The present invention relates to sensor die that are electrically and

mechanically coupled to provide a stable, linear output voltage in response to applied physical stimulus. Background Art

A variety of sensors are currently marketed and are well known to those skilled in the art. Sensor die may comprise pressure sensor, microphone, gyroscope, or accelerometer die. High performance sensor die maintain small measurement error as a percentage of output voltage over wide temperature range and long operating life.

Design of pressure sensors has converged over the past several years, such that most commercially available silicon-based pressure sensors share a good deal of commonality. Common elements include a diaphragm that may be round, square, or rectangular in shape, having sidelength (or diameter) to thickness ratio of roughly 20 - 200. For most silicon manufacturers, minimum diaphragm thickness is in the range of .005 - .020 mm. Surrounding the diaphragm is a relatively stiff constraint region that deflects minimally in response to applied pressure.

The diaphragm deflects proportionally to a difference in pressure on the top and bottom of the diaphragm. Various sensing components have been applied to extract an output voltage that is relatively linearly related to the diaphragm deflection, and hence to the applied pressure. For example, piezoresistive and capacitive sensing components have been widely applied.

Nominal design diaphragm thickness is limited by manufacturing variability. For example, with electrochemical etch stop, any leakage currents across a given wafer result in debiasing and localized variation in the stopping potential. Such leakage currents lead to variability in diaphragm thickness of perhaps +/- .001 mm. A reasonable diaphragm thickness tolerance is +/- 20% about nominal. Therefore, for this example manufacturing method, the minimum nominal design diaphragm thickness is about .005 mm. Sensor die cost is roughly proportional to die area. For a pressure sensor die, the output voltage in response to applied pressure is approximately proportional to the square of the ratio of diaphragm sidelength to thickness regardless of the sensing components used. For sensing at lower full-scale applied pressure, larger diaphragm sidelength-to-thickness ratio is required to maintain a given output voltage. Since diaphragms are subject to a minimum thickness due to manufacturing constraints, larger sidelength-to-thickness ratio dictates increasingly larger diaphragm size as full-scale applied pressure decreases. In turn, die size and cost is increased as full-scale applied pressure is decreased.

The minimum die size is defined by the size of the diaphragm plus

surrounding constraint region. For example, a 10 kPa full scale sensor die might have a diaphragm sidelength of 1 .0 mm, diaphragm thickness of 0.01 mm. With a constraint region width of 0.15 mm, the minimum die size prior to dicing is then 1 .3 X 1 .3 mm. However, many manufacturers apply wet anisotropic silicon etch to form diaphragms. As frequently applied, this etch procedure produces sidewalls angled at 54.74 degrees. Die size must be further increased to accommodate this slope.

Alternatively, deep reactive ion etch (DRIE) can be used to produce nearly vertical sidewalls in silicon. Unfortunately, DRIE equipment is expensive, and the DRIE cost per silicon wafer is proportional to the thickness of silicon being etched. When etching through the almost entire thickness of the wafer, as typically required to produce a diaphragm, the cost of DRIE is often considered to be prohibitive. A tradeoff exists such that smaller die can be produced but at a higher cost per silicon wafer. Therefore, many manufacturers avoid use of DRIE. A solution applying DRIE to produce vertical diaphragm sidewalls might lead to reduced die cost if, for example, the required thickness of silicon to be etched is less than about 20% of the entire thickness of the wafer.

Consider die size of 1 .3 X 1 .3 mm as a starting point. Cutting the diaphragm sidelength and thickness in half results in a die size of 0.8 X 0.8 mm prior to dicing. This is roughly the size of the smallest commercially available, high performance, silicon sensor die today. Such small die have not been widely used since high-cost vertical diaphragm sidewalls are required, but also because forming both electrical and mechanical interconnect is more challenging as die size is reduced. There is a need for innovative approaches to forming electrical and mechanical interconnects to small die. Sensing components and circuitry are formed in the top side of the wafer and must be electrically connected. Therefore, electrical connection is typically made to the top side of the die.

In a conventional design, mechanical connection is made to the bottom, or back, of the silicon die. By this connection, applied fluid pressure is also

communicated to the chamber, and hence to the diaphragm when required.

Mechanical interconnections might be formed on the front side of the wafer.

However, a significant concern is the coupling of mounting stress to the sensing diaphragm. With mechanical connection, a common problem is that stresses arising from difference in coefficient of thermal expansion or changes in material elastic constants over operating life are coupled into the diaphragm portion of the sensor die. Diaphragm deflection varies in response to any stresses. Deflection due to coupled mounting stresses cannot be distinguished from deflection due to stresses arising from the applied pressure to be measured. In effect, mounting stresses are a source of measurement error. Therefore, such mounting stresses must be minimized or isolated. A first line of defense is to mechanically connect to the bottom, or back, of the die, as far away as possible from the sensitive diaphragm.

Placing a stress buffering element between the silicon die and the port to which the die is mounted is a common approach to further isolate mounting stresses when the mechanical interconnection is to the bottom, or back, of the silicon die. For example, an anodically bonded borosilicate glass isolator is often applied. Although such isolators significantly reduce coupling of mounting stress into the sensitive diaphragm, the cost is substantially increased. In addition, inclusion of an isolator often leads to doubling or tripling the overall thickness of the mounted die. And, singulation by dicing is much more difficult and costly when using a thick glass isolator. There is a need for an approach of forming mechanical interconnection that accommodates small die while avoiding increased die area and cost, as well as increased overall thickness of die and isolator.

Even with an isolator present, the magnitude of mounting stress effects tends to increase when the temperature is changed, due to mismatch in the coefficient of thermal expansion (CTE) of the die, the isolator, adhesion layers, and the port to which the die is mounted. For example, borosilicate glass has a CTE that matches silicon reasonably well, but not perfectly. Therefore, a borosilicate glass isolator itself introduces mounting stresses as the temperature is changed. Many alternatives have been proposed for isolators. For example, mounting stresses can also be isolated by interposing between the die and the port a compliant member that flexes in response to applied stress. Although many such compliant members have been proposed, the solution is seldom applied due to increased cost and complexity. Coupling of mounting stresses can also be reduced by increasing the thickness of the isolator. In practice, isolator complexity and thickness are limited by assembly processes or by application requirements on size and cost of packaged sensor die.

Most typically, the port is formed of a metal alloy having a CTE that is substantially larger than the silicon CTE. To deal with the issue of large CTE mismatch between the die, isolator, adhesion layer and port, soft adhesion layer materials are often used to attach the die on isolator to the port. For example, room temperature vulcanizing (RTV) silicone materials having very low shear elastic constants are often used. The combination of minimizing stress coupling by such soft mount and inclusion of an isolator often delivers acceptable performance, albeit with a significant penalty on size and cost.

Epoxies, solders, eutectic alloys, and fritted glass are representative of mechanically hard adhesion layer materials. Such materials undesirably couple mounting stresses arising from CTE mismatch between the port, isolator and the die into the sensitive diaphragm portion of the sensor die. In addition, hard adhesion layer materials may themselves introduce stresses that affect the diaphragm. For example, the elastic constants of epoxy are known to change over operating life, while solders are subject to creep over temperature and operating life. However, there are significant motivations to apply such hard mount adhesion layer materials. For example, application of epoxy is desired for ease of assembly and for cost reduction. Application of solder or eutectic alloy is desired for hermetic applications. An approach is needed that accommodates an adhesion layer formed of hard materials without resulting in undue coupling of mounting stresses to the sensitive sensor die diaphragm.

A drawback common to various circuits formed of sensing components is that a non-zero electrical output voltage, termed an offset voltage, is seen at the output terminals. The offset voltage is undesirable at room temperature, since it complicates sensor calibration. However, the offset voltage also changes with temperature, partially due to CTE mismatch. It is challenging to directly compensate for the temperature change in offset (TCO). In fact, the specified device operating temperature range is often limited primarily by TCO considerations. Reproducible changes in offset voltage over temperature and non-reproducible changes in offset voltage over operating life together establish critical limitations on performance. It is difficult to correct for both of these error sources over operating life. And, it is well established empirically that as die size is decreased, performance as limited by TCO becomes steadily worse. But, there is a need for sensor die having both reduced die size and high performance. For high performance, both room temperature offset voltage and TCO must be minimized.

An option to further reduce mounting stress effects is to offset or cantilever the diaphragm away from the port. Mounting stresses are strongly coupled vertically, but weakly coupled laterally. For example, see Patent No. U.S. 5,412,994, in which the diaphragm is offset from the port. From the specification, "This offset

characteristic improves the stress isolation between the attachment of the composite structure to a fluid conduit and the location on the sensor die where the sensitive components are located." This approach is successful in isolating mounting stresses when implemented with an isolator. The approach is reasonably successful even when using hard mount materials such as epoxy or eutectic solder that readily couple mounting stresses arising from attachment of the isolator to the port. Even so, the approach has not been widely applied due to higher cost. The die area and therefore cost is increased by about 60 - 150% in order to cantilever the diaphragm away from the port. In addition, the sensor die diaphragm must be mechanically connected to a port, requiring that lateral connecting channels and/or through holes are also formed in the isolator. Additional cost is incurred to form these features. Finally, the isolator material is borosilicate glass as opposed to silicon. The CTE mismatch between silicon and borosilicate glass leads to increased TCO. The result is an expensive isolator that itself introduces mismatch stresses over temperature. The cantilever approach has merit, but there is a need for an inventive approach that simultaneously reduces cost and minimizes coupling of mounting stresses.

In U.S. Patent No. 6,993,975, a pressure sensor module incorporates a platform providing stress isolation to a pressure sensing element. This patent does not teach how to make a sensing element. From the specification and drawings it appears that the sensing element does not involve a diaphragm. And, it appears that there are no fluid conduits associated with the sensing element. In U.S. Patent No. 7,635,077, a method of flip-chip mounting pressure sensor die to a substrate is taught. The method makes no provision for stress isolation. A low-cost method of manufacturing pressure sensors suitable for price sensitive applications is provided, without regard to performance in terms of measurement error. To those skilled in the art it will be obvious that this method results in unacceptably high measurement error for the majority of applications. There is a need for a flip-chip mounting approach that simultaneously reduces cost and maintains or improves on established performance.

Based on the foregoing, it would be significantly beneficial if a means were provided to eliminate the need for a separate isolator by integrating stress isolation features into a sensor die, thereby preventing or reducing coupling of mounting stresses to the sensitive diaphragm. It would also be highly beneficial if such integration leads to smaller area, thinner, and less costly packaged sensor die.

Much of the foregoing discussion also applies to other sensor types. For example, the discussion applies equally to microphones, accelerometers, and gyroscopes. In each case, mounting stresses may be coupled into sensitive areas of the sensor, resulting in measurement error. In each case, there is a need for a mounting approach that simultaneously provides low cost, small size and high performance.

Summary of Invention

The invention enables integration of multiple stress isolation features into a single, silicon-based, sensor die, thereby eliminating the need for a separate isolator.

In one aspect, the invention is directed to a sensor comprising a sensor die, the die comprising a planar first surface comprising a first axis and second axis, wherein the second axis is perpendicular to the first axis and the first axis is longer than the second axis, and wherein the first axis comprises first and second ends; a second surface adjacent to the first surface; a cavity disposed between the first and second surfaces, wherein the cavity is proximate the first end of the first axis;

a diaphragm capping the cavity; and one or more stress isolation features disposed between the first and second surfaces, wherein the stress isolation features comprise cavities spaced apart from and not in fluidic communication with the cavity; a substrate proximate to the second surface; an adhesion layer mechanically bonding the second surface to the substrate, wherein the adhesion layer is nearer the second end of the first axis than the first end of the first axis and wherein the adhesion layer does not extend beneath the diaphragm; and a sensing component disposed on or proximate to the diaphragm, wherein the sensing component is connected in signal communication with an external circuit.

In another aspect, the invention is directed to a sensor comprising a sensor die, the sensor die comprising a planar first surface comprising a first axis and second axis, wherein the second axis is perpendicular to the first axis and the first axis is longer than the second axis, and wherein the first axis comprises first and second ends; a second surface adjacent to the first surface; a cavity disposed between the first and second surfaces, wherein the cavity is proximate the first end of the first axis; a diaphragm capping the cavity; and one or more stress isolation features disposed between the first and second surfaces, wherein the stress isolation features comprise cavities spaced apart from and not in fluidic

communication with the cavity; a substrate proximate to the second surface; an adhesion layer mechanically bonding the second surface to the substrate, wherein the adhesion layer is nearer the second end of the first axis than the first end of the first axis and wherein the adhesion layer does not extend beneath the diaphragm; a sensing component disposed on or proximate to the diaphragm; an electronic circuit connected to the sensing component; and an external circuit electrically connected to the electronic circuit.

In another aspect, the invention is directed to a sensor comprising a sensor die, the sensor die comprising a planar first surface comprising a first axis and second axis, wherein the second axis is perpendicular to the first axis and the first axis is longer than the second axis, and wherein the first axis comprises first and second ends; a second surface adjacent to the first surface; a first cavity disposed between the first and second surfaces, wherein the first cavity is proximate the first end of the first axis; a diaphragm capping the first cavity; a second cavity disposed between the first and second surface to form a chamber, wherein the chamber is proximate a second end of the first axis; a channel disposed between the first and second surface, wherein the channel connects the first cavity and the chamber whereby fluid may flow therebetween; and one or more stress isolation features disposed between the first and second surfaces, wherein the stress isolation features comprise cavities spaced apart from and not in fluidic communication with the first cavity; a via connecting the chamber to the second surface; a substrate proximate to the second surface, wherein the substrate comprises a port connected in fluidic communication with the via in the sensor die and a pressure source; an adhesion layer mechanically bonding the second surface to the substrate, wherein the adhesion layer is nearer the second end of the first axis than the first end of the first axis and wherein the adhesion layer does not extend beneath the diaphragm and does not extend beneath the via; and a sensing component disposed on or proximate to the diaphragm, wherein the sensing component is connected in signal communication with an external circuit.

In another aspect, the invention is directed to a sensor comprising a sensor die, the sensor die comprising a planar first surface comprising a first axis and second axis, wherein the second axis is perpendicular to the first axis and the first axis is longer than the second axis, and wherein the first axis comprises first and second ends; a second surface adjacent to the first surface; a first cavity disposed between the first and second surfaces, wherein the first cavity is proximate the first end of the first axis; a diaphragm capping the first cavity; a second cavity disposed between the first and second surface to form a chamber, wherein the chamber is proximate the second end of the first axis; a channel disposed between the first and second surface, wherein the channel connects the first cavity and the chamber whereby fluid may flow therebetween; and one or more stress isolation features disposed between the first and second surfaces, wherein the stress isolation features comprise cavities spaced apart from and not in fluidic communication with the first cavity; a via connecting the chamber to the second surface; a substrate proximate to the second surface, wherein the substrate comprises a port connected in fluidic communication with the via in the sensor die and a pressure source; an adhesion layer mechanically bonding the second surface to the substrate, wherein the adhesion layer is nearer the second end of the first axis than the first end of the first axis and wherein the adhesion layer does not extend beneath the diaphragm and does not extend beneath the via; a sensing component disposed on or proximate to the diaphragm; an electronic circuit connected to the sensing component; and an external circuit electrically connected to the electronic circuit.

In another aspect, the invention is directed to a sensor comprising a sensor die, the sensor die comprising a planar first surface comprising a first axis and second axis, wherein the second axis is perpendicular to the first axis and the first axis is longer than the second axis, and wherein the first axis comprises first and second ends; a second surface adjacent to the first surface; a cavity disposed between the first and second surfaces, wherein the cavity is proximate the first end of the first axis; a diaphragm capping the cavity; and one or more stress isolation features disposed between the first and second surfaces, wherein the stress isolation features comprise cavities spaced apart from and not in fluidic

communication with the cavity; a substrate proximate to the first surface; an adhesion layer mechanically bonding the first surface to the substrate, wherein the adhesion layer is nearer the second end of the first axis than the first end of the first axis and wherein the adhesion layer does not extend beneath the diaphragm; and a sensing component disposed on or proximate to the diaphragm, wherein the sensing component is connected in signal communication with an external circuit.

In another aspect, the invention is directed to a sensor comprising a sensor die, the sensor die comprising a planar first surface comprising a first axis and second axis, wherein the second axis is perpendicular to the first axis and the first axis is longer than the second axis, and wherein the first axis comprises first and second ends; a second surface adjacent to the first surface; a cavity disposed between the first and second surfaces, wherein the cavity is proximate the first end of the first axis; a diaphragm capping the cavity; and one or more stress isolation features disposed between the first and second surfaces, wherein the stress isolation features comprise cavities spaced apart from and not in fluidic

communication with the cavity; a substrate proximate to the first surface; an adhesion layer mechanically bonding the first surface to the substrate, wherein the adhesion layer is nearer the second end of the first axis than the first end of the first axis and wherein the adhesion layer does not extend beneath the diaphragm; a sensing component disposed on or proximate to the diaphragm; an electronic circuit connected to the sensing component; and

an external circuit electrically connected to the electronic circuit.

In another aspect, the invention is directed to a sensor comprising a sensor die, the sensor die comprising a planar first surface comprising a first axis and second axis, wherein the second axis is perpendicular to the first axis and the first axis is longer than the second axis, and wherein the first axis comprises first and second ends; a second surface adjacent to the first surface; a first cavity disposed between the first and second surfaces, wherein the first cavity is proximate the first end of the first axis; a diaphragm capping the first cavity; a second cavity disposed between the first and second surface to form a chamber, wherein the chamber is proximate a second end of the first axis; a channel disposed between the first and second surface, wherein the channel connects the first cavity and the chamber whereby fluid may flow therebetween; and one or more stress isolation features disposed between the first and second surfaces, wherein the stress isolation features comprise cavities spaced apart from and not in fluidic communication with the first cavity; a via connecting the chamber to a first surface; a substrate proximate to the first surface, wherein the substrate comprises a port connected in fluidic communication with the via in the sensor die and a pressure source; an adhesion layer mechanically bonding the first surface to the substrate, wherein the adhesion layer is nearer the second end of the first axis than the first end of the first axis and wherein the adhesion layer does not extend beneath the diaphragm and does not extend beneath the via; and a sensing component disposed on or proximate to the diaphragm, wherein the sensing component is connected in signal communication with an external circuit.

In another aspect, the invention is directed to a sensor comprising a sensor die, the sensor die comprising a planar first surface comprising a first axis and second axis, wherein the second axis is perpendicular to the first axis and the first axis is longer than the second axis, and wherein the first axis comprises first and second ends; a second surface adjacent to the first surface;

a first cavity disposed between the first and second surfaces, wherein the first cavity is proximate the first end of the first axis; a diaphragm capping the first cavity;

a second cavity disposed between the first and second surface to form a chamber, wherein the chamber is proximate the second end of the first axis; a channel disposed between the first and second surface, wherein the channel connects the first cavity and the chamber whereby fluid may flow therebetween; and one or more stress isolation features disposed between the first and second surfaces, wherein the stress isolation features comprise cavities spaced apart from and not in fluidic communication with the first cavity; a via connecting the chamber to the first surface; a substrate proximate to the first surface, wherein the substrate comprises a port connected in fluidic communication with the via in the sensor die and a pressure source; an adhesion layer mechanically bonding the first surface to the substrate, wherein the adhesion layer is nearer the second end of the first axis than the first end of the first axis and wherein the adhesion layer does not extend beneath the diaphragm and does not extend beneath the via; a sensing component disposed on or proximate to the diaphragm; an electronic circuit connected to the sensing component; and an external circuit electrically connected to the electronic circuit. Brief Description of Drawings

Fig. 1A is a side cross-sectional view illustration of a conventional prior art silicon die having sloped diaphragm sidewalls and stress isolation mount.

Fig. 1 B is a side cross-sectional view illustration of a conventional prior art silicon die having vertical diaphragm sidewalls formed by DRIE. Stress isolation mount is included.

Fig. 2 is a side cross-sectional view illustration of a prior art silicon die. The mechanical connection to a port is on one end of the die (right side in view), while the diaphragm is positioned on the opposite end of the die (left side in view).

Fig. 3A is a plan view illustration of a first embodiment of the inventive die. Fig. 3B is a sectional view illustration of a first embodiment of the inventive die.

Fig. 4A is a plan view illustration of a second embodiment of the inventive die incorporating buried stress isolation cavities.

Fig. 4B is a sectional view illustration of a second embodiment of the inventive die incorporating buried stress isolation cavities.

Fig. 4C is a plan view illustration of a second embodiment of the inventive die where connecting channel is serpentine.

Fig. 4D is a plan view illustration of a second embodiment of the inventive die where connecting channel is serpentine and coarse-feature buried stress isolation cavities are included.

Fig. 4E is a plan view illustration of a second embodiment of the inventive die connecting channel is serpentine and fine-feature buried stress isolation cavities are included.

Fig. 4F is a plan view illustration of a second embodiment of the inventive die where connecting channel is serpentine and arbitrary-shape buried stress isolation cavities are included.

Fig. 5A is a plan view illustration of a third embodiment of the inventive die including top-down stress isolation slots.

Fig. 5B is a side view illustration of a third embodiment of the inventive die including top-down stress isolation slots.

Fig. 5C is a plan view illustration of a third embodiment of the inventive die including both top-down stress isolation slots and bottom-up stress isolation slots.

Fig. 5D is a side view illustration of a third embodiment of the inventive die including both top-down stress isolation slots and bottom-up stress isolation slots.

Fig. 6A is a plan view illustration of a fourth embodiment of the inventive die incorporating flip-chip die attach and multiple stress isolation features, optionally including top-down stress isolation slots, bottom-up stress isolation slots, and buried stress isolation cavities.

Fig. 6B is a side view illustration of a fourth embodiment of the inventive die incorporating flip-chip die attach and multiple stress isolation features, optionally including top-down stress isolation slots, bottom-up stress isolation slots, and buried stress isolation cavities.

Fig. 7A is a plan view illustration of a fifth embodiment of the inventive die incorporating backstops.

Fig. 7B is a sectional view illustration of a fifth embodiment of the inventive die incorporating backstops.

Description of Embodiments

A sensor according to preferred embodiments of the invention comprises a sensor die having first and second surfaces. Means for forming both mechanical and electrical interfaces to the sensor die are provided. The first surface of the sensor die includes a diaphragm portion extending across a buried cavity, sensing components disposed on or near the diaphragm as needed, and optional accompanying circuitry. A connecting channel, chamber, and port allow for coupling of applied pressure to the buried cavity beneath the diaphragm as required.

Mounting to a port is disposed at one end of the die, while the diaphragm is disposed at the opposite end of the die. Mechanical attachment is formed only at the port end of the sensor die. The remainder of the sensor die containing the diaphragm is cantilevered away from the port mounting surface.

In a first embodiment, a buried cavity beneath diaphragm and optionally buried cavities comprising connecting channel and chamber are defined with a single photomask and DRIE step. In said first embodiment, electrical connections are formed to the first, or top, surface of the sensor die. For example, conventional wire bonding may be applied. Mechanical connections are made to the second, or bottom, surface of the sensor die. Since the sensitive diaphragm region is

cantilevered away from the port mounting surface, a separate isolator is not required to meet performance requirements for many applications.

Stress isolation is further improved by addition of lateral compliant structures.

And, additional stress isolation features comprised of slots cut into either the top surface, bottom surface, or both top and bottom surfaces may be included in order to reduce lateral coupling of stresses into the diaphragm region.

A second embodiment is similar to the first embodiment, with the addition of buried stress isolation cavities. Integration of buried cavities designed to further improve isolation of mounting stresses is accomplished by simply adding features to the photomask used during the DRIE step, with no added cost relative to the first embodiment.

A third embodiment is similar to the second embodiment, with the addition of top-down isolation slots, bottom-up isolation slots, or both top-down and bottom-up isolation slots formed in both first and second surfaces of the sensor die to further improve isolation of mounting stresses.

The invention enables flip-chip mounting of sensor die while maintaining performance meeting a majority of application requirements. In a fourth embodiment, similar to the first embodiment in that buried stress isolation cavities are included, the sensor die is flip-chip mounted with both the electrical, the mechanical and optionally the fluidic connections made to the first, or top, surface of the die. An electrically conductive adhesion layer is used to mechanically attach the sensor die to the port. Electrically isolated portions of said electrically conductive adhesion layer form electrical connections to each of several circuit interconnect pads on the die. It is understood that slots may optionally be cut into either the first surface, second surface, or both first and second surfaces to optimize mounting stress isolation and cost for a given application. Integration of stress isolation features enables flip-chip mounting to a port without incurring increased measurement error. In addition, with flip-chip mounting, application of DRIE is required to penetrate only a small percentage of the sensor die thickness, resulting in lower fabrication costs. In fact, flip-chip mounting is generally considered as being the lowest cost method of assembling sensor die. The method is infrequently applied with sensor die due to performance degradation. In a fifth embodiment, backstops are optionally included to protect against diaphragm breakage both during fabrication and during operating life. It is

understood that various combinations of buried isolation cavities, top-down and bottom-up isolation slots, and die orientation (normal vs. flip-chip) may be used simultaneously with backstops.

Generally, fabrication follows the method commonly known as silicon-on- insulator (SOI). Fabrication begins with a wafer that has been prepared to have one or more buried cavities. According to this procedure, a first cavity and optionally additional cavities forming connecting channels extending from the first cavity to second cavity defining a chamber are photolithographically defined and etched by DRIE into a first wafer. For example, the wafer may be 0.5 mm thick, while the etched features are 0.1 mm deep. A second wafer, preferably coated with a dielectric layer such as silicon dioxide, is direct fusion bonded to the first wafer. The direct fusion bond may optionally be performed under vacuum such that the pressure in the buried cavities is less than about 10 ^"5 bar. The thickness of the second wafer is then reduced to a target value equal to a planned diaphragm thickness. At this point, portions of the device layer formed by thinning the second wafer cover all of the buried cavities. Typically, the portion of the device layer covering the largest cavity is termed a diaphragm. Optionally, connecting channels leading from the cavity beneath the diaphragm to a port-connecting chamber and the chamber itself are formed simultaneously with other cavities. Stress isolation cavities may be formed simultaneously with the buried cavity beneath the diaphragm by simply adding features to a photomask. The various functions of diaphragm, connecting channel, chamber and stress isolation cavities are provided by a single preparation procedure on a sensor wafer.

Some non-uniformity in the device layer thickness results from thinning of the second wafer. Such non-uniformity may be minimized by maintaining a sidelength to thickness ratio of no more than 40:1 for all features. Alternatively, a layer transfer technique, well known to those skilled in the arts, may be applied to obtain sidelength to thickness ratios much greater than 40:1 while maintaining acceptable thickness uniformity.

Importantly, diaphragm thickness of 0.002 mm is achievable using

conventional thinning methods. Generally, diaphragm thickness variation can be controlled to within +/- .0005 mm of target value by use of these methods. Using alternative layer transfer methods, device layer thickness may be reduced to a few nanometers. Some care is required to ensure that all features formed by device layer over buried cavities are sufficiently robust to enable follow-on fabrication. Optionally, the continuity of buried cavities may be interrupted to include supporting posts. As is well known to those skilled in the arts, such supporting posts may be designed to provide diaphragm backstop, which enhances robustness.

With thinner diaphragm the sidelength can be reduced, resulting in smaller die. For example, with 0.2 mm diaphragm sidelength, and constraint width of 0.15 mm, the die size might be reduced to 0.5 X 0.5 mm. However, some relationship exists between the required constraint width and the diaphragm thickness. For example, to maintain sufficient stiffness, both the constraint width and thickness should be at least about 3 times the diaphragm thickness, and preferably at least about 10 times the diaphragm thickness. Therefore, for a diaphragm thickness of .002 mm, the constraint width may comfortably be reduced to .05 mm. In practice, a rectangular die sized 0.3 X 0.5 mm may meet all requirements. In this case, the chamber and via connecting to the port may be limited to about 0.1 mm diameter.

Subsequent fabrication procedures lead to formation of any required sensing components, along with associated interconnect metal and pads. Interconnect metal and pads typically enable electrical connection, but may also include, for example, an annular ring about the port to make fluidic and mechanical connection. Those skilled in the arts will recognize that many alternative procedures are satisfactory to form such sensing components and interconnect.

It is also understood that in addition to sensing components, other circuitry may be formed on the same surface. For example, added circuit elements may include resistors, capacitors, inductors, transistors, and isolation regions. In fact, such various circuit elements along with associated interconnect metal and pads are known to comprise an integrated circuit.

Finally, it is understood that said interconnect metal and pads must be compatible with the chosen method for electrical interconnection. For example, if wire bonding is the intended means of forming electrical connection, the top layer of the pads is typically formed of aluminum or gold. If conductive epoxy or solder is to be used, the top layer of the pads is typically formed of gold. With solder, the under layers of the interconnect pads must incorporate a solder stop. In each case, a bottom-most under layer of the interconnect pads may optionally be incorporated to enhance adhesion.

With sensing components, other circuit elements, and interconnection formed, the next steps are normally to electrically test, then singulate each die. The instant invention allows for two photopattern and DRIE steps prior to singulation. A first, optional photopattern and DRIE step is included to form slots extending from the first, or top, surface towards the second, or back, surface. In this case, the DRIE is targeted to remove .002 - .400 mm of silicon thickness in the areas not protected by photoresist. A second, optional, photopattern and etch step is included to form a fluidic connection between the chamber and the second, or bottom, surface of the die.

Following completion of first and second photopattern and DRIE steps, and removal of photoresist, the wafer can proceed to electrical test and singulation.

The first, slot-creating, photopattern and etch step can fulfill multiple purposes. First, silicon can be substantially removed as required to create a via bringing the chamber into fluidic communication with the port. Second, coupling of mounting stresses into the diaphragm portion can be further reduced, both at ambient temperature and across the specified temperature range, by incorporation of slots. Third, die can be partially or totally singulated by etching in the dicing lanes.

The second, chamber-connecting, photopattern and etch step can also fulfill multiple purposes. First, the step can complete any removal as required to bring the chamber into fluidic communication with the port. Second, additional slots may be created on the second, or bottom, surface of the die, to further reduce coupling of mounting stresses. Third, again die can be partially or totally singulated by etching in the dicing lanes.

To complete wafer-level fabrication, interconnect pads may optionally be selectively coated with a material that meets the requirements for both electrical and mechanical interconnections. For example, conductive epoxy may be applied by screen printing methods, covering interconnect pads and forming an annular ring about the port.

Practice of the present invention is expected to lead to substantially smaller area, thinner and lower cost sensor die having performance meeting most applications requirements. Further, such improvement will enable applications that cannot presently be serviced.

It will be appreciated that while silicon materials are predominantly used in forming sensor die, other substrate materials, for example silicon carbide, ll-VI, or III- V compound semiconductor materials, may be advantageously applied in practicing the present invention.

It will also be appreciated that various types of sensor die may benefit from the present invention. Microphone die are very similar to pressure sensing die in that sensor output is proportional to diaphragm deflection. Paddle-style accelerometer die may be viewed as being functionally similar to pressure sensing die, with at least three sides of a diaphragm cut to enable motion. Similarly, gyroscope die may be viewed as functionally similar to pressure sensing die, with the diaphragm

substantially free to move, and suspended by highly compliant members. In each case, measurement error can be reduced by practicing the instant invention.

Turning now to the drawing figures, Figure 1 A is an illustration of a prior art conventional die having a diaphragm formed by wet anisotropic etch. Silicon die 10 including sensing components 40 is mounted on isolator die 20, with optional use of adhesion layer 15. The assembly formed by silicon die 10 and isolator 20 is further mounted onto port 30 using adhesion layer 25. As can be seen, the diaphragm sloped sidewalls contribute to an increased die size. For example, with a wafer thickness of 0.5 mm, a diaphragm having 0.5 mm sidelength, and a design rule of minimum diaphragm etched feature to die edge spacing of 0.1 mm, the minimum die size is about 1 .4 X 1 .4 mm.

Figure 1 B is an illustration of a prior art improved silicon die 50 mounted in the same fashion to isolator 60. The diaphragm sidewalls are perpendicular, resulting in reduced die size for the same diaphragm size. For example, with a wafer thickness of 0.5 mm, a diaphragm having 0.5 mm sidelength, and a design rule of minimum diaphragm to die edge spacing of 0.1 mm (post-dicing), the minimum die size is 0.7 X 0.7 mm. However, in this case, the cost to extend DRIE through most of the thickness of the silicon wafer is significant. In addition, an etch stop approach must be selected. SOI is one convenient etch stop method.

Figure 2 is an illustration of a prior art sensor die design having an offset diaphragm. Isolator die 1 10 is formed of borosilicate glass. Silicon die 100 is attached to isolator 1 10 by anodic bonding of borosilicate glass to silicon. The assembly formed by silicon die 100 and isolator die 1 10 is further mounted onto port 130 using adhesion layer 120. Diaphragm 105 is included in a region of the silicon die 100 that is cantilevered above a surface of port 130 (offset from the port). Gap 140 maintains mechanical isolation between the surfaces. This approach results in reduced coupling of mounting stresses into diaphragm 105, and therefore in reduced measurement error. It is generally understood that a pressure to be measured may be applied to port 130 and hence to conduit 150, such that said pressure is coupled to diaphragm 105.

Fig. 3A is a plan view, Fig. 3B a sectional view illustrating a first embodiment of a die according to the present invention. Referring to Fig. 3, diaphragm 320, connecting channel 330, and chamber 335 are simultaneously formed during wafer fabrication photolithography and deep reactive ion etch (DRIE) steps. Subsequently, silicon layer 300 is formed by direct silicon fusion bond of a second wafer followed by thinning. Thickness of silicon layer 300 establishes the thickness of diaphragm 320. Following formation of sensing components and interconnect by conventional methods, conduit 305 is formed by photolithography and DRIE step. It is understood that the photopattern used to form conduit 305 is carefully aligned to chamber 335 using industry standard methods. Following completion of fabrication and die separation, silicon die 310 is mounted to port 315 using adhesion layer 380. Conduit 305 connects chamber 335 to port 315 allowing for coupling of pressure to connecting channel 330 and hence to diaphragm 320.

Importantly, adhesion layer 380 mounting is positioned on the opposite end of silicon die 310 from diaphragm 320, allowing for isolation of mounting stresses. The inventive approach results in significant cost (size) reduction both by incorporation of vertical diaphragm sidewalls and by elimination of the requirement for specialized epitaxial layers and a separate isolator formed of borosilicate glass. The all-silicon construction results in perfect matching of expansion coefficients over temperature. Port 315 includes conductive traces 350. Electrical connection between conductive traces 350 and circuit interconnect pads 340 on silicon die 310 are formed by wire bonds 360. As will be understood by those skilled in the art, conductive traces 350 may alternately be formed on a secondary structure, for example a printed circuit board, with wire bonds 360 connecting circuit interconnect pads 340 to said secondary structure. Although not shown, it is further understood that additional conductive traces will connect circuit interconnect pads 340 to sensing components on diaphragm 320.

Fig. 4A is a plan view, Fig. 4B a sectional view of Section B - B taken from Fig. 4A, together illustrating a second embodiment of the inventive die incorporating additional stress isolation features. Referring to Fig. 4, diaphragm 400, connecting channel 410, chamber 420, and buried stress isolation cavities 430 are

simultaneously formed during wafer fabrication photolithography and deep reactive ion etch steps. It is understood that rectilinear shapes are shown for illustration purposes, and that connecting channel 410 and buried stress isolation cavities 430 may be curvilinear and of arbitrary shape. There is a limitation on both the widths and spacings of buried stress isolation cavities 430. To minimize the possibility of breakage during fabrication, buried stress isolation cavities 430 should be no wider than about one-fifth of the diaphragm 400 sidelength. To minimize lateral flexing due to applied pressure in connecting channel 410 and diaphragm 400, buried stress isolation cavities 430 should be spaced apart from connecting channel 410, chamber 420, and the cavity beneath diaphragm 400 by a minimum of about 3 - 4 times the thickness of device layer 440.

Fig. 4C and Fig. 4D are plan views further illustrating a second embodiment of the inventive die. Referring to Fig. 4C, connecting channel 450 is shown as serpentine. Referring to Fig. 4D, connecting channel 450 is serpentine, and coarse- feature buried stress isolation cavities 460 are included.

Fig. 4E and Fig. 4F are plan views further illustrating a second embodiment of the inventive die incorporating additional stress isolation features. Referring to Fig. 4E, connecting channel 450 is shown as being serpentine. Fine-feature buried stress isolation cavities 470 are included. By reducing the feature width and spacing between adjacent features, a multiplicity of such cavities may be included. As will be apparent to those skilled in the art, feature widths may be as small as about .05 times feature etched depth, while feature spacing is limited by undercut to be about the same as feature width. For example, with a 0.2 mm etch depth, feature width and spacing may each be 0.01 mm. Such feature size is substantially larger than typical minimum geometry as defined by photolithography limitations. Referring to Fig. 4F, connecting channel 450 is serpentine, and arbitrary-shape buried stress isolation cavities 480 are included. As above, some \restrictions must be observed to minimize lateral flexing due to applied pressure.

Figure 5 is an illustration of a third embodiment of the invention. Fig. 5A is a plan view, Fig 5B a side view illustration of a third embodiment of the inventive die incorporating additional stress isolation features. Top-down stress isolation slots 520 are formed in silicon layer 500 and extend into silicon substrate 510. Top-down stress isolation slots 520 are formed by photolithography and DRIE. Such top-down stress isolation slots 520 further enhance stress isolation and prevent coupling of stress from the mounting platform to the sensitive diaphragm.

Fig. 5C is a plan view, Fig 5D a side view illustration of a third embodiment of the inventive die incorporating yet additional stress isolation features. Top-down stress isolation slots 520 are formed in silicon layer 500 and extend into silicon substrate 510. Top-down stress isolation slots 520 are formed by a first

photolithography and DRIE step. Additionally, bottom-up stress isolation slots 530 are formed by a second photolithography and DRIE step. Such top-down stress isolation slots 520 and bottom-up stress isolation slots 530 further enhance stress isolation.

Figure 6 is an illustration of a fourth embodiment of the invention. Fig. 6A is a plan view, Fig 6B a side view illustration of a fourth embodiment of the inventive die incorporating flip-chip die attach and multiple stress isolation features. An optimum combination of top-down stress isolation slots 600, bottom-up stress isolation slots 610, and buried stress isolation cavities can be applied. Inclusion of various stress isolation features is a critical enabler allowing the die to be flip-chip mounted to a port without degrading performance. With flip-chip, electrical and mechanical functions are combined into a single interconnection. Critical to this embodiment is a die attach material that both has low electrical resistance and is mechanically strong. For example, epoxy doped with silver, nickel, or gold has the appropriate die attach material properties. However, as is well known, epoxy does not form a hermetic seal. Hermetic materials are required in some applications. Solder and eutectic alloys in general are higher cost alternative flip-chip attachment materials that are hermetic. To apply solder or eutectic alloys, solderable metal must be applied to both the die and the port. And, it is well known that solder exhibits thermal hysteresis and tends to creep with time. In fact, usage of solder mounting without addition of the inventive lateral isolation features would result in significantly reduced performance.

Figure 7 is an illustration of a fifth embodiment of the invention. Fig. 7A is a plan view, Fig 7B a side view illustration of a fifth embodiment of the inventive die incorporating backstops 700. Backstops 700 are optionally included to protect diaphragm 730 against breakage during fabrication, which is key to enabling larger diaphragm sidelength-to-thickness ratios. However, backstops 700 are also beneficial over the operating life in protecting diaphragm 730 from breakage during accidental application of pressure that causes diaphragm 730 to be deflected towards said second surface of the sensor die. Formation of backstops 700 requires an additional photolithography and DRIE step prior to forming buried cavities 710 and prior to fusion bonding and thinning to form silicon layer 720. For example, a first photolithography pattern would include all buried cavity features, and DRIE would be performed perhaps only .001 - .005 mm deep. Said first photolithography pattern would be removed and a second photolithography pattern having all buried cavity features except for backstop 700 regions would be applied. Using said second photolithography pattern, DRIE would be performed to normal target depth.

Wafer thinning may be completed prior to singulation as a means of further miniaturizing sensor die and further optimizing lateral stress isolation.

As used herein, "comprising" is synonymous with "including," "containing," or "characterized by," and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, "consisting of excludes any element, step, or ingredients not specified in the claim element. As used herein, "consisting essentially of does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term "comprising", particularly in a description of components of a composition or in a description of elements of a device, is understood to encompass those

compositions and methods consisting essentially of and consisting of the recited components or elements. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. Thus, additional embodiments are within the scope of the invention and within the following claims. In general the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The preceding definitions are provided to clarify their specific use in the context of the invention.

All references cited herein are hereby incorporated by reference to the extent that there is no inconsistency with the disclosure of this specification.

The embodiments and examples set forth herein are presented to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. The foregoing description and examples have been presented for the purpose of illustration and example only. These descriptions as set forth are not intended to be exhaustive or to limit the scope of the invention. As just one example, although many included illustrations depict rectilinear geometries, it is understood that the teachings are valid when curvilinear geometries are substituted. Many variations and modifications of the present invention will be apparent to those of skill in the art. The present invention has been described with reference to certain preferred and alternative embodiments that are intended to be exemplary only, and not limiting to the full scope of the present invention as set forth in the appended claims.