Electric Field Induced Phenomena: Mass Flow and Chemical Reaction-based Patterning
Numerous electric field induced phenomena have been studied, for long, at various length scales. In particular, a concentrated electric field applied across a conductor, or equivalently an electric current of very high density passing through a conductor, can manifest in form of both destructive and constructive processes, depending on the requirements of an application. For example, electromigration, which is a diffusion-controlled electric field directed mass transport phenomenon, often leads to the formation of voids and hillocks near the cathode and the anode, respectively, metal interconnects in microelectronic devices. This results in failure of the device and hence this “destructive” manifestation of the electric field is considered as a “villain” in microelectronic interconnects. On the other hand, recently discovered electromigration in liquid metals may pave the path for various useful applications, such as maskless conformal coating, pattern formation, surface modification, etc. Besides the exploitation of the capability of the electric field for transporting matter (e.g., in liquid metals) in controlled and directed fashion in various applications, harnessing the unique potential of the electric field in inducing a chemical reaction in a controlled fashion in a confined region also provides new avenues for constructive usage. In particular, the electric field induced chemical reaction has been exploited for patterning at extremely small length scale, using scanning probe microscopes, such as atomic force microscope (AFM) and scanning tunneling microscope (STM). It is imperative to unambiguously understand the fundamentals of the concerned phenomenon before the aforementioned electric current induced phenomenon can be exploited to bear numerous technologies and applications. Here, we have studied two different electric field induced phenomena, namely electromigration in liquid metals and electric field-induced chemical reaction in solid thin metals1. The presentation of the study in form of this thesis is divided into three main parts, (i) Theoretical modelling of electromigration in liquid metals (or liquid electromigration), (ii) Study of the electric field induced chemical reaction in Cr film, including a detailed investigation of effects of ambient conditions on reaction kinetics, and (iii) Development of a tool for pattern drawing by the means of electric field-induced chemical reaction. As mentioned earlier, electromigration, irrespective of whether it is in solids or liquids, is a diffusion-controlled directional mass transport phenomenon that is driven by the applied electric field. The direction of the mass transport, in general, depends on the net force experienced by the positively charged ions due to the applied electric field and the momentum transferred from the colliding free-electrons. Hence, it is critical to understand the direction of this effective force in metal. Often it is from the cathode to the anode in a solid metal; however, it is not that straightforward in liquid metals. For example, the direction of the net mass transport in most of the liquid metals, e.g., Ga, In, Ga, etc., is in the direction of the electric field (i.e., from the anode to the cathode, which is contrary to the solid metals), whereas the direction of the flow is reversed in the liquid Pb. The reason for the dichotomy of the directionality in the liquid metal flow was not completely understood, and hence we performed detailed analytical and experimental work to resolve it. Here, we developed a theoretical model based on the cell model of liquids and incorporating Lennard-Jones interaction potential. The model considers the short-range order in liquid metals and calculates the force on the ions due to the momentum transferred by electrons during electron-ion collisions. The model not only successfully predicts the flow direction of numerous liquid metals, including liquid Pb, Ga, etc., but it also gives the value of effective charge number of liquid metals as the function of the temperature. Experiments were performed on selected liquid metals in order to validate the model. As mentioned earlier, electrical interaction between the tip and the metal (e.g., Cr) film may induce a chemical reaction in the localized region around the probe tip on the film. If the probe is translated over the metal film along a predefined path, then the chemical reaction induced controlled patterning of the film can be achieved. In the second segment of our work, the focus has been to understand the mechanism of the phenomenon of electric field-induced chemical reaction in Cr film by performing a series of experiments using a custom-built experimental setup, so that we can later exploit the gained understanding for lithography. The phenomenon was studied using a W-tip with a diameter of 20 μm under stationary tip condition. Although this length scale is relatively larger than that of using AFM or STM tip, it provides a significant amount of reaction product and allows easy maneuverability as well as better control of the experiments: Both of these are critical for an unambiguous understanding of the nature and kinetics of the chemical reaction. The study includes confirmation of observation of a chemical reaction induced process in presence of electric field (as per Faraday’s law) and the identification of the reactants (as Cr and H2O – in the form of both vapor and liquid) and the product (as CrO3) at the cathode. The ambient conditions affect the reaction kinetics at the cathode probe tip and hence the dimensions (as well as quality) of the patterns. Therefore, the phenomenon was studied under different ambient conditions, such as vacuum, gaseous (e.g., N2, O2 and air) environments, variable humidity, high and low temperature, etc. It was observed that the reaction did not occur in an environment unless water vapor (or water) was present. Furthermore, the reaction occurred without the generation of a significant amount of heat (and hence the negligible rise in the temperature was associated with this process). Finally, the reaction was favored at the locations of high current densities at or near the cathode. A study on the understanding of the nature of the reaction product revealed that CrO3 is highly hygroscopic and it quickly absorbs water from the air to become liquid. As the reaction product is soluble in water, the region where reaction had taken place could be easily removed by dipping the sample into water. The use of water in the reaction was further exploited to develop a new SPM based lithography process that can preclude the need to keep the sample and the tip into contact and proffers spontaneous removal of the reaction product. Overall, the results obtained in this segment of the work paves the path for developing a new tip-based lithography technique that is better suited to meet the challenges of tip wear, debris collection, low throughput, etc., which are often associated with other SPM based lithography techniques. In the last segment of the work, the understanding of the electric current induced chemical reaction in Cr film was applied to develop a tool for drawing patterns at the micrometer length scale. A considerable amount of effort was made to assemble a standalone lithography unit that can work in ambient as well as submerged in water conditions. Here, a micro-positioner was used to place the sample at the desired location relative to the tip, and a W-tip was traversed over the sample. The tip was brought into contact with (and detached from) sample using an “electromagnet-based lever-type drive.” A software-hardware interface was developed using LabView software, which was also capable of importing drawings made in third-party software, such as CleWin. Tool parameters, such as tip velocity, tip force, etc., were observed to have a significant impact on the pattern dimensions. Finally, several patterns, including closely spaced parallel lines, were generated using the developed tool in Cr films of different thicknesses and statistical information was obtained. In summary, this work, which includes both explorations of fundamentals and application of the learned fundamentals to develop new technology for lithography, confirms the constructive potential of the electric current and invites researchers to explore this area further.