TiO2-SiO2 glass is one of the leading candidates for optical elements of extreme ultraviolet lithography. TiO2-SiO2 glass synthesized by the soot method has shown striae related to inhomogeneity of TiO2 concentration formed in the planes perpendicular to soot growth direction in the synthesis process. It can induce CTE variation and localized surface roughness. Striae were characterized in three modes by polarization microscope. Such striae were improved with an improved gas condition and developing a modified material gas supply system. Specimen prepared from the improved TiO2-SiO2 glass was evaluated by a line-focus-beam ultrasonic material characterization system, using a surface-acoustic-wave mode. Improved glass had 43% striae level compared to conventional glass by birefringence measurement, 31% compared to conventional glass by the ultrasonic measurement. It was found that improved glass had good homogeneity to both directions perpendicular and parallel to striae plane.
This paper presents the development of a practical system for super-precise evaluation of zero-CTE temperatures T(zero-
CTE) of TiO2-SiO2 glasses for extreme ultraviolet lithography (EUVL) by measuring leaky surface-acoustic-wave
(LSAW) velocity VLSAW with a line-focus-beam ultrasonic material characterization (LFB-UMC) system. This new
system can evaluate T(zero-CTE) from 20 to 150°C on the surfaces of glass substrates for photomasks and optical
mirrors located at different positions in EUVL systems. This system operates in a stabilized temperature measurement
environment (e.g., 23.00°C). It was demonstrated at 225 MHz for homogenized TiO2-SiO2 glass specimens with
different annealing temperatures and realized an extremely homogeneous glass ingot with a ΔT(zero-CTE) of 1.6°C
around 23.2°C. This ultrasonic system enables both glass manufacturers and users to speedily inspect all glass substrates
with reliable data of T(zero-CTE).
This paper presents a calibration line between leaky surface acoustic wave (LSAW) velocities (VLSAW) and coefficient-of-thermal-expansion (CTE) characteristics for TiO2-doped SiO2 (TiO2-SiO2) glass to evaluate the absolute CTE by the line-focus-beam ultrasonic material characterization (LFB-UMC) system. Commercial TiO2-SiO2 ultra-low-expansion glass and synthetic silica glass were selected as specimens. We measured VLSAW by the LFB-UMC system and CTE characteristics by dilatometers, and obtained relationships among VLSAW, CTE at 22°C {CTE(22°C)} and zero-CTE temperature {T(zero-CTE)}. Resolutions of CTE(22°C) and T(zero-CTE) determined by the LSAW velocity measurement were estimated as ±0.72 (ppb/K) and ±0.14°C (±2σ, σ. standard deviation) at 225 MHz. Both manufacturers and users can precisely inspect T(zero-CTE) for all EUVL-grade ultra-low-expansion glass substrates by this indirect evaluation method using the calibration line.
In this paper, we tried to fabricate a TiO2-doped SiO2 (TiO2-SiO2) glass ingot by the soot method, and homogenized the
glass ingot by the zone-melting method. Homogeneities of the specimens were evaluated by measuring leaky surface
acoustic wave (LSAW) velocity using the line-focus-beam ultrasonic material characterization system at 225 MHz.
Two-dimensional LSAW velocity distributions having an average velocity of 3304.08 m/s with a maximum velocity
difference of 3.85 m/s were measured for a homogenized specimen. Striae were not observed for the specimen. The
velocity difference corresponds to 17.0 ppb/K from the sensitivity of the LSAW velocity to the CTE {4.41
(ppb/K)/(m/s)}. However, the velocity distributions excluding the edge parts were within ±1.13 m/s, corresponding to
the CTE specification of ±5 ppb/K required for EUVL-grade glass. We also discussed the relationship between LSAW
velocities and fictive temperatures by heat-treating a part of the homogenized specimen.
We proposed a new coefficient-of-thermal-expansion (CTE) evaluation method for ultra-low expansion glasses using the
line-focus-beam ultrasonic material characterization system. In this paper, we investigated evaluation procedures for
photomasks and optical mirrors with practical size used as reflective optics in extreme ultraviolet lithography (EUVL)
systems. Two specimens were prepared with their surfaces parallel to the striae plane from commercial TiO2-SiO2 ultralow-
expansion glass ingots. Homogeneities/inhomogeneities of specimens were evaluated at 225 MHz. Evaluation
procedures with sufficient accuracy were established for analysis of striae parameters such as striae periodicity and
variations. Our ultrasonic method should be standardized as a new evaluation method not only for development of the
EUVL-grade glass and evaluation of the production processes, but also for quality control and selection of the production
lots.
Super accurate evaluation method for TiO2-doped SiO2 ultra-low-expansion glass having periodic striae associated with its fabrication process was investigated using the line-focus-beam ultrasonic material characterization (LFB-UMC) system. To obtain absolute values of leaky surface acoustic wave (LSAW) velocities measured with the LFB-UMC system, proper standard specimens of the glass for system calibration was examined. Using a specimen with a surface perpendicular to the striae plane as the standard specimen, a reliable standard LSAW velocity of 3308.18 m/s within ±0.35 m/s for the calibration was obtained regardless of influence of velocity variations due to the periodic striae. Also, we determined the accurate relationship between the TiO2 concentrations and LSAW velocities (sensitivity: -0.0601 wt%/(m/s)) so that the TiO2 concentration of the standard specimen with a LSAW velocity of 3308.18 m/s was determined to be 7.09 wt%. Furthermore, to evaluate more reliably the more homogeneous ultra-low-expansion glasses in the near future, the measurement accuracy of the LSAW velocity was improved with a method using the LFB device with a larger curvature radius R operating at lower frequency from ±0.0053% for R=1.0 mm at 225 MHz to ±0.0020% for R=2.0 mm at 75 MHz.
A super-precision evaluation method of the coefficient of thermal expansion (CTE) of ultra-low expansion glasses was developed using the line-focus-beam ultrasonic material characterization (LFB-UMC) system. Evaluation was demonstrated for TiO2-SiO2 glass. The sensitivity and resolution in the velocity measurement of leaky surface acoustic waves (LSAWs) in CTE were estimated to be 4.40 (ppb/K)/(m/s) and ±0.77 ppb/K for ±2σ (σ: standard deviation) at 225 MHz. LSAW velocity differences caused by different TiO2 concentrations and distributions or striae in the specimens were successfully detected and evaluated, providing two-dimensional information under the nondestructive and noncontact measurement condition. This ultrasonic method is much more accurate than conventional methods, for evaluating CTE on the surface of ultra-low-expansion glass materials needed for extreme ultra-violet lithography (EUVL) systems.
Acoustic characterization of doped silica glasses with a Ge02, P205, F, Ti02, A1203 or B203 dopant having different concentrations is presented. The quantitative measurements are performed by a 225 MHz line-focus-beam scanning acoustic microscope. The acoustic velocity variation due to different dopant concentrations for each dopant is given. It has been found that the A1203 dopant increases but the other dopants decrease the acoustic velocity as compared to that of the pure fused silica. The fiber preforms having step and graded refractive index profiles also show step and graded acoustic velocity profiles respectively. We have also found that the acoustic velocity is more sensitive to the dopant concentration as compared with that of optical refractive index.
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.