This application claims the benefit of U.S. Provisional Application No. 61/475,432, filed on Apr. 14, 2011. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to membrane-based devices such as micromechanical electrical system (MEMS) pressure sensors devices or semiconductor devices incorporating a membrane. 
     BACKGROUND 
     The manufacture of micromechanical electrical system (MEMS) such as pressure sensors and other devices incorporating a membrane poses serious challenges because of the sensitivity of the devices. Typically, the devices which are made of silicon, (polysilicon or silicon-germanium) must exhibit low stress values or predetermined stress values along with low or specific stress gradient properties. Stress reduction is accordingly typically achieved during a stress/stress-gradient relief step during a high temperature annealing process. 
     MEMS devices however, can be very complicated devices with a number of mechanical parts that are integrated with other permanent and/or temporary (sacrificial) materials. The integrated parts may exhibit detrimental interactions due to the thermal budget. Thus, subsequent annealing steps could affect the layers previously deposited/annealed and therefore modify the film stress and stress gradient values of the devices. Thus, the timing and manner in which stress relief is accomplished must be carefully planned. This adds complexity and costs to the manufacturing process. 
     Various attempts have been made to control stress in the prior art. Some of those attempts include development of specialized films. While effective at reducing stress, these films suffer various shortcomings such as lack of conductivity, roughness, and irregular electrical properties. Other approaches include the use of doping or specific atmosphere control while depositing films. These approaches affect the chemical composition of the films. 
     What is needed, therefore, is a simple and effective approach to modification of stress characteristics within a membrane. A further need exists for an approach to modification of stress characteristics within a membrane that does not alter the chemical composition membrane. 
     SUMMARY 
     A method of modifying stress characteristics of a membrane in one embodiment includes providing a membrane layer, determining a desired stress modification, and forming at least one trough in the membrane layer based upon the determined desired stress modification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a side cross sectional view of a MEMS device with a released membrane wherein stress characteristics have been modified by planned incorporation of a mixture of partial and full depth stress modifying troughs in accordance with principles of the invention; 
         FIG. 2  depicts a top plan view of the MEMS device of  FIG. 1  showing overlapping troughs used to substantially completely isolate the released membrane from stress generated in the membrane layer outside of the released membrane area; 
         FIG. 3  depicts a top plan view of a MEMS device including overlapping troughs wherein some of the troughs are located within the released membrane area so as to modify both the stiffness of the released membrane and the stress characteristics of the released membrane; 
         FIG. 4  depicts a top plan view of a MEMS device with troughs extending about the corners of the released membrane, wherein the troughs of  FIG. 4  are significantly wider than the troughs of  FIG. 3 ; 
         FIG. 5  depicts the modeled results of the application of a pressure to a released membrane incorporating spaced apart troughs along one edge of the membrane resulting in a focusing of stress in the area between the space apart troughs resulting in increased stress levels over a smaller area as compared to edges without troughs; 
         FIG. 6  depicts a perspective view of a MEMS device with a released membrane incorporating spaced apart troughs along each edge of the membrane resulting in a focusing of stress in the areas between the spaced apart troughs which are occupied by a respective piezoresistor; 
         FIG. 7  depicts a side cross sectional view of a device with a released membrane including troughs which extend upwardly into the membrane layer both in the released membrane portion and the unreleased portion of the membrane layer so as to modify both the stiffness of the released membrane as well as the stress characteristics of the membrane layer; 
         FIG. 8  depicts a side cross sectional view of the substrate of  FIG. 8  with sacrificial ridges provided on the spacer layer prior to deposition of the membrane layer onto the spacer layer; 
         FIG. 9  depicts a side cross sectional view of the substrate of  FIG. 8  with sacrificial ridges provided on the spacer layer after deposition of the membrane layer onto the spacer layer; and 
         FIGS. 10-13  depict various stages in the manufacture of a device incorporating upwardly extending troughs within a bond ring. 
     
    
    
     DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
       FIGS. 1 and 2  depict a MEMS device  100  which may be, for example, a pressure detector. The MEMS device  100  includes a substrate layer  102  and a membrane layer  104  which is spaced apart from the substrate layer  102  by a spacer layer  106 . The membrane layer  104  may be a silicon layer and the spacer layer  106  may be an oxide layer. 
     The membrane layer  104  has a released membrane portion  108 . Stress within the membrane portion  108  is isolated by positioning of full stress troughs  110  and partial stress troughs  112  about the membrane portion  108 . In the embodiment of  FIGS. 1 and 2 , the partial troughs  112  overlap a non-troughed area  114  located between the opposing end portions of the full stress troughs  110 . Because the partial stress troughs  112  do not extend completely through the membrane layer  104 , the structural integrity of the membrane layer  104  is greater in the area about the partial stress troughs  112  as compared to the structural integrity of the membrane layer  104  in the area about the full stress troughs  110 . The stress relief, however, is not as great. 
     In the embodiment of  FIGS. 1 and 2 , all of the stress relief troughs  10  and  112  are located outside of the released membrane  108 . Thus, the released membrane  108  is fully supported by an overlying portion  118  that is positioned on an upper surface of the spacer layer  106 . Accordingly, the stiffness of the released membrane  108  is primarily dictated by the thickness and material of the released membrane  108 , although the width and proximity of the full stress troughs  110  will provide some reduction in the stiffness of the released membrane  108 . 
       FIG. 3  depicts an embodiment of a MEMS device  130  that provides increased stiffness reduction. The MEMS device  130  includes a plurality of troughs  132  and  134 . The troughs  132 , which may be full or partial troughs depending upon the desired strength and stress modification, which are located adjacent to a released membrane  136 . The troughs  132  will thus have a significant effect on stress modification, but a lesser effect on the stiffness of the membrane  136 . The troughs  134 , however, are located within the outer perimeter of the released membrane  136 . Accordingly, while the combination of the troughs  136  and  134  provide a significant isolation of the released membrane  136  from stresses originating outside of the released membrane  136 , the troughs  134  also significantly reduce the stiffness of the membrane  136 . 
     Accordingly, troughs can be used not only to reduce stress, but also to modify the stiffness of the membrane. By planning the orientation, depth, and location of the troughs, both stress characteristics and stiffness characteristics of a MEMS device can be optimized for a particular application. 
       FIG. 4  depicts a MEMS device  140  that includes troughs  142  and a released membrane  144 . The troughs  142  are significantly wider than the troughs in the embodiments of  FIGS. 1-3 . The troughs  142 , however, are located only at the corners of the membrane  144 . Thus, while the stiffness of the membrane  144  is not significantly reduced, stress patterns will be focused by the troughs  142 . Stress focusing is shown, for a different embodiment, in the stress simulation results depicted in  FIG. 5 . 
       FIG. 5  depicts a stress simulation performed on a porous silicon diaphragm  150 . The diaphragm  150  is 12 μm thick and includes two 6 μm troughs  152  and  154 . For the depicted simulation, a 100 kPa force was applied at location  156 , which is the center of the porous silicon diaphragm  150 . 
     The resulting stress pattern included a region of high stress (0.884E+08 kPa) in the area  158  immediately around the applied force. Stress was focused as a result of the support of the porous silicon diaphragm  150  at the edges  160 ,  162 , and  164  even without any troughs. The stress at the edges  160 ,  162 , and  164  reached 0.118E+09 kPa. 
     Stress was also focused at the remaining edge  166 . The stress pattern at the edge  166  is modified, however, by the troughs  152  and  154 . The stress is concentrated over a smaller area, resulting in a string of stress areas  168  that reach 0.147E+09 kPa. Thus, the troughs  152  and  154  provide stress/strain focusing at predetermined sites. By positioning a piezoresistor at the predetermined site, larger variations in piezoresistor output may be obtained for a given applied pressure. Of course, stress modification may be used in a variety of sensor types in addition to those incorporating piezoresistors including, for example, capacitive sensors. 
     The stress modification pattern affected by the troughs  152  and  154  thus show that precise geometry of corrugations (width, depth, shape, etc.) can be used to fine-tune the effect of the troughs. The embodiment of  FIG. 6  utilizes the basic arrangement of the troughs  152  and  154  of  FIG. 5  in order to maximize sensitivity of a device to a deflection of a membrane. In  FIG. 6 , a MEMS device  170  includes a released membrane  172 . Each edge of the released membrane  172  includes spaced apart trough groups  174 . A piezoresistor  176  is positioned in the area between the spaced apart trough groups  174 . 
     As is evident from  FIG. 5 , spaced apart troughs  152  and  154  focus stress in the area between the spaced apart troughs  152 / 154 . Likewise, the spaced apart trough groups  174  focus stress in the area occupied by the piezoresistors  178 . Thus, any stress in the membrane  172 , whether as a result of applied force or differential pressure across the membrane  172 , is focused by the spaced apart troughs  152  and  154  into the areas occupied by the piezoresistors  178 . If desired, more or fewer groupings of spaced apart troughs may be provided. 
     In the foregoing embodiments, the partial troughs are depicted as extending downwardly from an upper surface of the devices. If desired, troughs may also be formed which extend upwardly from a lower surface of a membrane layer. For example,  FIG. 6  depicts a MEMS device  180  which includes a substrate layer  182  and a membrane layer  184  which is spaced apart from the substrate layer  182  by a spacer layer  186 . The membrane layer  184  has a released membrane portion  188 . 
     Stress within the membrane portion  108  is modified by troughs  190  and  192 . The troughs  190  are positioned within the released membrane  188 . Thus, the troughs  190  also modify the stiffness of the released membrane  188 . The troughs  190  and  192  may be formed in a number of different approaches. For example, the troughs  190  and  192  may be etched into the membrane layer  184 , and the membrane layer  184  may then be bonded to the spacer layer  186 . 
     Alternatively, sacrificial ridges  194  and  196  may be formed on the spacer layer  106  prior to formation of the membrane layer  184  as depicted in  FIG. 7 . After deposition of the membrane layer  184  (see  FIG. 8 ), the sacrificial ridges  194  and  196  may then be etched. The sacrificial ridges  194  may be etched concurrent with the release of the membrane  188 . The sacrificial ridges  196  may be etched separately or at the same time as the membrane release using an etch stop positioned between the sacrificial ridges  196  and the spacer layer  186 . 
     Devices incorporating bond rings may also be provided with stress modification troughs. One approach to manufacturing such a device is discussed below with reference to  FIGS. 9-12 . In  FIG. 9 , a device  200  includes a substrate layer  202 , a spacer layer  204 , and a device layer  206 . The device layer  206  includes a released membrane  208 . A bond ring  210  is located on the lower surface of the substrate layer  202 . The bond ring  10  may be formed by soldering, eutectic, or any other approach useful in bonding one substrate to another substrate. 
     In this embodiment, two different depths are desired for stress modification troughs. Accordingly, in a first etching process, stress modification troughs  212  are etched at locations within the bond ring as depicted in  FIG. 10 . During an ensuing etching step, additional troughs  214  are etched ( FIG. 11 ). During the second etching step, additional material is etched out of the troughs  212 . If desired, troughs of different widths may be provided. The incorporation of troughs having different depths allows for increased substrate strength beneath the released membrane  208  while still providing stress modification. 
     When the desired troughs have been formed, the bond ring  210  is used to bond the substrate layer  202  to a base substrate layer  216 . If desired, the base substrate layer  216  may be, for example, a cap layer of another MEMS device. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.
A method of modifying stress characteristics of a membrane in one embodiment includes providing a membrane layer, determining a desired stress modification, and forming at least one trough in the membrane layer based upon the determined desired stress modification.