Patent Publication Number: US-2012034794-A1

Title: Enhancing the width of polycrystalline grains with mask

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of 12/644,273, filed Dec. 22, 2009 which is a continuation of Ser. No. 11/373,773, filed Mar. 10, 2006, and granted under U.S. Pat. No. 7,638,728, issued Dec. 29, 2009 which is a continuation of International Application Serial No. PCT/US04/030326, filed Sep. 16, 2004, published Mar. 31, 2005, and which claims priority to U.S. Provisional Application No. 60/503,437, filed on Sep. 16, 2003, each of which are incorporated by reference in their entireties herein and from which priority is claimed. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor processing techniques, and more particularly, techniques for fabricating semiconductors suitable for use as thin-film transistor (“TFT”) devices. 
     BACKGROUND INFORMATION 
     During the past several years, sequential lateral solidification (“SLS”) techniques have been developed to generate quality large grained polycrystalline thin films, e.g., silicon films, having a substantially uniform grain structure. For example, in U.S. Pat. No. 6,322,625, issued to Im and U.S. patent application Ser. No. 09/390,537 (the “537 application”), the entire disclosures of which are incorporated herein by reference, particularly advantageous apparatus and methods for growing large grained polycrystalline or single crystal silicon structures using energy-controllable laser pulses and small-scale translation of a silicon sample to implement sequential lateral solidification have been described. As described in these patent documents, at least portions of the semiconductor film on a substrate are irradiated with a suitable radiation pulse to completely melt such portions of the film throughout their thickness. 
     In order to increase throughput, continuous motion SLS processes have been proposed. Referring to FIG.  1 ., such system preferably includes an excimer laser  110 , an energy density modulator  120  to rapidly change the energy density of a laser beam  111 , a beam attenuator and shutter  130 , optics  140 ,  141 ,  142  and  143 , a beam homogenizer  144 , a lens and beam steering system  145 ,  148 , a masking system  150 , another lens and beam steering system  161 ,  162 ,  163 , an incident laser pulse  164 , a thin film sample on a substrate  170  (e.g., a silicon thin film) a sample translation stage  180 , a granite block  190 , a support system  191 ,  192 ,  193 ,  194 ,  195 ,  196 , and a computer  100  which manages X and Y direction translations and microtranslations of the film sample and substrate  170 . The computer  100  directs such translations and/or microtranslations by either a movement of a mask within masking system  150  or by a movement of the sample translation stage  180 . As described in U.S. Pat. No. 6,555,449 issued to Im, the entire disclosure of which is incorporated herein by reference, the sample  170  may be translated with respect to the laser beam  149 , either by moving the masking system  150  or the sample translation stage  180 , in order to grow crystal regions in the sample  170 . 
       FIG. 2  depicts the mask used in the continuous motion SLS process as described in International Publication No. 02/086954 (the “&#39;954 Publication”), the entire disclosure of which is incorporated herein by reference. This mask is divided into a first mask section  20  and a second mask section  22 . The first mask section  20  can be used for the first pass under the laser. The second mask section  22  is used on the second pass. The first mask section  20  may have corresponding opaque areas  24  and clear areas  25 . Throughout the specification of the &#39;954 Publication and the present application, “opaque areas” are referred to as areas of the mask that prevent associated regions of a thin film sample irradiated by beams passed through the mask from being completely melted throughout its thickness, while “clear areas” are areas of the mask that permit associated regions of a thin film sample irradiated by beams passed through the mask to be completely melted throughout its thickness. The clear areas can be actual holes in the mask or may be sections of the mask that allow the sample behind it to be completely melted throughout its thickness. The second mask section  22  also has corresponding opaque areas  26  and clear areas  27 . The opaque areas  24 ,  26  of both sections  20 ,  22  are areas that prevent radiation from a laser source from passing through to the sample. The shape of these clear areas, both in the second mask section  22  and in the first mask section  20 , generally have a shape of “straight slits.” The array of the clear areas  24  in the first mask section  20  are generally staggered from the array of clear areas  26  in the second mask section  22 . As indicated above, the clear areas  25 ,  27  of both sections allows radiation to pass through to melt the sample below the surface of the mask. 
       FIG. 3  depicts the radiation pattern passing through the mask of  FIG. 2  during processing of the film. The first pattern section  30  shows the pattern that results after the first pass of the irradiation by the pulses shaped using the mask. The pulse passing through the mask may have a first portion  34  that corresponds to the pattern of the first mask section  20 . The clear areas of the first mask section  20  in  FIG. 2  allow the radiation to pass therethrough, and melt the thin film throughout its thickness, thus resulting in a first melted region and an unmelted region  44  (see  FIG. 4 ) after the first pass of the sample processing. When the mask is translated in the direction of the arrow  33 , the second pattern section  32  of  FIG. 3  with the radiation pattern results after the second pass of processing the sample. The pulse passing through the mask may have a second portion  36  that corresponds to the pattern of the second mask section  22 . The clear areas of the second mask section  22  of the mask in  FIG. 2  allow the radiation to pass therethrough, and again melt the thin film throughout its thickness. This results in a second melted region and an tunnelled region over the grain boundary  45  (see  FIG. 4 ). 
       FIG. 4  depicts the resulting crystalline structure that is produced using the mask of  FIG. 2 . The first structure section  40  includes the structure  41  that results after the first pass of the sample processing. The opaque areas of the first mask section  20  of the mask of  FIG. 2  prevent the associated regions  44  from completely melting. A grain boundary  45  in the direction of the crystalline structure forms approximately halfway between the associated regions  44 . The second structure section  42  includes the crystalline structure  48  that results after the second pass of the sample processing. The grain boundary  45  from the first pass is not removed, while the individual grains expand in length until they meet one another, because all areas are exposed to the laser during the second pass except the area that corresponds to the grain boundary  45 . Thus, the grain length  46  (parallel to the direction of the crystalline structure) may be controlled by the properties and slit patterns of the mask of  FIG. 2 . The width  47  of the grain (perpendicular to the direction of the crystalline structure), however, is not very easily controlled. Indeed, it may be primarily dependent on the characteristics of the film. 
     As noted above, the aforementioned SLS techniques typically employ a straight slit mask pattern. This allows for the ease of control of the grain length (in the direction of the primary crystallization). In such case, the perpendicular grain spacing may be dependent on the properties of the film, and thus is not very easily manipulated. While the tailoring of the shaped areas to manipulate the microstructure has been employed in other SLS methods and systems, such as with the use of chevron-shaped openings in a mask, the techniques associated therewith may produce narrow grain areas. Accordingly, there is a need to control grain length in the thin film, as well as increase the area in which a smaller number of grains are present. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-mentioned problems by providing a mask having a row of point-type areas (e.g., diamond and/or dot patterned opaque regions) provided thereon. Such mask pattern that uses closely spaced circular or diamond-shaped areas is utilized in lieu of the straight slits in at least a portion of the mask in order to produce a microstructure with wider grain areas. Using the mask of this configuration according to the present invention advantageously affects a melt interface curvature on the evolution of grain boundaries to favorably increase the perpendicular grain boundary spacing. 
     According to one exemplary embodiment of the present invention, a masking arrangement, system and process are provided for processing a thin film sample, e.g., an amorphous silicon thin film, into a polycrystalline thin film. In particular, a mask can be utilized which includes a first section having at least one opaque areas arranged in a first pattern, e.g., diamond areas, oval areas, and/or round areas. The first section may be configured to receive a beam pulse thereon, and produce a first modified pulse when the beam pulse is passed therethrough. The first modified pulse may include at least one first portion having a pattern that corresponds to the first pattern of the first section. When the first portion is irradiated on the sample, at least one first region of the sample is prevented from being completely melted throughout its thickness. The mask may also includes a second section associated with the first section, with the second section including a further area arranged in a second pattern. The second section may be configured to receive a further beam pulse thereon, and produce a second modified pulse when the further beam pulse is passed therethrough. The second modified pulse can include at least one second portion having a pattern that corresponds to the second pattern of the second section. When the second portion is irradiated on the sample, at least one second region of the sample irradiated by the second portion is completely melted throughout its thickness. In addition, when the first region is irradiated by the second modified pulse, the second portion of the second modified pulse completely melts the first region throughout its thickness. 
     The accompanying drawings, which are incorporated and constitute part of this disclosure, illustrate preferred embodiments of the invention and serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional diagram of a conventional system for performing semiconductor processing including sequential lateral solidification of a thin film; 
         FIG. 2  is a top view of a conventional mask; 
         FIG. 3  is a schematic top view showing the radiation pattern associated with the mask of  FIG. 2 ; 
         FIG. 4  is a schematic top view showing grain spacing in the processed thin film that results from use of the mask of  FIG. 2 ; 
         FIG. 5  is a top view of a mask according to an exemplary embodiment according to the present invention; 
         FIG. 6  is a top view of an irradiation pattern generated by the mask of  FIG. 5 ; 
         FIG. 7  is a top view of a grain spacing produced by the mask of  FIG. 5 ; 
         FIG. 8  is a top view of a mask according to an exemplary embodiment according to the present invention; 
         FIG. 9  is a top view of a grain spacing produced by the mask of  FIG. 8 ; and 
         FIG. 10  is a flow diagram illustrating the steps according to the present invention implemented by the system of  FIG. 1 . 
     
    
    
     Throughout the Figs., the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the present invention will now be described in detail with reference to the Figs., it is done so in connection with the illustrative embodiments. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 5-7 , a presently preferred embodiment of the present invention will be described. This embodiment utilizes an exemplary mask pattern according to the present invention which uses preferably closely spaced circular or diamond-shaped areas in order to produce a microstructure with wider areas of limited number of grains provided therein. Those skilled in the art should understand that the systems, methods, and masks according to the present invention are applicable not only to single-shot motion SLS processes and systems, but also to thin films that have been processed with n-shot and 2n-shot SLS techniques. 
     Referring to  FIG. 5 , the mask which may be used in an exemplary embodiment of the present invention may be divided into a first mask section  50  and a second mask section  52 . Alternatively, two separate masks may be used instead of separate sections in one mask. The first mask section  50  may be used to process a selected area of the thin film as an initial shot. The second mask section  52  may be used as a second shot which immediately follows the first shot. The first mask section  50  may have corresponding opaque areas  54  and clear areas  55 . The second mask section  52  may also have corresponding opaque areas  56  and clear areas  57 . While the shape of these opaque areas in the second mask section  52  may be in the shape of traditional “straight slits” as described herein above in  FIGS. 2-4 , the opaque areas in the first mask section  50  are preferably provided in rows of diamonds, circular shaped, and/or oval shaped areas. The array of opaque areas  54  in the first mask section may be staggered from the array of opaque areas  56  in the second mask section. 
       FIG. 6  depicts the radiation pattern that may be shaped by the mask of  FIG. 5  upon passing a beam pulse therethrough. In particular, the first pattern section  60  includes the pattern that may result upon the first shaped pulse impacting the corresponding portions on the sample. A pulse shaped by the mask may have a first portion  64  that corresponds to the pattern of the first mask section  50 . The opaque mask areas  54  of the first mask section  50  in  FIG. 5  may block the radiation from passing through to the thin film sample, and thus result in a first unmelted region  74  in the first pass (see  FIG. 7 ). As shown in  FIG. 7 , the grains grow outwardly from the unirradiated areas because they seed the melted regions upon the resolidification of the melted areas. Thus, the width of the resolidified regions is based on the grain growth into two opposite directions. This is because the grains grow outward from the unmelted regions, e.g., in the opposite directions thereof. Parallel grain boundaries  75 , as shown in  FIG. 7 , are formed when the grain growth from neighboring regions produced by the pattern of the first mask section  50  impact one another. In this manner, approximately horizontal borders between resolidified regions may be formed. When the mask is shifted in the direction of the arrow  63 , the beam is translated and/or the sample may be translated in the opposite direction of the arrow  63  by the translation stage, the second pattern section  62  of  FIG. 6  shows the radiation pattern that may result after the second shot irradiates the corresponding portions of the thin film. In particular, a pulse passing through the mask may have a second portion  66  that corresponds to the pattern of the second mask section  52 . The opaque areas  56  of the second mask section  52  of the mask in  FIG. 5  may prevent the sample irradiated by pulses that are shaped by the mask from being completely melted throughout its thickness. This may result in a generation of second melted region, and an unmelted region which is provided over the unmelted grain boundary  75  (see  FIG. 7 ). 
       FIG. 7  depicts the resulting crystalline structure that may develop using the mask of  FIG. 5 . The first structure section  70  includes a structure  71  that may be produced after irradiation thereof by the first beam pulse. The opaque areas of the first section of the mask of  FIG. 5  prevent the associated regions  74  from completely melting. A parallel grain boundary  75  as well as a perpendicular grain boundary  73  may be formed approximately halfway between the associated regions  74 . The second structure section  72  includes a crystalline structure that may be formed after the irradiation by the second beam pulse. The crystal grained structures in this section  72  may grow radially outward from the associated regions  74 . The parallel grain boundary  75  as well as the perpendicular grain boundary  73  produced by the irradiation with the first pulse may remain in tact while the sample is exposed to the second beam pulse shaped by the second section  52  of the mask. Thus, the grain length  76  (parallel to the direction of the crystalline structure) as well as the grain width  77  (perpendicular to the direction of the crystalline structure) may be controllable by the properties of the mask (e.g. pattern), rather than merely being dependent on the characteristics of the film. The grain width  77  formed using the embodiment of the mask according to the present invention may be wider than the grain width  47  formed with a straight slit mask pattern, and can be controlled using the mask pattern. 
     Referring to  FIG. 8 , a mask that may be used in an exemplary embodiment of the present invention may be divided into a first mask section  80  and a second mask section  82 . Alternatively, two separate masks may be used instead of separate sections in one mask. The first mask section  80  may be used to process a selected area of the thin film as an initial shot. The second mask section  82  may be used as a second shot which immediately follows the first shot. The first mask section  80  may have corresponding opaque areas  84  and clear areas  85 . The second mask section  82  may also have corresponding opaque areas  86  and clear areas  87 . While the shape of the opaque areas may be in both the first and second mask section may be any shape as described herein above in  FIGS. 2-4 . The opaque areas in the first mask section  85  are preferably provided in rows of diamonds, circular shaped, dot shaped and/or oval shaped areas. In one exemplary embodiment, as shown in  FIG. 8 , the opaque areas of both the first and second mask sections are dots. Optionally, the array of opaque areas  84  in the first mask section maybe staggered from the array of opaque areas  86  in the second mask section. 
       FIG. 9  depicts the resulting crystalline structure that may develop using the mask of  FIG. 8 . The first structure section  90  includes a structure  91  that may be produced after irradiation thereof by the first beam pulse. The opaque areas of the first section of the mask of  FIG. 8  prevent the associated regions  94  from completely melting. A parallel grain boundary  95  as well as a perpendicular grain boundary  93  may be formed approximately halfway between the associated regions  94 . crostructures. In one exemplary embodiment, the opaque areas of the second section  86  may be located on the edge of two islands grown from regions produced by the first pulse. In another exemplary embodiment, the opaque areas of the second section  86  may be located on the corner of four islands grown from opaque areas of the first region  84 . 
     Referring next to  FIG. 10 , the steps executed by a computer to control the crystal growth process implemented with respect to  FIG. 7  will be described.  FIG. 8  is a flow diagram illustrating the basic steps implemented in the system of  FIG. 1 . The various electronics of the system shown in  FIG. 1  may be initialized  1000  by the computer to initiate the process. A thin film sample, e.g., a silicon thin film, may then be loaded onto the sample translation stage  1005 . It should be noted that such loading may be either manual or robotically implemented under the control of computer  100 . Next, the sample translation stage may be moved into an initial position  1015 , which may include an alignment with respect to reference features on the sample. The various optical components of the system may be focused  1020  if necessary. The laser may then be stabilized  1025  to a desired energy level and repetition rate, as needed to fully melt the sample in accordance with the particular processing to be carried out. If necessary, the attenuation of the laser pulses may be finely adjusted  1030 . 
     Next, the shutter maybe opened  1035  to expose the sample to a single pulse of irradiation through a masking arrangement including at least one of diamond shaped areas, oval shaped areas, and round shaped areas, and accordingly, to commence the sequential lateral solidification process. The sample may be translated in the horizontal direction  1040 . The shutter is again opened  1045  exposing previously unmelted regions to a single pulse of irradiation. The process of sample translation and irradiation  1040 ,  1045  may be repeated  1060  to grow the polycrystalline region. 
     Next, if other regions on the sample have been designated for crystallization, the sample is repositioned  1065 ,  1066  and the crystallization process is repeated on the new region. If no further regions have been designated for crystallization, the laser is shut off  1070 , the hardware is shut down  1075 , and the process is completed  1080 . Of course, if processing of additional samples is desired or if the present invention is utilized for batch processing, steps  1005 ,  1010 , and  1035 - 1065  can be repeated on each sample. 
     The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous systems and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the invention.