Laser-induced Backside Dry Etching Research Articles

Robust metallic micropatterns fabricated on quartz glass surfaces by femtosecond laser-induced selective metallization.

Quartz glass has a wide range of application and commercial value due to its high light transmittance and stable chemical and physical properties. However, due to the difference in the characteristics of the material itself, the adhesion between the metal micropattern and the glass material is limited. This is one of the main things that affect the application of glass surface metallization in the industry. In this paper, micropatterns on the surface of quartz glass are fabricated by a femtosecond laser-induced backside dry etching (fs-LIBDE) method to generate the layered composite structure and the simultaneous seed layer in a single-step. This is achieved by using fs-LIBDE technology with metal base materials (Stainless steel, Al, Cu, Zr-based amorphous alloys, and W) with different ablation thresholds, where atomically dispersed high threshold non-precious metals ions are gathered across the microgrooves. On account of the strong anchor effect caused by the layered composite structures and the solid catalytic effect that is down to the seed layer, copper micropatterns with high bonding strength and high quality, can be directly prepared in these areas through a chemical plating process. After 20-min of sonication in water, no peeling is observed under repeated 3M scotch tape tests and the surface was polished with sandpapers. The prepared copper micropatterns are 18 µm wide and have a resistivity of 1.96 µΩ·cm (1.67 µΩ·cm for pure copper). These copper micropatterns with low resistivity has been proven to be used for the glass heating device and the transparent atomizing device, which could be potential options for various microsystems.

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.

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.

Influence of the confinement on laser-induced dry etching at the rear side of fused silica

Laser-induced etching at the rear side of transparent material enables high-quality machining results. However, the mechanism is still not completely recognized which would allow further optimization. Therefore, multi-pulsed laser-induced backside dry etching with different thick photoresist films was studied experimentally for air (MP-LIBDE) and water confinements (cMP-LIBDE). The water confinement causes differences in photoresist ablation morphology and etching rate in dependence on laser fluence, film thickness and pulse number. Owing to the water confinement, the extent of photoresist film spallation and the etching rate slope difference in low and high fluence ranges are reduced. In particular, the etching rate of cMP-LIBDE keeps constant with different film thicknesses in contrast to MP-LIBDE. Two effects that are related to the water confinement, mechanical confinement and heat transfer alterations, are analysed and discussed in relation to the differences between MP-LIBDE and cMP-LIBDE.

Multi-pulse LIBDE of fused silica at different thicknesses of the organic absorber layer

Laser-induced etching techniques feature several unique characteristics that enable ultraprecise machining of transparent materials. However, LIBDE (laser-induced back side dry etching) and LIBWE (laser-induced back side wet etching) are preferentially studied due to experimental feasibilities either using a very thin or a bulk absorber at the rear side of the transparent material. This study aims to fill the gap by examining the thickness dependence of the absorbing material. Multi-pulse-LIBDE (MP-LIBDE) of fused silica using different thick photoresist absorber layers (dL=0.2–11.7μm) was performed with a KrF excimer laser (λ=248nm, tp≈20ns). The influence of several experimental parameters, such as laser fluence, pulse number, film thickness, on the ablation morphology and the etching rate were investigated. Especially at moderate fluences (F=0.7–1.5J/cm2) MP-LIBDE and LIBWE show several similar process characteristics such as the etching rate dependence on the laser fluence and the pulse number with a typical etching rate of approx. 12nm at 1J/cm2. However, the specific etching rate values depend on the absorber layer thickness, for instance. The morphology of the etched surface is smooth with a roughness of below 5nm rms. Further, the modification of the surface has been observed and will be discussed in relation to the multi-pulse laser etching mechanism.

Possible application of laser-induced backside dry etching technique for fabrication of SERS substrate surfaces

Laser-induced backside dry etching (LIBDE) method has proved to be capable of micro- and nanomachining of transparent materials. In present study the applicability of this technique was tested to prepare surface structures that are suitable for surface enhanced Raman scattering (SERS) spectroscopy. Fused silica plates were coated with 16nm thick silver layer by pulsed laser deposition (PLD) method. The metal absorbing film was irradiated and removed by 10 ArF excimer laser (λ=193nm, FWHM=20ns) pulses perpendicularly through the transparent samples at 1200mJ/cm2 fluence. The remaining part of the silver coating was dissolved chemically shortly after the experiment using nitric acid. After this the etched holes were re-coated with 10, 20 and 40nm thick silver layers by PLD, respectively. SERS activity of the different parts of the holes was investigated by a Thermo Scientific DXR Raman Microscope at 785nm wavelength using 10−3 M aqueous solution of Rhodamin 6G as reference sample. SERS spectra were recorded at different areas of the holes having various roughness parameters. It was found that the intensity of the main peaks depended significantly on the surface morphology of the tested area and the thickness of its silver coating.

Laser Induced Backside Dry Etching of BK-7 and Quartz in Vacuum

Laser Induced Plasma Assisted Ablation (LIPAA) has been used to carry out back side dry etching/ablation of different optical glasses under vacuum. It is observed that etching depth increases with decrease in target-substrate gap and becomes maximum (50 μm) at zero gap size for steel-quartz pair. The maximum ablation depth for brass-quartz pair was 25 μm. The effects of laser power on etching depth and laser scan speed has also been reported.

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.

Fabrication of microtransmittance grating using laser induced backside dry etching

Laser induced backside dry etching (LIBDE) is a novel technique, which applied in the micromachining and nanomachining of transparent dielectric materials used for optical elements by direct laser irradiation with the assistance of solid mediums as an absorbed layer. On the basis of exploring two different solid mediums, i.e. Al2O3 powder and alumina ceramic wafer as absorbers, LIBDE with 1064 nm central wavelength pulsed ytterbium doped fiber laser as the irradiation source was successfully applied for the fabrication of periodical microscaled structures on fused silica. The threshold of laser fluence to etch fused silica was 7.47 J/cm2 extrapolated from the experimental data. By using alumina ceramic wafer as the absorber, a microbinary dimensional transmittance grating with the constant of 25 μm was obtained on the silica. The depth of the trench is 4.2 μm, and the root mean square roughness of the trench bottom is below 40 nm according to the measurement of 3D profiler. Additionally, diffraction patterns were observed by the normal incidence of a 532 nm diode indicator laser to examine the optical features of the microbinary grating etched under the different laser fluences.

Time-resolved measurements during backside dry etching of fused silica

The laser-induced backside dry etching (LIBDE) investigated in this study makes use of a thin metal film deposited at the backside of a transparent sample to achieve etching of the sample surface. For the time-resolved measurements at LIBDE fused silica samples coated with 125 nm tin were used and the reflected and the transmitted laser intensities were recorded with a temporal resolution of about 1 ns during the etching with a ∼30 ns KrF excimer laser pulse. The laser beam absorption as well as characteristic changes of the reflection of the target surface was calculated in dependence on the laser fluence in the range of 250–2500 mJ/cm 2 and the pulse number from the temporal variations of the reflection and the transmission. The decrease of the time of a characteristic drop in the reflectivity, which can be explained by the ablation of the metal film, correlates with the developed thermal model. However, the very high absorption after the film ablation probably results in very high temperatures near the surface and presumably in the formation of an absorbing plasma. This plasma may contribute to the etching and the surface modification of the substrate. After the first pulse a remaining absorption of the sample was measured that can be discussed by the redeposition of portions of the ablated metal film or can come from the surface modification in the fused silica sample. These near-surface modifications permit laser etching with the second laser pulse, too.

Laser-induced backside dry and wet etching of transparent materials using solid and molten tin as absorbers

Laser-induced backside wet and dry etching (LIBWE and LIBDE) methods were developed for micromachining of transparent materials. Comparison of these techniques is helpful in understanding the etching mechanism but was not realized due to complications in setting up comparable experimental conditions. In our comparative investigations we used a solid tin film for dry and molten tin droplets for wet etching of fused-silica plates. A tin–fused-silica interface was irradiated through the sample by a KrF excimer laser beam (λ=248 nm, FWHM=25 ns); the fluence was varied between 400 and 2100 mJ/cm2. A significant difference between the etch depths of the two investigated methods was not found. The slopes of the lines fitted to the measured data (slLIBDE=0.111 nm/mJ cm−2, slLIBDE=0.127 nm/mJ cm−2) were almost similar. Etching thresholds for LIBDE and LIBWE were approximately 650 and 520 mJ/cm2, respectively. To compare the dependence of etch rates on the pulse number, target areas were irradiated at different laser fluences and pulse numbers. With increasing pulse number a linear rise of depth was found for wet etching while for dry etching the etch depth increase was nonlinear. Secondary ion mass spectroscopic investigations proved that this can be due to the reconstruction of a new thinner tin-containing surface layer after the first pulse.

Laser-induced backside dry etching: wavelength dependence

The laser-induced backside dry etching (LIBDE) method has been developed by analogy with the well-known LIBWE technique for micromachining of transparent materials. In our experiments tin thin films were applied as absorbing layers and the dependence of the etch depth on the applied laser wavelength was investigated. Multipulse irradiation experiments at different wavelengths have also been carried out. A fused silica plate coated by a tin layer was used as a target. The metal film–transparent material interface was irradiated through the fused silica plate by the most frequently used nanosecond excimer laser beams: ArF (193 nm), KrF (248 nm) and XeCl (308 nm). It was found that the etch depth increased linearly with decreasing wavelength in investigated range. Multipulse investigations proved that the energy of the absorbed photons has a strong influence on the mechanism of LIBDE. The etch depth–pulse number curves showed saturation at 1500 mJ cm−2 applied laser fluence at around 2, 3 and 30 pulses for 308 nm, 248 nm and 193 nm wavelengths, respectively. This indicates that the effectiveness of multipulse LIBDE increases when the wavelength of the applied laser source is decreased in the UV range. XPS and spectroscopic ellipsometric analyses showed that formation and embedding of tin-oxides into the upper surface layer can be responsible for the shift of the saturation in the case of ArF as compared with the other studied wavelengths.

Time-resolved transmission study of fused silica during laser-induced backside dry etching

Laser-induced backside dry etching (LIBDE) is a promising technique for micro- and nanomachining of transparent materials. Although several experiments have already proved the suitability and effectiveness of the technique, there are several open questions concerning the etching mechanism and the concomitant processes. In this paper time-resolved light transmission investigations of etching process of fused silica are presented. 125 nm thick silver coating was irradiated through the carrying 1 mm thick fused silica plate by single pulses of a nanosecond KrF excimer laser. The applied fluences were 0.38, 0.71 and 1 J/cm 2. During the etching process the irradiated spots were illuminated by an electronically delayed nitrogen laser pumped dye laser. The delay between the pump and probe pulses was varied in the range of 0 ns and 20 μs. It was found that the transmitted probe beam intensity strongly depends on the applied delays and fluences. Scanning electron microscopy and energy dispersive X-ray spectrometry of the etched surface showed the existence of silver droplets and fragments on the illuminated surfaces and silver atoms built into the treated surface layer influencing the transmission behavior of the studied samples.

Influence on the laser induced backside dry etching of thickness and material of the absorber, laser spot size and multipulse irradiation

Laser induced backside dry etching method (LIBDE) was developed – analogously to the well-known laser induced backside wet etching (LIBWE) technique – for the micromachining of transparent materials. In this procedure, the absorbing liquid applied during LIBWE was replaced with solid metal layers. Fused silica plates were used as transparent targets. These were coated with 15–120 nm thick layers of different metals (silver, aluminium and copper). The absorbing films were irradiated by a nanosecond KrF excimer laser beam through the quartz plate. The applied fluence was varied in the 150–2000 mJ/cm 2 range, while the irradiated area was between 0.35 and 3.6 mm 2. At fluences above the threshold values, it was found that the metal layers were removed from the irradiated spots and the fused silica was etched at the same time. In our experiments, we investigated the dependence of the main parameters (etch rate and threshold) of LIBDE on the absorption of the different metal layers (silver, copper, aluminium), on the size of the irradiated area, on the film thickness and on the number of processing laser pulses.

Laser induced backside dry etching of transparent materials

Laser induced backside wet etching (LIBWE) is a well-known procedure in the micromachining of transparent materials. We developed a modified technique in which the absorbing liquid of LIBWE was substituted by an absorbing solid thin film (laser induced backside dry etching, LIBDE). Fused silica plate having 1 mm thickness was coated by a 100 nm thick silver layer. The metal film was irradiated through the quartz plate by single pulses of a nanosecond KrF excimer laser. The applied fluence was varied in the range of 90–4030 mJ/cm 2, the illuminated area was 1.05 mm 2. At the appropriate fluences it was found that the silver was removed from the irradiated spots and here the fused silica was etched, simultaneously. The dependence of the etch rate (which corresponds to the etched depth, since this is a single-pulse technology) on the fluence was investigated. The results showed a linear relation between etching depth (0–600 nm/pulse) and applied fluence in the range of 0.2 (the etching threshold fluence) to 4 J/cm 2. With its higher etching rate, the LIBDE procedure is much more effective than LIBWE. The morphology of the etched holes was studied by an atomic force microscope. A thermal model was developed for the description of the etching mechanism which was in a good agreement with the experimental results.

Processing of transparent materials using visible nanosecond laser pulses

Laser micromachining of transparent materials is an intensively studied research area from the point of view of microoptical element fabrication. One of the most promising indirect processing methods is the laser-induced back-side dry etching (LIBDE). During this method, transparent targets are contacted with solid thin layers, which absorb and transform the pulse energy resulting in etching. The applicability of LIBDE technology for processing of fused silica using a visible nanosecond dye laser (λ=500 nm, FWHM=11 ns) and a 100-nm-thick aluminium absorbing layer was investigated. The applied fluence was varied in the range of 0–3050 mJ/cm2; the illuminated area was 0.1 mm2. The threshold fluence of the LIBDE etching of fused silica was found to be approximately 540 mJ/cm2. The chemical composition of the surface layers on and around the etched holes was investigated by field-emission scanning electron microscopy and energy-dispersive X-ray spectrometry. It was found that on average 0.4±0.3 at. % aluminium is built into the upper ∼1-μm-thick volume of the illuminated fused silica, while the aluminium content fell below the detection limit in the case of the original surface. Our experiments proved that the LIBDE procedure is suitable for microprocessing of transparent materials using visible nanosecond laser light.

Production of submicrometre fused silica gratings using laser-induced backside dry etching technique

Laser micromachining of transparent materials is a promising technique for producing micro-optical elements. Several types of both direct (e.g. ablation) and indirect (e.g. laser-induced backside wet etching: LIBWE) procedures have already been developed and presented in the last two decades. Here we present a new method (laser-induced backside dry etching (LIBDE)) in the analogy of LIBWE for the micro and nanoprocessing of transparent materials.In our experiments 1 mm thick fused silica plates were used as transparent work pieces. The plates were covered with 100 nm thick silver layers. The metal absorbing films were irradiated through the fused silica by a KrF excimer laser beam (λ = 248 nm, FWHM = 30 ns). The illuminated area was 1.05 mm2 and the fluence on the silver–quartz interface varied in the range 0–1800 mJ cm−2. We have provided evidence that LIBDE is more effective and simple than LIBWE, its etch rate being much higher at a given laser fluence. Our interference experiments proved that the LIBDE etching technique is suitable to fabricate gratings displaying submicrometre periods in transparent materials. On the basis of all these, it is suggested that this method may be useful to produce other nano and microoptical elements, too.