Abstract:
Microelectromechanical system (MEMS) devices and methods with controlled die bonding areas. An example device includes a MEMS die having a glass layer and a protective package. The glass layer includes a side facing the protective package with at least one mesa protruding from a recessed portion of the glass layer. The at least one mesa is attached to the protective package. An example method includes creating at least one mesa on a glass layer of a MEMS die and attaching the at least one mesa to a protective package.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Inertial microelectromechanical system (MEMS) sensors are sensitive to stresses from mounting the sensor in a protective package. A major source of die mounting stress is a mismatch in the speed at which the die and its package expand with temperature changes related to their coefficient of thermal expansion (CTE). Current methods of reducing stress such as using a single attachment point in the center of the die have weaknesses such as not providing enough strength and stability. Additionally, controlling an area where an attachment material such as solder is in contact with the die and package can be difficult and has other drawbacks. 
       SUMMARY OF THE INVENTION 
       [0002]    The present invention includes microelectromechanical system (MEMS) devices and methods with controlled die bonding areas. An example device includes a MEMS die having a glass layer and a protective package. The glass layer includes a side facing the protective package with at least one mesa protruding from a recessed portion of the glass layer. The at least one mesa is attached to the protective package. 
         [0003]    In accordance with further aspects of the invention, the glass layer includes a plurality of mesas. 
         [0004]    In accordance with other aspects of the invention, the device further includes an attachment material between the at least one mesa and the protective package. 
         [0005]    In accordance with still further aspects of the invention, an example method includes creating at least one mesa on a glass layer of a MEMS die and attaching the at least one mesa to a protective package. 
         [0006]    In accordance with yet other aspects of the invention, the method includes creating a plurality of mesas. 
         [0007]    In accordance with still another aspect of the invention, creating at least one mesa includes abrading a glass layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0009]      FIG. 1  is a diagram showing a cross-sectional side view of a packaged MEMS device formed in accordance with an embodiment of the invention; 
           [0010]      FIG. 2  is a diagram showing a bottom view of a MEMS die having mesas formed in accordance with an embodiment of the invention; 
           [0011]      FIG. 3  is a diagram showing a cross-sectional side view of a MEMS device formed in accordance with an embodiment of the invention; and 
           [0012]      FIGS. 4 and 5  are flowcharts of a method of reducing thermal stress in a packaged MEMS device in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]      FIG. 1  is a diagram showing a cross-sectional side view of a packaged MEMS device  20  formed in accordance with an embodiment of the invention. The packaged MEMS device  20  includes a protective package  22  having a first package component  24  and a second package component  26 . A MEMS device die  28  having a device layer  30 , a first glass layer  32 , and a second glass layer  34  is attached to the first package component  24 . The device layer  30  may be formed of silicon and the first and second glass layers  32 ,  34  may be formed of a borosilicate glass such as Pyrex®, for example. In other embodiments, differing numbers of layers and/or different layer materials may be used. The MEMS device die  28  includes an inertial sensor such as an accelerometer or a gyroscope in an example embodiment, but may include other types of MEMS devices in other embodiments. 
         [0014]    The first glass layer  32  includes a first mesa  36  and a second mesa  38 . The mesas  36 ,  38  are typically between 20 micrometers (μm) and 200 μm in height, but may have a different height in some embodiments. The mesas  36 ,  38  are attached to the first package component  24  with an attachment material  40 . In an example embodiment, the attachment material  40  is a gold-tin solder. However, in other embodiments, other types of solder or other types of materials such as glass frit or epoxy may be used. A recessed portion  42  of the first glass layer  32  is not attached to the protective package  22 . It should be understood that  FIG. 1  is not drawn to scale and that various components may have different relative sizes in some embodiments. The protective package  22  may be smaller than shown in relation to the MEMS die  28  and/or the attachment material  40  may be thinner in relation to the first glass layer  32  than shown, for example. 
         [0015]      FIG. 2  is a diagram showing a bottom view of the MEMS device die  28  shown in  FIG. 1 . In addition to the first and second mesas  36 ,  38 , a third mesa  44  and a fourth mesa  46  are also on the first glass layer  32 . Although the mesas  36 ,  38 ,  44 ,  46  are shown as having a square profile when viewed from the bottom, the mesas  36 ,  38 ,  44 , and/or  46  may have almost any shape such as rectangular, round, toroidal, or semi-circular for example. The mesas  36 ,  38 ,  44 ,  46  are typically between approximately 50 and 500 μm across, but may have dimensions outside this range in other embodiments. The mesas  36 ,  38 ,  44 ,  46  are used as bond areas for attachment to the protective package  22 . The recessed portion  42  exists in areas of the first glass layer  32  that do not have mesas. The recessed portion  42  is an area that is not desired to be in contact with the protective package  22 . In an example embodiment, the recessed portion  42  has an abraded surface. Although only a single die is shown, it should be understood that a plurality of dies will typically be formed at a wafer level followed by dicing the wafer to separate each individual die. 
         [0016]      FIG. 3  is a diagram showing a cross-sectional side view of the MEMS device die  28  attached to a substrate  50 . In some embodiments, the device die  28  may use a simple die carrier without sidewalls and lid or just be attached directly to a next level of a system, such as a board or intermediate substrate. In some embodiments, the device die  28  is sealed at the die level and does not need a package lid such as the second package component  26  shown in  FIG. 1 . The device die  28  may then be mounted directly to a system board or to a chip carrier or other intermediate substrate which relieves strain and redistributes signals. Signals in and out of the MEMS device die  28  can go through metal on the mesas  36 ,  38  or through wire bonds to pads on a package, such as may be used with “flip chip” mounting, for example. 
         [0017]      FIGS. 4 and 5  are flowcharts of a method  200  of reducing thermal stress in a packaged MEMS device in accordance with an embodiment of the invention. First, at a block  202 , at least one mesa is created on a layer of a MEMS die. The first mesa  36  may be created on the first glass layer  32  of the MEMS device die  28 , for example. Next, at a block  204 , the at least one mesa is attached to a protective package or substrate. The first mesa  36  may be attached to the protective package  22  or the substrate  50 , for example. 
         [0018]    As shown in  FIG. 5  creating at least one mesa at the block  202  may include abrading a glass layer of the MEMS die by applying a mask to the glass layer at the block  206 . The mask is generally formed of a metallic or very tough polymer material. Next, at a block  208 , exposed areas of the glass layer are abraded by accelerating an abrasive such as a ceramic material toward the glass layer using a gas. In an example embodiment, the gas is air. However, in other embodiments, other gases or a liquid may be used. Although other techniques may be used to form the mesas in some embodiments, abrasion is preferred with a glass layer because glass is generally resistant to chemical etching and is difficult to laser cut. Then, the mask is removed. Generally, metal masks are held by tooling or temporary adhesives and masks made of polymers are held by adhesives. Depending on the adhesive used, the mask is peeled off or the mask and/or adhesive layer is dissolved using a solvent. Although the method  200  is described with respect to a single MEMS device die, it should be understood that the mesas will typically be created on a plurality of MEMS device dies on a wafer, with the wafer being diced after the mesas have been created. In some embodiments, mesas may be created on a glass layer before the glass layer is attached to a device layer. In other embodiments, the mesas may be created after the glass layer has been attached to the device layer. 
         [0019]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, mesas having a different shape may be formed, and/or mesas may be formed in a material other than glass. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.