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Inventors
Kusuyama, Koichi
Tsukada, Masao
Application #
133536
Filed
Aug-13-1998
Published
May-9-2000
Current US Class
073/724
073/727
257/419
International Classes
G01L 009/08; G01L 009/12
Field of Search
73/724 73/726 73/727 338/42 361/283.4 257/419
Assignee
Unisia Jecs Corporation (Atsugi, JP)
Examiners
Felber; Joseph L.
Attorney, Agent or Firm
Foley & Lardner
US Patent References
| 4287501 |
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Pressure sensor |
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| 4314226 |
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Pressure sensor |
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| 4881056 |
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Facedown-type sem... |
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| 5186054 |
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Capacitive pressur... |
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| 5493470 |
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SOI diaphragm se... |
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| 5683594 |
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Method for making... |
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Referenced by:
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Citation
Cite This Patent
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Abstract
A pressure sensor including a substrate having a diaphragm portion. A diaphragm-defining recess is recessed inwardly from one surface of the substrate to define the diaphragm portion between a bottom surface of the recess and an opposite surface of the substrate. A recess is disposed within the diaphragm portion. A pressure sensitive arrangement is disposed in the substrate, a detector portion of which is disposed within the recess formed within the diaphragm portion.
Claims
What is claimed is:
1. A pressure sensor, comprising:
a diaphragm portion;
a substrate including said diaphragm portion, said substrate having one surface and an opposite surface and formed with a first recess recessed inwardly from said one surface to define said diaphragm portion between said first recess and said opposite surface;
a second recess disposed within the diaphragm portion;
a pressure sensitive arrangement having a detector portion disposed within the second recess; and
a lid fixed to the diaphragm portion to cover the second recess and the detector portion of the pressure sensitive arrangement.
2. A pressure sensor as claimed in claim 1, wherein said diaphragm portion and said lid cooperate to define a reference pressure chamber between the second recess and a surface of the lid opposed to the second recess.
3. A pressure sensor as claimed in claim 1, wherein said second recess has a planar bottom surface smaller than a bottom surface of the first recess.
4. A pressure sensor as claimed in claim 3, wherein said second recess includes a plurality of recesses, a total area of which is smaller than an area of the bottom surface of the first recess.
5. A pressure sensor as claimed in claim 3, wherein said second recess is defined by a recessed surface including a round beveled surface connected with a periphery of the bottom surface.
6. A pressure sensor as claimed in claim 5, wherein said recessed surface of the second recess has a peripheral side surface inclined relative to the bottom surface.
7. A pressure sensor as claimed in claim 6, wherein said peripheral side surface includes a beveled portion connected with the bottom surface and a curved portion connected with the beveled portion, said beveled portion and said curved portion cooperating to form an arcuate-shaped vertical section.
8. A pressure sensor as claimed in claim 6, wherein said peripheral side surface includes a planar portion substantially perpendicular to the bottom surface.
9. A pressure sensor as claimed in claim 5, wherein said recessed surface of the second recess includes a second round beveled surface disposed at an open end of the second recess.
10. A pressure sensor as claimed in claim 5, wherein said second recess has a generally rectangular shape and the round beveled surface at each corner thereof.
11. A pressure sensor as claimed in claim 10, wherein said recessed surface has a peripheral side surface including a planar portion interposed between the adjacent two of the round beveled surfaces.
12. A pressure sensor as claimed in claim 11, further comprising a film of spin-on-glass (SOG) deposited on the substrate, said SOG film being formed with the round beveled surface and the peripheral side surface connected with the round beveled surface.
13. A pressure sensor as claimed in claim 11, wherein said periphery of the bottom surface is a rectilinear periphery and the peripheral side surface includes a side surface portion having a length twice or more a length of the detector portion coextending with the side surface portion.
14. A pressure sensor as claimed in claim 10, wherein said round beveled surface has a radius of curvature of approximately 8-30% of a length of a line segment constituting the generally rectangular shape.
15. A pressure sensor as claimed in claim 1, wherein said diaphragm portion includes a thinned portion defined between the second recess and the one surface of said substrate, and a thicker portion surrounding the thinned portion.
16. A pressure sensor as claimed in claim 15, wherein said lid has a predetermined rigidity to reinforce the thicker portion of the diaphragm portion.
17. A pressure sensor as claimed in claim 15, wherein said thicker portion of the diaphragm portion has a width of at least not less than 10 .mu.m.
18. A pressure sensor as claimed in claim 1, wherein said pressure sensitive arrangement includes an electrically conductive film and diffused leads connecting the detector portion with the electrically conductive film.
19. A pressure sensor as claimed in claim 1, wherein said detector portion includes a piezoresistive element.
20. A pressure sensor as claimed in claim 1, wherein said detector portion includes a capacitive element comprising a first electrode disposed on a bottom surface of the second recess and a second electrode disposed on a surface of the lid in opposed relation to the first electrode.
21. A method for forming a pressure sensor according to claim 1, comprising the steps of:
a) preparing a substrate having opposed surfaces;
b) forming a first recess recessed inwardly from one of the opposed surfaces of the substrate to define a diaphragm portion between the first recess and the other of the opposed surfaces;
c) forming a second recess within the diaphragm portion;
d) placing a detector portion, forming a part of a pressure sensitive arrangement, within the second recess;
e) fixing a lid to the substrate to define a reference pressure chamber between the second recess and a surface of the lid opposed to the second recess.
22. A method as claimed in claim 21, further comprising the step of placing other parts of the pressure sensitive arrangement in the substrate, the step of placing other parts of the pressure sensitive arrangement comprising:
placing diffused leads connected with the detector portion, in the substrate; and
depositing an electrically conductive film connected with the diffused leads, on the substrate.
23. A method as claimed in claim 22, wherein the step of preparing a substrate comprises growing one layer of a first conductivity type onto a silicon base of a second conductivity type.
24. A method as claimed in claim 23, wherein the step of growing one layer onto a silicon base comprises in the following sequence:
providing the silicon base having opposed surfaces;
depositing a film of silicon dioxide on the opposed surfaces of the silicon base and a film of silicon nitride on the silicon dioxide film;
selectively removing the silicon nitride film on one of the opposed surfaces of the silicon base to partially expose the silicon base thereunderneath;
oxidizing the exposed portion of the silicon base by local oxidation of silicon (LOCOS);
removing the remainder of the silicon nitride film on the one of the opposed surfaces of the silicon base to expose the silicon dioxide film thereunderneath; and
selectively doping impurity into the silicon base underlying the exposed portion of the silicon dioxide film, to deposit the one layer on the silicon base.
25. A method as claimed in claim 24, wherein the step of forming a second recess comprises in the following sequence:
selectively removing the silicon dioxide film overlying the one layer to partially expose the one layer thereunderneath;
removing the exposed portion of the one layer to form the second recess recessed inwardly from one face of the one layer; and
removing the remainder portion of the silicon dioxide film on the one layer.
26. A method as claimed in claim 24, wherein the step of forming a first recess comprises in the following sequence:
selectively removing the silicon nitride film and the silicon dioxide film on the other of the opposed surfaces of the silicon base to partially expose the silicon base thereunderneath;
anisotropically etching the exposed portion of the silicon base over an area greater than an area of the first recess; and
stopping the removal of the silicon base at an interface between the silicon base and the one layer.
27. A method as claimed in claim 23, wherein the step of placing a detector portion comprises doping impurity into a portion of the one layer which defines the second recess, to form a piezoresistive element within the second recess.
28. A method as claimed in claim 23, wherein the step of placing other parts of the pressure sensitive arrangement comprises:
depositing an insulator film of electrically insulating material on the silicon base and the one layer; and
forming contact holes for connection of the detector portion and the electrically conductive film, in the insulator film.
29. A method as claimed in claim 28, wherein the step of placing other parts of the pressure sensitive arrangement comprises:
depositing a protective film of electrically insulating material on the electrically conductive film; and
forming contact holes to define bonding pad regions on the electrically conductive film, in the protective film.
30. A method as claimed in claim 21, wherein the step of preparing a substrate comprises:
providing a silicon-on-insulator (SOI) substrate including a base of single crystal silicon, an insulator film of electrically insulating material on the base, and one layer of silicon based material on the insulator film; and
depositing a film of silicon dioxide on opposed surfaces of the SOI substrate, and a film of silicon nitride on the silicon dioxide film.
31. A method as claimed in claim 30, wherein the step of providing the SOI substrate comprises doping impurity into the one layer of the SOI substrate.
32. A method as claimed in claim 30, wherein the step of forming a second recess comprises in the following sequence:
selectively removing the silicon nitride film on the silicon dioxide film on the one layer to partially expose the silicon dioxide film thereunderneath;
locally oxidizing the one layer underneath the exposed portion of the silicon dioxide film;
removing the remainder of the silicon nitride film on the silicon dioxide film on the one layer to expose the silicon dioxide film thereunderneath;
selectively removing the exposed silicon dioxide film on the one layer to partially expose the one layer thereunderneath;
removing the exposed portion of the one layer to form the second recess recessed inwardly from the one of the opposed surfaces of the SOI substrate; and
removing the entire remainder of the silicon dioxide film on the one layer.
33. A method as claimed in claim 30, wherein the step of forming a second recess comprises in the following sequence:
selectively removing the silicon nitride film on the silicon dioxide film on the one layer to partially expose the silicon dioxide film thereunderneath;
locally oxidizing the one layer underneath the exposed portion of the silicon dioxide film;
removing the remainder of the silicon nitride film on the silicon dioxide film on the one layer.
34. A method as claimed in claim 30, wherein the step of forming a first recess comprises:
selectively removing the silicon nitride film and the silicon dioxide film deposited on the base, respectively, to partially expose the base thereunderneath;
anisotropically etching the exposed portion of the base over an area greater than an area of the second recess; and
stopping the anisotropic etching at an interface between the base and the insulator film.
35. A method as claimed in claim 30, wherein the step of preparing a substrate comprises depositing an insulator film of electrically insulating material on the one of the opposed surfaces of the SOI substrate and a film of phosphosilicate glass (PSG) on the insulator film.
36. A method as claimed in claim 35, wherein the step of forming a second recess comprises selectively removing the PSG film by isotropic etching to form a curved portion of the recess which has a predetermined radius of curvature.
37. A method as claimed in claim 30, wherein the step of forming a second recess comprises selectively removing the one layer using anisotropic etching and isotropic etching after the anisotropic etching to form a second recess-defining surface including a planar bottom surface, a planar side surface substantially perpendicular to the bottom surface, and a round beveled surface interconnecting the bottom surface and the side surface.
38. A method as claimed in claim 30, wherein the step of preparing a substrate comprises depositing a film of spin-on-glass (SOG) on the one layer of the SOI substrate.
39. A method as claimed in claim 38, wherein the step of forming a second recess comprises selectively removing the SOG film using anisotropic etching and isotropic etching to form a second recess-defining surface including a planar bottom surface, a planar side surface inclined relative to the bottom surface, and a round beveled surface interconnecting the bottom surface and the side surface.
40. A method as claimed in claim 21, wherein the step of placing a detector portion comprises:
doping impurity into a portion of the substrate which defines the first recess, to form a first electrode within the second recess; and
prior to the step of fixing the lid to the substrate, depositing a film of electrically conductive material on the surface of the lid opposed to the second recess, to form a second electrode cooperating with the first electrode to form a capacitive element as the detector portion.
41. A pressure sensor as claimed in claim 1, wherein said substrate includes a base and one layer on the base, said base being formed with the first recess, said one layer including the diaphragm portion.
42. A pressure sensor as claimed in claim 41, wherein said one layer is electrically isolated from the base.
43. A pressure sensor as claimed in claim 41, wherein each of the base and the one layer is made of silicon based material.
44. A pressure sensor as claimed in claim 43, wherein the substrate is a silicon-on-insulator (SOI) substrate and said substrate further includes a film of electrically insulating material interposed between the base and the one layer.
45. A pressure sensor as claimed in claim 44, wherein the substrate further includes a film of phosphosilicate glass (PSG) deposited on the SOI substrate, said second recess being formed in the PSG film.
46. A pressure sensor formed by a method comprising the steps of:
a) preparing a substrate having opposed surfaces;
b) forming a first recess recessed inwardly from one of the opposed surfaces of the substrate to form a diaphragm portion between the first recess and the other of the opposed surfaces;
c) forming a second recess within the diaphragm portion;
d) placing a detector portion, forming a part of a pressure sensitive arrangement, within the second recess;
e) placing other parts of the pressure sensitive arrangement in the substrate, comprising:
placing diffused leads connected with the detector portion, in the substrate; and
depositing an electrically conductive film connected with the diffused leads, on the substrate; and
f) fixing a lid to the diaphragm portion to define a reference pressure chamber between the second recess and a surface of the lid opposed to the second recess.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a pressure sensor and a method for forming the pressure sensor, and more specifically to a pressure sensor empolying a semiconductor substrate and a method for forming the same.
Generally, it is well known to fabricate a pressure sensor by, for example, etching a semiconductor substrate.
Japanese Patent Application First Publication No. 2-132337 discloses a pressure sensor employing a semiconductor substrate which has a diaphragm portion.
An example of such kinds of pressure sensors conventionally proposed includes a silicon substrate having opposed surfaces each lying substantially in a (100) crystal plane. The substrate has a recess recessed inwardly from one of the opposed surfaces to form a diaphragm portion having a reduced thickness. The recess is formed of a generally rectangular shape in section by anisotropic etching to form side walls lying substantially in a (111) crystal plane. A piezoresistive element is disposed on the diaphragm portion, which is capable of sensing strain caused in response to a pressure, for instance fluid pressure, exerted on the diaphragm portion.
In the conventionally proposed process for fabricating the pressure sensor, it is important to position the piezoresistive element and the diaphragm portion in a precise manner with respect to each other in order to correctly detect a value of the pressure acting on the diaphragm portion. For the alignment, masks for the piezoresistive element and the diaphragm portion are placed on the opposite sides of the silicon substrate in alignment with each other by using a suitable equipment such as a double sided aligner. It is known that the mask alignment on the opposite sides of the substrate is less accurate than the mask alignment carried out on one of the opposite sides thereof. This is because the masks tends to be placed in misaligned positions due to an offset of optical axes in the double sided aligner, for example, an infrared radiation aligner, a reflection-type aligner and the like.
If the opposite surfaces of the silicon substrate which are cut in the cutting process of the silicon substrate are out of the (100) crystal plane due to errors in slicing, abrasion or the like, in the cutting process, then the recess will be formed in a position displaced from a given position. Therefore, the diaphragm portion cannot be disposed in an intended position in the silicon substrate. In such case, even though the masks are precisely aligned with each other, it is likely that the diaphragm portion and the piezoresistive element are displaced from the alignment position.
A magnitude of the strain which is sensed by the piezoresistive element, varies depending on the position relative to the diaphragm portion. Therefore, the displacement of the diaphragm portion and the piezoresistive element from the alignment position influences the magnitude of the strain sensed by the piezoresistive element, making it difficult to detect the pressure acting on the diaphragm portion with high accuracy.
Meanwhile, if the diaphragm portion and the piezoresistive element each having a large size are used, dispersion of the sensed magnitude of strain which is caused by the misalignment of the diaphragm portion and the piezoresistive element, may be reduced and thus the accuracy of the pressure detection can increase. However, in this case, the pressure sensor cannot be reduced in size as the whole.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a pressure sensor including a diaphragm portion and a detector portion, such as piezoresistive element, of a pressure sensitive arrangement accurately aligned with each other, and capable of detecting a pressure applied thereto with an increased accuracy.
It is a further object of the present invention to provide a method of fabricating the pressure sensor having advantages as described above, which serves for reducing the dimension thereof.
It is a still further object of the present invention to provide a pressure sensor formed by the method described above.
According to one aspect of the present invention, there is provided a pressure sensor, comprising:
a diaphragm portion;
a substrate including said diaphragm portion, said substrate having one surface and an opposite surface and formed with a first recess recessed inwardly from said one surface to define said diaphragm portion between said first recess and said opposite surface;
a second recess disposed within the diaphragm portion; and
a pressure sensitive arrangement having a detector portion disposed within the second recess.
According to further aspect of the present invention, there is provided a method for forming a pressure sensor, comprising the steps of:
a) preparing a substrate having opposed surfaces;
b) forming a first recess recessed inwardly from one of the opposed surfaces of the substrate to define a diaphragm portion between the first recess and the other of the opposed surfaces;
c) forming a second recess within the diaphragm portion; and
d) placing a detector portion forming a part of a pressure sensitive arrangement, within the second recess.
According to still further aspect of the present invention, there is provided a pressure sensor formed by a method comprising the steps of:
a) preparing a substrate having opposed surfaces;
b) forming a first recess recessed inwardly from one of the opposed surfaces of the substrate to define a diaphragm portion between the first recess and the other of the opposed surfaces;
c) forming a second recess within the diaphragm portion; and
d) placing a detector portion forming a part of a pressure sensitive arrangement, within the second recess.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-section of a pressure sensor of a first embodiment according to the present invention, taken along the line 1--1 of FIG. 2;
FIG. 2 is a cross-section of the pressure sensor, taken along the line 2--2 of FIG. 1;
FIG. 3 is a cross-section of a monocrystalline silicon base as a starting material used in the first embodiment;
FIGS. 4-7 are cross-sections showing the process of preparing a substrate of the pressure sensor of the first embodiment;
FIGS. 8-10 are cross-sections showing the process of forming a recess in the substrate shown in FIG. 7;
FIGS. 11-13 are cross-sections showing the process of placing a pressure sensitive arrangement in the substrate shown in FIG. 10;
FIG. 14 is a cross-section showing the process of forming a recess in the substrate shown in FIG. 13;
FIG. 15 is a cross-section similar to FIG. 1 but showing a pressure sensor of a second embodiment according to the present invention;
FIGS. 16-17 are cross-sections showing the process of preparing a substrate of the pressure sensor shown in FIG. 15;
FIGS. 18-23 are cross-sections showing the process of forming a recess in the substrate shown in FIG. 17;
FIGS. 24-26 are cross-sections showing the process of placing a pressure sensitive arrangement in the substrate shown in FIG. 23;
FIG. 27 is a cross-section showing the process of forming a recess in the substrate shown in FIG. 26;
FIG. 28 is a cross-section similar to FIG. 1 but showing a pressure sensor of a third embodiment according to the present invention;
FIG. 29 is an enlarged fragmentary view of FIG. 28, showing a recess formed in a diaphragm portion of a substrate;
FIGS. 30-31 are cross-sections showing the process of preparing the substrate of the pressure sensor shown in FIG. 28;
FIGS. 32-34 are cross-sections showing the process of forming a recess in the substrate shown in FIG. 31;
FIGS. 35-38 are cross-sections showing the process of placing a pressure sensitive arrangement in the substrate shown in FIG. 34;
FIG. 39 is a cross-section showing the process of forming a recess in the substrate shown in FIG. 38;
FIG. 40 is a cross-section similar to FIG. 1 but showing a pressure sensor of a fourth embodiment according to the present invention;
FIG. 41 is a cross-section similar to FIG. 1 but showing a pressure sensor of a fifth embodiment according to the present invention;
FIG. 42 is a cross-section similar to FIG. 1 but showing a pressure sensor of a sixth embodiment according to the present invention;
FIG. 43 is a fragmentary plan view of FIG. 42, showing a recess within which a detector portion of a pressure sensitive arrangement is disposed;
FIG. 44 is a view similar to FIG. 43 but showing a modification of the sixth embodiment;
FIG. 45 is a cross-section similar to FIG. 1 but showing a pressure sensor of a seventh embodiment according to the present invention;
FIG. 46 is an enlarged fragmentary view of FIG. 45, showing a recess formed in a diaphragm portion of a substrate of the pressure sensor; and
FIG. 47 is a modification of the seventh embodiment.
DETAILED DESCRIPTION OF THE INVENTION
The present invention describes a novel new structure and technique for fabricating a pressure sensor employing a semiconductor substrate.
The detailed description of the pressure sensor structures and the processes for fabricating them has been simplified by using the following predetermined and named process sequence or definition that is repetitively referenced.
The term "deposited" refers to any method of layer formation that is suitable to the materials as are generally practiced throughout the semiconductor industry. One or more of the following examples of deposition techniques can be used with the materials, such as sputtering, chemical vapor deposition, electro or electroless plating, oxidation, evaporation, sublimation, plasma deposition, anodization, anodic deposition, molecular beam deposition or photodeposition.
Referring to FIGS. 1 and 2, a first preferred embodiment of a pressure sensor according to the present invention, will be explained hereinafter.
As illustrated in FIGS. 1 and 2, the pressure sensor includes a substrate 11. The substrate 11 is made of a single crystal silicon and has a generally rectangular shape in section as shown in FIG. 2. The substrate 11 includes a base 11A having a P-conductivity type, and a surface layer 11B having an N-conductivity type and grown onto the base 11A. The surface layer 11B is deposited on a central portion of an upper surface as viewed in FIG. 1, of the base 11A. A silicon dioxide film 12 is deposited on a lower surface as viewed in FIG. 1, of the base 11A which forms a lower surface of the substrate 11. A silicon nitride film 13 is deposited on the silicon dioxide film 12. The surface layer 11B has a thickness, for instance, of approximately 5-20 .mu.m and forms a mesa portion of the substrate 11 raised outwardly, upwardly as viewed in FIG. 1, from the upper surface of the base 11A. An upper-side face of the surface layer 11B forms an upper surface of the substrate 11 in cooperation with the upper surface of the base 11A.
A diaphragm-defining recess 14 is formed in the substrate 11 to define a diaphragm portion 15 of the substrate 11 as explained later. The recess 14 also acts as a pressure port through which a pressure is applied to the pressure sensor. The recess 14 is recessed inwardly from the lower surface of the substrate 11 to define the diaphragm portion 15 between the recess 14 and the upper surface of the substrate 11. The recess 14 is disposed at substantially the center of the base 11A and has a planar bottom surface 14A of a generally rectangular shape as indicated by the broken line in FIG. 2. The generally rectangular shape of the bottom surface 14A is formed by line segments each having a predetermined length L0, for example, of approximately 150 .mu.m. The recess 14 extends through the base 11A to be opposed to a rearside face of the surface layer 11B. The recess 14 is formed by a suitable conventional etching technique such as wet etching, through apertures 12A and 13A respectively formed in the silicon dioxide film 12 and the silicon nitride film 13.
The diaphragm portion 15 is disposed in the surface layer 11B. As illustrated in FIG. 2, the diaphragm portion 15 is defined by the rectangular-shaped planar bottom surface 14A of the recess 14. The diaphragm portion 15 has a width coincident with the predetermined length L0 of the line segments of the rectangular shaped bottom surface 14A of the recess 14. The diaphragm portion 15 has a predetermined thickness T1 as shown in FIG. 1.
Disposed within the diaphragm portion 15 is a recess 16 where the pressure applied to the diaphragm portion 15 is detected. The recess 16 is recessed inwardly from an upper-side face of the surface layer 11B to define a thinned portion 17 of the diaphragm portion 15 between a planar bottom surface of the recess 16 and the planar bottom surface 14A of the recess 14. The diaphragm portion 15 also has a thicker portion 18 surrounding the thinned portion 17 and the recess 16.
The recess 16 has a generally rectangular shallow groove-like shape formed by dry etching as explained later. The recess 16 is defined by a recessed surface 16B formed in the surface layer 11B where the diaphragm portion 15 is disposed. The recessed surface 16B includes the planar bottom surface of a generally rectangular shape. The rectangular-shaped bottom surface of the recessed surface 16B is defined by four line segments each having a predetermined length L1, for example of approximately 100 .mu.m. The bottom surface of the recessed surface 16B of the recess 16 has a smaller area than that of the bottom surface 14A of the recess 14.
The thinned portion 17 of the diaphragm portion 15 has a width coincident with the predetermined length L1 of the line segments of the rectangular-shaped bottom surface of the recess 16, and a predetermined thickness T2 extending between the rearside face of the surface layer 11B and the bottom surface of the recess 16. The thinned portion 17 is flexible in response to a pressure applied to the diaphragm portion 15. The thicker portion 18 extends along a rectangular periphery of the thinned portion 17 and has a generally rectangular frame-like shape as shown in FIG. 2. The thicker portion 18 has a predetermined width L2 of not less than 10 .mu.m, for instance, of approximately 25 .mu.m. The thicker portion 18 has a predetermined thickness coincident with the thickness T1 of the diaphragm portion 15 and greater than the thickness T2 of the thinned portion 17.
Disposed within the recess 16 is a detector portion 19 of a pressure sensitive arrangement which detects flexure generated in the thinned portion 17 of the diaphragm portion 15. The detector portion 19 may be a piezoresistive element, a piezoelectric element or the like. In this embodiment, a plurality of piezoresistive elements 19, for instance four, are placed inside along the periphery of the bottom surface of the recess 16, namely, the periphery of the thinned portion 17. As illustrated in FIG. 2, the four piezoresistive elements 19 are of an elongated rectangular shape and positioned at the front, rear, left and right sides of the thinned portion 17 in spaced relation to each other. Each of the piezoresistive elements 19 is of a P-conductivity type formed by doping impurity such as boron into the surface layer 11B of the substrate 11, using ion implantation. The piezoresistive element 19 is strainable in response to the flexure of the thinned portion 17 to detect the flexure as the pressure applied to the diaphragm portion 15.
Diffused leads 20 are disposed on predetermined portions of the substrate 11 as shown in FIG. 2. The diffused leads 20 are formed by doping impurity into portions of the surface layer 11B, for instance, by using ion implantation as well as the piezoresistive elements 19. Each of the diffused leads 20 has one end portion disposed within the recess 16 of the diaphragm portion 15 and an opposite end portion extending outside the recess 16. The one end portion of the diffused lead 20 is connected with each piezoresistive element 19 and the opposite end portion thereof is connected with an electrically conductive film 22 as explained later. The diffused lead 20 has a conductivity higher than that of the piezoresistive element 19.
An insulator film 21 of an electrically insulating material, for instance silicon dioxide, is deposited on the upper surface of the substrate 11. The insulator film 21 is conformally deposited on the recess 16 to cover the piezoresistive elements 19 and the diffused leads 20. The insulator film 21 also covers the upper-side face of the surface layer 11B around the recess 16 and the upper surface of the base 11A connected with the upper-side face thereof via an inclined surface therebetween. The insulator film 21 has a thickness of approximately 0.3-0.9 .mu.m. The insulator film 21 has contact holes 21A each positioned on the opposite end portion of the diffused lead 20. The diffused leads 20 are connected with the electrically conductive films 22 selectively deposited over the upper surface of the base 11A, through the contact holes 21A.
Each of the electrically conductive films 22 may be made of metals such as aluminum. The electrically conductive film 22 has one end portion connected with the diffused lead 20 through the contact hole 21A of the insulator film 21. The electrically conductive film 22 has the other end outside the diaphragm portion 15 which forms an electrode 22A connected to an external detecting circuit, not shown.
With this pressure sensitive arrangement, an output signal indicative of the pressure detected by each piezoresistive element 19 is introduced into the external detecting circuit.
A protective film 23 of an electrically insulating material is deposited overall the upper surface of the substrate 11. The protective film 23 covers the electrically conductive films 22 and the diaphragm portion 15 of the surface layer 11B which is covered with the insulator film 21. The protective film 23 has contact holes 23A for electrical connection of the electrically conductive films 22 with the external detecting circuit, each being located corresponding to the electrode 22A of the electrically conductive film 22. FIG. 1 shows one of the electrodes 22A and the corresponding contact hole 23A, for simple illustration.
The insulator film 21 and the protective film 23 are extremely thinned and conformally deposited on the recess 16. Thus, an outer, upper as viewed in FIG. 1, recessed surface 16A is formed atop the protective film 23. The outer recessed surface 16A is similar in shape to the recessed surface 16B.
A lid 24 is fixed to the thicker portion 18 of the diaphragm portion 15 of the substrate 11 via the insulator film 21 and the protective film 23, covering the recess 16 and the detector portion 19 within the recess 16. The lid 24 is made of a suitable material having a predetermined rigidity to reinforce the thicker portion 18. The lid 24 increases mechanical strength of the thicker portion 18 to prevent the thicker portion 18 from being flexed when the diaphragm portion 15 is subject to a pressure within the recess 14. The lid 24 may be made of PYREX glass and be secured, by anode coupling, to the thicker portion 18. The lid 24 cooperates with the diaphragm portion 15 to define a reference pressure chamber S between the outer recessed surface 16A of the recess 16 and a surface of the lid 24 which is opposed to the recess 16. When a pressure is applied to the diaphragm portion 15 via the recess 14 as the pressure port, the thinned portion 17 of the diaphragm portion 15 is flexibly deformable by a differential pressure between the reference pressure chamber S and the recess 14.
It will be appreciated from the above description that, with the arrangement of the diaphragm portion 15 and the recess 16 within the diaphragm portion 15, the thinned portion 17 of the diaphragm portion 15 which is defined by the recess 16 can be more flexible than the thicker portion 18 thereof in response to application of a pressure to the diaphragm portion 15.
With the arrangement of the lid 24 covering the recess 16, the thicker portion 18 of the diaphragm portion 15 which is disposed around the recess 16 can be reinforced by the lid 24. Therefore, when a pressure is applied to the diaphragm portion 15, the thicker portion 18 can be prevented from being easily flexed while the thinned portion 17 defined by the recess 16 is more flexibly deformable. The detector portion 19 disposed within the recess 16 at the thinned portion 17 can detect the pressure applied to the diaphragm portion 15.
Further, since the diaphragm-defining recess 14 has the bottom surface 14A greater in area than the bottom surface 16B, a part of the inner recessed surface 16B, of the recess 16, the recess 16 can be readily placed within the diaphragm portion 15 defined by the bottom surface 14A of the recess 14. Therefore, the detector portion 19 of the pressure sensitive arrangement which is disposed within the recess 16 can be certainly subject to flexure generated in the thinned portion 17 by a pressure applied to the diaphragm portion 15. This serves for reducing the adverse influence on detection accuracy of the pressure sensor which is caused in a case where the recess 16 is placed outside the diaphragm portion 15.
Furthermore, the thicker portion 18 of the diaphragm portion 15 which has the width not less than 10 .mu.m, can permit the position of the bottom surface 14A of the recess 14 to be offset by approximately 10 .mu.m from the intended position relative to the recess 16, even though the cut surface of the substrate 11 is out of the (100) crystal plane due to errors in the cutting process of the substrate 11.
The thinned portion 17 of the diaphragm portion 15 which is defined by the recess 16, acts as an easily flexible part of the diaphragm portion 15. The thinned portion 17 and the detector portion 19 located at the thinned portion 17 can be positioned in alignment with each other with high accuracy.
This allows accurate detection of the pressure applied to the diaphragm portion 15, by the detector portion 19. In addition, dimensions of the thinned portion 17 and the detector portion 19 can be reduced. Then, the dimension of the diaphragm portion 15 can be reduced as compared with the conventionally used one. This contributes to reduction of the size of the pressure sensor.
The use of the piezoresistive elements 19 strainable in response to the flexure at the thinned portion 17 of the diaphragm portion 15 makes the pressure sensor to detect the pressure value variably applied to the diaphragm portion 15.
The formation of the piezoresistive elements 19 by doping serves for enhancing the accurate positioning of the piezoresistive elements 19 within the recess 16.
The following fabrication sequences and the related diagrams illustrate the formation of the structure of the pressure sensor of the present invention.
FIGS. 3 through 14, demonstrate the fabrication of the pressure sensor of the first embodiment shown in FIGS. 1 and 2.
FIG. 3 illustrates a base 25 made of single crystal silicon which serves as a starting material. The monocrystalline silicon base 25 is placed inside apparatus for fabricating the pressure sensor, not shown. The monocrystalline silicon base 25 is of a P-conductivity type which may be formed by doping impurity, for example, boron, thereinto.
A film of silicon dioxide 12 as a passivation film is deposited on opposite, upper and lower, surfaces of the monocrystalline silicon base 25 as shown in FIG. 4. A film of silicon nitride 13 as a passivation film is deposited on the silicon dioxide film 12 on each of the upper and lower surfaces of the monocrystalline silicon base 25.
As shown in FIG. 5, the silicon nitride film 13 over the upper surface of the monocrystalline silicon base 25 is selectively removed by lithographical and etching techniques to partially expose the monocrystalline silicon base 25 thereunderneath. In this process, a portion of the monocrystalline silicon base 25 which is formed into a surface layer 11B shown in FIG. 7, in the subsequent processes, is left covered with the remainder of the silicon nitride film 13.
A film of silicon dioxide 26 is grown onto the monocrystalline silicon base 25 by oxidizing the exposed portion of the monocrystalline silicon base 25 in an ambient containing oxygen, as shown in FIG. 6. This process is often referred to as LOCOS, for local oxidization of silicon. The grown silicon dioxide film 26 having an increased thickness is thus deposited on the upper surface of the monocrystalline silicon base 25.
The remainder of the silicon nitride film 13 on the upper surface of the monocrystalline silicon base 25 is then removed to expose the silicon dioxide film thereunderneath. The monocrystalline silicon base 25 underlying the exposed portion of the silicon dioxide film is now doped with impurity, for instance, phosphorous, to be formed into the surface layer 11B of the N-conductivity type. This doping process may be conducted using ion implantation. Thus, the surface layer 11B of the N-conductivity type is grown onto the base 11A of the P-conductivity type, cooperating with the base 11A to form the substrate 11 as shown in FIG. 7.
An aperture 26A is formed in the silicon dioxide film overlying a portion of the surface layer 11B where the recess 16 is formed in the subsequent process, by removing the corresponding portion of the silicon dioxide film using lithographical and etching techniques, as shown in FIG. 8.
The portion of the surface layer 11B which is exposed through the aperture 26A, is removed by dry etching to form the recess 16 in the surface layer 11B, as shown in FIG. 9. The silicon dioxide film 26 acts as a mask in this process.
The remainder of the silicon dioxide film 26 on the upper surface of the substrate 11 is entirely etched away as shown in FIG. 10.
A detector portion 19 of a pressure sensitive arrangement is formed by doping impurity, for instance, boron, into a portion of the surface layer 11B which defines the recess 16. The detector portion 19 is so disposed as to be within at least a part of the recess 16. The detector portion 19 may be formed of a plurality of elongated rectangular-shaped piezoresistive elements of the P-conductivity type which are disposed along inside a periphery of the bottom surface of the recess 16 in spaced relation to each other. In this process, diffused leads 20 are formed in predetermined regions of the substrate 11, substantially along the upper-side face of the surface layer 11B, as shown in FIG. 11. The insulator film of an electrically insulating material 21 is conformally deposited on the upper surface of the substrate 11. The insulator film 21 may be made of silicon dioxide.
As illustrated in FIG. 12, contact holes 21A are formed in the insulator film 21 for connection of the detector portion 19 with electrically conductive films 22 deposited in the following process.
The electrically conductive films 22 may be deposited on the insulator film 21 by vacuum deposition, as shown in FIG. 13.
A protective film 23 is conformally deposited over the entire upper surface of the substrate 11, as shown in FIG. 14. The protective film 23 has a portion which overlies the insulator film 21 in the recess 16 to form an outer recessed surface 16A. A contact hole 23A is formed to define a bonding pad region corresponding to an electrode 22A of the electrically conductive film 22, in the protective film 23. The silicon dioxide film 12 and the silicon nitride film 13 on the lower surface of the substrate 11 are selectively removed by lithographical and etching techniques to form apertures 12A and 13A, respectively, corresponding to a diaphragm-defining recess 14 formed in the following process.
The substrate 11 is now removed through the apertures 12A and 13A by anisotropic etching using, for instance, potassium hydroxide, hydrazine, ethylenediamine pyrocatechol or the like. The anisotropic etching is terminated at an interface between the base 11A and the surface layer 11B by using an electrochemical etch stopping technique in which positive voltage is applied to the surface layer 11B of the N-conductivity type. The diaphragm-defining recess 14 is thus formed in the substrate 11 as shown in FIG. 1.
A lid 24 is placed onto the thicker portion 18 of the diaphragm portion 15 which is covered with the insulator film 21 and the protective film 23, and then fixed thereto by a suitable manner such as anode coupling.
Referring now to FIGS. 15-27, a second embodiment of the pressure sensor and the method for forming the same, according to the present invention, is explained. This embodiment differs in use of a silicon-on-insulator (SOI) substrate as the substrate of the pressure sensor, from the above-described first embodiment.
As illustrated in FIG. 15, the pressure sensor includes the SOI substrate 31. The SOI substrate 31 is of a generally rectangular shape similar to the substrate 11 of the first embodiment. The SOI substrate 31 includes a base 31A made of single crystal silicon, an insulator film 31C of an electrically insulating material deposited on the base 31A, and a surface layer 31B of a silicon based material, for example, polysilicon, deposited on the insulator film 31C. The insulator film 31C is interposed between the base 31A and the surface layer 31B and may be made of silicon dioxide. The surface layer 31B may be of an N-conductivity type formed by doping impurity such as phosphorous thereinto and have a thickness of approximately 5-20 .mu.m. Deposited on a rearside surface of the base 31A is a silicon dioxide film 32 on which a silicon nitride film 33 is deposited.
A diaphragm-defining recess 34 is substantially centered on the base 31A. The recess 34 is recessed inwardly from the rearside surface of the base 31A to define a diaphragm portion 35 of the SOI substrate 31 between a generally rectangular-shaped planar bottom surface 34A of the recess 34 and an upper-side face of the surface layer 31B. The recess 34 extends through the base 31A to be open to an upper-side surface of the base 31A. The recess 34 is thus opposed to a rearside surface of the insulator film 31C. The recess 34 may be formed by etching the base 31A through apertures 32A and 33A which are formed in the silicon dioxide film 32 and the silicon nitride film 33, respectively. The recess 34 acts as a pressure port through which a pressure is applied to the pressure sensor, as well as the recess 14 of the aforementioned first embodiment.
The diaphragm portion 35 is defined by the generally rectangular-shaped bottom surface 34A of the recess 34. The diaphragm portion 35 has a width corresponding to the bottom surface 34A and a predetermined thickness, as well as the diaphragm portion 15 of the aforementioned first embodiment. The diaphragm portion 35 has a thinned portion 37 defined by a recess 36 as explained later, and a thicker portion 38 disposed around the thinned portion 37 and the recess 36.
The recess 36 is disposed within the diaphragm portion 35 and in the form of a shallow rectangular-shaped groove similar to the recess 16 of the aforementioned first embodiment. The recess 36 is recessed inwardly from the upper-side face of the surface layer 31B of the SOI substrate 31. The recess 36 is defined by a recessed surface 36B of the surface layer 31B. The recessed surface 36B includes a planar bottom surface of a generally rectangular shape which is defined by four line segments, each having a predetermined length as well as the recess 16 of the aforementioned first embodiment. The bottom surface of the recessed surface 36B of the recess 36 has a smaller area than that of the bottom surface 34A of the recess 34.
The thinned portion 37 of the diaphragm portion 35 extends between the bottom surface of the recessed surface 36B and the bottom surface 34A of the recess 34. Thus, the thinned portion 37 includes the surface layer 31B and the insulator film 31C which are disposed between the bottom surface of the recessed surface 36B and the bottom surface 34A of the recess 34. The thinned portion 37 has the predetermined width and thickness and is flexibly deformable in response to a pressure applied to the diaphragm portion 35, as well as the thinned portion 17 of the aforementioned first embodiment.
The thicker portion 38 of the diaphragm portion 35 has a generally rectangular frame-like shape having a predetermined width, for example, of approximately 10-30 .mu.m. The thicker portion 38 includes the surface layer 31B and the insulator film 31C which are disposed between an upper-side face of the diaphragm portion 35 and the bottom surface 34A of the recess 34. The thicker portion 38 has a predetermined thickness greater than the thickness of the thinned portion 37, similar to the thicker portion 18 of the aforementioned first embodiment.
A plurality of piezoresistive elements 39 as a detector portion of a pressure sensitive arrangement are disposed within the recess 36, which detect flexure generated in the thinned portion 37 of the diaphragm portion 35. In this embodiment, four piezoresistive elements 39 are placed inside along the periphery of the thinned portion 37, two of which are shown in FIG. 15. Each of the piezoresistive elements 39 is formed by doping impurity such as boron into the surface layer 31B of the substrate 31 and then being configured to a generally elongated rectangular shape.
Diffused leads 40 are disposed on predetermined portions of the surface layer 31B of the SOI substrate 31. The diffused leads 40 are formed by doping impurity into the predetermined portions of the surface layer 31B. Similar to the diffused lead of the aforementioned first embodiment, each of the diffused leads 40 has one end portion disposed within the recess 36 of the diaphragm portion 35 and an opposite end portion extending outside the recess 36. The one end portion of the diffused lead 40 is connected with each piezoresistive element 39 and the opposite end portion thereof is connected with an electrically conductive film 42 as explained later.
An insulator film 41 of an electrically insulating material is deposited on the upper-side face of the surface layer 31B of the substrate 31. The insulator film 41 may be made of silicon dioxide, as well as the insulator film 21 of the aforementioned first embodiment. The insulator film 41 is conformally deposited on the recess 36 to cover the piezoresistive elements 39 and the diffused leads 40. The insulator film 41 has contact holes 41A each positioned corresponding to the opposite end portion of the diffused lead 40.
The electrically conductive films 42 are deposited on predetermined regions of the upper-side face of the surface layer 31B through the insulator film 41. Each of the electrically conductive film 42 may be made of metals such as aluminum. The electrically conductive film 42 has one end portion connected with the diffused lead 40 through the contact hole 41A of the insulator film 41. The electrically conductive film 42 has the other end outside the diaphragm portion 35 which forms an electrode 42A connected to an external detecting circuit, not shown.
With the pressure sensitive arrangement as described above, an output signal indicative of the pressure detected by each piezoresistive element 39 is introduced into the external detecting circuit.
A protective film 43 of an electrically insulating material is conformally deposited overall on the upper-side face of the surface layer 31B of the SOI substrate 31. The protective film 43 covers the electrically conductive films 42 and overlaps the insulator film 41 on the diaphragm portion 35. The protective film 43 has contact holes 43A for electrical connection of the electrically conductive films 42 with the external detecting circuit, each being located corresponding to the electrode 42A of the electrically conductive film 42. FIG. 15 shows merely one of the contact holes 43A corresponding to the electrode 42A for simple illustration.
Thus, the recess 36 is covered with the insulator film 41 and the protective film 43 which are extremely thinned and conformally deposited on the recess 36. An outer recessed surface 36A similar in shape to the recessed surface 36B is located on the protective film 43.
A lid 44 is fixedly mounted to the thicker portion 38 of the diaphragm portion 35 of the substrate 31 via the insulator film 41 and the protective film 43. The lid 44 covers the recess 36 and the detector portion 39 within the recess 36. The lid 44 is made of such a rigid material as to reinforce the thicker portion 38. The lid 44 may be secured to the thicker portion 38 by anode coupling, as well as the lid 24 of the aforementioned first embodiment. The lid 44 and the diaphragm portion 35 cooperate to define a reference pressure chamber S between the outer recessed surface 36A of the recess 36 and a surface of the lid 44 which is opposed to the recess 36. When a pressure is applied to the diaphragm portion 35 via the recess 34, the thinned portion 37 of the diaphragm portion 35 is flexibly deformable by a differential pressure between the reference pressure chamber S and the recess 34.
Insulator walls 46 may be formed in the surface layer 31B as indicated by a phantom line in FIG. 15. The insulator walls 46 can define a region of the surface layer 31B which is electrically isolated from the remainder of the surface layer 31B. This serves for readily forming an active device, for instance, MOS transistor, on the SOI substrate 31. Such active device as wave-shaping circuit for amplification of the output signal from the piezoresistive elements 39, may be formed integrally with the pressure sensor.
The pressure sensor of the second embodiment can exhibit substantially same function and effects as those of the pressure sensor of the aforementioned first embodiment.
Referring to FIGS. 16-27, the method of fabricating the pressure sensor shown in FIG. 15 will be explained hereinafter. The fabrication processes are conducted in the following sequence.
As illustrated in FIG. 16, a silicon-on-insulator (SOI) substrate 31 is provided as a starting material, which includes a base of monocrystalline silicon 31A, a surface layer of a silicon based material 31B, and an insulator film of an electrically insulating material 31C deposited between the base 31A and the surface layer 31B.
As illustrated in FIG. 17, a film of silicon dioxide 32 as a passivation film is deposited on opposed surfaces of the SOI substrate 31. A film of silicon nitride 33 as a passivation film is deposited on the silicon dioxide film 32 on each of the opposed surfaces of the substrate 31.
As shown in FIG. 18, the silicon nitride film 33 over the upper surface of the SOI substrate 31 is selectively removed by lithographical and etching techniques such that a diaphragm portion 35 formed in the subsequent processes, is left covered with the remainder of the silicon nitride film 33.
A film of silicon dioxide 45 is grown onto a portion of the surface layer 31B which is uncovered with the silicon nitride film 33, using LOCOS of the portion of the surface layer 31B in an ambient containing oxygen, as shown in FIG. 19. The grown silicon dioxide film 45 having an increased thickness is thus deposited on the upper-side face of the surface layer 31B. A portion of the surface layer 31B raised as mesa is formed, in which the diaphragm portion 35 is formed by the subsequent processes.
The remainder of the silicon nitride film 33 over the upper-side face of the surface layer 31B is then removed to expose the silicon dioxide film thereunderneath, as shown in FIG. 20.
An aperture 45A is formed in the silicon dioxide film by selectively removing the exposed silicon dioxide film using lithographical and etching techniques, as shown in FIG. 21. The aperture 45A is open to an upper face of the mesa-like portion of the surface layer 31B.
The mesa-like portion of the surface layer 31B is removed by dry etching through the aperture 45A to form the recess 36 recessed inwardly from the upper-side face of the surface layer 31B, as shown in FIG. 22. The silicon dioxide film 45 acts as a mask in this process.
The remainder of the silicon dioxide film 45 on the upper-side face of the surface layer 31B is entirely etched away as shown in FIG. 23. The surface layer 31B may be doped with impurity, for instance, phosphorous, to be formed into an N-conductivity type.
A detector portion 39 of a pressure sensitive arrangement is formed by doping impurity, for instance, boron, into a portion of the surface layer 31B which defines the recess 36, as shown in FIG. 24. The detector portion 39 is so disposed as to be within at least a part of the recess 36. The detector portion 39 may be formed of a plurality of elongated rectangular-shaped piezoresistive elements of a P-conductivity type which are disposed along inside a periphery of a planar bottom surface of the recess 36 in spaced relation to each other. In this process, diffused leads 40 are formed in predetermined regions of the surface layer 31B, substantially along the upper-side face of the surface layer 31B. The insulator film of an electrically insulating material 41, for instance, silicon dioxide, is conformally deposited on the upper-side face of the surface layer 31B.
As illustrated in FIG. 25, contact holes 41A are formed in the insulator film 41 for connection of the detector portion 39 and the diffused leads 40 with electrically conductive films 42 deposited in the following process.
The electrically conductive films 42 may be deposited on the insulator film 21 by sputtering, vacuum deposition or the like, as shown in FIG. 26.
A protective film 43 is conformally deposited on the entire upper-side face of the surface layer 31B, as shown in FIG. 27. The protective film 43 has a portion which overlies the insulator film 41 in the recess 36 to form an outer recessed surface 36A. A contact hole 43A is formed to define a bonding pad region corresponding to an electrode 42A of the electrically conductive film 42, in the protective film 43. The silicon dioxide film 32 and the silicon nitride film 33 on the lower surface of the base 31A of the SOI substrate 31 are selectively removed by lithographical and etching techniques to form apertures 32A and 33, respectively, corresponding to a diaphragm-defining recess 34 formed in the following process.
The base 31A of the SOI substrate 31 is now removed through the apertures 32A and 33A by anisotropic etching. The anisotropic etching is stopped at an interface between the base 31A and the insulator film 31C. The diaphragm-defining recess 34 is thus formed in the base 31A as shown in FIG. 15.
A lid 44 is placed onto the upper-side face of the thicker portion 38 of the diaphragm portion 35 which is covered with the insulator film 31 and the protective film 33, and then fixed thereto by a suitable connecting manner such as anode coupling.
In the second embodiment of the pressure sensor employing the SOI substrate, the process of depositing the surface layer 11b of the N-conductivity type on the base 11A as explained in the first embodiment, can be omitted. This contributes to reduction of time required for fabricating the pressure sensor.
Further, since the etching of the base 31A for forming the diaphragm-defining recess 34 is stopped at the interface between the base 31A and the insulator film 31C, a large number of pressure sensors having the recess 34 of substantially equal depth may be formed from one SOI substrate 31. This allows the pressure sensors to generate substantially equivalent output signals upon a pressure being applied thereto.
Referring to FIGS. 28-39, a third embodiment of the pressure sensor and the method for forming the same, according to the present invention, is explained. This embodiment differs in that an SOI substrate 51 similar to the SOI substrate 31 of the aforementioned second embodiment is used and a recess 56 for the detector portion of the pressure sensitive arrangement is formed by LOCOS, from the above-described first embodiment.
As illustrated in FIG. 28, the pressure sensor includes the SOI substrate 51 of a generally rectangular shape similar to the SOI substrate 31 of the second embodiment. The SOI substrate 51 includes a base 51A made of single crystal silicon, an insulator film 51C of an electrically insulating material deposited on the base 51A, and a surface layer 51B of a silicon based material, for example, polysilicon, deposited on the insulator film 51C. The insulator film 51C is interposed between the base 51A and the surface layer 51B and may be made of silicon dioxide. The surface layer 51B is of an N-conductivity type formed by doping impurity such as phosphorous thereinto. Deposited on a rearside surface of the base 51A is a silicon dioxide film 52 on which a silicon nitride film 53 is deposited.
A diaphragm-defining recess 54 is substantially centered on the base 51A. The recess 54 is recessed inwardly from the rearside surface of the base 51A to define a diaphragm portion 55 of the SOI substrate 51 between a generally rectangular-shaped planar bottom surface 54A of the recess 54 and an upper-side face of the surface layer 51B. The recess 54 extends through the base 51A to be open to an upper-side surface of the base 51A. The recess 54 is opposed to a rearside surface of the insulator film 51C. The recess 54 may be formed by etching the base 51A through apertures 52A and 53A which are formed in the silicon dioxide film 52 and the silicon nitride film 53, respectively. The recess 54 acts as a pressure port through which a pressure is applied to the pressure sensor, as well as the recess 34 of the aforementioned second embodiment.
The diaphragm portion 55 is defined by the bottom surface 54A of the recess 54. The diaphragm portion 55 has a width corresponding to the bottom surface 54A and a predetermined thickness, as well as the diaphragm portion 35 of the aforementioned second embodiment. The diaphragm portion 55 has a thinned portion 57 defined by a planar bottom surface of the recess 56, and a thicker portion 58 surrounding the thinned portion 57 and the recess 56. The thinned portion 57 includes the surface layer 51B and the insulator film 51C which are disposed between the bottom surface of the recess 56 and the bottom surface 54A of the recess 54. The thinned portion 57 has the predetermined width and thickness and is flexibly deformable in response to a pressure applied to the diaphragm portion 55, as well as the thinned portion 37 of the aforementioned second embodiment. The thicker portion 58 has a generally rectangular frame-like shape having a predetermined width of approximately 10-30 .mu.m, similar to the second embodiment. The thicker portion 58 includes the surface layer 51B and the insulator film 51C which are disposed between an upper-side face of the diaphragm portion 55 and the bottom surface 54A of the recess 54. The thicker portion 58 has a predetermined thickness greater than the thickness of the thinned portion 37.
The recess 56 is disposed within the diaphragm portion 55 of the surface layer 51B of the SOI substrate 51. The recess 56 is formed by LOCOS into a rectangular-shaped groove. The recess 56 is recessed inwardly from the upper-side face of the surface layer 51B and is defined by a recessed surface 56B formed in the surface layer 51B. The recessed surface 56B includes the generally rectangular-shaped planar bottom surface 56B1 and four peripheral side surfaces 56B2 gently inclined relative to the bottom surface 56B1. The bottom surface 56B1 of the recess 56 has a smaller area than that of the bottom surface 54A of the recess 54, as well as the recess 36 of the aforementioned second embodiment.
Specifically, as illustrated in FIG. 29, the bottom surface 56B1 and the inclined peripheral side surfaces 56B2 of the recessed surface 56B are connected with each other through an inner round beveled surface 56B3 having an arcuate-shaped section. The recessed surface 56B also includes an outer round beveled surface 56B4 disposed at an open end of the recess 56 and connects the peripheral side surfaces 56B2 with the upper-side face of the surface layer 51B which surrounds the open end of the recess 56. The outer round beveled surface 56B4 has an arcuate-shaped section as well as the inner round beveled surface 56B3. The inner and outer round beveled surfaces 56B3 and 56B4 are positioned along the periphery of the bottom surface 56B1. The inner round beveled surface 56B3 serves for reducing stress concentration at the connection between the bottom surface 56B1 and the peripheral side surfaces 56B2. The outer round beveled surface 56B4 serves for reducing stress concentration at the connection between the peripheral side surfaces 56B2 and the upper-side face of the surface layer 51B around the open end of the recess 56.
A plurality of piezoresistive elements 59, for example, four, as the detector portion as explained in the second embodiment, are disposed within the recess 56. The four piezoresistive elements 59 are placed inside along the recessed surface 56B, two of which are shown in FIG. 29. Each of the piezoresistive elements 59 is formed by doping impurity such as boron into the surface layer 51B of the SOI substrate 51 and then being configured to a generally elongated rectangular shape.
Diffused leads 60 are disposed on predetermined portions of the surface layer 51B of the SOI substrate 51, as well as the diffused leads 40 of the aforementioned second embodiment. The diffused leads 60 are formed by doping impurity into the predetermined portions of the surface layer 51B. Each of the diffused leads 60 has one end portion disposed within the recess 56 of the diaphragm portion 55 and an opposite end portion extending outside the recess 56. The one end portion of the diffused lead 60 is connected with each piezoresistive element 59 and the opposite end portion thereof is connected with an electrically conductive film 63 as explained later.
A surface film 61 of silicon dioxide is deposited on the surface layer 51B of the SOI substrate 51 by oxidizing the upper-side face of the surface layer 51B. The silicon dioxide film 61 is conformally deposited on the recess 56 to cover the piezoresistive elements 59 and the diffused leads 60. The silicon dioxide film 61 has contact holes 61A each positioned corresponding to the opposite end portion of the diffused lead 60.
An insulator film 62 of an electrically insulating material is conformally deposited on the silicon dioxide film 61 overlying the surface layer 51B. The insulator film 62 may be made of silicon dioxide similar to the insulator film 41 of the aforementioned second embodiment. The insulator film 62 has contact holes 62A, each being positioned corresponding to the opposite end portion of the diffused lead 60 as well as the contact holes 61A of the silicon dioxide film 61.
The electrically conductive films 63 are deposited on predetermined regions of the upper-side face of the surface layer 51B through the surface film 61 and the insulator film 62. Each of the electrically conductive film 63 may be made of metals such as aluminum. The electrically conductive film 63 has one end portion connected with the diffused lead 60 through the contact holes 61A and 62A respectively formed in the surface film 61 and the insulator film 62. The electrically conductive film 63 has the other end outside the diaphragm portion 55 which forms an electrode 63A connected to an external detecting circuit, not shown.
A protective film 64 of an electrically insulating material is conformally deposited overall on the upper-side face of the surface layer 51B of the SOI substrate 51. The protective film 64 covers the electrically conductive films 63 and overlaps the surface film 61 and the insulator film 62 on the diaphragm portion 55. The protective film 64 has contact holes 64A for electrical connection of the electrically conductive films 63 with the external detecting circuit, each being located corresponding to the electrode 63A of the electrically conductive film 63. FIG. 28 shows merely one of the contact holes 64A corresponding to the electrode 63A for simple illustration.
The recess 56 is covered with the surface film 61, the insulator film 62 and the protective film 64 which are extremely thinned and deposited conformally on the recess 56. An outer recessed surface 56A is located on the protective film 64 and similar in shape to the recessed surface 56B.
A lid 65 is fixedly mounted to the thicker portion 58 of the diaphragm portion 55 of the SOI substrate 51 via the surface film 61, the insulator film 62 and the protective film 64. The lid 65 covers the recess 56 and the detector portion 59 within the recess 56. The lid 65 is made of such a rigid material as to reinforce the thicker portion 58. The lid 65 may be secured to the thicker portion 58 by anode coupling, as well as the lid 44 of the aforementioned second embodiment. The lid 65 and the diaphragm portion 55 cooperate to define a reference pressure chamber S between the outer recessed surface 56A and a surface of the lid 65 which is opposed to the outer recessed surface 56A.
With the provision of the inner round beveled surface 56B3 along the periphery of the bottom surface 56B1 of the recessed surface 56B, the peripheral portion of the inner round beveled surface 56B3 is flexibly deformable to reduce stress concentration on the periphery of the bottom surface 56B1, when a pressure is applied to the diaphragm portion 55. This serves for enhancing the pressure resistance of the pressure sensor.
Further, the outer round beveled surface 56B4 disposed on the open end of the recess 56 can prevent the peripheral portion of the open end of the recess 56 from suffering from stress concentration caused by an increasing pressure applied to the d |