Abstract:
In some embodiments, a method of processing a film is provided, the method comprising defining a plurality of spaced-apart regions to be pre-crystallized within the film, the film being disposed on a substrate and capable of laser-induced melting; generating a laser beam having a fluence that is selected to form a mixture of solid and liquid in the film and where a fraction of the film is molten throughout its thickness in an irradiated region; positioning the film relative to the laser beam in preparation for at least partially pre-crystallizing a first region of said plurality of spaced-apart regions; directing the laser beam onto a moving at least partially reflective optical element in the path of the laser beam, the moving optical element redirecting the beam so as to scan a first portion of the first region with the beam in a first direction at a first velocity, wherein the first velocity is selected such that the beam irradiates and forms the mixture of solid and liquid in the first portion of the first region, wherein said first portion of the first region upon cooling forms crystalline grains having predominantly the same crystallographic orientation in at least a single direction; and crystallizing at least the first portion of the first region using laser-induced melting.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. § 119(e) of the following application, the entire contents of which are incorporated herein by reference:
       U.S. Provisional Patent Application Ser. No. 60/742,276, filed Dec. 5, 2005 and entitled “Scheme for Crystallizing Films Using a Continuous-Wave Light Source Compatible With Glass Substrates And Existing Precision Stages.”       
 
     
    
     FIELD 
       [0003]    Systems and methods for processing a film, and thin films, are provided. 
       BACKGROUND 
       [0004]    In recent years, various techniques for crystallizing or improving the crystallinity of an amorphous or polycrystalline semiconductor film have been investigated. Such crystallized thin films may be used in the manufacture of a variety of devices, such as image sensors and active-matrix liquid-crystal display (“AMLCD”) devices. In the latter, a regular array of thin-film transistors (“TFTs”) is fabricated on an appropriate transparent substrate, and each transistor serves as a pixel controller. 
         [0005]    Crystalline semiconductor films, such as silicon films, have been processed to provide pixels for liquid crystal displays using various laser processes including excimer laser annealing (“ELA”) and sequential lateral solidification (“SLS”) processes. SLS is well suited to process thin films for use in AMLCD devices, as well as organic light emitting diode (“OLED”) devices. 
         [0006]    In ELA, a region of the film is irradiated by an excimer laser to partially melt the film, which subsequently crystallizes. The process typically uses a long, narrow beam shape that is continuously advanced over the substrate surface, so that the beam can potentially irradiate the entire semiconductor thin film in a single scan across the surface. The Si film is irradiated multiple times to create the random polycrystalline film with a uniform grain size. ELA produces small-grained polycrystalline films; however, the method often suffers from microstructural non-uniformities, which can be caused by pulse-to-pulse energy density fluctuations and/or non-uniform beam intensity profiles.  FIG. 6A  illustrates a random microstructure that may be obtained with ELA. This figure, and all subsequent figures, are not drawn to scale, and are intended to be illustrative in nature. 
         [0007]    SLS is a pulsed-laser crystallization process that can produce high quality polycrystalline films having large and uniform grains on substrates, including substrates that are intolerant to heat such as glass and plastics. SLS uses controlled laser pulses to fully melt a region of an amorphous or polycrystalline thin film on a substrate. The melted regions of film then laterally crystallize into a solidified lateral columnar microstructure or a plurality of location-controlled large single crystal regions. Generally, the melt/crystallization process is sequentially repeated over the surface of a large thin film, with a large number of laser pulses. The processed film on substrate is then used to produce one large display, or even divided to produce multiple displays, each display being useful for providing visual output in a given device.  FIGS. 6B-6D  shows schematic drawings of TFTs fabricated within films having different microstructures that can be obtained with SLS. SLS processes are described in greater detail below. 
         [0008]    The potential success of SLS systems and methods for commercial use is related to the throughput with which the desired microstructure and texture can be produced. The amount of energy and time it takes to produce a film having the microstructure is also related to the cost of producing that film; in general, the faster and more efficiently the film can be produced, the more films can be produced in a given period of time, enabling higher production and thus higher potential revenues. 
       SUMMARY 
       [0009]    The application describes systems and methods for processing thin films, and thin films. 
         [0010]    In some embodiments, a method of processing a film is provided, the method comprising defining a plurality of spaced-apart regions to be pre-crystallized within the film, the film being disposed on a substrate and capable of laser-induced melting; generating a laser beam having a fluence that is selected to form a mixture of solid and liquid in the film and where a fraction of the film is molten throughout its thickness in an irradiated region; positioning the film relative to the laser beam in preparation for at least partially pre-crystallizing a first region of said plurality of spaced-apart regions; directing the laser beam onto a moving at least partially reflective optical element in the path of the laser beam, the moving optical element redirecting the beam so as to scan a first portion of the first region with the beam in a first direction at a first velocity, wherein the first velocity is selected such that the beam irradiates and forms the mixture of solid and liquid in the first portion of the first region, wherein said first portion of the first region upon cooling forms crystalline grains having predominantly the same crystallographic orientation in at least a single direction; and crystallizing at least the first portion of the first region using laser-induced melting. 
         [0011]    Some embodiments include one or more of the following features. The laser beam is continuous-wave. Further comprising re-positioning the film relative to the laser beam in preparation for at least partially pre-crystallizing a second region of the plurality of spaced-apart regions; and moving the optical element so as to scan a first portion of the second region with the laser beam in the first direction at the first velocity, wherein the first portion of the second region upon cooling forms crystalline grains having predominantly the same crystallographic orientation in said at least a single direction. Said first velocity is further selected such that heat generated by the beam substantially does not damage the substrate. The moving optical element comprises a rotating disk that comprising a plurality of facets that reflect said laser beam onto the film. The first velocity is at least about 0.5 m/s. The first velocity is at least about 1 m/s. 
         [0012]    The method of claim  1 , further comprising, after redirecting the beam with the moving optical element so as to scan the first portion of the first region, translating the film relative to the laser beam in a second direction so as to scan a second portion of the first region with the laser beam in the first direction at the first velocity, wherein the second portion of the first region upon cooling forms crystalline grains having predominantly the same crystallographic orientation in said at least a single direction. The second portion of the first region partially overlaps the first portion of the first region. Continuously translating the film in the second direction with a second velocity selected to provide a pre-determined amount of overlap between the first and second portions of the first region. Continuously translating the film in the second direction with a second velocity for a period of time selected to sequentially irradiate and a plurality of portions of the first region, wherein each of said plurality of portions upon cooling forms crystalline grains having predominantly the same crystallographic orientation in said at least a single direction. Said crystallographic orientation in said at least a single direction is substantially normal to the surface of the film. Said crystallographic orientation in said at least a single direction is a &lt;100&gt; orientation. Crystallizing at least the first portion of the first region comprises performing uniform sequential lateral crystallization. The uniform sequential lateral crystallization comprises line-scan sequential lateral crystallization. Crystallizing at least the first portion of the first region comprises performing Dot sequential lateral crystallization. Crystallizing at least the first portion of the first region comprises performing controlled super-lateral growth crystallization. Crystallizing at least the first portion of the first region comprises forming crystals having a pre-determined crystallographic orientation suitable for a channel region of a driver TFT. Further comprising fabricating at least one thin film transistor in at least one of the first and second regions. Further comprising fabricating a plurality of thin film transistors in at least the first and second regions. Defining the plurality of spaced-apart regions comprises defining a width for each spaced-apart region that is at least as large as a device of circuit intended to be later fabricated in that region. Defining the plurality of spaced-apart regions comprises defining a width for each spaced-apart region that is at least as large as a width of a thin film transistor intended to be later fabricated in that region. The spaced-apart regions are separated by amorphous film. The film comprises at least one of a conductor and a semiconductor. The film comprises silicon. The substrate comprises glass. Shaping said laser beam using focusing optics. 
         [0013]    Some embodiments provide a system for processing a film, the system comprising a laser source providing a laser beam having a fluence that is selected to form a mixture of solid and liquid in the film and where a fraction of the film is molten throughout its thickness in an irradiated region; a movable at least partially reflective optical element in the path of the laser beam capable of controllably redirecting the path of the laser beam; a stage for supporting the film and capable of translation in at least a first direction; and memory for storing a set of instructions, the instructions comprising defining a plurality of spaced-apart regions to be pre-crystallized within the film, the film being disposed on a substrate and capable of laser-induced melting; positioning the film relative to the laser beam in preparation for at least partially pre-crystallizing a first region of said plurality of spaced-apart regions; moving the movable optical element so as to scan a first portion of the first region with the beam in the first direction at a first velocity, wherein the first velocity is selected such that the beam forms a mixture of solid and liquid in the film and where a fraction of the film is molten throughout its thickness in the first portion of the first region, wherein said first portion of the first region upon cooling forms crystalline grains having predominantly the same crystallographic orientation in at least a single direction. 
         [0014]    Some embodiments include one or more of the following features. The laser beam is continuous-wave. Re-positioning the film relative to the laser beam in preparation for at least partially re-crystallizing a second region of the plurality of spaced-apart regions; and moving the movable optical element so as to scan a first portion of the second region with the beam in the first direction at the first velocity, wherein the first portion of the second region upon cooling forms crystalline grains having predominantly the same crystallographic orientation in said at least a single direction. The first velocity is further selected such that heat generated by the beam substantially does not damage the substrate. The movable optical element comprises a disk comprising a plurality of facets that at least partially reflect said laser beam onto the film. The first velocity is at least about 0.5 m/s. The first velocity is at least about 1 m/s. The memory further includes instructions to, after moving the movable optical element so as to scan the first portion of the first region, translate the film relative to the laser beam in a second direction so as to scan a second portion of the first region with the laser beam in the first direction at the first velocity, wherein the second portion of the first region upon cooling forms crystalline grains having predominantly the same crystallographic orientation in said at least a single direction. The memory further includes instructions to partially overlap the first and second portions of the first region. The memory further includes instructions to continuously translate the film in the second direction with a second velocity selected to provide a pre-determined amount of overlap between the first and second portions of the first region. The memory further includes instructions to continuously translate the film in the second direction with the second velocity for a period of time selected to sequentially irradiate and partially melt a plurality of portions of the first region, wherein each of said plurality of portions upon cooling forms crystalline grains having predominantly the same crystallographic orientation in said at least a single direction. The memory further includes instructions to perform uniform sequential lateral crystallization in at least the first region. The memory further includes instructions for defining a width for each spaced-apart region that is at least as large as a device or circuit intended to be later fabricated in that region. The memory further includes instructions for defining a width for each spaced-apart region that is at least as large as a width of a thin film transistor intended to be later fabricated in that region. The film comprises at least one of a conductor and a semiconductor. The film comprises silicon. The substrate comprises glass. Further comprising laser optics to shape said laser beam. 
         [0015]    Some embodiments provide a thin film, the thin film comprising columns of pre-crystallized film positioned and sized so that rows and columns of TFTs can later be fabricated in said columns of pre-crystallized film, said columns of pre-crystallized film comprising crystalline grains having predominantly the same crystallographic orientation in at least a single direction; and columns of untreated film between said columns of pre-crystallized film. 
         [0016]    Some embodiments include one or more of the following features. Said crystallographic orientation in said at least a single direction is substantially normal to the surface of the film. Said crystallographic orientation in said at least a single direction is a &lt;100&gt; orientation. The columns of untreated film comprise amorphous film. 
         [0017]    Some embodiments provide a method of processing a film, the method comprising defining at least one region within the film, the film being disposed on a substrate and capable of laser-induced melting; generating a laser beam having a fluence that is selected to form a mixture of solid and liquid in the film and where a fraction of the film is molten throughout its thickness in an irradiated region; directing the laser beam onto a moving optical element that is at least partially reflective, said moving optical element directing the laser beam across a first portion of the first region in a first direction at a first velocity; moving the film relative to the laser beam in a second direction and at a second velocity to displace the film along the second direction during laser irradiation of the first portion while moving the optical element, wherein said first portion of the first region upon cooling forms crystalline grains having predominantly the same crystallographic orientation in at least a single direction, wherein the first velocity is selected such that the beam irradiates and forms a mixture of solids and liquid in the first portion of the film; and repeating the steps of moving the optical element and moving the film at least once to crystallize the first region. 
         [0018]    Some embodiments include one or more of the following features. The laser beam is continuous-wave. Further comprising re-positioning the film relative to the laser beam in preparation for at least partially pre-crystallizing a second region of the plurality of spaced-apart regions; and moving the optical element so as to scan a first portion of the second region with the laser beam in the first direction at the first velocity, wherein the first portion of the second region upon cooling forms crystalline grains having predominantly the same crystallographic orientation in said at least a single direction. Said first velocity is further selected to avoid heat generation by the beam that damages the substrate. Directing the moving optical element comprises rotating a disk that comprises a plurality of facets that reflect said laser beam onto the film. The first velocity is at least about 0.5 m/s. The first velocity is at least about 1 m/s. The steps of moving the optical element and moving the film provide first and second portions of the first region having predominantly the same crystallographic orientation and the second portion of the first region partially overlaps the first portion of the first region. Continuously translating the film in the second direction with a second velocity selected to provide a pre-determined amount of overlap between the first and second portions of the first region. Continuously translating the film in the second direction with a second velocity for a period of time selected to sequentially irradiate and partially melt a plurality of portions of the first region, wherein each of said plurality of portions upon cooling forms crystalline grains having predominantly the same crystallographic orientation in said at least a single direction. Said crystallographic orientation in said at least a single direction is substantially normal to the surface of the film. Said crystallographic orientation in said at least a single direction is a &lt;100&gt; orientation. Further comprising subjecting the film to a subsequent sequential lateral crystallization process to generate location controlled grains having wherein crystallizing at least the first portion of the first region comprises performing uniform sequential lateral crystallization. The uniform sequential lateral crystallization comprises line-scan sequential lateral crystallization. Crystallizing at least the first portion of the first region comprises performing Dot sequential lateral crystallization. Crystallizing at least the first portion of the first region comprises performing controlled super-lateral growth crystallization. Crystallizing at least the first portion of the first region comprises forming crystals having a pre-determined crystallographic orientation suitable for a channel region of a driver TFT. Further comprising fabricating at least one thin film transistor in at least one of the first and second regions. Further comprising fabricating a plurality of thin film transistors in at least the first and second regions. Defining the plurality of spaced-apart regions comprises defining a width for each spaced-apart region that is at least as large as a device or circuit intended to be later fabricated in that region. Defining the plurality of spaced-apart regions comprises defining a width for each spaced-apart region that is at least as large as a width of a thin film transistor intended to be later fabricated in that region. The spaced-apart regions are separated by amorphous film. The film comprises at least one of a conductor and a semiconductor. The film comprises silicon. The substrate comprises glass. Shaping said laser beam using focusing optics. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0019]    In the drawing: 
           [0020]      FIG. 1  illustrates a thin film with regions pre-crystallized with high throughput pre-crystallization according to some embodiments. 
           [0021]      FIG. 2  illustratively displays a method for the high throughput pre-crystallization of a thin film and optional subsequent TFT fabrication according to some embodiments. 
           [0022]      FIG. 3  is a schematic diagram of an apparatus for high throughput pre-crystallization of a thin film according to some embodiments. 
           [0023]      FIG. 4A-4B  illustrates the pre-crystallization of a TFT region using a high throughput pre-crystallization apparatus according to some embodiments. 
           [0024]      FIG. 5  is a schematic diagram of an apparatus for sequential lateral crystallization of a semiconductor film according to some embodiments. 
           [0025]      FIG. 6A  illustrates crystalline microstructures formed by excimer laser annealing. 
           [0026]      FIGS. 6B-6D  illustrate crystalline microstructures formed by sequential lateral crystallization. 
           [0027]      FIGS. 7A-7D  illustrate schematically processes involved in and microstructures formed by sequential lateral crystallization according to some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Systems and methods described herein provide pre-crystallized thin films having controlled crystallographic texture. The textured films contain grains having predominantly the same crystallographic orientation in at least a single crystallographic orientation. The thin films are suitable for further processing with SLS or other lateral growth processes, as discussed in greater detail below. In SLS, the crystal orientation of lateral growth during SLS depends on the orientation of the material at the boundary of the irradiated region. By pre-crystallizing the film before performing SLS, the crystals that laterally grow during SLS adopt the crystalline orientation generated during pre-crystallization, and thus grow with an improved crystalline orientation relative to crystal grains grown without pre-crystallization. The pre-crystallized and laterally crystallized film can then be processed to form TFTs, and ultimately be used as a display device. 
         [0029]    When a polycrystalline material is used to fabricate devices having TFTs, the total resistance to carrier transport within the TFT channel is affected by the combination of barriers that a carrier has to cross as it travels under the influence of a given potential. Within a material processed by SLS, a carrier crosses many more grain boundaries if it travels perpendicularly to the long grain axes of the polycrystalline material, and thus experiences a higher resistance, than if it travels parallel to the long grain axes. Thus, in general, the performance of TFT devices fabricated on SLS-processed polycrystalline films depends on the microstructure of the film in the channel, relative to the film&#39;s long grain axes. However, SLS is not able to fully define the crystallographic texture of those grains, because they grow epitaxially from existing grains that do not themselves necessarily have a well-defined crystallographic texture. 
         [0030]    Pre-crystallizing a thin film can improve the crystal alignment, e.g., texture, obtained during subsequent lateral crystallization processes, and can allow separate control and optimization of the texture and the microstructure of the film. Pre-crystallizing the film generates a textured film having crystal grains with predominantly the same crystallographic orientation in at least one direction. For example, if one crystallographic axis of most crystallites in a thin polycrystalline film points preferentially in a given direction, the film is referred to as having a one-axial texture. For many embodiments described herein, the preferential direction of the one-axial texture is a direction normal to the surface of the crystallites. Thus, “texture” refers to a one-axial surface texture of the grains as used herein. In some embodiments, the crystallites have a (100) texture. The degree of texture can vary depending on the particular application. For example, a high degree of texture can improve the performance of thin film transistor (TFT) being used for a driver circuit, but not provide as significant a benefit to a TFT that is used for a switch circuit. 
         [0031]    One method that can be used to pre-crystallize a film is known as mixed-phase zone-melt recrystallization (ZMR), which, in some embodiments, uses a continuous wave (CW) laser beam to partially melt a silicon film and thus produce a film having a desired texture, e.g., (100) texture. In ZMR, irradiation causes some parts of the film to completely melt while others remain unmelted, forming a “transition region” which exists as a result of a significant increase in reflectivity of Si upon melting (a semiconductor-metal transition). Crystal grains having (100) texture form in this transition region. For further details, see U.S. Patent Publication No. 2006/0102901, entitled “Systems and Methods for Creating Crystallographic-Orientation Controlled poly-Silicon Films,” the entire contents of which are incorporated herein by reference. The texture of a pre-crystallized film can be further improved by scanning the film multiple times, as preferably oriented grains get enlarged at the expense of less preferably oriented grains. For further details, see U.S. Provisional Patent Application No. 60/707,587, the entire contents of which are incorporated herein by reference. For further general details on ZMR, see M. W. Geis et al., “Zone-Melting recrystallization of Si films with a movable-strip-heater oven,” J. Electro-Chem. Soc. 129, 2812 (1982), the entire contents of which are incorporated herein by reference. 
         [0032]    However, pre-crystallizing an entire panel to get (100) large-grain material can be time consuming, as typical CW laser sources have limited power. Additionally, pre-crystallizing a silicon film with a CW laser can significantly heat the film and underlying substrate due to the continuous radiation. For glass substrates, sufficient heat can be generated to cause the substrate to warp or actually melt and damage the substrate. In general, a glass substrate benefits from a scan velocity of at least about 1 m/s in order to avoid damage. However, as substrate sizes increase, this velocity becomes increasingly difficult to achieve; for example, current panel sizes in so-called low-temperature polycrystalline silicon (LTPS) technology, commonly used for mobile (small-display) applications, are up to ˜720 mm×930 mm (which can be divided into 4 or more devices) or larger. Currently available stage technology typically limits scan velocities to a few cm/s or a few 10&#39;s of cm/s, as is used in normal SLS processes. Thus, conventional pre-crystallization using a CW laser is not readily applied to large substrates. Although heat-resistant substrates can be used, they are more costly and are less attractive for large-area electronic applications. 
         [0033]    The pre-crystallization systems and methods described herein allow the film to be scanned at high scan velocities, which helps to prevent heat damage to the underlying substrate. The systems can use conventional (e.g., relatively slow) handling stages to move large substrates, and at the same time can provide scan velocities of about 1 m/s, or even higher. Specifically, a handling stage moves the film and substrate at a typical scan velocity in one direction, while moving optics scan a laser beam across the film at a much higher velocity in a different, e.g., perpendicular, direction. The motions of the stage and laser beam are coordinated so that defined regions of the film are pre-crystallized, and other regions are left untreated. This increases the effective scanning velocity of the film above a threshold at which the substrate would be damaged, and greatly improves the efficiency of pre-crystallizing the film. 
         [0034]    The systems and methods also are capable of reducing the overall time to process the film. Specifically, the film is pre-crystallized in regions of the film where devices that benefit from controlled crystallographic texture, e.g., regions that contain the most demanding circuitry, will be fabricated. In some embodiments, these regions are on the periphery of a display, where the integration TFTs will be fabricated. Regions of the film where such devices will not be located, or devices not requiring controlled crystallographic texture, are not pre-crystallized. In some embodiments, the speed with which panels are pre-crystallized are approximately matched to the throughput rate of SLS systems and methods, with which the pre-crystallization systems and methods can be incorporated. 
         [0035]      FIG. 1  illustrates an embodiment of a silicon film  300  that is pre-crystallized in defined regions, and left untreated in other regions. The defined regions can be selected for a variety of reasons, such as that devices benefiting from improved crystalline texture will eventually be fabricated there. In some embodiments, the defined regions correspond to TFT channels. The film includes areas of pre-crystallized silicon  325 , and areas of untreated silicon  310 . The areas are positioned and sized so that rows and columns of TFTs can optionally be subsequently fabricated within the areas of pre-crystallized silicon  325 , e.g., with SLS and other processing steps. The untreated regions  310  can be uncrystallized silicon, e.g., amorphous silicon, or can be, e.g., polycrystalline silicon. 
         [0036]    Although the areas of untreated and pre-crystallized silicon are illustrated to have approximately the same width, the area widths and relative spacing can vary, depending on the desired area of the display and the width of the integration regions. For example, the integration regions can be only several mm wide for a display that can have a diagonal of several inches. In this case, the pre-crystallized silicon columns  325  can be fabricated to be substantially narrower than the untreated areas  310 . This will further improve the efficiency with which the film can be processed, because large regions of the film will not need to be pre-crystallized. In general, the width of the pre-crystallized regions needs only to be long enough to cover the area for integration circuits. 
         [0037]      FIG. 2  illustratively displays a method  400  for the high throughput pre-crystallization, and optional subsequent processing of a semiconductor film to form TFTs, according to certain embodiments. First, the regions to be pre-crystallized are defined ( 410 ). The defined regions optionally correspond to areas in which TFT circuits will be fabricated, as described above. The area widths and spacings are selected according to the requirements of the device that will eventually be fabricated using the film. 
         [0038]    Then, the film is pre-crystallized in the defined regions ( 420 ). In some embodiments, this is done with a continuous wave (CW) laser as described in greater detail below. The laser partially melts the film, which crystallizes to have a desired texture. The textured film contains grains having predominantly the same crystallographic orientation in at least a single direction. However, the grains are randomly located on the film surface, and are of no particular size. 
         [0039]    Then, the film is optionally laterally crystallized ( 430 ). In many embodiments, this is done with SLS processes, for example as described in greater detail below. For further details and other SLS processes, see U.S. Pat. Nos. 6,322,625, 6,368,945, 6,555,449, and 6,573,531, the entire contents of which are incorporated herein by reference. 
         [0040]    Then, TFTs are optionally fabricated within the defined regions ( 440 ). This can be done with silicon island formation, in which the film is etched to remove excess silicon except where the TFTs are to be fabricated. Then, the remaining “islands” are processed using techniques known in the art to form active TFTs, including source and drain contact regions as illustrated in  FIG. 6A . 
         [0041]    Note that in general, even if defined regions of a given film are pre-crystallized and the remaining regions left untreated, the SLS process need not be performed solely within the pre-crystallized regions. For example, the entire film, or portions thereof, can be laterally crystallized with SLS. Then, TFTs can be fabricated at desired locations within the laterally crystallized regions of the film, such that some or all of the TFTs are fabricated within the regions that were originally pre-crystallized. Determining which steps to perform on a given region of the film depends on the performance requirements of the finished device. 
         [0042]      FIG. 3  schematically illustrates an embodiment of a system that can be used for precrystallizing a thin film. The system includes a rotating disk with a plurality of facets, each of which is at least partially reflective for the laser beam wavelength. The laser beam is directed at the rotating disk, which is arranged such that the facets redirects the laser beam so that it irradiates the film. As the disk rotates, it causes the laser beam to scan the surface of the film, thus pre-crystallizing successive portions of the film. As the disk continues to rotate, each new facet that reflects the laser beam effectively “re-sets” the position of the beam relative to the film in the direction of rotation, bringing the laser beam back to its starting point on the film in that direction. At the same time, the film is translated in another direction, e.g., perpendicular to the scan direction, so that as the disk continues to rotate, new facets reflect the laser beam onto successive portions of the film that are displaced from each other in the second direction. Thus, an entire region of the thin film can be pre-crystallized. 
         [0043]    As illustrated in  FIG. 3 , pre-crystallization system  500  that can be used to pre-crystallize a thin film  515  within defined region  520 . A laser (not shown), e.g., an 18 W, 2ω Nd:YVO 4  Verdi laser from Coherent Inc., generates a CW laser beam  540 . One or more optics (also not shown) shape laser beam  540  so that it forms a thin line beam. In some embodiments, the beam has a length of between about 1-15 mm, a width of between about 5-50 μm, and a fluence of between about 10-150 W/mm of beam length. Note, however, that the beam may have any desired length, and in some cases may be a “line beam” having a very high length to width aspect ratio (e.g., about 50-10 5 ), and may even extend for the full length of the panel being irradiated. In this case, the film need not be scanned in the second direction, because the entire length of a given region will be irradiated at once. In some embodiments, the beam has approximately uniform energy along the long axis, although in other embodiments the beam will have other energy profiles such as Gaussian or sinusoidal. In some embodiments, the beam along the short axis has a “top hat” energy profile, i.e., having substantially equal energy across the short axis profile of the beam, and in other embodiments, the beam has a tightly focused Gaussian profile along the short axis. Other energy profiles, and other beam sizes, are possible and can be selected according to the performance requirements of the finished device. The overall beam power, as well as the size of the beam, is selected to provide a sufficient energy density to partially melt the film  515  so that it recrystallizes with the desired amount of texture. One of skill in the art would be able to readily select appropriate lasers and optics to achieve a desired beam profile, wavelength, and energy. Note that the laser beam need not be CW, but can also have any suitable temporal profile, for example sufficiently long pulses to partially melt the irradiated regions, or have a relatively high repetition rate (“quasi-CW”). 
         [0044]    The laser beam is directed towards a rotating disk  560  having a plurality of at least partially reflective surfaces or facets  580 . Reflective facets  580  of disk  560  are positioned relative to film  575  so as to direct laser beam  540  towards the film surface. Specifically facets  580  are arranged so as to redirect laser beam  540  so that it irradiates film  515  within defined region  520 . Where the laser beam irradiates region  520 , it partially melts the film, which crystallizes upon cooling as described in greater detail in U.S. Patent Publication No. 2006/0102901. Disk  560  rotates about axis  570 . This rotation moves facets  580  relative to laser beam  540 , so that they behave as a moving mirror for the laser beam, and guide the beam in a line across the substrate. The movement of facets  580  move laser beam  540  rapidly relative to film  515  in the (−y) direction. The relative velocity v scan  of the beam relative to the film  515  in the (−y) direction is determined by the speed of rotation of disk  560 . The velocity of the beam imparted by the disk is substantially higher than could be generated by moving the substrate with a typical mechanical stage. At the same time, stage  518  moves film  515  in the (+x) direction with a velocity v stage , perpendicular to the direction of beam motion. Thus, the total beam velocity relative to a given point of the film can be substantially higher than normally achievable using stage  518  alone. Furthermore the irradiation pattern of the film surface is defined by the state scanning speed and direction as well as the facet size and rotation rate of the disk, as well as the distance between the disk and the film. 
         [0045]    While  FIG. 3  shows faceted disk  560  with eight facets  580 , this number of facets is meant to be illustrative only. In general, other ways of deflecting the beam in order to provide high velocity scanning are contemplated, for example, a single movable mirror. Or, for example, other numbers of facets can be used, according to the desired processing speed and size of pre-crystallized regions  520 . 
         [0046]      FIG. 4A  illustrates a detailed view of the path of laser beam  540  relative to the substrate  610 . As disk  560  rotates, a first facet  580  reflects beam  540  so that it first irradiates substrate  610  at a first edge  621  of the defined film region  620  to be pre-crystallized, starting a “first scan” of the region. Disk  560  continues to rotate the given facet  580 , so that the beam moves across film region  620  in the (−y) direction with a velocity v scan . At the same time, stage  518  moves substrate  610  in the (+x) direction with a velocity v stage  resulting in a diagonal crystallization path. Wherever beam  540  irradiates defined film region  620 , it partially melts the film, which upon cooling recrystallizes with texture as described above. Thus, as can be seen in  FIG. 6A , the width w scan  of a particular scanned region is defined by the length of the laser beam in that region, and the edge of the scanned region follows a “diagonal” path relative to the substrate, defined by v scan  and v stage , as described in greater detail below. 
         [0047]    As disk  560  continues to rotate, the first facet  580  eventually rotates far enough that it no longer reflects beam  540 . When this happens, the beam stops irradiating the defined region  620  at second edge  622  which coincides with the other edge defining the preselected region  620 . With continued rotation of disk  560 , the laser beam  540  is directed onto a second facet  580 . Second facet  580  redirects laser beam  540  so that it irradiates substrate  610  at a first edge  621  of the defined film region  620 , starting a “second scan” of the region. At the beginning of the second scan, the stage has moved the substrate  610  by a predetermined distance (based on stage velocity) in the (+x) direction relative to where it was at the beginning of the first scan. This yields an offset in the (+x) between the edge of the first scan and the edge of the second scan that is determined by stage speed v stage . This offset can be chosen to provide a desired amount of overlap between the first and second scans. As mentioned above, pre-crystallizing a film multiple times can enlarge the size of preferably oriented grains, so it may be desirable to use a relatively small offset to provide a large amount of overlap between the first and second scanned areas. 
         [0048]    As disk  560  continues to rotate, second facet  580  moves the beam  540  across region  620  in the (−y) direction, and stage  518  moves substrate  610  in the (+x) direction. Eventually the second facet  580  moves out of the path of beam  540 , and a third facet  580  reflects the beam  540  to irradiate region  620 , again offset in the (+x) direction by an amount determined by speed v stage . In this way, as disk  560  continues to rotate and stage  518  moves the substrate  610 , defined film region  620  is substantially pre-crystallized, while other regions of substrate  610  are not pre-crystallized and remain, e.g., amorphous silicon. After completing the pre-crystallization of region  620 , the stage moves the substrate  610  in the (−x) and (+y) or (−y) direction, so that a new region can be pre-crystallized as described above. 
         [0049]    Although  FIG. 4A  shows a non-pre-crystallized area at the bottom of defined film region  620 , resulting from the “diagonal” motion of the beam relative to the substrate, this area can be pre-crystallized by simply starting the first scan below the edge of the substrate. Alternately, the bottom of the substrate can be trimmed, or TFTs simply not fabricated on that particular area. 
         [0050]    As illustrated in  FIG. 4B , the combination of beam velocity v scan  in the (−y) direction and stage velocity v stage  in the (+x) direction yields an effective scan velocity v scan,eff . This velocity v scan,eff  is selected so that the beam travels fast enough not to damage the substrate  610 , but slow enough to partially melt the defined film region  620  to the desired degree. 
         [0051]    Assuming that the beam  540  moves in only one direction (although it can be bidirectional), and continuously irradiates the film, the frequency of scanning f scan  is given by: 
         [0000]    
       
         
           
             
               f 
               scan 
             
             = 
             
               
                 v 
                 scan 
               
               
                 l 
                 scan 
               
             
           
         
       
     
         [0000]    where v scan  is the scan velocity, as described above, and l scan  is the length of the region to be scanned, i.e., the y-dimension of the pre-treated area. As an example, for a scan velocity v scan  of 1 m/s and a scan length lscan of 4 mm, the scan frequency will be 250 Hz. 
         [0052]    For a certain number of scans per unit area n, the beam width w scan  follows from: 
         [0000]    
       
         
           
             
               w 
               scan 
             
             = 
             
               
                 n 
                 · 
                 
                   v 
                   stage 
                 
               
               
                 f 
                 scan 
               
             
           
         
       
     
         [0000]    where v stage  is the velocity of the stage. So, in addition to the exemplary numbers above, if n=10 scans per unit area are desired and the stage velocity v stage  is approximately 20 cm/s, then the beam width w scan  is approximately 8 mm. 
         [0053]    In order to remain within the margins of the partial melting regime, the scanning velocity is held to be substantially constant, as opposed to following, for example, a sinusoid trace. In the described embodiment, disk  560  rotates at a substantially constant speed, which causes beam  540  to also move at a substantially constant speed. Translation of the stage allows a new region to be pre-crystallized. 
         [0054]    In some embodiments, the semiconductor film is first pre-crystallized in defined regions, and then laterally crystallized everywhere. The pre-crystallized regions will have more highly aligned crystals than the non-pre-crystallized regions, although all the regions of the film will be laterally crystallized. The regions that are both pre-crystallized and laterally crystallized can be used to fabricate devices that are particularly sensitive to microstructure, such as integration TFTs; the non-pre-crystallized regions can be used to fabricate devices that are less sensitive to microstructure, but still benefit from lateral crystals, such as pixel TFTs. Pre-crystallizing the film only in regions needing improved crystalline alignment can save time and energy over pre-crystallizing the entire semiconductor film. 
         [0055]    In some embodiments, the semiconductor film is laterally crystallized following pre-crystallization. One suitable protocol, referred to herein as “uniform-grain sequential lateral solidification,” or “uniform SLS,” may be used to prepare a uniform crystalline film characterized by repeating columns of laterally elongated crystals. Uniform crystal growth is described with reference to  FIGS. 7A-7D . The crystallization protocol involves advancing the film by an amount greater than the characteristic lateral growth length, e.g., δ&gt;LGL, where δ is the translation distance between pulses, and less than two times the characteristic lateral growth length, e.g., δ&lt;2 LGL. The term “characteristic lateral growth length” (LGL) refers to the characteristic distance the crystals grow when cooling. The LGL is a function of the film composition, film thickness, the substrate temperature, the laser pulse characteristics, the buffer layer material, if any, and the optical configuration. Fro example, a typical LGL for 50 nm thick silicon films is approximately 1-5 μm or about 2.5 μm. The actual growth may be limited by other laterally growing fronts, e.g., where two fronts collide as illustrated below. 
         [0056]    Referring to  FIG. 7A , a first irradiation is carried out on a film with a narrow, e.g., less than two times the lateral growth length, and elongated, e.g., greater than 10 mm and up to or greater than 1000 mm, laser beam pulse having an energy density sufficient to completely melt the film. As a result, the film exposed to the laser beam (shown as region  400  in  FIG. 7A ), is melted completely and then crystallized. In this case, grains grow laterally from an interface  420  between the unirradiated region and the melted region. As noted above, the grains grow epitaxially from the solidus boundaries on either side of the melted region. Thus, the laterally growing grains adopt the texture of the pre-crystallized film, formed as described above. By selecting the laser pulse width so that the molten zone width is less than about two times the characteristic LGL, the grains growing from both solid/melt interfaces collide with one another approximately at the center of the melted region, e.g., at centerline  405 , and the lateral growth stops. The two melt fronts collide approximately at the centerline  405  before the temperature of the melt becomes sufficiently low to trigger nucleation. 
         [0057]    Referring to  FIG. 7B , after being displaced by a predetermined distance δ that is at least greater than about LGL and less than at most two LGL, a second region of the substrate  400 ′ is irradiated with a second laser beam pulse. The displacement of the substrate, δ, is related to the desired degree of overlap of the laser beam pulse. As the displacement of the substrate becomes longer, the degree of overlap becomes less. It is advantageous and preferable to have the overlap degree of the laser beam to be less than about 90% and more than about 10% of the LGL. The overlap region is illustrated by brackets  430  and dashed line  435 . The film region  400 ′ exposed to the second laser beam irradiation melts completely and crystallizes. In this case, the grains grown by the first irradiation pulse serve as crystallizing seeds for the lateral growth of the grains grown from the second irradiation pulse.  FIG. 7C  illustrates a region  440  having crystals that are laterally extended beyond a lateral growth length. Thus, a column of elongated crystals are formed by two laser beam irradiations on average. Because two irradiation pulses are all that is required to form the column of laterally extended crystals, the process is also referred to as a “two shot” process. Irradiation continues across the substrate to create multiple columns of laterally extended crystals.  FIG. 7D  illustrates the microstructure of the substrate after multiple irradiations and depicts several columns  440  of laterally extended crystals. 
         [0058]    Thus, in uniform SLS, a film is irradiated and melted with a low number of pulses, e.g., two. The crystals that form within the melted regions preferably grow laterally and with a similar orientation, and meet each other at a boundary within the particular irradiated region of film. The width of the irradiation pattern is preferably selected so that the crystals grow without nucleation. In such instances, the grains are not significantly elongated; however, they are of uniform size and orientation. For further details on variations of uniform SLS processes, see U.S. Pat. No. 6,573,531, the contents of which are incorporated herein in their entirety by reference, and PCT Publication No. WO 2006/107926, entitled “Line Scan Sequential Lateral Solidification of Thin Films,” the entire contents of which are incorporated herein by reference. Other lateral crystallization methods that provide relatively short elongations of crystal grains are also suitable, for example so-called “Dot-SLS” methods as described in U.S. Patent Publication number 2006/0102901, as well as controlled super-lateral growth, or “C-SLG” methods, as described in PCT Publication No. WO US03/25947, the entire contents of which are incorporated herein by reference. 
         [0059]      FIG. 5  illustrates an SLS system according to some embodiments. A light source, for example, an excimer laser  710  generates a laser beam which then passes through a pulse duration extender  720  and attenuator plates  725  prior to passing through optical elements such as mirrors  730 ,  740 ,  760 , telescope  735 , homogenizer  745 , beam splitter  755 , and lens  765 . The laser beam pulses are then passed through a mask  770 , which may be on a translation stage (not shown), and projection optics  795 . The mask can be a slit, which shapes the laser beam into a “line beam,” although the system is capable of making more complex beam shapes depending on the choice of mask. The projection optics reduce the size of the laser beam and simultaneously increase the intensity of the optical energy striking substrate  799  at a desired location. The substrate  799  is provided on a precision x-y-z stage  800  that can accurately position the substrate  799  under the beam and assist in focusing or defocusing the image of the mask  770  produced by the laser beam at the desired location on the substrate. As described in U.S. Patent Publication No. 2006/0102901, the firing of the laser can be coordinated with the motion of x-y-z stage  800  to provide location-controlled firing of pulses. 
         [0060]    Although the discussion above refers to the processing of silicon thin films, many other kinds of thin films are compatible. The thin film can be a semiconductor or a conductor, such as a metal. Exemplary metals include aluminum, copper, nickel, titanium, gold, and molybdenum. Exemplary semiconductor films include conventional semiconductor materials, such as silicon, germanium, and silicon-germanium. Additional layers situated beneath or above the metal or semiconductor film are contemplated, for example, silicon oxide, silicon nitride and/or mixtures of oxide, nitride, or other materials that are suitable, for example, for use as a thermal insulator to further protect the substrate from overheating or as a diffusion barrier to prevent diffusion or impurities from the substrate to the film. See, e.g., PCT Publication No. WO 2003/084688, for methods and systems for providing an aluminum thin film with a controlled crystal orientation using pulsed laser induced melting and nucleation-initiated crystallization. 
         [0061]    In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are illustrative only, and should not be taken as limiting the scope of the present invention.