Patent Application
20110234218
N/A
Sensors to detect the earth's local magnetic field have been proposed and produced in large volume in the past. Some of these sensors feature two-axis sensing, while more sophisticated ones feature three-axis sensing. Different technologies are commonly used to detect such low strength magnetic fields. One of the two most common type of sensor is the magnetoresistive (MR) sensor.
The construction of a magnetoresistive sensor is well known where, generally, the resistivity of the sensor varies according to a local magnetic field oriented in the same plane as the magnetoresistance. “Barber-pole” structures are added to allow a sensing of the magnetic field along one axis to include direction, or vector, information. Magnetoresistive sensors have been used successfully in electronic compass applications, using two sensors to detect the magnetic field in the same plane as the surface they are mounted on, (X, Y), with an additional sensor mounted in a particular way so that the sensitive element is properly aligned to sense the component of the magnetic field orthogonal (Z) to the plane of the system.
In some known systems, the orthogonal (Z) axis sensitive sensor is mounted on a pre-cut printed circuit board (PCB) in the same plane as the other sensors and then folded orthogonally to that plane before being encapsulated. In some other known systems, the three sensors are encapsulated separately before being soldered on a PCB as a module. In this case, the orthogonal (Z) axis sensor is mounted along the axis orthogonal to the PCB directly rather than along the plane, as in, for example, U.S. Pat. No. 7,271,586. This particular orthogonal axis sensor mounting, however, can be technically challenging, and significantly increases the cost of manufacturing, as well as results in an increase in the thickness of the final product.
An alternative solution is to deposit magnetoresistive layers on an inclined plane on a substrate, as found in U.S. Patent Publication 2009/0027048. Microtechnology, however, is not well adapted to precisely control structure geometry on inclined planes, and renders the manufacturing of such sensors technically challenging.
An additional known approach consists of changing the fabrication process of the magnetic field sensor so that it becomes sensitive to the out-of-plane magnetic field as taught in U.S. Pat. No. 6,577,124. This solution increases both the cost and complexity of the device while requiring a trade-off, i.e., a decrease, of the resultant measurement sensitivity.
Another technology used in low magnetic field sensing is based on the Hall effect. As known, Hall sensors use the deviation of an electron flow caused by a local magnetic field to generate a voltage difference across a conductive element in a direction orthogonal to the current path and the magnetic field. Hall sensors generally consume more power than magnetoresistive sensors due to the high current required to generate a measurable Hall voltage.
Recently, the number of applications where it is desirable to have a low-cost three-axis magnetic field sensor capable of accurately measuring the earth's local magnetic field integrated in a small package has significantly increased. When produced in large volume, these devices can be embedded in consumer products such as mobile phones and navigation systems, for example, and are used in combination with the Global Positioning System (“GPS”) as well as other products where small size and low cost per unit are important.
There is a need, therefore, for a low profile, inexpensive, but high performance, three-axis magnetic field sensor that can be produced in large volume using a simple manufacturing process.
The present invention proposes to combine two magnetoresistive sensors to measure the two components X, Y of the magnetic field parallel to the system's plane and a Hall sensor for the Z component of the magnetic field orthogonal to the system's plane.
In one embodiment, the two magnetoresistive sensors are built together in one single chip, and then stacked on top of a CMOS die embedding the Hall sensor and associated electronics for the signal processing management of the three sensors and the system's interface.
In such an arrangement, the three-axis sensors can provide sufficient sensitivity within a very low profile device while keeping the unit costs low and avoiding manufacturing complexity as compared to single technology solutions.
Various aspects of at least one embodiment of the present invention are discussed below with reference to the accompanying figures. It will be appreciated that for simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn accurately or to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity or several physical components may be included in one functional block or element. Further, where considered appropriate, reference numerals may be repeated among the drawings to indicate corresponding or analogous elements. For purposes of clarity, not every component may be labeled in every drawing. The figures are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the invention. In the figures:
Embodiments of the present invention propose combinations of different sensor technologies, i.e., Hall effect and magnetoresistive, integrated in a single package to combine the advantages of the different technologies.
As a general overview, in various embodiments of the present invention, as will be explained in more detail below, two magnetoresistive sensors are used to measure planar components of the earth's magnetic field with respect to a plane defined by a substrate of the packaging. These two magnetoresistive sensors measure magnetic fields along two axes that are orthogonal to each other and co-planar, generally referred to as the X and Y directions or EX and EY. A Hall sensor, i.e., one that uses the Hall effect to measure a magnetic field along an axis, is used to measure the magnetic field that is orthogonal to the plane defined by the substrate of the packaging, generally referred to as the Z direction, or EZ. Various embodiments will be described below to provide more details about the different combinations and arrangements of the sensors and supporting circuitry.
One embodiment of the present invention will now be described with respect to the functional block diagram shown in
The two magnetoresistive sensors 102, 104 are oriented with respect to one another and the plane defined by the substrate 101 to measure orthogonal magnetic fields, i.e., EX and EY. The magnetoresistive sensors 102, 104 may be of the same construction and, in one embodiment, as detailed below, may be the same model of device. The Hall sensor is positioned to measure the Z direction, i.e., EZ.
In one embodiment, the ASIC 108 may be designed with 0.18 micrometer (μm) CMOS technology having a combination of metal layers and polysilicon layers. Further, the differential signals from each of the magnetoresistive sensors 102 and 104 may be fed, respectively, into two signal processing channels 110, 112 that provide, among other functions, low noise amplification, offset adjustment, sensitivity adjustment, temperature compensation and analog to digital conversion. The ASIC 108 may be designed such that the differential signal from the Hall sensor 106 is fed into a processing channel 114 to process those signals as is known in the art. The ASIC 108 may include an I2C digital communications module 116, operated in FAST mode, i.e., up to a 400 KHz clock rate, that eliminates the need for an external analog digital converter and provides a two-pin I2C interface to an external MCU (not shown).
One of ordinary skill in the art will understand that any one of a number of different technologies may be used to design a mixed-signal ASIC to operate in accordance with the embodiments described herein. Further, while an I2C interface has been described, any one or more of many known interfaces may be implemented instead. These interfaces can include, for example, one or more digital interrupt pins to communicate with an external MCU. The selection of a digital interface for the ASIC 108 is a design choice depending upon the needs of the system into which the ASIC will be placed.
The substrate 101, sensors 102-106 and the ASIC 108 may be covered with a potting material in order to provide a hybrid “system on a chip.” Thus, this complete three-axis sensor 100 can be inserted as a single part in a system that requires three-axis magnetic field measurements. Of course, although not shown, appropriate other connections would be provided such as power and ground/return.
Referring now to
In this system 200, the three sensors 102, 104 and 106 can use standard mounting processes on the PCB 101, e.g., using a die attach paste and wirebonding. The system 200 can further be encapsulated inside an encapsulant material using a molding or analog method to make a complete device. Some benefits of having all the chips mounted with respect to a plane include the possibility to thin down, i.e., reduce the height of the chips, to obtain a small profile for the system 200.
Embodiments of the present invention are not limited to the use of a PCB and encapsulant, as any encapsulant can be replaced by a ceramic package having a cavity, some pads, and a lid.
Those skilled in the art will also note that a two axis magnetic sensor system having one axis parallel to the plane made by the PCB 101 and one axis orthogonal to the plane made by the PCB 101 can be realized by removing, or disabling, one of the two magnetoresistive sensor chips 102, 104, for a specific application.
Finally, those skilled in the art will also note that the ASIC 108 can additionally integrate other types of CMOS compatible sensors, such as, for example, accelerometers and pressure sensors, to form a multi-sensor structure while maintaining the benefits of the present invention.
A three-axis magnetic field sensor system in a package according to a second embodiment of the present invention will be described with respect to
Similar to the discussion with respect to the first embodiment, the packaging, two axis solution, and multi-sensor system modifications are also applicable as variations of the second embodiment of the present invention shown in
A three-axis magnetic field sensor system according to a third embodiment of the present invention is presented in
Those skilled in the art will notice that such a sensor 404, combining two orthogonal magnetoresistive sensor elements, can be used in other embodiments of the present invention to replace the two magnetoresistive sensors.
Having a stack configuration, as shown in
The same remarks concerning the package, two axis solution, and multi-sensor systems mentioned with respect to the previous embodiments are still valid with respect to this embodiment.
A three-axis magnetic sensor system 500, according to a modification of the third embodiment of the present invention, is presented in
In this embodiment, the assembly complexity linked with the wirebonding together of the different devices is removed, and the total thickness is reduced by the height of the wires above the sensor surface compared to the device 400 in
Those skilled in the art will notice that a flip-chip method can be used in the previous embodiments in which the chips are mounted side-by-side so as to replace the wirebonding. Many combinations of devices and sensors mounted using the flip-chip method, or the die attach and wirebonding method, are possible and can be selected depending on different optimizations required by the final device.
Those skilled in the art will also notice that it is possible to keep a stack assembly of the different devices, either by die attach and wirebonding or flip-chip, and use some vias to connect the ASIC to the PCB to further reduce the lateral size of the complete system.
The same remarks with respect to the previous embodiments regarding the package, two axis solution and multi-sensor systems mentioned with respect to the previous embodiments are still valid in the current embodiment.
A schematic view of a magnetic field sensor system according to a fourth embodiment of the present invention will now be described with respect to
Each of the two magnetoresistive sensors in the MR device 608 has a high sensitivity for the amplitude of the component of the magnetic field along the respective axis but cannot detect the direction of the detected magnetic field. The respective direction is determined by the two Hall elements switches 606.1 and 606.2.
By eliminating the need for direction detection in the magnetoresistive sensors 608, the fabrication process is simpler and less expensive because there are no “barber pole” structures. The ASIC 604 combines the information from the two Hall switches 606.1, 606.2, in corresponding processing sections 610 and 612 with the information from the MR device 608 similar to that which is taught in U.S. Pat. No. 6,707,293 to detect the direction of the magnetic fields.
The same remarks as before concerning the packaging, two axis solutions, multi-sensor system, flip-chip combinations and through silicon via connections with the PCB 602 are applicable to this embodiment of the present invention.
In the above-described embodiments of the present invention, the combinations of different Hall sensor elements and magnetoresistive sensors allows for a low cost, small scale and high precision three-axis magnetic sensor.
Having thus described several features of at least one embodiment of the present invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.
