Strategies for inorganic incorporation using neat block copolymer thin films for etch mask function and nanotechnological application

Block copolymers (BCPs) and their directed self-assembly (DSA) has emerged as a realizable complementary tool to aid optical patterning of device elements for future integrated circuit advancements. Methods to enhance BCP etch contrast for DSA application and further potential applications of inorganic nanomaterial features (e.g., semiconductor, dielectric, metal and metal oxide) are examined. Strategies to modify, infiltrate and controllably deposit inorganic materials by utilizing neat self-assembled BCP thin films open a rich design space to fabricate functional features in the nanoscale regime. An understanding and overview on innovative ways for the selective inclusion/infiltration or deposition of inorganic moieties in microphase separated BCP nanopatterns is provided. Early initial inclusion methods in the field and exciting contemporary reports to further augment etch contrast in BCPs for pattern transfer application are described. Specifically, the use of evaporation and sputtering methods, atomic layer deposition, sequential infiltration synthesis, metal-salt inclusion and aqueous metal reduction methodologies forming isolated nanofeatures are highlighted in di-BCP systems. Functionalities and newly reported uses for electronic and non-electronic technologies based on the inherent properties of incorporated inorganic nanostructures using di-BCP templates are highlighted. We outline the potential for extension of incorporation methods to triblock copolymer features for more diverse applications. Challenges and emerging areas of interest for inorganic infiltration of BCPs are also discussed

The stability of “Ce2O3” nanodots in ambient conditions: a study using block copolymer templated structures

The stability of reduced cerium oxide in ambient conditions is clearly demonstrated in this paper. Well-defined, crystalline, cerium oxide nanodots (predominantly Ce4+ or Ce3+ material could be selectively prepared) were defined at silicon substrate surfaces by a method of block copolymer templating. Here, selective addition of the cerium ion into one block via solvent inclusion and subsequent UV/ozone processing resulted in the formation of well-separated, size mono-dispersed, oxide nanodots having a hexagonal arrangement mimicking that of the polymer nanopattern. The size of the dots could be varied in a facile manner by controlling the metal ion content. Synthesis and processing conditions could be varied to create nanodots which have a Ce2O3 type composition. The stability of the sesquioxide type structure under processing (synthesis) conditions and calcination was explored. Surprisingly, the sesquioxide type structure appears to be reasonably stable in ambient conditions with little evidence for extensive oxidation until heating to temperatures above ambient. Room temperature fluorescence is supposed to originate from a distribution of surface or defect states and depends on preparation conditions

Reduction and control of domain spacing by additive inclusion: morphology and orientation effects of glycols on microphase separated PS-b-PEO

Cylindrical phase polystyrene-b-polyethylene oxide (PS-b-PEO) block copolymer (BCP) was combined with lower molecular weight poly/ethylene glycols at different concentrations and their effect on the microphase separation of BCP thin films were studied. Well-ordered microphase separated, periodic nanostructures were realized using a solvent annealing approach for solution cast thin films. By optimizing solvent exposure time, the nature and concentration of the additives etc. the morphology and orientation of the films can be controlled. The addition of the glycols to PS-b-PEO enables a simple method by which the microdomain spacing of the phase separated BCP can be controlled at dimensions below 50nm. Most interestingly, the additives results in an expected increase in domain spacing (i.e. pitch size) but in some conditions an unexpected reduction in domain spacing. The pitch size achieved by modification is in the range of 16–31nm compared to an unmodified BCP system which exhibits a pitch size of 25nm. The pitch size modification achieved can be explained in terms of chemical structure, solubility parameters, crystallinity and glass transition temperature of the PEO because the additives act as PEO ‘stress cracking agents’ whereas the PS matrix remains chemically unaffected

Probing thermal flux in twinned Ge nanowires through Raman spectroscopy

We report a noninvasive optical technique based on micro-Raman spectroscopy to study the temperature-dependent phonon behavior of normal (nondefective) and twinned germanium nanowires (Ge-NWs). We studied thermophysical properties of Ge-NWs from Raman spectra, measured by varying excitation laser power at ambient condition. We derived the laser-induced temperature rise during Raman measurements by analyzing the Raman peak position for both the NWs, and for a comparative study we performed the same for bulk Ge. The frequency of the Ge–Ge phonon mode softens for all the samples with the increase in temperature, and the first-order temperature coefficient (χT) for defected NWs is found to be higher than normal NWs and bulk. We demonstrated that apart from the size, the lamellar twinning and polytype phase drastically affect the heat transport properties of NWs

Development of a facile block copolymer method for creating hard mask patterns integrated into semiconductor manufacturing

Our goal is to develop a facile process to create patterns of inorganic oxides and metals on a substrate that can act as hard masks. These materials should have high etch contrast (compared to silicon) and so allow high-aspect-ratio, high-fidelity pattern transfer whilst being readily integrable in modern semiconductor fabrication (FAB friendly). Here, we show that ultra-small-dimension hard masks can be used to develop large areas of densely packed vertically and horizontally orientated Si nanowire arrays. The inorganic and metal hard masks (Ni, NiO, and ZnO) of different morphologies and dimensions were formed using microphase-separated polystyrene-b-poly(ethylene oxide) (PS-b-PEO) block copolymer (BCP) thin films by varying the BCP molecular weight, annealing temperature, and annealing solvent(s). The self-assembled polymer patterns were solvent-processed, and metal ions were included into chosen domains via a selective inclusion method. Inorganic oxide nanopatterns were subsequently developed using standard techniques. High-resolution transmission electron microscopy studies show that high-aspect-ratio pattern transfer could be affected by standard plasma etch techniques. The masking ability of the different materials was compared in order to create the highest quality uniform and smooth sidewall profiles of the Si nanowire arrays. Notably good performance of the metal mask was seen, and this could impact the use of these materials at small dimensions where conventional methods are severely limited

Nanophase separation and structural evolution of block copolymer films: a "green" and "clean" supercritical fluid approach

Thin films of block copolymers (BCPs) are widely accepted as potentially important materials in a host of technological applications including nanolithography. In order to induce domain separation and form well-defined structural arrangements, many of these are solvent-annealed (i.e. solvent swollen) at moderate temperatures. The use of solvents can be challenging in industry from an environmental point of view as well as having practical/cost issues. However, a simple and environmentally friendly alternative to solvo-thermal annealing for the periodically ordered nanoscale phase separated structures is described herein. Various asymmetric polystyrene-b-poly(ethylene oxide) (PS-b-PEO) thin films were annealed in a compressible fluid, supercritical carbon dioxide (scCO2), to control nanodomain orientation and surface morphologies. For the first time, periodic well defined, hexagonally ordered films with sub-25 nm pitch size were demonstrated using a supercritical fluid (SCF) process at low temperatures and pressures. Predominant swelling of PEO domains in scCO2 induces nanophase separation. scCO2 serves as green alternative to the conventional organic solvents for the phase segregation of BCPs with complete elimination of any residual solvent in the patterned film. The depressurization rate of scCO2 following annealing was found to affect the morphology of the films. The supercritical annealing conditions could be used to define nanoporous analogues of the microphase separated films without additional processing, providing a one-step route to membrane like structures without affecting the ordered surface phase segregated structure. An understanding of the BCP self-assembly mechanism can be realized in terms of the deviation in glass transition temperature, melting point, viscosity, interaction parameter and volume fraction of the constituent blocks in the scCO2 environment

"In situ" hard mask materials: a new methodology for creation of vertical silicon nanopillar and nanowire arrays

A novel, simple and in situ hard mask technology that can be used to develop high aspect ratio silicon nanopillar and nanowire features on a substrate surface is demonstrated. The technique combines a block copolymer inclusion method that generates nanodot arrays on substrate and an inductively coupled plasma (ICP) etch processing step to fabricate Si nanopillar and nanowire arrays. Iron oxide was found to be an excellent resistant mask over silicon under the selected etching conditions. Features of a very high aspect ratio can be created by this method. The nanopillars have uniform diameter and smooth sidewalls throughout their entire length. The diameter (15–27 nm) and length of the nanopillars can be tuned easily. Different spectroscopic and microscopic techniques were used to examine the morphology and size, surface composition and crystallinity of the resultant patterns. The methodology developed may have important technological applications and provide an inexpensive manufacturing route to nanodimensioned topographical patterns. The high aspect ratio of the features may have importance in the area of photonics and the photoluminescence properties are found to be similar to those of surface-oxidized silicon nanocrystals and porous silicon

Fabrication of ordered, large scale, horizontally aligned Si nanowire arrays based on an in-situ hard mask block copolymer approach

A simple technique is demonstrated to fabricate horizontal, uniform, and hexagonally arranged Sinanowire arrays with controlled orientation and density at spatially well defined locations on a substrate based on an in situ hard-mask pattern-formation approach by microphase-separated block-copolymer thin films. The technique may have significant application in the manufacture of transistor circuitry

Development of ordered, porous (sub-25 nm dimensions) surface membrane structures using a block copolymer approach

In an effort to develop block copolymer lithography to create high aspect vertical pore arrangements in a substrate surface we have used a microphase separated poly(ethylene oxide) -b- polystyrene (PEO-b-PS) block copolymer (BCP) thin film where (and most unusually) PS not PEO is the cylinder forming phase and PEO is the majority block. Compared to previous work, we can amplify etch contrast by inclusion of hard mask material into the matrix block allowing the cylinder polymer to be removed and the exposed substrate subject to deep etching thereby generating uniform, arranged, sub-25 nm cylindrical nanopore arrays. Briefly, selective metal ion inclusion into the PEO matrix and subsequent processing (etch/modification) was applied for creating iron oxide nanohole arrays. The oxide nanoholes (22 nm diameter) were cylindrical, uniform diameter and mimics the original BCP nanopatterns. The oxide nanohole network is demonstrated as a resistant mask to fabricate ultra dense, well ordered, good sidewall profile silicon nanopore arrays on substrate surface through the pattern transfer approach. The Si nanopores have uniform diameter and smooth sidewalls throughout their depth. The depth of the porous structure can be controlled via the etch process

A general method for controlled nanopatterning of oxide dots: a microphase separated block copolymer platform

We demonstrate a facile, generic method for the fabrication of highly dense long range hexagonally ordered various inorganic oxide nanodots on different substrates by using a microphase separated polystyrene-b-poly(ethylene oxide) (PS-b-PEO) block copolymer (BCP) thin film as a structural template. The method does not require complex co-ordination chemistry (between metal precursors and the polymer) and instead involves the simple, solution based chemistry applicable to a wide range of systems. A highly ordered PS-b-PEO thin film with perpendicularly oriented PEO cylinders is fabricated by solvent annealing over wafer scale. PEO cylinders are activated by ethanol to create a functional chemical pattern for nanodot development via spin coating and block selective metal ion inclusion. Subsequent UV/ozone treatment forms an ordered arrangement of oxide nanodots and removes the polymer components. The phase purity, crystallinity and thermal stability of these materials coupled to the ease of production may make them useful in technological applications. This method is particularly useful because the feature sizes can be tuned by changing the concentration of the precursors without changing the molecular weight and concentration of the block copolymer