Abstract:
The disclosure is directed to electronic device displays which are constructed to withstand damage from an impact resistance test wherein a steel ball of 2 g having a diameter of 8 mm is dropped from a designated height greater than 1 ft, more preferably greater than 2 ft, even more preferably greater than 3 ft, still even more preferably greater than 4 ft, yet even more preferably greater than 5 ft and even more preferably greater than 6 ft. The displays are configured using, for example, ultrathin glass adhered to a base glass, wherein the adhesive layer is optimized for thinness and stiffness.

Description:
CROSS-REFERENCE 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/582,660, entitled Structure and Manufacturing Methods for Glass Covered Electronic Device by Paul Wickboldt and Eric Jones, filed Jan. 3, 2012, which application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    A variety of electronic displays are used with electronic devices. Displays can operate using either emissive (pixels generate light), transmissive (light transmitted through pixels) and reflective (ambient light reflected) approaches. Display types may include, for example, liquid crystal displays (LCDs) which use liquid crystal cells that change transmission, or reflection in an applied electric field, organic light emitting diode (OLED) devices which utilize a light emitting diode (LED) in which an emissive electroluminescent film of organic compounds emits light in response to an electric current, and different types of electrophoretic displays in which pigmented particles are moved in response to an electric field (e.g. Gyricon, E-ink, etc.). 
         [0003]    The LCD panel typically consists of two sheets of glass separated by a sealed-in liquid crystal material. Both sheets have a thin transparent coating of conducting material, with the viewing side etched into segments with leads going to the edge of the display. Voltages applied between the front and back coatings disrupt the orderly arrangement of the molecules sufficiently to darken the liquid and form visible patterns. 
         [0004]    Additionally, displays have been developed that can detect the presence and location of touch, e.g., by a finger, or passive object such as a stylus or digital pen, are commonly referred to as touch screens. Touch screens have become a component of many computer and electronic devices. Many LCD displays are manufactured to include touch screen functionality. Touch screens can be attached or incorporated into to computers, networks, mobile telephones, video games, personal digital assistants (PDA), tablets, or any digital device. A variety of technologies are currently used to produce a device with touch screen capabilities. Technologies that enable touch screen functionality include: resistive touch screen panels; surface acoustic wave technology; capacitive sensing panels (e.g., using surface capacitance technology or projective capacitive touch technology, which uses either mutual capacitive sensors or self-capacitive sensors); infrared; optical imaging; dispersive signal technology; and acoustic pulse recognition. Touch screen functionality can be combined with a display in a device in many configurations. The touch screen sensing circuits can be incorporated directly in or on the layers of the display (using, for example, “in-cell” or “on-cell” approaches), built on a separate substrate which is laminated onto the display (e.g., using an “out-cell” approach), or laminated on a cover lens which protects the display in the device, or the sensing circuits can be incorporated directly on the back-side of this cover lens (“Touch-on-Lens”). See, for example, US2011/0102567A1 to Erhart published May 5, 2011, US2011/0102569A1 to Erhart published May 5, 2011, US 2011/026729A1 to Erhart published Jul. 14, 2011, and US2012/0242635A1 to Erhart published Jun. 15, 2012. 
         [0005]    As will be appreciated by those skilled in the art, electronic devices can be configured to include a variety of components and features including: a display, a touch screen, a scratch-resistant cover (e.g., lens), storage, a system on a chip, a CPU core, a GPU core, memory, Wi-Fi connectivity (e.g., 902.11 b.g), Bluetooth connectivity (e.g., USB connector), camera, audio, battery (e.g., built-in, rechargeable lithium-ion polymer battery), power connector, computer readable media, instructions capable of being performed by computer readable media (e.g., software), etc. 
         [0006]    One drawback of currently available electronic device interfaces is that the screen can be damaged fairly easily. This can be particularly problematic for devices that are designed for increased mobility and may be subjected to harsh conditions. Currently, devices having an interface are manufactured to withstand a drop of ˜1 foot. As will be appreciated, portable devices such as smart phones, touch pads, and other devices having currently available interfaces may often be deployed in environments where the device will be subjected to physical trauma, such as having a user drop the device from a height greater than 1 foot or having a heavy object fall on the device What is needed, therefore, is a device interface that can withstand the force of an impact equivalent to an impact resistance drop test performed at a height greater than 1 foot, more preferably, greater than 4 feet and even more preferably greater than 6 feet. 
       SUMMARY OF THE INVENTION 
       [0007]    An aspect of the disclosure is directed to electronic device displays, methods of making the displays, and methods of using devices with the displays. Displays are formed from a thin glass (e.g., 30 um to 200 um in thickness), an adhesive (having a thickness d and a hardness K or Ym), and a base glass having a thickness of from 0.1 mm to 0.5 mm or greater. Either, neither or both the thin glass and base glass can be chemically strengthened. The total thickness of the adhesive layer between the thin glass and the base glass is preferably about 8-mil (200 um) or less, more preferably about 4-mil (100 um) or less, and even more preferably 3-mil (75 um or less) in thickness. Additionally, the modulus of the adhesive layer between the thin glass and the base glass has a modulus, when averaged by their thickness, of 1 GPa or greater, more preferably 2 GPa or greater, and even more preferably 3 GPa or greater. 
         [0008]    Another aspect of the disclosure is directed to electronic device displays with sensors and methods of making the displays. Electronic device displays include, for example, a thin glass layer, a clear adhesive layer, a sensor layer, a plastic film layer, a second clear adhesive layer, and a base glass layer. Either, neither or both the thin glass and base glass can be chemically strengthened. The total thickness of the one or more adhesive layers is preferably about 8-mil (200 um) or less, more preferably about 4-mil (100 um) or less, and even more preferably 3-mil (75 um or less) in thickness. Additionally, the average modulus of the one or more adhesive layers between the thin glass and the base glass has a modulus, when averaged by their thickness, of 1 GPa or greater, more preferably 2 GPa or greater, and even more preferably 3 GPa or greater. 
         [0009]    Still another aspect of the disclosure is directed to electronic device displays with sensors and methods of making the displays. Electronic device displays include, for example, a thin glass layer, a clear adhesive layer, a sensor layer, a second thin glass layer, a second clear adhesive layer, and a base glass layer. The total thickness of the one or more adhesive layers is preferably about 8-mil (200 um) or less, more preferably about 4-mil (100 um) or less, and even more preferably 3-mil (75 um or less) in thickness. Additionally, the average modulus of the one or more adhesive layers between the thin glass and the base glass has a modulus, when averaged by their thickness, of 1 GPa or greater, more preferably 2 GPa or greater, and even more preferably 3 GPa or greater. 
         [0010]    Yet another aspect of the disclosure is directed to electronic device displays which are constructed to withstand damage from a force equivalent to an impact resistance test wherein a steel ball of 2 g having a diameter of 8 mm is dropped from a designated height greater than 1 foot, more preferably greater than 2 foot, even more preferably greater than 3 foot, still even more preferably greater than 4 foot, yet even more preferably greater than 5 foot and even more preferably greater than 6 foot. 
         [0011]    Yet another aspect of the disclosure is directed to electronic device displays which are constructed to withstand damage from a force equivalent to an impact resistance test wherein a steel ball of 130 g (+/−5 g) having a diameter of 31.8 mm (+/−1 mm) is dropped from a designated height greater than 1 foot, more preferably greater than 2 foot, even more preferably greater than 3 foot, still even more preferably greater than 4 foot. 
         [0012]    Still another aspect of the disclosure is directed to an electronic display device that uses a transparent flex which can incorporate a wide variety of electronics into the display thereby creating a smart piece of glass. 
         [0013]    An aspect of the disclosure is directed to impact resistant display devices. The impact resistant display devices comprise: a first thin glass layer having a thickness less than 400 micrometers; a second base glass layer having a thickness of from 0.1 to 0.5 mm; an adhesive layer positionable between the first thin glass layer and the second base glass layer having a thickness of 200 micrometers or less wherein the adhesive layer is selected based on one or more of a GPa measurement and a Young&#39;s Modulus measurement for the adhesive; and a sensor layer positioned between the first thin glass layer and the second base glass layer dimensioned to cover at least a portion of the height and width of the first thin glass layer or the second glass layer wherein the sensor layer is adaptable to communicate with a printed circuit board, wherein the display device has a total thickness of less than 800 micrometers and is constructed to pass a ball drop test of 2 grams at greater than 4 feet. In some configurations, a second thin glass layer is provided, wherein the sensor layer is positioned between the first thin glass layer and the second thin glass layer. Additionally, a plastic layer and a second adhesive layer can be provided, wherein the plastic layer and the second adhesive layer are positioned between the first thin glass layer and the second glass layer such that the two adhesive layers are not adjacent. The thickness-averaged modulus of the first adhesive layer, the second adhesive layer and the plastic layer is, in at least some configurations, greater than 1 GPa. Additionally, one or more of a coating, a plastic film, an ink layer, an anti-fingerprint layer, and a scratch-resistant layer can be provided. In still other configurations, the interface is in communication with a one or more of a printed circuit board, storage, memory, a system on a chip, a CPU core, a GPU core, Wi-Fi connectivity, Bluetooth connectivity, camera, audio, battery, power connector, computer readable media, and instructions for operation on computer readable media. The adhesive layer can also be selected such that it comprises several layers of different materials, and the modulus averaged by thickness through the adhesive is greater than 1 GPa. Additionally, the adhesive layer can have a modulus of greater than 1 GPa. In some aspects, the adhesive is optically transparent. Additionally, the sensor layer can further comprise thin patterned conductive traces. Suitable sensors include one or more of each of a touch screen sensor and a biometric sensor, such as a one dimensional or two dimensional fingerprint sensor. In at least some configurations, the thickness of the adhesive layer is a function of the at least one of the modulus GPa value for the adhesive and the Young&#39;s Modulus value of the adhesive. An amount of deformation of the first thin glass layer relative to the glass layer can be a function of at least one of a thickness between the first thin glass layer and the second glass layer and at least one of the GPa value for the adhesive and the Young&#39;s Modulus value of the adhesive. A second sensor layer can also be provided that is positioned between the first thin glass layer and the second base glass layer dimensioned to cover at least a portion of the height and width of the first thin glass layer or the second glass layer wherein the second sensor layer is adaptable to communicate with a printed circuit board and to operate independently of the first sensor layer. 
         [0014]    Still another aspect of the disclosure is directed to impact resistant display devices. The impact resistant display devices comprise: a first thin glass layer having a thickness less than 400 micrometers; a second base glass layer having a thickness of from 0.1 to 0.5 mm; an adhesive layer positionable between the first thin glass layer and the second base glass layer having a thickness of 200 micrometers or less wherein the adhesive layer is selected based on one or more of a GPa measurement and a Young&#39;s Modulus measurement for the adhesive; and a sensor layer positioned between the first thin glass layer and the second base glass layer dimensioned to cover at least a portion of the height and width of the first thin glass layer or the second glass layer wherein the sensor layer is adaptable to communicate with a printed circuit board, wherein the display device has a total thickness of less than 1100 micrometers and is constructed to pass a ball drop test of 2 grams at greater than 4 feet. 
         [0015]    Another aspect of the disclosure is directed to a method for making an impact resistant display device. The method comprises the steps of: providing a first thin glass layer having a thickness less than 400 micrometers, an adhesive layer having a thickness of from 0.01 to 0.4 mm and a second base glass layer having a thickness of from 0.1 to 0.5 mm; providing a sensor layer dimensioned to cover at least a portion of the height and width of the first thin glass layer or the second base glass layer; forming the adhesive layer on a surface the thin glass layer; placing the second base glass layer on the adhesive layer, wherein the display device is less than 800 micrometers in thickness and is constructed to pass a ball drop test of 2 grams at greater than 4 feet. Additional steps include, for example, applying a sheet of plastic to the sensor layer. The thin sheet of plastic can also have a conductive traces patterned on one or both sides. Additionally, a second thin glass layer wherein the sensor layer is positioned between the first thin glass layer and the second thin glass layer. An additional step can include providing a second sensor layer dimensioned to cover at least a portion of a height and width of the first thin glass layer or the second base glass layer. 
         [0016]    Another aspect of the disclosure is directed to a method for making an impact resistant display device. The method comprises the steps of: providing a first thin glass layer having a thickness less than 400 micrometers, an adhesive layer having a thickness of from 0.01 to 0.4 mm and a second base glass layer having a thickness of from 0.1 to 0.5 mm; providing a sensor layer dimensioned to cover at least a portion of the height and width of the first thin glass layer or the second base glass layer; forming the adhesive layer on a surface the thin glass layer; placing the second base glass layer on the adhesive layer, wherein the display device is less than 1100 micrometers in thickness and is constructed to pass a ball drop test of 2 grams at greater than 4 feet. 
       INCORPORATION BY REFERENCE 
       [0017]    All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The novel features of the invention are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which: 
           [0019]      FIG. 1  is a top view of an exemplar electronic device having a display such as those currently available; 
           [0020]      FIG. 2  is a side view of an exemplar electronic device having a display such as those currently available; 
           [0021]      FIG. 3  is an illustration of a basic construction of an exemplar interface, such as a touch screen used with a smart phone, which uses a glass cover lens; 
           [0022]      FIGS. 4   A - B  are illustrations of a construction of a cover lens assembly which incorporates a capacitive sensor near the surface where thin glass is also incorporated; 
           [0023]      FIGS. 5   A - B  are illustrations of a cover lens assembly with thin glass as a top surface layer; 
           [0024]      FIGS. 6   A - B  are images of the failure height of the STR® 15420P at 1 foot ( FIG. 6   A ) and the DuPont PV5412 at 6.5 feet ( FIG. 6   A );  FIG. 6   C  is a chart showing the height at which the failure occurred during the impact resistance testing for the STR EVA 15420P˜400 um, and the DuPont PV5412˜380 um; and 
           [0025]      FIG. 7   A  illustrates impact test results using showing failure distance in inches vs. optically-clear adhesive (OCA) thickness;  FIG. 7   B  illustrates impact test results showing failure distance vs. OCA thickness. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. Devices 
       [0026]    For purposes of illustration a touch screen display currently employed by, for example, a smart phone is described. Such a touch screen typically comprises a 9 cm (3.5 in)×6 cm (2.4 in) liquid crystal display (LCD) with a scratch-resistant glass layer. Other size touch screens can be employed without departing from the scope of the disclosure. 
         [0027]    The capacitive touch screen of the LCD is typically optimized for a bare finger, or multiple finger multi-touch, sensing. However, as will be appreciated by those skilled in the art, a variety displays as well as a variety of touch screen configurations and touch screen operated devices can be used without departing from the scope of the disclosure. 
         [0028]    An LCD touch screen typically is an assembly that includes an LCD, a printed circuit board (PCB) on which input-output (I/O) connections and integrated circuits (ICs) performing various functions are mounted, a transparent touch screen circuit pattern on a transparent substrate, and a protective shield or coating applied on top of the touch screen circuitry. 
         [0029]    The touch screen circuitry is connected along with the LCD display to the PCB. The touch screen circuitry is typically incorporated into the assembly using one of two methods. In a first method, the touch screen circuitry is incorporated directly into or onto the LCD, then a protective shield or coating (e.g. cover lens) is located above the LCD/Touch screen combination. In a second method, the touch screen circuitry is applied onto the protective coating or shield (e.g. cover lens) and then the resulting structure is mounted above the LCD, with the touch screen circuitry mounted between the protective coating or shield and the LCD. In all cased the PCB is located below the LCD, out of view. 
         [0030]      FIG. 1  is an electronic device  100  from a top or upper surface view. The device is any suitable electronic device, such as a smart phone, having a device or display interface  120  which a user engages with their finger. Depending upon the nature of the device and display used, the interface  120  can further be comprised of a plurality of parts, as discussed in further detail below. 
         [0031]    The device itself has a top surface  102  and a bottom surface  104 . Moreover, each component of the device  100  has an upper surface (i.e. a surface that faces the top surface of the device) and a lower surface (i.e. a surface that faces the bottom surface of the device) as will be appreciated from the cross-sectional views. The housing  110  of the electronic device  100  can be configured to form a bezel or rim  112  which secures the interface  120  within the housing  110  of the device  100 . A mask  124 , such as an ink mask, can be provided which frames at least part of the interface  120 . The mask  124  is typically positioned such that it obscures device electronics located within the housing under a portion of the interface  120 . For a touch screen enabled interface, a portion of the interface  120  that is not covered by mask  124  has a plurality of touch screen sensors  134 . The plurality of touch screen sensors  134  can be any suitable conductor, including a transparent conductor, for example, from a layer of patterned indium tin oxide (ITO), carbon nanotubes, metal nanowires, conductive polymers or fine metal lines (e.g., copper lines). The dimensions of the device interface  120  can be from 3-25 cm width and from 5-30 cm in length. 
         [0032]      FIG. 2  is a cross-sectional view of an electronic device  200 , such as device  100  of  FIG. 1 , taken along the lines  2 - 2  of  FIG. 1 .  FIG. 2  illustrates a common cross-sectional view of a device, including the display. Electronic device  200  includes a housing  210 , a printed circuit board (PCB)  230  and a display  228 , such as an LCD or LCD module. The device can also include a touch sensor component  235 , such as a glass layer, onto which a conductive layer such as indium tin oxide (ITO) or similar materials are applied to form the touch screen circuitry. The conductive layer can be applied such that it forms a pattern on the surface of the glass layer, as will be appreciated by those skilled in the art. As shown in  FIG. 2 , a first conductive layer  234  covers an upper surface  206  of the touch sensor component  235  and a second conductive layer  234 ′ covers a lower surface  208  of the touch sensor component  235 . The cover lens  238  can be formed from suitable material including, for example, a chemically hardened glass. A touch circuit controller  226  is coupled to a touch screen circuit or digitizer which can be formed from conductive layers  234 ,  234 ′ of the touch circuit component  235  via a flexible circuit  232 . The controller  226  is mountable on the PCB  230 . A display  228  is positioned below the cover lens  238  and above the PCB  230 . As will be appreciated by those skilled in the art, devices can also be configured such that part of the control functions are incorporated into a chip on the flex (COF) itself. The display  228  can include, for example, a glass layer as well as any other components required to achieve a functional display including logic devices. The protective layer  222  and display layer  228  can be formed from any suitable non-conductive material (e.g., glass, PET or a suitable hard coating). 
         [0033]    Turning now to  FIG. 3 , a basic construction of a touch screen suitable for use with a smart phone or other similar device is shown. The device  300  has a cover lens  338  with a sensor  340  positioned such that it is sensable by an upper surface  302  of the cover lens  338 . The cover lens  338  is positioned such that it fits within a housing  310  that contains the remainder of the device components (including the display and electronics, as described above, for example, with respect to  FIG. 2 ). 
         [0034]    For various electronic and optical device products, including, but not limited to, capacitive fingerprint sensors, touch sensors and displays, it is desired to incorporate a layer (also referred to as a sheet) of thin glass as discussed below with respect to  FIGS. 4-5 . This glass may have a thickness of 400 micrometers or less, more specifically between 50 and 400 micrometers in thickness, and in some cases between 30 and 250 micrometers in thickness. Many of these products must be physically robust for their intended use. One test of this physical robustness or durability is to drop a steel ball onto the surface of the device to determine under what conditions (ball drop height, ball weight, substrate mounting, etc.) the product fails. Because of these requirements of durability, thin glass layers have been avoided in the structure of these devices. 
         [0035]    In two specific implementations provided for illustration, a capacitive fingerprint sensor and/or a capacitive touch sensor may need to be incorporated into a device which has a display, such as on a touch screen phone. These phones often have a cover lens which is made from a chemically strengthened glass or base glass, typically 0.5 mm thick or thicker. However, as will be appreciated by those skilled in the art, cover lenses having a thickness between 0.3 mm and 0.5 mm, such as an ultrathin glass (UT), can also be used. This cover lens can also have other thin layers such as inks printed on them for visual effects or coatings used to reduce fingerprint smudging or enhance scratch resistance. In this embodiment, the sensors are located close to the finger, in this case within either 50 micrometers, 100 micrometers, or values between 30 micrometers and 300 micrometers. Thus, the sensors can be placed above the cover lens, or incorporated as part of a cover lens assembly. It is desired to place a glass layer above the sensor to retain the feel and many of the properties of the glass surface for the user. 
       II. Methods of Manufacture 
       [0036]    As disclosed here, a method and structure has been developed which enables thin glass to be incorporated into these device products (and similar device products or components thereof) while achieving the durability that is required. Specifically, when this structure is subjected to steel ball impact tests, as discussed below, the results are similar to those obtained for existing commercial cover lenses made from strengthened glass. By providing the thin glass surface at or near the surface of the device product, the results of hardness and scratch resistance may also be similar to that for the existing commercial cover lenses made from strengthened glass. 
         [0037]      FIG. 3  shows a basic assembly cross-section of a touch screen phone or similar device. As can be seen, the cover lens  338  is mounted above the display and other device components (shown in  FIG. 2 ), onto the housing  310  (or bezel) of the device  300  has an interior cavity  350  sized to house the electronic components of the device. Typically sensors  340  are positioned at or near a top surface  302  of the cover lens  338 . 
         [0038]      FIGS. 4   A - B  show a cover lens assembly in cross-section, which incorporates a base glass layer  438  that may be similar to that used in a typical commercial cover lens construction, but also now incorporates one or more sensors  448  and a thin glass layer  422  above the base glass layer  438 . This base glass layer  438  may either be thin (between 30 um and 300 um thickness) or thicker (greater than 300 um up to 5 mm) and preferably strengthened. For all or any of the thin glass layers  422 ,  422 ′, and base glass layer  438  in these structures, the glass may or may not be strengthened, although strengthened glass layers may have benefits. Suitable strengthened glass includes, for example, Corning® Gorilla® Glass, available from Corning Incorporated, Corning, N.Y. 
         [0039]    In this example, sensors  448  can be made from thin patterned conductive traces on the surface (either top, bottom or both) of a thin sheet of plastic  464 . With appropriate patterns and connections, these patterns can comprise, for example, a fingerprint sensor, a touch sensor, or a combination of both, and may or may not be transparent (e.g. could be copper or ITO or a combination of both). This sensor layer may cover the entire area of the cover lens, or only part of the area. There may also be ink patterns  424  printed on the surfaces of one of the glass layers or the plastic sheet, as for example a masking layer typically used in a cell phone. One or more adhesive layers  452 ,  452 ′, can be used between and around the different layers to bond the assembly together. 
         [0040]    In  FIGS. 4   A - B  optically transparent adhesive(s), or optically clear adhesives (OCA), can be used between a sensor layer and a thin glass layer, and between a sensor and a base glass. 
         [0041]    Additionally, a thin glass laminate (having a thickness of from 30-500 micrometers)  422 ,  422 ′ can be positioned above and below the sensor  448 . 
         [0042]      FIGS. 5   A - B  shows a cross-section of a general concept of a cover lens assembly which incorporates thin glass. The thin glass layer  522  is positioned as a top layer with respect to the adhesive and base glass layer  538 . Note that this thin glass  522  could have additional layers on either the top or bottom side (e.g. ink patterns, anti-fingerprint coatings, etc.). Below the thin glass layer  522  is a base glass layer  538  which also may have additional coatings, if desired. This base glass layer  538  is preferably strengthened. This base glass layer  538  may have thicknesses in the range of 100 um to 300 um, or 300 um to 500 um, or be greater than 500 um. An adhesive layer  552  of thickness d and with a hardness given by the Bulk Modulus, K, is used to bond the thin glass layer  522  to the base glass layer  538 . It may be desired to also have other layers, device components and devices positioned between the base glass layer  538  and the thin glass layer  522 , wherein, for example, by doing so these additional components could be closer to an upper surface  502  than if they were located beneath the base glass. 
         [0043]    Additionally, a thin glass laminate (having a thickness of from 30-500 micrometers)  522 ,  522 ′ comprised of a first thin glass layer  522  and an second thin glass layer  522 ′ can be positioned above and below the sensor  548 . 
         [0044]    One primary problem which this disclosure addresses is that the thin glass layer in assemblies, like the ones illustrated in  FIGS. 3 and 4 , are normally susceptible to breaking In particular, one way to test the durability of these assemblies is to subject them to ball drop tests to determine impact resistance, wherein a specified steel ball is dropped from successively increasing heights onto the surface of the assembly until breakage occurs. This susceptibility to breaking is solved by providing all the elements between the thin glass and base glass as being below a certain thickness, and having hardnesses above certain values. During a ball drop test, when the ball impacts the front surface, the thin glass layer may deflect locally toward conforming to the radius of the ball. In doing so, local stresses occur in this region which can result in the formation of a crack. The deformation of the thin glass layer can be reduced by increasing the hardnesses of the materials between the thin glass layer and the base glass layer. It also can be reduced by reducing the thickness of the space between the thin glass layer and the base glass layer. By choosing an appropriate combination of thicknesses and hardnesses, breakage of the thin glass layer due to impact, particularly due to ball drop testing, can be reduced or eliminated to the point where the overall breakage of the cover glass assembly is the same as a typical commercial assembly based on the strengthened glass. This is supported by the attached figures, data. 
         [0045]    As will be appreciated by those skilled in the art, hardness can be defined many ways. The Bulk Elastic Modulus, K, is an example of a suitable measurement. However, this value is not always readily available, but for many of the materials of interest the Young&#39;s Modulus, Y M , can be used as a substitute as it is often measured. 
         [0046]    The configurations of this disclosure utilize a total thickness of the layers between the thin glass layer and base glass layer of about 8-mil (200 um) or less, more specifically 4-mil (100 um) or less, and even more specifically 3-mil (75 um or less) in thickness. Additionally, the modulus of these middle layers, averaged by their thicknesses, is 1 GPa or greater. When much of this thickness is defined by the adhesive thickness, the adhesive will be one of a high hardness. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Device Component 
                 From 
                 To 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Interface Thickness (mm) 
                 0.013 
                 0.075 
               
               
                   
                 Adhesive Layer Thickness (mm) 
                 0.025 
                 0.200 
               
               
                   
                 Modulus of Middle Layers (Y m,  GPa) 
                 0.5 
                 5 
               
               
                   
                   
               
             
          
         
       
     
         [0047]    Types of adhesives that are included are polyester-based resins, acrylic-based resins and polycarbonate-based resins. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Material 
                 Y m   
                 PSI 
               
               
                   
                   
               
             
             
               
                   
                 Glass 
                  ~50 GPa 
                 ~7 × 10 6   
               
               
                   
                 PET* 
                 ~3.5 GPa 
                 ~5 × 10 5   
               
               
                   
                 EVA** 
                  ~70 uPa 
                 ~1,000-10,000 
               
               
                   
                   
               
               
                   
                 *Polyethylene Terephthalate (PET) 
               
               
                   
                 **Ethelyne Vinyl Acetate (EVA) 
               
             
          
         
       
     
         [0048]    The device interfaces disclosed herein achieve an increased durability and performance of the electronic device interface by optimizing the thickness of the adhesive layer and the hardness. 
         [0049]    The cover lens assemblies can be fabricated using a variety of processes, which depend critically on the type of adhesive being used. In general, the stack of layers are built-up sequentially by applying each layer on top of the other. This can happen in whichever order is appropriate for the desired handling of materials and schedule of applying process conditions such as heating, exposure to UV or applying pressure. 
         [0050]    In one example, thermoforming and thermosetting adhesives are used, which are supplied as thin dry sheets. For these materials the full assembly is stacked up, which includes (by illustration of  FIG. 5 ) the base glass layer  538 , adhesive layer  552  and a thin glass layer  522 . This stack of material is placed in a vacuum laminator and undergoes a series of process steps which include evacuation of surrounding air (to eliminate trapped air between layers), heating, application of pressure, and cooling. Timing and sequence of these steps can be optimized to improve the desired properties of, for example, optical clarity, smoothness, uniformity, hardness and adhesion. Example assemblies which include the adhesives of EVA and DuPont&#39;s PV5412 have been assembled utilizing such a process. 
         [0051]    In another example, optically clear pressure sensitive adhesive (PSA) films, commonly referred to as optically clear adhesives (OCAs), are used. When using these adhesives, lamination is done by applying the film(s) with a roller laminator which applies pressure as the films are laminated. Heat may also be applied either during or after the physical lamination. The sequence of assembly may vary depending on many factors of material handling. In one example for these materials (by illustration of  FIGS. 5   A - B ), the adhesive layer  552  is first applied to the base glass layer  538 , then the thin glass layer  522  is applied to the stack of adhesive and base glass layer. In an alternative example for these materials, the adhesive layer  552  is first applied to the thin glass layer  522 , thereafter the combination of the thin glass layer and adhesive layer are applied to the base glass layer  538 . Following either of these lamination approaches, heat and/or pressure can be applied to accelerate bonding at the interfaces. 
         [0052]    Other adhesives may include liquid epoxies. These may be applied as layers using a variety of coating methods including spray, gravure, slot, dispense, spin methods. Once the stack of layers to be bonded by the epoxy are formed, the epoxy may be hardened and cured using heat and/or exposure to UV radiation. 
       III. Methods of Use 
       [0053]    An electronic device  100  configurable to position a sensor, such as a 1D sensor, a 2D sensor or a touch sensor, within sensing region is deployed by a user. The user applies and/or swipes his or her finger, or applies the element to be sensed, to a surface of the device in a position in communication with the sensor. 
         [0054]    For example, the element to be sensed (such as a finger) can be applied (e.g., by swiping across the surface) in a position that is in communication with the sensing element. Thus, for example, the fingerprint is sensed by the fingerprint sensor and/or biometric input is received. The sensed input (such as a fingerprint) is validated. Once validated, use of the electronic device  100  is permitted or other functionality controlled by the fingerprint authentication is permitted. 
         [0055]    As will be appreciated by those skilled in the art, the sensors disclosed can be used in a variety of ways including, for example:
       User authentication for device (e.g., phone, tablet, or computer) unlocking   User authentication for online transactions   User authentication for access to device systems and services, including websites and email   Replacement of Password and PINs   Physical access such as door locks   Time and attendance systems (prove you were at a certain place at a certain time)   Finger-based input devices/navigation for mobile phones and gaming   Finger-based shortcuts (authentication of a given finger results in the corresponding action to which that finger is mapped)   Non-repudiation for online transactions       
 
       IV. Examples 
     Impact Resistance Test: STR® 15420P vs. DuPont® PV5412 
       [0065]    The object of this test was to measure the difference between STR Photocap® 15420P EVA thermal adhesive (˜400 um), available from Specialized Technology Resources, Inc. (www.strsolar.com, Enfield, Conn.), and DuPont® PV5412 thermal adhesive (380 um), available from DuPont Photovoltaic Solutions (www2.dupont.com/photovoltaics/en_US/products_services!), via an Impact Resistance Test. To achieve this, a 2 g steel ball with a diameter of 8 mm was dropped from a designated height five times. The drop height was increased until failure occurred. The test package consisted of: glass, thermal adhesive, thin glass. A substantial difference was measured between the two thermal adhesives, the DuPont PV5412 adhesive increased impact strength over the STR EVA material. The impact resistance improvement is attributed to the greater hardness of the Dupont PV5412 material.  FIG. 6   A  illustrates an image of the device interface after the impact resistance test failure for 15420P at 1 foot, and  FIG. 6   B  illustrates an image of the device interface after the impact resistance test failure for the DuPont PV5412 at 6.5 feet. The breakage of the ultrathin (UT) glass for the standard configuration was due to too much deflection during impact which causes a tensile break. The PV 5412 material ( FIG. 6   B ) reduced deflection by using an adhesive layer that is harder than that of the EVA material. The DuPont® PV5412 is expected to have a Young&#39;s Modulus above 1 GPa, while the STR Photocap® 15420P EVA is expected to have a Young&#39;s Modulus in a range below 0.1 GPa.  FIG. 6   C  is a chart illustrating the relative impact resistance test results and the height at which failure occurred for the materials shown in  FIGS. 6   A - B . 
         [0066]      FIG. 7   A  illustrates impact test results using an OCA (available from 3M Company, St. Paul, Minn., such as OCA 8173, 8211, 8212, 8213, 8214, or 8215) which comprises the following layers: glass-OCA-ultrathin glass (UT). Failure distance (in inches) are provided for configurations having an OCA thickness of 1 mil-6 mil. Failure distances were well above 1 foot, and typically 4 feet (48 inches) or above. 
         [0067]      FIG. 7   B  illustrates impact test results using an OCA (available from 3M Company, such as OCA 8173, 8211, 8212, 8213, 8214, or 8215) which comprises a multi-layered structure having the following layers: glass-OCA-2 mil PET-OCA-UT. Failure distance (in inches) is provided for configurations having an OCA thickness of 1 mil-2 mil. Failure distance was 5 feet (60 inches) or above. 
         [0068]    While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.