Abstract:
A method is disclosed for scribing a contained crack or vent in a chemically strengthened glass sheet. The glass has shallow surface regions under compressive stress, bounding a central region under tensile stress. The vent is formed by rapidly bulk-heating the glass, using radiation from a carbon monoxide laser, to a depth just below a surface compressive region and extending marginally into the tensile stress region then rapidly cooling the heated region with a water mist spray. The glass sheet can be subsequently divided along the vent by application of mechanical or thermal stress.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to laser-cutting of glass and other brittle materials. The invention relates in particular to laser-scribing of chemically strengthened glass. 
       DISCUSSION OF BACKGROUND ART 
       [0002]    Radiation from a laser has been used, and still is being used, for cutting various materials including dielectric materials in general. Laser cutting is now increasingly used as a replacement for mechanical cutting of large sheets of glass into smaller shapes for various applications. 
         [0003]    Several techniques have been developed over the past several years using various types of laser alone or in combination, i.e., with two or more lasers required for a particular process. The laser types used include long-wavelength infrared IR lasers, particularly carbon-dioxide (CO 2 ) gas-discharge lasers, near IR (NIR) solid-state pulsed lasers, and pulsed, solid-state lasers with wavelengths in the visible and near ultraviolet (UV) region of the spectrum. This latter type typically involves NIR pulsed lasers with frequency conversion using optically nonlinear crystals. 
         [0004]    The choice of a laser (or lasers) for the glass-cutting depends, inter alia, on the glass type and thickness of glass being cut, the cutting speed required, and the quality desired of a cut edge. Cost of the laser cutting equipment can also be a factor in deciding which type of laser and process to select. 
         [0005]    In recent years there has been developed a type of glass generally categorized as chemically strengthened glass. This type of glass is in increasing demand for covering display screens on hand-held electronic devices such as “smart phones” and tablet computers. 
         [0006]    The chemical strengthening is achieved by an ion-exchange process in which a sheet of conventional glass is immersed in a solution containing potassium (K) salts in a manner which causes potassium ions to replace sodium ions in outer surface regions of the glass. This places the outer surface regions of the sheet in compression, with an inside (center) region in tension. By way of example in a sheet of glass having a total thickness of about 0.5 millimeters (mm), the compressive surface regions have a depth of about 21 micrometers (μm). The surface-compression regions make the chemically strengthened glass particularly resistant to cracking or breakage due to mechanical stresses or impact. The compressive and tensile stresses are in a stable balance which provides the stress and impact resistance properties. 
         [0007]    The compression-tension stresses also make this glass difficult to cut as most processes developed for conventional glass can upset the stable balance between the compressive and tensile stresses to some degree creating unwanted cracking or breakage and generally resulting in an unacceptably low cutting-yield. A most common approach to avoiding the problem of cutting the chemically strengthened glass is to cut desired shapes from glass prior to strengthening and then immerse the cut parts in the solution for strengthening individual parts. This however requires means for handling a high volume of individual parts during the strengthening process. 
         [0008]    There is a need for a laser cutting method for chemically strengthened sheet-glass wherein the laser-radiation interaction with the chemically strengthened glass can be controlled in a manner which preserves the stable balance between the outer compressive stress and the inner tensile stress. The method should be applicable to cutting relatively large sheets of the chemically strengthened glass. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to a method and apparatus scribing a sheet of chemically strengthened glass for subsequent separation into one or more parts. The chemically strengthened glass sheet is characterized as having opposite surface zones under compressive stress, with a central zone between the surface zones being under tensile stress. In one aspect of the invention, the scribing method comprises directing a beam of laser-radiation from a carbon monoxide (CO) laser onto a surface to be scribed, and while translating the laser-radiation beam with respect to a surface of the chemically strengthened glass sheet, heating the glass sheet to a depth extending below the surface zone, and extending partially into the central zone. A coolant spray is directed onto the surface at a predetermined distance behind the laser-radiation beam in the translation-direction of the beam. This cools the heated surface-zone and creates a contained crack following the translation-direction of the laser-radiation beam. The crack has a depth extending below the surface zone and partially into the central zone. 
         [0010]    In one preferred embodiment of apparatus for carrying out the inventive scribing method, an inventive feature is provided for converting a Gaussian intensity distribution of the beam from the laser into a flat-top “(top-hat” or uniform) intensity distribution on the glass sheet. In another embodiment of apparatus for carrying out the inventive scribing method, an inventive device provides that the coolant spray is an annular spray surrounding the laser beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
           [0012]      FIG. 1  is a three-dimensional view schematically illustrating a preferred embodiment of apparatus in accordance with the present invention for laser scribing grooves or vents in a sheet of chemically strengthened glass prior to separating the sheet into individual parts defined by the vents, the apparatus including a CO laser delivering a beam of radiation, optics for focusing the beam on a sheet of glass being scribed, a water spray atomizer for cooling the sheet in the vicinity of the focused laser beam, and a translation stage for translating the sheet relative to the focused beam and the water spray for forming one of the vents. 
           [0013]      FIG. 1A  is a three-dimensional view schematically illustrating one form of a vent formed by the apparatus of  FIG. 1  in the chemically strengthened glass sheet, relative to compressive-stress and tensile-stress zones of the sheet. 
           [0014]      FIG. 1B  is a three-dimensional view schematically further detail of the vent depicted in  FIG. 1A . 
           [0015]      FIG. 2  schematically illustrates scribing a glass sheet in accordance with the present invention using six vents to define four panels for subsequent separation, with each vent initiated at a corresponding purposefully created defect in the sheet. 
           [0016]      FIG. 3  schematically illustrates coordinating movement of a round laser beam and the spray-cooling atomizer of  FIG. 1  for forming an arc-shaped vent. 
           [0017]      FIG. 4  schematically illustrates forming an arc-shaped vent using a round laser beam surrounded by an annular water-spray. 
           [0018]      FIG. 5  is a cross-section view schematically illustrating an atomizer in accordance with the present invention for forming the annular water spray of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates a preferred embodiment  10  of apparatus in accordance with the present invention for laser scribing grooves or vents in a sheet of chemically strengthened glass prior to separating the sheet into individual parts defined by the vents. The apparatus includes a carbon monoxide (CO) laser  12  preferably operating in a continuous wave (CW) mode. Such a laser is commercially available from Coherent Inc. of Santa Clara, Calif., an assignee of the present invention. This laser is energized (driven) by radio frequency (RF) power. The term “CW” as used herein for the CO laser incudes what might be designated quasi-CW operation resulting from the RF-driving, wherein the laser radiation is modulated at the frequency of the RF radiation. This frequency is typically about 100 megahertz (MHz). 
         [0020]    Laser radiation is delivered as a beam  16  via a window  14  in the laser. Beam  16  can include CO characteristic laser-wavelengths in a range between about 4 micrometers (μm) and about 6 μm. Beam  16 , on exiting window  14 , has an about round cross-section, and an about Gaussian intensity-distribution (indicated in the drawing by curve G). A more preferred cross-section for the beam has a more uniform intensity-distribution commonly referred as a “flat top” or “top-hat” intensity-distribution. 
         [0021]    A flat-top intensity distribution in apparatus (indicated in  FIG. 1  by curve F) is provided by a simple inventive arrangement  18 , comprising a controlled-atmosphere tube  17  having windows  19  sealed to the tube at each end. Air having a controlled relative humidity is flowed through tube  17  via an inlet  20  and an outlet  22 . Passage through the humid air in the tube is sufficient to change the Gaussian energy distribution G of the beam entering the tube to the flat-top distribution F of the beam exiting the tube. 
         [0022]    By way of example, in one example of arrangement  18 , tube  17  has a length of about 25 centimeters (cm) and a diameter of about 5 cm. An input beam had a 1/e 2  diameter of about 6 mm and a power of about 90W. Air having a relative humidity of about 35% was flowed through the tube at a rate of about 20 liters per minute (LPM). This was sufficient to provide the desired flat-top distribution. Clearly, several variables, such as tube-length, air-flow rate, and relative humidity are available to accommodate beams having different parameters. Further, a carrier gas other than air may be used while still staying within the bounds of the inventive intensity-distribution transformation arrangement. 
         [0023]    Regarding humidifying the air, this may be accomplished by any well-known means such as passing dry-air through a water bath. Relative humidity could be controlled by varying the air-flow rate through the water bath or the temperature of the water in the bath. As such humidifying means are well-known, a detailed description or depiction thereof is not presented herein. 
         [0024]    It is not clear what mechanism is responsible for causing the transformation of intensity-distribution. As there are a number of hydroxyl-radical (OH − ) absorption bands within the wavelength range of emission of a CO laser, one possible explanation, without being limited to a particular hypothesis, is that the transformation is due to an absorption induced thermal-lensing effect. Whatever the mechanism, it is important that the humidified air is flowing, at least for maintaining the effective relative-humidity level, and also for carrying away any heating of the gas resulting from the absorption. It should also be noted that the mechanism is not due to any wave-guiding action of tube  17 , as the dimensions of the tube are selected such that only free-space propagation of the beam can occur. 
         [0025]    In order for this beam transformation to be effective, and indeed for minimizing ambient atmospheric absorption or uncontrolled transformation of the laser-beam, it is most preferable that the remainder of the beam-path from the laser to the focusing lenses is enclosed, by one or a plurality of enclosures, indicated by long-dashed lines  27  for simplicity of illustration, and that the enclosure or enclosures are purged with a dried flowing gas, such as dry nitrogen. 
         [0026]    Continuing with reference to  FIG. 1 , beam  16  having the flat-topped intensity-distribution is steered by plane-minors  24  and  26  to a beam focusing lens comprising spherical positive lens elements  28  and  30 . These elements focus beam  16  into a round focal-spot  34  on a sheet  38  of chemically-strengthened glass being scribed. Glass-sheet  38  is supported on a translation stage  40 . Translating stage  40  in the direction indicated by arrow C causes focal-spot  34  to scan over sheet  38  linearly in the direction indicated by arrow D. The focal-spot may be more generally referred to as the beam-spot. This is because in certain operations, described in detail further hereinbelow, the beam may be focused slightly above the surface of the sheet to create a beam-spot on the sheet having dimensions bigger than the actual (or theoretical) focal-spot. 
         [0027]    Provision for longitudinal and transverse reciprocal motion of stage  40 , indicated by arrow L and T respectively, can provide for scribing curved vents. Scribing curved vents has particular requirements which are described in detail further hereinbelow. 
         [0028]    An atomizer  43  is fed by water-flow through a conduit  44 , and by air-flow through a conduit  45 . The atomizer sprays a water-mist  46  on the vent at some predetermined distance, for example about 2 mm, from the focal-spot. The cooling spray effectively “follows” the focal-spot over the sheet. The CO laser-radiation at the focal spot is absorbed in the bulk of glass and locally heats the glass to a depth determined by the beam and focal spot parameters. Near-immediate cooling by the water-mist spray rapidly cools the heated glass and causes a controlled crack or vent  42 . The vent is actually initiated at a purposely-created defect (not shown) in the glass surface. The scale of the drawing does not permit accurate representation of the created defect. A more detailed description of the defect creation and vent-formation process is provided further hereinbelow. 
         [0029]    Regarding focal-spot dimensions, while apparatus  10  is described with reference to a round focal-spot, it may be found preferable in some applications, for example for making an extended straight vent, to have a focal-spot extended in one direction. With this in mind, in apparatus  10 , spherical lens  30  is replaceable by a cylindrical (optical power in one transverse axis only) lens  32 . Here the power is in a direction at 90° to the vent direction. Lens  30  is moved out of the beam as indicated by arrow A. Lens  32  is moved into the beam as indicated by arrow B. 
         [0030]    In this example, cylindrical lens  32  has positive power only in the y-axis, which is the vent-width direction, as depicted. If the y-axis power of lens  32  is the same as the power of lens  30  an elongated (elliptical) focal-spot  36  (depicted in phantom) will result. Focal-spot  36  will have the same width (y-axis dimension) as focal-spot  34 , but will have a longer x-axis dimension due to the reduced optical power of the combination of lens elements  28  and  32 . 
         [0031]    It will be evident to those skilled in the optical art, without further detailed description or illustration, that lens  32  could have a y-axis power different from that of lens  30 . Further, lens  28  may be made replaceable by another lens to provide for a wider variation of focal-spot dimensions. A particularly elongated focal spot can be generated by replacing both spherical lenses with a single cylindrical lens having twice the y-axis power or two cylindrical lenses having the same Y-axis power as the two spherical lenses. These and any other variations of focusing optics with adjustable focal-spot shape may be practiced without departing from the spirit and scope of the present invention. Regarding means for replacing the elements, this could be accomplished by a simple translation-stage, or stages, not shown in  FIG. 1  for simplicity of illustration. 
         [0032]    The depth of vent  42  relative to the surface compressive zone of the chemically strengthened glass is very important. This is illustrated schematically in  FIG. 1A  and  FIG. 1B . In  FIG. 1A , the upper and lower surface compressive regions are designated  50 A and  50 B respectively. Region  52  is the central tensile region between the compressive regions. In  FIG. 1B  the thickness of sheet  38  is designated t; the depth of the compressive regions is designated d c ; the thickness of the central tensile region is designated t t  and the depth of vent  42  is designated d v . The depth can be conveniently controlled by controlling beam power focal-spot size, intensity distribution; the speed which the focal-spot is scanned over the chemically strengthened glass sheet; and the distance of the water-mist spray from the focal-spot. 
         [0033]    In empirical terms, the vent must have a depth (d v ) sufficient to extend through compressive region  50 A into tensile region  52 , but not so far into the tensile region that instability of the stress balance causes a self-propagating crack to develop. With vents of the correct depth, a sheet, such as sheet  38 , can be scribed with a plurality of vents  42  to define a plurality of parts of a desired shape. The parts can then be separated by applying mechanic force to the vents, or introducing local thermal stress in the vents through heating the surface with a surface-absorbed laser beam, such as a CO 2  laser beam having a wavelength of about 10.6 μm. 
         [0034]    In absolute terms, by way of example, in a sheet of chemically strengthened CT-30 glass (available from Corning Inc, of Corning, N.Y.) having a thickness of about 0.55 mm compressive regions  50 A and  50 B have depth d c  of about 21 μm. The vent-depth dv should preferably be greater than 80 μm but preferably not greater than about 110 μm. A detailed description of scribing a CT-30 glass sheet for separating the sheet into multiple parts is set forth below with reference to  FIG. 2 . 
         [0035]    Here, a sheet  38  of CT-30 glass is scribed for later separation into four panels by vents  42 A,  42 B,  42 C,  42 D,  42 E, and  42 F. All of the vents are formed by the same process. Accordingly the description of the process is provided only for one of the vents, arbitrarily selected as vent  42 B. 
         [0036]    The first stage of the process is creating a line-defect (linear defect)  60  from which the vent is initiated. Line defect  60  is preferably formed in a direction transverse (perpendicular) to the direction in which the vent will be formed. In theory such a defect could be created by a diamond scribe or the like but the highly compressively stressed surface region of the chemically strengthened glass makes for relatively poor control of the defect depth and a propensity for self-propagating surface cracking. In this preferred method, the defect itself is laser-propagated from a directly-laser-damaged area  62 . 
         [0037]    The direct damage is formed by focusing the laser into a round spot quenched by the water-mist as described above. Beam parameters are selected such that that the damage is in the form of surface and sub-surface cracking, which is self-contained, i.e., not self-propagating. 
         [0038]    In one example of creating the direct-damage region, the laser beam, with a nominal wavelength of 5 μm had the flat-top intensity distribution discussed above, a diameter of about 12 mm as measured by burning a hole in a burn-card, and was focused on the surface of the sheet in a round spot having a diameter of 100 microns. The beam power was 80 Watts (W). The water-mist from the atomizer was directed at a distance about 2 mm behind the round beam, with water-flow of 3 milliliters per minute (mL/min), and air-flow of 1.2 liters per minute (L/min). The water temperature was about 15° C. The scan speed of the laser beam (and water-mist spray) was about 250 mm per second (mm/sec). The length of the direct-damage region was about 6 mm. 
         [0039]    Line defect  60  was created by propagating laser damage from the direct damage region  62  by scanning the laser beam and water-mist. The propagated damage is created by a similar process to the vent creation, i.e, by thermal-shock conditions resulting from the bulk laser-heating followed by the water-mist cooling. As in the vent formation, it is desirable that the depth of the propagated damage created does not extend into the central tensile region of the glass sufficient to cause one or more self-propagating cracks. 
         [0040]    In one example of creating defect  60  by propagating damage from direct damage region  62 , the laser beam, again with a nominal wavelength of 5 μm, had the flat-top intensity distribution discussed above. A cylindrical lens was arranged to focus the beam at a height of about 2 mm above the glass sheet with an elongated spot on the sheet having a width of about 0.4 mm and a length of about 12 mm as measured by the burn-card. The beam power was 80 Watts (W). The water-mist from the atomizer was directed at a distance about 2 mm behind the elongated beam, and the water-mist parameters were as described above. The scan-speed of the laser beam (and water-mist spray) was about 200 mm per second (mm/sec). The length of defect  60  was about 20 mm. The length of the beam spot was in the scan direction 
         [0041]    In one example of creating vent  42 B from defect  60 , the 5 μm-wavelength laser beam had the flat-top intensity distribution discussed above. The cylindrical lens was arranged to focus the beam at a height of about 5 mm above the glass sheet creating a beam with an elongated spot on the sheet having a width of about 0.8 mm and a length of about 12 mm, as measured by the burn-card. The beam power and water-mist parameters were as described above. The vent-scan was initiated with the beam behind the defect. The scan-speed of the laser beam (and water-mist spray) was about 180 mm per second (mm/sec), with the length of the elongated beam-spot in the scan direction. In most cases, the depth of the vent (contained crack) is about the same as the depth of the defect. A particular benefit of the inventive vent-formation (contained-crack formation) method is that it permits one vent to cross an existing vent without introducing any damage at the crossing point. 
         [0042]    It cannot be overemphasized how important the use of the high power CO laser having wavelengths in the region between about 4 micrometers (μm) and about 6 μm is to the inventive chemically strengthened glass scribing method. Applicants were motivated to develop the inventive method after many unsuccessful attempts to scribe or cut the chemically strengthened glass with 10.6 μm-wavelength CO 2  laser radiation. Absorption of glass for 10.6 μm-wavelength radiation is extremely high, so that there is essentially no penetration into the bulk that would create the vent formation method of the present invention. The glass absorption coefficients in the wavelength region between about 4 micrometers (μm) and about 6 μm allow for heating the glass, in bulk, to the controlled depth needed to extend below the surface compression region with minimal penetration into the central region, thereby maintaining the glass in a stable state. At the shorter (1 μm and less) wavelengths of solid-state lasers, glass absorption is almost zero. Even with sufficient power to cause heating, this heating would occur throughout the glass thickness and could not be effected at the controlled depth required of the inventive method. 
         [0043]    In the example provided above, an elongated beam is used for generating vents  42 A-F. While the elongated beam is convenient for forming straight vents, it is not practical for forming a curved vent, as would be necessary for scribing a rounded rectangle shape. This is because alignment of the elongated beam and the water would mean that the beam was only in tangential contact with an arc to be scribed, and the beam and the water spray could not both be constantly on the arc. 
         [0044]    At least one popular smart-phone brand has a cover glass in rounded rectangular form, so there is a motivation to adapt the inventive vent-scribing process to scribing arcs. This is facilitated by the fact that a round beam can be used to form a vent avoiding the tangential problem of the elongated beam. It is then only necessary to provide that the round beam and the water spray are always on the arc. 
         [0045]    One way of coordinating a round beam with the water spray for arc-scribing is depicted very schematically in  FIG. 3 . Here, an arrowhead tipped arcuate path depicts a vent-shape to be produced. An open circle represents a laser beam at various points on the arc. This could be effected by two-axis (x-y) movement of translation stage  40  in the apparatus of  FIG. 1 . The water-mist spray is depicted by a shaded circle. As the laser beam proceeds around the arcuate path, the water spray (the atomizer of  FIG. 1 ) is rotated around the laser beam, as indicated by arrow R, so that both the laser-beam and the water-mist spray remain on the arcuate path for forming a corresponding arcuate vent. While this method is certainly effective, a requirement for providing independent but coordinated motion of the atomizer relative to the translation stage adds considerable complexity to the apparatus of  FIG. 1  by. This complexity would increase if the apparatus were required to scribe arcuate-shaped vents of different curvature or even arbitrarily curved vents. 
         [0046]    An alternative method to providing arcuate shaped vents is schematically depicted in  FIG. 4 . Here the water-mist spray has an annular shape (shaded annulus) disposed, preferably concentrically, around the laser beam (open circle) in a fixed spatial relationship thereto. A gas, such as nitrogen flowing coaxially around the laser beam and exiting through the same orifice as the beam provides positive pressure and prevents the water mist from impinging onto the glass in the region of laser heating. A minor drawback of this method is that water mist is deposited ahead of the laser beam and the arcuate path. However, as this is deposited on not-yet-heated glass, no thermal shock results, and the deposited water is kept from interacting with the laser beam by the coaxial gas-pressure. A significant advantage is that this method can be implemented in the apparatus of  FIG. 1  merely by changing the conventional atomizer by a specially configured atomizer. A detailed description of one such atomizer is set forth below with reference to  FIG. 5 . 
         [0047]    Here an atomizer  70  in accordance with the present invention includes a central conduit  72  for air-flow and beam-transmission. Conduit  72  is in fluid communication with an air plenum chamber  74 , which is enclosed at one end thereof by a window  76 , transparent to the CO laser-radiation wavelengths. The laser beam is introduced into the atomizer through this window as indicated in the drawing. Air is supplied to the plenum chamber via an aperture  78  therein. Conduit  72  has a tapered region  72 A at the distal end thereof to force air-flow through the conduit into a jet. 
         [0048]    Surrounding central conduit  72  is a water-sleeve  80 . Sleeve  80  is sealed to conduit  72  near the proximal (input) end thereof and is open at the distal end. Sleeve  80  is configured such that a cylindrical conduit is formed around the central conduit leaving an annular aperture  82 A at the open end of the cylindrical conduit. The sleeve is tapered corresponding to the taper of the central conduit such that the cylindrical conduit is also correspondingly tapered. Water is input into the cylindrical conduit through input apertures  84 A and  84 B of sleeve  80  and exits the cylindrical conduit via aperture  82 A thereof. The juxtaposition of the water from aperture  82 A with the airflow from end  72 A of the central conduit creates the annular water mist spray of  FIG. 4 . 
         [0049]    Surrounding water sleeve  80  is a vacuum sleeve  92 . Vacuum sleeve  92  is sealed to water sleeve  80  at a proximal end thereof and open at a distal end thereof. 
         [0050]    Sleeve  90  is tapered in conformance with the tapering of central conduit  72  and water-sleeve  80  and forms a correspondingly tapered vacuum conduit  92  surrounding water sleeve  80 . 
         [0051]    Conduit  92  has an annular opening  92 A surrounding annular water outlet  82 A of sleeve  80 . A vacuum pump (exhaust pump) communicates with conduit  92  via apertures  94 A and  94 B in vacuum sleeve  90 . This provides for a vacuum or suction effect in an annular region surrounding the annular water-mist spray. One function of this vacuum or suction effect is to inhibit the incursion of the water mist spray into the region of the beam, reinforced by the air jet from the central conduit. Another function is to remove any steam resulting from the interaction of the laser beam with pre-sprayed regions (by the annular spray) of a sheet being scribed. 
         [0052]    The present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather the invention is limited only by the claims appended hereto.