Surface mountable transducer system

[0001] The present invention relates to a sensor system comprising a carrier member, a transducer element and an electronic device. The present invention relates in particular to condenser microphone systems assembled using flip-chip technology. The present invention further relates to condenser microphone systems adapted for surface mounting on e.g. printed circuit boards (PCB's).

[0002] In the hearing instrument and mobile communication system industry, one of the primary goals is to make components of small sizes while still maintaining good electroacoustic performance and operability giving good user friendliness and satisfaction. Technical performance data include sensitivity, noise, stability, compactness, robustness and insensitivity to electromagnetic interference (EMI) and other external and environmental conditions. In the past, several attempts have been made to make microphone systems smaller while maintaining or improving their technical performance data.

[0003] Another issue within these component industries concerns the ease of integration into the complete system.

[0004] EP 561 566 discloses a solid state condenser microphone having a field effect transistor (FET) circuitry and a cavity or sound inlet on the same chip. The techniques and processes for manufacturing a FET circuitry are quite different from the techniques and processes used in manufacturing transducer elements. Consequently, the transducer element and FET system disclosed in EP 561 566 requires two (or possibly more) separate stages of production which by nature makes the manufacturing more complicated and thereby also more costly.

[0005] The article The first silicon-based micro-microphone published in the Danish journal Elektronik og Data, No. 3, p. 4-8, 1998 discloses how silicon-based microphone systems can be designed and manufactured. The article discloses a three-layer microphone system where a transducer element is flip-chip mounted on an intermediate layer connecting the transducer element to an electronic device, such as an ASIC. The transducer element comprises a movable diaphragm and a substantially stiff back plate. On the opposite side of the transducer element a silicon-based structure forming a back chamber is mounted. It is worth noting that in order for the microphone system to be electrically connected to the surroundings wire bonding or direct soldering is required. The development of combined microelectromechanical systems (MEMS) has progressed significantly over the last years. This has primarily to do with the development of appropriate techniques for manufacturing such systems. One of the advantages of such combined systems relates to the size with which relative complicated systems involving mechanical micro-transducers and specially designed electronics may be manufactured.

[0006] It is an object of the present invention to provide a sensor system where the different elements forming the sensor system are flip-chip mounted, applying standard batch-oriented techniques.

[0007] It is a further object of the present invention to provide a sensor system suitable for mounting on e.g. PCB's using flip-chip or surface mount technologies and thereby avoid wire bonding or complicated single-chip handling.

[0008] It is a still further object of the present invention to provide a sensor system where the distance between the transducer element and the electronics is reduced so as to reduce parasitics and space consumption.

[0009] The above-mentioned objects are complied with by providing, in a first aspect, a sensor system comprising

[0010] a carrier member having a first surface, said first surface holding a first and a second group of contact elements,

[0011] a transducer element comprising an active member and at least one contact element, said at least one contact element being aligned with one of the contact elements of the first group so as to obtain electrical contact between the transducer element and the carrier member, and

[0012] an electronic device comprising an integrated circuit and at least one contact element, said at least one contact element being aligned with one of the contact elements of the second group so as to obtain electrical contact between the electronic device and the carrier member,

[0013] wherein at least one of the contact elements of the first group is electrically connected to at least one of the contact elements of the second group so as to obtain electrical contact between the transducer element and the electronic device.

[0014] The transducer element may in principle be any kind of transducer, such as a pressure transducer, an accelerometer or a thermometer.

[0015] In order for the sensor system to communicate with the surroundings the carrier member may further comprise a second surface, said second surface holding a plurality of contact elements. At least one of the contact elements of the first or second group is electrically connected to one of the contact elements being held by the second surface. The first and second surfaces may be substantially parallel and opposite each other.

[0016] The carrier member and the transducer element may be based on a semiconductor material, such as Si. In order to decouple thermal stresses, the carrier member, the transducer element and the electronic device may be based on the same semiconductor material. Again, the material may be Si.

[0017] In order to form a back chamber for microphone applications the carrier member may further comprise an indentation aligned with the active member of the transducer element. Also for microphone applications the active member of the transducer element may comprise a capacitor being formed by a flexible diaphragm and a substantially stiff back plate. Furthermore, the transducer element further comprises a cavity or sound inlet. The bottom of the cavity may be defined or formed by the active member of the transducer element. The flexible diaphragm and the substantially stiff back plate may be electrically connected to a first and a second contact element of the transducer element, respectively, in order to transfer the signal received by the transducer element to the carrier member.

[0018] The integrated circuit may be adapted for signal processing. This integrated circuit may be an ASIC. The integrated circuit is operationally connected to the at least one contact element of the electronic device.

[0019] In order to obtain directional sensitivity the sensor may further comprise an opening or sound inlet between the second surface of the carrier member and the indentation.

[0020] In order to protect the transducer element against e.g. particles or humidity an outer surface of the sensor is at least partly protected by a lid. The lid and the active member of the transducer element may define an upper and lower boundary of the cavity, respectively. Furthermore, at least one outer surface of the sensor system may hold a conductive layer.

[0021] The conductive layer may comprise a metal layer or a conductive polymer layer. The contact elements may comprise solder materials, such as a Sn, SnAg, SnAu or SnPb. Furthermore, the sensor system may comprise sealing means for hermetically sealing the transducer element.

[0022] In a second aspect, the present invention relates to a sensor system comprising

[0023] a carrier member having a first surface, said first surface holding a first, a second and a third group of contact elements,

[0024] a first transducer element comprising an active member and at least one contact element, said at least one contact element being aligned with one of the contact elements of the first group so as to obtain electrical contact between the first transducer element and the carrier member,

[0025] a second transducer element comprising an active member and at least one contact element, said at least one contact element being aligned with one of the contact elements of the second group so as to obtain electrical contact between the second transducer element and the carrier member, and

[0026] an electronic device comprising an integrated circuit and at least one contact element, said at least one contact element being aligned with one of the contact elements of the third group so as to obtain electrical contact between the electronic device and the carrier member,

[0027] wherein at least one of the contact elements of the first group is electrically connected to at least one of the contact elements of the third group, and wherein at least one of the contact elements of the second is electrically connected to at least one of the contact elements of the third group so as to obtain electrical contact between the first transducer element and the electronic device and between the second transducer element and the electronic device.

[0028] The sensor according to the second aspect may be suitable for directional sensing, such as for directional sensitive pressure transducers.

[0029] The carrier member, such as a Si-based carrier member, may further comprise a second surface holding a plurality of contact elements. In order to obtain electrical connection to the second surface at least one of the contact elements of the first, second or third group may be electrically connected to one of the contact elements being held by the second surface. The first and second surfaces may be substantially parallel and opposite each other. Preferably, the transducer elements and the electronic device are Si-based.

[0030] The carrier member may further comprise a first and a second indentation, the first indentation being aligned with the active member of the first transducer element, the second indentation being aligned with the active member of the second transducer element. The first and second indentations act as back chambers.

[0031] Each of the first and second transducer elements may further comprise a cavity, the bottom of said cavities being defined by the active members of the first and second transducer elements.

[0032] In order to measure e.g. pressure variations each of the active members of the first and second transducer elements may comprise a capacitor, said capacitor being formed by a flexible diaphragm and a substantially stiff back plate, said flexible diaphragm and said substantially stiff back plate being electrically connected to contact elements of the respective transducer elements.

[0033] Each of the first and second transducer elements further may comprise a lid for protecting the transducer elements. The lids and the active members of the first and second transducer elements may be positioned in such a way that they define an upper and a lower boundary of the respective cavities.

[0034] At least part of an outer surface of the sensor system may hold a conductive layer. This conductive layer may be a metal layer a conductive polymer layer. The contact elements may comprise a solder material, such as Sn, SnAg, SnAu or SnPb.

[0035] Solid state silicon-based condenser microphone systems according to the invention are suitable for batch production. The combination of the different elements forming the microphone system is more flexible compared to any other system disclosed in the prior art. The present invention makes it possible to provide a very well defined interface to the environment, e.g. by an opening on one side of the system. This opening can be covered by a film or filter preventing dust, moisture and other impurities from contaminating or obstructing the characteristics of the microphone. Electrical connections between the different elements of the microphone system are established economically and reliably via a silicon carrier using flip-chip technology.

[0036] The present invention uses an integrated electronic circuit chip, preferably an application specific integrated circuit (ASIC) which may be designed and manufactured separately and independent of the design and manufacture of the transducer element of the microphone. This is advantageous since the techniques and processes for manufacturing integrated electronic circuit chips are different from those used in manufacturing transducer elements, and each production stage can thus be optimised independently. Furthermore, testing of transducer elements and ASICs may be performed on wafer level.

[0037] The complete sensor system can be electrically connected to an external substrate by surface mount technology with the contacts facing one side of the system that is not in conflict with the above-mentioned interface to the environment. This allows the user to apply simple and efficient surface mount techniques for the assembly of the overall system.

[0038] The present invention will now be explained in further details with reference to the accompanying drawings, where

[0039] FIG. 1 is an illustration of a general application of a silicon-based sensor system,

[0040] FIG. 2 is an illustration of a general application of a silicon-based sensor system with a lid,

[0041] FIG. 3 is an illustration of a microphone application of the silicon-based sensor system,

[0042] FIG. 4 is an illustration of an encapsulated microphone application,

[0043] FIG. 5 is a close up of a lateral feed-through and sealing ring,

[0044] FIG. 6 is an illustration of a directional microphone application of the silicon-based sensor system, and

[0045] FIG. 7 is an illustration of a second directional microphone application of the silicon-based sensor system.

[0046] The process used for manufacturing the different elements of the sensor system involves mainly known technologies within the field of microtechnology.

[0047] In FIG. 1 a silicon carrier substrate 2 containing one or more vertical etched feed-through holes 20 is shown. The silicon carrier substrate 2, which is bulk crystalline silicon, has solder bumps 8, 22 on a first surface and a second surface, respectively. The electrical signal is carried from the first surface to the second surface via feed-through lines 23. On the first surface, one or more transducer elements 1 are flip-chip mounted onto the silicon carrier substrate 2, connected and fixed by a first group of solder bumps 8. Also on the first surface, one or more electronic devices, such as integrated circuit chips 3, are flip-chip mounted onto the silicon carrier substrate 2, connected and fixed by a second group of solder bumps 8. The solder bump 8 material is typically Sn, SnAg, SnAu, or SnPb, but other metals could also be used.

[0048] A solder sealing ring 9 provides sealing for the transducer element 1. In this case, feed-through lines 23 are used for carrying the electrical signals from the transducer element 1 under the sealing ring 9 to the electronic device 3. This is shown in greater detail in FIG. 5. The signal can also be carried to the electronic circuit by other conductive paths. Electrical conductive paths 23 are also formed through the carrier e.g. by etching holes 20 and subsequent metallization. The etching can be done by wet chemical etching or dry plasma etching techniques. This path 23 is called a vertical feed-through and can be used for carrying the electrical signal from either the transducer 1 or the electronic circuit 3 to the second surface of the carrier.

[0049] The second surface is supplied with solder bumps 22 for surface mounting onto e.g. a PCB or another carrier.

[0050] FIG. 2 shows a package like the one shown in FIG. 1, but in this embodiment the electronic device 3 has been connected and fixed by one group of solder bumps 8 as well as other means such as underfill or glue 21. Furthermore, the package is protected by a lid 5, which is fixed to the flip-chip mounted transducer element 1 or electronic device 3 or both. The lid 5 has an opening 4 providing a well-determined access to the environment, e.g. a sound-transmitting grid or filter as protection against particles or humidity for a microphone. The lid can be made separately, e.g. from metal or polymer by punching or injection moulding, respectively.

[0051] In FIGS. 3 and 4 a system for microphone applications is shown. In these embodiments the transducer element 1 is a microphone and a back chamber 11 has been etched into the silicon substrate 2. The back chamber is etched into the silicon carrier by wet etching processes using reactants as KOH, TMAH or EDP or by dry etching processes such as reactive ion etching. The cavity 11 can be etched in the same step as the feed-through hole 20.

[0052] The difference between FIGS. 3 and 4 is that the system, in FIG. 4, has been encapsulated with a filter 5 for providing EMI-shielding. The EMI-shield 16 is a conductive polymer layer, such as silver epoxy or a metal layer, such as electroplated or evaporated Cu or Au. Furthermore, the integrated circuit chip 3 and the filter 5 in FIG. 4 have been connected and fixed with additional means such as underfill or glue 21.

[0053] The function of the microphone is as follows. The opening 4 functions as a sound inlet, and ambient sound pressure enters through the filter 5 covering the opening 4 to the cavity 10 functioning as a front chamber for the microphone. The sound pressure deflects the diaphragm 12, which causes the air between the diaphragm 12 and the back plate 13 to escape through the perforations 19.

[0054] The diaphragm may be designed and manufactured in different ways. As an example the diaphragm may be designed as a three-layer structure having two outer layers comprising silicon nitride whereas the intermediate layer comprises polycrystalline silicon. The polycrystalline silicon comprised in the intermediate layer is doped with either boron (B) or phosphorous (P). The back plate also comprises B- or P-doped polycrystalline silicon and silicon nitride. The cavity 11 functions as a back chamber for the microphone.

[0055] When the diaphragm 12 is deflected in response to the incident sound pressure, the electrical capacity of the electrical capacitor formed by the diaphragm 12 and the back plate 13 will vary in response to the incident sound pressure. The circuit on the integrated circuit chip 3 is electrically connected to the diaphragm 12 and the back plate 13 through solder bumps 8. The circuit is designed to detect variations in the electrical capacity of the capacitor formed by the diaphragm 12 and the back plate 13. The circuit has electrical connections via the solder bumps 8 and the vertical feed-through lines 23 to the solder bumps 22 for electrically connecting it to a power supply and other electronic circuitry in e.g. a hearing instrument.

[0056] When operating the capacitor formed by the diaphragm 12 and the back plate 13, the back plate 13 is connected to a DC power supply in order to charge the back plate 13. When the capacitance varies due to distance variation between the diaphragm 12 and the back plate 13 in response to a varying sound pressure, an AC voltage is superimposed on top of the applied DC level. The amplitude of the AC voltage is a measured for the change in capacitance and thus also a measure for the sound pressure experienced by the diaphragm.

[0057] In FIG. 5 a close-up of a lateral feed-through line 24 and sealing ring 9 is shown. The feed-through 24 is electrically insulated from the sealing ring 9 and the substrate 2 by insulating layers 25. Insulating layers 25 similarly insulate the solder bumps 8 of the transducer 1 from the substrate 2. The solder bumps 8 of the transducer 1 and the solder bumps 8 of the circuit chip 3 are electrically connected via the feed-through line 24.

[0058] In FIG. 6, a microphone similar to the one in FIG. 3 is shown. However, an opening 24 has been introduced in the backchamber 11. The opening 24 causes a membrane deflection that reflects the pressure gradient over the membrane resulting in a directional sensitivity of the microphone.

[0059] In FIG. 7, a microphone similar to the one in FIG. 3 is shown. However, an additional transducer element has been added so that the microphone now uses two transducer elements 1, both containing a membrane 12 and a backplate 13. Both transducer elements are connected to the carrier member 3 by solder bumps 8 and seal ring 9 with an indentation 11 for each transducer element. The two transducer elements allow to measure the phase difference of an impinging acoustical wave resulting in a directional sensitivity of the microphone.

[0060] It will be evident for the skilled person to increase the number of sensing elements from two (as shown in FIG. 7) to an arbitrary number of sensing elementse.g. arranged in an array of columns and rows.