Abstract:
A non-volatile memory cell, comprising a semiconductor substrate having a first conductivity type. A control region is formed of said first conductivity type in the substrate and a control region oxide formed over the control region. The cell includes a program element having a first active region of a second conductivity type formed in said substrate, a doped or implanted region adjacent to said first active region, and a gate oxide overlying at least the channel region. An active region oxide covers a portion of the first active region. A floating gate is formed over said semiconductor substrate on said active region oxide and said control region oxide.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to nonvolatile memory, and particularly an EEPROM device suitable for use in programmable logic devices and a method of forming the device. 
     2. Description of the Related Art 
     Non-volatile memory devices of the type commonly referred to in the art as EPROM, EEPROM, or Flash EEPROM serve a variety of purposes, and are hence provided in a variety of architectures and circuit structures. 
     As with many types of integrated circuit devices, some of the main objectives of non-volatile memory device designers are to increase the performance of devices, while decreasing device dimensions and consequently increasing circuit density. Cell designers strive for designs which are reliable, scalable, cost effective to manufacture and able to operate at lower power, in order for manufacturers to compete in the semiconductor industry. EEPROM devices are one such device that must meet these challenges. In some applications, such as flash memory cards, density is at a premium, while in applications such as programmable logic devices (PLD&#39;s), reliability is more important and space is at less of a premium. Generally, arrays of individual memory cells are formed on a single substrate and combined with sense and read circuitry, and connected by row-wise and column-wise conductive regions or metallic conductors to allow for array wide bulk program and erase as well as selected bit programming. 
     Semiconductor process technology has continued to move toward defining smaller device features, and the conventional “stacked gate” EEPROM structure has given way to different cell designs and array architectures, all intended to increase density and reliability in the resulting circuit. In addition, in EEPROM devices used for programmable logic devices, designers strive to reduce power requirements of devices by reducing program and erase voltage requirements. 
     Conventionally, programmable logic EEPROMS were typically formed by stacked gate devices operating utilizing Fowler-Nordheim tunneling to program and erase the floating gate or in single polysilicon-based cells such as that set forth in U.S. Pat. No. 4,924,278. An alternative to the aforementioned Fowler-Nordheim tunneling-based cell structures is presented in Ranaweera, et al., “Performance Limitations of a Flash EEPROM Cell, Programmed With Zener Induced Hot Electrons,” University of Toronto Department of Electrical Engineering (1997). Discussed therein is a flash EEPROM cell which accomplishes programming and erase by establishing a reverse breakdown condition at the drain/substrate junction, generating hot electrons which are then injected into the floating gate to program the cell. To program the flash ZEEPROM cell, the PN junction is reverse-biased to create an electric field of approximately 10 6  volt/cm. and generate energetic hot electrons independent of the channel length. The P+ region adjacent to the drain enhances this generation. A low junction breakdown voltage can be used for programming by optimizing the PN junction depth and profiles. 
     A structure and method for programming an avalanche injection cell is detailed in co-pending U.S. patent application Ser. No. 08/871,589, inventors Hao Fang, et al., filed Jul. 24, 1998 and assigned to the assignee of the present application. 
     In Fang, et al. the non-volatile memory cell is formed of a P substrate having embedded therein an N+ source region, an N-type diffused drain region, a floating gate capacitively coupled to the P substrate through a tunnel oxide, or other gate dielectric such as nitride oxide; and a control gate coupled capacitively to the floating gate through an oxide/nitride/oxide, or other type of inter polysilicon dielectric, film. The diffused region is formed of a shallowly diffused but heavily doped N-type junction, while the source region is formed of a deeply diffused but lightly doped N junction. 
     To program the cell, electron injection is effected from the drain side. The programming operation is accomplished by applying +3 volts on the drain and −6 volts on the P substrate so as to shift upwardly the threshold voltage V t  by 4 volts in approximately 0.002 seconds. To erase, holes are injected from the drain side by applying +6.5 volts on the drain and −3 volts on the P substrate so as to shift down with the voltage threshold V t  by 4 volts. 
     The Fang, et al. application also teaches a single polysilicon layer embodiment wherein the stacked control gate is replaced with a diffusion region. FIG. 1 represents a schematic depiction of such embodiment. The control gate can be switched between 0 volts and V cc  to select and de-select the cell during the read period and between V jb  and 0 volts to program and erase the cells as set forth above. A select transistor is added at the source side to enable a fast read of the memory cell. Cell size is decreased in comparison to conventional single poly memory cells for programmable logic devices. 
     Even with the scaling advantages presented by the ZEEPROM-type cells, designers constantly seek to improve scalability, performance and cost advantages of cells. 
     Each of the aforementioned configurations presents advantages and disadvantages when used in particular applications. Nevertheless, improvements in both the structure of individual cells and the manner in which they are connected together will result in more reliable, stable, faster, and lower power devices which can be programmed and erased at lower voltages. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention, roughly described, comprises a non-volatile memory cell. The cell is formed on and in a semiconductor substrate having a first conductivity type and includes a control region formed of said first conductivity type in a well of a second conductivity type in the substrate. A control region oxide is formed over the control region. The cell includes a program element having a first active region of a second conductivity type formed in said substrate. A doped or implanted impurity region of the first conductivity type is positioned adjacent to said first active region, and an active region oxide overlies at least a portion of the first active region. A floating gate is formed over said semiconductor substrate on said active region oxide and said control gate oxide. 
     In one aspect, the channel region and the control region are formed simultaneously by, for example, an impurity implant of said first conductivity type into said control region and channel region. Using the cell of the present invention provides advantages in scaling and improves the quality of the oxide formed over the control region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with respect to the particular embodiments thereof. Other objects, features, and advantages of the invention will become apparent with reference to the specification and drawings in which: 
     FIG. 1 is a schematic diagram of the single polysilicon embodiment of the avalanche/Zener injection EEPROM cell of the prior art. 
     FIG. 2 is a schematic diagram of a first embodiment of an avalanche/Zener injection EEPROM cell of the present invention. 
     FIG. 3 is a semiconductor cross-section of the avalanche/Zener injection EEPROM cell of the present invention. 
     FIG. 4 is a schematic diagram of a second embodiment of an avalanche/Zener injection EEPROM cell of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A novel EEPROM structure, including a buried control gate formed of a p-type impurity, is hereafter disclosed. In the following description, numerous details, for example specific materials process steps, etc., are set forth in order to provide a thorough understanding of the invention. It will be readily understood, however, to one of average skill in the art that specific details need not be employed to practice the present invention. Moreover, specific details of particular processes or structures may not be specifically presented in order to not unduly obscure the invention where such details would be readily apparent to one of average skill in the art. Those having ordinary skill in the art and access to the teachings described herein will recognize additional modifications and applications and embodiments within the scope of the present invention. 
     FIG. 2 shows a schematic diagram of a first embodiment of a nonvolatile memory cell structure formed in accordance with one aspect of the present invention. 
     Structure  100  includes an (array) control gate ACG, floating gate FG, avalanche/Zener program element Q w , a read transistor Q r , and a sense element Q c . The array control gate (ACG)  240  is used to accelerate electrons or holes selectively to or from the floating gate FG by capacitively coupling a field across the oxide that separates the avalanche element from the floating gate. 
     As shown in FIG. 2, sense transistor Q c  and avalanche element Q w  share floating gate FG  112 . Floating gate FG  112  is capacitively coupled to array control gate (ACG)  240  voltage via capacitor  211 . As shown in FIG. 3, capacitor  211  comprises ACG region  240  and program gate oxide  115 . In the first embodiment shown in FIG. 2, program and erase by hot carrier generation occurs at one junction of the program element Q w , that is, at the p-n junction between region  122  and channel  230 . Program and erase can be separated over separate junctions (i.e. program over the junction between region  122  and channel  230  and erase at the junction between region  124  and channel  230 ) as shown in FIG.  4 . 
     Program element Q w  allows program and erase of EEPROM  100  through generation of hot electrons and hot holes which are swept onto the floating gate upon application of appropriate voltage to the program junction, as described below. 
     FIG. 3 shows an exemplary cross-section of the embodiment of the EEPROM cell  100  as formed on a semiconductor substrate  105 . It should be understood that numerous ones of such cells are formed in a single integrated circuit device in an array in accordance with well-known techniques. Silicon substrate  105  has a first conductivity type such as a P-type conductivity having a background doping concentration of about 1×10 15 -1×10 17  cm −3 . 
     Avalanche/Zener program element Q w  shares floating gate FG with sense element Q c , and includes a first active region  122  and second active region  124 . Floating gate FG  112  overlies the program element oxide layer  140 , the program junction oxide  115  and sense oxide layer  117 . Floating gate FG is formed of a conducting material, such as a polycrystalline silicon material. 
     In accordance with the invention, array control gate  240  is formed of a conductivity type the same as the substrate, in this embodiment a p-type impurity, and is provided on one side of the field oxide region  101 . A well region  290  is provided in the substrate  105  prior to formation of the control gate  240 . The n well  290  serves to isolate the control gate region  240  and allow specific selection of the control gate  240  during device operation. 
     Program element Q w  is electrically separated from the ACG  240  by isolation region  101 , and the ACG is separated from sense transistor Q c  by isolation region  102 , e.g. silicon dioxide, also formed in the semiconductor substrate  105 . Field oxidation regions  101  and  102  represent a device isolation structure formed in accordance with well known techniques such as LOCOS, trench isolation, shallow trench isolation and various equivalent alternatives. The shape of the isolation depicted in the figures of the present disclosure is not intended to limit the nature of the type of isolation used herein. 
     A channel  230  is positioned between regions  122  and  124 . Overlying the channel  230  is an oxide layer  140 . The oxide layer  140  is typically composed of an insulating material, such as silicon dioxide, and has a thickness of approximately 80 to 150 angstroms. Oxide layer  140  may be deposited or grown (using conventional oxide deposition techniques) in a single process step. 
     Prior to formation of the gate stacks (regions  112 / 140 ,  112 / 117  and  113 / 114 ), a p+ type impurity implant is made in the substrate to form a p+ region  155  in the channel  230  of the program element Q w  adjacent to regions  122  and  124 . Typically an implant of boron at an energy of 30 to 200 KeV, to a depth as great as 0.1 to 0.4 μm in a concentration of about 1×10 18  to 1×10 20  cm −3  is suitable. This p-type impurity implant allows for specific breakdown voltage engineering of the avalanche program element Q w  of the cell with a great deal of accuracy. In cell  100 , reverse breakdown voltages in a range of 3V to 10V may be used in order to generate energetic hot carriers independent of the channel length of the device. It should be recognized with reference to Ranaweera, et al., that the breakdown characteristics of the various P+ N+ junctions varies with the concentration of the P+ region  155 . 
     In an advantageous feature of the invention, the implant utilized to form impurity region  155  can likewise be utilized to form the control gate region  240  and save processing steps. It will be recognized that such implant is advantageous in either the single side embodiment of FIG. 2 or the two-side (program/erase) embodiment of FIG.  4 . 
     Gate oxide  115  and floating gate  112  are formed in accordance with conventional techniques by, for example, forming a thermal oxide on the surface of substrate  105 , depositing a polysilicon layer on top of the gate oxide, and etching the gate oxide and polysilicon layers to form oxides  114 ,  115 ,  117  and  140  and floating gate  112 . Various alternative methods are suitable for growing the gate oxide layer and are well within the knowledge of one of average skill in the art. For example, oxide  115  may be grown during the same step or separate steps. Likewise, numerous techniques for forming the floating gate layer may be used, including, but not limited to depositing polysilicon by chemical vapor deposition or sputtering and annealing techniques well known to one of average skill in the art. Etching of the polysilicon and gate oxide layers may be performed by any number of suitable wet or dry directional etch step in accordance with well-known techniques. 
     Sense transistor Q c  shares first active region  132  with read transistor Q r  Gate  113  of read transistor Q r  is connected to word line WL. Active region  136  of read transistor Q r  is connected to a read signal select (product term) PT, while region  134  of sense transistor Qc is connected to sense signal (product term ground) PTG. 
     A sense transistor channel is formed between region  134  and region  132 . The conductivity of the region  134  and region  132  is of the second conductivity type, for example, an N+ conductivity type. Overlying the sense channel is an oxide layer  117  having an approximate thickness of 80 angstroms. The sense gate oxide layer  117  may also be simultaneously formed with the oxide layer  140 . Depending on the mode of sense transistor Q c  (depletion or enhancement mode), the relevant voltages for operating the EEPROM cell  100  are adjusted. The sense transistor Q c  is, in one embodiment, a depletion mode transistor, as is commonly understood in the industry. In a further embodiment, the sense transistor Q c  is an enhancement mode transistor (also as commonly known in the industry). 
     The read transistor Q r  includes region  132  and region  136 , both formed of the second conductivity type, e.g. an N+ conductivity type. A channel is positioned between regions  132  and  136 . Overlying the read channel is an oxide layer  114  that is composed of an insulating material, such as silicon dioxide, and has an approximate thickness of 25-150 angstroms. Oxide layer  114  may be formed in the same step as the oxide layer  117 , or in a separate step. A read gate  113  overlies the read oxide layer  114  and is composed of a conducting material, such as a polycrystalline silicon material. 
     Regions  122 ,  124 ,  132 ,  134  and  136  may be formed by an impurity implant of a dopant having a conductivity type opposite to that of the substrate (arsenic or phosphorus, for example) to form self-aligned active regions in substrate  105 . Typical junction depths of 0.1 μm to 0.5 μm and doping concentration of about 5×10 18  to 1×10 21  cm −3  are suitable for regions  134 , 132  and  136 . Substrate  105  may optionally have a connection  107  to allow for biasing the substrate. 
     Exemplary operational characteristics for the device shown in FIGS. 2 and 3 are given as follows: to add electrons to floating gate FG  112 , the substrate is biased to 0V, region  124  is internally isolated, region  122  is at, for example, 8V and the FG is coupled to a positive voltage from control gate ACG, such as 8V. To remove electrons from FG, the substrate is biased to 0V, region  124  is floating or isolated, second region  122  is at 8V and FG is at a low voltage coupled to the ACG of about 0V. It should be understood that either adding electrons (or removing holes), or removing electrons (or adding holes) can constitute a “program” or “erase” operation, as such “program” or “erase” operation is defined by the context of the overall device in which the non-volatile memory cell is used. It should be noted that the voltage on the product-term ground connection may be varied in accordance with the EEPROM design constraint that a higher voltage will increase program/erase speed, but may induce greater oxide damage, while lower voltages will ensure better oxide qualities, and hence greater data retention integrity, over time. 
     Typical operating voltages for the foregoing lines in the cell shown in FIG. 2 are given in Table 1: 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 WBL 
                 SUBSTRATE 
                 ACG 
                 PT 
                 PTG 
                 WL 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Erase 
                 6 v 
                 0 v 
                 8 v 
                 Float 
                 &gt;0-8 v 
                 Vcc 
               
               
                 Program 
                 6 v 
                 0 v 
                 0 v 
                 Float 
                 0 v 
                 0 v 
               
               
                   
               
             
          
         
       
     
     In contrast with the cell disclosed in U.S. Pat. No. 4,924,278, the cell of the present invention utilizes the avalanche/Zener injection capacities of the aforementioned prior art to place electrons or holes on the floating gate in accordance with the techniques described therein. 
     Because of the separate formations for each of the elements, the diode doping gradient for transistor Q w  can be selected to control the avalanche breakdown voltage of cell Q w  and a scaling of the programming voltage below current known levels. 
     In a further unique aspect of the present invention, a reduction in program voltage coincident with the avalanche/Zener program element allows a reduction in oxide thickness for all floating gate elements. It should be recognized that the amount of oxide thickness reduction is limited by the necessity to maintain data retention integrity. Separation of the read path and program elements in the present embodiment further allows one to use differing oxides for the read and sense elements. 
     It should be recognized that the cell described herein may be utilized with any number of coupling arrangements in any number of matrix arrangements shown herein or in the prior art. It should be further recognized that the method of the present invention may be utilized to construct a non-volatile device wherein the operating parameters vary from the exemplary embodiment set forth above. 
     In the alternative embodiment of the present invention shown in FIG. 4, programming and erase using hot electrons or hot holes generated by Zener/avalanche breakdown performed over different regions of the cell oxide  140 , alternatively any regions  124 / 155  and  122 / 155  improves the quality of oxide  140  over prolonged program and erase. 
     The many features and advantages of the present invention will be apparent to one of average skill in the art in view of the illustrative embodiments set forth herein. The present invention has been described herein with respect to particular embodiments for a particular applications. It will be apparent to one of average skill in the art that numerous modifications and adaptations of the present invention may be made in accordance with the invention without departing from the spirit of the scope of the invention as disclosed herein and defined by the following claims.