Particle-wafer interactions in semiaqueous silicon cleaning systems

During the semiconductor chip manufacturing process, a silicon wafer goes through a number of cycles in both hydrophilic and hydrophobic environments. As silicon chips become more sophisticated, the manufacturing process becomes more involved and new challenges are imposed by size reduction, increase in the aspect ratio and the formation of multilayer structures. Wafer cleaning processes emerge several times in one manufacturing cycle. By rule of thumb, it is necessary to remove wafer contamination by particles which are half of a feature size. This is an enormous challenge, keeping in mind that currently wafer structures are of nanometer size.;The cleaning procedures which worked for the last 40 years are becoming ineffective and obsolete. The industry calls for more efficient cleaning procedures in terms of particle contamination removal, and at the same time less aggressive procedures to prevent damage/dissolution of the fragile and narrow wafer structures.;Atomic Force Microscopy (AFM), besides being an imaging tool with nano resolution, proves to be an indispensable instrument to characterize interaction forces, lateral forces, and adhesion between micron and submicron contaminant particles and the wafer surfaces both in air and liquid. Using the AFM colloidal probe technique interaction forces were measured between a contaminant particle and a wafer surface. These measurements were done for the silica---silica hydrophilic system and for the silanated silica---silanated silica hydrophobic system. The influence of the nonaqueous component in semiconductor wafer cleaning solution on interaction forces was also investigated under both hydrophilic and hydrophobic conditions. In addition the effect of particle size on the interaction forces as well as particle removal rate under both conditions is addressed. While force/radius normalization of measured interaction forces works great for hydrophilic systems, it was found to significantly underestimate the influence of particle size in a hydrophobic system. A correction to the normalization of hydrophobic forces with respect to particle size has been proposed.;The issue of nanobubble formation at hydrophobic surfaces under certain conditions has been investigated. The nanobubbles were observed under AFM and characterized with respect to size, shape and Laplace pressure.;As wafer structures become smaller and more fragile, it is crucial to confront the impact of different manufacturing steps, including the process of cleaning, with the strength of wafer structures. The force required to remove a contaminant should be close to the force that damages a structure in order to maximize particle removal efficiency, but not higher than this force in order to avoid structure damage. Such measurements have been done with the AFM using lateral force mode of operation.;Understanding and manipulation of the interaction forces has led to the design of novel semiconductor wafer cleaning solutions.