Rotational Precision Mems-based Clamping Mechanism for Stable Fixation of Elastic Mechanisms

A mechanical clamp is presented which clamps one of the actuators of the MEMS-based TEM sample manipulator. The clamp incorporates a relatively large clamp force of 0.5 mN and is able to maintain the clamp force without external power. Although compliant mechanism design in MEMS design requires extra attention because of the relatively large deflections, small actuations forces and relatively large actuation stiffness, the real challenge lays in designing within the limited fabrication techniques. Though still in development, the fabrication technique proposed offers 35 μm high mechanisms with a trench width bandwidth of 3-20μm. The fabrication technique is compatible with the fabrication of a 6 Degree-of-freedom manipulator. Introduction Conventional TEM sample manipulators often lack the crucial stability of 0.1 nm/min. A MEMS manipulator attached directly to the TEM pole would greatly increase both thermal and dynamic stability. However a stable E-beam requires no interference of electric or magnetic fields. Therefore the position of the manipulator should be maintained passively. To this end a mechanical clamp is presented which clamps one of the actuators of the MEMS-based TEM sample manipulator [1]. The clamp incorporates a relatively large clamp force of 0.5 mN and is able to maintain the clamp force without external power. A theoretical basis of the clamp has been presented in previous work [2]. In this paper the design and fabrication of a second generation rotational clamp is presented. This clamp design is part of a research project for a 6 Degrees-of-Freedom MEMS TEM sample manipulator. Clamp design Figure 1 shows part of the manipulator tail which has to be fixed in y-direction by the clamp mechanism. The clamp mechanism clamps the manipulator tail which is connected to the manipulator actuator shuttle. A transmission ratio is created to increase the clamp force and to decrease the influence of play in the rack and pin locking mechanism. The operation of the total clamp mechanism starts by closing the parallel plate actuator. The pin moves down and the clamp shuttles are free to rotate around the pivots. The two jaws are actuated by one comb-drive collectively which results in a rotation of the jaws and shuttles around the pivots. Once a desired clamp force is established the parallel plate actuator is switched off and the pin is locked in one of the gaps of the rack. The comb-drive actuator can be switched off. Figure 2 shows the clamp design. The jaw is suspended by leaf-springs A and B. In general, leaf-springs are very stiff in tensile direction while in bending direction they are compliant. When the two leaf-springs A and B deform together, the jaw will rotate around a virtual pivot which is the intersecting axis of leaf-springs A and B. The rotational comb operates at a larger radius than the jaw creating a force amplification. Leaf-springs C & D intersect at the same virtual pivot as leaf-springs A & B. For relatively small deflections, the rotational combdrive will rotate around the common virtual pivot. Reinforcement F, is a folded leaf-spring, supporting the comb shuttle in out-of-plane direction, and leaving the other degrees of freedom compliant. FIGURE 1. Schematic view of the clamp mechanism. The compliance of leaf-springs A, B, C, D and F are represented by 1D springs. The virtual pivots are represented by real pivots. During the first 1.64° rotation of the jaw and 1.87° rotation of the comb-drive, the comb-drive energy is stored in leaf-springs A, B, C, D and reinforcement F, as the jaw does not touch the manipulator tail. After the jaw touches the manipulator tail the clamp comb-drive energy is mainly stored in leaf-springs C, D and reinforcement F. At the same time the clamp force starts to build up over a 0.49° clamp actuator stroke. Once a desirable clamp force is obtained, the parallel plate actuator can be gradually switched off, raising the pin in the rack. The clamp combdrive can now be switched off. There will be some backlash due to movement of the pin in the rack until a stop is reached. Details of first generation clamping and fixation mechanism can be found in previous work [2]. The fixation mechanism freezes the clamp force without the necessity of sustaining an electrical field. The combination of the position uncertainty due to backlash of the pin in the rack and the fabrication uncertainty should not lead to considerable clamp force loss. Therefore the bending compliance of leaf-springs C, D and reinforcement F will allow pre-tension to be built up. Making the leaf-springs too compliant will require extra stroke and actuator energy for the comb-drive. Making leaf-springs too stiff leads to unacceptable loss of clamp force when locking. Because of large clamp forces, and therefore the risk of buckling, the relatively long and slender leaf-springs A & B are loaded with a tensile force during clamping. Leaf-springs A, B, C, & D are initially pre-curved so that at the maximum overlap of the comb-fingers, the deformation causes them to be straight. Exactly at this point the electrostatic pull-in force is greatest, and the leaf-spring’s longitudinal stiffness is largest. In order to minimize the parasitic influence of clamping on the manipulator position several measures have been taken: First, the virtual pivot is located in the middle of the manipulator-tail. Therefore nearly pure motion in xdirection of the jaw results, leaving the manipulator position unaffected. Second, the clamping force results in a tensile elongation of leaf-springs A and B. The leaf-spring tensile stiffnesses are tuned so the clamp force will result almost solemnly in an xdisplacement of the jaw. The residual y-displacement due to the above mentioned effect is less than 1 nm. The manipulator-tail is compliant in x-direction to obtain an equal distribution of the clamp force in the jaws. The Hertzian contact stress due to clamping of the manipulator-tail results in an elongation of the tail of 0.19 nm in y-direction, which is considered to be insignificant. Consequently by clamping, the position of the manipulator will only be affected on a sub-nanometer scale. Manipulator tail Double pivot Jaw Clamp shuttle Comb-drive Rack Pin Parallel plate actuator Fcomb Fcomb Fparallel plate F C & D A & B