Abstract:
Substrates having modified effective thermal conductivity for use in the sequential lateral solidification process are disclosed. In one arrangement, a substrate includes a glass base layer, a low conductivity layer formed adjacent to a surface of the base layer, a high conductivity layer formed adjacent to the low conductivity layer, a silicon compound layer formed adjacent to the high conductivity layer, and a silicon layer formed on the silicon compound layer. In an alternative arrangement, the substrate includes an internal subsurface melting layer which will act as a heat reservoir during subsequent sequential lateral solidification processing.

Description:
NOTICE OF GOVERNMENT RIGHTS 
     The U.S. Government has certain rights in this invention pursuant to the terms of the Defense Advanced Research Project Agency award number N66001-98-1-8913. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention. 
     The present invention relates to techniques for processing of semiconductor films, and more particularly to techniques for processing semiconductor films on glass or other substrates. 
     II. Description of the Related Art. 
     Techniques for fabricating large grained single crystal or polycrystalline silicon thin films using sequential lateral solidification are known in the art. For example, in U.S. patent application Ser. No. 09/390,537, the contents of which are incorporated by reference herein and which application is assigned to the common assignee of the present application, 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 are disclosed. Using the sequential lateral solidification technique, low defect density crystalline silicon films can be produced on those substrates that do not permit epitaxial regrowth, upon which high performance microelectronic devices can be fabricated. 
     The effectiveness with which sequential lateral solidification can be implemented depends on several factors, the most important of which corresponds to the length of lateral crystal growth achieved per laser pulse. Such lateral crystal growth depends on several parameters, including the duration of the laser pulses, film thickness, substrate temperature at the point of laser pulse irradiation, the energy density of the laser pulse incident on the substrate, and the effective thermal conductivity of the substrate. In particular, if all other factors are kept constant, reducing is the thermal conductivity of the substrate will have the effect of increasing lateral crystal growth. 
     While there have been attempts to utilize low thermal conductivity materials, such as porous glass, in connection with sequential lateral solidification for the purpose of enhancing lateral crystal growth, such attempts have not achieved commercially viable results. For example, when a porous glass layer is used under a silicon film in the sequential lateral solidification process densification, and subsequent physical distortion, of such glass has been observed. Accordingly, there exists a need in the art for a technique for fabricating substrates having a modified effective thermal conductivity in order to optimize the sequential lateral solidification process. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide substrates having modified effective thermal conductivity which can be later used in an optimized sequential lateral solidification process. 
     A further object of the present invention is to provide substrates having modified effective thermal conductivity. 
     Still a further object of the present invention is to provide substrates having a directionally optimized effective thermal conductivity. 
     Yet a further object of the present invention is to provide multi layer substrates where one or more of the subsurface layers act as a heat reservoir in order to optimize the effective thermal characteristics of the substrate. 
     In order to achieve these objectives as well as others that will become apparent with reference to the following specification, the present invention provides a substrate having modified effective thermal conductivity for use in the sequential lateral solidification process. The substrate includes a base layer, e.g., glass, a low conductivity layer formed adjacent to a surface of the base layer, a high conductivity layer formed adjacent to the low conductivity layer, and a silicon layer formed on the high conductivity layer. 
     In a preferred arrangement, the low conductivity layer is porous glass, and is in the range of 5,000 Angstroms to 2 microns thick. The high conductivity layer may be a metal, and should be sufficiently thin so as to not increase the overall vertical conductivity of the substrate, preferably in the range of 50 to 5,000 Angstroms thick. 
     An intermediate silicon compound layer is preferably formed between the silicon layer and the high conductivity layer. The silicon compound may be silicon dioxide, and should be sufficiently thick to prevent diffusion of impurities from the high conductivity layer. It is preferred that the silicon compound layer is in the range of 200 to 2,000 Angstroms thick. 
     In an alternative arrangement, the present invention provides a substrate having modified effective thermal conductivity for use in the sequential lateral solidification process, wherein the high conductivity layer is replaced by an internal subsurface melting layer. In this arrangement, the substrate includes a base layer, a low conductivity layer formed adjacent to the base layer, a subsurface melting layer having a melting point which is less than that of silicon and formed adjacent to the low conductivity layer, a silicon compound layer formed adjacent to the subsurface melting layer, and silicon layer formed on the silicon compound layer. 
    
    
     The accompanying drawings, which are incorporated and constitute part of this disclosure, illustrate a preferred embodiment of the invention and serve to explain the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a substrate in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is an illustrative diagram showing lateral solidification of silicon; 
     FIGS. 3 a  and  b  are graphs showing the relationship between the temperature of solidifying silicon and the position of such silicon around a liquid to solid interface; and 
     FIG. 4 is a schematic diagram of a substrate in accordance with a second preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a preferred embodiment of the present invention will be described. As shown in FIG. 1, the substrate  100  includes a bulk glass plate layer  110 , a low conductivity layer  120 , a high conductivity layer  130 , a silicon dioxide layer  140 , and a semiconducting film layer  150 . The multilayer structure of substrate  100  may be fabricated by any combination of thin film formation techniques, such as physical or chemical vapor deposition,,electrochemical deposition, or spin coating. 
     The low conductivity layer  120  may be porous glass or a polymer film layer. In addition, the layer  120  must have a conductivity which is less than the glass plate  110  and sufficiently thick so that the glass plate layer  110  will not participate when the substrate  100  is used in later processing. Layer  120  will be in the order of 5,000 Angstroms to 2 microns thick. 
     The high conductivity layer  130  may be a metallic layer such as copper or aluminum. The high conductivity layer must have a conductivity which is greater than that of the glass plate  110 , and sufficiently thin so as to not increase the overall vertical conductivity of the substrate  100 , i.e., conductivity in the direction which crosses layers  110 ,  120 ,  130 ,  140 ,  150 . Typically, layer  130  will be in the order of 50 to 5,000 Angstroms thick. 
     The silicon dioxide layer  140  should be sufficiently thick to prevent potential diffusion of unwanted impurities from the underlying layer  130  to the silicon cap  150 . The Layer  140  will be in the order of 200 to 2,000 Angstroms thick. Alternatively, the layer  140  may be fabricated from silicon nitride or a mixture of silicon dioxide and silicon nitride. 
     Alternatively, the high conductivity layer  130  may be formed from a material which is electrically and chemically compatible with the semiconducting film layer  150 , such as diamond, In this case, the silicon dioxide layer  140  may be omitted, with the semiconducting film layer.  150  formed directly on the high conductivity layer  130 . 
     Finally, the top semiconducting film layer may be either be amorphous, microcrystalline or polycrystalline silicon, or a mixture thereof. Typically, layer  150  will be in the order of 200 to 2,000 Angstroms thick. 
     When fabricated as described above, the substrate  100  will exhibit either a reduced overall effective thermal conductivity, or a reduced effective thermal conductivity in the vertical direction. Having such a modified thermal conductivity, the substrate  100  is highly useful in order to improve lateral crystal growth in the lateral solidification process, as will be now described. 
     Referring next to FIG. 2, the lateral solidification of silicon in accordance with the above-noted sequential lateral solidification technique is illustrated. FIG. 2 represents a cross sectional view of the silicon film  150  as it may appear during lateral solidification, with liquid silicon  210  solidifying into crystalline silicon  220  at a velocity Vg. As the liquid silicon solidifies through the motion of the interface  230 , latent heat is released at the interface  230  due to reduction in enthalpy associated with the liquid to solid transition. The lateral solidification will continue along moving boundary  230  until either impingement of the interface with another similar interface, or until nucleation. 
     Referring next to FIG. 3 a , a graphs showing the relationship between the temperature of solidifying silicon and the position of such silicon around a liquid to solid interface is shown, where T bulk  represents the temperature of the bulk liquid silicon as it cools, T int  represents the temperature of the silicon as the interface  230 , and T mp  represents the melting temperature of silicon. As those skilled in the art will appreciate, the temperature of T int  will impact the growth rate of the forming crystal, with a lower temperature leading to a faster growth rate. Likewise, when T bulk  reaches a certain temperature range, random nucleation will commence, ceasing the crystal growth process. 
     Referring to FIG. 3 b , two possible temperature profiles for solidifying silicon are shown, at a time t after laser irradiation. The temperature profile  310  represents a poor temperature profile, as the high interface temperature will cause slow lateral solidification, and the low temperature in the region away from the interface  230  will cause the temperature of those regions of liquid silicon to drop below the nucleation temperature range, ΔT N . In contrast the temperature profile  320  represents a optimal temperature profile, with a lower interface temperature causing more rapid lateral solidification, and a less cooling in the liquid silicon away from the interface  230  such that the temperature remains above the nucleation temperature range for a loner time., 
     Referring next to FIG. 4, a substrate in accordance with a second preferred embodiment of the present invention is now described. As shown in FIG. 4, the substrate  400  includes a bulk glass plate layer  410 , a low conductivity layer  420 , a subsurface melting layer  430 , a silicon dioxide layer  440  and a semiconductor layer  450  made from a predetermined semiconductor material. The low conductivity layer  420 , a silicon dioxide layer  440  and semiconductor layer  450  may be fabricated as described above in connection with substrate  100  by any combination of thin film formation techniques, such as physical or chemical vapor deposition, electrochemical deposition, or spin coating. 
     The subsurface melting layer  430  must have a melting point which is less than or equal to that of the predetermined semiconductor material, and preferably should exhibit an increased conductivity after melting. In addition, it is highly preferable to use a material having a high latent heat for the melting layer  430 , such as Silicon Germanium alloy. A 1000 Angstrom thick layer of Silicon Germanium alloy would be suitable for melting layer  430  Alternatively, an approximately 1000 Angstrom thick layer of certain metals such as Aluminum or Copper could be used for melting layer  430 . 
     When fabricated as described above, the substrate  400  will exhibit either a reduced overall effective thermal conductivity, or a reduced effective thermal conductivity in the vertical direction. When used in the sequential lateral solidification process, the melting layer  430  will partially or completely melt, thereby storing heat. Later, as the melting layer solidifies, heat will be released through the phase transformation from liquid to solid, thereby preventing rapid cooling of the overlying silicon layer  450 , and delaying nucleation. Thus, as shown in FIG. 3 b , the solidification of the melting layer  430  will have the effect of moving the temperature profile of the solidifying silicon layer up from profile  310  to profile  320  in the regions away from the boundary  230 . With such a modified thermal conductivity, the substrate  400  is likewise highly useful in order to improve lateral crystal growth in the lateral solidification process. 
     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. For example, the silicon layer  150 ,  450  may be replaced by other semiconductors such Germanium, Silicon Germanium, Gallium Arsenide, or Gallium Nitride, with, in the case of the second embodiment, suitable modifications to the melting layer  430 . Likewise, other metals may be used for the high conductivity layer  130 . Moreover, the high and low conductivity layers may be either a single unitary layer, or consist of multiple sub-layers. 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.