Asia/Pacific

03/01/1998

A viable resist for <100nm design rules is available in time for pilot line development of 193nm lithography. This resist has been optimized for specific device layers by engineering its organic chemistry. Practical application has produced 60nm features using 193nm exposure with phase shift masks and a unique antireflective coating technology that tailors thin-film optical properties.

Asia/Pacific

Historically, problems associated with chemically amplified (CA) resist systems, which were developed for 248nm DUV lithography, were not apparent until they were introduced into pilot line manufacturing [1]. The very high catalytic chain lengths in CA resists, which provided high sensitivity, were also problematic because the de-activation of generated photoacid molecules by airborne adventitious amines and wafer-surface effects resulted in "T-tops" and "scum" that reduced process margin and reproducibility.

China may be moving ahead on its promise to eliminate tariffs on information technology products, according to Japan`s Nihon Keizai Shimbun newspaper. Government sources indicated that the measures, when announced, may be retroactive to January 1. During a visit to the US late last year, China president Jiang Zemin said China would eliminate the tariffs, which run as high as 35% on semiconductor capital equipment. Questions about when and how extensive the tariff cuts would be were left unanswered.

High deprotective turnover rates also increased process sensitivity to resist-processing bake cycles, necessitating extremely tight control of bake temperatures and times. Systematic experiments traced these anomalies to amine concentrations as small as 5 ppb (T-tops) and to titanium and silicon nitride layers (scum). Collaborative efforts between resist chemists and process engineers led to a new generation of innovative resist materials.

Under a proposed joint venture agreement between DuPont Photomasks and United Microelectronics Corp., Taiwan may be home to a new $75 million photomask manufacturing operation. The two companies have signed a memorandum of understanding to establish the joint venture, DuPont Photomasks Taiwan Ltd., and plan to build a new photomask manufacturing facility there in the next four years. The joint venture will also manage and expand UMC`s existing photomask facility at its wafer foundries in Hsinchu.

The significant lesson is that CA principles of designing resists with low deprotective turnover rates (100-200 deprotection events/acid molecule) using partially protected poly(vinyl)phenol and weaker-acid-generating photoacid generators (PAGs), and buffering the resist with amines, have all positively impacted subsequent design of 193nm resists. This has helped reduce new-resist implementation cycle times from eight to 10 years, as was the case for 248nm DUV resists, to three to four years for 193nm lithography, which is the emerging solution for 150nm and 130nm design rules [2, 3] (Fig. 1a).

Sensitivity, contrast, etch resistance, and purity are ubiquitous to all resists (Table 1). The opacity of traditional UV and DUV organic matrix resins, photoresists, and photoresist components at 193nm, however, have necessitated a shift in resist materials design. With 193nm resists, the challenge has been to design a chemical system largely based on aliphatic components (polymers and dissolution inhibitors), but functionally identical to DUV and i- or g-line resists built on poly(4-hydroxy styrene) and novolac resins.

Sensitivity for 193nm resists can be readily dealt with by adopting CA-resist design principles. Contrast - the rate at which a given material responds to incident radiation - must be high to ensure high resolution patterning and is inherent to CA resists. Absorption, plasma etch resistance, and base solubility that were inherent to phenolic resins, however, have had to be intentionally engineered into aliphatic resins for use at 193nm because phenolic resins are simply too opaque for use in single-layer 193nm resist schemes. In addition, the use of polymers that contain any aromatic groups or ethylenic functionalities is also precluded because of intense absorption at 193nm.

For a material to be implemented into device fabrication, it must effectively withstand plasma and reactive-ion-etching environments. This has conventionally been achieved using aromatic moieties in the resist polymer. It has been proposed that a key factor affecting etch resistance is the "effective carbon content" of a material [4, 5]. Thus, it was predicted and later shown to a first approximation that high-alicyclic-content polymers should approach the performance of aromatic resins in plasma environments. The etch stability issue is further compounded by aspect ratio considerations as surface tension effects during development and the inherent poor mechanical stability of the polymer predicate usable film thickness (o4:1 aspect ratio). This translates to a resist thickness limit of 0.6 and 0.42?m for 150nm and 130nm design rules, thicknesses significantly below those used in manufacturing today.

Base solubility in a majority of matrix polymers used for lithography is due to either phenolic hydroxides or carboxylic (aliphatic or aromatic) acid moieties. Much of the initial effort in designing 193nm resists was focused on derivatized acrylate and methacrylate copolymers. While resists based on acrylates, methacrylates or derivatized acrylates and methacrylates with large alicyclic groups such as menthyl, adamantyl, isobornyl and tricyclodecyl in the side chain have demonstrated transparency, high resolution, and photospeed in select systems, they lacked aqueous base solubility [6]. Acrylates with low structural carbon, on the other hand, exhibited poor etch stability, incompatibility with industry standard 0.262N TMAH (tetramethyl ammonium hydroxide) developer, and high sensitivity to e-beam radiation that made SEM metrology unreliable.

Alicyclic polymers as matrix resins

Such materials are readily soluble in standard organic solvents used to spin-coat resist films, which are compatible with aqueous base media such as 0.262N TMAH, the developer of choice in semiconductor manufacturing. Aqueous base solubility and imaging functionalities could be introduced into the polymer by addition of acrylic acid or acrylates into the polymer backbone or by the use of appropriately substituted cyclo-olefins or maleic anhydride monomers (Fig. S1). The absorbance of these polymers typically ranges between 0.2 and 0.3AU/?m. The optimization of lithographic properties of these polymers is illustrated here (Fig. S1) using norbornene-maleic anhydride-acrylic acid-t-butyl acrylate copolymers as representative materials.

Dissolution inhibitor design

Multifunctional diazonaphthquinones (DNQs) have been shown to enhance dramatically the dissolution selectivity in i- and g-line resists by inhibiting dissolution in unexposed regions, while enhancing it in exposed regions [9]. The multifunctional nature of dissolution inhibitors (DIs) allows for strong interaction between polymer and DI, and imparts nonlinear dissolution behavior to the resist, contributing to high contrast and resolution.