Abstract:
A method for producing a component, especially a micromechanical, micro-electro-mechanical or micro-opto-electro-mechanical component, as well as such a component which has an active structure that is embedded in a layer structure. Strip conductor bridges are formed by etching first and second depressions having a first and second, different etching depth into a covering layer of a first layer combination that additionally encompasses a substrate and an insulation layer. The deeper depression is used for insulating the strip conductor bridge while the shallower depression provides a moving space for the active structure with the moving space being bridged by the strip conductor bridge.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention relates to micromechanical, micro-electromechanical (MEMS) or micro-opto-electromechanical (MOEMS) components. More particularly, this invention pertains to a method of manufacture of such a component as well as the resultant component. 
     2. Description of the Prior Art 
     Active structures of micro-electromechanical components (MEMS) or micro-opto-electromechanical components (MOEMS) are often hermetically encapsulated to minimize environmental influences such as moisture and contaminants (e.g. dust). (“Active structure” should be understood to mean, in particular, movable structures, optical structures or structures having both movable and optical components such as movable mirrors. The term “active region” denotes the region or volume of the component in which the active structure lies or moves.) Hermetic encapsulation can also be employed to set a specific internal pressure in the region of the active structures. This is particularly advantageous in components whose functioning is dependent on a defined internal pressure (e.g. acceleration sensors and gyroscopes (rate-of-rotation sensors)). Fabrication of MEMS or MOEMS components generally takes place at the wafer level so that production can be implemented as cost-effectively as possible. Joining processes that are often carried out can be performed, for example, on the basis of direct and anodic bonding processes. 
     Leading electrical contacts out from the hermetically tight region of the component to make contact with specific parts of the component (e.g. the active structure) is difficult from the standpoint of fabrication. Various possibilities exist. Electrical contacts can be realized, for example, by laterally extending semiconductor layers produced by implantation or diffusion methods that have a low sheet resistance. It is additionally possible to accomplish this by means of patterned conductive layers covered with a planarized passivation layer. Alternatively, the electrical contacts can be led out from the component through a plurality of vertically extending plated-through holes. 
     DE 102005015584 describes a method for production of a component in which the active region and, hence, the active structure, of the component is isolated from the environment (as far as contaminants and moisture are concerned) before contact holes are made. Electric current required for operation and signals generated by the active structure are respectively fed to the active structure and tapped off via the contact holes and the adjacent conductive structure layer. However, this does not permit any crossover of interconnects. In particular, it is not possible to make contact with regions (e.g. electrodes) lying within a movable structure that is closed (in a component layer plane) with a tolerably small area requirement. Therefore, the movable structures  6  in MEMS  2  realized by this technology often have openings  3  for the interconnects  4  to electrodes  5  (see  FIG. 3 ). 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is therefore the object of the invention to provide a method for producing a component, in particular a micromechanical, micro-electromechanical or micro-opto-electromechanical component, of the type in which interconnect crossovers and, in particular, bridges over movable structures, are realized. 
     The foregoing and other disadvantages of the prior art are addressed by the present invention that provides, in a first embodiment, a method for producing a micromechanical, micro-electromechanical or micro-opto-electromechanical component. 
     Such method is begun by producing a first layer assembly which has a first substrate, a first insulation layer on the first substrate and an at least partly conductive covering layer on the first insulation layer. 
     First and second depressions are produced in the covering layer. The first depressions have a first etching depth and the second depressions have a second etching depth that is smaller than the first etching depth. The first etching depth is at least equal to the thickness of the covering layer. An at least partly conductive structure layer is applied to the covering layer so the structure layer adjoins, at least regions of, the covering layer. 
     In a second embodiment, the invention provides a micromechanical, micro-electromechanical or micro-opto-electromechanical component. Such component includes a first layer assembly that has a first substrate with a first insulation layer and a covering layer. 
     An at least partly conductive structure layer is arranged on the covering layer. First and second depressions are provided in the covering layer that proceed from an interface with the structure layer. The first depressions have a first etching depth and the second depressions have a second etching depth smaller than the first etching depth. The first etching depth is at least equal to the thickness of the covering layer. 
     The preceding and other features of the invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the detailed written description, point to the features of the invention. Like numerals refer to like features of the invention throughout. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a series of side sectional views that illustrate steps  1 - 1 ,  1 - 2 ,  1 - 3 ,  1 - 4  and  1 - 5  for patterning depressions having different etching depths with the aid of a double mask; 
         FIG. 2  is a series of side sectional views that illustrate the method of the invention according to steps  2 - 1 ,  2 - 2 ,  2 - 3  and  2 - 4 ; 
         FIG. 3  is a schematic plan view of a micromechanical sensor structure with openings for interconnects in accordance with the prior art; 
         FIG. 4  is a schematic plan view of a sensor structure produced in accordance with the method of the invention; 
         FIG. 5  is a sectional elevation view of a component with a buried electrode; 
         FIG. 6  is a sectional elevation view of a component with a buried electrode and an interconnect bridge; 
         FIGS. 7   a  and  7   b  are sectional elevation and top plan views, respectively, illustrating a component with an interconnect bridge along the sectional area I-I′ ( FIG. 7   b ), and a component along the sectional area II-II′ superimposed by a section along the area III-III′ ( FIG. 7   a , light gray shade) and along the area IV-IV′ ( FIG. 7   a , dark gray shade). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the invention, a cover wafer  10 , in particular an SOI (Silicon on Insulator) wafer, may be employed (e.g., for a first layer assembly). Such a wafer is patterned by a two-stage patterning step such as a two-stage dry etching step (DRIE: Deep Reactive Ion Etching) using a double mask. In such case, the SOI wafer  10  comprises a first silicon substrate  11 , a first insulation layer  12  (generally silicon dioxide), and a covering layer  13  that is isolated from the first substrate  11  by the buried first insulation layer  12 . 
       FIG. 1  is a series of cross-sectional views that illustrate how a first depression  14  of first etching depth D 1  and a second depression  15  of second etching depth D 2  are created (the result illustrated by step  1 - 5 ). An oxide layer  16  is initially created on the SOI wafer  10  and patterned (step  1 - 1 ). A layer of photoresist  17  is then applied, exposed and developed (step  1 - 2 ). Regions are etched in the silicon of the covering layer  13  that have openings at the same location in the oxide layer  16  and in the photoresist mask  17  (i.e., at the lateral position of the subsequent first depression  14  (step  1 - 3 )). After this first patterning step, the photoresist  17  is removed (step  1 - 4 ). Openings in the oxide layer  16  previously covered with photoresist  17  are uncovered in the process. In a second patterning step, such regions and regions already patterned in the first patterning step are etched into the silicon of the covering layer  13 . Following the two etching steps, the regions of the first depressions  14  that had already been patterned in the first patterning step are opened down to the buried oxide  12  of the SOI wafer to enable electrical insulation of separate electrodes. The depth of the second etching step determines the distance between a bridge and the movable structure (or the interconnect in the structure layer  26 ) (step  1 - 5 ), as can be seen with reference to  FIG. 2 . The buried oxide  12  acts as an etching stop. 
     The oxide  16  is then removed. Since the silicon surface lying beneath the oxide  16  is bonded later, removal is preferably accomplished wet-chemically. The buried oxide  12  at the bottom of the first depressions  14  is also wholly or partly removed (see  FIG. 2-1 ). This does not have a disadvantageous effect on function. The cover wafer  10  then has the structure illustrated in  FIG. 2-1  with first depressions  14  of a first etching depth D 1  that corresponds to the thickness of the covering layer  13  and, thus, extend at least down to the buried oxide  12 , and with second depressions  15  of a second etching depth D 2  that is smaller than the first etching depth D 1 . 
     A patterned second insulation layer  21  is then produced on the surface of a second substrate  20 . Afterward, third depressions  22  having a third etching depth D 3  and fourth depressions  23  having a fourth etching depth D 4  are produced in the surface of the second substrate  20 . The widths B 1  of the third depressions  22  are narrower than the widths B 2  of the cutouts of the second insulation layer  21  above the third depressions  22 . In this way, break-off edges  24  occur in the regions that border the third depressions  22 , whose function will be described later. 
     A structure layer  26  is applied to the insulation layer  21  to produce a second layer assembly  25  in a next process step. The structure layer  26  bears on the individual regions of the second insulation layer  21 . 
     In a following step, the structure layer  26  is patterned to form an active structure  27  wherein outer regions  30  (the chip edge, i.e., the edge region of the component to be produced) of the structure layer  26  are electrically insulated from the conductive regions “within” the component by trenches  31 . The resultant structure is illustrated in  FIG. 2-2 . 
     The first layer assembly  10  and the second layer assembly  25  are then joined together so that the covering layer  13  adjoins the structure layer  26  and the second depressions  15  and the fourth depressions  23  are located respectively above and below the active structure  27 . Not illustrated, but likewise desired in part, is the fact that at least a portion of the first depressions  14  and of the third depressions  22  are also situated respectively above and below the active structure  27 . 
     During bonding of the first layer assembly  10  onto the second layer assembly  25  “SOI with buried cavities”, silicon is bonded onto a silicon, rather than a silicon being bonded onto oxide. Besides the hermetically tight mechanical bond, a connection of lowest possible electrical resistance is produced. 
     In a following process step, a bonding pad region  35  of the second substrate  20  is etched back to a vertical position corresponding to the vertical position of the bottoms of the third depressions  22 , uncovering the third depressions  22  and creating contact holes  36 . 
     In the next step, a metallization layer is deposited on the surface of the second substrate  20 . Due to the presence of the break-off edges  24 , the part of the metallization layer deposited within the third depressions  22  is electrically isolated from the rest of the metallization layer. As a result, metal contact-making areas  32  are created within the third depressions  22 . Thereafter, contact is made with the metal contact-making areas  32  by bonding wires  33 , creating the structure of  FIG. 2-4 . 
     If desired, in a further process step, an additional metallization layer (not illustrated) can be deposited on the surface of the first substrate  11  remote from the structure layer  26 . The added metallization layer as well as the metallization layer serve as shielding electrodes for shielding undesirable electromagnetic fields. The two metallization layers can be connected to a defined, common potential or to different potentials. 
     Accordingly, the invention discloses a method for producing micro-electromechanical or micro-opto-electromechanical components, in particular components having hermetically encapsulated active structures and areas for making electrical contact. The production method of the invention enables hermetically tight encapsulation of specific regions of the structure layer at the wafer level with an adjustable internal pressure. It also affords the possibility of connecting the electrodes  5  in the structure layer by interconnect bridges  34  over active structures  27  (shown, for example in  FIG. 2-4 ) without openings  3  as illustrated by the prior art interconnect structure of  FIG. 3 . As a result, it is possible to produce structures  1  as illustrated in  FIG. 4  in which the electrodes  5  can be contact-connected via the interconnect bridges  34  (not shown) and, thus, the structures  1  are not interrupted in comparison with the open structures  6  in  FIG. 3 . 
     In order to insulate the conductive material of the second substrate, use is advantageously made of break-off edges  24  that bring about electrical isolation of the conductive sidewalls of the contact hole  36  from the bottom of the contact hole, with the bottom being connected (often directly) to an electrode of the component. 
     Metallization of the contact regions is performed only after the conclusion of all the joining processes. It is thus possible to use methods such as, for example, silicon direct bonding (SDB) with temperature loads of greater than 400° C. provided that no doped active regions exist within the structure layer  26  whose doping profiles could be impaired at relatively high temperatures. 
     The invention can be applied to production of any (miniaturized) components, in particular for a micromechanical, micro-electromechanical or micro-opto-electromechanical component (e.g., acceleration sensors, rate-of-rotation sensors, pressure sensors, optical couplers, etc.) 
       FIGS. 2-2  to  2 - 4  illustrate the optional case in which the second substrate  20  is also patterned by means of a two-stage DRIE step (before completion of the structure layer  26 ). In this case, the first etching depth D 1  and the third etching depth D 3  are chosen to be identical while the second etching depth D 2  is chosen to be identical to the fourth etching depth D 4 . This has the advantage of symmetrical gas surroundings of the active structure  27 . It substantially suppresses the resulting damping forces perpendicular to the wafer plane and parasitic movements resulting therefrom. 
     If there is no need for hermetically tight encapsulation of the structures in the structure layer  26 , the structure layer  26  can be formed on the described first layer assembly  10  by means of SDB (Silicon Direct Bonding) and can be patterned (after the formation of bonding pads e.g. by aluminum sputtering and etching). 
     It is also possible to form a structure layer  26  on the above-described first layer assembly  10  and to subsequently pattern it. It is subsequently possible to form an encapsulation by means of an encapsulation layer (e.g. a second substrate  20 ) by SDB, anodic bonding, anodic bonding with e.g. a sputtered PYREX interlayer or other joining methods. In this case, the encapsulation layer (e.g. the layer  20 ) can be prepatterned to insure access to the metal contact-making areas  32 . This embodiment creates cross sections similar or identical to those shown in  FIG. 2-4 . In this way, the metal contact-making areas  32  can be applied to the structure layer  26  prior to encapsulation, and the active structure  27  can be tested. Low-temperature joining methods should then be used for the last joining process to prevent the metal contact-making areas  32  from being destroyed. 
       FIG. 5  is a sectional elevation view of a component with buried electrodes. This demonstrates that the two-stage patterning process can also form buried electrodes  40  that can be employed primarily to detect and impress movements and forces in the z direction (perpendicular to the wafer plane). 
     As is illustrated in  FIG. 6 , it is possible to form both buried electrodes  40  and interconnect bridges  34  by a three-stage patterning with fifth depressions  41  of a fifth etching depth D 5 . In this case, the buried electrodes  40  are formed, for example, by the material of the corresponding layer (the covering layer  13 ) itself, or by deposition of an additional metallization layer on the corresponding layer (the covering layer  13 ). 
       FIG. 7   a  illustrates a schematic section taken along the sectional area I-I′ of  FIG. 7   b , while  FIG. 7   b  illustrates a schematic cross section taken along the sectional area II-II′ of  FIG. 7   a , superimposed by a section along the area of  FIG. 7   a  (light gray shade) and a section along the area IV-IV′ of  FIG. 7   a  (dark gray shade). In this case, the cross section of  FIG. 7   b  illustrates particularly well the active structure  27  and the interconnect bridge  34  that connects an electrode  5  situated within the active structure to a connection  51  outside the active structure. In this case, the illustrated component also illustrates the fact that a portion of the first depressions  14  and of the third depressions  22  as well as the second depressions  15  and the fourth depressions  23  are situated symmetrically above and below the active structure respectively. In a symmetrical arrangement resulting from identical etching depths of the second and fourth depressions  15 ,  23  and of the first and third depressions  14 ,  22 , symmetrical gas environments of the movable active structure  27  occur. This substantially suppresses the resulting damping forces perpendicular to the plane of the layers and the resulting parasitic movements. 
     In the invention, interconnect bridges are formed by different etching depths with structures being bridged by the interconnect bridges. The method of the invention increases design freedom and variety since new structures become possible. By eliminating openings, a stiffer structure is achieved. This leads to the reduction of parasitic movements and effects. Moreover, the number of bonding pads can be reduced, giving rise to lower costs due to a smaller area requirement and an increase in yield or reliability. 
     In a preferred embodiment, the active structure of the component is produced by patterning the structure layer, where the patterning can be accomplished before or after application of the structure layer to the first layer assembly. The patterning can be accomplished, for example, by applying a mask on the surface of the structure layer and subsequently etching that layer. If the structure layer is not patterned until after application, then it is not necessary to take any joining tolerances into account during application of the structure layer. 
     In accordance with further advantageous embodiments, application of an encapsulation layer, or of a second layer assembly, enables hermetically tight encapsulation at the wafer level with adjustable internal pressure. It simultaneously affords the possibility of producing a shield electrically insulated from the other electrical contacts for protection against external electromagnetic interference fields. In this case, the structure layer can also be part of the second layer assembly which also includes a second substrate and a second insulation layer. Simple access to the metal contact-making areas through the encapsulation layer can be accomplished by contact holes produced in the encapsulation layer before application of the encapsulation layer to the structure layer. 
     When using the second layer assembly, preferably in the side of the second substrate facing the structure layer, before application of the structure layer to the second layer assembly, third depressions are produced. The lateral positions of the third depressions correspond, at least in part, to those of the contact holes formed later in the second substrate. The third depressions can be used as contact holes (or at least as parts of the contact holes) in a later process stage of the production method of the invention. 
     Fourth depressions are produced, in the side of the second substrate facing the structure layer, before application of the structure layer to the second layer assembly. The lateral positions of the fourth depressions correspond, at least in part, to those of the active structure or the active structure of the structure layer. The second depressions can likewise be produced at these lateral positions. The second and fourth depressions enable mechanical movement (e.g. vibration) of that region of the structure layer that lies within the active region. Also, the second and fourth depressions can be used to establish specific parameters of the component. Since mechanical vibration quality under specific conditions is dependent primarily on the pressure enclosed into the component, the geometry of the active (movable) structure and the direct surroundings, it is possible, for example, to influence the vibration quality of a vibratory active structure in a targeted way through the depths of the second and fourth depressions. Thus, vibration quality is greater the deeper the second and fourth depressions (for the same pressure within the component). 
     In the case of a symmetrical arrangement (as a result of identical etching depths of the second and fourth depressions), symmetrical gas surroundings of the movable active structure result. This substantially depresses resulting damping forces perpendicular to the plane of the layers and the parasitic movements that result. 
     If third depressions have been formed within the second substrate, it is then possible, in order to form the contact holes, to remove at least part of the second substrate as far as a vertical position corresponding to the vertical position of the bottoms of the third depressions proceeding from the surface of the second substrate remote from the structure layer. The third depressions are thus “opened” and available as contact holes. 
     It is also possible for a portion of the first depressions and a portion of the third depressions to be located above and below the active structure respectively. In a particularly preferred embodiment, the first and second substrates as well as the structure layer and the covering layer, are composed of silicon. However, the invention is not limited thereto as other materials/material combinations are also within the scope of the invention. Silicon generally possesses the advantages of good mechanical properties, high availability and well-developed processing methods. If the components mentioned above are composed of silicon, then this offers the advantages of low thermal stress (always present if the two substrates and the covering and structure layers are composed of the same material) and also little outgassing during thermal joining (compared with PYREX or SD2 (materials commercially available from Corning Glas and Hoya respectively)), whereby pressures of less than 0.01 mbar can be obtained within the component. 
     The different etching depths can be produced by a two-stage dry etching step using a double mask. By producing electrodes for the active structure at the positions of the second depressions in/on the covering layer, it is possible to form buried electrodes that can be used to detect and impress movements and forces perpendicular to the wafer plane. Both buried electrodes and interconnect bridges can be formed jointly in one component by a three-stage etching process. 
     Thus it is seen that the present invention provides a method for producing a component, in particular, a micromechanical, micro-electromechanical or micro-opto-electromechanical component of the type in which interconnect crossovers and, in particular, bridges over movable structures, are realized. 
     While this invention has been described with references to its presently-preferred embodiment, it is not limited thereto. Rather, the invention is limited only insofar as it is defined by the following set of patent claims and includes within its scope all equivalents thereof.