Patent Publication Number: US-7586116-B2

Title: Semiconductor device having a semiconductor-on-insulator configuration and a superlattice

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/089,950, filed Mar. 25, 2005 now U.S. Pat. No. 7,303,948, which is a continuation of U.S. patent application Ser. No. 10/647,069 filed Aug. 22, 2003, now U.S. Pat. No. 6,897,472, which in turn is a continuation-in-part of U.S. patent application Ser. Nos. 10/603,696 and 10/603,621, both now abandoned filed on Jun. 26, 2003, the entire disclosures of which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductors, and, more particularly, to semiconductors having enhanced properties based upon energy band engineering and associated methods. 
     BACKGROUND OF THE INVENTION 
     Structures and techniques have been proposed to enhance the performance of semiconductor devices, such as by enhancing the mobility of the charge carriers. For example, U.S. Patent application No. 2003/0057416 to Currie et al. discloses strained material layers of silicon, silicon-germanium, and relaxed silicon and also including impurity-free zones that would otherwise cause performance degradation. The resulting biaxial strain in the upper silicon layer alters the carrier mobilities enabling higher speed and/or lower power devices. Published U.S. Patent application No. 2003/0034529 to Fitzgerald et al. discloses a CMOS inverter also based upon similar strained silicon technology. 
     U.S. Pat. No. 6,472,685 B2 to Takagi discloses a semiconductor device including a silicon and carbon layer sandwiched between silicon layers so that the conduction band and valence band of the second silicon layer receive a tensile strain. Electrons having a smaller effective mass, and which have been induced by an electric field applied to the gate electrode, are confined in the second silicon layer, thus, an n-channel MOSFET is asserted to have a higher mobility. 
     U.S. Pat. No. 4,937,204 to Ishibashi et al. discloses a superlattice in which a plurality of layers, less than eight monolayers, and containing a fraction or a binary compound semiconductor layers, are alternately and epitaxially grown. The direction of main current flow is perpendicular to the layers of the superlattice. 
     U.S. Pat. No. 5,357,119 to Wang et al. discloses a Si—Ge short period superlattice with higher mobility achieved by reducing alloy scattering in the superlattice. Along these lines, U.S. Pat. No. 5,683,934 to Candelaria discloses an enhanced mobility MOSFET including a channel layer comprising an alloy of silicon and a second material substitutionally present in the silicon lattice at a percentage that places the channel layer under tensile stress. 
     U.S. Pat. No. 5,216,262 to Tsu discloses a quantum well structure comprising two barrier regions and a thin epitaxially grown semiconductor layer sandwiched between the barriers. Each barrier region consists of alternate layers of SiO 2 /Si with a thickness generally in a range of two to six monolayers. A much thicker section of silicon is sandwiched between the barriers. 
     An article entitled “Phenomena in silicon nanostructure devices” also to Tsu and published online Sep. 6, 2000 by Applied Physics and Materials Science &amp; Processing, pp. 391-402 discloses a semiconductor-atomic superlattice (SAS) of silicon and oxygen. The Si/O superlattice is disclosed as useful in a silicon quantum and light-emitting devices. In particular, a green electroluminescence diode structure was constructed and tested. Current flow in the diode structure is vertical, that is, perpendicular to the layers of the SAS. The disclosed SAS may include semiconductor layers separated by adsorbed species such as oxygen atoms, and CO molecules. The silicon growth beyond the adsorbed monolayer of oxygen is described as epitaxial with a fairly low defect density. One SAS structure included a 1.1 nm thick silicon portion that is about eight atomic layers of silicon, and another structure had twice this thickness of silicon. An article to Luo et al. entitled “Chemical Design of Direct-Gap Light-Emitting Silicon” published in Physical Review Letters, Vol. 89, No. 7 (Aug. 12, 2002) further discusses the light emitting SAS structures of Tsu. 
     Published International Application WO 02/103,767 A1 to Wang, Tsu and Lofgren, discloses a barrier building block of thin silicon and oxygen, carbon, nitrogen, phosphorous, antimony, arsenic or hydrogen to thereby reduce current flowing vertically through the lattice more than four orders of magnitude. The insulating layer/barrier layer allows for low defect epitaxial silicon to be deposited next to the insulating layer. 
     Published Great Britain Patent Application 2,347,520 to Mears et al. discloses that principles of Aperiodic Photonic Band-Gap (APBG) structures may be adapted for electronic bandgap engineering. In particular, the application discloses that material parameters, for example, the location of band minima, effective mass, etc, can be tailored to yield new aperiodic materials with desirable band-structure characteristics. Other parameters, such as electrical conductivity, thermal conductivity and dielectric permittivity or magnetic permeability are disclosed as also possible to be designed into the material. 
     Despite considerable efforts at materials engineering to increase the mobility of charge carriers in semiconductor devices, there is still a need for greater improvements. Greater mobility may increase device speed and/or reduce device power consumption. With greater mobility, device performance can also be maintained despite the continued shift to smaller device features. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, it is therefore an object of the present invention to provide a semiconductor device, such as a silicon-on-insulator (SOI) device, having relatively high charge carrier mobility and related methods. 
     This and other objects, features, and advantages in accordance with the present invention are provided by a semiconductor device which may include a substrate, an insulating layer adjacent the substrate, and a superlattice adjacent a face of the insulating layer opposite the substrate. More particularly, the superlattice may include a plurality of stacked groups of layers, with each group of layers including a plurality of stacked base semiconductor monolayers defining a base semiconductor portion and an energy band-modifying layer thereon. Furthermore, the energy band-modifying layer may include at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions. 
     More particularly, the semiconductor device may further include a semiconductor layer between the insulating layer and the superlattice. Moreover, source and drain regions may be on the semiconductor layer, and the superlattice may extend between the source and drain regions. A gate may also overly the superlattice, and a contact layer may be on at least one of the source and drain regions. By way of example, the substrate may comprise silicon, and the insulating layer may comprise silicon oxide. 
     Additionally, the superlattice may have a common energy band structure therein, and it may also have a higher charge carrier mobility than would otherwise be present without the energy band-modifying layer. Each base semiconductor portion may comprise at least one of silicon and germanium, and each energy band-modifying layer may comprise oxygen. Further, each energy band-modifying layer may be a single monolayer thick, and each base semiconductor portion may be less than eight monolayers thick. 
     The superlattice may further have a substantially direct energy bandgap, and it may also include a base semiconductor cap layer on an uppermost group of layers. In one embodiment, all of the base semiconductor portions may be a same number of monolayers thick. In accordance with an alternate embodiment, at least some of the base semiconductor portions may be a different number of monolayers thick. In addition, each energy band-modifying layer may include a non-semiconductor selected from the group consisting of oxygen, nitrogen, fluorine, and carbon-oxygen, for example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic cross-sectional view of a semiconductor device in accordance with the present invention. 
         FIG. 2  is a greatly enlarged schematic cross-sectional view of the superlattice as shown in  FIG. 1 . 
         FIG. 3  is a perspective schematic atomic diagram of a portion of the superlattice shown in  FIG. 1 . 
         FIG. 4  is a greatly enlarged schematic cross-sectional view of another embodiment of a superlattice that may be used in the device of  FIG. 1 . 
         FIG. 5A  is a graph of the calculated band structure from the gamma point (G) for both bulk silicon as in the prior art, and for the 4/1 Si/O superlattice as shown in  FIGS. 1-3 . 
         FIG. 5B  is a graph of the calculated band structure from the Z point for both bulk silicon as in the prior art, and for the 4/1 Si/O superlattice as shown in  FIGS. 1-3 . 
         FIG. 5C  is a graph of the calculated band structure from both the gamma and Z points for both bulk silicon as in the prior art, and for the 5/1/3/1 Si/O superlattice as shown in  FIG. 4 . 
         FIGS. 6A-6C  are a series of schematic cross-sectional diagrams illustrating a method for making the semiconductor device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternate embodiments. Also, the size of regions or thicknesses of various layers may be exaggerated in certain views for clarity of illustration. 
     The present invention relates to controlling the properties of semiconductor materials at the atomic or molecular level to achieve improved performance within semiconductor devices. Further, the invention relates to the identification, creation, and use of improved materials for use in the conduction paths of semiconductor devices. 
     Applicants theorize, without wishing to be bound thereto, that certain superlattices as described herein reduce the effective mass of charge carriers and that this thereby leads to higher charge carrier mobility. Effective mass is described with various definitions in the literature. As a measure of the improvement in effective mass Applicants use a “conductivity reciprocal effective mass tensor”, M e   −1  and M h   −1  for electrons and holes respectively, defined as: 
                 M     e   ,   ij       -   1       ⁡     (       E   F     ,   T     )       =         ∑     E   &gt;     E   F                 ⁢       ∫     B   .   Z   .               ⁢         (       ∇   k     ⁢     E   ⁡     (     k   ,   n     )         )     i     ⁢       (       ∇   k     ⁢     E   ⁡     (     k   ,   n     )         )     j     ⁢       ∂     f   ⁡     (       E   ⁡     (     k   ,   n     )       ,     E   F     ,   T     )           ∂   E       ⁢           ⁢       ⅆ   3     ⁢   k               ∑     E   &gt;     E   F                 ⁢       ∫     B   .   Z   .               ⁢       f   ⁡     (       E   ⁡     (     k   ,   n     )       ,     E   F     ,   T     )       ⁢           ⁢       ⅆ   3     ⁢   k                   
for electrons and:
 
                 M     h   ,   ij       -   1       ⁡     (       E   F     ,   T     )       =           -   ∑       E   &gt;     E   F                 ⁢       ∫     B   .   Z   .               ⁢         (       ∇   k     ⁢     E   ⁡     (     k   ,   n     )         )     i     ⁢       (       ∇   k     ⁢     E   ⁡     (     k   ,   n     )         )     j     ⁢       ∂     f   ⁡     (       E   ⁡     (     k   ,   n     )       ,     E   F     ,   T     )           ∂   E       ⁢           ⁢       ⅆ   3     ⁢   k               ∑     E   &gt;     E   F                 ⁢       ∫     B   .   Z   .               ⁢       f   ⁡     (       E   ⁡     (     k   ,   n     )       ,     E   F     ,   T     )       ⁢           ⁢       ⅆ   3     ⁢   k                   
for holes, where f is the Fermi-Dirac distribution, E F  is the Fermi energy, T is the temperature, E(k,n) is the energy of an electron in the state corresponding to wave vector k and the n th  energy band, the indices i and j refer to Cartesian coordinates x, y and z, the integrals are taken over the Brillouin zone (B.Z.), and the summations are taken over bands with energies above and below the Fermi energy for electrons and holes respectively.
 
     Applicants&#39; definition of the conductivity reciprocal effective mass tensor is such that a tensorial component of the conductivity of the material is greater for greater values of the corresponding component of the conductivity reciprocal effective mass tensor. Again Applicants theorize without wishing to be bound thereto that the superlattices described herein set the values of the conductivity reciprocal effective mass tensor so as to enhance the conductive properties of the material, such as typically for a preferred direction of charge carrier transport. The inverse of the appropriate tensor element is referred to as the conductivity effective mass. In other words, to characterize semiconductor material structures, the conductivity effective mass for electrons/holes as described above and calculated in the direction of intended carrier transport is used to distinguish improved materials. 
     Using the above-described measures, one can select materials having improved band structures for specific purposes. One such example would be a superlattice  25  material for a channel region in a semiconductor device. A silicon-on-insulator (SOI) MOSFET  20  including the superlattice  25  in accordance with the invention is now first described with reference to  FIG. 1 . One skilled in the art, however, will appreciate that the materials identified herein could be used in many different types of semiconductor devices, such as discrete devices and/or integrated circuits. 
     The illustrated SOI MOSFET  20  includes a silicon substrate  21 , an insulating layer (i.e., silicon oxide)  37  on the substrate, and a semiconductor (i.e., silicon) layer  39  on a face of the insulating layer opposite the substrate. Lightly doped source/drain extension regions  22 ,  23  and more heavily doped source/drain regions  26 ,  27  are formed in the semiconductor layer  39 , as shown, and a channel region extending between the lightly doped source/drain extension regions is provided by the superlattice  25 . Source/drain silicide layers  30 ,  31  and source/drain contacts  32 ,  33  overlie the source/drain regions, as will be appreciated by those skilled in the art. 
     A gate  35  illustratively includes a gate insulating layer  36  (e.g., silicon oxide) adjacent the channel provided by the superlattice  25 , and a gate electrode layer  38  (e.g., silicon) on the gate insulating layer. Sidewall spacers  40 ,  41  are also provided in the illustrated SOI MOSFET  20 , as well as a suicide layer  34  on the gate electrode layer  36 . For clarity of illustration, the insulating layer  37  and the gate insulating layer  36  are shown with stippling in the drawings. Moreover, the superlattice  25  is shown with dashes in regions where dopant from the source/drain implantations is present. 
     It should be noted that other source/drain and gate configurations may be also used, such as those disclosed in SEMICONDUCTOR DEVICE COMPRISING A SUPERLATTICE CHANNEL VERTICALLY STEPPED ABOVE SOURCE AND DRAIN REGIONS, U.S. patent application Ser. No. 10/940,426, and SEMICONDUCTOR DEVICE COMPRISING A SUPERLATTICE WITH UPPER PORTIONS EXTENDING ABOVE ADJACENT UPPER PORTIONS OF SOURCE AND DRAIN REGIONS, U.S. patent application Ser. No. 10/941,062. Both of these applications are assigned to the present Assignee and are hereby incorporated herein in their entireties by reference. 
     As will be appreciated by those skilled in the art, the insulating layer  37  of the above-described SOI device advantageously provides reduced capacitance adjacent the source and drain regions  26 ,  27 , thereby reducing switching time and providing faster device operation, for example. It should be noted that other materials may be used for the insulating layer  37 , such as glass or sapphire, for example. Moreover, the substrate  21  and semiconductor layer  39  may comprise other semiconductor materials, such as germanium, for example. 
     Applicants have identified improved materials or structures for the channel region of the SOI MOSFET  20 . More specifically, the Applicants have identified materials or structures having energy band structures for which the appropriate conductivity effective masses for electrons and/or holes are substantially less than the corresponding values for silicon. 
     Referring now additionally to  FIGS. 2 and 3 , the materials or structures are in the form of a superlattice  25  whose structure is controlled at the atomic or molecular level and may be formed using known techniques of atomic or molecular layer deposition. The superlattice  25  includes a plurality of layer groups  45   a - 45   n  arranged in stacked relation, as perhaps best understood with specific reference to the schematic cross-sectional view of  FIG. 2 . 
     Each group of layers  45   a - 45   n  of the superlattice  25  illustratively includes a plurality of stacked base semiconductor monolayers  46  defining a respective base semiconductor portion  46   a - 46   n  and an energy band-modifying layer  50  thereon. The energy band-modifying layers  50  are indicated by stippling in  FIG. 2  for clarity of illustration. 
     The energy-band modifying layer  50  illustratively includes one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions. In other embodiments, more than one such monolayer may be possible. It should be noted that reference herein to a non-semiconductor or semiconductor monolayer means that the material used for the monolayer would be a non-semiconductor or semiconductor if formed in bulk. That is, a single monolayer of a material, such as semiconductor, may not necessarily exhibit the same properties that it would if formed in bulk or in a relatively thick layer, as will be appreciated by those skilled in the art. 
     Applicants theorize without wishing to be bound thereto that energy band-modifying layers  50  and adjacent base semiconductor portions  46   a - 46   n  cause the superlattice  25  to have a lower appropriate conductivity effective mass for the charge carriers in the parallel layer direction than would otherwise be present. Considered another way, this parallel direction is orthogonal to the stacking direction. The band modifying layers  50  may also cause the superlattice  25  to have a common energy band structure. 
     It is also theorized that the semiconductor device described above enjoys a higher charge carrier mobility based upon the lower conductivity effective mass than would otherwise be present. In some embodiments, and as a result of the band engineering achieved by the present invention, the superlattice  25  may further have a substantially direct energy bandgap that may be particularly advantageous for opto-electronic devices, for example, as described in further detail below. 
     As will be appreciated by those skilled in the art, the source/drain regions  22 ,  23  and gate  35  of the MOSFET  20  may be considered as regions for causing the transport of charge carriers through the superlattice in a parallel direction relative to the layers of the stacked groups  45   a - 45   n . Other such regions are also contemplated by the present invention. 
     The superlattice  25  also illustratively includes a cap layer  52  on an upper layer group  45   n . The cap layer  52  may comprise a plurality of base semiconductor monolayers  46 . The cap layer  52  may have between 2 to 100 monolayers of the base semiconductor, and, more preferably between 10 to 50 monolayers. 
     Each base semiconductor portion  46   a - 46   n  may comprise a base semiconductor selected from the group consisting of Group IV semiconductors, Group III-V semiconductors, and Group II-VI semiconductors. Of courser the term Group IV semiconductors also includes Group IV-IV semiconductors, as will be appreciated by those skilled in the art. More particularly, the base semiconductor may comprise at least one of silicon and germanium, for example. 
     Each energy band-modifying layer  50  may comprise a non-semiconductor selected from the group consisting of oxygen, nitrogen, fluorine, and carbon-oxygen, for example. The non-semiconductor is also desirably thermally stable through deposition of a next layer to thereby facilitate manufacturing. In other embodiments, the non-semiconductor may be another inorganic or organic element or compound that is compatible with the given semiconductor processing as will be appreciated by those skilled in the art. More particularly, the base semiconductor may comprise at least one of silicon and germanium, for example. 
     It should be noted that the term monolayer is meant to include a single atomic layer and also a single molecular layer. It is also noted that the energy band-modifying layer  50  provided by a single monolayer is also meant to include a monolayer wherein not all of the possible sites are occupied. For example, with particular reference to the atomic diagram of  FIG. 3 , a 4/1 repeating structure is illustrated for silicon as the base semiconductor material, and oxygen as the energy band-modifying material. Only half of the possible sites for oxygen are occupied. 
     In other embodiments and/or with different materials this one half occupation would not necessarily be the case as will be appreciated by those skilled in the art. Indeed it can be seen even in this schematic diagram, that individual atoms of oxygen in a given monolayer are not precisely aligned along a flat plane as will also be appreciated by those of skill in the art of atomic deposition. By way of example, a preferred occupation range is from about one-eighth to one-half of the possible oxygen sites being full, although other numbers may be used in certain embodiments. 
     Silicon and oxygen are currently widely used in conventional semiconductor processing, and, hence, manufacturers will be readily able to use these materials as described herein. Atomic or monolayer deposition is also now widely used. Accordingly, semiconductor devices incorporating the superlattice  25  in accordance with the invention may be readily adopted and implemented, as will be appreciated by those skilled in the art. 
     It is theorized without Applicants wishing to be bound thereto, that for a superlattice, such as the Si/O superlattice, for example, that the number of silicon monolayers should desirably be seven or less so that the energy band of the superlattice is common or relatively uniform throughout to achieve the desired advantages. The 4/1 repeating structure shown in  FIGS. 2 and 3 , for Si/O has been modeled to indicate an enhanced mobility for electrons and holes in the X direction. For example, the calculated conductivity effective mass for electrons (isotropic for bulk silicon) is 0.26 and for the 4/1 SiO superlattice in the X direction it is 0.12 resulting in a ratio of 0.46. Similarly, the calculation for holes yields values of 0.36 for bulk silicon and 0.16 for the 4/1 Si/O superlattice resulting in a ratio of 0.44. 
     While such a directionally preferential feature may be desired in certain semiconductor devices, other devices may benefit from a more uniform increase in mobility in any direction parallel to the groups of layers. It may also be beneficial to have an increased mobility for both electrons or holes, or just one of these types of charge carriers as will be appreciated by those skilled in the art. 
     The lower conductivity effective mass for the 4/1 Si/O embodiment of the superlattice  25  may be less than two-thirds the conductivity effective mass than would otherwise occur, and this applies for both electrons and holes. Of course, the superlattice  25  may further comprise at least one type of conductivity dopant therein as will also be appreciated by those skilled in the art. 
     Indeed, referring now additionally to  FIG. 4 , another embodiment of a superlattice  25 ′ in accordance with the invention having different properties is now described. In this embodiment, a repeating pattern of 3/1/5/1 is illustrated. More particularly, the lowest base semiconductor portion  46   a ′ has three monolayers, and the second lowest base semiconductor portion  46   b ′ has five monolayers. This pattern repeats throughout the superlattice  25 ′. The energy band-modifying layers  50 ′ may each include a single monolayer. For such a superlattice  25 ′ including Si/O, the enhancement of charge carrier mobility is independent of orientation in the plane of the layers. Those other elements of  FIG. 4  not specifically mentioned are similar to those discussed above with reference to  FIG. 2  and need no further discussion herein. 
     In some device embodiments, all of the base semiconductor portions of a superlattice may be a same number of monolayers thick. In other embodiments, at least some of the base semiconductor portions may be a different number of monolayers thick. In still other embodiments, all of the base semiconductor portions may be a different number of monolayers thick. 
     In  FIGS. 5A-5C  band structures calculated using Density Functional Theory (DFT) are presented. It is well known in the art that DFT underestimates the absolute value of the bandgap. Hence all bands above the gap may be shifted by an appropriate “scissors correction.” However the shape of the band is known to be much more reliable. The vertical energy axes should be interpreted in this light. 
       FIG. 5A  shows the calculated band structure from the gamma point (G) for both bulk silicon (represented by continuous lines) and for the 4/1 Si/O superlattice  25  as shown in  FIGS. 1-3  (represented by dotted lines). The directions refer to the unit cell of the 4/1 Si/C structure and not to the conventional unit cell of Si, although the (001) direction in the figure does correspond to the (001) direction of the conventional unit cell of Si, and, hence, shows the expected location of the Si conduction band minimum. The (100) and (010) directions in the figure correspond to the (110) and (−110) directions of the conventional Si unit cell. Those skilled in the art will appreciate that the bands of Si on the figure are folded to represent them on the appropriate reciprocal lattice directions for the 4/1 Si/O structure. 
     It can be seen that the conduction band minimum for the 4/1 Si/O structure is located at the gamma point in contrast to bulk silicon (Si), whereas the valence band minimum occurs at the edge of the Brillouin zone in the (001) direction which we refer to as the Z point. One may also note the greater curvature of the conduction band minimum for the 4/1 Si/O structure compared to the curvature of the conduction band minimum for Si owing to the band splitting due to the perturbation introduced by the additional oxygen layer. 
       FIG. 5B  shows the calculated band structure from the Z point for both bulk silicon (continuous lines) and for the 4/1 Si/O superlattice  25  (dotted lines). This figure illustrates the enhanced curvature of the valence band in the (100) direction. 
       FIG. 5C  shows the calculated band structure from both the gamma and Z point for both bulk silicon (continuous lines) and for the 5/1/3/1 Si/O structure of the superlattice  25 ′ of  FIG. 4  (dotted lines). Due to the symmetry of the 5/1/3/1 Si/O structure, the calculated band structures in the (100) and (010) directions are equivalent. Thus the conductivity effective mass and mobility are expected to be isotropic in the plane parallel to the layers, i.e. perpendicular to the (001) stacking direction. Note that in the 5/1/3/1 Si/O example the conduction band minimum and the valence band maximum are both at or close to the Z point. 
     Although increased curvature is an indication of reduced effective mass, the appropriate comparison and discrimination may be made via the conductivity reciprocal effective mass tensor calculation. This leads Applicants to further theorize that the 5/1/3/1 superlattice  25 ′ should be substantially direct bandgap. As will be understood by those skilled in the art, the appropriate matrix element for optical transition is another indicator of the distinction between direct and indirect bandgap behavior. 
     Referring now additionally to  FIGS. 6A-6C , a method for making the SOI MOSFET  20  will now be described. The method begins with providing a first semiconductor (e.g., silicon) substrate  61 . By way of example, the substrate  61  may be an eight-inch wafer  21  of lightly doped P-type or N-type single crystal silicon with &lt;100&gt; orientation, although other suitable substrates may also be used. In accordance with the present example, a base undoped silicon layer  152  is epitaxially formed across the upper surface of the substrate  61 . In some embodiments, the layer  152  may include a plurality of base semiconductor monolayers  46  to define a cap layer for the superlattice  25 , which is then formed thereon. It should be noted that the superlattice here is being constructed in the reverse order as described above, since it will be “flipped” when combined with the SOI substrate  21 , as discussed further below. 
     The superlattice  25  material is deposited using atomic layer deposition, as will be appreciated by those skilled in the art. It should be noted that in some embodiments the superlattice  25  material may be selectively deposited in those regions where channels are to be formed, rather than across the entire substrate  61 , as will also be appreciated by those skilled in the art. The epitaxial silicon layer  39  is formed on the superlattice  25 , and this layer is preferably undoped or lightly doped. The thickness of the layer  39  should be appropriate for either a partially or fully depleted device, as the case may be. Planarization may be used as necessary after forming the above layers to achieve the desired thickness and surface characteristics. 
     Following the formation of the layer  39 , the substrate  61  is implanted with ions (e.g., hydrogen ions), as illustrated with the downward pointing arrows in  FIG. 6A , to form a separation layer  60  in the first substrate. The separation layer  60  is generally parallel to the upper surface of the substrate  61  and is located at a position corresponding to the mean penetration depth of the ion implantation, as will be appreciated by those skilled in the art. The implantation energy is chosen to give the desired overall layer thickness and the appropriate cap layer thickness in the finished device  20 . 
     The insulating layer  37  is formed on a second substrate  21 , which may be similar to the substrate  61  discussed above, using conventional semiconductor processing techniques. While shown side by side for reference in  FIG. 6A , it should be noted that the two substrates  21 ,  61  need not be processed simultaneously. 
     The substrate  61  is then flipped and the layer  39  is bonded to the insulating layer  37  as shown in  FIG. 6B . Once bonded, a thermal cycle is performed to fracture and cleave the first substrate  61  at the separation layer  60 , as will be appreciated by those skilled in the art. The separated substrate  61  may then be discarded, and the substrate  21  becomes the base substrate for the device  20 . 
     The SOI MOSFET  20  is shown in  FIG. 6C  after the gate oxide  36 , the gate electrode  38 , and spacers  40 ,  41  are formed. More particularly, a thin gate oxide  36  is deposited, and steps of poly deposition, patterning, and etching are performed. Poly deposition refers to low pressure chemical vapor deposition (LPCVD) of silicon onto an oxide (hence it forms a polycrystalline material). The step includes doping with P+ or As− to make it conducting, and the layer may be around 250 nm thick, for example. 
     In addition, the pattern step may include performing a spinning photoresist, baking, exposure to light (i.e., a photolithography step), and developing the resist. Usually, the pattern is then transferred to another layer (oxide or nitride) which acts as an etch mask during the etch step. The etch step typically is a plasma etch (anisotropic, dry etch) that is material selective (e.g., etches silicon ten times faster than oxide) and transfers the lithography pattern into the material of interest. The sidewall spacers  40 ,  41  may be formed after patterning of the gate stack, as will be appreciated by those skilled in the art. 
     Once the gate  35  and sidewall spacers  40 ,  41  are formed, they may be used as an etch mask to remove the superlattice  25  material and portions of the substrate  21  in the regions where the source and drain are to be formed, as will be appreciated by those skilled in the art. As may be seen in  FIG. 1 , this step forms an underlying step portion of the silicon layer  39  beneath the superlattice  25 . 
     The superlattice  25  material may be etched in a similar fashion to that described above for the gate  35 . However, it should be noted that with the non-semiconductor present in the superlattice  25 , e.g., oxygen, the superlattice may be more easily etched using an etchant formulated for oxides rather than silicon. Of course, the appropriate etch for a given implementation will vary based upon the structure and materials used for the superlattice  25  and substrate  21 , as will be appreciated by those of skill in the art. 
     The lightly doped source and drain (“LDD”) extensions  22 ,  23  are formed using n-type or p-type LDD implantation, annealing, and cleaning. An anneal step may be used after the LDD implantation, but depending on the specific process, it may be omitted. The clean step is a chemical etch to remove metals and organics prior to depositing an oxide layer. It should be noted that when the source and drain regions are said to be formed “on” the semiconductor layer  39  herein, this is meant to include implantations in the semiconductor layer as well as raised source/drain regions which may be formed on top of the semiconductor layer, as will be appreciated by those skilled in the art. An exemplary raised source/drain configuration is provided in co-pending U.S. application Ser. No. 10/941,062, which is assigned to the present Assignee and is hereby incorporated herein in its entirety by reference. 
     To form the source and drain  26 ,  27  implants, an SiO 2  mask is deposited and etched back. N-type or p-type ion implantation is used to form the source and drain regions  26 ,  27 . The structure is then annealed and cleaned. Self-aligned silicide formation may then be performed to form the silicide layers  30 ,  31 , and  34 , and the source/drain contacts  32 ,  33 , are formed to provide the final SOI MOSFET device  20  illustrated in  FIG. 1 . The silicide formation is also known as salicidation. The salicidation process includes metal deposition (e.g. Ti), nitrogen annealing, metal etching, and a second annealing. 
     The foregoing is, of course, but one example of a process and device in which the present invention may be used, and those of skill in the art will understand its application and use in many other processes and devices. In other processes and devices the structures of the present invention may be formed on a portion of a wafer or across substantially all of a wafer. Additionally, the use of an atomic layer deposition tool may also not be needed for forming the superlattice  25  in some embodiments. For example, the monolayers may be formed using a CVD tool with process conditions compatible with control of monolayers, as will be appreciated by those skilled in the art. 
     It should be noted that devices other than MOSFETs may be produced in accordance with the present invention. By way of example, one type of insulator-on-substrate device that may be produced using the above described techniques is a memory device, such as the one described in a co-pending application entitled SEMICONDUCTOR DEVICE INCLUDING A FLOATING GATE MEMORY CELL WITH A SUPERLATTICE CHANNEL, U.S. patent application Ser. No. 11/381,787, which is assigned to the present Assignee and is hereby incorporated herein in its entirety by reference. Other potential insulator-on-substrate devices include optical devices, such as those disclosed in U.S. patent application Ser. No. 10/936,903, which is also assigned to the present Assignee and is hereby incorporated herein in its entirety by reference. 
     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.