Abstract:
Measures are proposed by which the design freedom is significantly increased in the case of the implementation of the micromechanical structure of the MEMS element of a component, which includes a carrier for the MEMS element and a cap for the micromechanical structure of the MEMS element, the MEMS element being mounted on the carrier via a standoff structure. The MEMS element is implemented in a layered structure, and the micromechanical structure of the MEMS element extends over at least two functional layers of this layered structure, which are separated from one another by at least one intermediate layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a component at least including a carrier, in particular in the form of an ASIC (application-specific integrated circuit) element, an MEMS (micro-electromechanical system) element, and a cap. The MEMS element is mounted on the carrier via a standoff structure and the cap is situated above the micromechanical structure of the MEMS element. 
     Furthermore, the present invention relates to a method for manufacturing such a hybrid integrated component, in the case of which a standoff structure is produced on the carrier, the functional layer of an SOI wafer functioning as the MEMS substrate is structured, and the MEMS substrate is mounted face down, with the structured functional layer on the standoff structure of the carrier. 
     BACKGROUND INFORMATION 
     European Published Patent Application No. 0 773 443 describes a micromechanical acceleration sensor that is implemented in the form of a chip stack, including an ASIC substrate and an MEMS substrate. An SOI wafer, in the functional layer of which a rocker-shaped sensor structure is implemented, functions as the MEMS substrate. The MEMS substrate is mounted using flip-chip technology on the ASIC substrate via space holders, so that the sensor structure is located in a hermetically sealed cavity between the ASIC surface and the SOI substrate. Accelerations cause a deflection of the rocker structure, which is detected capacitively here with the aid of measuring electrodes on the sensor structure and stationary electrodes on the ASIC surface. In the case of the known acceleration sensor, the evaluation circuit for the measuring signals is additionally integrated on the ASIC substrate. 
     SUMMARY 
     Measures are proposed by the present invention, by which the design freedom is substantially increased in the case of the implementation of the micromechanical structure of the MEMS element within the scope of the known component concept. 
     This is achieved according to the present invention in that the micromechanical structure of the MEMS element extends over at least two functional layers of the layered structure of the MEMS element, these two functional layers being separated from one another by at least one intermediate layer, so that they may easily be structured independently of one another. Since in this way the third dimension may also be incorporated in the layout of the micromechanical structure of the MEMS element, not only is the design freedom increased in the implementation of the micromechanical structure, but the chip area of the MEMS element and therefore also of the component as a whole may be used more efficiently, which is associated with a reduction of the manufacturing costs. 
     In one preferred specific embodiment of the present invention, the two functional layers are made of monocrystalline silicon. In this case, only very low mechanical tensions occur within the layers—if at all—which has an advantageous effect on the micromechanical performance of the component. 
     According to a first manufacturing variant according to the present invention, a single SOI wafer is therefore used as the MEMS substrate. In this case, the SOI substrate functions as the second functional layer of the MEMS substrate. The SOI substrate is then structured as the second functional layer after mounting the MEMS substrate or the ASIC substrate on the carrier, so that a micromechanical structure results, which extends over the functional layer and the SOI substrate of the SOI wafer. A cap wafer is only then mounted above the micromechanical structure of the MEMS substrate. Since the carrier substrate of an SOI wafer is generally significantly thicker than the intended structural height of the micromechanical structure to be produced, the SOI substrate is usually also thinned back to a defined thickness after the mounting on the carrier and before the structuring. 
     In the case of a second manufacturing variant according to the present invention, a two-layer SOI wafer is used as the starting substrate for the MEMS element. Such an SOI wafer includes two monocrystalline functional layers, which are separated from one another and from the SOI substrate by insulating layers. The uppermost functional layer is structured even before mounting this SOI wafer on the carrier. The second functional layer of the SOI wafer functions as the second functional layer of the MEMS substrate and is structured only after mounting the MEMS substrate on the carrier, specifically after the SOI substrate has been removed. In this way, a micromechanical structure results, which extends over the two functional layers of the SOI wafer. Only then is a cap wafer mounted above the micromechanical structure of the MEMS element. 
     This second manufacturing variant is preferable in particular if a structural height which is predefined for the micromechanical structure is to be observed very precisely. This is because the thicknesses of the two functional layers of a two-layer SOI wafer may be set very precisely, while substantially higher thickness tolerances occur in the case of a thinning back process, as is carried out within the scope of the first manufacturing variant. 
     In the case of both manufacturing variants, the two functional layers of the MEMS substrate are structured independently of one another. The layouts of the two functional layers complement one another and together determine the three-dimensional shape of the micromechanical structure of the MEMS substrate. This micromechanical structure may, but does not have to be, designed symmetrically with the intermediate layer between the two functional layers. An asymmetry may be attributed, for example, to different layer thicknesses of the two functional layers. In addition, the layouts of the two functional layers may differ, so that at least one section of the micromechanical structure extends over both functional layers, while at least one other section only extends over one of the two functional layers. Manifold design variations for the micromechanical structure of the MEMS element result due to this third layout dimension. 
     The refined component concept according to the present invention is suited in particular for the implementation of inertial sensor components. In the case of this application, the micromechanical structure of the MEMS element includes at least one deflectable seismic mass and is equipped with a circuit for detecting the deflections of the seismic mass. The seismic mass is situated in a cavity between the cap and an ASIC element having an evaluation circuit for the sensor signals. The cavity is advantageously hermetically closed, so that the micromechanical sensor structure is protected from soiling and the influences of an aggressive measuring environment. In addition, defined pressure conditions may be produced within this cavity, in order to thus positively influence the damping behavior of the sensor. Since the micromechanical structure of the MEMS element extends over two functional layers according to the present invention, large seismic masses may be implemented on a comparatively small chip area. This contributes to a high measuring sensitivity at a small component size. In the case of some inertial sensor types, for example, in the case of sensor components for detecting accelerations perpendicular to the layer planes of the component structure, the mass distribution of the seismic mass is asymmetrical to its suspension or mounting. Due to the three-dimensional layout possibilities according to the present invention of the micromechanical structure, such a sensor design may also be implemented particularly simply on a very small chip area, in that the seismic mass includes at least one section which extends over both functional layers, and at least one section which only extends over one of the two functional layers. To increase the measuring sensitivity of the sensor structure, ventilation openings may additionally also be implemented in the seismic mass, which extend over the entire thickness of the seismic mass, i.e., over both functional layers and the intermediate layer or also over only one of the two functional layers, depending on which layers the seismic mass is implemented on. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic sectional view through the structure according to the present invention of an inertial sensor component. 
         FIG. 2   a  is a first schematic sectional view of the manufacturing of the inertial sensor component through the individual parts of the component structure. 
         FIG. 2   b  is a second schematic sectional view of the manufacturing of the inertial sensor component through the individual parts of the component structure. 
         FIG. 2   c  is a third schematic sectional view of the manufacturing of the inertial sensor component through the individual parts of the component structure. 
         FIG. 2   d  is a fourth schematic sectional view of the manufacturing of the inertial sensor component through the individual parts of the component structure. 
     
    
    
     DETAILED DESCRIPTION 
     Component  100  shown in  FIG. 1  is an inertial sensor for detecting z-accelerations, i.e., accelerations which are oriented perpendicularly to the layer planes of the component structure. For this purpose, component  100  includes an MEMS element  10  having a micromechanical sensor structure, in which a seismic mass  11  is implemented in the rocker design. This will be explained in greater detail hereafter. MEMS element  10  is additionally equipped with an arrangement for detecting the deflection of this seismic mass  11 , which are, however, not shown in detail here. Furthermore, component  100  includes an ASIC element  20 , having an evaluation circuit for the sensor signals of MEMS element  10 , and a cap  30 . 
     Component  100  is implemented in the form of a wafer stack. ASIC element  20  functions as the carrier of component  100 . MEMS element  10  is mounted via a standoff structure  21  on ASIC element  20 , so that the mobility of sensor structure  11  is ensured. Cap  30  is arranged above sensor structure  11  of MEMS element  10  and mounted on MEMS element  10  in such a way that sensor structure  11  is situated within a closed cavity  22  between ASIC element  20  and cap  30 . 
     According to the present invention, MEMS element  10  is implemented in a layered structure, the micromechanical sensor structure and in particular seismic mass  11  extending over two functional layers  3 ,  5  of this layered structure. These two functional layers  3 ,  5  are separated from one another by an insulating intermediate layer  4 , so that the two functional layers  3 ,  5  are electrically separated. 
     Seismic mass  11  is mounted here in one point like a rocker. For this purpose, it is connected via a single support point  23  of standoff structure  21  to ASIC element  20 . Since the mass distribution of seismic mass  11  is asymmetrical in relation to this mount, accelerations perpendicular to the layer planes of the component structure cause a rotational movement of seismic mass  11  around its mount, which is indicated in  FIG. 1  by an arrow. The asymmetrical mass distribution of seismic mass  11  in relation to its mount is not only to be attributed to the off-center arrangement of support point  23 , but rather also to the fact that seismic mass  11  is implemented on one side of support point  23  in both functional layers  3 ,  5 , while on the other side of support point  23 , it only extends over one functional layer  5 , since other functional layer  3  has been removed from this area. Accordingly, seismic mass  11  is approximately twice as thick on one side as on the other side. 
     In the exemplary embodiment shown here, ventilation openings  7 , which extend over the entire thickness of seismic mass  11 , i.e., over both functional layers  3  and  5  and intermediate layer  4 , are implemented on the one side of seismic mass  11 . These ventilation openings  7  contribute to reducing the damping of sensor structure  11 . 
     One preferred method variant for manufacturing above-described inertial sensor component  100  will be explained hereafter on the basis of  FIGS. 2   a  through  2   d . This method is directed to three substrates, which are initially processed independently of one another. 
     As already mentioned, in the exemplary embodiment described here, an ASIC element  20  functions as the carrier of the component structure. Since such ASIC elements are generally not manufactured individually but rather in a large number in the wafer composite, the teen “ASIC substrate  20 ” is used hereafter as a synonym for the term “ASIC element  20 ”.  FIG. 2   a  shows an ASIC substrate  20 , which has been provided with a signal processing and evaluation circuit for the MEMS sensor function of component  100 , which is not shown in detail here. In addition, ASIC substrate  20  may, however, also include MEMS-independent circuit functions. The CMOS processing of ASIC substrate  20  will not be described in detail here, since it is not specified in greater detail by the present invention. A silicon oxide layer  21  was deposited and structured on ASIC substrate  20  shown here. This structured silicon oxide layer  21  is used, on the one hand, as the electrical insulation for ASIC substrate  20  in relation to the further parts of the component structure and, on the other hand, forms a standoff structure for the mounting of an MEMS substrate. Standoff structure  21  includes a support point  23  for the seismic mass of the sensor structure and a peripheral bond frame  24  for the MEMS substrate. 
     Since MEMS element  10  is also manufactured in the wafer composite within the scope of the manufacturing of component  100 , the terms “MEMS element” and “MEMS substrate” are also used as synonyms hereafter. In the manufacturing variant described here, a two-layer SOI wafer is used as MEMS substrate  10 , which includes an SOI substrate  1 , a lower oxide layer  2 , a lower silicon functional layer  3 , a further oxide layer  4 , and an upper silicon functional layer  5 . Such an SOI wafer may be manufactured, for example, by a repeated sequence of thermal oxidation, silicon direct bonding, trenching steps (DRIE), and polishing steps (CMP). 
       FIG. 2   b  shows MEMS substrate  10  after the pre-processing, during which upper silicon functional layer  5  was structured, in order to expose a bond frame  51  for the mounting on ASIC substrate  20  or bond frame  24  of standoff structure  21  and the lower part of the micromechanical sensor structure having ventilation openings  7  in seismic mass  11 . Structuring processes may be used for this purpose, as are typical in microsystem technology, for example, lithography in combination with trenching processes. Upper oxide layer  4  may advantageously be used as the etch stop in this case. MEMS substrate  10  is then mounted face down, i.e., with structured upper functional layer  5 , on standoff structure  21  of ASIC substrate  20 , as shown in  FIG. 2   c.    
     The connection between MEMS substrate  10  and ASIC substrate  20  is preferably established in a bonding process, for example, by low-temperature silicon direct bonding. On the one hand, such connections are very stable and therefore permanent. On the other hand, micromechanical sensor structure  11  may thus easily be closed in a hermetically sealed manner in resulting cavity  22  between ASIC substrate  20  and cap  30 . The arrows on the rear side of MEMS substrate  10  illustrate that SOI substrate  1  is removed after the mounting, for example, by trenching or KOH etching. Oxide layer  2  may either also be removed, or it may be used as a hard mask for the following structuring step of lower functional layer  3 . 
     This structuring is carried out independently of the structuring of upper functional layer  5 , since the two functional layers  3  and  5  are separated from one another by oxide layer  4 , which functions as the etch stop. Accordingly, the layout of lower functional layer  3  may also be selected independently of the layout of upper functional layer  5 , which is illustrated by  FIG. 2   d . In contrast to functional layer  5 , functional layer  3  was completely removed from the area above support point  23  and to the right of support point  23 . Accordingly, the mass distribution of seismic mass  11 , which is composed of the two functional layers  3  and  5 , is clearly asymmetrical to support point  23 . Furthermore, a bond frame  31  for the mounting of a cap wafer was structured out of functional layer  3 .  FIG. 2   d  shows the component structure after oxide layer  4  was selectively opened, in order to electrically connect the two functional layers  3  and  5  by depositing a conductive layer, for example, aluminum or polysilicon. However, this layer is not shown here. 
     Subsequently, the sensor structure and in particular seismic mass  11  is finally exposed. For this purpose, the exposed areas of oxide layers  2  and  4  are removed, for example, by HF gas phase etching. This etching process should be sufficiently short or should be subject to timed monitoring, so as not to destroy standoff structure  21 . 
     A cap wafer which is pre-structured in a suitable way is then installed as the third substrate on MEMS substrate  10  thus processed. The connection between MEMS substrate  10  and the cap wafer may be established, for example, by eutectic bonding or also with the aid of glass solder. Only then are the individual components detached from the wafer composite. The separation takes place by sawing, for example. The result of the above-described manufacturing method is a component  100  as shown in  FIG. 1 . 
     Finally, it is also to be noted that the component concept according to the present invention is not restricted to sensor applications, but rather may also be used in the implementation of actuator components.