Amir Wachs

Contaminants Removal from Epi Substrates Using Vapor-Laser Process

Introduction: This paper describes the results of a joint development project between Applied Materials Inc. and Sumitomo Mitsubishi Silicon where an innovative dry cleaning technique for removing low levels of particulate contamination from a bare silicon surface was evaluated. Specifically the concept of local cleaning by using a steam-assisted laser cleaning was investigated. Vapor-laser cleaning or Steam Laser Cleaning as is commonly referred to in the literature has been studied in the past six years as a method for removing surface particles through “explosive evaporation. [1-3] The common approach is to indirectly heat a water film by absorbing the laser beam in the underlying substrate and then transferring the heat from the underlying substrate. The transferred heat leads to an explosive evaporation of the thin water film and detachment of the particle. Water is the preferred media because of its high latent heat and its critical point, as compared to other liquids, like alcohols. Here we introduce an innovative approach of local and direct heating of an adsorbed water film by illuminating the surface at a wavelength of 2.94 microns, which corresponds to a peak in the absorption spectrum of water. On the other hand silicon is almost transparent at this wavelength and free carriers only weakly absorb the IR laser pulse. The properties of direct heating decouple the efficiency of the vapor-laser cleaner from the properties of the underlying substrate. Another unique feature that is demonstrated here is the concept of local cleaning, which utilizes the X-Y coordinates of a surface inspection tool to individually target a particle with the laser beam, detach the particle by explosive evaporation, and apply local suction so the particle does not reattach to the surface. Applications of this approach can be of great interest to manufacturing processes where extremely clean surfaces are needed to suppress defect formation in a subsequent process step.