New Film Enables Gate Trimming Down to 40 nm

Photoresist trimming is an important method for shrinking linewidths without resorting to advanced lithography techniques. Although trimming does not offer increased circuit density, it's a valuable tool for narrowing gates in logic applications, for example.

But current trimming techniques have their limitations. Because of the need to avoid excessive loss of the photoresist, gates cannot be trimmed down further than 80 nm wide. Excessive trimming makes it difficult to control the trim profile for anti-reflective coatings (ARCs) and resists. And extending trimming techniques to 193 nm lithography — with its thinner, softer resists and tighter particle requirements — presents even more challenges, including more difficult trim control and the potential for resist poisoning.

Applied Materials (Santa Clara, Calif.) has come up with what it considers its next-generation lithography-enabling film, allowing lines patterned at 100 nm to be trimmed all the way down to 40 nm (Figure ). The Advanced Patterning Film (APF) is basically a strippable hard mask — an amorphous carbon/DARC (dielectric ARC) stack film that is designed to replace the spin-on ARC in typical trim procedures.

Conventional techniques use the photoresist to not only etch the ARC, but for the poly etch step as well. This process requires a relatively thick resist layer, which is what limits the trim (since trimming the width inevitably decreases resist height as well). Applied's new technique uses the photoresist to open the DARC layer, then uses the amorphous carbon as the mask for the poly etch step. Once the poly etch is complete, the amorphous carbon is easily removed with an oxygen-based resist strip process, eliminating the need for a post-hot-phosphoric acid wet clean step.

This technique requires <100 nm of photoresist to pattern a poly stack, compared with traditional approaches needing 4× that amount. This factor becomes particularly significant in 193 nm lithography, where resists are necessarily thinner because of depth of focus (DOF) issues.

Used as a strippable hard mask, the Advanced Patterning Film can trim 100 nm poly gates (top) down to 40 nm (bottom). (Source: Applied Materials)

Some key benefits of the APF technology include high etch selectivity (15:1 in the case of oxide contact etch), and good anti-reflective properties (<0.5% reflectivity), which help meet CD control requirements. Additionally, the amorphous carbon film does not contain nitrogen, making it compatible with 193 nm resists.

The main driver for this new film has been the ongoing transition to 193 nm lithography, according to Derek Witty, Applied's director of PECVD global product management. "There's a lot of customer pull to extend existing 248 nm lithography, to just go one more generation before switching to 193."

Applied already has some tools in place for poly gate trimming and oxide contact etch applications, but the company also has its eye on future applications for APF, Witty said, including ion implant mask technology, CMP stop layers and others.

For additional information on lithography, go to www.semiconductor.net/lithography.