Semiconductor memory device having isolation structure and...

Details Semiconductor memory device having isolation structure and... Semiconductor memory device having isolation structure and...

1998-03-24

2001-05-01

Smith, Matthew (Department: 2825)

Active solid-state devices (e.g., transistors, solid-state diode

With means to control surface effects

Insulating coating

C257S649000, C257S639000, C257S509000, C257S647000, C438S218000, C438S219000, C438S220000, C438S221000, C438S225000, C438S362000

Reexamination Certificate

active

06225682

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor memory device and a fabrication method for the same, and more particularly, to an improved semiconductor device fabrication method and a resulting semiconductor memory device having an isolation structure formed by a local oxidation of silicon (LOCOS) method for obtaining a decreased bird's beak and stress.
2. Description of the Conventional Art
In a conventional semiconductor memory device having a LOCOS isolation structure, a channel is formed between a source region and a drain region in the memory device when a predetermined voltage, which is higher than a threshold voltage, is applied to a gate. The magnitude of the threshold voltage required to form the channel is proportional to the thickness of a gate oxide film. Thus, when a field oxide film is about ten times thicker than the gate oxide film, a channel of a transistor provided on a field region is opened by applying a voltage to the gate that is ten times larger than the voltage ordinarily required for turning on the transistor. Thus, an electrical isolation between cells of a semiconductor memory device is sufficient to prevent the transistor from being turned on in response to a voltage ordinarily large enough to turn on the transistor when applied to a gate of an adjacent transistor separated by a field oxide film.
The fabrication method of the conventional LOCOS isolation structure will now be described with reference to
FIGS. 1A through 1C
.
As shown in
FIG. 1A
, an upper surface of an Si substrate
11
is oxidized to form a pad oxide film
12
having a thickness of 35 nm. A nitride film
13
having a thickness of 100 nm is deposited on the pad oxide film
12
. Nitride film
13
functions as an anti-oxidation film. As further shown in
FIG. 1B
, a photoresist film
14
is formed and patterned on the nitride film
13
. Using the patterned photoresist film
14
, the nitride film
13
and the pad oxide film
12
are patterned. Referring to
FIG. 1C
, the photoresist film
14
is removed and the entire structure is oxidized using a wet oxidation method. During the wet oxidation method, a portion of the substrate
11
covered with the nitride film
13
is not oxidized. By contrast, the exposed surface of the substrate
11
that is not covered with the nitride film
13
is oxidized. As a result of the oxidation of the exposed surface of substrate
11
, the field oxide film
15
swells to about an 800 nm thickness, thereby accomplishing an isolation between the cells in accordance with the field oxide film
15
.
In the above-described conventional LOCOS isolation structure for a semiconductor memory device, the pad oxide film
12
does not efficiently block the growth of the field oxide film
15
during the oxidation step for forming the field oxide film
15
, as shown in FIG.
1
C. Consequently, a bird's beak phenomenon is generated in which each end portion of the field oxide film
15
extends under the pad oxide film
12
.
In recent years, Y. Sambonsugi et al. published an article entitled, “Oxynitride Pad LOCOS(ON-LOCOS) Isolation Technology for Gigabit DRAMs” in SSDM p. 139, 1995, disclosing a method for decreasing a birds beak by substituting an oxynitride film for the pad oxide film
12
. A brief description of that article will now follow.
First, when an Si substrate is nitrated under an atmospheric gas of NH
3
/Ar for 10 minutes at temperatures ranging from 500° C. to 900° C. and then is dry-oxidized for 30 minutes at a temperature of 900° C., a 3 nm thickness of oxynitride film is grown on the substrate. The density of nitrogen in the oxynitride film can be varied by adjusting the temperature during the nitridation. After growing the oxynitride film, a nitride film is deposited. The nitride film is patterned and dry-etched and then the oxynitride film is etched using HF gas. The next steps are identical to those of the conventional fabrication method for a LOCOS isolation structure, as shown in
FIG. 1A-1C
. As described above, the oxynitride film is employed instead of the pad oxide film. Thus, when the oxidation process is carried out to form the field oxide film, an oxidant diffusion is prevented by the nitride component, which is included in the oxynitride film.
Stated differently, the oxynitride film efficiently blocks the growth of the field oxide film, thereby decreasing the bird's beak.
Moreover, the oxynitride film decreases the bird's beak when compared with the use of an oxide film. However, because the oxynitride film has a thermal coefficient larger than that of the oxide film, stress is increased during the oxidation process used to form the field oxide film.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the problem inherent in the above-described convention systems,
It is also an object of the present invention to provide a fabrication method for a semiconductor memory device having an isolation structure which is appropriate for obtaining a decreased bird's beak and stress, while forming an isolation structure according to a local oxidation of silicon (LOCOS).
To achieve the above and other objects, a method of fabricating an isolation structure for a semiconductor device includes the steps of forming more than one separated oxynitride region on a semiconductor substrate, and forming a field oxide film on an exposed surface portion of a semiconductor substrate positioned between the separated oxynitride regions. The step of forming more than one separated oxynitride region may include forming an oxynitride film on the semiconductor substrate, and dividing the oxynitride film into more than one oxynitride region using an etching process, revealing the exposed surface portion of the semiconductor substrate between the separated oxynitride regions. The method may also include removing the pad oxide film, first nitride film and oxynitride regions.
More than one separated oxynitride region may also be formed by sequentially forming a pad oxide film and first nitride film on the semiconductor substrate, patterning the first nitride film and the pad oxide film to expose a first surface portion of the semiconductor substrate, forming an oxynitride film on the first surface portion of the semiconductor substrate, and removing a portion of the oxynitride film to expose a second surface portion of the semiconductor substrate and to effectively form more than one separated oxynitride region. The portion of the oxynitride film may be removed using an etching method which employs an HF gas or a wet etching method.
A portion of the oxynitride film may be removed using an etching process which uses side walls of a second nitride film that are formed on sides of the first nitride film as a mask. The side walls are formed by etching the second nitride film after being formed on the sides of the first nitride film, the side walls being formed to have a thickness ranging from about 50-500 angstroms Alternatively, a portion of the oxynitride film may be removed by using the first nitride film as a mask while using one of a dry etching method and a wet etching method.
Alternatively, if the first nitride film and the pad oxide film are patterned by removing a region of the pad oxide film positioned under an undercut portion of the nitride film, the first surface portion of the semiconductor substrate includes a portion of the substrate positioned under the undercut portion of the nitride film, and the portion of the oxynitride film is removed using the undercut portion of the nitride film as a mask.
In either case, the field oxide film may be formed using a thermal oxidation process, the first nitride film may be formed using a chemical vapor deposition method, and the oxynitride film is formed by nitrating and dry-oxidizing the exposed portion of the substrate. Furthermore, the first nitride film and the pad oxide film may be patterned using a reactive ion etching method. The pad oxide film, oxynitride film and side walls may be formed to have thicknesses

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