DE102018211547A1 - Micromechanical sensor - Google Patents

Abstract

Micromechanical sensor (100), comprising:
- a substrate;
- A cap element arranged on the substrate; and
- At least one deflectable element (20) arranged in the cap element, an increased internal pressure defined in relation to the environment prevailing inside the cap element; marked by
- A symmetry element (40) arranged within the cap element and not connected to the deflectable element (20), which is designed to provide a fluid dynamic symmetry for the deflectable element (20) during operation of the micromechanical sensor (100).

Description

The present invention relates to a micromechanical sensor. The present invention further relates to a method for producing a micromechanical sensor.

State of the art

MEMS acceleration sensors with a geometrically symmetrical rocker structure (i.e. with rotating arms of the same length on both sides of an axis of symmetry) typically have a different hole structure on the two sides, as a result of which a different mass distribution is achieved. As a result of the different mass distribution of the rocker structure, a torque can occur when an acceleration is applied, which is recorded and evaluated.

Constructive variants with several functional layers are advantageous, which on the first side are made of a solid material (ie with two functional layers arranged one above the other) and on the second side apart from an outer frame and / or other reinforcing elements only from the lower, usually thinner, functional layer consist.

For these construction forms, too, the two sides of the rocker have vertical holes in the area of the seismic mass. This “perforation” facilitates process control when the sacrificial layer is etched. Due to the different perforations on both sides, the thermal sensitivity can also be reduced.

In addition to the use of differently structured but mechanically interconnected functional layers for the production of symmetrical rocker structures, several functional layers are also used for the production of differential capacitors in a vertical arrangement. For this purpose, these functional layers are mechanically and electrically separated and have different connections to the substrate.

US 2012/0227494 A1 discloses acceleration sensors with asymmetrical and symmetrical rockers based on two functional layers, which consist of solid material on a first side and only the lower, usually thinner functional layer on the second side except for an outer frame. For this purpose, the first thinner functional layer is arranged in the direction of the cap and the structured, thicker functional layer is arranged in the direction of the substrate.

US 2015/0041927 A1 discloses acceleration sensors with asymmetrical rocker structures based on two functional layers with separate substrate anchoring for the production of differential capacitors in a vertical arrangement.

US 20151/0192603 A1 discloses acceleration sensors with rocker structures based on three functional layers, the specific spring system of which causes a differently directed vertical deflection of the mass and enables a double capacitor arrangement. In the direction of the cap, the two differential capacitors have the same structure, so that a partial compensation of the thermal sensitivity can be expected.

JP 2015-068866 A discloses an acceleration sensor with a rocker structure based on two functional layers, mushroom-shaped vertical stops connected to the substrate being arranged in the stop area above a lower functional layer.

Disclosure of the invention

It is therefore an object of the present invention to provide a micromechanical sensor with improved fluid dynamic properties.

• - ein Substrat;
• - ein auf dem Substrat angeordnetes Kappenelement; und
• - wenigstens ein im Kappenelement angeordnetes auslenkbares Element, wobei innerhalb des Kappenelements ein gegenüber der Umgebung definiert erhöhter Innendruck herrscht; gekennzeichnet durch
• - ein innerhalb des Kappenelements angeordnetes, nicht mit dem auslenkbaren Element verbundenes Symmetrieelement, welches ausgebildet ist, im Betrieb des mikromechanischen Sensors eine fluiddynamische Symmetrie für das auslenkbare Element bereitzustellen.

According to a first aspect, the object is achieved with a micromechanical sensor, comprising:

• - a substrate;
• - A cap element arranged on the substrate; and
• - At least one deflectable element arranged in the cap element, an increased internal pressure prevailing in the cap element relative to the surroundings; marked by
• a symmetry element arranged within the cap element and not connected to the deflectable element, which is designed to provide a fluid dynamic symmetry for the deflectable element during operation of the micromechanical sensor.

The symmetry element is only intended to symmetrize the whole arrangement in terms of fluid dynamics. It can advantageously be achieved that fluid dynamic effects are harmonized for the deflectable element. The result of this is that differential pressures within the cap element are formed as uniformly as possible and thereby one Operating characteristics of the sensor can be significantly improved.

In this way, an improved insensitivity to thermal effects is advantageously realized for the micromechanical sensor. As a result, it is possible for the sensor to be deflected solely on the basis of mechanical inertial forces and not on the basis of force effects which have been generated due to thermal fluid effects.

• - Bereitstellen eines Substrats;
• - Bereitstellen eines auf dem Substrat angeordneten Kappenelements;
• - Bereitstellten wenigstens eines im Kappenelement angeordneten auslenkbaren Elements, wobei innerhalb des Kappenelements ein gegenüber der Umgebung definiert erhöhter Innendruck bereitgestellt wird; und
• - Bereitstellen eines innerhalb des Kappenelements angeordneten, nicht mit dem auslenkbaren Element verbundenen Symmetrieelements, welches ausgebildet wird, im Betrieb des mikromechanischen Sensors eine fluiddynamische Symmetrie für das auslenkbare Element bereitzustellen.

According to a second aspect, the object is achieved with a method for producing a micromechanical sensor, comprising the steps:

• - providing a substrate;
• - Providing a cap element arranged on the substrate;
• - Provided at least one deflectable element arranged in the cap element, an increased internal pressure being defined in relation to the surroundings being provided within the cap element; and
• Providing a symmetry element which is arranged within the cap element and is not connected to the deflectable element and which is designed to provide a fluid dynamic symmetry for the deflectable element during operation of the micromechanical sensor.

Preferred developments of the micromechanical sensor are the subject of dependent claims.

An advantageous development of the micromechanical sensor is characterized in that the deflectable element is a rocker. In this way, an advantageous application of fluid dynamic balancing for a z inertial sensor is provided.

Another advantageous development of the micromechanical sensor is characterized in that the symmetry element is fixed to the substrate by means of at least one connection element. This is preferably implemented in that the connecting element is guided through openings in the movable element and a type of “mushroom structure” is thereby implemented within the cap element.

Another advantageous development of the micromechanical sensor is characterized in that the symmetry element is fixed by means of at least one connecting element by a frame of the cap element. This provides an alternative embodiment, which provides that the symmetry element is fixed by means of at least one connecting element which is guided outwards through openings in a frame of the cap element. This advantageously supports a design-related variety of the micromechanical sensor.

A further advantageous development of the micromechanical sensor is characterized in that the symmetry element is fixed to the cap element by means of at least one connecting element. This provides a further implementation variant for the fluid dynamic symmetrization element.

A further advantageous development of the micromechanical sensor is characterized in that, by means of the symmetry element, masses on both sides of an axis of symmetry of the deflectable element are essentially identical. This realizes a specific dimensioning of the symmetry element with which the fluid dynamic symmetrization can be optimized.

Further advantageous developments of the micromechanical sensor are characterized in that the sensor is one of: acceleration sensor, rotation rate sensor, pressure sensor. As a result, the proposed concept can advantageously be used for different types of micromechanical sensors.

The invention is described in more detail below with further features and advantages using several figures. Identical or functionally identical elements have the same reference symbols. The figures are particularly intended to illustrate the principles essential to the invention and are not necessarily carried out to scale. For the sake of clarity, it can be provided that not all reference numbers are drawn in all the figures.

Process features disclosed arise analogously from corresponding disclosed device features and vice versa. This means in particular that features, technical advantages and designs relating to the method for producing a micromechanical sensor result analogously from corresponding designs, features and advantages relating to the micromechanical sensor and vice versa.

• 1 eine Draufsicht auf eine Ausführungsform eines vorgeschlagenen mikromechanischen Sensors;
• 2 eine Querschnittsansicht durch eine Wippenstruktur eines konventionellen mikromechanischen z-Inertialsensors;
• 3 eine Querschnittsansicht einer Ausführungsform des vorgeschlagenen mikromechanischen Sensors von 1; und
• 4 einen prinzipiellen Ablauf eines Verfahrens zum Herstellen eines vorgeschlagenen mikromechanischen Sensors.

The figures show:

• 1 a plan view of an embodiment of a proposed micromechanical sensor;
• 2 a cross-sectional view through a rocker structure of a conventional micromechanical z inertial sensor;
• 3 a cross-sectional view of an embodiment of the proposed micromechanical sensor of FIG 1 ; and
• 4 a basic sequence of a method for producing a proposed micromechanical sensor.

Description of embodiments

A key idea of the present invention is, in particular, to provide an improvement in a radiometric design for micromechanical sensors.

This is achieved by symmetrizing the structures of the sensor that are relevant to fluid dynamics, as a result of which time buoyancy effects in MEMS structures are minimized, which can arise if a heat source is present. As a result, undesirable changes in sensor parameters, such as e.g. Output offsets, avoided or at least minimized.

The above-mentioned buoyancy effects arise either in the case of asymmetrical geometries of the sensor due to uniformly acting thermal effects and / or in the case of symmetrical geometries of the sensor due to asymmetrical acting thermal effects. Mixed forms of both variants mentioned are also conceivable.

The invention relates in particular to MEMS elements which are hermetically sealed within a cap element and whose internal pressure is in the continuum range of fluid dynamics, which is characterized in that gas molecules interact with one another rather than with the walls of the cap element.

This is also the reason why a thermodynamic equilibrium is established in the molecular structure. When the gases are highly diluted, the molecules tend to interact with the walls of the cap member and the dynamics are dominated by these bumps. In such a case, gas states do not spread via the sensor. The local gas density is influenced by the application of a symmetrizing structure and thus exactly the area in which the sensor works.

If the thermal balance of such MEMS elements is disturbed (e.g. by switching an adjacent, electronically active circuit on and off), a new pressure balance must be established. If the structure is not symmetrical in its fluid dynamically relevant parts, transient, fluid dynamic effects can be observed in the time until the new equilibrium is established. As a result, different pressures, which can cause an incorrect signal from the sensor, arise in this way at different points within the sensor due to different heating.

2 shows a cross-sectional view of a rocker structure of a conventional micromechanical z-acceleration sensor, which is particularly susceptible to the unfavorable effects mentioned.

Another example of such an effect are characteristic curves of micromechanical sensors which are subject to run-in drift, as they are e.g. occur with MEMS acceleration sensors. Such sensors generally use an asymmetrical mass and a symmetrical detection area.

It is proposed, by attaching a perforated mass that is not connected to the movable mass, to symmetrize the entire arrangement in terms of fluid dynamics.

The following detailed description of the measure according to the invention takes place with reference to a typical z-acceleration sensor for automotive applications. However, it is also conceivable that the proposed measure can be used for other encapsulated sensors, not shown in the figures, which have movable micromechanical elements and which have an internal pressure that differs from the ambient pressure.

1 shows a top view of an embodiment of a proposed micromechanical sensor 100 in the form of a z-acceleration sensor. You can see one around a torsion spring 10 arranged movable rocker structure 20 whose deflection towards electrodes 30 eg capacitively recorded and evaluated. The seesaw structure 20 is with respect to a mass distribution compared to the torsion spring 10 asymmetrical. One recognizes an element of symmetry 40 , which on the substrate (not shown) by means of two connecting elements 41 is fixed. This is achieved through openings in the rocker structure 20 that an unimpeded movement of the seesaw structure 20 enable.

The movable MEMS structures (seismic mass in the form of the rocker) are sealed in a manufacturing process with a cap wafer (not shown). Depending on the application A suitable internal pressure, defined in relation to the environment, is enclosed within the volume closed thereby, the closure usually being carried out via a seal glass bonding process or via a eutectic bonding process, for example with AlGe. In order to produce a z-acceleration sensor in such a manufacturing process, a rocker structure is formed in the micromechanical functional layer, which is applied via the torsion spring 10 is anchored to the substrate. The mass distribution of the rocker structure is asymmetrical, with two electrode surfaces below the rocker structure 30 are arranged to deflect the rocker structure 20 to be able to measure capacitively.

3 shows a cross-sectional view of the proposed micromechanical sensor of FIG 1 , One can see that the symmetry element 40 on the right side of the seesaw 20 is designed such that the result is approximately the same mass as the mass of the rocker 20 on the left side of the torsion spring 10 has and via connection elements 41 is immovably connected to the substrate (not shown).

Another embodiment of the micromechanical sensor, not shown in the figures 100 can provide that the symmetry element 40 is fixed by the outer frame of the cap element by the protective frame of the rocker structure 20 is opened, in this way the symmetry element 40 by means of at least one connecting element 41 is fixed to the outside.

Another alternative embodiment provides that the symmetry element 40 by means of at least one connecting element 41 is attached to the cap element upwards (not shown).

As a result, all of the aforementioned variants enable the attachment of a mass element that is symmetrical in terms of fluid dynamics 40 for the micromechanical sensor 100 ,

Although the improvement of the radiometric design for design variants of acceleration sensors with a rocker structure based on a plurality of functional layers is disclosed above, which is laterally on a first side made of solid material (both functional layers one above the other) and on the second side, except for an outer frame and / or other reinforcing elements, consisting only of the lower, typically thinner functional layer, the proposed concept of the improved radiometric design cannot only be implemented for such sensors.
The invention can advantageously be implemented for all micromechanical sensors with movable elements enclosed in a cap under a defined increased internal pressure. For example, the invention can thereby be implemented for pressure sensors with a movable membrane, for acceleration sensors, rotation rate sensors, etc.

Dynamic effects that result from the fact that different temperatures or temperature gradients are present on the rocker between the top and bottom and gas flows are generated in the sensor can advantageously be symmetrized. As a result, tilting of the rocker due to thermal effects can at least be reduced, ideally avoided altogether.

An offset behavior of a z-inertial sensor designed in this way can be significantly improved in the result.

4 shows a basic sequence of the proposed method for producing a micromechanical sensor 100 ,

In one step 200 a substrate is provided.

In one step 210 a cap element arranged on the substrate is provided.

In one step 220 becomes at least one deflectable element arranged in the cap element 20 provided, wherein an increased internal pressure defined in relation to the environment is provided within the cap element.

In one step 230 is arranged inside the cap element, not with the deflectable element 20 connected symmetry element 40 provided, which is formed during operation of the micromechanical sensor 100 a fluid dynamic symmetry for the deflectable element 20 provide.

It goes without saying that the sequence of the steps mentioned can also be swapped in a suitable manner.

Although the invention has been described above on the basis of specific exemplary embodiments, the person skilled in the art can also implement embodiments which have not been disclosed or only partially disclosed without departing from the essence of the invention.

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• US 2012/0227494 A1 [0006]
• US 2015/0041927 A1 [0007]
• US 20151/0192603 A1 [0008]
• JP 2015068866 A [0009]

Claims (8)

Micromechanical sensor (100), comprising: - a substrate; - A cap element arranged on the substrate; and - at least one deflectable element (20) arranged in the cap element, an increased internal pressure prevailing in the cap element relative to the surroundings; characterized by - a symmetry element (40) which is arranged within the cap element and is not connected to the deflectable element (20) and which is designed to provide a fluid dynamic symmetry for the deflectable element (20) during operation of the micromechanical sensor (100). Micromechanical sensor (100) according to Claim 1 , characterized in that the deflectable element (20) is a rocker. Micromechanical sensor (100) according to Claim 2 , characterized in that the symmetry element (40) is fixed to the substrate by means of at least one connecting element (41). Micromechanical sensor (100) according to Claim 2 or 3 , characterized in that the symmetry element (40) is fixed by means of at least one connecting element (41) through a frame of the cap element. Micromechanical sensor (100) according to one of the Claims 2 to 4 , characterized in that the symmetry element (40) is fixed to the cap element by means of at least one connecting element (41). Micromechanical sensor (100) according to one of the Claims 2 to 5 , characterized in that by means of the symmetry element (40) masses on both sides of an axis of symmetry (10) of the deflectable element (20) are essentially identical. Micromechanical sensor (100) according to one of the preceding claims, characterized in that the sensor (100) is one of: acceleration sensor, rotation rate sensor, pressure sensor. Method for producing a micromechanical sensor (100), comprising the steps: - providing a substrate; - Providing a cap element arranged on the substrate; - Providing at least one deflectable element (20) arranged in the cap element, an increased internal pressure being provided in a defined manner in relation to the surroundings within the cap element; and - Providing a symmetry element (40) which is arranged within the cap element and is not connected to the deflectable element (20) and which is designed to provide a fluid dynamic symmetry for the deflectable element (20) during operation of the micromechanical sensor (100).

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