Paul Kwok Keung Ho

Challenges of damascene etching for copper interconnect

Dual damascene patterning is essential for the integration of copper into a high performance interconnect, hence the etching process becomes the most important challenge. This paper described the work on the dual damascene etching. The three most common schemes for patterning the dual damascene structure are trench-first, via-first (also known as counter-bore) and self-aligned etchings. Although only self- aligned etching requires the insertion of a stop layer, the stop layer is crucial to all schemes for a better control of the etching uniformity. The impact of using a stop layer with every dual damascene scheme was investigated. Lithography plays an important role in damascene etching. The use of negative-tone photoresist for metal trench masking and the challenge of forming a residue-free damascene structure in the presence of a bottom anti- reflecting coating were discussed.

Analysis on the metal etch resist selectivity measurement

Absence of a good resist selectivity is a key issue in the metal etch. It becomes increasingly critical when the geometry shrinks below the sub-half micron and the resist thickness reduces further for lithography to get smaller features resolved. Measured values are often quoted based on various techniques like surface profiler or cross section analysis, etc. For a multi step etch recipe we analyzed step by step the etched photoresist cross section for feature sizes between 0.40 . . . 0.65 micrometer and bondpads by using a scanning electron microscope (SEM). We found that the real photoresist margin is independent on the feature size which we explain using a simple geometrical model. However, this value is remarkably smaller than the height of the remaining photoresist on top of the center part of a bondpad measured using a surface profiler. Subsequently, the profiler measurements result in selectivity values which are considerably higher than those measured at small features with cross section SEM analysis. We discuss advantages and limitations of profiler measurements. The comparison between the results of both methods open the possibility to utilize the advantages of surface profiling (e.g. non-destructive approach) by reducing the risk to get overestimated selectivity values for small features. This method is very useful in metal etch process development and forecasting the requirements for the future technology as well as to interpret absolute selectivity often quoted.