Abstract:
The present invention relates to a manufacturing method for a recessed channel array transistor and a corresponding recessed channel array transistor. In one embodiment, the present invention uses a self-adjusting spacer on the substrate surface to provide the required distance between the gate and the source/drain regions. Thus, the requirements regarding the tolerances of the lithography in the gate contact plane are diminished.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention relates to a manufacturing method for a recessed channel array transistor and a corresponding recessed channel array transistor.  
       BACKGROUND OF THE INVENTION  
       [0002]     Although in principle applicable to arbitrary integrated circuits, the following invention and the underlying problems will be explained with respect to integrated memory circuits in silicon technology.  
         [0003]     U.S. Patent Publication 2005/0042833 A1, discloses a method of manufacturing an integrated circuit device including a recessed channel transistor. The method includes the steps of defining an active region by forming a trench device isolation region on an integrated circuit substrate, forming a mask pattern on the integrated substrate that exposes a channel sub-region of the active region and the trench device isolation region adjacent to the channel sub-region, etching the trench device isolation region which is exposed by the mask pattern to be recessed to a first depth using the mask pattern as an etch mask, etching the channel sub-region to form a gate trench having a second depth that is deeper than the first depth using the mask pattern as an etch mask, and forming a recessed gate that fills the gate trench.  
         [0004]      FIG. 15  shows schematic planar view of the geometric arrangement of a recessed channel array transistor as an example of the problems underlying the present invention.  
         [0005]     In  FIG. 15 a  schematic planar view of the active region RT and the isolation region of a recessed channel array transistor is shown. Two cross-sections of the planar view of  FIG. 15  are denoted A-A′ and B-B′, respectively.  
         [0006]      FIGS. 15A, 7B  show two different schematic cross-sections along lines A-A′ and B-B′ of  FIG. 15 , respectively, of a manufacturing method for a recessed channel array transistor and a corresponding recessed channel array transistor as an example of the problems underlying the present invention.  
         [0007]      FIG. 15A  shows a cross-section in parallel to the current flow direction, whereas  FIG. 15B  shows a cross-section perpendicular to the current flow direction.  
         [0008]     In  FIG. 15A , reference sign  1  denotes the silicon semiconductor substrate. Provided in the silicon semiconductor substrate  1  are isolation trenches IT filled with silicon oxide. In the middle of the transistor cell there is a trench  5  in the flow direction filled with a gate electrode  30  made of polysilicon. Not shown on the trench wall is a gate dielectric  20  made of silicon dioxide. Source and drain regions  40 ,  50  are provided in the surface area on both sides of the trench  5 . Moreover, reference sign  60  denotes a gate electrode contact made of tungsten, and  70  denotes a nitride spacer on both sides of the gate electrode  30  and the gate electrode contact  60 .  
         [0009]     Problems in such recessed channel array transistors are caused by the overlap of the vertical gate  30  with the highly doped source/drain regions  40 ,  50 . This overlap causes high electrical fields that generate leakage currents in the turned-off state of the transistor. Provided that the planar gate and thus the spacer  70  can be aligned sufficient above the recessed channel device, it can prevent high doping concentrations directly at the gate edge when used as source/drain implant mask. The scalability of the recessed channel array transistor is therefore limited by the alignment of the planar gate.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides an improved manufacturing method for a recessed channel array transistor and corresponding recessed channel array transistor which provide excellent scalability.  
         [0011]     In one embodiment, the present invention uses a self-adjusting spacer on the substrate surface to provide the required distance between the gate and the source/drain regions. Thus, the requirements regarding the tolerances of the lithography in the gate contact plane are diminished.  
         [0012]     According to a preferred embodiment, the steps of depositing and structuring a gate contact layer and an isolation layer above said gate electrode and the spacers are performed.  
         [0013]     According to another preferred embodiment, the step of forming second isolating spacers on the structured gate contact layer and isolation layer above the gate electrode and said spacers is performed.  
         [0014]     According to another preferred embodiment, the step of introducing impurities into the substrate in the forming area for providing lightly doped source/drain regions after the step of providing said gate electrode and before the step of forming the spacers is performed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention is described below in detail with reference to the drawings and exemplary embodiments.  
         [0016]     In the Figures:  
         [0017]      FIG. 1  shows a schematic planar view of the geometric arrangement of a recessed channel array transistor as a first embodiment of the present invention.  
         [0018]     FIGS.  1 A,B- 7 A,B show two different schematic cross-sections along lines A-A′ and B-B′, respectively, of  FIG. 1  of a manufacturing method for a recessed channel array transistor and a corresponding recessed channel array transistor as the first embodiment of the present invention.  
         [0019]      FIG. 8  shows a schematic planar view of the geometric arrangement of a recessed channel array transistor as a second embodiment of the present invention.  
         [0020]     FIGS.  8 A,B- 14 A,B show two different schematic cross-sections along lines A-A′ and B-B′, respectively, of  FIG. 1  of a manufacturing method for a recessed channel array transistor and a corresponding recessed channel array transistor as the second embodiment of the present invention.  
         [0021]      FIG. 15  shows a schematic planar view of the geometric arrangement of a recessed channel array transistor as an example of the problems underlying the present invention.  
         [0022]      FIGS. 15A,15B  show two different schematic cross-sections along lines A-A′ and B-B′ of  FIG. 15 , respectively, of a manufacturing method for a recessed channel array transistor and a corresponding recessed channel array transistor as an example of the problems underlying the present invention.  
         [0023]     In the Figures, identical reference signs denote equivalent or functionally equivalent components. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]      FIG. 1  shows a schematic planar view of the geometric arrangement of a recessed channel array transistor as a first embodiment of the present invention, and FIGS.  1 A,B show two different schematic cross-sections along lines A-A′and B-B′, respectively, of  FIG. 1 .  
         [0025]     In  FIG. 1 , reference sign  1  denotes a semiconductor substrate having a nitride sacrificial layer  3  on its surface and having isolation trenches IT′ adjoining a forming area RT for said recessed channel array transistor, said isolation trenches IT′ being filled with SiO 2  as an isolation material which extends to a same upper surface as said substrate  1 . In particular, such an arrangement may be obtained by a CMP (chemical mechanical polishing) process.  
         [0026]     Thereafter, a sacrificial layer opening  3   a  is formed in said sacrificial layer  3  which extends in the direction B-B′ and exposes said substrate  1  in the middle part of said forming area RT. The opening  3   a  defines the location of a trench  5  to be etched in the substrate  1  in the following step.  
         [0027]     FIGS.  2 A,B to  7 A,B show the two different schematic cross-sections along lines A-A′ and B-B′, respectively, of  FIG. 1  of a manufacturing method for a recessed channel array transistor and a corresponding recessed channel array transistor as the first embodiment of the present invention which starts from FIGS.  1 A,B.  
         [0028]     In the following process step which is depicted in  FIGS. 2A, 2B , the trench  5  of the recessed channel array transistor is provided by a dry etch process. Reference sign U denotes the bottom of the trench  5 . The dry etch process is a selective etching process which etches silicon with high selectivity with respect to the sacrificial layer  3  which in this step acts as a hard mask.  
         [0029]     In the next process step which is depicted in  FIGS. 3A, 3B , a wet etch is performed for removing a part of the silicon oxide of the isolation trenches IT′ adjacent to the trench  5  in the direction B-B′ as may be clearly obtained from  FIG. 3B . This wet etch step etches silicon oxide with high selectivity with respect to the silicon of the silicon substrate  1 . In this wet etch step, the trench  5  is broadened in the B-B′ direction and underetching areas  5   a  are formed along the B-B′ direction which underetching areas  5   a  are located below the bottom U of the trench  5  and which are adjacent to the trench  5 . By providing these under-etching areas  5   a , the control of the gate over the channel region is improved by the tri-gate arrangement, because the gate can be extended to below the corners at the bottom U.  
         [0030]     Subsequently, as shown in  FIGS. 4A, 4B , a gate dielectric  20  of silicon dioxide is formed along the substrate  1  in the trench  5 . Then, the trench  5  and the adjacent under-etched areas  5   a  in the isolation trenches IT are filled with the gate electrode  30 ′ made of polysilicon, preferably by a deposition and a following CMP process step. The gate electrode  30 ′ made of polysilicon then extends to the surface of the sacrificial layer  3 .  
         [0031]     With regard to  FIGS. 5A, 5B , the sacrificial layer layer  3  of silicon nitride is then removed in a selective etching step. Moreover, a first implantation I 1  is performed in a self-aligned manner in order to provide a lightly doped drain/source region  4  on both sides of the trench  5  as may be obtained from  FIG. 4A .  
         [0032]     In a following process step which is depicted in  FIGS. 6A, 6B , after a subsequent nitride deposition, spacers  70 ′ are formed adjacent to the polysilicon gate electrode  30 ′ which spacers extend along the A-A′ and B-B′ direction. These self-adjusting spacers  70 ′ made of silicon nitride prevent adverse electric field effects in the turned-off state of the recessed channel array transistor and provide the possibility of alignment tolerances in a later gate contact forming step.  
         [0033]     According to  FIGS. 7A, 7B , a gate contact layer  60 ′ and a cap nitride layer  80  are deposited and structured over the gate electrode  30 ′ and the self-adjusted nitride spacer  70 ′. This process step is not sensitive against slight misalignments of the sacrificial layer for structuring the layers  60 ′,  80 . In a next process step, second silicon nitride spacers  90  are formed on the sides of the layer  60 ′,  80  and on the first spacers  70 ′.  
         [0034]     When combining the deposition of said gate contact layer  60 ′ with the formation of the gate contact layer of planar support devices in memory applications, a gate dielectric which is formed before the deposition of said gate contact layer has to be removed from the gate electrode  30  to ensure electrical contact between gate contact layer  60 ′ and the gate electrode  30 .  
         [0035]     Finally, a second implantation I 2  is performed for providing source/drain regions  4 ′. Also, this implantation I 2  is self-adjusted by the isolation trenches IT and the spacers  70 ′. Due to the presence of the lightly doped source/drain areas  4 , it can be assured that the source/drain regions  4 ′ are suitably connected to the channel region along the trench  5  periphery. In order to expand the implanted source/drain areas  4 ′, it is possible to perform an additional thermal diffusion step. Thus, by the method of this first embodiment of the present invention, a recessed channel area transistor may be formed which has excellent scaling characteristics.  
         [0036]      FIG. 8  shows a schematic planar view of the geometric arrangement of a recessed channel array transistor as a second embodiment of the present invention, and FIGS.  8 A,B show two different schematic cross-sections along lines A-A′ and B-B′, respectively, of  FIG. 8 .  
         [0037]     In contrast to the above described first embodiment, here a sacrificial layer opening  3   a  is formed in said sacrificial layer  3  which extends in the direction B-B′ and exposes not only said substrate  1  in the middle part of said forming area RT, but also the adjoining isolation trenches IT′ in this direction. The opening  3   a  defines the location of a trench  5  to be etched in the substrate  1  and in the isolation trenches IT′ in the following step.  
         [0038]     FIGS.  9 A,B to  15 A,B show the two different schematic cross-sections along lines A-A′ and B-B′, respectively, of  FIG. 8  of a manufacturing method for a recessed channel array transistor and a corresponding recessed channel array transistor as the first embodiment of the present invention which starts from FIGS.  8 A,B.  
         [0039]     In the following process step which is depicted in  FIGS. 9A, 9B , the trench  5  of the recessed channel array transistor is provided by a dry etch process. Reference sign U denotes the bottom of the trench  5 . The dry etch process is a selective etching process which etches silicon and silicon oxide with high selectivity with respect to the sacrificial layer  3  which in this step acts as a hard mask. As may be seen from  FIG. 9B , the isolation trenches IT′ in the B-B′ direction are etched down to the same level as the bottom U of the trench.  
         [0040]     In the next process step which is depicted in  FIGS. 10A, 10B , a wet etch is performed for removing a part of the silicon oxide of the isolation trenches IT′ adjacent to the trench  5  in the direction B-B′ as may be clearly obtained from  FIG. 10B . This wet etch step etches silicon oxide with high selectivity with respect to the silicon of the silicon substrate  1 . In this wet etch step, underetching areas  5   a ′ are formed along the B-B′ direction which underetching areas  5   a ′ are located below the bottom U of the trench  5  and which are adjacent to the trench  5 . By providing these under-etching areas  5   a , the control of the gate over the channel region is improved by the tri-gate arrangement, because the gate can be extended to below the corners at the bottom U.  
         [0041]     Subsequently, as shown in  FIGS. 11A, 11B , a gate dielectric  20  of silicon dioxide is formed along the substrate  1  in the trench  5 . Then, the trench  5  and the adjacent under-etched areas  5   a ′ in the isolation trenches IT′ are filled with the gate electrode  30 ′ made of polysilicon, preferably by a deposition and a following CMP process step. The gate electrode  30 ′ made of polysilicon then extends to the surface of the sacrificial layer  3 .  
         [0042]     With regard to  FIGS. 12A, 12B , the sacrificial layer layer  3  of silicon nitride is then removed in a selective etching step. Moreover, a first implantation I 1  is performed in a self-aligned manner in order to provide a lightly doped drain/source region  4  on both sides of the trench  5  as may be obtained from  FIG. 12A .  
         [0043]     In a following process step which is depicted in  FIGS. 13A, 13B , after a subsequent nitride deposition, spacers  70 ′ are formed adjacent to the polysilicon gate electrode  30 ′ which spacers extend along the B-B′ direction. These self-adjusting spacers  70 ′ made of silicon nitride prevent adverse electric field effects in the turned-off state of the recessed channel array transistor and provide the possibility of alignment tolerances in a later gate contact forming step.  
         [0044]     According to  FIGS. 15A, 15B , a gate contact layer  60 ′ and a cap nitride layer  80  are deposited and structured over the gate electrode  30 ′ and the self-adjusted nitride spacer  70 ′. This process step is not sensitive against slight misalignments of the sacrificial layer for structuring the layers  60 ′,  80 . In a next process step, second silicon nitride spacers  90  are formed on the sides of the layer  60 ′,  80  and on the first spacers  70 ′. When combining the deposition of said gate contact layer  60 ′ with the formation of the gate contact layer of planar support devices in memory applications, a gate dielectric which is formed before the deposition of said gate contact layer has to be removed from the gate electrode  30  to ensure electrical contact between gate contact layer  60 ′ and the gate electrode  30 .  
         [0045]     Finally, a second implantation I 2  is performed for providing source/drain regions  4 ′. Also, this implantation I 2  is self-adjusted by the isolation trenches IT and the spacers  70 ′. Due to the presence of the lightly doped source/drain areas  4 , it can be assured that the source/drain regions  4 ′ are suitably connected to the channel region along the trench  5  periphery. In order to expand the implanted source/drain areas  4 ′, it is possible to perform an additional thermal diffusion step.  
         [0046]     Thus, by the method of this second embodiment of the present invention, also a recessed channel area transistor may be formed which has excellent scaling characteristics.  
         [0047]     Although the present invention has been described with respect to a preferred embodiment, it is not limited thereto, but can be modified in various manners which are obvious for the person skilled in the art.  
         [0048]     Particularly, the selection of the materials is only an example and can be varied variously.  
       LIST OF REFERENCE SIGNS  
       [0000]    
       
          RT forming area recessed channel array transistor  
           1  silicon semiconductor substrate  
          IT, IT′, IT″ isolation trenches  
           20  gate dielectric  
           30 , 30 ′ gate electrode  
           60  gate contact  
           40 , 50  source, drain  
           70  nitride spacer  
           5  trench  
          U bottom of trench  5   
           5   a ,  5   a ′ underetching area  
           4  lightly doped drain/source area (LDD)  
          I 1  LDD implantation  
           70 ′ nitride spacer  
           60 ′ gate contact  
           80  nitride cap  
           90  nitride spacer  
          I 2  S/D implantation  
           3  nitride sacrificial layer  
           3   a  sacrificial layer opening