Innovative Megasonic Cleaning Technology Evaluated Through Direct Wafer Bonding

A single-beam megasonic system for nano-particle cleaning which does not cause nano-scale pattern damage was designed. Considering ultrasonic wave propagation, a length of quartz waveguide was decided at a uniform acoustic wave region. The anti-resonance frequency of the piezoelectric actuator was measured to be 987 kHz, which agreed well with the fabricated quartz waveguide operating frequency of 983 kHz. The acoustic pressure of the single-beam megasonic system was measured, and the averaged value increased by 89% compared with a commercially available product (named an A-type). To compare uniformity, standard deviations of acoustic pressure values were calculated, and they were lower than A-type results by 20%. In addition, particle removal efficiency (PRE) tests were performed, and 91% of particles were removed after megasonic cleaning. These results explain our system's performance improvement in PRE and reduction in nano-pattern damage.

One of the main challenges in photomask cleaning is balancing particle removal efficiency (PRE) with pattern damage control. To overcome this challenge, a high frequency megasonic cleaning strategy is implemented. Apart from megasonic frequency and power, photomask surface conditioning also influences cleaning performance. With improved wettability, cleanliness is enhanced while pattern damage risk is simultaneously reduced. Therefore, a particle removal process based on higher megasonic frequencies, combined with proper surface pre-treatment, provides improved cleanliness without the unintended side effects of pattern damage, thus supporting the extension of megasonic cleaning technology into 10nm half pitch (hp) device node and beyond.

To maintain pace with evolving software, the density of circuits must increase, which first requires an atomically flat surface, devoid of defects. To achieve this, Chemical Mechanical Planarization (CMP) is employed to optimize the properties of the substrate surface. An important type of substrate in wafer manufacturing is dielectric, such as Tetraethyl orthosilicate (TEOS) and silicon carbide (SiC). For TEOS, Ceria (CeO2) is employed for polishing due to its compatibility attachment and TEOS being among the most widely utilized silicon-based materials for Shallow Trench Isolation (STI) applications. SiC, on the other hand, requires harsher chemistries (i.e., KMnO4) and a harder nanoparticle (i.e., Al2O3) to efficiently polish due to its inert surface. For both substrates, the CMP process leads to nanoparticles remaining on the substrate surface as a primary defect, requiring a post-CMP (p-CMP) cleaning. Traditionally, the p-CMP process utilizes a polyvinyl alcohol (PVA) brush to mechanically remove these defects. However, brush cleaning risks further defectivity due to the inherently high-stress nature of the process. Conversely, the application of high-frequency megasonic waves has been shown to effectively eliminate residual nanoparticles from the substrate while minimizing surface stress. Currently, megasonic cleaning is conducted in a static state, wherein the cleaning chemistry remains constant throughout the process and the wafer is affixed to a single location. This results in fixed localized cavitation, which limits the removal capability, and the risk of particle redeposition attributed to local concentration gradients from the lack of dynamic flow. This study aims to investigate the effects of a dynamic cleaning system by varying two critical parameters: the flow of cleaning chemistry and the rotational speed, in addition to the original power variable of the static system. The advantages of this approach include enhanced particle removal and uniform cleaning of the substrate, accompanied by a reduced risk of reattachment. From static megasonic cleaning, there’s an increase in particle removal rate as the power induced is increased. A mechanism illustrating the dynamic process as contrasted with the static process is presented, along with a discussion regarding the Ce+3/Ce+4 ratios observed between both cleaning methodologies for TEOS. Initial results of the SiC cleaning show an increase in particle removal efficiency (PRE) with the addition of rotation in comparison to the static system with the same power parameters.

Photomasks with various sub resolution assist feature (SRAF) were vigorously cleaned by a megasonic tool, and their pattern damage, "SRAF-missing", was investigated. As a result, it was found that SRAF-missing can occur at a low probability by the megasonic cleaning and the probability significantly depends on SRAF size, especially width. With smaller than 100 nm width, SRAF-missing probability rapidly increased with SRAF width reduction. In addition, the relationship between SRAF-missing and acoustic pressure was investigated, and at the same time that between particle removal efficiency (PRE) and acoustic pressure was also investigated. As a result, SRAF-missing and PRE showed a trade-off relationship. Using all results, an experimental equation was provided. After verification by additional experiments, some simulations were done, and megasonic cleaning subject was predicted for the 45nm and 32nm-node mask fabrication.

This paper describes a new surface cleaning approach for photomask cleaning. This non-conventional cleaning method uses momentum transfer between aerosolized, frozen, CO 2 particles and the contaminants for effective removal from surface. The purity of CO 2 used is an important component in the determination of cleaning efficiency. The authors will present two methods developed to analyze hydrocarbon contamination in CO 2 . These analytical methods were used to compare different grades of CO 2 including Ultra High Pure (UHP) grade developed for sub-micron particle removal from photomasks. Using the UHP grade CO 2 , it was shown that greater than 99% particle removal efficiency is possible from silicon wafer surfaces, with higher removal efficiency of sub-micron particles compared to larger size range. This particular characteristic of particle removal by cryogenic aerosol method is theoretically derived in an earlier paper. In this paper results of CO 2 cryogenic aerosol cleaning with respect to electrostatic discharges on two different binary masks are presented. The paper also shows the removal of 99.9% of the progressive defects such as haze of 0.5 to 1.0 μm size. Cleaning characterization of attenuating phase shift masks with MoSiON films indicate 0.04% change in transmission and 0.37% change in phase angle after 16 cleaning cycles, suggesting that cryogenic cleaning has minimal effect on transmission and phase of att-PSM.

With increasing filter efficiency, the need for energy-efficient methods to improve indoor air quality has grown. The investigated wet scrubber concept has lower pressure losses across the device compared to conventional wet scrubbers. The produced droplets in this wet scrubber are significantly larger than those produced in conventional wet scrubbers that use injectors to atomize the water. Larger droplets result in a reduced interface between water and particles. Therefore, while the new wet scrubber concept is more energy-efficient, it achieves lower particle removal efficiencies than conventional wet scrubber methods. Hence, various adaptions such as additives or surface modifications are used to increase the particle removal efficiency. Additives containing silicone and a rougher surface improve the particle removal efficiency by a factor of 3. However, combining the two does not improve the particle removal efficiency much more than the additives alone. Increasing the hydrostatic pressure on the perforated plate facilitates the water dripping and reduces the average droplet diameter, thereby increasing the water-particle interface. At twice the Weber number, the particle removal efficiency increases by a factor of 2.5 resulting in a particle removal efficiency of 67 % for larger particle diameters.

Megasonic cleaning has been a traditional approach for the cleaning of photomasks. Its feasibility as a damage free approach to sub 50 nm particulate removal is under investigation for the cleaning of optical and EUV photomasks. Two major mechanisms are active in a megasonic system, namely, acoustic streaming and acoustic cavitation. Acoustic streaming is instrumental in contaminant removal via application of drag force and rolling of particles, while cavitation may dislodge particles by the release of large energy during cavity implosion or by acting as a secondary source of microstreaming. Often times, the structures (substrates with or without patterns) subjected to megasonic cleaning show evidence of damage. This is one of the impediments in the implementation of megasonic technology for 45 nm and future technology nodes. Prior work suggests that acoustic streaming does not lead to sufficiently strong forces to cause damage to the substrates or patterns. However, current knowledge of the effects of cavitation on cleaning and damage can be described, at best, as speculative. Recent experiments suggest existence of a cavity size and energy distributions in megasonic systems that may be responsible for cleaning and damage. In the current work, we develop a two-dimensional atomistic model to study such multibubble cavitation phenomena. The model consists of a Lennard-Jones liquid which is subjected to sinusoidal pressure changes leading to the formation of cavitation bubbles. The current work reports on the effects of pressure amplitude (megasonic power) and frequency on cavity size distributions in vaporous and gaseous cavitation. The findings of the work highlight the role of multibubble cavitation as cleaning and damage mechanism in megasonic cleaning.

In this work, a compact-type megasonic cleaning system for semiconductor manufacturing was developed and its performance was verified. To design this megasonic system, firstly, an impedance analysis for a quartz megasonic waveguide with the piezoelectric actuator was performed with finite element method (FEM) software ANSYS. The obtained peak value of the anti-resonance frequency was 983 kHz, which showed good agreement with the design value of 982 kHz. In addition, acoustic pressures of the developed system was assessed by measuring acoustic pressures and comparing standard deviation values of them with a conventional megasonic system to evaluate the system performance. The analysis results showed that the standard deviation of the developed system was decreased by 38% compared to the commercial system. For another evaluation, the particle removal efficiency (PRE) test was performed with 80 nm particles. The experimental result revealed that the system has PRE of 91%. Based on the results, the developed megasonic system is regarded as an improved cleaning system which can irradiate more uniform acoustic pressures. As a consequence, it is thought to have higher energy efficiency and save the consumption of chemical and ultra pure water (UPW) in nano-particle cleaning.

Coal power plants are used extensively worldwide for electricity production due to its cheap rate. The major demerit is its adverse affect on the environment through emissions. The particulates and pollutant gases are the major ones, emitted by coal power plants. Venturi scrubber is one of those technologies that assist in simultaneous removal of particulates and toxic gases. To correctly understand and design these equipments, it is necessary to visualize the fluid dynamics inside these devices. In this regard, a numerical simulation is performed in ANSYS-CFX by using Eulerian-Lagrangian method to estimate particulate collection efficiency in the venturi scrubber. The collection efficiency is investigated at different gas and water mass flow rates. The potassium oxide (K2O) particles of one micron size with 2.35 g/cm3 density is used in simulation. Droplet diameter is also calculated in simulation by means of Cascade Atomization Breakup (CAB) model. Proper selection of breakup regime for droplet deformation in CAB model is determined by calculation of gas Weber and Ohnesorge number. Grade efficiency is calculated for throat gas velocity of 32 m/s with liquid to gas ratio of 7 l/m3. The affect of liquid to gas ratio at specific throat gas velocities on particulates removal efficiency and droplets diameter are also examined. The results shows that the increase in particulates removal efficiency occurred by increasing gas mass flow rate and liquid to gas ratio at particular gas velocities. The decrease in droplet size is observed during accession in liquid to gas ratio. Particulate removal efficiency of 99 % can be achieved at higher gas mass flow rates due to high inertial impaction parameter.

Hybrid DHF and N2 jet spray cleaning for silicon nitride and metal layer DRAM patterns

In this study, a typical workshop with internal water vapour sources was selected as the research object. A coupled Computational Fluid Dynamics model was established to simulate bio-particle diffusion, dry and wet collision and deposition. The study investigated the indoor humidity field and bioaerosol deposition distribution under different supply air parameters in transitional seasons when using a fresh air ventilation system. Additionally, quantitative analysis was conducted on the Mildew Risk Index (MRI) in the workshop under different airflow patterns and supply rates. The main findings of this study are as follows: 1) Higher indoor humidity led to larger MRI, with Beijing having an MRI of only 0.59% compared to Guangzhou’s 56.4%. Correlation analysis showed that MRI had a positive correlation with Absolute Humidity (AH) of supply air (coefficient = 0.92), while it had a negative correlation with Particle Removal Efficiency (PRE) (coefficient = −1.0). 2). There was a negative correlation between PRE and AH (coefficient = −0.93) which indicated that a higher humidity in the supply air could promote bioaerosol deposition indoors and would increase MRI. 3) The regression equations between MRI and PRE were also established in this study, and a variance analysis was performed on them.

Particle versus mercury removal efficiency of amalgam separators

Surface Contamination Genesis, Detection and Control: K.L. Mittal. Volumes 1 and 2

This work presents an experimental study of particle removal from surfaces by means of a pulsed air jet directed toward the particle-laden surface. During the experiments, solid particles were dispersed over the surface, forming a layer of particles that did not touch each other. Under these conditions, resuspension of an individual particle was independent of the number of particles and their location. We attempt to explain the observed phenomena by analogy to heat transfer enhancement by pulsed jets. It is expected that since pulsed jets are effective in surface cooling, their application to improved surface cleaning should be promising. For a pulsed jet, we investigated the effect of pulse frequency on particle removal. It was found that particle removal efficiency could be significantly affected by the frequency of the jet. In particular, for a fixed jet velocity, the efficiency increases with frequency, reaches a maximum, and then decreases.

Experimental study of fine particles removal in the desulfurated scrubbed flue gas