Anushka S. Gangnaik

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.

Novel germanium surface modification for sub-10 nm patterning with electron beam lithography and hydrogen silsesquioxane resist

Germanium is a promising high-mobility channel material for future nanoelectronic devices. Hydrogen silsesquioxane (HSQ) is a well known high-resolution electron beam lithography (EBL) resist, which is usually developed in aqueous based developers. However, this feature of HSQ causes troubles while patterning Ge surface as it is always shielded with native Ge oxides. GeO2 is a water soluble oxide, and since HSQ resist is developed in aqueous solvents, this oxide interferes with the patterning. After the EBL exposure, GeO2 is washed off during the development, lifting the patterned structures and making the high-resolution patterning impossible. To avoid this issue, it is necessary to either clean and passivate the Ge surface or use buffer layers between the native Ge oxides and the HSQ layer. In this article, a novel technique to clean the Ge surface prior to HSQ deposition, using simple “household” acids like citric acid and acetic acid, is reported. The acids are able to remove the native Ge oxides as w...

Correlation of lithographic performance of the electron beam resists SML and ZEP with their chemical structure

Study of topographical and structural changes occurring in a positive resist known as SML after electron beam lithography are presented in this article. The authors also defined its chemical structure, which is very important for understanding the lithographic performance of the resist. The structural and lithographic properties of SML have been compared to the traditional ZEP resist. First, the change in the surface roughness with respect to the electron dose of SML and ZEP resists was measured. It was found that both resists start off with similar initial roughness values. However, ZEP was observed to have a higher roughness at the apex electron dose, thereafter a reduction in roughness was observed. The roughness variation in the two resists reflected on the resolution of the gratings that were patterned in both the resists. Gratings in SML showed smoother line edge roughness, and the patterns transferred using SML resist showed more even features than the ones transferred with ZEP. Subsequently, to un...

Fabrication of Si and Ge nanoarrays through graphoepitaxial directed hardmask block copolymer self-assembly

Films of self assembled diblock copolymers (BCPs) have attracted significant attention for generating semiconductor nanoarrays of sizes below 100 nm through a simple low cost approach for device fabrication. A challenging abstract is controlling microdomain orientation and ordering dictated by complex interplay of surface energies, polymer-solvent interactions and domain spacing. In context, microphase separated poly (styrene-b-ethylene oxide) (PS-b-PEO) thin films is illustrated to fabricate nanopatterns on silicon and germanium materials trenches. The trenched templates was produced by simple electron beam lithography using hydrogen silsesquioxane (HSQ) resist. The orientation of PEO, minority cylinder forming block, was controlled by controlling trench width and varying solvent annealing parameters viz. temperature, time etc. A noticeable difference in microdomain orientation was observed for Si and Ge trenches processed under same conditions. The Ge trenches promoted horizontal orientations compared to Si due to difference in surface properties without any prior surface treatments. This methodology allows to create Ge nanopatterns for device fabrication since native oxides on Ge often induce patterning challenges. Subsequently, a selective metal inclusion method was used to form hardmask nanoarrays to pattern transfer into those substrates through dry etching. The hardmask allows to create good fidelity, low line edge roughness (LER) materials nanopatterns.