Sub-100 nm Features with Single-Layer 193 nm Resist

Staff -- Semiconductor International, 4/1/1998

Linewidths of 80 nm using a 193 nm exposure tool have been achieved by a research team at the University of Texas (Austin, Texas), headed by Dr. Grant Willson. The features were generated using an alternating aperture phase shift mask produced by DuPont Photomasks (Round Rock, Texas). Phase shift masks are designed to shift the phase of the exposure light when it passes through prescribed areas, producing interference effects for improved depth of focus and resolution. Exposures were made using Integrated Solutions Inc.'s (ISI, Tewksbury, Mass.) 193 nm ArF MicroStep.

The successful introduction of 193 nm lithography requires the availability of a high-performance, single-layer resist, upon which high-volume semiconductor device manufacturing is currently based. Alternative, more complex technologies are bilayer and top surface imaging, which separate resist layers for the pattern delineation from the etch barrier. The 193 nm resist, developed by the Willson group, marks the first demonstration of a single-layer resist to produce sub-100 nm features.

This new resist consists of a polymer that incorporates chemically amplified (CA) resist technology. Chemical amplification, used to enhance photosensitivity, is based on a catalyzed system where a catalyst, in this case an acid, is photochemically generated and subsequently regenerates another molecule of acid to spur chemical reactions. The acid catalyzes a deblocking reaction, which results in a solubility differential between the protected and unprotected polymer, between the exposed and unexposed areas.

Conventional polymers used for 248 nm resists, such as styrenes, are too opaque to be used at 193 nm. Styrene-based DUV polymers have good etch resistance but lack sensitivity because the transparency is too low. In contrast, acrylates have good transparency at 193 nm but poor etch resistance.

The new polymers that have been developed allow the light to pass through the polymer matrix and reach the photosensitive compounds, yet they are etch resistant. The alicyclic polymer used by the Willson group combines the good transparency characteristics of acrylates with the etch resistance of styrenes. An acid catalyzed deblocking reaction is incorporated into the alicyclic polymers. The result is a photoresist having the sensitivity and speed comparable to that of 248 nm DUV resists. In a single-layer process, the positive-tone resist was used to produce 80 nm device features, hitting the resolution limit of the photomask.

The UT group will be investigating post-exposure delays, contamination from airborne amines and optimization of the materials formulation and process in the months to come. They are collaborating with Professor Jean Frechet's group at the University of California, Berkeley, to explore the utility of new structures that offer promise for further performance improvement.