Photostructurable Glass Research Articles

Enhanced Mechanical Properties of Li2O-Al2O3-SiO2 Photostructurable Glass by SrO Doping

Li2O-Al2O3-SiO2 (LAS) photostructurable glasses (PSGs) are promising substrate materials with excellent three-dimensional shaping control. In this study, we prepared SrO-doped LAS-PSGs with different SrO content. The effect of SrO content on the dielectric and mechanical properties of the LAS-PSGs was investigated by deconvoluting (four-component) their Raman bands. The LAS-PSG sample with 1 wt% SrO exhibited enhanced mechanical properties: the enhanced surface micro-hardness of 568.20 HV, and the improved flexural strength of 157.23 MPa while maintaining a low dielectric loss of 4.1 × 10−3. These improvements of mechanical properties can be attributed to the increased relative fraction of Q3 units (Qn: n bridging oxygen bonds in a [SiO4] tetrahedral unit) in this sample. This is due to Q3 units being able to enhance the network structure of LAS-PSGs with little effect on [SiO4] tetrahedral units connections.

The effects of La2O3 doping on the photosensitivity, crystallization behavior and dielectric properties of Li2O-Al2O3-SiO2 photostructurable glass

Photostructurable Li2O-Al2O3-SiO2 glass is a promising material to fabricate complex three-dimensional structure with a high aspect ratio. However, its high dielectric loss at high frequencies has restrained its application in the field of integrated circuits packaging. In this research, La2O3, which has a large ionic radius, as well as strong polarization and bonding strength, was used to obstruct mobile ion migration to reduce the dielectric loss. The results indicated that moderate doping with La2O3 could effectively reduce the dielectric loss. When the dopant amount was 3%, the dielectric loss was successfully reduced to a minimum of 4 × 10−3 with a dielectric constant of 6.6 at 1 GHz, and this sample also possessed the optimal dielectric-temperature stability. Additionally, the effects of doping on the photosensitivity and crystallization behavior were also analysed. The results suggested that La2O3 doping did not affect the photosensitivity and selective crystallization characteristics. However, La2O3 restrained the precipitation of silicate from the [SiO4] tetrahedron, resulting in a decrease of nucleation rate and a delay of crystallization.

Picosecond laser fabrication of microchannels inside Foturan glass at CO2 laser irradiation and following etching

In this work, the formation of hollow-type microchannels in the bulk of a photostructurable glass of the brand Foturan is proposed. It is based on the chemical dissolution of crystallized areas, which are formed as a result of the action of picosecond laser pulses and photothermal treatment produced by CO2 laser irradiation. A speed of formation of microchannels was estimated. Microchannels formation features at each stage of laser irradiation are discussed, and the dependence of microchannels characteristics (size, shape and roughness of inner walls) on laser processing and chemical etching parameters were evaluated as well.

Reversible phase–structure modification of photostructurable glass ceramic by CO2 laser irradiation

Structural changes and phase transformations of the photostructurable glass (PhG) surfaces under the impact of 10.6-μm wavelength laser radiation are examined. The regimes initiating the formation and development of crystalline phase and its reversible (secondary) amorphization are determined. In addition, the kinetic of crystalline phase formation and its melting on the surface of PhG are investigated. The characteristics of multiple reversible phase–structure transformations in the temperature range of 470–800 °C are investigated.

Fast 3D laser-induced reversible phase-structure modification of photostructurable glass

Method of 3D processing of photosensitive glass (PhG) is demonstrated and researched in this work. Structural changes and phase transformation are initiated by CO2 laser radiation on defects, which were pre-generated in the bulk of the PhG by powerful picosecond Nd:YAG laser second harmonic pulses. Reversible phase transformations were realized inside PhG. CO2 laser irradiation allowed us to avoid completely heat treatment step in a furnace. As a result, direct writing of structures was implemented. As a part of the research, we have also carried out processes of crystallization visualization and return to amorphous state which allowed us to learn the kinetic of phase transformations in the bulk of the glass material.

Focus on materials, semiconductors, vacuum, and cryogenics

Focus on materials, semiconductors, vacuum, and cryogenics

Highly sensitive optofluidic chips for biochemical liquid assay fabricated by 3D femtosecond laser micromachining followed by polymer coating

The demand for increased sensitivity in the concentration analysis of biochemical liquids is a crucial issue in the development of lab on a chip and optofluidic devices. We propose a new design for optofluidic devices for performing highly sensitive biochemical liquid assays. This design consists of a microfluidic channel whose internal walls are coated with a polymer and an optical waveguide embedded in photostructurable glass. The microfluidic channel is first formed by three-dimensional femtosecond laser micromachining. The internal walls of the channel are then coated by the dipping method with a polymer that has a lower refractive index than water. Subsequently, the optical waveguide is integrated with the microfluidic channel. The polymer coating on the internal walls permits the probe light, which is introduced by the optical waveguide, to propagate along the inside of the microfluidic channel. This results in a sufficiently long interaction length between the probe light and a liquid sample in the channel and thus significantly improves the sensitivity of absorption measurements. Using the fabricated optofluidic chips, we analyzed protein in bovine serum albumin to concentrations down to 7.5 mM as well as 200 nM glucose-D.

Crystallisation of Foturan ® glass–ceramics

Photostructurable glass–ceramics are suitable for 3-dimensional microfabrication and, in some instances, can be used as an alternative material to silicon in the microfabrication of micro-electro-mechanical-system (MEMS) devices. Foturan ® is a photosensitive lithium–aluminium–silicate glass that can be structured by an exposure to UV light, followed by a thermal treatment and an etching step. In this work, the crystallisation kinetics and microstructure evolution of unexposed and UV-exposed Foturan ® are investigated by means of differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. The glass transition temperature T g , the apparent activation energy of crystallisation E a and the colour of UV-exposed Foturan ® are discussed. It is shown that nucleation and crystallisation of UV-exposed Foturan ® can be steered either as a function of temperature (non-isothermal) or as a function of time (isothermal). This is essential for the photostructuring and etching of Foturan ® regarding the achievable feature sizes of 25 μm for MEMS applications.

Efficient microwelding of glass substrates by ultrafast laser irradiation using a double-pulse train

Efficient microwelding of glass substrates by irradiation by a double-pulse train of ultrafast laser pulses is demonstrated. Temporal beam shaping techniques such as double-pulse irradiation enabled increased flexibility for high-quality, high-efficiency material processing. The bonding strength of two photostructurable glass substrates welded by double-pulse irradiation was evaluated to be 22.9 MPa, which is approximately 22% greater than that of a sample prepared by conventional irradiation by a single-pulse train.

Microchannel formation through Foturan® with infrared femtosecond and ultraviolet nanosecond lasers

Formation of microchannels produced through 1 mm thick photostructurable glass (Foturan®) samples with two different lasers, an IR laser that gives 450 fs pulses at 1027 nm wavelength and an UV laser delivering 10 ns pulses at 355 nm wavelength, has been studied. After laser irradiation, the samples were submitted to thermal treatment and selective etching with hydrofluoric acid (HF) for microchannel formation. Two different acid concentrations (5% and 10%) were tested. It has been found that photoexcitation with the femtosecond laser can be related to a multiphoton absorption process requiring eight photons, and the critical dose of radiation for this process to occur has been determined. Etch rates with 10% HF are about two times higher than with 5% HF under equal irradiation conditions, while similar etch contrast ratios are attained. High etch rate ratios, together with small modified areas, have been found to play a determinant role in obtaining microchannels with the best aspect ratios. Use of the femtosecond laser in the IR allows the modification of smaller zones, but to the detriment of high etch rates.

3D microfluidic chips with integrated functional microelements fabricated by a femtosecond laser for studying the gliding mechanism of cyanobacteria

Phormidium, a genus of filamentous cyanobacteria, forms endosymbiotic associations with seedling roots that accelerate the growth of the vegetable seedlings. Understanding the gliding mechanism of Phormidium will facilitate improved formation of this association and increased vegetable production. To observe the gliding movements, we fabricated various microfluidic chips termed nanoaquariums using a femtosecond (fs) laser. Direct fs laser writing, followed by annealing and successive wet etching in dilute hydrofluoric acid solution, can easily produce three-dimensional (3D) microfluidics with different structures embedded in a photostructurable glass. Using the fs laser, optical waveguides and filters were integrated with the microfluidic structures in the microchips, allowing the gliding mechanism to be more easily clarified. Using this apparatus, we found that CO(2) secreted from the seedling root attracts Phormidium in the presence of light, and determined the light intensity and specific wavelength necessary for gliding.

3D features of modified photostructurable glass–ceramic with infrared femtosecond laser pulses

The exclusive ability of laser radiation to be focused inside transparent materials makes lasers a unique tool to process inner parts of them unreachable with other techniques. Hence, laser direct-write can be used to create 3D structures inside bulk materials. Infrared femtosecond lasers are especially indicated for this purpose because a multiphoton process is usually required for absorption and high resolution can be attained. This work studies the modifications produced by 450 fs laser pulses at 1027 nm wavelength focused inside a photostructurable glass-ceramic (Foturan ®) at different depths. Irradiated samples were submitted to standard thermal treatment and subsequent soaking in HF solution to form the buried microchannels and thus unveil the modified material. The voxel dimensions of modified material depend on the laser pulse energy and the depth at which the laser is focused. Spherical aberration and self-focusing phenomena are required to explain the observed results.

Nano-aquarium with microfluidic structures for dynamic analysis of Cryptomonas and Phormidium fabricated by femtosecond laser direct writing of photostructurable glass

We demonstrate fabrication of microchips with microfluidic structures for dynamic analysis of living cells using a femtosecond (fs) laser. Fs laser direct writing followed by annealing and successive wet etching in dilute hydrofluoric (HF) acid solution resulted in formation of three dimensional (3D) hollow microstructures embedded in photostructurable glass. The embedded microchannel structure enabled us to analyze unique phenomenon of Cryptomonas, which suddenly swims very fast under certain condition. Vector analysis of the driving force for the rapid motion was also carried out by introducing nano-beads into the microchannel, in which Cryptomonas was encapsulated. We also fabricated a microchip for observation of Phormidium moving toward a seedling root, which accelerates growth of the seedling. Using the embedded microchannel in the microchip, observation of Phormidium assemblage to the seedling root was easily carried out. Such microchips with microfluidic structures, referred to as a nano-aquarium, realize the efficient and highly functional observation of living cells.

Nano-aquariums from ultrashort laser pulses

A large variety of organisms are presently living on the earth. Among these, microscopic-sized aquatic microorganisms have been surviving for more than 500 million years. Some of these microorganisms exhibit extremely rapid motion, which is unusual in the macro world in which we live, and can show unique 3D movements that appear to contradict gravity. Most are composed of a unit cell. Understanding the abilities and functions of these unit cells may offer insight into the cells that comprise more complex organisms, including human beings. The observation of microorganisms is currently a challenging subject for cell biologists. In the conventional microscopic observation system, a glass slide with a cover glass is generally used. However, the high numerical aperture of the objective lens limits both the field of the image and the depth of focus, thereby making it difficult to capture images of moving microorganisms. Indeed, it can take a very long time until a clear image is obtained. Amethod to reduce the amount of tracking required or the reliance on chance movements of the organisms is needed. To solve these problems, we applied our technique for fabricating 3D hollow microstructures inside photostructurable glass using femtosecond lasers to the manufacture of a special microchip, referred to as a nano-aquarium.1 The nano-aquarium can scale down the observation site by encapsulating the microorganism in a limited area while still providing enough space for motion. This makes it much easier to capture images of moving organisms in freshwater with a standard objective lens (see Figure 1). The nano-aquarium structures also have the advantage of keeping livingmicroorganisms fresh for a long time since such a structure prevents the evaporation of water as seen under the glass slide/cover glass system. To fabricate the nano-aquarium, we developed a technique that can be used to directly form 3D hollow microstructures with smooth internal surfaces inside photostructurable (Foturan) glass via femtosecond laser direct writing followed by anFigure 1. Illustration of microscopic observation of microorganisms using a nano-aquarium.

Micro Solid Oxide Fuel Cells on Glass Ceramic Substrates

AbstractMiniaturized solid oxide fuel cells are fabricated on a photostructurable glass ceramic substrate (Foturan) by thin film and micromachining techniques. The anode is a sputtered platinum film and the cathode is made of a spray pyrolysis (SP)‐deposited lanthanum strontium cobalt iron oxide (LSCF), a sputtered platinum film and platinum paste. A single‐layer of yttria‐stabilized zirconia (YSZ) made by pulsed laser deposition (PLD) and a bilayer of PLD–YSZ and SP–YSZ are used as electrolytes. The total thickness of all layers is less than 1 µm and the cell is a free‐standing membrane with a diameter up to 200 µm. The electrolyte resistance and the sum of polarization resistances of the anode and cathode are measured between 400 and 600 °C by impedance spectroscopy and direct current (DC) techniques. The contribution of the electrolyte resistance to the total cell resistance is negligible for all cells. The area‐specific polarization resistance of the electrodes decreases for different cathode materials in the order of Pt paste > sputtered Pt > LSCF. The open circuit voltages (OCVs) of the single‐layer electrolyte cells ranges from 0.91 to 0.56 V at 550 °C. No electronic leakage in the PLD–YSZ electrolyte is found by in‐plane and cross‐plane electrical conductivity measurements and the low OCV is attributed to gas leakage through pinholes in the columnar microstructure of the electrolyte. By using a bilayer electrolyte of PLD–YSZ and SP–YSZ, an OCV of 1.06 V is obtained and the maximum power density reaches 152 mW cm−2 at 550 °C.

Stability of NiO membranes on photostructurable glass substrates for micro solid oxide fuel cells

NiO thin films with thicknesses in the range of 100 to 900 nm were deposited by spray pyrolysis onto photostructurable glass substrates and, ultimately, free-standing membranes with diameters of 100, 200 and 300 µm were fabricated using these thin films. The membranes are intended to act as simplified anodes or anode current collectors in micro solid oxide fuel cells (µSOFCs) and their differential pressure and thermal stability were characterized. The membranes tolerated a differential pressure between 13,700 and 158,600 Pa. Smaller membranes showed more pressure tolerance than larger membranes. A membrane diameter of 100 µm and a film thickness of 400–500 nm turned out to be a promising geometry for µSOFC membranes. All membranes survived temperatures higher than the intended operating temperature of µSOFCs (350–600 °C). We attribute the good thermal stability to the match of the thermo-mechanical properties of the substrate and the NiO thin films for the lower temperature regime and the substrate softening at higher temperatures releasing stresses in the thin films. Furthermore, the thermal expansion of the substrate is close to thermal expansion of materials used in SOFCs and circular geometries can be realized using wet etching.

Nano-aquarium Fabrication by Femtosecond Laser Direct Writing for Microscopic Observation of Aquatic Microorganisms

We demonstrate the fabrication of three-dimensional (3-D) hollow microstructures embedded in photostructurable glass by a femtosecond (fs) laser direct writing. Fs laser direct writing followed by annealing and successive wet etching in dilute hydrofluoric (HF) acid solution resulted in the rapid manufacturing of microchips with 3-D hollow microstructures for the dynamic observation of living microorganisms in fresh water. The embedded microchannel structure enables us to analyze the continuous motion of flagellum movement of Euglena gracilis. Such microchips, referred to as nano-aquariums realize the efficient and highly functional observation of microorganisms.

Selective metallization of photostructurable glass by femtosecond laser direct writing for biochip application

We report the selective metallization of photostructurable glass by femtosecond (fs) laser direct writing followed by electroless copper (Cu) plating. It was found that a Cu thin film can be deposited only on the rough surface of glass ablated by the fs laser. The deposited Cu thin film exhibits strong adhesion and excellent electrical properties. A Cu film can even be deposited on the internal wall of a hollow microchannel inside photostructurable glass by the multiphoton absorption of the fs laser. To show the use of this technique for micro-total-analysis-system (μ-TAS) applications, the fabrication of a microheater operating at temperatures up to 200 °C was demonstrated.

Nano-aquarium for dynamic observation of living cells fabricated by femtosecond laser direct writing of photostructurable glass

We demonstrate the fabrication of three-dimensional (3-D) hollow microstructures embedded in photostructurable glass by a nonlinear multiphoton absorption process using a femtosecond (fs) laser. Fs laser direct writing followed by annealing and successive wet etching in dilute hydrofluoric (HF) acid solution resulted in the rapid manufacturing of microchips with 3-D hollow microstructures for the dynamic observation of living microorganisms and cells in fresh water. The embedded microchannel structure enables us to analyze the continuous motion of Euglena gracilis. A microchamber with a movable microneedle demonstrates its ability for the elucidation of the information transmission process in Pleurosira laevis. Such microchips, referred to as nano-aquariums realize the efficient and highly functional observation of microorganisms and cells.

Examination of the laser-induced variations in the chemical etch rate of a photosensitive glass ceramic

Previous studies in our laboratory have reported that the chemical etch rate of a commercial photosensitive glass ceramic (FoturanTM, Schott Corp., Germany) in dilute hydrofluoric acid is strongly dependent on the incident laser irradiance during patterning at λ=266 nm and λ=355 nm. To help elucidate the underlying chemical and physical processes associated with the laser-induced variations in the chemical etch rate, several complimentary techniques were employed at various stages of the UV laser exposure and thermal treatment. X-ray diffraction (XRD) was used to identify the crystalline phases that are formed in Foturan following laser irradiation and annealing, and monitor the crystalline content as a function of laser irradiance at λ=266 nm and λ=355 nm. The XRD results indicate the nucleation of lithium metasilicate (Li2SiO3) crystals as the exclusive phase following laser irradiation and thermal treatment at temperatures not exceeding 605 °C. The XRD studies also show that the Li2SiO3 density increases with increasing laser irradiance and saturates at high laser irradiance. For our thermal treatment protocol, the average Li2SiO3 crystal diameters are 117.0±10.0 nm and 91.2±5.8 nm for λ=266 nm and λ=355 nm, respectively. Transmission electron microscopy (TEM) was utilized to examine the microscopic structural features of the lithium metasilicate crystals. The TEM results reveal that the growth of lithium metasilicate crystals proceeds dendritically, and produces Li2SiO3 crystals that are ∼700–1000 nm in length for saturation exposures. Optical transmission spectroscopy (OTS) was used to study the growth of metallic silver clusters that act as nucleation sites for the Li2SiO3 crystalline phase. The OTS results show that the (Ag0)x cluster concentration has a dependence on incident laser irradiance that is similar to the etch rate ratios and Li2SiO3 concentration. A comparison between the XRD and optical transmission results and our prior etch rate results show that the etch rate contrast and absolute etch rates are dictated by the Li2SiO3 concentration, which is in turn governed by the (Ag0)x cluster concentration. These results characterize the relationship between the laser exposure and chemical etch rate for Foturan, and permit a more detailed understanding of the photophysical processes that occur in the general class of photostructurable glass ceramic materials. Consequently, these results may also influence the laser processing of other photoactive materials.