准分子激光电化学刻蚀镍的特性研究

提出一种新的准分子激光电化学加工工艺,在阳极钝化区内,通过准分子激光照射,以实现无屏蔽的各向异性的微加工。对该工艺进行了可行性分析,并对其进行了初步试验,证实在钝化区,准分子激光导致刻蚀电流增加的幅度比较明显,在此基础上,深入研究了在准分子脉冲激光作用过程中,刻蚀电流与脉冲数、脉冲频率之间的关系。表明准分子激光在钝化区具有增强电化学特性的能力。     

Photoablation of polyurethane films using UV laser pulses

Pulsed ultraviolet laser ablation of two polyurethane films has been studied in terms of ablation rate behaviour and time-of-flight mass spectroscopy of the positively charged photofragments. Three excimer laser wavelengths (193, 248 and 308 nm; 17–30 ns pulse duration) and short-pulse laser system (pulse duration 500 fs or 5 ps, at 248 nm) were employed. The results of the influence of energy fluence on the ablation rate are tested against other photoablation models and a table of fitted physical constants is presented. The upper limit of the mean activation energy for desorption is found to be considerably lower than the energy required to break single covalent bonds. The mass analysis of the positively charged species produced during the photoablation process provides valuable insight into the photofragmentation mechanism.     

Optical transmission and reflection of aluminum film irradiated by nanosecond laser beam and experimental studying of phase-explosion

In this paper we present evidence for a phase explosion during the laser-induced ablation process by studying the optical reflectivity of the ablated plume. The ablation was produced by irradiating thin film aluminum coated on a quartz substrate with a single pulse laser beam in ambient air. The laser pulse was provided by the second harmonic of a Q-switched Nd:YAG laser with ∼10 ns pulse duration. The transmission of a low power He–Ne laser beam through the hot ablated material plume and its reflection (from the front surface, and rear surface of aluminum film) were also monitored during the duration of the ablation event. The results show that the front surface reflectivity is enhanced at an early time of ablation which is described as strong evidence for the creation of a phase explosion in this process.     

Direct fabrication of microgratings in fused quartz by laser-induced plasma-assisted ablation with a KrF excimer laser

We report precision microfabrication of fused quartz by laser ablation with a conventional UV laser for what is believed to be the first time. A high-quality micrograting structure is fabricated in fused quartz by a novel technique of laser-induced plasma-assisted ablation with a single KrF excimer laser (248 nm). The plasma generated from a metal target by laser irradiation effectively assists in ablation of the fused-quartz substrate by the same laser beam, although the laser beam is transparent to the substrate. A grating with a period of 1.06 mum is achieved by use of a phase mask. We can control the grating depth to 300 nm by changing the pulse number. This technique permits high-quality microfabrication of electronic and optoelectronic devices based on fused quartz and related silicate materials by use of a conventional UV laser.     

Influence of laser fluence and irradiation timing of F2 laser on ablation properties of fused silica in F2-KrF excimer laser multi-wavelength excitation process

A collinear irradiation system of F₂ and KrF excimer lasers for high-quality and high-efficiency ablation of hard materials by the F₂ and KrF excimer lasers’ multi-wavelength excitation process has been developed. This system achieves well-defined micropatterning of fused silica with little thermal influence and little debris deposition. In addition, the dependence of ablation rate on various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number is examined to investigate the role of the F₂ laser in this process. The multi-wavelength excitation effect is strongly affected by the irradiation timing, and an extremely high ablation rate of over 30 nm/pulse is obtained between -10 ns and 10 ns of the delay time of F₂ laser irradiation. The KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing F₂ laser fluence. Moreover, the ablation rate shows a linear increase with the logarithm of KrF excimer laser fluence when the F₂ laser is simultaneously irradiated, while single KrF excimer laser ablation shows a nonlinear increase. The ablation mechanism is discussed based on these results. Received: 16 July 2001 / Accepted: 27 July 2001 / Published online: 2 October 2001     

A laser dry etch process for smooth continuous relief structures in InP

A laser induced etch process is described which uses a pulsed 248 nm KrF excimer laser and Cl₂ atmosphere for the fabrication of monolithic continuously curved reliefs in InP substrate. In a bakeable processing chamber with low base pressure a wide range of laser fluences is available for damage-free etching. Especially, by photothermal heating far above the melting point, mirrorlike smooth surfaces are obtained. The etch rate characteristics are correlated to the maximum surface temperature reached during the laser pulse. The etch rate is independent of pressure and gas flux in the ranges 0.1–10 mbar and 20–300 sccm, respectively. It increases, however, with the background substrate temperature. Etch rates of up to 3.6 nm/pulse or 4.3 lm/min are possible at 20 Hz pulse repetition rate without visible surface damage. The process exhibits a smooth increase of the etch rate from 1 to 3 nm/pulse between 200 and 300 mJ/cm², which could be used for making curved reliefs by optical transmission variations on the projection mask.     

Radiation effects on poly(methyl methacrylate) induced by pulsed laser irradiations

Poly(methyl methacrylate) (PMMA) was irradiated using a medical UV-ArF excimer laser operating at the fundamental wavelength of 193 nm. Characterized by a beam diameter of 1.8 mm and energy of 180 mJ with a Gaussian energy profile, it operates in a single mode or at 30 Hz repetition rate. Mechanical profilometry was carried out on ablation craters in order to study the rugosity and the ablation yield in the various operative conditions. Optical transmission and reflection measurements at six wavelengths were conducted in order to characterize the optical properties of the irradiated surfaces. Measured crater depths in PMMA were lower with respect to the forecasted ones in corneal tissue, while the lateral crater aperture was maintained. The rugosity produced at the crater bottom after irradiation was about 0.3 μm, and the ablation yield was about 10¹⁵ molecules/laser pulse, while etching depth and diameter show a roughly linear dependence on the number of laser shots. These experiments constitute a base for deeper clinical investigations.     

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/cm² 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.     

Deep etching of epitaxial gallium nitride film by multiwavelength excitation process using F2 and KrF excimer lasers

The deep etching of GaN(0001) thin films epitaxially grown on Al₂O₃(0001) has been investigated by laser ablation using F₂ and KrF excimer lasers. The simultaneous irradiation with F₂ and KrF excimer lasers markedly improves etching quality compared with single-KrF excimer laser ablation and provides almost the same quality as that in the case of single-F₂ laser ablation with a high etching rate. Additionally, the present method achieves the deep etching of a GaN film of more than 5 μm in depth with steep side walls at an angle of 87° by changing the laser incidence angle. PACS 52.38.Mf; 61.82.Fk; 81.65.Cf     

Projection ablation of glass-based single and arrayed microstructures using excimer laser

Ablation of single and arrayed microstructures using an excimer laser is studied. The single feature microstructures are fabricated for evaluating the ablation mechanism, threshold fluence, and associated material removing (ablation) rate. The morphology changes during ablation are investigated with the focus on the formation of the ablation defects, debris or recast. The possibility of removing these defects is also evaluated and demonstrated. The present study concentrates on the borosilicate glass, although ablation of polyimide and silicon are performed and discussed for comparison. Polyimide and silicon are the most popular polymer or semiconductor material used in the electronics industry. The arrayed microstructures are ablated to demonstrate the fact that, by repetition of a simple-patterned mask associated with synchronized laser pulses and substrate movement, arrayed and more complex structures can be cost-effectively manufactured. The potential applications of these arrayed microstructures are discussed and illustrated. A low-cost replication technique that uses the arrayed microstructure presently machined as the forming mold for making electroforming nickel microneedles is specifically presented. Finally, the potential areas of using excimer laser in micromachining of glass-based structures for future research are also briefly covered.     

Excimer laser patterned dielectric masks for the fabrication of diffractive optical elements by laser ablation

Masks for laser processing are generated by laser ablation patterning of dielectric layer systems. The application of these masks for the rapid fabrication of diffractive optical elements (DOEs) is presented. The diffractive optical elements are designed as phase-only elements, assuming an illumination with a plane wave. A continuous phase function is calculated using an iterative Fourier transform algorithm (IFTA). This continuous phase function is reduced to two or four levels by an iterative Fourier quantisation algorithm (IFQA) that is able to include focal power. The fabrication of the DOE is performed in a two-step process. First, a binary amplitude mask (or a set of masks for multi-level DOEs) is made by structured ablation of a highly reflective dielectric coating (HR 248 nm) from a fused silica substrate. This is accomplished by using an ArF excimer laser emitting at 193 nm, a wavelength that is sufficiently absorbed in the HfO₂/SiO₂-dielectric layer system, leading to precisely ablated mask structures. In the second step, this mask is used in a 4:1 projection configuration to generate a surface profile in a polymer substrate by ablation at 248 nm. The depth modulation can be defined by adjusting laser fluence and pulse number. Examples of DOEs ablated in polycarbonate are shown and their performance is characterised.     

Pulsed laser deposition—UV laser sources and applications

Progress in material research and processing industry is fueled by the technique of pulsed laser deposition (PLD). High energy excimer lasers enable this technique since every material is amenable to their high photon energies. Spectral properties, temporal pulse and laser beam parameters of state of the art excimer lasers will be compared with frequency converted Nd:YAG lasers. Both quality and longevity of the deposited layers strongly depend on the degree of accuracy achieved in the thin film ablation and subsequent deposition process.     

Structure formation in excimer laser ablation of stretched poly(ethylene therephthalate) (PET): the influence of scanning ablation

Multiple pulse laser ablation of stretched PET is performed with an ArF excimer laser (193 nm) in order to produce micro channels. The surface structure remaining after "scanning ablation", in which the sample is moved during irradiation, is compared to the known results upon "static ablation". We observed that the debris contribution is enhanced upon scanning ablation, which has a major impact for the channels used in micro-fluidic applications. A more fundamental change of the channel floor structure, as seen by SEM, occurs at high fluences and short mask lengths. The channel floor structure originates from the structure on the irradiated ramp at the end of the channel. The "scanning structure" appears only if the irradiated end ramp forms an angle higher than 10.55(ǂ.15)° with the non-irradiated sample surface. The angle of light incidence is not responsible for the structure changes, in agreement with literature reports. Other possible mechanisms are briefly discussed.     

XeCl准分子激光能量提取研究

在高功率准分子激光系统建设中,希望能获得较短的脉冲宽度和尽量多的激光能量。实验研究了不同注入水平下,脉冲时间间隔对脉冲链放大波形和放大器提取效率的影响;基于四能级速率方程和准分子反应动力学建立了准分子激光放大模型,计算了多种注入方式下种子光的放大过程,对关键参数给出了量化描述,得到与实验相符的计算结果。研究结果显示:脉冲序列间隔为9.3ns时,可获得约95%的连续注入情形下放大能量;对该准分子激光系统来讲,9.3ns是比较合适的脉冲间隔。     

Study of femtosecond ablation on aluminum film with 3D two-temperature model and experimental verifications

In this paper, a 3D two-temperature model is introduced to investigate femtosecond ablation on aluminum film. 3D temperature evolutions for both electrons and lattice are obtained, which present us a vivid view of the energy transformation process during femtosecond ablation. Simulated 3D ablation craters irradiated by a single pulse with different energy are acquired, from which we can easily and precisely predict crater depth and radius before ablation takes place. In the experiment we measure the radii of the craters ablated by pulses with different energy and numbers delivered from a chirped pulse amplification Ti: sapphire system. The threshold fluence for both single and multi pulses are obtained. Comparisons are made between results of the experiment and relative simulated calculations show the reliability of our proposed calculation model.     

Laser-induced etching of titanium by Br2 and CCl3Br at 248 nm

A quartz crystal microbalance (QCM) has been used to study the KrF* excimer laser-induced etching of titanium by bromine-containing compounds. The experiment consists of focusing the pulsed UV laser beam at normal incidence onto the surface of a quartz crystal coated with 1 m of polycrystalline titanium. The removal of titanium from the surface is monitored in real time by measuring the change in the frequency of the quartz crystal. The dependence of the etch rate on etchant pressure and laser fluence was measured and found to be consistent with a two-step etching mechanism. The initial step in the etching of titanium is reaction between the etchant and the surface to form the etch product between laser pulses. The etch product is subsequently removed from the surface during the laser pulse via a laser-induced thermal desorption process. The maximum etch rate obtained in this work was 6.2 Å-pulse^–1, indicating that between two and three atomic layers of Ti can be removed per laser pulse. The energy required for desorption of the etch product is calculated to be 172 kJ-mole^–1, which is consistent with the sublimation enthalpy of TiBr₂ (168 kJ-mole^–1). The proposed product in the etching of titanium by Br₂ and CCl₃Br is thus TiBr₂. In the etching of Ti by Br₂, formation of TiBr₂ proceeds predominantly through the dissociative chemisorption of Br₂. In the case of etching with CCl₃Br, TiBr₂ is formed via chemisorption of Br atoms produced in the gas-phase photodissociation of CCl₃Br.     

飞秒双脉冲激光照射金属薄膜的热行为

通过双温方程对飞秒单脉冲与双脉冲照射金薄膜进行了计算模拟分析,得到了金靶的电子温度和晶格温度随着时间空间的变化。在同样激光能量密度下,单脉冲与双脉冲使得金膜温度的变化表明双脉冲使得更多的激光能量渗透到靶材内部,这些能量可以使得烧蚀深度更深,有利于提高激光烧蚀靶材的效率。计算结果显示随着激光能量密度的增加熔化面深度逐渐增加,单脉冲与双脉冲熔化面深度的变化明显不同。在激光能量密度高于损伤阈值附近,单脉冲的烧蚀深度大于双脉冲的烧蚀深度,随着激光能量密度增加,双脉冲的烧蚀深度将大于单脉冲的烧蚀深度。     

Dopant-induced excimer laser ablation of Poly(tetrafluoroethylene)

Poly(tetrafluoroethylene) (PTFE) does not exhibit excimer laser etching behavior at conventional, e.g., single photon absorption, emissions of 193, 248, and 308 nm, due to the lack of polymer/photon interaction. This is not surprising since the electronic transitions available to the PTFE molecule are high energy and thus require short wavelength the radiation However, by incorporating a small quantity of material into the non-absorbing fluoropolymer matrix that interacts strongly with the emitted laser energy, e.g., a dopant, successful ablation, both in terms of etch rate and structuring quality occurs. Specifically, excimer laser ablation of PTFE films containing 5, 10, and 15% polyimide (wt/wt) as a dopant was achieved at 308 nm in a fluence range of 1 to 12 J/cm². Ablation rates for the materials increased with increasing fluence and, at the polyimide levels investigated, varied inversely with dopant concentration. All compositions exhibited excellent structuring quality.     

Dual-beam ablation of fused silica by multiwavelength excitation process using KrF excimer and F2 lasers

A new technique of dual-beam laser ablation of fused silica by multiwavelength excitation process using a 248-nm KrF excimer laser (ablation beam) coupled with a 157-nm F₂ laser (excitation beam) in dry nitrogen atmosphere is reported. The dual-beam laser ablation greatly reduced debris deposition and, thus, significantly improved the ablation quality compared with single-beam ablation of the KrF laser. High-quality ablation can be achieved at the delay times of KrF excimer laser irradiation shorter than 10 ns due to a large excited-state absorption. The ablation rate can reach up to 80 nm/pulse at the fluence of 4.0 J/cm² for the 248-nm laser and 60 mJ/cm² for the F₂ laser. The ablation threshold and effective absorption coefficient of KrF excimer laser are estimated to be 1.4 J/cm² and 1.2᎒⁵ cm^-1, respectively.     

Depth profiling a III–V multilayered structure with an excimer laser

A multilayered structure of GaAs and AlGaAs was depth profiled using the technique of digital etching. A single excimer laser (KrF) was used to control the etch rate and to identify each layer by monitoring the Ga ions generated during the desorption process. The Ga ions were the only ions observed and were only generated when the photon flux was in the GaAs layer. The etch rate, 0.9 monolayers (2.5 Å) per pulse, was constant with depth. The overall layer recognition resolution was 45 monolayers.