Abstract:
Provided is a micro-electromechanical assembly including an out-of-plane device formed on a device layer of a single crystal silicon substrate. A ribbon structure is formed on the device layer, where the ribbon structure has at least one of a width or depth, which is less than the width or depth of the out-of-plane device. A connection interface provides a connection point between a first end of the out-of-plane device and a first end of a ribbon structure, wherein the ribbon structure and out-of-plane device are integrated as a single piece.

Description:
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract no. 70NANB8H4014 awarded by NIST. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is directed to micro-hinges used in micro-electromechanical systems (MEMS) and micro-systems technology, and more particularly to an improved micro-hinge configuration and design adding robustness and strength to the hinging element. 
     The use of micro-hinges has become prevalent with the increased utilization and complexity of surface micro-machine components and systems. Typically used in the implementation of out-of-plane or vertically oriented micro-device designs, the micro-hinge is usually fabricated in a minimum two-layer, though typically three-layer, polysilicon process. Such a hinge  10 , known as a staple hinge, is illustrated in FIG. 1, integrally connected with micro-mirror  12 , and is used to attain out-of-plane motion. The multi-step fabrication process, includes depositing a first layer which is then patterned and etched. Next a second layer is deposited, patterned and etched in such a way that after removing any filling material, the first layer is free to move in a prescribed path, while being held in place by the second layer. This structure creates a rotating joint implemented in MEMS or micro-systems to permit for the mechanical movement required for micro-mirrors and other out-of-plane devices. 
     Drawbacks to existing micro-hinge designs include process complexity and cost of fabrication. 
     The inventors have also observed that the device layer of silicon-on-insulator (SIO) wafers are being used to form micro-structures such as mirrors, lenses and other out-of-plane or vertically oriented devices for integrated MEMS and micro-systems. The formation of such devices requires the use of micro-hinges to provide rotational freedom and mechanical support for these out-of-plane devices. 
     It is therefore considered useful to develop less complex and costly micro-hinges capable of providing the necessary mechanical integrity and strength to allow out-of-plane rotation or vertical movement of SOI device layer structures. 
     SUMMARY OF THE INVENTION 
     Provided is a micro-assembly including a micro-device formed on a device layer of a single crystal silicon substrate. A ribbon structure is formed on the device layer, where the ribbon structure is thinned to a thickness which is less than the thickness of the micro-device. A connection interface provides a connection point between a first end of the micro-device and a first end of a ribbon structure, wherein the ribbon structure and micro-device are integrated as a single piece. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is directed to a micro-mirrored device using multiple polysilicon layers for implementation of a micro-hinge; 
     FIG. 2 is an isometric view of a ribbon hinge attached to an out-of-plane device according to the teachings of the present invention; 
     FIG. 3 is a side view of the ribbon hinge and out-of-plane device of FIG. 2; 
     FIG. 4 sets forth the processing steps for formation of the ribbon structure attached to an out-of-plane device in accordance with the teachings of the present invention; 
     FIG. 5 is an illustration for one design in accordance with the teachings of the present invention; and 
     FIG. 6 illustrates an alternative embodiment of the present invention wherein the movement of the micro-mirrors is accomplished by an active operation. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     While FIG. 1 depicts a micro-device implementing a polysilicon staple hinge, FIGS. 2 and 3 illustrate a micro-assembly  18  having a ribbon hinge  20  configured according to the present invention, in an integrated arrangement with a micro-device  22 , such as a micro-mirror. The micro-device has been moved from an in-plane position to an angle of approximately 30°. Movement of the micro-device is achievable by a variety of mechanisms, including the use of a micro-probe or an actuator. 
     Ribbon hinge  20  is, therefore, designed to replace the widely used polysilicon staple-hinge design illustrated in FIG.  1 . Ribbon hinge  20  is a single-crystal-silicon (SCS) component which has mechanical stability, and which is configured using a simplified processing procedure. Thus, ribbon hinge  20  of the present invention provides a flexible mechanism as opposed to the jointed staple-hinge of FIG.  1 . 
     Ribbon hinge  20  is formed from the device layer of a silicon-on-insulator wafer, which has been thinned down to allow increased mechanical flexibility. This design produces a high quality mechanical structure having sufficient strength for its intended purpose. 
     FIGS. 2 and 3 emphasize the flexibility of ribbon hinge  20 . In this embodiment, ribbon hinge  20  is approximately 500 nm thick, approximately 50 μm wide and approximately 140 μm in length. Micro-device  18 , including ribbon hinge  20  and mirror  22  is fabricated using a silicon-on-insulator (SOI) wafer with a device layer thickness of approximately 3 μm and a buried oxide (BOX) layer thickness of approximately 2 μm. 
     In a two-mask process used to manufacture the micro-device  18 , an area to be thinned is first lithographically exposed, and surrounding areas are protected, before a timed wet etch reduces the thickness of the exposed silicon area  20  to  ˜ 500 nm. Then a subsequent lithography step is used to pattern the hinge  20  and mirror  22  areas exposing all surrounding areas to be etched. This leaves the mirror structure protected, by an oxide layer, and the thin silicon ribbon hinge resting on the sacrificial BOX layer. Following buried oxide removal using a Hydrofluoric Acid (HF) 49% etch process step and subsequent drying procedures, mirror  22  is freed to move. 
     As will be discussed in greater detail below, the present invention is a two-step process in the sense that in the first step the hinge area  20  is patterned and etched. Then a second procedure is used for lithographically defining and forming the mirror area  22  (or other out-of-plane or vertically oriented device). It is of course possible to inverse these processes by processing the out-of-plane device area first, then thinning the ribbon layer. An issue in this regard is that the out-of-plane device and ribbon hinge are all formed from the same material layer. The difference between the ribbon hinge and the out-of-plane device is the geometry of the patterning, and the physical thickness of the areas. Particularly, etching ribbon hinge  20  to a much thinner cross-section than the out-of-plane device, permits increased flexibility of the ribbon hinge. The flexibility of ribbon hinge  20  is illustrated by its almost S-shape (See FIG.  3 ). 
     The methodology that incorporates fabrication of the ribbon hinge structure in the same material as the out-of-plane device such as the mirror, has many advantages over existing hinge technologies, including a simplified fabrication process. For example, since the hinge is fabricated using the same material layer as that of the out-of-plane device, there is no adhesive joint or holding structure necessary between the hinge and the attached device. Such a design accommodates the high mechanical torque and forces delivered by the attached mechanical device without comprising the integrity of connection point  24  between the hinge structure and the attached micro-device. 
     FIG. 4 illustrates a process flow for fabrication of a single crystal silicon ribbon hinge according to the present invention. In step  28 , the process begins with a clean silicon-on-insulator (SOI) wafer  30  having a single crystal silicon device layer  32 , a buried oxide layer  34 , and a substrate layer  36 . In a first step of the process,  38 , a photo-resist layer  40  is deposited on device layer  32  using standard lithographic processes. Photo-resist layer  40  is patterned in such a way as to expose the area to be thinned into the ribbon hinge  42 . In a next step  44 , a wet etching process is undertaken such as wet etching with a potassium-hydroxide (KOH) 45% solution at 60° C. The wet etching causes the exposed ribbon hinge area  42  of device layer  32  to be removed to a thickness of approximately 500 nm. 
     In step  46 , previously applied resist layer  40  is removed prior to a repatterning for etching of the out-of-plane device, an island area and an anchor structure. Following removal of first photo-resist layer  40 , second resist layer  48  is applied on the top surface of SOI  30 . In step  50 , a dry etching process is undertaken on the exposed silicon of device layer  32  to form the out-of-plane device structure  52  as well as the island area  54  and anchor structure  56 . In step  58 , the second layer of photo-resist  48  has been removed, and an etching process is started to begin etching the exposed buried oxide layer  60 , using a Hydrofluoric Acid (HF) 49% solution. 
     Next, in step  62 , the third and final layer of photo-resist  64  is deposited and patterned on the SOI wafer  30 . This final photo-resist layer  64  is to be used during the buried oxide-release (BOX) operation, wherein the out-of-plane device  52  is released by etching all unprotected buried oxide. This process is shown completed in step  66  where remaining buried oxide layer material  68  and  70  are located under the island structure  54  and under the anchor section  56 . As can be seen in step  66 , a separation layer  72  and separation edge  74  are shown as being void of material. Removal of the material in these areas allows for the movement of the out-of-plane device  52  and ribbon hinge  42  in a manner similar to that shown in FIGS. 2 and 3. In step  66 , it is noted that all remaining photo-resist is removed, for example by a dry O 2  plasma-etch process. Thus, step  66  depicts the original SOI wafer  30  as a completed mirror and hinge structure. 
     Turning to FIG. 5, set forth is an implementation of a passive micro-mirror assembly using the ribbon-hinge methodology of the present invention. Dual mirror device  80  illustrates that by application of the discussed manufacturing steps a SOI wafer can be processed into a micro-device incorporating multiple mirrors and hinges. A first ribbon hinge  82  is fabricated so as to be integrated to an anchor portion  84  at one end and to a movable mirror structure  86  at a second end. First, ribbon hinge  82  and anchor portion  84  are joined at connection point  88 , and first ribbon hinge  82  and mirror  86  are joined at connection point  90 . Thereafter, a second ribbon hinge  92  is connectably fabricated to mirror  86  at connection point  94  and further integrated to second mirror  96  at connection point  98 . The mirrors and ribbon hinges of device  80  are fabricated in the same device layer of an SOI wafer. 
     Slots  100  may be formed in the same device layer as ribbon hinges  88 ,  92  and mirrors  86 ,  96 . Slots  100  are formed in an area behind the mirrors outside of the area of the ribbon hinges, and are made to run along both sides of mirror  96  (only one side of slots  100  are shown for convenience) allowing balanced fixture of mirrors  86 ,  96 . In such a passive design, mirrors  86 ,  96  are assembled using micro-probes, and once in place reside fixed and unaided. Particularly, as micro-probe (not shown) moves mirror  96  out of plane, the side edges  102  of mirror  96  may be placed into any one of slots of slot configuration  100 . 
     Once placed within a slot, mirror  96  as well as mirror  86  is maintained in a fixed position. It is noted that the flexibility of the ribbon hinges  82  and  92  allow for flexing in opposite directions. For example, ribbon hinge  82  is shown flexed in a concave position whereas ribbon  92  is in a convex position. 
     FIG. 6 sets forth an alternative micro-structure  104  embodiment implementing ribbon hinges according to the present invention. Particularly, the movement of mirrors is obtained via active operation as opposed to passive, e.g. movement of the mirrors by a probe. A controllable element, such as a comb-drive actuator assembly  106  is attached to mirror  96  via ribbon hinge  108 . Comb-drive assembly  106  includes interdigitated comb fingers  112 , a drive shuttle  114 , and suspension arms  116 . The mirror angles are then dynamically adjusted by application of an applied voltage from a voltage source (not shown) which results in the displacement of the comb-drive assembly  106  and hence the attached devices. It is to be appreciated other active actuators may also be used to move out-of-plane devices in accordance with the present invention. 
     While the present invention is described with respect to a preferred embodiment, it would be apparent to one skilled in the art to practice the present invention into other configurations and designs. Such alternate embodiments would not cause departure from the spirit and scope of the present invention.