Abstract:
A MEMS component for generating pressure pulses is provided, its micromechanical structure including at least three function levels: a first function level in which at least one stationary trench structure is implemented, a second function level, which is implemented above the first function level and includes at least one triggerable displacement element as well as through-openings as pressure outlet openings, the displacement element protruding into the trench structure and being movable in parallel with the function levels, whereby positive and negative pressure pulses are generated, and a third function level, which is implemented above the second function level and includes at least one triggerable cover element for at least one part of the pressure outlet openings in the second function level.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a MEMS (microelectromechanical system) component for generating pressure pulses. 
     BACKGROUND INFORMATION 
     Such MEMS components are used, for example, as loudspeakers as part of a wide variety of applications. Due to their miniaturized design and the possibility of integration of additional functionalities at very low manufacturing costs, MEMS loudspeaker components are becoming increasingly important economically. 
     Most of the MEMS components on the market for detection and/or generation of pressure pulses are equipped with a preferably large-scale diaphragm in parallel to the plane of the chip or substrate. In the configuration as a microphone, this diaphragm is excited to vertical (out-of-plane) vibrations by the sound pressure. Deflections of the diaphragm are then detected capacitively, for example, as a microphone signal. This principle is applied conversely in the case of the configuration as a loudspeaker. In this case, the diaphragm is excited capacitively, for example, to vertical (out-of-plane) vibrations, resulting in pressure pulses, i.e., sound waves. 
     These diaphragm-based MEMS components have proven to be problematical in many regards. 
     Suspension of the diaphragm necessitates bending or warping of the diaphragm, which results in nonlinearities of the microphone signal or the loudspeaker signal and thus has negative effects on the performance of the component. The thinner the diaphragm, the better is the performance of the component. Accordingly, the diaphragm is very fragile and sensitive to mechanical stresses such as impact and knocking effects. The performance of the component is thus in contrast with its robustness. 
     The detection direction or the drive direction and the displacement movement are similarly oriented in the case of diaphragm-based components. The given facts of the volume displacement are thus linked to the given facts of the detection or drive. In the case of a capacitive loudspeaker component, this results in the loudspeaker performance essentially depending on the energy consumption of the component. In other words, the loudspeaker performance is better, the larger the displacement volume, which is determined here by the gap distance between the diaphragm and the counter electrode. However, the greater the gap distance, the higher is also the energy consumption since high electrostatic forces are needed to trigger the diaphragm accordingly. 
     German Published Patent Appln. No. 10 2010 029 936 provides a capacitive MEMS microphone component whose layered structure includes at least three function layers. A sound opening, which opens into a cavity beneath the first function layer, is provided in the first function layer. This cavity extends essentially over the second function layer, in which a diaphragm and a counter element are formed, namely in such a way that the diaphragm delimits the cavity on at least one side and is deflectable in the plane of the layer. The diaphragm functions as the first electrode of a microphone capacitor, and the counter element functions as the carrier of a counter electrode of the microphone capacitor. At least one vent opening for the microphone structure is formed beneath the cavity in the third function layer. 
     In the case of the MEMS microphone component described here, the sound pressure acting on the component perpendicularly to the planes of the layers produces a diaphragm movement oriented in parallel to the planes of the layers of the component. 
     SUMMARY 
     The present invention provides a concept for MEMS components for generating pressure pulses, utilizing the deflection of the sound pressure described in German Published Patent Appln. No. 10 2010 029 936 to decouple the drive direction and the direction of the displacement movement; this also offers the option of controlling the polarity of the pressure pulses thereby generated. 
     The micromechanical structure of the MEMS component according to the present invention therefore has three function levels. At least one stationary trench structure is implemented in a first function level. The second function level is situated above the first function level and includes at least one triggerable displacement element as well as through-openings as pressure outlet openings for the pressure pulses generated. The displacement element protrudes into the trench structure and is movable in parallel with the function levels, whereby both positive and negative pressure pulses are generated. The third function level is implemented above the second function level and includes at least one triggerable cover element for at least one part of the pressure outlet openings in the second function level. 
     The component concept according to the present invention thus provides for pressure pulses to be generated by excitation of a displacement element, which is uncovered in the layer structure of the component within a trench structure and is set in vibration in parallel with the plane of the chip. The pressure pulses generated in this way emerge from the component structure through the pressure outlet openings, i.e., perpendicularly to the direction of movement of the displacement element. According to the present invention, the sequence of pressure pulses emerging from the pressure outlet openings is influenced with the aid of the cover elements in the third function level. The position of the cover elements is therefore regulated as a function of the movement or the direction of movement of the displacement element. For example, negative pressure pulses may be suppressed in a targeted manner in this way to prevent compensation of the sound waves thereby generated and undesirable heterodyne effects. 
     The concept according to the present invention has proven to be advantageous in many regards. For example, the component structure is comparatively robust since fragile diaphragm structures may be omitted. Furthermore, the trench structure of the first function level functions as overload protection for the displacement element of the second function level. Both the trench structure and the displacement element resemble the well-characterized sensor structures of micromechanical acceleration sensors and yaw rate sensors in both shape and aspect ratio. To manufacture such structures, it is thus possible to resort to known and tested manufacturing processes. Thus, for example, even very small gaps between the displacement element and the trench wall may be created, which is particularly advantageous from the standpoint of an electrostatic drive. In this case, the displacement volume required for the desired performance may be ensured by an increased structural depth. These measures do not detract from the stability and robustness of the component structure but instead promote them. 
     Since the design of the individual function levels may be determined lithographically, the component concept according to the present invention offers a very great design freedom. The component structure may be implemented exclusively by using the methods of surface micromechanics. However, it is also possible to implement individual structural elements, for example, the trench structure of the first function level, in the substrate using the methods of bulk micromechanics. In this case, the substrate contributes toward generation of the signal, whereby either a higher signal level is achieved or the chip size may be reduced. 
     There are fundamentally various options for implementation of the concept according to the present invention of a loudspeaker component, with regard to the layout of individual function levels and also with regard to the implementation of the triggering of the displacement element and cover elements. 
     In a preferred specific embodiment of the present invention, vent openings are formed in the first function level of the MEMS component. A local overpressure or underpressure, which occurs due to the movement of the displacement element in an area of the component structure, may be dissipated through these vent openings on the rear side of the component or in a housing, for example. This has proven to be advantageous in particular when the emergence of pressure pulses is prevented with the aid of the cover elements. These vent openings may be formed at the side of the trench structure or also in the bottom area of a trench, depending on the type of pressure outlet openings in the second function level and the design of the cover elements in the third function level. 
     The displacement element of a MEMS component according to the present invention may be excited to either a translatory movement or a rotatory “in-plane” movement. This depends essentially on the type of suspension or connection of the displacement element to the component structure. Thus, the displacement element may be tied into the layer structure of the component structure via a diaphragm or spring elements, for example. It may be suspended on all sides, multiple sides, two sides or only one side. The direction of movement is also determined by the triggering of the displacement element, i.e., by the type of triggering and the configuration of the corresponding electrical circuit elements. This excitation and triggering of the displacement element may take place electrostatically, piezoelectrically, magnetostatically and/or electromagnetically. 
     In one advantageous specific embodiment of the present invention, the displacement element is designed like a bar and the length and height of this bar-shaped displacement element are coordinated with the length and depth of the trench structure. This design variant is not only simple to create using known structuring methods in the layer structure of a semiconductor element, but also permits an uncomplicated triggering with low energy consumption and good loudspeaker performance. 
     Spacers are advantageously formed on the displacement element and/or on the trench wall. This makes it possible to prevent the displacement element from adhering to the trench wall in end-position contact. Such spacers ensure that a residual air cushion will always remain between the trench wall and the displacement element. 
     As mentioned previously, a MEMS component according to the present invention for generating pressure pulses includes at least one triggerable cover element with which the emergence of pressure pulses in at least one part of the pressure outlet openings is preventable. This cover element is therefore moved over the corresponding pressure outlet openings in the third function level. Here again, the type of movement and in particular the direction of movement depend on the type of suspension or connection of the cover element in the component structure and on the type of drive means or trigger means. These may be designed for an in-plane movement of the cover element within the third function level or for an out-of-plane movement perpendicular to the third function level. 
     For such an out-of-plane movement, the cover element could be implemented, for example, in the form of a triggerable rocker structure which is suspended via a torsion spring structure in the third function level and is deflectable out of this level. This variant is explained in greater detail below with reference to one exemplary embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 a , 1 b    show a detail of a schematic sectional view through the three function levels of a first MEMS component  10  according to the present invention at different points in time during the pressure pulse generation, and 
         FIG. 1 c    illustrates the triggering of the cover elements of this MEMS component  10 . 
         FIG. 2 a    shows a detail of a schematic sectional view through the three function levels of a second MEMS component  20  according to the present invention, and 
         FIG. 2 b    illustrates the triggering of the cover elements of this MEMS component  20 . 
         FIG. 3 a    shows a perspective partial view of another MEMS component  30  according to the present invention, and 
         FIG. 3 b    shows a top view of the trench structure in the first function level having a corresponding arrangement of displacement elements of this MEMS component  30 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 a  and 1 b    illustrate the design of the component structure of a MEMS loudspeaker component  10  according to the present invention in three function levels  1  through  3 . 
     First function level  1  here is formed in the carrier substrate of the component structure. A trench structure  11  having an essentially rectangular cross-sectional area was therefore introduced into the substrate surface. Within the scope of this structuring, vent openings  12  in the form of through-openings in substrate  1  were also created here. These vent openings  12  are situated at the side of trench structure  11  in the present exemplary embodiment. 
     Second function level  2  is implemented above first function level  1  in a layer structure on carrier substrate  1 . A triggerable displacement element  21 , which is connected to the layer structure via spring elements (not shown in detail here), is formed in this second function level  2 . Through-openings  23  which function as pressure outlet openings  23  are formed in the area of this spring suspension. Displacement element  21  protrudes into trench structure  11  of first function level  1 . It is bar-shaped in the exemplary embodiment shown here. The length and height of this bar-shaped displacement element  21  are coordinated with the length and depth of trench structure  11 . However, trench structure  11  is much wider than bar-shaped displacement element  21 , whereby displacement element  21  may be moved laterally back and forth within trench structure  11 . The triggering of displacement element  21  for this translatory movement in the component plane may be electrostatic, piezoelectric, magnetostatic and/or electromagnetic. However, the circuit means required for this are not described here. 
     Third function level  3  is implemented above second function level  2  in the layer structure of MEMS component  10 . In this function level  3 , at least one triggerable cover element  31  for at least one part of pressure outlet openings  23  is formed in second function level  2 . In the present exemplary embodiment, the suspension of cover element  31  and the circuit means for triggering an in-plane movement of cover element  31  are both designed within third function level  3 , which will be explained in greater detail below in conjunction with  FIG. 1   c.    
     The generation of pressure pulses with the aid of MEMS component  10  is illustrated by a combined view of  FIGS. 1 a  and 1 b   . The arrows show the direction of movement of displacement element  21  and of cover element  31 . Positive pressure pulses  4  are generated by the movement of displacement element  21  within trench structure  11  in the direction of movement of displacement element  21 , whereas an underpressure, i.e., negative pressure pulses  5 , occur(s) on the rear side of displacement element  21 . Displacement element  21  moves to the right in  FIG. 1 a   . An overpressure develops in trench structure  11  at the right of displacement element  21  accordingly, which emerges as a positive pressure pulse  4  from pressure outlet opening  23  at the right of displacement element  21 . Since pressure outlet opening  23  on the left side of displacement element  21  is covered by cover element  31  in third function level  3 , corresponding negative pressure pulse  5  on the left side of displacement element  21  cannot emerge from the front side of the component but instead is diverted onto the rear side of the component via vent opening  12  at the left of trench structure  11 . This prevents two pressure pulses  4  and  5  from mutually compensating one another. 
     Through appropriate triggering of cover element  31  out of cycle with displacement element  21 , negative pressure pulse  5  is also diverted onto the rear side of the component in the reverse movement of displacement element  21 , as shown in  FIG. 1 b   . Cover element  31  is moved to the right, while displacement element  21  is moved to the left. Pressure outlet opening  23  at the left of displacement element  21  is therefore uncovered, while pressure outlet opening  23  at the right of displacement element  21  is covered. Now a positive pressure pulse  4  emerging from uncovered pressure outlet opening  23  to the left of displacement element  21  is formed to the left of displacement element  21 , according to the direction of movement of displacement element  21 , while corresponding negative pressure pulse  5  at the right side of displacement element  21  is diverted via vent opening  12  at the right of trench structure  11  onto the rear side of the component. 
       FIG. 1 c    shows a top view of third function level  3  of a MEMS component  10 , which may also be referred to as a control level. The shape and position of rectangular trench structure  11  in first function level  1  beneath this level is shown with dashed lines. In the present exemplary embodiment, two plate-type cover elements  311  and  312  are formed in function level  3 , which covers the entire trench structure  11  in first function level  1  except for a central gap  33 . Cover elements  311  and  312  are each tied into the layer structure of third function level  3  on two opposite sides via U-spring elements  32  and may be moved to the right and left in the present exemplary embodiment in relation to trench structure  11  with the aid of electrostatic drive means  34 , for example, a plate or comb capacitor configuration. In the process, the size and, if necessary, also the position of central gap  33  change, whereby pressure outlet openings  23  in second function level  2  are optionally covered or may also be uncovered. The movement of cover elements  311  and  312  and thus the size and, if necessary, the position of central gap  33  are regulated independently of the movement of displacement element  21 , but in coordination with this movement to generate defined sound waves. 
     The component structure of a second MEMS loudspeaker component  20  according to the present invention, as illustrated in  FIG. 2 a   , differs from the structure of MEMS component  10  described above only in third function level  3 . Therefore, only this part of MEMS component  20  is described below. Reference is made to the description of  FIGS. 1 a  and 1 b    with respect to first and second function levels  1  and  2 . 
     In the case of MEMS component  20 , third function level  3  is also implemented above second function level  2  and includes at least one triggerable cover element for at least one part of pressure outlet openings  23  in second function level  2 . However, the suspension of cover element(s)  411 ,  412  and the circuit means for triggering in the case of MEMS component  20  are not designed for an in-plane movement within third function level  3  but instead are designed for an out-of-plane movement perpendicular to third function level  3 . Two partial plates  411  and  412 , which are suspended like a rocker via a torsion spring structure  42  in third function level  3 , each covering one-half of trench structure  11 , function here as cover elements  411  and  412 . This is illustrated in  FIG. 2 b    in particular. Each of two partial plates  411  and  412  together with an electrode  43  situated above it in the layer structure forms a trigger capacitor, whereby each of two partial plates  411 ,  412  is deflectable simply by applying a corresponding voltage perpendicular to the component plane. Since two partial plates  411  and  412  are linked via torsion spring  42 , the other partial plate is deflected in the opposite direction. Pressure outlet openings  23  may therefore optionally be covered or uncovered over half of trench structure  11 , while pressure outlet openings  23  over the other half of trench structure  11  accordingly are covered or uncovered. 
     In  FIG. 2 a   , displacement element  21  moves to the right. Accordingly, an overpressure develops in trench structure  11  at the right of displacement element  21 , which emerges as a positive pressure pulse  4  from pressure outlet opening  23  at the right of displacement element  21  since partial plates  412  above that were raised electrostatically while the other partial plate  411  was thereby lowered. Accordingly, pressure outlet opening  23  is covered on the left side of displacement element  21 , whereby the corresponding negative pulse  5  on the left side of displacement element  21  cannot emerge from the front side of the component. Instead of that, negative pressure pulse  5  is diverted onto the rear side of the component through vent opening  12  at the left of trench structure  11 . This prevents two pressure pulses  4  and  5  from mutually compensating one another. 
     With two MEMS components  10  and  20 , bar-shaped displacement element  21  is moved by a translatory movement within trench structure  11  of first function level  1 .  FIGS. 3 a  and 3 b    illustrate that the displacement element(s) of a MEMS component  30  according to the present invention may also be excited to a rotatory in-plane movement when the displacement elements are connected suitably to the layer structure of the component structure and suitable drive means are present. 
       FIG. 3 a    shows a bar-shaped displacement element  521 , which protrudes into a trench structure  511  in the form of a segment of a circle. Displacement element  521  is suspended on only one side, whereby it is pivotable about its suspension point within trench structure  511 . The top view of the first function level of a MEMS component  30  according to the present invention illustrated in  FIG. 3 b    shows a circular configuration of such trench structures  511  together with corresponding displacement elements  521 . It is thus possible, for example, to generate a succession of ultrasonic pulses, which are in the audible range when superimposed.