Patent Application
20130009295
This invention relates to a semiconductor device including a contact clip having protrusions and a method of manufacturing thereof.
Semiconductor device manufacturers are constantly striving to increase the performance of their products, while decreasing their cost of manufacture. A cost intensive area in the manufacture of semiconductor devices is packaging the semiconductor chips. As those skilled in the art are aware, integrated circuits are fabricated in wafers, which are then singulated to produce semiconductor chips. One or more semiconductor chips are placed in a package to protect them from environmental and physical stresses. Packaging also involves electrically coupling the semiconductor chips to a leadframe. This may be accomplished by using various coupling techniques, such as wire bonding, soldering or gluing. Further, contact clips may be used to electrically couple electrodes of the semiconductor chips to the leadframe.
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.
As employed in this specification, the terms “coupled” and/or “electrically coupled” are not meant to mean that the elements must be directly coupled together; intervening elements may be provided between the “coupled” or “electrically coupled” elements.
Devices containing one or more semiconductor chips are described below. The semiconductor chips may be of different types, may be manufactured by different technologies and may include, for example, integrated electrical, electro-optical or electro-mechanical circuits or passives. The integrated circuits may, for example, be designed as logic integrated circuits, analog integrated circuits, mixed signal integrated circuits, power integrated circuits, memory circuits or integrated passives. Furthermore, the semiconductor chips may be configured as so-called MEMS (micro-electro mechanical systems) and may include micro-mechanical structures, such as bridges, membranes or tongue structures. The semiconductor chips may be configured as sensors or actuators, for example, pressure sensors, acceleration sensors, rotation sensors, magnetic field sensors, electro-magnetic field sensors, microphones etc. The semiconductor chips need not be manufactured from specific semiconductor material, for example, Si, SiC, SiGe, GaAs, and, furthermore, may contain inorganic and/or organic materials that are not semiconductors, such as, for example, insulators, plastics or metals. Moreover, the semiconductor chips may be packaged or unpackaged.
In particular, semiconductor chips having a vertical structure may be involved, that is to say that the semiconductor chips may be fabricated in such a way that electric currents can flow in a direction perpendicular to the main surfaces of the semiconductor chips. A semiconductor chip having a vertical structure has electrodes on its two main surfaces, that is to say on its top side and bottom side. In particular, power semiconductor chips may have a vertical structure. The vertical power semiconductor chips may, for example, be configured as power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), IGBTs (Insulated Gate Bipolar Transistors), JFETs (Junction Gate Field Effect Transistors), power bipolar transistors or power diodes. By way of example, the source electrode and gate electrode of a power MOSFET may be situated on one main surface, while the drain electrode of the power MOSFET is arranged on the other main surface. Furthermore, the devices described below may include integrated circuits to control the integrated circuits of power semiconductor chips.
The semiconductor chips may have electrodes (or contact elements or contact pads) which allow electrical contact to be made with the integrated circuits included in the semiconductor chips. The electrodes may include one or more metal layers that are applied to the semiconductor material. The metal layers may be manufactured with any desired geometric shape and any desired material composition. The metal layers may, for example, be in the form of a layer covering an area. Any desired metal or metal alloy, for example, aluminum, titanium, gold, silver, copper, palladium, platinum, nickel, chromium or nickel vanadium, may be used as the material. The metal layers need not be homogenous or manufactured from just one material, that is to say various compositions and concentrations of the materials contained in the metal layers are possible.
The semiconductor chips may be placed on leadframes. The leadframes may be of any shape, size and material. The leadframes may include die pads and leads. During the fabrication of the devices the die pads and leads may be connected to each other. The die pads and leads may also be made from one piece. The die pads and leads may be connected among each other by connection means with the purpose of separating some of the die pads and leads in the course of the fabrication. Separation of the die pads and leads may be carried out by mechanical sawing, a laser beam, cutting, stamping, milling, etching or any other appropriate method. The leadframes may be electrically conductive. They may be entirely fabricated from metals or metal alloys, in particular copper, copper alloys, iron nickel, aluminum, aluminum alloys, steel, stainless steel or other appropriate materials. The leadframes may be plated with an electrically conductive material, for example copper, silver, iron nickel or nickel phosphorus. The leads of the leadframes may be bent during fabrication, for example, in an S-shaped manner.
One or more contact clips may be used to electrically couple components to each other within the device. For example, an electrode of a semiconductor chip may be electrically coupled to an external contact element, for example, the lead of a leadframe by a contact clip. Each of the contact clips has at least two contact areas, which are used to attach the contact clip to at least two components of the device. The contact areas include protrusions extending towards the components the contact clip is attached to. Each of the protrusions may have a height of at least 5 μm. Due to the protrusions the contact areas of the contact clip may have a comb structure. Thus, the contour of the contact areas include peaks and valleys, which may extend from one side surface of the contact clip to another side surface of the contact clip. The widths of the peaks and/or the widths of the valleys may be regular or, alternatively, irregular. Further, the heights of the peaks may be regular or, alternatively, irregular. The contact clip may be soldered to the components. For this, a diffusion soldering process may be used.
The devices described below include external contact elements (or external contact pads), which may be of any shape and size. The external contact elements may be accessible from outside the device and may thus allow electrical contact to be made with the semiconductor chips from outside the device. For this reason, the external contact elements may have external contact surfaces which can be accessed from outside the device. Furthermore, the external contact elements may be thermally conductive and may serve as heat sinks for dissipating the heat generated by the semiconductor chips. The external contact elements may be composed of any desired electrically conductive material, for example, of a metal, such as copper, aluminum or gold, a metal alloy or an electrically conducting organic material. Some of the external contact elements may be leads of a leadframe.
The devices may include an encapsulation material, for example, a mold material covering at least parts of the components of the devices. The mold material may be any appropriate thermoplastic or thermosetting material. Various techniques may be employed to cover the components with the mold material, for example, compression molding, injection molding, powder molding or liquid molding.
The devices may have mounting surfaces. The mounting surface may serve to mount the device onto another component, for example, a circuit board, such as a PCB (printed circuit board). External contact elements and, in particular, external contact surfaces may be disposed on the mounting surface to allow to electrically couple the device to the component on which the device is mounted. Solder deposits, such as solder balls, or other appropriate connection elements may be used to establish an electrical and, in particular, mechanical connection between the device and the component on which the device is mounted.
As illustrated in
In one embodiment, the leadframe 10 includes further die pads and leads. In one embodiment, the leads 12-14 are arranged on different sides of the die pad 11.
The leadframe 10 is manufactured from metals or metal alloys, in particular copper, copper alloys, iron nickel, aluminum, aluminum alloys, or other electrically conductive materials. In one embodiment, the leadframe 10 is plated with an electrically conductive material, for example, copper, silver, iron nickel or nickel phosphorus. The shape of the leadframe 10 is not limited to any size or geometric shape. For example, the leadframe 10 may have a thickness in the range from 100 μm to 1 mm or may be even thicker. The leadframe 10 may have been manufactured by punching, milling or stamping a metallic plate.
The semiconductor chip 15 is mounted on the die pad 11 with its second surface 20 facing the die pad 11 and its first surface 19 facing away from the die pad 11. The semiconductor chip 15 has a first electrode 16 on the first surface 19 and a second electrode 17 on the second surface 20. The first and second electrodes 16, 17 are load electrodes. Furthermore, the semiconductor chip 15 has a third electrode 18 on its first surface 19. The third electrode 18 is a control electrode. The top surface of the die pad 11 is larger than the second surface 20 of the semiconductor chip 15.
The semiconductor chip 15 is configured as a power transistor, for example, a power MOSFET, IGBT, JFET or power bipolar transistor, or a power diode. In the case of a power MOSFET or a JFET, the first electrode 16 is a source electrode, the second electrode 17 is a drain electrode, and the third electrode 18 is a gate electrode. In the case of an IGBT, the first electrode 16 is an emitter electrode, the second electrode 17 is a collector electrode, and the third electrode 18 is a gate electrode. In the case of a power bipolar transistor, the first electrode 16 is an emitter electrode, the second electrode 17 is a collector electrode, and the third electrode 18 is a base electrode. In the case of a power diode, the first and second electrodes 16, 17 are cathode and anode, and there is no third electrode. During operation, voltages higher than 5, 50, 100, 500 or 1000 V may be applied between the first and second electrodes 16, 17. The switching frequency applied to the third electrode 18 may be in the range from 1 kHz to 100 MHz, but may also be outside this range.
The second electrode 17 is electrically and mechanically coupled to the die pad 11 by diffusion soldering. For that, a solder material 30 is deposited on the second electrode 17 or the upper surface of the die pad 11, for example, by sputtering or other appropriate physical or chemical deposition methods. In one embodiment, the solder material 30 is deposited on the second electrode 17 when the semiconductor chip 15 is still in the wafer bond, which means that the solder material 30 is deposited on the semiconductor wafer before the semiconductor wafer is singulated in order to produce individual semiconductor chips. In one embodiment, the solder material 30 consists of AuSn, AgSn, CuSn, Sn, AuIn, AgIn, AuSi or CuIn.
For producing the soldered joint, the leadframe 10 is heated by a hot plate to a temperature above the melting temperature of the solder material 30. For example, the leadframe 10 is heated to a temperature in the range from 200 to 400° C. and, in particular, in the range from 280 to 320° C. In one embodiment, both the leadframe 10 and the semiconductor chip 15 are placed in an oven and are heated to an appropriate temperature. A pick-and-place tool is used capable of picking the semiconductor chip 15 and placing it on the die pad 11. During the soldering process the semiconductor chip 15 may be pressed onto the die pad 11 for an appropriate time in the range from 10 to 200 ms, in particular around 50 ms.
During the diffusion soldering process the solder material 30 produces a metallic joint between the die pad 11 and the semiconductor chip 15 which is able to withstand high temperatures through the fact that the solder material 30 forms a temperature-resistant and highly mechanically stable intermetallic phase with high-melting materials of the die pad 11 and the semiconductor chip 15. The intermetallic phase has a higher melting temperature than the solder material 30 used to generate the intermetallic phase. In the process, the low-melting solder material 30 is completely transformed, i.e. it passes completely into the intermetallic phase. The process is diffusion-controlled and its duration increases as the thickness of the layer of the solder material 30 rises. In one embodiment, the gap d2 between the die pad 11 and the semiconductor chip 15 is smaller than 1 μm after the soldering process.
The contact clip 25 may be manufactured from a metal or a metal alloy, in particular, copper, copper alloys, iron nickel or other appropriate electrically conductive materials. The shape of the contact clip 25 is not limited to any size or geometric shape. The contact clip 25 may have the shape as exemplarily illustrated in
In the oven, a diffusion soldering process is carried out. During the diffusion soldering process the solder material produces metallic joints between the first contact area 26 and the first lead 12 and between the second contact area 27 and the first electrode 16, which are able to withstand high temperatures through the fact that the solder material forms temperature-resistant and highly mechanically stable intermetallic phases 50 with high-melting materials of the contact clip 25, the first lead 12 and the first electrode 16. The intermetallic phases 50 have a higher melting temperature than the solder material used to generate the intermetallic phases 50. The temperature in the oven is set such that it is lower than the melting temperature of the intermetallic phases 50. In one embodiment, the melting temperature of the intermetallic phases 50 is higher than 300° C. Thus, the intermetallic phases 50 may solidify while they are still in the oven.
Since the process described above is diffusion-controlled, its duration increases as the thickness of the layer of the solder material rises. Due to the protrusions 28 extending from the contact areas 26, 27 towards the first lead 12 and the first electrode 16, respectively, there is only a thin layer of solder material between the bottom surfaces of the protrusions 28 and the top surface of the first lead 12 or the first electrode 16. In this areas the low-melting solder material is thus completely transformed into the intermetallic phases 50 at first. Since the components are placed in the oven for not longer than 60 seconds, the portions of the solder material between neighboring protrusions 28 are not transformed into the intermetallic phase 50. Thus, when the components are taken out from the oven, the interfaces between the contact clip 25 and the first lead 12 and between the contact clip 25 and the first electrode 16 have the structure as shown in the enlarged illustration at the bottom of
Instead of wire bonding, a contact clip is placed on the third electrode 18 and the third lead 14 in one embodiment. The contact clip may have protrusions extending from its contact areas similar to the protrusions 28 of the contact clip 25.
The mold material 52 may be composed of any appropriate thermoplastic or thermosetting material, in particular, it may be composed of a material commonly used in contemporary semiconductor packaging technology. Various techniques may be employed to cover the components of the device 300 with the mold material 52, for example, compression molding, injection molding, powder molding or liquid molding.
Before or after the encapsulation with the mold material 52, the individual devices 300 are separated from one another by separation of the leadframe 10, for example, by sawing or cutting the dams of the leadframe 10. Other separation methods, such as etching, milling, laser ablation or stamping, may also be employed.
The leads 12-14 and, in particular, the contact clip 25 serve as external contact elements of the device 300, wherein the leads 12-14 are coupled to the first electrode 16, the second electrode 17 and the third electrode 18 of the semiconductor chip 15, respectively. The surfaces 53-55 of the leads 12-14 and the exposed surface of the contact clip 25 may be used as external contact surfaces to electrically couple the device 300 to other components, for example, a circuit board, such as a PCB. The surface 56 of the device 300 may be used as a mounting surface to mount the device 300 on the circuit board.
It is obvious to a person skilled in the art that the device 300 is only intended to be an exemplary embodiment, and many variations are possible. Although the device 300 in the embodiment illustrated in
One method to produce the protrusions 28 on the contact areas 26, 27 of the contact clip 25 is to use a metal forming manufacturing process. In this process a stamping press produces the desired pattern on the contact clip 25.
Subsequently, the main surface 60 of the contact area is exposed to an appropriate etching agent for a suitable time. Thereby the portions of the contact clip 25 that are exposed from the etching mask 61 are etched and cavities 62 are formed as illustrated in
In one embodiment, the device 300 is placed in an oven and is exposed there to a temperature which is higher than the melting temperature of the solder material of the layers 31, 32. The temperature in the oven may be in the range from 280 to 320° C. and, in particular, around 300° C. The device 300 is kept in the oven for an appropriate time so that the solder material of the layers 31, 32 is completely transformed into the intermetallic phase 50. This process is carried out after the device 300 has been separated from the other devices by separation of the leadframe 10 and before the device 300 is placed on the circuit board 70 or, alternatively, after the device 300 has been placed on the circuit board 70.
In one embodiment, the device 300 is not placed in an oven as described above. Thus the layers 31, 32 of solder material are not completely transformed into the intermetallic phase 50 after the device 300 has been mounted on the circuit board 70. In this embodiment, during operation of the device 300, the heat generated by the semiconductor chip 15 and, in particular, other components of the device 300 or the system 500 produces a temperature that is high enough to completely transform the solder material of the layers 31, 32 into the intermetallic phase 50 as shown in the enlarged illustration at the top of
In addition, while a particular feature or aspect of an embodiment of the invention may have been 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”. Furthermore, it should be understood that embodiments of the invention may be implemented in discrete circuits, partially integrated circuits or fully integrated circuits or programming means. Also, the term “exemplary” is merely meant as an example, rather than the best or optimal. It is also to be appreciated that features and/or elements depicted herein are illustrated with particular dimensions relative to one another for purposes of simplicity and ease of understanding, and that actual dimensions may differ substantially from that illustrated herein.
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.
