Patent Application
20240302237
The subject disclosure is directed to pressure sensors, and more particularly, to a high temperature piezo-resistive pressure sensor and a packaging assembly for a high temperature piezo-resistive pressure sensor.
MEMS pressure sensors have a wide range of useful applications in many commercial and industrial systems including aerospace, automotive, oil and gas drilling and consumer electronics. In most applications, MEMS pressure sensors convert a pressure signal to an electrical signal through a piezo-resistive effect. This sensing principle has significant advantages when employed in high temperature environments in terms of stability, signal processing convenience and accuracy.
In a high temperature environment, not only does the pressure sensor and its individual components have to survive and function correctly, the sensor packaging assembly must also be designed to withstand high operating temperatures. It would be beneficial to provide a piezo-resistive pressure sensor and a packaging assembly therefor that can perform reliably in a high temperature operating environment.
The subject disclosure is directed to a new and useful packaging assembly and method for a piezo-resistive pressure sensor that is suitable for use in a wide variety of high-temperature applications and environments including aerospace, consumer electronics, oil and gas drilling and industrial controls.
The assembly includes a pressure sensor having an upper substrate and a lower substrate that are bonded to one another by way of a first glass frit having a first bonding temperature, so as to define a hermetically sealed pressure sensing chamber therebetween. The assembly further includes a housing defining an internal cavity having a base with a support surface for supporting the pressure sensor. Preferably, the housing is formed from a stainless steel material and the base is formed from a material such as Kovar (an iron-nickel-cobalt alloy) or the like.
The pressure sensor is bonded to the support surface of the base by a second glass frit having a bonding temperature that is lower than the bonding temperature of the first glass frit. In an embodiment of the subject disclosure, the lower substrate of the pressure sensor is bonded to a compliant pedestal substrate by way of a third glass frit having a bonding temperature equal to the bonding temperature of the first glass frit. The pedestal substrate provides a stress buffer for the pressure sensor.
A diaphragm is operatively associated with the pressure chamber for converting a mechanical stress to an electrical signal indicative of a sensed pressure. The diaphragm has at least a piezo-resistive layer thereon made from a single silicon material or a polysilicon material. Alternatively, the diaphragm may be made entirely from a single silicon material or a polysilicon material. In certain embodiments of the subject disclosure, the pressure sensor and the housing are adapted and configured to sense an absolute pressure. In other embodiments of the subject disclosure, the pressure sensor and the housing are adapted and configured to sense a differential pressure.
The housing of the packaging assembly includes a pair of electrical connectors for electrically communicating with the diaphragm. In certain embodiments of the subject disclosure, bonding wires extend between the electrical connectors of the housing and a conductive bond pad associated with the upper substrate, and the conductive bond pad communicates with electrical conductors connected to the diaphragm by way of a metallic layer deposited between the upper and lower substrates outboard from the first glass frit. The metallic layer has a melting temperature that is less than the first bonding temperature. The metallic layer is formed from an alloy such as, for example, a gold alloy or the like, having a melting temperature that is less than the first bonding temperature.
In another embodiment of the subject disclosure, bonding wires extend between the electrical connectors of the housing and a conductive bond pad associated with the upper substrate, and the conductive bond pad communicates with electrical conductors connected to the diaphragm by way of separate bonding wires outboard from the first glass frit. In yet another embodiment of the subject disclosure, bonding wires extend between the electrical connectors of the housing and electrical conductors connected to the diaphragm outboard from the first glass frit.
The subject disclosure is also directed to a high-temperature pressure sensor that includes an upper substrate, a lower substrate bonded to the upper substrate by way of a glass frit so as to define a hermetically sealed pressure sensing chamber therebetween, a diaphragm operatively associated with the pressure chamber for converting a mechanical stress to an electrical signal indicative of a sensed pressure, and a metallic layer deposited between the upper substrate and the lower substrate outboard from the glass frit. The metallic layer is electrically connected to the diaphragm and to a conductive bond pad associated with the upper substrate. In certain embodiments of the subject disclosure, the pressure sensor is adapted and configured to sense an absolute pressure. In other embodiments of the subject disclosure, the pressure sensor is adapted and configured to sense a differential pressure.
In an embodiment of the subject disclosure, the lower substrate of the pressure sensor is bonded to a compliant pedestal substrate by way of a glass frit having a bonding temperature equal to the bonding temperature of the glass frit glass frit used to bond the upper and lower substrates together. The pedestal substrate provides a stress buffer for the pressure sensor. The metallic layer is a gold alloy or the like having a melting temperature that is less than the bonding temperature of the glass two frits, and the diaphragm has at least a piezo-resistive layer thereon made from a single silicon material or a polysilicon material.
These and other features of the high-temperature piezo-resistive pressure sensor of the subject disclosure of the packaging assembly therefor will become more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the drawings.
So that those skilled in the art will readily understand how to make and use the pressure sensors of the subject disclosure without undue experimentation, embodiments thereof will be described in detail herein below with reference to the figures wherein:
Referring now to the drawings wherein like reference numeral identify similar features or components of the subject disclosure, there is illustrated in
Referring to
A piezo-resistive diaphragm 20 is operatively associated with the pressure chamber 18 for converting a mechanical stress to an electrical signal indicative of a sensed pressure. The diaphragm 20 has at least a piezo-resistive upper layer thereon made from a single crystal silicon material or a polysilicon material. Alternatively, the diaphragm 20 may be made entirely from a single crystal silicon material or a polysilicon material.
A patterned metallic layer 22 is deposited between the upper substrate 12 and the lower substrate 14 outboard from the glass frit 16. The metallic layer 22 is electrically connected to the diaphragm 20 by way of metal connectors 24a, 24b and to a conductive bond pad 26 that is associated with the upper substrate 12. More particularly, the conductive bond pad 26, which his deposited on the top surface of the upper substrate 12 extends through a series of passages or vias 28 that are outboard from the pressure chamber 18, so as to connect with the metallic layer 22. As best seen in
The patterned metallic layer 22 has a melting temperature that is less than the bonding temperature of the glass frit 16. Thus, it will advantageously melt and form a conductive path between the bond pad 26 and the connectors 24a, 24b before the glass frit 16 begins to melt.
Preferably, the patterned layer 22 is a gold-tin alloy (AuSn) which will begin to melt at 280° C. and the glass frit 16 is formed from a material that has a bonding temperature of about 400° C. These dual bonding materials will achieve both hermetic vacuum sealing and electrical connection. More particularly, the glass frit bonding achieves the vacuum and the AuSn bonding achieves the electrical connection.
Adhesion and barrier layers are preferably associated with piezo-resistive diaphragm 20 and the patterned AuSn layer. The barrier layer could be Nickel (Ni) and the adhesion layer could be Chromium (Cr) or Zirconium (Zr). Preferably, a dielectric insulation layer 30 is deposited on the top surface of the lower substrate, beneath the diaphragm, and the connectors 24a, 24b. This insulation layer can be Silicon Oxide (SiO2), Silicon Nitride (SiN) or both.
Pressure sensor 10 is adapted and configured to sense an absolute pressure. More particularly, there is a recessed cavity 32 etched into the bottom surface of the lower substrate 14 for pressure income. The incoming pressure exerts a force on the substrate 14, within the cavity 32, below the diaphragm 20. This causes the diaphragm 20 to deflect, which in turn generates a mechanical stress on the piezo-resistive surface layer of the diaphragm 20. More particularly, the piezo-resistive surface layer defines a Wheatstone bridge circuit that converts the mechanical stress to an electrical signal indicative of an absolute pressure measurement.
Referring now to
Once the base 54 is secured, the pressure sensor 10 is bonded to the upper surface of thereof by a second glass frit 58, which has a bonding temperature that is lower than the bonding temperature of the glass frit 16 that hermetically bonds the upper substrate 12 to the lower substrate 14 during the die fabrication process. Thus, during package assembly, the first glass frit 16 will not reflow. It should be understood that the maximum operating temperature of the packaged sensor assembly will be effectively limited by the bonding temperature of the second glass frit 58. However, by proper material selection this temperature can withstand operating temperatures of 350° C. or more. Moreover, because the package assembly is constructed from materials, such as Kovar® and stainless steel, it can tolerate operating temperatures of 350° C. or more without concern.
After securing the pressure sensor 10 to the supporting surface of the base 54, a stainless steel ring 60 is welded to the top of the housing 50. The ring 60 has a pair of through bores 62a, 62b to accommodate a pair L-shaped external electrical connection pins 64a, 64b. Glass insulators 66a, 66b support the pins 64a, 64b within bores 62a, 62b.
A central access port 68 is formed in the ring 60 to enable electrical connection of the connector pins 64a, 64b to the conductive bond pad 26 on the upper substrate 12 by way of bond wires 70a, 70b. A stainless steel spacer disc 72 is installed above ring 60 to close the central access port 68 in the disc 60. Thereafter, a final ring clamp 74 is threaded to the top of the housing 50 to secure all of the packaging components of together and finish the assembly. It is envisioned that the interior cavity 52 of housing 50 could be filled with filling material, such as Ecofoam®, to stabilize the bonding wires 70a, 70b.
Referring now to
Referring now to
Referring to
While the subject disclosure has been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the scope of the subject disclosure.
