Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1998-09-25
2001-07-17
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S647000
Reexamination Certificate
active
06262452
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and a manufacturing method therefor, and more particularly to an improvement in the number of rewriting information in a split-gate-type flash memory.
2. Description of the Relatrd Art
A method of manufacturing a split-gate-type flash memory, which is a conventional nonvolatile semiconductor memory device (refer to, for example, Japanese Patent Application No. 9-42478) will now be described with reference to the drawings.
As shown in
FIG. 37
, the foregoing split-gate-type flash memory is a flash memory incorporating a control gate
100
formed from an upper portion of the floating gate
102
to the side portion of the same through an insulating film
101
. Referring to
FIGS. 34
to
37
, the processes for manufacturing the foregoing flash memory will now be described.
Initially, a first gate insulating film
104
in the form of a SiO
2
film is formed on a semiconductor substrate
103
. Then, a polycrystalline silicon film
105
is laminated. Then, P ions are implanted to form the floating gate
102
shown in FIG.
37
and attempted to be formed (see FIG.
34
).
Then, a silicon nitride film
106
, which is an oxidation resistance film, is formed in such a manner that the polycrystalline silicon film
105
, which is a region in which the floating gate
102
will be formed, is exposed. The silicon nitride film
106
is used as a mask to form a mini LOCOS oxide film
107
(see FIG.
35
).
Then, the silicon nitride film is etched, and then the mini LOCOS oxide film
107
is used as a mask to etch and remove the polycrystalline silicon film
105
so that the floating gate
102
is formed.
Then, the first gate insulating film
104
is isotropically etched with hydrofluoric acid etching solution so that the first gate insulating film
104
is left in only a portion directly below the floating gate (or etched and removed in such a manner that the first gate insulating film
104
is somewhat left around the floating gate
102
).
Then, a second insulating film
101
made of silicon oxide (a thermal oxidation film or a CVD film) is formed on the entire surface (see FIG.
36
).
Then, a polycrystalline silicon film is formed on the second insulating film
101
, and then a patterning process is performed so that a control gate
100
extending from the upper portion of the floating gate
102
to the side portion of the same is formed. The thus-formed floating gate
102
and the control gate
100
are used as masks when impurities are implanted onto the semiconductor substrate
103
. Thus, a source region and a drain region (not shown) are formed. As a result, the split-gate-type flash memory can be formed.
A program has been written on the above-mentioned split-gate-type flash memory by turning transistors of memory cells (hereinafter called “selected cells”), on which data must be written, on to implant electrons into the floating gate
102
.
Moreover, as a region surrounded by a dashed line shown in
FIG. 37
, the polycrystalline silicon film
105
on the upper surface of the floating gate
102
is oxidized so that a mini LOCOS oxide film
107
is formed on the polycrystalline silicon film
105
. Then, a projection
108
is formed at the leading end of the bird's beak. Concentration of electric fields to the projection
108
is used to remove electrons of the floating gate
102
from the floating gate
102
toward the control gate
100
so that the program is erased.
FIG. 5
is a plan view showing the above-mentioned split-gate-type flash memory.
FIG. 37
is a cross sectional view taken along line B—B. A rectangular portion (hatched with dots) indicated with an elongated alternate long and short dash line is in the form of a LOCOS oxide film, while rectangular portions indicated with an alternate long and two short dashes line and a dashed line are the floating gate
102
and the mini LOCOS oxide film
107
. A solid line extending laterally indicates the control gate
100
.
The above-mentioned projection
108
is formed mainly by dint of the shape of the bird's beak of the mini LOCOS oxide film. That is, the bottom surface of the bird's beak has an inclination which is raised in a direction toward the periphery. Therefore, the sharp projection
108
is formed on the entire circumference of the floating gate.
However, the above-mentioned inclination of the mini LOCOS oxide film
107
is unsatisfactory in a viewpoint of the shape of the projection.
SUMMARY OF THE INVENTION
In view of the foregoing, a first aspect has a structure that an end of the floating gate is disposed in the inclined portions of the bird's beak of the LOCOS oxide so as to solve the problem.
A second aspect has a structure that an end of the mini LOCOS oxide film formed on the floating gate is disposed in the inclined portion of the bird's beaks of the LOCOS oxide film.
If an inclined portion is provided for the bird's beak portion of the LOCOS oxide film as shown in
FIG. 6
, the floating gate formed on the bird's beak has an inclined portion. Hitherto, an end T of the floating gate is formed in the horizontal portion formed between points Y and Z. Therefore, a corner C formed by a line YZ and a line T has an angle of substantially 90°. However, the present invention has a structure that an end S is formed in an inclined portion XY. Therefore, a corner B has an acute angle. Thus, the projection has a sharp shape.
A third aspect has the steps of: forming an oxidation resistance film in the form such that a silicon film disposed between a first LOCOS oxide film and a second LOCOS oxide film is exposed;
forming a mini LOCOS oxide film by oxidizing the silicon film through the oxidation resistance film; and
forming a floating gate by etching the silicon film such that the mini LOCOS oxide film is used as a mask.
Thus, the above-mentioned problem can be solved. Plurality of LOCOS oxide films are comprised, and the first LOCOS oxide film and a second LOCOS oxide film show two LOCOS oxide films adjacent to each other. That is, the mini LOCOS oxide film is grown from the end of the oxidation resistance film, as shown in FIG.
1
. Therefore, when the oxidation resistance film in the form such that the silicon film is exposed is formed between the first LOCOS oxide film and the second LOCOS oxide film juxtaposed each other, the end of the mini LOCOS oxide film is formed in the inclined portion of the LOCOS oxide film. Thus, the end of the floating gate can furthermore be sharpened.
A fourth aspect, as shown in
FIG. 4
, has a structure that a sharp acute portion
12
is downwards formed from a top flat portion of each of the LOCOS oxide films
2
A and
2
B to the inclined portion, as shown in FIG.
4
. Since a reverse tunneling phenomenon occurs because of the foregoing fact, a spacer
11
is formed to prevent the foregoing phenomenon.
A fifth aspect has a structure that an end of the floating gate is disposed in a recess provided for the semiconductor substrate by removing the LOCOS oxide film and formed to have an inclined portion.
A sixth aspect has a structure that an end of the floating gate is disposed in a recess provided for the semiconductor substrate by combining isotropic etching and anisotropic etching and formed to have an inclined portion.
According to another aspect, there is provided a method of manufacturing a semiconductor device of the foregoing type. The first and second LOCOS oxide films can be formed so as to be elongated in parallel each other in a direction, or so be island-like shaped.
REFERENCES:
patent: 5095344 (1992-03-01), Harari
patent: 5488244 (1996-01-01), Quek et al.
patent: 5502321 (1996-03-01), Matsushita
patent: 5583811 (1996-12-01), Van Houdt et al.
patent: 5889305 (1999-03-01), Choi et al.
Ogata Takashige
Ono Masahiro
Oya Toshiharu
Yoshii Masurao
Armstrong Westerman Hattori McLeland & Naughton LLP
Crane Sara
Sanyo Electric Co,. Ltd.
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