Pressure Sensor

The present invention relates to a differential pressure sensor using the piezoresistivity effect, and more particularly to a differential pressure sensor that has a diaphragm on which a diffused resistor is formed as a pressure-sensitive element and to which a fluid pressure subject to measurement is introduced.

One conventional semiconductor differential sensor includes two absolute pressure sensor units each having a diaphragm, to which fluid pressures are applied for measurement, and a device for determining the difference between the absolute pressure outputs of the two sensor units. Another conventional semiconductor differential sensor is the type having a diaphragm to opposite surfaces of which fluid pressures subject to measurement are applied to produce an output representative of the difference between the two applied fluid pressures.

In FIG. 1 of the accompanying drawings is shown an absolute pressure sensor unit of the former type, generally designated by 10. This sensor unit includes a diaphragm block 12 having a diaphragm 14 and made of a silicon chip on the front surface 16 of which diffused resistors 18 are formed while the opposite or back surface 20 of the diaphragm is subject to the fluid pressure to be measured. The diaphragm block 12 is bonded to an alumina base 22 by means of a bonding layer 24 and covered by a cap 26 secured to the base 22 such that the front surface 16 of the diaphragm 14 on which the diffused layers 18 are formed is positioned within a vacuum chamber 28 formed by the base 22 and the cap 26. The base 22 has a hole 30 formed therethrough communicating with an inlet pipe 32 connected to the outside surface of the base. When a fluid pressure is introduced through the inlet pipe 32 and the hole 30 to back 20 of the diaphragm 14, the introduced pressure is converted to a corresponding electrical signal according to the stress-resistance characteristics of the diffused resistors 18. Accordingly, if two fluid pressures are introduced so as to arrive at the respective diaphragms 14 of two such sensor units, these sensor units will produce corresponding outputs from which the pressure difference can be obtained. However, use of two sensor units in this way leads to making the sensor necessarily bulky.

FIG. 2 shows one example of the latter differential sensor which has a single diaphragm 14 and which is structurally the same as the sensor of FIG. 1 except that it has an inlet pipe 34 connected to a through hole in the cap 26 through which another fluid pressure is introduced to the front 16 of the diaphragm 14. When two fluid pressures P.sub.1 `and P.sub.2 are introduced through the inlet pipes 32 and 34 to opposite surfaces of the diaphragm 16, the diffused resistors 18 on the diaphragm 16 produce an electrical signal corresponding to the difference between the introduced fluid pressures P1 and P2. This pressure sensor 10 is smaller than the sensor of FIG. 1, but since the diffused resistors 18 are exposed to the fluid of one of the pressures subject to measurement, they are liable to deteriorate due to moisture and corrosive gases which may be contained in the fluid.

Therefore, it is an object of the present invention to provide a semiconductor differential pressure sensor of a type having a diaphragm which is not exposed to a fluid whose pressure is subject to measurement, thereby resulting in improved durability.

Another object of the present invention is to provide a diaphragm type semiconductor differential pressure sensor which is of a compact structure.

Other objects, features and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings.

FIG. 1 is a cross-sectional view of a prior art semiconductor differential pressure sensor;

FIG. 2 is a cross-sectional view of another prior art semiconductor differential pressure sensor;

FIG. 3 is a cross-sectional view of the preferred embodiment of a semiconductor differential pressure sensor according to the present invention;

FIG. 4 is an exploded view of the embodiment of FIG. 3; and

FIG. 5 is a perspective view of the embodiment of FIG. 3.

The same reference numeral denotes corresponding parts or members throughout the drawings.

In FIGS. 3, 4 and 5 is shown a preferred embodiment of a pressure sensor 10 according to the present invention. This sensor 10 includes two semiconductor diaphragm blocks having diaphragms 14a and 14b, obtained by forming diffused resistors 18a and 18b on the front surfaces 16a and 16b of flat silicon chips using known techniques and etching away the opposite surfaces of the chips so as to form the back surfaces 20a and 20b of the diaphragms 14a and 14b to which fluid pressures subject to measurement are introduced, and circumferential supports 17a and 17b. These diaphragm blocks are bonded at their circumferential supports 17a and 17b to opposite inner sides of a box-like package 27 by means of bonding layers 24a and 24b so that the back surfaces 20a and 20b of the diaphragms face the opposite inside surfaces 36a and 36b respectively, of the package. The package 27 consists of two base plates 38a and 38b on which the diaphragm blocks 12a and 12b are mounted, and two circumferential spacers 40a and 40b are also mounted on the corresponding mounting plates 38a and 38b, respectively. The package 27 is provided with a plurality of terminals 42a and 42b connected to opposite sides thereof. The base plate 38a and spacer 40a are connected to each other using a conventional firing technique, and also the base plate 38b and spacer 40b are connected to each other in the same way. In order to prevent thermal stress from being produced in the diaphragms due to thermal change, materials which are of substantially the same linear coefficient of thermal expansion as the silicon chip may be used for the plates and the spacers: for example, mullite, zircon, Pyrex (trademark) glass or the like may be used. The diaphragm blocks 12a and 12b and the corresponding base plates 38a and 38b are joined by bonding layers 24a and 24b each of which may consist of a W (tungsten) or Mo-Mn (molybdenum-manganese) layer metallized on the corresponding base plate 38a or 38b, an Ni (nickel) layer plated thereon and an Au (gold) layer plated on the Ni layer. The Au layers and the corresponding diaphragm blocks form an Au-Si (gold-silicon) eutectic alloy at a temperature of 380.degree. to 400.degree. C. The base plates have inlet pipes 34a and 34b connected to through holes 35a, 35b formed at the respective centers of the plates 38a and 38b through which fluid pressures P1 and P2 are introduced to the respective back surfaces 20a and 20b of the diaphragms. On the spacers 40a and 40b leads 44a and 44b are W (tungsten) or Mo-Mn (molybdenum-manganese) metallized and Au (gold) plated and are electrically connected to the terminals 42a and 42b by means of solder 46a and 46b. The leads 44a and 44b are electrically connected to the diffused resistors 18a and 18b on the diaphragms 14a and 14b through Au (gold) wires 48a and 48b, respectively.

In fabrication, the spacers 40a and 40b are connected to the corresponding base plates 38a and 38b, using a firing technique. The diaphragm blocks 12a and 12b are bonded to the corresponding base plates. The leads and wires 44a, 44b and 48a, 48b are connected to the corresponding terminals 42a and 42b and diffused resistors 18a and 18b, respectively, and to each other. Then, the spacers 40a and 40b are aligned and bonded hermetically to each other by low melting point glass 50a and 50b within a vacuum container, not shown, such that the front surfaces 16a and 16b of the diaphragms on which the diffused resistors are formed are within an evacuated chamber 28 formed by the base plates 38a and 38b and spacers 40a and 40b.

A benifit of the structure of the embodiment described above, is that apart from the orientation of the leads the construction is substantially symmetrical. Thus exactly the same process can be used to manufacture each half of the sensor. Without being restricted to the embodiment described, this feature is widely applicable to the present invention and can provide a valuable saving in manufacturing costs.

When the whole package including the two diaphragm blocks 14a and 14b is mounted on a base, not shown, and the inlet pipes 34a and 34b are connected through hoses 52a and 52b to the two fluid pressures to be compared, the fluid pressures are exerted on the back surfaces 20a and 20b of the diaphragms, and the diffused resistors 18a and 18b produce outputs corresponding to the fluid pressures P1 and P2 to be compared. To obtain the difference between these two outputs results in the measurement of the differential pressure. If, for example, the two pressures are obtained from two different points in a fluid passage, the differential pressure sensor of the present invention can particularly conveniently be used to measure the pressure difference, and then the flow rate can be computed.

Although the invention has been disclosed and described in terms of a preferred embodiment, it will be apparent that other embodiments and modifications of the invention are possible within the scope of the present invention. Thus the scope of letters patent which are here petitioned for is not to be limited by any details of the above described embodiment, nor the appended drawings, but only by the claims which follow.