Method of fabricating mask read only memory

Details Method of fabricating mask read only memory Method of fabricating mask read only memory

2001-08-08

2003-12-30

Pham, Long (Department: 2823)

Semiconductor device manufacturing: process

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

Having insulated gate

C438S275000, C438S276000, C438S277000

Reexamination Certificate

active

06670247

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application serial no. 90112760, filed on May 28, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to a semiconductor process. More particularly, the invention relates to a method of fabricating a masked read only memory (MROM).
2. Description of the Related Art
The masked read only memory is the most basic structure and has the most simple fabrication process of the various kinds of read only memories. The memory cell of the masked read only comprises a MOS transistor. While programming, the channel region under the gate is implanted with ions to change the threshold voltage V
T
. In the masked read only memory, the data value is represented by the threshold voltage of the memory cell. By turning on/off the channel region of the memory cell under the gate, the data value is accessed. The on/off state is determined by the threshold voltage of the memory cell.
A conventional method of fabricating a mask read only memory is shown in
FIGS. 1A
to
1
C. In
FIG. 1A
, embedded bit lines
110
(perpendicular to the paper surface) and the isolation oxide layers
120
on the bit lines
110
are formed on the substrate
100
. A gate oxide layer
130
is formed over the substrate
100
. A word line
140
perpendicular to the bit lines
110
is formed. The substrate between a pair of the bit lines
110
and under the word line
140
is defined as a memory unit
144
.
In
FIG. 1B
, a coding step is performed. A photoresist layer
150
patterned with a coding window
153
that exposes a part of the memory unit
144
is formed. The coding window
153
has a width larger than the width of the memory unit
144
. The memory unit
144
is implanted with ions
160
to enhance the threshold voltage thereof. The coding process is complete.
In
FIG. 1C
, the photoresist layer
150
is removed, a dielectric layer is formed over the substrate
100
, and the subsequent process is performed.
In the above conventional method, as the coding window
153
in the photoresist layer
150
is wider than the memory unit
144
as shown in
FIG. 2
, the implanted ions
160
easily diffuse to positions outside the memory unit
144
. The threshold voltage and the channel threshold current are thus altered, thereby affecting the accuracy of data access.
SUMMARY OF THE INVENTION
The invention provides a method of fabricating a mask read only memory. Embedded bit lines are formed in a substrate. A gate dielectric layer and a word line are formed on the substrate. The substrate between a pair of the bit lines and under the word line is referred as a memory unit. A first dielectric layer is formed to cover the substrate. Several coding windows are formed in the first dielectric layers over a portion of the first dielectric layer. Spacers are formed on sidewalls of the coding windows. Using the first dielectric layer and the spacers as a mask, ions are implanted into the memory unit. A second dielectric layer is formed to fill the coding windows.
The invention further provides a programming method of a masked read only memory. After forming the bit lines and the word line across each other, a dielectric layer is formed to cover the substrate. A plurality of coding windows is formed in the first dielectric layer. The coding windows expose the memory units between two neighbouring bit lines and under the word line. Spacers are formed on sidewalls of the coding windows. Using the spacers and the first dielectric layer as a mask, the memory units under the coding windows are implanted with ions.
As mentioned above, spacers are formed on sidewalls of the coding windows in the invention. The ion implantation coverage is thus reduced. Ions are thereby prevented from diffusing to positions other than the memory units. Therefore, while programming the mask read only memory, the accuracy of data storage and access is not affected.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.


REFERENCES:
patent: 5308777 (1994-05-01), Hong
patent: 5384478 (1995-01-01), Hong
patent: 5510288 (1996-04-01), Hong
patent: 5672532 (1997-09-01), Hsue et al.
patent: 6468869 (2002-10-01), Yang et al.
patent: 2002/0072242 (2002-06-01), Chang
patent: 403083369 (1991-04-01), None
patent: 405102434 (1993-04-01), None

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