U.S. patent application number 13/209861 was filed with the patent office on 2012-02-09 for method for manufacturing a semiconductor device.
This patent application is currently assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Takeshi FUKUNAGA, Akiharu MIYANAGA, Hisashi OHTANI, Hongyong ZHANG.
Application Number | 20120034766 13/209861 |
Document ID | / |
Family ID | 27473824 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120034766 |
Kind Code |
A1 |
OHTANI; Hisashi ; et
al. |
February 9, 2012 |
METHOD FOR MANUFACTURING A SEMICONDUCTOR DEVICE
Abstract
A process for fabricating a highly stable and reliable
semiconductor, comprising: coating the surface of an amorphous
silicon film with a solution containing a catalyst element capable
of accelerating the crystallization of the amorphous silicon film,
and heat treating the amorphous silicon film thereafter to
crystallize the film.
Inventors: |
OHTANI; Hisashi; (Isehara,
JP) ; MIYANAGA; Akiharu; (Hadano, JP) ;
FUKUNAGA; Takeshi; (Atsugi, JP) ; ZHANG;
Hongyong; (Yamato, JP) |
Assignee: |
SEMICONDUCTOR ENERGY LABORATORY
CO., LTD.
Atsugi
JP
|
Family ID: |
27473824 |
Appl. No.: |
13/209861 |
Filed: |
August 15, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12219026 |
Jul 15, 2008 |
7998844 |
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13209861 |
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11322660 |
Jan 3, 2006 |
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12219026 |
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10026802 |
Dec 27, 2001 |
6998639 |
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11322660 |
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08633307 |
Apr 15, 1996 |
6335541 |
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10026802 |
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08329644 |
Oct 25, 1994 |
5643826 |
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08633307 |
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Current U.S.
Class: |
438/486 ;
257/E21.09 |
Current CPC
Class: |
H01L 27/1277 20130101;
H01L 29/78675 20130101; H01L 29/78678 20130101; H01L 21/02532
20130101; H01L 21/02686 20130101; H01L 21/02422 20130101; H01L
27/1296 20130101; H01L 29/66757 20130101; H01L 29/78654 20130101;
H01L 29/66772 20130101; Y10S 148/016 20130101; H01L 29/045
20130101; H01L 21/02672 20130101 |
Class at
Publication: |
438/486 ;
257/E21.09 |
International
Class: |
H01L 21/20 20060101
H01L021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 29, 1993 |
JP |
5-294633 |
Nov 9, 1993 |
JP |
5-303436 |
Nov 12, 1993 |
JP |
5-307206 |
Jun 20, 1994 |
JP |
6-162705 |
Claims
1. A method for manufacturing a semiconductor device comprising the
steps of: forming a semiconductor film comprising amorphous silicon
over a substrate; forming an oxide film on the semiconductor film;
disposing a material which contains a metal on the oxide film, the
metal being capable of promoting crystallization of silicon;
heating the semiconductor film so that the metal diffuses through
the oxide film so as to be introduced into the semiconductor film
whereby the semiconductor film is crystallized; and patterning the
crystallized semiconductor film to form a patterned semiconductor
film including a channel region.
2. The method according to claim 1 wherein the metal is selected
from the group consisting of Ni, Pd, Pt, Cu, Ag, Au, In, Sn, As and
Sb.
3. The method according to claim 1 wherein the material which
contains a metal is a liquid.
4. The method according to claim 1 wherein a thickness of the oxide
film is 100 .ANG. or less.
5. The method according to claim 1 wherein the oxide film is formed
by oxidizing a surface of the semiconductor film.
6. A method for manufacturing a semiconductor device comprising the
steps of: forming a semiconductor film comprising amorphous silicon
over a substrate; forming an oxide film on a selected region of the
semiconductor film; disposing a material which contains a metal on
the oxide film, the metal being capable of promoting
crystallization of silicon; heating the semiconductor film so that
the metal diffuses through the oxide film so as to be introduced
into the semiconductor film whereby the semiconductor film is
crystallized in such a manner that crystallization proceeds
laterally from the selected region of the semiconductor film to a
region adjacent to the selected region; and patterning the
crystallized semiconductor film to form a patterned semiconductor
film including a channel region, wherein the channel region is
located distant from the selected region.
7. The method according to claim 6 wherein the metal is selected
from the group consisting of Ni, Pd, Pt, Cu, Ag, Au, In, Sn, As and
Sb.
8. The method according to claim 6 wherein the material which
contains a metal is a liquid.
9. The method according to claim 6 wherein a thickness of the oxide
film is 100 .ANG. or less.
10. The method according to claim 6 wherein the oxide film is
formed by oxidizing a surface of the semiconductor film.
11. A method for manufacturing a semiconductor device comprising
the steps of: forming a semiconductor film comprising amorphous
silicon on an insulating surface; forming a silicon containing film
on the semiconductor film; disposing a material which contains a
metal on the silicon containing film, the metal being capable of
promoting crystallization of silicon; heating the semiconductor
film so that the metal diffuses through the silicon containing film
so as to be introduced into the semiconductor film whereby the
semiconductor film is crystallized; and patterning the crystallized
semiconductor film to form a patterned semiconductor film including
a channel region.
12. The method according to claim 11 wherein the metal is selected
from the group consisting of Ni, Pd, Pt, Cu, Ag, Au, In, Sn, As and
Sb.
13. The method according to claim 11 wherein the material which
contains a metal is a liquid.
14. The method according to claim 11 wherein a thickness of the
silicon containing film is 100 .ANG. or less.
15. The method according to claim 11 wherein the silicon containing
film comprises silicon oxide.
16. A method for manufacturing a semiconductor device comprising
the steps of: forming a semiconductor film comprising amorphous
silicon on an insulating surface; forming a silicon containing film
on a selected region of the semiconductor film; disposing a
material which contains a metal on the silicon containing film, the
metal being capable of promoting crystallization of silicon;
heating the semiconductor film so that the metal diffuses through
the silicon containing film so as to be introduced into the
semiconductor film whereby the semiconductor film is crystallized
in such a manner that crystallization proceeds laterally from the
selected region of the semiconductor film to a region adjacent to
the selected region; and patterning the crystallized semiconductor
film to form a patterned semiconductor film including a channel
region, wherein the channel region is located distant from the
selected region.
17. The method according to claim 16 wherein the metal is selected
from the group consisting of Ni, Pd, Pt, Cu, Ag, Au, In, Sn, As and
Sb.
18. The method according to claim 16 wherein the material which
contains a metal is a liquid.
19. The method according to claim 16 wherein a thickness of the
silicon containing film is 100 .ANG. or less.
20. The method according to claim 16 wherein the silicon containing
film comprises silicon oxide.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for manufacturing
a semiconductor device having a crystalline semiconductor. The
present invention further relates to an electro-optical device such
as an active matrix liquid crystal device using the semiconductor
device.
[0003] 2. Prior Art
[0004] Thin film transistors (referred to simply 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(-addressed) liquid crystal display device as well as in
driver elements of the peripheral circuits thereof.
[0005] An amorphous silicon film can be utilized most readily as
the thin film semiconductor for a TFT. However, the electric
characteristics of the amorphous silicon film are disadvantageously
poor. The use of a thin film of polysilicon (polycrystalline
silicon), which is a crystalline silicon, can solve the problem.
Crystalline silicon is denoted as, 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.
[0006] 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.
[0007] 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.
[0008] The elements above (hereinafter referred to as "catalyst
elements capable of accelerating the crystallization of an
amorphous silicon film" or simply as "catalyst elements") 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.
[0009] However, the presence of the catalyst 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.
[0010] That is, the catalyst elements 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 tend
to be 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.
[0011] The crystallization process using nickel or the like was
studied in detail. The following findings were obtained as a
result:
[0012] (1) In case of incorporating nickel by plasma treatment into
an amorphous silicon film, nickel is found to intrude into the film
to a considerable depth of the amorphous silicon film before
subjecting the film to a heat treatment;
[0013] (2) The initial nucleation occurs from the surface from
which nickel is incorporated; and
[0014] (3) When a nickel layer is deposited on the amorphous
silicon film, the crystallization of an amorphous silicon film
occurs in the same manner as in the case of effecting plasma
treatment.
[0015] 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.
[0016] A trace amount of nickel, i.e., a catalyst element capable
of accelerating 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 of the
film, and is therefore not suitable for precisely controlling the
amount of the catalyst element to be incorporated in the amorphous
silicon film.
SUMMARY OF THE INVENTION
[0017] In the light of the aforementioned circumstances, the
present invention aims to fabricate with high productivity, a thin
film of crystalline silicon semiconductor by a heat treatment at a
relatively low temperature using a catalyst element, provided that
the catalyst element is incorporated by precisely controlling the
quantity thereof.
[0018] In accordance with one aspect of the present invention, the
foregoing objects can be achieved by providing an amorphous silicon
film with a catalytic element for promoting the crystallization
thereof or a compound including the catalytic element in contact
with the amorphous silicon film, and heat treating the amorphous
silicon with said catalytic element or said compound being in
contact therewith, thereby, the silicon film is crystallized.
[0019] More specifically, a solution containing the catalytic
element is provided in contact with an amorphous silicon film in
order to introduce the catalytic element into the amorphous silicon
film.
[0020] It is another feature of the present invention to add a
material selected from the group consisting of Ni, Pd, Pt, Cu, Ag,
Au, In, Sn, Pd, Sn, P, As and Sb into a silicon semiconductor film
at a trace amount by contacting a solution containing said material
with the silicon film and then crystallize the silicon
semiconductor film by heating at a relatively low temperature.
[0021] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The foregoing objects and features of the present invention
will be described in detail with reference to the attached figures
in which:
[0023] FIGS. 1A to 1D are cross sectional views for forming a
crystalline silicon film in accordance with the present
invention;
[0024] FIGS. 2A and 2B are cross sectional views showing a
formation of a crystalline silicon film in accordance with the
present invention;
[0025] FIG. 3 is a graph showing a relation of a lateral growth
length of crystals with respect to a concentration of nickel in a
solution;
[0026] FIG. 4 is a graph showing a SIMS data with respect to nickel
in a silicon region into which nickel is directly added;
[0027] FIG. 5 is a graph showing a SIMS data with respect to nickel
in a silicon region where crystals grow along the lateral direction
from the region into which nickel is directly added;
[0028] FIGS. 6A to 6E show cross sectional views showing a
manufacturing process of a semiconductor device in accordance with
Example 3 of the present invention;
[0029] FIG. 7 shows a Ni concentration in a silicon film subjected
to a plasma treatment;
[0030] FIG. 8 is a Raman spectroscopic diagram with respect to a
region into which nickel is directly added;
[0031] FIG. 9 is a Raman spectroscopic diagram with respect to a
region where crystals grow in a lateral direction;
[0032] FIGS. 10A-10F are cross sectional views showing a
manufacturing process of an electro-optical device in accordance
with Example 4 of the present invention.
[0033] FIGS. 11A-11D are cross sectional views showing a
manufacturing process of a TFT in accordance with Example 5 of the
present invention;
[0034] FIG. 12 shows a schematic diagram of an active matrix type
electro-optical device in accordance with Example 6 of the present
invention;
[0035] FIGS. 13A and 13B are cross sectional views showing the
formation of a crystalline silicon film in accordance with Example
7 of the present invention;
[0036] FIGS. 14A-14E are cross sectional views showing a
manufacturing process of a TFT in accordance with Example 8 of the
present invention; and
[0037] FIGS. 15A and 15B are schematic diagrams showing an
arrangement of an active layer of a TFT in accordance with Example
8 of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0038] The use of a solution for adding nickel or the like
according to the present invention is advantageous in the following
points:
[0039] (a) The concentration of the catalyst element (e.g. nickel)
in the solution can be accurately controlled in advance;
[0040] (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 brought into contact with the solution;
and
[0041] (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.
[0042] 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 solution, various aqueous solutions and organic
solvent solutions can be used. Those solvents can be roughly
classified into a polar solvent and a non-polar solvent.
[0043] Water, alcohol, acid or ammonium can be used as a 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.
[0044] 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.
[0045] 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 to be coated in advance of coating the solution.
[0046] Also, when using an elemental nickel (metal), it is
necessary to use an acid to dissolve it.
[0047] In the foregoing examples, the nickel can be completely
solved by the solvent. However, even if the nickel is not
completely solved, 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.
[0048] 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 A or less. Also, it is possible to add an interfacial active
agent to the solution in order to increase a wetting property.
[0049] Further, it is possible to conduct a rubbing treatment on
the surface of the thin oxide film in order to give the surface an
irregularity with a uniform gap, width and direction. Such
irregularity helps the solvent to permeate, thereby, increasing the
uniformity of the size and directions of crystal grains. Also, such
a crystalline semiconductor film in which crystals are oriented in
a particular direction is advantageous to be used for a
semiconductor device in order to increase a uniformity of device
characteristics.
[0050] 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, for example, OAP (containing hexamethyl disilazane as
a main component, produced by Tokyo Oka Kogyo) which is used to
increase adhesivity of a resist.
[0051] 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,
and more preferably 10 ppm or lower. The concentration is
determined based on the nickel concentration in the silicon film or
the resistance against hydrofluoric acid of the film upon
completion of the crystallization.
[0052] 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 can be
grown in 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.
[0053] It is also confirmed that this lateral growth region
contains the catalyst element at a lower concentration. It is
useful to utilize a crystalline silicon film as an active layer
region for a semiconductor device, however, in general, the
concentration of the impurity in the active region is preferably as
low as possible. Accordingly, the use of the lateral growth region
for the active layer region is useful in device fabrication.
[0054] The use of nickel as the catalyst element is particularly
effective in the process according to the present invention.
However, other useful catalyst elements include nickel (Ni),
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, Mb, IVb, and Vb of
the periodic table.
Example 1
[0055] The present example refers to a process for fabricating a
crystalline silicon film on the surface of a glass substrate.
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 100 mm.times.100 mm
in size is used.
[0056] An amorphous silicon film from 100 to 1,500 A 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 A by plasma CVD (FIG. 1A).
[0057] Then, the amorphous silicon film is subjected to
hydrofluoric acid treatment to remove impurities and a natural
oxide formed thereon, if necessary. This treatment is followed by
the deposition of an oxide film 13 on the amorphous silicon film to
a thickness of from 10 to 50 .ANG.. A natural oxide film may be
utilized as the oxide film. The precise thickness of the oxide film
13 is not available because the film is extremely thin. However,
the natural oxide film is assumably about 20 A in thickness. The
oxide film 13 is deposited 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.
[0058] 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 to prevent
nickel from being incorporated uniformly into the surface of the
amorphous silicon film.
[0059] An aqueous acetate solution containing nickel added therein
is prepared thereafter. More specifically, an aqueous acetate
solution containing nickel at a concentration of 10 to 200 ppm,
e.g. 100 ppm, is prepared. Two milliliters of the resulting 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
predetermined duration of time, preferably for a duration of 0.5
minutes or longer, e.g. for a duration of 5 minutes. Spin drying at
2,000 rpm using a spinner is effected for 60 seconds thereafter to
remove the unnecessary solution (FIGS. 1C and 1D).
[0060] The concentration of nickel in the acetate solution is
practically 1 ppm or more, preferably, 10 ppm or higher. The
solution needs not be only an acetate solution, and other
applicable solutions include those of hydrochlorides, nitrates, and
sulfates. Otherwise, those of organic ocrylates and toluene can be
used as well. In case of using the organic solutions, the oxide
film 13 need not be incorporated because the solution can be
directly applied to the amorphous silicon film to introduce the
catalyst elements into the film.
[0061] 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 angstrom to several hundreds angstrom 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 materials do not
necessarily have to be in the form of a completely continuous film,
that is, it may be discontinuous, for example, in the form of
number of clusters.
[0062] The amorphous silicon film coated with one of the above
solutions is kept as it is thereafter for a duration of 5 minutes.
The final 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 final concentration of nickel catalyst element in the
crystallized silicon film is the concentration of the nickel
catalyst element in the solution.
[0063] The silicon film coated with a nickel-containing solution
thus obtained is subjected to 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.
[0064] The heat treatment can be effected at any 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.
Example 2
[0065] The present example refers to a process similar to that
described in Example 1, except that a silicon oxide film 1,200 A in
thickness is provided selectively to incorporate nickel into
selected regions of the amorphous silicon film using the silicon
oxide film as a mask.
[0066] Referring to FIGS. 2A to 2C, the process for fabricating a
semiconductor according to the present example is described below.
A silicon oxide film is deposited to a thickness of 1,000 .ANG. or
more e.g. 1200 A as a mask on an amorphous silicon film 12. The
silicon oxide film 21, however, may be thinner than 1000 A, e.g.
500 A if 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. 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 about 20 to 50 A (FIG. 2A). The function
of the silicon oxide film thus formed for improving the wettability
of the amorphous silicon film might be occasionally provided by the
hydrophilic nature of the silicon oxide film formed as the mask in
case 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.
[0067] Then, similar to the process described in Example 1, 5
milliliters (with 10 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 state for a duration of 5
minutes, the resulting structure 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 100 rpm or lower (FIG. 2B).
[0068] 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 as shown by arrow 23 toward the region 25 into
which nickel is not directly introduced.
[0069] 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.
[0070] It was confirmed through transmission electron microscopy
(TEM) and electron diffraction that:
[0071] (a) the crystals grown in a lateral direction are
monocrystalline in the form of needle or column having uniform
widths;
[0072] (b) the growth direction of the crystals are approximately
parallel with the substrate surface although it depends upon the
film thickness; and
[0073] (c) the growth direction of the crystals are substantially
aligned with the [111] axis of the crystals.
[0074] From the foregoing facts, it can be concluded that the
surface of the lateral growth region 25 has a plane which is
perpendicular to the [111] direction, namely at least one of planes
{111} and those expressed by {hkl} (h+k=l), for example, {110},
{123}, {134}, {235}, {145}, {156}, {257}, or {167}, or the
neighborhood thereof.
[0075] It should be noted that since crystalline silicon has a
diamond structure of which space group is indicated by Fd3m, when
the above index hid is even-odd mixing, a forbidden reflection
occurs and it can not be observed with the electron
diffraction.
[0076] FIG. 3 shows the relation between the distance (.mu.m) of
the crystal growth to the region 23 along the transverse (lateral)
direction and the nickel concentration (ppm) in the aqueous acetate
solution.
[0077] FIG. 3 reads that a crystal growth for a distance of 25
.mu.m or longer can be realized by preparing a solution containing
nickel at a concentration of 100 ppm or higher. It can be also
assumed from FIG. 3 that a crystal growth along the lateral
direction of about 10 .mu.m can be obtained by using an aqueous
acetate solution containing nickel at a concentration of 10
ppm.
[0078] The datum plotted in FIG. 3 are for the case that the
structure was held for a duration of 5 minutes after applying the
nickel-containing aqueous acetate solution. However, the distance
of crystal growth along the lateral direction changes with the
retention time.
[0079] In case of using an aqueous acetate solution containing
nickel at a concentration of 100 ppm, for instance, longer distance
of crystal growth can be obtained with increasing retention time up
to 1 minute. However, once a retention time of 1 minute or longer
is set, the further increase becomes insignificant.
[0080] In case an aqueous acetate solution containing nickel at a
concentration of 50 ppm is used, the retention time increases
proportional to the distance of the crystal growth along the
lateral direction. However, the increment tends to saturate with
increasing retention time to 5 minutes or longer.
[0081] Furthermore, it should be noted that temperature greatly
influences the time necessary for a reaction to achieve an
equilibrium. Accordingly, the retention time is also subject to the
temperature, and a strict control of the temperature is
indispensable. Thus, the distance of crystal growth can be
increased in total by elevating the temperature of heat treatment
and by elongating the duration of the heat treatment.
[0082] FIGS. 4 and 5 show the nickel concentration in a silicon
film obtained by introducing nickel using an aqueous acetate
solution containing 100 ppm nickel and thereafter heat treating the
silicon film at 550.degree. C. for a duration of 4 hours. The
nickel concentration is obtained by secondary ion mass spectroscopy
(SIMS).
[0083] FIG. 4 shows the nickel concentration of the region 24 shown
in FIG. 2C, i.e., the region into which nickel is directly
incorporated. FIG. 5 shows the nickel concentration of the region
25 in FIG. 2C, i.e., the region in which crystal growth occurred
along the lateral direction from the region 22.
[0084] By comparing the data of FIG. 4 with that of FIG. 5, it can
be seen that the nickel concentration of the region in which the
crystal growth occurs along the lateral direction is lower by about
one digit as compared with that of the region into which nickel is
introduced directly.
[0085] It can be seen also that the nickel concentration in the
crystallized silicon film in the region into which nickel is
introduced directly can be suppressed to a level of 10.sup.18
cm.sup.-3 by using an aqueous acetate solution containing nickel at
a concentration of 10 ppm.
[0086] Conclusively, it is understood that the nickel concentration
in the crystalline silicon region in which the crystal growth
occurs along the lateral direction can be suppressed to 10.sup.17
cm.sup.-3 or lower by using an aqueous acetate solution containing
nickel at a concentration of 10 ppm and effecting the heat
treatment at 550.degree. C. or higher for a duration of 4 hours or
longer.
[0087] In conclusion, 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 by controlling the density of
the solution and the retention time and further to maintain the
concentration of the nickel in the lateral growth region 25 below
that.
[0088] For comparison, a sample is prepared through a process in
which, instead of using a nickel containing solution, an amorphous
silicon film is exposed to a plasma which is produced by using an
electrode containing an amount of nickel in order to add the nickel
into the silicon (this is called as a plasma treatment), and
further the silicon film is crystallized by a heat annealing at
550.degree. C. for 4 hours. The condition of the plasma treatment
is selected so that the same degree of a lateral crystal growth can
be obtained as in the case where an acetic acid containing nickel
at 100 ppm is used. The SIMS data with respect to this sample is
shown in FIG. 7. As can be seen, in the case of using a plasma
treatment, the nickel concentration in the lateral growth region is
higher than 5.times.10.sup.18 atoms/cm.sup.3 which is undesirably
high for an active region of a semiconductor device. Accordingly,
it is to be understood that the use of a solution is advantageous
for minimizing the concentration of the nickel in the lateral
growth region.
[0089] FIG. 8 shows a result of Raman spectroscopy with respect to
the region corresponding to FIG. 4, namely, the region where the
nickel is directly introduced. FIG. 8 indicates that the
crystallinity in this region is extremely high. Also, FIG. 9 shows
a result of Raman spectroscopy with respect to the region where the
crystal grows laterally. As can be seen, even in the lateral growth
area, the intensity of the Raman spectrum is more than 1/3 of the
intensity of the single crystal silicon. Accordingly, it is
concluded that the crystallinity in the lateral growth region is
also high.
[0090] 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 it is necessary to
pattern a silicon oxide film which is formed over the crystalline
silicon film for forming a contact hole therethrough, a
hydrofluoric acid is usually used as an etchant. If the crystalline
silicon film has a sufficiently high resistance against the
hydrofluoric acid, a large selection ratio (the difference in the
etching rate of the silicon oxide film and the crystalline silicon
film) can be objected so as to remove the silicon oxide film alone.
Accordingly, a crystalline silicon film having high resistance
against attack of hydrofluoric acid is of great advantage in the
fabrication process of a semiconductor device.
Example 3
[0091] The present example relates to a process for fabricating
TFTs which are provided to each of the pixels of an active matrix
liquid crystal display device, using a crystalline silicon film
fabricated by the process according to the present invention. The
TFTs thus obtained can be applied not only to liquid crystal
display devices, but also to a wide field generally denoted as thin
film integrated circuits (ICs).
[0092] Referring to FIGS. 6A to 6E, the process for fabricating a
TFT according to the present example is described below. A silicon
oxide film (not shown in the figure) is deposited to a thickness of
2,000 A as a base coating on a glass substrate. This silicon oxide
film is provided to prevent the diffusion of impurities into the
device structure from the glass substrate.
[0093] An amorphous silicon film is deposited thereafter to a
thickness of 1,000 A 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 A by means of UV irradiation under a gaseous
oxygen atmosphere.
[0094] The resulting amorphous silicon film having the oxide film
thereon is coated with an aqueous acetate solution containing
nickel at a concentration of 10 ppm. The resulting structure is
retained for a duration of 5 minutes, and is subjected thereafter
to spin drying using a spinner. The silicon oxide film is removed
thereafter using a buferred hydrofluoric acid, and a silicon film
is crystallized by heating the resulting structure 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.
[0095] The silicon film thus crystallized is patterned to form an
island-like region 104 as shown in FIG. 6A. The island-like region
104 provides the active layer for the TFT. A silicon oxide film 105
is formed thereafter for a thickness of from 200 to 1,500 A at a
thickness of 1,000 .ANG.. The silicon oxide film functions as a
gate insulating film (FIG. 6A).
[0096] 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 to 600.degree. C., preferably in the range of
300 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 to 600.degree. C., preferably, in the range of from 400 to
550.degree. C. The film thus deposited is annealed in oxygen or
under ozone in the temperature range from 400 to 600.degree. C. for
a duration of from 30 to 60 minutes.
[0097] The crystallization of the silicon region 104 can be
accelerated by 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 radiation 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.
[0098] Subsequently, an aluminum film is deposited to a thickness
of from 2,000 A to 1 .mu.m by electron beam vapor deposition, and
is patterned to form 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 A, more specifically, at a thickness of, for
example, 2,000 A in this manner (FIG. 6B).
[0099] Impurities (specifically in this case, phosphorus) are
implanted into the island-like silicon film of the TFT in a
self-aligned 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 cm.sup.-2.
[0100] The crystallinity of the portion whose crystallinity is
impaired by the introduction of impurities 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 asec. 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 (regions doped with phosphorus) 108. The sheet resistance
of the regions is found to be in the range of 200 to 800
.OMEGA./square.
[0101] This step of laser annealing can be replaced by an RTA
process, i.e., a rapid thermal annealing process using a flash
lamp, which comprises elevating the temperature of the silicon film
rapidly to a range of from 1,000 to 1,200.degree. C. (as measured
on the silicon monitor). This method of annealing is also called as
RTP (rapid thermal process).
[0102] A silicon oxide film is deposited thereafter to a thickness
of 3,000 A as an interlayer dielectric 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 to
450.degree. C., for instance, at 350.degree. C. A smooth surface is
obtained thereafter by mechanically polishing the resulting silicon
oxide film. An ITO coating is deposited thereon by sputtering, and
is patterned to provide a pixel electrode 111 (FIG. 6D).
[0103] The interlayer dielectric 110 is etched to form contact
holes in the source/drain as shown in FIG. 6E, and interconnections
112 and 113 are formed using chromium or titanium nitride to
connect the interconnection 113 to the pixel electrode 111.
[0104] In the process according to the present invention, nickel is
incorporated into the silicon film by using an aqueous solution
containing nickel at such a low concentration of 10 ppm.
Accordingly, a silicon film having a high resistance against
hydrofluoric acid can be realized and contact holes can be formed
stably and with high reproducibility.
[0105] A complete TFT can be formed by finally annealing the
silicon film in hydrogen in a temperature range of 300 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.
[0106] In accordance with the present example, the concentration of
the nickel contained in the active layer is in the range of
5.times.10.sup.16 to 3.times.10.sup.18 atoms/cm.sup.3.
[0107] As described above, the process according to the present
example comprises crystallizing the portion into which nickel is
introduced. However, the process can be modified as in Example 2.
That is, nickel can be incorporated to selected portions through a
mask, and crystals may be allowed to grow from the portions in a
lateral direction. This region of crystal growth is used for the
device. A device far more preferred from the viewpoint of electric
stability and reliability can be realized by further lowering the
nickel concentration of the active layer region of the device.
Example 4
[0108] This example is directed to a manufacture of a TFT used to
control a pixel of an active matrix. FIGS. 10A-10F are cross
sectional views for explaining the manufacture of the TFT in
accordance with this example.
[0109] Referring to FIG. 10A, a substrate 201, for example glass
substrate, is washed and provided with a silicon oxide film 202 on
its surface. 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 A, for example. Then, an amorphous
silicon film 203 of an intrinsic type having a thickness of
500-1500 A, for example, 1000 A is formed on the silicon oxide film
202, following which a silicon oxide film 205 of 500-2000 A, for
example 1000 A is formed on the amorphous silicon film
successively. Further, the silicon oxide film 205 is selectively
etched in order to form an opening 206 at which the amorphous
silicon film is exposed.
[0110] Then, a nickel containing solution (an acetic acid salt
solution here) is coated on the entire surface in the same manner
as set forth 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.
[0111] The amorphous silicon film 203 provided with the nickel
containing film in contact therewith is crystallized through a heat
annealing at 500-620.degree. C. for 4 hours in a nitrogen
atmosphere. The crystallization starts from the region under the
opening 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
axes of [111].
[0112] After the crystallization, the silicon oxide film 205 is
removed. At this time, an oxide film formed on the silicon film in
the opening 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. 10B. It should be noted that
the nickel is contained in the silicon film at a higher
concentration not only under the opening 206 where the nickel is
directly added but also at a portion 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
portions at which nickel is contained at a higher
concentration.
[0113] The patterned active layer 208 is then exposed to an
atmosphere containing 100% aqueous vapor of 10 atm at
500-600.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 A. 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. HCI may be added to
the atmosphere at 0.1 to 10%. 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 900-1200.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 50-200.degree. C./sec. and also the substrate is cooled
naturally at 20-100.degree. C./sec. Since the IR light can heat the
silicon film selectively, it is possible to minimize the heating of
the glass substrate.
[0114] Referring to FIG. 10C, an aluminum film is formed by
sputtering method to a thickness of 3000-8000 A, for example, 6000
A and then patterned into a gate electrode 210. The aluminum film
may preferably contain scandium at 0.01-0.2%.
[0115] Referring to FIG. 10D, 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 A, which will determine the
size of an offset gate area which is to be formed in a later step
as discussed below.
[0116] Referring then to FIG. 10E, using the gate electrode and the
surrounding anodic oxide film 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 MITT) 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.
[0117] 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
200-450.degree. C. to enhance the effect of the irradiation.
[0118] Referring to FIG. 10F, an interlayer insulating film 214 of
silicon oxide is formed through a plasma CVD to a thickness of 6000
A. Further, a transparent polyimide film 215 is formed by spin
coating to obtain a leveled surface. Then, a transparent conductive
film made of indium tin oxide for example is formed on the leveled
surface by sputtering to a thickness of 800 A and patterned into a
pixel electrode 216.
[0119] The interlayer insulating films 214 and 215 are provided
with contact holes therethrough, 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.
Example 5
[0120] This example is directed to a manufacture of a TFT and will
be described with reference to FIGS. 11A-11D. The same reference
numerals will be referred to for describing the same or similar
elements as those of the previous example.
[0121] Referring to FIG. 11A, a base film 202 of silicon oxide is
initially formed on a Corning 7059) substrate 201 by sputtering to
2000 A thick. The substrate is annealed at a temperature higher
than a distortion point of the substrate following which the glass
is cooled to a temperature less than the distortion point at a rate
of 0.1-1.0.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 facilitated. This
step may be performed either before or after the formation of the
base film 201 or it may be done both before and after the formation
of the base film 201. In the case of using the Corning 7059
substrate, the substrate may be heated at 620-660.degree. C. for
1-4 hours, following which it is cooled at 0.1-0.3.degree. C. and
taken out from a furnace when the temperature decreases to
400-500.degree. C.
[0122] Then, an intrinsic (I-type) amorphous silicon film is formed
to 500-1500 .ANG. thick, for example, 1000 A through plasma CVD.
The amorphous silicon film is crystallized in the same manner as in
Example 1. Therefore, the redundant explanation will be omitted.
After the crystallization, the silicon film is patterned into an
island form having a dimension of 10-1000 microns square.
Accordingly, a crystalline silicon film 208 in the form of an
island is formed as an active layer of a TFT as shown in FIG.
11A.
[0123] Referring to FIG. 11B, the surface of the silicon film is
oxidized by exposing the surface to an oxidizing atmosphere to form
an oxide film 209. The oxidizing atmosphere contains an aqueous
vapor at 70-90%. The pressure and the temperature of the atmosphere
is 1 atm and 500-750.degree. C., typically 600.degree. C. The
atmosphere is produced by a pyrogenic reaction from oxygen and
hydrogen gases with a hydrogen/oxygen ratio being 1.5-1.9. The
silicon film is exposed to the thus formed atmosphere for 3-5
hours. As a result, the oxide film 209 having a thickness of
500-1500 A, for example, 1000 A is formed. Since the surface of the
silicon film is reduced (eaten) by 50 A or more due to the
oxidation, an effect of a contamination of the upper most surface
of the silicon film does not extend to the silicon-silicon oxide
interface. In other words, by the oxidation, it is possible to
obtain a clean silicon-silicon oxide interface. Also, since the
thickness of the silicon oxide film is two times as the thickness
of the portion of the silicon film to be oxidized, when the silicon
film is originally 1000 A thick and the silicon oxide film obtained
is 1000 A, the thickness of the silicon film remaining after the
oxidation is 500 A.
[0124] Generally, the thinner a silicon oxide film (gate insulating
film) and an active layer are, the higher a mobility is and the
smaller an off current is. On the other hand, a preliminary
crystallization of an amorphous silicon film is easier when its
thickness is thicker. Accordingly, there was a contradiction in the
crystallization process and electrical characteristics with respect
to the thickness of the active layer. The present example
advantageously solves this problem. That is, the amorphous silicon
film having a larger thickness is initially formed so that a better
crystalline silicon film can be obtained, following which the
thickness of the silicon film is reduced by the oxidation,
resulting in an improvement of characteristics of the active layer
of a TFT. Moreover, an amorphous component or grain boundaries
contained in the crystalline silicon film tend to be oxidized
during the thermal oxidation, resulting in a decrease in
recombination centers contained the active layer.
[0125] After the formation of the silicon oxide film 209 through
thermal oxidation, the substrate is annealed in a 100% monoxide
dinitrogen atmosphere at 1 atm and 600.degree. C. for 2 hours.
[0126] Referring to FIG. 11C, a silicon containing 0.01 to 0.2%
phosphorous is deposited through low pressure CVD to 3000-8000 A
thick, for example, 6000 A, and then patterned into a gate
electrode 210. Further, using the gate electrode 210 as a mask, an
N-type conductivity impurity is added into a portion of the active
layer in a self-aligning manner by ion doping. Phosphine is used as
a dopant gas. The doping condition is substantially the same as in
the Example 4. The dose amount is, for example, 5.times.10.sup.15
cm.sup.-2. Thus, N-type impurity regions 212 and 213 are
formed.
[0127] Thereafter, an annealing is performed with a KrF excimer
laser in the same manner as set forth in Example 4. The laser
annealing may be replaced by a lamp annealing with a near infrared
ray. The near infrared ray is absorbed by crystalline silicon more
effectively than by amorphous silicon. Accordingly, the annealing
with the near infrared ray is comparable with a thermal annealing
at 1000.degree. C. or more. On the other hand, it is possible to
prevent the glass substrate from being detrimentally heated
inasmuch as the near infrared ray is not so absorbed by the glass
substrate. That is, although a far infrared ray can be absorbed by
a glass substrate, visible or near infrared ray of which wavelength
ranges from 0.5-4 .mu.m are not so absorbed.
[0128] Referring to FIG. 11D, an interlayer insulating film 214 of
silicon oxide is formed to 6000 .ANG. thick by a plasma CVD. A
polyimide may be used instead of silicon oxide. Further, contact
holes are formed through the insulating film. Electrode/wirings 217
and 218 are formed through the contact holes by using a multilayer
of titanium nitride and aluminum films. Finally, an annealing in a
hydrogen atmosphere is conducted at 350.degree. C. and 1 atm for 30
minutes. Thus, a TFT is completed.
[0129] The mobility of the thus formed TFT is 110-150 cm.sup.2/Vs.
The S value is 0.2-0.5 V/digit. Also, in the case of forming a
P-channel type TFT by doping boron into source and drain regions,
the mobility is 90-120 cm.sup.2/V s and the S value is 0.4-0.6
V/digit. The mobility in accordance with the present example can be
increased by 20% or more and the S value can be reduced by 20% or
more as compared with a case where a gate insulating film is formed
by a known PVD or CVD.
[0130] Also, the reliability of the TFT in accordance with the
present example is comparable to that of a TFT which is produced
through a thermal oxidation at a temperature as high as
1000.degree. C.
Example 6
[0131] FIG. 12 shows an example of an active matrix type liquid
crystal device in accordance with the present example.
[0132] In the figure, reference numeral 61 shows a glass substrate,
and 63 shows a pixel area having a plurality of pixels in the form
of a matrix each of which is provided with a TFT as a switching
element. Reference numeral 62 shows peripheral driver region(s) in
which driver TFTs are provided in order to drive the TFTs of the
pixel area. The pixel area 63 and the driver region 62 are united
on the same substrate 61.
[0133] The TFTs provided in the driver region 62 need to have a
high mobility in order to allow a large amount of electric currents
to pass therethrough. Also the TFTs Provided in the pixel area 63
need to have a lower leak current property in order to increase a
charge retention ability of pixel electrodes. For example, the TFTs
manufactured in accordance with Example 3 are suitable as the TFTs
of the pixel area 63.
Example 7
[0134] The present example is a modification of Example 1. That is,
before forming a nickel acetate aqueous solution, a rubbing
treatment is performed on a silicon oxide surface in order to form
number of minute scratches there.
[0135] Referring to FIG. 13A, a Corning 7059 substrate 11 having a
silicon oxide film as a base film 18 is provided. The silicon oxide
film is formed by sputtering to a thickness of 2000 A for example.
On the silicon oxide film, an amorphous silicon film 12 is formed
by plasma CVD to a thickness of 300-800 A, for example, 500 A.
Subsequently, the surface of the amorphous silicon film is treated
with a hydrofluoric acid in order to remove a contamination or a
natural oxide formed thereon. After that, a silicon oxide film of
10-100 A thick is formed by exposing the substrate in an oxygen
atmosphere with the surface being irradiated with a UV light (not
shown). The oxidation may be carried out with a hydrogen peroxide
treatment or thermal oxidation.
[0136] Then, fine scratches (unevenness or irregularity) are formed
on the silicon oxide film by a rubbing treatment as shown by
reference numeral 17. The rubbing treatment is carried out with a
diamond paste. However, a cotton cloth or a rubber may be used
instead of diamond paste. It is desirable that scratches have a
uniform direction, width and gap.
[0137] After the rubbing treatment, a film of nickel acetate is
formed by spin coating in the same manner as in Example 1. The
nickel acetate solution is absorbed by the scratches uniformly.
[0138] Referring to FIG. 13B, the amorphous silicon film is then
furnace annealed at 550.degree. C. for 4 hours in a nitrogen
atmosphere like in Example 1. Thus, a crystalline silicon film is
obtained. The grain sizes and orientation directions of the grains
19 in the thus obtained film are more uniform than that obtained in
Example 1. The grains 19 are extended in one direction and have an
approximately rectangular or ellipse shape or the like.
[0139] The dimension or number of scratches can be controlled by
changing a density of the diamond paste. Since it is difficult to
observe the scratches with a microscope, the rubbing condition is
determined in such a manner that the size of grains or density of
remaining amorphous silicon in the obtained crystalline silicon
film can be maximized. In this example, the condition of the
treatment is selected so that lengths of amorphous regions which
remain after the crystallization be 1 .mu.m or less, preferably,
0.3 .mu.m or less.
[0140] In the case of Example 1 in which a rubbing treatment is not
performed, there is a tendency that the nickel is not uniformly
diffused and non-crystallized regions in the form of 1-10 .mu.m
circles are observed. Accordingly, the rubbing treatment improves
the uniformity of the obtained crystals.
Example 8
[0141] The present example is directed to a manufacturing process
of TFTs for switching pixels of an active matrix in accordance with
Example 7. FIGS. 14A-14E are cross sectional views showing the
manufacturing process.
[0142] Referring to FIG. 14A, a silicon oxide film 202 is formed by
a plasma CVD to a thickness of 3000 A on a substrate 201 made of
Corning 7059 glass (10 cm square). Then, an amorphous silicon film
203 is formed by plasma CVD to a thickness of 300-1000 A, for
example, 500 A on the silicon oxide film 202.
[0143] The thus formed amorphous silicon film is crystallized by
the process as set forth in Example 7. After the thermal
crystallization, a laser annealing is performed in order to improve
the crystallinity with a Kr excimer laser (248 nm wavelength)
having a power density 200-350 mJ/cm.sup.2. As a result, amorphous
components which remain in the crystalline silicon film are
completely crystallized.
[0144] After the crystallization, the silicon film 203 is patterned
into an island form silicon film 208 as shown in FIG. 14B. At this
time, the location and the direction of the silicon island with
respect to grain boundaries can be selected in such a manner as
shown in FIGS. 15A and 15B.
[0145] When an electric current of a TFT crosses grain boundaries,
the grain boundaries function as a resistance. On the other hand,
the electric current is easy to flow along grain boundaries.
Accordingly, the electrical characteristics of a TFT is greatly
influenced by the number and direction of the grains (grain
boundaries) included in the channel region. For example, when there
are a number of TFTs, a leak current property of each TFT varies
depending upon the number and direction of the grains contained in
the channel region thereof.
[0146] The above problem becomes serious when the size of the
grains is approximately the same as the size of the channel or is
smaller than that. When the channel is sufficiently larger than
grains, this dispersion is averaged and is not observed
significantly.
[0147] For example, if there is no grain boundary in the channel,
it can be expected that the TFT has an electrical property which is
the same as that of a single crystalline TFT. On the other hand,
when grain boundaries extend through the island along a direction
of a drain current, the leak current becomes larger. In contrast,
when grain boundaries extend in a direction perpendicular to a
direction of a drain current, the leak current becomes smaller.
[0148] When TFTs are arranged in such a manner that its drain
current flows in a direction along the rubbing direction, since
crystals lengthen along the rubbing direction, the number of grain
boundaries included in a channel tends to be nonuniform and
therefore the leak current is likely to disperse. Moreover, the
intensity of the leak current becomes larger because the grain
boundaries are aligned with the direction of the drain current as
shown in FIG. 15A. On the other hand, as shown in FIG. 15B, if a
drain current flows in a direction perpendicular to the rubbing
direction, the off current property can be stabilized. This is
because the width of the grains 19 are approximately constant and
the number of grains existing in the channel region 26 can be made
constant. In conclusion, it is desirable to arrange the active
region 208 in such a way that a drain current of a TFT flows in a
direction perpendicular to the direction of grain boundaries, i.e.
the rubbing directions. Moreover, the rubbing treatment makes the
size of crystal grains uniform, which results in that
non-crystallized region can be epitaxially crystallized by a
subsequent laser irradiation.
[0149] As shown in FIG. 14B, a silicon oxide film of 200-1500 A
thick, for example, 1000 A thick is formed as a gate insulating
film 209 through plasma CVD.
[0150] Then, an aluminum containing Si at 1 weight % or Sc at 0.1
to 0.3 weight % is sputter formed to 1000 A to 3 .mu.m, for example
5000 A, following which it is patterned into a gate electrode 210.
The aluminum electrode is then subjected to an anodic oxidation
process using an ethylene glycol solution containing a tartaric
acid at 1-3%. The pH of the electrolyte is about 7. A platinum
electrode is used as a cathode while the aluminum electrode is used
as an anode. The voltage is increased with an electric current
maintained constant until it reaches 220 V and then this condition
is maintained for one hour. As a result, an anodic oxide film 211
is formed to a thickness of 1500-3500 A, for example 2000 A.
[0151] Referring to FIG. 14C, an impurity having one conductivity
type (boron) is introduced into the silicon island through an ion
doping method with the gate electrode 210 used as a mask in a
self-aligning manner. Diborane (B.sub.2H.sub.6) is used as a dopant
gas. The dose amount is 4-10.times.10.sup.15 cm.sup.-2. The
acceleration voltage is 65 kV. Thus, a pair of impurity regions
(p-type) 212 and 213 are obtained.
[0152] Thereafter, the impurity regions 212 and 213 are activated
by irradiating KrF excimer laser (248 nm wavelength, 20 nsec. pulse
width). The energy density of the laser beam is 200-400
mJ/cm.sup.2, preferably, 250-300 mJ/cm.sup.2.
[0153] Referring to FIG. 14D, an interlayer insulating film 214
made of silicon oxide is formed through plasma CVD to a thickness
of 3000 A. Then, a contact hole is formed on the impurity region
212 (source) through the interlayer insulating film 214 and the
gate insulating film 209 by etching. An aluminum film is then
formed by sputtering and patterned to form a source electrode
217.
[0154] Referring to FIG. 14E, silicon nitride is deposited through
plasma CVD to 2000-6000 .ANG. as a passivation film 215. A contact
hole is formed on the impurity region (drain) 213 through the
passivation film 215, interlayer insulating film 214 and the gate
insulating film 209 by etching. Finally, an indium tin oxide film
(ITO) is formed into a pixel electrode 216. Thus, a pixel TFT is
obtained.
[0155] While the present invention has been disclosed in preferred
embodiments, it is to be understood that the scope of the present
invention should not be limited to the specific examples of the
embodiments. Various modifications may be made.
[0156] For example, the nickel containing film may be formed by
using a non-aqueous solution such as alcohol. When using an
alcohol, the solution may be directly formed on the amorphous
silicon film without using an oxide film. Specifically, a nickel
containing compound such as nickel acetyl acetonate may be
dissolved by ethanol. This material can be decomposed during the
heating for the crystallization because the decomposition
temperature thereof is relatively low. The amount of the nickel
acetyl acetonate is selected so that the concentration of the
nickel in the solution is controlled to be 100 ppm. The nickel
containing film can be obtained by coating the solution and then
dried by a spin dry method at 1500 rpm for 1 minute. Also, since
the contact angle of the alcohol is smaller than that of water, the
amount of the solution used for forming the film may be smaller
than in the case when a water solution is used. In this case, a
drop of 2 ml with respect to 100 mm square is appropriate. The
subsequent steps for forming the crystalline silicon may be
entirely the same as those explained in the preferred
embodiments.
[0157] For another example, an elemental nickel may be dissolved by
an acid. That is, a nitric acid of 0.1 mol/l is used as an acid.
Nickel powder is dissolved in this acid at 50 ppm.
* * * * *