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Title: Generation and controlled etching of ion tracks in polycarbonate films
Other Titles: Ju tan suan zhi mo zhong li zi jing ji zhi zhi bei ji ke shi kong zhi
Authors: Man, Chi Tao (文志韜)
Department: Department of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2008
Publisher: City University of Hong Kong
Subjects: Particle track etching.
Thin films.
Notes: CityU Call Number: QC793.3.T67 M36 2008
vii, 78 leaves : ill. (some col.) 30 cm.
Thesis (M.Phil.)--City University of Hong Kong, 2008.
Includes bibliographical references (leaves 75-78)
Type: thesis
Abstract: Ion tracks (or nuclear tracks) have found many applications in different branches of science. In recent years, ion-track technologies have found applications in the design of track-etch templates for micro- and nano-fabrication, hole engineering and track membrane technology, etc. The technology is commonly based on the availability of accelerated heavy ions. Polymers (belong to the class of solid-state nuclear track detectors or SSNTDs) such as polycarbonate (commercially known as Makrofol) or polyallyldiglycol carbonate (commercially available as CR-39) are widely used for this technology due to their good mechanical and chemical strength, and their high susceptibility for selective ion track etching. By controlling the irradiation characteristics, etching time and the concentration of the etchant, micro- or nano-pores with high aspect ratios can be obtained. Chapter 1 gives an introduction to the present project. The objectives of the present research are two-fold. First, a method based on a fission fragment (FF) source is explored to generate micrometer-sized tracks in a polycarbonate (PC) film, which is covered in Chapter 2 of the present thesis. Current ion-track technology to design track-etch templates is usually based on heavy ions accelerated by accelerators. However, it would be more convenient for most laboratories if such track-etch templates can also be generated using FF sources. For this feasibility study, 5-μm thick polycarbonate films are irradiated by FFs from a 252Cf- FF source. Since the track etch rate for the FF tracks is much faster than the bulk etch rate in the PC films, tracks with high aspect ratios can be obtained. The etched tracks are examined using Scanning Electron Microscopy (SEM). In addition the etched channels are replicated by electroless plating. The etched PC films with FF tracks are first attached onto a zinc plate and then immersed into a copper sulphate solution. The copper ion in the copper sulphate solution will migrate towards the zinc surfaces through the etched channels and deposited onto the zinc surface. After a predetermined deposition time, the PC film is etched until it is completely dissolved. The deposited copper structures are then examined using SEM to help visualize the etched channels. The second objective of the present research is to explore a method to control the profile of the etched channels in the PC films, which is covered in Chapter 3 of the thesis. In order to obtain etched channels with smaller size, a shorter etching time or an etchant maintained at a lower temperature are preferable. The presence of a surfactant in the etchant can also help achieve the objective, which is the method explored in the present work. Chemically speaking, surfactants are composed of a hydrophobic tail and a hydrophilic head. The hydrophobic tail will adsorb on the hydrophobic surface of the polymer film while the hydrophilic head is immersed in the etchant. Adsorbed layers of the surfactant can slow down the diffusion of the etchant into the sample, thus affecting the etching rate at the respective surface. In this part of work, for simplicity, 30 μm PC irradiated by 238U heavy ions (11.3 MeV/u) are used. The methodology and findings should also generally apply to other PC films, such as those described in Chapter 2. In the current project, the influence of the alkali resistant surfactant Dowfax 2A1 on etching the 238U ion track in the 30 μm PC films is studied at low etch rate using electro conductivity measurements. At surfactant concentrations above 10-4 vol.-% break-through times are predictable. The track etch rate is decreased and stabilized by increasing the concentration of the surfactant. Formation of cylindrical channels is favored under this situation. By using an electrolytic cell in connection with a data acquisition system, the diameter of the etched tracks or the bulk etch rate can be monitored in real time. Thus the etching process can be terminated at any preset size. This can reduce the number of times the samples needed to be taken out to be observed under the microscope. To verify whether the electro conduction measurements give correct values for the etched track diameter, single ion tracks etched for 9.6 h in 5 M NaOH plus 1 vol.-% of Dowfax 2A1 surfactant at (41.5 ± 2.0) °C were selected for electro-replication. The resulting micro-wires were observed by SEM. The results showed agreement in the radii better than 0.1 μm, which provided a strong evidence that the electro conduction measurements were reliable. Depending on the current limit set during electro replication, compact or hollow cylinders can be obtained. A compact copper wire is obtained when electro replication proceeds at 10 nA limiting current while a hollow one is formed when the limiting current is set at 100 nA. Chapter 4 gives the conclusions.
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