Abstract:
A micromechanical pressure sensor includes a first diaphragm and a second diaphragm accommodated in a shared semiconductor substrate. The two diaphragms facilitate independent pressure sensing of one or more media, by the fact that a respective pressure variable is sensed by way of the deflection of the respective diaphragm. A cap above the first diaphragm defines a hollow space that is connected to the hollow space below the second diaphragm.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a pressure sensor for sensing the pressure of at least one medium, and also relates to a self-testing method. 
         [0003]    2. Description of Related Art 
         [0004]    Published German patent document DE 10 2004 021 041 A1 discloses a pressure sensor in which both an absolute-pressure sensor and a differential-pressure sensor are integrated into one shared substrate. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a micromechanical pressure sensor having a first and a second diaphragm that are housed in a shared semiconductor substrate, as well as a self-testing method with which the functionality of the pressure sensor can be checked. The two diaphragms serve for independent pressure sensing of one or more media by respectively sensing a pressure variable by way of the warping of the respective diaphragm. The essence of the invention is that a cap above the first diaphragm defines a hollow space that is connected to the hollow space below the second diaphragm. 
         [0006]    By way of such a connection between the two hollow spaces, both the leak-tightness of the hollow space and the functionality of both diaphragms can be checked by way of the other diaphragm. 
         [0007]    The configuration of the pressure sensor is particularly advantageous when the hollow spaces that are connected are completely delimited with respect to the environment. It is conceivable, for example, for the two hollow spaces to exhibit a vacuum or a defined pressure. 
         [0008]    In a refinement of the invention, provision is made that in addition, evaluation means are provided which ascertain pressure variables as a function of the deflection of one or both diaphragms. In order to minimize the space requirement of the micromechanical pressure sensor, provision can be made in this context that at least a portion of the evaluation means is integrated into the semiconductor substrate in the physical vicinity of the diaphragms. 
         [0009]    In order to sense the pressure of two media, provision can be made that suitable supply conduits lead from one side of the semiconductor substrate each to one diaphragm. It is, however, furthermore also possible for the diaphragms to be provided on different sides of the semiconductor substrate, so that the pressures of two media act on the semiconductor substrate from different sides. Disposition of the diaphragms on oppositely located sides of the semiconductor substrate is preferred in this context, since this enables easier construction in the context of structural and connection technology. 
         [0010]    The cavity that is associated with the first diaphragm is advantageously produced by way of an etching process, e.g. a trench etching process or KOH etching. 
         [0011]    Self-testing of the pressure sensor, which for example can also be integrated into the evaluation means directly on the semiconductor substrate, allows the leakage of the two hollow spaces to be inferred from the pressure variable of one of the two diaphragms, the second pressure variable of the second diaphragm preferably being utilized. 
         [0012]    It is particularly advantageous that a sealing failure of hollow space  170  can be recognized by way of the second pressure variable. It is moreover also possible to perform the evaluation of the first pressure variable as a function of the second pressure variable, for example by the fact that an adaptation is carried out. With this procedure, a pressure decrease in hollow space  170  can be compensated for by way of the second pressure variable. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0013]      FIG. 1  shows a schematic cross section through an example embodiment of the pressure sensor according to the present invention. 
           [0014]      FIG. 2  shows a plan view of the pressure sensor according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The construction of a pressure sensor in accordance with the present invention is illustrated in  FIG. 1 . A semiconductor substrate  100 , made e.g. of silicon, is used in this context, into which two diaphragms  110  and  140  are introduced by way of usual micromechanical methods. 
         [0016]    In the exemplifying embodiment in accordance with  FIG. 1 , provision is made to produce first diaphragm  110  by introducing into substrate  100  from back side  155 , by way of a suitable trench etching method, a trench hole that forms the subsequent cavity  130 . The use of a buried stop layer, for example in the form of an oxide layer, allows even very thin diaphragms to be manufactured in this context. Alternatively, it is also possible to produce first diaphragm  110  and cavity  130  by way of a different usual etching method such as KOH etching. 
         [0017]    Second diaphragm  140  is preferably produced using an APSM method; other etching methods that produce a continuous diaphragm  140  above a first open space  160  in semiconductor substrate  100  can also be used. In the present exemplifying embodiment, second diaphragm  140  is produced on front side  150  of semiconductor substrate  100 . This is, however, only one particular embodiment of the invention. It is entirely conceivable, for example, also to produce the second diaphragm on back side  155 . In this case, however, in order to sense the different pressures  210  and  220  it is necessary for two separate supply conduits to be provided from back side  155  to diaphragms  110  and  160 . 
         [0018]    In order to sense pressures  210  and  220 , piezoresistors  115  are provided in first diaphragm  140 , and piezoresistors  145  in second diaphragm  160 . Evaluation of the pressure variables that can be sensed with these piezoresistors is accomplished in an evaluation circuit  190  that, in a particular embodiment of the invention, is integrated directly into semiconductor substrate  100 . Alternatively, however, this evaluation circuit can also be provided externally, in which case corresponding electrical connections are provided for. 
         [0019]    Optionally, semiconductor substrate  100  having the two diaphragms  110  and  140  can be mounted on a glass base  200 , in which is introduced an access point to cavity  130  for the medium that is under pressure  210  and is to be monitored. 
         [0020]    In order to produce an absolute pressure sensor from the differential pressure sensor that is formed by the two-sided access of media to diaphragm  110 , a cap  120 , in particular a seal-glass cap, which covers diaphragm  110 , is mounted onto front side  150  of semiconductor substrate  100 . With this cap  120 , a second hollow space  170  is formed above first diaphragm  110  by way of the internal volume of cap  120 , so that a deflection of diaphragm  110  can be interpreted directly as an indicator pressure  210  applied to the diaphragm. 
         [0021]    In order to detect leakage in the two hollow spaces  160  and  170 , a connecting channel  180  is provided which connects the two hollow spaces to one another. This connecting channel is preferably integrated directly into semiconductor substrate  100  (see the cross section in  FIG. 1 , and the plan view in  FIG. 2 , in this connection). Integration of the connecting channel can likewise be accomplished via APSM technology, so that the buried channel  180  is formed even before cap  120  is applied onto the semiconductor substrate. Example methods for this are described in, among others, published German patent documents DE 100 32 579 A1 and DE 10 2004 043357 A1. Access from hollow space  170  to the buried channel can be produced by way of a trench etching process. The plan view in  FIG. 2  demonstrates graphically how the connection between the two hollow spaces  160  and  170  is provided by way of the buried channel. 
         [0022]    By way of the construction described above, the sensor signal of the absolute-pressure sensor can be checked for plausibility, and compensated for within certain limits if hollow spaces  160  and  170  become leaky. For example, during manufacture of the sensor, i.e. upon application of cap  120  onto substrate  100 , a vacuum or an atmosphere having a very low pressure becomes enclosed in hollow spaces  160  and  170 . With the aid of this predefined pressure in hollow space  170 , diaphragm  110  can be used as an absolute-pressure sensor for pressure  210 . The pressure enclosed in hollow space  170  is checked by way of diaphragm  140 . With the aid of the measuring devices on diaphragm  140 , e.g. piezoresistors on its surface, cap  120  can be checked for leak-tightness immediately after it is applied. This is because if no change in the measured signal of diaphragm  140  is detected upon application of an external pressure  220  onto front side  150  of the sensor, a loss of cap sealing must be inferred, since both the pressure in hollow space  170  and thus in hollow space  160 , and external pressure  220 , are the same. It is thus possible to detect, immediately after the manufacturing process, whether the sensor is functioning correctly. 
         [0023]    The proposed construction is, however, also advantageous during normal operation of the sensor. After the first function test of the sensor, for example, it is possible to store a measured value of the measurement device on diaphragm  140  that should occur at an external pressure  220  typical of the application. If this original value then changes during the service life of the sensor, a leak in the cap, in the cap connection, or in one of diaphragms  110  or  140  can the be inferred. If the original value changes only slowly, i.e. if the initial value decreases only over a long time interval, it can be assumed that the leak is small. In this case the deviation from the initial value can be used as a calibration variable for the measured value of diaphragm  110 . Exact sensing of pressure  210  is thus still possible for a certain time in the context of small rips in diaphragm  140  or a small leak in, for example, the cap adhesive. 
         [0024]    In addition, the present invention also allows a rip in diaphragm  110  to be inferred if the signal of the measuring device on diaphragm  110  decreases while the signal of the measuring device on diaphragm  140  continues to generate measured values. 
         [0025]    In a further exemplifying embodiment, provision can also be made additionally to apply one or more temperature elements onto one or both diaphragms. These temperature elements serve to compensate, in the context of the sensed pressure variables, for the temperature existing at the respective diaphragm.