Abstract:
A method for manufacturing a chip housing includes a first basis having a photolithograpically structurable layer on a main face, structured into a cover. A chip has the structure at a main face between first contact elements. A second photolithograpically structurable layer applied to the main face is structured forming a recess surrounded by a wall near the structure exposing the first contact elements. Then, the first basis and the chip are merged with the structure and the cover facing and aligned with each other, and the recess closed by the cover. Removing the first basis leads to an on-chip cavity. Afterwards, a second basis and the chip are merged with the first contact elements connected to the second basis via a conductive structure. Afterwards, the second basis is removed for exposing the conductive structure. The method is less subject to cost and size limitations of known housing technologies.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for manufacturing a housing for a chip having a micromechanical structure. 
   BACKGROUND OF THE INVENTION AND PRIOR ART 
   Chips having micromechanical structures or so-called micromechanical circuits, respectively, have an increasing market proportion with high frequency circuits and frequency filters. One of the main markets for chips of that kind having micromechanical structures is the mobile communication market. A chip having a micromechanical structure which is also referred to as a micromechanical circuit is a semiconductor device which is implemented with a micromechanical structure on its surface. For such circuits individual housing technologies are required, wherein the housing needs to determine a cavity around the micromechanical structure. 
   A common practice for housing the chip having a micromechanical structure known in the prior art is to use a housing element with a cavity consisting of ceramic. These ceramic housing structures are both too expensive and too large for the technology requirements resulting today. Typical dimensions of such ceramic housings for a chip having a micromechanical structure are about 3 mm×3 mm×1.3 mm. These dimensions may not be further reduced using the common ceramic housing technologies. 
   An alternative method is proposed by WO 9952209 A1 which discloses a method for housing an acousto wave device without the contamination of an active region arranged on a main face of same. A substrate, which comprises conductive pads and a dyke  26  on the top side of the same is connected to the acousto wave device, such that the main face of the acousto wave device  10  and the top side of the substrate are opposing, whereby a cavity around the active region is formed and the conductive pads are connected to contact bumps on the acousto wave device. The resulting housing structure includes the acousto wave device, the substrate and the cavity lying between the same, which is successively surrounded by an underfiller material. The considerable height due to the presence of a mounting substrate is a disadvantage of the resulting structure. 
   Based on this prior art the present invention is therefore based on the object to provide a method for manufacturing a housing for a chip having a micromechanical structure, which is no longer subject to the cost and size limitations of known housing technologies. 
   SUMMARY OF THE INVENTION 
   In accordance with a first aspect of the invention, this object is achieved by a method for manufacturing a housing for a chip having a micromechanical structure, comprising the following steps: 
   (a) providing a first basis which comprises a photolithographically structurable layer on at least a partial region of a main face; 
   (b) photolithographically structuring the first photolithographically structurable layer to obtain a cover for the micromechanical structure; 
   (c) providing a chip comprising the micromechanical structure, which is arranged at a main face of the chip between the first contact elements; 
   (d) applying a second photolithographically structurable layer on at least a partial region of the main face of the chip; 
   (e) photolithographically structuring the second photolithographically structurable layer for generating a recess surrounded by a wall in the photolithographically structurable layer in the region of the micromechanical structure and for exposing the first contact elements; 
   (f) merging the first basis and the chip in a way so that the micromechanical structure and the cover are facing each other and are aligned with each other, so that the recess is closed by the cover, whereby an on-chip cavity is obtained; 
   (g) removing the first basis in order to obtain a chip comprising an on-chip cavity; 
   (h) merging a second basis and the chip comprising the on-chip cavity in such a way that the first contact elements are connected to the second basis via a conductive structure; and 
   (i) removing the second basis for exposing the conductive structure. 
   According to an inventive method, on a main surface of a first basis a first photolithographically structurable layer is applied within at least one partial region of the main surface of the first basis and is structured photolithographically in order to obtain a cover for the micromechanical structure. A second photolithographically structurable layer is applied to at least one partial region of the main face of a chip with the micromechanical structure, which is arranged at a main face of the chip between first contact elements, and is photolithographically structured in a suitable way in order to generate a recess surrounded by a wall in the second photolithographically structurable layer in the region of the micromechanical structure and to expose the first contact elements. Subsequently, the first basis and the chip are merged in a way so that the micromechanical structure and the cover are facing each other and are aligned with each other, so that a recess is closed by the cover, whereby an on-chip cavity is obtained. By removing the basis a chip having an on-chip recess is thus obtained. A second basis is merged with the thus obtained chip having an on-chip cavity, in such a way, that the first contact elements are connected to the second basis via a conductive structure. Finally, the second basis is removed for exposing the conductive structure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, preferred embodiments of the present invention are explained in more detail with reference to the accompanying drawings, in which: 
       FIGS. 1A and 1B  show a first basis, which serves as a support of the cover for the cavity around the micromechanical structure, in two method steps according to one embodiment of the present invention; 
       FIGS. 2A–2C  show a chip having the micromechanical structure in three further method steps for manufacturing the housing according to an embodiment of the present invention; 
       FIGS. 3A–3C  show the first basis manufactured in  FIGS. 1A and 1B  after merging with the chip from  FIGS. 2A–2B  in three further method steps for manufacturing the housing according to an embodiment of the present invention; 
       FIGS. 4A–4C  show the structure resulting from the method steps of  FIGS. 3A–3C  after merging with a second basis in three further method steps for manufacturing the housing according to an embodiment of the present invention; 
       FIG. 5  shows a structure which is set in a step corresponding to the method step of  FIG. 4B  according to a transformed embodiment of the method; 
       FIG. 6A  shows a second basis provided with metal islands for manufacturing the housing according to a further embodiment of the present invention; 
       FIG. 6B  shows a structure which is set according to a method step corresponding to the step of  FIG. 4C  when using the second basis according to  FIG. 6A ; and 
       FIGS. 7A and 7B  show a front view and a bottom view of a structure resulting upon two method steps corresponding to the step of  FIG. 4B  according to a further embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Before preferred embodiments of the present invention are discussed in more detail with reference to the figures, it is noted that similar elements or elements with the same functions are provided with the same or similar reference numerals in the figures, respectively, and that for preventing repetitions a renewed description of these elements is omitted. 
   In the method steps of  FIGS. 1A and 1B  first of all the preparation of a support wafer or of a first basis is described, respectively, which is to serve as a support for a cover for a cavity which surrounds a micromechanical structure of a chip for which the housing is to be produced. 
   As it is shown in  FIG. 1A , first of all the support wafer  10  is provided, which consists for example of a semiconductor material, like e.g. Si. Alternatively, any other material may be used for the first basis  10 , which is compatible with the process steps to be discussed in the following. On a main face of the wafer  10  a photosensitive epoxy layer  12  is located, wherein between the epoxy layer  12  and the wafer  10  a sacrificial layer  14  is arranged. The sacrificial layer  14  serves to separate the wafer  10  from a cover formed from the epoxy layer  12 , as it is discussed in the following. 
   As it is shown in  FIG. 1B , in a next method step a photolithographic structuring of the photosensitive epoxy layer  12  is performed to obtain an epoxy region  16  which is to serve as the cover for the micromechanical structure. With this photolithography at least those regions of the photosensitive epoxy layer  12  need to be exposed, so that they remain after the developing, which are opposed to the “active” region around the micromechanical structure of the chip after the arrangement of the housing. 
   The method steps to be explained now with reference to  FIGS. 2A to 2C  are implemented at the chip  20  including the micromechanical structure. The term “chip” as defined by the present invention is any semiconductor device on which a micromechanical structure is implemented. As a micromechanical structure, for example a BAW filter (BAW=bulk acoustic wave) is conceivable. 
   As it is shown in  FIG. 2A , the chip comprises a micromechanical structure  22  at its bottom, which is connected to contact bumps  24 ,  26  arranged also at the bottom of the chip  20 . As far as the provided chip  20  comprising the micromechanical structure does not yet comprise these contact bumps  24 ,  26 , the performance of a corresponding metallization method step is required for generating the bottom contact bumps  24 ,  26  (“underbump metalization”). 
   In the method step shown in  FIG. 2B  a coating onto the surface of the chip or the semiconductor wafer  20 , respectively, is performed for example by spin coating using a photosensitive epoxy layer. This spin coating may be repeated several times for building up a desired layer thickness which determines the thickness of the cavity to be realized later, until a second photosensitive epoxy layer  28  of the desired thickness has built up on the bottom of the chip  20 . 
   As it is illustrated in  FIG. 2C , now a photolithographic structuring of the second epoxy layer  28  is performed for generating a recess  32  surrounded by a wall or a dyke  30 , respectively, and for exposing the contact bumps  24 ,  26 . The wall  30  encloses the “active region” around the micromechanical structure  22 . 
   The method steps shown in the following with reference to  FIGS. 3A–3C  are related to the merging of the chip prepared as described above and the basis prepared as described above and to the processing carried out at the resulting structure. 
   As it is shown in  FIG. 3A , after the preparation of the chip  20  and the basis  10  a first merging step takes place, in which the basis  10  and the chip  20  are merged in such a way that the micromechanical structure  22  and the cover  16  are facing each other and are aligned with each other, so that the recess  32  defined by the wall  30  is closed by the cover  16 , whereby an on-chip cavity  32  on the chip  20  around the micromechanical structure  22  is obtained. Consequently, the wall  30  together with the first epoxy layer  12  generates a closed cavity from the recess  32  which surrounds the micromechanical structure  22 . 
   In the following method step shown in  FIG. 3B , the support wafer  10  is detached from the epoxy region forming the cover by etching away the sacrificial layer  14  by a suitable etching method. The result is the chip shown in  FIG. 3B  comprising the on-chip cavity around the micromechanical structure  22 . For detaching, however, also an alternative method could be used, like e.g. grinding, wherein the sacrificial layer  14  may be abandoned. 
   In a method step illustrated in  FIG. 3C , soldering pellets  34 ,  36  are applied to the contact bumps  24 ,  26 . The soldering pellets are used at least as part of a conductive structure, to connect the contact bumps  24 ,  26  to a basis consisting of copper, which is subsequently removed again, whereby the conductive structure provides a conductive connection path between the contact bumps  24 ,  26  and the housing bottom or the pad side, respectively. An alternative proceeding for connecting is generating a stud bumping, as it is briefly explained with reference to  FIG. 7 . 
   The method steps explained in the following with reference to  FIGS. 4A–4C  refer to the processing of the on-chip cavity chip, prepared as described above, for closing this structure, wherein for this a further copper basis is used as the sacrificial substrate, to define the lower pad of the final housing when closing the structure and to be removed in the end. 
   As it is shown in  FIG. 4A , first of all the chip having an on-chip cavity, prepared as in  FIG. 3C , and a copper basis  40  are merged such that the soldering pellets  34 ,  36  form a conductive structure from the contact faces  24 ,  26  up to that main face of the copper basis  40 , which faces the chip  20 . This connection of the contact bumps  24 ,  26  to the facing main face of the copper basis  40  is performed by soldering or by a thermocompression process. In the present embodiment the merging is performed such that after merging the chip  20  contacts the copper base plate  40  with the cover  16  of the on-chip cavity  32 . The temperatures used in soldering or in the thermocompression process should lie above the temperatures as they are used in the subsequent step of  FIG. 4B  for closing the structure generated up to here. 
     FIG. 4B  shows a state as it is set after the next method step, i.e. after closing the so far generated structure of  FIG. 4A  with a covering layer  42 . This method step preferably takes place at an increased temperature level at which a plastics material forming the covering layer  42  is liquefied. In the final sinking of the temperature level a contraction of the plastic material results, which contributes to a solidification of the so far manufactured structure. 
   In the final method step shown in  FIG. 4C , the basis  40  is removed by a copper etching process, whereby the conductive structures formed by the soldering pellets  34 ,  36  become accessible for a later contacting at the exposed main surface of the structure comprised of the covering layer  42  and the chip  20  comprising the on-chip cavity  34 . The thus generated housing is generally indicated by  44  in  FIG. 4C . The bottom of the housing  44  generally serving as an attachment face or a contacting face, respectively, is indicated by  46 . The bottom  46  is comprised of three parts, i.e. one which is formed by the epoxy of the cover  16 , one which is formed by the conductive material of the soldering pellets  34 ,  36  and one which is formed by the plastic of the covering layer  42 . 
   After performing the copper etching step described with reference to  FIG. 4C , preferably a gold plating of the exposed contact regions of the conductive structures  34 ,  36  is performed at the bottom  46  of the housing  44 , which is now exposed. 
   According to one variation of the above-described method for manufacturing the housing, in the step of  FIG. 4A , i.e. the merging of the chip with the on-chip cavity and the copper base plate, the merging is performed such that after merging the soldering pellets  34 ,  36  result in conductive structures from the contact bumps  24 ,  26  to the main face of the copper base plate  40  facing the chip and between the epoxy region  16  forming the cover and the main face of the copper base plate  40  facing the chip  20  a gap remains, so that, as it is shown in  FIG. 5 , after closing the thus resulting structure by the covering layer  42  the structure shown in  FIG. 5  results. By the contraction resulting from the decrease of the temperature the plastics material used when closing causes that the wall  30  is firmly pressed to the opposing epoxy portion serving as a cover. For completing the housing according to  FIG. 5 , only the method steps described with reference to  FIG. 4C  are to be performed. A housing which is manufactured according to the variation of  FIG. 5  is different from the bottom of a housing according to  FIG. 4C  in that it is only divided into two parts, i.e. one formed by the plastics material of the covering layer  42  and one formed by the contacting structures defined by the soldering pellets  34 ,  36 . 
   A further variation of the method proceeding described above with reference to  FIGS. 1A–4C  is described with reference to  FIGS. 6A and 6B . In this embodiment instead of the pure copper base plate  40 , as it was used in the method step  4 A, an already prepared basis  40  is used in which on a main side, which should later be facing the chip having the on-chip cavity during merging, metal islands  50 ,  52  are formed. Preferably, the metal islands are embodied as nickel-plated islands on the copper basis  40 , which are coated with a gold-plating. The way of the arrangement of these islands  50 ,  52  and the size of these islands  50 ,  52  is chosen so that they correspond to the contacting bumps  24 ,  26  at the bottom of the chip  20 . In this variation, the copper base plate  40  prepared as shown in  FIG. 6A  is merged with the chip comprising on-chip cavity as described with reference to  FIG. 4A , such that their mentioned main faces are facing each other and that the respectively opposing metal islands  50 ,  52  and contact bumps  24 ,  26  are connected to each other via the soldering pellets  34 ,  36  by soldering or by a thermocompression process, in such a way, that either the epoxy portion  16  forming the cover contacts the copper base plate  40 , as it is shown in  FIG. 4A , or a gap between the portion  16  and the base plate  40  remains, as it is shown in  FIG. 5 . The housing resulting according to this variation after performing the steps according to  FIGS. 4A–4C  is shown in  FIG. 6B . The advantage of this proceeding is that the cross-section of the pads or the form of the contact regions, respectively, at the bottom of the housing may be varied as required, which makes it possible to arrange different chips in the housing which have the same contact region arrangement. The metal islands  50 ,  52  are for example generated or structured, respectively, in a common way by a photo process on the copper base plate  40 . 
   A special embodiment of a housing manufactured according to an inventive method is shown in  FIGS. 7A and 7B .  FIG. 7A  shows a front projection view, while  FIG. 7B  shows a bottom or mounting side, respectively, of the housing. The housing is generally indicated by  44 ′. It was manufactured according to the method of  FIGS. 1A–4C  with the variations of  FIGS. 5 and 6A  and  6 B, i.e. a gap exists between the epoxy  16  forming the cover and the bottom or mounting main face  46  of the housing  44 ′, respectively, and the conductive structures forming the conductive connection to the bottom or mounting side, respectively, of the housing  44 ′ for the contact bumps  24 ,  26  on the bottom of the chip  20  further include the metal islands  50 ,  52 . As a variation to the solder application step of  FIG. 3C , in the housing  44 ′ of  FIG. 7A  and  FIG. 7B  a stud bumping was used. The conductive structures providing a conductive connecting path from the contact bumps  24 ,  26  on the bottom of the chip  20  to the mounting side  46  of the mounting  44 ′ therefore include stud bumps  60 ,  62 , studs of nickel and gold  64 ,  66  and the metal islands  50 ,  52  of nickel as mentioned above. In  FIG. 7A  the housing  44 ′ is illustrated in this state, as the same is already attached to a lead frame or a printed circuit board  70 , respectively. In order to adjust the resulting contact area arrangement resulting at the bottom  46  of the housing  44 ′ to the terminal configuration provided on the board  70 , and to manufacture a printed circuit board  70  having uniform terminal configurations, the metal islands  50 ,  52  or  50 ′ and  52 ′, respectively (only visible in  FIG. 7B ) were suitably arranged and sized on the copper base  40 . Additionally, a dummy metal island  72  was formed when structuring the metal on the copper base plate  40 , which is not connected to one of the contact bumps on the bottom of the chip  20 , but only serves as a dummy terminal for soldering onto the board  70 . According to this embodiment of  FIGS. 7A and 7B  consequently for every chip  20  a terminal configuration may be achieved, as it is shown in  FIG. 7B , and which for example serves as a standard configuration. 
   In other words, for every chip a housing having a terminal configuration may be achieved, which is adjusted to the target terminal configuration on a desired board  70 , independent of the number of terminals and the type of the chip to be housed. 
   As the preceding embodiments showed, it is consequently possible by the present invention to manufacture terminal-compatible housings without additional redistribution layers being necessary for it. Additionally, it is made possible to variably adjust the terminal configuration on the bottom of the housing, i.e. the footprint, to the desires of the customer. Additionally, based on the inventive method, housings with small dimensions may be manufactured, in particular with a small size, like e.g. dimensions of 1.5 mm in the lateral direction and 0.4–0.6 mm in the height direction. Also the pad construction itself may be variably implemented, by providing the metal islands and the interconnecting studs, to be adjusted to the respective connecting technology regarding the board  70 , like e.g. soldering, bonding or adhering. In the case of the stud bonding according to  FIGS. 7A and 7B , studs without or with overgrowth are possible. Compared to the ceramic housings better anchoring possibilities for the stability of the housing result, and a better reliability, like e.g. on the JEDEC level (JEDEC=Joint Electronic Device Engineering Council) may be achieved. 
   With reference to the metal islands of  FIG. 6A  or  FIG. 7A , respectively, it is noted that they may comprise an exterior outline comprising protrusions and retreats or an overgrowth, respectively, for a better anchoring in the cover layer material. This exterior outline offers a better anchoring of the metal islands in the cover layer material. 
   The above-described method is based on a base plate consisting of copper. As the base plate only illustrates a sacrificial structure, instead of copper any other easily removable material may be used for the base plate  40 , preferably a material which is removable using etching. Similar things hold true for the support wafer of  FIGS. 1A and 1B , as mentioned above. 
   For the metal islands and contact bumps instead of using nickel as the base material with a gold plating as a cover any other contact material may be used. 
   In the described preferred embodiments the photolithographically structurable layers consist of a photosensitive epoxy material which is removed or remains by exposing or not exposing, respectively, parts of the epoxy material. Similarly, it is possible, however, to form the photolithographically structurable layers by any materials which may be etched which are covered with photo masks. Deviating from the above-described preferred embodiments, an enclosure of the manufactured housing structure may be provided using vacuum screen printing or molding.