Santa Clara, CA - Cell phones continue to evolve at a rapid pace. In their early years, the leading trend was one of overall miniaturization-eventually reaching an end point determined by the size of the human hand. Today, it’s an issue of increased functionality and performance-higher-resolution screens, cameras, and enhanced Internet connectivity-all delivered while maintaining a miniaturized overall packaging. Lasers are enabling this technological evolution to a remarkable extent because they offer increased device performance, reduced manufacturing costs, increased manufacturing yields, and greener manufacturing-for example replacing wet photochemistry with more eco-friendly laser machining.
Lasers impact cell phone manufacturing in three main areas: the FPDs (flat panel displays), the PCBs (printed circuit boards), and the chips (both memory and logic). At present, most cell phones use LCD (liquid crystal) type displays, but OLEDs (organic light emitting diodes) are now appearing in these products for the first time. For OLEDs and certain high-performance LCD types, a fundamental process is silicon annealing with excimer lasers to produce LTPS (low temperature polysilicon). This process converts the amorphous silicon deposited on the glass panels into poly-crystalline silicon, which has superior electrical and photonics properties for brighter displays. It eliminates the need for high-temperature annealing and the requirement for expensive thermal glass.
Another display-related process is glass substrate cutting. Here a medium-power (up to 500W) CO2 laser is scanned across the glass closely followed by a cold air jet. The thermal shock cracks the glass without the need for mechanical processing.
Display manufacturers also are starting to adopt Q-switched diode-pumped lasers to pattern the transparent ITO (indium tin oxide) front-side electrodes in FPDs, eliminating the problems of traditional photo-etch processes. Earlier adopters are mainly involved with flat panel televisions, but manufacturers of smaller displays for cell phones are expected to follow.
Lasers enter the PCB fabrication process at several points, most prominently for microvia drilling. Vias are tiny holes drilled in multilayer PCBs, which, after metal plating, provide electrical connection between the different layers of circuitry. The choice of laser tool depends on the via dimensions. For via diameters above 50 microns, CO2 lasers are generally preferred, whereas solid-state ultraviolet (355 nm) lasers are used for via diameters smaller than 50 microns.
An emerging PCB application is LDI (laser direct imaging), where a quasi-CW ultraviolet laser beam writes the circuit pattern on a layer of photoresist, rather than using conventional lamps and photomasks. The main obvious benefits of LDI are the time and cost savings associated with the creation, use, handling, and storage of phototools. In addition, LDI avoids any quality problems associated with film related defects. The technique even enables unique marking or serialization of boards. In a related technique, excimer lasers are being used to directly pattern thin film flex circuits with high resolution in a fast reel-to-reel process and without the need for photochemistry. Initially, this was developed for disposable medical sensors, but is now starting to be used for flex antenna circuits and related products.
In terms of logic and memory chips, lasers are used extensively in both front-end (wafer fab) and back-end processing (singulation and packaging). Excimer lasers have long been used for microlithography, but there are also many other laser-based processes that support lithography. This includes the use of CW and pulsed ultraviolet lasers to create and repair photomasks, as well as for calibration of the optical systems used in lithography. Lasers are also used to inspect both the patterned and unpatterned wafers.
Die singulation (cutting the wafer into individual chips) is increasingly depending on lasers. This is because traditional methods cannot cope with the porous substrate materials now used in high-speed logic chips and the back-thinned silicon wafers used for stacked memory chips. Further along the chip fab process, package singulation is also using lasers to cope with round corners and other geometric intricacies for removable memory devices such as SD and NAND Flash.
And, of course, laser marking is also extensively used throughout cell phones, to identify the chip packages and PCBs, as well as for non-electrical components such as the plastic casing. These marking applications rely on low-cost Q-switched diode-pumped infrared lasers.
So next time you use your cell phone, be aware how big a role laser processing is playing in these ubiquitous consumer products. - Sri Venkat([email protected]) Coherent Inc., www.coherent.com
Eat your heart out Jay
Back in February I complained that the laser industry was losing out in the celebrity PR game. So I decided to do something about it. This month’s cover is a first for ILS-a celebrity photo. And we didn’t just pick some late night show host; we went with an international tennis star, Maria Sharapova. So tough luck Jay; you lost out to a much more photogenic celebrity. We are still open to other high profile endorsements so keep us in mind laser industry. - DAB