Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
1999-05-17
2001-03-20
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S257000, C438S260000, C257S288000
Reexamination Certificate
active
06204124
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for fabricating a nonvolatile memory cell, and more especially, to a method for fabricating rugged tunnel oxide with high electron injection efficiency and a large charge-to-breakdown for low power nonvolatile memory.
BACKGROUND OF THE INVENTION
Nonvolatile memories, including mask read-only memories (Mask ROM), programmable ROM (PROM), erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM or E
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PROM) and flash memories, retain their memory data whenever the power is turned off, and have wide application in the computer and electronic industry. In recent years, the markets of portable computers and telecommunications have developed rapidly and have become a major driving force in the design and technology semiconductor integrated circuit. As stated by A. Bergemont, et al., in “Low Voltage NVG™: A New High Performance 3 V/5 V Flash Technology for Portable Computing and Telecommunications Application”, IEEE Trans. Electron Devices Vol. 43, p. 1510, (1996), it creates a great need for low power, high density, and electrically re-writable nonvolatile memories. That is, the memories programmable and erasable as EPROM, E
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PROM or flash memories are required for aforementioned systems to store operating systems or application software.
The basic storage cell of these programmable and erasable memories contain a double polysilicon storage transistor with a floating gate isolated in silicon dioxide and capacitively coupled to a second control gate which is stacked above it. The E
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PROM cell further comprises an access, or select, transistor. These memories execute the programming and erasure by charging or discharging their floating gates. For example, the EPROM is programmed by hot electron injection at the drain to selectively charge the floating gate and erased by discharging the floating gate with ultraviolet light or X-ray, which the latter has never been commercially applied for this purpose. The E
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PROM and most of the flash memories are programmed by hot electron injection or cold electron tunneling named Fowler-Nordheim tunneling, and erased mostly by Fowler-Nordheim tunneling from the floating gate to the source, with the control gate ground.
Fowler-Nordheim tunneling, or cold electron tunneling, is a quantum-mechanical effect, which allows the electrons to pass through the energy barrier at the silicon-silicon dioxide interface at a lower energy than required to pass over it. H. Shirai, et al., stated in their paper “A 0.54 &mgr;m
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Self-Aligned, HSG Floating Gate Cell for 256 Mbit Flash Memories”, IEDM Tech. Dig. Vol. 95, p. 653 (1995) that, because of its low current consumption, the Fowler-Nordheim program/erase scheme becomes indispensable for low power operation of the E
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PROM and flash memories. However, the Fowler-Nordheim program/erase scheme requires high voltage applied to control gate of the memory cell due to its need for a large reversible electric field to the thin oxide separating the floating gate from the substrate. Therefore, to lower the control gate bias, the memory cell must have a high capacitive-coupling ratio structure.
Y. S. Hisamune, et al., propose a method for fabricating a flash memory cell with contactless array and high capacitive-coupling ratio in “A High Capacitive-Coupling Ratio Cell for 3 V-Only 64 Mbit and Future Flash Memories”, IEDM Tech. Dig. Vol. 93 p. 19, (1993). However, this method achieves high capacitive-coupling ratio with four times of polysilicon deposition and has a complex fabrication. In addition, this cell structure makes it difficult to scale the size down and increase the integration of the memory due to its short tunnel oxides. Furthermore, as mentioned by C. J. Hegarty, et al., in “Enhanced Conductivity and Breakdown of Oxides Grown on Heavily Implanted Substrates”, Solid-State Electronics, Vol. 34, p. 1207 (1991), it is also difficult to fabricate a thin tunnel oxide on the heavily doped substrate with a high electron injection efficiency and a large charge-to-breakdown for low power nonvolatile memories. Thus, to reach high capacitive-coupling ratio, high electron injection efficiency and a large charge-to-breakdown with a simple manufacture is the subject of high density and low power nonvolatile memories today.
SUMMARY OF THE INVENTION
A method for fabricating a nonvolatile memory cell with rugged tunnel oxide is disclosed. First, the field oxide is formed, the active region is defined, and a semiconductor substrate is prepared. A stacked silicon oxide/silicon nitride layer is deposited on the substrate and then the tunnel oxide region is defined by a standard photolithography process followed by an anisotropic etching. A high temperature steam oxidation process is used to grow a thick thermal oxide on the non-tunnel region. After removing the masking silicon nitride layer, the phosphorus ions are implanted to form the doped regions and serve as source and drain, then a thermal annealing is performed to recover the implantation damage and to drive in the doped ions. After the pad oxide film is removed, a silicon film is deposited over the substrate
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and then etched back by a dry etching. A rugged topography is then formed on the doped substrate regions. Thereafter, a thin oxide is grown on the rugged doped substrate region to form a rugged tunnel oxide. Finally, the first n+ doped polysilicon film which serves as the floating gate, the interpoly dielectric such as NO or ONO, and the second n+ doped polysilicon film which serves as the control gate are sequentially formed, and the memory cell is finished.
REFERENCES:
patent: 4562638 (1986-01-01), Schwabe et al.
patent: 4806202 (1989-02-01), Tang et al.
patent: 4894353 (1990-01-01), Ibok
patent: 5429965 (1995-07-01), Shimoji
patent: 5504022 (1996-04-01), Nakanishi et al.
patent: 5814856 (1998-09-01), Bergemont et al.
patent: 5970342 (1999-10-01), Wu
patent: 6043124 (2000-03-01), Wu
Malsawma Lex H.
Smith Matthew
Texas Instruments - Acer Incorporated
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