Laser etching of transparent materials at a backside surface adsorbed layer

The laser etching using a surface adsorbed layer (LESAL) is a new method for precise etching of transparent materials with pulsed UV-laser beams. The influence of the processing parameters to the etch rate and the surface roughness for etching of fused silica, quartz, sapphire, and magnesium fluoride (MgF₂) is investigated. Low etch rates of 1 nm/pulse and low roughness of about 1 nm rms were found for fused silica and quartz. This is an indication that different structural modifications of the material do not affect the etching significantly as long as the physical properties are not changed. MgF₂ and sapphire feature a principal different etch behavior with a higher etch rate and a higher roughness. Both incubation effects as well as the temperature dependence of the etch rate can be interpreted by the formation of a modified near surface region due to the laser irradiation. At repetition rates up to 100 Hz, no changes of the etch rate have been observed at moderate laser fluences.     

Reactive ion etching of FePt using inductively coupled plasma

We propose a reactive ion etching (RIE) process of an L1₀-FePt film which is expected as one of the promising materials for the perpendicular magnetic recording media. The etching was carried out using an inductively coupled plasma (ICP) RIE system and an etching gas combination of CH₄/O₂/NH₃ was employed. The L1₀-FePt films were deposited on (1 0 0)-oriented MgO substrates using a magnetron sputtering system. The etching masks of Ti were patterned on the FePt films lithographically. The etch rates of ∼16 and ∼0 nm/min were obtained for the FePt film and the Ti mask, respectively. The atomic force microscopy (AFM) analyses provided the average roughness (R_a) value of 0.95 nm for the etched FePt surface, that is, a very flat etched surface was obtained. Those results show that the highly selective RIE process of L1₀-FePt was successfully realized in the present study.     

Laser etching of fused silica using an adsorbed toluene layer

A new method for laser etching of transparent materials with a low etch rate and a very good surface quality is demonstrated. It is based on the pulsed UV-laser backside irradiation of a transparent material that is covered with an adsorbed toluene layer. This layer absorbs the laser radiation causing the etching of the solid. The threshold fluence for etching of fused silica amounts to 0.7 J/cm². The constant etch rate of about 1.3 nm/pulse that has been observed in a fluence interval from 2 to 5 J/cm² is evidence of a saturated process. The limited thickness of the adsorbed layer causes the low etch rates and the rate saturation. The etched surface structures have well defined edges and low surface roughness values of down to 0.4 nm rms. PACS 81.65.Cf; 81.05.Kf; 79.20.Ds; 61.80.Ba; 42.55.Lt     

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     

刻蚀AlN缓冲层对硅衬底N极性n-GaN表面粗化的影响

研究了等离子体刻蚀AlN缓冲层对硅衬底N极性n-GaN表面粗化行为的影响. 实验结果表明, 表面AlN缓冲层的状态对N极性n-GaN的粗化行为影响很大, 采用等离子体刻蚀去除一部分表面AlN缓冲层即可以有效提高N极性n-GaN在KOH溶液中的粗化效果, AlN缓冲层未经任何刻蚀处理的样品粗化速度过慢, 被刻蚀完全去除AlN缓冲层的样品容易出现粗化过头的现象. 经X射线光电子能谱分析可知, 等离子体刻蚀能够提高样品表面AlN缓冲层Al 2p的电子结合能, 使得样品表面费米能级向导带底靠近, 原子含量测试表明样品表面产生了大量的N空位, N空位提供电子, 使得材料表面费米能级升高, 这降低了KOH溶液和样品表面之间的肖特基势垒, 从而有利于表面粗化的进行. 通过等离子体刻蚀掉表面部分AlN缓冲层, 改善了N极性n-GaN在KOH溶液中的粗化效果, 明显提升了对应发光二级管器件的出光功率.     

Effect of etching on the electrical and magnetic properties of writer shield material

Magnetron sputtered permalloy films are treated by wet chemical etchings with acid etchant as well as fluorine based reactive ion etch (RIE). Upon these treatments, the resistivity and coercivity of the permalloy film increase is within 10 %. No significant increase observed with prolonged etching time. The effective magnetization change of the permalloy films are within 5 % post the treatments. Atomic force microscope (AFM) and transmission electron microscope (TEM) are used to study the surface and interface evolution of permalloy film upon etching. The small impact on the electrical and magnetic properties of permalloy films can be correlated with the surface oxide protecting layer formation during the etch. Consequently, sputtered NiFe is a safe material to expose to these etching processes for write pole shield application.     

The effect of hydrogen as an additive in reactive ion etching of GaAs for obtaining polished surface

The effect of hydrogen on the reactive ion etching (RIE) of GaAs in the CF₂Cl₂ plasma is discussed. The addition of hydrogen into the reaction mixture improves the sharpness of etch borders; the etched surface is smooth for etching depth > 1 μm, etching rate is time-constant.     

In situ monitoring of uniformity by end point signal during plasma etching

A theory describing the etch rate uniformity based on the end point signal during the cleaning phase of etching is formulated for the case where the influence of residence time of the etching products and the loading effect can be neglected. An etch rate distribution function representing the probability density that the layer is etched at a given etch rate is introduced to calculate the relative root mean square of the etch rate across the wafer. The reduction in the size of the etched surface or the reduction of the flux of etching product molecules leaving the surface during the cleaning phase is used to determine the etch rate distribution function. A numerical simulation of the end point signal was used to illustrate possible applications of the theory. It is also shown that the slope of the end point curve itself cannot be used to characterize the etching nonuniformity as it is generally supposed. The theory was formulated with the aim of finding a simplein situ method applicable to statistical process control in semiconductor manufacturing when the nonuniformity of an etch process is characterized by a number (so-called 3σ method). Dedicated to Prof. Jan Janča on the occasion of his 60th birthday. This work was supported by the Slovak Grant Agency (No. 1/2312/95).     

Atomic force microscopy investigation of surface roughness generated between SiO2 micro-pits in CHF3/Ar plasma

The present paper investigates the surface roughness generated by reactive ion etching (RIE) on the location between silicon dioxide (SiO₂) micro-pits structures. The micro-pit pattern on polymethyl methacrylate (PMMA) mask was created by an electron beam lithography tool. By using PMMA as a polymer resist mask layer for pattern transfer in RIE process, the carbon (C) content in etching process is increased, which leads to decrease of F/C ratio and causes domination of polymerization reactions. This leads to high surface roughness via self-organized nanostructure features generated on SiO₂ surface which was analyzed using atomic force microscopy (AFM) technique. The etching chemistry of CHF₃ plasma on PMMA masking layer and SiO₂ is analyzed to explain the polymerization. The surface root-mean-square (RMS) roughness below 1 nm was achieved by decreasing the RF power to 150 W and process pressure lower than 10 mTorr.     

Real-time monitoring of SiO2/Si(1 1 1) interlayer etching by Brewster-angle reflectometry

A transitory etching regime after SiO₂ dissolution and before bulk Si(1 1 1) etching in neutral NH₄F solutions was monitored by in situ Brewster-angle reflectometry (BAR). An observed intermediate increase of the BAR reflectance signal is attributed to a fast dissolution of a stressed/strained interlayer beneath the SiO₂/Si(1 1 1) interface. Similar effects were observed on thin thermal oxides (18.2 nm), grown on float zone silicon, as well as on ultra-thin native oxides (1.2 nm) on Czochralsky silicon. Native oxide covered samples showed an increased surface roughness in the course of interlayer dissolution while the surface is progressively covered with compounds of fluorinated silicon. The etch rate, determined by atomic force microscopy (AFM) and compared to the etch rate of bulk silicon, is increased by a factor of four. In the limit of extended etching, the known low etch rates for silicon in 40% NH₄F are observed. Structural and chemical properties of the interfacial layer were analyzed by synchrotron radiation photoelectron spectroscopy (SRPES) which confirmed the presence of Si^3+/4+ valence states throughout the interlayer and by near open-circuit potential (N-OCP) dark current measurements. As a result, oxide etch rates in NH₄F in the pH-range 7–8 as well as the silicon interlayer depth can be assessed by in situ BAR.     

Comparison of plasma chemistries for the dry etching of bulk single-crystal zinc-oxide and rf-sputtered indium-zinc-oxide films

The dry etching characteristics of bulk, single-crystal zinc-oxide (ZnO) and rf-sputtered indium-zinc-oxide (IZO) films have been investigated using an inductively coupled high-density plasma with different plasma chemistries. The introduction of interhalogens such as ICl, IBr, BI₃, and BBr₃ to the Ar plasma produced no enhancement of the ZnO and IZO etch rates with respect to physical sputtering in a pure argon atmosphere under the same experimental conditions. In these plasma chemistries, the etch rate of both materials increased with source power and ion energy, indicating that ion bombardment plays an important role in enhancing desorption of etch products. Except in Ar/CH₄/H₂ discharges, the ZnO etch rate was very similar to that of IZO, which indicates that zinc and indium atoms are driven by a similar plasma etching dynamic. CH₄/H₂-containing plasmas produced higher etch rates for IZO than for ZnO due to the preferential desorption of the group III etch products. Application of the CH₄/H₂/Ar plasma to the etching of deep features in bulk, single-crystal ZnO produced highly anisotropic profiles although some trenches were observed near the sidewalls.     

Microlens arrays etched into glass and silicon

Microlens arrays fabricated by melting photoresist were transferred by reactive ion etching (RIE) into glass (SiO₂) and silicon (Si). By controlling the etching rates of the mask and the substrate material, radii of curvature and focal lengths within a wide range can be achieved. For example, glass lenses with diameter 120 μm and focal lengths between 500 μm and 1500 μm were made. The scaling possibilities of microlens arrays, given the use of RIE, are discussed.     

The influence of laser-induced surface modifications on the backside etching process

Spectroscopic measurements in the UV/VIS region show reduced transmission through laser-induced backside wet etching (LIBWE) of fused silica. Absorption coefficients of up to 10⁵ cm⁻¹ were calculated from the transmission measurements for a solid surface layer of about 50 nm. The temperatures near the interface caused by laser pulse absorption, which were analytically calculated using a new thermal model considering interface and liquid volume absorption, can reach 10⁴ K at typical laser fluences. The high absorption coefficients and the extreme temperatures give evidence for an ablation-like process that is involved in the LIBWE process causing the etching of the modified near-surface region. The confinement of the ablation/etching process to the modified near-surface material region can account for the low etch rates observed in comparison to front-side ablation.     

Effect of the ceramic grain size and concentration on the dynamical mechanical and dielectric behavior of poly(vinilidene fluoride)/Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript> composites

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 (sl_LIBDE=0.111 nm/mJ cm⁻², sl_LIBDE=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.     

Monte Carlo simulation studies of sidewall roughening during reactive ion etching

The evolution of sidewall roughness (SWR) during reactive ion etching (RIE) was simulated using a Monte Carlo method. It was discovered that the sidewall roughness established during an earlier etch period represents a historical archive of the moment during which the etch front passes by and is not affected by further etching. We also found that the behavior of SWR follows two distinct trends. At the initial stages, SWR increases with etch time, or its equivalent, the etch depth, but, beyond a certain etch depth, SWR of etched surfaces stabilizes and does not change with further etching. This is related to the change of the number of shadowing and reemitted particles as the etch depth increases. When the shadowing sticking coefficient decreases significantly, SWR increases beyond a certain critical depth due to reemission. Additionally, the noise also increases as the sticking coefficient decreases because of an increased number of reemitted particles. The simulated results support very well the assertions and mechanisms of the experimental findings and an etch model based on shadowing and first-order reemission effects at low RIE pressure. PACS 52.65.Pp; 52.77.Bn; 81.65.Cf     

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.     

Chemical etching of silicon by CO2-laser-induced dissociation of NF3

The pulsed infrared laser dissociation of NF₃ is reported for the first time, and is used to investigate silicon etching. The role played by collision-enhanced multiple-photon absorption and dissociation is considered, with data on the nonlinear decrease of the absorption cross-section with increasing pulse energy and increasing pressure presented. Using an experimental arrangement in which the laser beam is focussed parallel to the surface, the dissociation process induces spontaneous etching of silicon. Fluorinecontaining radicals diffuse from the focal volume to the surface where a heterogeneous chemical reaction occurs. Etching was monitored by use of a quartz-crystal microbalance upon which a thin film of amorphous silicon was deposited. For a surface with no previous exposure to the photolysis products, dissociation causes the formation of a surface layer prior to the onset of etching. X-ray photoelectron spectroscopy demonstrates this to be a fluorosilyl layer possessing a significant concentration of SiF₃ and SiF₄. In contrast, a surface already thickly fluorinated does not form a thicker layer once laser pulsing commences again. In this case, etching starts immediately with the first pulse. The etch yield dependencies on several parameters were obtained using silicon samples possessing a thick fluorosilyl surface layer. These parameters are NF₃ pressure, laser wavenumber, pulse energy, buffer gas pressure, and perpendicular distance from focal volume to surface. Modeling of the etch yield variation with perpendicular distance shows the time-integrated flux of radicals impinging on the surface to be inversely proportional to the distance. Attempts at etching SiO₂ under identical conditions were unsuccessful despite the evidence that thin native oxide films are removed during silicon etching.     

Influence of resists on reactive ion etching

In the course of plasma etching we can observe a loading effect, i.e. the etch rate depends on the size of the etched surface exposed to the plasma. This phenomenon was explained according to Mogab by the plasma active etch species depletion via a rapid etch reaction. But there exist more coomplicated systems, for example SiO₂-photoresist SCR17-CHF₃, where the SiO₂ surface can be etched and a polymer layer can grow on the photoresist surface. The etching of SiO₂ is also influenced by different resists in the case of differences in their chemical structure. The degree of electrode coating with a resist influences both the etch rate of the masking layer. This may be used for the control of the etching selectivity in the SiO₂-resist system independently of other process parameters.The author is grateful to Mr. Z. Pokorný for his help in preparing the SiO₂ layers used in all experiments.     

AFM measurement of atomic-scale Si surface etching by active oxidation

Atomic-scale etching of a clean Si surface by active oxidation with oxygen molecules was examined using ex-situ atomic force microscopy (AFM). The etch rate was directly determined by measuring the etch depth with AFM. A SiO₂ anti-etching mask was used on a H-terminated Si(001)-2 × 1:H surface prepared by low pH HF treatment followed by annealing in H₂. The etch rate under active oxidation conditions was almost proportional to the O₂ pressure, which was consistent with previous reports. The etch rate exhibited a weak temperature dependence with an apparent activation energy of 0.2 eV. A distinct transition of the reaction mode from etching to oxide formation was observed in detail as an abrupt decrease of the etch rate by lowering the temperature near the boundary condition between the etching and oxide formation conditions.     

Preferential etching of ion-bombarded GaAs

Ion bombardment of GaAs above fluences of the order of 2 × 10¹³ 400-keV Xe-ions/cm² produces a highly strained surface which is elevated several hundred Angstroms above the adjacent masked surface. The irradiated area etches rapidly to yield circular etch pits ～ 0.3 μ in depth and 0.2 μ in diameter. At lower fluences, observations show much less expansion, slower crystalline etching, and some evidence of triangular etch pits and slip planes. These results are attributed to expansion in the implanted layer which results in high surface strain and the generation of dislocations to accommodate the mismatch of lattice parameters.