Abstract:
A method of indicating the progress of a sacrificial material removal process, the method, comprising; freeing a portion of a member, the member being disposed in a cage and laterally surrounded by the sacrificial material; and preventing the freed portion of the member from floating away by retaining the freed member.

Description:
BACKGROUND 
   Sacrificial materials are often used in the fabrication of devices, such as MEMS (microelectromechanical system) devices. These sacrificial materials are removed in a later stage in a process flow to generate designed empty spaces below or around the devices. This removal process is also commonly called a release process, because the movable parts of a device are released and free to move in at least one dimension after the sacrificial material is removed. The sacrificial material is often removed using chemical processes, such as etching, near the end of wafer fabrication. Since the sacrificial material often occupies space underneath the movable devices, it is frequently difficult to determine when the removal process is complete using standard optical inspection of the wafer. 
   In the past, it has been difficult to accurately monitor or evaluate the completeness of the release process. In some processes, device structures are scratched or otherwise physically removed to reveal the progress of the release process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric diagram illustrating a metrology structure according to one embodiment of the present invention. 
       FIG. 2  shows a third layer of a cage according to one embodiment of the present invention. 
       FIG. 3  shows a second layer of a cage according to one embodiment of the present invention. 
       FIG. 4  shows a first layer of a cage according to one embodiment of the present invention. 
       FIG. 5  is a cross-section of a metrology structure before removal of a sacrificial material according to one embodiment of the present invention. 
       FIG. 6  is a cross-section of a metrology structure after removal of a sacrificial material according to one embodiment of the present invention. 
       FIG. 7  is a cross-section of a plurality of cages and members disposed on a substrate before removal of a sacrificial material according to another embodiment of the present invention. 
       FIG. 8  is a cross-section of a plurality of cages and members disposed on a substrate during removal of a sacrificial material according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIGS. 1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 , and  8  are provided for illustration purposes only and are not intended to limit the present invention; one skilled in the art would recognize various modifications and alternatives, all of which are considered to be a part of the present invention. 
   Referring to  FIG. 1 , one embodiment of a metrology structure  10  is shown. In the embodiment, the metrology structure  10  is disposed on a substrate  12 . In some embodiments a MEMS device may be formed on the substrate  12 . The metrology structure  10  comprises a cage  13  having sacrificial material (not shown) disposed therein. The cage  13  comprises a first layer  14 , a second layer  16 , and a third layer  18 , disposed successively on the substrate  12 . However, the number of layers are not limiting on the invention, and any other suitable number of layers can also be used. Typically, the substrate  12  comprises real MEMS devices (not shown) along with the metrology structure  10 . For the purposes of illustration,  FIG. 1  comprises only the metrology structure  10 . For example, the metrology structure  10  may cover an area of approximately 1% on the substrate  12 , with the real MEMS device covering approximately 99% on the substrate  12 . These percentages may, of course, vary. In one embodiment, the substrate  12  used can be a silicon wafer or glass, for example. When the fabrication of the MEMS device is finished, the metrology structure  10  can be left on the substrate  12  or diced away during a chip dicing process. 
   Referring to  FIG. 2 , in one embodiment, the third layer  18  of the metrology structure  10  comprises a third portion  22  of a member disposed in a third portion  24  of the cage  13 . In one embodiment, the third portion  22  of the member comprises a post  23  (shown in  FIGS. 5 and 6 ) and the third portion  24  of the cage  13  comprises structures such as tabs  26  and posts  28 . According to one embodiment, the third portion  24  of the cage of the layer  18  comprises four tabs  26  and four posts  28 . During the fabrication of the metrology structure  10 , prior to the deposition of the third layer  18 , a sacrificial material (not shown) is patterned with holes (corresponding to the post  23  and the posts  28 ) in such a way that when the third layer  18  is deposited, the post  23  and the posts  28  are formed. The post  23  and the posts  28  act as support columns and are used to provide structural support for the third layer  18 . In some example embodiments, of the metrology structure  10 , the height dimension for the post  23  and the posts  28  can be in the range of approximately 0.1 μm to 5 μm. According to one embodiment of the present invention, the height dimension for the post  23  and the posts  28  can be in the range of approximately 1 nm to 100 nm. This height dimension may vary. 
   Referring to  FIG. 3 , in one embodiment, the second layer  16  of the metrology structure  10  comprises a second portion  32  of the member and is connected to the third portion  22  of the member of the third layer  18  by the post  23 . The second layer  16  further comprises a second portion  34  of the cage  13  that supports the third portion  24  of the cage  13  of the layer  18 . The second portion  34  of the cage  13  comprises posts  36  that are used to provide structural height therefore and are formed as mentioned above for the layer  18 . According to one embodiment, the second portion  34  of the cage  13  comprises twelve posts  36 . The number and height of the posts may vary. 
   Referring to  FIG. 4 , in one embodiment, the first layer  14  of the metrology structure  10  comprises a first portion  42  of the cage  13  that supports the second portion  34  of the cage  13 , and a member  44 . In one embodiment, the first layer  14  is designed such that the member  44  is disposed on only a portion of the substrate  12 . According to one embodiment, the member  44  is disposed in one corner of portion  42  of the cage  13  on the substrate  12 . However, the layer  14  can be also be designed such that the member  44  can be disposed on other portions of the substrate  12 . The purpose of the member  44 , in some embodiments, is to tilt the member  22 ,  32  at an angle when the member  22 ,  32 , tips or falls on to the substrate  12 . Thus the tilted member  22 ,  32  ( FIG. 6 ) can be more readily discernable to an observer as freed than if the member was disposed flat on the substrate  12 . 
     FIG. 5  shows a cross-section of the metrology structure  10 , before removal of a sacrificial material according to one embodiment of the invention. The sacrificial material is shown by the hatched area.  FIG. 6  shows a cross-section of the metrology structure  10 , after removal of a portion of the sacrificial material according to one embodiment of the invention. Arrows  46  and  48  in  FIGS. 5 and 6  represent incident and reflected light, respectively on the top surface of the third portion  22  of the member to be freed of the third layer  18  of the metrology structure  10 . Referring to  FIGS. 2 ,  3 ,  4 ,  5 , and  6 , as the third portion  22  is freed from the sacrificial material underneath the layer  18  and the second portion  32  is freed from the sacrificial material underneath and surrounding the layer  16 , simultaneously, the freed member will be retained in the cage comprising the third portion  24 , the second portion  34  and the first portion  42 , but will be able to move and drop down within the cage. It should be noted that the posts  28  of the layer  18  and the posts  36  of the layer  16  not only provide support for the layers  18  and  16 , respectively, they also prevent the freed member  22 ,  32  from moving far in a horizontal direction. Also, during the sacrificial material removal process, when external forces try to pull the freed member  22 ,  32  to float up, the tabs  26  of the third portion  24  of the cage  13  block the second portion  32  of the freed member which, in turn, is connected to the third portion  22  of the freed member of the layer  18 , and hence the possibility of the freed member  22 ,  32  being pulled/floated up in a vertical direction and potentially damaging other devices on a wafer is prevented. 
     FIGS. 1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 , and  8  are not drawn to scale. They provide schematic illustration of certain example embodiments. 
   In one embodiment of the invention, the cage  13  and members  22 ,  32 , and  44  can be made of aluminum or an alloy of aluminum such as aluminum/titanium. However, the above materials are not limiting on the invention, and any other combination of metals, dielectrics, or polymeric materials can also be used. The sacrificial material used can be an organic polymer such as photoresist, or any other suitable material such as silicon, or silicon dioxide, for example. The cage  13  and members  22 ,  32  may comprise, in one embodiment, a model structure of a real MEMS device(s). 
   According to another embodiment of the invention, a plurality of cages and members of different sizes in area laterally surrounded by a sacrificial material may be disposed on the substrate  12  as shown in  FIGS. 7 and 8 . For the purposes of illustration,  FIGS. 7 and 8  comprise only three cages and members. The number of cages and members may vary.  FIG. 7  shows a cross-section of the cages and members before removal of a sacrificial material and  FIG. 8  shows a cross-section of the cages and members during removal of a sacrificial material. The sacrificial material is shown by the hatched area in  FIGS. 7 and 8 . During the sacrificial material removal process, a smallest area member may be freed first. As the removal process progresses in time, successively larger area members will become free. This is because it takes longer time for an etching material to go underneath the larger area member as compared to the smaller area member during the removal process. Referring to  FIG. 8 , during the sacrificial material removal process, the members  110  and  120  have been freed whereas the member  130  still has some sacrificial material left. By using a plurality of cages and members of increasing size in area, the progress of the removal process can be determined. It should be noted that the increase in size is done laterally as shown in  FIGS. 7 and 8  in order to comply with the fabrication techniques. In one embodiment the size of the cages and the members may comprise a range of 1 μm to 500 μm. In an alternate embodiment, the size of the cages and the members may comprise a range of 10 nm to 1000 nm. 
   According to one embodiment, an observer can determine the state of the removal process, by detecting a change in the characteristic of the freed member  22 ,  32  such as detecting a change in light reflection (shown by the arrow  48  in the  FIG. 6 ) from a top surface of the first portion  22  of the freed member as compared to the light reflection shown by the arrow  48  in  FIG. 5 . In an alternate embodiment, the state of the removal process may be determined by detecting a change in the characteristic of the freed member  22 ,  32  by detecting a change in reflection or scattering of electrons, ions, atoms, or photons. 
   The metrology structure  10  can be built using standard micro-electronic fabrication techniques such as photolithography, vapor deposition and etching. However, the above techniques are not limiting on the invention and any other suitable techniques can also be used. 
   It should be noted that the metrology structure  10  provides an easier evaluation to determine the state of removal processes when using manual or automated visual inspection. The metrology structure  10  can be easily scaled or otherwise modified to be useful for monitoring the removal processes for a wide variety of MEMS designs. The metrology structure can be used for various standard mask levels and fabrication steps typically used in fabricating the real MEMS devices, and hence, pursuant to some embodiments, no additional process steps are required to create the metrology structure. 
   The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.