Recent dramatic price volatility and assurance of supply concerns with cerium oxide have left many users of this material in an uncertain and vulnerable position. Since few viable alternatives to ceria for precision glass polishing exist, and much of the supply is very concentrated geographically, technology which conserves ceria, improves absolute removal rate and promotes slurry longevity becomes extremely attractive under these circumstances. Using a plasma-based process to produce cerium oxide confers some unique attributes to the particles which make them particularly well suited for precision glass polishing operations. Many of those same particle characteristics, such as full crystallinity, near theoretical density, very high surface and bulk purity and extremely high zeta potentials in water can also be useful in mitigating the risks associated with a limited and costly ceria supply. This paper will explore how plasma-derived particles, in combination with a high performance chemistry package, can together constitute a fully formulated precision glass polishing slurry with very high activity, extended slurry lifetime, ability to recycle, and excellent overall process economics. Results showing the effect of particle longevity and chemical additives on removal rate and process stability will be discussed in detail, and selected examples which distinguish the benefits of a fully formulated, plasma-derived cerium oxide polishing slurry over conventional milled ceria will be shown.
Nano-sized particles with well defined geometries and size distributions suitable for polishing glass and glass ceramics
are readily available. Understanding how effective these particles are at removing material and smoothening surfaces
during pitch polishing processes is essential for process optimization and achieving better surfaces. This paper details
work conducted to measure how effective sub-micron sized particles are at polishing and to isolate the influence of
process chemistry and slurry density on the material removal rate (MRR). The paper also details how modifying the
slurry pH affects the polishing coefficient of friction (CoF). Fused silica was polished on a synthetic pitch polishing tool
with a range of different polishing slurries. Slurries tested included 40nm diameter ceria based slurries with varying
density and pH, and both 20nm and 750nm diameter ceria based slurries with fixed density and pH values. Findings
include that a) the material removal rate decreases with particle size and decreasing slurry density, b) the surface finish is
not strongly dependent on particle size, c) slurries with a pH of 7 are most effective in removing material, while slurries
with a pH value of 4 have the lowest MRR, and finally that d) the polishing CoF is greatest at pH 4 and lowest at pH 10.
The results indicate that while process chemistry is very influential when polishing with submicron sized particles, the
actual nature of the interaction between the abrasive, the workpiece and the tool requires further investigation.
EUV lithography promises large gains in resolution as a result of the extremely short wavelength. However, the requirement of aspherical off-axis mirrors dramatically increases the challenge of the optics manufacture relative to refractive designs. For example, because of the short wavelength of only 13.5 nm, a homogenous roughness and RMS values of 2 angstroms and below are necessary for sufficient throughput and high uniformity on these parts, and these specifications can only be achieved obtained through complex polishing processes. Because of these exacting microroughness requirements, fabrication technology is being driven to the exploration of new areas.
An example of one of these new technology areas involves the use of nanocrystalline cerium oxide made using a patented plasma arc process that produces particles with very well defined physical properties. Because of the unique manufacturing process, these particles have highly controlled surface chemistry which results in the ability to prepare extremely stable dispersions in water. As such, these dispersion are useful in a variety of polishing processes where a small particle and a tightly controlled particle size distribution are required to access increasingly stringent surface roughness requirements.
Carl Zeiss SMT has evaluated a number of cerium oxide slurries manufactured by Nanophase Technologies Corporation for improving polishing processes. The objective was to obtain reproducible low roughness values over a wide range of spatial frequencies. Results show that a significant improvement of the surface roughness was achieved with Nanophase ceria slurry CE-6068 in all spatial frequencies.
Increasingly, a finer finish polish is being sought in a variety of technologically advanced glass polishing applications. From the production of photomask blanks to the manufacture of precision lenses and prisms, improvements in surface planarity and the reduction of incidents of defectivity are critical to achieving higher levels of device performance. Nanophase Technologies Corporation (NTC) uses a patented plasma arc synthesis technique to produce nanosized cerium dioxide (ceria) particles. The small particle size, narrow particle size distribution, and unique crystal morphology of this ceria allow new performance benchmarks to be realized in the areas of decreased surface roughness (increased planarity), and reduced residual defects and subsurface damage. However, the success or failure of a nanocrystalline material in a particular application depends on the ability to form and maintain a stable dispersion with controlled rheology. The preparation of stable dispersions of nanocrystalline cerium dioxide particles is discussed with the following performance figures of merit - particle size stability, dispersion settling stability, and the ability to manipulate the isoelectric point and the zeta potential of the particles. Aspects of the dispersion specifically engineered for glass polishing, including pH and rheology characteristics, are presented. To illustrate the application of the nanocrystalline ceria dispersion, a specific example of an integrated program to reduce defects in the manufacture of quartz glass substrates and EUV (Extreme Ultraviolet) mask blanks at Schott Lithotech AG in Meiningen is presented. Substrates made of Low Thermal Expansion Materials (LTEM) require modified process formulations to achieve the flatness, roughness and defectivity goals of EUVL (Extreme Ultraviolet Lithography) mask blanks. Results of these processes for several LTEM options establish direction for material improvements, define key polishing process parameters, and guide the identification of important opportunities for future process adaptations. Results of the low defect polishing process for quartz glass substrates will be reported, along with activities directed toward adapting this process for LTEM substrates.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.