Abstract:
A CMOS device formed with a Silicon On Insulator (SOI) technology with reduced Drain Induced Barrier Lowering (DIBL) characteristics and a method for producing the same. The method involves a high energy, high dose implant of boron and phosphorus through the p- and n-wells, into the insulator layer, thereby creating a borophosphosilicate glass (BPSG) structure within the insulation layer underlying the p- and n-wells of the SOI wafer. Backend high temperature processing steps induce diffusion of the boron and phosphorus contained in the BPSG into the p- and n-wells, thereby forming a retrograde dopant profile in the wells. The retrograde dopant profile reduces DIBL and also provides recombination centers adjacent the insulator layer and the active layer to thereby reduce floating body effects for the CMOS device.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a divisional application of application number 09/652,864 filed Aug. 31, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to the field of semiconductor devices and fabrication processes and, in particular, to CMOS devices formed in a silicon-on-insulator (SOI) technology with reduced drain induced barrier lowering (DIBL) and a method for fabricating the same.  
           [0004]    2. Description of the Related Art  
           [0005]    There is an ever-present desire in the semiconductor fabrication industry to achieve individual devices with smaller physical dimensions. Reducing the dimensions of devices is referred to as scaling. Scaling is desirable in order to increase the number of individual devices that can be placed on a given area of semiconductor material and to increase the process yield and to reduce the unit cost of the devices and the power consumption of the devices. In addition, scaling can result in performance increases of the individual devices as the charge carriers with a finite velocity have a shorter distance to travel and less bulk material has to accumulate or dissipate charges. Thus, the trend in the industry is towards thinner device regions and gate oxides, shorter channels, and lower power consumption.  
           [0006]    However, scaling often creates some performance drawbacks. In particular, a known category of performance limitations known as short channel effects arise as the length of the channel of CMOS devices is reduced by scaling. One particular short-channel effect in CMOS devices, known as Drain Induced Barrier Lowering (DIBL) is mainly responsible for the degradation of sub-threshold swing in deep submicron devices. DIBL is a reduction in the potential barrier between the drain and source as the channel length shortens as illustrated in FIG. 1 reflecting known prior art. When the drain voltage is increased, the depletion region around the drain increases and the drain region electric field reduces the channel potential barrier which results in an increased off-state current between the source and drain.  
           [0007]    In conventional CMOS devices, a retrograde channel dopant profile can be effectively used to control DIBL. In a CMOS process, n-type and p-type wells are created for NMOS and PMOS devices. In a typical diffusion process, dopant concentration profiles in these n- and p-type wells are at a peak near the surfaces and decrease in the depth direction into the bulk as illustrated in FIG. 2. A retrograde profile is one in which the peak of the dopant concentration profile is not at the surface but at some distance into the bulk as shown in FIG. 3. Such retrograde profiles are helpful in deep submicron CMOS devices since they reduce the lowering of the source/drain barrier when the drain is biased high and when the channel is in weak inversion. This limits the amount of subthreshold leakage current flowing into the drain. A lower level of subthreshold leakage current provides improved circuit reliability and reduced power consumption.  
           [0008]    A retrograde dopant profile also typically results in a lower dopant concentration near the surface of the wafer which reduces junction capacitances. Reduced junction capacitances allow the device to switch faster and thus increase circuit speed. Typically, retrograde profile dopant implants are done after formation of the gate. A halo (or pocket) implant is another known method used in deep submicron CMOS devices to reduce DIBL.  
           [0009]    However in some applications, such as in an SOI process, it is difficult to create a retrograde profile due to the thinness of the silicon layer and the tendency of the dopants to diffuse. A SOI process has a buried insulating layer, typically of silicon dioxide. State-of-the-art SOI devices have a very thin silicon (Si) film (typically &lt;1600Å) overlying the oxide in which the active devices are formed. Increasing the Si film thickness any further will increase the extent to which the devices formed therein get partially depleted. SOI devices also suffer from ‘floating body’ effects since, unlike conventional CMOS, in SOI there is no known easy way to form a contact to the bulk in order to remove the bulk charges.  
           [0010]    When the as-implanted retrograde dopant profiles diffuse during subsequent heat cycles in a process, they spread out and lose their ‘retrograde’ nature to some extent. In SOI, since the silicon film is very thin, creating a true retrograde dopant profile is very difficult. This is true even while using higher atomic mass elements like Indium (In) for NMOS and Antimony (Sb) as channel dopants. Diffusivity of these dopants in silicon is known to be comparable to lower atomic mass elements like boron (B) and phosphorus (P), when the silicon film is very thin, as in an SOI technology. Moreover, leakage current levels are known to increase when Indium is used for channel dopants (See “Impact of Channel Doping and Ar Implant on Device Characteristics of Partially Depleted SOI MOSFETs”, Xu et al., pp. 115 and 116 of the Proceedings 1998 IEEE International SOI Conference, October,  1998  and “Dopant Redistribution in SOI during RTA: A Study on Doping in Scaled-down Si Layers”, Park et al. IEDM 1999 pp. 337-340, included herein by reference).  
           [0011]    From the foregoing it can be appreciated that there is an ongoing need for a method of fabricating deep submicron SOI CMOS devices while minimizing short channel effects such as DIBL. There is a further need for minimizing DIBL in deep submicron CMOS devices without incurring significant additional processing steps and high temperature processing.  
         SUMMARY OF THE INVENTION  
         [0012]    The aforementioned needs are satisfied by the SOI CMOS device with reduced DIBL of the present invention. In one aspect, the invention comprises a semiconductor transistor device comprising: a semiconductive substrate; an insulative layer buried within the semiconductive substrate; an active layer of semiconductive material above the insulative layer; a plurality of doped device regions in the active layer; a gate structure formed on the device regions; source and drain regions formed in the device regions such that the doping type for the source and drain is complementary to the doping type of the corresponding device region; dopant diffusion sources placed within the buried insulator layer underlying the device regions wherein the dopant diffusion sources diffuse into the device regions so as to create a retrograde dopant profile in the device regions; a plurality of conductive layers electrically interconnecting the transistor devices; and a passivation layer overlying the conductive layers. In one embodiment, the semiconductive substrate, insulative layer buried within the semiconductive substrate, and the active layer of semiconductive material above the insulative layer comprise a SOI Separation by IMplanted OXygen (SIMOX) wafer.  
           [0013]    Another aspect of the invention comprises dopant atoms implanted through the device regions such that the dopant atoms come to reside within the Buried OXide (BOX) layer underlying the device regions creating a borophosphosilicate glass (BPSG) within the BOX layer. Formation of the passivation layer causes the dopant atoms contained within the BPSG to diffuse into the device regions so as to create the retrograde dopant profile in the device regions. The retrograde dopant profile has a peak concentration substantially adjacent the interface of the BOX and the active region. The retrograde dopant profile in the device region provides the transistor device with improved resistance to drain-induced barrier lowering (DIBL) and also provides the transistor device with recombination centers to reduce floating body effects.  
           [0014]    In another aspect, the invention comprises a method for creating semiconductor transistor devices comprising the steps of: providing a semiconductor substrate; forming a buried insulation layer in the semiconductor substrate; forming an active layer above the buried insulation layer by placing additional semiconductor material on the buried insulation layer; doping the active layer with dopant atoms so as to form device regions; implanting additional dopant atoms through the device regions such that the additional dopant atoms come to reside within the buried insulation layer underlying the device regions; implanting dopant atoms into gate regions of the device regions; forming a gate stack on the active layer adjacent the gate regions; implanting dopant atoms into the device regions such that the dopant atoms come to reside within the device regions adjacent the gate regions so as to form source and drain regions and wherein the gate stack substantially inhibits penetration of the dopant atoms into the gate regions of the device regions; forming conductive paths that electrically connect to the source, drain, and gate regions; and forming a passivating layer overlying the conductive paths. The method of the invention also includes implanting dopant atoms through the device regions wherein the dopant atoms come to reside within the BOX layer underlying the device regions thereby creating a borophosphosilicate glass (BPSG) within the BOX layer.  
           [0015]    In another aspect of the invention, formation of the passivation layer induces the dopant atoms contained within the BPSG to outdiffuse into the device regions thereby forming a retrograde dopant profile within the device regions. The retrograde dopant profile within the device regions reduces DIBL effects for the CMOS device and also provides recombination centers adjacent the BOX active region interface thereby reducing floating body effects. These and other objects and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    FIG. I is a graph illustrating prior art concerning DIBL as the relation of threshold voltage (V T ) to drain-source voltage (V DS ) for various sub-micron channel lengths;  
         [0017]    [0017]FIG. 2 is a graph illustrating prior art of a typical diffusion based dopant profile in CMOS devices;  
         [0018]    [0018]FIG. 3 is a graph illustrating prior art of a retrograde dopant profile in CMOS devices;  
         [0019]    [0019]FIG. 4 is a section view of the starting material of the SOI CMOS with reduced DIBL, a SIMOX wafer;  
         [0020]    [0020]FIG. 5 is a section view of the SIMOX wafer with n- and p-type wells formed therein and a high dose, high energy implant, into the buried oxide (BOX) forming a borophosphosilicate glass (BPSG) structure;  
         [0021]    [0021]FIG. 6 is a section view of the SIMOX wafer with gate stacks formed on the n-and p-wells with source and drain implants;  
         [0022]    [0022]FIG. 7 is a section view of the SOI CMOS devices with conductive and passivation layers in place with the dopants entrained within the BPSG outdiffuse into the n- and p-wells thereby forming a retrograde dopant profile within the wells that reduces DIBL;  
         [0023]    [0023]FIG. 8 is a graph illustrating the net dopant concentration in the channel (gate) region of a SOI CMOS of the present invention as a function of depth into the substrate from the surface to the buried oxide layer; and  
         [0024]    [0024]FIG. 9 is a graph illustrating the dopant concentration in the source/drain regions of a SOI CMOS of the present invention as a function of depth in the substrate from the surface to the buried oxide layer. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    Reference will now be made to the drawings wherein like numerals refer to like structures throughout. FIG. 4 is a section view of one embodiment of the SOI CMOS with reduced DIBL  100  of the present invention showing the starting SOI material, a Separation by IMplanted OXygen (SIMOX) wafer  102 . The, SIMOX wafer  102  is well known in the art and comprises a silicon substrate  104  in which a layer of the substrate  104  is converted to a buried silicon dioxide (BOX)  106  layer with a heavy oxygen implant and subsequent anneal. An epitaxial layer  110  of Si approximately 500Å to 2500Å thick is then grown on top of the BOX layer  106 . The BOX layer  106  of the SIMOX wafer  102  provides electrical insulation between the active region of the epitaxial layer  110  and the bulk silicon of the substrate  104 . Thus, active devices formed in the epitaxial layer  110  are electrically isolated from the semiconductive substrate  104 . The SIMOX wafer  102  also provides physical structure as well as reactive material for formation of the SOI CMOS with reduced DIBL  100  in a manner that will be described in greater detail below.  
         [0026]    In the description of the SOI CMOS with reduced DIBL  100  that follows, a single CMOS  130  structure comprising PMOS  132  and NMOS  134  (FIG. 7) devices will be used to illustrate the invention. It should be appreciated that the process herein described for one CMOS  130  device also applies to forming a plurality of SOI CMOS with reduced DIBL  100  devices. It should also be appreciated that the invention herein described can be modified by one skilled in the art to achieve a PMOS  132 , an NMOS  134 , or other technology employing the methods herein described without detracting from the spirit of the invention. It should also be understood that FIGS.  4 - 7  are illustrative and should not be interpreted as being to scale.  
         [0027]    The method of forming the SOI CMOS with reduced DIBL  100  then comprises creating n-well  112  and p-well  114  regions as shown in FIG. 5. The n-well  112  and p-well  114  regions are created, in this embodiment, by implanting a dose of approximately 1e13/cm 2  of P @ 60 keV to create the n-well  112  and a dose of approximately 1e13/cm 2  of B @ 30 keV to create the p-well  114 . The n-well  112  and p-well  114  are then driven at a temperature of approximately 800° C. for a period of approximately 30 minutes. The n-well  112  and p-well  114  provide regions for the subsequent formation of the PMOS  132  and NMOS  134  devices that comprise a CMOS  130  device (FIG. 7).  
         [0028]    The method of forming the SOI CMOS with reduced DIBL  100  then comprises high energy, high dose n-type diffusion source  116  and p-type diffusion source  120  implants into the p-well  114  and n-well  112  respectively as shown in FIG. 5. The n-type diffusion source  116  and p-type diffusion sources  120  comprise borophosphosilicate glass (BPSG). The n-type diffusion source  116  and p-type diffusion source  120  implant parameters should be tailored in such a way that the resultant n-type diffusion source  116  and p-type diffusion source  120  dopant profiles mainly reside in the BOX layer  106 . In one embodiment, the n-type diffusion source  116  implant comprises an implant of phosphorus through the n-well  112  of approximately 2.0e14 /cm 2  @ 220 keV into the BOX layer  106  and the p-type diffusion source  120  implant comprises an implant of boron through the p-well  114  of approximately 2.0e14/cm 2  @ 100 keV into the BOX layer  106 . In this embodiment, the final n-type diffusion source  116  and p-type diffusion source  120  dopant concentration in the BOX  106  is preferably at least 10 20 cm −3 . As will be described in greater detail below, the diffusion sources  116 ,  120  provide a source of dopant atoms that can diffuse into the wells  112 ,  114  respectively to create a retrograde dopant profile.  
         [0029]    The method of forming the SOI CMOS with reduced DIBL  100  then comprises threshold voltage (vt) adjust implants  122 ,  124  as shown in FIG. 5. The threshold voltage adjust implants  122 ,  124  adjust the threshold voltage of the PMOS  132  and NMOS  134  devices either upwards or downwards in a manner known in the art. The threshold voltage adjust implants  122 ,  124  comprise, in this embodiment, a PMOS gate adjust  122  implant of BF 2  at a dose of approximately 5e12 to 1e13 @ 25-35 keV and an NMOS gate adjust  124  implant of Arsenic at a dose of approximately 5e12 to 1e13 @ 35-50 keV. The PMOS gate adjust  122  and the NMOS gate adjust  124  modify the dopant concentration in the gate region of the PMOS  132  and NMOS  134  devices so as to adjust the resultant threshold voltage of the PMOS  132  and NMOS  134  devices to a desirable level.  
         [0030]    The method of forming the SOI CMOS with reduced DIBL  100  then comprises formation of a gate stack  136  as shown in FIG. 6. The gate stack  136  comprises a gate oxide  126 , sidewalls  140 , a nitride layer  142 , and doped polysilicon  144 . The gate oxide  126  in this embodiment comprises a layer of silicon dioxide approximately 50 Åthick. The gate oxide  126  electrically isolates the n-well  112  and p-well  114  regions of the epitaxial silicon  110  from overlying conductive layers that will be described in greater detail below. The sidewalls  140  comprise silicon dioxide that is grown and subsequently anisotropically etched in a known manner to create the structures illustrated in FIG. 6. The sidewalls  140  electrically isolate the gate stack  136  from source/drain conductive layers and facilitates formation of source/drain extensions in a manner that will be described in greater detail below. The nitride layer  142  comprises a layer that is substantially silicon nitride approximately 450 Å thick emplaced in a known manner. The nitride layer  142  inhibits subsequent passage of Boron from the p+ polysilicon layer  144 . The doped polysilicon  144  comprises heavily p-type doped polysilicon for the PMOS  132  device and heavily n-type doped polysilicon for the NMOS  134 . The doped polysilicon  144  provides a reduced work function for the gates of the PMOS  132  and NMOS  134  (FIG. 7) and thus a lower contact resistance and corresponding faster device response.  
         [0031]    The method of forming the SOI CMOS with reduced DIBL  100  then comprises formation of the source  146  and drain  150  as shown in FIG. 6. The source  146  and drain  150  are formed by implanting BF 2  with a dose of approximately 2e15/cm 2  @ 15 keV for the PMOS  132  and As with a dose of approximately 2e15/cm 2  @ 10 keV for the NMOS  134 . As can be seen from FIG. 6 the implantation of the source  146  and drain  150  is partially masked by the gate stack  136  and results in source/drain extensions  152 . The source/drain extensions  152  are lower concentration regions of the source  146  and drain  150  that partially extend under the sidewalls  140 . The source/drain extensions  152  reduce the peak electric field under the gate and thus reduce hot carrier effects in a known manner.  
         [0032]    The method of forming the SOI CMOS with reduced DIBL  100  then comprises formation of a conductive layer  154  (FIG. 7). In this embodiment, the conductive layer  154  comprises a layer of metallic silicide (titanium silicide or cobalt silicide) emplaced in a well known manner. The conductive layer  154  is placed so as to be in physical and electrical contact with the source  146 , the drain  150 , and the doped polysilicon  144  of the gate stack  136 . The conductive layer  154  interconnects the CMOS  130  with other circuit devices on the SIMOX wafer  102  in a known manner.  
         [0033]    The method of forming the SOI CMOS with reduced DIBL  100  then comprises formation of a passivation layer  156  (FIG. 7) overlying the structures previously described. In this embodiment, the passivation layer  156  comprises a layer of oxide, BPSG, or polysilicon approximately 3000 Å thick formed in a known manner. The formation of the passivation layer  156  involves a high temperature process.  
         [0034]    The n-type diffusion source  116  and the p-type diffusion source  120  previously implanted into the BOX layer  106  in the manner previously described serve as solid-sources for dopant diffusion. When the passivation layer  156  is formed on the SIMOX wafer  102  with attendant heat steps, dopants contained in the n-type  116  and the p-type  120  diffusion sources will outdiffuse into the epitaxial silicon  110 , creating a thin, highly doped retrograde profile region  160  as shown in FIG. 7. In the case of the p-well  114 , the retrograde profile region  160  will comprise boron and, in the n-well  112 , the retrograde profile region  160  will comprise phosphorus. The retrograde profile region  160  layer will act as a punchthrough prevention layer to control DIBL.  
         [0035]    [0035]FIG. 8 shows the net dopant profile in a vertical outline in the middle of the channel region. The boron concentration increases from 9.0e17/cm 3  to 2.0e18/cm 3 , which is nearly a 120% increase, at the BOX  106 /silicon substrate  104  interface. FIG. 9 shows the dopant profile in the source  146  and drain  150  regions. The source  146  and drain  150  implants in this embodiment of the SOI CMOS with reduced DIBL  100  reach close to the BOX layer  106  as can be seen from FIG. 9. As such the source  146  and drain  150  implants will compensate the outdiffused dopants from the n-type  116  and p-type  120  diffusion sources in the retrograde profile region  160  close to the interface of the BOX  106  and the silicon substrate  104 . This will reduce the junction capacitance of the SOI CMOS with reduced DIBL  100  even further as compared to a process with halo implants.  
         [0036]    The dopants contained within the retrograde profile region  160  will also create recombination centers near the BOX  106 /silicon substrate  104  interface. These recombination centers are an added benefit in the SOI CMOS with reduced DIBL  100  since the recombination centers tend to reduce the floating body effects in the SOI CMOS with reduced DIBL  100 .  
         [0037]    Hence, the process of the illustrated embodiment provides a method in which a retrograde doping profile can be created in thin semiconductor active areas such as the active areas used in silicon-on-insulator (SOI) applications. The process of the illustrated embodiment does not significantly add to the processing of the device as only discrete implantation steps are required and the diffusion is obtained through the additional thermal processing of the device. Thus, retrograde profiles can be created in a manner that does not significantly increase the processing costs of the device.  
         [0038]    Although the preferred embodiments of the present invention have shown, described and pointed out the fundamental novel features of the invention as applied to those embodiments, it will be understood that various omissions, substitutions and changes in the form of the detail of the device illustrated may be made by those skilled in the art without departing from the spirit of the present invention. Consequently, the scope of the invention should not be limited to the foregoing description but is to be defined by the appended claims.