 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to a method for obtaining crystalline semiconductors
for use in thin film devices such as thin film insulator gate type field
effect transistors (Thin Film Transistors, or TFTs).
Conventionally, crystalline silicon semiconductor thin films used in thin
film devices such as thin film insulator gate type field effect
transistors (TFTs) have been manufactured by forming an amorphous silicon
film on an insulating surface such as an insulator substrate by plasma CVD
or thermal CVD and then crystallizing this film in an electric furnace or
the like at a temperature of above 600.degree. C. over a long period of
twelve hours or more. In order to obtain particularly good performance
(high field effect mobility and high reliability), heat treatment for even
longer periods has been required.
However, there have been numerous problems associated with this kind of
conventional method. One problem has been that throughput is low and
therefore costs are high. For example, if 24 hours are required for this
crystallization process, and if the treatment time for each substrate is 2
minutes, it has been necessary to treat 720 substrates at the same time.
However, for example, in a commonly used tube furnace the number of
substrates that can be treated at one time is 50 at the most, and when one
only apparatus (reaction tube) is used the time taken per substrate has
been as long as 30 minutes. In other words, in order to make the treatment
time per substrate 2 minutes, it has been necessary to use as many as 15
reaction tubes. This has meant that the scale of the required capital
investment has been great and that the depreciation on that investment has
been large and has kept the cost of the product high.
Another problem has been the temperature of the heat treatment. Substrates
commonly used in the manufacture of TFTs can be generally divided into
those which consist of pure silicon oxide, like quartz glass, and
no-alkali borosilicate glass types, like Corning Co.'s No. 7059
(hereinafter referred to as Corning 7059). Of these two classes, in the
case of the former, because they have excellent resistance to heat and can
be handled in the same way that substrates are handled in ordinary
semiconductor integrated circuit wafer processes, there are no problems
relating to heat. However, they are expensive, and their cost rapidly
increases exponentially along with increases in surface area. Therefore,
at present, they are only being used in TFT integrated circuits of
relatively small surface area.
No-alkali glass, on the other hand, is of satisfactorily low cost compared
to quartz; however, its resistance to heat is a problem, and because its
distortion point is generally about 550.degree. to 650.degree. C., and in
the case of particularly easily acquired materials is below 600.degree.
C., with heat treatment at 600.degree. C. problems of irreversible
shrinkage and warping have arisen. These problems have been especially
conspicuous with large substrates of over 10 inches in diagonal. For
reasons like these it has been considered in connection with the
crystallization of silicon semiconductor films that heat treatment
conditions of below 550.degree. C. and less than 4 hours are indispensable
to reductions in cost. An object of this invention is to provide a method
for manufacturing a semiconductor which clears these conditions and a
method for manufacturing a semiconductor device in which such a
semiconductor is used.
SUMMARY OF THE INVENTION
This invention is characterized in that a crystalline silicon film is
obtained by forming a film, islandish film, dot, line, particles or
clusters or the like containing nickel, iron, cobalt, platinum or
palladium on or underneath a silicon film in a disordered state of a kind
which can be described as an amorphous state or a substantially amorphous
state (for example a state in which portions having good crystallinity and
amorphous portions exist together) and annealing this at a temperature
which is lower, and preferably 20.degree. to 150.degree. C. lower, than
the normal crystallization temperature of amorphous silicon, or at a
temperature which is lower than the glass transition point of the
substrate, for example at a temperature below 580.degree. C.
With regard to conventional silicon film crystallization, methods wherein
an island-shaped crystalline film is made to serve as a nucleus and solid
phase epitaxial growth is brought about with this as a seed crystal have
been proposed (for example Japanese Laid-Open Patent Publication
H1-214110). However, with this kind of method, at temperatures below
600.degree. C. almost no crystal growth progress has occurred. Moving a
silicon substance from an amorphous state into a crystalline state
generally involves a process wherein, with the state of the substance
having been made such that the molecule chains in the amorphous state are
cut and these cut molecules do not combine again with other molecules,
these molecules are introduced to molecules having some crystalline
character and rebuilt into constituent parts of crystals. However, in this
process a large amount of energy is required for cutting the first
molecule chains and maintaining the state wherein these cut molecules do
not combine with other molecules, and this has been a barrier in the
crystallization reaction. To provide this energy, several minutes at a
temperature of about 1000.degree. C. or several tens of hours at a
temperature of about 600.degree. C. are necessary, and because the time
required varies exponentially in relation to the temperature (=energy), at
temperatures below 600.degree. C., for example 550.degree. C., it has been
almost impossible to observe any crystallization reaction progress at all.
The idea of conventional solid phase epitaxial crystallization did not
provide an answer to this problem.
The present inventors thought of reducing the barrier energy of the above
process by means of the action of some kind of catalyst, completely
separately from the conventional solid phase crystallization idea. The
inventors noted that nickel, iron, cobalt, platinum and palladium readily
combine with silicon and for example in the case of nickel silicides
(chemical formula NiSi.sub.x, 0.4.ltoreq..times..ltoreq.2.5) are formed
and that the lattice constants of nickel silicides are close to those of
silicon crystals. Accordingly, by simulating the energies, etc, of the
ternary system crystalline silicon--nickel silicide--amorphous silicon, it
was found that amorphous silicon readily reacts at an interface with
nickel silicide and that the following reaction (1) occurs:
Amorphous Silicon (silicon A)+Nickel Silicide (silicon B).fwdarw.Nickel
Silicide (silicon A)+Crystalline Silicon (silicon B)
(A and B indicate the locations of the silicon)
The potential barrier of this reaction is satisfactorily low, and the
temperature of the reaction is also low.
This reaction formula shows that nickel reconstructs amorphous silicon into
crystalline silicon as it advances. In practice it was found that the
reaction was started at under 580.degree. C. and observed even at
450.degree. C. Typically, it was shown that crystallization is possible at
temperatures 20.degree. to 150.degree. C. lower than the normal
crystallization temperature of amorphous silicon. Naturally, the higher
the temperature the more quickly the reaction proceeds.
In the present invention, islands, stripes, lines, or dots of nickel, iron,
cobalt, platinum or palladium, silicide, acetate, or nitrate of nickel,
iron, cobalt, platinum or palladium, film, particles, or clusters
containing at least one of nickel, iron, cobalt, platinum, and palladium
can be used as starting materials. As the above-described reaction
progresses, nickel, iron, cobalt, platinum or palladium expands around the
starting material, thus enlarging the region of crystalline silicon.
Oxides are not desired materials containing nickel, iron, cobalt, platinum
or palladium because oxides are stable compounds and because they cannot
initiate the reaction described above.
In this way, the crystalline silicon expanding from a certain location is
different from the conventional solid phase epitaxy but has crystallinity
of high continuity. The structure approximates a single crystal structure.
This is advantageous to utilize semiconductor devices such as TFTs. Where
a material containing nickel, iron, cobalt, platinum or palladium is
dispersed uniformly over a substrate, innumerable starting points of
crystallization exist. Therefore, it has been difficult to derive a good
film of high crystallinity.
The lower the concentration of hydrogen in the amorphous silicon film which
serves as the departure material in this crystallization, the better were
the results (the crystallization speeds) that could be obtained. However,
because hydrogen is expelled as crystallization progresses, there was not
such a clear correlation between the hydrogen concentration in the silicon
film obtained and the hydrogen concentration of the amorphous silicon film
that was the departure material. The hydrogen concentration in crystalline
silicon obtained according to this invention was typically from
1.times.10.sup.15 atoms.cm.sup.-3 to 5 atomic %. Furthermore, in order to
obtain good crystallinity the concentrations of carbon, nitrogen and
oxygen in the amorphous silicon film should be as low as possible, and
preferably should be below 1.times.10.sup.19 cm.sup.-3. Accordingly, this
point should be taken into consideration in selecting the material
containing nickel, iron, cobalt, platinum or palladium to be used in
practicing this invention.
A feature of this invention is that crystal growth progresses circularly.
This is because the nickel of the reaction described above advances
isotropically, and this is different than conventional crystallization
wherein growth occurs linearly along the crystal lattice surfaces.
In particular, by setting the material containing nickel, iron, cobalt,
platinum or palladium selectively, it is possible to control the direction
of the crystal growth. Because, unlike crystalline silicon produced by
conventional solid phase epitaxial growth, crystalline silicon obtained
using this kind of technique has a structure in which the continuity of
the crystallinity over long distances is good and which is close to being
monocrystalline, it is well suited to use in semiconductor devices such as
TFTs.
In the present invention, nickel, iron, cobalt, platinum or palladium is
used. These materials are not desirable for silicon which is used as a
semiconductor material. If such a material is contained excessively in the
silicon film, it is necessary to remove the material. With respect to
nickel, when a growing crystal of nickel silicide arrives its final
points, i.e., the crystallization has been completed, as a result of the
above-described reaction, the nickel silicide is easily dissolved in
hydrofluoric acid or hydrochloric acid. The nickel contained in the
substrate can be reduced by treating the nickel with these acids.
In the case where a catalytic element such as nickel, iron, cobalt,
platinum and palladium is diffused almost uniformly throughout the silicon
film by the annealing for crystallization, a process wherein the nickel is
removed is necessary. To perform this nickel removal, it has been found
that annealing at 400.degree. to 650.degree. C. in an atmosphere
containing chlorine atoms in the form of chlorine or a chloride is
effective. An annealing time of 0.1 to 6 hours was suitable. The longer
the annealing time was the lower the concentration of nickel in the
silicon film became, but the annealing time may be decided according to
the balance between the manufacturing cost and the characteristics
required of the product. Examples of the chloride include hydrogen
chloride, various kinds of methane chloride (CH.sub.3 Cl, CH.sub.2
Cl.sub.2, CHCl.sub.3), various kinds of ethane chloride (C.sub.2 H.sub.5
Cl, C.sub.2 H.sub.4 Cl.sub.2, C.sub.2 H.sub.3 Cl.sub.3, C.sub.2 H.sub.2
Cl.sub.4, C.sub.2 HCl.sub.5), and various kinds of ethylene chloride
(C.sub.2 H.sub.3 Cl, C.sub.2 H.sub.2 Cl.sub.2, C.sub.2 HCl.sub.3).
Especially, the material which can be used most easily is
trichloroethylene (C.sub.2 HCl.sub.3). We have discovered by SIMS that
preferred concentration of nickel, iron, cobalt, platinum or palladium in
the silicon film (e.g. a silicon film used for a semiconductor device such
as a TFT) according to the present invention is 1.times.10.sup.15
cm.sup.-3 to 1 atomic %, more preferably 1.times.10.sup.15 to
1.times.10.sup.19 cm.sup.-3. At less concentrations, the crystallization
does not progress sufficiently. At higher concentrations, the
characteristics and the reliability deteriorate.
Filmlike bodies containing nickel, iron, cobalt, platinum or palladium can
be formed using various physical and chemical methods. For example methods
requiring vacuum apparatus, such as vacuum vapor deposition, sputtering
and CVD, and atmospheric pressure methods, such as spin coating and
dipping, etc, (coating methods), doctor blade methods, screen printing and
spray thermal decomposition.
Spin coating and dipping in particular, while not necessitating grand
equipment, are techniques which offer excellent film thickness uniformity
and with which fine concentration adjustment is possible. As solutions for
use in these techniques, acetates and nitrates of nickel, iron, cobalt
platinum and palladium, or various types of carboxylic acid chloride or
other organic acid chlorides dissolved or dispersed in water or some type
of alcohol (low level or high level), or petroleum (saturated hydrocarbon
or unsaturated hydrocarbon), etc, can be used.
However, there was concern that in this case oxygen and carbon contained in
those salts might diffuse into the silicon film and cause its
semiconductor characteristics to deteriorate. But, through research
pursued using thermal balancing and differential thermal analysis it has
been confirmed that suitable materials break down at temperatures below
450.degree. C. to oxides or simple substances and thereafter there is
almost no diffusion into the silicon film. In particular, when substances
which are of lower order like acetates and nitrates were heated in a
reducing atmosphere such as a nitrogen atmosphere they broke down at below
400.degree. C. and became the simple metal. Similarly, it was observed
that when they were heated in an oxygen atmosphere, first oxides were
formed and then at higher temperatures oxygen broke away and left behind
the simple metal.
A crystalline silicon film is fabricated according to the invention, and
this film is used in a semiconductor device such as a TFT. As can be seen
from the description made above, a large grain boundary exists at the ends
of a growing crystal where the front ends of the growing material starting
from plural points meet. Also, the concentration of nickel, iron, cobalt,
platinum, or palladium is high. For these reasons, it is not desired to
form a semiconductor device. Particularly, a channel of a TFT should not
be provided in a region having such a large grain boundary.
The region from which the crystallization starts, that is, the region in
which the substance containing nickel, iron, cobalt, platinum or palladium
is provided has a large concentration of nickel, iron, cobalt, platinum or
palladium. For this reason, attention should be paid to the formation of
the semiconductor device. Further, such a region is readily etched by a
solution containing a hydrofluoric acid group as compared with a silicon
film which does not contain nickel, iron, cobalt, platinum or palladium.
For this reason, such a region becomes a cause of formation of a defective
contact. Accordingly, where a semiconductor device is fabricated by making
use of the present invention, the pattern of a coating containing nickel,
iron, cobalt, platinum, or palladium forming a starting point for
crystallization and a pattern of the semiconductor device must be
optimized.
In addition, the present invention provides a process which is
characterized in that it comprises: forming, on an amorphous silicon film
or on a film which has such a disordered crystalline state that it can be
regarded as being amorphous (for example, a state which comprises
crystalline portions and amorphous portions in a mixed state), a film,
particles or clusters containing at least one of nickel, iron, cobalt,
platinum or palladium (which are referred to hereinafter as catalytic
materials); allowing the catalytic material to react with amorphous
silicon at first, and removing the catalytic material which remained
un-reacted; and annealing the resulting structure at a temperature lower
than the normal crystallization temperature of a amorphous silicon by,
preferably, 20.degree. to 150.degree. C., or at a temperature not higher
than the glass transition temperature of the glass material conventionally
used as a substrate, e.g., at 580.degree. C. or lower.
Even after the nickel, iron, cobalt, platinum or palladium atoms are
removed from crystalline silicon, crystallization can be initiated by
using as nuclei the remaining crystalline silicon which was formed by the
reaction (1). As mentioned in the foregoing, the silicon crystals thus
formed by the reaction has excellent crystallinity. Thus, it has been
found that the crystallization of amorphous silicon can be accelerated by
using these crystals as the nuclei. It has been shown that, typically, the
crystallization can be effected at a temperature lower than the normal
crystallization of amorphous silicon by 20.degree. to 150.degree. C.
Furthermore, the time necessary for the crystal growth is found to be
shortened. As a matter of course, the crystallization proceeds more
rapidly with increasing the temperature. A similar reaction, though less
actively to the case using nickel, is found to occur in the case using
iron, cobalt, platinum or palladium.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(A) to 1(D) show schematically drawn step sequential cross section
structures obtained in a process according to an embodiment of the present
invention (Example 1);
FIGS. 2(A) to 2(E) show schematically drawn step sequential cross section
structures obtained in another process according to another embodiment of
the present invention (Example 2);
FIG. 3 shows the result of Raman scattering spectroscopy of a crystalline
silicon film obtained in Example 1;
FIG. 4 shows the X-ray diffraction pattern of a crystalline silicon film
obtained in an Example;
FIG. 5 shows the crystallization rate of silicon (Example 2);
FIGS. 8(A) to 8(E) show schematically drawn step sequential cross section
structures obtained in a process for fabricating a semiconductor according
to a yet another embodiment of the present invention (Example 3);
FIGS. 7(A) to 7(C) show a step of introducing a catalyst element using a
solution (Example 4);
FIGS. 8(A) to 8(C) are top views of TFTs illustrating crystallization steps
for manufacturing the TFTs according to the invention and their
arrangement;
FIGS. 9(A-1), 9(A-2), 9(B), 9(C), and 9(D) are cross-sectional views of
TFTs illustrating steps for selectively crystallizing a film according to
the invention;
FIGS. 10(A) to 10(C) are cross-sectional views of TFTs illustrating steps
of Example 5 of the invention;
FIGS. 11(A) to 11(C) are cross-sectional views of other TFTs illustrating
steps of Example 5 of the invention;
FIGS. 12(A) to 12(C) are cross-sectional views of TFTs illustrating steps
of Example 6 of the invention;
FIGS. 13(A) to 13(C) are cross-sectional views of TFTs illustrating steps
of Example 7 of the invention;
FIGS. 14(A) to 14(D) are cross-sectional views of TFTs illustrating steps
of Example 8 of the invention;
FIGS. 15(A) to 15(D) are cross-sectional views of TFTs illustrating steps
of Example 9 of the invention;
FIG. 16 is a graph showing the concentration of nickel in a crystalline
silicon film in Example 9 of the invention;
FIGS. 17(A) to 17(C) are cross-sectional views of a substrate undergoing a
manufacturing process according to Example 10 of the invention;
FIGS. 18(A) and 18(B) are cross-sectional views of a substrate undergoing a
manufacturing process according to Example 11 of the invention;
FIGS. 19(A) to 19(E) are cross-sectional views of a substrate undergoing a
manufacturing process according to Example 12 of the invention;
FIGS. 20(A) to 20(E) are cross-sectional views of a substrate undergoing a
manufacturing process according to Example 13 of the invention;
FIGS. 21(A) to 21(D) are cross-sectional views of a substrate undergoing a
manufacturing process according to Example 14 of the invention;
FIGS. 22(A) to 22(D) are cross-sectional views of a substrate undergoing a
manufacturing process according to Example 15 of the invention;
FIGS. 23(A) to 23(C) are cross-sectional views of a substrate undergoing a
manufacturing process according to Example 16 of the invention;
FIGS. 24(A) to 24(C) are cross-sectional views of a substrate undergoing a
manufacturing process according to Example 17 of the invention;
FIG. 25 is a graph illustrating the nickel concentration in a crystalline
silicon film; and
FIGS. 26(A) to 26(F) are cross-sectional views of a substrate undergoing a
manufacturing process according to Example 18 of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is illustrated in greater detail referring to
non-limiting examples below. It should be understood, however, that the
present invention is not to be construed as being limited thereto.
EXAMPLE 1
Referring to FIG. 1, a process for fabricating a crystalline silicon film
by forming a nickel film on a Corning #7059 substrate, and crystallizing
an amorphous silicon film using this nickel film is described below. By
using plasma CVD, a 2,000 .ANG. thick silicon oxide film 12 as a base film
was deposited on the substrate 11, and further thereon an amorphous
silicon film 13 at a thickness of from 500 to 3,000 .ANG., for example, at
a thickness of 1,500 .ANG.. After removing hydrogen from the film by
keeping the film at a temperature of 430.degree. C. for a duration of from
0.1 to 2 hours, for example, 0.5 hour, a nickel film 14 was deposited
thereon by sputtering at a thickness of from 100 to 1,000 .ANG., for
example, 500 .ANG.. A favorable nickel film can be obtained by heating the
substrate in the temperature range of from 100.degree. to 500.degree. C.,
preferably in the range of from 180.degree. to 250.degree. C., because a
nickel film having an improved adhesion strength with respect to the
silicon film formed as the base is obtained. A nickel silicide film can be
used in the place of the nickel film.
The resulting structure was then heated in the temperature range of from
450.degree. to 580.degree. C. for a duration of from 1 to 10 minutes to
allow the nickel film 14 to react with the amorphous silicon film 13,
thereby obtaining a thin crystalline silicon film 15. The thickness of the
crystalline silicon film depends on the temperature and duration of the
reaction, and a film about 300 .ANG. in thickness can be obtained by a
reaction performed at 550.degree. C. for a duration of 10 minutes. The
resulting structure is shown in FIG. 1(B).
The nickel film and the nickel silicide film thus obtained from the nickel
film through the reaction were subjected to etching using hydrochloric
acid in a concentration of from 5 to 30%. No influence was found on
crystalline silicon which has been formed by the reaction between
amorphous silicon and nickel (silicide) by this treatment. Thus was
obtained a structure shown in FIG. 1(C).
The resulting structure was annealed under a nitrogen atmosphere in an
annealing furnace whose temperature was kept in a range of from
450.degree. to 580.degree. C., for example, 550.degree. C., for a duration
of 8 hours. A crystalline silicon film 16 was thus obtained in this step
by crystallizing the amorphous silicon film. The Raman scattering
spectrogram and the X-ray diffractogram of the resulting crystalline
silicon film are each shown in FIG. 3 and FIG. 4. In FIG. 3, the curve
indicated by C--Si corresponds to the Raman spectrum of a standard sample,
i.e., single crystal silicon. The curves indicated by (a) and (b) each
represent the Raman spectra for a silicon film obtained by the process
according to the present invention, and a film obtained by annealing a
conventional amorphous silicon by the conditions described above. It can
be seen clearly from the results that the process according to the present
invention provides a favorable silicon crystal.
EXAMPLE 2
Referring to FIG. 2, a process for fabricating a crystalline silicon film
is described below. A 2,000 .ANG. thick silicon oxide film 22 as a base
film was deposited on a Corning #7059 glass substrate 21, and an amorphous
silicon film 23 was deposited further thereon at a thickness of from 500
to 3,000 .ANG., for example, at a thickness of 500 .ANG. and 1,500 .ANG..
After removing hydrogen from the film by keeping the film at a temperature
of 430.degree. C. for a duration of from 0.1 to 2 hours, for example, 0.5
hour, a nickel film was deposited thereon by sputtering at a thickness of
from 100 to 1,000 .ANG., for example, 500 .ANG.. A nickel silicide film
can be used in the place of the nickel film. The nickel film thus obtained
was etched to form patterns 24a, 24b, and 24c as shown in FIG. 2(A).
Then, the structure was heated in the temperature range of from 450.degree.
to 580.degree. C. for a duration of from 1 to 10 minutes to allow the
nickel films 24a to 24c to undergo reaction with the amorphous film 23 to
form thin crystalline silicon regions 25a, 25b, and 25c as shown in FIG.
2(B).
The nickel film and the nickel silicide film thus obtained from the nickel
film through the reaction were subjected to etching using hydrochloric
acid in a concentration of from 5 to 30%. No influence was found on
crystalline silicon regions 25a to 25c which have been formed by the
reaction between amorphous silicon and nickel (silicide) by this
treatment. Thus was obtained a structure shown in FIG. 2(C).
The resulting structure was annealed under a nitrogen atmosphere in an
annealing furnace whose temperature was kept in a range of from
450.degree. to 580.degree. C., for example, 550.degree. C., for a duration
of 4 hours. FIG. 2(D) provides an intermediate state during the annealing
process, in which the crystallization is observed to proceed from the
previously formed crystalline silicon regions 25a to 25b, in such a manner
that the crystalline silicon regions 26a, 26b, and 26c are observed to
extend into the amorphous region 23.
A crystalline silicon film 27 was finally obtained by crystallizing the
entire amorphous silicon film. In contrast to the case of Example 1 in
which the crystal growth proceeds perpendicularly from the surface to the
substrate side, the crystal in the present example grows transversely from
the patterned nickel. For instance, the crystal structure of the
crystalline silicon regions 26a to 26c as shown in FIG. 2(D) is similar to
that of a single crystal silicon. Accordingly, the structure can be
suitably applied to semiconductor devices such as TFTs because the
formation of a potential barrier in these crystalline silicon regions
along the transverse direction is relatively rare. However, at portions in
which the crystalline silicon regions 26a and 26b collide with each other,
for example, crystals are greatly damaged and are thus not suitable for
the application.
FIG. 5 shows the relation between the crystallization rate and the
temperature of crystallization. It has been found that the crystallization
proceeds faster with increasing thickness of the silicon film.
EXAMPLE 3
The present example relates to a process for fabricating a silicon film
having an improved crystallinity by irradiating a laser beam to the
silicon film after once crystallizing it by heating. Furthermore, the
present example provides a process for fabricating a TFT using the thus
crystallized silicon film.
FIG. 6 shows the cross section view of the step-sequential structures
obtained in the present process. Referring to FIG. 6, a 2,000 .ANG. thick
silicon oxide film 602 as a base film was deposited on a Corning #7059
glass substrate 601, and an intrinsic (I type) amorphous silicon film was
deposited further thereon at a thickness of from 100 to 1,500 .ANG., for
example, at a thickness of 800 .ANG. in this case. A nickel film, i.e., a
catalytic material for accelerating the crystallization of the amorphous
silicon, was deposited selectively thereon by a process similar to that
described in Example 2 (refer to FIG. 2(A)). The resulting structure was
then heated in the temperature range of from 450.degree. to 580.degree. C.
for a duration of from 1 to 10 minutes to allow the nickel film to react
with the amorphous silicon film, thereby obtaining a thin crystalline
silicon film. The resulting structure is shown in FIG. 2(B).
The nickel film and the nickel silicide film structure obtained from the
nickel film through the reaction were subjected to etching using
hydrochloric acid in a concentration of from 5 to 30%. No influence by
this treatment was found on crystalline silicon which has been formed by
the reaction between amorphous silicon and nickel (silicide). Thus was
obtained a structure shown in FIG. 2(C).
A further annealing at 550.degree. C. for 12 hours under a nitrogen
atmosphere at atmospheric pressure provides a crystalline silicon film 603
covering the entire surface of the structure.
Then, a KrF excimer laser was operated to irradiate a laser beam at a
wavelength of 248 nm and at a pulse width of 20 nsec to the surface of the
resulting crystalline silicon film to further accelerate the
crystallization thereof. The laser beam was irradiated at an output energy
density of from 200 to 400 mJ/cm.sup.2, for instance 250 mJ/cm.sup.2, for
2 shots During the laser beam irradiation, the substrate was maintained at
a temperature of 300.degree. C. by heating to fully enhance the effect of
laser beam irradiation. In general, the substrate is preferably heated in
the temperature range of from 200.degree. to 450.degree. C. The present
step is illustrated in FIG. 6(A).
Usable laser light other than that of the KrF excimer laser above include
those emitted from a XeCl excimer laser operating at a wavelength of 308
nm and an ArF excimer laser operating at a wavelength of 193 nm.
Otherwise, an intense light may be irradiated in the place of a laser
light. In particular, the application of RTA (rapid thermal annealing)
which comprises irradiating an infrared light is effective because it can
selectively heat the silicon film in a short period of time.
Thus, a silicon film having a favorable crystallinity can be obtained by
employing any of the aforementioned methods. The crystallized silicon film
603 obtained as a result of thermal annealing was found to change into a
silicon film having a further improved crystallinity. Furthermore,
observation by transmission electron microscope revealed that relatively
large grains of oriented crystallites constitute the laser-irradiated
film.
The silicon film thus obtained upon the completion of crystallization was
patterned into squares 10 to 1,000 .mu.m in edge length to obtain
island-like silicon film 603' as the active layer of the TFT, as shown in
FIG. 8(B).
A silicon oxide film 804 which functions as a gate insulator film was
formed. Here, the silicon film was exposed to an oxidizing atmosphere in
the temperature range of from 500.degree. to 750.degree. C., preferably in
the temperature range of from 550.degree. to 650.degree. C., to form a
silicon oxide film 604 which functions as a gate insulator film on the
surface of the silicon region. In this heat treatment step, the oxidation
reaction can be more enhanced by incorporating water vapor, nitrous oxide,
and the like into the atmosphere. As a matter of course, the silicon oxide
film can be formed by using any of the known means for vapor phase crystal
growth, such as plasma CVD and sputtering. This step is illustrated in
FIG. 6(C).
Subsequently, a polycrystalline silicon film containing from 0.01 to 0.2%
of phosphorus was deposited by reduced pressure CVD to a thickness of from
3,000 to 8,000 .ANG., specifically 6,000 .ANG.. A gate contact 605 was
formed thereafter by patterning the silicon film. Furthermore, an impurity
(phosphorus in the present Example), to render the active layer regions
(source/drain which constitute the channel) N-conductive, was added by ion
doping (plasma doping) in a self-aligned manner using the silicon film
above as a mask. In the present Example, the ion doping was performed
using phosphine (PH.sub.3) as the doping gas to introduce phosphorus at a
dose of 1.times.10.sup.15 to 8.times.10.sup.15 cm.sup.-2 specifically for
example, 5.times.10.sup.15 cm.sup.-2, and at an accelerated voltage of 60
to 90 kV. Thus were obtained N-type conductive impurity regions 606 and
607 for the source/drain regions.
Laser was then irradiated for annealing. Though a KrF excimer laser
operated at a wavelength of 248 nm and a pulse width of 20 nsec was used
in the present Example, other lasers can be used as well. The laser light
was irradiated at an energy density of 200 to 400 mJ/cm.sup.2, for example
250 mJ/cm.sup.2 and from 2 to 10 shots, for example 2 shots, per site. The
effect of laser annealing can be further enhanced by heating the substrate
in the temperature range of from 200.degree. to 450.degree. C. during the
irradiation of laser light. This is illustrated in FIG. 6(D).
Otherwise, this step can be carried out by a so-called RTA (rapid thermal
annealing) process, i.e., lamp annealing using near infrared light. Since
near infrared light can be more readily absorbed by a crystallized silicon
than by amorphous silicon, an effective annealing well comparable to
thermal annealing at temperatures not lower than 1,000.degree. C. can be
effected. More advantageously, near infrared light is less absorbed by
glass substrates. The fact is that a far infrared light is readily
absorbed, but a light in the visible to near infrared region, i.e., a
light in the wavelength region of 0.5 to 4 .mu.m, is hardly absorbed by a
glass substrate. Accordingly, the annealing can be completed within a
shorter period of time, and yet, without heating the substrate to a high
temperature. It can be seen that this method is most suitable for a step
in which the shrinking of the glass substrate is unfavorable.
A 6,000 .ANG. thick silicon oxide film 608 was deposited as an interlayer
insulator by plasma CVD. A polyimide film can be used in the place of
silicon oxide. Contact holes were perforated to form contacts with
connection 609 and 610 using a metallic material, for example, a
multilayered film of titanium nitride and aluminum. Finally, annealing was
performed at 350.degree. C. for a duration of 30 minutes under a pressure
of 1 atmosphere to obtain a complete TFT structure as shown in FIG. 6(E).
As described in the present Example, an amorphous silicon film can be more
favorably crystallized than in the case of simply heating by introducing
nickel as a catalytic element for the crystallization, and yet, the
crystallinity of the thus crystallized silicon film can be further
ameliorated by irradiating a laser light. In this manner, a crystalline
silicon having particularly high crystallinity can be obtained. The use of
the resulting crystalline silicon film of good crystallinity provides a
high performance TFT.
More specifically, an N-channel TFT obtained through the same process steps
as in the process of the present Example except for not employing the
crystallization step described in Example 2 yields a field-effect mobility
of from 50 to 90 cm.sup.2 /Vs, and a threshold voltage of from 3 to 8 V.
These values are in clear contrast to a mobility of from 150 to 200
cm.sup.2 /Vs and a threshold voltage of from 0.5 to 1.5 V obtained for the
N-channel TFT fabricated in accordance with the present Example. The
mobility is considerably increased, and the fluctuation in the threshold
voltage is greatly reduced.
Previously, the aforementioned TFT characteristics of such a high level
were obtained from amorphous silicon film only by laser crystallization.
However, the silicon films obtained by a prior art laser crystallization
yielded fluctuation in the characteristics. Furthermore, the
crystallization process required an irradiation of a laser light at an
energy density of 350 mJ/cm.sup.2 or higher at a temperature of
400.degree. C. or higher, and it was therefore not applicable to mass
production. In contrast to the conventional processes, the process for
fabricating a TFT according to the present Example can be performed at a
lower substrate temperature and at a lower energy density than the values
| | |
