Michael J. Leeson

Acid distribution in chemically amplified extreme ultraviolet resist

Acid generators are sensitized by secondary electrons in chemically amplified resists for ionizing radiation. As acid generators react with low-energy electrons (as low as thermal energy), this sensitization mechanism generates a significant blur and an inhomogeneous acid distribution at the image boundary, which results in line edge roughness (LER) formation. The evaluation of resolution blur intrinsic to the reaction mechanisms is important in the optimization of resist processes for extreme ultraviolet (EUV) lithography, especially from the viewpoint of LER. In this study, the authors simulated acid generation induced by EUV photons in poly(4-hydroxystyrene) with 10wt% triphenylsulfonium triflate and clarified the extent of resolution blur in latent acid images and theoretical acid generation efficiency. The average distance between the EUV absorption point and the acid generation point (resolution blur) is 6.3nm. The theoretical acid generation efficiency through the ionization path is 2.6 per EUV photon in the model system. Considering the deprotonation efficiency of polymer radical cations and the contribution of electronic excited states, the acid generation efficiency including the excitation path is 2.0–2.8 in typical resist materials with 10wt% acid generator loading.Acid generators are sensitized by secondary electrons in chemically amplified resists for ionizing radiation. As acid generators react with low-energy electrons (as low as thermal energy), this sensitization mechanism generates a significant blur and an inhomogeneous acid distribution at the image boundary, which results in line edge roughness (LER) formation. The evaluation of resolution blur intrinsic to the reaction mechanisms is important in the optimization of resist processes for extreme ultraviolet (EUV) lithography, especially from the viewpoint of LER. In this study, the authors simulated acid generation induced by EUV photons in poly(4-hydroxystyrene) with 10wt% triphenylsulfonium triflate and clarified the extent of resolution blur in latent acid images and theoretical acid generation efficiency. The average distance between the EUV absorption point and the acid generation point (resolution blur) is 6.3nm. The theoretical acid generation efficiency through the ionization path is 2.6 per EUV pho...

Polymer-Structure Dependence of Acid Generation in Chemically Amplified Extreme Ultraviolet Resists

For chemically amplified photoresists, the increase in the absorption of incident radiation by polymers leads to a decrease in the absorption by acid generators. Therefore, the absorption by polymers generally degrades resist sensitivities. However, this is not true for chemically amplified electron-beam (EB) resists because of the difference in the sensitization mechanisms of the acid generators. Whether the reaction mechanism of extreme-ultraviolet (EUV) resists is analogous to that of photoresists or EB resists is critical for material design. The difference in the reaction mechanisms characteristically appears in the polymer-structure dependence of the acid yield. In this study, we evaluated the relative dependence of the acid yield generated in polystyrene derivatives on the exposure to EUV. The dependence was similar to that of EB resists.

High transmission pellicles for extreme ultraviolet lithography reticle protection

The authors present the results of a full-field extreme ultraviolet (EUV) pellicle for reticle protection and defect mitigation. Based on novel microelectromechanical systems based fabrication, it comprises a 50 nm Si membrane attached to a wire-grid. Two types of pellicle fabrication techniques are described. The authors present the first actinic results of extreme ultraviolet lithography reticle with pellicle exposed on IMEC Advanced Demo Tool. The impact of different pellicle types on imaging is evaluated as a function of pellicle standoff distance and mesh geometry. A new prototype pellicle has been developed with a measured transmission of 82% in EUV. Actinic exposures are complemented with aerial image modeling, thermal analysis, vacuum cycling, resist outgas tests, and >5 g repeated scan cycle robustness tests.

EUVL compatible LER solutions using functional block copolymers

Directed self assembly (DSA) of block copolymers is an emerging technology for achieving sub-lithographic resolution. We investigate the directed self assembly of two systems, polystyrene-block-poly-DL-lactic acid (PS-b-PDLA) and PSb- poly(methyl methacrylate). For the PS-b-PDLA system we use an open source EUVL resist and a commerciallyavailable underlayer to prepare templates for DSA. We investigate the morphology of the phase separated domains and compare the LER of the resist and the PS-PDLA interface. For the PS-b-PMMA system we again use an open source resist, but the annealing conditions in this case require crosslinking of the resist prior to deposition of the block copolymer. For this system we also investigate the morphology of the phase separated domains and compare the LER of the resist and the PS-PMMA interface.

Patterning of tailored polycarbonate based non-chemically amplified resists using extreme ultraviolet lithography

A series of high-performance polycarbonates have been prepared with glass-transition temperatures and decomposition temperatures that are tunable by varying the repeat-unit chemical structure. Patterning of the polymers with extreme UV lithography has been achieved by taking advantage of direct photoinduced chain scission of the polymer chains, which results in a molecular-weight based solubility switch. After selective development of the irradiated regions of the polymers, feature sizes as small as 28.6 nm have been printed and the importance of resist-developer interactions for maximizing image quality has been demonstrated.

Chain scission resists for extreme ultraviolet lithography based on high performance polysulfone-containing polymers

A series of polymers with a comb architecture were prepared where the poly(olefin sulfone) backbone was designed to be highly sensitive to extreme ultraviolet (EUV) radiation, while the well-defined poly(methyl methacrylate) (PMMA) arms were incorporated with the aim of increasing structural stability. It is hypothesized that upon EUV radiation rapid degradation of the polysulfone backbone will occur leaving behind the well-defined PMMA arms. The synthesized polymers were characterised and have had their performance as chain-scission EUV photoresists evaluated. It was found that all materials possess high sensitivity towards degradation by EUV radiation (E0 in the range 4–6 mJ cm−2). Selective degradation of the poly(1-pentene sulfone) backbone relative to the PMMA arms was demonstrated by mass spectrometry headspace analysis during EUV irradiation and by grazing-angle ATR-FTIR. EUV interference patterning has shown that materials are capable of resolving 30 nm 1 : 1 line : space features. The incorporation of PMMA was found to increase the structural integrity of the patterned features. Thus, it has been shown that terpolymer materials possessing a highly sensitive poly(olefin sulfone) backbone and PMMA arms are able to provide a tuneable materials platform for chain scission EUV resists. These materials have the potential to benefit applications that require nanopattering, such as computer chip manufacture and nano-MEMS.

EUV Resist Requirements: Absorbance and Acid Yield

The challenge in obtaining good resist performance in terms of resolution, line width roughness and sensitivity at EUV wavelength forces to make more efficient use of photons that reach the wafer plane than has been the case for traditional optical lithography. Theory demonstrates that the current absorbance levels of EUV resists are quite far from optimal and absorbance should be increased. The most attractive pathway to achieve this is by increasing the fluorine content of EUV resists. The viability of this approach has been demonstrated using non-chemically amplified PMMA as model resist and comparing its photospeed with a fluorinated analogue. It has been demonstrated that the photospeed increases due to improved resist absorbance by ~1.5X, which is close to 1.7X that is predicted by the difference in absorbance. Further modeling studies support the experimental results and indicate an optimum for total film absorbance of ~0.20- 0.25. Compared to current platforms this would correspond to an increase in photospeed by ~1.7X which is accompanied with an improvement in LWR of ~1.14X. Combining this approach with the trends in EUV resists to increase PAG loading and include sensitizer in order to improve photospeed will likely provide a path for EUV resists that will meet the specifications that are required for the 32nm and 22nm node.

Development of Polymers for Non-CAR Resists for EUV Lithography

Three strategies for approaching the design and synthesis of non-chemically amplified resists (non-CARs) are presented. These are linear polycarbonates, star polyester-blk-poly(methyl methacrylate) and comb polymers with polysulfone backbones. The linear polycarbonates were designed to cleave when irradiated with 92 eV photons and high Tg alicyclic groups were incorporated into the backbone to increase Tg and etch resistance. The star block copolymers were designed to have a core that is sensitive to 92 eV photons and arms that have the potential to provide properties such as high Tg and etch resistance. Similarly the polysulfone comb polymers were designed to have an easily degradable polymer backbone and comb-arms that impart favorable physical properties. Initial patterning results are presented for a number of the systems.

Exposure dose dependence on line edge roughness of a latent image in electron beam/extreme ultraviolet lithographies studied by Monte Carlo technique

Of great importance in post-optical lithographies, such as electron beam (EB) and extreme ultraviolet, is the improvement of line edge roughness or line width roughness of patterned resists. We provide an exposure dose dependence on LER of a latent image in chemically amplified EB resist from 1 to 50 µC/cm2. By using a Monte Carlo simulation and empirical equations, the effects of exposure dose and amine concentration on LER are investigated in terms of shot noise and image contrast. We make clear the correlation between LER and the fluctuation of the initial number of acid molecules generated in resists.

The rational design of polymeric EUV resist materials by QSPR modelling

We present the initial results of the development of a qualitative structure property relationship (QSPR) model to guide in the design and synthesis of high-sensitivity, non-CAR materials for EUV lithography. The model was developed using the fragmentation data of low molecular weight species at 70 eV using a mass spectrometer (MS) with an electron ionization source as the input parameter. The preliminary model has highlighted a number of structural elements which will be important in the future design of resists, however, limitations with the current set of input data for molecules which fragment readily have been identified and these are currently being addressed. Additionally, a correlation between &ggr; (1 MeV) and EUV (92 eV) radiolysis of selected polymers has been established and it is proposed that the higher energy (1 MeV) irradiation source is a suitable model process for EUV and can, therefore, be used in the future screening of polymeric materials.