Photoemission study of size selected InP nanocrystals: the relationship between luminescence yield and surface structure

The surfaces of luminescent InP nanocrystals were investigated by photoelectron spectroscopy (PES) technique with synchrotron radiation. Semiconductor samples were prepared by an organometallic approach using trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO) as stabilizing and size regulating agents. As prepared, InP nanocrystals exhibit poor photoluminescence (PL) yield, usually less than 1%. However the yield can be drastically enhanced to about 20-30% by photoetching of the nanoparticle surface with fluorine compounds. High resolution In 3d and P 2p core level spectra of etched and non-etched particles taken at different excitation energy reveal changes of the nanocrystal surface. Three different InP samples are analyzed and a simple model for the etching process is discussed.     

In situ reflectivity investigations of solid/liquid interface during laser backside etching

In situ reflectivity measurements of the solid/liquid interface with a pump-probe setup were performed during laser-induced backside wet etching (LIBWE) of fused silica with KrF excimer laser using toluene as absorbing liquid. The intensity, the temporal shape, and the duration of the reflected light measured in dependence on the laser fluence are discussed referring to the surface modification and the bubble formation.The vaporisation of the superheated liquid at the solid interface causes a considerable increase of the reflectivity and gives information about the bubble lifetime. The alterations of the reflectivity after bubbles collapse can be explained with the changed optical properties due to surface modifications of the solid surface. Comparative studies of the reflectivity at different times and the etch rate behaviour in dependence on the laser fluence show that the in situ measured surface modification begins just at the etch threshold fluence and correlates further with etch rate behaviour and the etched surface appearance. The already observed surface modification at LIBWE due to a carbon deposition and structural changes of the near surface region are approved by the changes of the interface reflectivity and emphasizes the importance of the modified surface region in the laser-induced backside wet etching process.     

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/cm² 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/cm², 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.     

Photoelectrochemical etching of ZnS: Further evidence for non-uniform flow of charge carriers in Schottky barriers

It is shown that short photoelectrochemical etching of two kinds of ZnS crystals leads to a reduction in the surface recombination velocity as evidenced by their increased photocurrent. The unique surface morphology (>10⁹ pits cm^–2) revealed after photoetching supports the hypothesis that the charge flow within the space charge layer is highly nonuniform.Incumbent of Helen & Milton A. Kimmelman Career Development Chair in perpetuity established by Helen and Milton A. Kimmelman, New York, NY     

Laser-induced high-quality etching of fused silica using a novel aqueous medium

Laser-induced backside wet etching of fused-silica plates using an aqueous solution of naphthalene-1,3,6-trisulfonic acid trisodium salt (Np(SO₃Na)₃) is reported. A KrF excimer laser was employed as a light source. The etch rate varied greatly with the concentration of the solution and the laser fluence. For lower concentration solutions, the etch rate increased linearly with laser fluence. For highly concentrated solutions, however, the etch rate increased abruptly at higher fluence. Well-defined line-and-space and grid micropatterns were fabricated using a low etch rate. The etched surface was as flat as the surface of the virgin plates and the etched pattern was free of debris and microcracks. The formation and propagation of shockwaves and bubbles in the solution during the etch process were monitored. High pressure, as well as the high temperature generated by the photothermal process, plays a key role in the etching process. Received: 8 April 2002 / Accepted: 12 April 2002 / Published online: 19 July 2002     

Laser-assisted etching of gallium arsenide in chlorine at 308 nm

Xenon chloride (308 nm) excimer laser-assisted etching of GaAs (100) in Cl₂ was demonstrated and characterized with respect to laser and gas parameters. The etch rate increased linearly with laser fluence from thresholds in the range of 50 to 75 mJ/cm² to the highest fluence studied, 650 mJ/cm². For a laser fluence of 370 mJ/cm², the etch rate varied with Cl₂ pressure reaching a maximum at a Cl₂ pressure of about 2 Torr. The etch rate decreased monotonically with Ar buffer gas pressure because of redeposition of GaCl₃ products into the etched channel. The redeposited GaCl₃ affected the etch rate and the etch morphology. The etch rate and morphology also varied with laser repetition rate. The mobility of chlorine on the surface also plays an important role in the etching mechanism.     

酸蚀与紫外激光预处理结合提高熔石英损伤阈值

采用HF酸刻蚀和紫外激光预处理相结合的方式提升熔石英元件的负载能力,用质量分数为1%的HF缓冲溶液对熔石英刻蚀1~100 min,综合透过率、粗糙度和损伤阈值测试结果,发现刻蚀时间为10min的熔石英抗损伤能力最佳。采用355 nm紫外激光对HF酸刻蚀10 min的熔石英进行预处理,结果表明:紫外预处理能量密度在熔石英零损伤阈值的60%以下时,激光损伤阈值单调递增;能量到达80%时,阈值反而低于原始样片的损伤阈值。适当地控制酸蚀时间和紫外激光预处理参数能有效提高熔石英的抗损伤能力。     

Laser stimulated etching of n-GaAs

The HeNe laser stimulated etching of n-GaAs in aqueous solution of H₂SO₄/H₂O₂ mixture and alcali hydroxides was studied. The effects of laser power density as well as on the composition and concentration of etching solutions on photoetching was examined. In addition, a theoretical model of semiconductor etching in hydroxides is suggested.The authors thank Mr. F. Kozak for his help in the course of experiment.     

Dopant-enhanced ablation of nitrocellulose by a nitrogen laser

The photoetching behavior of pure nitrocellulose and of nitrocellulose dyed with stilbene-420, coumarin-120 and rhodamine 6G by 337 nm nitrogen laser pulses has been studied. Ablation with a low power nitrogen laser is hereby reported for the first time. A two step photochemical mechanism is proposed to account for the ablation of the pure material. With the addition of dyes strongly absorbing at 337 nm the photoetching rate of nitrocellulose can be increased significantly. This increase is proportional to the molar extinction coefficient of the dye at 337 nm and its concentration in the polymer. The photoetching mechanism and the energy transfer processes from the dye to the polymer are discussed in detail.     

XUV成像系统中像传递函数研究用的Si刻蚀膜

 研制具有网格或条状图形的Si刻蚀膜靶，用于XUV系统中像传递函数的研究。在自截止腐蚀工艺制备Si平面薄膜的基础上，结合离子束刻蚀工艺，获得刻蚀深度为1.0 μm左右，网格尺寸为25 μm×25 μm，或条状线宽为5 μm的Si刻蚀膜；测量了Si刻蚀膜的形貌和刻蚀深度；研究了离子束刻蚀参数对图形形貌的影响。并介绍采用两种靶型获得的像传递函数信息。     

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. The normalization of the etch rates with respect to the reflectivity and the pulse length results in similar etch rates and threshold fluence for the different pulse widths and wavelengths. It is concluded that etching is a thermal process including the laser heating, the materials melting, and the materials etching by mechanical forces. The backside etching of fused silica with IR-Nd:YAG laser can be a promising approach for the industrial usage of the backside etching of a wide range of materials. PACS 81.65.C; 81.05.J; 79.20.D; 61.80.B; 42.55.L     

Post-growth tailoring of quantum-dot saturable absorber mirrors by chemical etching

We have designed and grown a resonant, low-finesse quantum-dot saturable absorber mirror and subsequently modified the important parameters using chemical etching. The modulation depth and saturation fluence at the design wavelength of 1064 nm were modified by etching the sample to tune the cavity resonance. The device properties were characterised using normal incidence spectroscopic reflectivity measurements, intensity dependent reflectivity measurements and modelled using a transfer matrix approach. The saturable absorber mirror was used to facilitate self-starting, passively mode locked pulses in a neodymium vanadate laser operating at 1064 nm. The etching was found to affect the duration of the pulses, leading to temporal width tuning over a range of 94 ps. The shortest pulse duration of 84 ps was achieved for the cavity resonance close to 1064 nm, with an output power of 3 W. This method is an effective technique for post-growth engineering of the properties of semiconductor saturable absorber mirrors (SESAMs) with nanometre precision.     

Laser-induced back-side wet etching of fused silica with an aqueous solution containing organic molecules

The laser-induced back-side wet etching of fused silica with aqueous solutions of pyranine (8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt) is reported. KrF and XeF excimer lasers were employed as light sources. Well-defined line-and-space and grid micropatterns, free of debris and microcracks, were obtained. Compared with other organic solutions, the aqueous pyranine etching medium etches more slowly but produces a higher quality etched surface. With the KrF laser, the etch rate ranged from 0.02 to 0.12 nm pulse^-1, depending on the dye concentration and the fluence of the laser. The etch rate decreased dramatically when the XeF laser was employed, which was partially attributed to the lower absorption efficiency of the aqueous pyranine solution at the XeF laser wavelength. Received: 20 November 2001 / Accepted: 21 November 2001 / Published online: 2 May 2002     

Ablation of poly(methyl methacrylate) and poly(2-hydroxyethyl methacrylate) by 308, 222 and 193 nm excimer-laser radiation

Data on the ablation of Poly(Methyl MetAcylate) (PMMA) and Poly(2-Hydroxyethyl MetAcylate) (PHEMA) with 0%, 1% and 20% of Ethylene Glycol DiMethAcrylate (EGDMA) as crosslinking monomer by 193, 222 and 308 nm laser radiation are presented. Direct photoetching of PMMA at 308 nm is demonstrated for laser fluences ranging from 2 to 18 J/cm². The ablation rate of PHEMA is lower than the corresponding to PMMA and decreases when the amount of EGDMA increases. The determination of the absorbed energy density required to initiate significant ablation suggests that the photoetching mechanism is similar for all the polymers studied and is a function of the irradiation wavelength. The Beer-Lambert law, the Srinivasan, Smrtic and Babu (SSB) theory and the kinetic model of the moving interface are used to analyze the experimental results. It is shown that only the moving interface theory fits well the etch rate for all the selected polymers at the three radiation wavelengths.     

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.     

驱动光束不均匀性研究中的硅平面薄膜

 介绍以氧化、扩散、光刻等现代半导体技术结合化学腐蚀工艺实现对Si片的定向自截止腐蚀,制备获得用于研究驱动光束不均匀性的自支撑Si平面薄膜的工艺。通过台阶仪测量厚度在3~4 μm的Si平面薄膜,在扫描范围为1000 μm时,它的表面粗糙度在几十nm;SEM测量表明,Si薄膜表面颗粒度在纳米量级;探讨采用控制扩散、腐蚀参数和表面修饰处理来降低Si膜表面粗糙度的方法。     

XeCl laser controlled chemical etching of aluminum in chlorine gas

The 308 nm XeCl laser assisted etching process of thin Al metal films on Si substrate in Cl₂ gas was investigated. Etch rates were measured versus the laser fluence on the sample, the laser repetition rate, the Cl₂ pressure and the sample temperature. Irradiation experiments under vacuum of films which were previously exposed to Cl₂, and laser assisted etching in rare gases, nitrogen and air mixtures with Cl₂ were also performed to elucidate the mechanism of the etching process. The surface morphology was investigated by scanning electron microscopy. The results show that a) Etch rates of up to about 1.5 m per pulse are obtained which are strongly dependent on the Cl₂ pressure and sample temperature. b) The etching mechanism is essentially a chemical chlorination of the Al in between the laser pulses which is followed by photo-ablation of the reaction products, c) AlCl₃ evaporation and redeposition processes can explain the observed results. d) The Al films can be etched fully and cleanly without damage to the smooth Si substrate. e) Etching through adjacent or imaged mask on the Al film yielded relatively smooth and well defined Al walls with structures of the order of 1 m.     

快重离子辐照金红石型TiO2的肿胀效应及化学蚀刻行为研究

二氧化钛（Titatium Dioxide,简称TiO₂）晶体在中能重离子辐照时表面会出现肿胀效应, 肿胀高度与入射离子的电子能损和辐照注量有关。 辐照后的TiO₂在一定条件下能够被氢氟酸溶液化学蚀刻,化学蚀刻的电子能损阈值为8.2keV/nm,未辐照TiO₂呈现几乎零蚀刻率。要达到饱和蚀刻深度,辐照离子的注量必须大于或等于1×1013ions/cm2。采用离子辐照的潜径迹理论分析研究了辐照损伤及对化学蚀刻的影响, 快重离子辐照结合化学蚀刻是制备TiO₂微结构的有效方法。 There appears volume swelling on the surface of the irradiated rutile TiO₂ crystal and the volume swelling is affected by the ion fluence and the electronic stopping power. To induce adequate irradiation damage for the chemical etching, the irradiation parameters must fulfill some requirement. There is minimum electronic stopping power for the chemical etching of the irradiated region in TiO₂ crystal, which is about 8. 2 keV/nm. If the ion fluence is below 1×1013ions/cm2, the saturated etching depth of the irradiated region in TiO₂crystal cannot be reached. The irradiation damage based on latent track formation frame and the theoretical linkage to the etching technique is investigated. It is hopeful to fabricate micro and nano scale structurce in rutile TiO₂ crystal by using the ion irradiation and chemical etching technique.      

Laser-induced backside wet etching of fluoride and sapphire using picosecond laser pulses

Laser-induced backside wet etching (LIBWE) is a promising process for microstructuring of rigid chemical resistant and inert transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated in a number of studies. LIBWE in a time scale of femtosecond and picosecond pulse durations has been investigated only in a few studies and just on fused silica. In the present study LIBWE of fluorides (CaF₂, MgF₂) and sapphire with a mode-locked picosecond (t _p=10 ps) laser at a UV wavelength of λ=355 nm using toluene as absorbing liquid has been demonstrated. The influence of the laser fluence and the pulse number on the etching rate and the achieved surface morphology was investigated. The etching rate grows linearly with the laser fluence in the low and high-fluence ranges with different slopes. The achieved etching rates for CaF₂ and for sapphire were in the same range. Contrary to CaF₂ and sapphire the etching rates of MgF₂ were one magnitude less. For backside etching on sapphire at high fluences smooth surfaces and at low fluences ripples pattern were found, whereas fluoride surfaces showed a trend towards crack formation.