Patent Abstract:
The present invention features double- or dual-gate logic devices that contain gate conductors that are consistently self-aligned and that have channels that are of constant width. A single-crystal silicon wafer is utilized as the channel material. Pillars or stacks of self aligned dual gate MOSFETs are generated by etching, via the juxtaposition of overlapping germanium-containing gate conductor regions. Vertically etching through regions of both gate conducting material and dielectric insulating material provides an essentially perfect, self-aligned dual gate stack.

Full Description:
RELATED PATENT APPLICATIONS 
   This application is a divisional application of Ser. No. 09/879,590, filed Jun. 12, 2001 now U.S. Pat. No. 6,596,597. 

   FIELD OF THE INVENTION 
   This invention generally relates to semiconductor devices formed on a bulk single-crystal semiconductor substrate and, more particularly, to dual gate logic semiconductor devices composed of germanium-containing gate conductors and manufactured by a self-aligning process. 
   BACKGROUND OF THE INVENTION 
   For complimentary metal on silicon integrated circuits (CMOS ICs) the main performance factors are speed, power dissipation, and device packing density. Therefore, over the past several decades, integrated chip manufacturers have had as one major goal the reduction in microelectronic device size. Both manufacturer and consumer benefit from this reduction in size either by reduced cost or greater performance characteristics. However, the mere reduction in size of the components in the IC will lead to undesirable IC performance problems. In particular, power dissipation due to increased device leakage currents may increase or circuit speed may be degraded. Reliability problems that can afflict metal on silicon field effect transistors (MOSFETs) might also be worsened, including hot-carrier degradation, gate-oxide wearout and electromigration. Clearly, if the degree of process control is not increased, variations in these parameters will-become larger (on a percentage basis) as the devices become even smaller. Therefore, it is necessary for the manufacturer of such devices to utilize novel designs and employ processes having tighter processing controls that will mitigate performance and reliability problems, while still providing higher packing densities. 
   One particular difficulty in the manufacturing processes of some planar double-gate MOSFET devices is that the top and bottom gate conductors may not be self-aligned to each other, and the gate conductors may be of varying widths. Device yield and performance can be significantly constrained by such misalignment of the gate-conductors, and by large deviations in relative channel length. For example, it is reported that misalignment will cause extra gate to source/drain overlap capacitance as well as loss of current drive. Additional information on the effect of misalignment is described by Tanaka of Fujitsu in the 1994 VLSI Symposium. 
   Another difficulty in the manufacturing processes of these planar double-gate MOSFET devices is that the channel thickness is not of uniform thickness and/or uniform purity. For example, double-gate MOSFET devices should have a uniform and thin (10 to 25 nm) silicon channel. Typically, previous manufacturing processes formed this channel using epitaxially grown silicon via such processes as chemical vapor deposition or sputtering. These processes however, do not necessarily provide sufficient uniformity in thickness or purity, the latter due to entrapment of impurities. As will be described in detail hereinbelow the present invention utilizes a single crystal silicon wafer that is ground and polished to high precision to provide a silicon channel having physical and electrical properties that are superior to the prior art epitaxially grown silicon channels. References to prior art dual-gate MOSFET manufacturing processes can be found in Jong-Ho Lee, et al. IEEE IEDM99-71 through IEDM99-74; Hon-Sum Philip Wong, et al., IEEE IEDM98-407 through IEDM98-410; and Hon-Sum Philip Wong, et al., IEEE IEDM97-427 through IEDM97-429. 
   Over the years the preeminent semiconductor material for use in integrated chip technology has been silicon. For example, S. Wolf and R. N. Tauber in SILICON PROCESSING (copyright 1986) volume 1 page 1 state “Germanium was the original semiconductor material used to fabricate diodes and transistors. The narrow bandgap of Ge (0.66 eV), however, causes reverse-biased p-n junctions in Ge to exhibit relatively large leakage currents. This limits the operation to temperatures below about 100° C. In addition, integrated circuit planar processing requires the capability of fabricating a passivation layer on the semiconductor surface. Germanium oxide could act as such a passivation layer but it is difficult to form, is water soluble, and dissociates at 800° C. These limitations make Ge an inferior material for the fabrication of integrated circuits, compared to silicon.” 
   The use of germanium and germanium alloys has been reported in prior references as gate conducting materials, for example see  GERMANIUM ETCHING IN HIGH DENSITY PLASMAS FOR  0.18  MICRON COMPLENTARY METAL - OXIDE - SEMICONDUCTOR GATE PATTERNING APPLICATIONS , C. Monget, A. Schiltz, O. Joubert, L. Vallier, M. Guillermet, B. Tormen, J. Vac. Sci. Technol. B, Vol 16, 1998, p1833-1840. However, none of these references describe, teach, or contemplate the instant inventive feature of selectively etching-back these germanium containing gate conducting materials vis-à-vis the silicon channel. 
   SUMMARY OF THE INVENTION 
   The present invention provides for novel manufacturing processes and double- or dual-gate logic devices therefrom that contain gate conductors that are consistently self-aligned and that have channels that are of constant width. These characteristics are important to the industry because device yield and performance can be significantly constrained by such misalignment of the gate conductors, and by large deviations in relative channel length. The inventive process also provides a method of selectively etching germanium-containing gate conductor materials without significantly etching the adjacent silicon channel material. In this manner, the gate conductor can be encased in a dielectric shell without changing the length of the silicon channel. As mentioned supra, changes to the dimensions of the channel can cause adverse performance characteristics. 
   Also, many prior art planar dual-gate structures rely on the formation of lateral epi-silicon layers for the fabrication of the channel area. Defects in this epi layer can significantly reduce device yield and performance. The present invention alleviates this problem by preferentially utilizing a single-crystal silicon wafer as the channel material. 
   Therefore, in one aspect of the present invention, a process is described for formation of a uniformly thin channel comprising single-crystal silicon. 
   In another aspect of the present invention, a process involves etching to generate pillars or stacks of self aligned dual gate MOSFETs via the juxtaposition of overlapping germanium-containing gate conductor regions and vertically etching through regions comprising both gate conducting material and dielectric insulating material. The edge formed by vertically etching through both germanium-containing gate conductor regions provides for essentially a perfect self-aligned dual gate stack. 
   In yet another aspect of the invention, a process is described wherein the gate conductor material can be selectively etched without etching the channel material. 
   BRIEF DESCRIPTION OF THE DRAWINGS 
   For practical reasons, only a portion of a layout for an array of the features on the semiconductor device of this invention is depicted. It is understood that the same element will be identified with like numerical references consistently maintained from  FIG. 1  to FIG.  2 . 
   KEY TO REFERENCE NUMBERS 
     10  first single-crystal silicon wafer 
     11  channel region or stratum formed by thinning first single-crystal silicon wafer  10   
     12  raised islands comprising silicon residing on an upper surface of a silicon substrate or wafer  10   
     13  single-crystal channel region or stratum separating gate dielectric regions  24  and  24   a    
     14  sidewalls of a silicon channel region  13   
     15  topmost surface of the first single-crystal silicon wafer  10   
     16  etched intermediate topmost surface of the first single-crystal silicon wafer  10   
     17  topmost surface of the intermediate conformal dielectric coating  110   
     18  bottommost surface of the first single-crystal silicon wafer  10   
     19  silicon oxide coating on a silicon channel sidewall  14   
     20  first gate dielectric layer 
     20   a  second gate dielectric layer 
     22  preliminary first gate dielectric regions or stratum 
     22   a  preliminary second gate dielectric regions or stratum 
     24  intermediary first gate dielectric regions or stratum 
     24   a  intermediary second gate dielectric regions or stratum 
     30  first gate conductor layer 
     30   a  second gate conductor layer 
     32  preliminary first gate conductor regions or stratum 
     32   a  preliminary second gate conductor regions or stratum 
     34  intermediary first gate conductor regions or stratum 
     34   a  intermediary second gate conductor regions or stratum 
     35  intermediary first exterior wall of the gate conductor region  34   
     35   a  intermediary second exterior wall of the gate conductor region  34   a    
     36  recessed exterior wall of first gate conductor region 
     36   a  recessed exterior wall of second gate conductor region 
     37  germanium oxide coatings on the first germanium gate conductor regions sidewall  36   
     37   a  germanium oxide coatings on the first germanium gate conductor regions sidewall  36   a    
     38  preliminary gate stack comprising gate dielectric  22  and gate conductor  32   
     39  the topmost surface of preliminary gate stack  38  and dielectric fill regions  52  (same as  17  above) 
     39   a  the topmost surface of gate stack  200  and dielectric fill regions  52   a  after CMP treatment 
     40  first photoresist layer 
     40   a  second photoresist layer 
     42  first photoresist undeveloped regions 
     42   a  second photoresist undeveloped regions 
     44  first photoresist developed regions 
     44   a  second photoresist developed regions 
     46   a  etched areas (voids) below second photoresist developed regions  44   a    
     50  first dielectric insulator fill 
     50   a  second dielectric insulator fill 
     52  preliminary first dielectric fill regions 
     52   a  preliminary second dielectric fill regions 
     54  final first dielectric fill regions 
     54   a  final second dielectric fill regions 
     60  second single-crystal silicon wafer 
     65  topmost surface of final gate stack  200  and dielectric stack  250   
     70  trimming mask comprising opaque regions  72   a  and  72   b  and transparent regions  74   
     72   a  opaque mask regions for forming dual gate regions 
     72   b  opaque mask regions for forming dielectric insulating regions 
     74  transparent mask regions for etching through underlayers 
     80  dielectric coating on exterior recessed walls of first/second gate conductor regions ( 36  and  36   a  respectively) 
     90  polysilicon fill regions 
     100  silicon dioxide insulator layer 
     102  intermediary silicon dioxide insulator regions 
     103  bottom surface of  102  silicon dioxide insulator regions 
     104  final silicon dioxide insulator regions 
     105  the upper surface of the silicon dioxide insulator layer  100   
     110  conformal dielectric coating of a first gate conductor region  32  and first thinned single-crystal silicon substrate  10   
     120  second dielectric insulator layer 
     122  intermediary second dielectric insulator regions 
     124  final second dielectric insulator regions 
     200  final gate stack or pillar comprising channel region  13 , first and second gate dielectric regions  24  and  24   a , and gate conductor regions  34  and  34   a    
     210  sidewall of gate stack  200  comprising gate conducting sidewalls  35  and  35   a  and channel sidewall  37   
     250  a composite stack or pillar comprising first and second dielectric fill regions  54  and  54   a    
     260  sidewall of composite first and second dielectric fill pillar  250   
     270  a final dual gate pillar comprising the recessed gate conductors  34  and  34   a  covered by an insulating layer  80   
     300  void areas between pillars  200  and  250   
     350  filled contact holes 
     380  landing pads 
     400  source/drain regions. 

   
       FIG. 1  is an enlarged cross-sectional representation of a portion of one embodiment of a dual gate logic device depicting an area containing to final gate pillars  200  separated by a dielectric fill insulator stack  250  as the device is processed through the steps of one embodiment of the present invention; and 
       FIG. 2  is an enlarged plat view and an cross-sectional segment thereof containing representations of a single gate element  200  flanked by polysilicon fill  90 , as it is processed through the steps of a second embodiment of the present invention. The gate element  200  further comprising land features  380  to assist in connecting the dual gate logic device to an external electrical power supply. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In order to fully understand the present invention each of the aspects of the processes, manufacturing intermediates, and products will be presented in detail with specific reference to the accompanying FIGURES. 
   Compositional elements of an Embodiment of the Present Invention 
   Substrate  10   
   The present invention uses standard substrates as typically used in the art of semiconductor manufacture. The most commonly used material to manufacture the substrate is single-crystal silicon wafer, wherein the silicon is optionally lightly doped. The conversion and purification of polysilicon to single-crystal silicon ingots and final formation of single-crystal silicon wafers is well documented in the art and will not be discussed further. A unique feature of the present invention is that the starting single-crystal substrate is converted during the manufacturing process into channel regions by etching through the upper or topmost surface  15  and lower surface  18  of the single crystal silicon wafer body  10 . 
   First Gate Dielectric or Gate Oxide  20   
   Gate dielectric materials for the present invention are selected from those used in the art. Examples of useful materials include silicon oxide, silicon nitride, aluminum oxide, hafnium oxide, tantalum oxide, titanium oxide or composites thereof. A most preferred gate dielectric material is silicon dioxide grown by standard methods known in the art. Other materials could be sputtered or deposited by chemical vapor deposition techniques, such as Jet Vapor Deposition, which is also known in the art. The thickness of the gate dielectric layer of the present invention is between about 10 and about 40 angstrom. 
   First Gate Conductor Layer  30   
   The preferred gate conductor for the present invention is germanium. It should be understood that although germanium can be used in its pure state, this invention also allows for the use of germanium-containing compositions. Such germanium-containing compositions include mixtures of germanium and silicon wherein the concentration of silicon can be as high as 50% by weight. The gate conductor can initially be deposited as a uniform layer and then selectively etched in a subtractive process or it can be deposited selectively onto exposed areas of the first gate dielectric in an additive process. Typically, germanium and its mixtures are applied by chemical vapor deposition or sputtering, as known in the art. A useful thickness of the germanium layer is between 0.01 and 1 micron. A preferred thickness for the germanium first layer is between 0.05 and 0.03, while a most preferred thickness for the first germanium layer is 0.05 and 0.3 microns. 
   First Dielectric Insulating Region  50   
   The present invention utilizes known insulating or fill materials as used in the art. These include the same materials as in the gate dielectric, namely silicon oxide, silicon nitride, aluminum oxide, hafnium oxide, tantalum oxide, titanium oxide, or composites thereof. These materials can be chosen independently of the gate dielectric  20  material. A preferred dielectric insulation material is silicon dioxide. A preferred process in the present invention utilizes a TEOS process (tetrakis ethoxy silane) for a thermal CVD process to deposit the oxide. 
   Conformal Coating  110   
   A coating of silicon nitride or silicon carbide is deposited on the exterior surfaces of the first intermediate germanium gate stack  38  and the exposed surfaces of the silicon wafer. The conformal layer  110  serves as an etch stop in the later processing steps of the inventive process and lateral portions may be optionally removed in a polishing step from the final germanium-containing dual gate article. 
   Channel Layer  11   
   The present invention utilizes a silicon channel layer separating the two conductor gates. The silicon channel layer may be formed from either a single-crystal silicon wafer bonded indirectly to gate conductor  30  or by the common practice of epitaxially growing silicon (otherwise known as epi-silicon) onto the top surface of a gate oxide region  20  of gate conductor layer  30 . Preferably, the silicon channel layer  10  in the present invention is provided by bonding a single-crystal silicon wafer to the conductor gate oxide regions  20 . In the present invention, the single-crystal wafer employed as the channel to separate the gates in the present invention can be the one used as the initial substrate  10 . This is accomplished by reversing the orientation of the in-process device (i.e., rotating the device such that the bottom single-crystal silicon substrate  10  now is the topmost layer). After being placed in this configuration it may be thinned to less than 0.1 micron and preferably thinned to between 0.03 and 0.1 micron. At this thickness there is optimum gate control to regulate the device, and also optimum device drive current. The process for applying, thinning/grinding and polishing this second wafer is discussed below in the processing section. One significant benefit for using a bonded single-crystal wafer compared to epi-silicon grown laterally over the insulator is the reduction in defects associated with epitaxially growing this channel layer  11 . 
   Second Gate Dielectric or Gate Oxide  20   a    
   The present invention utilizes the same materials from those discussed for the first gate dielectric  20  materials, but are independently selected. Likewise, the processes for application of the second gate dielectric layer are similar, but independently selected to those processes described for the first gate dielectric layer. 
   Second Gate Conductor  30   a    
   The present invention utilizes the same materials and processes, but are independently selected from those discussed for the first gate conductor  30  materials and processes. A useful thickness of the germanium layer is between 0.01 and 1 micron. A preferred thickness for the germanium first layer is between 0.05 and 0.03 micron, while a most preferred thickness for the first germanium layer is 0.05 and 0.3 microns. The thicknesses of the first and second germanium layers are independently selected. 
   Second Dielectric Insulating Region  50   a    
   The present invention utilizes the same materials, but are independently selected from those discussed for the first dielectric insulating  50  materials. 
   Photoresist Layer  40  and  40   a    
   Photoresists and the photolithographic process of using them are well known in the art of manufacturing semiconductor chips. Typically, the photoresist material is either positive or negative working and can be either in dry film or liquid form as applied to the intermediate manufacture article. As will be described in further detail the use of photoresist materials allows for the conversion of a portion or region of an underlying surface or layer to be modified in some fashion without modifying other portions or regions. This is accomplished by selectively removing regions of the photoresist thereby uncovering portions of the layers beneath it. These uncovered regions can then be modified by chemical or mechanical processes. Typically, chemical modification can be made either to the surface of the exposed layer or to the entire uncovered layer. For example, the uncovered surface can be modified by ion implantation or can be used as a growth site for additional layers such as using processes such as sputtering or chemical vapor deposition. Alternatively, the exposed region can be removed by etching to reduce thickness or even to completely remove the layer in the uncovered region thereby uncovering layers beneath it. 
   The process of selectively removing portions of the photoresist are well known in the art and typically are known as photolithographic means. This process usually entails exposing certain, predetermined areas of the photoresist using a mask to particle or electromagnetic radiation. The irradiation process induces a chemical change in the exposed photoresist portions such that a change in physical properties is obtained relative to the unexposed areas. The property of greatest significance is solubility in etching formulations. After irradiation the photoresist layer is subjected to an etchant that will discriminate between the exposed vs. unexposed areas. In the case of positive-working photoresists, areas that have been exposed become more soluble to the etchant formulation and are thereby removed. Negative-working photoresists produce regions that after exposure are less susceptible to etching than those in unexposed areas. In either case, the discriminating etching process is known in the art as “developing.” The present invention can use either positive- or negative-working photoresists. Positive-working wet photoresists are preferred in the present invention. 
   Trim Masks  70   
   Trim masks can be used in place of photoresist materials of the type discussed above especially for photoresist  40   a . In this case the mask is placed in physical contact with the surface of the intermediate and as described for the previous photoresists allows certain, predetermined areas to be treated. Treatment typically involves etching of the uncovered regions to a depth corresponding to the lower surface  103  of the first insulating region  102 . The first insulating region acting as an etch stop to prevent significant etching of the single-crystal silicon substrate  60 . The mask is then removed without the need for a developing step. Optionally, the trim mask can also assist in the process of selectively ion implanting dopants into the single crystal substrate adjacent to the gate electrode in order to form source and drain sites. 
   Intermediate and Final Product Designs 
   The present invention takes advantage of many of the processes and materials known in the art of semiconductor or integrated chip technology. However, the present invention differs from the prior art in that the final article is a dual gate semiconductor device wherein both final gate regions  34  and  34   a  comprise germanium-containing gate conductors and these gate conductors are separated by a silicon channel region  13 . Preferably, the silicon channel layer  10  and regions  13  are comprised of a single-crystal silicon wafer. Unique to this design is the requirement that as part of the process of making the final product the two gate conductors  34  and  34   a  are essentially perfectly aligned one on top of the other. In the art, a process that creates this configuration is referred to as a “self-aligning” process. Many examples of such processes are known in the art since the problem of alignment is pervasive in the industry. However, this invention employs a novel process for “self-alignment”, thereby producing novel “self-aligned” germanium-containing dual gate logic semiconductor devices. 
   It should be noted that prior art references to self-alignment processes do not necessarily relate to the particular process of the present invention. For example, the term “self-alignment” is oftentimes associated with the process of doping by ion implantation. However, in the case of the present invention the term self-alignment is meant to specifically refer to the vertical alignment of the top gate to the bottom gate in double-gate MOSFET stack. 
   In order to achieve the benefits of the present invention, certain design features are required. Specifically, the preliminary germanium-containing gate structures  32  and  32   a  must be positioned so that at least a portion of  32   a  overlaps at least a portion of  32 . Furthermore, the photoresist  40   a  or trim mask  70  must be prepared to create openings that are above both portions of the germanium-containing gate conductors  32  and  32   a  and also portions of the dielectric insulator regions  52  and  52   a . The photoresist  40   a  or trim mask  70  must also provide protection to two types of predetermined areas where no etching should occur. It is a requirement of this invention that the first type of covered areas  72   a  be smaller in size than the first and second germanium-containing gate conducting region  32  and  32   a  and completely reside exclusively over regions  32  and  32   a . The second type of covered areas  72   b  must reside completely over regions of dielectric insulating  52  and  52   a.    
   The side walls  210  of pillar  200  and the sidewalls  260  of dielectric composite regions  250 , created from the etching process, are essentially vertical. In this manner the pillars  200  will comprise sidewalls  210  that comprise germanium-containing gate conductor regions while the etched areas, also described as void areas  300 , will be bounded on one side by germanium-containing gate conductor pillar  200  while on the other side comprise the dielectric insulating composite region  250 . 
   The present invention further allows for the selective processing of the germanium-containing gate conductors without essentially altering the single-crystal channel that separates them. Specifically, the germanium-containing gate conductors, constituting portions of a self-aligned pillar  200  have a cross-section that initially is equal to the cross-section of the single-crystal silicon channel as is required by the vertical etching process described hereinbelow. After the vertical etching process the exposed sidewalls of the germanium-containing regions  34  and  34   a  in pillar  200  are preferentially etched compared to the single-crystal silicon channel in order to reduce their cross-section. The process is preferably performed by either isotropic etching or oxidation of the germanium-containing exposed surfaces generating new sidewall surfaces  36  and  36   a  respectively. Etching of a germanium-containing surface relative to a silicon surface can be accomplished with CF2Cl2, at 100 mtorr and power levels of about 500 to 1500 Watts. Under these conditions, the germanium etches 5-10 times faster than the silicon (see Materials Research Society Symposium Proceedings vol. 316, 1994, pages 1041 to 1046, Yue Kuo from IBM Research). Germanium can also be etched preferentially to silicon via an indirect process that proceeds by initially preferentially oxidizing germanium in the presence of silicon. This can be achieved either thermally or by a plasma process. Conditions for the plasma process are about 500° C, at about 0.5 to about 0.7 torr oxygen, and about 10-150 volts bias on the wafer. Under these conditions, oxidation times of 30 minutes provide about 800 Angstroms of oxidation. Thermal oxidation of germanium can be achieved at about 550° C. and about 0.5 to about 0.7 torr oxygen. Under these conditions about 630 Angstroms of germanium oxide can be grown after two hours. 
   The germanium oxide can be washed away by means of a water rinse at room or elevated temperatures. These oxidation and wash conditions are published in Semiconductor Science and Technology, vol. 8, September 1993 “Plasma Anodic Oxidation and Nitridation of Germanium Surface”, Sun Zhaoqi, Liu Chunrong, p1779-1782. 
   In the present invention, a typical etch recess distance for an exposed germanium-containing surface is about 0.01 to 1.0 micron. Most preferred etch depth is 0.03 micron. 
   The etched sidewall surfaces  36  and  36   a  of the first and second germanium-containing regions respectively are then encapsulated and thereby passivated in a dielectric coating material  80 . This dielectric coating layer preferably is composed of germanium nitride, germanium oxide, silicon dioxide, silicon nitride, hafnium oxide, aluminum oxide, tantalum oxide, or titanium oxide. Most preferably the dielectric coating material is germanium nitride. In a preferred process the germanium-containing layer is oxidized under low temperature using a nitrogen ion plasma to convert germanium oxide to germanium nitride or germanium oxynitride. 
   Void areas  300  subsequently have polysilicon or the like filling  90  between the pillars or regions comprising the encapsulated germanium-containing pillars  200  and pillars  250  comprising the first dielectric insulating fill layer, and the second dielectric insulating fill layer (generated during the vertical etch process). When polysilicon is used it is optionally doped by deposition of diborane, arsine, or phosphine. The deposition process can be performed by such processes as batch thermal chemical vapor deposition, plasma chemical vapor deposition, or plasma enhanced chemical vapor deposition, preferably at 300° C. to about 500° C. Doping can be performed either prior to or after deposition of the polysilicon fill in order to prepare source and drain sites. Doping with diborane, arsene, or phosphine will generate either P or N-type doped sites. 
   The polysilicon filled intermediate is further processed by reducing the height of the polysilicon fill  90  to less than the height of the pillars  200  and dielectric insulating composite regions  250  with the use of a plasma reactive ion etch (RIE) process. The RIE process preferably used in the present invention utilizes either a chlorine or fluorine based plasma, as is known in the industry. 
   After the polysilicon fill is recessed, the wiring necessary to electrically connect the source, drain, and gate is provided. 
   Embodiments if the Processing Steps for the Present Invention 
   One preferred embodiment of a process to manufacture a dual-logic device is depicted in FIG.  1  and includes the following steps:
         to a first outer surface  15  of a first single-crystal silicon wafer  10  is sequentially applied a uniform layer of a gate dielectric  20 , a uniform layer of a germanium-containing gate material  30 , a uniform layer of silicon dioxide  100  is formed, and a uniform layer of a photoresist material  40  is applied ( FIG. 1   a );   the photoresist layer  40  is then imaged and developed to generate developed regions  44  that expose regions of upper surface  105  of the silicon dioxide layer  100 , ( FIG. 1   b );   the openings  44  are then treated with etchant to completely remove the underlying regions of silicon dioxide  100 , germanium-containing gate conductor  30 , gate dielectric  20 , and partially etch the single-crystal silicon wafer  10 , to generate a new surface  16 , all lying beneath the openings  44 , ( FIG. 1   c );   removal of the photoresist then uncovers the upper surface  103  of the remaining silicon dioxide layer  102  covering the germanium-containing gate conductor regions  32 , and the remaining gate dielectric regions  22  thus forming gate stack  38 ;   a uniform, conformal layer of a dielectric coating material, silicon nitride or silicon carbide  110 , is applied to the uncovered regions of the single-crystal silicon wafer  16 , the topmost surface  103  and vertical sidewalls  35  of the pillar  38  comprising silicon dioxide region  102 , germanium-containing gate conductor regions  32  and the gate dielectric region  22 ;   an oxide fill  50  is coated over the silicon nitride  110  coated surfaces of the single-crystal silicon wafer  10  and the silicon dioxide  102  covered germanium-containing gate conductor regions or stratum  32  ( FIG. 1   d );   planarization is performed to uncover the topmost surface  103  of silicon dioxide coating on the germanium-containing gate conductor (the silicon nitride coating  110  has also been etched from the uppermost surface  103  of the silicon dioxide) and to create a uniform height for the germanium-containing gate conductor region  38  and the oxide fill region  52 , ( FIG. 1   e );   a second silicon wafer  60  having a layer of thermally grown silicon oxide of about 500 Angstroms (not shown), said thermally grown dioxide layer being bonded to the planarized surface  103 , ( FIG. 1   f );       

   For the purposes of clarity the configuration as described in the next steps are obtained by rotating the configuration of the in-process device in steps 1-8 by 180° (e.g., the topside has become the bottomside). This rotation is not necessarily performed in the actual manufacturing process but is utilized in the specification to continue the process of manufacture to the top-most side) ( FIG. 1   f ′).
         the first single-crystal silicon wafer  10  is ground to the depth of the now lower surface  17  ( 39 ) of the silicon nitride and polished in order to reduce the thickness of the single crystal silicon wafer  10  and thereby remove the silicon nitride  110  originally horizontally residing on the first single-crystal silicon wafer  10  and to further create at least one silicon channel  11  residing on the dielectric gate region  22  which separates the channel from the germanium-containing gate conductor region  32 , said channel having a thickness of about 300 to about 1000 Angstroms ( FIG. 1   g );   sequentially coating uniform layers of a second gate dielectric material  20   a , a second germanium-containing gate conductor  30   a , silicon nitride  120 , and photoresist  40   a  over both the channel  11  and the dielectric oxide fill (or dielectric insulation fill)  52  regions, ( FIG. 1   h );   the photoresist  40   a  is imaged and developed to create covered regions  42   a  and openings  44   a , the covered regions must at least overlap a portion of the first germanium-containing gate conductor regions  32  and a portion of the adjacent dielectric fill region  52 ; in the areas where the photoresist openings  44   a  are located, portions of the second dielectric insulating layer  120 , the second germanium-containing gate conductor layer  30   a , and the second gate dielectric layer  20   a  are all etched to uncover portions of the first dielectric fill  52  or first germanium-containing regions  32  to create unetched regions or stacks comprising dielectric insulating regions  122 , germanium-containing regions  32   a , and gate dielectric regions  22   a  and also creating void regions  46   a  ( FIGS. 1   i  and  1   j );   the remaining photoresist areas  42   a  are removed and an oxide fill  50   a  is coated over the topmost and outer surfaces of  122 , outer surfaces  35   a  of the second germanium-containing gate conductor regions  32   a  and outer surfaces of the gate dielectric region  22   a  and into the etched areas  46   a  formed during the preceding step, ( FIG. 1   k );   planarization is performed by standard chemical mechanical processing to uncover the silicon nitride coating  122  on the germanium-containing gate conductor  32   a  and to create a uniform height for the silicon nitride coated germanium-containing gate conductor region  32   a  and the oxide fill region  52   a , ( FIG. 1   l );   a trim mask or photoresist  70  is printed and applied to the planarized surface to create openings that are above and encompass both the germanium-containing gate conductors  32  and  32   a  and also the dielectric insulator layers  52  and  52   a . The photoresist or trim mask  70  must also provide coverage to two types of predetermined areas where no vertical etching occurs. It is a requirement of this invention that the first type of covered areas  72   a  be smaller in size than the first and second germanium-containing gate conducting region  32  and  32   a  and exclusively reside over regions  32  and  32   a  (i.e., no regions of  52  or  52   a  reside under  72   a. ) The footprint area of  72   a  must therefore be no larger than the smaller of the footprint areas of region  32  or  32   a . The second type of covered areas  72   b  must reside completely over regions of dielectric insulating laminate comprising  52  and  52   a , (i.e., no regions of  32  or  32   a  reside under  72   b  ( FIG. 1   m );   vertical etching (i.e., trimming) is performed to create openings  300  and form gate-stack pillars  200  that are composed of germanium-containing gate conductors and which have sidewalls  210 , these pillars are separated from regions of dielectric fill  250  by a distance along the silicon wafer  60 ; the pillars are disposed on the second silicon substrate  60 , and composed sequentially from that substrate starting with a silicon oxide region  104 , a first germanium-containing gate conductor  34 , a first gate dielectric region  24 , a silicon channel  13 , a second gate dielectric region  24   a , a second germanium-containing gate conductor region  34   a , and a silicon nitride top coat  124 , in order to align the prior lower gate stack,  32  and  22 , with the upper gate stack,  32   a  and  22   a  ( FIG. 1   n );   the germanium-containing gate conductors sidewalls  35  and  35   a  are recessed using either wet or dry isotropic etch, to a depth of about 200 Angstrom, to form  36  and  36   a  respectively, leaving the remaining sidewalls  210  comprising silicon nitride topcoat  124 , the silicon channel  13 , the first and second gate dielectric regions  24  and  24   a , and the silicon dioxide regions  104  unaffected, ( FIG. 1   o );   the sidewalls  36  and  36   a  of the recessed germanium-containing gate conductor stacks  200  are initially oxidized and then converted to a germanium nitride encasing layer  80 , ( FIG. 1   p );   the void regions  300  separating the recessed germanium-containing gate conductor stacks or pillars  200  from the oxide fill regions  54  and  54   a  are filled with polysilicon  90  N+ doped at about 10E19 to about 10E21 atoms/cm2 or As or P doped at optimally 10E20, to provide source/drain contacts to the channel (the dopant in the polysilicon diffuses into the single-crystal silicon channel thereby forming the source/drain extensions for the double gate device);   the height of the polysilicon fill areas is reduced below the height  65  of recessed germanium-containing gate conductor stacks  200  and the oxide fill regions  54  and  54   a  and then the polysilicon regions (source and drain) are electrically connected to the single crystal silicon channel regions ( FIG. 1   q ). In this process the dopant from the polysilicon is diffused into the single crystal silicon to form source/drain extensions for the double gate FET device.       

   The embodiments described hereinabove require electrical connection to an external power supply in order to function properly. An example of a useful method for forming electrical connections between the dual germanium gate regions and the source/drain regions includes the construction of a landing pad  300 . As can be seen in  FIG. 2   a , dual gate element  270  is flanked on both sides by polysilicon regions  90 . During the time of manufacture of these elements a landing pad  380  is constructed of the same components as the dual gate element  270  and is itself an integral element located at the end of the dual gate element  270  and having a width that includes the dual gate element  270  and the two flanking polysilicon fill regions  90 . A typical construct is shown in  FIG. 2   a  comprising a top-down view and a cross-sectional representation. The orientation of the top-down and cross-sectional representation views are orthogonal to the views presented in FIG.  1 . 
   Referring to  FIGS. 2   a  and  2   b , the process of making the electrical connections begins with coating a photoresist  340  over the top surface of an intermediary dual gate device  270  and imaging and developing the photoresist creating an opening  350  essentially having a predetermined cross-section. The opening  350  is over a portion of the dual gate pillar  200  referred to as the landing pad  380  and another portion of the opening is over a section  320  of the silicon dioxide fill region  54 . Etching by RIE through the opening  350  is performed to the topmost germanium gate  36   a , the topmost gate dielectric region  24   a , the silicon channel  13 , the lower gate dielectric region  24  and partially into the lower germanium gate  34 . In this manner the sides of the upper germanium gate  34   a , the silicon channel  13 , and the landing pad  380  are exposed ( FIG. 2   b ). 
   The undeveloped photoresist is stripped from the top surface and all exposed surfaces are subjected to oxidizing conditions that typically include an oxygen plasma at elevated temperatures. Useful temperatures range from 500° C. to 700° C., optimally 600° C. Under these conditions the exposed surfaces of the silicon channel  13  and both germanium gates  34  and  34   a  are converted into their respective oxides. The depth of the thus formed germanium oxide coatings  37  and  37   a  are typically about 50 Angstrom while the depth of the thus formed silicon dioxide coating  19  is typically about 20 Angstroms. ( FIG. 2   c ). 
   Removal of the germanium oxide coatings  37  and  37   a , without simultaneous removal of the silicon oxide coating  19  is achieved with a water rinse at ambient or elevated temperature. In this manner the silicon channel  13  is protected with an insulating layer  19  thereby electrically isolating it from the subsequent steps described below; ( FIG. 2   d ). 
   By conventional chemical vapor deposition (CVD), doped polysilicon  90  is then added to the contact hole or void  300  and the upper surface is planarized by chemical mechanical polishing (CMP) ( FIG. 2   e ). 
   The contact and wiring scheme is then completed by depositing dielectric, such as silicon dioxide, over the planarized wafer surface, and then patterning the silicon dioxide layer with a photoresist (not shown) and etching the resist pattern into the dielectric layer  55   a . The etched pattern  55   a  is designed to create a minimum of three via openings; the first via is positioned approximately over the filled contact hole  350 , and the second and third via are positioned partially over each side of the polysilicon fill  90  which flanks the dual germanium gate pillar  200  and partially over the surrounding silicon dioxide insulator  54 . The at least three via are filled with a conductive material such as tungsten or copper, using conventional techniques to complete the electrical connection between source/drain sites  400  and the landing pad  380 . Typically, conductive wiring is attached to the topmost surface of each via to electrically connect the dual gate device to an external power source. 
   Although two preferred embodiments are described in detail it can readily be understood that there exists obvious permutations that would still fall within the spirit of the invention. By way of example, it is possible to apply the various materials such as the insulator layer  102 , the gate conductor region  32 , and the gate dielectric region  22  (each one atop the other) on discrete areas of the substrate, while depositing an insulating region on the remaining discrete surfaces of the substrate. In this manner gate stacks can be formed directly. Alternatively, all gate stack materials can initially be applied as uniform coating on the substrate. In this case etching in specific areas would then be required to form the final gate stacks, followed by deposition of a insulator fill into the etched areas. In another embodiment, the silicon dioxide fill is initially applied, and after etching to create openings, the gate stack is formed in the etched areas. After application of the silicon channel, the same embodiments as described to form the first gate stack can be independently selected to form the second gate stack. A proviso to these alternative processes is that there must be an overlap of regions of the first and second gate stacks. 
   In another embodiment, it is desired to use a silicon wafer as the channel that is other than the original silicon substrate. In this process the original substrate remains in its initial configuration (i.e., the dual gate element resides on the upper surface of the original silicon substrate). A second silicon wafer is then used as the channel layer. It is preferred that this second wafer be composed of single-crystal silicon. 
   It is also specifically contemplated that the order of the manufacturing steps may be varied and still generate a final product that performs essentially the same function as the present invention. All these types of permutations are considered within the scope of the invention. 
   Other embodiments and modifications of the present invention may occur to those of ordinary skill in the art subsequent to a review of the present application and the information presented herein; these embodiments and modifications, as well as their equivalents, are also included within the scope of this invention.

Technology Classification (CPC): 7