Micromechanical device with a first cavern and a second cavern

Abstract

Micromechanical device comprising a first cavity (10) which includes a MEMS element (15) and a second cavity (20), wherein the second cavity (20) is connected to the first cavity (10) by means of a connecting channel (30), wherein the connecting channel (30) has a closure (40) by which the first cavity (10) and the second cavity (20) are hermetically sealed from each other, wherein the closure (40) can be opened electrically, and wherein the micromechanical device has a principal extension plane (60) as well as a bonding frame (70) and a cap (80) parallel to the principal extension plane (60), and wherein the bonding frame (70) and the second cavity (20) are arranged at least partially overlapping in a direction perpendicular to the principal extension plane (60).

Description

State of the art The invention is based on a micromechanical device with a first cavern which has a MEMS element, and with a second cavern. MEMS components require a defined environment to function properly. This applies particularly to the ambient pressure. Especially, but not exclusively, for gyroscopes, a low ambient pressure is necessary that remains stable throughout the sensor's lifetime, regardless of environmental influences. According to the current state of the art, a getter material can be used to adjust the pressure in the cavity of a sensor. This is particularly advantageous when multiple sensors with different cavity pressures are to be manufactured on a single chip, e.g., a combined accelerometer and gyroscope. In this case, a getter material is introduced into the cavity of the sensor with the lower pressure, e.g., the gyroscope. A disadvantage is that the getter activation occurs thermally during the bonding process. Therefore, separate optimization of the bonding process and the getter activation is not possible. Another possibility is to use the getter as a stabilizer of the cavern pressure over its lifetime. For this purpose, a getter material is introduced into a cavern, with the getter's function being limited to adsorbing outgassing from the sensor and / or the cap. Here too, the getter material is thermally activated. Essential getter materials and methods for their production are described, for example, in EP 1 410 433 A2, EP 1 412 550 A2, and EP 1 869 696 A2. A micromechanical device is known from WO 01 / 94 823 A1, comprising a first cavity with a MEMS element and a second cavity. The two cavities are connected via a connecting channel containing an electrically operated closure. Object of the invention The object of the invention is to create a micromechanical device with a cavern whose atmospheric internal pressure is controllable. Advantages of the invention The invention relates to a micromechanical device comprising a first cavity containing a MEMS element and a second cavity. The core of the invention lies in the fact that the second cavity is connected to the first cavity by means of a connecting channel, wherein the connecting channel has a seal by which the first and second cavities are hermetically sealed from each other, and the seal is electrically openable. An advantageous embodiment of the invention provides that the second cavern contains a getter material. Advantageously, the sorption of gases can be increased by additionally introducing a suitable getter material into the second cavern, which is different from the wall material of the second cavern. It is particularly advantageous for the getter material to be arranged on an inner surface of the second cavern, forming a coating there. An advantageous embodiment of the invention provides that the second cavern has a structured inner surface. This advantageously increases the inner surface area where gas sorption can occur. An advantageous embodiment of the invention provides that the micromechanical device has a principal extension plane and that the first and second caverns are arranged in a direction perpendicular to the principal extension plane, at least partially overlapping. Advantageously, this allows the footprint of the micromechanical device in the principal extension plane to be kept small. According to the invention, the micromechanical device also comprises a main extension plane, a bonding frame, and a cap parallel to the main extension plane, and the bonding frame and the second cavity are arranged to overlap at least partially in a direction perpendicular to the main extension plane. Advantageously, this allows the footprint of the micromechanical device in the main extension plane to be kept small, as the otherwise unused space under the bonding frame forms a volume for the second cavity. This invention allows for the adjustment of a cavern pressure using an electrically activated getter. The invention can also be used to stabilize a cavern pressure over its lifetime, for example, when the cavern pressure is set by laser resealing and pressure changes due to outgassing from the sensor or cap are to be eliminated. This is possible because the internal pressure can be adjusted with a different gas (e.g., N₂, Ar) than the gas escaping from the sensor and / or cap (e.g., H₂). This utilizes the selectivity of the getter material with respect to different gases. Alternatively, a plurality of caverns can be used, which can be activated as needed. From a technical perspective, the present invention offers extensive possibilities for adjusting the internal pressure in caverns and, moreover, stabilizing it over its lifetime. This is achieved by using a getter material that is activated electrically rather than thermally. The key advantages lie in decoupling the getter activation from subsequent process steps, particularly the bonding process between the sensor and the cap. drawing Figures 1a to 1c show a micromechanical device according to the invention in a first embodiment. Figure 2 shows a micromechanical device according to the invention with an open closure. Figure 3 shows a micromechanical device according to the invention in a second embodiment with three connecting channels and three closures. Figure 4 shows a micromechanical device according to the invention in a third embodiment with a first cavity and two second cavities. Figure 5 shows a micromechanical device according to the invention in a fourth embodiment with a structured inner surface of the second cavity. Figure 6 shows a micromechanical device according to the invention in a fifth embodiment with a second cavity containing a getter material.Figures 7 a and b show in a sixth embodiment a micromechanical device according to the invention with two second caverns, which are arranged at least partially under a bonding frame. Description The present invention uses electrical activation of the getter by changing a diffusion barrier (hereinafter referred to as a "closure") between at least two cavities. In addition to a first cavity, in particular a sensor cavity, at least a second cavity is provided for the getter. The second cavity, or getter cavity, is closed and has a lower internal pressure than the first cavity, or sensor cavity. The sensor cavity is connected to the getter cavity via at least one connecting channel, which is, however, closed. The closure of this connecting channel is connected to electrical contacts, so that an external voltage allows a current to flow through this closure. If a sufficiently large current flows through the closure, it heats up and changes its properties as a diffusion barrier. This can occur, for example, by opening the closure through deformation.Such a deformation opens the channel, so that the sensor cavern and the getter cavern are now connected. However, the structure of the seal can also change in such a way that the seal no longer acts as an effective diffusion barrier between the sensor cavity and the getter cavity. This is achieved, for example, by changing the seal's porosity through the application of an electric current. The closure material can be either semiconductor materials in single-crystal, polycrystalline, or amorphous structures, such as silicon or germanium, or metals, such as aluminum. Polysilicon is preferred. Multiple getter cavities can be connected to a single sensor cavity via independent closures. In this case, if a pressure increase is detected in the sensor cavity of the sensor element, the pressure in the sensor cavity can be reduced by opening a closure to a getter cavity. If multiple getter cavities are connected to a single sensor cavity via independent closures, each getter cavity can be connected to the sensor cavity individually and independently of time by opening its respective closure. This can improve the stability of a sensor element over its lifetime. The detection of the sensor element's internal pressure and the opening of a getter closure are performed by the evaluation ASIC, which is present independently of this functionality for evaluating the sensor signals. The getter cavity can be integrated into the sensor element without affecting its surface area. This is possible by placing the getter cavity completely or partially under the bond frame; see the detailed description below. Possible implementations with a sensor cavern, multiple connection channels, and multiple getter caverns are shown in Figures 1, 2, 3 to 4. The volume ratio between getter cavern and sensor cavern can be less than one, greater than one, or equal to one. In Figures 1-4, the volume ratio of getter cavern to sensor cavern is shown as less than one by way of example. Figures 1a to 1c show a micromechanical device according to the invention in a first embodiment. Figure 1a shows a top view of a micromechanical device according to the invention with a first cavity 10, which contains a MEMS element 15, and with a second cavity 20. The two cavities are hermetically sealed from the environment. The MEMS element requires a specific operating pressure. The second cavity 20 is connected to the first cavity 10 by means of a connecting channel 30. The connecting channel 30 has a closure 40, through which the first cavity 10 and the second cavity 20 are hermetically sealed from each other. The closure 40 can be opened electrically. For this purpose, it has electrical contacts 45. The second cavity has the ability to act as a getter for the first cavity by having a lower internal pressure than the first cavity before the closure is opened.Furthermore, the inner surfaces of the second cavern act as getters to adsorb gases. Fig. 1b shows a cross-section of the micromechanical device with a first cavern 10, a second cavern 20, and a closed connecting channel 30 with a closure 40, before the application of a cap 80 to seal the caverns. Fig. 1c shows a cross-section of the completed micromechanical device after the application of a cap 80 to seal the caverns 10 and 20. The micromechanical device has a main extension plane 60. The first cavern 10 and the second cavern 20 are arranged partially overlapping in a direction perpendicular to the main extension plane 60. The first cavern 10 extends under the cap 80 over the area of ​​the connecting channel 30 and partially over the second cavern 20.The cap 80 is connected to the rest of the micromechanical device by means of a bonding frame 70 in a plane parallel to the main extension plane 60. The bonding frame 70 and the second cavity 20 are arranged partially overlapping in a direction perpendicular to the main extension plane 60. Fig. 2 shows a micromechanical device according to the invention with an open closure. A top view of a micromechanical device according to the invention is shown, with a connecting channel 40 opened after current flow. Fig. 3 shows in a second embodiment a micromechanical device according to the invention with three connecting channels 30 and, by way of example, three closed closures 40. Fig. 4 shows a third embodiment of a micromechanical device according to the invention, comprising a first cavern 10 and two second caverns 20. The top view shows two second caverns 20 of different geometries, a first cavern 10, and several closed connecting channels 30. The second cavern 20 itself can contain further structuring to increase its sorption capacity. In particular, porous surfaces can increase the surface area and thus improve sorption capacity. The getter material in the second cavern 20, or getter cavern, can consist of different materials or combinations thereof. Silicon is particularly suitable because it is easy to manufacture and readily available for the sensor element due to the process flow used. Additionally, the getter cavern and its structuring can be coated with a getter material to further enhance sorption capacity. Possible implementations are shown in Figures 5 and 6. Fig. 5 shows, in a fourth embodiment, a micromechanical device according to the invention with a structuring 24 of an inner surface of the second cavern 20. Fig. 6 shows a fifth embodiment of a micromechanical device according to the invention with a second cavern containing a getter material 22. In this example, the feature of a structuring 24 of an inner surface of the second cavern 20 is also realized. To avoid increasing the size of the micromechanical device by means of the second cavern, or to minimize its size, the second cavern 20 can be partially or completely placed beneath the bonding frame 70. This is possible because the bonding frame is several tens of micrometers wide. By placing the second cavern 20 beneath the bonding frame 70, this otherwise unused area can be utilized for the second cavern. Figures 7a and 7b show, in a sixth embodiment, a micromechanical device according to the invention with two second cavities 20, which are arranged at least partially under a bonding frame 70. Figure 7a shows a top view of a micromechanical device with a second cavity 20 that is arranged partially under the bonding frame 70 (left) and with a further second cavity 20 that is arranged completely under the bonding frame 70 (right). Figure 7b shows this micromechanical device in cross-section. The second cavern 20 can also be arranged around the first cavern 10. Reference symbol list 10 First Cavern 15 MEMS Element 20 Second Cavern 22 Getter Material 24 Structuring of an Inner Surface of the Second Cavern 30 Connection Channel 40 Closure 45 Electrical Contacting 60 Main Extension Plane 70 Bonding Frame 80 Cap

Claims

Micromechanical device comprising a first cavity (10) which includes a MEMS element (15) and a second cavity (20), wherein the second cavity (20) is connected to the first cavity (10) by means of a connecting channel (30), wherein the connecting channel (30) has a closure (40) by which the first cavity (10) and the second cavity (20) are hermetically sealed from each other, wherein the closure (40) can be opened electrically, and wherein the micromechanical device has a principal extension plane (60) as well as a bonding frame (70) and a cap (80) parallel to the principal extension plane (60), and wherein the bonding frame (70) and the second cavity (20) are arranged at least partially overlapping in a direction perpendicular to the principal extension plane (60). Micromechanical device according to claim 1, characterized in that the second cavern (20) contains a getter material (22). Micromechanical device according to claim 1 or 2, characterized in that the second cavern (20) has a structuring (24) of an inner surface. Micromechanical device according to one of the preceding claims, characterized in that the micromechanical device has a principal extension plane (60) and that the first cavern (10) and the second cavern (20) are arranged at least partially overlapping in a direction perpendicular to the principal extension plane (60).

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