JP2011155266A - Nonvolatile memory cell, memory array having the same, and method of operating cell and array - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of designing a nonvolatile memory cell that is enhanced in data retention performance and improved in operation speed, and can be operated (programming/deletion/retrieval) a number of times and an array. <P>SOLUTION: A semiconductor substrate 101 includes a source region 102 and a drain region 104 that are disposed under the surface of the substrate 101 and separated by a channel region 106. A tunnel dielectric structure 120 is disposed above the channel region 106. A memory cell 100 includes: the tunnel dielectric structure 120 having at least one layer with a low hole tunneling barrier height, a charge storage layer 130 disposed above the tunnel dielectric structure 120, an insulation layer 140 disposed above the charge storage layer 130, and a gate electrode 150 disposed above the insulation layer 140. An array of the memory cell 100 and an operation method are provided. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application is based on US Provisional Patent Application 60 / 640,229 filed January 3, 2005, US Provisional Patent Application 60 / 647,012, filed January 27, 2005, June 6, 2005. Based on US Provisional Patent Application 60 / 689,231 filed on May 10, and United States Provisional Patent Application 60 / 689,314 filed June 10, 2005, Claim the priority of the US provisional patent application under paragraph e). Each of the above US provisional patent applications is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION Non-volatile memory (NVM) is a device having NVM cells and refers to a semiconductor memory that can continuously store information even when the power supply to the device is removed. . The NVM includes mask ROM (Mask Read-Only Memory), programmable ROM (PROM: Programmable Read-Only Memory), erasable programmable ROM (EPROM: Erasable Programmable Read-Only Memory Programmable). (EEPROM: Electrically Erasable Programmable Read-Only Memory) and flash memory (Flash Memory) are included. Non-volatile memories are widely used in the semiconductor industry and are classified as memories developed to prevent loss of programmed (written) data. Typically, non-volatile memory can be programmed, read and / or erased based on the end use requirements of the device, and programmed data can be stored over a long period of time.

  In general, non-volatile memory devices can take a variety of designs. One example of an NVM cell design is the so-called SONOS (silicon-oxide-nitride-oxide-silicon) device. The SONOS device can use a thin tunnel oxide layer (oxide film) and can perform a direct tunneling erase operation of holes. Such a design may have a good erase speed, but the data retention capability is generally low. This is partly because direct tunneling can occur even at low field strengths that may exist during the retention state of the memory device.

  Another NVM design is an NROM (Nitride Read-Only Memory) that uses a thicker tunnel oxide layer to prevent charge loss during the hold state. However, a thick tunnel oxide layer can affect the channel erase rate. As a result, a band-to-band tunneling hot-hole (BTBTHH) erasing method can be used to inject hole traps to supplement electrons. However, the BTBTHH erasure method can cause some reliability problems. For example, the characteristics of NROM devices that employ the BBTTHH erase method may degrade after a number of P / E (program / erase) cycles.

  Accordingly, there is a need in the art for non-volatile memory cell designs and arrays that can be operated (programmed / erased / read) multiple times with improved data retention and improved operating speed.

SUMMARY OF THE INVENTION The present invention relates to a non-volatile memory device, and more particularly, to facilitate self-convergence erasing and to retain the charge retention capability in the charge storage layer of the memory device during the retention state. The present invention also relates to a nonvolatile memory device having a tunnel dielectric structure.

  One embodiment of the present invention is a semiconductor substrate, comprising a semiconductor substrate having a source region and a drain region disposed below the surface of the substrate and separated by a channel region; and disposed above the channel region A tunnel dielectric structure comprising at least one layer having a low hole tunneling barrier height; a charge storage layer disposed above the tunnel dielectric structure; and the charge storage layer A memory cell having an insulating layer disposed above the insulating layer; and a gate electrode disposed above the insulating layer.

  Another embodiment of the present invention is a semiconductor substrate comprising a source region and a drain region disposed below the surface of the substrate and separated by a channel region; and disposed above the channel region A multilayer tunnel dielectric structure comprising at least one layer having a low hole tunneling barrier height; a charge storage layer disposed above the multilayer tunnel dielectric structure; A memory cell having an insulating layer disposed above the charge storage layer; and a gate electrode disposed above the insulating layer.

In a particular preferred embodiment, the layer providing the low hole tunneling barrier height may comprise a material (substance) such as silicon nitride (Si ₃ N ₄ ) or hafnium oxide (HfO ₂ ). In certain preferred embodiments of the present invention, the memory cell has a tunnel dielectric structure with multiple layers, such as silicon oxide, silicon nitride and silicon oxide stacked dielectric tri-layer structure (ONO). Such tunnel dielectric structures provide SONONOS (silicon-oxide-nitride-oxide-nitride-oxide-silicon) or superlattice SONONOS designs.

In certain preferred embodiments of the invention, the tunnel dielectric structure may have at least two dielectric layers each having a thickness of up to about 4 nm (about 4 nm or less). Also, in certain preferred embodiments of the present invention, the gate electrode comprises a material having a work function value greater than that of N ⁺ polysilicon.

  In certain preferred embodiments, the tunnel dielectric structure can have a layer that includes a material having a low hole tunneling barrier height, the material having a concentration of the material at a depth in the layer. It exists in that layer with a concentration gradient that maximizes.

  The present invention also includes non-volatile memory devices having a plurality of memory cells (ie, arrays) in accordance with one or more embodiments described herein. As used herein, “plurality” means two or more. Memory devices according to the present invention exhibit significantly improved operating characteristics including increased erase speed, improved charge retention capability, and a larger operating window.

  The present invention also includes a method for operating non-volatile memory cells and arrays. The operating method according to the invention comprises resetting a memory device group by applying a self-convergence method so as to tighten the Vt distribution of the memory device group; and at least one of the memory device groups by channel + FN injection. Reading at least one of the memory device groups by applying a voltage between the erased state level and the programmed state level of at least one of the memory device groups; And a process. As used herein, the expression “tighten” means to narrow the threshold voltage distribution between multiple memory cells in an array. In general, when the threshold voltage distribution is “tightened”, the threshold voltages of the cells are within a narrow range of each other so that the operation of the array is improved over conventional designs. In some preferred embodiments, such as, for example, a NAND array having memory cells according to one or more embodiments of the present invention, a “tightened” threshold voltage distribution is the threshold voltage of various memory cells. It shows that it exists in the range of 0.5V mutually. In other architectures using memory cells according to the present invention, the “tightened” threshold voltage distribution may have a range of about 1.0 V from an upper limit (maximum value) to a lower limit (minimum value).

  An embodiment of the operating method according to the invention comprises operating the array according to the invention by the following steps. That is, the operation includes applying a self-converging reset / erase voltage to the substrate and the gate electrode in each memory cell to be reset / erased; programming at least one of the plurality of memory cells; Reading at least one of the plurality of memory cells by applying a voltage between at least one erased state level and a programmed state level of the device group.

  The present invention also provides a semiconductor substrate, the semiconductor substrate comprising a source region and a drain region formed in the substrate below the surface of the substrate and separated by a channel region; Forming a tunnel dielectric structure above the channel region; forming a charge storage layer above the tunnel dielectric structure; forming an insulating layer above the charge storage layer; Forming a gate electrode above the insulating layer, wherein the step of forming the tunnel dielectric structure includes forming at least two dielectric layers, of the at least two dielectric layers. One includes a method of forming a memory cell, wherein the hole tunneling barrier height is lower than the other of the at least two dielectric layers.

  The expression “low (small) hole tunneling barrier height” as used herein generally means a value below the approximate hole tunneling barrier height of silicon dioxide. In particular, the low hole tunneling barrier height is preferably about 4.5 eV or less. More preferably, the low hole tunneling barrier height is about 1.9 eV or less.

  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. However, it should be understood that the invention is not limited to the arrangement and equipment as shown.

1a and 1b are a schematic cross-sectional view of an N-channel (N-type) memory cell according to an embodiment of the present invention and a schematic cross-sectional view of a P-channel (P-type) memory cell according to an embodiment of the present invention, respectively. FIG. 2 is a graph illustrating the threshold voltage (charge trapping capability) of a tunnel dielectric structure according to an embodiment of the present invention under various programming methods. FIG. 3 is a graph illustrating the threshold voltage of a SONONOS memory cell according to an embodiment of the present invention with respect to time elapsed during the erase period. FIG. 4 is a graph illustrating the threshold voltage of a SONONOS memory cell according to one embodiment of the present invention over time in a retention period. Figures 5a and 5b are band energy diagrams of ONO tunnel dielectric structures according to various embodiments of the present invention. Figures 5c and 5d are band energy diagrams of ONO tunnel dielectric structures according to various embodiments of the present invention. FIG. 5e is a band energy diagram of an ONO tunnel dielectric structure according to various embodiments of the present invention. FIG. 6 is a graph showing the relationship between hole tunneling current and field strength for three different tunnel dielectric structures. FIG. 7a is a graph illustrating the threshold voltage of a memory cell according to an embodiment of the present invention over time during various erase periods after programming. FIG. 7b is a graph illustrating the threshold voltage of a memory cell having a platinum gate according to an embodiment of the present invention over time during the erase period. FIG. 7c is a graph showing the relationship between the capacitance and voltage of the memory cell referred to in FIG. 7b. FIG. 7d is a graph showing the relationship between the capacitance and voltage of the memory cell referred to in FIG. 7b. FIG. 8 is a graph illustrating the threshold voltage of a memory cell according to an embodiment of the present invention when multiple program / erase cycles are performed under various operating conditions. FIG. 9 is a graph showing a current-voltage (IV) relationship after one cycle and after 103 cycles for a memory cell according to an embodiment of the present invention. FIG. 10 is a graph illustrating the threshold voltage of a memory cell according to an embodiment of the present invention when multiple program / erase cycles are performed under a set of programming and erase conditions. FIG. 11 is a graph showing a change with time of the threshold voltage in the VG accelerated holding test for the memory cell according to the embodiment of the present invention. 12a and 12b are an equivalent circuit diagram and a layout diagram, respectively, of a virtual ground array with memory cells according to an embodiment of the present invention. FIG. 13 is a schematic cross-sectional view of a virtual ground array with memory cells according to one embodiment of the present invention along line 12B-12B of FIG. 12b. FIGS. 14a and 14b are equivalent circuit diagrams of a memory array having memory cells according to one embodiment of the present invention, illustrating appropriate reset / erase voltages according to two embodiments of operation according to the present invention. . 15a and 15b are equivalent circuit diagrams of a memory array having memory cells according to an embodiment of the present invention, illustrating a programming method according to the present invention. FIGS. 16a and 16b are equivalent circuit diagrams of a memory array having memory cells according to an embodiment of the present invention, illustrating a bit read method according to the present invention. FIG. 17 is a graph illustrating the threshold voltage of a memory cell according to an embodiment of the present invention over time under various erase conditions. FIG. 18 is a graph illustrating the threshold voltage of a memory cell according to an embodiment of the present invention when multiple program / erase cycles are performed. FIGS. 19a and 19b are graphs representing the current at the drain of the memory cell according to one embodiment under various gate voltages, on a logarithmic scale and a linear scale, respectively. FIG. 20 is an equivalent circuit diagram of an array having memory cells according to an embodiment of the present invention, illustrating a bit programming method according to the present invention. 21a and 21b are a layout diagram and an equivalent circuit diagram of a virtual ground array according to an embodiment of the present invention. 22a and 22b are respectively an equivalent circuit diagram and a layout diagram of a NAND array including memory cells according to an embodiment of the present invention. FIGS. 23a and 23b are schematic cross-sectional views of NAND arrays with memory cells according to one embodiment of the present invention, taken along lines 22A-22A and 22B-22B of FIG. 22b, respectively. FIG. 24a is an equivalent circuit diagram of a NAND according to an embodiment of the present invention, illustrating one method of operation according to the present invention. FIG. 24b is a diagram illustrating threshold voltages over time during a reset operation according to an embodiment of the present invention for two memory cells having different initial threshold voltages. FIG. 25 is an equivalent circuit diagram illustrating one method of operation according to an embodiment of the present invention. FIG. 26 is a graph illustrating the threshold voltage of a memory cell according to an embodiment of the present invention over time under various erase conditions. FIG. 27 is an equivalent circuit diagram illustrating an operation method according to an embodiment of the present invention. FIG. 28 is a graph illustrating the threshold voltage of a memory cell according to an embodiment of the present invention when multiple program / erase cycles are performed under a set of programming and erase conditions. FIGS. 29a and 29b are graphs representing, respectively, on a logarithmic scale and a linear scale, the current at the drain of a memory cell according to one embodiment under various gate voltages at three different cycle numbers. FIG. 30 is a graph illustrating the threshold voltage of a memory cell according to an embodiment of the present invention over time during a holding period at three different temperatures and cycle conditions. FIG. 31 is a schematic cross-sectional view of a word line of a NAND array according to an embodiment of the present invention. FIG. 32 is a schematic cross-sectional view showing a word line forming technique of a NAND array according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention and its presently preferred embodiments are described in detail below. Examples of these are shown in the accompanying drawings. Wherever possible, the same or similar reference numbers have been used in the drawings and the description to refer to the same or like parts. It should be understood that the drawings other than the graph are in a fairly simplified form and are not to scale. In the context of this disclosure, expressions that indicate directions such as top (top), bottom (bottom), left, right, top, bottom, top, bottom, directly below, back, and front are merely simplifications. Therefore, it is used with reference to the accompanying drawings. Such orientation used in connection with the following description of the drawings should be construed as limiting the scope of the invention in any manner not explicitly set forth in the appended claims. is not. While the disclosure herein refers to particular illustrated embodiments, it is understood that these embodiments are presented for purposes of illustration and not limitation. I want. It should be understood that the process steps and structures described herein do not cover a complete process flow for the manufacture of a complete integrated circuit. The present invention can be implemented with various integrated circuit manufacturing techniques that are known or developed in the art.

  The memory cell according to the present invention can solve some of the reliability problems in SONOS and NROM devices. For example, the memory cell structure according to the present invention can enable a fast FN channel erase method while maintaining good charge retention characteristics. Also, various embodiments of the memory cell according to the present invention can reduce reliance on the BBTTHH erase method, thereby preventing device degradation after multiple P / E cycles.

  In one example, an ultra-thin tunnel dielectric, i.e., an ultra-thin oxide layer, along with a low hole tunneling barrier height layer (low hole tunneling barrier layer) is employed in embodiments where the tunnel dielectric structure is a multilayer structure. Can do. Thereby, more favorable stress (stress) tolerance can be provided. Also, the non-volatile memory cell according to the present invention shows little degradation after multiple P / E cycles.

  The memory cell according to the present invention can adopt either an n-channel (n-type) design or a p-channel (p-type) design as shown in FIGS. 1a and 1b. FIG. 1a is a cross-sectional view of an n-channel memory cell 100 according to one embodiment of the present invention. This memory cell has a p-type substrate 101 with at least two n-doped (added) regions 102, 104. Each of the doped regions 102, 104 can function as either a source or a drain depending on the applied voltage. For reference, as shown in FIG. 1a, doped region 102 can serve as a source and doped region 104 can serve as a drain. The substrate 101 further has a channel region 106 between the two n-doped regions. Above the channel region 106 and on the surface of the substrate 101 is a tunnel dielectric structure 120. In certain preferred embodiments, the tunnel dielectric structure 120 can have a three-layer thin ONO structure. In this structure, a low hole tunneling barrier height nitride layer (nitride coating) 124 is sandwiched between a thin lower oxide layer 122 and an upper thin oxide layer 126. The memory cell 100 further includes a charge trapping (ie, charge storage) layer 130 that is preferably nitride and disposed above the tunnel dielectric structure 120, and preferably includes a blocking oxide and is disposed above the charge trapping layer 130. And an insulating layer 140 to be formed. The gate 150 is disposed on the insulating layer 140.

  FIG. 1b is a cross-sectional view of a p-channel memory cell 200 in accordance with one embodiment of the present invention. This memory cell has an n-type substrate 201 with at least two p-doped regions 202, 204. Each of the doped regions 202, 204 can function as either a source or a drain. The substrate 201 further has a channel region 206 between the two p-doped regions. The p-channel memory cell 200 similarly includes a tunnel dielectric structure 220, a charge trapping (ie, charge storage) layer 230, an insulating layer 240, and a gate 250. The tunnel dielectric structure 220 is a three-layer thin ONO structure in which a low hole tunneling barrier height nitride layer 224 is sandwiched between a thin lower oxide layer 222 and an upper thin oxide layer 226. Have

  Thus, for example, as shown in FIGS. 1a and 1b, a memory cell according to the present invention comprises a multilayer comprising a first silicon oxide layer O1, a first silicon nitride layer N1, and a second silicon oxide layer O2. A thin film tunnel dielectric structure; a charge storage layer, such as a second silicon nitride layer N2, and an insulating layer, such as a third silicon oxide layer O3; on a substrate, such as a semiconductor substrate (eg, a silicon substrate) (On) or over (“above”) the substrate. The tunneling dielectric structure can cause tunneling of holes from the substrate to the charge storage layer during an erase / reset operation of the memory device. The tunnel dielectric structure in the non-volatile memory cell of the present invention preferably has a negligibly small (very small) charge trapping efficiency during memory operation, and more preferably does not trap any charge at all.

Charge storage materials such as silicon nitride layers, HfO ₂ , and Al ₂ O ₃ can be used as low hole tunneling barrier height layers in tunnel dielectric structures. In certain preferred embodiments of the present invention, materials with high charge storage efficiency such as silicon nitride can be used as the charge storage layer in the memory device. The blocking oxide that prevents charge loss can serve as an insulating layer, such as the third silicon oxide layer O3. The memory cell according to the present invention has a gate such as a polysilicon gate or a gate electrode above the insulating layer. The tunnel dielectric structure, the charge storage layer, the insulating layer, and the gate can be formed above the substrate and above at least a portion of the channel region. The channel region is defined by and disposed between the source region and the drain region.

  A memory cell according to various embodiments of the present invention can provide a tunnel dielectric that can provide a high FN erase rate of about 10 msec under a negative gate voltage (Vg), eg, Vg from about -10V to about -20V. Has a body structure. On the other hand, charge retention capability can also be maintained, and in some examples, charge retention capability can be better than many conventional SONOS devices. The memory cell according to the present invention can also avoid the use of the band-to-band hot hole erase operation commonly used in NROM devices. Such avoidance of the inter-band hot hole erasing operation can largely eliminate the damage due to the hot holes, and therefore such avoidance is desirable.

  Referring to FIG. 2, the measured threshold voltage for a tunnel dielectric structure according to one embodiment of the present invention is evidenced by the unchanged threshold voltage level under successive programming (write) pulses. Shows that an ultrathin O1 / N1 / O2 structure can have negligible capture efficiency. In the test example of FIG. 2, the O1 / N1 / O2 layers have thicknesses of 30, 30, and 35 angstroms, respectively. As shown in FIG. 2, the threshold voltage Vt is about 1 during several shots of programming using various programming methods, ie, -FN programming, + FN programming, and CHE (channel hot electron) programming. .9 volts constant. Thus, such an ultrathin O1 / N1 / O2 film can act as a modulated tunnel dielectric structure. All results under various charge injection methods including CHE, + FN and -FN suggest negligibly small charge trapping. The manufacturing process or device structure can be designed to minimize interface trapping and therefore neither the O1 / N1 interface nor the N1 / O2 interface is active.

  FIG. 3 shows erase characteristics of a memory cell having a SONONOS structure according to an embodiment of the present invention. The memory cell in the embodiment described in FIG. 3 has an n-MOSFET design with an ONO tunnel dielectric structure having a thickness of 15 mm, 20 mm, and 18 mm, respectively. The memory cell of this embodiment includes a silicon nitride charge storage layer having a thickness of about 70 mm, a silicon oxide insulating layer having a thickness of about 90 mm, and any suitable conductive material such as, for example, n-doped polycrystalline silicon. Has a gate. Referring to FIG. 3, high-speed FN erasure within 10 msec can be achieved, and excellent self-convergent erasure characteristics can be obtained.

  FIG. 4 shows the charge retention characteristics of a SONONOS device according to one embodiment of the memory cell according to the present invention described with reference to FIG. As shown, the retention characteristics can be better than that of conventional SONOS devices and can be several orders of magnitude better.

  FIGS. 5a and 5b are band diagrams illustrating the effect obtained by using a tunnel dielectric structure having at least one layer with a low hole tunneling barrier height. FIG. 5a shows a band diagram of a tunnel dielectric structure of three layers of O1 / N1 / O2 in this example under a low electric field that may exist during the memory data retention period. Under low electric field, direct tunneling indicated by dotted arrows can be eliminated, thereby providing good charge retention capability during the retention period. On the other hand, the band diagram offset under high electric field as shown in FIG. 5b can reduce the barrier effect of N1 and O2, which can cause direct tunneling through O1. A tunnel dielectric structure having at least one low hole tunneling barrier height layer can enable an efficient FN erase operation.

  5c and 5d show another set of band diagrams in one example. For better band offset conditions in one example, the thickness of N1 can be greater than the thickness of O1. A band diagram of the valence band at the same electric field E01 = 14 MV / cm is plotted. The tunneling probability by the WKB approximation correlates with the shadow area. In this example, if the thickness is N1 = O1, the band offset does not completely remove the O2 barrier. On the other hand, when N1> O1, the band offset can more easily remove O1. Therefore, under the same electric field in O1, the hole tunneling current can be further increased when the thickness is N1> O1.

  The measured and simulated hole tunneling current experiments shown in FIG. 6 further illustrate hole tunneling through a tunnel dielectric structure in accordance with certain embodiments of the present invention. For example, the hole tunneling current through an O1 / N1 / O2 dielectric can be a value between that for an ultra-thin oxide and that for a thick oxide. In one example, under a high electric field, the hole tunneling current may be approximately equal to that for an ultrathin oxide. However, direct tunneling can be suppressed under a low electric field. As shown in FIG. 6, hole tunneling current through a thin oxide layer is detected even at a low field strength of only 1 MV / cm. Even at relatively high field strengths, eg 11-13 mV / cm, the hole tunneling current through the thick oxide is negligibly small. However, in the presence of high field strength, the hole tunneling current through the ONO tunnel dielectric structure approaches that for a thin oxide layer. In FIG. 6, a large current leak due to hole tunneling through an ultrathin oxide under a low electric field can be seen in region A of the graph. In FIG. 6, the hole tunneling current through the O1 / N1 / O2 tunnel dielectric structure at high field strength can be seen in region B of the graph. In FIG. 6, the virtually nonexistent tunneling current through the O1 / N1 / O2 tunnel dielectric structure and thick oxide at low electric fields can be seen in region C of the graph.

  The memory cell design according to the present invention can be applied to various memory types including, but not limited to, NOR and / or NAND type flash memories.

  As described above, the tunnel dielectric layer may have more than one layer, including one layer that can provide a low hole tunneling barrier height. In one example, the layer that provides a low hole tunneling barrier height may include silicon nitride. This layer may be sandwiched between two silicon oxide layers, which can form an O / N / O tunnel dielectric when silicon nitride is used as an intermediate layer. In certain preferred embodiments of the present invention, the thickness of each layer in the tunnel dielectric structure is up to about 4 nm (about 4 nm or less). In some preferred embodiments, each layer in the tunnel dielectric structure may have a thickness of about 1 nm to 3 nm. In one exemplary device, the three-layer structure includes a bottom layer such as a silicon oxide layer of about 10 to 30 inches, an intermediate layer such as a silicon nitride layer of about 10 to 30 inches, and another silicon oxide of about 10 to 30 inches. It may have an upper end layer such as a layer. In one particular example, an O / N / O three-layer structure with a 15 下端 bottom silicon oxide layer, a 20 中間 intermediate silicon nitride layer, and an 18 上端 top silicon oxide layer can be used.

  In one example, a thin O / N / O trilayer structure exhibits negligibly small charge trapping. As described with reference to FIGS. 5 a, 5 b, and 6, the theoretical band diagram and the tunneling current analysis show that the tunnel dielectric structure such as the O1 / N1 / O2 structure in which the thickness of each layer is 3 nm or less is used. It can be suggested that the direct tunneling of holes under a low electric field during the holding period can be suppressed. Nevertheless, efficient hole tunneling can still be possible under high electric fields. This is considered to be because the tunneling barrier of N1 and O2 can be effectively removed by the band offset. Therefore, the proposed device can avoid the problem of the holding performance of the conventional SONOS device while enabling high-speed hole tunneling erasure. Experimental analysis shows the excellent endurance and retention characteristics of memory cells according to various embodiments of the present invention.

In certain preferred embodiments, the tunnel dielectric structure has at least one intermediate layer and two adjacent layers on both sides (opposite sides) of the intermediate layer. Each layer of the intermediate layer and the two adjacent layers includes a first material and a second material. The second material has a valence band energy level that is greater than the valence band energy level of the first material, and the second material is less than the conduction band energy level of the first material. It has a conduction band energy level. Also, the concentration of the second material is higher in the intermediate layer than in the two adjacent layers, and the concentration of the first material is higher in the two adjacent layers than in the intermediate layer. In the tunnel dielectric structure according to this embodiment of the present invention, preferably the first material comprises oxygen and / or an oxygen containing compound and the second material comprises nitrogen and / or a nitrogen containing compound. For example, the first material can include an oxide such as silicon oxide, and the second material can include a nitride such as Si ₃ N ₄ or Si _x O _y N _z .

  The tunnel dielectric according to this aspect of the invention may consist of more than two layers, as long as the concentration of the material having the smallest hole tunneling barrier height is higher in the intermediate layer than in the two adjacent layers. The layers may include similar components (eg, Si, N and O).

  In certain tunnel dielectric structures according to the above-described embodiments of the present invention, the second material has a concentration of the second material in the intermediate layer at a depth position in the intermediate layer from one adjacent layer / intermediate layer interface. Can be present in the intermediate layer at a gradient concentration that increases to a maximum concentration of and decreases from that maximum concentration depth position to a lower concentration at the other adjacent layer / interlayer interface. The concentration increase / decrease is preferably gradual.

In yet another embodiment of the invention, the tunnel dielectric structure has at least one intermediate layer and two adjacent layers on both sides (opposite sides) of the intermediate layer. The two adjacent layers include a first material and the intermediate layer includes a second material. The second material has a valence band energy level that is greater than the valence band energy level of the first material, and the second material is less than the conduction band energy level of the first material. It has a conduction band energy level. The second material also increases the concentration of the second material in the intermediate layer from one adjacent layer / intermediate layer interface to the maximum concentration at a certain depth position in the intermediate layer, and from the maximum concentration depth position to the other. Present in the intermediate layer at a gradient concentration that decreases to a lower concentration at the adjacent layer / interlayer interface. The concentration increase / decrease is preferably gradual. In the tunnel dielectric structure according to this embodiment of the present invention, preferably the first material comprises oxygen and / or an oxygen containing compound and the second material comprises nitrogen and / or a nitrogen containing compound. For example, the first material can include an oxide such as silicon oxide, and the second material can include a nitride such as Si ₃ N ₄ or Si _x O _y N _z .

  For example, in an embodiment of the invention where the tunnel dielectric layer has a three-layer ONO structure, the bottom oxide layer and the top oxide layer can include silicon dioxide, and the intermediate nitride layer can be, for example, silicon oxynitride ( Silicon oxynitride) and silicon nitride, and the concentration of silicon nitride (ie, of the two materials having a smaller hole tunneling barrier height) in the same layer is not constant, rather the layer A maximum is reached at a certain depth in the layer between the two interfaces with the oxide layer sandwiching the.

  The exact location in the intermediate layer where the material with the lowest hole tunneling barrier height reaches its maximum concentration is not critical, it exists within the concentration gradient and is tunneled at some location in the intermediate layer. It is only necessary to reach its maximum concentration in the dielectric layer.

  Increasing the concentration of the material having the minimum hole tunneling barrier height can be beneficial in improving various characteristics of non-volatile memory devices, particularly non-volatile memory devices having a SONONOS or SONONOS-like structure. . For example, charge loss in the holding state can be reduced, hole tunneling under high electric fields can be improved, and charge trapping in the tunnel dielectric can be prevented to the extent that it can occur.

  The band diagram of the tunnel dielectric layer shows that, according to this aspect of the invention, the energy level of the valence band and the energy level of the conduction band do not have constant values, but rather the minimum hole tunneling barrier height. It can change suitably so that it may change along the thickness direction of a layer with the density | concentration of the material which has. Referring to FIG. 5e, a three-layer tunnel dielectric modification of ONO according to this aspect of the invention is illustrated by a band diagram. The intermediate layer (layer-2) is made of silicon nitride. The outer layers (Layer-1 and Layer-3) consist of silicon dioxide. The concentration of silicon nitride in layer-2 is changed, so that the energy level of the valence band and the energy level of the conduction band are respectively maximum at the depth in layer-2 where the concentration of silicon nitride is maximum. Reach the minimum value. Three possible silicon nitride concentration gradients are shown in FIG. 5e. The dashed lines shown represent the energy levels of the changing valence and conduction bands obtained by the concentration gradient. As shown in FIG. 5e, the circle on the dashed line represents the three distinct maximum silicon nitride concentrations in layer-2, the lowest valence band energy level and the highest conduction band energy level being It is consistent with the maximum concentration of silicon.

  Multi-layer tunnel dielectric structures according to such embodiments of the invention can be made in a variety of ways. For example, the first silicon dioxide layer or silicon oxynitride layer includes, but is not limited to, chemical vapor deposition (CVD), and thermal oxidation, radical (ISSG) oxidation, and plasma oxidation / nitridation. It can be formed using any number of conventional oxidation techniques. The intermediate layer having a graded concentration of SiN may then be formed, for example, by chemical vapor deposition or, alternatively, plasma nitridation of excess oxide or oxynitride formed on top of the first layer. it can. Then, the upper oxide layer, which is the third layer, can be formed, for example, by oxidation or chemical vapor deposition.

  A charge storage layer can then be formed over the tunnel dielectric structure. In one example, a charge storage layer of about 5 nm to 10 nm can be formed over the tunnel dielectric structure. In one particular example, a silicon nitride layer having a thickness of about 7 nm or more can be used. The insulating layer above the charge storage layer may have a thickness of about 5 nm to 12 nm. For example, a silicon oxide layer having a thickness of about 9 nm or more can be used. The silicon oxide layer can be formed by heat-treating at least a part of the nitride layer so as to form a silicon oxide layer. Any method known or that will be developed may be used to form the layers of suitable materials described herein, such as a tunnel dielectric layer, a charge storage layer, and An insulating layer can be deposited or formed. Suitable methods include, for example, thermal growth methods and chemical vapor deposition methods.

  In one example, the heat conversion process can provide high density or concentration interface traps, which can improve the capture efficiency of the memory device. For example, heat conversion of nitride can be performed at 1000 (C, with a gate flow ratio of H2: O2 = 1000: 4000 sccm.

  Furthermore, silicon nitride generally has a very low hole barrier (about 1.9 eV) and can therefore be non-resistive (transparent) to hole tunneling under high electric fields. On the other hand, the overall thickness of a tunnel dielectric such as an ONO structure can prevent direct tunneling of electrons under a low electric field. In one example, this non-equilibrium behavior can provide a memory device that provides not only fast hot tunneling erase, but also a reduction or elimination of charge leakage during the retention period.

  A typical device can be fabricated by 0.12 μm NROM / NBit technology. Table 1 shows the device structure and parameters in one example. The tunnel dielectric with the proposed ultra-thin O / N / O can change the hole tunneling current. In one example, a thicker (7 nm) N2 layer can serve as a charge trapping layer, and an O3 (9 nm) layer can serve as a blocking layer. Both N2 and O3 can be manufactured using NROM / NBit technology.

In particular embodiments of the present invention, the gate can include a material having a work function greater than that of N ⁺ polysilicon. In certain preferred embodiments of the present invention, such high work function gate materials may include metals such as, for example, platinum, iridium, tungsten, and other noble metals. Preferably, the gate material in such an embodiment has a work function of about 4.5 eV or greater. In certain preferred embodiments, the gate material comprises a high work function metal such as, for example, platinum or iridium. Further, preferred high work function materials include, but are not limited to, P ⁺ polysilicon and metal nitrides such as titanium nitride and tantalum nitride. In a particularly preferred embodiment of the invention, the gate material comprises platinum.

  A typical device according to an embodiment of the present invention having a high work function gate material can also be fabricated by 0.12 μm NROM / NBit technology. Table 2 shows the device structure and parameters in one example. The tunnel dielectric with the proposed ultra-thin O / N / O can change the hole tunneling current. In one example, a thicker (7 nm) N2 layer can serve as a charge trapping layer, and an O3 (9 nm) layer can serve as a blocking layer. Both N2 and O3 can be manufactured using NROM / NBit technology.

Memory cells according to embodiments using the high work function gate material of the present invention exhibit improved erase characteristics over other embodiments. The high work function gate material suppresses gate electron injection into the trapping layer. In certain embodiments of the invention where the memory cell has an N ⁺ polysilicon gate, hole tunneling into the charge trapping layer during the erase period occurs simultaneously with gate electron injection. This self-converging erase effect results in a higher threshold voltage level in the erased state, which may be undesirable in NAND applications. Memory cells according to embodiments using the high work function gate material of the present invention can be used in various types of memory applications including, for example, NOR-type and NAND-type memories. However, memory cells according to embodiments using the high work function gate material of the present invention are particularly suitable for use in NAND applications where the elevated threshold voltage in the erased / reset state may be undesirable. Memory cells according to embodiments using the high work function gate material of the present invention can be erased by a hole tunneling method, and preferably by an -FN erase operation.

A typical device having an ONO tunneling dielectric and an N ⁺ polysilicon gate can be programmed by conventional SONOS or NROM methods and can be erased by channel FN hole tunneling. FIG. 7a shows the erase characteristics of a typical SONONOS device having an ONO tunneling dielectric in one example. Referring to FIG. 7a, a higher gate voltage results in a faster erase speed. Gate injection is also more powerful and has a higher saturation Vt because the resulting dynamic balance point (which determines Vt) is higher. This is shown on the right side of the graph, with the threshold voltage reaching a minimum at a voltage from about 3 volts to about 5 volts, depending on the erase gate voltage. The hole tunneling current can be extracted by a transient analysis method (unsteady analysis method) based on the differentiation of the curve in FIG. The Hall current extracted from the measured values in FIG. 7a is shown in FIG. For comparison, the hole tunneling current simulated using the WKB approximation is also plotted. The experimental results are in reasonable agreement with our predictions. The tunneling current through the O1 / N1 / O2 stack approaches that for ultrathin O1 under high electric fields, but is extinguished under low electric fields.

  In certain embodiments of the memory cell of the present invention having a high work function gate material, the high work function gate may suppress gate electron injection, resulting in a very low device threshold voltage in the erased or reset state. Yes, and can even be negative depending on the erase time. FIG. 7b shows the threshold voltage value of a memory device according to an embodiment of the present invention in which the gate is made of platinum and the tunnel dielectric layer has an ONO structure of 15/20/18 angstroms. As shown in FIG. 7b, the threshold voltage of the device can be set to −3V or lower with the same gate voltage (−18V) in the −FN erase operation period. The relationship between the corresponding capacitance and the gate voltage value for this device is shown in FIG.

  Furthermore, the retention characteristics of memory devices according to embodiments using the high work function gate material of the present invention are improved. The retention characteristics of a memory device having a platinum gate is shown in FIG. 7d, where the capacitance is graphed as a function of gate voltage after erasing and programming, after 30 minutes of each operation and after 2 hours of each operation. A very small deviation is observed.

  Memory cells according to various embodiments of the invention can be operated in at least two distinct ways (schemes). For example, CHE programming with reverse read (mode 1) can be used to perform 2 bits per cell (ie, 2 bits / cell) operation. In addition, low power (low power) + FN programming (mode 2) can be used for 2 bit operation per cell. In both modes, the same hole tunneling elimination method can be used. Mode 1 can be suitably used for a virtual ground array architecture for NOR flash memory. Mode 2 can be suitably used for NAND flash memory.

  As an example, FIG. 8 illustrates the excellent endurance characteristics of a virtual ground array architecture NOR flash memory according to an embodiment of the present invention in mode 1 operation. Erase degradation of such a memory device having a tunnel dielectric structure does not occur because hole tunneling erase (Vg = -15V) is a uniform channel erase method. The corresponding IV curve is also shown in FIG. The figure suggests that there is little device degradation after multiple P / E cycles. In one example, this may be because the ultra-thin oxide / nitride layer has good stress relief characteristics. Furthermore, this memory device is not damaged by hot holes. FIG. 10 shows durability characteristics of the NAND flash memory according to the embodiment of the present invention in mode 2 operation. A larger bias (Vg = −16V) can be used for faster convergence erase time. Also in this example, excellent durability can be obtained.

  FIG. 4 shows the charge retention performance of a typical SONONOS device according to one embodiment of the present invention. In the figure, a charge loss of only 60 mV is observed after 100 hours. The retention characteristics are improved by several orders of magnitude better than conventional SONOS devices. The VG accelerated hold test also shows that direct tunneling can be suppressed under low electric fields. FIG. 11 shows an example of a VG accelerated holding test for a device that has been subjected to 10K P / E cycles. The charge loss is small at −VG stress after 1000 seconds stress, indicating that direct tunneling of holes in a low electric field can be suppressed.

  Therefore, the SONONOS design specified in the above example can provide excellent durability characteristics for high speed hole tunneling erasure. As mentioned above, this design can be implemented in both NOR and NAND type nitride storage (memory) flash memories. Furthermore, a memory array according to the present invention may have multiple memory devices of similar or different configuration.

  In various embodiments of the array according to the present invention, the memory cells according to the present invention can be used in a virtual ground array architecture in place of conventional NROM or SONOS devices. By using FN hole tunneling instead of hot hole injection, reliability problems and erasure degradation can be solved or reduced. Although not intended to limit the scope of the present invention to the particular structures described below, various methods of operation with the memory array of the present invention will now be described with respect to a typical NOR virtual ground array architecture.

  CHE or CHISEL (channel initiated secondary electron) programming and reverse read can be used for a 2-bit memory array per cell. The erasing method may be uniform channel FN hole tunneling erasing. In one example, the array architecture may be a virtual ground array or a JTOX array. Referring to FIGS. 12a-20, the tunnel dielectric can use an O1 / N1 / O2 three-layer structure with each layer having a thickness of about 3 nm or less to provide direct tunneling of holes. With reference to FIGS. 12a-20, N2 may be thicker than 5 nm to provide high capture efficiency. The insulating layer, O3, is a silicon oxide layer formed by wet oxidation, such as a wet-converted top oxide (silicon oxide), to provide a large trap density at the interface between O3 and N2. It's okay. O3 may have a thickness of about 6 nm or more so as to prevent charge loss from the silicon oxide layer.

  FIGS. 12a and 12b show an example of a virtual ground array architecture incorporating the memory cells described above, such as memory cells having a three-layer ONO tunnel dielectric. In particular, FIG. 12a shows an equivalent circuit of a portion of the memory array, and FIG. 12b shows a typical layout of a portion of the memory array.

Further, FIG. 13 shows a schematic cross-sectional view of several memory cells incorporated in the array. In one example, the buried diffusion (BD) region may be an N ⁺ doped junction (junction) for the source or drain region of the memory cell. The substrate may be a p-type substrate. In one example, thick BDOX (> 50 nm) can be used to avoid breakdown (breakdown, breakdown) of the BDOX region (oxide above BD) that can occur during the -FN erase period.

  FIGS. 14a and 14b show possible electrical reset methods for a typical virtual ground array incorporating 2 bits per cell (ie, 2 bits / cell) memory cells having the tunnel dielectric design described above (ie, Scheme). Before performing further P / E cycles, all devices can first undergo an electrical “reset”. The reset process can ensure the uniformity of Vt in the memory cells in the same array, and can raise the Vt of the device to a state of being converged and erased. For example, the application of Vg = -15V for 1 second, as shown in FIG. 14a, can have the effect of injecting some charge into the silicon nitride charge trapping layer to reach a dynamic equilibrium. By resetting, for example, even if the memory cell group is charged non-uniformly due to the plasma charging effect during the manufacturing process period, the Vt can be converged (aligned). Another way to generate a self-focusing bias condition is to bias both the gate voltage and the substrate voltage. For example, referring to FIG. 14b, Vg = −8V and P-well (P-well) = + 7V can be applied.

  FIGS. 15a and 15b illustrate a programming method (scheme) for a typical virtual ground array that incorporates 2 bits of memory cells per cell having the tunnel dielectric design described above. Channel hot electron (CHE) programming can be used to program the device. For the bit-1 programming shown in FIG. 15a, electrons are locally injected into the junction edge above BLN (bit line N). For the bit-2 programming shown in FIG. 15b, electrons are stored on BLN-1. A typical programming voltage for WL (word line) is about 6V to 12V. A typical programming voltage for BL (bit line) is about 3V to 7V, and the P-well can be left grounded.

  FIGS. 16a and 16b illustrate a read method (scheme) for a typical virtual ground array that incorporates two bits of memory cells per cell having the tunnel dielectric design described above. In one example, reverse read is used to read the device and perform a 2-bit operation per cell. Referring to FIG. 16a, for reading bit-1, an appropriate read voltage such as 1.6V is applied to BLN-1. Referring to FIG. 16b, for reading bit-2, an appropriate read voltage such as 1.6V is applied to BLN. In one example, the read voltage may be in the range of about 1V to 2V. The word line and the P well can be kept grounded. However, other modified read methods (schemes) such as a raised-Vs reverse read method can also be performed. For example, in the reverse reading method by increasing Vs, Vd / Vs = 1.8 / 0.2V for reading bit-2 and Vd / Vs = 0.2 for reading bit-1. /1.8 can be used.

  14a and 14b also illustrate a sector erase method (scheme) for a typical virtual ground array that incorporates two bits of memory cells per cell having the tunnel dielectric design described above. In one example, sector erasure by channel hole tunneling erasure can be applied to simultaneously erase memory cell groups. An ONO tunnel dielectric in a memory cell having a SONONOS structure can provide fast erase that can occur at about 10-50 msec and a self-focusing channel erase rate. In one example, the sector erasing operation condition can be the same as the reset process. For example, referring to FIG. 14a, sector erase can be achieved by simultaneously applying VG = about −15 V to the WL group and leaving all the BL groups in a floating state. The P-well can be left grounded.

  Alternatively, referring to FIG. 14b, sector erase can also be achieved by applying approximately -8V to the WL group and applying approximately + 7V to the P-well. In some examples, a complete sector erase operation may be performed within 100 msec without having any over-erase or hard-to-erase cells. it can. The device design described above can facilitate channel erasure resulting in excellent self-convergence characteristics.

FIG. 17 shows erase characteristics in an example using a SONONOS device. An example of a SONONOS device may have an O1 / N1 / O2 / N2 / O3 thickness of about 15/20/18/70/90 angstroms, respectively, N ⁺ polysilicon gate and heat-converted top as O3 It may have an oxide. The erase rate for various gate voltages is shown. The higher the gate voltage, the faster the erase speed.

However, the converged Vt is also higher. This is because gate injection is more active under higher gate voltages. To reduce gate injection, a P ⁺ polysilicon gate or other metal gate with a high work function can be used instead as a gate material to reduce electrons injected from the gate during the erase period. .

  FIG. 18 shows endurance characteristics when a SONONOS device is used in a virtual ground array architecture. The durability characteristics in some examples are excellent. Programming conditions are Vg / Vd = 8.5 / 4.4V, 0.1 μsec for bit-1 and Vg / Vs = 8.5 / 4.6V, 0. 1 μsec. The FN erase can use Vg = −15 V for about 50 msec, and two bits can be erased simultaneously. Since FN erase is a self-focusing uniform channel erase, there are usually no under-erased or over-erased cells. In some examples, the devices presented above exhibit excellent endurance characteristics without using program / erase verification (verification) or stepping algorithms.

  19a and 19b show the IV characteristics during the P / E cycle in one example. Corresponding IV curves are shown on both a logarithmic scale (FIG. 19a) and a linear scale (FIG. 19b). In one example, SONONOS devices have little degradation after multiple P / E cycles. Therefore, both the subthreshold swing (SS) and transconductance (gm) are almost the same after many cycles. This SONONOS device has superior durability characteristics than the NROM device. One of the factors seems to be that hot hole injection is not used. Furthermore, as described above, ultrathin oxides can have better stress resistance than thick tunnel oxides.

  FIG. 20 illustrates a CHISEL programming method (scheme) in one example. Another way to program the device is to use the CHISEL programming method (scheme), which promotes impact ionization (impact ionization) with a negative substrate bias to increase hot carrier efficiency. . In addition, the programming current can be reduced by the body effect (the effect of the substrate bias). The figure shows typical conditions, where a negative voltage (-2V) is applied to the substrate and the junction voltage is lowered to about 3.5V. For conventional NROM devices and techniques, CHISEL programming cannot be applied because more electrons may be injected near the central region of the channel. Also, hot hole erasure is not effective in removing electrons near the channel center region in conventional NROM devices.

  21a and 21b show a JTOX virtual ground array design in one example. The JTOX virtual ground array provides another way to realize the use of SONONOS memory cells in the memory array. In one example, one of the differences between the JTOX structure and the virtual ground array is that the devices in the JTOX structure are separated by STI processing (processing). FIG. 21a shows a typical layout example. FIG. 21b shows the corresponding equivalent circuit, which is the same as that of the virtual ground array.

  As described above, the memory cell structure according to the present invention is suitable for both NOR type and NAND type flash memories. In the following, further examples of memory array design and methods of operation thereof will be described. While not intending to limit the scope of the present invention to the specific structures described below, various methods of operation with the memory array of the present invention are described below with respect to a typical NAND architecture.

  As described above, an n-channel SONONOS memory device having an ONO tunneling dielectric can be utilized for the memory device. 22a and 22b show an example of a NAND array architecture. Figures 23a and 23b are cross-sectional views of a typical memory array design from two different directions. In some examples, the method of operating the memory array may include + FN programming, self-convergent reset / erase, and read methods. Further, in some examples, a circuit operation method may be included to prevent program disturb.

  In addition to the single block gate structure design, a split gate array may be used, such as a NAND array using a SONONOS device positioned between two transistor gates located adjacent to the source / drain regions. In some examples, a split gate design can reduce device dimensions to F = 30 nm or less. In addition, designing the device to achieve good reliability, reduce or eliminate inter-floating-gate coupling effects, or both Can do. As described above, SONONOS memory devices can provide excellent self-convergent erase, which helps control the sector erase operation and Vt distribution. Furthermore, the tightened erased state distribution can facilitate multilevel applications (MLC).

  By using a specific design for the memory array structure, the effective channel length (Leff) can be increased to reduce or eliminate short-channel effects. Some examples can be designed without the use of diffusion junctions, thereby avoiding the challenges that arise in providing shallow junctions or using pocket implantation in the memory device manufacturing process.

FIG. 1 shows an example of a memory device having a SONONOS design. Further, Table 1 above shows an example of materials used for different layers and their thickness. In some examples, a P ⁺ polysilicon gate can be used to provide a lower saturation reset / erase Vt that can be achieved by reducing gate implantation.

  FIGS. 22a and 22b show an example of a memory array, such as a SONONOS-NAND array, having a diffusion junction and having memory cells according to the embodiments described in Table 1. FIG. In one example, the individual devices use shallow trench isolation (STI: shallow-trench isolation) or silicon-on-insulator (SOI) isolation technology. Can be separated from each other by various separation techniques. Referring to FIG. 22a, the memory array may have a number of bit lines such as BL1 and BL2 and a number of word lines such as WL1, WLN-1 and WLN. Further, the array includes a source line transistor (s) (ie, source line select transistor (s) or SLT (group)) and a bit line transistor (s) (ie, bit line select transistor (s) or BLT (group)). ). As shown, the memory cells in the array can use the SONONOS design, and SLT and BLT are n-type metal-oxide-semiconductor field-effect transistors (NMOSFETs). an effect transistor).

  FIG. 22b shows an example of the layout of a memory array such as a NAND array. Referring to FIG. 22b, Lg is the channel length of the memory cell, and Ls is the spacing between individual lines of the memory device. Further, W is the channel width of the memory cell, and Ws is the width of the isolation region or source / drain region between individual bit lines, which in one example may be the STI width.

  Referring again to FIGS. 22a and 22b, memory devices can be connected in sequence to form a NAND array. For example, a column of memory devices may have 16 or 32 memory devices and can provide 16 or 32 column numbers. BLT (s) and SLT (s) can be used as select transistors for controlling the corresponding NAND strings. In one example, the gate dielectric for BLT and SLT may be a silicon oxide layer that does not include a silicon nitride capture layer. Such a configuration is not required in all cases, but in some examples can prevent BLT and SLT Vt shifts that may occur during operation of the memory array. Alternatively, BLT and SLT can use a combination of ONONO layers as their gate dielectric layers.

  In some examples, the gate voltage applied to the BLT and SLT can be less than 10V, which is considered to cause little gate disturb. In cases where the BLT and SLT gate dielectric layers may be charged or charge trapped, an additional -Vg erase may be applied to the BLT or SLT gates to discharge the gate dielectric layers. be able to.

Referring back to FIG. 22a, each BLT may be connected to a bit line (BL). In one example, the BL may be a metal line having a pitch that is the same as or approximately the same as the pitch of the STI. Each SLT is connected to a source line (SL). The source line is parallel to WL and is connected to a sense amplifier for reading detection. The source line may be a metal such as tungsten, or a polysilicon line, or a diffused N ⁺ doped line.

  FIG. 23a shows a cross-sectional view along the channel length direction of a typical memory array, such as a SONONOS-NAND memory array. Typically, Lg and Ls are approximately equal to F, which generally represents the critical dimension of the device (or node). The critical dimension can vary depending on the technology used for manufacturing. For example, F = 50 nm means using a 50 nm node. FIG. 23b shows a cross-sectional view along the channel width direction of a typical memory array, such as a SONONOS-NAND memory array. Referring to FIG. 23b, the pitch in the channel width direction is approximately equal to or slightly larger than the pitch in the channel length direction. Accordingly, the size of the memory cell is approximately 4F2 (ie, 4F2 / cell) per cell.

  In some examples for the manufacture of memory arrays, such as the arrays described above, the manufacturing process may include using only two primary masks (primary masks) or lithographic processes, for example one One is for polysilicon (word line) and the other is for STI (bit line). In contrast, the fabrication of NAND floating gate devices will require at least two poly processes and another inter-poly ONO process. Therefore, it is considered that the structure and manufacturing process of the presented device is simpler than that of the NAND type floating gate memory.

Referring to FIG. 23a, in one example, the spacing (Ls) between word lines (WL) can be formed by a shallow junction, such as a shallow junction of an N ⁺ doped region, which is the source or drain region of a memory device. Can work as. As shown in FIG. 23a, additional implantation and / or diffusion processes such as tilt-angle pocket implantation can be performed, thereby adjoining one or more shallow junction regions, One or more “pocket” regions may be provided, ie, a pocket extension of the junction. In some examples, such a structure can provide better device characteristics.

  In some instances where STI is used to isolate individual memory devices, especially when the junction bias used is raised higher, the trench depth of the STI region is depleted in the P-well. Can be larger than the width. For example, for a program inhibit (block) bit line (group) (a bit line (group) that is not selected during programming), the junction bias can be increased to about 7V. In one example, the depth of the STI region can be in the range of about 200 nm to 400 nm.

After the memory array is manufactured, a reset operation can be performed first before other operations of the memory array in order to tighten the Vt distribution (to reduce Vt variation). FIG. 24a shows an example of such an operation. In one example, the array can be reset (VG and VP-well voltage drops) by first applying VG = approximately −7V and VP well (VP-well) = + 8V before other operations begin. Can be distributed within the gate voltage within each WL and P well.) During reset, BL (s) can be floated or raised to the same voltage as the P-well. As shown in FIG. 24b, the reset operation can provide excellent self-convergence characteristics. In one example, even though the SONONOS device group is initially charged to various Vt, the reset operation can “tighten” them into the reset / erase state. In one example, the reset time is about 100 msec. In this example, the memory array may use ONONO = 15/20/18/70/90 angstrom n-channel SONONOS devices with Lg / W = 0.22 / 0.16 μm N ⁺ polysilicon gates. it can.

  In general, conventional floating gate devices cannot provide self-converging erase. In contrast, SONONOS devices can be operated with a convergent reset / erase method. In some examples, this operation can be essential. This is because the initial Vt distribution is often wide due to certain processing problems such as processing non-uniformity or plasma charging effects. A typical self-convergence “reset” can help to tighten the initial Vt distribution of the memory devices, ie, narrow the range of the initial Vt distribution.

  In one example of a programming operation, a high voltage, such as a voltage of about + 16V to + 20V, can be applied to the selected WL to cause channel + FN injection. Other pass gates (other unselected WL (s)) can be turned on to produce inversion layers in the NAND string. In some examples, + FN programming may be a low power method. In one example, a parallel programming method such as page programming of parallel 4 Kbyte cells can increase the programming throughput to 10 MB / sec or more, but still can control the total current consumption to within 1 mA. In some examples, a high voltage, such as a voltage of about 7V, can be applied to other BLs to prevent program disturb in the other BLs. Thereby, the potential of the inversion layer is raised higher, and a voltage drop in the non-selected BL (such as the cell B in FIG. 25) can be suppressed.

  In some examples of read operations, the selected WL can be raised to a voltage between the erased state level (EV) and the programmed state level (PV). Other WLs can act as “pass gates” so that their gate voltage can be raised to a higher voltage than PV. In some examples, the erase operation may be similar to the reset operation described above, which may allow self-convergence to the same or similar reset Vt.

  FIG. 25 shows an example of the operation of the memory array. Programming may include channeling electrons + FN injection into the SONONOS nitride trapping layer. Some examples may include applying Vg = about + 18V to the selected WLN-1 and applying VG = about + 10V to the BLT and other WLs. In order to prevent channel hot electron injection in cell B, the SLT can be turned off. In this example, since all the transistors in the NAND string are turned on, the inversion layer passes through the string. Furthermore, since BL1 is grounded, the inversion layer in BL1 has a zero potential. On the other hand, the other BLs are raised to a high potential such as a voltage of about + 7V, so that the potential of the inversion layer of the other BL becomes higher.

  In particular, for cell A, the cell selected for programming, the voltage drop is about + 18V, which causes + FN injection. Also, Vt can be raised to PV. For cell B, the voltage drop is + 11V and this causes only a little + FN injection since the FN injection is sensitive to Vg. For cell C, only + 10V is applied, so it does not cause + FN injection or negligibly little + FN injection. In some examples, the programming operation is not limited to the illustrated technique. In other words, other appropriate program prohibition techniques can be applied.

  FIGS. 24a, 26, and 27 further illustrate some examples of array operation, and show endurance and retention characteristics in some examples. As shown, device degradation after multiple operating cycles may remain very small. FIG. 24a shows a typical erase operation, which may be similar to a reset operation. In one example, erasure is performed in units of sectors or blocks. As described above, this memory device may have good self-converging erase characteristics. In some examples, the erase saturation Vt may depend on Vg. For example, a higher Vg can cause a higher saturation Vt. As shown in FIG. 26B, the convergence time can be about 10-100 msec.

  FIG. 27 shows a typical read operation. In one example, the reading can be performed by applying a gate voltage between Vt (EV) in the erased state and Vt (PV) in the programmed state. For example, the gate voltage may be about 5V. On the other hand, a higher gate voltage such as a voltage of about +9 V is applied to the other WL, BLT, and SLT, turning on all other memory cells. In one example, if Vt of cell A is higher than 5V, the read current may be very small (<0.1 μA). If Vt of cell A is lower than 5V, the read current may be higher (> 0.1 μA). As a result, the state of the memory, that is, the stored information can be identified (determined).

  In some examples, the pass gate voltage for the other WL should be higher than the high Vt state, ie, the programmed state Vt, but not too high to cause gate disturb. . In one example, the pass gate voltage is in the range of about 7-10V. The voltage applied to BL may be about 1V. A larger read voltage can produce a larger current, but in some examples, read disturb may be more pronounced. In some examples, the sensing amplifier can be placed on the source line (source sensing) or on the bit line (drain sensing).

  Some examples of NAND columns may have 8, 16 or 32 memory devices per column. Larger NAND strings can reduce overhead (and save more) and increase array efficiency. However, in some examples, the read current may be smaller and the disturb may be more pronounced. Therefore, the appropriate number of NAND strings should be selected based on various design, manufacturing and operational factors.

  FIG. 28 shows the cycle endurance characteristics of certain typical devices. Referring to FIG. 28, a P / E cycle with + FN program and -FN erase can be performed and the results suggest good endurance characteristics. In this example, the erase condition is Vg = about −16 V for 10 msec. In some examples, erasing of only a single shot is required and status confirmation is not required. The Vt window of the memory is good without deterioration.

  Figures 29a and 29b show the IV characteristics of a typical memory device using different scales. In particular, FIG. 29a shows low swing degradation of the device and FIG. 29b shows low gm degradation of the device. FIG. 30 shows the retention characteristics of a typical SONONOS device. Referring to FIG. 30, good retention capability has been obtained, and the charge loss is less than 100 mV for a device that was operated after 10K cycles and left at room temperature (room temperature) for 200 hours. FIG. 30 also shows acceptable charge loss at high temperatures.

In some examples, a split gate design such as a split gate SONONOS-NAND design can be used to achieve more aggressive shrinking of the memory array. FIG. 31 shows an example using such a design. Referring to FIG. 31, the distance (Ls) between each word line, that is, between two adjacent memory devices sharing the same bit line can be reduced. In one example, Ls can be reduced to about 30 nm or less than 30 nm. As shown, memory devices using a split gate design along the same bit line share only one source region and one drain region. In other words, the split gate SONONOS-NAND array may not use junctions such as diffusion regions, ie N ⁺ doped regions, for some of the memory devices. In one example, this design can reduce or eliminate the need for shallow junctions and adjacent “pockets” that, in some examples, can include more complex manufacturing processes. Furthermore, in some examples, this design is less affected by short channel effects. This is because, in one example, the channel length has been increased, for example to Lg = 2F-Ls.

  FIG. 32 illustrates a typical manufacturing process for a memory array using a split gate design. This schematic diagram is merely an illustrative example, and the memory array can be designed and manufactured in a variety of different ways. Referring to FIG. 32, after a plurality of layers are formed with materials to provide a memory device, the silicon oxide structure as a hard mask formed on those layers is used to form those layers. Patterning can be performed. For example, the silicon oxide region can be defined by lithography and etching processes. In one example, the pattern used to define the initial silicon oxide regions may have a width of about F and a spacing between the silicon oxide regions of about F, resulting in a pitch of about 2F. After the initial silicon oxide regions are patterned, silicon oxide spacers can then be formed around the patterned regions to enlarge each silicon oxide region and reduce their spacing.

  Referring again to FIG. 32, after the silicon oxide region is formed, it is used as a hard mask to define, i.e., pattern, the underlying layer, and one or more memory devices, e.g., multiple A NAND string can be provided. Further, an insulating material such as silicon oxide can be used to fill a space between adjacent memory devices, such as the Ls spacing shown in FIG.

  In one example, the spacing Ls between adjacent memory devices along the same bit line can be in the range of about 15 nm to about 30 nm. As described above, in this example, the effective channel length can be expanded to 2F-Ls. In one example, if F is about 30 nm and Ls is about 15 nm, Leff is about 45 nm. For the operation of these typical memory devices, the gate voltage can be reduced to 15V or less. Furthermore, in order to avoid spacer breakdown in the Ls interval, it can be designed so that the voltage drop between polysilicons between word lines does not exceed 7V. In one example, this can be achieved by making the electric field between adjacent word lines less than 5 MV / cm.

  The Leff with diffusion junction in conventional NAND floating gate devices is about half of its gate length. In contrast, in one example, for the proposed design (split gate NAND), if F is about 50 nm and Leff is about 30 nm, Leff is about 80 nm. Longer Leff can provide better device characteristics by reducing or eliminating the effects of short channel effects.

  As described above, the split gate NAND design can further reduce the interval (Ls) between adjacent memory cells in the same bit line. On the other hand, the conventional NAND type floating gate device cannot provide a small interval because the memory window may be lost (narrowed) due to the coupling effect between the floating gates. Coupling between floating gates is when the coupling capacitance between adjacent floating gates is large (when the spacing between floating gates is so small that the coupling capacitance between adjacent floating gates becomes so large that read disturb occurs) Interference between memory cells. As previously mentioned, this design can eliminate the need for the creation of several diffusion junctions, and the inversion layers can be directly connected if all word lines are turned on. Thus, this design can simplify the memory device manufacturing process.

  As explained above, the above examples, including memory device structural design, array design and operation, provide desirable array dimensions, good reliability, good performance, or any combination thereof. be able to. Some of the embodiments described above can be applied to reduce the size of non-volatile flash memory such as NAND flash memory and flash memory for data applications. Some embodiments can provide a SONONOS device capable of uniform, self-convergent channel hot tunneling erasure. Some embodiments may also provide good durability of the memory device and may reduce some under-erase or over-erase problems. In addition, it is possible to provide good device characteristics such as little deterioration after the P / E cycle and good charge retention capability. The uniformity of the devices in the memory array can be provided without producing abnormal bits or cells. In addition, some embodiments can provide good short channel device characteristics due to the split gate NAND design, which can provide good sense margin during operation of the memory device.

  The above disclosure of preferred embodiments of the present invention has been made for purposes of illustration and description. This is not exhaustive and does not limit the invention to the precise form disclosed. Those skilled in the art will appreciate that changes can be made to the above embodiments without departing from the broad inventive concept of the invention. Accordingly, it is to be understood that the invention is not limited to the specific embodiments disclosed, but also encompasses modifications made within the spirit and scope of the invention as defined by the appended claims.

Claims (11)

A method of operating a memory array having a plurality of memory cells disposed on a semiconductor substrate, each of the memory cells comprising a source region and a drain region disposed below the surface of the substrate and separated by a channel region A tunnel dielectric structure disposed above the channel region, the tunnel dielectric structure comprising at least one layer having a low hole tunneling barrier height, and disposed above the tunnel dielectric structure The method comprising: a charge storage layer; an insulating layer disposed above the charge storage layer; and a gate electrode disposed above the insulating layer,
Applying a self-focusing reset / erase voltage to the gate electrode in the substrate and each memory cell to be reset / erased;
Programming at least one of the plurality of memory cells;
Reading at least one of the plurality of memory cells by applying a voltage between at least one erased state level and a programmed state level of the memory device group;
A method characterized by comprising:   The method of claim 16, further comprising applying a self-converging reset / erase voltage to the gate electrode in the substrate and the at least one programmed memory cell.   The step of applying the self-convergence reset / erase voltage includes applying a negative gate voltage Vg and a substrate voltage Vs, and has a potential difference Vg−Vs of about −20V to about −12V. The method of claim 16.   The step of applying a self-focusing reset / erase voltage to the at least one programmed memory cell includes applying a negative gate voltage Vg and a substrate voltage Vs, the potential difference being about −20V to about −12V. The method of claim 16, comprising Vg−Vs.   The method of claim 16, wherein applying a self-converging reset / erase voltage comprises applying a gate voltage of about -20V to about -12V and grounding the substrate.   The method of claim 16, wherein applying the self-converging reset / erase voltage comprises applying a gate voltage of about -10V to about -2V and a substrate voltage of about + 5V to about + 10V.   The method of claim 16, wherein the memory array has a NOR architecture and the step of programming includes channel hot electron injection.   The method of claim 16, wherein the memory array has a NAND architecture and the programming step includes channel + FN implantation.   The method of claim 16, wherein the memory array has a NOR architecture, and the step of programming includes channel initiated secondary electron injection.   The step of programming at least one of the group of memory cells includes applying a voltage of about + 16V to about + 20V to at least one selected word line and grounding the substrate. Item 24. The method according to Item 23.   26. The method of claim 25, wherein programming the at least one memory cell further comprises applying a voltage of about + 7V to at least one unselected bit line.