WO2001046665A1

WO2001046665A1 - Multivariate semiconductor pressure sensor with passageway

Info

Publication number: WO2001046665A1
Application number: PCT/US2000/034781
Authority: WO
Inventors: Clifford D. Fung; P. Rowe Harris; Deguang Zhu
Original assignee: The Foxboro Company
Priority date: 1999-12-20
Filing date: 2000-12-20
Publication date: 2001-06-28
Also published as: AU2446401A; WO2001046665A9; WO2001046665A8

Abstract

A multivariate semiconductor pressure sensor includes differential pressure sensing elements formed by a polysilicon membrane (14) spanning a sensor cavity (38) formed in a substrate (12). The sensor cavity (38) is in communication with fluid pressure on the back surface of the substrate (12) by way of a fluid passageway (46) that connects an exterior opening in the back surface of the substrate with an interior opening into either the sensor cavity (38) itself or into a feeder cavity (62) that is in fluid communication with the sensor cavity. Deep Reactive Ion Etching (DRIE) can be used to form the fluid passageway (46).

Description

MU TI SEMICONDUCTOR PRESSURE SENSOR WITH STEEP PASSAGEWAY

FIELD OF THE INVENTION

This application relates to differential pressure sensors, and in particular, to multivariate pressure sensors in which multiple pressure sensing elements are integrated into a single substrate.

BACKGROUND

A multivariate semiconductor pressure sensor integrates several sensors on a common substrate. For example, one type of multivariate semiconductor pressure sensor integrates a differential pressure sensor and an absolute pressure sensor on the same silicon substrate. This reduces costs. Another type of multivariate pressure sensor integrates several diaphragms having different geometries on the same silicon substrate. Because the pressure range of a diaphragm depends, to a great extent, on its geometry, this type of multivariate pressure sensor has an extended dynamic range. Other types of multivariate pressure sensors combine absolute and differential pressure sensors of a variety of geometries on the same substrate.

It is known in the art to fabricate a differential pressure sensor in which a polysihcon diaphragm spans a sensor cavity formed in the front surface of a silicon substrate. The diaphragm deflects in response to changes in the difference between the pressure above the diaphragm and the pressure in the cavity below the diaphragm. This deflection is sensed by piezoresistive elements mounted on the diaphragm. Under conditions of extremely high pressure, tile diaphragm is forced against the floor of tile sensor cavity. The cavity floor thus supports the diaphragm and prevents it from rupturing. In this type of differential pressure sensor, which is described in Fung et al., U.S.

Patent No. 5,220,838, a fluid passageway extends between an internal opening in the floor of the sensor cavity and an external opening in the back surface of the silicon substrate. This passageway provides fluid communication between the interior of the sensor cavity and the fluid whose pressure is to be measured. The diaphragm's response time to changes in pressure is in part determined by the size of this internal opening. A large internal opening communicates changes in pressure quickly. However, a large internal opening also reduces the sensor cavity floor area and thereby reduces the overrange protection provided by the sensor cavity floor. Conversely, a small internal opening allows a large cavity floor area, thereby increasing overrange protection, albeit at the cost of longer diaphragm response time.

The passageway in the conventional pressure sensor is formed by masking the back surface of the silicon substrate and leaving an unmasked portion whose extent determines the size of the external opening. The substrate is then immersed in an anistropic etchant, such as a potassium hydroxide (KOH) solution, which dissolves the exposed substrate. Unfortunately, the mechanism by which the etchant dissolves the silicon substrate is such that as the etchant works its way through the substrate, the passageway it forms becomes progressively narrower. As a result, the passageway formed by this process has a tapered profile. The slope of this taper, which is approximately 54.7 degrees, is determined by the crystalline structure of the silicon.

Because of the tapered profile of the passageway that the etchant forms, the external opening of the passageway is significantly larger than the minimal internal opening. Consequently, in order to form an internal opening having an appropriate size for the diaphragm spanning the sensor cavity, it is necessary to allocate space for an exterior opening that is much larger than the diaphragm. As a result, the minimum spacing between two diaphragms on a chip is dictated by the areas of the external openings rather than by the areas of the diaphragms. This, in turn, limits the number of diaphragms, and hence pressure sensors, that can be manufactured on a single substrate of a given size. For example, in the case of a 390 pm thick substrate, in order to form a 60μm square internal opening at the sensor cavity floor, it is necessary to allocate space for a 612μm square external opening on the back surface of the substrate. Consequently, to avoid having their respective passageways overlap, two diaphragms must be separated by a distance on the order of 552μm.

SUMMARY OF THE INVENTION

A semiconductor structure embodying features of the invention includes a substrate with a front surface on which is formed a first cavity having a cavity floor. The cavity floor has an internal opening into a fluid passage that extends through the substrate to an external opening on the back surface of the substrate. The internal and external openings have essentially the same area. As a result, the minimum spacing between adjacent differential pressure sensing elements on a multivariate pressure sensor is no longer dictated by the extent of the external opening. A multivariate pressure sensor embodying the invention can thus accommodate a large number of such differential pressure sensing elements in a relatively small area. In practice, the internal aperture has an area that is at least 80% of the area of the external aperture. This includes cases in which the internal aperture has an area that is larger than the area of the external aperture.

The semiconductor structure can further include a deformable membrane covering the front surface of the substrate. The membrane has two opposed faces: a back face exposed to a first fluid pressure and a front face exposed to a second fluid pressure. The deformable membrane deflects in response to the difference between the first and second fluid pressures. This deflection is sensed by a sensor coupled to the membrane. The output of the sensor, which depends on the pressure difference between the first and second fluid pressures, then modulates a signal. In one embodiment, the deformable layer defines a diaphragm spanning the cavity.

In this embodiment, the internal opening is formed on the floor of the cavity spanned by the diaphragm. An alternative embodiment includes a, second cavity that is spanned by a second diaphragm formed by the deformable layer. In this second embodiment, the internal opening is formed in a cavity that is different from the cavity spanned by the first diaphragm. This second embodiment provides the basis for a third embodiment, in which a single internal opening provides fluid communication between the first fluid pressure and a plurality of cavities, each of which is spanned by a separate diaphragm formed by the deformable layer.

A semiconductor structure embodying the principles of the invention is fabricated by forming a first cavity in the front surface of a semiconductor substrate and forming a passageway extending from an internal aperture in a cavity floor that a cavity to an external aperture in the back surface of the semiconductor substrate. The internal and external apertures are made to have essentially the same extent. In practice, the internal aperture has an area that is at least 80% of the area of the external aperture. This includes cases in which the internal aperture has an area that is larger than the area of the external aperture. The cavity can be formed by recessing the front surface of the substrate and depositing a sacrificial spacer material into the recess. A deformable layer is then deposited over the front surface and over the sacrificial spacer material. The sacrificial spacer material is later removed through the passageway that extends from the cavity floor to the back surface of the semiconductor substrate.

In a preferred method for fabricating the semiconductor structure, the passageway is formed by masking the back surface of the substrate and exposing the masked back surface to a plasma. This process, commonly referred to as a deep reactive ion etching process, forms a passageway having a constant cross section. This results in the external aperture in the back surface and the internal aperture in the cavity floor having essentially the same extent.

BRIEF DESCRIPTION OF FIGURES

The foregoing and other objects, features and advantages of the invention will be apparent from the following description and from the accompanying drawings. In these drawings, like reference characters refer to the same parts throughout the different views. Structures on the drawings are not to scale and the drawings as a whole are intended to illustrate principles of the invention rather than relative dimensions of structures shown therein.

FIG. 1 is an isometric view of a multivariate pressure sensor incorporating principles of the invention; FIG. 2 is a cross section of a differential pressure sensing element of the multivariate pressure sensor of FIG. I as viewed along section line 2-2 in FIG. 1;

FIG. 3 A is a plan view of a differential pressure sensor similar to that shown in FIG. 2 but with rectangular interior and exterior openings;

FIGS. 3B and 3C are cross-sectional views of the differential pressure sensor of FIG. 3 A taken along the section lines 313-313 and 3C-3C, respectively;

FIG. 4A is a plan view of a differential pressure sensor similar to that shown in FIG. 2 and having a plurality of interior and exterior openings;

FIG. 4B is a cross-sectional view of the differential pressure sensor of FIG. 4A along section line 413-413; FIG. 5 is a plan view of a multi-range differential pressure sensor;

FIG. 6 is a cross-sectional view of the multi-range differential pressure sensor of FIG. 5 along section line 6-6; and FIG. 7 is a flowchart of sequential steps in the formation of differential pressure sensors in accordance with the invention.

DETAILED DESCRIPTION A multivariate pressure sensor 10, as shown in FIG. 1, includes a silicon substrate

12 having a front surface 14 and a back surface 16. A deformable polysihcon membrane 18 covers the front surface 14 of the substrate 12.

The multivariate pressure sensor 10 includes a plurality of pressure sensing elements. These pressure sensing elements can be absolute pressure sensing elements or differential pressure sensing elements. Each sensing element, whether absolute or differential, includes an associated cavity formed on the front surface 14 of the substrate 12.

In particular, FIG. 1 shows first and second absolute pressure sensing elements 20, 22 having first and second evacuated cavities 24, 26. The polysihcon membrane 18 spans the first and second evacuated cavities 24, 26, thereby forming flexible diaphragms 28, 30 over each of the evacuated cavities 24, 26. As shown in FIG. 1, the first and second evacuated cavities have different sizes to correspond to different ranges of absolute pressure. The larger of the two cavities is suitable for measuring lower absolute pressures. The smaller of the two cavities is suitable for measuring higher absolute pressures.

In operation, the polysihcon membrane 18 is exposed to a front fluid pressure. In response to that pressure, the diaphragms 28, 30 deflect. The deflection is measured by piezoresistive elements mounted on the diaphragms 28, 30. Since the cavities 24, 26 under the diaphragms are evacuated, this deflection provides a measure of the absolute value of the front fluid pressure.

FIG. 1 also shows that the sensor 10 has first and second differential pressure sensing elements 32, 34 having associated first and second sensor cavities 36, 38. All four of the sensing elements 20, 24, 32 and 34 are fabricated on the same common semiconductor substrate 12. A cross-sectional view of the substrate 12 in FIG. 2 shows elements of the second differential pressure sensing element 34 in more detail. The elements of the first differential pressure sensing element 32 are similar to those shown in FIG. 2. The polysihcon membrane 18 spans the first and second sensor cavities 36, 38, thereby forming a flexible diaphragm 40, 42 over each of the sensor cavities 36, 38. As shown in FIG. 1, the first and second sensor cavities 36, 38 have different sizes to correspond to different ranges of differential pressure. The larger of the two sensor cavities is suitable for measuring lower differential pressures. The smaller of the two sensor cavities is suitable for measuring higher differential pressures.

In order to measure differential pressure, the diaphragms 40, 42 associated with the differential pressure sensing elements 32, 34 are exposed to two fluid pressures, in the illustrated embodiment, one fluid pressure is provided by exposing the polysihcon membrane 18 to a front fluid pressure. The second fluid pressure is provided by communicating a back fluid pressure to the interiors of the first and second cavities 36, 38 through first and second fluid passages 44, 46. These passages connect internal openings 48, 50 in the cavity floors 52, 54 to corresponding external openings 56, 58 in the back surface 16 of the substrate 12.

As shown in FIG. 1, the extents, or lateral areas, of the interior openings 48, 50, are approximately equal to the extents, or lateral areas, of their corresponding exterior openings 56, 58. As a result, the footprints of the differential pressure sensing elements 32, 34 on the back surface 16 are much smaller than the footprints of conventional differential pressure sensing elements. This feature of the invention enables many more differential pressure sensing elements to be formed on a substrate 12 of a given size then was the case with the prior structures having tapered fluid passages.

In operation, the differential pressure sensing elements 32, 34 operate in a manner similar to the absolute pressure sensing elements 20, 22. In response to a difference between the front pressure and the back pressure, each diaphragm 40, 42 deflects. This deflection is measured by piezoresistive elements 60, 61 mounted on the diaphragms 40, 42.

When the relatively narrow bore of the fluid passageway 46 of the embodiment shown in FIG. 2 causes the diaphragm 42 to respond too slowly to changes in pressure, the bore can be selectively enlarged to overcome the limitation. In particular, internal and external openings 50, 58 having approximately the same size and shape as the diaphragm can be provided. FIGS. 3A, 3B, and 3C show an example of this feature of the invention. FIG. 3 A is a view of a narrow rectangular exterior opening 58 of the differential pressure sensor as seen from the back surface of the substrate. A narrow rectangular internal opening of similar size and aspect ratio is visible through the external opening. The shapes and sizes of the internal and external openings in FIG. 3 A are chosen to correspond to a narrow rectangular cavity formed in th_ top surface of the substrate and a narrow rectangular diaphragm spanning the cavity, both of which are shown in the transverse and longitudinal cross-sections of FIGS. 3B and 3C.

In conventional differential pressures sensors, the response time is increased by providing a fluid passageway having a wider bore. However, this wider bore in the cavity floor compromises the ability of the cavity floor to function as a backstop to protect the diaphragm from rupture under conditions of excessively high pressure.

In a second embodiment, shown in FIGS. 4A and 4B, a differential pressure sensor 42' includes a plurality of fluid passageways 50a', 50b' ... 50e' each of which extends between one of a plurality of internal openings in the cavity and one of a corresponding plurality of external openings in the back surface of the substrate. This second embodiment 42' has the advantage of providing more rapid response time than the single-passage embodiment of FIG. 2. In addition, because there is no single large internal opening in this embodiment, the cavity floor retains the ability to effectively support the diaphragm and protect it from rupture under conditions of excessively high pressure.

As noted above, the range of pressures over which a pressure sensor can function is determined in part by the dimensions of each associated diaphragm. In general, considerable pressure is necessary to deflect a small diaphragm. Hence, small diaphragms are best suited for high pressure applications. Conversely, relatively little pressure is necessary to deflect a large diaphragm. Hence, large diaphragms are best suited for low-pressure applications.

A multivariate semiconductor pressure sensor having multi-range capabilities includes diaphragms of progressively increasing size. In such a multivariate pressure sensor, at a given pressure, it is more likely that at least one diaphragm will deflect by an amount that is conducive to the generation of an accurate pressure measurement. In the fabrication of such a device, it is more convenient to communicate fluid pressure to each diaphragm through a single exterior opening in the back surface of the substrate. This eliminates the difficulties associated with precisely aligning a plurality of exterior openings in the back surface of the substrate with a corresponding plurality of cavities formed on the front surface of the substrate.

FIG. 5 shows a plan view of a pressure sensor 10" having multi-range capabilities. The polysihcon membrane and the piezoresisitive elements have been removed from the plan view to show clearly the configuration of the diaphragms on the top surface 14" of the substrate 12". The pressure sensor includes a feeder cavity 62" having an interior opening 50" on its floor 63". As shown in FIG. 6, the interior opening 50" leads to a passageway 46" that connects the interior opening 50" to an exterior opening 58" on the backside 16" of the substrate 12".

The feeder cavity 62" is connected to a first, second, and third sensor cavities 38a", 38b", 38c" by way of first, second and third feeder tubes 64a", 64b", 64c". The cross section of the substrate 12" along a line passing through the first and second feeder tubes 64a", 64b" in FIG. 6 shows the polysihcon membrane 18" and piezoresistive elements 66a", 66b" omitted from FIG. 5.

Because the backside fluid pressure is communicated by first and second sensor tubes 64a", 64b" in the sensor 10" of FIG. 6, the first and second sensor cavities 38a", 38b" need not have openings on their respective cavity floors 66a", 66b". As a result, the overrange protection provided by the cavity floors 66a", 66b" is not diminished by the presence of any openings in the cavity floors. Meanwhile, the feeder cavity 62" can be sufficiently small so that no significant deflection occur in the diaphragm spanning the feeder cavity, even at high differential pressures. A further advantage of the sensor 10" configuration shown in FIGS. 5 and 6 is that a rapid rise in back fluid pressure is rapidly dissipated as the pressure wave traverses the feeder tubes 64a"-c". The feeder tubes 64a"-c" thus act as an anti-shock device for further protecting the diaphragms spanning the sensor cavities 38a"-c" from rupture.

A feature common to the three semiconductor sensor embodiments described thus far is that a fluid passageway connects an exterior opening with an interior opening having substantially the same extent as the exterior opening. The term "substantially the same extent" means an extent, or area, such that the minimum spacing between cavities on the substrate is not dictated by the size of the exterior opening.

Referring now to FIG. 7, the process 69 for the manufacture of a pressure sensor according to the invention begins with the step of covering the frontside surface and the backside surface of the substrate with an oxide layer (step 70), and the step of covering the oxide layer with a nitride layer (step 72). This is followed by the step of masking the nitride layer with a mask having cutouts corresponding to the desired cavities (step 74). The cavities are then formed by etching through the nitride and oxide layers and part way through the substrate (step 76). An oxide layer, referred to as the cavity oxide, is then grown on the resulting cavity (step 78). Next, the cavity oxide is covered with a photoresistive material (step 80) and the nitride and oxide layers are etched away (step 82). This results in an exposed substrate surface with islands of oxide corresponding to the desired cavities. The substrate surface and the oxide islands formed thereon are then covered by a polysihcon layer (step 84). This results in a plurality of buried oxide islands that are to be removed, in order to form the desired cavities. These steps are described in more detail in Fung et al., U.S. Patent No. 5,438,875, which is hereby incorporated by reference. The formation of the fluid passageway begins with the placement of a mask on the backside of the substrate (step 86) and the exposure of the backside of the substrate to an inductively coupled plasma source (step 88). This procedure, referred to as "deep reactive ion etching" (DRIE), is a known procedure for forming holes in a silicon substrate. A suitable machine for performing DRIE in this manner is available from Surface Technology Systems Ltd. of Gwent, U.K.

The DRIE process is carried out until the passageway thus formed reaches the cavity oxide from the backside of the substrate and forms the interior opening into what will shortly become the cavity. Once the passageway has reached the cavity oxide, an HF solution is passed through the passageway to etch away the cavity oxide, thereby forming a hollow cavity under the polysihcon layer (step 90).

It will thus be seen that a differential pressure sensor incorporating the principles of the invention results in a smaller footprint on the backside of the substrate because the exterior opening of the fluid passageway that communicates backside fluid pressure to the cavity can be made relatively small. This facilitates the placement of more diaphragms on the same substrate area. In addition, the use of the DRIE process for formation of the fluid passageway provides more control over the placement of the passageway. The increased accuracy in the placement of the passageway reduces the registration problems associated with targeting a small feeder cavity on the frontside of the substrate starting from the backside of the substrate. This facilitates the fort-nation of a multirange differential pressure sensor in which a small feeder cavity in communication with the backside pressure can then be used to feed several diaphragms of different sizes. Since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all generic and specific features of the invention described herein, and all statements of the scope of the invention which as a matter of language might be said to fall therebetween. Having described the invention, what is claimed as new and secured by Letters Patent is:

Claims

1. A method for fabricating a semiconductor structure for a differential-pressure sensor, said method comprising the steps of forming a first cavity in a front surface of a semiconductor substrate, said first cavity having a cavity floor, and forming a passageway extending from an internal aperture in said cavity floor to an external aperture in a back surface of said semiconductor substrate, said internal and external aperture having essentially the same extent.

2. The method of claim 1 further comprising the step of filling said first cavity with a sacrificial spacer material.

3. The method of claim 2 further comprising the step of depositing a deformable layer over said front surface and said sacrificial spacer material.

4. The method of claim 3 further comprising the step of removing said sacrificial spacer material through said passageway.

5. The method of claim 1 further comprising the step of fort-ning a second passageway providing fluid communication between said cavity and an external aperture in said back surface of said semiconductor substrate.

6. The method of claim 1 wherein said step of forming said passageway comprises the step of performing a deep reactive ion etching process.

7. The method of claim 2 wherein said step of filling said first cavity comprises the step of forming an oxide layer in said first cavity.

8. The method of claim 3 wherein said step of depositing a deformable layer comprises the step of depositing a polysihcon layer over said front surface and said sacrificial spacer material.

9. The method of claim 1 wherein said step of forming a passageway comprises the step of masking said back surface of said semiconductor substrate.

10. The method of claim 9 wherein said masking step comprises the step of defining a rectangular aperture on said back surface.

11. The method of claim 9 wherein said masking step comprises the step of defining a circular aperture on said back surface.

12. The method of claim 3 further comprising the step of coupling a sensor element to said deformable layer, said sensor element being responsive to deformation of said deformable layer.

13. The method of claim 12 wherein said step of coupling a sensor element to said deformable layer comprises the step of depositing, onto said deformable layer, a piezoresistive element having a resistance responsive to deflection of said deformable layer.

14. The method of claim 1 further comprising the steps of forming a second cavity on said front surface, and forming a passageway connecting said first cavity and said second cavity, thereby providing fluid communication between said first and second cavities.

15. The method of claim 1 further comprising the steps of forming a plurality of cavities on said front surface, and forming a passageway connecting each of said cavities from said plurality of cavities with said first cavity, thereby providing fluid communication between said first cavity and each of said cavities.

16. The method of claim 1 further comprising the step of forming a plurality of external openings in said back surface, and forming a corresponding plurality of internal openings into said first cavity, each of said internal openings having essentially the same extent as its corresponding external opening.

17. A semiconductor structure for a differential pressure sensor, said semiconductor structure comprising a semiconductor substrate having a front surface recessed with a first cavity having a cavity floor, a back surface exposed to a first fluid pressure, and a fluid passage extending between an internal opening in said cavity floor and an external opening in said back surface, said internal and external openings having essentially the same extent.

18. The semiconductor structure of claim 17, further comprising a deformable layer having a back face exposed to said first fluid pressure and a front face exposed to a second fluid pressure, said deformable layer being mounted over said substrate and deforming in response to a differential pressure defined by said first and second fluid pressures.

19. The semiconductor structure of claim 18 further comprising a sensor coupled to said deformable layer, said sensor being responsive to deformation of said deformable layer.

20. The semiconductor structure of claim 18 wherein said deformable layer forms a diaphragm spanning said first cavity.

21. The semiconductor structure of claim 18 wherein said front surface has a second cavity formed thereon, said second cavity being in fluid communication with said first cavity, and said polysihcon layer defines a diaphragm spanning said second cavity.

22. The semiconductor structure of claim 21 wherein said deformable layer defines a diaphragm spanning said second cavity.

23. The semiconductor structure of claim 18 further comprising pressure-shock dissipation means inteφosed between said first cavity and said external aperture.

24. The semiconductor structure of claim 21 further comprising pressure-shock dissipation means inteφosed between said first cavity and said second cavity.

25. The semiconductor structure of claim 18 wherein said front surface is recessed with a plurality of cavities, each cavity in said plurality of cavities being in fluid communication with said first cavity.

26. The semiconductor structure of claim 18 wherein said external opening, is circular.

27. The semiconductor structure of claim 18 wherein said external opening is rectangular.

28. The semiconductor structure of claim 18 wherein said deformable layer is a polysihcon layer.

29. The semiconductor structure of claim 18 wherein said sensor is a piezoresistive element having a resistance responsive to deflection of said deformable layer.

30. A semiconducto : structure for a differential pressure sensor, said semiconductor structure comprising a semiconductor substrate having a front surface recessed with a first cavity, a back surface, and means for communicating a fluid pressure between said front surface and said back surface, said means for communicating a fluid pressure including an internal opening in said first cavity and an external opening in said back surface, said internal opening and said external opening having essentially the same extent.

31. The semiconductor structure of claim 30 further comprising means for responding to a pressure difference between a second fluid pressure and said first fluid pressure, said means for responding to said pressure difference being mounted on said semiconductor substrate.

32. The semiconductor structure of claim 31 further comprising transducer means coupled to said means for responding to said pressure difference, said transducer means being responsive to said pressure difference.

33. The semiconductor structure of claim 30 wherein said front surface is recessed with a second cavity in fluid communication with said first cavity.

34. The semiconductor structure of claim 30 further comprising, disposed between said first cavity and said second cavity, means for dissipating a pressure shock.

35. The semiconductor structure of claim 30 wherein said front surface is recessed with a plurality of cavities, each of which is in fluid communication with said first cavity.

36. The semiconductor structure of claim 30 wherein said external opening is a circular opening.

37. The semiconductor structure of claim 30 wherein said external opening is a rectangular opening.

38. The semiconductor structure of claim 30 wherein said means for communicating a fluid pressure includes a plurality of external openings in said back surface and a corresponding plurality of internal openings into said first cavity, each of said internal openings having essentially the same extent as its corresponding external opening. ,

39. A method for fabricating a semiconductor structure for a differential-pressure sensor, said method comprising the steps of forming a first cavity in a front surface of a semiconductor substrate, said first cavity having a cavity floor, and forming a passageway extending from an internal aperture in said cavity floor to an external aperture in a back surface of said semiconductor substrate, said internal aperture having an extent that is at least 80% of the extent of the external aperture.

40. The method of claim 39 wherein said internal aperture has an extent that is greater than the extent of the external aperture.

41. A semiconductor structure for a differential pressure sensor, said semiconductor structure comprising a semiconductor substrate having a front surface recessed with a first cavity having a cavity floor, a back surface exposed to a first fluid pressure, and a fluid passage extending between an internal opening in said cavity floor and an external opening in said back surface, said internal aperture having an extent that is at least 80% of the extent of the external aperture.

42. The semiconductor structure of claim 41 wherein said internal aperture has an extent that is greater than the extent of the external aperture.