Optimization of Ammonia-Peroxide Water Mixture (APM) for High Volume Manufacturing through Surface Chemical Investigations

Abstract

Ammonia-peroxide mixture (APM) is a widely used wet chemical system for particle removal from silicon surfaces. The conventional APM solution in a volume ratio of 1:1:5 (NH4OH:H2O2:H2O) is employed at elevated temperatures of 70-80 °C. At these temperatures, APM solution etches silicon at a rate of ~3 Å/min, which is unacceptable for current technology node. Additionally, APM solutions are unstable due to the decomposition of hydrogen peroxide and evaporative loss of ammonium hydroxide resulting in the change in APM solution composition. This has generated interest in the use of dilute APM solutions. However, dilution ratios are chosen without any established fundamental relationship between particle-wafer interactions and APM solutions.Atomic force microscopy has been used to measure interaction forces between H-terminated Si surface and Si tip in APM solutions of different compositions. The approach force curves results show attractive forces in DI-water, NH4OH:H2O (1:100) and H2O2:H2O (1:100) solutions at separation distances of less than 10 nm for all immersion times (2, 10 and 60 min) investigated. In the case of dilute APM solutions, the forces are purely repulsive within 2 min of immersion time. During retraction, the adhesion force between Si surface and Si tip was in the range of 0.8 nN to 10.0 nN. In dilute APM solutions, no adhesion force is measured between Si surfaces and repulsive forces dominated at all distances. These results show that even in very dilute APM solutions, repulsive forces exist between Si surface and particle re-deposition can be prevented.The stability of APM solutions has been investigated as a function of temperature (24 - 65 °C), dilution ratio (1:1:5 - 1:2:100), solution pH (8.0 - 9.7) and Fe2+ concentration (0 - 10 ppb) using an optical concentration monitor. The results show that the rate of H2O2 decomposition increased with an increase in temperature, solution pH and Fe2+ concentration. The kinetic analysis showed that the H2O2 decomposition follows a first order kinetics with respect to both H2O2 and OH- concentrations. In the presence of Fe2+, hydrogen peroxide decomposition follows a first order reaction kinetics with respect to H2O2 concentration.