Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Physical deformation
Reexamination Certificate
1999-12-29
2001-03-13
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Physical deformation
C257S417000, C257S666000, C257S787000
Reexamination Certificate
active
06201285
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a sensor with a diaphragm sensor chip.
Acceleration sensors using diaphragm type pressure sensitive sensor chips have been proposed.
A pressure sensor chip used in such acceleration sensors has a relatively thick base and thin diaphragm. Stain gauges are formed on the surface of the diaphragm by thin-film forming technology. The strain gauges form a bridge circuit. The base of the sensor chip is flexibly adhered to a die pad having lead frames with an adhesive. The lead frames are entirely molded over with a mold material. A space exists about the sensor chip, which is die bonded. The space is filled with silicone gel, which seals the chip.
Each material of the sensor has its own property of thermal deformation. Metal used for the sensor chip and the lead frames has a relatively low coefficient of linear expansion. Resin used for the mold member and the adhesive has a greater coefficient of linear expansion than that of the metal. Great differences of thermal deformation among the materials tends to apply stress to the diaphragm when the ambient temperature changes. This deforms the strain gauges and changes the resistance of the gauges. As a result, the sensitivity of the sensor is changed. Accordingly, the temperature characteristics of the sensor deteriorate.
A few measures have been introduced to address the above draw back. A first measure is to adhere a small ceramic substrate on the die pad and to flexibly attach the sensor chip on the ceramic substrate with elastomer. A second measure is to provide a compensation circuit outside the sensor for electrically compensating the sensor detection signals. These measures have decreased the ratio of change of sensitivity from ±10 to 30% to ±5%, which is within the tolerance.
However, the first measure requires a ceramic substrate and therefore increases the material cost: In addition, the first measure requires a manufacturing step for adhering the ceramic substrate and therefore complicates the assembly. This lowers the production efficiency.
The second measure requires a compensation circuit and thus increases the cost.
Accordingly, it is an objective of the present invention to provide a sensor having a diaphragm sensor chip that is inexpensive and easy to manufacture, and the sensitivity of which is not greatly affected by temperature.
SUMMARY OF THE INVENTION
To achieve the forgoing and other objectives and in accordance with the purpose of the invention, a sensor is provided. The sensor has a lead frame including a die pad, and a sensor chip flexibly secured on the die pad by adhesive. The sensor chip includes a diaphragm and outputs signals corresponding to strain amount of the diaphragm. A mold member molds the lead frame and defines a space about the sensor chip. The coefficient of linear expansion of the mold member is between 1.4×10
−5
/° C. and 1.8×10
−5
/° C., and the modulus of elasticity of the mold member is between 100×10
3
kg/cm
2
and 130×10
3
kg/cm
2
.
A relatively great volume of the sensor is occupied by a mold member. Therefore, thermal stress generated in the mold member greatly affects the magnitude of thermal stress acting on a diaphragm. In this invention, the mold member has proper characteristics (the coefficient of linear expansion and the modulus of elasticity). Therefore, the difference of the thermal stress amounts among the parts is decreased, which eliminates deformation of the diaphragm. Thus, the ratio of change of sensitivity due to temperature changes is minimized.
Conventionally, a sensor chip is flexibly adhered to a ceramics substrate to decrease the ratio of change of sensitivity due to temperature changes. Alternatively, a separate compensation circuit is connected to a sensor. The present invention reduces the ratio of change of sensor sensitivity due to temperature changes without using these conventional measures. This reduces the manufacturing cost of the sensor and facilitates the manufacture.
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Iwata Hitoshi
Kanbe Masakata
Clark Sheila V.
Kabushiki Kaisha Tokai Rika Denki Seisakusho
Nawrocki, Rooney & Sivertson P.A.
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