Abstract:
The present invention relates to a semiconductor chip comprising a semiconductor element, at least a conducting line and a contact area being arranged on the semiconductor element, the conducting line being connected with the contact area, the contact area being disposed for contacting another electrical contact, characterised in that the semiconductor element comprises at a face of the semiconductor element a flexible arm formed out of the semiconductor element and the contacting area being arranged on the arm.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention refers to a semiconductor chip, and in particular, to a semiconductor chip having a semiconductor element and at least a conducting line that is electrically connected to a contact area. 
   BACKGROUND OF THE INVENTION 
   Conventionally, there are different approaches to provide semiconductor chips of integrated circuits with contact areas. The basic technique is to provide them on the surface of the semiconductor chip as a metal area that is connected by conducting lines with the integrated circuit. 
   For contacting several semiconductor chips arranged in the stack, it is well known using electrical feed-throughs inside the bulk material to connect the circuitries of the different semiconductor chips via contact pads from the top surface of a semiconductor chip to the bottom surface. The bottom face is also the interface to contact pads of a second chip that adjoins the bottom surface of the first chip with a top face. This approach, however, is disadvantageous since the second semiconductor chip comprises devices or electronic circuits buried underneath the contact pads on the top surface. 
   In another approach electrical wires are fabricated around the rim of each semiconductor chip of a stack, either along its side face or through a carrier in which the chip is embedded. U.S. Pat. No. 5,656,553 discloses such a stack and the fabrication method for a monolithic electronic module comprising a plurality of stacked planar extending arrays of integrated circuit chips. The fabrication method includes dicing a wafer of integrated circuit chips into a plurality of arrays of integrated circuit chips. The arrays of the integrated circuit chips are then stacked to form an electronic module. A metallization pattern is deposited on a side face of the electronic module to interconnect the various arrays of the integrated circuit chips. This side surface metallization is only used for connecting the semiconductor chips of the stack to each other. This method requires dicing of a wafer into chips in order to continue processing of the wires and other packaging steps, which is typically less cost-effective than a batch fabrication at wafer scale. 
   SUMMARY OF THE INVENTION 
   The present invention provides a chip comprising contact areas that improve the quality of contacting the semiconductor chip. 
   In one embodiment of the invention, there is a semiconductor chip comprising a semiconductor element and at least a conducting line that is electrically connected to a contact area. The semiconductor chip may be, but is not limited to, a simple sensing element or a semiconductor chip with a highly sophisticated integrated circuit, for example a DRAM. More particularly, the invention relates to a semiconductor chip that comprises a flexible arm with a contacting area at a face constituting a microspring contact. The semiconductor chips may be assembled in a stack, whereby each of the semiconductor chips comprises the flexible arm aligned on top of each other. Furthermore, the present invention relates to a method for producing the semiconductor chip with a flexible arm at a face of the semiconductor element. Furthermore, the invention relates to an electrical interconnection system of thinned semiconductor chips of integrated circuits within a stack and a method of packaging the semiconductor chips in a stack. 
   In another embodiment of the invention, there is a semiconductor chip comprising a semiconductor element, at least a conducting line and a contact area that is arranged on the semiconductor element. The conducting line is connected to the contact area and the contact area is disposed for contacting another electrical contact. The semiconductor element comprises a flexible arm formed out of the semiconductor element at a face of the semiconductor element and the contacting area is arranged on the arm. The contact area with the flexible arm constitute a microspring contact. Therefore, the flexible arm assists a contacting process as the flexible arm is able to compensate a failure in the correct distance between the contact area and the electrical contact that is brought into contact with the contact area. Furthermore, the flexible arm generates a force against the electrical contact by contacting with the electrical contact, said force arising the contacting force between the contact area and the electrical contact. The flexible arm made of the semiconductor element is rigidly fixed to the semiconductor element and therefore provides a high reliability. The flexible arm with the contact area includes a microspring contact that could be used for a temporary test procedure before the semiconductor chips is mounted in a housing. Furthermore, the microspring has the advantage that different temperature expanding coefficients of the chip and the housing could be compensated. Preferably, the microspring contact functions as a cantilever arm. 
   In addition the flexible arm compensates mechanical stress due to different thermal expansion coefficients of semiconductors and any interface, e.g. plastic sockets. Thus the reliability is in general superior compared to uncompensated devices. 
   In still another embodiment of the invention, there is a semiconductor element comprising an upper surface and a side surface, whereby the arm is arranged on the side surface. The configuration of the arm at the side surface has the advantage that the arm can be easily manufactured out of the semiconductor element and the upper surface of the semiconductor element is free of the contact area. Therefore, there is more space on the upper surface for conducting lines or electronic devices or integrated circuits. 
   In yet another embodiment of the invention, there is a semiconductor chip, whereby the semiconductor element comprises a recess in which the flexible arm can pivot contacting another electrical contact. This embodiment has one advantage that space is available for the flexible arm to compensate a misalignment between the semiconductor chip and the electrical contact. Furthermore, the semiconductor chip has a small overall size, although the flexible arm is able to pivot at a relatively large distance to the semiconductor element. 
   In another embodiment of the present invention, there is a semiconductor element, whereby the flexible arm and the recess are produced by an etching process. Using an etching process has the advantage of shaping the flexible arm with a well-known process that may be used for mass production of semiconductor chips. 
   In still another embodiment of the present invention, there is a semiconductor element with a recess that is at least as deep as the thickness of the flexible arm in the pivoting plane of the flexible arm. Thus, it is possible to pivot the flexible arm completely in the recess of the semiconductor element in relation to the side surface at which the semiconductor element is arranged. 
   In yet another embodiment of the present invention, there is a semiconductor chip, whereby the arm is arranged at a given height above a bottom side of the semiconductor element. Thus, the flexible arm may move freely in spite of the semiconductor chips being arranged in a stack. 
   In another embodiment of the present invention, there is a simple semiconductor chip that is one of several semiconductor chips of a wafer and the arm is shaped from a part of an area that is arranged between the semiconductor chips of the wafer and used for separating the semiconductor chips. Therefore, the area between the semiconductor chips, i.e. the kerf is used for fabricating the flexible arm. The surface of the semiconductor chip is advantageously not reduced by providing the flexible arm. 
   In still another embodiment of the present invention, there is a stack of semiconductor chips, whereby several semiconductor chips are piled up and electrically contacted by the respective arms of the semiconductor elements with disposed electrical contacts. This arrangement has the advantage that the electrical contacting of the semiconductor chips is achieved by the flexible arms of the semiconductor chips that are arranged at a side face of the stack. Due to the flexible arms, misalignments of the semiconductor chips can be compensated by the flexibility of the flexible arms. This leads to a higher quality of the electric contacts between the stack and the disposed electrical contacts. 
   In another embodiment of the present invention, there is a method for producing a semiconductor chip with a flexible arm, that is fabricated by an etching process. Semiconductor material can easily be etched and shaped with an etching process that is a well-known technology. Therefore, the semiconductor chip can be produced cheaply and easily. 
   According to one aspect the fabrication process, the flexible arm is first covered with an dielectric layer and then covered with a conducting layer that connects the contacting area and the conducting line. Thus, the contacting area and the conducting line are isolated against the semiconductor element. 
   In yet another embodiment of the present invention, there is a method for producing the semiconductor chip, whereby the arm, the conducting area and the conducting line are fabricated from an upper side of the semiconductor element, whereby an undercut is formed under the arm in the semiconductor element by etching and the semiconductor chip is subsequently thinned by a grinding and polishing process from a bottom side of the semiconductor element, until the undercut is reached by the bottom face of the semiconductor element. 
   In still another embodiment of the invention, there is a semiconductor chip with a semiconductor element with at least a conducting line and a contact area that can be easily brought into contact with an electrical contact. This is achieved by a flexible arm that is made of the semiconductor element comprising the contact area. Due to the flexible arm, larger production tolerances can be used for connecting the semiconductor chip with the electrical contact alone or, in the embodiment of a stack, with several semiconductor chips. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail below with reference to the exemplary drawings, in which: 
       FIG. 1  is a partial view of the top surface of a semiconductor chip. 
       FIG. 2  is a partial view of the top surface of a flexible arm with solder. 
       FIG. 3  depicts a top view of a flexible arm and a contact socket frame prior to soldering. 
       FIG. 4  depicts a top view after soldering the flexible arm to the contact socket frame. 
       FIG. 5  depicts a further embodiment of an electrical connection between a flexible arm and the contact socket frame. 
       FIG. 6  depicts a schematic side view of the flexible arm prior to soldering. 
       FIG. 7  depicts a side view of the flexible arm after soldering the contact area. 
       FIG. 8  depicts a side view on a stack of semiconductor chips that are arranged on a printed circuit board after soldering. 
       FIGS. 9 to 17  depict cross-sectional views of various process steps of a method of producing a semiconductor chip comprising a flexible arm. 
       FIG. 18  depicts a cross-sectional view of a stack of semiconductor chips that are glued to each other. 
       FIG. 19  depicts the stack after dipping it into liquid solder material. 
       FIG. 20  depicts a cross-sectional view of a stack of semiconductor chips that are pressed against a contact frame. 
       FIG. 21  depicts a cross-sectional view of a stack that is electrically contacted with a contact frame without solder. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts a partial cross-sectional view of a semiconductor chip  1 . The semiconductor chip  1  comprises a flexible arm  2  that is preferably arranged at a side surface of a semiconductor element  3 . The semiconductor chip  1  comprises the semiconductor element  3  with a circuit path  4  which is electrically connected to a contact area  5 . The contact area  5  is arranged at a contact face  6  of the arm  2 . The circuit path  4  is also connected to a device  7  that is arranged on the semiconductor element  3 . The device  7  may be a simple sensing element, such as a resistor, an electric circuit or an electronic circuit. Depending on the embodiment, however, the device  7  may be a dynamic random access memory. 
   The contact face  6  preferably comprises a contact tip  8 . The contact tip  8  is also part of the contact area  5  that is covered with a metal layer, for example consisting of copper, nickel and/or gold. The metal layer is deposited e.g. by means of a electroplating process. The circuit path  4  is preferably deposited by a electroplating process, as well. The semiconductor element  3  preferably consists of silicon, however, it may also be made of materials that may be used for constructing a chip and for fabricating a flexible arm  2 . 
   The flexible arm  2  is arranged at a surface of the semiconductor element  3 . Depending on the embodiment, the flexible arm  2  may be arranged on a top, bottom or side surface.  FIG. 1  depicts an embodiment of the semiconductor chip  1  that comprises the flexible arm  2  at a side surface  9 . The flexible arm is in one piece with the semiconductor element  3  comprising a cantilever portion  10  and a contact portion  11 . The cantilever portion  10  emerges from the semiconductor element  3  having a smaller diameter than the contact portion  11 . The contact portion  11  carries the contact area  5 . The cantilever portion  10  is made of a shape that provides a flexible retention system of the contact portion  11 . Between the flexible arm  2  and the semiconductor element  3 , a recess  12  is arranged. By contacting the arm  2  with an electrical contact, the arm  2  may pivot in the recess  12 . Therefore, recess  12  provides space for pivoting the arm  2  in the direction of semiconductor element  3 . In one embodiment of the semiconductor chip  1 , the recess  12  is large enough to receive the whole arm  2 . The flexible arm  2  constitutes the function of an electrical micro-spring contact. If the contact area  5  is pushed in the direction of the semiconductor element  3 , the flexible arm  2  generates a spring force against the motion of the contact area  5 . Thus, the contacting force between the contact area  5  and an electrical contact increases. 
     FIG. 2  depicts the flexible arm  2  with a contact area  5  on which a solder layer  13  is deposited. For covering the contact area  5  with the solder layer  13 , the contact area  5  is dipped into liquid solder. 
     FIG. 3  shows another method for electrically connecting the contact area  5  to an electrical contact  14  that is arranged on a contact socket frame  15 . In this embodiment, the electrical contact  14  is constituted as a metal layer that is arranged on a contact socket frame  15 . The electrical contact  14  is covered with a solder layer  13 . In order to obtain an electrical connection between the contact area  5  and the electrical contact  14 , the contact socket frame is pushed against the contact area  5  as shown in  FIG. 3 . In this situation already, the flexible arm  2  is pivoted in the recess  12 . Then the solder layer  13  is heated and cooled down as shown in  FIG. 4 . Depending on the embodiment, the contact area  5  and the contact socket frame  15  may also be pressed against each other after heating up the solder layer  13 . After cooling down the solder layer  13 , an electrical connection is achieved between the contact area and the electrical contact  14 . Thus, the circuit path  4  and the devices that are connected to the circuit path  4  are electrically connected to the electrical contact  14  of the contact socket frame  15 . Instead of the method shown in  FIG. 3 , the solder could be deposited on the contact area  5 , as shown in  FIG. 2  and the contact area  5  is then pressed to the electrical contact  14  of the contact socket frame  15 . After heating and cooling, an electrical connection is achieved between the contact area  5  and the electrical contact  14 . According to this method, the solder layer  13  is deposited on the contact area  5  before pressing the contact area  5  against the electrical contact  14 . 
   Depending on the embodiment, it might not be necessary to provide a solder layer  13  and the electrical contact between the contact area  5  and the electrical contact  14  could be achieved by pressing the contact socket frame against the contact tip of the contact area  5  with the electrical contact  14 , as shown in  FIG. 5 . As a consequence of the spring force of the flexible arm  4 , the contact tip  8  is biased against the electrical contact  14 . Due to the pre-tensional force, a good and reliable electrical contact is provided between the contact area  5  and the electrical contact  14 . 
     FIG. 6  depicts a schematic side view of the side surface of the semiconductor chip  1 . The circuit path  4  is arranged on the top surface of the semiconductor element  3  and electrically connected to the contact face  6  that is arranged on the contact area  5  at the side surface of the flexible arm  2 . The flexible arm  2  starts at the top surface of the semiconductor element  3  and reaches down along the side surface of the semiconductor element  3  at a given distance D from the bottom surface of the semiconductor element  3 . The distance D from the bottom surface  16  of the semiconductor element  3  has the advantage that the flexible arm  2  may also move freely, although the semiconductor element  3  is arranged on a top surface of another semiconductor chip  1 .  FIG. 6  shows a lateral view of  FIG. 1 . 
     FIG. 7  depicts a lateral view of  FIG. 2 . The difference between  FIG. 7  and  FIG. 6  is in that on the contact face  6 , a solder layer  13  is deposited. In the schematic views of  FIG. 6  and  FIG. 7 , the device  7  is depicted as a rectangle, whereas the devices  7  are realized as planar integrated circuits. 
     FIG. 8  depicts a schematic side view of a first stack  17  comprising two semiconductor chips  1 ,  19  that are arranged upon each other. The first semiconductor chip  1  is arranged on the second semiconductor chip  19 . The second semiconductor chip  19  is arranged on a printed circuit board  17 . On an upper surface, the printed circuit board  17  comprises an electrically conductive layer  18 . The second semiconductor chip  19  is arranged with its contact face  6  above the conductive layer  18 , whereby the contact face  6  of the second semiconductor chip  19  is covered with a solder layer  13  that is also in contact with the conductive layer  18 . Furthermore, the solder layer  13  also covers the contact face  6  of the first semiconductor chip  1 . The first semiconductor chip  1  is arranged on the second semiconductor chip  19  in the same position as the second semiconductor chip  19 . Thus, the flexible arms  2  of the first and the second semiconductor chip  1 ,  19  are arranged on top of each other. The solder layer  13  constitutes an electrical connection between the contact faces  6  of the first and second semiconductor chip  1 ,  19  and the conductive layer  18  of the printed circuit board  17 . 
   The following  FIGS. 9 to 17  depict process steps of a method of producing a semiconductor chip with a flexible arm as a micro-spring contact. 
     FIG. 9  depicts a cross-sectional view of a wafer  20  with a multitude of semiconductor chips  1 , whereby in  FIG. 9  only two semiconductor chips  1  are shown. The wafer  20  consists of a semiconductor material, e.g. silicon. On the surface of the wafer  20 , two conductive lines  21  are arranged. The conductive lines  21  may include contact pads. The other surface of the wafer  20  is covered by a protective layer  22  that is made of e.g. polyamide. The conductive lines  21  are e.g. made of aluminium. Between the two semiconductor chips  1 , a dicing area  23  is disposed which is not covered by the protective layer  22 . The dicing area  23  and the protection layer  22  and the conductive lines  21  are covered by a photoresist layer  24 . The photoresist layer  24  is structured in the dicing area  23  with a central opening  25  and a first and second side opening  26 ,  27 . For the structuring of the photoresist layer  24 , suitable photo masks are used. This process situation is shown in  FIG. 9 . 
   Thereafter, a first and second arm  2 ,  28  are produced out of the semiconductor material of the wafer  20  using an etching process. Beneath the first and the second arm  2 ,  28 , an undercut  29  is etched into the wafer  20 . For etching the first and the second arm  2 ,  28 , an anisotropic etch step is used that continues with an isotropic etch step in order to achieve the undercut  29  of the first and second arm  2 ,  28 . With the anisotropic etch process, the lateral sides of the first and the second arm  2 ,  28  are fabricated. The undercut  29  is fabricated by means of the isotropic etch step forming a vertical bottom side of the arms  2 ,  28 . Preferably, the anisotropic etch process is used until a depth of 50 μm and the following anisotropic etch process is used for a further depth of 5 μm. This process situation is shown in  FIG. 10 . 
   In a following step, a dielectric layer  30 , e.g. silicon nitride is deposited on the whole surface of the structured wafer  20 . Particularly, the surface of the first and second arm  2 ,  28  and the surface of the wafer  20  in the region of the first and second side openings  26 ,  27  and the surface of the undercut  29  are covered by the dielectric layer  30 . The silicon oxide layer  30  is preferably deposited by a plasma-enhanced chemical wafer deposition at a temperature of 150° C. This process situation is shown in  FIG. 11 . 
   In a further process step, a polymer layer  31  is printed onto the protective layer  22  and the dicing area  23  filling up the central opening  25 , the side openings  26 ,  27  and the undercut  29 . Thereafter, the silicon oxide layer  30  is stripped off the conductive lines  21 . This process step is shown in  FIG. 12 . 
   In a further process step, the polymer layer  31  is stripped off. For a simple embodiment of the process, the polymer layer  31  is made of the same material as the photoresist layer  24 . After this, a seed layer  32  is deposited on the surface of the structured wafer  20 . The seed layer is deposited on the whole surface and within the etching chamber  29  and on the surface of the first and second arm  2 ,  28 , as well. As a material for the seed layer, e.g. titanium-copper may be used. This situation is shown in  FIG. 13 . 
   In a further process step, a first and a second side region next to the conductive line  21  is covered by a electrophoretic photoresist layer  33 . This process step is shown in  FIG. 14 . In a further process step, a metal layer  34  is deposited in the region that is not covered by the second photoresist layer  33 , e.g. by an electroplating process. The metal layer  34  covers the conductive lines  21 , the protective layer  22  and the upper and side faces of the first and second arm  2 ,  28 . In addition, a central part of the bottom face of the undercut  29  is covered by the metal layer  34 . Advantageously, the faces of the first and second arm  2 ,  28  adjoining the central opening  25  are covered by the metal layer  34 . This process step is shown in  FIG. 15 . 
   In a following process, the wafer  20  is fixed by its upper surface to a carrier plate  35 , as shown in  FIG. 16 . The carrier plate may e.g. be an adhesive tape or a thin plate with an adhesive layer. The wafer  20  is subsequently thinned out from the bottom face until the etching chamber  29  is reached, as is shown in  FIG. 16 . 
   After this step, a polyamide layer  36  is deposited on the bottom side of the wafer  20 , as shown in  FIG. 17 . The polyamide layer  36  is e.g. deposited in a roll printing process. After removing the carrier plate  35  and subsequent curing of the polyimide layer, a multitude of semiconductor chips  1 ,  19  are obtained from the wafer  20 . The handling of the fabrication processes is more easily performed at the wafer level. Thus, the various semiconductor chips  1 ,  19  are separated after the fabrication processes. 
   As an etching process, e.g. a plasma-etching process is used for forming the first and second arm  2 ,  28  that constitute micro-springs at the rim side of each semiconductor chip  1 ,  19 . 
   Depending on the embodiment of the processes, the plasma etching used for forming the first and the second arm  2 ,  28  are used for dividing the wafer  20  up into several semiconductor chips  1 ,  19  that are separated by the following thinning process as shown in  FIG. 16 . The plasma etching process allows to round off the semiconductor chip corners by a design that prevents chipping particularly for thin dies compared to sawing. The subsequent handling of semiconductor chips may also be less critical. The proposed design allows the testing of dies before packaging in an stack. This increases the yield of the chip stacks. 
   The semiconductor chips  1  may be arranged in a stack  37  as shown in  FIG. 18 .  FIG. 18  depicts a schematic view on a stack of four semiconductor chips  1  in a cross-sectional view. The semiconductor chips  1  are aligned in the same direction, whereby the flexible arms  2  with the contact areas  5  and the contact faces  6  are arranged at the same side of the stack  37 . Beside the stack  37 , a second conductive layer  39  is arranged on the surface of the second printed circuit board  38 . 
   The stack  37  is dipped into a liquid solder  40  that wet the contact faces  6  of the arms  2  of the semiconductor chips  1 . Furthermore, the liquid solder covers the second conducting layer  39 , as well. After cooling down, an electrical connection between the semiconductor chips  1  and between the semiconductor chips  1  and the second conducting layer  9  is attained by the solder  40 , as shown in  FIG. 19 . Depending on the embodiment, an additional solder stop layer may be used for preventing the liquid solder from flowing into the chamber  12 . 
     FIG. 20  shows a cross-sectional view through a stack of semiconductor chips  1  with flexible arms  2  that are put into a socket frame  41 . The socket frame  41  comprises a further metal layer  43  at an inner side  42  that is electrically connected by the solder  40  to the contact faces  6  of the arms  2 . The stacking is brought into contact with the socket frame by a fastening clip  44 . Depending on the embodiment, the semiconductor chips  1  of the stack are optionally tested before and subsequently soldered to the further metal plane  43  of the socket frame  41 . 
     FIG. 21  depicts a cross-sectional view of a further embodiment of a stack of semiconductor chips  1  that is electrically connected to a further metal plane  43  of a socket frame  41  without solder  40 . In this embodiment, the electrical contact is directly achieved between the further metal plane  43  and the contact face  6  of the arms  2 . Therefore, the stack  37  is biased against the socket frame  41 . Therefore, a latching clip  44  is used. 
   The arms  2 ,  28  with the contact areas  5  and the contact faces  6  may be used as temporary electrical contact sockets for testing the chip  1 ,  19  or for a permanent anchorage inside of a frame. Moreover, the micro-springs act as a stress release to solder bumps or other hard contacts to the semiconductor chips  1 ,  19 .