Abstract:
A process for manufacturing a packaged microelectromechanical device includes: forming a lid having a face and a cavity open on the face; coating the face of the lid and walls of the cavity with a metal layer containing copper; and coating the metal layer with a protective layer.

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to a process for manufacturing a lid for an electronic device package, to a process for manufacturing a packaged microelectromechanical device, to a lid for an electronic device package, and to a packaged microelectromechanical device. 
     Description of the Related Art 
     MEMS (microelectromechanical systems) devices find increasingly extensive use in a wide range of sectors as miniaturized sensors or transducers. For example, microphones and pressure sensors are frequently used in mobile communication devices and filming apparatuses, such as cell phones and video cameras. 
     Given that the extremely marked miniaturization of MEMS devices entails a certain fragility of micromechanical structures, it is common to use protective lids that encapsulate the parts more readily subject to failure. Normally, MEMS sensors or transducers are mounted on substrates, possibly with control circuits. The substrates are coupled to respective lids and form packages within which the devices to be protected are located. 
     The protective lids also perform other functions, in addition to that of mere mechanical barrier. 
     In particular, in many cases, the transmission of the signals may be disturbed by the environment, and hence it is necessary to envisage a protection from light and electromagnetic interference. For this purpose, the cavities of the lids are coated internally by metal shielding layers. The lids may moreover have the function of determining optimal conditions of acoustic pressure for operation of the MEMS sensors. 
     The protective lids are in general bonded to the substrate on which the MEMS sensors are mounted by conductive glues, which enable grounding of the electromagnetic shielding layer. 
     Soldering pastes, for example with a base of tin-lead, tin-aluminum-copper, or tin-antimony, would in themselves be preferable to conductive glues, especially on account of the better resistance to impact demonstrated by the results of drop tests. However, soldering pastes melt during the steps of assembly of a package (comprising supporting board, MEMS sensor, and lid) to the boards of the electronic system in which the MEMS sensor is to be used. Molten soldering pastes tend to climb up the vertical conductive walls of the lid, invading the cavities in which the MEMS sensor is housed and leaving empty spaces in the soldering joints. The empty spaces in the soldering joints are particularly undesirable, because, on the one hand, they weaken soldering and, on the other, may cause leakages that affect the performance of the devices, especially when a controlled-pressure reference chamber is desired. 
     There is thus felt the need to allow the use of soldering pastes in the production of packaged electronic devices comprising microelectromechanical structures. 
     BRIEF SUMMARY 
     One or more embodiments of the present invention is to provide a process for manufacturing a lid for an electronic device package, a process for manufacturing a packaged microelectromechanical device, a lid for an electronic device package, and a packaged microelectromechanical device that allow to overcome the limitations described and, in particular, enable use of soldering pastes eliminating or at least reducing the risk of migration of molten soldering paste in cavities for housing the microelectromechanical devices during final assembly. 
     According to various embodiments of the present invention a process for manufacturing a lid for an electronic device package, a process for manufacturing a packaged microelectromechanical device, a lid for an electronic device package, and a packaged microelectromechanical device are provided. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the invention, some embodiments will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
         FIG. 1  shows a cross section through a first substrate, in an initial step of a process for manufacturing a packaged microelectromechanical device according to one embodiment of the present invention; 
         FIG. 2  shows the first substrate of  FIG. 1  in a subsequent processing step; 
         FIG. 3  shows a cross section through a second substrate in a step of the process; 
         FIG. 4  shows a cross section through a lid according to one embodiment of the present invention, obtained by joining the first substrate of  FIG. 2  and the second substrate of  FIG. 3 ; 
         FIGS. 5-7  show the lid of  FIG. 4  in successive steps of the process; 
         FIG. 8  is a cross section of a third substrate in a step of the process; 
         FIG. 9  shows a cross section through a composite structure obtained in an intermediate step of the process, by joining the lid of  FIG. 7  and the composite structure of  FIG. 8 ; 
         FIG. 10  shows the composite structure of  FIG. 9  in a subsequent step of the process; 
         FIG. 11  shows a packaged microelectromechanical device according to one embodiment of the present invention in a final step of the process; 
         FIG. 12  is a cross section through a packaged microelectromechanical device, obtained by a process according to a different embodiment of the present invention; 
         FIG. 13  is a cross section through a lid according to one embodiment of the invention, incorporated in the packaged microelectromechanical device of  FIG. 12 ; 
         FIG. 14  is a cross section through a substrate, in an initial step of a process for manufacturing a packaged microelectromechanical device according to one embodiment of the present invention; 
         FIG. 15  shows a cross section through a lid according to one embodiment of the present invention, obtained from the substrate of  FIG. 14 ; 
         FIG. 16  shows a packaged microelectromechanical device according to one embodiment of the present invention in a final step of the process; 
         FIG. 17  is a block diagram of a packaged microelectromechanical device; and 
         FIG. 18  is a block diagram of an electronic system incorporating the packaged microelectromechanical device of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION 
     In an initial step of a process for manufacturing a packaged microelectromechanical device, to which  FIG. 1  refers, a first face  1   a  and a second face  1   b  of a first protective substrate  1  are coated, respectively, with a first conductive layer  2   a  and a second conductive layer  2   b , both made of metal, in particular copper. In one embodiment, the first protective substrate is made of an organic material, for example bismaleimide triazine (BT). Moreover, an adhesive layer  3  is laminated on the second conductive layer  2   b.    
     Next ( FIG. 2 ), a through cavity  5  is formed in the first conductive layer  2   a , in the first protective substrate  1 , in the second conductive layer  2   b , and in the adhesive layer  3 , for example by through punching. 
     As illustrated in  FIG. 3 , a second protective substrate  7 , which has a smaller thickness than the first protective substrate  1  and is made of the same material, is prepared separately. In particular, a first face  7   a  and a second face  7   b  of the second protective substrate  7  are coated with a third conductive layer  8   a  and with a fourth conductive layer  8   b , made, for example, of the same material used for the first conductive layer  2   a  and for the second conductive layer  2   b , which in the embodiment described is copper. 
     The first protective substrate  1  is then bonded to the second protective substrate  7  (more precisely to the third conductive layer  8   a ) through the adhesive layer  3 , as illustrated in  FIG. 4 . In this way, a lid  9  is obtained, in which the first protective substrate  1  and the second protective substrate  7  form, respectively, side walls  5   a  and a covering of the cavity  5  on a side opposite to the first face  1   a  of the first protective substrate  1 . 
     After the first protective substrate  1  and the second protective substrate  7  have been bonded, the lid  9  is coated internally with conductive material by a process of plating, followed by a process of electrodeposition ( FIG. 5 ). In one embodiment, the conductive material is the same as the one used for forming the first layer  2   a , the second layer  2   b , and the third conductive layer  8   a , in particular copper. Residual portions of the first layer  2   a  and of the second layer  2   b  and the third conductive layer  8   a  hence remain incorporated in a shielding layer  10 . The shielding layer  10  coats the first face  1   a  of the first protective substrate  1  and the walls of the cavity  5 , i.e., the side walls  5   a  and a portion of the first face  7   a  of the second protective substrate  7  facing the cavity  5 . 
     Next, the lid  9  is washed and a protective organometal layer  11  made of organic surface protection (OSP) material is deposited on the shielding layer  10  before the surface of the shielding layer  10  itself is oxidized with copper (II) oxide (CuO). Following upon washing, in fact, a layer of copper (I) oxide (Cu 2 O) is formed on the surface of the shielding layer  10  and tends in a short time to oxidize further into copper (II) oxide. 
     The protective organometal layer  11  is formed both within the cavity  5  (on the side walls  5   a  and on the portion of the first face  7   a  of the second protective substrate  7  facing the cavity  5 ) and on the first face  1   a  of the first protective substrate  1 . In one embodiment, in particular, the protective organometal layer  11  is made of a one-pass OSP material. OSP materials, which are commonly used in the production of printed circuits, are obtained by depositing substances such as imidazole and imidazole derivatives, which, in contact with copper, form organometal compounds capable of preventing oxidation of the surface copper. OSP materials are can be removed thermally or else chemically, for example in acid. In the family of OSP materials, one-pass OSP materials form organometal compounds that present greater ease of removal by thermal cycles. In particular, the organometal compounds formed by one-pass OSP materials are substantially removed if subjected to the thermal stress determined by a single cycle of soldering during printed-circuit-board assembly. In one embodiment, the OSP material is obtained by depositing benzotriazole, which forms a compound of Cu(I) benzotriazole. 
     The protective organometal layer  11  thus prevents oxidation of the shielding layer  10 . 
     Processing of the second protective substrate  7  is then completed with opening of a through sound port  12  ( FIG. 7 ) so as to set the cavity  5  in communication with the outside world after closing of the lid  9  with another substrate. 
     It is to be appreciated that various steps of the method may be performed sequentially, in parallel, omitted or in an order different from the order that is described and illustrated. A supporting substrate  13  ( FIG. 8 ), mounted on which are a first chip, integrating a MEMS acoustic transducer  15  (for example, with capacitive variation), and a second chip, integrating an ASIC (application-specific integrated circuit) control circuit  16 , is prepared separately by deposition of a layer of soldering paste  17 , for example with a base of tin-lead, tin-aluminum-copper, or tin-antimony, on a soldering surface. 
     The supporting substrate  13  is a composite substrate made of organic material, for example BT, and comprises conductive paths  18  set on a plurality of levels and connected by interconnections  19  (represented purely by way of example). The MEMS acoustic transducer  15  and the control circuit  16  are mounted on a face  13   a  of the supporting substrate  13  to be fitted to the lid  9 . The layer of soldering paste  17  extends over the face  13   a  of the supporting substrate  13  around the MEMS acoustic transducer  15  and the control circuit  16 . 
     The lid  9  is then joined to the supporting substrate  13  as illustrated in  FIG. 9 , with the protective organometal layer  11  in contact with the layer of soldering paste  17  so that the MEMS acoustic transducer  15  and the control circuit  16  remain housed in the cavity  5 . 
     The lid  9  and the supporting substrate  13  are heated until melting of the layer of soldering paste  17  is obtained ( FIG. 10 ). The protective organometal layer  11  is thermally destroyed and releases the shielding layer  10 , enabling formation of a conductive soldering joint  20  with the shielding layer  10  itself. In particular, where the protective organometal layer  11  is in contact with the shielding layer  10 , the soldering paste penetrates into the protective organometal layer  11 , which is destroyed. The OSP material of the protective organometal layer  11  is removed by a flux that is contained in the soldering paste or, alternatively, is deposited prior to soldering. Within the cavity  5  the protective organometal layer  11  vaporizes. The molten soldering paste rises by capillarity into the protective organometal layer  11  also for a short stretch along the shielding layer  10  within the cavity  5 . Penetration within the cavity  5  is, however, negligible. 
     A packaged microelectromechanical device  25 , in particular a MEMS microphone, is thus formed, comprising the MEMS acoustic transducer  15 , the control circuit  16 , and a package  24 , forming part of which are the lid  9  and the supporting substrate  13 . 
     Finally, the shielding layer  10 , in direct contact everywhere with the atmosphere present in the cavity  5 , is coated with a protective layer of copper (II) oxide  26 . 
     Advantageously, the protective layer of copper (II) oxide  26  is permanent and has a very low wettability. For this reason, also during subsequent steps of assembly of the packaged microelectromechanical device  25  to a printed-circuit board, given that the molten soldering paste is unable to climb up the shielding layer  10 , which is protected by the protective layer of copper (II) oxide  26 , it remains confined in the region of the soldering joint  20  and does not invade the cavity  5 . 
     It is thus possible to use soldering paste instead of conductive glues, without any need to resort to costly solutions, such as Ni-Au plating processes. 
     According to the embodiment illustrated in  FIG. 12 , a packaged microelectromechanical device  125 , in particular a MEMS microphone, comprises a MEMS acoustic transducer  115 , integrated in a first chip, a control circuit  116 , integrated in a second chip, and a package  124 . 
     The package  124  comprises a lid  109  and a supporting substrate  113 , on which the MEMS acoustic transducer  115  and the control device  116  are mounted. The lid  109 , obtained by bonding a first protective substrate  101  and a second protective substrate  107 , has a blind cavity  105  and is without through openings. The supporting substrate  113  has a through opening that is formed previously and is in fluid communication with the MEMS acoustic transducer  115  and defines a sound port  112 . In this case, the cavity  105  defines a reference chamber for the MEMS acoustic transducer  115 . Moreover, a copper shielding layer  110  coats the walls of the cavity  105  and a face  101   a  of the first protective substrate  101  bonded to the supporting substrate  113 . 
     The packaged microelectromechanical device  125  is obtained as already described, except for the fact that the sound port  112  is obtained in the supporting substrate  113  instead of in the lid  109 . In particular, in a step of the process of production, the lid  109 , prior to being joined to the supporting substrate  113 , is coated with a protective organometal layer  111  made of OSP material, as illustrated in  FIG. 13 . When the lid  109  and the supporting substrate  113  are bonded by a layer of soldering paste  117 , the protective organometal layer  111  made of OSP material is thermally destroyed and exposes the shielding layer  110  both on the face  101   a  of the first substrate  101  and in the cavity  105 . The layer of soldering paste  117  melts and forms a soldering joint  120 . The atmosphere present in the cavity  105  causes oxidation of the copper in the exposed portions of the shielding layer  110 , which are thus coated with a protective layer of copper (II) oxide  126  (visible in  FIG. 12 ). 
     According to a different embodiment, illustrated in  FIGS. 14 and 15 , in a protective metal substrate  201 , for example brass, a cavity  205  is obtained by a molding process on a face  201   a.    
     The protective substrate  201  ( FIG. 15 ) is coated with a metal layer  210  of copper, both on the face  201  a and on the walls of the cavity  205 , and then with a protective organometal layer  211  made of OSP material. A lid  209  is thus completed. 
     As illustrated in  FIG. 16 , the lid  209  is then bonded to a supporting substrate  213 , mounted on which are a MEMS acoustic transducer  215 , integrated in a first chip, and a control circuit  216 , integrated in a second chip. The supporting substrate  213  is moreover provided with a sound port  212  for the MEMS acoustic transducer  215 . 
     A packaged microelectromechanical device  225  is thus formed, in particular a MEMS microphone, comprising the MEMS acoustic transducer  215 , the control circuit  216 , and a package  224 , forming part of which are the lid  209  and the supporting substrate  213 . 
     To bond the lid  209  and the supporting substrate  213 , a layer of soldering paste is used around the MEMS acoustic transducer  215  and the control circuit  216 , which remain housed in the cavity  205 . In this step, the protective organometal layer  211  is thermally destroyed and exposes the metal layer  210 , enabling formation of a conductive soldering joint  220 . In addition, the atmosphere present in the cavity  205  causes oxidation of the copper in the exposed portions of the metal layer  210 , which are thus coated by a protective layer of copper (II) oxide  226 . 
       FIG. 17  shows a simplified block diagram of a packaged microelectromechanical device  325 . 
     The packaged microelectromechanical device  325  comprises a capacitive MEMS acoustic transducer  315  and an integrated control circuit  316 , housed in a package  324  according to any one of the embodiments described previously. The integrated control circuit  316  is configured to properly bias the MEMS acoustic transducer  315 , to process input signals S IN  generated by capacitive variations of the MEMS acoustic transducer  315 , and to supply, on an output of the packaged microelectromechanical device  325 , a digital output signal S OUT , which can be then processed by a microcontroller of an associated electronic device. 
     In one embodiment, the integrated control circuit  316  comprises: a pre-amplifier circuit  330 , of an analog type, which is configured to directly interface with the MEMS acoustic transducer  315  and to amplify and filter the input signal S IN  supplied by the MEMS acoustic transducer  315 ; a charge pump  331 , which supplies appropriate voltages for biasing the MEMS acoustic transducer  315 ; an analog-to-digital converter  332 , for example of the sigma-delta type, configured to receive a clock signal CK and a differential signal amplified by the pre-amplifier circuit  330  and to convert the amplified differential signal into a digital signal; a reference generator  333 , connected to the analog-to-digital converter  332  and configured to supply a reference signal for the analog-to-digital converter  332 ; and a driving circuit  334 , configured to operate as interface with an external system, for example, a microcontroller of an associated electronic device. 
     In addition, the packaged microelectromechanical device  325  may comprise a memory  335  of a volatile or non-volatile type, which may be, for example, programmed externally so as to enable a use of the packaged microelectromechanical device  325  in different operating configurations. 
     The packaged microelectromechanical device  325  may be used in an electronic device  350 , as illustrated in  FIG. 18 . The electronic device  350  is, for example, a portable mobile communication device (for example, a cell phone), a PDA (personal digital assistant), a portable computer (notebook), a voice recorder, a reader of audio files with capacity of voice recording, an acoustic apparatus, etc. 
     The electronic device  350  comprises, in addition to the packaged microelectromechanical device  325 , a microprocessor  351  and an input/output interface  352 , connected to the microprocessor  351  and, for example, provided with a keyboard and a display. The packaged microelectromechanical device  325  communicates with the microprocessor  351  through a signal-processing module  353 . In addition, the electronic device  350  can comprise a loudspeaker  354  and an internal memory  355 . 
     Modifications and variations may be made to the lid, to the packaged microelectromechanical device, and to the process described, without thereby departing from the scope of the present invention. 
     In particular, the MEMS acoustic transducer could be replaced by a different MEMS sensor or transducer, in the case where there is the need for said devices to be packaged with a protective lid. The control device might not be present or might be incorporated in one and the same die with the MEMS device. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.