Patent Grant
7414889
1. Field of the Invention
The present invention relates generally to non-volatile memory devices, and more particularly, to nitride-based trapping-storage flash memories.
2. Description of Related Art
Electrically programmable and erasable non-volatile memory technologies based on charge storage structures known as Electrically Erasable Programmable Read-Only Memory (EEPROM) and flash memory are used in a variety of modem applications. A flash memory is designed with an array of memory cells that can be independently programmed and read. Sense amplifiers in a flash memory are used to determine the data value or values stored in a non-volatile memory. In a typical sensing scheme, an electrical current through the memory cell being sensed is compared to a reference current by a current sense amplifier.
A number of memory cell structures are used for EEPROM and flash memory. As the dimensions of integrated circuits shrink, greater interest is arising for memory cell structures based on charge trapping dielectric layers, because of the scalability and simplicity of the manufacturing processes. Memory cell structures based on charge trapping dielectric layers include structures known by the industry names Nitride Read-Only Memory, Poly-Silicon-Oxide-Nitride-Oxide-Silicon (SONOS), and PHINES, for example. These memory cell structures store data by trapping charge in a charge trapping dielectric layer, such as silicon nitride. As negative charge is trapped, the threshold voltage of the memory cell increases. The threshold voltage of the memory cell is reduced by removing negative charge from the charge trapping layer.
Nitride Read-Only Memory devices use a relatively thick bottom oxide, e.g. greater than 3 nanometers, and typically about 5 to 9 nanometers, to prevent charge loss. Instead of direct tunneling, band-to-band tunneling induced hot hole injection BTBTHH can be used to erase the cell. However, the hot hole injection causes oxide damage, leading to charge loss in the high threshold cell and charge gain in the low threshold cell. Moreover, the erase time must be increased gradually during program and erase cycling due to the hard-to-erase accumulation of charge in the charge trapping structure. This accumulation of charge occurs because the hole injection point and electron injection point do not coincide with each other, and some electrons remain after the erase pulse. In addition, during the sector erase of an Nitride Read-Only Memory flash memory device, the erase speed for each cell is different because of process variations (such as channel length variation). This difference in erase speed results in a large Vt distribution of the erase state, where some of the cells become hard to erase and some of them are over-erased. Thus the target threshold Vt window is closed after many program and erase cycles and poor endurance is observed. This phenomenon will become more serious when the technology keeps scaling down.
A typical flash memory cell structure positions a tunnel oxide layer between a conducting polysilicon tunnel oxide layer and a crystalline silicon semiconductor substrate. The substrate refers to a source region and a drain region separated by an underlying channel region. A flash memory read can be executed by a drain sensing or a source sensing. For source side sensing, one or more source lines are coupled to source regions of memory cells for reading current from a particular memory cell in a memory array.
A traditional floating gate device stores 1 bit of charge in a conductive floating gate. The advent of Nitride Read-Only Memory cells in which each cell provides 2 bits of flash cells that store charge in an Oxide-Nitride-Oxide (ONO) dielectric. In a typical structure of a Nitride Read-Only Memory memory cell, a nitride layer is used as a trapping material positioned between a top oxide layer and a bottom oxide layer. The ONO layer structure effectively replaces the gate dielectric in floating gate devices. The charge in the ONO dielectric with a nitride layer may be either trapped on the left side or the right side of a NROM cell.
Floating gate devices encounter substantial scaling challenges due to inter-floating gate coupling, while nitride trapping device is flexible from such limitations. There are two main types of nitride trapping device: NROM that stores charges locally and SONOS that uses channel program/erase. These two types of devices have drawbacks. A Nitride Read-Only Memory device is sensitive to hot-hole induced damages, and a SONOS device suffers from retention problems caused by direct tunneling leakage through the thin tunnel oxide.
A conventional AND-type floating gate flash memory is suitable for many commercial applications because the memory device possesses the characteristics of high-density, low-power and fast speed programming. However, due to the inter-floating gate coupling effect, the scaling of AND-type floating gate devices is limited. When the space parameter for the floating gate device is shrunk, a high floating gate coupling effect may cause undesirable and severe disturbance. The conventional AND-type floating gate device also suffers from tunnel oxide scaling issues and erratic bits where a local defect or trapped charge, in a tunnel oxide can result in the leakage of the charge in the floating gate.
To address the scaling issue in floating gate devices, charge trapping devices such as SONOS, MNOS or nano-crystal trapping devices are suggested. However, these devices all suffer serious charge retention problems. For a SONOS device, the ultra-thin tunnel oxide is unable to properly preserve a charge storage. For a MNOS device, the structure does not provide a top oxide to block the charge loss. A nano-crystal device cannot be well-controlled because of the randomly distributed nano particles.
Accordingly, it is desirable to design AND-type floating gate flash memories that provide scalability while overcoming the retention problems as well as maintaining efficient hole tunneling erase.
The present invention provides a bandgap engineered SONOS (referred to as “BE-SONOS” or “SONONOS”) device structure for design with various AND architectures to perform a source side injection (SSI) programming method. The BE-SONOS device structure comprises a spacer oxide disposed between a control gate overlaying an oxide-nitride-oxide-nitride-oxide (O3—N2—O2—N1—O1) stack and a sub-gate overlaying a gate oxide. In a first embodiment, a BE-SONOS SG-AND (sub-gate-AND) array architecture is constructed multiple columns of SONONOS devices with sub-gate lines and diffusion bitlines. In a second embodiment, a BE-SONOS SGIB-AND (sub-gate-inversion-bitline-AND) architecture is constructed multiple columns of SONONOS devices with sub-gate inversion bitlines and with no diffusion bitlines.
Broadly state, an integrated circuit device comprises a semiconductor substrate; a plurality of memory cells on the semiconductor substrate, each memory cell having a spacer oxide disposed between a gate and a sub-gate, each gate overlaying a blocking oxide-charge storage layer-modulated tunnel dielectric stack, each sub-gate overlaying a gate oxide; and an N+ buried diffusion disposed in the semiconductor substrate and positioned underneath between a first gate oxide and a first blocking oxide-charge storage layer-modulated tunnel dielectric stack that serves as a first diffusion bitline.
Advantageously, the BE-SONOS AND array architectures of the present invention provides greater scalability over floating gate and AND type of memory devices. The present invention also advantageously provides a uniform and self-converging channel hole tunneling erase operation. Moreover, the present invention eliminates the inter-floating gate coupling effect. The present invention further provides desirable reliability properties including predictable characteristic of excellent charge retention, predictable number of nearly no erratic bits, and predicable small degradation after program and erase cycles.
The structures and methods regarding to the present invention are disclosed in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. These and other embodiments, features, aspects, and advantages of the invention will become better understood with regard to the following description, appended claims and accompanying drawings.
Referring now to
The n-channel BE-SONOS device 100 is a five-terminal device with two gates, the control gate 110 and the sub-gate 130. Underneath the control gate 110, there is the O3—N2—O2—N1—O1 structure 140 for the charge storage. Underneath the sub-gate 130, there is the non-trapping gate oxide 150. The control gate 110 can control program, erase, and read the charge storage layer. The sub-gate 130 can provide source side injection (SSI) programming method. The source side injection is a low-power and high-speed programming method. The O1—N1—O2 layer can be implemented with ultra-thin oxide and nitride, typically within 3 nm to provide hole direct tunneling. The N2 layer 142 is thicker than 5 nm to provide higher trapping efficiency. In the layer 141 formation method, one technique is to use a wet converted top oxide to provide a large density of traps at the interface between O3 and N2. The O3 layer is typically thicker than 6 nm in order to prevent charge loss from top oxide. The O1—N1—O2 layers serve as a tunneling dielectric for the hole tunneling.
An exemplary set of device parameters for the n-channel BE-SONOS device 100 with the sub-gate 130 is shown below.
In
As shown in
In
To state in another way, a reset operation is carried out to tighten the Vt distribution before operations. In contrast to a floating gate device where there is no self-converging erase, the BE-SONOS provides a self-converging erase reset/erase methods, which is necessary because the initial Vt distribution is often widely distributed due to the process issues, such as plasma charging effect. The self-converging reset assists in tighten the initial Vt distribution.
When a cell-A 422 is selected programming, the SG is set to 1 volt so that the channel underneath SG is slightly turned on. Electrons are injected into the cell-A 422 by source side injection method to make the voltage threshold, Vt, higher than PV. The SG for a cell-B 424 is set to 0 volt which turns SG off so that there is no injection into the cell-B 424. As for a cell-C 426, the SG is set to 1 volt where the WL=0 volt which turns off cell-C 426 so that there is also no injection into the cell-C 426. As a result, programming can be randomly selected with adequate program inhibit technique.
To carry out an electrical program, the selected wordline is applied a high voltage, 10 volts, and the sub-gate is applied 1 volt to perform a source side injection. The source side injection is a low-power and high-speed programming method. One of skill in the art should recognize that parallel programming methods such as page programming with 2 kB cells in parallel can burst the programming throughput to more than 10 MB per second while the cell current consumption can be controlled within 2 mA. To avoid program disturbance to other bitlines, the sub-gate SG2231 is set to 0 volt and turns off the inhibit cell.
As illustrated in
The applied voltage at a bitline (BL) is typically around 1 volt. A larger read voltage will induce more current, but the read disturbance may be larger. The WL number of SG-AND string is typically 64, 128, or 256. A larger number of SG-AND string may save more overhead and increase the array efficiency. However, the program distribution may be larger. A trade-off is weighed in choosing an adequate number of SG-AND string.
Although the above read function describes a random access read operation, one of ordinary skill in the art should recognize that a page read of multiple cells are possible without departing from the spirits of the present invention.
Turning now to
The BE-SONOS array architecture 800 comprises a plurality of wordlines WL0810, WL1811, WL2812, WLm 813 intersecting a plurality of bitlines BL0820, BL1821, BL2822, BL3823 and BL4824. A corresponding sub-gate line is parallel to each bitline. A sub-gate SG0830 is placed in parallel to the bitline BL0820. A sub-gate SG1831 is placed in parallel to the bitline BL1821. A sub-gate SG2832 is placed in parallel to the bitline BL2822. A sub-gate SG3833 is placed in parallel to the bitline BL3823. A sample BE-SONOS (or SONONOS) device 840 that functions as a memory cell is shown in a circled area.
In the SGIB-AND array architecture 850, every fourth sub-gates are commonly electrically connected, i.e. the SG0830, SG4834, SG8, etc. are electrically connected together, the SG1831, SG5, SG9, etc are electrically connected together, the SG2832, SG6, SG10, etc are electrically connected together, and the SG3833, SG7, SG11, etc are electrically connected together.
As shown in the layout diagram 850 in
When the SG0830, the SG1831, the SG2832, the SG3833, the SG4834, and the SG 835 are turned on, each creates an N-channel inversion layer, which effectively serves as a barrier diffusion layer to function, respectively, as a source/drain 860, a source/drain 861, a source/drain 862, a source/drain 863, a source/drain 864 and a source/drain 865. Each sub-gate in the SG0830, the SG1831, the SG2832, the SG3833, the SG4834, and the SG 835 therefore serves dual functions. The first function that each sub-gate in the SG0830, the SG1831, the SG2832, the SG3833, the SG4834, and the SG 835 serves is a sub-gate for a source side injection programming. The second function that each sub-gate in the SG0830, the SG1831, the SG2832, the SG3833, the SG4834, and the SG 835 serves is an inversion bitline when a subgate is turned on. Each source/drain in the source/drain 860, the source/drain 861, the source/drain 862, the source/drain 863, the source/drain 864, and the source/drain 865 is for connecting to a metal bitline. The symbol Lg 870 denotes the drawn channel length. The symbol W 874 denotes the channel width. Typically, the parameters W 874, Ws 876, Lg 870, Ls 872 are approximately equal to the parameter F, where the parameter F represents the critical dimension in a technology node. For example, the parameter F is equal to 50 nm for a 50 nm node.
As shown in
In
Before operations, the memory circuit 1000 is reset by applying Vgb=−15V (or partition the gate voltage into each WL and p-well), which produces a desirable self-converging property, as shown in the graph 1050 in
To phrase in another way, a reset operation is carried out to tighten the Vt distribution before operations. In contrast to a floating gate device where there is no self-converging erase, the BE-SONOS provides a self-converging erase reset/erase methods, which is necessary because the initial Vt distribution is often widely distributed due to the process issues, such as plasma charging effect. The self-converging reset assists in tighten the initial Vt distribution.
As illustrated in
The bitlines BL0820, BL1821, BL2822, BL3823, and BL4824 provide a greater flexibility in programming than sub-gates SG0830, SG1831, SG2832, SG3833 and SG4834 because each bitline can be independently programmed. One type of electrical programming method is a source side injection. The source side injection programs a cell to a high voltage threshold Vt state. For example, the source injection applies Vg=10 V to the selected WL1, Vg=0V to other WL's, SG=1V for programming and SG=0V for inhibition. The voltage setting of the SG voltage at 1 volt is intended as an illustration such that in general it is 0.5 to 2 volts higher than the threshold voltage under SG gate.
To carry out an electrical program, the selected wordline is applied a high voltage, 10 volts, and the sub-gate SG3833 is applied 1 volt to perform a source side injection to program a target cell. The SG1831 is set to 0 volt for program inhibit and the SG4834 is set to 8 volts to provide sufficient overdrive to reduce a bitline resistance. One of skill in the art should recognize that parallel programming methods such as page programming with 2 kB cells in parallel can burst the programming throughput to more than 10 MB per second while the cell current consumption can be controlled within 2 mA. To avoid program disturbance to other bitlines, the sub-gate SG2231 is set to 0 volt and turns off the inhibit cell.
The electrical programming is to conduct a source side injection to program a cell to a high voltage threshold, Vt, state. For example in the electrical programming of the cell A 1110, the operations apply Vg=10 V to the selected WL1 811, apply Vg=0V to other wordlines including WL0810, WL2812 and WLm 813, set SG3=1V for programming, set SG1=0V for program inhibition, and set SG2=5V for pass gate. The sub-gate SG4834 is set to 8 volt to highly turn on the sub-gate SG4834 so that the inversion layer potential can be raised up to 5 volts. In programming the cell A 1110, the sub-gate SG3833 is set to 1 volt so that the source side injection occurs. The threshold voltage Vt is raised to above the programming voltage, PV. A program inhibition is provided to a cell-B 1012, a cell-C 1014 and a cell-D 1016.
The applied voltage at a bitline (BL) is typically around 1 volt. A larger read voltage will induce more current, but the read disturbance may be larger. The WL number of SG-AND string is typically 64, 128, or 256. A larger number of SG-AND string may save more overhead and increase the array efficiency. However, the program distribution may be larger. A trade-off is weighed in choosing an adequate number of SGIB-AND string.
Although the above read function describes a random access read operation, one of ordinary skill in the art should recognize that a page read of multiple cells are possible without departing from the spirits of the present invention. The invention has been described with reference to specific exemplary embodiments. Various modifications, adaptations, and changes may be made without departing from the spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded as illustrative of the principles of this invention rather than restrictive, the invention is defined by the following appended claims.
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