Micromechanical component

Abstract

Micromechanical component (100) comprising: - a carrier wafer (10) with at least one micromechanical structure (23) arranged in a cavity (21); - a thin-film cap (30) arranged on the carrier wafer (10), by means of which the cavity (21) is hermetically sealed; and - a cap wafer (50) arranged on the thin-film cap (30) in the region of the cavity (21) with the micromechanical structure (23);wherein a region of the thin-film cap (30) above the cavity (21) is hermetically sealed by means of the cap wafer (50), characterized in that two micromechanical structures (23) are formed in the carrier wafer (10), wherein one micromechanical structure (23) is arranged in each cavity (21, 21a), wherein regions above the cavities (21, 21a) are hermetically sealed by means of the cap wafer (50), wherein a fluid channel (53) is formed between a cavity (52a) of the cap wafer (50) and a cavity (21a) of the carrier wafer (10).

Description

The invention relates to a micromechanical component and a method for manufacturing a micromechanical component. State of the art Micromechanical sensors for measuring, for example, acceleration, rotation rate, magnetic field and pressure are known and are mass-produced for various applications in the automotive and consumer sectors. Conventional micromechanical sensors, such as inertial sensors, which require vacuum encapsulation, need a hermetically sealed cap. This can be implemented either as a wafer cap or as a thin-film cap. Wafer capping requires a bonding process, typically using metallic bonding compounds. These usually only allow for low process temperatures, which limits the internal pressures that can be contained. Furthermore, these bonding compounds have proven to be insufficiently moisture-tight over their service life. Thin-film capping is known, for example, from US 7,041,225 B2 and US 7,259,436 B2. These thin-film cappings can be produced at very high temperatures, allowing for particularly low internal pressures. However, at atmospheric pressure, the thin-film cappings flex, necessitating the integration of support columns for mechanical support. This prevents the use of small sensors and can lead to false signals during external pressure fluctuations. Patent specification US 8,786,130 B1 discloses a method for manufacturing an electromechanical circuit breaker. The patent application WO 2014 / 062273 A2 discloses a substrate with several encapsulated devices. The patent application DE 10 2013 102 206 A1 discloses a building element with stacked functional structures. The patent application DE 10 2012 208 031 A1 discloses a hybrid integrated component and a method for its manufacture. Disclosure US 2004 / 0108588 A1 discloses a package for microchips. Disclosure of the invention It is an object of the present invention to provide an improved micromechanical component with a thin-film capping. According to a first aspect, the problem is solved by a micromechanical component having the features of claim 1, comprising: - a carrier wafer with at least one micromechanical structure arranged in a cavity; - a thin-film cap arranged on the carrier wafer, by means of which the cavity is hermetically sealed; and - a cap wafer arranged on the thin-film cap in the region of the cavity with the micromechanical structure; wherein a region of the thin-film cap above the cavity is hermetically sealed by means of the cap wafer. According to a second aspect, the problem is solved by a method for manufacturing a micromechanical component with the features of claim 9, comprising the steps of: - providing a carrier wafer; - forming a micromechanical structure in a cavity of the carrier wafer; - arranging a thin-film cap on the carrier wafer, wherein the cavity is hermetically sealed; and - arranging a cap wafer on the thin-film cap above the cavity with the micromechanical structure, wherein a region of the thin-film cap above the cavity is hermetically sealed by means of the cap wafer. By means of the cap wafer positioned over the micromechanical structure, it can be advantageously achieved that the dependence of the micromechanical component on atmospheric pressure with the thin-film capping is largely eliminated. In particular, it can be largely prevented that the thin-film capping is indented by atmospheric pressure. In this way, an inertial sensor with a thin-film capping is created that is largely insensitive to external atmospheric pressure fluctuations and mechanical stress. As a result, advantageously compact inertial sensors can be realized because support structures for the thin-film capping are not required. The additional capping also advantageously supports reliable operation of the inertial sensor under variable environmental conditions. Preferred embodiments of the micromechanical component are the subject of dependent claims. A preferred embodiment of the micromechanical component is characterized in that the cap wafer is a glass cap wafer. This advantageously eliminates the need for an additional electrical contacting element (e.g., wire bond) for electrically contacting the micromechanical structure of the component. For example, a chip-scale package can be realized in this way. For this purpose, for example, an electrical via can be formed through the glass cap wafer, which has solder balls on one top side. Another preferred embodiment of the micromechanical component is characterized in that the cap wafer is an ASIC wafer. This allows elements of the ASIC wafer to be advantageously used for electrical contacting and evaluation purposes for a sensor. Another preferred embodiment of the micromechanical component is characterized by the fact that a buried wiring plane is formed in the support wafer below the micromechanical structure. In this way, electrical contacting for the micromechanical structure can be provided in a simple manner. The micromechanical component according to the invention is characterized in that two micromechanical structures are formed in the carrier wafer, each micromechanical structure being arranged in a separate cavity, wherein the cap wafer hermetically seals areas above the cavities, and wherein a fluid channel is formed between a cavity of the cap wafer and a cavity of the carrier wafer. The presence of two cavities is particularly important for inertial devices, as, for example, a gyroscope and an accelerometer can be formed in a single housing in this way. The two separate areas in the cap wafer make it easy to provide different pressures for the two cavities. Another preferred embodiment of the micromechanical component is characterized by the formation of filled insulation trenches in the thin-film cap. This allows advantageously separate areas to be created, to which different electrical potentials can be applied to control the micromechanical structure. Another preferred embodiment of the micromechanical component is characterized in that a metal layer is formed on the thin-film cap in the area of ​​the filled insulation trenches. The metal layer ensures that the filled insulation trenches are gas-tight, thereby maintaining the vacuum in the cavern over the long term. Another preferred embodiment of the micromechanical component is characterized in that the micromechanical component is an inertial sensor with at least one sensor element. In this way, the concept of additional encapsulation is applied particularly usefully for inertial sensors. Another preferred embodiment of the micromechanical component is characterized by the fact that the inertial sensor includes an accelerometer and a gyroscope. Advantageously, the additional encapsulation provided by the cap wafer with two separate cavities allows for the provision of different pressures in two cavities, which are required for the aforementioned sensor topologies. The invention is described in detail below with further features and advantages, illustrated by several figures. All disclosed features, regardless of their reference to the claims and regardless of their representation in the description and figures, constitute the subject matter of the present invention. Identical or functionally equivalent elements have the same reference numerals. The figures are intended in particular to clarify the essential principles of the invention and are not necessarily drawn to scale. Disclosed device features result analogously from corresponding disclosed process features and vice versa. This means, in particular, that features, technical advantages, and embodiments relating to the micromechanical component result analogously from corresponding embodiments, features, and advantages relating to the method for manufacturing a micromechanical component and vice versa. The figures show: Fig. 1 an embodiment of a micromechanical component; Fig. 2 another embodiment of a micromechanical component; Fig. 3 another embodiment of a micromechanical component; Fig. 4 another embodiment of a micromechanical component; Fig. 5 another embodiment of a micromechanical component; and Fig. 6 a basic sequence of an embodiment of the inventive method for manufacturing a micromechanical component. Description of embodiments Micromechanically based inertial sensors must meet a multitude of requirements, all of which must be fulfilled as effectively as possible. Highly accurate and energy-efficient micromechanical angular rate sensors are most cost-effectively implemented as so-called "high-quality oscillators," which represent systems with large resonance peaks. To achieve the highest possible quality factor, the micromechanical oscillator must be enclosed in a near-perfect vacuum to minimize damping caused by trapped gas. Furthermore, the encapsulation must remain moisture-proof throughout its service life. Additionally, thermomechanical influences from the component packaging must be minimized to largely eliminate temperature drift in the sensor signal. Last but not least, for combined inertial sensors (acceleration and gyroscope) two different internal pressures must be provided in the same component in adjacent caverns. To achieve the highest possible vacuum, the micromechanical structure is first sealed with a thin-film or thin-layer capping, for example in an epitaxial reactor at temperatures exceeding 1000°C. At these high process temperatures in a hydrogen atmosphere, virtually all surface-adhered molecules decompose and transition into the gas phase. Upon cooling after the cavern sealing, a very low internal pressure results, according to Gay-Lussac's second law. The resulting thin-film capping is also advantageously moisture-tight. Due to its small thickness, typically less than approximately 50 µm, the thin-film cap can deform significantly when the device is exposed to normal atmospheric pressure. Overmolding the components with molding compound can lead to further pressure stresses. Conventionally, excessive deformation of the thin-film cap is prevented by incorporating support columns, which, however, requires additional surface area. It is therefore proposed to avoid support measures by placing an additional wafer cap on top of the thin-film cap and creating a vacuum cavity within it. In this way, the wafer cap shields the thin-film cap from external pressure and mechanical stress. The bonding areas of the wafer cap are located in the same areas as the cavity walls of the underlying caverns. The wafer cap can be implemented either as a pure cap wafer or as an ASIC substrate or ASIC wafer. The second capping layer allows one of the thin-film-capped cavities to be opened, enabling a higher internal pressure to be set during the cap bonding process. Fig. 1 shows a cross-sectional view through a first embodiment of a proposed micromechanical component 100 in the form of an inertial sensor. A carrier wafer 10 is visible, on which a micromechanical functional layer 20 with a micromechanical structure 23 (MEMS structure) is formed. The micromechanical structure 23 is located within a cavity 21 containing a vacuum ("vacuum cavity"), for which different quality requirements are placed depending on the sensor type. The carrier wafer 10 with the micromechanical structure 23 is capped with a thin-film cap 30, for example, made of polysilicon. The cavity 21 is hermetically sealed by means of the thin-film cap 30. The thin-film cap 30 is preferably applied using the aforementioned high-temperature sealing process, which provides a low internal pressure in the cavity 21.Insulation trenches 31 filled with an insulating material in the thin-film cap 30 can be provided to electrically separate areas of the thin-film cap 30 from each other, in order to supply different electrical potentials to the different areas of the thin-film cap 30. Above the filled insulation trenches 31, a metal layer 32 is partially present for electrical contacting the aforementioned areas of the thin-film cap 30. In the area above the cavern 21 containing the micromechanical structure 23, a cap wafer 50 is attached by means of a bonding connection 40, thus creating a hermetic seal of the thin-film cap 30 against the environment. In this way, the thin-film cap 30 in the area above the cavern 21 is protected from atmospheric pressure and / or mechanical stress, preventing it from being depressed. As a result, the micromechanical structure 23 formed in the cavern 21 is better protected and therefore remains functional for a longer period. An insulating layer 33 is provided between the bonding frame 40 and the metal layer 32 to prevent electrical short circuits between the electrically conductive bonding frame 40 and the metal layer 32. The metal layer 32 serves as a wiring layer for realizing electrical conductor tracks for controlling or transmitting sensor signals of the movable micromechanical structure 23 in the functional layer 20 of the carrier wafer 10. In addition, the metal layer 32 also serves as a gas-tight seal for the insulating trenches 31 filled with insulating material, so that the vacuum in the cavern 20 can be maintained for a long time and in good quality. Fig. 2 shows a cross-sectional view through another embodiment of the micromechanical component 100. In this case, the cap wafer 50 is designed as a glass cap wafer. Advantageously, through-glass vias 51 are provided within the glass cap wafer 50, which, in combination with solder balls 60, enable electrical contacting of switching structures of the micromechanical component 100. Advantageously, this eliminates the need for additional external wiring compared to the topology of Fig. 1. Fig. 3 shows a cross-sectional view through another embodiment of a proposed micromechanical component 100. In this variant, the micromechanical component 100 contains a buried wiring level 24 below the micromechanical functional layer 20. The buried wiring level 24 allows for electrical power supply to the micromechanical structure 23. Fig. 4 shows a cross-sectional view through another embodiment of the micromechanical component 100. In this case, the cap wafer 50 is configured as an ASIC wafer, preferably a CMOS ASIC wafer with electronic evaluation circuits, digital circuits, memory, interfaces, etc., at the transistor level (not shown). The carrier wafer 10 is connected to the ASIC-configured cap wafer 50 by means of a bond connection 40. In this way, evaluation and processing capabilities can be advantageously integrated into the micromechanical component 100, enabling a compact design. Advantageously, external electrical contact of the micromechanical component 100 is achieved via through-silicon vias 51 (TSVs) in the ASIC wafer and solder balls 60 on the ASIC wafer. Fig. 5 shows a cross-sectional view through another preferred embodiment of the micromechanical component 100. In this case, two cavities 21, 21a are formed in the functional layer 20, each preferably containing a micromechanical structure 23. In this way, different sensor topologies can be realized with the micromechanical structures 23. It is advantageously possible to enclose different pressures in the two cavities 21, 21a. This is achieved by means of a cap wafer 50, which provides two separate cavities 52, 52a above the cavities 21, 21a. In this way, it is possible to first enclose an initial pressure within the two cavities 21, 21a of the carrier wafer 10. Subsequently, a fluid channel 53 is opened through the thin-film cap 30 by etching. At a suitable second pressure, the cap wafer 50 is then bonded onto the thin-film cap 30, thereby generating the second pressure via the fluid channel 53 in the second cavity 52a of the cap wafer 50. As a result, it is advantageously possible to enclose different internal pressures in both caverns 21 and 21a, thereby enabling the realization of an inertial sensor with two sensor topologies, for example, in the form of an acceleration and a gyroscope. For the gyroscope, a very low first pressure can be provided in the first cavern 21 in the manner described, and for the acceleration sensor, which requires defined damping, a defined higher second pressure can be provided in the second cavern 21a. Fig. 6 shows a basic flowchart of an embodiment of a method for manufacturing a micromechanical component. In step 200, a carrier wafer 10 is provided. In step 210, a micromechanical structure 23 is formed in a cavity 21 of the carrier wafer 20. In step 220, a thin-film cap 30 is placed on the carrier wafer 10, whereby the cavern 21 is hermetically sealed. In step 230, a cap wafer 50 is arranged on the thin-film cap 30 above the cavern 21 with the micromechanical structure 23, whereby a region of the thin-film cap 30 above the cavern 21 is hermetically sealed by means of the cap wafer 50. In summary, the present invention proposes a micromechanical component and a method for manufacturing such a component, which features a thin-film cap without additional support elements and is therefore compact. Compared to conventional micromechanical components with thin-film caps or wafer caps, smaller build volumes can be achieved in this way. Although the invention has been described above with reference to specific application examples, a person skilled in the art can also implement embodiments not disclosed above or only partially disclosed above without deviating from the core of the invention.

Claims

Micromechanical component (100) comprising: - a carrier wafer (10) with at least one micromechanical structure (23) arranged in a cavity (21); - a thin-film cap (30) arranged on the carrier wafer (10), by means of which the cavity (21) is hermetically sealed; and - a cap wafer (50) arranged on the thin-film cap (30) in the region of the cavity (21) with the micromechanical structure (23);wherein a region of the thin-film cap (30) above the cavern (21) is hermetically sealed by means of the cap wafer (50), characterized in that two micromechanical structures (23) are formed in the carrier wafer (10), wherein one micromechanical structure (23) is arranged in each of two caverns (21, 21a), wherein regions above the caverns (21, 21a) are hermetically sealed by means of the cap wafer (50), wherein a fluid channel (53) is formed between a cavern (52a) of the cap wafer (50) and a cavern (21a) of the carrier wafer (10). Micromechanical component (100) according to claim 1, characterized in that the cap wafer (50) is a glass cap wafer. Micromechanical component (100) according to claim 1, characterized in that the cap wafer (50) is an ASIC wafer. Micromechanical component (100) according to one of claims 1 to 3, characterized in that a buried wiring plane (24) is formed in the carrier wafer (10) below the micromechanical structure (23). Micromechanical component (100) according to one of the preceding claims, characterized in that insulation trenches (31) filled in the thin-layer cap (30) are formed. Micromechanical component (100) according to claim 5, characterized in that a metal layer (32) is formed on the thin-film cap (30) in the area of ​​the filled insulation trenches (31). Micromechanical component (100) according to one of the preceding claims, characterized in that the micromechanical component (100) is an inertial sensor with at least one sensor element. Micromechanical component (100) according to claim 7, characterized in that the inertial sensor comprises an accelerometer and a gyroscope. Method for manufacturing a micromechanical component (100) comprising the steps: - providing a support wafer (10); - forming a micromechanical structure (23) in a cavity (21) of the support wafer (20); - arranging a thin-film cap (30) on the support wafer (10), wherein the cavity (21) is hermetically sealed;and- arranging a cap wafer (50) on the thin-film cap (30) above the cavity (21) with the micromechanical structure (23), wherein a region of the thin-film cap (30) above the cavity (21) is hermetically sealed by means of the cap wafer (50),- characterized in that two micromechanical structures (23) are formed in the carrier wafer (10), wherein one micromechanical structure (23) is arranged in each cavity (21, 21a), wherein regions above the cavities (21, 21a) are hermetically sealed by means of the cap wafer (50), wherein a fluid channel (53) is formed between a cavity (52a) of the cap wafer (50) and a cavity (21a) of the carrier wafer (10).; Method according to claim 9, wherein a glass cap wafer is used as the cap wafer (50). Method according to claim 9, wherein an ASIC wafer is used as the cap wafer (50). Use of a micromechanical component (100) according to one of claims 1 to 8 for an inertial sensor with at least one sensor element.

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