N-channel field effect transistor having an oblique arsenic implant for lowered series resistance
First Claim
1. A process for fabricating an improved N-channel field effect transistor for use as an access device in a dynamic random access memory cell, said transistor having a first source/drain region which functions as a bitline contact region and a second source/drain region which functions as a storage node, said process comprising the following steps:
- (a) implanting a P-type impurity aligned to the physical limits of the gate electrode;
(b) forming a first dielectric layer on at least the exposed vertical surfaces of the gate electrode;
(c) performing a first implanting of an N-type impurity that is aligned to the exposed surfaces of portions of the dielectric layer covering the exposed vertical surfaces of the gate electrode;
(d) conformally depositing a second dielectric layer;
(e) anisotropically etching said first and second dielectric layers to form spacers on the vertical surfaces of the gate electrode;
(f) constructing a cell capacitor superjacent the storage node source/drain region;
(g) performing a second implanting of an N-type impurity that is aligned to the physical limits of the spacer in the bitline contact source/drain region, the cell capacitor preventing implanting of the N-type impurity in the storage node source/drain region during the second implanting; and
(h) performing a third implanting of an N-type impurity, said third implanting being performed obliquely, the cell capacitor preventing implanting of the N-type impurity in the storage node source/drain region during the third implanting.
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Abstract
An improved N-channel field-effect transistor is fabricated by performing a vertical N- implant, aligned to the vertical edges of the gate electrode, in both the source and drain regions of the device. In a first embodiment of the invention intended for use in dynamic random access memory access devices, a dielectric spacer is then formed on the sidewall of the gate electrode adjacent the drain (i.e., the regions which functions as the bitline contact in a DRAM memory cell). A vertical N+ implant, aligned to the exposed vertical edge of that spacer, is performed, in addition to an oblique implant of an N-type impurity. The oblique implant dosage is significantly greater than the N- implant dosage, but significantly less than the N+ implant dosage. In a second embodiment of the invention intended for use in applications where the transistor has no capacitive storage node, spacers are formed on both sidewalls of the gate electrode and the N+ implant, as well as the oblique N-type implant are performed in both the source and drain regions of the device. In preferred embodiments of the invention, phosphorus is utilized as the N- implant impurity, while arsenic is utilized for the other two N-type implants. The oblique implant provides not only reduced electric field strength in the channel region, but also reduced series resistance.
75 Citations
10 Claims
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1. A process for fabricating an improved N-channel field effect transistor for use as an access device in a dynamic random access memory cell, said transistor having a first source/drain region which functions as a bitline contact region and a second source/drain region which functions as a storage node, said process comprising the following steps:
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(a) implanting a P-type impurity aligned to the physical limits of the gate electrode; (b) forming a first dielectric layer on at least the exposed vertical surfaces of the gate electrode; (c) performing a first implanting of an N-type impurity that is aligned to the exposed surfaces of portions of the dielectric layer covering the exposed vertical surfaces of the gate electrode; (d) conformally depositing a second dielectric layer; (e) anisotropically etching said first and second dielectric layers to form spacers on the vertical surfaces of the gate electrode; (f) constructing a cell capacitor superjacent the storage node source/drain region; (g) performing a second implanting of an N-type impurity that is aligned to the physical limits of the spacer in the bitline contact source/drain region, the cell capacitor preventing implanting of the N-type impurity in the storage node source/drain region during the second implanting; and (h) performing a third implanting of an N-type impurity, said third implanting being performed obliquely, the cell capacitor preventing implanting of the N-type impurity in the storage node source/drain region during the third implanting. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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Specification