Jaehyuck Choi

Mechanism of particle removal by megasonic waves

We elucidate the major mechanism of microparticle removal in the megasonic cleaning process through the direct visualization experiments. It is revealed that particles sitting on solids are removed by adjacent microbubbles that oscillate near the substrates and exert interfacial and pressure gradient forces on the particles. Other pressure and streaming effects are shown to be too weak to detach the particles.

Visualization and minimization of disruptive bubble behavior in ultrasonic field

Although ultrasonic technology has been successfully adopted for semiconductor cleaning, a recent trend of extreme miniaturization of patterns calls for a novel process that can remove contaminant particles without damaging nanoscale patterns. Unstable bubble oscillations have been hypothesized to cause such surface damages, and here we show direct visualization results that a high acoustic pressure induces bubble instability leading to pattern damages. As a remedy for the conventional ultrasonic cleaning scheme, we introduce a novel cleaning system using dual transducers, in which one transducer generates bubbles with a high acoustic pressure in an acoustically isolated sub-chamber and the other drives the oscillation of bubbles around the cleaning area at a low acoustic pressure. The system is shown to achieve a high cleaning efficiency for submicron-sized particles while significantly suppressing the disruptive bubble instability thereby reducing the detachment of firmly attached nanoparticles. Comparison of the adhesion force of the firmly attached nanoparticles and the yield strength of nanopatterns allows us to anticipate that this scheme is capable of reducing damages of nanopatterns on semiconductor wafers and photomasks.

Lifetime of EUVL masks as a function of degree of carbon contamination and capping materials

Lifetime of EUVL masks which are intentionally contaminated with carbon is investigated by comparing Si and Ru capping layer. Carbon deposition is observed not only on the multilayer, but also on the absorber sidewall of the mask. Deposited carbon on the sidewall during EUV exposure gradually varies mask CD and also induces the changes in the wafer printability and dose in the scanner. In addition, we compare the effects of carbon contamination between Si and Ru capped blank. Ru capped blank shows longer mask mean time between cleaning (MTBC) than Si capped blank by 25% in our experiments.

Prevention of chemical residue from growing into Haze defect on PSM pattern edge after normal cleaning process

It is known that PSM pattern edge (MoSiON/Qz boundary) of EA-PSM mask is the weakest point against Haze occurrence in real mass production. Based on the understanding of these phenomena, we have developed very efficient ways to protect PSM pattern edge from Haze defect formation even after normal SPM cleaning processes. Oxide layer formulated on the PSM pattern (including pattern top and side) is actively trapping chemical ions existing on the surface and inside bulk of mask substrate, preventing their motion or diffusion toward Haze defect creation during laser exposure. As a result, we are able to reduce cleaning frequency of each EA-PSM mask set without Haze issues and thereby dramatically expand their life time in real mass production.

Substrate effects on the characteristics of Haze defect formation on the photomask surface under exposure condition

We have explored substrate effects upon the characteristics of haze creation on the mask surface by performing surface analysis for each of Cr, MoSiON, and Qz substrates of the mask before and after laser exposure. We found out chemical ions such as sulfur and ammonium ions should have different mobility behavior towards haze defect creation depending on each substrate during laser exposure. This fact can partially clarify the reason why haze occurrence on the mask in real mass production mainly comes up with Qz substrate surface even though it has the lowest level of chemical residue on it. We also realized that sulfur ions are penetrating into a sub layer of Qz substrate and even deeper during laser exposure, which signifies that we may have to remove a thin surface layer from Qz substrate to further improve haze issue from the current standpoint.

Effect of SPM-based cleaning POR on EUV mask performance

EUV masks include many different layers of various materials rarely used in optical masks, and each layer of material has a particular role in enhancing the performance of EUV lithography. Therefore, it is crucial to understand how the mask quality and patterning performance can change during mask fabrication, EUV exposure, maintenance cleaning, shipping, or storage. The fact that a pellicle is not used to protect the mask surface in EUV lithography suggests that EUV masks may have to undergo more cleaning cycles during their lifetime. More frequent cleaning, combined with the adoption of new materials for EUV masks, necessitates that mask manufacturers closely examine the performance change of EUV masks during cleaning process. We have investigated EUV mask quality and patterning performance during 30 cycles of Samsungs EUV mask SPM-based cleaning and 20 cycles of SEMATECH ADT exposure. We have observed that the quality and patterning performance of EUV masks does not significantly change during these processes except mask pattern CD change. To resolve this issue, we have developed an acid-free cleaning POR and substantially improved EUV mask film loss compared to the SPM-based cleaning POR.

Effect of EUV exposure upon surface residual chemicals on EUV mask surface

Photo-induced defect for optic mask mainly depends on the surface residual ions coming from cleaning process, pellicle outgassing, or storage environments. Similar defect for EUV mask triggered by accumulated photon energy during photolithography process has drawn interest recently but this defect is somewhat different from normal photo-induced defect for optic mask. The photo-induced defect for EUV mask is known to be created by the chemical deposition of Carbon atoms originating from cracking of hydrocarbons by EUV light and secondary electrons on capping layer. It is very likely that Carbon contamination would be dominant under normal EUV exposure condition. On the other hand, it is expected that another kind of photo-induced defects would rise to surface under controlled environment where Carbon contamination growth is severely suppressed. We may have to understand the behavior of surface residual ions under EUV light in order to cope with another probable EUV photo-induced defect. In this paper, we will investigate whether surface ions remaining after cleaning process like sulfate or ammonium ions would create printable defects or decompose into evaporable species under EUV light. In case they create certain defects on mask surface, their effect on EUV reflectivity and absorber pattern CD variation will be also examined. Finally, improved cleaning process to impede photo-induced defect creation on EUV mask will be introduced.

Real time analysis of the haze environment trapped between the pellicle film and the mask surface

With the use of 193nm lithography, time-dependent haze problem has become a critical issue for semiconductor industry. The understanding of the conditions that create haze defects is very crucial for the future development of haze-free cleaning processes. The gaseous environment trapped between the pellicle film and the mask surface triggers photochemical reaction under laser exposure, which could result in the formation of killer (printable) defects on the mask surface. Therefore, the real time analysis of the haze environment in the trapped space could provide essential clues to the characterization of haze defect growth mechanism. This fundamental study can be applied to the invention of real-time monitoring tools for the defect growth progress on the mask surface as well as the development of haze-free cleaning processes. Here, we propose a method to analyze the gaseous space trapped between the pellicle film and the mask surface that creates a highly reactive environment.

Evaluation of dry technology for removal of pellicle adhesive residue on advanced optical reticles

The fast pace of MOSFET scaling is accelerating the introduction of smaller technology nodes to extend CMOS beyond 20nm as required by Moore’s law. To meet these stringent requirements, the industry is seeing an increase in the number of critical layers per reticle set as it move to lower technology nodes especially in a high volume manufacturing operation. These requirements are resulting in reticles with higher feature densities, smaller feature sizes and highly complex Optical Proximity Correction (OPC), built with using new absorber and pellicle materials. These rapid changes are leaving a gap in maintaining these reticles in a fab environment, for not only haze control but also the functionality of the reticle. The industry standard of using wet techniques (which uses aggressive chemicals, like SPM, and SC1) to repel reticles can result in damage to the sub‐resolution assist features (SRAF’s), create changes to CD uniformity and have potential for creating defects that require other means of removal or repair. Also, these wet cleaning methods in the fab environment can create source for haze growth. Haze can be controlled by: 1) Chemical free (dry) reticle cleaning, 2) In‐line reticle inspection in fab, and 3) Manage the environment where reticles are stored. In this paper we will discuss a dry technique (chemical free) to remove pellicle adhesive residue from advanced optical reticles. Samsung Austin Semiconductors (SAS), jointly worked with Eco‐Snow System (a division of RAVE N.P., Inc.) to evaluate the use of Dry Reactive Gas (DRG) technique to remove pellicle adhesive residue on reticles. This technique can significantly reduce the impact to the critical geometry in active array of the reticle, resulting in preserving the reticle performance level seen at wafer level. The paper will discuss results on the viability of this technique used on advanced reticles.

A new paradigm for haze improvement: retardation of haze occurrence by creating mask substrate insensitive to chemical contamination level

Haze issues are getting more serious since size of Haze defect printable on the water surface that could matter is decreasing further with reduced pattern size. Many efforts have been made to reduce the contamination level on the photomask surface by applying wet or dry processes. We have successfully reduced surface contamination down to subppb level for organic and inorganic chemicals. No matter how well the mask surface is cleaned, chemical contaminant cannot be perfectly eliminated from the surface. As long as contaminants exist on the surface, they are getting aggregated around certain points with higher energy to create defects on it during laser exposure. Also, the cleaned mask surface could be contaminated again during following processes such as shipping and storage. Here, we propose a new paradigm for Haze retardation where we severely decelerate defect generation and growth rather than eliminate chemical contaminants on the mask surface. We have made mask surface on which chemical contaminants are hardly accumulated to generate Haze defects even during laser exposure. By creating mask surface insensitive to chemical impurity level up to a certain degree, we are able to retard Haze occurrence much better than by reducing surface impurities down to sub-ppb level. This approach has another advantage of allowing a freedom for mask environment during the process of shipping, storage, and exposure. We further investigate how the treated mask surface should have strong resistance against chemical contaminant aggregation towards Haze defect generation around specific points with high energy.