Abstract:
A method for coating a micro-electromechanical system (MEMS) device is provided. A coating material, such as a ceramic slurry, may be utilized to form a gas permeable enclosure or shell around the device after the coating material hardens. A vacuum may be applied near the device to exert an attractive force on the coating material to aid in homogenously distributing the coating material over the device. In addition, a vibration may be applied to the device to aid in distributing the coating material. If the device is attached to a substrate, a hole may be formed through the substrate with one opening near the device and a second opening located elsewhere. The vacuum may then be applied to the second opening to draw the coating material over the device and towards the first opening.

Description:
CROSS-REFERENCE  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/688,722, filed on Oct. 16, 2000, which is a continuation-in-part of U.S. application Ser. No. 09/483,640, filed Jan. 14, 2000, and issued on Mar. 6, 2001, as U.S. Pat. No.  6 , 197 , 610 . 
     
    
     
       BACKGROUND  
         [0002]    The present disclosure relates generally to semiconductor processing, and more particularly, to a system and method for making micro-electromechanical system (MEMS) devices with gas-permeable enclosures.  
           [0003]    Integrated circuit devices may need one or more small gaps placed within the circuit. For example, MEMS devices and other small electrical/mechanical devices may incorporate a gap in the device to allow the device to respond to mechanical stimuli. One common MEMS device is a sensor, such as an accelerometer, for detecting external force, acceleration or the like by electrostatically or magnetically floating a portion of the device. The floating portion can then move responsive to the acceleration and the device can detect the movement accordingly. In some cases, the device has a micro spherical body referred to as a core, and a surrounding portion referred to as a shell. Electrodes in the shell serve not only to levitate the core by generating an electric or magnetic field, but to detect movement of the core within the shell by measuring changes in capacitance and/or direct contact of the core to the shell. A coating may be applied to the MEMS device to provide a protective and insulating enclosure around the device and its components.  
           [0004]    Due in part to the size of MEMS devices, imperfections created by the manufacturing process may create problems in the structure of a MEMS device that might be insignificant in larger scale applications but may render the MEMS device unusable. Such problems may, for example, include flaws in a coating layer of a MEMS device.  
           [0005]    Accordingly, certain improvements are desired for MEMS devices and their manufacturing. For one, it is desirable to provide a coating that is relatively homogeneous and free of voids. Furthermore, it is desired to provide protection, to provide a coating of a desired thickness, to provide high productivity, and to provide a manufacturing process that is more flexible and reliable.  
         SUMMARY  
         [0006]    A technical advance is provided by a method for coating a micro-electromechanical system device. In one embodiment, the method comprises providing the device mounted on a substrate, where the substrate includes an aperture having a first opening proximate to the device and a second opening. A vacuum is applied to the second opening and a coating material is applied to the device. The vacuum aids in the homogeneous distribution of the coating material on the device by drawing a portion of the coating material over the device towards the first opening.  
           [0007]    In another embodiment, the method includes applying a vibration to the device to aid in the distribution of the coating material over the device.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a flowchart of a manufacturing process for implementing one embodiment of the present invention.  
         [0009]    FIGS.  2 - 5 ,  6   a ,  6   b ,  7   a , and  7   b  are cross sectional views of a spherical shaped accelerometer being manufactured by the process of FIG. 1.  
         [0010]    [0010]FIG. 8 is a flowchart of a manufacturing process for implementing one embodiment of the present invention of FIG. 1.  
         [0011]    [0011]FIG. 9 is a cross sectional view of a spherical shaped accelerometer being manufactured by the processes of FIGS. 1 and 8.  
         [0012]    [0012]FIG. 10 is a flowchart of a method for applying an enclosure around a micro-electromechanical device.  
         [0013]    [0013]FIG. 11 is a cross sectional view of a spherical shaped accelerometer without the enclosure provided by the method of FIG. 10.  
         [0014]    [0014]FIG. 12 is a cross sectional view of the accelerometer of FIG. 11 with the enclosure.  
         [0015]    [0015]FIG. 13 is a cross-sectional view of a spherical shaped accelerometer with multiple gas-permeable enclosures.  
         [0016]    [0016]FIG. 14 is a cross-sectional view of a spherical shaped accelerometer with a hermetic seal. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The present disclosure relates to semiconductor processing, and more particularly, to a system and method for making a micro-electromechanical system (MEMS) devices with gas-permeable enclosures. It is understood, however, that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.  
         [0018]    Referring to FIG. 1, the reference numeral  10  refers, in general, to a manufacturing process for making MEMS devices such as is described in U.S. Pat. No. 6,197,610, issued on Mar. 6, 2001, and also assigned to Ball Semiconductor, Inc., entitled “METHOD OF MAKING SMALL GAPS FOR SMALL ELECTRICAL/MECHANICAL DEVICES” and hereby incorporated by reference as if reproduced in its entirety. For the sake of example, FIGS.  2 - 7   b  will illustrate a spherical shaped accelerometer that is being made by the manufacturing process  10 . It is understood, however, that other MEMS devices can benefit from the process. For example, clinometers, ink-jet printer cartridges, and gyroscopes may be realized by utilizing a similar design.  
         [0019]    At step  12  of the manufacturing process  10 , a substrate is created. The substrate may be flat, spherical or any other shape. Referring also to FIG. 2, for the sake of example, a spherical substrate (hereinafter “sphere”)  14  will be discussed. The sphere  14  is one that may be produced according to presently incorporated U.S. Pat. No. 5,955,776, issued on Sep. 21, 1999, and also assigned to Ball Semiconductor, Inc., entitled “SPHERICAL SHAPED SEMICONDUCTOR INTEGRATED CIRCUIT,” and to continue with the present example, is made of silicon crystal. On an outer surface  16  of the sphere  14  is a silicon dioxide (SiO2) layer. It is understood that the presence of the SiO2 layer  16  is a design choice and may not be used in certain embodiments. For example, the SiO2 layer  16  may not be used if the substrate  16  will not react with an etchant.  
         [0020]    At step  18  of FIG. 1, a first group of processing operations are performed on the substrate. This first group of processing operations represents any operations that may occur before a sacrificial layer is applied (described below, with respect to step  22 ). Referring also to FIG. 3, in continuance with the example, a first metal layer  20  (hereinafter “metal 1”) is deposited on top of the SiO2 layer  16 . The metal 1 layer  20  may be a material such as a chromium film, although other materials may be used. This metal deposition may be created by sputtering. Several different methods, such as is described in U.S. Pat. No.  6 , 053 , 123 , issued on Apr. 25, 2000, and also assigned to Ball Semiconductor, Inc., entitled “PLASMA-ASSISTED METALLIC FILM DEPOSITION” and hereby incorporated by reference as if reproduced in its entirety.  
         [0021]    At step  22  of FIG. 1, a sacrificial layer is applied to the substrate. The sacrificial layer may be applied on top of the previous layers (if any). In continuance with the example of FIG. 3, a sacrificial polysilicon layer  24  is applied on top of the metal 1 layer  20 . The sacrificial layer  24  may be applied by sputtering or any conventional manner, such as is described in the presently incorporated patents. Polysilicon is chosen because it reacts well with an etchant discussed below with respect to step  50 , but it is understood that other materials can also be used.  
         [0022]    At step  26  of FIG. 1, a second group of processing operations is performed on the substrate. This second group of processing operations represents any operations that may occur after the sacrificial layer is applied. In continuance with the example of FIG. 3, a second metal layer  28  (hereinafter “metal 2”) is deposited on top of the sacrificial layer  24 . The metal 2 layer  28  may be a gold-chromium (Au/Cr) material, although other materials may be used and the metal 2 layer may have a different composition than the metal 1 layer  20 .  
         [0023]    At step  30  of FIG. 1, one or more layers of material applied in the second group of processing operations are patterned. The patterning occurs before the removal of the sacrificial layer (described below, with respect to step  50 ). Referring also to FIG. 4, the metal 2 layer  28  is patterned to produce a plurality of electrodes  28   a ,  28   b ,  28   c ,  28   d , and  28   e.    
         [0024]    The metal 2 layer  28  can be patterned by several different methods. For example, a resist coating may be applied to the metal 2 layer  28 , such as is shown in U.S. Pat. No. 6,179,922, issued on Jan. 30, 2001, entitled “CVD PHOTO RESIST DEPOSITION” and/or U.S. Pat. Ser. No. 09/584,913, filed on May 31, 2000, entitled “JET COATING SYSTEM FOR SEMICONDUCTOR PROCESSING,” which are both assigned to Ball Semiconductor, Inc., and hereby incorporated by reference as if reproduced in their entirety.  
         [0025]    Once the resist coating has been applied, the coating may be exposed using a conventional photolithography process. In the present embodiment, the etching should not remove the sacrificial layer  24 . For example, photolithography processes, such as shown in U.S. Pat. No. 6,061,118, issued on May 9, 2000, entitled “REFLECTION SYSTEM FOR IMAGING ON A NONPLANAR SUBSTRATE” and/or U.S. Pat. No. 6,251,550, issued on Jun. 26, 2001, entitled “MASKLESS PHOTOLITHOGRAPHY SYSTEM THAT DIGITALLY SHIFTS MASK DATA RESPONSIVE TO ALIGNMENT DATA,” which are both assigned to Ball Semiconductor, Inc., and hereby incorporated by reference as if reproduced in their entirety, may be used. In the present example, the metal 2 layer  28  is the only layer that is patterned. For this reason, there is no need for alignment. It is understood, however, that different embodiments may indeed require alignment. For example, if the sphere  14  is flat, or if the metal  1  layer  20  is also patterned, the metal 2 layer  28  may indeed need to be patterned. Also, if the entire resist coating cannot be exposed at the same time, alignment between exposures may be required.  
         [0026]    Once the resist coating has been fully exposed (to the extent required), the exposed surface can be developed and etched according to conventional techniques. For example, the exposed photo resist and Au/Cr metal 2 layer may be etched according to a technique such as shown in U.S. Pat. No. 6,077,388, issued on Jun. 20, 2000, and also assigned to Ball Semiconductor, Inc., entitled “SYSTEM AND METHOD FOR PLASMA ETCH ON A SPHERICAL SHAPED DEVICE” and hereby incorporated by reference as if reproduced in its entirety. Once etching is complete (and cleaning, if required), the electrodes  28   a ,  28   b ,  28   c ,  28   d , and  28   e  may be fully processed.  
         [0027]    At step  34  of FIG. 1, the substrate and processed layers are assembled, as required by a particular application. Referring also to FIG. 5, a plurality of bumps  36   a ,  36   b  are applied to the electrodes  28   a ,  28   b , respectively. In the present example, the bumps are gold, but it is understood that other materials may be used, such as solder. The bumps  36   a ,  36   b  may also be applied to electrodes  38   a ,  38   b , respectively, of a second substrate  40 . Because the sacrificial layer  24  still exists, the process of applying the bumps  36   a ,  36   b  to the electrodes  28   a ,  28   b  and  38   a ,  38   b  is relatively straight forward. For the sake of example, the bump application may be performed by the method described in U.S. Pat. No. 6,251,765, issued on Jun. 26, 2001, and also assigned to Ball Semiconductor, Inc., entitled “MANUFACTURING METAL DIP SOLDER BUMPS FOR SEMICONDUCTOR DEVICES” and hereby incorporated by reference as if reproduced in its entirety.  
         [0028]    Once the bumps have been applied and attached, a protective coating  42  may be applied as will be described in greater detail in reference to FIGS.  8 - 11 . In the present example of FIG. 5, the protective coating  42  covers all of the electrodes  28   a ,  28   b ,  28   c ,  28   d ,  28   e  (and thus the underlying layers and substrates), the bumps  36   a ,  36   b , and at least a portion of the electrodes  38   a ,  38   b . In the present example, the protective coating  42  is ceramic, but may be epoxy resin, polyimide, or any other material. The protective coating  42  may be applied in any manner, including dipping or spraying the coating onto the components to be coated.  
         [0029]    The above-described manufacturing process  10  uses conventional processing operations in a new and modified sequence. It is recognized that the processing operations referenced above, or different operations that better suit particular needs and requirements, may be used.  
         [0030]    At step  44  of FIG. 1, holes are created in one or more of the processed layers. Referring also to FIGS. 6 a  and  6   b , holes  46  are made through the protective coating  42  and extending between the electrodes  28   a ,  28   b ,  28   c ,  28   d ,  28   e  to the sacrificial layer  24 . In the preferred embodiment, these holes are made using a laser  48 . The laser  48  is positioned to burn the hole directly through the protective coating  42  to reach the sacrificial layer  24 . Other ablation methods include particle injection or other chemical and/or mechanical techniques.  
         [0031]    At step  50  of FIG. 1, the sacrificial layer is removed. Referring also to FIGS. 7 a  and  7   b , the sacrificial layer  24  is etched through the holes  46 . In continuance of the above examples where the sacrificial layer  24  is polysilicon, a xenon difluoride (XeF2) dry etchant  52  can be used. The XeF2 dry etchant  52  has extremely high selectivity. It readily reacts with crystalline silicon and polysilicon, but does not react with the metal  2  layer  28 , the protective coating  42 , or various other materials. It is understood that other etchants may be used.  
         [0032]    As a result, the sacrificial layer  24  is removed and a gap  54  is formed in its place. The gap  54  separates the sphere  14 , SiO2 layer  16 , and metal 1 layer  20  (collectively the “core”) from the metal 2 layer  28  (the “shell”). In the present embodiment, the gap  54  extends around the entire core to complete the construction of a three-axis accelerometer  56 .  
         [0033]    Referring now to FIGS. 8 and 9, in another embodiment, the reference numeral  60  refers, in general, to one embodiment of a manufacturing process for producing a gas permeable shell that surrounds MEMS devices. At step  62 , a first solid is dissolved in a solvent to form a solution. The first solid may be boron oxide (B 203 ) or any other material. The solvent may be iso-propyl (IPA) alcohol or any other solvent.  
         [0034]    At step  64 , the solution from step  62  is mixed with a second solid to form a slurry. The second solid may be alumina cement or any other material. By controlling the amount of mixing in step  64 , the size of the pores of the gas permeable shell  42  can be controlled. The size of the pores of the gas permeable shell  42  can be also be controlled by the composition of the slurry.  
         [0035]    At step  66 , the slurry from step  48  is poured onto the substrate and processed layers. The slurry covers all of the electrodes  28   a ,  28   b ,  28   c ,  28   d ,  28   e , and  28   f  (and thus the underlying layers and substrates), the bumps  36   a ,  36   b , and at least a portion of the electrodes  38   a ,  38   b . At step  68 , the slurry covered substrate and processed layers are dried at room temperature. The second solid may be dispersed in the gas permeable shell  42 .  
         [0036]    At step  70 , the substrate and processed layers are exposed to the solvent. The solvent re-dissolves the first solid leaving behind the gas permeable shell  42 . The gas permeable shell  42  has pores that are now interconnected and extend between the electrodes  28   a ,  28   b ,  28   c ,  28   d ,  28   e  and  28   f  to the sacrificial layer  24 .  
         [0037]    In yet another embodiment, alumina cement may be utilized without the need for a solvent to open the interconnected pores. This simplifies the creation of the protective layer  42 .  
         [0038]    Referring now to FIG. 9, the sacrificial layer  24  (as shown in FIG. 5) may be etched through the gas permeable shell  42 . In continuance of the above examples where the sacrificial layer  24  is polysilicon, a xenon difluoride (XeF2) dry etchant  52  can be used. The XeF2 dry etchant  52  has extremely high selectivity. It readily reacts with crystalline silicon and polysilicon, but does not react with the metal 2 layer  28 , the protective coating  42 , or various other materials. It is understood that other etchants may be used.  
         [0039]    As a result, the sacrificial layer  24  is removed and a gap  54  is formed in its place. The gap  54  separates the sphere  14 , SiO2 layer  16 , and metal 1 layer  20  (collectively the “core”) from the metal 2 layer  28  (the “shell”). In the present embodiment, the gap  54  extends around the entire core to complete the construction of a three-axis accelerometer  56 .  
         [0040]    Referring now to FIG. 10, in yet another embodiment, a method  72  for applying the coating  42  is illustrated in greater detail in steps  74 - 80 . At step  74 , a device over which the coating is to be applied and, if desirable, a substrate attached to the device are provided as described in greater detail with respect to FIGS. 11 and 12.  
         [0041]    Referring also to FIGS. 11 and 12, the accelerometer  56  described above is illustrated without the protective coating  42  that may be applied in step  34  of FIG. 1 (FIG. 11) and with the coating (FIG. 12). It should be noted that the exemplary accelerometer  56  of FIGS. 11 and 12 is illustrated without an outer surface  16  and with additional electrodes  28   f  and  28   g . It is understood that the accelerometer  56  is merely one example of a device that may utilize such a coating  42 , and many other devices of varying sizes and shapes may benefit from the application of the coating  42 . In the present example, the coating  42  forms a porous, gas permeable ceramic enclosure or shell around the accelerometer  56  and its associated layers.  
         [0042]    Due in part to the relatively small scale of the accelerometer  56  (e.g., approximately one millimeter), imperfections in the coating may create problems that might be insignificant in larger scale applications but may render the accelerometer  56  unusable. Accordingly, it may be desirable to achieve a relatively homogenous, void free layer over the accelerometer  56  with the ceramic coating  42 .  
         [0043]    In the present example, the substrate  40  may be made of a material such as borosilicate glass (e.g., PYREX material by CORNING GLASS WORKS CORPORATION, NEW YORK). An aperture  82  may be formed in the substrate  40  proximate to the bumps  36   a ,  36   b . The aperture  82  may be formed either before or after the accelerometer  56  is connected to the substrate  40 , depending on the particular manufacturing process used.  
         [0044]    In step  76  of the method of FIG. 10, a vacuum (indicated by arrows  84  in FIG. 12) may be applied to the aperture  82  on the side of the substrate  40  opposite the accelerometer  56  to create a suction. Vibrations may be induced in step  78 , as will be described later in greater detail. Accordingly, when a material such as a ceramic slurry is poured over the accelerometer  56  in step  80 , the suction draws a portion of the ceramic slurry over the accelerometer  56  and towards the aperture  82 . This may aid in the creation of a homogenous, void-free coating  42  over the accelerometer  56 . The amount of suction, which in turn may affect the flow of the coating  42 , may depend on a number of factors, such as the rate at which the ceramic slurry is applied to the accelerometer  56 , the dimensions of the aperture  82 , and similar factors.  
         [0045]    In still another embodiment, a vibrating device  85  may be attached to the substrate  40  to aid in the even distribution of the ceramic slurry over the accelerometer  56 . For example, the vibrating device  85  may be a piezoelectric transducer operable to create a 150 Hertz vibration. The vibrations created by the transducer may aid in homogenizing the coating  42  during application. In addition, this may aid in the prevention of voids in the coating  42 . The amount of vibration, which in turn may affect the flow of the coating  42 , may depend on a number of factors, such as the consistency of the ceramic slurry.  
         [0046]    Referring now to FIG. 13, in yet another embodiment, the protective coating  42  may comprise multiple layers of porous material. This may be desirable, for example, if the metal 2 layer  28  and the protective coating  42  do not adhere well to one another. In the present example, the protective layer  42  includes an inner protective layer  86  that provides a desired level of adhesion with the metal 2 layer  28 . An outer protective layer  88  can then be added that adheres well to the inner protective layer  86 , but that would not adhere well to the metal 2 layer  28 . The level of adhesion may vary with the porosity of the inner and outer protective layers  86  and  88 , and so the inner protective layer  86  may be less porous than the outer protective layer  88 . In this manner, both adhesion and gas permeability may be achieved by using multiple layers of protective coatings.  
         [0047]    Referring now to FIG. 14, in still another embodiment, a sealing layer  90  may be deposited on the single or multiple-layer protective coating  42  to provide a hermetic seal. As described previously, the protective coating  42  may comprise one or more gas-permeable layers that enable the sacrificial layer  24  to be etched after the protective coating  42  is applied. However, in certain MEMS applications, it may be undesirable to have a porous protective coating  42 . Accordingly, the sealing layer  90  may be deposited onto the protective layer  42  after the etching process to seal the pores provided in the protective layer  42  for the etching process.  
         [0048]    In another embodiment, referring still to FIG. 14, a material (a “getter”)  92  may be added proximate to the device  56 . For example, the getter  92  may be formed on the substrate  40  and within the protective layer  42  and/or the sealing layer  90 . The getter  92  may attract gas molecules during the etching process as well as gas molecules remaining after the etching process. If the sealing layer  90  is deposited on the protective layer  42  after the etching process, the getter  92  may attract gas molecules that are trapped inside the device  56  by the sealing layer  90 . Accordingly, the getter  92  may stabilize the device  56 .  
         [0049]    While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, it is within the scope of the present invention to use a MEMS device of non-spherical shape. Also, it may be desirable to use materials other than ceramic for the coating. Furthermore, the coating may enter certain openings in the MEMS device. Also, it may be desirable to have multiple coatings. Therefore, the claims should be interpreted in a broad manner, consistent with the present invention.