Vapor Deposition Of Thin Films Research Articles

Thermodynamic Analysis of Physical Vapor Deposited Inorganic Thin Films on Low Temperature Cofired Ceramic

Low temperature cofired ceramic (LTCC) has been established as an excellent packaging technology for high-reliability, high-density microelectronics. The functionality and robustness of rework have been increased through the incorporation of a physical vapor deposition (PVD) thin film Ti/Cu/Pt/Au metallization. PVD metallization is suitable for radio frequency (RF) applications as well as digital systems. Adhesion of the Ti “adhesion layer” to the LTCC as-fired surface is not well understood. Although previous work has established extrinsic parameters for delamination mechanisms of thin films on LTCC substrates, there is incomplete information regarding the intrinsic (i.e., thermodynamic) parameters in the literature. This article analyzes the thermodynamic favorability of adhesion between Ti, Cr, and their oxide coatings on LTCC (assumed as amorphous silica glass and Al2O3). Computational molecular calculations are used to determine interface energy as an indication of molecular stability between pair of materials at specific temperature. The end result will expand the understanding of thin film adhesion to LTCC surfaces and assist in increasing the long-term reliability of the interface bonding on RF microelectronic layers.

Thermodynamic Analysis of Physical Vapor Deposition (PVD) Inorganic Thin Films on Low Temperature Cofired Ceramic (LTCC)

Abstract Low temperature cofired ceramic (LTCC) has been established as an excellent packaging technology for high reliability, high density microelectronics. The functionality and robustness of rework has been increased through the incorporation of a Physical Vapor Deposition (PVD) thin film Ti/Cu/Pt/Au metallization. PVD metallization is suitable for RF (Radio Frequency) applications as well as digital systems. Adhesion of the Ti “adhesion layer” to the LTCC as-fired surface is not well understood. While past work has established extrinsic parameters for delamination mechanisms of thin films on LTCC substrates, there is incomplete information regarding the intrinsic (i.e. thermodynamic) parameters in literature. This paper analyzes the thermodynamic favorability of adhesion between Ti, Cr, and their oxides coatings on LTCC (assumed as amorphous silica glass and Al2O3). Computational molecular calculations are used to determine interface energy as an indication of molecular stability over a range of temperatures. The end result will expand the understanding of thin film adhesion to LTCC surfaces and assist in increasing the long-term reliability of the interface bonding on RF microelectronic layers.

Synthesis and characterization of polyimide thin films obtained by thermal evaporation and solid state reaction

AbstractIn this research polyimide films were prepared by physical vapor deposition (PVD), using solid state reaction of pyromellitic dianhydride (PMDA) and p-phenylene diamine (PDA) to form poly(amic acid) (PAA) films. The resultant films were converted to polyimide by thermal treatment, usually below 300 °C. For this study, a FT-IR spectrometer has been used to measure the effect of imidization temperature on the chemical structure of the vapor-deposited thin films of aromatic PI. When temperature increased, an increase in all absorption peaks was observed. This suggests that residual PAA monomers continued to be converted into PI. The surface topology of the PI films obtained at imidization temperatures of 150, 200, 250 °C for 1 hour was further examined by using AFM atomic force microscopy. It can be clearly seen that the surface became rougher with increasing imidization temperature. The thermal stability of polyimide was also studied by using thermogravimetric analysis (TGA).

Description and Test of a New Multilayer Thin Film Vapor Deposition Apparatus for Organic Semiconductor Materials

In this work the description, test, and performance of a new vacuum apparatus for thin film vapor deposition (ThinFilmVD) of organic semiconductor materials are presented. The apparatus is able to fabricate single, multilayer/composites, or hybrid thin films using four independent, organic or inorganic, vapor deposition sources (Knudsen cells type), and the vapor mass flow is condensed onto a substrate surface (temperature regulated). The same apparatus could be also used to measure vapor pressures according to the Knudsen effusion methodology. Vapor pressures and thermodynamic properties of sublimation measured by Knudsen effusion of some reference organic materials (benzoic acid, anthracene, triphenylene, benzanthrone, 1,3,5-triphenylbenzene, perylene) were used to evaluate and test the performance of the apparatus. Moreover, nanostructures of thin films and composite materials of relevant charge transport and electroluminescent materials were deposited onto an indium–tin oxide (ITO) surface, and the mo...

Thermally, Operationally, and Environmentally Stable Organic Thin-Film Transistors Based on Bis[1]benzothieno[2,3-d:2′,3′-d′]naphtho[2,3-b:6,7-b′]dithiophene Derivatives: Effective Synthesis, Electronic Structures, and Structure–Property Relationship

By developing an efficient synthetic route to the bis[1]benzothieno[2,3-d;2′,3′-d′]naphtho[2,3-b;6,7-b′]dithiophene (BBTNDT) framework, we have successfully synthesized new BBTNDT derivatives with phenyl (DPh-BBTNDT) or n-hexyl groups (C6-BBTNDT) at the 2 and 10 positions. Characterization of their vapor-deposited thin films revealed that, depending on the substituents introduced, their HOMO energy levels were slightly altered, and DPh-BBTNDT with the HOMO energy level of ca. 5.3 eV was supposed to be a stable organic semiconductor under ambient conditions. In fact, the DPh-BBTNDT-based OTFTs showed not only high mobility of up to 7.0 cm2 V–1 s–1 under ambient conditions but also excellent operational and thermal stabilities up to 300 °C, whereas the parent and the hexyl derivative were less stable against the thermal treatments at high temperatures. The high mobility observed for the DPh-BBTNDT-based OTFTs can be correlated to the interactive packing structure in the bulk single crystal and thin film sta...

Nanostructured Thin Film Silicon Anodes for Li-Ion Microbatteries.

Thin film microbatteries require electrode materials with high areal specific capacities and good cyclability. Use of vapor-deposited silicon thin films as anodes in Li-ion microbatteries offers the advantage of high capacity as well as compatibility with other processes used for microsystem fabrication. Unfortunately, monolithic silicon films greater than 200 nm in thickness pulverize during lithiation and delithiation. We have used metal-assisted-chemical-etching of sputter-deposited amorphous silicon films to make nanoporous silicon layers and arrays of silicon nanopillars as a means of achieving anodes with high areal capacity and good cyclability. We have compared the performance of these nanostructured layers with the performance of monolithic silicon films in Li half-cells. A reduced first cycle coulombic efficiency was observed in all cases and was attributed to the irreversible formation of Li2O due to the presence of oxygen in the sputter-deposited silicon films. This was controlled through modifications of the sputtering conditions. As expected, monolithic films thicker than 200 nm showed poor cycling performance due to pulverization of the film. Nanoporous silicon showed good initial cycling performance but the performance degraded due to porosity collapse and delamination. Arrays of silicon nanopillars made from 750 nm silicon films exhibited good cycling, rate performance and an areal capacity (0.20 mA h cm(-2)) 1.6 times higher than what could be obtained with monolithic Si films with similar cyclability.

Singlet Exciton Fission in Thin Films of tert-Butyl-Substituted Terrylenes

Two terrylene chromophores, 2,5,10,13-tetra(tert-butyl)terrylene (1) and 2,5-di(tert-butyl)terrylene (2), were synthesized and studied to determine their singlet exciton fission (SF) efficiencies. Compound 1 crystallizes in one-dimensional stacks, whereas 2 packs in a slip-stacked, herringbone pattern of dimers motif. Strongly quenched fluorescence and rapid singlet exciton decay dynamics are observed in vapor-deposited thin films of 1 and 2. Phosphorescence measurements on thin films of 1 and 2 show that SF is only 70 meV endoergic for these chromophores. Femtosecond transient absorption experiments using low laser fluences on these films reveal rapid triplet exciton formation for both 1 (τ = 120 ± 10 ps) and 2 (τ = 320 ± 20 ps) that depends strongly on film crystallinity. The transient absorption data are consistent with formation of an excimer state prior to SF. Triplet exciton yield measurements indicate nearly quantitative SF in thin films of both chromophores in highly crystalline solvent-vapor-annealed films: 170 ± 20% for 1 and 200 ± 30% for 2. These results show that significantly different crystal morphologies of the same chromophore can both result in high-efficiency SF provided that the energetics are favorable.

Horizontal molecular orientation in solution-processed organic light-emitting diodes

Horizontal orientation of the emission transition dipole moments achieved in glassy vapor-deposited organic thin films leads to an enhancement of the light out-coupling efficiency in organic light-emitting diodes (OLEDs). Here, our combined study of variable angle spectroscopic ellipsometry and angle dependent photoluminescence demonstrates that such a horizontal orientation can be achieved in glassy spin-coated organic films based on a composite blend of a heptafluorene derivative as a dopant and a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl as a host. Solution-processed fluorescent OLEDs with horizontally oriented heptafluorene emitters were then fabricated and emitted deep blue electroluminescence with an external quantum efficiency as high as 5.3%.

Enhanced diffusion and mobile fronts in a simple lattice model of glass-forming liquids.

The diffusion of mobility in bulk and thin film fluids near their glass transition is examined with a kinetic lattice model, and compared to recent experiments on bulk liquids and vapor-deposited thin film glasses. The "limited mobility" (LM) lattice model exhibits dynamic heterogeneity of mobility when the fluid is near its kinetic arrest transition; a finite-parameter second-order critical point in the LM model bearing strong resemblance to the glass transition in real fluids. The spatial heterogeneity of mobility near kinetic arrest leads to dynamics that violate the Stokes-Einstein relation. To make connections with experiment, LM model simulations of self-diffusion constants in fluids near kinetic arrest are compared to those in two organic glass-formers. In addition, simulations of mobility in films that have been temperature-jumped above kinetic arrest (starting from an arrested state) are carried out. The films develop a "front" of mobility at their free surface that progresses into the film interior at a constant rate, thereby mobilising the entire film to fluidity. The velocity of the front scales with the self-diffusion constant for analogous bulk systems-an observation consistent with experiments on vapor-deposited molecular thin films.

Transformation kinetics of vapor-deposited thin film organic glasses: the role of stability and molecular packing anisotropy.

While ordinary glasses transform into supercooled liquid via a homogeneous bulk mechanism, thin film glasses of higher stability transform heterogeneously by a front propagating from the surface and/or the interfaces. In this work, we use quasi-adiabatic fast scanning nanocalorimetry to determine the heat capacity of thin glassy layers of indomethacin vapor-deposited in a broad temperature range of 110 K below the glass transition temperature. Their variation in fictive temperature amounts to 40 K. We show that a propagating front is the initial transformation mechanism in all cases. Using an ad hoc surface normalization procedure we determine the corresponding growth front velocity for the whole range of deposition temperatures. Although the transformation rate changes by a factor of 10 between the most and less stable samples, the relation between the mobility of the front and the thermodynamic stability of the glass is not uniquely defined. Glasses grown above 280 K, which are at equilibrium with the supercooled liquid, present a different dependence of the growth front velocity on fictive temperature compared to glasses grown out of equilibrium at Tdep < 250 K. These glasses transform faster with increasing Tf. Our data clarify previous reports and support the evidence that the fictive temperature alone is not an absolute indicator of the properties of the glass, at least when its structure is not completely isotropic. To interpret the data, we propose that the growth front velocity depends on three terms: the mobility of the liquid at a given temperature, the mobility of the glass and the arrangement of the molecules in the glass.

Comparative life cycle assessment of laminated and vacuum vapor-deposited thin film solid-state batteries

Lamination technologies used to produce lithium ion batteries are limited by the capital investment needed for multiple unit operations, solvent use for electrolyte and electrode slurries, and an inability to exert control over active material particle morphology and homogeneity. Advancements in thin-film solid-state processing using vacuum coating hold promise to overcome these challenges for batteries with superior energy density and cycle life, if cost and scalability issues can be overcome. A comparative life cycle assessment is reported for battery production using lamination and thin-film vacuum vapor deposition. Lithium vanadium oxide solid-state cells are found to have the lowest impact, per unit energy storage, in cumulative energy demand (CED), global warming potential (GWP), and six other midpoint environmental indicators. Human health and resource depletion impacts are generally higher for lithium manganese oxide and lithium cobalt oxide solid-state cells than for their laminated counterparts, whereas CED and GWP per unit energy storage are 25–65% lower for solid-state cells across all cathode chemistries. Sensitivity analysis, taking into account uncertainties related to solid-state cell properties and vacuum vapor deposition process efficiencies, indicates that CED and GWP impacts for battery electric vehicle mobility using packs with solid-state cells will be lower than those incurred using laminated cells.

Role of nucleation layer morphology in determining the statistical roughness of CVD-grown thin films

During the chemical vapor deposition of thin films, a molecular inhibitor can be added to control the morphology during the nucleation stage and/or the conformal coating behavior during the growth stage. The authors use this control strategy to determine the separate influence of nucleation morphology and of conformal growth on the final surface roughness, evaluated through the power spectral density of AFM height data. The experimental system is HfB2 deposition from the precursor Hf(BH4)4 using NH3 as the inhibitor. For a nucleation layer consisting of mounds, the low frequency (long lateral range) roughness cannot be reduced by the overgrowth of film, even with the conformal growth conditions. Conversely, when the nucleation layer consists of a dense compact of islands, the low frequency roughness remains low throughout film growth, even when carried out in the nonconformal growth conditions. In all cases, the high frequency portion of the roughness decays in a similar manner, indicating that short-range smoothing mechanism is operative. The sensitivity of the final surface roughness to the morphology of the initial nucleation layer demonstrates that “shadowing” by peaks in the surface height is a strong kinetic driving force for roughening, consistent with previous theory. The use of an inhibitor molecule in CVD provides a means to obtain ultrasmooth films on relatively unreactive substrates, without the need for surface activation and without changing the film composition.

Molecular fouling resistance of zwitterionic and amphiphilic initiated chemically vapor-deposited (iCVD) thin films

Biofouling is a universal problem in various applications ranging from water purification to implantable biomedical devices. Recent advances in surface modification have created a rich library of antifouling surface chemistries, many of which can be categorized into one of the two groups: hydrophilic surfaces or amphiphilic surfaces. We report the straightforward preparation of antifouling thin film coatings in both categories via initiated chemical vapor deposition. A molecular force spectroscopy-based method is demonstrated as a rapid and quantitative assessment tool for comparing the differences in antifouling characteristics. The fouling propensity of single molecules, as opposed to bulk protein solution or bacterial culture, is assessed. This method allows for the interrogation of molecular interaction without the complication resulted from protein conformational change or micro-organism group interactions. The molecular interaction follows the same trend as bacterial adhesion results obtained previously, demonstrating that molecular force probe is a valid method for the quantification and mechanistic examination of fouling. In addition, the molecular force spectroscopy-based method is able to distinguish differences in antifouling capability that is not resolvable by traditional static protein adsorption tests. To lend further insight into the intrinsic fouling resistance of zwitterionic and amphiphilic surface chemistries, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, advancing and receding water contact angles, and atomic force microscopy are used to elucidate the film properties that are relevant to their antifouling capabilities.

Superconformal chemical vapor deposition of thin films in deep features

The authors report a new and potentially widely applicable method for the chemical vapor deposition (CVD) of films with a superconformal thickness profile in recessed features, i.e., the rate of growth increases with depth away from the opening. Provided that the aspect ratio of the feature is not too large, deposition initially affords a “V” shaped profile; continued deposition eventually fills the feature without leaving a void or seam of low-density material along the centerline. Superconformal deposition occurs under the following set of conditions: (1) growth involves two coreactants; (2) the deposition rate depends directly on the surface concentrations of both coreactants; (3) the molecular diffusivities of the coreactants are different; and (4) the partial pressures of the coreactants are chosen such that the surface coverage of the more rapidly diffusing coreactant is relatively small, and therefore rate-limiting, near the opening. The latter condition can be fulfilled if the more slowly diffusing coreactant is employed in excess or has an intrinsically higher sticking coefficient. Under these circumstances, the deposition rate will increase deeper in the feature for the following reason: the pressure of the slowly diffusing coreactant necessarily drops more quickly with depth than that of the rapidly diffusing coreactant, which increases the fractional surface coverage of the fast-diffusing coreactant and with it the growth rate. At sufficiently large depths, eventually the surface concentration of the more slowly diffusing coreactant will become rate limiting and the growth rate will begin to fall; to obtain superconformal growth, therefore, conditions must be chosen so that the growth rate does not surpass its peak value. As a specific example of how this new approach can be implemented, MgO is deposited at 220 °C using the aminodiboranate precursor Mg(DMADB)2 and H2O. Under properly chosen conditions, the growth rate increases from 1.0 nm/min at the trench opening to 1.8 nm/min at a depth/width ratio of 18. The authors propose a kinetic model that quantitatively explains these observations and, more generally, predicts the film profile as a function of the partial pressures of the coreactants in the gas feed, the molecular diffusivities, and the aspect ratio of the feature. An additional benefit of the model is that it can be used to predict conditions under which perfectly conformal CVD depositions will result. The present method should enable the fabrication of nanoscale devices in which high aspect ratio recessed features need to be completely filled. The method is intrinsic in nature and does not require special surface preparation, the use of a catalyst, or cycles of deposition and etching.

Broadband Reflection Minimization Using Silver Ultra Thin Film Sandwiched Between Silicon Nitride Layers for c-Si Solar Cell Application

Wafer thickness reduction is the main trend for cost reduction of crystalline silicon (c-Si) solar cells. For better light trapping in lower thickness wafers where traditional texturization method may not be promising, alternative techniques are being explored. Here, reflection minimization using silver ultra thin film sandwiched between silicon nitride (SiNx) antireflection coating (ARC) layers on c-Si substrate is reported. SiNx ARC layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) and silver ultra thin film by e-beam evaporation; 41 % reflection reduction for silver ultra thin film sandwiched in SiNx ARC layers as compared to standard 80 nm thin SiNx ARC on c-Si substrate is shown. The sandwiched structure gives 8.1 % weighted total reflectance for wavelength range of 300–1,200 nm as compared to standard 80-nm SiNx-based ARC which gives 13.8 %. Also, comparison of this Ag ultra thin film-based sandwiched geometry is done with randomly distributed nanoparticle (NP)-based ARC geometry. It is shown that optically, the sandwiched Ag ultra thin film-based device geometry is more promising than Ag NP-based device geometry and standard 80-nm SiNx-based geometry for broad wavelength range of 300–1,200 nm.

Constrained Swelling of Polymer Networks: Characterization of Vapor-Deposited Cross-Linked Polymer Thin Films

In certain real situations, polymer networks cannot swell freely but are subjected to constraints by action of external forces or geometric restrictions. In this contribution, the restriction to swelling by adhesion of a cross-linked polymer film deposited on a rigid substrate is considered. Such constraints prevent swelling in plane, and the osmotic force concentrates on extension of network chains in the direction normal to the surface. We analyze existing rubber elasticity models taking into account finite extensibility of network chains and derive the elastic contribution to the chemical potential of the solvent in swollen networks. For the mixing contribution to the change of the Gibbs energy, it is more appropriate to consider the network containing cross-links, but with undeformed network chains than an un-cross-linked polymer of infinite degree of polymerization, because continuing formation of bonds within an infinite molecule further decreases the entropy. The additional term is proportional to ...

Photo-initiated chemical vapor deposition of thin films using syngas for the functionalization of surfaces at room temperature and near-atmospheric pressure

Hydrophilic and hydrophobic thin films have been deposited onto flat metallic substrates through photo-initiated chemical vapor deposition (PICVD), using syngas as a precursor, and affordable UVC germicidal lamps as a source of light. This study is the first experimental investigation of what has been previously concluded to be the potential solution to the current widespread nanoparticle functionalization problem. This study addresses the current limiting factor, namely the cost issue, by using simple gas precursors, using an affordable initiation source and operating under normal conditions. This approach differs from the current approaches which use expensive solvents as precursors, energy consuming-sources of initiation (e.g. high temperature, plasma and VUV) and operate under high vacuum and/or high temperatures. While the current paradigm is to target the peak absorption of a molecule, the present study indicates that long chain polymerized products can be formed from off-peak wavelengths. It has been found that photo-initiated deposition occurs and that a wide range of water contact angles, from 5° to 118°, can be obtained by manipulating the experimental conditions. A multilinear empirical model has been derived, and it predicts fairly well the contact angles obtained as a function of the different experimental parameters.

Electron injection at the PTCDA/metal interface detected by electron paramagnetic resonance

Electron paramagnetic resonance absorption was observed for samples prepared by vacuum vapor deposition of thin films of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on metal films of magnesium (Mg), aluminum (Al) and silver (Ag) but not on gold (Au) or PTCDA on quartz. The resonance absorption is seen as a Lorentzian line with g-value of 2.0030 and a linewidth of 0.19mT. The signal is attributed to PTCDA anions at the metal organic interface. The relative values of the metal work function and the PTCDA electron affinity appear to determine the occurrence of the EPR absorption.

Selective ion-induced grain growth: Thermal spike modeling and its experimental validation

Ion-bombardment-induced selective grain growth is a process that allows for full control of the orientation of vapor-deposited thin films, which can be converted from fiber-textured polycrystalline to single-crystal films. The main mechanisms behind this phenomenon are explained by a new thermal spike model, which takes into account and compares the different driving forces governing the film microstructure evolution upon irradiation and emphasizes the importance of the thermal spike shape and volume. The strong agreement between model and experimental data confirms that selective grain growth is driven by the minimization of the volume free energy.

Formation of CuInSe2 and CuGaSe2 Thin‐Films Deposited by Three‐Stage Thermal Co‐Evaporation: A Real‐Time X‐Ray Diffraction and Fluorescence Study

AbstractThin film solar cells based on co‐evaporated Cu(In,Ga)Se2 absorber films present the highest efficiencies among current polycrystalline thin‐film technologies. Thanks to the development of a novel experimental setup for in situ growth studies, it was possible to follow the formation of the crystalline phases during such deposition processes for the first time. This synchrotron‐based energy‐dispersive X‐ray diffraction and fluorescence setup is suited for real‐time studies of thin film vapor deposition processes. Focusing on the growth of CuInSe2 and CuGaSe2 fabricated by three‐stage processing, we find that the phase transitions in the Cu‐In‐Se system follow the reported pseudo‐binary In2Se3‐Cu2Se phase diagram. This requires a transformation of the Se sublattice during the incorporation of Cu‐Se into the In2Se3 precursor film from the first process stage. In the Cu‐Ga‐Se system, besides an increase in the lattice spacings, we observe no transformation of the Se sublattice. Furthermore, the structural defects of the Ga‐Se precursor film are preserved until the CuGaSe2 stoichiometry is reached. By means of model calculations of the fluorescence signals, we confirm in both systems the segregation of Cu2Se at the surface near a concentration of 25 at.% Cu shortly after the recrystallization of the films. The modeling also reveals that Cu2Se penetrates into the CuInSe2 film, whereas it remains at the surface of the CuGaSe2 film.