Stress-isolated absolute pressure sensor

Suspending a microelectromechanical system (MEMS) pressure sensing element inside a cavity using spring-like corrugations or serpentine crenellations, reduces thermally-mismatched mechanical stress on the sensing element. Overlaying the spring-like structures and the sensing element with a gel further reduces thermally-mismatched stress and vibrational dynamic stress.

BACKGROUND

Microelectromechanical system (MEMS) pressure sensors are well known. Examples of such sensors are disclosed in various U.S. patents including but not limited to: U.S. Pat. No. 7,997,142 issued Aug. 16, 2011, entitled, “Low pressure sensor device with high accuracy and high sensitivity,” U.S. Pat. No. 8,215,176 issued Jul. 12, 2013, entitled “Pressure sensor for harsh media sensing and flexible packaging,” and U.S. Pat. No. 8,833,172 issued Sep. 16, 2014, entitled “Pressure sensing device with stepped cavity to minimize thermal noise,” the contents of each being incorporated by reference in their entireties.

Those of ordinary skill in the MEMS pressure sensing art know that the thermally-mismatched mechanical stress or vibrational dynamic stress on a MEMS pressure sensing element adversely affects the device's accuracy. Reducing or eliminating thermally-mismatched stress and vibrational dynamic stress is therefore important to improving MEMS pressure sensing element accuracy.

DETAILED DESCRIPTION

Referring now toFIG. 1Ain an exploded view, a MEMS pressure sensor100comprises a main housing102having a port104that conducts pressurized fluid to a MEMS pressure sensing element106located inside a housing cavity108, which is inside the plastic housing102. A layer of viscous gel110is overlaid both the MEMS pressure sensing element106and an application specific-integrated circuit (ASIC)112, also mounted inside the housing cavity108.

Electrical signals are provided to and received from the ASIC112through metal “lead frames”114that extend through the plastic housing102. The lead frames114are electrically coupled to the ASIC112through small bond wires116. On the other side of the ASIC112, bond wires118connect the ASIC112with the MEMS pressure sensing element106. The cross-sectional diagram of the MEMS pressure sensing element106in the prior art is depicted inFIG. 1B. The MEMS pressure sensing element106comprises a silicon substrate120with a diaphragm124and a cavity126and anodically bonded to a glass substrate122. The cavity126is near vacuum for topside pressure sensing by a plurality of piezoresistors128formed near the edge of the diaphragm124on the top side.

Inaccuracy or noise caused by thermally-induced and/or vibrational mechanical stress on the MEMS pressure sensing element106can be reduced by an improved MEMS pressure sensing element201with flexible “springs” or “suspenders”, which are formed from the same semiconductor material as shown inFIG. 2. In the preferred embodiment, the springs are made to be electrically conductive by doping and carry signals to and from a plurality of piezoresistors304as shown inFIG. 3. Additional mechanical stress isolation is provided to the stress-isolated MEMS pressure sensor200by overlaying the springs with the viscous gel110to damp out and reduce the vibrational dynamic stress.

FIG. 2is a cross-sectional diagram of a preferred embodiment of a stress-isolated pressure sensor200that comprises a MEMS pressure sensing element201with an internal vacuum cavity204. The cavity204has a bottom surface205and is defined by or bounded by sidewalls207, which are substantially orthogonal to the bottom surface205.

A thin semiconductor diaphragm206having piezoresistors, as described in the aforementioned prior art patents, is formed and attached to the substrate202by a thin layer of silicon dioxide209.

As used herein, the term “suspender” refers to a structure by which something is suspended or supported. As shown inFIG. 2, the MEMS pressure sensing element201having the diaphragm206is suspended in a cavity210by a plurality of suspenders208formed from the same semiconductor material from which the substrate202and the diaphragm206are made. The cavity210is formed by bonding a bottom surface213of a silicon rim212to a substrate214.

If the substrate214is a glass substrate, the silicon rim212can be anodically bonded onto the substrate214. If the substrate214is a silicon substrate, the silicon rim212can be fusion-bonded onto the substrate214or glass-frit-bonded onto the substrate214.

FIG. 3is a top or plan view of the stress-isolated pressure sensor200shown inFIG. 2. The diaphragm206is essentially square and thus has four edges305. Piezoresistors304are formed to be located adjacent to each edge305of the diaphragm206. The piezoresistors304are electrically connected with each other by a plurality of P+ interconnects306and bond pads215to form a Wheatstone bridge circuit320. As is well known, and explained in at least the issued patents noted above, deformation of the diaphragm206responsive to pressure applied to its top surfaces causes the piezoresistors304to deform and stressed. When they deform and are stressed, their resistance changes. When a constant-value input voltage is applied to input terminals of the Wheatstone bridge circuit, the voltage output from the Wheatstone bridge changes responsive to deformation and stresses, i.e., responsive to diaphragm deflection and stresses due to the applied pressure.

Each piezoresistor304has of course two terminal ends. Each end of each piezoresistor304is connected to a suspender208, each of which is doped to make the suspenders208electrically conductive.

Each suspender208is “connected” to a bond pad215on the top surface211of the diaphragm206. The suspenders208thus perform three functions: supporting the diaphragm206, providing stress isolation, and providing a conductive signal path to and from the piezoresistors304.

FIG. 4Ais a cross-sectional diagram of an overmolded stress-isolated pressure sensor400. The stress-isolated pressure sensor200is attached to a printed circuit board (PCB)412or the lead frames114, with adhesive411. An overmold402using materials such as thermoplastic materials to overmold the stress-isolated pressure sensor200and bond wires406, which connects the stress-isolated pressure sensor200to the ASIC112or the lead frame114. Gel404is dispensed to cover the top surface211of the stress-isolated pressure sensor200to protect the Wheatstone bridge circuit320. A through hole418is formed by the overmold402to allow the pressure media to pass through the hole418to reach the top surface of the gel404. The viscous gel404is soft enough to transmit pressure applied thereon to the diaphragm206of the MEMS pressure sensing element201. The gel404is also filled into the cavity210. The gel404can damp down the dynamic stress and reduce the noise sensed by the piezoresistors304on the diaphragm206during vibration or impact.

FIG. 4Bis a cross-sectional diagram of an overmoled stress-isolated pressure sensor401, which comprises a stress-isolated pressure sensor200using different electrical interconnections with through-vias408and solder bumps410to connect to a PCB412or the lead frames114for voltage signal input and output. The gel404fills in the cavity completely over the MEMS pressure sensing element201and is overmolded by the overmold402without an open space. A membrane420is formed by the overmold402on the top of the gel404. The membrane420is flexible to transmit pressure applied thereon to the gel and the diaphragm206of the MEMS pressure sensing element201.

The stress-isolated pressure sensor can be also contained in the injection-molded plastic housing102in the prior art as shown inFIG. 1Aby replacing the MEMS pressure sensing element106by the stress-isolated pressure sensor200inFIG. 2.

The suspenders208that support and isolate the MEMS pressure sensing element201from stress are formed by either etching the top surface211or the bottom surface213or etching both surfaces211and213of the stress-isolated pressure sensor200. The etching required to form a preferred-embodiment suspender is a multi-step process that forms corrugated suspenders or serpentine suspenders in the material from which the stress-isolated pressure sensor200is made. The corrugated suspenders or serpentine suspenders expand and contract responsive to acceleration or movement of the MEMS pressure sensing element201and thus act as springs, absorbing mechanical forces that would otherwise be applied to the diaphragm and distort its output signals. Stated another way, the suspenders208isolate or alleviate the diaphragm206from mechanical stress.

FIG. 5shows a corrugated suspender500, which can be one kind of the suspenders208used in the stress-isolated pressure sensor201as shown inFIG. 2andFIG. 3for stress isolation. The corrugated suspender500comprises a wavy top surface501and a wavy bottom surface502. The top surface501is boron-doped with P+ conductive material as an interconnect connecting the P+ interconnect306as shown inFIG. 3. The corrugated suspenders500have several substantially planar and horizontal merlons504, which are “attached” to inclined sections referred to herein as crenels506. The substantially planar merlons504are substantially parallel to the substantially planar top and bottom surfaces of the diaphragm206of the stress-isolated MEMS pressure sensing element201.

FIG. 6Ais a perspective view of a serpentine suspender600that is an alternate for the corrugated suspender500.FIG. 6Bis a top view of the serpentine suspender600andFIG. 6Cis a side view of the serpentine suspender600. The material from which the suspender600is formed can be doped to be conductive and carry electrical current. It can also support a pressure sensing element in a cavity. Unlike the corrugated suspenders shown inFIG. 5, which have vertically-oriented crenellations, the serpentine suspender600has crenellations considered herein to be horizontally-oriented. The serpentine suspender600shown inFIG. 6can be more easily formed by fewer etching steps than the corrugated suspenders shown inFIG. 5and can thus be considered “preferred.”

Each crenellated section610has a merlon612and a crenel614. In a preferred embodiment the suspenders600are doped to be P+ using conventional processes to make them electrically conductive while at the same time being mechanically flexible.

Referring now toFIG. 7there is depicted a cross section and method of forming a stress-isolated pressure sensor. A silicon-on-insulator (SOI) substrate701with an internal cavity704can be formed, which consists of a silicon support substrate702, a silicon dioxide layer706, and a silicon device layer708. The SOI silicon support substrate702is etched by deep reactive ion etching (DRIE) at the center of the bottom surface703to effectively form a shallow cavity712.

The corrugated or serpentine suspenders208, as described above, are formed after a further DRIE etching process framing a deeper cavity714surrounding the MEMS pressure sensing element201. The shallow cavity712and the deeper cavity714surrounding the MEMS pressure sensing element201constitute the cavity210. The suspenders208support and mechanically isolate the pressure sensing element201from the remaining SOI substrate701.

The etched SOI substrate consisting of the MEMS pressure sensing element201is attached to a substrate720. The entire structure is supported on the substrate720, which can be glass or silicon.

FIG. 8depicts an alternate method of forming a stress-isolated pressure sensor. A first silicon substrate802is etched to form a shallow cavity804and attached to a SOI substrate810to form a bonded substrate. The SOI substrate810comprises a silicon dioxide layer812, a silicon device layer814, a silicon dioxide layer816, and a silicon support substrate818. The rim or edge portion of the bonded substrate is etched from the topside to expose the silicon device layer814. A silicon or glass cap820with a deep cavity822formed is then attached to the top surface of the exposed silicon device layer814of the bonded substrate810. If the cap820is glass, the cap820can be anodically bonded onto the bonded substrate810. If the cap820is silicon, the cap820can be fusion-bonded onto the bonded substrate810or glass-frit-bonded onto the bonded substrate810.

The bottom surface of the SOI substrate810is etched to define a substantially square-shaped MEMS pressure sensing element201having a diaphragm206and a plurality of supporting suspenders208to form a stress-isolated pressure sensor.

FIG. 9depicts a stress-isolated pressure sensor900comprising a MEMS pressure sensing element901attached on top of a silicon or glass substrate904. A cavity906in which the MEMS pressure sensing element901is suspended by suspenders908is filled with a viscous gel910in which the MEMS pressure sensing element901is suspended.

Conductive vias914extend between the top surface916of the substrate902and the bottom surface918of the substrate904and provide conductive pathways to an ASIC920having its own conductive through-silicon vias922. Signals of the ASIC920can thus be conducted to and from the MEMS pressure sensing element901suspended in a viscous gel by corrugated or serpentine suspenders, which are doped to be electrically conductive.

Referring now toFIG. 10, the structure shown inFIG. 9is shown as being overmolded in an overmold1002. The stress-isolated pressure sensor1000and its overmold1002include PCB with conductive traces or conductive lead frames1004which extend from exterior surfaces of the overmold1002through the overmolding material to the bond pads1006on the ASIC920.

Those of ordinary skill in the art should know that a catenary is the curve assumed by a cord of uniform density and cross section that is perfectly flexible but not capable of being stretched to be horizontal and which hangs freely from two fixed points. Examples of catenaries are power lines and telephone lines suspended from towers or posts.

Those of ordinary skill in the art should recognize that the support or suspension of a MEMS pressure sensing element in a cavity by springs, regardless of the springs' shape will have a shape that is inherently catenary. The spring-like suspenders disclosed herein are thus considered herein to have a shape that is at least partly catenary. The shape assumed by the opposing spring and the MEMS pressure sensing element is also assumed to be at least partially catenary.

The foregoing description is for purposes of illustration only. The true scope of the invention is set forth in the following claims.