U.S. patent application number 11/787445 was filed with the patent office on 2007-08-16 for high order silane composition, and method of forming silicon film using the composition.
Invention is credited to Takashi Aoki, Masahiro Furusawa, Haruo Iwasawa, Yasuo Matsuki, Yasumasa Takeuchi.
Application Number | 20070190265 11/787445 |
Document ID | / |
Family ID | 28786756 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070190265 |
Kind Code |
A1 |
Aoki; Takashi ; et
al. |
August 16, 2007 |
High order silane composition, and method of forming silicon film
using the composition
Abstract
It is an object of the present invention to provide a high order
silane composition that contains a polysilane having a higher
molecular weight than conventionally, this being from the
viewpoints of wettability when applying onto a substrate, boiling
point and safety, and hence in particular enables a high-quality
silicon film to be formed easily, and also a method of forming an
excellent silicon film using the composition. The present invention
attains this object by providing a high order silane composition
containing a polysilane obtained through photopolymerization by
irradiating a solution of a photopolymerizable silane or a
photopolymerizable like-liquid silane with ultraviolet light.
Moreover, the present invention provides a method of forming a
silicon film comprising the step of applying such a high order
silane composition onto a substrate.
Inventors: |
Aoki; Takashi; (Suwa,
JP) ; Furusawa; Masahiro; (Chino, JP) ;
Matsuki; Yasuo; (Chuo, JP) ; Iwasawa; Haruo;
(Chuo, JP) ; Takeuchi; Yasumasa; (Chuo,
JP) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
28786756 |
Appl. No.: |
11/787445 |
Filed: |
April 16, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10420521 |
Apr 22, 2003 |
7223802 |
|
|
11787445 |
Apr 16, 2007 |
|
|
|
Current U.S.
Class: |
427/568 ;
257/E21.114; 522/148 |
Current CPC
Class: |
C09D 183/16 20130101;
H01L 21/02628 20130101; C08G 77/60 20130101; H01L 21/02422
20130101; H01L 21/02532 20130101; C01B 33/04 20130101 |
Class at
Publication: |
427/568 ;
522/148 |
International
Class: |
C08J 3/28 20060101
C08J003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 22, 2002 |
JP |
2002-119961 |
Claims
1. A method of forming a silicon film over a substrate comprising:
irradiating a first liquid material with a light to form a second
liquid material, the first liquid material including a silane
compound that is represented by the general formula
Si.sub.nX.sub.2n, each X independently representing a hydrogen atom
or a halogen atom, and n being an integer greater than or equal to
3, the second liquid material including a first polysilane
component and a second polysilane component, a molecular weight of
the first polysilane component being larger than a molecular weight
of the second polysilane component; and applying the second liquid
material to the substrate.
2. A method of forming a silicon film over a substrate comprising:
irradiating a first liquid material with a light to form a second
liquid material, the first liquid material including a silane
compound that is represented by the general formula SinX.sub.2n,
each X independently representing a hydrogen atom or a halogen
atom, and n being an integer greater than or equal to 3, the second
liquid material including a first polysilane component and a second
polysilane component, a molecular weight of the first polysilane
component being larger than a molecular weight of the second
polysilane component such that the first polysilance component is a
precipitate in the second liquid material; removing the precipitate
from the second liquid material by filtering to form a third liquid
material, a viscosity of the third liquid material being within the
range of 1 to 100 m Pa s; and applying the third liquid material to
the substrate.
3. A method of forming a silicon film over a substrate comprising:
irradiating a first liquid material with a light to form a second
liquid material, the first liquid material including a silane
compound that is represented by the general formula
Si.sub.nX.sub.2n, each X independently representing a hydrogen atom
or a halogen atom, and n being an integer greater than or equal to
3, the first liquid material including a substance that has at
least one of the group 3B elements of the periodic table and the
group 5B elements of the periodic table, the second liquid material
including a first polysilane component and a second polysilane
component, a molecular weight of the first polysilane component
being larger than a molecular weight of the second polysilane
component; and applying the second liquid material to the
substrate.
4. The method of forming a silicon film over a substrate according
to claim 1, further comprising: adding a substance to the first
liquid material before the irradiating the first liquid material
with the light, the substance including at least one of the group
3B elements of the periodic table and the group 5B elements of the
periodic table.
5. The method of forming a silicon film over a substrate according
to claim 1, the silane compound being photopolymerized by the
irradiating the first liquid material with the light.
6. The method of forming a silicon film over a substrate according
to claim 1, the boiling point of the second liquid material at
normal pressure being higher than the decomposition point of the
second liquid material.
7. The method of forming a silicon film over a substrate according
to claim 1, the first liquid material including a solvent that has
the boiling point lower than the decomposition point of the second
liquid material.
8. The method of forming a silicon film over a substrate according
to claim 1, the first liquid material including a solvent that is
not decomposable by the light.
9. The method of forming a silicon film over a substrate according
to claim 1, the light having a wavelength of at least 250 nm.
10. The method of forming a silicon film over a substrate according
to claim 1, the second liquid material being applied to the
substrate by a droplet discharging method.
Description
[0001] This application is a divisional patent application of U.S.
Ser. No. 10/420,521, filed Apr. 22, 2003 and claims priority to
Japanese Application No. 2002-119961 filed Apr. 22, 2002, all of
which are incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a high order silane
composition and a method of forming a silicon film that can be used
in particular with integrated circuits, thin film transistors,
photoelectric converters, photoreceptors, and soon. Specifically,
the present invention relates to a high order silane composition
that in particular enables a high-quality silicon film to be formed
easily, and a method of forming an excellent silicon film using the
composition.
[0004] 2. Description of the Related Art
[0005] Patterning of silicon thin films (amorphous silicon films,
polysilicon films, etc.) used with integrated circuits, thin film
transistors, and so on is generally carried out through a process
in which a silicon film is formed over the whole surface by a
vacuum process such as CVD (chemical vapor deposition), and then
unwanted parts are removed by photolithography. However, with this
method, there are problems such as a large apparatus being
required, the efficiency of usage of the raw material being poor,
the raw material being difficult to handle due to being a gas, and
a large amount of waste being generated.
[0006] Japanese Patent Application Laid-open No. 1-29661 discloses
a method in which a gaseous raw material is liquefied and attached
on a cooled substrate, and is then reacted with chemically active
atomic hydrogen, thus forming a silicon-based thin film. However,
there is a problem in that the raw material silane is continuously
vaporized and cooled, and hence a complicated apparatus is
required, and moreover it is difficult to control the film
thickness.
[0007] Moreover, Japanese Patent Application Laid-open No. 5-144741
and Japanese Patent Application Laid-open No. 7-267621 disclose
methods in which a liquid silane is applied onto a substrate, and
then a silicon film is produced through heating or UV irradiation.
However, with these methods, a low-molecular-weight material is
used, and hence the system is unstable and handling is problematic.
Moreover, with these methods, the wettability to the substrate of
the solution used is poor, and hence the application onto the
substrate is intrinsically difficult, and moreover the boiling
point is low due to the molecular weight being low, and hence
during the heating evaporation occurs more quickly than formation
of the silicon film, and thus it is extremely difficult to obtain
the desired film. That is, how high the molecular weight is (i.e.
how good the wettability is, how high the boiling point is, and how
good the safety is) of the polysilane (high order silane) used as a
material is an important point with regard to film formation.
[0008] As a method of resolving the above problem, Japanese Patent
Application Laid-open No. 10-321536 suggests a method in which the
wettability of a solution of a polysilane is improved by subjecting
a mixture of the solution and a catalyst to thermal decomposition
or photo decomposition as treatment before the application. With
this method, it is necessary to mix a catalyst such as nickel into
the solution, and hence there is a drawback that the properties of
the silicon film are markedly degraded.
[0009] Regarding methods of directly synthesizing silane having a
high molecular weight, synthesis procedures and refinement methods
are both extremely problematic in general; there have been attempts
at a method of directly synthesizing polysilane through thermal
polymerization as disclosed in Japanese Patent Application
Laid-open No. 11-260729, but at best Si.sub.9H.sub.20 is obtained
and at a low yield, and at such a molecular size, the
above-mentioned properties such as wettability are still
insufficient.
[0010] Moving on, as a method of forming a silicon film containing
an n-type or p-type dopant, in general the silicon film is first
produced, and then the dopant is introduced therein by ion
implantation. In contrast to this, Japanese Patent Application
Laid-open No. 2000-31066 discloses a method in which, during a
process of forming a silicon film from a polysilane solution as
described above, a dopant source is mixed into the material liquid,
whereby a doped silicon film is formed. However, with this method
as well, there is the fundamental problem for the case of using a
low-molecular-weight material that the polysilane solution
evaporates during the heating process and hence the amount thereof
drops, and accompanying this the dopant source also evaporates, and
hence it is difficult to add the dopant effectively.
SUMMARY OF THE INVENTION
[0011] In view of the above, it is an object of the present
invention to provide a high order silane composition that contains
a polysilane having a higher molecular weight than conventionally,
this being from the viewpoints of wettability when applying onto a
substrate, boiling point and safety, and hence in particular
enables a high-quality silicon film to be formed easily, and also a
method of forming an excellent silicon film using the
composition.
[0012] The present inventors carried out assiduous research, and as
a result discovered that the above object can be attained through a
composition containing a polysilane formed through a specific
polymerization process using a solution of a silane having a cyclic
silicon chain such as cyclopentasilane (Si.sub.5H.sub.10) as a
silane having specific properties.
[0013] The present invention was accomplished based on the above
discovery, and attains the above object by providing a high order
silane composition containing a polysilane obtained through
photopolymerization by irradiating a solution of a
photopolymerizable silane with ultraviolet light.
[0014] Moreover, the present invention also provides a high order
silane composition containing a polysilane obtained through
photopolymerization by irradiating a photopolymerizable like-liquid
silane with ultraviolet light.
[0015] Preferably, the boiling point of the polysilane at normal
pressure is higher than the decomposition point thereof.
[0016] Moreover, preferably, the high order silane composition
contains a solvent, wherein the boiling point of the solvent is
lower than the decomposition point of the polysilane.
[0017] Moreover, preferably, the ultraviolet light has a wavelength
such as not to decompose the solvent used in the solution of the
photopolymerizable silane.
[0018] Particularly preferably, the ultraviolet light has a
wavelength of at least 250 nm.
[0019] Moreover, preferably, the irradiation time of the
ultraviolet light is in a range of 0.1 seconds to 120 minutes.
[0020] Moreover, preferably, the photopolymerizable silane has at
least one cyclic structure in the molecule thereof.
[0021] Particularly preferably, the photopolymerizable silane is
one represented by the general formula Si.sub.nX.sub.2n, wherein
each X independently represents a hydrogen atom or a halogen atom,
and n is an integer greater than or equal to 3.
[0022] Moreover, preferably, the high order silane composition
further has a substance containing a group 3B element of the
periodic table or a substance containing a group 5B element of the
periodic table added thereto after irradiating with the ultraviolet
light.
[0023] Alternatively, a substance containing a group 3B element of
the periodic table or a substance containing a group 5B element of
the periodic table is added to the solution of the
photopolymerizable silane before irradiating with the ultraviolet
light.
[0024] Moreover, preferably, the high order silane composition is
for forming a silicon film.
[0025] The present invention also provides a method of forming a
silicon film comprising the step of applying a high order silane
composition as described above onto a substrate.
[0026] Preferably, in the method, after applying the high order
silane composition onto the substrate, heat treatment and/or light
treatment is/are carried out.
[0027] Alternatively, in the method, a composition containing a
solvent is used as the high order silane composition, and after
applying the composition onto the substrate, the solvent only is
selectively removed, and then heat treatment and/or light treatment
is/are carried out.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a MALDI-TOFMS spectrum of a polysilane obtained by
irradiating a xylene solution of cyclopentasilane
(Si.sub.5H.sub.10) with ultraviolet light to carry out
photopolymerization.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(High Order Silane Composition)
[0029] Following is a detailed description of the high order silane
composition of the present invention through preferred embodiments
thereof.
[0030] The high order silane composition of the present invention
contains a polysilane as photopolymers obtained through
photopolymerization by irradiating a solution of a
photopolymerizable silane with ultraviolet (hereinafter referred to
as `UV`) light. Because the polysilane in the present invention is
formed by photopolymerizing a specific photopolymerizable silane by
irradiating a solution of this silane with UV light in this way,
the polysilane has a molecular weight incomparably larger than that
of polysilane used in conventional silicon film formation methods
(for example, in the case of Si.sub.6H.sub.14, the molecular weight
is 182) Regarding the polysilane contained in the high order silane
composition of the present invention, one having a molecular weight
up to approximately 1800 have been identified (see the MALDI-TOFMS
spectrum of FIG. 1).
[0031] Note that a photopolymerizable like-liquid silane may be
used instead of the above-mentioned solution of a
photopolymerizable silane, i.e. the high order silane composition
of the present invention may also be a composition containing a
polysilane obtained through photopolymerization by irradiating a
photopolymerizable like-liquid silane with ultraviolet light.
[0032] Because the high order silane composition of the present
invention comprises a solution of a polysilane having a high
molecular weight in this way, when forming a silicon film in
particular, extremely good wettability to the substrate is
exhibited, and hence application onto the substrate can be carried
out much more nicely than with a conventional method. Moreover, the
polysilane contained in the high order silane composition of the
present invention have an extremely high molecular weight. The
higher the molecular weight of a polysilane, the lower the
reactivity, and hence handling is safer than with a conventional
polysilane.
[0033] Moreover, due to the very high molecular weight of the
polysilane contained in the high order silane composition of the
present invention, the boiling point of the polysilane at normal
pressure is higher than the decomposition point thereof, and hence
during heating and baking when forming a silicon film, there is no
problem of the polysilane evaporating before the silicon film is
formed.
[0034] Note that in actual practice if such a polysilane is heated,
then decomposition occurs before the boiling point is reached, and
hence the boiling point, which is higher than the decomposition
point, cannot be determined experimentally. However, here the
boiling point at normal pressure means the theoretical value
thereof as determined from the temperature dependence of the vapor
pressure or a theoretical calculation.
[0035] Moreover, if the high order silane composition of the
present invention is used, then due to this property of the boiling
point of the polysilane being higher than the decomposition point
thereof, it is not necessary to heat rapidly so that a high
temperature is reached before evaporation occurs as conventionally.
It is thus possible to use a process in which the temperature is
raised only gradually, or heating is carried out to a relatively
low temperature while reducing the pressure. This means that the
speed of bonding between silicons when forming the silicon film can
be controlled, and moreover by using a method in which the
temperature is maintained at a temperature that is not so high that
the silicon film is formed but is higher than the boiling point of
the solvent, it becomes possible to reduce the amount in the
silicon film of the solvent, which may cause degradation of the
properties of the silicon, more efficiently than with a
conventional method.
[0036] For reasons such as the above, it is preferable for the
boiling point of the polysilane contained in the high order silane
composition of the present invention to be higher than the
decomposition point thereof. Such a polysilane for which the
boiling point thereof is higher than the decomposition point
thereof can easily be obtained by selecting a preferable silane as
described later as the raw material silane, selecting a preferable
UV wavelength, irradiation time, irradiation method, and
irradiation energy as described later for the irradiated UV light,
and selecting a preferable solvent as the solvent used and a
preferable method of refinement after the UV irradiation.
[0037] According to the high order silane composition of the
present invention, due to the effects described above, a silicon
film can be formed more easily and to a higher quality than with a
conventional method. The amorphous silicon film formed in this way
can of course subsequently be crystallized using a method such as
heat treatment or excimer laser annealing, thus further improving
the properties.
[0038] In the present invention, the molecular weight distribution
of the polysilane can be controlled through the UV irradiation
time, irradiation dose, and irradiation method. Moreover, the
polysilane can be subjected to separation refinement using GPC,
which is a common polymer refinement method, or the like after the
silane has been irradiated with the UV light, whereby a polysilane
of a desired molecular weight can be extracted. Moreover,
refinement can also be carried out by utilizing the difference in
solubility between polysilane having different molecular weights.
Moreover, refinement can also be carried out through fractional
distillation, utilizing the difference in boiling point at either
normal pressure or under reduced pressure between polysilane having
different molecular weights. By controlling the molecular weight of
the polysilane in the high order silane composition in this way, it
becomes possible to obtain high-quality silicon films with
fluctuations in properties suppressed yet further.
[0039] The higher the molecular weight of the polysilane, the
higher the boiling point, and the lower the solubility in the
solvent. Consequently, depending on the UV irradiation conditions,
after the photopolymerization there may be a polysilane that will
not completely dissolve in the solvent but rather precipitate out;
in this case, the high order silane composition can be refined by
using a method of filtering with a microfilter or the like to
remove insoluble components.
[0040] The ultraviolet (UV) light irradiated onto the silane
solution preferably has a wavelength such as not to decompose the
solvent used in the solution; specifically, this wavelength is
preferably at least 250 nm, particularly preferably at least 300
nm. In the present invention, `a wavelength such as not to
decompose the solvent` means a wavelength such that chemical bonds
in the solvent molecules are not broken by the irradiation with the
ultraviolet light. By using UV light having a wavelength as
described above, it is possible to prevent impurity atoms such as
carbon atoms originating from the solvent being present in the
silicon film after heat treatment and/or light treatment, and hence
it becomes possible to obtain a silicon film having yet better
properties.
[0041] From the standpoint of being able to obtain a polysilane
having a desired molecular weight distribution, the UV irradiation
time is preferably in a range of 0.1 seconds to 120 minutes,
particularly preferably 1 to 30 minutes. Moreover, from the
standpoint of being able to obtain a polysilane having a desired
molecular weight distribution, as the UV irradiation method, it is
preferable to carry out the irradiation after diluting the silane
with a solvent, or carry out the UV irradiation uniformly over the
whole of the solution of the silane while stirring the
solution.
[0042] Moreover, the viscosity and surface tension of the high
order silane composition of the present invention can easily be
controlled through the adjustment methods described above with
regard to the molecular weight distribution of the polysilane, and
by adjusting the solvent. This is extremely advantageous in the
process of carrying out patterning using droplet discharging, which
is a great merit when forming a silicon film from a liquid.
[0043] There are no particular limitations on the silane for
preparing the high order silane composition of the present
invention, provided this silane is photopolymerizable, i.e. can be
polymerized through irradiation with UV light; examples include a
silane represented by the general formula Si.sub.nX.sub.m (wherein
n represents an integer greater than or equal to 3, m represents an
integer greater than or equal to 4, with n and m being independent
of one another, and each X independently represents a substituent
such as a hydrogen atom or a halogen atom).
[0044] Examples of such silanes are all photopolymerizable silane
that can be used in the process of photopolymerization through
irradiation with ultraviolet light in the present invention, for
example a cyclic silane represented by the general formula
Si.sub.nX.sub.2n (wherein n represents an integer greater than or
equal to 3, and each X independently represents a hydrogen atom or
a halogen atom), a silane having two or more cyclic structures
represented by the general formula Si.sub.nX.sub.2n-2 (wherein n
represents an integer greater than or equal to 4, and each X
independently represents a hydrogen atom or a halogen atom), and
other silanes having at least one cyclic structure in the molecule
thereof and halogen-substituted derivatives thereof.
[0045] Specific examples of such silanes include cyclotrisilane,
cyclobutasilane, cyclopentasilane, cyclohexasilane,
cycloheptasilane and so on as ones having one cyclic structure,
1,1'-bicyclobutasilane, 1,1'-bicyclopentasilane,
1,1'-bicyclohexasilane, 1,1'-bicycloheptasilane,
1,1'-cyclobutasilylcyclopentasilane,
1,1'-cyclobutasilylcyclohexasilane,
1,1'-cyclobutasilylcycloheptasilane,
1,1'-cyclopentasilylcyclohexasilane,
1,1'-cyclopentasilylcycloheptasilane,
1,1'-cyclohexasilylcycloheptasilane, spiro[2,2]pentasilane,
spiro[3,3]heptasilane, spiro[4,4]nonasilane, spiro[4,5]decasilane,
spiro[4,6]undecasilane, spiro[5,5]undecasilane,
spiro[5,6]undecasilane, spiro[6,6]tridecasilane and so on as ones
having two cyclic structures, and also silanes in which some of the
hydrogen atoms in the above compounds are substituted with
SiH.sub.3 groups or halogen atoms. Two or more of these silanes can
be used mixed together.
[0046] Of these silanes, ones having a cyclic structure in at least
one place in the molecule thereof have an extremely high reactivity
to light, and can thus be photopolymerized efficiently, and hence
it is preferable to use such a silane having a cyclic structure in
at least one place in the molecule thereof as a raw material. Of
such a silane having a cyclic structure in at least one place in
the molecule thereof, a silane represented by the general formula
Si.sub.nX.sub.2n (wherein n represents an integer greater than or
equal to 3, and each X independently represents a hydrogen atom or
a halogen atom such as a fluorine atom, a chlorine atom, a bromine
atom or an iodine atom) such as cyclotetrasilane, cyclopentasilane,
cyclohexasilane and cycloheptasilane are particularly preferable
from the viewpoint of synthesis and refinement being easy, as well
as for the reasons described above.
[0047] Note that although a silane having at least one cyclic
structure as described above is preferable as the silane, if
necessary a silane such as n-pentasilane, n-hexasilane or
n-heptasilane, a modified silane that has been modified with boron
atoms(s) and/or phosphorus atom(s), or the like may be used
together with the silane having at least one cyclic structure, so
long as the process of photopolymerization through irradiation with
ultraviolet light in the present invention is not inhibited.
[0048] Moreover, there are no particular limitations on the solvent
for forming the silane solution, so long as this solvent is one
that dissolves the silane but does not react with the silane; in
general a solvent having a vapor pressure in a range of 0.001 to
200 mmHg at room temperature is used. If the vapor pressure is
higher than 200 mmHg, then when forming the coating film the
solvent will evaporate before the coating film is formed, and hence
it will be difficult to form a good coating film. On the other
hand, if the vapor pressure is lower than 0.001 mmHg, then when
forming the coating film drying will be slow, and hence solvent
will become prone to remaining behind in the coating film of the
silicon compounds, and hence it will be difficult to obtain a
high-quality silicon film even after the subsequent heat treatment
and/or light treatment.
[0049] Moreover, as the solvent, it is preferable to use one having
a boiling point at normal pressure that is higher than room
temperature, but lower than the decomposition point of the
polysilane, i.e. 250.degree. C. to 300.degree. C. By using a
solvent having a boiling point lower than the decomposition point
of the polysilane, it is possible to selectively remove the solvent
only by heating after the application without decomposing the
polysilane, and hence it is possible to prevent solvent from
remaining behind in the silicon film, and thus a higher quality
silicon film can be obtained.
[0050] Specific examples of the solvent used in the silane solution
include hydrocarbon solvents such as n-hexane, n-heptane, n-octane,
n-decane, dicyclopentane, benzene, toluene, xylene, durene, indene,
tetrahydronaphtahalene, decahydronaphtahalene and squalane, and
also ether solvents such as dipropyl ether, ethylene glycol
dimethyl ether, ethylene glycol diethyl ether, ethylene glycol
methyl ethyl ether, diethylene glycol dimethyl ether, diethylene
glycol diethyl ether, diethylene glycol methyl ethyl ether,
tetrahydrofuran, tetrahydropyran, 1,2-dimethoxyethane,
bis(2-methoxyethyl)ether and p-dioxane, and moreover polar solvents
such as propylene carbonate, .gamma.-butyrolactone,
N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile and
dimethylsulfoxide. Of these solvents, from the standpoint of the
solubility of the silane and the stability of the solution,
hydrocarbon solvents and ether solvents are preferable, with
hydrocarbon solvents being particularly preferable. These solvents
can be used alone, or two or more may be used mixed together.
[0051] Moreover, before the irradiation with the ultraviolet light,
a substance containing a group 3B element of the periodic table or
a substance containing a group 5B element of the periodic table (a
dopant source) can be added to the silane solution. When forming a
silicon film, the process of irradiating with UV light after
putting such a dopant source into the silane solution is a novel
process not seen with conventional methods. According to this
process, bonding between the dopant and the polysilane at a
molecular level can be brought about through the irradiation with
the UV light, and hence by applying the solution onto a substrate
and carrying out heat treatment and/or light treatment, an n-type
or p-type doped silicon film having good properties can be formed.
The doped silicon film formed through this process can of course
then be subjected to a step such as heating, thus further improving
the properties. In particular, it is possible to apply onto a
substrate the high order silane composition formed from the silane
solution containing the substance (dopant source), and then carry
out heat treatment and/or light treatment as described later, thus
activating the substance (dopant).
[0052] Moreover, the concentration of the dopant source added
should be determined in accordance with the dopant concentration
ultimately required in the silicon film; the concentration may be
adjusted by diluting with a solvent after the UV irradiation, or
the dopant source may be mixed into a high order silane composition
that has been subjected to UV irradiation without adding the dopant
source.
[0053] Examples of such substances containing a group 3B element of
the periodic table and substances containing a group 5B element of
the periodic table (dopant sources) are substances containing an
element such as phosphorus, boron or arsenic, specifically
substances such as those cited in Japanese Patent Application
Laid-open No. 2000-31066.
[0054] The concentration of the polysilane in the high order silane
composition formed through photopolymerization by irradiating the
silane solution with UV light is preferably approximately 1 to 80
wt % from the standpoint of preventing uneven precipitation of the
polysilane after application onto a substrate when forming a
silicon film, and thus being able to obtain a uniform coating film;
this concentration can be adjusted as appropriate in accordance
with the desired silicon film thickness.
[0055] The high order silane composition of the present invention
comprises a solution containing the polysilane and the solvent, and
moreover other additives can be added as required.
[0056] A substance containing a group 3B element of the periodic
table or a substance containing a group 5B element of the periodic
table can be added as a dopant source to the high order silane
composition of the present invention. By appropriately selecting
and adding such a substance, a desired n-type or p-type silicon
film having a dopant introduced therein can be formed. In the
process of forming a silicon film using a high order silane
composition to which such a substance has been added, because the
polysilane has a high boiling point and hence is not prone to
evaporating, evaporation of the dopant source can also be
suppressed, and hence the introduction of the dopant into the film
can be carried out more efficiently than with a conventional
method. Note that in the case that the substance is added to the
silane solution before irradiating with UV light to form the
polysilane as described earlier, there is no need to add the
substance at the present stage (after the UV irradiation). Examples
of substances containing a group 3B element of the periodic table
and substances containing a group 5B element of the periodic table
are the same as those given for the case of adding such a substance
to the silane before the UV irradiation as described earlier.
Moreover, after the high order silane composition has been applied
onto a substrate, the substance (dopant) can be activated through
heat treatment and/or light treatment as described later.
[0057] Moreover, a small amount of a surface tension regulating
agent of a fluorinated type, a silicone type, a nonionic type or
the like can be added to the high order silane composition of the
present invention as required, so long as this is within a range
such that the desired functions of the high order silane
composition are not damaged. Such surface tension regulating agents
are useful in improving the wettability of the solution to the
article onto which the solution is applied, and thus improving the
leveling ability of the applied film, and preventing the occurrence
of lumps in the coating film, the occurrence of orange peel, and so
on.
[0058] The viscosity of the high order silane composition of the
present invention is generally in a range of 1 to 100 mPas, and can
be selected as appropriate in accordance with the coating apparatus
and the desired coating film thickness. If the viscosity is greater
than 100 mPas, then it becomes difficult to obtain a uniform
coating film.
[0059] The high order silane composition of the present invention
is useful for forming silicon films used in particular with
integrated circuits, thin film transistors, photoelectric
converters, photoreceptors, and so on.
(Method of Forming Silicon Film)
[0060] Next, a detailed description will be given of a method of
forming a silicon film of the present invention.
[0061] The method of forming a silicon film of the present
invention is characterized by applying a high order silane
composition as described above onto a substrate; with regard to
other points, the same procedure can be used as with a method of
forming a silicon film using an ordinary solution. The method of
forming a silicon film of the present invention preferably includes
a step of carrying out heat treatment and/or light treatment after
applying the high order silane composition onto the substrate.
Moreover, in the case of using a composition containing a solvent
as the high order silane composition, after applying the
composition onto the substrate, a step of selectively removing the
solvent only may be included before the step of carrying out heat
treatment and/or light treatment.
[0062] The method of forming a silicon film of the present
invention is not a method in which a gas is fed in as in the
commonly used CVD method, but rather is a method in which a high
order silane composition as described above is applied onto a
substrate, then the solvent is dried off to form a film of a
polysilane, and then the film is converted into a silicon film by
carrying out thermal decomposition and/or photo decomposition, or
is converted into a polycrystalline silicon film by carrying out
thermal decomposition and/or photo decomposition and then further
carrying out laser treatment. Furthermore, according to the method
of forming a silicon film of the present invention, a p-type or
n-type silicon film modified with boron atoms or phosphorus atoms
can be formed without carrying out ion implantation in a vacuum
system.
[0063] As the method of applying the high order silane composition,
a method such as spin coating, roll coating, curtain coating, dip
coating, spraying, or droplet discharging can be used. The
application is generally carried out at a temperature above room
temperature. At a temperature below room temperature, the
solubility of the polysilane will drop, and hence partial
precipitation may occur. The silane, polysilane, and high order
silane composition in the present invention degenerate through
reaction with water and oxygen, and hence it is preferable to carry
out the series of steps in a state in which water and oxygen are
not present. The series of steps is thus preferably carried out in
an atmosphere of an inert gas such as nitrogen, helium or argon.
Furthermore, it is preferable for a reducing gas such as hydrogen
to be mixed into the atmosphere as required. Moreover, it is
preferable for the solvent and additives used to be ones from which
water and oxygen have been removed.
[0064] Note that in the present invention a `droplet discharging
method` is a method in which droplets are discharged in desired
regions, thus forming a desired pattern containing the discharged
material, and is also known as an ink jet method. Note, however,
that in the present case, the discharged droplets are not of an ink
used on a printed article, but rather of a liquid containing a
material that constitutes a device; such materials include, for
example, materials that can function as a semiconducting material
or an insulating material constituting a device. Furthermore,
`droplet discharging` is not limited to the case of discharging as
a spray, but also includes the case that the liquid is discharged
so that the droplets of the liquid are continuous.
[0065] Moreover, in the case of using spin coating, the rotational
speed of the spinner is determined in accordance with the thickness
of the thin film to be formed and the composition of the coating
solution, but is generally in a range of 100 to 5000 rpm,
preferably 300 to 3000 rpm.
[0066] In the method of forming a silicon film of the present
invention, after the high order silane composition has been
applied, heating is carried out to remove the solvent. The heating
temperature varies according to the type and boiling point (vapor
pressure) of the solvent used, but is generally in a range of 100
to 200.degree. C. As with the application step described above, the
heating is preferably carried out in an atmosphere of an inert gas
such as nitrogen, helium or argon. At this time, by reducing the
pressure of the system as a whole, the removal of the solvent can
be carried out at a lower temperature. As a result, thermal
degradation of the substrate can be reduced.
[0067] Moreover, in the method of forming a silicon film of the
present invention, the polysilane on the substrate from which the
solvent has been removed is converted into a silicon film through
heat treatment and/or light treatment. The silicon film obtained
through the method of the present invention is amorphous or
polycrystalline; in the case of heat treatment, in general an
amorphous silicon film is obtained if the temperature reached is
less than approximately 550.degree. C., whereas a polycrystalline
silicon film is obtained if the temperature reached is higher than
this. In the case that one wishes to obtain an amorphous silicon
film, a temperature in a range of 300.degree. C. to 550.degree. C.
is preferably used, more preferably 350.degree. C. to 500.degree.
C. If the temperature reached is less than 300.degree. C., then the
thermal decomposition of the polysilane may not proceed
sufficiently, and hence it may not be possible to form a silicon
film of sufficient thickness.
[0068] In the present invention, the atmosphere when carrying out
the heat treatment is preferably an inert gas such as nitrogen,
helium or argon, or such an inert gas with a reducing gas such as
hydrogen mixed therein. In the case that one wishes to obtain a
polycrystalline silicon film, an amorphous silicon film obtained as
described above can be converted into a polycrystalline silicon
film by irradiating with a laser.
[0069] On the other hand, in the case of carrying out light
treatment, examples of light sources that can be used include a
low-pressure or high-pressure mercury lamp, a deuterium lamp, a
discharge lamp of a noble gas such as argon, krypton or xenon, a
YAG laser, an argon laser, a carbon dioxide laser, an excimer laser
of XeF, XeCl, XeBr, KrF, KrCl, ArF, ArCl or the like, and so on. In
general a light source with a power output in a range of 10 to 5000
W is used, with100 to 1000 W usually being sufficient. There are no
particular limitations on the wavelength of the light source,
provided this wavelength is a wavelength at which the polysilane
absorbs to some extent, but this wavelength is generally in a range
of 170 nm to 600 nm. Moreover, from the standpoint of the
efficiency of conversion into a polycrystalline silicon film, it is
particularly preferable to use laser light. The temperature during
the light treatment is generally between room temperature and
150.degree. C., and can be selected as appropriate in accordance
with the semiconductor properties of the silicon film to be
obtained.
[0070] There are no particular limitations on the substrate used in
the method of forming a silicon film of the present invention, but
ordinary quartz, borosilicate glass, soda glass, a transparent
electrode of ITO or the like, a substrate made of a metal such as
gold, silver, copper, nickel, titanium, aluminum or tungsten, a
glass or plastic substrate having such a metal on a surface
thereof, and so on can be used.
[0071] Silicon films obtained using the method of forming a silicon
film of the present invention can be used with integrated circuits,
thin film transistors, photoelectric converters, photoreceptors,
and so on.
EXAMPLES
[0072] Following is a more detailed description of the present
invention through examples; however, the present invention is not
limited to these examples.
[0073] All of the following examples were carried out under a
nitrogen atmosphere with an oxygen concentration of not more than 1
ppm.
Examples 1 to 5
[0074] 3 g of cyclohexasilane was dissolved in 10 ml of benzene,
thus preparing a solution. The solution was put into a 20 ml glass
beaker, was irradiated for 5 minutes with UV light of wavelength
308 nm at 20 mW/cm.sup.2 while being stirred, and was then filtered
using a 0.5 .mu.m filter, thus obtaining a high order silane
composition. The high order silane composition was taken as a
coating solution and was applied onto quartz substrates using a
spin coating method at 1000 rpm. The substrates onto which the high
order silane composition had been applied were then baked under
various conditions as shown in Table 1, thus forming a brown
amorphous silicon film on the quartz substrates. The results of
ESCA measurements (surface composition) and the results of Raman
spectroscopy measurements (crystallization rate) on the amorphous
silicon films obtained are also shown in Table 1. TABLE-US-00001
TABLE 1 Crystalli- Baking Silicon Oxygen Carbon zation conditions
(%) (%) (%) rate (%) Example 1 30 min at 100.degree. C., 87 12 0 5
then 10 min at 350.degree. C. Example 2 30 min at 100.degree. C.,
98 2 0 20 then 10 min at 500.degree. C. Example 3 10 min at
350.degree. C. 83 12 5 4 Example 4 10 min at 500.degree. C. 93 2 5
15 Example 5 Temperature 96 2 2 10 increased at 50.degree. C./min
up to 500.degree. C.
[0075] In Examples 1 and 2, heating was first carried out at
100.degree. C. to remove the benzene solvent, and then baking was
carried out at 350.degree. C. or 500.degree. C. to convert the
polysilane into an amorphous silicon film. At 350.degree. C., the
baking was insufficient, and hence the film partially oxidized
after the baking, but with Example 2 in which the baking was
carried out at 500.degree. C., a high-quality silicon film
containing hardly any oxygen could be formed.
[0076] Moreover, in Examples 3 and 4, a solvent removal step like
that used in Examples 1 and 2 was not carried out, and hence there
was a problem that carbon atoms from the benzene solvent remained
in the silicon film.
[0077] Moreover, in Example 5, the silicon film was formed by
raising the temperature from room temperature at a rate of
50.degree. C. per minute, and then stopping the heating after
approximately 10 minutes once the temperature had reached
500.degree. C. Because the rate of increasing the temperature was
high, the solvent could not be eliminated completely, and hence
there was residual carbon in the silicon film; nevertheless, it was
possible to carry out the removal of the solvent and the baking of
the polysilane in a short time of only 10 minutes, and a
high-quality silicon film containing at least 96% silicon could be
obtained.
[0078] A 308 nm excimer laser was irradiated at an energy density
of 300 mJ/cm.sup.2 onto an amorphous silicon film obtained as in
Example 2 in an argon atmosphere. As a result of Raman spectroscopy
measurements, it was found that a polycrystalline silicon film
having a crystallization rate of 90% was obtained.
[0079] An amorphous silicon film obtained as in Example 2 was
subjected to heat treatment for 10 hours at 800.degree. C. in an
argon atmosphere containing 3% of hydrogen, and then Raman
spectroscopy measurements were carried out, whereupon it was found
that the amorphous silicon film had been converted into a
polycrystalline silicon film having a crystallization rate of
95%.
Examples 6 to 11
[0080] 5 g of cyclopentasilane was dissolved in 20 ml of xylene,
thus preparing a solution. The solution was put into a 50 ml glass
beaker, and was irradiated with UV light under any of various
irradiation conditions as shown in Table 2 while being stirred.
Each of the solutions was then filtered using a 0. 5 .mu.m filter
to remove insoluble components, whereby high order silane
compositions were obtained. Each of the high order silane
compositions was taken as a coating solution and was applied onto a
quartz substrate using a spin coating method at 1500 rpm. Each of
the substrates onto which a high order silane composition had been
applied was then heated for 30 minutes at 120.degree. C. while
reducing the pressure to 5 Torr to remove the xylene solvent, and
was then baked for 10 minutes at 500.degree. C., thus forming a
brown amorphous silicon film on the quartz substrate.
[0081] The UV wavelength used, the irradiation time, the
irradiation dose, the results of ESCA measurements (surface
composition), and the results of film thickness measurements by
ellipsometry are shown in Table 2 for each case. TABLE-US-00002
TABLE 2 Film UV irradiation Silicon Oxygen Carbon thickness
conditions (%) (%) (%) (nm) Example 6 436 nm, 10 min: 98 2 0 180 20
mW/cm.sup.2 Example 7 436 nm, 20 min: 98 2 0 240 20 mW/cm.sup.2
Example 8 254 nm, 10 min: 98 2 0 260 15 mW/cm.sup.2 Example 9 254
nm, 20 min: 98 2 0 310 15 mW/cm.sup.2 Example 10 172 nm, 10 min: 87
1 12 360 10 mW/cm.sup.2 Example 11 172 nm, 20 min: 84 1 15 540 10
mW/cm.sup.2
[0082] From the results of Examples 6 to 11, it was found that the
film thickness varies according to the UV irradiation energy and
irradiation time, and hence that an amorphous silicon film of a
desired thickness can be obtained by suitably controlling the UV
irradiation energy and irradiation time. Moreover, it can be seen
that, with Examples 10 and 11, the methyl groups of the xylene
solvent were detached by the UV light of wavelength 172 nm, and
were taken into the amorphous silicon film.
Example 12
[0083] The viscosity of a high order silane composition prepared
using the same method as in Example 9 described above was measured,
and was found to be 70 mPas. Application using a droplet
discharging method would be difficult in this state, and hence 10
ml of the solution was diluted with 100 ml of hexane, and
long-chain polysilane that would not completely dissolve were
filtered off using a 0.2 .mu.m filter, thus preparing a new high
order silane composition. The viscosity of this solution was 3
mPas. The solution was applied using a droplet discharging method
over the whole of an upper surface of a quartz substrate that had
been made lyophilic in advance by irradiating for 20 minutes with
10 mW of 254 nm UV light. Baking was subsequently carried out as in
Example 9, whereby a uniform amorphous silicon film of thickness
120 nm could be obtained.
Example 13
[0084] 10 ml of a high order silane composition prepared using the
same method as in Example 9 described above was heated at
80.degree. C. under a reduced pressure of 10 Torr, thus
concentrating the high order silane composition down to 5 ml.
Long-chain polysilane that would not completely dissolve were
filtered off using a 0.2 .mu.m filter, and then spin coating and
baking were carried out as in Example 9, whereupon a silicon film
having a thickness of 500 nm was obtained.
Examples 14 to 17
[0085] 5 g of spiro[4,4]nonasilane was dissolved in 20 ml of
cyclohexane, thus preparing a solution. 20 ml of the solution was
put into a quartz beaker, and was irradiated for 10 minutes with UV
light of wavelength 254 nm at 20 mW/cm.sup.2 while being stirred.
50 g of one of the additives shown in Table 3 was added as a dopant
source to the solution and was dissolved, and then filtration was
carried out using a 0.5 .mu.m filter to remove insoluble
components, thus obtaining a high order silane composition. Each of
the high order silane compositions was taken as a coating solution
and was applied using a spin coating method at 1000 rpm onto a
glass substrate on which electrodes had been formed in advance by
photolithography. Each of the substrates onto which a high order
silane composition had been applied was then heated for 30 minutes
at 150.degree. C. to remove the solvent, and then baking was
carried out for 10 minutes at 400.degree. C. while irradiating with
UV light of wavelength 172 nm at 5 mW/cm.sup.2, whereby a doped
silicon film was obtained. The results of conductivity measurements
on the doped silicon films are as shown in Table 3; it was found
that high-quality doped silicon films doped to a high concentration
could be obtained easily without using a vacuum process.
TABLE-US-00003 TABLE 3 Additive Conductivity (S/cm) Example 14
B.sub.5H.sub.9 6.8*10.sup.-5 Example 15 B.sub.10H.sub.14
3.0*10.sup.-4 Example 16 P (yellow phosphorus) 8.7*10.sup.-4
Example 17 PH.sub.3 4.3*10.sup.-3
Examples 18 to 21
[0086] 10 g of silylcyclohexasilane was dissolved in 30 ml of
benzene, thus preparing a solution. 500 mg of one of the additives
shown in Table 4 was added to the solution and dissolved in the
benzene. Each of the mixtures was put into a 100 ml beaker, and was
irradiated for 30 minutes with UV light of wavelength 254 nm at 20
mW/cm.sup.2 while being stirred, thus carrying out
photopolymerization of the silane and at the same time bonding the
polysilane and the additive together. Next, each of the solutions
was filtered using a 0.5 .mu.m filter to remove insoluble
components, whereby high order silane compositions each containing
an additive were obtained. Each of the high order silane
compositions was taken as a coating solution and was applied using
a spin coating method at 2000 rpm onto a glass substrate on which
electrodes had been formed in advance. Each of the substrates onto
which a high order silane composition had been applied was then
heated for 20 minutes at 110.degree. C. to remove the solvent, and
then baking was carried out for 10 minutes at 400.degree. C.,
whereby a doped silicon film was obtained. The results of
conductivity measurements on the doped silicon films are as shown
in Table 4; it was found that, as with Examples 14 to 17,
high-quality doped silicon films doped to a high concentration
could be obtained easily without using a vacuum process.
TABLE-US-00004 TABLE 4 Additive Conductivity (S/cm) Example 18
B.sub.5H.sub.9 2.8*10.sup.-4 Example 19 B.sub.10H.sub.14
7.8*10.sup.-4 Example 20 P (yellow phosphorus) 1.5*10.sup.-3
Example 21 PH.sub.3 7.2*10.sup.-3
Example 22
[0087] A 308 nm excimer laser was irradiated at an energy density
of 360 mJ/cm.sup.2 onto a doped silicon film produced as in Example
20 described above, thus converting the film into a polycrystalline
silicon film. As a result of Raman spectroscopy measurements, it
was found that the polycrystalline silicon film had a
crystallization rate of 100%, and moreover the conductivity had
increased to 2.6.times.10.sup.-2 S/cm.
[0088] According to the high order silane composition of the
present invention, from the viewpoints of wettability when applying
onto a substrate, boiling point and safety, the high order silane
composition contains a polysilane having a higher molecular weight
than conventionally; as a result, in particular a high-quality
silicon film can be formed easily. Moreover, according to the
method of forming a silicon film of the present invention, an
excellent silicon film can be obtained.
* * * * *