Abstract:
A MEMS device and a method for making a MEMS device are described. The MEMS device includes a support member, an optical device, and a flexible member. In one aspect, the flexible member is formed separately from the support member and the optical device. In one aspect, the flexible member is dimensioned to enable flex in one direction while maintaining stiffness in two orthogonal directions. In one fabrication embodiment, the MEMS device is formed by etching an opening into the structural layer to create a structural support member and an optical device. The structural support member and optical device are mounted on a support substrate with a sacrificial layer. A flexible member is conformally deposited over the structural support member and the optical device and then etched. The sacrificial layer is partially etched away to leave the structural support member anchored to the support substrate.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is generally directed to micro-electro-mechanical system (MEMS) devices, and more particularly to a MEMS device with a flexure member and a method for making such a MEMS device.  
         BACKGROUND  
         [0002]    Occasions arise when devices are manufactured that contain surfaces whose shapes must be accurately controlled to maintain a necessary level of optical performance. Optical MEMS devices are one such device. For example, in an optical cross-connect design, a reflecting mirror must stay flat to suppress any disturbance to the focusing/collimating action of the lens system. Typically, the material which is utilized to fabricate the structural portion of the optical surface does not possess the required optical properties, and thus coatings generally must be applied to the optical surface. The coatings are frequently stressed, which can cause the optical surface to deform, resulting in a loss of performance.  
           [0003]    To inhibit this effect, it is desirable to make the structural portion of the optical surface as stiff as possible. Usually, greater stiffness is achieved by making the structural portion thicker. However, making the structural portion thicker leads to disadvantages when such a device is employed. FIG. 1 illustrates the dimensions (length, width, thickness) of a generic spring.  
           [0004]    In known optical MEMS devices, the same layer of material used to form the optical surface is also used to fabricate a flexure structure. The flexure structure is generally utilized to connect an optical device, such as a mirror, with a support structure. For example, FIG. 2 illustrates a known MEMS device  10  including a flexure structure  12  formed from the same layer of material used to form the optical surface  14  of an optical device  16 .  
           [0005]    A flexure structure, such as flexure structure  12 , allows the optical layer  14  of the optical device  16  to rotate in a direction A when the MEMS device  10  is actuated. Generally, electrostatic force is used to actuate MEMS devices. It is desired to fabricate the MEMS device  10  in such a way as to limit the amount of electrostatic force needed to actuate the device. The amount of actuating force necessary is that which can overcome the stiffness of the flexure structure  12 . Thus, it is known to make the flexure structure  12  relatively compliant. It is further known that a large degree of control over the compliance of the flexure structure  12  is needed to optimize the MEMS device  10  design.  
           [0006]    Highly compliant flexure structures can be fabricated by reducing at least one dimension of the flexure structure. For example, in the instance where the flexure structure is fabricated from the same layer of material as the optical structure, such as the flexure structure  12 , the only dimension which is reducible is the width. Making narrow but deep, i.e., high aspect ratio, structures, however, presents a processing challenge and tends to put a constraint on the thickness T (FIG. 1) of the optical layer  14 . Making the flexure structure  12  more compliant by extending its length L (FIG. 1) encounters other problems, such as requiring a great amount of space and could lead to undesirable deflection modes.  
           [0007]    Thus, the design requirements for the optical surfaces of known optical devices, which should be made as stiff as possible, are in conflict with those for flexure structures, which should be made as compliant as possible.  
           [0008]    As shown in FIG. 3, another MEMS device  110  is shown including a flexure structure  112  out of plane with an optical surface  114  of an optical device  116 . The flexure structure  112  is out of plane with the optical surface  114  by virtue of being mounted on posts  118 . Further, such a design, as described in U.S. Pat. No. 6,201,629 (McClelland et al.), is one in which the flexure structure  112 , and not the optical device  116 , is configured to be actuated.  
           [0009]    Conventionally, one way of fabricating a MEMS device involved fabricating the flexible layer and the mirror on one chip and the driver electronics on another chip and flip chip bonding the two chips together. The use of flip chip bonding has disadvantages. For example, alignment is not as accurate as fabricating the MEMS device from one wafer. Further, flip chip bonding adds an extra complicated, and hence expensive, step to the fabrication process which adds to fabrication costs and often leads to decreases in yield.  
           [0010]    There exists a need for devices having a flexure structure whose dimensions can be decoupled from the dimensions of other components of the optical device. There further exists a need for a MEMS optical device which is fabricated from two different materials planarly aligned and which does not require complicated flip chip bonding.  
         SUMMARY  
         [0011]    The invention provides a MEMS device that includes a support member, an optical device adapted to be electrostatically actuated and having an optical device support layer, and a member dimensioned to be flexible and interconnecting the support member and the optical device support layer. The member is formed separately from the optical device support layer.  
           [0012]    The invention further provides a MEMS device that has a support substrate, a structural support layer, and an optical device adapted to be electrostatically actuated and having a support layer. The support layer and the structural support layer are integrally formed of the same material and are deposited over the support substrate.  
           [0013]    The invention further provides a method for fabricating a MEMS device that includes forming a support member, forming an optical device separated from said support member and having a support layer, and forming a member which is dimensioned to be flexible and which interconnects the support member and the optical device support layer. The member is formed separately from the optical device support layer.  
           [0014]    The invention also provides a method for fabricating a MEMS device that includes forming a support substrate, forming a structural support layer, and forming an optical device adapted to be electrostatically actuated and having a support layer. The support layer and the structural support layer are integrally formed of the same material and are deposited over the support substrate.  
           [0015]    The foregoing and other advantages and features of the invention will be more readily understood from the following detailed description of the invention, which is provided in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 illustrates the dimensions of a generic spring device.  
         [0017]    [0017]FIG. 2 is a perspective view of a conventional MEMS device.  
         [0018]    [0018]FIG. 3 is a cross-sectional view of another conventional MEMS device.  
         [0019]    [0019]FIG. 4 is a perspective view of a MEMS device constructed in accordance with an embodiment of the invention.  
         [0020]    FIGS.  5 - 10  are side views illustrating the fabrication of the MEMS device of FIG. 4.  
         [0021]    [0021]FIG. 11 illustrates method steps for fabricating a MEMS device in accordance with an embodiment of the invention.  
         [0022]    FIGS.  12 - 15  are side views illustrating a partial fabrication of the MEMS device of FIG. 4 in accordance with another embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]    It should be appreciated that while the invention is described herein in relation to an optical MEMS device, the invention is applicable to all devices in which structural and flexibility constraints are at odds with each other. As illustrated in FIG. 4, a MEMS device  210  according to an exemplary embodiment of the invention includes a free-standing support layer  224   a  in flexible attachment with a structural support layer  224   b.  The free-standing support layer  224   a  includes a mirror  242 . Instead of a mirror  242 , another optical device may be utilized. The free-standing support layer  224   a  and the structural support layer  224   b  are flexibly attached to one another through a flexible layer  218 . The flexible layer  218  is attached to respective surfaces of and extends over opposing ends of the free-standing support layer  224   a  and the structural support layer  224   b.    
         [0024]    The flexible layer  218  is dimensioned in such a way as to enable flex of the flexible layer  218  in the direction A about an axis of rotation  100 . Further, the flexible layer  218  is dimensioned in such a way as to enable stiffness with respect to motion in directions B and C. Suitable materials which may be used to form the flexible layer  218  include polysilicon, silicon nitride, titanium nitride, silicon carbide, metallic film, diamond, diamond-like carbon, or other materials suitable in the fabrication of MEMS devices. Additionally, the flexible layer  218  extends partially over the free-standing support layer  224   a  and the structural support layer  224   b  and flexes at a location which is within the same plane as the free-standing support layer  224   a  and the structural support layer  224   b.    
         [0025]    Introducing a flexible layer  218 , which is separate from the free-standing support layer  224   a  and the structural support layer  224   b  makes possible the optimization of the design of the MEMS device  210 . Through the use of patterning and deposition, precise control may be maintained regarding the thickness of the flexible layer  218 . Compliance of the component made from the material is generally controlled by its minimum dimension, and so heightened control over the thickness of the flexible layer  218  allows greater control over its compliance.  
         [0026]    The stiffness of a flexible layer, which acts as a spring, may be described mathematically. For example, the stiffness of a torsion, or twisting, spring k torsion  is described by Equation 1 below:  
           k   torsion   =ab   3 [16/3−3.36( b/a )(1−( b   4 /12 4 ))]( G/L )  
         [0027]    where a represents the greater dimension (be it width or thickness), b represents the lesser dimension, G represents the shear modulus of the spring, and L represents the length of the spring. Equation 1 shows that the k torsion  is proportional to the first order to the cube of the smaller dimension b of the flexible layer. The stiffness of a flexible layer, with a concentrated load on its free end as a cantilever spring, is described by Equation 2 below:  
         k bending =Ecd 3 /4L 3    
         [0028]    where d is the thickness of the flexible layer and c is either width or length, depending upon the direction of the bending plane. In both equations, the stiffness of the flexible layer is proportional to the smaller dimension (b or d) cubed of the flexible layer while only being linearly related to the larger dimension (a or c) of the flexible layer. For example, the deposition of the flexible layer  218  at a uniform thickness T (FIG. 1) of 0.1 micrometer provides such a flexible layer with approximately 6.5 percent of the stiffness of a flexible layer having a thickness T (FIG. 1) of 0.25 micrometers and a similar width W (FIG. 1). Typical semiconductor processing can pattern and etch a flexible layer to a width W (FIG. 1) of 0.25 micrometer, whereas depositing flexible layers as separate films can- easily produce a uniform thickness T (FIG. 1) of 0.05 micrometer.  
         [0029]    In addition to the stiffness achieved in a spring due to torsion and/or bending, stiffness may also be achieved through stretching. By exerting a force in a direction that creates a piston-type motion in a spring, a certain degree of stiffness may be achieved. By depositing a structural layer separate from an optical layer, the stiffness achieved can be adjusted to be at a desired level.  
         [0030]    The flexible layer  218  may be a thin material, on the order of about  50  nanometers thick or thicker. It is desired that the flexible layer  218  not be as thick as the free-standing support layer  224   a  and the structural support layer  224   b.  The upper limit of the thickness dimension T (FIG. 1) of the flexible layer  218  is dependent upon the desired width W (FIG. 1) of the flexible layer. Through use of the invention, a spring may be manufactured with a decreased thickness T and a greater width W with very little or no change in the stiffness than springs manufactured via conventional methods described in the Background. For example, to obtain a flexible layer  218  having a width W twice that of a conventionally formed flexible layer, one can reduce the thickness T to be eighty percent of the thickness T of the conventionally formed flexible layer. The thinness of the flexible layer  218  allows high levels of compliance in a desired direction, in this instance the direction A, while maintaining stiffness to motion in other directions, namely directions B and C.  
         [0031]    [0031]FIG. 11 illustrates a method for fabricating the MEMS device of FIG. 4 while FIGS.  5 - 10  illustrate the fabricated structure during various stages of fabrication. Referring to FIGS. 4 and 5, a support substrate  220 , formed from a silicon wafer, serves as a base. Driver electronics  246  are schematically shown in FIG. 4 as fabricated on a surface of the support substrate  220 . It should be appreciated that the driver electronics  246  may be positioned elsewhere, and that only the electrodes of the driver electronics  246  need be positioned on the support substrate  220 . Actuating the driver electronics  246  electrostatically actuates the mirror  242  and causes an attractive force between the mirror  242  (FIG. 4) and the driver electronics  246 , leading to bending of the flexible layer  218  about the axis of rotation  100 .  
         [0032]    After the support substrate  220  is fabricated with the driver electronics  246 , the support substrate  220  is overlain with an intermediate sacrificial layer  222 . The sacrificial layer  222  may be formed of an oxide. A silicon-on-insulator structure is formed by depositing a polysilicon layer  224  over the sacrificial layer  222 . It is to be appreciated that instead of a silicon wafer, quartz or a polymer material may be utilized for the support substrate  220  instead. In such a situation, the driver electronics  246  may be located elsewhere (with only the electrodes on the support substrate  220 ), or an extra layer of silicon or other semiconductor material will be required over the quartz or polymer support substrate  220  to allow location of the driver electronics  246  thereon. Further, the support substrate  220  and the polysilicon layer  224  may be formed of any materials which may be differentiated from the sacrificial layer  222  through etching. The polysilicon layer  224  serves as a structural support layer for the MEMS device. A resist layer is then patterned on the structural support layer  224  in step  300  (FIG. 10). As illustrated in FIG. 4, the resist layer is patterned into resist layer portions  226   a  and  226   b.  An opening  225  separates the resist layer portions  226   a  and  226   b.    
         [0033]    In step  305  (FIG. 11), the structural layer  224  is etched beneath the opening  225 . Etching of the structural layer  224  separates the structural support layer  224  into structural support layer portions  224   a  and  224   b  separated by the opening  225  (FIG. 6). After etching of the structural layer  224 , the resist layer portions  226   a  and  226   b  are removed at step  310 . The exposed free-standing support layer  224   a  may be further processed to prepare it as a mirror  242  (FIG. 4). Such further processing may include applying a coating of aluminum or other suitable optical coating that serves as the mirror  242 .  
         [0034]    At step  315  (FIG. 11), a flexible layer  218  (FIG. 7), is deposited over the free-standing support layer  224   a  and the structural support layer  224   b.  The flexible layer  218  is conformally deposited over the surface of the free-standing support layer  224   a  and the structural support layer  224   b  and within the opening  225 . A resist  230  (FIG. 8) is next patterned on the flexible layer  218  at step  320  (FIG. 11). The resist  230  is patterned over the opening  225  and laterally outside the area of the opening.  
         [0035]    At step  325  (FIG. 11), the flexible layer  218  is etched. The resist  230  serves as a mask and protects the flexible layer  218  in and around the opening  225 . After etching of the flexible layer  218 , the resist  230  is removed. The sacrificial oxide layer  222  is then partially removed at step  330  (FIG. 11) such that a portion of layer  222  remains beneath the structural support layer  224   b.  One preferred way for removing the sacrificial layer  222  is subjecting it to a timed etch which removes a portion of the sacrificial layer  222  underlying the optical device  224   a,  leaving a portion of the sacrificial layer  222  in place beneath the structural support layer  224   b.  Holes  244  (FIG. 4) are provided through the free-standing support layer  224   a  to allow etchant to contact and etch the sacrificial layer  222  underneath. While the holes  244  are shown to extend through the mirror  242 , the mirror  242  may instead be narrower and the holes  244  may instead be outside the outline of the mirror  242 .  
         [0036]    As illustrated in FIG. 10, a portion of the sacrificial layer  222  remains, mechanically grounding the structural support layer  224   b  to the support substrate  220  to form a support member  240 , while the support layer  224   a  now cantilevers from the support member  240 .  
         [0037]    Although a timed etch of the sacrificial layer  222  has been described as one process for creating the free-standing support layer  224   a,  this is not the only method that may be used to create a cantilevered support layer. FIGS.  12 - 15  illustrate an alternative fabrication process for the MEMS device  210 . The MEMS device begins with the overlaying of the sacrificial layer  222  over the support substrate  220  (FIG. 12). The sacrificial layer  222  is then subjected to a partial etch which removes a portion of the sacrificial layer (FIG. 13). Then, as illustrated in FIG. 14, the structural layer  224  is overlain over the support substrate  220  and the sacrificial layer  222 . An optical element, such as a mirror, may be fabricated on the structural layer  224  in the manner described above. Finally, an etch is used to remove the remaining sacrificial layer  222  to create the free-standing support layer  224   a  and the structural support layer  224   b.    
         [0038]    The MEMS device illustrated in and described with reference to FIGS.  4 - 15  offers a variety of benefits. One benefit is protection against thin-film stress that causes mirror curvature. Further, the illustrated MEMS device allows low voltage operation which increases reliability while concurrently lowering costs of manufacture and operation.  
         [0039]    Further, a monolithic process may be utilized to fabricate such MEMS devices, which eliminates the need for complicated and expensive flip chip bonding. A monolithic process as described above may greatly increase the alignment accuracy between the optical device, such as a mirror, and the driver electronics.  
         [0040]    The design of the MEMS devices of the invention conserves space, allowing such MEMS devices to be packed efficiently on a substrate. The design also ensures adequate contact between the flexible layer  218  and the adjacent structural support layer  224   b  and the free-standing support layer  224   a.  The creation of a more compliant flexible layer  218  through the described and illustrated process opens up the design space. Specifically, the flexible layer  218  may be made thin enough to allow for the fabrication of a greater number and variety of MEMS device designs. For example, a MEMS device which requires a soft spring and which is fabricated conventionally would require a spring which his thin and long. Such a spring is difficult to fabricate via conventional lithography and etching. However, by depositing a thin film as described above, such a soft spring may be more easily and more accurately fabricated. Furthermore, using thin film deposition, smaller surface areas for the optical device may be utilized, allowing devices with smaller electrode surface areas to be activated. This in turn allows for closer packing of such MEMS devices.  
         [0041]    Also, the low stiffness of the flexible layer  218 , caused by the thinness of the flexible layer, permits actuation of the mirror  242  using lower voltages than for conventional MEMS devices. Also, by controlling the thickness of the flexible layer  218  to a high degree of accuracy during film deposition, the sensitivity of the structure to film thickness is minimized. Since thickness dominates the stiffness of the flexible layer  218 , optical devices such as mirrors  242  can be designed such that their lateral dimensions are large enough that errors during fabrication have a negligible effect on their performance.  
         [0042]    While the invention has been described in detail in connection with the preferred embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.