JPH02275601A - Manufacture of thin-film thermistor - Google Patents

Abstract

PURPOSE:To reduce condensation of Au-Pt electrode film and to improve thermal stability at the contact part between an SiC thin film and the Au-Pt electrode film by using the Au-Pt electrode film where the mixture of a Ca oxide and a Ti oxide is added slightly. CONSTITUTION:A pair of Au-Pt calcination electrode film 22 of a specified shape and an SiC sputter resistance thin film 23 are formed on an insulation substrate 21 and a small amount of a mixed oxide of a Ca oxide and a Ti oxide is added into this Au-Pt calcination electrode film 22 slightly. When the Au-Pt calcination electrode film 22 including a small amount of oxide is used, it becomes difficult for condensation even at a high temperature of 500 deg.C to proceed. Thus, it becomes possible to obtain a practical SiC thin film thermistor 1 which can operate for a long time even at a high temperature of 500 deg.C.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a thin film thermistor with high heat resistance.
This thin film thermistor is used as a temperature sensor for electric ovens, gas ovens, etc. with a thermal self-cleaning function.

Conventional technology SiC''FllI'A thermistors are described, for example, in the National Technical Report by Nagai et al.
technical report) Vol, 29. (
1983) P, 145, the structure was as shown in Figure 1. That is, a practical 5iCl film thermistor I is composed of a thermistor element 2, a lead wire 3, and a glass coating Fi4. The thermistor element 2 includes an insulating substrate 21 on which a comb-shaped fired electrode W122 is formed in advance.
A sputtered 5iCi film 23 is formed on one surface of the 5iCi film 23. Alumina substrate 2 as a representative insulating substrate 21
1 is used. The alumina substrate 21 usually has a surface roughness of 2 to 3 μm and a purity of about 95%. As a typical fired electrode film 22, an Au-Pt fired electrode film 22 is used. The heat resistance of the thermistor element 2 is determined by this fired electrode 11.
! 22, so it will be described in detail later. Si
The C thin film 23 is formed using a normal parallel plate type high frequency sputtering device. After the thermistor element 2 is formed, a lead wire 3 is connected, and a glass coating layer 4 is formed to protect the element from humidity, dust, etc.
The C thin film thermistor 1 is completed.

Problems to be Solved by the Invention However, although such conventional practical 5iCi film thermistors have the characteristic of being suitable for detecting a wide temperature range, their maximum operating temperature is approximately 400°C.
The problem was that it could not be used as an open temperature sensor with a thermal self-cleaning function. In other words, the above-mentioned open uses a temperature sensor that can be used over a wide temperature range from the normal cooking temperature range (40 to 300 degrees Celsius) to the self-cleaning temperature range (450 to 500 degrees Celsius) that burns off food stains on the internal walls of the refrigerator. I need. Therefore, there has been a need for a temperature sensor that has a wide detection temperature range and can withstand high temperatures of 500°C.

Therefore, the first object of the present invention is to provide a practical SiC thin film thermistor that can operate for a long time even at a high temperature of 500°C.

A second object of the present invention is to provide a method for stabilizing the thermistor characteristics of the above-mentioned high heat-resistant SiC thin film thermistor.

When a thermistor element is tested at high temperatures, the resistance value increases, while the B constant decreases. The reason for this is that during the test at high temperature, the fired electrode film aggregates and a high interfacial impedance layer is formed between the SiC thin film and the fired electrode film. Since the above-mentioned aggregation tends to progress in the conventional electrode structure, a high interfacial impedance layer easily grows. For this reason, conventional thermistors cannot be used at temperatures above about 400° C. even if they are coated with protective glass. Since the thermistor of the present invention uses a fired Au--Pt electroplated film containing a small amount of oxide, agglomeration is difficult to proceed even at the above-mentioned high temperature of 500°C, and therefore a heat resistance of 500'C was obtained.

EXAMPLE Hereinafter, an example of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a perspective view showing the structure of a 5iCR11Q thermistor of the present invention, and this structure itself is the same as a conventional SiC thin film thermistor.

Below, the thermistor element 2 of the Au-Pt fired electrode film 22
The heat resistance is not as good as that of the previous one, but the effect will be explained in detail. The conventional Au-Pt fired electrode film 22 was formed as follows. Au-PL paste was printed on the alumina substrate 21 in a predetermined pattern. After drying the printed alumina substrate 21, 9
It was fired at a temperature of 00 to 1000°C. The conventional A'u-Pt fired electrode film 22 after firing thus formed is composed of Au, PL, and glass (SiOi). Glass is necessary to firmly adhere Au and PL to the alumina substrate 21, and has a weight of approximately 10w relative to the total weight of Au and PL.
t% is added. It has been found that the heat resistance of the thermistor element 2 can be improved by adding a small amount of oxide to such a conventional Au-Pt fired electrode film 22. In the following description, the thermistor element 2A of the present invention is defined as the thermistor element 2 using the Au-Pt fired electrode film 22A to which a trace amount of oxide is added. The conventional thermistor element 2B is defined as a thermistor element 2B using a conventional Au-pt fired electrode film 22B.

FIG. 2 is a diagram showing the rate of change in B constant (ΔB/B) with respect to the elapse of annealing time when the thermistor elements 2A and 2B are annealed at 825° C. in air. In the thermistor element 2A of the present invention, a mixture of (Ca oxide + Ti oxide) was added as an oxide in an amount of about 0.1% based on the total weight of Au and PL. In order to avoid variations in heat resistance between sputter lofts, both thermistor elements 2A and the SiC thin film 28 were made with the same sputter loft. The B constant before annealing is 2400-245 for both thermistor elements 2A and 2B.
It was 0. Note that the B constant is expressed by the formula 1n(R+/Rt)/
(1/T+-i/lt), R1
is T, DC resistance value at (50℃・323K), Rt is T
This is the DC resistance value at t (160"c/433K). From the figure, it can be seen that the thermistor element 2A of the present invention is more stable than the conventional thermistor element 2B.

Figure 3 shows the XM of Au-Pt fired electrode films 22A and 228.
An example of composition analysis using A (X-ray Micro-Analyser) is shown. The Au-Pt fired electrode film 22A of the present invention contains CaT in addition to AuPL and Si (main components of glass) contained in the conventional Au-Pt fired electrode film 22B.
It contained an i. Note that it is unclear from FIG. 3 whether Ca and Ti are in an oxide state. However, as mentioned above, the Au-Pt fired electrode film 22 of the present invention is also
Since the -Pt fired electrode film 22B is also formed by firing in high temperature air, it is clear that Ca and Ti are in an oxide state.

In order to clarify the reason why the Au-Pt fired electrode film 22A of the present invention improves the heat resistance of the thermistor element 2A,
The surface structure of the u-Pt fired electrode films 22A and 22B is 825
Analysis was performed before and after annealing in air for 6 hours at °C. Figure 4 shows the S of the refired electrode films 22A and 22B before and after the above annealing.
An EM image is shown. It can be seen that the Au--Pt fired electrode film 22A of the present invention has clearly less agglomeration than the conventional Au--Pt fired electrode film 22B.

Moreover, FIG. 5 shows Cole-Cole plots of thermistor elements 2A and 2B before and after annealing in air at 825° C. for 3 hours. Before annealing, thermistor elements 2A and 2B
Both had almost the same resistance value and B constant. After annealing, the thermistor element 2A of the present invention showed an increase in resistance of about 70% and a decrease in B constant of about -1%. However, after annealing, the conventional thermistor element 2B showed an approximately 5-fold increase in resistance value and a decrease in B constant of approximately -1e% or more. Co1e-Cot
As shown in FIG. 5, the e-plot is defined as the relationship between resistance and actance of complex impedance. The complex impedance of thermistor elements 2A and 2B is 2-1
Measurements were made at room temperature in the frequency range of 000 kHz. Co1e-Cole of thermistor elements 2A and 2B before annealing
The plots were nearly identical to each other and showed almost perfect semicircular arcs. After annealing, Co1 of the thermistor element 2A of the present invention
The e-Cole plot showed a nearly complete semicircular arc, although the radius was larger than the radius of the semicircular arc before annealing.

However, after annealing, the Co of the conventional thermistor element 2B
The 1e-Cole plot was not a semicircular arc. Approximately 50k
In the high frequency region above tlz, its C.

1e-Cole plot shows the thermistor element 2 of the present invention.
It was a semicircular arc almost similar to that of A. On the other hand, 50k
In the low frequency region below Hz, the reactance decreased slowly with increasing resistance and increased again below 10 kHz. Such behavior indicates that the conventional thermistor element 2B after annealing is equivalently represented by the circuit shown in FIG. This equivalent circuit is composed of two series-connected composite circuits in which a resistor and a capacitor are connected in parallel. If this equivalent circuit is one composite circuit consisting of r and C, the Cole-Cole plot shows a complete semicircular arc and a maximum reactance r/2 when ωcr=1.

Here, ω=2πf, [ is the frequency, C is the capacitance value, and r is the resistance value. C measured before annealing.

In the Ie-Cole plot, r is the resistance value of the SiC3 film, and C is the capacitance value between the comb-shaped Au-Pt fired electrode film 22A or 22B formed on the alumina-based Fi, 21. Co1e-Col of one composite circuit
It matched well with the e-plot.

When the equivalent circuit is constructed by connecting two composite circuits in series, the Cole-Cole plot shows a composite curve of two semicircular arcs corresponding to each composite circuit. One of the two composite circuits consists of one parallel connection of r and C. Another composite circuit is defined by one parallel connection of r′ and Co. cr(
c'r'', the Cole-Cole plot depends on C and r in the high frequency region and on Co and r'' in the low frequency region. Co1e of conventional thermistor element 2B
The increase in reactance below 10 kHz in the -Co1e plot is due to a composite circuit consisting of Co and r', and this composite circuit of r' and Co is thought to have been formed during annealing. On the other hand, in the conventional thermistor element 2B after annealing, the conventional Au-Pt fired electrode film 22B
When the contact area between the electrode and the SiC thin film 23 was lightly rubbed with an insulating material such as alumina, the resistance value was reduced by several tens of percent. This decrease in resistance shows that the composite circuit of Co and r゛ is different from the conventional Au-Pt
This shows that it corresponds to the interface impedance between the fired electrode film 22B and the SiC thin film 23.

For these reasons, conventional Au-Pt fired electric digging 11!2
It can be said that 2B easily aggregates during annealing, and as a result, an interfacial impedance is formed that causes an increase in resistance value and a decrease in B constant. On the other hand, the Au-Pt fired electrode film 2 of the present invention
In 2A, aggregation during annealing is slightly reduced due to the addition of oxide, so no interfacial impedance is formed.

This improved the heat resistance of the thermistor element 22A of the present invention.

Next, in order to confirm the practical operating temperature, a life test was conducted in air at temperatures of 400'C, 500C, and 1600C. Practical 3iCI film thermistor IA using the SiC thin film thermistor element 2A of the present invention and practical 5icl film thermistor IB using the conventional SiC thin film thermistor element 2B
were used respectively. For the practical thermistors tested, p
The T wire is welded to the thermistor element 2, and the temperature is about 660'C.
A glass coating layer 4 was formed having a transition point temperature of . Conventional practical thermistor IB has resistance value change rate (Δr/r) <±5%, B constant change rate (ΔB/B) <±2%, and 100~2% at 500'C after 1000 hours test at 400'C.
After 00 hours test, Δr/r>10%, ΔB/B<5%,
After further testing at 500'C for 1000 hours, Δr/r>5
0%, ΔB/B<10%. However, after testing the practical SiC thin film thermistor IA of the present invention at 500'C for 1000 hours, (Δr/r)<+5%, B constant change rate (ΔB
/B)<±2%, and similar results were obtained after testing at 600°C for 100 hours. From these results, it was confirmed that the practical thermistor IA of the present invention can operate at 500°C.

Next, the Au-Pt fired electrode film 22 of the present invention was added with a mixture of (Ca oxide + Ti oxide) at various weight concentrations.
A test was carried out on the practical thermistor IA of the present invention using A at 500'C in air. The results are shown in Table 1. The desirable addition amount of the mixture of (Ca oxide + TI oxide) is 0.0
It was in the range of 1-0.1%.

Table 1 Au of the present invention formed at various weight ratios A u / P t under the condition of adding a constant amount of 0.1 wt % of the mixture of oxide addition effects
A practical thermistor IA using -pt fired electrode M22A was also tested in air at 500°C. The result is the second
Shown in the table.

Table 2 Effect of weight ratio (Au:Pt) Also, as oxides (Ca oxide, Ti oxide) Au and P
The practical thermistor IA using the fired electrode film 22A made of a single metal of L had poor heat resistance. However, Au-P
Baked electrode II consisting of a metal mixture of t! IA using 22A had excellent heat resistance. The detailed reason why the practical thermistor IA of the present invention using the fired electrode film 22A of the metal mixture has excellent heat resistance is not clear. However, its excellent heat resistance is thought to be deeply related to the stable existence of two phases (at, az) in the binary alloy of Au and pt. These two phases (at, ax) are Con5
posture binary alloy, Kano,
Max Hansen, McGrawill Bo
OK Company, pp226-229.195
8, it has been reported to exist in a temperature range of about 600°C or higher. The two phases prevent each phase from thermally coalescing separately. Since the Au-Pt fired electrode film 22 of the present invention contains an oxide added thereto, this thermal aggregation is even more difficult. On the other hand, since an elemental metal has only a single phase, thermal aggregation is extremely easy, and even if an oxide is added, thermal aggregation of an elemental metal cannot be effectively reduced. From these considerations, the preferred A u /P ratio is within a range where two phases exist.

In the process of manufacturing the practical thermistor IA of the present invention, the thermistor element 2A of the present invention is handled with stainless steel tweezers. For example, when the Pt thin wire 3 is welded, the surface of the thermistor element 2 is occasionally scratched with tweezers. The practical thermistor IA of the present invention, which uses the thermistor element 2A of the present invention that has been scratched with metal tweezers during manufacturing, has a temperature of 100°C in air at 500°C.
After 300 hours test, ΔB/B<±2%, but Δr
/r=-5% to -20%. Such a decrease in resistance value is considered to be caused by thermal diffusion of metal atoms attached to the surface of the SiC thin film 23 when scratched with tweezers.

However, this decrease in resistance value saturated at a certain value, so
It has been found that post-manufacturing boss annealing can stabilize the material. A damaged practical thermistor IA has a temperature of 500℃.
After testing for about 230 hours in air, Δr/r x ~-1
It showed 0%. However, that same practical thermistor IA then showed Δ
r/r~-8%.

In addition, several other damaged practical thermistors IA are 60
After testing in 0℃ air for 3-10 hours, Δr/'==5% ~
-10% was shown. However, that same number of practical thermistors IA had Δr/r <±2% after being subsequently tested in air at 500° C. for about 800 hours.

These results indicate that boss annealing can stabilize the decrease in resistance of a damaged practical thermistor IA.

It is actually difficult to completely remove the thermistor element 2A that has been damaged during the manufacturing process. This indicates that all fabricated practical thermistors IA are preferably bottom annealed. Bost annealing may be performed in air, vacuum, or inert gas, but in-air heat treatment is preferable in view of ease of operation and the need for no special equipment. Further, the boss annealing conditions are (500 to 600)'C and (3 to 300
) time is preferred.

Effects of the Invention As described above, the thin film thermistor of the present invention provides the following effects.

(1) Since we use an Au-Pt electrode film to which a small amount of a mixture of (Ca oxide and Ti oxide) is added, the Au-Pt
The agglomeration of the t-electrode film can be reduced, and for this reason, the SiC thin film and A
Thermal stability of the contact area of u-P t '@ scapula was improved.

(2) As a result, the heat resistance of the SiC thin film thermistor can be improved from the conventional 400°C to 500°C.

(3) By introducing Bost annealing, the resistance value can be stabilized even if the element surface is scratched with a pin during the manufacture of SiC thin tli thermistors.

[Brief explanation of drawings]

Fig. 1 is a perspective view of a thin film thermistor showing one embodiment of the present invention, Fig. 2 is a characteristic diagram showing the B constant stability of the same Au-Pt electrode film, and Fig. 3 A and B are the same Aupt electrocuring film. Fig. 4 is a SEM image showing the surface structure of the Au-Pt electrode film, Fig. 5 is a Co1e-Cole plot showing the effect of the Au-Pt electrode film, and Fig. 6 is an analysis example showing the composition of the same. is an equivalent circuit diagram of a thermistor element. 1...Practical SIC thin film thermistor, 2...
...Thermistor element, 21...Insulating substrate, 2
2...Au-Pt electrode film, 23...S
iC thin film, 3... lead wire, 4... glass coating layer. Agent's name Patent attorney Shigetaka Awano Haka 1st person - Anil Tsuba (h) IIa Diagram special 粁X sacrilegious energy (ev) Characteristic

Claims (5)

[Claims] (1) An insulating substrate, a pair of Au-Pt fired electrode films of a predetermined shape formed on the insulating substrate, and an Au-Pt fired electrode film formed on the insulating substrate and the pair of Au-Pt fired electrode films. The Au-Pt
A thin film thermistor characterized in that a small amount of a mixed oxide of Ca oxide and Ti oxide is added to the fired electrode film. (2) The thin film thermistor according to claim (1), wherein the mixed oxide is added in an amount of 0.01 to 0.1 wt% based on the total weight of Au and Pt. (3) A step of preparing an insulating substrate, and forming an Au-Pt fired electrode film to which a small amount of a mixed oxide of Ca oxide and Ti oxide is added onto the insulating substrate into a pair of predetermined shapes. a step of firing the insulating substrate and the pair of Au-P;
A step of forming an SiC resistance film on the T-fired electrode film by sputtering, a step of connecting a lead wire to the Au-Pt fired electrode film, and a step of forming the Au-Pt fired electrode film and the SiC sputtered film. A method for manufacturing a thin film thermistor, comprising the steps of: coating the surface of the insulating substrate with a low melting point glass coating layer; and heat-treating the surface of the insulating substrate. (4) The method for manufacturing a thin film thermistor according to claim (3), wherein the heat treatment step is an in-air heat treatment. (5) The heat treatment step is performed at a temperature of 500 to 600°C.
Claims characterized in that it is made for 300 hours (
4) The method for manufacturing a thin film thermistor described in section 4).