The technology has been proven especially viable in the field of flat panel display (FPD), where the quality and speed of cut are becoming increasingly important as the industry battles to gain market share from cathode ray tubes.
Because of the controlled application of a precise amount of heat to the glass, the residual thermal stresses present in the material are negligible. Also, because there is no micro-mechanical damage to the material's surface, as there is with mechanical scribing, the residual, mechanically induced internal stresses are nonexistent. The geometry of the crack produces a perfectly smooth profile with no loss of material (also called "kerf"). The cleaved surface is so smooth that it is many times stronger than a sawed or scribed edge. It is also perfectly square and regular. This results in the elimination of the necessity for stress control seaming and grinding and/or heat treating (normalizing) for a majority of applications. These benefits result in the reduction of many process steps in many types of glass products, including FPDs.
Laser cutting of glass is, on the other hand, quite different. Laser cutting ablates or evaporates glass material from the parts to be separated. Material is consumed as the cut line is burned or evaporated away. Besides the physical loss of material, ablative cutting puts a tremendous amount of heat into the glass. This heating results in high internal residual thermal stress and unpredictable fracture failures over time. Because of the unpredictability of failure, parts created by this process need to be annealed, or "thermally stabilized." This is a costly and time-consuming process. Edge and cut plane surface geometry of thermally ablated cut glass is also not optimum. The corners are rounded due to the evaporation of the surface and the plane faces are "glassy" but not smooth.
This method is based on the equation d = fk (P/Ã), where d is the depth of the crack, Ã is the relative displacement speed of the laser beam, P is the power density and T is the glass substrate thickness. d is a function of P/Ã. This means that as the laser power increases, the depth of the Micro-Crack increases. In comparison, decreasing the speed also increases the depth of the Micro-Crack. A "full body cut" is achieved when d > T.
Unfortunately, due to the physical properties of glass, such as energy absorption, thermal conductivity, Young's Modulus and the melting point, d is typically limited to 0.4-0.7 mm. A further decrease of Ã converts the process into conventional thermal breaking.
Laser splitting technology research has been focused on perfecting the method of increasing the depth of the Micro-Crack without losing cutting speed and accuracy. At the same time, FPD began moving toward 0.4-0.5 mm thicknesses-well within the d range. To capitalize on these developments, a new generation of machines* was introduced that enables the laser splitting technology to be used to successfully cut FPDs. These machines feature advances in glass processing technology, thermal stress generation, laser scribing, thermal distortion resistance and laser mounting technology.
Several parameters are controllable on the LCIT that do not exist with mechanical cut initiation tools. Mechanical tools wear with use, their points become blunt, and the geometry of the defect they create changes. This results in deteriorating defect uniformity, which leads to poor and unpredictable splitting. The LCIT never changes geometry, on its own, through wear or damage. It remains completely the same 100% of the time. Should the defect geometry need to be adjusted for some special purpose, this can be achieved by changing the lens characteristics to create the required defect shape. The defect geometry cannot be changed with mechanical systems with any degree of accuracy or reliability.
Mechanical cut initiation tools rely on mechanisms to meter the force with which the tool impacts the glass surface in order to create the defect. Impact pressure and tool point geometry are the primary determinants of defect quality. It is almost impossible to implement a system of measurement and feed back control that can compensate for wear, aging and non-linearities inherent with these mechanisms. The LCIT, on the other hand, is a non-contact method. The energy delivered to the glass to create the defect is a focused beam of heat. It is constant, consistent and easy to calibrate to deliver a precise amount of energy to a precise point on the glass and create a precise defect, time and time again.
Alternatively, other appropriate cooling gases are relatively expensive consumables and may not be available in the required quantities on the factory floor. CO2, however, is a common industrial gas and is available in the manifold as part of the standard gases used in high-tech manufacturing. Generally these include CDA (clean dry air), vacuum, chilled water and CO2. CO2's heat uptake is excellent for this process type and it is relatively inert at processing temperatures. In addition, it contains no water and therefore will not contaminate moisture-sensitive components.