Today's binary and EAPSMs need advanced mask cleaning methods
02/01/2004
The increased demands of high-performance photomasks required for lithography advancement have brought with them a need to tweak and update the various aspects of mask processing. For example, engineers have resolved some of the critical issues associated with cleaning binary and embedded attenuated phase-shift masks (EAPSM). These technologies combine hardware modification with conventional wet chemistry optimization.
Recently, a significant effort has focused on developing and improving chromium-based binary and molybdenum-silicon-based attenuated phase-shift masks (MoSiON) and their commercial applications. The Mo-Si phase-shift layer has chemical and physical properties distinct from those of the conventional chromium binary masking layer, however, including high chemical reactivity against most alkaline solutions, a high degree of affinity to particles, and ease of patterning with dry etchers.
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Easy patterning with dry etching technology for EAPSMs has brought in a new era of mask cleaning because post-etch resist stripping and polymer residue removal become increasingly critical for the phase-shift layers compared to chromium absorber layers. The high particle affinity and defect printability of the shifter surface require more aggressive or more frequent rework cleaning, whereas critical dimension and optical properties can be significantly altered due to the high chemical reactivity between the substrate and cleaning chemistries.
Although wet cleaning has long been the common process for controlling defect density on photomasks, both the cleaning process and cleaning equipment (tools) must advance to meet existing and future technological challenges. Thus, Akrion process engineers have been developing cleaning technology to improve and optimize the performance of binary photomasks and EAPSMs.
Performance vs. substrate integrity
Masks were cleaned with Akrion's immersion mask-cleaning equipment using H2SO4-based chemistries — either sulfuric-acid hydrogen-peroxide mixture (SPM) or sulfuric-acid ozone mixture (SOM), ozonated deionized water (DIO3), and ammonium-hydroxide-hydrogen-peroxide mixture (APM) in tanks with filtered recirculation loops, deionized water rinse tanks, and Akrion's low IPA-consumption dryer.
 Figure 1. The effect of different cleaning chemistries on phase and transmittance of EAPSMs. |
Traditionally, SPM at elevated temperatures (>100°C) followed by APM has been used in reticle cleaning to remove photoresist and organic residue from previous processes. However, SPM applications have drawbacks such as difficulty maintaining bath concentration. Moreover, test results have shown that SPM produces relatively great variations of phase-shift angle and transmittance compared to other oxidizing chemistries (see Fig. 1). Given these concerns and others, ozone-involved chemistries have been introduced to replace SPM for mask-cleaning processes.
 Figure 2. The effect of megasonics on photoresist removal efficiency of SOM chemistry. |
Although the strong oxidizing power of ozone has made ozone chemistries (SOM and DIO3) excellent candidates for cleaning light organic contaminants, the efficiency of the chemistries for heavy polymer removal such as photoresist stripping is sometimes limited and depends on the throughput of presently available ozone generators. By incorporating megasonic technology, we have demonstrated enhanced resist-residue removal (see Fig. 2). Experimental study also indicated that there was no negative impact of adding megasonics in SOM to the phase and transmittance of Mo-Si films.
APM chemistry in combination with megasonic irradiation is well known for its capability to remove particles in silicon wafer and photomask industries. Due to the chemical sensitivity of EAPSM films, however, process parameters such as chemical concentrations, bath temperature, and process time vs. the optical characteristics and cleaning performance of photomasks need to be evaluated for process optimization. Our investigation has shown that diluting APM, controlling bath temperature, and fine-tuning megasonic hardware all contribute to better results.
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With properly tailored SOM, DIO3, and APM combinations, data collected from a mask manufacturer's site demonstrated that our mask-cleaning technology can achieve excellent cleaning performance without significantly compromising the substrate's optical requirements (see table).
Chemical residues on reticles after cleaning
Ionic residues left on the reticle surface are a concern in the photomask industry as the lithography wavelength continues to decrease. The high energy associated with a short wavelength of ultraviolet lasers could readily form crystallized substances through optical-chemical reactions on the mask surface, if ionic species such as SO4-2 and NH4+ associated with wet cleaning processes that use sulfuric acid and ammonia are not controlled in acceptable ranges after the chemical treatments. The ionics may volatilize in the pellicle cavity, changing a particle-free reticle to one with particles after repeated exposure to short-wavelength lithography, which will cause a significant yield loss.
 Figure 3. Surface ionic residue levels and phase-angle loss vs. different cleaning recipes. Extraction testing with ion chromatographic analysis. |
Typically, hot DI water application in the post-sulfuric rinse step and the final rinse step of a mask cleaning process is suggested to solve the problem. Otherwise, a sulfuric-absent cleaning process such as DIO3 followed by APM is a fundamentally effective solution. Figure 3 shows that all of our cleaning recipes result in lower levels of ionic residues than the control samples, suggesting a cleaning rather than contaminating effect. Note that the recipe with hot DI final rinse indeed produces the lowest level of ionic residues, but also yields the greatest phase loss for EAPSM; a trade-off needs to be taken into account in considering the overall performance. In general, we recommend hot DI water quick-dump rinse after the sulfuric acid step and leaving the hot final rinse after APM as an option.
Acknowledgments
DIO3 is a trademark of Akrion LLC. STARlight is a trademark of KLA-Tencor Corp.
Richard Novak is VP of advanced technology and CTO at Akrion LLC, 6330 Hedgewood Dr., Allentown, PA 18106; ph 610/530-3449, fax 610/530-3616, e-mail [email protected].
Ismail Kashkoush is director of applications and process engineering at Akrion.
Gim S. Chen is a senior process engineer at Akrion.