Experimental Study On Laser Cleaning Of Particle Contaminants From Silicon Wafer Surface

As current ULSI technology advances toward higher densities and smaller circuit dimensions, contamination control becomes one of the most critical problems in industry, submicron-sized contaminants adhering to the surface can cause a substantial number of defects, with a serious impact on the manufacturing yield. Traditional cleaning method, such as RCA cleaning, mechanical wiping and scrubbing, ultrasonic cleaning, are not adequate for removal of submicron-sized particle contaminants; moreover, they are prone to damaging the silicon wafer surface because they rely on mechanical contact forces and chemical reaction. Laser cleaning is a novel non-contact cleaning method, which is characterized by powerful removing force, high flexibility and environment-friendly method. Therefore, laser cleaning has been demonstrated as a potentially promising means to meet highly demanding cleaning needs.Based on the analysis of the adhesion forces between the particle contaminants and the substrate, the paper build the removing model to analyze the particle cleaning mechanism by means of dry laser cleaning. The main cleaning mechanism is fast thermal expansion of the silicon wafer surface owing to its temperature rises induced by pulse laser irradiation. The temperature field is simulated by building laser-heating models, describing them by heat-transfer equations and solving them by finite element method. Then, the adhesion force and cleaning force acting on the particles are calculated, and the theoretical laser cleaning threshold of 1μmAl₂O₃, 1μmCeO₂, 0.5μmAl₂O₃ are 60mJ/cm2, 50mJ/cm2, >150mJ/cm2, respectively.Under the guidance of the mechanism analysis, a serial of laser dry cleaning experiments are carried out on silicon wafer surface by using 1064nm, 0.5ms Nd:YAG laser and 248nm, 30ns, KrF excimer laser. The dependence of laser cleaning efficiency on pulse width, laser fluence, numbers of pulse, and laser beam incidence angle are studied, and laser cleaning threshold and damage threshold are acquired. The cleaning threshold of 1μmAl₂O₃, 1μmCeO₂, 0.5μmAl₂O₃ are 30⁴0mJ/cm2, 30⁴0mJ/cm2, 10²0mJ/cm2, respectively; and the damage threshold are 320mJ/cm2, 300mJ/cm2, and near 30mJ/cm2, respectively.Optical microscope and 3D surface morphology are used to analyze the surface damage of silicon wafer, and dry laser cleaning will not increase the surface roughness of silicon wafer when laser fluence is lower than damage threshold.In order to evaluate the cleanliness of silicon wafer surface before and after laser cleaning rapidly and precisely, a program is developed to value it through processing the surface images by means of MATLAB.In this paper, some key problems in dry laser cleaning particle contaminants from silicon wafer surface are studied. This will help generalizing the technology of laser cleaning of the silicon wafer surface.