Patent Document

CROSS-REFERENCE TO RELATED PATENT APPLICATION 
   This application claims priority to Korean Patent Application No. 10-2006-0090147, filed on Sep. 18, 2006, and all the benefits accruing therefrom under 35 U.S.C. §119, the content of which is incorporated herein by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of crystallizing a semiconductor layer, and more specifically, to a method of manufacturing a laterally crystallized semiconductor layer having good electron mobility and good electrical characteristics using a simple, easy process. 
   2. Description of the Related Art 
   Transistors are used, for example, as switching devices in flat panel displays (FPDs), such as organic light emitting diodes (OLEDs) or liquid crystal displays (LCDs). In general, a thin film transistor (TFT) comprises a channel area, a source, and a drain formed beside both sides of the channel area, and a gate formed above the channel area. 
   The channel area in the TFT comprises amorphous silicon or polycrystalline silicon. Polycrystalline silicon (poly-Si) generally has higher mobility than amorphous silicon (a-Si), and thus is advantageous for operating a TFT at high-speed. Amorphous silicon may be crystallized through annealing to obtain polycrystalline silicon, and some electrical characteristics of the channel area are determined by the grain size of the polycrystalline silicon. For example, if the grain size of the polycrystalline silicon is large, the mobility of the electrons becomes greater in the channel area. Thus, some electrical characteristics of the TFT are improved. 
   Excimer laser annealing (ELA) has been recently used to crystallize amorphous silicon. However, increasing the grain size is limited, i.e., it is difficult to obtain a grain size of 0.5 μm or more, and it is not easy to control uniformity of the grain size. 
   Accordingly, crystallization methods using sequential lateral solidification (SLS), an optical phase shift mask (OPSM), a pre-patterned laser beam mask (PLBM), or the like, have been suggested. However, the crystallization methods require an apparatus for accurately adjusting substrates and multi-laser beams. Thus, it is very difficult to apply the crystallization methods to a TFT process. 
   SUMMARY OF THE INVENTION 
   In an embodiment, there is provided a method of manufacturing a laterally crystallized semiconductor layer, the method comprising: forming a semiconductor layer on a substrate; irradiating a plurality of laser beams on the semiconductor layer; splitting the laser beams using a prism sheet comprising an array of a plurality of prisms, advancing the laser beams toward the semiconductor layer to alternately form first and second areas in the semiconductor layer so as to fully melt the first areas, wherein the laser beams are irradiated onto the first areas, and the laser beams are not irradiated onto the second areas; and inducing the first areas to be laterally crystallized using the second areas as seeds. 
   In another embodiment, each of the prisms comprises a first slope and a second slope, which refract the irradiated laser beams in different directions. 
   In another embodiment, the first slopes may refract the irradiated laser beams at an angle from about 30° to about 40° clockwise from the incident directions of the irradiated laser beams. 
   In another embodiment, the second slopes may refract the irradiated laser beams at an angle from about 30° to about 40° counterclockwise from the incident directions of the irradiated laser beams. 
   In another embodiment, facing slopes of two adjacent prisms selected from the plurality of prisms may refract and transmit laser beams so that the laser beams overlap with one another in the first areas. 
   According to another embodiment, there is provided a method of manufacturing a TFT, the method comprising: forming a semiconductor layer on a substrate; irradiating a plurality of laser beams onto the semiconductor layer; splitting the laser beams using a prism sheet comprising an array of a plurality of prisms, advancing the laser beams toward the semiconductor layer to alternately form first and second areas in the semiconductor layer so as to fully melt the first areas, wherein the laser beams are irradiated onto the first areas, and the laser beams are not irradiated onto the second areas; inducing the first areas to be laterally crystallized using the second areas as seeds so as to form a channel area between the second areas; sequentially forming a gate insulating layer and a gate electrode on the channel area comprising at least two sides; and doping dopant ions into the second areas to form a source and a drain beside two sides of the channel area. 
   According to another embodiment, laser beams can be easily split using a prism sheet, and positions of areas of the semiconductor layer on which the laser beams are to be irradiated can be easily controlled in a laser annealing process for crystallizing the semiconductor layer. Thus, since it is very easy to control a position of a laterally crystallized area, semiconductor devices can be easily and uniformly manufactured. Specifically, if the laser annealing process is performed using the prism sheet, all of the laser beams can penetrate the semiconductor layer. Thus, the laser beams cannot be lost. As a result, use efficiency of the laser beams can be further improved in the laser annealing process for crystallizing the semiconductor layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIGS. 1A through 1C  are cross-sectional views illustrating a method of manufacturing a laterally crystallized semiconductor layer according to an embodiment; 
       FIG. 2  is a scanning electron microscope (SEM) photograph illustrating an upper surface of the laterally crystallized semiconductor layer of  FIG. 1C ; 
       FIG. 3  illustrates results of a simulation performed on differences between laser intensities of areas of the laterally crystallized semiconductor layer of  FIG. 1B  on which laser beams are irradiated and areas of the laterally crystallized semiconductor layer on which laser beams are not irradiated; and 
       FIGS. 4A through 4C  are cross-sectional views illustrating a method of manufacturing a thin film transistor (TFT) according to an embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A method of manufacturing a laterally crystallized semiconductor layer and a method of manufacturing a thin film transistor (TFT) using the method will now be described in detail with reference to the accompanying drawings. In the drawings, the thickness of the layers and regions is exaggerated for clarity. 
   It will be understood that when an element is referred to as being “on”, “beside”, or “above” another element, it can be directly on, beside, or above the other element, or intervening elements may be present therebetween. 
   The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
   Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     FIGS. 1A through 1C  are cross-sectional views illustrating a method of manufacturing a laterally crystallized semiconductor layer according to an embodiment of the present invention. Referring to  FIG. 1A , a semiconductor layer  12  is formed on a substrate  10  using a deposition method such as a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method. The material for forming the semiconductor layer  12  is not limited. For example, the semiconductor layer  12  may be formed of silicon (Si), germanium (Ge), a compound of Si and Ge, or a group III-V semiconductor material. The semiconductor layer  12  may be formed in a crystal phase or an amorphous phase. The substrate  10  is not limited, and it may be a plastic substrate, a glass substrate, a quartz substrate, or the like. 
   Referring to  FIGS. 1B and 1C , laser beams are irradiated on the semiconductor layer  12 . The irradiated laser beams are split using a prism sheet  110  comprising a plurality of prisms  102 , and advance toward the semiconductor layer  12 . First and second areas  12   a  and  12   b  are alternatively formed in the semiconductor layer  12  through the splitting of the laser beams to selectively fully melt the first areas  12   a . Here, the laser beams are irradiated onto the first areas  12   a  but not irradiated onto the second areas  12   b . Lateral crystallization of the first areas  12   a  is induced using the second areas  12   b  as seeds to obtain a laterally crystallized area  12   c . The induction of the lateral crystallization will now be described in detail. 
   The first areas  12   a  are fully melted but the second areas  12   b  are not melted or less melted than the first areas  12   a  in a laser annealing process. Thus, thermal gradients and solidification velocities of the first areas  12   a  are different from those of the second area  12   b . Thus, nuclei are generated and grown at boundaries among the first and second areas  12   a  and  12   b , and the solidification velocities of the second areas  12   b  are faster than those of the first areas  12   a . Thus, a grain growth is directed from the boundaries among the first and second areas  12   a  and  12   b  toward centers of the first areas  12   a . As a result, the laterally crystallized area  12   c  can be obtained. 
   Each of the first and second areas  12   a  and  12   b  may be formed to a width between 0.5 μm and 20 μm. The width is an appropriate numeral range for easily performing lateral crystallization. Reference characters A and B denote the first and second areas  12   a  and  12   b , respectively. 
   The plurality of prisms  102  may be formed on a surface of an optical film  101  to manufacture the prism sheet  110 . The shape of the prism sheet  110  and the method of manufacturing the prism sheet  110  are well known in the art, and thus the prism sheet  110  can be easily manufactured. For example, the plurality of prisms  102  may be formed in a striped pattern. Each of the prisms of the plurality of prisms  102  may include first and second slopes,  102   a  and  102   b  respectively, which refract the irradiated laser beams in different directions. The first slopes  102   a  refract the irradiated laser beams at an angle from about 30° to about 40° clockwise from the incident directions of the irradiated laser beams. The second slopes  102   b  refract the irradiated laser beams at an angle from about 30° to about 40° counterclockwise from the incident directions of the irradiated laser beams. Thus, the laser beams are split. 
   The facing slopes of two adjacent prisms selected from the plurality of prisms  102  refract and transmit the laser beams so that the laser beams overlap with one another in the first areas  12   a . Thus, intensities of the laser beams irradiated onto the first areas  12   a  can be increased. Specifically, the laser beams irradiated from the second slope  102   b  of a first prism selected from the plurality of prisms  102  overlap with the laser beams irradiated from the first slope  102   a  of a second prism adjacent to the first prism. Thus, the overlapped laser beams may be irradiated onto the first areas  12   a . Intensities of the laser beams irradiated on the first areas  12   a  can be increased using the overlapping of the laser beams to shorten a time required for melting and laterally crystallizing the first areas  12   a.    
   The laser beams may be excimer laser beams or YAG laser beams. The excimer laser beams may be, for example, 308 nm xenon chloride (XeCl) laser beams. The semiconductor layer  12  has a high absorption coefficient with respect to these laser beams. If these laser beams are used, the semiconductor layer  12  may be easily heated. 
   The laser beams may be easily split using the prism sheet  110 , and positions of areas of the semiconductor layer  12  onto which laser beams are irradiated may be easily controlled in a laser annealing process of crystallizing the semiconductor layer  12 . Thus, since it is very easy to control a position of the laterally crystallized area  12   c , semiconductor devices can be easily and uniformly manufactured. Specifically, if a laser annealing process is performed using the prism sheet  110 , all of the laser beams may penetrate the semiconductor layer  12 . Thus, the laser beams are not lost. Therefore, use efficiency of the laser beams can be improved in the laser annealing process for crystallizing the semiconductor layer  12  when compared to the prior art. Also, since the prism sheet  110  is very low in cost, the cost for manufacturing a semiconductor device can be lowered according to the above method. 
   In addition, the laterally crystallized semiconductor layer  12  may be manufactured using a simple, easy process. Since the semiconductor layer  12  is formed in a lateral grain structure having a size of 1 μm or more, the semiconductor layer  12  has good electron mobility and good electrical characteristics. Thus, if a semiconductor device such as TFT is manufactured using the laterally crystallized semiconductor layer  12 , the performance of the semiconductor device can be more improved when compared to the prior art. 
     FIG. 2  is a scanning electron microscope (SEM) photograph illustrating an upper surface of the laterally crystallized semiconductor layer  12  of  FIG. 1C . 
     FIG. 3  illustrates results of a simulation performed on differences between laser intensities of areas of the laterally crystallized semiconductor layer  12  of  FIG. 1B  onto which the laser beams are irradiated and areas of the laterally crystallized semiconductor layer onto which the laser beams are not irradiated. 
     FIGS. 4A through 4C  are cross-sectional views illustrating a method of manufacturing a TFT according to an embodiment. 
   Processes of  FIGS. 4A through 4C  are similar to those illustrated in reference to  FIGS. 1A through 1C , and thus their repeated descriptions will be omitted herein. 
   Referring to  FIG. 4A , a gate insulating layer  22  and a gate electrode  24  are sequentially formed in the laterally crystallized area  12   c  of  FIG. 1C , which is referred to as a channel area in a process of manufacturing a TFT. The material and method for forming the gate insulating layer  22  are well known in the art, and thus their detailed descriptions will be omitted herein. The material and method for forming the gate electrode  24  are also well known, and thus their detailed descriptions will be omitted herein. For example, the gate insulating layer  22  may be formed of silicon dioxide (SiO 2 ) using a PVD or CVD method. The gate electrode  24  may also be formed of at least one material selected from the group consisting of nickel (Ni), aluminum (Al), platinum (Pt), gold (Au), cobalt (Co), iridium (Ir), chromium (Cr), molybdenum (Mo), tungsten (W), rhodium (Rh), or the like, or a combination comprising at least one of the foregoing materials, using a PVD or CVD method. 
   Referring to  FIGS. 4B and 4C , dopant ions are doped into the second areas  12   b  positioned beside both sides of the channel area  12   c  to form a source  12   s  and a drain  12   d . The dopant ions may be selected from a conductive material group consisting of p-type dopants, n-type dopants, metal ions, and the like. An ion implantation method may be suitable as a method of doping the dopant ions but is not specifically limited. Specifically, the gate electrode  24  may be used as a mask in the ion implantation process, and thus an additional mask is not required. Thus, dopant ions may be easily selectively doped into the second areas  12   b , which are not covered with the gate electrode  25 , without the additional mask. An excimer laser annealing (ELA) or furnace annealing process may be additionally performed after the ion implantation process in order to activate dopant ions implanted into the source  12  and the drain  12   d . The ELA or furnace annealing process is well known in the art, and thus its detailed description will be omitted herein. 
   A laterally crystallized semiconductor layer can be manufactured using a simple, easy process. The laterally crystallized semiconductor layer can have a lateral grain structure having a size of about 1 μm or more and thus good electron mobility and good electrical characteristics. Thus, if a semiconductor device such as a TFT is manufactured using the laterally crystallized semiconductor layer, the performance of the semiconductor device can be further improved. 
   Also, laser beams can be easily split using a prism sheet, and positions of areas of the semiconductor layer onto which the laser beams are to be irradiated can be easily controlled in a laser annealing process for crystallizing the semiconductor layer. Thus, since it is relatively easy to control a position of a laterally crystallized area, semiconductor devices can be easily and uniformly manufactured. Specifically, if the laser annealing process is performed using the prism sheet, all of the laser beams can penetrate the semiconductor layer. Thus, the laser beams cannot be lost. As a result, use efficiency of the laser beams can be further improved in the laser annealing process for crystallizing the semiconductor layer. In addition, since the prism sheet is relatively low-priced, cost for manufacturing the semiconductor device can be lowered. 
   While the present invention has been specifically shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Technology Category: 5