Abstract:
A micromechanical component and method for its manufacture, in particular an acceleration sensor or a rotational speed sensor, includes: function components suspended movably above a substrate; a first insulation layer provided above the substrate; a first micromechanical function layer including conductor regions provided above the first insulation layer; a second insulation layer provided above the conductor regions and above the first insulation layer; a third insulation layer provided above the second insulation layer; a second micromechanical function layer including first and second trenches provided above the third insulation layer, the second trenches extending to the third insulation layer above the conductor regions and the first trenches extending to a cavity beneath the movably suspended function components in the second micromechanical function layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a micromechanical component, in particular an acceleration sensor or a rotational speed sensor including function components suspended movably above a substrate, and a corresponding manufacturing method for the micromechanical component. 
     BACKGROUND INFORMATION 
     Although it may be applied to any micromechanical components and structures, in particular to sensors and actuators, the present invention and the underlying problem are elucidated with reference to a micromechanical acceleration sensor that may be manufactured using silicon surface micromachining technology. 
     Acceleration sensors, in particular micromechanical acceleration sensors manufactured using surface or volume micromachining technology, have an increasing market share in the automotive equipment industry and are increasingly replacing the piezoelectric acceleration sensors customarily used. 
     Conventional micromechanical acceleration sensors normally operate so that a flexibly mounted seismic mass device, which is deflectable in at least one direction by an external acceleration, on deflection causes a change in the capacitance of a differential capacitor device that is connected to it. This change in capacitance is a measure of the acceleration. 
     German Published Patent Application No. 195 37 814 describes a method of manufacturing surface micromechanical sensors. 
     A first insulation layer of a thermal oxide (approximately 2.5 μm thick) is first deposited on a silicon substrate. Then, a thin (approximately 0.5 μm thick) polysilicon layer is deposited on this insulation layer. The polysilicon layer is subsequently doped from the gas phase (POCl 3 ) and structured by a photolithographic process. This conducting polysilicon layer to be buried is thus subdivided into individual regions that are insulated from one another and function as conductors or as surface electrodes arranged vertically. 
     A second insulation layer is deposited above the layers applied previously. This insulation layer is made up of an oxide generated from the gas phase. In a photolithographic process, the top insulation layer is structured by introducing contact holes into the top insulation layer, through which the underlying polysilicon layer may be contacted. 
     Then, a thin polysilicon layer, which functions as a seed for subsequent deposition of silicon, is applied. This is followed in another step by deposition, planarization and doping of a thick polycrystalline silicon layer. The deposition occurs in an epitaxial reactor. Then, a structured metal layer is applied to the thick silicon layer. 
     The thick silicon layer is structured in another photolithographic process, in which a photoresist mask is applied to the top side of the layer to protect the metal layer in the subsequent etching. Then, plasma etching of the thick silicon layer is performed through openings in the photoresist mask according to the method described in German Published Patent Application No. 42 410 45, in which trenches having a high aspect ratio are produced in the thick silicon layer. These trenches extend from the top side of the thick silicon layer to the second insulation layer. The layer is thus subdivided into individual regions that are insulated from one another, unless they are interconnected by the buried conducting layer. 
     The two sacrificial layers are then removed through the trenches in the area of the freely movable structures of the sensor. The oxide layers are removed by a vapor etching method using media containing hydrofluoric acid according to a method described in German Published Patent Application No. 43 172 74 and German Published Patent Application No. 197 04 454. 
     However, there are disadvantages with the removal of the sacrificial layer by the hydrofluoric acid vapor etching method. With this etching method, it is very difficult to achieve a defined undercutting, i.e., the oxide is removed not only beneath the functional or freely movable sensor structures, but also above and beneath the buried polysilicon conductors. This requires very wide conductors because the possibility of lateral undercutting must be assumed. Due to the undercutting, no conductors are allowed to pass beneath the functional structure. Another disadvantage is corrosion of the metal layer due to the hydrofluoric acid vapors. 
     If the water content of the gas phase is too high, there may be sticking problems, i.e., the freely movable sensor elements may adhere to the substrate. Due to the limited oxide thickness (due to the deposition method) of the insulation layers, the distance between the functional structure and the substrate is also limited. 
     Since the hydrofluoric acid vapor etching method is not compatible with the materials used in CMOS technology, there may not be any integration of the sensor element and the analyzer circuit. 
     SUMMARY 
     The micromechanical component according to an example embodiment of the present invention and a corresponding manufacturing method may have the advantage that both the buried conductors and the sacrificial layer beneath the freely movable structures may be made of the same layer. Therefore, fewer layers and photolithographic processes may be needed. 
     The present invention provides a layer structure and a corresponding method for manufacturing micromechanical components, e.g., acceleration sensors having a lateral sensitivity, sacrificial layer regions being made of the same material, e.g., polysilicon, as the buried conductor regions. A defined etching of the sacrificial polysilicon layer regions is achieved with the method according to the present invention, thus preventing undercutting of the buried conductor regions. 
     The method according to the present invention allows a simple method of manufacturing a sensor element using only method steps that are conventional in semiconductor technology. In addition, only a few layers and photolithography steps are necessary with the method according to the present invention. 
     The first micromechanical function layer and the second micromechanical function layer may include polysilicon layers. 
     According to another example embodiment of the present invention, the first through fourth insulation layers are oxide layers. 
     In removing the sacrificial layer, if the first micromechanical function layer is made of polysilicon and the insulation layers are oxide layers, then etching media based on fluorine compounds (e.g., XeF 2 , ClF 3 , BrF 3 , etc.) may be used. The etching media have a very high selectivity with respect to silicon dioxide, aluminum and photoresist. Due to this high selectivity, the polysilicon conductor regions that are not to be etched, in contrast with the sacrificial polysilicon layer regions, are sheathed with silicon dioxide. This prevents etching or undercutting of the polysilicon conductor regions. 
     This also permits a conductor to be guided beneath the freely movable structures. Since the buried polysilicon conductor regions are no longer being undercut, they may be narrower. Reproducible removal of the sacrificial polysilicon layer regions that is well-defined both laterally and vertically is possible with the sacrificial polysilicon layer technology described above. Due to the high selectivity of the etching medium with respect to silicon dioxide, it may be possible to implement a multilayer system of polysilicon conductors and insulation layers, and crossing of lines may also be possible. Large lateral undercutting widths may be feasible in sacrificial layer etching, so that the number of etching holes in the seismic mass may be reduced or omitted entirely. This yields an increase in the seismic mass. 
     Since the etching process for removing the sacrificial polysilicon layer occurs in the gas phase, no problems may exist with regard to corrosion and sticking. The silicon sacrificial layer technology may be compatible with materials used in CMOS technology, thus permitting integration of the sensor element and analyzer circuit. 
     It may be possible to structure the insulation layers by dry etching processes through the choice of layer thicknesses and sequence, thus eliminating wet etching processes and yielding improved process tolerances. The distance between the freely movable structure and the silicon substrate layer may be adjustable as needed through the thickness of the polysilicon layer. 
     According to another example embodiment of the present invention, the conductor regions and the sacrificial layer regions are provided through local implantation and subsequent photolithographic structuring. 
     According to yet another example embodiment of the present invention, contact holes for connecting the second micromechanical function layer to the conductor regions are provided in the second and third insulation layers. 
     According to even another example embodiment of the present invention, contact holes for connecting the second micromechanical function layer to the substrate are provided in the first, second and third insulation layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  11  are schematic cross-sectional views illustrating the manufacturing process for an acceleration sensor according to an example embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the Figures, identical symbols denote identical or functionally equivalent components. 
     FIGS. 1 to  11  are schematic cross-sectional views illustrating the manufacturing process for an acceleration sensor according to an example embodiment of the present invention. 
     FIG. 1 illustrates a silicon substrate  1  to which are applied a first insulation layer  2  and then a polysilicon layer  3  on top of the first insulation layer  2 . Conventional deposition processes from semiconductor technology for depositing dielectric layers may be used for depositing first insulation layer  2 . In addition to silicon dioxide, thus silicon nitride, dielectric layers having a lower dielectric constant than silicon dioxide, various types of glass or other ceramic layers may also be deposited. For the remaining description, it is assumed that first dielectric layer  2  is made of silicon dioxide formed by thermal oxidation of silicon substrate  1  and has a thickness between 10 nm and 2.5 μm. 
     Polysilicon layer  3  has a thickness between 0.5 μm and 5 μm. After its subsequent structuring, polysilicon layer  3  yields both buried polysilicon conductor regions  4  and sacrificial polysilicon regions  5 . 
     A high conductivity is required for buried polysilicon conductor regions  4 , so polysilicon layer  3  is doped from the gas phase (POCl 3 ) over the entire surface area. Any other method of producing a sufficiently highly doped polysilicon layer may also be used. If doping of polysilicon layer  3  is desired only in polysilicon conductor regions  4 , the high conductivity in these regions may be produced by local implantation, which requires an additional photolithographic process. 
     Then, a structuring of doped or partially doped polysilicon layer  3  occurs through a photolithographic process, as illustrated in FIG.  2 . This structuring of polysilicon layer  3  occurs by dry etching (plasma etching). Polysilicon layer  3  is thus subdivided into individual, mutually insulated regions  4 ,  5  that function as buried polysilicon conductor regions  4  and/or as sacrificial polysilicon layer regions  5 . 
     As illustrated in FIG. 3, a second insulation layer  6  is deposited on the structure illustrated in FIG.  2  and structured. The second insulation layer  6  in this example embodiment of the present invention is made of silicon dioxide, which is produced from the gas phase, e.g., by decomposition of silane. The thickness of second insulation layer  6  may be greater than or equal to the thickness of first insulation layer  2 . 
     In another photolithographic process, second insulation layer  6  is structured. The oxide of second insulation layer  6  is removed in region  7  above sacrificial polysilicon layer regions  5  and in region  8  of the substrate contact. Structuring of second insulation layer  6  may also occur by dry etching (plasma etching). 
     As illustrated in FIG. 4, a third insulation layer  9  is deposited on the structure illustrated in FIG.  3 . Insulation layer  9  is configured to protect or passivate structures  25 , which are later to be made freely movable (see FIG.  11 ), on the lower side with respect to the etching medium used in sacrificial layer etching. Third insulation layer  9  may be made of silicon dioxide produced from the gas phase, e.g., by decomposition of silane. Insulation layer  9  is needed only in regions  7  where second insulation layer  6  above sacrificial polysilicon layer regions  5  is removed. 
     Third insulation layer  9  may also be produced by a local thermal oxidation only in region  7 . The layer thickness of third insulation layer  9  may be between 5 nm and 500 nm. 
     Then, a polysilicon starting layer  10  is deposited on the surface of the structure illustrated in FIG. 4, as illustrated in FIG.  5 . Polysilicon starting layer  10  covers the surface of third insulation layer  9  and functions as a seed for the subsequent deposition of polysilicon. To deposit polysilicon starting layer  10 , any conventionally used method in semiconductor technology for deposition of thin polysilicon layers on dielectric layers may be suitable. 
     In a subsequent process step, a photolithographic structuring of polysilicon starting layer  10  and underlying insulation layers  2 ,  6 ,  9  or  6 ,  9  may occur by dry etching (plasma etching). 
     In the regions above buried polysilicon conductor regions  4 , contact holes  11  are introduced into polysilicon starting layer  10  and second and third insulation layers  6 ,  9 , through which underlying polysilicon conductor regions  4  may be contacted. 
     In the regions where a substrate contact hole  12  is to be produced, polysilicon starting layer  10  and first, second and third insulation layers  2 ,  6 ,  9  are structured. 
     As illustrated in FIG. 6, a thick silicon layer  13  is deposited in another process step. This deposition occurs in a conventional epitaxial reactor. Such an epitaxial reactor is an installation for deposition of silicon layers that is used in semiconductor technology for the production of single-crystal silicon layers on a single-crystal silicon substrate. In the example embodiment according to the present invention, deposition in the epitaxial reactor does not occur on a single-crystal silicon substrate, but instead occurs on polycrystalline silicon starting layer  10 , so that no single-crystal silicon layer develops, but instead there is a thick polycrystalline silicon layer  13 . Polysilicon starting layer  10  becomes a part of thick polycrystalline silicon layer  13  in this step of the process. 
     Since polycrystalline silicon layer  13  has a rough surface after this deposition, it is subsequently planarized. An electric connection to buried polysilicon conductor regions  4  is established through thick polycrystalline silicon layer  13  so that thick polycrystalline silicon layer  13  is doped. 
     Then, a structured metal layer  14  is provided on the top side of thick polycrystalline silicon layer  13 . Metal layer  14  may be applied over the entire surface, for example, and then structured. 
     Deposition of a silicon dioxide layer  15  from the gas phase occurs in a subsequent process step, e.g., by decomposition of silane as illustrated in FIG.  7 . Silicon dioxide layer  15  may have a thickness between 0.5 μm and 5.0 μm. The layer thickness of silicon dioxide layer  15  may be greater than the layer thickness of insulation layer  9 . Silicon dioxide layer  15  is structured by a subsequent photolithographic process. The structuring of silicon dioxide layer  15  may also occur by a dry etching process (plasma etching). Silicon dioxide layer  15  functions as a mask for the subsequent etching process for structuring thick polycrystalline silicon layer  13 . It also provides protection for metal layer  14  during subsequent etching. Then, dry etching (plasma etching) of thick polycrystalline silicon layer  13  is performed through openings  16  in silicon dioxide layer  15 , which functions as a mask, thus forming trenches  17 ,  18 . 
     The etching process stops on reaching third insulation layer  9  because it has a very high selectivity of silicon with respect to silicon dioxide. This yields exposed regions  19 ,  20  at the bottom of trenches  17 ,  18 . Trenches  17 ,  18  having a high aspect ratio, i.e., a large depth and a low lateral dimension, may be produced by the anisotropic etching process. Trenches  17 ,  18  extend from the surface of thick polycrystalline silicon layer  13  to third insulation layer  9 . Polycrystalline silicon layer  13  is thus subdivided into individual regions that are insulated from one another so that they are not linked together by buried polysilicon conductor regions  4 . 
     The functional structure is produced through trenches  17  which are located above sacrificial polysilicon layer regions  5 , or freely movable structures  25  (see FIG. 11) are produced after removal of underlying sacrificial polysilicon layer regions  5 . The connecting regions are defined and insulated by trenches  18 . 
     As illustrated in FIG. 8, a fourth insulation layer  21  protecting or passivating the side walls of trenches  22  with respect to the etching medium used in sacrificial layer etching is deposited. This fourth insulation layer  21 , which functions as a side wall passivation, may be produced from silicon dioxide deposited from the gas phase, e.g., by decomposition of silane. Since insulation layer  21  is required only on side walls  22  of trenches  17 ,  18 , it may also be produced by local thermal oxidation or by an oxide formed in the oxygen plasma. The layer thickness of insulation layer  21  may be between 5 nm and 500 nm. 
     To allow the etching medium to be introduced through trenches  17  to sacrificial polysilicon layer  5  to remove sacrificial polysilicon layer regions  5 , third and fourth insulation layers  9 ,  21  are removed at the bottom of trenches  19 ,  20 . This yields trenches  17  having exposed regions  23  of sacrificial polysilicon layer regions  5 . 
     FIG. 9 illustrates the result after removal of third and fourth insulation layers  9 ,  21  at the bottom of trenches  19 ,  20 . Insulation layers  9 ,  21  may be removed, for example, by a plasma etching process directed vertically. In this etching step, fourth insulation layer  21  is removed not only at the bottom of trenches  17 ,  18  but also at surface  24  of the structure illustrated in FIG.  8 . Fourth insulation layer  21  thus remains only on side walls  22  of trenches  17 ,  18 . 
     Silicon dioxide layer  15  is also partially removed in this etching process. Therefore, the silicon dioxide layer  15  may have a greater layer thickness than third insulation layer  9 . Since second insulation layer  6  is located between trenches  18  and buried polysilicon conductor regions  4 , no exposed regions to the buried polysilicon conductor regions  4  are obtained after removing third and fourth insulation layers  9 ,  21  at the bottom of trenches  18 . Therefore, buried polysilicon conductor regions  4  remain completely enclosed by insulation layer  9 . 
     After opening third and fourth insulation layers  9 ,  21  at the bottom of trenches  17 ,  18 , isotropic etching is performed to remove sacrificial polysilicon layer regions  5  illustrated in FIG.  10 . An etching medium such as xenon difluoride, chlorine trifluoride or bromine trifluoride is brought to sacrificial polysilicon layer regions  5  by introducing it through trenches  17 . These etching media have a very high selectivity with respect to a non-silicon such as silicon dioxide. 
     A cavity  26  having predefined lateral and vertical dimensions is produced by removing sacrificial polysilicon layer regions  5 , with freely movable structures  25  of the resulting sensor located above this cavity. Freely movable structures  25 , buried polysilicon conductor regions  4  and the other regions of thick polysilicon layer  13  are not etched by the etching media because they are protected by the oxide on all sides. 
     FIG. 11 illustrates the layer structure after removing fourth insulation layer  21  on the side walls of trenches  17 ,  18 , second insulation layer  9  on the lower side of freely movable structures  25  as well as silicon dioxide layer  15  by a vapor etching method using media containing hydrofluoric acid. First insulation layer  2  beneath freely movable structures  25  may also be removed completely if desired. 
     FIG. 11 thus is a cross-sectional view through an example embodiment of a sensor element. Various function regions have been structured from thick polysilicon layer  13 . A terminal region  27 ,  28 , completely surrounded by trenches  18 , has been structured out beneath metal layer  14 . These terminal regions  27 ,  28  are thus completely insulated by trenches  18  from the remainder of thick polysilicon layer  13 . Terminal region  27  is in direct contact with buried polysilicon conductor region  4 , so that contact may be established with the other regions of thick polysilicon layer  13 , namely the second neighboring region on the right side. Terminal region  28  is in direct contact with silicon substrate  1 , thus implementing a substrate contact. Freely movable structures  25 , e.g., parts of interdigital capacitors, are located above cavity  26 . 
     Although the present invention has been described above on the basis of example embodiments, the present invention is not limited to these example embodiments but is, instead, modifiable in a variety of manners. 
     In particular the choice of layer materials described above is only an example and may be varied as desired. The present invention is also not limited to acceleration sensors and rotational speed sensors.