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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for manufacturing a
semiconductor device having a crystalline semiconductor.
2. Prior Art
Thin film transistors (referred to hereinafter as "TFTs") are well known
and are widely used in various types of integrated circuits or an
electro-optical device, and particularly used for switching elements
provided to each of pixels of an active matrix liquid crystal display
device as well as in driver elements of the peripheral circuits thereof.
An amorphous silicon film can be readily utilized as a thin film
semiconductor of a TFT. However, electric characteristics of the amorphous
silicon film are poor. A silicon film having a crystallinity may be used
for solving this problem. The silicon film having a crystallinity is, for
example, polycrystalline silicon, polysilicon, and microcrystalline
silicon. The crystalline silicon film can be prepared by first forming an
amorphous silicon film, and then heat treating the resulting film for
crystallization.
The heat treatment for the crystallization of the amorphous silicon film
requires heating the film at a temperature of 600.degree. C. or higher for
a duration of 10 hours or longer. Such a heat treatment is detrimental for
a glass substrate. For instance, a Corning 7059 glass commonly used for
the substrate of active matrix liquid crystal display devices has a glass
distortion point of 593.degree. C., and is therefore not suitable for
large area substrates that are subjected to heating at a temperature of
600.degree. C. or higher.
According to the study of the present inventors, it was found that the
crystallization of an amorphous silicon film can be effected by heating
the film at 550.degree. C. for a duration of about 4 hours. This can be
accomplished by disposing a trace amount of nickel or palladium, or other
elements such as lead, onto the surface of the amorphous silicon film.
The foregoing elements which promote crystallization can be introduced into
the surface of the amorphous silicon film by depositing the elements by
plasma treatment or vapor deposition, or by incorporating the elements by
ion implantation. The plasma treatment more specifically comprises adding
the catalyst elements into the amorphous silicon film by generating a
plasma in an atmosphere such as gaseous hydrogen or nitrogen using an
electrode containing catalyst elements therein in a plasma CVD apparatus
of a parallel plate type or positive columnar type.
However, the presence of the foregoing elements in a large quantity in the
semiconductor is not preferred, because the use of such semiconductors
greatly impairs the reliability and the electric stability of the device
in which the semiconductor is used.
That is, the foregoing elements such as nickel which promotes a
crystallization (in this invention, these elements will be called as
catalyst elements hereinbelow) are necessary in the crystallization of the
amorphous silicon film, but are preferably not incorporated in the
crystallized silicon. These conflicting requirements can be accomplished
by selecting an element which is more inactive in crystalline silicon as
the catalyst element, and by incorporating the catalyst element at a
minimum amount possible for the crystallization of the film. Accordingly,
the quantity of the catalyst element to be incorporated in the film must
be controlled with high precision.
The crystallization process using nickel or the like was studied in detail.
The following findings were obtained as a result:
(1) In case of incorporating nickel by a plasma treatment into an amorphous
silicon film, nickel is found to penetrate into the film to a considerable
depth of the amorphous silicon film before subjecting the film to a heat
treatment;
(2) The initial nucleation occurs from the surface from which nickel is
incorporated: and
(3) When a nickel layer is deposited on the amorphous silicon film by
evaporation, the crystallization of an amorphous silicon film occurs in
the same manner as in the case of effecting plasma treatment.
In view of the foregoing, it is assumed that not all of the nickel
introduced by the plasma treatment functions to promote the
crystallization of silicon. That is, if a large amount of nickel is
introduced, there exists an excess amount of the nickel which does not
function effectively. For this reason, the inventors consider that it is a
point or face at which the nickel contacts the silicon that functions to
promote the crystallization of the silicon at lower temperatures. Further,
it is assumed that the nickel has to be dispersed in the silicon in the
form of atoms. Namely, it is assumed that nickel needs to be dispersed in
the vicinity of a surface of an amorphous silicon film in the form of
atoms, and the concentration of the nickel should be as small as possible
but within a range which is sufficiently high to promote the low
temperature crystallization.
A trace amount of nickel i.e., a catalyst element capable of promoting the
crystallization of the amorphous silicon, can be incorporated in the
vicinity of the surface of the amorphous silicon film by, for example,
vapor deposition. However, vapor deposition is disadvantageous concerning
the controllability, and is therefore not suitable for precisely
controlling the amount of the catalyst element to be incorporated in the
amorphous silicon film.
Also, it is required to minimize the amount of the catalyst element but
there occurs a problem that a crystallization does not occur sufficiently.
SUMMARY OF THE INVENTION
It is an object of the present invention to manufacture a crystalline thin
film silicon semiconductor device through a heat process at less than
600.degree. C. using a catalyst element, wherein (1) the amount of the
catalyst element to be introduced is controlled and minimized, (2) the
productivity is increased, and (3) the crystallinity is improved as
compared with the case in which a heat treatment is applied.
In order to achieve the foregoing objects, the method of the present
invention comprises the steps of disposing a catalyst element for
promoting a crystallization or a compound containing the catalyst element
in contact with an amorphous silicon film, heat treating the amorphous
silicon film with the catalyst element or the compound kept in contact
with the silicon film in order to crystallize a part or a whole of the
film, and further improving or enlarging the crystallinity by irradiating
the film with a laser light or a light having an equivalent strength
thereto. Thus, a silicon film having an excellent crystallinity is formed.
As a method for introducing the catalyst element, it is advantageous to
coat an amorphous silicon film with a solution containing the catalyst
element. In particular, in accordance with the present invention, the
solution should contact the surface of the silicon film in order that the
amount of the catalyst element to be introduced should be accurately
controlled.
The catalyst may be introduced from either upper or lower surface of the
amorphous silicon film. Namely, in the former case, a catalyst containing
solution may be applied onto the amorphous silicon film after the
formation thereof. Also, in the latter case, the solution may be applied
first onto a base surface and then the amorphous silicon film is formed
thereon.
By utilizing the silicon film having a crystallinity thus formed, it is
possible to form an active region including therein at least one electric
junction such as PN, PI or NI junction. Examples of semiconductor devices
are thin film transistors (TFT), diodes, photo sensor, etc.
The foregoing construction of the present invention has the following basic
advantages:
(a) The concentration of the catalyst element in the solution can be
accurately controlled in advance, and it is possible to improve the
crystallinity while the amount of the catalyst element can be minimized;
(b) The amount of the catalyst element incorporated into the amorphous
silicon film can be determined by the concentration of the catalyst
element in the solution so long as the surface of the amorphous silicon
film is in contact with the solution;
(c) The catalyst element can be incorporated at a minimum concentration
necessary for the crystallization into the amorphous silicon film, because
the catalyst element adsorbed by the surface of the amorphous silicon film
principally contributes to the crystallization of the film; and
(d) A crystalline silicon having an excellent crystallinity can be obtained
without a high temperature process.
As a solution, various aqueous solutions and organic solvent solutions can
be used in the present invention. The word "including" or "containing"
mentioned in the present specification may be understood as either (a)
that the catalytic element is simply dispersed in a solution or (b) that
the catalytic element is contained in a solution in a form of a compound.
As a solvent, it is possible to use water, alcohol, acid, or ammonium which
are polar solvent.
Examples of nickel compounds which are suitable for the polar solvent are
nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel
chloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate,
nickel acetyl acetonate, 4-cyclohexyl butyric acid, nickel oxide and
nickel hydroxide.
Also, benzene, toluene, xylene, carbon tetrachloride, chloroform or ether
can be used as a non-polar solvent. Examples of nickel compounds suitable
for a non-polar solvent are nickel acetyl acetonate and 2-ethyl hexanoic
acid nickel.
Further, it is possible to add an interfacial active agent to a solution
containing a catalytic element. By doing so, the solution can be adhered
to and adsorbed by a surface at a higher efficiency. The interfacial
active agent may be coated on the surface in advance of coating the
solution.
Also, when using an elemental nickel (metal), it is necessary to use an
acid to dissolve it.
While nickel is completely dissolved in the above listed solutions, it is
possible to use a material such as an emulsion in which elemental nickel
or nickel compound is dispersed uniformly in a dispersion medium.
Furthermore, it is possible to use a solution which is used for forming an
oxide film. An example of such a solution is OCD (Ohka Diffusion Source)
manufactured by Tokyo Ohka Co. Ltd. In this case, after forming the OCD on
a surface, a silicon oxide film can be easily formed simply by baking at
about 200.degree. C. Also, an impurity can be added arbitrarily.
The foregoing explanations apply to the case of using a material other than
nickel as the catalyst element.
When using a polar solvent such as water for dissolving nickel, it is
likely that an amorphous silicon film repels such a solution. In such a
case, a thin oxide film is preferably formed on the amorphous silicon film
so that the solution can be provided thereon uniformly. The thickness of
the oxide film is preferably 100 .ANG. or less. Also, it is possible to
add an interfacial active agent to the solution in order to increase a
wetting property.
Also, when using a non-polar solvent such as toluene for obtaining a
solution of 2-ethyl hexanoic acid nickel, the solution can be directly
formed on the surface of an amorphous silicon film. However, it is
possible to interpose between the amorphous silicon film and the solution
a material for increasing the adhesivity therebetween which is used to
increase adhesivity of a resist. However, if the amount of the coating of
this material is too much, it would prevent the catalyst element from
being introduced into the amorphous silicon film.
The concentration of the catalyst element in the solution depends on the
kind of the solution, however, roughly speaking, the concentration of the
catalyst element such as nickel by weight in the solution is 1 ppm to 200
ppm, and preferably, 1 ppm to 50 ppm. The concentration is determined
based on the nickel concentration in the silicon film or the resistance
against hydrofluoric acid of the film after the completion of the
crystallization.
It is possible to further improve the crystallinity of the silicon film by
irradiating the silicon film with a laser light after the heat
crystallization thereof. Also, if the crystallization is caused to a
portion of the silicon film by the heat treatment, it is possible to
expand the crystallization therefrom by means of a laser light, resulting
in achieving a higher crystallinity.
For example, when the amount of the catalyst element is small, the
crystallization locally occurs at small spot regions of the silicon film.
This condition can be regarded as a mixture of a crystalline component and
an amorphous component. By using the laser light in this condition,
crystal growths occur from the crystal nuclei which exist in the
crystalline component and thus it is possible to obtain a higher
crystallinity. In other words, small crystal grains are grown into larger
crystal grains. The effect of the use of the laser irradiation is more
apparent with respect to a silicon film of which crystallinity is
incomplete.
Also, in place of laser light, it is possible to use other intense lights
such as an infrared ray. The IR ray is difficult to be absorbed by glass
while it is easy to be absorbed by a silicon thin film. Therefore, the use
of IR light is advantageous for selectively heating a silicon film formed
on a glass substrate. The method in which this IR ray is used is called
rapid thermal annealing (RTA) or rapid thermal process (RTP).
The crystal growth can be controlled by applying the solution containing
the catalyst element to a selected portion of the amorphous silicon film.
In particular, the crystals grow within the silicon film by heating, the
silicon film in a direction approximately parallel with the plane of the
silicon film from the region onto which the solution is directly applied
toward the region onto which the solution is not applied. The region in
which the crystals grow in this manner will be referred to in the present
invention as a lateral crystal growth region or simply as a lateral growth
region.
It is also confirmed that this lateral growth region contains the catalyst
element at a lower concentration. While it is useful to utilize a
crystalline silicon film as an active layer region of a semiconductor
device, in general, the concentration of the impurity in the active region
should be reduced as much as possible. Accordingly, the use of the lateral
growth region for the active layer region is useful in a device
fabrication.
The use of nickel as the catalyst element is found most effective in the
process according to the present invention. However, other catalyst
elements may be used in place of nickel, for example, palladium (Pd),
platinum (Pt), copper (Cu), silver (Ag), gold (Au), indium (In), tin (Sn),
phosphorus (P), arsenic (As), and antimony (Sb). Otherwise, the catalyst
element may be at least one selected from the elements belonging to the
Group VIII, IIIb, IVb, and Vb of the periodic table.
Also, the solution for introducing the catalyst element should not be
limited to water solution or alcohol solution. Various materials may be
used which contains the catalyst element. For example, a metal compound or
an acid which contains a catalyst element may be used.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other features of the present invention will b e
described in further detail in the preferred embodiments of the present
invention with reference to the attached drawings in which:
FIGS. 1A-1D show a method for forming a crystalline silicon film in
accordance with EXAMPLE 1 of the invention;
FIGS. 2A-2C show a method for forming a crystalline silicon film in
accordance with EXAMPLE 2 of the invention;
FIGS. 3A-3E show a method for manufacturing a TFT in accordance with
EXAMPLE 3 of the invention;
FIGS. 4A-4F show a method for manufacturing a TFT in accordance with
EXAMPLE 4 of the invention;
FIGS. 5A-5D show a method for manufacturing a TFT in accordance with
EXAMPLE 5 of the invention;
FIGS. 6A-6F show a method for manufacturing a TFT in accordance with
EXAMPLE 6 of the invention; and
FIG. 7 show a block diagram of an electro-optical device in accordance with
EXAMPLE 6 of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
EXAMPLE 1
In this example, a catalyst element contained in a water solution is
applied onto an amorphous silicon, following which it is crystallized by
heating and further by laser irradiation.
Referring to FIGS. 1A-1D, the process for incorporating a catalyst element
(nickel In this case) into the amorphous silicon film is described below.
A Corning, 7059 glass substrate of 100 mm.times.100 mm in size is used
Initially, an amorphous silicon film of 100 to 1,500 .ANG. in thickness is
deposited by plasma CVD or LPCVD. More specifically in this case, an
amorphous silicon film 12 is deposited at a thickness of 1,000 .ANG. by
plasma CVD (FIG. 1A).
Then, the amorphous silicon film is subjected to hydrofluoric acid
treatment to remove contaminants and a natural oxide formed thereon. This
treatment is followed by the deposition of an oxide film 13 on the
amorphous silicon film to a thickness of 10 to 50 .ANG.. A natural oxide
film may be utilized as the oxide film instead of the oxide film 13 if the
contaminants can be disregarded.
The precise thickness of the oxide film 13 is not available because the
film is extremely thin. However, it is presumed to be about 20 .ANG. in
thickness. The oxide film 13 is formed by irradiating an ultraviolet (UV)
radiation in an oxygen atmosphere for a duration of 5 minutes. The oxide
film 13 can be formed otherwise by thermal oxidation. Furthermore, the
oxide film can be formed by a treatment using aqueous hydrogen peroxide.
The oxide film 13 is provided with an aim to fully spread the acetate
solution containing nickel, which is to be applied in the later step, on
the entire surface of the amorphous silicon film. More briefly, the oxide
film 13 is provided for improving the wettability of the amorphous silicon
film. If the aqueous acetate solution were to be applied directly, for
instance, the amorphous silicon film would repel the aqueous acetate
solution and prevent nickel from being incorporated uniformly into the
surface of the amorphous silicon film. This means that a uniform
crystallization can not be done.
An acetate solution containing, nickel added therein is prepared
thereafter. More specifically, an acetate solution containing nickel at a
concentration of 25 ppm, is prepared. Two milliliters of the acetate
solution is dropped to the surface of the oxide film 13 on the amorphous
silicon film 12, and is maintained as it is for a duration of 5 minutes.
Then, spin drying at 2,000 rpm using a spinner is effected for 60 seconds.
(FIGS. 1C and 1D).
The concentration of nickel in the acetate solution is practically 1 ppm or
more, preferably, 10 ppm or higher. In case of using a nonpolar solvent
such as toluene for obtaining a solution of 2-ethyl hexanoic acid nickel,
the oxide 13 is unnecessary and the solution can be directly formed on the
amorphous silicon Film.
The coating of the solution is carried out at one time or may be repeated,
thereby, it is possible to form a film containing nickel on the surface of
the amorphous silicon film 12 uniformly to a thickness of several
angstroms to several hundreds angstroms after the spin dry. The nickel
contained in this film will diffuse into the amorphous silicon film during
a heating process carried out later and will function to promote the
crystallization of the amorphous silicon film. By the way, it is the
inventors, intention that the film containing nickel or other catalyst
elements do not necessarily have to be in the form of a completely
continuous film.
The amorphous silicon film coated with the above solution is kept as it is
thereafter for a duration of 1 minute. The concentration of nickel
catalyst element in the crystallized silicon film 12 can be controlled by
changing this retention time, however, the most influencing factor in
controlling the concentration is the concentration of the nickel catalyst
element in the solution.
The silicon film coated with a nickel-containing solution thus obtained is
subjected to a heat treatment at a temperature of 550.degree. C. for a
duration of 4 hours in a nitrogen atmosphere in a heating furnace. Thus, a
thin film of crystalline silicon 12 is formed on the substrate 11.
The heat treatment can be effected at a temperature of 450.degree. C. or
higher. If a low temperature is selected, however, the heat treatment
would consume much time and result in a poor production efficiency. If a
heat treatment temperature of 550.degree. C. or higher were to be
selected, on the other hand, the problem of heat resistance of the glass
substrate must be considered.
Also, it is possible to form the solution containing the catalyst element
on a substrate surface prior to the formation of the amorphous silicon
film as said before.
After the crystallization by heating, the crystallinity of the silicon film
12 is further improved by irradiating the film with several shots of a KrF
excimer laser (wavelength: 248 nm, pulse width: 30 nsec) in a nitrogen
atmosphere with a power density of 200-350 mJ/cm.sup.2. An IR ray may be
used instead of the laser as said before.
EXAMPLE 2
The present example relates to a process similar to that described in
Example 1, except that a silicon oxide film 1,200 .ANG. in thickness is
provided selectively as a mask to incorporate nickel into selected regions
of the amorphous silicon film.
Referring to FIG. 2A, a silicon oxide film 21 to be used as a mask is
formed on a glass substrate (Corning 7059, 10 centimeters square) is
formed to a thickness of 1000 .ANG. ore more, for example, 1200 .ANG.. It
may be thinner than this, for example, 500 .ANG. as long as the film is
sufficiently dense as a mask.
The silicon oxide film 21 is patterned into a predetermined pattern
thereafter by means of a conventional photolithography technique.
Thereafter, a thin silicon oxide film 20 is formed by irradiating, a UV
radiation in oxygen atmosphere for 5 minutes. The thickness of the silicon
oxide film 20 is presumably from 20 to 50 .ANG.. The function of the
silicon oxide film 20 thus formed for improving the wettability of the
amorphous silicon film may be provided by the hydrophilic nature of the
silicon oxide film formed as the mask if the solution is matched with the
size of the mask pattern. However, this is a special case, and, in
general, a silicon oxide film 20 is safely used.
Then, similar to the process described in Example 1,5 milliliters (with
respect to a substrate 10 cm.times.10 cm in size) of an acetate solution
containing 100 ppm of nickel is dropped to the surface of the resulting
structure. A uniform aqueous film is formed on the entire surface of the
substrate by effecting spin coating using a spinner at 50 rpm for a
duration of 10 seconds. Then, after maintaining the solution for a
duration of 5 minutes on the surface, it is subjected to spin drying using
a spinner at a rate of 2,000 rpm for a duration of 60 seconds. During the
retention time, the substrate may be rotated on the spinner at a rate of
150 rpm or lower (FIG. 2B).
The amorphous silicon film 12 is crystallized thereafter by applying heat
treatment at 550.degree. C. for a duration of 4 hours in gaseous nitrogen.
It can be seen that the crystal growth proceeds along a lateral direction
from the region 22 into which nickel is introduced toward the region 25
into which nickel is not directly introduced as shown by arrow 23. In FIG.
2C, the reference numeral 24 shows a region in which the nickel is
directly introduced to cause the crystallization and the reference numeral
25 shows a region in which the crystallization proceeds laterally from the
region 24. It has been confirmed by the inventors that the crystal growth
is along the axis of [111].
After the crystallization by the above heat treatment, the crystallinity of
the silicon film is further improved by means of a XeCl excimer laser
(wavelength: 308 nm). In particular, the crystallinity in the region 25 in
which the lateral growth occurs can be remarkably improved.
Also, it is advantageous to heat the substrate or the surface to be
irradiated during the laser irradiation at a temperature of 200.degree. C.
to 450.degree. C.
By controlling the concentration of the solution or the retention time, it
is possible to control the concentration of nickel in the region 24 of the
silicon film where the nickel is directly added within a range of
1.times.10.sup.16 atoms/cm.sup.3 to 1.times.10.sup.19 atoms/cm.sup.3. At
the same time, the concentration of the nickel in the lateral growth
direction can be controlled to be lower than that.
The crystalline silicon film thus fabricated by the process according to
the present invention is characterized in that it exhibits an excellent
resistance against hydrofluoric acid. To the present inventors' knowledge,
if the nickel is introduced by a plasma treatment, the resistivity of the
crystallized silicon against a hydrofluoric acid is poor.
When a silicon oxide film is formed on a crystalline silicon film as a gate
insulator or an interlayer insulator, there is a case where the silicon
oxide film is provided with a contact hole through which an electrode is
to be formed. In such a case, a buffered hydrofluoric acid is usually used
to etch the silicon oxide. However, when the crystalline silicon film does
not have a sufficient resistance against the hydrofluoric acid, it is
difficult to selectively remove the silicon oxide without etching the
crystalline silicon.
However, in the present invention, the difference in an etching rate
(selection ratio) between the silicon oxide film and the crystalline
silicon film is large enough to remove only the silicon oxide film since
the crystalline silicon film of the present invention has a sufficient
resistance against the hydrofluoric acid.
As said before, the concentration of the catalyst element in the lateral
growth area can be made small and has an excellent crystallinity. For this
reason, the lateral growth region is suitable for an active region of a
semiconductor device, for example, a channel region of a thin film
transistor.
EXAMPLE 3
The present example is directed to a manufacturing of a TFT using a
crystalline silicon film in accordance with the present invention. The TFT
of this example is suitable for a driver circuit or pixels in an active
matrix type liquid crystal display device. The TFT of the present
invention is also suitable in other types of thin film integrated
circuits.
Referring to FIGS. 3A to 3E, the process for fabricating a TFT according to
the present example will be described. A silicon oxide film (not shown in
the figure) is deposited to a thickness of 2,000 .ANG. as a base film on a
glass substrate. This silicon oxide film is provided to prevent the
diffusion of impurities from the glass substrate.
An amorphous silicon film is then deposited to a thickness of 1,000 .ANG.
in a manner similar to that used in Example 1. After removing the natural
oxide film by a treatment using hydrofluoric acid, a thin film of an oxide
film is formed to a thickness of about 20 .ANG. by means of UV irradiation
under a gaseous oxygen atmosphere. The formation of this oxide film may be
carried out by a hydrolysis treatment or thermal oxidation.
Then, the amorphous silicon film having the oxide film thereon is coated
with an aqueous acetate solution containing nickel at a concentration of
10 ppm. The substrate is maintained for 5 minutes after the coating and
thereafter, the solution is dried by dry spinning. The silicon oxide films
20 and 21 is removed thereafter using a buffered hydrofluoric acid. Then,
the silicon film is crystallized by heating at 550.degree. C. for a
duration of 4 hours. The process up to this step is the same as that
described in Example 1.
After the foregoing steps, a silicon film in which an amorphous component
and a crystalline component is mixed is obtained. Crystal nuclei exists
within the crystalline component. This structure is then irradiated with a
KrF excimer laser at a power density of 200-300 mJ in order to improve the
crystallinity. At this time, the substrate is maintained at 400.degree. C.
In this way, crystal growth based on the crystal nuclei existing in the
crystalline component occurs.
Then, the crystalline silicon film is patterned into an island form 104 as
shown in FIG. 3A. The island form silicon functions as a channel region of
a TFT. Then, a silicon oxide film 105 is formed to a thickness of 200-1500
.ANG., for example, 1000 .ANG.. This oxide film functions as a gate
insulating layer.
The silicon oxide film 105 is deposited by means of RF plasma CVD process
using TEOS (tetraethoxysilane). That is, TEOS is decomposed and then
deposited together with oxygen at a substrate temperature of 150.degree.
to 600.degree. C., preferably in the range of 300.degree. to 450.degree.
C. TEOS and oxygen are introduced at a pressure ratio of 1:1 to 1:3 under
a total pressure of 0.05 to 0.5 Torr, while applying an RF power of 100 to
250 W. Otherwise, the silicon oxide film can be fabricated by reduced
pressure CVD or normal pressure CVD using TEOS as the starting gas
together with gaseous ozone, while maintaining the substrate temperature
in the range of from 350.degree. to 600.degree. C., preferably, in the
range of from 400.degree. to 550.degree. C. The film thus deposited is
annealed in oxygen or ozone in the temperature range from 400.degree. to
600.degree. C. for a duration of 30 to 60 minutes.
The crystallization of the silicon region 104 can be further improved b y
irradiating a laser beam using a KrF excimer laser (operating at a
wavelength of 248 nm at a pulse width of 20 nsec) or an intense light
equivalent thereto. The application of RTA (rapid thermal annealing) using
infrared ray is particularly effective because the silicon film can be
heated selectively without heating the glass substrate. Moreover, RTA is
especially useful in the fabrication of insulated gate field effect
semiconductor devices because it decreases the interface level between the
silicon layer and the silicon oxide film.
Subsequently, an aluminum film is deposited to a thickness of from 2,000
.ANG. to 1 .mu.m by electron beam evaporation, and is patterned into a
gate electrode 106. The aluminum film may contain from 0.15 to 0.2% by
weight of scandium as a dopant. The substrate is then immersed into an
ethylene glycol solution controlled to a pH of about 7 and containing 1 to
3% tartaric acid to effect anodic oxidation using platinum as the cathode
and the aluminum gate electrode as the anode. The anodic oxidation is
effected by first increasing the voltage to 220 V at a constant rate, and
then holding the voltage at 220 V for 1 hour to complete the oxidation. In
case a constant current is applied as in the present case, the voltage is
preferably increased at a rate of from 2 to 5 V/minute. An anodic oxide
109 is formed at a thickness of from 1,500 to 3,500 .ANG., more
specifically, at a thickness of, for example, 2,000 .ANG. in this manner
(FIG. 3B).
Impurities (specifically in this case, phosphorus) are introduced into the
island-form silicon film of the TFT in a self-aligning manner by ion
doping (plasma doping) using the gate electrode portion as a mask.
Phosphine (PH.sub.3) is used as a doping gas to implant phosphorus at a
dose of from 1.times.10.sup.15 to 4.times.10.sup.15 atoms/cm.sup.2.
The crystallinity of the portion whose crystallinity is impaired by the
introduction of the impurity is cured by irradiating a laser beam using a
KrF excimer laser operating at a wavelength of 248 nm and a pulse width of
20 nsec. The laser is operated at an energy density of from 150 to 400
mJ/cm.sup.2, preferably, in a range from 200 to 250 mJ/cm.sup.2. Thus are
formed N-type impurity regions doped with phosphorus) 108 and 109. The
sheet resistance of the regions is found to be in the range of 200 to 800
.OMEGA./square.
This step of laser annealing can be replaced by an RTA process, i.e., a
rapid thermal annealing process using a flash lamp where the temperature
of the sample is rapidly increased to 1000.degree.-1200.degree. C. (in
terms of a silicon monitor).
A silicon oxide film is deposited thereafter to a thickness of 3,000 .ANG.
as an interlayer insulator 110 by means of plasma CVD using TEOS together
with oxygen, or by means of reduced pressure CVD or normal pressure CVD
using TEOS together with ozone. The substrate temperature is maintained in
the range of 250.degree. to 450.degree. C., for instance, at 350.degree.
C. A smooth surface is obtained thereafter by mechanically polishing the
resulting silicon oxide film. (FIG. 3D).
The interlayer insulator 110 is etched to form contact holes at the
source/drain as shown in FIG. 3E, and interconnections 112 and 113 are
formed using chromium or titanium nitride.
While it was difficult to form the contact holes by etching without etching
the silicon film in the case of introducing the nickel by a plasma
treatment of a conventional method, the use of the low concentration
solution of 10 ppm for introducing the nickel is particularly advantageous
for obtaining the contact holes.
A complete TFT can be formed by finally annealing the silicon film in
hydrogen in a temperature range of 300.degree. to 400.degree. C. for a
duration of from 0.1 to 2 hours to accomplish the hydrogenation of the
silicon film. A plurality of TFTs similar to the one described
hereinbefore are fabricated simultaneously, and are arranged in a matrix
to form an active matrix liquid crystal display device. The TFT includes
source and drain regions 108 and 109 and a channel region 114. Also, the
reference numeral 115 shows an electrical junction of NI.
In accordance with the present example, the concentration of the nickel
contained in the active layer can be kept lower than 3.times.10.sup.18
atoms/cm.sup.3, more specifically, in the range of 5.times.10.sup.16 to
3.times.10.sup.18 atoms/cm.sup.3.
The mobility of the N-channel TFT formed in the present example can be
increased to 150 cm.sup.2 /Vs or higher. Also, a threshold voltage Vth can
be reduced and have an excellent characteristics. Further, a variation of
the mobility can be kept within a range of .+-.10%. This small variation
is assumed to be caused by an improvement of the crystallinity due to the
laser light irradiation which follows an incomplete crystallization by a
heat treatment. Although it is possible to obtain a crystalline film
having a mobility of 150 cm.sup.2 /Vs or higher even with only the laser
irradiation, the uniformity of such a film is not so good.
EXAMPLE 5
In this example, nickel is selectively introduced as described in Example 2
and an electronic device is formed using the lateral growth region. The
nickel concentration in the channel region of the device can be lowered.
This is particularly advantageous in terms of electrical stability or
reliability of the device.
Referring to FIG. 4A, a substrate 201 is washed and provided with a silicon
oxide film 202 thereon. The silicon oxide film 202 is formed through a
plasma CVD with oxygen and tetraethoxysilane used as starting gases. The
thickness of the film is 2000 .ANG., for example. Then, an amorphous
silicon film 203 of an intrinsic type having a thickness of 500-1500
.ANG., for example, 1000 .ANG. is formed on the silicon oxide film 202,
following which a silicon oxide film 205 of 500-2000 .ANG., for example
1000 .ANG. is formed on the amorphous silicon film successively. Further,
the silicon oxide film 205 is selectively etched in order to form an
exposed region 206.
Then, a nickel containing solution (an acetic acid salt solution here) is
coated on the entire surface in the same manner as in Example 2. The
concentration of nickel in the acetic acid salt solution is 100 ppm. The
other conditions are the same as in Example 2. Thus, a nickel containing
film 207 is formed.
The amorphous silicon film 203 provided with the nickel containing film in
contact therewith is crystallized through a heat annealing at
500.degree.-620.degree. C. for 4 hours in a nitrogen atmosphere. The
crystallization starts from the region 206 where the silicon film directly
contacts the nickel containing film and further proceeds in a direction
parallel with the substrate. In the figure, a reference numeral 204
indicates a portion of the silicon film where the silicon film is directly
added with nickel and crystallized while a reference numeral 203 indicates
a portion where the crystal grows in a lateral direction. The crystals
grown in the lateral direction are about 25 .mu.m. Also, the direction of
the crystal growth is approximately along an axis of [111]. (FIG. 4A)
After the above crystallization, the crystallinity of the silicon film is
further improved by an infrared ray irradiation. An IR light having a
wavelength of 1.2 .mu.m is used. An effect achieved by this step is
equivalent with that obtainable using a high temperature treatment for
some minutes.
A halogen lamp is used as a light source of the infrared light. The
intensity of the IR light is controlled so that a temperature on the
surface of a monitoring single crystalline silicon wafer is set between
900.degree.-1200.degree. C. More specifically, the temperature is
monitored by means of a thermocouple embedded in a single crystal silicon
wafer and is transferred back to the IR light source (feed back). In the
present example, the temperature rising rate is kept constant in the range
of 50.degree.-200.degree. C./sec. and also the substrate is cooled
naturally at 20.degree.-100.degree. C./sec. Since the IR light can heat
the silicon film selectively, it is possible to minimize the heating of
the glass substrate.
After the crystallization, the silicon oxide film 205 is removed. At this
time, an oxide film formed on the silicon film on the region 206 is
simultaneously removed. Further, the silicon film 204 is patterned by dry
etching to form an active layer 208 in the form of an island as shown in
FIG. 4B. It should be noted that the nickel is contained in the silicon
film at a higher concentration not only in the region 206 where the nickel
is directly added but also in a region where top ends of the crystals
exist. The patterning of the silicon film should be done in such a manner
that the patterned silicon film 208 should not include such regions in
which nickel is contained at a higher concentration.
The patterned active layer 208 is then exposed to an atmosphere containing
100% aqueous vapor at 10 atm and at 500.degree.-600.degree. C., typically,
550.degree. C. for one hour in order to oxidize the surface thereof and
thus to form a silicon oxide film 209 of 1000 .ANG.. After the oxidation,
the substrate is maintained in an ammonium atmosphere (1 atm, 100%) at
400.degree. C. At this condition, the silicon oxide film 209 is irradiated
with an infrared light having an intensity peak at a wavelength in the
range of 0.6-4 .mu.m, for example, 0.8-1.4 .mu.m for 30-180 seconds in
order to nitride the silicon oxide film 209. HCl may be added to the
atmosphere at 0.1 to 10%.
Referring to FIG. 4C, an aluminum film is formed on the oxide film by a
sputtering method to a thickness of 3000-8000 .ANG., for example, 6000
.ANG. and then patterned into a gate electrode 210. The aluminum film is
preferably added with scandium at 0.01-0.2%.
Referring to FIG. 4D, the surface of the aluminum electrode 210 is anodic
oxidized to form an anodic oxide film 211 in an ethylene glycol solution
containing a tartaric acid at 1-5%. The thickness of the oxide film 211 is
2000 .ANG., which will determine the size of an offset gate area which is
to be formed in a later step as discussed below.
Referring then to FIG. 4E, using the gate electrode 210 and the surrounding
anodic oxide film 211 as a mask, an N-type conductivity impurity
(phosphorous, here) is introduced into the active layer in a self-aligning
manner by ion doping method (also called as plasma doping method) in order
to form impurity regions 212 and 213. Phosphine (PH.sub.3) is used as a
dopant gas. The acceleration voltage is 60-90 kV, for example, 80 kV. The
dose amount is 1.times.10.sup.15 -8.times.10.sup.15 cm.sup.-2, for
example, 4.times.10.sup.15 cm.sup.-2. As can be seen in the drawing, the
impurity regions 212 and 213 are offset from the gate electrode by a
distance "x". This configuration is advantageous for reducing a leak
current (off current) which occurs when applying a reverse bias voltage
(i.e. a negative voltage in the case of an NTFT) to the gate electrode. In
particular, since it is desired that electric charges stored in a pixel
electrode be maintained without leaking in order to obtain an excellent
display, the offset configuration is particularly advantageous when the
TFT is used for controlling a pixel of an active matrix as is the case in
the present example.
Thereafter, an annealing is performed with a laser irradiation. As a laser,
a KrF excimer laser (wavelength: 248 nm, pulse width: 20 nsec.) or other
lasers may be used. The conditions of the laser irradiation in the case of
KrF excimer laser are: energy density is 200-400 mJ/cm.sup.2, for example,
250 mJ/cm.sup.2, a number of shots is 2-10 shots per one site, for
example, 2 shots. Preferably, the substrate is heated to
200.degree.-450.degree. C. to enhance the effect of the irradiation.
Referring to FIG. 4F, an interlayer insulating film 214 of silicon oxide is
formed through a plasma CVD to a thickness of 6000 .ANG.. Further, a
transparent polyimide film 215 is formed by spin coating to obtain a
leveled surface.
The interlayer insulating films 214 and 215 are provided with contact
holes, through which electrode/wirings 217 and 218 can reach the impurity
regions of the TFT. The electrode/wirings 217 and 218 are formed of a
metallic material, for example, a multi-layer of titanium nitride and
aluminum. Finally, an annealing in a hydrogen atmosphere of 1 atm is
carried out at 350.degree. C. for 30 minutes in order to complete a pixel
circuit of an active matrix circuit having TFTs.
The TFT of this example has a high mobility so that it is usable for a
driver circuit of an active matrix type liquid crystal device.
EXAMPLE 5
This example is directed to a manufacture of a TFT and will be described
with reference to FIGS. 5A-5D. Referring to FIG. 5A, a base film 502 of
silicon oxide is initially formed on a Corning 7059 substrate 501 by
sputtering to 2000 .ANG. thick. The substrate is annealed at a temperature
higher than a distortion point of the substrate and then the glass is
cooled to a temperature less than the distortion point at a rate of
0.1.degree.-1.0.degree. C./minute. Thereby, it is possible to reduce a
contraction of the substrate due to a substrate heating which occurs later
(for example, thermal oxidation, thermal annealing). As a result, a mask
alignment process will be eased. This step may be performed either before
or after the formation of the base film 201. In the case of using the
Corning 7059 substrate, the su | | |
