Research On Characteristics Of IPA Evaporation Process Inside Vertical Pores Under Micro/nano Scales

Hundred times of cleaning and drying process is applied during wafers manufacturing. Isopropyl alcohol (IPA) remains inside nano-pore structure on the wafer surface after wafer is cleaned. And wafers will be put in cassettes under Nitrogen environment to make IPA evaporate by high speed revolution. IPA liquid evaporates at different rate inside the different pores of different diameters and different positions on the wafer surface. Different liquid level in adjacent pores is caused by different evaporation rate, which will lead to disequilibrium condition of stress on the structure between pores. In worst conditions, configurations on the wafer surface will be collapse and the wafer will be scrapped. Therefore, to predict and control the evaporation rate of IPA inside nano-pores on the wafer surface has great significance for wafer manufacturing.The purpose of this paper is to investigate IPA evaporation process inside nano-pores and to predict the evaporation rates. To achieve this purpose, researches were started with IPA evaporation process under micro-scale by experiment and numerical analysis and further investigation under nano-scale was made based on the results of micro-scale. The main works and conclusions are briefly reviewed as follows:(1) Experimental rig of IPA evaporation process inside macro and micro pores under Nitrogen condition was designed and established according to the evaporation process on wafer surface. The smallest diameter of pore used in the experimental rig is 50μm.(2) Experimental researches on the characteristics of IPA evaporation process inside micro-pores were investigated. The experimental results show that the evaporation rate decreases with the decrease of flow velocity at the orifice. However, the influence of flow velocity decreases with the increase of evaporated IPA height. The fast the flow velocity is, the more obvious the decrease of evaporation rate. The IPA evaporation rate and diffusion coefficient also increases with the increase of ambient temperature. And the IPA evaporation rate decreases with the decrease of pore diameter. However, the influence of pore diameter becomes limited in the range of 0.05 mm to 0.2 mm. Two correlations of local mass transfer coefficient were developed for mm-scale pores (d > 3 mm) andμm-scale pores (d < 0.5 mm) respectively. For mm-scale pores, the correlation can agree 88% of the experiment data within deviation of±20%, and the average deviation is 12.6%. Forμm-scale pores, the correlation can agree 86% of the experiment data within deviation of±30%, and the average deviation is 17.6%.(3) Evaporation model of IPA inside vertical micro-pore has been established according to the experimental results. The evaporation model is divided into two kinds due to the different mass transfer mechanism caused by different ranges of flow velocity:a. When the flow velocity is low (v < 1 m/s), molecular diffusion dominates the evaporation process. The influence of the curved shape of vapor-liquid interface and the variation of vapor partial pressure at the orifice on the evaporation rate has been concerned in the low velocity condition model. The mean difference between the prediction value of the model and the experimental results is about 10.5%.b. When the flow velocity is high (v > 10 m/s), the IPA evaporation process consists of three main steps orderly: strong convection mass transfer near the orifice (liquid been blown off directly), convection mass transfer inside pore and molecular diffusion inside pore. And the evaporation rate decrease continually in these three steps. The difference of molecular diffusion step between low velocity model and high velocity model is that vapor molecules will be influenced by convection mass transfer all the time under high velocity condition. The mean difference between the prediction value of the model and the experimental results is about 5%.(4) Based on the research results of micro scale, evaporation model under nano scale has been established by introducing absorption effects between silicon wall and IPA molecules. The mutual diffusion coefficients between IPA and Nitrogen inside nano-pores were also investigated.