Hermetic attachment method for pressure sensors

A pressure sensor includes a sensor die coupled to a glass support tube and located within a metallic intermediate sleeve. The glass support tube is bonded or otherwise attached to the metallic intermediate sleeve with solder or other adhesive material. End portions of the glass support tube and the metallic intermediate sleeve are configured to have complementary, contoured surfaces such that at least a portion of an attachment layer located there between is placed in compression during operation of the pressure sensor.

BACKGROUND OF THE INVENTION

Many pressure sensors or transducers, such as micro-machined silicon pressure sensors, that require high reliability and stable performance for long periods of time are “absolute” pressure sensors that measure applied pressure relative to an integral reference vacuum reference chamber. Many are typically sealed within a hermetic hollow, outer body within which the reference vacuum resides. For example, a conventional high pressure silicon pressure sensor requires a hermetic attachment interface that connects an outer packaging element with an inner tube that supports the pressure sensing integrated circuit (IC) (i.e., pressure sensor “die”). Traditionally, the hermetic attachment interface includes solder or other adhesive material and takes the form of either a “butt” or a “sleeve” joint, as explained in greater detail below.

FIG. 1shows a conventional absolute pressure sensor100having a sensor die102coupled to a glass support member104. As illustrated, the glass support member104is adhered with an adhesive106to the package header124. A package cover108includes a pressure inlet port110. The sensor die102includes a sensor diaphragm112located between the pressure inlet port110and a vacuum reference chamber114. In addition, the sensor die in this typical embodiment includes an active side116that faces away from the vacuum reference chamber114. Electrical connections to and from the sensor die102are typically provided via bond wires118or an equivalent “lead frame” coupled to electrical interconnect pins120. The pins120are positioned within insulated seals122that extend through a header portion124of the package cover108. In this embodiment, the packaging material may be either metal or plastic since the only requirement for near-perfect hermeticity is for the seal between the sensor die112and the glass support member104. That is required to maintain the integrity and hence stability of the reference vacuum. This configuration, however suffers mainly from the exposure of the active side (side with circuitry and interconnects on it) of the sensor IC die to atmospheric contaminants such as moisture, dust, and other fluidic and particulate contaminants, many of which are conductive to some extent. Such conductive contamination bridging between the electrical interconnects on the surface of the die and the pins and the package induces stray conduction paths that cause errors in the measurement. In an attempt to counter that, many sensors use a flexible silicone gel or parylene overcoat125as a barrier to contamination.

The silicone gel approach has several disadvantages, for example: 1) It mass-loads the top of the diaphragm causing increased sensitivity to G forces; 2) the gel is hygroscopic and slowly absorbs water over time thereby negating its potential benefit; 3) the gel deteriorates with time and environmental exposure, thereby changing its physical characteristics; and 4) the gel adds extreme thermal and pressure hysteresis to the measurement, thereby limiting its use for precision applications.

Parylene, being a gaseous-deposition ultra-thin coating performs orders of magnitude better in regard to most of the errors it induces relative to the silicone gel, however, it still suffers the following drawbacks: 1) It has limited environmental compatibility vs. time, temperature, and compatibility with the pressure media, especially oxygen and ozone; and 2) the parylene film deposited over the sensor diaphragm creates both thermal and pressure hysteresis due to the mismatch of the thermal coefficient of expansion between materials.

FIG. 2shows a conventional differential pressure sensor200having a sensor die202coupled to a perforated glass support member204. The perforated glass support member204is adhered with an adhesive206to a package208. The package208includes a first pressure port210and a second pressure port212configured to permit a sensor diaphragm214to be displaced by the differential pressure acting on the diaphragm214. Electrical connections to and from the sensor die102are provided via bond wires218coupled to electrical interconnect pins220. The pins220are positioned within insulated seals222that extend through the header portion224of the package208. In this configuration, the higher pressure must always be applied to pressure port210because the adhesive206used to attach the glass support member to the package is often insufficiently robust in tension to maintain a seal at the interface between package224and the glass support member204if the higher pressure were to be applied to port212. In addition, a flexible silicone gel or parylene overcoat225operates as a barrier to contamination.

The main drawbacks for the conventional absolute and differential pressure sensors described above are the previously discussed performance, reliability and environmental robustness of the sensors due to their packaging approach.FIG. 3schematically shows a portion of a pressure sensor300that alleviates at least some of the previously discussed problems since the pressure sensing die302is not coated with any material that would affect its performance and it also resides within a pristine vacuum environment303.

In this embodiment, the sensor die302is coupled to an inner glass support tube304. An adhesive or soldered “butt” joint306hermetically attaches the inner glass support tube304to a metallic package308. The inner glass support tube304includes a passageway310where a pressurized media “P” loads the sensor die302when the sensor300is in operation. Loading the sensor die302with a positive pressure places the butt joint306in tension. In addition, torsional or bending loads applied to the sensor die302may tend to induce shear loads across the interface between the sensor die302and the inner glass support tube304, for example the shear loads would be along lateral or radial axes as defined by the inner glass support tube304. An overload of any one of these load conditions may result in a failure of the pressure sensor300. Further, repeated loading of the sensor die302and resultant stressing of the solder or adhesive material may eventually degrade the structural integrity of the butt joint306, causing a non-instantaneous degradation in sensor performance, and in some instances may lead to an instantaneous failure of the pressure sensor300. This sensor configuration also has limited life in high vibration environments. This is due to the limited cross-sectional area of the attachment combined with the cantilevered configuration of the sensor and glass support tube assembly.

FIG. 4schematically shows a portion of an improved pressure sensor configuration400having a sensor die402coupled to an inner glass support tube404. A soldered “sleeve” joint406attaches the inner glass support tube404to an intermediate metallic sleeve408as described in U.S. Pat. No. 4,509,880 and is further located within a pristine vacuum environment403. The inner glass support tube404includes a passageway410where a pressurized media “P” loads the sensor die402when the sensor400is in operation. Loading the sensor die402with a positive pressure (up the tube) places the sleeve joint406in shear and tension. Again and similar to the pressure sensor ofFIG. 3, repeated loading of the sensor die402and stressing of the solder material in shear or tension may eventually degrade the structural integrity of the sleeve-type hermetic seal406, causing a non-instantaneous degradation in sensor performance, or, in some instances, may lead to an instantaneous failure of the pressure sensor400. This sleeve joint is stronger than the above-described butt joint in both vibration and pressure environments due to the increased support area of the joint, with resulting reliability improvements. In addition, the joint may include a metallic layer412to help the inner glass support tube404bond to the metallic sleeve408. However, one drawback of the sleeve joint406is that it is still loaded in shear, which limits the amount of pressure up-the-tube to applications of 1000 psi or less.

SUMMARY OF THE INVENTION

The present invention provides a more robust attachment between the sensor's IC's mounting tube and a hermetic package, which addresses the above-mentioned problem by placing at least a portion of the attachment joint in compression to substantially reduce, if not eliminate, the potential failure modes in traditional butt and sleeve joint types of pressure sensors or transducers.

In one aspect of the invention, a pressure sensor includes a sensor die; a glass support tube having an expanded or portion and a narrow or necked-down portion. The expanded portion is distally located from the sensor die. The perimeter of the expanded portion is significantly larger than the perimeter of the necked-down portion. A glass support tube is located within the intermediate sleeve and includes an expanded perimeter portion and a reduced perimeter portion. The expanded perimeter portion is sized to be closely received by the expanded portion of the intermediate sleeve the reduced perimeter portion is sized to be closely received by the necked-down portion of the intermediate sleeve. The glass support tube further includes a bore for receiving a pressurized fluid and a contoured surface that is substantially complementary to a corresponding inner surface of the intermediate sleeve. An attachment joint, typically taking the form of a solder based material, is located between the glass support tube and the intermediate sleeve to attach the two components together. The attachment is configured such that when pressure is applied through the tube to the sensor die, at least a portion of the attachment is placed in compression.

In another aspect of the invention, a method of reacting applied pressure in a pressure sensor includes applying pressure to a pressure sensor die mounted on a first end of an inner glass tube (commonly referred to as a “chip tube”). The glass tube includes a expanded distal end portion. During operation, the applied pressure generates strain on the solder-based attachment of the inner glass tube relative to the metallic intermediate sleeve. In this embodiment, that strain is reacted through converting the strain at the attachment due to high applied pressure primarily into a compressive strain in lieu of the weaker tension and shear strain of the prior configuration shown inFIG. 4. The sensor configuration shown inFIG. 5still utilizes the shear stress of the sleeve joint to rigidly captivate the glass tube during vibration, thus providing a substantially more robust assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details or with various combinations of these details. In other instances, well-known structures and methods associated with pressure sensors and the operation thereof may not be shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.

The following description is generally directed to an attachment joint for a pressure sensor where at least a portion of the joint is under compression during operation. By arranging at least a portion of the joint to be under compression, the failure modes of the previously-described butt and sleeve joints may be substantially reduced, if not eliminated. In a preferred embodiment, the pressure sensor includes a bulbous or expanded end portion sized and contoured to place at least a portion of solder material or adhesive located in the joint into compression when pressure is applied to the pressure sensor.

FIG. 5schematically shows a cross-sectional view of a portion of a pressure sensor500having a sensor die502coupled to a glass support tube504. A solder or adhesive506attaches the glass support tube504to an intermediate sleeve508. The function of this intermediate sleeve is described in U.S. Pat. No. 4,509,880. The glass support tube504includes a bore or passageway510through which the pressure media “P” loads the sensor die502.

In a preferred embodiment, the intermediate sleeve508includes a first portion512with a first perimeter514extending into and coupled to a second portion516with a second perimeter518. The first portion512is distally located from the sensor die502and the first perimeter514is preferably larger than the second perimeter516. In the illustrated embodiment, the first portion512takes the form of an expanded or bulbous end portion and the second portion516takes the form of a recessed or necked-down portion. In addition, first portion512is coupled to the recessed portion516via a sloped or tapered shoulder520. In another embodiment, the first portion512and the second portion516may be connected by a stepped surface (not shown). By way of example, the intermediate sleeve508may made from a KOVAR® metallic alloy.

The glass support tube504is located within the intermediate sleeve508and includes a surface522contoured to be closely received by the first and second portions514,516of the intermediate sleeve508. This conformal faying surface522of the glass tube is typically plated with metal538to enhance attachment with solder when the adhesive is employed.

In particular and as illustrated, the glass support tube504includes an expanded perimeter portion524having a perimeter526sized to be closely received by the first portion512of the intermediate sleeve508. In addition, the glass support tube504includes a necked-down portion528having a perimeter530sized to be closely received by the second portion516of the intermediate sleeve508. The expanded perimeter portion524is distally located from the sensor die502. The surface522of the glass support tube504is complementarily contoured with respect to an inner surface532defined by the first and second portions514,516of the intermediate sleeve508. As will be described in greater detail below, the solder or other adhesive attachment layer506is located between the surface522of the glass support tube504and the inner surface532of the intermediate sleeve508.

The glass support tube504and the intermediate sleeve508are coupled together via the attachment layer506, which, as noted above, is located between the outer surface522of the glass support tube504and the inner surface532of the intermediate sleeve508. Due to the configuration of the complementary surfaces522and532, applied pressure on the sensor die502places at least a portion of the attachment layer506in compression.

In a preferred embodiment, the glass support tube504is a cylindrically shaped glass support tube504where the first portion512takes the form of an expanded or bulbous end portion513. The glass support tube504may be made of any appropriate glass material. In the preferred embodiment, the glass support tube504is made from PYREX® glass material. In addition, the glass support tube504may have a rounded, flame polished surface536at the distal end537. Preferably, the longitudinal length538is longer than the thickness540of the expanded or bulbous end portion513.

In one embodiment, a thin metallic layer542may be located on the inner surface522of the glass support tube504adjacent the attachment layer506by plating, coating, or some other equivalent process. The metallic layer542assists in solder bonding of the glass support tube504to the intermediate sleeve508. In one embodiment, the metallic layer542is a platinum-silver layer having an approximate thickness of about 0.001 inches. In other embodiment, the metallic layer542may include amounts of titanium, tungsten, nickel, gold, or other materials as generally described in U.S. Pat. No. 4,509,880.