Media-compatible electrically isolated pressure sensor for high temperature applications

A pressure sensor is described with sensing elements electrically and physically isolated from a pressurized medium. An absolute pressure sensor has a reference cavity, which can be at a vacuum or zero pressure, enclosing the sensing elements. The reference cavity is formed by bonding a recessed cap wafer with a gauge wafer having a micromachined diaphragm. Sensing elements are disposed on a first side of the diaphragm. The pressurized medium accesses a second side of the diaphragm opposite to the first side where the sensing elements are disposed. A spacer wafer may be used for structural support and stress relief of the gauge wafer. In one embodiment, vertical through-wafer conductive vias are used to bring out electrical connections from the sensing elements to outside the reference cavity. In an alternative embodiment, peripheral bond pads on the gauge wafer are used to bring out electrical connections from the sensing elements to outside the reference cavity. In various embodiments, a regular silicon-on-insulator wafer or a double silicon-on-insulator wafer may be used as the gauge wafer, and appropriate micromachining steps are adopted to define the diaphragm. A layer of corrosion resistant material is deposited on the surface of the diaphragm that is accessed by the pressurized medium.

TECHNICAL FIELD

The present disclosure relates generally to pressure sensing, and more specifically to pressure sensing in a harsh and/or electrically conducting pressurized medium at high temperature.

BACKGROUND

Diaphragm-based pressure sensors have been used for a variety of applications, where pressure exerted by a pressurized medium deflects a diaphragm, and sensing elements (such as strain gauges) coupled to the diaphragm sense the deflection and provide a signal correlating the deflection of the diaphragm with the amount of pressure.

There are two major types of pressure sensors. The first type is called a gauge pressure sensor, which measures pressure with respect to atmospheric pressure. The second type is called an absolute pressure sensor, which typically measures pressure with respect to a vacuum or zero pressure.

FIGS. 1A and 1Brespectively show a conventional gauge pressure sensor100and a conventional absolute pressure sensor110, both by Silicon Microstructures, Inc. of Milpitas, Calif. (pressure sensor models SM5102). Both the pressure sensors100and110use a silicon micromachined structure (also known as a gauge wafer) with a diaphragm having sensing elements (not shown) on the top or outer surface. The micromachined structure is mounted on a support structure (also known as a spacer). Typical dimensions of the pressure sensors are shown in millimeters. Both the pressure sensors100and110are top side pressure sensors, i.e., the top or outer side of the diaphragm is accessed by the pressurized medium. The support structure in the gauge pressure sensor100has an opening to expose the opposite side (i.e., the bottom side or the inner side) of the diaphragm to atmospheric pressure. On the other hand, the support structure in the absolute pressure sensor110has no opening, and defines a vacuum reference cavity underneath the diaphragm. Both the pressure sensors100and110are not ideal for applications involving harsh pressure media (such as fuel mixtures, acidic solution, and the like), as the sensing elements on the top side of the diaphragm may come in contact with the harsh pressurized medium if a protective coating on the sensing elements is damaged.

The gauge pressure sensor reads pressure with respect to atmospheric pressure. Atmospheric pressure varies over elevation and weather conditions. Thus, an absolute pressure sensor is often preferred where high accuracy is needed. For example, gauge pressure reading can change by 2-3 psi due to variations in atmospheric pressure. Thus, it can contribute to 2% to 3% error in the pressure reading if the full scale is 100 psi. Most new pressure sensor applications below 500 psi require +/−1% accuracy over operational temperature range, pressure range, and over the life of the product. Thus absolute pressure sensing is becoming very important in such applications.

FIG. 2shows a conventional absolute pressure sensor200, by Silicon Microstructures, Inc. of Milpitas, Calif. (pressure sensor model SM5112). Pressure sensor200uses a silicon gauge wafer with a micromachined diaphragm. A boro-silica glass cap sealed to the gauge wafer creates a reference vacuum cavity on top of the diaphragm. The pressurized medium exerts pressure from the bottom side of the diaphragm. The support structure (spacer) is made of boro-silica glass with a drilled hole in the center to allow the pressurized medium to access the bottom side of the diaphragm. The top side of the diaphragm has strain gauge sensing elements, interconnecting diffused resistors, and electrical interconnect metallizations to bring electrical signals out from the strain gauge sensing elements. This configuration is better for applications involving harsh pressurized media, as the sensing elements are separated from the pressurized medium. However, attaching a boro-silica glass cap wafer to a silicon gauge wafer to create a vacuum cavity is a very expensive process.

Another problem encountered by pressure sensor200is failure to withstand high temperature because of electrical leakage in the sensing elements. Pressure sensor200uses piezoresistors as sensing elements (configured as a strain gauge). In a typical piezoresistive strain gauge, four piezoresistors are connected in a Wheatstone bridge configuration (seeFIG. 4) on top of the diaphragm. When the applied pressure deflects the diaphragm, induced stress in the diaphragm causes the piezoresistors to change their respective resistance values, resulting in an imbalance in the Wheatstone bridge. The imbalanced piezoresistor bridge produces an electrical signal output that is proportional to the applied pressure. In a silicon diaphragm, piezoresistors can be integrated at a low cost by using standard photolithographic processes. As shown inFIG. 3(only the gauge wafer is shown here), piezoresistors may be defined as diffused wells of opposite-polarity regions embedded in the bulk material of the diaphragm. In the inset ofFIG. 3, p-type diffused piezoresistors are created in the n-type bulk silicon diaphragm by photolithographically opening windows in the top insulator layer, and then doping with p-type material (e.g., boron) to achieve a desired sheet resistivity. The piezoresistors create diode-like p-n junctions with the diaphragm substrate, as shown in the equivalent circuit inFIG. 4. At temperatures beyond about 125° C., the p-n junctions behave like leaky diodes with the leakage current increasing exponentially as the temperature rises. As shown inFIG. 4, if the surrounding pressurized medium is electrically conductive, current leaks to the ground through the pressurized medium (represented by the resistor R in the equivalent circuit). The current leakage is substantial at higher temperatures, and causes sensing malfunction and may cause irreversible physical damage. Although not shown inFIG. 3for the sake of clarity, persons of ordinary skill in the art will now understand that interconnecting diffused resistors (as shown inFIG. 2) coupled to the strain gauge piezoresistors will also contribute to current leakage at high temperature. Moreover, contamination during fabrication processes may cause diode current leakage even at lower temperatures such as room temperature.

In the previously discussed examples, a silicon diaphragm is used with integrated sensing elements. Silicon diaphragms and integrated sensing elements are popular because of ease of manufacturing using batch processing. However, depending on particular applications, in some conventional pressure sensors, the diaphragm and the sensor may be separated. This may be useful from a harsh pressurized media compatibility standpoint, as the diaphragm can be made of a corrosion-resistant material, such as stainless steel, and the sensing element can be made of silicon and can be kept isolated from pressurized medium exposure in a sealed chamber filled with an additional pressure transfer medium. One example of this type of sensor is referred to as an oil-filled sensor, where the pressure transfer medium is oil. This process can be relatively expensive, as the oil filling has to be performed in a vacuum. Errors arise in this approach because there is usually a small amount of residual air in the chamber after sealing. Thermal effects on the oil volume and air bubble also act to increase the error in the pressure reading. For reference, readers are encouraged to read U.S. Pat. No. 6,311,561 to Bang et al.

In the case of a corrosion-resistant metal diaphragm, strain gauges are usually defined by depositing and patterning thin metal films on the diaphragm. For example, a titanium oxy nitride (TiON) strain gauge layer may be deposited on a silicon dioxide coated stainless steel diaphragm. However, typically, this type of strain gauge has lower gauge factors than micromachined silicon piezoresistive strain gauges, affecting pressure measurement accuracy.

By adopting a hybrid configuration, where a micromachined silicon piezoresistive strain gauge is bonded to an oxide-coated metal diaphragm, one can address the low gauge factor problem. However, the hybrid pressure sensor suffers from a thermal expansion mismatch problem between the sensing elements and the diaphragm. Moreover, the hybrid construction may not be efficient for batch processing. Adding a vacuum sealed cavity on top of the sensing elements can be prohibitively expensive. Even depositing oxide on the stainless-steel diaphragm requires an expensive fine polishing process. The operating temperature is typically limited to 140° C. in the hybrid pressure sensor.

Absolute pressure sensors, where the sensing elements are enclosed in a sealed reference cavity, offer the advantage of protection of sensing elements from harsh pressurized media. However, special design and processing steps are required to bring out electrical connections from the sensing elements to outside the sealed chamber, i.e. packaging of the sensor becomes costly. U.S. Pat. Nos. 5,929,497 and 6,109,113 show one way of bringing out electrical connections from a vacuum cavity. The process is complicated and uses capacitive sensors and poly-silicon connections. Wafers are bonded using an electrostatic bonding technique. The same technique with additional circuits is described in U.S. Pat. No. 6,713,828. These techniques are suited for ambient and sub-atmospheric pressure levels in motor vehicle applications.

Accordingly, a new pressure sensor, that offers a combination of a plurality of desired features, including, but not limited to, high accuracy absolute pressure sensing, wide pressure range, chemical and electrical compatibility with harsh pressurized media, reliable operation at all ranges of temperature including high temperatures, ease of manufacturing and packaging, compact size, and low cost would be desirable.

OVERVIEW

A pressure sensor is described with sensing elements electrically and physically isolated from a pressurized medium. An absolute pressure sensor has a reference cavity, which can be at a vacuum or zero pressure, enclosing the sensing elements. The reference cavity is formed by bonding a recessed cap wafer with a gauge wafer having a micromachined diaphragm. Sensing elements are disposed on a first side of the diaphragm. The pressurized medium accesses a second side of the diaphragm opposite to the first side where the sensing elements are disposed. A spacer wafer may be used for structural support and stress relief of the gauge wafer.

In one embodiment, vertical through-wafer conductive vias are used to bring out electrical connections from the sensing elements to outside the reference cavity.

In an alternative embodiment, peripheral bond pads on the gauge wafer are used to bring out electrical connections from the sensing elements to outside the reference cavity.

In various embodiments, a regular silicon-on-insulator wafer or a double silicon-on-insulator wafer may be used as the gauge wafer, and appropriate micromachining steps are adopted to define the diaphragm.

In certain embodiments, a layer of corrosion resistant material is deposited on the surface of the diaphragm that is accessed by the pressurized medium.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Those of ordinary skill in the art will realize that the following description is illustrative only and is not intended to be in any way limiting. Other embodiments will readily suggest themselves to skilled artisans having the benefit of this disclosure. Reference will now be made in detail to implementations of the example embodiments as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following description to refer to the same or like items.

In accordance with this disclosure, the components and process steps described herein may be implemented using various types of semiconductor manufacturing equipment. It is understood that the phrase “an embodiment” encompasses more than one embodiment and is thus not limited to only one embodiment.

Embodiments describe pressure sensors that can be used for a wide range of temperature and pressure, including automobile applications, as described with respect toFIGS. 10 and 11. Though absolute pressure sensors are described in detail for illustrative purposes, persons of ordinary skill in the art will appreciate that similar processes may be used to make other type of pressure sensors, such as gauge pressure sensors. In the case of an absolute pressure sensor, a reference cavity may be at actual vacuum or zero pressure, or filled with a chemically compatible fluid (such as gas or liquid) with a known reference pressure, and calibrated to determine the absolute pressure.

Although silicon is often shown as the material of choice for making a micromachined gauge wafer including the diaphragm, and a cap wafer that creates a reference cavity on top of the diaphragm, the scope of the invention is not limited by the choice of material. Similarly, although a spacer as shown can be made of silicon or Pyrex or other types of glasses or ceramic materials, the invention is not intended to be limited by the choice of spacer material.

Persons of ordinary skill in the art will now appreciate that embodiments of the present invention are likely to have better thermal performance and longetivity if the material of the gauge wafer and the material of the sensing elements have similar coefficients of thermal expansion.

FIG. 5shows a pressure sensor embodiment500, of the present invention, wherein sensing elements502are electrically and physically isolated from a pressurized medium that exerts pressure from the back side of a diaphragm506. Gauge wafer518may be made of bulk silicon. Diaphragm506may be defined by using various micromachining techniques, including, but not limited to anisotropic wet chemical etching, and dry etching (such as Deep Reactive Ion Etching). Micromachining thins down a central portion of gauge wafer518to define the diaphragm, while the unetched pedestal portions508remain intact, and provide structural support. In one embodiment Diaphragm506may be about 27 microns thick. However, thickness of the diaphragm will depend upon the range of pressure to be measured. An inner surface507of the diaphragm region of the micromachined structure/gauge wafer518is accessed by the pressurized medium that deflects the diaphragm506. A layer of electrically insulating material504is deposited or thermally grown on the outer surface509of the diaphragm region of the micromachined structure/gauge wafer518. Insulating material504may be silicon dioxide or another suitable insulating material. As no p-n junction is formed in embodiment500, as shown in detail in the inset, there is no possibility of current leakage at high temperature, as in the case of the conventional pressure sensor shown inFIG. 3. Therefore, this configuration is suitable for high temperature operation (350° C. or even higher), even if the pressurized medium is electrically conductive. Embodiment500is shown without the reference cavity enclosing the sensing elements502. However, those of ordinary skill in the art will now be able to modify the structure shown in embodiment500to create an absolute pressure sensor. Also, for the sake of clarity in this view, interconnections for getting electrical signals out from the sensing elements502are not shown.

Sensing elements502may be piezoresistors arranged in an electrical configuration to act as a piezoresistive strain gauge. Alternatively, other types of strain gauges may be used without departing from the scope of the invention.

Sensing elements502may be made of single-crystalline or poly-crystalline silicon or thinned down bulk silicon, or other types of strain gauge material, such as TiON. However, for a silicon gauge wafer518, silicon sensing elements502are the logical choice of material. Poly-crystalline silicon (for the sensing element502layer) can be formed on top of the thermally grown silicon dioxide layer504by using a low pressure chemical vapor deposition (LPCVD) process. Typically, single crystal silicon has a higher gauge factor compared to poly-silicon. The gauge factor of poly-silicon depends on its grain size. To make a silicon-on-insulator (SOI) gauge wafer, single-crystal silicon film can be formed either by the separation by Implantation of oxygen (SIMOX) process or by fusion bonding. In the SIMOX process, oxygen ions are implanted through a thin layer of silicon, and then heat treated to convert to silicon dioxide with a thin film of single crystal silicon on top. The silicon-on-oxide (SOI) can also be formed by bonding a sacrificial silicon wafer to another oxide coated silicon wafer using fusion bonding (direct bonding). The sacrificial wafer is later ground, lapped and/or polished to a relatively thin film on top of the oxide insulator layer. The thin single-crystal or poly-crystalline silicon is then doped with boron (or other dopants) using ion implantation or thermal diffusion to achieve the necessary sheet resistance. Photolithography and reactive ion etching (RIE) are used to pattern the single-crystal or poly-crystalline silicon film to form the sensing elements502. Patterned metal film (not shown) connects the sensing elements502to form a Wheatstone bridge. The temperature expansion of the sensing element502is thus well matched with the silicon diaphragm506so that there are no significant thermally induced stresses.

The micromachined gauge wafer518is bonded to a spacer wafer520using fusion bonding, anodic bonding, eutectic bonding, solder preform bonding, glass frit bonding or an alternative bonding technique. The bonding should be able to survive high-temperature operation or any subsequent processing or packaging steps that occur after bonding. As shown inFIG. 5, the bonding surface includes bottom surface510of the pedestal508of the micromachined gauge wafer518and the top surface512of the spacer520. Typically, anodic bonding is used for a pyrex spacer wafer, and fusion bonding, glass frit bonding, eutectic bonding, or thermo-compression bonding is used for a silicon spacer wafer. Other types of spacer materials and bonding techniques can be employed without diverting from the scope of the invention. The spacer wafer provides stress relief when the gauge wafer is mounted in a housing or a package that may have a different coefficient of thermal expansion from the gauge wafer.

Example Embodiment #1 of Absolute Pressure Sensor

FIG. 6shows a first embodiment600of the present invention. The absolute pressure sensor600shown inFIG. 6is similar to the pressure sensor500shown inFIG. 5, but shows additional structural components not shown inFIG. 5.

Pressure sensor600comprises three main structural components: gauge wafer518, cap wafer530, and spacer wafer520. Example dimensions for the thickness of the gauge wafer b=0.4 mm, thickness of the spacer wafer c=0.5 mm, thickness of the cap wafer a=0.5 mm. Other dimensions, i.e. width of the gauge wafer g, inner and outer diameters of the spacer wafer (e and (e+20 respectively), width of the diaphragm region d etc. are also typically in the millimeter range. An example thickness of the thinned down silicon diaphragm is 27 microns. Other dimensions are possible depending on the materials and configurations.

Gauge wafer518may include silicon interconnectors503, planarization/passivation layer558, barrier metal layer556and bonding metal layer554. Interconnectors503are typically made of the same material as the sensing elements502. Typical composition and thickness of layer554is 1 um gold. Barrier metal layer556may be 2000 Å Ti/Pt or Ti/W. In the embodiment shown inFIG. 6, eutectic gold bonding is used to couple cap wafer530with gauge wafer518. Other bonding techniques may be used and other interfacial layers may be employed according to the bonding technique. Planarization/passivation layer558may be 500 Å silicon dioxide and 2000 Å silicon nitride. As shown inFIG. 6, windows are opened in the planarization/passivation layer558to make contact with on-wafer silicon interconnects503. Sensing elements502may be exposed as shown inFIG. 6, or covered by planarization/passivation layer858(similar to layer558), as shown inFIG. 8.

An external metal layer560can be deposited at the bottom of diaphragm506. For example, a Ti/Pt/Au 500 Å/1000 Å/1500 Å metal layer can be deposited if spacer wafer520is blanket coated for bonding to a housing/package (not shown) that houses the pressure sensor600. The housing may be made of Kovar or other materials, compatible to the pressurized medium.

Cap wafer530, when bonded to gauge wafer518, creates reference cavity535. Cap wafer530has a bottom surface537that includes a recess532. The position and dimension of the recess532is such that it encloses sensing elements502. Cap wafer530includes embedded vias550that are electrically conductive. Vias550may have insulating sidewalls552if the cap wafer is made of an electrically conducting material. As will be described with respect toFIG. 7A, in one embodiment, vias may be defined as portions of the conductive cap wafer surrounded by insulating sidewalls. In other embodiments, the cap wafer may be an electrically insulating material, and vias are conductive pathways through the cap wafer. Electrical connections to sensing elements502are brought out through the vias550to the outer surface541of the cap wafer530. Outer surface of cap wafer541has an insulator layer542with windows defined in it to pattern metal pads540. Metal pads540may be made of 1 um thick Al/Si/Cu or other interconnect or bonding metallization.

In the example shown inFIG. 6, material of the cap wafer is silicon. Insulating sidewall552of each via550ensures that the vias are not shorted through the bulk silicon body of cap530. When through-wafer silicon vias are used to bring out electrical connectivity from the vacuum cavity, cap wafer530uses silicon material with relatively low bulk resistivity. Other type of conductive materials can be used too for the cap wafer.

Spacer wafer520has a central hole562to give access to the pressurized medium to the diaphragm. Hole562may be of any geometric shape, including circular, square, rectangular, polygonal etc.

FIG. 7Ashows an example method of making a cap wafer530. InFIG. 7A, in step1, a silicon wafer530A is dry etched to define deep trenches710a-b. Trench710ais a continuous trench encircling via550a, and trench710bis a continuous trench encircling via550b. In step2, trenches710a-bare filled with electrical insulator, such as silicon dioxide to create insulating sidewalls552a-b. In step3, lower portion732of the wafer530A is polished or etched off to complete the through-wafer vias. In step4, the central recess532is defined by another etching step.

FIG. 7Bshows an example method of forming spacer wafer520. In step1, a silicon/pyrex or other material wafer520A is polished to a desired thickness c. In step2, a hole562is made at the center. The hole can be made by drilling or through-wafer etching. Upper surface512may be further processed for bonding with the gauge wafer518. Note that spacer wafer520A may be polished after being bonded to the gauge wafer518.

FIG. 7Cshows example steps of making gauge wafer518. Gauge wafer518processing may begin with a silicon-on-insulator wafer518A, having a silicon layer502A on top of an silicon dioxide layer504. Alternatively, in step1, layer502A may be formed on top of an oxide-coated silicon wafer, as described with respect toFIG. 5. In step2, the layer502A is patterned to define sensing elements502and interconnects503. A passivation layer705is deposited to protect the sensing elements on top, and a window of width dl is opened in the bottom in preparation for wet etching. Passivation layer may have silicon nitride, and acts as a wet etch mask. Width dl is determined by the desired final width d of the diaphragm region. In the wet etch step3, KOH solution is used for anisotropic etching of silicon. Time of the wet etch process is controlled precisely to obtain the desired thickness (for example 27 um) of the diaphragm. Note that dry etching can be used too to define the diaphragm (as shown inFIG. 7F). In step4, the passivation layer is stripped off.

FIG. 7Dshows a combination720with the gauge wafer518and the spacer wafer520bonded to each other. As discussed before, various bonding techniques, such as anodic bonding, fusion bonding, glass frit bonding, eutectic bonding, solder preform bonding, or thermo-compressive bonding may be used, and an interfacial surface preparation for the gauge wafer and the spacer wafer is done accordingly.

FIG. 7Eshows how the gauge-spacer combination720is bonded to the cap wafer530. In step1, a planarization/passivation layer558is formed, and windows750are opened on interconnections503to bring out electrical connection from sensing elements502. In step2, barrier metal556and bonding metal layer554are formed. Typical composition and thickness of layer554is 1 um gold. Barrier metal layer556may be 2000 A Ti/Pt or Ti/W. These steps are for eutectic bonding. Different interface preparation may be needed for other forms of bonding, such as solder preform bonding, fusion bonding, flip-chip bonding, thermo-compression bonding etc. It has to be noted that the bonding process should be reliable enough to create and sustain a vacuum reference cavity535. In step3, processed cap wafer530is bonded to the gauge wafer518.

Variations of the structure and the process steps are possible within the scope of the invention. For example, in step3ofFIG. 7E, a layer762of corrosion-resistant material, such as aluminum oxide, can be deposited on the diaphragm backside. Layer762will protect the diaphragm backside506in a very corrosive pressurized medium. The deposition of the corrosion-resistant material layer762has to be done before putting down metal layer560(shown inFIG. 6). Aluminum oxide may be deposited with a Liquid Phase deposition process or other types of deposition processes.

Although not shown inFIG. 7E, adding metallization540on top of insulation layer542and adding metal layer560creates the structure similar to pressure sensor600shown inFIG. 6.

In the above process, only one set of example steps are shown and described. Intermediate steps may be added or deleted, as required. The sequence of the processing steps is not limiting to the scope of the invention. For example, persons skilled in the art will appreciate that the gauge wafer518and the cap wafer530may be bonded first, and then the cap-gauge combination may be bonded to the spacer wafer520. In that case, cap wafer and gauge wafer may be bonded using a higher temperature bonding process (for example, eutectic bonding), that remains intact while the gauge and the spacer wafers are bonded using a relatively lower temperature bonding process (for example, solder preform bonding). However, the lower temperature bonding process should still be able to sustain the operational temperature of the pressurized media. Also, wet etching of the gauge wafer (to define the diaphragm) may be done after the cap wafer is bonded to the gauge wafer.

FIG. 7Fshows another pressure sensor embodiment700that is similar to the embodiment600shown inFIG. 6with the exception of how the gauge wafer is micromachined to define the diaphragm506. In this example embodiment, a double silicon-on-insulator (SOI) wafer718is chosen as the gauge wafer. A top silicon layer718A of wafer718is sandwiched between two layers of oxide504and704. Thickness of the layer718A is determined by the desired thickness of the diaphragm. The lower silicon layer718B of wafer718is dry etched to create the opening through which the pressurized medium reaches the bottom side of the diaphragm506. Deep Reactive Ion Etching (DRIE) can be used to etch through the layer718B with the oxide layer704acting as an etch stop layer. Corrosion-resistant layer762may or may not be used depending on the application.

Example Embodiment #2 of Absolute Pressure Sensor

FIG. 8shows another example embodiment800of a pressure sensor, in which instead of vertical vias, peripheral bond pads840are used to get electrical signals out from sensing elements502. The cap wafer830inFIG. 8does not need to be made of a conductive material, such as silicon, as no electrical connection is done using the cap830. However, from a batch processing perspective, it may be advantageous to use a silicon wafer for the cap. Cap wafer830has a recess832in its bottom surface. Cap wafer830has a sidewall831which may or may not be slanted or sloped. When cap wafer830is bonded with gauge wafer518, a sealed reference cavity835is created. Cap wafer may have a bonding layer860that is mated with a planarization layer858on top of the gauge wafer.

Like pressure sensor600and700, pressure sensor800also does not employ any p-n junction, and is suitable for high-temperature applications. Additionally, as sensing elements502are away from direct contact with the pressurized medium, this configuration is compatible with harsh pressurized media, similar to pressure sensor600and700.

FIG. 9Ashows example processing steps for making a gauge wafer518shown in pressure sensor800. In step1, oxide layers504are formed on a silicon wafer518A. In step2, sensing element layer502A is formed, and doped with a dopant to obtain desired sheet resistance. In step3, sensing elements502and interconnects503are patterned photolithographically. In step4, a planarization layer858(such as PSG) is formed to encapsulate the sensing elements502and interconnects503. The layer858is planarized. In step5, windows859are opened in the planarization layer858to access interconnects503. In step6, peripheral bond pads840are created making contact with interconnects503, so that electrical connection from sensing elements502can be brought out from the vacuum cavity. In step7, the diaphragm506is defined by wet KOH etch. Dry etching can be used too to define the diaphragm. Nitride mask705may be used as a mask for the wet etch process. Though not shown in step7, a nitride mask may protect metal pad840during wet etch.

FIG. 9Bshows example processing steps for making the cap wafer830. In step1, oxide layers842are formed on a silicon wafer830A. In step2, recess832is defined by dry or wet etch. In step3, part of the wafer830A is removed by etching through. This process may create a sloped sidewall831. In step4, wafer830A is re-oxidized. In step5, a glass frit layer860is deposited if glass frit bonding is adopted. For other bonding techniques, other type of interface preparation may be needed.

FIG. 9Cshows the steps for preparing spacer wafer520. The steps are identical to the steps shown inFIG. 7B.

FIG. 9Dschematically shows how the cap wafer830, the gauge wafer518, and the spacer wafer520are assembled to create the pressure sensor embodiment800.

As described before, the sequence of bonding does not limit the scope of the invention, but the process steps need to be designed to support the bonding techniques and sequences.

Persons of ordinary skill in the art will now appreciate that a double SOI gauge wafer (as shown inFIG. 7F) can be used in embodiment800as well for defining the diaphragm506. Moreover, a corrosion-resistant layer762(as shown inFIGS. 7E-7F) may be used on the bottom surface of the diaphragm506.

Embodiments of the present invention may be used in automotive applications, such as in an intake manifold, in transmission lines, in exhaust pipes, in or near tires, in or near engines, and the like. It can also be applied in biomedical instrumentations, aerospace, defense and other fields as will now be apparent to those of ordinary skill in the art.

The environments for automotive electronic products differ depending on location within the vehicle. In general, these environments are harsher than the consumer electronic products that are used in more benign home or office environments. The harshness stems from higher temperature, high humidity, vibration, and the like.FIG. 10shows typical high temperatures encountered at different locations under the hood of an automobile [Reference: The National Electronics Manufacturing Initiative (NEMI) Roadmap, December 2000]

Table-1inFIG. 11shows the temperature extremes that would be encountered by the automotive pressure sensor depending on its location in the automobile. The temperatures shown are the temperatures of the sensor packaging surface. The internal temperature of the sensor electronic components is usually 10 to 15 degrees Celsius higher than the base plate temperature. Nowadays, more and more electronic accessories are being added to automobiles in each model year. When under-the-hood-temperatures became an issue and reliability was of major concern, many auto companies changed from 0 to 25° C. specifications to −40 to +125° C. Now, many electronic components, including automotive pressure sensors, have to pass extremely long operating tests at 140° C. or higher. Embodiments of the present invention are developed with the higher temperature and corrosive pressurized medium (exhaust gas, transmission fluid, and the like) in mind.

While embodiments and applications have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts disclosed herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.