Abstract:
A non-volatile memory device includes a substrate having a first active region and a second active region. A first floating gate is provided over the first active region and having an edge, the first floating gate being made of a conductive material. A first spacer is connected to the edge of the first floating gate and being made of the same conductive material as that of the first floating gate. A control gate is provided proximate to the floating gate.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application is a division of U.S. application Ser. No. 10/431, 172, filed on May 6, 2003, which claims priority to U.S. Provisional Patent Application No. 60/383,470, filed on May 24, 2002, which disclosures are incorporated by reference. 
     
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     NOT APPLICABLE  
       REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK  
       [0003]     NOT APPLICABLE  
       BACKGROUND OF THE INVENTION  
       [0004]     This invention relates to integrated circuit nonvolatile memories, and in particular to flash memories. Flash memories are electrically erasable nonvolatile memories in which groups of cells can be erased in a single operation.  
         [0005]     Numerous types of integrated circuit memory are now well known, as are processes for manufacturing them. One particular type of integrated circuit memory is nonvolatile memory. Nonvolatile memory is referred to as such because it does not lose the information stored in the memory when power is removed from the memory. Nonvolatile memory has many applications in products where the supply of electricity is interruptable. For example, one well known product employing flash memory is PCMCIA or PC cards. PC cards are small credit card-sized packages that contain nonvolatile memory within which a computer program or other information is stored. Such devices allow the user to connect and disconnect the memory card from a computer or other electronic apparatus, without losing the program stored within the memory card.  
         [0006]     Nonvolatile memory devices include read only memories (ROM), programmable read only memories (PROM), electrically erasable read only memories (EEPROM), as well as other types. Within the field of electrically erasable programmable memories, a certain class of devices is known as flash memory, or flash EEPROMs. Such memories are selectively programmable and erasable, typically with groups of cells being erasable in a single operation.  
         [0007]     In conventional flash memories, each memory cell is formed from a transistor having a source, drain, control gate and floating gate. The floating gate is formed between the control gate and the substrate. The presence, or absence, of charge trapped on the floating gate can be used to indicate the contents of the memory cell. Charge trapped on the floating gate changes the threshold voltage of the transistor, enabling detection of its binary condition.  
         [0008]     In most flash memories, charge is placed on, or removed from, the floating gate by operating the memory at conditions outside its normal operating conditions for reading its contents. For example, by adjusting the relative potentials between the gate and the source, drain or channel regions, charge, in the form of electrons, can be caused to be injected onto the floating gate, or removed from the floating gate.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     According to one embodiment of the present invention, a non-volatile memory device includes a substrate having a first active region and a second active region. A first floating gate is provided over the first active region and having an edge, the first floating gate being made of a conductive material. A first spacer is connected to the edge of the first floating gate and being made of the same conductive material as that of the first floating gate. A control gate is provided proximate to the floating gate.  
         [0010]     In another embodiment, a flash memory device includes a substrate having a first active region and a second active region. A field trench oxide separates the first and second active regions. A floating gate is provided over the first active region and having an edge, the first floating gate being made of polysilicon. A spacer is coupled to the edge of the floating gate and being made of polysilicon, the spacer having a slope less than about 60 degrees. A control gate overlies the floating gate. A metal layer is provided over the control gate, wherein the spacer reduces formation of a void in the metal layer.  
         [0011]     In another embodiment, a method of fabricating a non-volatile memory device includes forming a polysilicon floating gate over a substrate, the floating gate having an edge; forming a polysilicon spacer joined to the edge of the floating gate, the spacer having a sloping edge having a slope less than 60 degrees; and forming a polysilicon control gate over the floating gate and the spacer.  
         [0012]     In another embodiment, a non-volatile memory device includes a substrate having a first active region and a second active region; an isolation structure separating the first and second active regions; a first floating gate provided over the first active region and having a first edge, the first floating gate being made of a conductive material; a first spacer connected to the first edge of the first floating gate and having a first sloping edge, the first spacer being of a conductive material and overlying the isolation structure; and a control gate provided proximate to the floating gate. The first sloping edge of the first spacer forms an angle of less than 65 degrees to facilitate deposition of material over the first spacer and the isolation structure. The device further includes a second floating gate provided over the second active region and having a second edge, the second floating gate being of the same conductive material as the first floating gate; a second spacer connected to the second edge of the second floating gate and having a second sloping edge, the second spacer being of a conductive material and overlying the isolation structure and electrically isolation from the first spacer; a metal layer overlying the first and second floating gates and the isolation structure, wherein each of the first and second sloping edges forms an angle of less than 65 degrees, so that a portion of the metal layer overlying the isolation structure is substantially free of a void. The metal layer includes tungsten or aluminum.  
         [0013]     In yet another embodiment, a method for fabricating a non-volatile memory device includes forming a first polysilicon layer over an isolation structure and first and second regions of a substrate, the first and second regions being defined by the isolation structure; forming a dielectric layer overlying the first polysilicon layer; etching the first polysilicon layer and the dielectric layer to expose a portion of the isolation structure, the etching step defining a first edge associated with the first region and a second edge associated with the second region; forming a second polysilicon layer over the exposed portion of the isolation structure, the second polysilicon layer contacting the first and second edges; etching the second polysilicon layer to form a first spacer joined to the first edge and a second spacer joined to the second edge; forming an interpoly dielectric layer overlying the first and second spacers and the first polysilicon layer; forming a third polysilicon layer overlying the interpoly dielectric layer; and forming a metal layer over lying the third polysilicon layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  illustrates a schematic top view of a flash memory device.  
         [0015]      FIGS. 2A-2E  illustrate a conventional method of fabricating a control gate and a tungsten silicide thereon.  
         [0016]      FIGS. 3A-3I  illustrate a method of fabricating a control gate and a tungsten silicide without a void or seam therein according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  illustrates a schematic top view of a flash memory device  100 . The device  100  includes a plurality of active regions  102  and  104  separated by a field trench oxide region  106 . A plurality of floating gates  108  is provided overlying the active regions  102  and  104 . The floating gates are generally formed using polysilicon and commonly referred to as a “poly 1 layer” or “P1.” The floating gates generally extend over the field trench oxide region  106  to define a P1-to-P1 spacing  110  that is less than the width of the field trench oxide region  106 . A plurality of control gates  112  is provided over the floating gates. The control gates are generally formed using polysilicon and also referred to as a “poly 2 layer” or “P2.” A drain region  114  is provided at one side of the floating gate. A source region  116  is provided at the other side of the floating gate.  
         [0018]      FIGS. 2A-2E  illustrate a conventional process flow of forming a metal layer, e.g., a tungsten silicide layer, overlying a control gate. A void or seam may be formed within the metal layer due to step coverage problem, described below, which is undesirable since the void would increase the sheet resistance of the metal layer.  
         [0019]     A field trench oxide  202  is formed to electrically isolate adjacent active regions ( FIG. 2A ). A first polysilicon layer  204  having a thickness of about 500-1,000 Åis deposited on the trench oxide  202 . The first polysilicon layer  204  is etched to define a floating gate ( FIG. 2B . Along with the unwanted portions of the first polysilicon layer, a portion of the trench oxide  202  is etched to form a groove  206 . The groove  206  defines edges  208  and  210 . The unetched portion of the first polysilicon layer  204  defines the floating gate. An interpoly dielectric layer  212  is then formed over the first polysilicon layer and the substrate ( FIG. 2C ). The layer  212  is often called the interpoly dielectric film since it is sandwiched between the first polysilicon layer and another polysilicon layer which defines the control gate for each cell, as will be explained later.  
         [0020]     Referring to  FIG. 2D , after the formation of the interpoly dielectric layer  212 , a second polysilicon layer  214  having a thickness of about 1000-2000 Åis deposited over the dielectric layer and the substrate using one of many techniques. Due to the conformal nature of the polysilicon deposition, the second polysilicon layer is unable to fill the groove  206 . Accordingly, a P2 groove  216  having edges  218  and  220  is formed after the deposition of the second polysilicon layer. The edges  218  and  220  are relatively high because each successive layer formed over the groove  206  adds to the height.  
         [0021]     A metal layer  222 , e.g., a tungsten silicide (Wsi), is deposited over the second polysilicon layer ( FIG. 2E ). As a result of the relatively high edges  218  and  220 , a void or seam  224  may be formed within the tungsten silicide provided between the edges  218  and  220 . The void  224  decreases the conductivity of the tungsten silicide that is undesirable since it may reduce the operational speed of the device.  
         [0022]      FIGS. 3A-3I  illustrate a process flow of forming a metal layer, e.g., a tungsten silicide layer, overlying a control gate according to one embodiment of the present invention. The process flow described reduces formation of a void or seam within the metal layer and improves the coupling coefficient of the floating gate.  
         [0023]     A field trench oxide  302  is formed on a substrate  300  to electrically isolate adjacent active regions ( FIG. 3A ). Although the figure shows a single trench oxide, numerous trench oxides are formed simultaneously on the substrate. The substrate is a silicon substrate, preferably of 8-10 ohm centimeter resistivity, and of crystal orientation &lt;100&gt;. A first polysilicon layer  304  having a thickness of about 500-1,000 Åis deposited on the trench oxide  302 . Generally, the first polysilicon layer is deposited using a low pressure chemical vapor deposition (“LPCVD”) process and is lightly doped. The methods used to dope the first polysilicon include diffusion doping, in-situ doping, and ion implantation doping techniques. The polysilicon layer is doped with n-type dopants to a concentration level of about 1×10 19  dopants per cubic centimeter.  
         [0024]     A dielectric layer  306  is formed over the first polysilicon layer  304  ( FIG. 3B ). The dielectric layer  306  is relatively thin, e.g., about 500 Åor less. The dielectric layer may be PSG or other suitable materials.  
         [0025]     Thereafter, the dielectric layer  306  and the first polysilicon layer  304  are etched, preferably in a single etch step ( FIG. 3C ). The etch step includes forming a masking layer (not shown) over the dielectric layer and the first polysilicon layer, patterning the mask layer to expose an unwanted portion  308  of the dielectric layer that is overlying the trench oxide  302 . The exposed portion  308  of the dielectric layer  306  and a portion  310  of the first polysilicon layer underlying the exposed dielectric layer are removed using a dry etch method such as a reactive ion etching process (“RIE”) using a plasma ignited from a gas mixture of HBr and O 2  or HBr, Cl 2  and O 2 .  
         [0026]     Along with the unwanted portions  308  and  310 , a portion  312  of the trench oxide  302  is etched as well in an over etch since a precise etch control is difficult ( FIG. 3C ). Generally, a slight over etch is desired to ensure electrical isolation between two portions  314  and  316  of the polysilicon layer  304  defined by the etch step. These portions of the polysilicon layer  304  define floating gates for the adjacent flash memory transistors or cells. A groove  318  is formed on the trench oxide  302  as a result of the over etch. The groove and the polysilicon portions  314  and  316  together define edges  320  and  322  that are substantially vertical or have relatively high slopes since the etch step used to removed the unwanted portions generally is an anisotropic etch.  
         [0027]     Referring to  FIG. 3D , a sacrificial or second polysilicon layer  324  is deposited over the dielectric layer  306 , the groove  318 , and edges  320  and  322  to form polysilicon spacers (see  FIG. 3E ). The polysilicon layer  324  is deposited using a low pressure chemical vapor deposition (“LPCVD”) process. Due to the conformal nature of the polysilicon deposition, portions  326  and  328  of the sacrificial layer  324  are contacting the edges of the first polysilicon layer  306 . The sacrificial layer is lightly doped. The methods used to dope the first polysilicon include diffusion doping, in-situ doping, and ion implantation doping techniques. The sacrificial polysilicon layer is doped with n-type dopants to a concentration level of about 1×10 19  dopants per cubic centimeter. The doping level of the layer  324  is substantially similar to that of the first polysilicon layer  304  since the layer  324  will be used to form spacers for the floating gates. Alternatively, the different doping levels may be used for the sacrificial layer. In one implementation, the layer  324  has a thickness of about 300-1,000 Å. Alternatively, the layer  324  may be greater or lesser in thickness according the thickness of spacers desired.  
         [0028]     Polysilicon or poly spacers  330  and  332  are formed by blanketly etching away the sacrificial polysilicon layer  324  ( FIG. 3E ). The poly spacers  330  and  332  are electrically  
         [0029]     coupled to the floating gates  314  and  316 , respectively. A separation  334  is provided between the two poly spacers  330  and  332 , so that the electrical isolation of the floating gates  314  and  316  is maintained. The spacers  330  and  332  have sloping edges  336  and  338  that is substantially less than 90 degrees. In one implementation, the slopes of the edges  336  and  338  are about 70 degrees or less, 65 degrees or less, 60 degrees or less, 50 degrees or less, 40 degrees or less, or 30 degrees or less. The etch step used to remove the sacrificial polysilicon layer may be controlled to obtain different slopes for the poly spacers, as desired for different applications. For example, the gas composition and/or bias power (when RIE is used) can be adjusted for control the slope of the spacers. The angle of the sloping edge is defined by a plane  333  that is substantially parallel to the upper surface of the substrate  300  and a line  331  that is tangent to the sloping edge  336  or  338 , i.e., an angle  335 .  
         [0030]     Thereafter, the dielectric layer  308  is removed ( FIG. 3F ). An interpoly dielectric layer  340  is then formed over the first polysilicon layer  304 , the spacers  330  and  332 , and the trench oxide  302  ( FIG. 3G ). The layer  340  is often called the interpoly dielectric film since it is sandwiched between the first polysilicon layer and another polysilicon layer which defines the control gate for each cell, as will be explained later. The interpoly dielectric layer can be a silicon oxide or an ONO layer having a thickness of about 150-400 Å, where the ONO layer has oxide, nitride, and oxide layers stacked in sequence. As a result of the underlying spacers, the ONO layer  340  is also provided with sloping edges  342  and  344  since the layer  340  are deposited conformally. In implementation, the slopes of the edges  342  and  344  are about 70 degrees or less, 60 degrees or less, 50 degrees or less, 40 degrees or less, or 30 degrees or less.  
         [0031]     Referring to  FIG. 3H , a third polysilicon layer  346  having a thickness of about 700-2000 Å, generally about 1000 Å, is deposited over the dielectric layer to form a control gate. Generally, the third polysilicon layer is deposited using a LPCVD process and is heavily doped in contrast to the first polysilicon layer. The methods used to dope the third polysilicon layer include diffusion doping, in-situ doping, and ion implantation doping techniques. In one embodiment, the polysilicon layer  346  is doped with n-type dopants to a concentration level of about 1×10 21  dopants per cubic centimeter or to another concentration level suitable for a control gate. As a result of the sloping ONO layer  340  and the spacers  330  and  332 , the polysilicon layer  346  is provided with smoother surface than otherwise possible. For example, a groove  348  provided on the polysilicon layer  346  between the spacers  330  and  332  has substantially less depth than the groove  216  ( FIG. 2D ) formed under the conventional method.  
         [0032]     A metal layer  350 , e.g., a tungsten silicide (Wsi), is deposited over the third polysilicon layer ( FIG. 2E ). The metal layer is free of a void or seam unlike in the conventional method due to the relatively smooth surface of third polysilicon layer, thereby providing a higher conductivity for the metal layer.  
         [0033]     While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. For example, specific dimensions discussed above are for the specific embodiments. These dimensions may depend on the particular application. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.