Method of fabricating a flash memory cell using angled implant

Details Method of fabricating a flash memory cell using angled implant Method of fabricating a flash memory cell using angled implant

2002-05-15

2003-09-16

Chaudhari, Chandra (Department: 2813)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

Having insulated gate

C438S302000, C438S532000

Reexamination Certificate

active

06620689

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a memory device, and more particularly to the fabrication of a memory cell of a flash memory.
2. Description of the Prior Art
Complementary metal-oxide-semiconductor (CMOS) memory can be divided into two main categories: random access memory (RAM) and read-only memory (ROM). ROM's market share has been continuously growing in the past few years, and further growth in the near future is foreseen, especially for flash memory in which a single cell can be electrically programmable and a block, sector or page of cells that are electrically erasable at the same time. Due to the flexibility of flash memory against electrically programmable read-only memory (EPROM), electrically programmable but erasable via ultraviolet exposure, the market share of flash memory is also experiencing rapid growth. Electrically erasable and programmable read-only memory (EEPROM), electrically erasable and programmable per single byte, will be manufactured for specific applications only, since they use larger area and are more expensive. In recent years, flash memory has found interesting applications in electrical consumer products such as: digital cameras, digital video cameras, cellular phones, laptop computers, mobile MP
3
players, and Personal Digital Assistants (PDA's). Since portability of these electrical consumer products is strongly prioritized by consumers, the products' size must be minimal. As a result, the capacity of the flash memory must be enlarged, and functions have to be maximized while size is reduced. The capacity of flash memory has increased from 4 to 256 MB, and will increase to even 1 GB in the near future. With the increase in packing density for flash memory, floating gates and control gates have to be made as small as possible. In conventional processes, masks are usually used to define the gates in flash memory.
FIGS. 1A
to
1
G show the manufacturing processes of a conventional split gate flash memory device.
As shown in
FIG. 1A
, a semiconductor substrate
100
is provided. An LOCOS Oxidation process is performed to form a field insulating layer (not shown) on the substrate
100
. The field insulating layer isolates each active area. A first insulating layer
110
is formed on the substrate
100
within the active area. The first insulating layer
110
can be made of oxide formed by oxidation and has a thickness of from 50 to 200 angstroms. Then, a first conductive layer
115
, which has a thickness of about 100 to 2000 angstroms, is formed on the first insulating layer
110
. The first conductive layer
115
is usually made of doped polycrystalline silicon formed by chemical vapor deposition (CVD) process. Then, a first masking layer
120
, with a thickness of about 500 to 2000 angstroms, is formed on the first conductive layer
115
by depositing a silicon nitride layer.
As shown in
FIG. 1B
, the first masking layer
120
is removed by performing an etching process to define the first opening
125
and to expose the surface of the first conductive layer
115
.
As shown in
FIG. 1C
, a floating gate insulating layer
130
is formed on the exposed surface of the first conductive layer
115
by an oxidation process. Bird's peaks
137
of the floating gate insulating layer
130
are formed at the tip of the left and right sides of the floating gate insulating layer
130
.
As shown in
FIG. 1D
, the first masking layer
120
is removed by performing an etching process. Then, using the floating gate insulating layer
130
as a hard mask, a portion of the first conductive layer
115
and the first insulating layer
110
are sequentially removed to expose the surface of the substrate
100
by anisotropic etching. The portions of the first conductive layer
115
and the first insulating layer
110
under the floating gate insulating layer
130
remain. The remaining first conductive layer
115
forms a floating gate
136
. The remaining first insulating layer
110
will be expressed as a first gate insulating layer
112
. Poly tips
138
situated at the tip of the two sides of the floating gate
136
under the bird's peaks
137
are used for tip discharging when flash memory is erasing. Then, a second insulating layer
132
is formed on the surface of the substrate
100
, the oxide layer
130
, the floating gate
136
and the first gate insulating layer
112
. The second insulating layer
132
, which has a thickness of about 50 to 250 angstroms, is an oxide and is formed by oxidation or CVD.
As shown in
FIG. 1E
, a second conductive layer
135
is formed on the second insulating layer
132
. The second conductive layer
135
has a thickness of about 1000 to 2000 angstroms and is usually made of the doped polycrystalline silicon formed by CVD.
As shown in
FIG. 1F
, portions of the second conductive layer
135
and the second insulating layer
132
are removed by photolithography and etching to form a second opening
142
and a third opening
144
. The remaining second conductive layer
135
forms the control gate
170
. The remaining second insulating layer
132
will be expressed as a second gate insulating layer
155
.
As shown in
FIG. 1G
, a source region
180
is formed on the exposed substrate
100
by implanting N-type ions, such as phosphorus or arsenic into the substrate
100
, which is exposed in the second opening
142
. Then, an oxide layer (not shown) is formed to cover the surface and the sidewalls of the control gate
170
, the surface of the floating gate insulating layer
130
, the sidewalls of the second gate insulating layer
155
, floating gate
136
, and the first gate insulating layer
112
. Etching is performed to remove portions of the oxide layer and form the sidewall spacers
150
on the sidewalls of the floating gate
136
, the first gate insulating layer
112
, the control gate
170
and the second gate insulating layer
155
. A drain region
190
is formed on the exposed substrate
100
by implanting N-type ions, such as phosphorus or arsenic into the substrate
100
, which is exposed in the third opening
144
. The manufacture of a cell of flash memory is thus completed.
As memory devices become more integrated, the minimization of feature size of flash memory is needed. The conventional processes for fabricating flash memory usually use an oxidation process to form the floating gate
136
, as shown in FIG.
1
D. The bird's beaks
137
on the two sides of the floating gate insulating layer
130
are thinner and longer. Using the floating gate insulating layer
130
as a hard mask to form the floating gate
136
, the poly tips
138
are formed on the two sides of the floating gate
136
under the bird's beaks
137
.
SUMMARY OF THE INVENTION
An object according to the present invention is to provide a method of fabricating a flash memory cell characterized by improvement of tip discharge efficiency.
A method of fabricating a flash memory cell first provides a semiconductor substrate. An active area is defined on the substrate, and a first insulating layer is formed within the active area. A first conductive layer is formed on the first insulating layer. Then, a barrier layer is formed on the first conductive layer. A first opening is formed by removing a portion of the barrier layer. Afterwards, an angled implant is performed on the exposed surface of the first conductive layer. Next, a floating gate insulating layer is formed on the exposed surface of the first conductive layer. The barrier layer is removed by performing an etching process. Then, using the floating gate insulating layer as a hard mask, a portion of the first conductive layer and the first insulating layer are sequentially removed to expose the surface of the substrate. The remaining first conductive layer forms a floating gate. The remaining first insulating layer forms a first gate insulating layer. Then, a second insulating layer is formed on the surface of the substrate, the oxide layer, the floating gate and the first gate insulating layer. A second con

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