AVS2012 Session PS-WeM: Advanced BEOL/Interconnect Etching

Back-End-of-Line (BEOL) etch processes have undergone a significant transition in recent years as the requirements of advanced technology nodes have shifted the advantage from via-first-trench-last (VFTL) to trench-first-metal-hard-mask (TFMHM) dual damascene patterning schemes throughout the industry. This shift has been primarily driven by the TFMHM scheme’s ability to enable self-aligned vias (SAV), as well as inherently be more multi-patterning friendly through the use of a dissimilar hard mask material. However, typically key advances don’t come without other complexities, and TFMHM is no exception. The role and impact that the metal hard mask material has on all of the etches where it is involved is significant and sometimes very subtle. At a high level, the metal hard mask influences can be split into two areas: patterning of the MHM itself, and etches where the material acts as the hard mask. The patterning of the MHM itself is where all of the challenges associated with multi-patterning come into play. The MHM profile largely determines the overall critical dimension behavior across all patterns, and so pattern control and fidelity in both x and y dimensions are critical. The material properties of the MHM play a significant role in etch profile control and line edge/width roughness (LER/LWR). When considered as a hard mask in the etch, the MHM impacts are far-reaching. First, the selectivity of the MHM material vs. the underlying films must be considered. This is a key focus of etch process optimization when implementing an SAV patterning scheme and often drives all facets of the etch process. Second, the very properties that make it attractive as a hard mask (etch resistivity) can also make it the source of many other issues, typically due to the non-volatile nature of most MHM materials (Ti, TiN, TaN, etc.). Some examples include metal-containing sidewall and/or via hole residues, temperature sensitive profiles, ability to shape the MHM to help metallization, and chamber contamination effects (i.e., non-repeatability or slot-order effects). The MHM material properties have a large influence on these etches, as well, with tradeoffs between etch resistivity (good for SAV) and metal material stress (bad for pattern integrity --> can induce line wiggling). This talk will review some of the benefits of utilizing a metal hard mask, as well as some of the challenges that come hand-in-hand with it and how plasma etch process optimization is advancing in learning how to work with these materials.

This work was performed by the Research Alliance Teams at various IBM Research and Development Facilities.

The advanced microelectronic architecture utilizes more conductive Cu interconnect insulated by porous low-k interlayer dieelctrics (ILD) to minimize the overall RC delay. To decrease the dielectric constant, the porosity of organosilicate glass was increased by partial replacing Si-O with Si-C bonds during plasma-enhanced chemical vapor deposition. However, carbon stripping by subsequent plasma etching/patterning processes can make porous low-k interlayer dielectrics, with higher carbon content, more prone to water damage during subsequent wet cleans processes. In this presentation, we report the development of a new characterization approach that utilizes Multiple Internal Reflection Infrared Spectroscopy (MIR-IR) as a sensitive characterization tool to guide the development of cleans-friendly plasma etches with minimal ILD damages. Previously, we have successfully utilized MIR-IR to characterize chemical bonding of hydrogen termination, trace organic adsorption and plasma deposited polymer thin film on silicon wafer surface with sub-monolayer sensitivity. MIR-IR utilizes the silicon wafer (including patterned ILD wafer) itself as an IR waveguide to enable multiple total internal reflections which greatly enhances IR measuring sensitivity. Therefore, MIR-IR possesses a unique capacity of identifying specific functional groups of deposited thin films (sub 10 nm) and monitoring their corresponding reactivity evolution influenced by plasma process. We will discuss new characterization results of post-etch residues formation (fluoro-polymers), plasma etch induced water damage (silanols formation) and oxygen plasma etch of organic thin film on patterned porous low-k nanostructure using MIR-IR spectroscopy, x-ray photoelectron spectroscopy and other surface characterization techniques.

As feature sizes continue to decrease, significant issues are found using high selectivity fluorocarbon gases to etch interconnect low-k dielectrics including low selectivity to organic masks, line wiggling and low-k damage. We have evaluated novel etch gases that enables the optimized fabrication of BEOL interconnects for 14nm and 22nm nodes. Compared to conventional fluorocarbon etch gases, experiments with a novel hydrofluorocarbon etch gas have shown a substantial increase in low-k dielectric etch selectivity to organic hard mask and improved selectivity to the capping layer. In addition, lower line-edge and line-width roughness values were observed. We investigate the etch behavior and selectivity between the low-k dielectric and organic hard mask, metal hard mask, and capping layer through optical emission spectroscopy and x-ray photoelectron spectroscopy. Etch performance is assessed for trench and via patterns and also full dual-damascene structures. Low-k damage is also investigated by post-etch HF treatment to measure critical dimension loss from dissolution of plasma-damaged dielectric. Finally, we propose a mechanism for high selectivity, low damage dielectric etch at sub-80nm pitch structures using rationally-designed novel etch gases.