Abstract:
A micromechanical component having a substrate, a cavity formed in the substrate, a membrane provided on the surface of the substrate, which seals the cavity, the membrane having at least two layers between which one or several piezoelectric or piezoresistive circuit trace strip(s) is/are embedded, and a corresponding manufacturing method therefor.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a micromechanical component, and also relates to a corresponding manufacturing method.  
       BACKGROUND INFORMATION  
       [0002]     The structure of piezoelectric or piezoresistive pressure sensors today is based exclusively on the use of monocrystalline silicon membranes into which piezoelectric resistors are introduced using doping technology. In particular, the piezoelectric or piezoresistive circuit traces may be formed in the silicon layer of an SOI substrate.  
         [0003]     Overall, these manufacturing methods may be expensive and complicated.  
       SUMMARY OF THE INVENTION  
       [0004]     Although applicable to any number of micromechanical components and structures, particularly sensors and actuators, the present invention (as well as its basic underlying problem) definition are explained with reference to a piezoelectric or piezoresistive pressure sensor.  
         [0005]     The micromechanical component of the present invention, and the manufacturing method of the present invention have the advantage of allowing simple manufacturability.  
         [0006]     One advantage of the present invention is that a piezoelectric or piezoresistive sensor, in particular a pressure sensor, is provided with a dielectric membrane and monocrystalline piezoelectric resistors, using surface micromechanic technology.  
         [0007]     In the method according to the present invention or the micromechanical component according to the present invention, piezoelectric or piezoresistive circuit traces are defined by being patterned out of a layer of the circuit-trace material, which is separated from a substrate by an intermediate layer.  
         [0008]     A membrane layer is then deposited above the piezoelectric or piezoresistive circuit traces and etching access to the substrate is created. By selective removal of a substrate area underneath the membrane, a cavity is produced which will be sealed again in a later step, a predefined reference pressure being able to be enclosed.  
         [0009]     A micromechanical component according to the present invention has a membrane that has at least two layers between which one or several piezoelectric or piezoresistive circuit trace strip(s) are embedded. The piezoelectric or piezoresistive circuit trace strips may be made of monocrystalline silicon. A reference pressure may be enclosed in the cavity, the substrate having the form of a tub.  
         [0010]     According to an exemplary embodiment, the piezoelectric or piezoresistive circuit trace strip(s) is/are at least partially arranged above the hollow cavity.  
         [0011]     According to another exemplary embodiment, the piezoelectric or piezoresistive circuit trace strip(s) is/are arranged above or near an edge region of the hollow cavity. According to another exemplary embodiment, the two layers are made of an insulating material and the piezoelectric or piezoresistive circuit trace strip(s) is/are made of a monocrystalline material.  
         [0012]     According to another exemplary embodiment, the piezoelectric or piezoresistive circuit trace strip(s) is/are connected by means of a contacting device, which extends through the upper layer of the membrane.  
         [0013]     According to another exemplary embodiment, the membrane has a lowest first layer, a second-lowest second layer, a third-lowest third layer and an uppermost fourth layer, the piezoelectric or piezoresistive circuit trace strip(s) being embedded between the first and the second layer, the third layer being made of a permeable material, and the fourth layer being produced in such a way that it hermetically seals the third layer.  
         [0014]     According to another exemplary embodiment, the first and second layers have at least one through-hole above the hollow cavity, the at least one through-hole being sealed by the third and fourth layers. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1   a  shows a schematic cross-sectional view of the essential manufacturing steps of a micromechanical component according to an exemplary embodiment and/or exemplary method of the present invention.  
         [0016]      FIG. 1   b  shows another schematic cross-sectional view of the essential manufacturing steps of a micromechanical component according to an exemplary embodiment and/or exemplary method of the present invention.  
         [0017]      FIG. 1   c  shows another schematic cross-sectional view of the essential manufacturing steps of a micromechanical component according to an exemplary embodiment and/or exemplary method of the present invention.  
         [0018]      FIG. 1   d  shows another schematic cross-sectional view of the essential manufacturing steps of a micromechanical component according to an exemplary embodiment and/or exemplary method of the present invention.  
         [0019]      FIG. 1   e  shows another schematic cross-sectional view of the essential manufacturing steps of a micromechanical component according to an exemplary embodiment and/or exemplary method of the present invention.  
         [0020]      FIG. 1   f  shows another schematic cross-sectional view of the essential manufacturing steps of a micromechanical component according to an exemplary embodiment and/or exemplary method of the present invention.  
         [0021]      FIG. 1   g  shows another schematic cross-sectional view of the essential manufacturing steps of a micromechanical component according to an exemplary embodiment and/or exemplary method of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]     In the figures, identical reference numbers designate the same or functionally comparable or corresponding components. According to  FIG. 1   a , the starting point of the process sequence according to the exemplary embodiment of the present invention is an SOI substrate stack, which includes a monocrystalline silicon substrate  1 , a superposed silicon oxide layer  3  and a monocrystalline silicon layer  5  being situated above. Such an SOI substrate stack may be produced by various manufacturing processes known from the related art and can be purchased from the manufacturer in large quantities at the required quality. According to this specific embodiment, the SOI substrate stack may also be already preprocessed with IC circuit elements prior to the start of the process sequence, so that the sensor is able to be integrated into a circuit environment. It is only useful that the region where a membrane and piezoelectric resistors will later be formed is not interrupted by circuit elements.  
         [0023]     Additionally, with reference to  FIG. 1   b , piezoelectric or piezoresistive circuit trace strips  5   a ,  5   b  are patterned in monocrystalline silicon layer  5  as subsequent strain gauges in a first process step. The resistance of circuit trace strips  5   a ,  5   b  is able to be adjusted via their lateral geometry, the layer thickness of layer  5  and via the doping of layer  5 . The etching procedure for patterning circuit trace strips  5   a ,  5   b  selectively stops on silicon oxide layer  3 , which expediently has a thickness of approximately 5 to 500 nm. A practical layer thickness for layer  5  is between 100 and 1000 nm.  
         [0024]     Furthermore, with reference to  FIG. 1   c , at least one dielectric layer  7  is deposited above piezoelectric or piezoresistive circuit trace strip  5   a ,  5   b  and the exposed regions of layer  3 . This layer  7  assumes the supporting function of a membrane M (cf.  FIG. 1   g ) and should therefore be under tensile stress in order to prevent uncontrolled arching. It is especially preferred to provide a layer  7  of LPCVD silicon nitride, which has a thickness of between 100 and 300 nm.  
         [0025]     In a subsequent process step, which is illustrated in  FIG. 1   d , layers  3  and  7  are patterned with through-holes  10   a ,  10   b  by an etching process, through-holes  10   a ,  10   b  lying between piezoelectric or piezoresistive circuit-trace strips  5   a ,  5   b . Both an anisotropic, one-stage etching process and a selective anisotropic, two-stage etching process may be used as etching process.  
         [0026]     In a subsequent process step, which is illustrated in  FIG. 1   e , a permeable, porous layer  15  is deposited across the entire surface above the resulting structure. Permeable layer  6  is penetratable by a predefined etching agent, such as an etching gas, and reduces the effective cross section of through-holes  10   a ,  10   b . This not only considerably increases the etching rate in a subsequent ClF3 etching step, but also provides for easily resealing the nanopatterned pore structure of layer  15  at a later point utilizing a CVD process. Permeable layer  6  may also be a dielectric.  
         [0027]     The following, for example, may be used for permeable layer  15 : 10 to 200 nm thin PECVD oxide on silane basis (SiH 4 ) with 02, N 2 O or other oxidation agents, 10 to 200 nm thin PECVD or LPCVD silicon nitride. The desired permeability of the nitride is able to be obtained by an appropriate aftertreatment, for instance in that the nitride is subsequently converted—completely or partially—to silicate such as SiF 6  (NH 4 ) 2  under hydrogen fluoride (HF), which is permeable as continuous layer. Furthermore, 10 to 200 nm thin polymer films from a plasma deposition such as aluminum fluoride may be used as material for layer  15 , or organic compounds, for instance from C 4 F 8  processes or similar thin, sputtered or vapor-deposited metals such as gold, aluminum, AlSiCu etc. When AlSiCu is used, the selective solving out of silicon and copper deposits may be used to produce micro-porosity.  
         [0028]     Finally, photoresists with a silicon portion in the polymer chain, BCB (butyl cyclobutane) porous dielectric layers from vapor deposition or centrifugal processes may be used as material for permeable layer  15 .  
         [0029]     Finally, permeable layer  15  may be formed as cover layer by deposition of two different polymers, which are dissolved in a common or in (a) different solvent(s), whereupon the one or plurality of solvents is evaporated and one of the two polymers is selectively removed from the other from the formed layer.  
         [0030]     Furthermore, with reference to  FIG. 1   f , substrate  1  is now selectively removed with respect to dielectric layers  3 ,  7  and  15  by a ClF 3  etching step or some other gas etching step that spontaneously etches silicon. In the process, a cavity  20  is produced which is able to be controlled in its lateral and vertical extension in a satisfactory manner. The control of the etching front is simple, in particular when the edge region R of cavity  20  is suitably oriented with respect to particular crystal directions of silicon substrate  1  since this provides for forming smooth etching fronts. An orientation of the edge region of cavity  20  may be in parallel to the (100) surfaces. The geometry of the cavity is defined by the arrangement of through-holes  10   a ,  10   b.    
         [0031]     Piezoelectric or piezoresistive circuit trace strips  5   a ,  5   b  may be arranged in such a way that they lie above the edge region of cavity  20  since the greatest deformation occurs at the particular edge, that is, the subsequent sensor signal or the sensor sensitivity is at a maximum. After etching of cavity  20 , permeable layer  15  is sealed by at least one further dielectric layer  30  according to  FIG. 1   g . Layer  30  in turn may be under tensile stress or receives the tensile stress of layer stack  3 ,  7 ,  15 ,  30 , which together forms membrane M.  
         [0032]     The deposition of dielectric layer  30  may be implemented in a PVD or CVD process during which a defined process pressure prevails, so that a reference pressure may be enclosed below the membrane. The material of dielectric layer  30  may be PECVD-TEOS or thermal TEOS, PECVD-silane oxide, PECVD nitride or a similar material, for instance. Finally, with reference to  FIG. 1   g , piezoelectric or piezoresistive circuit trace strips  5   a ,  5   b  are contacted via a contacting device  40 , for example a Wolfram contact provided in a contact hole.  
         [0033]     Piezoelectric circuit trace strips  5   a ,  5   b  are able to be connected to parts of the evaluation circuit or bond pads via this contacting device  40 .  
         [0034]     Although the present invention was described above in light of the exemplary embodiments, it is not so restricted to it, but is able to be modified in various ways.  
         [0035]     In particular, any micromechanical basic materials such as germanium, may be used and not only the silicon substrate which was cited as an example.  
         [0036]     Likewise, a variety of sensor structures and not only the illustrated pressure sensor may be formed. Instead of ClF 3 , XeF 2  or BrF 3  may be used as etching agents.  
         [0037]     The reference numeral list is as follows: 
     1  Silicon wafer substrate;      3  Silicon oxide layer;      5  Monocrystalline silicon layer;      5   a ,  5   b  Piezoelectric circuit trace strips;      7  Silicon nitride layer;      10   a ,  10   b  Through holes;      15  Permeable layer;      20  Cavity;     R Edge area;     M Membrane;      30  TEOS layer; and      40  Contacting device.