A PRESSURE TRANSDUCER

Abstract: The present invention relates to compact solid state silicon-based condenser microphone systems suitable for batch production. The combination of the different elements forming the microphone system is easier and more economical to manufacture compared to any other system disclosed in prior art. In addition the invention is compatible with electronic equipment manufacturing processes, such as SMD pick and place techniques. The invention uses a transducer chip comprising a chamber, a diaphragm that is positioned at the first lower surface and covering the second opening of the transducer chip. The transducer chip is flip-chip mounted onto a post-processed chip also comprising a chamber. The microphone system can be electrically connected to an external substrate by conventional techniques such as wire bonding.

Description: It is known to package micro-machined transducers in SMD-packages (surface mount devices) in which a chip is die bonded on an organic or inorganic carrier (chip facing up) and wire bonded to leads which connect by soldering techniques to a printed circuit board (PCB) metallisation. Such packages tend to be large and expensive. US 5,889,872 discloses a hybrid system consisting of a silicon microphone and an integrated circuit chip mounted onto it using wire bonding for the electrical connection between the silicon microphone and the integrated circuit chip. This solution has the disadvantage of requiring additional protection and space for the bonding wires. US 5,856,914 discloses a flip-chip mounted micro-mechanical device, such as a condenser microphone, where part of the carrier upon which the micro-device is mounted, forms part of the final system. A disadvantage of this system is the fact that the micromechanical device may not be tested prior to the mounting on the carrier. Another disadvan- tage of the disclosed system relates to the chosen materials. The micromechanical device is comprised of Si whereas the carrier is made of a PCB or ceramic material. Differences in thermal expansion coefficients may easily complicate the integration of such different materials. It is an object of the present invention to provide a transducer that 1. is easy to manufacture, 2. can be made available at a low cost, 3. includes an integrated circuit device, 4. has a small die size, and 5. is compatible with electronic equipment manufacturing processes, such as SMD pick and place techniques. It is a further object of the present invention to provide a fully functional and encap- sulated transducer that can be operated independently of its final position on e.g. a PCB. It is a still further object of the present invention to provide a fully functional and encapsulated transducer that can be tested prior to mounting. 3 It is a still further object of the present invention to provide a transducer where the distance between the transducer element and the electronic circuit is minimised so as to minimise parasitics. SUMMARY OF THE INVENTION According to the invention the above and other objects are fulfilled by providing, in a first aspect, a transducer, such as a capacitive transducer, a pressure transducer, a sound transducer, a loudspeaker, etc, comprising - a first Si substrate with a first upper surface and a first lower surface opposite the first upper surface and enclosing a first chamber extending from a first opening of the first chamber at the first upper surface and through the first Si substrate to a second opening at the first lower surface, - a first diaphragm that is positioned at the first lower surface and covering the second opening, - a second Si substrate with a second upper surface and a second lower surface, the second Si substrate enclosing a second chamber extending into the second Si substrate from a third opening at the second upper surface, the first Si substrate being positioned on the second Si substrate with the second opening aligned with the third opening, and - a Si-based integrated circuit, said Si-based integrated circuit being operationally connected to the first diaphragm and said Si-based integrated circuit being positioned at the second upper surface of the second Si substrate. The transducer may further comprise a first back plate that is arranged adjacent and sub- stantially parallel to the first diaphragm. The first diaphragm and the first back plate may be conductive and form a capacitor in combination whereby a miniature condenser microphone is formed. In a preferred embodiment of the invention, the first substrate is flip-chip mounted on the second substrate. The first diaphragm and the first back plate is connected to contact elements, such as solder bumps, on the first substrate. The second substrate has contact elements thereon corresponding to the contact elements of the first substrate. The first substrate is placed with its first lower surface on the second upper surface of the second substrate with the contacts in corresponding positions. When in position, the contacts are attached, e.g. by reflow soldering. Further, the first substrate and the second substrate may have seal surfaces which surround the second and third openings, respectively. When the first substrate is secured to the second substrate the seal surfaces are connected to form a seal or a surrounding bar- rier. The seal may be an airtight seal, a hermetic seal, etc. The seal may be formed by a solder ring, an epoxy ring, etc. Directional sensitivity may be obtained by introducing a fourth opening between the second chamber and the second lower surface. Preferably, the integrated circuit comprises an ASIC, that is operationally connected to the first diaphragm. For example, in a condenser microphone according to the present invention, a commercially available ASIC chip may be further processed by etching the second chamber into the chip. Subsequently, the ASIC is connected to the first diaphragm and the first back plate via the above-mentioned contact elements. Preferably, the seal also surrounds the integrated circuit. It is preferred to encapsulate the transducer in a polymer or in a metal layer that is sprayed, evaporated, or electroplated onto the transducer. The encapsulation should not cover the first opening in the first substrate allowing the environment to communicate with the diaphragm. Preferably, the first opening is covered by a filter that is transparent to pressure or to pressure changes. This method of encapsulation eliminates the need for a container made of metal or ceramic whereby weight and size of the transducer is reduced the transducer is thermally matched to the substrate. A second way of obtaining directional sensitivity is by using two or more sensing elements such as an array of sensors e.g. with separated back chambers in the second substrate in order to detect a phase difference of an acoustic wave. Thus, in a second aspect, the present invention relates to directional transducers, such as directional pressure transducers, applying more than one pressure transducer. In order to obtain directional sensitivity, the directional transducer may, in addition to the transducer according to the fist aspect, further comprise - a third chamber being enclosed in the first Si substrate and extending from a fifth opening of the third chamber at the first upper surface and through the first Si substrate to a sixth opening at the first lower surface, - a second diaphragm that is positioned at the first lower surface and covering the sixth opening of the third chamber, and - a fourth chamber extending into the second Si substrate from a seventh opening at the second upper surface, the sixth opening of the fourth chamber being aligned with the seventh opening. The transducer may further comprise a second back plate that is arranged adjacent and substantially parallel to the second diaphragm. The second diaphragm and the second back plate may be conductive and form a capacitor in combination. Furthermore, the sec- ond diaphragm and the second back plate may be operationally connected to the integrated circuit via solder bumps. Preferably, a filter covers the fifth opening. The transducer may further comprise an encapsulation enclosing the first and second Si substrates and having an opening aligned with the first and fifth opening. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be explained with reference to the drawing, in which: Figure 1 is a cross section of a microphone according to the invention, Figure 2 is a cross section of a directional microphone with one pressure grading sensing element, and Figure 3 is a cross section of a directional microphone formed by an array of absolute pressure microphones using the detected phase difference. DETAILED DESCRIPTION OF THE INVENTION In Fig. 1 , 2, and 3, dimensions such as material thickness and mutual distances and possibly other proportions are not necessarily drawn to the same scale. The size of the microphones is about 1-4 mm. The microphone illustrated in Fig. 1 has the following structure. A silicon transducer chip 1 with a first chamber 10 etched therein extending from a first opening 4 at the upper surface and through the transducer to a second opening 6, carries a diaphragm 12 and a back plate 13 covering the second opening 6 in the transducer chip. The transducer chip 1 with the diaphragm 12 and a back plate 13 may be manufactured using traditional tech- niques for micro-machining/manufacturing. The diaphragm 12 and the back plate 13 are both electrically conductive and are arranged parallel in close proximity to each other, so that they form an electrical capacitor. The back plate 13 has a plurality of perforations 19 making it acoustically transparent, and the diaphragm 12 has a tiny vent hole 15 for equalising the static pressure on both sides of the diaphragm 12. The transducer chip 1 is flip-chip mounted via solder bumps 8 onto a post-processed silicon chip 3 comprising an electric circuit 17, such as an integrated circuit, on a surface thereof. The post-processed silicon chip 3 also has a second chamber 11 etched into it having a third opening 7. The transducer chip 1 is secured to the silicon chip 3 by means of an electrically conductive solder sealing ring 9 or by other means making an airtight seal. It is an advantage of the invention that the transducer chip 1 and the silicon chip 3 are both based on silicon substrates, thereby avoiding thermally-induced stresses. The diaphragm 12 and the back plate 13 are electrically connected to a number of separate solder bumps 8, which connect the diaphragm 12 and the back plate 13 to the silicon chip 3. The first opening 4 is covered with a filter 5 or a flexible sheet or diaphragm of acoustically transparent material. The whole structure is encapsulated in a polymer or metal encapsulation 16 leaving the filter 5 and pad 18 free. The function of the above-described structure is as follows. The first opening 4 functions as a sound inlet, and ambient sound pressure enters through the filter 5 covering the first opening 4 to the first chamber 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. The chamber 11 func- tions as a back chamber for the microphone. The diaphragm 12 is movable relative to the back plate 13 in response to the sound pressure. When the diaphragm 12 is moved in response to the incident sound, the electrical capacitance of the electrical capacitor formed by the diaphragm 12 and the back plate 13 will vary in response to the incident sound. The integrated circuit 17 on the silicon chip 3 is electrically connected to the diaphragm 12 and the back plate 13 via the separate solder bumps 8. The integrated circuit 17 is designed to detect variations in the electrical capacitance of the capacitor formed by the diaphragm 12 and the back plate 13. The integrated circuit 17 has electrical connections to the pad 18 for electrically connecting it to a power supply and other electronic circuitry in e.g. a hearing instrument. Variations and modifications in the herein described system, within the scope of transducers such as pressure transducers and capacitive transducers, will undoubtedly suggest themselves to those skilled in the art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense. The microphone illustrated in Fig. 2 is a directional microphone sensing a pressure gradient. The microphone has the same basic structure as the microphone in Fig. 1 , but in order to obtain directional sensitivity an opening 24 has been introduced between to the backchamber 11 so as to allow for an acoustic wave to enter the backchamber 11. The deflection of the membrane 12 is therefore a measure for the pressure gradient over the membrane 12, resulting in a directional sensitivity. The microphone illustrated in Fig. 3 is also a directional microphone. This microphone senses a phase difference of an impinging acoustical wave. The microphone has the same basic structure as the microphone in Fig. 1 , however, the microphone in Fig. 3 uses 8 two sensing elements in the first substrate 1 , both containing a membrane 12 and a back- plate 13 and both being connected to the second substrate 3 by solder bumps 8 and seal ring 9 with a cavity 11 for each sensing element and both covered by a filter 5. It will be evident for the skilled person to increase the number of sensing elements from two (as shown in Fig. 3) to an arbitrary number of sensing elements - e.g. arranged in an array of columns and rows.

Claims: CLAIMS 1. A transducer comprising - a first Si substrate with a first upper surface and a first lower surface opposite the first upper surface and enclosing a first chamber extending from a first opening of the first chamber at the first upper surface and through the first Si substrate to a second opening at the first lower surface, - a first diaphragm that is positioned at the first lower surface and covering the second opening, - a second Si substrate with a second upper surface and a second lower surface, the second Si substrate enclosing a second chamber extending into the second Si substrate from a third opening at the second upper surface, the first Si substrate being positioned on the second Si substrate with the second opening aligned with the third opening, and - a Si-based integrated circuit, said Si-based integrated circuit being operationally connected to the first diaphragm and said Si-based integrated circuit being positioned at the second upper surface of the second Si substrate. 2. A transducer according to claim 1 , further comprising a first back plate that is arranged adjacent and substantially parallel to the first diaphragm. 3. A transducer according to claim 2, wherein the first diaphragm and the first back plate are conductive and form a capacitor in combination. 4. A transducer according to claim 2 or 3, further comprising solder bumps for connection of the first diaphragm and the first back plate to the Si-based integrated circuit. 5. A transducer according to claim 4, wherein the solder bumps forms an airtight seal between the first and second Si substrate. 10 6. A transducer according to claim 5, wherein the airtight seal forms a ring comprising ep- oxy. 7. A transducer according to any of the preceding claims, further comprising a fourth opening between the second chamber in the second Si substrate and the second lower surface. 8. A transducer according to any of the preceding claims, wherein the first diaphragm is attached to the first Si substrate before assembly of the first and second Si substrates. 9. A transducer according to any of the preceding claims, further comprising a filter covering the first opening. 10. A transducer according to any of the preceding claims, further comprising an encap- sulation, said encapsulation surrounding the first and second Si substrates and having an opening that is aligned with the first opening. 11. A transducer according to any of the preceding claims, further comprising - a third chamber being enclosed in the first Si substrate and extending from a fifth opening of the third chamber at the first upper surface and through the first Si substrate to a sixth opening at the first lower surface, - a second diaphragm that is positioned at the first lower surface and covering the sixth opening of the third chamber, and - a fourth chamber extending into the second Si substrate from a seventh opening at the second upper surface, the sixth opening of the fourth chamber being aligned with the seventh opening. 12. A transducer according to claim 11 , further comprising a second back plate that is arranged adjacent and substantially parallel to the second diaphragm. 13. A transducer according to claim 12, wherein the second diaphragm and the second back plate are conductive and form a capacitor in combination. 1 1 14. A transducer according to any of claims 11-13, further comprising solder bumps for connection of the second diaphragm and the second back plate to the Si-based integrated circuit. 15. A transducer according to any of claims 11-14, further comprising a filter covering the fifth opening. 16. A transducer according to any of claims 11-15, further comprising an encapsulation, said encapsulation surrounding the first and second Si substrates and having an opening that is aligned with the first and fifth opening.

Inventors: MUELLENBORN MATTHIAS, SCHEEL PETER U, MUELLENBORN, MATTHIAS, SCHEEL, PETER, U.
Applicants: MICROTRONIC AS, MUELLENBORN MATTHIAS, SCHEEL PETER U, MICROTRONIC A/S, MUELLENBORN, MATTHIAS, SCHEEL, PETER, U.
Application year: 2000
Main classification: H04R19
IPC codes: G01L9, H01L29, H04R19