Electric heating – Metal heating – By arc
Reexamination Certificate
2002-08-27
2004-06-01
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121720
Reexamination Certificate
active
06744009
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for cutting sheets of brittle materials into desired configurations or shapes utilizing a combined laser-scribing and laser-breaking technique. The present invention has particular applicability in cutting or separating brittle, non-magnetic sheets along curvilinear paths to produce substrates for use in the manufacture of magnetic and/or magneto-optical (MO) recording media.
BACKGROUND OF THE INVENTION
Two techniques are conventionally employed for cutting or shaping a sheet of brittle material, such as a glass, amorphous glass, glass-ceramic or ceramic material, to form a sheet or substrate with a desired configuration or geometry. A first such conventional method involves mechanical scribing of the sheet employing a hard device, such as a diamond tip, to score the surface of the brittle material, which is then broken along the score line or pattern. The second of such conventional techniques involves laser-scribing. Currently employed laser-scribing differs from traditional high power (i.e., >1 KW) laser-drilling/cutting and utilizes a lower power (i.e., <500 W) for achieving scribing with less material removal and better edge quality subsequent to breaking/separation. Such laser-scribing typically utilizes a continuous wave (“CW”) laser, such as a CO
2
laser of 10.6 &mgr;m wavelength, to heat a localized zone of a brittle material, such as an amorphous-glass sheet (similar to float glass), up to a temperature below the softening point of the material, and then immediately quenching the heated zone by applying a fluid coolant, e.g., a gas, such as air, a liquid, such as water, or a combination of a gas and a liquid, such as air/water.
In a typical process for laser-scribing an amorphous glass sheet, the output beam of a 10.6 &mgr;m CW CO
2
laser, or a high frequency pulse repetition rate 10.6 &mgr;m CO
2
laser, is re-shaped into a beam with an elongated spot shape, which beam is utilized in an unfocussed manner for locally heating the glass. The locally heated zone is then chilled by spraying thereon cool air or an air/liquid (e.g., air/water) mixture. When the localized heating/cooling process starts from a small surface defect or micro-crack made in the glass, e.g., by a means of a mechanical scriber or indenter, or by application of suitable laser pulses, the defect or micro-crack propagates to form a scribing line due to the combination of localized heating-quenching which initiates tiny surface cracks arising from compression-tension stress effects. The sheet of material is then separated, i.e., broken, along the scribing line by applying an external thermal or mechanical stress.
A conventional laser-scribing technique utilizing a low power CO
2
laser is disclosed by Kondratenko in U.S. Pat. No. 5,609,284, wherein an elliptical target area is impinged with a beam of coherent radiation along the intended direction of the crack, while a stream of fluid coolant is directed at a point on the heated surface on the intended line of the crack. U.S. Pat. No. 6,259,058 B1 to Hoekstra discloses a modification of U.S. Pat. No. 5,609,284 wherein dual laser beams are utilized after cooling in order to assist separation along the laser-scribing line. Allaire et al. in U.S. Pat. No. 5,776,220 disclose a laser-scribing technique for brittle materials wherein the laser spot has an extremely elongated elliptical shape such that its major axis is greater than 20 mm to enable rapid scribing.
Conventional substrates for use in manufacturing magnetic recording media include various brittle materials, such as glasses, ceramics and glass-ceramics. In order to form annular disk-shaped substrates suitable for use in magnetic and/or magneto-optical (MO) recording media, two circular scribings must be performed with high precision, one defining the outer diameter (e.g., ranging from about 65 to about 95 mm, such as 84 mm) and one defining the inner diameter (e.g., ranging from about 20 to about 25 mm). However, applicability of current linear laser-scribing techniques, such as utilized with flat panels, to circular scribing for producing annularly-shaped substrates suitable for manufacture of disk-shaped magnetic and/or magneto-optical recording media, is limited, for at least the following reason: laser-scribing is very sensitive to variations of the glass material, including optical reflectivity of the surface, glass composition, surface and thickness uniformity, etc., resulting in that the CO
2
laser-based scribing process requires very precise control of defect initialization, laser power distribution, and cooling stream. As a consequence, current laser-scribing technology of amorphous glass substrates is generally restricted to linear scribing.
Another drawback/disadvantage of conventional laser-scribing technology is associated with the methodology for separating/breaking the brittle substrate (e.g., of amorphous glass) subsequent to laser-scribing. Specifically, because of the nature of the localized heating/cooling of the laser-scribing process, and due to the formation of a compression layer on the surface of the amorphous glass sheet, the propagation of micro-cracks during the laser-scribing process occurs in the layer nearest the glass surface. As a consequence, the scribe line provided by a single laser beam at the surface of a glass surface is insufficiently deep, and application of additional mechanical force to the glass sheet is typically required during the laser-scribing process or subsequent thereto, disadvantageously resulting in edge defects, residual stresses, increased risk of cracking resulting in product loss (i.e., low yield), reduced product throughput, and poor cost-effectiveness arising from a requirement for complicated, thus expensive, processing.
More specifically,
FIG. 1
shows an example of a linear laser-scribing process performed on a glass sheet for separating the latter into two segments, wherein a stationary, elongated, elliptically-shaped CO
2
laser beam and a H
2
O/air cooling spray are successively supplied to a moving glass sheet along a substantially straight line of defect initialization markers (indicated by dark circles in the drawing) previously formed in the surface of the sheet, as by mechanical scribing/indentation or pulse laser irradiation, to form a laser scribe line (indicated by the dashed line in the drawing) within the glass sheet.
FIG. 2
illustrates an example of a similar, curvilinear laser-scribing process performed on a square or rectangular glass sheet for shaping the latter into an annular disk-shaped substrate, e.g., for use in the manufacture of disk-shaped, thin film magnetic and/or magneto-optical (MO) recording media. In this instance, the sheet is preliminarily provided with defect initialization markers (again indicated by dark circles in the drawing) arranged in a pair of concentric circles having diameters generally corresponding to the desired inner diameter (ID) and outer diameter (OD) of the medium, and proportionately-sized, arcuately extending, elliptically-shaped, stationary CO
2
laser-scribing beams, along with a stationary H
2
O/air cooling spray, are utilized for scribing each of the inner and outer diameters along the corresponding circular lines of defect initialization markers while the sheet is rotated about a central axis to effect relative movement between the sheet and each of the laser-scribing beam +H
2
O/air spray combinations.
However, as indicated supra, because the thus-described conventional laser-scribing process: (1) involves localized heating/cooling, and (2) formation of compression layer on an amorphous glass surface, micro-crack formation occurs essentially only within the compression layer adjacent the glass surface. As a consequence, the laser scribing line which is formed is insufficiently deep, as shown in
FIG. 3
, and a post-scribing thermal-breaking step (refer to
FIG. 4
) is typically required to effect separation along the scribing line, in which step a portion of the sheet on one side of the scribing
Shih Chung-Yuang
Xuan Jialuo Jack
Elve M. Alexandra
McDermott & Will & Emery
Seagate Technology LLC
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