Abstract:
A new class of tilting micromechanical mechanisms have been developed. These new mechanisms use floating pivot structures to relieve some of the problems encountered in the use of solid flexible pivots.

Description:
This invention was made with Government support under Contract DE-AC04-94DP85000 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     The present invention relates generally to micromechanisms, and more specifically to micromechanisms comprising pivoting structures. 
     Although there is no precise referent for the term “micromechanical”, its common use is to machinery whose size scale makes invalid most of our assumptions about machinery of ordinary dimensions. For example, whereas at ordinary dimensions the strength of materials is usually limited by mechanisms involving motion and multiplication of dislocations in response to applied stress, in the micromechanical regime surface erosion and cleavage modes of failure are more common. Again, whereas at ordinary dimensions fluid lubrication of frictionally related elements (e.g., an axle and a shaft bearing) is one of a handful of practical choices, in the micromechanical regime the viscous shear stress of a lubricant increases in inverse proportion to the size scale of the apparatus, increasing the energy dissipation rate to unusable levels. 
     When the change in size scale renders conventional practice invalid, it also enables new approaches toward micromechanical functionality. An effective replacement for fluid lubricants can be a surface layer of a solid which is (usually) either amorphous or polycrystalline, is reasonably strong, forms a smooth film on the component surfaces, and has a surface structure that resists bonding with itself. 
     Consistent with common practice, in this specification the term ‘micromechanical’ is associated with mechanical apparatus whose functional components have sizes ranging from about 1 mm to about 1 μm. Such micromechanical apparatus can be made of a wide variety of materials, but perhaps the most common system is a combination of polycrystalline silicon, amorphous silicon, silicon oxides, and silicon nitride. This material system has high material strength and rigidity, high resistance to fracture, can be doped to provide electrically conducting regions, and is relatively easy to directly integrate with microelectronics at a chip level. 
     A common mechanism in micromechanical apparatus is a tilt stage, whose function is to provide an attached element with a controllable amount of tilt along a predetermined axis or axes. The attached element can be an intrinsic part of the apparatus (e.g., as in an escapement), or an external element which is to be positioned and directed by the apparatus (e.g., a micromirror). The implementations described in this specification will be for micromirror tilt stages, but this should not be taken as an implicit limitation on the scope of the present invention, which can be used in a wide range of pivoting mechanisms. 
     A typical prior art micromirror tilt stage is shown in FIGS. 1 a - 1   d . FIG. 1 a  shows a transparent top view of the tilt stage, and FIG. 1 b  shows a side view. Micromirror  11  is mounted on top of base  10  by flexible pivot  12 . (Note that  11  could as easily be a mounting platform for a micromirror or for some other component. The present identification is made for simplicity of description, and not to limit the scope of the present invention.) The top surface of micromirror  11 , if necessary, is polished flat and/or coated with a reflective layer. 
     Capacitor plates  13  and  14  provide the forces which tilt the micromirror. In one working arrangement the micromirror is doped to the point of being a good conductor, and is grounded through flexible pivot  12  and base  10 , both of which are also appropriately doped. Capacitor plates  13  and  14  are electrically insulated from the base  10 . When a voltage is applied to capacitor plate  13 , the micromirror  11  tilts to the left. When a voltage is applied to capacitor plate  14 , the micromirror  11  tilts to the right. 
     If the applied voltage is large enough, the micromirror  11  will tilt until it hits a solid stop. This condition is of interest for designers, as it provides for a precise amount of tilt, and at the same time helps prevent vibration of the micromirror. Two common solid stops are illustrated here. FIG. 1 c  shows a maximum leftward tilt, the magnitude of the tilt being limited by contact between the micromirror  11  and the base  10 . This provides a limitation in many designs, because the micromirror structure is fabricated by growing layers of structural and sacrificial material on top of the base, and then removing the sacrificial material. Practical limitations of such processes limit the thickness of the sacrificial layers to a few microns at most. A typical micromirror can be perhaps 100 μm across, with a gap separating the micromirror from the base of about 1 μm. In this case, the maximum tilt angle is limited to about 1°. 
     FIG. 1 d  shows a maximal rightward tilt, where the magnitude of the tilt is limited by the presence of stop post  15 . Such a stop post can be used if a smaller maximum tilt is desired than results from contact between the micromirror and the base. 
     In another implementation of this prior art device, a second pair of capacitor plates can be added to the base, thereby allowing an arbitrary tilt direction to be driven electrostatically, as shown in FIG.  2 . 
     Many equivalent mechanisms are known in the art. However, what they share is pivoting about a structure which also provides a permanent physical connection between the moving parts of the tilting structure. For example, whereas the pivoting motion was provided above by the bending of a flexible pivot connecting the micromirror to the base, an equivalent motion can be obtained using a pair of torsional equivalent motion can be obtained using a pair of torsional pivots attached to the sides of the micromirror and to a mounting frame which is attached to the base. Similarly, the top surface of a cantilever beam can be used as a micromirror or as a micromirror mount. Such a cantilever mount can provide multiaxis tilt capability when electrostatic actuators are arranged so that both bending of the cantilever and rotation about the cantilever are driven. 
     The common factor to these mechanisms is that the pivoting members are solid bodies which pivot through bending and/or torsion. A second prior art mechanism which allows pivoting is the fabrication of actual hinges. A tilt mechanism which illustrates this approach appears in FIGS. 3 a - 3   b.    
     In the figure, a device to tilt a micromirror is assembled from carefully patterned material and sacrificial layers on a base  500 . A micromirror  504  is mounted on top of a mirror frame  503 . Mirror frame  503  is rotably fixed to base  500  by a first set of axles  502  attached to mirror frame  503  and rotating in a first pair of bearing blocks  501  which are at the surface of base  500 . 
     Mirror frame  503  is rotably fixed to driving frame  507  by a second set of axles  505  attached to mirror frame  503  and rotating in a second pair of bearing blocks  506  which are attached to driving frame  507 . Driving frame  507  is secured to base  500  by a third pair of axles  509  which are attached to the driving frame, and are constrained to slide on the surface of the base by a pair of sliding bearings  508 . 
     Driving frame  507  is moved along the surface of the base by the action of linear transfer beam  513 , which transfers motion and force from a bi-directional linear electrostatic actuator comprising comb electrodes  514 ,  515 ,  517 , and  518 . The linear transfer beam  513  is restricted to move substantially along its long axis through the combined action of support bushing  516 , beam guides  512 , and rotable connection  510  and  511  to the driving frame. 
     The function of this prior art tilt stage can be understood by comparing the side view of FIG. 3 a  with FIG. 3 b . FIG. 3 a  shows the ‘flat’ configuration, in which the electrostatic actuator has pulled linear transfer beam  513  as far to the right as is possible. In the flat configuration the micromirror  504  is in a well-defined position and orientation approximately parallel to the surface of base  500 . The exact position can be adjusted in the design phase by placing additional material under either between the micromirror and the mirror frame, or by placing additional material under the mirror frame. 
     FIG. 3 b  shows the ‘tilted’ configuration in which the electrostatic actuator has pushed the linear transfer beam  513  as far to the right as possible. In the tilted configuration micromirror  504  achieves a well-defined position which is tilted at a large angle relative to the surface of base  500 . 
     Although hinged mechanisms of the sort shown in FIG. 3 can be fabricated, their mechanism and structure is quite complex, and rather delicate in operation. Because of the enormous material strength associated with silicon at micromechanical size scales, flexible pivots have proven quite useful for limited tilt angles, but represent a major restriction to the design process, thereby severely limiting the types of mechanisms which can be implemented. 
     A new approach to the problem of micromechanical pivots is badly needed in this realm of the useful arts. 
     SUMMARY 
     The present invention is of a new type of micromechanical pivot, in which initially disjoint components come into contact when a tilting motion is initiated, and thereafter act as a conventional mechanical pivot. Such floating pivots are extremely simple to fabricate, and can replace conventional micromechanical pivots in many instances. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIGS. 1 a - 1   d  show a schematic diagram of a prior art flexible-pivot tilt stage. 
     FIG. 2 shows a schematic diagram of a multiaxis prior art flexible-pivot tilt stage. 
     FIGS. 3 a - 3   b  show a schematic diagram of a prior art sliding and hinged tilt stage driven by a linear comb drive. 
     FIGS. 4 a - 4   b  show a schematic diagram of a tilt stage comprising a floating pivot according to the present invention. 
     FIGS. 5 a - 5   d  show a schematic diagram of various floating pivots according to the present invention. 
     FIGS. 6 a - 6   b  show a schematic diagram of a multiaxis tilt stage comprising a floating pivot according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     A simple version of a floating pivot micromechanism according to the present invention is shown in FIG. 4 a  (side view) and FIG. 4 b  (top view). Here a micromirror  41  is floated over a substrate  40  by the action of suspension springs  45 . (As before, although the present invention is being described in terms of a tilt stage for a micromirror, any application suitable for a floating pivot mechanism can be substituted without changing the fundamental nature of the present invention.) These springs are secured to substrate  40  by mounting posts  44 , and to the micromirror by spring attachment  43 . 
     As shown, spring attachment  43  and pivot base  42  are positioned so that their ends are aligned and nearly touching. (They can in fact touch, but for the present invention cannot be directly connected.) This alignment is compatible with and sustained by the combined action of the suspension springs  45 . 
     FIG. 4 b  illustrates the function of the floating pivot apparatus described above. As the micromirror  41  is tilted relative to substrate  40 , the edge of spring attachment  43  makes contact with the top surface of pivot base  42 , thereby forming a mechanical and functional pivot at point  46 . 
     In general, a pair of structural elements which satisfy the following conditions will be called a floating pivot—one of the elements is functionally attached to the micromirror, the other is functionally attached to the substrate, and the two elements are aligned so that they make contact and form a mechanical pivot when the micromirror tilts. The use of such floating pivots can relieve both design and fabrication constraints which are associated with the use of conventional flexible or hinged pivots. 
     Many types of structures can make floating pivots, as shown in FIGS. 5 a - 5   d . In FIG. 5 a , a rod  50  and a pivot cup  51  form a floating pivot, if they replace components  42  and  43  in FIG.  4 . In FIG. 5 b , ring  53  and plate  54  form a floating pivot if they replace  42  and  43 . In FIG. 5 c , the combination of ring  56  and sphere  57  form a floating pivot if they replace  42  and  43 . Finally, in FIG. 5 d , the combination of channel walls  58  and cylinder  59  combine to give a single axis floating pivot when they replace  42  and  43 . Many other configurations can form floating pivots. 
     Clearly, however, a floating pivot only exists in the context of the remainder of the apparatus. As the components which will form the mechanical turning point when they come into contact are held in close proximity indirectly by the remainder of the apparatus, such devices as a ball and socket pivot, where the ball is held directly on the socket (typically using a retaining ring), are not floating pivots. 
     Finally, an multi-axis application for a floating pivot is illustrated in FIGS. 6 a - 6   b . This is a tilt stage capable of being tilted in any direction through the action of integral electrostatic actuators. FIG. 6 a  shows a side view of this apparatus, and FIG. 6 b  shows a top view. Tilt platform  601  is suspended above table  600  by a set of suspension springs  605 , each of which is attached to a platform mount  604  and to one of the spring anchor rods  606 . Pivot base  603  is positioned below platform mount  604 , and when the tilt platform  601  begins to tilt, the floating pivot comprising  603  and  604  will make contact and provide a mechanical pivot for the tilt stage. 
     The electrostatic actuators comprise capacitor pads  602 , to each of which is applied a voltage by a power supply (not shown) relative to the tilt platform  601 , which is held at ground potential. By applying suitable combinations of voltages to the capacitor plates, and thereby combining the forces generated thereby, this tilt stage can be made to tilt in arbitrary directions. 
     The examples and implementations described above are intended to illustrate various aspects of the present invention, not to limit the scope thereof. The scope of the invention is set by the claims interpreted in view of the specification.