Han-Shin Lee

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.

Damage/organic free ozonated DI water cleaning on EUVL Ru capping layer

The adaption of EUVL requires the development of new cleaning method for the removal of new contaminant without surface damage. One of the harsh contaminants is the carbon contamination generated during EUV exposure. This highly dense organic contaminant is hardly removed by conventional SPM solution on Ru capped Mo/Si multilayer. The hopeful candidate for this removal is ozonated water (DIO3), which is not only well-known strong oxidizer but also environmentally friendly solution. However, this solution might cause some damage to the Ru capping layer mostly depending on its concentration. For these reasons, DIO3 cleaning solutions, which are generated with various additive gases, were characterized to understand the correlation between DIO3 concentration and damages on 2.5 nm thick ruthenium (Ru) surface. An optimized DIO3 generation method and cleaning condition were developed with reduced surface damage. These phenomena were explained by electrochemical reaction.

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.

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.

Investigation of EUV haze defect: molecular behaviors of mask cleaning chemicals on EUV mask surfaces

Photo-induced defects (or haze defects) on 193nm optic masks (haze defects) have been a serious problem not only to reticle engineers working for mask manufacturing and handling but also to photo-lithography engineers. The most widely accepted explanation of the root causes of haze defects is the cleaning chemical residues remaining on the mask surface and unavoidable outgassed molecules that outgas from pellicle materials when exposed to 193nm radiation. These have been significant challenges for reticle cleaning engineers who need to use cleaning chemicals whose residues do not lead to progressive defect formation on the mask and to find improved materials to minimize pellicle outgassing. It is assumed that contamination generation on EUV masks would have a higher probability than on optic masks, primarily since EUV masks are not protected by a pellicle and amorphous carbon films can accumulate during exposure to EUV light. While there is potential to mitigate the generation of carbon contamination by improving the exposure tool environment and removing carbon films using in-situ atomic hydrogen cleaning, it is not yet clear whether the reaction of mask cleaning chemicals to EUV radiation will lead to creation of progressive defects on EUV mask surfaces. With the work to being done it has been observed that carbon contamination on EUV masks dominates any effects of solvent chemicals under normal environmental or exposure conditions (from atmospheric pressure up to a vacuum level of 10-6 Torr) during EUV exposure. However, it is still unknown whether residual cleaning chemicals will provide a nucleus for progressive defect formation during exposure. This lack of understanding needs to be addressed by the industry as EUV masks are expected to undergo more frequent cleaning cycles. In this work, we will report on an investigation of the molecular behavior of cleaning chemicals on EUV mask surfaces during EUV exposure. Movement (e.g., migration or aggregation) of cleaning chemical molecules near EUV exposure spots on the top surface and beneath the mask will be examined under high vacuum (~10-8 Torr). We will also investigate whether EUV exposure can trigger the evaporation of cleaning chemical residues from the EUV mask surface, possibly contaminating the exposure environment. Better understanding of the influences of the mask cleaning chemicals during exposure, coupled with knowledge about mask tolerance and patterning performance affected by the cleaning chemicals, should enable the proper selection of mask cleaning processes and chemicals to meet EUV requirements.

Formation mechanism of the photomask blanks material related haze

We have observed a new type haze of which formation deviates from the generally accepted models with respect to the size, shape, and removability by chemicals. It has very small size of 50~100nm and are crowded around the cell boundary, while the typical haze doesn’t prefer a special region on mask in the majority of cases. It is hard to remove by general cleaning, while the typical haze is easily removed by general cleaning process and even de-ionized water. It is confirmed that the source of the haze is blank material related ions which are formed by chemical etching of blanks during mask cleaning process or the photomask blanks itself.

Progressive defects caused by crosstalk between mask fabrication processes

Most of defects generated in mask fabrication processes have been mainly created during each unit process. It becomes more important to detect and remove smaller defects on mask as pattern nodes keep shrinking. Each unit processes are getting not only more challenging to sustain mask quality and defect level but also more influencing on other processes for smaller pattern nodes. New type of defects based on such influences (crosstalk) between different processes is starting to emerge, which is requesting for a revision of defect reduction strategy because dealing with crosstalk defects is directly related with quality and TAT of mask manufacturing. It is relatively difficult to properly understand root-cause or working mechanism of defects generated by crosstalk between different processes. This is because interaction between different processes from defect generation perspectives has hardly been studied. In this paper, we introduce emerging progressive defects created while etched masks are undergoing cleaning process or subsequent events of moving to next process or temporary storage. We will investigate how etch gas residues on mask surface remaining after etching process interact with cleaning chemicals or moisture from subsequent process or environment to trigger defect generation and its growth. We will also examine effects of POD outgassing on generation of crosstalk progressive defect. Based on this understanding, appropriate solutions to mitigate defects caused by crosstalk between mask fabrication processes will be proposed. It is believed that new type of progressive defects caused by crosstalk between different mask fabrication processes will be more flourishing in the near future where mask blank materials, mask manufacturing processes, and chemicals need to diversify in order to meet much tighter specifications of mask quality. Therefore, it is very crucial to have right understandings on the interactions between various processes and eradicate possible root-causes of defect generations.