Microstructure and optical properties of submicron porous silicon thin films grown at low current densitiesAn evaporation process has been developed for depositing highly porous insulator or metal films with densities as low as 15% of bulk. The process utilizes either multiple evaporation sources or substrate movement to provide a symmetrical but very oblique ͑Ͼ80%͒ flux incident on the substrate. Extreme self-shadowing produced a vertical columnar microstructure consisting of isolated and evenly spaced columns including a unique zigzag structure in a number of insulator films. Features of the film are often anisotropic, leading to conductivity differences of as much as a factor of two along perpendicular axes in the plane of the film surface. The direction of anisotropic growth was observed to switch orientation as the incident flux angle was increased to very oblique, beyond approximately 80°. A line segment simulator incorporating ballistic deposition and minimization of chemical potential has been used to aid in the understanding of the growth mechanisms of these films and to optimize the evaporation process. The simulator helped to confirm that self-shadowing was the dominant mechanism in this porous structure formation.

No abstract

We report a three-dimensional (3D) simulation model based on the kinetic transport theory for calculating the distribution of PMMA fragments after an exposure to electron impact. The conditions employed for the modeling were chosen to resemble a typical electron beam lithography exposure. The model accounts for inelastic collisions of electrons in PMMA and resulting random main-chain scissions. We have considered gratings composed of parallel lines distanced by 10–50nm and exposed to electrons with energies of 10–60keV. By the model simulations, we have generated and analyzed the detailed 3D distributions of small PMMA fragments (one to ten monomers) that are soluble at the development stage and thus are responsible for the clearance in the gratings. In terms of the spatial distributions of soluble fragments, we have formulated the criteria that define the total clearance as well as the local grating development and investigated their dependence on the grating period, electron dose, and energy.

A novel three-dimensional (3D) ballistic deposition simulator 3D-FILMS has been developed for the modeling of thin film deposition and structure. The simulator features a ballistic transport algorithm to model incident species with angular distributions appropriate to physical vapor deposition systems. Two-tiered data structuring is employed in order to enable the simulator to run using memory resources available to workstations. The simulator has been applied to a unique class of thin films grown by the technique of glancing angle deposition (GLAD). These films exhibit low bulk density due to an internal structure consisting of isolated microcolumns, which can be engineered into a variety of 3D forms. Because of their inherent 3D morphology, created by a combination of complex substrate motion and 3D shadowing, GLAD films represent an ideal test subject for 3D simulation. Scanning electron microscope images of films are presented together with simulation results, which correctly reproduce aspects of column morphology, column growth competition and extinction, and film bulk density.

A modern alternative to the positive-tone PMMA resist is the ZEP 520A (Nippon Zeon) brand co-polymer resist, which offers a higher sensitivity and etch durability for electron beam lithography. However, the molecular mechanisms are not entirely understood, and the relative performance of two resists for various process conditions of nanofabrication is not readily predictable. The authors report a thorough experimental comparison of the performance of PMMA 950k and ZEP 520A resists in MIBK:IPA, ZED, and IPA:water developers. Interestingly, ZEP resist performance was found to depend significantly on the developer. ZED developer increases the sensitivity, whereas IPA:water optimizes line edge roughness and conceivably the resolution at the expense of sensitivity. The authors also describe two alternative numerical models, one assuming an enhancement of the main chain scission in ZEP as a result of electronic excitations in side groups, and another without such enhancement. In the second case, the differences in ZEP and PMMA resists performance are attributed to their different interaction with the developers. Using both approaches, the authors parameterize the respective models of ZEP development by fitting numerical results to the experimental resist morphologies, and analyze the outcomes.