A Non-Reactive Excimer-Based Surface Preparation and Cleaning Tool for Broad-Based Industrial Applications
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335 Mat. Res. Soc. Symp. Proc. Vol. 397 0 1996 Materials Research Society
MATERIALS PROCESSING USING PHOTONS AND A FLOWING INERT GAS Thermodynamic mechanisms such as temperature, pressure and force or chemical cleaning have traditionally been used for many applications. The prevention of sub-surface damage is critical to the acceptance of any cleaning or surface preparation process. Additionally, in many industrial applications, surface contamination tolerances have transcended from the macroscopic (gross contamination) to the microscopic. An alternative approach views the surface as a series of complex bonds which can be broken by applying a sufficient radiation force. The precise interaction details when using a high energy photon source such as a krypton fluoride (KrF) excimer laser and a laminar flowing inert gas are not clear. It is assumed that a multiple-photon bond-breaking interaction occurs. When the surface-contaminant bonds have been broken, the contaminant gains kinetic energy from the interaction and penetrates through the flowing inert gas's boundary layer into the bulk gas flow. If the gas flow is laminar, the probability of the contaminant readhering to the surface is greatly reduced and the cleaning efficiency enhanced. The combination of high energy photons and a laminar fowing inert gas is called the Radiance Process. It uses an average energy flux (J/cm 2) and peak power flux (MW/cm 2) that are significantly below the ablation threshold of the substrate so that the underlying surface structure cannot be damaged. These are very simplified views of some of the possible interactions; several other mechanisms may contribute to the bond-breaking process. A Lambda Physik LEXtra 200 KrF excimer laser (maximum energy per pulse at 600 mJ and a maximum repetition rate of 30 Hz) was used to develop the process.. The beam was held stationary and the workpiece scanned using translation stages. Fixturing and optical configuration were changed to accommodate various substrate sizes and shapes. A mass flow controller monitored the gas flow. The inert gas lines used 0.003 gtm particles point-of-use filters and an oxygen/moisture scrubber to 0.01 ppb. Energy dose and peak power flux to the surface were adjusted to optimize efficient contaminant removal. These adjustments were made by controlling laser output energy and repetition rate, beam length and width, workpiece scan velocity, gas composition and gas flow velocity, and laser beam polarization. For a given contaminant and substrate, a process recipe could be optimized very rapidly for efficient contaminant removal. Table 1 lists the optimum irradiation conditions for a variety of materials. The energy fluence is determined by the instantaneous laser energy incident on the workpiece divided by the area of the incident beam. The average energy flux (or fluence) considers the number of laser pulses incident per unit area. Peak power flux is calculated from the energy fluence divided by the laser pulse duration. For this calculation the laser pulse is approxi
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