AVS2011 Session PS-TuM: Advanced BEOL / Interconnect Etching I

Cu resistivity increase at smaller geometries and the use of lower κ dielectrics forced several changes on BEOL integration. Susceptibility of ULK material to strip damage drove the adoption of TiN hard mask integration starting with a few players at 65nm node to full adoption at 32nm for advance logic.

TiN hard mask demonstrated good selectivity during dielectric etch but concerns arise as we move to smaller pitch: 1- TixFy residues may compromise metallization of small features and 2- Residual compressive stress in the TiN may induce line wiggling. This last concern is the focus of this work.

We develop a simple analytical model based on Energy balance to predict wiggling onset based on geometrical and mechanical properties of the materials. We showed that the wiggling onset aspect ratio reduces with the pitch. We also shown that even when at the end of the process the no-wiggling condition is satisfied, wiggling might have occurred during etch causing feature distortions.

Increasingly there are more challenges of controlling the plasma processes for BEOL integration, which include the Cu/low-k interconnects, contact hole etching, etc. The presentation will give an overview of the surface reaction control during plasma processes. The main focus is damage reduction and suppression of process fluctuations.

Reducing the damage to low-k dielectrics caused by plasma exposure is one of the key issues. O₂-based plasma has been widely used for ashing the photoresist on low-k SiOCH. H₂-based plasma is employed for the etching of organic low-k film as well as the ashing of photoresist. The origin of damage generation is classified by ions, radicals, and UV/VUV radiation. It was found that not only the ions but also the synergy of radicals and UV/VUV radiation cause a significant amount of damage in the SiOCH, measured by using the pallet for plasma evaluation (PAPE).¹ Hence, precise control of incident ions, radicals, and UV/VUV radiation is required for controlling the surface reactions.

In addition to the precise control of incident species, the optimization of subsequent processes (wet treatment, annealing, etc.) are very important to obtain sufficient electrical yields and reliabilities. The remaining damaged layer after wet treatment degrades the Cu and SiOCH surface and corrupts the interface between the Cu and barrier metal. For instance, the desorbed H₂O from the SiOCH damaged layer causes the oxidation of barrier metal, which results in a shorter EM lifetime. The surface modification during SiCN and SiN etching by CH_xF_y-based plasma and its impact on the electrical yields will also be discussed.

The suppression of fluctuation is also required in the advanced interconnects. Although statistical prediction is one of the approaches to realize stable processes, there is a limitation of prediction accuracy for purely statistical predictions due to the lack of physical models. Thus, the combination of statistical and physical models for highly accurate prediction has become an emerging trend in mass production. We will demonstrate a novel statistical etch rate prediction model by considering the fluctuation caused by the plasma-wall interactions.²

The developed prediction model is one of the approaches to realize stable processes. Changes in the spatial distribution of reactive species in the etching chamber are, however, very difficult to detect in current mass production tools. Thus, greater progress of the in-situ monitoring tools and prediction methods based on the physical model (simulation) are strongly required in the near future.

1. S. Uchida et al., JAP 103, 073303 (2008).

2. M. Fukasawa et al., JJAP 48, 08HC01 (2009).

A high etch selectivity hydrogen-containing fluorocarbon gas, C5HF7, was evaluated for high aspect ratio dielectric etch. Plasma etching with Ar/C5HF7/O2 chemistry was shown to have significant advantages over Ar/C4F6/O2 in terms of oxide-to-organic mask etch selectivity and line-edge roughness. The mechanism behind the high etch selectivity of C5HF7 originated from the different thickness and composition of steady-state fluorocarbon (FC) layers generated on oxide and organic mask materials during plasma etch. We also determined that hydrogen addition to the Ar/C4F6/O2 feedgas did not reproduce C5HF7 etch behavior, presumably due to the difference in atomic hydrogen formation between molecular H2 dissociation versus intramolecular H dissociation (from C5HF7). This latter phenomenon facilitated a wider window for “etch stop” margin. Profile evaluation showed larger bowing for C5HF7, compared to C4F6, and was linked to a higher sticking coefficient of CxFyHz radicals. This was verified and remedied by increasing the substrate temperature, which reduced the radical sticking coefficient and eliminated bowing, while maintaining the high etch selectivity. We also demonstrated reduced low-frequency line-edge roughness when etching with C5HF7. We showed that less surface roughness was generated using C5HF7 since less organic mask thickness was removed. In addition, since a major contribution of line-edge roughness was the transfer of organic mask surface roughness into the sidewalls of the dielectric, overall lower line-edge roughness was observed.

As device dimensions continue to shrink, uniformity of etch rate/feature depth and critical dimension becomes very important. Capacitively coupled plasma (CCP) sources have advantage in terms of uniformity over non-planar sources in addition to design simplicity, reliability and wide process window. Wide-gap CCPs have been used for front-end etch applications where as small-gap CCPs are work-horse for back-end dielectric etch. Recently, studies on mid-gap CCPs indicates that inter-electrode spacing of ~100mm is best suited for etch rate and CD uniformity. In our studies, for tri-layer mask etch and TiN hard-mask etch, mid-gap CCP achieved uniformity of <1nm (3σ) CD and ~1% (3σ) etch rate. Mid-gap CCPs have 20-30% higher etch rate compared to wide-gap CCPs. Power on top electrode can be divided to center and edge for control of plasma density distribution. Design changes in pumping port assembly effectively create uniform confined plasma without plasma leaking through pump-port. Plasma confinement helps in creating denser plasma at relatively lower RF power. Using above-mentioned “knobs”, flat etch rates and CDs were achieved in mid-gap CCPs.

This work was performed by the Research and Development team at TEL Technology Center America in joint development with IBM Research Alliance Teams in Albany, NY 12222. This work has also been supported by the independent Bulk CMOS and SOI technology development projects at the IBM Microelectronics Div. Semiconductor Research & Development Center, Hopewell Junction, NY 12533.