Abstract:
A method is provided in which for fabricating a complementary metal oxide semiconductor (CMOS) circuit on a semiconductor-on-insulator (SOI) substrate. A plurality of field effect transistors (FETs) are formed, each having a channel region disposed in a common device layer within a single-crystal semiconductor layer of an SOI substrate. A gate of the first FET overlies an upper surface of the common device layer, and a gate of the second FET underlies a lower surface of the common device layer remote from the upper surface. The first and second FETs share a common diffusion region disposed in the common device layer and are conductively interconnected by the common diffusion region. The common diffusion region is operable as at least one of a source region or a drain region of the first FET and is simultaneously operable as at least one of a source region or a drain region of the second FET.

Description:
CROSS-REFERENCE TO RELATED APPLICATONS 
   This application is a division of U.S. patent application Ser. No. 10/832,894 filed Apr. 27, 2004, now U.S. Pat. No. 7,141,853 which in turn, is a division of U.S. patent application Ser. No. 09/879,530 filed Jun. 12, 2001 now U.S. Pat. No. 6,759,282. The disclosures of said applications are hereby incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention involves fabrication of semiconductor devices using Silicon-on-Insulator (SOI) technology. More specifically the invention is directed to the use of the SOI Buried Oxide (BOX) layer as an integral component of electronic devices and circuits. 
   2. Description of the Related Art 
   Silicon-On-Insulator (SOI) technology has emerged as an electronic fabrication technique that improves characteristics such as latch-up and speed, although typically at higher manufacturing cost. The term SOI typically describes structures where devices are fabricated in single-crystal Si layers formed over an insulating film or substrate. 
     FIGS. 11A and 11B  show a typical conventional SOI structure, where a thin silicon device layer  110  formed on an insulator  111  is supported over substrate  112 . For current technology the substrate is most commonly silicon and the insulator is most commonly silicon dioxide. Devices  113  are formed in device layer  110  and interconnected by surface conductors  114 . The conventional SOI structure is predominantly created by one of two techniques. 
   The first process, known as SIMOX (Separation by IMplanted OXygen), consists of implantation of oxygen into an Si substrate at a prescribed depth and heating it to form a continuous layer of SiO 2 . The SIMOX process requires only a single wafer. The alternate process, shown in greater detail later, is commonly referred to as “Bonded SOI” and starts with two wafers, preferably with at least one having an oxide surface. The first wafer is the carrier wafer which is joined together with the second wafer, and the second wafer is “thinned” to leave a layer of silicon bonded onto the carrier wafer, separated by an insulator layer. 
   Both of the techniques have experienced many variations and enhancements over the years for improvement of yield and lower cost and to achieve desirable device layer quality for uniformity and defects. An important characteristic of conventional SOI that is obvious from  FIG. 11B  is that the insulator layer  111  is used primarily for isolating the silicon device layer  110  with its active devices  113  from the silicon substrate  112 . Thus, the conventional wisdom forms devices on the device layer  110  on only one side of the insulator layer  111 . 
   The problem with this approach is that, although devices and interconnects are formed similar to conventional substrates, SOI techniques introduce newer problems such as floating body effects. Additionally, conventional SOI structure takes up considerably more chip “real estate” than required in corresponding non-SOI structure, since floating body effects which not an issue with conventional substrates require additional connections to the channel regions. There are also added process steps to provide ground interconnections to the substrate. More important, the conventional approach fails to recognize that the insulator layer could provide more functionality than merely separating predetermined groups of devices from the substrate. 
   SUMMARY OF THE INVENTION 
   The inventors have recognized that the SOI insulator layer, or BOX (Buried OXide), can be an integral part of a specific device, and further, even circuits can be advantageously built around this innovative approach. That is to say, the BOX can be considered more than a mere passive isolation mechanism separating layers of devices. It can become an integral component even of an entire circuit. As will be demonstrated, by adopting this innovative approach, a whole new possibility opens up for SOI technology that provides improved device density and speed and fewer conductor interconnects between devices. 
   According to an aspect of the invention a method is provided for fabricating a complementary metal oxide semiconductor (CMOS) circuit on a semiconductor-on-insulator (SOI) substrate, where the SOI substrate includes a single-crystal semiconductor layer separated from a bulk semiconductor region by a buried oxide layer. In such method, a plurality of field effect transistors (FETs) are formed including a first FET and a second FET. Each of the plurality of FETs has a channel region disposed in a common device layer within the single-crystal semiconductor layer. A gate of the first FET overlies an upper surface of the common device layer, and a gate of the second FET underlies a lower surface of the common device layer remote from the upper surface. In addition, the first and second FETs share a common diffusion region disposed in the common device layer and are conductively interconnected by the common diffusion region. The common diffusion region is operable as at least one of a source region or a drain region of the first FET and is simultaneously operable as at least one of a source region or a drain region of the second FET. 
   According to a second aspect of the invention, a method is disclosed of fabricating an electronic circuit using an SOI technique, said SOI technique resulting in formation of at least one buried oxide layer, the electronic circuit comprising a plurality of interconnected electronic devices, each electronic device comprising a respective plurality of components. The method includes fabricating a predetermined first set of respective plurality of components to be on a first side of the buried oxide layer and fabricating a predetermined second set of respective plurality of components to be on a second side of the buried oxide layer, where the second side is the opposite side of the first side, and where the buried oxide layer performs a function integral to the functioning of at least one of the electronic devices. 
   According to a third aspect of the invention, a method is disclosed of SOI fabrication in which a buried oxide layer is formed, where the method includes forming a first set of device components to be on a first side of the buried oxide layer and forming a second set of device components to be on the side opposite the first side, where the buried oxide layer performs a function integral to the functioning of at least one device comprised of components from the first set of components and components from the second set of components. 
   According to a fourth aspect of the invention, a method and structure are disclosed of fabricating a DRAM cell using an SOI technique on a substrate, where the SOI technique results in formation of at least one buried oxide layer. The method includes forming a buried capacitor beneath the buried oxide layer, subsequently forming an FET source and drain regions on top of the buried oxide layer, and interconnecting the capacitor to one of the source region or drain region with a via penetrating the buried oxide layer, where the via is a conductive material. 
   According to a fifth aspect of the invention, a method and structure are disclosed of fabricating a DRAM cell using an SOI technique, where the SOI technique results in formation of at least one buried oxide (BOX) layer, whereby a capacitor for the DRAM cell is formed by a process including forming a buried electrode in a substrate, wherein the buried electrode serves as a lower capacitor charge plate and forming a diffusion link between the diffusion region of a transistor located on the upper side of the BOX and a region to comprise an upper charge plate of the capacitor, where the upper charged plate of the capacitor is formed on the upper side of the BOX when impressing a bias voltage on the buried electrode. 
   According to a sixth aspect of the invention, a method and structure are disclosed of fabricating an electronic circuit having a plurality of electronic devices using an SOI technique, the SOI technique resulting in formation of at least one buried oxide layer. The method includes forming an interconnector of conductive material to interconnect at least two of said plurality of electronic devices, the interconnector at least partially enclosed by said buried oxide. 
   According to a seventh aspect of the invention, a method and structure are disclosed of fabricating a dynamic two-phase shift register. The method includes forming a buried oxide layer using an SOI technique, forming a plurality of FET transistors to be in a device layer above the buried oxide layer, forming a first clock signal conductor on top of the device layer, and forming a second clock signal conductor below the device layer, the second clock signal conductor at least partially enclosed by the buried layer. 
   According to an eighth aspect of the invention, a method and structure are disclosed of fabricating a CMOS circuit. The method includes forming a buried oxide layer using an SOI technique and forming a plurality of FET transistors to be in a device layer above the buried oxide layer, wherein at least two of the FET transistors share a common diffusion region, thereby electrically interconnecting the two FET transistors without using a separate interconnecting conductive material. 
   According to a ninth aspect of the invention, a method and structure are disclosed of fabricating a FET using an SOI technique, the SOI technique resulting in formation of at least one buried oxide layer. The method includes forming a first gate beneath the buried oxide layer and forming a second gate on top of the buried oxide layer. 
   According to a tenth aspect of the invention, a structure is disclosed of an electronic device including at least one SOI buried oxide layer, where the at least one buried oxide layer performs a function integral to the device. 
   According to an eleventh aspect of the invention, a structure is disclosed of an electronic device comprising at least one SOI buried oxide layer, where the at least one SOI buried oxide layer becomes a structural element integral to the device. 
   According to a twelfth aspect of the invention, a structure is disclosed of an electronic circuit comprising a plurality of interconnected devices, the circuit mounted on a wafer having at least one SOI buried oxide layer, wherein the at least one SOI buried oxide layer is a functional element integral to at least one of the devices. 
   According to a thirteenth aspect of the invention, a structure is disclosed of an electronic circuit comprising a plurality of interconnected devices, the circuit mounted on a wafer having at least one SOI buried oxide layer, where the at least one SOI buried oxide layer comprises a structural element integral to at least one of the devices. 
   According to a fourteenth aspect of the invention, a structure is disclosed of an electronic circuit comprising a plurality of interconnected devices, the circuit mounted on a wafer having at least one SOI buried oxide layer, where the two adjacent devices share at least one device component, thereby electrically interconnecting the two devices without an interconnecting conductor, and where the SOI buried oxide layer serves to isolate components of the two interconnected devices other than the shared component. 
   According to a fifteenth aspect of the invention, a method is disclosed of SOI fabrication wherein a buried oxide layer is formed. The method includes forming a first set of device components to be on a first side of the buried oxide layer and forming a second set of device components to be on the side opposite, where the buried oxide layer is used for an active functioning of at least one buried device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
       FIGS. 1A-1C  show three exemplary kinds of structure formed in the supporting silicon body which illustrate how the BOX can be advantageously used; 
       FIG. 2  shows exemplary device structures using the techniques taught in the invention; 
       FIGS. 2A-2E  show exemplary structures formed in the lower section silicon body prior to forming SOI substrate; 
       FIGS. 3A-3D  illustrate the bonded SOI process for completing the process of  FIGS. 2A-2E  to form the device illustrated by  FIG. 2 ; 
       FIGS. 4A-4E  show an exemplary set of steps using the SIMOX process for forming a device illustrated by  FIG. 2 ; 
       FIGS. 5A-5D  illustrate examples of different device elements formed using the invention that illustrate advantages of the invention; 
       FIGS. 6A-6C  illustrate an advantage of the invention of using the BOX to interconnect components without having to use connectors; 
       FIGS. 7A-7B  illustrate another example of the invention, as used to implement DRAM cells; 
       FIGS. 8A-8C  illustrate a second implementation of DRAM cells using the invention; 
       FIGS. 9A-9C  illustrate an example of the invention for a dynamic two phase shift register circuit, which example demonstrates the BOX as a circuit element; 
       FIGS. 10A-10B  illustrate the invention used for a NOR circuit; and 
       FIGS. 11   a,    11   b  show conventional SOI structures. 
   

   Note that the drawings are drawn more to illustrate the inventive processes and structures and are not drawn to scale. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Going back to  FIGS. 11A-11B  illustrating the conventional SOI device, wherein oxide layer  111  separates device layer  110  from substrate  112 . FET devices  113  are built into device layer  110 . One conventional technique forms FET transistors with the following steps: a gate oxide is formed by a surface oxidation of layer  110 , a gate electrode is formed by deposition and patterning of polysilicon, and source and drain regions are formed by implantation of a dopant. These source/drain regions, gate electrodes can then be surface wired  114  by common interconnection processes. 
   Turning now to the invention,  FIGS. 1A-1C  illustrate respectively a buried gate  13 , a buried wire  14 , and a buried capacitor  15  which are exemplary structures resulting from the present invention to use the BOX  12  as an integral part of devices and even entire circuits. Either the SIMOX technique or the bonded technique can be used. Substrate  10  receives device components which are then complemented with components  16  in the device layer  11  above BOX  12 . Similar to conventional SOI structures of  FIGS. 11A-11B , surface conductors  17  could still be used to interconnect devices if desired, although the invention permits interconnections in a different manner. Details of forming these structures and the advantages of the invention will become obvious to one skilled in the art after an understanding of the following sections 
     FIG. 2  shows an exemplary SOI structure  20  in which two FETs  20 A,  20 B are constructed with the BOX  24  as an integral part at the device level. Buried elements  21 , 22  have been formed in the lower section  23 . In this discussion element  21  is a body contact and element  22  is a buried gate. BOX  24  separates lower section  23  from upper section  25  containing additional source and drain regions  26 ,  27 . 
     FIGS. 2A through 3D  show an exemplary formation using the bonded SOI techniques to result in the structure  20  shown in  FIG. 2 . An exemplary formation using SIMOX is illustrated in  FIGS. 4A-4E . The buried elements  13 ,  14 ,  15  shown in  FIGS. 1A-1C  demonstrate that the buried elements  21 ,  22  in  FIG. 2  could be variously a gate, capacitor, or wire, depending on the process/material used in forming the elements. Therefore, it should be obvious that a great variety of devices can be constructed using the concepts taught by this invention. 
   Concerning the bonded technique,  FIG. 2A  shows a method of constructing lower section  23  whereby a silicon dioxide layer  200  having thickness of 250-2500 A is formed on a silicon carrier substrate  201 . This layer  200  and its thickness is not critical since it is used as a selective mask in etching trenches  202 . It is quite likely that the insulator etch and later on polysilicon polish process will remove some of the oxide layer. In a preferred process, the BOX layer will be reformed after removing any residual mask layer at the same step as forming trench sidewall insulator. If needed, a silicon nitride layer  203  (not shown) of thickness in the range of 500-2500 A is used in addition to silicon dioxide  200 . Silicon nitride, although not intended as part of the BOX layer, can provide good selectivity for etching and chemical mechanical polishing and will protect the underlying oxide or the substrate. When nitride is used on top of an oxide layer, any remaining nitride layer after completion of buried structures in substrate  23  will be removed prior to bonding it to the device wafer. The final insulator stack for the mask layer preferably comprises oxide/nitride/oxide layers, although not all layers are essential. The thicknesses of the insulators are chosen depending on the depth of the trench  202  which in turn depends upon the specific component to be placed in the trench, but typically the combined thickness of the insulator stack is less than 5000 A. For forming a buried gate  13  (see  FIG. 1A ), the trench depth  204  is typically about 2000-5000 A, similar to the typical thickness of gate electrodes. For forming a buried wiring layer  14  ( FIG. 1B ), the depth of the trench is typically in the range of 5000 Angstroms to 2 micron. For forming a trench capacitor  15  ( FIG. 1C ), a larger depth of the order of 2-6 microns is chosen. 
   The process of etching vertical trenches in silicon substrate is well known. For example, for the exemplary buried gate process a standard lithography can be used to create the pattern in a resist mask, followed by a directional etching using a Cl 2 /Ar plasma such as described in U.S. Pat. No. 4,139,442, assigned to the assignee and incorporated herein by reference. Other commercially available etch processes are also satisfactory for the trench etch. After removing the resist mask, the substrate is similar to that shown in  FIG. 2B . 
   Subsequently, as shown in  FIG. 2C , to further develop the buried gate structure, an insulation layer  205 ,  206  will be incorporated on the sides/bottom of the trenches  202 . This insulator layer  205 ,  206  typically would be an oxide or nitride layer or a combination thereof and is formed by depostion, in situ conversion of silicon, or a combination of processes. If thermal oxidation is chosen, it can use conventional steam or dry oxygen in a furnace, a rapid thermal heating in an oxidizing ambient, or any equivalent methods. Deposited oxides providing good conformality can also be used. For buried gates or buried wires, it is desired to have these conductors (yet to be formed) fully encased on the sides and bottom  207  with insulator. For other applications such as body contacts, the insulator in the bottom of the trench is not desirable  208 , and for removing the bottom insulator section, a directional etching using fluorine-containing gases such CF 4  or SF 6  can be used in a directional mode to selectively etch the newly formed insulator (oxide)  205  from the horizontal bottom surface  208 , leaving only the insulator along the trench vertical side walls. 
     FIG. 2D  shows that the trenches are then overfilled and planarized back to result in a selected conductor  209 ,  210  embedded in the trench. The conductor  210  can be selected from polysilicon, tungsten or molybdenum and alike for close thermal matching with silicon and stability at the follow-on high process temperatures. An epitaxial Si  209  process can also be used. In one preferred process polysilicon  210  is formed by depositing in an LPCVD reactor at about 600-700 C, using dichlorosilane and a dopant precursor such as phosphine. 
   The width of the gate pattern for the buried gates is restricted by the specific design ground rule. The polysilicon conductor  210  when deposited typically will fill and provide approximately a planar top surface. The polysilicon is then preferably polished by chemical-mechanical polishing (“CMP”) using, for example CABOT SC-I, a colloidal silica in an aqueous KOH solution with pH i10. Other polishing slurries commercially available and known in the field for polishing polysilicon with good selectivity to silicon nitride or silicon dioxide can also be used. 
   At the end of polishing, the polysilicon  210  in the trench may be slightly recessed with respect to the insulator  206  but has a high degree of smoothness, typically a few nanometers. Specifically, the polishing process described in the publication “Characterization of Polysilicon Oxides Thermally Grown and Deposited on the Polished Polysilicon Films”, by Tan Fu Lei et al., IEEE Transactions on Electron Devices, vol. 45, No.4, April 1998, pages 912-917 is extremely attractive for producing a highly smooth polysilicon surface. The polish stop layer silicon nitride  203 , if it was used, is now removed from the top horizontal surfaces by wet etching selective to silicon and silicon dioxide, as is well known in the art. 
     FIG. 2D  represents approximately the appearance in cross section of the substrate after the polishing, with thermal oxide  200  on the top horizontal surfaces and polysilicon  210 . All surfaces are then subjected to post CMP clean using a dilute 50:1 ammonia in a megasonic cleaner. An additional RCA clean process could be used. At this point the height differences between the polysilicon and the silicon substrate is typically less than 500 A Next, as shown in  FIG. 2E , an oxide layer  211  of about 500-1000 A is formed over the polysilicon and remnants of thermal oxide layer  200 . When  211  is formed by thermal oxidation, the thickness of oxide over doped polysilicon is expected to be somewhat thicker than the oxide growth in the surrounding Si areas. If thermal oxidation is used, 1000 A of oxide will consume about 400 A of polysilicon, whereas a slightly thinner oxide layer is formed over the silicon. The surface of the oxide is polished by CMP using a stiff pad and suitable oxide slurry such as Cabot SC-1 so as to form a continuous and smooth oxide layer. If needed, other thinning processes such as etching can be used to compliment the polishing to achieve the desired oxide thickness over the polysilicon gate electrode. The process is typically designed to leave about 100-250 A of silicon dioxide  211  over the polysilicon gate. 
   Alternatively, a high quality CVD silicon dioxide of about 200-1000 A is deposited and polished back to leave a desired thinner oxide layer over the polysilicon gate region. Because of the method described above for the formation of the polysilicon in the trenches, the resulting structure shown in  2 E will have a thinner oxide over the polysilicon gate region  212  and a thicker oxide  213  over the silicon substrate regions. 
   As a possible alternative, if CVD tungsten is used as the buried gate electrode. Instead of depositing polysilicon, a seed layer of TiN or Ti/TiN or TiW is deposited and followed by CVD W deposition using well established techniques with silane, hydrogen and WF 6  gases in a thermal reactor. The blanket metal film will appear similar to the polysilicon planar structure after deposition, which can be treated by CMP or plasma etched back to remove the W and the seed layers from the top surfaces. In one preferred process, the W layer will be recessed by using a plasma etch followed by forming a cap of silicide or silicon. The purpose of forming a tungsten silicide or polysilicon cap is again to form a thin oxide surface over the electrode. If a buried body contact is being formed, then there is no need to form the additional oxide on the surface of the encased conductor. Any oxide formed on the encased conductor is selectively removed. Other known variations of processes can be used to achieve essentially the structure shown in  FIG. 2E  with a variety of materials to form the components. 
   Continuing with the bonded technique,  FIG. 3A  shows the development of the upper section  25 . Substrate  30  is prepared to become a temporary carrier. First, as an optional but one preferred technique to facilitate the removal of excess wafer material after the lower section  23  and upper section  25  have been joined (reference  FIG. 2 ), hydrogen is implanted  31  into the silicon substrate  30 . Epitaxial layers  32  of silicon with different dopants from substrate or silicon-germanium may be deposited over the silicon substrate. Optionally, in the absence of a deposited epitaxial layer, the top surface region of the device substrate will become the device layer. The “Smart-Cut” process utilizing the epitaxially deposited layer is described in greater detail in U.S. Pat. No. 5,882,987, hereby incorporated by reference. The process of hydrogen implantation forms a silicon hydride layer  31  on suitable annealing, that becomes the basis of the Smart-Cut technique to allow separation of the unwanted layers of carrier wafer  30  after the top section  25  is bonded to the bottom section  23 . Although Smart-Cut is the exemplary process for transferring the device layer  32 , alternate processes of combining etching and polishing, such as those described in U.S. Pat. Nos. 4,601,779 and 4,735,679, can also be used. 
   Device layer  32  is deposited epitaxially using, for example, SiGe, but the specific material depends upon the device to be fabricated. An etch stop layer is optionally added on top of the device layer, which could be simply a highly doped silicon layer or a silicon-germanium layer, as per the teaching of the above mentioned U.S. Pat. No. &#39;987. A thin thermal oxide  33  of thickness 50-200 A is optionally grown on the monocrystalline surface. When the end device will include body contact, a bare silicon without oxide layer  33  is used. 
   Hydrogen  31  is implanted under conditions taught in &#39;987, preferably at a depth below the deposited device layer. As shown in  FIGS. 3B and 3C , the device wafer  25  is then flipped and attached to the carrier substrate  23  prepared in  FIGS. 2A-2E . By way of exemplary technique, the oxide surfaces are joined using surface treatments to make oxide surfaces  33 ,  211  hydrophilic. Such attached wafers have sufficient bonding to withstand most handling. The wafers are now heated at about 300-600 C to complete the Smart-Cut process as shown in  FIG. 3D , in which the excess wafer section  34  is then removed. In one variation of the Smart-Cut process, the wafer is heated to a temperature range 250-400 C to segregate hydrogen to the device layer interface (in the case of SiGe deposited layer), followed by cleaving the substrate  34  along the hydrogen implanted surface using water jets. 
   The transferred device layer surface  35  is now finished to a smooth surface by polishing or etching or along the teaching of U.S. Pat. No. &#39;987 using an optional etch-stop layer Thus, an SOI wafer  20  (see also  FIG. 2 ) with buried body contact  21  and buried gate electrode  22  has now been formed. The gate oxide  36  on the buried gate electrode is roughly equal to the thickness of oxide  33  or to the sum of the two surface oxides  33 ,  211 , and can be between 100-500 A, depending upon the selection of thickness of individual oxide layers. As discussed earlier, one of the oxide layers  33  can also be conveniently not formed since bare Si surface can also be effectively bonded to silicon dioxide. As discussed in the IEEE publication mentioned above, the polyoxide formed over polished polysilicon, either thermally formed or deposited, is very high quality, approaching that required for gate oxide applications. 
   Referring now back to  FIG. 2  showing the completed SOI structure, top gate electrodes  214 ,  215  are formed on top of a gate oxide layer  216 . To achieve this, typically a polysilicon layer deposited on top of the top gate insulator is patterned to create top gate electrodes  214 ,  215 . The device layer  25  is now the channel or body layer for both the top gates  214 ,  215  and bottom gate structures  21 , 22 . The top gate electrodes  214 , 215  could be a polycide layer if the application would require a lower resistance. 
     FIGS. 4A-4E  illustrate an alternate formation using the SIMOX process of a corresponding buried contact and buried gate electrode structure. The process steps to create the buried structures  209 ,  210  is same as used in  FIGS. 2A-2D . Thus,  FIG. 4A  starts as being the same structure shown in  FIG. 2D  with trench/sidewall/conductor  209  and trench/sidewall/bottom/conductor  210  structures filled with doped polysilicon or other suitable refractory conductor material. Polysilicon will arbitrarily be assumed here as the conductor.  FIG. 4B  shows surface insulator  200  having been removed and gate insulator  401  having thickness of 50-200 angstroms being formed over the electrode  210  to be used a buried gate. In a preferred process, this gate insulator  401  is formed by oxidizing the polysilicon with the oxide insulator  200  still in place and then the insulator  200  is removed by a polish or etch process. In one preferred embodiment of this polysilicon oxidation process, the insulator  200  has an additional SiN layer to allow only the polysilicon to be exposed and thereby oxidized in a controlled manner. 
   Thereafter, oxide layer  200  is removed and any insulating layer  402  over the buried contact  209  is selectively removed by means of a block-out mask ( FIG. 4B ). A device layer  403  is deposited under epitaxial condition, which forms a single crystal over the all silicon surface ( FIG. 4C ), except that small regions of polycrystalline Si  404 ,  405  are formed over polysilicon and oxide surfaces. The regions  404  and  405  can be formed single crystalline if epitaxial conditions for lateral growth can be used, such as taught in U.S. Pat. No. 5,646,958, the contents of which are incorporated herein by reference. In  FIG. 4D  an implantation mask  406  is formed over the buried regions and oxygen ions  407  are implanted into substrate  201 , using typical SIMOX conditions such as taught in U.S. Pat. No. 6,043,166, the contents of which are incorporated herein by reference. 
   The energetics of the implantation controls the depth of the implanted ions  407 . For a buried gate  210  or body contact  209 , the implant depth is chosen to be slightly beneath the device layer. For buried wires and capacitors, since the structures are fully encased in insulator, this implantation location is less critical but preferably the implant depth is chosen to be near the device layer and substrate interface so that at least part of the BOX layer formed can cover the top of the wire and capacitor elements. 
   Using anneal conditions and timing such as taught in U.S. Pat. No. 6,043,166, the implanted oxygen is converted into a buried oxide layer  408  as shown in  FIG. 4E . Transistors are formed with gate oxide  409  and gate electrodes  410  using standard masking and deposition techniques to result in the structure similar to that shown in  FIG. 2 . 
   Even though the SIMOX process has been described using a set of preferred process steps with a view to forming buried gate electrode and buried body contact elements, it should be obvious to one skilled in the art, the above described process steps can be used as well to form other elements such as buried wiring layer or capacitor elements by small variations to the above process. 
     FIGS. 5A-5D  show a magnified view of three exemplary SOI structures, buried gate electrode  50 A ( FIG. 5A ), body contact  50 C ( FIG. 5C ), and buried wire  50 D ( 5 D), for of demonstrating additional advantages of the invention. 
     FIG. 5A  shows the resultant structure  50 A when the lower section  53  and upper section  54  are formed so as to result in a buried gate electrode  58 A. Of particular interest in this structure  50 A, and which differs from the prior art, is that the buried oxide (BOX) layer  55 A is now an integral part of the second gate device  58 A. Specifically, the SOI buried oxide layer  55 A acts as the second gate insulator for the buried FET and also as isolation of the device layer  54  from the substrate  53 . 
   Also of interest in the  FIG. 5A  structure  50 A is that the buried oxide layer  55 A forming the second gate insulator will generally be a different thickness than the upper oxide layer  59  forming the upper gate insulator structure. This different thickness can be a useful technique for controlling the dual gated device characteristics. 
     FIG. 5B  shows an example of a top view of the dual gates layout. The effective shapes  501 ,  502  of the two gates  56  and  58 A can have different length, width or shapes to facilitate easier contact to respective gates or to obtain a device of different channel lengths so that a dual gate and single gate channel regions can be combined in parallel to achieve different gains. The top and bottom gates  56 ,  58 A can be positioned to small variations such as different angles (bent gates) to facilitate for example, better lay out of wiring tracks on the top or easier contact to bottom. 
     FIGS. 5A and 5B  also show the technique of connecting the upper and lower gates  501 ,  502  by vias  503 ,  504  so that when the gate voltage is impressed on the top, it acts on both top and bottom, improving the device performance. In the inventive process, this connection can be achieved using a simple process of using two layers of polysilicon for the top gate electrode, such as described in U.S. Pat. No. 4,341,009, which is incorporated herein by reference. Using this referenced process, formation of the via  503  and  504  is straightforward. &#39;009 describes a process using dual polysilicon to form buried contacts. First a thin layer of polysilicon or polycide is deposited on the gate oxide, followed by etching a contact hole through the thin electrode, gate oxide and body channel layer, and buried gate oxide to the buried gate electrode. A second gate electrode layer is now deposited and patterned to make the first and second electrode contact. During this process, it is also possible to make other connections such as body contact, as additional contact can be made to the carrier substrate. This technique is used here where the gate electrode is formed in two steps. In step  1 , a first polysilicon layer is blanket deposited over the gate oxide in forming the top device, followed by etching the via. A second polysilicon layer now is deposited on the first polysilicon which makes the contact to the body layer or bottom electrode while providing additional thickness to the top gate electrode. This stack is now patterned to include top gate electrode and the via connection. A more traditional process step can be used whereby the top electrode is formed, via  403  or  404  is etched in a separate step and a local interconnect or a contact stud metallization used to connect the top and bottom electrodes. 
   In SOI devices, there is a strong need to connect the body silicon region to a common ground or substrate potential to stabilize the threshold voltage.  FIG. 5C  shows one such structure  50 C having device layer  54 , BOX layer  55 C, and substrate  53 . Region  58 C which is a polysilicon electrode that contacts directly the device layer  54  at the body region of the gate  56 . Forming such a polysilicon electrode has been discussed already relative to  FIGS. 2-4 . This preferred embodiment provides a required body contact with no additional space needs, without any need for additional photo process, layer depositions, etc. This embodiment therefore represents an attractive process for forming an SOI buried contact. 
     FIG. 5D  shows a buried wire  52 , which can be used for making local interconnect between a contact of a transistor to an adjacent transistor or to a resistor or capacitor. For schematic simplicity one via contact is shown extending a via from a buried wire to the top surface above the device layer. In typical applications multiple vias are provided from the same buried wire which can be used to connect devices at the top surface. Since the buried wire layer  52  is at a different plane than the devices, wireability is easily achieved, without concerning of crossing over other devices or other connections on the top surface. 
   One of the important features of this invention is the ability to use the SOI buried layer to form separate devices while still retaining a commonality between the devices. This feature allows devices to be interconnected without having to provide interconnection conductors, thereby improving device density. This feature is exemplarily illustrated in  FIG. 6A  and  FIG. 6B  for the case of forming separate FETs  61 ,  62  sharing a common body layer  64 . Additional specific examples will be discussed later and many more should be obvious to one of ordinary skill in the art, but the examples in  FIGS. 6A ,  6 B will demonstrate the important concept that entire circuits can be more effectively fabricated by considering the BOX as an important component at not only the device level but also at the circuit level, as will be discussed in more detail shortly. 
   In  FIG. 6A  is shown the general case of two devices  61  and  62  isolated by SOI buried layer  63  and sharing a common body layer  64 . This feature enables formation of many more FETs, with each layer of FET design being optimized by separate layout restraints. As discussed earlier, buried electrodes and body contacts can be advantageously used to interconnect these devices to form circuits. When the buried gate  62  is laterally separated from the top gate, the source/drain regions for the buried gate can be formed by patterning dummy gates over the buried gate as a masking layer and implanting selective regions to complete the buried FET device. 
   For many applications, the source/drain of adjacent devices can be advantageously shared, as shown in  FIG. 6B , to provide specific circuit interconnections. This technique increases the density of device layout since this configuration becomes a series connection at node  68  between FETs  65 ,  66  without having to use additional interconnectors. It should be obvious that parallel connections are similarly possible. 
     FIG. 6C  illustrates the degree of freedom in the layout of the top electrodes  61 , 66  and bottom electrodes  62 , 65  resulting from this invention. For example, one or both of the gates can have bends in order to meet other requirements or provide other advantages such as ease of wireability. 
     FIG. 7A  illustrates a schematic of a conventional DRAM cell using a single FET  75  and a single capacitor  70 . One electrode of the capacitor is connected to the drain region of the FET  75  and the other electrode is grounded.  FIG. 7B  shows the SOI device embodying two of these DRAM cells and taking advantage of the invention, the first using a top gate FET  75 A and the second a buried gate FET  75 B. Buried capacitors  70 A,  70 B are formed in the substrate  78  and top gate for  75 A and buried gate for  75 B are connected to the capacitors using vias  74 A,  74 B. The structure of  FIG. 7   b  is formed by the combination of substrate  78  with a device layer  77  through an intermediate BOX layer  79 . Various possible processes that can be used to form these structures have been described already with the aid of  FIGS. 2-4 . 
   The formation of capacitors  70 A, 70 B in a substrate, for example, has been specifically discussed with the aid of  FIGS. 2A-2D . The buried conductor  73 A,  73 B with the right choice of trench dimensions and capacitor node dielectric  71 A,  71 B (oxide or oxide/nitride formed on the trench walls) will determine the capacitance value of the buried capacitors. The capacitor ground electrodes can be formed either by use of a highly doped substrate  78 , or by diffusion drive-in of dopants to form a highly doped external regions  72 A,  72 B in the substrate along the perimeter of the capacitors  70 A, and  70 B prior to forming the node insulator. This step and additional process steps for forming such a structure is known and described in U.S. Pat. No. 5,770,484, which contents are herein incorporated. 
   In contrast to processes where the buried capacitor is formed subsequent to the SOI substrate, the process described here, where the buried capacitor was formed prior to the SOI structure, offers process simplicity in comparison with other SOI trench capacitor processes and can provide better yields and lower cost. 
     FIGS. 8A-8C  show a variation of the DRAM cells discussed in  FIGS. 7A-7B .  FIG. 8A  is a well known prior art schematic diagram of a single device storage capacitor circuit which uses a single transistor Q 1  and a storage capacitor C 1 . Use of depletion capacitors are well known in the art (see for example U.S. Pat. Nos. 4,163,243 and 4,259,729). The gate of Q 1  is activated by a high voltage to turn Q 1  on, thus allowing the data signal level on bit-line BL 0  to be transferred to the capacitor C 1 . The schematic shown in  FIG. 8A  is similar to the schematic in  FIG. 7A , except that the capacitor node labeled VDD was at ground potential.  FIG. 8B  illustrates one embodiment of using a single depleted capacitor  80  utilizing a positive bias voltage impressed on the buried electrode to create an accumulation region  81  (counter electrode) in the device layer  82 . An important novelty of this circuit application is in the physical arrangement of the transistor Q 1  ( 83 ) located on the top of the common shared semiconductor region  82  and the capacitor C 1  ( 80 ) located on the bottom side of that same shared region. This structure is made possible by the semiconductor teaching of this invention.  FIG. 8B  will be further described in the following paragraph but it should be explained that a multiple variations on this scheme are easily visualized. 
   In the embodiment of  FIG. 8B  the data bit to be stored is presented to the cell on bit line BL 0 . Transistor  83  (Q 1 ) is activated, as previously stated, by a high signal applied to its gate  84 , thus allowing the voltage level of BL 0  to be transferred to capacitor  80  (C 1 ). As is well known in the art, the DRAM cell is read out by preconditioning BL 0  to a predetermined voltage level that is between a logical 1 high and a logical 0 low voltage level. Bit line BL 0  is connected to a sense amplifier (not shown) which will differentially sense the voltage between BL 0  and a reference voltage. A high voltage is applied to WL 1  the gate of transistor Q 1 . This turns Q 1  on and the signal stored on capacitor C 1  will be transferred to BL 0 . This signal will be very small compared to the signal that was originally written into the cell using BL 0 . The sense bit line BL 0  will be disturbed electrically in either the positive voltage direction or negative voltage direction from its predetermined intermediate level depending on the state stored in capacitor C 1 . The sense amplifier attached to BL 0  will sense and amplify this small voltage disturbance. 
     FIG. 8B  shows that one side of capacitor Cl is connected via a diffusion  85  to transistor Q 1 . The other electrode of C 1  is a plate formed with polysilicon electrode of capacitor  80 . The insulator  86  overlying the electrode of capacitor  80  (C 1 ) is the capacitor dielectric. This dielectric could be the same or similar material SiO 2  as in BOX layer  87 . It could also be a different material such as a high dielectric material allowing a larger value of capacitance for C 1  using the same plate area as this material can be formed during the formation of the buried capacitor electrode by deposition. 
   The arrangement of electrodes of capacitor becomes clear by comparing  FIGS. 8A and 8B . The diffusion region  85  connects the top electrode of the capacitor C 1  to a plate-like region formed by inducing charge on the top surface of the thin dielectric  86  (oxide or high dielectric material) by applying a positive potential to the lower plate of C 1 . The positive potential causes negative carriers to be attracted to the top side of C 1  making it conductive and forming the top plate. The bottom plate of the capacitor is simply the buried electrode of capacitor  80 . 
   One aspect of novelty in this structure is the location of C 1  horizontally relative to Q 1 . C 1  may be located substantially under Q 1  which produces a minimum total cell area, allowing maximum DRAM memory density on a unit area of silicon wafer. It may, however, be located substantially outside the region covered by the gate of Q 1  for a minimum density result and still operate. The point is that the location of C 1  relative to Q 1  is non critical, so long as C 1  does not come closer to bit line BL 0  than some minimum dimension established by a leakage current/storage cell retention time criteria. 
     FIG. 8C  is an extension of  FIG. 8B , wherein the capacitor is provided by forming the structures  80 B and  80 T, where  80 T is now formed on top of the device layer  82 . The advantage of this is that the area of capacitor  80  can be cut in half allowing for greater overall packing density. In addition to using high dielectric constant insulators for the capacitors, one can also use roughened surface electrodes to increase the capacitor electrode area. Both these techniques are well known in the art. Additional variations of structure and materials are possible within the general concepts of forming buried structures taught in here. 
     FIGS. 9A-9C  illustrate an application of building and operation of a dynamic two phase shift register using the invented structure.  FIG. 9A  shows a conceptual vertical structure utilizing the semiconductor processing teaching of this invention to construct four N-type transistors that is connected as per the Figure C schematic to provide a two phase dynamic shift register with the  FIG. 9B  timing diagram. These dynamic shift registers have been a classical circuit technique to store data. 
     FIG. 9A  shows the cross section of one possible SOI structure created by a substrate  91 , a device layer  92 , and an oxide layer  90  separating the two. Further, along the teachings of this invention, two buried gate transistors  941  and  943  are formed in the substrate region  91 . Two top surface FETs  942 ,  944  are formed using additional process steps on the device layer. All the FETs are N-type, as determined by the choice of dopants in the device layer and Source/Drain regions, and all sharing the same body layer  92 . By use of overlapping source and drain regions  95  between adjacent FETs, the series connection of the transistors as in  FIG. 9C  is achieved without a need for any external wiring. 
   In a two phase dynamic shift register two transistors are used to store one bit of data. In the case of  FIG. 9C , transistor  941  and  942  together store bit  1  and transistors  943  and  944  store bit  2 . Referring to  FIG. 9B , clock C 1  signal  96  is applied to the gate of transistors  941 ,  943 , and clock C 2  signal  97  is applied to wire connected to gates of transistors  942 ,  944 . The data bit is actually stored on the parasitic capacitance of the circuit, such as the diffusion capacitance. Two clock signals  96 ,  97  are used to control the shifting of the data from one bit location to the next. One bit is shifted one position by applying clock signal C 1  (a high) followed by clock signal C 2  (a high). The clocks are non-overlapping meaning that C 1  and C 2  are never both high at the same time. Eventually the data entered into the shift register is attenuated and lost after some number of shift positions unless it is restored in amplitude by a gain stage. Variations of the two phase shift registers can be constructed with more transistors than shown in  FIG. 9A  so as to restore or amplify the data at each bit position in the serial string. The circuits for shift registering and amplification are known in the art and the novel aspect of the present invention is the two phase shift register structure shown in  FIG. 9A , which provide space saving and greater density. 
   The two phase shift register structure of this invention register is based upon the very important semiconductor processing teaching of this invention that allows transistors to be isolated by BOX layer  90  to be formed on top and in bottom of a shared region  92  of semiconducting material. 
   In the structure shown in  FIG. 9A  the transistors do not lie one above another but are staggered such that the source of one transistor is shared with the drain of a second transistor, an embodiment earlier discussed with  FIG. 6B . As can be readily seen, one of the novelties in  FIG. 9A  is that the two transistors of this invention  941  and  942 , unlike prior art, do not reside on the same vertical level, typically both on top. In this invention, one of the transistor  941  (Q 1 ) is in the bottom, and the next transistor  942  (Q 2 ) is on the top. The wiring of the clock signal C 1  to the gate of Q 1  takes place below the common layer  92  structure, at least in part, where it is necessary to connect to gate region, i.e., via polysilicon. Similarly, the corresponding wiring to Q 2  takes place above the common layer  92  structure providing a means to connect clock signal C 2  to the gate region of transistor Q 2 . In this manner the necessary wiring to gates on either the top side or the bottom side is substantially reduced in utilization of available real-estate on any one side. 
   Further, if geometries of the transistors, diffusions, and gate wiring were such that a conflict for available real-estate existed when attempting to wire the gate regions of two sequential transistors in the shift register chain, such conflict would be substantially reduced or eliminated by constructing the shift register in an alternating fashion of top/bottom transistor location as shown in  FIG. 9A . The circuit chosen to demonstrate this concept is the two phase dynamic shift register because it is a well known application of classical MOSFET function. However other circuit applications would obviously benefit equally well with reduced gate wireability congestion thus allowing for improved device/circuit density. 
     FIGS. 10A and 10B  show the application of the subject disclosure to a CMOS NOR logic circuit. The  FIG. 10A  schematic shows a two-way logical NOR circuit. Input signals A &amp; B are connected to the gates of transistors Q 2  &amp; Q 4  and Q 1  &amp; Q 3 , respectively. Transistors Q 1  and Q 2  are P-type transistors and transistors Q 3  and Q 4  are N-type. This schematic is well known and one of the most widely used logic circuits. The other widely used CMOS circuits are the NAND and the simple inverter circuit, and the implementation of the invention into these well known circuits would be obvious to one of ordinary skill in the art. 
   The structure of the NOR circuit in  FIG. 10B  represents a vertical cross-section of a semiconductor chip utilizing the subject invention. A substrate  101  and a device layer  103  are separated by a BOX layer  102 . The transistor Q 4  is formed within the substrate (buried) using the process steps taught in the preferred embodiments. The transistors Q 1 , Q 2  and Q 3  are formed on the device layer using conventional processes of oxidation, gate electrode deposition, patterning etc. The scale of the semiconductor geometry is simplified here to assist the understanding of how the NOR circuit of  FIG. 10A  is realized. The most dramatic benefit and novel benefit apparent in  FIG. 10B  is seen in the location of transistor Q 3  directly above transistor Q 4 . It should be noted that transistors Q 1  and Q 2  are constructed on the same horizontal axis. Since the transistors Q 1 , Q 2  are P-type and Q 3  and Q 4  are N-type, the device layer has isolation regions to separate the different dopants in the device layer corresponding N-type and P-type regions. The current industry practice is to have the placement or physical location of all transistors on the same horizontal plane. 
   However, this invention allows a unique means to fabricate transistor Q 3 , Q 4 , one above the other, thus allowing for a significant reduction in chip size for a given logical function. It should be noted that this is the technique discussed earlier in which components are connected in parallel without requiring separate interconnection conductors. 
   Additional benefit will be apparent in this structure in the area required for the commonly shared source drain diffusions shared by transistors Q 3 , Q 4 . In particular the area of the common drain diffusion of Q 3  and Q 4  shared with the source diffusion of Q 2  is reduced in area such that the switching time on the NOR circuit is significantly reduced. This common node or diffusion also serves as the output node of the circuit. Since any capacitance reduction results in a reduced circuit delay (switching time), the speed is additionally increased. The concept here is shown for a NOR circuit but is also readily applied to the popular NAND logic circuit and many other circuit types found in the current CMOS logic technology industry that produces today&#39;s microprocessor chips and ASIC chips. 
   These are but some examples of circuits that can be formed utilizing buried devices in conjunction with traditional FETs and other devices. Many ASIC applications can benefit with the additional design ground rules allowed by the inventive devices being available in the buried substrate. 
   The examples discussed also demonstrate that with these techniques the buried oxide can be used for more than simple isolation. The BOX has been shown to be available for other functions such as the gate oxide for a buried transistor and the pass-through for a body contact. 
   While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.