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Optimization of Cupric Chloride Subtractive Etching for Cu High Density Interconnects
Keywords: Cu Etching , Cu High Density Interconnect , Printed Circuit Boards
Integrated circuits (ICs) play an important role in the microelectronics industry. The past several decades have demonstrated substantial improvements in IC manufacturing process with a significant reduction in the IC feature sizes (feature scaling) resulting in reduced cost and improved functionality. Feature scaling is important in all stages of IC manufacturing. In the manufacturing of printed circuit boards (PCBs) substrates, one area where scaling has become critical recently is the core patterning step for fabrication of Cu high-density interconnect (HDI). The subtractive etching of the copper foils to form HDI for PCBs can be accomplished though several means, including etchants of ferric chloride [1] and ammonium chloride [2]. The most common method, however, is to use highly concentrated (2-2.5 M) CuCl2 in hydrochloric acid [3]. With acidic cupric chloride, the fundamental reaction of the Cu etching process generates Cu(I) ions can be described as : Cu2+ + Cu → 2Cu+. The primary advantage of CuCl2 as an etchant is that resulting Cu(I) can be oxidized back to Cu(II) via H2O2 addition, among other methods, thus regenerating the post-etch solution so that it can be reused. The elimination of large quantities of waste and improvements in efficiency via this method is of great importance to the microelectronics industry, which contributes to the common use of CuCl2 as an Cu etchant. The ever tighten design rule of Cu HDI requires stringent control of undercut during etching process to achieve the desirable etch factor. As etch features getting smaller and closer, the usual artwork compensation of undercut by increasing the widths of resist to improve etch factor is no longer possible. Also, the gain in undercut performance is usually offset by undesirable Cu etch rate decreases. Furthermore, precise control of the Cu etch rate of a given cupric chloride solution can prove difficult even under the stringent engineering controls of common parameters like 1) Specific gravity, 2) Oxidation-Reduction potential, and 3) Conductivity. We previously reported that these semi-chemical indicators are ineffective of monitoring the actual dynamic equilibrium between multiple chemical processes occurred simultaneously during etching and regeneration [4]. As such, precise Cu etch rate control cannot be achieved. In this paper, we examined the sources of difficulties on each common parameter in predicting the Cu etch rate and etch factor in various cupric chloride solutions. We developed new metrology including sapphire based Attenuated Total Reflection (ATR) UV-Vis spectroscopy to study the Cu etching chemical equilibrium by exploring the roles of copper oxidation states, mixed-valence copper complexes, and chloride coordination to the overall etching system. Our data indicated that the mechanism of acidic cupric chloride etching, regeneration and recovery is complex, and illustrated why the current monitoring strategies have difficulty controlling the complex interlocking chemical equilibria. Our UV-Vis characterization metrology also revealed various underlying mechanistic reasons for Cu etch bath behaviors and demonstrated the important roles of H+ and Cl- to the etch bath while also providing a means to monitor the Cl-. With this more comprehensive understanding of the complex changes in the multiple chemical equilibrium for cupric chloride etch system, we anticipate that better monitoring and further improvement of the HDI PCBs fabrication process can be achieved. .
Oliver Chyan, Professor
University of North Texas
Denton, Texas

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