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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. By using this
technique, TFTs having necessary characteristics can be formed selectively
on a substrate.
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
proceeds in a columnar shape in 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 are 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 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 of a 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 effects 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 (Corning
7059 glass), on which a thin film containing a very small amount of nickel
is not formed, by heating in a nitrogen atmosphere at 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 (lower
side of amorphous silicon film), and in the case of ion implantation or
plasma treatment as described later in detail. Accordingly, such a series
of process shall be called as "nickel micro-adding". Technically it is
also possible to perform the nickel micro-adding during when the amorphous
silicon film is formed.
At first, the method of nickel micro-adding will be explained. The method
of forming the thin film of a small amount of nickel on the substrate and
forming the film of amorphous silicon thereafter and the method of forming
the film of amorphous silicon at first and 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. It is important for the low temperature
crystallization of the present invention that silicon and nickel directly
contact and it is considered 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 less than
1.times.10.sup.15 atoms/cm.sup.3, the action as a catalyst such as nickel
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 it is found that the growth increases 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) 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 crystallizes 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
(carriers) pass through because the current passing direction and the
crystal lateral growth direction coincide; 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 thermally
annealing 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 matrix type liquid crystal display. That is, the TFTs used for the
picture element in the active matrix type liquid crystal display are not
required to have so much mobility and rather than that, there is more
merit for the off current to be smaller. Then in the present invention, by
directly performing the nickel micro-adding to the region which is to
become the TFTs used for the picture element, it is possible to reduce an
off current by growing crystals in a direction vertical to a surface of
the substrate and by forming a number of crystal boundaries in a channel
direction (a direction when a source region and a drain region are
connected each other by a line). 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 into which carriers move, that
is, direction when a source region and a drain region are connected each
other by a line).
That is, an object of the present invention is to provide a crystalline
silicon semiconductor film constituting desired TFTs which a crystal
growth direction is controlled, to selectively fabricate TFTs satisfying
necessary characteristics on a substrate in a semiconductor device in
which a large number of thin film transistors are formed on the substrate
such as a glass substrate.
The feature of the present invention is, in an active matrix type liquid
crystal display having a peripheral circuit portion and a pixel element
portion, to provide TFTs having a crystalline silicon film crystal-grown
in a direction vertical to a substrate in the pixel element portion and
TFTs having a crystalline silicon film crystal-grown in a direction
parallel to a substrate in the peripheral circuit portion. In the pixel
element portion, by using a crystalline silicon film crystal-grown in a
direction vertical to a substrate, a structure that carriers moving
between a source and a drain cross crystal boundaries can be obtained so
that the off current is low in TFTs. On the other hand, in the peripheral
circuit portion, TFTs having a high mobility (that is, a large on current)
can be obtained by forming the source and drain in parallel to the crystal
growth direction. In operation of TFTs, since the carriers flow between
the source and drain, possibility which the carriers cross the crystal
boundaries becomes low by forming the source and drain in crystal growth
direction, therefore, resistance to the carriers can be reduced.
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 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.
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 in a direction vertical or parallel to a surface of a
substrate, 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. In this process, characteristics of both
silicon films crystal-grown in vertical or parallel to the surface of the
substrate can be improved.
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
(TFTs) 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 TFT (PTFT) and N channel type TFT (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 silicon films
having a crystallinity grown in the longitudinal direction.
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. 4A 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 formed on a glass substrate (Corning 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..ltoreq.2.5, x=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. In the
silicon film 104 in the picture element section 10 (see FIG. 4B) where
nickel silicide film is formed, crystal grows in a direction vertical to
the substrate 101. 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 in the above crystal growth.
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) shown in
FIG. 2B in the peripheral circuit section 20 and the crystal grows in a
direction vertical to the substrate 101 in the picture element section 10,
as shown in FIG. 4B.
After that, TFTs are separated between the elements, and the silicon film
104 at unnecessary part is removed to form an island-shape element
regions. In this process, if a length of an active layer of the TFT
(source/drain regions and channel forming region) is within 40 micron, the
source/drain regions and channel forming region in the peripheral circuit
portion may be structured by the crystalline silicon film grown in the
direction parallel to the substrate 101. Further, if the channel forming
region is structured by the crystalline silicon film, the length of the
active layer may be prolonged further.
Then a silicon oxide film 106 having thickness of 1000 angstrom is formed
as a gate insulating film by a sputtering method. Silicon oxide is used as
a target in the sputtering. A temperature of the substrate during the
sputtering is 200.degree. to 400.degree. C., e.g. 350.degree. C. Oxygen
and argon are used as an atmosphere of the sputtering and a ratio of the
argon/oxygen=0 to 0.5, e.g. less than 0.1. Following to that, an aluminum
film (containing silicon by 0.1 to 2%) having a thickness of 6000 to 8000
angstrom, e.g. 6000 angstrom, is formed by a sputtering method. By the
way, it is desirable to consecutively carry out the processes for forming
the silicon oxide film 106 and aluminum film.
Gate electrodes 107 and 109 are formed by patterning the formed aluminum
film. As mentioned above, the process shown in FIG. 2C and that shown in
FIG. 4C are carried out simultaneously.
The surface of the gate electrodes 107 and 109 is anodized to form oxide
layers 108 and 110 on the surface ther | | |
