Patent Publication Number: US-2011068419-A1

Title: Micromechanical system

Description:
FIELD OF THE INVENTION 
     The present invention relates to a micromechanical system and a method for manufacturing a micromechanical system. 
     BACKGROUND INFORMATION 
     In manufacturing electromechanical microstructures (MEMS), it is known that conductive layers of polycrystalline silicon may be placed one above the other vertically. The layers may be used as conductor path layers, electrodes or function layers. This is described in German Patent Application No. DE 10 2007 060 878, for example. The conductive layers, which are initially separated by sacrificial layers, may be exposed by etching processes. It is also known that conductive connections may be created between individual conductive layers. To do so, openings are created in the underlying insulation layer before applying a conductive layer situated at a higher level, so that a conductive connection to the deeper conductive layer is formed simultaneously when the conductive layer is applied. This results in an irregular elevation profile (topography) on the surface of the newly applied conductive layer, thus hindering the manufacturing of high-resolution structures. If the connecting elements are designed to be smaller, this reduces the interfering influences of the topography. However, there is a marked decline in the mechanical stability of the connecting elements at the same time. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a micromechanical system having an improved connection between two conductive layers. This object is achieved by a micromechanical system according to the present invention. In addition, an object of the present invention is to provide a method for manufacturing a micromechanical system having an improved connection between two conductive layers. This object is achieved by a method according to the present invention. 
     A micromechanical system according to the present invention includes a substrate, a first conductive layer situated above the substrate and a second conductive layer situated above the first conductive layer. The first conductive layer and the second conductive layer are conductively interconnected by a first connecting element. The first connecting element has a first conductive edge which surrounds a first nonconductive region. The second conductive layer advantageously has only a low topography over the connecting element. The connecting element nevertheless has a very high mechanical stability. One particular advantage is that mechanical elastic and torsion properties of the connecting element are adjustable by varying the volume and the material composition of the nonconductive region. 
     In a specific embodiment of the micromechanical system, the first nonconductive region has an oxide. This advantageously produces a particularly stable connection between the first and second conductive layers. 
     The first conductive edge expediently has a ring shape. 
     In one refinement, the first conductive edge surrounds another conductive region extending from the first conductive layer to the second conductive layer. This advantageously makes it possible to increase the conductivity of the connecting element. Furthermore, the further conductive region may also border a nonconductive region. Such a chamber structure makes it possible to design the mechanical properties of the connecting element as desired. 
     A wall thickness of the first conductive edge parallel to the substrate surface is preferably smaller than twice the thickness of the second conductive layer in the direction perpendicular to the substrate surface. The surface of the second conductive layer then advantageously has only a slight variation in height (topography). 
     In one refinement, the micromechanical system has a third conductive layer situated above the second conductive layer in such a way that the second conductive layer and the third conductive layer are conductively interconnected by a second connecting element. The second connecting element has a second conductive edge surrounding a second nonconductive region. The additional conductive layer of this micromechanical system may advantageously be used for manufacturing conductor path intersections, for example. The surface of the third conductive layer advantageously has only a low topography. 
     The second conductive edge is in particular preferably situated with an offset relative to the first conductive edge in a direction parallel to the substrate surface. The creation of an excessively strong topography in the surfaces of the conductive layers is advantageously prevented by such an offset placement, for example, a cascading placement of the connecting elements. 
     A method according to the present invention for manufacturing a micromechanical system has method steps for providing a substrate with a first conductive layer, for depositing and structuring a second insulating layer, creating, in the second insulating layer, a trench extending from the surface of the second insulating layer to the first conductive layer and bordering a section of the second insulating layer, for depositing a second conductive layer and for removing a portion of the second insulating layer. This method advantageously allows the manufacture of a mechanically stable connection between the first and second conductive layers and therefore creates only minor differences in height in the surface of the second conductive layer. Another advantage is that the mechanical properties of the conductive connection between the conductive layers are adaptable to the particular requirements. 
     For providing the substrate with the first conductive layer, method steps are expediently performed for providing a substrate, for depositing and structuring a first insulating layer, and for depositing and structuring the first conductive layer. 
     It is also expedient if at least one through opening is created in the second conductive layer and if the second part of the second insulating layer is removed by an etching process. The region of the second insulating layer bordered by the resulting conductive edge between the first and second conductive layers may advantageously be either removed or retained. This allows the mechanical properties of the connecting element to be varied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a micromechanical system according to a first specific embodiment. 
         FIG. 2  shows a micromechanical system according to a second specific embodiment. 
         FIG. 3  shows a section through a connecting element of the micromechanical system. 
         FIG. 4  shows a micromechanical system according to a third specific embodiment. 
         FIG. 5  shows a section through a connecting element of the micromechanical system. 
         FIG. 6  shows a micromechanical system according to a fourth specific embodiment. 
         FIG. 7  shows a micromechanical system according to a fifth specific embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a section through a layer structure of a micromechanical system  100  in a highly schematic diagram. Micromechanical system  100  may be part of a micromechanical sensor structure such as an acceleration sensor or a yaw sensor, for example. Micromechanical system  100  includes a substrate  110 , which functions as the carrier. Substrate  110  may be a silicon substrate, for example. A first insulating layer  120  is provided on the surface of substrate  110 . First insulating layer  120  is preferably embodied as a sacrificial layer and is made of a silicon oxide, for example. A first conductive layer  130  is situated on first insulating layer  120 . First conductive layer  130  may be a buried polysilicon layer, for example. For example, conductor paths may be defined in first conductive layer  130 . First conductive layer  130  may also function as an electrode. A second insulating layer  140  is situated above first conductive layer  130 . Second insulating layer  140  is preferably also designed as a sacrificial layer and may also be made of a silicon oxide, for example. A second conductive layer  150  is provided above second insulating layer  140 . Second conductive layer  150  may be a polysilicon function layer, for example. Second conductive layer  150  may have a greater thickness than first conductive layer  130 . For example, movable elements of a sensor structure of micromechanical system  100  may be manufactured from second conductive layer  150 . Second conductive layer  150  has one or more trench openings  180 , which run perpendicularly to the substrate surface through second conductive layer  150 . 
     First conductive layer  130  and second conductive layer  150  are interconnected by a conductive connecting element  200 . Conductive connecting element  200  has a sleeve-shaped edge  210  made of a conductive material extending from first conductive layer  130  to second conductive layer  150 . Conductive edge  210 , first conductive layer  130  and second conductive layer  150  surround a nonconductive region  220  of connecting element  200 . In the example shown in  FIG. 1 , nonconductive region  220  is formed by a part of second insulating layer  140 . Parallel to the surface of substrate  110 , connecting element  200  may have a circular cross section, for example. However, other cross sections are also possible, for example, rectangular or polygonal cross sections. Connecting element  200  establishes a mechanically stable connection between first conductive layer  130  and second conductive layer  150 . The wall thickness of edge  210  of connecting element  200  in a direction parallel to the surface of substrate  110  is less than twice the thickness of second conductive layer  150  in the direction perpendicular to the surface of substrate  110 . In other words, second conductive layer  150  is more than half as thick as the wall thickness of edge  210 . The surface of second conductive layer  150  facing away from substrate  110  has only minor differences in height, i.e., only a low topography. The surface of second conductive layer  150  in particular has only a slight recess in the region above edge  210  of connecting element  200 . The recess in the surface of second conductive layer  150  perpendicularly above edge  210  is smaller than the thickness of second insulating layer  140 . 
     To manufacture micromechanical system  100  of  FIG. 1 , first insulating layer  120  is first deposited on the surface of substrate  110  and is suitably structured. First conductive layer  130  is deposited and structured next. Second insulating layer  140  is deposited in the next step. A trench is then created in second insulating layer  140 , bordering a section of second insulating layer  140 . The trench extends perpendicularly to the surface of substrate  110  throughout the entire second insulating layer  140 . The depth of the trench thus corresponds to the thickness of second insulating layer  140 . The section of second insulating layer  140  bordered by the trench later forms nonconductive region  220  of connecting element  200 . The shape of edge  210  of connecting element  200  is defined by the shape of the trench. Second conductive layer  150  is deposited in the subsequent method step. At the same time, the trench created in second insulating layer  140  is filled, thus forming edge  210  of connecting element  200 . Edge  210  and second conductive layer  150  are therefore deposited simultaneously. Only a low topography is formed in the surface of second conductive layer  150  above edge  210  of connecting element  200  because the thickness of second conductive layer  150  is more than half as great as the wall thickness of edge  210 , i.e., the width of the trench created in second insulating layer  140 . In particular, the surface of second conductive layer  150  is recessed perpendicularly above edge  210  by less than the thickness of second insulating layer  140 . Second conductive layer  150  is structured subsequently. One or more trench openings  180  in particular are created, extending through second conductive layer  150  perpendicularly to the surface of substrate  110 . Next, in a sacrificial layer process, parts of first insulating layer  120  and of second insulating layer  140  may be removed selectively. The etching medium penetrates through trench openings  180  to insulating layers  120 ,  140 . Nonconductive region  220  of second insulating layer  140  surrounded by edge  210  is protected from the etching medium by edge  210  and is therefore not removed. 
       FIG. 2  shows a schematic view of a micromechanical system  1100  according to a second specific embodiment, in which connecting element  200  is replaced by a connecting element  1200 . Connecting element  1200  has a conductive edge  1210  surrounding a nonconductive region  1220 .  FIG. 3  shows a section through connecting element  1200  and parallel to the surface of substrate  110 . It is discernible here that edge  1210  of connecting element  1200  has a plurality of struts  1215  pointing outward like rays, made, like edge  1210 , of the conductive material of second conductive layer  150 . Struts  1215  increase the conductivity and mechanical stability of connecting element  1200 . The thickness of each strut  1215  corresponds approximately to the wall thickness of edge  1210  and is thus less than twice the thickness of second conductive layer  150 . Struts  1215  therefore also produce only a low topography of second conductive layer  150 . 
       FIG. 4  shows a micromechanical system  2100  according to a third specific embodiment. Connecting element  200  of the first specific embodiment has been replaced here by a conductive connecting element  2200  between first conductive layer  130  and second conductive layer  150 .  FIG. 5  shows a section through connecting element  2200  parallel to the surface of substrate  110 . Connecting element  2200  has a hollow cylindrical inner edge  2210  and a hollow cylindrical outer edge  2215 , each being made of the conductive material of second conductive layer  150  and extending between first conductive layer  130  and second conductive layer  150 . Outer edge  2215  is concentric around inner edge  2210 . Inner edge  2210  surrounds an inner nonconductive region  2220 , which is filled with the insulating material of second insulating layer  140 . An outer nonconductive region  2225  in which the insulating material of second insulating layer  140  has been removed is situated between inner edge  2210  and outer edge  2215 . There is thus a vacuum or a gas such as air in outer nonconductive region  2225 . 
     Manufacturing of micromechanical system  2100  differs from the method explained with reference to  FIG. 1  in that two concentric trenches are created after applying second insulating layer  140 , inner edge  2210  and outer edge  2215  of connecting element  2200  being formed subsequently in these trenches. Furthermore, in structuring of second conductive layer  150 , one or more trench openings  180  are also created perpendicularly above outer nonconductive region  2225 . These trench openings  180  thus extend from the surface of second conductive layer  150  facing away from substrate  110  through second conductive layer  150  into the region of second insulating layer  140  bordered by outer edge  2215 . During the sacrificial layer process for dissolving out portions of first insulating layer  120  and second insulating layer  140 , the etching medium may therefore also penetrate into outer nonconductive region  2225  and remove second insulating layer  140  there. It is of course possible to also remove the material of second insulating layer  140  in inner nonconductive region  2220  or to also retain the material of second insulating layer  140  in outer nonconductive region  2225 . This results in different mechanical properties of connecting element  2200 . The material of second insulating layer  140  may also be removed by creating suitable trench openings  180  from nonconductive region  220  of connecting element  200  of  FIG. 1  and nonconductive region  1220  of connecting element  1200  of  FIG. 3 . 
       FIG. 6  shows a micromechanical system  3100  according to a fourth specific embodiment, in which, in contrast with micromechanical system  100  of  FIG. 1 , a third insulating layer  160  and a third conductive layer  170  are situated above second conductive layer  150 . These three conductive layers  130 ,  150 ,  170  of micromechanical system  3100  allow more complex sensor systems to be manufactured, in which conductor path intersections, for example, are possible. A first conductive connecting element  3200  is situated between first conductive layer  130  and second conductive layer  150 . Second conductive layer  150  and third conductive layer  170  are conductively interconnected by a second connecting element  3300 . First connecting element  3200  includes a first edge  3210 , which is made of the conductive material of second conductive layer  150  and surrounds a first nonconductive region  3220 , in which some material of second insulating layer  140  remains. Second connecting element  3300  has a second edge  3310 , which is made of the conductive material of third conductive layer  170  and surrounds a second conductive region  3320 , in which insulating material of third insulating layer  160  remains. First edge  3210  and second edge  3310  may each have a hollow cylindrical sleeve shape, for example. The wall thickness of first edge  3210  is less than twice the thickness of second conductive layer  150 . The wall thickness of second edge  3210  is less than twice the thickness of third conductive layer  170 . Second edge  3310  is not situated directly above first edge  3210  in the direction perpendicular to the surface of substrate  110 . In the example shown here, second connecting element  3300  has a smaller diameter than first connecting element  3200 , so that second edge  3310  is situated perpendicularly above first nonconductive region  3220 . The placement of edges  3210 ,  3310  so they are not directly one above the other has the advantage that topographic recesses in second conductive layer  150  and third conductive layer  170 , which are formed in manufacturing connecting elements  3200 ,  3300 , are not additive. Therefore, third conductive layer  170  also has only minor topographical recesses. Third conductive layer  170  also has one or more trench openings  190  through which an etching medium is able to penetrate during a first sacrificial layer process to first, second and third insulating layers  120 ,  140 ,  160 . 
       FIG. 7  shows a micromechanical system  4100  according to a fifth specific embodiment. The layer sequence of micromechanical system  4100  corresponds to that of micromechanical system  3100  in  FIG. 6 . However, first conductive layer  130  and second conductive layer  150  of micromechanical system  4100  are conductively interconnected by a first connecting element  4200 . Second conductive layer  150  and third conductive layer  170  are conductively interconnected by a second connecting element  4300 . First connecting element  4200  has an inner edge  4210 , which is made of the conductive material of second conductive layer  150  and borders an inner nonconductive region  4220 . Furthermore, first connecting element  4200  has an outer edge  4215 , which concentrically surrounds inner edge  4210 , and is made of the material of second conductive layer  150  and borders an outer nonconductive region  4225  situated between inner edge  4210  and outer edge  4215 . Second connecting element  4300  has a hollow cylindrical edge  4310 , which is made of a material of third conductive layer  170  and connects it conductively to second conductive layer  150 . Edge  4310  surrounds a nonconductive region  4320 . Furthermore, a cylindrical pin or ram  4315  situated in nonconductive region  4320  is also made of the material of third conductive layer  170  and runs from second conductive layer  150  to third conductive layer  170 . 
     The insulating material of third insulating layer  160  has been removed in nonconductive region  4320 . Third conductive layer  170  therefore has one or more trench openings  190  extending from the surface of third conductive layer  170  facing substrate  110  through third conductive layer  170  into nonconductive region  4320 . During the sacrificial layer process, the etching medium has been able to penetrate through trench openings  190  into nonconductive region  4320  and remove third insulating layer  160  there. Inner nonconductive region  4220  and outer nonconductive region  4225  of first conducting element  4200  are also not filled with the material of second insulating layer  140 . Second insulating layer  150  therefore has one or more trench openings  180 , extending from nonconductive region  4320  of second connecting element  4300  through second conducting layer  150  into inner nonconductive region  4220  and outer nonconductive region  4225 . The etching medium was also able to penetrate through trench openings  180  into nonconductive regions  4220 ,  4225  of first connecting element  4200  during the sacrificial layer process and remove the material of second insulating layer  140  there. In alternative specific embodiments, nonconductive regions  4220 ,  4225 ,  4320  may of course also remain filled with the insulating material of insulating layers  140 ,  160 . 
     According to the present invention, the exact shape of the connecting element and their conductive edges may be selected differently. In particular, rectangular or other cross sections are also possible in addition to the circular cross sections shown here. The decisive factor is only that the conductive edge of the particular connecting element surrounds a nonconductive region. The nonconductive region may remain filled with insulating material of a sacrificial layer, resulting in a particularly high mechanical stability of the connecting element. Alternatively, the sacrificial layer material may be removed from the nonconductive region. The edge of the connecting element may advantageously be selected to be so thin that only a low height topography is established in the layer situated above the connecting elements.