The effect of DI water and intermediate rinse solutions on past metal etch residue removal using semi-aqueous cleaning chemistries

As semiconductor processing complexity increases with decreasing geometries, the ability to pattern metal interconnects becomes more difficult. The photolithography, metal etch, and post etch cleaning steps each must render robust, repeatable processes in order for a manufacturing operation to be successful. The successful implementation of DUV photoresists, coupled with etching tools incorporating advanced chamber designs (i.e. HDP, TCP, ECR) allows successful patterning of metal lines associated with sub 0.25 mu design technologies. The subsequent cleaning step also demands merging consistency with process latitude in order to deliver a production worthy process. As metal etch processes have evolved, cleaning has included resist stripping, "sidewall polymer" removal, and in today's arena encompasses surface cleaning which can include mobile ion reduction and "damaged" metal removal. As devices shrink, chip architecture also poses challenges that have caused paradigm shifts in chemical cleaning. For example, it has been observed that certain devices, where tungsten plugs are exposed during the metal cleaning step, are susceptible to unconventional corrosion schemes (tungsten plug erosion) when cleaned with commonly used "sidewall polymer" removal chemistries. This type of scenario ushered in the recent popularity of fluoride based semi-aqueous cleaning (SAC) chemistries for post metal etch cleaning. Certain aspects of these chemistries ase very attractive to end users, which can include low or room temperature operation, very short process times, easily rinseable in water, and ease of disposal. As well, these chemistries have allowed manufacturers of single wafer cleaning tools an avenue to compete with batch processing equipment. These chemistries appear to be well suited to fill in the gay needed for current metal cleaning requirements, and depending on the type of dielectric used, may be applicable for dual damascene copper cleaning. There still however, seems to be a strong industry dependence on continued use of hydroxylamine-based residue removers for contact and via cleaning applications associated with tungsten plug based technologies. This study looks at the effects of cleaning metal structures with semi-aqueous cleaning (SAC) chemistries. The primary focus is on metal integrity when looking at 1) process latitude of the cleaning chemicals; 2) the effect of DI water rinsing; 3) the effect on isolated versus dense metal structures; and 4) the effect of utilizing different intermediate rinses. The study shows that several factors can impact metal integrity during these cleaning processes and attempts to partition these factors. A proposed mechanism for the cleaning and rinsing process can be seen in Figure 1. DI water rinsing can have a significant effect on metal integrity, In particular, the study shows that use of CO2 sparging in the DI water, which is commonly used to prevent galvanic corrosion after cleaning chemistries, can cause linewidth loss in metal structures when compared to DI water without CO2 As well. it is shown that buffered intermediate rinse solutions that work with hydroxylamine-based chemistries are ineffective, and in fact can enhance corrosion when used with certain semi-aqueous chemistries. The study shows the effect of pH adjustments of the intermediate rinse solutions and how a robust cleaning process can be developed for device structures that are very sensitive to the chemistry and DI rinse sequence The SEM pictures in Figures 2 through 7 detail some of the aforementioned studies. In this specific case, an extremely corrosion-sensitive isolated structure is observed for effects during the cleaning process, in which the cleaning chemistry time and temperature are kept constant. Figure 2 shows the sample after metal etch and in-situ ash. Figures 3, 4, and 5 show the effect of varying the DI water rinse time after the SAC chemistry, The SEM image in figure 6 shows the effect of a commercially available intermediate rinse (pH 4.2) which is commonly used with hydroxylamine chemistries, when used in sequence after the cleaning chemistry and before the DI water rinse. Figure 7 shows the effect of raising the pH (7.5) of the commercially available intermediate rinse. While semi-aqueous cleaning chemistries can successfully fulfill process requirements for sub 0.25 mu technologies, this study shows that it is necessary to investigate and optimize all aspects of the wet cleaning process in order to obtain the desired results.