Patent Publication Number: US-2019181009-A1

Title: Apparatus for annealing a layer of semiconductor material, a method of annealing a layer of semiconductor material, and a flat panel display

Description:
The invention relates to apparatus and methods for efficiently annealing a semiconductor material, for example to convert amorphous silicon to polysilicon by annealing or to convert IGZO to annealed IGZO, particularly for manufacturing the thin film transistors required in large flat panel displays (FPDs) based for example on liquid crystal (LC) or organic light-emitting diode (OLED) materials 
     To provide polysilicon for the electronics (e.g. TFTs) in each pixel of an LC display (LCD) or OLED display (or other FPD), it is known to provide a layer of amorphous silicon and use annealing to convert the amorphous silicon to polysilicon. In one process, as depicted in  FIG. 1 , a long, narrow line laser beam  4  is scanned slowly over a layer of amorphous silicon on a substrate  2  to provide a single, continuous region of polysilicon. The line laser beam may be formed using a UV (e.g. 308 nm) excimer laser or a multi-mode green DPSS laser, for example. The line laser beam may typically be up to about 750 mm in length and about 30 microns wide. The speed of the scanning and the pulse repetition rate are controlled so that all of the irradiated region receives substantially the same radiation dose and is converted reliably to polysilicon. By converting all of the amorphous silicon to polysilicon in the continuous region, polysilicon will be available in sub-regions  6  where TFTs need to be provided, for driving individual pixels (and colours within pixels) of the display. 
     Similar processing may be required for annealing alternative semiconductor materials such as indium gallium zinc oxide (IGZO) to improve their properties, for example to improve spatial uniformity of their electrical properties and/or carrier mobility. 
     As displays become larger it is becoming increasingly difficult to perform the above processing sufficiently quickly and in a cost-effective manner. It is difficult for example to increase the length of individual line laser beams and provide the required increase in laser pulse energy. 
     It is an object of the invention to provide improved methods and apparatus for providing regions of annealed semiconductor material, particularly for manufacturing large FPDs. 
     According to an aspect of the invention, there is provided an apparatus for annealing a layer of semiconductor material, comprising: a laser source configured to generate a laser beam; and a beam scanning arrangement configured to scan the laser beam, or a plurality of sub-beams generated from the laser beam, relative to the layer of semiconductor material in such a way as to selectively irradiate a plurality of regions of the layer of semiconductor material and thereby generate a corresponding plurality of regions of annealed semiconductor material by annealing, wherein each of the regions of annealed semiconductor material is separated from all of the other regions of annealed semiconductor material. 
     The semiconductor material to be annealed may comprise amorphous silicon or IGZO for example. The annealed semiconductor material may comprise polysilicon or an annealed form of IGZO (e.g. a form of IGZO in which electrical properties have been made more uniform by annealing and/or in which carrier mobility has been improved by annealing). 
     In an embodiment, there is provided an apparatus for annealing a layer of amorphous silicon, comprising: a laser source configured to generate a laser beam; and a beam scanning arrangement configured to scan the laser beam, or a plurality of sub-beams generated from the laser beam, relative to the layer of amorphous silicon in such a way as to selectively irradiate a plurality of regions of the layer of amorphous silicon and thereby generate a corresponding plurality of regions of polysilicon by annealing, wherein each of the regions of polysilicon is separated from all of the other regions of polysilicon. 
     By providing an apparatus capable of selectively irradiating a plurality of separated regions, it is possible to perform the annealing of the semiconductor material (e.g. amorphous silicon or IGZO) using a much lower total energy. The proportion of the original layer of semiconductor material can be much closer to the proportion that is actually needed to support the electronic devices (e.g. TFTs) to be fabricated. For example, in the case of an LCD or OLED display, the proportion of the total area of the display in which TFTs may need to be formed is typically of the order of 3% of the total area. If a line laser beam were used to provide the polysilicon, as in the prior art, substantially 100% of the total area would be annealed. The selective irradiation of the invention would typically require irradiation of a proportion much nearer to the 3%, typically in the region of about 10% (to provide a safety margin around each of the TFT regions). This approach reduces power requirements, increases processing speed and reduces processing cost 
     In an embodiment, the laser beam is split into a plurality of sub-beams. The plurality of sub-beams are scanned over the layer of semiconductor material (e.g. amorphous silicon or IGZO). This approach has been found to provide a particularly efficient way of providing the selective irradiation. The technique can be implemented at low cost and provides the basis for rapidly processing large areas of semiconductor material. Multiple lasers and corresponding beam splitters can be used to process particularly large areas or multiple areas in parallel. 
     In an embodiment, the laser beam is a pulsed laser beam and the beam scanning arrangement is configured so that each sub-beam of the plurality of sub-beams is scanned relative to the layer of semiconductor material in such a way that successive pulses of the sub-beam irradiate different respective ones of the plurality of regions of the layer of semiconductor material to be irradiated. This approach provides a degree of flexibility in how radiation dose is applied to each region that is not available in the prior art. For example, in prior art arrangements using a line laser beam, the intensity profile within the line laser beam parallel to the direction of scanning of the line laser beam will generally be Gaussian. This means that each region being irradiated by the line laser beam will receive pulses that increase and then decrease in intensity and no other arrangement will be easily possible. Varying the pulse intensity in this manner will not be optimal for annealing the semiconductor material, further increasing the total amount of radiation that needs to be applied using the prior art approach relative to the invention. 
     In one particular embodiment, the energy per pulse received by each of the plurality of regions is substantially the same for each pulse. In an alternative embodiment, the energy per pulse received by each of the plurality of regions increases progressively for each pulse received by the region. The efficiency of the annealing process is thereby improved further relative to the Gaussian variation provided by prior art arrangements. 
     According to an alternative aspect, there is provided a method of annealing a layer of semiconductor material, comprising: generating a laser beam; and scanning the laser beam, or a plurality of sub-beams generated from the laser beam, over the layer of semiconductor material in such a way as to selectively irradiate a plurality of regions of the layer of semiconductor material and thereby generate a corresponding plurality of regions of annealed semiconductor material, wherein each of the regions of annealed semiconductor material is separated from all of the other regions of annealed semiconductor material. 
     According to an embodiment, there is provided a method of annealing a layer of amorphous silicon, comprising: generating a laser beam; and scanning the laser beam, or a plurality of sub-beams generated from the laser beam, over the layer of amorphous silicon in such a way as to selectively irradiate a plurality of regions of the layer of amorphous silicon and thereby generate a corresponding plurality of regions of polysilicon, wherein each of the regions of polysilicon is separated from all of the other regions of polysilicon. 
     The method may be used as part of a method of manufacturing a flat panel display, particularly an LCD or OLED display. 
    
    
     
       The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  depicts scanning of a line laser beam over a layer of semiconductor material to anneal the semiconductor material; 
         FIG. 2  depicts an apparatus for annealing a layer of semiconductor material comprising a beam scanner; 
         FIG. 3  depicts an alternative apparatus for annealing a layer of semiconductor material without a beam scanner; 
         FIG. 4  depicts an individual irradiated region relative to a TFT region; 
         FIG. 5  depicts an intensity profile along line X-X′ in the irradiated region of  FIG. 4 ; 
         FIG. 6  depicts an intensity profile along line Y-Y′ in the irradiated region of  FIG. 4 ; 
         FIG. 7  depicts scanning of a plurality of sub-beams over a layer of semiconductor material to selectively irradiate a plurality of regions of the semiconductor material; 
         FIG. 8  depicts a bow-tie type scanning pattern; 
         FIG. 9  depicts a first embodiment of raster scanning of a plurality of sub-beams over a layer of semiconductor material; 
         FIG. 10  depicts a second embodiment of raster scanning of a plurality of sub-beams over a layer of semiconductor material; 
         FIG. 11  is a bar chart showing an example variation of energy density received at a region as a function of time (corresponding to an intensity profile across a plurality of sub-beams); 
         FIG. 12  is a bar chart showing a further example variation of energy density received at a region as a function of time (corresponding to an intensity profile across a plurality of sub-beams); 
         FIG. 13  is a bar chart showing a further example variation of energy density received at a region as a function of time (corresponding to an intensity profile across a plurality of sub-beams); and 
         FIG. 14  depicts a gantry comprising multiple laser systems for processing plural substrates in parallel. 
     
    
    
     As mentioned in the introductory part of the description, as displays get larger it is becoming increasingly difficult efficiently to provide polysilicon (or other annealed semiconductor material) for the TFTs for each pixel. Consider for example typical requirements for a 70 inch  8 K resolution display. Such a display will have overall dimensions of 1550×872 mm. 7680 pixels would be required along the length. 4320 pixels would be required along the width. Each pixel would have a width of about 67 microns and a height of about 202 microns. The number of TFT units for such a display would be 23040 along the length (one TFT unit being required for each of the three colours) and  4320  along the width. Nearly 100 million TFT units are therefore required. 
     In the prior art substantially all of the 1550×872 mm display area would need to be subjected to annealing radiation to provide the annealed semiconductor material (e.g. polysilicon or annealed IGZO). The embodiments described below greatly reduce the total amount of annealing that is carried out while still providing all of the annealed semiconductor material (e.g. polysilicon or annealed IGZO) required for the nearly 100 millions TFTs. 
     In an embodiment, examples of which are depicted in  FIGS. 2 and 3 , there is provided an apparatus  1  for annealing a layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). The layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) may be conveyed by a layer transport device  42 . The layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) may be supported on a substrate  40 . The substrate  40  may in turn be supported (and conveyed) by the layer transport device  42 . The layer transport device  42  may comprise a movable table supporting and/or gripping the substrate  40 . 
     The apparatus  1  comprises a laser source  30  that generates a laser beam  31 . The laser source  30  may be a pulsed laser source  30 . Any laser source that is capable of annealing the semiconductor material (e.g. amorphous silicon or IGZO) can be used. Details of the laser source may vary according to the particular characteristics of the semiconductor material to be annealed. In an embodiment, the laser source  30  is a low M 2  high repetition rate DPSS laser. In an embodiment, the laser source  30  is a UV laser source generating pulses of radiation at about 355 nm (particularly suitable for annealing amorphous silicon). In an alternative embodiment, the laser source  30  is a green laser source generating pulses of radiation at about 532 nm (also suitable for annealing amorphous silicon). In an alternative embodiment, the laser source  30  is a DUV laser source generating pulses at about 266 nm (particularly suitable for annealing IGZO). The laser source  30  may comprise a multi-mode high power laser, optionally a high M 2  low repetition rate DPSS laser. This latter embodiment may be particularly applicable where a two-dimensional array of beam spots are generated, due to the higher power requirements. An example of such an arrangement is described below with reference to  FIG. 10 . The laser source  30  may comprise a Q switched laser source. In an embodiment, the laser source  30  is configured to provide pulses having pulse lengths of 200 ns or less, optionally 150 ns or less, optionally 100 ns or less. 
     In the embodiments shown in  FIGS. 2 and 3 , an optical element  32  (e.g. a diffractive optical element, DOE) generates a plurality of sub-beams  33  by splitting the laser beam  31 . 
     A beam scanning arrangement is provided that scans the laser beam  31 , or a plurality of sub-beams  33  generated from the laser beam  31  (as in the embodiments of  FIGS. 2 and 3 ), relative to (over) the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) to be annealed. The scanning is performed in such a way as to selectively irradiate a plurality of regions of the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). A corresponding plurality of regions of annealed semiconductor material (e.g. polysilicon or annealed IGZO) are produced by the irradiation. Each region of annealed semiconductor material is separated from every other region of annealed semiconductor material. 
     In one embodiment, the semiconductor material comprises, consists essentially of, or consists of, amorphous silicon and the irradiation is such as to anneal the amorphous silicon to form polysilicon. 
     In an alternative embodiment, the semiconductor material comprises, consists essentially of, or consist of, IGZO and the irradiation is such as to anneal the IGZO to form annealed IGZO. In an embodiment, the annealed IGZO has significantly different electrical properties than the IGZO prior to the annealing, including for example higher spatial uniformity of electrical properties and/or increased carrier mobility. 
     In an embodiment, an example of which is depicted in  FIG. 2 , the beam scanning arrangement comprises a beam scanner  34 . The beam scanner  34  provides movement relative to the laser source  30  of one or more beam spots  9  generated by the laser beam  31  or by the plurality of sub-beams  33 , thereby at least partially performing the scanning of the laser beam  31  or plurality of sub-beams  33  relative to the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). The controlled movement of the one or more beam spots  9  may be achieved for example by controlled deflection or steering of the laser beam  31  or sub-beams  33 , for example using moving mirrors, scanning refractive optics, acousto-optic deflectors, or electro-optic deflectors, or any other technique known in the art of beam scanners. The beam scanner  34  may further comprise optics (e.g. f-theta lens) to focus the laser beam  31  or sub-beams  33  onto the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). 
     The beam scanning arrangement may additionally or alternatively comprise a layer transport device  42  that moves the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO), and thereby at least partially performs the scanning of the laser beam  31  or plurality of sub-beams  33  relative to the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). 
     The beam scanning arrangement may additionally or alternatively comprise an optics transport device  50 , as shown for example in  FIG. 3 . The optics transport device  50  moves either or both of the laser source  30  and optics (or a portion of optics) for directing the laser beam  30  or plurality of sub-beams  33  onto the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO), and thereby at least partially performs the scanning of the laser beam  31  or plurality of sub-beams  33  relative to the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). In the particular example of  FIG. 3 , the optics moved by the optics transport device  50  includes laser source  30 , a beam shaping optical element  32 ′ (see below), a beam splitting optical element  32 , and optics  52  (e.g. f-theta lens) to focus sub-beams  33  onto the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). 
     As depicted schematically in  FIG. 4 , in an embodiment each of the plurality of regions  8  of annealed semiconductor material (e.g. polysilicon or annealed IGZO) contains a region  6  in which a single electronic unit (e.g. TFT device) needed for a pixel of a display device (e.g. LCD or OLED display) will be provided. In an embodiment, the laser beam  31  or each sub-beam  33  is shaped by an optical element  32 ′ (see  FIGS. 2 and 3 ) such as a diffractive optical element (DOE) to form a substantially rectangular spot  9  on the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). In an embodiment, each spot  9  is substantially the same size and shape as each of the plurality of regions  8 . In an embodiment, each laser beam pulse has a substantially top-hat cross-sectional intensity profile. Thus, for the region  8  of  FIG. 4 , the intensity profile along line X-X′ would be as shown in  FIG. 5 . The intensity profile along line Y-Y′ would be as shown in  FIG. 6 . In an embodiment, the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) is positioned at the far field of a focussing lens. It is not necessary to form an accurate image at the layer  2  of semiconductor material because high spatial accuracy is not required. Regions of the semiconductor material (whether annealed or not) that are not needed to form part of the final manufactured device can be removed accurately using later processing techniques, such as optical lithography. 
     In contrast to prior art methods which convert substantially 100% of the amorphous silicon to polysilicon, at least in a region corresponding to a display region of a display to be manufactured, embodiments disclosed herein are configured to convert less than 20% of the layer of semiconductor material (e.g. amorphous silicon or IGZO) to annealed semiconductor material (e.g. polysilicon or annealed IGZO), optionally less than 10%, optionally less than 8%, optionally less than 6%, optionally less than 4%. 
     In an embodiment, each region  8  is slightly larger than the minimum size of the region  6  needed to create the electronic unit for each pixel (e.g. TFT device). For example, each region  8  may have a surface area equal to between 110% and 2000% of the surface area of the region  6  that it contains, optionally between 150% and 1000%, optionally between 200% and 800%, optionally between 300% and 600%. In one particular embodiment, for a region  6  for a TFT of 10×35 microns, regions  8  of 30×55 microns are provided. 
     In embodiments in which the laser beam  31  is split into a plurality of sub-beams  33 , each sub-beam  33  may produce an individual spot  9  with each pulse of the laser beam  31 . Each of the sub-beams  33  is focussed onto the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). Providing a plurality of sub-beams  33  makes it possible simultaneously to irradiate a plurality of regions  8  using a corresponding plurality of spots  9 . The beam scanning arrangement (e.g. beam scanner  34 ) scans the sub-beams  33  over the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). In an embodiment the laser beam  31  is a pulsed laser beam and the scanning arrangement (e.g. beam scanner  34 ) is configured so that each sub-beam  33  is scanned relative to (over) the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) in such a way that successive pulses of the sub-beam  33  irradiate different respective ones of the plurality of regions  8  of the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) to be irradiated. 
       FIG. 7  depicts example trajectories  10  of a line of spots  9  across a portion of a layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) (in the reference frame of the layer  2  of semiconductor material). The speed of scanning along the trajectories  10  and the pulse rate of the laser beam  31  are configured such that each sub-beam  33  generates a spot  9  of radiation at each point along the trajectory  10  corresponding to one of the regions  6  in which a TFT is to be formed, one spot being formed for each successive pulse of the laser beam  31 . At a subsequent time, a different one of the sub-beams  33  follows the same trajectory  10  and provides a further spot  9  of radiation at each of the same points. The process is repeated until a plurality of regions  8 , each containing one of the regions  6 , is fully annealed, for example to form polysilicon or annealed IGZO. Thus, each of the plurality of regions  8  receives one pulse of radiation from each of two or more (different ones) of the sub-beams  33 . In an embodiment, each of the plurality of regions  8  receives a single pulse (i.e. one and only one pulse) of radiation from each and every one of the sub-beams  33 . 
     In an embodiment, the plurality of regions  8  to be irradiated comprises one or more sets of regions  8  (each containing a region  6 ) that are spaced apart from each other along a first direction with a first pitch  12 . In the example of  FIG. 7 , the first direction is the vertical direction within the page, and each set of regions  8  comprises a vertically aligned column of regions  8 . A plurality of the sets of regions  8  (columns) are provided, each set of regions  8  being aligned with a corresponding set of the regions  6  (so that each region  8  contains one of the regions  6 ). The plurality of sub-beams  33  comprises at least one set of sub-beams  33  that are spaced apart from each other in the first direction with the same first pitch  12  at the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO), thereby producing a corresponding set of spots  9  that are spaced apart from each other in the first direction with the same first pitch  12  (as shown in  FIG. 7 ). This enables multiple sub-beams  33  to simultaneously irradiate multiple corresponding regions  8  (each region  8  lying on a different one of the horizontal trajectories  10 ). The plurality of sub-beams  33  in each set of sub-beams are aligned with each other along the first direction. 
     In the example of  FIG. 7  the plurality of sub-beams  33  comprises only one of the abovementioned sets of sub-beams  33  (aligned along the first direction). In other embodiments further such sets of sub-beams  33  may be provided that are separated from each other in a perpendicular direction to form a two-dimensional array of sub-beams  33 . An example is discussed below with reference to  FIG. 10 . In an embodiment each of the plurality of regions  8  receives a single pulse of radiation from each of the sub-beams  33  in at least one of the abovementioned sets of sub-beams  33 . 
     In an embodiment, the beam scanning arrangement moves the layer of semiconductor material (e.g. amorphous silicon or IGZO) in the first direction during the scanning of the sub-beams  33  relative to the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO), for example along the trajectories  10  of  FIG. 7 . In an embodiment, the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) is moved relative to a beam scanner  34  along the first direction and the beam scanner  34  scans the sub-beams  33  (and therefore spots  9 ) in a direction that is oblique relative to the first direction in order to compensate for the movement of the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). In  FIG. 7  the trajectories  10  are shown in the reference frame of the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). In the reference frame of the beam scanner  34  each trajectory  10  would move diagonally (i.e. at an oblique angle relative to the vertical) upwards so as to follow the upwards motion of each of the regions  6  and position the spot  9  over a respective region  6  each time the laser beam  31  pulses. 
     In an embodiment, each region  8  receives a single pulse (i.e. one and only one pulse) of radiation from each and every one of the sub-beams  33  of radiation in at least one of the abovementioned sets of sub-beams (i.e. from each and every one of the sub-beams  33  when only one of the sets of sub-beams  33  is provided). Thus, where each region  8  needs to receive N pulses of radiation, N sub-beams  33  will be provided in each set of sub-beams  33 . In an embodiment, N=20, but other values of N may be used. 
     A bow-tie type scanning arrangement, an example of which is depicted in  FIG. 8 , may be used to efficiently move the set of sub-beams  33  across the surface of the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). For example, in a scan involving movement of each sub-beam  33  (and associated spot  9 ) along the trajectory from point  21  to point  22 , a set of N sub-beams  33  is scanned along N lines of regions  8  (each region  8  containing one of the TFT regions  6 ). At point  22  each sub-beam  33  (and associated spot  9 ) is moved down to point  23 , which corresponds to a distance equivalent to the first pitch  12 , and is then scanned along the trajectory from point  23  to point  24  to irradiate another N lines of regions  8  (overlapping with the previous N lines of regions). Each sub-beam  33  (and associated spot) is then moved back to point  21 , which corresponds again to a distance equivalent the first pitch  12 , ready for scanning a further N lines of regions  8 . The process continues in this embodiment until all of the regions  8  on the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) have been irradiated by N successive laser pulses to form the annealed semiconductor material (e.g. polysilicon or annealed IGZO) in each of the regions  8 . 
     In the scanning process described above with reference to  FIGS. 7 and 8 , the beam scanning arrangement provides a raster scan in the reference frame of the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) of a beam spot  9  from each of a set of sub-beams  33  aligned along the first direction over all of the plurality of regions  8  to be irradiated. Thus, each and every one of the set of sub-beams  33  is scanned over each and every one of the regions  8  to be irradiated. The scanning path  46  is illustrated schematically (in the reference frame of the layer  2  of semiconductor material to be annealed) in  FIG. 9 . The set of sub-beams  33  aligned along the first direction produces a corresponding set  44  of beams spots  9 . The first direction  48  is vertically upwards in the plane of the page. The long axis of the raster scan is perpendicular to the first direction  48  (horizontal in the plane of the page). 
     In an embodiment, the plurality of sub-beams  33  comprises a plurality of the sets of sub-beams  33  aligned along the first direction (producing a corresponding plurality of sets  44  of beam spots  9 ). Each of the sets  44  is separated from each other set  44  in a direction perpendicular to the first direction by a second pitch. A two-dimensional array of sub-beams  33  is thereby formed, defined by the first pitch and the second pitch. The two-dimensional array of sub-beams  33  produces a corresponding two-dimensional array of beam spots  9  (illustrated schematically in the upper left portion of  FIG. 10 ). In an embodiment each set comprises N sub-beams  33  as described above (but other values of N may be used). The number M of sets is not particularly limited. Optionally M is larger than N, optionally larger than 20, optionally larger than 30, optionally larger than 40. 
       FIG. 10  depicts an example scanning path  46  for an embodiment comprising an M×N array of sub-beams producing an M×N array of beam spots  9 . The scanning path comprises a raster scan in the reference frame of the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) of the array of sub-beams  33  (and beam spots  9 ) over the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). In embodiments of this type a long axis of the raster scan may be parallel to the first direction  48  (vertical in the example of  FIG. 10 ). Embodiments of this type may be implemented by a beam scanning arrangement which does not use a beam scanner  34 . In other words, the scanning is achieved without using deflection or steering of the laser beam to provide the scanning. Instead, the scanning is provided by moving either or both of 1) the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO), and 2) the laser source  30  and optics (or a portion of optics) for directing the laser beam  30  or plurality of sub-beams  33  onto the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). In the example shown in  FIG. 10 , for example, the scanning may be implemented by using a layer transport device to move the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) along each of the vertical portions of the scanning path  46  while holding the sub-beams  33  stationary (by holding the laser source  30  and/or associated optics stationary). An optics transport device may then be used to step the laser source and/or associated optics in the horizontal direction to move the sub-beams  33  and thereby provide each of the horizontal portions of the scanning path  46 . Alternatively all of the scanning path  46  could be provided solely by movement of the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) (i.e. in a two-dimensional scan) or all of the scanning path  46  could be provided solely by movement of the laser source  30  and/or associated optics. 
     In an embodiment, all of the sub-beams  33  have the same intensity and the energy per pulse delivered to each sub-region  8  is therefore constant (each pulse delivers the same energy to the region  8 ). This is illustrated schematically by the bar chart in  FIG. 11  showing the variation of energy density received at a region  8  as a function of time (in the case where each region receives a pulse from 25 different sub-beams  33 ). 
       FIG. 12  depicts an alternative embodiment in which the sub-beams  33  have progressively increasing intensities, such that the energy per pulse delivered to each sub-region  8  progressively increases as a function of time (each pulse delivers a higher energy per pulse than the preceding pulse). The intensity of each sub-beam  33  remains constant during the scanning. The progressive increase in energy per pulse received by each region  8  is provided by the differences in intensity between different sub-beams  33 , which can in turn be controlled by suitable design of the diffractive optical element. An example in which the energy per pulse progressively (monotonically) increases is illustrated by the bar chart in  FIG. 12 . Other arrangements are possible. Any variation which encourages efficient (e.g. using a low total amount of laser energy) and/or high quality (e.g. providing a quality of polysilicon that is particularly well adapted for forming reliable and long-lived electronic devices and/or which achieves high uniformity across the different regions  8 ) can be envisaged. 
     A progressively increasing energy density arrangement such as that shown in  FIG. 12  is desirable compared to a constant arrangement such as shown in  FIG. 11  since it leads to a more gradual annealing and, where applicable, crystallization of the semiconductor material (e.g. amorphous silicon or IGZO) and hence a reduction in the likelihood of film disruption. 
       FIG. 13  depicts an example in which the variation in energy pulse is configured to imitate the variation that is intrinsic to prior art approaches using scanning of a line laser beam, i.e. an approximate Gaussian variation. This approach allows the method to produce annealed semiconductor material (e.g. polysilicon or annealed IGZO) of a quality corresponding to prior art approaches 
     A progressively increasing energy density arrangement such as that shown in  FIG. 12  is also desirable compared to a rising and falling arrangement such as shown in  FIG. 13  since all of the successively increasing energy density pulses contribute fully to the progressive annealing and, where applicable, crystallization of the semiconductor material (e.g. amorphous silicon or IGZO) whereas pulses with reducing energy density as occur after the peak in  FIG. 13  make significantly less contribution to the annealing and, where applicable, crystallization process. 
     In the arrangements discussed above, each of the regions  8  receives plural pulses of radiation (e.g. one from each of the sub-beams  33  provided). In an alternative embodiment the apparatus  1  is configured such that each of the plurality of regions  8  receives a single pulse of radiation from the radiation beam. The single pulse of radiation converts the semiconductor material (e.g. amorphous silicon or IGZO) to annealed semiconductor material (e.g. polysilicon or annealed IGZO) without any further pulses being required. Optionally, an optical element  32  is provided to split the laser beam into a plurality of sub-beams. In this case the scanning of the laser beam comprises scanning of the sub-beams and the single pulse of radiation received by each of the plurality of regions  8  is received from one of the sub-beams. Providing plural sub-beams may speed up processing of the layer  2  of semiconductor material in comparison to where only one radiation beam spot can be incident on the layer  2  at any one time. 
       FIG. 14  depicts schematically how the apparatus  1  can be scaled up to process larger layers  2  of semiconductor material (e.g. amorphous silicon or IGZO), for example for larger displays, or multiple laterally adjacent layers  2  of semiconductor material (e.g. for multiple displays), as shown in the  FIG. 14 . In the example configuration shown, the apparatus  1  comprises a gantry comprising a plurality of laser sources  30  (ten in the particular example shown). Each source  30  provides radiation simultaneously to two optical systems  36  (such that 20 optical systems  36  are provided). Each optical system  36  comprises an optical element  32  configured to split a laser beam  31  into a plurality of sub-beams  33 , an optical element  32 ′ to shape the sub-beams  33 , and a corresponding beam scanner  34  (including focussing optics such as an f-theta lens). The beam scanner  34  scans the sub-beams  33  over a layer  2  of semiconductor material (e.g. amorphous silicon or IGZO). In the configuration shown the layers  2  of semiconductor material (e.g. amorphous silicon or IGZO) will be moved vertically downwards (as depicted in the page) underneath the gantry while the sub-beams  33  are scanned substantially left and right (e.g. in a bow-tie type pattern as described above). 
     In an embodiment, further steps of a method of manufacturing a display are performed after processing the layer  2  of semiconductor material (e.g. amorphous silicon or IGZO) to produce the regions  8  of polysilicon. In an embodiment, an electronic device such as a TFT for driving a pixel of a display, is formed in each of the regions  8 . In an embodiment a flat panel display such as an LCD or OLED display is manufactured that includes the electronic devices. 
     Embodiments of the disclosure are also described by the following numbered clauses. 
     1. An apparatus for annealing a layer of amorphous silicon, comprising: 
     a laser source configured to generate a laser beam; and 
     a beam scanner configured to scan the laser beam in such a way as to selectively irradiate a plurality of regions of the layer of amorphous silicon and thereby generate a corresponding plurality of regions of polysilicon by annealing, wherein each of the regions of polysilicon is separated from all of the other regions of polysilicon. 
     2. The apparatus of clause 1, further comprising an optical element configured to split the laser beam into a plurality of sub-beams, wherein the scanning of the laser beam comprises scanning of the sub-beams.
 
3. The apparatus of clause 2, wherein the laser beam is a pulsed laser beam and the beam scanner is configured so that each sub-beam is scanned over the layer of amorphous silicon in such a way that successive pulses of the sub-beam irradiate different respective ones of the plurality of regions of the layer of amorphous silicon to be irradiated.
 
4. The apparatus of clause 2 or 3, wherein the plurality of regions to be irradiated are spaced apart from each other with a pitch and the sub-beams generated by the optical element are spaced apart from each other with the same pitch.
 
5. The apparatus of any of clauses 2-4, configured to move the layer of amorphous silicon relative to the beam scanner during the irradiation of the plurality of regions.
 
6. The apparatus of clause 5, wherein:
 
     the layer of amorphous silicon is moved relative to the beam scanner along a first direction; and 
     the sub-beams generated by the optical element are aligned parallel to the first direction and the beam scanner is configured to scan the sub-beams in a direction that is oblique relative to the first direction in order to compensate for the movement of the layer of amorphous silicon. 
     7. The apparatus of any of clauses 2-6, configured such that each of the plurality of regions receives one pulse of radiation from each of at least two of the sub-beams.
 
8. The apparatus of clause 7, configured such that each of the plurality of regions receives a single pulse of radiation from each of the sub-beams.
 
9. The apparatus of any of clauses 2-8, wherein the laser source is a pulsed laser source and the apparatus is configured such that the energy per pulse received by each of the plurality of regions is substantially the same for each pulse.
 
10. The apparatus of any of clauses 2-8, wherein the laser source is a pulsed laser source and the apparatus is configured such that the energy per pulse received by each of the plurality of regions is substantially different for at least two of the pulses received by the region.
 
11. The apparatus of clause 10, wherein the energy per pulse received by each of the plurality of regions increases progressively for each pulse received by the region.
 
12. The apparatus of any of clauses 2-11, wherein each sub-beam of radiation has a substantially top-hat cross-sectional intensity profile.
 
13. The apparatus of any preceding clause, configured to convert less than 20% of the layer of amorphous silicon to polysilicon.
 
14. The apparatus of any preceding clause, configured such that each of the plurality of regions receives a single pulse of radiation from the laser beam.
 
15. The apparatus of clause 14, further comprising an optical element configured to split the laser beam into a plurality of sub-beams, wherein the scanning of the laser beam comprises scanning of the sub-beams, and the single pulse of radiation received by each of the plurality of regions is received from one of the sub-beams.
 
16. A method of annealing a layer of amorphous silicon, comprising:
 
     generating a laser beam; and 
     scanning the laser beam over the layer of amorphous silicon in such a way as to selectively irradiate a plurality of regions of the layer of amorphous silicon and thereby generate a corresponding plurality of regions of polysilicon, wherein each of the regions of polysilicon is separated from all of the other regions of polysilicon. 
     17. The method of clause 16, wherein the selective irradiation is performed by splitting the laser beam into a plurality of sub-beams and scanning the sub-beams over the layer of amorphous silicon.
 
18. The method of clause 17, wherein the laser beam is a pulsed laser beam and each sub-beam is scanned over the layer of amorphous silicon in such a way that successive pulses of the sub-beam irradiate different respective ones of the plurality of regions of the layer of amorphous silicon to be irradiated.
 
19. The method of clause 17 or 18, wherein the sub-beams are spaced apart from each other with the same pitch as the plurality of regions to be irradiated.
 
20. The method of any of clauses 17-19, wherein the layer of amorphous silicon is moved during the irradiation of the plurality of regions.
 
21. The method of clause 20, wherein:
 
     the layer of amorphous silicon is moved along a first direction during the irradiation of the plurality of regions; and 
     the sub-beams are aligned parallel to the first direction and scanned in a direction that is oblique relative to the first direction in order to compensate for the movement of the layer of amorphous silicon. 
     22. The method of any of clauses 17-21, wherein each of the plurality of regions receives one pulse of radiation from each of at least two of the sub-beams.
 
23. The method of claim  22 , wherein each of the plurality of regions receives a single pulse of radiation from each of the sub-beams.
 
24. The method of any of clauses 17-23, wherein each sub-beam of radiation has a substantially top-hat cross-sectional intensity profile.
 
25. The method of any of clauses 16-22, wherein the laser beam is pulsed and the energy per pulse received by each of the plurality of regions is substantially the same for each pulse.
 
26. The method of any of clauses 16-24, wherein the laser beam is pulsed and the energy per pulse received by each of the plurality of regions is substantially different for at least two of the pulses received by the region.
 
27. The method of clause 26, wherein the energy per pulse received by each of the plurality of regions increases progressively for each pulse received by the region.
 
28. The method of any of clauses 16-27, wherein less than 20% of the layer of amorphous silicon is converted to polysilicon.
 
29. The method of any of clauses claims  16 - 28 , wherein each of the plurality of regions receives a single pulse of radiation from the laser beam.
 
30. The apparatus of clause 29, further comprising an optical element configured to split the laser beam into a plurality of sub-beams, wherein the scanning of the laser beam comprises scanning of the sub-beams, and the single pulse of radiation received by each of the plurality of regions is received from one of the sub-beams.
 
31. The method of any of clauses 16-30, further comprising manufacturing an electronic device in each of the regions of polysilicon.
 
32. The method of clause 31, wherein each region of polysilicon has a surface area at least 10% larger than the surface area of the region occupied by the electronic device in each region.
 
33. The method of clause 32, wherein each electronic device comprises a thin film transistor.
 
34. The method of any of clauses 16-33, further comprising manufacturing a flat panel display using the regions of polysilicon.
 
35. A flat panel display manufactured using the method of any of clauses 16-34.