A device and a method of forming a floating gate memory transistor of very small area, thereby allowing a high-density integrated circuit chip, more specifically for Erasable Programmable Read-Only memory (EPROM) or similar non-volatile devices.
In a first embodiment, a method is disclosed that fabricates a programmable memory cell described as a "diffusion cut" cell where a plug-type floating gate contact hole cuts through a diffusion region and partially into a substrate region. In a second embodiment, a method is disclosed that fabricates a programmable memory cell described as an "oxide cut" cell, where the plug-type floating gate contact hole only penetrates a silicon oxide c2 g0">layer. This "oxide cut" cell is formed in a similar fashion except penetration does not go into the diffusion region or substrate.
|
1. A method of fabricating a minimum area floating gate memory cell within an IC chip contact well comprising the steps of:
a. growing and patterning a field oxide on a substrate by a locos process; b. implanting an impurity element in the substrate to form a diffusion region adjacent the field oxide; c. depositing an oxide c2 g0">layer over the diffusion region and field oxide; d. patterning and etching a contact well in the oxide c2 g0">layer, field oxide, diffusion region, and substrate further comprising: i. performing a cell plug masking; ii. etching the oxide c2 g0">layer; iii. etching the silicon and diffusion region, thereby forming an extended channel; e. growing a gate/tunnel oxide c2 g0">layer within the contact well; f. depositing and doping a first c1 g0">polysilicon c2 g0">layer with within the contact well, thereby forming a floating gate and capacitor c1; g. etching the first c1 g0">polysilicon c2 g0">layer until the only c1 g0">polysilicon is within the contact well forming a c1 g0">polysilicon plug; h. depositing a second c1 g0">polysilicon c2 g0">layer within the contact well; i. etching back the second c1 g0">polysilicon c2 g0">layer leaving a thin c2 g0">layer of the second c1 g0">polysilicon on a perimeter of the contact well; j. depositing an ono dielectric c2 g0">layer over the second c1 g0">polysilicon c2 g0">layer wherein the ono dielectric c2 g0">layer is about 100 Å thick; k. depositing and doping a third c1 g0">polysilicon c2 g0">layer over the contact hole, thereby forming a second capacitor c2 between the third c1 g0">polysilicon c2 g0">layer and the second c1 g0">polysilicon c2 g0">layer wherein a capacitance ratio of c2/c1 is maximized by controlling a depth of the oxide c2 g0">layer of step c, thereby providing a maximum voltage and charge on the floating gate during electrical programming; l. depositing a tungsten silicide c2 g0">layer over the third c1 g0">polysilicon c2 g0">layer; and then m. masking and etching the third c1 g0">polysilicon c2 g0">layer and tungsten silicide to form a word line.
2. A method of fabricating a minimum area floating gate memory cell within an IC chip contact well comprising the steps of:
a. growing and patterning a field oxide on a substrate by a locos process; b. implanting an impurity element in the substrate to form a diffusion region adjacent the field oxide; c. depositing an oxide c2 g0">layer over the diffusion region and field oxide; d. patterning and etching a contact well in the oxide c2 g0">layer further comprising: i. performing a cell plug masking; ii. etching the oxide c2 g0">layer; iii. etching the silicon and diffusion region, thereby forming an extended channel; e. growing a gate/tunnel oxide c2 g0">layer within the contact well, thereby forming a floating gate and a capacitor c1 between the first c1 g0">polysilicon c2 g0">layer and the substrate, said capacitor c1 having a minimal capacitance; f. depositing and doping a first c1 g0">polysilicon c2 g0">layer within the contact well, thereby forming a floating gate and capacitor c1; g. etching the first c1 g0">polysilicon c2 g0">layer until the only c1 g0">polysilicon is within the contact well forming a c1 g0">polysilicon plug; h. depositing a second c1 g0">polysilicon c2 g0">layer within the contact well; i. etching back the second c1 g0">polysilicon c2 g0">layer leaving a thin c2 g0">layer of the second c1 g0">polysilicon on a perimeter of the contact well; j. depositing an ono dielectric c2 g0">layer over the second c1 g0">polysilicon c2 g0">layer wherein the ono dielectric c2 g0">layer is about 100 Å thick; k. depositing and doping a third c1 g0">polysilicon c2 g0">layer over the contact hole, thereby forming a second capacitor c2 between the third c1 g0">polysilicon c2 g0">layer and the second c1 g0">polysilicon c2 g0">layer c2 having a larger capacitance than c1; l. depositing a tungsten silicide c2 g0">layer over the third c1 g0">polysilicon c2 g0">layer; and then m. masking and etching the third c1 g0">polysilicon c2 g0">layer and tungsten silicide to form a control gate and word line. 3. A method of fabricating a floating gate memory cell comprising the steps of: a. forming a doped region on a substrate; b. disposing a first c2 g0">layer of dielectric material over said doped region, said first c2 g0">layer of dielectric material having a thickness; c. forming a contact well extending through said first c2 g0">layer of dielectric material and through said doped region, said contact well having inner walls and a bottom portion; d. disposing a second c2 g0">layer of dielectric material within said contact well over said inner walls and bottom portion; e. disposing conductive material within said contact well over said second c2 g0">layer of dielectric material, thereby forming a floating gate and a first capacitor; f. disposing a third c2 g0">layer of dielectric material within said contact well over said conductive material; and g. disposing conductive material over said third c2 g0">layer of dielectric material within said contact well, thereby forming a second capacitor, wherein a capacitance ratio between said first capacitor and said second capacitor is maximized by controlling said thickness of said first c2 g0">layer of dielectric material, thereby providing a maximum voltage and charge on said floating gate during electrical programming. 4. The method, as set forth in (h) forming a conductive line coupled to said conductive material disposed in step (g). 5. The method, as set forth in claim 3, wherein step (a) comprises the steps of: (a1) disposing a field oxide on said substrate, said field oxide having a window therein; and (a2) implanting an impurity in said substrate through said window.
6. The method, as set forth in claim 5, wherein step (b) comprises the step of: (b1) disposing an oxide c2 g0">layer over said doped region and over said field oxide. 7. The method, as set forth in claim 3, wherein step (e) comprises the steps of: (e1) disposing a first c2 g0">layer of conductive material within said contact well; (e2) removing a portion of said first c2 g0">layer of conductive material to leave a plug of conductive material disposed in said bottom portion of said contact well; (e3) disposing a second c2 g0">layer of conductive material within said contact well; and (e4) removing a portion of said second c2 g0">layer of conductive material to leave a c2 g0">layer of conductive material disposed on said inner walls of said contact well. 8. The method, as set forth in claim 3, wherein step (f) comprises the step of: (f1) disposing a c2 g0">layer of ono dielectric approximately 100 angstroms thick within said contact well over said conductive material. 9. The method, as set forth in claim 3, wherein said conductive material disposed in steps (e) and (g) is c1 g0">polysilicon. 10. The method, as set forth in claim 4, wherein step (h) comprises the steps of: (h1) depositing a tungsten silicide c2 g0">layer over said conductive material disposed in step (g); and (h2) masking and etching said conductive material disposed in step (g) and said tungsten silicide c2 g0">layer to form a word line. 11. A method of fabricating a floating gate memory cell comprising the steps of: a. forming a doped region on a substrate; b. disposing a first c2 g0">layer of dielectric material over said doped region, said first c2 g0">layer of dielectric material having a thickness; c. forming a contact well extending through said first c2 g0">layer of dielectric material to said doped region without etching into said doped region, said contact well having inner walls and a bottom portion; d. disposing a second c2 g0">layer of dielectric material within said contact well over said inner walls and bottom portion; e. disposing conductive material within said contact well over said second c2 g0">layer of dielectric material, thereby forming a floating gate and a first capacitor; f. disposing a third c2 g0">layer of dielectric material within said contact well over said conductive material; and g. disposing conductive material over said third c2 g0">layer of dielectric material within said contact well, thereby forming a second capacitor. 12. The method, as set forth in claim 11, further comprising the step of: (h) forming a conductive line coupled to said conductive material disposed in step (g). 13. The method, as set forth in claim 11, wherein step (a) comprises the steps of: (a1) disposing a field oxide on said substrate, said field oxide having a window therein; and (a2) implanting an impurity in said substrate through said window. 14. The method, as set forth in claim 13, wherein step (b) comprises the step of: (b1) disposing an oxide c2 g0">layer over said doped region and over said field oxide. 15. The method, as set forth in claim 11, wherein step (e) comprises the steps of: (e1) disposing a first c2 g0">layer of conductive material within said contact well; (e2) removing a portion of said first c2 g0">layer of conductive material to leave a plug of conductive material disposed in said bottom portion of said contact well; (e3) disposing a second c2 g0">layer of conductive material within said contact well; and (e4) removing a portion of said second c2 g0">layer of conductive material to leave a c2 g0">layer of conductive material disposed on said inner walls of said contact well. 16. The method, as set forth in claim 11, wherein step (f) comprises the step of: (f1) disposing a c2 g0">layer of ono dielectric approximately 100 angstroms thick within said contact well over said conductive material. 17. The method, as set forth in claim 11, wherein said conductive material disposed in steps (e) and (g) is c1 g0">polysilicon. 18. The method, as set forth in claim 12, wherein step (h) comprises the steps of: (h1) depositing a tungsten silicide c2 g0">layer over said conductive material disposed in step (g); and (h2) masking and etching said conductive material disposed in step (g) and said tungsten silicide c2 g0">layer to form a word line. 19. The method, as set forth in claim 12, wherein a capacitance ratio between said first capacitor and said second capacitor is maximized by controlling said thickness of said first c2 g0">layer of dielectric material, thereby providing a maximum voltage and charge on said floating gate during electrical programming. |
This invention relates to a device and a method of forming a floating gate memory transistor of very small area, thereby allowing a high-density integrated circuit chip, more specifically for Erasable Programmable Read-Only Memory (EPROM) or similar non-volatile devices.
The non-volatile memory function of EPROM memories is based on the storage of charge. EPROM memory transistors are all of the floating gate type, which means that the charge is stored on an additional polysilicon layer, located between the control gate and the substrate of a transistor. This type of memory cell can consist of only one transistor. The presence or absence of charge on the floating gate determines whether the cell will conduct current or not, which defines the two logic states. Present EPROM devices all use channel hot electron (CHE) injection to bring electrons onto the floating gate, and ultra-violet (UV) light is used to erase the memory. CHE injection programming is accomplished by applying a higher voltage (12 V) to the cell control gate. CHE programming is also used to store charge on flash-EPROMs. Another non-volatile memory floating gate device is the electrically erasable read-only memory (EEPROM) which is also programmed with the 12 volt gate signal.
In the IC chip industry, it is a continuing objective to reduce the size of each individual cell to allow packing more devices on a single chip, i.e., higher density. Since it has become clear that CHE programmable transistors are to be used in high-density EPROMs, technology has been adapted in order to yield fast-programmable, high-density floating gate EPROM structures. The efficiency of the CHE programming process is largely dependent on substrate (and drain junction) doping level, effective channel length, and floating gate-drain junction overlap. One method that can achieve high density is by using the self-aligned double polysilicon stacked gate structure, in which the drain/source junctions are self-aligned with respect to the floating gate and in which the double polysilicon stacked gate structure is etched in one step, usually by means of an anisotropic dry etching process.
There are certain problems associated with extremely short channels. Included in these so-called "short channel effects" are: (1) failure of the drain current to saturate at large VDS ; (2) increased subthreshold current; (3) reduction of threshold voltage Vt ; and (4) in the extreme, a loss of the ability of the gate to control gain. Consequently, much of the present-date MOSFET design effort is devoted to producing transistors that are physically small but still exhibit the electrical characteristics similar to those of physically long channels.
It is the purpose of this invention to provide a memory cell of minimal surface area, i.e., one square micron or less, yet maintain the electrical characteristics of a larger area device.
In first embodiment, a method is disclosed that fabricates a programmable memory cell described as a "diffusion cut" cell where a plug-type floating gate contact hole cuts through a diffusion region and partially into a substrate region.
This diffusion cut cell is formed in an IC chip contact well by the following method:
growing and patterning a field oxide on a substrate by a LOCOS process;
implanting an impurity element in the substrate to form a diffusion region adjacent the field oxide;
depositing an oxide layer over the diffusion region and field oxide;
patterning and etching a contact well in the oxide layer, field oxide, diffusion region, and substrate;
growing a gate/tunnel oxide layer within the contact well;
depositing and doping a first polysilicon layer within the contact well thereby forming a floating gate and capacitor, C1, between the first polysilicon layer and the substrate, said capacitor C1 having a minimal capacitance;
etching the first polysilicon layer until the only polysilicon is within the contact well forming a poly plug;
depositing a second polysilicon layer within the contact well;
etching back the second poly layer leaving a thin layer of the second poly on a perimeter of the contact well;
depositing an ONO dielectric over the second polysilicon layer;
depositing and doping a third polysilicon layer over the contact well, thereby forming a second capacitor, C2, between the third polysilicon layer and the second polysilicon layer, C2, having a larger capacitance than C1;
depositing a tungsten silicide layer over the third polysilicon layer; and then
masking and etching the third polysilicon layer to form a control gate and word line.
In this embodiment, the channel length Ieff under the floating gate can be controlled based on the depth of penetration into the substrate by the contact well. This also controls the floating gate to substrate capacitance, C1. The floating gate to control gate capacitance C2 can be controlled by the depth of an oxide and second polysilicon layer. These effects will be described later.
In a second embodiment, a method is disclosed that fabricates a programmable memory cell described as an "oxide cut" cell, where the plug-type floating gate contact hole only penetrates a silicon oxide layer. This "oxide cut" cell is formed in a similar fashion except penetration does not go into the diffusion region or substrate. In this embodiment, the capacitance C2 (control gate to floating gate) is again a function of depth of the oxide and second polysilicon layer.
It is the purpose of this invention to disclose a minimum area programmable floating gate memory cell of the MOS FET type of device that is based on a plug-type construction and has maximum charge storing capacity while maintaining this minimum area per cell.
Other objects, advantages, and capabilities of the present invention will become more apparent as the description proceeds.
FIG. 1 is a plan view of a diffusion cut floating gate memory cell of the present invention:
FIG. 2 is a section view of the memory cell taken along lines 2--2 of FIG. 1;
FIG. 3 is a section view of the memory cell taken along lines 3--3 of FIG. 1;
FIG. 4 is a plan view of an oxide cut floating gate memory cell of the present invention;
FIG. 5 is a section view of the oxide cut memory cell taken along lines 5--5 of FIG. 4;
FIG. 6 is a section view of the oxide cut memory cell taken along lines 6--6 of FIG. 4, and
FIG. 7 is a schematic representation of the floating gate memory cell.
A first embodiment of the floating gate memory cell 8 will be described by referring to FIGS. 1-3 which disclose a "diffusion cut" cell. FIG. 1 in plan view shows the "active" layers on a cell having diffusion regions 10, field oxide 12, and word line 13. Until the polysilicon plug contact well 14 is etched, the diffusion regions 10 are bridged by bridging diffusion 16. After patterning plug well 14, a layer of gate/tunnel oxide, three polysilicon layers, and a very thin layer of dielectric ONO (oxide, nitride, oxide) 18 are formed above the substrate within the well 14. The ONO layer is about 100 Å and its thickness varies the capacitance as will be explained later.
Referring to cell section view (FIG. 2), these layers can be more clearly seen over silicon substrate 20. A very thin gate/tunnel oxide layer 22 is formed within the well 14. Above this is the first plug-shape polysilicon layer forming floating gate 24 (labeled as Poly 1 in FIG. 1). Directly above floating gate 24 is a thin cylindrical second polysilicon layer 26 (labeled Poly 2). Within this second polysilicon layer 26 is plug-shaped third polysilicon layer 28 forming the control gate (labeled Poly 3). A layer of tungsten silicide (WSix), 30 with polysilicon layer 28 forms word line 32. In this embodiment, an oxide layer 34 is formed over the field oxide 12 and diffusion (FIGS. 2 and 3).
Superimposed over the substrate 20 and floating gate 24 is a capacitor Cl schematically shown at 36 formed between gate (capacitor plate) 36 and substrate 20 (second capacitor plate) and separated by gate/tunnel[oxide (dielectric) 22. In a similar manner, a capacitor C2 is formed between the second and third poly layers 26 and 28 as at 38.
It is important that the ratio of C2 to C1 be maximized to provide the highest voltage Vfg for programming the cell, based on the relationship: ##EQU1## The capacitors are shown in FIG. 7. The charge on the floating gate is proportional to Vfg, and Vcg is the programming voltage, i.e., about 12 volts.
It can be seen that the floating gate voltage Vfg is proportional to the capacitance of C2 which in turn is proportional to the surface area of the second polysilicon layer 26. Consequently, the capacitance of C2 is controlled by, and is proportional to the depth 40 of oxide layer 34 and inversely proportional to the thickness of ONO layer 18.
Another important electrical characteristic of a floating gate FET is the channel and channel length Leff. As described supra, it is desirable to avoid short channel effects. The semicircular channel is shown at 42 and is bounded on the ends at 44 and 46 where it intersections diffusion 16. It can again be seen that the Leff of channel 42 can be controlled by the depth of penetration of the well 14 into substrate 20. In this case, there is a trade off, i.e., while it may be desirable to increase Leff (channel length), it is also desirable to minimize C1 which unfortunately increases with the length of channel 42. This effect can be compensated by increasing length 40.
The final steps and materials consist of an insulating layer of silicon dioxide which is etched to provide contact windows followed by depositing and etching of aluminum conductors. These two surfaces are not shown in the figures.
A second embodiment is described as an oxide cut method, named for the process of only cutting a well into the oxide layer and not into the underlying substrate. FIGS. 4-6 illustrate this configuration. In this case, the plug contact well 50 is contained within an oxide layer 52 between continuous N+ diffusion lines 54 and 56 and over a field oxide 58. The first layer polysilicon forms floating gate 60 which is electrically insulated from substrate 62 by a thin gate/tunnel oxide layer 64. Again, in this case, the second polysilicon layer 66, a cylindrical shape, is formed above the floating gate 60 and has an insulating ONO layer 68, acting as a dielectric between the second polysilicon layer 66 and a third polysilicon layer control gate 70, which is covered by tungsten silicide conductor 72.
As previously described, capacitor C2 would be formed between polysilicon layer 66 and polysilicon gate 70, and C1 would be formed between floating gate 60 and substrate channel 74. In this case, we have formed a shorter channel Leff at 76 and a smaller capacitance C2 as controlled by oxide 52 height above the floating gate 60 as at 78.
FIG. 7 shows schematically the capacitor C1, at 36 between the floating gate 24 and substrate 20, and C2, at 38 between control gate 28 and floating gate 24 located above the channel 42 and between source and drain 80 and 82.
Referring back to the diffusion cut cell of FIGS. 1-3, this cell has some interesting potential for programming options. The region where floating gate 24 and drain in diffusion 16 at 90 can be used to tunnel (a quantum mechanics phenomenon) electrons between this gate and the drain for programming (write function) or erase function.
It is also possible to perform drain or channel engineering functions to improve the cell electrical characteristics by adding implants and masks during N+ diffusion formation, i.e., implant a dopant to only one of the N+ diffusions.
While a preferred embodiment of the invention has been disclosed, various modes of carrying out the principles disclosed herein are contemplated as being within the scope of the following claims. Therefore, it is understood that the scope of the invention is not to be limited except as otherwise set forth in the claims.
Patent | Priority | Assignee | Title |
5952690, | Sep 13 1994 | LG Semicon Co., Ltd. | Thin film transistor and fabrication method of the same |
5960282, | Dec 04 1998 | United Microelectronics Corp | Method for fabricating a dynamic random access memory with a vertical pass transistor |
6057574, | Sep 30 1996 | NEC Electronics Corporation | Contactless nonvolatile semiconductor memory device having buried bit lines surrounded by grooved insulators |
6084265, | Mar 30 1998 | TSMC-ACER Semiconductor Manufacturing Corporation; TAIWAN SEMICONDUCTOR MANUFACTURING CO , LTD | High density shallow trench contactless nonvolitile memory |
6147377, | Mar 30 1998 | MONTEREY RESEARCH, LLC | Fully recessed semiconductor device |
6147378, | Mar 30 1998 | MONTEREY RESEARCH, LLC | Fully recessed semiconductor device and method for low power applications with single wrap around buried drain region |
6225161, | Mar 30 1998 | MONTEREY RESEARCH, LLC | Fully recessed semiconductor method for low power applications with single wrap around buried drain region |
6274432, | Sep 30 1996 | NEC Electronics Corporation | Method of making contactless nonvolatile semiconductor memory device having buried bit lines surrounded by grooved insulators |
6605860, | Sep 29 1999 | SAMSUNG ELECTRONICS CO , LTD | Semiconductor structures and manufacturing methods |
6717205, | Aug 27 2000 | Qimonda AG | Vertical non-volatile semiconductor memory cell and method for manufacturing the memory cell |
6740555, | Sep 29 1999 | SAMSUNG ELECTRONICS CO , LTD | Semiconductor structures and manufacturing methods |
8022489, | May 20 2005 | Macronix International Co., Ltd. | Air tunnel floating gate memory cell |
9240345, | Aug 27 2008 | Intellectual Ventures II LLC | Shallow trench isolation structure having air gap, CMOS image sensor using the same and method of manufacturing CMOS image sensor |
Patent | Priority | Assignee | Title |
4964143, | Mar 02 1988 | Cypress Semiconductor Corporation | EPROM element employing self-aligning process |
4975384, | Jun 02 1986 | Texas Instruments Incorporated | Erasable electrically programmable read only memory cell using trench edge tunnelling |
5045490, | Jan 23 1990 | Texas Instruments Incorporated | Method of making a pleated floating gate trench EPROM |
5049515, | Mar 09 1990 | Intel Corporation, Inc. | Method of making a three-dimensional memory cell with integral select transistor |
5063172, | Jun 28 1990 | National Semiconductor Corporation | Manufacture of a split-gate EPROM cell using polysilicon spacers |
5071782, | Jun 28 1990 | Texas Instruments Incorporated | Vertical memory cell array and method of fabrication |
5135879, | Mar 26 1985 | Texas Instruments Incorporated | Method of fabricating a high density EPROM cell on a trench wall |
5141886, | Apr 15 1988 | Texas Instruments Incorporated | Vertical floating-gate transistor |
5399516, | Mar 12 1992 | International Business Machines Corporation | Method of making shadow RAM cell having a shallow trench EEPROM |
5460989, | Jun 20 1991 | Mitsubishi Denki Kabushiki Kaisha | Electrically erasable and programmable semiconductor memory device with trench memory transistor and manufacturing method of the same |
Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
Oct 27 1994 | MICRON SEMICONDUCTOR, INC | Micron Technology, Inc | MERGER SEE DOCUMENT FOR DETAILS | 008097 | /0339 | |
Jan 25 1996 | Micron Technology, Inc. | (assignment on the face of the patent) | / |
Date | Maintenance Fee Events |
Jun 23 1998 | ASPN: Payor Number Assigned. |
Jul 05 2001 | M184: Payment of Maintenance Fee, 8th Year, Large Entity. |
Jun 28 2005 | M1553: Payment of Maintenance Fee, 12th Year, Large Entity. |
Date | Maintenance Schedule |
May 26 2001 | 4 years fee payment window open |
Nov 26 2001 | 6 months grace period start (w surcharge) |
May 26 2002 | patent expiry (for year 4) |
May 26 2004 | 2 years to revive unintentionally abandoned end. (for year 4) |
May 26 2005 | 8 years fee payment window open |
Nov 26 2005 | 6 months grace period start (w surcharge) |
May 26 2006 | patent expiry (for year 8) |
May 26 2008 | 2 years to revive unintentionally abandoned end. (for year 8) |
May 26 2009 | 12 years fee payment window open |
Nov 26 2009 | 6 months grace period start (w surcharge) |
May 26 2010 | patent expiry (for year 12) |
May 26 2012 | 2 years to revive unintentionally abandoned end. (for year 12) |