Extending The Hardmask

In chip production, the backend-of-the-line (BEOL) is where the critical interconnects are formed within a device. Interconnects—those tiny wiring schemes in devices—are becoming more compact at each node.

This, in turn, is causing a degradation in performance and an increase in the resistance-capacitance (RC) delay in chips. “The scaling roadblocks that the interconnect faces need to be addressed,” said Sree Kesapragada, global product manager at Applied Materials.

In the BEOL, there are many process steps, which fall into two basic categories—patterning and the dual damascene flow. Initially, in the flow, each level of a given chip structure must be patterned to create the wiring schemes. For this, chipmakers use 193nm immersion and multiple patterning.

The patterning challenges are daunting. Today’s chips can have 20 kilometers of copper wiring in a 100 square millimeter area. On top of that, a leading-edge, 10-level metal process could have up to 10 billion vias, or vertical connections, between the various layers.

Aligning the various vias is key in the process flow. Needless to say, alignment errors in the via flow could be disastrous. The chip could simply fail.

To ensure the vias are aligned property, chipmakers for years have used a removable metal hardmask, based on a titanium nitride (TiN) material. The hardmask is devised using a traditional physical vapor deposition (PVD) process. The hardmask resembles a stencil. “The hardmask ensures perfect via alignment,” Kesapragada said. “It removes the overlay error component out of the fabrication process.”

In scaling, however, the compressive stress from traditional TiN hardmask layers can cause the narrow lines patterned in ultra low-k films to collapse. “The pattern is becoming more and more delicate,” he said.

In other words, the industry needs new breakthroughs to extend the TiN hardmask. To solve the problem, Applied has rolled out the Endura Cirrus HTX PVD, a next-generation PVD technology. The tool boasts high plasma density and ion energy control features. It also enables surface atom mobility.

All told, the tool enables TiN hardmasks to scale at 10nm and beyond. “The film stress and film density can be tuned (within the tool),” he said. “The density that you can obtain in this chamber is close to the bulk density.”

The technology is based on a VHF-based source. “There is an ionization mechanism inside the chamber,” he said. “You can get close to 100% metal ionization. We have mechanisms to control the energy of the ions.”