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MODIFYING THE ELECTRO-MECHANICAL
BEHAVIOR OF DEVICES
CROSS REFERENCE TO RELATED
control material in the package, where the environmental control material is selected based upon the material's effect on operation of the element.
BRIEF DESCRIPTION OF THE DRAWINGS
This application is a continuation of, and claims priority to, U.S. patent application Ser. No. 10/839,307, filed May 4, 2004 now U.S. Pat. No. 7,060,895, which is incorporated by reference in its entirety. 10
BACKGROUND OF THE INVENTION
Microelectromechanical systems (MEMS) typically involve at least one movable element in a self-contained 15 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. MEMSbased switches may be encased in packages having protec- 20 tive 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.
This packaging type results in an operating environment 25 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 her- 30 metic process is discussed in U.S. 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.
Another accommodation to the package environment is 35 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. 40
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. 45
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 50 performance, as the elements may move more quickly than in environments without reduced partial pressure.
SUMMARY OF THE INVENTION
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 60 device of this embodiment further includes an environmental control material inside the package to affect an operation of the movable element.
Another embodiment provides a method of packaging a device, the method including forming at least one movable 65 element on a substrate, enclosing the element and the substrate in a package, and including an environmental
The invention may be best understood by reading the disclosure with reference to the drawings, wherein:
FIG. 1 shows a hysteresis curve of an operating microelectromechanical device.
FIG. 2 shows hysteresis curves of operating microelectromechanical devices in different environmental conditions.
FIG. 3 shows one embodiment of a device package for a microelectromechanical device including an environmental control material.
FIG. 4 shows one embodiment of a module package for a microelectromechanical device including an environment control material.
FIG. 5 shows a flowchart of an embodiment to manufacture a microelectromechanical device including an environmental control material.
DETAILED DESCRIPTION OF THE
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.
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.
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.
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.
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 5 device. A change in the switching voltage may lead to a change in the current consumption of the device.
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 10 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 15 result in a significant decrease in the time it takes for the movable element to achieve its actuated state.
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 20 stiffness. The altered mechanical stiffness may alter the response time or release time of the movable element.
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 25 to C02. If the device is intended to last 5 years, the C02 adsorption can be controlled. Assume the permeation rate of CO2 is (X) grams/square meter/24 hours. If A is an area through which C02 can permeate, an environmental control material having a pore size of 5 A may be selected. The total 30 amount of C02 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:
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 40 elements in the presence of moisture, thereby changing the optical properties of the device.
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. 45 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 adsorb- 50 ing 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.
Other aspects of the device that can be controlled by the 55 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 60 damage the device. This effect can be avoided by the selection of the environmental control material.
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 65 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.
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.
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 50a through 50e. The manufacture of the movable elements, step 40 could include the environmental control material at step 50a. 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.
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 50b, 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 50c.
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 50d. The joining material is then applied to form the module package at step 48, where the environmental control material may be included at step 50e. 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.
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