Abstract:
A method of manufacturing a flash memory device with blunted corners of the floating gate. The blunted corners of the floating gate allow a reduction in the required gate edge lifting that is designed into flash memory design and allows a shortening of the flash memory device to increase the density of flash memory devices that can be formed in a given area.

Description:
This application claims benefit to provisional application 60/163,761 Nov. 5, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to floating gate semiconductor memory devices such as EEPROMs. More specifically, this invention relates to a method of manufacturing floating gate semiconductor memory devices such as EEPROMs. Even more specifically, this invention relates to a method of manufacturing floating gate semiconductor memory devices such as EEPROMs that reduce the necessity for gate edge lifting. 
     2. Discussion of the Related Art 
     A class of non-volatile memory devices known as “flash” EEPROM (Electrically Erasable Programmable Read Only Memory) devices combines the advantages of EPROM density with the electrical erasability of an EEPROM. One feature that distinguishes flash EEPROM memory cells from standard EEPROM memory cells is that unlike standard EEPROM memory cells, flash EEPROM memory cells do not contain a select transistor on a one-for-one basis with each floating gate memory cell. A select transistor is a transistor that allows the selection of an individual memory cell within the memory device and is used to selectively erase a specific memory cell. Because flash EEPROMs do not have a select transistor for each floating gate transistor, flash EEPROM memory cells cannot be individually erased and therefore must be erased in bulk, either by erasing the entire chip or by erasing paged groups or banks of cells. Elimination of the select transistor allows for smaller cell size and gives the flash EEPROM an advantage in terms of manufacturing yield (in terms of memory capacity) over comparably sized standard EEPROMs. 
     Typically, a plurality of flash EEPROM cells is formed on a semiconductor substrate, which is also known as a silicon wafer. FIG. 1 illustrates a single conventional flash EEPROM memory cell having a double-diffused source region. As shown in FIG. 1, flash memory cell  100  is formed on a p-type substrate  110  and includes an n type double-diffused source region  102  and an n+ drain region  104 . The drain region  104  and the source region  102  are spaced apart from each other forming a channel region  122 . A source electrode  114  and a drain electrode  112  are connected to the source region  102  and the drain region  104 , respectively. 
     The double-diffused source region  102  is formed of a lightly doped n region  128  (phosphorous doped) and a more heavily doped but shallower n+ region  130  (arsenic doped) embedded within the deep n region  128 . The phosphorus doping within n region  128  reduces the horizontal electric field between the source region  102  and the substrate  110 . 
     The floating gate  106  is disposed a short distance above the source region  102 , the drain region  104  and the channel region  122  over a dielectric layer  118 , which is also known as a tunnel oxide region. Above the floating gate  106  and disposed over the dielectric layer  116  is a control gate  108 . The dielectric layer  116  is typically formed of an oxide/nitride/oxide layer known in the semiconductor manufacturing art as an ONO layer. A control gate electrode  120  is attached to control gate  108 . The dimension L GATE    132  represents the gate length for the gates contained in flash memory cell  100 . 
     In a conventional method of operation, the programming of a flash EEPROM memory cell is achieved by inducing “hot electron” injection from a section of the channel  122  near the drain  104  into the floating gate  106 . The injected electrons cause the floating gate  106  to carry a negative charge. Grounding the source region  102 , biasing the control gate  108  to a relatively high positive voltage and biasing the drain region  104  to a moderate positive voltage induce the hot electrons. 
     For example, to program the flash memory cell  100 , the source electrode  114  is connected to ground, the drain electrode  112  is connected to a relatively high voltage (typically +4 volts to +9 volts) and the control gate electrode  120  is connected to a relatively high voltage level (typically +8 volts to +12 volts). Electrons are accelerated from source region  102  to drain region  104  via the channel  122  and the “hot electrons” are generated near the drain region  104 . Some of the hot electrons are injected through he relatively thin gate dielectric layer  118  and become trapped in the floating gate  106  thereby giving floating gate  106  a negative potential. 
     After sufficient negative charge accumulates on floating gate  106 , the negative potential of floating gate  106  raises the threshold voltage of the stacked gate transistor and inhibits current flow through the channel  122  during a subsequent “read” mode. The magnitude of the read current is used to determine whether a memory cell has been programmed. 
     Conversely, to erase a flash memory device, electrons are typically driven out of the floating gate  106  by biasing the control gate  108  to a large negative voltage and biasing the source region  102  to a low positive voltage in order to produce a sufficiently large vertical electric field in the tunnel oxide. The large vertical field  136  in the tunnel oxide produces Fowler-Nordheim (F-N) tunneling of electrons stored in the floating gate  106  through the tunnel oxide into the source region  102 . Arrows  105  indicate the tunneling of the electrons from the floating gate  106  to the source region  102 . The charge removed from the floating gate  106  produces a threshold voltage shift. 
     For example, during erasure a relatively low positive voltage (typically from +0.5 volts to +5 volts) is applied to source electrode  114  and a relatively large negative voltage (typically from −7 volts to −13 volts) is applied to control gate electrode  120 . The voltage of the substrate electrode  126  is grounded and the drain electrode  112  is allowed to float. The vertical electric filed established between the control gate  108  and the source region  102  induces electrons previously stored in floating gate  106  to pass through dielectric layer  118  and into source region  102  by way of Fowler-Nordheim tunneling. 
     In order to produce a sufficient electric field in the tunnel oxide, it is typically necessary to bias the control gate  108  to a large enough negative voltage such that the floating gate  106  reaches a voltage of approximately −5.5 volts. A typical potential difference V SF  between the source region  102  and floating gate  106  is on the order of 10 volts and accordingly, when the source voltage V S  is made less positive, the control gate voltage V CG  should be made more negative. Once the source to floating voltage V SF  is selected, the remaining factors are preferably constrained according to the equation: 
     
       
           V   FG =α CG ( V   CG   −ΔV   T )+α S   V   S +α B   V   B   
       
     
     where: 
     V FG =the floating gate voltage; 
     V CG =the control gate voltage; 
     V S =the source voltage; 
     V B =the substrate or p-well bias; 
     ΔV T =the threshold voltage difference arising from negative charge added to the floating gate as measured from the control gate; 
     α CG =the capacitive coupling coefficient from the control gate to the floating gate; 
     α S =the capacitive coupling coefficient between the source and the floating gate; 
     α B =the capacitive coupling coefficient between the substrate or p-well and the floating gate. 
     As technology advances, a continuing goal throughout the industry is to increase the density of memory devices. By reducing the size of a flash EEPROM device a greater memory capacity can be achieved. As can be appreciated, the more die per wafer, the cost per die can be reduced. In addition, using higher density memory devices may provide for a reduction in the overall power consumption. 
     In order to increase the memory density of flash EEPROM devices, the memory cells are typically scaled down in size, for example the reduction in overall footprint of the device, is accomplished by reducing the gate length (L GATE )  132 . However, a problem with reducing the length of the memory cell gates is that the distance L CHHANNEL    122  between the source region  102  and the drain region  104  must also be reduced. As the source region  102  approaches the drain region  104 , the lateral diffusion of the phosphorous in the source region  128  causes a leakage between the source region  102  and the drain region  104  resulting in detrimental short channel effects. Short channel effects produce serious problems in the flash memory cells and are typically evident when the gate length (L GATE )  132  is reduced below 0.4 microns. 
     One method for reducing the short-channel effect would be to eliminate the double-diffused phosphorous region  128 . By using a single-diffused source region, the phosphorous diffusion overlap distance L DD    124  would no longer present and the short channel effect problem would be significantly reduced. Eliminating the phosphorous diffusion overlap distance L DD    124  would allow for a gate length (L GATE ) reduction below 0.4 microns and would provide for an increased packing density of the memory cells. 
     However, eliminating the phosphorous-doped N region  128  produces the unwanted side effect of increasing the horizontal electric field E H  in the pn junction between the source region  102  and the substrate  110  during erasure of the memory cell. This increase in the horizontal electric field E H  contributes to an increase in the band-to-band current since it is generally accepted that: 
     
       
           J   b-t-b   =A   b-t-b ƒ( E )e −(B   b-t-b   /E)   
       
     
     where: 
     J b-t-b =band-to-band current density (amps/cm 2 ) 
     A b-t-b ,B b-t-b =constants 
     ƒ(E) sometimes modeled as E 2    
     E=SQRT (E V   2 +E H   2 ) (the tunneling field in the junction). 
     Because of the source-to-substrate biasing during the erasure of the memory cell device, a reversed-biased pn junction is formed that produces band-to-band currents (also known as Zener currents) in the source junction. The band-to-band currents are normally several orders of magnitude larger than the Fowler-Nordheim current. This band-to-band current is hard to sustain from a circuit design point of view and is also believed to generate detrimental reliability problems such as hole trapping in the tunnel oxide. 
     One barrier to further decrease the size of flash memory cells is that during manufacture of the flash memory cells, there is a phenomenon called gate-edge lifting that occurs during oxidation procedures. Gate edge lifting causes the tunnel oxide to thicken at each end and is a particular problem at the source end of the tunnel oxide through which the electrons must tunnel during Fowler-Nordheim erasure. Because the gate edge lifting is not the same for each memory cell, the erase characteristics of each memory cell may differ and as a result the erase procedure has to be extended to ensure that all memory cells are erased. In addition, in order for each memory cell to be erased at the lowest possible voltage the source region has to be extended under the gate region until the heavily doped region of the double diffused source region reaches a region under the tunnel oxide that has not been thickened. This ensures that the erase characteristics for all the memory cells are the same. However, the extension of the source region under the gate until the heavily doped region reaches a non-thickened portion under the tunnel region prevents the memory cell from being shortened. 
     Methods have been developed to virtually eliminate the gate edge lifting, however when the gate edge lifting has been eliminated or substantially eliminated, it has been determined that a certain amount of gate edge lifting is necessary to stabilize erase characteristics. However, the required amount of gate edge lifting limits the amount the memory cell can be shortened. 
     Therefore, what is needed is a method of manufacturing flash memory devices that substantially eliminates the necessity to provide gate edge lifting and further provides uniform erase characteristics. 
     SUMMARY OF THE INVENTION 
     According to the present invention, the foregoing and other objects and advantages are achieved by a method of manufacturing a flash memory device having blunted floating gate corners. 
     In an aspect of the invention, a gate stack is formed on a substrate wherein the gate stack comprises a layer of tunnel oxide, a floating gate, a layer of ONO and a control gate. Portions of the tunnel oxide are etched forming cavities underneath each end of the floating gate thus exposing corners of each end of the floating gate. In the same process, the corners of the floating gate are oxidized and etched thus blunting the comers of the floating gate. 
     In another aspect of the invention, a layer of oxide is anisotropically deposited on the surfaces of the gate stack that fill the cavities. The layer of oxide is etched to form spacers on the side of the gate stack. 
     In another aspect of the invention, a double diffused source region and a drain region are formed in the substrate. 
     The described method of manufacturing a flash memory cell thus provides a flash memory cell having blunted corners on the floating gate thus allowing less required gate edge lifting. 
    
    
     The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications in various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, and further objects and advantages hereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 shows the structure of a typical flash memory cell; 
     FIGS. 2A-2D illustrate a portion of a prior art process for the manufacture of flash memory devices; 
     FIG. 2A shows a portion of a flash memory device including a substrate, a layer of tunnel oxide formed on the substrate and a “gate stack” formed on the layer of tunnel oxide comprising a floating gate, a layer of ONO and a control gate; 
     FIG. 2B shows the portion of the flash memory device as shown in FIG. 2A after an oxidation process to form a thin layer of oxide on the gate stack to protect the gate stack from a subsequent ion implantation process; 
     FIG. 2C shows the portion of the flash memory device as shown in FIG. 2B undergoing an ion implantation to form source and drain regions; 
     FIG. 2D shows the portion of the flash memory device as shown in FIG. 2C after an anneal process to drive the implanted ions into the substrate to form the source and drain regions; 
     FIG. 2E shows the Fowler-Nordheim tunneling paths of electrons during erase in a memory cell with no gate edge lifting; 
     FIG. 2F shows the Fowler-Nordheim tunneling paths of electrons during erase in a memory cell with gate edge lifting; 
     FIGS. 3A-3G illustrate a portion of a process for the manufacture of flash memory devices in accordance with the present invention; 
     FIG. 3A shows a portion of a flash memory device including a substrate, a layer of tunnel oxide formed on the substrate and a “gate stack” formed on the layer of tunnel oxide comprising a floating gate, a layer of ONO and a control gate; 
     FIG. 3B shows the portion of the flash memory device as shown in FIG. 3A after a hydrogen fluoride cleaning/oxidizing process that etches a portion of the layer of tunnel oxide under the floating gate forming cavities in the tunnel oxide and blunts the corners of the gate edges; 
     FIG. 3C shows the portion of the flash memory device as shown in FIG. 3B after an oxide has been anisotropically deposited on the flash memory device and etched to form a protective coating on the sides of the gate stack and that fills the cavities formed in the tunnel oxide; 
     FIG. 3D shows the portion of the flash memory device as shown in FIG. 3C with a patterned mask formed on the surface of the flash memory device and being implanted with phosphorus ions that will form the phosphorus implanted region of the double diffused source; 
     FIG. 3E shows the portion of the flash memory device as shown in FIG. 3D being implanted with arsenic ions to form the arsenic implanted region of the double diffused source and the drain region; and 
     FIG. 3F shows the portion of the flash memory device as shown in FIG. 3E undergoing an anneal process to drive the implanted ions into the substrate and to repair damage caused by the implantation process. 
     FIG. 3G shows the portion of the flash memory device as shown in FIG. 3F undergoing an anneal procedure. 
    
    
     DETAILED DESCRIPTION 
     Reference is now made in detail to specific embodiments of the present invention that illustrate the best mode presently contemplated by the inventors for practicing the invention. 
     FIGS. 2A-2D illustrate a portion of a prior art process for the manufacture of flash memory devices. FIG. 2A shows a portion  200  of a flash memory cell that includes a substrate  202  with a layer  204  of tunnel oxide formed on the surface of the substrate  202 . A structure  206  known as a gate stack is formed on the layer  204  of tunnel oxide. As is known in the semiconductor manufacturing art, the gate stack  206  structure is formed by forming a layer of polysilicon that will be the floating gate  208 , layer  210  of ONO, which is a thin layer of silicon oxide, a thin layer of silicon nitride, and a thin layer of silicon oxide, and a layer of polysilicon that will be the control gate  212 . After the layers of the various materials are formed, an etch process forms the gate stack as shown in FIG.  2 A. 
     FIG. 2B shows the portion  200  of the flash memory device as shown in FIG. 2A after an oxidation process that serves to form a protective layer  214  of oxide on the gate stack to specifically protect the exposed ends of the polysilicon material that will form the floating gate  208  and the control gate  212  by forming the thin layer of oxide  214  that forms on exposed silicon surfaces. However, the oxidation process causes gate edge lifting as shown at  216  and causes oxide to form underneath a portion of the layer  204  of tunnel oxide as indicated at  218 . As is known in the art of semiconductor manufacturing, the substrate, which is formed from lightly doped silicon, will oxidize as shown by the solid lines  220  and  222 . The dashed lines  224  show the original outline of the silicon substrate  202 . The dashed lines  226  show the original outline of the tunnel oxide  204 . 
     FIG. 2C shows the portion  200  of the flash memory device as shown in FIG. 2B with arrows  228  representing various ion implantations into the substrate  202  that will form the double diffused source region and the drain region. As is known in the semiconductor manufacturing art, there may be more than one ion implantation to implant phosphorus ions in the substrate to form the n region of the double diffused source region and another ion implantation to implant arsenic ions into the substrate to form the n+ region of the double diffused source region and the drain region of the flash memory device. 
     FIG. 2D shows the portion  200  of the flash memory device as shown in FIG. 2C after an anneal process to drive the phosphorus and arsenic ions into the substrate  202  to form the phosphorus implanted source region  230  and the arsenic implanted source region  232 . The anneal process causes further oxidation, which causes the layer of oxide to increase in thickness. The anneal process is an oxidizing process in order to repair the damage done during the implantation of dopant ions. This oxidation causes increased gate edge lifting because the oxidation is enhanced by the high impurity concentration of dopants from the implantations. The dashed line  238  indicates the original thickness of the oxide layer  214  resulting from the first oxidation and the line  240  indicates the outline of the increased thickness of the layer of oxide. The anneal process also causes increased gate edge lifting as indicated at  242 . As is known in the semiconductor manufacturing art, phosphorus ions migrate more readily than arsenic ions and a single anneal in a region that contain both phosphorus ions and arsenic ions can result in a double diffused region as shown by regions  230  and  232 . As shown within circle  234 , the tunnel oxide at the source end has been thickened in the region at which the electrons indicated at  236  must tunnel from the floating gate  204  to the source region  232  across the tunnel oxide  204  during the Fowler-Nordheim erase procedure. As known in the flash memory art, the electrons tunnel across the tunnel oxide  204  to the portion of the double diffused source region that has arsenic ions implanted. As discussed above, the electrons have to tunnel through a thicker tunnel oxide, which as also discussed above, may vary from memory cell to memory cell and may cause each memory cell to have differing erase characteristics. In addition, because the electrons tunnel to the arsenic implanted portion of the double diffused source region, the phosphorus implanted region of the double diffused source region must be driven further under the tunnel oxide and to maintain the minimum required channel length the size of the memory cell cannot be decreased. Therefore, it should be obvious that a process that does not include the second anneal/oxidation process would allow shorter memory cells and less gate edge lifting. 
     FIG. 2E illustrates the path taken by electrons during a Fowler-Nordheim erase in a memory cell with no gate edge lifting. The regions  230  and  232  are the phosphorus and arsenic doped regions of the double diffused source. The boundary line  250  denotes the extent of the phosphorus-doped region. The boundary line  252  denotes the extent of the arsenic-doped region and the lines  254 ,  256  and  258  denote increasing concentration profiles in the arsenic-doped region. Without gate edge lifting the thickness of the tunnel oxide  204  is the same in the region of the arsenic doped region  232 . The electrons tend to tunnel from the floating gate  208  to the highest concentration regions of the arsenic doped region  232 . This tendency is indicated by the thick dashed arrow  260 , the medium dashed arrow  262 , and the thin dashed arrow  264 . The tunneling falls off exponentially in the direction of the arrow  266 . The problem with the majority of the tunneling occurring from the corner  267  is that the tunnel oxide in that region becomes stressed and can cause early failure of the device. 
     FIG. 2F shows the Fowler-Nordheim tunneling paths of electrons during erase in a memory cell with gate edge lifting. The gate edge lifting is indicated at  242  both above and below the tunnel oxide  204 . When the tunnel oxide varies in thickness, the electrons tend to tunnel through the thinnest part of the oxide. The arrow  266  indicates the falling off exponentially of electron tunneling as described above in FIG.  2 E. The arrow  268  indicates the falling off exponentially of electron tunneling as the thickness increases. As can be appreciated, if only the gate edge lifting is reduced leaving a sharp corner, then almost all of the tunneling current goes through the sharp corner. For this reason, some designs purposely use gate edge lifting to attempt to achieve an even distribution of tunneling current, however, what typically is achieved is the tunneling current distribution across the tunnel oxide  204  as indicated by dashed arrows  269 . What is needed is to eliminate most of the gate edge lifting and blunt the corner so that the electric fields there are not as high. However, given the same source diffusion this will increase the tunneling current, probably too much. Therefore, the source region is moved back towards the gate edge until approximately the same tunneling current as was present with gate edge lifting is present. Because the moving of the source region back towards the gate edge makes the channel longer, the device can be shrunken allowing an increased density of devices for a given area. 
     FIGS. 3A-3G illustrate a portion of a process for the manufacture of flash memory devices in accordance with the present invention. FIG. 3A shows a portion  300  of a flash memory device that includes a substrate  302  with a layer  304  of tunnel oxide formed on the surface of the substrate  302 . A structure  306  known as a gate stack is formed on the layer  304  of tunnel oxide. As is known in the semiconductor manufacturing art, the gate stack  306  structure is formed by forming a layer of polysilicon that will be the floating gate  308 , layer  310  of ONO, which is a thin layer of silicon oxide, a thin layer of silicon nitride, and a thin layer of silicon oxide, and a layer of polysilicon that will be the control gate  312 . After the layers of the various materials are formed, an etch process forms the gate stack as shown in FIG.  3 A. 
     FIG. 3B shows the portion  300  of the flash memory device as shown in FIG. 3A after a process that removes portions of the layer  304  forming cavities  301  in the tunnel oxide underneath the floating gate  308 . The process that forms the cavities  301  in the tunnel oxide can be a process such as a hydrogen fluoride cleaning process or other isotropic etch process suitable for etching oxide. A weak oxidizing agent is added to the cleaning agent that oxidizes the surface of the floating gate  308 , which is then attacked by the cleaning agent. The combined oxidizing/etching process causes the corner  309  of the floating gate  308  to be “blunted” or rounded. The portion of the flash memory device within dashed circle  311  is shown enlarged in FIG.  3 C. 
     FIG. 3C shows the corner  309  of the floating gate  308 , the cavity  301  and the shape  313  of the corner  309  after the combined oxidizing/etching process. 
     FIG. 3D shows the portion  300  of the flash memory device as shown in FIG. 3B after a process of isotropically depositing oxide on the flash memory device and anisotropically etching the oxide to form spacers  305  and  307  as shown in FIG.  3 D. 
     FIG. 3E shows the portion  300  of the flash memory device as shown in FIG. 3D being implanted with phosphorus (P) ions as indicated by arrows  311 . A patterned mask layer  313  protects portions of the flash memory device that are not to receive phosphorus ions. The phosphorus ions are to form the phosphorus implanted region of the double diffused source region of the device. The phosphorus ions are implanted beneath the surface of the substrate  302  and initially are concentrated generally in a region indicated by the dashed shape  315 . It is noted that the spacer  305  protects the ends of the control gate  312  and floating gate  308  from damage that would be caused by the implantation of the phosphorus ions. 
     FIG. 3F shows the portion  300  of the flash memory device as shown in FIG. 3E with the mask layer  313  removed and being implanted with arsenic ions (As) as indicated by arrows  317 . The arsenic ions are to form the arsenic implanted region of the double diffused source region and to form the drain region of the flash memory device. Like the phosphorus ions, the arsenic ions are implanted beneath the surface of the substrate  302  and initially are concentrated generally in regions indicated by the dashed shapes  319 . It is noted that the spacers  305  and  307  protects the ends of the control gate  312  and floating gate  308  from damage that would be caused by the implantation of the arsenic ions. 
     FIG. 3G shows the portion  300  of the flash memory device as shown in FIG. 3F undergoing an anneal procedure, indicated by the wavy arrows  321 , the purpose of which is to drive the implanted arsenic ions and phosphorus ions into the substrate  302  to form the double diffused source region  323  and the drain region  325 . The anneal procedure also repairs damage to the substrate caused by the implantation procedure. 
     In summary, the present invention overcomes the limitations of the prior art and fulfills the need for a flash memory cell that provides blunted corners on the floating gate thus allowing less required gate edge lifting. 
     The foregoing description of the embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.