Large scale copper microstructures generation using electro-deposition through nuclear track filters

Abstract

Microstructure fabrication is not only of immense importance for microelectronics technology but also plays a vital role as a potent tool for carrying out investigations relating to behavior of materials at sub micron scales. In the recent years, it has broadened to encompass more and more areas of multidiscipline. A variety of possible applications include high power microwave generation, ultra fast computer and tetra hertz amplifier devices fabrication, radiation and temperature insensitive electronics. Fields where well-established applications already exist include field emitters; electrochemistry; conductive polymer nanofibers fabrication; transparent metal structures and macroscopic quantum tunneling phenomena. Of the many possible geometrical shapes and growth patterns, the simplest structure is probably an ensemble of wires. A variety of techniques like optical, X-rays, electron and ion beam lithography have been used for fabrication of such ensembles. However, electrochemical methods involving electrodeposition of metals into the etched pores of nuclear track filters (NTFs) of mica and polymers, are a convenient and simple technique [1–6]. The morphological study of such structures produced through electrochemical methods and of replicas of etched tracks in NTFs used as templates, has two-fold purpose. One, it provides the finest and critical details of the geometry and dimensions of microstructural constituent elements and the second, as a by product it enables to study the various aspects of interaction of a nuclear particle with given material leading to formation of tracks in NTF. It is well known that parameters, which control the shapes of tracks in NTFs, include the nature of the material; the ion beam and energy deposition rate; pre, post-irradiation storage and environment and the etching conditions. In the present work nuclear track filters of Makrofol (KG) polycarbonate having pore size ca. 2.6 μm and pore density 1 × 106 cm−2 have been used as templates for synthesis of copper microstructures using the technique of electrodeposition (template synthesis). Because the membranes used contain cylindrical pores of uniform diameter, crops of monodisperse microstructures of the desired material, whose dimensions can be carefully controlled, are obtained. Track etch membranes in the form of nuclear track filters have emerged as a spin-off from solid state nuclear track detectors (SSNTDs)-solid dielectric materials capable of storing tracks of energetic, heavily ionizing ions which can subsequently chemically amplified for optical observations as pores or channels of well defined geometry and pore density [3–6]. The size and dimensions of the pore depend upon various factors, viz. the nature and energy of the incident ions, the target material, etch conditions e.g., temperature, nature of the etchant. The pore size which is controllable, may range from few nm to mm. NTFs have been put to numerous applications besides their use in the synthesis of nano/microstructures. The underlying principle of template synthesis technique is akin to that of producing components through the use of replication e.g., die-casting or mold casting, like making ice candies out of molds. In this technique, materials can be deposited with in the template membranes by either electrochemical or chemical (electroless) reduction of the appropriate metal ion. The generated structures can both be homogeneous or heterogeneous depending on the pore size and geometries, with complete control over the aspect ratio (length and diameter ratio). The simple and well known underlying concept of electrodeposition of metals through electroplating is described as an electrochemical process in which metallic ions in a supporting solution are reduced to the metallic state at the cathode, which is closely covered by an NTF membrane, would lead to formation of growth of plated film as the embodiment of micro or nano structure. The etched pores of the membranes would act as templates [1–10]. Electrodeposition (template synthesis) is a versatile technique combining low processing cost with ambient conditions that can be used to prepare metallic, polymeric and semi conducting microstructures. In the case of microstructures arrays, electro deposition is the cost effective method with which such structures have been successfully fabricated. Samples of polycarbonate of thickness 30 μm were irradiated to 13.64 MeV/n 238U ions (fluence 106 ions/cm2)utilizing the heavy ion accelerator UNILAC facility at GSI, Darmstadt, Germany. Samples were etched in 6N NaOH solution at 70 ◦C for 40 min and pore size of 2.6 μm was obtained. After etching nuclear track filter having pore size 2.6 μm and pore density 106/cm2 is used for synthesis of iron microstructures using the technique of electrodeposition. Because the membranes used contain cylindrical pores of uniform diameter, crops of