Abstract:
A microelectromechanical systems (MEMS) device includes a structural layer having a top surface. The top surface includes surface regions that are generally parallel to one another but are offset relative to one another such that a stress concentration location is formed between them. Laterally propagating shallow surface cracks have a tendency to form in the structural layer, especially near the joints between the surface regions. A method entails fabricating the MEMS device and forming trenchesin the top surface of the structural layer of the MEMS device. The trenches act as a crack inhibition feature to largely prevent the formation of deep cracks in structural layer which might otherwise result in MEMS device failure.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to microelectromechanical systems (MEMS) devices. More specifically, the present invention relates to methodology for inhibiting the propagation of cracks in the surface of a MEMS device. 
       BACKGROUND OF THE INVENTION 
       [0002]    Microelectromechanical Systems (MEMS) devices are widely used in applications such as automotive, inertial guidance systems, household appliances, protection systems for a variety of devices, cellular communication devices, and many other industrial, scientific, and engineering systems. Some MEMS devices may be used to sense a physical condition such as acceleration, pressure, angular rotation, or temperature, and to provide an electrical signal representative of the sensed physical condition to the applications and/or systems employing the MEMS sensors. Other MEMS devices may be utilized as actuators, switches, pumps, and so forth. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the 
           [0004]    Figures (not necessarily drawn to scale), wherein like reference numbers refer to similar items throughout the Figures, and: 
           [0005]      FIG. 1  shows a side view of a portion of a MEMS device; 
           [0006]      FIG. 2  shows an enlarged top view of a portion of the MEMS device; 
           [0007]      FIG. 3  shows a flowchart of a fabrication process for fabricating a MEMS device incorporating features for inhibiting the propagation of surface cracks in the MEMS device in accordance with an embodiment; 
           [0008]      FIG. 4  shows a partial cross-sectional view of a structure at an initial stage of manufacture for producing a MEMS device having features incorporated therein for inhibiting the propagation of surface cracks; 
           [0009]      FIG. 5  shows a partial cross-sectional view of the structure of  FIG. 4  at a subsequent stage of processing; 
           [0010]      FIG. 6  shows a partial cross-sectional view of the structure of  FIG. 5  at a subsequent stage of processing; 
           [0011]      FIG. 7  shows a partial cross-sectional view of the structure of  FIG. 6  at a subsequent stage of processing; 
           [0012]      FIG. 8  shows a partial cross-sectional view of the structure of  FIG. 7  at a subsequent stage of processing; 
           [0013]      FIG. 9  shows a partial cross-sectional view of the structure of  FIG. 8  at a subsequent stage of processing; 
           [0014]      FIG. 10  shows a partial cross-sectional view of a MEMS device fabricated from the structure of  FIGS. 4-9  having features formed therein that inhibit the propagation of cracks in the surface of the MEMS device; and 
           [0015]      FIG. 11  shows an enlarged partial top view of the MEMS device of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Numerous MEMS devices have been developed which use polycrystalline silicon (polysilicon) as a primary structural material. It has been observed, however, that polysilicon can crack during MEMS device fabrication, as well as under potentially severe mechanical and environmental loading conditions. The cracks tend to form at the polysilicon surface and propagate a relatively long distance laterally across the surface of the polysilicon before propagating deeply into the polysilicon material, resulting in MEMS device failure. 
         [0017]    Referring to  FIGS. 1 and 2 ,  FIG. 1  shows a side view of a portion of an exemplary MEMS device  20 , and  FIG. 2  shows an enlarged top view of a portion of MEMS device  20 . In this example, MEMS device  20  may be formed by depositing, patterning, and etching a series of layers onto an underlying substrate  22 . For example, MEMS device  20  may include electrodes  24  formed on substrate  22 , a layer of nitride film  26 , and a polysilicon structural layer  28 , each of which are suitably patterned and etched to yield MEMS device  20 . In some devices, polysilicon structural layer  28  may be suspended above, i.e., separated by an air or vacuum gap from, the underlying electrodes  24  and/or nitride film  26 . In such a design, polysilicon structural layer  28  interconnects to the underlying material layers at anchor regions  30  or by a system of anchors and springs (not shown). In other devices, additional material layers may be formed on top of polysilicon structural layer  28 . 
         [0018]    Due to the conventional surface micromachining processes, of deposition, patterning, and etching, a top surface  32  of polysilicon structural layer  28  may not lie in one plane. That is, top surface  32  may not be flat or planar. For example, a step-down  34  may occur at the intersection of a surface region  36  and a surface region  38  of top surface  32  near anchor regions  30 . Step-down  34  represents an inconsistency, i.e., a change in planarity of top surface  32 . As such, surface region  36  may generally lie in a plane  40  and surface region  38  may generally lie generally in a plane  42  that is parallel to, but is offset from, plane  40 . 
         [0019]    It has been observed that surface cracks  44  (one shown in  FIG. 2 ) tend to initiate at top surface  32  of polysilicon structural layer  28  near anchor regions  30 . Unfortunately, these surface cracks  44  can propagate toward the suspended structures within polysilicon structural layer  28  of MEMS device  20 . Surface cracks  44  may tend to initiate at anchor regions  30  because the nonplanar surface topology of top surface  32  creates a tensile stress concentration point. In general, surface cracks  44  can start when the tensile stress exceeds the material strength. In addition, the grain structure of polysilicon structural layer  28  may not be cohesive around the anchor regions  30 . The merging of two different grain orientations of polysilicon can result in a seam at the anchor edge, i.e., at step-downs  34 , that may be weaker than the rest of polysilicon structural layer  28 . 
         [0020]    Unfortunately, polysilicon cracking can result in a yield loss. In some examples, the yield loss can run around approximately five percent. Tests may be run on MEMS devices to detect cracks in the polysilicon. While testing might be viewed as a practical solution to reliability assurance of MEMS devices, the cost for developing effective and reliable testing remains high, which ultimately increases the cost for the MEMS devices. Furthermore, the tests may not be one hundred percent effective at detecting polysilicon cracks. Thus, some MEMS devices can still get shipped into the field, resulting in customer returns of the cracked units. 
         [0021]    Embodiments described herein entail methodology for fabricating a MEMS device by incorporating features for inhibiting the propagation of surface cracks in the MEMS device, and a MEMS device that includes features that inhibit the propagation of cracks in its polysilicon surface. Such methodology and MEMS device structure can result in reduced tests costs, MEMS device yield improvement, and improved quality and reliability of the MEMS devices in the field. 
         [0022]      FIG. 3  shows a flowchart of a fabrication process  50  for fabricating a MEMS device incorporating features for inhibiting the propagation of surface cracks in the MEMS device in accordance with an embodiment. The various method steps depicted in  FIG. 3  will be described in more detail below in connection with  FIGS. 4-11 . Accordingly, the following discussion of fabrication process  50  should be considered a summary of the method, and the various embodiment details discussed below in connection with  FIGS. 4-11  apply to the discussion of the method steps of  FIG. 3 . 
         [0023]    In general, the method begins, at a task  52 , at which a MEMS device is fabricated. The MEMS device may be MEMS device  20  mentioned above, or any other MEMS device design in which surface cracking of a polysilicon structural layer may occur. Next, at a task  54 , trenches are formed in the surface of the MEMS device.  FIGS. 4-9  generally show schematic cross-sectional views illustrating a number of MEMS operations encompassed within the summary task  52  of fabrication process  50 , and  FIGS. 10-11  generally show schematic cross-sectional views illustrating trenches  56  (see  FIGS. 10-11 ) formed in a MEMS device  58  (see  FIGS. 10-11 ) in accordance with the operations encompassed within the summary task  54  of fabrication process  50 . 
         [0024]    The trenches may be formed by implementing a suitable manufacturing process, such as by etching, although the particular trench formation manufacturing process is not a limitation. The trenches are formed anywhere in the surface of the polysilicon structural layer at stress concentration points at which cracks may likely initiate in the future, such as at the location of step-downs  34  ( FIG. 1 ) or step-ups. The trenches may allow small, i.e., relatively short, surface cracks to form. However, suitably arranged trenches largely prevent the small surface cracks from propagating laterally far enough to cause MEMS device failure. Following the formation of the trenches at task  54 , MEMS device fabrication process  50  ends. Of course, there may be additional operations prior to the completion of MEMS device fabrication process  50  that are omitted herein for brevity. 
         [0025]    Referring now to  FIG. 4 ,  FIG. 4  shows a partial cross-sectional view of a structure  60  at an initial stage  62  of manufacture for producing a MEMS device, e.g., MEMS device  58  ( FIG. 10 ), having features incorporated therein for inhibiting the propagation of surface cracks  44  ( FIG. 2 ). Different shading and/or hatching is utilized in the illustrations to distinguish the different material layers and/or the different elements within the structural layers. 
         [0026]    In general, at initial stage  62 , a polysilicon layer  64  is deposited on a substrate  66  and patterned using, for example, a photolithographic process, and etched using, for example, reactive ion etching (RIE). A high conductivity may be desired for polysilicon layer  64  in some embodiments. Hence, polysilicon layer  64  may be doped over the entire surface area, or may otherwise be made highly conductive to yield, for example, electrodes (such as electrodes  24  shown in  FIG. 1 ). Those skilled in the art will readily recognize that prior to initial stage  62  at which polysilicon layer  60  is formed over substrate  66 , various surface preparation operations may be performed that are omitted herein for brevity. 
         [0027]      FIG. 5  shows a partial cross-sectional view of structure  60  at a subsequent stage  68  of processing. At stage  68 , nitride  70  is deposited over polysilicon layer  64  as well as any exposed portions of substrate  66 . Nitride  70  may be patterned using, for example, a photolithographic process, and etched using, for example, RIE, to produce a patterned nitride layer. Nitride  70  insulates various regions of polysilicon layer  64  from one another. 
         [0028]      FIG. 6  shows a partial cross-sectional view of structure  60  at a subsequent stage  72  of processing. At stage  72 , a sacrificial oxide  74  is deposited over substrate  66 , polysilicon layer  64 , and nitride  70 . Sacrificial oxide  74  may then be patterned using, for example, a photolithographic process, and etched using, for example, an oxide wet etch process. 
         [0029]      FIG. 7  shows a partial cross-sectional view of structure  60  at a subsequent stage  76  of processing. At stage  76 , thick polysilicon deposition is performed. As shown, a polysilicon structural layer  78  is formed overlying the various structures previously built up on substrate  66 . Polysilicon structural layer  78 , including any openings extending through layer  78 , may be suitably formed using various processes for thick film deposition, patterning, and etching. 
         [0030]      FIG. 8  shows a partial cross-sectional view of structure  60  at a subsequent stage  80  of processing. At stage  80 , etching is performed to remove sacrificial layer  74 , which was illustrated in  FIGS. 6 and 7 , but is no longer visible in  FIG. 8 . Following removal of sacrificial layer  74 , the microstructures of MEMS device  58  ( FIG. 10 ) are released and are spaced apart from the underlying substrate  66 , polysilicon layer  64 , and nitride  70 . Accordingly, any movable structures (not expressly labeled) of MEMS device  58  are now movably suspended in accordance with a particular design of MEMS device  58 . 
         [0031]      FIG. 9  shows a partial cross-sectional view of structure  60  at a subsequent stage  82  of processing. At stage  82 , a sealing material  84 , e.g., a packaging material, may be deposited over portions of structure  60  to seal otherwise protect at least portions of structure  60  from the external environment. However, remaining portions of structure  60  may not be encapsulated in sealing material  84  when the movable microstructures are to be exposed to an external environment in order to detect an external stimulus (e.g., pressure). 
         [0032]      FIGS. 4-9  are discussed in connection with surface micromachining operations implemented to form a MEMS device. It is to be understood that certain operations depicted in  FIGS. 4-9  may be performed in parallel with each other or with performing other processes, or may be omitted in accordance with particular MEMS device fabrication methodologies. Furthermore, the particular deposition, patterning, and etching techniques mentioned herein are not a limitation. Rather, any suitable technique for deposition, patterning, etching, and so forth may be implemented in accordance with alternative embodiments. In addition, it is to be understood that the particular ordering of the operations depicted in connection with  FIGS. 4-9  may be modified, while achieving substantially the same result. 
         [0033]    Referring now to  FIGS. 10-11 ,  FIG. 10  shows a partial cross-sectional view of MEMS device  58  fabricated from structure  60  having features formed therein that inhibit the propagation of cracks in a top surface  86  of MEMS device  58 , and  FIG. 11  shows an enlarged partial top view of MEMS device  58 . In accordance with block  54  ( FIG. 3 ) of fabrication process  50  ( FIG. 3 ), trenches  56  are formed in a top surface  86  of MEMS device  58 . In  FIG. 10 , trenches  56  are represented in dashed line form extending into top surface  86 . In  FIG. 11 , trenches  56  are illustrated using rightwardly and upwardly wide hatching to distinguish them from the surrounding top surface  86  of polysilicon structural layer  78 . 
         [0034]    Due to the surface micromachining processes, of deposition, patterning, and etching as shown above, top surface  86  of polysilicon structural layer  78  may not lie in one plane. That is, top surface  86  may not be flat or planar. In this example, a step-down  90  may occur at the intersection of a surface region  92  and a surface region  94  of top surface  86  near anchor regions  96  (one being visible in  FIG. 10 ). As such, surface region  92  lies generally in a plane  98  and surface region  94  lies generally in a plane  100  that is parallel to, but is offset from, plane  98 . In the illustrated embodiment, surface region  94  extends upwardly from surface region  92 . The intersection of surface regions  92  and  94  is a tensile stress concentration location where polysilicon structural layer  78  may crack during fabrication or in the future. Of course, other step-down regions and/or other tensile stress concentration location may be present in top surface  86 . 
         [0035]    Trenches  56  are formed in top surface  86  of polysilicon structural layer  78 . In particular, trenches  56  are formed across one or more longitudinal joints  102  between surface regions  92  and  94 , and extend into each of surface regions  92  and  94 . Trenches  56  may be formed by etching, saw cutting, or any other suitable technique. In an embodiment, a length  104  of each of trenches  56  is oriented approximately perpendicular to the longitudinal joint  102  across which it spans. Thus, trenches  56  spanning the same one of longitudinal joints  102  are parallel to one another. In some embodiments, length  104  of each of trenches  56  is in a range of three to seven microns, and a width  106  of each of trenches  56  is in a range of three to five microns into top surface  86 . A spacing  108  between adjacent trenches  56  may also be in a range of three to five microns. In an embodiment, width  106  and spacing  108  are as small as can be manufactured. 
         [0036]    In general, shallow surface cracks  44  ( FIG. 2 ) have been observed to be less than 0.25 microns deep. Accordingly, trenches  56  are formed to extend into top surface  86  by a depth  110  in a range of 0.25-0.75 microns. In this example embodiment, trenches  56  may extend into surface region  92  by approximately  0 . 5  microns and trenches  56  may extend into surface region  94  by 0.5 microns plus the height of surface region  94  above surface region  92  (e.g., 0.25 microns). In some embodiments, depth  110  can be any suitable amount such that a bottom surface  112  of trench  56  extending across longitudinal joint  102  and into surface regions  92  and  94  may be generally flat, i.e., is without step-downs or step-ups. In other embodiments, bottom surface  112  of trench  56  may be uneven as a result of a particular etch process used, e.g., an isotropic etch process. 
         [0037]    In some embodiments, one of longitudinal joints  102  is continuous with another one of longitudinal joints  102 , but they are oriented out of alignment with one another, i.e., nonparallel relative to one another. Such a configuration can occur at certain corners  114  of surface regions  92  and  94  of top surface  86 . Accordingly, a subset  116  of trenches  56  may extend across one of longitudinal joints  102 , e.g., a longitudinal joint  102 A, oriented perpendicular to that longitudinal joint  102 A, and another subset  118  of trenches  56  may extend across the other longitudinal joint  102 , oriented perpendicular to that longitudinal joint  102 . As such, trenches  56  extending across longitudinal joints  102  that are out of alignment (nonparallel) relative to one another will likewise be out of alignment (nonparallel) relative to one another. In some embodiments, these nonparallel trenches  56  in close proximity with one another may intersect to form a contiguous, i.e., single, extended trench  120  having a portion that is perpendicular to the longitudinal joint  102  that it spans and having another portion that is perpendicular to the longitudinal joint  102  that it spans. 
         [0038]    The multiple trenches  56  are formed anywhere in top surface  86  of polysilicon structural layer  78  where surface cracks  44  ( FIG. 2 ) may be likely to form in top surface  86  of polysilicon structural layer  78 . In some embodiments, the surface of trenches  56  may be filled with a packaging material, i.e., an encapsulant. Nevertheless, when trenches  56  are located near anchor regions  96 , small (relatively short), shallow surface cracks  44  may be allowed to form. However, the presence of trenches  56  prohibits the short surface cracks  44  from propagating laterally where they could go deep enough to breach the suspended structures and cause MEMS device failure. Thus, trenches  56  can limit the length of the propagation path of shallow surface cracks  44 . 
         [0039]    Embodiments described herein entail methodology for fabricating a MEMS device by incorporating trench features that can inhibit the propagation of surface cracks in the MEMS device, and a MEMS device that includes the trench features. Although particular trench configurations are described above, MEMS devices may include trench features having other shapes, depths, orientations, locations, and so forth. These and other variations are intended to be included within the scope of the inventive subject matter. Such methodology and MEMS device having trenches formed therein can result in reduced tests costs, MEMS device yield improvement, and improved quality and reliability of the MEMS devices in the field. 
         [0040]    While the principles of the inventive subject matter have been illustrated and described above in connection with specific devices and methods, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the inventive subject matter. The foregoing description of specific embodiments reveals the general nature of the subject matter sufficiently so that others can, by applying current knowledge, readily modify and/or adapt it for various applications without departing from the general concept. Therefore, such adaptations and modifications are within the meaning and range of equivalents of the disclosed embodiments. The inventive subject matter embraces all such alternatives, modifications, equivalents, and variations as fall within the spirit and scope of the appended claims.