Vapor Deposition Of Thin Films Research Articles

Nanoscale View of Alignment and Domain Growth in a Hexagonal Columnar Liquid Crystal.

Highly ordered liquid crystalline (LC) phases have important potential for organic electronics. We studied the molecular alignment and domain structure in a columnar LC thin film with nanometer resolution during in situ heating using four-dimensional scanning transmission electron microscopy (4D STEM). The initial disordered vapor-deposited LC glass thin film rapidly ordered at its glass transition temperature into a hexagonal columnar phase with small (<10 nm), well-aligned, planar domains (columns oriented parallel to the surface). Upon further heating, the domains coarsen via bulk diffusion, then the film crystallizes, then finally transforms back to an LC phase at an even higher temperature. The LC phase at high temperature shows straight columns of molecules, which we attribute to structure inherited from the intermediate crystalline phase. Nanoscale 4D STEM offers direct insight into the mechanisms of domain reorganization, and intermediate crystallization is a potential approach to manipulate orientational order and texture at the nano- to mesoscale in LC thin films.

Remarkable difference in structural relaxation dynamics of conventionally prepared bulk glass and vapor-deposited thin films.

The structural relaxation dynamics of conventionally prepared bulk glass of N,N'-bis(3-methylphenyl)-N,N'-diphenyl-benzidine (TPD) was measured by differential scanning calorimetry. The calorimetric data were quantitatively described in terms of the Tool-Narayanaswamy-Moynihan (TNM) model. The TNM parameters were evaluated using a combination of linearization and non-linear optimization methodologies: h*/R = 109.5 kK, ln(A/s) = -321, β = 0.37, x = 0.64. In addition, the TNM phenomenology was used to describe recently reported normalized thickness data of stable and aged thin TPD films measured by ellipsometry. The structural relaxation was found to proceed at a markedly higher rate in these thin films prepared by the physical vapor deposition compared to that of conventional bulk glass. This feature appears to be associated with the significantly narrower distribution of relaxation times (β ≅ 0.8) observed for stable thin film in "as-deposited" form with uniquely dense molecular packing. Interestingly, very similar attributes of the relaxation kinetics were also found in the aged thin film with a previously erased thermal history associated with the deposition.

Duplex surface engineering of cold spray Ti coatings and physical vapor-deposited TiN and AlTiN thin films

The feasibility of a duplex coating based on cold spray technology and magnetron sputtering was evaluated for repair applications requiring a ‘thin-on-thick’ layered structure. Commercially pure angular-shaped Ti grade 4 particles are fed to a cold spray gun and accelerated toward a Ti alloy substrate to deposit thick coatings (∼4.5 mm). TiN and AlTiN thin films are deposited on polished cold spray coatings using a four-source closed-field unbalanced magnetron sputtering (CFUBMS) system. Microstructure was characterized using focused ion beam (FIB) lift-out, scanning electron microscopy (SEM), and electron channeling contrast imaging (ECCI). The nanoindentation technique was used to evaluate the mechanical properties of coatings. The H/E ratios and H3/E2 ratios for TiN films were found to be 0.098 and 0.26 GPa, respectively, while those for AlTiN films were measured at 0.066 and 0.052 GPa, respectively, suggesting higher capacity of TiN films to withstand both elastic and plastic deformation. Using scratch testing, the adhesion of TiN and AlTiN thin films to cold spray Ti was investigated, with TiN-Ti duplex coatings exhibiting better performance compared to AlTiN-Ti coatings. Tribological testing was performed on duplex coatings using a reciprocating tribometer equipped with an alumina ball counterface. The wear rate for AlTiN-Ti coatings after 2000 sliding cycles was found to be (1.0 × 10−3 ± 0.1 × 10−3 mm3/Nm), three orders of magnitudes higher than that for TiN-Ti (8 × 10−6 ± 2 × 10−6 mm3/Nm. SEM was used to reveal worn surface morphologies and cross-sectional analysis of the wear track. Subsurface microstructural changes due to wear were examined using focused ion beam cross-sectioning, revealing bending cracks and tribofilm formation.

Volatile N,N′-dialkyl-β-dialdiminate complexes of magnesium and zinc as possible chemical vapor deposition precursors

A series of volatile magnesium(II) and zinc(II) bis(β-dialdiminato) complexes (of stoichiometry M[RN(CH)3NR]2 where R = methyl, ethyl, isopropyl, or tert-butyl) have been prepared and investigated as potential precursors for the chemical vapor deposition of thin films. Several of the compounds have been characterized crystallographically; the metal centers adopt pseudo-tetrahedral geometries with the two ligands forming six-membered rings with the metal center. All eight compounds sublime cleanly both in vacuum and under 1 atm of N2, and the most volatile compounds (those having N-methyl substituents) have vapor pressures of 1 Torr at 88 (Mg) and 90 °C (Zn). In general, metal complexes of N,N′-dialkyl-β-dialdiminate ligands are more volatile than their corresponding N,N′-dialkyl-β-diketiminate and N,N-dialkylamidinate analogues.

Methylation of Si(111) Modulates Molecular Orientation in Perylenediimide Thin Films.

Singlet fission produces a pair of low-energy spin-triplet excitons from a single high-energy spin-singlet exciton. While this process offers the potential to enhance the efficiency of silicon solar cells by ∼30%, meeting this goal requires overlayer materials that can efficiently transport triplet excitons to an underlying silicon substrate. Herein, we demonstrate that the chemical functionalization of silicon surfaces controls the structure of vapor-deposited thin films of perylenediimide (PDI) dyes, which are prototypical singlet fission materials. Using a combination of atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS), we find terminating Si(111) with either a thin, polar oxide layer (SiOx) or with hydrophobic methyl groups (Si-CH3) alters the structures of the resulting PDI films. While PDI films grown on SiOx are comprised of small crystalline grains that largely adopt an "edge-on" orientation with respect to the silicon surface, films grown on Si-CH3 contain large grains that prefer to align in a "face-on" manner with respect to the substrate. This "face-on" orientation is expected to enhance exciton transport to silicon. Interestingly, we find that the preferred mode of growth for different PDIs correlates with the space group associated with bulk crystals of these compounds. While PDIs that inhabit a monoclinic (P21/c) space group nucleate films by forming tall and sparse crystalline columns, PDIs that inhabit triclinic (P1̅) space groups afford films comprised of uniform, lamellar PDI domains. The results highlight that silicon surface functionalization profoundly impacts PDI thin film growth, and rational selection of a hydrophobic surface that promotes "face-on" adsorption may improve energy transfer to silicon.

A hierarchical model-based method for wafer level virtual metrology under process information deficiency

Online inspection is one of the most critical processes of quality control in semiconductor manufacturing. The physical inspection methods for wafers are time-consuming and unable to achieve wafer level metrology. In order to improve production efficiency and expand inspection coverage, virtual metrology (VM) methods have recently received widespread attention; they utilize process parameters to estimate wafer metrology results. However, due to process drift and other reasons, the process information contained in real-time signal data (RTS data) used for VM modeling in industrial production is insufficient. This work proposed a hierarchical modeling method for machine learning-based virtual wafer metrology, leveraging RTS and post-process quality characteristics. The hierarchical model consists of an multiway principle analysis (MPCA) sub-model for RTS feature extracting and two separate long short-term memory (LSTM) networks for wafer-to-wafer dynamics in RTS and quality characteristics, respectively. A case study on the thickness VM of chemical vapor deposition thin film is conducted, and the proposed method has achieved better results than other methods in comparison.

Sub-500 nm patterned synthesis of corrosion resistant SnO2 and Pt@SnO2 using metal-organic precursor

In this study, a thin film of transparent conducting metal oxide, SnO2, is obtained using a novel metal-organic precursor, Sn-TOABr complex. The precursor is synthesized by phase-transferring tin from the aqueous layer to the liquid-liquid interface. The formed precursor is used for depositing SnO2 thin films over arbitrary and even large substrates via the modified atmospheric pressure chemical vapor deposition (MAPCVD) method in a simple furnace at 450 °C. The vapor-deposited SnO2 thin films are crystalline, semiconducting, highly transparent with excellent mechanical robustness against scratch and scotch tape tests, and resistant towards corrosive media. The obtained robust thin films via this MAPCVD technology expand applications towards protective coatings for arbitrary substrates. Using a simple direct micromolding technique, sub-500 nm patterns of SnO2 and Pt@SnO2 were obtained. This single-step soft lithography patterning technique, besides being cost-effective, offers a large throughput.

Preparation of High-quality Vapor-deposited Crystalline Organic Thin Films through Crystallization from Their Liquid Crystal Phase

High-quality crystalline organic thin films with large crystalline domain size were fabricated by vacuum deposition, followed by a post-annealing and cooling crystallization from their respective smectic A (SmA) liquid crystal precursor states. Crystallinity, surface smoothness and air exposure stability of ionic liquid crystalline thin films were much improved by this process, and the electric double layer action was for the first time observed at the SmA phase. Moreover, a seeded temperature gradient cooling process from the SmA phase was proposed and its capability was demonstrated in the fabrication of benzothienobenzothiophene derivative thin films, achieving their directional lateral crystal growth with high precision controllability.

Pressure sensing of Ga2O3 thin film

In this paper, a Ga2O3-based pressure sensor is introduced via metalorganic chemical vapor deposition thin film growth technique. As an important semiconductor materials, it could achieve some functional electronics and optoelectronics, while Ga2O3-based pressure sensor is less investigated, inspite of that the natural property endows it the possibility. Here, the fabricated Ga2O3-based pressure sensor displayed decent sensing performance responding to various pressures. Under pressure of 5 kPa, the current increase two of orders with respect to the current without any additional pressure. Moreover, the response/release times were 0.18 s/0.15 s, 0.27 s/0.21 s, 0.32 s/0.23 s, and 0.38 s/0.24 s under different pressures of 1, 5, 10 and 20 kPa. In all, this work provides a possible route for constructing smart pressure sensor based on functional Ga2O3 along with good sensing behaviors.

Hydrolysis of Poly(fluoroacrylate) Thin Films Synthesized from the Vapor Phase.

The post-synthesis surface reaction of vapor-deposited polymer thin films is a promising technique in engineering heterogeneous surface chemistry. Because the existing research has neglected marginally reactive precursor films in preference of their highly reactive counterparts, our knowledge of kinetics and loss of film integrity during the reaction are limited. To address these limitations, we characterize hydrolysis of two fluoroacrylates, poly(1H,1H,2H,2H-perfluorooctyl acrylate) (pPFOA) and poly(2,2,3,4,4,4-hexafluorobutyl acrylate) (pHFBA), with sodium hydroxide using X-ray photoelectron spectroscopy. Without crosslinking with di(ethylene glycol)divinyl ether (DEGDVE) and grafting with trichlorovinyl silane, the films degrade rapidly during hydrolysis. An SN2 mechanism describes hydrolysis well, with rate constants of 0.0029 ± 0.0004 and 0.011 ± 0.001 L mol-1s-1 at 30 °C for p(PFOA-co-DEGDVE) and p(HFBA-co-DEGDVE), respectively. Our detailed study of hydrolysis kinetics of marginally reactive fluoroacrylates demonstrates the full capability and limitations of the post-synthesis reaction. Importantly, copolymers are characterized using a density correction new to polymer chemical vapor deposition.

Large-area vapor-deposited Cs3Cu2I5 perovskite thin films for highly effective third-order nonlinear optics

Zero-dimensional (0D) lead-free perovskites hold promise for efficient optoelectronic and photonic devices, but there are still critical challenges in fabricating large-area, high-quality thin films for practical applications. Herein, we report a simple vapor route to prepare 0D lead-free Cs3Cu2I5 perovskite thin films of high optical quality over large areas. Importantly, the deposition approach can be applied to diverse substrates, including mica, fused silica, MgO, and indium tin-oxide (ITO) coated glass. The deposited Cs3Cu2I5 thin films are phase pure and highly uniform. These Cs3Cu2I5 thin films exhibit strong blue emission upon UV light illumination, a broad band centered at 445 nm assigned to the self-trapped exciton emission. Furthermore, third-order nonlinear optical properties of the as-prepared thin films were determined by means of the Z-scan method. These thin films showed the third-order nonlinear absorption coefficient β as ∼ 1.07 × 10−7 m/W, which is five orders of magnitude larger than three-dimensional counterpart, CsPbI3. Thus, we demonstrate that large-area, high-quality Cs3Cu2I5 thin films can be vapor-deposited and have great potentials in optical switching and optical limiting applications.

Influence of Surface Roughness on the Dynamics and Crystallization of Vapor-Deposited Thin Films.

The substrate roughness is a very important parameter that can influence the properties of supported thin films. In this work, we investigate the effect of surface roughness on the properties of a vapor-deposited glass (celecoxib, CXB) both in its bulk and in confined states. Using dielectric spectroscopy, we provide experimental evidence depicting a profound influence of surface roughness on the α-relaxation dynamics and the isothermal crystallization of this vapor-deposited glass. Besides, we have verified the influence of film confinement on varying values of surface roughnesses as well. At a fixed surface roughness value, the confinement could alter both the dynamics and crystallization of vapor-deposited CXB.

Inferring topological transitions in pattern-forming processes with self-supervised learning

The identification of transitions in pattern-forming processes are critical to understand and fabricate microstructurally precise materials in many application domains. While supervised methods can be useful to identify transition regimes, they need labels, which require prior knowledge of order parameters or relevant microstructures describing these transitions. Instead, we develop a self-supervised, neural-network-based approach that does not require predefined labels about microstructure classes to predict process parameters from observed microstructures. We show that assessing the difficulty of solving this inverse problem can be used to uncover microstructural transitions. We demonstrate our approach by automatically discovering microstructural transitions in two distinct pattern-forming processes: the spinodal decomposition of a two-phase mixture and the formation of binary-alloy microstructures during physical vapor deposition of thin films. This approach opens a path forward for discovering unseen or hard-to-discern transitions and ultimately controlling complex pattern-forming processes.

Shedding light on the initial growth of ZnO during plasma-enhanced atomic layer deposition on vapor-deposited polymer thin films

Interest in atomic layer deposition (ALD) processes on polymer substrates is fueled by the increasing rise of organic electronics and polymer-based nanodevices. This study provides new insights into the initial growth and interface formation during plasma-enhanced ALD (PE-ALD) of ZnO on poly ethylene glycol dimethylacrylate (pEGDMA) and poly 2-hydroxyethyl methacrylate (pHEMA) thin films, both deposited by initiated chemical vapor deposition (iCVD). In-situ spectroscopic ellipsometry showed that PE-ALD growth on the investigated polymers is a result of two competing processes: plasma etching of the polymer substrate and ZnO nucleation and growth. During the first 10–15 ALD cycles, polymer etching was found to prevail until at a certain point (depending on plasma power and type of polymer) ZnO growth takes over and the regime of linear ALD growth is entered. On pHEMA, though more sensitive to etching, ZnO film formation starts early on, whereas on pEGDMA, subsurface nucleation and island growth appear to dominate the initial stage of deposition. Despite the initial etching, resulting ZnO films are smooth and of comparable structural quality to those grown on silicon. These findings contribute to a deeper understanding of PE-ALD growth on polymers providing knowledge essential for the successful development of new processes and applications.

Arc energy minimization in high-power impulse magnetron sputtering

High-power impulse magnetron sputtering (HiPIMS) is rather a new and rapidly developing method for physical vapor deposition of thin films. Due to a high ionization degree of the sputtered material, the HiPIMS coatings have better properties than those produced by conventional DC magnetron sputtering (DCMS) and mid-frequency magnetron sputtering (MFMS). Along with its advantages, HiPIMS has some disadvantages, which make difficult its application. HiPIMS is characterized by the high pulsed discharge current, which provides higher arc frequency and energy than in MFMS and DCMS. This leads to larger defect size and quantity that appear in the coating obtained. This paper proposes the analytical simulation model to describe the discharge current and discharge voltage during arcing. This model helps to calculate the arc energy, lifetime, and maximum current. It also identifies what parameters of the power supply exert the highest effect on the arc parameters. The paper considers the types of the HiPIMS power supply and compares them with the view of arc quenching effectiveness. The model adequacy is verified by the experimental results of measurement the arc parameters in HiPIMS. It is shown the effect of arc energy on the size and number of defects in the carbon coatings obtained in HiPIMS.

Vapor-Deposited Thin Films: Studying Crystallization and α-relaxation Dynamics of the Molecular Drug Celecoxib.

Crystallization is one of the major challenges in using glassy solids for technological applications. Considering pharmaceutical drugs, maintaining a stable amorphous form is highly desirable for improved solubility. Glasses prepared by the physical vapor deposition technique got attention because they possess very high stability, taking thousands of years for an ordinary glass to achieve. In this work, we have investigated the effect of reducing film thickness on the α-relaxation dynamics and crystallization tendency of vapor-deposited films of celecoxib (CXB), a pharmaceutical substance. We have scrutinized its crystallization behavior above and below the glass-transition temperature (Tg). Even though vapor deposition of CXB cannot inhibit crystallization completely, we found a significant decrease in the crystallization rate with decreasing film thickness. Finally, we have observed striking differences in relaxation dynamics of vapor-deposited thin films above the Tg compared to spin-coated counterparts of the same thickness.

Sequential surface passivation for enhanced stability of vapor-deposited methylammonium lead iodide thin films

Understanding the correlation between surface passivation and degradation kinetics in vapor-deposited organic–inorganic hybrid perovskite thin films is crucial for developing strategies to enhance their structural integrity for long-term operational stability. Here, we demonstrate that the sequential introduction of vapor-phase phenethylammonium iodide (PEAI) and methylammonium (MAI) into methylammonium lead iodide (MAPbI3) thin films, which is obtained from PbI2, suppresses their degradation and enhances the ambient stability. Compared with the simultaneous PEAI introduction during the conversion of PbI2 to MAPbI3, the MAPbI3 films with sequential treatment exhibited higher PEAI surface coverage, which imparts superior resistance to degradation. Combined analyses of the exposure-time-dependent crystal structure evolution, photoemission profiles, and photocurrent generation characteristics of the films reveal that conformal PEAI passivation suppresses the volatile release of methylammonium (MA) species and the inclusion of O2/H2O molecules. This study highlights the importance of developing a vapor-phase passivation strategy for innate polycrystalline perovskite thin films and their optoelectronic applications with long-term stability.

Characterization of the Interfacial Orientation and Molecular Conformation in a Glass-Forming Organic Semiconductor.

The ability to control structure in molecular glasses has enabled them to play a key role in modern technology; in particular, they are ubiquitous in organic light-emitting diodes. While the interplay between bulk structure and optoelectronic properties has been extensively investigated, few studies have examined molecular orientation near buried interfaces despite its critical role in emergent functionality. Direct, quantitative measurements of buried molecular orientation are inherently challenging, and many methods are insensitive to orientation in amorphous soft matter or lack the necessary spatial resolution. To overcome these challenges, we use polarized resonant soft X-ray reflectivity (p-RSoXR) to measure nanometer-resolved, molecular orientation depth profiles of vapor-deposited thin films of an organic semiconductor Tris(4-carbazoyl-9-ylphenyl)amine (TCTA). Our depth profiling approach characterizes the vertical distribution of molecular orientation and reveals that molecules near the inorganic substrate and free surface have a different, nearly isotropic orientation compared to those of the anisotropic bulk. Comparison of p-RSoXR results with near-edge X-ray absorption fine structure spectroscopy and optical spectroscopies reveals that TCTA molecules away from the interfaces are predominantly planar, which may contribute to their attractive charge transport qualities. Buried interfaces are further investigated in a TCTA bilayer (each layer deposited under separate conditions resulting in different orientations) in which we find a narrow interface between orientationally distinct layers extending across ≈1 nm. Coupling this result with molecular dynamics simulations provides additional insight into the formation of interfacial structure. This study characterizes the local molecular orientation at various types of buried interfaces in vapor-deposited glasses and provides a foundation for future studies to develop critical structure-function relationships.

Numerical investigation of SiO&lt;sub&gt;2&lt;/sub&gt; film deposition enhanced by capacitively coupled discharge plasma

Higher requirements for the performances of thin films need to be fulfilled in the rapid development of integrated circuit technology, due to the more complicate structure and smaller size of chips. In plasma-enhanced chemical vapor deposition , high-density and high-performance thin films can be deposited at low temperature, compared with traditional chemical vapor deposition. In this work, a two-dimensional fluid/MC model coupled with the deposition module is used to describe the capacitively coupled SiH&lt;sub&gt;4&lt;/sub&gt;/N&lt;sub&gt;2&lt;/sub&gt;O/Ar discharges as well as the deposition processes, focusing on the influences of the radial position, gas ratio and gas pressure on the deposition of silicon oxide films. The results show that the edge effect which leads the plasma density to rise near the electrode edges gives rise to the non-uniform deposition rate along the radial direction. It is also found that the more N&lt;sub&gt;2&lt;/sub&gt;O and less Ar content in the gas mixture, as well as an increased gas pressure will improve this uniformity. However, an excessive deposition rate will lead to a series of undesirable phenomena, such as “key hole structure”, vacancies and excessive impurities in films. These problems are also troublesome in the microelectronics manufacture processes. More detailed investigation into the deposition mechanism can be expected in the future .

Material-dependent submicrometer particle trapping in capacitively-coupled plasma sheaths in an intermediate collision regime

Understanding submicrometer particle behavior in non-thermal capacitively coupled plasmas (CCPs) is important in the application of CCP reactors in thin-film vapor deposition; nucleated and resuspended particles can deposit on thin films, forming defects. Prior studies of supermicrometer particle behavior in CCP reactors have revealed that particles are trapped in the pre-sheath or sheath regions near electrodes, but have examined in detail neither the trapping of submicrometer particles, nor the influence of particle material properties on trapping. Using laser light scattering (LLS), we examined trapping of submicrometer metal oxide particles (radii in the 211 nm–565?nm range) of 6 distinct material compositions in the pre-sheath/sheath region of a CCP reactor operated at pressures in the 0.5–2.0 Torr range. We specifically focus on trapping near the upper electrode of a horizontally-oriented reactor. In this instance, trapping is brought about by a balance between electrostatic forces and gravitational forces driving particles away from the electrode, with ion drag forces driving particles toward the electrode. LLS measurements reveal that submicrometer particles are trapped near the upper electrode for all particle sizes, types, and operating pressures, with the trapping location at an increased distance away from the electrode with decreased CCP reactor pressure. Interestingly, we find the trapping location shifts slightly farther from the top electrode with increasing material dielectric constant. This suggests that the ion drag force is influenced by particle material properties, though in an unclarified manner. Measured trapping locations are also compared to model predictions where particle charge levels and the ion drag force are calculated using expressions based on ion trajectory calculations in a plasma sheath accounting for ion–neutral collisions. Predicted ion densities required for trapping are a factor of 6–16 higher than calculated at the observed particle trapping locations when applying a dissipative ion–particle encounter model, with more substantial disagreement found when considering a non-dissipative encounter model. In total, our results confirm that submicrometer particle trapping occurs at the upper electrode of CCP reactors, which must be facilitated by a balance largely between electrostatic and gravitational forces opposed by ion drag forces, but suggest future studies will be required to understand how particle material properties affect forces on particles on the plasma volume boundary, and how the ion drag force is sufficiently high to facilitate trapping.