Acceleration sensor

A very small and economical acceleration sensor which can detect acting acceleration with high sensitivity and high accuracy by precisely processing a semiconductor substrate and the like by using the photoengraving technique in a semiconductor manufacturing process to accurately form the elements themselves such as a sensor case, a cavity, a heater, a temperature-sensing resistor element, and a heat-type temperature-sensing resistor element, and the relative placement of each element. One embodiment of the acceleration sensor can detect acceleration acting from any of the three-dimensional directions.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to an acceleration sensor for detecting 
acceleration acting on the sensor body, and, more particularly, to an 
acceleration sensor for detecting acceleration in the form of changes in 
the temperature distribution of gas in a closed space. 
Description of the Related Art 
Disclosed in Japanese Published Unexamined Patent Application No. 3-176669 
is an acceleration sensor in which the equilibrium of the temperature 
distribution is formed by heating a gas enclosed in a closed space in a 
case, and the phenomenon in which the temperature distribution is changed 
while a gas flow is generated by the action of acceleration is detected as 
a change in the resistance of a resistance temperature sensor disposed 
within the case. 
In the above conventional acceleration sensor, a thin-film-resistor 
temperature sensor serving as a heater is disposed in a resin case, and 
heated by applying current thereto. Resistance values are previously 
detected for the heated thin-film-resistor temperature sensor at various 
temperatures. 
When acceleration acts on the acceleration sensor, a gas flow is generated 
in the case. The generated gas flow takes heat from the thin-film-resistor 
temperature sensor, thereby reducing its temperature so that the 
resistance of the thin-film-resistor temperature sensor sensor is changed. 
Because the change in the resistance of the thin-film-resistor temperature 
sensor corresponds to the acceleration acting on the acceleration sensor, 
the acceleration is detected by converting the resistance of the 
thin-film-resistor temperature sensor to an electric signal. 
The conventional acceleration sensor has, however, a problem that, although 
it can detect the absolute value of acceleration, it cannot detect the 
direction in which acceleration acts because it uses a heater heating the 
gas in the case and a thin-film-resistor temperature sensor for the heater 
also serving as a temperature-sensing element which detects the 
temperature change due to the action of acceleration. 
In addition, the conventional acceleration sensor has a problem that, 
because the thin-film-resistor temperature sensor for the heater serves as 
both a heater and a temperature-sensing element, aging from deterioration 
is caused in the thin-film-resistor temperature sensor for the heater if 
the generated temperature is high, leading to variations in heat 
generation and reduced sensitivity to temperature so that the sensor 
cannot accurately detect acceleration. 
Moreover, the conventional acceleration sensor has a relatively bulky size 
for the closed space, which results in a degraded response to the 
temperature change by the gas flow. Reduction of the size of the 
acceleration sensor is limited by its structure. 
Furthermore, while the accuracy of acceleration detection depends on the 
positional accuracy of the thin-film-resistor temperature sensor for the 
heater, the conventional acceleration sensor has a structure in which the 
thin-film-resistor temperature sensor for the heater is directly installed 
on the case, which makes it difficult to accurately position the 
thin-film-resistor temperature sensor for the heater. 
SUMMARY OF THE INVENTION 
The present invention provides a very small acceleration sensor which is 
constructed by etching a semiconductor board or insulating substrate using 
a photoengraving process in a semiconductor manufacturing process, joining 
two sensor cases formed with a cavity therein to form a closed space, 
forming a bridge at the center of one of the sensor cases at the time the 
cavity is formed, and integrally placing a temperature-sensing resistor 
element and a heater in the bridge with the photoengraving process, and 
which accurately detects imbalances in the temperature distribution 
generated in the closed space by acceleration acting on the sensor case as 
a change in resistance of the temperature-sensing resistor element. 
In addition, the present invention provides an acceleration sensor which 
can accurately detect the absolute value of acceleration acting on the 
sensor case in any two- or three-dimensional direction and the direction 
of action by arranging a pair of temperature-sensing resistor elements in 
the sensor case in one-dimensional (X axis), two-dimensional (X and Y 
axes) or three-dimensional (X, Y, and Z axes) axial directions. 
As described, because the acceleration sensor is constructed by using the 
photoengraving process in the semiconductor manufacturing process, 
resistance values of the temperature-sensing resistor element and the 
heater and their positioning in the sensor case can be attained with high 
accuracy. 
Therefore, the present invention can attain an acceleration sensor which 
eliminates problems inherent in the conventional acceleration sensor of 
poor response to the detection of acceleration due to limitations in the 
reduction of the size, poor accuracy of acceleration detection due to an 
inability to accurately position the temperature-sensing resistor element 
and the heater, time consumed for selecting the resistance for the 
temperature-sensing resistor element and the heater, and the occurrence of 
waste parts, and which present invention is very small in size, has 
excellent accuracy in acceleration detection, and is suitable for mass 
production.

DETAILED DESCRIPTION 
Preferred embodiments of the present invention will be explained in the 
following with reference to the attached drawings; 
FIG. 1 is a cross-sectional view of a first embodiment of the acceleration 
sensor according to the present invention, and FIG. 2 is a plan view of a 
lower sensor case of the first embodiment of the acceleration sensor 
according to the present invention. They show the basic arrangement of the 
acceleration sensor according to the present invention. 
Referring to FIGS. 1 and 2, an acceleration sensor 1 comprises a lower 
sensor case 2 formed therein with a cavity 4 and a bridge 5 by, for 
example, etching a semiconductor substrate with a fine-processing 
technique in the semiconductor manufacturing process, and an upper sensor 
case 3 formed therein with a cavity 7 by etching a semiconductor 
substrate, a closed space 8 being formed by joining the cases so that the 
cavities 4 and 7 face each other. 
The closed space 8 is filled with an inert gas with a low heat transfer 
coefficient such as nitrogen or argon under pressure. 
In addition, metal such as platinum is vapor-deposited on the surface of 
the bridge 5 of the lower sensor case 2, and etched to form a heat-type 
temperature-sensing resistor element 6 with a desired pattern. 
The bridge 5 is formed to bridge the center of the cavity 4 of the lower 
sensor case 2. Etching and vapor evaporation are controlled to accurately 
position the heat-type temperature-sensing resistor element 6 at the 
center of the closed space 8 when the acceleration sensor 1 is 
constructed. 
An electrode 9 formed by the same material as the heat-type 
temperature-sensing resistor element 6 is provided on each end of the 
extension of the heat-type temperature-sensing resistor element 6 to 
electrically determine changes in the resistance of the heat-type 
temperature-sensing resistor element 6 which is caused by detecting the 
electrical supply from an external power supply and the action of 
acceleration (G). 
For the acceleration sensor 1 thus constructed, a thermal equilibrium state 
with a steep temperature gradient is previously attained by heating the 
heat-type temperature-sensing resistor element 6 with the electrical 
supply from the external power supply through the electrodes 9. 
In the thermal equilibrium state, when acceleration (G) (shown by the arrow 
in FIG. 2) acts on the acceleration sensor 1 in the direction 
perpendicular to the longitudinal direction of the heat-type 
temperature-sensing resistor element 6, a gas flow corresponding to the 
absolutes value of the acceleration (G) and its direction of action is 
generated in the closed space 8, which makes the temperature distribution 
in the closed space 8 unbalanced and lowers the surface temperature of the 
heat-type temperature-sensing resistor element 6. 
Lowering of the surface temperature changes the resistance of the heat-type 
temperature-sensing resistor element 6 so that the absolute value of the 
acceleration (G) acting on the acceleration sensor 1 can be detected by 
electrically detecting the change in resistance. 
As described above, in the acceleration sensor 1 according to the present 
invention, the sensor cases 2 and 3, the bridge 5, and the heat-type 
temperature-sensing resistor element 6 are formed by etching or vacuum 
evaporation with a fine-processing technique in the semiconductor 
manufacturing process so that a very small acceleration sensor several 
millimeters square can be constructed and so that the acceleration can be 
detected with a quick response and high accuracy. 
Although the first embodiment has the sensor case constructed of a 
semiconductor substrate, it may also be constructed of an insulating 
substrate such as glass or ceramic. 
In addition, the upper sensor case 3 is not limited to the semiconductor 
substrate, but may be constructed of glass or metal. 
Furthermore, the lower sensor case 2 with the heat-type temperature-sensing 
resistor element 6 may be disposed in a closed space, in which case the 
upper sensor case 3 is not necessarily required. 
In the following embodiments, it is also assumed that the sensor case, the 
bridge, the heater, the temperature-sensing resistor element, and the 
heat-type temperature-sensing resistor element are formed by using 
fine-processing technology of the semiconductor manufacturing process. 
FIG. 3 is a cross-sectional view of a second embodiment of the acceleration 
sensor according to the present invention, while FIG. 4 is a plan view of 
a lower sensor case of the second embodiment of the acceleration sensor 
according to the present invention. They show an embodiment for detecting 
the absolute value and the direction of acceleration action (G) (shown by 
the arrows in FIG. 4) acting on the acceleration sensor. 
Referring to FIGS. 3 and 4, an acceleration sensor 11 comprises a lower 
case 2 formed therein with a cavity 4 and bridges 5 and 15 by, for 
example, etching in a semiconductor substrate with a fine-processing 
technique in the semiconductor manufacturing process, and an upper sensor 
case 3 formed therein with a cavity 7 by etching a semiconductor 
substrate, a closed space 8 being formed by joining the cases so that the 
cavities 4 and 7 about each other. 
Formed on the bridge 5 laying in the direction of acceleration action (G) 
are a heater 12 at the center of the bridge 5 in the longitudinal 
direction, temperature-sensing resistor elements 13, 14 opposite each 
other at a predetermined distance from the center of the bridge 5 in the 
longitudinal direction, and a lead pattern formed by using vacuum 
evaporation and etching of a fine-processing technique. 
Moreover, the bridge 15 formed by placing it coplanar with and 
perpendicular to the bridge 5, and by depositing the same metal as the 
heater 12 on its surface, and etching it to form a lead pattern of the 
heater 12. 
The pair of temperature-sensing resistor elements 13 and 14 is formed in 
such a manner that a metal pattern is formed by depositing 
high-melting-point metal such as Pt, Mo, Ni, Au, or Ti, and then etched to 
form a resistance temperature sensor with a pattern having a predetermined 
resistance. 
When thermal stability and durability are required for the 
temperature-sensing resistor elements 13, 14, the surface of the 
temperature-sensing resistor elements 13, 14 formed by etching is coated 
by an oxide coating which is formed by an oxide of a material such as SiN. 
The pair of temperature-sensing resistor elements 13, 14 and the heater 12 
are connected through the lead pattern to respective electrodes 9 provided 
on the periphery of the lower sensor case 2, which electrodes 9 are then 
connected to an external power supply or a detector circuit. 
In the acceleration sensor 11 thus constructed, the heater 12 is heated by 
supplying external power through two of the electrodes 9 to create a 
thermal equilibrium state with a steep temperature gradient in the closed 
space 8. 
In the thermal equilibrium state where no acceleration (G) acts, the pair 
of temperature-sensing resistor elements 13 end 14 are in the same 
temperature atmosphere so that each of them exhibits the same resistance 
value if their resistance and characteristics of temperature coefficient 
are matched (paired). 
When acceleration (G) acts in the equilibrium state in the direction shown 
in FIG. 4 (indicated by a solid arrow or broken arrow along bridge 5), a 
gas flow is generated in the inert gas, such as nitrogen or argon, 
enclosed in the closed space 8 to create a thermally unbalanced state in 
the direction of acceleration action (G) so that the temperature-sensing 
resistor elements 13 and 14 are subject to different temperatures, 
respectively. 
Since the temperature-sensing resistor elements 13 and 14 generate opposite 
changes in resistance in the thermally unbalanced state, the absolute 
value and the direction of acceleration action (G) are detected by 
electrically detecting the change in resistance. 
FIG. 5 shows the arrangement of a detection circuit of the acceleration 
sensor of FIG. 4. 
Referring to FIG. 5, a detector circuit comprises a bridge circuit 16, a 
reference resistor R1, and a reference resistor R2, and an amplifier 18 
which amplifies the differential output of the bridge circuit 16, wherein 
the bridge circuit 16 constitutes a resistor bridge by connecting the 
temperature-sensing resistor elements 13 and 14, which is externally 
provided through the electrodes 9, and reference resistors R1 and R2. A 
power supply 17 applies power to the bridge circuit 16. 
When no acceleration (G) occurs, the bridge circuit 16 maintains the 
equilibrium state (R.sub.x1 *R2=R.sub.x2 *R1) because the 
temperature-sensing resistor elements 13 and 14 have the same resistance 
(R.sub.x1, R.sub.x2, R.sub.x1 =R.sub.x2), and the bridge circuit 16 sets 
the reference resistors to the same resistance. Then, the bridge output 
(voltage) becomes zero so that the output V.sub.a of the amplifier 18 also 
becomes zero. 
When acceleration (G) occurs, the temperature-sensing resistor elements 13 
and 14 exhibit different resistance values (R.sub.x1 &gt;R.sub.x2 or R.sub.x1 
&lt;R.sub.x2), and the bridge circuit 16 supplies to the amplifier 18 a 
bridge output (voltage) with negative (-) or positive (+) polarity, 
depending on the absolute value and the direction of acceleration action 
(G), and detects the output V.sub.a corresponding to the bridge output 
from the amplifier 18. 
Because the temperature-sensing elements 13 and 14 are formed by a 
fine-processing technique of the semiconductor manufacturing process, it 
is possible to accurately provide resistance (R.sub.x1, R.sub.x2) in a 
paired state so that the offset of the bridge output (voltage) can be set 
to a value as close to zero as possible. 
The acceleration sensor 11 employs nitrogen gas with a heat transfer 
coefficient of about 0.024 (kcal/m.multidot.h.multidot..degree.C.) as the 
gas enclosed in the closed space 8. When it is assumed that the heater 12 
generates heat of about 0.01 (kcal/h) per square millimeter, the 
temperature gradient (=heat generation/heat transfer coefficient) for the 
heater 12 can be set to 400 (.degree.C./mm) to make the temperature 
distribution in the closed space 8 steep so that acceleration in a very 
small space on a millimeter order can be detected with high sensitivity. 
It is possible to further improve the sensitivity of acceleration detection 
by pressurizing the gas to be enclosed in the closed space 8 to pressure 
of more than one atmosphere. 
As described, the acceleration sensor 11 can detect acceleration (G) acting 
on the acceleration sensor with high sensitivity because the closed space 
8, the heater 12, and the temperature-sensing resistor elements 13 and 14 
can be constructed by using a fine-processing technique of the 
semiconductor manufacturing process, and their positions can be precisely 
determined so that a very small acceleration sensor on a millimeter order 
can be easily obtained. 
In addition, the acceleration sensor 11 separately provides the heater 12 
and the temperature-sensing resistor elements 13 and 14, which are 
positioned opposite to the direction to which acceleration (G) occurs, so 
that the absolute value and the direction of acceleration (G) acting on 
the acceleration sensor can be detected with high accuracy. 
If the acceleration sensor is constructed without using the fine-processing 
technique of the semiconductor manufacturing process as in the 
conventional sensor, the ability to reduce its size as limited, and the 
positioning accuracy of the heater and the temperature-sensing resistor 
elements in the closed space and the pairing of temperature-sensing 
resistor elements cannot be fully satisfied, with the result that the 
sensitivity and accuracy in the detection of acceleration (G) are 
deteriorated. 
FIG. 6 shows a plan view of a lower sensor case of a third embodiment of 
the acceleration sensor according to the present invention. 
Referring to FIG. 6, an acceleration sensor 21 differs from the 
acceleration sensor 11 of FIG. 4 in that, in place of the 
temperature-sensing resistor elements 13 and 14, heat-type 
temperature-sensing resistor elements 19 and 20 are positioned opposite to 
each other to eliminate the heater 12 and the bridge 15. 
Because the heat-type temperature-sensing resistor elements 19 and 20 
themselves serve as both heaters and temperature-sensing resistor 
elements, it is possible to create a thermal equilibrium state in 
temperature distribution with two heaters, and to detect an unbalanced 
state in the temperature distribution caused by the action of acceleration 
(G) (shown by the arrows in FIG. 6) with two temperature-sensing resistor 
elements. 
FIGS. 7 and 8 are views of the arrangement of a fourth embodiment of the 
acceleration sensor according to the present invention. 
Referring to FIGS. 7 and 8, an acceleration sensor 21 comprises an upper 
sensor case 3 formed therein with a cavity (not shown), a lower sensor 
case 2 having a cavity 4, a heater 32, and a temperature-sensing resistor 
element 33, the latter two components being provided on a bridge formed in 
the cavity 4. A gas 34 is enclosed in a closed space formed by the upper 
and lower cavities. 
The heater 32 and the temperature-securing resistor element 33 are 
resistors which are formed by depositing and etching platinum or tungsten 
on a bridge (not shown). The gas 34 is a pressurized gas with a low heat 
transfer coefficient such as nitrogen gas or argon. The lower sensor case 
2 and the upper sensor case 3 are closely attached and joined. 
The heater 32 is driven by an external power supply through lead wires 32a 
and 32b, and generates a temperature sufficiently higher than the ambient 
temperature. 
In addition, the temperature-sensing resistor 33 is previously adjusted to 
have a predetermined resistance value by supplying a small current from 
the external power supply through lead wires 33a and 33b. 
The heat generated from the heater 32 is transferred through the gas 34 to 
create a temperature distribution corresponding to the distance from the 
heater 32. 
Moreover, use of a pressurized gas with a low heat transfer coefficient 
such as nitrogen gas or argon creates temperature distribution with a 
steep temperature gradient corresponding to the distance from the heater 
32. 
When acceleration (G) acts in the direction indicated by an arrow (G) shown 
in FIG. 8 (to the left) under a stable temperature distribution in the 
closed space, the heated gas 34 moves in the direction of arrow P (to the 
right) to raise the temperature of the temperature-sensing resistor 
element 33. 
As the temperature of the temperature-sensing resistor element 33 is 
raised, the resistance also rises, if the temperature coefficient is 
positive, to increase the value of the voltage to be detected on the lead 
wires 33a and 33b. 
On the contrary, if acceleration (G) acts in the opposite direction (to the 
right), the heated gas 34 moves to the left to reduce the temperature of 
the temperature-sensing resistor element 33 and also to lower the 
resistance value of the temperature-sensing resistor element 33 so that 
the value of the voltage to be detected on the lead wires 33a and 33b is 
reduced. 
The heater 32 and the temperature-sensing resistor element 33 are 
positioned on a bridge (not shown) at a predetermined distance D. 
As described, since the acceleration sensor 31 is constructed by 
positioning the heater 32 and the temperature-sensing resistor element 33, 
enclosing the pressurized gas with a low heat transfer coefficient such as 
nitrogen gas or argon in the closed space to increase the temperature 
gradient and to detect the temperature with a steep temperature gradient 
from the heater 32, it can detect the temperature change from the change 
of acceleration (G) with high accuracy. 
FIG. 9 shows a functional block diagram for the detection of acceleration 
by using the acceleration sensor of FIGS. 7 and 8. 
Referring to FIG. 9, the heater 32 is driven by a constant current source 
(I.sub.H) 35, and generates heat at a high temperature corresponding to 
the power (R.sub.H *I.sub.H.sup.2) based on the resistance R.sub.H of 
heater 32 and the current I.sub.H. 
In addition, the temperature-sensing resistor element 33 is driven by a 
constant current source (I.sub.L) 36, set to a resistance value R.sub.c 
which is the sum of a resistance value at the ordinary temperature and a 
resistance value corresponding to the temperature transferred from the 
heater 32, and outputs the voltage at a value V.sub.C (R.sub.C *I.sub.L). 
the voltage V.sub.C is an output of the acceleration sensor 31 
corresponding to the resistance R.sub.C when no acceleration (G) acts, and 
input to one terminal (for example, the positive input terminal) of a 
comparator 37 which is constituted by, for example, an operational 
amplifier. 
The other terminal (for example, a negative input terminal) of the 
comparator 37 is input with a voltage V.sub.S from a reference resistor 
R.sub.S. The voltage V.sub.C is set to be equal to the voltage V.sub.S 
(V.sub.C =V.sub.S), for example, when no acceleration (G) occurs. 
The comparator 37 calculates and outputs the difference .DELTA.V between 
the voltage V.sub.C and the voltage V.sub.S (=V.sub.C --V.sub.S). 
Therefore, the difference .DELTA.V is zero when no acceleration (G) 
occurs. 
When the acceleration (G) occurs in the direction shown by an arrow (G) in 
the figure, heat transfer is generated from the heater 32 to the 
temperature-sensing resistor element 33 so that the resistance R.sub.C of 
the temperature-sensing resistor element 33 increases, and the difference 
.DELTA.V exceeds zero (.DELTA.V&gt;0) to provide a positive voltage. 
On the contrary, when the acceleration (G) occurs in the direction opposite 
to that shown in the figure, the resistance R.sub.C of the 
temperature-sensing resistor element 33 decreases, and the difference 
.DELTA.V becomes less than zero (.DELTA.V&lt;0) to provide a negative 
voltage. 
An acceleration conversion means 38 has a memory such as a ROM which 
stores, beforehand, a value of the acceleration G.sub.0 corresponding to 
the difference .DELTA.V, and is arranged to output an acceleration signal 
G.sub.0 in response to the input of the difference .DELTA.V. 
As described above, the acceleration sensor 31 according to the present 
invention comprises the heater which heats the gas to create a temperature 
distribution in the space, and the temperature-sensing resistor element 
which, when acceleration acts on the sensor case, detects a temperature 
change from the movement of the gas with the temperature distribution so 
that it can detect the absolute value and the direction of acceleration 
action. In addition, because the temperature-sensing resistor element is 
positioned at a predetermined distance from the heater to detect a 
temperature lower than that of the heater, the acceleration sensor can 
maintain stable sensitivity while preventing the deterioration or aging 
which may be caused by high temperature. 
Furthermore, the acceleration sensor 31 according to the present invention 
employs a pressurized gas with a low heat transfer coefficient as the gas 
to be enclosed, and can heighten the sensitivity to the temperature 
detected with the steep temperature gradient so that the acceleration 
corresponding to the detected temperature can be detected with high 
accuracy. 
FIGS. 10 and 11 are views of the arrangement of a fifth embodiment of the 
acceleration sensor according to the present invention. 
Referring to FIGS. 10 and 11, an acceleration sensor 41 differs from the 
acceleration sensor 31 shown in FIGS. 7 and 8 in that a 
temperature-sensing resistor element 42 for temperature compensation is 
disposed in a cavity 44 formed in a lower sensor case 2 or outside the 
lower sensor case 2. 
The cavity 44 is formed separately from a cavity 43 to avoid the influence 
of the heat from the heater 32. Gas is enclosed in the closed space of 
cavity 44 so that the temperature-sensing resistor element 42 for 
temperature compensation detects the ambient temperature. 
When the temperature-sensing resistor element 42 for temperature 
compensation is arranged on a bridge in the cavity 44, it is formed on the 
same semiconductor substrate as for the heater 32 and the 
temperature-sensing resistor element 33 by using a fine-processing 
technique of the semiconductor manufacturing process. 
As the temperature-sensing resistor element 42 for temperature compensation 
and the temperature-sensing resistor element 33 are formed on the same 
semiconductor substrate with the same semiconductor manufacturing process, 
temperature-sensing resistor elements with the same characteristics can be 
formed so that variations in the characteristics and aging between both 
temperature-sensing resistor elements can be compensated for. 
When the temperature-sensing resistor element 42 for temperature 
compensation is disposed outside the lower sensor case 2 (the upper sensor 
case 3 being included), it is attained by attaching, for example, by 
bonding, to the case the temperature-sensing resistor element 42 for 
temperature compensation with temperature characteristics matching those 
of the temperature-sensing resistor element 33. 
When left for a sufficient period of time in a predetermined ambient 
temperature without supplying current to the heater 32, the heater 32, the 
temperature-sensing resistor element 33, and the temperature-sensing 
resistor element 42 for temperature compensation are set to the ambient 
temperature. 
In such state, when current is supplied to the heater 32, the heater 32 
generates heat corresponding to the power consumption determined by the 
supplied current and resistance, and thus is heated. It has a temperature 
equal to the sum of the temperature from the consumed power and the 
ambient temperature. 
The heat generated from the heater 32 is transferred to the gas 34 such as 
nitrogen gas or argon to create a temperature distribution with a steep 
temperature gradient, which is detected by the temperature-sensing 
resistor element 33. Since the gas 34 and the temperature-sensing resistor 
element 33 are previously set to the ambient temperature, the temperature 
detected by the temperature-sensing resistor 33 is also the sum of the 
temperature transferred from the heater 32 and the ambient temperature. 
For example, if the reference ambient temperature is 20.degree. C., and the 
ambient temperature is higher than 20.degree. C. by a predetermined 
temperature .DELTA.T, the temperature of the heater 32 is the temperature 
from the consumed power added to the predetermined temperature .DELTA.T, 
and the temperature detected by the temperature-sensing resistor element 
33 is also the temperature of temperature distribution at the location 
where the temperature-sensing resistor element 33 is positioned added to 
the predetermined temperature .DELTA.T. 
In addition, because the temperature-sensing resistor element 42 for 
temperature compensation is also set to the ambient temperature (the 
reference temperature 20.degree. C. plus the predetermined temperature 
.DELTA.T), it is possible to compensate for the temperature detected by 
the temperature-sensing resistor element 33 based on the temperature 
detected by the temperature-sensing resistor element 42 for temperature 
compensation. 
Likewise, also, when heat movement is generated by the movement of the gas 
34 in the closed space formed by the cavity 43 under the action of 
acceleration (G), temperature compensation can be attained based on the 
temperature detected by the temperature-sensing resistor element 42 for 
temperature compensation. 
FIG. 12 shows a functional block diagram for the detection of acceleration 
by using the acceleration sensor of FIGS. 10 and 11. 
Referring to FIG. 12, the heater 32 and the temperature-sensing resistor 
element 33 are driven by a constant-current source (I.sub.H) 35 and a 
constant-current source (I.sub.L) 36, respectively, and the 
temperature-sensing resistor element 42 for temperature compensation is 
driven by a constant-current source (I.sub.L) 45 with the same current as 
that for the temperature-sensing resistor element 33. 
When the heater 32 is not driven (I.sub.H =0), the temperature-sensing 
resistor element 33 and the temperature-sensing resistor element 42 for 
temperature compensation detect the ambient temperature. Because the same 
temperature characteristics are arranged to be provided for both 
temperature-sensing resistor elements, the resistance R.sub.C is equal to 
the resistance R.sub.F, and the detection outputs (voltage) V.sub.C and 
V.sub.R from the temperature-sensing resistor element 33 and the 
temperature-sensing resistor element 42 for temperature compensation 
become equal (V.sub.C =V.sub.R). 
On the contrary, when the heater 32 is driven by the current source 
(I.sub.K), the rise in temperature from the heater 32 is detected, and the 
resistance R.sub.C of the temperature-sensing resistor element 33 is 
increased (R.sub.C &gt;R.sub.F) so that the detection output (voltage) 
V.sub.C is also increased (V.sub.C &gt;V.sub.R). 
An acceleration correction means 46 comprising a temperature comparator 47, 
a correction value output means 48, and a correction value memory 49, 
corrects the detection output V.sub.C from the temperature-sensing 
resistor element 33 based on the detection output V.sub.R from the 
temperature-sensing resistor element 42 for temperature compensation to 
compensate for the variation in the acceleration (G) due to the ambient 
temperature, and detects the actual acceleration G.sub.O acting on the 
acceleration sensor. 
The temperature comparator 47 consists of a comparator circuit such as a 
comparator, and stores, beforehand, the detection output V.sub.R provided 
by the temperature-sensing resistor element 42 for temperature 
compensation. It compares the detection output V.sub.H with the reference 
voltage V.sub.f corresponding to 20.degree. C., and outputs the difference 
.DELTA.V.sub.R between the detection output V.sub.H and the reference 
voltage V.sub.f to the correction value output means 48. 
The correction value output means 48 determines the detection output 
V.sub.C from the temperature-sensing resistor element 33 and the 
difference .DELTA.V.sub.R, reads an acceleration correction value .DELTA.G 
corresponding to the detection output V.sub.C and the difference 
.DELTA.V.sub.H from the correction value memory 49, and provides it to the 
acceleration conversion means 38. 
The correction value output means 48 is set to make the acceleration 
correction value .DELTA.G to be output as zero when no acceleration (G) 
acts at an ambient temperature of, for example, 20.degree. C. 
The correction value memory 49 consists of a memory such as a ROM and sets, 
in a table, a correction value .DELTA.G when the difference .DELTA.V.sub.R 
changes, which correction value is previously determined through 
experiments for a reference of a difference (V.sub.C -.DELTA.V.sub.R) 
between the detection output V.sub.R and the difference .DELTA.V.sub.R 
based on the detection output V.sub.R and the difference .DELTA.V.sub.R. 
The acceleration conversion means 38 comprises a memory such as a ROM for 
converting the detection output V.sub.C to corresponding acceleration 
G.sub.O, and a subtractor, converts the detection output V.sub.C to 
acceleration G.sub.O, the calculates the difference (G.sub.O -.DELTA.G) of 
the correction value .DELTA.G from the acceleration G.sub.O, and outputs 
the difference as the acceleration G.sub.O. 
The acceleration conversion means 38 is set to make the acceleration 
G.sub.O to be output to zero when no acceleration (G) acts at an ambient 
temperature of, for example, 20.degree. C. 
Thus, as the acceleration G.sub.O to be output is set to zero when no 
acceleration (G) acts at an ambient temperature of 20.degree. C., and the 
acceleration correction means 46 compensates for the temperature according 
to the change in the ambient temperature, it is possible to set the 
acceleration G.sub.O output from the acceleration conversion means 38 
always to zero. 
When acceleration (G) acts, as the detection voltage V.sub.C of the 
temperature-sensing resistor element 33 increases or decreases, the 
acceleration G.sub.O corresponding to the detection output V.sub.C and 
corrected for the acceleration with respect to the ambient temperature 
(correction value .DELTA.G) can be obtained from the acceleration 
conversion means 38. 
The acceleration correction means 46 is an embodiment of an arrangement 
assuming a case where the ambient temperature changes from the temperature 
detected at a state where the temperature-sensing resistor element 33 is 
at the ambient temperature of 20.degree. C. and no acceleration (G) 
occurs, and where the output G.sub.O from the acceleration conversion 
means 38 corresponding to the detection output V.sub.C is nonlinear. 
FIG. 13 shows another embodiment of the acceleration correction means in 
FIG. 12. 
This embodiment represents a case in which the output G.sub.O from the 
acceleration conversion means 50 and the detection output V.sub.CC from 
the acceleration correction means 49 are linear. 
The acceleration correction means 49 comprises a comparator circuit such as 
a comparator and an arithmetic circuit such as a subtractor, and 
calculates and outputs a difference V.sub.CC (V.sub.C -V.sub.R) between 
the detection output V.sub.C from the temperature-sensing resistor element 
33 and the detection output V.sub.R from the temperature-sensing resistor 
element 42 for temperature compensation. 
The acceleration conversion means 50 has a memory such as a ROM for 
converting the difference output V.sub.CC from the acceleration correction 
means 49 in to a corresponding acceleration G.sub.O, converts the 
difference output V.sub.CC into acceleration G.sub.O, and outputs it. 
As described above, since the acceleration sensor 41 according to the 
present invention comprises the temperature-sensing resistor element 42 
for temperature compensation for detecting the ambient temperature, and 
the acceleration correction means for correcting the output signal output 
from the temperature-sensing resistor element 33 based on the output 
signal from the temperature-sensing resistor element 42 for temperature 
compensation, it can detect the accurately acting acceleration by 
compensating for the influence from the ambient temperature. 
FIGS. 14 and 15 are views of the arrangement of a sixth embodiment of the 
acceleration sensor according to the present invention. 
Referring to FIGS. 14 and 15, an acceleration sensor 51 comprises an upper 
sensor case 3 formed therein with a cavity (not shown), a lower sensor 
case 2 having a cavity 4, a temperature-sensing resistor elements 52 and 
53 provided on a bridge (not shown) in the cavity 4, and a pair of 
reference resistors 54 and 55 externally connected to the 
temperature-sensing resistor elements 52 and 53 to form a bridge circuit 
56, a gas 34 being enclosed in a closed space formed by the upper and 
lower sensor cases. 
The pair of the heat-type temperature-sensing resistor elements 52 and 53 
comprises resistors, each of which is formed by depositing and etching 
platinum or tungsten on a bridge (not shown). The gas 34 employed is a 
pressurized gas with a low heat transfer coefficient such as nitrogen gas 
or argon. The lower sensor case 2 and the upper sensor case 3 are closely 
attached and joined. 
In addition, the pair of the heat-type temperature-sensing resistor 
elements 52 (resistor R1) and 53 (resistor R2) are connected by lead wires 
which lead to outside of the sensor case for connecting to the reference 
resistors 54 (resistor r1) and 55 (resistor r2), which are disposed 
outside the sensor case, to form a bridge circuit 56. The bridge circuit 
56 has terminals (56a-56d), as shown in FIG. 15, and consists of four 
resistors. 
When power (for example, from a power supply V.sub.1 shown in FIG. 16) is 
applied across the terminals (56a-56b), the heat-type temperature-sensing 
resistor elements 52 and 53 generate heat corresponding to the power 
consumed, and generate, as a heat source with a temperature sufficiently 
higher than the ambient temperature, a temperature distribution 
corresponding to the distance between the heat-type temperature-sensing 
resistor elements 52 and 53 in the closed space. 
In this state, the heat-type temperature-sensing resistor elements 52 and 
53 have resistances R1 and R2, respectively, so that voltage V.sub.X 
divided by the resistances R1 and R2 is generated at the terminal (56c). 
In contrast, voltage V.sub.Y divided by resistances r1 and r2 of the 
reference resistors 54 and 55 is generated at the terminal (56d). Thus, 
resistances R1, R2, r1 and r2 are set in such a manner that the output of 
the bridge circuit 56 (potential difference V.sub.X -V.sub.Y) is in the 
equilibrium state (output voltage=0V) when no acceleration (G) occurs 
(R1=R2, r1=r2). 
In this state, the temperature distribution in the closed space caused by 
heat generated from the heat-type temperature-sensing resistor elements 52 
and 53 is also in the equilibrium state. 
In addition, the employment of a pressurized gas with a low heat transfer 
coefficient, such as nitrogen gas or argon, as the gas 34 results in a 
temperature distribution corresponding to the distance between the 
heat-type temperature-sensing resistor elements 52 and 53 in the closed 
space with a steep temperature gradient. 
When acceleration (G) acts in the direction indicated by the arrow (G) 
shown in FIG. 15 in the equilibrium state, the gas 34 moves in the 
direction of P to cause heat movement from the heat-type 
temperature-sensing resistor element 52 to the heat-type 
temperature-sensing resistor element 53. 
When the heat movement (direction of P) occurs, the thermal equilibrium in 
the closed space is destroyed, the temperature of the heat-type 
temperature-sensing resistor element 52 decreases, and the temperature of 
the heat-type temperature-sensing resistor element 53 increases so that 
the resistance R1 of the heat-type temperature-sensing resistor element 52 
decreases and the resistance R2 of the heat-type temperature-sensing 
resistor element 53 increases. 
When, with the occurrence of acceleration (G), the resistance R2 increases 
and the resistance R1 decreases, the output of the bridge circuit 56 also 
becomes unbalanced so that the potential difference V.sub.X -V.sub.Y 
detects a positive value (V.sub.X -V.sub.Y &gt;0) corresponding to the 
acceleration (G). 
In contrast, when acceleration (G) acts in the direction opposite to the 
arrow (G) shown in FIG. 15, a reverse phenomenon occurs, that is, the 
resistance R1 increases and the resistance R2 decreases so that the output 
of the bridge circuit 56 detects a negative potential value (V.sub.X 
-V.sub.Y &lt;0) corresponding to the acceleration (G). 
Thus, the acceleration sensor 51 detects the magnitude of the acting 
acceleration (G) with the absolute value of the output of the bridge 
circuit 56 (potential difference V.sub.X -V.sub.Y), and the acting 
direction of acceleration (G) with the sign of the output (positive or 
negative of potential difference V.sub.X -V.sub.Y). 
The output of the bridge circuit 56 (potential difference V.sub.X -V.sub.Y) 
will be explained in the following: 
FIG. 16 shows a bridge circuit diagram of the acceleration sensor of FIGS. 
14 and 15. 
Referring to FIG. 16, the bridge circuit 56 consists of the resistances R1 
and R2 of the heat-type temperature-sensing resistor elements 52 and 53, 
the resistances r1 and r2 of the reference resistors 54 and 55, and a 
power supply (for example, the voltage source V.sub.I) being connected 
between the terminals (56a-56b), with the output occurring across the 
terminals (56c-56d). 
The voltage V.sub.X at the terminal (56c) and the voltage V.sub.Y at the 
terminal (56d) with respect to ground (GND) is calculated by using 
equation 1, as follows: 
EQU V.sub.X =R2*V.sub.I /(R1+R2), and V.sub.Y =r2*V.sub.I /(r1+r2)(1) 
Based on equation 1, the output potential difference of the bridge circuit 
56 V.sub.O (=V.sub.X -V.sub.Y) can be represented by the following, 
equation 2: 
##EQU1## 
In equation 2, by setting R1=R2 and r1=r2, the output potential difference 
V.sub.O =0 can be obtained in the equilibrium state where no acceleration 
(G) occurs. 
When, in the equilibrium state, the acceleration (G) shown in FIG. 15 
occurs, and the heat movement (direction of arrow P) causes a decrease in 
the resistance R1 by .DELTA.R and an increase in the resistance R2 by 
.DELTA.R, the output potential difference V.sub.O (=V.sub.X -V.sub.Y) 
becomes the value represented by 
##EQU2## 
Thus, the output potential difference V.sub.C can provide a value 
proportional to the variation .DELTA.R of resistance corresponding to 
acceleration (G). 
However, when acceleration (G) acts in a direction opposite to that shown 
in FIG. 15, the output potential difference V.sub.C provides a value with 
the opposite sign to equation 3 (-.DELTA.R*V.sub.I /2R). 
As described above, because in the acceleration sensor 51 according to this 
embodiment, a pair of heat-type temperature-sensing resistor elements is 
disposed with a predetermined distance in a closed space containing a 
pressurized gas with a low heat transfer coefficient, and a bridge circuit 
is constituted by a pair of heat-type temperature-sensing resistor 
elements and a pair of external reference resistors, it is possible to 
detect heat movement in the sensor case, which is caused by the action of 
acceleration, with the change in resistance of the pair of heat-type 
temperature-sensing resistor elements, to detect the absolute value of 
acceleration at a high accuracy with the output voltage of the bridge 
circuit and its sign, and to detect the direction in which acceleration is 
acting. 
FIGS. 17 and 18 are views of the arrangement of a seventh embodiment of the 
acceleration sensor according to the present invention. 
Referring to FIGS. 17 and 18, the acceleration sensor 61 differs from the 
acceleration sensor 51 shown in FIG. 14 in that heat-type 
temperature-sensing resistor elements 62 and 63 are disposed at a 
predetermined distance D in a closed space in a lower sensor case 2, and a 
plurality of heaters 64 (R.sub.11 -R.sub.1n, R.sub.21 -R.sub.2n, R.sub.31 
-R.sub.3n) are positioned on both sides of the heat-type 
temperature-sensing resistor elements 62 and 63. 
Furthermore, similar to the embodiment of FIG. 15, the heat-type 
temperature-sensing resistor elements 62 and 63, and the reference 
resistors 54 and 55 constitute the bridge circuit 56 shown in FIG. 15. 
The heaters 64 (R.sub.11 -R.sub.1n, R.sub.21 -R.sub.2n, R.sub.31 -R.sub.3n) 
are heated by, for example, connecting the elements in series and 
connecting a power supply across the terminals (56e-56f). 
In addition, as in the embodiment of FIG. 16, a voltage source V.sub.1 is 
connected across the terminals (56a-56b) of the bridge circuit 56 to heat 
the heat-type temperature-sensing resistor elements 62 and 63. 
It is assumed that the heat-type temperature-sensing resistor elements 62 
and 63 have resistances R1 and R2 that are equal when the temperature 
distribution in the closed space caused by the heat generation of the 
heat-type temperature-sensing resistor elements 62 and 63, and the heaters 
64 (R.sub.11 -R.sub.1n, R.sub.21 -R.sub.2n, R.sub.31 -R.sub.3n) reaches 
equilibrium. Then, the bridge circuit 56 constituted by resistances r1 and 
r2 of the standard resistors 54 and 55, and resistances R1 and R2 is set 
in that state in which its output (output potential difference V.sub.o) is 
in an equilibrium state (V.sub.x -V.sub.y =0). 
When heat movement is generated in the closed space in the direction of the 
arrow (direction of arrow P) by the action of acceleration (G) in the 
direction arrow (G) shown in FIG. 18, heat moves from the heat-type 
temperature-sensing resistor element 62 to the heat-type 
temperature-sensing resistor element 63 as described for FIG. 15 to 
decrease the resistance R1 and to increase the resistance R2. 
Since the decrease in the resistance R1 and the increase in the resistance 
R2 provide a value larger than the variation in FIG. 15 (for example, 
k*.DELTA.R) because of the heat movement in the heaters 64 (R.sub.11 
-R.sub.1n, R.sub.21 -R.sub.2n, R.sub.31 -R.sub.3n), the bridge output 
V.sub.o (V.sub.X -V.sub.Y) becomes k*.DELTA.R*V.sub.1 /2R times the 
relationship of equations 1-3. 
Thus, because of the acceleration sensor 61 is provided with a plurality of 
heaters 64, the movement of heat (temperature change) can be set at a 
linear value, and a larger bridge output than that of the arrangement of 
FIG. 14 can be obtained for the sense acceleration (G) so that the 
detection accuracy can be improved. 
In contrast, when the direction of acceleration (G) is reversed, the bridge 
output V.sub.o of -k*.DELTA.R*V.sub.1 /2R is obtained. 
It is preferred as in the arrangement of FIG. 14, to employ a pressurized 
gas with a low heat transfer coefficient such as nitrogen gas or argon as 
the gas 34 used. 
As described, since the acceleration sensor 61 according to this embodiment 
provides a plurality of heaters together with a pair of heat-type 
temperature-sensing resistor elements in the closed space, the heat 
movement (temperature change) in the closed space can be increased so that 
the absolute value of acceleration can be detected with a high accuracy. 
FIGS. 19 and 20 are views of the arrangement of a eighth embodiment of 
acceleration sensor according to the present invention. 
Referring to FIGS. 19 and 20, an acceleration sensor 71 differs from the 
arrangements shown in FIGS. 14 and 15 in that heat-type 
temperature-sensing resistor elements 74 and 75 corresponding to the 
standard resistor (r1) 54 and the standard resistor (r2) 55 shown in FIG. 
15 are provided in the closed space in the lower sensor case 2 together 
with the heat-type temperature-sensing resistor elements 72 and 73, and in 
that a bridge circuit 56 is constituted by the heat-type 
temperature-sensing resistors elements 72, 73, 74 and 75. 
A power supply V.sub.1 is connected across the terminals (56a-56b) to heat 
the type-type temperature-sensing resistor elements 72, 73, 74, and 75. 
The resistances R1, R2, r2, and r1 are set to values to cause equilibrium 
(V.sub.X -V.sub.Y =0) of the output V.sub.o (across the terminals 56c-56b) 
of the thermally balanced bridge circuit 56 in the closed space. 
When the movement of heat is generated in the closed space as acceleration 
(G) acts in the direction of the arrow (direction of P), heat moves from 
the heat-type temperature-sensing resistor elements 72 and 74 to the 
heat-type temperature-sensing resistor elements 73 and 75 to decrease the 
resistances R1 and r2 and to increase the resistances R2 and r1. 
When it is assumed that the decrease in the resistances R1 and r2 is 
-.DELTA.R and -.DELTA.r, and the increase in the resistances R2 and r1 is 
.DELTA.R and .DELTA.r, the bridge output V.sub.o (V.sub.X -V.sub.Y) is 
represented by equation 4: 
##EQU3## 
Thus, the acceleration sensor 71 provided with the heat-type 
temperature-sensing resistor elements 74 and 75 can set the movement of 
heat (temperature change) to a large value, and a larger bridge output can 
be obtained for the same acceleration (G) than for the arrangements of 
FIGS. 14 and 15 so that the detection accuracy of the sensor can be 
increased. 
It is preferred as in the arrangement of FIG. 14, to employ a pressurized 
gas with a low heat transfer coefficient such as nitrogen gas or argon as 
the gas 34 used, and to position the heat-type temperature-sensing 
resistor elements 72 and 73 at a relative distance D. 
As described above, since in the acceleration sensor 71 according to this 
embodiment, two pairs of heat-type temperature-sensing resistor elements 
are provided in the closed space, and the bridge circuit is constituted by 
four heat-type temperature-sensing resistor elements, the absolute value 
of acceleration can be detected at a higher accuracy. 
FIG. 21 is an exploded perspective view of the arrangement of essential 
components of a ninth embodiment of an acceleration sensor according to 
the present invention. 
Referring to FIG. 21, an acceleration sensor 81 is an embodiment of a 
three-axis acceleration sensor, and is constituted by four types of 
semiconductor substrates 82-85. 
The semiconductor substrates 82-85 are formed therein with a cavity, a 
closed space, heaters, and temperature-sensing resistor elements by a 
fine-processing technique in the semiconductor manufacturing process such 
as etching or depositing. 
Dimensions and placement of the closed space, the heaters, the 
temperature-sensing elements, and external connection pads can be very 
finely determined by a mask so that they can be attained with a high 
accuracy. 
The semiconductor substrate 82 constitutes the cover for a sensor case for 
the acceleration sensor 81 in which a cavity 86 is very finely processed 
and formed by etching. 
The semiconductor substrate 83 is formed with a temperature-sensing 
resistor element Z1 positioned upward in the direction of the Z axis, 
external connection pads (91a) and (92b) for connecting the 
temperature-sensing resistor element Z1 to the outside, and a space 87 
extending through the Z axis. 
The temperature-sensing resistor element Z1 is formed by etching based on a 
fine pattern diagram which represents a mask for manufacturing conductors 
to be formed on a bridge (not shown), which is formed on the top of the 
semiconductor substrate by, for example, etching, by depositing or crystal 
growing metal such as platinum or tungsten. 
The temperature-sensing resistor element Z1 is formed on a diagonal line, 
in the X direction, or in the Y direction so that it is positioned at the 
center of the top of the substrate. 
The external connection pads (91a) and (91b), and a lead pattern for 
electrically connecting the temperature-sensing resistor element Z1 and 
the external connection pads (91a) and (91b) are also formed by depositing 
or crystal growing metal such as platinum or tungsten as in the 
temperature-sensing resistor element Z1. 
The semiconductor substrate 83 is formed of a sufficiently large size so 
that the external connection pads (91a) and (91b) appear on the surface of 
the substrate when it is joined together with the semiconductor substrate 
82, and the upper surface of the space 87 aligns with the plane of the 
cavity 86 in the semiconductor substrate 82. 
The semiconductor substrate 84 is formed with a heater H, two pairs of 
temperature-sensing resistor elements X1 and X2, and Y1 and Y2 facing the 
heater H and respectively disposed in the directions of the X and Y axes, 
then external connection pads (93a, 93b, 94a, 94b, 95a, 95b, 96a, 96b, 
97a, and 97b) for connecting the heater H, and the temperature-sensing 
resistor elements X1, X2, Y1, and Y2 to the outside, and a space 88 
extending through the semiconductor substrate 84 in the Z axis. 
The heater H is formed so as to be positioned at the center of the space 88 
on the top of the semiconductor substrate 84. 
The heater H, and the pairs of temperature-sensing resistor elements X1 and 
X2, and Y1 and Y2 are formed by depositing and etching metal such as 
platinum and tungsten on a bridge (not shown) which is formed by etching 
as in the temperature-sensing resistor element Z1 on the semiconductor 
substrate 83. 
The heater H and the temperature-sensing resistor elements X1, X2, Y1, and 
Y2 differ in that the resistance of the heater H is lower than the 
temperature-sensing resistor elements X1, X2, Y1, and Y2. 
A lead pattern is also formed by deposition or the like and etching metal 
such as platinum or tungsten for electrically connecting the heater H and 
the temperature-sensing resistor elements X1, X2, Y1, and Y2 to the 
external connection pads (93a, 93b, 94a, 94b, 95a, 95b, 96a, 96b, 97a, and 
97b). 
The space 88 is formed in the same size as the space 87 to pass through the 
Z axis. 
The semiconductor substrate 84 is formed of a sufficiently large size so 
that the external connection pads (93a, 93b, 94a, 94b, 95a, 95b, 96a, 96b, 
97a, and 97b) appear on the surface of the substrate when it is joined 
together with the semiconductor substrate 83, and the upper surface of the 
space 88 aligns with the plane of the space 87 in the semiconductor 
substrate 83. 
The semiconductor substrate 85 is formed with a temperature-sensing 
resistor element Z2 positioned downward in the direction fo the Z axis, 
external connection pads (92a) and (92b) for connecting the 
temperature-sensing element Z2 to the outside, and a space 89 which forms 
the bottom of the case of the sensor 81 in the downward direction of the Z 
axis. 
The temperature-sensing resistor element Z2 is arranged to be paired with 
the temperature-sensing resistor element Z1 on the semiconductor substrate 
83 with respect to the heater H on the semiconductor substrate 84. 
The temperature-sensing resistor element Z2 is formed in a manner similar 
to that for the temperature-sensing resistor element Z1 on the 
semiconductor substrate 83. The semiconductor substrate 85 is formed of a 
sufficiently large size so that the external connection pads (92a) and 
(92b) appear on the surface of the substrate when it is joined together 
with the semiconductor substrate 84, and the upper surface of the space 89 
aligns with the plane of the space 88 in the semiconductor substrate 84. 
After the semiconductor substrates 82-85 are completely arranged, the 
semiconductor substrates 83, 84 and 85 are stacked and joined in the 
direction of the Z axis, so as to align the spaces 86-89, and a 
pressurized gas with a low heat transfer coefficient such as nitrogen or 
argon is introduced into the space while joining the semiconductor 
substrate 82 to form a pyramid-shaped acceleration sensor 81. 
Positioning recesses may be provided in the upper surface of the 
semiconductor substrates 83-85 for accurately joining the semiconductor 
substrates 82-85. 
Although the embodiment is described for a three-axis (X, Y, and Z axes) 
acceleration sensor, a two-axis (X and Y axes) acceleration sensor can be 
constituted by eliminating the semiconductor substrates 83 and 85, and 
providing a bottom on the space 88 in the semiconductor substrate 84 
similar to the space 89 in the semiconductor substrate 85. 
The thus-constituted acceleration sensor 81 can attain accurate positioning 
of the heater H and the temperature-sensing resistor elements X1, X2, Y1, 
Y2, Z1, and Z2 by using a fine-processing technique in the semiconductor 
manufacturing process so that alignment can be accurately achieved. 
Also, since the temperature-sensing resistor elements X1, X2, Y1, Y2, Z1, 
and Z2 are formed on the same semiconductor substrate or the semiconductor 
substrates of the same production lot, the acceleration sensor 81 can be 
provided with good pairing (resistivity and temperature coefficient) of X1 
and X2, Y1 and Y2, and Z1 and Z2. 
Furthermore, because the acceleration sensor 81 is formed by the 
fine-processing technique in the semiconductor manufacturing process, it 
can be produced in the minimum size necessary for a sensor so that 
miniaturization can be attained. 
FIGS. 22A and 22B are views of the external appearance of the acceleration 
sensor shown in FIG. 21. 
Referring to FIGS. 22A and 22B, the acceleration sensor 81 has the 
semiconductor substrates 82-85 described with respect to FIG. 21, and is 
constituted in a pyramid-shaped acceleration sensor by stacking and 
joining the semiconductor substrates 82-85 in the direction of the Z axis. 
Disposed in the closed space 90 formed by spaces 86-89 are the heater H, 
and the temperature-sensing resistor elements X1, X2, Y1, Y2, Z1, and Z2 
for detecting acceleration in the three axes (X, Y, and Z axes). The 
pressurized gas 91 with a low heat transfer coefficient such as nitrogen 
or argon is also enclosed in the space 90. 
In addition, disposed on the surface of the acceleration sensor 81 are the 
external connection pads (bonding pads 91a-97b) of the temperature-sensing 
resistor elements X1, X2, Y1, Y2, Z1, and Z2 and heater H for connection 
with an external acceleration detector circuit (not shown). 
Detection of acceleration by the acceleration sensor 81 will be explained 
in the following section: 
FIG. 23 is a diagram illustrating the operation of the acceleration sensor 
shown in FIG. 21. 
Referring to FIG. 23, in the closed space 90 of the acceleration sensor 81 
shown in FIGS. 21, 22A, and 22B, the heater H is disposed at the center of 
the X-Y plane and at the center of the Z axis, and a pair of 
temperature-sensing resistor elements X1 (resistance R1) and X2 
(resistance R2) and a pair of temperature-sensing resistor elements Y1 
(resistance R3) and Y2 (resistance R4) are disposed opposite to each other 
with respect to the X and Y axes, respectively, in symmetry with the 
heater H (resistance R). 
In addition, a pair of temperature-sensing resistor elements Z1 (resistance 
R5) and Z2 (resistance R6) are disposed opposite to each other with 
respect to the Z axis in symmetry with the heater H (resistance R). 
The heater H generates Joule heating corresponding to the electrical power 
(V.sub.o.sup.2 /R or I.sub.o.sup.2 *R) by driving it with an external 
power supply (for example, a voltage source V.sub.o or a current source 
I.sub.o). 
The gas 91 is heated by Joule heating, and a temperature distribution 
inversely proportional to the distance from the heater H is created in the 
closed space 90 with a steep temperature gradient. 
Because the temperature-sensing resistor elements X1, X2, Y1, Y2, Z1, and 
Z2 are positioned at equal distances from the heater H in the X, Y and Z 
axes, respectively, when no acceleration (G) acts on the acceleration 
sensor 81, the temperature-sensing resistor elements are in an equal 
temperature environment and thermally balanced, and the resistances R1-R6 
are in the relationships of R1=R2, R3=R4, and R5=R6. 
When acceleration (G) occurs, for example, in the direction of the X (-X) 
axis as shown by arrow (G) in FIG. 23 in the thermally balanced state, the 
temperature distribution in the closed space 90 moves in the direction 
opposite to the direction of acceleration (G), the temperature balance 
between the temperature-sensing resistor elements X1 and X2 is destroyed 
to increase the temperature of the temperature-sensing resistor element 
X1, and to decrease that of the temperature-sensing resistor element X2. 
Because the increase in the temperature of the temperature-sensing resistor 
element X1 increased the resistance R1, while a decrease in the 
temperature of the temperature-sensing resistor element X2 decreases the 
resistance R2, when an arrangement is made to detect acceleration (G) with 
a value corresponding to the resistance difference (R1-R2) between the 
resistances R1 and R2, the resistance difference (R1-R2) becomes a 
positive value (R1-R2&gt;0) so that the magnitude of acceleration (G) can be 
detected from the value corresponding to the resistance difference, and 
the direction of acceleration (G) can be detected from the sign (+ or -) 
of the resistance difference, as described with respect to preceding 
embodiments of the present invention. 
In contrast, when acceleration (G) acts in a direction opposite to the 
direction of the arrow in FIG. 23, the resistance difference (R1-R2) takes 
a negative value (R1-R2&lt;0) so that the magnitude and the direction of 
acceleration (G) can be detected from a value corresponding to the 
resistance difference and its sign (+ or -), respectively. 
When acceleration (G) acts only in the direction of the X axis, the 
temperature-sensing resistor elements Y1 and Y2, as well as Z1 and Z2 in 
the directions of the Y and Z axes are thermally balanced, therefore both 
the resistance difference (R3-R4) and (R5-R6) becomes zero, and the 
temperature-sensing resistor elements are not affected by acceleration 
(G). 
Similarly, when acceleration (G) acts in the direction of the Y or Z axis, 
the thermal balance is destroyed in the temperature-sensing resistor 
elements Y1 and Y2 or Z1 and Z2 positioned in the direction of 
acceleration (G) so that the acceleration (G) can be detected from a value 
corresponding to the resistance difference (R3-R4) or (R5-R6) and their 
sign (+ or -). 
FIGS. 24-27 are diagrams of acceleration detector circuits of the 
acceleration sensor shown in FIG. 21. 
FIG. 24 shows a diagram of the heater driving circuit, FIG. 25 is a diagram 
of the acceleration detector circuit in the direction of the X axis, FIG. 
26 is a diagram of the acceleration detector circuit in the direction of 
the Y axis, and FIG. 27 is a diagram of the acceleration detector circuit 
in the direction of the Z axis. 
Although FIGS. 24-27 show examples of the driving power supply which is 
constituted by a current source (I.sub.o), the driving power supply may be 
constituted by a voltage source (V.sub.o). 
The heater driving circuit shown in FIG. 24 supplies current to the 
resistance R of the Heater H from a current source (I.sub.o) to generate 
Joule heating corresponding to the electrical power (I.sub.o.sup.2 *R). 
The acceleration detector circuit in the direction of the X axis shown in 
FIG. 25 constitutes a resistor bridge circuit with the temperature-sensing 
resistor element X1 (resistance R1) and X2 (resistance R2), and standard 
resistors Ro1 and Ro2, and is arranged to output difference V.sub.oX 
(V.sub.x1 -V.sub.x2) based on the bridge output voltage V.sub.x1 and 
V.sub.x2 through a differential amplifier A.sub.x, and to obtain a 
detection output corresponding to acceleration (G) acting in the direction 
of the X axis. 
FIGS. 26 and 27 similarly constitute a resistor bridge circuit, and are 
arranged to output difference V.sub.oY (V.sub.Y1 -V.sub.Y2) and V.sub.oZ 
(V.sub.Z1 -V.sub.Z2) based on the bridge output voltage V.sub.Y1 and 
V.sub.Y2, as well as V.sub.Z1 and V.sub.Z2, and to obtain a detection 
output corresponding to acceleration (G) acting in the direction of the Y 
or Z axis direction. 
As described, because the absolute values and the directions of 
acceleration (G) acting in the X, Y and Z axis directions can be detected 
from the difference (V.sub.oX, V.sub.oY and V.sub.oZ) and their sign (+ or 
-), it is possible to detect acceleration (G) acting in any of three 
dimensional directions by providing a calculation means for calculating 
the mean square .sqroot.(V.sub.oX.sup.2 +V.sub.oY.sup.2 +V.sub.oZ.sup.2) 
and an orientation determination means for determining the quadrant of a 
three-dimensional XYZ coordinate system from the sign (+ or -) of each 
difference. 
As described above, since the acceleration sensor 81 of FIG. 21 is arranged 
by forming the sensor case with the closed space, the heaters, a pair of 
temperature-sensing resistor elements disposed in each of multiple axis 
directions on a plurality of separate semiconductor substrate with the 
semiconductor manufacturing process, and by joining the plurality of 
semiconductor substrates in one direction, it is possible to easily align 
the plurality of temperature-sensing resistor elements with the multiple 
axes which are orthogonal to each other, and to form the 
temperature-sensing resistor elements with matched characteristics so that 
a very small acceleration sensor can be attained with less variation and 
higher accuracy. 
Furthermore, since the acceleration sensor 81 employs pressurized gas with 
a low heat transfer coefficient as the gas to be enclosed in the closed 
space, it is possible to crease a steep temperature gradient and to attain 
a sensor with high sensitivity. 
As described above in detail for various embodiments, because embodiments 
of the present invention very finely process the semiconductor substrates 
and the like with a fine-processing technique in the semiconductor 
manufacturing process, accurately form the sensor case, the heater, the 
temperature-sensing resistor element, the heat-type temperature-sensing 
resistor element and the like, and accurately determine their relative 
placement, a miniaturized and economic acceleration sensor is provided 
which can detect acceleration acting on the sensor with a high accuracy. 
Furthermore, because embodiments of the present invention enclose a 
pressurized inert gas with a low heat transfer coefficient in the closed 
space formed by the sensor case, an acceleration sensor is provided which 
can detect acceleration acting on the sensor with a high sensitivity by 
increasing the temperature gradient in the closed space. 
Furthermore, because embodiments of the present invention dispose a pair of 
temperature-sensing resistor elements with good pairing (resistivity, and 
temperature coefficient) in the direction of acceleration, it can provide 
an acceleration sensor which can detect the absolute value and the 
direction of acceleration more accurately. 
Furthermore, because one disclosed embodiment the present invention 
disposes a pair of temperature-sensing resistor elements with good pairing 
(resistivity, and temperature coefficient) in each of the 
three-dimensional axis directions, it can provide an acceleration sensor 
which can accurately detect acceleration acting in any of the 
three-dimensional directions.