Bilayer Resists Overcome Outgassing

As the industry reaches the 90 nm technology node and below, the smaller features will require increasingly thin resists. The approach of such ultrathin resists is in turn making lithographers reconsider the use of multilayer schemes, which they've previously shied away from largely because of complexity and outgassing concerns.

Photoresist outgassing is a serious concern because the contamination on the exposure lens can cause transmission loss and distorted images. Outgassing has already been an issue related to 248 and 193 nm lithography, and is an even bigger concern for 157 nm lithography, where the exposure energy is much higher, therefore presenting a greater potential for contamination problems. A bilayer resist approach, which makes use of a thin silicon top layer, has the potential to outgas silicon — a major optics contamination issue.

At SPIE Microlithography 2001, Stefan Hien presented results of outgassing studies from International SEMATECH (Austin, Texas) and Massachusetts Institute of Technology's Lincoln Laboratory (Lexington, Mass.) that noted the location of the silicon atom in the polymer as critical in resist development. This year's Microlithography conference included presentations from resist developers demonstrating silsesquioxane (SSQ) polymers, in which the silicon is incorporated into the polymer backbone, as a means to minimize silicon outgassing. Shipley Co. LLC (Marlborough, Mass.), in a paper titled, "Bilayer Technology for ArF and F₂ Lithography: The Development of Resists to Minimize Si Outgassing," presented its SSQ-based bilayer scheme, showing that some SSQ species showed no detectable silicon outgassing.

The bilayer resist, which has been demonstrated for KrF (248 nm) lithography, as well as ArF (193 nm) and F₂ (157 nm) techniques, includes a thin silicon top layer (1000-2000 Å) over an etch-resistant underlayer (3000-6000 Å). The pattern from the silicon imaging layer is transferred to the underlayer with a plasma etch. The scheme offers some benefits that are becoming more important with successive chip generations. The ultrathin top resist enables improved resolution, a larger process window, and reduced pattern collapse; and the underlayer offers improved etch resistance, reflection control and planarization, noted George Barclay, a principal scientist with Shipley.

The underlayer can be tailored for a variety of applications, including dual damascene, which presents several challenges for single-layer resists, such as fencing, complex topography, and resist poisoning from low-k dielectrics. The standard attributes of the underlayer come in handy in dual-damascene processing, as well as its use as a barrier to resist poisoning, and its uniform via fill capability. Together, the underlayer's characteristics serve to better control the etch process.

Without denying its benefits, the industry has nonetheless been reluctant to risk the outgassing commonly associated with the bilayer approach. In his presentation, however, Barclay pointed out key advantages of the SSQ polymer that result in minimal outgassing: The silicon is integrated with the polymer's backbone structure, there are no pendant silicon groups, and it's a very stable polymer. SSQ also provides a high silicon content for improved etch. Shipley's studies — which used a system developed to measure the amount of volatile compounds released when exposing a material to laser radiation — detected no silicon contamination. This is in contrast to side chain silicon polymers and siloxane polymers, which did exhibit outgassing.

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