Galvanic corrosion during wet processing of BEOL nano-interconnect structures

Over the past decades, the semiconductor industry managed to continuously increase performance at an impressive steady rate by scaling the dimensions of all parts of the integrated devices to nanometer-size dimensions, thereby increasing the number of transistors on the chip as well as decreasing the size of metallization lines enormously.  For next-generation advanced interconnects, the material properties of Cu render it unreliable for producing ever more performant chips. Recently, various alternative metals, such as ruthenium, rhodium, cobalt and molybdenum, have been studied and start to be implemented by the industry to tackle the resistive losses related to the small dimensions of the nano-interconnect structures. To further lower the resistivity losses at even more aggressively scaled dimensions, metal alloys are currently being explored.

For the fabrication of metal nanolines, patterning of metal layers is a critical step. Various nanofabrication techniques have been investigated of which the process of dry etching has proven to be very successful. However, dry etching results often in the build-up of nonvolatile etching products that remain on the side walls of the metal lines or at the bottom of the etched nanotrenches. A wet-chemical processing step is performed to clean up these etch residues. As multiple metals are present in the metallization stack, wet-chemical cleaning can result in local damage because of galvanic effects. When metals are brought into contact, in this case in the device stack, potential differences between the layers can drive redox reactions through the principle of the galvanic cell. The resulting local flow of electrons induces undesired etching reactions at specific locations in the advanced metallization pattern. In the proposed PhD work, the electrochemistry of metals and metal alloys will be investigated as a method to obtain insight into corrosion reactions that can occur between metal layers of different work function. The target materials will be at start pure elements (e.g. Cu, Ru, Co, Mo, and W) and will be extended to binary and ternary metal alloys. Of special interest is the role of acid, redox species, chelating and passivating agents during metal corrosion. Basic insights on the surface chemistry of metal oxide formation and dissolution will be used in a proof-of-concept study using advanced nano interconnect device structures. The learning of this PhD work will provide a basis for the development of selective wet cleaning processes for BEOL processing.

As a PhD student, you will learn to work in a highly dynamic and multicultural environment and be exposed to a large variety of analytical techniques and experimental methods. Electrochemical measurements combined with inductively coupled plasma mass spectrometry (ICP-MS) will be used to study the metal/electrolyte interface in the parameter space. The basic insights obtained on atomic-scale oxidation/dissolution kinetics allow to predict galvanic effects between metals in a stack and corresponding etching selectivity. Surface chemistry will be studied both ex situ and post operando by x-ray photoelectron spectroscopy (XPS) and high-resolution synchrotron radiation photoemission spectroscopy (SRPES). Other complementary physical characterization techniques like elastic recoil detection analysis (ERDA), electrical measurements, atomic force microscopy (AFM), scanning and transmission electron microscopy (SEM, TEM) are available to support your mechanistic studies on galvanic etching reactions.