Abstract:
A method for suppressing the formation of leakage-promoting defects in a crystal lattice following dopant implantation in the lattice. The process provides a compressive layer of atoms, these atoms having a size greater than that of the lattice member atoms. The lattice is then annealed for a time sufficient for interstitial defect atoms to be emitted from the compressive layer, and in that manner energetically stable defects are formed in the lattice at a distance from the compressive layer.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of semiconductor fabrication. In particular, it relates to the fabrication of field effect transistors (FETs), involving the formation of semiconductor materials of selected conductivity, carried on by implantation of dopants. 
     Fabrication of metal oxide semiconductor (MOS) FETs requires the formation of source and drain regions in a substrate of generally pure silicon (Si). The Si is provided in the form of a wafer, grown as a single crystal. Zones of the Si lattice are converted into regions of N or P conductivity by the addition of donor-type dopants, such as arsenic, for N regions, and acceptor-type dopants, such as boron, for P regions. These dopants are generally introduced by ion bombardment, in which ionized dopant atoms are energized and fired at the lattice, penetrating the crystal structure to a depth largely dependent on the bombardment energy and the ion mass. 
     It can be immediately gathered that such bombardment introduces crystal damage, in which lattice atoms are knocked out of lattice sites, while at the same time a certain number of the newly-introduced atoms will likewise come to rest in positions outside the lattice positions. Such out-of-position phenomena are termed defects. A vacant lattice site is termed a vacancy defect, while an atom located at a non-lattice site is referred to as an interstitial defect. The restorative method generally employed in the art consists of annealing the crystal, applying heat to the lattice to mildly energize the atoms, allowing them to work themselves back into the lattice structure, which provides the arrangement having the lowest overall energy level. 
     SUMMARY OF THE INVENTION 
     An aspect of the claimed invention is a method for suppressing the formation of leakage-promoting defects in a crystal lattice following dopant implantation in the lattice. The process provides a compressive layer of atoms, these atoms having a size greater than that of the lattice member atoms. The lattice is then annealed for a time sufficient for interstitial defect atoms to be emitted from the compressive layer, and in that manner energetically stable defects are formed in the lattice at a distance from the compressive layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates manner in which the problem of stress-induced defect formation occurs in a semiconductor substrate. 
         FIG. 2  schematically depicts the solution of the claimed invention. 
         FIG. 3  illustrates the mechanism by which the method of the presently claimed invention suppresses defects in a semiconductor substrate. 
         FIG. 4  depicts an embodiment of a semiconductor formed in accordance with the claimed invention. 
         FIG. 5  depicts an alternate embodiment of a semiconductor formed in accordance with the claimed invention. 
         FIG. 6  depicts a further alternate embodiment of a semiconductor formed in accordance with the claimed invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. 
     The problem addressed by the present disclosure is seen in  FIG. 1 , which depicts a typical MOSFET  100  after undergoing ion implantation. The transistor is formed on a silicon substrate  101  and includes source  102 , drain  104  and gate  106 . The depletion layer  108  adjacent each electrode and extending across the channel between the source and drain, is well known in the art. The depicted cell is one member of an array that can encompass millions of cells, as known in the art, and the cell is separated from other members of the array by Shallow Trench Isolators (STI)  109 , filled with dielectric fill material. 
     The source and drain are formed in the silicon substrate by the implantation of dopants, as known in the art, and the implantation process produces defects in the lattice, which defects are addressed by annealing the substrate after implantation. While most defects are removed by annealing, some will coalesce into larger defects  112 , often in the form of dislocation loops or area defects. These defects are generally collections of interstitial silicon atoms, knocked out of their lattice positions by dopant atoms. It has been found that large defects  112  tend to form more readily in portions of the substrate that either have no mechanical stress on the lattice, or preferably, where the lattice is subjected to tensile stress. For example, locations where the lattice passes around a convex point in an underlying strained structure, such as around corners in the SiGe source, SiGe drain and STI&#39;s, tend to favor the accumulation of defects, resulting in the situation seen in  FIG. 1 , where defects  112  are seen at the corners of the source and drain as well as the corners of the STI&#39;s. 
       FIG. 2  illustrates an analogy to the mechanism at work in this situation. In each portion of the drawing, the lattice  200  is represented as having lattice sites, corresponding to the peaks in the figure, where an interstitial atom  201  can come to rest in between the lattice atoms, requiring some expenditure of energy to move to another interstitial position. In the upper portion of the drawing, the energy required to move to either side is identical, corresponding to the situation in an unstressed lattice. Probabilities of moving either left or right in the drawing are equal. The bottom portion of the drawing, however, shows one end of the line is higher than the other, analogous to the situation in which one portion of the lattice is subjected to compressive stress and the other is relaxed. Here, movement “uphill” toward the compressive stress clearly requires more energy than movement “downhill” toward the relaxed portion of the lattice. Thus, movement toward relaxed portions of the lattice, such as locations where the lattice is stressed by bending around a corner, are energetically favorable and will tend to collect more defects than compressively stressed areas. 
     A conceptual view of the operation of the claimed invention is seen in  FIG. 3 . There, along with the implantation of dopants, a compressive layer  103  of atoms having a larger size than silicon, being higher on the periodic table, is implanted in the substrate. Germanium, lying directly below silicon in the periodic table, is a preferred element for this purpose. That layer produces an area of higher compression, causing a net migration of defects away from the compressive layer, indicated by arrows  105 . After annealing, the structure formed by this process contains a number of large defects  112 , in the form of area, line or dislocation loop defects, but all such defects occur outside the compressive layer. 
       FIG. 4  illustrates an embodiment of the presently claimed invention in a transistor  100 , with elements as set out above in connection with  FIG. 1 . Here, however, a compressive layer  103  of germanium is formed along with the dopant atoms. The germanium can be co-implanted—implanted at the same time—along with the dopants, or a layer of Ge can be epitaxially grown during formation of the substrate. Both of these techniques are effective in the illustrated context, and both are well-known in the art. Neither is particularly preferred, and thus the user can choose which process to employ based on other factors. 
     Most importantly, defects  112  are all located outside the compressive layer, and thus away from the depletion layer  108 . Confined to portions of the substrate where they cannot form a leakage path, these defects are rendered entirely harmless. 
     An alternative embodiment locates the compressive layer at least partially within the depletion layer, as seen in  FIG. 4 . The exact position can lie entirely within the layer, or overlapping it. The primary goal of the compressive layer is to protect the depletion layer from defect formation, so having the compressive layer within the depletion layer altogether prevents the formation of defects there. 
     The optimum solution calls for the compressive layer  103  to lie entirely outside the depletion layer, between the depletion layer and the defects  112 , as shown in  FIG. 5 . There, the compressive layer is positioned to prevent the movement of intersitials toward or into the depletion layer, completely precluding defect formation there. 
     Another embodiment, shown in  FIG. 6 , positions the compressive layer  103  at a level shallower than that of the depletion layer  108 . Here, those in the art will understand that it would not be desirable to have the compressive layer extend into channel, and thus the compressive layer would be implanted after implantation of the source and drain regions  102  and  104 , and like those operations, the gate structure would be employed as a mask, as known in the art. Thus, the compressive layer would lie wholly within the source and drain regions, as shown. As will be understood, the shape of the compressive layer will have substantially the same profile as does the source or drain region containing that layer, a fact that may not be clear from the drawing. Here, defects would form in the source and drain regions. 
     While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.