A Study Of Mechanism And Performance Of Electrogenerated Chemical Polishing Of Copper

Ultra-smooth and damage-free copper surfaces are extensively required in the microelectronic components and micro-electromechanical systems (MEMS) devices. As a common ultra-precision machining technology, chemical mechanical polishing (CMP) is widely used for achieving ultra-smooth and ultra-flat surface, but a damage-free copper surface is difficult to be obtained due to the mechanical force of CMP processing. Because there is no effect of the mechanical force, chemical polishing and electrochemical polishing (ECP) and other stress-free polishing method can obtain a high integrity and damage-free copper surface. However, the surface roughness by the chemical polishing method can only be reduced to submicron, and the surface flatness can not be improved. Althrough nanometer surface roughness can be achieved by the ECP, but the high precision surface planarization can not be realized. Therefore, it is necessary to develop a new non-contact and stress-free polishing method. A new polishing method, called electrogenerated chemical polishing (EGCP), is proposed based on the principle of the electrochemical oxidation and the diffusion controlled chemical reaction. Through studying the polishing mechanism of EGCP, the basic requirements are put forward, and the experimental system and the polishing solution are developed. The polishing performance of EGCP is analyzed by the numerical model and verified by experiments. Rough surface and micropatterned surface are also polished with damage-free by EGCP process test. The main contents and conclusions are as follows:1) Based on the principle of EGCP, the analytical model of EGCP process is established, and the polishing ability index? (defined as the ratio of the peak material removal rate to the valley material removal rate) is proposed. The processing efficiency and the polishing ability index of EGCP are studied. Through the analysis of processing efficiency and the polishing ability index, ultra-smooth working electrode and as small as possible control processing distance d are the keys to realize the efficient polishing. Therefore, the preparation methods of ultra-smooth working electrode are studied, and the experimental system is developed. Through the displacement feedback and visual feedback and force feedback of experimental system, the precision control of processing distance d is realized, and the Z direction control accuracy is 40 nm.2) For realizing EGCP of Cu, the FeSO4 (redox mediator), H2SO4 (pH adjusting agent) and BTA (lateral charge propagation inhibitor) are chosen as the polishing solution. The effect of temperature and the viscosity of the polishing solution are also studied, and the results show that the processing efficiency and polishing ability are decreasing with increasing viscosity. With increasing temperature, the processing efficiency will also increase, but the polishing ability change little. Through analyzing the surface components of polished workpiece by XPS and XRD, the action mechanisms of H2SO4 and BTA are studied. The results show that the H2SO4 is benefit to promoting the etching reaction between Fe3+ and Cu through removing the Cu oxide layer. BTA can be adsorbed on the Cu surface, and then suppresses surface lateral charge propagation.3) A numerical model of EGCP process is established, and the effects of the special wavelength L, the peak-to-valley height h of the workpiece surface profile and the working distance d between working electrode and workpiece surface on the polishing ability are studied. The results show that the polishing ability index is decreasing with increasing the working distance and finally approaches the value of (d+h)/d when L is much larger than d. However, the effect of the working distance on polishing ability is negligible while L is close to or less than d. Based on the numerical analysis results, an empirical equation of the polishing ability index and an forecast formula of average material removal depth of the PV value reduced to a specified value are given, and their effectiveness and feasibility are verified by experiment, and therefore the quantitative analysis of the polishing ability of EGCP is realized.4) Single crystal and polycrystal copper workpiece are polished by EGCP, and the surface roughness variations are analyzed, and the results show that EGCP have a good polishing effect for copper workpiece of two different organizational structures. Patterned copper is also polished by EGCP, and the PV value is reduced from 4.70?m to 0.059?m, and therefore planarization processing of copper can be realized by EGCP. In order to study whether EGCP method can achieve a large area polishing, electrode of 6 mm diameter is used for dynamic polishing, and electrode of 50 mm diameter is used for static and dynamic polishing. The result shows that large area workpiece can be polished by the dynamic EGCP of a small area electrode. In order to solve problems of 50 mm uniform gap control and ultra-smooth electrode preparation, a novel micro-gap control method is proposed based on the principle of the hydrostatic support. A micro-gap control apparatus is developed, and large area (50 mm diameter) workpiece polishing are realized in static and dynamic processing conditions.