Madhav Datta

Fundamental aspects and applications of electrochemical microfabrication

Abstract The theory and applications of electrochemical microfabrication technology are reviewed focusing on electrodeposition and dissolution processes. Electrochemical microfabrication offers some unique advantages over competing vapor phase technologies and therefore finds increasing use in the electronics and microsystems industries. The present paper discusses the underlying principles of electrochemical microfabrication processes. The important role of mass transport and current distribution is stressed and it is shown how numerical modeling contributes to the present understanding of critical process parameters. The application of electrochemical microfabrication technology in the electronics industry is illustrated with selected examples.

Electrochemical machining under pulsed current conditions

Abstract The possibility of using pulsed current in electrochemical machining at low electrolyte flow rate has been investigated. Theoretical aspects of predicting electrolyte heating and limiting rate of mass transport are discussed in terms of simplified models. High rate dissolution of nickel in sodium chloride solutions under pulsed current conditions was investigated in a flow channel cell by studying the influence of different pulse parameters on anode potential, surface microtexture, surface roughness and current efficiency of metal dissolution. Obtained results indicate that anode potential and surface finish are controlled by mass transport in agreement with steady state behavior. Maximum current density applicable under pulsed current conditions is limited by the occurrence of sparking.

Thickness of natural oxide films determined by AES and XPS with/without sputtering

The thickness of natural air‐formed oxides on Al, Si, Fe, Ni, and Ta is determined by Auger electron spectroscopy (AES), depth profiling, and angle‐resolved photoelectron spectroscopy (XPS). The sputter rate of the oxides is measured under the same conditions and their values are given with respect to a certified standard 100 nm Ta2O5 reference. Data from AES depth profiling are corrected for the influence of the electron mean free path. XPS data are evaluated from area intensities of the bands after peak synthesis (fitting). Using literature values of mfp for oxides, their thickness is evaluated. AES and XPS data are in reasonable agreement giving oxide thickness ranging from 0.2 to 5 nm increasing in the order of SiO2

Anodic dissolution of metals at high rates

Electrochemical metal shaping and finishing processes involve anodic dissolution of metals at high rates. This paper presents a review of some fundamental aspects related to the understanding of such processes. Included are discussions of the phenomena of passive film breakdown that lead to the transpassive dissolution of metals, some of the available information on anodic reaction stoichiometry, and the role of convective mass transport and salt precipitation layers on metal removal rate and surface finish. The use of pulsating current permits the altering of anodic mass transport rates and transpassive dissolution behavior, thereby making it possible to obtain high dissolution efficiencies even at low average current densities.

On the role of mass transport in high rate dissolution of iron and nickel in ECM electrolytes—I. Chloride solutions

Abstract High rate anodic dissolution of iron and nickel in 5 M NaCl was studied in a flow channel cell under controlled hydrodynamic conditions. Galvanostatic experiments were aimed at investigating the influence of current density and electrolyte flow rate on anode potential, current efficiency for metal dissolution and surface texture resulting from dissolution. Active dissolution at low current densities leads to surface etching and transpassive dissolution at high current densities leads to surface brightening. Transition from active to transpassive dissolution is mass transport controlled and is accompanied by a change in anode potential, surface microtexture and in case of iron by a change in the valence of metal dissolution.

Electrochemical micromachining: An environmentally friendly, high speed processing technology

Wet chemical etching processes are employed in the manufacturing of a variety of microelectronic components. These processes use etchants that generally contain aggressive and toxic chemicals, generate hazardous waste and have limited resolution. Electrochemical metal removal is an evolving alternate processing technique that involves controlled metal shaping by an external current, thereby requiring less aggressive and nontoxic electrolytes. The application of controlled electrochemical metal removal in the fabrication of microstructures and microcomponents is referred to as electrochemical micromachining (EMM). In this paper a recently developed EMM process and tool for metal mask fabrication is discussed. EMM performance is compared to that obtained by the conventional chemical etching process. Obtained results demonstrate the opportunities offered by EMM particularly as a high-speed, environmentally friendly processing technology.

Microfabrication by electrochemical metal removal

Recent advances in the development of electrochemical metal-removal processes for microfabrication are reviewed in this paper. After a brief description of the process, several important parameters are identified that determine the material-removal rate, shape control, surface finishing, and uniformity. The influence of surface film properties, mass transport, and current distribution on microfabrication performance are discussed. Several examples of microelectronic component fabrication are presented. These examples demonstrate the challenges and opportunities offered by electrochemical metal removal in microfabrication.

Experimental investigation of mass transport in pulse plating

Abstract Mass transport during the electrodeposition of copper from acidified sulphate solution using single and multiple pulses was studied. Experimental results obtained in a diffusion cell and with a rotating hemispherical electrode are compared with published theoretical calculations of various degrees of mathematical sophistication and complexity. The simple duplex diffusion-layer model proposed by Ibl gives adequate values of the pulse limiting current density for many practical purposes. The agreement with experiment can be further improved by using a semi-empirical modification of the model.

Jet and Laser‐Jet Electrochemical Micromachining of Nickel and Steel

Experimental results on jet and laser-jet electrochemical micromachining of nickel and steel in neutral solutions of sodium chloride and sodium nitrate are reported

On the role of mass transport in high rate dissolution of iron and nickel in ECM electrolytes—II. Chlorate and nitrate solutions

Abstract Transpassive dissolution of iron and nickel in 5 M NaClO 3 and 6 M NaNO 3 has been investigated in a flow channel cell allowing for controlled hydrodynamic conditions. Applied current density ranged up to 33 A/cm 2 , applied flow rates up to 1760 cm/s. The influence of mass transport on current efficiency for metal dissolution, apparent anode potential and surface microtextures resulting from dissolution was investigated. Obtained results confirm that all these parameters are influenced by mass transport processes but secondary effects such as gas evolution, hydroxide precipitation, homogeneous chemical reactions, or joule beating in many cases interfere with convective transport phenomena in the solution. Optimisation of ECM operating conditions and modelling for tool design applications require intimate knowledge of the influence of hydrodynamic conditions on the electrochemical behavior of the metal-electrolyte combination employed.