Semiconductor device manufacturing: process – Gettering of substrate
Reexamination Certificate
2000-05-26
2002-01-08
Everhart, Caridad (Department: 2825)
Semiconductor device manufacturing: process
Gettering of substrate
C438S660000, C438S486000, C438S166000
Reexamination Certificate
active
06337259
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for fabricating semiconductor devices and, more particularly, a method for fabricating a TFT (Thin Film Transistor) using crystalline silicon film as an active region.
TFTs using crystalline silicon film as an active region have been widely used for active matrix type liquid crystal devices, contact type image sensors and the like. Further, in recent years, these devices have been increasingly growing toward higher resolutions, and what is more, there has been made an attempt to build in, on the same board, not only driver circuits but also IC (Integrated Circuit) functions of control circuits, data processing circuits and the like that have been externally provided in the prior arts. For these reasons, TFTs are demanded to attain further enhancement in performance (higher-speed operations, lower leak current and lower-voltage operations).
Also, by enhancing the TFT performance up to an equivalency to MOS (Metal Oxide Semiconductor) transistors of single crystal silicon, it becomes possible to achieve newly functioned devices and so-called 3D ICs, which take advantage of characteristics as SOI (Silicon On Insulator). Like this, for enhancement in TFT performance, it is indispensable to achieve higher qualities of crystalline silicon films constituting the active region, i.e., scale-up of crystal grains, improvement in orientation characteristics, reduction in defect density and reduction in impurities.
Conventionally, as a method for achieving higher qualities of crystalline silicon films, there has been used a method in which a metallic element serving for acceleration of crystallization is introduced into an amorphous silicon film formed on an insulating substrate so that amorphous silicon is crystallized, and thereafter the metallic element is removed or reduced by gettering process. Such methods for achieving higher qualities of crystalline silicon films are disclosed in, for example, Japanese Patent Laid-Open Publications HEI 7-192998 and HEI 10-223533. These are described below in detail and separately.
(A) Japanese Patent Laid-Open Publication HEI 7-192998
After a metallic element serving for acceleration of crystallization is introduced into an amorphous silicon film so that the amorphous silicon film is crystallized, the amorphous silicon film is heated in an oxidative atmosphere and thereby oxidized, by which the metallic element is gettered into the oxide. More specifically, this method is carried out as follows (see
FIGS. 6A
to
6
F):
(1) As shown in
FIG. 6A
, on an insulating substrate (glass substrate)
1
, oxide as a base coat film
2
is deposited to a thickness of 2000 Å by sputtering process, and then amorphous silicon
3
is deposited to a thickness of about 1000 Å.
(2) As shown in
FIG. 6B
, on the amorphous silicon
3
, oxide
4
is deposited to a thickness of 1000 Å or more by sputtering process, and patterning is performed, by which a mask is formed.
(3) A metallic element of Ni element
6
or the like is introduced into the amorphous silicon
3
at an opening portion
5
.
(4) As shown in
FIG. 6C
, the oxide (mask layer)
4
on the amorphous silicon
3
is removed by etching process, and heated at 550° C., so that surrounding amorphous silicon
3
is crystallized out of a Ni-element high concentration region
7
, by which a crystalline silicon film
8
is obtained.
(5) The crystalline silicon film
8
is patterned into an island shape as shown in
FIG. 6D
, and the surface of the crystalline silicon film
8
is oxidized as shown in
FIG. 6E
to thereby form oxide
10
. In this way, by incorporating the Ni element into the oxide
10
, Ni is gettered into the oxide
10
. As a result, the concentration of Ni in the crystalline silicon film
8
is reduced. It is noted that reference numeral
9
denotes a crystal growth end.
(6) Finally, as shown in
FIG. 6F
, the surface oxide
10
is completely removed.
(B) Japanese Patent Laid-Open Publications HEI 10-223533
After a metallic element serving for acceleration of crystallization is introduced into an amorphous silicon film so that the amorphous silicon film is crystallized, a mask is formed selectively on the amorphous silicon film. Then, one kind or a plurality of kinds of elements among nitrogen, phosphorus, arsenic, antimony and bismuth are added, and the metal in regions where the element or elements are not added is gettered into the region where the element or elements are added. More specifically, this is carried out as follows (see
FIGS. 7A
to
7
F):
(1) As shown in
FIG. 7A
, on a glass substrate
11
with a 2000 Å silicon oxide (not shown) formed thereon, 500 Å amorphous silicon film
12
is deposited by plasma CVD (chemical vapor deposition) process.
(2) Further, Ni acetate
13
is formed by spin-coating a Ni acetate solution having a Ni concentration of 100 ppm, and a metal of Ni is introduced into the surface of the amorphous silicon film
12
.
(3) A 4 hour heating process is performed at a temperature of 600° C. so that the amorphous silicon film
12
is crystallized, by which a crystalline silicon film
14
is obtained as shown in FIG.
7
B.
(4) With KrF (krypton fluoride) excimer laser beam (wavelength: 248 nm) applied, a laser annealing process is performed.
(5) As shown in
FIG. 7C
, 1000 Å silicon nitride
15
is deposited on the crystalline silicon film
14
by plasma CVD process.
(6) By etching the silicon nitride
15
, as shown in
FIG. 7D
, a mask
16
for implantation of phosphorus is formed.
(7) By plasma doping process, phosphorus is implanted at a dose of 5×10
14
atom/cm
2
.
(8) As shown in
FIG. 7E
, a
2
hour heating process is performed at a temperature of 600° C. in a nitrogen atmosphere, by which Ni of a crystalline silicon film
18
under the mask
16
is moved in the arrow direction and thereby gettered into a region
17
where phosphorus has been implanted. As a result, the concentration of Ni in the crystalline silicon film
18
, in which phosphorus has not been implanted, is reduced.
(9) As shown in
FIG. 7F
, the region
17
, in which phosphorus has been implanted by using the mask
16
(to which Ni has moved), is removed, by which the mask
16
is removed. Finally, peripheral part
19
of the gettered region
18
is removed by using a mask (not shown) smaller than the mask
16
, and the used mask is also removed.
However, the conventional method including the gettering of a metallic element has the following problems:
(1) Japanese Patent Laid-Open Publication HEI 7-192998
In the growth of a crystalline silicon film using a metallic element serving for acceleration of crystallization, since enough amount of a metallic element is necessitated before crystallization in order to obtain enough crystalline growth, a large amount of the metallic element is contained in the film just before the crystallization. However, after the crystallization, it is desirable that the concentration of the metallic element be as low as possible in terms of the quality of crystalline silicon film and TFT characteristics. That is, the remaining metallic element serves as impurities that adversely affect the TFT characteristics, so that high mobility cannot be obtained and the resulting leak current is large.
Unfortunately, in the crystalline silicon film that has been crystallized by heating process performed after the implementation of a metal serving for acceleration of crystallization into amorphous silicon film, the metal serving for acceleration of crystallization is distributed not uniformly but unevenly. In particular, at grain boundaries at which a plurality of crystal grains contact one another, high-concentration metals are present in a state of compound with silicon. In Japanese Patent Laid-Open Publication HEI 7-192998, the crystalline silicon film is oxidized in order to reduce the remaining metals, in which case oxidation considerably proceeds at grain boundaries where metals are present at high concentrations, and after the oxidation, irregularities of the cryst
Fukushima Yasumori
Higami Yoshinori
Ueda Tohru
Everhart Caridad
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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