Abstract:
This invention provides structures and a fabrication process for incorporating thin film transistors in back end of the line (BEOL) interconnect structures. The structures and fabrication processes described are compatible with processing requirements for the BEOL interconnect structures. The structures and fabrication processes utilize existing processing steps and materials already incorporated in interconnect wiring levels in order to reduce added cost associated with incorporating thin film transistors in the these levels. The structures enable vertical (3D) integration of multiple levels with improved manufacturability and reliability as compared to prior art methods of 3D integration.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is divisional of U.S. application Ser. No. 11/358,183 filed Feb. 21, 2006 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to the fields of semiconductor integrated circuits and electrical interconnect technology, and more particularly relates to vertical or 3D integration of devices such as thin film transistors (TFTs) into back end of the line (BEOL) interconnect structures. 
     2. Description of the Related Art 
     In recent years 3D integration has gained significant attention as a possible pathway for increasing IC density and for reducing interconnect delays and ac power consumption (by reducing interconnect distances). 
     In one prior art technique for fabricating 3D integrated circuits, a process called “smart cut” wafer bonding is used to form a single crystal germanium layer above passivated metal interconnect levels on a silicon device level. This method is described, for example, in Yu. D. S et al, “Three-Dimensional Metal Gate-High-k-GOI CMOSFETs on 1-Poly-6-Metal 0.18-mm Si Devices,”  IEEE Electron Device Lett ., vol. 26, no. 2, pp. 118-120, February 2005. This method utilizes germanium as an additional device layer stacked over the device layer in the base substrate. Ge offers the advantage of lower temperature processing compared to silicon, a critical factor for vertically integrated device structures that are formed after the first silicon device layer and metal interconnect layers. 
     However, this method is associated with significant manufacturing problems, which arise from the requirement for wafer bonding above an already-formed interconnect structure. In addition to the cost of wafer bonding, there are concerns with reliability of bonding above the already-formed layers. The cost of losing all of the chips on a 300 mm wafer due to a problem during bonding would be tremendous. Additionally, this type of 3D integration is limited in that it is not easily imbedded in multiple back end of the line (BEOL) wiring levels along with the interconnect structures. 
     In another prior art 3D vertical integration structure, multiple levels of devices are placed one above the other utilizing single crystal silicon formed by lateral epitaxial growth from a vertical column of silicon seed originating from the Si substrate. This structure is described, for example, in Wei, L. et al. “Vertically Integrated SOI Circuits for Low-Power and High-Performance Applications,”  IEEE Transactions on Very Large Scale Integration  ( VLSI )  systems , vol. 10, no. 3, pp. 351-362, June 2002. 
     This epitaxial growth method of vertical integration has the disadvantage that it is limited to a location close to a seed column. 
     In addition, Silicon devices require high temperatures for both forming the silicon layer and for later processing steps such as dopant activation. These high temperatures can cause significant degradation to the first device level and prevent the possibility of incorporating these structures in the same level as the back end of the line interconnect levels which are typically limited to a processing temperature of less than 400-450° C. 
     Therefore, there is a need for a simplified, cost-effective, 3D vertical integration structure and method that could be formed from primarily existing steps and would be compatible with the processing requirements of the BEOL interconnect levels. Implementation of devices into the BEOL wiring levels using primarily standard BEOL processing steps would enable a more cost effective path to 3D integration as compared with the existing prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a vertically (3D) integrated structure that is formed without using any high temperature (&gt;450° C.) processes that could damage the underlying silicon device level or the BEOL interconnect levels. The structure allows for the incorporation of nFETs, pFETs, and/or other devices as needed by the targeted application. 
     These devices can be in multiple levels, selected from those just above the silicon device layer to those separated from the silicon device layer by multiple levels of wiring. The structure includes devices in the same levels with BEOL interconnect wiring and incorporates many processing steps that are already used to form the metal interconnects, therefore reducing added cost of forming the devices. The devices, thin film transistors (TFTs), are formed on thin polycrystalline semiconductor films that can be deposited at any level of the interconnect structure. 
     Although these devices can have an inferior mobility and Ion/Ioff ratio compared to advanced single crystal silicon devices due to the grain boundaries in the polycrystalline film, these devices are targeted toward applications that do not require the high performance of the standard single crystal silicon devices in the bottom level. The materials in the present invention have been selected to optimize the performance of the polycrystalline devices while at the same time allowing for lower temperature (&lt;450° C.) processing. 
     The present invention achieves significant circuit area/footprint reduction of the single crystal device level by enabling the incorporation of selected circuits, i.e., those that do not require the high performance of the single crystal device level, into upper levels of the chip. 
     Thus, it is an object of the present invention to provide an electrical interconnect structure containing thin film transistors within one or more of interconnect wiring levels. 
     The thin film transistor is comprised of a metal gate and metal source and drain contacts that contain the same materials as the metal interconnect wiring. 
     The semiconductor material in the thin film transistor is a polycrystalline material that can be formed by deposition or deposition plus annealing steps at temperatures below 450° C. 
     The structure can be prepared with minimal additional processing steps in a standard single or dual damascene interconnect structure. 
     The structure and method of the preferred embodiment minimizes additional processing steps and allows implementation in a copper plus low k dielectric back end of the line (BEOL) interconnect structure. 
     Further, the structure of the preferred embodiment incorporates semiconductor materials including polycrystalline germanium and cadmium selenide, which have significantly higher bulk mobilities than polycrystalline or amorphous silicon. 
     Devices formed from these materials are also compatible with processing temperatures at or below 450° C., temperatures significantly lower than those required in polycrystalline or amorphous silicon devices to achieve close to equivalent performance. In addition, the structure of the preferred embodiment incorporates copper as the metal gate and source/drain contacts. The copper can be deposited simultaneously with the copper wiring in the interconnect structure reducing additional processing steps and added costs. 
     The method of the preferred embodiment incorporates several existing dual damascene BEOL process steps in the formation of the thin film transistors. In many cases, these processes are performed simultaneously with formation of the line and via interconnect structures. 
     Accordingly, it is an object of this invention to provide a thin film transistor structure within a low-k dielectric plus Cu interconnect structure of the single or dual damascene type. 
     It is another object of this invention to provide a self-aligned thin film transistor structure within a low-k dielectric plus Cu interconnect structure of the single or dual damascene type. 
     It is still another object of this invention to provide an electrical interconnect structure containing p-type thin film transistors in one BEOL wiring level and n-type thin film transistors in a second BEOL wiring level. 
     It is yet another object of this invention to provide a method to make the inventive structures described herein. 
     Accordingly, the present invention provides a electrical interconnect structure having thin film transistors including: 
     a first dielectric containing a plurality of conductors wherein some of the conductors form conducting lines and/or vias, and other conductors form gate electrodes of the thin film transistors; 
     an insulating material atop the gate electrodes; 
     a semiconductor having spaced-apart doped source and drain regions with a channel disposed there between atop the insulating material; and 
     a second dielectric having a plurality of conductors where some conductors form conducting lines and/or vias, and other conductors form contacts to the source and drain regions of the thin film transistors. 
     The present invention provides an integrated circuit structure including: 
     a layer of active circuit devices on a substrate; 
     a plurality of layers having random or regular layouts of interconnecting line and/or via structures above the layer of active circuit devices; wherein the plurality of layers have at least a layer having both interconnecting line and/or via structures and a multiplicity of thin film transistors with self-aligned overlap between the source and drain regions and the gate electrode, which layer includes at least a first dielectric containing conducting line and/or via interconnect structures and a self aligned thin film transistor structure having a semiconductor material, a gate dielectric, a gate electrode, spaced apart doped source and drain regions within the semiconductor material that extend just to the edges of the gate electrode with a self-aligned controlled degree of overlay conducting metal contacts contacting the source and drain regions; and 
     optionally at least one of: 
     a second dielectric material between the source and drain contacts and the gate electrode; 
     a conducting diffusion barrier materials on at least one side of any or all of the conducting line or via interconnect structures, the gate electrode, and the conducting metal contacts contacting the source and drain regions; 
     a region between the source and drain contacts and the doped source and drain regions which acts for improving the contacts to the source and drain regions; wherein the region includes metal germanides, metal silicides, or mixtures of metal germanides and metal silicides; wherein the metal is selected from: Ni, Co, Pd, Pt, Nb, Ti, Zr, Hf, Ta, Cr, Mo, W, Er and Ir. 
     The present invention still further provides a thin film transistor with germanium-containing semiconductor region, including: 
     spaced-apart doped source and drain regions with a channel region disposed there between; 
     a gate dielectric in contact with the channel region; and 
     a conductive Cu-containing gate. 
     The present invention additionally provides a method of forming a damascene electrical interconnect structure containing thin film transistors including the steps of: 
     forming a first interlayer dielectric on a substrate; 
     forming conducting metal structures in the first interlayer dielectric by standard single of dual damascene processing; 
     depositing and insulating material or materials; 
     depositing a semiconductor material; 
     patterning the semiconductor material; 
     depositing a second planarizing interlayer dielectric material; 
     patterning the second interlayer dielectric material forming openings to expose the semiconductor material; 
     forming doped regions in the semiconductor material; 
     etching the doped regions in the semiconductor material; 
     filling the etched regions with a sacrificial planarizing material; 
     patterning and etching to form openings that will become line and via interconnect structures; and 
     metallizing the openings to form source and drain contacts and interconnect structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing illustrating a cross-sectional view of the inventive structure in a first embodiment with utilization of an insulating diffusion barrier. 
         FIG. 2  is a schematic drawing illustrating a cross-sectional view of the inventive structure in a second embodiment with utilization of a selectively aligned diffusion barrier. 
         FIGS. 3   a  and  3   b  are schematic drawings illustrating cross-sectional views of two variations of the inventive structure in a third embodiment with a double gated structure. 
         FIG. 4  is a schematic drawing illustrating a cross-sectional view of the inventive structure in a fourth embodiment with a self aligned source and drain region. 
         FIG. 5  is a schematic drawing illustrating a cross-sectional view of the inventive structure in a fifth embodiment with a cross-point structure. 
         FIG. 6  is a schematic drawing illustrating a cross-sectional view of the inventive structure in a sixth embodiment with a dual channel structure. 
         FIGS. 7   a -L are schematic drawings illustrating a cross-sectional view of the structure of the first embodiment (Structure L) and the intermediate structures (structures a-k) leading thereto as they are being constructed according to the steps of the method of the present invention. 
         FIG. 8  is a list of steps in the method to make the structure of the first embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Structure According to the Invention 
     Referring to  FIG. 1 , an electrical interconnect structure containing thin film transistors includes a substrate  1 , a first interlayer dielectric layer  3  containing conducting line and/or via interconnect structures  21  and a gate electrode  17 , an insulating diffusion barrier  7  to prevent diffusion of the metal in the gate electrode or interconnect structures and act as the gate dielectric material, a second interlayer dielectric material  5 , containing conducting line and/or via interconnect structures  29 , a semiconductor material  11  above the metal gate electrode, spaced apart doped regions  13  within the semiconductor material which act as the source and drain regions of the thin film transistor, and conducting metal contacts  23  and  25  contacting the source and drain regions. 
     The structure can further include a conducting diffusion barrier liner  19  on at least one surface of the gate electrode  17 . The structure can further include a conducting diffusion barrier liner  27  on at least one surface of the conducting metal contacts  23  and  25 . The structure can further include a conducting diffusion barrier liner  31  on at least one surface of the line and via structures  29  and  21 . 
     The structure can also include an additional thin layer  9  above the insulating diffusion barrier  7 , which can be a layer which improves interface properties of the gate insulator stack, or improves nucleation of overlying semiconductor material  11 . 
     The structure can further include a region  15  between the source and drain contacts and the doped source and drain regions which acts to improve the contacts to the source and drain regions. The region  15  can be comprised of metal germanides, metal silicides, and mixtures of metal germanides and metal silicides, where metal is selected from the group including but not limited to Ni, Co, Pd, Pt, Nb, Ti, Zr, Hf, Ta, Cr, Mo, W, Er, and Ir. 
     The structure can further include a thin capping layer atop the semiconductor material to protect the semiconductor material from oxidation or degradation. 
     The electrical interconnect structure can include multiple interconnect levels with multiple levels of thin film transistors. In one embodiment, the electrical interconnect structure contains n-type thin film transistors in one set of interconnect levels and p-type thin film transistors in a second set of interconnect levels. These n-type and p-type thin film transistors can include the same or different semiconductor materials. In one specific embodiment, the n-type transistors can be formed with CdSe as the semiconductor material and the p-type transistors can be formed with polycrystalline Ge as the semiconductor material. 
     This structure allows for incorporation into a standard BEOL process flow with minimal additional processing steps. This structure utilizes damascene processing and can incorporate standard BEOL materials including Cu metallization and Ta containing liners to form the gate and source drain contacts. 
     The gate can be formed simultaneously with the line and via wiring of that dual damascene level with no additional processing steps. One additional masking step will be required to form the isolation trenches and remove any poly-Ge or other semiconductor material from regions outside the TFT structure. A second additional masking step would typically be required to define the source and drain regions. However, it should be feasible to deposit liner, plate Cu, and CMP the source drain contacts in the same step as the line and via wiring. The additional processing steps not typically encountered in BEOL processing include the following: deposition and patterning of semiconductor  11 , doping of source and drain regions (for example, by ion implantation), and the metal deposition, anneal, and wet etch removal steps associated with germanide or silicide formation. 
     In this structure standard BEOL Cu barrier materials such as SiN or SiCN, SiCHN, can be used as the gate dielectric. This allows the minimal amount of changes to the standard BEOL process flow. 
     Referring to  FIG. 2 , an alternative structure can incorporate all of the components described in  FIG. 1  except for the insulating diffusion barrier material. This structure can include a selective metal diffusion barrier  35  atop the gate electrode and the line and/or via patterns instead of the insulating diffusion barrier material. This structure also includes a thin insulating material  39  atop the gate electrode to act as the gate dielectric of the thin film transistor structure. 
     This structure has the advantage of enabling a thinner gate dielectric with more flexibility on the material choices for the gate dielectric material. Selective metal diffusion barriers of this type are described in U.S. Pat. No. 5,695,810 entitled “Use of Cobalt Tungsten Phosphide as a barrier Material for Copper Metallization” by Valery M. Dubin et al., and the commonly owned U.S. Patent Application Publication Number US 2005/0127518 A1 entitled “Electroplated CoWP Composite Structures as Copper barrier layers” by Cyril Cabral Jr. et al., the contents of which are incorporated herein by reference in their entirety as fully set forth herein. 
     Selective metal caps, such as CoWP have been under investigation to replace the dielectric cap in the BEOL wiring levels for several years in order to reduce the capacitance of the structure. Incorporation of a selective metal cap would prevent the need for the thicker insulating barrier layer and would enable the use of a very thin gate dielectric, which could significantly improve the properties of the device. 
     The first interlayer dielectric layer  3  and second interlayer dielectric layer  5  can be the same or different materials and can be comprised of but not limited to an insulating oxide, a low k dielectric material, a porous low k dielectric material, a dielectric containing air gaps. The insulating diffusion barrier material  7  can be comprised of SiN; materials containing Si, C, N, and H; materials containing Si, C, and H; or other insulating materials that have barrier properties that prevent metal diffusion of the gate metal  17 . The conducting line and/or via interconnect structures  21  and  29 , can be comprised of Cu, Al, W, Ag or other like metals which are typically used in interconnect structures. The gate electrode  17  can be comprised of but is not limited to Cu, Al, W, Ag, Er, Ni, Co, Au, Sn, poly-Si, poly-Ge, or other materials which are typically used in interconnect structures or gate electrodes. The source and drain contacts  23  and  25  can be comprised of but are not limited to, Cu, Al, W, Ag, Er, Ni, Co, Au, Sn or other like metals which are typically used in interconnect structures or contacts. 
     Preferably, the metal gate electrode  17  and source drain contacts  23  and  25  are formed from the same material which forms the conducting line and/or via structures  21  and  29 . 
     The conducting diffusion barrier liners  19 ,  27 , and  31  can be the same or different materials and can be comprised of, but are not limited to: TiN, TaN, TiSiN, other metal nitrides and metal silicon nitrides, conductive metal carbides, Ti, Ta, W, WN, Cr, Nb and other like materials including combinations thereof. The semiconductor material  11  can be comprised of, but is not limited to, polycrystalline Ge, polycrystalline SiGe, CdSe, polycrystalline Si, amorphous Si, amorphous Ge. These materials can further include carbon, InAs, InAlAs, InGaAs or other III-V compounds. 
     Preferably the semiconductor material is a polycrystalline material with a bulk mobility of greater than 100 cm 2 /Vs, is formed at temperatures below 450° C., from which devices can be fabricated with a maximum processing temperature of less than 450° C. More preferably the semiconductor material is polycrystalline Ge, polycrystalline SiGe, or CdSe. 
     The dopant in the doped semiconductor region  13  can be comprised of, but is not limited to, B, As, P, Ga, In, Al, Zn or other like materials. The selective metal diffusion barrier  35  can be comprised of but is not limited to CoWP, Ta, W, Mo, TiW, TiN, TaN, WN, TiSiN, TaSiN, and other like materials including combinations thereof. The thin material  9  includes one or more layers of a material, such as, SiO2, silicon nitride, silicon oxynitride, silicon-containing oxides, insulating metal oxides, insulating metal nitrides, insulating metal silicon oxides, insulating metal silicon oxynitrides, germanium oxynitride, germanium-containing oxide, insulating metal germanium oxides, insulating metal germanium oxynitrides, amorphous silicon, and Si or Ge-containing seed layers, without being limited thereto. 
     The thin insulating material  39  can be SiO2, silicon oxynitride, silicon-containing oxides, insulating metal oxides, insulating metal nitrides, insulating metal silicon oxides, insulating metal silicon oxynitrides, germanium oxynitride, germanium-containing oxides, insulating metal germanium oxides, insulating metal germanium oxynitrides, but are not limited thereto. 
     Referring to  FIG. 3 , in another embodiment of the invention the structure can further include a second gate electrode  43  above the semiconductor region  11 , which is separated from the semiconductor region by an insulating material  41  or  51 . Referring to  FIG. 3   a  the insulating material  41  can cover the entire semiconductor region or referring to  FIG. 3   b  the insulating material  51  can surround the gate electrode. The structure can further include a conducting diffusion barrier liner  45  surrounding the gate electrode  43 . 
     Referring to  FIG. 4 , in another embodiment of the invention the source and drain regions are self aligned by the gate allowing a very controlled degree of overlap between the gate and the source and drain. The electrical interconnect structure containing self-aligned thin film transistors, includes a substrate  61 , a first interlayer dielectric layer  63  containing conducting line and/or via interconnect structures  85 , and a self aligned thin film transistor structure containing a semiconductor material  65 , spaced apart doped source and drain regions within the semiconductor material  67 , a gate insulator material  77 , a gate electrode  73 , and conducting metal contacts  79  and  81  contacting the source and drain regions. 
     The structure can further include a second dielectric material  71  between the source and drain contacts and the gate electrode. 
     The structure can still further include conducting diffusion barrier materials ( 87 ,  83 ,  75 ) on at least one side of any or all of the conducting line or via interconnect structures  85 , the gate electrode  73 , or the conducting metal contacts  79  and  81  contacting the source and drain regions. 
     The structure can further include a region  69  between the source and drain contacts and the doped source and drain regions which acts to improve the contacts to the source and drain regions. The region  69  can be comprised of metal germanides, metal silicides, and mixtures of metal germanides and metal silicides, where metal is selected from the group including but not limited to Ni, Co, Pd, Pt, Nb, Ti, Zr, Hf, Ta, Cr, Mo, W, and Ir. 
     Referring to  FIG. 5 , in another embodiment of the invention the structure can include a cross-point thin film transistor structure within an electrical interconnect structure. The structure includes a substrate  1 , a first interlayer dielectric layer  3  containing conducting line and/or via interconnect structures and a gate electrode  17 , a thin insulating material  39  atop the gate electrode, the insulating material acting as the gate dielectric of the thin film transistor, a second interlayer dielectric material  5 , containing conducting line and/or via interconnect structures, a semiconductor material  11  above the metal gate electrode, spaced apart doped regions  13  within the semiconductor material which act as the source and drain regions, and conducting metal contacts  23 ,  25 , and  123  contacting the source and drain regions. 
     The structure can further include a third interlayer dielectric layer  103  containing conducting line and/or via interconnect structures  105 , a semiconductor material  111  above at least two of the source and drain contacts, spaced apart doped regions  113  within the semiconductor material which act as source and drain regions, a gate dielectric material  107 , and a gate electrode  117  overlapping partially with the source and drain regions  113 . 
     The structure can still further include a selective metal diffusion barrier  35  atop the gate electrode  17  or  117 , and the line and/or via patterns  105 . The structure can further include a selective metal diffusion barrier  135  atop the conducting metal contacts  23 ,  25 , and  123 . 
     The structure can further still include a thin seed layer  109  to improve the deposition of the semiconductor material. The seed layer can also have doped regions. 
     The structure can additionally include a conducting diffusion barrier liner  19 ,  119  on at least one side of the gate electrodes, the source and drain contacts, or the conducting line and/or via interconnect structures. 
     The structure also includes conducting contacts in contact with the source and drain contacts and the gate electrodes. These contacts are out of the plane illustrated in this figure and therefore are not represented in the figure. 
     The structure can further include a region  15  between the source and drain contacts and the doped source and drain regions which acts to improve the contacts to the source and drain regions. The region  69  can be of metal germanides, metal silicides, and mixtures of metal germanides and metal silicides, where metal is selected from the group including, but not limited to, Ni, Co, Pd, Pt, Nb, Ti, Zr, Hf, Ta, Cr, Mo, W, and Ir. 
     The structure can further include a thin capping layer atop the semiconductor material to protect the semiconductor material from oxidation or degradation. 
     Referring to  FIG. 6 , which depicts another embodiment of the invention, the structure can include a dual channel thin film transistor within an electrical interconnect structure. The structure includes a substrate  201 , a first interlayer dielectric layer  203  containing conducting line and/or via interconnect structures  229 , and conducting metal contacts  209  and  211 , a second interlayer dielectric layer  205  containing conducting line and/or via interconnect structures  231 , a semiconductor material  217  above the conducting metal contacts, spaced apart doped regions  239  within the semiconductor material, the spaced apart doped regions  239  acting as source and drain regions, a gate dielectric  221  atop the semiconductor material, and a gate electrode  225  atop the gate dielectric, a second gate dielectric  223  atop the gate electrode, a third interlayer dielectric layer  207  containing conducting line and/or via interconnect structures  233 , a semiconductor material  219  above the gate electrode, spaced apart doped regions  327  within the semiconductor material, the spaced apart doped regions  327  acting as source and drain regions, and conducting metal contacts  213  and  215  in contact with the spaced apart doped regions. 
     The structure can further include a selective metal diffusion barrier  235  atop one of the conducting metal contacts  209 ,  211   213 , and  215 , the interconnect structures  229 ,  231 , and  233 , and the gate electrode  225 . The structure can further include a conducting diffusion barrier liner  227  on at least one surface of any of the conducting line and/or via interconnect structures  229 ,  231 ,  233 , conducting metal contacts  209 ,  211 ,  213 ,  215 , and gate electrode  225 . 
     The structure can still further include regions  241  between the source and drain contacts and the doped source and drain regions which acts to improve the contacts to the source and drain regions. The regions  241  can be comprised of metal germanides, metal silicides, and mixtures of metal germanides and metal silicides, where the metal is selected from the group including but not limited to Ni, Co, Pd, Pt, Nb, Ti, Zr, Hf, Ta, Cr, Mo, W, and Ir. 
     The structure can further include a thin seed layer  243  to improve the deposition of the semiconductor material. The seed layer can also have doped regions. 
     The structure can further include a thin capping layer atop the semiconductor material to protect the semiconductor material from oxidation or degradation. 
     The structure further includes conducting contacts in contact with the source and drain contacts and the gate electrodes. These contacts are out of the plane illustrated in the figure and therefore are not represented in the figure. 
     Method According to the Invention 
     Referring to  FIG. 7  and  FIG. 8 , a method of forming an electrical interconnect structure including thin film transistors is described. 
     The method of forming the interconnect structure includes the steps of: forming a first interlayer dielectric  3  on a substrate  1  ( FIG. 7   a ), forming conducting metal structures  17  and  21  in the first interlayer dielectric by standard single of dual damascene processing ( FIG. 7   b ), depositing and insulating material or materials  7  and  9  then depositing a semiconductor material  11 , preferably at a temperature below 450° C. ( FIG. 7   c ), patterning the semiconductor material ( FIG. 7   d ), depositing a second planarizing interlayer dielectric material  5  ( FIG. 7   e ), patterning the second interlayer dielectric material  5  forming openings  323  and  325  to expose the semiconductor material ( FIG. 7   f ), forming doped regions  13  in the semiconductor material by ion implantation ( FIG. 7   g ), filling the etched regions with a sacrificial planarizing material  303  ( FIG. 7   j ), patterning and etching to form openings  329  that will become line and via interconnect structures ( FIG. 7   k ), metallizing the openings to form source and drain contacts  23  and  25  and interconnect structures  29 . 
     The method can further include annealing the semiconductor material  11  at a temperature below 450° C. to crystallize or recrystallize the material. 
     The method can further include depositing patterning  301  and photoresist  305  layers atop the second planarizing interlayer dielectric material. 
     The method can further include forming germanide or silicide regions  15  by depositing a metal  315  ( FIG. 7   h ), annealing the metal to react with the semiconductor material preferably at a temperature below 450° C., and thereafter removing any unreacted metal ( FIG. 7   i ). 
     The method can further include depositing a conducting liner material prior to the ion implantation to form the doped regions or prior to the metal deposition to form the germanide or silicide regions. 
     The method can further include removing the liner from only the bottom of the etched regions prior to ion implantation or to metal deposition to form germanide or silicide regions. 
     The present invention has been described with particular reference to the preferred embodiments. It should be understood that variations and modifications thereof can be devised by those skilled in the art without departing from the spirit and scope of the present invention. Accordingly, the present invention embraces all such alternatives, modifications and variations that fall within the scope of the appended claims.