Numerical study of edge condensation in wafer to wafer bonding process with lattice Boltzmann approach

Direct wafer bonding process is applied in semiconductor production because of the advantage of attaching several chips at once. During the bonding process, the top wafer is deformed and the wafer surface is oblique from the bonding front to the edge, thereby forming a wedge-shaped flow field with a wider gap toward the edge. In this flow field, acceleration of the flow rate occurs and expansion due to pressure drop occurs. This expansion also lowers the temperature. When the temperature is sufficiently low, the vapor aggregates and the droplet condenses at the edge for wafer. Experiment on condensation reduction is difficult because of the inability to observe the flow space between wafers. It is only through numerical analysis to observe phenomena occurring in the wafer flow filed and use them to improve process recipe. In this study, numerical analysis is used to observe the condensation occurring at the edge. Numerical method enables the phase change of microscale and interface capture based on lattice Boltzmann, and added the theoretical basis for molecular behavior by adding interatomic potential including electrostatic potential of hydrogen bond. This numerical model was used to analyze the flow between wafers during bonding process and to simulate temperature drop and vapor condensation. It was observed that the vapor density decreased with time and then increased again. When the temperature is sufficiently low, the vapor aggregates and the density of the vapor increases again. A rapid rising in vapor density was observed below 2.5°C. The higher the wettability of the surface, the lower the vapor density at the cold spot. Due to the high wettability of the wafer surface, vapor is dispersed throughout the surface, and thus the amount of vapor that aggregates at the cold spot is reduced. If the wettability is higher than a certain level, a regime in which the width of the generated droplet increases with wettability appears. As a result, an optimum contact angle with minimum droplet width appears, but this may not be optimal in terms of bonding strength.