Solid State Phenomena Vol. 187

Abstract: Physical cleaning uses the physical force for particle removing process and the physical force can be represented by PRE(%) and pattern damage. Using the damage proving pattern, which was composed with conductive material for electrical detection, the damage of physical cleaning was quantitatively analyzed. And pattern damage was calculated with a form of damage density and plotted with pattern CD. Using PRE(%) and three parameters, which were derived in the damage density plot, the comparison of various physical cleaning was performed..

Abstract: High speed spray cleaning which utilize droplets impact has been used for removing contaminants from wafer surface. When a droplet impacts a solid surface at high speed, the contact periphery expands very quickly and liquid compressibility plays an important role in the initial dynamics and the formation of lateral jets. Impact results in high pressures that can clean or damage the surface. In this study, we numerically investigated a high speed droplet impacts on a solid wall. In order to compare the available theory and experiments, 1D, 2D and axisymmetric solutions are obtained. The generated pressures, shock speeds, and the lateral jetting mechanism are investigated; especially the effect of target compliance is focused.

Abstract: We have been developing an innovative ultra-low environmental load cleaning technique by the use of steam-water mixed spray. We showed that this technique is quite effective in both cleaning and photo-resist stripping. We also found that the physical force associated with steam-water mixed spray is greater than that with air-water mixed spray; hence we proposed that the condensation plays an important role in this cleaning technique. In order to discuss further this mechanism, we perform the order estimation of physical processes in dynamics of liquid droplet moving in vapor flow impacting on a solid interface in this study. Results show that droplet impact velocity can be reduced while the droplet approaches to the solid surface. However, the vapor in the gap can condensate to either the liquid droplet or the solid surface with the velocity whose order of magnitude cannot be negligible compared to the impact velocity; hence the amount of vapor that should be pushed out from gap can be drastically reduced, This condensation results in the significant reduction of viscous force. This reduction of force with the existence of condensation reduces the impact velocity deceleration. Consequently significantly large impact pressure is generated.

Abstract: The fundamental characteristics of the resist removal-cleaning system using cryogenic micro-solid nitrogen spray flow were investigated by a new type of integrated measurement technique. The present system utilizes the micro-solid nitrogen (SN₂) which consists of the fine solid nitrogen particle produced by the high-speed collision of subcooled liquid nitrogen and the cryogenic gaseous helium (cryogen). According to present study, the effect of ultra-high heat flux cooling on the resist removal performance due to the rapid thermal contraction of resist material is clarified in detail. Furthermore, the effect of ultrasonic atomization of micro-solid nitrogen on ultra-clean performance of the wafer is newly founded.

Abstract: In this work the dynamics of particle removal by aerosol spray is investigated. Local dwell time of spray cleaning is calculated numerically from the process conditions, and some striking topological similarities between the particle removal efficiency and dwell time profiles are observed. The particle removal rates, defined as the normalized speed of particle removal, are not constant during a typical process, with the highest removal rate for the first tens of milliseconds and a temporal decay as time elapses. Increasing N₂ flow rate results in an enhancement in both the particle removal efficiency and the particle removal rate.

Abstract: A novel advanced spray technology, in which droplet size and velocity are accurately and tightly controlled, has been developed to realize the damage-free cleaning for next generation device manufacturing. The influence of droplet characteristics on pattern collapse/damage was quantitatively investigated using this technology. It was shown that the amount of damage was correlated to the droplet energy density on the wafer. The mechanism of pattern damage generated by the conventional dual fluid spray was revealed by the damage threshold curve, which was obtained from the theoretical consideration. Finally higher particle removal efficiency without any pattern damage was achieved by controlling the distribution of effective droplets for cleaning.

Abstract: Removal of gold particles (40 nm and 100 nm) from fused silica and thermal oxide surfaces in dilute ammonium hydroxide solutions has been investigated. The particle removal efficiency (PRE) from fused silica surface has been found to be a strong function of ammonium hydroxide concentration and bath temperature. PRE increases from 0 to 85 % with increase in bath temperature from 30 to 80 °C for ammonium hydroxide concentration of 1 %. Addition of megasonic energy to the ammonium hydroxide bath at 30 °C has also shown to improve the PRE significantly. In the case of thermal oxide, the removal of gold particles is much easier compared to that from fused silica. Even for cleaning at 30 °C, the PRE for oxide surface increases from 10 to 90 % with increase in ammonium hydroxide concentration from 0 % to 4 %. Atomic force microscopy measurements reveal that an adhesion force of 10 mN/m exists between fused silica and gold particles in 4 % ammonium hydroxide solution as opposed to only repulsive force in the case of thermal oxide.

Abstract: The megasonic cleaning efficiency is evaluated as a function of the angle of incidence of acoustic waves on a Si wafer. Acoustic Schlichting streaming alone is not able to remove nanoparticles smaller than 400 nm. It is shown that oscillating or collapsing behavior of bubbles are responsible for removing nanoparticles smaller than 400 nm during a cleaning process with ultrasound. Optimal particle removal efficiency is obtained around the angle of acoustic transmission of the silicon wafer.

Abstract: In IC manufacturing, particle removal from a wafer's back side (BS) has become as important as that from the front side (FS). For example, during lithography, BS particles can cause a variation on the topside surface topography. This may result in a focus-spot failure due to the reduced process window for depth of focus (DOF) as shown in Fig. 1. This problem increases as the feature size decreases. BS particles may cause other problems in wet benches, where BS particles can be transferred to the adjacent front side of wafers. Fig. 2 shows these FS particles, which usually appear as flow or streak patterns on the wafer [.

Abstract: An improved fundamental understanding of the megasonic cleaning process is necessary to optimize cleaning efficiency and minimize the unwanted damage to fragile structures. Argon sonoluminescence (SL) measurements are done to achieve an improved insight in the collapse threshold and behavior of microbubbles. This paper reports on acoustic cavitation by means of Ar Sonoluminescence (SL) investigation achieved with a dedicated test cell, a photomultiplier tube (PMT) and a gasification system. The results show an increase in SL signal as a function of the applied acoustic power density. An increase in Ar concentration results in a decrease in SL signal. Furthermore, a clear hysteretic behavior in the SL signal is identified when ramping the acoustic power up and down. This hysteresis effect can be attributed to the nucleation of bubbles during the increasing branch of the power loop. Finally the time evolution of SL light after the switching on of the acoustic transducers revealed the existence of a delay time.