Yordan M. Georgiev

Study of a high contrast process for hydrogen silsesquioxane as a negative tone electron beam resist

An extensive study of parameters pertinent to electron beam lithography with hydrogen silsesquioxane as a negative tone electron beam resist is presented. With higher developer concentrations contrast and reproducibility are improved significantly at the expense of lower sensitivity. In a similar way extended delays between the baking and exposure degrade the sensitivity but increase the contrast. In contrast, at higher baking temperatures the sensitivity is improved but the contrast and reproducibility deteriorate. These results are discussed within a microscopic model. Contrast values as high as 10 and good reproducibility have been obtained with a developer concentration of 25% tetramethyl ammonium hydroxide and a baking temperature of 90 °C. With these optimal parameters an experimental lithographic pattern of 50 nm lines and spaces could be resolved in 220 nm thick HSQ resist film exposed at 50 keV.

Interferometric in situ alignment for UV-based nanoimprint

A high precision alignment concept is evaluated for suitability in UV-based nanoimprint lithography. Through three consecutive alignment steps an overlay accuracy of 50nm is obtained with ample room for further improvements.

Organo-arsenic Molecular Layers on Silicon for High-Density Doping

This article describes for the first time the controlled monolayer doping (MLD) of bulk and nanostructured crystalline silicon with As at concentrations approaching 2 × 10(20) atoms cm(-3). Characterization of doped structures after the MLD process confirmed that they remained defect- and damage-free, with no indication of increased roughness or a change in morphology. Electrical characterization of the doped substrates and nanowire test structures allowed determination of resistivity, sheet resistance, and active doping levels. Extremely high As-doped Si substrates and nanowire devices could be obtained and controlled using specific capping and annealing steps. Significantly, the As-doped nanowires exhibited resistances several orders of magnitude lower than the predoped materials.

Access resistance reduction in Ge nanowires and substrates based on non-destructive gas-source dopant in-diffusion

To maintain semiconductor device scaling, in recent years industry has been forced to move from planar to non-planar device architectures. This alone has created the need to develop a radically new, non-destructive method for doping. Doping alters the electrical properties of a semiconductor, related to the access resistance. Low access resistance is necessary for high performance technology and reduced power consumption. In this work the authors reduced access resistance in top–down patterned Ge nanowires and Ge substrates by a non-destructive dopant in-diffusion process. Furthermore, an innovative electrical characterisation methodology is developed for nanowire and fin-based test structures to extract important parameters that are related to access resistance such as nanowire resistivity, sheet resistance, and active doping levels. Phosphine or arsine was flowed in a Metalorganic Vapour Phase Epitaxy reactor over heated Ge samples in the range of 650–700 °C. Dopants were incorporated and activated in this single step. No Ge growth accompanied this process. Active doping levels were determined by electrochemical capacitance–voltage free carrier profiling to be in the range of 1019 cm−3. The nanowires were patterned in an array of widths from 20–1000 nm. Cross-sectional Transmission Electron Microscopy of the doped nanowires showed minimal crystal damage. Electrical characterisation of the Ge nanowires was performed to contrast doping activation in thin-body structures with that in bulk substrates. Despite the high As dose incorporation on unpatterned samples, the nanowire analysis determined that the P-based process was the better choice for scaled features.

Solvent vapor annealing of block copolymers in confined topographies: commensurability considerations for nanolithography.

The directed self-assembly of block copolymer (BCP) materials in topographically patterned substrates (i.e., graphoepitaxy) is a potential methodology for the continued scaling of nanoelectronic device technologies. In this Communication, an unusual feature size variation in BCP nanodomains under confinement with graphoepitaxially aligned cylinder-forming poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) BCP is reported. Graphoepitaxy of PS-b-P4VP BCP line patterns (CII ) is accomplished via topo-graphy in hydrogen silsequioxane (HSQ) modified substrates and solvent vapor annealing (SVA). Interestingly, reduced domain sizes in features close to the HSQ guiding features are observed. The feature size reduction is evident after inclusion of alumina into the P4VP domains followed by pattern transfer to the silicon substrate. It is suggested that this nano-domain size perturbation is due to solvent swelling effects during SVA. It is proposed that using a commensurability value close to the solvent vapor annealed periodicity will alleviate this issue leading to uniform nanofins.

Megasonic-assisted development of nanostructures

The effect of high frequency (1MHz) acoustic agitation (megasonic agitation) on development of electron beam exposed poly(methylmethacrylate) (PMMA) nanostructures is investigated. Test patterns consisting of dense holes, isolated lines, and gratings with high aspect ratios have been used. Compared to conventional dip development, the sensitivity of the development process is increased and the homogeneity of nanopatterns is improved considerably. Furthermore, experiments towards ultimate aspect ratios and resolution of PMMA in the range of 2–3nm with megasonically assisted development have been carried out. The physical mechanisms for the observed enhanced development performance which is particularly attractive for nanostructuring are discussed.

Porous to Nonporous Transition in the Morphology of Metal Assisted Etched Silicon Nanowires

A single step metal assisted etching (MAE) process, utilizing metal ion-containing HF solutions in the absence of an external oxidant, has been developed to generate heterostructured Si nanowires with controllable porous (isotropically etched) and non-porous (anisotropically etched) segments. Detailed characterisation of both the porous and non-porous sections of the Si nanowires was provided by transmission electron microscopy studies, enabling the mechanism of nanowire roughening to be ascertained. The versatility of the MAE method for producing heterostructured Si nanowires with varied and controllable textures is discussed in detail.

Megasonic-assisted development of nanostructures: Investigations on high aspect ratio nanoholes

The influence of megasonic agitation on the development of nanostructures with high aspect ratio is investigated thoroughly. The improvements in homogeneity, depth, and quality of nanostructures are related to specific interactions of the sound wave with the resist and developer. Two phases in the development process are operative. The specific role of microstreaming providing physical supply of fresh developer is identified and the reduction of viscosity by megasonic interaction is derived. The advantage of megasonic agitation as a nondestructive development of high aspect ratio nanostructures is demonstrated.

Resist-substrate interface tailoring for generating high density arrays of Ge and Bi2Se3 nanowires by electron beam lithography

The authors report a chemical process to remove the native oxide on Ge and Bi2Se3 crystals, thus facilitating high-resolution electron beam lithography (EBL) on their surfaces using a hydrogen silsesquioxane (HSQ) resist. HSQ offers the highest resolution of all the commercially available EBL resists. However, aqueous HSQ developers such as NaOH and tetramethylammonium hydroxide have thus far prevented the fabrication of high-resolution structures via the direct application of HSQ to Ge and Bi2Se3, due to the solubility of components of their respective native oxides in these strong aqueous bases. Here we provide a route to the generation of ordered, high-resolution, high-density Ge and Bi2Se3 nanostructures with potential applications in microelectronics, thermoelectric, and photonics devices.

Top-down process of Germanium nanowires using EBL exposure of Hydrogen Silsesquioxane resist

An initial top-down process of Germanium nanowires is developed in this work. The Silicon Nitride (Si3N4) is used as a hard mask to obtain a stable surface for lithography and a resistive mask for etch. The electron-beam lithography (EBL) is utilized for patterning the nanowires with Hydrogen Silsesquixane (HSQ) as a negative photoresist. Several different etch conditions are examined to transfer the patterns into the substrate.