US20060191351A1

US20060191351A1 - Sealed capacitive sensor

Info

Publication number: US20060191351A1
Application number: US11/353,359
Authority: US
Inventors: Peter Meehan; William Hunt; Eamon Hynes; John O'Dowd; Oliver Kierse
Original assignee: Analog Devices Inc
Current assignee: Analog Devices Inc
Priority date: 2005-02-25
Filing date: 2006-02-14
Publication date: 2006-08-31
Also published as: US7938014B2; US20080210013A1

Abstract

A sealed capacitive sensor includes a substrate having a diaphragm forming a first plate of a capacitor; a second fixed plate of the capacitor spaced from the diaphragm and defining a predetermined dielectric gap and a sealing medium connecting together the substrate and fixed plate in an integrated structure and hermetically sealing the gap.

Description

RELATED APPLICATIONS

This invention claims the benefit of U.S. Provisional Application No. 60/656,238, filed Feb. 25, 2005, incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to a sealed capacitive sensor.

BACKGROUND OF THE INVENTION

In conventional capacitive sensors the fixed plate, typically silicon, is connected to the diaphragm, typically formed in a ceramic substrate, using solder balls. One problem with this is that the solder balls are typically about 150-200 μm in height. This defines at least in part the dimension of the capacitive gap. With such a large height the gap, too, is large and this reduces the sensitivity. Therefore the diaphragm has to deflect more under pressure to obtain the desired sensitivity. To accommodate this the diaphragm must be made thinner to get the desired sensitivity. This makes the diaphragm more susceptible to cracking and fracturing. One solution is to provide holes to receive the solder balls and thus reduce their effective height. But the addition of these holes adds considerable cost and must be done extremely accurately. Even assuming a proper balance between solder ball height and diaphragm thickness can be obtained, the solder balls themselves contribute to inaccuracy as they may fracture, break and are subject to creep from temperature and time.
Another shortcoming is that solder balls do not hermetically seal the gap: contaminants and conditions such as changes in ambient pressure can effect the gap and change the capacitance, independent of any change in the parameter being measured by displacement of the diaphragm. To accommodate this the entire device can be placed in a sealed package but this, too, adds to the cost.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an improved sealed capacitive sensor.
It is a further object of this invention to provide such an improved sealed capacitive sensor which provides a hermetically sealed dielectric gap.
It is a further object of this invention to provide such an improved sealed capacitive sensor which is more robust, reliable and accurate, yet lower in cost.
It is a further object of this invention to provide such an improved sealed capacitive sensor in which the dielectric gap is not vulnerable to fracture, breakage or creep of solder balls.
It is a further object of this invention to provide such an improved sealed capacitive sensor which is less susceptible to contamination.
The invention results from the realization that a simpler, less costly, more accurate, reliable and robust capacitive sensor can be achieved by using a sealing medium such as a frit or glass frit to both connect together and hermetically seal a substrate having a diaphragm forming a first plate of a capacitor and a second fixed plate of a capacitor spaced from the diaphragm and defining a predetermined dielectric gap.
The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives.
This invention features a sealed capacitive sensor including a substrate having a diaphragm forming a first plate of a capacitor and a second fixed plate of the capacitor spaced from the diaphragm and defining a predetermined dielectric gap. There is a sealing medium which connects together the substrate and the fixed plate in an integral structure and hermetically seals the gap.
In a preferred embodiment, the substrate may include a ceramic, or it may include silicon. The fixed plate may include silicon. The sealing medium may be a frit; it may be a glass frit. At least one of the first and fixed plates may include a metal layer. At least one of the first and fixed plates may include silicon. Each of the silicon plates may be doped. The fixed plates may bear an active electronic circuit associated with the capacitive sensor. The active electronic circuit may be on a separate chip. There may be an insulation layer between the active electronic circuit and the fixed plate. The fixed plate may include a recess which defines at least a part of the gap dimension. The sealing medium may define at least a part of the gap dimension. The fixed plate may be electrically accessed through a wire bonding lead. The first plate may be electrically accessed through a conductor buried in the substrate traversing the sealing medium. The fixed plate may include a second recess for housing an active electric circuit associated with a capacitive sensor. There may be a cover and a second sealing medium for connecting the cover to the fixed plate and hermetically sealing the second recess. The sealing medium may include a frit; it may be a glass frit. There may be an over mold covering the fixed plate and attached to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:
FIG. 1 is a side sectional diagrammatic view of a sealed capacitive sensor according to this invention;
FIG. 2 is a top view of the sealed capacitive sensor of FIG. 1;
FIG. 3 is a view similar to FIG. 1 with the substrate diaphragm reinforced;
FIG. 4 is a view similar to FIG. 1 with the active circuit as a silicon on insulation layer;
FIG. 5 is a view similar to FIG. 4 without a recess in the fixed plate and the gap dimensions defined solely by the sealing medium;
FIG. 6 is a view similar to FIG. 3 with metal plates on the fixed plate and both sides of the diaphragm.
FIG. 7 is a view similar to FIG. 1 made entirely of silicon;
FIG. 8 is a side sectional diagrammatic view of a sealed capacitive sensor according to this invention with a second recess for housing associated circuitry or a second sensor e.g. an accelerometer.
FIG. 9 is a view similar to FIG. 1 showing both upper and lower moats or wells and buried conductor traversing the sealing medium;
FIG. 10 is a view similar to FIG. 9 showing barrier walls in place of wells or moats; and
FIG. 11 is a view similar to FIG. 1 showing the active circuit borne by the fixed plate as a separate chip.

DISCLOSURE OF THE PREFERRED EMBODIMENT

Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer.
There is shown in FIG. 1, a sealed capacitive sensor 10 including a ceramic substrate 12, for example, Al₂O₃having a flexible diaphragm 14 and a fixed plate silicon die 16 spaced from diaphragm 14 defining a dielectric gap 18. Substrate 12 is connected to fixed plate or die 16 to form an integral structure by means of a sealing medium 20, such as, a frit or glass frit, for example, a glass frit sold by Ferroelectronics Material Systems, Inc. of Vista, Calif., under the name of Seal Glass Paste, DL11036 Model FX-11-036. Other sealing mediums can be used, such as organic or inorganic adhesives, for example, Ablestik 660 available from Ablestick Laboratories, Rancho Domingues, Calif. Sealing medium or frit 20 not only connects together fixed plate 16 and substrate 12, but it also forms a hermetic seal about gap 18 so that no contaminates from the outside can enter and no spurious changes of pressure can enter that would change the normal conditions in the gap. In this way only the parameter, such as, pressure or force which is being sensed by the capacitive sensor 10 will flex diaphragm 14 and change the dimension of gap 18, thereby, changing the capacitance which change is used as an indication of the parameter being measured.
In addition to diaphragm 14 the first plate of the capacitor includes a metal layer 22 which may be diffused into the ceramic which extends out through or under the frit 20 to trace 28 for connection to circuitry this may be seen as a top view in FIG. 2. Fixed plate 16, FIG. 1, formed from a silicon die forms the plate of the capacitor doped or undoped and may in addition in some cases have its own metal plate 30. Although not a necessary limitation of the invention a recess 32 is formed in fixed plate 16. In this case dimension 34 of gap 18 is determined by the size of recess 32, the sealing medium or frit 20 and the height, small though it be, of metal layer 22. An active circuit 36, such as, signal processing circuitry, accelerometers and the like associated with the capacitive sensor 10 may be fabricated right in the silicon die of fixed plate 16. Metal layer 22 may be circular or another shape as required. Ceramic substrate 12 with diaphragm 14 may be 100 to 200 microns thick for sensing tire pressures and perhaps approximately 400 microns thick for sensing braking pressure.
Ceramic substrate 12 a, FIG. 3, may be formed with reinforcing supports 40, 42 where necessary. Alternatively, ceramic 12 a can be formed as a unit with reinforcing supports 40, 42 and have portion 44 etched or machined away to the same effect. Alternatively, diaphragm 44 and supports 40,42 may be made integral and co-fired with substrate 12. Even though gap 18 is fully hermetically sealed and substrate 12 is well secured to silicon die fixed plate 16 by sealing medium frit 20, a further protection may be added in the nature of a over mold or cover, such as, a glob top or plastic 50, FIG. 3. This may be made of any suitable material such as DYM 9001-E-v3.1 from DYMAX Corporation, 51 Greenwoods Road, Torrington, Conn. 06790 or Sumitomo 6300H G770 from Sumitomo Corporation.
The active circuit need not be fabricated directly on fixed plate silicon die 16, FIG. 4, it may be a silicon on insulation (SOI) circuit 36 b mounted on insulation 52 on die 16. Although thus far FIGS. 1 through 4 depict gap 18 as formed at least partially by recess 32, this is not a necessary limitation of the invention, for as shown in FIG. 5, the full dimension 34 of gap 18 may be defined by the height of the sealing medium, frit 20. Various layers of metal 22, 30 and layer 60 on diaphragm 14, FIG. 6, may be used to modify the mechanical (stiffness) and electrical (conductivity) responses of capacitive sensor 10 d. Although thus far the substrate 12 has been indicated as a ceramic, this is not a limitation of the invention, for as shown in FIG. 7, substrate 12 in sensor 10 e may be silicon, just as fixed plate silicon die 16. In that case, metal plate 22 would be optional since now the silicon in substrate 12 as well as that in die 16 can act as a conductor plate of the capacitor. In that case, FIG. 7, metal plate conductor 22 is optional.
Capacitive sensor 10 f may be formed as a dual recess device, FIG. 8, having a second recess 70 in which is disposed active circuit 36 and or other sensors, for example, accelerometer or capacitive sensors . Active circuit 36 may be connected to metal plate conductor 30 by means of via 72. Trace 74 passing under frit 76 to bond wire 28 and eventually to trace lead 24. Frit 76 acts to fix cover 78, made of for example ceramic or Kovar, to fixed plate silicon die 16 f and hermetically seal second recess 70. Sealing medium 76 may be a frit, such as, a glass frit as used for sealing medium 20. The connection between metal plate conductor 22 and trace conductor 24 may use a buried conductor 80, FIG. 9, which traverses and passes beneath sealing medium 20 to improve the hermetic seal since the frit seals better to the ceramic 12 than to the metal 22, 24. Also shown in FIG. 9 are the use of wells or moats 90, 92 in fixed plate silicon die 16 and/or wells and moats 94, 96 in ceramic 12 to prevent spread of the frit during sealing. The material may be removed in either case in any suitable manner to create individual wells or a continuous moat to prevent the glass frit from moving, wicking, or spreading into the sensing area where it would interfere with the accuracy of the sensor. Alternatively, instead of moats or wells, barrier walls 98, FIG. 10, may be used for the same purpose. The walls, moats and wells are disclosed in more detail in co-pending application, U.S. Patent Application, entitled, IMPROVED CAPACITIVE SENSOR AND METHOD OF FABRICATING, by Meehan et al. (AD-444J), filed on even date herewith, which is herein incorporated in its entirety by this reference. If the active circuit 36, FIG. 11, is quite expensive as formed on silicon die 16, it may be more inexpensively produced on its own chip 110, FIG. 11, and then, for example, flip chip bonded using solder bumps 112 directly to silicon die 16. That contact may be sufficient or a via 114, for example, may be used to interconnect active circuit 36 with fixed plate conductor 30. Throughout here specific techniques of connections, such as, bond wires and vias, metal layers and traces, have been shown but these are not limitations of the invention, as any of the connection devices shown as well as other devices, not shown, may be used to interconnect the various components. The unit may also be incorporated into a ceramic package.
Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments.
In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended.
Other embodiments will occur to those skilled in the art and are within the following claims.

Claims

1. A sealed capacitive sensor comprising:

a substrate having a diaphragm forming a first plate of a capacitor;

a second fixed plate of the capacitor spaced from said diaphragm ad defining a dielectric gap; and

a sealing medium connecting together said substrate and fixed plate in an integral structure and hermetically sealing said gap.

2. The sealed capacitive sensor of claim 1 in which said substrate includes ceramic.

3. The sealed capacitive sensor of claim 1 in which said substrate includes silicon.

4. The sealed capacitive sensor of claim 1 in which said fixed plate includes silicon.

5. The sealed capacitive sensor of claim 1 in which said sealing medium includes a frit.

6. The sealed capacitive sensor of claim 5 in which said frit includes a glass frit.

7. The sealed capacitive sensor of claim 1 in which at least one of said first and fixed plates includes a metal layer

8. The sealed capacitive sensor of claim 1 in which at least one of said first and fixed plates includes silicon.

9. The sealed capacitive sensor of claim 8 in which each of said silicon plates is doped.

10. The sealed capacitive sensor of claim 1 in which said fixed plate bears an active electronic circuit associated with said capacitive sensor.

11. The sealed capacitive sensor of claim 1 in which said fixed plate bears on a separate chip, an active electronic circuit associated with said capacitive sensor.

12. The sealed capacitive sensor of claim 1 1 in which there is an insulation layer between said active electronic circuit and said fixed plate.

13. The sealed capacitive sensor of claim 1 in which said fixed plate includes a recess which defines at least a part of the gap dimension.

14. The sealed capacitive sensor of claim 1 in which the sealing medium defines at least a part of the gap dimension.

15. The sealed capacitive sensor of claim 1 in which said fixed plate is electrically accessed through a wire bonding lead.

16. The sealed capacitive sensor of claim 1 in which said first plate is electrically accessed through a conductor buried in the substrate traversing said sealing medium.

17. The sealed capacitive sensor of claim 1 in which said fixed plate includes a second recess for housing an active electric circuit associated with said capacitive sensor.

18. The sealed capacitive sensor of claim 17 further including a cover and second sealing medium for connecting said cover to said fixed plate and hermetically sealing said second recess.

19. The sealed capacitive sensor of claim 18 in which said second sealing medium includes a frit.

20. The sealed capacitive sensor of claim 18 in which said second sealing medium includes a glass frit.

21. The sealed capacitive sensor of claim 1 further including an over mold covering said fixed plate and attached to said substrate.