# Patent

## Title
Non-volatile memory semiconductor device having an oxide-nitride-oxide (ONO) top dielectric layer

## Abstract

A non-volatile memory (NVM) cell includes a silicon substrate having a main surface, a source region in a portion of the silicon substrate, a drain region in a portion of the silicon substrate, and a well region disposed in a portion of the silicon substrate between the source and drain regions. The cell includes a bottom oxide layer formed on the main surface of the substrate. The bottom oxide layer is disposed on a portion of the main surface proximate the well region. The cell includes a charge storage layer disposed above the bottom oxide layer, a dielectric tunneling layer disposed above the charge storage layer and a control gate formed above the dielectric tunneling layer. The dielectric tunneling layer includes a first oxide layer, a nitride layer and a second oxide layer. Erasing the NVM cell includes applying a positive gate voltage to inject holes from the gate.

## Background
RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 11/197,668 filed on 4 Aug. 2005, which application is incorporated herein by reference.


BACKGROUND OF THE INVENTION
The present invention relates to a non-volatile memory semiconductor device and a method for manufacturing a non-volatile memory semiconductor device, and more particularly, to a non-volatile memory semiconductor device having an oxide-nitride-oxide (ONO) top dielectric layer and a method for manufacturing a non-volatile memory semiconductor device having an ONO top dielectric layer.
Non-volatile memory (“NVM”) refers to semiconductor memory which is able to continually store information even when the supply of electricity is removed from the device containing such an NVM memory cell. NVM includes Mask Read-Only Memory (Mask ROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM) and Electrically Erasable Programmable Read-Only Memory (EEPROM). Typically, NVM can be programmed with data, read and/or erased, and the programmed data can be stored for a long period of time prior to being erased, even as long as ten years.
Nitride read only memory (NROM) is a type of EEPROM that uses charge-trapping for data storage. An NROM cell is typically composed of a metal-oxide-silicon field effect transistor (MOSFET) having an ONO (oxide-nitride-oxide) layer disposed between the gate and the source/drain of the semiconductor material. The nitride layer in the ONO layer is able to “trap” charge (electrons) when the device is “programmed.” Charge localization is the ability of the nitride material to store the charge without significant lateral movement of the charge throughout the nitride layer. NROM utilizes a relatively thick tunnel oxide layer, which typically negatively impacts the time it takes to erase a memory cell. NROM can be contrasted with conventional “floating gate” memory cells wherein the floating gate is conductive and the charge is spread laterally throughout the entire floating gate and charge is transferred through a tunnel oxide layer. Programming (i.e. charge injection) of the charge-trapping layer in NROM cells can be carried out by various hot carrier injection methods such as channel hot electron injection (CHE), source side injection (SSI) or channel initiated secondary electron (CHISEL), which all inject electrons into the nitride layer. Erasing is performed by applying a positive gate voltage, which permits hole tunneling through the ONO top dielectric layer from the gate. Erasing (i.e., charge removal) in NROM devices is typically carried out by band-to-band hot hole tunneling (BTBHHT). However, BTBHHT erasing causes many reliability issues with NROM devices and causes degradation of the NROM devices and charge loss after many program/erase cycles. Reading is carried out in a forward or reverse direction. Localized charge-trapping technology allows two separate bits per cell, thus resulting in a doubling of memory density. The NROM can be repeatedly programmed, read, erased and/or reprogrammed by known voltage application techniques.
Another EEPROM is a metal-nitride-oxide-silicon (MNOS) memory cell as shown in FIG. 4B. A typical MNOS 40 includes a very thin layer of insulating material 50 like silicon dioxide (SiO2) to separate a silicon nitride charge storage region 54 from a gate 55 and from a well region 45 of the semiconductor device. An MNOS 40 also includes a substrate 41 in which a source 44 and a drain 42 are formed. The well region 45 is disposed between the source 44 and the drain 42 under the gate 55. Erasing an MNOS 40 includes injecting holes into the nitride region 54 from the gate 55. This is accomplished by using a large positive gate voltage while grounding the source 44, the drain 42 and the substrate 41. In order to facilitate hole injection from the gate 55, there is nothing between the nitride 54 and the gate 55. However, such MNOS devices 40 suffer from charge retention problems in the nitride region 54 because electrons can easily “de-trap” from the nitride region 54 into the gate 55.
Yet another EEPROM is a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell. U.S. Pat. No. 6,011,725 (Eitan), the entire content of which is incorporated by reference herein, provides a detailed comparison of several of the prior art NVMs including respective programming, erasing and reading techniques. The Eitan patent also discloses a type of SONOS memory cell capable of storing two data bits by localized charge storage techniques.
A typical conventional SONOS device 10 is shown in FIG. 4A. The conventional SONOS device 10 includes a silicon substrate 11, a source 14, a drain 12, a well region 15 and a first oxide layer 20 on top of the well region 15 extending over portions of the source 14 and the drain 12. A nitride charge storage layer 24 is provided above the first oxide layer 20, and a second oxide layer 30 is provided above the nitride charge storage layer 24. A polysilicon (poly) gate 25 is disposed on top of the ONO stack 20, 24, 30. By providing the second oxide layer 30 on top of the nitride layer 24 there is an improvement in the ability to control where the charge is placed or stored within the nitride layer 24 during programming operations. Additionally, the addition of the second oxide layer 24 prevents holes from entering from the overlying gate 25. In order for electrons to be able to tunnel through the oxide layer 20, the oxide layer 20 must be relatively thin, e.g., 20-30 Angstroms (Å). However, direct tunneling at retention still occurs which causes unsatisfactory retention results.
It is desirable to provide a non-volatile memory semiconductor device that has improved charge retention compared to conventional SONOS and MNOS devices. It is desirable to provide a non-volatile memory semiconductor device having a band-gap engineered oxide-nitride-oxide (ONO) top dielectric layer which serves as a blocking oxide or top oxide layer. It is also desirable to provide a non-volatile memory which can be erased using gate-injected holes by applying a positive gate voltage.
BRIEF SUMMARY OF THE INVENTION
Briefly stated, the present invention comprises a non-volatile memory cell including a silicon substrate having a main surface, a source region in a portion of the silicon substrate proximate the main surface, and a drain region in a portion of the silicon substrate proximate the main surface. The drain region is spaced apart from the source region. A well region is disposed in a portion of the silicon substrate proximate the main surface between the source region and the drain region. The cell includes a bottom oxide layer formed on the main surface of the substrate. The bottom oxide layer is disposed on a portion of the main surface proximate the well region. A charge storage layer is disposed above the bottom oxide layer relative to the main surface of the silicon substrate. A dielectric tunneling layer is disposed above the charge storage layer relative to the main surface of the silicon substrate. The dielectric tunneling layer includes, in order relative to the main surface of the silicon substrate and outward, a first dielectric oxide layer, a dielectric nitride layer and a second dielectric oxide layer. A control gate is disposed above the dielectric tunneling layer relative to the main surface of the silicon substrate.
The present invention also comprises a method of forming a memory cell including providing a silicon substrate having a main surface, forming a source region in a portion of the silicon substrate proximate the main surface and forming a drain region in a portion of the silicon substrate proximate the main surface. The drain region is spaced apart from the source region. A well region is defined in a portion of the silicon substrate proximate the main surface between the source region and the drain region. A bottom oxide layer is deposited on the main surface of the substrate. The bottom oxide layer is disposed on a portion of the main surface proximate the well region. A charge storage layer is formed above the bottom oxide layer relative to the main surface of the silicon substrate. A first dielectric oxide layer is deposited above the charge storage layer relative to the main surface of the silicon substrate. A dielectric nitride layer is deposited above the first dielectric oxide layer relative to the main surface of the silicon substrate. A second dielectric oxide layer is deposited above the dielectric nitride layer relative to the main surface of the silicon substrate. A control gate is formed above the dielectric tunneling layer relative to the main surface of the silicon substrate.
In yet another aspect, the present invention comprises a method of erasing a non-volatile memory cell. The non-volatile memory cell includes a silicon substrate, a control gate, a source, a drain, a well region between the source and the drain, a bottom oxide layer above the well region, a charge storage layer above the bottom oxide layer and an oxide layer-nitride layer-oxide layer (ONO) dielectric layer between the charge storage layer and the control gate. The method includes applying a positive gate erasing voltage to the control gate sufficient to cause hole tunneling from the control gate toward the charge storage layer and applying a source/drain programming voltage to one of the source region and the drain region and coupling the other of the source region and the drain region to reference. The source/drain programming voltage is sufficient to cause electron tunneling from the well region toward the charge storage nitride region.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:
FIG. 1A is a partial sectional side elevational view of an n-channel non-volatile memory (NVM) cell having an oxide-nitride-oxide (ONO) dielectric layer in accordance with a first preferred embodiment of the present invention;
FIG. 1B is a partial sectional side elevational view of a p-channel non-volatile memory (NVM) cell having an oxide-nitride-oxide (ONO) dielectric layer in accordance with a second preferred embodiment of the present invention;
FIG. 2 is a partial sectional side elevational view of the NVM cell of FIG. 1A showing hole tunneling and electron tunneling directional pathways;
FIG. 3A is a band energy diagram of an oxide-nitride-oxide (ONO) dielectric layer in a charge retention state;
FIG. 3B a band energy diagram of an oxide-nitride-oxide (ONO) dielectric layer when voltage applied to a control gate is non-zero;
FIG. 4A is a partial sectional side elevational view of a conventional non-volatile memory (NVM) silicon-oxide-nitride-oxide-silicon (SONOS) memory cell;
FIG. 4B is a partial sectional side elevational view of a conventional non-volatile memory (NVM) metal-nitride-oxide-silicon (MNOS) memory cell;
FIG. 5A is a partial sectional side elevational view of the n-channel NVM cell of FIG. 1A being programmed by channel hot electron (CHE) injection;
FIG. 5B is a partial sectional side elevational view of the p-channel NVM cell of FIG. 1B being programmed by band-to-band tunneling induced hot electron (BBHE);
FIG. 6A is a graph demonstrating experimental data regarding programming and erasing the n-channel NVM cell of FIG. 1A; and
FIG. 6B is a graph demonstrating experimental data regarding programming and erasing the p-channel NVM cell of FIG. 1B.



DETAILED DESCRIPTION OF THE INVENTION
Certain terminology is used in the following description for convenience only and is not limiting. The words “right,” “left,” “lower” and “upper” designate directions in the drawing to which reference is made. The words “inwardly” and “outwardly” refer direction toward and away from, respectively, the geometric center of the object described and designated parts thereof. The terminology includes the words above specifically mentioned, derivatives thereof and words of similar import. Additionally, the word “a,” as used in the claims and in the corresponding portions of the specification, means “at least one.”
One (1) micron or micrometer (μm) is 10,000 Angstroms (Å) or 1000 nanometers (nm).
As used herein, reference to conductivity will be limited to the embodiment described. However, those skilled in the art know that p-type conductivity can be switched with n-type conductivity and the device would still be functionally correct (i.e., a first or a second conductivity type). Therefore, where used herein, the reference to n or p can also mean that either n and p or p and n can be substituted therefor.
Furthermore, n+ and p+ refer to heavily doped n and p regions, respectively; n++ and p++ refer to very heavily doped n and p regions, respectively; n− and p− refer to lightly doped n and p regions, respectively; and n−− and p−− refer to very lightly doped n and p regions, respectively. However, such relative doping terms should not be construed as limiting.
Referring to the drawings in detail, wherein like numerals reference indicate like elements throughout, there is shown in FIGS. 1A and 2 a non-volatile memory (NVM) cell 100 in accordance with a first preferred embodiment of the present invention. The NVM cell 100 includes a silicon substrate 102 having a main surface 102a, a source region 104 in a portion of the silicon substrate 102 proximate the main surface 102a, and a drain region 112 in a portion of the silicon substrate 102 proximate the main surface 102a. The drain region 112 is spaced apart from the source region 104. A well region 105 is disposed in a portion of the silicon substrate 102 proximate the main surface 102a between the source region 104 and the drain region 112. The NVM cell 100 includes a bottom oxide layer 120 formed on the main surface 102a of the silicon substrate 102. The bottom oxide layer 120 is disposed on a portion of the main surface 102a proximate the well region 105. A charge storage layer 124 is disposed above the bottom oxide layer 120 relative to the main surface 102a of the silicon substrate 102. A dielectric tunneling layer 130 is disposed above the charge storage layer 124 relative to the main surface 102a of the silicon substrate 102. The dielectric tunneling layer 130 includes, in order relative to the main surface 102a of the silicon substrate 102 and outward, a first dielectric oxide layer 131, a dielectric nitride layer 132 and a second dielectric oxide layer 133, thus the dielectric tunneling layer 130 is an oxide-nitride-oxide (ONO) layer 130. A control gate 125 is disposed above the dielectric tunneling layer 130 relative to the main surface 102a of the silicon substrate 102.
As shown, the NVM cell 100 is an N-channel device (FIG. 1A) because the source region 104 and drain region 112 are both n-type regions and the well region 105 is a p-type region.
Thus, a p-type substrate 102 can be doped or implanted with an n-type material in the areas where the desired source and drain regions 104, 112 are to be located. Alternately, trenches (not shown clearly) may be formed in the p-type substrate 102 for desired source and drain regions 104, 112, and the trenches can be refilled with an n-type material such as with a heavily doped n-type polysilicon (n+) and the like.
The bottom oxide layer 120 is preferably between about 30-90 Angstroms (Å) in thickness. The bottom oxide layer 120 is formed of an oxide such as silicon dioxide (SiO2), Aluminum Oxide (Al2O3) and the like. The bottom oxide layer 120 is preferably thicker than 30 Å in order to prevent direct-tunneling charge loss from the charge storage layer 124 after the NVM cell 100 has been programmed (i.e., charge has been intentionally stored on the charge storage layer 124). Thus, the bottom oxide layer 120 functions as an insulating layer.
The charge storage layer 124 is preferably between about 50-150 Å in thickness. The charge storage layer 124 is preferably formed of a nitride such as Silicon Nitride (Si3N4), Silicon Oxynitride (SiOXNX) and the like. The charge storage layer 124 may be alternate materials instead of nitride such as a trapping insulating material such as Aluminum Oxide (Al2O3), Hafnium Oxide (HfO2) and the like. The charge storage layer 124 provides the programmable memory storage portion of the NVM cell 100. The charge storage layer 124 is preferably a material that efficiently traps or stores charge after a programming operation which entails applying a programming voltage potential to the control gate 125 and one of the source region 104 and the drain region 112 in order to induce charge into the charge storage layer 124.
The ONO top dielectric 130 is a bandgap-engineered tunneling dielectric layer that provides an efficient tunneling barrier for hole injection from the gate 125, yet prevents direct tunneling leakage during desired charge retention. The ONO top dielectric layer 130 is a multi-layer structure (sandwiched) where each of the first and second dielectric oxide layers 131 and 133 and the dielectric nitride layer 132 is preferably between about 10-30 Å in thickness. For example, the first dielectric oxide layer 131 can be about 18 Å in thickness, the dielectric nitride layer 132 can be about 20 Å in thickness and the second dielectric oxide layer 133 can be about 15 Å in thickness. The dielectric nitride layer 132 is preferably formed of silicon nitride or other insulating dielectric materials with a low hole-barrier height of less than about 3 electron-volts (eV). Silicon nitride has a relatively low hole barrier on the order of 1.9 eV, so it may become “transparent” to hole tunneling under a relatively high electric field such as when voltage applied to the control gate 125 is greater than zero.
The dielectric tunneling layer 130 functions as a tunneling dielectric for hole tunneling from the control gate 125 during an erase operation. Due to the reduced thickness of the first and second dielectric oxide layers 131, 133 and the dielectric nitride layer 132, the dielectric tunneling layer 130 has negligible charge trapping efficiency itself. Thus, the relatively thin dielectric nitride layer 132 of the ONO top dielectric layer 130 does not trap charge during memory programming and retention operations.
Thus, the NVM cell 100 is preferably a silicon-oxide-nitride-oxide-nitride-oxide-silicon (SONONOS) device wherein the gate 125 is formed of polysilicon, the top dielectric layer 130 is formed of oxide-nitride-oxide (ONO), the charge storage layer 124 is formed of nitride and the blocking layer 120 is formed of oxide which are all on top of the silicon substrate 102.
The control gate 125 may be formed of undoped polysilicon, doped polysilicon or a metal. For example, the control gate 125 may be lightly or heavily n-doped polysilicon, lightly or heavily p-doped polysilicon, platinum (Pt), Tantalum Nitride (TaN) and the like.
The operation of the NVM cell 100 is as follows. An erasing operation entails applying a gate erasing voltage VG (FIG. 2) to the control gate 125 and a source/drain erasing voltage VSJD (FIG. 2) to one of the source region 104 and the drain region 112 in order to induce charge into the charge storage layer 124. The NVM cell 100 is erased by applying a positive direct current voltage (+VDC) to the control gate 125 in order to provide hole tunneling from the control gate 125 toward the charge storage layer 124. The positive gate erasing voltage VG is preferably between about +10 to about +18 VDC. Additionally, a negative source/drain erasing voltage (−VDC) is applied to one of the source region 104 and the drain region 112 to reference (i.e., zero potential or ground). The negative source/drain erasing voltage VS/D is sufficient to cause electron tunneling from the well region 105 toward the charge storage region 124. However, there is no need to utilize band-to-band hot hole erase (BTBHHE), thereby reducing or eliminating hot-hole introduced damage.
The NVM cell 100 is programmed by an electron injection method. For n-channel devices 100 (FIG. 1A), channel hot electron (CHE) injection, source side injection (SSI), channel initiated secondary electron (CHISEL) injection and the like, can be employed. For p-channel devices 200 (FIG. 1B), band-to-band tunneling hot electron (BBHE) injection, channel hot-hole initiated hot electron injection or simply negative Fowler-Nordheim (−FN) gate injected electrons or the like can be employed. FIG. 5A shows n-channel programming by CHE injection and FIG. 5B shows p-channel programming by BBHE injection. Preferably, the NVM cell 100 can be programmed with 2-bits per cell 100 with a first bit in the charge storage layer 124 closer to the source region 104 and a second bit in the charge storage layer 124 closer to the drain region 112, as is known in the art.
The NVM cell 100 is read by conventional reading methods such as applying a gate read voltage to the control gate 125 and a source/drain read voltage to one of the source region 104 and the drain region 112 and grounding the other of the source region 104 and the drain region 112. The well region 105 is also grounded during a read operation. For example, the gate read voltage and the source/drain read voltage may each be between about 1-2 VDC.
FIG. 6A is a graph demonstrating experimental data regarding programming and erasing the n-channel NVM cell of FIG. 1A. FIG. 6B is a graph demonstrating experimental data regarding programming and erasing the p-channel NVM cell of FIG. 1B.
FIG. 3A is a band energy diagram of the ONO dielectric layer 130 in a charge retention state (VG≈0). Band offset of the ONO dielectric layer 130 does not occur when there is a low electric field, so tunneling is suppressed. FIG. 3B is a band energy diagram of the ONO dielectric layer 130 when voltage VG applied to the control gate 125 is non-zero (VG>0). When there is a relatively high electric field, band offset of the ONO dielectric layer 130 occurs thereby providing hole tunneling from the control gate 125 to the charge storage layer 124.
FIG. 1B depicts a non-volatile memory (NVM) cell 200 in accordance with a second preferred embodiment of the present invention. The NVM cell 200 includes a silicon substrate 202 having a main surface 202a, a source region 204 in a portion of the silicon substrate 202 proximate the main surface 202a, and a drain region 212 in a portion of the silicon substrate 202 proximate the main surface 202a. The drain region 212 is spaced apart from the source region 204. A well region 205 is disposed in a portion of the silicon substrate 202 proximate the main surface 202a between the source region 204 and the drain region 212. The NVM cell 200 includes a bottom oxide layer 220 formed on the main surface 202a of the silicon substrate 202. The bottom oxide layer 220 is disposed on a portion of the main surface 202a proximate the well region 205. A charge storage layer 224 is disposed above the bottom oxide layer 220 relative to the main surface 202a of the silicon substrate 202. A dielectric tunneling layer 230 is disposed above the charge storage layer 224 relative to the main surface 202a of the silicon substrate 202. The dielectric tunneling layer 230 includes, in order relative to the main surface 202a of the silicon substrate 202 and outward, a first dielectric oxide layer 231, a dielectric nitride layer 232 and a second dielectric oxide layer 233, thus the dielectric layer 230 is an oxide-nitride-oxide (ONO) layer 230. A control gate 225 is disposed above the dielectric tunneling layer 230 relative to the main surface 202a of the silicon substrate 202.
As shown, the NVM cell 200 is a p-channel device (FIG. 1B) because the source region 204 and drain region 212 are both p-type regions and the well region 105 is an n-type region. Thus, an n-type substrate 202 can be doped or implanted with a p-type material in the areas where the desired source and drain regions 204, 212 are to be located. Alternately, trenches may be formed in the n-type substrate 202 for desired source and drain regions 204, 212, and the trenches can be refilled with a p-type material such as with a heavily doped p-type polysilicon (p+) and the like.
Similar to the first preferred embodiment, the bottom oxide layer 220 is preferably between about 30-90 Å in thickness. The bottom oxide layer 220 is formed of an oxide material such as Aluminum Oxide (Al2O3). The charge storage layer 224 is preferably between about 50-150 Å in thickness. The charge storage layer 225 is preferably formed of a nitride such as Silicon Nitride (Si3N4), Silicon Oxynitride (SiOXNX) and the like. The charge storage layer 225 may be alternate materials instead of nitride that can function as a trapping insulating material such as Aluminum Oxide (Al2O3), Hafnium Oxide (HfO2) and the like. Each of the first and second dielectric oxide layers 231 and 233 and the dielectric nitride layer 232 is preferably between about 10-30 Å in thickness. For example, the first dielectric oxide layer 231 can be about 18 Å in thickness, the dielectric nitride layer 232 can be about 20 Å in thickness and the second dielectric oxide layer 233 can be about 15 Å in thickness. The control gate 225 may be formed of undoped polysilicon, doped polysilicon or a metal. For example, the control gate 225 may be lightly or heavily n-doped polysilicon, lightly or heavily p-doped polysilicon, platinum (Pt), Tantalum Nitride (TaN) and the like.
Thus, the NVM cell 200 is preferably a silicon-oxide-nitride-oxide-nitride-oxide-silicon (SONONOS) device wherein the gate 225 is formed of polysilicon, the top dielectric layer 230 is formed of oxide-nitride-oxide (ONO), the charge storage layer 224 is formed of nitride and the blocking layer 220 is formed of oxide which are all on top of the silicon substrate 202.
The gate materials of 125 and 225 are preferably a p+-polysilicon. In this way, a large density of holes can be provided by applying a positive gate voltage VG.
The present invention also includes methods of forming the NVM cells 100, 200 in accordance with the preferred embodiments. The method of forming the memory cell 100, 200 includes providing a silicon substrate 102, 202 having a main surface 102a, 202a, forming a source region 104, 204 in a portion of the silicon substrate 102, 202 proximate the main surface 102a, 202a and forming a drain region 112, 212 in a portion of the silicon substrate 102, 202 proximate the main surface 102a, 202a. The drain region 112, 212 is spaced apart from the source region 104. A well region 105, 205 is defined in a portion of the silicon substrate 102, 202 proximate the main surface 102a, 202a between the source region 104, 204 and the drain region 112, 212. A bottom oxide layer 120, 220 is deposited on the main surface 102a, 202a of the substrate 102, 202 through the mask 50. The bottom oxide layer 120, 220 is disposed on a portion of the main surface 102a, 202a proximate the well region 105, 205. A charge storage layer 124, 224 is formed above the bottom oxide layer 120, 220 relative to the main surface 102a, 202a of the silicon substrate 102, 202. A first dielectric oxide layer 131, 231 is deposited above the charge storage layer 124, 224 relative to the main surface 120a, 202a of the silicon substrate 102, 202. A dielectric nitride layer 132, 232 is deposited above the first dielectric oxide layer 131, 231 relative to the main surface 102a, 202a of the silicon substrate 102, 202. A second dielectric oxide layer 133, 233 is deposited above the dielectric nitride layer 132, 232 relative to the main surface 102a, 202a of the silicon substrate 102, 202. A control gate 125, 225 is formed above the second dielectric oxide layer 133, 233 relative to the main surface 102a, 202a of the silicon substrate 102, 202. A mask 50 (shown in phantom in FIGS. 1A-1B) is placed proximate the main surface of the silicon substrate 102, 202. The mask 50 leaves exposed at least a portion of the main surface 102a, 202a proximate the well region 105, 205. Appropriate portions of layers 120, 124, 131, 132, 133 and 220, 224, 231, 232, 233 and gates 125, 225 may be removed by etching or by a different removal technique.
As mentioned above, the bottom oxide layer 120, 220 is preferably between about 30-90 Å in thickness, the charge storage layer 124, 224 is preferably between about 50-150 Å in thickness, and each of the first and second dielectric oxide layers 131, 231 and 133, 233 and the dielectric nitride layer 132, 232 is preferably between about 10-30 Å in thickness.
The NVM cell 100 may be an N-channel device (FIG. 1A) by making, likely by doping and/or implanting, the source region 104 and drain region 112 both n-type regions and by leaving the well region 105 a p-type region.
Alternatively, the NVM cell 200 may be a P-channel device (FIG. 1B) by making, likely by doping and/or implanting, the source region 204 and drain region 212 both p-type regions and by leaving the well region an n-type region. Alternately, trenches may be formed in the n-type substrate 202 for desired source and drain regions 204, 212, and the trenches can then be refilled with a p-type material such as with a heavily doped p-type polysilicon and the like.
The various layers 120, 124, 131, 132, 133 and 220, 224, 231, 232, 233 and gates 125, 225 may be formed in any of a variety of ways known in the art. For example, the layers 120, 124, 132, 133 and 220, 224, 231, 232, 233 and gates 125, 225 maybe grown or deposited. Deposition may be by chemical vapor deposition (CVD), physical vapor deposition (PVD), evaporation, sputtering and the like. Patterns may be formed on the surface of the semiconductor substrate by photolithography or photomasking (“masking”) techniques. The various layers 120, 124, 131, 133 and 220, 224, 231, 232, 233 and gates 125, 225 may be etched back by mechanical etching or by chemical etching and/or chemical mechanical polishing (CMP) and the like. Additionally, known methods of doping, heat treating, diffusing, etching, layering, trenching, polishing and the like, may be utilized in the fabrication process of the a NVM cell 100, 200 without departing from the present invention.
The NVM cells 100, 200 may be applied to various memory types including NOR and NAND memories. The NVM cells 100, 200 can be used in a virtual ground array architecture.
The top dielectric layer 130, 230 may include additional layers of oxide and/or nitride and/or may include additional layers of other materials.
From the foregoing, it can be seen that the present invention is directed to a non-volatile memory semiconductor device having an ONO top dielectric layer and a method for manufacturing a non-volatile memory semiconductor device having an ONO top dielectric layer. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

## Claims

1. A method of forming a memory cell comprising:
providing a silicon substrate having a main surface;
forming a source region in a portion of the silicon substrate proximate the main surface;
forming a drain region in a portion of the silicon substrate proximate the main surface, the drain region being spaced apart from the source region;
depositing a bottom oxide layer on the main surface of the substrate, the bottom oxide layer disposed on a portion of the main surface;
forming a dielectric charge storage layer above the bottom oxide layer relative to the main surface of the silicon substrate;
depositing a first dielectric oxide layer above, and contacting, the dielectric charge storage layer relative to the main surface of the silicon substrate;
depositing a dielectric nitride layer above, and contacting, the first dielectric oxide layer relative to the main surface of the silicon substrate;
depositing a second dielectric oxide layer above, and contacting, the dielectric nitride layer relative to the main surface of the silicon substrate; and
forming a control gate above the second dielectric oxide layer relative to the main surface of the silicon substrate;
wherein the bottom oxide layer is between about 30-90 Angstroms (Å) in thickness, the charge storage layer is between about 50-150 Å in thickness, and each of the first and second dielectric oxide layers and the dielectric nitride layer is between about 10-30 Å in thickness.

2. The method according to claim 1, wherein the source and drain regions are both one of an n-type and a p-type region and a well region between the source and drain regions in the silicon substrate is the other of an n-type region and a p-type region.

3. A method of forming a memory cell having a threshold voltage that is a function of trapped charge in the memory cell, comprising: providing a substrate; forming a storage structure over the substrate, the storage structure including a dielectric layer adjacent and on the substrate, a multilayer tunneling structure on the dielectric layer, and a dielectric charge trapping layer located between the dielectric layer and the multilayer tunneling structure; and forming a control gate over the storage structure, wherein the multilayer tunneling structure and the dielectric layer are arranged so that an electric field across the substrate and the control gate induces hole tunnel current through the multilayer tunneling structure that exceeds electron current through the dielectric layer to reduce threshold voltage of the memory cell.

4. The method according to claim 3, wherein the dielectric layer is between about 30-90 Angstroms (Å) in thickness, the charge storage layer is between about 50-150 Å in thickness, and the multilayer tunneling structure includes first and second dielectric oxide layers and a dielectric nitride layer the first and second dielectric oxide layers, and wherein the first dielectric oxide layer is between about 10-30 Å in thickness.

5. A method of forming a dielectric charge trapping memory cell having a threshold voltage that is a function of trapped charge in the memory cell, comprising:
providing a substrate;
forming a bottom insulating layer over the substrate;
forming a dielectric charge storage layer over the bottom insulating layer;
forming a top dielectric structure over the dielectric charge storage layer, the top dielectric structure including
a first and a second oxide layers, and a nitride layer between the first and the second oxide layers; and
forming a poly layer over the top dielectric structure, wherein the top dielectric structure and bottom insulating layer are arranged so that an electric field across the substrate and the poly layer induces hole tunnel current through the top dielectric structure that exceeds electron current through the bottom insulating layer to reduce threshold voltage of the memory cell.