UV-Optics for Excimer Laser based Crystallization Processes
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UV-Optics for Excimer Laser based Crystallization Processes H.-J. Kahlert, Frank Simon and Berthold Burghardt MicroLas Lasersystem GmbH, Robert-Bosch-Breite 10, 37079 Göttingen, Germany Abstract Laser based crystallization of thin amorphous films on glass substrates have entered into industrial applications since several years. The excimer laser based process provides a low temperature procedure to obtain polycrystalline silicon films on flat panel display substrates to fabricate thin film transistors (TFT’s). The key to this application is a uniform illumination of the. Line Beam systems provide up to 365mm long homogeneous exposure fields operated with up to 300 W average power 308nm excimer lasers. The paper covers a technical overview of Line Beam Optics layout, recent developments and results. Further high resolution optics are described and discussed for sequential lateral solidification (SLS) (1,2,3) processes. The SLS application has demonstrated to efficiently produce directionally solidified microstructures or even grain-boundary –free regions on Si-films. Diffraction limited resolution in the range of several micrometers and high optical throughput are important parameters to this application. General considerations are presented to describe technical limits which compromise laser beam related coherence effects, optimum uniform illumination, adequate resolution and depth of focus and optical efficiency for the practical application. Introduction Excimer lasers emit at the following wavelengths dependent on the gas mixture: 351nm 308nm 248nm 193nm 157nm
XeF XeCl KrF ArF F2
The wavelength of interest for the recrystallization is 308nm. The state of the art high power LS 1000 excimer laser provides 1000mJ pulse energy @ 300Hz repetition rate (300W average power). Excimer Laser Based Recrystallization The recrystalization process is initiated by the absorption of 308nm laser light which causes melting of the a-Si film. A controlled “freezing” finally allows to obtain a crystalline film with properties adequate to the application of thin film transistors. The 20 to 30ns pulse length of the D6.2.1
laser pulse provides the substrate processing at room temperature. Thus normal glass can be used as a substrate for display applications. The crystallization control is provided by means of optical resolution. Both line beam and SLSapplications (sequential lateral solidification) take advantage of a homogenuous exposure, SLS further requires the resolution of an illumination pattern at 2-3µm feature size. The amorphous Si film typically reflects 56% of the incident 308nm laser beam. The rest is absorbed within a thin layer of 3-4nm (absorption coefficient α ~ 6 x 106 cm-1). Thus a 50nm film starts to melt at an energy density above 200 mJ/cm2. The typicall operating conditions are described in the following table Energy Density Exposure Line Beam Homogeneity SLS Exposure Field
300...400...600mJ/cm2 single pulse or multiple 2σ =30mm) for the SLS process. Acknowledgments The authors want to thank J. Im and co-worke
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