Abstract:
Electrostatic actuation arrangements are disclosed comprising of at least two wafers and having electrodes formed on their facing surfaces. One of the wafers has holes in it while the other wafer has posts extending therefrom. The holes and the posts are arranged so as to face each other and a voltage is applied to the electrodes across the wafers so that they can be moved toward and away from each other allowing such a movement to control fluid or optical parameters.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of official duties by an employee of the Department of the Army and may be manufactured, used, licensed by or for the Government for any governmental purpose without the payment of any royalty thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     1.0 Field of the Invention 
     The present invention relates to a control system and, more particularly, to an electrostatic actuating control system that utilizes miniature silicon wafers and finds applications in fluid control systems and optical applications. 
     2.0 Description of the Prior Art 
     Microfabrication techniques employed in standard integrating circuit processing industry have lead to the fabrication of miniature devices, such as wafers formed of silicon. 
     Miniature devices because of their small dimensions and relatively light weight may be advantageously used in control systems, such as an electrostatic control system, that utilizes the movement of members to accomplish a control function. It is desired that an electrostatic actuating arrangement be provided that utilizes relatively small wafers to accomplish control functions and that find application in a fluid control system or in an optical application. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to provide for an electrostatic actuating system that employs miniature wafers that are physically moved to accomplish a desired control operation. 
     It is another object of the present invention to provide means for ensuring that movable wafers are returned to their home position whenever moved therefrom. 
     It is yet another object of the present invention to provide for an electrostatic actuating control system that is responsive to high-frequency actuations. 
     In accordance with these and other objects, the present invention provides an electrostatic actuating system comprising at least first and second wafers and a first source of voltage. The first wafer as at least one hole in it and the second wafer has at least one post extending upward from one of its surfaces and which is dimensioned so as to be insertable into the at least one hole. The first and second wafers have faces that are arranged so that the post faces the hole. The first and second wafers have a layer of conductive material placed on one of their facing surfaces which serve as an electrode. The hole and the post are free of the conductive material. The source of voltage is applied across the electrodes of the first and second wafers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects and advantages of the present invention will be more fully understood from the following detailed description having reference to the appended drawings wherein: 
     FIG. 1 illustrates one of the wafers used in the practice of the present invention; 
     FIG. 2 illustrates the second wafer used in the practice of the present invention; 
     FIG. 3 illustrates one embodiment of an electrostatic actuation control system of the present invention; 
     FIG. 4 illustrates another embodiment of an electrostatic actuation control system of the present invention; 
     FIG. 5,  6  and  7  illustrates various embodiments that ensure that the wafers of the present invention are returned to their home position when moved therefrom; 
     FIG. 8 schematically illustrates a control system that is used to control fluid functions; and 
     FIG. 9 schematically illustrates a control system used for optical applications. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides an electrostatic actuation arrangement  10  that finds application in many fields, such as those involved with fluid control or optical interactions. 
     Referring to the drawings, wherein the same reference number indicates the same element throughout, there is shown in FIG. 1 a wafer  12  used in the electrostatic actuation arrangement  10  to be further discussed with reference to FIG.  3  and having a typical diameter  14  of up to 200 millimeters (mm), and a typical thickness  16  from about 100 micrometers to about 200 micrometers. The wafer  12  has an electrode  18 . The wafer  12  can be of any shape and does not have to be a circle. The wafer  12  can be of any diameter and can be as small as a few millimeters or as large as a full 4″, 6″, or 8″ standard silicon wafer. 
     The wafer  12  is preferably comprised of silicon and has a series of holes  20 ,  22 ,  24 ,  26 ,  28  and  30 , arranged in columns as shown in FIG. 1, with the holes  20  . . .  30  being provided by deep reactive ion etching (DRIE), known in the art of silicon fabrication. It should be noted that the holes  20 ,  22 ,  24 ,  26 ,  28  and  30  are shown in FIG. 1 as being both on the surface of the wafer  12  and also in cross section (lower portion of FIG.  1 ). The holes  20  . . .  30  can have any cross-sectional shape to suit any given application, and wherein circles and squares cross-sectional shapes are commonly used in fluid boundary-layer applications, and triangles and rectangles cross-sectional shapes are commonly used in optical applications. The electrostatic actuating system  10  further utilizes a second wafer  32  that may be described with reference to FIG.  2 . 
     FIG. 2 illustrates the second wafer  32  as having the same diameter  14  as that of the first wafer  12 , a thickness  34  that has a range of about  100  micrometers to  300  micrometers, and an electrode  38  that is used in different applications for the present invention. The electrode  38 , as well as the electrode  18  of the first wafer  12  and the electrode of the third wafer to be described hereinafter, comprises a metal or a layer of conductive material sputtered or applied over one of the entire surfaces thereof. The electrodes are formed so that the holes  20 ,  22 ,  24 ,  26 ,  28  and  30  of FIG. 1, as well as the post  40 ,  42 ,  44 ,  46 ,  48  and  50  of FIG. 2, are free of conductive material. 
     The posts  40 ,  42 ,  44 ,  46 ,  48  and  50  are shown in FIG. 2 as both being located in the surface of the second wafer  32  and in the lower portion of FIG. 2 as extending upward from the upper surface of wafer  32  as viewed in FIG. 2 by distance  52  having a typical value of 0.4 mm. The posts  40 ,  42 ,  44 ,  46 ,  48  and  50  have complementary dimensions relative to holes  20 ,  22 ,  24 ,  26 ,  28  and  30  respectively. More particularly, when the first and second wafers  12  and  32  are arranged so as to be facing each other, the posts  40 ,  42 ,  44 ,  46 ,  48  and  50  are respectively insertable into the holes  20 ,  22 ,  24 ,  26 ,  28  and  30 . A comparison between FIGS. 1 and 2 also reveals that cross-section of the holes  20  . . .  30  respectively match  40  . . .  50  and, accordingly, the selection of the cross-sectional shapes of holes  20  . . .  30  for particular applications, previously discussed, are the same for the posts  40  . . .  50 . 
     Each of the wafers  12  and  32  preferably has a minimum thickness of 100 micrometers so as to prevent their damage during handling. A typical wafer arrangement, comprising wafers  12  and  32 , has a thickness of 500 micrometers, wherein the first wafer  12  has a thickness of about 100 to about 200 micrometers and the second wafer  32  has a thickness of about 100 to 300 micrometers so that the overall thickness is 500 micrometers. For a three wafer arrangement, to be described with reference to FIG. 4, this overall thickness may be about 1000 micrometers. An electrostatic actuation arrangement, sometimes referred to as a parallel-plate plate configuration, utilizing the wafers  12  and  32  may be further described with reference to FIG.  3 . 
     FIG. 3 illustrates the wafers  12  and  32  as being connected to a source of voltage  56  applied to wafer  12 , by way of connection  58  and electrode  18  and to wafer  32  by way of connection  60  and electrode  38 . For such connectors, pads for wire bonding or other conductor attachment are located at a suitable location on each of the wafers  12  and  32 , but the wire bondings are placed on the electrodes  18  and  36  so that the bondings do not interfere with the motion of either of the wafers  12  and  32 . 
     In operation, the electrostatic actuation arrangement  10  of FIG. 1 is used to raise and lower the posts  40 ,  42 ,  44 ,  46  and  48  through holes  20 ,  22 ,  24 ,  26 ,  28  and  30  respectively. The arrangement  10  utilizes the pair of electrodes  18  and  38  in a parallel-plate configuration. The electrostatic force associated with the source of voltage  56  that is applied across the electrodes  18  and  38  of the first and second wafers  12  and  32 , respectively, moves wafers  12  and  32  towards each other. The amount of separation  62  between the wafers  12  and  32  is determined by the value of the voltage of source  56 , to be further described hereinafter. An alternative electrostatic actuation arrangement  64  may be further described with reference to FIG.  4 . 
     FIG. 4 illustrates the electrostatic actuation arrangement  64  as having a third wafer  66  that has an electrode  66 A which is the same as electrodes  18  and  38 . Similarly, the third wafer  66  has the same dimensions as wafer  32 . However, the third wafer  66  does not have posts although it may need a pattern or series of random holes to reduce squeeze-film damping, known in the art. The third wafer  66  has a full electrode,  66 A, covering the surface facing the wafer  32  having the posts  40  . . .  50 . A second voltage supplied from a source  68  of voltage is applied across the electrode  66 A of wafer  66  and an electrode  36  of wafer  32  and operates so that the third wafer  66  pulls the wafer  32  and separates it from the top wafer  12 . In most applications, the first wafer  12  may be fixed to a rigid surface and the second wafer  32  with the post may serve as the moving wafer. The third wafer  66  is also preferably fixed to a rigid surface in most applications. 
     In operation, the wafers  12 ,  32 , and  66  in any arrangement thereof should return to their home position, and in most applications thereof, gravity assists in such a return. However, to ensure that the wafers do not contact each other or remain contact therebetween which would otherwise hinder the wafers  12 ,  32  and  66  from returning to each respective home position, different embodiments are provided by the practice of the present invention. The different embodiments may be further described with reference to FIGS. 5,  6  and  7  which provide for bumps or stops to prevent any two facing wafers from contacting each other. 
     FIG. 5 illustrates the wafer  32  having a post, such as post  40 , extending upward from its surface. The post  40  has at least one protrusion  74 , but preferably two protrusions  74  and  76  extending outward from the surface of the post  40 . It is preferred that the protrusions  74  and  76  be placed at a distance of about not more than 200 micrometers from the surface shown in FIG. 5 of the wafer  32 . The protrusions  74  and  76  prevent the possibility of the wafer  32  from physically contacting another wafer, such as wafer  12 . Without such a suitable stop provided by protrusions  74  and  76 , it may be difficult to separate the wafers  12  and  32  once they contact each other. An embodiment for a motion stop that utilizes an elastic material may be further described with reference to FIG.  6 . 
     FIG. 6 illustrates that the post  40  has a piece of elastic material  78  applied thereto. The elastic material may have a thickness of not more than 200 micrometers. A further embodiment that provides for a motion stop, may be further described with reference to FIG.  7 . 
     FIG. 7 shows the wafers  12 ,  32  or  66  having a piece of elastic material  80  along with one of its surfaces. The elastic material  80  may extend upward from the surface by an amount similar to that of stop  74 - 76  and  78 , that is, by no more than 200 micrometers. The elastic materials  80  act as springs or electrical insulators to prevent the mating between the surfaces of the wafers  12 ,  32  and  66 . 
     The maximum protrusion of the posts  40 ,  42 ,  44 ,  46 ,  48 , and  50 , extending through the holes  20 ,  22 ,  24 ,  26 ,  28  and  30 , respectively, depends upon the thickness of the wafer  12  and the length of the posts  40 ,  42 ,  44 ,  46 ,  48 , and  50 . For a post having a length of 400 micrometers, and a thickness of 100 micrometers for the top wafer  12 , the maximum protrusion of the posts  40 ,  42 ,  44 ,  46 ,  48 , and  50  respectively into holes  20 ,  22 ,  24 ,  26 ,  28  and  30  is approximately 300 micrometers. This 300 micrometer displacement requires an initial gap, such as generally indicated by reference number  62  of FIG. 3, between the wafers of about 300 micrometers. 
     Since electrostatic forces are virtually proportional to the square of the distance of the gap  62 , large displacements require correspondingly large voltages. The force for an electrostatic charge applied to a pair of parallel-plate electrodes, such as those of wafers  12  and  32 , having an area A, gap x, and voltage V, may be expressed by relationship (1) given below:              f   =       ɛ                 A                   V   2         2        x   2                 (   1   )                                
     where ε is the permeativity of the gap (ε=8.8×10− 12  f/m for air). The weight, w, of a wafer, such as wafer  12  or  32 , of diameter d, density p, and thickness t may be expressed by relationship (2) given below:              w   =     p                 g                 t                     π                   d   2       4               (   2   )                                
     where g is the acceleration of gravity. 
     Equating the electrostatic force and the weight of the wafer and solving for the voltage yields relationship (3) given below:                V   2     =         2                 p                 g                 t                   x   2       ɛ     .             (   3   )                                
     This equation of relationship (3) can be used to estimate the voltage required to bring the two wafers, such as  12  and  32 , together for any given initial gap  62 . For a 100 μm thick moving wafer of silicon (p=2330 kg/m 3),  60 volts is required for an initial gap  62  of 100 μm. Other values are easily determined since voltage is linearly proportional to the gap  62 . 
     The electrostatic actuating arrangements  10  and  64  of FIGS. 3 and 4, respectively, may find applications such as that schematically illustrated in FIG.  8 . 
     FIG. 8 illustrates an arrangement  82  having some of the features previously described with reference to FIG.  3  and is particularly suited to provide a fluid boundary-layer control and fluid heat-transfer augmentations. For either application, the wafer  12  is flush mounted to a wall or surface  84  and the holes (not shown) as well as the posts  40  and  42  preferably has a circle or square cross-sectional shape. In such an application, the posts  40  and  42  are part of the wafer  32  and extend into the fluid,generally indicated by directional arrow  86 , when boundary-layer control is required. The posts  40  and  42  preferably extend into flow  86  so as to be perpendicular to the axis  88  of the flow  86 . 
     The amount of voltage supplied by source  56  is in accordance with the relationships (1), (2), and (3). An additional embodiment of the present invention particularly suited for optical switching may be further described with reference to FIG.  9 . 
     FIG. 9 illustrates the first and second wafers  12  and  32  arranged in the parallel-plate configuration similar to that of FIG. 8, but with the post  40  of wafer  32  preferably having a triangular or rectangular cross-sectional shape and being retracted or protruding from the complement hole (not shown) in the first wafer  12  so as to intercept or deflect a laser beam  90 . 
     It should now be appreciated that the practice of the present invention provides for electrostatic actuation arrangements that find applications in fluid control and optical control situations. 
     Although the previous discussion was directed to the wafers  12 ,  32 , and  66  as having a complete shape, such as shown in FIGS. 1 and 2, this is not necessary in that the use of a less than full wafer  2  may be utilized in all of the previously described applications. 
     Various additional modifications will become apparent to those skilled in the art, and all such variations, which basically rely on the teaching of which this invention advanced the art, are properly considered within the scope of this invention.