Patent Document

FIELD OF THE INVENTION 
     The present invention relates to a micromechanical component and an appropriate manufacturing method. The present invention as well as the underlying problem are explained with respect to a micromechanical pressure sensor in the manufacturing technology of surface micromechanics, although they could theoretically be applied to any micromechanical structural components. 
     BACKGROUND INFORMATION 
     In known methods heretofore, there is a capacitative pressure sensor in surface micromechanical technologies (OMM) which uses a costly and tedious etching process for etching out sacrificial oxide, in order to prevent sticking of the diaphragm to the underlying cavity bottom (see also, T. Scheiter et al., Sensors and Actuators A 67 (1998), 211 -214). This etching process consists in a repetitive sequence of 10 sec etching intervals in HF gas and subsequent rinsing in nitrogen. 
     Piezoresistive pressure sensor elements in OMM technology with structured polycrystalline resistors have not been published up to now. In the known pressure sensor elements, the piezoresistive resistors are diffused into a monocrystalline silicon layer. 
     The known pressure sensors up to now are adapted to various pressure regions by varying the diaphragm size, since the thickness of the diaphragm is preselected by the particular process used. 
     SUMMARY OF THE INVENTION 
     The micromechanical component according to the present invention or the corresponding manufacturing method according to the present invention has the advantage compared to known attempts of a solution, that a simple design of a pressure sensitive micromechanical component having a membrane is created. Adaptation to different pressure regions can occur by changing a single process step, namely of the epitaxy thickness of the functional layer. Adaptation of the lithography masks, as with known methods, is not required. 
     One idea on which the present invention is based, is that, between the substrate and the functional layer a cavity is provided, which defines a diaphragm region of the functional layer, and below the diaphragm region on the substrate, one or a plurality of spacers are provided, to prevent adhesion of the diaphragm region to the substrate during deformation. 
     By using such expediently pyramid-shaped spacers in the cavity, the sticking problem during gas phase etching can be prevented. The spacers in the cavity even permit doing without costly gas phase etching processes for dissolving out the sacrificial layer from the cavity. During the etching process, the spacers prevent the diaphragm from being drawn to the bottom of the cavity by the surface tension of the water produced during etching, and sticking to it. Through this, the etching rate can be markedly increased, and thus the processing time reduced. This permits also arriving at the large lateral etching depths for this design in acceptable time. 
     According to a preferred further refinement, in the diaphragm region on the functional layer, and insulated by an insulating layer, polycrystalline, piezoresistive printed circuit traces are provided, made of semiconductor material. 
     According to another preferred improvement thereto, in the diaphragm region and/or the periphery of the diaphragm region, stoppered etching channels are provided for etching a sacrificial layer defining the cavity, the insulating layer in the region of the etching channel having corresponding holes whose sidewalls are covered by the material of the printed circuit trace. The insulating layer under the polycrystalline resistors is covered laterally by polycrystalline silicon in the region of the etching channels. Without this covering, the insulating layer under the polycrystalline resistors would be etched away too, during the etching away of the sacrificial layer of the cavity, whereby the resistors would lift off. 
     According to yet another preferred further refinement, the semiconductor material is silicon. 
     According to still another preferred further refinement, the sacrificial layer and the insulating layer are a first and second silicon dioxide layer. 
     A further underlying idea of the present invention is that the following steps are carried out for manufacturing a micromechanical component having a diaphragm, as for instance a pressure sensor: Preparation of a substrate from a semiconductor material; providing a sacrificial layer on the substrate; structuring the sacrificial layer so as to define a later-formed cavity having an overlying diaphragm region; epitactic provision of a functional layer made of the semiconductor material on the substrate having the structured sacrificial layer; providing an insulating layer on the functional layer; providing etching channels in the diaphragm region and/or in the periphery of the diaphragm region for etching the sacrificial layer; etching the sacrificial layer; sealing the etching channels; and providing one or more spacers to prevent sticking of the diaphragm region to the substrate caused by deformation below the diaphragm region onto the substrate. 
     During etching away the sacrificial layer of the cavity, the design according to the present invention requires great lateral etching depth. In order to reach acceptable processing time, a high etching rate is desirable. Subject to the process, this produces relatively much water. Without special measures being taken, this would cause the diaphragm to be drawn to the cavity bottom by surface tension. Because of the close touching of the two surfaces over a large area, strong cohesion forces would be created, which would prevent releasing of the diaphragm from the cavity bottom after evaporation of the water. The spacers proposed within the framework of the present invention prevent sticking of the diaphragm to the bottom of the cavity. The surface tension of the water can draw the diaphragm down only up to the point where it rests on the spacers. The area over which diaphragm and spacers touch is very small. The small cohesion forces resulting from this can be overcome by the inner tension of the diaphragm, i.e., the diaphragm snaps back after evaporation of the water. 
     The method delineated here makes possible relatively simple and cost-effective manufacturing, using existing OMM process steps. Using this design, a clear reduction in size of the sensor element is possible. A considerable advantage comes about because the sensor element is adapted to other pressure ranges merely by changing layer thickness. The epitaxy thickness essentially determines the thickness of the diaphragm, and thus, how much the diaphragm is bent by an applied pressure. A thicker diaphragm requires a higher pressure for attaining a certain amount of deformation, and thereby a certain output signal. Particularly, the sensor element is also suitable for higher pressures. 
     One design element represents the structuring forward of the sacrificial material. It creates an etching stop during etching out of the cavity sacrificial material. The lateral dimension of the cavity is defined by the sacrificial layer. That stops the etching process laterally, whereby the position of the diaphragm edges is exactly defined. The forward structuring of the sacrificial material permits, in addition, the definition of lateral etching channels outside the cavity. The channels speed up the etching out of the sacrificial material, because, in addition to the etching channels in the middle of the diaphragm, the sacrificial material is also etched out by the lateral channels. 
     According to a preferred further refinement, provision is made of polycrystalline, piezoresistive printed circuit traces made of the semiconductor material in the diaphragm region on the insulating layer. 
     According to a preferred further refinement, the etching channels are provided using the following steps: Forming of holes in the insulating layer; providing a layer made of the printed circuit trace material on the insulating layer having the holes; depositing a protective layer on the layer made of the printed circuit trace material; forming of holes in the protective layer within the holes; and transferring the holes into the functional layer to form the etching channels. 
     In keeping with another preferred further refinement, the protective layer, the insulating layer and the sacrificial layer will be made of the same material. 
     According to yet another preferred further refinement, the semiconductor material is silicon. Before providing the sacrificial layer, the following steps are executed: Providing a silicon nitride layer on the substrate; structuring the silicon nitride layer in such a way that spots of the silicon nitride layer remain in the cavity to be formed later; thermally oxidizing the substrate with the spots of the silicon nitride layer, so that, under the spots of the silicon nitride layer, spacers for preventing adhesion of the diaphragm region to the substrate, during deformation, are formed from non-oxidized substrate material; and removing the silicon nitride layer. Optionally, then, thermal oxidizing can still be performed, in order to increase the clearance between the tip of the pyramids and the upper edge of the oxide. 
     In accordance with still another preferred further development, after etching the sacrificial layer above the layer made of the printed circuit trace material, a sealing layer for sealing the etching channels is deposited, and structured in such a way that the etching channels are sealed by plugs made of the sealing layer. 
     According to another preferred further refinement, the printed circuit traces are structured from the layer made of the printed circuit trace material, after the sealing of the etching channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  shows a schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to a first specific embodiment of the present invention. 
     FIG. 1 b  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 c  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 d  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 e  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 f  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 g  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 h  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 i  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 j  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 k  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 l  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 1 m  shows another schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to the first specific embodiment of the present invention. 
     FIG. 2 shows a schematic cross-sectional illustration of a micromechanical component according to a second specific embodiment of the present invention, at the process stage corresponding to FIG. 1 m.    
     FIG. 3 shows a top view of the micromechanical component according to the first specific embodiment. 
     FIG. 4 shows a top view of the micromechanical component according to the second specific embodiment. 
    
    
     DETAILED DESCRIPTION 
     In the figures, the same reference numerals denote the same or functionally the same component parts. 
     FIGS. 1 a-m  shows a schematic cross-sectional illustration of the process steps for manufacturing a micromechanical component according to a first specific embodiment of the present invention. 
     With reference to FIG. 1 a , a substrate  10  in the form of a silicon wafer is first provided. On the entire area of the surface of substrate  10  silicon nitride  20  is deposited, and in a subsequent photolithography step it is structured so as to form silicon nitride spots  20 . 
     Following that, as illustrated in FIG. 1 b , thermal oxidation of the surface of substrate  10  is performed by the spots of silicon nitride. This causes pyramids of monocrystalline silicon to form under the spots of silicon nitride  20 , since, during this selective oxidation, oxygen diffuses into the silicon substrate  10 , but does not diffuse through the nitride spots  20 . And so, in the area of the spots of silicon nitride  20 , there is only a lateral underdiffusion, which leads to the formation of pyramids  40 . Following that, the nitride is removed. 
     The silicon dioxide  30 , thus formed, is later used as sacrificial layer for forming a hollow space or cavity under the diaphragm of the pressure sensor. 
     As shown in FIG. 1 c , the sacrificial oxide  30  is structured in a subsequent process step. The remaining portion of sacrificial oxide  30 , shown in FIG. 1 c , exactly defines the cavity of the pressure sensor. 
     According to FIG. 1 d , in a subsequent step silicon is grown epitactically over the entire surface, and covered with a second oxide layer  60 . The thickness of the grown silicon layer  50 , which is also denoted as functional layer, essentially determines the thickness of diaphragm region M of the pressure sensor. 
     With reference to FIG. 1 e , in a further process step, holes  70  in the second oxide layer  60  are then formed above the diaphragm region M. Subsequently, a polycrystalline silicon layer  50  is superposed on the resulting structure, and doped either in situ or later. In this polycrystalline silicon layer  80  the piezoelectric resistors of the pressure sensor are structured in the further course of the process, as is explained further down. 
     With reference to FIG. 1 f , a metallization of the entire area of the resulting structure is then performed, e.g. with aluminum, and in this metal layer contact pads  90  are formed, at the edge of the sensor element, for later connection to the piezoresistive resistors  400  to be structured from polycrystalline layer  80 . 
     In relation to FIG. 1 g , a protective layer  100  is deposited over the entire area of the resulting structure in a subsequent process step, which also consists, for example, of oxide or nitride. This is, thereafter, structured in such a way, that within holes  70  of second oxide layer  60  holes  70 ′ of this protective layer  100  are formed. The purpose of protective layer  100  is to protect the remaining surface during a subsequent etching process. 
     With reference to FIG. 1 h , in this subsequent etching process, which can be an anisotropic plasma etching process, the holes  70 ′ are transferred into the diaphragm region down to the sacrificial oxide. This produces etching channels  110  for the subsequent etching of the sacrificial layer of sacrificial oxide  30 , which at this moment still fills the cavity. 
     With reference to FIG. 1 i , in the next step the sacrificial oxide  30  is etched out, in order to form the hollow space, or rather the cavity. If protective layer  100  should also consist of silicon dioxide, then this protective layer will also be etched away during this etching step. Otherwise this upper protective layer  100  would have to be etched away by a separate etching process. Since doped polycrystalline silicon layer  80  covers, in holes  70  of second oxide layer  60 , the edges of oxide layer  60  lying below these, this second oxide layer  60  is protected during the etching of the sacrificial layer. During this etching step the pyramid-shaped spacers  40  are of great importance. For, an important problem in the field of surface micromechanics is this “sticking” during the etching out of sacrificial oxide  30 . Because, as it happens, during etching, drops of water form, which, based on their surface tension, draw together neighboring silicon regions, i.e. the silicon of diaphragm region M and of substrate  10 . Without any spacers  40 , diaphragm region  10  would be drawn to the diaphragm bottom. Even after drying of the water, diaphragm region M would adhere to the bottom on account of cohesion forces now acting over a large area. The pyramid-shaped spacers  40  formed here, prevent this sticking or adhesion of diaphragm region M to the bottom, since after drying of the water, diaphragm region M adheres exclusively to the pyramid tips of spacers  40 . The restoring force of diaphragm region M is here large enough to overcome the cohesion forces effective in the small contact area diaphragm/pyramid tip. 
     Furthermore, the etching boundary is stopped by the forward structuring of sacrificial oxide  30  in the lateral direction, as soon as it meets the interface sacrificial oxide/silicon, which here forms an etching stop AS. 
     With reference to FIG. 1 j , a sealing layer  120  is then deposited on the entire area of resulting structure. This sealing layer  120  can consist of oxide, nitride or another suitable material Simultaneously with this process step, the internal pressure or atmosphere of hollow space  300  is established 
     Furthermore, protective layer  120  is then structured in such a way, that in the central region of diaphragm region M, where etching channels  110  are arranged, a bulge is created. Of course, it would also be possible to provide an understructuring of this bulge in such a way that only individual etching channels  110  are plugged by individual plugs from sealing layer  120 . 
     With reference to FIG. 11, thereafter the position and shape of the piezoresistive resistors  400  are defined, by customary photolithographic structuring. 
     Finally, according to FIG. 1 m , the resulting structure is covered with a passivating layer  130 , which is opened in the region of the contact pads  90  by a terminating photolithographic structuring. 
     FIG. 2 shows a schematic cross-sectional illustration of a micromechanical component according to a second specific embodiment of the present invention, at the process stage corresponding to FIG. 1 m.    
     As illustrated in FIG. 2, additional etching channels  115  can also be formed in the edge region, or rather in the periphery of diaphragm region M, which then can be sealed by protective layer  120  with plugs  120 ″, analogously to bulge  120 ′ in the first specific embodiment. Of course, such etching channels  115  with plugs  120 ″ should have sufficient clearance from piezoresistive resistors  400  so as not influence their behavior. 
     FIG. 3 shows a top view of the micromechanical component according to the first specific embodiment. 
     FIG. 3 makes clear the planar design of the pressure sensor according to the first specific embodiment. Four piezoresistive resistors  400  are provided, which are each U-shaped. Two neighboring piezoresistive resistors  400  are connected with each other diagonally by a common contact pad  90  to form a Wheatstone bridge. The dotted line in FIG. 3 indicates diaphragm edge MK. In other words, the hollow space lies within the region surrounded by the dotted line. 
     FIG. 4 shows a top view of the micromechanical component according to the second specific embodiment. 
     The top view according to FIG. 4 corresponds to the second specific embodiment according to FIG. 2, in which additional etching channels  115  are provided in the periphery of diaphragm region M, and which are sealed by plugs of protective layer  120 ″. Other than that, the structures shown in FIG.  3  and FIG. 4 are the same. 
     Although the method of manufacture according to the present invention is described based on the aforementioned preferred exemplary embodiments, the method is not limited thereto, but can be modified in various ways. 
     Of course, it is possible to give the piezoresistive resistor elements any arbitrary form that departs from the U-shape. Etching channels  110  or  115  can also be arranged in a manner different from the illustrated form. The design of the connection of the piezoresistive resistor elements via contact pads  90  can, of course, also be varied at will. 
     As further variants, the pyramid-shaped spacers, the etching stop due to the forward structuring of the sacrificial oxide and the lateral etching channels can be used for a capacitative pressure sensor in OMM technology.

Technology Category: 7