Nanoparticle Removal Mechanisms during Post-CMP Cleaning

Abstract

In this research, the synergistic interaction of a brush, an adhered particle, and a wafer surface was evaluated during post chemical mechanical polishing ͑CMP͒. The approach includes theoretical analysis combined with experiments. A theoretical model was used to predict the particle/wafer contact area and contact force in order to identify the maximum tangential force required to remove particle agglomerates. Experiments are carried out to validate the analysis and to further study the removal mechanisms. This research suggests that a combined effort between a brush and surfactant would be effective in removing small particles during cleaning. For large particles, the mechanical force would be the dominating factor.Chemical mechanical polishing ͑CMP͒ was introduced as a method to planarize interlevel dielectrics ͑ILD͒ and damascene metal wiring for on-chip multilevel interconnects. As the size of microelectronic device decreases, planarization of both front and back end layers becomes crucial for interconnects smaller than 0.5 m. 1-3 CMP processes leave a large amount of contamination on the surface, such as residual particles, metallic contamination, and a damaged surface layer. Therefore, a cleaning process called post-CMP cleaning has become a standard manufacturing step. One of the major concerns on the CMP is the high level of residual particles. Depending on the type of CMP and the polishing conditions, these particles can be physically adsorbed on the surface or partially embedded in the substrate due to the mechanical pressure exerted by the polishing pad.It has been accepted that the van der Waals force plays an important role in the adhesion process. To overcome the adhesion force, the mechanical scrubbing from the brush or chemical loosening using solutions or additives is necessary. Chemical additives are used to control the electrostatic repulsion in order to prevent particle readhesion. This can be done by controlling the charges on the substrate and particles, which can be altered by modifying the solution pH or the addition of surfactant. The objective of using surfactant is that it can be used to modify the apparent charges on particles and on the substrate surface. Another important characteristic is that particles can be kept apart during the occupation of the organic surfactant molecules when attached to the particle.The post-CMP cleaning has become a standard process step for local and global planarization. Reports have been found in understanding the mechanisms. These include the basic cleaning principles, 4,5 the effect of hydrodynamic force, 6,7 modeling parameters to study the adhesion force, 8-13 studying the lubrication behavior and friction forces, 14 and using additives and different chemicals during cleaning. 15,16 These results, among others, have discussed the mechanical nature of surface material removal in a micrometerlength scale and larger. The abrasive particles used in CMP are in nanometers. Interesting questions arose when considered sizes become smaller. Thus, understandin...

In situ observation of colloidal particle behavior between two planar surfaces

“…The interactions between colloidal particles and planar surfaces in aqueous solutions are highly relevant to many natural and industrial processes, such as water purification [1], nematic liquid colloidal behavior [2][3][4], motion and adhesion of cells [5][6][7], lubrication [8], chemical mechanical polishing (CMP) process [10][11][12][13][14][15][16]. For example, the research of interaction between colloidal particles and different structural surfaces in shear fluids is of help to understand a series of physiological phenomena [5][6][7].…”

“…Colloidal particles behavior in liquid environment can dramatically affect the tribological characteristics of liquid lubricants [8,9]. A variety of work has proven that the pH value and the ionic strength of the polishing slurry have a great effect on the motion of abrasive particles, which are closely related with material removal rate [10][11][12][13][14][15]. Apparently, the interaction between colloidal particles and surfaces in contact situations is a very important factor in these phenomena.…”

Interfacial Forces in Chemical-Mechanical Polishing

“…During CMP, contaminations have been found to be impregnated in to the wafer surface due to the presence of adhesion forces (36)(37)(38). The most commonly reported contaminations were abrasive particles, debris from the copper surface being polished, and debris from the polishing pad.…”

“…The explanation of how these instruments work has been provided (57). Despite a number of shortcomings, AFM has been a preferred instrument to measure adhesion forces in a complex environment (36,(66)(67)(68). Table 3.1 lists reported removal forces conducted using an AFM in a vacuum environment.…”

Comparative Surface Study on Hydrogen Terminated Si Surface Covered with Alcohols

“…Ever since wet cleaning tools were first used for RCA standard clean processes based on hot alkaline and acidic hydrogen peroxide solutions, 1 many challenges have been reported over the years in improving the performance of wet cleaning processes on metal impurities, 2-13 particles, [14][15][16][17][18][19][20][21][22] and organic molecule (contamination) removal [23][24][25][26] from Si wafer surfaces. One cause of organic contamination is the long exposure time of the cleaned wafers in the clean room environment, wafer box, standard mechanical inter face (SMIF) or front opening unified pod (FOUP).…”