Patent Application
20120267730
The invention relates to a micro-electronic device, preferably a micro-electromechanical system (MEMS) device (e.g. a sensor for measuring acceleration or rotation speed or an actuator) comprising at least two substrates, at least one micro-electronic structure, preferably a movable structure in a MEMS-layer, wherein said micro-electronic structure is arranged in a cavity which is encapsulated between the at least two substrates. The invention also relates to a method for manufacturing such a device.
There is a wide range of MEMS devices for detecting accelerations e.g. U.S. Pat. No. 5,220,835 (Ford), U.S. Pat. No. 5,488,864 (Ford), U.S. Pat. No. 5,900,550 (Ford), U.S. Pat. No. 6,000,287 (Ford), U.S. Pat. No. 6,082,197 (Zexel), U.S. Pat. No. 6,308,568 (Murata). There are also sensor devices that are sensitive to accelerations in different directions.
U.S. Pat. No. 5,065,628 (Fraunhofer Gesellschaft) discloses a MEMS (Micro-ElectroMechanical System) device for measuring accelerations in several different directions. The accelerations are measured by three micromechanical sensors, each consisting of an inert mass (proof mass) that is sensitive to a motion in only one of the different directions e.g. x, y and z. The sensors are integrated in a single crystal surface layer.
Typically, a MEMS device comprises at least one movable structure such as a mobile mass or a mobile cantilever encapsulated in a cavity.
U.S. Pat. No. 5,623,099 (Temic Telefunken) shows a basic structure for packaging a capacitive acceleration sensor. Two semiconductor substrates are bonded together at their major surfaces by a thermally grown oxide. A cavity is formed between the two substrates for encapsulating a mobile mass sensing acceleration in one direction. Each substrate provides a part of the box-shaped cavity and the mobile element, which defines a variable capacity within the cavity, moves perpendicularly to the substrate. The thermally grown oxide provides an electrical insulation between the substrate plates.
U.S. Pat. No. 5,381,300 (Sextant Avionique) and U.S. Pat. No. 6,153,917 (Akebone Brake) propose an acceleration sensor that is made of three silicon plates welded together by an annealing process. Instead of using a single silicon oxide layer for bonding the substrates a sandwich consisting of first insulating layer, a conducting layer and a second insulating layer is applied. The sandwich structure avoids stray capacities influencing the measurements.
For multiple axes sensor devices, one of the key points is cross-axis sensitivity. The cross-axis sensitivity determines how much acceleration for an accelerometer (or rotation speed for a gyrometer), perpendicular to the measuring axis, couples to the output. Typically, cross-axis sensitivity should be not more than 1% and preferably substantially less than 1% on best performing devices, and less than 5% for low-performing devices.
It is an object of the invention to provide a micro-electronic device, preferably a micro-electromechanical system (MEMS) device which is insensitive to electro-magnetic interferences. Preferably, the device should be adapted for integrating two or more movables structures or two or more micro-electric circuits (or combinations thereof) in a single chip. The chip may comprise a sensor and/or an actuator and/or an electronic circuit. It is a further object to reach electrical decoupling of separate devices (preferably of different sensors or actuators or electronic circuits) arranged in close proximity by a minimum consumption of substrate area. It is a further object of the invention to provide a process for manufacturing such a device.
According to the invention, the above mentioned objectives are achieved by a micro-electronic device, preferably a MEMS device comprising:
The method for manufacturing such a device comprises the steps of:
In the framework of the present invention a “micro-electronic” device is meant to be a general term encompassing “micro-electromechanical systems (MEMS)” as well as “micro-electronic circuits”.
According to the invention a substrate may be a single or multi-layer body that can be handled and processed as a self-containing element and that can be bonded face-to-face to the other substrate in the course of a bonding step. Each substrate of the invention may be a single unit or may consist of two substrates or may be a layered element such as an SOI (Silicon On Insulator) structure or an element coated with thin layers. All these types of substrates are known in the prior art (see also introductory chapter “background, prior art” of this specification). Typically the substrates mainly consist of a semiconductor material such as crystalline silicon defining the bulk material of the substrate. The outer surface of the substrate (i.e. the surface that is not at the interface to the frame and not inside the cavity) should have a resistance less than 5 Ohm in order to enable the Faraday cage effect. With typical thickness of several 100 μm, preferably in the range of 300-450 μm, this is leading to a resistivity less than 50 mOhm×cm.
In the framework of the present invention, a movable structure comprises a mobile mass or a mobile cantilever supported by a flexible suspension element at a distance to the walls of the cavity. The movable structure may therefore oscillate or rotate about one or more geometric axis (x, y, z) or may be deflected from a still (inactive) position by electrostatic forces. The movement of the mobile element may be initiated by inertial forces or by electrical fields. According to the invention, the movable structure is implemented in a MEMS layer. The MEMS layer may be a separate substrate or may be formed on top of one of the two substrates.
The electrically conducting frame is at an interface between the two substrates. It defines a substantially closed ring or hedge around the movable structure in the MEMS-layer. In the top plan view, the ring or hedge may have a rectangular shape or any polygonal shape running around the movable structure.
The electrically conducting frame is part of a so-called Faraday cage formed by the two substrates and the frame. The Faraday cage does not have to be perfectly closed in order to perform the intended electromagnetic shielding effect. It is to be remembered that the electromagnetic shielding affect of a Faraday cage may also be achieved by a mesh-type structure, meaning that a local opening in the Faraday should not be a problem as long as its typical dimension is less than the wavelength of the electromagnetic field (i.e. less than 300 μm for a frequency of 1 THz). Such openings may be necessary for electrically contacting the mobile structure or the electrodes inside the cavity from outside of the Faraday cage.
Generally speaking, the proposed structure has the effect of a Faraday cage keeping electrical stray fields and electromagnetic disturbances away from the movable structure inside the cavity and from the electrical measurements and/or from electrical control elements inside the cavity. Preferably, by using an electrically conducting wafer sealing (such as a metallic sealing) it is possible to use a seal width as low as 30 μm Therefore, it is possible to ensure a perfect seal around each movable structure (which may comprise a proof mass or a cantilever switch) and to achieve (if desired) multi-movable structures with minimum cross-axis sensitivity by a minimum amount of substrate area.
Indeed, according to the prior art the standard seal between active wafer and cap wafer is performed with glass-frit technology, with a thickness of 5 μm and a width of around 200 μm (micron). Implementing a Faraday cage around each sensor has a huge impact on the die area and can decrease the cost of the MEMS devices (e.g. sensors or actuators) very substantially.
The invention results in devices that have higher performance and that can also be used in harsh environment (with a lot of electromagnetic perturbations), such as in automotive applications, without sacrificing die area, i.e. unit cost.
Optionally, the frame is part of the same MEMS-layer as the movable structure inside the cavity. This has the advantage that the production of the frame does not need an additional step in the production process. The MEMS-layer is typically made of silicon, preferably of mono crystalline silicon.
Optionally, the frame consists of crystalline silicon that is sufficiently high doped for achieving an adequate conductivity. The conductivity may be in the range of e.g. 10-20 mOhm×cm. The frame may have a width in a direction parallel to a main surface of the substrate of not more than 150 μm (micron). From the point of view of mechanical stability the width of the frame should be not less than 50 μm (micron). This provides sufficient mechanical stability during handling of the MEMS layer. An optimal range of the frame width for silicon structures is 50-150 μm (micron)
It is also possible to use a different material for the frame. For example, if the MEMS structure is not made from crystalline silicon, it may be advantageous not to use crystalline silicon for the frame but to use the same material as the MEMS structure. Of course, it is not mandatory to make the frame from the same material as the MEMS structure.
The width of the frame (i.e. of the beams of the frame) may be as small as possible to save die area. However, under certain conditions the width may also be above 50 or above 100 μm when the conductivity of the frame material is only moderate or if the metal sealing technology requires a larger area.
Optionally, the frame has a thickness (measured in a direction normal to the surface of the substrate, i.e. z-direction) that is within a thickness range defined by the mobile mass of the sensor. This means that the frame may be produced from the same layer as the mobile mass of the MEMS structure. For instance, the frame may be etched into the silicon layer at the periphery of the MEMS structure.
However, it is also possible to use the invention in combination with any type of mobile mass, in particular with a mobile mass that is not on the same level as the frame.
Connection Between Frame and First Substrate:
Optionally, there is a first electrically conducting connection at the interface plane between the frame and the first substrate. Said interface plane substantially corresponds to the plane of the main surface of the substrate. Metal stripes placed on the main surface of the first substrate may form the conducting connection and ensure the electrical contact between the first substrate and the frame. In the top plan view, the stripes are quasi buried under the frame. The stripes may define a closed rectangle or polygon line surrounding the MEMS structure of a sensor in the same way as the frame does.
Alternatively, the conducting connection between the first substrate and the frame may also be placed at the outside periphery of the frame. Further more, the conducting connection does not have to be formed as continuous metal stripes that are so to speak a foot-print of the shape of the frame. Electrical sealing may also be achieved by several separate contacting elements (contact pads).
Optionally, the second electrically conducting connection is provided by a conducting layer applied at the outer periphery of the frame and at a peripheral face of the second substrate. The peripheral face is a surface that is orthogonal to or inclined to the main face i.e. the x-y-face of the substrate. This connection is therefore not interposed between the mating main surfaces of the second substrate and of the frame. Rather more, it is arranged at the periphery of the device (i.e. outside the area of the frame) for electrically connecting the frame and the substrates to ground voltage.
Alternatively, it is possible to use a similar connection as described between the frame and the first substrate.
Inclined Peripheral Face:
Optionally, the peripheral surface of the second substrate has a section that is inclined at an angle of <90° with respect to the main surface of the substrate. The electrical connection is applied to the inclined section of the peripheral surface and to the peripheral face of the frame.
Nevertheless an alternative solution is to avoid usage of the above mentioned connection in order to save die surface. Of course in this case the Faraday cage surrounding the cavity containing the movable structure is not completely closed in terms of conductivity, but the gap of isolation is so small that the interferences are sufficiently small.
Optionally, the frame and the second substrate are connected (or bonded) at their mating main surfaces by an insulating bonding material, preferably by an oxide. This insulating layer may also be used to electrically isolate the MEMS structure (mobile mass, cantilever etc.) from the top substrate defining the cavities of the device. Alternatively, the insulating layer may also be an oxide layer. The bonding material may also be a polymer or a silicon or glass fit,
Optionally, the mobile mass and the frame are integrated in a third substrate, which is interposed between the first and the second substrate. The device may then consist of three substrates: The first and/or the second substrates are provided with cavities in the area where the third substrate has its mobile mass. So the mobile mass can move according to the acceleration applied or according to electromagnetic fields deflecting the movable structure.
Optionally, the frame and the MEMS structure are separated by a separation space (a trench in the MEMS layer, or a slot). This trench may define (in a top plan view of the frame) a rectangle or a polygon line running around the MEMS structure and it provides for electrical isolation. The separation space may be as narrow as a few microns (in a direction parallel to the surface).
In the z-direction (i.e. perpendicular to the surface of the substrate) the trench penetrates the MEMS layer completely.
Alternatively, the electrical isolation may be achieved by areas of non-conducting material between the frame and the MEMS structure.
Preferably, the device comprises two or more movable structures arranged side by side in close proximity. The frame surrounds the two or more movable structures and has a separating section (i.e. a separating beam) arranged between the movables structures. So the movable structures are electrically isolated form each other and from further external influences. Three movables structures may be arranged in one row, in a square area or rectangular area encompassing the movables structures. Other arrays are also possible, but they might be less efficient in using the available chip area.
Preferably the device is an acceleration sensor integrated in at least one of said at least two substrates. The patents U.S. Pat. No. 5,220,835 (Ford), U.S. Pat. No. 5,488,864 (Ford), U.S. Pat. No. 5,900,550 (Ford), U.S. Pat. No. 6,000,287 (Ford), U.S. Pat. No. 6,082,197 (Zexel), U.S. Pat. No. 6,308,568 (Murata), U.S. Pat. No. 5,377,544 (Motorola) show linear acceleration sensors and the patents EP 0 623 807 A1 (GM), U.S. Pat. No. 5,329,815 (Motorola) show rotation rate sensors. The whole variety of designs may be used for the movable structure. However, the invention is not limited to the designs shown in those patents.
Multi-Axis Sensor:
Optionally, the movable structures of a multi-axis device are sensitive to mutually different directions. Each movable structure is sensitive to only one acceleration direction. Combining three single-axis movable structures, each being sensitive to only one of the directions x, y and z, results in a most preferred embodiment.
It is also possible to use the frame in combination with rotation rate MEMS sensors. One or more of the movable structure may also be sensitive to a rotation (gyro) or to other type of physical variable that can be detected by capacitive measurements.
The device may also be an actuator, e.g. a switch. The movable structure may be a cantilever that is attracted to an electrode for making contact. When the attracting electrical field is removed, the cantilever moves back to its still position.
Movement Detection Electrodes:
Preferably, the first substrate comprises electrodes for detecting a movement of the mobile mass. The second substrate is then only a cap for protecting the MEMS structure. If the MEMS structure is an actuator there are control electrodes for changing the position of the cantilever element (still position vs. deflected position). The invention does not propose new electrode structures but refers to well known prior art electrode structures.
The method for manufacturing the device may involve one or more of the following steps.
Silicon direct bonding is a method well known in the art (see e.g. U.S. Pat. No. 7,442,622). The following detailed description and the combination of all claims present further advantageous embodiments and element combinations of the invention.
Any optional element that has been explained in relation to the MEMS system may also be used in combination with an electronic circuit in place of the movable structure.
The accompanying drawings further illustrate the invention and serve to explain the preferred embodiments in combination with the detailed specification:
a, b a schematic representation of a device with two movable structures arranged side by side;
a-c a schematic cross-section of a device with two movable structures in more detail;
Generally speaking, like reference numerals refer to identical parts.
a, b illustrate a possible embodiment of the invention.
The frame may have a width w (measured in x-direction) of the section 1.2. of e.g. 50 μm (micron). The sections 1.4 and 1.5 may have a different width (measured in y-direction). The same is true for the sections 1.1 and 1.3.
As shown in
The frame 1 is attached to the second substrate 6 by an insulating bonding layer 9 (e.g. silicon dioxide). The bonding layer 9 substantially covers all sections 1.1-1.5 of the frame 1. In the top plan view the area of the bonding layer 9 corresponds to the top plan view of the area of the frame 1 shown in
The MEMS structures of substrate 7 are connected to driving and sensing electrodes 12 arranged at the outside of the device on insulating layer 10.3 of substrate 5 via metallic sealing 11.1, 11.2. The details of the electrode configuration and the conductor path between the metallic sealing 11.1, 11.2 and the driving and sensing electrodes 12 are not shown, but can be implemented as known in the prior art. The invention is not directed to those details.
The frame 1 (in
In order to establish an electrical connection between the frame 1 and the substrate 6 there is a metallic pad 14 at the peripheral face 15 bridging the non-conducting bonding layer 9 between the main face of the substrate 6 and the frame 1. In
The cavities 8.1, 8.2 shown in
a-c illustrate a preferred method of manufacturing the device of the invention.
The frame sections 1.1-1.3 are separated from the MEMS structure of the sensors 3.1, 3.2 by narrow slots or trenches 16.1-16.4. The trenches 16.1 und 16.2 correspond to the separation space 2.1 of
b shows the substrate 5 carrying the electrodes 17.1, 17.2 for detecting the movement of the mobile mass of the MEMS structure. The metallic sealings 11.1, 11.2 are placed at the position where the MEMS structure needs to be contacted. In
c shows the device after bonding the two substrates 5 and 7 at their mating main surfaces 5.1 and 7.2 along the metallic sealings 13.1, 13.2, 13.2, 11.1, 11.2. The peripheral face 18 has been etched in the area of the frame 1 to get an inclined face. A metal layer may then be deposited through a mask M to provide the electrical contact between the frame and the top substrate 6.
The preferred embodiments illustrate the following three measures:
A cap wafer substrate 38 is fixed on top of the substrate 36 and forms for each electronic circuit 35.1, 35.2 a cavity 39.1, 39.2. A silicon frame 40 is bonded to the cap wafer substrate 38 and has two openings 41.1, 41.2 aligned with the cavities 39.1, 39.2 and having the same size as the cavities 39.1, 39.2. The openings 41.1, 41.2 are larger than the area of the electronic circuits 35.1, 35.2 so that there is no mechanical or electrical contact between the frame 40 and the electronic circuits 35.1, 35.2. The bonding layer 42 between the frame 40 and the cap wafer substrate 38 may be an insulator. In fact, the frame 40 may be implemented on the substrate 38 in the manner of a SOI (silicon on insulator) structure.
The frame 40 and the substrate 36 are electrically connected by a metallic sealing 43. The metallic sealing 43 forms a closed line around each electronic circuit 35.1, 35.2. Only at local spots, where there is an electric connection 45.1, 45.2 from the electronic circuit 35.1, 35.2 to an electrode pad 44.1, 44.2 arranged outside of the frame 40, is there a small opening in the Faraday cage structure. To electrically separate the electric connection 45.1 from the frame 40 and the sealing there is e.g. a thin insulating layer 46 between the electric connection 45.1 and the metallic sealing 43.
The electric connection between the structure (e.g. mobile mass, electronic circuit) inside the cavity and the contact pad at the outside and in particular the manner in which the Faraday cage is made around this local electrical opening may be the same for MEMS structure and for electronic circuits. Accordingly, the details shown in
The invention is not limited to the embodiments shown in the drawings. The geometric arrangement of the sensors depends on the characteristics of the sensors (required chip area, particular two-dimensional shape, place of contact electrodes etc.) and on the desired efficient use of die area. Instead of using SOI-technology for providing the MEMS structure with cavities below the mobile masses it is also possible to use other techniques that require only one substrate instead of two. The invention can be applied to many different movable structures e.g. as shown in the prior art cited in the “Background, prior art” section of this specification. The movable mass may also be an actuator e.g. an RF switch (see (see e.g. U.S. Pat. No. 7,501,911).
While it is preferable that each movable structure is in a separate cavity which is protected by a Faraday cage structure according to the invention, it is also possible to have two movable structures within the same cavity. More generally speaking, the invention can be applied whenever a cavity on a microelectronic level has to be shielded against external electromagnetic fields.
The external metal layer for electrically connecting the frame may be arranged on one or several spots along the connection between the second and third substrate. It may also be a continuous line around the frame.
In summary, the invention provides a technical solution to minimize electrical interference between adjacent sensors or actuators.
| Number | Date | Country | Kind |
|---|---|---|---|
| 11 003 325.5 | Apr 2011 | EP | regional |
