Patent Publication Number: US-2012026451-A1

Title: Tunable liquid crystal lens with single sided contacts

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application U.S. 61/368,863 filed Jul. 29, 2010, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of liquid crystal optical devices and, more particularly, to tunable liquid crystal lenses. 
     BACKGROUND 
     Tunable liquid crystal (LC) optical devices, such as lenses, beam steering devices and shutters are known in the art. Typically, these devices use a spatially modified electric field generated by electrodes within the device. These electrodes require electrical connections to allow contact with external elements. Other electrical components may also be included within some of these devices, which may likewise require external electrical connections. Different package designs may be used to provide a device which is appropriately compact and which may be easily integrated into an external system. 
     SUMMARY 
     In accordance with the proposed solution, a liquid crystal lens is provided that comprises a liquid crystal layer, a plurality of conductive elements, such as electrodes, and a surrounding housing. The device makes use of contacts on an exterior of the housing, each of which are in electrical communication with at least one of the conductive elements and which are positioned adjacent to one another in a first region of the housing. In an exemplary embodiment of the proposed solution, the contacts are all positioned along one side of the housing, simplifying the electrical connection of the device to external components. For example, the contacts may be arranged in a single row. The contacts may also be surface contacts, and vertical conductive portions may be provided that provide electrical connection between the surface contacts and the conductive elements in different layers of a lens device. 
     While electrodes are typically among the conductive elements in a liquid crystal lens device, other conductive elements may also be used. For example, a heater element may be used for increasing the operating temperature of the device, or an electrical sensor may be used to detect electrical properties of device components indicative of parameters such as temperature. Such elements are better described in co-pending commonly assigned U.S. provisional patent application 61/384,962 filed on Sep. 21, 2010 the subject matter of which is incorporated herein by reference. Other components requiring electrical connection may also be present. 
     In an exemplary embodiment of the proposed solution, the lens may be produced as part of a lens array manufactured using a wafer-scale process. In such a process, multiple lenses are constructed using the same wafer level layers, and are then singulated to form individual lens devices. The layers of the array correspond to layers in each of the resulting lens devices. During fabrication of the array, conductive bands may be applied to a substrate in a first layer, each band corresponding to a different row (column) of lenses in the array, and each extending across all of the individual devices of its respective row (column) such that simultaneous electrical contact may be made to all of the devices in that row (column). When the individual devices are singulated, the separated portions of a conductive band may function as electrodes for each of the lens devices. Conductive busbars that run perpendicular to the conductive bands may also be applied to the first layer during the wafer level fabrication, and make electrical contact with the conductive bands. These busbars provide a common connection point at either end of the conductive bands to allow a single testing signal to be applied simultaneously to all of the bands for testing the devices of the array. It is also possible to use a second set of conductive bands in the same layer that separate the columns (rows) of the individual lens devices and make contact with the first set of conductive bands. 
     In one embodiment of the proposed solution, a second layer of the array includes secondary, non-planar electrodes that work in concert with the planar electrodes of the first layer during operation of the singulated liquid crystal lenses. Each of the non-planar electrode devices is associated with a different one of the tunable lens devices and, together with its corresponding planar electrode, will generate an electric field for changing the optical properties of the liquid crystal layer of its respective device. The second layer can include conductive bands that each interconnect the secondary electrodes of a different one of the rows (columns) of devices in the array. In this embodiment, individual devices of the array may be tested during the wafer stage by selectively applying a drive signal between one or more conductive bands of the first layer and one or more conductive bands of the second layer. The result of this selective application is to provide a signal between the planar and non-planar electrodes of a single device, and to thereby change the optical properties of the liquid crystal layer for only one isolated device of the array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which: 
         FIG. 1  is a schematic top view of a tunable liquid crystal lens device structure according to the proposed solution; 
         FIG. 2  is a schematic top view of a wafer-level array of tunable liquid crystal lens devices prior to their singulation into individual components; 
         FIG. 3  is a schematic, perspective exploded view of the structure shown in  FIG. 2 , showing the individual layers of the array; 
         FIG. 3A  is a schematic, perspective exploded view of a structure similar to that of  FIG. 3 , but for which testing of the tunable liquid crystal lens devices may be done one at a time in the wafer stage; 
         FIG. 4A  is a schematic perspective view of a singulated tunable liquid crystal device according to the proposed solution; 
         FIG. 4B  is a schematic perspective view of the singulated tunable liquid crystal device with vertical contacts in place between internal elements and contact pads on a base of the device; and 
         FIG. 4C  is a schematic bottom view of the singulated tunable liquid crystal device of  FIG. 4B  showing the arrangement of the contact pads on the bottom of the device. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a top, schematic view of a tunable liquid crystal lens for which all electrical contact to the lens may be made along a single side of the device. In this embodiment, the lens includes two liquid crystal layers that are controlled by an electric field to form the desired lens configuration. The electric field is generated by electrodes to which a drive signal is connected. The electrodes include two planar electrodes and one centralized, hole-patterned control electrode. In this embodiment, heating and temperature sensing elements are also provided. 
     In the figure, the location of certain components in a horizontal dimension of the device is shown relative to the contact points. The planar electrodes  10  are in electrical contact with conductive strips  12  (highly conductive bands), each of which runs along an opposite edge of the (device layered) structure, thereby forming two contact points along the contact side  14  of the device. The control electrode  16  is, as discussed below, located between the planar electrodes  10  in a vertical dimension of the device, and has a contact point  18  along the contact side  14  of the device. A conductive sensor  20  is also provided, and has an electrical contact  22  along the contact side. The control structure  16 / 20  is better described in co-pending commonly assigned U.S. provisional patent application 61/384,962 filed on Sep. 21, 2010 the subject matter of which is incorporated herein by reference. 
     In an exemplary embodiment, the device of  FIG. 1  is manufactured using a wafer scale process in which many such devices are formed simultaneously on a wafer scale structure, and are subsequently singulated to form the individual devices. This is demonstrated by the schematic top view of  FIG. 2 , which shows nine such devices formed from a single wafer structure. Those skilled in the art will understand that the number of devices shown in  FIG. 2  is for descriptive purposes only and that the actual number of devices formed from a single wafer may, in fact, be much larger.  FIG. 2  also shows dicing lines  24  (dashed lines) which indicate the lines along which dicing would be used to singulate the individual devices. 
     To better understand the layered structure of the liquid crystal lenses described herein,  FIG. 3  shows a schematic, perspective view of the layers of the device during wafer fabrication, the layers being depicted in an exploded view. These layers include top and bottom planar electrode layers  26   a ,  26   b , liquid crystals  28   a ,  28   b , glass substrate  30  and control layer  32 . Each of the planar electrode layers includes a glass substrate  33  and bands  34  of electrode material, each of which provides one planar electrode for each of the devices in the row corresponding to that band  34 . In this embodiment, the electrode material is index matched indium tin oxide, and is deposited with a relatively small thickness (e.g., less that 100 nm). Also deposited on the substrates  26   a ,  26   b  are highly conductive strips  12 , which provide electrical contact to the bands  34  and, after singulation, provide conductive paths for the planar electrodes  10  of the individual devices. These deposited strips  12  include busbars  36  that are deposited along two opposing edges of the array, and serve as primary contacts during array testing. 
     Also shown in  FIG. 3  are strips  27  that are deposited on control layer  32  and serve to interconnect the contact points  18 ,  22  of each row of devices, thereby allowing common electrical connection to all of the devices of a row during array testing. Perpendicular conductors  29  are also deposited along the edges of the layer  32 , perpendicular to, and in contact with, the strips  27 . The perpendicular conductors  29  interconnect the strips  27  and allow common electrical connection to all of the components of the control layer. The strips  27  are also shown in  FIG. 2 , and may be removed during singulation of the devices. In general, the strips  12 ,  27  are of a highly conductive material, typically metal, and allow uniform contact to the planar electrodes of all of the devices of the array. In an exemplary embodiment, the strips  12 ,  27  and (busbars)  36  are deposited with a relatively large thickness (e.g., greater than 500 nm). 
     While the foregoing embodiment allows the simultaneous testing of all of the components in the wafer level array, an alternative embodiment may be used in which the individual devices may be individually addressed. An example of such an embodiment is shown in  FIG. 3A , which is an exploded perspective view similar to that of  FIG. 3 . Those skilled in the art will understand that, while the figure is an exploded view, the testing actually takes place with all of the layers of the device in contact with each other. In the  FIG. 3A  embodiment, there are no perpendicular conductors along the edges of layer  32 , and strips  27  of layer  32  are used along with strips  12  of layer  33  to power a desired one of the devices in the array. The application of different signal potentials to busbars  36  and strips  27  and  12  will provide power to the desired device. For example, to provide power to just the device  35  indicated in  FIG. 3A , each of the planar electrode layers  26   a ,  26   b  has its busbars  36  and strips  12   a  connected to ground. Likewise, strips  27   a ,  27   b  and  27   d  of layer  32  are also connected to ground. A test signal may then be provided between strip  27   c  of the control layer  32  and strips  12   b  and  12   c  of each of planar electrode layers  26   a ,  26   b . The presence of an electric potential between the control electrode and the planar electrode material of the device  35  thereby activates the liquid crystal lens for that device, the optical change in which is then detectable. Since the other strips  12   a  and the busbars  36  are connected to ground, the driver circuit must have sufficient capacity to drive the device and sustain a leakage current from the drive strips  12   b ,  12   c  to the next adjacent strips  12   a  or busbar  36  through the bands  34 , which have a limited conductivity. By changing which of the strips  27  and which of the strips  12  or busbars  36  are connected to the signal source, individual devices may be powered and tested one at a time in the wafer stage. 
     Referring again to  FIG. 3 , each of the liquid crystals  28   a ,  28   b  is contained between one of the planar electrode layers  26   a ,  26   b  and an inner layer. On a first side of the device, the liquid crystal  28   a  is located between planar electrode layer  28   a  and glass substrate  30 . The glass substrate  30  is an optically transparent glass material that provides support for the liquid crystal  28   a . Each of the glass substrate  30  and the planar electrode layer  28   a  has an alignment layer coating (not shown), such as a polyimide, that provides the liquid crystal  28   a  with a desired pre-tilt, as is known in the art. 
     On a second side of the device, the liquid crystal  28   b  is contained between planar electrode layer  26   b  and control layer  32 . The control layer  32  includes a glass substrate on which is deposited a frequency dependent material, that is, a material that is optically uniform, but which is electrically non-uniform for a predetermined set of electrical frequencies. This frequency dependent material behaves like a conductor at certain frequencies of the electric field, while appearing nonconductive at other frequencies. Thus, by adjusting the frequency of a drive signal applied to the electrodes, a spatial profile of the electric field may be modified. On top of the frequency dependent material the control electrode  16  and conductive sensor  20  are patterned. For each of the planar electrode layer  26   b  and the control layer  32 , the side of the layer facing the liquid crystal  28   b  is coated with an alignment coating, such as polyimide. In addition, the planar electrodes may also serve as heater elements, and have a non-negligible finite resistance that results in resistive heating when an appropriate current is passed through them. 
     The wafer-level fabrication of the proposed solution produces devices that have all of their electrical contacts on a single side of the package. This allows the overall package to be smaller and simplifies the contact arrangement. The configuration of the metal strips  12 ,  27  and busbars  36  on the structure also improves wafer-level testing of the devices. If the only contact points were at the busbars  36  along the edges of the array, there would be a significant difference in how the devices near the interior of the array were driven as opposed to those along the edges. In the present embodiment, however, the metal strips  12  make contact with each of the planar electrodes, allowing them all to be driven in a relatively uniform manner during array level testing. 
     An example of a final device structure according to the present embodiment is shown in  FIGS. 4A-4C . As shown in  FIG. 4A , a package base  40  has a hole in the center to allow light to pass through it, and provides support for the other component layers. The base  40  also has contact pads  42   a - 42   d  that allow for easy electrical contact to the device. The different layers of the device have their contact points all along the same side of the device. Thus, the busbars  36   a ,  36   b ,  36   c ,  36   d  all have a contact edge at the front of the device. Likewise, electrical contacts  18  and  22  for the control electrode  16  and conductive sensor  20 , respectively, are located at the same side. However, each of these contacts is at a different relative height along the front side of the device (package). 
       FIG. 4B  shows the device structure (package) of  FIG. 4A  with full package contacts  44   a - 44   d  in place. Each of the full contacts  44   a - 44   d  makes contact, respectively, with one of the contact pads  42   a - 42   d  and extends vertically to make an electrical connection with the appropriate device contacts. Thus, contact  44   a  provides an electrical path between the contact pad  42   a  and the busbars  36   c ,  36   d , contact  44   b  provides an electrical path between contact pad  42   b  and conductive sensor contact  22 , contact  44   c  provides an electrical path between contact pad  44   c  and control electrode contact  18 , and contact  44   d  provides an electrical path between contact pad  42   d  and busbars  36   a ,  36   b.    
     It is understood that providing an electrical path between busbars  36   c  and  36   d  via contact  44   a  and between busbars  36   a  and  36   b  via contact  44   d  corresponds to the device drive mode illustrated in  FIG. 3A  wherein the control ring electrode  16  is driven with respect to planar electrodes  10  shown in  FIG. 1  planar electrodes  10  which are driven at the same potential. The invention is not limited to the connectivity illustrated in  FIG. 4B , differently configured contacts  44   a  and  44   d  can be employed to drive each top and bottom planar electrode  10  at a different potential via corresponding separate contact pads  42   a  and  42   d , for example to account for differences in top and bottom liquid crystal layers  28   a  and  28   b . The invention is not limited to the device illustrated in  FIG. 4A  which shows a single device with busbars  36  on both sides, a device singulated from a wafer having multiple devices thereon would have either strips  12  on both sides or strips  12  and busbars  36  on opposite sides. 
     The contact pads  42   a - 42   d  extend through the base  40  of the device (package) such that they are accessible on the other side of the base  40 . Thus, as shown in  FIG. 4C , when the package is fully enclosed in its exterior housing, the bottom of the device provides a simple set of surface contacts for connecting the lens to an appropriate device. The contact pads  42   a - 42   d  are easily accessible on the underside of the base  40 . In this way, making contact with the conductive elements of the device may be achieved with relatively simple connections along just one side of the device and the device benefits from increased compactness. As shown, the central portion of the base also has a circular opening to allow the transmission of light therethrough. It should be noted that, while light will pass through the center of the lens structure, the orientation of the device relative to the direction of the light may be either sense, as the application requires. That is, light may pass through the rest of the device before passing through the base  40 , or the base  40  may face the direction of the incoming light, such that light passes through it before the rest of the structure. 
     Regarding base  40 , in the above reference has been made to a package base  40  and to the device package being fully enclosed in its exterior housing particularly with reference to  FIGS. 4A ,  4 B and  4 C. 
     As mentioned hereinabove, devices can have different package designs intended for integration into different external systems providing a corresponding form factor. In some embodiments the devices are required to be compact in general, in other embodiments the devices are required to be flat, while in other embodiments the devices are required to be slender. 
     In the context of the tunable liquid crystal lens devices presented herein, integration into external systems generally requires mechanical integration as well electrical integration. 
     Mechanical integration aspects concern providing sufficient structure to integrate the device into an external system including, but not limited to: positioning and orienting the devices with respect to the overall external system. As well mechanical integration aspects can also relate to structural integrity of the overall external system and mechanical protection of the device from environmental factors such as but not limited to shock and vibration. 
     Accordingly, the (package) base  40  can be shaped to provide form factors which enable mechanical device integration for example into a barrel assembly, a lens assembly, etc. The hole on the base  40  can be positioned with respect to the edges of base  40  to locate the optical axis of the device, while for example a pattern of notches or a pattern of holes (not shown) in the base  40  can dictate a specific orientation in mechanically integrating the device into the external system. 
     It is appreciated that positioning and orienting aspects are not limited to the (package) base  40 . It is appreciated that (package) base  40  can be part of an overall package into which the device is provided for integration into external systems, base  40  acting as an interposer. The base  40  can by itself provide such a package, however the invention is intended to include other types of packaging such as, but not limited to: a barrel assembly, a lens assembly, an encasing material (resin), a mould, a coating, etc. In some embodiments base  40  is oversized with respect to the device to enable mechanical integration. 
     Electrical integration aspects concern providing sufficient structure to electrically interconnect the device to the external system including, but not limited to: powering, conditioning and driving the device. Powering and driving aspects relate to the actuation of the device within the overall system into which it is integrated, whereas conditioning aspect can relate to providing the environmental conditions (for example temperature control) for the device to operate as well to providing protection for example from: electrical shock, thermal shock, static electricity discharges, over-currents, under-currents, capacitive/inductive coupling, etc. and/or electrical shielding. 
     Accordingly, the (package) base  40  can be configured act as an electrical interconnect between the device and the external system into which the device is integrated, defining and simplifying electrical interconnection between the device and the external system. 
     It is appreciated that protection for example from: electrical shock, thermal shock, control of capacitive/inductive coupling, etc. and/or electrical shielding can be provided by other than the (package) base  40 . It is appreciated that (package) base  40  can be part of an overall package into which the device is provided for integration into external systems, base  40  acting as an interposer. The base  40  can by itself provide such a package, however the invention is intended to include other types of packaging such as, but not limited to: a barrel assembly, a lens assembly, an encasing material (resin), a mould, a coating, etc. In some embodiments base  40  is oversized with respect to the device to enable electrical integration. For example the base  40 , besides contact pads  42   a - d , can also include shunt resistors for example to control over-currents, static electricity discharge etc., and signal conditioning electrical components to provide protection from: under-currents, capacitive/inductive coupling, etc., whereas the overall packaging can be configure to provide thermal shielding, thermal dissipation, capacitive/inductive coupling, etc. For example, the overall packaging can contain Zinc oxide thermal paste for temperature control, or invar for electrical shielding. 
     It is appreciated then that the device being an optical device, such as a tunable liquid crystal lens device cannot be totally encased in (opaque) packaging. While the base  40  is integrated into the stack of the singulated device, the base  40  and packaging can form part of the device housing together with other components of the device such as, but not limited to glass substrates  33  which provide an open optical path through the device. As such the housing includes the base, any packaging and components providing optical access to the device. In some embodiments, housing/packaging components can be optically transparent and/or provide optical conditioning, for example part of the housing/packaging can be made from a transparent material configured have an optical power (lenticular, graduated index lens, etc.) For clarity, in some embodiments the housing is the base  40 . 
     While the invention has been shown and described with referenced to preferred embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.