A gas phase and surface chemistry study of inductively coupled plasmas fed with C4F6/Ar and C4F8/Ar intended for SiO2 etching processes was performed. Adding Ar to those fluorocarbon gases results in a strong increase of the ion current, by up to a factor of 5 at 90% Ar relative to the pure fluorocarbon gases. The fluorocarbon deposition rate is higher for C4F6/Ar than for C4F8/Ar, whereas the fluorocarbon etching rate is lower, and both quantities decrease as the amount of Ar is increased. For both C4F6/Ar and C4F8/Ar, the CF2 density is more than an order of magnitude greater than the CF density. The CF2 partial pressure decreases as more Ar is added to the C4F6/Ar plasmas. A comparison of these data with corresponding results obtained with C4F8/Ar shows that the CF2 partial pressure in C4F6 is higher for Ar-lean gas mixture than for C4F8/Ar. This remains true up to 40% Ar. Above 40% Ar the CF2 partial pressure in C4F8 is higher than for C4F6. The CF and COF2 partial pressures in C4F8 are higher than for C4F6. The SiO2 etch rate is higher for C4F8/Ar than for C4F6/Ar. This may be attributed in part to the higher F/C ratio of the steady-state fluorocarbon film formed on SiO2 surfaces for C4F8/Ar which was determined by x-ray photoemission spectroscopy (XPS). The etching selectivity of SiO2 over resist and silicon is increased by the addition of Ar to the fluorocarbon gases. Overall, the SiO2/resist and SiO2/Si etching selectivity are higher for C4F6/Ar (i.e., 4 and 9, respectively) at 90% Ar than for C4F8/Ar (i.e., 2 and 5, respectively) at 90% Ar and otherwise identical conditions. Both ellipsometry and XPS measurements show that the steady-state fluorocarbon layer thickness is greater for C4F6/Ar (∼4 nm) than for C4F8/Ar (∼2.8 nm). Argon addition leads to a strong decrease of the fluorine content of the steady-state fluorocarbon layers on both Si and SiO2 surfaces relative to films produced in pure fluorocarbon discharges, and this effect is related to the increase of the SiO2/Si and SiO2/resist etching selectivity.
Gas mixtures based on C4F8 are promising for the development of high-performance SiO2 plasma etching processes. Measurements of important gas phase species, thin film etching rates and surface chemistry changes were performed for inductively coupled plasmas fed with C4F8/Ar and C4F8/O2 gas mixtures. The addition of Ar to C4F8 causes a strong increase of the plasma density relative to that of pure C4F8 (by up to a factor of 4× at 90% Ar). For O2 addition the changes in plasma density are small up to 90% O2 relative to pure C4F8. Infrared laser absorption spectroscopy was used to determine the absolute densities of neutral CF, CF2 and COF2 radical species as a function of the gas composition. The densities of CF and CF2 were enhanced for certain operating conditions when Ar was added to C4F8 as long as the amount of Ar remained below 20%. For instance, the partial pressure of CF was 0.1 mTorr for a 20 mTorr 1400 W source power discharge for pure C4F8, and increased to 0.13 mTorr at 20% Ar. Above 20% Ar it decreased, roughly following the gas dilution. The CF2 partial pressure was about 5 mTorr for the same conditions, and increased by about 10% at 20% Ar. Above 20% Ar the CF2 partial pressure decreased roughly linearly with the amount of Ar added, to about 2 mTorr at 50% Ar. Of particular interest was the analysis of the difference in behavior of CF, CF2 and COF2 partial pressures over SiO2 and Si surfaces, with and without rf bias power (in the latter case a self-bias voltage of −100 V was used). For pure C4F8 discharges at 20 mTorr and 1400 W inductive power without rf bias the partial pressures of CF, CF2 and COF2 radicals are comparable over SiO2 and Si surfaces. Upon applying a rf bias, the CF2 partial pressure over a SiO2 surface is reduced much more strongly than for a Si surface. The overall reduction appears to be consistent with the relative SiO2/Si etch rate ratios observed for these conditions. These results indicate that CF2 is consumed during the etching of SiO2 and Si. We also measured fluorocarbon deposition rates without rf bias and etching rates of blanket SiO2, silicon, resist and deposited fluorocarbon films as a function of the rf bias and feed gas composition. Important differences in the response of the etching rates of those materials upon the addition of O2 and Ar to C4F8 were observed. In particular, we show that the SiO2/Si and SiO2/resist etching selectivities can be doubled by adding up to 90% Ar to C4F8, without inducing an unacceptably large reduction of the SiO2 etching rate. The change in etch rate ratios is at least in part due to strong surface chemical changes seen for Ar-rich fluorocarbon gas mixtures. The surface chemical changes of Si and SiO2 surfaces were investigated by real-time ellipsometry and x-ray photoelectron spectroscopy. A strong reduction of the fluorine content of the fluorocarbon steady-state layer and an increase in thickness is seen when up to 90% Ar was added to C4F8, and this coincides with an increase of the SiO2/Si etching selectivity. The change in fluorocarbon surface chemistry can be explained by the strongly increased ion/neutral flux ratio that is characteristic of Ar-rich C4F8/Ar gas mixtures.
Anodized aluminum has recently attracted much attention because of its interesting pore structure.[1] The pore structure is a self-ordered hexagonal array of cells with cylindrical pores of variable sizes with diameters of 25±250 nm and with depths exceeding 100 lm depending on the anodizing conditions employed. It can be fabricated through electrochemical anodization of aluminum in various acidic electrolytes, [2±4] and has been demonstrated to provide the basis for an inexpensive, highthroughput fabrication of nanostructures. These properties make anodic aluminum oxide (AAO) a desirable material for many applications, which include microfabricated fluidic devices, [5] quantum-dot arrays, [6±9] magnetic memory arrays, [10] high-aspect-ratio microelectro-mechanical systems, [11] and photonic crystals.[12] The applications of AAO films that have been explored also include template growth of nanotubes, nanowires, creation of nanoholes, nanodots, and nanopillars.[13±15]The ability to pattern porous films is important for a number of technological applications, including sensor arrays, catalysis, optics, and microfluidic devices. Doshi et al. have developed a novel method for patterning mesoporous silica films by employing photosensitive groups.[16] The pore sizes available in such materials are in the range of 2±5 nm, which are small compared to those achievable through anodization of aluminum. Development of patterned AAO membranes, which exhibit high aspect ratio microstructure, is more challenging and the most promising approach for the above mentioned applications. It can be achieved by patterning the aluminum surfaces with an anodization barrier prior to anodization. During anodization of the patterned surfaces, the anodization barrier prevents anodization in the patterned areas while the oxide grows in the unpatterned areas. In addition, it also addresses the fragility problem [17] of AAO membranes due to the presence of aluminum metal that serves as the robust support for nanopore channel arrays. These nanopore channel arrays can then be integrated into microfabricated devices with less fragility problem. Huang et al. [18] have reported the first anodization barrier of poly(methylmethacrylate) deposited on aluminum films. Due to the penetration of the electrolyte through this polymeric layer, the underlying surface was partially anodized to form AAO up to 10 nm deep. In addition, the procedure requires optical lithography, polymer nanoprinting, ionbeam exposure, and milling. Recently, Sun and Kim have reported the formation of alumina nanopore arrays on holographically patterned aluminum films.[19] They patterned aluminum films by using holographic lithography process on silica substrate and anodized the aluminum to get the pores. The pores are not circular and ordered and the pore channels and non-porous substrate are not an integral part of the material. Li et al. [20] and Bae et al. [21] have adopted an indirect method for preparing patterned AAO. First they prepared ordered AAO and patterned the porous ...
Freestanding anodic aluminum oxide ͑AAO͒ membranes were fabricated using a two-step anodization procedure, voltage reduction method, and inductively coupled plasma polymerization. The fabrication process was monitored with electrochemical and spectroscopic ellipsometry measurements, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy. Static contact angle measurements demonstrated that the two surfaces of the fabricated AAO membranes have dramatically different degrees of hydrophobicity. The contact angle of water on the side modified with the fluorocarbon film was 150°, whereas the contact angle on the hydrophilic side, which was initially attached to the aluminum substrate, was less than 5°. The fabricated freestanding AAO membranes with only one hydrophobic surface are expected to find applications in the design of new systems for separation and filtration and as gas and liquid transport devices such as gas-diffusion electrodes for miniature power sources and sensors.Nanoporous anodic aluminum oxide ͑AAO͒ membranes and nanopore arrays attached to aluminum substrates have great potential for numerous applications in the rapidly developing nanotechnology field. 1,2 Both freestanding membranes and arrays, having hexagonally ordered pores, have been extensively used in the fabrication of a variety of nanostructures. The modification of pores with metals or semiconductor materials is primarily motivated by the numerous possibilities to fabricate novel materials and nanodevices ͑e.g., magnetic recording media, optical and electroluminescent display devices͒. 3 This modification can be accomplished by a number of methods, including electro-and electroless deposition, 3-8 chemical vapor deposition, 9 and sol-gel deposition. 10 The potentially useful optical, photoelectrochemical, electrical, and transport properties of modified AAO arrays make them very promising for application in design of new nanotechnology products such as: sensing devices, adsorbants, preconcentrators, gas and liquid separation and transportation devices, and membranes in microreactors or micropower sources.The highly ordered AAO arrays are typically fabricated using a two-step anodization procedure. 11 This method involves porous-type anodization of aluminum in acidic electrolytes and sequential chemical etching of resulting porous and barrier aluminum oxide layers. The second anodization leads to formation of highly ordered AAO arrays. 11-13 The preparation of freestanding AAO membranes requires the underlying aluminum substrate to be removed. This can be accomplished, for example, by dissolving the whole aluminum substrate in a mercuric chloride solution. Alternatively, the separation of the AAO membrane from the aluminum substrate can be achieved by the voltage reduction method. 14 The application of this method results in gradual thinning and complete dissolution of the barrier oxide layer, which connects porous aluminum oxide and aluminum. [14][15][16] The focus of recent research has ...
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