ALD resist formed by vapor-deposited self-assembled monolayers

Enhanced Deposition Selectivity of High-k Dielectrics by Vapor Dosing and Selective Removal of Phosphonic Acid Inhibitors

Area-selective atomic layer deposition (AS-ALD), which provides a bottom-up nanofabrication method with atomic-scale precision, has attracted a great deal of attention as a means to alleviate the problems associated with conventional top-down patterning. In this study, we report a methodology for achieving selective deposition of high-k dielectrics by surface modification through vapor-phase functionalization of octadecylphosphonic acid (ODPA) inhibitor molecules accompanied by post-surface treatment. A comparative evaluation of deposition selectivity of ZrO2 thin films deposited with the O2 and O3 reactants was performed on SiO2, TiN, and W substrates, and we confirmed that high enough deposition selectivity over 10 nm can be achieved even after 200 cycles of ALD with the O2 reactant. Subsequently, the electrical properties of ZrO2 films deposited with O2 and O3 reactants were investigated with and without post-deposition treatment. We successfully demonstrated that high-quality ZrO2 thin films with high dielectric constants and stable antiferroelectric properties can be produced by subjecting the films to ozone, which can eliminate carbon impurities within the films. We believe that this work provides a new strategy to achieve highly selective deposition for AS-ALD of dielectric on dielectric (DoD) applications toward upcoming bottom-up nanofabrication.

Droplet slipperiness despite surface heterogeneity at molecular scale

Friction determines whether liquid droplets slide off a solid surface or stick to it. Surface heterogeneity is generally acknowledged as the major cause of increased contact angle hysteresis and contact line friction of droplets. Here we challenge this long-standing premise for chemical heterogeneity at the molecular length scale. By tuning the coverage of self-assembled monolayers (SAMs), water contact angles change gradually from about 10° to 110° yet contact angle hysteresis and contact line friction are low for the low-coverage hydrophilic SAMs as well as high-coverage hydrophobic SAMs. Their slipperiness is not expected based on the substantial chemical heterogeneity of the SAMs featuring uncoated areas of the substrate well beyond the size of a water molecule as probed by metal reactants. According to molecular dynamics simulations, the low friction of both low- and high-coverage SAMs originates from the mobility of interfacial water molecules. These findings reveal a yet unknown and counterintuitive mechanism for slipperiness, opening new avenues for enhancing the mobility of droplets.

Vapor-Phase Halogenation of Hydrogen-Terminated Silicon(100) Using N-Halogen-succinimides

The focus of this study was to demonstrate the vapor-phase halogenation of Si(100) and subsequently evaluate the inhibiting ability of the halogenated surfaces toward atomic layer deposition (ALD) of aluminum oxide (Al2O3). Hydrogen-terminated silicon ⟨100⟩ (H-Si(100)) was halogenated using N-chlorosuccinimide (NCS), N-bromosuccinimide (NBS), and N-iodosuccinimide (NIS) in a vacuum-based chemical process. The composition and physical properties of the prepared monolayers were analyzed by using X-ray photoelectron spectroscopy (XPS) and contact angle (CA) goniometry. These measurements confirmed that all three reagents were more effective in halogenating H-Si(100) over OH-Si(100) in the vapor phase. The stability of the modified surfaces in air was also tested, with the chlorinated surface showing the greatest resistance to monolayer degradation and silicon oxide (SiO2) generation within the first 24 h of exposure to air. XPS and atomic force microscopy (AFM) measurements showed that the succinimide-derived Hal-Si(100) surfaces exhibited blocking ability superior to that of H-Si(100), a commonly used ALD resist. This halogenation method provides a dry chemistry alternative for creating halogen-based ALD resists on Si(100) in near-ambient environments.

Computational Investigation of Precursor Blocking during Area-Selective Atomic Layer Deposition Using Aniline as a Small-Molecule Inhibitor

Area-selective atomic layer deposition using small-molecule inhibitors (SMIs) involves vapor-phase dosing of inhibitor molecules, resulting in an industry-compatible approach. However, the identification of suitable SMIs that yield a high selectivity remains a challenging task. Recently, aniline (C₆H₅NH₂) was shown to be an effective SMI during the area-selective deposition (ASD) of TiN, giving 6 nm of selective growth on SiO₂ in the presence of Ru and Co non-growth areas. In this work, using density functional theory (DFT) and random sequential adsorption (RSA) simulations, we investigated how aniline can effectively block precursor adsorption on specific areas. Our DFT calculations confirmed that aniline selectively adsorbs on Ru and Co non-growth areas, whereas its adsorption on the SiO₂ growth area is limited to physisorption. DFT reveals two stable adsorption configurations of aniline on the metal surfaces. Further calculations on the aniline-functionalized surfaces show that the aniline inhibitor significantly reduces the interaction of Ti precursor, tetrakis(dimethylamino)titanium, with the non-growth area. In addition, RSA simulations showed that the co-presence of two stable adsorption configurations allows for a high surface inhibitor coverage on both Co and Ru surfaces. As the surface saturates, there is a transition from the thermodynamically most favorable adsorption configuration to the sterically most favorable adsorption configuration, which results in a sufficiently dense inhibition layer, such that an incoming precursor molecule cannot fit in between the adsorbed precursor molecules. We also found that, as a result of the catalytic activity of the metallic non-growth area, further reactions of inhibitor molecules, such as hydrogenolysis, can play a role in precursor blocking.

Characterizing Self-Assembled Monolayer Breakdown in Area-Selective Atomic Layer Deposition

To enable area-selective atomic layer deposition (AS-ALD), self-assembled monolayers (SAMs) have been used as the surface inhibitor to block a variety of ALD processes. The integrity of the SAM throughout the ALD process is critical to AS-ALD. Despite the demonstrated effectiveness of inhibition by SAMs, nucleation during ALD eventually occurs on SAM-protected surfaces, but its impact on SAM structures is still not fully understood. In this study, we chose the octadecyltrichlorosilane (ODTS) SAM as a model system to investigate the evolution of crystallinity and structure of SAMs before and after ALD. The breakdown behavior of SAMs when exposed to ZnO and Al₂O₃ ALD was systematically studied by combining synchrotron X-ray techniques and electron microscopy. We show that the crystallinity and structure of ODTS SAMs grown on Si substrates remain intact until a significant amount of material deposition takes place. In addition, the undesired ALD materials that grow on ODTS SAMs present contrasting morphologies: dispersed nanoparticles for ZnO while relatively continuous film for Al₂O₃. Lastly, substrate dependency was explored by comparing a Si substrate to single-crystal sapphire. Similar results in the evolution of SAM crystallinity and formation of ALD nuclei on top of SAM are observed in the ODTS-sapphire system. This study provides an in-depth view of the influence of ALD processes on the SAM structure and the nucleation behavior of ALD on SAM-protected surfaces.

Selective Gas-Phase Functionalization of SiO2 and SiNx Surfaces with Hydrocarbons

Selective functionalization of dielectric surfaces is required for area-selective atomic layer deposition and etching. We have identified precursors for the selective gas-phase functionalization of plasma-deposited SiO₂ and SiN_x surfaces with hydrocarbons. The corresponding reaction mechanism of the precursor molecules with the two surfaces was studied using in situ surface infrared spectroscopy. We show that at a substrate temperature of 70 °C, cyclic azasilanes preferentially react with an -OH-terminated SiO₂ surface over a -NH_x-terminated SiN_x surface with an attachment selectivity of ∼5.4, which is limited by the partial oxidation of the SiN_x surface. The cyclic azasilane undergoes a ring-opening reaction where the Si-N bond cleaves upon the reaction with surface -OH groups forming a Si-O-Si linkage. After ring opening, the backbone of the grafted hydrocarbon is terminated with a secondary amine, -NHCH₃, which can react with water to form an -OH-terminated surface and release CH₃NH₂ as the product. The surface coverage of the grafted cyclic azasilane is calculated as ∼3.3 × 10¹⁴ cm^-2, assuming that each reacted -OH group contributes to one hydrocarbon linkage. For selective attachment to SiN_x over SiO₂ surfaces, we determined the reaction selectivity of aldehydes. We demonstrate that aldehydes selectively attach to SiN_x over SiO₂ surfaces, and for the specific branched aliphatic aldehyde used in this work, almost no reaction was detected with the SiO₂ surface. A fraction of the aldehyde molecules reacts with surface -NH₂ groups to form an imine (Si-N═C) surface linker with H₂O released as the byproduct. The other fraction of the aldehydes also reacts with surface -NH₂ groups but do not undergo the water-elimination step and remains attached to the surface as an aminoalcohol (Si-NH-COH-). The surface coverage of the grafted aldehyde is calculated as ∼9.8 × 10¹⁴ cm^-2 using a known infrared absorbance cross-section for the -C(CH₃)₃ groups.

Area-selective atomic layer deposition of noble metals: Polymerized fluorocarbon layers as effective growth inhibitors

The increasingly complex nanoscale three-dimensional and multilayered structures utilized in nanoelectronic, catalytic, and energy conversion/storage devices necessitate novel substrate-selective material deposition approaches featuring bottom-up and self-aligned precision processing. Here, we demonstrate the area-selective atomic layer deposition (AS-ALD) of two noble metals, Pt and Pd, by using a plasma-polymerized fluorocarbon layer as growth inhibition surfaces. The contact angle, x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy measurements were performed to investigate the blocking ability of polymerized fluorocarbon (CFx) layers against ALD-grown metal films. Both Pt and Pd showed significant nucleation delays on fluorocarbon surfaces. Self-aligned film deposition is confirmed using this strategy by growing Pt and Pd on the microscale lithographically patterned CFx/Si samples. CFx blocking layer degradation during ozone exposure was analyzed using XPS measurements, which confirmed the oxygen physisorption as the main responsible surface reaction with further hydroxyl group formation on the CFx surface. Our work reveals that the CFx layer is compatible with an ozone coreactant until the blocking polymer cannot withstand oxygen physisorption. Our results could potentially be used to investigate and develop radical-assisted AS-ALD processes for a wider selection of materials.

Precise Definition of a "Monolayer Point" in Polymer Brush Films for Fabricating Highly Coherent TiO2 Thin Films by Vapor-Phase Infiltration

In this work, we show that in order to fabricate coherent titania (TiO₂) films with precise thickness control, it is critical to generate a complete polymer brush monolayer. To date, demonstrations of such dense polymer monolayer formation that can be utilized for inorganic infiltration have been elusive. We describe a versatile bottom-up approach to covalently and rapidly (60 s processing) graft hydroxyl-terminated poly(2-vinyl pyridine) (P2VP-OH) polymers on silicon substrates. P2VP-OH monolayer films of varying thicknesses can subsequently be used to fabricate high-quality TiO₂ films. Our innovative strategy is based upon room-temperature titanium vapor-phase infiltration of the grafted P2VP-OH polymer brushes that can produce TiO₂ nanofilms of 2-4 nm thicknesses. Crucial parameters are explored, including molecular weight and solution concentration for grafting dense P2VP-OH monolayers from the liquid phase with high coverage and uniformity across wafer-scale areas (>2 cm²). Additionally, we compare the P2VP-OH polymer systems with another reactive polymer, poly(methyl methacrylate)-OH, and a relatively nonreactive polymer, poly(styrene)-OH. Furthermore, we prove the latter to be effective for surface blocking and deactivation. We show a simple process to graft monolayers for polymers that are weakly interacting with one another but more challenging for reactive systems. Our methodology provides new insight into the rapid grafting of polymer brushes and their ability to form TiO₂ films. We believe that the results described herein are important for further expanding the use of reactive and unreactive polymers for fields including area-selective deposition, solar cell absorber layers, and antimicrobial surface coatings.

Effect of Multilayer versus Monolayer Dodecanethiol on Selectivity and Pattern Integrity in Area-Selective Atomic Layer Deposition

Monolayer and multilayer dodecanethiols (DDT) can be assembled onto a copper surface from the vapor phase depending on the initial oxidation state of the copper. The ability of the copper-bound dodecanethiolates to block atomic layer deposition (ALD) and the resulting behavior at the interfaces of Cu/SiO₂ patterns during area-selective ALD (AS-ALD) are compared between mono- and multilayers. We show that multilayer DDT is ∼7 times more effective at blocking ZnO ALD from diethylzinc and water than is monolayer DDT. Conversely, monolayer DDT exhibits better performance than does multilayer DDT in blocking of Al₂O₃ ALD from trimethylaluminum and water. Investigation into interfacial effects at the interface between Cu and SiO₂ on Cu/SiO₂ patterns reveals both a gap at the SiO₂ edges and a pitch size-dependent nucleation delay of ZnO ALD on SiO₂ regions of multilayer DDT-coated patterns. In contrast, no impact on ZnO ALD is observed on the SiO₂ regions of monolayer DDT-coated patterns. We also show that these interfacial effects depend on the ALD chemistry. Whereas an Al₂O₃ film grows on the TaN diffusion barrier of a DDT-treated Cu/SiO₂ pattern, the ZnO film does not. These results indicate that the structure of the DDT layer and the ALD precursor chemistry both play an important role in achieving AS-ALD.

Detecting and Removing Defects in Organosilane Self-Assembled Monolayers

Defects occur as self-assembled monolayers form, and the number and type of defects depend on the surface preparation and deposition solvent, among other parameters. Indirect measures to detect defects using a layer property, such as the thickness or bond vibrational frequency, are used routinely for process development but often lack sensitivity. Direct measures using an atomic probe offer a glimpse of defect structures but over a small fraction of the layer. Direct detection after reacting defects by etching or deposition is more common, and this approach has advanced our understanding of how monolayers form and has led to improved monolayers for a variety of applications. Here we show that a series of TiCl₄ gas pulses reacts with defects in organosilane layers on SiO₂ depositing TiO, which was measured by X-ray photoelectron spectroscopy. The defects were silanol groups and siloxane bridge bonds at the interface between the layer and the SiO₂ surface and on agglomerates physisorbed to the layer. As the TiO saturation coverage or the total number of defects decreased, the incubation period in which no TiO was detected became longer. Cleaning the layer by solvent extraction to remove nonpolar agglomerates followed by an aqueous mixture of ammonium hydroxide and hydrogen peroxide, which is Standard Clean 1, a common particle removal step for silicon surfaces, produced an organosilane monolayer without agglomerates based on atomic force microscopy. After a second organosilane immersion, the monolayer density rose to 3.8 molecules/nm². This monolayer inhibited the deposition of TiO on the SiO₂ surface for 250 pulses of TiCl₄ and 200 complete TiO₂ atomic layer deposition cycles using TiCl₄ and water vapor, and it failed at 300 complete cycles. The Standard Clean 1 solution not only removed defects left by solvent extraction but also led to the reorganization of the organosilane layer.

Local Structure and Point-Defect-Dependent Area-Selective Atomic Layer Deposition Approach for Facile Synthesis of p-Cu2O/n-ZnO Segmented Nanojunctions

Area-selective atomic layer deposition (AS-ALD) has attracted much attention in recent years due to the possibility of achieving accurate patterns in nanoscale features, which render this technique compatible with the continuous downscaling in nanoelectronic devices. The growth selectivity is achieved by starting from different materials and results (ideally) in localized growth of a single material. We propose here a new concept, more subtle and general, in which a property of the substrate is modulated to achieve localized growth of different materials. This concept is demonstrated by selective growth of high-quality metallic Cu and semiconducting Cu₂O thin films, achieved by changing the type of majority point defects in the ZnO underneath film exposed to the reactive species using a patterned bilayer structure composed of highly conductive and highly resistive areas, as confirmed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The selective growth of these materials in a patterned ZnO/Al-doped ZnO substrate allows the fabrication of p-Cu₂O/n-ZnO nanojunctions showing a nonlinear rectifying behavior typical of a p-n junction, as confirmed by conductive atomic force microscopy (C-AFM). This process expands the spectra of materials that can be grown in a selective manner by ALD and opens up the possibility of fabricating different architectures, taking advantage of the area-selective deposition. This offers a variety of opportunities in the field of transparent electronics, catalysis, and photovoltaics.

Chemical Vapor Deposition of Azidoalkylsilane Monolayer Films

N₃-functionalized monolayers on silicon wafer substrates are prepared via the controlled vapor-phase deposition of 11-azidoundecyltrimethoxysilanes at reduced pressure and elevated temperature. The quality of the layer is assessed using contact angle, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and ellipsometry measurements. At 60 °C, longer deposition times are needed to achieve monolayers with similar N₃ density compared to depositions at 145 °C. The monolayers formed via the vapor phase are denser compared to those formed via a solvent-based deposition process. ATR-FTIR measurements confirm the incorporation of azido-alkyl chains in the monolayer and the formation of siloxane bridges with the underlying oxide at both deposition temperatures. X-ray photon spectroscopy shows that the N₃ group is oriented upward in the grafted layer. Finally, the density was determined using total reflection X-ray fluorescence after a click reaction with chlorohexyne and amounts to 2.5 × 10¹⁴ N₃ groups/cm². In summary, our results demonstrate the formation of a uniform and reproducible N₃-containing monolayer on silicon wafers, hereby providing a functional coating that enables click reactions at the substrate.

Selective Deposition of Dielectrics: Limits and Advantages of Alkanethiol Blocking Agents on Metal-Dielectric Patterns

Area selective atomic layer deposition has the potential to significantly improve current fabrication approaches by introducing a bottom-up process in which robust and conformal thin films are selectively deposited onto patterned substrates. In this paper, we demonstrate selective deposition of dielectrics on metal/dielectric patterns by protecting metal surfaces using alkanethiol blocking layers. We examine alkanethiol self-assembled monolayers (SAMs) with two different chain lengths deposited both in vapor and in solution and show that in both systems, thiols have the ability to block surfaces against dielectric deposition. We show that thiol molecules can displace Cu oxide, opening possibilities for easier sample preparation. A vapor-deposited alkanethiol SAM is shown to be more effective than a solution-deposited SAM in blocking ALD, even after only 30 s of exposure. The vapor deposition also results in a much better thiol regeneration process and may facilitate deposition of the SAMs on porous or three-dimensional structures, allowing for the fabrication of next generation electronic devices.

Nanoscale selective area atomic layer deposition of TiO2 using e-beam patterned polymers

Self aligned nano patterning of TiO₂ using area selective atomic area deposition.

Quantum confinement effects in transferrable silicon nanomembranes and their applications on unusual substrates

Two dimensional (2D) semiconductors have attracted attention for a range of electronic applications, such as transparent, flexible field effect transistors and sensors owing to their good optical transparency and mechanical flexibility. Efforts to exploit 2D semiconductors in electronics are hampered, however, by the lack of efficient methods for their synthesis at levels of quality, uniformity, and reliability needed for practical applications. Here, as an alternative 2D semiconductor, we study single crystal Si nanomembranes (NMs), formed in large area sheets with precisely defined thicknesses ranging from 1.4 to 10 nm. These Si NMs exhibit electronic properties of two-dimensional quantum wells and offer exceptionally high optical transparency and low flexural rigidity. Deterministic assembly techniques allow integration of these materials into unusual device architectures, including field effect transistors with total thicknesses of less than 12 nm, for potential use in transparent, flexible, and stretchable forms of electronics.

Advances in top-down and bottom-up surface nanofabrication: techniques, applications & future prospects

This review highlights the most significant advances of the nanofabrication techniques reported over the past decade with a particular focus on the approaches tailored towards the fabrication of functional nano-devices. The review is divided into two sections: top-down and bottom-up nanofabrication. Under the classification of top-down, special attention is given to technical reports that demonstrate multi-directional patterning capabilities less than or equal to 100 nm. These include recent advances in lithographic techniques, such as optical, electron beam, soft, nanoimprint, scanning probe, and block copolymer lithography. Bottom-up nanofabrication techniques--such as, atomic layer deposition, sol-gel nanofabrication, molecular self-assembly, vapor-phase deposition and DNA-scaffolding for nanoelectronics--are also discussed. Specifically, we describe advances in the fabrication of functional nanocomposites and graphene using chemical and physical vapor deposition. Our aim is to provide a comprehensive platform for prominent nanofabrication tools and techniques in order to facilitate the development of new or hybrid nanofabrication techniques leading to novel and efficient functional nanostructured devices.

Deposition of dense siloxane monolayers from water and trimethoxyorganosilane vapor

A convenient, laboratory-scale method for the vapor deposition of dense siloxane monolayers onto oxide substrates was demonstrated. This method was studied and optimized at 110 °C under reduced pressure with the vapor of tetradecyltris(deuteromethoxy)silane, (CD(3)O)(3)Si(CH(2))(13)CH(3), and water from the dehydration of MgSO(4)·7H(2)O. Ellipsometric thicknesses, water contact angles, Fourier transform infrared (FTIR) spectroscopy, and electrochemical capacitance measurements were used to probe monolayer densification. The CD(3) stretching mode in the FTIR spectrum was monitored as a function of the deposition time and amounts of silane and water reactants. This method probed the unhydrolyzed methoxy groups on adsorbed silanes. Excess silane and water were necessary to achieve dense, completely hydrolyzed monolayers. In the presence of sufficient silane, an excess of water above the calculated stoichiometric amount was necessary to hydrolyze all methoxy groups and achieve dense monolayers. The excess water was partially attributed to the reversibility of the hydrolysis of the methoxy groups.

Monitoring and mapping imperfections in silane-based self-assembled monolayers by chemical amplification

This paper describes an innovative and simple technique for analyzing defects in silane-based self-assembled monolayers. The assembly of monolayers is a simple method for chemically modifying surfaces, which can be important for resisting chemical attack or adhesion of biomolecules. Measuring the molecular scale properties of monolayers and the reproducibility of their ability to uniformly modify a surface requires tools that provide limits of detection at the level of at least a few atoms per million with specificity to the top couple nanometers of the surface. To achieve this level of sensitivity a new technique is developed that combines spectroscopy and microscopy techniques (particularly atomic force microscopy) with chemical amplification of exposed silicon in self-assembled monolayers of silane molecules. This development is an important achievement for monitoring the quality of monolayers as a function of modifications to the method(s) used to deposit the silane molecules. Techniques presented here could be easily extended to assessing the molecular scale quality of other surface modifications.

Formation of alkanethiolate self-assembled monolayers at halide-terminated Ge surfaces

We have studied Ge halide passivation and formation of 1-octadecanethiolate self-assembled monolayers (SAMs) at Cl- and Br-terminated Ge(100) and Ge(111) surfaces. The results of water contact angle measurements, ellipsometry, transmission infrared spectroscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy show that good quality 1-alkanethiolate SAMs can be achieved at both Cl- and Br-terminated surfaces via direct Ge-S bonds. The quality of the SAMs depends on the concentration and the solvent of the 1-alkanethiol solution. Moreover, SAMs formed at Ge(100) surfaces have higher water contact angles, thicknesses, and ambient stability than those formed at Ge(111) surfaces. Surface passivation and light are found to play an important role in the packing and stability of the SAMs. Furthermore, well-packed SAMs can be retrieved by repassivation after degradation due to ambient exposure. This work presents novel routes for Ge surface passivation.

TEM-based metrology for HfO2 layers and nanotubes formed in anodic aluminum oxide nanopore structures

Nanotubes are fabricated by atomic layer deposition (ALD) into nanopore arrays created by anodic aluminum oxide (AAO). A transmission electron microscopy (TEM) methodology is developed and applied to quantify the ALD conformality in the nanopores (thickness as a function of depth), and the results are compared to existing models for ALD conformality. ALD HfO2 nanotubes formed in AAO templates are released by dissolution of the Al2O3, transferred to a grid, and imaged by TEM. An algorithm is devised to automate the quantification of nanotube wall thickness as a function of position along the central axis of the nanotube, by using a cylindrical model for the nanotube. Diffusion-limited depletion occurs in the lower portion of the nanotubes and is characterized by a linear slope of decreasing thickness. Experimentally recorded slopes match well with two simple models of ALD within nanopores presented in the literature. The TEM analysis technique provides a method for the rapid analysis of such nanostructures in general, and is also a means to efficiently quantify ALD profiles in nanostructures for a variety of nanodevice applications.