Sensor package and method

A sensor package and method are described. The sensor package includes an enclosure, a diaphragm coupled to the enclosure. The diaphragm is configured to receive vibrations from an ambient environment. Further, the sensor package includes a pressure sensing element disposed inside the enclosure, and a pressure transfer medium disposed inside the enclosure and proximate the pressure sensing element, where the pressure transfer medium includes a fluid, and a plurality of filler particles suspended in the fluid. The filler particles serve to reduce a coefficient of thermal expansion of the pressure transfer medium.

BACKGROUND

The invention relates generally to a system and method for protecting a pressure sensing element from an ambient atmosphere whose pressure is being sensed. More particularly, the invention relates to a sensor package employing a pressure sensing element and method for manufacturing the sensor package.

Pressure sensors made of semiconductor materials are employed in a variety of applications because of their small size and compatibility with other electronic systems. Semiconductor pressure sensors or dies are generally used as pressure sensing elements in applications, such as combustion engines or in marine applications. For example, when employed in combustion engines, semiconductor pressure sensors are used to measure pressure variations in combustion fuel. However, semiconductor materials are sensitive to contamination caused by the harsh environment in such applications. Accordingly, if the surface of a semiconductor pressure sensor is exposed directly to an ambient environment whose pressure is being measured the pressure sensor may be adversely affected.

Therefore, typically pressure sensors are sealed in a metal container having a metal diaphragm which receives the pressure variations in the ambient environment and transfers it to the oils or fluids employed as a pressure transmitting medium, which in turn transfers the pressure to the pressure sensing element. This flow of pressure from the ambient environment to the diaphragm, and pressure transmitting medium and subsequently to the pressure sensor leads to measurement errors due to material mismatch. Also, due to wide temperature ranges involved in these applications, it is desirable to have a close match between the values of coefficient of thermal expansion (CTE) for the various materials involved, to prevent pressure fluctuations caused by thermal mismatch. For example, in case of an oil with high CTE, expansion of oil with temperature causes increased and undesirable stress on the diaphragm. Additionally, low CTE of the oil is desirable to reduce the errors from other components of the pressure sensor package, such as the diaphragm, from propagating to the sensing element. Hence, it is desirable to employ an oil which has a close CTE match with the other components of the assembly, such as diaphragm.

There exists a need for a suitable pressure transmitting medium which protects the pressure sensing element from the ambient environment without substantial loss in accuracy by having a close match of the CTE with other materials employed in the package.

SUMMARY

Embodiments of the invention are directed to a system and a method for protecting a pressure sensing element from an ambient atmosphere whose pressure is being sensed.

One exemplary embodiment of the invention is a sensor package. The sensor package includes an enclosure and a diaphragm coupled to the enclosure. The diaphragm is configured to receive vibrations from an ambient environment. Further, the sensor package includes a pressure sensing element disposed inside the enclosure and a pressure transfer medium disposed inside the enclosure and proximate the pressure sensing element. The pressure transfer medium includes a fluid, and a plurality of filler particles suspended in the fluid. The filler particles serve to reduce a coefficient of thermal expansion of the pressure transfer medium.

Another exemplary embodiment of the invention is a fluidic medium having a coefficient of thermal expansion no greater than 500 ppm/° C. The fluidic medium comprises a fluid, and a plurality of filler particles suspended in the fluid. A coefficient of thermal expansion of the plurality of filler particles is no greater than about 5 ppm/° C.

Another exemplary embodiment of the invention is a method for manufacturing a sensor package that includes the steps of providing an enclosure having a base, coupling a pressure sensing element to the base, disposing a pressure transfer medium in the enclosure proximate the pressure sensing element, and disposing a diaphragm on the enclosure to seal the enclosure. A coefficient of thermal expansion of the pressure transfer medium is no greater than 500 ppm/° C.

These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring now toFIGS. 1–3, a sensor package10is illustrated. The illustrated sensor package10includes an enclosure12coupled to a diaphragm14. The illustrated enclosure12has a generally circular profile and includes an inner surface16and an outer surface18. It should be appreciated that the configuration of the enclosure12may take any suitable form. Further, the enclosure12includes an extended portion20, such that the extended portion has a diameter greater than the diameter of the inner surface16, but smaller than the diameter of the outer surface18. The enclosure12is employed to house a pressure sensing element22to isolate the pressure sensing element22from the harsh ambient environment. The sensor package10may be employed in, for example, automotive applications, process transmissions, aerospace applications, and depth measurements. The enclosure12and diaphragm14may be made of stainless steel in instances where the stainless steel is compatible with the ambient environment. In marine applications, such as depth measurements, the diaphragm14may be made of copper-nickel. In other applications, the diaphragm14may be made of tantalum. As will be appreciated, for the same amount of pressure, the sensor package10or the diaphragm14having a smaller diameter will experience relatively lesser amount of force as compared to the sensor package10or the diaphragm14having a larger diameter. Accordingly, in some cases the sensor package may have a diameter of less than about 25 mm. In these embodiments, the pressure values in the ambient environment may range to upto about 1500 psi. In some embodiments, the pressure sensing element14may include a resonant sensor. In an exemplary embodiment, the pressure sensing element22may include a microelectromechanical system (MEMS) device, such as, for example, a sensor. For some applications, the pressure sensing element22may include a semiconductor material such as silicon.

The pressure sensing element22is mounted on a substrate24, such as a glass substrate, and disposed in a cavity26located within the inner surface16of the enclosure12. Typically, an adhesive layer28is employed to couple the glass substrate24to the enclosure surface around the cavity26. In other words, the adhesive layer28is disposed between the glass substrate24and the enclosure material. In some embodiments, the adhesive layer28may include an epoxy. It should be appreciated that silicon and glass have a relatively close CTE match. Hence, no operational limitations are generated by simultaneous use of silicon and glass. However, the pressure variations seen by the pressure sensing element22lead to development of stress between the pressure sensing element22and the glass substrate24. Also, the pressure sensing element22faces interfacial stresses generated due to the bonding between the adhesive layer28and materials having dissimilar coefficient of thermal expansions (CTEs). Accordingly, as described in detail with respect toFIGS. 4–8, the glass substrate24may include a block, or may include columnar structures, or other shapes.

The sensor package10employs leads30disposed within sockets32adjacent the cavity26and within the inner surface16. The leads30are employed to electrically couple the pressure sensing element22to an output device, such as a display. The leads30are electrically coupled to an external or internal electrical source. The sensor package10further has a reference tube34protruding out from the bottom of the enclosure12. The reference tube34serves to ground the sensor package10.

As will be appreciated, the diaphragm14is configured to receive vibrations from an ambient environment. The received vibrations are then passed on to a pressure transmitting medium, such as a pressure transfer medium or a fluidic medium38(FIG. 3) disposed inside an internal hollow37in the enclosure12housing. For some applications, the ambient environment of the enclosure12is harsh for the pressure sensing element22, making it necessary to isolate the pressure sensing element22from the ambient environment. The pressure sensing element22is housed in a closed container, such as the enclosure12employing the diaphragm14. The diaphragm14communicates with the pressure sensing element22through the fluidic medium38(FIG. 2). As will be appreciated, the pressure transfer medium38is employed to transfer the vibrations of the diaphragm14to the pressure sensing element22. Accordingly, it is desirable to employ a pressure transfer medium38, which does not contaminate the pressure sensing element and has such properties as a low coefficient of thermal expansion and a manageable viscosity. As will be appreciated, to avoid errors generated from the material mismatch, it is desirable to have relatively close match between the CTEs of the material of the diaphragm14and the pressure transfer medium38. Typically, the diaphragm14includes stainless steel, which has a CTE varying in a range from about 10 to about 30 ppm/° C. Accordingly, it is desirable to have a low CTE for the pressure transfer medium. For example, the coefficient of thermal expansion of the pressure transfer medium is no greater than 500 ppm/° C. In some embodiments, the CTE of the pressure transfer medium is no greater than about 350 ppm/° C.

The pressure transfer medium or the fluidic medium38includes a fluid and a plurality of filler particles39(FIG. 3) suspended in the fluid. Typically, the CTE of the plurality of filler particles39is significantly lower than the CTE of the fluid. Sometimes the CTE of the plurality of filler particles39may even be a negative value. Specifically, the CTE of the plurality of filler particles is no greater than 5 ppm/° C., whereas the CTE of the fluid may be of the order of 900 ppm/° C. or higher. It should be appreciated that the presence of the plurality of filler particles39in the fluidic medium38serves to tailor the CTE of the fluidic medium38by reducing the overall CTE of the fluidic medium38, thereby matching it more closely to the other components of the sensor package10, such as pressure sensing element22and the diaphragm14. The fluid in the fluidic medium38may include silicone oil or polyethylene glycol blends. The plurality of filler particles39may include fused silica particles, colloidal silica particles, organofunctionalized colloidal silica particles, hollow spheres of glass, solid spheres of glass, alumina particles, titania particles, zirconium tungstate particles, or combinations thereof.

In the illustrated embodiment ofFIG. 1, the diaphragm14employs a diaphragm ring36, which may serve to secure the diaphragm14to the enclosure12. In other embodiments, the diaphragm ring36may have a diameter smaller than the diameter of the diaphragm and may be employed co-centrically above the diaphragm14such that when the diaphragm14deflects due to the expansion of the fluidic medium38, the diaphragm14comes in contact with the diaphragm ring36and the stress experienced by the diaphragm is distributed.

It may be desirable to prevent sedimentation of the filler particles39suspended in the fluid in the fluidic medium38. It should be appreciated that rate of sedimentation is directly proportional to the size and density of the particles. Accordingly, to inhibit sedimentation of the filler particles39, the size of the plurality of filler particles39should be no greater than 20 microns. The size of the plurality of filler particles39also may be varied depending on its density. In other words, particles39having a higher density may be employed in smaller sizes, and particles39having relatively lesser density may be employed in larger sizes. In an exemplary embodiment, the fluid includes a silicone oil and the plurality of filler particles include a mixture of fused combination of solid and hollow spheres of glass. In some embodiments, the separation time (time for sedimentation to occur) of the filler particles39from the fluid varies in a range from about 2 weeks to about 5 weeks at standard gravity.

Additionally, it is desirable to have a manageable viscosity for the fluidic medium38to enable reduced errors in the pressure readings taken by the pressure sensing element22. Also, it is easier to fill the enclosure12when the fluidic medium has low viscosity, as the fluidic medium38is generally filled in the enclosure12by means of a small cavity. Suspending more filler particles39in the fluid, in other words, loading the fluid beyond a certain value especially at low temperatures, may adversely affect the viscosity of the fluidic medium38, which in turn may affect the pressure readings indicated by the pressure sensing element22. It is desirable to have a fluidic medium38that is incompressible and inseparable in a temperature range of about 125° C. to about −55° C.

Additionally, the plurality of filler particles39of the fluidic medium38may be functionalized to achieve desirable properties, such as increased hydrophobicity, low CTE, and better compatibility with the fluid. For example, filler particles39that are sized in the nanoscale range may be functionalized by treatment with organosilanes to make them relatively more hydrophobic. In another example, the plurality of filler particles39may be functionalized using organoalkoxysilanes, organochlorosilanes or organosilazanes; or combinations thereof to further lower the CTE of the plurality of filler particles39.

The glass substrate24may include a block, or may include columnar structures, or other shapes to mount the pressure sensing element22and also to be coupled to the adhesive layer28.FIGS. 4–8illustrate different embodiments of a glass substrate, which are employed in the sensor package10to reduce the stress in the sensor package10. In these illustrated embodiments, the glass substrate employs columnar structures, which may be coupled to the pressure sensing element22and/or the adhesive layer28. With specific reference toFIG. 4, a glass structure40includes a base42divided into an array of pixels44. The array of pixels44employs columns46in a triangular pattern to reduce the stress in the pressure sensing element22. Referring toFIG. 5, an alternate glass structure48including a base50employing a columnar structure is illustrated. The base50is divided into array of pixels52. Further, the glass structure48employs four columns54positioned at four corners of the rectangular base50.

Similarly, inFIG. 6the structure56employing a glass substrate58is illustrated. The glass substrate58is divided into an array of pixels60. For the reasons of simplicity, inFIGS. 6 and 7the structures56and64are illustrated as top views as compared to perspective views of structures40and48inFIGS. 4 and 5. In the illustrated embodiment, the columns62are disposed on the pixels located at the perimeter of the array of pixels60, and the pixels63of the array of pixels60are empty. It should be appreciated that although in the illustrated embodiment, the array of pixels60displays evenly sized pixels, as will be appreciated, the size and shape of the individual pixels may vary. Next, with reference toFIG. 7a structure64having a glass substrate66divided into an array of pixels68is illustrated. The glass substrate66employs columns70positioned at alternate pixels of the array of pixels68leaving pixels72empty. The illustrated structure64may be used either to support the pressure sensing element22or to be coupled with the adhesive layer28.

FIG. 8illustrates a glass structure74with a two-tier structure76of a glass substrate78employing the triangular pattern ofFIG. 4. In the illustrated embodiment, columns80are present on either side of the base78. Furthermore, the structure74includes an adhesive layer28disposed between the two-tier structure76and a metal base around the cavity16.

In addition to the geometry of the glass substrate, various other factors such as thickness of the adhesive layer28and hardness of the adhesive layer28may also contribute to the stress experienced by the pressure sensing element22. The adhesive layer28may have a thickness varying in a range from about 5 mils to about 15 mils. Specifically, the thickness of the adhesive layer may be around 10 mils.

With reference toFIG. 9, next will be described a method for displaying pressure changes taking place in an ambient environment surrounding the sensor package10(FIGS. 1–3) onto an output device. At Step84, pressure changes taking place in the ambient environment are transferred onto the diaphragm14of the enclosure12. Upon experiencing the pressure changes of the ambient environment, the diaphragm generates vibrations. At Step86, the vibrations generated in the diaphragm14are transmitted to the fluidic medium38in the form of pressure changes. At Step88, the fluidic medium38transmits these pressure changes to the pressure sensing element22to be sensed and quantified. At Step90, the pressure changes sensed/experienced by the pressure sensing element22are then converted into signals, such as electrical signals. At Step92, the electrical signals are displayed on an output device such as, a monitor.

With reference toFIG. 10, next will be described a method for manufacturing a sensor package10of the present technique. At Step94, an enclosure12is provided, as mentioned above, the enclosure12may include stainless steel. At Step96, a pressure sensing element22is disposed inside the enclosure12. As discussed above with reference toFIGS. 1,2and3, the pressure sensing element22is mounted on a substrate, such as a glass substrate24and coupled to the substrate using an adhesive layer28. At Step98, a fluidic medium38is disposed inside the enclosure12, proximate the pressure sensing element22. As discussed in detail above, the fluidic medium includes a fluid having a plurality of filler particles suspended therein. At Step100, the diaphragm14is disposed on the enclosure12to enclose the enclosure12to shield the pressure sensing element22from the harsh ambient environment.

Various fluidic mediums having filler particles of different commercial grades of fused silica or glass suspended in different commercial grades of silicone oil were prepared as mentioned in the table 1 below. Table 1 provides the list of various silicone oils and name and amount of filler particles, which were added in 100 grams of the listed silicone oils to make the fluidic medium. The various grades of silicone oils used for the purpose of the experiment are mentioned in Table 1, all the silicone oils were manufactured by GE Silicones at Waterford, N.Y. Similarly, various grades of commercially available filler particles were employed, the fillers particles were manufactured by Denka Corp. Tokyo, Japan. 100 grams of silicone oil was charged to a mixer Ross Mixer (Hauppauge, N.Y. 11788) equipped with steam heated jacket and vacuum port. For all the samples prepared, a predetermined amount of filler particles was divided into three portions. The three portions were added to the silicone oil at room temperature one at a time and the mixture was mixed for 5 minutes after each addition. After the addition of the filler particles, the mixture was stirred at room temperature for 15 minutes. Subsequently, the mixture was heated in vacuum for one hour to 100° C. Then the fluidic medium was cooled down to the room temperature.

1035 grams of isopropanol (Aldrich, Milwaukee, Wis. 53233) was mixed with 67 grams of Snowtex OL (Nissan, Chemical, Houston, Tex., 77042) having 21 weight percent silica to form a dispersion. Further, 20.1 grams of trimethoxy benzene (Aldrich, Milwaukee, Wis. 54481) was added and the dispersion was stirred and heated at a temperature varying from about 70° C. to about 80° C. for about 1–2 hours. The dispersion was then cooled and stored for about 2 hours before completing the preparation of the concentrated dispersion.

Further, 540 grams of the aliquot of the dispersion having 42 grams nanosilica was diluted with 750 grams of 1-methoxy-2-propanol to form a relatively clear dispersion. The clear dispersion was then concentrated by employing rotary evaporation at the temperature of about 60° C. per at 60 mm Hg Pressure to obtain a final weight of 84 grams of the clear dispersion. During the removal of the last portion of solvent from the dispersion, rapid agitation is desirable to prevent agglomeration of silica. Two sets of experimental data was gathered by mixing the 50 wt. % dispersion with one of the three solvents, poly-(ethylene glycol) (PEG), Dow Corning 550(SP-1) (DC550) and Service Pro 6012 (SP6012) to make blends or fluidic medium.

In the first set, the 115 ml of one of the solvents PEG, DC550, or SP6012 were mixed with 50 wt. % dispersion in appropriate ratio to prepare blends as summarized in Tables 2. This combined solution was subjected to rotary evaporation at 100° C. and about 10 mm Hg for 2 hours to remove the 1-methoxy-propane-2-ol solvent. CTE measurements were made following this reduction.

Alternatively, in the second set of experiments, 50 wt. % sol of the nanosilica particles was mixed in appropriate amounts with one of the three fluids, PEG, DC550, or SP6012, such that the total volume of the fluidic medium was 115 mil. These samples were subjected to CTE measurements without evaporation. These blends are summarized in Table 4.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, while the sensor package is described in conjunction with automotives applications, process transmitters, aerospace applications, and depth measurements, it should be appreciated that the sensor package may find utility for any application in which a pressure difference in an ambient environment is transferred from the environment through the diaphragm onto the pressure sensing element, such as, for example, in flow sensing. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.