Innovative Process Control Technology to Meet the Challenges of Today’s PWB Electrolytic Copper Plating Requirements

摘要

To meet the challenges of increasing PWB complexity, an improvement is needed beyond the traditional electrolyticcopper plating process. Specifically all the components of the process including the process chemistries, specificchemical performance and the chosen process control tools must work together in a synergistic manner. Theoptimization of these parameters which include the specific control of all chemical components of the copperadditive systems is what will provide the PWB fabricator with reliable and repeatable improvements in surfacedistribution and plated through hole throwing power. These improvements are especially critical for large panelswith high hole counts (30,000+) and in thick panels (over 0.250 inch thick) with aspect ratios of 12:1 or higher. Thebaseline chemistry performance is only one of the critical aspect to achieving these improvements in distribution andthrowing power on a sustained basis. It has been observed that such improvements can be gained initially from manyof the new electrolytic copper additive systems, especially those employed in the pulse periodic reverse platingsystems. However, it has also been observed that over time, with the wide range of applied current densities and incombinations with the various pulse periodic reverse plating cycles that may be utilized to meet specific PWBdesigns, the performance of many of these chemistries begin to deteriorate. Through improvements in processcontrol of the individual components of the additive system a significant sustained improvement in overallperformance and plating consistency can be achieved. Utilizing real time analysis methods that are routinelyemployed in semiconductor copper plating to maintain optimum performance throughout the bath life, processconsistency can be achieved in the PWB fabrication facility as well. These semiconductor process control tools havebeen successfully adapted and installed into the PWB environment and are used to monitor and control theelectrolytic copper process to insure predictability and consistency of chemical performance. Utilizing a DC- andAC-voltammetric technique in an in-situ, on-line monitoring system, the Real Time Analyzer (RTA) provides a fullchemical analysis of all the components of the copper plating solution. This tool is comprised of an in-tank probepositioned in the plating solution which samples and reports all inorganic and organic components of the platingbath. Using computerized instrumentation the tool requires no calibration, and minimizes the need for manualanalysis in the analytical laboratory. The technique measures potential (voltage) which is varied in a manner, suchthat the electroactive species is forced to be reduced (plated) or oxidized (stripped off) at the electrode. The resultantcurrent is proportional to the concentration of the chemical species in solution. The range of potential applications ofvoltammetry is very wide. Electroanalytical techniques are among the very few techniques that are equally suitablefor analyzing inorganic, organic and organometallic compounds over a very broad range of concentration levels. Thepower of electroanalytical techniques (AC and DC) allows for a deep look into the mechanism of an electrochemicalprocess that may include initial stages of nucleation, crystal growth, chemical reactions coupled to electrochemicalchanges, diffusion, anodic dissolution, and so on. The effect of organic additives on these processes can bequantified and correlated with their concentration.The RTA uses algorithms for both modeling and analysis of voltammetric responses (chemometric approach).Design of experiments (DOE) techniques are used to optimize experimental conditions for voltammetricmeasurement. Calibration procedures include training set design, determination of the optimal-for-analysis portionof DC/AC-signal, outlier detection, regression calculation (PCR - Principal Component Regression), cross- andexternal validation.