Patent Publication Number: US-11028010-B2

Title: Machining of fusion-drawn glass laminate structures containing a photomachinable layer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/094,353, filed Apr. 8, 2016, which is a continuation of U.S. application Ser. No. 13/798,479, filed Mar. 13, 2013, which claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No. 61/770,454, filed Feb. 28, 2013, the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present disclosure generally relates to fusion-drawn glass laminate structures and, more particularly, to methods for machining fusion-drawn glass laminate structures that include at least one photomachinable layer. 
     TECHNICAL BACKGROUND 
     Fusion-drawn core-clad glass laminates have numerous uses in the electronics and optics industries. The formation of structures such as holes and through-holes through the laminates can be challenging and imprecise, particularly using techniques such as laser drilling. Accordingly, ongoing needs exist for fusion-drawn core-clad glass laminates having properties amenable to creating simple and complex structures including but not limited to holes and through-holes, and also for methods of machining the structures into the fusion-drawn core-clad glass laminates. 
     SUMMARY 
     According to various embodiments, methods for machining glass structures are disclosed. The methods according to the various embodiments may be performed on glass structures including, but not limited to, fusion-drawn laminates having a core layer interposed between a first cladding layer and a second cladding layer. In the fusion-drawn laminates, the core layer may be formed from a core glass composition having a core photosensitivity, the first cladding layer may be formed from a first-clad glass composition having a first-clad photosensitivity different from the core photosensitivity, and the second cladding layer may be formed from a second-clad glass composition having a second-clad photosensitivity that is different from the core photosensitivity. At least one of the core layer, the first cladding layer, and the second cladding layer is a photomachinable layer. The methods may include exposing at least one selected region of at least one photomachinable layer in the fusion-drawn laminate to ultraviolet radiation for a predetermined exposure time; heating the glass structure until the at least one selected region forms a crystallized region of crystallized material in the photomachinable layer; and removing the crystallized region selectively from the photomachinable layer. 
     Additional features and advantages of the embodiments described herein will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts a cross section of a glass structure according to one or more embodiments shown and described herein; 
         FIG. 2  schematically depicts a fusion draw process for making the glass structure of  FIG. 1 ; 
         FIGS. 3A-3D  schematically depict an embodiment of a method for machining the glass structure of  FIG. 1 ; 
         FIGS. 4A-4B  schematically depict an embodiment of additional machining steps performed on the glass structure of  FIG. 3D ; 
         FIGS. 5A-5D  schematically depict embodiments of additional machining steps performed on the glass structure of  FIG. 3D , by which one embodiment is shown in  FIGS. 5A and 5B  in combination and another embodiment is shown in  FIGS. 5A, 5C, and 5D  in combination; 
         FIGS. 6A-6E  schematically depict an embodiment of an ion-exchange processes performed on the glass structure of  FIG. 4A ; 
         FIGS. 7A-7E  schematically depict an embodiment of a method for machining the glass structure of  FIG. 1 , in which the core layer has a higher photosensitivity than the cladding layers; 
         FIGS. 8A-8F  schematically depict an embodiment of a method for machining the glass structure of  FIG. 1  to form complex structures including through-holes; 
         FIGS. 9A-9B  schematically depict a surface roughening step performed in an embodiment of a method for machining the glass structure of  FIG. 1 ; and 
         FIGS. 10A-10E  schematically depict an embodiment of a method for machining the glass structure of  FIG. 1 , in which the glass structure is machined into a component that may be used in a tactile interface. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of methods for machining glass structures. First the glass structures themselves will be described with reference to  FIGS. 1 and 2 . Various methods for machining the glass structures will be described below with reference to  FIGS. 3A-10E . 
     As used herein, the term “liquidus viscosity” refers to the shear viscosity of the glass composition at its liquidus temperature. 
     As used herein, term “liquidus temperature” refers to the highest temperature at which devitrification occurs in the glass composition. 
     As used herein, the term “CTE” refers to the coefficient of thermal expansion of the glass composition averaged over a temperature range from about 20° C. to about 300° C. 
     The term “substantially free,” when used to described the absence of a particular oxide component in a glass composition, means that the component is present in the glass composition as a contaminant in a trace amount of less than 1 mol. %. 
     For glass compositions described herein as components of glass structures, the concentration of constituent components (e.g., SiO 2 , Al 2 O 3 , Na 2 O and the like) of the glass compositions are given in mole percent (mol. %) on an oxide basis, unless otherwise specified. Glass compositions disclosed herein have a liquidus viscosity which renders them suitable for use in a fusion draw process and, in particular, for use as a glass cladding composition or a glass core composition in a fusion laminate process. As used herein, unless noted otherwise, the terms “glass” and “glass composition” encompass both glass materials and glass-ceramic materials, as both classes of materials are commonly understood. Likewise, the term “glass structure” should be understood to encompass structures containing glasses, glass ceramics, or both. 
     Examples of glass structures for use in methods for machining glass structures now will be described. Embodiments of methods for machining the glass structures will be described below. Referring to  FIG. 1 , glass compositions suitable for use in fusion-draw processes, including but not limited to those described herein, may be used to form an article, such as the glass structure  100  schematically depicted in cross section in  FIG. 1 . The glass structure  100  generally comprises a core layer  102  formed from a core glass composition. The core layer  102  may be interposed between a pair of cladding layers such as a first cladding layer  104   a  and a second cladding layer  104   b . The first cladding layer  104   a  and the second cladding layer  104   b  may be formed from a first cladding glass composition and a second cladding glass composition, respectively. In some embodiments, the first cladding glass composition and the second cladding glass composition may be the same material. In other embodiments, the first cladding glass composition and the second cladding glass composition may be different materials. 
       FIG. 1  illustrates the core layer  102  having a first surface  103   a  and a second surface  103   b  opposed to the first surface  103   a . A first cladding layer  104   a  is fused directly to the first surface  103   a  of the core layer,  102  and a second cladding layer  104   b  is fused directly to the second surface  103   b  of the core layer  102 . The glass cladding layers  104   a ,  104   b  are fused to the core layer  102  without any additional materials, such as adhesives, polymer layers, coating layers or the like being disposed between the core layer  102  and the cladding layers  104   a ,  104   b . Thus, the first surface  103   a  of the core layer  102  is directly adjacent the first cladding layer  104   a , and the second surface  103   b  of the core layer  102  is directly adjacent the second cladding layer  104   b . In some embodiments, the core layer  102  and the glass cladding layers  104   a ,  104   b  are formed via a fusion lamination process. Diffusive layers (not shown) may form between the core layer  102  and the cladding layer  104   a , or between the core layer  102  and the cladding layer  104   b , or both. 
     In some embodiments, the cladding layers  104   a ,  104   b  of the glass structures  100  described herein may be formed from a first glass composition having an average cladding coefficient of thermal expansion CTE clad , and the core layer  102  may be formed from a second, different glass composition which has an average coefficient of thermal expansion CTE core . In some embodiments, the glass compositions of the cladding layers  104   a ,  104   b  may have liquidus viscosities of at least 20 kPoise. In some embodiments, the glass compositions of the core layer  102  and the cladding layers  104   a ,  104   b  may have liquidus viscosities of less than 250 kPoise. 
     Specifically, the glass structure  100  according to some embodiments herein may be formed by a fusion lamination process such as the process described in U.S. Pat. No. 4,214,886, which is incorporated herein by reference. Referring to  FIG. 2  by way of example and further illustration, a laminate fusion draw apparatus  200  for forming a laminated glass article may include an upper isopipe  202  that is positioned over a lower isopipe  204 . The upper isopipe  202  may include a trough  210 , into which a molten cladding composition  206  may be fed from a melter (not shown). Similarly, the lower isopipe  204  may include a trough  212 , into which a molten glass core composition  208  may be fed from a melter (not shown). In the embodiments described herein, the molten glass core composition  208  has an appropriately high liquidus viscosity to be run over the lower isopipe  204 . 
     As the molten glass core composition  208  fills the trough  212 , it overflows the trough  212  and flows over the outer forming surfaces  216 ,  218  of the lower isopipe  204 . The outer forming surfaces  216 ,  218  of the lower isopipe  204  converge at a root  220 . Accordingly, the molten core composition  208  flowing over the outer forming surfaces  216 ,  218  rejoins at the root  220  of the lower isopipe  204 , thereby forming a core layer  102  of a laminated glass structure. 
     Simultaneously, the molten composition  206  overflows the trough  210  formed in the upper isopipe  202  and flows over outer forming surfaces  222 ,  224  of the upper isopipe  202 . The molten composition  206  has a lower liquidus viscosity requirement to be run on the upper isopipe  202 , and will have a CTE either equal to or less than the glass core composition  208  when present as a glass. The molten cladding composition  206  is outwardly deflected by the upper isopipe  202  such that the molten cladding composition  206  flows around the lower isopipe  204  and contacts the molten core composition  208  flowing over the outer forming surfaces  216 ,  218  of the lower isopipe, fusing to the molten core composition and forming cladding layers  104   a ,  104   b  around the core layer  102 . 
     In the laminated sheet so formed, the clad thickness will also be significantly thinner than the core thickness so that the clad goes into compression and the core into tension. But because the CTE difference is low, the magnitude of the tensile stress in the core will be very low (for example, on the order of 10 MPa or lower) which will allow for the production of a laminated sheet that will be relatively easy to cut off the draw due to its low levels of core tension. Sheets can thus be cut from the laminate structure that is drawn from the fusion draw apparatus. After the sheets are cut, the cut product can then be subjected to a suitable UV light treatment(s), as will be described below in the context of methods for machining the glass structure  100 . 
     As illustrative embodiments, the processes for forming glass structures by fusion lamination described herein with reference to FIGS. 1 and 2 and in U.S. Pat. No. 4,214,886 may be used for preparing glass structures  100  in which the glass cladding layers  104   a ,  104   b  have the same glass composition. In other embodiments, the glass cladding layers  104   a ,  104   b  of the glass structure  100  may be formed from different glass compositions. Non-limiting exemplary processes suitable for forming glass structures having glass cladding layers of different compositions are described in commonly-assigned U.S. Pat. No. 7,514,149, which is incorporated herein by reference in its entirety. 
     The laminated glass articles and glass structures disclosed herein may be employed in a variety of consumer electronic devices including, without limitation, mobile telephones, personal music players, tablet computers, LCD and LED displays, automated teller machines and the like. In some embodiments, the laminated glass article may comprise one or more layers that are opaque, transparent, or translucent, such as a clad derived from a glass composition wherein the clad layer is opaque, transparent or translucent after heat treatment(s). In some embodiments, the glass structures may be sheet-glass structures. 
     Having described non-limiting exemplary forms of glass structures  100  containing fusion-drawn laminates with a core layer  102  and cladding layers  104   a ,  104   b , methods for machining the glass structures  100  will now be described. Referring to  FIG. 1 , in exemplary methods for machining a glass structure  100 , the glass structure  100  may include a fusion-drawn laminate of a core layer  102  interposed between a first cladding layer  104   a  and a second cladding layer  104   b . The core layer  102  may be formed from a core glass composition having a core photosensitivity. The first cladding layer  104   a  may be formed from a first-clad glass composition having a first-clad photosensitivity different from the core photosensitivity. The second cladding layer may be formed from a second-clad glass composition having a second-clad photosensitivity that is also different from the core photosensitivity. In some embodiments, the first-clad glass composition and the second-clad composition may be identical. In other embodiments, the first-clad glass composition and the second-glad glass composition may be different. In such embodiments, the first-clad photosensitivity and the second-clad photosensitivity may be the same or different. 
     In some embodiments, any or all of the core glass composition, the first-clad glass composition, and the second-clad glass composition may be photosensitive glass compositions. Photosensitive glass compositions compose a class of glass or glass ceramic materials that undergo a change in crystallinity properties when the photosensitive glass composition is exposed to radiation such as UV radiation, for example. In some photosensitive glass compositions, the change in crystallinity may result directly from the exposure to the radiation. In other photosensitive glass compositions, the exposure to the radiation may cause undetectable physical changes to the glass composition, such as the formation of nucleation centers. In such photosensitive glass compositions, once the nucleation centers are formed, the change to crystallinity may be completed by applying a heat treatment to the glass composition. 
     The photosensitivity of a particular glass varies with respect to the actual composition of the photosensitive glass. Not all glass compositions are photosensitive and, as such, truly non-photosensitive glasses shall be described herein as having a photosensitivity of zero. Likewise, glass compositions that do exhibit photosensitivity shall be defined as having a nonzero photosensitivity. Unless stated otherwise, glass compositions herein said to have a “core photosensitivity,” a “first-clad photosensitivity,” or a “second-clad photosensitivity do not necessarily exhibit photosensitivity and may have a zero photosensitivity (i.e., may be non-photosensitive) or a nonzero photosensitivity (i.e., may be exhibit photosensitivity). 
     Relative photosensitivities of two glass compositions having nonzero photosensitivities may be determined objectively. For example, sheets of each composition with equal thicknesses may be exposed to radiation such as UV radiation for various periods of time, followed by heat treatment, to determine the minimum radiation exposure times that enable the secondary crystalline phase to form through the entire thickness of each sheet after the heat treatment. As applicable to embodiments described herein, a first photosensitive glass composition having a shorter minimum radiation exposure time than a second photosensitive glass composition shall be considered to have a photosensitivity greater than that of the second photosensitive glass composition. Conversely, a first photosensitive glass composition having a longer minimum radiation exposure time than a second photosensitive glass composition shall be considered to have a photosensitivity less than that of the second photosensitive glass composition. 
     The photosensitive and/or photomachinable glass compositions suitable for use herein may include, as non-limiting examples, alkaline-earth aluminoborosilicate glasses, zinc borosilicate glasses, and soda-lime glass. The photosensitive and/or photomachinable glass compositions may also include glass ceramics such as glasses enriched with magnesium oxide, yttria, beryllia, alumina, or zirconia. Illustrative photosensitive glass compositions suitable for use in embodiments herein include those described in U.S. Pat. Nos. 7,241,559; 7,262,144; and 7,829,489, all of which are incorporated herein by reference. In some embodiments, FOTOFORM®, available from Corning Incorporated, may be a suitable photosensitive glass composition. The FOTOFORM® glass has a composition of 79.3 wt. % SiO 2 , 1.6 wt. % Na 2 O, 3.3 wt. % K 2 O, 0.9 wt. % KNO 3 , 4.2 wt. % Al 2 O 3 , 1.0 wt. % ZnO, 0.0012 wt. % Au, 0.115 wt. % Ag, 0.015 wt. % CeO 2 , 0.4 wt. % Sb 2 O 3 , and 9.4 wt. % Li 2 O. Other nonlimiting exemplary photosensitive glasses suitable for use in embodiments described herein are provided in TABLE 1 below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Exemplary Photosensitive Glass Compositions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Composition 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (wt %) 
               
               
                 SiO 2   
                 76.5 
                 78.7 
                 74.21 
                 73.86 
                 71.57 
                 71.2 
                 77.63 
                 71.70 
                 72.39 
                 73.09 
                 73.78 
               
               
                 Al 2 O 2   
                 8.78 
                 4.82 
                 6.18 
                 7.58 
                 8.05 
                 8 
                 8.95 
                 8.14 
                 8.27 
                 8.40 
                 8.53 
               
               
                 Li 2 O 
                 9.23 
                 7.5 
                 6.55 
                 6.53 
                 6.19 
                 5.78 
                 8.56 
                 6.93 
                 7.96 
                 8.99 
                 10.02 
               
               
                 Na 2 O 
                 1.21 
                 1.51 
                 1.67 
                 1.62 
                 2.63 
                 2.6 
                 1.10 
                 2.38 
                 2.14 
                 1.90 
                 1.66 
               
               
                 K 2 O 
                 2.34 
                 6.53 
                 6.62 
                 6.01 
                 4.92 
                 5.72 
                 2.15 
                 5.37 
                 4.72 
                 4.07 
                 3.42 
               
               
                 BaO 
                 0 
                 0 
                 3.11 
                 3.11 
                 6.23 
                 6.17 
                 0 
                 4.90 
                 3.68 
                 2.45 
                 1.23 
               
               
                 ZnO 
                 1.64 
                 0.47 
                 1.18 
                 0.89 
                 0.29 
                 0.28 
                 1.61 
                 0.57 
                 0.83 
                 1.09 
                 1.35 
               
               
                 CeO2 
                 0.01 
                 0.007 
                 0.017 
                 0.011 
                 0.013 
                 0.014 
                 0.013 
                 0.015 
                 0.014 
                 0.014 
                 0.013 
               
               
                 SnO 2   
                 0 
                 0.03 
                 0.03 
                 0.03 
                 0.03 
                 0.03 
                 0 
                 0.002 
                 0.002 
                 0.001 
                 68.04 
               
               
                 Sb 2 O 3   
                 0.22 
                 0.46 
                 0.58 
                 0.52 
                 0.5 
                 0.5 
                 0.22 
                 0.442 
                 0.387 
                 0.331 
                 0.276 
               
               
                 Ag 
                 0.1 
                 0.088 
                 0.088 
                 0.087 
                 0.086 
                 0.077 
                 0.081 
                 0.10 
                 0.09 
                 0.09 
                 0.08 
               
               
                 Au 
                 0.0001 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
               
               
                 Composition 
               
               
                 (mole %) 
               
               
                 SiO2 
                 73.5 
                 76.7 
                 74.9 
                 74.7 
                 73.6 
                 73.8 
                 74.8 
                 72.7 
                 72.1 
                 71.4 
                 70.8 
               
               
                 Al2O3 
                 5.0 
                 2.8 
                 3.7 
                 4.5 
                 4.9 
                 4.9 
                 5.1 
                 4.9 
                 4.8 
                 4.8 
                 4.8 
               
               
                 Li2O 
                 17.7 
                 14.6 
                 13.2 
                 13.2 
                 12.7 
                 12.0 
                 16.5 
                 14.1 
                 15.9 
                 17.6 
                 19.2 
               
               
                 Na2O 
                 1.1 
                 1.4 
                 1.6 
                 1.6 
                 2.6 
                 2.6 
                 1.0 
                 2.3 
                 2.1 
                 1.8 
                 1.5 
               
               
                 K2O 
                 1.4 
                 4.1 
                 4.3 
                 3.9 
                 3.2 
                 3.8 
                 1.3 
                 3.5 
                 3.0 
                 2.5 
                 2.1 
               
               
                 BaO 
                 0 
                 0 
                 1.2 
                 1.2 
                 2.5 
                 2.5 
                 0 
                 2.0 
                 1.4 
                 0.9 
                 0.5 
               
               
                 ZnO 
                 1.2 
                 0.3 
                 0.9 
                 0.7 
                 0.2 
                 0.2 
                 1.2 
                 0.4 
                 0.6 
                 0.8 
                 1.0 
               
               
                 CeO2 
                 0.003 
                 0.003 
                 0.006 
                 0.004 
                 0.005 
                 0.005 
                 0.004 
                 0.005 
                 0.005 
                 0.005 
                 0.004 
               
               
                 SnO2 
                 0 
                 0.012 
                 0.012 
                 0.012 
                 0.012 
                 0.012 
                 0 
                 0.001 
                 0.001 
                 0.004 
                 0.0002 
               
               
                 Sb2O3 
                 0.04 
                 0.090 
                 0.120 
                 0.109 
                 0.106 
                 0.107 
                 0.040 
                 0.092 
                 0.079 
                 0.067 
                 0.055 
               
               
                 Ag 
                 0.050 
                 0.048 
                 0.049 
                 0.049 
                 0.049 
                 0.044 
                 0.043 
                 0.056 
                 0.050 
                 0.049 
                 0.043 
               
               
                 Au 
                 0.00003 
                 0.0003 
                 0.0003 
                 0.0003 
                 0.0003 
                 0.0003 
                 0.0003 
                 0.0003 
                 0.0003 
                 0.0003 
                 0.0003 
               
               
                 0-300° C. CTE 
                 75 
                 80.9 
                 80.2 
                 77.1 
                 80.0 
                 81.3 
                 68.4 
                 80.4 
                 80.1 
                 80.1 
                 79.9 
               
               
                 24 hour Liquidus 
                 940 
                 880 
                 850 
                 860 
                 840 
                 840 
                 950 
                 855 
                 870 
                 885 
                 890 
               
               
                 (° C.) 
               
               
                 Liquidus 
                 20 
                 71 
                 123 
                 119 
                 220 
                 140 
                 38 
                 99 
                 56 
                 34 
                 25 
               
               
                 Viscosity (kP) 
               
               
                   
               
            
           
         
       
     
     According to some embodiments, in the fusion-drawn laminate of the glass structure, at least one of the core layer  102 , the first cladding layer  104   a , and the second cladding layer  104   b  is a photomachinable layer. In this regard, the glass composition from which the at least one photomachinable layer among the core layer  102 , the first cladding layer  104   a , and the second cladding layer  104   b , is a photomachinable glass composition. As used herein, the term “photomachinable glass composition” refers to a glass composition having a nonzero photosensitivity, such that the glass composition forms a secondary crystalline phase after exposure of the glass composition to radiation (such as UV radiation, for example) and, optionally, a heat treatment. 
     Additionally, in photomachinable glass compositions, the secondary crystalline phase that forms after radiation exposure and optional heat treatment is capable of being selectively removed by a physical or chemical procedure such as selective etching. To illustrate, selective removal of the secondary crystalline phase may be enabled by differences in solubility in an etchant medium such as hydrofluoric acid of the secondary crystalline phase to the portions of the glass composition unexposed to radiation. The solubility difference may result in an etch-rate difference, whereby the secondary crystalline phase may etch at least 1.5 times faster, at least 2 times faster, at least 5 times faster, at least 10 times faster, at least 20 times faster, or even at least 100 times faster than the portions of material not exposed to radiation. This feature of etch-rate and/or solubility differentiation may or may not be present in all photosensitive glass compositions. Thus, as the terms are used herein, all photomachinable glass compositions are photosensitive glass compositions with a nonzero photosensitivity, but photosensitive glass compositions are not necessarily photomachinable. Moreover, though in some embodiments one or more of the core layer  102 , the first cladding layer  104   a , and the second cladding layer  104   b  may be neither photosensitive nor photomachinable, in such embodiments at least one of the core layer  102 , the first cladding layer  104   a , and the second cladding layer  104   b  is both photosensitive and photomachinable. 
     According to various embodiments of methods for machining a glass structure  100  such as those described above, the machining may include exposing at least one selected region of at least one photomachinable layer in the fusion-drawn laminate to ultraviolet radiation for a predetermined exposure time. The machining may further include heating the glass structure until the at least one selected region forms a crystallized region of crystallized material in the photomachinable layer. The machining may further include removing the crystallized region selectively from the photomachinable layer. The above components of the methods for machining the glass structure  100  will be described in general now, and specific illustrative embodiments of the components to the methods will be described in detail below. 
     In illustrative embodiments, the at least one selected region of at least one photomachinable layer in the fusion-drawn laminate is exposed to ultraviolet radiation for a predetermined exposure time. In some embodiments, the at least one selected region may include one contiguous region or multiple non-contiguous regions. In other embodiments, the at least one selected region may include the entire photomachinable layer. In illustrative embodiments, the at least one selected region may include a portion of the first cladding layer  104   a , a portion of the second cladding layer  104   b , the entire first cladding layer  104   a , the entire second cladding layer  104   b , or a combination of these. It should be understood that when an entire layer is exposed to the UV radiation, after a heat treatment and subsequent selective removal described below, the layer or layers that are completely exposed may be completely removed from the glass structure  100  and function as sacrificial layers. 
     During the UV exposure, according to some embodiments the ultraviolet radiation  120  may have a wavelength of from about 100 nm to about 400 nm, for example from about 290 nm to about 330 nm. The predetermined exposure time may range from 5 seconds to several hours, such as from about 3 minutes to about 2 hours. In some embodiments, the intensity of the ultraviolet radiation may be varied to affect the kinetics of the physical processes that enable the secondary phase formation during the subsequent heat treatment. The depth to which the ultraviolet radiation enables the secondary phase formation in a given glass structure may depend on the wavelength of the UV radiation, the length of the exposure time, and the intensity of the exposure. 
     In illustrative embodiments, the heating of the glass structure may be conducted after the exposure to the ultraviolet radiation. The heating may proceed at least until the at least one selected region forms a crystallized region of crystallized material in the photomachinable layer. In some embodiments, the heating may be conducted at temperatures of from about 300° C. to about 900° C., depending on the composition of the photomachinable layer. For example, some photomachinable compositions may be heat treated at temperatures of from about 300° C. to about 500° C., or from about 500° C. to about 650° C. 
     In illustrative embodiments, removing the crystallized region selectively from the photomachinable layer includes taking advantages of one or both of a differential solubility or a differential etch-rate of the secondary crystalline phase compared to that of the unexposed photomachinable material. Removing the crystallized region may include etching techniques such as immersion, ultrasonic etching, or spraying, for example, in a suitable etchant such as hydrofluoric acid, for example. With regard to etchants, it should be understood that any etchant may be used, in which the secondary crystalline phase has a solubility or etch-rate differential from that of the unexposed photomachinable material such as 5 times greater, 10 times greater, 20 times greater, or even 100 times greater. As described above, if an entire layer such as the first cladding layer  104   a , the second cladding layer  104   b , or both is exposed to the UV radiation, the selective removal process may result in the removal of the entire layer, which thereby functions as a sacrificial layer. 
     Specific illustrative embodiments of methods for machining glass structures and including the general exposing, heating, and removing actions described above will now be described with reference to  FIGS. 3A-10E . 
     Referring first to  FIGS. 3A and 3C , the glass structure  100  may include a core layer  102  interposed between a first cladding layer  104   a  and a second cladding layer  104   b . The core layer may have a core photosensitivity that is less than a first-clad photosensitivity of the first cladding layer  104   a  and a second-clad photosensitivity of the second cladding layer  104   b . The first-clad photosensitivity and the second-clad photosensitivity may be the same or different. In the embodiments of  FIGS. 3A and 3B , at least the first cladding layer  104   a  is photomachinable. In the embodiments of  FIGS. 3C and 3D , at least the first cladding layer  104   a  and the second cladding layer  104   b  are photomachinable. The core layer  102  in each embodiment may have a photosensitivity of zero or a nonzero photosensitivity and may or may not be photomachinable. 
     In the embodiment of  FIG. 3A , the first cladding layer  104   a  is exposed to the UV radiation  120  through a photomask  110  having apertures  115  that define the selected regions of the first cladding layer  104   a  to be exposed to the UV radiation  120 . Heat treatment of the glass structure  100  may result in the glass structure  100  of  FIG. 3B , in which crystallized regions  130  have formed in locations corresponding to the apertures  115  in the photomask  110 . Because the core layer photosensitivity is less than the first-clad photosensitivity, at least one of the exposure to the UV radiation  120  depicted in  FIG. 3A  or the heat treatment parameters used to form the glass structure  100  of  FIG. 3B , is insufficient to enable formation of crystallized regions in the core layer  102 . It should be understood that the UV radiation  120  may penetrate into or through the core layer  102  and also may penetrate into or through the second cladding layer  104   b . As such, in some embodiments if the first-clad photosensitivity and the second-clad photosensitivity are significantly greater than the core photosensitivity, a single UV exposure from one side of the glass structure  100  as shown in  FIG. 3A  and followed by heat treatment may result in the glass structure  100  of  FIG. 3D  with crystallized regions  130   a ,  130   b  in both the first cladding layer  104   a  and the second cladding layer  104   b.    
     In the embodiment of  FIG. 3C , the first cladding layer  104   a  is exposed to the UV radiation  120   a  through a first photomask  110   a  having apertures  115   a  that define the selected regions of the first cladding layer  104   a  to be exposed to the UV radiation  120   a . Likewise, the second cladding layer  104   b  is exposed to the UV radiation  120   b  through a second photomask  110   b  having apertures  115   b  that define the selected regions of the second cladding layer  104   b  to be exposed to the UV radiation  120   b . Though the apertures  115   a ,  115   b  of the first photomask  110   a  and  110   b  may be vertically aligned in some embodiments, they do not need to be vertically aligned. It should be understood that the UV radiation  120   a ,  120   b  may penetrate into or even through the core layer  102 . Heat treatment of the glass structure  100  may result in the glass structure  100  of  FIG. 3D , in which crystallized regions  130   a ,  130   b  have formed in locations corresponding to the apertures  115   a ,  115   b  in the first photomask  110   a  and the second photomask  110   b . Because the core layer photosensitivity is less than the first-clad photosensitivity, at least one of the exposure to the UV radiation  120   a ,  120   b  depicted in  FIG. 3C  or the heat treatment parameters used to form the glass structure  100  of  FIG. 3D , is insufficient to enable formation of crystallized regions in the core layer  102 . 
     The glass structure  100  of  FIG. 3D  may be further processed by selectively removing the crystallized regions  130   a ,  130   b  from the first cladding layer  104   a  and the second cladding layer  104   b  by a suitable technique such as etching, for example. The resulting structure after such a removal is shown in  FIG. 4A , in which hole structures  140   a ,  140   b  remain in the first cladding layer  104   a  and the second cladding layer  104   b . In some embodiments, the hole structures  140   a ,  140   b  are substantially circular in shape. Without intent to be bound by theory, it is believed that the selective removal of crystalline regions of photomachinable glass compositions may facilitate formation of substantially circular hole structures  140   a ,  140   b  with precision unattainable from customary selective removal or etching techniques. 
     Referring to  FIG. 4B , if the hole structures  140   a ,  140   b  are vertically aligned in the glass structure  100 , a physical or chemical etching technique may be used to remove the portions of the core layer  102  between the hole structures  140   a ,  140   b , thereby forming through-holes  150   a ,  150   b , which may function as via holes, for example. Thus, in some embodiments, the methods for machining the glass structure  100  may include etching the core layer  102  through hole structures  140   a ,  140   b  in the first cladding layer  104   a  and the second cladding layer  104   b  to form through-holes  150   a ,  150   b  in the glass structure  100 . 
     In some embodiments of methods for machining glass structures, a glass structure  100  having crystallized regions  130   a ,  130   b , as depicted in  FIGS. 3D and 5A , may be exposed again to radiation such as UV radiation  120   a ,  120   b . When the glass structure  100 , already having crystallized regions  130   a ,  130   b , is further exposed to the radiation, the crystallized regions  130   a ,  130   b  may shield portions of the core layer  102  beneath or directly adjacent to the crystallized regions  130   a ,  130   b  from any exposure to the radiation. In some embodiments, the additional exposure to the UV radiation is conducted with suitable parameters such as intensity, wavelength, and duration that enable the core layer  102 , the first cladding layer  104   a , and the second cladding layer  104   b  to crystallize after heat treatment. For example, the glass structure  100  of  FIG. 5B  illustrates one embodiment of a structure that may result after heat treatment of the glass structure of  FIG. 5A  after exposure to radiation. In the glass structure  100  of  FIG. 5B , the first cladding layer  104   a  and the second cladding layer  104   b  have crystallized entirely in all directions. The core layer  102  contains core crystallized regions  135  and core uncrystallized regions  155   a ,  155   b . The core uncrystallized regions  155   a ,  155   b  may represent portions of the core layer  102  that were shadowed from UV radiation exposure by the crystallized regions  130   a ,  130   b  in the first cladding layer  104   a  and the second cladding layer  104   b.    
     In other embodiments, the crystallized regions  130   a ,  130   b  of the first cladding layer  104   a  and the second cladding layer  104   b  may be removed from the glass structure  100  of  FIG. 5A  after the depicted UV exposure but before any heat treatment to result in the glass structure  100  of  FIG. 5C . In such embodiments, the compositions of the first cladding layer  104   a , the second cladding layer  104   b , or both, may be chosen such that crystallization occurs only with UV exposure and subsequent heat treatment, not with UV exposure alone. The glass structure  100  of  FIG. 5C  formed in this manner may contain hole structures  140   a ,  140   b . Thereupon, the glass structure  100  of  FIG. 5C , in which all layers have already been exposed to UV radiation  120  as shown in  FIG. 5A , may be subjected to heat treatment to result in the glass structure  100  of  FIG. 5D . Similar to the glass structure  100  of  FIG. 5B , in the glass structure  100  of  FIG. 5D , all of the first cladding layer  104   a  and the second cladding layer  104   b  are crystallized, and the core layer  102  includes both core crystallized regions  135  and core uncrystallized regions  155   a ,  155   b . Unlike the glass structure of  FIG. 5B , however, in the glass structure  100  of  FIG. 5D , hole structures  140   a ,  140   b  are present in the first cladding layer  104   a  and the second cladding layer  104   b.    
     Referring now to  FIGS. 6A-6E , further embodiments of methods for machining glass structures may additionally comprise subjecting the glass structure to an ion-exchange process. In some embodiments, the ion-exchange process may include replacing certain elements in the glass composition, such as sodium, for example, with other elements such as potassium, for example, to strengthen all or a portion of the glass composition. In some embodiments, the glass structure  100  of  FIG. 6A , having hole structures  140   a ,  140   b  formed as described above, may be subjected to an ion-exchange process. As used herein, the term “ion-exchanged” is understood to mean that the glass is strengthened by ion exchange processes that are known to those skilled in the glass fabrication arts. Such ion exchange processes include, but are not limited to, treating the heated alkali aluminosilicate glass with a heated solution containing ions having a larger ionic radius than ions that are present in the glass surface, thus replacing the smaller ions with the larger ions. Potassium ions, for example, could replace sodium ions in the glass. Alternatively, other alkali metal ions having larger atomic radii, such as rubidium or cesium could replace smaller alkali metal ions in the glass. Similarly, other alkali metal salts such as, but not limited to, sulfates, halides, and the like may be used in the ion exchange process. In one embodiment, the down-drawn glass is chemically strengthened by placing it a molten salt bath comprising KNO 3  for a predetermined time period to achieve ion exchange. In one embodiment, the temperature of the molten salt bath is about 430° C., and the predetermined time period is about eight hours. 
     In the embodiment of  FIG. 6B , for example, the ion-exchange process may continue until ion-exchanged regions  160   a ,  160   b  form in portions of the first-cladding layer  104   a , the second cladding layer  104   b , and even the core layer  102  near the hole structures  140   a ,  140   b , but only for a predetermined time that does not result in ion exchange through the entire depth of the cladding layers  104   a ,  104   b . In the embodiment of  FIG. 6C , for example, the ion-exchange process may continue until the first-cladding layer  104   a  and the second cladding layer  104   b  are ion-exchanged regions  160   a ,  160   b , and the ion-exchanged regions  160   a ,  160   b  optionally may extend into the core layer  102 . In the embodiment of  FIG. 6D , for example, the ion-exchange process may continue until the ion-exchanged layer  160  is contiguous throughout the glass structure  100 , such that all portions of the first-cladding layer  104   a  and the second cladding layer  104   b  are ion-exchanged and regions of the core layer  102  extending through the entire depth of the core layer  102  are also ion exchanged. In some embodiments, referring to  FIG. 6E , the ion-exchanged region  160  is contiguous throughout the glass structure  100 , and in the regions of the glass structure  100  in which the core layer  102  is ion-exchanged throughout its entire depth, the glass structure may be broken at break lines  170  that may readily form in the ion-exchanged portions of the core layer  102 . Ion exchange processes and suitable exemplary glass compositions amenable to the ion exchange processes are described in U.S. Pat. No. 7,666,511, which is incorporated herein by reference in its entirety. 
     Referring now to  FIGS. 7A-7E , in some embodiments of methods for machining glass structures, the core layer  102  may have a core photosensitivity greater than the first-clad photosensitivity of the first cladding layer  104   a . In such embodiments, the core photosensitivity may also be greater than the second-clad photosensitivity of the second cladding layer  104   b . Thereby, when the glass structure  100  of  FIG. 7A  is exposed to radiation such as UV radiation  120   a ,  120   b  through photomasks such as first photomask  110   a  with apertures  115   a  and second photomask  110   b  with apertures  115   b , for a suitable time and at a suitable wavelength and intensity, heat treatment of the glass structure may result in the glass structures of  FIG. 7B  or  FIG. 7D , for example. In such embodiments, optionally the UV radiation  120  may be focused through the first cladding layer  104   a  to result in a narrow column of UV exposure to the core layer  102 . 
     As illustrative of the embodiments in which the core photosensitivity greater than the first-clad photosensitivity of the first cladding layer  104   a , in the glass structure  100  of  FIG. 7B , the radiation exposure was not sufficient to enable crystallization of the core layer  102  to form core crystallized regions  135  extending all the way through the core layer  102 . In some embodiments, the core crystallized regions  135  may be removed by a suitable technique such as etching, for example, to form the glass structure  100  of  FIG. 7C  having removed core portions  145 . If etching is used, in some embodiments it may be necessary to provide a route for the etchant to reach the core crystallized regions  135 . In such embodiments, optionally an additional physical treatment such laser drilling may be conducted to form a hole or void in the cladding layers  104   a ,  104   b . In the glass structure  100  of  FIG. 7D , the radiation exposure provided to the glass structure  100  of  FIG. 7A  was sufficient to enable crystallization of the core layer  102  to form core crystallized regions  135  extending all the way through the core layer  102 . Thus, the removed core portions  145  extend through the entire depth of the core layer  102  from the first cladding layer  104   a  to the second cladding layer  104   b.    
     Referring now to  FIGS. 8A-8F , exemplary embodiments of methods for machining glass structures may incorporate multiple radiation exposures and/or heat treatments. to form complex shapes and through-holes. In the non-limiting illustrative embodiments of  FIGS. 8A-8F , the core photosensitivity is less than the first-clad photosensitivity and the second-clad photosensitivity. Preparation of the glass structure  100  of  FIG. 8C  by exposing the glass structure  100  of  FIG. 8A  to radiation, then heat treating to form the glass structure  100  of  FIG. 8B , then removing crystallized regions  130   a ,  130   b , has been described in detail above. According to some embodiments, the methods may include exposing the glass structure of  FIG. 8C  to radiation a second time, as shown in  FIG. 8D , such that the radiation traverses the hole structures  140   a ,  140   b  in the cladding layers  104   a ,  104   b  and penetrates only the core layer  102 . 
     In the photomasks  110   a ,  110   b  of  FIG. 8A , the apertures  115   a ,  115   b  are wider than the apertures  117   a ,  117   b  of the photomasks  112   a ,  112   b  of  FIG. 8D . When the exposure process of  FIG. 8D  is conducted, the UV radiation no longer enters the cladding layers  104   a ,  104   b , because the portions of the cladding layers  104   a ,  104   b  visible through the photomasks  112   a ,  112   b  have already been removed. Thereby, when the glass structure  100  of  FIG. 8D  is heat treated, the glass structure  100  of  FIG. 8E  may be formed. In the glass structure of  FIG. 8E , the cladding layers  104   a ,  104   b  contain hole structures  140   a ,  140   b , and the core layer  102  contains core crystallized regions  135   a ,  135   b  extending from hole structures  140   a ,  140   b  on opposite sides of the glass structure  100 . In some embodiments, the core crystallized regions  135   a ,  135   b  may be removed by a suitable technique such as etching, for example, to form the glass structure  100  of  FIG. 8F . In the glass structure  100  of  FIG. 8F , removed core portions  145   a ,  145   b  are through-holes or via holes that connect the hole structures  140   a ,  140   b  in the cladding layers  104   a ,  104   b  on opposite sides of the glass structure  100 . 
     Referring now to  FIGS. 9A and 9B , exemplary embodiments of methods for machining glass structures may further include surface treatments such as etching or surface roughening to the glass structures. Illustrative routes for preparing the glass structure  100  of  FIG. 9A  have been described above. In some embodiments, the glass structure  100  of  FIG. 9A  may be etched or roughened to produce the glass structure  100  of  FIG. 9B . In the glass structure  100  of  FIG. 9B , the cladding layers  104   a ,  104   b  contain roughened cladding surfaces  106   a ,  106   b . The hole structures  140   a ,  140   b  include roughened hole surfaces  142   a ,  142   b . Depending on an etching time, the etching treatment used, and the identity and concentration of the etchant, the hole structures  140   a ,  140   b  that existed only in the cladding layers  104   a ,  104   b  of the glass structure  100  of  FIG. 9A  may be made to extend to a desired depth into the core layer  102 , as in the glass structure  100  of  FIG. 9B . 
     Referring now to  FIGS. 10A-10E , in an illustrative embodiment, the methods described herein may be used to form a glass structure  100  into a fluidic component such as the tactile interface described in U.S. Pat. No. 8,179,375, incorporated herein by reference. In the embodiments of  FIGS. 10A-10E , however, the cladding layers  104   a ,  104   b  may be formed from a fast-etch glass that may be, but need not be, photosensitive or photomachinable. The fast-etch glass may be any glass composition that can be fusion-drawn with a photomachinable glass composition as described above, in particular with a photomachinable core layer. In some embodiments, the fast-etch glass composition may have an etch rate or solubility in an etchant such as hydrofluoric acid that is at least 1.5 times greater, at least 2 times greater, at least 5 times greater, at least 10 times greater, at least 20 times greater, or at least 100 times greater than the same characteristic of the photomachinable core layer. In some embodiments, suitable fast-etch glasses may include those described in U.S. Pat. No. 4,880,453, which is incorporated herein by reference in its entirety. Each of the fast-etch glasses of U.S. Pat. No. 4,880,453 is believed to have a photosensitivity of zero or nearly zero. Other exemplary fast-etch glass compositions suitable for use herein are listed in TABLE 2, of which some compositions may be photosensitive and/or photomachinable and others may be neither photosensitive nor photomachinable: 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Exemplary fast-etch glass compositions 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
               
               
                   
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
                 (Wt %) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 SiO2 
                 49.3 
                 46.2 
                 47.5 
                 45.6 
                 43.8 
                 42.0 
                 49.3 
                 49.3 
                 49.8 
                 50.3 
                 50.7 
                 51.2 
                 49.6 
               
               
                 Al2O3 
                 27.6 
                 25.8 
                 27.5 
                 27.5 
                 27.4 
                 27.4 
                 27.6 
                 27.6 
                 27.9 
                 28.1 
                 28.4 
                 28.7 
                 27.7 
               
               
                 B2O3 
                 5.0 
                 4.7 
                 7.0 
                 9.0 
                 10.9 
                 12.9 
                 5.0 
                 5.0 
                 5.1 
                 5.1 
                 5.2 
                 5.2 
                 5.0 
               
               
                 Li2O 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.9 
                 1.8 
                 2.7 
                 3.6 
                 0.0 
               
               
                 Na2O 
                 16.7 
                 18.6 
                 16.6 
                 16.6 
                 16.5 
                 16.5 
                 16.7 
                 16.7 
                 15.0 
                 13.3 
                 11.6 
                 9.9 
                 16.8 
               
               
                 K2O 
                 0.7 
                 0.6 
                 0.7 
                 0.7 
                 0.7 
                 0.7 
                 0.7 
                 0.7 
                 0.7 
                 0.7 
                 0.7 
                 0.7 
                 0.7 
               
               
                 SnO2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.0 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
               
               
                 ZrO2 
                 0.04 
                 0.03 
                 0.04 
                 0.04 
                 0.04 
                 0.04 
                 0.04 
                 0.04 
                 0.04 
                 0.04 
                 0.04 
                 0.04 
                 0 
               
               
                   
               
            
           
         
       
     
     The glass structure  100  of  FIG. 10A  includes a core layer  102  of a photomachinable glass composition interposed between a first cladding layer  104   a  and a second cladding layer  104   b . Both cladding layers  104   a ,  104   b  are formed from the fast-etch glass compositions described above and may be formed from the same material or different materials. Masking layers  113   a ,  113   b  may be applied to cover the cladding layers  104   a ,  104   b . In some embodiments, the masking layers  113   a ,  113   b  may be photomasks, whereby the cladding layers  104   a ,  104   b  may be entirely coated with the masking layers  113   a ,  113   b , the masking layers may be cured under radiation such as UV radiation, and portions of the masking layers  113   a ,  113   b  may be removed to expose portions of the cladding layers  104   a ,  104   b  to be etched. In other embodiments, the masking layers  113   a ,  113   b  may be selectively applied over portions of the cladding layers  104   a ,  104   b  not intended to be etched away. Regardless, portions of the cladding layers  104   a ,  104   b  not covered by the masking layers  113   a ,  113   b , such as those portions within apertures  115  of the masking layer  113   a , may be removed by a suitable technique such as laser drilling or wet or dry etching. The removal of the portions of the cladding layers  104   a ,  104   b  may result in the glass structure of  FIG. 10B , in which hole structures  140  are present in the first cladding layer  104   a  and the core layer  102  is exposed after a portion of the second cladding layer  104   b  has been removed. In the embodiment of  FIG. 10C , a photomask  112  having an aperture  117  may be placed over the first cladding layer  104   a , and the core layer  102  may be exposed to radiation such as UV radiation  120  through the aperture  117 . Heat treatment of the glass structure of  FIG. 10C  may produce the glass structure  100  of  FIG. 10D , in which a core crystallized region  135  has formed in the portion of the core layer  102  that has been exposed to the UV radiation  120 . The core layer  102  of  FIG. 10D  also includes core non-crystallized regions  155  in portions of the core layer  102  underneath the photomask  112  during the exposure to the UV radiation. 
     The core crystallized region  135  may be removed from the glass structure  100  of  FIG. 10D  by a suitable technique, such as the selective etching described above, to result in the glass structure of  FIG. 10E . The glass structure of  FIG. 10E  includes the hole structures  140  in the first cladding layer  104 , a hollow chamber  180 , and a fluidic channel  185 . When used as the tactile user interface described in U.S. Pat. No. 8,179,375, the hollow chamber  180  may be filled with a fluid such as water, for example and covered with an elastomeric sheet. The cavity may be designed to have two volumetric settings: a retracted volume setting and an extended volume setting. When the displacement device expands the cavity outward, a button-like shape is formed. With the button-like shape, the user will have tactile guidance when providing input to the touch-enabled electronic device. 
     Thus, methods have been described for machining glass structures that include fusion-drawn core-clad laminates that include at least one photomachinable layer. The methods described herein may be used in numerous ways to fabricate machined laminate structures that may be useful in optical and electronic applications, for example. 
     It should be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.