Edge Treatment Important for Hard Masks

Hard masks are becoming more commonplace in today's fabs because of the use of thinner photoresist for advanced lithography and the need to maintain etch selectivity. A new process was designed to specifically deliver organic chemical vapor deposition (CVD) hard masks for front-end applications. One of the differentiating features of the system is an additional edge bead removal (EBR) process to reduce particle defects and improve die-edge yield. “That's the key reason we are seeing improved yield during customer demos,” said Julian Hsieh, senior director of product management for the dielectrics business group at Novellus Systems (San Jose). Novellus offers the ashable hard mask on the Vector Express plasma-enhanced chemical vapor deposition (PECVD) platform, which was introduced in March of this year.

Hsieh explained the advantages of using an amorphous carbon hard mask: “An ashable hard mask allows customers to utilize 193 nm litho better, because depth of focus is not that great, so the photoresist can not be made thicker,” he said. “In order to etch finer geometries with a high aspect ratio, photoresist is also not a good solution because of sidewall striation and low etch selectivity.”

The amorphous carbon flow proceeds in a similar manner to other hard mask processes that might use oxide, nitride or TiN films. The hard mask is deposited — followed by conventional lithography with a photoresist — the pattern is transferred into the hard mask, and the hard mask is used to perform the substrate etch. Then the mask is stripped away using atomic oxygen generated by remote plasma.

Novellus expects the market for PECVD ashable hard mask processes to grow beyond $400M by 2011. The key market right now is DRAM, with associated gate, shallow trench isolation (STI), bit-line contact and other front-end layers. Novellus estimates four layers using the process at the 80 nm node, six at the 60 nm node and eight at the 40 nm node. Hsieh expects logic manufacturers to be slower to adopt the new process, because the front-end topography is more planar. The company expects one layer at the 65 nm node and three at the 45 nm node. Adoption for flash should be somewhere in between.

In designing the process, Hsieh said that a key problem was achieving the needed selectivity while maintaining transparency. In particular, the film needs to be transparent to the 633 nm light used for alignment. “All these films will lose sufficient transparency if they are very thick. So, in the past, the process would have to be detuned to allow transparency to improve, which is a compromise.” Selectivities of 20:1 have been achieved. To improve selectivity and transparency simultaneously, the company uses a different process regime chemistry and plasma confinement method. “Because the carbon/hydrogen plasma is easy to form, problems of deposition on the chamber walls and other hardware can be problems,” Hsieh said. Typically, a gate might be patterned with a 2000 Å amorphous carbon film, while a deep trench capacitor contact might require an 8000 Å film. The company is claiming 2× better selectivity with high-aspect-ratio features relative to the industry's benchmark process provided by Applied Materials (Santa Clara, Calif.).

To optimize the process out to the wafer's edge, the tool performs the bevel clean in a loadlock chamber after deposition and before going back into the FOUP. Rather than using an exclusion ring to prevent precursors from coming to the wafer's edge and bevel, this system uses an oxygen generator for the plasma, which flows over a shield to selectively remove the bevel/edge film (Figure ). The concentric strip process allows repeatable 2 mm edge exclusion control performance. A larger chamber also reduces sidewall deposition. Pinhole-free films, also important for high yield, are partially a result of the company's multistage sequential platform.

In a loadlock chamber integrated on the PECVD platform (left), a selective strip of the hydro­carbon film is performed using atomic oxygen that flows over a shield (right). Advantages include lower defectivity and process flexibility. (Source: Novellus Systems)

Novellus's Vector Express PECVD platform, where the ashable hard mask process is released on, performs wafer heat-up independent from the film deposition to reduce cycle time and provide a more consistent wafer temperature during thin-film deposition. The estimated throughput gain with this approach is 40%.

Novellus currently has more than 20 Vector Express systems installed worldwide, and expects the installed base number growing to 50 by the end of 3Q07. The company is entering the organic hard mask market late, considering Applied Materials introduced its advanced patterning film (APF) back in 2002.¹