Abstract:
A device for irradiating a laser beam onto an amorphous silicon thin film formed on a substrate. The device includes: a stage mounting the substrate; a laser oscillator for generating a laser beam; a projection lens for focusing and guiding the laser beam onto the thin film; a reflector for reflecting the laser beam guided onto the thin film; a controller for controlling a position of the reflector; and an absorber for absorbing the laser beam reflected by the reflector.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. application Ser. No. 10/532,459 filed Nov. 2, 2005, which is based on International Application Serial No. PCT/KR2003/002212 filed Oct. 21, 2003, which claims priority of Korean Patent Application No. 10-2002-0064511 filed Oct. 22, 2002, the disclosures of which are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     (a) Field of the Invention  
         [0003]     The present invention relates to a method of polycrystallization, a method of manufacturing a thin film transistor, and a laser irradiation device therefor.  
         [0004]     (b) Description of the Related Art  
         [0005]     In general, a liquid crystal display (“LCD”) includes two panels with electrodes and a liquid crystal layer interposed therebetween. The two panels are combined with a sealant for sealing the liquid crystal layer, which is printed around the edges of the panels. The panels are supported by spacers distributed therebetween.  
         [0006]     This LCD displays desired images by applying electric field using the electrodes to the liquid crystal layer with dielectric anisotropy and adjusting the strength of the electric field to control the amount of light passing through the panels. In this case, thin film transistors (TFTs) are used for controlling signals transmitted to the electrodes.  
         [0007]     The most commonly used TFTs for an LCD adapts amorphous silicon as a semiconductor layer.  
         [0008]     An amorphous silicon TFT has mobility of about 0.5 to 1 cm 2 /Vsec, which is suitable for a switching element of an LCD. However, it is not sufficient for directly forming a driving circuit on an LCD panel.  
         [0009]     In order to overcome such a problem, a TFT LCD including polysilicon with electron mobility of 20 to 150 cm 2 /Vsec has been developed. The relatively high electron mobility polysilicon TFT enables to implement a chip in glass technique that a display panel embeds its driving circuits.  
         [0010]     Techniques for obtaining polycrystalline silicon thin film include a deposition technique depositing polycrystalline silicon directly on a substrate at high temperature, a solid phase crystallization technique depositing amorphous silicon and crystallizing at high temperature of about 600° C., a technique depositing amorphous silicon and crystallizing by laser, and so forth. However, since those techniques require a high temperature process, it is not proper for application of glass substrates for LCDs. Also, they have a disadvantage that electrical characteristics are not uniform between TFTs due to non-uniform grain boundaries.  
         [0011]     To solve these problems, a sequential lateral solidification process capable of adjusting the distribution of the grain boundaries has been developed. The process is based on the fact that the grains of polysilicon at the boundary between a liquid phase region exposed to laser beam and a solid phase region not exposed to laser beam grow in a direction perpendicular to the boundary surface. A mask having a slit pattern is provided, and a laser beam passes through transmittance areas of the mask to completely melt amorphous silicon, thereby producing liquid phase regions arranged in a slit pattern. Thereafter, the melted amorphous silicon cools down to be crystallized, and the crystal growth starts from the boundaries of the solid phase regions not exposed to the laser beam, and proceeds in the directions perpendicular to the boundary surface. The grains stop growing when they encounter each other at the center of the liquid phase region. The sequential lateral solidification process is performed with moving a die, which mounts a panel including the amorphous silicon film thereon, in a horizontal direction when irradiating the laser beam and such a scanning step is repeated along the horizontal direction to cover all areas of the panel.  
         [0012]     The laser beam irradiation in the sequential lateral solidification process is made through a projection lens. At this time, the laser beam may be precisely focused on desired locations.  
         [0013]     However, the focus of the laser beam varies depending on the temperature of the projection lens such that the crystallization of the polysilicon layer for the thin film transistor is non-uniform. In order to solve such a problem, it is most important to develop a technique of keeping the temperature of the projection lens constant when irradiating the laser beam.  
       SUMMARY OF THE INVENTION  
       [0014]     It is a motivation of the present invention to provide a laser irradiation device capable of precisely controlling the focus of a laser beam during the sequential lateral solidification process, and a method of manufacturing a thin film transistor using the same.  
         [0015]     According to an aspect of the present invention, a device for irradiating a laser beam onto an amorphous silicon thin film formed on a substrate is provided, which includes: a stage mounting the substrate; a laser oscillator for generating a laser beam; a projection lens for focusing and guiding the laser beam onto the thin film; a reflector for reflecting the laser beam guided onto the thin film; a controller for controlling a position of the reflector; and an absorber for absorbing the laser beam reflected by the reflector.  
         [0016]     A method of manufacturing a thin film transistor using a laser irradiation device including a projection lens is also provided, which includes: depositing an amorphous silicon thin film on a substrate; irradiating a laser beam from the laser irradiation device onto the thin film through an exposure mask having a slit pattern to form a polysilicon layer after preheating the projection lens; patterning the polysilicon layer to form a semiconductor layer; depositing a first insulating layer on the semiconductor layer; forming a gate electrode on the first insulating layer; implanting impurities into the semiconductor layer to form source and drain regions; depositing a second insulating layer on the gate electrode; forming contact holes exposing the source and the drain regions in the first and the second insulating layers; and forming source and drain electrodes respectively connected to the source and the drain regions through the contact holes.  
         [0017]     The polysilicon layer is preferably formed by lateral sequential solidification.  
         [0018]     A pixel electrode, preferably made of a transparent conductive material or a reflective conductive material, connected to the drain electrode may be additionally formed.  
         [0019]     A method of polycrystallization of an amorphous silicon thin film using a laser irradiation device including a projection lens is provided, which includes: depositing an amorphous silicon thin film on a substrate; preheating the projection lens without irradiating a laser beam from the laser irradiation device onto the thin film; and irradiating a laser beam from the laser irradiation device onto the thin film to form a polysilicon layer after the preheating.  
         [0020]     The laser beam from the laser irradiation device is preferably reflected away from the thin film during the preheating. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The present invention will become more apparent by describing preferred embodiments thereof in detail with reference to the accompanying drawings in which:  
         [0022]      FIG. 1A  is a schematic diagram showing a sequential lateral solidification process for crystallizing amorphous silicon into polysilicon by irradiating laser beam;  
         [0023]      FIG. 1B  schematically shows a detailed structure of a polycrystalline silicon thin film during crystallization from amorphous silicon to polycrystalline silicon in the sequential lateral solidification process;  
         [0024]      FIG. 1C  schematically shows a scanning step in a sequential lateral solidification process for crystallizing amorphous silicon into polysilicon;  
         [0025]      FIGS. 2A and 2B  illustrate schematic diagrams of a laser irradiation device for polycrystallization according to an embodiment of the present invention;  
         [0026]      FIG. 3  is a sectional view of a polysilicon thin film transistor; and  
         [0027]      FIGS. 4A  to  4 E are sectional views of the polysilicon thin film transistor shown in  FIG. 3  in intermediates steps of a manufacturing method thereof. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.  
         [0029]     In the drawings, the thickness of layers, films, panels, regions, etc. are exaggerated for clarity. Like numerals refer to like elements throughout. It will be understood that when an element such as a layer, film, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.  
         [0030]     A laser irradiation device and a method of manufacturing a thin film transistor using a laser irradiation device according to an embodiment of the present invention will be now described in detail with reference to the accompanying drawings.  
         [0031]      FIG. 1A  is a schematic diagram showing a sequential lateral solidification process for crystallizing amorphous silicon into polysilicon by irradiating laser beam,  FIG. 1B  schematically shows a detailed structure of a polycrystalline silicon thin film during crystallization from amorphous silicon to polycrystalline silicon in the sequential lateral solidification process, and  FIG. 1C  schematically shows a scanning step in a sequential lateral solidification process for crystallizing amorphous silicon into polysilicon.  
         [0032]     As shown in  FIG. 1A , according to the sequential lateral solidification process, a laser beam is applied to a plurality of local regions of an amorphous silicon layer  200  formed on an insulating substrate using a mask  300  having a transmission area  310  with a slit pattern to completely melt the amorphous silicon in the local regions such that a plurality of liquid phase regions are formed in an area of the amorphous silicon layer  200  corresponding to the transmission area  310 .  
         [0033]     At this time, a grain of polycrystalline silicon grows from a boundary surface between the liquid phase region  210  exposed to the laser beam and a solid phase region  220  where the laser beam is not applied along a direction perpendicular to the boundary surface as shown in  FIG. 1B . The grains stop growing when they meet at the center of the liquid phase region. They are grown to have a various size of a desired degree by performing the step along the growing direction of the grains to continue the lateral growth of the grains.  
         [0034]     For instance, the sequential lateral solidification process illustrated in  FIG. 1C  uses a mask  300  including a plurality of transmissive areas  301  and  302  having slits. Each slit in the transmissive areas  301  and  302  is elongated in a transverse direction, and the transmissive areas  301  and  302  form a plurality of columns. The transmissive areas  301  and  302  in each column are arranged with a predetermined pitch, and the transmissive areas  301  and  302  in adjacent two columns are offset by about half of the pitch. The sequential lateral solidification moves the substrate by a width of the column in the transverse direction (i.e., x direction) with respect to the mask  300  after irradiating laser beams through the mask (referred to as a shot). Since the transmissive areas  301  and  302  are elongated in the x direction, the grain growth proceeds in the y direction by a width of the transmissive areas  301  and  302  as shown in  FIG. 1B .  
         [0035]     The movement of the substrate is performed by a stage mounting the substrate while a laser irradiation device is fixed.  
         [0036]      FIGS. 2A and 2B  illustrate schematic diagrams of a laser irradiation device for polycrystallization according to an embodiment of the present invention.  
         [0037]     A laser irradiation device according to an embodiment of the present invention generates a laser beam by frequency oscillation and irradiates the laser beam onto an amorphous silicon thin film formed on an insulating substrate  100  such as glass. Referring to  FIGS. 2A and 2B , the laser irradiation device includes a stage  400  for fixing and supporting the substrate  100 , a laser oscillator  500  for generating a uniform laser beam with a predetermined frequency, an optical unit  600 , a projection lens  700 , a reflector  820 , an absorber  830 , and a controller  810 .  
         [0038]     The optical unit  600  imparts a desired energy to the generated laser beam, removes the afterimage of the laser beam, and makes the frequency of the laser beam uniform. The projection lens  700  condenses the laser beam such that the laser beam is correctly focused onto the amorphous silicon thin film on the substrate  100 .  
         [0039]     The reflector  820  reflects the laser beam irradiated from the optical unit  600  through the projection lens  700  toward the absorber  830 , which absorbs the reflected laser beam, under the control of the controller  810  so that the amorphous silicon layer of the substrate  100  is not exposed to the laser beam during the preheating of the protection lens  700  with a predetermined temperature as shown in  FIG. 2A  and it moves away from the substrate  100  under the control of the controller  810  such that the amorphous silicon layer is timely exposed to the laser beam when the temperature of the projection lens  700  is stabilized and the focusing of the projection lens  700  is completed as shown in  FIG. 2B .  
         [0040]     The laser irradiation device according to the embodiment of the present invention enables to perform uniform polycrystallization by irradiating the laser beam onto the amorphous silicon layer when the projection lens reaches a predetermined temperature to make the laser beam be exactly and uniformly focused on the amorphous silicon thin film.  
         [0041]     A thin film transistor and a manufacturing method thereof using the laser irradiation device according to embodiments of the present invention will be now described in detail.  
         [0042]      FIG. 3  is a sectional view of a polysilicon thin film transistor according to an embodiment of the present invention, and  FIGS. 4A  to  4 E are sectional views of the polysilicon thin film transistor shown in  FIG. 3  in intermediates steps of a manufacturing method thereof according to an embodiment of the present invention. Although the figures and the description thereof illustrates a thin film transistor for a pixel electrode, a thin film transistor for driving circuits on the substrate is also formed by the similar method.  
         [0043]     As shown in  FIG. 3 , a semiconductor layer  20  made of polysilicon is formed on an insulating substrate  10 . The semiconductor layer  20  includes a channel region  21  and source and drain regions  22  and  23  opposite each other with respect to the channel region  21 . Here, the source and the drain regions  22  and  23  are doped with n type or p type impurity and may include a silicide layer.  
         [0044]     A gate insulating layer preferably made of Si02 or SiN x  and covering the semiconductor layer  20  is formed on the substrate  10 , and a gate electrode  40  is formed on the gate insulating layer  30  opposite the channel region  21 .  
         [0045]     An interlayer insulating layer  50  covering the gate electrode  40  is formed on the gate insulating layer  30 , and the gate insulating layer  30  and the interlayer insulating layer  50  have contact holes  52  and  53  exposing the source and the drain regions  22  and  23 .  
         [0046]     A source electrode  62  and a drain electrode  63  are formed on the interlayer insulating layer  50 . The source electrode  62  is connected to the source region  22  via the contact hole  52 , and a drain electrode  63  is opposite the source electrode  62  with respect to the gate electrode  40  and connected to the drain region  23  via the contact hole  53 .  
         [0047]     The interlayer insulating layer is covered with a protective layer  70  having a contact hole  73  exposing the drain electrode  63 . A pixel electrode  80  is formed on the protective layer  70 . The pixel electrode  80  is made of a transparent conductive material such as indium tin oxide (ITO) and indium zinc oxide (IZO), or a reflective conductive material, and it is connected to the drain electrode  63  through the contact hole  73 .  
         [0048]     In a method of manufacturing a thin film transistor according to an embodiment of the present invention, as shown in  FIG. 4A , an amorphous silicon thin film  25  is formed on an insulating substrate  10  by depositing amorphous silicon on the substrate  10  using low pressure chemical vapor deposition, plasma enhanced chemical vapor deposition or sputtering.  
         [0049]     Thereafter, as shown in  FIG. 1C , a polysilicon thin film  25  is formed by a sequential lateral solidification process using a mask with a slit pattern shown in  FIG. 1C  and a laser irradiation device shown in  FIGS. 2A and 2B . In detail, a projection lens  700  of the irradiation device is preheated until the temperature of the projection lens  700  reaches a predetermined temperature. During the preheating of the projection lens  700 , the controller  810  controls the reflector  820  such that the reflector  820  reflects the laser beam into the absorber  830  for preventing the laser beam from being irradiated onto the amorphous silicon thin film  25 . The laser beam is focused and irradiated onto the amorphous silicon thin film  25  to start crystallizing the amorphous silicon by moving away the reflector  820  from the substrate  100  after the temperature of the projection lens  700  is kept uniform. The grains of the polysilicon layer  25  formed in this way can be uniform formed to make the performance characteristic of the thin film transistors be uniform.  
         [0050]     As shown in  FIG. 4B , the polycrystalline silicon layer  25  is patterned by a photo-etching with a mask to form a polycrystalline silicon semiconductor layer  20 .  
         [0051]     As shown in  FIG. 4C , silicon oxide or silicon nitride is deposited to form a gate insulating layer  30 . Subsequently, a conductive material for a gate wire is deposited and patterned to form a gate electrode  40 . As shown in  FIG. 4C , n or p-type impurities are then ion-implanted into the semiconductor layer  20  using the gate electrode  40  as a mask, and activated to form source and drain regions  22  and  23 . The region between the source and the drain regions  22  and  23  is defined as a channel region  21 .  
         [0052]     As shown in  FIG. 4D , an interlayer insulating layer  50  covering the gate electrode  40  is formed on the gate insulating layer  30 , and then, the interlayer insulating layer  50  as well as the gate insulating layer  30  and the planarization layer  90  is patterned to form contact holes  52  and  53  exposing the source and the drain regions  22  and  23  of the semiconductor layer  20 .  
         [0053]     As shown in  FIG. 4E , a metal for a data wire is deposited on the insulating substrate  10  and patterned to form a source electrode  62  and a drain electrode  63  connected to the source region  22  and the drain region  23  via the contact holes  52  and  53 , respectively.  
         [0054]     Thereafter, as shown in  FIG. 3 , a protective layer  70  is deposited thereon, and patterned to form a contact hole  73  exposing the drain electrode  63 . A transparent conductive material such as ITO or IZO, or a reflective conductive material is deposited and patterned to form a pixel electrode  80 .  
         [0055]     As described above, the laser irradiation device according to the embodiment of the present invention enables to perform uniform polycrystallization by irradiating the laser beam onto the amorphous silicon layer when the projection lens reaches a predetermined temperature to make the laser beam be exactly and uniformly focused on the amorphous silicon thin film.  
         [0056]     While the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.