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FIELD OF THE INVENTION
The present invention relates to a method of fabricating a semiconductor
device in the form of a thin film such as a thin-film transistor (TFT) or
a thin-film diode and, more particularly, to a method of fabricating a
semiconductor device using a crystalline semiconductor material. A
semiconductor device fabricated according to the invention can be formed
either on an insulating substrate made of glass or the like or on a
semiconductor substrate made of a single crystal of silicon or the like.
BACKGROUND OF THE INVENTION
Thin-film semiconductor devices such as thin-film transistors and thin-film
diodes are classified into amorphous devices and crystalline devices,
depending on the kind of silicon used. Since amorphous silicon is inferior
in physical characteristics such as field mobility and conductivity to
crystalline silicon, crystalline semiconductor devices are required in
order to obtain excellent operating characteristics.
However, to crystallize a silicon film, a high temperature exceeding
600.degree. C. is needed. Also, it takes a long time to crystallize the
film. Where crystalline silicon devices are mass-produced in practice,
several crystallizing machines are required. Consequently, huge investment
in equipment results in increased costs.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of fabricating
a semiconductor device by crystallizing a silicon film below 600.degree.
C. in a substantially negligibly short time. In the present invention, a
trace amount of catalytic material is added to a substantially amorphous
silicon film to promote crystallization and to lower the crystallization
temperature, thus shortening the crystallization time. Adequate examples
of the catalytic material include metal elements such as nickel (Ni), iron
(Fe), cobalt (Co), and platinum, and compounds such as silicides. In
particular, a film, particles, clusters, or the like containing any one of
these elements form a layer on or under an amorphous silicon film so as to
be in intimate contact with this amorphous silicon film. Alternatively,
any one of these elements is implanted into an amorphous silicon film by
ion implantation and then the film is thermally annealed at an appropriate
temperature to crystallize the film.
When the amorphous silicon film is formed by CVD, the catalytic material is
added to the raw material gas. When the amorphous silicon film is formed
by physical vapor deposition such as sputtering, the catalytic material is
added to the target or evaporation source which forms a film. Of course,
as the anneal temperature rises, the crystallization time decreases.
Furthermore, as the concentrations of nickel, iron, cobalt, and platinum
are increased, the crystallization temperature drops, and the
crystallization time is shortened. Our research has revealed that if the
concentration of at least one of these elements is in excess of
1.times.10.sup.17 cm.sup.-3, favorable results are obtained. Preferably,
the concentrations of these elements are determined, using the minimum
values in the film measured by SIMS (secondary ion mass spectroscopy).
Since all of the aforementioned catalytic materials are not desirable for
silicon, it is desired their concentrations be made as low as possible.
Our research has shown that the total concentration of these catalytic
materials is preferably not in excess of 1.times.10.sup.20 cm.sup.-3. To
improve the characteristics further, the surface of the silicon film which
has been crystallized by thermal annealing is etched to a depth of 20 to
200 .ANG. or to a depth of not more than 50% of the thickness of the
silicon film, preferably between 1/100 and 1/5 of the thickness of the
silicon film, because excessive portions of these catalytic materials tend
to be deposited on the surface. The surface cleaned in this way is coated
with an insulating film by a CVD method such as plasma CVD, photo-assisted
CVD, or LPCVD or a physical vapor deposition method such as sputtering,
the insulating film consisting mainly of silicon oxide. As a result, the
clean interlace is preserved. If necessary, phosphorus or other element
may be added to the insulating film. This semiconductor-insulating film
structure can be directly used for a MOS structure. Where TFTs were
fabricated by the method described above, leakage current (OFF current)
decreased, and the sub threshold characteristics were improved.
Other objects and features of the invention will appear in the course of
the description thereof, which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(A) to 1(E) are cross-sectional views of TFTs, illustrating steps
for manufacturing the TFTs according to Example 1 of the present
invention;
FIG. 2 is a graph showing the characteristics of the TFTs shown in FIGS.
1(A)to 1(E); and
FIGS. 3 (A) to 3(E) are cross-sectional views of TFTs, illustrating steps
for manufacturing the TFTs according to Example 2 of the present invention
.
DETAILED DESCRIPTION OF THE INVENTION
EXAMPLE 1
FIG. 1, (A)-(E), are cross-sectional views of TFTs, illustrating steps for
fabricating the TFTs according to the present example. In the present
example, two kinds of TFTs were fabricated. First, silicon oxide was
sputtered as a base film 11 to a thickness of 2000 .ANG., on a substrate
10 made of Corning 7059. An amorphous silicon film 12 having a thickness
of 500 to 1500 .ANG., e.g., 800 .ANG., was formed on the silicon oxide
film 11 by plasma CVD. Subsequently, nickel silicide was deposited as a
film 13 having a thickness of 5 to 200 .ANG.,, e.g., 20 .ANG., by
sputtering (FIG. 1(A)). The nickel silicide film 13 is given by the
chemical formula NiSi.sub.x ;
0.4.ltoreq.x.ltoreq.2.5, e.g., x=2.0.
The laminate was annealed at 500.degree. C. for four hours in a reducing
ambient to crystallize the amorphous silicon film. Up to this step, the
two TFTs were treated in the same way. The surface of one TFT was etched
to a depth of 20 to 200 .ANG., e.g., 100 .ANG., with an etchant containing
hydrofluoric acid. In this way, a clean surface 14 was exposed. The other
TFT was merely cleaned with pure water and not etched (FIG. 1(B)).
Thereafter, the two TFTs underwent the same steps. The obtained silicon
film was photolithographically patterned to form island regions 15.
Silicon oxide was sputtered as a gate-insulating film 16 having a
thickness of 1000 .ANG.. A target consisting of silicon oxide was used in
the sputtering step. In this sputtering step, the substrate temperature
was 200.degree. to 400.degree. C., e.g., 350.degree. C. The sputtering
ambient contained oxygen and argon. The ratio of the argon to the oxygen
was 0 to 0.5, e.g., less than 0.1 (FIG. 1(C)).
Subsequently, silicon containing 0.1 to 2% phosphorus was deposited to a
thickness of 6000 to 8000 .ANG., e.g., 6000 .ANG. by LPCVD. Preferably,
the steps for forming the silicon oxide film and the silicon film are
carried out in succession. The silicon film was patterned to form gate
electrodes 17.
Then, phosphorus ions were implanted into the silicon region by plasma ion
implantation, using the gate electrodes 17 as a mask. Phosphine (pH) was
used as a dopant gas. The accelerating voltage was 60 to 90 kV, e.g., 80
kV. The dose was 1.times.10.sup.15 to 8.times.10.sup.15 cm.sup.-2, e.g.,
5.times.10.sup.15 cm.sup.-2. As a result, N-type doped regions 18a and 18b
were formed (FIG. 1(D)).
Thereafter, the laminate was annealed at 500.degree. C. for four hours to
activate the impurities. Since the nickel atoms were diffused into the
silicon film, recrystallization was promoted by the anneal, and the doped
regions 18a and 18b were activated. Subsequently, an interlayer insulator
19 having a thickness of 6000 .ANG. was formed from silicon oxide by
plasma CVD. Contact holes were formed in the silicon oxide film 19.
Conductive interconnects 20a and 20b were formed from a multi-layer film
of titanium nitride and aluminum. Finally, the laminate was annealed at
350.degree. C. for 30 minutes in a hydrogen ambient at 1 atm. Thus, a
semiconductor circuit was completed (FIG. 1(E)).
The ID-VG characteristics of the two kinds of TFTs obtained in the present
example are shown in FIG. 2. During the measurement, the source-drain
voltage was 1 V. Curve a indicates the characteristic of the TFT which was
derived by etching the silicon surface to a depth of 100 .ANG. after the
crystallization and then forming the silicon oxide film. Curve b indicates
the characteristic of the TFT which was fabricated by forming the silicon
oxide film immediately after the crystallization. It can be seen from
curve a that the leakage current I.sub.OFF a when a negative voltage was
applied to the gate was small, and that a steep rising characteristic
(S.sub.a) occurred when a positive voltage was applied. Furthermore, the
ON/OFF ratio is given by 9 digits. In this way, this device is an ideal
field-effect transistor. The device indicated by curve b acts also as a
field-effect transistor. However, the leakage current I.sub.OFF b is
larger than the leakage current I.sub.OFF a of the former device. The
rising characteristic (S.sub.b) occurring when a positive voltage was
impressed is milder. Also, the ON/OFF ratio is given by about 6 digits.
The former device has a less threshold voltage than that of the latter
device. This suggests that the density of the trap levels existing in the
semiconductor film of the former device is smaller. In this manner, the
present invention offers TFTs having novel characteristics.
EXAMPLE 2
FIG. 3, (A)-(E), are cross-sectional views of a semiconductor device,
illustrating manufacturing steps according to the present invention.
Silicon oxide was sputtered as a base film 31 to a thickness of 2000 .ANG.
on a substrate 30 made of Corning 7059. Nickel was deposited as a nickel
film 33 having a thickness of 5 to 200 .ANG., e.g., 10 .ANG., by
electron-beam evaporation. Then, an amorphous silicon film 32 having a
thickness of 500 to 1500 .ANG., e.g., 500.ANG., was deposited by plasma
CVD (FIG. 3(A)).
The laminate was annealed at 480.degree. C. for 8 hours to crystallize the
amorphous silicon film 32. Then, the surface of the silicon film was
lightly etched to a depth of 20 to 200 .ANG. with a plasma of carbon
tetrachloride (CCl.sub.4) or carbon tetrafluoride (CF.sub.4). The laminate
was subsequently treated at a temperature of 350.degree. to 480.degree. C.
for 30 minutes in an ambient containing 1 to 10% hydrogen chloride (HCl).
In this way, a clean surface 34 was formed (FIG. 3(B)).
Then, this silicon film was patterned to form island silicon regions 35. A
gate-insulating film 36 having a thickness of 1000 .ANG., was fabricated
from silicon oxide by plasma CVD which used TEOS (tetraethoxysilane,
Si(OC.sub.2 H.sub.5).sub.4 and oxygen as raw materials. Trichloroethylene
(C.sub.2 HCl.sub.3) was added to the raw material gases. Before the
formation of the film, oxygen was supplied into the chamber at a flow rate
of 400 SCCM. A plasma was created at a substrate temperature of
300.degree. C., at a total pressure of 5 Pa, and at an RF power of 150 W.
This state was maintained for 10 minutes. Then, oxygen, TEOS, and
trichloroethylene were introduced into the chamber at flow rates of 300
SCCM, 15 SCCM, and 2 SCCM, respectively. Under this condition, a silicon
oxide film was formed. The substrate temperature was 300.degree. C. The RF
power was 75 W. The total pressure was 5 Pa. After the completion of the
film, hydrogen was introduced into the chamber at a pressure of 100 torr.
The laminate was annealed at 350.degree. C. for 35 minutes in a hydrogen
ambient.
Subsequently, aluminum containing 2% silicon was deposited as an aluminum
film having a thickness of 6000 to 8000 .ANG., for example 6000 .ANG., by
sputtering. Preferably, the silicon oxide film 36 and the aluminum film
are formed in succession. The aluminum film was photolithographically
patterned to form conductive interconnects 37a, 37b, and 37c. The
interconnects 37a and 37b acted as gate electrodes. The surfaces of the
aluminum interconnects were anodized to form oxide layers 39a, 39b, and
39c on the surfaces. Before the anodization, a polyimide mask 38 was
selectively formed from photosensitive polyimide (Photoneece) on those
portions on which contacts would later be formed. During the anodization,
no anodic oxide was formed on these portions because of the presence of a
mask.
The anodization was conducted in an ethylene glycol solution containing 1
to 5% tartaric acid. The thickness of the obtained oxide layer was 2000
.ANG.. Then, phosphorus ions were implanted into the silicon region by
plasma ion implantation. Phosphine (PH.sub.3) was used as a dopant gas.
The accelerating voltage was 60 to 90 kV, for example 80 kV. The dose was
1.times.10.sup.15 to 8.times.10.sup.15 cm.sup.-2, for example
2.times.10.sup.15 cm.sup.-2. In this way, N-type doped regions 40a were
formed. Only the left TFT, or an N-channel TFT, was masked with a
photoresist. Boron ions were implanted into the silicon region of the
right TFT, or a P-channel TFT, again by plasma ion implantation. Diborane
(B.sub.2 H.sub.6) was used as a dopant gas. The accelerating voltage was
50 to 80 kV, for example 65 kV. The dose was 1.times.10.sup.15 to
8.times.10.sup.15 cm.sup.-2, for example 5.times.10.sup.15 cm.sup.-2,
which was larger than the dose of the phosphorus previously implanted. In
this way, the P-type doped regions 40b were formed.
Then, the impurities were activated by laser annealing. A KrF excimer laser
having a wavelength of 248 nm and a pulse duration of 20 nsec was used as
the above-described laser. Other lasers such as an XeF excimer laser
emitting a wavelength of 353 nm, an XeCl excimer laser emitting a
wavelength of 308 nm, and an ArF excimer laser emitting a wavelength of
193 nm may be employed. The energy density of the laser radiation was 200
to 400 mJ/cm.sup.2, for example 250 mJ/cm.sup.2. Two to ten shots, for
example 2 shots, were emitted for each one location. During the laser
irradiation, the substrate can be heated to about 200.degree. to
450.degree. C. It is to be noted that the best energy density is varied
when the substrate is heated. In the laser illumination step, the
polyimide "Photoneece" mask 38 was left behind because the exposed
aluminum would be damaged by laser irradiation. This mask 38 can be easily
removed by exposing it to an oxygen plasma. As a result, the doped regions
40a and 40b were activated (FIG. 3(D)).
Subsequently, a silicon oxide film 41 was formed from TEOS by plasma CVD to
form an interlayer insulator having a thickness of 2000 .ANG.. Contact
holes were formed in this insulator. Conductive interconnects 42a, 42b,
and 42c were formed from a metal material such as a multi-layer film of
titanium nitride and aluminum. The interconnects 42c connect the
interconnects 37c with one 41 of the doped regions 40b of the right TFT,
or a P-channel TFT. In this manner, a semiconductor circuit was completed
(FIG. 3(E)).
The semiconductor circuit was completed by the steps described thus far.
The characteristics of the fabricated TFTs were by no means inferior to
those of TFTs fabricated by the prior art method in which crystallization
is carried out by an annealing step at 600.degree. C. We have confirmed
that a shift register fabricated according to the present example operates
at 11 MHz at a drain voltage of 15 V and at 16 MHz at a drain voltage of
17 V. Furthermore, a reliability test showed that the novel TFTs did not
differ in reliability from the TFTs fabricated by the prior art method.
The present invention permits fabrication of TFTs having improved
characteristics and improved reliability. In the present invention, as
described in connection with Example 2, silicon is crystallized at a low
temperature, for example below 500.degree. C, and in a short time, for
example 4 hours. In addition, the obtained characteristics and reliability
are never inferior to the characteristics and reliability obtained
heretofore. Obviously, the throughput is enhanced, and the cost is
reduced. Where a conventional 600.degree. C.-process is adopted, the glass
substrate shrinks and warps, leading to a decrease in the production
yield. The present invention enables execution of a crystallization
process below 550.degree. C., which fully solves this problem. This means
that a substrate of a large area can be treated simultaneously. That is,
the single substrate of a large area is sawn into numerous ICs and hence
the cost of each IC can be reduced greatly. In this way, the invention is
industrially advantageous.
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