Semiconductor memory device and fabrication method thereof

Details Semiconductor memory device and fabrication method thereof Semiconductor memory device and fabrication method thereof

2001-07-23

2003-07-15

Everhart, Caridad (Department: 2825)

Semiconductor device manufacturing: process

Making passive device

C438S244000, C438S396000, C438S626000, C438S645000, C257S311000, C257S306000, C257S752000

Reexamination Certificate

active

06593202

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a structure and a method of fabricating a DRAM cell, or more in particular to a method of fabricating a memory cell of capacitor on bit-line type (COB) in which at least a bit line is formed first, and then at least a memory capacitor is formed on the bit line.
2. Description of the Related Art
The integration scale of semiconductor memory devices, especially, that of the dynamic RAM is ever on the increase every year. The resulting tendency is toward a further reduced area occupied by a unit memory element. A three-dimensional cell structure is therefore essential for securing a memory cell capacity sufficient for read and write operations (more than 20 fF). This has promoted general applications of cell structures using a trench-type capacitor and a stack-type capacitor.
A conventional method of fabricating a stack-type capacitor concerns a COB (capacitor over bit-line) memory cell as disclosed in, for example, “A CAPACITOR-OVER-BIT-LINE (COB) CELL WITH A HEMISPHERICAL-GRAIN STORAGE NODE FOR 64 Mb DRAMs,” by M. Sakao et al., IEDM Technical Digest, pp. 655, 658, 1990.
This device will be described in detail below with reference to drawings.
FIG. 27
is a plan view showing a COB memory cell,
FIG. 28
a perspective view taken diagonally from above the memory cell of
FIG. 27
,
FIGS. 29
,
30
,
32
sectional views taken in line XXIX—XXIX in
FIG. 27
, showing semiconductor structures in the fabrication steps of the COB memory cell, and
FIG. 31
a sectional view taken in line XXXI—XXXI in
FIG. 27
, showing a semiconductor structure in the fabrication step of the COB memory cell.
As shown in
FIG. 29
, first, an element-isolating oxide film
13
is formed on a silicon substrate
11
using the LOCOS process, while at the same time forming at least an element region
14
. Next, a gate oxide film (not shown) is formed. Polysilicon gate electrodes
19
each having the upper surface and the sidewalls thereof covered with a silicon oxide film are formed on the gate oxide film. Contact holes are opened to the gate electrodes
19
in self-alignment on the element region
14
, and a polysilicon film is deposited to form interconnects
50
(FIG.
29
).
As shown in
FIG. 30
, a first interlayer insulating film
51
is deposited over the whole surface. The first interlayer insulating film
51
is patterned using a bit line contact pattern
52
thereby to form a bit line contact hole
53
. Polysilicon
54
with impurities introduced therein is filled in the bit line contact hole
53
. Then, a bit line
55
is formed using a tungsten polycide (FIG.
30
).
As shown in
FIG. 31
, a second interlayer insulating film
56
is deposited over the whole surface of the bit line
55
and flattened. At least a storage node contact hole
57
is opened, thereby forming HSG (hemispherical grain) storage nodes
58
connecting to the interconnects
50
(FIG.
31
).
As shown in
FIG. 32
, a capacitor insulating film (not shown) made of a silicon oxide thin film and a silicon nitride thin film is formed over the whole surface, followed by forming plate electrodes
59
by depositing polysilicon. Then, a third interlayer insulating film
60
is deposited on the whole surface thereby to form metal wires
61
(FIG.
32
).
The DRAM memory cell fabricated using the prior art has the following problems:
(1) The interconnects
50
are inserted under the bit line
55
, so that the height of the bit line contact
52
is increased, resulting in an increased aspect ratio.
(2) The bit line contact
52
extends from the bit line to a diffusion layer, and therefore the aspect ratio of the bit line contact increases.
(3) The interconnects
50
are formed in the vicinity of the bit line contact
62
of an adjacent element region. Therefore, the interconnects
50
are liable to short with the polysilicon
54
in the bit line contact by misregistration.
(4) Since the bit line contact
52
is not formed in self-alignment with respect to the gate electrodes
19
, a short is liable to occur between the bit line and the gate electrodes.
(5) A new lithography process is required for forming the interconnects
50
, leading to an increased number of fabrication steps.
SUMMARY OF THE INVENTION
In view of the above-mentioned problems of the prior art, it is an object of the present invention to provide a memory cell structure and a method of fabricating a memory cell, in which the aspect ratio of the capacitor contact and the bit line contact can be reduced in forming a COB DRAM cell, misregistration has a lesser effect and the number of fabrication steps is not unduly increased.
According to the present invention, there is provided a semiconductor memory device comprising: a semiconductor substrate; an element-isolating oxide film formed on the semiconductor substrate for defining an element region; a first interlayer insulating film formed on the element region and the element-isolating oxide film; a first contact hole formed in the first interlayer insulating film and extending over the element region and the element-isolating oxide film, the first contact hole reaching the semiconductor substrate; and a first conductive film connected to the element region for filling the first contact hole. The semiconductor memory device may further comprise: a second interlayer insulating film formed on the first interlayer insulating film and the first conductive film; a second contact hole formed in the second interlayer insulating film and reaching the first conductive film above the element-isolating oxide film; and a wiring layer connected to the first conductive film through the second contact hole.
The semiconductor memory device may further comprise: a third contact hole formed in the first interlayer insulating film and reaching the semiconductor substrate at the element region; and a second semiconductive film filling the third contact hole.
The semiconductor memory device may further comprise a gate insulating film formed on the element region between the first contact hole and the third contact hole and a gate electrode extending over the gate insulating film.
The semiconductor memory device may further comprise: a third interlayer insulating film formed on the second interlayer insulating film and the wiring layer; a fourth contact hole formed through the third interlayer insulating film and the second interlayer insulating film and reaching the second conductive film; and a first electrode formed on the third interlayer insulating film and connected with the second conductive film through the fourth contact hole.
The semiconductor memory device may further comprise: a capacitor insulating film formed on the first electrode; and a second electrode formed on the capacitor insulating film.
According to the present invention, there is further provided a semiconductor memory device comprising: a semiconductor substrate; an element-isolating oxide film formed on the semiconductor substrate for defining an element region; a data transfer MOSFET formed on the element region and having a gate electrode thereof connected to a word line; a first interlayer insulating film covering the element-isolating oxide film and the MOSFET; a first contact hole formed in the first interlayer insulating film and extending over the element region and the element-isolating oxide film, the first contact hole reaching one of the source/drain diffusion layers of the MOSFET; a second contact hole formed in the first interlayer insulating film and reaching the other of the source/drain diffusion layers of the MOSFET on the element region; a first conductive plug filling the first contact hole; a second conductive plug filling the second contact hole; a second interlayer insulating film covering the first interlayer insulating film, the first conductive plug and the second conductive plug; a bit line contact hole formed in the second interlayer insulating film and reaching the first conductive plug above the element-isolating oxide film; a third conductive plug filling the bi

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