Abstract:
A method for producing microelectromechanical structures in a substrate includes: arranging at least one metal-plated layer on a main surface of the substrate in a structure pattern; leaving substrate webs open beneath a structure pattern region by introducing first trenches into the substrate perpendicular to a surface normal of the main surface in a region surrounding the structure pattern; coating the walls of the first trenches perpendicular to the surface normal of the main surface with a passivation layer; and introducing cavity structures into the substrate at the base of the first trenches in a region beneath the structure pattern region.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for producing MEMS structures, in particular using CMOS processes, and a method for combined production of a CMOS structure and a MEMS structure. 
         [0003]    2. Description of the Related Art 
         [0004]    At the present time, microelectromechanical (MEMS) structures are frequently produced using CMOS technology. The metal printed conductors employed in a CMOS process may be used as movable structures as well as for forming electrodes. The dielectric layers, in particular the oxide layers, situated beneath the metal printed conductors may be used as a sacrificial layer in order to partially or completely leave the metal printed conductors open. With the aid of the CMOS technology, together with the MEMS structures, standard switching elements may also be produced which take over the control and evaluation of the MEMS element within a chip. 
         [0005]    One important aspect in the use of metal printed conductors in MEMS structures is typically their mechanical properties, such as plasticity, brittleness, flexural strength, breaking strength, and the like. Another aspect is that vertically high structures having a height of greater than 5 μm and high aspect ratios (ratio of the vertical to the lateral extension) are not producible using conventional CMOS technology. 
         [0006]    Another approach concerns the design of metal printed conductors which remain connected to a silicon substrate via oxide layers. With the aid of etching processes, the silicon in the substrate may be appropriately structured in order to form supporting silicon webs beneath the metal printed conductors. Published German patent application document DE 10 2008 054 553 A1, for example, discloses an acceleration sensor having movable and stationary electrodes in a substrate. 
         [0007]    Published German patent application document DE 10 2006 051 597 A1 discloses a semiconductor system having a cantilevered microstructure above a substrate. 
         [0008]    Front- and back-side processes are frequently necessary for producing MEMS structures, in particular in the production of undercut silicon structures for cantilevered elements. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a method for producing microelectromechanical structures which is based on CMOS processes and which may be carried out using one-sided, in particular front-side, processes. In particular, with the aid of the method it is the aim for mechanical and electrostatic properties of the microelectromechanical structures to be adjustable in a simple and efficient manner. 
         [0010]    For this purpose, it is a concept of the present invention to allow webs made of substrate material which are electrically insulated from the remainder of the substrate and electrically contactable to be provided beneath the microelectromechanical structures. 
         [0011]    According to one specific embodiment, a method for producing microelectromechanical structures in a substrate includes arranging at least one metal-plated layer on a main surface of the substrate in a structure pattern, and leaving substrate webs open beneath a structure pattern region by introducing first trenches into the substrate perpendicular to a surface normal of the main surface in a region surrounding the structure pattern, coating the walls of the first trenches perpendicular to the surface normal of the main surface with a passivation layer, and introducing cavity structures into the substrate at the base of the first trenches in a region beneath the structure pattern region. In this way, it may be advantageously achieved that the structure patterns of the metal-plated layer on the surface of the substrate are extended into the interior of the substrate via the substrate webs. Since the substrate webs are left open and are not directly electrically coupled to the remainder of the substrate, the substrate webs together with the particular associated structure pattern region may act as an electrode, which significantly increases the electrode surface area compared to an electrode formed only from the structure pattern region. In addition, mechanical and electrostatic properties of the substrate webs may be adjusted in a flexible manner via the dimensioning of the first trenches and of the cavity structures. 
         [0012]    It is particularly advantageous to provide metal-plated webs or metal webs within the metal-plated layer for mechanically connecting the structure pattern region to the substrate, the width of the metal-plated webs being smaller than the width of the structure pattern region, and to introduce second trenches into the substrate beneath the metal-plated webs. With the aid of the metal-plated webs, on the one hand a controlled electrical coupling to the circuit regions, which may be provided on the substrate, for example, may be created, and on the other hand the metal-plated webs establish a mechanically stable connection of the structure pattern regions and the substrate webs to the remainder of the substrate. 
         [0013]    According to another specific embodiment, a semiconductor device includes a substrate having a main surface, a first microelectromechanical structure element having a first metal-plated layer and a first substrate web, situated beneath the first metal-plated layer, which is electrically decoupled from the substrate, and at least one metal-plated web which is narrower than the first microelectromechanical structure element and which electrically couples the first microelectromechanical structure element to the substrate. The semiconductor device offers the advantage of providing a microelectromechanical structure element which has a large electrode surface area, and which may advantageously be interconnected to form a capacitive sensor element via a second microelectromechanical structure element having a second metal-plated layer and a second substrate web, situated beneath the second metal-plated layer, which is electrically decoupled from the substrate. The second microelectromechanical structure element is advantageously movable with respect to the first microelectromechanical structure element, thus allowing a reliable and accurate acceleration sensor to be provided. 
         [0014]    The above embodiments and refinements may be combined with one another, if appropriate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIGS. 1   a  through  1   f  show schematic diagrams for illustrating method steps of a method for producing MEMS structures according to one specific embodiment of the present invention. 
           [0016]      FIG. 2  shows a top view of a MEMS structure according to another specific embodiment of the present invention. 
           [0017]      FIG. 3  shows a top view of a MEMS structure according to another specific embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Unless stated otherwise, in each case identical and functionally equivalent elements, features, and components are provided with the same reference numerals in the figures. It is understood that for reasons of clarity and understandability, components and elements are not necessarily illustrated to scale relative to one another in the drawings. 
         [0019]      FIGS. 1   a  through  1   f  show schematic diagrams for illustrating method steps of a method for producing MEMS structures in a substrate according to one specific embodiment of the present invention. A first method step  100  is shown in  FIG. 1   a . A substrate  1  having a main surface  1   a  is provided. One or multiple metal-plated layers  4 ,  4   a ,  4   b ,  4   c  is/are applied on main surface  1   a  of the substrate. In particular, metal-plated layers  4 ,  4   a ,  4   b ,  4   c  may include printed conductors on substrate  1 . The structure of metal-plated layers  4 ,  4   a ,  4   b ,  4   c  may include different patterns, and may depend on the particular use of the MEMS structure to be produced. Metal-plated layers  4 ,  4   a ,  4   b ,  4   c  may be metal-plated levels in a CMOS process. Metal-plated layers  4 ,  4   a ,  4   b ,  4   c  may contain, for example, conductive materials such as aluminum, tungsten, titanium, copper, gold, platinum, or similar metals. 
         [0020]    In addition, one or multiple dielectric layers  2  may be applied on main surface  1   a  of substrate  1 . Dielectric layers  2  may be applied in particular between the structures of metal-plated layers  4 ,  4   a ,  4   b ,  4   c , and may terminate flush with the metal-plated layer. Dielectric layers  2  may include oxide layers, for example, such as silicon oxide layers, for example. 
         [0021]    A metal-plated layer  3  is applied on the side of main surface  1   a  of substrate  1 . Metal-plated layer  3  may be applied, for example, on metal-plated layer  4   c  and/or dielectric layer  2 . Metal-plated layer  3  may be structured in order to meet predefined constraints imposed by the use of the MEMS structure. For example, thin metal-plated webs may be provided in a region  5  which electrically connect two metal-plated regions of metal-plated layer  3  to one another. Metal-plated layer  3  may also be structured in such a way that regions  11  remain open, and a surface of dielectric layer  2  is exposed to the outside. 
         [0022]    It may be provided that regions (not shown) of substrate  1  on which, for example, CMOS circuit regions or other regions, not required as a MEMS structure, are covered by a protective layer. 
         [0023]    A second method step  200  is shown in  FIG. 1   b . With metal-plated layer  3  as a mask layer, depressions or trenches are initially introduced into dielectric layers  2 . A plasma etching process may be used, with the aid of which preferably vertical etching flanks may be produced. In region  11  of exposed dielectric layers  2 , trenches  21  are introduced down to main surface  1   a  of substrate  1 . Trenches  21  may undercut an overhang  23  of metal-plated layer  3 ; i.e., the width of trenches  21  may be slightly greater than the width of the opening in region  11 . 
         [0024]    A trench  22  is likewise introduced into dielectric layers  2  in region  5  of metal-plated layer  3 , in which thin metal-plated webs may be applied. Trench  22  is produced by undercutting the metal-plated webs in region  5  of trench structures which are situated outside the plane of the drawing of  FIG. 1   b . In this way, it may be achieved that a region of dielectric layers  2  between two trenches  22  and  21  is spatially separated from the remaining region of dielectric layers  2 , while a connection of metal-plated layer  3  between these regions is still ensured by metal-plated webs  5 . For this purpose, it may also be provided that an isotropic etching process or a combination of anisotropic and isotropic etching processes is used to completely or partially remove dielectric layers  2  beneath region  5 . 
         [0025]    When metal-plated webs  5  are located at a relatively great distance from main surface  1   a  of substrate  1 , for example due to a relatively large number of metal-plated layers  4 ,  4   a ,  4   b ,  4   c  connected therebetween, complete undercutting of metal-plated webs  5  for forming trench  22  may be dispensed with. This is possible in particular when introducing trenches into substrate  1  beneath metal-plated webs  5  may be ensured in a subsequent method step. 
         [0026]    A further method step  300  is shown in  FIG. 1   c . With metal-plated layer  3  as a mask layer, further trenches  31  are introduced into substrate  1  in the region of trenches  21  and  22 . Depth  33  of the trenches may be selected depending on the MEMS structures to be produced and their mechanical and/or electrostatic properties. For example, for introducing trenches  31 , an anisotropic etching process such as a deep reactive ion etching (DRIE) process, for example, may be used. It is provided that trenches  31  are designed in such a way that metal-plated layer  3  is completely undercut in substrate  1  in region  5  of the metal-plated webs. 
         [0027]    A further method step  400  is shown in  FIG. 1   d . Starting from the side of main surface  1   a  of substrate  1 , a passivation layer is applied to the intermediate product. A passivation layer  43  is applied on a surface of metal-plated layer  3 , a passivation layer  41  is applied on the side walls of trenches  31 , and a passivation layer  42  is applied on base  32  of trenches  31 . The passivation layer may, for example, include an oxide layer, or a polymer layer made of octafluorocyclobutane, for example. 
         [0028]    A further method step  500  is shown in  FIG. 1   e . Regions  42  and  43  of the passivation layer are removed. This may be achieved using an anisotropic etching process, for example. In particular, base  32  of trenches  31  is exposed. 
         [0029]    A further method step  600  is shown in  FIG. 1   f . Cavity structures  61 ,  62 ,  63  are formed inside substrate  1  via base  32  of trenches  31 . An isotropic etching process or an isotropic gas phase process may be used for etching substrate  1 . Starting from base  32  of a trench  31 , an essentially spherical cavity structure  61  is formed which extends essentially uniformly beneath trench  31  inside substrate  1 . It may be provided that cavity structures  61  of adjacent trenches  31  abut one another in a region  63  and form an opening. Such an opening may be provided to separate regions  65  above cavity structures  61  from the remaining substrate material, in particular to electrically insulate and to create exposed regions  65  in the process. It may also be provided that beneath regions  66 , cavity structures  62  are produced which result from openings (not shown) in cavity structures  61  which abut one another outside the plane of the drawing of  FIG. 1   f  in a direction perpendicular to the plane of the drawing. This may be ensured, for example, by metal-plated layer  3  having a sufficiently small width above region  66 . Structures  66 , made of substrate material, which are left open may thus be produced which are mechanically separated and electrically insulated from the remainder of substrate  1 . Structure  66  is connected to the remainder of substrate  1 , solely via metal-plated layer  3  in region  5  of the metal-plated webs and optionally, via other remainders of dielectric layers  2 . 
         [0030]    In another specific embodiment, it may be provided that cavity structures  61 ,  62 ,  63  are introduced into substrate  1  via a back-side process, i.e., from a side facing away from main surface  1   a  of substrate  1 . It may also be provided that passivation layers  41  on the side walls of trenches  31  are removed after forming cavity structures  61 ,  62 ,  63 . 
         [0031]      FIG. 2  shows a top view of a MEMS structure according to another specific embodiment of the present invention. Intersection line I-II shows a cross section which is similar to the cross section shown in  FIGS. 1   a  through  1   f . In particular, the MEMS structure in  FIG. 2  may be produced using a method according to the present invention, according to the method steps in  FIGS. 1   a  through  1   f.    
         [0032]    The MEMS structure in  FIG. 2  may, for example, be applied to a substrate. Structure patterns of a metal-plated layer  3  are shown which, for example, may correspond to metal-plated layer  3  in  FIGS. 1   a  through  1   f . However, it may also be provided that the MEMS structure is formed completely or partially from other metal-plated layers, for example metal-plated layers  4 ,  4   a ,  4   b ,  4   c  in  FIGS. 1   a  through  1   f . The MEMS structure may include webs  204   a ,  204   b ,  205   a ,  205   b  which are aligned parallel to one another. It may be provided that a plurality of webs  204   a ,  204   b ,  205   a ,  205   b , in each case arranged in pairs, is provided. Three web pairs  204   a ,  205   a , and  204   b ,  205   b  are shown in  FIG. 2  as an example, although any other number of web pairs is also possible. It may also be provided that webs  204   a  and  204   b , which are not paired with a web  205   a  and  205   b , respectively, are provided. Webs  204   a ,  204   b  are advantageously connected to a main region  208   a  or  208   b , respectively, of metal-plated layer  3  via metal-plated webs  206 . Metal-plated webs  206  may correspond to the metal-plated webs in region  5  of  FIGS. 1   a  through  1   f . It may be provided that metal-plated webs  206  are arranged not only in a row, but also in such a way that they cover the largest possible area, within which webs  204   a ,  204   b  may be suspended in a mechanically stable manner. 
         [0033]    Webs  204   a ,  204   b ,  205   a ,  205   b  span regions of the substrate material which are electrically and mechanically separated from the remainder of the substrate. These regions may in particular be regions which are similar to regions  66  in  FIGS. 1   a  through  1   f . Thus, webs  204   a ,  204   b ,  205   a ,  205   b  together with the substrate regions therebeneath may be used as electrodes having a large electrode surface area. Webs  204   a  and  204   b  and  205   a  and  205   b  may each be used as an electrode of a capacitive MEMS structure. In particular, the MEMS structures in  FIG. 2  may be used as acceleration sensors. Electrodes  204   a ,  204   b ,  205   a ,  205   b  may be acted on by voltage via terminals  201 ,  202 ,  203 . Two webs  204   a ,  205   a  and  204   b ,  205   b  form a capacitive sensor element. A capacitance of the sensor element between two webs  204   a ,  205   a  and  204   b ,  205   b  is a function, among other things, of the distance between the two webs in the y direction. The middle electrode having terminals  202  is mechanically freely mounted via an equalizing structure  207 . Thus, during an acceleration in the y direction, structure  209  may be deflected in the y direction due to the mass inertia of structure  209 , while webs  204   a  and  204   b  undergo little or no deflection in the y direction due to their rigid suspension via metal-plated webs  206 . As a result, the distance between a web pair  204   a ,  205   a  decreases to a degree that is comparable to the distance between a web pair  204   b ,  205   b . This change in distance is reflected in a change in capacitance, which may be evaluated by differential analysis of electrode terminals  201  and  202  or  202  and  203 . 
         [0034]      FIG. 3  shows a top view of a MEMS structure according to another specific embodiment of the present invention. The MEMS structure in  FIG. 3  differs from the MEMS structure in  FIG. 2  essentially in that webs  304   a ,  304   b ,  304   c ,  304   d  of the stationary electrodes in two regions  9  are connected to main regions  308   a ,  308   b  of the stationary electrodes via metal-plated webs. This advantageously results in higher mechanical stability of webs  304   a ,  304   b ,  304   c ,  304   d  of the stationary electrodes.