated, if so desired, by reversing the design of the air bearings, placing pockets  24  along tracks  2 , 3  and  4  and making members  18  and  5  a smooth passive surface. The penalty is increased air consumption, as some of the pockets will not be covered at all times. 
     Turning now to FIG. 3-a, we provide an alternative embodiment of the reciprocating device, replacing the linear motor with an air-piston-and-cylinder arrangement. Air pressure supply  28  is routed via an air valve  29 , the timing of which is controlled by timing signals from the linear encoder  19  (see FIG.  1  and FIG.  2 ). Air cylinders  30  are affixed to the base  1  of the scanner (See FIG.  1 ). Air pistons  31  are affixed to the shuttle carriage  5  (see FIG.  1  and FIG.  2 ). Unlike conventional cylinders, there is no contact between piston  31  and cylinder  30 . This eliminates the creation of particulate matter. At either end of its range of travel the shuttle carriage  5  will cause the relevant piston  31  to enter the air cylinder  30  at that particular end of the base  1 . The pressurized air will be switched on or off or to any intermediate value by a control circuit driven by the timing signal from linear encoder  19 . In this way the air supply to the cylinders will be shut off when the shuttle carriage is at the opposite end of its stroke or travel and it will be switched on when the piston  31  moves into the cylinder  30  or is sufficiently near it. Manipulation of this air supply and timed switching of the valves  29  allow the motion of the shuttle carriage to be controlled. Since there is no contact between piston  31  and cylinder  30 , some air will leak. The low losses of the air bearings require very little energy to be added in each stroke, these losses can be tolerated. 
     Turning now to FIG. 3-b we depict yet another alternative embodiment of the reciprocating device. In this case springs  32  affixed to base  1  have buffer plates  33  at their ends facing the shuttle carriage  5 . The shuttle carriage in its turn has at its ends air bearings  34  fed with pressurized air by duct  23  (See FIG.  2 ). We do not depict the means to drive the shuttle in this embodiment and either of the previous two means depicted in FIG.  1  and FIG. 2 or in FIG. 3-a may be employed. In this case the shuttle again does not physically contact either the base or the spring buffer plates  33 . The air bearing  34  prevents direct contact between the shuttle  5  and buffer plate  33 , thus preventing generation of particulate matter 
     In FIG. 4 we depict a further embodiment in which two sides of the flat plate object are scanned simultaneously. This requires the addition of a second scanner head  12 A further air bearings  35  and  35 A that, together, serve to maintain the flatness of the plate in the vicinity of the area being scanned.