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Claims  |
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What is claimed is:
1. A semiconductor device, comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein a first part of the plurality of thin film transistors has a
crystalline silicon film including crystals having a crystal growth
direction approximately in parallel to a surface of the substrate and a
second part of the plurality of thin film transistors has an amorphous
silicon film.
2. A semiconductor device according to claim 1 wherein each of the thin
film transistors formed on the substrate includes a source region, a
channel region, and a drain region and wherein said crystalline silicon
film extends through at least the channel region of each of said thin film
transistors of said first part of the plurality of thin film transistors.
3. A semiconductor device, comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein a first part of the plurality of thin film transistors is provided
as a peripheral circuit section of an active matrix type liquid crystal
display and a second part of the plurality of thin film transistors is
provided as a picture element section of the active matrix type liquid
crystal display, and
the thin film transistors provided as the peripheral circuit section have a
crystalline silicon film including crystals having a crystal growth
direction parallel to a surface of the substrate and the thin film
transistors provided as the picture element section have an amorphous
silicon film.
4. A semiconductor device according to claim 3 wherein each of the thin
film transistors formed on the substrate includes a source region, a
channel region, and a drain region and wherein said crystalline silicon
film extends through at least the channel region of each of said thin film
transistors of said first part of the plurality of thin film transistors.
5. An active matrix type liquid crystal display, comprising:
a picture element section having a plurality of thin film transistors
respectively associated with a plurality of picture element electrodes and
driving circuit means including a plurality of thin film transistors for
driving the thin film transistors of the picture element section,
wherein the thin film transistors of the driving circuit means each has a
substrate and has a crystalline silicon film including crystals having a
crystal growth direction approximately parallel to the surface of the
substrate and the thin film transistors of the picture element section
each has an amorphous silicon film.
6. An active matrix type liquid crystal display according to claim 5
wherein each of the thin film transistors of the driving circuit means
includes a source region, a channel region, and a drain region and wherein
said crystalline silicon film extends through at least said channel
region.
7. A semiconductor device comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a crystalline silicon
film including crystals having a crystal growth direction in parallel to
the substrate and at least another one of the thin film transistors has an
amorphous silicon film, and
wherein the crystalline silicon film includes a metal element.
8. The device of claim 7 wherein the metal element includes one of Ni, Fe,
Co, Pd and Pt.
9. A semiconductor device comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a crystalline silicon
film including crystals having a crystal growth direction in parallel to
the substrate and at least another one of the thin film transistors has an
amorphous silicon film, and
wherein the crystalline silicon film and the amorphous silicon film each
includes a metal element and a concentration of the metal element included
in the crystalline silicon film is higher than that included in the
amorphous silicon film.
10. The device of claim 9 wherein the metal element includes one of Ni, Fe,
Co, Pd and Pt.
11. A semiconductor device comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a crystalline silicon
film including crystals having a crystal growth direction in parallel to
the substrate and at least another one of the thin film transistors has an
amorphous silicon film, and
wherein the crystalline silicon film include a metal element and has a
concentrations of 1.times.10.sup.15 to 5.times.10.sup.19 atoms/cm.sup.3.
12. The device of claim 11 wherein the metal element includes one of Ni,
Fe, Co, Pd and Pt.
13. A semiconductor device comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a crystalline silicon
film including crystals having a crystal growth direction in parallel to
the substrate and at least another one of the thin film transistors has an
amorphous silicon film, and
wherein the crystalline silicon film and the amorphous silicon film each
includes a metal element, a concentration of the metal element included in
the crystalline silicon film is higher than that included in the amorphous
silicon film, and the concentration of the metal element included in
crystalline silicon film is 1.times.10.sup.15 to 5.times.10.sup.19
atoms/cm.sup.3.
14. The device of claim 13 wherein the metal element includes one of Ni,
Fe, Co, Pd and Pt.
15. A semiconductor device comprising:
a substrate having a glass strain point of 593.degree. C. or less; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a crystalline silicon
film including crystals having a crystal growth direction in parallel to
the substrate and at least another one of the thin film transistors has an
amorphous silicon film.
16. The device of claim 15 wherein the substrate includes Corning 7059.
17. A semiconductor device comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a crystalline silicon
film including crystals having a crystal growth direction which coincides
with a carrier moving direction and at least another one of the thin film
transistors has an amorphous silicon film.
18. A semiconductor device for an active matrix type liquid crystal display
having an peripheral driving circuit portion and a picture element
portion, comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors is provided in the
peripheral driving circuit portion and at least another one of the thin
film transistors is provided in the picture element portion, and
wherein the thin film transistor provided in the peripheral driving circuit
portion has a crystalline silicon film including crystals having a crystal
growth direction which coincides with a carrier moving direction and the
thin film transistors provided in the picture element portion has an
amorphous silicon film.
19. A semiconductor device comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a semiconductor film
which has a first crystallinity and includes crystals having a crystal
growth direction in parallel to the substrate and at least another one of
the thin film transistors has a semiconductor film having a second
crystallinity lower than the first crystallinity.
20. A semiconductor device comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a crystalline silicon
film including crystals having a crystal growth direction in parallel to
the substrate and at least another one of the thin film transistors has an
amorphous silicon film, and
wherein the crystalline silicon film and the amorphous silicon film each
includes hydrogen.
21. A semiconductor device comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a crystalline silicon
film including crystals having a crystal growth direction in parallel to
the substrate and at least another one of the thin film transistors has an
amorphous silicon film, and
wherein the crystalline silicon film includes nickel.
22. The device of claim 21 wherein the nickel has a concentration of
1.times.10.sup.15 to 5.times.10.sup.19 atoms/cm.sup.3.
23. A semiconductor device comprising:
a substrate; and
a plurality of thin film transistors formed on the substrate,
wherein at least one of the thin film transistors has a crystalline silicon
film including crystals having a crystal growth direction in parallel to
the substrate and at least another one of the thin film transistors has an
amorphous silicon film, and
wherein the crystalline silicon film and the amorphous silicon film each
includes nickel and a concentration of nickel included in the crystalline
silicon film is higher than that included in the amorphous silicon film.
24. The device of claim 23 wherein the nickel has a concentration of
1.times.10.sup.15 to 5.times.10.sup.19 atoms/cm.sup.3 in the crystalline
silicon film. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device using TFTs (thin
film transistor) mounted on an insulating substrate such as a glass and
more particularly to a semiconductor device utilizable for an active
matrix type liquid crystal display.
2. Description of the Related Art
A semiconductor device having TFTs on an insulating substrate such as a
glass is known to be utilized in an active matrix type liquid crystal
display, image sensor and the like using such TFTs for driving picture
elements.
Generally a thin film silicon semiconductor is used for the TFT used in
such devices. The thin film silicon semiconductor may be roughly
classified into two semiconductors; those composed of amorphous silicon
(a-Si) semiconductor and those composed of silicon Semiconductor having a
crystallinity. The amorphous silicon semiconductor is most generally used
because its fabrication temperature is low, it can be fabricated
relatively easily by a vapor phase method and it has a mass-producibility.
However, because it is inferior as compare to the silicon semiconductor
having a crystallinity in terms of physical properties such as an
electrical conductivity, it has been strongly demanded to establish a
method for fabricating a TFT composed of the silicon semiconductor having
a crystallinity to obtain a faster characteristic. By the way, as the
silicon semiconductor having a crystallinity, there are known to exist a
polycrystal silicon, microcrystal silicon, amorphous silicon containing
crystal components, semi-amorphous silicon having an intermediate state
between crystallinity and amorphousness.
The following method is known to obtain those thin film silicon
semiconductors having a crystallinity: (1) directly form a film having a
crystallinity, (2) form an amorphous semiconductor film and crystallize it
by energy of laser light, and (3) form an amorphous semiconductor film and
crystallize it by applying thermal energy.
However, it is technically difficult to form a film having favorable
physical properties of semiconductor on the whole surface of a substrate
by the method of (1). Further, it has a problem in terms of cost that
because its film forming temperature is so high as more than 600.degree.
C., a low cost glass substrate cannot be used. The method (2) has a
problem that its throughput is low because an irradiation area is small
when an eximer laser which is presently most generally used is used.
Further, the laser is not stable enough to homogeneously treat the whole
surface of a large area substrate. Accordingly, it is thought to be a next
generation technology. Although the method (3) has a merit that it allows
to accommodate with a large area as compare to the methods (1) and (2), it
is also necessary to apply such a high temperature as more than
600.degree. C. as the heating temperature. Accordingly, the heating
temperature needs to be reduced in a case of using a low cost glass
substrate. In particular, because the screen of present liquid crystal
display is enlarged more and more, a large size glass substrate needs to
be used accordingly. When such a large size glass substrate is used, its
contraction and strain caused during the heating process indispensable in
fabricating the semiconductor produce a large problem that they reduce an
accuracy of mask positioning and the like. In particular, because the
strain point of the 7059 glass which is presently most generally used is
593.degree. C., it deforms largely by the conventional heating
crystallization method. Further, beside the problems concerning to the
temperature, it takes more than tens of hours as the heating time required
for the crystallization in the conventional process, so that such time
needs to be shortened.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to solve the
aforementioned problems by providing a process which realizes both the
reduction of the temperature necessary for the crystallization and the
shortening of the heating time in a method for fabricating a thin film
composed of silicon semiconductor having a crystallinity using a method
for crystallizing a thin film composed of amorphous silicon by heating.
The silicon semiconductor having a crystallinity fabricated by using the
process of the present invention has physical properties equal to or
superior than those of the silicon semiconductor fabricated by the prior
art and is usable for an active layer region of TFTs.
The inventors of the present invention perform the following experiments
and study concerning to the method of forming a silicon semiconductor film
by a CVD method or sputtering method and crystallizing the film by heat.
When, after an amorphous silicon film is formed at first on a glass
substrate, a mechanism of crystallizing the film by heating is studied
through experiments, it is recognized that the crystal growth begins from
the interface between the glass substrate and the amorphous silicon and it
proceeded in a columnar shape vertical to the surface of the substrate if
the thickness of the film is more than a certain value.
The above phenomenon is considered to have been caused because crystal
nuclei (seeds) which would become bases of crystal growth existed at the
interface of the glass substrate and the amorphous silicon film and the
crystals grow from the nuclei. Such crystal nuclei are considered to have
been impurity metal elements and crystal components (crystal components of
silicon oxide is considered to be existing on the surface of glass
substrate as it is called as a crystallized glass) which existed on the
surface of the substrate in a very small amount.
Then the inventors consider that it is possible to lower the
crystallization temperature by positively introducing the crystal nuclei.
In order to confirm that effect, the inventors try experiments by forming
a film of a very small amount of another metal on the glass substrate,
forming a thin film composed of amorphous silicon thereon and then heating
and crystallizing it. As a result, it is confirmed that the
crystallization temperature reduces when several metals are formed on the
substrate and therefore it is presumed that crystal grows centering on
foreign materials as the crystal nuclei. Then the inventors study the
mechanism in more detail on the plurality of impurity metals for reducing;
the temperature. The plurality of impurity elements are Ni, Fe, Co, Pd and
Pt.
It can be considered that the crystallization has two stages; an initial
stage of producing a nucleus and a stage of crystal growth from the
nucleus. While the speed of the initial nucleus production stage may be
observed by measuring a time until a spot microcrystal is produced in a
fixed temperature, this time is shortened any time when an amorphous
silicon thin film is formed using the impurity metals as the base and the
effect of the introduction of the crystal nucleus on the lowering of the
crystallization temperature is verified. Further, unexpectedly, it is
observed that the speed of the crystal growth after the production of
nucleus also remarkably increases in the crystallization of the amorphous
silicon thin film formed on a certain metal after forming it when the
growth of the crystal grain after the production of nucleus is studied by
varying heating time. Although this mechanism is not clarified yet, it is
presumed that some catalytic effect is acting.
In any case, it is found that when the thin film composed of amorphous
silicon is formed on the film of a very small amount certain metal formed
on the glass substrate and is then heated and crystallized, a sufficient
crystallinity can be obtained due to the two effects described above at a
temperature less than 580.degree. C. and in about 4 hours which have been
impossible in the past. Nickel has the most remarkable effects among
impurity metals having such effect and is an element selected by the
inventors.
How nickel is effective will now be exemplified. Although more than 10
hours of heating time is necessary in crystallizing a thin film composed
of amorphous silicon formed by a plasma CVD method on a substrate (Coning
7059 glass), on which a thin film containing a very small amount of nickel
is not formed, by heating in a nitrogen atmosphere when the heating
temperature is 600.degree. C., the same crystallization state can be
obtained by heating at 580.degree. C. for about 4 hours when the thin film
composed of amorphous silicon formed on the substrate on which the thin
film containing a very small amount of nickel is formed is used. By the
way, Raman spectroscopic spectrum is used in the judgment (determination)
of the crystallization at this time. It can be seen that the effect of
nickel is very great even only from this fact.
As it is apparent from the above explanation, it is possible to lower the
crystallization temperature and to shorten the time required for the
crystallization when the thin film composed of amorphous silicon is formed
on the thin film of a very small amount of nickel. Now a more detailed
explanation will be made assuming that this process is used in fabricating
TFTs. By the way, the nickel thin film has the same effect even if it is
formed on the amorphous silicon film, not only on the substrate, and in
the case of ion implantation as described later in detail. Accordingly,
such a series of process shall be called as "nickel micro-adding".
Technically it is also possible to implement the nickel micro-adding
during when the amorphous silicon film is formed.
At first, the method of nickel micro-adding will be explained. The both of
the method of forming the thin film of a small amount of nickel on the
substrate and of forming the film of amorphous silicon thereafter and of
the method of forming the film of amorphous silicon at first and of
forming the thin film of a small amount of nickel thereon have the same
effect of lowering the temperature by adding a small amount of nickel.
Further, it is clarified that any of the methods of sputtering method,
vapor deposition method, spin coating method and a method using plasma may
be used in forming the film. However, when the thin film containing a
small amount of nickel is formed on the substrate, the effect is more
remarkable when a thin film (base film) of silicon oxide is formed on the
substrate and then the thin film of a small amount of nickel is formed on
the base film rather than when the thin film of a small amount of nickel
is formed directly on the 7059 glass substrate. The reason is that it is
important for the low temperature crystallization of the present invention
that silicon and nickel directly contact and that components other than
silicon may disturb the contact or reaction of the both in the case of the
7059 glass.
Further, as for the method of nickel micro-adding, it is confirmed that
almost the same effect can be obtained by adding (introducing) nickel by
ion implantation, other than the methods of forming the thin film
contacting above or under the amorphous silicon. For nickel, it is
confirmed that the temperature can be lowered when an amount of more than
1.times.10.sup.15 atoms/cm.sup.3 is added. However, because a shape of
peak of Raman spectroscopic spectrum becomes apparently different from
that of simple substance of silicon when the added amount is more than
5.times.10.sup.19 atoms/cm.sup.3, an actual usable range is considered to
be from 1.times.10.sup.15 atoms/cm.sup.3 to 1.times.10.sup.19
atoms/cm.sup.3. When the nickel concentration is 1.times.10.sup.15
atoms/cm.sup.3, the action as a catalyst for the crystallization
decreases. Further, when the concentration is more than 5.times.10.sup.19
atoms/cm.sup.3, NiSi is locally produced, losing the characteristics of
semiconductor. In the crystallized state, the lower the nickel
concentration, the more favorably the semiconductor may be used.
Next, the configuration of the crystal when the nickel micro-adding is
performed will be explained. It is known that when no nickel is added,
nuclei are produced at random from the crystal nuclei at the interface of
the substrate and the like, that the crystals grow at random from the
nuclei until a certain thickness and that columnar crystals in which (110)
direction is arranged in a direction vertical to the substrate generally
grow in a thicker thin film as described above and an almost uniform
crystal growth is observed across the whole thin film as a matter of
course. Contrary to that, when a small amount of nickel is added, the
crystal growth is different at a region into which the nickel is added and
at the surrounding section. That is, it is clarified through pictures of a
transmission electron beam microscope that in the region into which nickel
is added, the added nickel or a compound of nickel and silicon become the
crystal nucleus and columnar crystal grows almost vertical to the
substrate similarly to one into which no nickel is added. It is also
confirmed that the crystallization proceeds in a low temperature also in
the surrounding region where no nickel is added. A peculiar crystal growth
that the direction vertical to the substrate is arrayed in (111) in that
portion and needle or columnar crystal grows in parallel to the substrate,
is seen. It is observed that some large crystals among the crystals grown
in the lateral direction parallel to the substrate grow as long as several
hundreds micron from the region where a small amount of nickel is added
and is found that the growth increased in proportional to the increase of
time and rise of temperature. For example, a growth of about 40 micron is
observed in heating at 550.degree. C. for 4 hours. Further, it is
clarified that the large crystals in the lateral direction are all
single-crystal like according to pictures taken by the transmission
electron beam microscope. When the nickel concentration is examined at the
portion where a small amount of nickel is added, at the nearby lateral
growth portion and at the further distant amorphous portion (the low
temperature crystallization does not occur at the considerably distant
portion and the amorphous portion remains as it is) by SIMS (secondary ion
mass spectrometry), less amount of nickel by about 1 digit from the
portion where a small amount of nickel is added is detected from an amount
of the lateral growth portion and it is observed that it diffuses within
the amorphous silicon. Further less amount of nickel by about 1 digit is
observed in the amorphous portion. Although the relationship between this
fact and the crystal configuration is not clear yet, it is possible to
form a silicon thin film having a crystallinity of desired crystal
configuration at a desired section by controlling a nickel adding amount
and an adding position.
Next, electrical characteristics of the nickel micro-added portion where a
small amount of nickel is added and the nearby lateral growth portion will
be explained. Among the electrical characteristics of the nickel
micro-added portion, an electrical conductivity is almost the same with
the film into which no nickel is added, i.e. the film crystallized at
about 600.degree. C. for tens of hours. When an activation energy is found
from the temperature dependency of the electrical conductivity, no
behavior considered to have been caused by the level of nickel is observed
when the nickel added amount is 10.sup.17 atoms/cm.sup.3 to 10.sup.18
atoms/cm.sup.3. As far as this fact is concerned, it can be concluded that
there is no problem in the operation of TFT if the nickel concentration
within the film used in an active layer of TFT and others is less than
around 10.sup.18 atoms/cm.sup.3.
Contrary to that, the electrical conductivity of the lateral growth portion
is higher than that of the nickel micro-added portion by more than 1
digit, which is considerably high for a silicon semiconductor having a
crystallinity. This fact is considered to have been caused by that less or
almost no crystal boundaries existed between electrodes where electrons
pass through because the current passing direction and the crystal lateral
growth direction coincides; it coincides with the result of the pictures
of the transmission electron beam microscope without contradiction. That
is, it coincides with the observation fact that the needle or columnar
crystals grow in the direction parallel to the substrate.
Here, based on the various characteristics described above, an applying
method for a TFT will be explained. As an application field of the TFT, an
active type liquid crystal display in which TFTs are used for driving
picture elements will be assumed here.
While it is important to suppress a contraction of the glass substrate in
the late large screen active type liquid crystal display as described
above, the use of the nickel micro-adding process of the present invention
allows to crystallize at a fully lower temperature as compare to the
strain point of glass and is especially suitable. The present invention
allows to replace a conventionally used amorphous silicon with silicon
having a crystallinity by adding a small amount of nickel and by
crystallizing in about 450.degree. to 550.degree. C. for about 4 hours.
Although it may be necessary to change design rules and others
corresponding to that, it can be fully accommodated with the conventional
equipments and process and its merit is considered to be great.
Furthermore, the present invention allows to form TFTs used for picture
elements and those forming the drivers of the peripheral circuit
separately utilizing the crystal configurations corresponding to each
characteristic and hence is useful when it is applied especially for the
active type liquid crystal display. That is, the TFTs used for the picture
element are not required to have so much mobility and rather than that,
there is more merit for the off current to be smaller. Then the present
invention allows to remain the region which is to become the TFT used for
the picture element as amorphous without performing the nickel
micro-adding and to grow a crystalline silicon film on the region for
forming the driver of the peripheral circuit by adding a small amount of
nickel. That is, although so much mobility is not required in the TFT
formed in the picture element section, the off current needs to be reduced
to improve the yield and to hold electric charge. Accordingly, it is
useful to use TFTs using an amorphous silicon film whose production
technology has been accumulated from the past and whose characteristics
can be readily controlled in the picture element section. On the other
hand, considering to apply the liquid crystal display for a workstation
for the future, a very high mobility is required for the TFTs structuring
the peripheral circuit. Then it is effective to fabricate TFTs having a
very high mobility by adding a small amount of nickel near the TFTs which
form the drivers of the peripheral circuit to grow crystals in one
direction (growth in lateral direction) from there and to cause the
crystal growth direction to coincide with the current passing direction
(direction into which carriers move).
That is, an object of the present invention is to provide the region in
which the silicon semiconductor film is selectively crystallized and the
region left as being amorphous to selectively fabricate TFTs satisfying
necessary characteristics on a substrate in a semiconductor device in
which a large number of thin film transistors (generally called as TFT)
are formed on the substrate such as a glass substrate. Another object of
the present invention is to selectively provide the TFT having a higher
mobility by causing the crystal growth direction to coincide with the
carrier moving direction within the TFT by paralleling it with the
substrate.
According to the present invention, the TFT using a crystalline silicon
film and the TFT using an amorphous silicon film may be selectively
obtained at each necessary region by selectively obtaining the TFT having
a high mobility by causing the direction into which carriers move during
operation of the TFT to approximately coincide with the crystal growth
direction and by selectively providing the TFT using the amorphous silicon
film in the other region.
As described above, the crystal growth direction may be freely selected in
the direction either vertical to the substrate or parallel to the
substrate by adding a small amount of nickel. Further, the relationship of
the direction into which carriers flow during operation of the TFT and the
crystal growth direction may be determined selected by selecting a
direction (direction connecting a source and drain) and position of the
TFT to be formed. The direction into which carriers flow described above
is the direction connecting the source and drain when an insulated gate
type field effect semiconductor device is used for example as the TFT.
The present invention may be used for an active matrix type liquid crystal
display. Further, the TFT having a high mobility may be obtained by using
the crystalline silicon film whose crystal has grown in the direction
parallel to the surface of substrate.
Further, the present invention relates to a fabrication process for
obtaining such TFTs as described above. The present invention utilizes a
technology for selectively providing crystallized regions by adding a
small amount nickel.
Furthermore, the present invention is characterized in that the TFTs using
the crystalline silicon film are formed at the peripheral circuit section
of the liquid crystal display and that the crystal of the crystalline
silicon film composing the TFT has been grown approximately in the same
direction with the moving direction of carriers within the TFT. Further,
the TFTs composing the picture element section of the liquid crystal
display is structured in the same time using the amorphous silicon film.
The selective formation of the crystalline silicon film and amorphous
silicon film on the same substrate may be realized by selectively adding
nickel. That is, because the temperature required for the crystallization
can be less than 550.degree. C. in the region where nickel micro-adding is
performed, the amorphous region which does not crystallize at 550.degree.
C. (although it is considered to crystallize in more than hundreds of
hours, it will not crystallize at 550.degree. C. in several hours) can be
left in the region where no nickel is added.
Although it is typically useful to use nickel as a small amount of metal
element for promoting the crystallization, the similar effect can be
obtained even by cobalt, iron and platinum in the present invention.
Further, although a kind of substrate is not specifically limited, the
usefulness of the present invention that the crystalline silicon film can
be obtained in a low temperature less than 600.degree. C. as compare to
the conventional method become remarkable when it is used for a glass
substrate and particularly for a large area glass substrate.
While the crystalline silicon film may be thus obtained by selectively
crystallizing it, the characteristics of such crystalline silicon film may
be improved further by irradiating laser or an equivalent strong light
after the crystallization process. That is, insufficiently crystallized
components left at the crystal boundaries and others may be crystallized
due to that. By the way, it is necessary for the region in which the TFT
using the amorphous silicon film is formed not to be irradiated by such
strong light, because the amorphous silicon is crystallized by the
irradiation of such strong light.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a drawing showing a schematic construction of a liquid crystal
display according to an embodiment of the present invention;
FIGS. 2A through 2D are drawings showing a process for fabricating a
circuit in which NTFT and PTFT which compose a peripheral circuit section
of the liquid crystal display are formed complementarily according to the
embodiment of the present invention;
FIG. 3 is a drawing showing the configuration shown in FIG. 2D seen from
the above;
FIGS. 4A through 4D are drawings showing a process for fabricating a NTFT
formed in a picture element section in the liquid crystal display
according to the embodiment of the present invention;
FIGS. 5A through 5E are drawings showing a process for fabricating TFT
circuits in the peripheral circuit section and picture element section in
the liquid crystal display according to another embodiment of the present
invention; and
FIGS. 6A and 6B are SEM pictures around the distal end of crystallized
region of a silicon film crystallized by a growth in a lateral direction
in the fabricated TFT.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to the drawings, preferred embodiments of the present
invention will be explained.
First Embodiment
FIG. 1 is a top plan view showing a construction of a liquid crystal
display of the embodiment of the present invention in outline, wherein a
picture element section 10 having a plurality of picture element
electrodes provided in matrix (not shown) and a peripheral circuit section
20 as a driving circuit for driving each of the picture element electrodes
are shown. According to the present embodiment, thin film transistors
(TFT) for driving the picture elements and those composing the peripheral
circuit are formed on an insulated substrate (e.g. a glass substrate). In
concrete, the peripheral circuit section is a circuit structured as a CMOS
in which P channel type TFTs (PTFT) and N channel type TFTs (NTFT) using
silicon films having a crystallinity grown in the lateral direction
(called as a crystalline silicon film) are provided complementarily and
the picture element section is TFTs formed as NTFT using amorphous silicon
films.
FIGS. 2A to 2D are drawings showing a process for fabricating the circuit
in which the NTFT and PTFT structuring the peripheral circuit section 20
are formed complementarily. FIGS. 4A to 4D described later are drawings
showing a process for fabricating the NTFT formed on the picture element
section. Because the both fabricating processes are performed on the same
substrate, common processes are executed simultaneously. That is, the
processes shown in FIGS. 2A to 2D and those shown in FIGS. 4 A to 4D
correspond each other, so that they are carried out in the same time,
respectively.
At first, a silicon oxide base film 102 having a thickness of 2000 angstrom
is form on a glass substrate (Coning 7059) 101 by a sputtering method. A
mask 103 formed by a metal mask or silicon oxide film is provided only on
the peripheral circuit section 20 as shown in FIG. 2A. By the way, because
nickel introduced in a later process easily diffuses also within the
silicon oxide film, a thickness of more than 1000 angstrom is necessary
when the silicon oxide film is used as the mask 103. The base film 102 is
exposed in a slit shape by the mask 103. That is, seeing the state of FIG.
2A from above, the base film 102 is exposed in the slit shape by a slit
shape region 100 while the other region is masked. The mask 103 is covered
on the whole surface of the picture element section 10 shown in FIG. 4A
and the base film 102 is masked by the mask 103.
After providing the mask 103, a nickel silicide film (chemical formula:
NiSi.sub.x, 0.4.ltoreq..times.2.5,.times.=2.0 for example) having a
thickness of 5 to 200 angstrom, e.g. 20 angstrom, is formed by a
sputtering method. As a result, the nickel silicide film is formed over
the whole area of the peripheral circuit section 20 and the picture
element section 10. After that, the mask 103 is removed to selectively
form the nickel silicide film only on the region 100. That is, it means
that the nickel micro-adding has been selectively made on the region 100.
Next, after removing the mask 103, an intrinsic (I type) amorphous silicon
film 104 having a thickness of 500 to 1500 angstrom, e.g. 1000 angstrom,
is deposited by a plasma CVD method. After that, it is crystallized by
annealing for 4 hours at 550.degree. C. under a hydrogen reducing
atmosphere (preferably a partial pressure of hydrogen is 0.1 to 1
atmospheric pressure). Although the annealing temperature may be selected
within a range of about 450.degree. C. to 700.degree. C., a preferably
temperature range is 450.degree. C. to 550.degree. because it takes time
for the annealing if the annealing temperature is low and the same result
as that in the prior art is obtained about if the temperature is high. By
the way, this annealing may be carried out in an inactive atmosphere (e.g.
a nitrogen atmosphere) or air.
The silicon film 104 is crystallized in a direction vertical to the
substrate 101 in the region 100 where the nickel silicide film has been
selectively formed. On the other hand, crystal grows in a lateral
direction (direction parallel to the substrate) from the region 100 as
shown by arrow 105 in the peripheral region of the region 100. The
amorphous silicon film is left at the picture element section 10 (see FIG.
4B) where the mask 103 is provided, because the amorphous silicon film
will not crystallize by such annealing of four hours at 550.degree. C. By
the way, a distance of crystal growth in the direction shown by the arrow
105 which is parallel to the substrate 101 is about 40 micron.
The amorphous silicon film at the peripheral circuit section 20 may be
crystallized by the process described above. Here, the crystal grows in
the lateral direction (direction parallel to the substrate 101) as shown
in FIG. 2 B in the peripheral circuit section 20 and the amorphous silicon
film remains without being crystallized in the picture element section 10.
After that, TFTs are separated between the elements and the silicon film
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