Abstract:
A MEMS device is packaged with a control material that is included in the package to affect an operation of a moveable element of the device. The control material may affect operational characteristics including actuation and release voltages and currents, mechanical affects including damping and stiffness, lifetime of the device, optical properties, thermal affects and corrosion. The control material may be inserted into the package as part of any of several structural components of the package or the MEMS device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of, and claims priority to, U.S. patent application Ser. No. 10/839,307, filed May 4, 2004 which is incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Microelectromechanical systems (MEMS) typically involve at least one movable element in a self-contained package. For example, spatial light modulators such as interferometric or deflectable, reflective modulators may be contained in a package having a clear window offset from a substrate by a ring or other packaging support. MEMS-based switches may be encased in packages having protective caps offset from a substrate by similar components. Generally, MEMS devices will lie between the substrate and an offset cap or cover, as the devices need room to move when activated by an actuation signal.  
         [0003]     This packaging type results in an operating environment for the MEMS devices that needs to be controlled. In many instances, the solution to controlling the environment inside the package involves establishing the environment inside the package and then hermetically sealing it to ensure that the environment remains constant. An example of such a hermetic process is discussed in US Pat. No. 6,589,625, issued Jul. 8, 2003. The process of hermetically sealing packages contributes to the cost of the devices, as well as the complexity of the manufacturing process.  
         [0004]     Another accommodation to the package environment is the use of getters. A getter is a material that is inserted into the package environment to adsorb moisture from the devices. A getter may adsorb water as it forms from high temperature operation, or may adsorb other materials that outgas from the operation of the device, as examples.  
         [0005]     Control of the humidity of the interior of the package may have dramatic effects on the operation of the devices inside. High humidity may cause problems with stiction, a combination of sticking and friction, while completely dry environments may have other effects.  
         [0006]     Other aspects of the environment may cause problems or result in increased performance. For example, reducing the partial pressure inside the package may speed the devices, as any damping effects of air between the movable element and the substrate may be reduced. This may result in increased performance, as the elements may move more quickly than in environments without reduced partial pressure.  
       SUMMARY OF THE INVENTION  
       [0007]     An embodiment provides a device that includes a first surface and a second surface offset from the first surface to form a package. The device of this embodiment further includes at least one movable element within the package having a movable surface to contact another surface. The device of this embodiment further includes an environmental control material inside the package to affect an operation of the movable element.  
         [0008]     Another embodiment provides a method of packaging a device, the method including forming at least one movable element on a substrate, enclosing the element and the substrate in a package, and including an environmental control material in the package, where the environmental control material is selected based upon the material&#39;s effect on operation of the element. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention may be best understood by reading the disclosure with reference to the drawings, wherein:  
         [0010]      FIG. 1  shows a hysteresis curve of an operating microelectromechanical device.  
         [0011]      FIG. 2  shows hysteresis curves of operating microelectromechanical devices in different environmental conditions.  
         [0012]      FIG. 3  shows one embodiment of a device package for a microelectromechanical device including an environmental control material.  
         [0013]      FIG. 4  shows one embodiment of a module package for a microelectromechanical device including an environment control material.  
         [0014]      FIG. 5  shows a flowchart of an embodiment to manufacture a microelectromechanical device including an environmental control material. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Microelectromechanical (MEMS) devices include a large class of devices in which mechanical structures come into contact with other structures of a different material. For example, a movable element may contact a surface coated with dielectric. The presence of moisture in these types of MEMS devices may result in altered behaviors that may or may not be desirable.  
         [0016]     An example of an undesired behavior occurs when humidity in the device operating environment becomes too high. When the humidity level exceeds a level beyond which surface tension from the moisture becomes higher than the restoration force of a movable element, the movable element may become permanently stuck to the surface. If the humidity level is too low, the moisture charges up to the same polarity as the movable element when the element comes into contact with the coated surface. This may cause a collapse of the hysteresis curve  10  shown in  FIG. 1 .  FIG. 1  shows a typical operating curve for a MEMS device in normal operating conditions.  
         [0017]     If the device is operating in a low moisture or completely dry environment, the device behavior may become dependent upon the design of the device and the materials, such as the movable element and the coated surface coming into contact. This may result in a drastic alteration of the device operating behavior.  FIG. 2  compares a device operating in a typical environment,  10 , with one operating in a low or no humidity environment  12 .  
         [0018]     In some instances, depending mostly on the application, these different environments may produce the desired results. In the alternative, they may produce results that are to be avoided. Controlling the environment of the movable element or elements can control the behavior of the elements. This has effects in several different aspects of the operating environment.  
         [0019]     For example, the electrical behavior can be altered as discussed above. The amount of charging observed in MEMS devices can be controlled with the amount of moisture, as can the control of electro-static discharge (ESD). The presence or absence of moisture can be controlled to change the actuation voltage, that voltage which causes the movable element to move from an initial position. The release voltage, that voltage which causes the movable element to release from its actuated position, may also be controlled, or controlled instead of the actuation voltage. The amount of voltage needed to switch a MEMS device also relates to the amount of current needed to run the device. A change in the switching voltage may lead to a change in the current consumption of the device.  
         [0020]     Mechanical affects may be controlled as well. Different environment control materials can change the partial pressure of gases inside the package. This can change the mechanical response times of the movable element. In MEMS devices, the gaps between the mechanical membranes and the substrate result in movement resistance similar to that of the mean free path of air. Reducing the partial pressure linearly decreases the damping and may result in a significant decrease in the time it takes for the movable element to achieve its actuated state.  
         [0021]     In addition, the mechanical stiffness of the structural film may be altered. Controlling the humidity inside the package may cause the film to oxidize, changing its mechanical stiffness. The altered mechanical stiffness may alter the response time or release time of the movable element.  
         [0022]     The general properties of the device can be altered as well. The lifetime of the device may be tailored, for example. In one scenario, a device could fail after exposure to CO 2 . If the device is intended to last 5 years, the CO 2  adsorption can be controlled. Assume the permeation rate of CO 2  is (X) grams/square meter/24 hours. If A is an area through which CO 2  can permeate, an environmental control material having a pore size of 5A may be selected. The total amount of CO 2  adsorbed by the material by Y, where Y is between 0 and 1. The formula for how much environmental control material to be placed in the package becomes: 
 
(X*5*365*24*A)/Y. 
 
         [0023]     For MEMS devices used in optical applications, such as photo electronic switches or display elements, the optical properties may be controlled as well. One example may involve a material placed in the device to cause the formation of aluminum oxide on the surfaces of the movable elements in the presence of moisture, thereby changing the optical properties of the device.  
         [0024]     The environmental control material may alter the thermal effects as well. In one example, a device that is normally packaged by hermetic sealing of the device in a vacuum. Dissipation of heat only occurs by radiation. Using an environmental control material that has good convection heat dissipation, no further hermetic sealing or vacuums are necessary. The environmental control material could be comprised of several different materials capable of adsorbing different molecular sizes, or different materials, such as water, oxygen and nitrogen. Regeneration of the environmental control material may also be controlled inside the package.  
         [0025]     Other aspects of the device that can be controlled by the environmental control material include corrosion. Oxidation in presence of moisture and formation of an acidic environment can damage the elements. For example, chlorine may outgas from the device at high temperatures. When combined with moisture, hydrochloric acid may form that may damage the device. This effect can be avoided by the selection of the environmental control material.  
         [0026]     Once selected, the environmental control material can be inserted either into the device package or a module package. A device package  20  is shown in  FIG. 3 . The MEMS device  22  is shown as a layer of thin films, merely as an example, formed on a substrate  24 . In this particular example, the device is a display device, using interferometric effects to form images from incident light. The interferometric modulator elements are formed of a movable element that comes into contact with another surface, such as the substrate or part of the thin film stack shown. The viewer  19  would see the images formed by the modulator through the substrate  24 . A back plate, or second surface,  26  may be offset and joined to the substrate  24 , the first surface, by sides  30  comprising a joining material that forms the package. Alternatively, the back plate may be formed of a structure having a second surface  26  and sides  30 , where the back plate  26  and the sides  30  would be one unit. This forms a recessed back plate to accommodate the modulator. The environmental control material  28  is inserted in the package to cause or avoid the effects as discussed above.  
         [0027]     In an alternative embodiment, the environmental control material can be inserted into a module package, which as defined here is a package in which the device package is contained shown in  FIG. 4 . The module package  31  has a first surface  32  offset from a second surface  36  by a joining material  34 . Inside the module package  31  is the device package  20 . The environmental control material  28  is inserted into the module package  31 . In addition, the environmental control material can be made part of the package by being made part of the joining material  34 , the substrate, or as part of the device manufacture. In one embodiment, the window of the modulator, and the first surface  32  of the module package  31  may be the same surface, or mounted such that they form one structure. In this case, the first surface would be the first surface of both the module package  31  and the device package  20 .  
         [0028]     An embodiment of a manufacturing process is shown in  FIG. 5 . The process has three basic steps, manufacture of the movable elements, step  60 , packaging of the device, step  70  and including the environmental material, steps  50   a  through  50   e.  The manufacture of the movable elements, step  40  could include the environmental control material at step  50   a.  The environmental control material could be made as part of the substrate, the manufacture of the elements themselves or during processing of the various materials during the formation of the elements, such as deposition as thin films.  
         [0029]     The packaging process may or may not include a module package. The movable elements are enclosed in the device package, step  42 . The including environmental control materials may be included in the device package at step  50   b,  as discussed before. The joining material is applied at step  44  to form the device package between the first and second surfaces. Again, as mentioned before the environmental control material could be inserted into the package as part of the joining material at step  50   c.    
         [0030]     If the device is to be enclosed into a module package at step  46 , the module package can then have the environmental control materials at step  50   d.  The joining material is then applied to form the module package at step  48 , where the environmental control material may be included at step  50   e.  The process may include a device package, a module package or both. The packaging process involves joining a first surface or second surface with a joining material, whether that first surface and second surface be a device package or a module package.  
         [0031]     The materials used in each portion of the process may vary depending upon the MEMS device and its application, as well as the environmental effect desired. The device package will generally include a substrate and a back plate. The substrate, or first surface of the device or module package, may include glass, plastic, metal, silicon, or ceramic, among others. The back plate, or second surface of the device or the module package, may be glass, plastic, metal, metal foil, silicon, ceramic, fluid, etc. The joining material used in either package may be an epoxy-based adhesive, o-ring sealants, PIBs, poly-urethanes, adhesives with environmental control materials, thin film metal-to-metal welded joints, liquid spin-on glass, solders, or insitu growth of polymers or plastic layers to form seals.  
         [0032]     The environmental control materials may be zeolites, which is the common name for sodium aluminosilicate, having various pore sizes, molecular sieves, surface or bulk adsorbents that absorb other molecules on the surface or in the bulk, or dessicants. The environmental control material may be a chemical reactant that reacts with the unwanted material to form an innocuous compound, such as one that adsorbs moisture to form another compound that is inert. The environmental control materials may be fabricated as devices that physically obstruct the leakage of molecules once the environmental control materials get inside the molecules. The molecule sieves could be incorporated into thin films deposited or spun on polymers. The molecular sieves could be sprayed on the surface of the device, or prepared in advance and regenerated prior to use, as well as applied as a dip coat.  
         [0033]     In addition to different types of materials and different methods to apply them, the environmental control materials could be of different shapes, sizes and forms. The environmental control material could be a solid, powder that are dumped directly inside the package or mixed with adhesive for application. The materials could be formed into shapes as cylinder, sheets, etc., and then applied inside the package. The material could be screen printed or liquid dispensed inside the package or on the substrate. The application of the material is left up to the system designer.  
         [0034]     Thus, although there has been described to this point a particular embodiment for a method and apparatus for MEMS devices having environmental control materials, it is not intended that such specific references be considered as limitations upon the scope of this invention except in-so-far as set forth in the following claims.