Abstract:
Electrical and visual test structures monitor the degree of removal of conductive sacrificial layers used in micromachining processes that fabricate micro-electromechanical systems (MEMS).

Description:
FIELD OF THE INVENTION 
       [0001]    This disclosure relates to micromachining processes used to make micro-electromechanical systems (MEMS). More specifically, this disclosure relates to monitoring the removal of a sacrificial layer in a process of making one or more MEMS devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    A current process to manufacture MEMS devices, such as micro-machined relays, involves the steps of depositing and patterning a copper or other electrically conductive sacrificial layer on a substrate, depositing and patterning a nickel or other electrically conductive structural layer at least partially over the sacrificial layer, and depositing and patterning a polymer insulating structural layer over the nickel structural layer. Then the copper sacrificial layer is removed to release the micro-machined parts. Since the copper is an electric conductor, incomplete etching of the copper can cause undesired electric shorts in the system in addition to preventing the structures from moving. 
       SUMMARY OF THE INVENTION 
       [0003]    A test structure, preferably, but not limited to, an electrical test structure, which allows for simple monitoring of the degree of removal of the conductive sacrificial layers used in micromachining processes would alleviate the problem identified above. 
         [0004]    In one example of the invention, a micro-electromechanical test apparatus comprises an insulating substrate. A first conductive structural element has a first anchor portion fixed to the substrate and a first bridge portion cantilevered from the anchor portion over the substrate. A second conductive structural element is insulated from the first conductive structural element and has a second anchor portion fixed to the substrate and a second bridge portion cantilevered from the second anchor portion over the substrate. An electrical resistance measuring device connected to the first and second conductive structural elements detects the presence of an incompletely removed conductive sacrificial layer contacting the first and second conductive structural elements. 
         [0005]    In a preferred example of the invention, the first bridge portion comprises a first plurality of conductive fingers and the second bridge portion comprises a second plurality of conductive fingers interleaved with the first plurality of conductive fingers. In other examples of the invention, an insulating structural layer is situated between the first and second bridge portions. 
         [0006]    In one specific example of the invention involving electrical monitoring of the degree of removal, a test structure is defined by a pair of interdigitated nickel fingers covered by a polymer layer. Initially, the pairs of interdigitated fingers are electrically connected by the copper sacrificial layer. Once the copper layer is completely removed this electrical connection is broken. The polymer layer covering the fingers region is used to produce a broader structure that takes longer to underetch or undercut. It is also possible to create a test structure that permits a visual inspection of the extent to which a sacrificial layer has been removed. In this case, a transparent polymer structure can be used if only a visual test structure is desired. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]      FIG. 1  depicts an illustrative partially completed structure formed in a typical micromachining process involving the use of sacrificial layers. 
           [0008]      FIG. 2  illustrates incomplete release of the sacrificial layer shown in  FIG. 1 . 
           [0009]      FIG. 3  shows incomplete release of a sacrificial layer in another partially completed structure formed in a typical micromachining process involving the use of sacrificial layers. 
           [0010]      FIG. 4  depicts a side view of an illustrative test structure in accordance with the invention before removal of a sacrificial layer. 
           [0011]      FIG. 5  depicts an illustrative test structure in accordance with the invention with an incomplete release of the sacrificial layer shown in  FIG. 4  and detection of the presence of the residual sacrificial material. 
           [0012]      FIG. 6  is a top view of the test structure of  FIG. 5 . 
           [0013]      FIG. 7  is a top view of a preferred example of an electrical test structure involving interleaved conductive fingers that permit highly reliable electrical sensing of the presence of a sacrificial layer underneath conductive structural elements suspended over an insulating substrate. 
           [0014]      FIG. 8  is a top view of an illustrative visual test structure in accordance with the invention. 
           [0015]      FIG. 9  is a side view of an illustrative test structure involving non-cantilevered conductive structures. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  depicts a partially completed structure made in accordance with a micro-machining process used to fabricate MEMS devices. The process involved the deposition of a conductive sacrificial layer  10  on a non-conductive substrate  12 . The conductive sacrificial layer  10  illustratively may be made of an electrically conductive material such as copper. A conductive structural layer  14  is deposited over the sacrificial layer  10  and the substrate  12 . The conductive structural layer  14  illustratively may be made of an electrically conductive material such as nickel. A non-conductive layer  16  is deposited over the conductive structural layer  14 . The non-conductive layer  16  illustratively may be made of a polymer such as polyimide. The sacrificial layer  10  is subsequently removed to release the portions of the conductive layer  14  shown in  FIG. 1  so that those portions of the layer  14  are movably suspended above the substrate  12  at a height above the substrate  12  defined by the thickness of the sacrificial layer  10 . Portions of the layer  14  not shown in  FIG. 1  deposited directly over the substrate  12  anchor and support the suspended portions of the layer  14  to the substrate  12 . For example, the anchor portion of the layer  14  cantilevers the suspended portions over the substrate  12 . 
         [0017]    The sacrificial layer  10  may be removed by using any suitable removal technique, such as chemical etching.  FIG. 2  illustrates a problem that may occur in removing the sacrificial layer  10 . In  FIG. 2 , the sacrificial layer  10  has not been completely removed by the etching process, leaving a remnant  18  of conductive material that may electrically short together the sections of the layer  14  shown in  FIG. 2  and may prevent them from moving appropriately in the finished MEMS device. 
         [0018]      FIG. 3  shows a situation where no insulating layer  16  is used. In this case, the etching step has left two residues  28  and  30  of the sacrificial layer  10  underneath the two sections of conductive layer  14 . Although there are no electrical shorts, the sections of layer  14  are anchored to the substrate  12  and substantially immobilized. 
         [0019]    Test structures in accordance with this invention reliably detect the presence of residues of a sacrificial layer used in a micromachining process that have not been removed by etching or other removal technique. They preferably are located on the same substrate on which the actual MEMS parts are being fabricated. The test structures are placed so that incomplete removal of a sacrificial layer under the test structure in all likelihood would indicate the incomplete removal of the sacrificial layer under the actual MEMS part being fabricated. When such residues are detected, additional etching or other steps can be performed to completely remove the sacrificial layer. 
         [0020]      FIG. 4  is an exemplary test structure that electrically tests for the presence of a residual sacrificial conductive layer removed from a substrate during a micromachining process. The test structure of  FIG. 4  is a mechanical bridge structure designed to have an open electrical path that is only closed by the residual conductive material left on the substrate  12  in the event of an incomplete release step.  FIG. 4  shows this test structure prior to removal of the sacrificial layer  10  from the substrate  12 . The test structure comprises a first conductive structural element  30  having a first anchor portion  32  fixed to the substrate  12 . The first conductive structural element  30  also has bridge portion  34  extending from and supported by the anchor portion  32  and resting on the sacrificial layer  10 . The bridge portion  34  is to be supported or cantilevered a predetermined distance above the substrate  12  by the anchor portion  32  after the removal of the sacrificial layer  10 . The test structure also comprises a second conductive structural element  36  having a second anchor portion  38  fixed to the substrate  12  in a fashion similar to that of anchor portion  32 . The second conductive structural element  36  also has bridge portion  40  extending from, and supported by, the anchor portion  38  and resting on the sacrificial layer  10 . The bridge portion  40  is to be supported or cantilevered a predetermined distance above the substrate  12  by the anchor portion  38  after the removal of the sacrificial layer  10 . Like the apparatus of  FIG. 1 , the test structure of  FIG. 4  also comprises an insulating structural layer  16  that insulates the conductive structure  30  from the conductive structure  36 . 
         [0021]      FIG. 5  shows a side view, and  FIG. 6  shows a top view, of the test structure of  FIG. 5  after removal of the sacrificial layer  10 . Due to an incomplete etching of the sacrificial layer  10 , a residual portion  42  of the layer  10  remains under the bridge portions  34  and  40  and the insulating layer  16 . This residual portion  42  not only interferes with the motion of the bridge portions  34  and  40 , but it also electrically shorts the two bridge portions  34  and  40  together. This electrical short between portions  34  and  40 , and the presence of the residual portion  42  indicating an incomplete release of the sacrificial layer  10 , is detected by an electrical resistance measuring device  44  connected to structural elements  30  and  36  as shown in  FIG. 6 . 
         [0022]      FIG. 7  shows a preferred embodiment of an electrically based test structure that detects the presence of a residual piece of conductive sacrificial layer  10  remaining after an incomplete release of the sacrificial layer  10  by an etching step in a micromachining process. The structure of  FIG. 7  involves an interdigitated finger-like structure that will increase the bridge width and decrease the chance of a false negative. The structure of  FIG. 7  is similar to that of  FIGS. 4 to 6 , except that the bridge portion of the conductive structural element  30  comprises a row of narrow strips  46  of conductive material extending from the anchor portion  32  and suspended or cantilevered over the substrate  12 ; similarly, the bridge portion of the conductive structural element  36  comprises a row of narrow strips  48  of conductive material extending from the anchor portion  38  and suspended or cantilevered over the substrate  12 . As shown in  FIG. 7 , the row  46  of conductive strips is separated from and insulated from the row  48  of conductive strips by the insulating layer  16 . The interleaved finger-like strips  46  and  48  separated by the insulating layer  16  define an increased length serpentine interface between the two conductive elements  30  and  36  as compared with the interface between the conductive elements  30  and  36  shown in  FIG. 6 . This arrangement increases the likelihood of detecting small pieces of residual sacrificial layer as compared with the apparatus of  FIG. 6 . 
         [0023]      FIG. 8  shows a test apparatus for visually detecting the presence of residual sacrificial layer. If only a visual test structure is desired to evaluate the release of a MEMS sacrificial layer, the finger structure of  FIG. 7  may be removed leaving only a transparent window  50  through the insulating layer  16  under which residue  42  can be visually observed. Although  FIG. 8  shows two structural elements  30  and  36  supporting the window  50  above the substrate  12  and anchoring it to the substrate  12 , only one such structural element is needed to support the window  50  above the substrate  12  in the  FIG. 8  embodiment of the invention. 
         [0024]      FIG. 9  is another embodiment of the invention involving electrical detection of conductive sacrificial layer residue. In the apparatus of  FIG. 9 , conductive layers  52  and  54  are formed on insulating substrate  12 . The conductive layers  52  and  54  are separated and insulated from one another on the substrate  12 . Preferably, conductive layers  52  and  54  include interdigitated fingers like those shown in  FIG. 7 . A conductive sacrificial layer, only a residue  44  of which is shown in  FIG. 9 , is formed over the conductive layers  52  and  54  so that they are shorted together by the sacrificial layer. The test structure of  FIG. 9  also includes additional structural elements  56 ,  58 , and  60  which may be electrically conductive or insulating and shaped similarly to that of elements  30 ,  36 , and  16  described above. The electrical measuring device  46  in  FIG. 9  is connected to conductive layers  52  and  54  and indicates the presence of residue  44  which shorts the layers  52  and  54  together when the sacrificial layer has not been completely removed. This also indicates that the movable MEMS parts are not likely to have been released. Although  FIG. 9  shows two structural elements  52  and  54  supporting the insulating layer  60  and anchoring it to the substrate  12 , only one such structural element is needed to support the layer  60  above the substrate  12  in the  FIG. 9  embodiment of the invention. 
         [0025]    It should be pointed out that any number and configuration of structural elements like those described above that suspend other structures above the substrate may be used depending on the MEMS structures being fabricated and the nature of the sacrificial layer being removed during the fabrication process. 
         [0026]    The Title, Technical Field, Background, Summary, Brief Description of the Drawings, Detailed Description, and Abstract are meant to illustrate the preferred embodiments of the invention and are not in any way intended to limit the scope of the invention. The scope of the invention is solely defined and limited by the claims set forth below.