Silicon Annealing with Lambda STEEL Excimer Lasers
The high volume consumer market is driving the production of flat panel displays such as camera viewfinders, car navigation systems, laptops and TV monitors. The Active-Matrix Liquid Crystal Displays (AMLCD) are most common, using a matrix of Thin Film Transistor (TFT) switches to control each single pixel of the screen, as shown in Figure 1.

Excimer laser induced crystallization of amorphous silicon (a-Si) to low temperature poly-silicon is a pivotal technology for high performance TFT devices, offering excellent resolution and brightness, large angle of view, high pixel refresh rates, and the possibility of display driver circuitry integration on the panel for the next step in the ongoing miniaturization process.

Excimer Laser Annealing Process
The excimer laser light (308 nm) is very efficiently absorbed in an ultra thin amorphous silicon surface layer without heating the underlying substrate. A line-shaped and homogenized excimer laser beam is scanned across the surface. See Figure 2. Within the laser pulse duration (approx. 25 ns) the amorphous-silicon layer is rapidly heated and melted. As it cools down the re-crystallization into poly-silicon occurs.

The Excimer Laser Annealing (ELA) process is proving to be much more flexible and economical than previous techniques. Not only can the substrate sizes be easily changed, the selective annealing of designated areas can also be achieved. Due to no thermal affects on the underlying substrate, it becomes possible to use inexpensive glass substrates instead of quartz. This process allows displays of almost any size to be manufactured at reduced costs. The results are clear-thinner, lighter, brighter and more reliable displays.

Laser Requirements
The ELA process places stringent demands on laser performance since the properties of poly-silicon films are critically dependent upon the dose stability and homogeneity of the applied laser light. It is absolutely necessary to deliver a very precise amount of laser energy per pulse to the substrate, Figure 3.

Lambda Physik® developed a powerful (315W) generation of the Lambda STEEL especially for use in high volume industrial production environments. This laser is designed and optimized for high-energy stability to meet the specific requirements for industrial ELA. The Lambda STEEL design incorporates NovaTube® technology for outstanding gas and tube lifetime, a tube adjustment device, and a cleanroom compatible stainless steel cabinet. Pulse energy, as well as temporal and spatial laser beam distribution is optimally stabilized. The Lambda STEEL is highly reliable for large volume industrial production with minimum maintenance downtime and low operating costs.

Line Beam Optics
In addition to the laser, the most important part of a ELA tool is the optical system, forming and guiding the laser beam to the panel. MicroLas® , a product line of Lambda Physik, optical system comprising beam telescopes, homogenizers and high resolution imaging systems. The system shapes the excimer beam into a line beam with a length up to 465 mm and the flexibility to adjust the width from 0.1 to 0.5 mm. The beam profile at the substrate plane is a "top hat" intensity distribution in both directions with a long axis plateau uniformity of >98.75%, see Figure 4.

Scanning the line beam across the TFT panel produces uniform annealing right up to the edge of the panel. Since all of the optics are enclosed in a sealed and purged housing, the optics’ lifetime are increased (>10 billion pulses), significantly decreasing running costs.

SLS Optics
Conventional Line Beam ELA still has its limitations. It can not produce polycrystalline silicon layers with CMPS architecture compatible physical properties, higher field effect mobility in the poly-silicon layer is required, this an even more elaborate crystallization technique must be employed. Sequential Lateral Solidification (SLS), a technique developed by James S. Im from Columbia University, seemed appropriate and MicroLas developed an industry compatible optical system, which shapes and homoginizes a Lambda STEEL laser beam for stitching illumination of a 4 mm x 15 mm recrystallization area with a mask technique. The SLS process includes another advantage that should be of great interest for the industrial market, the substrate illumination time is significantly reduced. 

Conclusion
The excimer laser annealing systems developed by Lambda Physik and the Japan Steel Works are established for creation of LTPS at major display manufacturers. The systems have been proved to fulfill the most stringent design and manufacturing quality standards, to be reliable, user-friendly and economic for LTPS LCD manufacturing. Due to the remarkable progress in display technology over the last few years a continuous improvement and highly sophisticated development of the total annealing setup is required. With the aim of fulfilling today’s and future demands, Lambda Physik is continuously improving the laser and optics for that process. The recent result of investigations is a high power excimer laser and a line beam
optics system for wide beam width. This annealing system opens the door to achieve high quality and throughput for the next generation of large AM-LCDs and OLED displays. In addition, a new recrystallization technology, the sequential lateral solidification, has now become available for use on an industrial scale.

References
L. Herbst et al: “New Technology for Creation of LTPS with Excimer Laser Annealing”, IMID 2004 Proceedings, 17-4 (2004) J. Shida, “JSW FLX Series SLS Method ELA System”, presentation at The Display Search 2004 Taiwan FPD International Conference, (2004) D. Basting et al., “Excimer Laser Technology: laser sources, optics, systems and applications“, Lambda Physik AG, pp 125 – 134, (2001) I.-J. Chung et al: “Manufacturing Issues for Low Cost TFT-LCD”, IDMC 2002 Proceedings, pp 415 – 419, (2002) J. Shida et al., “Crystallization of a-Si Films by Excimer Laser Annealing Method”, Japan Steel Works Technical Review, (1997) James S. Im: „Sequential Lateral Solidification“, Phys. Stat. Sol. A 166, 1998, pp. 603