Laser-induced Backside Wet Etching Research Articles

Etching of quartz crystals in liquid phase environment: A review

Quartz crystals are the most widely used material in resonant sensors, owing to their excellent piezoelectric and mechanical properties. With the development of portable and wearable devices, higher processing efficiency and geometrical precision are required. Wet etching has been proven to be the most efficient etching method for large-scale production of quartz devices, and many wet etching approaches have been developed over the years. However, until now, there has been no systematic review of quartz crystal etching in liquid phase environments. Therefore, this article provides a comprehensive review of the development of wet etching processes and the achievements of the latest research in this field, covering conventional wet etching, additive etching, laser-induced backside wet etching, electrochemical etching, and electrochemical discharge machining. For each technique, a brief overview of its characteristics is provided, associated problems are described, and possible solutions are discussed. This review should provide an essential reference and guidance for the future development of processing strategies for the manufacture of quartz crystal devices.

Sound-Based Depth Estimation of Glass Microchannel in Laser-Induced Backside Wet Etching Using Wavelet Transform

Laser-induced backside wet etching (LIBWE) has been proposed to fabricate high-quality micromachined components on transparent materials. However, the process is limited by poor repeatability when fabricating high-aspect-ratio structures, even under the same conditions due to uncertainties arising from the thermal process and the complex mechanisms associated with the indirect irradiation of the etching process. Such errors could lead to redundant trials and wastages when trying to achieve the desired dimension. To identify the factors causing these variations, we targeted the process sounds generated during the etching. This study uses a microphone to measure factors that result in variations in material removal quantity during the etching process under the same conditions. The sound was filtered at frequencies between 3 and 6 kHz, which were selected as characteristic frequencies for the process under various laser conditions. By integrating the root mean squared value of the detail coefficient of the wavelet transform, the depth estimation closely matched the measured depth of the fabricated part. This finding suggests that determining the etching rate from sound at a certain characteristic frequency during the LIBWE process is feasible; this approach can improve the accuracy and repeatability of the process. Based on this estimation mechanism, we designed a closed-loop feedback control system capable of fabricating highly accurate microchannels in the range of 80–120 μm with a maximum error of 5.6%.

One-Stage Femtosecond Laser-Assisted Deposition of Gold Micropatterns on Dielectric Substrate.

In this study, a simple one-stage laser-assisted metallization technique based on laser-induced backside wet etching and laser-induced chemical liquid-phase deposition is proposed. It allows for the fabrication of gold micropatterns inside the laser-written trace on a glass substrate. The reduction and deposition of gold inside and outside the laser-ablated channel were confirmed. The presence of Au nanoparticles on the surface of the laser-written micropattern is revealed by atomic force microscopy. The specific resistivity of the gold trace formed by ultrafast light-assisted metal micropatterning on a dielectric glass substrate is estimated as 0.04 ± 0.02 mΩ·cm. The obtained results empower the method of the selective laser-assisted deposition of metals on dielectrics and are of interest for the development of microelectronic components and catalysts, heaters, and sensors for lab-on-a-chip devices.

Thermoplasmonic laser-induced backside wet etching

The article is devoted to one of the effective technologies for processing solid transparent materials—thermoplasmonic laser-induced backside wet etching (TP LIBWE). This technology involves aqueous solutions of metal precursors as a working medium. The dependence of the efficiency of sapphire TP LIBWE micromachining on the parameters of laser action is studied with the aqueous solution of the AgNO3 precursor as a working media. The near-optimal range of laser intensities from the point of the etching speed and quality is found. Utilizing the optoacoustic methods, high-speed video, and an optical integrating sphere, the initial stage of the TP LIBWE process is studied in detail. A four-stage model of the TP LIBWE beginning process is proposed, which explains the effects from the beginning of Ag nanoparticle formation in the region of laser exposure to the transition of the TP LIBWE process to a stationary laser microstructuring mode. It is shown that effective microstructuring occurs due to the appearance on the sapphire surface of a thin modified layer in the region of laser action. This thin modified layer is an amorphous Al2O3 with numerous plasmonic Ag nanoparticles inside it and at the sapphire/liquid interface.

In-situ enhanced laser absorption in aqueous transition metal salt solution enables high-quality backside wet etching of optical glass by near-infrared lasers

A laser-induced backside wet etching technique using near-infrared laser sources (N-LIBWE) has been proposed for high-quality micromachining of optical glass. Compared with conventional LIBWE using organic dye solutions with high absorption coefficients (∼104–105 cm−1) as the liquid absorbent, the transition metal salt solution used in N-LIBWE shows the absorption coefficient of only 10-1-101 cm−1. Moreover, as we proved in this study, the etching rate of N-LIBWE is independent of the absorptivity of the liquid absorbent. How the etching occurs during N-LIBWE is still controversial. Herein, interface images with a temporal resolution of 4 ns after the start of laser irradiation are captured by a stroboscopic shadowgraphy system. Afterward, the density, pressure, and temperature distribution in the solution are calculated by a physics-based two-dimensional hydrodynamic model to explain the observed interface images. We prove that the localized laser absorption is enhanced with laser irradiation because of the in-situ formation of solid substances. It is the properties of the formed solid products instead of the normal absorption coefficient of the solution that determines the final interface status. We further prove that the formed solid substances in aqueous CuSO4 solutions are primarily amorphous nanoclusters, which benefit the enhancement of laser absorption and tend to attach to the transparent substrates, resulting in the incubation effect in N-LIBWE. Our results provide systematic mechanisms explaining the N-LIBWE process, which also benefit the deep understanding of other liquid-assisted laser materials processing technologies.

Backside wet etching of sapphire substrate by laser-induced carbothermal reduction

Laser-induced backside wet etching is a promising technology to process sapphire substrate. However, the conventional working media including toxic organic and heavy metal salt solutions is not environmentally friendly. In this paper, the backside wet etching of sapphire substrate with ink as a working solution by laser-induced carbothermal reduction is achieved. Raman and XRD analyses demonstrate that ink can provide a stable carbon source and the carbothermal reduction of sapphire occurs during the etching. Numerical calculation reveals that the deposition of the carbon layer plays a crucial role in heating the sapphire to the temperature triggering the carbothermal reduction. The grooves etched by the laser-induced carbothermal reduction have smooth walls and good shape reproducibility. The linearly increase in etching depth with pulse number at the same laser fluence. The high-quality etching based on the carbothermal reduction as the etching mechanism is beneficial for fabricating microstructures on sapphire.

Crackless glass through-structure fabrication with laser-induced backside wet etching using detachably bonded cover

We propose a novel process for fabricating crackless high aspect ratio glass through-structures in this study. A glass material was machined using laser-induced backside wet etching (LIBWE) and a detachably bonded glass cover. The uncontrolled spread of the liquid absorbent was prevented, which inhibited the crack generation by plasma-activated bonding and local fusion between the cover and workpiece. High aspect ratio crackless through-structures could be fabricated with inexpensive machining devices. The machined quality was investigated based on the machining principle of LIBWE. In the final step, various glass structures were machined to verify the range of machinable geometry.

Precise glass microstructuring with laser induced backside wet etching using error-compensating scan path

Laser induced backside wet etching (LIBWE), a simple-setup process capable of processing transparent materials, has been studied to overcome difficulties in glass micromachining. However, LIBWE still show practical difficulty in machining various glass applications due to the crack occurrence and unprecise final geometry. This study proposes the error-compensating scan path generation method for the precise fabrication of glass microstructures without additional devices. In conventional scan paths, the overlap of scan path’s initial and final points, constant scan path patterns and uneven distributions of laser irradiation were the leading causes of geometric errors. The scan path generation method was developed to minimize or eliminate the causes of geometric errors in conventional scan path methods. Machined results, with error-compensating scan path, showed the removal of significant error from conventional paths with proper material removal rates. The effects of scan path generation parameters on the machining characteristics were also investigated. By adjusting the scan duty ratio and laser irradiation distribution of the entire scan path, micropockets with an average surface roughness of 0.26 μm could be processed at a material removal rate of 29,700 μm 3 /s. Based on machining characteristics of the error-compensating scan path, various glass microstructures were fabricated and the feasibility of the proposed method was verified.

From nanoparticles generation to nanostructures diversity at thermoplasmonics laser-induced backside wet etching of sapphire

The nanoparticles and nanostructures formed during the thermoplasmonics laser-induced backside wet etching of sapphire were studied using electron microscopy and X-Ray energy dispersive analysis. It is shown that the generation and subsequent modification of nanoparticles result in the formation of various plasmonic structures, such as crystalline silver nanoaggregates and hybrid structures obtained due to the interaction of the sapphire decomposition products with silver nanoparticles, such as randomly mixed nanoalloys and Al/O nanoaglomerates surrounded by silver nanoparticles. Possible mechanisms for the creation and modification of nanomaterials depending on the localization of the thermoplasmonics process in space and time are discussed.

Deposition of Durable Micro Copper Patterns into Glass by Combining Laser-Induced Backside Wet Etching and Laser-Induced Chemical Liquid Phase Deposition Methods.

Glass is a well-known non-conductive material that has many useful properties, and considerable research has been conducted into making circuits on glass. Many deposition techniques have been studied, and laser-induced chemical liquid phase deposition (LCLD) is a well-known and cost-effective method for rapid prototyping of copper deposition on glass. However, the deposition results from the LCLD method on the surface of glass, which shows an issue in its detachment from the substrates because of the relatively low adhesion between deposited copper and the nontreated glass surface. This problem undermines the usability of deposited glass in industrial applications. In this study, the laser-induced backside wet etching (LIBWE) method was performed as a preceding process to fabricate microchannels, which were filled with copper by LCLD. Additional durable copper wire was produced as a result of the enhanced adhesion between the glass and the deposited copper. The adhesion was enhanced by a rough surface and metal layer, which are characteristics of LIBWE machining. Furthermore, the proposed method is expected to broaden the use of deposited glass in industrial applications, such as in stacked or covered multilayer structures with built-in copper wires, because the inserted copper can be physically protected by the microstructures.

Microfabrication of polymer with laser-induced backside wet etching

A new study demonstrates that laser-induced backside wet etching can successfully work on a polymer substrate, which has implications for manufacturing microfluidic chips.

Surface micromachining on a polymethylmethacrylate substrate using visible laser-induced backside wet etching with a KMnO4 solution as an absorber

In this paper, the authors report a method for continuous trench micromachining on polymethylmethacrylate (PMMA). Visible laser-induced backside wet etching (v-LIBWE) on PMMA using a potassium permanganate (KMnO4)-based absorber liquid was studied. PMMA is widely used in microfluidic devices for chemical and biological applications. Conventional micromachining of channels in the PMMA substrate using CO2 lasers achieves the smallest feature of approximately 85 μm. In this study, a continuous 12 μm-wide trench etching on PMMA was achievable by v-LIBWE using a 532 nm nanosecond pulsed laser. The etching threshold is ∼10 J/cm2, which corresponds to the average power of 58 mW for the repetition rate of 10 kHz. The authors also determined that the lowest scanning speed of 2 mm/s was necessary for the v-LIBWE of PMMA to generate the crack-free surface. Our study provides a new perspective and a convenient approach for the micromachining of the polymer substrate using v-LIBWE.

A Nanoscale Modification of Materials at Thermoplasmonic Laser-Induced Backside Wet Etching of Sapphire

Laser-induced backside wet etching using water solution of plasmon precursor (AgNO3) as an absorbing medium provides effective structuring of such a complex material as sapphire. At the same time, this process is accompanied by the formation of silver nanoparticles and a significant modification of a surface layer of sapphire. Hybrid plasmon structures are formed on the surface of sapphire. In this paper, the structure and phase composition of surface layer formed during the transformation of the materials in the process of etching have been studied using electron microscopy methods, electron diffraction, and EDX analysis; possibilities of creating of new plasmon nanostructures have been analyzed.

High aspect ratio channel fabrication with near-infrared laser-induced backside wet etching

Abstract Laser-induced backside wet etching (LIBWE) is one of the most promising manufacturing processes for fabricating high aspect ratio microfeatures of glass. However, cracks inside the glass during the LIBWE process due to high thermal stress limits the manufacturable shape. In this study, we propose a new technique to prevent the crack generation in the near-infrared (NIR) LIBWE process by adding phosphoric acid to the absorbent. It was possible to increase the maximum fabricable depth by approximately five times compared to the absorbent without phosphoric acid. The nature of the crack suppression mechanism in the absorbent with added phosphoric acid was investigated by in situ observation of the process and chemical analysis. To investigate the effects of phosphoric acid on LIBWE with added phosphoric acid, the influences of phosphoric acid concentration on maximum fabricable depth, fabrication speed and sidewall roughness were studied. Based on the investigated principle and process characteristics, we found that high aspect ratio microchannels of different shapes could be produced.

Powerful 3-μm lasers acousto-optically Q-switched with KYW and KGW crystals.

We report, to the best of our knowledge, the first use of KY(WO4)2 and KG(WO4) acousto-optical Q-switches in 3-μm powerful lasers. Q-switches of two different designs (normal incidence with antireflection coatings, and Brewster-angle cut crystals) operate in lasers based on Er:YAG, Cr:Yb:Ho:YSGG, and Cr:Er:YSGG laser crystals. Gain and lifetime of laser crystals significantly influence the regime of Q-switched generation. Er:YAG and Cr:Yb:Ho:YSGG lasers deliver pulses with 10.8mJ and 17.5mJ energy, respectively. Pulses with energy of 29.6mJ and duration of 75ns in the TEM00 mode are obtained in a Cr:Er:YSGG laser. The energy is scaled up to 85.7mJ in the two-stage master oscillator power amplifier system. Powerful laser systems of this kind are in the region of interest for pumping other mid-IR laser media (e.g., Fe:ZnSe and Fe:CdSe), OPOs, CO2 lasers, and amplifiers. Preliminary experiments on microstructuring of transparent materials by the laser-induced backside wet etching method demonstrate the potential of such lasers to build the foundation for dye-free tissue and cell engineering concepts.

Nanosecond Nd:YAG laser induced backside wet etching of NaCl with eutectic gallium-indium alloy as absorbing liquid

Laser-induced back-side etching of NaCl (100) with eutectic gallium-indium (EGaIn, Ga 78% / In 22% weight percent) as the liquid absorber is demonstrated using nanosecond pulsed Nd:YAG laser with pulse width τp = 7 ns. The influences of the laser fluence and the pulse number on the etch rate are studied. The etch rate rises linearly with the laser fluence and is characterized by a high etch rate e (between 13.2 µm/pulse to 0.25 µm/pulse)in the laser fluence range of Φ = 15 J/cm2 to 0.6 J/cm2.Meanwhile, the etch rate is constant up to 6 pulses and a low threshold fluence for etching of Φth = 0.27 J/cm2 is found. According to the SEM image, cleavage cracks appear at the NaCl surface after laser radiation. The thermal stress of NaCl is estimated and compared with the rupture modulus to explain the mechanisms of the cleavage cracks of NaCl. In order to explain the high etch rate of NaCl, the maximum molten layer thickness during laser heating process is calculated to estimate the etch rate, which is significantly lower than the experimental value. Here, the thermal effects like cleavage cracks, fracture of material and defect production are most likely the reason for the high etch rate of NaCl. However, further experiments need to be completed for a detailed understanding of the etching mechanisms.

Efficiency of laser-induced backside wet microstructuring of sapphire increases with pressure

The study is directed to the development of the complex machining of hard transparent materials such as quartz, sapphire, and diamond using pulsed laser light (5 ns pulse duration, 532 nm wavelength, 1 kHz repetition rate). The deepening rate of the laser-induced backside etching of sapphire in a water solution of silver nitrate was studied as a function of liquid pressure in the range of 0.1–25 MPa. Its linear increase of about 1.5–2 times with pressure was demonstrated, which associated with the stronger microbubbles cavitation impact on hard material and chemical etching by sub- and super-critical water. The increase in the etching efficiency of sapphire at a higher pressure for this particular case can be generalized to other types of laser-induced backside wet etching and other materials.

Fabrication of robust metallic micropatterns on glass surfaces by selective metallization in laser-induced porous surface structures

Conductive metallic micropatterns on transparent glass substrates are desired in lots of applications, e.g. electrofluidic devices and micrototal analysis systems. Due to the significant differences in material properties between metal and glass, the fabricated metallic micropatterns prone to show limited bonding strength to the glass substrates. In this paper, microgrooves with porous edge areas are prepared by a laser-induced backside wet etching (LIBWE) method. In the subsequent electroless plating process, this peculiar surface structure enables fast deposition of copper on these areas without further treatment of the surface. Moreover, due to the strong anchor effect caused by the porous microstructures, the obtained copper micropatterns show high bonding strength to the glass substrate. No damage or peeling is observed under 20-min sonication in water and repeated scotch tape test. These micropatterns can even survive from surface polishing by sandpapers. We also demonstrated that this copper micropatterns with low resistivity (1.9 μΩ·cm) could be used for rapid heating of localized surface areas, which may be a potential choice for various microsystems.

Thermoplasmonic laser-induced backside wet etching of sapphire

The technology of thermoplasmonic laser-induced backside wet etching is proposed to microstructure such solid and difficult-to-process materials as sapphire. In this technology, the laser absorbing medium is silver nanoparticles obtained from a precursor (AgNO3) and providing significant laser radiation absorption due to the presence of plasmonic absorption with the formation of a substantially localised region with extremely high temperatures and pressures. This approach makes it possible to process sapphire both in the ‘gentle’ regime with the formation of nanometre structures and in the ‘strong’ regime to allow for the formation of ‘deep’ structures with etching rates up to several micrometers per pulse and high aspect ratio. The possibility of formation of periodic structures in sapphire in the above-threshold regime is shown.

Laser machining of transparent brittle materials: from machining strategies to applications

Transparent brittle materials such as glass and sapphire are widely concerned and applied in consumer electronics, optoelectronic devices, etc. due to their excellent physical and chemical stability and good transparency. Growing re-search attention has been paid to developing novel methods for high-precision and high-quality machining of transparent brittle materials in the past few decades. Among the various techniques, laser machining has been proved to be an effective and flexible way to process all kinds of transparent brittle materials. In this review, a series of laser machining methods, e.g. laser full cutting, laser scribing, laser stealth dicing, laser filament, laser induced backside dry etching (LIBDE), and laser induced backside wet etching (LIBWE) are summarized. Additionally, applications of these techniques in micromachining, drilling and cutting, and patterning are introduced in detail. Current challenges and future prospects in this field are also discussed.