Back-side Ablation Research Articles

Laser-Induced Periodic Surface Structures and Their Application for Gas Sensing.

Semiconducting metal oxides are widely used for solar cells, photo-catalysis, bio-active materials and gas sensors. Besides the material properties of the semiconductor being used, the specific surface topology of the sensors determines device performance. This study presents different approaches for increasing the sensing area of semiconducting metal oxide gas sensors. Micro- and nanopatterned laser-induced periodic surface structures (LIPSSs) are generated on silicon, Si/SiO2 and glass substrates. The surface morphologies of the fabricated samples are examined by FE SEM. We selected the nanostructuring and characterization of nanostructured source Ni/Au and Ti/Au films prepared on glass using laser ablation as the most suitable of the investigated approaches. Surface structures produced on glass by backside ablation provide 100 nm features with a high surface area; they are also transparent and have high resistivity. The value of the hydrogen sensitivity in the range concentrations from 100 to 500 ppm was recorded using transmittance measurements to be twice as great for the nanostructured target TiO2/Au as compared to the NiO/Au. It was found that such transparent materials present additional possibilities for producing optical gas sensors.

Crystalline Flat Surface Recovered by High-Temperature Annealing after Laser Ablation

Ultra-short laser pulses (1030 nm/230 fs) were used to laser ablate the surface of crystalline sapphire (Al2O3) at high intensity per pulse 20–200 TW/cm2/pulse. Laser-ablated patterns were annealed at a high temperature of 1500 °C. Surface reconstruction took place, removing the ablation debris field at the edges of ablated pits in oxygen flow (O2 flow). Partial reconstruction of ripples was also observed when multi-pulse ablated surfaces were annealed at high temperature in O2 flow. Back-side ablation of a 0.5-mm-thick Al2O3 produced high surface roughness ∼1μm which was reduced to ∼0.2μm by high-temperature annealing at 1500 °C for 2 h in O2. Improvement of surface quality was due to restructuring of the crystalline surface and sublimation, while the defined 3D shape of a micro-lens was not altered after HTA (no thermal morphing).

Laser-Induced Cavitation-Assisted True 3D Nano-Sculpturing of Hard Materials.

Femtosecond lasers enable flexible and thermal-damage-free ablation of solid materials and are expected to play a critical role in high-precision cutting, drilling, and shaping of electronic chips, display panels, and industrial parts. Although the potential applications are theoretically predicted, true 3D nano-sculpturing of solids such as glasses and crystals, has not yet been demonstrated, owing to the technical challenge of negative cumulative effects of surface changes and debris accumulation on the delivery of laser pulses and subsequent material removal during direct-write ablation. Here, a femtosecond laser-induced cavitation-assisted true 3D nano-sculpturing technique based on the ingenious combination of cavitation dynamics and backside ablation is proposed to achieve stable clear-field point-by-point material removal in real time for precise 3D subtractive fabrication on various difficult-to-process materials. As a result, 3D devices including free-form silica lenses, micro-statue with vivid facial features, and rotatable sapphire micro-mechanical turbine, all with surface roughness less than 10nm are readily produced. The true 3D processing capability can immediately enable novel structural and functional micro-nano optics and non-silicon micro-electro-mechanical systems based on various hardsolids.

Ultrashort pulsed laser backside ablation of fused silica.

We report on the fabrication of rectangular microchannels with vertical sidewalls in fused silica by laser backside ablation. A 515 nm femtosecond laser is focused by an objective with a NA of 0.5 through the sample on the glass/air interface, allowing processing from the backside into the bulk material. Experimental investigations reveal a logarithmically increasing depth of the channels with an increasing number of scans, while keeping the focal position fixed. A certain number of scans has to be applied to generate rectangular shaped channels while their depth can be controlled by the applied fluence from 2.64 µm to 13.46 µm and a corresponding ablation roughness Ra between 0.20 µm and 0.33 µm. The channel width can be set directly via the number of parallel ablated lines demonstrated in a range from 10 µm to 50 µm. By adjusting the focal position after each scan the channel depth can be extended to 49.77 µm while maintaining a rectangular channel geometry. Finally, concentric rings are ablated to demonstrate the flexibility of the direct writing process.

Fast Fabrication of Fishnet Optical Metamaterial Based on Femtosecond Laser Induced Stress Break Technique.

To speed up the fabrication of optical metamaterials by making use of the fast speed advantage of femtosecond laser preparation, a metamaterial appropriate for femtosecond laser processing was designed, and the interaction between femtosecond laser and metal-dielectric-metal fishnet stacks was investigated in detail. Two kinds of processing mechanisms, thermal melting and stress break, were revealed during the fabrication. The thermal melting process, dominated by the interaction of femtosecond laser with metals, makes the upper and lower metal layers adhere to each other, which leads to the magnetic resonance impossible. The stress break process, dominated by the interaction of femtosecond laser with dielectrics, can keep the upper and lower metal coatings isolated. Fishnet optical metamaterial was fabricated by femtosecond laser-induced stress break technique, using back side ablation, high numerical aperture and super-Gaussian beam. The resolution and speed can reach 500 nm, and 100 units/s, respectively. Spectrophotometer measurement results proved that the magnetic resonances were found in the fishnet nanostructure. The theoretical refractive index of the metamaterial on a glass substrate reached −0.12 at the wavelength of 3225 nm. It proved that femtosecond laser-induced stress break was a good and fast tool during the fabrication of optical metamaterials.

Ultraviolet Laser Patterning of Fluorine-Doped Tin Oxide with Different Radiation Directions

Selective laser patterning of thin films in a multilayered structure is an emerging technology for fabricating optoelectronics and microelectronics devices. As the application of ultraviolet (UV) lasers is more and more popular in electronics manufacturing, in this study, we compare UV laser patterning of fluorine-doped tin oxide (FTO) films from the front and back sides. We summarize a formula for calculating the focus offset in back-side ablation (BSA) and analyze systematically the relationships between the laser ablation crater size, the damage on glass substrate, and the laser ablation parameters. In addition, the laser ablation process and mechanisms of the BSA and the front-side ablation (FSA) are also elaborated in this paper.

Femtosecond Laser Backside Ablation of Gold Film on Silicon Substrate

Femtosecond laser ablation of gold thin film on the front and backside of silicon substrate is investigated, with backside ablation being the focus and front side ablation for comparison. The experiments are performed using 100 fs pulses delivered through an ultrafast laser source combined with an OPA for wavelength conversion at 1300nm. We create a single shot ablation matrix by varying focus position and pulse energy. The laser beam is characterized using an IR imaging technique at both the front and backside of the substrate. It is found that the pulse profile experiences little distortion after passing though the 1mm silicon substrate, despite the high pulse energy used. However, a comparison of the front and back ablation site indicates significant attenuation of pulse energy due to nonlinear absorption. Two types of damage happen depending on laser fluence: ablation and burst. Burst damage is confirmed with finite element simulation.

Scribe Width Optimization of Absorber Laser Ablation for Thin-film Silicon Solar Modules

The scribe width reduction of the absorber removal process for the integrated series connection of thin-film silicon solar modules was investigated with laser-induced back-side ablation. Depending on the used scribing beam spot radius and absorber thickness changes of ablation behavior lead to limitations of the scribe width reduction. The influence of the laser processes on the electrical properties between front- and back-contact was characterized with Transmission Line Method measurements. The values of the interconnection resistance between front- and back-contact are used in electrical calculations to estimate the optimal trade-off between increase of active cell area with thinner scribe lines and the increase of series resistance losses from interconnection resistance formation. Calculations predict a ~1% relative decrease of module losses for the optimized processes compared to commonly used processes from industry.

Hybrid Laser-Plasma Micro-Structuring of Fused Silica Based on Surface Reduction by a Low-Temperature Atmospheric Pressure Plasma

In this contribution, we report on a novel hybrid laser-plasma method for material processing applications. This method is based on the combination of both an ArF excimer laser (λ = 193 nm) and a low-temperature atmospheric pressure plasma jet source for the chemical reduction of glass surfaces. Here, a hydrogen-containing plasma gas was applied. Due to the layer of silicon suboxide that is generated in this vein, the absorption of the incoming machining laser beam is significantly increased after 15 minutes of plasma-treatment. Several machining experiments in terms of frontside ablation were performed on fused silica. Here, both pure and plasma-treated surfaces were ablated using single laser pulses with a pulse duration of 20 ns. By introducing the presented hybrid technique, the ablation threshold for micro-structuring was reduced significantly by a factor of 4.6 whereas the peak-to-valley height Rz of the machined area was decreased by a factor of 2.3. Further, back-side ablation using the presented method was considered. By a terminal tempering process, the initial transmission characteristics of fused silica can be reconstituted.

Mechanism of backside etching of transparent materials with nanosecond UV-lasers

In consequence of high interest in micro- and nanomachining of transparent materials by laser irradiation, studies on the mechanism of laser-induced backside wet etching (LIBWE) are presented. To reveal the role of the surface modification due to LIBWE the backside ablation (BSA) of LIBWE-modified fused silica (mFS) surfaces at 248 nm was investigated. The threshold fluence and the etch rate of BSA are similar to that of LIBWE and amount ∼250 mJ/cm2 and 30 nm for 1 J/cm2, respectively. The sample transmission after backside ablation of mFS increases and proves the decreasing thickness of the absorbing layer. Time-resolved reflection studies at LIBWE and BSA of mFS show similar patterns in the backside reflection that can be assigned an ablation process as the comparison to thin polymer films demonstrates. By fitting the BSA data to an exponential decay absorption model a modification depth and a surface absorption of about 38 nm and α S ∼1.3×107 m−1 were calculated, respectively. In conclusion of the results a new model for LIBWE is proposed.

Experimental study on infrared nanosecond laser-induced backside ablation of sapphire

Experimental study has been performed on infrared nanosecond laser-induced backside ablation (LIBA) of sapphire with black paint as the backing layer, where laser energy absorption and material removal occur around the sapphire–backing-layer interface. The effect of laser fluence and pulse duration on ablation rate has been revealed. It has been found that LIBA using nanosecond lasers can produce ∼ 3 to ∼ 80 times higher ablation rate than direct laser ablation using femtosecond or picosecond laser pulses with the same fluence per pulse. Reasonably good ablation quality can be obtained using LIBA without chemical contamination from the backing layer. It has also been found that under the studied conditions the ablation rate in LIBA is not sensitive to laser pulse duration, and the damage threshold is not sensitive to laser pulse number. The experimental data in this work provides very important and useful information for practical applications of LIBA in sapphire micromachining and for the fundamental understanding and quantitative modeling of the process.

Energy Balance in a Laser-Induced Forward Transfer Process Studied by Shadowgraphy

Laser-induced forward transfer is a powerful technique for the patterned deposition of thin layers. Sensitive materials can be transferred using an intermediate sacrificial layer, which protects the material from direct laser irradiation. Among various materials, triazene polymers are very promising sacrificial layers, and therefore, their ablation behavior has a particular importance for the transfer. The ablation behavior of a single sacrificial layer of a specific triazene polymer was investigated using time-resolved shadowgraphy to understand and optimize the deposition process. We analyzed the evolution of the shock wave generated by the decomposition of the triazene polymer to estimate the thermodynamics of the process. The energy balance shows that a large part of the energy is released into the shock wave but that thermal and mechanical losses predominate. On the other hand, the kinetic energy of the flyer corresponds only to a few percent of the total energy. We also observed a difference in the shock wave energy between front side and back side ablation when a flyer of undecomposed triazene polymer is ejected. The presence of the flyer diminishes the shock wave energy by a factor of 2 in the most extreme case.

Self-organization in a chromium thin film under laser irradiation

Self-organization of chromium on glass was observed during laser ablation of the metal film with partially overlapping laser pulses. The beam of a nanosecond pulse laser tightly focused to a line was applied to the back-side ablation of the chromium thin film on a glass substrate. While the line ablated with a single laser pulse had sharp edges on both sides with ridges of the melted metal, the use of partially overlapping pulses formed a complicated structure made of the metal remaining from the ridges. Regular structures of ripples were developed in a certain range of laser fluence and pulse overlap. The ripple period could be controlled from 2.5 to 4 μm by variation of the processing parameters. Various experimental techniques were applied to test the structures, and different models of the ripple formation in the thin metal film were considered. The initial quasi-periodical formation started because of dewetting of thin liquid metal films on the glass substrate after its melting. Similar to the evaporation of liquid films, the small perturbation in the ridge thickness was able to induce instability in evaporation of the thin melted metal film. Freezing of the nonequilibrium state between laser pulses was one of the stabilizing factors in self-organization of the metal.

Ripple formation in the chromium thin film during laser ablation

The beam of a nanosecond pulse laser tightly focused to a line was applied for the back-side ablation of the chromium thin film on a glass substrate. The stripe ablated with a single laser pulse had sharp edges on both sides and ridges of the melted metal around it. The partially overlapping pulses formed a wide cleaned area with a complicated structure made of the metal remaining from the ridges. Regular structures, ripples, were developed when laser fluence was slightly above the single-pulse removal threshold and the shift between pulses was less than half width of the line ablated with a single laser pulse. The ripples were located periodically (∼4 μm) and were orientated perpendicularly to the long axis of the beam spot. Their orientation did not depend on the laser beam polarization. Different models of the ripple formation in the thin metal film were considered, and instability of the moving vapor–liquid–solid contact line during evaporation of thin liquid films appears to be the most probable process responsible for the observed phenomena. Formation of regular gratings with the unlimited line length was experimentally implemented by using the above-mentioned technique.

Laser backside etching of fused silica due to carbon layer ablation

The backside ablation of a absorbing carbon layer onto fused silica is studied in air and water confinement in comparison. The confinement influences the etch rate and the laser fluence dependence of the etch rate significantly while the threshold fluence is almost the same. The different confinement of the laser induced plasma results in the observed rate saturation in the case of air and in a linear growing rate in the case of water confinement at medium laser fluences. The less dense air confinement permits a faster plasma expansion of the laser plume than in the case of water confinement and effects consequently the interaction time and interaction strength of the laser plume with the fused silica surface. The differences in the laser-plasma-substrate interaction cause the observed rate saturation at weak interaction (air) and the growing etch rate at strong interaction (water). Thus, the confinement situation controls the interaction process in the case of backside ablation and should be considered in indirect material processing methods such as LIBWE and LESAL, too.