Apparatus for Mounting an Absolute Pressure Sensor

The present invention is directed toward solving the problem of stress related to the packaging configuration for a diaphragm-type absolute pressure sensor which includes a vacuum reference.

The prior art is descriptive of a silicon based absolute sensor structure which employs implantable semiconductor strain sensitive resistors (utilizing the commonly referred to piezoresistive effect), a stress isolating structure, for example, a Pyrex tube to which the silicon pressure die is anodically bonded, and a vacuum reference, which is made during encapsulation of the sensor within the sensor package. The vacuum reference is maintained over time by the use of a discrete getter attached to the cover of the sensor package and "activated" during the encapsulation process. The package provides the mechanical pressure port interconnect between the strain sensor and the outside world and the electrical interconnect between the silicon piezoresistors (strain resistors) on the silicon die and the outside world.

The strain sensitive piezoresistors are arranged on the silicon die and typically interconnected so as to form a full Wheatstone bridge on one surface. The underside of the silicon die is usually etched to form a thin diaphragm, in this case circular. The edges of the diaphragm correspond to the location of the piezoresistors, where strain is the greatest when the diaphragm undergoes bending as a result of pressure loading on one side. The desired sensitive axis is perpendicular to the diaphragm.

Since the sensor is also sensitive to package induced strain effects, a mechanical strain isolator must be employed to maintain pressure sensitivity selectivity. A Pyrex tube or washer, or a secondary and/or tertiary silicon strain isolation chip is often employed between the sensor and package.

The absolute pressure sensor accuracy over time and temperature is a function of the quality of the vacuum. The lower and more stable the vacuum, the better the accuracy and repeatability of the transducer over time. The vacuum is established during package cover seal at which time a getter is installed and activated. The cover is usually sealed by laser welding in a vacuum.

The attachment of the sensor die (chip) to the Pyrex tube, insertion of the chip/tube into the package, and vacuum processing of the package is done in discrete steps, which are labor intensive and therefore expensive.

Thus, a need exists for a reduced-cost apparatus for isolating a diaphragm-type absolute pressure sensor from the stress introduced by the sensor package.

The present invention solves these and other needs by providing a stress-isolating apparatus for mounting an absolute pressure sensor having a self-contained vacuum reference, including a housing surrounding a cavity with the cavity having opposing and having a pressure port. In one embodiment, a vacuum is pulled on the pressure port to hold the pressure sensor against the pressure port while wire bonds are made from the pressure sensor to the internal leads of the package. Alternatively a sublimeable adhesive may be used to hold the pressure sensor to the housing during wirebond. A drop of adhesive is then placed on the central portion of the housing cover and when the cover is received on the housing base, the adhesive contacts the pressure sensor. As the adhesive cures, it moves the pressure sensor so that it is spaced from the housing and is supported only by the adhesive.

FIG. 1 is a sectional drawing of an absolute pressure sensor subassembly.

FIGS. 2-6 are sectional drawings illustrating an apparatus in accordance with the present invention for mounting the absolute pressure sensor of FIG. 1.

FIG. 7 is a first alternate embodiment of the present invention.

FIG. 8 is a second alternate embodiment of the present invention.

FIG. 9 is a third alternate embodiment of the present invention.

An apparatus for mounting an absolute pressure sensor including a self-contained vacuum reference within a housing is shown in the drawings and is generally designated 10. A general description of the wafer level processes to provide pressure sensor die 20 will be provided now.

To start with, the aforementioned vacuum processing, formerly done at the packaging level, is now proposed to be performed at the wafer level, before wafer saw, where the vacuum reference is now considered part of the sensor die structure. The vacuum would be encapsulated by bonding within a vacuum a secondary wafer of silicon or another suitable material to the top of a sensor wafer. FIG. 1 shows the resulting sensor subassembly or sensor die 20 which includes a lower portion or base 22 having a thinned central diaphragm 24 and vacuum capsule or top cap 30 which is bonded to base 22. Base 22 has an upper surface 26 and a lower surface 28 and top cap 30 is bonded to upper surface 26. Upper surface 26 also includes bonding pads 32. Vacuum cavities 34 would be formed in the top cap wafer corresponding in position to the diaphragms 24 in the sensor wafer. Top cap 30 has top surface 36. The aforementioned getter used in the past would be replaced by a suitable getter material 38 sputtered to the inside of the vacuum cavity wafer prior to attachment to the sensor wafer. The bonding of the two wafers in a vacuum would be performed using standard anodic techniques or glass reflow techniques currently in practice. These require first the sputtering of thin film soft glasses, or anodically bonded thick film glasses on the vacuum cavity wafer that act as the adhesive layer, binding the two wafers together. The bonding itself requires heat, which serves to activate the getter by driving off adsorbed gases. The "clean" getter surface is now prepared to adsorb gases released over time by the sensor material in the encapsulated vacuum cavity 34. While the vacuum encapsulation process has been described with reference to bonding a secondary silicon wafer, it is not limited to this. For example, Pyrex or glass, or other materials having thermal expansion properties that match closely with silicon may be used. For example, materials such as sapphire or ruby may be used. The material to be used must be amenable to etching. One material having suitable properties is a glass identified as HOYA SD-2.

Housing 40 includes body 42 and cover 44. Body 42 as shown in FIG. 2 has a pressure connection 46, a passage 48 in which the pressure to be measured is present, and a chamber 50. Pressure port 52 is within chamber 50 and is surrounded by surface 54. Pressure port 52 makes the pressure to be measured available within cavity or chamber 50. Leads 56 extend through body 42 and have external end portions 58 and internal end portions 60. Body 42 includes peripheral surface 62.

Cover 44 as shown in FIG. 4 has a central portion 65 including central surface 66 and a peripheral portion 68 including surface 70. Surface 62 of body 42 is configured for complementary receipt of surface 70 of cover 44. For example, by providing recess 72 in surface 62 and protrusion 74 in surface 70.

After batch fabrication of the silicon sensor and vacuum capsule has been completed and the wafer sandwich has been sawed into discrete sensor subassemblies they are now ready for the next step, wire bonding. Sensor subassembly 20 is placed on body member 42 with its diaphragm 24 directly above pressure port 52 and clamped firmly in place by pulling a vacuum on pressure connection 46. Wire bonding performed in the usual known way results in wire bonds 76 as shown in connecting bonding in FIG. 3 pads 32 and internal leads 60. After completion of wire bonds 76, the vacuum is released allowing sensor subassembly 20 to "float" held in place only by wire bonds 76. Alternatively, sensor subassembly 20 could be held in place during wire bonding by using a sublimeable adhesive.

Next, cover 44 is prepared with a drop of adhesive 80 on central surface 66, as shown in FIG. 4. Adhesive drop 80 should be relatively inviscid, perhaps elastomeric, and have good wetting as well as high surface tension characteristics. It should shrink during cure. An example of a satisfactory adhesive material is Ablestik Laboratories, Rely-Imide 72254. The adhesive can be loaded with ceramic powders to alter the amount of shrinkage during cure. The shape of adhesive drop 80 is variable, as is the volume, to adjust the amount of lift-back. As Cover 44 is lowered onto body 42 as shown in FIG. 5 adhesive drop 80 contacts sensor subassembly 20 at top surface 36. During cure, adhesive drop 80 shrinks pulling sensor assembly 20 toward cover 44 and off of surface 54 where it was resting, exposing pressure port 52 to chamber 50. Adhesive drop 80 is located between fixed surface 66 and top surface 36. As curing occurs the contracting of adhesive drop 80 moves sensor subassembly 20 in a direction toward surface 66. When the curing process is completed, lower surface 28 will be in a position spaced from pressure port 52 and the pressure to be measured will be within chamber 50. Sensor subassembly 20 will then be supported by wire bonds 76 and adhesive drop 80 as shown in FIG. 6 and will be stress isolated from housing 40.

FIG. 7 is a first alternate embodiment of the apparatus 10, using plastic as the packaging material. FIG. 7 shows header or body 42a with leads 56a extending through header 42a internal end portions 60a and external lead portions 58a. Pressure sensor subassembly or die 22a includes vacuum reference cavity 34a. Header 42a includes a surface 54a surrounding pressure port 52a. Cover 44a includes a central surface 66a where adhesive drop 80a is placed and cover 44a is complementarily received on header 42a. Cover 44a can be ultrasonically or chemically bonded to header 42a.

FIG. 8 is a second alternate embodiment using a commonly available TO-type housing or metal package 40b with glass-metal electrical feedthroughs 84. Where the attachment method of cover 44b to body 42b at 86 can be either metal-metal welding or epoxy adhesive type. FIG. 8 shows sensor subassembly 20b having vacuum cavity 34b and top cap 30b having a top cap top surface 36b. Adhesive drop 80b is located between cover central surface 66b and top surface 36b.

FIG. 9 is a third alternate embodiment of the apparatus 10. Sensor subassembly 20 is of the same construction as in the other embodiments. Housing 40c includes a body 42c and a cover 44c. A pressure connection 46c to a passageway 48c extends through cover 44c into chamber 50c at pressure port 52c. Leads 56c have internal lead portions 60c. Body 42c includes a peripheral surface 62c and a central surface 63c. Cover 44c includes a peripheral surface 68c and a central surface 66c. FIG. 9 illustrates that the pressure port may be located in the cover as well as in the base. Thus, the first and second alternate embodiments could also have a pressure port in the cover. It is only necessary that a passageway for an external pressure extend into the cavity.

Now that the construction and operation of apparatus have been set forth, certain advantages can be explained and appreciated. The expensive vacuum processing which in the past was done at the time that the sensor die was placed in the housing is no longer necessary because the vacuum reference is on chip. The present invention simplifies the design by combining the vacuum encapsulation chip function and the packaging strain isolation function into one sensor subassembly 20, and assembling it in a few steps, amenable to automation, into an inexpensive nonhermetic package.

Apparatus 10 provides mounting of sensor subassembly 20 with low package induced strain due to the small bond area between top cap 30 and base 22 of subassembly 20 and the small bond area between adhesive drop 80 and top cap 30.

Also, the need for a hermetic package is obviated because the sensor subassembly is the hermetic element. Therefore, lower cost plastic packages and packaging epoxies which were precluded in the past because of high outgassing levels may now be used.

In addition, the mounting apparatus 10 requires only low cost, low precision, discrete assembly operations which are amenable to automated assembly.