Robert Sabia

PERFORMANCE CHARACTERIZATION OF CERIUM OXIDE ABRASIVES FOR CHEMICAL-MECHANICAL POLISHING OF GLASS

Abstract Investigation of the nature of chemomechanical polishing (CMP) for glass requires an in-depth knowledge of the influences of abrasive physical properties and slurry chemistry. In the present work, eight cerium oxide abrasives were used to polish fused silica on a polyurethane pad. Results from pre-polished surfaces re-polished in aqueous and non-aqueous suspensions and from corrosion testing of solutions recovered from abrasive suspensions were used to discern chemical, chemomechanical, and mechanical equivalents for each abrasive. Performance indices (i.eabrasive quality values) were generated from the equivalents and related to conventional polishing of lapped surfaces. Optimally performing abrasives were found to require a balance of equivalents (i.eset ratio) as dictated by inherent chemical activities, heat treatments, and milling. From solution chemistry testing, particle/workpiece attraction via surface charge was found to increase the chemomechanical effect. The CMP mechanism is concluded to be a chemical reaction at the particle/workpiece interface promoted primarily by mechanical interactions such as abrasion, with both cerium oxide and rare-earth oxyfluoride constituents promoting the reaction.

The effect of abrasive hardness on the chemical-assisted polishing of (0001) plane sapphire

A series of abrasives with various hardness values including monocrystalline and polycrystalline diamond, α- and γ-alumina, zirconia, ceria, and silica were used to examine the concept of chemical-assisted polishing for finishing the (0001), c-plane (basal plane), of sapphire. Diaspore, a monohydrate of alumina, was also evaluated. Atomic force microscopy suggested that the hydrated layer of the c-plane surface was about 1 nm thick. Polishing experiments were designed to determine whether the chemically modified surface hydration layer forms on the basal plane in water. The results indicate that harder abrasives do not necessarily cause faster material removal and better surface finish for similar abrasive particle size. Abrasives with hardness equal to or less than sapphire such as α-Al2O3 and γ-Al2O3 achieved the best surface finish and greatest efficiency of material removal. It is proposed that the (0001) c-plane sapphire surface was modified by water to form a thin hydration layer with structure and hardness close to diaspore. This reaction layer can be removed by an abrasive that is softer than sapphire but harder than the reaction layer. α-Al2O3 was particularly effective. This result is attributed to adhesion between identical reaction layers on the basal planes of the alumina abrasive and the sapphire. This demonstrates that high removal rates and good surface finish can be achieved without costly diamond polishing.

Surface chemistry of SiO2 and TiO2–SiO2 glasses as determined by titration of soot particles

Abstract Surface charge is a critical issue for various types of glass processing including surface finishing (polishing) and cleaning. Unlike monomeric silicic acid with a pKa of 9.8, glass surfaces can have variable pKa values as determined by extent of Si–O–Si bonding, composition, and structure. The purpose of this study was to determine the surface charge, point of zero charge, and pKas for high purity fused SiO2 and TiO2–SiO2 (∼7 wt% TiO2) glasses. To achieve necessary surface areas for titration, colloidal soot particles were used in this study rather than glass monolith. Soot particles have the same inherent physical and chemical properties as the product glasses that are made by the same chemical vapor flame hydrolysis deposition process. Titration experiments performed in 10−1–10−5 M NaCl solutions revealed dissociation constants (i.e., intrinsic pKa1 and pKa2 values) of 0.0±0.2 and 7.0±0.1, respectively for the fused SiO2 particles, and 0.0±0.1 and 5.0±0.2 , respectively for the TiO2–SiO2 particles. Points of zero charge for each material were calculated as 3.5±0.1 and 2.5±0.1 for the fused SiO2 and TiO2–SiO2 particles, respectively. The role of TiO2 in lowering point of zero charge and pKa2 values is hypothesized to be the result of four-fold coordination as compared to the six-fold coordinated TiO2 used in surface chemistry studies reviewed by this paper.

Characterization and characteristics of a ULE glass tailored for EUVL needs

The EUVL industry has unique material requirements, which are being addressed. Implementation of metrology methods new to ULE Glass will be discussed along with material characteristics altered to meet the needs of EUVL. Metrology methods include multiple means of evaluating the striae, CTE and inclusions. Material characteristics have been altered to better meet the demands of the industry. The reduction in inclusion levels along with other improvements such as in the area of striae will be discussed here. Improvements of greater than 4x were achieved in these preliminary striae reduction trials.

Characterization of striae in ULE for EUVL optics and masks

Compositional striations in Cornings Ultra Low Expansion (ULE®) glass are thought to affect the surface roughness when the glass is polished. For EUV Lithography photomask blanks, it is important for the polished surface to be as smooth as possible. Therefore, since the compositional striations may impact photomask polishing, Corning has undertaken an effort to better characterize the striae and its impact on surface roughness, improve the fundamental understanding of its origin during boule formation, and develop methods and procedures to reduce its potential impact on polishing. This work has verified that striae can vary quite a bit throughout a single ULE glass boule. Characterization has shown that there are two main types of striae. These can be described as high frequency (secondary) striae and lower frequency (primary) striae. Due to the new understanding of the striae origin, two methods have recently been identified and used to greatly reduce or eliminate the high frequency striae component. Currently, new modeling efforts have helped identify potential process changes that may reduce the impact of the primary striae frequency. Experiments are in process to determine their effectiveness.

Corning ULE® glass can meet P-37 specifications

Corning ULE® glass is a binary SiO2 + TiO2 composition formed directly using a flame hydrolysis process. ULE possesses a very low thermal expansion range that can also be accurately adjusted for various applications including EUV photolithography. For ULE to be used for mask blanks and optics applications, it is also necessary that the material be capable of meeting stringent flatness and roughness specifications. For ULE, small compositional striations have been shown to affect the surface quality by inducing mid-spatial frequency roughness during polishing. Therefore, the main challenge has been to reduce mid-spatial frequency roughness to an acceptably low level by diminishing compositional striations present in the glass. Recently, a combination of predictive modeling and experimentation has resulted in a process that reduces striae to the levels needed for EUV masks and optics. These models have enhanced the fundamental understanding of the glass forming process, leading to process adjustments both in oscillation patterns and additional thermal treatments producing glass with improved striae characteristics. ULE masks with reduced striae have been polished to mid-spatial frequency roughness peak-to-valley levels of less than 8 nm. This sub-8 nm topography accounts for less than 20% of the total 50 nm flatness error budget allowable for EUVL masks. These results indicate that Cornings ULE product can meet the P-37 surface finishing specifications, and combined with ULEs superior CTE performance is positioned as the material of choice for EUV mask blanks.

Corning 7972 ULE material for segmented and large monolithic mirror blanks

Ultra-Low Expansion (ULE®) glass has been and continues to be a significant material for astronomical applications. With a nominal composition of 7 wt. %TiO2 in SiO2, Corning Code 7972 ULE® has a mean room temperature coefficient of thermal expansion (CTE) of 0 ± 30 ppb/°C with a typical CTE range of less than 15 ppb/°C, properties vital to the manufacture of high resolution optics requiring extreme thermal stability. Combined with lightweighting techniques developed at Corning during the past 30 years, ULE® has been successfully employed for numerous monolithic and lightweight mirror applications including the 2.4 meter Hubble Space Telescope lightweight primary mirror, the Airborne Laser (ABL) primary mirrors, and most recently the Discovery Channel Telescope 4 meter mirror blank. ULE® maintains its strong candidacy for future ELT applications. Recent challenges in mirror surface specifications and the development of alternative material choices calls for a comparison with ULE®. The objective of this article is to review ULE® properties and manufacturing capabilities, and to compare relevant material properties to those of alternative material options, thus allowing designers to properly execute material selection. Finally, recent development efforts directed toward improving ULE® will be discussed.

Acidic dispersion of fused SiO2 particles

The acidic dispersion behavior of fused SiO 2 particles was investigated and compared to two fumed SiO 2 particles. In contrast, the fused SiO 2 particles have a larger particle size, broader size distribution, and lower surface area. Fluoride adsorption was used to study surface activity, and acid-base titration was used to study surface charge in 10 - 1 to 10 - 3 M NaCI solutions over the pH range of 2-7.5. Each of the three SiO 2 particles exhibited similar titration behavior, with the fused SiO 2 particles displaying a higher intrinsic pK a 2 value of 7.0 as compared to 6.8 and 6.1 for the two fumed SiO 2 particles. Rheological experiments designed to test for dispersion and agglomeration/ gellation at 3 and 6 wt% solids loading in 10 - 3 M NaCl solutions adjusted to pH 2, 4, and 6 showed the fused SiO 2 particles to be more stable in suspension, exhibiting lower viscosity results for all test conditions. Results show that the fused SiO 2 particles tested in this report display superior dispersion properties as compared to conventional fumed SiO 2 particles for slurry applications under acidic conditions.