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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic device and a method of
producing the device, and more particularly, to an electronic device in
which a silicon nitride insulating film is laid on a conductive member
formed on the surface of an insulating substrate, and a method of
producing the device.
2. Description of the Related Art
As an example of a conventional electronic device, a thin-film transistor
(referred to as a TFT hereinafter) used to drive an active matrix liquid
crystal display device shown in FIG. 5 will be described.
FIG. 6 is a sectional view taken along line A-A' shown in FIG. 5 and
conceptually illustrates a TFT array. Since FIG. 6 is a conceptual view,
the dimensions therein are different from those of an actual device.
Referring to FIG. 6, numerals 9 and 10 respectively denote a gate electrode
(wiring pattern) and a gate wire (wiring pattern) patterned on a substrate
5. An insulating film 3 includes a gate insulating film 3a and an
interlayer insulating film 3b at the intersection of wires.
Numerals 11, 12 and 13 respectively denote a source electrode, a source
wire and a drain electrode.
A silicon nitride thin film is frequently used as the gate insulating film
3a and the interlayer insulating film 3b at the intersection of multilevel
metal wires of the TFT array.
In a desirable composition of the silicon nitride thin film, the element
ratio of silicon to nitrogen is approximately 3 to 4 and a trace quantity
of hydrogen is contained therein to stabilize the quality thereof. Such
insulating layer is mainly formed in plasma enhanced chemical vapor
deposition (referred to as PECV-D hereinafter). As a material gas,
silane-nitrogen, silane-ammonia-nitrogen, silane-ammonia-hydrogen,
silane-nitrogen-hydrogen, and silane-ammonia-nitrogen-hydrogen gases are
normally used.
However, in an electronic device like such TFT using silicon nitride
insulating films as the gate insulating film 3a and the interlayer
insulating film 3b at the intersection of multilevel wires, an electrical
short circuit sometimes occurs between the gate electrode 9, the gate wire
10 and wires formed through the insulating film (the source wire 12, the
source electrode 11 and so on). More particularly, the probability such
short circuit will occur is very high in a highly integrated or large-area
substrate. Such short circuit may occur during use or the production
process of an ultimate product. If the short circuit occurs during use of
the ultimate product, the reliability of the product is lowered.
Furthermore, a short circuit occurring during the production process
lowers the yield. For example, dielectric breakdown sometimes occurs
between the source wire 12 and the gate wire 10 or the gate electrode 9
during a photoresist process for forming contact holes and so on after the
formation of the source wire 12 and so on.
Considering that these problems result from the existence of pinholes in
the insulating film, Japanese Laid-open Patent No. 58-190042 attempts to
solve the problems by adopting a so-called multilayered insulating film
structure in which a non-doped amorphous silicon layer is laid on the
intersection of a gate wire and a source wire. However, since this
technique is used on the assumption that the insulating layer will not
have a single layer structure, but a multilayered insulating film
structure, it is hard to avoid complication of the production process.
Accordingly, an electronic device such as a TFT of a single-layer structure
which is excellent in the dielectric characteristics and capable of being
easily produced is desired.
Particularly, since the electronic device is now frequently used in harsh
environments, it is desired that the break-down voltage of the insulating
film be more than 100 V. Furthermore, it is desirable from the viewpoint
of miniaturization of the electronic device that the thickness of the
insulating film be limited to less than 500 nm, preferably, 200 to 400 nm.
Therefore, an electronic device having a thin insulating film, which has a
thickness of approximately 200 nm and a break-down voltage of more than
100 V, is desired.
No electronic device which satisfies such desire has been developed up to
now.
A silicon nitride thin film is frequently used as an insulating film in an
electronic device, for example, an insulating film 103 at the intersection
of multilevel metal wires of a TFT array used to drive an active matrix
liquid crystal display device shown in FIGS. 12 and 13. Such film is
mainly formed in the PECV-D method, and silane-nitrogen,
silane-ammonia/nitrogen, silane-ammonia-hydrogen, silane-nitrogen-hydrogen
and silane-ammonia-nitrogen-hydrogen gases are well known as a material
gas of the film. The film is deposited at a temperature of more than
300.degree. C. to stabilize the quality thereof. This temperature is
several tens of degrees higher than the temperature at which an amorphous
silicon layer frequently used in the electronic device is formed. When a
similar insulating film is formed in sputtering, argon of more than 0.50
Pa is put into an atmosphere.
However, the silicon nitride thin film formed in the PECV-D or sputtering
method using the above mixed gas has more pinholes and lower break-down
voltage than an oxidized film formed by high temperature oxidization of
silicon. Even if the silicon nitride thin film is used in an electronic
device, the single layer structure of the film makes it difficult to
obtain the necessary break-down voltage. Therefore, in the TFT array shown
in FIGS. 12 and 13, the necessary break-down voltage is obtained from a
multilayered film formed by adding another film at the intersection of the
multilevel metal wires. This complicates the structure and production
process of the film.
Furthermore, since the temperature for depositing an insulating film in the
PECV-D method is high, many foreign particles are generated from the inner
wall of a film forming device in the forming operation, and these
sometimes have a bad influence on the quality of the film.
Still furthermore, in an electronic device such as a TFT having an
amorphous silicon film in direct contact with an insulating film, since
the insulating film and the amorphous silicon film are different in film
depositing temperature, it is necessary to increase or decrease the
temperature of a substrate so as to achieve the depositing temperature of
one of the films after forming the other film, and such adjustment of
temperature takes a long time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an electronic device
like a TFT including a thin insulating film of a single layer structure
which is approximately 0.2 .mu.m in thickness and has an excellent
break-down voltage more than 100 V, and a method of producing the
electronic device with efficiency and reliability.
In order to achieve the above object, according to a first aspect of the
present invention, there is provided an electronic device having a
conductive wiring pattern formed on a surface of a substrate, at least the
surface being electrically insulated, and an insulating layer formed to
cover the substrate and the wiring pattern partly or wholly, in which the
insulating layer is made of a silicon nitride insulating film, a contact
angle .theta. between the wiring pattern and the substrate equals
60.degree. or more, a value Tn1/Tg of a thickness Tn1 of the silicon
nitride insulating film and a thickness Tg of the wiring pattern equals 2
or more, and Tn1 and a horizontal distance Tn2 between a rise start
position where the silicon nitride insulating film rises because of a step
of the wiring pattern and the top portion of the wiring pattern is in a
relation 0.6.ltoreq.Tn2/Tn1.
According to a second aspect of the present invention, in an electronic
device provided with a conductive wiring pattern formed on a surface of a
substrate, at least the surface being electrically insulated, and an
insulating layer formed to cover the substrate and the wiring pattern
partly or wholly, main components of the insulating film are silicon and
nitrogen, the element ratio of the silicon to the nitrogen is
approximately 3:4, and the content of a rare gas element in the insulating
film is within a range of 0.01 atm % to 3 atm %.
According to a third aspect of the present invention, the electronic device
in the second aspect is a thin film transistor.
According to a fourth aspect of the present invention, the rare gas element
is argon in the electronic device in the second or third aspect.
According to a fifth aspect of the present invention, there is provided a
method of producing an electronic device provided with a conductive wiring
pattern formed on a surface of a substrate, at least the surface being
electrically insulated, and an insulating layer formed to cover the
substrate and the wiring pattern partly or wholly, in which a gas for
forming the insulating film in PECV-D is a mixed gas containing at least
silane, ammonia, nitrogen and a rare gas element, and the mole ratio of
the rare gas element/nitrogen is within a range of 0.1 to 10.
According to a sixth aspect of the present invention, the insulating film
is formed at a temperature ranging from 220.degree. C. to 280.degree. C.
in the method of the fifth aspect.
According to a seventh aspect of the present invention, in the method of
the fifth or sixth aspect, the electronic device has an amorphous silicon
film in direct contact with the insulating film, and the insulating film
and the amorphous silicon film are deposited at almost the same
temperature.
According to an eighth aspect of the present invention, there is provided a
method of producing an electronic device provided with a conductive wiring
pattern formed on a surface of a substrate, at least the surface being
electrically insulated, and an insulating film mainly composed of silicon
and nitrogen and formed to cover the substrate and the wiring pattern
partly or wholly, in which a target for forming the insulating film in
sputtering contains at least silicon, an atmospheric gas for the
sputtering is a mixture of at least a rare gas element gas, a hydrogen gas
and a nitrogen gas or an ammonia gas, the partial pressures of the rare
gas element gas and the hydrogen gas are respectively 0.20 Pa to 0.40 Pa
and 0.02 Pa to 0.15 Pa.
According to a ninth aspect of the present invention, in the method of one
of the fifth to eighth aspects, the rare gas element is argon.
The operation of the present invention will now be described based on our
knowledge obtained in working on the present invention.
The inventor of the present invention reviewed a conventional electronic
device.
Many causes of lowering of the break-down voltage of the electronic device
can be thought of, for example, the thickness, quality and so on of a
silicon nitride insulating film.
The inventor thought that the quality of the silicon nitride insulating
film was a main cause of lowering the dielectric voltage. Japanese
Laid-open Patent Application No. 58-190042 said that pinholes in the
insulating film had a bad influence on the break-down voltage. In order to
confirm whether such pinholes were the only cause of lowering the
break-down voltage, the inventor formed a nitride film in which the number
of pinholes were reduced. In other words, a silicon nitride insulating
film was formed in PECV-D while controlling the substrate temperature, the
RF electric power, the composition of a material gas and so on, the
presence of pinholes was checked, and a silicon nitride insulating film
containing few pinholes was obtained. However, even the thus formed
silicon nitride insulating film having few pinholes could not ensure a
sufficient dielectric voltage.
Then, experiments were further conducted repeatedly to examine under what
conditions the break-down voltage was lowered. The results of the
examination revealed that the angle between a wiring pattern and a
substrate (contact angle: .theta. in FIG. 1) was one factor. In other
words, for example, in a case of a gate wire in a TFT used to drive a
liquid crystal display device, the leak characteristic was relatively good
when the contact angle .theta. was less than 60.degree., while the
probability the break-down voltage would be lowered was increased rapidly
when .theta. exceeded 60.degree.. However, since the break-down voltage
was not so bad in some cases even if .theta. exceeded 60.degree., various
kinds of experiments were conducted for further study on a condition in
which .theta. was more than 60.degree..
First, the break-down voltage was measured while changing the thickness of
the insulating film in order to examine the influence of the film
thickness. From a common sense standpoint, the break-down voltage of a
thicker insulating film without any pinholes are better than that of a
thinner one. Statistics of measurement results of many samples showed that
the break-down voltage of the thicker insulating film was indeed better
than that of the thinner film. However, contrary to expectations, even the
thick film was inferior to the thin film in break-down voltage in some
cases.
In order to put renewed focus on the sample films each having a low
break-down voltage, etching was conducted on the sample films. As a result
of etching, the etching speed in a region between a rise start position (A
in FIG. 1) where the film rises because of a step of an electrode and a
corner portion (B or C in FIG. 1) of the electrode is extraordinarily
higher than that of other portions.
As shown in FIG. 6, an insulating film 3 is exposed to an etchant in some
step during the production of the electronic device. The inventor
concluded that etching was selectively conducted from the above-mentioned
rise start position toward the corner portion of the electrode when the
insulating film 3 is exposed to the etchant and that such etching was a
main cause of lowering the break-down voltage.
Furthermore, the inventor conceived that the generation of such region
subject to be etched might depend on the rise start position, and
developed another film depositing method capable of controlling the rise
start position. Tests carried out in this method while changing the rise
start position revealed that the break-down voltage was remarkably
increased when 0.6.ltoreq.Tn2/Tn1 even if 60.degree..ltoreq..theta. and
that a break-down voltage of more than 100 V could be obtained even if the
insulating film was less than 200 nm in thickness and had a single layer
structure. These findings led to the present invention.
According to the second aspect of the present invention, the break-down
voltage of a silicon nitride thin film as an insulating film is greatly
improved and the break-down voltage of an electronic device using the
insulating film is also improved. Preferable characteristics can be
obtained when a rare gas element, though an unwelcome impurity in itself,
is contained, even if by a small amount within a fixed range. Though that
reason has not been clarified, it can be supposed that the insulating film
is stabilized by a kind of wedge effect caused by the injection of an
appropriate amount of a more inactive and stable element than silicon and
nitrogen. On the other hand, it can be also supposed that an excellent
characteristics of silicon nitride are diminished and the break-down
voltage is lowered when the amount of the rare gas element is excessive.
According to the third aspect, since the electronic device is a TFT, the
above insulating film functions as a gate insulating film and an
insulating film at the intersection of multilevel wires in the TFT.
Therefore, the characteristics of the insulating film on a gate electrode
of the TFT are stabilized, and the gate voltage and the break-down voltage
between the multilevel wires are increased.
According to the fourth aspect, argon, which can be less expensive and be
obtained more stably compared with other rare gas elements, is used as a
rare gas element. Therefore, the present invention according to the second
or third aspect can be carried out inexpensively and stably. This is
extremely effective from an industrial point of view.
According to the fifth aspect, when an electronic device is produced with
forming an insulation film mainly composed of silicon and nitrogen in
PECV-D nitrogen and a rare gas element are used as a dilution gas for
depositing the insulating film. Since the mixing ratio of nitrogen and the
rare gas element is in a relation where the rare gas element/nitrogen=0.1
to 10, a predetermined amount of rare gas element can be injected into the
insulating film, and therefore, the break-down voltage of the electronic
device can be increased.
Since the ionization characteristic of the rare gas element gas,
representing the aptness to ionize, is different from that of nitrogen,
the composition of the mixed gas is controlled by adding the rare gas
element into the dilution gas, thereby independently controlling the
plasma density and the potential of plasma during the PECV-D film
formation. According to tests carried out by the inventor of the present
invention, when the nitrogen content of the dilution gas is increased, the
potential of plasma is also increased. In other words, though a proper
nitrogen content makes it possible to obtain a dense and high-quality
insulating film, an excessive nitrogen content causes damage to the film
being formed. On the other hand, the composition of the rare gas element
is related to the control of the plasma density. When the content of the
rare gas element is small, the distribution of the plasma density is good,
but the density is low. To the contrary, the content of the rare gas
element is excessive, the distribution of the plasma density is not
preferable, and abnormal discharge is liable to occur at the end of the
substrate.. Therefore, the composition ratio of nitrogen and the rare gas
element in the dilution gas has an optimal range. In a composition range
according to the fifth aspect of the present invention, a dense and
high-quality insulating film having a high break-down voltage can be
obtained without damaging the film during the process and causing abnormal
discharge.
According to the sixth aspect, since the temperature for depositing the
insulating film is low, 220.degree. C. to 280.degree. C., the number of
foreign particles generated from the inner wall of a film forming
apparatus during film deposition is reduced.
According to the seventh aspect, since the insulating film and the
amorphous silicon film are deposited at almost the same temperature, one
of the films can be deposited immediately after depositing the other film,
without cooling or heating the substrate. In short, a process for
adjusting the temperature of the substrate does not need so long a time.
According to the eighth aspect, a predetermined amount of rare gas element
to be injected is smaller than that of a conventional insulating film
deposited in sputtering, and the structure of the film is stabilized.
Therefore, the break-down voltage of the electronic device is increased.
Furthermore, since an appropriate amount of hydrogen gas is contained,
hydrogen atoms are connected to dangling bonds of silicon atoms in the
deposited film, defects in the film are reduced, and the break-down
voltage of the electronic device is increased.
Still furthermore, since a silicon crystal or a sintered compound of
silicon nitride is used as a target, it is unnecessary to use a dangerous
silane gas which is required to be treated carefully.
According to the ninth aspect, since argon, which is less expensive and
capable of being obtained more stably than other rare gas elements, is
used as a rare gas element, the present invention according to one of the
fifth to eighth aspects can be carried out inexpensively and stably. This
is extremely effective from an industrial point of view.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged conceptual sectional view showing the vicinity of a
wiring pattern;
FIG. 2 is a plan view of a TFT produced in an embodiment;
FIG. 3 is a sectional view taken along line A-A' in FIG. 2;
FIG. 4 is a graph showing a relationship between the etching amount leading
to cracks in an insulating film and Tn2/Tn1;
FIG. 5 is a plan view of a conventional TFT;
FIG. 6 is a sectional view taken along line A-A' in FIG. 5;
FIG. 7 is a graph showing a relationship between the content of argon and
the break-down voltage of an insulating film used in another embodiment;
FIG. 8(A) is a schematic plan view of a capacitative element in the
embodiments of the present invention, and FIG. 8(B) is a sectional view
taken along an alternate long and short dash line in FIG. 8(A);
FIG. 9 is a schematic plan view of a part of an active matrix substrate
using the TFT according to the embodiment of the present invention;
FIG. 10 is a schematic sectional view taken along an alternate long and
short dash line in FIG. 9;
FIG. 11 is a graph showing a relationship between the content of argon and
the internal stress of an insulating film used in an electronic device
according to the embodiment of the present invention;
FIG. 12 is a view of a conventional TFT array for an active matrix
substrate;
FIG. 13 is a schematic sectional view taken along an alternate long and
short dash line in FIG. 12;
FIG. 14 is a graph showing a relationship between the content of argon and
the peak density of sodium ions in the film;
FIG. 15 is a graph showing a relationship between the film depositing
temperature and the number of foreign particles; and
FIG. 16 is a view showing the temperature profiles of three-layered films
in the embodiment and a compared example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The construction of the present invention will now be divisionally
described with reference to embodiments.
[Electronic Device]
In an electronic device according to the present invention, a conductive
wiring pattern is formed on a surface of a substrate in which at least the
surface is electrically insulated, and a silicon nitride insulating film
covers the substrate and the wiring pattern partly or wholly. Such
electronic device is, for example, a TFT device, a capacitative element, a
light emitting element, a photosensor, a solar battery and so on.
It is only necessary that at least the surface of the substrate is
electrically insulated. A substrate which is insulated itself (e.g. a
ceramic substrate like glass) may be used as it is, or an insulating film
(e.g. a SiO.sub.2 film, a Si.sub.3 N.sub.4 film) may be formed on the
surface of a conductive or semiconductive substrate. Either of the above
ways may be appropriately selected according to the type of the electronic
device.
Though the present invention is effective particularly when the insulating
film has a single layer structure, the break-down voltage thereof is
increased in a multilayered insulating film, and this case is also
included in the scope of the present invention.
A material of the conductive pattern is not specially limited, and, for
example, Cr, Al, Cu or the like, or an alloy of the metals is properly
selected. A preferable width of the wiring pattern at a gate electrode is
less than 10 .mu.m, a more preferable one is less than 7 .mu.m, and the
most preferable one is 5 .mu.m. A preferable thickness of the pattern is
100 nm to 200 nm. The thickness in this range makes it possible to
decrease the height of steps on the surface of an alignment film on a TFT
array and to improve the display performance of a LCD.
In the present invention, a contact angle .theta. which the wiring pattern
forms with the surface of the substrate is set at more than 60.degree..
If a silicon nitride insulating film covers the wiring pattern at a contact
angle of less than 60, the break-down voltage between electrodes is
increased. However, a special etching process is required to achieve such
contact angle.
Furthermore, it is necessary in the case of the contact angle of less than
60.degree. to make either the width or thickness of the electrode
excessively larger. Though a cross section area larger than a
predetermined one is needed to prevent heat generation of the electrode or
the wire, the width or thickness of the electrode must be increased to
make up for cut portions in the case of the contact angle of less than
60.degree..
Excessive increase of the width or thickness of the electrode opposes the
desire to miniaturize the electronic device, particularly, the desire that
the thickness of an electrode (wire) in a TFT used to drive a liquid
crystal display device be less than 7 .mu.m in order to decrease the area
in which extraneous light is shielded by the electrode.
Therefore, a contact angle of more than 60.degree. is desired. However,
when the contact angle is more than 60.degree., the isolation
characteristics remarkably become worse. The present invention aims to
solve this problem.
In order to make the contact angle more than 60.degree., for example, it is
only necessary to perform anisotropic etching by wet or dry etching.
The step is inversely tapered (.theta..gtoreq.90.degree.) in some etching,
and the present invention applies to such case.
[0.6.ltoreq.Tn2/Tn1]
The most characteristic point of the present invention is that a horizontal
distance T.sub.n2 between a rise start position, where the silicon nitride
insulating film rises because of a step of the wiring pattern, and the top
portion of the wiring pattern and a thickness T.sub.n1 of the silicon
nitride insulating film are in a relation where 0.6.ltoreq.Tn2/Tn1.
This point will be described with reference to FIG. 1. FIG. 1 is an
enlarged conceptual view of the vicinity of a wiring pattern. However, the
dimensions therein are not precise.
Referring to FIG. 1, A denotes a rise start position in which a silicon
nitride insulating film 25 rises because of a step of a wiring pattern
(gate electrode) 9. On the other hand, the top portion of the wiring
pattern 9 is denoted by B in the figure. In other words, the upper surface
of the wiring pattern 9 starts to descend toward a substrate 5 at the
position B. T.sub.n2 denotes a horizontal distance between the rise start
position A and the descend start position B.
When Tn2/Tn1 is more than 0.6, the dielectric characteristics are
remarkably enhanced. Though the reason of the enhancement is not clear,
the enhancement was confirmed from the result of an experiment in an
embodiment described below.
In the case of film formation in PECV-D, Tn2/Tn1 can be controlled by
appropriately changing the RF electric power, the gas composition and the
temperature of the substrate. Therefore, conditions for controlling
Tn2/Tn1 can be previously found in an experiment or the like while
changing these values.
For example, when a silane-ammonia-Ar-N.sub.2 gas is used as a material
gas, the substrate temperature is approximately 250.degree. C., the RF
electric power is more than 400 W and the argon gas content is more than
25%, Tn2/Tn1 can exceed 0.6. It is needless to say that the value is also
influenced by other gas components.
[2.gtoreq.Tn1/Tg]
Tg denotes the thickness of the wiring pattern 9. In the present invention,
Tn1/Tg equals 2 or more, and preferably, equals 4 or more.
Tn1/Tg is set at more than 2 because the increase of the break-down voltage
is not noticed if less than 2.
Even if Tn1/Tg exceeds 4, since the increase of the break-down voltage per
unit is brought to a standstill, Tn1/Tg less than 4 is desirable. To the
contrary, when Tn1/Tg exceeds 4, the film thickness becomes too large,
which is not preferable from the viewpoint of miniaturization of the
electronic device.
[Thickness of Insulating Film]
In the present invention, a preferable thickness of the insulating film is
less than 500 nm, and more preferably, 200 nm to 400 nm. It is a great
characteristic that such a thin film having a thickness of 200 nm can have
a break-down voltage of more than 100 V.
The present invention will now be described in detail according to
embodiments. It is needless to say that the scope of the present invention
is not limited to the following embodiments.
[Embodiment 1]
In this embodiment, the present invention is applied to the intersection of
multilevel metal wires of a TFT in an active matrix liquid crystal display
device.
The TFT in this embodiment has an inverse stagger structure, and will be
described with reference to FIGS. 2 and 3.
After a film of indium-tin oxide (ITO) was formed on the surface of a glass
substrate (Corning #7059) 5 and patterning was conducted thereon, thereby
forming a transparent electrode (ITO electrode) 7 functioning as a pixel
electrode.
Then, a Cr film having a thickness of 100 nm was formed, and a gate wire 10
having a width of 3 .mu.m and a gate electrode 9 having a width of 15
.mu.m were formed by resist application, development and etching.
In the etching, the angle which the substrate 5 formed with the gate wire
10 or the gate electrode 9 was determined by controlling anisotropy of wet
etching or dry etching. In this embodiment, the angle was set at
approximately 90.degree..
Then, a silicon nitride insulating film was formed in PECV-D to cover the
substrate 5, the gate electrode 9 and the gate wire 10. The silicon
nitride insulating film functions as a gate insulating film and an
interlayer insulating film of the TFT.
The conditions for forming the silicon nitride insulating film were as
follows.
Film Depositing Apparatus: Parallel plate in-line type from Anelva
Injection Gas (sccm)
SiH.sub.4 : 50
NH.sub.3 : 150
N.sub.2 : 500
Ar: 500
Pressure: 150 Pa
Substrate Temperature: 250.degree. C.
RF Electric Power: 700 W
Excitation Frequency: 13.56 MHz
Film Thickness: 300 nm
Then, a semiconductive layer (i: a-Si layer) made of amorphous silicon
containing no impurities was deposited in a thickness of approximately 100
nm in a normal method, and an amorphous silicon layer 17 (n+:a-Si layer)
having a thickness of approximately 20 nm, in which P was added, was laid
as an ohmic contact layer in the same chamber. After the resist process,
other parts of the amorphous silicon layer than a channel portion 15 of
the TFT were removed by wet etching. An etchant containing hydrofluoric
acid (HF) was used for the etching. The surface of the silicon nitride
insulating film bared by removing the amorphous silicon film was exposed
to the etchant during overetching, and therefore, slightly etched.
Then, contact holes were formed by dry etching on the transparent electrode
(ITO) 7 after the resist process. Subsequently, a Cr film having a
thickness of 300 nm was formed, and a source electrode 11, a source wire
1, a drain electrode 13 and a drain wire 18 were formed by patterning. In
this embodiment, the source wire 12 was so designed as to cross the gate
wire 10 as shown in FIG. 2. The drain wire 18 and the transparent
electrode 7 were connected to each other through a contact hole formed in
a proper position on the transparent electrode 7. The channel length and
width of the TFT were respectively 7 .mu.m and 12 .mu.m.
[Evaluation]
The following items of the TFT device formed as described above were
measured and various kinds of characteristics thereof were evaluated.
(Rise Start Position)
The rise start position was observed under a cross section transmission
electron microscope (TEM).
Tn2/Tn1 was 1.0 in this embodiment.
(break-down voltage)
The break-down voltage was measured by applying voltage between the gate
electrode and the drain electrode. The apparatus #4142B from Hewlett
Packard was used for the measurement.
The measured isolation voltage was 243 V.
(Etching Characteristic)
After the insulating film was exposed by removing the drain wire and the
source wire, etching was conducted on the insulating film. It was
examined, by using a buffered hydrofluoric solution (NH.sub.4 F40%
solution:HF48% solution=10:1) what amount of etching caused cracks on the
insulating film.
As a result of the examination, cracks arose on the insulating film after
etching by 280 nm was performed.
(Other Characteristics)
Threshold Value 1.0 V
ON-state Current 1.5.times.10.sup.-6 A
OFF-state Current 1.0.times.10.sup.-15 A
[Embodiment 2]
In this embodiment, an experiment was conducted while changing Tn2/Tn1.
Other conditions of the experiment were the same as those of Embodiment 1.
The conditions and the evaluation result of the experiment are shown in
Table 1.
TABLE 1
______________________________________
Film deposit-
No. 1 No. 2 No. 3 No. 4 No. 5 No. 6
ing condition
Injection gas
(sccm)
Silane 50 50 50 50 50 50
Ammonia 150 150 150 150 150 150
Nitrogen 1000 900 754 600 500 500
Argon 0 100 250 400 500 500
Pressure (Pa)
150 150 150 150 150 150
Excitation
13.56 13.56 13.56 13.56 13.56 13.56
frequency
(MHz)
RF power 300 400 400 450 500 600
(W)
Substrate
300 300 250 250 250 250
temperature
(.degree.C.)
Insulating
300 300 300 300 300 300
film
thickness
(nm)
T.sub.n2 /T.sub.n1
0.3 0.4 0.6 0.8 0.9 1.0
Break-down
60 62 153 225 237 240
voltage (V)
______________________________________
In Table 1, Conditions Nos. 3 to 6 are used in this embodiment of the
present invention, and Conditions Nos. 1 and 2 are samples for comparison.
Table 1 reveals that the break-down voltage increases rapidly when Tn2/Tn1
exceeds 0.6.
The etching characteristic of each sample was evaluated in the same manner
as Embodiment 1. The evaluation results are shown in FIG. 4. As shown in
FIG. 4, the etching amount causing cracks on the insulating film rapidly
increases and the etching characteristic is enhanced at the point where
Tn2/Tn1 exceeds 0.6.
[Embodiment 3]
In this embodiment, the influence of .theta. was examined.
The value of .theta. was changed to 0.degree., 60.degree., 90.degree. and
100.degree.. .theta. equals 0.degree. in a case in which a Cr film is
deposited on a substrate and an insulating film is laid on the Cr film
without patterning. .theta. equals 100.degree. when the step is inversely
tapered.
As a film forming condition in PECV-D, Condition No. 3 in Table 1 was used.
In this embodiment, the thicknesses of the Cr film and the insulating film
were respectively 100 nm and 200 mn.
The result of the above examination is shown in Table 2. Tn2/Tn1 was
approximately 0.6.
TABLE 2
______________________________________
.theta. Break-down voltage (V)
______________________________________
0.degree.
104
60.degree.
100
90.degree.
98
100.degree.
95
______________________________________
As shown in Table 2, a surprising result was obtained that the dielectric
voltage in the case of .theta. of 60.degree. can be almost the same as
that when the insulating film was formed on the flat Cr film
(.theta.=0.degree.).
[Embodiment 4]
In this embodiment, the relationship between the thickness ratio of the
wiring pattern to the insulating film in thickness and the break-down
voltage was examined.
The film forming condition No. 3 in Table 1 was adopted.
The result of the examination is shown in Table 3.
TABLE 3
______________________________________
Thickness of
Thickness of
Insulating film Break-down
Cr film (nm)
(nm) T.sub.n1 /T.sub.g
Voltage (V)
______________________________________
300 300 1 32
200 300 1.5 45
150 300 2 143
100 300 3 153
75 300 4 165
60 300 5 168
______________________________________
Table 3 reveals that the break-down voltage increases rapidly when Tn1/Tg
exceeds 2 and that the increase of the break-down voltage tends to be
brought to saturation when Tn1/Tg exceeds 4.
[Embodiment 5]
In this embodiment, the thickness of the insulating film was examined.
An experiment was conducted while changing the thicknesses of the Cr film
and the insulating film as shown in Table 4 so that Tn1/Tg was
approximately 2.5. The film depositing condition No. 3 in Table 1 was used
for the experiment. Tn2/Tn1 was approximately 0.65.
TABLE 4
______________________________________
Thickness of
Thickness of
Insulating film Break-down
Cr film (nm)
(nm) T.sub.n1 /T.sub.g
Voltage (V)
______________________________________
35 80 2.3 41
40 100 2.5 51
80 200 2.5 102
125 300 2.4 148
150 400 2.7 202
200 500 2.5 252
______________________________________
As shown in Table 4, even a thin insulating film having a thickness of 200
nm has a break-down voltage of more than 100 V in this embodiment, and
this is extremely effective in miniaturizing an electronic device.
Furthermore, thinning of the gate insulating film can achieve a
quick-response device.
[Embodiment 6]
An embodiment in which the present invention is applied to a capacitative
element will be described with reference to FIG. 8. FIG. 8(A) is a
schematic plan view of a capacitative element 127, and FIG. 8(B) is a
schematic sectional view taken along an alternate long and short dash line
in FIG. 8(A). In the capacitative element 127, a lower wiring terminal 128
and a lower wire 121 thereto, and an upper wiring terminal 129 and an
upper wire 123 thereto are formed through an insulating film 125 on a
substrate 105 of 1 cm in length and breadth. The lower wire 121 and the
upper wire 123 intersect at 100 points (10 lines.times.10 rows), and each
of the wires is made of Cr and has a thickness of 0.1 .mu.m and a width of
7 .mu.m. The insulating film 125 was deposited in the PECV-D method
described in detail below, and the thickness thereof was 0.2 .mu.m.
A silicon nitride thin film used as the insulating film 125 mainly contains
silicon and nitrogen at a ratio of approximately 3:4, and has an argon
content of 0 atm % (conventional insulating film) to more than 3 atm %.
The quantitative analysis of argon was carried out by SIMS6600 from Perkin
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