Electrical device with covering

The invention relates to a device comprising a sensor chip and a structure housing the sensor chip. The structure is covered by a mold compound and is fabricated from a ceramic or a glass material.

FIELD OF THE INVENTION

This invention relates to an electrical device in general and more particularly to a sensor chip.

BACKGROUND OF THE INVENTION

Sensors are used in everyday life. Applications include automobiles, machines, aerospace, medicine, industry and robotics. Technological progress allows more and more sensors to be manufactured on the microscopic scale included in semiconductor chips.

DETAILED DESCRIPTION OF THE INVENTION

In the following embodiments of the invention are described with reference to the drawings, wherein like reference numerals are generally utilized to refer to like elements throughout, and wherein the various structures are not necessarily drawn to scale. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more aspects of embodiments of the invention. It may be evident, however, to one skilled in the art that one or more aspects of the embodiments of the invention may be practiced with a lesser degree of these specific details. In other instances, known structures and devices are shown in block diagram form in order to facilitate describing one or more aspects of the embodiments of the invention. The following description is therefore not to be taken in a limiting sense, and the scope of the invention is defined by the appended claims.

The devices described in the following contain sensor chips. The specific embodiment of these sensor chips is not important in this case. The sensor chips may contain electromechanical or electrooptical functional elements. An example of an electromechanical sensor is a microphone. Examples of electrooptical sensors include photodiodes or diode lasers. The sensor chips may also function fully electrically, for example, as Hall Effect sensors. The sensor chips may be embodied as so-called MEMS (Micro-Electro-Mechanical System), wherein micromechanical movable structures such as, for example, bridges, membranes or reed structures may be provided. Such sensor chips may be motion sensors, which may be embodied as acceleration sensors (detecting accelerations in different spatial directions) or rotation sensors. Sensors of this type are also referred to as gyrosensors, roll-over sensors, impact sensors, inertial sensors, etc. They are used for example in the automotive industry for signal detection in ESP (Electronic Stability Program) systems, ABS (Anti-lock Braking Systems), airbags and the like. Usually such sensor chips are made of a semiconductor material. However, the sensor chips are not limited to be fabricated from a specific semiconductor material. They may additionally contain non-conductive inorganic and/or organic materials.

The described devices further contain a structure housing the sensor chip. The structure may be made of a ceramic or a glass material or combinations thereof. For example, the structure may be fabricated using cofired ceramic multilayer structures, which may contain (depending on the respective application) up to 40 or more dielectric layers. Between adjacent layers electrically conductive vias may be arranged. For example, the layers may contain metallized traces or solder-filled vias, which are conventionally made by thick-film metallization techniques including screen-printing. Using such techniques, the structure housing the sensor chip may then contain one or more contact elements providing an electrical connection through the walls of the structure. Including contact pads on the inner and outer surface of the structure, an electrical connection between applications inside and outside the structure can be established.

During the fabrication process, the multiple layers may be joined together by a burnout process (at about 350° C.-600° C.), which is followed by a firing process at elevated temperatures (depending on the applied materials). Conventionally used systems are low temperature cofired ceramic (LTCC) or high temperature cofired ceramic (HTCC) multilayered systems. HTCC systems may be fabricated by using aluminum substrates; they are printed with molybdenum-manganese or tungsten conducting traces and are fired at temperatures of about 1300° C.-1800° C. For LTCC systems various glass-ceramic substrates are used, which are printed with gold, silver or copper metallizations and are fired at temperatures of about 600° C.-1300° C.

The structure housing the sensor chip may have a thermal expansion coefficient similar or close to the thermal expansion coefficient of the sensor chip. The structure may be of optional shape and geometric form, it may particularly be sealed, for example, by a cover also fabricated from a ceramic or glass material. The structure housing the sensor chip and the cover sealing the structure may also be fabricated from other materials than ceramic or glass if these materials have thermal expansion coefficients in the range from 0.3·10−6/K to 8.2·10−6/K and, in particular, in the range from 4.0·10−6/K to 4.5·10−6/K.

Devices described herein further contain a mold compound that partly or fully covers the structure housing the sensor chip. Said mold compound may, for example, be made of a thermoplastic resin or a thermosetting plastic (e.g. epoxy resin).

The devices may further comprise a semiconductor chip, which may serve to control the functionality of the sensor chip or to process signals that are sensed and/or generated by the sensor chip. By way of example, in the case of the sensor chip being a motion sensor, the deflection of a movable element comprised in the sensor chip may be read piezoresistively or capacitively and may then be processed by the semiconductor chip. The semiconductor chip may be coupled to the sensor chip for the purpose of a (bidirectional) data exchange. The semiconductor chip may, for example, be embodied as an ASIC (Application Specific Integrated Circuit).

FIG. 1shows a sectional side view of a device100as a first embodiment. The device100contains a sensor chip1housed in a structure2made of a ceramic or a glass material. The structure2is covered by a mold compound3. In the illustrated case, the mold compound3only partly covers the structure2and does not fully enclose it.

FIG. 2shows a sectional side view of a device200as a second embodiment. In comparison to the device100ofFIG. 1, the structure2of the device200fully encapsulates the sensor chip1. The structure2itself is covered by the mold compound3. As for the device100, the structure2of the device200may be fabricated from a ceramic or a glass material. Furthermore, the structure2may also be fabricated from a semiconductor material or any other material having a thermal expansion coefficient in the range from 0.3·10−6/K to 8.2·10−6/K and, in particular, in the range from 4.0·10−6/K to 4.5·10−6/K.

FIGS. 3 to 7illustrate devices300to700representing further implementations of the devices100and200described above. The configurations of the devices300to700, which are described in the following, can therefore likewise be applied to the devices100and200.

FIG. 3shows a sectional side view of a device300as a third embodiment.FIG. 3, as well asFIGS. 4-7, includes components similar to those that are positioned and connected as shown, in one implementation, in the steps depicted inFIG. 8(althoughFIG. 8literally shows the fabrication of a device500illustrated inFIG. 5). In comparison to the devices100and200, the device300further comprises a semiconductor chip4. The semiconductor chip4may, for example, be an ASIC that processes signals received from the sensor chip1. The semiconductor chip4may also control the sensor chip1. In the implementation ofFIGS. 3-6, the sensor chip1is mounted in the structure2, but the semiconductor chip4is not. The semiconductor chip4and the structure2are both mounted on a carrier5,6. In the case of the device300, the carrier5,6is implemented in form of a leadframe comprising at least one die pad5and several leads6(or pins) surrounding the die pad5. The die pad5and the leads6may be fabricated from a metal, for example, copper. Due to the chosen perspective,FIG. 3only shows two leads6. In practice, the number of leads6for example depends on the number of electrical contacts of the semiconductor chip4and/or the sensor chip1. In the example ofFIG. 3, the semiconductor chip4is placed on the die pad5, whereas the structure2containing the sensor chip1is placed on some of the leads6. Alternatively, the structure2may be mounted on a separate die pad. It is however understood that the carrier5,6is not restricted to embodiments as described above.

The structure2, the semiconductor chip4, and the die pad5are completely embedded in a mold compound3, while portions of the leads6protrude out of the mold compound3. The portions of the leads6that are not covered by the mold compound3may be bent as illustrated inFIG. 3.

The structure2further comprises a contact element7having contact pads on its inner and outer surface. Inside the structure2, the contact element7is electrically coupled to the sensor chip1via a bond wire8. In the embodiment shown inFIG. 3, the structure2is placed on one of the leads6in such a manner that the contact element7is in direct contact with one of the leads6. A good electrical contact between the contact element7and the lead6may be assured by joining the contact element7with the lead6using electrically conductive glue or by soldering the contact element7with the lead6.

The contact element7provides the possibility of an electrical connection through the structure2. The semiconductor chip4is connected to several leads6via bond wires8. One of these bond wires8is connected to the lead6the contact element7is connected to. This bond wire8establishes an electrical connection between the sensor chip1and the semiconductor chip4. Accordingly, a bidirectional data exchange between the sensor chip1and the semiconductor chip4is possible. It is to be noted that the structure2may comprise more than one contact element7and that the structure2(and thus the sensor chip1) may be coupled to the semiconductor chip4via several bond wires8.

Further, it is to be noted that the sensor chip1and the semiconductor chip4do not necessarily have to be wire bonded, alternative types of mounting, such as flip-chip technology, may be also used. Since the leads6protrude out of the mold compound3, they provide the possibility of the semiconductor chip4and the sensor chip1being connected to an external system, for instance, a circuit board.

One advantage of housing the sensor chip1in the structure2and covering the structure2with the mold compound3is that stress effects on the sensor chip1are reduced. The reason is that the thermal expansion coefficient of the structure2made of a ceramic or glass material is similar to the thermal expansion coefficient of the sensor chip1. Accordingly, signals sensed and/or generated by the sensor chip1and thus the overall functionality of the device are less influenced by stress effects. Due to their sensitivity, such influences on the sensing process may particularly be considerable in the case of MEMS and Hall Effect sensor chips.

It is to be noted that, in principle, any material may be used for the fabrication of the structure2if the thermal expansion coefficient of the chosen material matches the thermal expansion coefficient of the sensor chip1. In practice, the thermal expansion coefficient of the structure2preferably lies in the range from 0.3·10−6/K to 8.2·10−6/K. It is however understood that the material composition and embodiment of the structure2should be related to the respective case.

Housing the sensor chip1within the structure2further reduces the risk of the bond wires8, which are connected to the sensor chip1, being damaged. As can be seen inFIG. 3, due to the complete enclosure of the sensor chip1and the bond wires8to the sensor chip1by the structure2, the mold compound3does not contact the bond wires8and therefore no stress effects between these components can occur. In case of a hermetically sealed structure2, the sensor chip1and the bond wires8housed in the structure2are protected against any kind of undesired environmental influences, such as intruding moisture.

The sensor chip1may be produced on a semiconductor wafer with microstructures applied on the semiconductor wafer via planar techniques. Therefore the sensing unit of the sensor chip1, such as movable elements in the case of a MEMS, is oriented within a main surface of the sensor chip1. For example, a micromechanical movable membrane used for sensing of an acceleration is usually oriented parallel to the main surface of the sensor chip1.

In some cases, the physical value to be sensed by the sensor chip1may depend on the spatial orientation of its main surface, for example, when various spatial components of an acceleration are to be detected. In the device300, the structure2is mounted onto the carrier5,6in such a way that the spatial orientation of the sensor chip1, i.e. its main surface, supports the functional requirements of the device300. InFIG. 3, the main surface of the sensor chip1and the surface of the carrier5,6are tilted by a tilt angle of about 90°. (InFIG. 8, previously referenced, the tilt angle of the sensor chip1to the surface of the carrier5,6is 0°; inFIG. 4, the sensor chip is mounted at a tilt angle approximately 0° as shown inFIG. 8.) It is however understood that the main surface of the sensor chip1and the surface of the carrier5,6can be arranged with any tilt angle. The tilt angle should be chosen in agreement with the desired functionality of the device300.

FIG. 4shows a sectional side view of a device400as a fourth embodiment. In contrast to the device300, the structure2contained in the device400is placed on the carrier5,6in such a manner that the main surface of the sensor chip1is oriented parallel to the surface of the carrier5,6. In case of the device400, there is no tilt angle between the main surface of the sensor chip1and the surface of the carrier5,6.

FIG. 5shows a sectional side view of a device500as a fifth embodiment. The difference between the devices400and500lies within the respective design of their contact elements7. The contact element7of the device400is in direct contact with one of the leads6, whereas the contact element7of the device500is arranged on an outer surface of the structure2, which does not contact the carrier5,6. In this way, an electrical connection between the contact element7and the semiconductor chip4can be established via a bond wire9, one end of which is attached to the contact element7and the other end of which is attached to the semiconductor chip4. The bond wire9does not contact the carrier5,6. As a result, the additional connection between the contact element7and the lead6(cf.FIG. 4) is omitted.

FIG. 6shows a sectional side view of a device600as a sixth embodiment. The device600differs from the device300in the way the contact element7is designed. InFIG. 6, the contact element7is a metallization layer which is applied to the structure2. Moreover, the contact element7is directly wire bonded to the semiconductor chip4.

FIG. 7Ashows a top plan view of a device700as a seventh embodiment.FIG. 7Bshows another view of the device700. The internal structure of the device700may be the same as the internal structure of one of the devices300to600. Due to the chosen perspective ofFIG. 7A, the internal structure of the device700, such as the sensor chip1, the structure2and the semiconductor chip4, are not shown. The visible components of the device700are the mold compound3and the portions of leads6and10protruding out of the mold compound3. The device700has leads not on four, but only on three sides of the device700. As indicated inFIG. 7Aby an axis A12, the leads6are bent by 90°. Further, each of the two outer leads10are bent by 90° in two places (cf. axes A12, B14, and C16). The bending of the leads6and10results in end points11of the leads6and10, lying within a plane. This plane is the mounting plane of the device700, which can, for example, be used to mount the device onto a circuit board. Since the main surface of the sensor chip1lies within the drawing plane ofFIG. 7A, the main surface of the sensor chip1and the mounting plane of the device700are tilted by a tilt angle of 90°. If, for some reason, the circuit board is positioned so as not to be quite perpendicular to the desired attitude of the device, the leads6may be bent at a different angle to allow for orienting the device700at a desired operating angle. For example, the leads6may be bent within +45° to −45° of the circuit board to effect a desired position of the device700regardless of the angular position of the circuit board.

Besides a tilt angle between the main surface of the sensor chip1and the surface of the carrier5,6as shown inFIGS. 3 and 6, the bending of the leads6and10as proposed inFIGS. 7A and 7Bprovides a further possibility to adjust the spatial orientation of the sensor chip1. It is understood that one or more of the leads6and10may have further bents. Specifically, the design of the leads6and10may depend on the external application type as well as on the desired functionality of the device700.

FIG. 8illustrates process steps of an exemplary fabrication of the device500. The components and properties of the device500were already described above. In a first step S120, a structure2made of a ceramic or glass material or any other material having a thermal expansion coefficient in the range from 0.3·10−6/K to 8.2·10−6/K is provided. The structure2comprises at least one contact element7which has contact pads inside and outside of the structure2. In a second step S222, a sensor chip1is mounted onto the structure2, for example by using a conventional die attach method, such as gluing. Furthermore, the sensor chip1is electrically connected to the contact element7via a bond wire8. In a third step S324, the structure2is closed and hermetically sealed with a cover12, which may be made of the same material as the structure2.

In a fourth step S426, a leadframe comprising a die pad5and leads6is provided. A semiconductor chip4is mounted on the die pad5and is electrically connected to the leads6via bond wires8. The step S426further comprises mounting the structure2onto the leadframe. This mounting process is not restricted to a certain technique and may for example be carried out by gluing the structure2to the leadframe. In a fifth step S528, the structure2and the semiconductor chip4are covered with a mold compound3in such a way that portions of the leads6protrude out of the mold compound3. Depending on the type of a possible external application and the desired functionality of the device500, the leads6may be bent accordingly.

It is understood that all devices shown inFIGS. 1 to 7may be manufactured in a process similar to the one illustrated inFIG. 8. Moreover, the described process steps may be interchanged in any reasonable way. For example, it is possible to perform the step S428before the steps S120to S326, i.e. the structure2may be mounted onto the carrier5,6first and the steps S120to S324may be performed afterwards with the structure2already attached to the carrier5,6. It is also to be noted that further fabrication steps may be added to the method illustrated inFIG. 8.