Abstract:
An inspection method for evaluating the hermeticity of integrated circuits and micro-electromechanical devices having a cavity. The method includes submerging one or more of the devices in a liquid, under pressure, for a given time period in order to allow the liquid to seep into any cavities that have leak paths. After the pressure soak, the devices are transferred to another liquid and observed under a microscope for signs of the liquid in any of the devices cavities, thereby determining if hermeticity is present.

Description:
FIELD OF THE INVENTION 
     The present invention relates to inspection methods for integrated circuit (IC) and micro-electromechanical (MEM) devices and more particularly to inspection of these devices for hermeticity. 
     BACKGROUND OF THE INVENTION 
     For certain IC and MEM devices, the environment in which they will be operating requires that they are hermetically sealed in order to operate properly. As used herein, when referring to the term “devices”, it is meant to refer to MEM devices or IC devices that contain enclosed cavities. For these devices, then, some type of test or inspection is needed to assure that the ones which are not sealed properly are rejected. A common practice in the integrated circuit industry is to test for hermeticity using helium leak detectors, but this does not work for every type of package and substrate. While this method generally works well for ceramic packages, it does not work well with glass substrates since the glass will readily absorb the helium. The result is that the helium leak detector will detect a leak when in fact there may not be one. Consequently, the need arises for a reliable and inexpensive method to test for hermeticity of the devices, particularly for those applications that employ glass substrates. 
     SUMMARY OF THE INVENTION 
     In its embodiments, the present invention contemplates a method for inspecting for hermeticity of a device containing a cavity. The method comprises the steps of: submerging the device in a liquid; pressurizing the liquid to greater that one atmosphere for a given time period; removing the pressure from the liquid; and inspecting the device under magnification for indications of the liquid in the cavity. 
     Accordingly, an object of the present invention is to soak devices in high pressure liquid to force the liquid into the cavities of defective devices and then inspect the devices for hermeticity. 
     Another object of the present invention is to inspect the devices while in a liquid in order to avoid evaporation of the liquid that may be within the cavities. 
     An advantage of the present invention is that hermeticity can be assured, thus preventing defective devices from being assembled into finished electronic components. 
     A further advantage of the present invention is that hermeticity testing can be conducted for those devices employing glass substrates. 
     Another advantage of the present invention is that multiple devices can be subjected to the inspection process at one time, thus assuring an efficient and cost effective inspection process. 
     An additional advantage of the present invention is that the inspection for leakage can take place with the devices underwater to assure that the liquid which leaks into any of the cavities will not evaporate prior to inspection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of an example MEM device that can be subjected to the hermeticity inspection in accordance with the present invention; 
     FIG. 2 is a schematic plan view of the MEM device of FIG. 1; 
     FIG. 3 is a schematic view of a wafer containing MEM devices undergoing a portion of the inspection process in accordance with the present invention; 
     FIG. 4 is a schematic view of the wafer of the MEM devices undergoing another portion of the inspection process in accordance with the present invention; 
     FIG. 5 is a view of a portion of a sample MEM device illustrating a gas bubble in the device; 
     FIG. 6 is a view of a portion of another sample MEM device illustrating water droplets on a lid surface inside a cavity; 
     FIG. 7 is a view of a portion of yet another sample MEM device illustrating beads of water on a lid surface inside a cavity; and 
     FIG. 8 is a view illustrating a portion of still another sample MEM device illustrating water in a corner of a cavity. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 illustrate a typical MEM device  20 . This device  20  includes a glass substrate  22 , with a silicon lid  24  mounted thereon having a cavity  26  between the glass substrate  22  and the lid  24 . While this is illustrated having a glass substrate, the substrate can also be formed from a ceramic; the device can also be some other type of integrated circuit (IC) containing a cavity, not necessarily a MEM device. A sensor or other type of element, not shown, will then reside in the cavity  26 . Conventional bond pads  28  are also mounted to the substrate  22 . 
     The MEM device that one may wish to inspect can be, for example, an accelerometer as is disclosed in U.S. Pat. No. 5,404,749 to Spangler, incorporated herein by reference. Also, see U.S. Pat. No. 5,264,075 to Zanini-Fisher et al., incorporated herein by reference, for an example of how one can seal such a device during its fabrication. 
     It is important for certain applications to assure that the silicon lid  24  is sealed to the glass substrate  22 . The method disclosed herein provides such assurance. FIG. 3 illustrates the first steps in the hermeticity inspection. A container  34  is filled partially with liquid  36 . This can be any of various liquids, such as fluorescent dies, but preferably is water in order to minimize cost. A wafer  38  of MEM devices  20  is placed in the water  36 . Individual MEM devices  20  can be inspected, but it is more cost efficient to conduct these steps with the whole wafer  38 . In fact, one may use a container with a deeper chamber and put more than one wafer in at a time in order to make the process more efficient. 
     The container  34  is then sealed with a lid  40 , that is attached to the container  34  by a conventional method that will maintain a seal up to the desired pressure. After sealing, the space above the water  36  is pressurized. One method is to pump nitrogen  42  into the volume above the water  36  up to the desired pressure level. Another preferred method is to employ a hydraulic system to pressurize the water  36  itself with the container filled essentially completely with water  36 . After pressurization, the wafer  36  then remains in the pressurized container  34  for a predetermined amount of soaking time. This will allow for the water  36  to seep into any cavities that are not fully sealed. An added benefit of the high pressure water soaking is that it inherently cleans some of the contaminates off of the surfaces of the devices  20 . So, while water is not the only liquid that can be employed for the high pressure soaking, it does possess this added advantage. 
     An example of the pressurized soaking portion of the process can be described with respect to the MEM device  20  of FIGS. 1 and 2. If the glass substrate  22  has overall length and width dimensions of about 0.070 inches and a thickness of about 0.023 inches, and the silicon lid  24  has length and width dimensions of about 0.065 inches and a thickness of about 0.016 inches, then a satisfactory test may include pressurizing the container  34  to about 1200 pounds per square inch and holding it there for about 24 hours. Of course, higher pressures may provide a more robust test, but then one must accommodate the higher pressures; and longer soak times may be preferred for allowing the water  36  to seep into the cavities, but then the test time is extended for each batch of devices. The length of time and pressures desired will also depend upon the size and shape of the device and cavity being tested for hermeticity. 
     After the pressurized soaking portion of the process, the wafer  38  is then removed from the container  34  and can be placed under a microscope  44  for observation by a technician. Preferably this is done with a precision X-Y table to assure accuracy in locating the wafer. If the devices  20  on the wafer  38  are observed by the technician in the air, then the observation must take place very soon after the wafer  38  is removed from the water  36  in order to avoid having the water evaporate from the cavities. 
     A preferred way to conduct the inspection, in order to avoid the time constraint and assure more consistent results, is to transfer the wafer  38  from the container  34  and place it in a second container  46  of water  48  under the microscope  44 , as is illustrated in FIG.  4 . This avoids the concern with evaporation of the water that may have leaked into any of the cavities. A technician will then look through the microscope  44  at each cavity to determine if there are signs of water leakage into that particular cavity. There are several ways that the technician can then determine which devices are defective. One indicator is the shading of a particular cavity, with darker areas indicating the presence of water and relatively lighter areas indicating a gas, such as air. Other signs of possible water leakage are discussed below with respect to FIGS. 5-8. 
     As an alternative to moving the wafer  38  after the high pressure portion of the procedure, the high pressure container can be configured to allow for the wafer  38  to be left in the original water and inspected. But this then ties up the high pressure container until the visual inspection is done rather than allowing a new batch of wafers to begin the high pressure soak, and may require a more complicated design for the high pressure container, so it is preferred to move the wafer to another location. 
     FIGS. 5-8 illustrate various signs of water leakage within the cavities of MEM devices that are present after a high pressure soaking but before the water can evaporate. These figures show that it is relatively easy, through magnified visual inspection, to determine which cavities are not hermetically sealed and reject the devices as defective. 
     FIG. 5 illustrates a portion of a sample MEM device  58  under the microscope after the high pressure water soaking. A gas bubble  60  is surrounded by water  62  under the lid  64 , indicating that there was leakage. This device  58  does not have a proper hermetic seal and will be rejected as defective. 
     FIG. 6 illustrates a portion of another sample MEM device  66  under the microscope after the high pressure water soaking. One will note that water droplets  68  are adhering to a lid  70  inside a cavity  72 , again indicating a defect. 
     FIG. 7 is a view of a portion of yet another sample MEM device  74  illustrating beads of water  76  adhering to a lid surface  78  inside a cavity  80 . And, FIG. 8 is a view illustrating a portion of still another sample MEM device  82  illustrating water  84  in a corner  86  of a cavity  88 . Note that the corner  86  appears rounded, this indicates water  84  in the corner; a dry corner would have sharp edges. Again, FIGS. 7 and 8 indicate defective devices. While FIGS. 5-8 illustrate some signs of water in device cavities, they are not meant to be exhaustive since there are other indicators of water leakage, which sometimes depend upon the particular geometry of a given device, its features, and the shape and size of the cavity. 
     Thus, while certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.