Abstract:
In one embodiment, a method of forming an insulating spacer includes providing a base layer, providing an intermediate layer above an upper surface of the base layer, etching a first trench in the intermediate layer, depositing a first insulating material portion within the first trench, depositing a second insulating material portion above an upper surface of the intermediate layer, forming an upper layer above an upper surface of the second insulating material portion, etching a second trench in the upper layer, and depositing a third insulating material portion within the second trench and on the upper surface of the second insulating material portion.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/475,438, filed on Apr. 14, 2011. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to wafers and substrates such as are used in micromechanical electrical system (MEMS) devices or semiconductor devices. 
       BACKGROUND 
       [0003]    Device isolation typically is achieved by utilizing local oxidation of silicon (“LOCOS”) or shallow trench isolation (“STI”) techniques. In the STI device isolation technique, isolation is typically achieved by forming a recess or trench in a layer that is destined to become two adjacent active areas, and filling the trench with an isolation material. The material in the trench, typically a nitride material, is referred to as a spacer. Nitride spacers, in addition to electrical isolation, may also be used as a fluid barrier. 
         [0004]    STI is beneficial in providing higher packing density, improved isolation, and greater planarity, by avoiding the topographical irregularities encountered when using conventional thick film oxide isolation (LOCOS). In particular, the growth of thermal field oxide using a mask, such as nitride, creates an encroachment of the oxide into the active areas; this encroachment is referred to as the bird&#39;s beak effect. 
         [0005]    High aspect ratio trenches, while theoretically desirable in reducing the footprint of a nitride spacer, present various technical problems. One significant problem is that when depositing nitride to fill a high aspect ratio trench, a vertical seam inherently occurs along the center of the trench, where the outer surfaces of deposited nitride layers on opposing vertical trench walls meet. The vertical seam generally includes gaps in which no nitride material is present. While the gaps may not degrade the electrical isolation function of a nitride spacer, the gaps are problematic in other respects. For example, in fluid barrier applications, the gaps are essentially a short circuit in the isolation capability of the nitride spacer. Moreover, the gaps are flaws which can reduce the material strength of the nitride spacer. 
         [0006]    What is needed therefor is a spacer and method of forming a spacer that overcomes one or more problems in known spacers. It would be beneficial if the spacer and method of forming a spacer could include high aspect ratio trench forming processes while providing increased spacer strength. It would be further beneficial if the spacer and method of forming a spacer could include high aspect ratio trench forming processes while providing improved spacer isolation characteristics. 
       SUMMARY 
       [0007]    In one embodiment, a method of forming an insulating spacer includes providing a base layer, providing an intermediate layer above an upper surface of the base layer, etching a first trench in the intermediate layer, depositing a first insulating material portion within the first trench, depositing a second insulating material portion above an upper surface of the intermediate layer, forming an upper layer above an upper surface of the second insulating material portion, etching a second trench in the upper layer, and depositing a third insulating material portion within the second trench and on the upper surface of the second insulating material portion. 
         [0008]    In a further embodiment, a wafer includes a base layer, a first layer portion above an upper surface of the base layer, a first trench in the first layer portion, a first insulating material portion within the first trench, a second insulating material portion extending horizontally above an upper surface of the first layer portion and connected to the first insulating material portion, a second layer portion above an upper surface of the second insulating material portion, a second trench in the second layer portion, and a third insulating material portion within the second trench and on the upper surface of the second insulating material portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  depicts a partial side cross sectional view of a nitride spacer between adjacent segments of a layer, the nitride spacer having a laterally extending portion at a location between two trench portions in accordance with principles of the invention; 
           [0010]      FIG. 2  is a surface electron microscope (SEM) microphotograph of a nitride spacer between adjacent segments of a layer, the nitride spacer having a laterally extending portion at a location between two trench portions in accordance with principles of the invention; 
           [0011]      FIGS. 3-10  depict a procedure which can be used to form a nitride spacer between adjacent segments of a layer, the nitride spacer having a laterally extending portion at a location between two trench portions; 
           [0012]      FIG. 11  depicts a side cross sectional view of a nitride spacer between adjacent segments of a layer, the nitride spacer having a laterally extending portion at a location between two trench portions, each of the trench portions in a different type of material; 
           [0013]      FIG. 12  depicts a side cross sectional view of a nitride spacer between adjacent segments of a layer, the nitride spacer having two laterally extending portions separated by a middle trench portion with additional trench portions extending above and below a respective one of the two laterally extending portions; 
           [0014]      FIG. 13  depicts a side cross sectional view of a nitride spacer between adjacent segments of a layer, the nitride spacer having a laterally extending portion at a location between two trench portions, and two hook portions extending downwardly from the outer end portions of the laterally extending portion; 
           [0015]      FIG. 14  depicts a side cross sectional view of a released nitride spacer between adjacent segments of a layer, the nitride spacer having a laterally extending portion at a location between two trench portions, and two hook portions extending downwardly from the outer end portions of the laterally extending portion; and 
           [0016]      FIG. 15  depicts a side cross sectional view of a nitride spacer between adjacent segments of a layer, the nitride spacer having a laterally extending portion at a location between two trench portions, the two trench portions axially offset from each other. 
       
    
    
     DESCRIPTION 
       [0017]    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. 
         [0018]      FIG. 1  depicts a wafer  100  which includes a substrate layer  102 , a device layer  104 , and a cap layer  106 . The device layer  104  may comprise silicon or another structural material. A spacer  108 , which in this embodiment is a nitride material, divides the device layer  104  into two adjacent layer segments  110 / 112 . The spacer  108  may be used to electrically isolate the adjacent layer segments  110 / 112  from each other. The spacer  108  may additionally or alternatively be used to isolate the adjacent layer segments  110 / 112  such that gasses cannot diffuse from one of the layer segments  110 / 112  to the other of the layer segments  110 / 112 . 
         [0019]    The spacer  108  includes two trench portions  116  and  118  which are axially aligned with an axis  120 . Two laterally extending portions  122  and  124  extend outwardly away from the axis  120 . Located generally along the axis  120  are various gaps  126 . 
         [0020]    The gaps  126  are void areas in the nitride spacer  108  that result from the process used to form the spacer  108 . This phenomenon is visible in  FIG. 2 , which is a SEM photomicrograph of a wafer  140 . The wafer  140  is similar to the wafer  100 , including a substrate layer  142 , and a device layer  144 . The device layer  144  is a membrane with an air gap  146  above the device layer  144  and an air gap  148  between the substrate layer  142  and the device layer  144 . A spacer  150  separates the device layer  144  into adjacent membrane segments  152  and  154 . The spacer  150  provides electrical isolation within an otherwise continuous membrane. 
         [0021]    The spacer  150  includes an upper trench portion  156  which in this embodiment extends into the air gap  146 , a lower trench portion  158 , and two laterally extending portions  160  and  162 . Gaps  164  can be seen at about the centerline of the lower trench portion  158 . The upper trench portion  156 , however, does not indicate a significant amount of gap formation. One reason for this is that the upper trench portion  156  is visibly wider than the lower trench portion  158 . Thus, for some deposition techniques, as the width of a trench decreases for a given aspect ratio, the potential for gap formation increases. This phenomenon is discussed more fully with reference to  FIGS. 3-10 . 
         [0022]      FIGS. 3-10  depict a process for forming a spacer in a wafer. Initially, a substrate layer  200  is provided and a lower device layer portion  202  is formed on the substrate layer  200  ( FIG. 3 ). The lower device layer portion  202  is then etched to form a trench  204  which is depicted in  FIG. 4 . The trench  204  may be a high aspect ratio trench formed using any desired technique. In one embodiment, the trench  204  is formed using deep reactive ion etching (DRIE). 
         [0023]    Next, as depicted in  FIG. 5 , a nitride layer  206  is formed on the exposed portions of the device layer  200  and the substrate layer  202 . As the nitride layer is deposited, the nitride material “grows” from each exposed surface. Within the trench  204 , the growth rate may not be uniform along the sidewalls of the trench  204 , particularly if the trench  204  is relatively narrow. Accordingly, the nitride material from opposing sides of the trench at a first height may connect before nitride material from opposing sides of the trench at a lower height connect, thereby isolating a void area at a height below the first height. This results in gaps  208  in the lower trench portion  210  of the nitride layer  206 . 
         [0024]    Because the nitride grows outwardly from the sidewalls (lateral growth), the gaps  208  are generally close to the centerline of the trench  204 . A laterally extending portion  212  of the nitride layer  206  which is directly above the upper surface of the device layer  202 , however, is typically free of any significant gaps since the growth is primarily upward growth. Likewise, the laterally extending portion  212  above the trench  204  is filled primarily by upward growth once the trench  204  has been closed. Moreover, the upper portion of the lower trench portion  210  is filled primarily by upward growth. Thus, while a slight depression may be developed directly above the trench  204 , the nitride layer  206  directly above the trench  204  and the upper portion of the trench portion  210  are typically free of any significant gaps. The depression, if any may be removed by CMP. 
         [0025]    Referring now to  FIG. 6 , the laterally extending portion  212  is then etched to a desired shape. The shape may be selected based upon the desired final configuration of the spacer. Once the laterally extending portion  212  is in the desired shape, an upper device layer portion  214  is formed on the exposed upper surface of the lower device layer portion  202  and on the upper surface of the laterally extending portion  212  (see  FIG. 7 ). In  FIG. 7 , the upper device layer portion  214  is depicted differently from the lower device portion  202  for ease of discussion. In this embodiment, however, both materials are identical such that the upper device layer portion  214  and the lower device layer portion  202  form a single integrated layer of material such as silicon. The upper device layer portion  214  is then planarized and, referring to  FIG. 8 , a trench  216  is etched through the upper device layer  214  to the laterally extending portion  212 . 
         [0026]    Next, as depicted in  FIG. 9 , a nitride layer  218  is formed on the exposed portions of the upper device layer portion  214  and the laterally extending portion  212 . In alternative embodiments, the layer  218  is a type of material different from the layer  206 . The nitride layer  218  includes a laterally extending portion  220  and an upper trench portion  222 . In  FIG. 9 , the laterally extending portion  220  and the upper trench portion  222  are depicted differently for ease of discussion. In this embodiment, both materials are identical such that the upper trench portion  222  and the laterally extending portion  220  form a single integrated nitride structure. 
         [0027]    In the same manner discussed above with respect to the lower trench portion  210 , gaps  224  are formed in the upper trench portion  222 , but not within the laterally extending portion  220  or the upper portion of the trench portion  222 . Thus, after removing the laterally extending portion  220 , the configuration of  FIG. 10  is realized. 
         [0028]    In  FIG. 10 , a device layer  226  includes two adjacent layer segments  228  and  230 . The layer segments  228  and  230  are separated by a nitride spacer  232 . While gaps including gap  224  are present within the nitride spacer  232 , the upper surface and lower surface of the spacer  232  are free of gaps. Additionally, a laterally extending portion  212  which is positioned between the upper trench portion  222  and the lower trench portion  210  is free of gaps. The laterally extending portion  212  extends laterally beyond the outer edges of both the upper trench portion  222  and the lower trench portion  210 . Thus, the laterally extending portion  212  provides increased strength and isolation capability. 
         [0029]    Those of skill in the art will recognize that the process described with reference to  FIGS. 3-10  may be modified in order to provide a variety of spacer configurations. One such configuration is depicted in  FIG. 11  wherein a wafer  240  includes a substrate  242  and a spacer  244 . While the substrate  242  and spacer  244  may be substantially identical to the substrate  200  and spacer  232  of  FIG. 10 , the wafer  240  further includes a lower layer portion  246  and an upper layer portion  248  which are formed from different types of material. 
         [0030]    In a further embodiment, a spacer  250  may be formed with two laterally extending portions  252  and  254  as depicted in  FIG. 12 . The laterally extending portions  252  and  254  are separated by a middle trench portion  256 . 
         [0031]    Another embodiment is depicted in  FIG. 13 . The wafer  260  of  FIG. 13  includes a spacer  262  with a laterally extending portion  264 . Two hook portions  266  and  268  of the spacer  262  extend downwardly from the laterally extending portion  264 . The hook portions  266  and  268  may be formed by etching shallow trenches prior to deposition of the nitride layer used to form the laterally extending portion  264 . The hook portions  266  and  268  provide increased mechanical strength for resisting lateral movement of layer segments  270  and  272  away from each other. Additional strength may be realized by forming the spacer  262  partially within the substrate layer  274  and/or a cap layer (not shown). 
         [0032]    Referring to  FIG. 14 , a wafer  280  includes a spacer  282  which allows for movement of adjacent layer segments  284  and  286 . The spacer in  FIG. 14  is released from the layer segments  284  and  286  such as by use of a sacrificial coating which is later etched, resulting in a gap  288  between the spacer  282  and the layer segments  284  and  286 . The spacer  282  thus provides stress isolation between the layer segments  284  and  286 . By using a sacrificial layer between the layer segments  284  and  286  and the substrate  290 , the layer segments  284  and  286  may be released while the spacer  282  movably interlocks the layer segments  284  and  286 . If desired, the spacer  282  may be partially formed within the substrate  290  and/or a cap layer (not shown), thereby anchoring the spacer  282  while allowing controlled movement of the layer segments  284  and  286 . 
         [0033]    In addition the foregoing arrangements, each of which may be combined with one or more aspects of the other arrangements, depending upon the desired application, the orientation and sizes of the spacer components may also be varied. Thus, an upper trench may be shorter, taller, narrower, or wider than a lower trench. The laterally extending portion in a particular embodiment may be wider and/or thicker than a laterally extending portion in another embodiment. Moreover, while the trench portions in the preceding embodiments have been shown as aligned with each other, the trench portions may be laterally offset. For example, the wafer  294  of  FIG. 15  includes a spacer  296  with an upper trench portion  298  that defines a longitudinal axis that is laterally offset from a longitudinal axis defined by a lower trench portion  300 . 
         [0034]    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.