ERASEABLE NONVOLATILE MEMORY WITH SIDEWALL STORAGE
A nonvolatile storage cell, integrated circuit (IC) including the cells and method of manufacturing the cells. A layered spacer (ONO) is formed at least at one sidewall of cell gates. Source/drain diffusions at each layered spacer underlap the adjacent gate. Charge may be stored at a layer (an imbedded nitride layer) in the layered spacer.
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The present application is a divisional of allowed U.S. patent application Ser. No. 10/905,475, (Attorney docket No. BUR920040102US1) entitled “ERASEABLE NONVOLATILE MEMORY WITH SIDEWALL STORAGE” to Chung H. Lam, filed Jan. 6, 2005, which is assigned to the assignee of the present invention and incorporated herein by reference.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention is related to nonvolatile storage and more particularly to integrated circuit chips including nonvolatile storage such as one or more cells or an array of nonvolatile random access memory (NVRAM) cells.
2. Background Description
Nonvolatile floating gate storage devices, such as may be used for memory cells in a nonvolatile random access memory (NVRAM), are well known in the industry. In a typical such NVRAM cell, the cell's conductive state is determined by charge or lack thereof on the storage device's floating gate. The floating gate is an electrically isolated gate of a field effect transistor (FET) stacked in a two device NAND-like structure with the gate of a select device. Charge is forced onto or removed from the floating gate through a thin insulator layer that, during a normal read, isolates the gate electrically from other adjoining conductive layers. For example, a negatively (or positively) charged floating gate may be representative of a binary one state, while an uncharged floating gate may be representative of a binary zero state or, vice versa.
Typically, the select device in the NAND-like structure is connected to a word line. In typical state of the art designs, adjacent cells are connected to a common bit line. Each of the word lines is uniquely addressable and physically distinct. Intersection of each word line with each bit line provides unique cell selection for reading and writing the selected cell. For reading, a read voltage (e.g., Vhi or ground) is applied to a control gate (or program gate) that is capacitively coupled to floating gates of the nonvolatile devices of devices being read. Typically, the bit lines are pre-charged high. Thus, when a word line is raised, bit lines discharge for those devices programmed for zeros and do not those programmed for ones. For writing, a write voltage applied to the control gate (or program gate) is capacitively coupled to floating gates of the nonvolatile devices and, when the gate, source and drain voltages are biased properly, the charge changes on the floating gate, i.e., to write selected cells. Similarly, cells are biased to remove the charge from the floating gates during each erase.
The typically high voltages needed to write and erase each cell normally require a very complicated fabrication process. So, to minimize cell write voltages and for adequate read performance, the floating gate is large. Consequently, large floating gates account for much of the cell area for a typical NVRAM cell. While, reduced cell size cannot come at the expense of unacceptably degraded performance, designers normally strive for minimum cell size to achieve maximum cell density for reduced storage costs.
Thus, there is a need for smaller, denser NVRAM cells.
SUMMARY OF THE INVENTIONIt is a purpose of the invention to improve nonvolatile storage density;
It is another purpose of the invention to increase eraseable nonvolatile storage cell density;
It is yet another purpose of the invention to double the density of nonvolatile storage arrays.
The present invention relates to a nonvolatile storage cell, integrated circuit (IC) including the cells and method of manufacturing the cells. A layered spacer (oxide-nitride-oxide) is formed at least at one sidewall of cell gates. A metallurgical junction of subsequently formed source/drain diffusion regions at each layered spacer lies under the layer spacer rather than under the polysilicon gate of cell. This alignment of the metallurgical junction under the layered spacer is referred to herein as underlap. Charge may be stored at a layer (an imbedded nitride layer) in the layered spacer.
The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:
Turning now to the drawings and, more particularly,
So, cell formation begins in
Next, as shown in
Finally, in
The trapped electrons alter the characteristic of the programmed half-cell such that when biased for a read with a low voltage (e.g., 0.1-0.5V and 0.1V in this example) on the previously written terminal LB, the FET channel resistance is substantially higher than it would otherwise be. Thus, contents of each half-cell (i.e., LB=0.1V, RB=0.0V and RB=0.1V, LB=0.0V) may be read by sensing a high or lower device resistance as a 1 or a 0 or vice versa. It should be noted that read and write voltages are provided for example only and are fabrication process dependent, e.g., thickness, dopant, dopant type and material dependent. Thus, fabrication process changes are typically accompanied with corresponding voltage changes.
Similarly, each half-cell may be erased by grounding the gates 106, floating unerased sides, clamping the side being erased to the same high voltage (3.5V) and clamping the channel 102 back bias (SX) to an equally high negative voltage, −3.5V. This reverse biases the source/drain diffusion 118 at the side being erased. This also forces holes into the sidewall spacers 116 that neutralize the previously trapped electrons at the side being erased.
Advantageously, preferred embodiment nonvolatile storage cells can be made using typical FET manufacturing process steps without requiring complicated process changes for a very dense NVRAM cell.
While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. It is intended that all such variations and modifications fall within the scope of the appended claims. Examples and drawings are, accordingly, to be regarded as illustrative rather than restrictive.
Claims
1. A method of forming nonvolatile storage cells in an integrated circuit, said method comprising the steps of:
- a) forming gates in cell locations;
- b) forming a first isolation layer on said cell locations;
- c) forming charge storage sidewall spacers at said gates;
- d) forming a second isolation layer on said cell locations; and
- e) forming an outer sidewall spacer adjacent to said gates, each of said charge storage sidewall spacers being between one of said gates and one said outer sidewall spacer.
2. A method of forming nonvolatile storage cells as in claim 1, wherein said first isolation layer and said second isolation layer are oxide layers and said charge storage sidewall spacers and outer sidewall spacers are nitride sidewall spacers.
3. A method of forming nonvolatile storage cells as in claim 2, further comprising forming source/drain diffusions on either side of each of said gates, said source/drain diffusions at said outer sidewall spacers underlapping said gates.
4. A method of forming nonvolatile storage cells as in claim 3, wherein said source/drain diffusions at said outer sidewall spacers underlap said gates at least to said charge storage layer.
5. A method of forming nonvolatile storage cells as in claim 4, wherein said charge storage sidewall spacers and outer sidewall spacers are formed at both end of said gates in ones of said nonvolatile storage cells, said ones of said nonvolatile storage cells storing two bits each.
Type: Application
Filed: Aug 13, 2008
Publication Date: Dec 4, 2008
Applicant: INTERNATIONAL BUSINESS MACHINES CORPORATION (Armonk, NY)
Inventors: Chung H. Lam (Peekskill, NY), Jeffery B. Johnson (Essex Junction, VT)
Application Number: 12/190,657
International Classification: H01L 21/336 (20060101);