Abstract:
A method for applying anti-stiction material to a micro device includes encapsulating a micro device in a chamber, vaporizing anti-stiction material in a container to form vaporized anti-stiction material, transferring the vaporized anti-stiction material from the container to the chamber, and depositing the vaporized anti-stiction material on a surface of the micro device.

Description:
BACKGROUND 
       [0001]    The present disclosure relates to the packaging of micro devices. 
         [0002]    Assuring reliability and yield are two critical tasks for the manufacturing of micro devices, such as integrated circuits and micro electro-mechanical systems (MEMS). Typically, in manufacturing micro devices, multiple micro devices are fabricated on a semiconductor wafer. The semiconductor wafer is then separated into individual dies each containing one or more individual micro devices. The electrical and optical performance of the micro devices are often tested for quality assurance on the individual dies in an ambient environment. For testing purposes, electrical and optical signals need to be properly input into the circuits in each micro device. Output electric and optical signals from the micro devices need to be properly detected and measured to analyze the functional performance of the micro devices. During testing and handling of the micro devices, the micro devices must not be contaminated by dust and pollutants in the ambient environment. Electrical and optical input and output, as well as protecting the micro devices from the environment, all need to be considered when designing packaging for the micro devices. A need therefore exists for improved packaging for micro devices to ensure desired and robust device performance. 
       SUMMARY 
       [0003]    In one general aspect, the present invention relates to a method for applying anti-stiction material to a micro device. The method includes encapsulating a micro device in a chamber, vaporizing anti-stiction material in a container to form vaporized anti-stiction material, transferring the vaporized anti-stiction material from the container to the chamber, and depositing the vaporized anti-stiction material on a surface of the micro device. 
         [0004]    In another general aspect, the present invention relates to a micromechanical system that includes a chamber comprising an inlet to permit the transfer of a vaporized anti-stiction material into the chamber, a micro device encapsulated in the chamber, wherein the micro device comprises a first component and a second moveable component configured to contact the first component, and anti-stiction material coated on a surface of the first component or the second moveable component to prevent stiction between the first component and the second moveable component. 
         [0005]    Implementations of the system may include one or more of the following. The method can further include evacuating the chamber before the step of transferring. The step of transferring can include diffusing the vaporized anti-stiction material into the chamber. The step of transferring can include connecting an outlet of the container with an inlet of the chamber to permit fluidic communication between the container and the chamber. The step of transferring can include opening a valve at the outlet of the container. The method can further include sealing the inlet of chamber after the step of transferring. The step of vaporizing can include heating the anti-stiction material. The step of vaporizing can include evaporating the anti-stiction material. The step of vaporizing can include subliming the anti-stiction material. The micro device can include a first component and a second moveable component configured to contact the first component. The method can further include depositing the vaporized anti-stiction material on a surface of the first component or a surface of the second moveable component to prevent stiction between the first component and the second moveable component. The second moveable component can be a micro mirror plate configured to tilt. The chamber can include a window transparent to at least one of visible, UV, or IR light. The anti-stiction material can include tridecafluoro-1,1,2,2,-tetrahydrooctyltrichlorosilane (FOTS) or heptadecafluoro-1,1,2,2,-tetrahydrooctyltrichlorosilane (FDTS). 
         [0006]    Implementations may also include one or more of the following advantages. A potential advantage of the disclosed systems and methods is simplification of the fabrication process of the micro-device. Anti-stiction material can be applied to a plurality of micro devices after the micro devices are encapsulated in micro chambers on a semiconductor wafer (i.e., in situ). The anti-stiction material can be vaporized in a container. The vapor phase anti-stiction material can be transferred to a micro chamber containing a micro device through an inlet to the micro chamber. The evaporated anti-stiction material can be deposited on the surfaces of the micro devices to prevent stiction between components that can contact each other in the operation of the micro device. The inlet to the micro chamber can be subsequently sealed. In contrast, anti-stiction material is conventionally deposited on the surface of the components during the fabrication of the micro devices. The in situ application of anti-stiction material disclosed in the present specification may reduce the device development and testing times. 
         [0007]    Furthermore, the chamber encapsulating the micro device can be evacuated, receive the anti-stiction material in vaporized form in the same vacuum environment, and sealed all in the same vacuum environment. No valve is needed in the inlet of the chamber, which also simplifies the design and the fabricating of the encapsulation chamber. 
         [0008]    Another potential advantage of the disclosed systems and methods is that the anti-stiction materials can be heated and vaporized in a container separate from the chamber. Thus the micro device and the associated control circuit in the chamber as well as the sealing to the chamber will not be affected by the heating process. 
         [0009]    Another potential advantage of the disclosed systems and methods is that anti-stiction materials may be applied to contact areas that are hidden in a micro device. For example, the contact surfaces between a tiltable mirror plate and a landing tip on a substrate can be hidden underneath the mirror plate. The contact surfaces are often formed at the final stage of the device fabrication. The disclosed methods and system may provide a way to isotropically deposit anti-stiction material on the contact surfaces that are hidden by other components of the micro device. 
         [0010]    Yet another potential advantage of the systems and method described herein is the prevention of particles being applied to the surfaces of the micromirrors. When particles are prevented from landing on the mirrors, the production yield can be increased. 
         [0011]    Although the invention has been particularly shown and described with reference to multiple embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The following drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles, devices and methods described herein. 
           [0013]      FIG. 1  is a flowchart for packaging and applying anti-stiction material to micro devices. 
           [0014]      FIG. 2  is a cross sectional view of an exemplified micro device. 
           [0015]      FIG. 3  illustrates the transfer of vapor-phase anti-stiction material to an encapsulated micro device. 
           [0016]      FIG. 4  is a cross-sectional view of an encapsulated micro device along line A-A in  FIG. 3 . 
           [0017]      FIG. 5  illustrates the transfer of vapor-phase anti-stiction material to several encapsulated micro devices. 
           [0018]      FIG. 6  is a cross sectional view of a micro device after it has received the anti-stiction material. 
           [0019]      FIG. 7  illustrates a wafer with a plurality of encapsulated micro devices. 
           [0020]      FIG. 8  illustrates a system for transferring vapor-phase anti-stiction material to a plurality of encapsulated micro devices. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring to  FIGS. 1 and 2 , a micro device  200  is formed on a substrate  210  (step  110 ). The substrate  210  can be semiconductor wafer including addressing and control electric circuit in a complementary metal-oxide semiconductor (CMOS) layer. The micro device  200  can include a microstructure that can produce a mechanical movement, or can produce electromagnetic signals, acoustic signals, or optical signals in response to an input signal. The micro device can include micromechanical electrical systems (MEMS) such as an array of tiltable micro mirrors, integrated circuits, micro sensors, micro actuators, and light emitting elements. A plurality of micro devices  200  can be formed on the substrate  210 . 
         [0022]    In one embodiment of a MEMS micro device, the micro device  200  includes a mirror plate  202  that is tiltable around a hinge component  206 . The hinge component  206  is supported by a post  205  that is connected to the substrate  210 . The mirror plate  202  can include a hinge layer  203   c,  a spacer layer  203   b,  and a reflective layer  203   a.  The reflective layer can reflect an incident light beam in a direction  230  to a direction  240 . A pair of electrodes  221   a  and  221   b  can be formed on a hinge support frame  208  on the substrate  210 . A pair of mechanical stops  222   a  and  222   b  can also be formed on the substrate  210  for stopping the tilt movement of the mirror plate  202  and defining precise tilt angles for the mirror plate  202 . The hinge layer  203   c  can be made of an electrically conductive material. The hinge layer  203   c  and the mechanical stops  222   a  and  222   b  can be electrically connected to a common electrode  233 . The electrodes  221   a  and  221   b  can be separately connected to electrodes  231  and  232 . The substrate  210  can include an electric circuit in connection with the electrodes  231 - 233 . 
         [0023]    Electric signals can be applied to the electrodes  231 - 233  to produce electric potential differences between the hinge layer  203   c  and the electrodes  221   a  or  221   b.  Properly designed voltage signals can produce electrostatic torques that can tilt the mirror plate  202  away from an un-tilt direction (which is normally parallel to the upper surface of the substrate  210 ). The tilting of the mirror plate  202  produces a distortion in a hinge (not shown) connected with the hinge component  205  and an elastic restoring force associated with the distortion. The elastic restoring force pulls the tilted mirror plate  202  back to the un-tilted position. The electrostatic torque can overcome the elastic restoring force to tilt the mirror plate  202  to come into contact with one of the mechanical stops  222   a  and  222   b.  The position of the mirror plate  202  when in contact with the mechanical stops  222   a  or  222   b  can determine the “on” or the “off” position of the mirror plate and determine the direction  240  of the reflected light. Optionally, the micro devices  200  formed on the substrate  210  are tested by applying external signals to the micro device  20  and measuring mechanical movement of the micro device  200  or output signals produced by the micro device  200 . 
         [0024]    The micro devices  200  can then be encapsulated (step  120 ) by bonding an encapsulation cover to the substrate  210 . Encapsulation as described herein is not merely covering a device, but permanently enclosing a micro device within one or more layers, such as by adhering the layers together or causing them to be connected in such as way that the encapsulation cannot be pulled away from other layers or parts surrounding the device unless cut or broken. The encapsulation may include an inlet that allows the fluidic communication between inside and outside of the encapsulation in the packaging process of the micro device, as described below. The inlet can be sealed to fully enclose the micro device in the encapsulation. The micro devices  200  and encapsulation cover can then be diced and cut into individual dies  300  each containing one or more micro devices  200  in a chamber  260  (step  130 ). Details about the encapsulation and dicing of the micro devices are disclosed in the pending U.S. patent application Ser. No. 11/379,932, titled “Micro device encapsulation”, filed Apr. 24, 2006, which is incorporated by reference herein for all purposes. 
         [0025]    A common problem for micro devices is stiction between components that contact each other during operation. For example, a mirror plate  202  can tilt to an “on” position, wherein the micro mirror plate directs incident light to a display device, and an “off” position, wherein the micro mirror plate directs incident light away from the display device. The mirror plate  202  can be stopped by mechanical stops  222   a  and  222   b  at the “on” or the “off” positions to precisely define tilt angles of the mirror plate  202  at these two positions. The mirror plate  202  stopped at the “on” or the “off” position must be able to overcome stiction between the mirror plate  202  and the mechanical stops  222   a  and  222   b.  A delay in the response of the mirror plate  202  can affect the proper operation of the micro mirror  202 . 
         [0026]    Referring to  FIGS. 3 and 4 , each die  300  includes one or more micro devices  200  encapsulated in a chamber  260 . The chamber  260  is defined by a cover  310  and spacer walls  320 . The cover  310  can be transparent to visible, UV, or IR light to allow optical signals to be sent to or received from the micro device  200  through the cover  310 . One or more electric contacts  340  can be formed on the substrate  210  outside of the chamber  260 . The electric contacts  340  are provided for sending electric signals to the micro device  200  or receiving electric signals from the micro device  200 . An inlet  350  is in fluid communication with the chamber  260  and in some embodiments, is directly adjacent to the chamber. Optionally, the die  300  is placed in a vacuum environment to exhaust the air or gas in the chamber  260  (step  140 ). The devices can be cleaned, such as by a dry clean process after the chamber  260  has been evacuated (step  150 ). 
         [0027]    The inlet  350  to the chamber  260  is configured to be connected with the outlet  365  of a container  360 . The outlet  365  of the container  360  can be opened or closed by a valve  370 . The container  360  contains an anti-stiction material. Examples of the anti-stiction material compatible with the disclosed system and methods can include tridecafluoro-1,1,2,2,-tetrahydrooctyltrichlorosilane (FOTS) or heptadecafluoro-1,1,2,2,-tetrahydrooctyltrichlorosilane (FDTS). 
         [0028]    If the anti-stiction material is in a non-vapor form, the anti-stiction material is heated by a heat source  380  while the valve  370  is in a closed position. The vaporized anti-stiction material is in the container  360  (step  160 ). Before heating, the anti-stiction material can be in a solid state, a liquid state, or a polymer melt. The vaporization process can thus include evaporation or sublimation of the anti-stiction material. The outlet  365  of the container  360  is then moved in the direction  355  to be coupled with the inlet  350  of the chamber  260  to allow fluidic communication between the chamber  260  and the container  360 . 
         [0029]    The vaporized anti-stiction material is transferred from the container  360  to the chamber  260  (step  170 ). For example, the vaporized anti-stiction material can diffuse from the container  360  to the chamber  260 , which can be driven by the higher vapor concentration in the container  260  compared to the low-pressure degassed environment in the chamber  260 . The vaporized anti-stiction material cools and deposits on the surface of the micro device  200 . For example, as shown in  FIG. 6 , anti-stiction material  250  can be deposited on the lower surface of the hinge layer  203   c.  Anti-stiction material  251   a  and  251   b  can be deposited respectively on the upper surfaces of the mechanical stops  222   a  and  222   b.  The anti-stiction material  250 ,  251   a  and  251   b  coated on the contact surfaces of the mirror plate  202  and the mechanical stops  222   a  and  222   b  can help the mirror plate  202  to overcome the stiction at the contact surface and ensure timely tilt response by the mirror plate  202 . The anti-stiction material may also be deposited on the surface of the reflective layer  203   a.  In some embodiments, the deposition can be controlled such that the layer thickness of the anti-stiction material is kept much shorter than the wavelength of light (visible, UV, or IR light). For example, the layer thickness of the anti-stiction material deposited on the reflective layer  203   a  can be controlled at 1-50 nanometers, or in one or a few monolayers. The layer thickness can be controlled for example by the time and temperature at which the container  360  is heated and the valve  370  is opened during the vapor transfer. 
         [0030]    An advantage of the disclosed process is that the vaporization of the anti-stiction material does not require the heating of the micro devices. The micro device, the electric circuit in the (CMOS) substrate, and the encapsulation sealing of chamber  260  thus are not be affected by the heating process. 
         [0031]    Another advantage of the disclosed process is that anti-stiction material can be applied to contact areas that are hidden in a micro device after the micro device is fully formed. The disclosed methods of application of the anti-stiction material do not require additional steps in the fabrication of the micro device. For example, the lower surface of the hinge layer  203   c  and the upper surfaces of the mechanical stops  222   a  and  222   b  are hidden under the mirror plate  202  and are not readily accessible if the anti-stiction material were applied from above the mirror plate  202 . It can thus be difficult to apply anti-stiction material from above the mirror plate  202 . Using the disclosed methods, vaporized anti-stiction material can be isotropically deposited on the contact surfaces that are hidden by other components of the micro device. 
         [0032]    The inlet  350  is subsequently sealed (step  180 ). In some embodiments, the inlet  350  is sealed with an epoxy seal. The micro device  200  having the deposited anti-stiction material can be further tested in the encapsulated environment in the chamber  260  by applying or receiving electric signals to the electric contacts  340  or using optical communications through a transparent cover  310  (step  190 ). An advantage of the disclosed system and methods is that the chamber  260  can stay in a same vacuum environment for the application of the anti-stiction material and the subsequent sealing of the inlet  350 . 
         [0033]    In some embodiments, as shown in  FIG. 5 , the container  360  can be coupled to a plurality of chambers  260 ,  260   a  and  260   b  on a multiple of dies  300 ,  300   a  and  300   b.  Each die  300 ,  300   a  and  300   b  can include electric contracts  340 ,  340   a  and  340   b  for electrical communications from outside of the chambers  260 ,  260   a  and  260   b.  The container  360  includes a conduit  390  that can be multiplexed to a plurality of outlets  365 ,  365   a,  and  365   b.  The outlets  365 ,  365   a,  and  365   b  can be connected to the inlets  350 ,  350   a  and  350   b  in the chambers  260 ,  260   a  and  260   b  to allow fluidic communication between the chamber  260 ,  260   a,  or  260   b  and the container  360 . The vaporized anti-stiction material produced in the container  360  can thus be simultaneously transferred to a plurality of chambers  260 ,  260   a  and  260   b.    
         [0034]    In some embodiments, the transfer of the vaporized anti-stiction material is conducted on a single substrate that includes a plurality of chambers each containing one or more micro devices. The plurality of outlets  365 ,  365   a,  and  365   b  can be aligned and engaged with the inlets of the plurality of chambers on the common substrate. The vaporized anti-stiction material can be transferred to the chambers and the respectively encapsulated micro devices. The chambers can then be sealed and are cut into individual dies each containing one or more encapsulated micro devices. The processes described herein allow for applying the anti-stiction material at either the die level or the wafer level. It is understood that the disclosed systems and methods are compatible with a variety of anti-stiction materials. The disclosed system and methods are also compatible with different configurations of the device-encapsulation chambers and containers for holding the vaporized anti-stiction materials. The micro device can generally include micromechanical electrical systems (MEMS) such as tiltable micro mirrors, integrated circuits, micro sensors, micro actuators, and light emitting elements. 
         [0035]    In some embodiments, referring to  FIGS. 7 and 8 , a plurality of chambers  260   a - 260   f  are formed on a wafer  700 . Each chamber  260   a - 260   f  includes spacer walls  320   a - 320   f  and a cover  310   a - 310   f  that encapsulates a micro device  200   a - 200   f.  Each chamber  260   a - 260   f  can also include an inlet  350   a - 350   f.  Each micro device  200   a - 200   f  is connected with electric contracts  340   a - 340   f  that provide electrical communications from outside of the chambers  260   a - 260   f.  The chambers  260   a - 260   f  can be formed by bonding a cover having a plurality of spacer walls to the wafer  700 . The cover can then be selectively cut to expose areas  710  and the electric contracts  340   a - 340   f  on the wafer  700 . 
         [0036]    The wafer  700  including the chambers  260   a - 260   f  and the respective encapsulated micro devices  200   a - 200   f  can be placed in a chamber  800  for the transfer of anti-stiction material to the micro devices  200   a - 200   f.  In some embodiments, the wafer is placed on a temperature controlled substrate  810 . An outlet  820  in the chamber  800  can be connected with a vacuum pump that evacuates air or fluid from the chamber  800  when a valve  825  is opened. A vacuum state can be maintained in the chamber  800  when the valve  825  is closed. Vaporized anti-stiction material is produced in the container  360 . The vaporized anti-stiction material can be transferred from the container  360  to the chamber  800  when the valve  370  is opened. The vaporized anti-stiction material is subsequently transferred into individual chambers  260   a - 260   f  through inlets  350   a - 350   f  and deposited on the surfaces of the micro devices  200   a - 200   f.  After the transfer of the anti-stiction material, the inlets  350   a - 350   f  can be sealed in vacuum by epoxy that can be applied to the inlets  350   a - 350   f,  for example, by a dispenser. 
         [0037]    The methods and systems described herein can provide advantages in terms of manufacturing the MEMS devices. During manufacturing, the risk of particles, such as dust or other debris from the air, of landing on the MEMS device is typically present. Particles of about 1 micron or greater on the MEMS device surface, particularly on the surface of a micromirror, can reduce the functionality of the device, even to the point that the device is not useful. Reducing the likelihood of particles landing on the MEMS device surfaces can create cleaner devices. In turn, the manufacturing yield may be increased using the methods and systems described herein.