Stress isolated semiconductive pressure sensor

Abstract: An electronic pressure sensor (10) is enhanced by attaching a sensor die (18) to a stress isolation platform (12) using an adhesive (42) having a similar thermal coefficient of expansion. The adhesive provides a hermetic seal between the stress isolation platform and the pressure sensor die. A via (20) in the stress isolation platform provides an opening for pressure to be applied to the sensor die. The stress isolation platform is attached to a plastic package body (16) via a semi-rigid adhesive (40) for providing stress isolation and a hermetic seal between the package body and the stress isolation platform. Any hostile chemical entering the via contacts an exposed diaphragm (50) of the sensor die to assert pressure against its piezoelectric network (52) to generate the electrical signals representative of the applied pressure but are keep away from the sensitive interconnects by the hermetic seals.

Description: Background of the Invention The present invention relates in general to pressure sensors and, more particularly, to an electronic pressure sensor with the sensing die protected from hostile environments by a hermetically sealed stress isolated platform. By 1996, federal government regulations will require that automotive fuel system vapor pressure sensors be placed in every automobile and light truck sold in the United States. The petrochemical environment of a fuel system is extremely hostile. It is known that mechanical pressure sensors can be manufactured to be impervious to adverse chemical environments. However, the mechanical sensor is very expensive and characterized by a bulky metallic package with a strain gauge-piezoelectric sensor. The size and cost of such a sensor make it impractical for use in automotive applications. Due to cost, size, and reliability advantages of integrated electronics, it is desirable to adapt electronic sensor technology to petrochemical environments in automobile applications. Electronic pressure sensors are used in ambient and atmospheric applications. However, adaptation of electronic pressure sensors to petrochemical environments presents new problems to the technology. One such problem lies in the integrity of the sensor associated interconnects. Present uses of electronic pressure sensors permit intimate contact between sensor wirebonds, package leads, and the sensed non-hostile pressure medium. Any contact with hostile chemicals has proven to cause premature failure of the sensor. It is further desired to extend electronic pressure sensor technology to higher pressure applications involving gases. In this instance, the fragile nature of the sensing die and interconnects creates stress problems that often lead to untimely failure of the sensor. European patent application no. EP-A-0033749, German patent application no. DE-OS-4227893 and European patent application no. EP-A-0509798 provide examples of different types of pressure sensors. Hence, a need exists for a low cost and reliable hermetically sealed pressure sensor that can be used in hostile chemical and pressure environments. Brief Description of the Drawings FIG. 1 illustrates a top view of a hermetically sealed electronic pressure sensor; FIG. 2 illustrates a cross sectional view of the electronic pressure sensor of FIG. 1; and FIG. 3 illustrates further detail of sensor die of FIG. 2. Detailed Description of the Preferred Embodiment The present invention provides a hermetically sealed electronic pressure sensor for detecting a wide range of fluid or gas pressures and supplying an electrical signal representative of the applied pressure. A pressure sensitive semiconductor die is attached to a stress isolation platform using an adhesive that has matching thermal coefficient of expansion and provides a hermetic seal between the platform and the sensor die. The stress isolation platform includes a via port that provides an opening for pressure to be applied to the semiconductor sensor die. The stress isolation platform is attached to a package body with a semi-rigid adhesive providing stress isolation and a hermetic seal between the plastic package body and the stress isolated platform. Subsequent electrical connections via wirebonds are made between the sensor die and the package leads to carry electrical signals to external connections on the package body. The pressure sensor device is capable of sensing wide ranges of pressures from various types of fluids and gases, that are commonly considered to be hostile to the components of the sensor, without affecting reliability or performance the device. Examples of such hostile chemicals include calcium chloride, copper, distilled water, ethyl alcohol, ethylene glycol, formic acid, hydraulic fluid, indolene, iso-octane, methanol, sodium chloride, nitric acid, sulfuric acid, t-butyl hydroperoxide, tert-butyl methyl ether, toluene, xylene, and zinc. Referring to FIG. 1, a top view of pressure sensor 10 is shown including a stress isolation platform 12 peripherally mounted on edge 14 of package body 16. Sensor die 18 is mounted to the underside of stress isolating platform 12. Stress isolation platform 12 is shown as transparent so that edge 14 and sensor die 18 are visible from the top view. Via 20 located in the center of stress isolating platform 12 provides an opening for the external pressure to reach sensor die 18. Sensor die 18 is attached to stress isolated platform 12 with a glass bonding adhesive agent to provide a hermetic seal around via 20. Sensor die 18 senses a differential pressure between the pressure applied through via 20 and a reference pressure inside the cavity of package body 16. Sensor die 18 converts the sensed pressure to an electrical signal representative of the differential pressure across its body. Package body 16 is molded from a novalac epoxy and provides a housing and electrical interconnect for sensor die 18. Other potential materials for package body 16 include organic epoxies, elastomers filled with inorganic matter, and ceramic that provide polar and non-polar chemical resistance. Bondwires 22, 24, 26, and 28 are preferably made of gold and attached to aluminum bonding pads on the underside of sensor die 18 using conventional integrated circuit bonding techniques. Bondwires 22, 24, 26 and 28 are further electrically coupled to package body copper leads 30, 32, 34 and 36, respectively. Stress isolating platform 12 is composed of a material having a thermal coefficient of expansion similar to that of sensor die 18. Suitable materials for stress isolating platform 12 that provide stress isolation by matching thermal coefficients include borosilicate glass, various grades of alumina, cordierite, and steatite. Stress isolating platform 12 mounts on edge 14 of package body 16 using semi-rigid bonding adhesive agent 40 that hermetically seals the inner cavity of package body 16 to protect bonding wires 22-28 and a piezoelectric pressure sensing network (not shown in FIG. 1) located on the backside of sensor die 18 from hostile chemicals. Stress isolating platform 12 must also have non-porosity characteristics such that hermetic qualities are preserved while providing polar and non-polar chemical resistance. The material used to form stress isolating platform 12 preferably has the capability of molding or boring via 20. Turning to FIG. 2, a cross-sectional view of pressure sensor 10 is shown. A thermal adhesive agent 40 between stress isolating platform 12 and edge 14 provides stress isolation by allowing independent thermal expansion of package body 16 and provide polar and non-polar chemical resistance. Suitable materials for adhesive agent 40 that provide stress isolation by matching thermal coefficients include rigid and semi-rigid epoxy and elastomers. Sensor die 18 is attached to the underside of stress isolating platform 12 using glass bonding adhesive agent 42 around its perimeter. Adhesive 42 must also have non-porosity characteristics to maintain the hermetic seal and stress matching between stress isolating platform 12 and sensor die 18. Adhesive 42 is preferably a material such as lead titanate glass and various grades of lead borate glasses with thermal coefficients of expansion that are the same as sensor die 18 and stress isolating platform 12 while providing polar and non-polar chemical resistance. Bondwire 26 in FIG. 2 is coupled between a conventional bond pad on the backside of sensor die 18 and package body lead 34. The backside of package body 16 includes a pressure inlet 48 open to the inner cavity of pressure sensor 10. Pressure inlet 48 is typically connected to an external reference pressure source. The external gauge pressure is applied against the backside of sensor die 18 so as to measure a differential pressure between that applied through via 20 and the pressure entering inlet 48. FIG. 3 illustrates further detail of sensor die 18. Sensor die 18 is made from silicon with one side micro-machined to provide transducer diaphragm 50. A piezoelectric pressure sensing network 52 is formed on the underside of sensor die 18. Transducer diaphragm 50 flexes upon application of a fluid or gaseous differential pressure causing a change in the resistance film of piezoelectric network 52 and results in a corresponding change in the current flow in the respective resistor. Current tap points or electrical connections are made along the resistive path of piezoelectric network 52. The electrical connections are defined at the juncture of the attachment point of bondwires 22-28 and sensor die 18 at conventional bond pads. The change in current flow appears as a potential difference on current taps of piezoelectric network 52. The current taps are electrically coupled via bondwires 22-28 to electrical package body leads 30-36, respectively. Piezoelectric network 52 comprises an implanted transducer, thin-film deposited resistors, evaporated metallization, and aluminum evaporated bond pads. The transducer, resistors, and bond pads are interconnected using the aluminum metallization. The function of piezoelectric network 52 is to sense the flexing of transducer diaphragm 50 in response to the applied differential pressure and to provide a variable resistance path as a function of the applied differential pressure for the direct current induced by the voltage applied between two of the package body leads. A key feature of the present invention is to protect piezoelectric network 52 and bondwires 22-28 and associated bonding pads from hostile chemicals introduced through via 20. The hermetic seal between edge 14 and stress isolation platform 12 using adhesive 40 and the hermetic seal between sensor die 18 and stress isolation platform 12 using adhesive 42 provide the needed isolation for piezoelectric network 52 and bondwires 22-28. The hostile chemicals entering via 20 contact diaphragm 50 to assert pressure against piezoelectric network 52 to generate the electrical signals as described above but are keep away from the sensitive interconnects by the hermetic seals. By now it should be appreciated that the pressure sensor includes a stress isolation platform hermetically sealed to the package for protecting the piezoelectric network and interconnects from harsh conditions. The pressure sensor package provides protection for the electronic pressure sensing die in many hostile environments including fuel systems, hydraulic systems, aggressive vapor and liquid chemicals such as cooling/heating systems, and chemicals that may be either polar, non-polar or combinations of both. A glass bonding agent adhesive with thermal coefficients of expansion that are the same as the sensor die and the stress isolating platform provide a hermetic seal and stress matching between the stress isolating platform and the sensor die. The via in the stress isolation platform provides access for pressure source to the sensor die. The pressure sensor package includes an inlet to gain access to the intemal cavity of the cavity to establish the differential pressure. Alternately, the sensor package may contain multiple sensor die sharing a common gauge pressure applied to the internal cavity through an inlet.

Claims: A pressure sensor (10) having a sensor die (18) mounted with a first adhesive (42) to a stress isolation platform (12) that has an opening (20) for receiving a first pressure, the stress isolation platform being located on an edge of a package body (16), characterized by : the sensor die (18) and the first adhesive (42) having thermal coefficients of expansion that are matching a coefficient of expansion of the stress isolation platform; and a seal formed by a second adhesive between the stress isolation platform and the edge of the package body to protect the sensor die located within an inner cavity of the package body. The pressure sensor of claim 1 wherein said package body includes a second opening (48) for receiving a second pressure to establish a differential pressure across said sensor die. The pressure sensor of claim 2 wherein said sensor die includes: a transducer diaphragm (50) that flexes in response to said differential pressure; a piezoelectric network (52) mounted to said transducer diaphragm that changes resistance in response to said flexing of said transducer diaphragm; and a plurality of bonding wires (22-28) coupled between tap points of said piezoelectric network and conductive leads of said package body. The pressure sensor of claim 1, 2 or 3 wherein the stress isolation platform is made of glass. A method of sealing a pressure sensor having a sensor die (18) mounted to a stress isolation platform (12) that has an opening (20) for receiving a pressure, the stress isolation platform being located on an edge of a package body (16), wherein the method comprises the steps of: selecting a first adhesive (42) having a thermal coefficient of expansion that matches coefficients of expansion of the stress isolation platform (12), the sensor die (18) and the stress isolation platform for mounting the sensor die (18) over the opening of the stress isolation platform; and sealing the stress isolation platform to the package body with a second adhesive to protect the sensor die located within an inner cavity of the package body. The method of claim 5 further including the step of providing a second opening (48) in said package body for receiving a second pressure to establish a differential pressure across said sensor die.

Inventors: BROWN CLEM H, VOWLES DAVID L, BROWN, CLEM H., VOWLES, DAVID L.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1995
Main classification: G01L9
IPC codes: H01L29, G01L9