Abstract:
A semiconductor structure comprises a silicon substrate ( 10 ), one or more layers of single crystal oxides or nitrides ( 26 ), and an interface ( 14 ) between the silicon substrate and the one or more layers of single crystal oxides or nitrides, the interface manufactured with a crystalline material which matches the lattice constant of silicon. The interface comprises an atomic layer of silicon, nitrogen, and a metal in the form MSiN 2 , where M is a metal. In a second embodiment, the interface comprises an atomic layer of silicon, a metal, and a mixture of nitrogen and oxygen in the form MSi[N 1−x O x ] 2 , where M is a metal and X is 0≦X&lt;1.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to a semiconductor structure including a crystalline alkaline earth metal nitride-based interface between a silicon substrate and oxides or other nitrides, and more particularly to an interface including an atomic layer of an alkaline earth metal, silicon, and nitrogen. 
     BACKGROUND OF THE INVENTION 
     An ordered and stable silicon (Si) surface is most desirable for subsequent epitaxial growth of single crystal thin films on silicon for numerous device applications, e.g., ferroelectrics or high dielectric constant oxides for non-volatile high density memory and logic devices. It is pivotal to establish an ordered transition layer on the Si surface, especially for subsequent growth of single crystal oxides, e.g., perovskites. 
     Some reported growth of these oxides, such as BaO and BaTiO 3  on Si(100) was based on a BaSi 2  (cubic) template by depositing one fourth monolayer of Ba on Si(100) using reactive epitaxy at temperatures greater than 850° C. See for example: R. McKee et al.,  Appl. Phys. Lett.  59(7), pp 782-784 (Aug. 12, 1991); R. McKee et al.,  Appl. Phys. Lett.  63(20), pp. 2818-2820 (Nov. 15, 1993); R. McKee et al.,  Mat. Res. Soc. Symp . Proc., Vol. 21, pp. 131-135 (1991); R. A. McKee, F. J. Walker and M. F. Chisholm, “Crystalline Oxides on Silicon: The First Five Monolayers”,  Phys. Rev. Lett.  81(14), 3014-7 (Oct. 5, 1998). U.S. Pat. No. 5,225,031, issued Jul. 6, 1993, entitled “Process for Depositing an Oxide Epitaxially onto a Silicon Substrate and Structures Prepared with the Process”; and U.S. Pat. No. 5,482,003, issued Jan. 9, 1996, entitled “Process for Depositing Epitaxial Alkaline Earth Oxide onto a Substrate and Structures Prepared with the Process”. However, atomic level simulation of this proposed structure indicates that it likely is not stable at elevated temperatures. 
     Growth of SrTiO 3  on silicon (100) using an SrO buffer layer has been accomplished. T. Tambo et al.,  Jpn. J. Appl. Phys.,  Vol. 37 (1998), pp. 4454-4459. However, the SrO buffer layer was thick (100 Å), thereby limiting application for transistor films, and crystallinity was not maintained throughout the growth. 
     Furthermore, SrTiO 3  has been grown on silicon using thick metal oxide buffer layers (60-120 Å) of Sr or Ti. B. K. Moon et al.,  Jpn. J. Appl. Phys ., Vol. 33 (1994), pp. 1472-1477. These thick buffer layers would limit the application for transistors. 
     Therefore, a thin, stable crystalline interface with silicon is needed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-2 illustrate a cross-sectional view of a clean semiconductor substrate having an interface formed thereon in accordance with the present invention; 
     FIGS. 3-6 illustrate a cross-sectional view of a semiconductor substrate having an interface formed from a silicon nitride layer in accordance with the present invention; and 
     FIGS. 7-8 illustrate a cross-sectional view of an alkaline-earth-metal nitride layer formed on the structures illustrated in FIGS. 1-6 in accordance with the present invention. 
     FIGS. 9-12 illustrate a cross-sectional view of a perovskite formed on the structures of FIGS. 1-8 in accordance with the present invention. 
     FIG. 13 illustrates a side view of the atomic structure of one embodiment of the layers of FIG. 12 in accordance with the present invention. 
     FIG. 14 illustrates a top view along view line AA of FIG. 13 of the interface. 
     FIG. 15 illustrates a top view along view line AA of FIG. 13 including the interface and the adjacent atomic layer of the substrate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     To form the novel interface between a silicon (Si) substrate and one or more layers of a single crystal oxide or nitride, various approaches may be used. Several examples will be provided for both starting with a Si substrate having a clean surface, and a Si substrate having silicon nitride (Si 3 N 4  or the like) on the surface. Si 3 N 4  is amorphous rather than single crystalline and it is desirable for purposes of growing additional single crystal material on the substrate that a single crystal nitride be provided as the interface. 
     Turning now to the drawings in which like elements are designated with like numbers throughout, FIGS. 1 and 2 illustrate a semiconductor structure including a Si substrate  10  having a clean surface  12 . A clean (2×1) surface  12  may be obtained with any conventional cleaning procedure, for example, with thermal desorption of SiO 2 at a temperature greater than or equal to 850° C., or by removal of the hydrogen from a hydrogen terminated Si(1×1) surface at a temperature greater than or equal to 300° C. in an ultra high vacuum. Hydrogen termination is a well known process in which hydrogen is loosely bonded to dangling bonds of the silicon atoms at surface  12  to complete the crystalline structure. The interface  14  of a crystalline material may be formed by supplying (as shown by the arrows in FIG. 1) controlled amounts of a metal, Si, and activated nitrogen, either simultaneously or sequentially to the surface  12  at a temperature less than or equal to 900° C. in a growth chamber with N 2  partial pressure less than or equal to 1×10 −6  mBar. The metal applied to the surface  12  to form the interface  14  may be any metal, but in the preferred embodiment comprises an alkaline-earth-metal, such as barium (Ba) or strontium (Sr). 
     As the application of the Ba, Si, and activated nitrogen form BaSiN 2  as the interface  14 , the growth is monitored using Reflection High Energy Electron Diffraction (RHEED) techniques which are well documented in the art and which can be used in situ, i.e., while performing the exposing step within the growth chamber. The RHEED techniques are used to detect or sense surface crystalline structures and in the present process change rapidly to strong and sharp streaks by the forming of an atomic layer of the BaSiN 2 . It will of course be understood that once a specific manufacturing process is provided and followed, it may not be necessary to perform the RHEED techniques on every substrate. 
     The novel atomic structure of the interface  14  will be described in subsequent paragraphs. 
     It should be understood by those skilled in the art that the temperatures and pressures given for these processes are recommended for the particular embodiment described, but the invention is not limited to a particular temperature or pressure range. 
     Alternatively, in forming the interface  14 , oxygen may be supplied along with the metal, silicon, and nitrogen to form a mixture. The ratio of nitrogen to oxygen may vary substantially, but preferably would be approximately 80%. 
     Referring to FIGS. 3-6, another approach comprises forming a Si substrate  10  having a surface  12 , and a layer  16  of silicon nitride thereupon. The layer  16  of silicon nitride can be formed purposely in a controlled fashion known in the art, e.g., by applying (arrows) active nitrogen onto the surface  12 . The silicon nitride layer can also be formed on Si substrate using both silicon and active nitrogen in an ultra high vacuum. See for example, R. Droopad, et. al., U.S. Pat. No. 5,907,792, issued May 25, 1999, entitled “Method of Forming a Silicon Nitride Layer”. The novel interface  14  may be formed at least in one of the two suggested embodiments as follows: By applying an alkaline-earth-metal to the surface  18  of silicon nitride layer  16  at 700-900° C., under an ultra high vacuum. More specifically, the Si substrate  10  and the amorphous silicon nitride layer  16  are heated to a temperature below the sublimation temperature of the silicon nitride layer  16 . This can be accomplished in a molecular beam epitaxy chamber or Si substrate  10  can be at least partially heated in a preparation chamber after which it can be transferred to the growth chamber and the heating completed. Once the Si substrate  10  is properly heated and the pressure in the growth chamber has been reduced appropriately, the surface  12  of the Si substrate  10  having silicon nitride layer  16  thereon is exposed to a beam of metal, preferrably an alkaline-earth-metal, as illustrated in FIG.  5 . In a preferred embodiment, the beam is Ba or Sr which is generated by resistively heating effusion cells or from e-beam evaporation sources. In a specific example, Si substrate  10  and silicon nitride layer  16  are exposed to a beam of Ba. The Ba joins the silicon nitride and converts the silicon nitride layer  16  into the interface  14  comprising BaSiN 2  in a crystalline form. Alternatively, an alkaline-earth-metal may be provided to the surface  18  at lower temperatures, annealing the result at 700-1000° C., in an ultra high vacuum. In another embodiment, oxygen may be supplied with the nitrogen to form the interface  14 , resulting in a ordered form of BaSi[N 1−x O x ] 2 . 
     Once the interface  14  is formed, one or more layers of a single crystal oxide, nitride, or combination thereof, may be formed on the surface of the interface  14 . However, an optional layer of an alkaline-earth-metal oxide, such as BaO or SrO, may be placed between the interface  14  and the single crystal oxide. This alkaline-earth-metal oxide provides a low dielectric constant (advantageous for certain uses such as memory cells) and also prevents oxygen from migrating from the single crystal oxide to the Si substrate  10 . 
     Referring to FIGS. 7 and 8, the formation of alkaline-earth-metal nitride layer  22  may be accomplished by either the simultaneous or alternating supply to the surface  20  of the interface  14  of an alkaline-earth-metal and active nitrogen at less than or equal to 700° C. and under N 2  partial pressure less than or equal to 1×10 −5  mBar. This alkaline-earth-metal nitride layer  22  may, for example, comprise a thickness of 50-500 Å. 
     Referring to FIGS. 9-12, a single crystal oxide layer  26 , such as an alkaline-earth-metal perovskite, may be formed on either the surface  20  of the interface  14  or the surface  24  of the alkaline-earth-metal nitride layer  22  by either the simultaneous or alternating supply of an alkaline-earth-metal oxide, oxygen, and a transition metal, such as titanium, at less than or equal to 700° C. under an oxygen partial pressure less than or equal to 1×10 −5  mBar. This single crystal oxide layer  26  may, for example, comprise a thickness of 50-1000 Å and will be substantially lattice matched with the underlying interface  14  or alkaline-earth-metal oxide layer  22 . It should be understood that the single crystal oxide layer  26  may comprises one or more layers in other embodiments. 
     Referring to FIG. 13, a side view (looking in the &lt;{overscore (l)}10&gt; direction) of the atomic configuration of the Si substrate  10 , interface  14 , and alkaline-earth-metal metal oxygen layer  26  is shown. The configuration shown comprises, in relative sizes, for illustrative purposes, from larger to smaller, strontium atoms  30 , silicon atoms  32 , nitrogen atoms  34 , and titanium atoms  36 . The Si substrate  10  comprises only silicon atoms  32 . The interface  14  comprises metal atoms (which in the preferred embodiment are illustrated as strontium atoms  30 ), silicon atoms  32 , and nitrogen atoms  34 . The alkaline-earth-metal nitrogen layer  26  comprises strontium atoms  30 , nitrogen (or a combination of nitrogen and oxygen) atoms  34 , and titanium atoms  36 . 
     Referring to FIG. 14, a top view of the interface along view line AA of FIG. 13, shows the arrangement of the strontium, silicon, and nitrogen atoms  30 ,  32 ,  34 . 
     Referring to FIG. 15, a top view along line AA of FIG. 13, shows the interface  14  and the top atomic layer  11  of the Si substrate  10 . 
     For this discussion, a monolayer equals 6.8×10 14  atoms/cm 2  and an atomic layer is one atom thick. It is seen that the interface  14  shown in the FIGs. comprises a single atomic layer, but could be more than one atomic layer, while the Si substrate  10  and the alkaline-earth-metal metal nitrogen layer may be many atomic layers. Note that in FIG. 13, only four atomic layers of the Si substrate  10  and only two atomic layers of the alkaline-earth-metal metal nitride layer  26  are shown. The interface  14  comprises a half monolayer of the alkaline-earth-metal, a half monolayer of silicon, and a monolayer of nitrogen. Each strontium atom  30  is substantially equally spaced from four of the silicon atoms  32  in the Si substrate  10 . The silicon atoms  32  in the interface  14  are substantially on a line and equally spaced between the alkaline-earth-metal atoms in the &lt;110&gt; direction. Each silicon atom  32  in the top layer of atoms in the Si substrate  10  is bonded to a nitrogen atom  34  in the interface  14  and each silicon atom  32  in the interface  14  is bonded to two nitrogen atoms  34  in the interface  14 . The three-fold bonding coordination of the nitrogen atoms at the interface  14  is satisfied in this interface structure, which greatly lowers the total energy of the interface layer  14 , thus enhancing its stability. The interface  14  comprises rows of strontium, silicon, and nitrogen atoms  30 ,  32 ,  34  in a 2×1 configuration on a (001) surface of the Si substrate  10 , 1× in the &lt;{overscore (l)}10&gt; direction and 2× in the &lt;110&gt; direction. The interface  14  has a 2×1 reconstruction. 
     A thin, crystalline interface  14  with silicon  10  has been described herein. The interface  14  may comprise a single atomic layer. Better transistor applications are achieved by the interface  14  being thin, in that the electrical coupling of the overlying oxide layers to the Si substrate  10  is not compromised, and in that the interface  14  is more stable since the atoms will more likely maintain their crystallinity in processing. This alkaline earth metal-Si-nitrogen-based interface also acts as a diffusion barrier to oxygen and possibly other elements.