James Thomas Fahey

Design of a bottom antireflective layer for optical lithography

The advent of deep-UV(DUV), chemically amplified, acid catalyzed photoresists as successors to positive diazoquinones photoresists has brought about a new set of process environment concerns directed towards all materials in contact or absorbed by the photoresists. In addition to the application of DUV bottom anti-reflective coatings (BARCs) to suppress optical reflection and subsequent linewidth distortion, we must consider the properties and interaction of the BARC layer with the labile photoacid of the latent image. In this regard, we have examined the physico-chemical aspects of the DUV BARC with regards to acting as a barrier layer to substrate poisoning, and as an optical absorbing layer that does not interact and/or distort the deep-UV profile. Various single component polymeric BARCs were synthesized and examined. Considerations will be discussed of the optical absorbance, the coating quality, dry etch rate, and the impermeability of the BARC layer to photoacid diffusion to fulfill the performance requirements of BARCs for DUV lithography.

Shot-size reduction of photoresist formulations

The cost of expendable chemicals in the resist process is increasing and with this the economic impetus to conserve usage. The volume of liquid resist dispensed (shot size) determines the consumption rate and disposal volumes of liquid resist. The choice of resist solvent can influence the shot volume. Three formulation factors influence the shot size: (1) the surface tension of the resist and the interfacial energy of the coating surface, (2) the viscosity of the resist formulation, and (3) the evaporation rate of the solvent. The suitable resist formulation and subsequent solvent choice should be of the lowest surface tension and lowest viscosity and be balanced by an evaporation rate which allows a minimum shot volume to be spread on the surface without significant solvent loss. Of all the solvents examined, ethyl 3-ethoxy propionate (EEP) gave the lowest shot size relative to the old resist solvent standard of 2- ethoxy ethyl acetate (ECA).

Investigation into the origin of microbridging in chemically amplified negative-tone photoresists

Microbridge formation in a CAMN photoresist we have developed is dependent on the ratio of dose to print (DTP) to dose to gel (DTG) as well as resist contrast. Photoresists formulated with poly(p-hydroxystyrene) (PHS) have a very high tendency to form microbridges when developed in 2.38 wt% TMAH due to high contrast and high DTP/DTG ratio. When photoresists formulated from PHS were developed in 1.2 wt% TMAH contrast and DTP/DTG ratio were reduced resulting in microbridging being nearly eliminated. Using this observation we developed an I-line CAMN photoresist with PHS type thermal stability and high resolution capabilities which can be developed in industry standard 2.38 wt% TMAH.

Improved reflectivity control of APEX-E positive tone deep-UV photoresist

A study optimizing the actinic absorbance of APEX-E positive deep UV photoresist was performed using a variety of dye additives. The selection of a dye and the optimization of dye content for APEX-E positive photoresist has led to substantial process enhancements in reduction of reflective notching and of thin film interference effects. The usual side effects as found in dyed I- line resists such as significant loss of photospeed, decreased focus latitude and sidewall angle decrease were not apparent with selected conjugated aromatic dyes. The benefit of added absorbance has allowed the direct use of dyed APEX-E to counteract the step interference (notching) problems over the severe topography of CMOS gate level and eliminate the reflective notching of surface strap level in the fabrication of 16 Mb devices. In addition, the depth of focus window was enhanced and process latitude was maintained. Geometries of 250nm were printed, with dyed APEX-E for optical densities ranging from 0.4 to 0.8 per micron with a DUV optical scanner.