Laser-induced Backside Etching Research Articles

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.

Studies of the confinement at laser-induced backside dry etching using infrared nanosecond laser pulses

In the present study, laser-induced backside etching of SiO2 at an interface to an organic material using laser pulses with a wavelength of λ=1064nm and a pulse length of τ=7ns have been performed in order to investigate selected processes involved in etching of the SiO2 at confined ablation conditions with wavelengths well below the band gap of SiO2. Therefore, in between the utilized metallic absorber layer and the SiO2 surface, a polymer interlayer with a thickness between 20nm to 150nm was placed with the aim, to separate the laser absorption process in the metallic absorber layer from the etching process of the SiO2 surface due to the provided organic interlayer. The influence of the confinement of the backside etching process was analyzed by the deposition of different thick polymer layers on top of the metallic absorber layer. In particular, it was found that the SiO2 etching depth decreases with higher polymer interlayer thickness. However, the etching depth increases with increasing the confinement layer thickness. SEM images of the laser processed areas show that the absorber and confinement layers are ruptured from the sample surface without showing melting, and suggesting a lift off process of these films. The driving force for the layers lift off and the etching of the SiO2 is probably the generated laser-induce plasma from the confined ablation that provides the pressure for lift off, the high temperatures and reactive organic species that can chemically attack the SiO2 surface at these conditions.

Laser-induced back-side etching with liquid and the solid hydrocarbon absorber films of different thicknesses

Laser-induced backside wet and dry etching (LIBWE and LIBDE) are methods for high-quality surface patterning of transparent dielectrics that making use of an additional absorber material attached to the rear side that is ablated in a confined configuration. Due to the manifold of the involved processes, the mechanism of the etching process and the parameter influence on the material removal process are multifaceted and not fully understood yet. In the present paper, we investigate the influence of the confinement to the backside etching process by studying the impact of the thickness of the attached liquid or solid absorber within a range of 12–125 and 0.2–11.7 μm, respectively. It was found that for the liquid and solid absorbers, the etching rate increases with the thickness of the absorber layer and saturates exceeding a certain value, which depends on the used laser fluence. Moreover, the incubation of etching depends on the absorber thickness. The comparison of the etching results of a similar thickness of the liquid and the solid absorber layers shows that the phase of the absorber (liquid or solid) does not influence the back-side etching process. Time-resolved shadowgraph images of the process indicate that with higher absorber layer thickness, the interaction time and strength of the laser-induced processes at the sample surface increase. The results suggest that confinement of the rear side attached absorber ablation influences the impact of the laser-induced secondary processes to the strength of the material modifications and, therefore, the etching rate.

Laser induced backside wet and dry etching of solar glass by short pulse ytterbium fiber laser irradiation

Microstructures fabricated on the surface of solar glass have the potential to improve the performance of solar cells. In this paper, in order to overcome the high transmittance to 1064 nm center wavelength fiber laser irradiation and realize high efficiency process on transparent glass substrates, different absorber materials, including alumina powder, alumina ceramic wafers, and copper sulphate solutions, were applied for dry and wet etching under the irradiation of 1064 nm pulsed fiber laser respectively. The laser fluence was varied from 7 to 10 J/cm2 with a pulse repetition rate of 20 kHz. The morphology of trenches etched by means of laser induced backside dry etching (LIBDE) and by laser induced backside wet etching was measured using a scanning electronic scope, and compared from the aspects of etch depth and width, as well as the roughness. On the basis of this comparison, a higher etch rate can easily be obtained by dry etching, while lower roughness is a feature of wet etching. The mechanism of LIBDE of solar glass was investigated by demonstrating the procedure of dry etching using alumina ceramic wafer. Moreover, the etch threshold fluence was estimated to be 7.47 J/cm2 by extrapolation. Both types of laser induced backside etching techniques, wet and dry, show the evidence of effective microprocessing on solar glass.

Using IR laser radiation for backside etching of fused silica

Laser-induced backside etching of fused silica with gallium as highly absorbing liquid is demonstrated using pulsed infrared laser radiation. The influences of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography were studied and the results are compared with these of excimer laser etching. The high reflectivity of the fused silica-gallium interface at IR wavelengths results in the measured high threshold fluences for etching of about 3 J/cm2 and 7 J/cm2 for 18 ns and 73 ns pulses, respectively. For both pulse lengths the etch rate rises almost linearly with laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm2, respectively. The etching process is almost free from incubation processes because etching with the first laser pulse and a constant etch rate were observed. The etched surfaces are well-defined with clear edges and a Gaussian-curved, smooth bottom. A roughness of about 1.5 nm rms was measured by AFM at an etch depth of 0.95 μm.

Backside etching of fused silica with UV laser pulses using mercury

Laser-induced backside wet etching (LIBWE) is an indirect etching technique that was first demonstrated for fused silica applying a pyrene doped hydrocarbon solution and was recently demonstrated for gallium as a highly absorbing liquid. In this study the etching of fused silica with mercury as liquid absorber is demonstrated using a nanosecond UV laser (excimer laser λ = 248 nm, 25 ns pulses) radiation. The high absorption coefficient of mercury enforces the absorption of the UV laser radiation within a 100 nm region near the solid interface so that, despite the high reflectivity, the threshold for etching was measured to be 0.76 J cm−2. With rising laser fluence the etch rate increases nearly linearly in the whole laser fluence range and reaches a value of 650 nm/pulse at a laser fluence of about 10 J cm−2. The etched surfaces were investigated by scanning electron microscopy, atomic force microscopy and interference microscopy and show well defined etch groove edges and a flat, smooth etched bottom that features an rms roughness of about 2.2 nm. The laser-induced backside etching of fused silica with mercury combines a low etching threshold, high etch rates and a still smooth etching of well-defined patterns into material surfaces.

Backside etching of fused silica with Nd:YAG laser

The laser-induced backside etching of fused silica with gallium as highly absorbing backside absorber using pulsed infrared Nd:YAG laser radiation is demonstrated for the first time. The influence of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography was studied. The comparable high threshold fluences of about 3 and 7 J/cm 2 for 18 and 73 ns pulses, respectively, are caused by the high reflectivity of the fused silica–gallium interface and the high thermal conductivity of gallium. For the 18 and 73 ns long pulses the etch rate rises almost linearly with the laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm 2, respectively. Incubation processes are almost absent because etching is already observed with the first laser pulse at all etch conditions and the etch rate is constant up to 30 pulses. The etched grooves are Gaussian-curved and show well-defined edges and a smooth bottom. The roughness measured by interference microscopy was 1.5 nm rms at an etch depth of 0.6 μm. The laser-induced backside etching with gallium is a promising approach for the industrial application of the backside etching technique with IR Nd:YAG laser.