Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/708,307, filed Feb. 18, 2010, which is a continuation of U.S. patent application Ser. No. 10/524,809, filed on Feb. 15, 2005, which is a national phase of International Patent Application No. PCT/US03/02594, filed Aug. 19, 2003, published on Feb. 26, 2004 as International Patent Publication No. WO 04/017381, which claims priority from U.S. Application No. 60/404,447, which was filed on Aug. 19, 2002, each of which are incorporated by reference in their entireties herein, and from which priority is claimed. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to semiconductor processing methods, and more particularly, to methods for making semiconductors materials in a form suitable for fabrication of thin-film transistor (“TFT”) devices. 
         [0003]    Flat panel displays and other display units are used as visual imaging interfaces for the common and ubiquitous electronic devices and appliances such as computers, image sensors, and television sets. The displays are fabricated, for example, from thin films of liquid crystal and semiconductor material placed on glass or plastic substrates. Each display is composed of a grid (or matrix) of picture elements (“pixels”) in the liquid crystal layer. Thousands or millions of these pixels together create an image on the display. TFT devices fabricated in the semiconductor material layer are used as switches to individually turn each pixel “on” (light) or “off (dark). The semiconductor materials used for making the TFTs, traditionally, are amorphous or polycrystalline silicon thin films. These films are deposited on to the substrates by physical or chemical processes at relatively low deposition temperatures in consideration of the low melting temperatures of the substrate materials used (e.g., glass or plastic). The relatively low deposition temperatures degrade the crystallinity of the deposited silicon films and cause them to be amorphous or polycrystalline. 
         [0004]    Unfortunately, the device characteristics of a TFT fabricated in a silicon thin film undesirably degrade generally in proportion to the non-crystallinity of the silicon thin film. For industrial TFT device applications, silicon thin films of good crystalline quality are desirable. The crystallinity of a thin film of silicon deposited at low temperatures on a substrate may be advantageously improved by laser annealing. Maegawa et al. U.S. Pat. No. 5,766,989, for example, describes the use of excimer laser annealing (“ELA”) to process amorphous silicon thin films deposited at low temperatures into polycrystalline silicon thin films for LCD applications. The conventional ELA processes, however, are not entirely satisfactory at least in part because the grain sizes in the annealed films are not sufficiently uniform for industrial use. The non-uniformity of grain size in the annealed films is related to the beam shape of the laser beam, which is used in the ELA process to scan the thin film. 
         [0005]    Im et al. U.S. Pat. No. 6,573,531 and Im U.S. Pat. No. 6,322,625 (hereinafter “the &#39;531 patent” and “the &#39;625 patent”, respectively), both of which are incorporated by reference herein in their entireties, describe laser annealing apparatus and improved processes for making large grained polycrystalline or single crystal silicon structures. The laser annealing processes described in these patents involve controlled resolidification of target portions of a thin film that are melted by laser beam irradiation. The thin film may be a metal or semiconductor material (e.g., silicon). The fluence of a set of laser beam pulses incident on the silicon thin film is modulated to control the extent of melting of a target portion of a silicon thin film. Then, between the incident laser beam pulses, the position of the target portion is shifted by slight physical translation of the subject silicon thin film to encourage epitaxial lateral solidification. This so-called lateral solidification process advantageously propagates the crystal structure of the initially molten target portion into grains of large size. The apparatus used for the processing includes an excimer laser, beam fluence modulators, beam focussing optics, patterning masks, and a motorized translation stage for moving the subject thin film between or during the laser beam irradiation. (See e.g., the &#39;531 patent,  FIG. 1 , which is reproduced herein). 
         [0006]    Consideration is now being given to ways of further improving laser annealing processes for semiconductor thin films, and in particular for recrystallization of thin films. Attention is directed towards apparatus and process techniques, with a view to both improve the annealing process, arid to increase apparatus throughput for use, for example, in production of flat panel displays. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides systems and methods for recrystallizing amorphous or polycrystalline semiconductor thin films to improve their crystalline quality and to thereby make them more suitable for device applications. The systems and processes are designed so that large surface area semiconductor thin films can be processed quickly. 
         [0008]    Target areas of the semiconductor thin film may be intended for all or part semiconductor device structures. The target area may, for example, be intended for active regions of the semiconductor devices. The target areas are treated by laser beam irradiation to recrystallize them. The target areas are exposed to a laser beam having sufficient intensity or fluence to melt semiconductor material in the target areas. A one shot laser beam exposure may be used—the melted semiconductor material recrystallizes when the laser beam is turned off or moved away from the target area. 
         [0009]    A large number of target areas in a region on the surface of the semiconductor thin film may be treated simultaneously by using laser radiation that is patterned. A projection mask can be deployed to suitably pattern the laser beam. The mask divides an incident laser beam into a number of beamlets that are incident on a corresponding number of target areas in a surface region of the semiconductor thin film. Each of the beamlets has sufficient fluence to melt the semiconductor material in target area on which it (beamlet) is incident. The dimensions of the beamlets may be chosen with consideration to the desired size of the target areas and the amount of semiconductor material that can be effectively recrystallized. Typical beamlet dimensions and corresponding target area dimensions may be of the order of the order of about 0.5 um to a few um. 
         [0010]    An exemplary mask for patterning the laser beam radiation has a number of rectangular slits that are parallel to each other. Using this mask, an incident laser beam can be divided into a number of parallel beamlets. The target areas corresponding to these beamlets are distributed in the surface region in a similar parallel pattern. Another exemplary mask has a number of rectangular slits that are disposed in a rectangular pattern of sets of parallel and orthogonal slits. The slits may for example, be arranged in pairs along the sides of squares. Using this mask the resultant radiation beamlets and the corresponding target areas also are distributed in a similar rectangular pattern (e.g., in sets of parallel and orthogonal target areas). 
         [0011]    The laser beam may be scanned or stepped across the surface of the semiconductor thin film to successively treat all regions of the surface with a repeating pattern of target areas. Conversely, the semiconductor thin film can be moved relative to a laser beam of fixed orientation for the same purpose. In one embodiment of the invention, a motorized linear translation stage is used to move the semiconductor thin film relative to the laser beam in linear X-Y paths so that all surface regions of the semiconductor thin film can he exposed to the laser beam irradiation. The movement of the stage during the process can be continuous across a width of the semiconductor thin film or can be stepped from one region to the next. For some device applications, the target areas in one region may be contiguous to target areas in the next region so that extended strips of semiconductor material can be recrystallized. The recrystallization contiguous target areas may benefit from sequential lateral solidification of the molten target areas. For other device applications, the target areas may be geometrically separate from target areas in the adjoining areas. 
         [0012]    The generation of laser beam pulses for irradiation of the target areas may be synchronized with the movement of the linear translation stage so that the laser beam can be incident on designated target areas with geometric precision. The timing of the generated laser beam pulses may be indexed to the position of the translation stage, which supports the semiconductor thin film. The indexing may be occur in response to position sensors that indicate in real time the position of the stage, or may be based on computed co-ordinates of a geometrical grid overlaying the thin film semiconductor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Further features of the invention, its nature, and various advantages will be more apparent from the following detailed description of the preferred embodiments and the accompanying drawings, wherein like reference characters represent like elements throughout, and in which: 
           [0014]      FIG. 1  is a schematic and block diagram of a semiconductor processing system for the laser annealing of semiconductor thin films for recrystallization; 
           [0015]      FIG. 2  is a top exploded view of an exemplary thin film workpiece; 
           [0016]      FIGS. 3   a  and  3   b  are top views of exemplary masks in accordance with the principles of present invention; 
           [0017]      FIG. 4  is a schematic diagram illustrating a portion of the thin film silicon workpiece of  FIG. 2  that has been processed using the mask of  FIG. 3   a,  in accordance with the principles present invention; 
           [0018]      FIG. 5  is a schematic diagram illustrating an exemplary processed thin film silicon workpiece that has been processed using the mask of  FIG. 3   b  in accordance with the principles present invention; and 
           [0019]      FIG. 6  is a schematic diagram illustrating an exemplary geometrical pattern whose co-ordinates are used to trigger radiation pulses incident on a silicon thin film workpiece in accordance with the principles present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The present invention provides processes and systems for recrystallization of semiconductor thin films by laser annealing. The processes for recrystallization of semiconductor thin films involve one-shot irradiation of regions of a semiconductor thin film workpiece to a laser beam. The systems direct a laser beam to a region or spot on the surface of the semiconductor thin film. The incident laser beam has sufficient intensity or fluence to melt targeted portions of the region or spot of the semiconductor thin film on which the laser beam is incident. After the targeted incident areas or portions are melted, the laser beam is moved or stepped to another region or spot on the semiconductor thin film. The molten semiconductor material recrystallizes when the incident laser beam is moved away. The dwell time of the laser beam on a spot on the semiconductor thin film may be sufficient small so that the recrystallization of an entire semiconductor thin film workpiece can be carried out quickly with high throughput rates. 
         [0021]    In order that the invention herein described can be fully understood the subsequent description is set forth in the context of laser annealing of silicon thin films. The annealed silicon thin films may be intended for exemplary TFT device applications. It will, however, be understood that the invention is equally applicable to other types of materials and/or other types of device applications. 
         [0022]    An embodiment of the present invention is described herein with reference to  FIGS. 1-6 . Thin film silicon workpieces (see e.g., workpiece  170 , FIGS.  2  and  4 - 6 ) are used herein as illustrative workpieces. Workpiece  170  may, for example, be a film of amorphous or randomly expanding and collimating lenses  141  and  142 , homogenizer  144 , condenser lens  145 , a field lens  148 , eye piece  161 , controllable shutter  152 , multi-element objective lens  163 ), also may, for example, he any suitable commercially available optical components sold by the by Lambda Physik USA, or by other vendors. 
         [0023]    The suitable optical components  120 - 163  for shaping and directing the radiation beam may include a masking system  150 . Masking system  150  may be a projection masking system, which is used for patterning incident radiation ( 149 ) so that radiation beam ( 164 ) that is ultimately incident on workpiece  170  is geometrically shaped or patterned. 
         [0024]    Stage assembly  180 , on which workpiece  170  rests during processing, may be any suitable motorized translation stage capable of movement in one or more dimensions. A translation stage capable of high translation speeds may be advantageous for the high throughput single-shot processing described herein. Stage assembly  80  may he supported on suitable support structures to isolate the thin film silicon workpiece  170  from vibrations. The support structures may, for example, include conventional optical benches such as a granite block optical bench  190  mounted on a vibration isolation and self-leveling system  191 ,  192 ,  193  and  194 . 
         [0025]    A computer  100  may be linked to laser  110 , modulator  120 , stage assembly  180  and other controllable components of apparatus  1000 . Computer  100  may be used to control the timing and fluence of the incident laser beam pulses and the relative movement of the stage assembly  180 . Computer  100  may be programmed to controllably move stage assembly translation stage  180  in X, Y and Z directions. Workpiece  170  may be moved, for example, over predetermined distances in the X-Y plane and as well as in the Z direction in response to instruction from computer  1000 . In operation, the position of workpiece  170  relative to the incident radiation beam  164  may be continuously adjusted or intermittently reset during the single-shot laser annealing process at suitable times according to preprogrammed process recipes for single shot recrystallization of workpiece  170 . The movement of workpiece  170  may be synchronized or co-ordinated with the timing of radiation beam pulses generated by laser  100 . 
         [0026]    In apparatus  1000 , the movement of stage assembly  180  translates the workpiece  170  and the radiation beam ( 164 ) relative to each other. In the processing described herein the radiation beam ( 164 ) is held fixed in a position or orientation while stage  180  is moved. Alternative configurations or arrangements of optical components may be used to move incident radiation beam  164  and workpiece  170  relative to each other along defined paths. For example, a computer-controlled beam steering mirror may be used to deflect radiation beam  164  while stage  180  is held fixed in position. By such beam deflecting arrangements it may be possible to completely or partially dispense with the use of mechanical projection masks (e.g., masking system  150 ) and instead use electronic or optical beam guiding mechanisms to scan or step selected portions of workpiece  170  at a rapid pace. 
         [0027]    Using apparatus  1000 , sequential lateral solidification of molten semiconductor material may be achieved using, for example, the processes that involve incremental movement or shifting the position of stage  180  between excimer laser pulses as described in the &#39;531 patent. The movements of stage  170  are small, so that the portions of the silicon thin film that are molten by sequential pulses are proximate to each other. The proximity of the two molten portions allows the first portion to recrystallize and propagate its crystal structure into the adjacent portion, which is melted by the next pulse. 
         [0028]    In the single shot recrystallization processes described here, apparatus  1000  may be used to scan or step a laser beam across the surface of a semiconductor thin film by moving of stage assembly  180 . The laser beam has sufficient intensity or fluence to melt target areas in the regions or spots at which the laser beam pulses are incident. To process an entire workpiece  170 , stage assembly  180  may be moved predetermined distances to cause the laser beam to move along paths across semiconductor thin film  175 /workpiece  170 .  FIG. 2  also schematically shows paths  230 ,  255  etc. that may be traced by incident radiation beam  164  as it is moved across the surface of the workpiece  170 . 
         [0029]    The number of paths and their geometrical orientation may be determined by the cross sectional dimensions of the laser beam and the target area requirements of the circuit or device applications for which workpiece  170  is being processed. Accordingly, the surface of a. semiconductor thin film  175 /workpiece  170  may be partitioned in a geometric array of regions for generating processing recipes for computer  1000  or otherwise controlling the operation of apparatus  1000 .  FIG. 2  shows an exemplary geometrical partitioning of the surface of a semiconductor thin film  175  on workpiece  170 . In the exemplary geometrical partitioning shown in  FIG. 2 , the surface is divided into a number of rows (e.g.,  205 ,  206 ,  207 , etc.) each having a width W. The widths of rows W may be selected with consideration to the cross sectional width of incident radiation beam  164 . Each row contains one or more regions. As an illustrative numerical example, workpiece  170  may have x and y dimensions of about 30 cms and 40 cms, respectively. Each of rows  205 ,  206 ,  207 , . . . etc., may, for example, have a width W of about Vi cm in the Y direction. This value of W may, for example, correspond a laser beam width of about the same size. Thus, the surface of workpiece  170  can be divided into eighty (80) rows each with a length of about 30 cms in the X direction. Each row contains one or more regions whose combined length equals 30 cms (not shown). 
         [0030]    The co-ordinates of each row may be stored in computer  100  for use by the processing recipes. Computer  1000  may use the stored co-ordinates, for example, to compute the direction, timing and travel distances of stage  180  during the processing. The co-ordinates also may be used, for example, to time the firing of laser  110  so that designated regions of semiconductor thin film  175  are irradiated as stage  180  is moved. 
         [0031]    Workpiece  170  may be translated in linear directions while silicon thin film  175  is being irradiated so that a linear strip of silicon thin film  175  is exposed to radiation beams of melting intensity or fluence. The translation paths traced by the radiation beams may be configured an that the desired portions of the entire surface of thin film silicon  175  are successively treated by exposure to laser beams. The translation paths may be configured, for example, so that the laser beam traverses rows  205 ,  206 ,  207 , etc. sequentially. In  FIG. 2 , the radiation beam is initially directed to a point  220  off side  210 ′ near the left end of row  205 . Path  230  represents, for example, the translation path traced by the center of the radiation beam through row  205  as stage  180  is moved in the negative X direction, 
         [0032]    The movement of stage  180  may be conducted in a series of steps in an intermittent stop-and-go fashion, or continuously without pause until the center of the radiation beam is directed to a point  240  near the right end of row  205 . Path segments  225  and  235  represent extensions of path  230  that may extend beyond edges  210 ′ and  210 ″ of workpiece  170  to points  220  and  240 , respectively. These segments may be necessary to accommodate acceleration and deceleration of stage assembly  180  at the ends of path  230  and/or may be useful for reinitializing stage  180  position for moving stage  180  in another direction. Stage  180  may, for example, be moved in the negative Y direction from point  240 , so that the center of the radiation beam traces path  245  to point  247  next to the right end of row  206  in preparation for treating the silicon material in row  206 . From point  247  in manner similar to the movement along path  230  in row  205  (but in the opposite direction), stage  180  is moved in the X direction so that the center of the radiation beam moves along path  255  irradiating thin film silicon material in row  206 . The movement may be continued till the center of radiation beam is incident at spot  265  that is near the left end of row  206 . Path extensions  260  and  250  represent segments of path  255  that may extend beyond edges  210 ′ and  210 ″ to spots  247  and  265 , respectively. Further linear movement of stage  180  in the Y direction moves the center of the incident radiation beam along path  270  to a point  272  next to row  207 . Then, the thin film silicon material in row  207  may be processed by moving stage  180  in the negative X direction along path  275  and further toward the opposite side  210 ″ of workpiece  170 . By continuing X and Y direction movements of stage  180  in the manner described for rows  205 .  206 , and  207 , all of the rows on the surface of thin film silicon  175  may be treated or irradiated. It will be understood that the particular directions or sequence of paths described above are used only for purposes of illustration, other directions or sequences may be used as appropriate. 
         [0033]    In an operation of apparatus  1000 , silicon thin film  175  may be irradiated by beam pulse  164  whose geometrical profile is defined by masking system  150 . Masking system  150  may include suitable projection masks for this purpose. Masking system  150  may cause a single incident radiation beam (e.g., beam  149 ) incident on it to dissemble into a plurality of beamlets in a geometrical pattern. The beamlets irradiate a corresponding geometrical pattern of target areas in a region on the thin film silicon workpiece. The intensity of each of the beamlets may be chosen to be sufficient to induce complete melting of irradiated thin film silicon portions throughout their (film) thickness. 
         [0034]    The projection masks may be made of suitable materials that block passage of radiation through undesired cross sectional areas of beam  149  but allow passage through desired areas. An exemplary projection mask may have a blocking/unblocking pattern of rectangular stripes or other suitable geometrical shapes which may be arranged in random or in geometrical patterens. The stripes may, for example, be placed in a parallel pattern as shown in  FIG. 3   a , or in a mixed parallel and orthogonal pattern as shown in  FIG. 3   b , or any other suitable pattern. 
         [0035]    With reference to  FIG. 3   a , exemplary mask  300 A includes beam-blocking portions  310  which has a number of open or transparent slits  301 ,  302 ,  303 , etc. Beam-blocking portions  310  prevent passage of incident portions of incident beam  149  through mask  300 A. In contrast, open or transparent slits  301 ,  302 ,  303 , etc. permit passage of incident portions of radiation beam  149  through mask  300 . Accordingly, radiation beam  164  exiting mask  300  A has a cross section with a geometrical pattern corresponding to the parallel pattern of the plurality of open or transparent slits  301 ,  302 ,  303 , etc. Thus when positioned in masking system  150 , mask  300 A may be used to pattern radiation beam  164  that is incident on semiconductor thin film  175  as a collection of parallel rectangular-shaped beamlets. The beamlets irradiate a corresponding pattern of rectangular target areas in a region on the surface of the on semiconductor thin film  175 . The beamlet dimensions may be selected with a view to promote recrystallization or lateral solidification of thin film silicon areas melted by a beamlet. For example, a side length of a beamlet may be chosen so that corresponding target areas in adjoining regions are contiguous. The size of the beamlets and the inter beamlet separation distances may be selected by suitable choice of the size and separation of transparent slits  301 ,  302 ,  303 , etc. Open or transparent slits  301 ,  302 ,  303 , etc. having linear dimensions of the order of a micron or larger may, for example, generate laser radiation beamlets having dimensions that are suitable for recrystallization processing of silicon thin films in many instances. 
         [0036]      FIG. 3   b  shows another exemplary mask  300 B with a pattern which is different than that of mask  300 A. In mask  300 B, a number of open or transparent slits  351 ,  352 ,  361 ,  362 . etc. may, for example, be arranged in pairs along the sides of squares. This mask  300 B also may be used in masking system  150  to pattern the radiation beam  164  that is incident on semiconductor thin film  175 . The radiation beam  164  may be patterned, for example, as a collection of beamlets arranged in square-shaped patterns. The beamlet dimensions may be selected with a view to promote recrystallization or lateral solidification of thin film silicon areas melted by a beamlet. Open or transparent slits  351 ,  352 ,  361 ,  362 , etc. having linear dimensions of about 0.5 micron may generate laser radiation beamlets of suitable dimensions for recrystallization of thin film silicon areas 
         [0037]    It will be understood that the specific mask patterns shown in  FIGS. 3   a  and  3   b  are exemplary. Any other suitable mask patterns may be used including, for example, the chevron shaped patterns described in the &#39;625 patent. A particular mask pattern may be chosen in consideration of the desired placement of TFTs or other circuit or device elements in the semiconductor product for which the recrystallized thin film silicon material is intended. 
         [0038]      FIG. 4  shows, for example, portions of workpiece  170  that has been processed using mask  300 A of  FIG. 3   a . (Mask  300 A may be rotated by about 90 degrees from the orientation shown in  FIG. 3   a ). The portion shown corresponds to a row, for example, row  205  of workpiece  170  ( FIG. 2 ). Row  205  of processed workpiece  170  includes recrystallized polycrystalline silicon linear regions or strips  401 ,  402 , etc. Each of the linear strips is a result of irradiation by a radiation beamlet formed by a corresponding mask slit  301 ,  302 , etc. The continuous extent of recrystallized silicon in the linear strips across row  205  may he a consequence, for example, of a continuous movement of the stage  180  along path  230  under laser beam exposure ( FIG. 2 ). Strips  401 ,  402 , may have a microstructure corresponding to the one shot exposure with colliding liquid/solid growth fronts in the center creating a long location-controlled grain boundary. Alternatively, in a directional solidification process the continuous extent may be a result of closely spaced stepped movements of stage  180  along path  230  that are sufficiently overlapping to permit formation of a continuous recrystallized silicon strip, hi this alternative process, the microstructure of the recrystallized material may have long grains parallel to the scanning direction. The recrystallized polycrystalline silicon (e.g. strips  401 ,  402 , etc.) may have a generally uniform structure, which may he suitable for placement of the active region of one or more TFT devices. Similarly,  FIG. 5  shows, exemplary results using mask  300 B of  FIG. 3   b . Exemplary processed workpiece  170  includes recrystallized polycrystalline silicon strips  501 ,  502 , etc. Recrystallized polycrystalline silicon strips  501 ,  502 , etc. like strips  401  and  402  may have a uniform crystalline structure, which is suitable for placement of the active regions of TFT devices. Strips  501  and  502  that are shown to be generally at right angles to each other may correspond to radiation beamlets formed by orthogonal mask slits (e.g.,  FIG. 3   b  slits  351 ,  361 ). The distinct geometrical orientation and physical separation of strips  501  and  502  (in contrast to extended length of strips  401  and  402 ) may be a consequence, for example, of physically separated exposure to laser radiation during the processing of workpiece  170 . The separated radiation exposure may be achieved by stepped movement of stage  180  (e.g., along path  230   FIG. 2 ) during the processing. Additionally or alternatively, the separated exposure may be achieved by triggering laser  110  to generate radiation pulses at appropriate times and positions of stage  180  along path  230  while stage  180  and laser beam  164  are moved or scanned relative to each other at constant speeds. 
         [0039]    Computer  100  may be used control the triggering of laser  110  at appropriate times and positions during the movement of stage  180 . Computer  100  may act according to preprogrammed processing recipes that, for example, include geometrical design information for a workpiece-in-process.  FIG. 6  shows an exemplary design pattern  600  that may be used by computer  1000  to trigger laser  110  at appropriate times. Pattern  600  may be a geometrical grid covering thin film silicon  175 /workpiece  170 . The grid may, for example, be a rectangular x-y grid having co-ordinates (x 1 , x 2 , . . . etc.) and (y 1 , y 2 , . . . etc.). The grid spacings may be regular or irregular by design. Pattern  600  may be laid out as physical fiducial marks (e.g., on the thin film workpiece) or may be a mathematical construct in the processing recipes. Computer  100  may trigger laser  110  when stage  180  is at the grid coordinates (xi, yi). Computer  100  may do so in response, for example, to conventional position sensors or indicators, which may be deployed to sense the position of stage  180 . Alternatively, computer  100  may trigger laser  110  at computed times, which are computed from parameters such as an initial stage position, and the speeds and direction of stage movements from the initial stage position, Computer  100  also may be used advantageously to instruct laser  110  to emit radiation pulses at a variable rate, rather than at a usual even rate. The variable rate of pulse generation may be used beneficially to accommodate changes in the speed of stage  180 , for example, as it accelerates or decelerates at the ends of paths  230  and the like. 
         [0040]    It will be understood that the foregoing is only illustrative of the principles of the invention and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention, which is limited only by the claims that follow.

Technology Category: 7