Platinum thin film resistance element and production method therefor

A platinum thin film is formed by sputtering platinum onto an insulating substrate and heat aging the platinum thin film in a stairstep manner. A kerf is formed in the platinum thin film to produce a desired resistance, and a metal oxide semiconductor film is thereafter deposited on the platinum thin film to produce a gas sensor.

BACKGROUND OF THE INVENTION 
The present invention relates to a platinum thin film resistance element 
for use in a temperature sensor and a gas sensor and a method for the 
manufacture of such a platinum thin film resistance element. 
As a resistance thermometer for use in temperature sensors, there has 
heretofore been employed a platinum winding resistance element or negative 
temperature coefficient temperature sensitive resistance element (for 
example, a thermistor). These elements, however, have the following 
defects: 
Main defects of platinum winding resistance element: 
(1) Since its impedance is usually as small as 50 or 100.OMEGA., the output 
sensitivity is low. 
(2) It is susceptible to mechanical vibration or shock; namely, since a 
platinum wire is wound on a glass rod, the platinum wire is likely to slip 
off from the glass rod due to vibrations. 
(3) Since the resistance value is small, a three-core or four-core lead 
wire is needed for avoiding the influence of a lead wire. 
Main defects of thermistor: 
(1) Compatibility is poor because of difficulty in the production of 
thermistors of the same characteristics. 
(2) Characteristic variations with time are substantial. 
(3) The temperature-resistance characteristic is negative and exponential 
rather than linear. 
Gas sensors employing such conventional resistance elements have the 
following defects: 
A sensor using a platinum wire coil is called a hot-wire gas sensor. A 
catalyst is laid on the platinum wire coil and, upon arrival of a gas, the 
catalyst promotes its combustion to cause a change in the resistance value 
of the platinum winding, and this resistance variation is detected. 
(1) Because of the winding, its resistance value cannot be increased. 
Therefore, a voltage of a bridge circuit for detecting the resistance 
variation is as low as 2 V or so on an average and the gas sensitivity is 
also poor; it is impossible to detect a low concentration of a gas the 
molecular heat of combustion of which is low, such as carbon monoxide. 
(2) Since it is necessary that the platinum wire be wound into uniform 
coils, with their catalyst coated surfaces held in the same condition, the 
productivity is poor and the mechanical strength is low. 
A sensor using a thermistor is called a gas thermal conductivity system. 
This makes use of a difference in thermal conductivity between air and a 
gas to be sensed and requires two thermistors of the same characteristics. 
(1) It is difficult to select two thermistors of the same characteritics. 
(2) The balance between the two thermistors is lost owing to characteristic 
variations with time. 
(3) The output is not linearly proportional to the gas concentration. 
A sensor using a metal oxide is called a semiconductor sensor and is 
intended to directly read out a resistance variation which is caused by 
the adsorption of a gas to a metal oxide such as SnO.sub.2, ZnO, V.sub.2 
O.sub.5 or the like. This sensor also has the following drawbacks: 
(1) Much time is required until it becomes stable after the connection of 
the power source. 
(2) The zero point is very unstable even in the absence of the gas to be 
sensed. 
(3) The low-concentration sensitivity is good but an output change with a 
concentration change at a high concentration is very small. 
(4) Reproducibility is very poor and hence reliability is low. 
Heretofore, there has not been put to practical use a gas sensor which is 
capable of accurately detecting carbon monoxide even at such a low 
concentration as 50 PPM without being affected by other gases. 
Similarly, there has not been available a highly stable and sensitive 
sensor for detecting a nitrogen oxide, in particular, nitrogen monoxide. 
Further, there have not been proposed a sensor capable of stably detecting 
only ammonia even at a low concentration or a sensor capable of stably 
detecting only an inflammable gas. 
A conventional platinum resistance element has employed a winding resistor 
and it has been said that a thin film resistance element could not be 
produced. That is, even if a platinum thin film is deposited by sputtering 
on an insulating substrate as is the case with the fabrication of the 
conventional thin film resistance element, the platinum thin film is not 
held stably and disappears during heat aging. 
It is an object of the present invention to provide a platinum thin film 
resistance element the resistance value of which can easily be made large 
and which does not require a three-core or four-core lead wire but is 
stable. 
Another object of the present invention is to provide a method for the 
manufacture of a stable platinum thin film resistance element. 
Another object of the present invention is to provide a stable and reliable 
platinum thin film resistance element capable of accurately detecting a 
gas to be detected. 
Another object of the present invention is to provide a platinum thin film 
resistance element which is capable of accurately detecting carbon 
monoxide of low concentration. 
Another object of the present invention is to provide a platinum thin film 
resistance which is capable of stably detecting a nitrogen oxide. 
Another object of the present invention is to provide a platinum thin film 
resistance element which is capable of detecting an ammonia gas even at a 
low concentration. 
Yet another object of the present invention is to provide a platinum thin 
film resistance element which is capable of stably detecting an 
inflammable gas alone. 
SUMMARY OF THE INVENTION 
According to the present invention, a platinum thin film is deposited on 
the surface of an insulating substrate of, for example, a cylindrical or 
columnar configuration, and a pair of lead wires are electrically 
connected to both end portions of the platinum thin film and fixed to the 
insulating substrate. The platinum thin film is deposited by sputtering to 
a thickness of, for example, about 200 to 1000 A. The insulating substrate 
is required to have a smooth surface and stand heat aging at 1000.degree. 
C. The power for the sputtering is selected to be 0.8 W/cm.sup.2 or more 
so as to ensure the adhesion of the platinum thin film to the insulating 
substrate. The platinum thin film thus deposited on the insulating 
substrate is stabilized by heat aging, raising temperature from about 
100.degree. C. up to around 1000.degree. C. in a stepwise manner. 
Thereafter, a spiral kerf is formed in the platinum thin film to obtain 
thereacross a required resistance value. The abovesaid lead wires are 
attached to both end portions of the platinum thin film. In the case of 
obtaining a mere temperature sensor, the platinum thin film is covered 
with a protective film of an insulating paint of the polyimid or silicon 
system. 
In this way, a stable platinum thin film resistance element is obtained 
which has a resistance value of several tens of ohms to scores of 
kilo-ohms. The platinum thin film resistance element thus obtained is 
combined, as a temperature sensor, with a resistance element having no 
temperature coefficient to form a bridge circuit, by which temperature can 
be measured with high accuracy. Further, a temperature sensor free from 
the influence of the lead wires can be obtained with a simple 
construction. 
By forming on the platinum thin film a metal oxide semiconductor film 
capable of adsorbing and releasing a gas to be sensed and selecting the 
resistance value of the metal oxide semiconductor film to be sufficiently 
larger than the resistance value of the platinum thin film, it is possible 
to obtain a platinum thin film resistance element which is capable of gas 
detection with a linear and hence reproducible detection sensitivity 
characteristic. Further, by forming a thin film of the copper oxide system 
on the platinum thin film, a platinum thin film resistance element capable 
of accurately detecting carbon monoxide even at a low concentration can be 
obtained. Moreover, a platinum thin film resistance element capable of 
detecting a nitrogen oxide can be produced by forming on the platinum thin 
film a film of a mixture including 10 to 30 wt% of rare earth oxide and 
0.5 to 5 wt% of silver nitrate with respect to vanadium pentoxide. Also it 
is possible to obtain a platinum thin film resistance element capable of 
detecting ammonia by forming on the platinum thin film a film of a mixture 
including 3 to 10 wt% of rare earth oxide, 1 to 5 wt% of antimony trioxide 
and 0.5 to 5 wt% of silver nitrate with respect to vanadium pentoxide. 
When such a metal oxide film is thus formed on the platinum thin film, a 
protective layer as of alumina cement or beryllia cement is interposed 
therebetween, by which it is possible to obtain a gas sensor which detects 
only a specified gas and has small characteristic variations with time. 
Further, in the case of using the platinum thin film resistance element 
for gas detection, heating means is provided in the element for improving 
its sensitivity. That is, a coiled nichrome wire heater is housed, for 
example, in a tubular insulating substrate and a current is applied to the 
heater to heat the platinum thin film resistance element up to a proper 
temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the present invention, a platinum thin film is deposited on an 
insulating substrate of a smooth surface, which is formed by transparent 
fused quartz, hard glass capable of standing temperatures higher than 
1000.degree. C. or porcelain. The unevenness of the surface of the 
insulating substrate, if any, is made smaller than the thickness of the 
platinum film to be formed thereon. As the insulating substrate, use is 
made of a cylindrical insulating substrate 11 as shown in FIG. 1A. After 
being sufficiently washed and dried, the insulating substrate 11 is heated 
at 1000.degree. C. or so in a furnace so that adsorbed gases and water are 
completely released from the substrate 11. Then, the insulating substrate 
11 is put in a sputtering equipment, wherein it is subjected to sputtering 
of platinum while being rotated about its axis by means of a rotating jig, 
by which a platinum thin film 12 is deposited uniformly all over the outer 
peripheral surface of the insulating substrate 11, as shown in FIG. 1B. 
The platinum thin film 12 has a purity of 99.999% or more. For the above 
sputtering, a sputtering equipment can be employed and the sputtering 
condition is such that when a platinum target and the insulating substrate 
11 are spaced 1 cm apart, use is made of an ionic current of 10 mA or more 
with 1.4 KV, that is, the sputtering power of 0.7 W/cm.sup.2, preferably, 
0.8 W/cm.sup.2 or more. The sputtering time depends on the thickness of 
the platinum thin film 12 desired to obtain; usually, the sputtering is 
carried out for approximately an hour to an hour and a half. 
The platinum thin film thus formed by sputtering is unstable if it is left 
untreated. The insulating substrate 11 deposited with the platinum thin 
film 12 is accordingly subjected to heat aging in an electric furnace, in 
which it is heated up to 1000.degree. C. raising the temperature, for 
example, from 100.degree. C. by steps of 100.degree. C. at one-hour 
intervals. 
After the heat aging, a spiral-shaped kerf 13 is cut by a diamond cutter or 
laser cutter in the platinum thin film 12 to increase its resistance 
value. The pitch of the kerf 13 is dependent on the resistance value 
desired to obtain. By the formation of the kerf 13 the resistance value 
can be raised on the order of 1000 times. From the viewpoint of increasing 
the resistance value by the formation of the kerf 13, it is preferred that 
the thickness of the platinum thin film 12 be at least about 100 A or 
more. Too large a thickness of the platinum thin film 12 takes much time 
for sputtering which lowers productivity and increases the amount of 
platinum used, and hence is not preferred from the economical point of 
view. Further, for raising the resistance value, too, it is desirable that 
the thickness of the platinum thin film 12 not be too large; it is 
considered that a maximum thickness is approximately 1000 A. 
Following the formation of the kerf 13, lead wires are connected to both 
ends of the platinum thin film 12. For example, as depicted in FIG. 1D, 
caps 14 and 15 of a corrosion resisting metal such as stainless steel are 
press-fitted and fixed onto the marginal portions of the platinum thin 
film 12 on both end portions of the cylindrical insulating substrate 11. 
Then, heatproof lead wires 16 and 17, each produced, for example, by 
plating an iron wire with copper and then nickel, are connected at one 
end, as by spot welding, to the centers of the outer end faces of the caps 
14 and 15, respectively, through which the lead wires 16 and 17 are 
electrically connected to the platinum thin film 12. As illustrated in 
FIG. 1E, a protective film 18 is formed to a thickness of about 10 to 15 
.mu.m all over the platinum film 12 and the caps 14 and 15 by baking 
thereon a heat-resisting wet-proof, insulating resin paint as of the 
polyimid or silicon system. 
The lead wires 16 and 17 may be attached, for instance, in the manner shown 
in FIG. 1F, too, in which the lead wires 16 and 17 of platinum are wound 
on the platinum thin film 12 on both end portions of the insulating 
substrate 11 and then a platinum paste is baked thereon to connect the 
lead wires 16 and 17 to the platinum thin film 12 and fix them to the 
insulating substrate 11. 
In the sputtering of platinum, when the sputtering power was smaller than 
0.7 W/cm.sup.2, for example, 0.53 W/cm.sup.2 with an ionic current of 8 mA 
and a voltage of 1.4 KV, platinum particles were not firmly deposited by 
sputtering on the insulating substrate. As compared with the case of an 11 
mA ionic current, the quantity of gas adsorbed to the platinum particles 
was large to make the platinum film sparse and thick, containing many 
pores around the platinum particles, and the resistance value was as large 
as 70 to 80.OMEGA. (in the case of the 11 mA ionic current, 20 to 
30.OMEGA.). And in the course of heat aging, the platinum particles were 
dispersed together with the adsorbed gas, resulting in the resistance 
value becoming 200.OMEGA. to infinity. In the case where the ionic current 
was 11 mA, however, the adsorbed gas in the platinum thin film was 
released by the heat aging and the platinum thin film became a thin, 
continuous or solid film with a resistance value of 1.7 to 2.0.OMEGA.. As 
will be appreciated from the above, the platinum thin film 12 cannot be 
formed with the sputtering power of less than 0.70 W/cm.sup.2. 
The platinum thin film deposited by sputtering on the insulating substrate 
is an assembly of platinum particles that contains gas, the gas being 
released by the heat aging from the platinum thin film to make it a 
continuous, solid film. Shown in the following are variations in the 
resistance value and temperature coefficient of the platinum thin film 
during the heat aging in the case of sputtering platinum on the outer 
peripheral surface of an insulating substrate 2.5 mm in diameter and 7 mm 
long using a voltage of 1.4 KV and an ionic current of 11 mA. 
______________________________________ 
Resistance 
Temperature 
value coefficient 
______________________________________ 
Immediatedly after 
24 .OMEGA. 
2260 PPM 
sputtering 
100.degree. C. an hour 
24 .OMEGA. 
2370 PPM 
100 to 400.degree. C. 
16 .OMEGA. 
2700 PPM 
(raised by steps of 100.degree. C. 
at 1-hour intervals) 
100 to 600.degree. C. 
6 .OMEGA. 
2860 PPM 
(raised by steps of 100.degree. C. 
at 1-hour intervals) 
100 to 800.degree. C. 
2.0 .OMEGA. 
3660 PPM 
(raised by steps of 100.degree. C. 
at 1-hour intervals) 
100 to 1000.degree. C. 
2.0 .OMEGA. 
3680 PPM 
(raised by steps of 100.degree. C. 
at 1-hour intervals) 
______________________________________ 
As is evident from the above, when the heat aging is carried out up to 
800.degree. C. raising the heating temperature by steps of 100.degree. C. 
at one-hour intervals, the resistance value becomes constant and, in this 
respect, such heat aging is satisfactory; in terms of the temperature 
coefficient, however, it is preferred that the heat aging be conducted up 
to 1000.degree. C. 
For testing the stability of the resistance value of the platinum thin film 
12, samples were produced by heat-aging platinum thin films of the same 
lot through various methods, attaching the caps 14 and 15 and the lead 
wires 16 and 17, forming the spiral kerf 13 to provide a resistance value 
of about 1000.OMEGA. and then forming the protective film 18. The samples 
were each subjected to a temperature cycle test for 30 minutes at 
-50.degree. to 200.degree. C. five times and their resistance values at 
0.degree. C. were measured before and after the tests to check the 
stability of the resistance value. 
______________________________________ 
Result of 
Dispersion in 
stability 
temperature 
Heat aging method test coefficient 
______________________________________ 
100 to 1000.degree. C. 
-0.3% .+-.0.6% 
(raised by steps of 100.degree. C. 
at 40-minute intervals) 
100 to 1000.degree. C. 
-0.01% .+-.0.3% 
(raised by steps of 100.degree. C. 
at 1-hour intervals) 
100 to 1000.degree. C. 
-0.01% .+-.0.2% 
(raised by steps of 100.degree. C. 
at 1.5-hour intervals) 
100 to 1000.degree. C. 
-0.01% .+-.0.2% 
(raised by steps of 100.degree. C. 
at 2-hour intervals) 
200 to 800.degree. C. 
-0.7% .+-.1.0% 
(raised by steps of 200.degree. C. 
at 2-hour intervals) 
200 to 1000.degree. C. 
-0.4% .+-.1.0% 
(raised at steps of 200.degree. C. 
by 2-hour intervals) 
400 to 800.degree. C. 
-0.8% .+-.2.3% 
(raised by steps of 100.degree. C. 
at 2-hour intervals) 
400 to 1000.degree. C. 
-0.5% .+-.1.8% 
(raised by steps of 100.degree. C. 
at 1-hour intervals) 
______________________________________ 
The above indicates that the aging methods 2 , 3 and 4 provide a high 
degree of stability in the resistance value and hence are preferred. 
As referred to previously, it is preferred that the platinum thin film be 
about 100 to 1000 A (0.01 to 0.1.mu.m) thick; therefore, the unevenness of 
the surface of the insulating substrate 11 is held less than 1.1 .mu.m. 
For example, in the case of an alumina substrate, crystals of alumina are 
several .mu. meters in size and even if lapped, the surface of the alumina 
substrate still has an unevenness of 0.5 .mu.m or so. FIG. 2A is a 
photo-micrograph of a platinum thin film formed on such an alumina 
substrate and heat-aged. FIG. 2B is a photo-micrograph of a platinum thin 
film deposited on a transparent fused quartz substrate and heat-aged. It 
appears from FIG. 2B that the platinum thin film is formed uniformly as 
compared with the thin film shown in FIG. 2A. The platinum thin films were 
about 200 A in either case. 
The platinum thin film resistance element obtained as described in the 
foregoing is stable chemically and exhibits a positive linear 
temperature-resistance characteristic and, in addition, it can be set by a 
suitable selection of the pitch of the kerf 13 to a resistance value, for 
example ranging from several tens of ohms to scores of kilo-ohms. By 
setting such a high resistance value, the accuracy of temperature 
measurement can be enhanced and the resistances of the lead wires can be 
neglected; accordingly, no compensation is needed for the lead-wire 
resistances, permitting simplification of the measuring circuit 
arrangement. Moreover, since the platinum thin film 12 is deposited on the 
insulating substrate 11, there is no likelihood that the film slips out of 
position due to mechanical vibration and shock unlike a winding wrapped 
around an insulating substrate as in the prior art; furthermore, the 
deposition of the platinum thin film on the insulating substrate is better 
for mass-production purposes than the winding of a thin platinum wire on 
the insulating body and allows ease in the production of resistance 
elements free from dispersion in the resistance value and temperature 
coefficient. 
For example, the platinum thin film resistance element of the present 
invention, which has a resistance value of 10 K.OMEGA. at 0.degree. C. and 
in which the resistance ratio between 100.degree. and 0.degree. C. is 
1.3000, has the following temperature-resistance characteristic: 
______________________________________ 
-100.degree. C. 
6,998 .OMEGA. 
+50.degree. C. 
11,500 .OMEGA. 
-50.degree. C. 
8,499 .OMEGA. 
+75.degree. C. 
12,250 .OMEGA. 
-25.degree. C. 
9,250 .OMEGA. 
+100.degree. C. 
13,000 .OMEGA. 
0.degree. C. 
10,000 .OMEGA. 
+125.degree. C. 
13,750 .OMEGA. 
+25.degree. C. 
10,750 .OMEGA. 
+150.degree. C. 
14,500 .OMEGA. 
______________________________________ 
In this case, the temperature-resistance characteristic is almost linear, 
as indicated by the line 21 in FIG. 3. In a thermistor heretofore employed 
for measuring a small temperature change, a resistance 
variation/.degree.C. at -25.degree. C. and a resistance 
variation/.degree.C. at 50.degree. and 100.degree. C. are entirely 
different, as is evident from the curve 22 in FIG. 3. In addition to this, 
because of secular change and hysteresis, the conventional thermistor is 
poor in reproducibility and hence is not utterly reliable and not accurate 
as a temperature measuring instrument. In contrast thereto, the platinum 
thin film resistance element has substantially the same temperature 
coefficient over the temperature range from &lt;100.degree. to 200.degree. 
C., as mentioned above; accordingly, a resistance variation/.degree.C. at 
any temperature within the range of -100.degree. to 200.degree. C. is 
30.OMEGA. and it is 3.0.OMEGA. per 0.1.degree. C. This indicates that even 
if the temperature coefficient slightly decreases, the resulting 
resistance variation remains below an error range. Such a large resistance 
variation could not have been taken out by a conventional 100.OMEGA. 
platinum resistance thermometer. 
In the case of applying the platinum thin film resistance element of the 
present invention to a temperature measuring instrument, use in made of a 
bridge circuit arrangement such, for example, as shown in FIG. 4, as is 
the case with kind of temperature measuring instrument hitherto employed. 
In FIG. 4, a platinum thin film resistance element 23 and a variable 
resistor 24 the temperature coefficient of which is substantially zero are 
connected in series; a series circuit of resistors 25 and 26 is connected 
in parallel to the series circuit 23, 24; a power source 27 is connected 
across the series circuit 25, 26; an ammeter, voltmeter or like indicator 
is connected between the junction of the resistance element 23 and the 
resistor 24 and the junction of the resistors 25 and 26. The resistance 
value of the resistor 24 is set, for instance, to 0.degree. C. in 
agreement with the resistance value of the platinum thin film resistance 
element 23. The resistance values of the resistors 25 and 26 are selected 
equal to each other. The resistor 24 is formed, for example, of a manganin 
wire (Cu 83 to 86%, Mn 12 to 15% and Ni 2 to 4%). The temperature 
coefficient of this wire is about 50 PPM in the temperature range of 
-100.degree. to +200.degree. C., and consequently, when the resistance 
value of the resistance element 23 is 10.OMEGA., a resistance variation 
per 0.1.degree. C. is less than 0.01.OMEGA., which is negligible relative 
to the 3.0.OMEGA. resistance change of the platinum thin film resistance 
element 23 per 0.1.degree. C. Thus the use of the platinum thin film 
resistance element of the present invention permits highly accurate 
temperature measurements. In the case of employing the platinum thin film 
resistance element for temperature measurements, the insulating substrate 
11 is formed, for example, about 2.8 mm in diameter and about 10 mm long 
so as to provide for enhanced accuracy in the measurement. 
The present invention allows ease in the fabrication of platinum thin film 
resistance elements of such a high resistance value as 1 to 10.OMEGA. and 
of uniform characteristics, with the dispersion thereof held less than 
.+-.0.1%. Accordingly, for instance, as shown in FIG. 5, platinum thin 
film resistance elements 23 and 29 of the same characteristics are 
connected in series and a bridge circuit is formed using the resistance 
elements 23 and 29 and the resistors 25 and 26 as respective arms, and 
then the power source 27 and the indicator 28 are connected to the bridge 
circuit. With such a bridge circuit arrangement, it is possible to detect 
gas by sealing the one resistance element 29 in a gas-tight envelope 31 as 
of glass and disposing the other resistance element in the air at the 
place where it is desired to detect the arrival of a gas. The thermal 
conductivities of main gases are as follows: 
______________________________________ 
Thermal conductivity of gas 
Cal cm.sup.-1 sec.sup.-1 (.degree.C.) .times. 10.sup.-5 
Gas 0.degree. C. 
100.degree. C. 
______________________________________ 
Air 5.83 7.4 
Hydrogen 41.6 54.7 
Oxygen 5.8 7.6 
Methane 7.2 -- 
Ethane 4.3 7.7 
Propane 3.5 -- 
Alcohol 3.4 5.0 
Carbon dioxide 2.3 
______________________________________ 
Accordingly, if the output from the bridge circuit is pre-adjusted to zero 
with the resistance element 23 in air, when the resistance element 23 
comes into contact with a gas the thermal conductivity of which greatly 
differs from that of the air, such as, for example, hydrogen, methane, 
propane, carbon dioxide or the like, the surface of the resistance element 
23 is cooled or heated to cause a current to flow through the ammeter 28. 
Thus a specified gas can be detected providing that the existence of only 
that gas is possible. Futhermore, under such condition since the 
temperature change by such cooling or heating is proportional to the gas 
concentration, it is also possible to measure the gas concentration by 
checking the gas concentration-output characteristic of the bridge circuit 
and calibrating the ammeter 28 in advance. 
In this case, in order to improve the detection sensitivity, the platinum 
thin film resistance element 23 is adapted to be heated up to a certain 
temperature by the current flowing in the element itself. Therefore, it is 
desirable for the reduction of power consumption that the platinum thin 
film resistance element 23 be small in size and in heat capacity. For 
example, the insulating substrate 11 is about 1 mm in diameter and about 3 
mm in length and has a resistance value of 200 to 300.OMEGA. and the 
voltage of the power source 27 is approximately 6 to 8 V. 
The platinum thin film resistance element of the present invention can be 
employed not only for the detection of a gas through utilization of a 
difference in thermal conductivity between the air and the gas, but also 
as a gas sensor which makes use of the change in heat generation depending 
upon an amount of gas or the kind of gas adsorbed to the surface of a 
metal oxide. That is, as described previously in respect to FIG. 1, the 
platinum thin film 12 is deposited on the insulating substrate 11 and the 
kerf 13 is formed in the platinum thin film 12. Then, as depicted in FIG. 
6, a semiconductor oxide film 31 is uniformly deposited by high-frequency 
sputtering to a thickness of 1 to 2 .mu.m over the entire area of the 
platinum thin film 12, while rotating the insulating substrate 11; in the 
alternative, the semiconductor oxide film 31 may be formed 10 to 20 .mu.m 
thick by a painting method. Thereafter, the semiconductor oxide film 31 is 
heat-aged at 500.degree. to 800.degree. C. for several hours, by which the 
oxide film is stabilized. Finally, a heater 32 formed by a nichrome wire 
is inserted into the body of the cylindrical insulating substrate 11 to 
produce a resistance element 35. As shown in FIG. 7, the lead wires 16 and 
17 and both ends of the heater 32 are respectively connected to four 
terminal pins 34 inserted into and fixed to a stem 33 as of steatite or 
bakelite and then the resistance element assembly is covered with a net 
cap 36. The semiconductor oxide film 31 can be made of SnO.sub.2, ZnO and 
V.sub.2 O.sub.5. The resistance value of the platinum thin film 12 is 
selected to range from about 100 to 500.OMEGA.. 
In our experiment in which the insulating substrate 11 was 2.3 mm in 
diameter and 7 mm long, the resistance value of the platinum thin film 11 
was 100.OMEGA., the semiconductor oxide film 31 was formed of SnO.sub.2, 
the bridge circuit of FIG. 4 was used, the heater 32 with a 90.OMEGA. 
resistance value was energized by a current of 10 mA at a voltage of 12 V, 
the voltage of the power source 27 was 4 V, a sensor of the and following 
sensitivity to methane was thereby obtained: 
______________________________________ 
CH.sub.4 
10 50 100 200 500 1,000 
5,000 
10,000 PPM 
3 14 25 41 69 94 178 
230 mV 
______________________________________ 
Drift: lower than 1 mV within 24 hr. When the methane conentration was 
changed from 10,000 PPM to zero, the meter 28 returned to the zero point 
within three minutes. 
The gas sensing mechanism in this case is to detect a variation in the 
resistance value of the platinum thin film 12 which is caused by a 
temperature rise of the SnO.sub.2 film 31 due to the adsorption thereto of 
methane. Incidentally, the resistance value R.sub.1 of the platinum thin 
film 12 and the resistance value R.sub.2 of the SnO.sub.2 film 31 undergo 
such changes as follows: 
______________________________________ 
Temperature (.degree.C.) 
R.sub.1 (.OMEGA.) 
R.sub.2 (K.OMEGA.) 
______________________________________ 
20 150 95 
100 199 44 
200 260 4.9 
300 310 9.5 
400 355 30 
______________________________________ 
In this way, the resistance value of the SnO.sub.2 film 31 also varies with 
the temperature change. A prior art semiconductor gas sensor detects a gas 
through utilization of a variation in the resistance value of the 
SnO.sub.2 itself which is caused by the gas. The variation characteristic 
in this case is nonlinear. In the resistance element at the present 
invention, for use in the gas sensor shown in FIG. 6, the resistance value 
across the lead wires 16 and 17 becomes a parallel resistance value R of 
the resistance value R.sub.1 of the platinum thin film 12 and the 
resistance value R.sub.2 of the SnO.sub.2 film 31 as follows: 
EQU R=(R.sub.1 .times.R.sub.2)/(R.sub.1 +R.sub.2) 
However, by selecting the resistance value R.sub.2 to be larger than the 
resistance value R.sub.1, for example, by two orders of magnitude, as 
shown in the foregoing table, so that the resistance value R across the 
lead wires 16 and 17 may be substantially dependent on the platinum thin 
film, the resistance value R can be made linear and stable. For example, 
in the case of the methane detecting element mentioned previously, the 
unbalanced voltage characteristic of the bridge circuit with respect to 
the methane concentration becomes nonlinear when the resistance value 
R.sub.1 of the platinum thin film 12 used as a parameter increases up to 
about 250.OMEGA., as shown in FIG. 8. As the resistance value R.sub.1 
decreases, the unbalanced voltage characteristic becomes linear but the 
sensitivity drops. Thus, by selecting the resistance value R.sub.2 to be 
sufficiently larger than the resistance value R.sub.1, the sensitivity 
exhibits linearity and this sensitivity rises with an increase in the 
voltage E.sub.1 of the power source 27 of the bridge circuit, as depicted 
in FIG. 9. 
The gas sensor employing the platinum thin film resistance element shown in 
FIGS. 6 and 7 has the following features: 
(1) The zero point is very stable in the absence of a gas. 
(2) The sensor becomes stabilized in a short time after connecting thereto 
the power source. (The initial stabilization characteristic is excellent.) 
(3) It is possible to accurately detect methane from a low concentration of 
10 PPM or so to a high concentration of 10% or more. 
(4) Since a relatively large platinum thin film resistance can be used, a 
high voltage can also be applied to the sensor when it is incorporated in 
the bridge circuit and the sensor can be used with its output freely 
adjusted by selecting the bridge voltage E.sub.1. 
(5) Since a stable layer of the platinum thin film 12 underlies the 
semiconductor oxide thin or thick film 31, the sensor suffers no 
temperature loss and is capable of detecting temperature variations with 
high sensitivity and hence it is very excellent in response speed and in 
sensitivity to gas. 
(6) Since the resistance of the element is designed to depend on the 
variation in the resistance of the platinum, the reproducibility of the 
gas sensitivity is also excellent. 
(7) Since the gas sensitivity characteristic is also almost linear, the 
sensor is easy to use. 
For the detection of a gas, in particular, carbon monoxide, the metal oxide 
semiconductor film 31 is formed of copper monoxide CuO in FIG. 6. From the 
viewpoints of stability, gas sensitivity and response speed, it is 
preferred that the CuO film 31 is formed about 1 to 0.5 .mu.m thick by the 
high-frequency sputtering or painting. When the resistance elements of 
various values were heated up to about 200.degree. C. by applying a 
current to the heater 32 and the bridge voltage E.sub.1 was set to 6 V and 
12 V, the bridge unbalanced voltages with respect to 50 PPM of carbon 
monoxide were as follows: 
______________________________________ 
Unbalanced 
Unbalanced 
Resistance value of 
voltage voltage 
platinum thin film 
(E.sub.1 = 6 V) 
(E.sub.2 = 12 V) 
______________________________________ 
452 .OMEGA. 13 mV 25 mV 
523 .OMEGA. 21 mV 42 mV 
871 .OMEGA. 30 mV 60 mV 
1700 .OMEGA. 63 mV 120 mV 
______________________________________ 
The sensor provides large outputs in response to carbon monoxide of low 
concentration and is sensitive only to carbon monoxide. That is, with this 
resistance element, the sensitivities to carbon monoxide, methyl alcohol 
and hydrogen are respectively such as indicated by curves 41, 42 and 43 in 
FIG. 10 and, by heating the resistance element below 240.degree. C., it is 
possible to detect carbon monoxide alone. With a conventional sensor of 
the type using a lead oxide for the semiconductor film 31, the 
sensitivities to carbon monoxide, alcohol and hydrogen are respectively 
such as indicated by curves 44, 45 and 46 in FIG. 11 and, in this case, 
even if the element temperature is suitably selected, it is impossible to 
detect carbon monoxide and hydrogen independently of each other. Further, 
in the case of a conventional sensor of the type using platinum black for 
the semiconductor film 31, the sensitivities to carbon monoxide, alcohol 
and hydrogen are respectively such as indicated by curves 47, 48 and 49 in 
FIG. 12 and these gases cannot be detected separately. The sensitivities 
shown in FIGS. 10, 11 and 12 were measured in the case of the 
concentration of each gas being 500 PPM. 
By a suitable selection of the metal oxide for the semiconductor film 31, 
the resistance element of FIG. 6 can be employed for detecting a nitrogen 
oxide gas. In this case, 10 to 30 wt% of a rare earth oxide (for example, 
samarium trioxide Sm.sub.2 O.sub.3) and 0.5 to 5 wt% of silver nitrate 
AgNO.sub.3 are added to vanadium pentoxide V.sub.2 O.sub.5 and the mixture 
is sufficiently kneaded with pure water into a paste. The paste, after 
being dried, is pulverized and baked in a crucible at 500.degree. to 
550.degree. C. for more than two hours, thus obtaining a semiconductor 
powder. The semiconductor powder thus obtained is deposited by 
high-frequency sputtering, or coated by the aforementioned painting 
method, on the platinum thin film 12. In our experiment, such 
semiconductor films were sufficiently heat-aged at 400.degree. to 
500.degree. C. and further subjected to electrical aging for four to seven 
days. When the resistance elements were heated up to 300.degree. to 
320.degree. C. and the bridge voltage E.sub.1 was 6 V, the sensitivities 
to an NO gas were as follows: 
______________________________________ 
Resistance 
of platinum 
Thickness of Unbalanced 
thin film semiconductor voltage (mV) 
(.OMEGA.) film 31 20 PPM 40 PPM 
______________________________________ 
150 thick 8 15 
200 thick 15 29 
300 thick 18 34 
150 thin 24 45 
220 thin 35 68 
350 thin 42 80 
______________________________________ 
In the case where the rare earth oxide is 0%, the resistance element is 
insensitive to NO but sensitive to NO.sub.2 alone. As nitrogen oxide gas 
sensors, there have been known those of the types using V.sub.2 O.sub.5 
-Ag and phthalocyanine-copper systems; though capable of detecting the 
NO.sub.2 gas, they are not stable and their sensitivity to NO is not 
sufficient. In contrast thereto, the platinum thin film resistance element 
permits the detection of low-concentration NO gas, too. 
Further, for the detection of ammonia, the semiconductor film 31 of the 
resistance element of FIG. 6 was formed using a mixture of 3 to 10 wt% of 
Sm.sub.2 O.sub.3, 1 to 5 wt% of Sb.sub.2 O.sub.3 and 0.5 to 5 wt% of 
AgNO.sub.3 with respect to V.sub.2 O.sub.5. As shown in the following 
table, this element is excellent in that it is several times higher in 
sensitivity to ammonia than conventional ammonia detecting elements and is 
almost insensitive to perfume and ethyl alcohol. 
______________________________________ 
NH.sub.3 40 PPM 
Perfume C.sub.2 H.sub.5 OH 100 PPM 
______________________________________ 
SnO.sub.2 system semi- 
10 mV 36 mV 42 mV 
conductor 
(for ammonia) 
SnO.sub.2 --Pd semi- 
3 5 12 
conductor 
(for methane) 
V.sub.2 O.sub.5 --Ag semi- 
18 15 26 
conductor 
(for NO.sub.2) 
ZnO system semi- 
4 12 18 
conductor 
Element of this 
35 0 2 
invention 
______________________________________ 
When Sb.sub.2 O.sub.3 is out of the range from 1 to 5%, the sensitivity to 
ammonia abruptly lowers. 
As described above, the resistance element having the metal oxide 
semiconductor film 31 formed on the platinum thin film 12 can be employed 
for the detection of a specified gas according to the material used for 
the formation of the semiconductor film 31. Variations in the 
characteristics of such an element can markedly be reduced, for instance, 
by forming a protective layer 51 of alumina cement or beryllia cement on 
the platinum thin film 12 and, further, forming the semiconductor film 31 
on the protective layer 51, as shown in FIG. 13. For example, in the case 
of the resistance element having the semiconductor film 31 of the 
SnO.sub.2 system formed directly on the platinum thin film 12, the 
resistance value increased about 10 to 15% when the element was held at 
400.degree. C. for seven days, whereas, in the case of the resistance 
element having the protective layer 51, no resistance variations were 
observed when the element was held at 400.degree. C. for 20 days. This is 
considered due to the fact that the protective layer 51 prevents diffusion 
of the platinum from the thin film 12 into the semiconductor film 31 (or 
vice versa). Moreover, by the provision of such a protective layer 51, it 
is possible to specify the gas to which the resistance element is 
sensitive. The gas sensitivity of various elements is as follows: 
______________________________________ 
CH.sub.4 
iC.sub.4 H.sub.10 
H.sub.2 C.sub.2 H.sub.5 OH 
CO 
0.1% 0.1% 0.1% 0.1% 0.02% 
______________________________________ 
Pt--SnO.sub.2 
15.about.30 
40.about. 
80.about.150 
40.about.80 
5.about.15 
140 mV 
Pt--alumina 
10.about.20 
40.about.80 
40.about.70 
0.about.3 
0.about.1 
cement-SnO.sub.2 
Pt--beryllia 
8.about.10 
11.about.16 
20.about.25 
0.about.1 
0.about.1 
cement-SnO.sub.2 
Pt--SiO.sub.2 --SnO.sub.2 
15.about.30 
40.about. 
80.about.150 
40.about.80 
5.about.15 
140 
______________________________________ 
As will be understood from the above table, by combining the SnO.sub.2 film 
31 with the beryllia cement layer 51 and the alumina cement layer 51, 
respectively, there can be obtained resistance elements which are almost 
insensitive to alcohol and smoke but sensitive mainly to inflammable 
gases, that is, natural gas, coke gas, propane gas and so forth. 
In the gas sensors of the type utilizing the resistance variation, a 
temperature compensating element is usually employed for avoiding the 
influence of ambient temperature. To this end, in the case of sensing a 
gas by the element 35 having the metal oxide semiconductor film 31, use is 
made of a bridge circuit such as shown in FIG. 14 which employs, in 
addition to the element 35, a temperature compensating element 52 which is 
identical in characteristics with the element 35 except that it is 
insensitive to the gas. For increasing the sensitivity, a current is 
applied to the heater 32 of the element 35 to heat it, for example, up to 
150.degree. to 450.degree. C. for burning the gas; in this case, a current 
is also applied to the heater of the temperature compensating element 52 
to heat it up to the same temperature as the element 35. In such a case, 
power consumption is increased by the heaters 32 of the two elements 35 
and 52. But in the case where a platinum thin film resistance element 53 
with no heater is used as the temperature compensating element and a 
platinum thin film having a resistance value, for example, 150.OMEGA. at 
20.degree. C. is used as the element 35 at 350.degree. C. to provide a 
resistance value of 300.OMEGA. as shown in FIG. 15, the resistance value 
of the platinum thin film resistance element 53 is selected to be equal, 
at room temperature, to the resistance value of the element 35 at the 
working temperature, i.e. 300.OMEGA. in this example. According to this 
arrangement, temperature is sufficiently compensated by the platinum thin 
film resistance element 53 for compensation use; furthermore, since no 
heater is needed for the temperature compensation, power consumption is 
small. 
In the foregoing, the insulating substrate 11 need not always be 
cylindrical but may be plate-shaped, too. 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel concepts of this invention.