WO2008151972A2

WO2008151972A2 - Capacitative and piezoresistive differential pressure sensor

Info

Publication number: WO2008151972A2
Application number: PCT/EP2008/056900
Authority: WO
Inventors: Dieter Stolze; Anh Tuan Tham; Friedrich Schwabe
Original assignee: Endress+Hauser Gmbh+Co.Kg
Priority date: 2007-06-11
Filing date: 2008-06-04
Publication date: 2008-12-18
Also published as: WO2008151972A3; DE102007027274A1

Abstract

The invention relates to a differential pressure sensor, comprising a first measuring diaphragm (5a), arranged on a first support (3a), on the exterior of which facing the support (3a) a first pressure (p1) acts, on the interior of which facing away from the first support (3a) at least one electrode (E1, E1') and at least one piezoresistive sensor element (R) for detecting a differential pressure-dependent deflection of the first measuring diaphragm (5a) are arranged, a second measuring diaphragm (5b), arranged on a second support (3b), which extends in parallel to the first measuring diaphragm (5a), on the exterior of which facing the support (3b) a second pressure (p2) acts, on the interior of which facing away from the second support (3b) at least one second electrode (E2, E2') is arranged which together with a dedicated first electrode (E1, E1') arranged on the first measuring diaphragm (5a) forms a capacitative sensor element, the first and the second measuring diaphragm (5a, 5b) being arranged in parallel and being firmly interconnected via a mechanical connection (P) that connects an outer edge (21) of the interior of the first measuring diaphragm (5a) to an outer edge (21) of the interior of the second measuring diaphragm (5b) and a spacer (7) of the same height connected to the diaphragm centers of the two measuring diaphragms (5a, 5b).

Description

description

Differential Pressure Sensor

The invention relates to a differential pressure sensor.

Differential pressure sensors are used to detect a pressure difference between two pressures acting on the differential pressure sensor and are used for example in differential pressure gauges that are used in industrial metrology.

In pressure measurement technology, so-called semiconductor sensors, e.g. Silicon sensors used. There are various types of semiconductor sensors on the market, which are usually distinguished by the measuring principle used. The two most common sensor types are capacitive and piezoresistive differential pressure sensors.

Piezoresistive differential pressure sensors have piezoresistive elements whose resistance changes as a function of a pressure acting thereon or a mechanical stress to which they are exposed. The piezoresistive elements are usually connected together to form a bridge circuit, in particular a Wheatstone bridge, and arranged on a measuring diaphragm, which is exposed to the differential pressure to be measured. A differential pressure-dependent deflection of the measuring diaphragm is detected by means of the piezoresistive elements and converted into an electrical output signal dependent on the differential pressure.

Capacitive pressure sensors contain at least one capacitor formed by two oppositely arranged electrodes. Typically, the electrodes are arranged on the inner sides of two mutually parallel membranes. In measuring mode, a first pressure acts on the outside of the first diaphragm and a second pressure on the outside of the second diaphragm. The pressures cause a deflection of the membranes and thus a differential pressure dependent change in the distance between the two electrodes. This change in distance causes a differential pressure dependent change in the capacitance of the capacitor. The capacity is detected by means of a capacitance measuring circuit and in a converted by the differential pressure dependent electrical output signal.

In many applications very high demands

Differential pressure sensors provided. They should esp. Have a very high accuracy and a low susceptibility to errors.

These requirements can be met for example by the use of redundant measuring devices. Redundant measuring devices offer the advantage that two independently obtained measuring signals are available for the derivation of the quantity to be measured. This makes it possible on the one hand by external influences, such as. Temperature, humidity or static pressure, better compensated for measuring errors, on the other hand, it is possible to select that measurement signal that is more suitable under the given conditions of use. Another advantage is that the measuring operation can still be maintained even if one of the two measuring devices fails.

In conventional piezoresistive silicon differential pressure sensors, two variants with redundant measurement technology are already used today. One variant consists of arranging two or more independent piezoresistive measuring bridges on a measuring membrane. However, this variant has the disadvantage that fail in a fraction of the measuring diaphragm both bridges. The second variant consists in arranging two independent measuring membranes with piezoresistive measuring bridges arranged in a measuring mechanism. However, this variant requires a usually very complicated and therefore expensive construction of the measuring unit.

An example of a micromechanical capacitive

Silicon differential pressure sensor is described in EP 0 896 658 B1. This differential pressure sensor comprises a base plate and two membrane plates arranged on both sides of the base plate parallel to the base plate and spaced from the base plate. The membrane plates are connected by at least one coupling element in terms of force such that a pressure difference between the outside of the Membrane plates adjacent pressures leads to a movement of the membrane plates, which causes a change in the distance between the membrane plates and arranged between the membrane plates base plate. This change is detected by means of capacitive sensor elements, and converted into a dependent of the differential pressure electrical output signal. The capacitive sensor elements each comprise at least one electrode arranged on the base plate and an associated counterelectrode arranged opposite one another on the inside of the respective membrane plate.

It is an object of the invention to provide a differential pressure sensor which offers a high degree of measurement reliability.

For this purpose, the invention consists in a differential pressure sensor with a arranged on a first carrier first measuring diaphragm, on the outer side facing the carrier during measurement operation, a first pressure acts on the side facing away from the first carrier inside at least a first electrode and at least one piezoresistive sensor element Detection of a differential pressure-dependent deflection of the first

Measuring diaphragm are arranged, arranged on a second carrier second measuring diaphragm which extends parallel to the first measuring diaphragm, acting on the outside of the carrier during measurement a second pressure acting on the side facing away from the second carrier inside at least one second electrode is arranged, which together with an associated arranged on the first measuring membrane first electrode forms a capacitive sensor element for detecting a present between the first and the second pressure differential pressure, wherein the first and the second measuring membrane via an outer edge of the inside of the first measuring membrane with a outer edge of the inside of the second measuring membrane connecting mechanical connection and connected to the membrane centers of the two measuring membranes spacers of the same height are arranged parallel to each other and are firmly connected.

According to a development is on the inside of the second

Measuring diaphragm at least one further piezoresistive sensor element for detecting a differential pressure dependent deflection of the second diaphragm arranged.

[0013] According to a further development

- Provided on the first measuring diaphragm two identical first electrodes, which are arranged on opposite sides of the spacer,

- Provided on the second measuring diaphragm two identical-shaped second electrodes, which are arranged on opposite sides of the spacer, and

- Each first electrode is associated with a same shape of the second electrode, which is arranged parallel to the associated first electrode of this directly opposite one another.

According to a preferred embodiment, each piezoresistive sensor element consists of several piezoresistive elements, which are joined together to form a resistance bridge.

According to one embodiment of this embodiment, the piezoresistive elements of a sensor element are arranged in a line symmetrical to both sides of the spacer.

According to a further development of the differential pressure sensor consists of two identical sections, which are connected to each other via the mechanical connection and the spacer, wherein the mechanical connection and the spacer are each composed of two identical elements, one of which is part of the first section and is a part of the second part and which are firmly connected to each other.

According to a further development, the carrier with the measuring membranes arranged thereon each consist of a BESOI wafer, the one first silicon layer, a thinner second silicon layer and an oxide layer disposed therebetween, in which

a recess is provided in the first silicon layer which is surrounded on the outside by a part of the first silicon layer forming the respective carrier, and over which a region of the BESOI wafer forming the respective measuring diaphragm is exposed,

- In the second silicon layer, a recess is provided, through which the oxide layer is exposed to an outer edge and a region in the middle of the oxide layer, wherein at least portions of the remaining outer edge of the second silicon layer are part of the mechanical connection and in the middle remaining

Portion of the second silicon layer forms a part of the spacer,

at least one electrode is applied to the exposed oxide layer,

on at least one of the two BESOI wafers a piezoresistive sensor element is arranged on the exposed oxide layer, and

- the two BESOI wafers together over one

Bonding material are firmly attached, which is applied to serving as connecting surfaces Teilbreichen the outer edges of the second silicon layers and on the remaining areas in the middle of the second silicon layers.

According to a further development, the connecting surfaces are arranged with the same shape and symmetrically on the respective outer edge of the respective second silicon view.

According to a development, the first and second exist

Electrodes of the same material, esp. Of gold, which is also used as a connecting material.

According to a further development connecting lines for the capacitive and the piezoresistive sensor elements are applied to the inner sides of the two measuring membranes, and guided out between the connecting surfaces of the differential pressure sensor. The differential pressure sensor according to the invention has the advantage that it has at least two mutually independent sensor elements for detecting the differential pressure. This ensures a reliable measurement even in case of failure of a sensor element by the remaining independent thereof sensor elements, and thus ensures a high degree of measurement security.

Another advantage is that at least two independent

Measuring signals are available, the optimal compensation of external influences, such. temperature, humidity or static pressure, allow for dependent measurement errors. Accordingly, a very high measuring accuracy can be achieved with the differential pressure sensor according to the invention.

Another advantage is that the inventive

Differential pressure sensor has at least one capacitive sensor element and at least one piezoresistive sensor element. As a result, the advantages of both measuring principles can be exploited. This increases the achievable measurement accuracy and the field of application in which the differential pressure sensor according to the invention can be used is expanded.

Another advantage of the differential pressure sensor according to the invention is that the differential pressure sensor is a microelectromechanical system based on silicon, which can be produced with standard methods currently used in semiconductor technology.

The invention and its advantages will now be explained in more detail with reference to the figures of the drawing, in which two embodiments are shown. Identical elements are provided in the figures with the same reference numerals.

Fig. 1 shows a section through a differential pressure sensor according to the invention along a cutting plane containing a first central axis A-A 'of the differential pressure sensor;

Fig. 2 shows a section through the invention

Differential pressure sensor along a second central axis BB 'containing sectional plane of the differential pressure sensor, the vertical opposite to the sectional plane shown in Figure 1;

Fig. 3 shows a view of the inside of the first measuring diaphragm of the differential pressure sensor of Fig. 1 and 2;

Fig. 4 is a view of the inside of the second measuring diaphragm of the differential pressure sensor of Figs. 1 and 2;

FIG. 5 shows a BESOI wafer of a portion of the FIG

Differential pressure sensor with a measuring membrane exposing recess in the first silicon layer;

FIG. 6 shows the BESOI wafer of FIG. 5 with a recess in its second silicon layer; FIG.

Fig. 7 shows a section through a portion of the differential pressure sensor in the sectional plane shown in Fig. 1;

FIG. 8 shows a section through a section of the differential pressure sensor in the sectional plane shown in FIG. 2; FIG. and

Fig. 9 shows a differential pressure sensor according to the invention, in which the first and the second measuring diaphragm in each case a pressure transmitter is connected upstream.

Fig. 1 shows a section through an inventive

Differential pressure sensor along a first central axis A-A 'of the differential pressure sensor containing cutting plane. FIG. 2 shows a section through the differential pressure sensor according to the invention along a sectional plane containing a second central axis B-B 'of the differential pressure sensor, which is perpendicular to the sectional plane shown in FIG. The central axes A-A 'and B-B' are perpendicular to each other and intersect at the center of the differential pressure sensor.

The differential pressure sensor consists of two preferably identical interconnected sections 1a, 1b. The first section 1a comprises a first measuring diaphragm 5a arranged on a first carrier 3a and the second section 1b a second measuring diaphragm 5b arranged on a second carrier 3b. The sections 1a, 1b are connected to one another such that the two measuring membranes 5a, 5b are arranged parallel to one another and spaced apart from one another. In measuring mode acts on one A first pressure p1 is applied to the outside of the first measuring diaphragm 5a facing the first carrier 3a and a second pressure p2 to an outside of the second measuring diaphragm 3b facing the second carrier 3b.

On a side facing away from the first carrier 3a inside of the first

Measuring diaphragm 5a, at least one first electrode E1 and at least one piezoresistive sensor element R is arranged. 3 shows a view of the inside of the first measuring diaphragm 5a.

In the exemplary embodiment illustrated here, two rectangular electrodes E1 and EV of the same shape are provided, which are arranged symmetrically on both sides of the central axis A-A 'passing through the middle of the measuring diaphragm. The central axis A-A 'lies centrally in the sectional plane of the section shown in FIG.

The piezoresistive sensor element R comprises at least one piezoresistive element R11, R12, R13, R14 applied to the first measuring diaphragm 5a and serves to detect a deflection of the first measuring diaphragm 5a which is dependent on the differential pressure. Preferably, the piezoresistive sensor element R consists of a plurality of piezoresistive elements R11, R12, R13, R14, which are connected together to form a resistance bridge. In the illustrated embodiment, four piezoresistive elements R11, R12, R13, R14, which are connected together to form a resistance bridge, are provided. The connection takes place via printed conductors L applied to the inside of the first measuring diaphragm 5a. The piezoresistive sensor element R is connected to a connecting lines KR1, KR2 and KR3, KR4 which are not shown in FIGS. 1 to 4 and are applied externally from the differential pressure sensor to the first measuring diaphragm 5a Connected measuring circuit which provides a differential pressure corresponding measurement signal .DELTA.PR.

On the side facing away from the second carrier 3b inside the second

Measuring diaphragm 5b is arranged at least a second electrode E2. 4 shows a view of the inside of the second measuring diaphragm 5b. In the embodiment shown here, two identical rectangular electrodes E2 and E2 'are provided, which are symmetrical to both sides of a guided through the differential pressure sensor center central axis BB 'are arranged. The central axis BB 'is located centrally in the sectional plane of the section shown in Fig. 2 and is perpendicular to the central axis A-A'.

The second electrode E2 forms, together with the same opposite the same form on the first measuring diaphragm 5a arranged first electrode E1 a capacitive sensor element E for detecting a between the first and the second pressure p1, p2 existing differential pressure.

Analogously, the second electrode E2 'forms, together with the first electrode EV disposed opposite the same shape on the first measuring diaphragm 5a, a further capacitive sensor element E' for detecting a differential pressure existing between the first and the second pressure p1, p2.

As an alternative to the electrodes E1, E1 'and E2, E2' described here, it is of course also possible to use other electrode pairs which differ from those described here by their number, their arrangement and their geometry.

Preferably, at least one further piezoresistive sensor element R 'for detecting a differential pressure-dependent deflection of the second measuring diaphragm 5b is arranged on the inside of the second measuring diaphragm 5b. This further piezoresistive sensor element R 'is preferably constructed identically to the piezoresistive sensor element R arranged on the first measuring diaphragm 5a. For this purpose, it comprises at least one piezoresistive element R21, R22, R23, R24 applied to the second measuring membrane 5b. Preferably, it consists of a plurality of piezoresistive elements R21, R22, R23, R24, which are connected together via applied on the second measuring membrane 5b interconnects L to a resistance bridge. In the illustrated embodiment, four piezoresistive elements R21, R22, R23, R24 are provided, which are connected together to form a resistance bridge. The piezoresisitive sensor element R 'is applied to the second measuring diaphragm 5b connected outside of the differential pressure sensor connecting lines KR1 ', KR2', KR3 ', KR4' connected to a measuring circuit, not shown in the figures 1 to 4, which provides a differential pressure corresponding to the measurement signal ΔPR '.

In addition to or instead of the piezoresistive described here

Of course, sensor elements R, R 'can also be used with additional or differently dimensioned piezoresistive sensor elements. It is e.g. It is possible to design individual piezoresistive sensor elements by positioning and dimensioning their piezoresistive elements for different differential pressure measuring ranges.

The measuring ranges of the capacitive and the piezoresistive

Sensor elements R, R ', E, E' can additionally be adjusted by a corresponding dimensioning of the thickness of the individual measuring membranes 5a, 5b.

The first and the second measuring membrane 5a, 5b are connected via an outer edge of the inside of the first measuring membrane 5a with an outer edge of the inside of the second measuring membrane 5b connecting mechanical connection P and one connected to the membrane centers of the two measuring membranes 5a, 5b Spacer 7 of the same height parallel to each other from each other and firmly connected.

Preferably, the spacer 7 is made of two identical shape

Elements 7a and 7b composed of which is a part of the first portion 1a and a part of the second portion 1 b and which are fixedly connected to each other. The same applies analogously to the mechanical connection P. described in more detail below.

The two measurement membranes 5a, 5b mechanically coupled in this way form a membrane composite. This symmetrical double diaphragm construction significantly increases the bursting strength of the differential pressure sensor, so that it is also suitable for measuring very high differential pressures. The resulting high mechanical stability offers a high degree of measurement reliability.

A differential pressure acting on this membrane composite leads to a substantially synchronous identical deflection of the two measuring membranes 5a, 5b. Each of the two measuring membranes 5a, 5b thus experiences a deflection dependent on the differential pressure. Accordingly, both the piezoresistive sensor element R arranged on the first measuring diaphragm 5a and the further piezoresistive sensor element R 'arranged on the second measuring diaphragm 5b detect the differential pressure acting on the differential pressure sensor. The piezoresistive sensor elements R and R 'are connected via connecting leads KR1, KR2, KR3, KR4 or KR1', KR2 \ KR3 \ KR4 ', which are brought out of the differential pressure sensor on the first and the second measuring diaphragm 5a, 5b, to one in the figures 1 to 4 measuring circuit, not shown, each of which provides a pressure signal corresponding to the differential pressure. Preferably, a separate measuring circuit is provided for each sensor element R, R '. Thus, two differential pressure measuring signals ΔPR and ΔPR 'generated independently of one another are generated by two completely independent sensor elements R, R'.

A differential pressure-induced deflection of the two measuring membranes 5a, 5b leads to a parallel displacement of the respective first electrodes E1, EV of the capacitive sensor elements E, E 'relative to the respective associated second electrodes E2, E2' of the respective capacitive sensor element E or E. 'Which has a substantially linear differential pressure dependent change in the capacitance of the respective sensor element E2, E2' result.

[0052] The capacitive sensor elements E and E 'are connected to a measuring circuit, not shown in FIGS. 1 to 4, via connecting leads KE1, KE2 and KEV, KE2' applied outside the differential pressure sensor and applied to the first and second measuring diaphragm 5a, 5b which supplies a measurement signal corresponding to the differential pressure. Preferably, a separate measuring circuit is also provided here for each capacitive sensor element E, E '. Thus, two more of two completely independent sensor elements E, E 'generated independently prepared differential pressure measurement signals .DELTA.PE and .DELTA.PE' to Available.

Overall, the differential pressure sensor according to the invention shown here thus provides four completely independent differential pressure measuring signals ΔPR, ΔPR ', ΔPE and ΔPE'. As a result, there is a high degree of redundancy, which significantly increases the measurement reliability. Should one of the sensor elements R, R ', E, E' fail, a reliable differential pressure measurement is still guaranteed by the remaining intact sensor elements.

Furthermore, there is a high degree of flexibility with regard to the preparation and / or evaluation of the available differential pressure measuring signals ΔPR, ΔPR ', ΔPE and ΔPE'. The four available differential pressure measuring signals ΔPR, ΔPR ', ΔPE and ΔPE' can be processed and / or processed completely independently of one another and subjected to different compensation and / or treatment methods both as individual signals and in combination with each other. For example, the differential pressure to be measured can be derived from an average value of the four differential pressures available via the four differential pressure measurement signals ΔPR, ΔPR ', ΔPE and ΔPE'. Alternatively, the characteristic curves of the four sensor elements R, R ', E', E can be measured as part of a calibration and subsequently individual differential pressure measuring signals for the derivation of the differential pressure to be measured are selected, for example, appear particularly reliable or whose characteristic curve has the most linear course. This significantly simplifies subsequent compensation of measurement errors and achieves higher measurement accuracy.

In addition, the measurement results derived by means of the individual sensor elements R, R ', E, E' can be compared with one another and, if appropriate, with their mean value. If a single measurement result shows a clear deviation from the other measurement results or from the mean value, then this measurement result can be sorted out as outliers and from the derivative of the differential pressure to be measured be excluded. In this way, damage to a single sensor element can be detected early and their influence on the measurement accuracy can be excluded.

Preferably, the differential pressure sensor according to the invention has a highly symmetrical structure. For this purpose, it preferably consists of two completely identical sections 1a / 1b and is as symmetrical as possible with regard to the arrangement of the components of the sensor elements R, R 'and E, E'. This leads to a significant reduction in the nonlinearity of the measurement results caused by the geometry of the structure of the differential pressure sensor and by the physical properties of the sensor elements R, R ', E, E' and thus to an increase in the measurement accuracy achievable.

Another advantage of the highly symmetrical structure is that this reduces the influence of the static pressure. In this case, the static pressure refers to a reference or output pressure acting equally on both measuring membranes 5a / 5b, to which the pressure difference to be measured is superimposed. The static pressure thus corresponds to the lower of the two pressures p1, p2, the difference of which is to be measured. Due to the highly symmetrical structure caused by the static pressure mechanical stresses are largely avoided.

To achieve the highly symmetrical structure, those on the first

Measuring diaphragm 5a arranged first electrodes E1 and EV equal in shape and symmetrical to the central axis A-A 'on opposite sides of the spacer 7 is arranged. Correspondingly, the second electrodes E2 and E2 'arranged on the second measuring diaphragm 5b are arranged with the same shape and symmetrical to the central axis A-A' on opposite sides of the spacer 7. In addition, the first and second electrodes E1, EV and E2, E2 'have the same shape and each first electrode E1, EV is assigned a shape-identical second electrode E2, E2', which is arranged parallel to the associated first electrode E1, EV this immediately opposite ,

This high degree of symmetry is preferably also with respect to the Arrangement of the piezoresistive elements R11, R12, R13, R14 and R21, R22, R23, R24 of the piezoresistive sensor elements R, R 'complied with. For this purpose, the piezoresistive elements R11, R12, R13, R14 or R21, R22, R23, R24 of each piezoresistive sensor element R or R 'are preferably arranged in a line symmetrical to the central axis BB' on both sides of the spacer 7.

This high degree of symmetry causes all sensor elements R, R ', E, E' have a very low linearity error. The linearity error denotes a measure of the deviation of the differential pressures measured with the individual sensor elements R, R ', E, E' from their ideal linear dependence on the differential pressure.

Another advantage is that the piezoresistive elements R11, R12, R13 and R14 of the piezoresistive sensor R with respect to each in the same position oppositely disposed piezoresistive elements R21, R22, R23 and R24 of the piezoresistive sensor R 'in each case with the opposite sign behavior. A differential pressure acting on the differential pressure sensor, which causes an expansion of the piezoresistive elements R11, R12, R13 and R14 of the piezoresistive sensor R, causes a compression of the oppositely arranged piezoresistive elements R21, R22, R23 and R24 of the piezoresistive sensor R ', and vice versa. This opposite behavior can be exploited, for example, in the compensation of static pressure and / or temperature-dependent measurement errors. Likewise, this can compensate for measurement errors caused by asymmetries caused by manufacturing tolerances. The same applies to the linearity error of the individual piezoresistive sensor elements R, R '.

Furthermore, the symmetrical structure causes an opposing movement of the electrodes E1, E2 and EV, E2 'at an acting differential pressure. A differential pressure causes either a parallel displacement of the electrode pairs, in which first electrodes E1, EV are displaced outwards relative to the opposing second electrodes E2, E2 ', or a parallel displacement, in which the second electrodes E2, E2 'are shifted outward with respect to the opposing first electrodes E1, EV. This opposite behavior can be exploited, for example, in the compensation of static pressure and / or temperature-dependent measurement errors. Likewise, this can compensate for measurement errors caused by asymmetries caused by manufacturing tolerances. The same applies to the linearity error of the individual capacitive sensor elements E, E '.

Another advantage of the differential pressure sensor according to the invention is that it can be formed as a microelectromechanical system (MEMS) based on silicon, which can be produced by standard methods known from semiconductor technology. As a result, a cost-effective production of the differential pressure sensor according to the invention is possible.

Semiconductor sensors are now regularly silicon-based, e.g. manufactured using silicon-on-insulator (SOI) technology. In this case, BESOI wafers (bonded and etched silicon on insulator) are preferably used as the starting material. BESOI wafers are manufactured using silicon direct bonding. For this purpose, two oxidized silicon wafers are aligned against each other and bonded under pressure and high temperature. This results in a three-layered wafer, in which there is an oxide layer between two silicon layers. The buried oxide layer known as BOX (buried oxide layer) has a thickness of a few nm to a few μm. This composite is thinned and polished from one side. The thinned polished side later forms the active layer. The active layer may be a few microns thick and is known in the English speaking art e.g. referred to as device wafer or as silicone overlayer (SOL). The thickness of the active layer can already be produced very accurately and uniformly and with high reproducibility using today's production methods.

A significant advantage of the use of BESOI wafers for the production of pressure sensors is that the buried oxide layer (BOX) forms a reliable etch stop. This will happen all used for the production of movable electrodes of capacitive pressure sensors.

However, methods are also known in which BESOI wafers are used for the production of piezoresistive pressure sensors.

Such a method is, for example, in the article published in 2000 in the Journal of Micromechanical Engineering, Volume 10, pages 204 to 208: 'Optimized technology for the fabrication of piezoresistive pressure sensors', by A. Merlos, J. Santander , MD Alvares and F. Campabadal.

The two sections 1a and 1b of the invention

Differential pressure sensors are also preferably each made of a BESOI wafer, which has a first silicon layer 9, a thinner second silicon layer 11 and an oxide layer 13 arranged between the first and the second silicon layer 9, 11. The oxide layer 13 preferably has a thickness of at least 1 μm. The carriers 3a and 3b, respectively, with the measuring membranes 5a and 5b disposed thereon are each made from a BESOI wafer. The section 1a / 1b has a recess 15 in the first silicon layer 9, which is surrounded on the outside by a part of the first silicon layer 9 forming the respective carrier 3a / 3b, and via which a region of the BESOI wafer forming the respective measuring membrane 5a / 5b is released. The recess 15 preferably has an inner lateral surface 17, which tapers conically in the direction of the oxide layer 13. This shape offers the advantage of increased mechanical stability of the differential pressure sensor. 5 shows the BESOI wafer with the recess 15. The recess 15 is produced, for example, by means of an etching process.

In the second silicon layer 11, one shown in Fig. 6 is shown

Recess 19 is provided, through which the oxide layer 13 is exposed to an outer edge 21 and a region 23 in the middle of the oxide layer 13. The recess 19 is produced, for example, by an etching process with which the silicon present in the region of the recess 19 is removed. The buried oxide layer is used for this purpose 13 as an etch stop. Preferably, a KOH etching solution is used, since this has a very different selectivity between silicon and silicon dioxide.

At least portions of the remaining outer edge 21 of the second silicon layer 11 form part of the mechanical connection P. The remaining in the middle region 23 of the second silicon layer 11 forms a part of the spacer. 7

Subsequently, a piezoresistive sensor element, in the illustrated embodiment, the two piezoresistive sensor elements R and R ', on the respective exposed oxide layer 13, as shown in Fig. 7, arranged on at least one of the two BESOI wafer. For this purpose, the piezoresistive elements R11, R12, R13, R14 and R21, R22, R23, R24 of the respective sensor element R, R 'applied to the oxide layer 13, for example by means of chemical vapor deposition. The piezoresistive elements R11, R12, R13, R14 and R21, R22, R23, R24 consist for example of poly-silicon, silicon carbide (SiC) or diamond-like carbon (DLC). The oxide layer 13 forms an insulating layer between the piezoresistive elements and the underlying first silicon layer 9. This insulating layer causes the differential pressure sensor can be used even at relatively high temperatures of up to 350 ° C, since even at these high temperatures by insulation layer sufficient insulation the piezoresistive elements R11, R12, R13, R14 and R21, R22, R23, R24 is guaranteed. Without the oxide layer 13 serving as the insulating layer, a pn junction would exist in the region of the piezoresistive elements R11, R12, R13, R14 or R21, R22, R23, R24, which would effect a sufficient insulation only at significantly lower temperatures.

The piezoresistive elements R11, R12, R13, R14 and R21, R22, R23, R24 can of course also be produced in other ways known from semiconductor technology. Thus, for example, by using corresponding masks, it is possible to obtain the second silicon layer 11 in the regions of the piezoresistive elements R11, R12, R13, R14 and R21, R22, R23, R24 and subsequently to include these regions to provide a corresponding doping, for example with boron.

Likewise, at least one electrode is applied to the exposed oxide layer 13 of each section 1a, 1b. In the exemplary embodiment described, the two first electrodes E1 and E1 'are applied to the exposed oxide layer 13 on the first measuring diaphragm 5a, and the two second electrodes E2 and E2' are applied to the exposed oxide layer 13 of the second measuring diaphragm 5b. This is shown in FIG. 8. The electrodes E1, E1 \ E2, E2 'are preferably made of a gold layer with a thickness of at least 500 nm, which is sputtered onto the respective oxide layer 13 or vapor-deposited.

The oxide layer 13 serves again as an insulating layer, which causes an electrical insulation of the electrodes E1, E1 \ E2, E2 'relative to the respective measuring membrane 5a, 5b.

Finally, the two sections 1a and 1a are firmly connected to each other by the two BESOI wafers are firmly connected to each other via a bonding material serving as connecting surfaces P1, P2, P3, P4 Teilbreichen the outer edges 21 of the second silicon layers 11 and is applied to the area remaining in the middle areas 23 of the second silicon layers 11 surface. Preferably, the first and second electrodes E1, E1 '; E2, E2 'of the same material, which is also used as a connecting material. In this way, it is possible to apply the connecting material for the connection of the two sections 1a, 1b together with the electrodes E1, E1 \ E2, E2 'in one operation. Preferably, the bonding material is also gold, which is applied with a layer thickness of at least 500 nm.

Preferably, there are also for the combination of the piezoresistive elements R11, R12, R13, R14 or R21, R22, R23, R24 of the piezoresistive sensor elements R and R 'provided conductor tracks L and the connecting lines KE1, KE2 and KE1', KE2 'for the capacitive sensor elements E, E' and the connecting lines KR1, KR2, KR3, KR4, KR1 ', KR2', KR3 ', KR4' for the piezoresistive sensor elements R, R 'of the same Werststoff, preferably also for the electrodes E, E ' and the mechanical connection P is used.

In addition, a joining material G is applied to the central portion 23 of the second silicon layer 11 of each measuring membrane 5a, 5b. Again, preferably the same material, here a gold layer with a thickness of 500 nm, used that is also used for the mechanical connection P. The connecting material G, together with the region 23 of the respective measuring diaphragm 5a, 5b, respectively form an element 7a or 7b of the spacer 7.

The geometry of the connecting surfaces P1, P2, P3, P4 on the outer edges 21 is selectable. Preferably, the connecting surfaces P1, P2, P3, P4 are arranged in the same shape and symmetrically on the respective outer edge 21 of the respective second silicon view 11. This has an advantageous effect on the linearity of the achievable measurement results. In the illustrated embodiment, the four connecting surfaces P1, P2, P3, P4 are provided, which, as seen from the views of the inner sides of the first and second measuring diaphragm 5a, 5b shown in Figures 3 and 4, in the four corners of the respective sections 1a, 1b are applied to the outer edge 23. In this case, the connection surface P1 in the representation of FIG. 3 is located at the top left on the first measuring diaphragm 5a, the connecting surface P2 at the top right, the connecting surface P3 at the bottom left and the connecting surface P4 at the bottom right. As shown in FIG. 4, the respective opposing surfaces are arranged mirror-symmetrically on the second measuring diaphragm 5b so that the connecting surface P1 is on the upper right, the connecting surface P2 on the upper left, the connecting surface P3 on the lower right and the connecting surface P4 on the lower left on the second Measuring diaphragm 5b is located.

Subsequently, the two sections 1a and 1b are brought together so that the applied on the first measuring diaphragm 5a connecting surfaces P1, P2, P3 and P4 on the same name on the second measuring diaphragm 5b applied connecting surfaces P1, P2, P3 lie flat and the connecting surfaces G of the elements 7a and 7b of the spacer 7 are superimposed. The mechanical connection P the two sections 1a, 1b is then preferably by bonding. In this case, the sections 1a, 1b are pressed together under forces in effect and exposed to a temperature of about 400 ° C to 420 ° C. As a result, a conductive eutectic phase is formed, and a mechanically stable connection of the respectively opposite connection surfaces P1, P2, P3, P4 and G is formed. Preferably, the differential pressure sensor formed in this way is stored for several hours at this high temperature. This leads to an additional diffusion of gold into the silicon, so that a very strong mechanical connection is formed.

Due to the symmetrical arrangement of the connecting surfaces P1, P2, P3, P4 and G, the influence of the different thermal expansion coefficients of gold and silicon is completely compensated.

The connection lines KE1, KE2, KE1 'and KE2' for the capacitive

Sensor elements E, E 'and KR1, KR2, KR3, KR4, KRf, KRZ, KR3' and KR4 'for the piezoresistive sensor elements R, R', which are applied to the inner sides of the respective measuring membranes 5a, 5b, are centered between each two the connection surfaces P1, P2, P3 and P4 outwardly guided out of the differential pressure sensor, and each end have a widened portion disposed on the respective outer edge 21 of the second silicon layer 11, which serves as a contact surface. Each individual contact surface is assigned to the respective opposite measuring membrane 5a, 5b an identical identical shape consisting of the same material counter surface GR1, GR2, GR3, GR4, GR1 ', GR2', GR3 ', GR4 \ GE1, GE2, GE1 \ GE2', each connected exclusively with the respectively associated contact surface of the connecting lines KE1, KE2, KEV KE2 'KR1, KR2, KR3, KR4, KRV, KR2', KR3 'KR4', but not with the sensor elements R, R ', E, E' is. This connection takes place at the same time and in the same way as the connection of the connecting surfaces P1, P2, P3, P4 and G. The contacts formed in this way are electrically, thermally and mechanically stable, since only firmly defined proportions of gold and silicon in enter into the eutectic connection. The differential pressure sensor according to the invention is extremely stable due to the highly symmetrical double membrane and thus also suitable for the measurement of very high differential pressures.

Furthermore, the double membrane construction according to the invention offers the advantage that the sensitive capacitive and piezoresistive sensor elements R, R ', E, E' are protected against external influences inside the differential pressure sensor. This makes it possible to use the differential pressure sensor directly, without having to be connected upstream of diaphragm seal. The measuring membranes 5a, 5b can be exposed directly to the media whose pressure difference is to be measured. As a result, the disadvantages associated with the use of upstream diaphragm seals typically filled with oil as a pressure transmission medium, such as e.g. the temperature dependence of the pressure transmission, avoided.

Alternatively, the differential pressure sensor according to the invention can of course also be used in conjunction with upstream diaphragm seals. Fig. 9 shows an embodiment thereof. There, the differential pressure sensor according to the invention is inserted into a measuring unit 25. The measuring unit 25 comprises a first, the first measuring diaphragm 5a upstream of a pressure transmitter 27a and an identical second measuring diaphragm 5b upstream second pressure transmitter 27b. The pressure averagers 27a and 27b each comprise a pressure-receiving chamber 31a, 31b which is closed off from a separating diaphragm 29a, 29b and which is connected via a pressure transmission line 33a, 33b to a pressure measuring chamber 35a, 35b adjacent to the respective measuring diaphragm 5a, 5b from the respective carrier 3a, 3b limited recess 15 is formed. The Druckempfangskammem 31a, 31b, the pressure transmission lines 33a, 33b and the pressure measuring chambers 35a, 35b are filled with a pressure-transmitting liquid, which preferably has a low coefficient of thermal expansion, and is as incompressible. For this purpose, e.g. a silicone oil.

The measuring unit 25 further comprises a measuring mechanism housing 37 in the Differential pressure sensor and the pressure averagers 27a, 27b, leaving the separation membranes 29a, 29b are used. Preferably, a bore 39 is provided in the measuring mechanism housing 37, via which there is a gas-permeable connection between the interior of the measuring mechanism housing 37 and the environment. The interior is preferably protected by a bore 39 to the outside covering splash water protection 41 and / or a filter inserted into the bore 39 43 from penetrating moisture and from penetrating particles. In measuring operation, the first separation membrane 29a is supplied with the first pressure p1 and the second separation membrane 29b with the second pressure p2. The pressures p1 and p2 are transmitted through the diaphragm seals 27a and 27b to the measuring diaphragms 5a, 5b, which thereby experience a deflection dependent on the differential pressure, which is detected by means of the differential pressure sensor.

Table 1

Claims

claims

1. 1. Differential pressure sensor with - a first carrier (3a) arranged first measuring diaphragm (5a), - on whose the carrier (3a) facing outside in the measuring operation, a first pressure (p1) acts - on the first carrier (3a ) facing away from the inside at least one first electrode (E1, EV) and at least one piezoresistive sensor element (R) for detecting a differential pressure dependent deflection of the first measuring diaphragm (5a) are arranged,

a second measuring diaphragm (5b) arranged on a second carrier (3b),

which runs parallel to the first measuring diaphragm 5a,

- On whose the carrier (3b) facing outside in measuring operation, a second pressure (p2) acts,

- At the second carrier (3b) facing away from the inside at least one second electrode (E2, E2 ') is arranged, which together with an associated on the first measuring diaphragm (5a) arranged first electrode (E1, EV) a capacitive sensor element for detecting a forms between the first and the second pressure (p1, p2) existing differential pressure, in which

- The first and the second measuring diaphragm (5a, 5b) via an outer edge (21) of the inside of the first measuring diaphragm (5a) with an outer edge (21) of the inside of the second measuring diaphragm (5b) connecting mechanical connection (P) and a with the membrane centers of the two measuring membranes (5a, 5b) connected spacers (7) of the same height are arranged parallel to each other and are firmly connected.

2. Differential pressure sensor according to claim 1, wherein on the inside of the second measuring diaphragm (5b) at least one further piezoresistive sensor element (R ') for detecting a differential pressure dependent deflection of the second measuring diaphragm (5b) is arranged.

3. 3. Differential pressure sensor according to claim 1, wherein

- On the first measuring diaphragm (5a) two identical first electrodes (E1, EV) are provided, which are on opposite sides of the spacer (7) are arranged,

- On the second measuring diaphragm (5b) two identical second electrodes (E2, E2 ') are provided, which are arranged on opposite sides of the spacer (7), and

- Each first electrode (E1, ET) is associated with a same shape of the second electrode (E2, E2 '), which is arranged parallel to the associated first electrode (E1, EV) of this directly opposite one another.

4. A differential pressure sensor according to claim 1 or 2, wherein each piezoresistive sensor element (R, R ') consists of a plurality of piezoelectric elements (R11, R12, R13, R14, R21, R22, R23, R24) which are connected to form a resistance bridge are.

5. The differential pressure sensor according to claim 4, wherein the piezoresistive elements (R11, R12, R13, R14, R21, R22, R23, R24) of a sensor element (R, R ') in a line symmetrical to both sides of the spacer (7 ) are arranged.

6. The differential pressure sensor according to claim 1, wherein the differential pressure sensor consists of two identical sections (1 a, 1 b), which are connected to each other via the mechanical connection (P) and the spacer (7), wherein the mechanical connection (P) and the spacers (7) are each composed of two identical elements (7a, 7b), one of which is part of the first part (1a) and part of the second part (1b), and which are fixedly connected to each other.

7. 7. Differential pressure sensor according to claim 1, wherein the carrier (3a, 3b) with the measuring membranes arranged thereon (5a, 5b) each consist of a BESOI wafer having a first, a thinner second silicon layer (9, 11) and a has interposed oxide layer (13), in which

- In the first silicon layer (9) has a recess (15) is provided, the outside of a respective carrier (3a, 3b) forming part of the first silicon layer (9) is surrounded, and on a the respective measuring membrane (5a, 5b ) forming area of the BESOI wafer is exposed, - In the second silicon layer (11) is provided a recess through which the oxide layer (13) except for an outer edge (21) and a region (23) in the middle of the oxide layer (13) is exposed, wherein at least portions of the remaining outer edge (21) of the second silicon layer (11) are part of the mechanical connection (P) and the central region (23) of the second silicon layer (11) forms part of the spacer (7),

at least one electrode (E1, E1 ', E2, E2') is applied to the exposed oxide layer (13),

on at least one of the two BESOI wafers a piezoresistive sensor element (R, R ') is arranged on the exposed oxide layer (13), and

- the two BESOI wafers together over one

Connection material are fixed, serving as connecting surfaces (P1, P2, P3, P4) Teilbreichen the outer edges (21) of the second silicon layers (11) and on the remaining areas in the center (23) of the second silicon layers (11) is applied.

8. Differential pressure sensor according to claim 7, wherein the connecting surfaces (P1, P2, P3, P4) are identical in shape and are arranged symmetrically on the respective outer edge (21) of the respective second silicon view (11).

9. The differential pressure sensor according to claim 7, wherein the first and second electrodes (E1, E1 ', E2, E2') are made of the same material, esp. Of gold, which is also used as a connecting material.

10. The differential pressure sensor according to claim 7, wherein the connecting lines (KE1, KEV, KE2, KE2 \ KR1, KR2, KR3, KR4, KR1 \ KRZ, KR3 ', KR4') for the capacitive and the piezoresistive sensor elements (E, E ', R, R') on the inner sides of the two measuring membranes (5a, 5b) are applied, and are guided out of the differential pressure sensor between the connecting surfaces (P1, P2, P3, P4).