Micromechanical component and pressure sensor having a component of this type

A micromechanical component in which lateral deformations, i.e., deformations of the component parallel to its two main surfaces, are concentrated in a defined area of the component structure, making it possible to decouple lateral and vertical stresses in the component. The component structure includes at least one bellows-like structure in which lateral deformations of the component are concentrated. A pressure sensor having a micromechanical component of this type may be used, for example, for measured-value detection.

FIELD OF THE INVENTION

The present invention relates to a micromechanical component for mounting on a carrier as well as a pressure sensor comprising a component of this type, the pressure sensor including a diaphragm and the component at least being partially located on the diaphragm.

BACKGROUND INFORMATION

PCT Application No. WO 9739320 describes a pressure sensor having a steel diaphragm onto which is mounted a micromechanical component in the form of a silicon bridge. Piezoresistive measuring elements that are integrated into the silicon bridge are used to detect the diaphragm deflections.

In practice, the sensor concept described in WO 9739320 has proven to be problematic because the micromechanical component and steel diaphragm have different coefficients of thermal expansion. Because the sensor is ordinarily exposed to temperature fluctuations, lateral mechanical stresses that can significantly distort the sensor measurement results occur in the silicon bridge. To prevent this, WO 9739320 proposes a complex mounting concept that is intended to absorb the lateral mechanical stresses.

SUMMARY

In accordance with an example embodiment of the present invention, a micromechanical component is provided in which lateral deformations, i.e., deformations of the component parallel to its two main surfaces, are concentrated in a defined area of the component structure, which makes it possible to decouple lateral and vertical stresses in the component.

This is achieved according to the present invention by providing the component structure with at least one bellows-like structure in which lateral deformations of the component are concentrated.

According to the present invention, it has been recognized that bellows-like structures, including those in micromechanical components, are easy to produce and, if oriented in the direction of movement, are able to efficiently absorb deformations. Bellows-like structures may thus be advantageously used to compensate for thermally induced and mounting-related deformations.

A micromechanical component according to the present invention and, in particular, its bellows-like structure, may in principle be implemented in a number of different ways.

In a first advantageous embodiment, the bellows-like structure includes at least one thinned section that extends largely across the entire thickness of the component structure and is therefore perpendicular to the two main surfaces of the component. This thinned section is provided between two largely parallel notches in the component structure, one notch extending away from one main surface of the component and the other notch extending away from the other main surface of the component. The thinned section easily yields to lateral deformations of the component, while it is able to efficiently transfer vertical deformations, i.e., deformations that are perpendicular to the two main surfaces of the component, to the component structure.

In a second advantageous embodiment, the bellows-like structure includes at least one undulated web that extends largely across the entire thickness of the component structure and interconnects two areas of the component structure. The longer the undulated web, or the more loops it has, the better it is able to absorb lateral deformations. Vertical deformations in this case are also largely transferred to the component structure, since the web extends across the entire thickness of the component structure.

The component structure according to the present invention may be easily implemented in silicon, i.e., using standard microsystem techniques. Thus, the bellows-like structure of the first embodiment described above may be produced, for example, by both sawing and anisotropic etching, for example, plasma etching, trench etching or KOH etching, a more accurate position and depth of the structures being achievable using anisotropic etching techniques. The bellows-like structure of the second embodiment described above may be easily produced by sawing. Anisotropic etching techniques are mainly used for this purpose.

The present invention also proposes a pressure sensor having a micromechanical component according to the present invention. The pressure sensor includes a diaphragm on which the component is at least partially located. In addition to the bellows-like structure, in which lateral deformations of the component are concentrated, the component in this case also includes means for detecting deformations that are caused by deflections of the diaphragm, i.e., means for detecting vertical deformations in the component. Because the lateral and vertical stresses are decoupled in the component according to the present invention, thermally produced deformations of the component relative to the diaphragm do not affect the pressure measurement results.

In an advantageous embodiment of the pressure sensor according to the present invention, the component structure includes at least one deformation area in which vertical deformations of the component, i.e., mainly the deformations caused by diaphragm deflections, are concentrated. The design of a deformation area of this type supports the decoupling of lateral and vertical stresses in the component. The deformation area is designed to be thinner than the other parts of the component structure and may therefore be implemented, for example, in the form of a web in the component structure that is oriented largely parallel to the two main surfaces of the component.

In the case of the component used in connection with the pressure sensor according to the present invention, the deformation area is also integratable into the bellows-like structure. One example of a bellows-like structure of this type has at least one undulated web that extends largely across the entire thickness of the component structure and interconnects two areas of the component structure. The central area of the undulated web extending along the entire length of the web acts neutrally toward lateral deformations of the component, but represents the point of maximum stress in the case of vertical deformations of the component so that this central area forms a deformation area in the manner described above.

Because the vertical deformations of the component are concentrated in the deformation area, it has been proven to be advantageous if the means for detecting the deformations include at least one piezoelectric resistor, metallic resistor or wire resistance strain gauge that is located in the deformation area. This resistor is advantageously connectable to reference resistors in a Wheatstone bridge.

In addition to having a bellows-like structure that absorbs the lateral deformations of the component and a deformation area where vertical deformations are concentrated, the component structure of the component of a pressure sensor according to the present invention may also include areas where no deformations at all occur. At least some parts of an evaluation circuit may be advantageously located in these reinforced areas of the component structure.

In one example embodiment of the pressure sensor according to the present invention, the component is implemented in the form of a bridge that is mounted on the diaphragm via a first junction point in the central area of the diaphragm and via a second junction point in the area of the diaphragm edge. The bellows-like structure and the deformation area are located between the two junction points. If the deformation area is not integrated into the bellows-like structure, the bellows-like structure is advantageously located in the area of the diaphragm, while the deformation area is located in the vicinity of the second junction point. In this case, a reinforced central area, on which, for example, part of the evaluation circuit is locatable, may be provided between the bellows-like structure and the deformation area. In this regard, note that both a resistor located in the deformation area and parts of the evaluation circuit are integratable into the micromechanical component, provided the component is implemented in a suitable substrate, such as a silicon substrate.

As explained in detail above, the subject matter of the present invention may be advantageously improved and refined in a number of different ways. To this end, a number of exemplary embodiments of the present invention are explained below.

DETAILED DESCRIPTION

FIGS. 1aand1bshow a pressure sensor, i.e., part of diaphragm1of a pressure sensor. In this case, diaphragm1having diaphragm edge2is made of steel. On it is mounted a micromechanical component10that, in the exemplary embodiment illustrated here, is made of silicon. According to the present invention, the component structure of this component10includes a bellows-like structure11that is designed so that lateral deformations of component10are concentrated therein. Such deformations, located parallel to the two main surfaces of component10and to diaphragm1, occur, for example in the presence of temperature fluctuations, when the materials of micromechanical component10and diaphragm1have different coefficients of thermal expansion.

Bellows-like structure11in this case includes a thinned section12that is oriented perpendicular to the two main surfaces of component10and extends largely across the entire thickness of the component structure and is provided between two parallel notches13and14in the component structure. Notch13extends away from the main surface of component10facing diaphragm1, while notch14extends away from the other main surface of component10facing away from diaphragm1.

Lateral deformations of component10, in particular thermally induced mechanical stresses, are absorbed by bellows-like structure11and result in a deformation of the thinned section12, as shown inFIG. 1b. Conversely, vertical deformations of component10, in particular mechanical stress caused by a compressive load on diaphragm1, are efficiently transferred, as explained in greater detail below.

In this case, micromechanical component10is implemented in this case in the form of a bridge. It is connected to diaphragm1, and diaphragm edge2, respectively, only via a first junction point15in the central area of diaphragm1and via a second junction point16in the area of diaphragm edge2.

In the exemplary embodiment illustrated here, the component structure contains a deformation area17in which vertical deformations of component10are concentrated, i.e., deformations that are oriented perpendicular to the two main surfaces of component10and are caused, for example, by a deflection of diaphragm1. Deformation area17, in this case, is implemented in the form of a web that is produced by a third notch18in the component structure. This notch18extends away from the main surface of component10that faces diaphragm1, and it is deeper than notches13and14in bellows-like structure11so that web17is thinner than the other parts of the component structure. At least one piezoelectric resistor3that is used to detect the deformations of the component structure concentrated in web17is integrated into web17.

Both bellows-like structure11and deformation area17are located between the two junction points15and16. Bellows-like structure11is located next to first junction point15in the area of diaphragm1, while deformation area17is positioned next to second junction point16in the area of diaphragm edge2. A reinforced central area19, which transfers the mechanical stresses in the presence of a compressive load on diaphragm1to piezoelectric resistor3in deformation area17and thermally induced stresses to thinned section12of the bellows-like structure, is provided between bellows-like structure11and deformation area17. Because central area19does not, in principle, undergo any deformation, at least some parts of an evaluation circuit may be positioned here and, if necessary, integrated therein. This also enables this part of the chip surface to be used.

A contact pad4for a wire5to connect piezoelectric resistor3and a possible evaluation circuit is positioned over second junction point16. This arrangement has proven to be especially advantageous during the wire bonding process, when wire5is firmly pressed onto contact pad4, because the forces that this produces in the area of second junction point16are absorbed by rigid diaphragm edge2.

When pressure acts upon diaphragm1, as indicated by arrow6inFIG. 1a, the central area of diaphragm1is deflected upward. First junction point15, and thus also the left edge of component10, is also pressed upward, while the right edge of component10is firmly connected to diaphragm edge2via second junction point16. This produces a vertical deformation of component10, resulting in a deformation of the vertically oriented thinnest area of the component structure. This is deformation area17having piezoelectric resistor3. In this connection, it has proven to be advantageous to apply a tensile load to thinned section12of bellows-like structure11so that the stresses are optimally transferred to piezoelectric resistor3. The pressure-dependent variation in the resistance of piezoelectric resistor3is detected as a measure of the compressive load on diaphragm1.

FIG. 1bshows the behavior of component10under thermally induced stresses. If the expansion of silicon component10in the presence of temperature changes is different than that of the steel substrate, lateral mechanical stresses are produced, since silicon component10is unable to expand according to the temperature change because it is connected to the steel substrate. These lateral stresses are concentrated in the laterally oriented thinnest area of the component structure, namely in thinned section12of bellows-like structure11. A deformation that reduces the lateral stresses occurs here. This prevents component10from bending in the vertical direction, which would result in a change in the resistance of piezoelectric resistor3without pressure acting upon diaphragm1.

Component10illustrated inFIGS. 1aand1bmay be manufactured in a number of different ways. Notches13and14in bellows-like structure11and notch18in the area of deformation area17may be produced, for example, by sawing prior to separating the components. Another possibility is to use anisotropic etching techniques, such as plasma etching, high-speed trench etching or KOH etching, which allow a more accurate notch position and depth to be achieved. Predetermined breaking points, for example in the form of perforations, may be created for separating the component chips.

FIG. 2shows the layout of component10illustrated inFIGS. 1aand1b, viewed from above. The illustration shows—from left to right—first junction point15for attaching component10in the central area of the steel diaphragm; bellows-like structure11; reinforced central area19, including space for parts of an evaluation circuit; deformation area17, in which the piezoelectric resistors are locatable; and second junction point16, having an area for contact pads, second junction point16being located over diaphragm edge2.

It should be noted at this point that both the number and position of the resistors implemented on or in the component according to the present invention are variable. In an advantageous embodiment, two piezoelectric resistors are provided in the deformation area. In addition, two non-piezoelectric resistors are positioned as reference resistors next to the deformation area. The piezoelectric resistors and the reference resistors are easily connectable to a Wheatstone bridge. The vertical deformations of the component are also advantageously detectable by two piezoelectric resistors that are positioned perpendicular to each other in the deformation area. A further possibility is to provide, in the deformation area, two longitudinally oriented piezoelectric resistors as well as two non-piezoelectric reference resistors that are oriented at a 45-degree angle thereto.

In the embodiment of a pressure sensor according to the present invention illustrated inFIGS. 3aand3b, component30is also designed in the form of a bridge and, as illustrated inFIGS. 1aand1b, mounted on diaphragm1and diaphragm edge2, respectively. The main difference between component30illustrated inFIGS. 3aand3band component10illustrated inFIGS. 1a,1band2lies in the implementation of bellows-like structure31. In this case, bellows-like structure31is implemented in the form of two undulated connecting webs32and33that connect first junction point15to reinforced central area19. Undulated connecting webs32and33extend vertically, i.e., perpendicular to the two main surfaces of component30, across the entire thickness of the component structure. This allows them to absorb lateral deformations, while vertical deformations of the component structure are transferred to deformation area17, where, in the exemplary embodiment illustrated here, two piezoelectric resistors3are located to detect measured values.

FIG. 4ashows a further embodiment of a component40according to the present invention in which bellows-like structure41is integrated into the deformation area. In this case, bellows-like structure41is implemented in the form of two undulated connecting webs42and43, each having four loops, that connect first junction area15to second junction area16. This component structure therefore does not have a reinforced central area, but is stabilized solely by an intermediate beam44. In this case as well, undulated connecting webs42and43extend vertically across the entire thickness of the component structure. Piezoelectric resistors45and46, which are positioned so that they undergo a change in resistance only in the presence of deformations perpendicular to the component structure, but not in the presence of lateral stresses in the component structure, are provided on thin connecting webs42and43. This arrangement is described in greater detail below on the basis ofFIGS. 4band4c.

Arrows7and8inFIG. 4billustrate, based on the example of connecting web42, the mechanical stresses that occur when component40undergoes a lateral deformation in bellows-like structure41. When bellows-like structure41is compressed, a tensile load acts upon one side of connecting web42—arrow7—while a compressive load occurs on the other side—arrow8. In the middle of connecting web42is an area that remains largely stress-neutral and is therefore referred to as neutral fiber47. Piezoelectric resistor45is positioned here so that it does not undergo any change in resistance in the presence of a lateral deformation of the component structure. However, if component40is deformed perpendicular to the direction of movement of bellows-like structure41, a tensile load occurs on the bottom of bellows-like structure41, while a compressive load occurs on the top thereof, as again illustrated by arrows7and8inFIG. 4c. In this case, piezoelectric resistor45positioned on the top of connecting web42is located at the point of maximum stress and therefore also undergoes a change in resistance.

FIG.5andFIGS. 6aand6bshow two example ways to implement a bellows-like structure51and61in a component50and60, respectively, according to the present invention, in the event that printed conductors52and62, respectively, need to be routed over bellows-like structure51and61, respectively. In both embodiments, bellows-like structure51and61, respectively, is implemented by two notches13and14in the component structure, which extend away from the two diametrically opposed main surfaces of the component structure.

Bellows-like structure51illustrated inFIG. 5includes a notch13that extends across the entire width of component50and begins at the bottom of the component structure. Parallel thereto is a first notch14that extends away from the top of the component structure and is interrupted in the middle, leaving a web53for routing printed conductors52. This web53extends along the entire depth of first notch14and is oriented perpendicular to notches13and14. A second notch14extending away from the top of the component structure is provided parallel to first notch14. This second notch14does not extend across the entire width of component50, leaving webs54in the two edge areas over which printed conductors52may be routed. In the exemplary embodiment illustrated inFIG. 5, two printed conductors52are routed from the left area of the component surface over notch13and over web53bridging first notch14on web55remaining between first and second notches14. On this web55, the printed conductors are then routed between both notches14to the outside and via webs54to reinforced central area19of component50.

Bellows-like structure61illustrated inFIGS. 6aand6balso includes a notch13that extends across the entire width of component60and begins at the bottom of the component structure. In contrast to the example embodiment illustrated inFIG. 5, only one notch14extending away from the top of the component structure is positioned parallel to notch13. In this case as well, bellows-like structure61has a web63for routing printed conductors62. This web62has an undulated design, is positioned in the central area of notch14and extends along the entire depth of notch14. In the exemplary embodiment illustrated inFIGS. 6aand6b, a printed conductor62is routed from the left area of the component surface over notch13and over undulated web63bridging notch14to reinforced central area19of component60.

FIG. 7shows an advantageous way to implement second junction area16of a component10according to the present invention on diaphragm edge2of a pressure sensor. In this case, spacers21are provided in junction area16on the bottom of component10. These spacers21may be produced, for example, by etching techniques or by a wide, flat sawing cut. Connecting material23, for example, solder, is provided in this case in notch22between spacers21.

Resistors, feed lines and contact pads may also be recessed. This has proven to be especially advantageous when mounting the component according to the present invention, since the component is able to be pressed onto the carrier across the entire surface without scratches or pressure marks damaging sensitive areas.

To increase the distance between the component and the diaphragm, the diaphragm may be provided with a groove that is positioned centro-symmetrically to the central supporting point of the component.

It should also be noted in connection with the exemplary embodiments described above that the component according to the present invention may have not only an asymmetrical design, as described, but also a symmetrical one, for example having a centrally located supporting point and bellows-like structures positioned to the left and right thereof. For example, it may also have a round shape, including a round bellows-like structure, and a round notch for concentrating the mechanical stresses at the location of the piezoelectric resistors.

The primary advantages of the component according to the present invention and the pressure sensor according to the present invention are summarized below:The example component according to the present invention is able to efficiently absorb parasitic, thermally induced, mechanical stresses in the pressure sensor according to the present invention. At the same time, it efficiently transfers mechanical stresses that are caused by compressive loads on the sensor diaphragm.The construction techniques needed to produce the pressure sensor according to the present invention as well as the necessary connecting techniques are simple. A high chip yield may be expected in manufacturing the components. In addition, the surface of the sensor diaphragm need not meet strict standards. On the whole, this makes the pressure sensor according to the present invention very economical to produce.Despite the compact size of the component according to the present invention, an evaluation circuit is integratable.