Abstract:
A micromechanical capacitive converter and a method for manufacturing a micromechanical converter comprise a movable membrane and an electrically conductive face element in a carrier layer. The electrically conductive face element is arranged opposite the membrane above a cavity. The electrically conductive face element and the carrier layer are perforated by perforation openings. The opening width of the perforation openings corresponds approximately to the thickness of the carrier layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of co-pending U.S. patent application Ser. No. 10/991,350, filed Nov. 15, 2004. 
     
    
     FIELD  
       [0002]     The present invention relates to a micromechanical capacitive converter and methods for manufacturing the same.  
       BACKGROUND  
       [0003]     In a micromechanical capacitive converter for which a silicon microphone is an example, frequently an air-filled cavity with a small volume is present. In a microphone, this is for example an air-filled sensor capacity consisting of a sensitive membrane and a rigid counter electrode. Due to this small air volume, the enclosed air exerts a strong restoring force on the sensor membrane. The enclosed air causes a damping of the membrane deflection and reduces the sensitivity or bandwidth, respectively, of the sensor.  
         [0004]     For increasing the bandwidth it is known to provide discharge facilities for air, wherein this is done by a perforation of the counter electrode in silicon microphones. By such a perforation, the air may escape from the capacitor gap, i.e. the cavity between the sensitive membrane and the rigid counter electrode.  
         [0005]     Well-established commercial elecret microphones comprise geometries with dimensions so great that the rigidity of the air cushion is neglectable. These microphones have, however, not the advantages of a temperature-stable silicon microphone in mass production.  
         [0006]     In micromechanically manufactured microphones, ones with electroplated counter-electrodes are known, wherein the counter-electrode is electroplated in the last step of the manufacturing process on the microchip. With regard to such microphones, reference is for example made to Kabir et al., High sensitivity acoustic transducers with p +  membranes and gold black-plate, Sensors and Actuators 78 (1999), pages 138-142; and J. Bergqvist, J. Gobet, Capacitive Microphone with surface micromachined backplate using electroplating technology, Journal of Micromechanical Systems, Vol. 3, No. 2, 1994. In manufacturing processes for such microphones the perforation openings may be selected so large that the acoustic resistance is very small and has no influence on the damping of the membrane deflection. Disadvantageous is the expensive process of electroplating.  
         [0007]     From the prior art, further two-chip-microphones are known, in which the membrane and the counter electrode are respectively manufactured on separate wafers. The microphone capacity is then obtained by “bonding” the two wafers. With regard to such a technology, reference is made to W. Kühnel, Kapazitive Silizium-Mikrofone, Series 10, Informatik/Kommunikationstechnik, No. 202, Fortschrittsberichte, VDI, VDI-Verlag, 1992. Dissertation; J. Bergqvist, Finite-element modeling and characterization of a silicon condenser microphone with highly perforated backplate, Sensors and Actuators 39 (1993), pages 1991-2000; and T. Bourouina et al., A new condenser microphone with a p +  silicon membrane, Sensors and Actuators A, 1992, pages 149-152. Also with this type of microphone it is technologically possible to select sufficiently large diameters for the perforation openings of the counter-electrode. For cost reasons, however, one-chip solutions are preferred. In addition to that, with the two-chip microphones, the alignment of the two wafers to each other is problematic.  
         [0008]     With the one-chip microphones, the counter-electrode is manufactured in an integrated way, i.e. only one wafer is required. The counter-electrode consists of one silicon substrate or is formed by deposition or epitaxy, respectively. Examples for such one-chip microphones are described in A. Torkkeli et al., Capacitive microphone with low-stress polysilicon membrane and high-stress polysilicon backplate, Physica Scripta, Vol. T79, 1999, pages 275-278; Kovacs et al., Fabrication of single-chip polysilicon condenser structures for microphone applications, J. Micromech. Miroeng. 5 (1995) pages 86-90; and Füldner et al., Silicon microphone with high sensitivity diaphragm using SOI substrate, Proceedings Eurosensors XIV, 1999, pages 217-220. In the manufacturing methods for those one-chip microphones it is generally required to close the generated perforation openings in the counter-electrode again for the following processing in order to balance the topology.  
         [0009]     One manufacturing method for such one-chip microphones is known from WO 00/09440. In this manufacturing method, initially perforation openings are generated in an epitactic layer formed on a wafer. In the following, among others for generating a sacrificial layer an oxide deposition is performed on the front side of the epitaxy layer, so that on the one hand the perforation openings are closed and on the other hand a spacing layer whose thickness defines the later spacing between membrane and counter-electrode, is formed. On this layer, a silicon membrane with the required thickness is deposited then. After the required processing of the electronic devices, in the area of the perforation openings the wafer is etched from the backside up to the epitaxy layer. In the following, from the backside an etching of the oxide is performed for opening the perforation openings and the cavity between membrane and counter-electrode. One part of the sacrificial layer between membrane and epitaxy layer thus remains as a spacing layer between the membrane and the counter-electrode.  
         [0010]     One disadvantage of this hitherto known manufacturing method for one-chip microphones is that the hole diameter in the counter-electrode may not be larger than twice the thickness of the layer deposited thereon, so that the perforation openings may still be securely closed when depositing the sacrificial layer with the desired thickness. This is disadvantageous in particular insofar as the width of the individual perforation openings may not be realized so large that the acoustic resistance and thus e.g. the top cut-off frequency of the microphone sensitivity may be optimized.  
       SUMMARY  
       [0011]     It is advantageous according to at least one embodiment of the present invention to provide a high-sensitive micromechanical capacitive converter with a minimum attenuation of the membrane and a maximum bandwidth and a method for manufacturing such a micromechanical capacitive converter.  
         [0012]     In accordance with a first aspect, at least one embodiment of the present invention provides a micromechanical capacitive converter, having a movable membrane; an electrically conductive face element, wherein the electrically conductive face element is arranged across a cavity and is opposite the membrane; and a carrier layer in which the electrically conductive face element is arranged, wherein the carrier layer and the electrically conductive face element are perforated by perforation openings, characterized in that the opening width of the perforation openings approximately corresponds to the thickness of the carrier layer.  
         [0013]     In accordance with a second aspect, at least one embodiment of the present invention provides a method for manufacturing a micromechanical capacitive converter with the steps of providing a substrate, applying a carrier layer onto the substrate, applying a mask layer over the surface of the carrier layer facing away from the substrate, structuring the mask layer such that it comprises first openings whose smallest expansion corresponds at maximum to double the later distance between a membrane and the surface, generating perforation openings in the area below the first openings in the mask layer reaching through the carrier layer, wherein the smallest opening width of the perforation openings corresponds to more than double the later distance between the membrane and the surface, generating a substantially planar sacrificial layer over the structured mask layer with a thickness, which is dependent on the later desired distance between the carrier layer and a membrane, applying the membrane onto the substantially planar sacrificial layer, exposing at least one part of the side of the carrier layer abutting the substrate, removing the sacrificial layer and the mask layer for opening the perforation openings and for generating a cavity between the membrane and the carrier layer in which the perforation openings are formed.  
         [0014]     In at least one embodiment, the present invention provides an arrangement and a method for manufacturing micromechanical capacitive converters, in particular microphones, but also other micromechanical capacitive converters having a cavity arranged between two faces. As an example, here acceleration sensors, pressure sensors, and the like are mentioned.  
         [0015]     As a substantial advantage of at least one embodiment of the invention may be regarded that the processing of large perforation openings may easily be integrated in a conventional overall process for manufacturing a micromechanical capacitive converter.  
         [0016]     In one alternative implementation of the inventive arrangement, the electrically conductive face element is arranged on the carrier layer.  
         [0017]     In one advantageous implementation of the inventive arrangement, the smallest opening width of the perforation opening is more than 2 μm. Thereby, a decrease of the acoustic resistance is achieved.  
         [0018]     In a further advantageous implementation of the invention, the perforation openings occupy 10% to 50% of the overall face from the interface between the cavity and the carrier layer and the interface between the cavity and the electrically conductive face element. By this dimensioning, a sufficient stability of the perforated element is guaranteed.  
         [0019]     In an advantageous implementation of the invention, the carrier layer is deposited epitactically onto the substrate and may serve as an etch stop layer.  
         [0020]     In the developments of the inventive method it is regarded as particularly advantageous when after applying the carrier layer an electrically conductive face element is introduced into the carrier layer or applied to the carrier layer, because this face element may then serve as an electrode in particular in a silicon microphone.  
         [0021]     In a further advantageous embodiment, before applying the electrically conductive face element onto the carrier layer an electrically insulating layer is generated.  
         [0022]     In a further advantageous embodiment, when generating the substantially planar sacrificial layer, the perforation openings are lined with the sacrificial layer at their interior wall. This gives additional stability to the perforation openings.  
         [0023]     It is especially advantageous when the interior walls of the perforation openings are lined with a material, which is etching-resistant against the substrate. Thereby, a selective removing of the substrate for exposing at least one part of the side of the carrier layer abutting the substrate is enabled. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     Further embodiments of the present invention are described in detail with respect to the following figures, in which:  
         [0025]      FIG. 1  shows a schematical sectional view of a micromechanical capacitive converter;  
         [0026]      FIG. 2  shows a diagram that illustrates the dependence of the microphone sensitivity of an inventive microphone on the hole diameter of the perforation openings;  
         [0027]      FIG. 3 a ) to i) show schematical sectional illustrations for explaining a method for manufacturing an individual perforation opening. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     In  FIG. 1 , a general set-up of a one-chip silicon microphone is illustrated schematically.  
         [0029]     The one-chip silicon microphone comprises a moveable membrane  10 . The membrane  10  lies above a cavity  12  and opposite a counter-electrode  14 . This counter-electrode  14  is formed by areas of an epitaxy layer  15  applied to a substrate  11 . In the counter-electrode  14  a doping area  18  and perforation openings  20  are formed.  
         [0030]     The membrane  10  is applied to the epitaxy layer  15  via a spacing layer  22 . A first terminal electrode  24  is connected to the membrane  10  in an electrically conductive way, while a second terminal electrode  26  is connected to the doping area  18  of the counter-electrode  14 . On the epitaxy layer  15  outside the membrane area an insulating layer  28  is provided.  
         [0031]     In the substrate  11  below the portion of the epitaxy layer  15  serving as a counter-electrode  14  an opening  30  is provided, so that the perforation openings  20  fluidically connect the cavity  12  to the opening  30 . The opening  30  may be etched into the substrate  11 .  
         [0032]     As the functioning of the illustrated capacitive converter should be obvious for a person skilled in the art, it is merely noted that by the acoustic waves hitting the membrane  10 , a deformation of the membrane takes place, so that a capacity change resulting due to the changed spacing between the membrane  10  and the counter-electrode  14  may be detected between the terminal electrodes  24  and  26 .  
         [0033]     In order to reduce the influence of the air contained within the cavity  12  on the sensitivity and the response of the converter, the perforation openings  20  serving as discharge openings are provided in the counter-electrode  14 . By these perforation openings  20 , when the membrane is deformed, the air may escape from the capacitor gap, i.e. escape from the cavity and enter trough the same, wherein the resulting acoustic resistance determines the top cut-off frequency of the microphone sensitivity depending on the perforation density and the size of the individual perforation openings.  
         [0034]     In a diagram  FIG. 2  shows the dependence of the microphone sensitivity on the hole diameter of the perforation openings  20  plotted over the frequency using  6  curves.  
         [0035]     A first curve  40  shows an almost constant microphone sensitivity across the maximum bandwidth of the frequency response with a hole diameter of 8 μm, while the second, third, and forth curves  37 ,  38 , and  39  with a smaller hole diameter of 1 μm or 2 μm or 4 μm, respectively, and the fifth and sixth curves  41  and  42  with a larger hole diameter of 16 μm or 32 μm, respectively, show a clearly worse microphone sensitivity at higher frequencies. In all cases, the perforation area is respectively approx. 25% of the overall face of the counter-electrode  14  (see  FIG. 1 , dashed zone).  
         [0036]     In  FIG. 3 , a number of successively running technology steps a) to i) when manufacturing a single perforation opening in a one-chip microphone are illustrated.  
         [0037]     In the first step a) using epitaxy an approx. 5 μm thick layer  150  is applied to a silicon substrate  110 . On this layer  150  first of all an insulating layer  200  covering the complete surface  120  of the layer  150  and on top of that a patterned electrically conductive layer  300  are applied. Subsequently, over the insulating layer  200  and the electrically conductive layer  300  a mask layer  350  is applied and patterned such that it comprises small openings  400  at the location where the mask layer  350  directly covers the insulating layer  200 . Preferably, this mask layer  350  is an oxide.  
         [0038]     In the second step b) using a dry etching process a hole  190  is etched through the insulating layer  200  and into the layer  150  approximately up to the interface of layer  150  and substrate  110 .  
         [0039]     In the third step c), then by a selective isotropic etching process, the hole  190  is expanded to the desired final diameter of 5 μm below the mask layer  350 . Thereby, the perforation opening  180  results. The etching process may preferably be either dry-chemical or wet-chemical.  
         [0040]     In a forth step d) now the overall surface and the perforation opening  180  is provided with a thin dielectric layer  250 .  
         [0041]     In a fifth step e) using a dry etching method the dielectric layer  250  is selectively removed on the surface of the mask layer  350  so that this dielectric layer  250  only remains at the surface of the perforation opening  180 .  
         [0042]     In a sixth step f), now a sacrificial layer  380 , preferably an oxide sacrificial layer, is deposited. This deposition causes the perforation opening  180  to be lined with a layer until the small opening  400  in the mask layer  350  is closed. The deposition of the sacrificial layer  380  takes place until the thickness of the sacrificial layer  380  has reached the desired value. In this process, the surface of the wafer is almost completely planarized, so that subsequent processes may be performed with conventional means of semiconductor technology. When using a material as a sacrificial layer  380  which is etch-resistant against the silicon substrate  110 , the forth and fifth step d) and e) may be omitted.  
         [0043]     In a seventh step g) the membrane  500  is deposited onto the sacrificial layer  380 . In further steps which are not important for the explanation of the embodiment and therefore omitted here, any other processes required for the manufacturing of a functional one-chip microphone are performed, for example for forming the terminals  24  and  26 .  
         [0044]     In an eighth step h), the silicon substrate  110  is removed in the area below the membrane  500  using so-called volume micromechanics. This process is selectively against the layer  150  and against the lining of the perforation opening  180 . This way, the surface  170  of the layer  150  facing the substrate  110  is exposed.  
         [0045]     In a final step i) the insulating layer  200 , the possibly present dielectrics layer  250 , the sacrificial layer  380  and the mask layer  350  are wet- or dry-chemically removed in so far that by doing this the perforation opening  180  is opened and a cavity  450  results between the surface  120  and the membrane  500 .  
         [0046]     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.