Abstract:
A microphone in a miniaturized form is described herein. The microphone includes a flat carrier substrate having a first recess extending through the carrier substrate. The microphone includes a first electro-acoustic transducer on a first surface of the carrier substrate and at least partially overlapping the first recess. The microphone also includes a cap on a second surface opposite the first surface having a tight seal with the second surface and spanning the first recess. The cap includes at least one metallic layer for electromagnetic shielding.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Pursuant to 35 USC §120, this application claims the benefit of PCT/DE2006/001946 filed Nov. 6, 2006 which claims the benefit of German Patent Application No. 102005053767.7 filed Nov. 10, 2005. Each of these applications is incorporated by reference in its entirety. 
     BACKGROUND 
     There is great demand to miniaturize the electronic components of mobile communications devices. This especially applies also, e.g., to microphones, because they have a relatively high-profile design and thus represent limitations for the device design of mobile communications devices. 
     From the published American Patent Application No. US 2005/0185812A1, a microphone housing is known, in which a microphone constructed as a MEMS component is arranged together with a semiconductor chip on a base plate and is covered with a common cap against the base plate. The base plate can have, on its bottom side opposite the cap, a sound entrance opening, so that the entire component can be soldered onto the back side of the printed-circuit board, which is turned away from the sound source. For this purpose, a corresponding borehole must be provided in the printed-circuit board. In another construction, the sound entrance opening can be provided in a conventional way on the top side in the cap, so that the component with the base plate can be deposited onto the surface of the printed-circuit board facing the sound source. 
     SUMMARY 
     The problem of the present invention is to specify another design for miniaturized microphones, with which the total structural height of a printed-circuit board equipped with a microphone can be reduced. 
     A disadvantage of the known microphone described above is that the total height of the component cannot be further reduced, because the height of the MEMS chip essentially determines the total height of the component. In order to solve this problem, a microphone is currently proposed, with which the thickness of the printed-circuit board can be used for holding the height-determining MEMS chip. For this purpose, for a flat carrier substrate, which has a recess reaching through the carrier substrate, an electro-acoustic transducer at least partially covering the recess is arranged on a first surface. On the second surface opposite the first surface sits a cap, which sits directly on the carrier substrate covering the recess and which forms a tight seal with its surface and which spans the recess. The cap has at least one metallic layer for the electromagnetic shielding of the electro-acoustic transducer. 
     This microphone is suitable for mounting on the back side of the printed-circuit board facing away from the sound source, wherein a borehole in the printed-circuit board can hold the electro-acoustic transducer. Therefore, for the conventional thicknesses of printed-circuit boards used today of ca. 0.5 to 1.2 millimeters, it is possible that the electro-acoustic transducer, which is constructed, in particular, as a MEMS component, is embedded completely in the borehole of the printed-circuit board. Therefore, the total height of the microphone measured above the surface of the printed-circuit board after installation is only determined by the height of the cap plus the thickness of the carrier substrate. This can lie in the range of a few tenths of a millimeter. 
     The electro-acoustic transducer is preferably a MEMS component built on a silicon wafer. For the electro-acoustic conversion, all of the known principles are suitable, capacitive transducers in particular, and those operating especially on the piezoelectric principle. Such components are known and distinguish themselves through the common property that the acoustic energy of the sound wave sets a membrane or another moving part in oscillation; the oscillations are converted by means of a suitable transducer principle into electrical signals. 
     The carrier substrate can be equipped with additional active and/or passive electrical components, which can directly interact with the microphone function and are directly connected to the microphone. Such components can be, in particular, impedance transducers or amplifiers, low-pass filters, signal processors, analog/digital converters, or ESD protective components against electrostatic overcharging. Advantageously, these additional electrical components are arranged on the surface of the carrier substrate lying opposite the electrical transducer and, in particular, under the cap. In addition, the cap spans a hollow space, which is formed between the cap and the carrier substrate and which can be used as a rear volume and thus as a reference for the electro-acoustic transducer. 
     The cap can be made completely from metal and soldered or bonded onto the carrier substrate. Alternatively, it can also be made from a non-metallic material, which is coated with a metallic layer. Preferably, but not absolutely necessary, the metallic coating is formed on the outer side of the cap. As a non-metallic carrier material, the cap can comprise a plastic layer; for example, a plastic film, which is coated onto one side with a metallic layer. 
     The carrier substrate preferably has at least one metallization layer, which can be structured to form corresponding electrical connection surfaces and electrical connection lines. Contacts connected to the electro-acoustic transducer and preferably also to the other electrical component are arranged at least on the first surface carrying the electro-acoustic transducer. 
     The electrical connection to the component arranged on the second surface under the cap can be realized via lines integrated into the carrier substrate and, in particular, by means of via contacts. However, it is also possible that the electro-acoustic transducer built preferably on the silicon wafer has corresponding terminals, which are connected via a wire contact to the connection surfaces on the first surface of the carrier substrate. The component can then also be connected through the recess in the carrier substrate by means of bonding wires to the electro-acoustic transducer. The component can also be connected, however, via bonding wires to corresponding connection surfaces on the second surface of the carrier substrate, which are connected by means of via contacts through the carrier substrate to the corresponding contacts on the first surface of the carrier substrate. However, it is also possible to connect the component directly to the corresponding contacts on the second surface, for example, in a flip-chip arrangement. The electrical connection can be realized by means of conductive adhesive or by means of a solder connection. Likewise, the electro-acoustic transducer can be affixed and wire-bonded or soldered onto corresponding contact surfaces or affixed by means of conductive adhesive. 
     The carrier substrate can also have multiple layers and can have at least one other metallization layer in its interior. Wiring or circuitry can be realized in this layer. However, through suitable structuring of the metallization layer it is also possible to realize passive components in one or more metallization layers; for example, capacitors, inductors, or resistors. Accordingly, these passive electrical components can be connected to circuitry, with which other electrical functions of the microphone or components interacting with this microphone are realized. Individual metallization layers can be connected by via contacts. In a carrier substrate made from organic laminates, active semiconductor components—Ics—can also be embedded. 
     For conversion, the electro-acoustic transducer can have at least one piezoelectric layer provided with transducer electrodes. The transducer electrodes can be constructed in such a way that they can convert bulk acoustic waves generated in the piezoelectric layer by the acoustic energy into electrical signals. Preferably, the transducer electrodes are then arranged on both sides of the piezoelectric layer. However, a one-sided arrangement of transducer electrodes spaced apart from each other for transforming bulk waves and also a suitably constructed transducer for transforming surface acoustic waves or shear waves is also possible. 
     The cap sitting on the second surface represents electromagnetic rear-side shielding for the microphone. It is also advantageous to provide the surface of the electro-acoustic transducer pointing toward the sound source with electromagnetic shielding. For on the first surface of the carrier substrate and on parts of the electro-acoustic transducer. The shielding layer on the electro-acoustic transducer can be deposited on all of the electro-acoustic transducer not for the membrane or on the part of the electro-acoustic transducer receiving the oscillation. 
     Alternatively, it is also naturally possible to provide this shielding on the surface of the printed-circuit board facing the sound source. In this case, it is even possible to cover the opening of the printed-circuit board borehole directed toward the sound source with an especially mesh-like shielding that is permeable to sound waves, for example, with a metallic or metal-coated mesh or a metallic or metal-coated perforated film or a metal-coated porous membrane or plate. 
     Additional electro-acoustic shielding can be provided in the interior of the printed-circuit board. For example, the inside of the borehole for the electro-acoustic transducer in the printed-circuit board can be coated with conductive material. It is also possible to provide a series of metallic structures within the printed-circuit board; for example, a crown of via contacts, at a distance from the borehole but surrounding it. 
     Preferably, all of the metallization layers provided for electromagnetic shielding are connected to ground potential in order to safely drain away dangerous voltages and charges generated electrostatically or in some other way, and in this way to protect the component and/or to prevent electromagnetic interference in the measured signal. 
     Below, the microphone and also the method for fabricating the microphone will be explained in more detail with reference to embodiments and the associated figures. The figures are shown purely schematically and not true to scale, so that neither absolute nor relative dimensional information can be taken from these figures. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Shown in detail are: 
         FIG. 1 , a microphone with a metallic cap, 
         FIG. 2 , a microphone with a metal-coated cap and an electrical component, 
         FIG. 3 , a microphone with another electrical component, which is connected by means of bonding wires, 
         FIG. 4 , a component mounted on a printed-circuit board, 
         FIG. 5 , a component mounted on a printed-circuit board with additional shielding above the electro-acoustic transducer, 
         FIG. 6 , processing steps of a method for fabricating an encapsulated microphone, 
         FIG. 7 , processing steps of a method for generating additional shielding on the electro-acoustic transducer, and 
         FIG. 8 , a microphone with two electro-acoustic transducers on a common carrier substrate. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a simple microphone according to the invention in schematic cross section. The system is formed on a carrier substrate TS, which is built from an organic or ceramic material and in particular, from a laminate with at least one metallization layer, but preferably with several metallization layers embedded between electrically insulating material layers. In these structured metallization layers, active and passive components can be realized, which is advantageous, in particular, for ceramic laminates made from HTCC or LTCC. Glass or silicon is also suitable for the carrier substrate TS. Preferred thicknesses lie between 0.1 and 0.5 millimeters. Larger layer thicknesses are naturally possible, but increase the costs and oppose further miniaturization of the components or the microphone. 
     In the carrier substrate, a recess AN is provided. On the first surface, an electro-acoustic transducer WA is arranged so that it completely covers the recess. The electro-acoustic transducer WA is, in particular, a MEMS component (micro-electromechanical system), which is a sensor that is produced with micro-mechanical methods preferably on a single-crystal material, such as, for example, silicon, and that works according to a capacitive or piezoelectric principle. In a crystalline substrate material, through anisotropic etching of the substrate, the transducer can have an opening that tapers inwardly and on whose base a membrane or a correspondingly shaped tongue is arranged, which is set to oscillate by the acoustic energy. On the second surface sits a cap K, which similarly covers the recess and which can form a good seal all around and in particular, hermetically with the surface of the carrier substrate TS. However, selective leaks for slow pressure equalization with the ambient pressure can also be provided. In this way, a closed rear-side volume RV is created, which is used as a reference by the electro-acoustic transducer. 
     The cap can have a round, polygonal, or a different, arbitrarily shaped cross section and comprises at least one metal layer or is made completely from metal. 
       FIG. 2  shows another implementation of a microphone in which a semiconductor component HLB is also arranged under the cap on the second surface of the carrier substrate TS. In addition, here one possible modification of the cap K is shown, which is made from a plastic film KF, in particular, a laminate film and a metal layer MS deposited above this film. Preferably, the plastic film KF is structured in such a way that the larger surface area metal layer MS completely covers the plastic film and the plastic film KF forms an all-around, flush seal with the surface of the carrier substrate TS. However, the metal layer is connected at one or more points to ground potential, which contacts the carrier substrate at metallization structures located there. 
     In addition, in  FIG. 2  on the first surface of the carrier substrate, solderable contacts LK are shown, which are connected in a suitable way both to the electro-acoustic transducer WA and to the semiconductor component HLB. The connection can be realized by means of printed conductors, which are formed on the surface. Connections leading through the carrier substrate are also possible. 
     Not shown are additional implementations in which several, optionally also different semiconductor components or passive components, are arranged on the second surface underneath the cap K and are connected to the electro-acoustic transducer WA. In an especially preferred way, the semiconductor component or components, together with the electro-acoustic transducer WA, which is then formed, for example, as a resonator on the basis of a surface wave component or as a BAW resonator, form an oscillator circuit. Furthermore, in the carrier substrate or in the form of a component on the second surface of the carrier substrate, measures can be realized that are used for temperature compensation of the electro-acoustic transducer and whose temperature changes are compensated through active control or passively through suitable compensation means. 
       FIG. 3  shows another implementation of the microphone, in which both the semiconductor component HLB and also the electro-acoustic transducer WA, or at least one of these two, is mounted on the carrier substrate TS by means of an adhesive layer KS. The electrical connection can be realized as shown, for example, by means of bonding wires BD, wherein a bonding wire guided through the recess AN in the carrier substrate TS can electrically interconnect the semiconductor component HLB and electro-acoustic transducer. However, it is also possible to realize at least one of the electrical connections to a metallization layer formed on the surface of the carrier substrate TS by means of an electrically conductive adhesive layer KS. In this case, especially preferred are anisotropic conductive adhesives, which exhibit conductivity exclusively in the Z-direction, that is, perpendicular to the surface of the carrier substrate. 
     In the schematic cross section,  FIG. 4  shows how a microphone can be mounted on a printed-circuit board LP in a way that reduces the total overall height. For this purpose, an opening OE is provided in the printed-circuit board LP, a single-layer or multi-layer plate with one or more metallization layers, which is large enough to hold the electro-acoustic transducer WA. The carrier substrate is then placed and fixed with the first surface on the printed-circuit board in such a way that the electro-acoustic transducer WA extends into the opening OE. At conventional thicknesses of the printed-circuit board and electro-acoustic transducer, it is possible to conceal the transducer WA completely in the opening OE, so that it does not project beyond the opposing surface of the printed-circuit board. The electrical connection between the contacts of the carrier substrate and the printed-circuit board can be realized, in turn, by electrically conductive adhesive or by soldering. With the arrangement shown only schematically in the figure, a microphone is obtained, which is electromagnetically shielded by the second surface against external fields and therefore can be operated without interference from these fields. 
       FIG. 5  shows additional implementations of the microphone according to the installation in a printed-circuit board LP, with which a more reliable operation and additional shielding of the component from the opposite side is enabled. For this purpose, for example, the intermediate space between the transducer WA and inner side of the opening OE can be filled with a sealing material DM, for example, with a hardened reactive resin. On the side of the printed-circuit board LP opposite the carrier substrate TS, the opening OE can be covered with a porous layer, a mesh, or netting, which are each metallized on the surface or made completely from a metallic material. Not shown in the figure are additional metallization layers, which are deposited on the same surface as the porous layer PS on the surface of the printed-circuit board LP. Preferably, all of the shielding layers are grounded. 
     Alternatively or additionally, the electro-acoustic transducer WA can be provided on its surface pointing downward in  FIG. 5  with another shielding layer AS (see  FIG. 7 ). In addition, the inner walls of the opening OE can also be metallized. Furthermore, it is possible to provide a shielding lattice made from via contacts surrounding the opening within the printed-circuit board around the opening. 
     With reference to schematic cross sections,  FIG. 6  shows different possible processing steps according to one variant for fabricating a microphone. Here, the starting point is the already described carrier substrate TS. This can be a single-layer or multi-layer construction and has at least one metallization layer, which comprises printed conductors and circuitry formed therefrom. On a first surface (shown at the bottom in the figure), solderable contacts LK are generated, which are connected to corresponding printed conductors on the surface of the carrier substrate TS. On the opposite second surface, a semiconductor component HLB is arranged and preferably also contacted to printed conductors. This can be realized as already described by means of a flip-chip contact or a bonding wire (not shown in the figure). Advantageously, the flip-chip contact can be produced with an anisotropic conductive adhesive, which exhibits conductivity only in the vertical direction relative to the adhesive layer. Thus, with a single adhesive layer, several electrical connections can be produced in parallel, without short-circuiting through the adhesive layer.  FIG. 6   a  shows the arrangement at this processing step. 
     In the next step, on the second surface, a reactive layer OS is deposited above the semiconductor component. This reactive layer is made from a suitable material that can again be dissolved with a mild process, for example, a paste or an organic material and, in particular, from a resist film. The reactive layer OS can be structured directly, for example, in a printing method, for which pastes or corresponding viscous synthetic resins are suitable. However, it is also possible to apply the reactive layer over a large surface area and only then to structure it. 
     If a photoresist and in particular, a dry resist film that can be laminated, for example, is used for the reactive layer OS, then the structuring can be realized in a simple way photolithographically through exposure and development of the photoresist. The structuring takes place so that the volume of the reactive layer corresponds to the later rear volume enclosed by the covering cap.  FIG. 6   b  shows the arrangement on this processing step. 
     In the next step, a first layer of a multi-layer cap is generated, for example, a plastic film KF. Preferably, this is deposited in a plastically deformable state, e.g., through lamination or film casting, and then hardened. The lamination can be supported by increased temperature and also mechanical pressure from the top side or a vacuum from the bottom side. The plastic film can have, for example, a thickness of approximately 20 μm. Films of such small thickness are easy to process and generate a dense surface, which can later be metallized. Higher layer densities are naturally also possible, but not necessary. 
     In the next step, according to  FIG. 6   d , a metal layer MS is generated above the plastic film KF. This is realized preferably in a two-step process, wherein at least one base metallization layer or a suitable adhesive layer is sputtered and then reinforced. As the base or adhesive layer, in particular, one or more of the metals titanium, nickel, chromium, tungsten, and copper are suitable. The reinforcement is realized preferably in a galvanic or current-less method by means of copper or nickel. A gold or nickel layer can also be provided as a coating layer. In one variant of the method, the plastic film can also be eliminated if the reactive layer OS can be metallized, so that the metal layer can be deposited directly on the reactive layer. 
     A suitable end thickness, which is suitable as good shielding for the microphone, lies between 10 and 100 μm. Higher layer thicknesses are naturally also possible, but are not absolutely necessary for the desired purpose of the shielding. 
     The metal layer is formed so that it seals the second carrier substrate TS. In this way, a hermetic shield to the carrier substrate can be formed, if necessary. The structuring of the metal layer can be realized after the generation of the base or adhesive metallization layer. It is also possible to deposit a lift-off layer under the metallization layer and to remove it again together with the metal layer lying above this lift-off layer after the metallization or the fabrication of the metal layer MS in a lift-off process. 
     The cap generated in this way and made from the plastic film and metal layer lies on all sides on the structured reactive layer OS, which is now removed. For this purpose, in this processing step a recess AN can be generated in the carrier substrate TS, through which the reactive layer is accessible from the first surface.  FIG. 6   e  shows the arrangement after the fabrication of the recess. 
     However, it is also possible to generate the recess in an earlier processing stage, for example, before depositing the semiconductor component HLB and in each case before depositing the reactive layer OS. This becomes possible if the reactive layer can span the recess, which is possible, in particular, for a dry resist film as a reactive layer. 
     The recess can be generated according to the material of the carrier substrate through boring, milling, etching, laser beams, sandblasting, punching, or also ultrasonic erosion. The removal of the reactive layer through the recess can take place preferably with a solvent, for example, an organic solvent for an organic reactive layer or an aqueous solvent for a paste. The breaking up of the reactive layer can also be supported with ultrasonic energy. 
     Finally, the electro-acoustic transducer is deposited on the first surface above the recess and electrically connected to the printed conductors on the carrier substrate. The electrical connection can be realized with conductive adhesive and, in particular, with anisotropic conductive adhesive. Other flip-chip methods are also possible by means of bump technology, in this case, any gaps at the contacts between the transducer and the carrier substrate are then sealed with a suitable material, in particular with an underfiller. The electro-acoustic transducer is preferably deposited on the carrier substrate with the side carrying the thin-layer construction with the MEMS structures. However, it is also possible to deposit the transducer with the carrier side, in particular, with the silicon chip on the carrier substrate, as long as via contacts are available. 
       FIG. 6   f  shows the finished microphone, in which a rear-side volume RV, which is used as reference pressure for the electro-acoustic transducer, is enclosed between the cap, the carrier substrate, and the electro-acoustic transducer WA sealing the recess. 
       FIG. 7  shows one possibility of metallizing a plurality of electro-acoustic transducers on a panel, and in this way of protecting the sound entrance opening and especially the membrane of the transducer in a simple way or to eliminate the metallization. For this purpose, a corresponding number of electro-acoustic transducers WA are advantageously mounted on a carrier substrate or on an auxiliary carrier, as shown, for example, in  FIG. 7   a.    
     Then small spheres of matching diameter are placed in the only schematically shown openings of the transducer leading to the membrane. The diameter is designed so that it can generate sufficient shading on the electro-acoustic transducer. Suitable sphere diameters can lie, for example, between 0.5 and two millimeters, wherein the spheres are preferably glass spheres. In a simple way, these can also be shaken into the rear-side opening of the electro-acoustic transducer. Since they are not fixed in the transducer opening, excess spheres can be easily removed.  FIG. 7   b  shows the transducer with the glass spheres GK arranged in its openings. 
     In the next step, a metallization layer is deposited in the form of a shielding layer AS that is preferably anisotropic and vertical to the surface in a desired layer thickness. The cover layer AS can be deposited through sputtering or vaporization. 
     If the shielding layer AS is deposited directly onto the finished microphones, still interconnected on the panel above the carrier substrate TS, then it may be necessary to protect and, for example, to cover in advance electrical connections before the large surface-area deposited shielding layer. Here, for the metallization layer, a template can be used, which is placed on the carrier substrate. 
     If the shielding layer is deposited in a sufficiently anisotropic process, regions underneath the equator of the sphere remain free of the shielding layer, so that no connection is produced between the shielding layer on the sphere surface and the layer on the surface of the electro-acoustic transducer. Therefore, the spheres can be easily removed after the shielding layer is generated.  FIG. 7   d  shows the arrangement with the electro-acoustic transducers metallized at the desired positions. 
     In  FIG. 8  another construction of a microphone is shown, which has two or more electro-acoustic transducers in one component. These are each generated above a separate recess in the carrier plate. The recesses can be covered on the second surface with a common cap K. Here it is advantageous to divide the rear-side volume RV, so that for each transducer WA a separate rear-side volume is available. The partitioning can also be realized with the semiconductor component HLB as shown in  FIG. 8 . For this purpose, the reactive layer OS is structured in such a way that it does not project beyond the top edge of the semiconductor component. For the use of several transducers, it can also be necessary to provide several semiconductor components, of which only one is shown in  FIG. 8  for the sake of clarity. However, it is also possible to provide a microphone with a plurality of transducers, wherein a separate cap, which creates a separate rear-side volume above the recess, is deposited on the second surface of the carrier substrate in the region of each individual transducer WA. 
     The invention is not limited to the shown embodiments and permits a series of possible modifications, which cannot be described here in detail. The same applies to the production method, for which only advantageous implementations were proposed. Thus, in particular, the sequence of individual processing steps usually can be arbitrarily replaced, as long as they are not mutually dependent.