Pressure sensor element with glass barrier material configured for increased capacitive response

A method for increasing capacitive response to applied pressure by including a glass layer in the air gap of an improved capacitive sense element. The glass layer has additional thickness compared to previously known capacitive sense elements. The additional thickness is selected to achieve a predetermined increase in dielectric constant across the gap between electrodes. In an illustrative embodiment, the resulting capacitive sense element includes a gap having two dielectric media, air and glass, between the electrodes, wherein the thickness of the glass media in the gap and the thickness of the remaining air gap are predetermined based on the dielectric constant of glass and air to achieve desired capacitive response characteristics of the sensor design.

FIELD OF THE DISCLOSURE

The present disclosure is in the field of pressure transducers and more particularly to capacitor type pressure sense elements.

BACKGROUND

Pressure transducers are used in a wide variety of machines and systems to sense fluid pressure and applied forces at transducer locations. Pressure transducers generally convert a magnitude of the applied pressure to an electrical signal representing the magnitude. For example, previously known capacitive pressure sensors convert a magnitude of applied pressure to a magnitude of electrical capacitance.

Some previously known pressure sense elements that can be used in pressure transducer applications are based on a capacitive sensing technology. An example of such a sense element uses a pair of ceramic plates and electrodes separated by an air gap, which form a parallel plate capacitor on a rigid ceramic plate substrate. A deformable ceramic plate forms a diaphragm, which adjusts the distance between two electrodes across the gap. A nominal capacitance of the sense element and an amount of change in capacitance under application of pressure loading are characteristics of the sense element that are determined by several mechanical factors. These factors include the thickness of the diaphragm, the area of the diaphragm that is allowed to deform, the area of the electrodes, and the thickness of the air gap between the electrodes.

In certain applications, the air gap distance between electrodes of a sense element is in the order of micrometers. Sense elements that are manufactured with such a small gap thicknesses can have high defect rates due to contamination by conductive particles in the air gap that can provide a conductive path between the electrodes. The conductive particles are difficult to completely eliminate during manufacturing.

To reduce manufacturing defects due to conductive particle contamination in the air gap of a sense element, some previously known capacitive sense elements have included a glass layer over one of the electrodes for providing electrical insulation between the electrodes. However, because glass layer has a different dielectric constant than air in the gap, glass layer has the detrimental effect of altering the sense element characteristics such as the nominal capacitance and the amount of change in capacitance under application pressure loading. Moreover, by including a glass barrier over one of the electrodes, the amount of deflection that the deformable ceramic plate can achieve before contacting the glass barrier is limited. In previously known capacitive sense elements that include an extra glass layer over one of the electrodes, changes to the sensor's capacitive response characteristics are minimized or reduced by minimizing the thickness of the glass layer or increasing the thickness of the air gap.

SUMMARY

Aspects of the present disclosure include a capacitive sensor apparatus. The capacitive sensor apparatus includes a rigid insulator plate, a first electrode formed on the rigid insulator plate, a deformable insulator plate opposite the rigid insulator plate and a second electrode formed on the deformable insulator plate and facing the first electrode. An air gap is formed between the first electrode and the second electrode. A glass barrier layer is also formed between the first electrode and the second electrode. The glass barrier layer is configured by using a predetermined glass barrier layer thickness and/or by selecting a glass barrier layer material to achieve a predetermined increase in capacitive performance. The deformable insulator plate has an area and a thickness selected to deform through the air gap in response to an applied pressure;

Another aspect of the present disclosure includes method for sensing pressure. The method includes providing a first electrode layer on a rigid dielectric substrate and providing a second electrode layer facing the first electrode layer on a flexible dielectric substrate. The method further includes providing a first dielectric layer in the gap between the first electrode layer and the second layer. The first dielectric layer includes a solid dielectric material, such as glass, for example, covering the first electrode layer. According to an aspect of the present disclosure, the thickness and dielectric constant of the first dielectric layer selected to provide a predetermined increase in capacitance between the first electrode layer and the second electrode layer. The method further includes providing a second dielectric layer in the gap. The second dielectric layer includes a compressible dielectric material adjacent the second electrode layer. The second dielectric layer may be air or other gas, for example. The method includes exposing the flexible dielectric substrate to a pressure, and measuring a capacitance between the first electrode layer and the second dielectric layer. The second dielectric layer has a thickness selected to allow a predetermined deflection distance of the flexible dielectric substrate into the gap.

DETAILED DESCRIPTION

FIG. 1shows a previously known capacitive sense element having two electrodes forming a capacitor along with a layer of boundary glass providing separation between the electrodes, a rigid substrate, and a thin diaphragm capable of predictably deflecting in response to applied pressure.

As shown inFIG. 1, the previously known pressure transducer100includes a substrate102, a diaphragm layer104and a boundary glass layer106between the substrate102and the boundary glass diaphragm layer104. A void108extends through the boundary glass layer106. A first electrode layer110is formed on the substrate102in the void108, and a second electrode layer112is formed on the diaphragm layer104in the void108. An air gap114is formed in the void108between the first electrode110and the second electrode112.

FIG. 2shows the same capacitive sense element100shown inFIG. 1, with a pressure applied to deflect the diaphragm. The reduced distance between the electrodes increases the capacitance of the sense element and can be calculated from material properties and dimensions.

When a pressure118is applied to the diaphragm layer104, the diaphragm layer104is deflected into the void108thereby decreasing the air gap thickness between the first electrode layer110and the second electrode layer112. The decreased air gap thickness results in a difference in capacitance between the first electrode layer110and second electrode layer112. Changes in capacitance are easily measurable and provide a signal that is representative of the applied pressure118.

Such previously known pressure transducers100have been prone to failure when the air gap114is not free from conductive particles. Conductive particles that can enter the air gap114and provide a conductive bridge between the first electrode and the second electrode are a natural byproduct of the manufacturing process and have been difficult or impossible to completely eliminate.

FIG. 3shows a previously known capacitive sense element300which is similar to the sense element100shown inFIGS. 1 and 2, but also includes a glass barrier layer316over one of the electrodes310.

The capacitive sense element300includes a substrate302, a diaphragm layer304and a boundary glass layer306between the substrate302and the diaphragm layer304. The boundary glass layer306includes a void308that creates a gap between the substrate302and the diaphragm layer304. A first electrode layer310is formed on the substrate302in the void308, and a second electrode layer312is formed on the diaphragm layer304in the void308. An air gap314is formed in the void308between the first electrode310and the second electrode312.

The glass barrier layer316prevents bridging between electrode layers310,312by conductive particles in the air gap314. The glass barrier layer316between the first electrode310and the second electrode312has a higher dielectric constant than the air it replaces and accordingly increases the capacitance of the sense element300as compared to the sense element100shown inFIGS. 1 and 2.

The different dielectric constant of the glass barrier layer316affects the capacitance of the sensor element300in response to an applied pressure318. The dielectric behavior of the glass barrier layer316also changes with temperature at a different rate than air. These differences can affect the resulting sensor element accuracy with changes in temperature. The previously known capacitive sense element300is configured to minimize the capacitive influence of the glass barrier layer by increasing the target gap dimension. For example, in the capacitive sense element300the minimum capacitor gap target is increased to about 21 to 25 micrometers compared to a minimum capacitor gap target of about 14 to 17 micrometers in capacitive sense elements100(FIGS. 1 and 2), which do not include a glass barrier layer. In the capacitive sense element300, the glass barrier layer316has a thickness of 7 micrometers.

Although the glass barrier layer316can prevent shorting between electrodes, its capacitive effect has prevented further reduction of the gap between electrodes, and therefore has not previously been effective to further reduce the size of capacitive sense elements.

According to an aspect of the present disclosure, a glass barrier layer or other dielectric material separating the electrodes in place of the glass barrier layer can be selected to increase capacitive response of a capacitive sense element rather than being treated as a side effect to be minimized.

FIG. 4shows a capacitive sense element400according to an aspect of the present disclosure. The capacitive sense element400overcomes limitations of previously known sense elements, such as the sense element300shown inFIG. 3.

The capacitive sense element400includes a substrate402, a diaphragm layer404and a boundary glass layer406between the substrate402and the diaphragm layer404. The boundary glass layer406includes a void408that creates a gap between the substrate402and the diaphragm layer404. A first electrode layer410is formed on the substrate402in the void408, and a second electrode layer412is formed on the diaphragm404in the void408. An air gap414is formed in the void308between the first electrode310and the second electrode412.

The glass barrier layer416prevents bridging between electrode layers410,412by conductive particles in the air gap414. The capacitive influence of the glass barrier layer416increases the capacitive response of the capacitive sense element400.

The different dielectric constant of the glass barrier layer416affects the capacitance of the sensor element400in response to an applied pressure418. The dielectric behavior of the glass barrier layer416also changes with temperature at a different rate than air. These differences can affect the resulting sensor element accuracy with changes in temperature.

According to an aspect of the present disclosure, the glass barrier layer416prevents shorting between electrodes and is selected and dimensioned such that the capacitive influence of the glass barrier layer416increases the capacitive response of the capacitive sense element400without increasing the gap between electrodes. For example, in the capacitive sense element400the minimum capacitor gap target was reduced to about 17 micrometers compared to a minimum capacitor gap target of about 21 to 25 micrometers in the previously known capacitive sense element300(FIG. 3) having a glass barrier layer. This gap dimension favorably compares to a minimum capacitor gap target of about 14 to 17 micrometers in capacitive sense elements100(FIGS. 1 and 2), which do not include a glass barrier layer.

The target air gap of the presently disclosed capacitive sensor element400is about 10 micrometers. The dielectric constant of the glass barrier layer416provides an increased capacitance and increased capacitive response to deflection of the diaphragm layer404, which allows the target air gap to be reduced to less than 15 micrometers, or about 10 micrometers, for example. The addition of the 7 micrometer thick glass barrier layer increases the overall gap between electrodes to about 17 micrometers.

In comparison, the target air gap in the previously known capacitive sensor element300was about 14 micrometers to provide similar operative characteristics to sensor elements100which do not have glass barrier layers. The addition of a 7 micrometer glass barrier layer in the previously known capacitive sensor element300increases the overall gap between electrodes to about 21 micrometers.

FIG. 5shows measured capacitances of two groups of sense elements differing only in that a first group502represents the sense element100shown inFIGS. 1 and 2and a second group504represents the sense element400shown inFIG. 4having a glass barrier layer416over one of the electrodes410in accordance with an aspect of the present disclosure.

For sense elements in the first group502of the example ofFIG. 5, (without glass barrier material in the sense element gap), a capacitor gap target of 14 micrometers was used. This represents the smallest gap that can be practically achieved without incurring significant manufacturing yield loss due to conductive particles bridging the gap between electrodes. For sense elements in the second group504of the example inFIG. 5(with glass barrier material in the sense element gap) a capacitor gap target of 17 micrometers between electrodes was used. In the second group504, the glass barrier thickness target was 7 micrometers which leaves an air gap of less than 15 micrometers. Other design parameters of the sense elements100,400used in the example ofFIG. 5are identical between the two groups.

Each point shown in the graph500ofFIG. 5represents a single manufactured sense element. The horizontal scale506represents measured capacitance of the sense element100,400with no pressure applied. The vertical scale represents the measured capacitance of the same sense element100,400at a set application-representative pressure. As described herein, the second group504representing the improved sense element400described herein demonstrates an increase in capacitive response over the previously known sense element100.

Although measured capacitance data for the previously known capacitive sense element300is not included inFIG. 5, the increased gap of about 21 micrometers between electrodes in the previously known capacitive sense element300results in capacitive performance that is similar to or worse than the group shown in502, ofFIG. 5, for example.

Another aspect of the present disclosure includes a method for increasing capacitive response to applied pressure by including a glass layer in the air gap wherein the glass layer has additional thickness compared to previously known capacitive sense elements. The additional thickness is selected to achieve a predetermined increase in dielectric constant across the gap between electrodes. In an illustrative embodiment, the resulting capacitive sense element includes a gap having two dielectric media, air and glass, between the electrodes, wherein the thickness of the glass media in the gap and the thickness of the remaining air gap are predetermined based on the dielectric constant of glass and air to achieve the desired capacitive response characteristics of the sensor design.

Referring toFIG. 6, a method600of sensing pressure according to an aspect of the present disclosure includes providing a first electrode layer on a rigid dielectric substrate at block602, and providing a second electrode layer on a flexible dielectric substrate at block604such that second electrode layer faces the first electrode layer. A separation distance between the first electrode layer and the second electrode layer defines a gap. At block606, the method includes providing a first dielectric layer in the gap from a solid dielectric material covering the first electrode layer. According to an aspect of the present disclosure, the first dielectric layer has a thickness and a dielectric constant selected to provide a predetermined increase in capacitance between the first electrode layer and the second electrode layer. At block608, the method includes providing a second dielectric layer in the gap from a compressible dielectric material adjacent the second electrode layer. At block610, the method includes exposing the flexible dielectric substrate to a pressure, and at block612, the method includes measuring a capacitance between the first electrode layer and the second dielectric layer.

According to an aspect of the present disclosure, the rigid dielectric substrate and/or the flexible dielectric substrate is/are made of a ceramic material. According to another aspect of the present disclosure, the flexible dielectric substrate comprises an area and a thickness selected to deform through the second dielectric layer in response to the pressure. In an illustrative embodiment, according to an aspect of the present disclosure, the first electrode and the second electrode are respective metal layers which are each about 0.4 micrometers thick.

According to an aspect of the present disclosure the second dielectric layer has a thickness selected to allow a predetermined deflection distance of the flexible dielectric substrate into the gap. The second dielectric layer is a gas layer, in which the gas may be air or another dielectric material selected for its dielectric properties to increase capacitive response of the sensor element.

According to another aspect of the present disclosure the first dielectric layer is a glass layer. In an illustrative embodiment, the second dielectric layer has a thickness of about 10 micrometers and the first dielectric layer has a thickness of about 7 micrometers. In an illustrative embodiment, according to this aspect of the present disclosure, the capacitance is between about 5 pF and about 8 pF between when the flexible dielectric substrate is in an undeformed state. In the illustrative embodiment the capacitance changes by between 0.8 pF and 2.0 pF in response to a deflection of the flexible dielectric substrate from an undeformed state to a maximally deformed state. In different embodiments, the capacitance may be in a range of about 5 pF to about 50 pF and the capacitive changes may be in a range of between about 0.5 pF and about 40 pF, for example.

By increasing the capacitive response of a capacitive pressure sense element, the disclosed method and apparatus facilitates the manufacture of smaller sense elements and sense elements in presently available sizes with expanded pressure sensing ranges, for example.

Although the disclosed sense element and method are described herein in terms of a glass dielectric layer between the first and second electrode, it should be understood that other solid dielectric layers may be substituted for glass in various alternative embodiments of the present disclosure.

A number of implementations of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure.