WO1996018872A1 - Capacitive pressure transducer and fabrication method thereof - Google Patents
Capacitive pressure transducer and fabrication method thereof Download PDFInfo
- Publication number
- WO1996018872A1 WO1996018872A1 PCT/GB1995/002923 GB9502923W WO9618872A1 WO 1996018872 A1 WO1996018872 A1 WO 1996018872A1 GB 9502923 W GB9502923 W GB 9502923W WO 9618872 A1 WO9618872 A1 WO 9618872A1
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- WO
- WIPO (PCT)
- Prior art keywords
- substrate
- membrane
- spacer
- transducer
- cavity
- Prior art date
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
- G01L9/0041—Transmitting or indicating the displacement of flexible diaphragms
- G01L9/0072—Transmitting or indicating the displacement of flexible diaphragms using variations in capacitance
- G01L9/0073—Transmitting or indicating the displacement of flexible diaphragms using variations in capacitance using a semiconductive diaphragm
Definitions
- This invention concerns improvements relating to capacitive force-sensing transducers or sensors and more particularly, though not exclusively, to a silicon capacitive pressure sensor of a single capacitor cavity type.
- single capacitor cavity type encompasses all simple capacitive transducers or sensors which utilize only a single capacitor as the transducer.
- This type of transducer does not include the more complicated and expensive dual capacitor transducer as shown in US 5,317,922 where one capacitor senses any applied force and the other capacitor is not force-sensing and which simply provides a reference signal for cancelling the effects of temperature drift etc.
- Capacitive pressure sensors are well known and are increasingly used in a number of different applications due to their high pressure sensitivity and relatively low temperature sensitivity.
- Various pressure sensor designs have been developed and the need for improved performance has resulted in a growing interest in silicon capacitive sensors.
- a membrane that is pressure sensitive forms one of two spaced apart plates of a capacitor.
- the deflection of this pressure sensitive membrane by an applied pressure varies the spacing between the two plates and thus the capacitance of the sensor.
- This change in capacitance influences an electrical output signal of an associated interface circuit.
- the plates of the capacitor are formed by silicon wafers which have been sufficiently doped to be suitably conductive.
- the spacing between the two plates is maintained by an insulating dielectric layer, typically glass or silicon dioxide to which the two wafers are bonded
- the insulating layer is formed in the shape of a frame such that a cavity is formed between the two plates when the sensor is fabricated.
- EP-A-0 380 885 addresses the problem of dielectric drift due to the large parasitic capacitance by the reduction of the dielectric spacer wall thickness at the outer edges of the device.
- the fabrication processes involve complex processing techniques that make this device expensive to manufacture.
- An object of the present invention is to provide a generic design and simple batch process method for the production of low cost capacitive force sensing sensors with low long term drift.
- a force sensing transducer comprising: a conductive substrate; an insulating dielectric spacer on said substrate; and a flexible conductive member on said dielectric spacer to form with said conductive substrate a capacitor, wherein the insulating spacer is so formed as to define a cavity between said substrate and said membrane so that the capacitance of said capacitor at the cavity is dependent upon force applied to the conductive membrane, and the insulating spacer comprises a multiplicity of supporting formations bounding said cavity and being so tessellated as to minimise capacitance between the membrane and the substrate at the spacer.
- a method of forming a force sensing transducer comprising: forming an insulating dielectric spacer on a conductive substrate; and forming a flexible conductive member on said dielectric spacer to form with said conductive substrate a capacitor; wherein the insulating spacer is so formed as to define a cavity between the substrate and the member so that the capacitance of the capacitor at the cavity is dependent upon the force applied to the conductive membrane, and as to comprise a multiplicity of supporting formations bounding the cavity which are so tessellated as to minimise the capacitance between the membrane and the substrate at the spacer.
- the invention enables the parasitic capacitance of the dielectric spacer of the transducer to be effectively reduced thereby significantly reducing the potential electrical drift.
- the simple structure of the transducer enables its method of manufacture to require a minimal number of uncomplicated processing steps thereby making the invention suitable for batch processing. Furthermore, by decreasing the capacitance of the transducer, it is made much more suitable for low power applications.
- the substrate may also include a plurality of supporting formations being so tessellated as to correspond to those of the spacer. This advantageously, further reduces the parasitic capacitance of the transducer.
- Figures 1 through 6 are schematic cross sectional views of a pair of silicon wafers showing the steps in fabricating a silicon capacitive pressure sensor embodying the present invention
- Figure 7 is a schematic cross sectional view showing bonded wafers after dicing to create the individual pressure sensor of Figure 6;
- Figure 8 is a schematic top view of the substrate wafer of Figure 6 prior to bonding showing the oxidised and non-oxidised regions of the wafer. Detailed Description of An Embodiment.
- a substrate 12 is formed from a 4" (100mm) monocrystalline ⁇ 100> silicon wafer that has been P doped with boron to a resistivity of O.Ol ⁇ cm "1 and is of standard thickness.
- a membrane 10 is formed from a 4" (100mm) silicon wafer of the same specification as the substrate wafer 12, but of a precisely defined reduced thickness, typically 250 ⁇ m. The membrane 10 is thinner than this substrate to enable the membrane to be flexible when a force is applied thereto.
- the substrate wafer 12 is oxidised on its upper surface to form a polycrystalline silicon dioxide (Si0 2 ) layer 11 of a precisely defined thickness of 1/xrn ( Figure 2) .
- Figure 3 shows how the oxide layer 11 on the substrate wafer 12 is processed using standard microengineering techniques such as chemical etching to create a precisely defined cavity 13.
- This cavity 13 is laterally circular and has a given diameter of 2700mm and vertical depth of l ⁇ m. Further etching is then undertaken on the surrounding oxide support structure to form a multiplicity of supportive formations 14 arranged in a tessellated structure 21 (see Figure 8) . This acts to reduce the parasitic capacitance of the oxide layer 11.
- each of the supportive formations 14 is a square of dimensions 50 ⁇ m and wall thickness 2 ⁇ m. However, it is possible to use any tessellatable shape for the support formations 14 each as hexagons or triangles.
- Figures 5 and 6 show how the substrate 12 and membrane 10 wafer are joined together using direct silicon bonding in a oxygen ambient atmosphere and at a relatively high temperature so as to obtain a vacuum reference cavity 16.
- This cavity 16 is hermetically sealed.
- the cavity 16 can be formed in a vacuous or pressurised environment to give different operational characteristics to the transducer 1.
- transducers having a vacuous cavity 13 will tend to be more sensitive to low pressures than would transducers having a pressurised cavity 13.
- Aluminium contacts 18, 19 are deposited on the outer surfaces of the thus formed transducer 1 so as to provide electrical contact to the membrane 10 and the substrate 12 of the capacitor (see Figure 7) .
- any other suitable metal may alternatively be used in place of aluminium, such as gold or silver.
- the above described embodiment is susceptible to modification and alteration without departure from the spirit and scope of the invention as determined in the appended claims.
- the substrate 10 and membrane 12 do not have to be formed out of silicon and other semiconductor materials may alternatively be used.
- the above described transducer 1 senses changes in pressure but it is possible for the transducer to be an accelerometer which senses the forces generated by acceleration.
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Measuring Fluid Pressure (AREA)
Abstract
A silicon capacitive pressure transducer (1) of a single capacitor cavity type for sensing pressure is described. The transducer or sensor comprises a flexible conductive doped silicon membrane (10) and a conductive doped silicon substrate (12). An insulating dielectric oxide layer (11) is provided between the membrane (10) and substrate (12), and the dielectric layer (11) is etched to define a single capacitor cavity (13, 16) in the transducer (1). The insulating dielectric layer (11) includes a multiplicity of supporting formations (14) which are tessellated so as to minimise the capacitance contribution of the dielectric layer (11).
Description
CAPACITIVE PRESSURE TRANSDUCER AND FABRICATION METHOD THEREOF.
Field of the Invention
This invention concerns improvements relating to capacitive force-sensing transducers or sensors and more particularly, though not exclusively, to a silicon capacitive pressure sensor of a single capacitor cavity type. It should be noted that the term "single capacitor cavity type" encompasses all simple capacitive transducers or sensors which utilize only a single capacitor as the transducer. This type of transducer does not include the more complicated and expensive dual capacitor transducer as shown in US 5,317,922 where one capacitor senses any applied force and the other capacitor is not force-sensing and which simply provides a reference signal for cancelling the effects of temperature drift etc.
Background to the Invention
Capacitive pressure sensors are well known and are increasingly used in a number of different applications due to their high pressure sensitivity and relatively low temperature sensitivity. Various pressure sensor designs have been developed and the
need for improved performance has resulted in a growing interest in silicon capacitive sensors.
In known capacitive pressure sensors for example as shown in EP-A-0380885 and US 4586109, a membrane that is pressure sensitive forms one of two spaced apart plates of a capacitor. The deflection of this pressure sensitive membrane by an applied pressure varies the spacing between the two plates and thus the capacitance of the sensor. This change in capacitance influences an electrical output signal of an associated interface circuit. In the silicon capacitive sensor the plates of the capacitor are formed by silicon wafers which have been sufficiently doped to be suitably conductive. The spacing between the two plates is maintained by an insulating dielectric layer, typically glass or silicon dioxide to which the two wafers are bonded The insulating layer is formed in the shape of a frame such that a cavity is formed between the two plates when the sensor is fabricated.
There is a need in many applications for the long-term drift of the sensor to be minimised and also for a reduction in size and cost of the sensor.
This reduction in size and cost of the sensor can be made by the use of a small number of simple batch processing steps. Until fairly recently batch
processing techniques have been employed in the production of piezoresistive pressure sensors and capacitive sensors employing a conductive silicon membrane on a glass or ceramic substrate. However, silicon-glass capacitive sensors have significant drawbacks in that, the electrical connection to the inner capacitive plate on the glass substrate involves complicated fabrication processes, and the substrate material must be selected with care to avoid stresses caused by thermal mismatch.
US 4,586,109 addresses some of these problems but itself suffers from problems such as: a high complexity of fabrication for manufacturing the pressure sensitive membrane to the required tolerance; a requirement for double sided etching; a need for two wafer alignment; and a high dielectric drift due to the large parasitic capacitance of the silicon dioxide dielectric spacer.
EP-A-0 380 885 addresses the problem of dielectric drift due to the large parasitic capacitance by the reduction of the dielectric spacer wall thickness at the outer edges of the device. However the fabrication processes involve complex processing techniques that make this device expensive to manufacture.
Objects and Summary of the Invention
Thus it can be seen there is a need for a minimal batch processing method for the manufacturing of capacitive pressure sensors with small physical dimensions, low cost and reduced long term drift. The sensors produced should ideally be suitable for use in high temperature environments and in environments where large changes in temperature may occur.
An object of the present invention is to provide a generic design and simple batch process method for the production of low cost capacitive force sensing sensors with low long term drift.
According to one aspect of the present invention, there is provided a force sensing transducer comprising: a conductive substrate; an insulating dielectric spacer on said substrate; and a flexible conductive member on said dielectric spacer to form with said conductive substrate a capacitor, wherein the insulating spacer is so formed as to define a cavity between said substrate and said membrane so that the capacitance of said capacitor at the cavity is dependent upon force applied to the conductive membrane, and the insulating spacer comprises a multiplicity of supporting formations bounding said cavity and being so tessellated as to minimise capacitance between the membrane and the substrate at
the spacer.
According to another aspect of the present invention, there is provided a method of forming a force sensing transducer said method comprising: forming an insulating dielectric spacer on a conductive substrate; and forming a flexible conductive member on said dielectric spacer to form with said conductive substrate a capacitor; wherein the insulating spacer is so formed as to define a cavity between the substrate and the member so that the capacitance of the capacitor at the cavity is dependent upon the force applied to the conductive membrane, and as to comprise a multiplicity of supporting formations bounding the cavity which are so tessellated as to minimise the capacitance between the membrane and the substrate at the spacer.
Advantageously, the invention enables the parasitic capacitance of the dielectric spacer of the transducer to be effectively reduced thereby significantly reducing the potential electrical drift. In addition, the simple structure of the transducer enables its method of manufacture to require a minimal number of uncomplicated processing steps thereby making the invention suitable for batch processing. Furthermore, by decreasing the capacitance of the transducer, it is made much more suitable for low
power applications.
The substrate may also include a plurality of supporting formations being so tessellated as to correspond to those of the spacer. This advantageously, further reduces the parasitic capacitance of the transducer.
The above and further features of the invention are set forth with particularity in the appended claims and, together with the advantages thereof, will become clearer from consideration of the following detailed description of an exemplary embodiment of the invention given with reference to the accompanying drawings.
Brief Description of The Drawings Figures 1 through 6 are schematic cross sectional views of a pair of silicon wafers showing the steps in fabricating a silicon capacitive pressure sensor embodying the present invention;
Figure 7 is a schematic cross sectional view showing bonded wafers after dicing to create the individual pressure sensor of Figure 6; and
Figure 8 is a schematic top view of the substrate wafer of Figure 6 prior to bonding showing the oxidised and non-oxidised regions of the wafer.
Detailed Description of An Embodiment.
Referring now to Figures l to 6, there is shown a method of manufacturing a transducer 1 embodying the present invention. A substrate 12 is formed from a 4" (100mm) monocrystalline <100> silicon wafer that has been P doped with boron to a resistivity of O.OlΩcm"1 and is of standard thickness. A membrane 10 is formed from a 4" (100mm) silicon wafer of the same specification as the substrate wafer 12, but of a precisely defined reduced thickness, typically 250μm. The membrane 10 is thinner than this substrate to enable the membrane to be flexible when a force is applied thereto. The substrate wafer 12 is oxidised on its upper surface to form a polycrystalline silicon dioxide (Si02) layer 11 of a precisely defined thickness of 1/xrn (Figure 2) .
Figure 3 shows how the oxide layer 11 on the substrate wafer 12 is processed using standard microengineering techniques such as chemical etching to create a precisely defined cavity 13. This cavity 13 is laterally circular and has a given diameter of 2700mm and vertical depth of lμm. Further etching is then undertaken on the surrounding oxide support structure to form a multiplicity of supportive formations 14 arranged in a tessellated structure 21 (see Figure 8) . This acts to reduce the parasitic
capacitance of the oxide layer 11.
Further reduction in parasitic capacitance is undertaken by etching the substrate 12 to form supportive formations 15 corresponding to those of the oxide layer 11 (see Figure 4) . This reduces the bonded area to 10% of the non-etched structure. The formations 14, 15 are etched in a tessellatable shape so as to obtain the best mechanical performance for the given area reduction. In this embodiment, each of the supportive formations 14 is a square of dimensions 50μm and wall thickness 2μm. However, it is possible to use any tessellatable shape for the support formations 14 each as hexagons or triangles.
Figures 5 and 6 show how the substrate 12 and membrane 10 wafer are joined together using direct silicon bonding in a oxygen ambient atmosphere and at a relatively high temperature so as to obtain a vacuum reference cavity 16. This cavity 16 is hermetically sealed. Alternatively, the cavity 16 can be formed in a vacuous or pressurised environment to give different operational characteristics to the transducer 1. In particular, transducers having a vacuous cavity 13 will tend to be more sensitive to low pressures than would transducers having a pressurised cavity 13. Aluminium contacts 18, 19 are deposited on the outer surfaces of the thus formed transducer 1 so as
to provide electrical contact to the membrane 10 and the substrate 12 of the capacitor (see Figure 7) . However, any other suitable metal may alternatively be used in place of aluminium, such as gold or silver. The above method of manufacturing a capacitive transducer 1 is applied to each wafer 10, 12 such that a plurality of transducers are manufactured in several simple batch processing steps. The individual silicon capacitive pressure sensors 22 are then formed by the dicing of the wafers at 20 as shown in Figures 7 and 8.
Having thus described the invention by reference to a particular embodiment, it is to be appreciated that the above described embodiment is susceptible to modification and alteration without departure from the spirit and scope of the invention as determined in the appended claims. For example, the substrate 10 and membrane 12 do not have to be formed out of silicon and other semiconductor materials may alternatively be used. Furthermore, the above described transducer 1 senses changes in pressure but it is possible for the transducer to be an accelerometer which senses the forces generated by acceleration.
Claims
1. A force sensing transducer comprising: a conductive substrate; an insulating dielectric spacer on said substrate; and a flexible conductive member on said dielectric spacer to form with said conductive substrate a capacitor, wherein the insulating spacer is so formed as to define a cavity between said substrate and said membrane so that the capacitance of said capacitor at the cavity is dependent upon force applied to the conductive membrane, and the insulating spacer comprises a multiplicity of supporting formations buonding said cavity and being so tessellated as to minimise capacitance between the membrane and the substrate at the spacer.
2. A transducer according to claim 1, wherein said conductive substrate comprises a multiplicity of supporting formations being so tessellated as to correspond to those of said spacer.
3. A transducer according to claim 1 or 2, wherein the spacer is so formed with the substrate and the membrane to define a hermetically sealed cavity.
4. A transducer according to any preceding claim, wherein said substrate and said membrane each comprise a doped semi-conducting material.
5. A transducer according to any preceding claim, wherein said substrate and said membrane each comprise silicon.
6. A transducer according to any preceding claim, further comprising a metal electrode provided on each of said membrane and said substrate.
7. A transducer according to any preceding claim, wherein said insulating dielectric spacer comprises an oxide layer.
8. A method of forming a force sensing transducer said method comprising: forming an insulating dielectric spacer on a conductive substrate; and forming a flexible conductive member on said dielectric spacer to form with said conductive substrate a capacitor; wherein the insulating spacer is so formed as to define a cavity between the substrate and the member so that the capacitance of the capacitor at the cavity is dependent upon the force applied to the conductive membrane, and as to comprise a multiplicity of supporting formations bounding the cavity which are so tessellated as to minimise the capacitance between the membrane and the substrate at the spacer.
9. A method according to claim 8, wherein said insulating spacer forming step comprises: oxidising said substrate and selectively etching the oxidised layer to remove respective portions thereof.
10. A method according to claim 8 or 9, further comprising fusing said substrate, spacer and membrane together to create a hermetically sealed cavity.
11. A method according to claim 10 wherein said fusing step is carried out in an atmosphere of oxygen and said method further comprises heating said transducer.
12. A method according to claim 10, wherein said fusing step is either carried out in a vacuum or under pressure to provide a vacuous or pressurised sealed cavity.
13. A method according to any of claims 8 to 12, further comprising forming metal electrodes on each of said membrane and substrate.
14. A method according to any of claims 8 to 13, further comprising removing portions of said substrate to provide a multiplicity of supporting formations being so tessellated as to correspond to those in said spacer.
15. A capacitive force sensing transducer of a single capacitor cavity type for sensing physical changes such as changes in pressure, said transducer comprising: a flexible conductive membrane; a conductive substrate; and an insulating dielectric spacer provided between the substrate and membrane, the spacer being arranged to define a single capacitor cavity in said transducer; wherein the insulating dielectric spacer includes a plurality of supportive formations arranged which are tessellated so as to minimise the capacitance contribution of the dielectric spacer.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GBGB9425250.9A GB9425250D0 (en) | 1994-12-14 | 1994-12-14 | Silicon capacitive pressure sensor designed for manufacturability and low long term drift |
| GB9425250.9 | 1994-12-14 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| WO1996018872A1 true WO1996018872A1 (en) | 1996-06-20 |
Family
ID=10765942
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/GB1995/002923 WO1996018872A1 (en) | 1994-12-14 | 1995-12-14 | Capacitive pressure transducer and fabrication method thereof |
Country Status (2)
| Country | Link |
|---|---|
| GB (1) | GB9425250D0 (en) |
| WO (1) | WO1996018872A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2000036385A1 (en) * | 1998-12-15 | 2000-06-22 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method of producing a micromechanical structure for a micro-electromechanical element |
| US7273763B1 (en) | 1998-12-15 | 2007-09-25 | Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V. | Method of producing a micro-electromechanical element |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1993017313A1 (en) * | 1992-02-28 | 1993-09-02 | Siemens Aktiengesellschaft | Pressure difference sensor |
| US5317922A (en) * | 1992-04-30 | 1994-06-07 | Ford Motor Company | Capacitance transducer article and method of fabrication |
| WO1995020754A1 (en) * | 1994-01-28 | 1995-08-03 | United Technologies Corporation | A capacitive pressure sensor having a reduced area dielectric spacer |
-
1994
- 1994-12-14 GB GBGB9425250.9A patent/GB9425250D0/en active Pending
-
1995
- 1995-12-14 WO PCT/GB1995/002923 patent/WO1996018872A1/en active Application Filing
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1993017313A1 (en) * | 1992-02-28 | 1993-09-02 | Siemens Aktiengesellschaft | Pressure difference sensor |
| US5317922A (en) * | 1992-04-30 | 1994-06-07 | Ford Motor Company | Capacitance transducer article and method of fabrication |
| WO1995020754A1 (en) * | 1994-01-28 | 1995-08-03 | United Technologies Corporation | A capacitive pressure sensor having a reduced area dielectric spacer |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2000036385A1 (en) * | 1998-12-15 | 2000-06-22 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method of producing a micromechanical structure for a micro-electromechanical element |
| US7273763B1 (en) | 1998-12-15 | 2007-09-25 | Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V. | Method of producing a micro-electromechanical element |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9425250D0 (en) | 1995-02-08 |
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