Patent Application
20250042721
The present invention relates to a membrane spring, in particular a micromechanical membrane spring for a MEMS component (MEMS: microelectromechanical system, microsystem), such as a MEMS transducer. The present invention furthermore relates to a MEMS component, in particular a MEMS sensor or MEMS actuator, having such a membrane spring. Furthermore, methods for producing membrane springs for MEMS components are provided.
A fundamental difficulty of many MEMS sensors is that conventional, mechanically elastically deformable elements, such as beams or membranes, are typically only slightly deflected under the forces acting on them. The small deflections result in only low electrical signals generated by transducer elements. With MEMS actuators, only limited forces can thus usually be generated for the desired large deflections. In principle, it is technologically possible to produce very soft beams or membranes with the desired dimensions for use in MEMS components, but, due to their fineness, the structures produced can easily be destroyed during the production process or in use, for example due to an overload.
The mechanical stiffness of elastically deformable elements is described by the spring constant or spring characteristic curve. In the field of mechanics, springs or spring elements of different designs with different spring characteristic curves, which can, for example, have a linear, progressive, degressive-progressive, degressive-horizontal-progressive or degressive-negative-progressive course, are sufficiently known. Within the framework of this disclosure, linear spring characteristic curves are referred to as “nonlinear” in parts. Usually, MEMS sensors and MEMS actuators are designed to operate in a linear stiffness range.
In the case of mechanical springs, it is generally conventional to change the course of the spring characteristic curve in wide ranges by structural modifications, for example by a mechanical combination of spring elements with different spring characteristic curves. For example, a slotted disk spring, or a tongue disk spring, generally has an increased spring travel and a reduced spring force in comparison to a conventional disk spring. The production of such structures within the framework of microelectromechanical systems (MEMS) is largely unknown.
Conventional mechanical spring elements are not necessarily suitable for use in MEMS components. Disk springs are typically not clamped inside or outside, i.e., they are mounted loosely. When they are deformed, friction occurs at these points, which is often undefined and excludes them from use in a MEMS sensor or MEMS actuator.
U.S. Patent Application Publication No. US 2020/0284940 A1 discloses a MEMS gravimeter with a spring-mass system, which comprises a spring with a partly negative stiffness (also rigidity) and a further spring with a positive stiffness. The spring with the partly negative stiffness is provided by a one-dimensionally curved beam whose opposite ends are firmly anchored.
The present invention aims to provide micromechanical spring elements and methods for their production, which are particularly suitable for use in MEMS components, in particular MEMS actuators and/or MEMS sensors.
The aforementioned object may be achieved by features of the present invention. Advantageous configurations of the present invention are disclosed herein.
According to an example embodiment of the present invention, a MEMS component, in particular a MEMS actuator or sensor, comprises a micromechanical membrane spring or a micromechanical membrane spring element with an at least regional, convex or concave, two-dimensional curvature. The membrane spring is suitable for imparting a spring force in the layer sequence direction of a layer composite of the MEMS component into which the membrane spring can be integrated.
According to an example embodiment of the present invention, the structure of the micromechanical membrane spring is two-part and comprises a first membrane spring element, preferably with a linear characteristic curve, and a second membrane spring element, preferably with a nonlinear characteristic curve. The second membrane spring element has the at least regionally convex or concave, two-dimensional curvature and is mechanically coupled to the first membrane spring element in such a way that the spring forces imparted by the first and the second membrane spring element act as a resulting spring force in the layer sequence direction of a layer structure of the MEMS component. For this purpose, the membrane spring is integrated into the layer structure of the MEMS component.
By structural modifications of the first and/or second membrane spring element, the spring characteristic of the membrane spring can be adapted to specific requirements of the application. In particular, it is provided in embodiments for a first membrane spring element with a linear spring characteristic curve and a second membrane spring element with a nonlinear spring characteristic curve to be mechanically coupled to one another, preferably in such a way that the resulting spring characteristic curve has a slight gradient in a partial region.
According to an example embodiment of the present invention, the MEMS component comprises a device for preloading the membrane spring. The preloading device is configured to set a permanent elastic deflection of the membrane spring, in particular in such a way that an operating point is set with respect to the resulting spring characteristic curve of the membrane spring. Preferably, the membrane spring is preloaded in such a way that the operating point is in an at least approximately linear spring characteristic curve range of the membrane spring with a slight gradient. For this purpose, the membrane spring is slightly deflected from its rest position, in particular by the application of force. With suitable dimensioning, in particular of the first and/or second membrane spring element, the high spring stiffness which the membrane spring as a spring combination element preferably has in the non-deflected state can be significantly reduced by the preloading so that a linear range of the spring characteristic curve with a small positive or negative gradient can in particular be achieved. This in particular makes it possible to produce MEMS sensors and actuators in silicon, which are very robust against destruction by an overload during production. Preloading also reduces the stiffness of the membrane spring during operation so that large movement amplitudes for detection by sensors and/or for actuation are made possible.
In advantageous exemplary embodiments of the present invention, for generating the resulting spring force, the first membrane spring element is mechanically connected in a central region of the second membrane spring element. Preferably, the first membrane spring element is connected in a curved region of the second membrane spring element and/or in the region of a maximum spring travel of the second membrane spring element in order to provide a membrane spring element with the desired spring characteristic along the resulting spring travel.
In advantageous embodiments of the membrane spring of the present invention, a membrane plane of the first membrane spring element is arranged in parallel with a membrane plane of the second membrane spring element. Preferably, the first membrane spring element is designed to generate a spring force which acts in a direction perpendicular to the membrane plane and follows a linear characteristic curve. The second membrane spring element is preferably designed to generate a spring force which follows a nonlinear spring characteristic curve in the direction perpendicular to the membrane plane.
In exemplary embodiments of the present invention, the first membrane spring element and/or the second membrane spring element is substantially annular. The first membrane spring element and the second membrane spring element are preferably arranged concentrically with one another. In alternative embodiments, the first and/or second membrane spring element is substantially designed as a lens-shaped raised structure.
Preferably, according to an example embodiment of the present invention, the first and/or second membrane spring element is designed to be rotationally symmetrical about an axis of symmetry and is arranged concentrically with respect to this axis of symmetry.
Preferably, according to an example embodiment of the present invention, the membrane spring consists at least in part, particularly preferably completely, of silicon. In particular, the first and/or the second membrane spring element can consist of silicon. Silicon is a material with very good elastic properties. The spring properties of such a membrane spring are very homogeneous and stable due to the high purity of the material. In general, membrane springs made of silicon do not show significant plastic deformation up to a temperature of several hundred degrees Celsius. Silicon can be precisely structured using established MEMS processes and makes parallelized production of a large number of miniaturized components on a wafer at low cost possible.
In advantageous embodiments of the present invention, the preloading device comprises at least one cavity which is subjected to negative pressure or positive pressure and which is at least regionally delimited by the membrane spring, in particular at least regionally by the first and/or second membrane spring element. Subjecting the cavity to pressure causes a force to be applied to the membrane spring or to at least one of the membrane spring elements so that a pneumatic preloading of the membrane spring as a whole and, accordingly, a reduced spring stiffness after joint preloading can be set. This makes it possible to implement a wide range of applications for MEMS sensors and actuators.
Preferably, according to an example embodiment of the present invention, the first membrane spring element is mechanically connected at an outer circumference to the layer structure and to the second membrane spring element in such a way that a functional membrane suspended on the inner circumference of the first membrane spring element is movably guided along a spring path parallel to the first direction.
In preferred embodiments of the present invention, the second membrane spring element is mechanically connected at its inner circumference and outer circumference to the layer structure. The second membrane spring element is mechanically connected to the first membrane spring element in a two-dimensionally curved region located between the inner circumference and the outer circumference.
The production of a curved membrane spring, in particular made of silicon, represents a major challenge in terms of process technology since it cannot be implemented using conventional MEMS planar processes and is therefore difficult to integrate into the process sequences commonly used for the production of MEMS components.
In the following, methods for producing two-dimensionally convexly or concavely curved membrane springs, in particular made of silicon, are disclosed.
Within the framework of this disclosure, two-dimensionally curved membrane springs or membrane spring elements in particular refer to those that have a curvature with respect to two mutually orthogonal directions at least in one region of their surface and are thus to be distinguished from spring elements which are only curved in one dimension, such as beam springs.
An at least regionally two-dimensionally convexly or concavely curved membrane spring can, for example, be produced in an additive, subtractive or forming manufacturing step.
According to a variant of the production method according to the present invention, in a forming manufacturing step, a layer which is at least partly exposed in the layer structure, in particular of the MEMS component already described above, is subjected to pressure and plastically deformed.
Preferably, the at least partly exposed layer is heated locally during the plastic deformation, in particular by means of laser radiation, in order to locally limit heating and in particular thus to largely avoid damaging heat input to any heat-sensitive components or structures of the MEMS component.
In advantageous embodiments of the present invention, the first membrane spring element is mechanically connected to the second membrane spring element by means of laser welding.
In preferred embodiments of the production method of the present invention, for permanent elastic, in particular pneumatic, preloading of the membrane spring, at least one cavity introduced into the layer structure of the MEMS component is subjected to negative pressure or positive pressure and is subsequently sealed in a pressure-tight and gas-tight manner. The cavity is delimited at least regionally by the membrane spring, in particular at least regionally by the first and/or second membrane spring element, so that the membrane spring is deflected by the set pressure difference. The deflection of the membrane spring, in particular for setting a function corresponding to the particular application, preferably takes place at the end of the production process. Preferably, the membrane spring has a spring characteristic curve range with high rigidity in its rest position, i.e., without the influence of external forces. In its rest position, the membrane spring is in a spring characteristic curve range with a large gradient and is thus very robust against destruction by an overload during production. The deflection of the membrane spring preferably takes place in such a way that a flat spring characteristic curve range is achieved and a reduced force is thus required for further deflection of the membrane spring. Depending on the geometry of the curved membrane spring, in particular of the first and/or second membrane spring element, a common spring constant and/or resonance frequency can be varied via the coupled preloading and the membrane spring can thus be structurally adapted to the specific micromechanical application.
According to another variant of the production method of the present invention, the at least regionally convex or concave, two-dimensional curvature of the membrane spring or the membrane spring element is formed in an additive manufacturing step by depositing regionally overlapping layers, in particular made of silicon or polysilicon. Particularly preferred, multiple individual layers are stacked on top of one another and structured appropriately. For example, trenches are etched into an applied layer and are filled with a filler, in particular an oxide, before the subsequent layer is applied.
According to another variant of the production method of the present invention, the at least regionally convex or concave, two-dimensional curvature of the membrane spring is formed in a subtractive manufacturing step by material ablation of a substantially flat substrate, wherein material is removed from opposite end faces of the substrate. The material ablation takes place, for example, by laser ablation or by isotropic etching.
The production of such membrane springs, in particular membrane springs with robust membrane thickness and low spring stiffness after preloading, makes it possible to implement a wide range of applications of MEMS transducers, in particular MEMS sensors and MEMS actuators, such as:
Further details and advantages of the present invention are explained in more detail below with reference to the exemplary embodiments shown in the figures.
Identical or corresponding elements are provided with the same reference signs in all figures.
The exemplary embodiments illustrated in the figures have a rotationally symmetrical, in particular circular or annular, membrane geometry, but it is understood that this is not to be interpreted as restrictive and that deviations are possible.
With reference to
With reference to
The methods described here are in particular suitable for providing membrane springs 1 that have lnl spring characteristic curves and are made of silicon and in a wafer composite, for example in a layer structure 600 of a MEMS component 700 to be produced. The presented methods can in particular be combined with conventional production processes for producing further microelectromechanical functional elements, such as sensor elements or actuator elements.
By way of example, the figures schematically show methods for producing a membrane spring 1 with a two-dimensional curvature. It is understood that the presented methods are equally suitable for providing the aforementioned and below-mentioned membrane spring elements 10, 20 as individual components.
The method illustrated in
The removal of the material from the front side 1001 and the rear side 1002 of the substrate 1000 can take place simultaneously or in separate method steps, as shown in
The resulting geometry of the produced component can be designed almost arbitrarily using subtractive technology; in particular, a membrane thickness can be varied locally, for example in the radial direction and/or in the tangential direction.
In particular, it is provided to introduce local surface structuring, for example in the form of depressions, grooves or the like, radially symmetrically or rotationally symmetrically on the front and/or rear side 1001, 1002 of the membrane spring 1 or the membrane spring element 10, 20. In particular, it is thus possible to produce membrane springs 1 or membrane spring elements 10, 20 the geometries of which are comparable to disk springs of different variants and are, for example, slotted, locally reinforced and/or provided with other surface contours. On the basis of the surface structuring, the spring characteristic curves of the produced membrane springs 1 or membrane spring elements 10, 20 can be modified and in particular adapted to the technical application.
In the exemplary embodiment of
A further possibility for suitably removing the semiconductor material of the substrate 1000 from both the front side 1001 and the rear side 1002 is to apply a three-dimensionally structured etching mask 1011, 1012, in particular a polymer etching mask, as shown by way of example in
In the method illustrated in
The etching masks 1011, 1012 can in particular be provided by dispensing resist, by means of grayscale lithography or by means of holographic lithography. The production of a suitable, three-dimensional etching mask 1011, 1012 can take place by means of replication in the wafer composite, as shown schematically in
The at least regionally convex or concave, two-dimensional curvature of the membrane spring 1 is formed in the additive manufacturing step by depositing regionally overlapping layers 1001, . . . , 2005. The geometry of the thus produced membrane spring 1 or membrane spring elements 10, 20 and the resulting spring characteristic curve can be predetermined by the thickness of the individual layers 2001, . . . , 2005, the overlap of the individual layers 2001, . . . , 2005, in particular in the layer sequence direction 2050, and/or by the structuring in the layers 2001, . . . , 2005. For this purpose, slots may, for example, be introduced in the radial direction. After the curved component 1, 10, 20 has been exposed, for example by means of XeF2 etching, the cross-section thereof has a stepped structure corresponding to the thickness of the individual layers 2001, . . . , 2005, as shown in particular in
In a subsequent optional method step, as shown in
In the method step of
Optionally, as shown in
For forming, a pressure difference is established between the front side and rear side of the exposed layer 100, for example 1 bar ambient pressure on the front and vacuum on the rear side. In particular, a cavity 140 formed between the rear side of the exposed layer 100 and a further substrate 150 can be evacuated. In a hot process, optionally under a hydrogen atmosphere, the exposed layer 100 is plastically deformed under pressure into the membrane spring 1 or the membrane spring element 10, 20. In so doing, any remaining insulation layer 110 in the rear center of the membrane can be used as a mechanical stop, as shown in particular in
Before the hot process, the front side and/or rear side of the exposed layer 100 can be structured locally. In order to set the spring properties and to define zones with greater plastic deformation during the hot process, local depressions, such as locally thinned regions or the like, can be introduced. After cooling and after eliminating the pressure difference, a two-dimensionally curved membrane 1 or a two-dimensionally curved membrane spring element 10, 20 with reduced intrinsic mechanical stresses results.
The plastic deformation of the exposed layer 100 can be achieved by softening the material locally, in particular by means of laser treatment, as shown in
The laser beam is repeatedly directed along trajectories 3010 onto the exposed layer 100 via a controllable optical deflection unit (scanner), as shown in
Pulsed but also continuous laser radiation, in particular a continuous wave laser, is suitable for treating the exposed layer 100. The degree of local plastic deformation can also be set, for example, via the laser energy density, the duration of the irradiation and/or the pressure application. In a development, it is provided to regulate such parameters depending on the deformation measured in situ during the process, in order to bring about a desired deformation of the exposed layer 100 according to a predetermined, two-dimensional curvature. An advantage of this forming method is that the membrane is planar before the deformation, so that functional sensor elements or actuator elements, such as piezoresistive or piezoelectric elements, wiring levels and the like, can be produced on a planar wafer surface using the usual MEMS processes. However, these elements could be destroyed at the high temperatures that are typically necessary to plastically deform the exposed layer 100. The use of laser radiation advantageously makes a local temperature input on the exposed layer 100 to be formed possible, which is substantially limited to the irradiated region. The membrane spring 1 or the membrane spring element 10, 20 can thus be produced in a wafer composite or layer composite in such a way that destruction of the other functional elements of the MEMS component 700 to be produced can be largely avoided and waste is reduced.
In a first step, a first, a second and a third wafer 41, 42, 43, in particular made of silicon, are provided, cf. in particular
The third wafer 43 is provided with a rotationally symmetrical through-opening 44 in the region of the functional membrane. Outside the through-opening 44, smaller, further through-openings 45, 46 are introduced into the third wafer 43. The further through-openings 45, 46 can be countersunk on the front side with respect to the surface of the third wafer 43.
On the rear side of the third wafer 43, a bonding agent 47 for hermetically bonding is optionally applied in regions. Wafer bonding can take place using conventional MEMS processes and equipment, such as seal glass bonding or eutectic bonding.
In the illustrated exemplary embodiment, the second wafer 42 is composed of three layers 48, 49, 50, wherein the thickness of the first layer 48 of the second wafer 42 is preferably selected to ensure the handling of the second wafer 42 during the production method.
The second layer 49 and/or the third layer 50 of the second wafer 42 are optionally polysilicon layers. Thin oxide layers 51, 52 are introduced between the layers 48, 49, 50. The curved second membrane spring element 20 with a nonlinear characteristic curve is produced from the second layer 49 of the second wafer 42. The thickness of the second layer 49 of the second wafer 42 is preferably designed according to the desired spring characteristic curve. The thickness of the third layer 50 of the second wafer 42 defines the desired curvature height of the second membrane spring element 20, which is formed from the second layer 49. A recess 53 is introduced into the third layer 50 of the second wafer 42 at the position of the further through-opening 46. A further, rotationally symmetrical, in particular annular, recess 54 defines the clamping or the width of the second membrane spring element 20.
The first wafer 41 is composed of three layers 55, 56 and 57. The thickness of the first layer 55 of the first wafer 41 is preferably selected to ensure the handling of the first wafer 41 during the production method. The second layer 56 and/or the third layer 57 of the first wafer 41 optionally consist of polysilicon. Thin oxide layers 58, 59 are introduced between the layers 55, 56, 57. The first membrane spring element 10 with a linear characteristic curve is produced from the second layer 56 of the first wafer 41. The thickness of this second layer must be designed according to the desired spring characteristic curve. The thickness of the third layer 57 of the first wafer 41 defines the desired curvature height of the first membrane spring element 10, which is formed from the second layer 56. Annular recesses 60, 61 are introduced into the third layer 57. The recess 60 defines the clamping or the width of the first membrane spring element 10 with a linear characteristic curve. A remaining, in particular annular, connecting element 62 between the recesses 60, 61 serves to mechanically connect or mechanically couple the first and the second membrane spring element 10, 20 to one another. The connecting element 62 is interrupted at least at one position along its circumference (cf. in particular
The third layer 57 of the first wafer 41 and the third layer 50 of the second wafer 42 are aligned to face one another and are bonded to one another (cf. in particular
In a subsequent method step, the first layer 48 of the second wafer 42 is removed (cf. in particular
In a further method step, a central region 80 of the second layer 49 formed into the second membrane spring element 20 is firmly connected to the connecting element 62. This takes place locally, for example, by means of laser welding (cf. in particular
Subsequently, the cavity 66 is ventilated by locally removing the second layer 49 of the second wafer 42 in the region of the through-opening 46 (cf. in particular
The second layer 56 of the first wafer 41 forms both a functional membrane 70 and the first membrane spring element 10 with a linear spring characteristic curve. The first membrane spring element 10 with a linear spring characteristic curve is mechanically coupled to the curved membrane spring element 20 via the connecting element 62 and thus, as a whole, forms the membrane spring 1.
The cavity 67 is evacuated via the through-opening 45 and subsequently permanently and hermetically sealed by a seal 460 by means of a laser process, in particular laser reseal (cf. in particular
The first membrane spring element 10 of the MEMS component 700 is mechanically connected at its outer circumference to the layer structure 600 and furthermore to the second membrane spring element 20 in such a way that the functional membrane 70 suspended on the inner circumference of the first membrane spring element 10 is movably guided along a spring path parallel to the first direction 601. The annular second membrane spring element 20 of the MEMS component 700 is clamped on both sides in order to impart a substantially linear-nonlinear or nonlinear force depending on the deflection. The second membrane spring element is mechanically connected to the layer structure 600 both at its inner circumference and at its outer circumference and is mechanically connected to the first membrane spring element 10 in an intermediate, two-dimensionally curved region. The first membrane spring element 10 has a substantially linear characteristic curve so that the spring characteristic curve of the coupled membrane spring substantially follows a nonlinear spring characteristic curve, in particular a linear-nonlinear spring characteristic curve, with a range of a small positive or negative spring constant, in particular after preloading.
Alternatively, the curved second membrane spring element 20 can be produced in the second wafer 42 by means of additive or subtractive technology as a separate component, in particular by means of one of the methods shown in
The production methods shown are inter alia characterized by the following features:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2023 207 523.7 | Aug 2023 | DE | national |
