Patent Application
20110127998
Magnetic field sensors, such as magneto-resistive (XMR) sensors, are used in a variety of applications for sensing the rotation of a wheel and/or shaft, such as in anti-lock braking systems, crank-shaft sensors, cam-shaft sensors, etc., and for sensing vertical and/or angular movement, for example. XMR sensors include, for example, anisotropic magneto-resistive (AMR) type sensors, tunneling magneto-resistive (TMR) sensors, giant magneto-resistive (GMR) sensors, and colossal magneto-resistive (CMR) sensors. Typically, XMR type magnetic field sensors include one or more sensor elements, such as GMR sensor elements, which are formed as part of a semiconductor die which further includes integrated circuitry for evaluating parameters of the sensor (e.g. resistance of the GMR sensor elements). In turn, the semiconductor die is typically bonded to a carrier, such as a copper leadframe, for example, to form a magnetic field sensor package.
In the case of speed and/or angle sensors, the package further includes a permanent magnet which provides a back bias magnetic field for the XMR sensor elements. The magnetic field sensor is placed in front of a magnetically permeable toothwheel, the rotation of which generates a sinusoidally varying magnetic field on the magnetic field sensor. The XMR sensor elements detect variations in the component of the magnetic field parallel to the surface of the magnetic field sensor, with the detected magnetic field variations providing information about the angular position, rotational direction, and rotational speed of the toothwheel.
Often, the permanent magnetic is a permanent magnet attached to the backside of the magnetic field sensor, such as to a surface of the leadframe opposite the semiconductor die. However, attaching a permanent magnet in this fashion has a number of disadvantages. For example, the permanent magnet has position tolerances with respect to the magnetic field sensor package since the permanent magnet is typically attached after the completion of the fabrication of the magnetic field sensor package. Also, the glue used to attach the permanent magnet to the magnetic field sensor has to be selected carefully since the sensor assembly is typically exposed to a wide temperature range (e.g., −50° C.-170° C.). Additionally, the permanent magnet is typically attached to the magnetic field sensor package by someone other than the semiconductor manufacturer who fabricated the magnetic field sensor package, so that the permanent magnet is attached to the magnetic field sensor package after the final test of the magnetic field sensor package at the semiconductor manufacturer. Furthermore, the semiconductor manufacturer typically tests the magnetic field sensor package at several temperatures. However, the permanent magnet is typically not tested at several temperatures since the thermal mass of the entire sensor assembly is usually too large to perform an economical multi-temperature test.
In attempts to overcome such disadvantages, in lieu of attaching a permanent magnet to the backside of the leadframe, the semiconductor die and leadframe of some XMR sensors are embedded within a molded magnetic material. While such an approach eliminates many of the above described problems, it has been found that the ferromagnetic material of the leadframe (e.g. copper) produces distortions in the magnetic field or flux lines at the XMR elements which results in a horizontal component of the magnetic field, parallel to the surface of the magnetic field sensor (i.e. the sensed component), having a magnitude that result the XMR sensors being in saturation, thereby rendering the sensor inoperable.
For these and other reasons, there is a need for the present invention.
One embodiment provides an integrated circuit. The integrated circuit includes a leadframe, and a die having a top surface, a bottom surface, and a plurality of perimeter sides and including at least one magnetic field sensor element disposed proximate to the top surface, wherein the bottom surface is bonded to the leadframe. A molded magnetic material encapsulates the die and at least a portion of the leadframe, and provides a magnetic field substantially perpendicular to the top surface of the die. A non-magnetic material is disposed between the die and the molded magnetic material at least along perimeter sides of the die intersecting a lateral magnetic field component which is parallel to the top surface of the die.
The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain principles of embodiments. Other embodiments and many of the intended advantages of embodiments will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.
In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.
It is to be understood that the features of the various exemplary embodiments described herein may be combined with each other, unless specifically noted otherwise.
In addition, while a particular feature or aspect of one embodiment may be disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features or aspects of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “include,” “have,” “with,” or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprise.” Also, the term “exemplary” is merely meant as an example, rather than the best or optimal. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.
The embodiments of a method of fabricating a semiconductor device may use various types of semiconductor chips or semiconductor substrates, among them logic integrated circuits, analog integrated circuits, mixed signal integrated circuits, sensor circuits, MEMS (Micro-Electro-Mechanical Systems), power integrated circuits, chips with integrated passives, discrete passives and so on. In general the term “semiconductor chip” as used in this application can have different meanings one of which is a semiconductor die or semiconductor substrate including an electrical circuit.
In several embodiments layers are applied to one another or materials are applied or deposited onto layers. It should be appreciated that any such terms as “applied” or “deposited” are meant to cover literally all kinds and techniques of applying layers onto each other. In one embodiment, they are meant to cover techniques in which layers are applied at once as a whole, in for example, laminating techniques; as well as techniques in which layers are deposited in a sequential manner, in for example, sputtering, plating, molding, chemical vapor deposition (CVD) techniques. One example for a layer to be applied is a redistribution layer (RDL) patterned in electrical connection with contacts of a chip. The redistribution layer can be in the form of a multilayer, in particular a multilayer including a repeating layer sequence.
The semiconductor chips may include contact elements or contact pads on one or more of their outer surfaces wherein the contact elements serve for electrically contacting the semiconductor chips. The contact elements may be made from any electrically conducting material, e.g., from a metal as aluminum, gold, or copper, for example, or a metal alloy, e.g., solder alloy, or an electrically conducting organic material, or an electrically conducting semiconductor material.
In some embodiments the semiconductor chips are covered with an encapsulant material. The encapsulant material includes any electrically insulating material like, for example, any kind of molding material, any kind of epoxy material, or any kind of resin material with or without any kind of filler materials. In special cases it could be advantageous to use a conductive encapsulant material. In the process of covering the semiconductor chips or dies with the encapsulant material, a fan-out of embedded dies is fabricated. The fan-out of embedded dies is arranged in an array having the form of a wafer and is referred to as a “re-configured wafer.” However, it should be appreciated that the fan-out of embedded dies is not limited to the form and shape of a wafer but can have any size and shape and any suitable array of semiconductor chips embedded therein.
In the claims and in the following description different embodiments of a method of fabricating a semiconductor device are described as a particular sequence of processes or measures, in particular in the flow diagrams. It is to be noted that the embodiments should not be limited to the particular sequence described. Particular ones or all of different processes or measures can also be conducted simultaneously or in any other useful and appropriate sequence.
Embodiments described herein provide a magnetic field sensor employing non-magnetic material to space a molded magnetic material providing a bias magnetic field from one or more magnetoresistive (XMR) elements so that a magnitude of a component of the bias magnetic field parallel to the one or more XMR elements is less than a threshold level, such as a saturation point of the one or more XMR elements.
According to one embodiment, leadframe 102 is formed of a ferromagnetic material, such as copper, for example. According to one embodiment, XMR elements 106a and 106b, are giant magnetoresistive (GMR) elements fabricated on a top surface 116 of magnetic field sensor die 104. In other embodiments, XMR elements 106a and 106b comprise other types of XMR elements such as anisotropic magneto-resistive (AMR) elements, giant magneto-resistive (GMR) sensors, colossal magneto-resistive (CMR) elements, and tunneling magneto-resistive (TMR) elements, for example.
Leadframe 102, magnetic field sensor die 104, bond wires 112, and portions of pins 110 are encased within a molded magnetic material 120 which, as described below, is magnetized to form a plastic bonded magnet. For illustrative purposes, it is noted that only portions of molded magnetic material 120 are shown in the top view of
According to one embodiment, molded magnetic material 120 includes a fine powder of a permanent magnet material, such as a hard ferrite material with Ba or Sm, a rare earth metal (e.g. NdFeB, Sm2Col7), or other suitable magnetic material, which is mixed with an epoxy binder such as a polymide (PA), poly-phenylen-sulfide (PPS), or other suitable epoxy binder. According to one embodiment, the mixture includes approximately 60% by volume of magnetic material, although other suitable percentages may be employed. The molded magnetic material may be applied using any suitable molding process including injection molding, compression molding, and transfer molding, for example.
After being applied about leadframe 102 and magnetic field sensor die 104, molded magnetic material 120 is magnetized in a direction substantially perpendicular to top surface 116 of field sensor die 104. The magnetization of molded magnetic material 120 results in molded magnetic material 120 providing a bias magnetic field B 124 having magnetic field or flux lines 126 which are substantially perpendicular to top surface 116 when proximate to a centerline 128 of molded magnetic material 120, but which diverge in the positive and negative x-directions on the right and left sides of centerline 128, as respectively illustrated by flux lines 130 and 132.
As a result, the magnetic field at GMR elements 106a and 106b has both a y- or vertical component, By 136, which is perpendicular to top surface 116, and an x- or lateral component, Bx 138, which is parallel to top surface 116. The vertical components By 136 are substantially equal at GMR elements 106a and 106b. Similarly, the magnitudes of lateral components Bx 138 at GMR elements 106a and 106b are substantially equal, but the component Bx 138 at GMR element 106a is positive while the component Bx 138 at GMR element 106b is negative.
Molded magnetic material 120 is applied such that a thickness 142 above a centerline 140 of GMR elements 106a and 106b is substantially the same as a thickness 144 below centerline 140. By having molded magnetic material 120 disposed equally above and below centerline 140, the vertical component By 136 of magnetic field B 124 is substantially uniform, and the magnetic field or flux lines, such as illustrated by flux lines 126, pass through magnetic sensor die 104 in a more perpendicular fashion relative to top surface 116. The more perpendicular the flux lines, the more the lateral component Bx 138 of magnetic field B 124 at GMR elements 106a, 106b (i.e. the vector component of magnetic field B 124 which is parallel to top surface 116) will be reduced. Reducing the magnitude of lateral component Bx 138 helps to prevent saturation of GMR elements 106a, 106b and helps to insure the proper functioning and operation of GMR elements 106a, 106b and magnetic field sensor 100.
However, regardless of the care taken to apply molded magnetic material 120 in an equal fashion above and below GMR sensor elements 106a, 106b, ferromagnetic leadframe 102 causes distortions in magnetic field B 124, especially along the perimeter sides of field sensor die 104. Of most concern are magnetic field distortions along perimeter sides of magnetic field sensor die 104 which are perpendicular to the lateral component Bx 138 of magnetic field B 124, such as sides 150 and 152, which distort magnetic field B 124 at GMR elements 106a, 106b and increases the magnitude of lateral component Bx 138. If the magnitude of lateral component Bx 138 becomes too large, it can cause the saturation of GMR elements 106a and 106b and render magnetic field sensor 100 inoperable.
Employing non-magnetic material 170 to space or shift molded magnetic material away from magnetic field sensor die 104 has the effect of shifting the magnetic field lines of magnetic field B 124 toward the sides of magnetic field sensor die 104. As a result, divergent flux lines, such as flux lines 132 and 134, are shifted away from magnetic field sensor die 104, thereby reducing the lateral component Bx 138 across surface 116. According to embodiments described herein, non-magnetic material 170 can be configured to reduce the magnitude of lateral component Bx 138 to a level that ensures proper operation of magnetic field sensor 200. According to one embodiment, non-magnetic material 170 has a dimension Dx 172 which spaces molded magnetic material 120 from GMR elements 106 in the direction of lateral component Bx 138 by an amount which maintains the magnitude of lateral component Bx 138 across top surface 116 of magnetic field sensor die 104 to a level at least below a saturation point of GMR elements 106a, 106b.
The required dimension Dx 172 of non-magnetic material 170 may vary depending upon a variety of factors including the strength of magnetic field B 124, the physical size of the magnet formed by molded magnetic material 120, and the design and configuration of GMR elements 106a, 106b. According to one embodiment, non-magnetic material 170 is disposed in a symmetrical fashion about centerline 128 of magnetic field sensor die 104. According to one embodiment, non-magnetic material 170 has a dimension Dy 174 so as to approximately align with top surface 116 of magnetic field sensor die 104.
Non-magnetic material 170 may comprise any suitable non-magnetic material. Examples of materials which may be suitable for use as non-magnetic material 170 include silicon, glass, ceramic, polyamide plastic, polyphenylenesulfid plastic, cross-linked silicon glues, hardened novalak materials, polyimides, hardened Cresol materials, polybenzoxazoles, and epoxy based materials. According to one embodiment, non-magnetic material 170 comprises silicon, which is selected for its temperature stability characteristics.
Although the magnetic particles within molded magnetic material 120 are bonded with a polymer material, molded magnetic material 120 is still electrically conductive so that the magnetic particles need to be separated or isolated from the surfaces of leadframe 102, magnetic field sensor die 104, bonding wires 112, and non-magnetic material 170 from molded magnetic material 120. According to one embodiment, prior to application of molded magnetic material 120, an isolation layer 180 is applied so as to cover and isolate leadframe 102, magnetic field sensor die 104, bonding wires 112, and non-magnetic material 170 from molded magnetic material 120. According to one embodiment, isolation layer 180 is an isolating resin material. According to one embodiment, isolation layer 180 is a SiOx layer deposited over the surfaces such as via plasma deposition processes, for example. According to one embodiment, isolation layer 180 is applied prior to application of non-magnetic material 170. According to one embodiment, not illustrated herein, as an alternative to applying isolation layer 180, the magnetic particles within molded magnetic material 120 are coated with an isolating material, such as silane, for example, to electrically isolate the magnetic particles from the device surfaces. It is noted that, for ease of illustration, isolation layer 180 is not shown in
According to one embodiment, non-magnetic material 170 can be dispensed and hardened during an assembly process, such as in the form of silicon or epoxy glues, for example. According to such an embodiment, non-magnetic material 170 hardens into a final “globtop” or “globetop” shape, such as illustrated above by the embodiment of
Although primarily described above as being disposed along only those perimeter sides 150 and 152 of magnetic field sensor die 104 which are perpendicular to or intersect lateral component Bx 138 of magnetic field B 124, according to other embodiment, non-magnetic material 170 may be applied about magnetic field sensor die 104 in other configurations as well. For example, according to one embodiment (not explicitly illustrated herein), non-magnetic material 170 may be disposed, in a fashion similar to that illustrated by
At
At
At 244, non-magnetic material 170 is disposed at least along selected perimeter sides of magnetic field sensor 104, such as illustrated by
At 246, magnetic material 120, using any suitable molding technique, is molded so as to encase magnetic field sensor die 104, non-magnetic material 170, and at all but the pins 110 of leadframe 102. At 248, molded magnetic material 120 is magnetized so as to provide a magnetic field which is substantially perpendicular to a top surface of magnetic field sensor die 104 and so that any lateral component thereof is in a direction toward perimeter sides of magnetic field sensor die 104 along which non-magnetic material 170 has been disposed, such as illustrated by
According to one embodiment, as illustrated by
In operation, as toothed wheel 252 rotates, as indicated at 254, the teeth pass in front of GMR sensing elements 106 and generate sinusoidal magnetic field variations in lateral magnetic field B 138 which are detected by GMR sensing elements 106. The detected magnetic field variations provide information about the angular position and rotational speed of toothed wheel 252.
According to one embodiment, as illustrated by
By employing non-magnetic material 170 to space molded magnetic material 120 away from at least selected perimeter sides of magnetic field sensor die 104 to reduce and/or eliminate magnetic field distortions caused by ferromagnetic leadframe 102, as described above, the physical dimensions of molded magnetic material 120 may be reduced while still providing a magnetic field having a lateral component which is below a saturation level of the magnetoresistive sensing elements. As such, the size, weight, and cost of magnetic field sensor 200 can be reduced relative to conventional sensors.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.
