GMT: Laser Splitting Flat Panel Displays
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.
Laser Splitting vs. Laser Cutting
Laser splitting and laser cutting, although commonly confused, are two distinctly different processes. Laser splitting of glass necessitates the development of a sub-microscopic crack within the body of the glass by the controlled generation of stress. The stress is generated by precisely heating and then rapidly cooling the glass material. This processes causes internal stresses that are stronger than the molecular bonds of the SiO2 groups. The result of this stress generation is that a crack-a molecular level split-is formed in the body of the glass. The crack follows the laser heating/chill beam as it traces its programmed contour over the glass surface. The glass, still intact but internally broken, is then separated by application of a simple couple to the break line. This is done in the same way that conventional "scribed line" breaking is accomplished.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.
Applying Laser Splitting to FPDs
Until just a few years ago, the laser splitting technology could only be used to cut certain types of glass. To broaden its applications, an R&D and manufacturing center was established. Thousands of tests were conducted, and more than 20 additional patents were filed using Fonon's method of controlling the propagation of a Micro-CrackT (also known as a "blind crack") through the subsurface layer of the glass (see Figure 1).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 à 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.
Glass Processing Advances
The new machines feature a patent-pending laser cut initiation tool (LCIT)-a critical advancement in glass processing technology. It creates the break initiation stress defect in the glass edge by ablating a precisely controlled amount of material from the surface to create the defect. Unlike mechanical cut initiators, which break chunks of glass out of the surface and cause uncontrolled surface and sub-surface stresses, as well as particle debris, the LCIT is clean. The laser's ablative action creates an area of "engineered stress" in the glass that makes the break not only more predictable, but also easier to start.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.
Thermal Stress Generation
The carbon dioxide chill jet (patent pending), or CO2J, is another major advance in thermal stress generation for Micro-Crack creation. The conventional method relied on a compressed air jet containing atomized water (to improve heat uptake) or other exotic gases, which can be costly, in order to quickly remove the heat that was applied by the laser cutting beam as part of the process. These methods are generally sufficient in performing the task, but with moisture-sensitive components, the humidified compressed air is not efficacious.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.
Accurate Laser Scribing
The new machines also have the capability to operate with an accuracy and repeatability of I1.0 micron. For the FPD industry, this means that substrate splitting can be done as few as 5.0 microns from surface features. The ability to hold this level of accuracy permits manufacturers to use more glass real estate, save on material costs and fabricate more densely populated displays.Resistance to Heat Distortion
There has long been a need for a high temperature vacuum plate that would resist thermal distortion in high volume production lines. After numerous experiments on many alloys, it was discovered that titanium is the best all-around material for this component. Titanium is strong, very hard and can withstand very high temperatures without being subject to distortion and thermal degradation.New Structural Design
The new machines are designed in an "inverted L" shape to provide a more stable platform on which to mount the lasers. Their minimum cantilever design minimizes the moment of inertia of the optical head, preventing most resonant vibrations from affecting position. This means that the lasers will hold their positions much more accurately and are more immune to the effects of mechanical shock, vibration and displacement due to operator inattention. The "inverted L" also minimizes the footprint of the machines, allowing them to occupy less space on the production floor. Additionally, the lasers are mounted vertically, making them less likely to sustain damage.For More Information
For more information about the laser splitting machines, contact PTG-Precision Technology Group, 100 Technology Park, Lake Mary, FL 32746; (407) 804-1000; or fax (407) 804-1002.*The SBM Series, based on PTG's Zero Width Cutting Technology, or 0WCTR.