A pellicle is employed to protect a photomask from particle contamination. The pellicle is positioned at a distance from the photomask pattern such that particle contaminants are out of focus. To extend the lifetime of a photomask, the pellicle must be replaced when it becomes heavily contaminated or damaged. The replacement process generally consists of the following three steps, 1) removal of the existing pellicle, 2) cleaning of the photomask, and 3) installation of a new pellicle. The cleaning step is primarily required for the removal of pellicle-glue residue unintentionally left behind during the pellicle removal process. This cleaning step has traditionally employed aggressive chemistries, such as a long process cycle, utilizing a Sulfuric Acid and Hydrogen Peroxide mix (SPM). The major drawback from this approach is that it inadvertently exposes the photomask pattern to aggressive chemistries, risking CD shift, damage to sub-resolution assist features (SRAFs), and accelerated haze growth, thus, ultimately reducing a masks lifetime . To overcome these shortcomings, this paper presents a localized approach for removal of pellicle-glue residue, which does not require long SPM cycles for post-clean.
Megasonic cleaning remains the industry’s workhorse technology for particle removal on advanced 193i and extreme ultraviolet (EUV) photomasks. Several megasonic cleaning technologies and chemistries have been proposed and implemented over the years in diverse production environments. The operational range of these process technologies, over a wide array of applications, is ultimately defined by measurable capability limits. As geometries continue to scale-down and new materials are introduced, existing cleaning technologies will naturally fade out of range and new capability is ultimately required. This paper presents a novel fundamental approach for expanding cleaning capability by use of high-frequency megasonics and tenside-based additives (BASF SELECTIPUR C-series). To this end, a sonoluminescence-based experimental test bench was configured to characterize and study the effects of various process parameters on cleaning performance, with a particular emphasis on cavitation-induced damage and enhancement of particle removal capabilities. The results from the fundamental studies provide a path forward towards delivering new cleaning capability by enabling high-frequency megasonic systems and tenside-based additives.
In the absence of pellicle a EUVL reticle is expected to withstand up to 100x cleaning cycles. Surface damage upon wet and dry cleaning methods has been investigated and reported in recent years.  Thermal stress, direct photochemical oxidation and underlying Silicon layer oxidation are reported as the most relevant root-causes for metal damage and peeling off. [2,3] An investigation of final clean performance is here reported as a function of operating pH; the results show increased Ruthenium durability in moderately alkaline environment. The electrochemical rationale and the dependency of the reducing strength of the media with the pH will be presented as possible explanations for reduced damage.
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.
SPM (Sulfuric acid peroxide mixture) which has been extensively used for acid cleaning of photomask and wafer has serious drawback for EUV mask cleaning. It shows severe film loss of tantalum-based absorber layers and limited removal efficiency of EUV-generated carbon contaminants on EUV mask surface.
Here, we introduce such novel cleaning chemicals developed for EUV mask as almost film loss free for various layers of the mask and superior carbon removal performance. Combinatorial chemical screening methods allowed us to screen several hundred combinations of various chemistries and additives under several different process conditions of temperature and time, eventually leading to development of the best chemistry selections for EUV mask cleaning.
Recently, there have been many activities for the development of EUV pellicle, driven by ASML and core EUV scanner customer companies. It is still important to obtain film-loss free cleaning chemicals because cleaning cycle of EUV mask should be much faster than that of optic mask mainly due to EUV pellicle 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 changes and film losses during 50 cleaning cycles using new chemicals as well as particle and carbon contaminant removal characteristics. We have observed that the performance of new chemicals developed is superior to current SPM or relevant cleaning chemicals for EUV mask cleaning and EUV mask lifetime elongation.
In recent years, photomask resist strip and cleaning technology development was substantially driven by the industry's need to prevent surface haze formation through the elimination of sulfuric acid and ammonium hydroxide from these processes. As a result, conventional SPM (H2SO4 + H2O2) was replaced with Ozone water (DIO3) for resist stripping and organic removal to eliminate chemical haze formation [1, 2]. However, it has been shown that DIO3 basted strip and clean process causes oxidative degradation of photomask materials [3, 4]. Such material damage can affect optical properties of funcitional mask layers, causeing CD line-width, phase, transmission and reflection changes, adversely affecting image transfer during the Lithography process. To overcome Ozone induced surface damage, SUSS MicroTec successfully developed a highly efficient strip process, where photolysis of DIO3 is leading to highly reactive hydroxyl radical formation, as the main contribution to hydrocarbon removal without surface damage . This technology has been further extended to a final clean process, which is utilizing pure DI water for residual organic material removal during final clean . Recently, SUS MicroTec did also successfully release strip and clean processes which completely remove NH4OH, eliminating any chemicals known today to induce haze . In this paper we show the benefits of these new technologies for highly efficient sulfate and ammonium free stripping and cleaning processes.
In the absence of a pellicle, an EUVL reticle is expected to withstand up to 100 cleaning cycles. EUVL reticles
constitute a complex multi-layer structure with extremely sensitive materials which are prone to damage during
cleaning. The 2.5 nm thin Ru capping layer has been reported to be most sensitive to repeated cleaning, especially
when exposed to aggressive dry etch or strip chemicals . Such a Ru film exhibits multiple modes of failure under
wet cleaning processes. In this study we investigated the Ru peeling effect. IR-induced thermo-stress in the multilayer
and photochemical-induced radical attack on the surface are investigated as the two most dominant
contributors to Ru damage in cleaning. Results of this investigation are presented and corrective actions are
We present results on the photorefractive performance of cyclometalated complexes in which a central metal atom (Pd or Pt) coordinates two different molecular sub-units in a single species. Depending on the details of their structure, these molecules aggregate in crystals, glasses or liquid crystalline phases. The photorefractive properties of the complexes are discussed by treating separately results obtained in different phases. Crystalline compounds can be dissolved in suitable polymers and we show how phase separation in polymeric composites, which is usually detrimental for sample stability, can be controlled and used to increase photorefractive performance parameters by orders of magnitude. In addition, we present a method for estimating the itensity of the space-charge field in chiral smectic phases without using any of the standard models developed for crystalline or amorphous materials.
We present results on the photorefractive performance of two different classes of organic materials. One of them is based on the space-charge field induced reorientation of the optical axis of chiral smectic A phases. In this case the orientational effect is linear in the field and it is due to the so-called electroclinic effect, in contrast with the quadratic effect present in nematics and associated with dielectric anisotropy. Besides presenting data on the photorefractive properties of these new mesophases, we will consider a simple model which describes their performance as a function of several material and geometrical parameters. In the second part of the paper we introduce cyclopalladated complexes as a new class of multifunctional photorefractive materials. Such molecules form amorphous phases which are photoconducting and exhibit a field dependent refractive index. Their efficiency is among the best known to date for organic materials and the simple synthetic route makes us foresee a fast optimization of cyclometallated compounds for photorefractive applications.