Abstract:
A non-volatile memory is provided which provides a floating gate (42) disposed over control gate (38) in order to increase the coupling therebetween. The degree of coupling may be varied by adjusting the area of the floating gate formed over the control gate relative to the area of the floating gate over the substrate.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to semiconductor devices, and more particularly to a non-volatile memory having improved coupling between gates for more efficient programming. 
     BACKGROUND OF THE INVENTION 
     Conventional EPROM devices are structured such that a control gate overlies a plurality of floating gates which are disposed between bitlines formed in a semiconductor surface. To program a standard EPROM memory cell, a voltage of approximately 14 volts is applied to the control gate and a voltage of approximately 10 volts is applied to one of the source/drain regions while the other source/drain is held at ground. The voltage on the control gate results in a depletion region between the two source/drain regions. When one source/drain is pulsed at 10 volts, a flow of electrons between source/drain regions the charge on the control gate, some of the electrons flowing between the source/drain regions will gain sufficient energy to penetrate an oxide layer and drift into the floating gate. This process is often termed &#34;hot-electron channel injection.&#34; 
     The rate at which electrons are attracted to the floating gate will depend upon the coupling between the floating gate and the control gate. A higher degree of coupling will produce a higher voltage on the floating gate, resulting in a greater number of electrons attracted thereto. 
     In conventional EEPROM devices, a thin tunnel oxide window is positioned in the gate dielectric near the source or drain region. To program the cell, the gate is pulsed to a voltage of approximately 17 volts while the source or drain is held at ground. The high electric field generated across the thin tunnel oxide causes a current to flow due to Fowler-Nordheim tunneling which charges the floating gate with electrons. 
     In both EPROMs and EEPROMs, higher coupling between the control gate and the floating gate increases the electric field across the gate oxide enhancing programming. 
     Typically, coupling between the control gate and the floating gate is on the order of 0.65-0.70. By increasing the coupling, the programmability of the device is improved, and therefore, a lower programming voltage may be used. 
     In order to increase coupling, present-day EPROMs and EEPROMs use a floating gate which extends over a thick oxide to maximize the area between the control gate and the floating gate. However, a fairly strong coupling component exists between the floating gate and the underlying substrate. Thus, a long floating gate also produces a high capacitance between the floating gate and the substrate, thereby negating some of the beneficial effects of a long floating gate. Additionally, extending the floating gate over the field oxide greatly increases the size of the cell. 
     Therefore, a need has arisen to provide a non-volatile memory with improved coupling within a small area between the floating gate and the control gate to enhance programmability. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method and apparatus for storing information is provided which substantially eliminates or prevents the disadvantages and problems associated with prior non-volatile memories. 
     In a first aspect of the invention, a non-volatile memory cell is provided having a substrate with source/drain regions formed therein. A control gate is formed over a first portion of the area between the source and drain regions. A floating gate is formed such that it covers both a portion of the control gate and the remaining portion of the area between source and drain. During programming, the effective coupling between the control gate and floating gate is increased relative to present day devices since the presence of the control gate decreases the coupling between the floating gate and the underlying substrate. 
     This aspect of the present invention provides the technical advantage that the efficiency of the memory cell is increased during programming, thereby allowing the memory cell to operate at a lower programming voltage and to occupy much less area. 
     In a second aspect of the present invention, a tunnel oxide is provided between the floating gate and the underlying substrate. This structure may be used as either a FLASH EPROM, FLASH EEPROM or an EEPROM. The increased coupling provided by the present invention also enhances the speed at which EEPROM programming and erase may be accomplished. 
     In a third aspect of the present invention, a polysilicon wordline is disposed over the floating gate and electrically connected to the control gate. This aspect of the invention further increases the coupling to the floating gate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a cross-sectional side view of a prior art non-volatile memory cell; 
     FIGS. 2a-b illustrates a cross-sectional side view and a top plan view of the non-volatile memory cell of the present invention; 
     FIG. 3 illustrates a cross-sectional side view of the memory cell of the present invention after a first processing stage; 
     FIG. 4 illustrates a cross-sectional side view of the memory cell of the present invention after a second processing stage; 
     FIG. 5a illustrates a cross-sectional side view of the memory cell of the present invention after a third processing stage; 
     FIG. 5b, illustrates a cross-sectional side view of an optional embodiment of the memory cell of the present invention after a third processing stage; 
     FIG. 6 illustrates a cross-sectional side view of the memory cell of the present invention after a fourth processing stage; and 
     FIG. 7 illustrates a cross-sectional side view of a second embodiment of the present invention using a third polysilicon layer to further improve coupling. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the present invention is best understood by referring to FIGS. 1-7 of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIG. 1 illustrates a conventional prior art EEPROM memory cell with a merged pass gate transistor. The EPROM memory cell 10 comprises N. source/drain regions 12 and 14 formed in a P- substrate 16. A dielectric oxide layer 18 having a tunnel oxide window 19 is formed above the surface of the substrate 16. A second dielectric layer 20 encloses a floating gate 22. A control gate 24 is formed above the floating gate 22, being separated from the floating gate 22 by the dielectric layer 20. 
     The floating gate 22 is programmed (written to) by accummulating negative charge on the floating gate 22 through Fowler-Nordheim tunnel injection (for FLASH EPROMs or EEPROMs) or by hot-electron channel injection (for EPROMs). Typically, to charge a floating gate, a voltage of approximately 14 volts is placed on the control gate 24 to form a channel between the source/drain regions 12 and 14 thereunder. Source/drain region 14 is pulsed at approximately 10 volts, while the region 12 is held at ground. This procedure produces a flow of electrons between the source/drain regions 12 and 14, as in an N-channel transistor. Because the high electric field created by the charge on the control gate 24, some of the electrons flowing between the source/drain regions 12 and 14 will gain sufficient energy to penetrate the dielectric layer 18 and drift into the floating gate 22. The number of electrons which drift into the floating gate 22 will be dependent upon the coupling between the control gate 24 and floating gate 22, since a larger voltage on the floating gate 22 will attract more electrons. 
     The speed at which the memory cell 10 can be programmed is dependent upon the rate at which electrons are attracted to the floating gate 22. Therefore, if more electrons can be drawn to the floating gate 22 in a given time, then the programming speed may be increased. Further, by increasing coupling between the control gate 24 and the floating gate 22, the voltage on the control gate 24 may be reduced. 
     FIGS. 2a-b illustrate a cross-sectional side view of the memory cell 26 of the present invention. The memory cell 26 comprises source/drain regions 28 and 30 disposed in a semiconductor surface 32. The source/drain regions 28 and 30 and the top portion of the semiconductor surface 32 are covered by a dielectric layer 34 having areas of increased thickness 36 over the source/drain regions 28 and 30. A control gate 38 overlies a first portion 40 of the semiconductor surface 32 between the source/drain regions 28 and 30. A floating gate 42 overlies a second portion 44 of the semiconductor surface between the source/drain regions 28 and 30 and also overlies a portion of the control gate 38. The control gate 38 is separated from the floating gate 42 by an interlevel oxide (ILO) layer 46 and an interlevel nitride (ILN) layer 48 and a sidewall oxide regions 49. A sealing oxide layer 50 is disposed over the structure and a relatively planar oxide 52 covers the sealing oxide 50. A metal layer 54 is disposed over the planar oxide 52 and contacts the control gate 38. 
     FIG. 2b illustrates a top plan view of the memory cell 26 of the present invention. As shown, the floating gate 42 overlaps the control gate 38 over the substrate 32, between bitlines 28 and 30. Contact 56 indicates the region in which the metal layer 54 contacts the control gate 38. 
     The memory cell 26 of the present invention increases coupling between the control gate 38 and the floating gate 42 by effectively decoupling a large portion of the floating gate 42 from the underlying substrate 32. 
     The increased coupling between the control gate 38 and floating gate 42 can be shown by mathematical expression: 
     
         K=C.sub.2 /(C.sub.1 +C.sub.2), 
    
     where 
     C 1  =capacitance between the floating gate 42 and the substrate 32 
     C 2  =capacitance between the floating gate 42 and the control gate 38 
     K=coupling between the control gate 38 and the floating gate 42 
     neglecting capacitances between the floating gate 42 and the source/drains 28 and 30, which should be small. 
     Assuming the dielectric constants for the dielectric layer 34 and the ILO/ILN 46 and 48 are equal, as are the electrical thicknesses thereof, then C 1  should be proportional to the area between the floating gate and the substrate (A 1 ), and C 2  should be proportional to the area between the floating gate and the control gate (A 2 ). Since A 1  =L 1  w and A 2  =L 2  w (where w=width), the equation for coupling simplifies to: 
     
         K=L.sub.2 /(L.sub.1 +L.sub.2) since L=L.sub.1 +L.sub.2, L.sub.1 =L-L.sub.2 
    
     and thus 
     K=L 2  /L 
     Table I illustrates the coupling per various combinations. T1 TABLE I- COUPLING FOR VARIOUS CONFIGURATIONS? - L? L 2?  K? - 1.5 0.75 .5  - 1.5 1.00 .67 - 2.0 1.50 .75 - 1.25 0.75 .60 - 1.25 1.00 .80 - 
     The assumptions that the dielectric constants and electrical thicknesses are equal for the dielectrics disposed between the floating gate 42 and control gate 38 and between the floating gate 42 and the semiconductor surface 32 are very conservative. In practice, the dielectric constant for the material between the floating gate 42 and the control gate 38 will be substantially greater than that for the material between the floating gate 42 and semiconductor surface 32. Hence, the capacitance between the floating gate 42 and control gate 38 will be substantially higher, thereby increasing the coupling. 
     Typical coupling figures for conventional EEPROMs range between 0.65-0.70. As shown in Table I, for a configuration in which the area of the floating gate overlying the control gate is four times that of the area of the floating gate 42 overlying the semiconductor surface 32, a coupling coefficient of 0.80 is achieved, a substantial improvement over the prior art. An overlap of three-to-one results in a coupling coefficient of 0.75, also an improvement over the prior art. 
     FIGS. 3-6 illustrate the process flow for fabricating the memory cell of the present invention. FIG. 3 illustrates the memory cell after the first processing stage. The semiconductor surface 32, typically a P- silicon layer, is patterned with photoresist, followed by implantation of an N+ dopant to form the source/drain regions or bitlines 28 and 30. A thermal oxide layer 34 of about 500 Angstroms is grown over the silicon surface 32. The N+ regions 28 and 30 enhance the growth of the oxide layer 34 thereover, creating areas of increased thickness 36. Typically, the areas of increased thickness 36 are about 2500 Angstroms thick. A polysilicon layer 58 is formed over the dielectric layer 34, and ILO and ILN layers 46 and 48 are formed over the polysilicon layer 58. 
     In FIG. 4, a cross-sectional side view of the memory cell of the present invention is shown after a second processing stage. The polysilicon layer 58 and ILO/ILN layers 46 and 48 are masked to define the control gate 38 in a first direction. At this point, long strips of the polysilicon layer 58 remain, the control gate 38 being etched in a second direction as described in connection with FIG. 6. At this point, the oxide beneath where the floating gate will be formed is typically stripped. 
     In FIG. 5a, a cross-sectional side view of the present invention is illustrated after a third processing stage. The previously stripped oxide is regrown to a thickness of about 300 Angstroms simultaneously with sidewall oxide regions 49 formed on the exposed edges of the control gate 38. At this point, a tunnel oxide 61 is formed as illustrated in FIG. 5b. To form the tunnel oxide 61, a portion of the oxide layer is removed down to the substrate 32 and approximately 100 angstroms of oxide is regrown. This results in about 50 Angstroms of additional oxide being added to the 300 Angstrom regrowth. 
     Referring again to FIG. 5a, a second polysilicon layer 62 is formed over the device. 
     In FIG. 6, the second polysilicon layer 62 is patterned and etched to define the floating gate in a first direction. Thereafter, a stack etch is performed to define the floating gate 42 and control gate 38 in a second direction, such that rectangular gates are created. 
     The sealing oxide 50 is grown over the device, forming a thin layer over exposed portions of oxide and polysilicon. Thereafter, the thick planar oxide 52 is deposited over the device and planarized. A window 64 is etched through the planar oxide 52, ILN layer 48, and ILO layer 46 to expose a portion of control gate 38. Thereafter, the metal layer 54 is deposited, patterned and etched to provide electrical contact to the control gate 38. 
     FIG. 7 illustrates a cross-sectional side view of an alternative embodiment of the present invention. The process steps performed in connection with FIGS. 3-5 are repeated. The second polysilicon layer 62 is patterned and etched to define the floating gates in a first direction. A sealing oxide layer 50 is deposited or grown over the device. A contact opening 66 is patterned and etched in through the ILN layer 48 and ILO layer 46 to expose a portion of the control gate 38. A third layer of polysilicon 68 is deposited over the device and a stack 5 etch is performed. The stack etch forms wordlines in the third polysilicon layer 68 and defines the floating gates 42 and control gates 38. The wordlines 68 contact the control gates 38 through the contact opening 66. 
     This embodiment of the present invention provides additional technical advantages over the prior art. The wordline provides additional capacitive coupling to the floating gate, thereby increasing the rate at which electrons are attracted to the floating gate. 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.