Abstract:
A microelectromechanical sensor is configured to measure a force, a pressure, or the like. The sensor includes a substrate and a measuring element. The measuring element includes at least two electrically conductive regions, and at least one of the electrically conductive regions is at least partly connected to the substrate. The sensor also includes at least one changing region, and the changing region lies at least partly between the electrically conductive regions. The changing region is configured in a substantially electrically insulating manner in an unloaded state and in a substantially electrically conductive manner in a loaded state.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/066393, filed on Sep. 21, 2011, which claims the benefit of priority to Serial No. DE 10 2010 043 277.6, filed on Nov. 3, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The disclosure relates to a microelectromechanical sensor for measuring a force, a pressure or the like, to a corresponding method and to a corresponding production method. 
     BACKGROUND 
     Microelectromechanical systems have gained great economic importance in the meantime. The majority is formed here by microelectromechanical sensors which are used, in particular, in the consumer sector and in the automotive sector as acceleration and pressure sensors. 
     Such pressure sensors are based on substantially the same functional principle: a pressure difference results in deformation of a diaphragm in the pressure sensor. This deformation of the diaphragm, which is proportional to the pressure difference, is measured. In this respect, two corresponding methods for the evaluation are already known: 
     In the capacitive method, the diaphragm is configured in such a manner that a capacitance changes as a result of the diaphragm deformation. The corresponding capacitance change is recorded and this capacitance change is then used to calculate the pressure difference or the corresponding pressure. 
     The second evaluation method is based on the so-called piezoresistive effect. Piezoresistors are arranged on or in the corresponding diaphragm. A voltage is then applied to the piezoresistors. If the diaphragm is deformed, the electrical resistance of the piezoresistors is also changed thereby, that is to say the diaphragm deformation is measured as the resistance change. A pressure sensor based on the piezoresistive effect is disclosed, for example, in DE 10 2008 033 592 A1. 
     SUMMARY 
     The microelectromechanical sensor for measuring a force, a pressure or the like, as defined in the description below, comprises a substrate with a measuring element, the measuring element comprising at least two electrically conductive regions, at least one of the electrically conductive regions being at least partially connected to the substrate, and at least one changing region, the changing region being at least partially arranged between the electrically conductive regions, the changing region being substantially electrically insulating in an unloaded state and being substantially electrically conductive in the loaded state. 
     The method for measuring a force, a pressure or the like, in particular suitable for being carried out with a microelectromechanical sensor as described herein, comprises the steps of
         measuring an electrical variable which is applied to a changing region in the unloaded state, the changing region being arranged between at least two electrically conductive regions,   deforming the changing region on the basis of a force, a pressure or the like,   measuring the electrical variable in the loaded state, and   determining the force, the pressure or the like using the measured electrical variable.       

     The method for producing a microelectromechanical sensor, in particular as described herein, comprises the steps of
         applying a thin oxide layer to a first electrically conductive layer, preferably using thermal oxidation or chemical vapor deposition, in particular using atomic layer deposition,   applying a second electrically conductive layer to the thin oxide layer, preferably using chemical and/or physical vapor deposition, and   arranging the layers arranged above one another on a substrate in such a manner that at least some of the layers arranged above one another—first electrically conductive layer, oxide layer and second electrically conductive layer—can be deformed.       

     The microelectromechanical sensor defined in the description below, the method defined in the description below and the method for producing a microelectromechanical sensor defined in the description below have the advantage that a microelectromechanical sensor can be produced in a cost-effective manner, is simultaneously more insensitive to interference and can be used in a wider temperature range. In addition, the microelectromechanical sensor and the corresponding method have a higher degree of accuracy. 
     According to another preferred development of the disclosure, the changing region has a thickness of less than 25 nanometers, in particular less than 10 nanometers, preferably less than 5 nanometers, in the unloaded state. The advantage here is thus that the quantum-mechanical tunnel effect for electrons can be used particularly well to measure the pressure and force, that is to say the sensitivity of the microelectromechanical sensor is accordingly high with a simultaneously high degree of accuracy. If the thickness of the changing region is reduced on account of a force acting indirectly on it, the tunnel barrier becomes smaller and the likelihood of electrons tunneling through is increased; the tunnel current through the changing region increases exponentially, in particular. 
     According to another advantageous development of the disclosure, the changing region comprises silicon dioxide and/or silicon nitride. The advantage achieved here is thus that the microelectromechanical sensor can be provided with an electrically insulating layer in a simple and cost-effective manner. 
     According to another advantageous development of the disclosure, the measuring element comprises a plurality of electrically conductive regions and changing regions arranged alternately. The advantage here is thus that the sensitivity and accuracy of the microelectromechanical sensor are increased further if, for example, a plurality of the regions are arranged above one another in such a manner. 
     According to another advantageous development of the disclosure, the microelectromechanical sensor comprises a diaphragm having a respective measuring element on at least two, in particular four, adjacent sides. The advantage here is thus that the sensitivity and accuracy of the microelectromechanical sensor can be increased yet further; for example, the measured values of the respective measuring elements can be used to average a force or pressure measurement. 
     According to another advantageous development of the method, the force, the pressure or the like is determined using a monotonous, in particular exponential, profile of the electrical variable. The advantage here is thus that a particular force or a particular pressure or the like can be unambiguously associated with the electrical variable in a simple and reliable manner, with the result that it is possible to reliably determine the pressure, the force or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure are illustrated in the drawing and are explained in more detail in the following description. In the drawing: 
         FIG. 1  shows a basic circuit diagram of a method of operation of a microelectromechanical sensor according to a first embodiment; 
         FIG. 2  shows a basic circuit diagram of a microelectromechanical sensor according to a second embodiment of the present disclosure; 
         FIG. 3  shows a schematic illustration of a microelectromechanical sensor according to a third embodiment of the present disclosure; 
         FIG. 4  shows a schematic illustration of a microelectromechanical sensor according to a fourth embodiment of the present disclosure; 
         FIG. 5  shows steps of a method for producing a microelectromechanical sensor according to the first embodiment of the present disclosure, and 
         FIG. 6  shows steps of a method according to the first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the figures, identical reference symbols denote the same or functionally identical elements. 
       FIG. 1  shows a basic circuit diagram for the method of operation of a microelectromechanical sensor according to a first embodiment. 
     In  FIG. 1 , reference symbol  1  denotes a measuring element of a microelectromechanical sensor. The measuring element  1  comprises a lower electrically conductive region  2   a  and an upper electrically conductive region  2   b  with a substantially rectangular cross section. A changing region  3  which likewise substantially has a rectangular cross section is arranged between the two electrically conductive regions  2   a ,  2   b . The thickness D of the changing region is measured in a vertical direction in this case according to  FIG. 1 . Furthermore, in  FIG. 1   a , the lower electrically conductive region  2   a  and the upper electrically conductive region  2   b  are connected to a voltage source  5  and to a current measuring device M for a tunnel current  4  through the electrically conductive regions  2   a ,  2   b  and the changing region  3 . If the thickness D of the changing region  3  is now reduced, the tunnel current  4  flowing between the conductive regions  2   a ,  2   b  increases exponentially according to the profile V in  FIG. 1   b . In the diagram according to  FIG. 1   b , the tunnel current  4  is thus plotted against the thickness D of the changing region  3 . In the profile, V is exponential in this case, and the scales on the abscissa and the ordinate are linear in this case. With a thickness D of only one nanometer, the tunnel current according to  FIG. 1   b  has the magnitude 0.67, whereas it has the value 0.1 with a thickness D of 6 nanometers. In this case, the tunnel current  4  is provided with any desired unit. 
       FIG. 2  shows a basic circuit diagram of a microelectromechanical sensor according to a second embodiment of the present disclosure.  FIG. 2   a  shows a measuring element  1  of a microelectromechanical sensor; said element substantially corresponds to the structure of the measuring element  1  according to  FIG. 1   a . A substrate  7  on which the electrically conductive region  2   a  is arranged is also shown. A substantially T-shaped force stamp  6  is arranged on the electrically conductive region  2   b . In  FIG. 2   a , a force is not applied to the force stamp  6 . 
     If a force F now acts on the force stamp  6  from above, the thickness D of the changing region  3  is indirectly reduced thereby. On account of the reduced thickness D of the changing region  3 , a larger tunnel current  4  can now flow through the changing region  3  between the electrically conductive regions  2   a ,  2   b  on account of the voltage applied to the electrically conductive regions  2   a ,  2   b . The force F acting on the force stamp  6  can then be determined using the tunnel current  4 . 
       FIG. 3  shows a schematic illustration of a microelectromechanical sensor according to a third embodiment of the present disclosure.  FIG. 3   a  shows a measuring element  1  of a microelectromechanical sensor. In this case, the structure of the measuring element  1  again substantially corresponds to the corresponding structure according to  FIGS. 1 and 2 . In contrast to  FIG. 1   a , the lower first electrically conductive region  2   a  is arranged with an offset to the right in the horizontal direction with respect to the changing region  3  and the second electrically conductive region  2   b  according to  FIG. 3   a . In addition, the first electrically conductive region  2   a  is connected to a substrate  7 . The substrate  7  also has a bendable region  7   a  in which a neutral fiber  7   b  is arranged. 
     The changing region  3  is at least partially connected to the substrate  7  on its left-hand side, whereas the first electrically conductive region  2   a  is connected to the substrate  7  on the right-hand side. As described in the preceding  FIGS. 1-2 , the first electrically conductive region  2   a  and the second electrically conductive region  2   b  are connected to a voltage source  5  and to a measuring device M for the tunnel current  4 . 
     In  FIG. 3   a , a pressure p 1  above the electrically conductive region  2   b  is equal to a pressure p 2  below the bendable region  7   a . The bendable region  7   a  may be in the form of a diaphragm, for example. The electrically conductive regions  2   a ,  2   b  and the changing region  3  are arranged horizontally and parallel to one another. The measuring device M for the tunnel current  4  measures a particular magnitude of a tunnel current  4  flowing through the electrically conductive regions  2   a ,  2   b  and through the changing region  3 . 
     In  FIG. 3   b , the pressure p 1  above the electrically conductive region  2   b  is now greater than the pressure p 2  below the bendable region  7   a  of the substrate  7  and therefore also below the electrically conductive region  2   a . The bendable region  7   a , the neutral fiber  7   b , the electrically conductive regions  2   a ,  2   b  and the changing region  3  are accordingly bent downward according to  FIG. 3   b . As a result of the higher pressure p 1  on the top side of the electrically conductive region  2   b , the electrically conductive regions  2   a ,  2   b  and the changing region  3  are compressed in the substantially horizontal plane of the diaphragm or the bendable region  7   a . On account of this compression or this transverse contraction, the electrically conductive regions  2   a ,  2   b  and the changing region  3  are expanded in the perpendicular direction thereto. As a result, the thickness D of the changing region  3  is increased and the tunnel current  4  consequently decreases. The pressure or the pressure difference can be determined in this manner. 
       FIG. 4  shows a schematic illustration of a microelectromechanical sensor according to a fourth embodiment of the present disclosure. In  FIG. 4   a , the microelectromechanical sensor has four measuring elements  1   a ,  1   b ,  1   c ,  1   d . The structure of each of the measuring elements  1   a ,  1   b ,  1   c ,  1   d  corresponds to that of the measuring element  1  in  FIG. 1 . The arrangement of the four measuring elements  1   a ,  1   b ,  1   c ,  1   d  is as follows:  FIG. 4   a  shows a part  7 ′,  7 ″ of a substrate  7  on the left-hand and right-hand sides, respectively. An intermediate space  8  is arranged between the two parts  7 ′,  7 ″ of the substrate  7 . The measuring element  1   b  is now arranged on the left-hand part of the substrate  7 ′, the first electrically conductive region  2   a  being arranged on the left-hand part  7 ′ of the substrate. The measuring element  1   c  is accordingly arranged on the right-hand part  7 ″ of the substrate  7 . In this case, the respective lower conductive electrical regions  2   a  of the measuring elements  1   b ,  1   c  are larger, with respect to their horizontal extent, than the respective changing regions  3  and the electrically conductive regions  2   b . In this case, the measuring elements  1   b ,  1   c  partially extend into the intermediate space  8 . A diaphragm  9  which is connected to the electrically conductive regions  2   b  of the measuring elements  1   b ,  1   c  is arranged on the two measuring elements  1   b ,  1   c . In this case, the diaphragm  9  has a bendable region  9   a  and comprises a neutral fiber  9   b  in its interior. In this case, the diaphragm  9  extends completely from the left to the right, that is to say from the left-hand part  7 ′ to the right-hand part  7 ″ of the substrate  7 . The two further measuring elements  1   a ,  1   d  are now arranged on the respective left-hand and right-hand end regions of the diaphragm  9  on the top side of the latter. The first electrically conductive regions  2   a  of the measuring elements  1   a ,  1   d  are connected to the diaphragm  9  in this case. In this case, the measuring elements  1   a ,  1   d  again partially extend into the intermediate space  8  between the two parts  7 ′,  7 ″ of the substrate  7 . Overall, the horizontal extent of the measuring elements  1   a ,  1   d  is shorter in each case than the horizontal extent of the measuring elements  1   b ,  1   c . The diaphragm  9  is likewise arranged in such a manner that it is only partially connected to the second electrically conductive regions  2   b  of the measuring elements  1   b ,  1   c.    
       FIG. 4   a  now shows the corresponding pressure situation according to  FIG. 3   b , that is to say the pressure above the diaphragm  9  is greater than the pressure below the diaphragm  9 . The diaphragm  9  according to  FIG. 4   a  is thus pushed downward by the pressure difference. If correspondingly applied respective voltages  5  are applied to the respective measuring elements  1   a , ab,  1   c ,  1   d , as shown in  FIG. 1 , a decrease in the tunnel current  4  is measured in the measuring elements  1   a ,  1   d , whereas the tunnel current  4  increases in the measuring elements  1   b ,  1   c , compared in each case with the tunnel currents  4  flowing through the measuring elements  1   a ,  1   b ,  1   c ,  1   d  when the diaphragm  9  is not deflected or when there is no pressure difference between the top side and underside of the diaphragm  9 . 
       FIG. 4   b  shows a connection of the measuring elements  1   a ,  1   b ,  1   c ,  1   d  in the form of a Wheatstone bridge. In this case, the resistors R 1a , R 1b , R 1c , R 1d  shown there correspond to the electrical resistances of the measuring elements  1   a ,  1   b ,  1   c ,  1   d . If a voltage  5  is now applied to the correspondingly connected resistors R 1a , R 1b , R 1c , R 1d  according to the principle of the Wheatstone bridge, the measuring device M for the tunnel current  4  can detect a monotonous increase in the tunnel current  4  with increasing diaphragm deflection  9 . The accuracy of the measurement of the pressure difference is increased in this manner by the deflection of the diaphragm  9  since this enables differential evaluation by the four measuring elements  1   a ,  1   b ,  1   c ,  1   d.    
       FIG. 5  shows steps of a method for producing a microelectromechanical sensor according to the first embodiment of the present disclosure. In this case, according to  FIG. 5   a , the first electrical region  2   a  is formed by a silicon wafer  2   a  on which a native oxide layer  10  is arranged, and this native oxide layer  10  is now removed in the next step according to  FIG. 5   b , for example by means of hydrofluoric acid, with the result that only the silicon wafer  2   a  remains according to  FIG. 5   b . According to  FIG. 5   c , an ultra-thin layer of silicon dioxide is applied (step T 1 ), as the changing region  3 , to this silicon wafer  2   a , for example by means of thermal oxidation. In a further step T 2  according to  FIG. 5   d , a further electrically conductive layer  2   b  of silicon is now applied, for example by means of a chemical vapor deposition process of a silicon-containing base material, and the layer stack  2   a ,  3 ,  2   b  is applied to a substrate  7  in a further step T 3 . 
       FIG. 6  shows steps of a method according to the first embodiment.  FIG. 6  shows a flowchart of a method for measuring a pressure, a force or the like: in a first step S 1 , an electrical variable is measured. In a further step S 2 , the changing region is deformed on account of a force, a pressure or the like. In a third step S 3 , the electrical variable is measured in the loaded state. In a further step S 4 , the force, the pressure or the like is determined using the measured electrical variable. 
     Although the present disclosure was described above using preferred exemplary embodiments, it is not restricted thereto, but rather can be modified in various ways. For example, not only silicon but also silicon carbide, for example, is possible as the substrate material.