Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
1999-12-22
2001-05-01
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S268000, C438S270000
Reexamination Certificate
active
06225161
ABSTRACT:
FIELD OF THE INVENTION
The subject matter of this invention relates to semiconductor devices and methods of manufacture, and more particularly, to semiconductor devices and methods of manufacture having a trenched floating gate and a trenched control gate.
BACKGROUND OF THE INVENTION
Conventional semiconductor non-volatile memories, such as read-only memories (ROMs), erasable-programmable ROMs (EPROMs), electrically erasable-programmable ROMs (EEPROMs) and flash EEPROMs are typically constructed using a double-gate structure.
FIG. 1
is a cross-sectional view of the device structure of a conventional nonvolatile memory device
100
including a substrate
102
of a semiconductor crystal such as silicon. The device
100
further includes a channel region
104
, a source region
106
, a drain region
108
, a floating gate dielectric layer
110
, a floating gate electrode
112
, an inter-gate dielectric layer
114
, and a control gate electrode
116
, The floating gate dielectric layer
110
isolates the floating gate electrode
112
from the underlying substrate
102
while the inter-gate dielectric
114
isolates the control gate electrode
116
from the floating gate electrode
112
. As shown in
FIG. 1
, the floating gate dielectric layer
110
, the floating gate electrode
112
, the inter-gate dielectric layer
114
, and the control gate electrode
116
are all disposed on the surface of the substrate
102
.
As semiconductor devices and integrated circuits are scaled down in size, demands for the efficient use of space have increased. Heretofore, conventional semiconductor memories have utilized a double-gate structure in which both gates being formed on the surface of the silicon substrate as shown in FIG.
1
. This type of device structure for non-volatile devices is limited to the degree to which active devices can be made smaller in order to increase packing density. Moreover, when the double gates are stacked on top of the substrate surface as shown in
FIG. 1
, difficulties in the subsequent contact etch process are created due to the uneven and non-uniform topology.
SUMMARY OF THE INVENTION
In accordance with the present invention, a semiconductor device for low power applications is fabricated to include a fully recessed cell structure comprising a trenched floating gate, a trenched control gate, and a single wrap around buried drain region. A fully recessed trenched gate structure embodying the principles of the present invention provides a substantially planar topography that improves the packing density and scaleability of the device. Additionally, the present invention provides low substrate current programming and an enhanced erase operation.
In one embodiment of the present invention, a fully recessed trenched gate device structure for a non-volatile semiconductor device includes a well junction region and a trench etched into the well junction region. The fully recessed trenched gate structure comprises a trenched floating gate and a trenched control gate both formed in the trench. The trenched floating gate is electrically isolated from the trench by a trench-to-gate dielectric layer formed on substantially vertical sidewalls and on a bottom surface inside the trench. An inter-gate dielectric layer is formed on the trenched floating gate and electrically isolates the trenched floating gate from the trenched control gate. The trenched control gate is formed inside the trench on the inter-gate dielectric layer and in a preferred embodiment, has a top surface which is substantially planar with a surface of the substrate. A buried source region and a buried drain region are also formed in the well junction region and are laterally separated by the fully recessed trenched gate structure. The upper boundaries of the buried source region and the buried drain region are of approximately the same depth as the top surface of the trenched floating gate. In one embodiment of the present invention the buried drain region has a lower boundary which partially extends laterally underneath the bottom surface of the trench to form a drain junction disposed along portions of the sidewall and bottom of the trench. The buried source region has a lower boundary which is approximately less than the depth of the trench. In another embodiment of the present invention the buried source region has a lower boundary which partially extends laterally underneath the bottom surface of the trench to form a source junction disposed along portions of the sidewall and bottom of the trench. The buried drain region has a lower boundary which is approximately less than the depth of the trench.
In one embodiment of the present invention. sidewall dopings of one conductivity type are formed in the semiconductor substrate. The sidewall dopings are immediately contiguous the vertical sidewalls of the trench and immediately contiguous the substrate surface. The depth of the sidewall dopings is approximately equal to or greater than the depth of the trenched control gate and partially extend into the buried source and buried drain regions.
In another embodiment of the present invention, an implanted region of one conducting type is formed in the semiconductor substrate. The implanted region is laterally separated by the trench and is immediately contiguous the substantially vertical sidewalls of the trench, the substrate surface and the upper boundaries of the buried source region and the buried drain region.
In accordance with the present invention, a fully recessed device structure is formed in a semiconductor substrate using an MOS fabrication process according to which a well junction region is formed in the substrate. A trench is then etched into the well junction region. A trench-to-gate insulating layer is formed on substantially upright vertical sidewalls and on a bottom surface inside the trench. A trenched floating gate is fabricated by first depositing a layer of polysilicon over the substrate and then etching the polysilicon layer. An inter-gate dielectric is then deposited on the trenched floating gate inside the trench to isolate the two gate electrodes. The trenched control gate is formed by first depositing a layer of polysilicon over the substrate and then planarizing the polysilicon layer until it is substantially planar with the substrate surface. Finally, a buried source region and a buried drain region are formed in the well junction region. In one embodiment, sidewall dopings are formed in the substrate and are immediately contiguous the vertical sidewalls of the trench and the substrate surface.
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Liu Yowjuang W.
Wollesen Donald L.
Advanced Micro Devices , Inc.
Fenwick & West LLP
Le Dung A
Nelms David
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