Abstract:
A MEMS device which utilizes a capacitive sensor or actuator is enhancement by initially fabricating the capacitive assembly which comprises the sensor or actuator as two sets of interdigitated fingers in a noninterdigitated configuration. One of the two sets of fingers is coupled to a movable stage. The stage is moved from an initial position to a post-release position in which the two sets of interdigitated fingers are interdigitated with each other. The stage is carried by two pairs flexures which maintain the stability of motion of the stage and when in the post-release position provide stiffness which prevents deflection of the set of fingers coupled to the stage. The stage and hence the assembled sets of fingers are then locked into the post-release position.

Description:
BACKGROUND OF THE INVENTION  
   1. Field of the Invention 
   The invention relates to the field of micromachined sensors and actuators and in particular to post-release enhancement of detection and actuation capacitances in micromachined sensors and actuators. 
   2. Description of the Prior Art 
   Capacitive detection and actuation are commonly used in micromachined devices due to simplicity of implementation and effectiveness. However, the performance of capacitive sensors and actuators highly depends on the nominal capacitance of the microsystem. For example, in capacitive micromachined inertial sensors (i.e. accelerometers, gyroscopes, etc.) the performance is generally defined by the nominal capacitance of the sensing electrodes. Furthermore, in electrostatically actuated devices, the nominal actuation capacitance determines the required drive voltages. For a small actuation capacitance, large voltages are needed to achieve sufficient forces, which in turn results in a large drive signal feed-through. Thus, it is desired to maximize the sensing capacitance, and minimize the actuation voltages by increasing the actuation capacitance. However, the sensing and actuation capacitances of micromachined devices are limited by the minimum-gap requirement of the fabrication process. 
   In the micro-domain, capacitive sensing and actuation offer several benefits when compared to other sensing and actuation means (piezoresistive, piezoelectric optical, magnetic, etc.) with their ease of fabrication and integration, good DC response and noise performance, high sensitivity, low drift, and low temperature sensitivity. However, the performance of micromachined sensors employing capacitive detection is generally determined by the nominal capacitance of the sensing electrodes. Even though increasing the overall sensing area provides improved sensing capacitance, the sensing electrode gap is the foremost factor that defines the upper bound. Various advanced fabrication technologies have been reported (e.g. oxidation machining) that provide minimal electrode gap. 
   However, all of these approaches require additional expensive fabrication steps. In electrostatically actuated devices such as micromachined gyroscopes, the nominal actuation capacitance determines the required drive voltages. For a small actuation capacitance, large voltages are needed to achieve sufficient forces, which in turn results in a large drive signal feed-through. The drive signal feed-through is generally a major noise source, and often a larger signal than the measured Coriolis signal. Thus, these devices are conventionally operated in vacuum to achieve large amplitudes with low actuation voltages to minimize the drive feed-through, which results in an extremely narrow response bandwidth. Similarly, the force generated by the electrostatic actuation electrodes (comb-drives or parallel-plates) is limited by the minimum gap attainable in the used fabrication process. MEMS designers are facing challenges similar to those exemplified above while implementing other electrostatic MEMS sensors and actuators. 
   The following section analytically illustrates the dependence of sensing and actuation capacitances on the design and fabrication parameters. Then, prior techniques to enhance capacitance are presented. 
   BRIEF SUMMARY OF THE INVENTION  
   The disclosed post-release assembly technique increases the sensing and actuation capacitances in micromachined inertial sensors, in order to enhance the performance and noise characteristics beyond the fabrication process limitations. The approach is based on attaching the stationary electrodes of the device to a moving stage that locks into the desired position to minimize the electrode gap before operation. The explored locking mechanisms include, but are not limited, to ratcheted structures and bistable mechanisms. Thermal actuators are employed for displacing the moving stage, but other actuation means are included with the contemplation and scope of the invention. The illustrated embodiment of the invention has been implemented in bulk-micromachined prototype gyroscopes, and the experimental results have successfully demonstrated the feasibility of the design concept. 
   The invention is an improvement in a MEMS device which provides capacitive enhancement of the electrostatic actuators and capacitive sensors in the device. The MEMS device illustratively comprises a capacitive assembly having two sets of interdigitated fingers fabricated initially in a noninterdigitated configuration. A movable stage is coupled to one of the two sets of interdigitated fingers. An actuator selectively moves the stage from an initial position to a post-release position in which the two sets of interdigitated fingers are interdigitated with each other. A lock then maintains the stage in the post-release position. 
   In the preferred embodiment the noninterdigitated configuration is characterized by a minimum, fabricated gap size between adjacent fingers in each of the two sets, so that when in the post-release position the fingers in the two sets are spaced from each by less than the minimum, fabricated gap size. 
   The capacitive assembly may comprise a capacitive sensor assembly or an electrostatic actuator assembly. The lock comprises a ratchet lock mechanism or a bistable lock mechanism. The actuator comprises a thermal actuator. The MEMS device itself may comprises a gyroscope or accelerometer. 
   The device is fabricated in or on a substrate. The stage comprises a stage body to which the one set of fingers are coupled, and two pairs of flexures coupling the stage body to the substrate. The two pairs of flexures function as a four-bar linkage to restrict movement of the stage body in a stable linear direction. The two pairs of flexures are arranged and configured to provide high stiffness when the stage is locked into the post-release position so that the one set of fingers coupled to the stage body is substantially prevented from deflecting. 
   The invention also comprises a method of assembling a MEMS device to provide an enhanced capacitive function as described above. 
   While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram of a capacitive structure illustrating the physical principles in which the capacitive enhancement of the invention is realized. 
       FIG. 1  is a diagram of a prior art capacitive sensor. 
       FIGS. 3   a – 3   d  are microphotographs of an interdigitated capacitive structure which has been fabricated and assembled according to the invention. 
       FIGS. 4   a – 4   c  are diagrams illustrating the prior art assembly in  FIG. 4   a  and the assembly of the invention in  FIGS. 4   b  and  4   c  before assembly and after assembly respectively. 
       FIG. 5  is a simplified functional diagram of the material components of the invention as assembled. 
       FIGS. 6   a  and  6   b  are simplified diagrams of the material components of the invention in its configuration before assembly and after assembly respectively. 
       FIGS. 7   a  and  7   b  are perspective view microphotographs of the material components of the invention in its configuration before assembly and after assembly respectively as diagrammatically depicted in  FIGS. 5 ,  6   a  and  6   b  and showing assembly and locking of the stage using a ratchet mechanism. 
       FIGS. 8   a – 8   d  are top plan view microphotographs of the assembly of the invention as described in  FIGS. 5 ,  6   a ,  6   b ,  7   a  and  7   b  showing assembly and locking of the stage using a ratchet mechanism. 
       FIGS. 9   a  and  9   b  are top plan view enlarged microphotographs of the assembled invention in a quiescent state and operated in resonant state respectively. 
       FIGS. 10   a – 10   c  are close-up top plan view microphotographs of the ratchet locking mechanism. 
       FIGS. 11   a  and  11   b  are perspective microphotographs of a bistable locking mechanism in two different devices. 
       FIGS. 12   a – 12   d  are top plan view microphotographs of the bistable locking mechanism of  FIGS. 11   a  and  11   b  in a capacitive sensor. 
       FIGS. 13   a  and  13   b  are perspective view microphotographs of the ratchet locking mechanism in a capacitive sensor where the sensing electrodes are laterally moved together to decrease the inter-electrode gap before and after assembly respectively. 
       FIGS. 14   a  and  14   b  are an overall perspective view and close-up microphotographic perspective view of a capacitive sensor where the sensing electrodes are laterally moved together to decrease the inter-electrode gap. 
       FIGS. 15   a  and  15   b  are top plan view microphotographs of a capacitive sensor of  FIGS. 14   a  and  14   b  before and after assembly respectively. 
   

   The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
   Consider first some basics of electrostatic actuation and detection by way of general background. The electrostatic actuation and sensing components of micromachined devices can be modeled as a combination of parallel-plate capacitors  10  as shown in diagrammatic perspective view in  FIG. 1 . In the most general case, the capacitance between two parallel plates can be expressed as 
   
     
       
         
           
             
               
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   where ε 0 =8.854×10 12  F/m is the dielectric constant, x 0 ×z 0 =A overlap  is the total overlap area, y 0  is the electrode gap. 
   In parallel-plate electrodes, the electrostatic force is generated due to the electrostatic conservative force field between the plates  12 . Thus, the force can be expressed as the gradient of the potential energy U stored on the capacitor 
   
     
       
         
           
             
               
                 
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   In the case of comb-drives, the actuation force is generated through a combination of parallel plates sliding parallel to each other which in the illustration of  FIG. 1  is in the x-direction. The electrostatic force generated in the x-direction as two plates slide parallel to each other in the x-direction reduces to 
   
     
       
         
           
             
               
                 
                   
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   It should be noticed that this force is independent of displacement in the x-direction and the overlapping area of the capacitor fingers. However, the electrostatic force generated in the y-direction as the plates approach to each other in the y-direction, which is the case for parallel-plate actuation, depends on the overlap area and is a nonlinear function of displacement y: 
   
     
       
         
           
             
               
                 
                   
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   Electrostatic actuation comb-drives are one of the most common actuation structures used in MEMS devices. The primary advantages of comb-drives are the linearity of the generated forces, and the ability of applying displacement-independent forces for high-stability actuators. Linearized drive forces along the x-axis can be achieved by appropriate selection of voltages applied to the opposing comb-drive sets. A balanced interdigitated comb-drive scheme is imposed by applying V 1 =V DC +V AC  to one set of comb drives, and V 2 =V DC −V AC  to the other set, where V DC  is a constant bias voltage, and V AC  is a time-varying voltage. Assuming negligible deflections along the y-axis, the net electrostatic force reduces to 
                 F   =     4   ⁢         ɛ   0     ⁢     z   0     ⁢   N       y   0       ⁢     V   DC     ⁢       v   AC     .               (   5   )               
where z 0  is the finger thickness, and y 0  is the finger separation. It should be noticed that, the net force along the x-axis is independent of the displacement along the x-axis, and the overlap length. The force is proportional to the drive voltages and the device thickness, however, the net force increases exponentially with decreasing gap between the fingers.
 
   Consider now capacitive sensing by building parallel-plate sense capacitors connected to a moving proof-mass  14  of a sensor  10  as diagrammatically depicted in  FIG. 2 . A mass  14  has a perpendicularly extending finger  20  disposed between two parallel electrode fingers  16  and  18 , which collectively form the differential capacitance bridge  26 . The deflection is sensed as a difference in capacitance. The deflection of the mass  14  changes the electrode gap, and the resulting capacitance change is detected. The fingers  16 ,  18  and  20  serve as differential air-gap sense capacitors for response sensing. Differential capacitance sensing is generally employed to linearize the capacitance change with deflection. 
   For a positive displacement, the finger  20  attached to the mass  14  approaches finger  18  increasing the capacitance Cs+,  22  and moves away from finger  16  decreasing the capacitance Cs−,  24 . By building a differential capacitive bridge  26 , the deflection is translated to a change in capacitance. Defining y 0  as the finger separation, l as the length of the fingers, and t as the thickness of the fingers; the differential capacitance values can be calculated as 
                     C     s   +       =     N   ⁢         ɛ   0     ⁢   tl         y   0     -   y           ,       C     s   -       =     N   ⁢         ɛ   0     ⁢   tl         y   0     +   y           ,       Δ   ⁢           ⁢   C     =         C     s   +       -     C     s   -         =     2   ⁢   N   ⁢         ɛ   0     ⁢   tl       y   0   2       ⁢   y                 (   6   )               
It is observed that the capacitance change is inversely proportional to the square of the initial gap. Thus, the performance of the sensor, i.e. sensitivity, resolution, and signal to noise ratio, is improved quadratically by decreasing the initial gap of the sensing electrodes  16 ,  18  beyond the minimum-gap capabilities of the fabrication process.
 
   Consider now prior art capacitance enhancement techniques. The sensing electrode gap is the foremost factor that defines the upper bound on the sensing performance. Various advanced fabrication technologies have been reported to minimize electrode gap, based on deposition of thin layers on electrode sidewalls. For example, in one reported approach, high aspect-ratio polysilicon structures are created by refilling deep trenches with polysilicon deposited over a sacrificial oxide layer. Thick single-crystal silicon structures are released from the substrate through the front side of the wafer by means of a combined directional and isotropic silicon dry etch and are protected on the sides by refilled trenches. This process involves one layer of low-pressure chemical vapor deposited (LPCVD) silicon nitride, one layer of LPCVD silicon dioxide, and one layer of LPCVD polysilicon. See F. Ayazi, and K. Najafi. High Aspect-Ratio Combined Poly and Single-Crystal Silicon (HARPSS) MEMS Technology Journal of Microelectromechanical Sytems, Vol. 9, 2000, pp. 288–294. However, this example process and all other similar approaches require additional expensive fabrication steps and alignment, and increase the resulting cost. In this approach, high aspect-ratio polysilicon structures are created by refilling deep trenches with polysilicon deposited over a sacrificial oxide layer. 
   Consider enhancement of actuation capacitance according to the illustrated embodiment of the invention. The illustrated embodiment of the post-release assembly technique of the invention aims to increase the capacitances in micromachined devices, in order to achieve low actuation voltages, and enhance the performance and noise characteristics beyond the fabrication process limitations. For the purpose of illustration and demonstration, the concept is implemented on MEMS vibratory gyroscopes, but can be applied to any type of MEMS sensors. 
   The approach is based on attaching the stationary electrodes  16  and  18  of the device  10  to a moving stage  28  that locks into the desired position to minimize the electrode gap before operation. The scanning-electron-microscope images of post-release positioned comb-drives integrated in a micromachined gyroscope are presented in  FIG. 3  in a micromachined gyroscope. The minimum gap requirement of the fabrication process is approximately 10 μm. The resulting gap between the stationary fingers  16 ,  18  and moving fingers  20  after the assembly is approximately 1 μm. In conventional interdigitated comb-drives, the gap between each stationary finger  16 ,  18  and moving finger  20  is determined by the minimum-gap requirement of the fabrication process. For example, if the minimum gap is 10 μm, the gap between the conventional comb-drive fingers is 10 μm. However, in the presented post-release positioning approach, the fingers attached to opposing electrodes are designed initially apart, and interdigitated after the release. Thus, the gap between the fingers after interdigitating can be much smaller than the minimum-gap requirement. 
   Consider for example a comparison of a conventional comb-drive structure, and the post-release positioning approach before and after assembly designed for the same fabrication process as illustrated in the diagrams of  FIGS. 4   a – 4   c .  FIG. 4   a  is a diagrammatic depiction of the prior art topology during the fabrication of stationary fingers  16  and  18  interdigitated between the proof of mass fingers  20 . Here, if the minimum finger gap which can be fabricated is 10 μm then the overall spacing of the assembly of fingers  16 ,  18  and  20  will necessarily be at least 20 μm across.  FIGS. 4   b  and  4   c  are diagrams illustrating the interdigitated finger assembly of the invention before assembly and after assembly respectively. Again, before assembly as seen in  FIG. 4   b  the minimum gap between fingers  16  and  18  is assumed to be 10 μm and the minimum gap between adjacent ones of fingers  20  is 10 μm. Notice the substantial decrease in the size of the resulting gap and the number of fingers per unit area in the assembled configuration of  FIG. 4   c  when the fingers are moved toward each other in an enmeshed or interdigitated configuration. 
   For the same example, if the width of one finger is approximately 8 μm and the minimum gap is 10 μm; the resulting gap between the stationary and moving fingers after the assembly is approximately 1 μm. This results in 10 times increase in the force per finger. Furthermore, the number of fingers per unit area is increased by allowing smaller gaps. In this example, exactly twice the number of fingers can be used in the same area, resulting in a total of 20 times increase in the drive-force. 
     FIG. 5  is a diagram of the flexure system suspending the moving stage  28  which provides very high stiffness after assembly, so that the stationary fingers  16 ,  18  are prevented from deflecting, and the required lateral stability is achieved.  FIG. 5  functionally illustrates a locking mechanism  30  which will lock stage  28  in the interdigitated position, bearings or flexures  32  which allow the laterally stable motion of stage  28  and a stiffness flexure  34  which provides high lateral stiffness after assembly. 
     FIGS. 6   a  and  6   b  are structural diagrams of the post-release positioning comb-drives assembly before and after assembly respectively. A pair of U-shaped resilient flexures  32   a  and  32   b  between the substrate  36  and stage  28  provide for stable lateral movement of stage  28 . In  FIG. 6   b  lock  30  is shown diagrammatically an engaged with stage  28  and retaining stage  28  in the deployed or interdigitated configuration. The moving stage flexure system  32   a  and  32   b  provides a straight line of displacement, and excellent alignment of the fingers  16 ,  18  and  20 .  FIGS. 7   a  and  7   b  are scanning-electron-microscope images of post-release positioning comb-drives integrated in a micromachined gyroscope before assembly, and after assembly respectively as diagrammatically depicted in  FIGS. 5 ,  6   a  and  6   b . A comparison of  FIGS. 7   a  and  7   b  shows movement of stage  28  from a completely unmeshed configuration in  FIG. 7   a  and an enmeshed configuration in  FIG. 7   b . As will be shown in greater detail below lock  30  is comprised of a leaf spring with a ratchet which resiliently rides over the side of stage  28  and then collapses inward to form a blocking stop when stage  28  is in the interdigitated configuration. The concept has been implemented in bulk-micromachined gyroscopes, and the experimental results have successfully demonstrated the feasibility of the design concept. Again it must be understood that the invention contemplates application in any capacitive MEMS sensor. 
     FIGS. 8   a – 8   d  and  10   a – 10   c  illustrate the locking sequence in a series of electron scanning microphotographs. In  FIGS. 8   a  and  10   a  stage  28  and lock  30  are in an initial unlocked configuration. A triangular stop  40  is provided on the distal end of the flexible finger which forms lock  30 . Stop  40  contacts and resiliently rides us a cam  38  defined on the side surface of stage  28  as best seen in  FIG. 8   b . Stage  28  continues to move to the left in the figure reaching a maximum flexure point of stop  40  on cam  38  as seen in  FIGS. 8   c  and  10   b . Further lateral movement of stage  28  results in the resilient inward movement of stop  40  behind stage  28  thereby locking stage  28  in its fully enmeshed lateral configuration as seen in  FIGS. 8   d  and  10   c.    
     FIGS. 9   a  and  9   b  are microscopic photographs of the assembled post-release positioning comb-drives integrated in a micromachined gyroscope. It is observed that excellent positioning is achieved, providing uniform gaps across the comb-drive structure as depicted in  FIG. 9   a . Also, the gyroscope proof-mass  14  is successfully driven into resonance in the drive-mode with the assembled comb fingers as depicted by the blurred image in  FIG. 9   b  caused by the relative movement of mass  14  and fingers  20  relative to fingers  16  and  18 . In atmospheric pressure, the assembled comb fingers with 15V DC bias and 7V AC provided the same oscillation amplitude as conventional comb-drives with 60V DC bias and 15V AC. 
   A thermal actuators is used in the illustrated embodiment for displacing the moving stage  28  and assembling the device  10 . However, it is to be expressly understood that all actuation means now known or later devised for providing displacement in MEMS structures are deemed equivalent and included within the scope of the invention. 
   The explored locking mechanism  30 , includes but not limited to the ratched structure  30  in  FIGS. 10   a – 10   c , or a bistable lock mechanism  30  in  FIGS. 11   a  and  11   b . The principle of the bistable lock mechanism  30   FIGS. 11   a  and  11   b  is comprised of a pair of flexures  44  attached to a support spring  42  connected to substrate  36  at one end of flexure  44  and connected to stage  28  at the opposing end of flexure  44  or at least an extension of stage  28 . Movement by a thermal actuator moves  28  laterally forward from an initial position shown in  FIGS. 12   a  and  11   a  where flexures  44  are rearwardly directed to a compressed configuration shown in  FIG. 12   b , which is permitted by compression of springs  42 . Continued lateral movement of stage  28  brings flexures  44  and springs  42  is a final locked or stable configuration as shown in  FIGS. 12   c  and  11   b  where flexures  44  are forwardly directed.  FIG. 12   d  shows resonance of mass  14  with the bistable lock  30  of  FIGS. 11   a ,  11   b ,  12   a – 12   c  in the locked configuration. 
   In the gyroscopes of the illustrated embodiment, the gyroscope proof-masses  14  were successfully driven into resonance in the drive-mode with the assembled comb fingers, verifying that both the ratchet and bistable lock-in mechanisms  30  provide the required alignment and uniform finger spacing. 
   Enhancement of sensing capacitance as derived analytically in the background discussion above shows that the capacitance change due to deflection in a differential capacitive bridge is inversely proportional to the square of the initial gap. Thus, the sensitivity and resolution of the sensor  10  are improved quadratically by decreasing the initial gap of the sensing electrodes. Similar to enhancement of actuation capacitance, the stationary fingers  16 ,  18  of the sensing electrodes are attached to a moving stage  28  that locks into the desired position before operation, to minimize the electrode gap. This concept also has been implemented in bulk-micromachined gyroscopes, and the experimental results have successfully demonstrated the feasibility of the design concept.  FIGS. 13   a  and  13   b  presents scanning-electron-microscope images of the post-release positioning sensing electrodes integrated in a micromachined gyroscope, before and after assembly respectively. It is observed that excellent positioning is achieved, providing uniform gaps across the sensing electrode structure. Electrode gaps in the order of 1–2 μm have been achieved with 100 μm thick structures as better shown in  FIGS. 14   a  and  14   b  in an overall view and close up view respectively, while the minimum-gap requirement of the fabrication process is 10 μm. 
   Thermal actuators are used in the illustrated embodiment for assembling the sensing electrodes  16 ,  18  and  20  as best depicted in the microphotographs of  FIGS. 15   a  and  15   b . The same locking mechanisms  30  were also successfully employed as described above. In the illustrated gyroscopes, the proof-mass  14  was driven into resonance in the sense-mode with the assembled parallel-plate sensing electrodes, verifying that the required alignment and uniform electrode spacing are achieved.  FIGS. 15   a  and  15   b  show the assembly sequence of the post-release positioning comb-drives using thermal actuators integrated in a micromachined gyroscope before and after assembly respectively. 
   Immediate applications of the design concept include capacitive micromachined inertial sensors, especially accelerometers and gyroscopes, where the performance of the sensor is generally defined by the nominal capacitance of the sensing electrodes. The idea can also be implemented in various electrostatically actuated devices, where the nominal actuation capacitance has to be enhanced to minimize the required drive voltages. 
   Hence, it can now be appreciated that a post-release assembly technique is disclosed that aims to increase the sensing and actuation capacitances in micromachined sensors and actuators, in order to enhance the performance and noise characteristics beyond the fabrication process limitations. The approach is based on attaching the stationary electrodes  16 ,  18  of the device  10  to a moving stage  28  that locks into the desired position to minimize the electrode gap before operation. The concept has been implemented in bulk-micromachined gyroscopes, and the experimental results have successfully demonstrated the feasibility of the design concept. In the illustrated gyroscopes, capacitive enhancement by post-release positioning is performed using lock-in mechanisms; and very robust and fast positioning is achieved with thermal actuators. 
   It can now also be appreciated that the invention is a design concept in MEMS devices which offers the following advantages and differences over existing devices:
         1. The performance of capacitive sensors (i.e. sensitivity, resolution, and signal to noise ratio) is improved by enhancing the nominal capacitance of the sensing electrodes beyond the minimum-gap capabilities of the fabrication process.   2. The drive signal feed-through is a major noise source in active devices such as micromachined gyroscopes. By increasing the actuation capacitances using the post-release assembly technique, the actuation voltages are minimized. Thus, the drive signal feed-through is minimized, and the noise characteristics are improved beyond the fabrication process limitations.   3. Capacitive enhancement by post-release positioning is performed using lock-in mechanisms; and very robust and fast positioning is achieved with thermal actuators.   4. The lock-in system and the flexure system suspending the moving stage provide very high stiffness after assembly, so that the stationary fingers are prevented from deflecting, and the required lateral stability is achieved.       

   Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. 
   The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
   The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination. 
   Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
   The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.