Patent Publication Number: US-7710371-B2

Title: Variable volume between flexible structure and support surface

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to techniques in which a flexible structure is attached to a support surface. More particularly, the invention relates to techniques in which a variable volume is defined between a flexible structure and a support surface. 
   Techniques have been previously proposed in which a flexible material such as polymer is deposited on a substrate. For example, Doany, F. E., and Narayan, C., “Laser release process to obtain freestanding multilayer metal-polyimide circuits,” IBM J. Res. Develop., Volume 41, No. 1-2, January/March 1997, pp. 151-157, describe deposition of polymer films with metal wiring features, after which the structure is removed from the substrate by a laser separation process that ablates a polymeric layer, forming a freestanding structure. Bakir, M. S., Reed, H. A., Mulé, A. V., Jayachandran, J. P., Kohl, P. A., Martin, K. P., Gaylord, T. K., and Meindl, J. D., “Chip-to-Module Interconnections Using ‘Sea of Leads’ Technology,” MRS Bulletin, January 2003, pp. 61-63 and 66-67, describe application and patterning of a sacrificial polymer on a wafer, followed by deposition of an overcoat polymer; the sacrificial polymer is then thermally decomposed to form an air gap embedded within the overcoat polymer, after which vias are fabricated to expose die pads and allow electrical connection of leads on the overcoat polymer to a chip in the wafer. 
   Previous techniques, however, are limited in the variety of articles that can be produced with a flexible structure attached to a support surface. It would be advantageous to have additional techniques for flexible structures attached to support surfaces. 
   SUMMARY OF THE INVENTION 
   The invention provides various exemplary embodiments of cells, arrays, apparatus, and methods. In general, each embodiment involves a variable volume between a flexible structure and a support surface to which it is attached. 
   These and other features and advantages of exemplary embodiments of the invention are described below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of a cell with variable volume showing circuitry schematically. 
       FIG. 2  is a schematic cross-sectional view of the cell of  FIG. 1  taken along the line  2 - 2 ′, with additional structure above the variable volume. 
       FIG. 3  is a schematic top view of an array of cells with variable volume 
       FIG. 4  is a cross-sectional view of an array as in  FIG. 3 , along the line A-A′, implemented as an optical modulator. 
       FIG. 5  is a cross-sectional view of an array as in  FIG. 3 , along the line A-A′, implemented as another optical modulator. 
       FIG. 6  is a top view of the unattached region of the flexible structure for a cell in the implementation of  FIG. 5 , taken along the line  6 - 6 ′ in  FIG. 5 . 
       FIG. 7  is a cross-sectional view of an array as in  FIG. 3 , along the line A-A′, implemented as a display. 
       FIG. 8  is a cross-sectional view of an array as in  FIG. 3 , along the line A-A′, implemented as a printer. 
       FIG. 9  is a timing diagram of signals to cell regions of an array as in  FIG. 8 . 
       FIG. 10  is a cross-sectional view of an array as in  FIG. 3 , along the line A-A′, implemented as a microphone. 
       FIG. 11  is a schematic diagram of a circuit that could be used with the array of  FIG. 10 . 
       FIG. 12  shows cross-sectional views of stages in a process that produces a variable volume cell. 
       FIG. 13  shows cross-sectional views of stages in another process that produces a variable volume cell. 
       FIG. 14  is a cross-sectional view of a stage in another process that produces a variable volume cell. 
       FIG. 15  is a cross-sectional view of a stage in another process that produces a variable volume cell. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, numeric ranges are provided for various aspects of the implementations described. These recited ranges are to be treated as examples only, and are not intended to limit the scope of the claims hereof. In addition, a number of materials are identified as suitable for various facets of the implementations. These recited materials are to be treated as exemplary, and are not intended to limit the scope of the claims hereof. 
   Various techniques have been developed for producing structures with one or more dimensions smaller than 1 mm. In particular, some techniques for producing such structures are referred to as “microfabrication.” Examples of microfabrication include various techniques for depositing materials such as growth of epitaxial material, sputter deposition, evaporation techniques, plating techniques, spin coating, and other such techniques; techniques for patterning materials, such as photolithography; techniques for polishing, planarizing, or otherwise modifying exposed surfaces of materials; and so forth. 
   In general, structures, elements, and components described herein are supported on a “support structure” or “support surface”, which terms are used herein to mean a structure or a structure&#39;s surface that can support other structures; more specifically, a support structure could be a “substrate”, used herein to mean a support structure on a surface of which other structures can be formed or attached by microfabrication or similar processes. 
   The surface of a substrate or other support structure is treated herein as providing a directional orientation as follows: A direction away from the surface is “up” or “over”, while a direction toward the surface is “down” or “under”. The terms “upper” and “top” are typically applied to structures, components, or surfaces disposed away from the surface, while “lower” or “underlying” are applied to structures, components, or surfaces disposed toward the surface. In general, it should be understood that the above directional orientation is arbitrary and only for ease of description, and that a support structure or substrate may have any appropriate orientation. 
   A process that produces a layer or other accumulation of material on structures or components over a substrate&#39;s surface can be said to “deposit” the material, in contrast to processes that attach a part such as by forming a wire bond or that mechanically transfer an existing layer from one substrate to another. A structure is “fabricated on” a surface when the structure was produced on or over the surface by microfabrication or similar processes. 
   A structure or component is “attached” to another when the two have surfaces that are substantially in contact with each other and the contacting surfaces are held together by more than mere mechanical contact, such as by an adhesive, a thermal bond, or a fastener, for example. A structure or component is “directly on” a surface when it is both over and in contact with the surface. 
   As used herein, “flexible structure” refers to a structure that can be deformed without breaking; specifically, as used herein, a flexible structure can be stretched from an unstretched position to other positions by a force, referred to herein as a “stretching force”. A flexible structure is referred to herein as “unstretched” when it is subject to approximately zero stretching force. 
   An “elastically flexible structure” is a flexible structure that returns elastically to substantially its unstretched position when released after being stretched; this elastic behavior is a materials property, and is true, for example, of many polymer materials. As used herein, “polymer” refers to any material that includes one or more compounds formed by polymerization and that has properties resulting from presence of those compounds. An elastically flexible structure may also have plastic deformation, especially if subject to extraordinary stretching force, but across some useful range of stretching forces its deformation is substantially elastic. 
   The invention provides certain implementations that are characterized as “cells” and “arrays”, terms that have related meanings herein: An “array” is an arrangement of “cells”. An array may also include circuitry that connects to electrical components within the cells such as to select cells or transfer signals to or from cells, and such circuitry is sometimes referred to herein as “array circuitry”. In contrast, the term “peripheral circuitry” is used herein to refer to circuitry on the same support surface as an array and connected to its array circuitry but outside the array. The term “external circuitry” is more general, including not only peripheral circuitry but also any other circuitry that is outside a given cell or array. 
     FIG. 1  shows support structure  10  with surface  12  on which is supported cell  20 . Cell  20  includes an elastically flexible structure  22  that is attached to surface  12  in region  24  and unattached to surface  12  in region  26 .  FIG. 1  also shows region  26  surrounded by region  24  in the sense that region  24  bounds region  26  all along its outer margin. When unstretched, flexible structure  22  lies in a “flat position”, meaning a position in which there is substantially no space, and therefore no gaseous or liquid fluid, between it and the underlying surface; more specifically, the lower side of flexible structure  22  is directly on surface  12  or other surfaces within region  26  that form the support surface. 
   As described below in relation to  FIG. 2 , cell  20  also includes “electrodes”, a term used herein to refer to a component within which charge carriers such as electrons or holes have nonzero mobility; electrodes can function, for example, as components through which current flows or as components within which charge can be concentrated in regions, such as within a capacitor electrode. Circuitry  28  provides conductive paths between at least some of the electrodes and external circuitry  30 . More specifically, circuitry  28  provides at least one “signal path”, meaning a conductive path through which information is transferred from one component to another, such as from an electrode to external circuitry  30  or vice versa. 
     FIG. 2  shows a cross-section of cell  20  taken along the line  2 - 2 ′ in  FIG. 1 , with flexible structure  22  in one of its possible stretched positions in response to a stretching force (not shown). As shown, cell  20  further includes variable volume  40  between flexible structure  22  and surface  12 ; as used herein, the term “variable volume” refers generally to a substantially enclosed volume that can change in response to one or more forces. As illustrated by the double arrow in  FIG. 2 , variable volume  40  increases and decreases in volume as flexible structure  22  rises and falls, respectively. More generally, variable volume  40  varies as flexible structure  22  moves in region  26 . 
     FIGS. 1 and 2  suggest a useful approach to measuring cells. An important feature of cell  20  is the area of region  26 , which is also the area of variable volume  40 . As used herein, the term “micro-cell” refers to a cell with a variable volume whose area on a support surface is not greater than approximately 1 mm 2 . 
   In  FIG. 2 , spacers  42  support top structure  44 , which extends over volume  46  above flexible structure  22 . Although volume  46  would also vary as flexible structure  22  moves in region  26 , it may not be substantially enclosed as a variable volume would be, as described below in relation to some implementations. 
   Flexible structure  22  is illustratively a layered structure with one or more layers of material that may have been differently patterned. The main part of flexible structure  22  is an elastically flexible material, such as a polymer film or other thin layered structure of polymer material. Polyimide, for example, can be deposited by a spin coating process to produce an elastically flexible polymer film on a support surface. Movable electrode  50  is illustratively shown as a separate, differently patterned layer on the elastically flexible material. Movable electrode  50  is part of flexible structure  22  and therefore moves with it. 
   Cell  20  also includes a set of stationary electrodes, including electrodes  52  and  54 . Electrode  52  is illustratively on surface  12  with its upper surface being part of the support surface on which flexible structure  22  lies when in the flat position, but electrode  52  could instead be a conductive region under surface  12 . Electrode  54  is illustratively part of top structure  44 . Movable electrode  50  is illustratively shown on the upper side of flexible structure  22 , but could be implemented within or on the lower side of flexible structure  22  if appropriate modifications are made to avoid electrical contact between electrodes  50  and  52 . 
   Since region  24  surrounds region  26 , variable volume  40  is enclosed with the possible exception of one or more ducts for fluid communication with variable volume  40 , schematically represented in  FIG. 2  by duct emblem  60 . The term “duct” is used herein to refer to a channel for fluid flow from region to region. In actual implementations, a duct could permit fluid flow between variable volume  40  and an exterior region; for example, one or more ducts could be defined in support structure  10  or in flexible structure  22 , as described below in relation to implementations. Furthermore, fluid under pressure can through a duct and produce a stretching force away from surface  12  on flexible structure  22 ; in response, flexible structure  22  moves out of its flat position to provide variable volume  40 . 
   Stationary electrodes  52  and  54  are insulated from movable electrode  50 . As a result, charge levels on electrodes  50 ,  52 , and  54  produce electrical fields that interact mechanically with flexible structure  22  through electrode  50 . In addition, flexible structure  22  has pressure interactions at its lower surface with fluid in variable volume  40  and at its upper surface with fluid in volume  46 . As used herein, charge levels on electrodes are described as “coupling with” a variable volume if signals changing one or more of the charge levels tend to provide or change the variable volume or if variations in the variable volume, such as in response to pressure interactions, tend to provide signals through one or more of the electrodes. Similarly, charge levels on electrodes are described as “coupling with each other” if the charge levels result in attractions or other interactions between the electrodes; for example, attraction between electrodes  50  and  54  would provide a stretching force away from surface  12  on flexible structure  22 , and flexible structure  22  would respond by moving out of its flat position to provide variable volume  40 . Various examples of coupling between charge levels are described below in relation to implementations. 
   A structure with features as shown in  FIGS. 1 and 2  could be produced in various ways using various materials. In general, the choice of particular materials and manufacturing techniques depends on the application, but the following indicate the range of available materials and techniques. 
   Support structure  10  could be a glass substrate on which lower electrode  52  has been photolithographically patterned from a layer of conductive material; the conductive material could include sputter coated chromium to a depth of 10 nm and gold to a depth of 100 nm. Instead of glass, support structure  10  could be a silicon wafer coated with insulating silicon dioxide or a flexible substrate material such as Mylar® from DuPont. If transparent electrodes are desired, the conductive material could be sputtered indium-tin-oxide (ITO). 
   Flexible structure  22  could be a membrane with a suitable polymer layer. For example, it could be made from spin-coated polyimide such as one of the polyimides available from HD MicroSystems, e.g. HD-4000, PI-2600, or another. Such a material has a modulus of elasticity (Young&#39;s modulus) in the range of 3-8 GPa and the membrane could have a thickness of 1 μm. A more elastic membrane material could be chosen, such as silicone, e.g. Sylgard® 184 from Dow Corning Corporation, with a modulus of elasticity around 2 MPa. Due to the much lower modulus of electicity for silicones, the membrane may be thicker, e.g. 10 μm. The diameter of the unattached area of the membrane may be 400 μm, but depending on the application could be as small as 50 μm, or as large as 10 mm. 
   The middle electrode  50  on flexible structure  22  could include sputter coated chromium/gold; a transparent conductor such as ITO; or a more flexible conductor such as one of the carbon nanotube-based polymers developed by Eikos, Inc., Franklin, Mass. Electrode  50  could be patterned into stripes, a spiral shape, or another similar shape for stress relief during bowing or flexing of flexible structure  22 . 
   Spacers  42  could be photolithographically patterned onto flexible structure  22  from a layer of a photopolymer such as SU-8 from MicroChem, Corp. Spacer walls could be formed by other techniques such as by printing of polymers, laser ablation, or plating techniques. The height of spacers  42  may be between 5 μm and 100 μm, or even as high as several hundred microns if appropriate. 
   Top structure  44  could be a counter plate bonded to spacers  42  in any appropriate way. Structure  44  could, for example, be a glass plate with a patterned top electrode  54  made of ITO for transparency. Also, rather than being formed on flexible structure  22 , spacers  42  could be patterned on structure  44 , in which case the assembly including structure  44  and spacers  42  could be bonded onto flexible structure  22 . 
     FIG. 3  illustrates array  80  on surface  12  of support structure  10 . Array  80  includes cells  82 ,  84 ,  86 , and  88 , each of which could be implemented as shown in  FIGS. 1 and 2 . As suggested by the ellipses, array  80  could be a two-dimensional array, the cells of which could be individually addressed by appropriate circuitry, such as circuitry that addresses each cell by row and column. Peripheral circuitry  90  on surface  12 , but outside array  80 , can have signal communication with each electrode through array circuitry  92 , connected to each electrode. 
     FIG. 4  shows a cross-section of an optical modulator implementation of array  80 , taken along the line A-A′ in  FIG. 3 . In  FIG. 4 , ducts in support structure  10  serve as breathing holes. Support structure  10  illustratively includes three general layers—substrate  100 , device layer  102 , and insulating layer  104 . Device layer  102  can include control and signal lines for cells in array  80 , and can also include active switches to control charge transfer to or from lower electrode  106  through interconnecting material  108 , illustratively labeled for cell  82  but similarly structured for other cells. Interconnecting material  108  could, for example, be sputter coated metal, plated metal, or plasma deposited doped amorphous silicon, deposited in each case within a via or other opening defined in insulating layer  104  or a region of conductive material produced by modifying insulating layer  104  in some other way. 
   Flexible structure  22  is attached to support structure  10  by adhesive material  110  in the attached region  24  ( FIG. 1 ) of each cell. Adhesive material  110  is an example of an “adhesion structure”, used herein to refer to a layer, layered structure, part of a layer or layered structure, or another structure that adheres to surfaces of each of two or more other components, attaching the surfaces to each other; an adhesion structure could be or include a thin layer of material from the surface of one of the components, melted or otherwise modified so that it adheres to the surface of the other component. In this case, adhesive material  110  adheres to both the lower surface of flexible structure  22  and the upper surface of support structure  10 , attaching them to each other. 
   Lower electrode  106  is in unattached region  26  ( FIG. 1 ), directly under flexible structure  22  and part of the support surface that is in contact with flexible structure  22  in its flat position. Each cell&#39;s lower electrode is independently addressable. 
   Flexible structure  22  can, for example, include polyimide film  112  on top of which is middle electrode  114 , a movable electrode that illustratively extends throughout array  80  and is therefore common to all cells. As in  FIG. 2 , top structure  44  is separated from flexible structure  22  by spacers  42 . Top structure  44  includes an upper stationary electrode (not shown), which can, for example, be independently addressable for each cell or common to all cells. 
   In operation, charge carriers concentrated in lower electrode  106 , middle electrode  114 , and the upper electrode (not shown) interact through electric fields, causing flexible structure  22  to move between its flat position, illustrated for cell  84 , and an open position, illustrated for cells  82 ,  86 , and  88 . These interactions provide examples of charge levels on electrodes coupling with each other and with a variable volume. For example, all cells can be reset to their flat positions by grounding all lower electrodes while applying the same voltage potential to the upper and middle electrodes. Then the upper electrode can be grounded and charges can be applied to selected lower electrodes to change their cells to their open positions. When flexible structure  22  moves from its flat position to the open position, fluid such as air is drawn into the cell&#39;s variable volume through duct  120  defined in support structure  10 , as illustratively labeled for cell  82 . Similarly, when flexible structure  22  moves from a cell&#39;s open position to its flat position, fluid is expelled from the cell&#39;s variable volume through duct  120 . 
   In general, the volume between flexible structure  22  and top structure  44  forms a plenum that communicates with the exterior of array  80 . Spacers  42  do not continuously surround the cells, so that fluid such as air is relatively free to flow in and out of the plenum region above each cell. 
   Top structure  44  is substantially transparent, while middle electrode  114  is reflective. For an appropriate wavelength, the change in position of middle electrode  114  between flat and open positions of flexible structure  22  is sufficient to change between constructive and destructive interaction between incident and reflected light. Arrows  130  indicate substantially monochromatic incident light arriving at each of cells  82 ,  84 ,  86 , and  88 . Due to destructive interaction, however, light is not effectively reflected by cells  82 ,  86 , and  88 , but arrow  132  indicates that a constructive interaction permits effective reflection of light from cell  84 . More specifically, if the difference between the flat and open positions of flexible structure  22  is one-fourth the wavelength of incident light, a transition between constructive and destructive interaction can be obtained. For example, for wavelengths between 1300-1500 nm, used in optical fiber communication, one-quarter wavelength would be approximately 300 nm. 
   The approach of Francais, O., and Dufour, I., “Enhancement of elementary displaced volume with electrostatically actuated diaphragms: application to electrostatic micropumps,”  J. Micromech. Microeng ., Vol. 10, 2000, pp. 282-286, incorporated herein by reference, can be used to obtain the voltage requirement to deflect a membrane such as flexible structure  22  a given distance. If it is assumed that the internal stress of polyimide film  112  is 2 MPa, a cell&#39;s unattached membrane surface area is 0.16 mm 2 , the thickness of the membrane is 3 μm, and the air gap between the membrane and lower electrode  106  in the open position is 3 μm, approximately 20 V are required to deflect the membrane by 300 nm. This voltage level can be applied using currently available active matrix addressing techniques through appropriate circuitry in device layer  102 . 
   At small cell sizes, problems may arise with curvature-induced divergence. Therefore, the size of the cell should be much larger than the optical beam size. For a 10 μm diameter laser beam and 400 μm cell diameter, the deviation of the height from the beam edge to the center is only about 0.06%. 
   To fabricate the structure of  FIG. 4 , device layer  102  can first be fabricated on the surface of substrate  100 , using any suitable techniques such as conventional deposition and photolithographic patterning techniques. Insulating layer  104  can then be deposited over device layer  102 ; layer  104  could, for example, include a photopolymer such as SU-8 from MicroChem, Corp., deposited to a thickness between approximately 1 μm and several 10 s of microns and patterned to include openings or through-holes for subsequent formation of ducts  120 . Interconnecting material  108  and lower electrode  106  can then be formed, such as by sputtering and plating techniques. More specifically, a plasma deposition method such as plasma deposited (PECVD) doped amorphous silicon may give a high quality conformal coating of narrow through-holes in layer  104 . To prevent the subsequent membrane coating from filling the through-holes, the through-holes may be temporarily filled with a wax or another polymer such as PVA that can be dissolved at a later stage. 
   Flexible structure  22  can then be produced and selectively adhered to the exposed surface such as by any of the selective adhesion techniques described in greater detail below. Middle electrode  114  can be produced on top of polyimide film  112  by deposition and photolithographic patterning of conductive material. 
   Spacers  42  can be fabricated by depositing an insulating material to a height of approximately 3 μm and then performing photolithographic patterning. Top structure  44 , produced separately with similar techniques, can then be attached to the top surfaces of spacers  42 , such as with an adhesive material or an appropriate bonding process. 
   Ducts  120  can be etched from the lower surface of substrate  100  through device layer  102 , through interconnecting material  108  in insulating layer  104 , and through lower electrode  106 , stopping at polyimide layer  112 . For example, if substrate  100  is silicon, deep reactive ion etching could be used; if substrate  100  is polymer material, laser ablation could be used; and other etching methods could be used as appropriate. 
     FIG. 5  shows a variation of the optical modulator in  FIG. 4  in which duct  120  is not defined in support structure  10 , but instead ducts  140  are defined in flexible structure  22 . The operation of the optical modulator in  FIG. 5  is substantially as described above in relation to  FIG. 4 . In fabrication, ducts  140  can be constructed after flexible structure  22  is fabricated, such as by photolithographically patterning a resist layer and by then etching through openings in the resist layer. In other respects, fabrication can be the same as described above.  FIG. 6  shows an example of a pattern of ducts  140  in unattached region  26  of flexible structure  22  in cell  88 , viewed along the line  6 - 6 ′ in  FIG. 5 . 
     FIGS. 4-6  illustrate examples of optical modulators in which a flexible structure is attached to a surface of a support structure in an array. In each of two or more cell regions within the array, however, the flexible structure is unattached to the support surface. In each cell region, the flexible structure and the support surface define a respective variable volume between them. In each cell region, the flexible structure includes a movable electrode portion and the cell region also includes a set of at least one stationary electrodes. As a result, charge levels on each cell region&#39;s movable electrode portion and stationary electrodes couple with each other and with the cell region&#39;s variable volume. 
   The optical modulator also includes array circuitry that connects to at least one electrode and peripheral circuitry at the support surface outside the array region as illustrated in  FIG. 3 . The peripheral circuitry thus has signal communication with at least one electrode through the array circuitry. An optical modulator as in  FIGS. 4-6  also includes a transparent top structure over the flexible structure, and the flexible structure has a reflective upper surface area for each cell region. The peripheral circuitry provides signals to each cell region&#39;s lower electrode through the array circuitry, and the signals produce charge levels causing the flexible structure to move between a flat position and an open position in which a variable volume is provided. As a result, the cell region&#39;s reflective upper surface area reflects differently in the flat and open positions, modulating incident light. 
     FIG. 7  shows a cross-section of a display implementation of array  80 , taken along the line A-A′ in  FIG. 3 . As in  FIG. 4 , ducts in support structure  10  allow fluid to flow in and out of each cell region&#39;s variable volume, but the fluid in this implementation is a dye or other light absorbent fluid that can interact with light when flexible structure  22  is in the open position. For a black and white or other monochrome display, the dye can be black or another monochrome color; for a multicolor display, separate dye reservoirs (such as red, green, blue, and black dyes) can be connected with the ducts of different sets of cells, and the cells of the colors can be arranged in an appropriate pattern. Support structure  10  can be implemented as described above in relation to  FIG. 4 . 
   There are several differences between the implementation in  FIG. 7  and that of  FIG. 4 . Flexible structure  22  in this implementation includes three general layers—a thin polyimide layer  160 ; an elastic polymer layer  162  such as a silicon rubber-like material; and an upper electrode layer  164 . The flexibility of structure  22  allows much larger volume change for each cell region than in the implementation of  FIGS. 4 and 5 . Also, lower electrode  106  is highly reflective material, such as an appropriately chosen metal. The implementation of  FIG. 7  illustratively does not include a top structure as in  FIGS. 4 and 5 , although a top structure could be provided. 
   In operation, fluid  170  is kept under a slight positive pressure by a fluid pressure system (not shown) and is available from a fluid reservoir (not shown) through ducts  120 . When charge carriers of the same polarity are concentrated in upper electrode layer  164  and lower electrode  106 , flexible structure  22  is held in its flat position with fluid  170  expelled through duct  120 , as illustrated for cell  84 . In this position, incident light is reflected from lower electrode  106 . When lower electrode  106  is then connected to ground, fluid  170  can enter through duct  120 , providing the variable volume of a cell region, as illustrated for cells  82 ,  86 , and  88 . In this open position, fluid  170  absorbs incident light, so that the cell region appears dark. 
   To fabricate the structure of  FIG. 7 , the same techniques can be used as described above in relation to  FIG. 4 , except that a different combination of layers can be deposited to form flexible structure  22 , as described above. The materials chosen can be nearly transparent, to maximize the contrast between light and dark cell regions of the display. 
     FIG. 7  therefore illustrates an example of a display in which a flexible structure is attached to a surface of a support structure in an array with cell regions and electrodes as summarized above for  FIGS. 4-6 . In the display, each cell region&#39;s stationary electrodes include a reflective lower electrode on the support surface, and each cell region&#39;s variable volume has fluid communication through a duct with a fluid reservoir that contains a light absorbent fluid. As a result, the cell region&#39;s reflective lower electrode reflects incident light in the flat position, while the light absorbent fluid prevents reflection in the open position in which it provides the variable volume. 
     FIG. 8  shows a cross-section of a printhead implementation of array  80 , taken along the line A-A′ in  FIG. 3 . As in  FIG. 4 , ducts in support structure  10  allow fluid communication with each cell region&#39;s variable volume, but an important difference is that the volume between top structure  44  and flexible structure  22  holds another fluid, droplets of which are ejected through apertures in top structure  44 . Support structure  10  can be implemented as described above in relation to  FIG. 4 . 
   One difference between the implementation in  FIG. 8  and that of  FIG. 4  is the presence of apertures  190  defined in top structure  44 . As noted above, top structure  44  can be produced separately with techniques such as deposition and photolithographic patterning, and can include an upper electrode (not shown) in each cell region. After deposition and patterning of layers in top structure  44 , a layer of photoresist can be patterned to include an opening corresponding to the position of each cell region. An etching operation through these openings can then produce apertures  190  as shown in  FIG. 8 . 
   Another difference between the implementation in  FIG. 8  and that of  FIG. 4  is in the layers of flexible structure  22 . To protect middle electrode  192  from contact with other electrodes and fluids, flexible structure  22  includes lower polyimide film  194  below middle electrode  192  and upper polyimide film  196  over middle electrode  192 . In the resulting structure, spacing between middle electrode  192  and the top electrode (not shown) is a few microns. If the effective area of a cell region&#39;s variable volume is 70 μm×70 μm, a volume change of (70 μm×70 μm×1 μm) provides a droplet  198  containing approximately 5 pl of fluid  200 . For an ink-jet printer, for example, fluid  200  can be an appropriate ink or other marking fluid. 
   In operation, fluid  200  is provided to the plenum region between top structure  44  and flexible structure  22  under a slight positive pressure so that the entire plenum fills. Then voltage signals under the control of peripheral circuitry  90  ( FIG. 3 ) are provided through array circuitry  92  ( FIG. 3 ). 
     FIG. 9  illustrates an example of voltage signals that could be provided to perform a printing operation with the apparatus of  FIG. 8 .  FIG. 9  illustratively shows frames T 1  and T 2 . As shown, middle electrode  192  is connected to a constant voltage (illustratively referred to as a “ground”) during the sequence of signals shown in  FIG. 9 ; as will be understood, however, the middle electrode must be more attracted by a low voltage on lower electrode  106  than by a low voltage on the upper electrode (not shown). The upper electrode (not shown) in top structure  44  is pulsed by the voltage signal V top , with one pulse being provided during each frame. Lower electrode  106  in each cell region is independently addressable, and therefore receives a specific signal V pixel , with the signals to the lower electrodes  106  of pixels M and N being shown in  FIG. 9 . The signals to lower electrodes  106  are provided through device layer  102  and interconnecting material  108  in support structure  10 . 
   Each frame begins with an interval during which V top  and V pixel  are both low for all cell regions, so that flexible structure  22  remains in its flat position. Then, at the end of the initial interval, the V pixel  signal goes high for each cell region that is ejecting a droplet of fluid during the current frame; as a result, middle electrode  192  is attracted by the upper electrode into an open position, as illustrated for cells  82 ,  86 , and  88  in  FIG. 8 . For pixels that are not ejecting during the current frame, V pixel  remains low through the frame, and flexible structure  22  remains in its flat position, as illustrated for cell  84  in  FIG. 8 . Then, V top  is pulsed high to more strongly attract middle electrode  192 , causing a brief deflection of flexible structure  22  toward top structure  44  and producing an ejected droplet  198  through aperture  190  from each ejecting cell region. 
   In  FIG. 9 , pixel M does not eject during frame T 1  but ejects during frame T 2 , while pixel N ejects during frame T 1  but does not eject during frame T 2 . In other words, during frame T 1 , the voltage of lower electrode  106  in pixel M holds flexible structure  22  flat, so that the voltage pulse on the upper electrode (not shown) does not produce an ejected droplet  198 . The high voltage on lower electrode  106  of pixel N releases flexible structure  22  into the open position, however, so that a droplet is ejected from pixel N in response to the pulse to the upper electrode (not shown). Each pulse of the upper electrode (not shown) therefore produces a printing operation from all ejecting cell regions, and other appropriate operations can be performed between frames, such as to move the paper sheet or other substrate onto which droplets  198  are ejected. 
   The structure of  FIG. 8  can be fabricated with the same techniques described above in relation to  FIG. 4 , except for a few changes. A different combination of layers can be deposited to form flexible structure  22 , as described above. Also, apertures can be defined in top structure  44 , also as described above. Appropriate additional structures (not shown) can supply fluid  200  under slight positive pressure to the plenum between top structure  44  and flexible structure  22 . 
     FIG. 8  therefore illustrates an example of a printhead in which a flexible structure is attached to a surface of a support structure in an array with cell regions and electrodes as summarized above for  FIGS. 4-6 . In the printhead, each cell region&#39;s electrodes include first and second electrodes that receive signals from peripheral circuitry. The first electrode, when signaled, changes the cell region between its flat position and an open position in which a variable volume is provided. The second electrode, when signaled while the cell region is in the open position, causes droplet ejection. The printhead also has a top structure over the flexible structure with an aperture defined therein for each cell region, and droplets of fluid from a plenum region between the top structure and the flexible structure are ejected through the apertures in response to signals from the peripheral circuitry. 
     FIG. 10  shows a cross-section of a microphone implementation of array  80 , taken along the line A-A′ in  FIG. 3 . As in  FIG. 4 , ducts in support structure  10  allow fluid communication with each cell region&#39;s variable volume, but an important difference is that the flow of fluid is part of a resonance phenomenon in each cell region&#39;s variable volume. More specifically, the ducts permit each cell&#39;s portion of flexible structure  22  to vibrate freely in response to incident pressure waves arriving at the lower surface of support structure  10 . Another difference is that signals from lower electrode  106  are received by peripheral circuitry  92  in order to obtain information about vibration frequencies and intensities. Support structure  10 , lower electrodes  106 , and flexible structure  22  can be implemented as described above in relation to  FIG. 4 . 
   Top structure  44  in  FIG. 10  is not supported on spacers as in  FIG. 4 , but rather is supported at the edge of array  80 . As suggested by the hatching in  FIG. 10 , top structure  44  can be a single top electrode that is conductive, such as an appropriate metal structure. Top structure  44  and middle electrode  114  can be biased and each cell&#39;s variable volume can have a diameter or other dimension sized so that flexible structure  22  resonates in response to incoming pressure waves in a specific wavelength range. The resulting vibration at the cell region&#39;s resonance frequency can then be detected. 
   Device layer  102  can include readout circuitry that allows peripheral circuitry  92  to read the capacitance change for each cell region. Peripheral circuitry  92  can then use the readout signals to obtain an acoustic spectrum for the incoming pressure waves. 
   To fabricate the structure of  FIG. 10 , the same techniques can be used as described above in relation to  FIG. 4 , except that top structure  44  can be attached to or mounted on substrate  10  at the periphery of array  80  rather than on spacers. In addition, the specific circuitry in device layer  102  will be suitable for readout of capacitive changes, as described above. 
     FIG. 10  therefore illustrates an example of a microphone in which a flexible structure is attached to a surface of a support structure in an array with cell regions and electrodes as summarized above for  FIGS. 4-6 . In the microphone, each region&#39;s set of electrodes includes a lower electrode on the support surface from which the peripheral circuitry receives readout signals. In addition, the microphone includes a top electrode, and each cell region has a resonance frequency at which it converts received sound waves into readout signals. 
     FIG. 11  shows circuit  210 , a simple circuit that could be used to measure deflection of flexible structure  22  in  FIG. 10 , similar to circuitry described by Senturia, S. C.,  Microsystem Design , Boston, Kluwer, 2001, pp. 502-507, incorporated herein by reference. Circuit  210  illustratively senses capacitance between lower electrode  106  and middle electrode  114  for one cell, but could be readily modified to measure capacitance for cells in sequence. 
   Amplifier  212  provides output signal V O =−R F i C  in response to the current i c  through displacement sensing capacitance C x , i.e. the capacitor formed by electrodes  106  and  114 . The current is caused by deflection or stretching of flexible structure  22  which in turn changes capacitance. Voltage source  214  acts as a driver. Parasitic capacitance C P  arises from the interconnect between electrode  114  and amplifier  212 . 
   The implementations described above in relation to  FIGS. 4-11  are merely exemplary, and cells and arrays as described above in relation to  FIGS. 1-3  could be implemented in a wide variety of other ways for a wide variety of other applications. Furthermore, cells and arrays as described above could be produced in many different ways. In general, conventional fabrication techniques and their foreseeable future variations can all be used to implement support structures, flexible structures, top structures, and other components. 
     FIGS. 12-15  illustrate several ways in which flexible structure  22  can be attached to surface  12  of support structure  10  to implement features described above. In general, the techniques of  FIGS. 12-15  include selective adhesion of a polyimide film to another material at surface  12  ( FIGS. 1-3 ). 
   The techniques in  FIGS. 12-15  could also be implemented to produce other structures, such as free-standing polyimide films with microelectronic devices on or in the polyimide. These techniques can overcome problems encountered when using a Kapton® film from DuPont bonded to a glass substrate using BCB solution as an adhesive. Although BCB material is stable up to approximately 220 degrees C., the seal between the film and the substrate is poor, resulting in impurity trapping there. Also, it is difficult to hold the film flat with BCB glue. As a result, the critical dimension of amorphous silicon p-i-n devices on such a film has been larger than 10 μm. 
   These problems have been overcome in a selective adhesion implementation in which a wafer&#39;s rim region is made adhesive to polyimide film; a polyimide solution is twice spin-coated to a thickness of approximately 15 μm; the polyimide is post annealed to obtain a film ready for standard wafer processing; chromium metal is deposited on the film and patterned by etching through a suitable photoresist patterned with a suitable mask; the center, non-adhesive portion of the film is released from the wafer; and a plastic disk is attached to the released polyimide film to avoid severe curving due to stress gradient in the film. Adhesion in the rim region seals the film very well and keeps the film flat during processing, allowing production of features as small as 2-3 μm. 
     FIG. 12  illustrates an approach to selective adhesion by modifying an adhesion promoter that promotes adhesion of polyimide to a support surface; more generally, the term “adhesion promoter” refers to any material that promotes adhesion of two surfaces. The polyimide can, for example, be P2610 Series from HD MicroSystems™. This polyimide film has a low stress, such as 2 MPa tensile stress for 10 μm thick, cured 2611 film. It also has a high decomposition temperature, greater than 620° C. With multiple coatings, a film thickness as great as 30 μm can be obtained, providing sufficient mechanical strength to support devices built above it. 
   In general, adhesion between P2610 Series polyimide and various materials is poor, including materials such as titanium-tungsten, silicon, carbon, and silicon dioxide. To obtain better adhesion, an adhesion promoter is typically applied to a substrate before coating with a P2610 polyimide. Some adhesion promoters for polyimide include a combination of a silane group and an aromatic group. After the adhesion promoter is coated and subjected to a thermal cycle, the silane group is coupled to the support surface or substrate and the aromatic group is ready to bond to polyimide. A layer of adhesion promoter including these coupling agents can remain stable on a substrate for one to two days. When a P2610 polyimide is applied over the adhesion promoter, the imide groups in the polyimide are tightly bonded to the coupling groups after a curing process. 
     FIG. 12  illustrates, more specifically, a form of selective adhesion in which an adhesion promoter as described above is selectively modified to obtain attached and unattached regions. In cross-section  220 , glass substrate  222  has a thin layer of an adhesion promoter  224  spin-coated on its surface and baked on a hotplate at 115° C. for 60 seconds, then subsequently baked in an oven at 120° C. for 15 minutes. The adhesion promoter can, for example, be VM 652, a product of HD MicroSystems™. 
   Cross-section  230  shows shadow mask  232 , with an appropriate pattern, positioned over adhesion promoter  224  while an oxygen plasma treatment is applied at 50 W for 5 seconds. The oxygen plasma  234  removes adhesion promoter  224  in the exposed areas not covered by shadow mask  232 . 
   In cross-section  240 , polyimide layer  242  has been formed, such as by spin-coating onto substrate  222  a layer of P2611 and then baking on a hotplate at 90° C. for 3 minutes, then at 150° C. for 3 minutes. After deposition, polyimide layer  242  is cured at 450° C. for about one hour. Then, device layer  244  is fabricated on polyimide layer  242 , such as with movable electrodes as described above. 
   Finally, cross-section  250  shows how the areas in which adhesion promoter  224  remains produce good attachments between polyimide layer  242  and substrate  222 , while volume  252  can be produced in unattached regions where adhesion promoter  224  has been removed. Although it would be possible to completely separate the unattached region of polyimide  242  from substrate  222 , such as by cutting off part of substrate  222  with attached portions of polyimide layer  224 , the above applications illustrate the usefulness of volume  252  enclosed between polyimide  242  and substrate  222 . 
   In addition to glass, other substrate materials suitable for a process like that in  FIG. 12  include silicon, titanium-tungsten, doped amorphous silicon, and sputter carbon. In addition, the technique shown in  FIG. 12  could be modified in various other ways, such as by removing adhesion promoter  224  with a different agent or selectively changing it in a way that makes it ineffective in attaching to polyimide layer  242 . Another approach would be to cover regions of adhesion promoter  224  with a material that prevents adhesion of polyimide layer  242 . 
     FIG. 13  illustrates another approach in which a material with poor adhesion is used between a substrate and an adhesion promoter. An example of such a material is fluorocarbon compound, which has a low surface energy and therefore poor adhesion to most materials. 
   Cross-section  260  in  FIG. 13  shows substrate  262 , which could be one of the materials mentioned above in relation to substrate  222  in  FIG. 12 . Fluorocarbon layer  264  has been deposited on a surface of substrate  262 , such as in a MARCH plasma system with approximately 300 mtorr CHF 3  gas at 100 W plasma power for 4 minutes at room temperature, with no intentional heating. 
   In cross-section  270 , mask  272  is positioned over fluorocarbon layer  264 , such as by deposition and photolithographic patterning of a layer of photoresist. Then, fluorocarbon layer  264 , where exposed, has been removed, such as with an oxygen plasma as in cross-section  230  in  FIG. 12 . 
   Cross-section  280  shows a stage in which mask  272  has been removed, and adhesion promoter  282  has been applied, which can be done in the same manner as in cross-section  220  in  FIG. 12 . At this point, adhesion promoter  282  is in direct contact with substrate  262  except in areas in which fluorocarbon layer  264  was not removed. 
   Finally, cross-section  290  shows polyimide layer  292  deposited over adhesion promoter  282 . Polyimide layer  292  can be composed of P2611 as described above. After polyimide layer  292  is cured, it has good adhesion to promoter  282 , but the regions in which fluorocarbon  262  are present have poor adhesion. Therefore, polyimide layer  292  can be released from substrate  262  in those areas by an appropriate technique, producing a variable volume as described above. 
   The technique in  FIG. 13  could be modified in various ways, including the use of a carbon release layer as described in U.S. Pat. No. 5,034,972, incorporated herein by reference. In addition, similar techniques employing a sacrificial material such as a polymer could be used, as described in Bakir, M. S., Reed, H. A., Mulé, A. V., Jayachandran, J. P., Kohl, P. A., Martin, K. P., Gaylord, T. K., and Meindl, J. D., “Chip-to-Module Interconnections Using ‘Sea of Leads’ Technology,” MRS Bulletin, January 2003, pp. 61-63 and 66-67, incorporated herein by reference. 
     FIG. 14  illustrates another example of selective adhesion, but with an inorganic material that has good adhesion to polyimide. Most inorganic materials, including oxides, semiconductors, and most metals, do not stick to polyimide films well. But certain materials have been found to adhere to polyimide, including gold and indium tin oxide (ITO). Therefore, selective adhesion can be obtained by depositing and patterning a layer of an inorganic material that adheres to polyimide on an appropriate substrate. 
   In  FIG. 14 , substrate  300  can be a suitable material to which gold or ITO adheres, such as silicon, glass, titanium-tungsten, or another metal. A layer of inorganic material such as gold or ITO has been deposited and photolithographically patterned to produce adhesion regions  302 . Then, polyimide layer  304  has been deposited, such as a layer of P2611 as described above. Since polyimide layer  304  adheres well to adhesion regions  302  but does not adhere to substrate  300 , unattached regions between regions  302  can be released, producing variable volumes as described above. 
   The technique in  FIG. 14  could be modified in various ways, such as by using a poor adhesion film over substrate  300  to facilitate release of unattached areas between adhesion regions  302 . 
     FIG. 15  illustrates yet another approach, employing a release layer similar to the sacrificial material technique of Bakir, et al., incorporated by reference above. 
   In  FIG. 15 , substrate  310  is transparent to ultraviolet light. On its surface is a pattern of an ultraviolet light absorbing layer  312 , such as a-Si:H. This layer can be deposited and patterned photolithographically or it could be sputtered or evaporated through a shadow mask. Then, a layer of adhesion promoter  314  is deposited over substrate  310 , and finally polyimide layer  316  is deposited. Adhesion promoter  314  and polyimide layer  316  can be deposited and processed as described above. Finally, ultraviolet light  318 , such as from an excimer laser, is applied through substrate  310 , causing layer  312  to heat up and release polyimide layer  316  from substrate  310  in the areas where layer  312  is present. Because the volume of layer  312  is small, the energy required to release polyimide layer  316  is also small, so that the releasing process is highly efficient, whether performed by laser ablation or not. 
   The technique in  FIG. 15  could similarly be modified, such as by using different types of exposure or laser scanning through the substrate and by using different materials. It may also be possible to use materials that are absorbent at different wavelengths to produce a similar effect. 
   Various other selective adhesion techniques may be used in addition to those described in relation to  FIGS. 12-15 . For example, it may be possible to use flexible substrates other than polyimide. 
   In addition to the applications described above, the techniques described above may be used in various other applications. For example, selective adhesion may be useful for various applications in which circuitry is formed on a flexible substrate, such as with the techniques described by Doany, F. E., and Narayan, C., “Laser release process to obtain freestanding multilayer metal-polyimide circuits,” IBM J. Res. Develop., Volume 41, No. 1-2, January/March 1997, pp. 151-157, incorporated herein by reference. The applications described above generally provide a common electrode on a flexible substrate, but more complicated circuitry could be produced on the flexible substrate related to the positions of the cells of an array or to connections with peripheral circuitry. 
   In addition, selective adhesion may be useful for applications of micro-cells, including those described above in relation to  FIGS. 4-11  and various others including micro-electro-mechanical systems (MEMS). Selective adhesion may be easier and less complicated than conventional techniques that integrate surface micromachining and/or bulk micromachining including building and etching sacrificial materials to produce three-dimensional structures. 
   Some of the above exemplary implementations involve specific materials, such as polyimide, but the invention could be implemented with a wide variety of materials and with layered structures with various combinations of sublayers. In particular, other polymer materials could be used to form flexible structures and a wide variety of materials could be used in substrates, device layers, insulating layers, electrodes, spacers, and top structures. 
   Some of the above exemplary implementations involve two-dimensional arrays of micro-cells, but the invention could be implemented with a single cell or with a one-dimensional array. Furthermore, the above exemplary implementations generally involve cells with movable electrodes on or in a flexible structure and with stationary electrodes above or below, but various other electrode arrangements could be used, such as with different numbers of electrodes, with different positioning, different operations, and so forth. The above exemplary implementations generally provide at least one duct for fluid communication with a variable volume, but implementations could be provide without a duct or with various other arrangements or combinations of ducts. 
   The above exemplary implementations generally involve production of cells following particular operations, but different operations could be performed, the order of the operations could be modified, and additional operations could be added within the scope of the invention. For example, as noted above, flexible structures and ducts could be produced in any of several different ways. 
   While the invention has been described in conjunction with specific implementations, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.