Rinsing Of Wafers After Wet Processing: Simulation And Experiments

Abstract

In semiconductor manufacturing, a large amount (50 billion gallons for US semiconductor fabrication plants in 2006) of ultrapure water (UPW) is used to rinse wafers after wet chemical processing to remove ionic contaminants on surfaces. Of great concern are the contaminants left in narrow (tens of nm), high-aspect-ratio (5:1 to 20:1) features (trenches, vias, and contact holes). The International Technology Roadmap for Semiconductors (ITRS) stipulates that ionic contaminant levels be reduced to below ~ 10¹⁰ atoms/cm². Understanding the bottlenecks in the rinsing process would enable conservation of rinse water usage. A comprehensive process model has been developed on the COMSOL platform to predict the dynamics of rinsing of narrow structures on patterned SiO₂ substrates initially cleaned with NH₄OH. The model considers the effect of various mass-transport mechanisms, including convection and diffusion/dispersion, which occur simultaneously with various surface phenomena, such as adsorption and desorption of impurities. The influences of charged species in the bulk and on the surface, and their induced electric field that affect both transport and surface interactions, have been addressed. Modeling results show that the efficacy of rinsing is strongly influenced by the rate of desorption of adsorbed contaminants, mass transfer of contaminants from the mouth of the feature to the bulk liquid, and the trench aspect ratio. Detection of the end point of rinsing is another way to conserve water used for rinsing after wet processing. The applicability of electrochemical impedance spectroscopy (EIS) to monitor rinsing of Si processed in HF with and without copper contaminant was explored. In the first study, the effect of the nature of surface state (flat band, depletion, or accumulation) of silicon on rinsing rate was investigated. The experimental results show that the state of silicon could affect rinsing kinetics through modulation of ion adsorption. In the second study, silicon was intentionally contaminated by spiking HF with copper ions, cleaned in dilute HCl and then rinsed, and the entire process was followed by continuous impedance measurements. The measured impedance values at different stages have been correlated to the nature of the silicon surface, as characterized by scanning electron microscope (SEM) and inductively coupled plasma mass spectrometry (ICP-MS) methods.