PacSurf2014 Session NM-MoM: Nano Fabrication

We have explored self-assembling proteins and block co-polymers, rationally designed peptides, and switchable polymer brushes as design elements for functional nanostructures.

Block co-polymers can self-assemble to form interesting regular nanostructures. Indeed, micro-phase separation in block copolymers has been fairly extensively explored as a means of patterning high-density memory elements. Similarly, proteins self-assemble: individual subunits can assemble into doughnuts, stacks, fibres and scaffolds. We have explored the idea of combining the two, using micro-phase separation of a block copolymer as a means to organise a protein stack which may then be used to organise something else. In the RNA-binding protein Lsm-α, monomeric subunits assemble into doughnuts. The doughnuts can then be induced to form stacks by a combination of protein engineering and changes in solution conditions such as pH and metal ion concentration. These nanoscale tunnels can then potentially acts as a template to organise metal ion or nanoparticle columns. In order to organise the protein stacks, we have explored the segregation of the protein into the hydrophilic domains of a hydrophobic-hydrophilic block copolymer. We illustrate the idea using thin films of poly(styrene)-b-poly(ethyleneoxide) – PS-b-PEO, with the Lsm-α doughnut incorporated into self-assembled hexagonally packed cylinders of PEO. The issues are choice of a solvent system that promotes structuring of the polymer, and functionalization of the protein to convey compatibility with the solvent system necessary for formation of the micro-phase separated structure, whilst still retaining the structure, function and organisation of the protein.

Silicon, carbon and SiC_x nano structures were fabricated using liquid phase electron beam induced deposition (LP-EBID) technology . SiCl₄, CH₂Cl₂, and SiCl₄ in CH₂Cl₂ solutions of different concentrations were used as the liquid precursors, which were sealed between two Si₃N₄ window grids in home made in situ TEM liquid cells. JEOL TEM systems operating under a 200 keV electron acceleration voltage were used for the deposition. Focused electron beams of 0.28 to ~40 nA were used to decompose the precursors and deposit the nano structures on the Si₃N₄ window substrates.

With the beam focused on a fixed location for a certain time, nano dots have been deposited, with sizes ranging from <60 nm to ~500 nm depending on the deposition parameters, with well size controllability. Generally, the nano dot diameter increases with beam exposure time and beam intensity, but was insensitive to the composition ratio of these precursors. Under the higher beam current, the nano particle growth was observed to be retarded. The general growth trend is attributed to a secondary electron effect, while the retarded growth is attributed to the influence of the primary electrons.

By using scanning electron beams, nano wires of different sizes have been deposited. Besides a uniform straight line growth, we have also observed a branched growth behavior under certain deposition conditions. The secondary electron mechanism can be used to explain these growth behaviors.

The in situ cells were later dissembled, with platinum nano electrodes deposited on the two ends of the SiC_x nano wires using a FEI Dual Beam 235 focused ion beam system, forming nano electronic devices. SEM and AFM imaging analysis showed good structural morphology of the devices, and I-V property test have been made on the devices. Issues of liquid bubbling under electron beam irradiation, image resolution and structural stability of the deposited nano structures made by in situ liquid cell TEM technology have been further discussed.

Gallium nitride (GaN) is a next-generation semiconductor material with a wide band gap and high electron conductivity. Although the atomic-level planar polished surface is essential for practical GaN devices, it is difficult to polish efficiently the GaN substrate because of its high hardness and chemical stability. The chemical mechanical polishing (CMP) is promising for efficient polishing of the GaN substrate. However, the detailed CMP mechanisms are unclear, and then the design of the processes is difficult. In this study, in order to design the efficient and precise GaN CMP processes, we investigate the GaN CMP via our tight-binding quantum chemical molecular dynamics (TB-QCMD) method.

We perform CMP simulations of a GaN(0001) surface by a SiO₂ abrasive grain in aqueous H₂O₂ solution and aqueous NaOH solution to clarify the chemical reactions of each solution. We reveal that OH radicals and OH^- ions are adsorbed on the GaN surface in aqueous H₂O₂ solution and aqueous NaOH solution, respectively. According to the analysis of the atomic bond population between the Ga atoms in the first layer and the N atoms in the second layer, we elucidate that Ga-N bonds of the GaN substrate in aqueous H₂O₂ solution are weaker than those in aqueous NaOH solution. Therefore, we suggest that the OH radicals are effective for GaN CMP. To confirm the effectivity of OH radicals, we add one OH radical into the solution every 4.0 ps until 64.0 ps during polishing simulation under pure water environment. After 8 OH radicals are added, the 8 added OH radicals are adsorbed on the GaN surface. After 10 OH radicals are added, a surface-adsorbed O atom is generated by the chemical reaction between the surface-adsorbed OH species and a OH radical in the solution. At the friction interface between the GaN substrate and the abrasive grain, the surface-adsorbed O atom is mechanically pushed into the GaN substrate by the abrasive grain. This O atom intrusion induces the dissociation of Ga-N bonds of the GaN substrate. The N-N bond in the GaN substrate is generated due to the Ga-N bonds dissociation. After 16 OH radicals are added, the Ga atom in the first layer binds with 3 OH radicals. Subsequently, Ga(OH)₃ is generated and desorbed from the surface. N₂ molecules are also generated and desorbed from the surface due to the dissociation of Ga-N bonds. We suggested that the GaN CMP process efficiently proceeds by the mechanically induced chemical reactions: a surface-adsorbed O atom is generated and pushed into the GaN bulk by the abrasive grain.

Massively parallel scanning-probe based methods have been used to address the challenges of nanometer to millimeter scale printing, and thus revolutionize the conventional scanning probe-based nanofabrication. Such tools enable simple, high-throughput, and low-cost nano-patterning, which allow researchers to rapidly synthesize and study systems ranging from nanoparticle synthesis to biological processes. In this regard, we have developed a novel scanning probe-based cantilever-free printing method termed hard-tip, soft-spring lithography (HSL), which uses an massive array of Si tips to transfer materials and energy in a direct-write manner onto a variety of surfaces. Various related techniques such as graphene-coated and actuation of HSL are also discussed.

References

1) Shim, W.; Braunschweig, A. B.; Liao, X.; Chai, J.; Lim, J. K.; Zheng, G.; Mirkin, C. A. Nature469, 516 (2011).

2) Shim, W.; Brown, K. A.; Rasin, B.; Liao, X.; Zhou, X.; Mirkin, C. A. Proc. Natl. Acad. Sci. USA109, 18312 (2012).

3) Eichelsdoerfer, D. J.; Brown, K. A.; Boya, R.;Shim, W.; Mirkin, C. A. Nano Letters 13, 664 (2013).

4) Shim, W.; Brown, K. A.; Zhou, X.; Rasin, B.; Liao, X.; Schmucker, A. L.; Mirkin, C. A. Small9, 3058 (2013).

5) Brown, K. A.; Eichelsdoerfer D. J.; Shim, W.; Rasin, B.; Boya, R.; Liao, X.; Schmucker, A. L.; Liu, G.; Mirkin, C. A. Proc. Natl. Acad. Sci. USA110, 12921 (2013).

Nanoimprint lithography (NIL), in which features on a prepatterned mold are transferred directly into a polymer material, represents a promising technique with the potential for high resolution and throughput as well as low cost. Although symmetric imprint resist profiles are expected in most cases, errors could occur in actual NIL processes and will result in undesired asymmetry and pattern transfer fidelity loss in downstream processes. Detection of imprint resist asymmetric defects leads to improvement of the template, imprint process, and imprint tooling design, and therefore guarantees pattern transfer fidelity in template replication. Both cross-sectional scanning electron microscopy (X-SEM) and atomic force microscopy (AFM) are capable of identifying imprint resist profile asymmetry, but they are in general time-consuming, expensive, complex to operate, and problematic in realizing in-line integrated measurements. Being nondestructive, inexpensive and time-effective, optical scatterometry, which is traditionally based on reflectometry and ellipsometry, has been successfully introduced to characterize nanoimprinted grating structures. However, conventional optical scatterometry techniques have difficulties in measuring asymmetric grating structures due to the lack of capability of distinguishing the direction of profile asymmetry.

In this work, we apply Mueller matrix ellipsometry (MME, sometimes also referred to as Mueller matrix polarimetry) to characterize nanoimprinted grating structures with foot-like asymmetric profiles that were encountered in our NIL processes when the processes were not operated at optimum conditions. Compared with conventional optical scatterometry, which at most obtains two ellipsometric angles Y and D, MME-based scatterometry can provide up to 16 quantities of a 4×4 Mueller matrix in each measurement and can thereby acquire much more useful information about the sample. In our recent work, MME was applied to characterize nanoimprinted grating structures with symmetric profiles. We experimentally demonstrated that improved accuracy can be achieved for the line width, line height, sidewall angle, and residual layer thickness measurement by using the additional depolarization information contained in the measured Mueller matrices. The present work will further show that MME not only has good sensitivity to both the magnitude and direction of profile asymmetry, but also can be applied to accurately characterize asymmetric nanoimprinted gratings by fully exploiting the rich information hidden in the measured Mueller matrices.

For flip-chip packaging technologies, which gain popularity in semiconductor packaging, forming fine solder bumps with a high aspect ratio at a low cost is an integral part. A promising alternative to the conventional methods (screen printing and electroplating) is reverse offset printing owing to its high throughput.

In the present study, we developed Sn-Ag-Cu paste customized for the reverse offset printing process to use as solder bump with a high aspect ratio.

Gold nanoparticles have attracted enormous attention owing to their interesting optical properties arising from the surface plasmon resonance. The plasmon peaks are very sensitive to the size and shape of the nanoparticles. For anisotropic nanoparticles there are multiple plasmon peaks due to the shape anisotropy. For instant, for a rod-shaped gold nanoparticle, there are two plasmon peaks, owing to the difference in resonance frequencies of electrons along the length and width of nanorods. The relative intensity of these peaks can be controlled by controlling the orientation of nanorods with respect to the incident electromagnetic wave’s polarization. Therefore, for application purposes, it is crucial to control the orientation of nanorods.

In this work we describe a simple and elegant method to obtain wafer scale alignment of gold nanorods. We have used hydrophilic-hydrophobic contrast patterned substrates to selectively deposit gold nanorods in desired regions of substrate. The gold nanorods were grown in solution and then deposited on the substrate simply by drop casting. As the nanorods were hydrophilic, they only deposited in hydrophilic regions. When the hydrophilic stripe width becomes comparable to the length of the nanorod, the nanorod aligned along the length of the hydrophilic stripe. The degree of alignment increased with decrease in the stripe width. The alignment is influenced by various entropic and energetic forces such as orientational entropy, excluded volume entropy, van der Waals forces and electrostatic forces [1]. We were able to tune the strength of these forces simply by tuning the concentration of nanorods in solution. Our results agree well with the two dimensional Monte Carlo simulations of confined rectangles.

Ref: [1] W. Ahmed, C. Glass, J. van Ruitenbeek, Nanotechnology 25, 035301, 2014.