1Mann Juin Kao, 1Shao Fu Chang, 1Chin Guo Kuo, 2
Ker Jer Huang,
3Chien Chon Chen*1Department of Industrial Education, National Taiwan Normal University, Taipei, Taiwan
2Chung-Shan Institute of Science and Technology, Taoyuan, Taiwan
3Department of Energy Engineering, National United University, Miaoli, Taiwan
Abstract
Most electronic and optolelctronic devices are based on high-quality semiconductors process. It will be highly if the self-assembly nanofabrication techniques can be combined with traditional micro-fabrication technologies in the pursuit of next generation high performance nano-scale devices. Based on the technique of AAO forming on Al foil, AAO on Si wafer and glass substrate can be achieved in our experiment. The AAO bottom of barrier layer on the Si, glass/AAO interface always decreases the electron conductivity transparency. For this reason, the barrier was removed by the process of applied 10 V pulse voltage.
Introduction
Nanotechnology is not so much a new technology, as it is a new concept for materials and device fabrication processes. The key to successful fabrication of nano-materials is understanding how to control process parameters correctly. When anodized in an acidic electrolyte and controlled under suitable conditions, aluminum forms a porous oxide called anodic aluminum oxide layer with approximately hemispherical geometry.
Aluminum in the presence of air or aqueous electrolytes is always covered with a thin natural layer of alumina. When a positive voltage is applied to an aluminum substrate in a suitable electrolyte, pores form on the surface in almost random positions. However, under specific conditions, almost perfect hexagonally ordered pores in anodic alumina can be obtained.
When aluminum reacts with acid for example, sulfur acid and under an applied voltage the reaction includes Eqns. 1~4. The Al Pourbaix diagram can therefore be created using the equations.
used to estimate pore size and pore distance in AAO. The formula for estimating pore size with voltage is: C=mV, area can be computed based on AAO structural parameters.
These parameters include AAO thickness, pore size, pore density, and sample size. It is generally accepted that the thickness of barrier-type alumina is mainly determined by the applied voltage (1~1.4 nm/V) [6], even though there are slight deviations depending on the electrolytes and temperature. The maximum attainable thickness in the barrier-type alumina film was reported to be less than 1 μm, corresponding to the breakdown voltage in the range of 500~700 V (DC) [7-9]. Akahori [10] has demonstrated that the melting point of this inner oxide layer is 1000 C, also the AAO template is stable around 800°C [11], which is much lower than that of the bulk alumina.
Figure 1 The schematic diagrams of AAO; (a) the TiN and Al film was deposited onto 6 inch Si wafer using a 4-inch TiN and Al targets with a purity of 99.999%. The
71 base pressure of the deposition chamber was kept at 1×10-6 Torr. Working pressure was 5×10-4 Torr, and sputtering power during deposition was 100 W, 50 V bias, applied for 20 min. A 15 nm pore diameter template was the surface, was then subjected to a second anodization for several minutes to form various AAO films of varying thickness. Finally, the sample was placed in a 5% vol.
H3PO4 solution at 25 ℃ for 10 min. to widen the nanotubes to an ordered array and form a good quality AAO film. A similar process was used to form a 80 nm pore diameter AAO template; the electrolyte was 3% vol.
(COOH)2 at 10 ℃, the applied voltage was 40 V, and the time for pore widening is 30 min [12-16].
Results and Discussion
When aluminum is immersed in the H2SO4 or H3PO4 electrolyte, H2SO4 is ionized to SO24 and 2H (reaction 5). Based on the Al Pourbaix diagram, applying a voltage to the aluminum ionizes Al to Al3+ (reaction 6), establishing the basic conditions (Al3+, acid region) for anodizing aluminum. In aqueous solution, water ionizes to H+ and OH-. A H+ ion gains an electron formation H atom, then a pair of H combines to form H2 after gaining double electrons from Al, escaping from the sample surface.
During the anodization, hydrogen gas escapes through the alumina tube, and Al3+ can be extracted from the Al surface, and the Al/oxide interface to form the nano-pattern structure. Therefore, Al3+ associates with OH- (reaction 7) or O2- which comes from air (reaction 8) exothermic reaction, where H0f is standard enthalpy.
kJ
In above equations, the heat of exothermic in Eqn. 6 can be removed before anodization when cooled down the solution in the isothermal tank. The local heat in Eqns. 7 and 8 should be removed by cycling or agitating
electrolyte, otherwise the local cracking, pits, defects present on the AAO surface.
Different architectures of AAO are developed, including AAO attached to Al foil, and free-standing AAO glass or Si wafer. The characteristics of AAO that the pore channels penetrate through the whole oxide film without interconnections, and the pore diameters are controllable between 10 to 500 nm. AAO forms on a Si wafer which is compatible with Si planar techniques and hence can be used to fabricate various nanostructure of integrated circuits (IC) devices by the semiconductor processes.
However, the process unsuitable includes the lasting heat treatment step because Si would solve in Al film.
According to Al-Si binary phase diagram the maximum solubility of Si in Al is 1.5 at.% at the eutectic temperature (577℃), and it decreases to 0.05 at% at 300℃. When the second phase of Si solved in Al, it will make sub-holds in AAO. As well as, the coefficient of linear thermal expansion between Si (22 × 10-6/K) and ceramic AAO (very low) is very different at high temperature it makes AAO falling off Si wafer easily. Therefore the interlayer of TiN between Si wafer and Al film should be formed.
Figure 2 showed the images of AAO on Si wafer; (a) optical image of AAO on a 6 inch Si wafer, (b) SEM can growth on the Al grains even on the micron size of Al grain boundaries of wafer substrate. Figure 3 images of AAO on Si wafer; (a) and (b) SEM image of AAO formation on the Al micro-grains, (c) AAO with 80 nm pore diameter, (d) side view images of straight AAO channel without barrier layer on Si wafer.
The thickness of barrier layer is mainly determined by the anodizing voltage although there is a slight deviation depending on the anodization electrolyte and temperature.
Early experimental studies on the morphology and mechanism of pore formation on aluminum films showed that the barrier layer thickness is proportional to the anodization voltage. Accordingly, the thicknesses of the barrier layer of the templates used in this study are estimated to be ~18 nm and ~40 nm for AAO (φ15 nm) and AAO (φ60 nm) templates respectively and hence the significant difference in etching time. A thin film of barrier layer has similar chemical composition with AAO.
Also, the thickness of barrier layer is similar to AAO pore wall. The isotropic etching would be happen during wet etching. It is difficult to keep AAO well but remove or dissolve barrier layer by just chemical etching method.
However, when applied pulse voltage to the specimen the barrier layer is closer to electrode (anodic) than AAO pore wall. Therefore, the short time electrochemical etching by pulse voltage method can remove barrier layer but retains AAO.
72 Figure 2 images of AAO on Si wafer; (a) optical image of AAO on a 6 inch Si wafer, (b) SEM image of AAO formation on the Al micro-grains, (c) and (d) high magnification SEM images of AAO with 10 nm pore diameter.
Figure 3 images of AAO on Si wafer; (a) and (b) SEM image of AAO formation on the Al micro-grains, (c) AAO with 80 nm pore diameter, (d) side view images of straight AAO channel without barrier layer on Si wafer.
73 Conclusions
In nano-technology research, fabricating functional nanoscale structures and devices in a well-controlled way represents one of the most difficult challenges facing researchers and engineers. Due to the small dimensions of these nanoelements, the AAO template process provides a viable approach to overcoming such technological challenges. AAO with 10 and 80 nm pore sizes were formed on the Si wafer by 10 vol.% H2SO4 (10V) and 3%
vol. (COOH)2 (40V) anodization method. The T interlayer of TiN between Si wafer and Al film can prevented Al-Si alloy and sub-holds form on the AAO. The straight AAO channel without barrier layer on Si wafer was achieved by pulse voltage method.
Acknowledgment
The authors gratefully appreciate the financial support of the National Science Council of ROC under the contract No.101-2627-M-239-001-, 101-3113-S-262-001-, and Chung-Shan Institute of Science and Technology (CSIST) under the contract No.(CSIST-442-V202).
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