Mark J. Davis

Influence of water on nucleation kinetics in silicate melt

Abstract Steady-state rates and induction times for crystal nucleation in lithium disilicate melt depend exponentially on water content (up to 975 ppm). Time-dependent crystal number densities exhibit self-similar behavior as a function of water content, thereby allowing for a description of all data by a universal curve. This behavior provides a powerful tool for predicting crystallization histories, implies that mechanisms involved in nucleation are accelerated by water, and suggests that no new reaction pathways arise with the addition of water. The influence of water on crystal nucleation cannot be solely explained by the effect of water on viscosity; in addition, the free energy barrier to nucleation is ∼ 8% with an addition of water from 130 to 975 ppm.

Heterogeneous crystal nucleation on bubbles in silicate melt

Abstract Experiments reported herein document heterogeneous crystal nucleation on bubbles in supercooled lithium disilicate melt. Crystalline lithium disilicate (Li2Si2O5) nucleated and grew on small bubbles (∼1 μm) with a one-to-one correspondence between the number of bubbles and crystals (ranging from <102 to ∼105 bubbles/mm3). Crystals grew on large bubbles (>100 μm) only in samples fused in N2, suggesting a chemical control on nucleating efficiency. Bubbles ∼1 μm in diameter served as nucleation sites for polycrystalline lithium disilicate spherulites; bubbles smaller than ∼1 μm served as nucleation sites for the more common ellipsoidal crystalline form. This difference in behavior might be due to the additional surface area available for crystal nucleation on the 1 μm bubbles. Our findings suggest that superliquidus thermal history can influence crystal nucleation via bubble formation induced by supersaturation, and has implications for both natural samples and experimental studies. Heterogeneous crystal nucleation on bubbles may serve as an efficient nucleation mechanism in natural degassing magmas and may aid in the formation of fine-grained groundmasses common to many volcanic rocks. Furthermore, we have documented a new mechanism of spherulite formation in highly supercooled silicate melt, similar to conditions thought to exist during devitrification of natural glasses. The ability of crystals to nucleate on bubbles can be exploited in the production of commercial glass-ceramic materials.

Effects of thermal history on crystal nucleation in silicate melt: Numerical simulations

[1]xa0We have used a numerical treatment to investigate time-dependent phenomena associated with transient crystal nucleation in a model silicate melt. Our approach is the first to explicitly account for both thermodynamic (surface and bulk free energies) and kinetic (equilibrium and nonequilibrium transport) driving forces in molten silicates. The degree of undercooling, transient nucleation behavior, and structural relaxation effects are included in our model. Our model can simulate a variety of natural processes, ranging from the rapid cooling of fragmenting melt during a volcanic eruption to the slow devitrification of a chilled lava flow. To illustrate the utility and generality of our approach, we present numerical solutions for T–t paths that mimic the conditions of experimental studies on the one-component lithium disilicate system [Davis et al., 1997]. Our numerical simulations suggest that the formation of a finite number of crystalline clusters occurs on quench of starting materials. Nucleation is enhanced initially by preexisting nuclei when quenched glasses are subjected to a nucleation treatment. The simulations further suggest that the effect of preexisting nuclei, cooling rate, and nonequilibrium viscosity becomes negligible for longer nucleation treatments (>1000 min). We also provide simulation results for the geologically relevant physical model of the devitrification of a lava flow following burial by a new flow, including the calculation of the crystal size distribution (CSD). This example exhibits several interesting features, indicating multiple nucleation events, negative nucleation rates, and widely disparate final CSDs depending on the position within the flow.

Influence of hydroxyl on glass transformation kinetics in lithium disilicate melt and a re-evaluation of structural relaxation in NBS 710 and 711

Differential scanning calorimetry (DSC) experiments are presented that document the strong effect that ppm-level hydroxyl concentrations have on glass transformation kinetics in silicate melt. In particular, relaxation times decrease by an order of magnitude and limiting fictive temperatures (Tf∞) decrease ~20 K with an increase in hydroxyl content from 30 to ~1400 ppmw in the lithium disilicate system. Activation energies derived from Arrhenius plots of reciprocal Tf∞ values versus log(quench rate) show no dependence on hydroxyl content. Heat capacity scans of samples with various hydroxyl contents quenched at the same rate collapse to single curves when scans are plotted using (T−Tf∞). Such behavior can be incorporated most simply into the Tool–Narayanaswamy (TN) model of structural relaxation by allowing only the pre-exponential term in the relaxation function to be a function of hydroxyl content. Fits of our data to the TN model were obtained using three different activation energy values: (1) 757 kJ/mol (from cooling rate dependence of Tf∞); (2) 599 kJ/mol (from published viscosity data); and (3) 526 kJ/mol (4-parameter fit). The fit using the activation energy from viscosity measurements provided the best overall fit. In addition, structural relaxation phenomena of NBS 711 and NBS 710, lead silicate and soda-lime silicate compositions, respectively, were re-examined and a significant discrepancy in activation energies for NBS 710 and 711 between this study and earlier work was documented.

Optimized glass-ceramic substrate materials for EUVL applications

EUV substrate materials have to meet enhanced requirements with respect to extreme low thermal expansion, high homogeneity and superior surface quality. A SCHOTT R&D program aims at the development of advanced materials covering these various aspects. The glass-ceramic Zerodur (registered trademark) of SCHOTT represents a substrate material currently used for EUV masks and optics of first generation tools due to its extremely low coefficient of thermal expansion (CTE) and its excellent homogeneity. Zerodur(registered trademark) even allows continuous shifting of the position of zero crossing of the CTE-slope to control the thermal expansion behavior according to varying customer requirements: As a result of specifically adjusted process parameters, samples of Zerodur (registered trademark) exhibit a coefficient of thermal expansion CTE < 5 ppb/K corresponding to the lowest expansion class of the SEMI standard P37 (19 to 25°C) for EUV mask blanks. By further variation of process parameters, the position of zero crossing, e.g. at 22.5°C or 30°C, can be varied, revealing an attractive attribute feature of Zerodur (registered trademark). A new dilatometer type reveals an improved reproducibility of ~ 1ppb/K in the temperature range of 0 to 50°C. A series of CTE(0;50°C) measurements with a test-cube of Zerodur (registered trademark) provides information on CTE homogeneity on a cm-scale: no CTE variation was observed within the error of measurements (1ppb/K) for a block exhibiting ± 3.5*10-6 variation in refractive index. CTE variation can cause surface deformations during changing temperature conditions. A Fizeau-Interferometer was used to record surface roughness at two different temperatures. This non- destructive metrology is regarded as a method to distinguish CTE variation < 1ppb/K. The surface deformation of Zerodur (registered trademark) due to elevated temperature was determined to be lower than the resolution. Both methods to analyze the CTE homogeneity of Zerodur (registered trademark) lead to the result of CTE variation below 1 ppb/K. Surface treatment of glass-ceramic material is a major challenge as final finishing of EUV substrates may increase roughness of super-polished surfaces significantly. Improved new glass-ceramic materials demonstrate optimization of glass-ceramic compositions to nearly meeting the specification of surface roughness after a standard finishing process. Recent achievements of material development reveal CTE-performance of this new glass-ceramic to also be adjustable to varying customer needs as already known for Zerodur (registered trademark). These results are regarded as a promising milestone to develop an optimized glass-ceramic material, because the features of the modified New-Glass Ceramic now better match the key requirements of EUVL substrate materials.

Plutonism across the Tujunga-North American terrane boundary: A middle to upper crustal view of two juxtaposed magmatic arcs

Opportunities to study middle crust magmatic arc construction are uncommon. Petrologic studies in southern California have revealed middle to upper crustal portions of two juxtaposed Mesozoic magmatic arcs, one being a native terrane of the Cordilleran orogen and the other the “suspect” Tujunga (or San Gabriel) terrane. Crystallization thermobarometry of successively intruded and dated plutons provide “crustal nails” that track the variable depth history of orogenic crust in both terranes. Deep crust is now exposed at the surface by virtue of two contrasting tectonic settings — lower plates of extensional detachment faults in Tertiary metamorphic core complexes and upper plates of Mesozoic basement-involved thrust faults.

Improved materials meeting the demands for EUV substrates

The enhanced demands for substrate materials for next-generation optics and masks have initiated detailed investigations on Zerodur as a proposed EUVL substrate material with focus on thermal expansion behavior and surface roughness. As a result of specifically adjusted process parameters, the coefficient of thermal expansion (CTE) was tailored to be a minimum at 22.5°C. Laboratory samples of Zerodur exhibit a CTE < 5 ppb/K corresponding to the lowest expasnion class of the SEMI standard P37 (19 to 25°C) for EUV mask blanks. By further variation of process parameters, the position of zero crossing, e.g. at 30°C, can be varied, revealing an attractive attribute feature of Zerodur. A new dilatometer type was mounted in 2002 with first operatinoal results revealing an improved reproducibility of ~1ppb/K in the temperature range of 0 to 50°C. A series of CTE measurements with a small block of Zerodur provides information on CTE homogeneity on a cm-scale: No CTE variation was observed within the error of measurements for a block exhibiting ± 3.5*10-6 vairtion in refractinve index. CTE variation can cause surface deformations during changing temperature conditions. A first setup of Fizeau-Interferometer with a current resolution of 0.3 nm rms was used to record surface deformation of Zerodur due to elevated temperature was determined to be lower than the current resolution. Both methods to analyze the CTE homogeneity of Zerodur lead to the result of CTE variation below 1 ppb/K, still identifying todays need to improve metrology further. Final finishing of EUV substrates may increase roughness of super-polished surfaces significantly. Using appropriate processes a to surface roughness < 0.25 nm rms under production conditions can be achieved after final finishing of Zerodur. As an improved Zerodur-type material, recent achievements of material development demonstrate the optimization of glass-ceramic composition to nearly meeting the specification of surface roughness after a standard finishing process. These results are regarded as a promising milestone to develop an optimized glass-ceramic material providnig adjusted thermal expansion behavior and surface processability according to the specific demands of EUV technology.

Thermal expansion behavior of proposed EUVL substrate materials

The enhanced demands for substrate materials for next- generation optics and masks have initiated detailed investigations on Zerodur as a proposed EUVL substrate material. The dependence of thermal expansion of Zerodur on process parameters is illustrated herein as well as its utility for EUV substrate material demands. As a result of specifically adjusted process parameters, the coefficient of thermal expansion (CTE) was tailored to be a minimum at 22.5 degrees C. Laboratory samples of Zerodur exhibit a CTE corresponding to the lowest expansion class of the SEMI standard P37. By further variation of process parameters, the position of zero crossing, e.g. at 30 degrees C, can be varied, revealing an attractive attribute feature of Zerodur. A series of CTE measurements with a small block of Zerodur provides information on CTE homogeneity on a cm- scale: No CTE variation was observed within the error of measurements for a block exhibiting +/- 2 * 10-6 variation in refractive index. A new dilatometer type is in the course of development. First operational results are expected in Summer 2002 with an increased accuracy < ppb/K in the temperature range of 17 to 30 degrees C.

Thermal expansion and internal quality of a proposed EUVL mask substrate material: Zerodur

Detailed thermal expansion measurements and internal homogeneity measurements of the glass-ceramic material Zerodur were undertaken to examine its usefulness for EUVL. Repeat measurements on 100-mm long samples from three castings exhibit an expansion of approximately 12 +/- 2 ppb/K 2 (sigma) in the temperature range of interest for EUVL, corresponding to Class C of the draft SEMI 3148 standard. Internal homogeneity measurements reveal extremely small refractive index variations, suggesting comparably small compositional variations. This in turn is viewed as a necessary but not sufficient condition for high CTE uniformity, a factor required by EUVL applications.

New controlled rapid quenched technique in a 1 atm infrared image furnace

Abstract We describe a new quench technique that allows for controlled and reproducible constant quench rates exceeding 100 °C/s in the temperature range of 1400-700 °C and ~ 10 °C/s in the range 1400-200 °C at 1 atm total pressure. Our technique uses a 1 atm infrared image furnace and a blower unit capable of discharging cooling air through the infrared furnace at speeds approaching 100 m/s. The control protocol consists of operating the furnace at a constant power setting, sufficient to reach the highest desired temperature, and modulating the blower output continuously; blower output is controlled via a siliconcontrolled rectifier (SCR) using a proportional-integral-derivative (PID) algorithm on a personal computer (PC). A special autotuning procedure was used that enables the finetuning of PID parameters necessary for precise temperature control. Temperatures are controlled to ± 1 °C over the entire temperature range under isothermal conditions and to within 10 °C of the setpoint during quench. The range of accessible quench rates using our technique opens up new temperature-time paths for quantitative study. Potential applications include detailed studies on chemical diffusion and the kinetics of bubble and crystal formation under conditions of rapid temperature changes. Such studies have direct relevance to the crystallization, degassing, and structural relaxation of silicate melts during rapid temperature changes such as those encountered during volcanic eruptions.