Novel concept for extreme-UV photoresist materials

For several years, extreme-UV (EUV) lithography—i.e., at a wavelength of 13.5nm—has been talked about as the next enabling technology for lithographic patterning. However, a number of technological stumbling blocks (i.e., issues with EUV optics, photomask infrastructures, and photoresist materials) have delayed the widespread introduction and implementation of this technique. For instance, the scanner optics that are used in patterning systems and photomasks have been changed from transmissive optics to reflective optics. This change has proven to be a rather challenging transition, but tremendous progress has now been made and EUV scanner shipments are taking place at an accelerated pace. EUV pellicle development is also progressing (a mitigation step necessary to address defect concerns), and mask infrastructures are being developed at both merchant and in-house mask shops. To meet the requirements for new EUV-suitable photoresist materials, photoresist manufacturers originally reformulated extant 193nm resist systems—via the use of formulation adjustments, additives, and photoacid generator (PAG) loading—for EUV use. Although this is a cost-effective approach, it brings line width roughness (LWR), sensitivity, and resolution limitations. LWR is defined by the random fluctuations in the width of a patterned lithographic feature along its length. As photoresists are used to print smaller and smaller patterns, the imperfections in the sidewall become a larger part of the patterning error. Moreover, in several previous studies, these high LWR values have been attributed to the use of polymers for the photoresist matrix. Other contributing factors to the LWR values are shot noise (e.g., flux variations, which are increasingly important because the dose per photon increases substantially in the EUV regime), PAG location in the bulk film (relative to the acid-sensitive protecting group), acid diffusion (or blur) during the chemical amplification process, and the level of developer selectivity. Figure 1. Schematic representation of (a) the traditional chemical amplification approach used for 193 and 248nm photoresist extension materials in extreme-UV patterning. The multitrigger concept for (b) a high-dose area and (c) a low-dose area is also illustrated.