Megasonic cleaning strategy for sub-10nm photomasks

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.

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.

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.

Particle versus mercury removal efficiency of amalgam separators

Adhesion and removal of particles to and from substrates were investigated in water, ethanol and ethanol/water mixtures. Spherical polyethylene and nylon particles of 4-5 ,um in diameter were used. Plates of polyethylene, polyvinylidene chloride, polyvinylidene fluoride, polyethylene terephthalate, and nylon 6 were used as substrates. A plate was perpendicularly immersed in the suspension, and the number of particles adhering to the plate was counted using a microscope video system. Since polyvinylidene chloride (PVDC) substrate showed the maximum particle adhesion in water for both kinds of particles, the particle removal experiments were carried out using the PVDC plates. The PVDC plate to which particles in water adhered was immersed in an ethanol/water mixture. The number of particles remaining on the plate was counted, and this determined the particle removal efficiency. The number of adhering particles decreased with increasing volume ratio of ethanol in the ethanol/water mixtures, and only a few particles adhered to all the substrates in the ethanol volume ratio above 30%. The particle removal efficiency increased with increasing volume ratio of ethanol and reached 90% in the ethanol volume ratio above 30%. The experimental results obtained are discussed in terms of electrokinetic potentials, Hamaker constants, and surface free energies. The particle adhesion and removal phenomena were essentially explained in terms of the free energy changes which were estimated from the dispersive and nondispersive surface free energies of the particle, substrate and liquid.

Different microelectronic processes require very clean surfaces in terms of particulate contamination. Among them, direct wafer bonding has very aggressive requirement in terms of particulate cleanliness. Current work describes development, testing and verification of a single wafer megasonic cleaning method utilizing a transducer design that meets extreme particle neutrality, particle removal efficiency and repeatability requirements of production scale wafer bonding and other very low particle applications. Wafer bonding will also be used as a characterization technique.

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.

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.

Introduction As semiconductor devices become smaller and more highly integrated, it becomes necessary to remove increasingly smaller particles. Currently, various processes such as atomized spray cleaning are being used for particle removal. This method is good for removing larger particles, but it is difficult to remove smaller particles with this kind of process. During atomized spray cleaning there is a boundary layer at the substrate surface which reduces the shear stress that ultimately provides the force to move particles away from the surface. Similarly, if the carrier gas flow rate is increased in order to get higher particle removal efficiency (PRE), above a certain amount there is the possibility of pattern damage. In the case of chemical cleaning, for example SC1 (ammonia + peroxide) cleaning on Si substrates, there is the concern of film loss due to etching of the substrate as well. In this paper, we propose a new cleaning method for removing smaller particles without pattern damage or film loss. New process Solid Phase Clean is a process that uses a polymer to coat the surface of the substrate. The polymer film covers the particles and then directly removes the particles when the film is removed. Furthermore, it does not attack fine patterns and does not etch the substrate material. Figure 1 shows the Solid Phase Clean process. The first step is polymer coating. Particles are covered with the polymer and encapsulated by it. The second step is treatment. The polymer is removed by a treatment liquid, and the particles are removed with the polymer during this step. Experimental Figure 2 shows particle removal evaluation. Smaller particles down to (30nm) were successfully removed. This process achieved higher particle removal performance than atomized spray, especially with particles under 100nm. Figures 3 and 4 show pattern damage and substrate etching results. They show that there is low risk of substrate damage occurring. Mechanism Figure 5 shows the cleaning energy model for this process. We considered the interface energies of each of the touching phases: polymer, particle, substrate, and treatment liquid. Figure 5 E(a) is the condition where the particle is lifted off of the substrate, and Figure 5 E(b)shows the condition where the particle stays attached to substrate. By calculating the total interface energy of these systems, we modeled the cleaning mechanism of this process. Energy to remove particle; Δ\U0001d438 (mN/m2) = \U0001d438 (a)−\U0001d438(b) = (1−\U0001d719)(\U0001d6fe14+\U0001d6fe24−\U0001d6fe12 )+\U0001d719(\U0001d6fe23−\U0001d6fe34) If Δ\U0001d438<0, particles would be removed. - \U0001d6fe\U0001d456\U0001d457: surface energy of interface between phases \U0001d456 and \U0001d457 -\U0001d719: fraction of particle surface area that is not touching substrate (0.5<\U0001d719<1). - (a) : total surface energy of interface particle is lifted off of substrate - (b) : total surface energy of interface particle stays attached to substrate Figure 6 shows the calculation of the interface energy. As this figure shows that −Δ\U0001d438 is over 0, it predicts that particles will be removed. Conclusion Solid Phase Clean is a new cleaning technology that allows removal of smaller particles. By using a polymer to coat the substrate and particles, the polymer is able to directly apply force to the particles and remove them. With this process we achieved high particle removal performance without pattern damage or significant etching of the substrate. We also estimate the removal force and developed a model for particle removal performance of this process by use of an interface energy model. Figure 1

Particle removal from silicon wafer surfaces was studied using acid and alkaline solutions employed in wet cleaning processes found in semiconductor manufacturing. It was demonstrated that alkaline solutions are superior to acid solutions in terms of particle removal efficiency. The following particle removal mechanism in the alkaline solutions was confirmed: the solutions etch the wafer surfaces to lift off particles, and the particles are then electrically repelled by the wafer surfaces. It was determined experimentally that an etch rate of 0.25 nm/min or more is required to lift off the particles adsorbed on the wafer surfaces. It was also confirmed that when the pH value of NH/sub 4/OH-H/sub 2/O/sub 2/-H/sub 2/O solution becomes higher, polystyrene latex spheres and natural organic particles are oxidized, with their surface turning into a gel and their shape changing. The particle removal efficiency was demonstrated to be degraded by the oxidation of organic particles. The results suggest that the mixing ratio of the NH/sub 4/OH-H/sub 2/O/sub 2/-H/sub 2/O solution should be set at 0.05:1:5. >

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

Chapter 2 - Megasonic Cleaning

To improve the cleanliness of coal-fired power plants' particulate matter emissions, a novel device (single-channel slit bubbling particle removal device (SCSB-PRD)) is proposed to improve the wet flue gas desulfurization system's (WFGDs) collaborative particle removal effect. Actual coal-fired flue gas was used to test the particle removal performance. The results showed that the flue gas temperature had no obvious effect on the scrubbing effect of the SCSB-PRD. The scrubbing space, scrubbing liquid volume, and flue gas flow rate effectively changed the gas-liquid flow state, and the bubbling state was the key factor in particle removal. The jet-bubbling contact state was more conducive to removing particles than the foam bubbling state. The jet-bubbling state improved the removal efficiency of fine particles by approximately 30% compared to the foam bubbling state. The device operated in a single stage, and the removal performance of the particulate matter reached more than 60%. Even the submicron particles had a satisfactory removal performance of greater than 50%. The particulate matter concentration at the outlet of the WFGDs was reduced to less than 10 mg/m3, which provides a feasible transformation path for ultraultra-low emissions of particulate matter from coal-fired power plants.

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.

This paper provides numerical study of the effects of mixed convection on particles removal from a cavity using multi-relaxation time thermal lattice Boltzmann method (LBM) for compute the flow and isotherm characteristics in the bottom heated cavity located on a floor of horizontal channel. A point force scheme was applied for particles-fluid interactionand double-distribution function (DFF) was coupled with MRT thermal LBM to study the effects of various grashof number (Gr) and Aspect Ratio (AR) on the efficiency of particles removal. The results show that change in Grashof number and Aspect ratio causes a dramatic different in the flow pattern and particles removal efficiency.