Substrate orientation effects on nanoelectrode lithography : ReaxFF molecular dynamics and experimental study

The crystallographic orientation of the substrate is an essential parameter in the kinetic mechanism for the oxidation process. Hence, the choice of substrate surface orientation is crucial in nanofabrication industries. In the present work, we have studied qualitatively the influence of substrate orientation in nanoelectrode lithography using ReaxFF reactive molecular dynamics simulation. We have investigated the oxidation processes on (100), (110) and (111) orientation surfaces of silicon at different electric field intensities. The simulation results show the thickness of the oxide film and the initial oxygen diffusion rate follow an order of (100) > (110) > (111) at lower electric field intensities. It also confirms that surfaces with higher surface energy are more reactive at lower electric field intensity. Crossovers occurred at a higher electric field intensity (7 V nm -1) under which the thickness of the oxide film yields an order of T(110) > T(100) > T(111). These types of anomalous characteristics have previously been observed for thermal oxidation of silicon surfaces. Experimental results show different orders for the (100) and (111) substrate, while (110) remains the largest for the oxide thickness. A good correlation has been found between the oxide growth and the orientation-dependent parameters where the oxide growth is proportional to the areal density of the surfaces. The oxide growth also follows the relative order of the activation energies, which could be another controlling factor for the oxide growth. Less activation energy of the surface allows more oxide growth and vice versa. However, the differences between simulation and experimental results probably relate to the empirical potential as well as different time and spatial scales of the process

Rolling nanoelectrode lithography

Non-uniformity and low throughput issues severely limit the application of nanoelectrode lithography for large area nanopatterning. This paper proposes, for the first time, a new rolling nanoelectrode lithography approach to overcome these challenges. A test-bed was developed to realize uniform pressure distribution over the whole contact area between the roller and the silicon specimen, so that the local oxidation process occurred uniformly over a large area of the specimen. In this work, a brass roller wrapped with a fabricated polycarbonate strip was used as a stamp to generate nanopatterns on a silicon surface. The experimental results show that a uniform pattern transfer for a large area can be achieved with this new rolling nanoelectrode lithography approach. The rolling speed and the applied bias voltage were identified as the primary control parameters for oxide growth. Furthermore, the pattern direction showed no significant influence on the oxide process. We therefore demonstrated that nanoelectrode lithography can be scaled up for large-area nanofabrication by incorporating a roller stamp

Promising lithography techniques for next generation logic devices : a review

Continuous rapid shrinking of feature size made the authorities to seek alternative patterning methods as the conventional photolithography comes with its intrinsic resolution limit. In this regard, some promising techniques have been proposed as next generation lithography (NGL) that have the potentials to achieve both high volume production and very high resolution. This article reviews the promising next generation lithography techniques and introduces the challenges and a perspective on future directions of the NGL techniques. Extreme Ultraviolet Lithography (EUVL) is considered as the main candidate for sub-10 nm manufacturing and it could potentially meet the current requirements of the industry. Remarkable progress in EUVL has been made and the tools will be available for commercial operation soon. Maskless lithography techniques are used for patterning in R&D, mask/mold fabrication and low volume chip design. Directed Self Assembly (DSA) has already been realized in laboratory and further effort will be needed to make it as NGL solution. Nanoimprint Lithography has emerged attractively due to its simple process-steps, high-throughput, high-resolution and low-cost and become one of the commercial platforms for nanofabrication. However, a number of challenging issues are waiting ahead and further technological progresses are required to make the techniques significant and reliable to meet the current demand. Finally, a comparative study is presented among these techniques

ReaxFF molecular dynamics simulation study of nanoelectrode lithography oxidation process on silicon (100) surface

The nanoelectrode lithography has been strengthened in recent years as one of the most promising methods due to its high reproducibility, low cost and ability to manufacture nano-sized structures. In this work, the mechanism and the parametric influence in nanoelectrode lithography have been studied qualitatively in atomic scale using ReaxFF MD simulation. This approach was originally developed by van Duin and co-workers to investigate hydrocarbon chemistry. We have investigated the water adsorption and dissociation processes on Si (100) surface as well as the characteristics (structure, chemical composition, morphology, charge distribution, etc.) of the oxide growth. The simulation results show two forms of adsorption of water molecules: molecular adsorption and dissociative adsorption. After breaking the adsorbed hydroxyls, the oxygen atoms insert into the substrate to form the Si−O−Si bonds so as to make the surface oxidized. The influence of the electric field intensity (1.5 – 7 V/nm) and the relative humidity (20 – 90%) on the oxidation process have also been discussed. Nevertheless, the results obtained from the simulations have been compared qualitatively with the experimental results and they show in good agreements. Variable charge molecular dynamics allowed us to characterize the nanoelectrode lithography process from an atomistic point of view

Flexible single-step fabrication of programmable 3D nanostructures by pulse-modulated local anodic oxidation

A flexible single-step nanofabrication approach was developed using programmable pulse modulation local anodic oxidation for the efficient generation of various 3D nanostructures. The dependence of oxidation growth on pulse parameters was derived from parametric studies, from which an analytical process model was developed for the first time to link the pulse parameters with the geometry of 3D nanostructure with precision. The nanofabrication approach was implemented on an atomic force microscope. Experimental results show that this approach can effectively create 3D nanostructures with minimum feature sizes of sub-10 nm (lateral) and sub-nm (vertical) with nm and sub-nm level precision, respectively

Nanoelectrode lithography of silicon surface by brass stamp

The stamps used in the nanoelectrode lithography (NEL) process require conductive layer deposition, which makes them a bit expensive. This paper reports the feasibility of using brass materials as the conductive stamps for NEL to shorten the process step and reduce the production cost. In this paper, the fabrication of nanostructures on the brass stamp was performed on a single point diamond turning (SPDT) machine. Some burrs were formed during the machining process, that prohibit the stamps from achieving a homogeneous contact with the substrates. Introduction of a thin layer of polymer (PS-OH) on the silicon substrate showed an improvement in contact uniformity so as the oxidation. However, some areas of the substrate remained unoxidized as few of the burrs were quite large. The brass stamps could be advantageous as they show no degradation after many uses. Nevertheless, the issues of the burr formation and non-uniformity should be alleviated first to make these stamps appropriate to the NEL process

Atomistic insights into bias-induced oxidation on passivated silicon surface through ReaxFF MD simulation

The study investigated the bias-induced oxidation through ReaxFF molecular dynamics simulations in order to bridge the knowledge gaps in the understanding of physical-chemical reaction at the atomic scale. Such an understanding is critical to realise accurate process control of bias-induced local anodic oxidation nanolithography. In this work, we simulated bias-induced oxidation by applying electric fields to passivated silicon surfaces and performed a detailed analysis of the simulation results to identify the primary chemical components involved in the reaction and their respective roles. In contrast to surface passivation, bias-induced oxidation led mainly to the creation of Si–O–Si bonds in the oxide film, along with the consumption of H2O and the generation of H3O+ in the water layer, whereas the chemical composition on the oxidised surface remained essentially unchanged with a mixture of Si–O–H, Si–H, Si–H2, H2O–Si and Si–O–Si bonds. Furthermore, parametric studies indicated that increased electric field strength and humidity did not significantly alter the surface chemical composition but notably enhanced the bias-induced oxidation, as indicated by the increased number of Si–O–Si bonds and oxide thickness in simulation results. A good agreement is achieved between the simulation and experimental results

Nanoelectrode lithography : modelling, experimental validation and instrumentation

This thesis was previously held under moratorium from 02/11/20 to 02/11/22Continuous rapid shrinking of feature size made the authorities to seek alternative patterning methods as the conventional photolithography process is reaching its intrinsic resolution limit. In this regard, some promising techniques have been proposed as the next generation lithography (NGL) that have the potentials to achieve both high volume production and very high resolution. Among them, several methods such as Extreme Ultraviolet Lithography (EUVL), Electron Beam lithography (EBL), Nanoimprint Lithography (NIL), Directed Self Assembly (DSA) and Scanning Probe Lithography (SPL) have demonstrated excellent potentials as promising candidates for future industrial nanofabrication. However, all these technologies are in their development phases and still need further work to overcome some challenges in terms of flexibility, uniformity, high throughput, high resolution, high reliability, high- efficiency, defectivity, and cost of ownership. On the other hand, nanoelectrode nanolithography (NEL) has been developed in the laboratory and demonstrated as an efficient lithographic tool. It has been strengthened in recent years as one of the most promising methods due to its high reproducibility, low cost, and ability to manufacture nano-sized structures. This method is based on the spatial confinement of the anodic oxidation between a conductive stamp and the sample surface. However, the non-uniformity issue severely limits the existing nanoelectrode lithography to be applied for large area nanopatterning. Besides, other issues such as stamp lifetime and low-cost stamp fabrication method need to be addressed to make this lithography technique viable for commercial applications. A clear and explicit understanding of the mechanism at a molecular level helps to improve this technique. Therefore, this PhD thesis firstly aims to gain an in-depth understanding of nanoscale mechanisms involved in the anodic oxidation process and the parametric influence in nanoelectrode lithography through molecular dynamics (MD) simulations. To do this, three-dimensional MD models of oxidation nanocell were developed, and a reactive force field (ReaxFF) was adopted to describe the interactions between atoms. The MD simulations were implemented in LAMMPS software and were performed by using a High-Performance Computing (HPC) service, ARCHIE-WeSt. The simulation results demonstrated two forms of adsorption of water molecules: molecular adsorption and dissociative adsorption. After breaking the adsorbed hydroxyls, the oxygen atoms insert into the substrate to form the Si−O−Si bonds so as to make the surface oxidized. A linear dependency of the electric field intensity on oxidation growth was observed. The relative humidity also showed the same linear behavior after a certain value (40%). The simulation results have been compared qualitatively with the experimental results, and they show in good agreement. MD simulation results also showed that the crystallographic orientation of the substrate has a great impact on the oxidation process. It was revealed that the thickness of the oxide film and the initial oxygen diffusion rate follow an order of (100) > (110) > (111) at lower electric field intensities. It also confirmed that surfaces with higher surface energy are more reactive at lower electric field intensity. Crossovers occurred at a higher electric field intensity (7 V/nm) under which the thickness of the oxide film yields an order of T(110) > T(100) > T(111). Atomic force microscope (AFM) oxidation experiments were performed to validate these results, which showed different orders for the (100) and (111) substrates, while (110) remained the largest for the oxide thickness. A good correlation has been found between the oxide growth and the orientation-dependent parameters where the oxide growth is proportional to the areal density of the surfaces. The oxide growth also follows the relative order of the activation energies, which could be another controlling factor for the oxide growth. However, the differences between simulation and experimental results probably relate to the empirical potential as well as different time and spatial scales of the process. Another objective of this thesis is to develop a new NEL process with a brass stamp that does not require conductive layer deposition. The brass material was chosen as it has high elastic modulus and high breaking strength, which ensures higher life expectancy. Therefore, this thesis reports the feasibility of using brass materials as the conductive stamps for NEL to shorten the process steps and reduce the production cost. The fabrication of nanostructures on the brass stamp was performed on a single point diamond turning (SPDT) machine. Some burrs were formed during the machining process, that prohibit the stamps from achieving a homogeneous contact with the substrates. Oxidation experiments were carried out with a home built NEL system. The results showed that an introduction of a thin layer of polymer (PS-OH) on the silicon substrate could improve the contact uniformity so as the oxidation. Finally, a rolling nanoelectrode lithography process was proposed, for the first time, to scale up the nanoelectrode lithography technique for large-area nanofabrication. A test-bed was developed to realize uniform pressure distribution over the whole contact area so that the local oxidation process occurs uniformly over a large area of the samples. A brass roller wrapped with a fabricated polycarbonate strip has been used as a stamp to generate nanopatterns on a silicon surface. The experimental results indicated that a significant improvement in pattern uniformity compared to the other results was obtained with the conventional NEL process. Moreover, the impact of pattern direction has been investigated, which shows no significant variation in the oxide pattern. Lastly, the rolling speed and the applied bias voltage were identified as the primary control parameters for the oxide growth.Continuous rapid shrinking of feature size made the authorities to seek alternative patterning methods as the conventional photolithography process is reaching its intrinsic resolution limit. In this regard, some promising techniques have been proposed as the next generation lithography (NGL) that have the potentials to achieve both high volume production and very high resolution. Among them, several methods such as Extreme Ultraviolet Lithography (EUVL), Electron Beam lithography (EBL), Nanoimprint Lithography (NIL), Directed Self Assembly (DSA) and Scanning Probe Lithography (SPL) have demonstrated excellent potentials as promising candidates for future industrial nanofabrication. However, all these technologies are in their development phases and still need further work to overcome some challenges in terms of flexibility, uniformity, high throughput, high resolution, high reliability, high- efficiency, defectivity, and cost of ownership. On the other hand, nanoelectrode nanolithography (NEL) has been developed in the laboratory and demonstrated as an efficient lithographic tool. It has been strengthened in recent years as one of the most promising methods due to its high reproducibility, low cost, and ability to manufacture nano-sized structures. This method is based on the spatial confinement of the anodic oxidation between a conductive stamp and the sample surface. However, the non-uniformity issue severely limits the existing nanoelectrode lithography to be applied for large area nanopatterning. Besides, other issues such as stamp lifetime and low-cost stamp fabrication method need to be addressed to make this lithography technique viable for commercial applications. A clear and explicit understanding of the mechanism at a molecular level helps to improve this technique. Therefore, this PhD thesis firstly aims to gain an in-depth understanding of nanoscale mechanisms involved in the anodic oxidation process and the parametric influence in nanoelectrode lithography through molecular dynamics (MD) simulations. To do this, three-dimensional MD models of oxidation nanocell were developed, and a reactive force field (ReaxFF) was adopted to describe the interactions between atoms. The MD simulations were implemented in LAMMPS software and were performed by using a High-Performance Computing (HPC) service, ARCHIE-WeSt. The simulation results demonstrated two forms of adsorption of water molecules: molecular adsorption and dissociative adsorption. After breaking the adsorbed hydroxyls, the oxygen atoms insert into the substrate to form the Si−O−Si bonds so as to make the surface oxidized. A linear dependency of the electric field intensity on oxidation growth was observed. The relative humidity also showed the same linear behavior after a certain value (40%). The simulation results have been compared qualitatively with the experimental results, and they show in good agreement. MD simulation results also showed that the crystallographic orientation of the substrate has a great impact on the oxidation process. It was revealed that the thickness of the oxide film and the initial oxygen diffusion rate follow an order of (100) > (110) > (111) at lower electric field intensities. It also confirmed that surfaces with higher surface energy are more reactive at lower electric field intensity. Crossovers occurred at a higher electric field intensity (7 V/nm) under which the thickness of the oxide film yields an order of T(110) > T(100) > T(111). Atomic force microscope (AFM) oxidation experiments were performed to validate these results, which showed different orders for the (100) and (111) substrates, while (110) remained the largest for the oxide thickness. A good correlation has been found between the oxide growth and the orientation-dependent parameters where the oxide growth is proportional to the areal density of the surfaces. The oxide growth also follows the relative order of the activation energies, which could be another controlling factor for the oxide growth. However, the differences between simulation and experimental results probably relate to the empirical potential as well as different time and spatial scales of the process. Another objective of this thesis is to develop a new NEL process with a brass stamp that does not require conductive layer deposition. The brass material was chosen as it has high elastic modulus and high breaking strength, which ensures higher life expectancy. Therefore, this thesis reports the feasibility of using brass materials as the conductive stamps for NEL to shorten the process steps and reduce the production cost. The fabrication of nanostructures on the brass stamp was performed on a single point diamond turning (SPDT) machine. Some burrs were formed during the machining process, that prohibit the stamps from achieving a homogeneous contact with the substrates. Oxidation experiments were carried out with a home built NEL system. The results showed that an introduction of a thin layer of polymer (PS-OH) on the silicon substrate could improve the contact uniformity so as the oxidation. Finally, a rolling nanoelectrode lithography process was proposed, for the first time, to scale up the nanoelectrode lithography technique for large-area nanofabrication. A test-bed was developed to realize uniform pressure distribution over the whole contact area so that the local oxidation process occurs uniformly over a large area of the samples. A brass roller wrapped with a fabricated polycarbonate strip has been used as a stamp to generate nanopatterns on a silicon surface. The experimental results indicated that a significant improvement in pattern uniformity compared to the other results was obtained with the conventional NEL process. Moreover, the impact of pattern direction has been investigated, which shows no significant variation in the oxide pattern. Lastly, the rolling speed and the applied bias voltage were identified as the primary control parameters for the oxide growth

Dynamic behaviours of water droplets impacting on laser ablated surfaces

In order to reveal the underlying mechanism of surface microstructure-determined wetting states, this paper adopted Volume of Fluid (VOF) method to investigate the dynamic behaviours of water droplets impacting on surfaces with different structures at low and high Weber numbers. The simulation results showed that the high and stable pressure of air pockets is critical for the formation of the superhydrophobicity. A superhydrophobic substrate will result in shorter recoiling time and longer rebound time for water droplet than the hydrophobic substrate. Furthermore, superhydrophobic surface resulted in higher kinetic energy for water droplet than hydrophobic surfaces, which is the underlying mechanism of microstructure-enabled self-cleaning function. High-speed camera tests of laser processed surface microstructures were conducted to validate the observation in dynamic impacting simulation. The results in both high-speed camera testing and VOF simulation proved that water droplet will have a lower adhesion force when impacting superhydrophobic surface than hydrophobic surface

Three-dimensional nanostructures enabled by customised voltage waveform-induced local anodic oxidation lithography

Atomic force microscope-based local anodic oxidation nanolithography has been recognised as an effective approach for the fabrication of nanoscale patterns for next-generation electronic devices. Particularly, the use of oscillating tips can reduce the spatial size of the processed patterns and significantly increase the tool lifetime. However, it is still very challenging to achieve three-dimensional nanostructures as oxidation occurs only in a nanoscale water bridge formed between the tip and the sample, where multiple parameters must be precisely and strategically controlled to obtain nanopatterns with different heights. In this paper, a new local anodic oxidation lithographic technique is researched with the application of customised voltage waveforms to generate 3D nanostructures on silicon wafers. The relationship between pulse waveform parameters and oxidation growth was investigated. Through adjusting the tip-scan frequency and modulation of pulses, it was shown that waveform-modulated local anodic oxidation could fabricate 3D nanostructures across a region in a single scan