Crystallization of thin Si-films using excimer lasers is a well introduced technique to the manufacturing process of flat panel display poly-Si back plates. The crystallization is performed by a Line Beam exposure which is scanned with a typical overlay of 95% over the Si-coated glass plate. The poly-Si film is obtained with a very good homogeneity of typically 100-150 cm2/Vs electron mobility. Thus this performance is of importance for the system on glass (SOG) driver microelectronic circuits and is a key technology for high resolution (200ppi) LCD displays for mobile phones, personal digital assistance (PDA) and for the near future OLEDs (organic light emitting display).
A recently considered process to obtain even improved performance poly-Si films is the sequential lateral solidification proposed by J. Im of Columbia University. This process requires a high resolution lines and spaces pattern which is designed to grow linear crystals or even single crystal like areas. High resolution optics for high power throughput is the relevant technique to introduce the SLS-process (sequential lateral solidification) into next generation production lines.
Industrial production of low temperature p-Si back plates for LCDs by high power excimer laser annealing was introduced several years ago. Regarding the economy of the process, one of the major advantages of excimer laser annealing is the opportunity to make use of low cost glass substrates due to the low temperature of the annealing process. The Lambda Physik high power excimer laser series are operated with the MicroLas 370 mm line beam optics, integrated by Japan Steel Works into industrial systems. The MicroLas line beam optics for conventional excimer laser annealing (ELA) process converts the raw laser beam profile into a stable and homogeneous rectangular illumination field with high aspect ratio. The excimer laser light source, the LAMBDA STEEL 1000, delivers stabilized pulse energies up to 1 Joule at repetition rates up to 300Hz. The crystallization using excimer lasers allows to produce films with electron mobility of 100-150 cm2/Vsec with the Line beam technique. The new SLS-method, which is currently under industrial investigation, even allows to obtain electron mobility between 200-400 cm2/Vsec.
The most recent development in industrial fabrication processes of low temperature poly-silicon by means of excimer laser and optics system is presented: . The recently developed high performance excimer laser, which delivers the highest output power currently commercially available in combination with two different beam shaping methods, the commonly known ELA-technique and the new SLS-technique for enhanced Field effect mobiliiy in the LTPS, is demonstrated.
The productive and accurate ablation of microstructures demands the precise imaging of a mask pattern onto the substrate under work. The job can be done with high performance wide field lenses as a key component of ablation equipment. The image field has dimensions of 20 to 30 mm. Typical dimensions and accuracy of the microstructures are in the order of some microns. On the other hand, the working depth of focus (DOF) has to be in the order of some 10 microns to be successful on drilling through 20 to 50 μm substrates. All these features have to be reached under the conditions of high power laser UV light. Some design principles for such systems are applied, such as optimum number of elements, minimum tolerance sensitivity, material restrictions for the lens elements as well as mechanical parts (mounting), restrictions of possible power densities on lens surfaces (including ghosts), matched quality for the manufactures system. The special applications require appropriate performance criteria for theoretical calculation and measurements, which allow to conclude the performance of the application. The base is wave front calculation and measurement (using Shack- Hartmann sensor) in UV. Derived criteria are calculated and compared with application results.
KEYWORDS: Excimer lasers, Micromachining, Excimers, Optical components, Deep ultraviolet, High power lasers, Imaging systems, Optical design, Photomasks, Ray tracing
Excimer laser micromachining applications have gained more and more interest in fabrication of commercial microstructure products (e.g. multi chip modules, printed circuit boards, excimer laser stripped wires and others). The unique properties of the pulsed excimer laser radiation are the deep UV wavelength and the high peak power which result in the special material interaction for high precision micromachining. The key elements in taking full advantage of the excimer laser beam and in providing reproducible and controlled micromachining is the illumination and imaging optics. Detailed investigations were performed to understand microstructure formation (e.g. wall angle and microstructure position accuracy) in regard to illumination and imaging optics parameters. Qualitative and quantitative relations were found to explain microstructure feature performance in regard to optical and processing parameters.
The bandwidth requirements 1, 2, 3, 4, 5 on DUV 248 nm lithography laser sources, in general, are a function of a number of physical parameters. For example, the stepper lens, the image field size and the required resolution.
The trend towards larger field sizes in new DUV-stepper lenses has resulted in a bandwidth need of below 2 ?m for a lorentzian spectral distribution.
Since 1987 commercial line narrowed 248 nm excimer lasers have been used with R&D DUV steppers. Several resonator concepts were employed to match linewidth power and lifetime needs for a DUV microlithography laser light source. The physics of different line narrowing resonators like prisms, grating, etalons and combinations are described and experimental lifetime and power capabilities reviewed. The directions of DUV-laser light source development are discussed in regard to 193 and 157 nm lithography.
Commercially used lithography lasers are line narrowed KrF excimer lasers (Ref. 1) operated at (248.38 0.2) nm at 2 W (10 mJ, 200 Hz) with a bandwidth of typically 3 pm (Ref.2). A system control concept for alternative control at two different locations including a remote RS 232 C operation has been developed. The on-line controlled parameters are: pulse energy, absolute wavelength, bandwidth and maintenance requests. Two years field experience with Lambda 248 L excimer lasers and long term test results are summarized. Lifetime issues and scheduled service actions are also presented. The 4 W Lambda 248 L-400 excimer laser was operated with modified optics, which led to an integral bandwidth below 2 pm. A narrower bandwidth provides the advantage of producing smaller feature sizes, e.g., 0.4 tm even with higher numerical aperture lenses. Bandwidth measurements and methods are compared. Lineshape measurements and corresponding energy portions were calculated to describe the spectral distribution. Long-term bandwidth monitoring results (over 8 hours continuous operation, burst mode operation) are described. Furthermore the high rep.-rate 248 L-400 laser was evaluated relative to pulse energy, bandwidth and pulse to pulse energy fluctuations. A maximum average power of more than 1 1 W (400 Hz, 28 nil) was achieved. Stabilized operation at 15 mJ, 400 Hz over several hours was demonstrated. The experimental results are presented and discussed.
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