Photolithographic patterning of organic materials and plasma-based transfer of photoresist patterns into other materials have been remarkably successful in enabling the production of nanometer scale devices in various industries. These processes involve exposure of highly sensitive polymeric nanostructures to energetic particle fluxes that can greatly alter surface and near-surface properties of polymers. The extension of lithographic approaches to nanoscale technology also increasingly involves organic mask patterns produced using soft lithography, block copolymer self-assembly, and extreme ultraviolet lithographic techniques. In each case, an organic film-based image is produced, which is subsequently transferred by plasma etching techniques into underlying films/substrates to produce nanoscale materials templates. The demand for nanometer scale resolution of image transfer protocols requires understanding and control of plasma/organic mask interactions to a degree that has not been achieved. For manufacturing of below 30 nm scale devices, controlling introduction of surface and line edge roughness in organic mask features has become a key challenge. In this article, the authors examine published observations and the scientific understanding that is available in the literature, on factors that control etching resistance and stability of resist templates in plasma etching environments. The survey of the available literature highlights that while overall resist composition can provide a first estimate of etching resistance in a plasmaetch environment, the molecular structure for the resist polymer plays a critical role in changes of the morphology of resist patterns, i.e., introduction of surface roughness. Our own recent results are consistent with literature data that transfer of resist surface roughness into the resist sidewalls followed by roughness extension into feature sidewalls during plasmaetch is a formation mechanism of rough sidewalls. The authors next summarize the results of studies on chemical and morphological changes induced in selected model polymers and advanced photoresistmaterials as a result of interaction with fluorocarbon/Ar plasma, and combinations of energetic ion beam/vacuum ultraviolet (UV) irradiation in an ultrahigh vacuum system, which are aimed at the fundamental origins of polymer surfaceroughness, and on establishing the respective roles of (a) polymer structure/chemistry and (b) plasma-process parameters on the consequences of the plasma-polymer interactions. Plasma induced resist polymer modifications include formation of a thin dense graphitic layer at the polymer surface due to ion bombardment and deeper-lying modifications produced by plasma-generated vacuum ultraviolet (VUV) irradiation. The relative importance of the latter depends strongly on initial polymer structure, whereas the ion bombardment induced modified layers are similar for various hydrocarbon polymers. The formation of surface roughness is found to be highly polymer structure specific. Beam studies have revealed a strong ion/UV synergistic effect where the polymer modifications introduced at various depths by ions or ultraviolet/UV photons can interact. A possible fundamental mechanism of initial plasma-induced polymersurface roughness formation has been proposed by Bruce et al. [J. Appl. Phys.107, 084310 (2010)]. In their work, they measured properties of the ion-modified surface layer formed on polystyrene (PS)polymer surfaces, and by considering the properties of the undamaged PS underlayer, they were able to evaluate the stressed bilayer using elastic buckling theory. Their approach was remarkably successful in reproducing the wavelength and amplitude of measuredsurface roughness introduced for various ion bombardment conditions, and other variations of experimental parameters. Polymer material-dependent VUV modifications introduced to a depth of about 100 nm can either soften (scission) or stiffen (cross-linking) this region, which produce enhanced or reduced surface roughness.
The authors thank S. Engelmann, R. L. Bruce, F. Weilnboeck, T. Kwon, T.C. Lin, H.-C. Kan, X. Hua, L. Ling, M. Sumiya, D. Nest, J. J. Vegh, T.-Y. Chung, B. Long, P. Lazzeri, and M. Anderle for their collaboration and contributions to the work described. They also would like to express their gratitude to S. Engelmann, R. L. Bruce, and F. Weilnboeck for a critical reading of this article and helpful suggestions. Some of the research described benefited greatly from collaboration with C. Grant Willson, University of Texas, Austin, TX 78712 and Azar Alizadeh, Polymer Division, General Electric Global Research Center, Niskayuna, and the authors express their appreciation for the wonderful cooperation. They also like to acknowledge the generosity of the Dow Chemical Company (Rohm & Haas Electronic Materials) for supporting part of this work, and they specifically thank Y. C. Bae, C. Andes, D. Wang, and Minqi Li for their unwavering partnership. They thank E. A. Hudson of Lam Research Corporation for strong collaboration over many years. The authors gratefully acknowledge the financial support of this work by the National Science Foundation under Award Nos. DMR-0406120, DMR-0705953, and NIRT CTS-0506988.
A. Challenges of plasma-resist interactions for nanometer scale pattern transfer
II. BRIEF REVIEW OF PHOTORESIST(PR)MATERIALS, PROCESSING, AND ISSUES WITH PR MASK PLASMA ETCHING RESISTANCE
A. Resist materials and considerations for pattern transfer
B. Models of plasma etching resistance of advanced photoresists
C. Surface roughness and line edge roughness introduced by plasma and energetic beam exposures
1. Energetic beam induced changes of resist chemistry and surface morphology
2. Examples of plasma-induced surface/line edge roughness in resists
3. Silicon-containing resists
III. RESULTS OF PLASMA AND ENERGETIC BEAM STUDIES OF MODEL PHOTORESISTS AND POLYMERS
A. 193 nm photoresistetching,surface, and line edge roughness
1. Examination of etching model
2. Time dependence of surface roughness and spectral size distribution
3. Relationship of roughening rate to deposited energy density
4. Role of energetic photons
5. Are line edge roughness and surface roughness correlated?
B. Results obtained with simple model polymers
1. Model of initial surface roughness development
C. Vacuum beam experiments
D. Molecular dynamics simulations of ion-polymer interactions
E. Intrinsic roughening of polymermaterials: Scaling approaches and experimental tests
IV. DEVELOPMENTS AND OTHER APPLICATIONS OF PLASMA-POLYMER SURFACE INTERACTIONS
A. Developments of photolithography and EUV
B. Soft lithography and self-assembly
C. Relevance to other applications of plasma-polymer surface interactions
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