U.S. patent application number 12/078832 was filed with the patent office on 2009-04-23 for thin-film transistor.
Invention is credited to Shunpei Yamazaki, Hongyong Zhang.
Application Number | 20090101910 12/078832 |
Document ID | / |
Family ID | 26473051 |
Filed Date | 2009-04-23 |
United States Patent
Application |
20090101910 |
Kind Code |
A1 |
Zhang; Hongyong ; et
al. |
April 23, 2009 |
Thin-film transistor
Abstract
A gate-insulated thin film transistor is disclosed. One
improvement is that the thin film transistor is formed on a
substrate through a blocking layer in between so that it is
possible to prevent the transistor from being contaminated with
impurities such as alkali ions which exist in the substrate. Also,
a halogen is added to either or both of the blocking lay r and a
gate insulator of the transistor.
Inventors: |
Zhang; Hongyong; (Kanagawa,
JP) ; Yamazaki; Shunpei; (Tokyo, JP) |
Correspondence
Address: |
NIXON PEABODY, LLP
401 9TH STREET, NW, SUITE 900
WASHINGTON
DC
20004-2128
US
|
Family ID: |
26473051 |
Appl. No.: |
12/078832 |
Filed: |
April 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10642305 |
Aug 18, 2003 |
7355202 |
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12078832 |
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08611571 |
Mar 6, 1996 |
6607947 |
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10642305 |
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08219286 |
Mar 28, 1994 |
5523240 |
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08611571 |
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08044883 |
Apr 9, 1993 |
5313075 |
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08219286 |
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07704103 |
May 22, 1991 |
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08044883 |
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Current U.S.
Class: |
257/66 ;
257/E29.117 |
Current CPC
Class: |
H01L 29/66757 20130101;
H01L 29/4908 20130101; H01L 27/1214 20130101; Y10S 148/118
20130101 |
Class at
Publication: |
257/66 ;
257/E29.117 |
International
Class: |
H01L 29/72 20060101
H01L029/72 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 1990 |
JP |
2-140580 |
Oct 29, 1990 |
JP |
2-293264 |
Claims
1. A semiconductor device comprising: a glass substrate; a
protective film formed over the glass substrate; a
non-single-crystalline semiconductor film formed over the
protective film and including source and drain regions and a
channel region formed between the source and drain regions, and a
gate electrode adjacent to the channel region with a gate insulator
therebetween, wherein the protective film includes a halogen
element.
2. The semiconductor device of claim 1, wherein the
non-single-crystalline semiconductor film comprises crystalline
silicon.
3. The semiconductor device of claim 1, wherein the protective film
comprises silicon oxide.
4. The semiconductor device of claim 1, wherein the gate electrode
is located over the channel region.
5. The semiconductor device of claim 1, wherein the protective film
is 500 .ANG.-5000 .ANG. thick.
6. A semiconductor device comprising: a glass substrate; a
protective film formed over the glass substrate; a semiconductor
film formed over the protective film and including source, drain
and channel regions; a gate electrode formed over the channel
region with a gate insulator therebetween; and wherein the
protective film contains a halogen element at a concentration not
higher than 5 atom %.
7. The semiconductor device of claim 6, wherein the gate insulator
and the protective film comprises a same insulating material with
each other.
8. A semiconductor device comprising: a glass substrate; a
protective film formed over the glass substrate; a
non-single-crystalline semiconductor film formed over the
protective film and including source, drain and channel regions; a
gate electrode adjacent to the channel region with a gate insulator
therebetween, wherein each of the gate insulator and the protective
film contains a halogen element.
9. The semiconductor device of claim 8, wherein the halogen element
is selected from the group consisting of fluorine or chlorine.
10. The semiconductor device of claim 8, wherein the channel region
comprises polycrystalline silicon.
11. The semiconductor device of claim 8, wherein the channel region
comprises microcrystalline silicon.
12. The semiconductor device of claim 8, wherein the halogen
element is contained at a concentration not higher than 5 atom
%.
13. A semiconductor device comprising: a glass substrate; a
protective film comprising silicon oxide formed on and in contact
with the glass substrate; a semiconductor film formed over the
protective film and including source and drain regions and a
channel region extending therebetween; a gate insulator formed over
the channel region; and a gate electrode formed over the gate
insulator, wherein the protective film contains a halogen element
and directly contacts the semiconductor film.
14. The semiconductor device of claim 13, wherein the halogen
element is contained in the protective film at a concentration of
not lower than 0.1 atom %.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a thin-film transistor (from here
on will also be referred to as a TFT) which is made of
non-single-crystal semiconductor, for example an IG-FET, and its
manufacturing process, and in more particular, to a highly reliable
thin-film transistor which is suitable for use as a driving element
of a display image sensor or liquid crystal device or the like.
[0002] Thin-film transistors can be formed by a chemical vapor
deposition method on an insulated substrate in a comparatively low
temperature atmosphere, with a maximum temperature of 500.degree.
C., and the substrate being made of an inexpensive material such as
soda glass or boron-silicate glass.
[0003] This thin-film transistor is a field-effect transistor and
has the same features as a MOSFET. In addition, as mentioned above,
it has the advantage that it can be formed on an inexpensive
insulated substrate at a low temperature. Also the thin film
transistors can be formed on a large substrate by the use of CVD
techniques. It is therefore a very good prospect for use as
switching elements of a matrix type liquid crystal display having a
lot of picture elements, or as switching elements of a
one-dimensional or two-dimensional image sensor.
[0004] Also, the thin-film transistors can be formed using already
established photolithography technology, by which a very minute
process is possible, and transistors can be integrated just as
making an IC and so on. FIG. 1 shows the construction of a typical
prior art TFT.
[0005] In FIG. 1, the thin-film transistor is comprised of an
insulated substrate 20 made of glass, a semiconductor thin film 21
made of a non-single-crystal semiconductor, a source 22, a drain
23, a source electrode 24, a drain electrode 25, gate insulating
film 26, and a gate electrode 27.
[0006] In this type of thin-film transistor, the current flow
between the source 22 and the drain 23 is controlled by applying a
voltage to the gate electrode 27. The response speed of the
thin-film transistor is given by the equation;
[0007] S=.mu.V/L.sup.2 where L is a channel length, .mu. is a
carrier mobility, and V is the gate voltage.
[0008] In this type of thin-film transistor, the non-single-crystal
semiconductor layer contains many grain boundaries. The
non-single-crystal semiconductor, when compared to the
single-crystal semiconductor, has disadvantages that the carrier
mobility is very low and thus the response speed of the transistor
is very slow due to the many grain boundaries. Especially if an
amorphous silicon semiconductor is used, the mobility is only about
0.1-1 (cm.sup.2/V.sec) and is too short to function for use as a
TFT.
[0009] It is obvious that to solve this problem the channel length
needs to be shortened and the carrier mobility increased. Many
improvements are being made.
[0010] When the channel length L is decreased, the effect it has on
the response speed is as the square of the length, and so it is a
very effective means. However, when forming elements on a large
area substrate, it is apparently difficult to use the
photolithography technique in order that the space between the
source and drain (this is essentially the channel length) should 10
.mu.m or less, due to the precise process, yield, and manufacturing
cost problems. Consequently, effective means for shortening the
channel length of the TFT have not been found.
[0011] On the other hand, to increase the mobility (.mu.) of the
semiconductor layer, single-crystal semiconductor or poly-crystal
semiconductor material is used, and when using amorphous
semiconductor material, after the semiconductor is formed, the
active region of the TFT should be crystallized using a process
such as heat treatment.
[0012] In this case, a temperature higher than what is normally
required to form a-Si is necessary. For example;
(1) For a thin-film transistor made of amorphous semiconductor
material, the amorphous silicon film is made at a temperature of
about 250.degree. C. and then a maximum temperature of 400.degree.
C. is required for thermal annealing. (2) When a poly-crystal
silicon film is formed by a low pressure CVD method, the maximum
temperature required for forming the film and then for
recrystallization is 500 to 650.degree. C. (3). For a thin-film
transistor where only an active layer is converted to a
poly-crystalline structure, the required CVD temperature for
forming the semiconductor layer is 250 to 450.degree. C., however
the temperature exceeds 600.degree. C. during a recrystallization
step of the active layer by CW laser.
[0013] The TFT is formed on a substrate made of a material such as
soda glass and the active region comes in direct contact with the
glass substrate, especially in the case of stagger-type and
coplanar-type transistors. When making a TFT that has sufficiently,
fast response speed, the heat treatment mentioned above is
necessary, and so the metallic alkali impurities such as sodium and
potassium which exist, in the glass substrate are externally
diffused and forced into the semiconductor layer which forms the
active layer or TFT. This lowers the mobility of the semiconductor
layer and changes the threshold value, making the characteristics
of the device worse and has an adverse effect on the long-term
reliability of the device.
[0014] Also, through operation of the TFT, the TFT produces heat
which causes the temperature of the glass substrate to rise thus
causing-impurities to be diffused from the substrate, which also
has an adverse effect on the TFT.
[0015] Generally, a gate-insulator of the IG-FET is made of a
silicon oxide film which is formed by a sputtering method with
argon (Ar) gas used as a sputtering gas. In the sputtering process,
the argon atoms are inherently introduced into the gate insulator
and generates a fixed charge in the semiconductor film. Also, ions
that exist in a reaction space during the sputtering collide with
the surface of the active layer of the thin-film transistor, which
causes a damage to the active layer. As a result, a mixed layer of
the active layer and the insulation layer is formed in the boundary
region of the gate insulation layer and the active layer of the
transistor. In producing a TFT as described above, the problems of
response speed and reliability need to be solved.
SUMMARY OF THE INVENTION
[0016] It is therefore an object of the present invention to
produce a high speed TFT which uses non-single-crystal
semiconductor. It is another object of the present invention to
solve the problem of reliability mentioned above.
[0017] In order to solve the above problems, in this invention an
insulation layer 500 .ANG. to 5000 .ANG. thick is formed on the
glass substrate as a bottom protective film before the TFT elements
are formed, and the TFT elements are formed on top of this
protective film. In this structure, it is possible to keep the
impurities existing in the glass substrate from going into the
active layer of a thin-film transistor or into the transistor
elements themselves, and to provide a thin-film transistor that has
high mutual conductance and high field-effect mobility. Also it
suppresses the diffusion of impurities from the substrate which
occurs when heat is generated during operation of the device. It
also provides a thin-film transistor that can control degeneration
of the electrical characteristics and has long-term stability and
reliability.
[0018] Also by adding a halogen element to the protective film or
to the gate insulator, impurities intruded from the outside or
impurities in the film can be neutralized. Interface states between
the insulation layer and the semiconductor layer can also be
reduced by the halogen element. This increases stability and
reliability of the TFT.
BRIEF EXPLANATION OF THE DRAWINGS
[0019] FIG. 1 shows a cross sectional view of a part of a prior art
thin film transistor;
[0020] FIGS. 2(A) to 2(C) show a first embodiment of a
manufacturing process of a thin-film transistor in accordance with
the present invention;
[0021] FIGS. 3(A) to 3(C) show a second embodiment of a
manufacturing process of the thin-film transistor in accordance
with the present invention;
[0022] FIGS. 4(A) to 4(D) show a third embodiment of a
manufacturing process of the thin-film transistor in accordance
with the present invention;
[0023] FIG. 5 is a graph to show a relationship between the
flatband voltage of an insulation film formed by a sputtering
method and the percentage of argon in the sputtering gas;
[0024] FIG. 6 is a graph to show a relationship between the
flatband voltage of the insulation film formed by a sputtering
method and the percentage of fluoride gas in the sputtering
gas;
[0025] FIG. 7 is a graph to show a relationship between the
withstand voltage of the insulation film formed by a sputtering
method and the percentage of fluoride gas in the sputtering
gas;
[0026] FIG. 8 is a graph to show a relationship between the
mobility of the non-single-crystal semiconductor formed by a
sputtering method and the partial pressure of hydrogen in the
sputtering gas;
[0027] FIG. 9 shows a relationship between the partial pressure of
hydrogen in the sputtering gas and the threshold voltage;
[0028] FIGS. 10 to 14 show the characteristics of the TFT source
current and the source voltage;
[0029] FIG. 15 shows a Raman spectrogram of the semiconductor layer
formed in the present invention;
[0030] FIG. 16 is a cross sectional view of a part of the structure
produced by a fourth embodiment of a manufacturing process of the
thin-film transistor in the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0031] Below the preferred embodiments of this invention will be
used to explain the above and other characteristics of this
invention.
Embodiment 1
[0032] The manufacturing process of the planar type thin-film
transistor in accordance with a first embodiment of the present
invention is shown in FIG. 2(A) to FIG. 2(C).
[0033] First a glass substrate 1 is made of soda glass and on an
entire surface of the substrate 1, a 300 nm thick silicon oxide
bottom protective film 2 is formed by sputtering. The formation
conditions of the film are shown below.
TABLE-US-00001 Sputtering Gas oxygen 100% Reaction Pressure 0.5 Pa
RF Power 400 W Substrate Temperature 150.degree. C. Film Formation
Speed 5 nm/min
[0034] Next, an approximately 100 nm thick I-type conductivity
non-single-crystal silicon semiconductor film 3 is formed by a CVD
method on the protective film 2. The manufacturing conditions are
shown below
TABLE-US-00002 Substrate Temperature 300.degree. C. Reaction
Pressure 0.05 Torr Rf Power (13.56 MHz) 80 W Gas Used SiH.sub.4
[0035] After this, a predetermined etching step is performed, so
that the structure shown in FIG. 2(A) is obtained.
[0036] Next, in at least one region of the semiconductor film 3 the
active layer is formed using an excimer laser to perform laser
anneal in this region allowing poly-crystallization. The conditions
are as follows.
TABLE-US-00003 Laser energy density 200 mJ/cm.sup.2 Number of
Irradiation Shots 50 times
[0037] Then a non-single-crystalline silicon layer 4 which has an
N-type conductivity is formed on the above structure by a CVD
method as a low resistance non-single-crystal semiconductor layer.
The formation conditions are as follows.
TABLE-US-00004 Substrate Temperature 220.degree. C. Reaction
Pressure 0.05 Torr Rf Power (13.56 MHz) 120 W Gas Used SiH.sub.4 +
PH.sub.3 Film Thickness 1500 .ANG.
[0038] When making this N-type non-single-crystal silicon
semiconductor layer 4, a large quantity of H.sub.2 gas can be used
and the RF power can be increased to form micro-crystals which
lowers the electrical resistance.
[0039] Then, a part of the N-type semiconductor layer 4 is etched
by using a photolithography so that it is patterned into source and
drain regions 4 and a channel region 7 is defined therebetween as
shown in FIG. 2B.
[0040] After that, hydrogen plasma processing is performed under
the following conditions to activate the channel region 7.
TABLE-US-00005 Substrate Temperature 250.degree. C. RF Power 100 W
Processing Time 60 minutes
[0041] On top of the structure, shown in FIG. 2B, a 100 nm thick
gate insulation film 5 is formed using the same material and same
method as the bottom protective film 2. The contact holes for the
source and drain regions are formed using an etching method and
then the source, drain, and gate electrodes 6 are formed using
aluminum. Through the above process, the IG-FET shown in FIG. 2(C)
is made.
[0042] In this embodiment, the gate insulation film 5 and the
bottom protective film 2 are made of the same material and are made
using the same method. Therefore during heat treatment of the
thin-film transistor, or when heat is generated during operation of
the transistor, there is no difference in the heat expansion of the
two and so there is no breakage or pealing of the aluminum or metal
electrodes on top, giving the transistor long-term reliability.
Embodiment 2
[0043] FIGS. 3A to 3C show a manufacturing process of an IG-FET in
accordance with a second embodiment of the present invention.
First, a 500 .ANG. to 5000 .ANG. thick silicon oxide film 2 is
formed by a sputtering method on top of the soda glass substrate 1
as a protective film in a same manner as in Embodiment 1. Next, on
the bottom protective film 2, a 200 nm thick molybdenum metallic
layer 10 is formed. Formed on top of this structure is a
non-single-crystal silicon film 8 which has a P-type conductor and
has a low resistance. The formation conditions this time are as
follows.
TABLE-US-00006 Substrate Temperature 230.degree. C. Reaction
Pressure 0.05 Torr Rf Power (13.56 MHz) 150 W Gas Used Si.sub.4 +
B.sub.2H.sub.6 Film Thickness 200 .ANG.
[0044] This semiconductor layer can have ohmic contact with the
I-type semiconductor layer that will be formed later in the
process.
[0045] Next, a predetermined pattern is etched, and the structure
shown in FIG. 3(A) is obtained. On top of this structure, a 200 nm
thick I-type non-single-crystal silicon semiconductor film 3 is
formed by a sputtering method. The formation conditions are as
follows.
TABLE-US-00007 Substrate Temperature 250.degree. C. Reaction
Pressure 0.2 Pa Rf Power (13.56 MHz) 80 W Gas Used Ar
[0046] Then, using the same process as described in Embodiment 1,
the I-type semiconductor layer 3 is heat treated causing
poly-crystallization and using a hydrogen plasma process it is
activated and the structure shown in FIG. 3(B) is obtained.
[0047] Further, SiO.sub.2 is formed by sputtering to be 100 nm
thick as a gate insulator 5 in the same manner as in the Embodiment
1, after which molybdenum gate electrode 9 is formed in the
predetermined pattern. Thus a thin-film transistor is formed as
shown in FIG. 3(C).
[0048] In this embodiment, because there is a metallic electrode
underneath the low resistance semiconductor layer 8, the wire
resistance is very low. For a TFT that is used as the switching
element of a large area liquid crystal device, if the wire
resistance is low, the drive signal wave form is not distorted and
the liquid crystal device can be driven at a high speed.
[0049] The silicon oxide film of this embodiment is formed using
the sputtering method but may also be formed using photo CVD,
plasma CVD, or thermal CVD.
Embodiment 3
[0050] This embodiment will be explained referring to FIG. 4(A) to
FIG. 4(D). In this embodiment a halogen element is added to the
protective film on the glass substrate or to the gate insulator of
the IG-FET or more preferably to the both.
[0051] In FIG. 4(A) a 200 nm thick SiO.sub.2 film 12 is formed on a
glass substrate 11 using a magnetron-type RF sputtering method with
the following formation conditions.
TABLE-US-00008 Reaction Gas O.sub.2 95% volume NF.sub.3 5% volume
Film Formation Temperature 150.degree. C. RF Power (13.56 MHz) 400
W Pressure 0.5 Pa Silicon is used as a target.
[0052] On top of this film 12, a 100 nm thick a-Si film 13 is
formed by a magnetron RF sputtering in order to form a channel
region, so that the structure shown in FIG. 4(A) is obtained. The
film formation is done in an atmosphere of inert gas of argon and
hydrogen and in the conditions shown below.
TABLE-US-00009 H.sub.2/(H.sub.2 + Ar) = 80% (partial pressure
ratio) Film Formation Temperature 150.degree. C. RF Power (13.56
MHz) 400 W Total Pressure 0.5 Pa
Single crystal silicon is used as the target.
[0053] After this, at a temperature of 450.degree. C. to
700.degree. C. for example at 600.degree. C. and in an atmosphere
of hydrogen or inactive gas, in this embodiment 100% nitrogen is
used, the a-Si film 13 is heat-crystallized for 10 hours, so that a
silicon semiconductor layer having a high crystallinity is
obtained. Besides, if a non-single crystalline silicon target is
used and the input power is lowered, the crystal size becomes
smaller and the crystalline condition becomes dense and therefore
the subsequent heat-crystallization of the film will be
facilitated.
[0054] Patterning is performed on this heat crystallized silicon
semiconductor, and the structure shown in FIG. 4(B) is obtained. In
a portion of the semiconductor layer 13, the channel formation
region of the insulated-gate semiconductor will be formed.
[0055] Next, a 100 nm thick silicon oxide film (SiO.sub.2) 15 is
formed by the magnetron-type RF sputtering method in the following
formation conditions.
TABLE-US-00010 Oxygen 95% volume; NF.sub.3 5% volume Pressure 0.5
Pa Film Formation Temperature 100.degree. C. RF Power (13.56 MHz)
400 W
A silicon target or synthetic quartz target is used.
[0056] If an amorphous silicon target is used and the applied power
is lowered, a densified silicon oxide film is obtained where it is
difficult for a fixed charge to exist.
[0057] When the silicon oxide film used in this invention, for
example the gate insulation film, is formed using the sputtering
method, it is preferable that the percentage of the inert gasses is
lower than 50% with respect to the halogen and oxide gasses,
desirably no inert gas.
[0058] Also, if a halogen containing gas is mixed with an oxygen
containing gas at 2-20% volume, it is possible to neutralize the
alkali ions that are incidentally mixed into the silicon oxide film
15, and at the same time makes it possible to neutralize the
silicon dangling bonds.
[0059] On the silicon oxide film 15 is formed a semiconductor layer
e.g. Si by sputtering, CVD or the like, doped with an impurity e.g.
phosphorous for giving one conductivity type thereto, following
which the layer is patterned in accordance with a prescribed mask
pattern so that a gate electrode 20 is formed as shown in FIG. 4C.
The gate electrode 20 is not limited to a doped semiconductor but
metals or other materials may also be used.
[0060] Next, using the gate electrode 20 or a mask on top of the
gate electrode 20, self-aligning impurity regions 14 and 14' are
formed by ion implantation. In so doing, the semiconductor layer 17
underneath the gate electrode 20 is made into a channel region of
the insulated-gate type semiconductor device.
[0061] After an insulating layer 18 is formed to cover the entire
surface of the above structure, holes are made in the layer 18 for
source and drain electrode contacts and on these holes an aluminum
film is formed by sputtering, and then by using a predetermined
pattern, the source electrode 16 and the drain electrode 16 are
formed whereby the insulated-gate type semiconductor device is
completed.
[0062] In this invention, the semiconductor layer that forms the
channel region 17 and the semiconductor layers that form the source
14 and the drain 14' are made, of the same material simplifying the
manufacturing process. Also, semiconductor is crystallized in the
source and drain regions as well as in the channel region, thus the
carrier mobility is enhanced, which makes it possible to make an
insulated-gate type semiconductor device that has high electrical
characteristics.
[0063] Finally, this embodiment is completed by performing hydrogen
thermal anneal in a 100% hydrogen atmosphere, at a temperature of
375.degree. C. for 30 minutes. This hydrogen thermal anneal lowers
the grain boundary potential in the poly-crystalline semiconductor
improving the characteristics of the device.
[0064] The size of the channel 17 of the thin-film transistor shown
in FIG. 4(D) of this embodiment is 100.times.100 .mu.m.
[0065] As explained in the above, the thin film transistors are
formed using the poly-crystalline semiconductor in this
embodiment.
[0066] For the sputtering method used in this embodiment, either RF
sputtering or direct-current sputtering can be used, however, if
the sputter target is made of an oxide with poor conductivity such
as SiO.sub.2, in order to maintain a constant electrical discharge,
the RF magnetron sputtering method is desired.
[0067] The oxide gas can be oxygen, ozone, or nitrous oxide,
however, if ozone or oxygen is used, the silicon oxide film does
not take in unnecessary atoms making it possible to obtain a very
good insulation film, for example the gate insulation film. Also it
is easy to decompose ozone into O radical and so the number of O
radical generated in a unit area is large contributing to the
improvement of the film formation speed.
[0068] The halogen containing gas can be fluoride gas such as
nitrogen fluoride (NF.sub.3, N.sub.2F.sub.4), or hydrogen fluoride
gas such as (HF), fluorine gas (F.sub.2) or fleon gas. The NF.sub.3
gas easy to chemically decompose and to handle is desirable. For
chlorine gas, it can be carbon chloride (CCl.sub.4), chlorine
(Cl.sub.2), or hydrogen chloride (HCl). The quantity of, halogen
gas, for example nitrogen fluoride, is 2 to 20% volume with respect
to the quantity of the oxide gas, for example oxygen. The halogen
elements, during heat treatment, neutralize the alkali ions such as
sodium in the silicon oxide and has an effect on neutralizing the
silicon dangling bond, however if the quantity of the halogen
elements is too large, the compound SiF.sub.4 is formed in the
film, which is a gas component and would lower the film quality and
therefore is not desired. Normally, the quantity of halogen
elements mixed into the film is 0.1 to 5 atomic % with respect to
the silicon.
[0069] In forming the gate insulation film by the sputtering method
as is done in the prior art, the quantity of the inert gas argon is
more than oxygen. Conventionally, oxygen is 0 to about 10% volume.
In the prior art sputtering method, it is natural to think that the
argon gas hits the target material, resulting in that the target
grains are generated to form the film on the surface. This is
because the probability that the argon gas will hit the target
material (sputtering yield) is high. We the inventors, earnestly
examined the characteristics of the gate insulation film formed by
the sputtering method and found that the shift from the ideal value
of flatband voltage, which reflects the number of fixed charges in
the gate insulation film, and the interface states between the
activation layer and the gate insulation film, indicating the gate
insulation film performance, largely depends on the proportion of
argon gas in sputtering. The flatband voltage is the voltage
required to oppose the effect of the fixed charge in the insulation
film, the lower this voltage the better the characteristics of the
insulation film are.
[0070] When the SiO.sub.2 film is formed by the sputtering method
on the non-single-crystal semiconductor prepared in accordance with
the present invention, the relationship between the proportion of
argon gas with respect to oxygen and the flatband voltage is shown
in FIG. 5. The objects observed in this experiment is prepared in
the following manner, an SiO.sub.2 film is formed by sputtering on
the poly-crystalline semiconductor layer shown in FIG. 4A and then
an Al electrode is formed on it by electron beam evaporation.
[0071] When the volume of argon is less than that of oxidizing gas
(oxygen in the case of FIG. 5), for example 50% or less, the
flatband voltage is apparently reduced when compared to 100% argon
gas. The shift from the ideal value of the flatband voltage depends
largely on the proportion of argon gas. If the percentage of argon
gas is less than 20%, the flatband voltage is very close to the
ideal voltage. The activated argon atoms in the reactive atmosphere
when forming the film by the sputtering method, have an effect on
the film quality of the gate insulation film, and so it is desired
for the sputtering film forming to lower the amount of argon atoms
as much as possible.
[0072] The reason is that the film formation surface is damaged by
argon ions or by activated argon atoms colliding thereon, which
results in forming interface states or fixed charges.
[0073] FIG. 6 shows the relationship between the shift
.DELTA.V.sub.FB from the ideal flatband voltage and the percentage
of fluorine with respect to oxygen in the sputtering gas
(O.sub.2/NF.sub.3 volume %).
[0074] In the experiment, a 1 mm diameter aluminum electrode is
formed on top of the silicon oxide film 15 doped with, halogen
elements on the poly-crystalline silicon semiconductor 13 prepared
in accordance with this invention, (FIG. 4A) then a thermal
annealing is done at 300.degree. C. followed by a B-T
(bias-temperature) process. Further a negative bias voltage of
2.times.10.sup.6 V/cm is applied to the gate electrode at a
temperature of 150.degree. C. for 30 minutes, then in the same
conditions, a positive bias voltage is applied and in this state
the shift of the flatband voltage .DELTA.V.sub.FB is measured.
[0075] As can be clearly seen in FIG. 6, when a silicon oxide was
formed by a magnetron RF-sputtering in an atmosphere in which
NF.sub.3 is 0%, .DELTA.V.sub.FB was as much as 9V. However, if just
a few halogen elements such as fluorine are added during film
formation, this value is suddenly reduced. This is because the
positive sodium ions contaminating the film during formation
combine with the fluorine and neutralized as follows:
Na.sup.++F.sup.---->NaF
Si.sup.++F.sup.---->Si-F
[0076] On the other hand, it is known, that adding hydrogen
neutralizes the silicon, however, the Si--H bond is likely to be
separated again by a strong electric field (BT processing) and
causes silicon dangling bonds and causes boundary levels to be
formed, and so it is desired to use fluorine for neutralization.
Also, there always is a Si--H bond in the silicon oxide film. When
this bond is separated again, the fluorine atoms neutralize the
separated hydrogen atoms, which is effective in preventing the
formation of boundary levels. Moreover, due to the existence of
fluorine, the hydrogen bonded to the silicon bonds also with the
fluorine, and thus the silicon prevents a fixed charge from
developing.
[0077] FIG. 7 shows the withstand voltage of the SiO.sub.2 film
when more fluoride gas is added. The withstand voltage is the
voltage measured, using a 1 mm diameter aluminum electrode, when
the leak current exceeds 1 .mu.A. Depending on the test materials,
there is disparity and so in the Figure, the value is shown by X
and .sigma. (dispersion sigma value). The withstand voltage becomes
lower as the percentage of fluorine gas is increased to more than
20% and the .sigma. value becomes larger. Therefore it is best if
the added halogen element is less than 20% volume, normally 2 to
20% is good. Incidentally, when halogen gas was added at 1 volume %
with respect to oxygen gas during the film formation, measuring by
SIMS (secondary ion mass analysis), it was found that the density
of halogen in the film was 2.times.10.sup.20 atoms/cm.sup.3. It was
found that when added simultaneously, during the sputtering method
of film formation, the fluorine element is very easily taken in by
the film. However, if too much is added (more than 20%), the
silicon oxide film tends to become porous and degraded because of
the formation of SiF.sub.4, and as a result the withstand voltage
becomes poor and very disperse.
[0078] Also, it is desired that the materials used in sputtering be
highly pure. For example, a sputtering target made of 4N or more
synthetic quartz, or high grade silicon as used for the LSI
substrate is very desired. The sputtering gas used is very pure (5N
or more), and mixing of impurities with the silicon oxide film is
avoided as much as possible.
[0079] In this embodiment, the silicon oxide film, which is the
gate insulation film formed by the sputtering method in an oxygen
atmosphere with fluorine added, is irradiated by an excimer laser,
and flash anneal is performed. As a result, it is effective that
halogen elements such as fluorine introduced in the film are
activated, to neutralize the silicon dangling bonds, so that the
cause of the fixed charge in the film is removed. At this time, by
selecting a suitable excimer laser power and shot number,
activation of both the above halogen element and the semiconductor
layer underneath the gate insulation film can be performed
simultaneously.
[0080] Then, following is an explanation regarding the formation of
the a-Si semiconductor layer 13 in FIG. 4(A) by sputtering in an
atmosphere with hydrogen added, and its heat recrystallization.
[0081] The channel formation region of this embodiment is obtained
by applying heat of 450 to 700.degree. C., e.g. 600.degree. C. for
crystallization to a non-crystalline, i.e. amorphous or close to
amorphous semiconductor (referred as a-Si hereinafter) obtained by
the sputtering method in a hydrogen atmosphere or inert gas
atmosphere with hydrogen mixed in. The semiconductor after the
crystallization had an average grain diameter of about 5 to 400
.ANG., and the quantity of hydrogen mixed in the semiconductor film
was 5 atomic % or less. Also, the crystals of this semiconductor
has a distorted lattice and the boundaries of all of the crystal
grains are bonded tightly at a microscopic view point, and the
barriers to the carriers in the boundary regions are substantially
eliminated. In a conventional poly-crystalline semiconductor
without a distorted lattice, impurities such as oxygen tends to be
separated at grain boundaries, which forms barriers against
carriers, however, in the present invention, the barriers are
substantially eliminated by virtue of the distorted lattice and
thus the mobility of electrons is 5-300 cm.sup.2/Vs, which is very
preferable.
[0082] Furthermore, in a semiconductor film obtained through the
plasma CVD method, the proportion of amorphous elements is large.
Portions of this amorphous element tends to be oxidized naturally
and the inside of the semiconductor is oxidized. On the other hand,
the sputtering film is very densified and natural oxidation does
not advance inside the semiconductor film, only the surface and a
region closer to surface are oxidized. This densified
micro-structure makes it possible for the distorted lattice crystal
grains to be pressed up very close together, not allowing the
energy barrier against carriers to be formed along the crystal
grain boundaries.
[0083] Using SIMS analysis, the quantity of oxygen impurities in
the semiconductor film formed with this method is found to be
2.times.10.sup.20 atomscm.sup.-3 the quantity of carbon was
5.times.10.sup.18 atomscm.sup.-3, and the quantity of hydrogen
mixed in is less than 5%. (The concentration value of the
impurities measured using the SIMS method was taken in the
direction of depth of the semiconductor, and because the
concentration changes in that direction, the values recorded are
the minimum values in that direction. The reason for this is
thought to be the naturally oxidized film on or closer to the
surface of the semiconductor film. The concentration value of the
impurities does not change even after crystallization took
place.)
[0084] It is of course preferable if the concentration of
impurities is as low as possible for forming semiconductor devices,
however, in the case of the present invention, even if oxygen is
included in the semiconductor at 2.times.10.sup.20 atomscm.sup.-3,
the property of the semiconductor such as carrier mobility is not
hindered because the semiconductor has a crystalline structure with
a distorted lattice so that grain boundaries can be reduced.
[0085] As can be seen from the laser Raman analysis data of this
semiconductor film, shown in FIG. 15, the peak indicating the
existence of crystals, has shifted to a lower wavenumber when
compared to the peak of normal single-crystal silicon (520
cm.sup.-1), proving the existence of a distorted lattice.
[0086] The conditions required during the RF magnetron sputtering
for forming the non-single-crystal semiconductor are made clear by
the comparison test described below.
[0087] In order to investigate the relationship between the
hydrogen partial pressure in the sputtering gas used when forming
the non-single crystal silicon, and the electrical characteristics
of the film, the following 6 comparison tests are performed with
the hydrogen partial pressure changed.
TABLE-US-00011 Example number 1 2 3 4 5 6 Partial pressure % 0 5 20
30 50 80
[0088] The partial pressure is calculated as the percentage of
hydrogen in the total sputtering gas,
H.sub.2/(H.sub.2+Ar).times.100%. Test 6 corresponds to Embodiment
3. The other conditions are substantially the same as the
conditions of Embodiment 3.
[0089] FIG. 8 is a graph showing the relationship between the
mobility .mu. of a non-single crystal silicon and the partial
"pressure" ratio (P.sub.H/P.sub.TOTAL=H.sub.2/(H.sub.2+Ar)) of
hydrogen in the sputtering gas. According to FIG. 8, it is seen
that remarkably high mobility is obtained when the hydrogen partial
pressure is 20% or more.
[0090] In the graph of FIG. 9, curve A shows the relationship
between the threshold voltage Vth and the hydrogen partial pressure
ratio. Curve B is used for comparison with the construction of this
invention and the case similar to this embodiment except that the
oxidized gate film does not have fluorine mixed in.
[0091] According to FIG. 9, it can be seen that when a gate
insulation film with fluorine mixed in is used, as in the
construction of this invention, a lower threshold voltage is
obtained when compared with the insulated-gate field-effect
transistor which uses the prior art gate insulation film.
[0092] The lower the threshold voltage, the lower the voltage
needed to operate the thin-film transistor becomes, and is
considered to have good characteristics for use as a device.
Accordingly, the result in FIG. 9, shows that with a condition of
high hydrogen partial pressure in the sputtering gas, a threshold
voltage of 2 V or less, in normally off condition, can be obtained.
FIG. 9 also shows that the higher the partial pressure of hydrogen
the lower the threshold voltage is. In all of the above tests, it
is found that when the a-Si film, which becomes the channel
formation region, is formed by the sputtering method, and as the
hydrogen partial pressure is increased, the electrical
characteristics of the device are improved.
[0093] FIGS. 10 to 14 show the relationship between the drain
voltage and the drain current with a gate voltage as a parameter in
the IG-FET formed in the comparison test above.
[0094] Curves a, b, and c of FIGS. 10 to 14 correspond to gate
voltages VG of 20 V, 25 V, and 30 V. The effects of the hydrogen
partial pressure can be seen in comparing FIG. 11 (partial pressure
5%) and FIG. 12 (partial pressure 20%). In FIGS. 11 and 12, when
the drain currents (curve c) are compared to each other at the gate
voltage, of 30V, it can be seen that the drain current when the
hydrogen partial pressure is 20% is 10 times larger or more than
when the partial pressure is 5%.
[0095] From this it is known that when a-Si film. 13 in FIG. 4(A)
is made, if the partial pressure ratio of hydrogen, added during
sputtering, increases from 5% to 20%, the electrical
characteristics of the thin-film transistor greatly improve.
[0096] FIG. 15 is a Raman spectrogram of the semiconductor layer of
the heat crystallized a-Si film with hydrogen partial pressure
ratios of 0, 5, 20, and 50%. The curves 91, 92, 93, and 94
correspond to the partial pressure ratios 0, 5, 20, and 50%,
respectively.
[0097] Looking at FIG. 15 and comparing curve 92 with curve 93, or
in other words, comparing hydrogen partial pressure ratios of 5%
and 20%, it can be seen that when heat crystallization is performed
and the hydrogen partial pressure ratio of the sputtering gas is
20%, the Raman spectrogram remarkably shows the crystal
characteristics of the silicon semiconductor.
[0098] The average diameter of the crystal grains were, from
half-value width, 5 to 400 .ANG., e.g. 50 to 300 .ANG.. The peak
position of the Raman spectrograph is shifted to the lower
wavenumber side a little off from the 520 cm.sup.-1 location of the
single crystal silicon peak, which clearly indicates that there is
distortion in the lattice. These results remarkably show the
characteristics of this invention. That is, the effects of making
the a-Si film using the sputtering method with hydrogen gas added,
appears only when heat crystallization of the a-Si film takes
place.
[0099] When the crystalline structure is distorted in the above
manner, the barriers which exists at grain boundaries can be
eliminated, therefore, the carrier mobility can be improved. Also,
the segregation of impurities such as oxygen at the boundaries
becomes very difficult to be formed, resulting in that high carrier
mobility is possible. For this reason, even if the concentration of
impurities in the semiconductor film is in a degree of
2.times.10.sup.20 atomscm.sup.-3, no barriers against the carrier
are formed, and the film can be used as the channel region of an
insulated-gate semiconductor.
[0100] In comparing FIGS. 12, 13, and 14, as the hydrogen partial
pressure in the sputtering gas increases when forming the a-Si film
mentioned above, the drain current becomes large. This is very
clear if curves c in FIGS. 12, 13, and 14 are compared to each
other.
[0101] Generally, in a thin-film field-effect transistor, when the
drain voltage V.sub.D is low, the relationship between the drain
current I.sub.D and the drain voltage V.sub.D is given by the
following equation:
I.sub.D=(W/L).mu.C(V.sub.G-V.sub.T)V.sub.D (i)
(Solid. State electronics. Vol. 24. No. 11. pp. 1059. 1981. Printed
in Britain)
[0102] In the above equation, W is the channel width, L is the
channel length, .mu. is the carrier mobility, C is the
electrostatic-capacitance of the gate oxide film, V.sub.G is the
gate voltage, and V.sub.T is the threshold voltage. In the curves
of FIG. 10 through 14 the regions near the origin are represented
by the above equation (i).
[0103] If the hydrogen partial pressure is fixed, the carrier
mobility .mu. and the threshold voltage V.sub.T are fixed, and
also, because W, L, and C are values that are fixed depending upon
the structures of the thin-film transistor, the variables in
equation (i) are I.sub.D, V.sub.G, and V.sub.D. In the region near
the origin of the curves shown in FIG. 10 through 14, V.sub.G is
fixed, and so it is seen that the curves are given by equation (i),
and this equation describes the curves near the origin of FIGS. 10
through 14. This is because this equation was approximately
developed for when the drain voltage V.sub.D is low.
[0104] According to equation (i), as the threshold voltage V.sub.T
is lower and, the mobility .mu. gets larger, the slope of the
curves increases. This is clearly shown when the curves of FIG. 10
through 14 are compared based on the mobility and threshold
voltages of FIGS. 8 and 9.
[0105] According to equation (i), it can be seen that the
electrical characteristics of the thin-film transistor depend on
and V.sub.T. Therefore, the device characteristics cannot be
decided from FIGS. 8 and 9 separately. When the slopes of the
curves near the origin of FIG. 10 through 14 are compared to each
other, it is clearly seen and concluded that it is good if the
hydrogen partial pressure ratio of the sputtering gas, used when
forming the a-Si film that will become the channel formation
region, is 20% or more, if possible 100%.
[0106] Data showing the effects of this invention is shown below in
Table 1.
TABLE-US-00012 TABLE 1 Hydrogen Partial Pressure Ratio S Value Vth
Mobility On/Off Ratio 0 2.5 10.6 0.30 5.4 5 2.4 7.9 0.46 5.7 20 1.6
4.9 2.11 6.7 30 1.1 4.5 3.87 6.9 50 0.78 2.5 10.1 6.9 80 0.49 1.9
35.1 6.2
[0107] In Table 1, the hydrogen partial pressure ratio is the
atmosphere condition in the magnetron RF sputtering method used
when forming the a-Si film 13 of FIG. 4(A) which becomes the
channel formation region 17 of FIG. 4(D) of this embodiment.
[0108] The S value is the minimum value of
[d(I.sub.D)/d(V.sub.G)].sup.-1 of the initial rise slope of the
curves of the graphs that show the relationship between the gate
voltage (V.sub.G) and the drain current (I.sub.D), which describes
the characteristics of the device. As this value gets smaller, the
inclination of the curves showing the (V.sub.G-I.sub.D)
characteristics becomes sharper, and the electrical characteristic
of the device is high.
[0109] The on/off characteristic is the log of the minimum ratio
value of the drain current, which occurs at a certain gate voltage
and fixed drain voltage, and the drain current when the gate
voltage is varied at the same fixed drain voltage.
[0110] According to Table 1, considering everything, it can be seen
that in order to obtain a high performance semiconductor using the
method of this embodiment, a condition of hydrogen partial pressure
ratio of 80% or more is adequate to be adopted.
[0111] This invention has been explained using the silicon
semiconductor of this embodiment, however, using germanium
semiconductor, and a silicon-germanium mixture semiconductor is
also possible, and in this case the temperature for heat
crystallization can be lowered by about 100.degree. C.
[0112] Also, in forming a more densified semiconductor film or
silicon oxide film in the above mentioned hydrogen atmosphere or in
a hydrogen and inert gas atmosphere during sputtering, intense
light or laser irradiation, of 1000 nm or less, can also be applied
continuously or in pulses, to the substrate or the sputtered and
flying target particles.
Embodiment 4
[0113] In this embodiment, an insulated-gate type semiconductor
device is formed as shown in FIG. 16.
[0114] Coating the insulated substrate with a silicon oxide film is
done in the same process as in Embodiment 1, however, in this
embodiment the formation of the gate insulation film is finished
before the formation of the semiconductor layer which forms the
channel region. On a surface of an insulation film 12, 3000 .ANG.,
thick metallic molybdenum is formed by a sputtering method, then a
prescribed patterning is performed, so that gate electrode 20 is
formed.
[0115] Then, a 100 nm thick gate oxide film (SiO.sub.2) 15 is
formed by a magnetron RF sputtering method in the conditions
below.
TABLE-US-00013 Oxygen 95% NF.sub.3 5% Pressure: 0.5 Pa Formation
Temperature: 100 C. RF (13.56 MHz) Power Output: 400 W
A silicon target or synthetic quartz target is used.
[0116] On a surface of the silicon oxide film, a 100 nm thick a-Si
film 13, which will become a channel formation region, is formed by
a magnetron RF sputtering. The conditions of formation are as shown
below in an inert argon and hydrogen gas atmosphere.
H.sub.2/(H.sub.2+Ar) 80% (partial pressure ratio)
Formation Temperature: 150.degree. C.
RF (13.56 MHz) Output: 400 W
Total Pressure: 0.5 Pa
[0117] The target used is made of poly-crystalline or non-single
crystalline silicon.
[0118] After the formation of the a-Si film 13, the laminar
structure is annealed for 10 hours in an atmosphere of hydrogen or
inactive gas, for example, in an N.sub.2 atmosphere at a
temperature in the range of 450-700.degree. C., specifically, at
600.degree. C., as a result, the a-Si film 13 is crystallized. When
the semiconductor layer formed by this method is analyzed by SIMS
analysis, the quantity of oxide impurities existing in the
semiconductor layer is found to be 1.times.10.sup.20
atomscm.sup.-3, the quantity of carbon is 4.times.10.sup.18
atomscm.sup.-3, and the amount of hydrogen is 5% or less. In so
doing, the channel region 17 is formed over the gate electrode
20.
[0119] Next a 50 nm thick n.sup.+ a-Si film 14 is formed in the
following conditions by a magnetron RF sputtering method.
[0120] The conditions of film formation are as follows and in an
atmosphere of hydrogen partial pressure ratio of 10 to 99% or more
(in this example 80%), and argon partial pressure ratio 10 to 99%
(in this example 19%).
Formation Temperature: 150.degree. C.
RF (13.56 MHz) Power Output: 400 W
Total Pressure: 0.5 Pa
[0121] The target used is single-crystal silicon doped with
phosphorus.
[0122] Next on the semiconductor layer 14, an aluminum layer as
source and drain electrodes is formed, patterning is performed, and
the source and drain impurity regions 14 and 14' as well as the
source and drain electrodes 16 and 16' are formed, wherein the
semiconductor device is completed.
[0123] In this embodiment, because the gate insulation is formed
before the semiconductor layer for the channel formation region,
the boundary regions between the gate insulation film and the
channel region are moderately heat annealed during the heat
crystallization process, thus making it possible to lower the
density of boundary levels.
[0124] Also in the aforementioned sputtering method, the inert gas
used is argon, however other inert gasses such as helium can be
used, or reactive gasses such as SiH.sub.4 or Si.sub.2H.sub.6 which
have been made plasmatic can also be used.
[0125] Also in the magnetron RF sputtering method used for forming
the a-Si film, the concentration of hydrogen is in the range of 20
to 100%, the film formation temperature is in the range of 50 to
500.degree. C., the RF power output is in the range of 1 W to 10 MW
at a frequency in the range of 500 Hz to 100 GHz. The values within
these ranges can be freely selected, in addition it is possible to
use a pulse energy source.
[0126] Also, the hydrogen gas used for the sputtering can be
converted to plasma more effectively by the use of an intense light
(having wavelength 1000 nm or less) or an electron cyclotron
resonance (ECR). By making the hydrogen more plasmatic, the
efficiency of the positive ions in sputtering is higher and thus
micro structures in the film formed by sputtering can be prevented,
in the case of this embodiment, micro structures in the a-Si film,
can be prevented. This is also applicable to the other process
gasses.
[0127] In the embodiments, the a-Si is utilized as the
non-crystalline semiconductor, however, other semiconductors such
as germanium or a silicon-germanium mixture Si.sub.xGe.sub.1-x
(0<x<1) can also be used.
[0128] Also it need not to be said that, the present invention can
be used in stagger-type, coplanar-type, reverse-stagger-type, and
reverse-coplanar-type insulated-gate field effect transistors.
[0129] Furthermore, FET is mentioned here but this invention is not
limited to FET but also be used in the insulated film of other
semiconductor devices such as DRAM. In the above embodiments, in
order for the Na or K neutralization, the halogen gasses such as
fluorine are used, however, other gasses such as phosphorus,
carbon, or nitrogen with a density of 1.times.10.sup.19 to
5.times.10.sup.20 atomic % can also be used. Also in the above
embodiments the insulation film used is SiO.sub.2, however,
according to specific needs, alumina, tantalum oxide, barium
titanate, or silicon nitride can be used in the same way.
* * * * *