Ultra-low Expansion Glass Research Articles

A Fabry–Pérot Etalon with an Ultralow Expansion Ceramic Spacer

We have developed a Fabry–Pérot etalon with an ultralow expansion ceramic spacer and investigated the thermal properties of the ceramic. We found that the ceramic cavity has a “zero crossing temperature” at room temperature where the thermal expansion coefficient crosses zero. The sensitivity of the thermal expansion coefficient of the ceramic to temperature change is about five times greater than that of ultralow expansion (ULE) glass at around the zero cross temperature. Nevertheless, owing to the high specific rigidity and relatively low thermal expansion at room temperature, the ceramic should be an attractive material for long optical cavities that can reduce the effect of the thermal noise relatively.

Chemical bonding of ultra low expansion glass substrates with aqueous NaOH solution

We report on our research towards determining a reliable and reproducible method of bonding ULE ® components for space-based instruments. Such bonded components are required to withstand considerable accelerations during launch and can be exposed to significant temperature variations. The results of tensile and shear testing of a large number of bonded samples are reported, as well as the impact of thermal cycling on bond strength. A study on the feasibility of de-bonding and re-bonding ULE parts is also described.

Low Energy Xe+ Ion Beam Machining of the Ultralow Expansion Glass Substrates for Aspherical Extreme Ultraviolet Lithography Projection Optics

Aspherical substrates for extreme ultraviolet lithography (EUVL) optics require an ultrahigh shape accuracy of less than 0.15 nm rms and a high-spatial frequency roughness (HSFR; spatial wavelength: less than 1 µm) of 0.12 nm rms. Generally, the ultra low expansion glass (ULE®) substrate with HSFR of 0.06–0.08 nm rms can be produced by mechanical machining methods. However, it is difficult to obtain the shape accuracy of less than 0.12 nm rms using mechanical machining methods. Therefore, ion beam figuring (IBF) may be adapted to final shape correction of the substrates for the projection optics of EUVL tools. In this study, we investigated the HSFR and machining rate of the ULE® substrate machined by a 0.3–1.0 keV Xe+ ion beam at off normal ion incidence angles and obtained the following results: the HSFRs of the ULE® substrate machined by a 1.0 keV Xe+ ion beam at a ion incidence angle of lower than 30° and a 0.3–0.5 keV Xe+ ion beam at an ion incidence angle of 0–45° are below 0.12 nm rms, which is smaller than the required HSFR specification of EUVL projection optics. From our experimental result and discussion, we concluded that the scan fine beam and tilt target mode smoothing for processing of the ULE® substrate meets the required specification of the HSFR (0.12 nm rms) of hemispherical ULE® substrates of EUVL projection optics.

Low Energy Xe+Ion Beam Machining of the Ultralow Expansion Glass Substrates for Aspherical Extreme Ultraviolet Lithography Projection Optics

Low Energy Xe⁺Ion Beam Machining of the Ultralow Expansion Glass Substrates for Aspherical Extreme Ultraviolet Lithography Projection Optics

Stable Narrow Linewidth 689 nm Diode Laser for the Second Stage Cooling and Trapping of Strontium Atoms

We report stable narrow linewidth laser systems based on self-developed Littman configuration external cavity diode lasers (ECDLs). The frequency of the ECDL is stabilized to a high fineness ultralow-expansion glass reference cavity with the Pound-Drever-Hall technique. By heterodyne beating of two identical systems, we conclude that the linewidth of each ECDL is reduced to lower than 150 Hz and its frequency stability reaches 4.3 × 10-14 at an averaging time of 1s, the averaged long-term frequency drift is less than 0.2 Hz/s over 30 h measurement time.

Multilayer Control of an Optical Reference Cavity for Space Applications

This paper deals with multilayer and fine thermal control of an optical reference cavity for space applications. The cavity, made of ultralow-expansion glass, must be kept close to the zero-expansion temperature (ZET) of the glass (near room temperature). The target can only be met by active control, while leaving the cavity free of sensors and actuators. This is achieved by applying two concepts: thermal bath and reference thermal sink, the latter allowing the ZET to be reached by heaters in a wide range of the environment. Guidelines for cavity design and thermal control implementation are detailed together with preliminary experimental results.

Diode-laser system for high-resolution spectroscopy of the2S1/2→2F7/2octupole transition in171Yb+

A diode-laser system at $467$ nm is built in order to drive the ${}^{2}{S}_{1/2}\ensuremath{\rightarrow}{}^{2}{F}_{7/2}$ electric octupole transition at $467$ nm in ${}^{171}{\mathrm{Yb}}^{+}$. The frequency of the laser is stabilized to a reference cavity made of ultra low expansion glass and is demonstrated to have a relative instability of better than $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ at $1$ s and a stable linear drift rate with variations below $10$ mHz/s over several hours. The system is applied for spectroscopy of a single trapped laser-cooled ${}^{171}{\mathrm{Yb}}^{+}$ ion. We obtain excitation spectra of the octupole transition with a resonant excitation probability of about $65%$ and an essentially Fourier transform-limited resolution of $13$ Hz.

Long-term stabilization of a heterodyne metrology interferometer down to a noise level of 20 pm over an hour

A heterodyne metrology interferometer was stabilized down to a noise level of 20 picometers (pm) as a root-mean-square (RMS) value integrated between 0.3 mHz and 1 Hz. This noise level was achieved by employing active and passive interferometer stabilization techniques. The heterodyne interferometer was built on a 50 mm square ultralow expansion glass plate in order to reduce an optical path length change caused by temperature variation. An optical configuration of the interferometer is a Mach-Zehnder interferometer with a design as symmetric as possible so that a detection signal can be insensitive to homogeneous thermal expansion of the glass plate. The heterodyne frequency is actively controlled in order to suppress residual noises caused by optical path length changes outside of the glass plate as well as phase fluctuations of the heterodyne frequency source. Our stabilization scheme is considered useful in achieving the 20 pm noise level without a stable heterodyne frequency source, as well as temperature stabilization around a whole apparatus. This interferometer can be used in precise metrology applications, such as characterization of deformation for satellite optical components against thermal exposure.

Sub-Nanometer Heterodyne Interferometry and Its Application in Dilatometry and Industrial Metrology

In this article, we present the current status of our high-sensitivity compact heterodyne interferometer and highlight its application in high-accuracy dilatometry and industrial surface metrology. The linear coefficient of thermal expansion (CTE) of dimensionally highly stable carbon-fiber reinforced plastic (CFRP) was measured with 10−7 K−1 accuracy. A very compact and quasi-monolithic interferometer setup is currently developed utilizing ultra-low expansion glass material. We present first investigations on the corresponding integration technologies, hydroxide-catalysis bonding and adhesive bonding. For high resolution 3-D profilometry and surface property measurements, a low-noise actuator will be implemented in the interferometer, which enables a scan of the measurement beam over the surface under investigation.

Compact, Ti:sapphire-based, methane-stabilized optical molecular frequency comb and clock

A compact optical clock and frequency comb system is presented, based on a 1 GHz octave-spanning Ti:sapphire laser referenced to the F2(2)(P(7),v3) optical transition in methane at 3.39 microm. The reference methane transition is accessed by a stabilized HeNe laser in a compact transportable format. The output from the octave-spanning Ti:sapphire laser is used to generate a mid-IR comb at the reference wavelength 3.39 microm. Direct comparison of the stabilized optical frequency comb with an ultralow expansion glass cavity-stabilized diode laser at 674 nm shows an Allan deviation of the frequency comb of 3x10(-14) in 20 s.

Study of Critical Dimensions of Printable Phase Defects Using an Extreme Ultraviolet Microscope

We constructed an extreme ultraviolet microscopy (EUVM) system for actinic mask inspection that consists of Schwarzschild optics and an X-ray zooming tube. This system was used to inspect completed extreme ultraviolet lithography (EUVL) masks and Mo/Si coated substrates on ultralow expansion (ULE) glass. We also have fabricated programmed phase defects on the blanks used for inspection. The EUVM system was capable of resolving a programmed line-pit defect with a width of 40 nm and a depth of 10 nm and also that with a width of 70 nm and a depth of 2 nm. However, a 75-nm-wide, 1.5-nm-deep pit defect was not resolved. The EUVM system was also capable of resolving programmed hole-pit defects with widths ranging from 35 to 170 nm and depths ranging from 2.2 to 2.5 nm. However, 20-nm-wide, 1.5-nm-deep hole-pit defects were not resolved. These results agree with the simulation results perfectly. Thus, in this study, critical dimensions of a pit defects on mask blanks were determined to be a width of 20 nm and a depth of 2 nm.

High speed phase shifting interferometry using injection locking of the laser frequency to the resonant modes of a confocal Fabry-Perot cavity

We present a high speed phase shifting interferometer which utilizes the self injection locking of a frequency tunable laser diode. By using a confocal Fabry-Perot cavity made of ultra low expansion glass, and linearly modulating the laser diode current, the laser frequency could be injection locked to the resonant modes of the Fabry-Perot cavity consecutively. It provided equal phase steps to the interferograms which are ideal to be analyzed by the Carré algorithm. The phase step error was evaluated to be about 3 MHz which corresponds to 0.2 nm in length measurement. With this technique, profile measurements are insensitive to external vibration since four 640x480 pixels images can be acquired within 4 ms. Difference of two profile measurements, each made with and without vibration isolation, respectively, was evaluated to be 0.5 nm (rms).

Intense femtosecond laser pulse interaction with ultra-low expansion glass: nonlinear responses and optical limiting effect

In this study, the nonlinear responses and optical limiting behaviour of titanium doped silicate glass (7.5TiO2–92.5SiO2), known as ultra-low expansion (ULE) glass, are investigated. The measurements were performed by use of the Z-scan technique and monitoring of the transmitted energy of 200 fs Ti:sapphire laser pulses at a central wavelength of 800 nm. The optical power limiting threshold was measured to be about 37 µJ (peak intensity of 0.55 × 1012 W cm−2) in this sample. The decrease of transmitted intensity was about 60% compared to the theoretical linear transmission for incident pulse energies of 320 µJ (peak intensity of 21 × 1012 W cm−2). For the prediction of the experimental nonlinear behaviour results, we used the three-photon absorption and three-photon-generated free-carrier absorption model within the sample.

Semiconductor laser with the subhertz linewidth

A semiconductor laser emitting at 972 nm is stabilised with respect to a vibrationally and thermally compensated reference Fabry—Perot resonator with the vertical axis. The supporting points lie in the horizontal symmetry plane of the resonator, the influence of vibrations in the vertical direction being substantially suppressed in this case. This configuration provides a low sensitivity of the laser emission frequency to vertical accelerations of the reference resonator. To reduce the influence of temperature fluctuations, the resonator is made of an ULE (ultra-low-expansion) glass and is kept at temperature at which the expansion coefficient of this glass is close to zero. The laser linewidth is smaller than 0.5 Hz and the frequency drift is ≈0.1 Hz s-1. The minimum of the Allan deviation achieved for 3 s is 2×10-15. The laser was used to record the spectra of the 1S—2S transition in atomic hydrogen.

Narrow-Line and Frequency Tunable Diode Laser System for S–D Transition of Ca+ Ions

A new type of diode laser is used for the development of an extremely narrow linewidth and frequency-tunable clock laser system for a Ca+ ion optical frequency standard. The required reduction of linewidth was achieved by locking the laser to an ultrahigh-finesse ultralow-expansion glass (ULE) reference cavity. The long-term frequency drift is reduced by stabilizing a cavity temperature at the point that the thermal expansion coefficient of the ULE is zero. As a result, the laser linewidth is decreased to lower than 25 Hz and the long-term drift is less than 0.025 Hz/s. The other laser offset locked to the stabilized laser is continuously frequency tunable in the entire free spectral range of the reference cavity without degradation of stability and linewidth. This laser system is actually applied to optical frequency standard development and used for the observation of the 2S1/2–2D5/2 transition of Ca+ ions.

Subhertz linewidth diode lasers by stabilization to vibrationally and thermally compensated ultralow-expansion glass Fabry-Pérot cavities

We achieved a 0.5 Hz optical beat note linewidth with $\ensuremath{\sim}0.1\text{ }\text{Hz}/\text{s}$ frequency drift at 972 nm between two external cavity diode lasers independently stabilized to two vertically mounted Fabry-P\'erot (FP) reference cavities with a finesse of 400 000. Vertical FP reference cavities are suspended in midplane such that the influence of vertical vibrations to the mirror separation is significantly suppressed. This makes the setup virtually immune for vertical vibrations that are more difficult to isolate than horizontal vibrations. To compensate for thermal drifts the FP spacers are made from ultralow-expansion (ULE) glass which possesses a zero linear expansion coefficient. A design using Peltier elements in vacuum allows operation at an optimal temperature where the quadratic temperature expansion of ULE could be eliminated as well. The measured linear drift of such ULE FP cavity of 63 mHz/s was due to material aging and the residual frequency fluctuations were less than $\ifmmode\pm\else\textpm\fi{}20\text{ }\text{Hz}$ during 16 h of measurement. Some part of the temperature-caused drift is attributed to the thermal expansion of the mirror coatings. Thermally induced fluctuations that cause vibrations of the mirror surfaces limit the stability of our cavity. By comparing two similar laser systems we obtain an Allan instability of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ between 0.1 and 10 s averaging time, which is close to the theoretical thermal noise limit.

Development of superprecise evaluation method for EUVL-grade ultralow-expansion glass using line-focus-beam ultrasonic material characterization system

Development of superprecise evaluation method for EUVL-grade ultralow-expansion glass using line-focus-beam ultrasonic material characterization system

A New Reactive Atom Plasma Technology (RAPT) for Precision Machining: the Etching of ULE® Surfaces

AbstractResults on reactive atom plasma etching performed on ULE® (Corning Ultra Low Expansion) glass samples at atmospheric pressure are presented for the first time. A reactive atomic plasma technology (RAPT®), has been developed by RAPT Industries and employed for the finishing of optical surfaces. An atmospheric pressure argon inductively coupled plasma (ICP) excites a reactive gas injected through its centre. The plume of hot neutral excited species reacts at the substrate yielding controlled and repeatable trenches. In the case of ULE a material removal (up to 0.55 mm3/s) is obtained without pre‐heating the samples. Among the factors influencing the results, an increase in gas concentration at the same power does not change the sample temperature, indicating that thermo‐chemical effects do not influence the removal rates. Due to the plasma constructive constrains, increasing the gas concentration is more practical and of wider effect than increasing the power. The benefits of the process are illustrated and the extension of the technology to large optical surfaces discussed.

Anodic bonding using the low expansion glass ceramic Zerodur®

Using anodic bonding, a strong bond has been created between the ultralow expansion glass ceramic Zerodur® from Schott and an Al coated piece of ultralow expansion glass (ULE® from Corning). The bond strength has been tested using a tensile testing machine. The bond strength is comparable to the fracture strength of the ULE® material. Using anodic bonding it will be possible to make complicated electrostatic clamps for the next generation lithography machines based on an all ultralow expansion design.

Micro machining glass with polycrystalline diamond tools shaped by micro electro discharge machining

Brittle materials are difficult to mechanically micro machine due to damage resulting from material removal by brittle fracture, cutting force-induced tool deflection or breakage and tool wear. This paper demonstrates the feasibility of micro machining glass materials with polycrystalline diamond (PCD) micro tools that are prepared in a variety of shapes using non-contact micro electro discharge machining. The PCD tools contain randomly distributed sharp protrusions of diamond with dimensions around 1 µm that serve as cutting edges for micro machining grooves in soda-lime glass and pockets in ultra-low expansion glass. Results indicate that smooth surfaces are obtained with process conditions allowing material removal by ductile regime cutting instead of brittle fractures, and the PCD tools show very little wear. With further improvements in material removal rate, micro machining with PCD tools is a promising approach for producing micro molds and micro fluidic devices in glass materials.