WO2001032571A1

WO2001032571A1 - Laser driven glass cut-initiation

Info

Publication number: WO2001032571A1
Application number: PCT/US2000/029279
Authority: WO
Inventors: Dmitri Nikitin; Thomas Michel
Original assignee: Ptg Precision Technology Center LLC
Current assignee: Ptg Precision Technology Center LLC
Priority date: 1999-11-01
Filing date: 2000-10-24
Publication date: 2001-05-10
Anticipated expiration: 2002-05-01
Also published as: AU1227501A

Abstract

A method for initiating a fracture of a brittle material along a micro-crack by thermally ablating the surface of the brittle material to form a notch in the brittle material at the desired starting point of fracture.

Description

LASER DRIVEN GLASS CUT-INITIATION The present invention relates to a method of cutting brittle materials. The cutting of glass has been done for centuries. The techniques developed many years ago are still in use today and remain fundamentally unchanged. The method generally consists of scribing a line, conforming to the shape desired, onto the surface to be cut with a material that is much harder than the glass itself, and then breaking the glass along the scribe line. The scribing material is typically made from diamond or zirconia. The scribing action chips away the surface and creates tiny fragments of glass from the glass surface leaving a small groove in the wake of the scribe. This groove creates a localized area of high stress in the glass. Because of this stress, the glass tends to fracture along this line when it is stressed beyond its strength threshold. Thus, to break a piece of glass, one first scribes it and then "bends" it until it breaks. The problem with this method is that the break line is somewhat unpredictable because when the scribe chips the glass flakes away, it does so in an unpredictable and irregular geometry. The best way to control the break line predictability is to make the scribe line as narrow and as deep as possible. There are, however, certain practical limitations as to just how narrow and deep the scribe line can be made. Some of these limitations are: scribe point diameter, scribe point geometry, scribing pressure, homogeneity of the glass substrate material and the velocity of the scribing. The practical limits of a diamond point diameter, for present day industrial diamond scribers, is in the range of { {.0015" (0.00381 cm) radius} }. Smaller size points can be made but increased wear factors and higher degrees of point fragility make their use infeasible or impractical. The larger point sizes, though more robust, create larger glass flake sizes and correspondingly, a larger stressed area and a shallower groove. This condition induces an unpredictable and more irregular break line. Scribing wheels have the same problems as stilii. The scribe point geometry also influences the break line qualities. As points wear they become faceted, i.e., flat spots are worn on the spherical diamond tip. These facets change the pressures applied to the glass as they mark it. This change in pressure (force) causes variations in the degree of scribing action that is applied to the glass, which in turn affects the uniformity of the stress field created and thereby influences the break characteristics, edge quality, etc. Scribing pressure variations are not easily controllable, even with machine automation, because of the "amorphous" nature of the glass and variations in the actual (microscopic) point of contact between the scribe point and the glass surface. Homogeneity of the glass material is critical to a clean conventional mechanical break because unless the scribing stresses are created evenly and in a symmetrical pattern, the glass will not fracture predictably. This will cause poor edge geometry and cut accuracy. Poor edge geometry results in fragile edges. Fragile edges limit the ability to safely handle the glass and restrict the use of certain processing steps and equipment. When a fragile edge is stressed (and there is no predictable stress threshold) it can cause the glass to develop a microscopic errant crack, which will grow larger with time. It is not possible to reliably predict how long the crack will take to sufficiently weaken the glass and induce failure. Thermal cycling and exposure to vibration accelerate the crack propagation but not at a predictable rate or along a predictable path. Each glass part has its own individual set of variables. This presents the worst of all possible scenarios, dealing with an unpredictable randomized failure mode. Changes in the scribing velocity, caused by variations of the glass surface (hills and valleys) will vary the effective applied scribing pressure, causing variations in the depth and width of the scribed line. This, then, impacts the repeatability and predictability of the glass break path and therefore the edge geometry, quality, fragility and accuracy. Some of these problems are illustrated in FIGS. 1 and 2, wherein the action of the mechanical scribe 10 moving across the surface of the glass produces airborne glass particles 12, and residual shards of glass 14 on the surface of the glass substrate. Further, the scribed groove 16 is illustrated, showing the ragged edges 18 resulting from the mechanical scribing action. Another disadvantage of scribing is that it creates volumes of tiny glass particles. Unless these particles are collected (adding more equipment and expense to the operation) they may find their way into the air and eventually onto a work surface, or more critically, a device surface. These tiny glass flakes are both abrasive and contaminating and may not be cleaned or controlled by conventional low cost means. Mechanical scribing has been the preferred method of glass cutting for centuries and it has also been the method for starting (initiating) a break at/on the edge of a glass substrate. Edge scribing, although common, is not the most reliable method of starting a break because of the above stated reasons. Edge starts, or cut- initiation, implemented by scribing has the same unpredictability and irregularity as the general scribing method does due to the same influences and limiting characteristics of the glass and the scribing implement. Recently lasers have been adapted to cut glass by thermally ablating through the glass material. This method can work but has several undesirable characteristics. First, in thermal ablative laser cutting, the glass is burned away or evaporated by the heat generated by the laser's beam. The material is severed, one part from the other. This process actually consumes material requiring dimensional parameters to be adjusted for cut losses. Second, the cut-edge of the glass is a melted edge. Melted edges have an unpredictable or irregular geometry. This necessitates post-cut edge processing such as by grinding to the required geometry with a diamond or zirconia abrasive wheel. Such processing is costly in both time and materials and because of vibration, caused by the grinding process, additional shear stresses may be imparted to the glass further increasing the risk of fracture or errant micro-crack formation. Third, heat induced stress, set up by the laser's thermal ablation (or evaporation) of the glass, in the heat effected zone at the margins of the cut, may create fragility on the edges which may greatly increase the propensity for edge damage. This randomized stress may further complicate the cutting process when these parts must be re-cut as part of another processing cycle or put through an edge finishing process. Because of these unpredictable characteristics, the scribe and break method of glass cutting is still preferred to thermal ablation laser cutting and used in most applications. In U.S. Patent # 5,609,284 (Kondratenko), a technique is disclosed that enables the splitting of glass with no debris or cutting waste. This new, laser based, glass (or other brittle material) cutting method, called Zero Width Cutting Technology 0WCT^®, does not rely on burning or melting the glass in order to cut it. The method, which relies on the thermo-physical properties of glass, uses a laser, in a controlled manner, to heat the glass to a specific temperature stressing it in a controlled manner, and then re-stressing it with a cooling jet. The method embodies the creation of controlled sub-surface stresses within the glass which are induced by precise laser heating (or other appropriate energy transfer method) and immediate controlled cooling with a water/cool air mist. The heat capacity of the water/air mist quickly removes the localized heat from the glass surface, which was caused by the laser, and thereby induces high tensile stresses deep in the glass body. These stresses overcome the molecular binding forces within the glass and result in the creation of a micro-crack within the molecular structure where the molecules bond, one-to-the- other, in the glass body. In other words, the heating and cooling creates stress that generates a micro-crack within the body of the glass with a controlled size (height) which is propagated through the body of the glass, in a plane normal to the glass surface and following the heat/chill path described by the translation of the laser beam/cooling jet across the glass surface which follows the outline of, and describes the shape of, the pattern to be cut from the glass. (The result of this process is roughly analogous to the conventional mechanical scribing process.) A bending moment is then applied to the glass, one vector being applied to either side of the "scribed line" on that surface and a pivotal vector being applied in the opposite direction on the opposite surface of the "scribed line". The glass, with the propagated crack, can then be split clean, having none of the disadvantages of the mechanical scribe and break process. In addition, there are many other advantages to this process like high-speed cutting and the ability to make complex geometric cuts. The glass neatly separates (breaks) following the laser-induced micro-crack that was propagated along and inside the body of the subject glass material with no kerf loss or generated particles. However, greater advances in cutting efficiency have been hindered by the continued necessity to use the archaic mechanical scribing techniques (described above) to initiate the cut. Even though the laser's thermal ablation "scribe and break line" is clean and repeatable, problems are still encountered during the cut-initiation process. Because cut-initiation is based on the diamond scribe technique, it brings to the process all of the problems of: glass surface contamination, uncontrolled stresses created in the glass, high variability in notch characteristics, and unpredictability of the break. In accordance with the present invention, there is provided a method for cutting a brittle material comprising forming a stress plane in the body of the brittle material by applying an energy beam along a cut path, followed by a cooling jet applied to the surface along the cut path to form a micro-crack in the material. The start or initiation point is done by thermal ablation of a tiny point on the surface of the brittle material, at the edge or within the field, at the start of the line to initiate the micro-crack fracture in the material. Preferably the surface of the brittle material is thermal ablated using a laser beam, that beam being of C0₂ or UV wavelengths, however it must be recognized that other appropriate or effective energy sources (beams) may be used. Further features and advantages of the present invention could be seen from the following detailed description of the invention, taken with the accompanying drawings wherein like numerals depict like parts, and wherein: FIG. 1 illustrates a conventional mechanical scribe method of cutting a brittle material. FIG 2 illustrates an enlarged end view of a mechanically scribed brittle substrate. FIG. 3 illustrates various placements of the laser ablated notch consistent with the present invention. FIG. 4 illustrates a comparison of the geometry of both mechanical crack initiation and laser crack initiation. FIG. 5 illustrates the ablated notch and the micro-crack separation plane. FIG. 6 illustrates the tensile forces generated within the glass when the micro- crack is formed.

Detailed Description of the Invention With reference to FIG. 6, a method for initiating the micro-crack in a brittle material 20 is illustrated. This is done by directing a C0₂ (or other laser) beam at the target while maintaining the spot size as small as possible. A micro-crack 24 corresponding to the desired cut line 22 is first formed in the body of the brittle material 20, for example, using a laser beam in accordance with the teachings of US Patent 5,609,284, the content of which is incorporated herein by reference. Thereafter, actual fracture of the brittle material 20 is by thermally or mechanically stressing a surface 28 of the brittle material 20, adjacent an edge thereof. The crack is initiated by the thermal ablation of a notch 26 disposed along the line of, but preceding the micro-crack 24, as is depicted in FIG. 5. The brittle material 20 to be cut may be, but is not limited to, mineral glasses, metal glasses, vitreous silica, ceramics, and crystalline materials. Preferably the micro-crack 24 formed within the body of the brittle material 20 is formed by directing a thermal energy beam along a cut path or cut line 22 on the surface of the brittle material 20 followed by a cooling jet to form a micro-crack 24 in a brittle material 20 whereby to heat the brittle material 20 along the desired cut line 22, and subsequently cool the brittle material 20 along the cut line 22. Preferably the brittle material 20 is heated by a laser containing appropriate optics to project a beam incident on the surface 28 of the brittle material 20. Also preferably, the brittle material 20 is cooled by a cooling jet traveling in the wake of the laser beam. The resultant thermal stress caused by the heating action of the laser beam and the subsequent cooling of the cooling jet produces a very fine and accurate micro-crack 24 along the path traced by the laser, i.e. the desired cut line 22, in a plane generally perpendicular to the surface 28 of the brittle material 20. In accordance with the present invention, a fracture initiation notch 26, illustrated in FIGS. 3, 4 and 6, is formed in the brittle material 20 adjacent an edge thereof disposed along the line of the micro-crack 28 by thermal ablating the surface 28 of the brittle material 20 adjacent its edge with an appropriately configured energy source. In a preferred embodiment, the energy source employed to thermal ablate the surface 28 of the brittle material 20 comprises a laser, and in an even more preferred embodiment the laser comprises a carbon dioxide laser, a NdYAG laser or a deep UV laser. The surface 28 of the brittle material preferably is ablated by the application of an incident laser energy beam focused to a controlled geometric shape so as to form an ablated region in the form of a notch 26. The fracture initiation notch 26 is ablated into the surface 28 of the brittle material 20 adjacent one edge of the brittle material and at the desired point of start of the fracture. That is to say, the ablated fracture initiation notch 26 is formed at the intersection of the edge boundary plane 30 and the surface plane 28, i.e. at a corner of an exterior surface of the brittle material 20. The combined effects of ablative reduction in cross-sectional thickness of the brittle material, thermally induced stress at the ablation site is enough to weaken the brittle material sufficiently for it to allow the laser scribing beam to easily form a micro-crack. The amount of stress produced and the concentration of the stress depends on the geometry and relative size of the notch relative to the brittle material being processed. If the notch 26 is formed with a rounded leading edge, relative to the desired cut line 22, and a rounded bottom, i.e. the surface defining potion of the notch 26 projecting into the body of the brittle material 20, the stress concentration will not be as great as if the leading edge and bottom are sharply angled. It will therefore be appreciated that the stress concentrating effect, and therefore the required initiation stress, may be controlled by altering the cross-sectional profile and aspect ratio of the notch 26. The preferred method involves a Guassian laser beam which results in a conical notch Guassian profile. As discussed above, the ablation of the surface 28 of the brittle material 20 creates a break in the surface of the brittle material and results in a reduction in cross- sectional thickness of the brittle material, (and the creation of localized high levels of stress), i.e. at the notch 26, much the same as if a notch 26 were mechanically scribed into the surface 28 of the brittle material 20. However, unlike a mechanically scribed notch, thermal ablating the surface 28 of the brittle material 20 with a controlled energy beam does not produce chips, airborne particles, or uncontrolled random stress razers. Therefore, contamination of the brittle material 20, or any optics or working surfaces, is greatly reduced, if not completely eliminated, and the breaking stress in the notch is predictable and controllable. In addition to the creation of a break in the surface of the brittle material and the reduction in the cross sectional thickness of the brittle material 20 resulting from the ablative loss due to the formation of the notch 26, the application of the controlled energy beam also induces thermal stress in the brittle material 20. The induced thermal stress creates a thermal stress zone 32 that produces an instability in the gross-molecular structure of the brittle material 20 in the immediate, and therefore controllable, area of the energy beam's incident foot print on the surface 28 of the brittle material 20. This thermal stress is converted to mechanical stress. The resultant mechanical stress assists the scribing beam, originating at the notch, to propagate the micro-crack through brittle material 20 along the scribing beam's path. That is to say, the combined effects of ablative reduction of the cross-sectional area of the brittle material 20 and the thermally induced mechanical stress in the material created by the cut initiation laser's coherent light wave front colliding with the target surface, is enough to weaken the brittle material 20 sufficiently to allow easy micro- crack initiation and sustained propagation by the scribing beam. Once the fracture (micro-crack) has been initiated, it rapidly propagates through the brittle material 20 as a running fracture or micro-crack 24. The resultant separation occurs along the fracture with no loss of material. Furthermore, the separation is found to be clean and smooth with no ripples or rough edges. While this invention has been disclosed and illustrated with reference to particular embodiments, the principles involved are susceptible for the use in numerous other embodiments. The invention is, therefore, not to be limited by the exemplary embodiments described in detail hereinabove, but only by the claims appended hereto.

Claims

What is claimed is: 1. A method for initiating the micro-crack cutting of a brittle material which comprises the steps in sequence of: (a) forming a stress point in said brittle material by applying an energy beam to a point at the location of the start of a desired cut path across said surface of said brittle material; (b) initiating fracture of said material by thermal ablating a portion of the surface of said material along said cut path; and ( c ) applying a scribing beam and a cooling jet to said surface of said material along said cut path downstream of and away from said energy beam initiation point. 2. A method according to claim 1, wherein said thermal ablating is performed using an energy beam from a laser. 3. A method according to claim 2, wherein said laser comprises a carbon dioxide laser. 4. A method according to claim 2, wherein said laser comprises a NdYAG, a deep UV laser, or other wavelength appropriate for the material to be cut. 5. A method according to claim 1, wherein said brittle material comprises glass, crystalline silica, ceramics, or other brittle material. 6. A method according to claim 1, wherein said material is thermal ablated portion is adjacent an edge of said material. 7. A method according to claim 1, wherein said location is in a central area of said brittle material. 8. A method according to claim 5, wherein said thermal ablating forms a notch. 9. A method according to claim 6, wherein said notch has a V-shaped cross-section. 10. A method according to claim 6, wherein said notch has a Guassian cross-section. 11. A method according to claim 6, wherein said notch has a modified Guassian cross-section resembling a cone or a stretched oval or a knife edge shape. AMENDED CLAIMS

[received by the International Bureau on 16 March 2001 (16.03.01 ); original claim 1 amended; remaining claims unchanged (1 page)] 1. A method for initiating the micro-crack cutting of a brittle material which comprises the steps in sequence of: (a) forming an initial stress point in said brittle material by applying a laser energy beam to a point at the location of the start of a desired cut path across said surface of said brittle material; (b) forming a fracture initiation notch at said location by thermal ablating a portion of the surface of said material along said cut path; and (c) applying a scribing beam and a cooling jet to said surface of said material along said cut path downstream of and away from said energy beam initiation point. 2. A method according to claim 1 , wherein said thermal ablating is performed using an energy beam from a laser. 3. A method according to claim 2, wherein said laser comprises a carbon dioxide laser. 4. A method according to claim 2, wherein said laser comprises a NdYAG, a deep UV laser, or other wavelength appropriate for the material to be cut. 5. A method according to claim 1 , wherein said brittle material comprises glass, crystalline silica, ceramics, or other brittle material. 6. A method according to claim 1, wherein said material is thermal ablated portion is adjacent an edge of said material. 7. A method according to claim 1 , wherein said location is in a central area of said brittle material. 8. A method according to claim 6, wherein said notch has a V-shaped cross-section. 9. A method according to claim 6, wherein said notch has a Guassian cross-section. 10. A method according to claim 6, wherein said notch has a modified Guassian cross-section resembling a cone or a stretched oval or a knife edge shape.