Advancements in EUV Optics Technology

Extreme ultraviolet lithography (EUVL) is the most likely lithography solution at 22 nm half-pitch and beyond. Therefore, in addition to aggressive immersion and double patterning programs, Nikon is also thoroughly engaged in EUVL development and is working closely with industry partners ASET, EUVA and Selete, as well as government organizations and academia, to resolve the multitude of technical challenges presented by this new way of imaging patterns.

Projection optics

The significant decrease in wavelength associated with EUVL creates substantial differences in optics compared with existing ArF (193 nm) systems. Since conventional lens materials are not transmissive at the 13.5 nm EUV wavelength, only reflective optics can be used. As with conventional lens elements, the mirrors must be exceptionally smooth to produce high-quality images. However, the very short wavelength necessitates far greater polishing accuracy than with conventional transmissive optics. The reduced wavelength also leads to much higher flare that must be minimized with very smooth optics.

To achieve the best reflectivity, the industry commonly manufactures EUV projection optic mirrors by coating low-expansion material with numerous layers of molybdenum and silicon. New aspheric mirror polishing technologies including ion-beam figuring (IBF) and elastic emission machining (EEM) are used in conjunction with traditional wet polishing processes¹ to achieve optimal surface smoothness. These advancements have yielded simultaneous improvements in low spatial frequency roughness (LSFR), mid spatial frequency roughness (MSFR) and high spatial frequency roughness (HSFR). LSFR is commonly known as surface figure error, and mainly determines the optical aberrations. MSFR is a metric closely associated with system flare, and HSFR affects surface reflectivity. Adoption of these new techniques has resulted in MSFR measurements that fully satisfy Nikon EUV1 Beta system objectives. Recent flare calculations show EUV1 flare levels of 8%,² which surpasses the 10% EUV1 program target and approaches the 7% flare goal for our next-generation EUV2 systems planned for 2010.

Projection optic coating processes have also been a key area of focus, as the requirement for high reflectance is critical to EUV throughput and thermal management. Mo/Si multilayer coatings deposited using sputtering processes (such as ion beam and magnetron) have very high compressive stress, which can degrade the surface figure of the precisely polished mirror substrates. Since the multilayer coatings are several hundred nanometers thick, the associated thickness distribution causes non-negligible figure errors on the reflective surface, thereby necessitating precise control of the internal stress and graded coating. Ongoing work in this area has resulted in Mo/Si multilayer coatings that are now simultaneously optimized for high reflectivity, low internal stress, and graded coating (where the thickness changes at a gradient). Measurements of the projection optic aspheric mirrors before and after multilayer coating confirmed that the optimized process essentially eliminated this surface figure issue.

As a result of significant manufacturing improvements, multiple EUV projection optics have been assembled and aligned with wavefront errors surpassing all program targets. At the latest SEMICON West show in July, Nikon shared projection optic measurement data over an exposure ring field size of 26 × 2 mm on the wafer. The results demonstrated an average wavefront error of 0.4 nm RMS with a maximum wavefront error of <0.5 nm RMS and a minimum of 0.3 nm RMS (Fig. 1).

1. EUV1 projection optic measurement data demonstrated an average wavefront error of 0.4 nm RMS.

Illumination optics

As with projection optics, EUV illumination optics present a variety of challenges. Reflection-type optical integrators, known as fly’s eye mirrors, have a vital impact on illumination optics performance. To further improve the fly’s eye surface roughness, new machining and a novel smoothing process were adopted. Additionally, while conventional fly’s eye mirrors were easier to manufacture, they essentially “blocked” the center of the pupil, resulting in significant power loss. To combat this, Nikon developed a more complex fly’s eye design, which offers much better power efficiency (Fig. 2). EUV illumination optics are also exposed to a very high thermal load, with the mirror closest to the source impacted the most. Efficient internal mirror cooling mechanisms have been developed to resolve this issue.

2. While conventional fly’s eye mirrors essentially blocked the center of the pupil, resulting in significant power loss, Nikon’s new fly’s eye designs result in minimal power loss.

To eliminate unwanted out-of-band (OoB) radiation (typically in the deep ultraviolet wavelength range), spectral purity filters (SPFs) are installed in the Nikon EUV1 system illumination optics unit. Although membrane-type filters are fragile and experience low EUV radiation transmittance, reflection-type SPFs do not experience these same issues. With minimal degradation in reflected EUV power, these specially designed multilayer mirrors greatly reduce the reflection of OoB radiation (Fig. 3).

3. Reflection-type spectral purity filters (SPFs) greatly reduce reflection of unwanted out-of-band radiation with minimal degradation in reflected EUV power.

Contamination control

Effective contamination control is also vital for successful EUVL applications. One key challenge is the decrease in relative reflectivity of the multilayer projection optic mirrors as a result of oxidation and carbon contamination of the mirror coatings. Such effects limit the usable lifetime of the optical components.³ Whereas carbon contamination blankets the coating and can potentially be cleaned, oxidation degrades the coating and is irreversible (Fig. 4).

4. The optics usable lifetime is limited as a result of coating oxidation (left), which is irreversible, and carbon contamination of the coating (right), which can potentially be cleaned.

Anti-oxidation capping layers and carbon film removal processes, as well as new methods and materials, are being studied to extend the optics lifetime. Nikon has brought online a new materials test station at the Saga synchrotron light source (SLS) facility in Kyushu, Japan, that uses a lightsource ~4× brighter than previous sources, enabling Nikon to evaluate several new coating materials. Several capping layers have demonstrated very high anti-oxidation capabilities — maintaining reflectivity over a significant dose range (Fig. 5). One such material will be implemented on Nikon optics. The reflectance change experienced when using pulsing vs. synchrotron lightsources also was investigated, but showed no significant difference between the two methods. The degree of oxidation change (ΔSi DoO) for the irradiated vs. reference condition was comparable for both SR (4.1%) and pulse (4.5%) sources.

5. A multitude of potential capping layers were evaluated, with several demonstrating high anti-oxidation capabilities.

To counter carbon film formation, the vacuum system design was also enhanced using materials with reduced outgassing. In addition, hydrocarbon film deposition on the multilayer mirrors can be suppressed and even removed using oxygen gas introduction under EUV irradiation (Fig. 6). Work is ongoing to enable less stringent photoresist outgassing limits. Refurbishment technologies of EUV optics at the end of their lifetime have also been an important area of investigation. These capabilities have the potential to provide significant cost of ownership (CoO) savings.

6. Carbon film deposition on the multilayer mirrors can be suppressed as well as removed using oxygen gas introduction under EUV irradiation.

Conclusion

The transition to EUV technology presents a significant challenge in the areas of projection and illumination optics, as well as contamination control. Through a variety of successful development partnerships, Nikon has made marked improvements in these areas. The adoption of sophisticated aspheric mirror polishing techniques and optimized multilayer coatings has enabled world-class projection optics. EUV1 has demonstrated wavefront error <0.5 nm RMS, with flare levels approaching targets for next-generation systems. Illumination optics development is also progressing well with enhanced fly’s eye designs, internal cooling systems, and the use of reflection-type SPFs.

Considerable advancements have also been made in the area of contamination control. Several potential candidates for anti-oxidation capping layers have already been identified, and problematic carbon films are suppressed and removed using newly developed in situ cleaning methods. Although challenges still remain for EUV technology, measurable progress has been made in several of the most vital areas, enabling the industry to continue on this critical path.