Adsorption and desorption kinetics of airborne ammonia on chromium-coated wafer in cleanroom depending on humidity and NH3 concentration

Abstract

Ammonia (NH3) is widely used in semiconductor fabrication as an indispensable chemical substance in chemical mechanical planarization (CMP), etching and cleaning processes. The undesired outgassing of residual NH3 gas results in many severe defects on integrated circuits. The front opening unified pods (FOUPs), which are made of polymers, were introduced as a controlled-microenvironment to protect the processed wafers during transport and storage steps. However, the FOUPs potentially outgas the previous sorbed-NH3 to FOUPs' atmosphere, leading to NH3 cross-contamination. The development of NH3-free liquid phase extraction (LPE) set-up coupled to ionic chromatography (IC) analysis allows sensitive NH3 measurement on wafer surfaces in avoiding the artifacts of airborne NH3 from cleanroom environment. The adsorption and desorption kinetics of NH3 on Cr-coated surface have been determined in cleanroom conditions (21 ± 2°C, 1.0 atm, different NH3 gas concentration (10–35 ppbv) and relative humidity (<1–40%RH). The NH3 adsorption kinetics on Cr-coated wafer correspond to modified Langmuir-type adsorption model. The deposited NH3 amount at saturation level (Cmax 1.94 × 1014 molecule/cm2) and the adsorption constant (kads 0.041 ppbv−1.h−1) are independent of the NH3 airborne concentration. On the contrary, the Cmax and kads linearly depend on the humidity of exposure environment. The desorption kinetics from saturated Cr-wafer are much slower than the adsorption kinetics. Mathematical model of modified Langmuir-type is established and served as prediction tools for the study of cross-contamination between FOUPs–wafer, optimal conditions for wafer/photomask storage.