Abstract:
A method of manufacturing a semiconductor wafer, the method comprising: providing a base wafer comprising a semiconductor substrate; preparing a first monocrystalline layer comprising semiconductor regions; preparing a second monocrystalline layer comprising semiconductor regions overlying the first monocrystalline layer; and etching portions of said first monocrystalline layer and portions of said second monocrystalline layer as part of forming at least one transistor on said first monocrystalline layer.

Description:
CROSS-REFERENCE OF RELATED APPLICATION 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 12/901,902, which was filed on Oct. 11, 2010, the contents of which are incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention describes applications of monolithic 3D integration to semiconductor chips performing logic and memory functions. 
         [0004]    2. Discussion of Background Art 
         [0005]    Over the past 40 years, one has seen a dramatic increase in functionality and performance of Integrated Circuits (ICs). This may have largely been a result from the phenomenon of “scaling” i.e. component sizes within ICs have been reduced (“scaled”) with every successive generation of technology. There may be two main classes of components in Complementary Metal Oxide Semiconductor (CMOS) ICs, namely transistors and wires. With “scaling”, transistor performance and density typically improve and this may have contributed to the previously-mentioned increases in IC performance and functionality. However, wires (interconnects) that connect together transistors degrade in performance with “scaling”. The situation today is that wires dominate performance, functionality and power consumption of ICs. 
         [0006]    3D stacking of semiconductor chips may be one avenue to tackle issues with wires. By arranging transistors in 3 dimensions instead of 2 dimensions (as was the case in the 1990s), one can place transistors in ICs closer to each other. This reduces wire lengths and keeps wiring delay low. However, there may be many barriers to practical implementation of 3D stacked chips. These include:
       Constructing transistors in ICs typically require high temperatures (higher than about 700° C.) while wiring levels may be constructed at low temperatures (lower than about 400° C.). Copper or Aluminum wiring levels, in fact, can get damaged when exposed to temperatures higher than about 400° C. If one may like to arrange transistors in 3 dimensions along with wires, it may have the challenge described below. For example, a 2 layer stack of transistors and wires, i.e. Bottom Transistor Layer, above it Bottom Wiring Layer, above it Top Transistor Layer and above it Top Wiring Layer, may be arranged. When the Top Transistor Layer is constructed using Temperatures higher than about 700° C., it can damage the Bottom Wiring Layer.   As a result from the above mentioned problem with forming transistor layers above wiring layers at temperatures lower than about 400° C., the semiconductor industry has largely explored alternative architectures for 3D stacking In these alternative architectures, Bottom Transistor Layers, Bottom Wiring Layers and Contacts to the Top Layer may be constructed on one silicon wafer. Top Transistor Layers, Top Wiring Layers and Contacts to the Bottom Layer may be constructed on another silicon wafer. These two wafers may be bonded to each other and contacts may be aligned, bonded and connected to each other as well. Unfortunately, the size of Contacts to the other Layer may be large and the number of these Contacts may be small. In fact, prototypes of 3D stacked chips today utilize as few as about 10,000 connections between two layers, compared to billions of connections within a layer. This low connectivity between layers may be because of two reasons: (i) Landing pad size may need to be relatively large as a result from alignment issues during wafer bonding. These may result from many reasons, including bowing of wafers to be bonded to each other, thermal expansion differences between the two wafers, and lithographic or placement misalignment. This misalignment between two wafers may limit the minimum contact landing pad area for electrical connection between two layers; (ii) The contact size may need to be relatively large. Forming contacts to another stacked wafer typically involves having a Through-Silicon Via (TSV) on a chip. Etching deep holes in silicon with small lateral dimensions and filling them with metal to form TSVs may not be easy. This places a restriction on lateral dimensions of TSVs, which in turn impacts TSV density and contact density to another stacked layer. Therefore, connectivity between two wafers may be limited.       
 
         [0009]    It may be highly desirable to circumvent these issues and build 3D stacked semiconductor chips with a high-density of connections between layers. To achieve this goal, it may be sufficient that one of three requirements must be met: (1) A technology to construct high-performance transistors with processing temperatures below about 400° C.; (2) A technology where standard transistors may be fabricated in a pattern, which allows for high density connectivity despite the misalignment between the two bonded wafers; and (3) A chip architecture where process temperature increase beyond about 400° C. for the transistors in the top layer does not degrade the characteristics or reliability of the bottom transistors and wiring appreciably. This patent application describes approaches to address (1), (2) and (3) in the detailed description section. In the rest of this section, background art that has previously tried to address (1), (2) and (3) may be described. 
         [0010]    U.S. Pat. No. 7,052,941 from Sang-Yun Lee (“S-Y Lee”) describes methods to construct vertical transistors above wiring layers at less than about 400° C. In these single crystal Si transistors, current flow in the transistor&#39;s channel region is in the vertical direction. Unfortunately, however, almost all semiconductor devices in the market today (logic, DRAM, flash memory) utilize horizontal (or planar) transistors as a result from their many advantages, and it may be difficult to convince the industry to move to vertical transistor technology. 
         [0011]    A paper from IBM at the Intl. Electron Devices Meeting in 2005 describes a method to construct transistors for the top stacked layer of a 2 chip 3D stack on a separate wafer. This paper is “Enabling SOI-Based Assembly Technology for Three-Dimensional (3D) Integrated Circuits (ICs),”  IEDM Tech. Digest , p. 363 (2005) by A. W. Topol, D. C. La Tulipe, L. Shi, et al. (“Topol”). A process flow is utilized to transfer this top transistor layer atop the bottom wiring and transistor layers at temperatures less than about 400° C. Unfortunately, since transistors may be fully formed prior to bonding, this scheme suffers from misalignment issues. While Topol describes techniques to reduce misalignment errors in the above paper, the techniques of Topol still suffer from misalignment errors that limit contact dimensions between two chips in the stack to greater than about 130 nm. 
         [0012]    The textbook “Integrated Interconnect Technologies for 3D Nanoelectronic Systems” by Bakir and Meindl (“Bakir”) describes a 3D stacked DRAM concept with horizontal (i.e. planar) transistors. Silicon for stacked transistors may be produced using selective epitaxy technology or laser recrystallization. Unfortunately, however, these technologies have higher defect density compared to standard single crystal silicon. This higher defect density degrades transistor performance. 
         [0013]    In the NAND flash memory industry, several organizations have attempted to construct 3D stacked memory. These attempts predominantly use transistors constructed with poly-Si or selective epi technology as well as charge-trap concepts. References that describe these attempts to 3D stacked memory include “Integrated Interconnect Technologies for 3D Nanoelectronic Systems”, Artech House, 2009 by Bakir and Meindl (“Bakir”), “Bit Cost Scalable Technology with Punch and Plug Process for Ultra High Density Flash Memory”, Symp. VLSI Technology Tech. Dig. pp. 14-15, 2007 by H. Tanaka, M. Kido, K. Yahashi, et al. (“Tanaka”), “A Highly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried Channel BE-SONOS Device,” Symposium on VLSI Technology, 2010 by W. Kim, S. Choi, et al. (“W. Kim”), “A Highly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried Channel BE-SONOS Device,” Symposium on VLSI Technology, 2010 by Hang-Ting Lue, et al. (“Lue”) and “Sub-50 nm Dual-Gate Thin-Film Transistors for Monolithic 3-D Flash”, IEEE Trans. Elect. Dev., vol. 56, pp. 2703-2710, November 2009 by A. J. Walker (“Walker”). An architecture and technology that utilizes single crystal Silicon using epi growth is described in “A Stacked SONOS Technology, Up to 4 Levels and 6 nm Crystalline Nanowires, with Gate-All-Around or Independent Gates (ΦFlash), Suitable for Full 3D Integration”, International Electron Devices Meeting, 2009 by A. Hubert, et al (“Hubert”). However, the approach described by Hubert may have some challenges including the use of difficult-to-manufacture nanowire transistors, higher defect densities may result from the formation of Si and SiGe layers atop each other, high temperature processing for long times, and difficult manufacturing. 
         [0014]    It is clear based on the background art mentioned above that invention of novel technologies for 3D stacked chips may be useful. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is an exemplary illustration of process temperatures needed for constructing different parts of a single-crystal silicon transistor. 
           [0016]      FIG. 2A-E  are exemplary illustrations of a layer transfer flow using ion-cut in which a top layer of doped Si may be layer transferred atop a generic bottom layer. 
           [0017]      FIG. 3A-E  are exemplary illustrations of a process flow for forming a 3D stacked IC using layer transfer which requires greater than about 400° C. processing for source-drain region construction. 
           [0018]      FIG. 4  is an exemplary illustration of a junction-less transistor as a switch for logic applications (prior art). 
           [0019]      FIG. 5A-F  are exemplary illustrations of a process flow for constructing 3D stacked logic chips using junction-less transistors as switches. 
           [0020]      FIG. 6A-D  are exemplary illustrations of different types of junction-less transistors (JLT) that could be utilized for 3D stacking applications. 
           [0021]      FIG. 7A-F  are exemplary illustrations of a process flow for constructing 3D stacked logic chips using one-side gated junction-less transistors as switches. 
           [0022]      FIG. 8A-E  are exemplary illustrations of a process flow for constructing 3D stacked logic chips using two-side gated junction-less transistors as switches. 
           [0023]      FIG. 9A-V  are exemplary illustrations of process flows for constructing 3D stacked logic chips using four-side gated junction-less transistors as switches. 
           [0024]      FIG. 10A-D  are exemplary illustrations of types of recessed channel transistors. 
           [0025]      FIG. 11A-F  are exemplary illustrations of a procedure for layer transfer of silicon regions for recessed channel transistors. 
           [0026]      FIG. 12A-F  are exemplary illustrations of a process flow for constructing 3D stacked logic chips using standard recessed channel transistors. 
           [0027]      FIG. 13A-F  are exemplary illustrations of a process flow for constructing 3D stacked logic chips using RCATs. 
           [0028]      FIG. 14A-I  are exemplary illustrations of construction of CMOS circuits using less than about 400° C. transistors (e.g., junction-less transistors or recessed channel transistors). 
           [0029]      FIG. 15A-F  are exemplary illustrations of a procedure for accurate layer transfer of thin silicon regions. 
           [0030]      FIG. 16A-F  are exemplary illustrations of an alternative procedure for accurate layer transfer of thin silicon regions. 
           [0031]      FIG. 17A-E  are exemplary illustrations of an alternative procedure for low-temperature layer transfer with ion-cut. 
           [0032]      FIG. 18A-F  are exemplary illustrations of a procedure for layer transfer using an etch-stop layer controlled etch-back. 
           [0033]      FIG. 19  is an exemplary illustration of a surface-activated bonding for low-temperature less than about 400° C. processing. 
           [0034]      FIG. 20A-E  are exemplary illustrations of a description of Ge or III-V semiconductor Layer Transfer Flow using Ion-Cut. 
           [0035]      FIG. 21A-C  show laser-anneal based 3D chips (prior art). 
           [0036]      FIG. 22A-E  are exemplary illustrations of a laser-anneal based layer transfer process. 
           [0037]      FIG. 23A-C  are exemplary illustrations of a window for alignment of top wafer to bottom wafer. 
           [0038]      FIG. 24A-B  are exemplary illustrations of a metallization scheme for monolithic 3D integrated circuits and chips. 
           [0039]      FIG. 25A-F  are exemplary illustrations of a process flow for 3D integrated circuits with gate-last high-k metal gate transistors and face-up layer transfer. 
           [0040]      FIG. 26A-D  are exemplary illustrations of an alignment scheme for repeating pattern in X and Y directions. 
           [0041]      FIG. 27A-F  are exemplary illustrations of an alternative alignment scheme for repeating pattern in X and Y directions. 
           [0042]      FIG. 28  show floating-body DRAM as described in prior art. 
           [0043]      FIG. 29A-H  are exemplary illustrations of a two-mask per layer 3D floating body DRAM. 
           [0044]      FIG. 30A-M  are exemplary illustrations of a one-mask per layer 3D floating body DRAM. 
           [0045]      FIG. 31A-K  are exemplary illustrations of a zero-mask per layer 3D floating body DRAM. 
           [0046]      FIG. 32A-J  are exemplary illustrations of a zero-mask per layer 3D resistive memory with a junction-less transistor. 
           [0047]      FIG. 33A-K  are exemplary illustrations of an alternative zero-mask per layer 3D resistive memory. 
           [0048]      FIG. 34A-L  are exemplary illustrations of a one-mask per layer 3D resistive memory. 
           [0049]      FIG. 35A-F  are exemplary illustrations of a two-mask per layer 3D resistive memory. 
           [0050]      FIG. 36A-F  are exemplary illustrations of a two-mask per layer 3D charge-trap memory. 
           [0051]      FIG. 37A-G  are exemplary illustrations of a zero-mask per layer 3D charge-trap memory. 
           [0052]      FIG. 38A-D  are exemplary illustrations of a fewer-masks per layer 3D horizontally-oriented charge-trap memory. 
           [0053]      FIG. 39A-F  are exemplary illustrations of a two-mask per layer 3D horizontally-oriented floating-gate memory. 
           [0054]      FIG. 40A-H  are exemplary illustrations of a one-mask per layer 3D horizontally-oriented floating-gate memory. 
           [0055]      FIG. 41A-B  are exemplary illustrations of a periphery on top of memory layers. 
           [0056]      FIG. 42A-E  are exemplary illustrations of a method to make high-aspect ratio vias in 3D memory architectures. 
           [0057]      FIG. 43A-F  are exemplary illustrations of an implementation of laser anneals for JFET devices. 
           [0058]      FIG. 44A-D  are exemplary illustrations of a process flow for constructing 3D integrated chips and circuits with misalignment tolerance techniques and repeating pattern in one direction. 
           [0059]      FIG. 45A-D  are exemplary illustrations of a misalignment tolerance technique for constructing 3D integrated chips and circuits with repeating pattern in one direction. 
           [0060]      FIG. 46A-G  are exemplary illustrations of using a carrier wafer for layer transfer. 
           [0061]      FIG. 47A-K  are exemplary illustrations of constructing chips with nMOS and pMOS devices on either side of the wafer. 
           [0062]      FIG. 48  is an exemplary illustration of using a shield for blocking Hydrogen implants from gate areas. 
           [0063]      FIG. 49  is an exemplary illustration of constructing transistors with front gates and back gates on either side of the semiconductor layer. 
           [0064]      FIG. 50A-E  are exemplary illustrations of polysilicon select devices for 3D memory and peripheral circuits at the bottom. 
           [0065]      FIG. 51A-F  are exemplary illustrations of polysilicon select devices for 3D memory and peripheral circuits at the top. 
           [0066]      FIG. 52A-D  are exemplary illustrations of a monolithic 3D SRAM according to some embodiments of the current invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0067]    Embodiments of the invention are now described with reference to  FIGS. 1-52 , it being appreciated that the figures illustrate the subject matter not to scale or to measure. Many figures describe process flows for building devices. These process flows, which may be essentially a sequence of steps for building a device, have many structures, numerals and labels that may be common between two or more adjacent steps. In such cases, some labels, numerals and structures used for a certain step&#39;s figure may have been described in previous steps&#39; figures. 
         [0000]    Section 1: Construction of 3D Stacked Semiconductor Circuits and Chips with Processing Temperatures Below about 400° C. 
         [0068]    This section of the document describes a technology to construct single-crystal silicon transistors atop wiring layers with less than about 400° C. processing temperatures. This allows construction of 3D stacked semiconductor chips with high density of connections between different layers, because the top-level transistors may be formed well-aligned to bottom-level wiring and transistor layers. Since the top-level transistor layers may be very thin (less than about 200 nm), alignment can be done through these thin silicon and oxide layers to features in the bottom-level. 
         [0069]      FIG. 1  shows different parts of a standard transistor used in Complementary Metal Oxide Semiconductor (CMOS) logic and SRAM circuits. The transistor may be constructed out of single crystal silicon material and may include a source  0106 , a drain  0104 , a gate electrode  0102  and a gate dielectric  0108 . Single crystal silicon layers  0110  can be formed atop wiring layers at less than about 400° C. using an “ion-cut process.” Further details of the ion-cut process may be described in  FIG. 2A-E . Note that the terms smart-cut, smart-cleave and nano-cleave may be used interchangeably with the term ion-cut in this document. Gate dielectrics can be grown or deposited above silicon at less than about 400° C. using a Chemical Vapor Deposition (CVD) process, an Atomic Layer Deposition (ALD) process or a plasma-enhanced thermal oxidation process. Gate electrodes can be deposited using CVD or ALD at less than about 400° C. temperatures as well. The part of the transistor that requires temperatures greater than about 400° C. for processing may be the source-drain region, which receives ion implantation that may need to be activated. It is clear based on  FIG. 1  that novel transistors for 3D integrated circuits that do not need high-temperature source-drain region processing may be useful (to get a high density of inter-layer connections). 
         [0070]      FIG. 2A-E  describes an ion-cut flow for layer transferring a single crystal silicon layer atop any generic bottom layer  0202 . The bottom layer  0202  can be a single crystal silicon layer. Alternatively, it can be a wafer having transistors with wiring layers above it. This process of ion-cut based layer transfer may include several steps, as described in the following sequence: 
         [0000]    Step (A): A silicon dioxide layer  0204  may be deposited above the generic bottom layer  0202 .  FIG. 2A  illustrates the structure after Step (A) is completed.
 
Step (B): The top layer of doped or undoped silicon  206  to be transferred atop the bottom layer may be processed and an oxide layer  0208  may be deposited or grown above it.  FIG. 2B  illustrates the structure after Step (B) is completed.
 
Step (C): Hydrogen may be implanted into the top layer silicon  0206  with the peak at a certain depth to create the hydrogen plane  0210 . Alternatively, another atomic species such as helium or boron can be implanted or co-implanted.  FIG. 2C  illustrates the structure after Step (C) is completed.
 
Step (D): The top layer wafer shown after Step (C) may be flipped and bonded atop the bottom layer wafer using oxide-to-oxide bonding.  FIG. 2D  illustrates the structure after Step (D) is completed.
 
Step (E): A cleave operation may be performed at the hydrogen plane  0210  using an anneal. Alternatively, a sideways mechanical force may be used. Further details of this cleave process may be described in “Frontiers of silicon-on-insulator,” J. Appl. Phys. 93, 4955-4978 (2003) by G. K. Celler and S. Cristoloveanu (“Celler”) and “Mechanically induced Si layer transfer in hydrogen-implanted Si wafers,” Appl. Phys. Lett., vol. 76, pp. 2370-2372, 2000 by K. Henttinen, I. Suni, and S. S. Lau (“Hentinnen”). Following this, a Chemical-Mechanical-Polish (CMP) may be done.  FIG. 2E  illustrates the structure after Step (E) is completed.
 
         [0071]    A possible flow for constructing 3D stacked semiconductor chips with standard transistors is shown in  FIG. 3A-E . The process flow may include several steps in the following sequence: 
         [0000]    Step (A): The bottom wafer of the 3D stack may be processed with a bottom transistor layer  0306  and a bottom wiring layer  0304 . A silicon dioxide layer  0302  may be deposited above the bottom transistor layer  0306  and the bottom wiring layer  0304 .  FIG. 3A  illustrates the structure after Step (A) is completed.
 
Step (B): Using a procedure similar to  FIG. 2A-E , a top layer of p− or n-doped Silicon  0310  may be transferred atop the bottom wafer.  FIG. 3B  illustrates the structure after Step (B) is completed.
 
Step (C) Isolation regions (between adjacent transistors) on the top wafer may be formed using a standard shallow trench isolation (STI) process. After this, a gate dielectric  0318  and a gate electrode  0316  may be deposited, patterned and etched.  FIG. 3C  illustrates the structure after Step (C) is completed.
 
Step (D): Source  0320  and drain  0322  regions may be ion implanted.  FIG. 3D  illustrates the structure after Step (D) is completed.
 
Step (E): The top layer of transistors may be annealed at high temperatures, typically in between about 700° C. and about 1200° C. This may be done to activate dopants in implanted regions. Following this, contacts may be made and further processing occurs.  FIG. 3E  illustrates the structure after Step (E) is completed.
 
         [0072]    The challenge with following this flow to construct 3D integrated circuits with aluminum or copper wiring is apparent from  FIG. 3A-E . During Step (E), temperatures above about 700° C. may be utilized for constructing the top layer of transistors. This can damage copper or aluminum wiring in the bottom wiring layer  0304 . It is therefore apparent from  FIG. 3A-E  that forming source-drain regions and activating implanted dopants forms the primary concern with fabricating transistors with a low-temperature (less than about 400° C.) process. 
       Section 1.1: Junction-Less Transistors as a Building Block for 3D Stacked Chips 
       [0073]    One method to solve the issue of high-temperature source-drain junction processing may be to make transistors without junctions i.e. Junction-Less Transistors (JLTs). An embodiment of this invention may use JLTs as a building block for 3D stacked semiconductor circuits and chips. 
         [0074]      FIG. 4  shows a schematic of a junction-less transistor (JLT) also referred to as a gated resistor or nano-wire. A heavily doped silicon layer (typically above 1×10 19 /cm 3 , but can be lower as well) forms source  0404 , drain  0402  as well as channel region of a JLT. A gate electrode  0406  and a gate dielectric  0408  may be present over the channel region of the JLT. The JLT may have a very small channel area (typically less than about 20 nm on one side), so the gate can deplete the channel of charge carriers at 0V and turn it off I-V curves of n channel ( 0412 ) and p channel ( 0410 ) junction-less transistors are shown in  FIG. 4  as well. These indicate that the JLT can show comparable performance to a tri-gate transistor that may be commonly researched by transistor developers. Further details of the JLT can be found in “Junctionless multigate field-effect transistor,” Appl. Phys. Lett., vol. 94, pp. 053511 2009 by C.-W. Lee, A. Afzalian, N. Dehdashti Akhavan, R. Yan, I. Ferain and J. P. Colinge (“C-W. Lee”). Contents of this publication are incorporated herein by reference. 
         [0075]      FIG. 5A-F  describes a process flow for constructing 3D stacked circuits and chips using JLTs as a building block. The process flow may include several steps, as described in the following sequence: 
         [0000]    Step (A): The bottom layer of the 3D stack may be processed with transistors and wires. This may be indicated in the figure as bottom layer of transistors and wires  502 . Above this, a silicon dioxide layer  504  may be deposited.  FIG. 5A  shows the structure after Step (A) is completed.
 
Step (B): A layer of n+ Si  506  may be transferred atop the structure shown after Step (A). It starts by taking a donor wafer which may be already n+ doped and activated. Alternatively, the process can start by implanting a silicon wafer and activating at high temperature forming an n+ activated layer. Then, H+ ions may be implanted for ion-cut within the n+ layer. Following this, a layer-transfer may be performed. The process as shown in  FIG. 2A-E  may be utilized for transferring and ion-cut of the layer forming the structure of  FIG. 5A .  FIG. 5B  illustrates the structure after Step (B) is completed.
 
Step (C): Using lithography (litho) and etch, the n+ Si layer may be defined and may be present in regions where transistors are to be constructed. These transistors may be aligned to the underlying alignment marks embedded in bottom layer of transistors and wires  502 .  FIG. 5C  illustrates the structure after Step (C) is completed, showing structures of the gate dielectric material  511  and gate electrode material  509  as well as structures of the n+ silicon region  507  after Step (C).
 
Step (D): The gate dielectric material  510  and the gate electrode material  508  may be deposited, following which a CMP process may be utilized for planarization. The gate dielectric material  510  could be hafnium oxide. Alternatively, silicon dioxide can be used. Other types of gate dielectric materials such as Zirconium oxide can be utilized as well. The gate electrode material could be Titanium Nitride. Alternatively, other materials such as TaN, W, Ru, TiAlN, polysilicon could be used.  FIG. 5D  illustrates the structure after Step (D) is completed.
 
Step (E): Litho and etch may be conducted to leave the gate dielectric material and the gate electrode material in regions where gates are to be formed.  FIG. 5E  illustrates the structure after Step (E) is completed. Final structures of the gate dielectric material  511  and gate electrode material  509  are shown.
 
Step (F): An oxide layer may be deposited and polished with CMP. This oxide region serves to isolate adjacent transistors. Following this, rest of the process flow continues, where contact and wiring layers could be formed.  FIG. 5F  illustrates the structure after Step (F) may be completed. Note that top-level transistors may be formed well-aligned to bottom-level wiring and transistor layers. Since the top-level transistor layers may be made very thin (less than about 200 nm), the lithography equipment can see through these thin silicon layers and align to features at the bottom-level. While the process flow shown in  FIG. 5A-F  gives the key steps involved in forming a JLT for 3D stacked circuits and chips, it is conceivable to one skilled in the art that changes to the process can be made. For example, process steps and additional materials/regions to add strain to junction-less transistors can be added or a p+ silicon layer could be used. Furthermore, more than two layers of chips or circuits can be 3D stacked.
 
         [0076]      FIG. 6A-D  shows that JLTs that can be 3D stacked fall into four categories based on the number of gates the JLT&#39;s may use: One-side gated JLTs as shown in  FIG. 6A , two-side gated JLTs as shown in  FIG. 6B , three-side gated JLTs as shown in  FIG. 6C , and gate-all-around JLTs as shown in  FIG. 6D . The JLT shown in  FIG. 5A-F  falls into the three-side gated JLT category. As the number of JLT gates increases, the gate gets more control of the channel, thereby reducing leakage of the JLT at 0V. Furthermore, the enhanced gate control can be traded-off for higher doping (which improves contact resistance to source-drain regions) or bigger JLT cross-sectional areas (which may be easier from a process integration standpoint). However, adding more gates typically increases process complexity. 
         [0077]      FIG. 7A-F  describes a process flow for using one-side gated JLTs as building blocks of 3D stacked circuits and chips. The process flow may include several steps as described in the following sequence: 
         [0000]    Step (A): The bottom layer of the two chip 3D stack may be processed with transistors and wires. This may be indicated in the figure as bottom layer of transistors and wires  702 . Above this, a silicon dioxide layer  704  may be deposited.  FIG. 7A  illustrates the structure after Step (A) is completed.
 
Step (B): A layer of n+ Si  706  may be transferred atop the structure shown after Step (A). The process shown in  FIG. 2A-E  may be utilized for this purpose as was presented with respect to  FIG. 5 .  FIG. 7B  illustrates the structure after Step (B) is completed.
 
Step (C): Using lithography (litho) and etch, the n+ Si layer  706  may be defined and may be present in regions where transistors are to be constructed. An oxide  705  may be deposited (for isolation purposes) with a standard shallow-trench-isolation process. The n+ Si structure remaining after Step (C) may be indicated as n+ Si  707 .  FIG. 7C  illustrates the structure after Step (C) is completed.
 
Step (D): The gate dielectric material  708  and the gate electrode material  710  may be deposited. The gate dielectric material  708  could be hafnium oxide. Alternatively, silicon dioxide can be used. Other types of gate dielectric materials such as Zirconium oxide can be utilized as well. The gate electrode material could be Titanium Nitride. Alternatively, other materials such as TaN, W, Ru, TiAlN, polysilicon could be used.  FIG. 7D  illustrates the structure after Step (D) is completed.
 
Step (E): Litho and etch may be conducted to leave the gate dielectric material  708  and the gate electrode material  710  in regions where gates are to be formed. It is clear based on the schematic that the gate may be present on just one side of the MT. Structures remaining after Step (E) may be gate dielectric  709  and gate electrode  711 .  FIG. 7E  illustrates the structure after Step (E) is completed.
 
Step (F): An oxide layer  713  may be deposited and polished with CMP.  FIG. 7F  illustrates the structure after Step (F) is completed. Following this, the rest of the process flow continues, with contact and wiring layers being formed.
 
Note that top-level transistors may be formed well-aligned to bottom-level wiring and transistor layers. Since the top-level transistor layers may be made very thin (less than about 200 nm), the lithography equipment can see through these thin silicon layers and align to features at the bottom-level. While the process flow shown in  FIG. 7A-F  illustrates several steps involved in forming a one-side gated JLT for 3D stacked circuits and chips, it is conceivable to one skilled in the art that changes to the process can be made. For example, process steps and additional materials/regions to add strain to junction-less transistors can be added. Furthermore, more than two layers of chips or circuits can be 3D stacked.
 
         [0078]      FIG. 8A-E  describes a process flow for forming 3D stacked circuits and chips using two side gated JLTs. The process flow may include several steps, as described in the following sequence: 
         [0000]    Step (A): The bottom layer of the 2 chip 3D stack may be processed with transistors and wires. This may be indicated in the figure as bottom layer of transistors and wires  802 . Above this, a silicon dioxide layer  804  may be deposited.  FIG. 8A  shows the structure after Step (A) is completed.
 
Step (B): A layer of n+ Si  806  may be transferred atop the structure shown after Step (A). The process shown in  FIG. 2A-E  may be utilized for this purpose as was presented with respect to  FIG. 5A-F . A nitride (or oxide) layer  808  may be deposited to function as a hard mask for later processing.  FIG. 8B  illustrates the structure after Step (B) is completed.
 
Step (C): Using lithography (litho) and etch, the nitride layer  808  and n+ Si layer  806  may be defined and may be present in regions where transistors are to be constructed. The nitride and n+ Si structures remaining after Step (C) may be indicated as nitride hard mask  809  and n+ Si  807 .  FIG. 8C  illustrates the structure after Step (C) is completed.
 
Step (D): The gate dielectric material  820  and the gate electrode material  828  may be deposited. The gate dielectric material  820  could be hafnium oxide. Alternatively, silicon dioxide can be used. Other types of gate dielectric materials such as Zirconium oxide can be utilized as well. The gate electrode material  828  could be Titanium Nitride. Alternatively, other materials such as TaN, W, Ru, TiAlN, polysilicon could be used.  FIG. 8D  illustrates the structure after Step (D) is completed.
 
Step (E): Litho and etch may be conducted to leave the gate dielectric material  820  and the gate electrode material  828  in regions where gates are to be formed. Structures remaining after Step (E) may be gate dielectric  830  and gate electrode  838 .  FIG. 8E  illustrates the structure after Step (E) is completed.
 
Note that top-level transistors may be formed well-aligned to bottom-level wiring and transistor layers. Since the top-level transistor layers may be made very thin (less than about 200 nm), the lithography equipment can see through these thin silicon layers and align to features at the bottom-level. While the process flow shown in  FIG. 8A-E  gives the key steps involved in forming a two side gated JLT for 3D stacked circuits and chips, it is conceivable to one skilled in the art that changes to the process can be made. For example, process steps and additional materials/regions to add strain to junction-less transistors can be added. Furthermore, more than two layers of chips or circuits can be 3D stacked. A note in respect to the JLT devices been presented may be that the layer transferred used for the construction may be a thin layer of less than about 200 nm and in many applications even less than about 40 nm. This may be achieved by the depth of the implant of the H+ layer used for the ion-cut and by following this by thinning using etch and/or CMP.
 
         [0079]      FIG. 9A-J  describes a process flow for forming four-side gated JLTs in 3D stacked circuits and chips. Four-side gated JLTs can also be referred to as gate-all around JLTs or silicon nanowire JLTs. Four-side gated JLTs may also offer excellent electrostatic control of the channel and provide high-quality I-V curves with low leakage and high drive currents. The process flow in  FIG. 9A-J  may include several steps in the following sequence: 
         [0000]    Step (A): On a p− Si wafer  902 , multiple n+ Si layers  904  and  908  and multiple n+ SiGe layers  906  and  910  may be epitaxially grown. The Si and SiGe layers may be carefully engineered in terms of thickness and stoichiometry to keep defect density which may result from lattice mismatch between Si and SiGe low. Some techniques for achieving this include keeping thickness of SiGe layers below the critical thickness for forming defects. A silicon dioxide layer  912  may be deposited above the stack.  FIG. 9A  illustrates the structure after Step (A) is completed.
 
Step (B): Hydrogen may be implanted at a certain depth in the p− wafer, to form a cleave plane  999  after bonding to bottom wafer of the two-chip stack. Alternatively, some other atomic species such as He can be used.  FIG. 9B  illustrates the structure after Step (B) is completed.
 
Step (C): The structure after Step (B) may be flipped and bonded to another wafer on which bottom layers of transistors and wires  914  may be constructed. Bonding occurs with an oxide-to-oxide bonding process.  FIG. 9C  illustrates the structure after Step (C) is completed.
 
Step (D): A cleave process occurs at the hydrogen plane using a sideways mechanical force. Alternatively, an anneal, such as thermal, could be used for cleaving purposes. A CMP process may be conducted till one reaches the n+ Si layer  904 .  FIG. 9D  illustrates the structure after Step (D) is completed.
 
Step (E): Using litho and etch, Si regions  918  and SiGe regions  916  may be defined to be in locations where transistors may be needed. Oxide may be deposited to form isolation oxide regions  920  and to cover the Si regions  918  and SiGe regions  916 . A CMP process may be conducted.  FIG. 9E  illustrates the structure after Step (E) is completed.
 
Step (F): Using litho and etch, oxide regions  920  may be removed in locations where a gate may need to be present. It may be clear that Si regions  918  and SiGe regions  916  may be exposed in the channel region of the MT.  FIG. 9F  illustrates the structure after Step (F) is completed.
 
Step (G): SiGe regions  916  in channel of the JLT may be etched using an etching recipe that does not attack Si regions  918 . Such etching recipes are described in “High performance 5 nm radius twin silicon nanowire MOSFET(TSNWFET): Fabrication on bulk Si wafer, characteristics, and reliability,” in  Proc. IEDM Tech. Dig.,  2005, pp. 717-720 by S. D. Suk, S.-Y. Lee, S.-M. Kim, et al. (“Suk”).  FIG. 9G  illustrates the structure after Step (G) is completed.
 
Step (H): For example, this is a step where a hydrogen anneal may be utilized to reduce surface roughness of fabricated nanowires. The hydrogen anneal can also reduce thickness of nanowires. Following the hydrogen anneal, another step, for example, of oxidation (using plasma enhanced thermal oxidation) and etch-back of the produced silicon dioxide, may be used. This process may thin down the silicon nanowire further.  FIG. 9H  illustrates the structure after Step (H) is completed.
 
Step (I): Gate dielectric and gate electrode regions may be deposited or grown. Examples of gate dielectrics include hafnium oxide, silicon dioxide. Examples of gate electrodes include polysilicon, TiN, TaN, and other materials with a work function that permits acceptable transistor electrical characteristics. A CMP may be conducted after gate electrode deposition. Following this, rest of the process flow for forming transistors, contacts and wires for the top layer continues.  FIG. 9I  illustrates the structure after Step (I) is completed.
 
 FIG. 9J  shows a cross-sectional view of structures after Step (I). It is clear that two nanowires may be present for each transistor in the figure. It may be possible to have one nanowire per transistor or more than two nanowires per transistor by changing the number of stacked Si/SiGe layers.
 
Note that top-level transistors may be formed well-aligned to bottom-level wiring and transistor layers. Since the top-level transistor layers may be very thin (less than about 200 nm), the top transistors can be aligned to features in the bottom-level. While the process flow shown in  FIG. 9A-J  gives the key steps involved in forming a four-side gated JLT with 3D stacked components, it is conceivable to one skilled in the art that changes to the process can be made. For example, process steps and additional materials/regions to add strain to junction-less transistors can be added. Furthermore, more than two layers of chips or circuits can be 3D stacked. Also, there may be many methods to construct silicon nanowire transistors and these may be described in “High performance and highly uniform gate-all-around silicon nanowire MOSFETs with wire size dependent scaling,”  Electron Devices Meeting  ( IEDM ), 2009  IEEE International , vol., no., pp. 1-4, 7-9 Dec. 2009 by Bangsaruntip, S.; Cohen, G. M.; Majumdar, A.; et al. (“Bangsaruntip”) and in “High performance 5 nm radius twin silicon nanowire MOSFET(TSNWFET): Fabrication on bulk Si wafer, characteristics, and reliability,” in  Proc. IEDM Tech. Dig.,  2005, pp. 717-720 by S. D. Suk, S.-Y. Lee, S.-M. Kim, et al. (“Suk”). Contents of these publications are incorporated herein by reference. Techniques described in these publications can be utilized for fabricating four-side gated JLTs without junctions as well.
 
         [0080]      FIG. 9K-V  describes an alternative process flow for forming four-side gated JLTs in 3D stacked circuits and chips. It may include several steps as described in the following sequence. 
         [0000]    Step (A): The bottom layer of the 2 chip 3D stack may be processed with transistors and wires. This may be indicated in the figure as bottom layer of transistors and wires  950 . Above this, a silicon dioxide layer  952  may be deposited.  FIG. 9K  illustrates the structure after Step (A) is completed.
 
Step (B): A n+ Si wafer  954  that may have its dopants activated may be taken. Alternatively, a p− Si wafer that may have n+ dopants implanted and activated can be used.  FIG. 9L  shows the structure after Step (B) is completed.
 
Step (C): Hydrogen ions may be implanted into the n+ Si wafer  954  at a certain depth.  FIG. 9M  shows the structure after Step (C) is completed. The hydrogen plane  956  may be formed and may be indicated as dashed lines.
 
Step (D): The wafer after step (C) may be bonded to a temporary carrier wafer  960  using a temporary bonding adhesive  958 . This temporary carrier wafer  960  could be constructed of glass. Alternatively, it could be constructed of silicon. The temporary bonding adhesive  958  could be a polymer material, including, such as, polyimide DuPont HD3007.  FIG. 9N  illustrates the structure after Step (D) is completed.
 
Step (E): A anneal or a sideways mechanical force may be utilized to cleave the wafer at the hydrogen plane  956 . A CMP process may be then conducted.  FIG. 90  shows the structure after Step (E) is completed.
 
Step (F): Layers of gate dielectric material  966 , gate electrode material  968  and silicon oxide  964  may be deposited onto the bottom of the wafer shown in Step (E).  FIG. 9P  illustrates the structure after Step (F) is completed.
 
Step (G): The wafer may then be bonded to the bottom layer of transistors and wires  950  using oxide-to-oxide bonding.  FIG. 9Q  illustrates the structure after Step (G) is completed.
 
Step (H): The temporary carrier wafer  960  may then be removed by shining a laser onto the temporary bonding adhesive  958  through the temporary carrier wafer  960  (which could be constructed of glass). Alternatively, an anneal could be used to remove the temporary bonding adhesive  958 .  FIG. 9R  illustrates the structure after Step (H) is completed.
 
Step (I): The layer of n+ Si  962  and gate dielectric material  966  may be patterned and etched using a lithography and etch step.  FIG. 9S  illustrates the structure after this step. The patterned layer of n+ Si  970  and the patterned gate dielectric for the back gate (gate dielectric  980 ) may be shown. Oxide may be deposited and polished by CMP to planarize the surface and form a region of silicon dioxide oxide region  974 .
 
Step (J): The oxide region  974  and gate electrode material  968  may be patterned and etched to form a region of silicon dioxide  978  and back gate electrode  976 .  FIG. 9T  illustrates the structure after this step.
 
Step (K): A silicon dioxide layer may be deposited. The surface may then be planarized with CMP to form the region of silicon dioxide  982 .  FIG. 9U  illustrates the structure after this step.
 
Step (L): Trenches may be etched in the region of silicon dioxide  982 . A thin layer of gate dielectric and a thicker layer of gate electrode may be then deposited and planarized. Following this, a lithography and etch step may be performed to etch the gate dielectric and gate electrode.  FIG. 9V  illustrates the structure after these steps. The device structure after these process steps may include a front gate electrode  984  and a dielectric for the front gate  986 . Contacts can be made to the front gate electrode  984  and back gate electrode  976  after oxide deposition and planarization.
 
Note that top-level transistors may be formed well-aligned to bottom-level wiring and transistor layers. While the process flow shown in  FIG. 9K-V  shows several steps involved in forming a four-side gated JLT with 3D stacked components, it is conceivable to one skilled in the art that changes to the process can be made. For example, process steps and additional materials/regions to add strain to junction-less transistors can be added.
 
         [0081]    Substantially all the types of embodiments of this invention described in Section 1.1 may utilize single crystal silicon or monocrystalline silicon transistors. Thicknesses of layer transferred regions of silicon may be less than about 2 um, and many times can be less than about 1 um or less than about 0.4 um or even less than about 0.2 um. Interconnect (wiring) layers may be constructed substantially of high conductivity material, including, for example, copper or aluminum. 
       Section 1.2: Recessed Channel Transistors as a Building Block for 3D Stacked Circuits and Chips 
       [0082]    Another method to solve the issue of high-temperature source-drain junction processing may be an innovative use of recessed channel inversion-mode transistors as a building block for 3D stacked semiconductor circuits and chips. The transistor structures described in this section can be considered horizontally-oriented transistors where current flow occurs between horizontally-oriented source and drain regions. The term planar transistor can also be used for the same in this document. The recessed channel transistors in this section may be defined by a process including a step of etch to form the transistor channel. 3D stacked semiconductor circuits and chips using recessed channel transistors may have interconnect (wiring) layers including, for example, copper or aluminum or a material with higher conductivity. 
         [0083]      FIG. 10A-D  shows different types of recessed channel inversion-mode transistors constructed atop a bottom layer of transistors and wires  1004 .  FIG. 10A  depicts a standard recessed channel transistor where the recess may be made up to the p− region. The angle of the recess, Alpha  1002 , may have a range of about 90° to about 180°. A standard recessed channel transistor where angle Alpha&gt;90° can also be referred to as a V-shape transistor or V-groove transistor.  FIG. 10B  depicts a RCAT (Recessed Channel Array Transistor) where part of the p-region may be consumed by the recess.  FIG. 10C  depicts a S-RCAT (Spherical RCAT) where the recess in the p− region may be spherical in shape.  FIG. 10D  depicts a recessed channel Finfet. 
         [0084]      FIG. 11A-F  shows a procedure for layer transfer of silicon regions for recessed channel transistors. Silicon regions that may be layer transferred may be less than about 2 um in thickness, and can be thinner than about 1 um or even about 0.4 um. The process flow in  FIG. 11A-F  may include several steps as described in the following sequence: 
         [0000]    Step (A): A silicon dioxide layer  1104  may be deposited above the generic bottom layer  1102 .  FIG. 11A  illustrates the structure after Step (A).
 
Step (B): A p− Si wafer  1106  may be implanted with n+ near its surface to form a layer of n+ Si  1108 .  FIG. 11B  illustrates the structure after Step (B).
 
Step (C): A p− Si layer  1110  may be epitaxially grown atop the layer of n+ Si  1108 . A layer of silicon dioxide  1112  may be deposited atop the p− Si layer  1110 . An anneal (such as a rapid thermal anneal RTA or spike anneal or laser anneal) may be conducted to activate dopants. Note that the terms laser anneal and optical anneal may be used interchangeably in this document.  FIG. 11C  illustrates the structure after Step (C). Alternatively, the n+ Si layer  1108  and p− Si layer  1110  can be formed by a buried layer implant of n+ Si in the p− Si wafer  1106 .
 
Step (D): Hydrogen H+ may be implanted into the n+ Si layer  1108  at a certain depth to form hydrogen plane  1114 . Alternatively, another atomic species such as helium can be implanted.  FIG. 11D  illustrates the structure after Step (D).
 
Step (E): The top layer wafer shown after Step (D) may be flipped and bonded atop the bottom layer wafer using oxide-to-oxide bonding.  FIG. 11E  illustrates the structure after Step (E).
 
Step (F): A cleave operation may be performed at the hydrogen plane  1114  using an anneal. Alternatively, a sideways mechanical force may be used. Following this, a Chemical-Mechanical-Polish (CMP) may be done. It may be noted that the layer-transfer including the bonding and the cleaving could be done without exceeding about 400° C. This may be the case in various alternatives of this invention.  FIG. 11F  illustrates the structure after Step (F).
 
         [0085]      FIG. 12A-F  describes a process flow for forming 3D stacked circuits and chips using standard recessed channel inversion-mode transistors. The process flow in  FIG. 12A-F  may include several steps as described in the following sequence: 
         [0000]    Step (A): The bottom layer of the 2 chip 3D stack may be processed with transistors and wires. This may be indicated in the figure as bottom layer of transistors and wires  1202 . Above this, a silicon dioxide layer  1204  may be deposited.  FIG. 12A  illustrates the structure after Step (A).
 
Step (B): Using the procedure shown in  FIG. 11A-F , a p− Si layer  1205  and n+ Si layer  1207  may be transferred atop the structure shown after Step (A).  FIG. 12B  illustrates the structure after Step (B).
 
Step (C): The stack shown after Step (A) may be patterned lithographically and etched such that silicon regions may be present in regions where transistors are to be formed. Using a shallow trench isolation (STI) process, isolation regions in-between transistor regions may be formed. These oxide regions may be indicated as  1216 .  FIG. 12C  illustrates the structure after Step (C). Thus, n+ Si region  1209  and p− Si region  1206  may be left after this step.
 
Step (D): Using litho and etch, a recessed channel may be formed by etching away the n+ Si region  1209 . Little or none of the p− Si region  1206  may be removed.  FIG. 12D  illustrates the structure after Step (D).
 
Step (E): The gate dielectric material and the gate electrode material may be deposited, following which a CMP process may be utilized for planarization. The gate dielectric material could be hafnium oxide. Alternatively, silicon dioxide can be used. Other types of gate dielectric materials such as Zirconium oxide can be utilized as well. The gate electrode material could be Titanium Nitride. Alternatively, other materials such as TaN, W, Ru, TiAlN, polysilicon could be used. Litho and etch may be conducted to leave the gate dielectric material  1210  and the gate electrode material  1212  in regions where gates are to be formed.  FIG. 12E  illustrates the structure after Step (E).
 
Step (F): An oxide layer  1214  may be deposited and polished with CMP. Following this, rest of the process flow continues, with contact and wiring layers being formed.  FIG. 12F  illustrates the structure after Step (F).
 
It is apparent based on the process flow shown in  FIG. 12A-F  that no process step greater than about 400° C. may be needed after stacking the top layer of transistors above the bottom layer of transistors and wires. While the process flow shown in  FIG. 12A-F  gives the key steps involved in forming a standard recessed channel transistor for 3D stacked circuits and chips, it is conceivable to one skilled in the art that changes to the process can be made. For example, process steps and additional materials/regions to add strain to the standard recessed channel transistors can be added. Furthermore, more than two layers of chips or circuits can be 3D stacked. Note that top-level transistors may be formed well-aligned to bottom-level wiring and transistor layers. This, in turn, may result from top-level transistor layers being very thin (less than about 200 nm). One can see through these thin silicon layers and align to features at the bottom-level.
 
         [0086]      FIG. 13A-F  depicts a process flow for constructing 3D stacked logic circuits and chips using RCATs (recessed channel array transistors). These types of devices may be typically used for constructing 2D DRAM chips. These devices can also be utilized for forming 3D stacked circuits and chips with no process steps performed at greater than about 400° C. (after wafer to wafer bonding). The process flow in  FIG. 13A-F  may include several steps in the following sequence: 
         [0000]    Step (A): The bottom layer of the 2 chip 3D stack may be processed with transistors and wires. This may be indicated in the figure as bottom layer of transistors and wires  1302 . Above this, a silicon dioxide layer  1304  may be deposited.  FIG. 13A  illustrates the structure after Step (A).
 
Step (B): Using the procedure shown in  FIG. 11A-F , a p− Si layer  1305  and n+ Si layer  1307  may be transferred atop the structure shown after Step (A).  FIG. 13B  illustrates the structure after Step (B).
 
Step (C): The stack shown after Step (A) may be patterned lithographically and etched such that silicon regions may be present in regions where transistors are to be formed. Using a shallow trench isolation (STI) process, isolation regions in-between transistor regions may be formed.  FIG. 13C  illustrates the structure after Step (C). n+ Si regions after this step may be indicated as n+ Si region  1308  and p− Si regions after this step may be indicated as p− Si region  1306 . Oxide regions may be indicated as Oxide  1314 .
 
Step (D): Using litho and etch, a recessed channel may be formed by etching away the n+ Si region  1308  and p− Si region  1306 . A chemical dry etch process is described in “The breakthrough in data retention time of DRAM using Recess-Channel-Array Transistor (RCAT) for 88 nm feature size and beyond,”  VLSI Technology,  2003 . Digest of Technical Papers.  2003  Symposium on , vol., no., pp. 11-12, 10-12 Jun. 2003 by Kim, J. Y.; Lee, C. S.; Kim, S. E., et al. (“J. Y. Kim”). A variation of this process from J. Y. Kim can be utilized for rounding corners, removing damaged silicon, etc after the etch. Furthermore, Silicon Dioxide can be formed using a plasma-enhanced thermal oxidation process, this oxide can be etched-back as well to reduce damage from etching silicon.  FIG. 13D  illustrates the structure after Step (D). n+ Si regions after this step may be indicated as n+ Si  1309  and p− Si regions after this step may be indicated as p− Si  1311 ,
 
Step (E): The gate dielectric material and the gate electrode material may be deposited, following which a CMP process may be utilized for planarization. The gate dielectric material could be hafnium oxide. Alternatively, silicon dioxide can be used. Other types of gate dielectric materials such as Zirconium oxide can be utilized as well. The gate electrode material could be Titanium Nitride. Alternatively, other materials such as TaN, W, Ru, TiAlN, polysilicon could be used. Litho and etch may be conducted to leave the gate dielectric material  1310  and the gate electrode material  1312  in regions where gates are to be formed.  FIG. 13E  illustrates the structure after Step (E).
 
Step (F): An oxide layer  1320  may be deposited and polished with CMP. Following this, rest of the process flow continues, with contact and wiring layers being formed.  FIG. 13F  illustrates the structure after Step (F).
 
It is apparent based on the process flow shown in  FIG. 13A-F  that no process step at greater than about 400° C. may be needed after stacking the top layer of transistors above the bottom layer of transistors and wires. While the process flow shown in  FIG. 13A-F  gives several steps involved in forming RCATs for 3D stacked circuits and chips, it is conceivable to one skilled in the art that changes to the process can be made. For example, process steps and additional materials/regions to add strain to RCATs can be added. Furthermore, more than two layers of chips or circuits can be 3D stacked. Note that top-level transistors may be formed well-aligned to bottom-level wiring and transistor layers. This, in turn, may result from top-level transistor layers being very thin (less than about 200 nm). One can look through these thin silicon layers and align to features at the bottom-level. As a result from their extensive use in the DRAM industry, several technologies exist to optimize RCAT processes and devices. These may be described in “The breakthrough in data retention time of DRAM using Recess-Channel-Array Transistor (RCAT) for 88 nm feature size and beyond,”  VLSI Technology,  2003 . Digest of Technical Papers.  2003  Symposium on , vol., no., pp. 11-12, 10-12 Jun. 2003 by Kim, J. Y.; Lee, C. S.; Kim, S. E., et al. (“J. Y. Kim”), “The excellent scalability of the RCAT (recess-channel-array-transistor) technology for sub-70 nm DRAM feature size and beyond,”  VLSI Technology,  2005. ( VLSI - TSA - Tech ). 2005  IEEE VLSI - TSA International Symposium on , vol., no., pp. 33-34, 25-27 Apr. 2005 by Kim, J. Y.; Woo, D. S.; Oh, H. J., et al. (“Kim”) and “Implementation of HfSiON gate dielectric for sub-60 nm DRAM dual gate oxide with recess channel array transistor (RCAT) and tungsten gate,”  Electron Devices Meeting,  2004 . IEEE International , vol., no., pp. 515-518, 13-15 Dec. 2004 by Seong Geon Park; Beom Jun Jin; Hye Lan Lee, et al. (“S. G. Park”). It is conceivable to one skilled in the art that RCAT process and device optimization outlined by J. Y. Kim, Kim, S. G. Park and others can be applied to 3D stacked circuits and chips using RCATs as a building block.
 
         [0087]    While  FIG. 13A-F  showed the process flow for constructing RCATs for 3D stacked chips and circuits, the process flow for S-RCATs shown in  FIG. 10C  may be not very different. The main difference for an S-RCAT process flow may be the silicon etch in Step (D) of  FIG. 13A-F . An S-RCAT etch may be more sophisticated, and an oxide spacer is used on the sidewalls along with an isotropic dry etch process. Further details of a S-RCAT etch and process may be given in “S-RCAT (sphere-shaped-recess-channel-array transistor) technology for 70 nm DRAM feature size and beyond,”  Digest of Technical Papers.  2005  Symposium on VLSI Technology,  2005 pp. 34-35, 14-16 Jun. 2005 by Kim, J. V.; Oh, H. J.; Woo, D. S., et al. (“J. V. Kim”) and “High-density low-power-operating DRAM device adopting 6F 2  cell scheme with novel S-RCAT structure on 80 nm feature size and beyond,”  Solid - State Device Research Conference,  2005 . ESSDERC  2005 . Proceedings of  35 th European , vol., no., pp. 177-180, 12-16 Sep. 2005 by Oh, H. J.; Kim, J. Y.; Kim, J. H, et al. (“Oh”). The contents of the above publications are incorporated herein by reference. 
         [0088]    The recessed channel Finfet shown in  FIG. 10D  can be constructed using a simple variation of the process flow shown in  FIG. 13A-F . A recessed channel Finfet technology and its processing details may be described in “Highly Scalable Saddle-Fin (S-Fin) Transistor for Sub-50 nm DRAM Technology,”  VLSI Technology,  2006 . Digest of Technical Papers.  2006 Symposium on, vol., no., pp. 32-33 by Sung-Woong Chung; Sang-Don Lee; Se-Aug Jang, et al. (“S-W Chung”) and “A Proposal on an Optimized Device Structure With Experimental Studies on Recent Devices for the DRAM Cell Transistor,”  Electron Devices, IEEE Transactions on , vol. 54, no. 12, pp. 3325-3335, December 2007 by Myoung Jin Lee; Seonghoon Jin; Chang-Ki Baek, et al. (“M. J. Lee”). Contents of these publications are incorporated herein by reference. 
       Section 1.3: Improvements and Alternatives 
       [0089]    Various methods, technologies and procedures to improve devices shown in Section 1.1 and Section 1.2 may be given in this section. Single crystal silicon (this term used interchangeably with monocrystalline silicon) may be used for constructing transistors in Section 1.3. Thickness of layer transferred silicon may be typically less than about 2 um or less than about 1 um or could be even less than about 0.2 um, unless stated otherwise. Interconnect (wiring) layers may be constructed substantially of copper or aluminum or some other higher conductivity material. The term planar transistor or horizontally oriented transistor could be used to describe any constructed transistor where source and drain regions may be in the same horizontal plane and current flows between them. 
         [0000]    Section 1.3.1: Construction of CMOS Circuits with Sub-400° C. Processed Transistors 
         [0090]      FIG. 14A-I  show procedures for constructing CMOS circuits using sub-400° C. processed transistors (i.e. junction-less transistors and recessed channel transistors) described thus far in this document. When doing layer transfer for junction-less transistors and recessed channel transistors, it may be easy to construct just nMOS transistors in a layer or just pMOS transistors in a layer. However, constructing CMOS circuits requires both nMOS transistors and pMOS transistors, so it requires additional ideas. 
         [0091]      FIG. 14A  shows one procedure for forming CMOS circuits. nMOS and pMOS layers of CMOS circuits may be stacked atop each other. A layer of n-channel sub-400° C. transistors (with none or one or more wiring layers)  1406  may be first formed over a bottom layer of transistors and wires  1402 . Following this, a layer of p-channel sub-400° C. transistors (with none or one or more wiring layers)  1410  may be formed. This structure may be important since CMOS circuits typically require both n-channel and p-channel transistors. A high density of connections may exist among different layers  1402 ,  1406  and  1410 . The p-channel wafer  1410  could have its own optimized crystal structure that improves mobility of p-channel transistors while the n-channel wafer  1406  could have its own optimized crystal structure that improves mobility of n-channel transistors. For example, it is known that mobility of p-channel transistors is maximum in the (110) plane while the mobility of n-channel transistors is maximum in the (100) plane. The wafers  1410  and  1406  could have these optimized crystal structures. 
         [0092]      FIG. 14B-F  shows another procedure for forming CMOS circuits that utilizes junction-less transistors and repeating layouts in one direction. The procedure may include several steps, in the following sequence: 
         [0000]    Step (1): A bottom layer of transistors and wires  1414  may be first constructed above which a layer of landing pads  1418  may be constructed. A layer of silicon dioxide  1416  may be then constructed atop the layer of landing pads  1418 . Size of the landing pads  1418  may be W x + delta (W x ) in the X direction, where W x  may be the distance of one repeat of the repeating pattern in the (to be constructed) top layer. delta(W x ) may be an offset added to account for some overlap into the adjacent region of the repeating pattern and some margin for rotational (angular) misalignment within one chip (IC). Size of the landing pads  1418  may be F or 2F plus a margin for rotational misalignment within one chip (IC) or higher in the Y direction, where F may be the minimum feature size. Note that the terms landing pad and metal strip may be used interchangeably in this document.  FIG. 14B  is a drawing illustration after Step (1).
 
Step (2): A top layer having regions of n+ Si  1424  and p+ Si  1422  repeating over-and-over again may be constructed atop a p− Si wafer  1420 . The pattern repeats in the X direction with a repeat distance denoted by W. In the Y direction, there may be no pattern at all; the wafer may be uniform in that direction. This ensures misalignment in the Y direction does not impact device and circuit construction, except for any rotational misalignment causing difference between the left and right side of one IC. A maximum rotational (angular) misalignment of about 0.5 um over a 200 mm wafer results in maximum misalignment within one 10 by 10 mm IC of about 25 nm in both X and Y direction. Total misalignment in the X direction may be much larger, which is addressed in some embodiment of the invention as shown in the following steps.  FIG. 14C  shows a drawing illustration after Step (2).
 
Step (3): The top layer shown in Step (2) receives an H+ implant to create the cleaving plane in the p− silicon region and may be flipped and bonded atop the bottom layer shown in Step (1). A procedure similar to the one shown in  FIG. 2A-E  may be utilized for this purpose. Note that the top layer shown in Step (2) may have had its dopants activated with an anneal before layer transfer. The top layer may be cleaved and the remaining p− region may be etched or polished (CMP) away until substantially only the N+ and P+ stripes remain. During the bonding process, a misalignment can occur in X and Y directions, while the angular alignment may typically be small. This may be because the misalignment may result from factors such as wafer bow, wafer expansion may result from thermal differences between bonded wafers, etc; these issues do not typically cause angular alignment problems, while these issues may impact alignment in X and Y directions.
 
Since the width of the landing pads may be slightly wider than the width of the repeating n and p pattern in the X-direction and there&#39;s no pattern in the Y direction, the circuitry in the top layer can shifted left or right and up or down until the layer-to-layer contacts within the top circuitry may be placed on top of the appropriate landing pad. This is further explained below:
 
After the bonding process, the co-ordinates of the alignment mark of the top wafer may be (x top , y top ) while co-ordinates of the alignment mark of the bottom wafer may be (x bottom , y bottom ).  FIG. 14D  shows a drawing illustration after Step (3).
 
Step (4): A virtual alignment mark may be created by the lithography tool. X co-ordinate of this virtual alignment mark may be at the location (x top +(an integer k)*W x ). The integer k may be chosen such that modulus or absolute value of (x top +(integer k)*W x −x bottom )&lt;=W x /2. This guarantees that the X co-ordinate of the virtual alignment mark may be within a repeat distance (or within the same section of width W x ) of the X alignment mark of the bottom wafer. Y co-ordinate of this virtual alignment mark may be y bottom  (since silicon thickness of the top layer may be thin, the lithography tool can see the alignment mark of the bottom wafer and compute this quantity). Though-silicon connections  1428  may be now constructed with alignment mark of this mask aligned to the virtual alignment mark. The terms through via or through silicon vias can be used interchangeably with the term through-silicon connections in this document. Since the X co-ordinate of the virtual alignment mark may be within the same ((p+)-oxide-(n+)-oxide) repeating pattern (of length W x ) as the bottom wafer X alignment mark, the through-silicon connection  1428  may always fall on the bottom landing pad  1418  (the bottom landing pad length may be W x  added to delta (W x ), and this spans the entire length of the repeating pattern in the X direction).  FIG. 14E  is a drawing illustration after Step (4).
 
Step (5): n channel and p channel junction-less transistors may be constructed aligned to the virtual alignment mark.  FIG. 14F  is a drawing illustration after Step (5).
 
         [0093]    From steps (1) to (5), it is clear that 3D stacked semiconductor circuits and chips can be constructed with misalignment tolerance techniques. Essentially, a combination of 3 key ideas—repeating patterns in one direction of length W x , landing pads of length (W x + delta (W x )) and creation of virtual alignment marks—may be used such that even if misalignment occurs, through silicon connections fall on their respective landing pads. While the explanation in  FIG. 14B-F  may be shown for a junction-less transistor, similar procedures can also be used for recessed channel transistors. Thickness of the transferred single crystal silicon or monocrystalline silicon layer may be less than about 2 um, and can be even lower than about 1 um or about 0.4 um or about 0.2 um. 
         [0094]      FIG. 14G-I  shows yet another procedure for forming CMOS circuits with processing temperatures below about 400° C., such as, for example, the junction-less transistor and recessed channel transistors. While the explanation in  FIG. 14G-I  may be shown for a junction-less transistor, similar procedures can also be used for recessed channel transistors. The procedure may include several steps as described in the following sequence: 
         [0000]    Step (A): A bottom wafer  1438  may be processed with a bottom transistor layer  1436  and a bottom wiring layer  1434 . A layer of silicon oxide  1430  may be deposited above it.  FIG. 14G  is a drawing illustration after Step (A).
 
Step (B): Using a procedure similar to  FIG. 2A-E  (as was presented in  FIG. 5A-F ), layers of n+Si  1444  and p+ Si  1448  may be transferred above the bottom wafer  1438  one after another. The top wafer  1440  may therefore include a bilayer of n+ and p+ Si.  FIG. 14H  is a drawing illustration after Step (B).
 
Step (C): p-channel junction-less transistors  1450  of the CMOS circuit can be formed on the p+ Si layer  1448  with standard procedures. For n-channel junction-less transistors  1452  of the CMOS circuit, one may need to etch through the p+ layer  1448  to reach the n+ Si layer 1444. Transistors may be then constructed on the n+ Si  1444 . As a result from depth-of-focus issues associated with lithography, separate lithography steps may be needed while constructing different parts of n-channel and p-channel transistors.  FIG. 141  is a drawing illustration after Step (C).
 
Section 1.3.2: Accurate Transfer of Thin Layers of Silicon with Ion-Cut
 
         [0095]    It may often be desirable to transfer very thin layers of silicon (less than about 100 nm) atop a bottom layer of transistors and wires using the ion-cut technique. For example, for the process flow in  FIG. 11A-F , it may be desirable to have very thin layers (less than about 100 nm) of n+ Si  1109 . In that scenario, implanting hydrogen and cleaving the n+ region may not give the exact thickness of n+ Si desirable for device operation. An improved process for addressing this issue may be shown in  FIG. 15A-F . The process flow in  FIG. 15A-F  may include several steps as described in the following sequence: 
         [0000]    Step (A): A silicon dioxide layer  1504  may be deposited above the generic bottom layer  1502 .  FIG. 15A  illustrates the structure after Step (A).
 
Step (B): An SOI wafer  1506  may be implanted with n+ near its surface to form n+ Si layer  1508 . The buried oxide (BOX) of the SOI wafer may be silicon dioxide layer  1505 .  FIG. 15B  illustrates the structure after Step (B).
 
Step (C): A p− Si layer  1510  may be epitaxially grown atop the n+ Si layer  1508 . A silicon dioxide layer  1512  may be deposited atop the p− Si layer  1510 . An anneal (such as a rapid thermal anneal RTA or spike anneal or laser anneal) may be conducted to activate dopants. Alternatively, the n+ Si layer  1508  and p− Si layer  1510  can be formed by a buried layer implant of n+ Si in a p− SOI wafer.
 
Hydrogen may then be implanted into the SOI wafer  1506  at a certain depth to form hydrogen plane  1514 . Alternatively, another atomic species such as helium can be implanted or co-implanted.  FIG. 15C  illustrates the structure after Step (C).
 
Step (D): The top layer wafer shown after Step (C) may be flipped and bonded atop the bottom layer wafer using oxide-to-oxide bonding.  FIG. 15D  illustrates the structure after Step (D).
 
Step (E): A cleave operation may be performed at the hydrogen plane  1514  using an anneal. Alternatively, a sideways mechanical force may be used. Following this, an etching process that etches Si but does not etch silicon dioxide, such as a KOH solution, may be utilized to remove the p− Si layer of SOI wafer  1506  remaining after cleave. The buried oxide (BOX) silicon dioxide layer  1505  acts as an etch stop.  FIG. 15E  illustrates the structure after Step (E).
 
Step (F): Once the etch stop silicon dioxide layer  1505  may be reached, an etch or CMP process may be utilized to etch the silicon dioxide layer  1505  till the n+ silicon layer  1508  may be reached. The etch process for Step (F) may be chosen so that it may etch silicon dioxide but does not attack Silicon. For example, a dilute hydrofluoric acid solution may be utilized.  FIG. 15F  illustrates the structure after Step (F).
 
It is clear from the process shown in  FIG. 15A-F  that one can get excellent control of the n+ layer  1508 &#39;s thickness after layer transfer.
 
         [0096]    While the process shown in  FIG. 15A-F  results in accurate layer transfer of thin regions, it may have some drawbacks. SOI wafers may be typically quite costly, and utilizing an SOI wafer just for having an etch stop layer may not typically be economically viable. In that case, an alternative process shown in  FIG. 16A-F  could be utilized. The process flow in  FIG. 16A-F  may include several steps as described in the following sequence: 
         [0000]    Step (A): A silicon dioxide layer  1604  may be deposited above the generic bottom layer  1602 . 
         [0097]      FIG. 16A  illustrates the structure after Step (A). 
         [0000]    Step (B): A n− Si wafer  1606  may be implanted with boron doped p+ Si near its surface to form a p+ Si layer  1605 . The p+ layer may be doped above 1E20/cm 3 , and may be above 1E21/cm 3 . It may be possible to use a p− Si layer instead of the p+ Si layer  1605  as well, and still achieve similar results. A p− Si wafer can be utilized instead of the n− Si wafer  1606  as well.  FIG. 16B  illustrates the structure after Step (B).
 
Step (C): A n+ Si layer  1608  and a p− Si layer  1610  may be epitaxially grown atop the p+ Si layer  1605 . A silicon dioxide layer  1612  may be deposited atop the p− Si layer  1610 . An anneal (such as a rapid thermal anneal RTA or spike anneal or laser anneal) may be conducted to activate dopants.
 
Alternatively, the p+ Si layer  1605 , the n+ Si layer  1608  and the p− Si layer  1610  can be formed by a series of implants on a n− Si wafer  1606 .
 
Hydrogen may then be implanted into the n− Si wafer  1606  at a certain depth to form hydrogen plane  1614 . Alternatively, another atomic species such as helium can be implanted.  FIG. 16C  illustrates the structure after Step (C).
 
Step (D): The top layer wafer shown after Step (C) may be flipped and bonded atop the bottom layer wafer using oxide-to-oxide bonding.  FIG. 16D  illustrates the structure after Step (D).
 
Step (E): A cleave operation may be performed at the hydrogen plane  1614  using an anneal. Alternatively, a sideways mechanical force may be used. Following this, an etching process that etches the remaining n− Si layer of n− Si wafer  1606  but does not etch the p+ Si etch stop layer  1605  may be utilized to etch through the n− Si layer of n− Si wafer  1606  remaining after cleave. Examples of etching agents that etch n− Si or p− Si but do not attack p+ Si doped above 1E20/cm 3  include KOH, EDP (ethylenediamine/pyrocatechol/water) and hydrazine.  FIG. 16E  illustrates the structure after Step (E).
 
Step (F): Once the etch stop  1605  is substantially reached, an etch or CMP process may be utilized to etch the p+ Si layer  1605  till the n+ silicon layer  1608  is substantially reached.  FIG. 16F  illustrates the structure after Step (F).
 
It is clear from the process shown in  FIG. 16A-F  that one can get excellent control of the n+ layer  1608 &#39;s thickness after layer transfer.
 
         [0098]    While silicon dioxide and p+ Si were utilized as etch stop layers in  FIG. 15A-F  and  FIG. 16A-F  respectively, other etch stop layers such as SiGe could be utilized. An etch stop layer of SiGe can be incorporated in the middle of the structure shown in  FIG. 16A-F  using an epitaxy process. 
       Section 1.3.3: Alternative Low-Temperature (Sub-300° C.) Ion-Cut Process for Sub-400° C. Processed Transistors 
       [0099]    An alternative low-temperature ion-cut process may be described in  FIG. 17A-E . The process flow in  FIG. 17A-E  may include several steps as described in the following sequence: 
         [0000]    Step (A): A silicon dioxide layer  1704  may be deposited above the generic bottom layer  1702 .  FIG. 17A  illustrates the structure after Step (A).
 
Step (B): A p− Si wafer  1706  may be implanted with boron doped p+ Si near its surface to form a p+ Si layer  1705 . A n− Si wafer can be utilized instead of the p− Si wafer  1706  as well.  FIG. 17B  illustrates the structure after Step (B).
 
Step (C): A n+ Si layer  1708  and a p− Si layer  1710  may be epitaxially grown atop the p+ Si layer  1705 . A silicon dioxide layer  1712  may be grown or deposited atop the p− Si layer  1710 . An anneal (such as a rapid thermal anneal RTA or spike anneal or laser anneal) may be conducted to activate dopants.
 
Alternatively, the p+ Si layer  1705 , the n+ Si layer  1708  and the p− Si layer  1710  can be formed by a series of implants on a p− Si wafer  1706 .
 
Hydrogen may then be implanted into the p− Si layer of p− Si wafer  1706  at a certain depth to form hydrogen plane  1714 . Alternatively, another atomic species such as helium can be (co-) implanted.  FIG. 17C  illustrates the structure after Step (C).
 
Step (D): The top layer wafer shown after Step (C) may be flipped and bonded atop the bottom layer wafer using oxide-to-oxide bonding.  FIG. 17D  illustrates the structure after Step (D).
 
Step (E): A cleave operation may be performed at the hydrogen plane  1714  using a less than about 300° C. anneal. Alternatively, a sideways mechanical force may be used. An etch or CMP process may be utilized to etch the p+ Si layer  1705  till the n+ silicon layer  1708  is substantially reached.  FIG. 17E  illustrates the structure after Step (E).
 
The purpose of hydrogen implantation into the p+ Si region  1705  may be because p+ regions heavily doped with boron are known to need the lower anneal temperature which may be needed for ion-cut. Further details of this technology/process may be given in “Cold ion-cutting of hydrogen implanted Si, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms”, Volume 190, Issues 1-4, May 2002, Pages 761-766, ISSN 0168-583X by K. Henttinen, T. Suni, A. Nurmela, et al. (“Hentinnen and Suni”). The contents of these publications are incorporated herein by reference.
 
       Section 1.3.4: Alternative Procedures for Layer Transfer 
       [0100]    While ion-cut may have been described in previous sections as the method for layer transfer, several other procedures exist that fulfill the same objective. These include:
       Lift-off or laser lift-off: Background information for this technology is given in “Epitaxial lift-off and its applications”, 1993 Semicond. Sci. Technol. 8 1124 by P Demeester et al. (“Demeester”).   Porous-Si approaches such as ELTRAN: Background information for this technology is given in “Eltran, Novel SOI Wafer Technology”, JSAP International, Number 4, July 2001 by T. Yonehara and K. Sakaguchi (“Yonehara”) and also in “Frontiers of silicon-on-insulator,” J. Appl. Phys. 93, 4955-4978, 2003 by G. K. Celler and S. Cristoloveanu (“Celler”).   Time-controlled etch-back to thin an initial substrate, Polishing, Etch-stop layer controlled etch-back to thin an initial substrate: Background information on these technologies is given in Celler and in U.S. Pat. No. 6,806,171.   Rubber-stamp based layer transfer: Background information on this technology is given in “Solar cells sliced and diced”, 19 May 2010, Nature News.
 
The above publications giving background information on various layer transfer procedures are incorporated herein by reference.
       
 
         [0105]      FIG. 18A-F  shows a procedure using etch-stop layer controlled etch-back for layer transfer. The process flow in  FIG. 18A-F  may include several steps in the following sequence: 
         [0000]    Step (A): A silicon dioxide layer  1804  may be deposited above the generic bottom layer  1802 . 
         [0106]      FIG. 18A  illustrates the structure after Step (A). 
         [0000]    Step (B): SOI wafer  1806  may be implanted with n+ near its surface to form n+ Si layer  1808 . The buried oxide (BOX) of the SOI wafer may be silicon dioxide layer  1805 .  FIG. 18B  illustrates the structure after Step (B).
 
Step (C): A p− Si layer  1810  may be epitaxially grown atop the n+ Si layer  1808 . A silicon dioxide layer  1812  may be grown/deposited atop the p− Si layer  1810 . An anneal (such as a rapid thermal anneal RTA or spike anneal or laser anneal) may be conducted to activate dopants.  FIG. 18C  illustrates the structure after Step (C).
 
         [0107]    Alternatively, the n+ Si layer  1808  and p− Si layer  1810  can be formed by a buried layer implant of n+ Si in a p− SOI wafer. 
         [0000]    Step (D): The top layer wafer shown after Step (C) may be flipped and bonded atop the bottom layer wafer using oxide-to-oxide bonding.  FIG. 18D  illustrates the structure after Step (D).
 
Step (E): An etch process that etches Si but does not etch silicon dioxide may be utilized to etch through the p− Si layer of SOI wafer  1806 . The buried oxide (BOX) of silicon dioxide layer  1805  therefore may act as an etch stop.  FIG. 18E  illustrates the structure after Step (E).
 
Step (F): Once the etch stop of silicon dioxide layer  1805  is substantially reached, an etch or CMP process may be utilized to etch the silicon dioxide layer  1805  till the n+ silicon layer  1808  may be reached. The etch process for Step (F) may be chosen so that it may etch silicon dioxide but does not attack Silicon.  FIG. 18F  illustrates the structure after Step (F).
 
At the end of the process shown in  FIG. 18A-F , the desired regions may be layer transferred atop the bottom layer 1802. While  FIG. 18A-F  shows an etch-stop layer controlled etch-back using a silicon dioxide etch stop layer, other etch stop layers such as SiGe or p+ Si can be utilized in alternative process flows.
 
         [0108]      FIG. 19  shows various methods one can use to bond a top layer wafer  1908  to a bottom wafer  1902 . Oxide-oxide bonding of a layer of silicon dioxide  1906  and a layer of silicon dioxide  1904  may be used. Before bonding, various methods can be utilized to activate surfaces of the layer of silicon dioxide  1906  and the layer of silicon dioxide  1904 . A plasma-activated bonding process such as the procedure described in US Patent 20090081848 or the procedure described in “Plasma-activated wafer bonding: the new low-temperature tool for MEMS fabrication”, Proc. SPIE 6589, 65890T (2007), DOI:10.1117/12.721937 by V. Dragoi, G. Mittendorfer, C. Thanner, and P. Lindner (“Dragoi”) can be used. Alternatively, an ion implantation process such as the one described in US Patent 20090081848 or elsewhere can be used. Alternatively, a wet chemical treatment can be utilized for activation. Other methods to perform oxide-to-oxide bonding can also be utilized. While oxide-to-oxide bonding may have been described as a method to bond together different layers of the 3D stack, other methods of bonding such as metal-to-metal bonding can also be utilized. 
         [0109]      FIG. 20A-E  depict layer transfer of a Germanium or a III-V semiconductor layer to form part of a 3D integrated circuit or chip or system. These layers could be utilized for forming optical components or form forming, for example, higher-performance or lower-power transistors.  FIG. 20A-E  describes an ion-cut flow for layer transferring a single crystal Germanium or III-V semiconductor layer  2007  atop any generic bottom layer  2002 . The bottom layer  2002  can be a single crystal silicon layer or some other semiconductor layer. Alternatively, it can be a wafer having transistors with wiring layers above it. This process of ion-cut based layer transfer may include several steps as described in the following sequence: 
         [0000]    Step (A): A silicon dioxide layer  2004  may be deposited above the generic bottom layer  2002 .  FIG. 20A  illustrates the structure after Step (A).
 
Step (B): The layer to be transferred atop the bottom layer (top layer of doped germanium or III-V semiconductor  2006 ) may be processed and a compatible oxide layer  2008  may be deposited above it.  FIG. 20B  illustrates the structure after Step (B).
 
Step (C): Hydrogen may be implanted into the Top layer doped Germanium or III-V semiconductor  2006  at a certain depth  2010 . Alternatively, another atomic species such as helium can be (co-)implanted.  FIG. 20C  illustrates the structure after Step (C).
 
Step (D): The top layer wafer shown after Step (C) may be flipped and bonded atop the bottom layer wafer using oxide-to-oxide bonding.  FIG. 20D  illustrates the structure after Step (D).
 
Step (E): A cleave operation may be performed at the hydrogen plane  2010  using an anneal or a mechanical force. Following this, a Chemical-Mechanical-Polish (CMP) may be done.  FIG. 20E  illustrates the structure after Step (E).
 
       Section 1.3.5: Laser Anneal Procedure for 3D Stacked Components and Chips 
       [0110]      FIG. 21A-C  describes a prior art process flow for constructing 3D stacked circuits and chips using laser anneal techniques. Note that the terms laser anneal and optical anneal may be utilized interchangeably in this document. This procedure is described in “Electrical Integrity of MOS Devices in Laser Annealed 3D IC Structures” in the proceedings of VMIC 2004 by B. Rajendran, R. S. Shenoy, M. O. Thompson &amp; R. F. W. Pease. The process may include several steps as described in the following sequence: 
         [0000]    Step (A): The bottom wafer  2112  may be processed with transistor and wiring layers. The top wafer may include silicon layer  2110  with an oxide layer above it. The thickness of the silicon layer  2110 , t, may be typically greater than about 50 um.  FIG. 21A  illustrates the structure after Step (A).
 
Step (B): The top wafer  2114  may be flipped and bonded to the bottom wafer  2112 . It can be readily seen that the thickness of the top layer may be greater than about 50 um. As a result from this high thickness, and that the aspect ratio (height to width ratio) of through-silicon connections may be limited to about less than 100:1, it can be seen that the approximate minimum width of through-silicon connections possible with this procedure may be 50 um/100=500 nm. This may be much higher than dimensions of horizontal wiring on a chip.  FIG. 21B  illustrates the structure after Step (B).
 
Step (C): Transistors may then be built on the top wafer  2114  and a laser anneal may be utilized to activate dopants in the top silicon layer. As a result from the characteristics of a laser anneal, the temperature in the top layer, top wafer  2114 , may be much higher than the temperature in the bottom layer, bottom wafer  2112 .  FIG. 21C  illustrates the structure after Step (C).
 
An alternative procedure described in prior art may be the SOI-based layer transfer (shown in  FIG. 18A-F ) followed by a laser anneal. This process is described in “Sequential 3D IC Fabrication: Challenges and Prospects”, by Bipin Rajendran in VMIC 2006.
 
         [0111]    An alternative procedure for laser anneal of layer transferred silicon may be shown in  FIG. 22A-E . The process may include several steps as described in the following sequence. 
         [0000]    Step (A): A bottom wafer  2212  may be processed with transistor, wiring and silicon dioxide layers.  FIG. 22A  illustrates the structure after Step (A).
 
Step (B): A top layer of silicon  2210  may be layer transferred atop it using procedures similar to  FIG. 2 .  FIG. 22B  illustrates the structure after Step (B).
 
Step (C): Transistors may be formed on the top layer of silicon  2210  and a laser anneal may be done to activate dopants in source-drain regions  2216 . Fabrication of the rest of the integrated circuit flow including contacts and wiring layers may then proceed.  FIG. 22C  illustrates the structure after Step (C).
 
 FIG. 22(D)  shows that absorber layers  2218  may be used to efficiently heat the top layer of silicon  2224  while ensuring temperatures at the bottom wiring layer  2204  are low (less than about 500° C.).  FIG. 22(E)  shows that one could use heat protection layers  2220  situated in-between the top and bottom layers of silicon to keep temperatures at the bottom wiring layer  2204  low (less than about 500° C.). These heat protection layers could be constructed of optimized materials that reflect laser radiation and reduce heat conducted to the bottom wiring layer. The terms heat protection layer and shield can be used interchangeably in this document.
 
         [0112]    Most of the figures described thus far in this document assumed the transferred top layer of silicon may be very thin (less than about 200 nm). This may enable light to penetrate the silicon and may allow features on the bottom wafer to be observed. However, that may be not typically the case.  FIG. 23A-C  shows a process flow for constructing 3D stacked chips and circuits when the thickness of the transferred/stacked piece of silicon may be so high that light does not penetrate the transferred piece of silicon to observe the alignment marks on the bottom wafer. The process to allow for alignment to the bottom wafer may include several steps as described in the following sequence. 
         [0000]    Step (A): A bottom wafer  2312  may be processed to form a bottom transistor layer  2306  and a bottom wiring layer  2304 . A layer of silicon oxide  2302  may be deposited above it.  FIG. 23A  illustrates the structure after Step (A).
 
Step (B): A wafer of p− Si  2310  may have an oxide layer  2308  deposited or grown above it. Using lithography, a window pattern may be etched into the p− Si  2310  and may be filled with oxide. A step of CMP may be done. This window pattern may be used in Step (C) to allow light to penetrate through the top layer of silicon to align to circuits on the bottom wafer  2312 . The window size may be chosen based on misalignment tolerance of the alignment scheme used while bonding the top wafer to the bottom wafer in Step (C). Furthermore, some alignment marks also exist in the wafer of p− Si  2310 .  FIG. 23B  illustrates the structure after Step (B).
 
Step (C): A portion of the p− Si  2310  from Step (B) may be transferred atop the bottom wafer  2312  using procedures similar to  FIG. 2A-E . It can be observed that the window  2316  can be used for aligning features constructed on the top wafer  2314  to features on the bottom wafer  2312 . Thus, the thickness of the top wafer  2314  can be chosen without constraints.  FIG. 23C  illustrates the structure after Step (C).
 
         [0113]    Additionally, when circuit cells may be built on two or more layers of thin silicon, and enjoy the dense vertical through silicon via interconnections, the metallization layer scheme to utilize this dense 3D technology may be improved as follows.  FIG. 24A  illustrates the prior art of silicon integrated circuit metallization schemes. The conventional transistor silicon layer  2402  may be connected to the first metal layer  2410  thru the contact  2404 . The dimensions of this interconnect pair of contact and metal lines generally may be at the minimum line resolution of the lithography and etch capability for that technology process node. Traditionally, this may be called a “1X’ design rule metal layer. Usually, the next metal layer may also be at the “1X’ design rule, the metal line  2412  and via below  2405  and via above  2406  that connects metal line  2412  with  2410  or with  2414  where desired. Then the next few layers may be often constructed at twice the minimum lithographic and etch capability and called ‘2X’ metal layers, and have thicker metal for current carrying capability. These may be illustrated with metal line  2414  paired with via  2407  and metal line  2416  paired with via  2408  in  FIG. 24A . Accordingly, the metal via pairs of  2418  with  2409 , and  2420  with bond pad opening  2422 , represent the ‘4X’ metallization layers where the planar and thickness dimensions may be again larger and thicker than the 2X and 1X layers. The precise number of 1X or 2X or 4X layers may vary depending on interconnection needs and other requirements; however, the general flow may be that of increasingly larger metal line, metal space, and via dimensions as the metal layers may be farther from the silicon transistors and closer to the bond pads. 
         [0114]    The metallization layer scheme may be improved for 3D circuits as illustrated in  FIG. 24B . The first crystallized silicon device layer  2454  may be illustrated as the NMOS silicon transistor layer from the above 3D library cells, but may also be a conventional logic transistor silicon substrate or layer. The ‘1X’ metal layers  2450  and  2449  may be connected with contact  2440  to the silicon transistors and vias  2438  and  2439  to each other or metal  2448 . The 2X layer pairs metal  2448  with via  2437  and metal  2447  with via  2436 . The 4X metal layer  2446  may be paired with via  2435  and metal  2445 , also at 4X. However, now via  2434  may be constructed in 2X design rules to enable metal line  2444  to be at 2X. Metal line  2443  and via  2433  may be also at 2X design rules and thicknesses. Vias  2432  and  2431  may be paired with metal lines  2442  and  2441  at the 1X minimum design rule dimensions and thickness. The thru silicon via  2430  of the illustrated PMOS layer transferred silicon layer  2452  may then be constructed at the 1X minimum design rules and provide for maximum density of the top layer. The precise numbers of 1X or 2X or 4X layers may vary depending on circuit area and current carrying metallization requirements and tradeoffs. The illustrated PMOS layer transferred silicon layer  2452  may include any of the low temperature devices illustrated herein. 
         [0115]      FIGS. 43A-G  illustrate the formation of Junction Gate Field Effect Transistor (JFET) top transistors.  FIG. 43A  illustrates the structure after n− Si layer  4304  and n+ Si layer  4302  may be transferred on top of a bottom layer of transistors and wires  4306 . This may be done using procedures similar to those shown in  FIG. 11A-F . Then the top transistor source  4308  and drain  4310  may be defined by etching away the n+ from the region designated for gates  4312  and the isolation region between transistors  4314 . This step may be aligned to the bottom layer of transistors and wires  4306  so the formed transistors could be properly connected to the underlying bottom layer of transistors and wires  4306 . Then an additional masking and etch step may be performed to remove the n− layer between transistors, shown as  4316 , thus providing transistor isolation as illustrated in  FIG. 43C .  FIG. 43D  illustrates an example formation of shallow p+ region  4318  for the JFET gate formation. In this example there might be a need for laser or other optical energy transfer anneal to activate the p+.  FIG. 43E  illustrates how to utilize the laser anneal and minimize the heat transfer to the bottom layer of transistors and wires  4306 . After the thick oxide deposition  4320 , a layer of light reflecting material, such as, for example, Aluminum, may be applied as a reflective layer  4322 . An opening  4324  in the reflective layer may be masked and etched, allowing the laser light  4326  to heat the p+ implanted area  4330 , and reflecting the majority of the laser energy from laser light  4326  away from bottom layer of transistors and wires  4306 . Typically, the open area  4324  may be less than 10% of the total wafer area. Additionally, a reflective layer  4328  of copper, or, alternatively, a reflective Aluminum layer or other reflective material, may be formed in the bottom layer of transistors and wires  4306  that may additionally reflect any of the laser energy of laser light  4326  that might travel to bottom layer of transistor and wires  4306 . This same reflective &amp; open laser anneal technique might be utilized on any of the other illustrated structures to enable implant activation for transistors in the second layer transfer process flow. In addition, absorptive materials may, alone or in combination with reflective materials, also be utilized in the above laser or other optical energy transfer anneal techniques. A photonic energy absorbing layer  4332 , such as amorphous carbon of an appropriate thickness, may be deposited or sputtered at low temperature over the area that needs to be laser heated, and then may be masked and etched as appropriate, as shown in  FIG. 43F . This may allow the minimum laser energy to be employed to effectively heat the area to be implant activated, and thereby may minimize the heat stress on the reflective layers  4322  &amp;  4328  and the bottom layer of transistors and wires  4306 . The laser or optical energy reflective layer  4322  may then be etched or polished away and contacts can be made to various terminals of the transistor. This flow may enable the formation of fully crystallized top JFET transistors that could be connected to the underlying multi-metal layer semiconductor device without exposing the underlying device to high temperature. 
         [0000]    Section 2: Construction of 3D Stacked Semiconductor Circuits and Chips where Replacement Gate High-k/Metal Gate Transistors can be Used. Misalignment-Tolerance Techniques May be Utilized to Get High Density of Connections. 
         [0116]    Section 1 described the formation of 3D stacked semiconductor circuits and chips with sub-400° C. processing temperatures to build transistors and high density of vertical connections. In this section an alternative method may be explained, in which a transistor may be built with any replacement gate (or gate-last) scheme that may be utilized widely in the industry. This method allows for high temperatures (above about 400 C) to build the transistors. This method utilizes a combination of three concepts:
       Replacement gate (or gate-last) high k/metal gate fabrication   Face-up layer transfer using a carrier wafer   Misalignment tolerance techniques that utilize regular or repeating layouts. In these repeating layouts, transistors could be arranged in substantially parallel bands.
 
A very high density of vertical connections may be possible with this method. Single crystal silicon (or monocrystalline silicon) layers that may be transferred may be less than about 2 um thick, or could even be thinner than about 0.4 um or about 0.2 um.
       
 
         [0120]    The method mentioned in the previous paragraph may be described in  FIG. 25A-F . The procedure may include several steps as described in the following sequence: 
         [0000]    Step (A): After creating isolation regions using a shallow-trench-isolation (STI) process  2504 , dummy gates  2502  may be constructed with silicon dioxide and poly silicon. The term “dummy gates” may be used since these gates may be replaced by high k gate dielectrics and metal gates later in the process flow, according to the standard replacement gate (or gate-last) process. Further details of replacement gate processes may be described in “A 45 nm Logic Technology with High-k+Metal Gate Transistors, Strained Silicon, 9 Cu Interconnect Layers, 193 nm Dry Patterning, and 100% Pb-free Packaging,” IEDM Tech. Dig., pp. 247-250, 2007 by K. Mistry, et al. and “Ultralow-EOT (5 Å) Gate-First and Gate-Last High Performance CMOS Achieved by Gate-Electrode Optimization,” IEDM Tech. Dig., pp. 663-666, 2009 by L. Ragnarsson, et al.  FIG. 25A  illustrates the structure after Step (A).
 
Step (B): Transistor fabrication flow may proceed with the formation of source-drain regions  2506 , strain enhancement layers to improve mobility, a high temperature anneal to activate source-drain regions  2506 , formation of inter-layer dielectric (ILD)  2508 , and additional conventional steps.  FIG. 25B  illustrates the structure after Step (B).
 
Step (C): Hydrogen may be implanted into the wafer at the dotted line regions indicated by  2510 .  FIG. 25C  illustrates the structure after Step (C).
 
Step (D): The wafer after step (C) may be bonded to a temporary carrier wafer  2512  using a temporary bonding adhesive  2514 . This temporary carrier wafer  2512  could be constructed of glass. Alternatively, it could be constructed of silicon. The temporary bonding adhesive  2514  could be a polymer material, such as, for example, polyimide DuPont HD3007. A anneal or a sideways mechanical force may be utilized to cleave the wafer at the hydrogen plane  2510 . A CMP process may be then conducted.  FIG. 25D  illustrates the structure after Step (D).
 
Step (E): An oxide layer may be deposited onto the bottom of the wafer shown in Step (D). The wafer may be then bonded to the bottom layer of wires and transistors  2522  using oxide-to-oxide bonding. The bottom layer of wires and transistors  2522  could also be called a base wafer. The temporary carrier wafer  2512  may be then removed by shining a laser onto the temporary bonding adhesive  2514  through the temporary carrier wafer  2512  (which could be constructed of glass). Alternatively, an anneal could be used to remove the temporary bonding adhesive  2514 . Through-silicon connections  2516  with a non-conducting (e.g. oxide) liner  2515  to the landing pads  2518  in the base wafer could be constructed at a very high density using special alignment methods to be described in  FIG. 26A-D  and  FIG. 27A-F .  FIG. 25E  illustrates the structure after Step (E).
 
Step (F): Dummy gates  2502  may be etched away, followed by the construction of a replacement with high k gate dielectrics  2524  and metal gates  2526 . Essentially, partially-formed high performance transistors may be layer transferred atop the base wafer (may also be called target wafer) followed by the completion of the transistor processing with a low (less than about 400° C.) process.  FIG. 25F  illustrates the structure after Step (F). The remainder of the transistor, contact and wiring layers may then be constructed.
 
It will be obvious to someone skilled in the art that alternative versions of this flow may be possible with various methods to attach temporary carriers and with various versions of the gate-last process flow.
 
         [0121]      FIG. 26A-D  describes an alignment method for forming CMOS circuits with a high density of connections among 3D stacked layers. The alignment method may include moving the top layer masks left or right and up or down until substantially all the through-layer contacts may be on top of their corresponding landing pads. This may be done in several steps in the following sequence: 
         [0000]      FIG. 26A  illustrates the top wafer. A repeating pattern of circuit regions  2604  in the top wafer in both X and Y directions may be used. Oxide isolation regions  2602  in-between adjacent (identical) repeating structures may be used. Each (identical) repeating structure may have X dimension=W x  and Y dimension=W y , and this includes oxide isolation region thickness. The top alignment mark  2606  in the top layer may be located at (x top , y top ).
 
 FIG. 26B  illustrates the bottom wafer. The bottom wafer may have a transistor layer and multiple layers of wiring. The top-most wiring layer may have a landing pad structure, where repeating landing pads  2608  of X dimension W x + delta(W x ) and Y dimension W y + delta(W y ) may be used. delta(W x ) and delta(W y ) may be quantities that may be added to compensate for alignment offsets, and may be small compared to W x  and W y  respectively. Alignment mark  2610  for the bottom wafer may be located at (x bottom , y bottom ). Note that the terms landing pad and metal strip may be utilized interchangeably in this document.
 
After bonding the top and bottom wafers atop each other as described in  FIG. 25A-F , the wafers look as shown in  FIG. 26C . Note that the repeating pattern of circuit regions  2604  in-between oxide isolation regions  2602  are not shown for clarity in illustration and understanding. It can be seen the top alignment mark  2606  and bottom alignment mark  2610  may be misaligned to each other. As previously described in the description of  FIG. 14B , rotational or angular alignment between the top and bottom wafers may be small and margin for this may be provided by the offsets delta(W x ) and delta(W y ).
 
Since the landing pad dimensions may be larger than the length of the repeating pattern in both X and Y direction, the top layer-to-layer contact (and other masks) may be shifted left or right and up or down until this contact may be on top of the corresponding landing pad. This method is further described below:
 
Next step in the process may be described with  FIG. 26D . A virtual alignment mark may be created by the lithography tool. The X co-ordinate of this virtual alignment mark may be at the location (x top +(an integer k)*W x ). The integer k may be chosen such that modulus or absolute value of (x top +(integer k)*W x −x bottom )&lt;=W x /2. This guarantees that the X co-ordinate of the virtual alignment mark may be within a repeat distance of the X alignment mark of the bottom wafer. Y co-ordinate of this virtual alignment mark may be at the location (y top +(an integer h)*W y ). The integer h may be chosen such that modulus or absolute value of (y top +(integer h)*W y −y bottom )&lt;=W y /2. This guarantees that the Y co-ordinate of the virtual alignment mark may be within a repeat distance of the Y alignment mark of the bottom wafer. Since the silicon thickness of the top layer may be thin, the lithography tool can observe the alignment mark of the bottom wafer. Though-silicon connections  2612  may be now constructed with alignment mark of this mask aligned to the virtual alignment mark. Since the X and Y co-ordinates of the virtual alignment mark may be within the same area of the layout (of dimensions W x  and W y ) as the bottom wafer X and Y alignment marks, the through-silicon connection  2612  may always fall on the bottom landing pad  2608  (the bottom landing pad dimensions may be W x  added to delta (W x ) and W y  added to delta (W y )).
 
         [0122]      FIG. 27A-F  show an alternative alignment method for forming CMOS circuits with a high density of connections between 3D stacked layers. The alignment method may include several steps in the following sequence: 
         [0000]      FIG. 27A  describes the top wafer. A repeating pattern of circuit regions  2704  in the top wafer in both X and Y directions may be used. Oxide isolation regions  2702  in-between adjacent (identical) repeating structures may be used. Each (identical) repeating structure may have X dimension=W x  and Y dimension=W y , and this includes oxide isolation region thickness. The top alignment mark  2706  in the top layer may be located at (x top , y top ).
 
 FIG. 27B  describes the bottom wafer. The bottom wafer may have a transistor layer and multiple layers of wiring. The top-most wiring layer may have a landing pad structure, where repeating landing pads  2708  of X dimension W x + delta(W x ) and Y dimension F or  2 F may be used. delta(W x ) may be a quantity that may be added to compensate for alignment offsets, and may be smaller compared to W. Alignment mark  2710  for the bottom wafer may be located at (x bottom , y bottom ).
 
After bonding the top and bottom wafers atop each other as described in  FIG. 25A-F , the wafers look as shown in  FIG. 27C . Note that the repeating pattern of circuit regions  2704  in-between oxide isolation regions  2702  are not shown for easy illustration and understanding. It can be seen the top alignment mark  2706  and bottom alignment mark  2710  may be misaligned to each other. As previously described in the description of  FIG. 14B , angular alignment between the top and bottom wafers may be small and margin for this may be provided by the offsets delta(W x ) and delta(W y ).
 
 FIG. 27D  illustrates the alignment method during/after the next step. A virtual alignment mark may be created by the lithography tool. X co-ordinate of this virtual alignment mark may be at the location (x top +(an integer k)*W x ). The integer k may be chosen such that modulus or absolute value of (x top +(integer k)*W x −x bottom )&lt;=W x /2. This guarantees that the X co-ordinate of the virtual alignment mark may be within a repeat distance of the X alignment mark of the bottom wafer. Y co-ordinate of this virtual alignment mark may be at the location (y top +(an integer h)*W y ). The integer h may be chosen such that modulus or absolute value of (y top +(integer h)*W y −y bottom )&lt;=W y /2. This guarantees that the Y co-ordinate of the virtual alignment mark may be within a repeat distance of the Y alignment mark of the bottom wafer. Since silicon thickness of the top layer may be thin, the lithography tool can observe the alignment mark of the bottom wafer. The virtual alignment mark may be at the location (x virtual , y virtual ) where x virtual  and y virtual  may be obtained as described earlier in this paragraph.
 
 FIG. 27E  illustrates the alignment method during/after the next step. Though-silicon connections  2712  may be now constructed with alignment mark of this mask aligned to (x virtual , y bottom ). Since the X co-ordinate of the virtual alignment mark may be within the same section of the layout in the X direction (of dimension W x  as the bottom wafer X alignment mark, the through-silicon connection  2712  may always fall on the bottom landing pad  2708  (the bottom landing pad dimension may be W x  added to delta (W x )). The Y co-ordinate of the through silicon connection  2712  may be aligned to y bottom , the Y co-ordinate of the bottom wafer alignment mark as described previously.
 
 FIG. 27F  shows a drawing illustration during/after the next step. A top landing pad  2716  may be then constructed with X dimension F or  2 F and Y dimension W y + delta(W y ). This mask may be formed with alignment mark aligned to (x bottom , y virtual ). Essentially, it can be seen that the top landing pad  2716  compensates for misalignment in the Y direction, while the bottom landing pad  2708  compensates for misalignment in the X direction.
 
The alignment scheme shown in  FIG. 27A-F  can give a higher density of connections between two layers than the alignment scheme shown in  FIG. 26A-D . The connection paths between two transistors located on two layers therefore may include: a first landing pad or metal strip substantially parallel to a certain axis, a through via and a second landing pad or metal strip substantially perpendicular to a certain axis. Features may be formed using virtual alignment marks whose positions depend on misalignment during bonding. Also, through-silicon connections in  FIG. 26A-D  may have relatively high capacitance as a result from the size of the landing pads. It will be apparent to one skilled in the art that variations of this process flow may be possible (e.g., different versions of regular layouts could be used along with replacement gate processes to get a high density of connections between 3D stacked circuits and chips).
 
         [0123]      FIG. 44A-D  and  FIG. 45A-D  show an alternative procedure for forming CMOS circuits with a high density of connections between stacked layers. The process utilizes a repeating pattern in one direction for the top layer of transistors. The procedure may include several steps in the following sequence: 
         [0000]    Step (A): Using procedures similar to  FIG. 25A-F , a top layer of transistors  4404  may be transferred atop a bottom layer of transistors and wires  4402 . Landing pads  4406  may be utilized on the bottom layer of transistors and wires  4402 . Dummy gates  4408  and  4410  may be utilized for nMOS and pMOS. The key difference between the structures shown in  FIG. 25A-F  and this structure may be the layout of oxide isolation regions between transistors.  FIG. 44A  illustrates the structure after Step (A).
 
Step (B): Through-silicon connections  4412  may be formed well-aligned to the bottom layer of transistors and wires  4402 . Alignment schemes to be described in  FIG. 45A-F  may be utilized for this purpose. Substantially all features constructed in future steps may be also formed well-aligned to the bottom layer of transistors and wires  4402 .  FIG. 44B  illustrates the structure after Step (B).
 
Step (C): Oxide isolation regions  4414  may be formed between adjacent transistors to be defined. These isolation regions may be formed by lithography and etch of gate and silicon regions and then fill with oxide.  FIG. 44C  illustrates the structure after Step (C).
 
Step (D): The dummy gates  4408  and  4410  may be etched away and replaced with replacement gates  4416  and  4418 . These replacement gates may be patterned and defined to form gate contacts as well.  FIG. 44D  illustrates the structure after Step (D). Following this, other process steps in the fabrication flow proceed as usual.
 
         [0124]      FIG. 45A-D  describe alignment schemes for the structures shown in  FIG. 44A-D .  FIG. 45A  describes the top wafer. A repeating pattern of features in the top wafer in Y direction may be used. Each (identical) repeating structure may have Y dimension=W y , and this includes oxide isolation region thickness. The alignment mark  4502  in the top layer may be located at (x top , y top ). 
         [0000]      FIG. 45B  describes the bottom wafer. The bottom wafer may have a transistor layer and multiple layers of wiring. The top-most wiring layer may have a landing pad structure, where repeating landing pads  4506  of X dimension F or  2 F and Y dimension W y + delta(W y ) may be used. delta(W y ) may be a quantity that may be added to compensate for alignment offsets, and may be smaller compared to W y . Alignment mark  4504  for the bottom wafer may be located at (x bottom , y bottom ).
 
After bonding the top and bottom wafers atop each other as described in  FIG. 44A-D , the wafers look as shown in  FIG. 45C . It can be seen the top alignment mark  4502  and bottom alignment mark  4504  may be misaligned to each other. As previously described in the description of  FIG. 14B , angle alignment between the top and bottom wafers may be small or negligible.
 
 FIG. 45D  illustrates the next step of the alignment procedure. A virtual alignment mark may be created by the lithography tool. The X co-ordinate of this virtual alignment mark may be at the location (x bottom ). The Y co-ordinate of this virtual alignment mark may be at the location (y top +(an integer h)*W y ). The integer h may be chosen such that modulus or absolute value of (y top +(integer h)*W y −y bottom )&lt;=W y /2. This guarantees that the Y co-ordinate of the virtual alignment mark may be within a repeat distance of the Y alignment mark of the bottom wafer. Since the silicon thickness of the top layer may be thin, the lithography tool can observe the alignment mark of the bottom wafer. The virtual alignment mark may be at the location (x virtual , y virtual ) where x virtual  and y virtual  may be obtained as described earlier in this paragraph.
 
 FIG. 45E  illustrates the next step of the alignment procedure. Though-silicon connections  4508  may be now constructed with alignment mark of this mask aligned to (x virtual , y virtual ). Since the X co-ordinate of the virtual alignment mark may be perfectly aligned to the X co-ordinate of the bottom wafer alignment mark and since the Y co-ordinate of the virtual alignment mark may be within the same section of the layout (of distance W y ) as the bottom wafer Y alignment mark, the through-silicon connection  4508  may always fall on the bottom landing pad (the bottom landing pad dimension in the Y direction may be W y  added to delta (W y )).
 
         [0125]      FIG. 46A-G  illustrate using a carrier wafer for layer transfer.  FIG. 46A  illustrates the first step of preparing dummy gate transistors  4602  on first donor wafer  4600  (or top wafer). This completes the first phase of transistor formation.  FIG. 46B  illustrates forming a cleave line  4608  by implant  4616  of atomic particles such as H+.  FIG. 46C  illustrates permanently bonding the first donor wafer  4600  to a second donor wafer  4626 . The permanent bonding may be oxide to oxide wafer bonding as described previously.  FIG. 46D  illustrates the second donor wafer  4626  acting as a carrier wafer after cleaving the first donor wafer off; leaving a thin layer  4606  with the now buried dummy gate transistors  4602 .  FIG. 46E  illustrates forming a second cleave line  4618  in the second donor wafer  4626  by implant  4646  of atomic species such as H+.  FIG. 46F  illustrates the second layer transfer step to bring the dummy gate transistors  4602  ready to be permanently bonded on top of the bottom layer of transistors and wires  4601 . For the simplicity of the explanation the steps of surface layer preparation done for each of these bonding steps are left out of the illustrative explanation.  FIG. 46G  illustrates the bottom layer of transistors and wires  4601  with the dummy gate transistors  4602  on top after cleaving off the second donor wafer and removing the layers on top of the dummy gate transistors. The dummy gates may be replaced with the final gates, metal interconnection layers may be formed, and the 3D fabrication process may continue. 
         [0126]    An illustrative alternative may be available when using the carrier wafer flow described in  FIG. 46A-G . In this flow the two large area sides of the transferred layer may be utilized to build NMOS on one side and PMOS on the other side. Timing properly the replacement gate step such flow could enable full performance transistors properly aligned to each other. As illustrated in  FIG. 47A , an SOI (Silicon On Insulator) donor wafer  4700  may be processed in the normal state of the art high k metal gate gate-last manner with adjusted thermal cycles to compensate for later thermal processing up to the step prior to where CMP exposure of the polysilicon dummy gates  4704  takes place.  FIG. 47A  illustrates a cross section of the SOI donor wafer  4700 , the buried oxide (BOX)  4701 , the thin silicon layer  4702  of the SOI wafer, the isolation  4703  between transistors, the polysilicon dummy gates  4704  and gate oxide  4705  of n-type CMOS transistors with dummy gates, their associated source and drains  4706  for NMOS, and the NMOS interlayer dielectric (ILD)  4708 . Alternatively, the PMOS device may be constructed at this stage. This completes the first phase of transistor formation. At this step, or alternatively just after a CMP of NMOS ILD  4708  to expose the polysilicon dummy gates  4704  or to planarize the NMOS ILD  4708  and not expose the polysilicon dummy gates  4704 , an implant of an atomic species  4710 , such as H+, is done to prepare the cleaving plane  4712  in the bulk of the donor substrate, as illustrated in  FIG. 47B . The SOI donor wafer  4700  may be permanently bonded to a carrier wafer  4720  that may have been prepared with an oxide layer  4716  for oxide to oxide bonding to the donor wafer surface  4714  as illustrated in  FIG. 47C . The details have been described previously. The SOI donor wafer  4700  may then be cleaved at the cleaving plane  4712  and may be thinned by chemical mechanical polishing (CMP) thus forming donor wafer layer  4700 ′, and surface  4722  may be prepared for transistor formation. The donor wafer layer  4700 ′ at surface  4722  may be processed in the normal state of the art gate last processing to form the PMOS transistors with dummy gates. During processing the wafer may be flipped so that surface  4722  may be on top, but for illustrative purposes this may be not shown in the subsequent  FIGS. 47E-G .  FIG. 47E  illustrates the cross section with the buried oxide (BOX)  4701 , the now thin silicon donor wafer layer  4700 ′ of the SOI substrate, the isolation  4733  between transistors, the polysilicon dummy gates  4734  and gate oxide  4735  of p-type CMOS dummy gates, their associated source and drains  4736  for PMOS, and the PMOS interlayer dielectric (ILD)  4738 . The PMOS transistors may be precisely aligned at state of the art tolerances to the NMOS transistors as a result from the shared substrate donor wafer layer  4700 ′ possessing the same alignment marks. At this step, or alternatively just after a CMP of PMOS ILD  4738  to expose the PMOS polysilicon dummy gates or to planarize the PMOS ILD  4738  and not expose the dummy gates, the wafer could be put into high temperature cycle to activate both the dopants in the NMOS and the PMOS source drain regions. Then an implant of an atomic species  4787 , such as H+, may prepare the cleaving plane  4721  in the bulk of the carrier wafer  4720  for layer transfer suitability, as illustrated in  FIG. 47F . The PMOS transistors may now be ready for normal state of the art gate-last transistor formation completion. As illustrated in  FIG. 47G , the PMOS ILD  4738  may be chemical mechanically polished to expose the top of the polysilicon dummy gates  4734 . The polysilicon dummy gates  4734  may then be removed by etch and the PMOS hi-k gate dielectric  4740  and the PMOS specific work function metal gate  4741  may be deposited. An aluminum fill  4742  may be performed on the PMOS gates and the metal CMP&#39;ed. A dielectric layer  4739  may be deposited and the normal gate  4743  and source/drain  4744  contact formation and metallization. The PMOS layer to NMOS layer via  4747  and metallization may be partially formed as illustrated in  FIG. 47G  and an oxide layer  4748  may be deposited to prepare for bonding. The carrier wafer and two sided n/p layer may be then permanently bonded to bottom wafer having transistors and wires  4799  with associated metal landing strip  4750  as illustrated in  FIG. 47H . The carrier wafer  4720  may then be cleaved at the cleaving plane  4721  and may be thinned by chemical mechanical polishing (CMP) to oxide layer  4716  as illustrated in  FIG. 471 . The NMOS transistors may be now ready for normal state of the art gate-last transistor formation completion. As illustrated in  FIG. 47J , the oxide layer  4716  and the NMOS ILD  4708  may be chemical mechanically polished to expose the top of the NMOS polysilicon dummy gates  4704 . The NMOS polysilicon dummy gates  4704  may be removed by etch and the NMOS hi-k gate dielectric  4760  and the NMOS specific work function metal gate  4761  may be deposited. An aluminum fill  4762  may be performed on the NMOS gates and the metal CMP&#39;ed. A dielectric layer  4769  may be deposited and the normal gate  4763  and source/drain  4764  contact formation and metallization. The NMOS layer to PMOS layer via  4767  to connect to  4747  and metallization may be formed. As illustrated in  FIG. 47K , the layer-to-layer contacts  4772  to the landing pads in the base wafer may be now made. This same contact etch could be used to make the connections  4773  between the NMOS and PMOS layer as well, instead of using the two step ( 4747  and  4767 ) method in  FIG. 47H . 
         [0127]    Another alternative may be illustrated in  FIG. 48  whereby the implant of an atomic species  4810 , such as H+, may be screened from the sensitive gate areas  4803  by first masking and etching a shield implant stopping layer of a dense material  4850 , for example, about 5000 angstroms of Tantalum, and may be combined with, for example, about 5,000 angstroms of photoresist  4852 . This may create a segmented cleave plane  4812  in the bulk of the donor wafer silicon wafer and may require additional polishing to provide a smooth bonding surface for layer transfer suitability. 
         [0128]    Using procedures similar to  FIG. 47A-K , it may be possible to construct structures such as  FIG. 49  where a transistor may be constructed with front gate  4902  and back gate  4904 . The back gate could be utilized for many purposes such as threshold voltage control, reduction of variability, increase of drive current and other purposes. 
       Section 3: Monolithic 3D DRAM. 
       [0129]    While Section 1 and Section 2 describe applications of monolithic 3D integration to logic circuits and chips, this Section describes novel monolithic 3D Dynamic Random Access Memories (DRAMs). Some embodiments of this invention may involve floating body DRAM. Background information on floating body DRAM and its operation is given in “Floating Body RAM Technology and its Scalability to 32 nm Node and Beyond,”  Electron Devices Meeting,  2006 . IEDM &#39; 06 . International , vol., no., pp. 1-4, 11-13 Dec. 2006 by T. Shino, N. Kusunoki, T. Higashi, et al., Overview and future challenges of floating body RAM (FBRAM) technology for 32 nm technology node and beyond, Solid-State Electronics, Volume 53, Issue 7, Papers Selected from the 38th European Solid-State Device Research Conference—ESSDERC&#39;08, July 2009, Pages 676-683, ISSN 0038-1101, DOI: 10.1016/j.sse.2009.03.010 by Takeshi Hamamoto, Takashi Ohsawa, et al., “New Generation of Z-RAM,”  Electron Devices Meeting,  2007 . IEDM  2007 . IEEE International , vol., no., pp. 925-928, 10-12 Dec. 2007 by Okhonin, S.; Nagoga, M.; Carman, E, et al. The above publications are incorporated herein by reference. 
         [0130]      FIG. 28  describes fundamental operation of a prior art floating body DRAM. For storing a ‘1’ bit, holes  2802  may be present in the floating body  2820  and change the threshold voltage of the cell, as shown in  FIG. 28(   a ). The ‘0’ bit corresponds to no charge being stored in the floating body, as shown in  FIG. 28(   b ). The difference in threshold voltage between  FIG. 28(   a ) and  FIG. 28(   b ) may give rise to a change in drain current of the transistor at a particular gate voltage, as described in  FIG. 28(   c ). This current differential can be sensed by a sense amplifier to differentiate between ‘0’ and ‘1’ states. 
         [0131]      FIG. 29A-H  describe a process flow to construct a horizontally-oriented monolithic 3D DRAM. Two masks may be utilized on a “per-memory-layer” basis for the monolithic 3D DRAM concept shown in  FIG. 29A-H , while other masks may be shared among substantially all constructed memory layers. The process flow may include several steps in the following sequence. 
         [0000]    Step (A): A p− Silicon wafer  2901  may be taken and an oxide layer  2902  may be grown or deposited above it.  FIG. 29A  illustrates the structure after Step (A).
 
Step (B): Hydrogen may be implanted into the p− silicon wafer  2901  at a certain depth denoted by  2903 .  FIG. 29B  illustrates the structure after Step (B).
 
Step (C): The wafer after Step (B) may be flipped and bonded onto a wafer having peripheral circuits  2904  covered with oxide. This bonding process occurs using oxide-to-oxide bonding. The stack may be cleaved at the hydrogen implant plane  2903  using either an anneal or a sideways mechanical force. A chemical mechanical polish (CMP) process may be then conducted. Note that peripheral circuits  2904  may be able to withstand an additional rapid-thermal-anneal (RTA) and still remain operational, and may retain good performance. For this purpose, the peripheral circuits  2904  may have not had their RTA for activating dopants or may have had a weak RTA for activating dopants. Also, peripheral circuits  2904  utilize a refractory metal such as tungsten that can withstand temperatures greater than about 400° C.  FIG. 29C  illustrates the structure after Step (C).
 
Step (D): The transferred layer of p− silicon after Step (C) may be then processed to form isolation regions using a STI process. Following, gate regions  2905  may be deposited and patterned, following which source-drain regions  2908  may be implanted using a self-aligned process. An inter-level dielectric (ILD) constructed of oxide (silicon dioxide)  2906  may be then constructed. Note that no RTA may be done to activate dopants in this layer of partially-depleted SOI (PD-SOI) transistors. Alternatively, transistors could be of fully-depleted SOI type.  FIG. 29D  illustrates the structure after Step (D).
 
Step (E): Using steps similar to Step (A)-Step (D), another layer of memory  2909  may be constructed. After substantially all the desired memory layers are constructed, a RTA may be conducted to activate dopants in substantially all layers of memory (and potentially also the periphery).  FIG. 29E  illustrates the structure after Step (E).
 
Step (F): Contact plugs  2910  may be made to source and drain regions of different layers of memory. Bit-line (BL) wiring  2911  and Source-line (SL) wiring  2912  may be connected to contact plugs  2910 . Gate regions  2913  of memory layers may be connected together to form word-line (WL) wiring.  FIG. 29F  illustrates the structure after Step (F).
 
 FIG. 29G  and  FIG. 29H  describe array organization of the floating-body DRAM. BLs  2916  in a direction substantially perpendicular to the directions of SLs  2915  and WLs  2914 .
 
         [0132]      FIG. 30A-M  describe an alternative process flow to construct a horizontally-oriented monolithic 3D DRAM. This monolithic 3D DRAM utilizes the floating body effect and double-gate transistors. One mask may be utilized on a “per-memory-layer” basis for the monolithic 3D DRAM concept shown in  FIG. 30A-M , while other masks may be shared among different layers. The process flow may include several steps that may occur in the following sequence. 
         [0000]    Step (A): Peripheral circuits  3002  with tungsten wiring may be first constructed and above this oxide layer  3004  may be deposited.  FIG. 30A  illustrates the structure after Step (A).
 
Step (B):  FIG. 30B  shows a drawing illustration after Step (B). A p− Silicon wafer  3006  may have an oxide layer  3008  grown or deposited above it. Following this, hydrogen may be implanted into the p− Silicon wafer at a certain depth indicated by  3010 . Alternatively, some other atomic species such as Helium could be (co-)implanted. This hydrogen implanted p− Silicon wafer  3006  may form the top layer  3012 . The bottom layer  3014  may include the peripheral circuits  3002  with oxide layer  3004 . The top layer  3012  may be flipped and bonded to the bottom layer  3014  using oxide-to-oxide bonding.
 
Step (C):  FIG. 30C  illustrates the structure after Step (C). The stack of top and bottom wafers after Step (B) may be cleaved at the hydrogen plane  3010  using either a anneal or a sideways mechanical force or other means. A CMP process may be then conducted. At the end of this step, a single-crystal p− Si layer exists atop the peripheral circuits, and this may have been achieved using layer-transfer techniques.
 
Step (D):  FIG. 30D  illustrates the structure after Step (D). Using lithography and then implantation, n+ regions  3016  and p− regions  3018  may be formed on the transferred layer of p− Si after Step (C).
 
Step (E):  FIG. 30E  illustrates the structure after Step (E). An oxide layer  3020  may be deposited atop the structure obtained after Step (D). A first layer of Si/SiO 2    3022  may be therefore formed atop the peripheral circuits  3002 .
 
Step (F):  FIG. 30F  illustrates the structure after Step (F). Using procedures similar to Steps (B)-(E), additional Si/SiO 2  layers  3024  and  3026  may be formed atop Si/SiO 2  layer  3022 . A rapid thermal anneal (RTA) or spike anneal or flash anneal or laser anneal may then be done to activate implanted layers  3022 ,  3024  and  3026  (and possibly also the peripheral circuits  3002 ). Alternatively, the layers  3022 ,  3024  and  3026  may be annealed layer-by-layer using a laser anneal system after their implantations are done.
 
Step (G):  FIG. 30G  illustrates the structure after Step (G). Lithography and etch processes may be then utilized to make a structure as shown in the figure.
 
Step (H):  FIG. 30H  illustrates the structure after Step (H). Gate dielectric  3028  and gate electrode  3030  may be then deposited following which a CMP may be done to planarize the gate electrode  3030  regions. Lithography and etch may be utilized to define gate regions over the p− silicon regions (eg. p− Si region after Step (D)). Note that gate width could be slightly larger than p− region width to compensate for overlay errors in lithography. Thus transistors with side gates may be formed.
 
Step (I):  FIG. 301  illustrates the structure after Step (I). A silicon oxide layer  3032  may be then deposited and planarized. For clarity, the silicon oxide layer may be shown transparent in the figure, along with word-line (WL) and source-line (SL) regions.
 
Step (J):  FIG. 30J  illustrates the structure after Step (J). Bit-line (BL) contacts  3034  may be formed by etching and deposition. These BL contacts may be shared among substantially all layers of memory.
 
Step (K):  FIG. 30K  illustrates the structure after Step (K). BLs  3036  may be then constructed. Contacts may be made to BLs, WLs and SLs of the memory array at its edges. SL contacts can be made into stair-like structures using techniques described in “Bit Cost Scalable Technology with Punch and Plug Process for Ultra High Density Flash Memory,”  VLSI Technology,  2007  IEEE Symposium on , vol., no., pp. 14-15, 12-14 Jun. 2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., following which contacts can be constructed to them. Formation of stair-like structures for SLs could be done in steps prior to Step (K) as well.
 
 FIG. 30L  shows cross-sectional views of the array for clarity. The double-gated transistors in  FIG. 30  L can be utilized along with the floating body effect for storing information.
 
 FIG. 30M  shows a memory cell of the floating body RAM array with two gates on either side, i.e., side gates, of the p− Si layer  3019 .
 
A floating-body DRAM may have thus been constructed, with (1) horizontally-oriented transistors—i.e., current flowing in substantially the horizontal direction in transistor channels, (2) some of the memory cell control lines, e.g., source-lines SL, constructed of heavily doped silicon and embedded in the memory cell layer, (3) side gates simultaneously deposited over multiple memory layers, and (4) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut.
 
         [0133]      FIG. 31A-K  describe an alternative process flow to construct a horizontally-oriented monolithic 3D DRAM. This monolithic 3D DRAM utilizes the floating body effect and double-gate transistors. No mask may be utilized on a “per-memory-layer” basis for the monolithic 3D DRAM concept shown in  FIG. 31A-K , and substantially all other masks may be shared among different layers. The process flow may include several steps in the following sequence. 
         [0000]    Step (A): Peripheral circuits with tungsten wiring  3102  may be first constructed and above this oxide layer  3104  may be deposited.  FIG. 31A  shows a drawing illustration after Step (A).
 
Step (B):  FIG. 31B  illustrates the structure after Step (B). A p− Silicon wafer  3108  may have an oxide layer  3106  grown or deposited above it. Following this, hydrogen may be implanted into the p− Silicon wafer at a certain depth indicated by  3114 . Alternatively, some other atomic species such as Helium could be (co-)implanted. This hydrogen implanted p− Silicon wafer  3108  forms the top layer  3110 . The bottom layer  3112  may include the peripheral circuits  3102  with oxide layer  3104 . The top layer  3110  may be flipped and bonded to the bottom layer  3112  using oxide-to-oxide bonding.
 
Step (C):  FIG. 31C  illustrates the structure after Step (C). The stack of top and bottom wafers after Step (B) may be cleaved at the hydrogen plane  3114  using either a anneal or a sideways mechanical force or other means. A CMP process may be then conducted. A layer of silicon oxide  3118  may be then deposited atop the p− Silicon layer  3116 . At the end of this step, a single-crystal p− Silicon layer  3116  exists atop the peripheral circuits, and this may have been achieved using layer-transfer techniques.
 
Step (D):  FIG. 31D  illustrates the structure after Step (D). Using methods similar to Step (B) and (C), multiple p− silicon layers  3120  may be formed with silicon oxide layers in-between.
 
Step (E):  FIG. 31E  illustrates the structure after Step (E). Lithography and etch processes may be then utilized to make a structure, such as, for example, as shown in the figure.
 
Step (F):  FIG. 31F  illustrates the structure after Step (F). Gate dielectric  3126  and gate electrode  3124  may be then deposited following which a CMP may be done to planarize the gate electrode  3124  regions. Lithography and etch may be utilized to define gate regions.
 
Step (G):  FIG. 31G  illustrates the structure after Step (G). Using the hard mask defined in Step (F), p− regions not covered by the gate may be implanted to form n+ regions. Spacers may be utilized during this multi-step implantation process and layers of silicon present in different layers of the stack have different spacer widths to account for lateral straggle of buried layer implants. Bottom layers could have larger spacer widths than top layers. A thermal annealing step, such as a RTA or spike anneal or laser anneal or flash anneal, may be then conducted to activate n+ doped regions. Thus transistors with side gates may be formed.
 
Step (H):  FIG. 31H  illustrates the structure after Step (H). A silicon oxide layer  3130  may be then deposited and planarized. For clarity, the silicon oxide layer may be shown transparent, along with word-line (WL)  3132  and source-line (SL)  3134  regions.
 
Step (I):  FIG. 311  illustrates the structure after Step (I). Bit-line (BL) contacts  3136  may be formed by etching and deposition. These BL contacts may be shared among substantially all layers of memory.
 
Step (J):  FIG. 31J  illustrates the structure after Step (J). BLs  3138  may be then constructed. Contacts may be made to BLs, WLs and SLs of the memory array at its edges. SL contacts can be made into stair-like structures using techniques described in “Bit Cost Scalable Technology with Punch and Plug Process for Ultra High Density Flash Memory,”  VLSI Technology,  2007  IEEE Symposium on , vol., no., pp. 14-15, 12-14 Jun. 2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., following which contacts can be constructed to them. Formation of stair-like structures for SLs could be done in steps prior to Step (J) as well.
 
 FIG. 31K  shows cross-sectional views of the array for clarity. Double-gated transistors may be utilized along with the floating body effect for storing information.
 
A floating-body DRAM may have thus been constructed, with (1) horizontally-oriented transistors—i.e. current flowing in substantially the horizontal direction in transistor channels (2) some of the memory cell control lines, e.g., source-lines SL, constructed of heavily doped silicon and embedded in the memory cell layer, (3) side gates simultaneously deposited over multiple memory layers, and (4) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut.
 
         [0134]    With the explanations for the formation of monolithic 3D DRAM with ion-cut in this section, it is clear to one skilled in the art that alternative implementations may be possible. BL and SL nomenclature has been used for two terminals of the 3D DRAM array, and this nomenclature can be interchanged. Each gate of the double gate 3D DRAM can be independently controlled for increased control of the memory cell. To implement these changes, the process steps in  FIG. 30A-M  and  31  may be modified. Moreover, selective epi technology or laser recrystallization technology could be utilized for implementing structures shown in  FIG. 30A-M  and  FIG. 31A-K . Various other types of layer transfer schemes that have been described in Section 1.3.4 can be utilized for construction of various 3D DRAM structures. Furthermore, buried wiring, i.e. where wiring for memory arrays may be below the memory layers but above the periphery, may also be used. This may permit the use of low melting point metals, such as aluminum or copper, for some of the memory wiring. 
       Section 4: Monolithic 3D Resistance-Based Memory 
       [0135]    While many of today&#39;s memory technologies rely on charge storage, several companies may be developing non-volatile memory technologies based on resistance of a material changing. Examples of these resistance-based memories may include phase change memory, Metal Oxide memory, resistive-RAM (RRAM), memristors, solid-electrolyte memory, ferroelectric RAM, conductive bridge RAM, and MRAM. Background information on these resistive-memory types is given in “Overview of candidate device technologies for storage-class memory,”  IBM Journal of Research and Development , vol. 52, no. 4.5, pp. 449-464, July 2008 by Burr, G. W.; Kurdi, B. N.; Scott, J. C.; Lam, C. H.; Gopalakrishnan, K.; Shenoy, R. S. 
         [0136]      FIG. 32A-J  describe a novel memory architecture for resistance-based memories, and a procedure for its construction. The memory architecture utilizes junction-less transistors and may have a resistance-based memory element in series with a transistor selector. No mask may be utilized on a “per-memory-layer” basis for the monolithic 3D resistance change memory (or resistive memory) concept shown in  FIG. 32A-J , and substantially all other masks may be shared among different layers. The process flow may include several steps that may occur in the following sequence. 
         [0000]    Step (A): Peripheral circuits  3202  may be first constructed and above this oxide layer  3204  may be deposited.  FIG. 32A  shows a drawing illustration after Step (A).
 
Step (B):  FIG. 32B  illustrates the structure after Step (B). N+ Silicon wafer  3208  may have an oxide layer  3206  grown or deposited above it. Following this, hydrogen may be implanted into the n+ Silicon wafer at a certain depth indicated by  3214 . Alternatively, some other atomic species such as Helium could be (co-)implanted. This hydrogen implanted n+ Silicon wafer  3208  forms the top layer  3210 . The bottom layer  3212  may include the peripheral circuits  3202  with oxide layer  3204 . The top layer  3210  may be flipped and bonded to the bottom layer  3212  using oxide-to-oxide bonding.
 
Step (C):  FIG. 32C  illustrates the structure after Step (C). The stack of top and bottom wafers after Step (B) may be cleaved at the hydrogen plane  3214  using either a anneal or a sideways mechanical force or other means. A CMP process may be then conducted. A layer of silicon oxide  3218  may be then deposited atop the n+ Silicon layer  3216 . At the end of this step, a single-crystal n+ Si layer  3216  exists atop the peripheral circuits, and this may have been achieved using layer-transfer techniques.
 
Step (D):  FIG. 32D  illustrates the structure after Step (D). Using methods similar to Step (B) and (C), multiple n+ silicon layers  3220  may be formed with silicon oxide layers in-between.
 
Step (E):  FIG. 32E  illustrates the structure after Step (E). Lithography and etch processes may be then utilized to make a structure as shown in the figure.
 
Step (F):  FIG. 32F  illustrates the structure after Step (F). Gate dielectric  3226  and gate electrode  3224  may be then deposited following which a CMP may be performed to planarize the gate electrode  3224  regions. Lithography and etch may be utilized to define gate regions.
 
Step (G):  FIG. 32G  illustrates the structure after Step (G). A silicon oxide layer  3230  may be then deposited and planarized. The silicon oxide layer may be shown transparent in the figure for clarity, along with word-line (WL)  3232  and source-line (SL)  3234  regions.
 
Step (H):  FIG. 32H  illustrates the structure after Step (H). Vias may be etched through multiple layers of silicon and silicon dioxide as shown in the figure. A resistance change memory material  3236  may be then deposited (for example, with atomic layer deposition (ALD)). Examples of such a material include hafnium oxide, well known to change resistance by applying voltage. An electrode for the resistance change memory element may be then deposited (for example, by using ALD) and may be shown as electrode/BL contact  3240 . A CMP process may be then conducted to planarize the surface. It can be observed that multiple resistance change memory elements in series with junction-less transistors with side gates may be created after this step.
 
Step (I):  FIG. 321  illustrates the structure after Step (I). BLs  3238  may be then constructed. Contacts may be made to BLs, WLs and SLs of the memory array at its edges. SL contacts can be made into stair-like structures using techniques described in in “Bit Cost Scalable Technology with Punch and Plug Process for Ultra High Density Flash Memory,”  VLSI Technology,  2007  IEEE Symposium on , vol., no., pp. 14-15, 12-14 Jun. 2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., following which contacts can be constructed to them. Formation of stair-like structures for SLs could be achieved in steps prior to Step (I) as well.
 
         [0137]      FIG. 32J  shows cross-sectional views of the array for clarity. 
         [0000]    A 3D resistance change memory, such as resistive RAM, may have thus been constructed, with (1) horizontally-oriented transistors—i.e. current flowing in substantially the horizontal direction in transistor channels, (2) some of the memory cell control lines, e.g., source-lines SL, constructed of heavily doped silicon and embedded in the memory cell layer, (3) side gates that may be simultaneously deposited over multiple memory layers for transistors, and (4) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut. 
         [0138]      FIG. 33A-K  describe an alternative process flow to construct a horizontally-oriented monolithic 3D resistive memory array. This embodiment may have a resistance-based memory element in series with a transistor selector. No mask may be utilized on a “per-memory-layer” basis for the monolithic 3D resistance change memory (or resistive memory) concept shown in  FIG. 33A-K , and substantially all other masks may be shared among different layers. The process flow may include several steps as described in the following sequence. 
         [0000]    Step (A): Peripheral circuits with tungsten wiring  3302  may be first constructed and above this a oxide layer  3304  may be deposited.  FIG. 33A  shows a drawing illustration after Step (A).
 
Step (B):  FIG. 33B  illustrates the structure after Step (B). P− Silicon wafer  3308  may have an oxide layer  3306  grown or deposited above it. Following this, hydrogen may be implanted into the p− Silicon wafer at a certain depth indicated by  3314 . Alternatively, some other atomic species such as Helium could be (co-)implanted. This hydrogen implanted p− Silicon wafer  3308  forms the top layer  3310 . The bottom layer  3312  may include the peripheral circuits  3302  with oxide layer  3304 . The top layer  3310  may be flipped and bonded to the bottom layer  3312  using oxide-to-oxide bonding.
 
Step (C):  FIG. 33C  illustrates the structure after Step (C). The stack of top and bottom wafers after Step (B) may be cleaved at the hydrogen plane  3314  using either a anneal or a sideways mechanical force or other means. A CMP process may be then conducted. A layer of silicon oxide  3318  may be then deposited atop the p− Silicon layer  3316 . At the end of this step, a single-crystal p− Silicon layer  3316  exists atop the peripheral circuits, and this may have been achieved using layer-transfer techniques.
 
Step (D):  FIG. 33D  illustrates the structure after Step (D). Using methods similar to Step (B) and (C), multiple p− silicon layers  3320  may be formed with silicon oxide layers in-between.
 
Step (E):  FIG. 33E  illustrates the structure after Step (E). Lithography and etch processes may be then utilized to make a structure as shown in the figure.
 
Step (F):  FIG. 33F  illustrates the structure on after Step (F). Gate dielectric  3326  and gate electrode  3324  may be then deposited following which a CMP may be done to planarize the gate electrode  3324  regions. Lithography and etch may be utilized to define gate regions.
 
Step (G):  FIG. 33G  illustrates the structure after Step (G). Using the hard mask defined in Step (F), p− regions not covered by the gate may be implanted to form n+ regions. Spacers may be utilized during this multi-step implantation process and layers of silicon present in different layers of the stack have different spacer widths to account for lateral straggle of buried layer implants. Bottom layers could have larger spacer widths than top layers. A thermal annealing step, such as a RTA or spike anneal or laser anneal or flash anneal, may be then conducted to activate n+ doped regions.
 
Step (H):  FIG. 33H  illustrates the structure after Step (H). A silicon oxide layer  3330  may be then deposited and planarized. The silicon oxide layer may be shown transparent in the figure for clarity, along with word-line (WL)  3332  and source-line (SL)  3334  regions.
 
Step (I):  FIG. 331  illustrates the structure after Step (I). Vias may be etched through multiple layers of silicon and silicon dioxide as shown in the figure. A resistance change memory material  3336  may be then deposited (for example, with atomic layer deposition (ALD)). Examples of such a material include hafnium oxide, which is well known to change resistance by applying voltage. An electrode for the resistance change memory element may be then deposited (for example by using ALD) and may be shown as electrode/BL contact  3340 . A CMP process may be then conducted to planarize the surface. It can be observed that multiple resistance change memory elements in series with transistors with side gates may be created after this step.
 
Step (J):  FIG. 33J  illustrates the structure after Step (J). BLs  3338  may be then constructed. Contacts may be made to BLs, WLs and SLs of the memory array at its edges. SL contacts can be made into stair-like structures using techniques described in “Bit Cost Scalable Technology with Punch and Plug Process for Ultra High Density Flash Memory,”  VLSI Technology,  2007  IEEE Symposium on , vol., no., pp. 14-15, 12-14 Jun. 2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., following which contacts can be constructed to them. Formation of stair-like structures for SLs could be done in steps prior to Step (I) as well.
 
 FIG. 33K  shows cross-sectional views of the array for clarity.
 
A 3D resistance change memory, such as resistive RAM, may have thus been constructed, with (1) horizontally-oriented transistors—i.e. current flowing in substantially the horizontal direction in transistor channels, (2) some of the memory cell control lines—e.g., source-lines SL, constructed of heavily doped silicon and embedded in the memory cell layer, (3) side gates simultaneously deposited over multiple memory layers for transistors, and (4) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut.
 
         [0139]      FIG. 34A-L  describes an alternative process flow to construct a horizontally-oriented monolithic 3D resistive memory array. This embodiment may have a resistance-based memory element in series with a transistor selector. One mask may be utilized on a “per-memory-layer” basis for the monolithic 3D resistance change memory (or resistive memory) concept shown in  FIG. 34A-L , and substantially all other masks may be shared among different layers. The process flow may include several steps as described in the following sequence. 
         [0000]    Step (A): Peripheral circuit layer  3402  with tungsten wiring may be first constructed and above this oxide layer  3404  may be deposited.  FIG. 34A  illustrates the structure after Step (A).
 
Step (B):  FIG. 34B  illustrates the structure after Step (B). P− Silicon wafer  3406  may have an oxide layer  3408  grown or deposited above it. Following this, hydrogen may be implanted into the p− Silicon wafer at a certain depth indicated by  3410 . Alternatively, some other atomic species such as Helium could be (co-)implanted. This hydrogen implanted p− Silicon wafer  3406  forms the top layer  3412 . The bottom layer  3414  may include the peripheral circuit layer  3402  with oxide layer  3404 . The top layer  3412  may be flipped and bonded to the bottom layer  3414  using oxide-to-oxide bonding.
 
Step (C):  FIG. 34C  illustrates the structure after Step (C). The stack of top and bottom wafers after Step (B) may be cleaved at the hydrogen plane  3410  using either a anneal or a sideways mechanical force or other means. A CMP process may be then conducted. At the end of this step, a single-crystal p− Si layer exists atop the peripheral circuits, and this may have been achieved using layer-transfer techniques.
 
Step (D):  FIG. 34D  illustrates the structure after Step (D). Using lithography and then implantation, n+ regions  3416  and p− regions  3418  may be formed on the transferred layer of p− Si after Step (C).
 
Step (E):  FIG. 34E  illustrates the structure after Step (E). An oxide layer  3420  may be deposited atop the structure obtained after Step (D). A first layer of Si/SiO 2    3422  may be therefore formed atop the peripheral circuit layer  3402 .
 
Step (F):  FIG. 34F  illustrates the structure after Step (F). Using procedures similar to Steps (B)-(E), additional Si/SiO 2  layers  3424  and  3426  may be formed atop Si/SiO 2  layer  3422 . A rapid thermal anneal (RTA) or spike anneal or flash anneal or laser anneal may be then done to activate implanted layers  3422 ,  3424  and  3426  (and possibly also the peripheral circuit layer  3402 ). Alternatively, the layers  3422 ,  3424  and  3426  may be annealed layer-by-layer using a laser anneal system after their implantations are done.
 
Step (G):  FIG. 34G  illustrates the structure after Step (G). Lithography and etch processes may be then utilized to make a structure as shown in the figure.
 
Step (H):  FIG. 34H  illustrates the structure after Step (H). Gate dielectric  3428  and gate electrode  3430  may be then deposited following which a CMP may be done to planarize the gate electrode  3430  regions. Lithography and etch may be utilized to define gate regions over the p− silicon regions (eg. p− Si region  3418  after Step (D)). Note that gate width could be slightly larger than p− region width to compensate for overlay errors in lithography.
 
Step (I):  FIG. 341  illustrates the structure after Step (I). A silicon oxide layer  3432  may be then deposited and planarized. It may be shown transparent in the figure for clarity. Word-line (WL) and Source-line (SL) regions may be shown in the figure.
 
Step (J):  FIG. 34J  illustrates the structure after Step (J). Vias may be etched through multiple layers of silicon and silicon dioxide as shown in the figure. A resistance change memory material  3436  may be then deposited (for example with atomic layer deposition (ALD)). Examples of such a material include hafnium oxide, which is well known to change resistance by applying voltage. An electrode for the resistance change memory element may be then deposited (for example using ALD) and may be shown as electrode/BL contact  3440 . A CMP process may be then conducted to planarize the surface. It can be observed that multiple resistance change memory elements in series with transistors with side gates may be created after this step.
 
Step (K):  FIG. 34K  illustrates the structure after Step (K). BLs  3436  may be then constructed. Contacts may be made to BLs, WLs and SLs of the memory array at its edges. SL contacts can be made into stair-like structures using techniques described in “Bit Cost Scalable Technology with Punch and Plug Process for Ultra High Density Flash Memory,”  VLSI Technology,  2007  IEEE Symposium on , vol., no., pp. 14-15, 12-14 Jun. 2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., following which contacts can be constructed to them. Formation of stair-like structures for SLs could be achieved in steps prior to Step (J) as well.
 
 FIG. 34L  shows cross-sectional views of the array for clarity.
 
A 3D resistance change memory, such as resistive RAM, may have thus been constructed, with (1) horizontally-oriented transistors—i.e. current flowing in substantially the horizontal direction in transistor channels, (2) some of the memory cell control lines, e.g., source-lines SL, constructed of heavily doped silicon and embedded in the memory cell layer, (3) side gates simultaneously deposited over multiple memory layers for transistors, and (4) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut.
 
         [0140]      FIG. 35A-F  describes an alternative process flow to construct a horizontally-oriented monolithic 3D resistive memory array. This embodiment may have a resistance-based memory element in series with a transistor selector. Two masks may be utilized on a “per-memory-layer” basis for the monolithic 3D resistance change memory (or resistive memory) concept shown in  FIG. 35A-F , and substantially all other masks may be shared among different layers. The process flow may include several steps as described in the following sequence. 
         [0000]    Step (A): The process flow starts with a p− silicon wafer  3500  with an oxide coating  3504 .  FIG. 35A  illustrates the structure after Step (A).
 
Step (B):  FIG. 35B  illustrates the structure after Step (B). Using a process flow similar to  FIG. 2 , p− silicon layer  3502  may be transferred atop a layer of peripheral circuits  3506 . The peripheral circuits  3506  may use tungsten wiring.
 
Step (C):  FIG. 35C  illustrates the structure after Step (C). Isolation regions for transistors may be formed using a shallow-trench-isolation (STI) process. Following this, a gate dielectric  3510  and a gate electrode  3508  may be deposited.
 
Step (D):  FIG. 35D  illustrates the structure after Step (D). The gate may be patterned, and source-drain regions  3512  may be formed by implantation. An inter-layer dielectric (ILD)  3514  may be also formed.
 
Step (E):  FIG. 35E  illustrates the structure after Step (E). Using steps similar to Step (A) to Step (D), a second layer of transistors  3516  may be formed above the first layer of transistors  3514 . A RTA or some other type of anneal may be performed to activate dopants in the memory layers (and potentially also the peripheral transistors).
 
Step (F):  FIG. 35F  illustrates the structure after Step (F). Vias may be etched through multiple layers of silicon and silicon dioxide as shown in the figure. A resistance change memory material  3522  may be then deposited (for example with atomic layer deposition (ALD)). Examples of such a material include hafnium oxide, which is well known to change resistance by applying voltage. An electrode for the resistance change memory element may be then deposited (for example using ALD) and may be shown as electrode  3526 . A CMP process may be then conducted to planarize the surface. Contacts may be made to drain terminals of transistors in different memory layer as well. Note that gates of transistors in each memory layer may be connected together perpendicular to the plane of the figure to form word-lines (WL). Wiring for bit-lines (BLs) and source-lines (SLs) may be constructed. Contacts may be made between BLs, WLs and SLs with the periphery at edges of the memory array. Multiple resistance change memory elements in series with transistors may be created after this step.
 
A 3D resistance change memory may have thus been constructed, with (1) horizontally-oriented transistors—i.e. current flowing in substantially the horizontal direction in the transistor channels, and (2) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut.
 
         [0141]    While explanations have been given for formation of monolithic 3D resistive memories with ion-cut in this section, it is clear to one skilled in the art that alternative implementations may be possible. BL and SL nomenclature has been used for two terminals of the 3D resistive memory array, and this nomenclature can be interchanged. Moreover, selective epi technology or laser recrystallization technology could be utilized for implementing structures shown in  FIG. 32A-J ,  FIG. 33A-K ,  FIG. 34A-L  and  FIG. 35A-F . Further, various other types of layer transfer schemes that have been described in Section 1.3.4 can be utilized for construction of various 3D resistive memory structures. Moreover, buried wiring, i.e. where wiring for memory arrays may be below the memory layers but above the periphery, may be utilized. Moreover, other technologies may utilize the described architectures, structures, and process flows, such as phase change memory and material. Other variations of the monolithic 3D resistive memory concepts may be possible. 
       Section 5: Monolithic 3D Charge-Trap Memory 
       [0142]    While resistive memories described previously form a class of non-volatile memory, others classes of non-volatile memory exist. NAND flash memory forms one of the most common non-volatile memory types. It can be constructed of two main types of devices: floating-gate devices where charge may be stored in a floating gate and charge-trap devices where charge may be stored in a charge-trap layer such as Silicon Nitride. Background information on charge-trap memory can be found in “ Integrated Interconnect Technologies for  3 D Nanoelectronic Systems” , Artech House, 2009 by Bakir and Meindl (“Bakir”) and “A Highly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried Channel BE-SONOS Device,” Symposium on VLSI Technology, 2010 by Hang-Ting Lue, et al. The architectures shown in  FIG. 36A-F ,  FIG. 37A-G  and  FIG. 38A-D  may be relevant for any type of charge-trap memory. 
         [0143]      FIG. 36A-F  describes a process flow to construct a horizontally-oriented monolithic 3D charge trap memory. Two masks may be utilized on a “per-memory-layer” basis for the monolithic 3D charge trap memory concept shown in  FIG. 36A-F , while other masks may be shared among substantially all constructed memory layers. The process flow may include several steps that may occur in the following sequence. 
         [0000]    Step (A): A p− Silicon wafer  3600  may be taken and an oxide layer  3604  may be grown or deposited above it.  FIG. 36A  illustrates the structure after Step (A). Alternatively, p− silicon wafer  3600  may be doped differently, such as, for example, with elemental species that may form a p+, or n+, or n− silicon wafer, or substantially absent of semiconductor dopants to form an undoped silicon wafer.
 
Step (B):  FIG. 36B  illustrates the structure after Step (B). Using a procedure similar to the one shown in  FIG. 2 , p− Si region  3602  may be transferred atop a peripheral circuit layer  3606 . The periphery may be designed such that it can withstand the RTA temperatures for activating dopants in memory layers formed atop it.
 
Step (C):  FIG. 36C  illustrates the structure after Step (C). Isolation regions may be formed in the p− Si region  3602  atop the peripheral circuit layer  3606 . This lithography step and substantially all future lithography steps may be formed with good alignment to features on the peripheral circuit layer  3606  since the p− Si region  3602  may be thin and reasonably transparent to the lithography tool. A dielectric layer  3610  (eg. Oxide-nitride-oxide ONO layer) may be deposited following which a gate electrode layer  3608  (eg. polysilicon) may be then deposited.
 
Step (D):  FIG. 36D  illustrates the structure after Step (D). The gate regions deposited in Step (C) may be patterned and etched. Following this, source-drain regions  3612  may be implanted. An inter-layer dielectric  3614  may be then deposited and planarized.
 
Step (E):  FIG. 36E  illustrates the structure after Step (E). Using procedures similar to Step (A) to Step (D), another layer of memory, a second NAND string  3616 , may be formed atop the first NAND string  3614 .
 
Step (F):  FIG. 36F  illustrates the structure after Step (F). Contacts may be made to connect bit-lines (BL) and source-lines (SL) to the NAND string. Contacts to the well of the NAND string may be also made. Substantially all these contacts could be constructed of heavily doped polysilicon or some other material. An anneal to activate dopants in source-drain regions of transistors in the NAND string (and potentially also the periphery) may be conducted. Following this, wiring layers for the memory array may be conducted.
 
A 3D charge-trap memory may have thus been constructed, with (1) horizontally-oriented transistors—i.e. current flowing in substantially the horizontal direction in transistor channels, and (2) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut. This use of monocrystalline silicon (or single crystal silicon) using ion-cut can be a key differentiator for some embodiments of the invention vis-àvis prior work. Past work described by Bakir in his textbook used selective epi technology or laser recrystallization or polysilicon.
 
         [0144]      FIG. 37A-G  describes a memory architecture for single-crystal 3D charge-trap memories, and a procedure for its construction. It utilizes junction-less transistors. No mask may be utilized on a “per-memory-layer” basis for the monolithic 3D charge-trap memory concept shown in  FIG. 37A-G , and substantially all other masks may be shared among different layers. The process flow may include several steps as described in the following sequence. 
         [0000]    Step (A): Peripheral circuits  3702  may be first constructed and above this oxide layer  3704  may be deposited.  FIG. 37A  shows a drawing illustration after Step (A).
 
Step (B):  FIG. 37B  illustrates the structure after Step (B). A wafer of n+ Silicon  3708  may have an oxide layer  3706  grown or deposited above it. Following this, hydrogen may be implanted into the n+ Silicon wafer at a certain depth indicated by  3714 . Alternatively, some other atomic species such as Helium could be implanted. This hydrogen implanted n+ Silicon wafer  3708  forms the top layer  3710 . The bottom layer  3712  may include the peripheral circuits  3702  with oxide layer  3704 . The top layer  3710  may be flipped and bonded to the bottom layer  3712  using oxide-to-oxide bonding. Alternatively, n+ silicon wafer  3708  may be doped differently, such as, for example, with elemental species that may form a p+, or p−, or n− silicon wafer, or substantially absent of semiconductor dopants to form an undoped silicon wafer.
 
Step (C):  FIG. 37C  illustrates the structure after Step (C). The stack of top and bottom wafers after Step (B) may be cleaved at the hydrogen plane  3714  using either a anneal or a sideways mechanical force or other means. A CMP process may be then conducted. A layer of silicon oxide  3718  may be then deposited atop the n+ Silicon layer  3716 . At the end of this step, a single-crystal n+ Si layer  3716  exists atop the peripheral circuits, and this may have been achieved using layer-transfer techniques.
 
Step (D):  FIG. 37D  illustrates the structure after Step (D). Using methods similar to Step (B) and (C), multiple n+ silicon layers  3720  may be formed with silicon oxide layers in-between.
 
Step (E):  FIG. 37E  illustrates the structure after Step (E). Lithography and etch processes may be then utilized to make a structure as shown in the figure.
 
Step (F):  FIG. 37F  illustrates the structure after Step (F). Gate dielectric  3726  and gate electrode  3724  may be then deposited following which a CMP may be done to planarize the gate electrode  3724  regions. Lithography and etch may be utilized to define gate regions. Gates of the NAND string  3736  as well as gates of select gates of the NAND string  3738  may be defined.
 
Step (G):  FIG. 37G  illustrates the structure after Step (G). A silicon oxide layer  3730  may be then deposited and planarized. It may be shown transparent in the figure for clarity. Word-lines, bit-lines and source-lines may be defined as shown in the figure. Contacts may be formed to various regions/wires at the edges of the array as well. SL contacts can be made into stair-like structures using techniques described in “Bit Cost Scalable Technology with Punch and Plug Process for Ultra High Density Flash Memory,”  VLSI Technology,  2007  IEEE Symposium on , vol., no., pp. 14-15, 12-14 Jun. 2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., following which contacts can be constructed to them. Formation of stair-like structures for SLs could be performed in steps prior to Step (G) as well.
 
A 3D charge-trap memory may have thus been constructed, with (1) horizontally-oriented transistors—i.e. current flowing in substantially the horizontal direction in transistor channels, (2) some of the memory cell control lines—e.g., bit lines BL, constructed of heavily doped silicon and embedded in the memory cell layer, (3) side gates simultaneously deposited over multiple memory layers for transistors, and (4) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut. This use of single-crystal silicon obtained with ion-cut may be a key differentiator from past work on 3D charge-trap memories such as “A Highly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried Channel BE-SONOS Device,” Symposium on VLSI Technology, 2010 by Hang-Ting Lue, et al. that used polysilicon.
 
         [0145]    While  FIG. 36A-F  and  FIG. 37A-G  give two examples of how single-crystal silicon layers with ion-cut can be used to produce 3D charge-trap memories, the ion-cut technique for 3D charge-trap memory may be fairly general. It could be utilized to produce any horizontally-oriented 3D mono crystalline-silicon charge-trap memory.  FIG. 38A-D  further illustrate how general the process can be. One or more doped silicon layers  3802  can be layer transferred atop any peripheral circuit layer  3806  using procedures shown in  FIG. 2 . These may be indicated in  FIG. 38A ,  FIG. 38B  and  FIG. 38C . Following this, different procedures can be utilized to form different types of 3D charge-trap memories. For example, procedures shown in “A Highly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried Channel BE-SONOS Device,” Symposium on VLSI Technology, 2010 by Hang-Ting Lue, et al. and “Multi-layered Vertical Gate NAND Flash overcoming stacking limit for terabit density storage”, Symposium on VLSI Technology, 2009 by W. Kim, S. Choi, et al. can be used to produce the two different types of horizontally oriented single crystal silicon 3D charge trap memory shown in  FIG. 38D . 
       Section 6: Monolithic 3D Floating-Gate Memory 
       [0146]    While charge-trap memory forms one type of non-volatile memory, floating-gate memory is another type. Background information on floating-gate flash memory can be found in “Introduction to Flash memory”, Proc. IEEE 91, 489-502 (2003) by R. Bez, et al. There may be different types of floating-gate memory based on different materials and device structures. The architectures shown in  FIG. 39A-F  and  FIG. 40A-H  may be relevant for any type of floating-gate memory. 
         [0147]      FIG. 39A-F  describe a process flow to construct a horizontally-oriented monolithic 3D floating-gate memory. Two masks may be utilized on a “per-memory-layer” basis for the monolithic 3D floating-gate memory concept shown in  FIG. 39A-F , while other masks may be shared among substantially all constructed memory layers. The process flow may include several steps as described in the following sequence. 
         [0000]    Step (A): A p− Silicon wafer  390  may be taken and an oxide layer  3904  may be grown or deposited above it.  FIG. 39A  illustrates the structure after Step (A). Alternatively, p− silicon wafer  3900  may be doped differently, such as, for example, with elemental species that may form a p+, or n+, or n− silicon wafer, or substantially absent of semiconductor dopants to form an undoped silicon wafer.
 
Step (B):  FIG. 39B  illustrates the structure after Step (B). Using a procedure similar to the one shown in  FIG. 2 , p− Si region  3902  may be transferred atop a peripheral circuit layer  3906 . The periphery may be designed such that it can withstand the RTA temperatures for activating dopants in memory layers formed atop it.
 
Step (C):  FIG. 39C  illustrates the structure after Step (C). After deposition of the tunnel oxide  3910  and floating gate  3908 , isolation regions may be formed in the p− Si region  3902  atop the peripheral circuit layer  3906 . This lithography step and substantially all future lithography steps may be formed with good alignment to features on the peripheral circuit layer  3906  since the p− Si region  3902  may be thin and reasonably transparent to the lithography tool.
 
Step (D):  FIG. 39D  illustrates the structure after Step (D). A inter-poly-dielectric (IPD) layer (eg. Oxide-nitride-oxide ONO layer) may be deposited following which a control gate electrode  3920  (eg. polysilicon) may be then deposited. The gate regions deposited in Step (C) may be patterned and etched. Following this, source-drain regions  3912  may be implanted. An inter-layer dielectric  3914  may be then deposited and planarized.
 
Step (E):  FIG. 39E  illustrates the structure after Step (E). Using procedures similar to Step (A) to Step (D), another layer of memory, a second NAND string  3916 , may be formed atop the first NAND string  3914 .
 
Step (F):  FIG. 39F  illustrates the structure after Step (F). Contacts may be made to connect bit-lines (BL) and source-lines (SL) to the NAND string. Contacts to the well of the NAND string may be also made. Substantially all these contacts could be constructed of heavily doped polysilicon or some other material. An anneal to activate dopants in source-drain regions of transistors in the NAND string (and potentially also the periphery) may be conducted. Following this, wiring layers for the memory array may be conducted.
 
A 3D floating-gate memory may have thus been constructed, with (1) horizontally-oriented transistors—i.e. current flow in substantially the horizontal direction in transistor channels, (2) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut. This use of monocrystalline silicon (or single crystal silicon) using ion-cut may be a key differentiator for some embodiments of the invention vis-à-vis prior work. Past work used selective epi technology or laser recrystallization or polysilicon.
 
         [0148]      FIG. 40A-H  show a novel memory architecture for 3D floating-gate memories, and a procedure for its construction. The memory architecture utilizes junction-less transistors. One mask may be utilized on a “per-memory-layer” basis for the monolithic 3D floating-gate memory concept shown in  FIG. 40A-H , and substantially all other masks may be shared among different layers. The process flow may include several steps that as described in the following sequence. 
         [0000]    Step (A): Peripheral circuits  4002  may be first constructed and above this oxide layer  4004  may be deposited.  FIG. 40A  illustrates the structure after Step (A).
 
Step (B):  FIG. 40B  illustrates the structure after Step (B). A wafer of n+ Silicon  4008  may have an oxide layer  4006  grown or deposited above it. Following this, hydrogen may be implanted into the n+ Silicon wafer at a certain depth indicated by hydrogen plane  4010 . Alternatively, some other atomic species such as Helium could be implanted. This hydrogen implanted n+Silicon wafer  4008  forms the top layer  4012 . The bottom layer  4014  may include the peripheral circuits  4002  with oxide layer  4004 . The top layer  4012  may be flipped and bonded to the bottom layer  4014  using oxide-to-oxide bonding. Alternatively, n+ silicon wafer  4008  may be doped differently, such as, for example, with elemental species that may form a p+, or p−, or n− silicon wafer, or substantially absent of semiconductor dopants to form an undoped silicon wafer.
 
Step (C):  FIG. 40C  illustrates the structure after Step (C). The stack of top and bottom wafers after Step (B) may be cleaved at the hydrogen plane  4010  using either an anneal or a sideways mechanical force or other means. A CMP process may be then conducted. A layer of silicon oxide  4018  may be then deposited atop the n+ Silicon layer  4016 . At the end of this step, a single-crystal n+ Si layer  4016  exists atop the peripheral circuits, and this may have been achieved using layer-transfer techniques.
 
Step (D):  FIG. 40D  illustrates the structure after Step (D). Using lithography and etch, the n+ silicon layer  4007  may be defined.
 
Step (E):  FIG. 40E  illustrates the structure after Step (E). A tunnel oxide layer  4008  may be grown or deposited following which a polysilicon layer for forming future floating gates may be deposited. A CMP process may be conducted, thus forming polysilicon region for floating gates  4030 .
 
Step (F):  FIG. 40F  illustrates the structure after Step (F). Using similar procedures, multiple levels of memory may be formed with oxide layers in-between.
 
Step (G):  FIG. 40G  illustrates the structure after Step (G). The polysilicon region for floating gates  4030  may be etched to form the polysilicon region  4011 .
 
Step (H):  FIG. 40H  illustrates the structure after Step (H). Inter-poly dielectrics (IPD)  4032  and control gates  4034  may be deposited and polished.
 
While the steps shown in  FIG. 40A-H  describe formation of a few floating gate transistors, it will be obvious to one skilled in the art that an array of floating-gate transistors can be constructed using similar techniques and well-known memory access/decoding schemes.
 
A 3D floating-gate memory may have thus been constructed, with (1) horizontally-oriented transistors—i.e. current flowing in substantially the horizontal direction in transistor channels, (2) monocrystalline (or single-crystal) silicon layers obtained by layer transfer techniques such as ion-cut, (3) side gates that may be simultaneously deposited over multiple memory layers for transistors, and (4) some of the memory cell control lines may be in the same memory layer as the devices. The use of monocrystalline silicon (or single crystal silicon) layer obtained by ion-cut in (2) may be a key differentiator for some embodiments of the invention vis-à-vis prior work. Past work used selective epi technology or laser recrystallization or polysilicon.
 
       Section 7: Alternative Implementations of Various Monolithic 3D Memory Concepts 
       [0149]    While the 3D DRAM and 3D resistive memory implementations in Section 3 and Section 4 have been described with single crystal silicon constructed with ion-cut technology, other options exist. One could construct them with selective epi technology. Procedures for doing these will be clear to those skilled in the art. 
         [0150]    Various layer transfer schemes such as, for example, those described in Section 1.3.4, can be utilized for constructing single-crystal silicon layers for memory architectures described in Section 3, Section 4, Section 5 and Section 6. 
         [0151]    The architecture that may include having the peripheral transistors below the memory layers, as depicted in, for example,  FIG. 28  to  FIG. 40A-H , may have an alternative variation of construction wherein the peripheral transistors could also be constructed above the memory layers, as shown, for example, in  FIG. 41B . This periphery layer above the memory layers may utilize technologies described in Section 1 and Section 2, and could utilize transistors including, for example, junction-less transistors or recessed channel transistors. 
         [0152]    The double gate devices shown in  FIG. 28  to  FIG. 40A-H  have both gates connected to each other. Alternatively, each gate terminal may be controlled independently. 
         [0153]    One of the concerns with using n+ Silicon as a control line for 3D memory arrays may be its high resistance. Using lithography and (single-step or multi-step) ion-implantation, one could dope heavily the n+ silicon control lines while not doping transistor gates, sources and drains in the 3D memory array. This high concentration doping may mitigate the concern of high resistance. 
         [0154]    In many of the described 3D memory approaches, etching and filling high aspect ratio vias may form a serious difficulty. One way to circumvent this obstacle may be by etching and filling vias from two sides of a wafer. A procedure for doing this is shown in  FIG. 42A-E . Although  FIG. 42A-E  describe the process flow for a resistive memory, such as resistive RAM, implementation, similar processes may be used for DRAM, charge-trap memories, phase change, conductive bridge, and floating-gate memories. The process may include several steps that may proceed in the following sequence: 
         [0000]    Step (A): 3D resistive memories may be constructed as shown in  FIG. 34A-K  but with a bare silicon wafer  4202  instead of a wafer with peripheral circuits on it. The resistance change memory and BL contact  4236  may be formed to the top layers of the memory, as illustrated in  FIG. 42A .
 
Step (B): Hydrogen may be implanted into the silicon wafer  4202  at a certain depth to form hydrogen implant plane  4242 .  FIG. 42B  illustrates the structure after Step B.
 
Step (C): The wafer with the structure after Step (B) may be bonded to a bare silicon wafer  4244 . Cleaving may be then performed at the hydrogen implant plane  4242 . A CMP process may be conducted to polish off the silicon wafer.  FIG. 42C  illustrates the structure after Step C.
 
Step (D): Resistance change memory material and BL contact layers  4241  may be constructed for the bottom memory layers. They may connect to the partially made top resistance change memory and BL contacts  4236  with state-of-the-art alignment.  FIG. 42D  illustrates the structure after Step D.
 
Step (E): Peripheral transistors  4246  may be constructed using procedures shown previously in this document.  FIG. 42E  illustrates the structure after Step E. Connections may be made to various wiring layers.
 
         [0155]    The charge-trap and floating-gate architectures shown in FIG.  36 A-F- FIG. 40A-H  may be based on NAND flash memory. It will be obvious to one skilled in the art that these architectures can be modified into a NOR flash memory style as well. 
       Section 8: Poly-Silicon-Based Implementation of Various Memory Concepts 
       [0156]    The monolithic 3D integration concepts described in this patent application can lead to some novel embodiments of poly-silicon-based memory architectures as well. Poly silicon based architectures could potentially be cheaper than single crystal silicon based architectures when a large number of memory layers may need to be constructed. While the below concepts may be explained by using resistive memory architectures as an example, it will be clear to one skilled in the art that similar concepts can be applied to NAND flash memory and DRAM architectures described previously in this patent application. 
         [0157]      FIG. 50A-E  shows one embodiment of the current invention, where polysilicon junction-less transistors may be used to form a 3D resistance-based memory. The utilized junction-less transistors can have either positive or negative threshold voltages. The process may include the following steps as described in the following sequence: 
         [0000]    Step (A): As illustrated in  FIG. 50A , peripheral circuits  5002  may be constructed above which oxide layer  5004  may be made.
 
Step (B): As illustrated in  FIG. 50B , multiple layers of n+ doped amorphous silicon or polysilicon
 
         [0158]    5006 may be deposited with layers of silicon dioxide  5008  in-between. The amorphous silicon or polysilicon layers  5006  could be deposited using a chemical vapor deposition process, such as Low Pressure Chemical Vapor Deposition (LPCVD) or Plasma Enhanced Chemical Vapor Deposition (PECVD). 
         [0000]    Step (C): As illustrated in  FIG. 50C , a Rapid Thermal Anneal (RTA) may be conducted to crystallize the layers of polysilicon or amorphous silicon deposited in Step (B). Temperatures during this RTA could be as high as about 500° C. or more, and could even be as high as about 800° C. The polysilicon region obtained after Step (C) is indicated as  5010 . Alternatively, a laser anneal could be conducted, either for substantially all amorphous silicon or polysilicon layers  5006  at the same time or layer by layer. The thickness of the oxide layer  5004  may be optimized if that process were conducted. Further, seeded polysilicon recrystallization, utilizing techniques such as, for example, contacts and via connections to a monocrystalline substrate, or nanographioepitaxy, may be utilized.
 
Step (D): As illustrated in  FIG. 50D , procedures similar to those described in  FIG. 32E-H  may be utilized to construct the structure shown. The structure in  FIG. 50D  may have multiple levels of junction-less transistor selectors for resistive memory devices. The resistance change memory may be indicated as  5036  while its electrode and contact to the BL may be indicated as  5040 . The WL may be indicated as  5032 , while the SL may be indicated as  5034 . Gate dielectric of the junction-less transistor may be indicated as  5026  while the gate electrode of the junction-less transistor may be indicated as  5024 , this gate electrode also serves as part of the WL  5032 .
 
Step (E): As illustrated in  FIG. 50E , bit lines (indicated as BL  5038 ) may be constructed. Contacts may be then made to peripheral circuits and various parts of the memory array as described in some embodiments described previously.
 
         [0159]      FIG. 51A-F  illustrate another embodiment of the invention, wherein polysilicon junction-less transistors may be used to form a 3D resistance-based memory. The utilized junction-less transistors can have either positive or negative threshold voltages. The process may include the following steps occurring in sequence: 
         [0000]    Step (A): As illustrated in  FIG. 51A , a layer of silicon dioxide  5104  may be deposited or grown above a silicon substrate without circuits  5102 .
 
Step (B): As illustrated in  FIG. 51B , multiple layers of n+ doped amorphous silicon or polysilicon  5106  may be deposited with layers of silicon dioxide  5108  in-between. The amorphous silicon or polysilicon layers  5106  could be deposited using a chemical vapor deposition process, such as LPCVD or PECVD.
 
Step (C): As illustrated in  FIG. 51C , a Rapid Thermal Anneal (RTA) or standard anneal may be conducted to crystallize the layers of polysilicon or amorphous silicon deposited in Step (B). Temperatures during this RTA could be as high as about 700° C. or more, and could even be as high as about 1400° C. The polysilicon region obtained after Step (C) may be indicated as  5110 . Since there may be no circuits under these layers of polysilicon, very high temperatures (such as, for example, about 1400° C.) can be used for the anneal process, leading to very good quality polysilicon with few grain boundaries and very high mobilities approaching those of single crystal silicon. Alternatively, a laser anneal could be conducted, either for substantially all amorphous silicon or polysilicon layers  5106  at the same time or layer by layer at different times. Further, seeded polysilicon recrystallization, utilizing techniques such as, for example, contacts and via connections to a monocrystalline substrate, or nanographioepitaxy, may be utilized.
 
Step (D): This may be illustrated in  FIG. 51D . Procedures similar to those described in  FIG. 32E-H  may be utilized to get the structure shown in  FIG. 51D  that may have multiple levels of junction-less transistor selectors for resistive memory devices. The resistance change memory may be indicated as  5136  while its electrode and contact to the BL may be indicated as  5140 . The WL may be indicated as  5132 , while the SL may be indicated as  5134 . Gate dielectric of the junction-less transistor may be indicated as  5126  while the gate electrode of the junction-less transistor may be indicated as  5124 , this gate electrode also serves as part of the WL  5132 .
 
Step (E): This is illustrated in  FIG. 51E . Bit lines (indicated as BL  5138 ) may be constructed. Contacts may be then made to peripheral circuits and various parts of the memory array as described in some embodiments described previously.
 
Step (F): Using procedures described in Section 1 and Section 2 of this patent application, peripheral circuits  5198  (with transistors and wires) could be formed well aligned to the multiple memory layers shown in Step (E). For the periphery, one could use the process flow shown in Section 2 where replacement gate processing may be used, or one could use sub-400° C. processed transistors, such as, for example, junction-less transistors or recessed channel transistors. Alternatively, one could use laser anneals for peripheral transistors&#39; source-drain processing. Various other procedures described in Section 1 and Section 2 could also be used. Connections can then be formed among the multiple memory layers and peripheral circuits. By proper choice of materials for memory layer transistors and memory layer wires (e.g., by using tungsten and other materials that withstand high temperature processing for wiring), even standard transistors processed at high temperatures (greater than about 1000° C.) for the periphery could be used.
 
       Section 9: Monolithic 3D SRAM 
       [0160]    The techniques described in this patent application can be used for constructing monolithic 3D SRAMs as well. 
         [0161]      FIG. 52A-D  an illustrative embodiment of the invention, where ion-cut may be utilized for constructing a monolithic 3D SRAM. Peripheral circuits may be first constructed on a silicon substrate, and above this, two layers of nMOS transistors and one layer of pMOS transistors may be formed using ion-cut and procedures described earlier in this patent application. Implants for each of these layers may be performed when the layers are being constructed, and finally, after substantially all layers have been constructed, a RTA may be conducted to activate dopants. If high k dielectrics are utilized for this process, a gate-first approach may be utilized, for example. 
         [0162]      FIG. 52A  illustrates a standard six-transistor SRAM cell. There may be two pull-down nMOS transistors  5202  in  FIG. 52A-D . There may be also two pull-up pMOS transistors, each of which may be represented by  5216 . There may be two nMOS pass transistors  5204  connecting bit-line wiring  5212  and bit line complement wiring  5214  to the pull-up transistors  5216  and pull-down nMOS transistors  5202 , and these may be represented by  5214 . Gates of nMOS pass transistors  5214  may be represented by  5206  and may be connected to word-lines (WL) using WL contacts  5208 . Supply voltage VDD may be denoted as  5222  while ground voltage GND may be denoted as  5224 . Nodes n 1  and n 2  within the SRAM cell may be represented as  5210 . 
         [0163]      FIG. 52B  illustrates a top view of the SRAM. For the SRAM described in  FIG. 52A-D , the bottom layer may be the periphery. The nMOS pull-down transistors may be above the bottom layer. The pMOS pull-up transistors may be above the nMOS pull-down transistors. The nMOS pass transistors  5204  may be above the pMOS pull-up transistors. The nMOS pass transistors  5204  on the topmost layer are displayed in  FIG. 52B . Gates  5206  for pass transistors  5204  may be also shown in  FIG. 52B . Substantially all other numerals have been described previously in respect of  FIG. 52A . 
         [0164]      FIG. 52C  illustrates a cross-sectional view of the SRAM. Oxide isolation using a STI process may be indicated as  5200 . Gates for pull-up pMOS transistors may be indicated as  5218  while the vertical contact to the gate of the pull-up pMOS and nMOS transistors may be indicated as  5220 . The periphery layer may be indicated as  5298 . Substantially all other numerals have been described in respect of  FIG. 52A  and  FIG. 52B . 
         [0165]      FIG. 52D  illustrates another cross-sectional view of the SRAM. The nodes n 1  and n 2  may be connected to pull-up, pull-down and pass transistors by using a vertical via  5210 .  5226  may be a heavily doped n+ Si region of the pull-down transistor,  5228  may be a heavily doped p+ Si region of the pull-up transistor and  5230  may be a heavily doped n+ region of a pass transistor. Substantially all other symbols have been described previously in respect of  FIG. 52A ,  FIG. 52B  and  FIG. 52C . Wiring connects together different elements of the SRAM as shown in  FIG. 52A . 
         [0166]    It can be seen that the SRAM cell shown in  FIG. 52A-D  may be small in terms of footprint compared to a standard 6 transistor SRAM cell. Previous work has suggested building six-transistor SRAMs with nMOS and pMOS devices on different layers with layouts similar to the ones described in  FIG. 52A-D . These may be described in “The revolutionary and truly 3-dimensional 25F 2  SRAM technology with the smallest S 3  (stacked single-crystal Si) cell, 0.16 um 2 , and SSTFT (stacked single-crystal thin film transistor) for ultra high density SRAM,” VLSI Technology, 2004. Digest of Technical Papers. 2004 Symposium on, vol., no., pp. 228-229, 15-17 Jun. 2004 by Soon-Moon Jung; Jaehoon Jang; Wonseok Cho; Jaehwan Moon; Kunho Kwak; Bonghyun Choi; Byungjun Hwang; Hoon Lim; Jaehun Jeong; Jonghyuk Kim; Kinam Kim. However, these devices are constructed using selective epi technology, which suffers from defect issues. These defects severely impact SRAM operation. The embodiment of this invention described in  FIG. 52A-D  may be constructed with ion-cut technology and may be thus far less prone to defect issues compared to selective epi technology. 
         [0167]    It is clear to one skilled in the art that other techniques described in this patent application, such as use of junction-less transistors or recessed channel transistors, could be utilized to form the structures shown in  FIG. 52A-D . Furthermore, alternative layouts for 3D stacked SRAM cells may be possible as well, where heavily doped silicon regions could be utilized as GND, VDD, bit line wiring and bit line complement wiring. For example, the region  5226  (in  FIG. 52D ), instead of serving just as a source or drain of the pull-down transistor, could also run along the length of the memory array and serve as a GND wiring line. Similarly, the heavily doped p+ silicon region of the pull-up transistors  5228  (in  FIG. 52D ), instead of serving just as a source or drain of the pull-up transistor, could run along the length of the memory array and serve as a VDD wiring line. Moreover, the heavily doped n+ region of pass transistor  5230  could run along the length of the memory array and serve as a bit line. Thus the invention is to be limited only by the appended claims. 
         [0168]    It will also be appreciated by persons of ordinary skill in the art that the invention is not limited to what has been particularly shown and described hereinabove. For example, drawings or illustrations may not show n or p wells for clarity in illustration. Rather, the scope of the invention includes both combinations and sub-combinations of the various features described herein above as well as modifications and variations which would occur to such skilled persons upon reading the foregoing description. Thus the invention is to be limited only by the appended claims.