Laser Scoring Improves Glass

Laurel M. Sheppard

Three decades ago it was proposed that a CO₂ laser could be used to draw a crack in a sheet of glass to separate it into smaller pieces. Further developed in Russia, the technique was brought to Corning Inc. in 1989 to scale it up for manufacturing for such applications as liquid crystal displays.

The process Corning has developed uses a 10.6-µm laser and precisely controlled heating and cooling to thermally drive a vent across a sheet of glass. This involves putting a nick in the edge of the glass, heating the glass to just above the annealing point and chilling the surface with water. The cooling step puts the surface into tension so that the crack follows the tensile zone across the glass.

The resulting vent permits the mechanical separation of the sheet without creating the chips or microcracks that occur with conventional wheel scoring. Rectangular pieces can be cut using a cross-vent pattern with the laser method. Dimensional tolerances are excellent, and the laser-scored glass strength can be higher than that of mechanically scored glass.

Because the original speed (125 mm/s) was too slow for production, Corning has increased it by putting as much heat into the glass as possible. The scoring speed is a function of the beam width, length and depth, so Corning uses two cylindrical lenses perpendicular to each other to lengthen the beam into an elliptical shape. A dual laser mode is also used to distribute energy uniformly. By combining these methods, more heat penetrates deeper into the glass, and the speed increases to 700 mm/s.

The laser scoring method has been used on sheet glass that is 1.1 mm thick. The crack depth is typically 0.1 mm, and the effective heating zone is 3 to 4 mm. This method is applicable to other higher-expansion materials, including silicon, according to Harrie J. Stevens, manager of finishing research at Corning.