Laser direct patterning induced the tunable optical properties of indium tin oxide micro-hole arrays films

Here we introduce a facile method to fabricate patterned indium tin oxide (ITO) thin films via selective laser ablation at ambient conditions. By scanning the ITO thin films with focused Nd: YAG pulsed laser, the ITO thin films were selective ablated and patterned without using any conventional chemical etching or photolithography steps. Then we investigated the effects of scanning rate for the structure, morphology and optical properties of patterned ITO thin film. These results indicate that the epsilon-near-zero (ENZ) wavelength of ITO thin films can be tuned from 1100 nm to 1340 nm by adjusting the period of the micro-hole array in microstructure. The nonlinear absorption response of patterned ITO films was about 2.85 time than of the as-deposited ITO thin film. Additionally, the results of the Finite-Difference Time-Domain (FDTD) simulation are in good agreement with those of the experiments.     

1 064 nm激光对氧化铟锡薄膜的损伤研究

 液晶光学器件在激光光束精密控制上具有重要应用前景,氧化铟锡（ITO）薄膜作为液晶光学器件的透明导电电极,是液晶器件激光损伤的薄弱环节。为此,建立了ITO薄膜激光热损伤物理模型。理论计算结果表明：1 064 nm激光对ITO薄膜的损伤主要为热应力损伤;连续激光辐照下,薄膜损伤始于靠近界面的玻璃基底内;脉冲激光辐照下,温升主要发生在光斑范围内的膜层,薄膜损伤从表面开始。利用泵浦探测技术,研究了ITO薄膜的损伤情况,测量了不同功率密度激光辐照后薄膜的方块电阻,结合1-on-1法测定了ITO薄膜的50%损伤几率阈值。实验结果表明：薄膜越厚,方块电阻越小,激光损伤阈值越低;薄膜未完全损伤前,方块电阻随激光功率密度的增加而增大。理论计算与实验结果吻合较好。设计液晶光学器件中的ITO薄膜电极厚度时,应综合考虑激光损伤、透光率及薄膜电阻的影响。     

Femtosecond laser patterning of Ta0.1W0.9Ox/ITO thin film stack

Selective laser patterning of thin films in a multilayered structure is an emerging technology for process development and fabrication of optoelectronics and microelectronics devices. In this work, femtosecond laser patterning of electrochromic Ta_0.1W_0.9O_x film coated on ITO glass has been studied to understand the selective removal mechanism and to determine the optimal parameters for patterning process. A 775 nm Ti:sapphire laser with a pulse duration of 150 fs operating at 1 kHz was used to irradiate the thin film stacks with variations in process parameters such as laser fluence, feedrate and numerical aperture of objective lens. The surface morphologies of the laser irradiated regions have been examined using a scanning electron microscopy and an optical surface profiler. Morphological analysis indicates that the mechanism responsible for the removal of Ta_0.1W_0.9O_x thin films from the ITO glass is a combination of blistering and explosive fracture induced by abrupt thermal expansion. Although the pattern quality is divided into partial removal, complete removal, and ITO film damage, the ITO film surface is slightly melted even at the complete removal condition. Optimal process window, which results in complete removal of Ta_0.1W_0.9O_x thin film without ablation damage in the ITO layer, have been established. From this study, it is found that focusing lens with longer focal length is preferable for damage-free pattern generation and shorter machining time.     

VO2光学薄膜的脉冲激光损伤特性测试

为评价VO2光学薄膜在光电器件中的工作可靠性,搭建了可输出连续渐变激光能量密度的脉冲激光照射实验平台,运用1对1与s对12种激光损伤测试手段进行激光辐射照射实验,采用线性外推法和测量计算法2种方法对实验结果进行了处理并得出VO2薄膜在重复频率10 kHz、中心波长532 nm、脉冲宽度15 ps脉冲激光辐射下的损伤特性。结果表明:VO2薄膜损伤几率与脉冲激光的单脉冲能量密度呈线性关系,重复辐射的激光脉冲对VO2薄膜造成的损伤具有积累效应,且重复辐射的激光脉冲次数越多损伤积累效果越明显。     

Femtosecond laser processing of indium-tin-oxide thin films

Micro- and nano-scale crystalline indium-tin-oxide (c-ITO) patterns fabricated from amorphous ITO (a-ITO) thin films on a glass substrate using a (low NA 0.26) femtosecond laser pulse that is not tightly focused are demonstrated. Different types of c-ITO patterns are obtained by controlling the laser pulse energies and pulse repetition rate of a femtosecond laser beam at a wavelength of 1064 nm: periodic micro c-ITO dots with diameters of ~1.4 μm, two parallel c-ITO patterns with/without periodic-like glass nanostructures at a laser scanning path and nano-scale c-ITO line patterns with a line width ~900 nm, i.e. ~1/8 of the focused beam׳s diameter (7 μm at 1/e²).     

ps-laser scribing of CIGS films at different wavelengths

Continuous growth of the thin-film electronics market stimulates the development of versatile technologies for large-scale patterning of thin-film materials on rigid and flexible substrates, and laser technologies are a promising method to accomplish the scribing processes. Lasers with picosecond pulse duration were applied in scribing of complex multilayered CuIn _x Ga_(1−x)Se₂ (CIGS) solar cells deposited on a polyimide substrate. The ablative properties of the films were examined as a function of the wavelength of laser radiation, pulse energy, and the irradiation dose. The selective removal of ITO and CIGS layers was achieved with 355 nm irradiation without any significant damage to the underlying layers in the ITO/CIGS/Mo/PI solar cell system. The 355 nm wavelength was also found to be favorable for scribing of absorber layer in a ZnO/CIGS/Mo/PI solar cell system. 266 nm radiation significantly modified the film structure due to high absorption. Extensive melt formation in the CIGS layer was found when 532 nm radiation was applied, though the trenches were smooth and crack-free.     

Ablation of transparent solids during self-limited deposition of diamond-like films from liquid hydrocarbons

A novel effect is studied of self-limitation of the diamond-like film thickness during laser irradiation of the interface of transparent substrates with liquid aromatic hydrocarbons. The interface is exposed through the transparent substrate to radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The thickness of diamond-like film increases linearly to 80-100 nm with the number of laser pulses and then saturates, while the substrate is ablated with nearly constant rate. This ablation rate depends on the thermal expansion coefficient of the substrate (glass, fused silica, sapphire, or CaF₂). The absorption of extinction coefficient of deposited films measured by ellipsometry is of order of 10⁴ cm^-1 and is sufficient to cause the significant heating of the interface. The ablation of the transparent substrates is due to their unequal thermal expansion compared to the diamond-like film having different thermal expansion coefficient. The measured ablation rates scale from 0.2 Å/pulse for glass to 4.5 Å/pulse for CaF₂. A 7m spatial resolution of the ablation process has been demonstrated for fused silica.     

Parametric study on femtosecond laser pulse ablation of Au films

Ablation process of 1 kHz rate femtosecond lasers (pulse duration 148 fs, wavelength 775 nm) with Au films on silica substrates has been systemically studied. The single-pulse threshold can be obtained directly. For the multiple pulses the ablation threshold varies with the number of pulses applied to the surface due to the incubation effect. From the plot of accumulated laser fluence N × ?_th(N) and the number of laser pulses N, incubation coefficient of Au film can be obtained (s = 0.765). As the pulse energy is increased, the single pulse ablation rate is increasing following two ablation logarithmic regimes, which can be explained by previous research.     

Dual-wavelength investigation of laser-induced damage in multilayer mirrors at 532 and 1064 nm

Thin film beam splitters with high reflectivity at 532 nm and high transmittance at 1064 nm were deposited via reactive electron-beam evaporation with optimized parameters. The damage performance of the samples was investigated under irradiations of 532 nm laser only, 1064 nm laser only, and various combined laser fluences. The damages induced by the 1064 nm laser were primarily attributed to the initiators at the interface between the coatings and substrate. Under 532 nm laser irradiation only, two distinctive damage pits initiated by the submicron absorptive defects located at different coating depths and correlated to interfaces were observed. The damage effect under simultaneous irradiation in multilayer films was also investigated. The respective sensitive defects of the two lasers remained the precursors for causing damage. However, the dominant damage factors in simultaneous irradiation changed with the 1064 nm laser fluence, which also determined the coupling effect between the two lasers in terms of causing damage. Finally, correlative analysis methods were used to discuss the different coupling effects.     

Acoustically induced light gyration in nickel nanoparticles

Circularly polarized acoustically induced light gyration (AILG) in nickel nanoparticles (NiNPs) attached to indium tin oxide (ITO) substrates was observed to be enhanced by nanosecond UV laser excitation at a wavelength in the surface plasmon resonance region. The AILG was observed during exposure to two acoustical waves with frequencies of 2 and 4 MHz and power densities of up to 5 W/cm². The maximum value of the AILG observed for NiNPs of average size ca. 8.7 nm, attached to an ITO substrate was about 2.8°/mm without UV-light illumination. Additional irradiation by 5 ns pulse UV laser light (λ: 337 nm) at the surface plasmon resonance region was found to favour the additional enhancement of the AILG up to 11°/mm. The effect was optimized at a temperature of 120 K. This increase was not observed when the size of NiNPs was 16.8 nm.     

Damage performance of TiO2/SiO2 thin film components induced by a long-pulsed laser

In order to study the long-pulsed laser induced damage performance of optical thin films, damage experiments of TiO₂/SiO₂ films irradiated by a laser with 1 ms pulse duration and 1064 nm wavelength are performed. In the experiments, the damage threshold of the thin films is measured. The damages are observed to occur in isolated spots, which enlighten the inducement of the defects and impurities originated in the films. The threshold goes down when the laser spot size decreases. But there exists a minimum threshold, which cannot be further reduced by decreasing the laser spot size. Optical microscopy reveals a cone-shaped cavity in the film substrate. Changes of the damaged sizes in film components with laser fluence are also investigated. The results show that the damage efficiency increases with the laser fluence before the shielding effects start to act.     

Picosecond pulse laser ablation of yttria-stabilized zirconia from kilohertz to megahertz repetition rates

Thin films of yttria stabilized zirconia were deposited onto silicon substrates using high repetition rate picosecond pulse lasers. The applied lasers covered the repetition rate range from 10 kHz to 4 MHz. We found that the laser pulse overlapping which results from increased repetition rates led to considerable changes in the ablation process. Defect formation and local heating lead to lower ablation thresholds and, with sufficiently high repetition rates, to melting of the target and even to thermal evaporation. We found that yttria-stabilized zirconia (YSZ) films deposited with picosecond pulses at 1064 nm wavelength below repetition rates of 2 MHz have rough, nanostructured morphology and the same atomic ratio of yttrium and zirconium as the target. Films deposited with 2 MHz and higher repetition rates with high number of overlapping pulses are very smooth, but are yttrium deficient, providing evidence of the importance of the thermal processes.     

Enhanced machining of steel using femtosecond pulse pairs

We report the use of a mode-locked fiber laser in pulsed laser deposition (PLD) of yttria-stabilized zirconium oxide. The fiber laser produces picosecond pulses with megahertz repetition rates at the wavelength of 1060 nm. We have investigated the effects of the time delay and the physical overlapping of the consecutive pulses on the ablation thresholds and the properties of the deposited films. Our results show existence of two distinct evaporation modes: (1) a single pulse evaporation mode observed for low overlapping and long time delays between the pulses and (2) a high repetition rate evaporation mode for high overlapping with short delays. The first mode is characterized by evaporation of nanoparticles and clusters and yields structured films with high surface area. The second mode yields smooth films, with evaporation characteristics closer to those of thermal evaporation than traditional PLD.     

F2-laser patterning of indium tin oxide (ITO) thin film on glass substrate

This paper reports the controlled micromachining of 100 nm thick indium tin oxide (ITO) thin films on glass substrates with a vacuum-ultraviolet 157 nm F₂ laser. Partial to complete film removal was observed over a wide fluence window from 0.49 J/cm² to an optimized single pulse fluence of 4.5 J/cm² for complete film removal. Optical microscopy, atomic force microscopy, and energy dispersive X-ray analysis show little substrate or collateral damage by the laser pulse which conserved the stoichiometry, optical transparency and electrical conductivity of ITO coating adjacent to the trenches. At higher fluence, a parallel micron sized channel can be etched in the glass substrate. The high photon energy and top-hat beam homogenized optical system of the F₂ laser opens new means for direct structuring of electrodes and microchannels in biological microfluidic systems or in optoelectronics. PACS 79.20.Ds; 42.62.Cf; 42.55.Lc     

Metal thin film ablation with femtosecond pulsed laser

Micromachining thin metal films coated on glass are widely used to repair semiconductor masks and to fabricate optoelectrical and MEMS devices. The interaction of lasers and materials must be understood in order to achieve efficient micromachining. This work investigates the morphology of thin metal films after machining with femtosecond laser ablation using about 1 μm diameter laser beam. The effect of the film thickness on the results is analyzed by comparing experimental images with data obtained using a two-temperature heat transfer model. The experiment was conducted using a high numerical aperture objective lens and a temporal pulse width of 220 fs on 200- and 500-nm-thick chromium films. The resulting surface morphology after machining was due to the thermal incubation effect, low thermal diffusivity of the glass substrate, and thermodynamic flow of the metal induced by volumetric evaporation. A Fraunhofer diffraction pattern was found in the 500-nm-thick film, and a ripple parallel to the direction of the laser light was observed after a few multiple laser shots. These results are useful for applications requiring micro- or nano-sized machining.     

高温合金不同脉宽超快激光作用下多脉冲去除阈值

实验研究了激光脉冲宽度和脉冲个数对镍基高温合金材料去除阈值的影响,分别在290 fs,1 ps和7 ps脉宽的激光下,使用1,10,50,100,300,500和1000个不同能量的激光脉冲辐照高温合金样品表面。实验结果表明,烧蚀坑尺寸会随脉冲数的增加而增加,而脉冲宽度的增加会加大脉冲个数对烧蚀坑直径的影响。通过烧蚀坑直径的平方值与激光脉冲能量之间存在的对数关系,得到了不同脉冲宽度下镍基高温合金的多脉冲材料阈值。3种不同脉宽下的高温合金多脉冲去除阈值都存在显著的累积效应。根据去除阈值计算得到290 fs,1 ps和7 ps脉宽下的累积效应系数分别为0.88,0.86和0.78。     

Effect of electron beam on structural,linear and nonlinear properties of nanostructured Fluorine doped ZnO thin films

Electron beam induced effects on Fluorine doped ZnO thin films (FZO) grown by chemical spray pyrolysis deposition technique were studied. The samples were exposed to 8 MeV electron beam at different dose rate ranging from 1 kGy to 4 kGy. All films exhibit a polycrystalline nature which shows an increase in crystallanity with irradiation dosages. The electron beam irradiation effectively controls the films surface morphology and its linear optical characteristics. Z-Scan technique was employed to evaluate the sign and magnitude of nonlinear refractive index and nonlinear absorption coefficient using a continuous wave laser at 632.8 nm as light source. Enhancement in the third order nonlinear optical properties was were noted due to electron beam irradiation. Tailoring the physical and NLO properties by electron beam, the FZO thin films becomes a promising candidate for various optoelectronic applications such as phase change memory devices, optical pulse compression, optical switching and laser pulse narrowing.     

Crystal growth of phosphor perovskite titanate thin films under excimer laser irradiation

Ca_0.997Pr_0.002TiO₃ (CPTO) thin films that show strong red luminescence were successfully prepared by means of an excimer laser assisted metal organic deposition (ELAMOD) process with a KrF laser at a fluence of 100 mJ/cm², a pulse duration of 26 ns, and a repetition rate of 20 Hz at 100°C in air. The CPTO films grew on the silica, borosilicate, and indium-tin-oxide (ITO) glasses. The crystallinity of the CPTO films depended on the substrates; the films were well grown on the borosilicate and ITO glasses compared to the silica glass. To elucidate the key factors for the crystallization of the CPTO films in this process, we carried out numerical simulations for the temperature variation at the laser irradiation, using a heat diffusion equation, and compared the experimental data with thermal simulations. According to the results, we have shown that a large optical absorbance of the film and a small thermal conductivity of the substrate provide effective annealing time and temperature for the crystallization of the CPTO films, and polycrystalline intermediate layer which has a large optical absorption such as the ITO also plays a key role for the nucleation of the CPTO crystals in the ELAMOD process.     

532 nm激光辐照下ZnSe薄膜光学特性研究

采用电子束热蒸发技术制备了ZnSe薄膜,研究了532 nm波长的不同能量(2.0 mJ、2.5 mJ、3.0 mJ)、不同脉冲数(3、10、15)激光诱导前后,ZnSe薄膜的透射率、折射率、消光系数、损伤阈值(LIDT)的变迁。研究结果显示,在能量为2.0 mJ激光辐照后,ZnSe薄膜折射率提高,透射率下降。相比较能量为2.5 mJ、3.0 mJ激光辐照,在能量为2.0 mJ激光辐照后折射率提高最明显,由2.489 4提高到2.501 6。薄膜损伤阈值从0.99 J/cm2提高到1.39 J/cm2(10脉冲辐照);薄膜的损伤经过了无损伤到严重损伤突变的损伤演变过程。采用原子力显微镜对预处理后薄膜表面粗糙度进行检测,发现激光预处理后的薄膜表面粗糙度Ra有所下降,从0.563 nm降低到0.490 nm(15脉冲激光辐照)。     

CO2 laser-induced crystallization of sol–gel-derived indium tin oxide films

Indium tin oxide (ITO) thin films prepared by the sol–gel method have been deposited by the dip-coating process on silica substrates. CO₂ laser is used for annealing treatments. The electrical resistivity of sol–gel-derived ITO thin films decreased following crystallization after exposure to CO₂ laser beam. The topological and electrical properties of the irradiated surfaces have been demonstrated to be strongly related to the coating solution and to the laser processing parameters. Optimal results have been obtained for 5 dip-coating layers film from 0.4 mol/l solution irradiated by 0.6 W/m² laser power density. In this case, homogeneous and optically transparent traces were obtained with a measured sheet resistance of 1.46×10² Ω/□.