Patent Description:
Generally, MEMS devices (also referred to as Micro-Electro-Mechanical, microelectronic and microelectromechanical systems, micro-mechatronics, etc.) are known. Conventional MEMS mirrors are of great interest for a wide range of applications such as imaging devices, optical networks, adaptive light sources, Light Detection and Ranging (LIDAR) applications, material processing, etc. In such MEMS mirrors, it is necessary to monitor the position (e.g., the tilt angle) of the beam steering element (mirror) with high precision. For example, in optical network applications of the MEMS, it is required to control the tilt angle with the highest accuracy. However, in the case of larger angles, as well as for the case of closely packaged MEMS mirror arrays, a robust and low cost technology for precise tilting and monitoring the tilt is not available yet.

Conventionally, capacitive position sensing is a widely used integrated technique. <FIG> schematically illustrates a cross-section through a conventional MEMS mirror device <NUM> utilizing parallel-plate capacitive sensing with fixed electrodes <NUM> beneath the movable mirror <NUM> included in a fixed frame <NUM>.

<FIG> schematically illustrates a 3D view of a conventional MEMS mirror device <NUM> including comb-type capacitive electrodes <NUM> for both of the actuation and the sensing being arranged in a plane with the mirror <NUM>.

Typically, the conventional MEMS mirrors are based on either planar sensing electrodes located beneath the movable mirror (e.g., device <NUM> in <FIG>) or the comb-type electrodes arranged in the plane with the mirror (e.g., device <NUM> in <FIG>).

The main advantages of the capacitive sensing are the easy integration of the required electrodes into MEMS processes, and the matureness of the readout electronics that allow resolutions down to the femtofarad (fF) range. However, the conventional MEMS mirrors have the disadvantage that they are not suitable for large tilt angles. For example, in the case of the parallel-plate configuration, a large tilt angle requires a correspondingly large gap between the movable mirror plate and the sensing electrodes. The measured capacitance is inversely proportional to this spacing. Moreover, in order to ensure a good sensitivity in the relative readout capacitance of C1/C2, and despite the large gap size, the very large electrode areas are needed. Consequently, the large electrodes areas may increase the overall size of the device.

Conventionally, the comb-type electrodes are applied since they can be relatively compact, e.g., due to the constant small gap between the opposite electrodes. Furthermore, for the low tilt angles a good sensitivity may be achieved. For instance, for the tilt angles, a reproducibility below <NUM>° is reported for a range of ±<NUM>°. For the larger tilt angles, the sensitivity may significantly decrease due to the reduced overlap of the electrodes.

Alternatively, some conventional devices are known that are using the piezoresistive sensing. In such devices, the piezoresistors may be integrated into springs that support the mirror plate. This technologically is by far more complex as the fabrication of electrodes for the capacitive sensing. Another disadvantage is the larger device size. However, a good tilt angle resolution may be obtained.

<FIG> schematically illustrates a conventional MEMS mirror <NUM> based on the electromagnetic actuation, and, <FIG> schematically illustrates a conventional MEMS mirror <NUM> based on the piezoresistor placement.

In the conventional MEMS mirror <NUM> illustrated in <FIG>, four wheatstone bridges with sixteen piezoresistors are required, in total, in order to ensure a full in-plane control with the high sensitivity.

Moreover, a minimum detectable angle of approximately <NUM>° (<NUM>µrad) may be measured. The reproducibility of the tilt angle may be, for example, as good as <NUM>° within a <NUM> mrad range (about <NUM>°). This rather small tilt angle may be due to the particular applied actuation mechanism. In addition, a significantly larger ranges may be obtained with the comparable high resolution using piezoelectric sensing.

In addition, apart from the described techniques, the tilt angle may be monitored using an external Position Sensitive Device (PSD), for example, photodiode-based. Furthermore, for applications that require large mirror arrays, using an external PSD is typically very expensive. In order to overcome this restrain, an integrated on-chip optical PSD may been used. Alternatively, the sound produced by the MEMS mirror may be utilized for the position sensing by applying a microphone as the PSD.

Conventional devices (on the scale of mm to m) are known that use magnetic position sensing. For example, the magnetic principles are known to be applied for e.g., various actuators, MEMS resonators, and tactile sensors, etc. Magnetic position sensing provides several advantages, for example, it is precise, cheaper than the optical sensing, and is insensitive to contaminations. However, the conventional devices have several drawbacks due to, for example, the lack of suitable micro-magnets (e.g., since magnetic forces scale with the volume, larger magnets are advantageous which cannot be easily produced). The traditional sintering techniques are suitable only for larger magnets and not for the micro-magnets. The common deposition processes of semiconductor technology provide only thin layers and the volume of obtained magnets may be very low. Moreover, the integration of the micro-magnets on planar substrates is not known.

AND NANOPHOTONICS (OMN), IEEE, <NUM> August <NUM> (<NUM>-<NUM>-<NUM>), pages <NUM>-<NUM>, discloses a 2D MEMS scanner with a rotation-angle detector for a time-of-flight image sensor.

<CIT> discloses a MEMS device. The MEMS device applies a magnetic field to the magnetic material, and detects the magnetic field of the magnetic material.

The objective of the present invention is achieved by the solution provided in the enclosed independent claims.

The present invention is defined by a Micro-Electro-Mechanical-System (MEMS) mirror according to independent claim <NUM>, and a method for determining a position of a moveable mirror according to independent claim <NUM>.

A first aspect of the invention, as defined by claim <NUM>, provides a device, in particular a Micro-Electro-Mechanical-System, MEMS, mirror, comprising a movable structure configured to rotate around at least one axis of rotation; one cylindrical micro-magnet connected to the moveable structure;.

For example, in some embodiments, the distance between the micro-magnet and the arrangement of magnetic field sensors may be between <NUM> to <NUM>. In some embodiments, the distance between the micro-magnet and the arrangement of magnetic field sensors may be between <NUM> and <NUM>, or between <NUM> and <NUM>, etc..

The device of the first aspect may provide a magnetic sensing (e.g., for the MEMS mirrors), for example, based on a high-flux micro-magnet in combination with an arrangement of the magnetic field sensors.

The micro-magnet may have a predefined size, volume and structure. The micro-magnet is connected to the moveable structure. For example, it may be integrated in the moveable structure, it may be mechanically connected to the moveable structure, fixed to the moveable structure, etc. Moreover, the micro-magnet included in the device may produce a magnetic field. The magnetic field produced by the micro-magnet may be sensed by the arrangement of the magnetic field sensors. For example, a direction and/or a magnitude of the magnetic field distribution may be sensed depending on the amount of rotation of the movable axis around the axis of rotation.

In an implementation form of the first aspect, at least two magnetic field sensors from the arrangement of the two or more magnetic field sensors are arranged symmetrically with respect to a default position of the micro-magnet.

For example, in some embodiments, the arrangement of the magnetic field sensors may be such that the at least two magnetic field sensors may be arranged arbitrarily. For instance, the arrangement may be symmetrical with respect to the micro-magnet. The default position of the micro-magnet may be a predefined position, e.g., in which the direction of the magnetic field distribution and/or the magnitude of the magnetic field distribution is known.

In a further implementation form of the first aspect, at least two magnetic field sensors from the arrangement of the two or more magnetic field sensors are arranged in a two-dimensional array.

In a further implementation form of the first aspect, the arrangement of the two or more magnetic field sensors comprises two or more magnetic elements integrated on one circuit and/or fabricated in parallel on a substrate.

In a further implementation form of the first aspect, the circuit is fabricated by a common semiconductor process.

For example, the integrated micro-magnets may be fabricated in parallel on the wafer level.

In a further implementation form of the first aspect, the arrangement of the two or more magnetic field sensors is based on one or more of:.

For example, the magnetic field sensors may be of any known type (Hall, AMR or GMR sensors as well as the MAGFETs). In some embodiments, it may be an array of many single elements integrated on one circuit. In some embodiments, the circuit may be fabricated using a common semiconductor process such as an Atomic Layer Deposition (ALD), a Chemical Vapor Deposition (CVD), a Physical Vapor Deposition (PVD), a Plasma Enhanced Chemical Vapor Deposition (PE-CVD), etc., without limiting the invention to a specific semiconductor process. For example, all sensor elements of the arrangement may be within one plane.

In a further implementation form of the first aspect, the movable structure comprises a reflective surface configured to reflect a beam of light.

In a further implementation form of the first aspect, a determined angle of rotation of the movable structure around the at least one axis of rotation corresponds to a determined magnetic field distribution sensed by the arrangement of the two or more magnetic field sensors.

In a further implementation form of the first aspect, the micro-magnet has a length in the range between <NUM> and <NUM>.

In a further implementation form of the first aspect, the micro-magnet has a width in the range between <NUM> and <NUM>.

A second aspect of the invention provides a method for determining a position of a moveable structure.

<FIG> schematically illustrates a device <NUM> according to various embodiments of the invention.

The device <NUM> is exemplary based on a MEMS mirror. The device <NUM> comprises a movable structure <NUM> configured to rotate around at least one axis of rotation <NUM> indicated by the dotted line, exemplarily into the plane.

The device <NUM> further comprises a micro-magnet <NUM> connected to the moveable structure <NUM>; wherein a rotation of the moveable structure <NUM> around the at least one axis of rotation <NUM> rotates the micro-magnet <NUM> around the same axis of rotation <NUM>.

The device <NUM> further comprises a fixed structure <NUM> comprising an arrangement of two or more magnetic field sensors <NUM> positioned at a certain distance (hmag) below the micro-magnet <NUM>, wherein the arrangement of the magnetic field sensors <NUM> is configured to sense a change of a magnetic field distribution caused by a rotation of the micro-magnet <NUM>.

The device may be a MEMS mirror, a micro mirror device, etc. In some embodiments, the micro-magnet may be integrated into the moveable structure. The micro-magnet may produce a magnetic field. Moreover, when the moveable structure (e.g., the integrated micro-magnet to the moveable structure) rotates, the magnetic field distribution may change. The change of the magnetic field distribution may be sensed by the arrangement of the magnetic field sensors, and the position of the moveable structure may be determined.

<FIG> schematically illustrates a device <NUM> in the form of a MEMS mirror according to various embodiments of the invention.

The schematic cross-section through the micro mirror <NUM> illustrates one micro-magnet <NUM> and an arrangement of the magnetic field sensors <NUM>, which may be used for magnetic position sensing, determining the position of the moveable structure, adjusting the tilt angle, etc..

In the device <NUM> of the <FIG>, the micro-magnet <NUM> is integrated at the lower side of the movable structure <NUM> of the micro mirror device <NUM>. Moreover, due to the rigid connection of the micro-magnet <NUM> to the movable mirror <NUM>, the micro-magnet <NUM> rotates around the same axis as the moveable mirror, e.g., during the operation and/or the rotation, etc. The different component of the device are included in a frame <NUM>.

The moveable structure <NUM> of the device <NUM> comprises a reflective surface (e.g., a moveable mirror plate) configured to reflect a beam of light.

In addition, the change in the position and/or the orientation of the moveable structure <NUM> (e.g., the mirror plate) may result in a change in the magnetic field seen by the arrangement of the magnetic field sensors <NUM>, for example, the arrangement of the magnetic field sensors <NUM> may sense the change of the magnetic field distribution caused by the rotation of the micro-magnet <NUM>.

Furthermore, the magnetic field detected by the arrangement of magnetic field sensors <NUM> may be utilized, and the position and/or the orientation of the micro-magnet and/or the mirror plate may be tracked.

<FIG> schematically illustrates the device <NUM> in the form of MEMS mirror for sensing magnetic field distribution by an arrangement of two Hall sensors, according to various embodiments of the invention.

The exemplary realization of the proposed sensing solution is illustrated in <FIG> in which two Hall sensors HS1 and HS2 are utilized in the arrangement of the magnetic field sensors <NUM>. The Hall sensors are located in the sensor plane at a distance hmag from the lower end of the micro-magnet <NUM> in the non-deflected state. Moreover, the distance between the lower end of the micro-magnet <NUM> and the rotation center of the movable mirror plate <NUM> (e.g., the moveable structure) is rmag.

<FIG> schematically illustrates the device <NUM> in the form of MEMS mirror with two axes of rotation <NUM>, according to various embodiments of the invention. The device <NUM> includes a single micro-magnet <NUM> integrated into the movable structure <NUM> and four Hall sensors beneath <NUM>.

The arrangement of the magnetic field sensors (e.g., as it is illustrated in the embodiment of the <FIG>) may be extended, in order to monitor the tilt angle of, for example, the micro-magnet <NUM> and/or the moveable structure <NUM> and/or the moveable mirror) included in the micro mirror device <NUM> in the two independent axis <NUM>.

In addition, two Hall sensors of HS3 and HS4, are placed in the arrangement of the magnetic field sensors. The two Hall sensors HS3 and HS4 are further rotated by <NUM>° with respect to the other two sensors (e.g., HS1 and HS2) within the sensor plane, in order to enable measuring in the second axis <NUM>.

In the following, in order to estimate the performance of the proposed sensing solution (i.e., the change of the magnetic field distribution caused by the rotation of the micro-magnet), the magnetic field of a cylindrical micro-magnet having a diameter of <NUM> and with different lengths are simulated, numerically, e.g., based on the arrangement and/or the configuration of the device <NUM> illustrated in <FIG>, without limiting the invention to a specific configuration and/or a specific arrangement of the magnetic field sensors, the micro-magnet, etc..

In <FIG> the normalized magnetic flux along the axis of the cylindrical shape (which is parallel to the magnetization direction) is plotted starting at the lower end of the micro-magnet. The micro-magnet <NUM> is considered to be based on an NdFeB magnet, without limiting the invention to a specific micro-magnets.

<FIG> illustrates the simulated normed magnetic flux density (B) as a function of the distance from the lower edge of the micro-magnet with <NUM> diameter and with various length (L) being fabricated from the NdFeB powder using agglomeration by Atomic Layer Deposition (ALD).

As can be derived from <FIG>, even the smallest powder-based NdFeB magnets generate a considerable magnetic field over tens of microns distance. The generated magnetic field is sufficient for the detection with the Hall sensors.

In some embodiments, the volume shaped micro-magnets may be used. Moreover, the effects of using volume shaped micro-magnets may also be derived. For example, in the illustration of the <FIG>, it may be derived that the magnetic field strength increases significantly changing the aspect ratio of the cylindrical micro-magnet from <NUM>:<NUM> to <NUM>:<NUM>. A further increase may be observed for an aspect ratio of <NUM>:<NUM>. However, the benefits may be less. In some embodiments, micro-magnets with aspect ratio of at least <NUM>:<NUM> may be employed for the proposed sensing solution (e.g., sensing the change of the magnetic field distribution caused by the rotation of the micro-magnet). Producing such a micro-magnets require a thickness which is not achievable by using thin film technologies.

In the following, a cylindrical micro-magnet with diameter of <NUM> and an aspect ratio of <NUM>:<NUM> is considered, as an exemplary illustration of sensing the change of the magnetic field distribution. <FIG>, <FIG> illustrate the magnetic field distribution in the sensing plane at the distance of hmag= <NUM> for different tilt angles.

<FIG> schematically illustrate the micro mirror device in the non-titled ground state (e.g., the idle state), according to various embodiments of the invention. <FIG> illustrate the magnetic field distribution <NUM> for the micro mirror device <NUM> in the non-titled ground state, according to various embodiments of the invention.

The z-component of the magnetic flux (e.g., the magnetic field distribution) is exemplarily illustrated in conjunction with the aforementioned Hall sensors. The method for sensing the change of the magnetic field distribution also works for other magnetic field sensors, which may be sensitive to the x- and/or the y-component of the magnetic flux.

As can be derived from the <FIG>, the micro-magnet yields a strongly focused magnetic flux in the sensing plane with a peak values of up to <NUM> mT. The magnetic field is concentrated within a spot of about <NUM>. In some embodiments, Hall sensors with a geometrical extend of approximately <NUM> may be used. Moreover, the two Hall sensors conceived for this exemplary illustration of the sensing concept are placed <NUM> apart from each other and are indicated by <NUM> (white lines) in <FIG>. In the non-titled ground state of the micro mirror device (illustrated in <FIG>), both sensors see a low magnetic flux with equal magnitudes, in the z direction.

<FIG> schematically illustrate the micro mirror device with a tilt angle of <NUM>°, according to various embodiments of the invention. <FIG> illustrate the magnetic field distribution <NUM> for the micro mirror device with a tilt angle of <NUM>°, according to various embodiments of the invention.

For a center of rotation positioned at a distance of rmag = <NUM> from the lower end of the micro-magnet, a rotation of <NUM>° of the micro mirror device (and thus the micro-magnet) results in a lateral shift of <NUM> at the lower end of the micro-magnet. This rotation may strongly alter the magnetic field distribution in the sensing plane, as depicted in <FIG>. The spot of concentrated magnetic flux in z direction may now coincide with the sensing area of HS1, which may lead to a strong asymmetry distribution, in the signals measured by the two sensors. The decrease in the peak values of the z-component of the magnetic flux may be, for example, due to the increase in the distance to the sensing plane and the relative tilt (e.g., angle) of the micro-magnet caused by rotation.

In some embodiments, the position of the micro-magnet may be measured from the signals of the Hall sensor in a differential readout scheme. For example, the utilization of a differential signal may strongly reduce the impact of the environmental influences, such as the magnetic stray fields or the temperature fluctuations on the accuracy of the sensing the change of the magnetic field distribution and/or determining the position of the moving structure.

In some embodiments, the achievable resolution of sensing the change of the magnetic field distribution and/or determining the position of the moving structure may depends on the performance of the employed magnetic field sensors, the geometry of the utilized micro-magnet, the geometric parameters e.g., the rmag and hmag.

In some embodiments, different magnetic field sensors, e.g., the AMR and the GMR sensors may be used. Moreover, the sensor elements may be integrated on the chip-level.

<FIG> shows a method <NUM> according to an embodiment of the invention for determining a position of a moveable structure <NUM>. The method <NUM> may be carried out by using and/or by means of the device <NUM>, as it described above.

The method <NUM> comprises a step <NUM> of sensing, by an arrangement of two or more magnetic field sensors <NUM> of a fixed structure <NUM> positioned at a certain distance (hmag) below a micro-magnet <NUM> connected to the moveable structure <NUM>, a change of a magnetic field distribution caused by a rotation of the micro-magnet <NUM> around at least one axis of rotation <NUM>.

The method <NUM> further comprises a step <NUM> of determining the angle of rotation of the movable structure <NUM> around the at least one axis of rotation <NUM> and/or the position of the moveable structure <NUM>, based on the sensed change of the magnetic field distribution.

Claim 1:
Device (<NUM>), in particular a Micro-Electro-Mechanical-System, MEMS, mirror, comprising:
a movable structure (<NUM>) configured to rotate around at least one axis of rotation (<NUM>);
one micro-magnet (<NUM>) connected to the moveable structure (<NUM>), wherein the micro-magnet (<NUM>) is cylindrical and has an aspect ratio larger than <NUM>:<NUM>;
wherein a rotation of the moveable structure (<NUM>) around the at least one axis of rotation (<NUM>) rotates the micro-magnet (<NUM>) around the same axis of rotation (<NUM>); and
a fixed structure (<NUM>) comprising an arrangement of two or more magnetic field sensors (<NUM>) positioned at a certain distance (hmag) below the micro-magnet (<NUM>), wherein the distance (hmag) between the fixed structure (<NUM>) and the micro-magnet (<NUM>) is in the range between <NUM> to <NUM>,
wherein the arrangement of the magnetic field sensors (<NUM>) is configured to sense a change of a magnetic field distribution caused by a rotation of the micro-magnet (<NUM>).