Thin Film Deposition Techniques Research Articles

Cooled infrared filters and dichroics for the James Webb Space Telescope Mid-Infrared Instrument

The cooled infrared filters and dichroic beam splitters manufactured for the Mid-Infrared Instrument are key optical components for the selection and isolation of wavelengths in the study of astrophysical properties of stars, galaxies, and other planetary objects. We describe the spectral design and manufacture of the precision cooled filter coatings for the spectrometer (7 K) and imager (9 K). Details of the design methods used to achieve the spectral requirements, selection of thin film materials, deposition technique, and testing are presented together with the optical layout of the instrument.

Using fence post designs to speed the atomic layer deposition of optical thin films

Atomic layer deposition (ALD) at this time is much slower than conventional optical thin-film deposition techniques. A more rapid ALD process for SiO(2) has been developed than for other ALD materials. A fence post design for optical thin films has thin layers of high-index posts standing above a broad low-index ground. If a design for ALD can be predominantly composed of SiO(2) layers with thin high-index layers, the deposition times can be correspondingly shortened, and it is shown that the required performance can still be nearly that of more conventional designs with high- and low-index layers of equal thickness. This combination makes the ALD benefits of conformal coating and precise thickness control more practical for optical thin-film applications.

Role of defect states on electrical and optical properties in CdSe nanorod thin films

In this work, we present the morphological, optical and electrical transport properties of CdSe colloidal nanorods as building blocks in electro-optical conversion devices, as a function of the thin-film deposition technique and of substrate functionalization. In particular, we show the role of the ligand molecules and of the defect states in phototransport and optical properties.

Thin Film Microbatteries Prepared by Pulsed Laser Deposition

There is a growing interest in thin film microbatteries due to miniaturization of electronic devices. The reduction in current and power consumptions of electronic devices makes microbatteries possible as power sources. Hence, if micro-scale thin-film batteries are to be developed, a suitable fabrication processing technique for thin-film cathodes, anodes and electrolytes becomes crucial. Among various thin film deposition techniques, pulsed laser deposition (PLD) is a relatively flexible and powerful technique for preparing high quality and dense thin-film electrodes on a laboratory scale. This paper reviews recent developments in the fabrication of thin-film electrodes by using PLD.

Microstructured vacuum gauges and their future perspectives

New prototypes and concepts of microstructured vacuum gauges have been developed by using the fabrication technologies for micro electro mechanical systems (MEMS). The realization of such microstructured gauges requires sophisticated fabrication processes such as thin film deposition, photolithography and etching techniques. This approach of MEMS vacuum gauges is demonstrated by a few examples. Micro-Pirani gauges are based on the principle that the heat transfer between two surfaces is proportional to the number of molecules (and hence the pressure) transferring the heat, when the mean free path in the gas is larger than the distance between the surfaces. In contrast to conventional Pirani gauges with heated thin wires, in a micro-Pirani gauge the heat transfer takes place between an extremely thin heated membrane and the surrounding. The thin membrane (200-nm thick) is realized by deposition of siliconoxide/siliconnitride, photolithography and anisotropic etching of silicon wafers. Heating is performed by a meander-shaped aluminum thin film heater in the center of the membrane. This micro-Pirani gauge shows a high sensitivity in the pressure range between 10 −4 and 100 mbar. By using a silicon “microbridge” with 10 μm small gap between heated membrane and surrounding, the pressure sensitivity of the chip is extended up to 1000 mbar. Similar concepts are presented and discussed with respect to the miniaturization of spinning rotor gauges. The new concept is based on the application of microfabricated disks (instead of spheres) and of electrostatic instead of magnetic driving forces. The extension of the sensitivity range for miniaturized spinning rotor gauges is also discussed. Finally, new perspectives for mechanical vacuum gauges are demonstrated. By application of micromechanical processes, very thin stress-compensated membranes can be fabricated which enable sensitive mechanical gauges even for pressures in the high vacuum range. First, experimental results with respect to these membranes are represented.

Integrating bulk piezoelectric materials into MEMS for high authority actuators

A new technique was developed under the European Aeromems II project to produce integrated, thin (<100 µm) piezoelectric bimorph structures to be used in MEMS applications requiring large forces and displacements. The process employs machining, low-temperature bonding and standard microfabrication techniques to build up a laminated actuator structure. The process is believed to be a major improvement over existing techniques used to produce PZT layers in the sub-100 µm thickness range. It is an inherently low-temperature (<250 °C) process and allows the high piezoelectric activities of bulk-processed ceramics to be utilized on a MEMS scale. This is in contrast with conventional thick and thin film deposition techniques, which commonly result in reduced piezoelectric properties, use high processing temperatures and can lead to undesirable tensile stresses due to shrinkage. Prototype microvalves were fabricated using this process and the testing and evaluation of these devices is the subject of this paper. A microvalve based on a 4 mm × 1 mm × 70 µm bimorph cantilever element achieved tip displacements in excess of 100 µm at a driving voltage of 90 V. As the device does not depend on resonance it is possible for it to operate over a wide range of frequencies and duty cycles. Tests showed the microvalve to be capable of modulating a supply pressure of 80 kPa from dc to 1 kHz and to be operable at pressure differentials in excess of 100 kPa at frequencies up to 400 Hz. When operated at high pressure differentials with a 200 µm diameter orifice the microvalve was capable of producing jet velocities in excess of 300 m s−1. These high authority capabilities suggest that this device would meet the requirements for a wide range of practical applications.

Transparent conducting Al-doped ZnO thin films prepared by magnetron sputtering with dc and rf powers applied in combination

A newly developed Al-doped ZnO (AZO) thin-film magnetron-sputtering deposition technique that decreases resistivity, improves resistivity distribution, and produces high-rate depositions has been demonstrated by dc magnetron-sputtering depositions that incorporate rf power (dc+rf-MS), either with or without the introduction of H2 gas into the deposition chamber. The dc+rf-MS preparations were carried out in a pure Ar or an Ar+H2 (0%–2%) gas atmosphere at a pressure of 0.4Pa by adding a rf component (13.56MHz) to a constant dc power of 80W. The deposition rate in a dc+rf-MS deposition incorporating a rf power of 150W was approximately 62nm∕min, an increase from the approximately 35nm∕min observed in dc magnetron sputtering with a dc power of 80W. A resistivity as low as 3×10−4Ωcm and an improved resistivity distribution could be obtained in AZO thin films deposited on substrates at a low temperature of 150°C by dc+rf-MS with the introduction of hydrogen gas with a content of 1.5%. This article describes the effects of adding a rf power component (i.e., dc+rf-MS deposition) as well as introducing H2 gas into dc magnetron-sputtering preparations of transparent conducting AZO thin films.

A dielectric-modulated field-effect transistor for biosensing

Interest in biosensors based on field-effect transistors (FETs), where an electrically operated gate controls the flow of charge through a semiconducting channel, is driven by the prospect of integrating biodetection capabilities into existing semiconductor technology. In a number of proposed FET biosensors, surface interactions with biomolecules in solution affect the operation of the gate or the channel. However, these devices often have limited sensitivity. We show here that a FET biosensor with a vertical gap is sensitive to the specific binding of streptavidin to biotin. The binding of the streptavidin changes the dielectric constant (and capacitance) of the gate, resulting in a large shift in the threshold voltage for operating the FET. The vertical gap is fabricated using simple thin-film deposition and wet-etching techniques. This may be an advantage over planar nanogap FETs, which require lithographic processing. We believe that the dielectric-modulated FET (DMFET) provides a useful approach towards biomolecular detection that could be extended to a number of other systems.

Fabrication and characterization of sub-45 nm multiple linewidth samples

A method of fabricating nanometre scale multiple linewidth standards based on multilayer thin film deposition techniques is presented. Three layers of chromium (Cr) thin films and three layers of silicon nitride (Si3N4) thin films were alternately deposited on a silicon substrate in the radio frequency (RF) magnetron sputtering system and the low-temperature plasma enhanced chemical vapour deposition (PECVD) system. The deposition thicknesses of the Cr thin films are equal to the desired linewidths. The silicon substrate was then cut into small pieces, and the cross-section of each piece was metallographically polished. Perpendicular to the cross-section Si3N4 was etched with reactive ion etching (RIE) and nanometre scale multiple linewidth samples with nominal linewidths of 20 nm, 25 nm and 35 nm were fabricated. The fabricated samples were characterized with a scanning electron microscope (SEM). The line edge roughness (LER) and the linewidth roughness (LWR) of the lines of the fabricated samples were evaluated by an offline image analysis algorithm. The lines of the fabricated samples have uniform attributes and good linearity over hundreds of microns. The results show that the presented method is a promising method for the fabrication of nanometre scale multiple linewidth standards.

Thin films for micro solid oxide fuel cells

Thin film deposition as applied to micro solid oxide fuel cell (μSOFC) fabrication is an emerging and highly active field of research that is attracting greater attention. This paper reviews thin film (thickness ≤1 μm) deposition techniques and components relevant to SOFCs including current research on nanocrystalline thin film electrolyte and thin-film-based model electrodes. Calculations showing the geometric limits of μSOFCs and first results towards fabrication of μSOFCs are also discussed.

Matrix-Assisted Pulsed Laser Evaporation of polythiophene films

Organic poly-conjugated systems have recently attracted great interest as semi-conducting materials and, among poly-conjugated systems, substituted polythiophenes have given relevant results in PVs applications. The high conductivity required is affected by both the polymer conjugation length and the chain packing. Thus, highly region-regular polymers must be used and deposited as thin films with some technique which favours orientation and crystallization of the polymer chains.A deposition technique often used for its flexibility and high control over film characteristics is Pulsed Laser Deposition (PLD). In PLD, largely applied for inorganic thin film deposition, the material is ablated from a solid target by a focused pulsed laser beam and is deposited on the substrate placed at a small distance. Although some addition polymers have been successfully deposited the deposition seems to proceed via a “depolymerization–monomer ablation–repolymerization” mechanism, this is clearly not possible in general for organic molecules and condensation polymers.On the contrary MAPLE (Matrix-Assisted Pulsed Laser Evaporation) is a recently developed PLD based thin film deposition technique, particularly well suited for organic/polymer thin film deposition. Up to now MAPLE depositions have been carried out mainly by means of modified PLD systems, using excimer lasers operating in UV, but use of less energetic radiations can minimize the photochemical decomposition of the polymer molecules.We have used a deposition system explicitly designed for MAPLE technique connected to a Q-switched Ng:YAG pulsed laser which can be operated at different wavelength ranging from IR to UV in order to evaluate the effect of the choice of laser radiation on the deposition of POOPT thin films.From DRIFT-IR spectroscopy, all deposited films showed structural order; it was determined that the better wavelength for POOPT deposition is 532 nm. With this value of the laser wavelength the local chemical structure of the polymer was retained and the film appeared more homogeneous.

Substitution of transparent conducting oxide thin films for indium tin oxide transparent electrode applications

The present status and prospects for further development of reduced or indium-free transparent conducting oxide (TCO) materials for use in practical thin-film transparent electrode applications such as liquid crystal displays are presented in this paper: reduced-indium TCO materials such as ZnO-In2O3, In2O3-SnO2 and Zn-In-Sn-O multicomponent oxides and indium-free materials such as Al- and Ga-doped ZnO (AZO and GZO). In particular, AZO thin films, with source materials that are inexpensive and non-toxic, are the best candidates. The current problems associated with substituting AZO or GZO for ITO, besides their stability in oxidizing environments as well as the non-uniform distribution of resistivity resulting from dc magnetron sputtering deposition, can be resolved. Current developments associated with overcoming the remaining problems are also presented: newly developed AZO thin-film deposition techniques that reduce resistivity as well as improve the resistivity distribution uniformity using high-rate dc magnetron sputtering depositions incorporating radio frequency power. In addition, stability tests of resistivity in TCO thin films evaluated in air at 90% relative humidity and 60 °C have demonstrated that sufficiently moisture-resistant AZO thin films can be produced at a substrate temperature below 200 °C when the film thickness was approximately 200 nm. However, improving the stability of AZO and GZO films with a thickness below 100 nm remains a problem.

Molecular Electrodes at the Exposed Edge of Metal/Insulator/Metal Trilayer Structures

Producing reliable electrical contacts of molecular dimensions has been a critical challenge in the field of molecule-based electronics. Conventional thin film deposition and photolithography techniques have been utilized to construct novel nanometer-sized electrodes on the exposed vertical plane on the edge of a thin film multilayer structure (metal/insulator/metal). Via thiol surface attachment to metal leads, an array of paramagnetic, cyanide-bridged octametal complexes, [(pzTp)FeIII(CN)3]4[NiII(L)]4[O3SCF3]4 (1) [(pzTp) = tetra(pyrazol-1-yl)borate; L = 1-S(acetyl)tris(pyrazolyl)decane], were covalently linked onto the electrodes forming a dominant conduction pathway. A series of molecule-based devices were fabricated using Ni, NiFe, Ta, and Au as metal electrodes separated by insulating Al2O3 spacers, followed by treatment with 1. A series of control experiments were also performed to demonstrate that the conduction path was through tethered metal clusters. The molecular current was analyzed via the Simmons tunnel model, and calculations are consistent with electron tunneling through the alkane ethers to the central metal core. With a Ni/Al2O3/Au molecular electrode, the tether binding was found to be reversible to the top Au layer, allowing for a new class of chemical detection based on the steric bulk of coordinating analytes to disconnect the molecular current path. Simple and economical photolithography/liftoff/self-assembly fabrication techniques afford robust molecular junctions with high reproducibility (>90%) and long operational lifetimes (>1 year).

Biomaterial thin film deposition and characterization by means of MAPLE technique

Polyethylene glycol (PEG) is a polymer with technologically important applications, especially as a biomaterial. Several biomedical applications (such as tissue engineering, spatial patterning of cells, anti-biofouling and biocompatible coatings) require the application of high quality PEG thin films. In order to have a good adhesion to substrate chemically modified polymer molecules have been used, but for some “in vivo” applications it is essential to deposit a film with the same chemical and structural properties of bulk PEG. Pulsed laser deposition (PLD) technique is generally able to produce high quality thin films but it is inadequate for polymer/organic molecules. MAPLE (Matrix Assisted Pulsed Laser Evaporation) is a recently developed PLD based thin film deposition technique, particularly well suited for organic/polymer thin film deposition. Up to now MAPLE depositions have been carried out mainly by means of modified PLD systems, using excimer lasers operating in UV, but the use of less energetic radiations can minimize the photochemical decomposition of the polymer molecules. We have used a deposition system explicitly designed for MAPLE technique connected to a Q-switched Ng:YAG pulsed laser which can be operated at different wavelength ranging from IR to UV in order to optimise the deposition parameters. The capability of MAPLE technique to deposit PEG has been confirmed and preliminary results show that visible (532 nm wavelength) radiation gives better results with respect to UV (355 nm) radiation. Despite usually UV wavelengths have been used and even if more systematic tests must be performed, it is important to underline that the choice of laser wavelength plays an important role in the application of MAPLE thin film deposition technique.

Measurement of Elastic Modulus and Residual Stress of Diamond Thin Films

Despite great advancements in diamond thin film growth and deposition techniques, determination of the residual stress and Young’s modulus for diamond films has continued to be a challenge. The bulge test is a potentially powerful tool for characterizing the mechanical properties of diamond film. In a bulge tester, pressure is applied on a thin membrane and the out-of-plane deflection of the membrane center is measured. The Young’s Modulus and the residual stress are simultaneously determined by using the load-deflection behavior of a membrane. By means of electron-enhanced hot filament chemical vapor deposition (HFCVD), a diamond film was deposited on silicon slice (100), and the free-standing window sample of diamond thin films was fabricated by means of photolithography and anisotropic wet etching. The deflection of the membranes is measured using a laser interferometry system. The elastic modulus and residual stress were measured using a self-designed bulge equipment. In addition, the distortion of diamond thin films under different pressure was simulated using finite element analysis and the contrast was made with experimental data. The research indicated that the Young’s Modulus of diamond thin films is 937.8GPa and the residual stress is -10.53MPa. The elastic modulus and the residual stress coincide with the report in the literature and the value tested by X-ray diffraction, respectively. This method uses a simple apparatus, and the fabrication of samples is very easy, and it has provided an effective means for precise measure the mechanical properties of other thin films.

Low Temperature Deposition of Metal Oxide Thin Films in Supercritical Carbon Dioxide Using Metal-organic Precursors

ABSTRACTA novel chemical route in thin film formation that includes the use of inorganic and organic peroxides and metal organic complexes soluble in supercritical carbon dioxide has been investigated for the deposition of alumina, titania and zirconia thin films at low temperatures (&lt;150°C). The metal organic precursors used include: Al(acac)3, OTi(tmhd)2, and Zr(acac)4. Tert-butyl peroxide, and a 30% aqueous solution of hydrogen peroxide were used as oxidants. Depositions were carried out in a 25 ml hot wall reactor at pressures ranging from 2100 to 3900 psi at 80-140°C. The deposited thin films were investigated by using X-ray photoelectron spectroscopy (XPS) and transmission Fourier transform infrared spectroscopy (FTIR). XPS and FTIR results indicate the formation of metal oxides thin films with some bonded carbon. The deposition temperatures achieved in this process are substantially lower than those used in conventional vacuum deposition techniques making feasible the deposition on temperature sensitive substrates and organic materials required for the development of hybrid organic/inorganic devices. Processing at low temperatures in supercritical carbon dioxide may provide the basis for the development of an alternative, environmentally friendly, thin film deposition technique for the processing of nanostructures.

Synthesis, Characterization, and Modeling of Nitrogen-Passivated Colloidal and Thin Film Silicon Nanocrystals

We review our main experimental and theoretical results on the optical emission properties of Si-nanocrystals (Si-nc's) fabricated by colloidal and thin film deposition techniques, showing that the surface chemistry of the Si-nc's or the surrounding matrix play a crucial role in determining both the structural stability and the optoelectronic properties of Si-nc. Our results, guided by first-principles simulations indicate that nitrogen, oxygen, and carbon atoms bonded to the surface of nanometer silicon clusters play a crucial role in the emission mechanism of small Si quantum dots. The control and the understanding of the Si-nc surface structure and composition can provide alternative routes toward the fabrication of optically active complementary metal/oxide semiconductor devices

Polycrystalline CdTe thin films for photovoltaic applications

The CdTe/CdS thin film solar cells can be most conveniently fabricated in the form of thin films. Solar cells based on CdTe have been studied for several years now and the technology seems to be ripe for starting significant industrial production. Recently CdTe thin film solar cells have achieved an energy conversion efficiency record of 16.5% on a laboratory scale. In this paper we review the process needed to prepare high efficiency CdTe/CdS solar cells from the point of view of thin film deposition techniques. Furthermore we show that thin film CdTe technology is sufficiently mature to be easily transferred to form the basis of a large-scale module production.

Development of electron reflection suppression materials for improved thermionic energy converter performance using thin film deposition techniques

Nonideal electrode surfaces cause significant degree of electron reflection from collector during thermionic converter operation. The effect of the collector surface structure on the converter performance was assessed through the development of several electron reflection suppression materials using various thin film deposition techniques. The double-diode probe method was used to compare the J-V characteristics of converters with polished and modified collector surfaces for emitter temperature and cesium vapor pressure in the ranges of 900–2000K and 0.02–1.5torr, respectively. The coadsorption of cesium and oxygen with respective partial vapor pressures of ∼1.27torr and a few microtorrs reduced the emitter work function to a minimum value of 0.99eV. It was found that the collector surfaces with matte black appearance such as platinum black, voided nickel from radio-frequency plasma sputtering, and etched electroless Ni–P with craterlike pore morphology exhibited much better performance compared with polished collector surface. For these thin films, the increase in the maximum output voltage was up to 2.0eV. For optimum performance with minimum work function and maximum saturation emission current density, the emitter temperature was in the range of 1100–1500K, depending on the collector surface structure. The use of these materials in cylindrical converter design and/or in combination with hybrid mode triode configuration holds great potential in low and medium scale power generators for commercial use.

PZT thin films for low voltage actuation: Fabrication and characterization of the transverse piezoelectric coefficient

Transverse piezoelectric coefficient d31 is a very important characteristic of lead zirconate titanate (PZT) thin films, which have great significance for enabling MEMS applications. In this work, the d31 characteristics of PZT thin-films actuated at low voltages of less than 1V are investigated. Square-shaped bending-type PZT thin film microactuators comprising 600nm thick sol–gel derived 52/48 PZT thin film on SiO2/Ta/Pt layers are fabricated using thin film deposition techniques and experimentally characterized for their piezoelectric behavior. Specifically, the characteristic of the in-plane transverse piezoelectric coefficient d31 is studied as a function of the PZT poling electric fields as well as the applied actuation voltages both at low voltages (<1V) and high voltages (1–10V). The d31 values of the PZT material are estimated via an experiment-model correlation where experimentally measured deflections and modal characteristics of the thin film actuator are corroborated with deflections and modal characteristics of the actuator in a finite element model. The results of the work presented show that d31 values of the PZT thin film material are considerably lower at low actuation voltages (<1V) than at high actuation voltages (5–10V) and that these values generally improve with the poling electric fields of the PZT sample. For the PZT thin films investigated in this work, a maximum effective d31 value for low actuation voltages was estimated to be −30pC/N, and a maximum effective d31 value for higher actuation voltages that approach the coercive field of the PZT film was estimated to be −55pC/N.