Post-RIE BEOL Cleaning Gains Attention
Cleaning leading-edge copper interconnect layers has become a major challenge, particularly as cobalt-based via capping layers are introduced. At the Sematech Surface Preparation and Cleaning Conference (SPCC), researchers from several companies presented new BEOL cleaning techniques that are tolerated by 32 nm copper interconnects and CoWP-based caps.
David Lammers, News Editor -- Semiconductor International, 4/8/2009
The challenge of cleaning copper interconnect layers with cobalt-based capping has attracted R&D attention from the leading suppliers, creating a face-off session on BEOL cleaning techniques at the Sematech Surface Preparation and Cleaning Conference (SPCC), held in Austin, Texas, in late March.
Cobalt-based via caps are extremely sensitive to galvanic corrosion. Minimizing low-k, copper and capping material losses from cleaning steps after reactive ion etch (RIE) is a focus of the International Technology Roadmap for Semiconductors (ITRS) at the 32 nm and beyond. The ITRS is calling for copper/low-k post-etch cleans to have &2.5% effect on the low-k value, and &1.5% effect on critical dimensions.
Traditional fluoride strip steps work well with the traditional via chain, according to Aiping Wu, research manager at Air Products and Chemicals Inc. (Allentown, Pa.). However, when a cobalt/tungsten/phosphorus (CoWP) metal cap is added, fluoride-based compounds no longer are compatible, she said.
“Traditional fluoride strippers severely damage the CoWP alloy,” Wu said during her SPCC presentation. That is leading Air Products and other chemistry suppliers to offer new and proprietary formulations, which none of the presenters during the session identified in detail. Working with GlobalFoundries, the former Advanced Micro Devices (AMD) fab in Dresden, Germany, Air Products studied a solvent that Wu said minimizes CoWP undercut, and is compatible with porous low-k dielectrics.
“By changing the solvent concentration, we can minimize CoWP undercut,” Wu said. “We found that if the galvanic current density is too high, that leads to severe CoWP damage. If the galvanic current density is too low, it prohibits cleaning.” The company has come up with a developmental formulation that provides good cleaning efficiency on patterned wafers both with and without CoWP capping layers.
Jeff Lauerhaas, an R&D manager at FSI International (Chaska, Minn.), presented a hardware-based solution, a specially designed chamber and inline monitoring system that keeps the levels of oxygen in hydrofluoride (HF) cleaning agents down to extremely low, parts-per-billion (ppb) levels.
At the 32 nm node, companies are likely to cap copper interconnects with self-aligned barriers of CoWP to improve reliability. However, these more complex structures make it difficult to use HF for post-etch cleaning, due to copper loss and the galvanic corrosion that occurs in the less-noble cobalt-based capping layer.
“If you use HF as it is used now, it is easy to lose the cobalt layer, due to the presence of oxygen,” Lauerhaas said. FSI developed a chamber that limits oxygen to &100 ppb, as measured by an inline monitor, in a dilute HF solution free from corrosion inhibitors and oxygen scavengers. Removing the oxygen limits copper corrosion, and eliminates galvanic corrosion at the copper/CoWP interface. With oxygen control, copper loss was held to &0.4 Å/min, compared with a copper loss of >18 Å/min without oxygen control, Lauerhaas said.
FSI also developed a spray bar atomizer that dispenses the solution with better results than a fixed nozzle approach. The chamber is part of a single-wafer tool FSI is testing now, he added.
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| BASF chemists have developed a new post-etch residue removal (PERR) chemistry. |
BASF (Ludwigshafen, Germany) and IMEC (Leuven, Belgium) researchers teamed up to develop and test all-wet post-etch residue remover (PERR) chemistries. Companies traditionally have used both a resist stripper to clear away resist, and a PERR to remove post-plasma etch residues, which occur after plasma etching and the subsequent ashing process, said Andreas Klipp, an R&D manager at BASF Electronic Materials.
BASF developed an all-wet PERR to clean residues that occur after plasma etching, without requiring an ashing process. The formulation also works to clean the antireflective coating (ARC) layers.
“The ARC layers are not easy to clean,” Klipp said. The solvent is compatible with both copper and the second-generation Black Diamond (BDII) low-k dielectric from Applied Materials Inc. “We found that the solvent can go into the pores of the low-k, but it recovers its k-value after the cleaning step is completed.”
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