The cutting of glass has been done for centuries. The techniques developed many years ago are still in use today and remain fundamentally unchanged. Generally the cutting of glass 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, carbide, sapphire or zirconia.

The action of the scribing chips away creates tiny fragments of glass from the glass surface leaving a small groove in its wake. This groove creates a localized area of high stress in the glass. Because of these stresses, 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 diamond point diameter, for present day industrial diamond scribes, 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. 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.

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 with 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. Illustrated in FIG. 1 is an exemplary mechanical scribing process showing some of these problems.

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 will 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 unreliability 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 burning through the glass material. This method, thermal ablative cutting, can work but has several undesirable characteristics.

First, in conventional 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 uses up 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 geometry. This necessitates post-cut edge processing (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 are 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 melt-cutting (or evaporation) of the glass, in the heat effected zone at the margins of the cut, create fragility on the edges which greatly increase the propensity for edge damage. This randomized stress further complicates 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 ablative laser cutting and used in most applications.

In U. S. Patent No. 5,609,284 (Kondratenko), a technique was 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 OWCT"", 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 and then stress 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 glasses'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"and a pivotal vector being applied in the opposite direction along and underneath the"scribed line". The glass, with the propagated crack, can then be split clean, having none of the disadvantages of the 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.

Critical to the laser micro-crack propagation method is the cooling function, presently being accomplished by a water/cool air mist. The present invention provides an improvement over Kondratenko.

In accordance with the present invention, there is provided a system, i. e. method and apparatus, for cutting a brittle material comprising heating the brittle material along a cut path, and subsequently cooling the brittle material. Preferably the brittle material is heated along the cut path using a laser. The brittle material is cooled by a stream of a cooling medium directed at the surface of the brittle material, wherein the cooling medium comprises a super cooled gas. Preferably the super cooled gas comprises carbon dioxide, and more preferably the cooling medium comprises a solid powder phase of carbon dioxide, or a slurry comprising finely divided solid phase carbon dioxide.

Exemplary embodiments of the invention are set forth in the following description as shown in the drawings, wherein: FIG. 1 illustrates a conventional mechanical scribe method of cutting a brittle material.

FIG. 2 schematically illustrates an embodiment consistent with the present invention.

With reference to FIG. 2, an exemplary brittle material cutting process is illustrated in schematic representation. The brittle material 10, which may be, but is not limited to, mineral glass, silica, metal glasses, crystalline materials, and ceramics, is heated along a desired cut path 12 by the application of radiation, for example from a laser 14. The heating of the brittle material 10 along the cut pat 12 may be accomplished by movement of the laser 14 or tracing the laser beam across the surface of the brittle material 10 or by movement of the brittle material 10 relative to a stationary laser beam.

Preferably, the laser 14 contains appropriate optics to project an elongated, elliptical, or other appropriate heating field 16 on the surface of the brittle material 10.

The elongated heating field 16 allows optimum heating along the desired cut path 12 by preheating the brittle material with the less intense periphery of the laser heating field 16 and by increasing the heating duration along the cut path 12 for a given linear sweep speed.

Trailing along the cut path 12 behind the heating field 16 of the laser 14 is a cooling jet nozzle 18. The cooling jet nozzle 18 produces a chill jet 20 which impinges the surface of the brittle material 10 behind the laser heating field 16. The pressure and volume flow rate of the chill jet 20 is controlled by the nozzle 18 and by the chill jet control valve 24, wherein the cooling medium is supplied to the control valve 24 from a cooling medium supply (not illustrated). The resultant thermal stress caused by the heating action of the laser 14 and the subsequent cooling of the chill jet produces a very fine and accurate micro-crack 22 in the brittle material 10 along the cut path 12 in a plane perpendicular to the surface of the brittle material 10.

In establishing the necessary thermal stress in the brittle material 10, the cooling parameter is very critical, and therein the cooling medium used to achieve the cooling of the brittle material 10 is also very critical. The cooling medium must be capable of effecting a rapid heat transfer. In most cases this means that the cooling medium must be cold. For many applications, the cooling medium preferably is also anhydrous. For example, most specialized semiconductor applications require that there be no water present in the immediate atmosphere. Furthermore, practical considerations must also be considered, such as cost, toxicity, pollution concerns, availability, etc.

The preferred cooling medium for use with the present invention is carbon dioxide, although gaseous nitrogen (near its liquefaction point) or air (similarly cooled) is also effective. Carbon dioxide possesses several critical and unique characteristics. First it becomes a very cold gas when decompressed. Second it can exist as a solid powder or as a liquid or a solid powder suspended in a liquid; yet the solid will sublimate to a gas rapidly, absorbing large quantities of heat. It leaves no residue as it gasifies and will not coat or adhere to most surfaces including glass, vitreous silica, metal glasses, many crystalline materials, and most ceramic surfaces.

Third, CO2 is non-reactive at these temperatures and with these materials.

Of particular utility is the solid powder phase of the CO2. By balancing pressure and temperature it is possible to maintain the CO2 in its solid powder phase as it exits the nozzle 18. The solid powder phase C02 ils then propelled by pressure

onto the surface of the brittle material 10 where it mechanically scrubs the surface of the brittle material 10 as it is sublimating and cooling the brittle material, carrying contaminates with it as it is blown away. Since it is a solid, though finely divided, its mass can be substantial. Therefore, it can act as an effective mechanical cleaner.

And, since it is a finely divided powder it will impact upon and scrub out very small features on the surface of the brittle material 10. The powder imparts a"scrubbing" action to the area that it cools as it simultaneously sublimates; absorbing the local laser induced heat as it facilitates creation and propagation of the micro-crack.

The super cooled CO2 is an inherently efficient medium for effecting thermal transfer from the brittle material's surface to itself. Its efficiency allows for economical operation and since CO2 is a commonly used industrial material, it is readily available with good field support. However, other gases such as super cooled N2 or liquid air, either in gas or liquid phase, may also be used achieving some or all of the benefits realized with CO2. Other gases such as super cooled helium also may be used but is less desirable due to cost and handling issues. In all cases, with the use of an appropriate cooling medium, the separation of the brittle material 10 along the cut path 12 is found 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.