Manufacturing method of mim nonlinear device, mim nonlinear device, and liquid crystal display device

In a manufacturing method of a MIM nonlinear device (50) having a Ta electrode layer (16), an anodic oxidation film (18) and a Cr electrode layer (20), tantalum oxidation film (14) is first formed on the transparent substrate (12). The Ta electrode layer (16) is formed on the tantalum oxidation film (14) and the anodic oxidation film (18) is formed on the Ta electrode layer (16). Then, heat treatment is performed to the substrate. The final temperature drop in the heat treatment process is carried out in the atmosphere that contains water vapor. After that, the Cr electrode layer (20) is formed to complete the MIM nonlinear device (50). By conducting the heat treatment in the atmosphere that contains water vapor, the nonlinear characteristics of the MIM device can be improved as well as the improvement of the resistance characteristic in the OFF state.

This is a national stage application of PCT/JP96/00903 filed Apr. 1, 1996. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a manufacturing method of a MIM 
(Metal-Insulator-Metal) nonlinear device, and to an MIM nonlinear device 
and a liquid crystal display device. 
2. Related Art 
Generally, active matrix liquid crystal display devices comprise two 
substrates, between which liquid crystal is filled. On one substrate, a 
switching device is provided for each pixel region to form a matrix array, 
and on the other substrate, a color filter is formed. The orientation of 
the liquid crystal is controlled in each pixel region, thereby displaying 
prescribed information. As the switching device, a three-terminal device, 
such as a TFT (Thin Film Transistor), or a two-terminal device, such as 
MIM nonlinear device, are typically used. A MIM nonlinear device is 
advantageous in responding to the demand for a large-sized screen and 
reduced manufacturing cost. Moreover, a MIM nonlinear device has another 
advantage of eliminating crossover short-circuit between the scan line and 
the data line because scan lines and data lines are separately provided on 
different substrates. That is, scan lines are provided on the substrate 
having a matrix array formed thereon while data lines are provided on the 
other substrate. 
FIG. 14 shows an example of the conventional active matrix liquid crystal 
display device 100 using an MIM nonlinear device, in which a matrix is 
formed by a plurality of scan lines 74 connected to the scan line driving 
circuit 72, and a plurality of data lines 78 connected to the data line 
driving circuit 76. Pixel region 80 is formed in each element of the 
matrix. Pixel region 80 includes a MIM nonlinear device 50 connected to 
the data line 78 at one end, and a liquid crystal display element 60 
connected between the MIM nonlinear device 50 and the scan line 74. Liquid 
crystal display element 60 is driven based on the differential voltage 
between the signal applied to the scan line 74 and the signal applied to 
the data line 78. If the threshold voltage of liquid crystal element 60 is 
represented as (Vb), and the threshold voltage of MIM nonlinear device 50 
is represented as (Vth), and if the voltage at both terminals of the 
liquid crystal display element 60, which turns on the liquid crystal 
display element 60, is represented as (Vb+.DELTA.V), then the liquid 
crystal display element 60 is in the ON state when the differential 
voltage is (Vb+Vth+.DELTA.V) during a selected period, while the liquid 
crystal display element 60 is in the OFF state when the differential 
voltage is (Vb+Vth). During a non-selected period, the differential 
voltage is set to less than (Vth) to maintain the state decided during the 
selected period. 
FIG. 15 is a cross-sectional view of the active matrix liquid crystal 
display device 100 using an MIM nonlinear device. Liquid crystal layer 40 
is positioned between the electrode substrates 10 and 30. Electrode 
substrate 10 comprises a transparent board 12, MIM nonlinear devices 50 
formed on the transparent board 12, and pixel electrodes 22 connected to 
the corresponding MIM nonlinear device 50. MIM nonlinear device 50 is 
composed of a Ta electrode layer 16 formed on the transparent electrode 
12, a Ta.sub.2 O.sub.5 film 18 formed on the Ta electrode 16, and a Cr 
electrode layer 20 formed on the Ta.sub.2 O.sub.5 film 18. Ta.sub.2 
O.sub.5 film 18 is formed on the surface of the Ta electrode layer 16 
through anodic oxidation of the Ta electrode layer 16 so that the film 
thickness becomes uniform without generating pin holes. (See Japanese 
Patent Application Laid-Opens 5-297389 and 5-313207.) 
With the conventional method, MIM nonlinear device 50 having such a 
structure is manufactured as follows: forming a tantalum oxide layer 14 
with a thickness of about 1000 .ANG. by depositing a tantalum layer on the 
transparent substrate 12 by sputtering, followed by heat oxidation; 
depositing a tantalum layer up to about 3000 .ANG. by sputtering, and 
patterning the tantalum layer to form a Ta electrode layer 16; performing 
anodic oxidation to the Ta electrode layer 16 to form a Ta.sub.2 O.sub.5 
anodic oxidation film 18; and depositing chromium film with a thickness 
1500 .ANG. by sputtering and patterning the chromium film to form a Cr 
electrode layer 20, thereby completing a MIM nonlinear device 50. 
IEEE Trans Electron Device, Vol. ED28, pp. 736-739, June 1981, proposes a 
technique for doping nitrogen into Ta electrode layer 16 composing an MIM 
nonlinear device, in order to improve the nonlinear characteristics of the 
MIM nonlinear device. However, this technique requires highly advanced 
technology of tantalum sputtering, and makes it difficult to manufacture 
the MIM nonlinear device with good repeatability. 
Japanese Patent Application Laid-Open 63-50081 proposes a technique of 
anodic oxidation of the tantalum film, which is followed by heat treatment 
at a temperature from 400.degree. C. to 600.degree. C. in the nitrogen 
atmosphere, for improving the nonlinear characteristic of the MIM 
nonlinear device. However, simply conducting heat treatment at 
400.degree.-600.degree. C. in the nitrogen atmosphere after the anodic 
oxidation of the tantalum film can not achieve an adequate nonlinear 
characteristic and a satisfactory resistance characteristic in the OFF 
state, which are required for obtaining an excellent image quality. Thus, 
further improvement of the nonlinear characteristic and resistance 
characteristic has been desired. 
Therefore, the objective of the invention is to provide a method for 
manufacturing an MIM nonlinear device, which is capable of improving the 
nonlinear characteristic and resistance characteristic in the OFF state of 
a MIM nonlinear device, and to provide a liquid crystal display device 
using an MIM nonlinear device with improved a nonlinear characteristic and 
resistance characteristic in the OFF state. 
SUMMARY OF THE INVENTION 
According to the present invention, a manufacturing method is provided of 
an MIM nonlinear device comprising a first conductive layer, an oxidation 
film and a second conductive layer. The method comprises the steps of 
forming the first conductive layer on the substrate, forming the oxidation 
film on the first conductive layer, performing heat treatment to the 
substrate on which the first conductive layer and the oxidation film have 
been formed in the atmosphere containing water vapor, and after the heat 
treatment, forming the second conductive layer on the oxidation film. 
By conducting heat treatment to the substrate having the first conductive 
layer and the oxidation film in the atmosphere which contains water vapor, 
the nonlinear characteristic of the MIM nonlinear device is improved, as 
well as improving the resistance characteristic in the OFF state. If the 
MIM nonlinear device is used as a switching device of a liquid crystal 
display device, excellent image quality with high contrast can be 
achieved. 
Since the resistance value in the OFF state is adequately high, a margin 
can be taken for the increased off-leak at a high temperature. Thus, MIM 
nonlinear device having a good temperature characteristic, and a liquid 
crystal display device using such MIM nonlinear device, can be provided. 
This manufacturing method is suitable to manufacture of a MIM nonlinear 
device in which the first conductive layer is made of Ta achieving a great 
effect. The same effect can be obtained from the application to MIM 
nonlinear device in which the first conductive layer contains Ta as the 
major component, with at least one element selected from the group of W, 
Re and Mo added thereto. 
Especially when the invention is applied to a MIM nonlinear device in which 
the anodic oxidation film is formed on the first conductive layer, 
significant effects can be obtained. In particular, when the first 
conductive layer is composed of Ta, or contains Ta as the major component, 
with at least one element selected from the group of W, Re and Mo added 
thereto, a remarkable effect is obtained. 
The oxidation film formed on the first conductive layer may be formed by 
CVD, sputtering, a sol-gel process, heat oxidation, etc., other than 
anodic oxidation. 
The second conductive layer is preferably a metal layer. Cr, Ti, Al or Mo 
are preferable as the material of the metal layer, and a Cr layer is 
especially preferable. 
Preferably, the gas which contains water vapor is the air because it 
simplifies the structure of the heat treatment furnace. 
It is also preferable that the gas which contains water vapor is mixed gas 
of water vapor and inert gas. When using the mixed gas of water vapor and 
inert gas, the ratio of the water vapor in the mixed gas can be easily 
controlled, which further facilitates controlling the heat treatment 
conditions in the gas atmosphere which contains water vapor. The inert gas 
is preferably nitrogen gas. This also makes the structure of the heat 
treatment furnace simple. 
The gas which contains water vapor may be introduced to the reactor by 
ejecting water as a mist into the inert gas and introducing the gas with 
water vapor through a narrow tube. Alternatively, water may be dropped 
into the reactor directly, and water vapor evaporated in the reactor may 
be used to give water vapor to the gas. 
The concentration of water vapor in the gas is preferably equal to or more 
than 0.014 mol % with regard to the entire gas which contains water vapor. 
However, water vapor content of more than 0.005 mol % also has the similar 
effect, and water vapor content of more than 0.001 mol % still has the 
effect. 
The time taken for the heat treatment of the substrate having the first 
conductive layer and the oxidation film formed thereon in the atmosphere 
which contains water vapor is preferably more than 10 seconds. More 
preferably, it is more than 2 minutes, and further preferably is more than 
5 minutes. 
The step of heat-treating the substrate having the first conductive layer 
and oxidation film formed thereon in the atmosphere which contains water 
vapor is preferably the final temperature drop stage of the entire heat 
treatment process for the substrate on which the first conductive layer 
and the oxidation film are formed. 
Preferably, the final temperature drop step (i.e., heat treatment in the 
atmosphere which contains water vapor) includes at least a temperature 
drop to 220.degree. C. Taking into account the temperature distribution or 
the margin of the apparatus, it is preferable to continue the heat 
treatment in the atmosphere which contains water vapor until the 
temperature drops to below 200.degree. C. It is also preferable for the 
temperature for forming the second conductive layer on the oxidation film 
after the heat treatment to be below 220.degree. C. 
The temperature drop rate during the final temperature drop step is 
preferably from 0.1.degree. C./min to 60.degree. C./min, more preferably 
from 0.5.degree. C./min to 40.degree. C./min, and further preferably, 
0.5.degree. C./min to 10.degree. C./min. During the final temperature drop 
step, the temperature may be maintained constant for a while, or the 
temperature may be slightly increased halfway. The temperature drop rate 
mentioned above is the average rate including these variations in the 
temperature drop step. 
If the final temperature drop step is a step of lowering the temperature 
from the first temperature to the second temperature, the method further 
comprises a step of performing heat treatment to the substrate, on which 
the first conductive layer and the oxidation film were formed at a 
temperature above the first temperature, in the atmosphere which contains 
water vapor. 
If the final temperature drop step is a step of lowering the temperature 
from the first temperature to the second temperature, the method may 
further comprise a step of performing heat treatment to the substrate on 
which the first conductive layer and the oxidation film were formed at a 
temperature above the first temperature, in the atmosphere of inert gas. 
If this is the case, the inert gas is preferably nitrogen gas. 
By conducting the heat treatment to the substrate, on which the first 
conductive layer and the oxidation film were formed at a temperature 
higher than the first temperature (final temperature drop starting 
temperature), in the atmosphere which contains water vapor, or in the 
atmosphere of inert gas such as nitrogen, higher .beta. value and higher 
resistance value in the OFF state can be obtained. The heat treatment 
temperature above the first temperature is preferably below 600.degree. 
C., more preferably, below 500.degree. C., further preferably, below 
450.degree. C. 
If the temperature drop rate is made small, .beta. value, which is a 
nonlinear parameter of the MIM nonlinear device, and the resistance value 
in the OFF state can be remarkably improved. Generally, the higher the 
heat treatment temperature, the higher the .beta. value and the OFF state 
resistance value. However, since the .beta. value and the OFF state 
resistance value are remarkably improved by making the temperature drop 
rate small, satisfactory .beta. value and the OFF state resistance value, 
which are suitable for practical use, can be obtained even when the heat 
treatment temperature over the first temperature (i.e., temperature drop 
starting point) in the atmosphere containing water vapor or in the inert 
gas atmosphere is lowered. Lowering the heat treatment temperature reduces 
the compaction (contraction) of the substrate (glass substrate, etc.), and 
as a result, the preciseness in fine processing and assembling of the 
liquid crystal display device is improved. The lower heat treatment 
temperature can also restrain the damage on the MIM nonlinear device due 
to the thermal stress. 
In another aspect of the invention, an MIM nonlinear device, which 
comprises a first conductive layer, an oxidation film, and the second 
conductive layers, is provided. The oxidation film has such a 
characteristic that the peak P2 shown in FIG. 12 is clearly observed 
through the measurement by thermal desorption spectrum. 
The MIM nonlinear device having a first conductive layer, an oxidation film 
and a second conductive layer is manufactured by the method comprising the 
steps of forming the first conductive layer on the substrate; forming the 
oxidation film on the first conductive layer; performing heat treatment to 
the substrate, on which the first conductive layer and the oxidation film 
have been formed, in the atmosphere containing water vapor; and after 
that, forming the second conductive layer on the oxidation film. In the 
step of forming the second conductive layer, the second conductive layer 
is formed on the oxidation film having the peak P2 shown in FIG. 12, which 
is clearly observed through the measurement by thermal desorption 
spectrum. 
The MIM nonlinear device has a superior nonlinear characteristic and a high 
resistance value in the OFF state. Using this MIM nonlinear device, an 
excellent liquid crystal display device is provided, which has a high 
contrast and a good temperature characteristic. 
The liquid crystal display device of the invention is characterized in 
using a MIM nonlinear device as a pixel switching device, the MIM 
nonlinear device being fabricated by a method comprising the steps of 
forming the first conductive layer on the substrate; forming the oxidation 
film on the first conductive layer; conducting heat treatment to the 
substrate, on which the first conductive layer and the oxidation film have 
been formed, in the atmosphere which contains water vapor; and after the 
heat treatment, forming the second conductive layer on the oxidation film. 
The liquid crystal device has a high contrast and a good temperature 
characteristic.

The preferred embodiment of the invention will be described below based on 
the actual examples with reference to the drawings. 
(EXAMPLE 1) 
As shown in FIG. 2, tantalum film was deposited by sputtering on the 
transparent substrate 12 made of non-alkali glass, which was then 
subjected to heat oxidation to form a tantalum oxide film 14 with a 
thickness of about 1000 .ANG.. The tantalum oxide film 14 is provided for 
the purpose of improving the contact between non-alkali glass transparent 
substrate 12 and Ta electrode layer 16. 
Next, tantalum film was deposited by sputtering up to 2000 .ANG., which was 
patterned to form Ta electrode layer 16. Anodic oxidation was performed to 
the Ta electrode layer 16 to form Ta.sub.2 O.sub.5 anodic oxidation film 
18 with a thickness 600 .ANG.. In the example, citric acid aqueous 
solution having a concentration of 0.05 weight percent was used as 
electrolytic solution. The anodic oxidation voltage was 31V, and the 
electric current density was 0.04mA/cm.sup.2. 
Then, heat treatment was performed to the transparent substrate 12 on which 
the Ta electrode layer 16 and Ta.sub.2 O.sub.5 anodic oxidation film 18 
were formed. 
The heat treatment was carried out using the lengthwise heat treatment 
furnace 200 shown in FIG. 3. Bell jar 202 of the heat treatment furnace 
200 contains a boat 206 to hold a plurality of transparent substrates 12. 
Heater 204 heats the bell jar 202, and gas is introduced into the bell jar 
202 from the top thereof and discharged from the side and bottom of the 
bell jar 202. 
In this example, forty (40) transparent substrates 12 were loaded onto the 
boat 206, and the boat 206 supporting the transparent substrates 12 was 
inserted into bell jar 202 from the bottom of the bell jar 202. N.sub.2 
gas was introduced into bell jar 202 from the top to create nitrogen 
atmosphere inside the bell jar 202 prior to starting the heat treatment. 
Heat treatment was performed while rotating the boat 206. Heater 204 
started heating, while maintaining N.sub.2 gas flow at a rate of 201/min, 
to raise the temperature until the temperature of the transparent 
substrate 12 reached 435.degree. C., at a temperature increasing rate 
5.degree. C./min. Then, the temperature of the transparent substrate 12 
was maintained at 435.degree. C. for two hours, still maintaining the 
N.sub.2 gas flow at a rate of 201/min. After that, the boat 206 supporting 
the transparent substrates 12 was removed from the bottom of the bell jar 
202 into the atmosphere for rapid cooling of the transparent substrate in 
the air. 
Then, as shown in FIG. 1, Cr film with a thickness 1000 .ANG. was formed by 
sputtering on the Ta.sub.2 O.sub.5 anodic oxidation film 18. The Cr film 
was patterned to form a Cr electrode layer 20. Thus, the MIM nonlinear 
device 50 was completed comprising Ta electrode layer 16, Ta.sub.2 O.sub.5 
anodic oxidation film 18 and Cr electrode layer 20. 
Using the MIM nonlinear device 50 formed on the transparent substrate, the 
nonlinear parameter .beta., the ON state resistance and the OFF state 
resistance were measured. In this context, the nonlinear parameter .beta. 
is the slope of the line plotting the logarithm of the quotient of 
electric current I and applied voltage V (i.e., log(I/V)), as a function 
of the root of the applied voltage V (i.e., V.sup.1/2). The ON state 
resistance is a resistance (.OMEGA.) measured with 10V voltage applied to 
the MIM nonlinear device and is represented as R10V. The OFF state 
resistance is a resistance (.OMEGA.) measured with 4V voltage applied to 
the MIM nonlinear device and is represented as R4V. In this example, the 
parameter .beta. and ON and OFF state resistance values were measured for 
three transparent substrates and the average of three substrates was 
obtained. As a result of the measurement, .beta. was 4.10, ON state 
resistance was 5.00.times.10.sup.9 .OMEGA., and OFF state resistance was 
3.50.times.10.sup.12 .OMEGA.. 
(EXAMPLE 2) 
The transparent substrate 12, on which the Ta electrode layer 16 and the 
Ta.sub.2 O.sub.5 anodic oxidation film 18 were formed thereon, was 
prepared under the same conditions as the first example. Then, heat 
treatment was performed to the transparent substrate 12 in similar manner 
to the first example. Although, in the first example the transparent 
substrate 12 was maintained at 435.degree. C. for two hours in the N.sub.2 
gas atmosphere the temperature of the transparent substrate 12 was 
maintained at 455.degree. C. for two hours in the N.sub.2 gas atmosphere 
in the second example. All the other conditions were the same as the first 
example including the rapid cooling in the air. 
The MIM nonlinear device 50 was formed similarly to the first example. 
Then, the nonlinear parameter .beta. and the resistance values of the ON 
state and OFF state were measured in the same manner as the first example 
to obtain the average of three transparent substrates 12. .beta. was 5.06, 
the ON state resistance was 1.07.times.10.sup.10 .OMEGA., and the OFF 
state resistance was 1.19.times.10.sup.13 .OMEGA.. 
In the first and second examples, the water vapor concentration in the air 
was 1.2 mole % with respect to the entire air which contains water vapor. 
(EXAMPLE 3) 
The transparent substrate 12 having the Ta electrode layer 16 and Ta.sub.2 
O.sub.5 anodic oxidation film 18 formed thereon was prepared under the 
same conditions as the first example. 
Heat treatment was performed to the transparent substrate 12 on which the 
Ta electrode layer 16 and the Ta.sub.2 O.sub.5 anodic oxidation film 18 
are formed. The heat treatment was carried out using a sideways heat 
treatment furnace 300 of FIG. 4. As shown in FIG. 4, boat 306 is provided 
within the reactor tube 302 in the heat treatment furnace. Plural 
transparent substrates 12 are loaded on the boat 306 lengthwise. Heater 
304 is used for the heat treatment, and gas is introduced into the reactor 
tube 302 through the gas induction pipe 308 provided on the upper part of 
the reactor tube 302, and discharged through the exhaust pipe 312 on the 
lower part of the reactor tube 302. 
In the example, the reactor tube 302 was heated while introducing N.sub.2 
gas in the reactor tube 302 until the temperature reached 250.degree. C. 
The N.sub.2 gas atmosphere was maintained within the reactor tube 302 at 
250.degree. C. Then, the boat 306 supporting one hundred transparent 
substrates 12 was inserted in the reactor tube 302 in which the N.sub.2 
gas atmosphere is maintained at 250.degree. C., from the left of the 
figure. After that, the valve 310 was closed to evacuate the N.sub.2 gas 
from the reactor tube 302 through the exhaust pipe 312 while maintaining 
the temperature at 250.degree. C. The valve 310 was opened and N.sub.2 gas 
was introduced again into the reactor tube 302 through the gas induction 
pipe 308 to fill the reactor tube 302 with N.sub.2 gas. Heater 304 started 
heating again to raise the temperature at a rate of 5.degree. C./min until 
the temperature of the transparent substrate 12 reached 450.degree. C., 
while introducing N.sub.2 gas through the gas induction pipe 308 at a flow 
rate of 501/min and discharging through the exhaust pipe 312 provided on 
the lower portion of the reactor tube 302. The transparent substrate 12 
was maintained at 450.degree. C. for two hours, still maintaining the 
N.sub.2 gas flow at a rate of 501/min. After that, valve 310 was closed at 
450.degree. C. and N.sub.2 gas was evacuated from the reactor tube 302 
through the exhaust pipe 312. Then, the valve 310 was opened to introduce 
the air in the reactor tube 302 through the gas induction pipe 308 to fill 
the reactor tube 302 with the air atmosphere. The temperature was lowered 
to 250.degree. C. at a rate of 1.3.degree. C./min, while maintaining the 
air atmosphere and the atmospheric pressure in the reactor tube 302. When 
the temperature of the transparent substrate 12 became lower than 
150.degree. C., the boat 306 supporting the transparent substrates 12 was 
taken out from the reactor tube 302 from the left of the figure. 
Then, similarly to the first example, the MIM nonlinear device 50 was 
completed comprising Ta electrode layer 16, Ta.sub.2 O.sub.5 anodic film 
18 and Cr electrode layer 20. 
The nonlinear parameter .beta. and the ON state and OFF state resistance 
values of the MIM nonlinear device 50 formed on the transparent substrate 
12 were measured in the same manner as the first example and the average 
of the three transparent substrates 12 was obtained. The value of .beta. 
was 9.06 , the ON state resistance was 2.02.times.10.sup.11 .OMEGA., and 
the OFF state resistance was 2.45.times.10.sup.14 .OMEGA.. The variation 
of the values of .beta., the ON state resistance and the OFF state 
resistance was very small both on the surface of the transparent substrate 
and among the transparent substrates compared with the first and second 
examples. 
By using the atmospheric air as the gas which contains water vapor, the 
apparatus can be simplified. 
Since the heat treatment in the N.sub.2 gas atmosphere and the cooling 
(temperature drop) in the air are continuously carried out in the same 
heat treatment furnace 300, controllability under the substrate cooling 
conditions is remarkably improved, and as a result, the variation in the 
characteristics of the MIM nonlinear device can be restrained within a 
substrate, among substrates, and among heat treatment batches. 
During the continuous process in the same heat treatment furnace 300, 
N.sub.2 gas is evacuated from the heat treatment furnace after the heat 
treatment of the substrate in the N.sub.2, and then, the air is introduced 
in the heat treatment furnace 300 for cooling the substrate in the air. 
This makes the gas replacement easy and the heat treatment atmosphere can 
be changed-over reliably in a short time, resulting in facilitated control 
of the heat treatment conditions and improved controllability. As a 
result, variation in the characteristics of the MIM nonlinear device can 
be further restrained within a substrate, among substrates and among heat 
treatment batches. 
(EXAMPLE 4) 
The transparent substrate 12 on which the Ta electrode layer 16 and 
Ta.sub.2 O.sub.5 anodic oxidation film 18 were formed was prepared under 
the same conditions as the first example. Then, the heat treatment of the 
transparent substrate 12 was carried out in the similar manner to the 
third example. Although, in the third example, the transparent substrate 
12 was maintained at 450.degree. C. for two hours in the N.sub.2 
atmosphere, the temperature of the transparent substrate 12 was maintained 
at 410.degree. C. for two hours in the N.sub.2 atmosphere. All the other 
conditions were the same as the third embodiment. 
The MIM nonlinear device 50 was formed similarly to the first example, and 
the nonlinear parameter .beta., the ON state resistance and the OFF state 
resistance of the MIM nonlinear device 50 formed on the transparent 
substrate 12 were measured in the same manner as the first example to 
obtain the average of the three transparent substrates 12. The value of 
.beta. was 9.22, the ON state resistance was 4.94.times.10.sup.10 .OMEGA. 
and the OFF state resistance was 9.28.times.10.sup.13 .OMEGA.. The 
variation in the values of .beta., the ON state resistance and the OFF 
state resistance was very small both on the surface of the transparent 
substrate and among the transparent substrates compared with the first and 
second examples. 
(EXAMPLE 5) 
The transparent substrate 12 on which the Ta electrode layer 16 and 
Ta.sub.2 O.sub.5 anodic oxidation film 18 were formed was prepared under 
the same conditions as the first example. Then, the heat treatment of the 
transparent substrate 12 was carried out in similar manner to the third 
example. Although, in the third example the transparent substrate 12 was 
maintained at 450.degree. C. for two hours in the N.sub.2 atmosphere, the 
temperature of the transparent substrate 12 was maintained at 380.degree. 
C. for two hours in the N.sub.2 atmosphere. All the other conditions were 
the same as the third example. 
The MIM nonlinear device 50 was formed similarly to the first example and 
the nonlinear parameter .beta., the ON state resistance and the OFF state 
resistance of the MIM nonlinear device 50 formed on the transparent 
substrate 12 were measured in the same manner as the first example to 
obtain the average of the three transparent substrates 12. The value of 
.beta. was 7.84, the ON state resistance was 1.96.times.10.sup.10 .OMEGA., 
and the OFF state resistance was 2.61.times.10.sup.14 .OMEGA.. The 
variation in the values of .beta., the ON state resistance and the OFF 
state resistance was very small both on the surface of the transparent 
substrate and among the transparent substrates compared with the first and 
second examples. 
(EXAMPLE 6) 
The transparent substrate 12 on which the Ta electrode layer 16 and the 
Ta.sub.2 O.sub.5 anodic oxidation film 18 were formed was prepared under 
the same conditions as the first example. Then, the heat treatment of the 
transparent substrate 12 was carried out in similar manner to the third 
example. Although, in the third example, the transparent substrate 12 was 
maintained at 450.degree. C. for two hours in the N.sub.2 atmosphere, the 
temperature of the transparent substrate 12 was maintained at 350.degree. 
C. for two hours in the N.sub.2 atmosphere. All the other conditions were 
the same as the third example. 
The MIM nonlinear device 50 was formed similarly to the first example and 
the nonlinear parameter .beta., the ON state resistance and the OFF state 
resistance of the MIM nonlinear device 50 formed on the transparent 
substrate 12 were measured in the same manner as the first example to 
obtain the average of the three transparent substrates 12. The value of 
.beta. was 6.07, the ON state resistance was 1.01.times.10.sup.10 .OMEGA. 
and the OFF state resistance was 7.45.times.10.sup.13 .OMEGA.. The 
variation in the values of .beta., the ON state resistance and the OFF 
state resistance was very small both on the surface of the transparent 
substrate and among the transparent substrates compared with the first and 
second examples. 
(EXAMPLE 7) 
The transparent substrate 12 on which Ta electrode layer 16 and Ta.sub.2 
O.sub.5 anodic oxidation film 18 were formed was prepared under the same 
conditions as the first example. Then, the heat treatment of the 
transparent substrate 12 was carried out in similar manner to the third 
example. Although, in the third example, the transparent substrate 12 was 
maintained at 450.degree. C. for two hours in the N.sub.2 atmosphere, the 
temperature of the transparent substrate 12 was maintained at 320.degree. 
C. for two hours in the N.sub.2 atmosphere. All the other conditions were 
the same as the third example. 
The MIM nonlinear device 50 was formed similarly to the first example and 
the nonlinear parameter .beta., the ON state resistance and the OFF state 
resistance of the MIM nonlinear device 50 formed on the transparent 
substrate 12 were measured in the same manner as the first example to 
obtain the average of three transparent substrates 12. The value of .beta. 
was 4.40, the ON state resistance was 3.17.times.10.sup.9 .OMEGA., and the 
OFF state resistance was 2.95.times.10.sup.12 .OMEGA.. The variation in 
the values of .beta., the ON state resistance and the OFF state resistance 
was very small both on the surface of the transparent substrate and among 
the transparent substrates compared with the first and second examples. 
(EXAMPLE 8) 
The transparent substrate 12 on which Ta electrode layer 16 and Ta.sub.2 
O.sub.5 anodic oxidation film 18 were formed was prepared under the same 
conditions as the first example. Then, the heat treatment of the 
transparent substrate 12 was carried out in similar manner to the third 
example. Although, in the third example, the transparent substrate 12 was 
maintained at 450.degree. C. for two hours in the N.sub.2 atmosphere, the 
temperature of the transparent substrate 12 was maintained at 290.degree. 
C. for two hours in the N.sub.2 atmosphere. All the other conditions were 
the same as the third example. 
The MIM nonlinear device 50 was formed similarly to the first example and 
the nonlinear parameter .beta., the ON state resistance and the OFF state 
resistance of the MIM nonlinear device 50 formed on the transparent 
substrate 12 were measured in the same manner as the first example to 
obtain the average of the three transparent substrates 12. The value of 
.beta. was 3.62, the ON state resistance was 1.90.times.10.sup.9 .OMEGA. 
and the OFF state resistance was 3.93.times.10.sup.11 .OMEGA.. The 
variation in the values of .beta., the ON state resistance and the OFF 
state resistance was very small both on the surface of the transparent 
substrate and among the transparent substrates compared with the first and 
second examples. 
In the first through eighth examples, the concentration of water vapor in 
the air introduced in the reactor tube 302 was 1.2 mole % with respect to 
the entire air which contains water vapor. 
(EXAMPLE 9) 
The transparent substrate 12 on which the Ta electrode layer 16 and the 
Ta.sub.2 O.sub.5 anodic oxidation film were formed was prepared under the 
same conditions as the first example. 
Then, heat treatment was performed to the transparent substrate 12 having 
the Ta electrode layer 16 and the Ta.sub.2 O.sub.5 anodic oxidation film 
formed thereon. The lengthwise heat treatment furnace 400 shown in FIG. 5 
was used for the heat treatment. 
The temperature in the bell jar 402 was raised to 250.degree. C., while 
introducing the N.sub.2 gas in the bell jar 402 through the gas induction 
pipe 462, mass flow controller 452, and pipes 464, 468 so that the inside 
of the bell jar 402 was maintained at 250.degree. C. in the N.sub.2 gas 
atmosphere. 
Boat 408 holding twenty (20) transparent substrates 12 was inserted in the 
bell jar 402 which contains the N.sub.2 gas at 250.degree. C., through the 
bottom of the bell jar 402. 
Then, the heater (not shown) started heating the bell jar 402 while 
introducing the N.sub.2 gas at a rate of 201/min from the top 404 of the 
bell jar 402 through the gas induction pipe 462, mass flow controller 452, 
and pipes 464, 468 to raise the temperature at a rate of 3.degree. C./min 
until the temperature of the transparent substrate 12 reached 350.degree. 
C. 
The temperature of the transparent substrate 12 was maintained at 
350.degree. C. for two hours while maintaining the flow rate of the 
N.sub.2 gas 201/min. 
After that, mass flow controller 452 reduced the N.sub.2 gas flow rate in 
the pipe 464 to 101/min at 350.degree. C. At the same time, mass 
controller 454 reduced the N.sub.2 gas flow rate in the pipe 466 for 
introducing the N.sub.2 gas into the bubbler 430 containing pure water 432 
to 101/min at 350.degree. C. so that the N.sub.2 gas which contains water 
vapor flows out at 101/min through the pipe 434. N.sub.2 gas of 101/min 
from the pipe 464 and N.sub.2 gas, which contains water vapor, of 101/min 
from the pipe 434 were mixed together in the pipe 468 and the mixed gas 
was introduced in the bell jar 402 from the top 404 of the bell jar 402. 
In this state (with the flow of N.sub.2 gas which contains water vapor), 
the temperature was lowered from 350.degree. C. to 250.degree. C. at a 
rate of 0.8.degree. C./min. 
In this example, the temperature of the bubbler 430 was maintained at 
22.degree. C., and the water vapor concentration in the N.sub.2 gas, which 
was introduced into the bell jar 402 from the top 404 thereof through the 
pipe 468, was set to 2.6 mole % with respect to the entire N.sub.2 gas 
which contains water vapor. 
When the temperature of the transparent substrate 12 reached 250.degree. 
C., the boat 408 supporting the transparent substrates 12 was pulled down 
from the bottom of the bell jar 402. 
Then, the MIM nonlinear device 50 comprising Ta electrode layer 16, the 
Ta.sub.2 O.sub.5 anodic oxidation film 18 and the Cr electrode layer 20 
was completed in the same manner as the first example. 
Using the MIM nonlinear device 50 formed on the transparent substrate 12, 
the nonlinear parameter .beta., the ON state resistance and the OFF state 
resistance of the MIM nonlinear device 50 were measured for three 
transparent substrates and the average was calculated. The .beta. value 
was 6.50, the ON state resistance was 1.60.times.10.sup.10 .OMEGA., and 
the OFF state resistance was 1.02.times.10.sup.14 .OMEGA.. The variation 
in the values of .beta., the ON state resistance and the OFF state 
resistance was very small both on the surface of the transparent substrate 
and among the transparent substrates compared with the first and second 
examples. 
By using the mixture of water vapor and N.sub.2 gas as the gas which 
contains water vapor, the ratio of the water vapor contained in the mixed 
gas is easily controlled, which facilitates the control of the heat 
treatment conditions in the water vapor containing gas atmosphere. In the 
example, a bubbler is used to supply water vapor, and the ratio of the 
water vapor in the mixed gas is easily set by controlling the temperature 
of the bubbler. 
(Comparison Example) 
The transparent substrate 12 on which the Ta electrode layer 16 and the 
Ta.sub.2 O.sub.5 anodic oxidation film 18 were formed was prepared under 
the same conditions as the first example. Then, heat treatment was 
performed to the transparent substrate 12 using the lengthwise heat 
treatment furnace 200 shown in FIG. 3. In this comparison example, forty 
(40) transparent substrates 12 were loaded on the boat 206 and the boat 
was inserted into the bell jar 202 from the bottom thereof. After the bell 
jar 202 was filled with the nitrogen atmosphere by introducing N.sub.2 gas 
from the top of the bell jar 202, heat treatment started. The heat 
treatment was carried out using the heater 204, while rotating the boat 
206 in the bell jar 202. The heater 104 started heating to raise the 
temperature at a rate of 5.degree. C./min until the temperature of the 
transparent substrate 12 reached 450.degree. C., while maintaining the 
N.sub.2 gas flow at a rate of 201/min. The temperature of the transparent 
substrate 12 was maintained at 450.degree. C. for two hours still 
maintaining the N.sub.2 gas flow at a rate of 201/min. Then, the 
temperature was lowered to 250.degree. C. at a rate of 1.degree. C./min 
under the N.sub.2 gas flow at a rate of 201/min. When the temperature 
reached 250.degree. C., the boat 206 supporting the transparent substrates 
12 was removed from the bell jar 202 through the bottom thereof. 
After that, the Cr electrode layer 20 was formed in the same manner as the 
first example to complete the MIM nonlinear device comprising the Ta 
electrode layer 16, the Ta.sub.2 O.sub.5 anodic oxidation film 18 and the 
Cr electrode layer 20. 
Similarly to the first example, the nonlinear parameter .beta., the ON 
state resistance and the OFF state resistance of the MIM nonlinear device 
50 were measured for the three transparent substrates and the average was 
calculated. The .beta. value was 3.10, the ON state resistance was 
1.2.times.10.sup.9 .OMEGA. and the OFF state resistance was 
4.05.times.10.sup.10 .OMEGA.. 
FIG. 6 is a chart in which the .beta. values of the first through ninth 
examples and the comparison example are plotted. The horizontal axis 
represents the heat treatment temperature in the N.sub.2 gas. Symbol 
.oval-solid. denotes the .beta. values obtained in the first and second 
examples, symbol .box-solid. denotes the .beta. values of the third 
through eighth examples, the black triangle denotes the .beta. value of 
the ninth example, and symbol .quadrature. denotes the .beta. value of the 
comparison example. From the chart, it can be seen that lowering the 
temperature of the transparent substrate in the gas atmosphere containing 
water vapor can obtain improved .beta. values as in the first through 
ninth examples compared with the comparison example in which the 
temperature was lowered only in the N.sub.2 gas atmosphere without 
containing water vapor. Furthermore, as the heat treatment temperature is 
high, the .beta. value becomes high. Slow cooling in the N.sub.2 
atmosphere which contains water vapor as is in the third through ninth 
examples indicates a higher .beta. value even with the lower heat 
treatment temperature compared with the first and second examples in which 
rapid cooling in the air was performed. With the heat treatment 
temperature of 350.degree.-450.degree. C., the measured OFF state 
resistance values were close to each other because these values were all 
beyond the measurement limit. 
With a high .beta. value, the contrast of liquid crystal display device can 
be improved. If the OFF state resistance value is high, an adequate margin 
can be taken against the increase of OFF-leak at a high temperature, 
thereby achieving a MIM nonlinear device having a superior temperature 
characteristic, and a liquid crystal display device using such MIM 
nonlinear device. 
In view of FIGS. 6 through 8, after the heat treatment in the nitrogen gas 
atmosphere, the heat treatment temperature itself can be lowered to 
350.degree. C. by slow cooling in the air or in the N.sub.2 gas which 
contains water vapor. This allows less-expensive soda glass to be used as 
a substrate. As a result, manufacturing cost of the liquid crystal device 
using the MIM nonlinear device manufactured in this way is also reduced. 
If the heat treatment temperature is low, compaction (contraction) of the 
substrate (e.g. glass substrate) is reduced and preciseness in minute 
processing or assembling can be improved. Lower heat treatment temperature 
can also achieve reduced damages onto the MIM nonlinear device due to the 
thermal stress. 
Slow cooling in the air, or in the N.sub.2 gas which contains water vapor, 
performed after the heat treatment in the N.sub.2 gas atmosphere also 
facilitates the temperature control during the temperature drop, whereby 
MOM nonlinear devices are easily manufactured with small variations in 
characteristics of the device among heat treatment batches. 
(EXAMPLE 10) 
The transparent substrate 12 on which the Ta electrode layer 16 and the 
Ta.sub.2 O.sub.5 anodic oxidation film 18 were formed was prepared under 
the same conditions as the first example. 
Heat treatment was performed to the transparent substrate 12 on which Ta 
electrode layer 16 and Ta.sub.2 O.sub.5 anodic oxidation film 18 were 
formed. The heat treatment was carried out using the lengthwise heat 
treatment furnace 400 shown in FIG. 5. 
In this example, the inside of the bell jar was maintained in the N.sub.2 
gas atmosphere at 250.degree. C. in advance similar to the ninth example. 
Boat 12 supporting twenty (20) transparent substrates 12 was inserted into 
the bell jar 402 which is filled with the N.sub.2 gas atmosphere at 
250.degree. C. through the bottom of the bell jar 402. 
Then, the heater (not shown) started heating the bell jar 402 while 
introducing the N.sub.2 gas at a rate of 201/min from the top 404 of the 
bell jar 402 through mass-flow controller 452, pipes 464, and 468, to 
raise the temperature at a rate of 3.degree. C./min until the temperature 
of the transparent substrate 12 reached 350.degree. C. 
When the temperature of the substrate 12 reached 350.degree. C., the 
N.sub.2 gas which contains water vapor was then introduced into the bell 
jar 402 using bubbler 430, through the pipe 468 under the same conditions 
as Example 9. The temperature of the transparent substrate 12 was 
maintained at 350.degree. C. for 90 minutes with the N.sub.2 gas flow 
which contains water vapor. 
After that, the temperature was lowered from 350.degree. C. to 250.degree. 
C. at a rate of 0.8.degree. C./min still introducing the N.sub.2 gas which 
contains water vapor. 
When the temperature of the transparent substrate 12 reached 250.degree. 
C., the boat 408 supporting the transparent substrates 12 was pulled down 
from the bottom of the bell jar 402. 
Then, the MIM nonlinear device 50 comprising the Ta electrode layer 16, the 
Ta.sub.2 O.sub.5 anodic oxidation film 18 and the Cr electrode layer 20 
was completed in the same manner as Example 1. 
Using the MIM nonlinear device 50 formed on the transparent substrate 12, 
the nonlinear parameter .beta., the ON state resistance and the OFF state 
resistance of the MIM nonlinear device 50 were measured for three 
transparent substrates and the average was calculated. The .beta. value 
was 6.87, the ON state resistance was 1.94.times.10.sup.10 .OMEGA. and the 
OFF state resistance was 7.76.times.10.sup.13 .OMEGA.. The variation in 
the values of .beta., the ON state resistance and the OFF state resistance 
was very small both on the surface of the transparent substrate and among 
the transparent substrates compared with Examples 1 and 2. 
In this example, the N.sub.2 gas which contains water vapor was introduced 
into the bell jar at the time of starting the temperature drop in the heat 
treatment process. However, the N.sub.2 gas which contains water vapor may 
be introduced during the period of constant temperature prior to the 
temperature drop. It can be seen from the measurement result that the 
characteristic values of the nonlinear device are also desirable similar 
to the examples in which the N.sub.2 gas which contains water vapor was 
introduced only during the cooling (temperature drop) period. This 
eliminates the necessity to use a complicated process controller for 
introducing the N.sub.2 gas which contains water vapor only during the 
temperature drop period and can facilitate the gas control system. This 
also allows an adequate margin to be obtained with respect to the 
introduction timing of the N.sub.2 gas which contains water vapor and 
simplifies the apparatus structure and process controlling resulting in 
reduced cost. 
(EXAMPLE 11) 
The transparent substrate 12 on which the Ta electrode layer 16 and the 
Ta.sub.2 O.sub.5 anodic oxidation film 18 were formed was prepared under 
the same conditions as the first example. 
Heat treatment was performed to the transparent substrate 12 on which the 
Ta electrode layer 16 and the Ta.sub.2 O.sub.5 anodic oxidation film 18 
were formed. The heat treatment was carried out using the lengthwise heat 
treatment furnace 400 shown in FIG. 5. 
First of all, the N.sub.2 gas which contains water vapor was introduced 
into the bell jar 402 using bubbler 430, through the pipe 468 under the 
same conditions as Example 9. The inside of the bell jar 402 was 
maintained at 250.degree. C. in the N.sub.2 gas atmosphere which contains 
water vapor with the gas flow of N.sub.2 which contains water vapor. 
Then, boat 408 supporting twenty (20) transparent substrates 12 was 
inserted into the bell jar 402 which is filled with the N.sub.2 gas which 
contains water vapor at 250.degree. C. through the bottom of the bell jar 
402. 
Then, heater (not shown) started heating the bell jar 402 while introducing 
the N.sub.2 gas which contains water vapor at a rate of 201/min from the 
top 404 of the bell jar 402 to raise the temperature at a rate of 
3.degree. C./min until the temperature of the transparent substrate 12 
reached 350.degree. C. 
The temperature of the transparent substrate 12 was maintained at 
350.degree. C. for 90 minutes while introducing the N.sub.2 gas which 
contains water vapor. 
After that, the temperature was lowered from 350.degree. C. to 250.degree. 
C. at a rate of 0.8.degree. C./min still introducing the N.sub.2 gas which 
contains water vapor. 
When the temperature of the transparent substrate 12 reached 250.degree. 
C., the boat 408 supporting the transparent substrates 12 was pulled down 
from the bottom of the bell jar 402. 
Then, the MIM nonlinear device 50 comprising the Ta electrode layer 16, the 
Ta.sub.2 O.sub.5 anodic oxidation film 18 and the Cr electrode layer 20 
was completed in the same manner as Example 1. 
Using the MIM nonlinear device 50 formed on the transparent substrate 12, 
the nonlinear parameter .beta., the ON state resistance and the OFF state 
resistance of the MIM nonlinear device 50 were measured for three 
transparent substrates and the average was calculated. The .beta. value 
was 5.43, the ON state resistance was 8.77.times.10.sup.9 .OMEGA. and the 
OFF state resistance was 2.76.times.10.sup.13 .OMEGA.. The variation in 
the values of .beta., the ON state resistance and the OFF state resistance 
was very small both on the surface of the transparent substrate and among 
the transparent substrates compared to Examples 1 and 2. 
In this example, the N.sub.2 gas which contains water vapor was introduced 
from the beginning of the heat treatment process. This can achieve the 
same desirable characteristic values of the nonlinear device as compared 
with the case in which the N.sub.2 gas which contains water vapor was 
introduced only during the cooling (temperature drop) period. This 
eliminates the necessity to use a complicated process controller for 
introducing the N.sub.2 gas which contains water vapor only during the 
temperature drop period and can facilitate the gas control system. Since 
the heat treatment can be started in the atmosphere which contains water 
vapor from the first temperature raising step, less expensive 
open-structured heat treatment furnace may be used instead of the 
diffusion furnace used in this example. 
The heat treatment furnace 400 used in Examples 9 through 11 has a 
diffusion plate 406 comprising a disc with a plurality of holes. The 
furnace 400 is designed so that the gas introduced from the top 404 of the 
bell jar 402 passes through the diffusion plate 406 and flows toward the 
transparent substrates 12 loaded on the boat 408. The boat 408 is put on 
the quartz stage 410 which functions as a heat barrier as well as a lid. 
The gas introduced into the bell jar 402 from the top 404 flows out of the 
furnace through the gap between the bell jar 402 and the quartz stage 410. 
(EXAMPLE 12) 
As shown in FIG. 2, tantalum oxide film 14 with a thickness of 1000 .ANG. 
was deposited by sputtering on the transparent substrate 12 made of 
non-alkali glass. Alternatively, the tantalum oxide film 14 may be formed 
by sputtering tantalum film with a thickness of 1000 .ANG. on the 
transparent substrate 12 made of non-alkali glass followed by thermal 
oxidation. 
Then, tantalum film, which contains 0.2 weight % of tungsten (W) with 
respect to tantalum (Ta), was formed up to 2000 .ANG.. The tantalum film 
was then patterned to form a Ta electrode layer 16. The Ta electrode layer 
16 was subjected to anodic oxidation to form anodic oxidation film 18 
having a thickness of 48 .ANG., 54 .ANG. and 60 .ANG., respectively. 
Citric acid aqueous solution was used as electrolytic solution. The anodic 
oxidation voltage values used for this process were 25V, 28V and 31V, 
respectively. 
The transparent substrate 12 on which the Ta electrode layer 16 and the 
Ta.sub.2 O.sub.5 anodic oxidation film 18 were formed was subjected to 
heat treatment. 
The heat treatment was performed using a sideways heat treatment furnace 
shown in FIG. 4. Gas was introduced into the reactor tube 302 through the 
gas induction pipe 308 provided on the top of the reactor tube 302 and was 
discharged from the exhaust pipe 312. 
In this example, the temperature was raised while introducing N.sub.2 gas 
into the reactor tube 302 and the inside of the reactor tube 302 was 
filled with the N.sub.2 gas atmosphere and maintained at 250.degree. C. 
Then, boat 306 supporting a plurality of transparent substrates 12 
vertically was inserted into the reactor tube 302 which is maintained at 
250.degree. C. in the N.sub.2 gas atmosphere from the left of the figure. 
Valve 310 was then closed and the gas was evacuated from the reactor tube 
302 through the exhaust pipe 312 while maintaining the temperature at 
250.degree. C. After the evacuation, the valve was opened to introduce 
N.sub.2 gas again into the reactor tube 302 through gas induction pipe 308 
to fill the reactor tube 302 with N.sub.2 gas atmosphere. Heater 304 began 
heating to raise the temperature at a rate of 5.degree. C./min until the 
temperature of the transparent substrate 12 reached 320.degree. C. while 
introducing N.sub.2 gas through the gas induction pipe 308 at a rate of 
501/min and discharging from the exhaust pipe 312 provided on the bottom 
of the reactor tube 302. The temperature of the transparent substrate 12 
was maintained at 320.degree. C. for 1/2 hour while maintaining the flow 
rate of the N.sub.2 gas at 501/min. Then, valve 310 was closed at 
320.degree. C. and the gas was evacuated from the reactor tube 302 again 
through the exhaust pipe 312. Valve 310 was opened to introduce mixed gas 
of the air and nitrogen into the reactor tube 302 through gas induction 
pipe 308. When the reactor tube 302 was filled with mixed gas, the 
temperature was lowered to 200.degree. C. at a rate of 1.0.degree. C./min 
under the atmospheric pressure. When the temperature of the transparent 
substrate 12 became under 150.degree. C., boat 306 supporting transparent 
substrates 12 was removed from the reactor tube 302 from the left of the 
figure. 
After that, the MIM nonlinear device 50 comprising Ta electrode layer 16, 
the Ta.sub.2 O.sub.5 anodic oxidation film 18 and the Cr electrode layer 
20 was completed in the same manner as the first example. 
Similarly to the first example, the nonlinear parameter .beta., the ON 
state resistance and the OFF state resistance of the MIM nonlinear device 
50 formed on the transparent substrate 12 were measured. 
FIG. 9 is a chart showing the relationship between the air ratio in the 
mixed gas of the air and nitrogen introduced into the reactor 302 through 
the gas induction pipe 308 and the ON and OFF state resistance values. The 
air ratio is represented as (air flow)/(air flow+N.sub.2 gas flow). The 
white square (.quadrature.), white triangle (.DELTA.) and white diamond 
denote the OFF state resistance values of the MIM nonlinear device 50 
formed with the anodic oxidation voltage values 25V, 28V and 31V, 
respectively. The black square (.box-solid.), black triangle and black 
diamond denote the ON state resistance values of the MIM nonlinear device 
50 formed with the anodic oxidation voltage values 25V, 28V and 31V, 
respectively. 
Even when the Ta electrode layer 16 is formed from the tantalum film which 
contains 0.2 weight % of tungsten (W) with respect to tantalum (Ta), the 
adequately high OFF state resistance can be obtained through a relatively 
low heat treatment temperature (320.degree. C.) by using the gas which 
contains the air for the cooling step during the heat treatment. The 
.beta. value is also desirable, which was higher than 4.3, indicating a 
good nonlinear characteristic sufficient for obtaining a good image 
quality. 
The concentration of the water vapor contained in the air was 1.2 mol % 
with respect to the entire air. Accordingly, when the air ratio is 1, the 
water vapor concentration becomes 1.2 mol % with respect to the mixed gas 
of the air and nitrogen. When the air ratio is 0.1, the water vapor 
concentration becomes 0.12 mol % with respect to the mixed gas of the air 
and nitrogen and with the air ratio 0.01, the water vapor concentration 
becomes 0.0112 mol % . The lower limit of the air ratio experimental data 
is 0.012, and the corresponding water vapor concentration is 0.014 mol % 
with respect to the mixed gas of the air and nitrogen. 
Although, in the example 0.2 weight % of tungsten (W) was contained in the 
Ta electrode layer 16, the Ta electrode layer 16 may contain 0.1 weight % 
of Re or 0.2 weight % of Mo. Also, the Ta electrode layer 16 which does 
not contain additional material added to tantalum can also achieve the 
same tendency of voltage-current characteristic as effective as this 
example. 
(EXAMPLE 13) 
Measurement was executed using a thermal desorption spectrum (TDS) 
technique to study the tantalum oxide film formed by the heat treatment 
process of the invention. Thermal desorption spectrometer 500 shown in 
FIG. 10 was used including a quadruple spectrometer 502 and an infrared 
heater 504 in the vacuum chamber 510. The sample 520 was heated from its 
rear surface by the infrared heater 504. The gas emitted from the sample 
520 was measured by the quadruple spectrometer 502 to obtain the thermal 
desorption spectrum. Thermocouple TC1 was provided on the bottom side of 
the sample 520 for the temperature control of the sample 520 in the aspect 
of the thermal controllability. Thermocouple TC2 was also provided on the 
top side of the sample 520 to measure the surface temperature of the 
sample 520. Since the heat conductivity of the quartz substrate 522 used 
as the sample 520 is not so good and the thickness of the substrate is as 
thick as 1.1 mm, there was a difference in the temperature between the 
thermocouples TC1 and TC2. 
The temperature in the actual MIM nonlinear device forming process is equal 
to that indicated by TC2. Although, in the actual examples the MIM 
nonlinear device is formed on the non-alkali glass, quartz glass was used 
for the TDS measurement for the purpose of ensuring the heat-resistant 
ability to put up with the measurement with a temperature as high as 
1000.degree. C. Even if the substrate material is changed, the 
voltage-current characteristic of the MIM nonlinear device formed thereon 
is the same. 
How the sample 520 used for the measurement was obtained will be described 
below. Tantalum oxide film 524 with a thickness 1000 .ANG. was formed by 
sputtering on the quartz substrate 522 of a thickness 1 mm. Then, tantalum 
film 526 was formed on the tantalum oxide film 524 by sputtering. The 
tantalum film 526 was subjected to anodic oxidation to form anodic 
oxidation film 528. The thickness of the tantalum film 426 after the 
anodic oxidation was 1600 .ANG. and the thickness of the tantalum 
oxidation film 528 was 850 .ANG.. 
The sample 520 was then subjected to heat treatment in the same manner as 
the twelfth example. That is, the temperature was raised in the N.sub.2 
gas atmosphere until the temperature of the sample 520 reached 320.degree. 
C. The temperature was maintained at 320.degree. C. for 1/2 hours in the 
N.sub.2 atmosphere. Then, the temperature was lowered to 200.degree. C. at 
a rate of 1.0.degree. C./min in the air atmosphere containing 1.2 mol % of 
water vapor. The sample 520 was taken out from the heat treatment furnace 
and was used for measurement of the thermal desorption spectrum. 
The measurement result is shown in FIG. 12. 
The horizontal axis represents the temperature of the controllable 
thermocouple TC1 positioned on the rear side and the vertical axis 
represents the intensity of the gas measurement at atomic weight 18 
(H.sub.2 O), which corresponds to water vapor. There are peaks P1, P2 and 
P3 appearing in the chart. As the peak values, the temperatures measured 
by the thermocouple TC2 which measures the surface temperature of the 
sample are cited in the chart because there is a slight difference in the 
temperature values between thermocouples TC1 and TC2. 
As a comparison example, a sample was prepared under the same heat 
treatment conditions except for the fact that the cooling step was also 
performed in the nitrogen atmosphere without containing water vapor. The 
thermal desorption spectrum was measured for the comparison sample. The 
result is shown in FIG. 13. 
As is clearly seen from FIGS. 12 and 13, when the cooling (temperature 
drop) in the heat treatment process was conducted in the air atmosphere 
which contains 1.2 mol % of water vapor, peaks P1 (surface temperature 
120.degree. C.), P2 (surface temperature 220.degree. C.) and P3 (surface 
temperature 410.degree. C.) were observed. On the other hand, when the 
cooling in the heat treatment process was performed in the nitrogen 
atmosphere without containing water vapor, only peaks P1 (120.degree. C.) 
and P3 (410.degree. C.) were observed, and peak P2 (220.degree. C.) was 
not observed. Peak P1 in FIG. 13 results from the water attached to the 
surface of the sample by physical adsorption. 
In order to identify the peak from among the three peaks, which is affected 
by the gas atmosphere during the cooling step performed after anodic 
oxidation, experimentation was performed. Nitrogen gas that passed through 
heavy water (D20) contained in the bubbler 430 shown in FIG. 5 was added 
during the cooling in the heat treatment process. Other than the gas 
atmosphere during the cooling, the experimentation was conducted under the 
same conditions as in example 13 for the sample having a characteristic 
shown in FIG. 12. Then, the obtained sample was measured by TDS to observe 
the spectrum with mass number 20 that corresponds to the peak of the heavy 
water. As a result, a peak was observed only in the same temperature range 
as P2. 
From above, it can be seen that what causes the peak P2 (220.degree. C.) is 
a substance introduced during the heat treatment in the atmosphere which 
contains water vapor. When measuring the anodic oxidation film formed in 
the examples 1 through 12 by TDS, peak P2 is clearly observed. 
Thus, it is preferable to perform the heat treatment in the atmosphere 
containing water vapor under the temperature control to at least lower 
than 220.degree. C. 
(EXAMPLE 14) 
After the MIM nonlinear device 50 was manufactured in the same manner as in 
Examples 6 and 9, ITO (Indium-Tin-Oxide) film with a thickness of 500 
.ANG. was formed by sputtering, which was then patterned to form a pixel 
electrode 22, as shown in FIG. 1. Thus, the electrode board 10 that 
comprises a transparent substrate 12, the MIM nonlinear device 50 formed 
on the transparent substrate 12 and pixel electrode 22 connected to the 
MIM nonlinear device 50 was completed. On the other hand, the ITO film was 
formed by sputtering on non-alkali glass transparent substrate 32. The ITO 
film was patterned to form an opposed signal electrode 32 and the other 
electrode substrate 30 was completed. Liquid crystal layer 40 was held 
between the electrode substrates 10 and 30 (FIG. 15). 
Then, data line 78 composed of a Ta electrode layer 16 was connected to the 
data line driving circuit 76 while connecting scan line 74 composed of the 
opposite signal electrode 34 to scan line driving circuit 72 to make up a 
liquid crystal display device 100. According to the measurement of the 
display characteristic, the liquid crystal display device 100 showed high 
contrast and excellent image quality. 
Another liquid crystal display device 100 was made using the MIM nonlinear 
device 50 formed in the same manner as in Examples 4, 7 and 9. After the 
measurement, the same high contrast and good image quality were obtained. 
Although the present invention has been described in conjunction with the 
preferred embodiments, the invention is not limited to those embodiments. 
In the embodiments the cooling step in the air or in the N.sub.2 gas 
atmosphere containing water vapor was started at the same temperature as 
the heat treatment temperature in the N.sub.2 gas atmosphere. However, the 
temperature may be lowered to a predetermined value in the same N.sub.2 
gas atmosphere after the heat treatment in the N.sub.2 gas atmosphere, and 
after that the atmosphere may be changed to the air or to the N.sub.2 gas 
which contains water vapor to further continue cooling. Moreover, Nb, W, 
Al or Mo may be added to the Ta electrode which contains Ta as the major 
component. Cr electrode layer 20 may be replaced by an electrode layer 
made of Ti, Mo, or Al. Furthermore, Cr electrode layer 20 may be omitted. 
If this is a case, pixel electrode 22 serves as Cr electrode layer 20. In 
FIG. 14, the connecting order of liquid crystal display element 60 and MIM 
nonlinear device 50 may be reversed between the scan line 74 and signal 
line 78. 
Industrial Applicability 
The manufacturing method of the MIM nonlinear device in accordance with the 
invention is suitably applied to manufacture of MIM nonlinear devices used 
in the liquid crystal display apparatus which requires superior image 
quality.