Abstract:
A non-volatile memory cell includes a first insulating layer over a substrate region, and a floating gate. The floating gate includes a first polysilicon layer over the first insulating layer and a second polysilicon layer over and in contact with the first polysilicon layer. The first polysilicon layer has a predetermined doping concentration and the second polysilicon layer has a doping concentration which decreases in a direction away from an interface between the first and second polysilicon layers. A second insulating layer overlies and is in contact with the second polysilicon layer. A control gate includes a third polysilicon layer over and in contact with the second insulating layer, and a fourth polysilicon layer over and in contact with the third polysilicon layer. The fourth polysilicon layer has a predetermined doping concentration, and the third polysilicon layer has a doping concentration which decreases in a direction away from an interface between the third and fourth polysilicon layers.

Description:
BACKGROUND OF THE INVENTION 
     FIG. 1 a  shows a cross section view of a conventional stacked-gate non-volatile memory cell  100  at an intermediate processing stage. Cell  100  has a polysilicon (gate) stack which includes floating gate  106  and control gate  110  insulated from each other by a composite oxide-nitride-oxide (ONO) dielectric layer  108 . A tungsten layer (WSi x )  112  overlies control gate  110 . Floating gate  106  is insulated from the underlying silicon substrate  102  by a tunnel oxide layer  104 . FIG. 1 b  shows a cross section view of cell  100  after formation of: (a) oxide spacers  116 - a ,  116 - b  along sidewalls of the gate stack, and (b) source region  114 - a  and drain region  114 - b  in substrate  102 . 
     A simplified conventional process sequence to form memory cell  100  includes: forming tunnel oxide layer over substrate  102 ; depositing a first layer polysilicon over the tunnel oxide layer; forming an interpoly composite ONO dielectric layer over the first layer polysilicon; depositing a second layer polysilicon over the ONO dielectric; forming a tungsten silicide layer over the second layer polysilicon; and self-aligned mask and self-aligned etch (SAE) to form the gate stack as shown in FIG. 1 a . In modern technologies, the control gate is often formed simultaneously with the gates of peripheral (CMOS) transistors, followed by cell self-aligned etch (SAE) of the first layer polysilicon and the ONO dielectric using the control gate as a mask. After formation of the gate stack, in some processes, polysilicon re-oxidation is performed. DDD mask and implant steps are then carried out to form the cell source DDD region (if for example a source DDD region is employed) and DDD regions for peripheral high voltage (HV) NMOS and PMOS transistors. Next, cell source/drain mask and implant steps are carried out to form cell source and drain regions  114 - a ,  114 - b , followed by oxidation and anneal cycles. LDD mask and implant steps may then be carried out to form LDD junctions for the low voltage (LV) NMOS and PMOS transistors. Spacers (e.g., spacers  16 - a ,  116 - b  in FIG. 1 b ) are then formed along sidewalls of the gate stack in the cell and along the side-walls of the gates of the periphery transistors. This is followed by N +  and P +  mask and implant steps to complete the junction formation of the peripheral transistors. 
     The first and second polysilicon layers are deposited by means of Chemical Vapor Deposition (CVD). Both first and second polysilicon layers are in-situ doped (usually by phosphorus P31) to a relatively high level (e.g., 2×10 19  to 5×10 20  cm −3 ). The level of polysilicon doping is usually controlled by gas flow rate and pressure of the gas compound containing P31, such as PH3. An example of a set of parameters associated with the polysilicon deposition of a conventional process is provided below. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 PH3 
                   
                   
               
               
                   
                   
                   
                 flow 
               
               
                 Thickness 
                 Temperature; 
                 SiH4 flow 
                 rate; 
                 Pressure; 
               
               
                 Target; Å 
                 ° C. 
                 rate; sccm 
                 sccm 
                 mTorr 
                 Rs; Ω/□ 
               
               
                   
               
             
             
               
                 600-1000 
                 580-620 
                 1200-1400 
                 80-120 
                 350-450 
                 200-1000 
               
               
                   
               
             
          
         
       
     
     There are a number of reasons for the high polysilicon doping. First, the high doping prevents or minimizes polysilicon depletion when gate bias is applied to the control gate of the memory cell or to the gate of the MOS transistor. Polysilicon depletion decreases gate capacitance thus reducing gate control in a MOS transistor channel region, and impairs other transistor/cell electrical characteristics. Second, the high doping helps maintain a proper value of polysilicon work function which impacts such important transistor/cell parameters as the threshold voltage. Third, the high doping reduces the world line resistance in the memory array, thus improving the memory performance. Fourth, the high doping reduces time delay associated with the peripheral transistor gate capacitance and resistance. 
     However, there are also drawbacks to the high polysilicon doping. The high doping leads to higher oxidation rate of polysilicon crystals. Higher oxidation rate in turn leads to a more pronounced “smiling” effect, i.e., an increased gate oxide thickness at the edges of the gate in MOS transistors, and similar increase of tunnel oxide thickness and ONO dielectric at the edges of the cell gate stack as shown in FIG. 1 b  by circles marked by reference numerals  118  and  120 . Although some minimal “smiling” effect can serve a useful reliability purpose by rounding corners of polysilicon thus reducing the electric field peak at polysilicon edges, excessive “smiling” effect impairs gate control of the channel and the drive current of MOS transistors. In memory cells, a pronounced “smiling” effect of ONO dielectric impairs gate coupling ratio, gate channel control, and program, erase, and read efficiency. 
     A further drawback of the high doping is that it leads to a larger polysilicon grain size which in turn leads to a more rugged interface between the gate oxide and the polysilicon gate in MOS transistors, and similarly between each of the tunnel oxide and the floating gate, bottom of the ONO dielectric and the floating gate, and top of the ONO dielectric and the control gate in a memory cell. In extreme cases, it may lead to gate oxide and/or tunnel oxide pinch-off or otherwise impact the integrity and reliability characteristics of the gate oxide in MOS transistors and the tunnel oxide and the ONO dielectric in memory cells. 
     In conventional processes, the room to achieve the necessary trade-off between the desirable and undesirable effects of the polysilicon doping is limited to only regulating the level of doping and uniformity of the doping profile across the polysilicon layers. Achieving the desired trade off thus often proves to be a difficult task from process and device optimization point of view. 
     Accordingly, there is a need for polysilicon layers structure and method of forming the same whereby an optimum polysilicon doping profile can be achieved, the depletion of the polysilicon and its associated adverse effects are prevented or minimized, the quality and uniformity of the polysilicon-oxide interface are improved, while the “smiling” effect in the dielectric layers interfacing polysilicon is minimized. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the present invention, a doped polysilicon layer interfaces a dielectric layer through an undoped polysilicon layer. In this manner, the drawbacks of the prior art structures wherein the doped polysilicon layer is in direct contact with the insulating layer are minimized or eliminated, while the advantages of a doped polysilicon layer is maintained. 
     In one embodiment, a semiconductor structure includes an undoped polysilicon layer, a doped polysilicon layer in contact with the undoped polysilicon layer, and an insulating layer in contact with the undoped polysilicon layer. The undoped polysilicon layer is sandwiched between the doped polysilicon layer and the insulating layer. 
     In accordance with an embodiment of the present invention, a semiconductor non-volatile memory cell includes a first insulating layer over a substrate region. A floating gate includes a first polysilicon layer over the first insulating layer and a second polysilicon layer over and in contact with the first polysilicon layer. The first polysilicon layer has a predetermined doping concentration and the second polysilicon layer has a doping concentration which decreases in a direction away from an interface between the first and second polysilicon layers. A second insulating layer overlies and is in contact with the second polysilicon layer. A control gate includes a third polysilicon layer over and in contact with the second insulating layer, and a fourth polysilicon layer over and in contact with the third polysilicon layer. The fourth polysilicon layer has a predetermined doping concentration, and the third polysilicon layer has a doping concentration which decreases in a direction away from an interface between the third and fourth polysilicon layers. 
     In another embodiment, the floating gate further includes a fifth polysilicon layer over and in contact with the first insulating layer. The first polysilicon layer overlies and is in contact with the fifth polysilicon layer. The fifth polysilicon layer having a doping concentration which decreases in a direction away from an interface between the first and fifth polysilicon layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a  and  1   b  show cross section views of a conventional stacked-gate non-volatile memory cell at different processing stages; 
     FIG. 2 shows a cross section view of a stacked-gate non-volatile memory cell at an intermediate processing stage, in accordance with one embodiment of the present invention; 
     FIG. 3 a  shows a cross section view of a variation of the FIG. 2 memory cell structure in accordance with another embodiment of the present invention; 
     FIG. 3 b  shows a cross section view of the cell structure of FIG. 3 a  after source/drain formation, side-wall spacers formation, and all thermal oxidation and anneal cycles; 
     FIGS. 4 a ,  4   b ,  4   c  show cross section views of an MOS transistor at two different processing stages in accordance with an embodiment of the present invention; and 
     FIG. 5 shows an exemplary doping profile through the floating gate in FIG. 3 a.    
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with an embodiment of the present invention, conventional semiconductor device structures wherein a doped polysilicon layer comes in contact with an insulating layer, such as silicon-dioxide, are modified so that the polysilicon layer comprises a doped and an undoped polysilicon layer with the undoped polysilicon layer interfacing with the insulating layer. In this manner, the drawbacks of the prior art structures wherein doped polysilicon layers are in direct contact with insulating layers are minimized or eliminated while the advantages of a doped polysilicon are maintained, as discussed in more detail below. 
     FIG. 2 shows a cross section view of a stacked-gate non-volatile memory cell  200  at an intermediate processing stage, in accordance with an embodiment of the present invention. Memory cell  200  includes a tunnel oxide  204  over a silicon substrate  202 . Floating gate  206  which comprises two polysilicon layers  206 - a ,  206 - b  overlies tunnel oxide  204 . A composite oxide-nitride-oxide (ONO) dielectric layer  208  insulates floating gate  206  from an overlying control gate  210 . Control gate  210  comprises two polysilicon layers  210 - a ,  210 - b , and in some processes, is overlaid by a tungsten (WSi x ) layer  212 . As shown, of the two polysilicon layers forming floating gate  206 , the lower layer  206 - a  is doped while the upper layer  206 - b  is undoped, and of the two polysilicon layers forming control gate  210 , the lower layer  210 - a  is undoped while the upper layer  210 - b  is doped. In this manner, ONO dielectric  208  is sandwiched between two undoped polysilicon layers  206 - b  and  210 - a.    
     In one embodiment, the structure of FIG. 2 is formed as follows. Tunnel oxide  204  is formed over substrate  202  in accordance with conventional methods. In forming floating gate  206 , two successive polysilicon deposition steps are carried out. First, an in-situ doped polysilicon deposition is performed to deposit doped polysilicon layer  206 - a , followed by an undoped polysilicon deposition step to deposit undoped polysilicon layer  206 - b . The polysilicon doping concentration (using for example phosphorus P31 as the dopant) is controlled by temperature and gas (e.g. PH3) flow rate and pressure, and the thickness of each polysilicon layer is controlled by deposition time and temperature. 
     After deposition of polysilicon layers  206 - a ,  206 - b , interpoly ONO dielectric  208  is formed in accordance with conventional methods. Next, in forming control gate  210 , two successive polysilicon deposition steps are performed. First, an undoped polysilicon deposition step is carried out, followed by an in-situ doped polysilicon deposition step. 
     The tables below show the temperature, gas flow rate and pressure, doping concentration, and polysilicon thickness for each of the doped and undoped polysilicon layers in accordance with an exemplary embodiment of the present invention. This table reflects a thickness ratio of doped polysilicon to undoped polysilicon of in the range of 2:1 to 5:1, with a preferred ratio of 3:1. Note that the values in these tables are merely illustrative and not intended to be limiting. Varying these values to achieve the target parameters and the desired cell performance would be obvious to one skilled in this art in view of this disclosure. 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Doped Polysilicon 
               
             
          
           
               
                 Doping con. 
                 Thickness 
                 Temperature; 
                 SiH4 flow 
                 PH3 flow 
                 Pressure; 
               
               
                 target; cm −3   
                 target; Å 
                 ° C. 
                 rate; sccm 
                 rate; sccm 
                 mTorr 
               
               
                   
               
               
                 2 × 10 19 -1 × 10 20   
                 450-750 
                 580-620 
                 1200-1400 
                 50-100 
                 350-450 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Undoped Polysilicon 
               
             
          
           
               
                 Thickness 
                 Temperature; 
                 SiH4 flow 
                 PH3 flow 
                 Pressure; 
               
               
                 target; Å 
                 ° C. 
                 rate; sccm 
                 rate; sccm 
                 mTorr 
               
               
                   
               
               
                 150-250 
                 580-620 
                 1200-1400 
                 0 
                 350-450 
               
               
                   
               
             
          
         
       
     
     After deposition of the polysilicon layers in forming control gate  210 , a tungsten (WSi x ) layer  212  is optionally deposited in accordance with conventional methods. In some processes, an ARC oxynitride layer (not shown) is deposited over the tungsten layer to complete gate layer formation. This is followed by gate mask and gate etch to form the control gate of the memory cell and the gate of peripheral transistors, and then self-aligned mask and self-aligned etch (SAE) is carried out to form the gate stack as it appears in FIG.  2 . All subsequent steps, including source/drain formation and side-wall spacer formation, are carried out in accordance with conventional methods. 
     During thermal oxidation and anneal cycles, such as ONO steam anneal (after ONO deposition), polysilicon re-oxidation after gate stack formation, and source/drain oxidation cycle(s), the top and bottom oxide layers in ONO dielectric  208  at the periphery of the gate stack (side walls) grow at a lower rate due to the lower oxidation rate of undoped polysilicon layers which interface the two oxide layers. Thus, ONO dielectric “smiling” effect is substantially reduced. The tunnel oxide “smiling” effect can similarly be reduced by including another undoped polysilicon layer as the bottom polysilicon layer of the floating gate. This is shown in the FIG. 3 a  embodiment. An undoped polysilicon layer  306 - c  forms the bottom layer of floating gate  306 , interfacing tunnel oxide  304 . 
     Other than the reduction in “smiling” effect, the smaller grain size of undoped polysilicon yields a polysilicon-oxide interface which is more uniform leading to improved tunnel oxide and ONO dielectric quality and integrity. Further, by selecting proper doping concentration in the doped polysilicon layers and proper thickness ratio between adjacent doped and undoped polysilicon layers, by the end of the thermal cycles, a uniform and high enough doping concentration can be achieved throughout the whole floating gate and control gate so as to prevent polysilicon depletion effects. 
     By the end of the oxidation/anneal thermocycle, depending on the thermal budget, the impurity (e.g., phosphorus) profile in the undoped polysilicon layers may be of diffusion character. FIG. 5 shows an exemplary impurity profile through floating gate  306  in FIG. 3 a . The horizontal axis represents the impurity concentration, and the vertical axis represents the dimension along the stack, from top to bottom, of ONO dielectric  308 , floating gate  306 , and tunnel oxide  304 . Solid lines  510 ,  512 ,  514  indicate the impurity concentration in the respective undoped polysilicon layer  306 - b , undoped polysilicon layer  306 - c , and doped polysilicon layer  306 - a  before carrying out the thermal cycles. The dashed, curved line  516  shows the impurity profile through the three polysilicon layers after the thermal cycles. Note that even though polysilicon layers  306 - b ,  306 - c  are undoped, they acquire some impurities during their deposition process. Solid lines  510  and  512  represent this impurity concentration before the thermal cycles are carried out. 
     After the thermal cycles, as shown by dashed line  516 , the impurity concentration is highest in the doped polysilicon layer  306 - a  and gradually reduces at the boundaries between the doped and undoped polysilicon layer and through the undoped polysilicon layers  306 - b ,  306 - c , and reaches its lowest concentration level at the interface between the undoped polysilicon layers  306 - b ,  306 - c  and the corresponding tunnel oxide  304  and ONO dielectric layer  308 . The thickness of the polysilicon layers and the thermocycles need to be optimized such that the final polysilicon doping concentration and its gradient at the polysilicon-dielectric interface is high enough to prevent or minimize polysilicon depletion effects. 
     Note that despite the high final doping concentration at the polysilicon-dielectric interface, the benefits of using undoped polysilicon layers are maintained. The diffusion of dopants from the doped polysilicon layer to the undoped polysilicon layers occurs slowly during the thermocycles. Thus, because the doping concentration at the polysilicon-dielectric interface is relatively low during a significant part of the oxidation processes, a reduced smiling effect is achieved. At the same time, the undoped polysilicon layers retain smaller size and more uniform grain structure, resulting in better uniformity and quality of polysilicon-dielectric interface. 
     FIG. 3 b  shows the cell structure of FIG. 3 a  after source/drain  314 - a ,  314 - b  and side-wall spacers  316 - a ,  316 - b  formation and all thermal oxidation and anneal cycles. As shown, by using multi-layers of doped/undoped polysilicon in both the floating gate and the control gate, the “smiling” effect (shown by circles  118  and  120  in FIG. 1 b ) is reduced. 
     Accordingly, by providing a combination of doped and undoped polysilicon layers in each of the floating gate and the control gate, a more flexible process is obtained whereby much of the adverse effects associated with the trade-offs in the polysilicon doping concentration present in conventional processes is eliminated. By providing undoped polysilicon at the dielectric interfaces, an ONO dielectric and a tunnel oxide layer having uniform thicknesses and improved dielectric quality and integrity are achieved while a high enough doping concentration in most of the floating gate and the control gate is maintained. Also, after all the thermal cycles, a relatively homogeneous polysilicon doping across the whole floating gate and control gate is obtained. Further, the tunnel oxide and the ONO dielectric are more uniform both in terms of their geometrical thickness and in terms of their dielectric quality and integrity. High uniformity of tunnel oxide and ONO result in better gate control over the channel, higher coupling ratio between the control gate and the floating gate, enhanced program, erase, and read efficiency, tighter erase distribution, and allow use of lower operating voltages. Further the improved quality of the ONO dielectric and tunnel oxide results in improved charge retention characteristics and overall reliability of the memory cell. Thus, a memory cell with a much improved electrical and reliability characteristics is achieved. 
     FIGS. 4 a ,  4   b ,  4   c  show cross section views of a MOS transistor at two different processing stages in accordance with another embodiment of the present invention. In FIG. 4 a , gate oxide  404  is formed over substrate  402  in accordance with conventional methods. Next, two successive polysilicon deposition steps are carried out in forming transistor gate  406 . First, an undoped polysilicon layer is deposited in forming undoped polysilicon layer  406 - a , followed by an in-situ doped polysilicon deposition step in forming doped polysilicon layer  406 - b . Similar to the above memory cell embodiments, the polysilicon doping concentration (using for example phosphorus P31 as the dopant) is controlled by temperature and gas (e.g. PH3) flow rate and pressure, and the thickness of each polysilicon layer is controlled by deposition time and temperature. 
     All subsequent processing steps are carried out in accordance with conventional methods. FIGS. 4 b  and  4   c  show two different cross sections of the transistor structure after source/drain  408 - a ,  408 - b  formation, side-wall spacers  410 - a ,  410 - b  formation, and all thermal oxidation and anneal cycles. FIG. 4 b  shows the cross section of the transistor along the gate length of the transistor, and FIG. 4 c  shows the cross section of the transistor along the gate width of the transistor through the channel region (the transistor width is the size of active area between two isolation areas). 
     As shown, by using the doped/undoped polysilicon layers, the “smiling” effect is reduced not only at the outer edges of gate oxide  404  near the drain and source regions (FIG. 4 b ), but also at the periphery of the active and isolation areas  440 - a ,  440 - b  (FIG. 4 c ). Although the increase in the thickness of the gate oxide at the transitional area between the active and isolation regions is mainly determined by the so called “bird&#39;s beak” effect of the isolation regions (which is stronger for LOCOS isolation and smaller for shallow trench isolation), high polysilicon doping of the gate in conventional processes, and the resulting “smiling” effect, results in an even thicker gate oxide at the transitional areas. Controlling the polysilicon doping at the gate oxide interface minimizes the contribution to the thicker gate oxide by the “smiling” effect, and thus improves transistor performance (gate control, drive current). This is specially advantageous for short active width devices. More importantly, the smaller grain size and uniformity of grain structure in the initially undoped polysilicon layer helps improve uniformity and quality of polysilicon-oxide interface and gate oxide thickness, leading to improved reliability. 
     The MOS transistor polysilicon gate can be formed simultaneously (i.e., using the same mask step) with the control gate of the memory cell. That is, the same two successive deposition steps in forming an undoped polysilicon layer followed by a doped polysilicon layer may be carried out to simultaneously form the control gate of the memory cells and the gate of periphery transistors. In another embodiment, the MOS transistor polysilicon gate can be formed simultaneously with the floating gate rather than with the control gate. In the memory cell embodiment wherein the floating gate comprises three polysilicon layers (FIG. 3 a ), the top undoped polysilicon layer is not useful in the periphery transistor, but may be present if a sufficiently uniform final doping could be obtained. In yet another embodiment, the gate of the peripheral transistors is formed simultaneously with the combined floating gate and control gate polysilicon layers. This embodiment, as in many flash EPROM processes, requires that the interpoly ONO dielectric be removed from the periphery. 
     The present invention is not limited in application to MOS transistors and stacked gate non-volatile memories. Any structure wherein doped polysilicon comes in contact with an insulating layer can benefit from the doped/undoped multi-layer polysilicon approach described herein. Examples of other structures include N-channel or P-channel non-volatile memory cells such as ROM, EPROM, EEPROM, and flash EEPROM cells, volatile memory cells such as DRAM and SRAM cells, NMOS and PMOS transistors, and depletion and enhancement transistors. Further, the present invention is not limited to any specific parameters or values indicated herein. For example, the values indicated in the tables above correspond to one particular process and set of targets, and may be varied to accommodate other processes and cell technologies. 
     While the above is a complete description of preferred embodiments of the present invention, it is possible to use various alternatives, modifications, and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents.