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Abstract:

An NTC thermistor having a metal base material, a thermistor film layer
formed on the metal base material, and a pair of split electrodes formed
on the thermistor film layer. A ceramic slurry is applied onto a carrier
film to form the thermistor film layer, a metal powder containing paste
is applied onto the thermistor film layer to form the metal base
material, and further an electrode paste is applied onto the metal base
material to form the split electrodes. Thereafter, the three substances
are integrally fired.

Claims:

1. A thermistor, comprising: a metal base material; a thermistor film
layer adjacent the metal base material; and a pair of split electrodes
adjacent the thermistor film layer.

2. The thermistor according to claim 1, wherein a thickness of the metal
base material is 10 to 80 μm and a thickness of the thermistor film
layer is 1 to 10 μm.

3. The thermistor according to claim 1, wherein when a distance between
the split electrodes is defined as Lp and a thickness of the thermistor
film layer is defined as Tt, Lp≧Tt+5 μm.

4. The thermistor according to claim 1, wherein a distance from an end
portion of the split electrodes to an end portion of the thermistor film
layer is 5 μm or more.

5. The thermistor according to claim 1, wherein a coefficient of linear
expansion ratio of the metal base material and the thermistor film layer
is 0.75 to 2.17.

6. The thermistor according to claim 1, wherein the metal base material
is in a sheet shape and formed from a metal powder paste and the
thermistor thin film layer is in a sheet shape and formed from a ceramic
slurry.

7. The thermistor according to claim 6, wherein the sheet-shaped metal
base material and the sheet-shaped thermistor thin film layer are fired
in an integrally laminated state.

8. The thermistor according to claim 1, further comprising a protection
layer containing an insulation material on a surface of the thermistor
film layer to which at least the split electrodes are adjacent.

9. The thermistor according to claim 8, wherein a difference in
resistivity between a thermistor material of the thermistor film layer
and an insulator material of the protection layer is 100 times or more.

10. The thermistor according to claim 1, wherein the thermistor film
layer is divided into first and second thermistor film layers
corresponding to each of the split electrodes and a width of the first
and second thermistor film layers is greater than that of the
corresponding split electrodes.

11. The thermistor according to claim 10, further comprising a protection
layer containing an insulation material located between the first and
second thermistor film layers.

12. The thermistor according to claim 11, wherein a peripheral portion of
the protection layer extends between a peripheral portion of the first
and second thermistor film layers and a peripheral portion of the
corresponding split electrodes.

13. The thermistor according to claim 1, wherein the thermistor film
layer is divided into first and second thermistor film layers
corresponding to each of the split electrodes and a width of the first
and second thermistor film layers is less than that of the corresponding
split electrodes.

14. The thermistor according to claim 13, further comprising a protection
layer containing an insulation material located between the first and
second thermistor film layers.

15. The thermistor according to claim 8, wherein a peripheral portion of
the protection layer extends between a peripheral portion of the split
electrodes and the thermistor film layer.

16. A method for manufacturing a thermistor having a metal base material,
a thermistor film layer adjacent the metal base material, and a pair of
split electrodes adjacent the thermistor film layer, the method
comprising: applying a ceramic slurry onto a carrier film with a
predetermined thickness to form a ceramic green sheet serving as the
thermistor film layer; applying a metal powder containing paste onto the
ceramic green sheet with a predetermined thickness to form a metal base
material sheet serving as the metal base material; applying an electrode
paste onto a surface of the ceramic green sheet with a predetermined
thickness to form a split electrode pattern serving as the split
electrodes; and integrally firing the metal base material sheet, the
ceramic green sheet, and the split electrode pattern.

17. The method for manufacturing a thermistor according to claim 16,
wherein a thickness of the metal base material after firing is 10 to 80
μm, and a thickness of the thermistor film layer after firing is 1 to
10 μm.

18. The method for manufacturing a thermistor according to claim 16,
wherein when a distance between the split electrodes after firing is
defined as Lp and a thickness of the thermistor film layer after firing
is defined as Tt, Lp≧Tt+5 μm.

19. The method for manufacturing a thermistor according to claim 16,
wherein a distance from an end portion of the split electrodes after
firing to an end portion of the thermistor film layer after firing is 5
μm or more.

20. The method for manufacturing a thermistor according to claim 16,
further comprising forming a protection layer containing an insulation
material at least between the split electrodes.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of International
application No. PCT/JP2010/064089, filed Aug. 20, 2010, which claims
priority to Japanese Patent Application No. 2009-198024, filed Aug. 28,
2009, the entire contents of each of which are incorporated herein by
reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a thermistor and a method for
manufacturing the same, and particularly relates to a thermistor in which
a metal base material, a thermistor thin film layer, and an electrode are
laminated and a method for manufacturing the same.

BACKGROUND OF THE INVENTION

[0003] As an NTC thermistor or a PTC thermistor used as a temperature
sensor or the like in a protection circuit heretofore, one disclosed in
Patent Document 1 is known. The thermistor has a plate-like metal
substrate which also serves as an electrode, a themosensitive resistor
film formed on one surface of the plate-like metal substrate, and an
electrode film formed on the themosensitive resistor film.

[0004] However, since the thermistor has a structure in which the
plate-like metal substrate is used as one electrode and the electrode
film formed on the top layer is used as another electrode, there is no
choice but to use wire bonding for electrical connection to the electrode
film. Therefore, it has been impossible to mount the same on a minimal
space. For example, when used as a temperature sensor of IC component
mounted on a printed-circuit board, there is a 150 to 200 μm minute
space between the printed-circuit board and the IC component, and it is
preferable to mount the thermistor in the space. However, the mounting by
wire bonding does not allow mounting on such a minimal space.

[0005] Moreover, the themosensitive resistor film (thermistor thin film)
has been formed by a gas phase method, such as sputtering, in the
thermistor, which has caused problems of an increase in the cost and poor
productivity. Furthermore, the thermistor has had problems that, when
cracking or the like occurs in the metal substrate or the themosensitive
resistor film, the resistance has fluctuated to change the
characteristics as a temperature sensor.

[0007] It is an object of the present invention to provide a thermistor
which can be mounted by reflow and can be mounted on a minimal space and
a method for manufacturing the same. It is another object of the
invention to provide a thermistor in which a reduction in the height can
be achieved and the occurrence of cracking can be suppressed as much as
possible and which can be manufactured at a low cost and a method for
manufacturing the same.

[0008] A thermistor which is a first aspect of the invention has a metal
base material, a thermistor thin film layer formed on the metal base
material, and a pair of split electrodes formed on the thermistor thin
film layer.

[0009] In the thermistor, the pair of split electrodes can be soldered by
reflow to lands of a printed-circuit board and mounting by wire bonding
is not required. Therefore, the thermistor can be mounted even on a
minimal space of 200 μm or lower.

[0010] In particular, when the thickness of the metal base material is 10
to 80 μm and the thickness of the thermistor thin film layer is 1 to
10 rim, a reduction in the height can be achieved and also flexibility is
imparted due to the fact that the thin film thermistor and the metal base
material are integrated. Therefore, even when a stress is applied to a
thermistor, cracking is hard to occur in a thermistor thin film layer
portion. Even when there are irregularities, level differences, and the
like in the mounting space, the thermistor described above can be
mounted.

[0011] Moreover, even when an excessive stress is applied to the
thermistor to cause bending, so that cracking occurs in the central part
of the thermistor thin film layer, the electrical characteristics as the
thermistor are hard to be affected because the thermistor employs split
electrodes and the central portion of the thermistor thin film layer is
not an energizing path.

[0012] A method for manufacturing a thermistor which is a second aspect of
the invention is a method for manufacturing a thermistor having a metal
base material, a thermistor thin film layer formed on the metal base
material, and a pair of split electrodes formed on the thermistor thin
film layer, and the method includes a process of applying a ceramic
slurry onto a carrier film with a predetermined thickness to form a
ceramic green sheet serving as the thermistor thin film layer, a process
of applying a metal powder containing paste onto the ceramic green sheet
with a predetermined thickness to form a metal base material sheet
serving as the metal base material, a process of applying an electrode
paste onto a surface of the ceramic green sheet facing the surface, on
which the metal base material sheet is formed, with a predetermined
thickness to form a split electrode pattern serving as the split
electrodes, and a process of integrally firing the metal base material
sheet, the ceramic green sheet, and the split electrode pattern.

[0013] In the manufacturing method, since the thermistor thin film layer
is formed by a solid phase method, the thermistor thin film layer can be
manufactured at a lower cost than that of a case where the thermistor
thin film layer is manufactured by a gas phase method and also since the
metal base material, the thermistor thin film layer, and the split
electrodes are integrally fired, the occurrence of cracking in the metal
base material or the thermistor thin film layer can be suppressed as much
as possible.

[0014] According to the present invention, a thermistor can be obtained in
which a reduction in the height or mounting by reflow can be achieved and
which can be mounted on a minimal space. Moreover, since the thermistor
thin film layer is formed by a solid phase method, the thermistor thin
film layer can be manufactured at a low cost and the occurrence of
cracking can be suppressed as much as possible by integrally firing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIGS. 1(A) and 1(B) illustrate a thermistor which is a first
example, in which FIG. 1(A) is a plan view and FIG. 1(B) is a front view.

[0017] FIG. 3 is an explanatory view for illustrating the energization
state of the thermistor.

[0018] FIGS. 4(A) to 4(E) are explanatory views for illustrating a
manufacturing processes of the thermistor.

[0019] FIG. 5 is a cross sectional view illustrating a thermistor which is
a third example.

[0020] FIG. 6 is a cross sectional view illustrating a modification of the
thermistor which is the third example.

[0021] FIG. 7 is a cross sectional view illustrating a thermistor which is
a fourth example.

[0022] FIG. 8 is a cross sectional view illustrating a thermistor which is
a fifth example.

DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, Examples of a thermistor and a method for
manufacturing the same according to the invention are described with
reference to the attached drawings. In each drawing, common parts and
common portions are designated by the same reference numerals, and the
same explanation is omitted.

FIRST EXAMPLE

FIGS. 1 to 3

[0024] As illustrated in FIG. 1, an NTC thermistor 1A which is a first
example is constituted by a metal base material 11, a thermistor thin
film layer 15 formed on the metal base material 11, and a pair of split
electrodes 21 and 22 formed on the thermistor thin film layer 15. The
metal base material 11 is formed into a sheet shape from a metal powder
paste. The thermistor thin film layer 15 is formed into a sheet shape
from a ceramic slurry. The split electrodes 21 and 22 are obtained by
forming an electrode material paste into a predetermined shape. These
three substances are integrally fired. At least the metal base material
11 and the thermistor thin film layer 15 may be fired.

[0025] The thickness of the metal base material 11 is about 10 to 80
μm. The thickness of the thermistor thin film layer 15 is about 1 to
10 μm. The thickness of the split electrodes 21 and 22 is about 0.1 to
10 μm. The thickness as the entire thermistor 1A is about 10 to 100
μm. Herein, the entire length size of the thermistor 1A is defined as
L, the entire width size thereof is defined as W, the distance between
the split electrodes 21 and 22 is defined as Lp, and the short side
length, the dimension to the end surface of the thermistor 1A, the long
side length, and the dimension to the side surface of the thermistor 1A
of the split electrodes 21 and 22 are defined as L1, Lg, W1, and Wg,
respectively. The height dimension of the metal base material 11 is
defined as Tb and the thickness of the thermistor thin film layer 15 is
defined as Tt.

[0026] As the thermistor thin film layer 15, various ceramic materials
containing Mn, Ni, Fe, Ti, Co, Al, Zn, and the like in an arbitrary
combination and in an appropriate amount can be used. In practice, oxides
of the transition metal elements mentioned above are mixed for use.
However, carbonates, hydroxides, and the like of the elements may be used
as a starting material. As the metal base material 11 and the split
electrodes 21 and 22, simple substances of precious metals, such as Ag,
Pd, Pt, and Au, base metals, such as Cu, Ni, Al, W, and Ti or alloys
containing the same can be used.

[0027] As a method for forming the metal base material 11 or the
thermistor thin film layer 15 into a sheet shape, a doctor blade method
is common. However, screen printing, gravure printing, and an ink jet
method may be used. The formation of the split electrodes 21 and 22 can
be performed by printing methods, such as screen printing, a sputtering
method, or a vapor deposition method. Materials and manufacturing
processes are described in detail later.

[0028] Here, an equivalent circuit of the thermistor 1A is described with
reference to FIG. 2. The split electrodes 21 and 22 serve as input/output
terminals, and resistances R1 and R2 are formed by the thermistor thin
film layer 15 and are electrically connected in series through the metal
base material 11. More specifically, the split electrodes 21 and 22
constitute a thermistor circuit through the resistances R1 and R2 formed
by the thermistor thin film layer 15 in a direct contact state.

[0029] Since the split electrodes 21 and 22 are formed on the surface of
the thermistor thin film layer 15, an energization state forms a path
passing the thermistor thin film layer 15 of portions contacting the
split electrodes 21 and 22 and the metal base material 11 as indicated by
the arrow in FIG. 3. In bending or mounting with a mounter of the
thermistor 1A, cracking is likely to occur in the central portion of the
thermistor thin film layer 15. However, even when cracking occurs in the
central portion of the thermistor thin film layer 15, the portion where
cracking occurs is not an energization path. Therefore, the electrical
characteristics as the thermistor 1A are not affected.

[0030] The NTC thermistor 1A having the above-described configuration is
used for a temperature sensor of IC component mounted on a
printed-circuit board, for example. In this case, the thermistor 1A is
mounted by soldering the split electrodes 21 and 22 by reflow onto the
lands of the printed-circuit board. Since the height of the thermistor 1A
which is the first example is reduced to about 10 to 100 μm, the
thermistor 1A can be mounted on an about 150 to 200 μm space formed
between the printed-circuit board and the IC component. Due to the fact
that the thermistor 1A is mounted on the space, the thermistor 1A can
immediately respond to an increase in heat of the IC component as a
temperature sensor.

[0031] Moreover, due to the fact that the thin film thermistor and the
metal base material are integrated, flexibility is imparted. Therefore,
even when a stress is applied to the thermistor, cracking is hard to
occur in the thermistor thin film layer portion. Even when there are
irregularities, level differences, and the like in a mounting space, the
thermistor 1A can be mounted.

[0032] (Manufacturing Process, FIGS. 4(A) to 4(E))

[0033] Next, a manufacturing process of the thermistor 1A is described.
First, as a raw material of the thermistor thin film layer 15, an
Mn--Ni--Fe--Ti oxide was weighed in such a manner as to have a
predetermined proportion (in such a manner that the resistivity is 104
Ωcm), sufficiently wet-grounded in a ball mill using a grinding
medium, such as zirconia, and thereafter fired at a predetermined
temperature, thereby obtaining ceramic powder.

[0034] An organic binder was added to the ceramic powder, followed by wet
mixing treatment to form a slurry. Then, the obtained slurry was formed
into a ceramic green sheet, in which the thickness after firing was 1 to
15 μm, by a doctor blade method. A metal base material paste
containing Ag--Pd as the main component was formed into a metal base
material sheet, in which the thickness after firing was 5 to 100 μm,
on the obtained ceramic green sheet by a doctor blade method.
Furthermore, for comparative examination, a 0.5 μm thick thermistor
thin film layer was formed by a sputtering method on a 30 μm thick
metal base material sheet to be used as a mother sheet for comparative
examination. Thereafter, on the ceramic green sheet, an Ag--Pd paste was
screen-printed to thereby form split electrodes.

[0035] Next, each mother sheet on which the split electrodes were formed
was cut into a 1 unit thermistor, accommodated in a zirconia sagger,
subjected to debinding treatment, and then fired at a predetermined
temperature (e.g., 900 to 1300° C.). Thus, the thermistor 1A of a
laminated type having the metal base material 11, the thermistor thin
film layer 15, and the split electrodes 21 and 22 was obtained.

[0036] As specific processes, the ceramic slurry was applied onto a PET
carrier film 31 to thereby form the ceramic green sheet 15 serving as a
thermistor thin film layer by a doctor blade method, and further the
metal base material paste was applied thereon to thereby form the metal
base material sheet 11 serving as a metal base material by a doctor blade
method as illustrated in FIG. 4(A). The film 31 and the sheets 15 and 11
are cut into a dimension for obtaining a multi-piece mother sheet (FIG.
4(B)), and the sheets 15 and 11 are separated from the film 31 (FIG.
4(C)). Thereafter, an Ag-Pd paste is screen-printed onto the sheet 15 to
thereby form the split electrodes 21 and 22 (FIG. 4(D)), and then the
sheet 15 is cut into a predetermined chip size (FIG. 4(E)). The chip is
fired to thereby obtain the laminated type thermistor 1A.

[0037] The thermistors obtained in the above-described processes were
subjected to various tests described below. Thereafter, the thermistors
were observed under an optical microscope, a scanning electron microscope
(SEM), and the like for the occurrence of defects (cracking). The room
temperature resistance (which refers to the resistance at room
temperature of 25° C., the same applies to the following
description) was measured before and after the tests, and the occurrence
of resistance change in the room temperature resistance by a load test
was verified. With respect to the resistance change, it was judged that
one in which the change ratio before and after performing the load test
is lower than ±1% had no resistance change.

[0038] (Evaluation Test)

[0039] First, a winding test was carried out. In the winding test,
thermistor test pieces having a length of 500 mm and a width of 5 mm were
wound around cylinders having a diameter of 0.71 cm, 1.30 cm, 5.07 cm,
and 10.13 cm equivalent to the curvature of the thermistors at a bending
length of 1 mm, 2 mm, 8 mm, and 16 mm, and then held for 10 seconds. In
the test pieces, the thickness Tb of the metal base material was 30 μm
and the thickness Tt of the thermistor thin film layer was 0.5 to 15.0
μm.

[0040] The test results are as illustrated in Table 1. The evaluation was
performed by observing cracking in the surface of the metal base material
and measuring the resistance before and after the test. The occurrence of
cracking was confirmed by observing the entire surface of the samples
under an optical microscope with a magnification of 50 times and 100
times, and further observing the entire surface of the sample under a
scanning electron microscope (SEM) with a magnification of 1000 times. In
the evaluation column of Table 1, .circle-w/dot. represents that cracking
was not observed and the resistance change ratio before and after the
test was lower than ±1%. ◯ represents that cracking was
observed but the resistance change ratio before and after the test was
lower than ±1%. × represents that cracking was observed and the
resistance change ratio before and after the test was ±1% or more.

[0041] According to the winding test described above, when the thickness
Tt of the thermistor thin film layer is larger than 10 μm, cracking
occurs in a test in which the bending length is about 1 mm. When the
thickness is lower than 1 μm, the test results can be sufficiently
appreciated but it is difficult to form a thermistor thin film layer
whose thickness is lower than 1 μm by a solid phase method. A solid
phase method is advantageous in terms of a manufacturing cost and
productivity. When supposing that a solid phase method is used, the
thickness Tt of the thermistor thin film layer is optimally 1.0 to 10
μm.

[0042] Thus, by integrating the thin film thermistor and the metal base
material, flexibility is imparted to the thermistor. It has been found
that, in particular, when the thickness Tt of the thermistor thin film
layer is 10 μm or lower, the thermistor has flexibility which allows
the thermistor to be wound around a cylinder having a diameter of 10.13
cm. More preferably, when the thickness Tt of the thermistor thin film
layer is 2 μm or lower, the thermistor has excellent flexibility which
allows the thermistor to be wound around a cylinder having a diameter of
0.71 cm.

[0043] Next, a tensile test was carried out. In the tensile test,
thermistor test pieces having a length of 50 mm and a width of 5 mm were
set in a tensile testing machine (Shimazu Autograph), and the load in
cutting was measured. In the test pieces, the width dimension W of the
metal base material was 500 μm, the thickness Tb thereof was 5 to 100
μm, and the thickness Tt of the thermistor thin film layer was 3
μm.

[0044] The test results are as shown in Table 2. When the thickness Tb of
the metal base material is smaller than 10 μm, the tensile strength is
remarkably low. For example, when mounted on a printed-circuit board,
there is a possibility that the thermistor may be broken due to a solder
stress between lands. Moreover, the handling in terms of manufacturing is
difficult. When the thickness Tb is larger than 80 μm, the tensile
strength is enough but the used amount of the metal materials increases
to increase the cost and a reduction in the height of the thermistor is
impaired. Therefore, the thickness Tb of the metal base material is
preferably 10 to 80 μm. However, the upper limit of the thickness Tb
is not necessarily limited in terms of strength.

[0045] Next, the resistance at room temperature (25° C.) at the
distance Lp between the split electrodes was calculated by simulation
using FEM (finite element method). The applied voltage in this case was 1
V. The resistance change ratio ΔR/R (%/μm) from the room
temperature resistance R in accordance with the change in the distance Lp
when the distance Lp between the split electrodes was 2.0 to 200 μm
and the thickness Tt of the thermistor thin film layer was changed in the
range of 1.0 to 10.0 μm (kΩ) was calculated by the following
equation. When the value is larger, the variation in the resistance value
is larger. The other numerical values are L=600 μm, W=300 μm,
L1=200 μm, W1=260 μm, Tb=30 μm, and Wg=20 μm.

ΔR/R(%/μm)={(R131 R2)/R2}/(Lp1-Lp2) [0046] R1: Resistance value
when the distance between the split electrodes is Lp1 [0047] R2:
Resistance value when the distance between the split electrodes is Lp2

[0048] Lp1 and Lp2 are continuous and adjacent numerical values in the
table and Lp1>Lp2 is established. For example, when Lp1 is 200 μm,
Lp2 is 190 μm and when Lp1 is 190 μm, Lp2 is 180 μm.
Accordingly, since a comparison target does not exist in the case of the
lowest column of the table (Lp is 2.0 μm in Table 3), - is indicated.
When a value exceeding 1.00 is obtained in the calculation of ΔR/R
(%/μm), ΔR/R (%/μm) becomes larger than 1.00 even when the
values of Lp1 and Lp2 are made smaller than the value. Therefore, the
experiment is omitted and - is indicated.

[0049] The simulation results are as shown in Table 3. It is preferable
that the resistance change ratio ΔR/R is lower than ±0.2%. More
specifically, the distance Lp is preferably Tt+5 μm or more. When the
distance is smaller than Tt+5 μm, the element resistance is affected
not only in the thickness direction but in the surface direction. As a
result, the contribution to the resistance of the distance Lp becomes
large, and the resistance value varies due to processing error. Moreover,
when cracking or the like occurs between the split electrodes, the
resistance value changes.

[0050] Similarly as above, the end surface distance Lg of the split
electrodes was also determined by simulation for the resistance at room
temperature (25° C.). The results of calculating the room
temperature resistance R (kΩ) and the resistance change ratio
ΔR/R (%/μm) when the end surface distance Lg was 0.0 to 20.0
μm and the thickness Tt of the thermistor thin film layer was changed
in the range of 1.0 to 10.0 μm are shown in Table 4. The side surface
distance Wg is 20 μm and other numerical values are indicated in the
margin of Table 4.

[0051] Moreover, the side surface distance Wg of the split electrode
resistances was also determined by simulation for the resistance value.
The results of calculating the room temperature resistance R (kΩ)
and the resistance change ratio ΔR/R (%/μm) when the side
surface distance Wg was 0.0 to 20.0 μm and the thickness Tt of the
thermistor thin film layer was changed in the range of 1.0 to 10.0 μm
are shown in Table 5. The end surface distance Lg is 20 μm and other
numerical values are indicated in the margin of Table 5.

[0052] It is preferable that the resistance change ratio ΔR/R is
lower than ±0.2% also about the distance Lg and the distance Wg. More
specifically, it is preferable to secure the distance Lg and the distance
Wg to be 5 μm or more, and the influence of the resistance change
resulting from the surface leak at the end surface and the side surface
can be prevented.

SECOND EXAMPLE

[0053] As a second example, thermistors having the same configuration as
that of the first example and having Tb=30 μm, L=600 μm, W=300
μm, L1=200 μm, W1=260 μm, Lg=20 μm, Wg=20 μm, Lp=160
μm, and Tt=5 μm were produced by preparing materials shown in
Tables 6 and 7 and by the same manufacturing process as the
above-described manufacturing process. The coefficient of linear
expansion shown in Tables 6 and 7 are the results of manufacturing a
square column having a cross section of 2.0 mm×2.0 mm and a length
of 5.0 mm from a material of the metal base material and a material of
the thermistor thin film, and measuring the coefficient of linear
expansion by TMA in the air atmosphere. The value of the coefficient of
linear expansion at 800° C. is indicated on the basis of
30° C. With respect to the measurement conditions, the temperature
elevation rate was 10° C./min and a load was 10 gf.

[0055] As is clear from Table 8, by adjusting the coefficient of linear
expansion ratio of the material of the metal base material and the
material of the thermistor thin film to 0.75 to 2.17, the occurrence of
cracking due to a linear expansion difference in integrally firing
(particularly in a reduction in temperature after firing) can be
suppressed. Since a ceramic material is vulnerable to a tensile stress,
cracking is likely to occur when shrunk earlier than the material of the
metal base material (when the coefficient of linear expansion of the
thermistor thin film material is high). Moreover, by adjusting the
coefficient of linear expansion ratio of both materials numerical value
in the above-described value range mentioned above, the occurrence of
cracking due to thermal stress when the thermistor is mounted on the
substrate by reflow can also be suppressed.

THIRD EXAMPLE

FIG. 5

[0056] Similarly as in the first example above, an NTC thermistor 1B which
is a third example has the metal base material 11, the thermistor thin
film layer 15, and the split electrodes 21 and 22 as illustrated in FIG.
5, and, in addition, a protection layer 16 is formed on the thermistor
thin film layer 15 and an Ni plating layer 23 and an Sn plating layer 24
are formed on the split electrodes 21 and 22.

[0057] An Ni plating layer 23' and an Sn plating layer 24' are formed also
on the surface of the metal base material 11. However, the layers are
formed simultaneously with the formation of the plating layers 23 and 24.
By these plating layers 23' and 24', an effect of preventing the
migration of Ag can be expected when the metal base material 11 is Ag/Pd
or the like.

[0058] The protection layer 16 is one which suppresses the corrosion of
the thermistor thin film layer 15 by plating in the formation of the
plating layers 23 and 24 and which may be an insulator material, such as
glass, resin, or insulator ceramic, which is not corroded by plating. In
particular, when insulator ceramic is used as the protection layer 16, by
forming an insulator ceramic green sheet on the thermistor thin film
layer 15 beforehand when integrally firing the metal base material 11 and
the thermistor thin film layer 15, the metal base material 11, the
thermistor thin film layer 15, and the protection layer 16 can be formed
by integrally firing, so that the manufacturing process is simplified and
the adhesion of the thermistor thin film layer 15 and the protection
layer 16 becomes good.

[0059] (Modification, FIG. 6)

[0060] A thermistor 1B' illustrated in FIG. 6 is one in which a protection
layer 16 is formed also on the back surface or the side surfaces of the
metal base material 11, as compared with the thermistor 1B illustrated in
FIG. 5. This kind of the thermistor is mounted by reflow on lands 41
formed on the surface of a printed-circuit board 40 through solders 42.
In this case, when the metal base material 11 is exposed to the surface,
there is a possibility that conductive parts, wiring, and the like which
are not illustrated may be electrically conductive to the metal base
material 11. Thus, by covering the entire surface of the thermistor
except the split electrodes 21 and 22 with the protection layer
(insulating layer) 16, such a short circuit accident can be prevented
beforehand.

FOURTH EXAMPLE

FIG. 7

[0061] A thermistor 1C which is a fourth example is one in which the
thermistor thin film layers 15 are formed immediately under the split
electrodes 21 and 22 in a rectangular shape slightly smaller than the
split electrodes 21 and 22 as illustrated in FIG. 7. The configuration
such that the Ni plating layer 23 and the Sn plating layer 24 are formed
on the split electrode 21 and 22 is the same as in the third example.

[0062] In the third example (FIG. 5), since the protection layer 16 is
formed on the split electrodes 21 and 22, it is surely necessary to
laminate the protection layer 16 on the circumference of the split
electrodes 21 and 22 in order to completely cover the thermistor thin
film layers 15 with the protection layer 16 (Section A of FIG. 5). In
this case, the firing condition and the sintering behavior of the
protection layer 16 changes in the A section due to a difference of the
foundation, which results in a possibility that cracking may occur in the
A section. Then, by providing the thermistor thin film layers 15
immediately under the split electrodes 21 and 22 and providing the
protection layer 16 on the same plane as the thermistor thin film layer
15 as in the fourth example, the entire foundation of the protection
layer 16 is the metal base material 11, so that the presence of the A
section which is an overlapped portion is canceled. Therefore, there is
no possibility of the occurrence of cracking also disappears and the
total thickness as a thermistor becomes small.

[0063] Here, in the thermistor 1C which is the fourth example, the results
of measuring samples No. 1 to No. 4 shown in Table 9 for the resistance
value change resulting from the variation in the area of the split
electrodes 21 and 22 are shown. In each of the samples No. 1 to No. 4,
the area (L1×W1) of the split electrodes 21 and 22 was set to 310
μm square, 300 μm square, and 290 μm square (three types), the
resistivity ρ1 of the thermistor material was 10 kΩcm, and the
resistivity ρ2 of the protection layer 16 was set 10 kΩcm in
the sample No. 1, 100 kΩcm in the sample No. 2, 1000 kΩcm in
the sample No. 3, and 10000 kΩcm in the sample No. 4. The area of
the thermistor thin film layer 15 is 250×250 μm and the
thickness (Tt) is 3 μm. L was 1000 μm, W was 500 μm, Tb was 30
μm, Lg was 20 μm, Lp was 960 μm -L direction dimension of the
split electrodes (290, 300, or 310 μm), and Wg was 20 μm. ρ1
represents the resistivity of a thermistor material (specifically,
Mn--Ni--Fe--Ti thermistor material) formed into the thermistor thin film
layer. ρ2 represents the resistivity of an insulator material
(specifically, Fe--Mn ferrite material) formed into the protection layer.
The resistivity is changed by changing the composition ratio. The
resistance value change (%) was calculated by the following equation.

Resistance value change=(R2-R1)/R1×100

[0064] R1: Element resistance value when the split electrode area is 290
μm.

[0065] R2: Element resistance value when the split electrode area is 310
μm.

[0066] As is clear from Table 9, the resistance value change (%) was 14.27
when ρ2/ρ1 was 1 (Sample No. 1), 1.86 when ρ2/ρ1 was 10
(Sample No. 2), 0.19 when ρ2/ρ1 was 100 (Sample No. 3), and 0.02
when ρ2/ρ1 was 1000 (Sample No. 4). When ρ2/ρ1 is 100
times or more, the resistance value change can be suppressed to 0.2% or
lower even when the area of the split electrodes 21 and 22 varies.
Therefore, the ρ2/ρ1 ratio is preferable.

FIFTH EXAMPLE

FIG. 8

[0067] A thermistor 1D which is a fifth example is one in which the area
of the thermistor thin film layers 15 is made larger than the area of the
split electrodes 21 and 22 as illustrated in FIG. 8. In other words,
peripheral portions B of the thermistor thin film layers 15 were located
outside to the peripheral portion of the split electrodes 21 and 22 and
the protection layer 16 covers a region from the peripheral portions B to
a part of the inner side of the thermistor thin film layer 15. The other
configurations are the same as those of the fourth example.

[0068] In the fifth example, the protection layer 16 covers the peripheral
portions B of the thermistor thin film layers 15. Therefore, by bringing
the protection layer 16 into close contact with the metal base material
11, the thermistor thin film layers 15 are held, so that the thermistor
thin film layers 15 are prevented from separating from the metal base
material 11. Supposing that the thermistor thin film layers 15 are
separated from the metal base material 11, the area to which the
resistance value contributes decreases, so that the resistance value
tends to increase. However, the invention is free from the problem. Since
the protection layer 16 does not contribute to the thermistor
characteristics, a material having high adhesion with the metal base
material 11 may be selected.

[0069] As in the fifth Example, in a case where the thermistor thin film
layers 15 were formed, the protection layer 16 was formed in such a
manner that a part thereof is overlapped with the peripheral portions of
the thermistor thin film layers 15, and then the split electrodes 21 and
22 were formed, peripheral portions of the protection layer 16 are formed
between the peripheral portions at the side of the thermistor thin film
layer 15 of the split electrodes 21 and 22 and the thermistor thin film
layers 15. In this case, even when cracking occurs between the protection
layer 16 and the thermistor thin film layer 15 in the A section of the
protection layer 16, so that the plating layers 23 and 24 grow during
plating, the split electrodes 21 and 22 and the metal base material 11 do
not cause a short circuit because a portion (A section) where cracking
may occur is apart from the split electrodes 21 and 22.

OTHER EXAMPLES

[0070] The thermistor and the method for manufacturing the same according
to the invention are not limited to the Examples above, and can be
modified in various manners within the scope thereof.

[0071] In particular, various sizes of the thermistor shown in the
Examples above are merely examples. Furthermore, the shape and the like
of the details of the metal base material, the thermistor thin film
layer, and the split electrodes are arbitrary.

[0072] As described above, the invention is useful for a thermistor and a
method for manufacturing the same and particularly is excellent in that a
reduction in the height or mounting by reflow or the like can be
achieved, the thermistor can be manufactured at a low cost, and the
occurrence of cracking can be suppressed as much as possible.