Abstract:
A method of producing a semiconductor component, e.g., a multilayer semiconductor component, and a semiconductor component produced by this method, where the semiconductor component has, e.g., a mobile mass, i.e., an oscillator structure. 
     A method easily and inexpensively produce a micromechanical component having monocrystalline oscillator structures, such as an acceleration sensor or a rotational rate sensor for example, by surface micromechanics, a first porous layer is formed in the semiconductor component in a first step and a cavity, i.e., a cavern, is formed beneath or out of the first porous layer in the semiconductor component in a second step.

Description:
BACKGROUND INFORMATION 
   The present invention is directed to a method for producing a semiconductor component, e.g., a multilayer semiconductor component, and to a semiconductor component produced by this method, the semiconductor component having a mobile mass, i.e., an oscillator structure. 
   Some semiconductor components such as micromechanical acceleration sensors or rotational rate sensors in particular have a mobile mass, i.e., an oscillator structure. Such sensors are usually produced from polycrystalline silicon by surface micromechanics, the oscillator structure being created by etching away a sacrificial silicon oxide layer by gas phase etching so the oscillator structure is freely mobile. 
   Surface micromechanics for production of acceleration sensors or rotational rate sensors may be complex and therefore expensive. In comparison with an oscillator structure of monocrystalline silicon, it may be possible to produce oscillator structures of polycrystalline silicon with a greater range of variation in the mechanical properties. Furthermore, they may have inferior long-term stability. 
   The methods of producing such sensors by surface micromechanics are not generally compatible with the typical methods of producing semiconductor circuit elements. 
   SUMMARY 
   An example embodiment of the method according to the present invention may have the advantage over the related art that a micromechanical component having monocrystalline oscillator structures, e.g., an acceleration sensor or a rotational rate sensor, may be produced easily and inexpensively by surface micromechanics. 
   An example embodiment of the present invention may include a cavern, i.e., a cavity, in a semiconductor substrate such as a silicon substrate by using an etching medium. To do so, the cover layer of the substrate may be etched in the area of the cavern to be produced subsequently, so that openings, i.e., etching openings such as pores, i.e., cavities, are formed in it. The etching medium or one or more other etching media may pass through the etching openings, i.e., the pores which are open to the outside, and reach deeper areas of the substrate. The part of the semiconductor substrate decomposed by the etching medium and/or the additional etching media in this area may be removed through the openings, i.e., the pores in the cover layer and/or through an external access opening to this area. The cover layer may have a thickness of, e.g., approx. 2 μm to 10 μm, e.g., 3 μm to 5 μm. A mostly monocrystalline epitaxial layer may be deposited on the porous cover layer. 
   In the case of an access opening, a porous cover layer may be used, having a thickness of, e.g., approx. 40 μm to 80 μm, e.g., 50 μm to 60 μm instead of a porous cover layer of approx. 2 μm to 10 μm. The purpose of the greater thickness is that the cover layer may function as an etching stop layer in etching the access opening and thus permitting a reliable etching stop before reaching a mostly monocrystalline epitaxial layer deposited on the cover layer. 
   To produce a semiconductor component having a mobile mass, i.e., an oscillator structure, such as an acceleration sensor or a rotational rate sensor, the mostly monocrystalline epitaxial layer deposited on the cover layer, i.e., the porous cover layer, may be structured by one or more operations so that a mobile mass, i.e., an oscillator structure of the sensor is formed entirely or partially from it. The structuring may be performed by using known dry etching techniques. 
   In an example embodiment of the present invention, electric and/or electronic semiconductor components may be produced in the monocrystalline epitaxial layer and/or in the monocrystalline mobile mass, i.e., mobile structure, i.e., oscillator structure formed from the monocrystalline epitaxial layer, e.g., by suitable doping. In a conventional manner, electric and/or electronic circuit elements may be integrated into a monocrystalline epitaxial layer, i.e., into a monocrystalline mobile mass. 
   In comparison with an oscillator structure conventionally formed from polycrystalline silicon, a mobile mass, i.e., an oscillator structure formed from monocrystalline silicon of the epitaxial layer, may be formed such that a mobile mass of monocrystalline silicon may be produced to have a small range of variation in the mechanical properties. Furthermore, such oscillator structures produced from monocrystalline silicon according to the present invention may have a good long-term stability. 
   In an example embodiment of the present invention, the operation of etching the porous cover layer may ensure that the rate of expansion of the pores in the cover layer is lower, possibly much lower, than the rate of expansion of the pores, i.e., cavities, in the area of the substrate which forms the subsequent cavity, i.e., cavern. 
   According to an example embodiment of the present invention, this may be achieved by selecting the etching parameters and/or the etching medium or media in etching the pores in the cover layer to be different from the etching parameters and/or the etching medium or etching media in etching the pores, i.e., cavities in the area of the subsequent cavern. 
   The porosity of the cover layer for removal of the silicon to be decomposed for production of the caverns may be adjustable to be adequately large in a manner that is well controllable in terms of the process technology. On the other hand, however, the cavern may be produced rapidly and thus inexpensively. 
   According to an example embodiment of the present invention, the etching parameters may be adjusted and/or the etching medium or media may be selected in etching the cavern so that the expansion rate of the pores, i.e., cavities is so high that the pores, i.e., cavities very rapidly “overlap” one another. This results first in a single, largely superficial starting cavity in the substrate, which expands in depth over a period of time and forms the cavern. 
   As an example embodiment of the present invention, which is an alternative to the immediately preceding embodiment, the etching parameters and/or the etching medium or media may be selected in etching the cavern so that the porosity of the area of the substrate which forms the subsequent cavern is greater than the porosity of the cover layer. The substrate may be a monocrystalline silicon substrate. The precursor of the subsequent cavern may have a porosity of more than 80%. The cavern may be formed subsequently from the porous area of the substrate by performing one or more controlled heating steps, e.g., at a temperature above approx. 900° C. 
   In controlled heating, e.g., in an atmosphere of hydrogen, nitrogen or a noble gas, e.g., at temperatures above approx. 900° C., the pores in the area of the silicon which will form the subsequent cavern are rearranged at a porosity of more than approx. 80%, so that a single large pore, i.e., a cavity, i.e., a cavern is formed beneath the low-porosity cover layer, i.e., starting layer, for an epitaxial layer to be deposited subsequently. The pores on the top side of the low-porosity layer, i.e., the starting layer, are mostly sealed in this high-temperature step, so that a largely monocrystalline silicon layer may be deposited on the starting layer to form a starting layer for producing one or more mobile masses. 
   According to an example embodiment of the present invention, the etching medium or the etching media for producing the openings and/or pores in the cover layer and/or for producing the cavern is hydrofluoric acid (HF) or a liquid mixture or a chemical compound that contains hydrofluoric acid. 
   In an example embodiment of the present invention, a readily volatile component, e.g., an alcohol such as ethanol and/or purified water, is added to the etching medium or the etching media to dilute the etching medium or etching media. 
   Ethanol reduces the surface tension of an etching medium to which it is added, thus permitting better wetting of the silicon surface and better penetration of the etching medium into the etched pores, i.e., openings, i.e., cavities. Furthermore, the bubbles which are formed during the etching process are smaller than those formed without the addition of ethanol to the etching medium, and thus the bubbles are better able to escape through the pores of the cover layer. It is therefore possible to keep the pore size, i.e., the porosity of the cover layer smaller than without the addition of the alcohol. 
   In another example embodiment of the present invention, the openings, i.e., pores in the cover layer and/or in the area of the subsequent cavern are produced by an electrochemical method, e.g., using the etching medium or the etching media mentioned above. 
   Furthermore, in an example embodiment of the present invention using an electrochemical etching method, e.g., an etching method using hydrofluoric acid (HF), the rate of expansion of the pores, i.e., cavities formed in the etching process may be influenced by applying an electric voltage and an electric current produced thereby through the etching medium or the etching media. The rate of expansion of the pores, i.e., cavities may depends, e.g., on the doping of the silicon substrate to be etched, the current density, the HF concentration in the etching medium, if present, and the temperature. It is self-evident that these are only examples of relevant parameters of an exemplary etching method according to the present invention. 
   According to an example embodiment of the present invention, the etching medium, the HF concentration in the etching medium and/or the doping of the area to be etched and/or the temperature and, if appropriate, other process parameters of the etching method may be selected, so that the etching process, i.e., the formation of pores, i.e., cavities, may be adjusted in a suitable manner and/or stopped when the electric voltage is turned off, possibly rather abruptly. 
   In an exemplary electrochemical etching method according to the present invention using a single etching medium and/or two or more etching media, a first current density, which is not necessarily constant over time, may be established in the etching medium in a first period of time during which the etching medium is in the area of the cover layer. During a second period of time when the particular etching medium is in the area of the cavern to be created, a second current density may be established which is not necessarily constant over time and is higher or much higher than the or a current density established during the first period of time. This results in formation of the cavern or a precursor of the cavern through pores, i.e., cavities, having a rate of expansion during the etching of the cavern that is higher or much higher than the rate of expansion of the pores for producing the porous cover layer. 
   In another example embodiment of the present invention, the area of the cover surface of the substrate to be etched to form pores may be surrounded by a mask layer, i.e., a supporting layer, before the etching process, thereby permitting free access of the etching medium or the etching media to the area to be etched with pores and shielding the areas of the cover surface of the substrate which are not to be etched to form pores to prevent etching attack there. 
   According to an example embodiment of the present invention, the supporting layer may be designed so that it mechanically secures the area, which is to be etched to form pores and/or the layer of the cover surface which is to be etched to form pores, on the unetched portion of the substrate during and after etching of the cavern. 
   In an example embodiment of the present invention, the supporting layer may be created before etching the area which is to be etched to form pores and/or the layer which is to be etched by providing at least the nearest surrounding area around the layer of the cover surface which is to be etched to form pores in a p-doped silicon substrate with an n-doping. In this way it is possible to largely prevent “undercutting” of the substrate, e.g., in the area where the layer which is etched to form pores is mechanically joined to the silicon substrate. Otherwise there may be the risk, e.g., in the case of a thin porous layer, i.e., starting layer, that it would become detached from the substrate. In addition, a silicon nitride layer may be used as a mask and, e.g., for protection against an etching attack of electronic circuits situated beneath the mask. 
   Alternatively or additionally, instead of the n-doping, i.e., an n-doped layer, a metal layer or metal mask may also be provided, likewise to largely prevent undercutting of the substrate. Use of a metal layer, i.e., metal mask, is usually expedient, however, if no circuits are to be provided in the substrate because otherwise metal atoms remaining in the substrate even after removal of the metal layer, i.e., metal mask, might impair the functioning of the circuits. 
   In another example embodiment of the present invention, a cover layer which is etched to form pores, such as a silicon layer, is to be pretreated before an epitaxial layer, e.g., a largely monocrystalline silicon layer, is applied, i.e., deposited on it. The pretreatment is performed with the goal of partially or entirely closing the pores in the cover layer, i.e., starting layer which is etched to form pores, to further improve the quality of the largely monocrystalline silicon layer, if necessary or expedient. 
   A pretreatment according to the present invention may include heating the cover layer, i.e., starting layer which is etched to form pores, in a controlled manner to a high temperature, e.g., at a temperature in the range of approx. 900° C. to approx. 1100° C. This controlled heating may be performed in an atmosphere of hydrogen, nitrogen and/or noble gas. 
   As an alternative or in addition to the pretreatment mentioned above, (slight) oxidation of the silicon starting layer which is etched to form pores may be provided. This oxidation may be performed with (slight) addition of oxygen into the atmosphere to which the starting layer is exposed in the reactor, the oxidation may be performed at a temperature in the range of approx. 400° C. to 600° C. The term “slight” is understood to refer to oxidation which causes some or all of the pores of the starting layer to be closed and forms an approximately mesh-like oxide structure. According to an example embodiment of the present invention, the oxide structure may cover as little of the surface of the porous etched starting layer as possible in order to ensure that a possibly monocrystalline silicon layer is deposited on the starting layer, so that a mobile mass may be formed from it subsequently, e.g., by dry etching techniques. The oxidation may be removed, if necessary, in a process step following the oxidation process until this desired state is obtained. 
   In an example embodiment of the present invention, the thickness of the starting layer may be much smaller than the thickness of the silicon layer deposited on it, so the physical properties of the at least one mobile mass, i.e., oscillator structure, thus created are determined largely by the silicon layer, the thickness of which is easily adjustable through the process technology. 
   According to an example embodiment of the present invention, the low-porosity layer, i.e., starting layer, for the deposition of an epitaxial layer may be created using an etching medium having a hydrofluoric acid (HF) concentration in the range of approx. 20% to approx. 50%, e.g., approx. 30% to approx. 40%, e.g., approx. 33%. 
   In another example embodiment of the present invention, the porous layer which forms a precursor of the subsequent cavity, i.e., cavern, is etched with an etching medium having a hydrofluoric acid (HF) concentration in the range of approx. 0% to approx. 40%, e.g., approx. 5% to 20%, e.g., less than approx. 20%. The remaining amount of etching medium which is not formed by hydrofluoric acid is composed mostly of an alcohol, e.g., ethanol. 
   An etching medium according to the present invention may be provided in an example embodiment of the present invention to achieve a high rate of expansion of the pores, i.e., cavities, in the layer which is to be decomposed during an aforementioned etching step according to the present invention to form a cavity, i.e., a cavern, so that the pores, i.e., cavities, will very rapidly “overlap” with one another and thus form a single “giant pore.” The etching medium according to an example embodiment of the present invention may have a hydrofluoric acid (HF) concentration in the range of approx. 0% to approx. 5%, preferably approx. 1% to approx. 3%, e.g., less than approx. 5%. The remainder of this etching medium not formed by hydrofluoric acid may be made up mostly of an alcohol, e.g., ethanol and/or purified water. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross section of a precursor of an acceleration sensor having two mobile masses, i.e., two oscillator structures. 
       FIG. 2  shows a cross section of a precursor for forming a first acceleration sensor according to an example embodiment of the present invention. 
       FIG. 3  shows a cross section of another precursor of the first acceleration sensor according to an example embodiment of the present invention, produced on the basis of the precursor illustrated in  FIG. 2  and having a monocrystalline porous layer and a cavern, i.e., a cavity, formed beneath the porous layer. 
       FIG. 4  shows a cross section of another precursor of the first acceleration sensor according to an example embodiment of the present invention, produced on the basis of the precursor shown in  FIG. 3  and having an epitaxial layer and an electronic circuit element or circuits integrated into it. 
       FIG. 5  shows a cross section of another precursor of the first acceleration sensor according to an example embodiment of the present invention, produced on the basis of the precursor illustrated in  FIG. 4  and having the mobile masses, i.e., oscillator structures, formed from the epitaxial layer. 
       FIG. 6  shows a top view of a precursor of a second acceleration sensor according to an example embodiment of the present invention, produced on the basis of the precursor illustrated in  FIG. 4  and having a mobile mass, i.e., an oscillator structure, the acceleration or deflection of which is detected by piezoresistive resistors. 
       FIG. 7  shows a cross section (along line A-A in  FIG. 8 ) of a precursor of a third acceleration sensor according to an example embodiment of the present invention, produced on the basis of the precursor illustrated in  FIG. 4  and having a mobile mass, i.e., oscillator structure, the acceleration or deflection of which is determined capacitively. 
       FIG. 8  shows a top view of the precursor of the third acceleration sensor according to an example embodiment of the present invention having capacitive analysis, as shown in  FIG. 7 . 
       FIG. 9  shows a cross section of another precursor of the first, second or third acceleration sensor according to the present invention, produced on the basis of the known precursor shown in  FIG. 2 , as an alternative to the precursor shown in  FIG. 3 . 
       FIG. 10  shows a cross section of another precursor produced on the basis of the precursor illustrated in  FIG. 9 . 
       FIG. 11  shows a cross section of another precursor formed on the basis of the precursor illustrated in  FIG. 10 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows precursor  100  of a known acceleration sensor. Precursor  100  has a silicon substrate  101  of monocrystalline silicon, a sacrificial silicon oxide layer  102  deposited on silicon substrate  101  and a polysilicon layer  103  of polycrystalline silicon deposited on sacrificial silicon oxide layer  102 . An etching mask (not shown) is applied to precursor  100  in a known way, so that etching openings  104  are not covered by the etching mask. The top side of precursor  100  shown in  FIG. 1  is subsequently etched in a known way, thus creating, i.e., forming, mobile masses  105  and  106  in polysilicon layer  103  and a cavern, i.e., a cavity  107  in sacrificial silicon oxide layer  102 , as shown in  FIG. 1 . 
   One disadvantage of this method of producing precursor  100  of a conventional acceleration sensor as shown in  FIG. 1  is that the mobile masses have fluctuations in their mechanical properties in mass production due to the polysilicon structure. Furthermore, there are great fluctuations in the dimensions of the cavity. 
     FIG. 2  shows a cross section of a conventional precursor  200  for forming a first acceleration sensor according to an example embodiment of the present invention. Conventional precursor  200  has a monocrystalline silicon substrate  101 , electronic circuit elements, i.e., circuits  201  integrated into monocrystalline silicon substrate  101 , and an etching mask  202  on the top of monocrystalline silicon substrate  101 , an etching opening  203  being provided in etching mask  202 . 
     FIG. 3  shows another precursor  300  of the first acceleration sensor according to an example embodiment of the present invention produced on the basis of the known precursor shown in  FIG. 2 . To produce precursor  300  from precursor  200  shown in  FIG. 2 , the area defined by etching opening  203  is etched electrochemically to make it porous by using one or more etching media containing hydrofluoric acid, as explained in detail above. The porosity is controlled by the current density in the etching medium, the doping of the silicon, and the composition of the etching medium. To form porous monocrystalline silicon layer  301 , the etching process is controlled so that porous monocrystalline silicon layer  101  has a low porosity. After porous monocrystalline silicon layer  301  has been produced, the current density in the etching medium is increased above a critical level and/or the composition of the etching medium is altered so that the “pores” beneath porous layer  301  become so large that the material of silicon substrate  101  is completely etched out of area  302 , and the cavern, i.e., cavity  302 , is created beneath porous monocrystalline silicon layer  301 . The silicon of silicon substrate  101  decomposed by the etching medium may be removed through the pores of the porous layer or through a separate access opening. 
     FIG. 4  shows another precursor  400  of the first acceleration sensor according to an example embodiment of the present invention, which was produced on the basis of precursor  300  shown in  FIG. 3 . Precursor  400  has silicon substrate  101 , electronic circuit elements, i.e., circuits  201 , integrated into silicon substrate  101 , porous monocrystalline silicon layer  301 , and the cavern, i.e., cavity  302 . An epitaxial monocrystalline silicon layer  401  has been deposited on porous monocrystalline silicon layer  301 . Deposition of an epitaxial monocrystalline silicon layer on porous monocrystalline silicon layer  301  according to the present invention is made possible by the fact that with a suitably low porosity of porous silicon layer  301 , it is possible to deposit a mostly monocrystalline epitaxial layer on porous monocrystalline silicon layer  301 . Epitaxial monocrystalline silicon layer  401  seals the cavern, i.e., cavity  302 , so that the pressure prevailing in the epitaxial process for deposition of epitaxial monocrystalline silicon layer  401  determines the pressure enclosed in cavity  302 . In the example embodiment illustrated in  FIG. 4 , additional electronic circuit elements, i.e., circuits  402  or the like are produced by standard semiconductor methods, e.g., by suitable doping of epitaxial monocrystalline silicon layer  401 . 
   To improve the quality of epitaxial monocrystalline silicon layer  401 , porous monocrystalline silicon layer  301  may, if necessary, be pretreated as already explained above. 
     FIG. 5  shows another precursor  500  of the first acceleration sensor according to an example embodiment of the present invention, which was formed on the basis of precursor  400  illustrated in  FIG. 4 . 
   Precursor  500  has silicon substrate  101 , electronic circuit elements, i.e., circuits  201  integrated into silicon substrate  101 , cavern, i.e., cavity  302 , and two mobile masses  501  and  502  which may be formed by conventional dry etching techniques from epitaxial monocrystalline silicon layer  401  and porous monocrystalline silicon layer  301 . Furthermore, the electronic circuit elements, i.e., circuits  402 , have been integrated into epitaxial monocrystalline silicon layer  401  by appropriate standard semiconductor processes, e.g., suitable doping. 
   When precursor  100  of a conventional acceleration sensor having two mobile masses  105  and  106 , as shown in  FIG. 1 , is compared with precursor  500  of a first acceleration sensor according to the present invention, as shown in  FIG. 5 , it is seen that mobile masses  501  and  502 —in contrast with mobile masses  105  and  106  made of polysilicon—have been formed from monocrystalline silicon of epitaxial monocrystalline silicon layer  401  and to a slight extent also from porous monocrystalline silicon layer  301 . On the basis of the defined material parameters of monocrystalline silicon, mobile masses  501  and  502  may be formed in a reproducible manner with only minor fluctuations in their mechanical properties. Furthermore, electronic circuit elements, i.e., circuits  402 , may be integrated into epitaxial monocrystalline silicon layer  401  of precursor  500 , which may not be possible with a polysilicon layer  103  using standard semiconductor processes. 
   Movements of the mobile masses, i.e., oscillator structures  501  and  502 , and optionally other masses may be analyzed capacitively, for example. For a capacitive analysis, mobile masses  501  and  502  and possibly also other mobile masses, may be formed as interdigital structures from epitaxial monocrystalline silicon layer  401 . Interdigital structures are understood to refer to structures composed of at least one first structure and one second structure. Each first and second structure has a plurality of finger-shaped masses, some of them mobile, with one finger of the first structure being situated between two adjacent fingers of the second structure. The first structure forms a first stationary capacitor plate, and the second structure forms a second mobile capacitor plate. Such interdigital structures have a high sensitivity for determining acceleration acting on the second structure. 
   As an alternative, however, piezoresistive resistors may also be provided on mobile masses  501  and  502  as well as other masses to determine the acceleration or deflection of the mobile masses, i.e., oscillator structures. In addition, it is also possible to provide a capacitor in precursor  500  shown in  FIG. 5  to deflect mobile masses  501  and  502  in a controlled manner when a voltage is applied, e.g., for test purposes. This deflection or acceleration is then determined by capacitive or piezoresistive means in the manner described above. 
     FIG. 6  shows a top view of a precursor  600  of a second acceleration sensor according to an example embodiment of the present invention, formed on the basis of precursor  400  shown in  FIG. 4 . In contrast with precursor  500  of a first acceleration sensor according to an example embodiment of the present invention as shown in  FIG. 5 , precursor  600  of the second acceleration sensor according to an example embodiment of the present invention has a single mobile mass  601 , which has a large area in relation to masses  501  and  502  and is connected by fastening arms  602  and  603  to epitaxial monocrystalline silicon layer  401 . The cavern, i.e., cavity  302 , is situated beneath large-area mobile mass  601 . Mobile mass  601  is elastically suspended on epitaxial monocrystalline silicon layer  401  due to an appropriate design of fastening arms  602  and  603 , so that large-area mobile mass  601  is able to oscillate in X direction, i.e., in the direction of the top or bottom edges of the page, as well as in Z direction, i.e., into and out of the page. It is possible to implement an acceleration sensor by using precursor  600  shown in  FIG. 6 , so that it detects acceleration in both X and Z directions and thus also detects the associated deflection of large-area mobile mass  601 . The deflection or acceleration of large-area mobile mass  601  is analyzed by using piezoresistive resistors  604  through  607 , piezoresistive resistors  604  and  605  being situated in fastening arm  602 , which functions as the first elastic suspension of mobile mass  601 , and piezoresistive resistors  606  and  607  being situated in second fastening arm  603 , which functions as the second elastic suspension of mobile mass  601 . Dotted line  608  shows the edge of the area of porous etching, i.e., the edge of porous monocrystalline silicon layer  301 , which is adjacent to silicon substrate  101 . 
   When large-area mobile mass  601  is accelerated in the X direction, i.e., in the direction of the top edge or the bottom edge of the page, both upper piezoresistive resistors  604  and  606  undergo the same change in resistance, this change being opposite the change in resistance of the two lower piezoresistive resistors  605  and  607 . When large-area mobile mass  601  is accelerated in the Z direction, i.e., into or out of the plane of the page, all piezoresistive resistors  604 ,  605 ,  606  and  607  undergo the same change in resistance. For example, the piezoresistive resistors may be wired to form a Wheatstone bridge for detecting the acceleration or deflection of large-area mobile mass  601 . 
   Large-area mobile mass  601 , which is made of monocrystalline silicon of epitaxial monocrystalline silicon layer  401 , may be created from it by conventional dry etching techniques, e.g., by trench etching. 
     FIG. 7  shows a cross section (along line A-A in  FIG. 8 ) of precursor  700  of a third acceleration sensor according to an example embodiment of the present invention, produced on the basis of precursor  400  shown in  FIG. 4 . Precursor  700  of the third acceleration sensor according to the present invention has silicon substrate  101 , a bottom electrode  701 , a cavern, i.e., a cavity  302 , a porous monocrystalline silicon layer  301 , an epitaxial monocrystalline silicon layer  401 , and a cover electrode  702 . Bottom electrode  701  has a doped area in which the doping has been introduced into silicon substrate  101  before the porous etching of silicon substrate  101 . The doped region forming bottom electrode  701  may extend deeper into silicon substrate  101  than the porous etched region, i.e., porous monocrystalline silicon layer  301 . Cover electrode  702  is formed by a doped region in which the doping is performed before deposition of epitaxial monocrystalline silicon layer  401 . 
     FIG. 8  shows a top view of precursor  700  of the third acceleration sensor according to an example embodiment of the present invention as illustrated in  FIG. 7 . Top view  800  of precursor  700  shows cover electrode  702 , which is a mobile mass having a large area in relation to masses  501  and  502 . Cover electrode  702  is suspended by elastic suspension on silicon substrate  101  by fastening arms  703  and  704 . Outer dotted line  705  indicates the edge of porous etched region  302  adjacent to silicon substrate  101 . Inner dotted line  706  shows bottom electrode  701 , which is essentially concealed beneath cover electrode  702  and is provided in silicon substrate  101 . An electric terminal  707  is provided to detect movement of the cover electrode due to an acceleration acting on the cover electrode and a resulting change in capacitance between the cover electrode and the bottom electrode which form a capacitor, this terminal extending from cover electrode  702  to silicon substrate  101  via fastening arm  704 . Furthermore, a terminal  708  is provided which contacts bottom electrode  701  and is connected to silicon substrate  101 . Terminals  707  and  708  are preferably formed by suitably doped regions in epitaxial monocrystalline silicon layer  401  and in silicon substrate  101 . In comparison with stationary bottom electrode  701 , cover electrode  702  is deflectable in the Z direction, i.e., into and out of the plane of the page, when an acceleration acts on the cover electrode, i.e., the third acceleration sensor according to an example embodiment of the present invention. The deflection or acceleration of the cover electrode may be detected and analyzed capacitively via the capacitor system formed by the cover electrode and the bottom electrode. 
     FIG. 9  shows an alternative to precursor  300  shown in  FIG. 3  for the first, second, or third acceleration sensor according to an example embodiment of the present invention. In contrast with the precursor shown in  FIG. 3 , precursor  900  shown in  FIG. 9  has a porous monocrystalline silicon layer  901  whose thickness largely corresponds to the total thickness of the combination of monocrystalline silicon layer  301  and the cavern, i.e., cavity  302 . Porous monocrystalline silicon layer  901  may be formed, e.g., by the measures explained above in detail. 
   The alternative shown in  FIG. 10  to precursor  400  shown in  FIG. 4  for formation of the first, second or third acceleration sensor according to an example embodiment of the present invention differs from precursor  400  shown in  FIG. 4  in that epitaxial layer  401  has been deposited on porous monocrystalline silicon layer  901  and on the top of monocrystalline silicon substrate  101  of precursor  900 . 
   The alternative shown in  FIG. 11  to precursor  500  shown in  FIG. 5  differs from precursor  500  shown in  FIG. 5  in that porous monocrystalline silicon layer  901  of precursor  1000  has been removed, i.e., etched away, in producing mobile masses  501  and  502  as described in conjunction with  FIG. 5 . A cavern, i.e., a cavity  1101 , is formed due to the removal of porous monocrystalline silicon layer  901 . 
   The alternatives shown in  FIGS. 9 through 11  to the precursors shown in  FIGS. 3 through 5  have the advantage over the related art that the total complexity for producing mobile masses  501  and  502  and the cavern, i.e., cavity  1101 , is reduced due to the measures described above. To form the cavern, i.e., cavity  1101 , shown in  FIG. 11 , it may not be necessary to adjust the etching parameters so that first a porous monocrystalline silicon layer  101  is formed and then a cavern, i.e., a cavity  102 , is formed by changing the etching parameters. Instead, without change in etching parameters, an entire porous monocrystalline silicon layer  901  may be formed and then removed or etched away from epitaxial layer  401  in etching, i.e., forming, mobile masses  501  and  502  described above.