Thin Film Deposition Process Research Articles

A Study of Fabrication and Characterization of Ultra-Small Polycrystalline Silicon Islands for Advanced Display and Microsensor Applications

AbstractPolycrystalline silicon (polysilicon) is a key component of integrated circuit technology, micromechanical systems and displays. High-mobility polysilicon thin-film transistors are replacing amorphous-silicon transistors in advanced active-matrix displays, and are also vital to the emerging fields of microsensors and microactuators [1]. In this study, the deposition processes (LPCVD) of polysilicon thin films were carefully studied and the basic kinetics were analyzed. The films were patterned into submicron islands and the relation of the deposition temperature versus the crystallinity of polysilicon islands was observed. A “threshold” deposition temperature for polysilicon crystals larger than 0.35 μm was found. The relationships of polysilicon crystal sizes vs. annealing temperature and vs. annealing duration were also systematically studied. Polysilicon islands of ∼ 0.1 μm lateral dimensions were successfully fabricated by ebeam lithography and several types of annealings were performed to increase micro-crystal size, with the goal of creating single-crystal high-mobility islands. Theoretically, transistors built from such islands should match the performance of state-of-the-art bulk CMOS devices. The crystal size was measured using TEM and AFM analysis. Moreover, EXAFS (Extended X-ray Absorption Fine Structure) measurements at SSRL (Stanford Synchrotron Radiation Lab) were employed to study the crystallization processes of polysilicon films and the correlation between the absorption curves, the thin-film electrical properties and the polysilicon grain sizes.

Thermal domain stability of advanced digital recording (ADR) thin film heads

In this paper the thermal domain stability of electroplated Nickel–Iron flux guides is studied. During the thin film deposition process of ADR magnetic heads these magnetic structures are heated up to about 300°C on a few occasions and it is shown that this has a pronounced effect on the stress state of the deposited flux-guide layers. Domain observations are carried out on both NiFe strips of varying widths as well as on as-deposited test structures and annealed NiFe on product level of a data head.

Yttrium sesquioxide, Y 2O 3, thin films deposited on Si by ion beam sputtering: microstructure and dielectric properties

Y 2O 3 thin films were in-situ deposited by ion beam sputtering on Si substrate. The influence of the deposition parameters are studied by X-ray diffraction, electrical measurements and high resolution transmission electron microscopy observations. The stress sate of the oxide layers is investigated by the sin 2ψ method as a function of the deposition parameters and the post-annealing treatments. Oxygen ion beam assisted deposition process or post-annealing of as-deposited thin films lead to the same relaxation of the internal compressive stress within the oxide layer. An SiO 2 layer sandwiched between Si and Y 2O 3 is always observed and should play a role both in the growing process and electrical properties of the MOS structure based on Y 2O 3 oxide layer. The results are interpreted in terms of diffusional process in the oxide, which are directly related to the temperature and the oxygen partial pressure during the growth process.

Amorphous-crystalline phase transition during the growth of thin films: The case of microcrystalline silicon

Thin silicon films of varying thickness were deposited on foreign substrates by electron-cyclotron resonance chemical vapor deposition from ${\mathrm{SiH}}_{4}{\ensuremath{-}\mathrm{H}}_{2}$ mixtures at 600 K. Optical thickness measurements, Rutherford backscattering, and transmission electron microscopy reveal that a thin amorphous interlayer of some 10 nm thickness has formed upon the substrate, before the growth of a microcrystalline layer begins. The amorphous layer is found to be deposited with a higher rate than the crystalline phase. Since similar effects have been observed for a large variety of deposition techniques, the amorphous-crystalline phase transition is considered as an inherent property of the growth of thin silicon films on foreign substrates at low homologous temperatures. The change in growth mode is interpreted in terms of Ostwald's rule of stages, which predicts the evolution of film growth to proceed via a set of phases of descending metastability and nucleation rate. In applying capillarity theory a criterion is derived from the ratio of amorphous-phase and crystalline-phase nucleation rates ${J}_{a}{/J}_{c}.$ This ratio is developed into basic thermodynamic functions and is shown to govern the formation of either the stable or metastable phase. The approach is of general validity for thin-film deposition processes. In the case of microcrystalline silicon, experimental measures can be derived from the developed model to directly design the evolution of film structure.

Characterisation studies of the pulsed dual cathode magnetron sputtering process for oxide films

Pulsed magnetron sputtering has become the leading industrial production process for large area thin film deposition due to its versatility, low environmental impact and ability to provide uniform coatings across large substrate areas. Such applications commonly employ oxides, including silica (SiO 2), alumna (Al 2O 3) and titania (TiO 2). Although all of these materials can be produced by reactive DC-magnetron sputtering, until recently the deposition process was highly problematic. The industrial exploitation of the pulsing process is impeded by the fact that, during long-term deposition runs, eventually all surfaces will be covered with the insulator. Once the chamber and anode are covered with insulating material, there can be no average (DC) current flowing to the power supply leads. Moreover, if the anode/chamber becomes covered with the dielectric film it will cease to operate as an effective ground (anode). A more viable approach is to use two magnetrons in a dual bipolar arrangement (dual cathode). In this arrangement each magnetron acts alternatively as an anode and a cathode. The broad aim of this study is to investigate the inter-relationship between ‘global’ parameters, pulse parameters, plasma parameters and in particular, film properties in relation to the dual cathode system. The spatial distribution of measured parameters will be considered. This paper describes the production of Al 2O 3 films by dual cathode unbalanced reactive magnetron sputtering, in particular, the effects of spatial distribution and pulse frequency on coating properties. In general, it was observed that, once hard arcs have been removed, all coating structures, coating surfaces and hardness show little variation. However, more variation was observed in critical loads during scratch adhesion testing for coatings deposited at different pulse frequencies.

Quantitative modeling of reactive sputtering process for MgO thin film deposition

Reactive sputtering process of magnesium target in d.c. planar magnetron discharge using argon and oxygen gases as buffer and reactive gases, respectively, has been investigated. A drastic mode transition between metallic and oxide modes has been observed due to a large difference in the secondary electron emission coefficients of magnesium and magnesium oxide. To describe the experimental results quantitatively, a new reactive sputtering model has been developed. The model is fundamentally based on a simple reactive gas balance model proposed by Berg et al. in 1988, but includes the change in the secondary electron emission coefficient of target. The modified model can deal with the change of plasma properties through the change of ion to electron current ratio at the target, and can quantitatively describe experimental results such as oxygen flow rate dependence of deposition rate and discharge voltage, which were obtained at a constant discharge current.

Deposition of high quality CMR thin films by rf magnetron sputtering under pure argon gas

La 0.67Ca 0.33MnO 3 epitaxial thin films on SrTiO 3 (100) substrates were deposited by using an off-axis rf magnetron sputtering under pure argon gas. The as-grown films were annealed under flowing oxygen at different temperatures. The La 0.67Ca 0.33MnO 3 films obtained with and without post annealing were studied by x-ray diffraction, electrical and magnetic measurements, surface profiling and scanning electron microscopy. A very small value of the full width at half maximum (∼0.15 o) of the (004) diffraction peak was obtained by rocking curve measurements indicating good crystallinity. The pictures of scanning electron microscopy show that our thin films are extremely smooth and outgrowth free. The average roughness over the scanning region of 2000μm is 1∼2 nm. The post annealing shows almost no effect on the crystallinity and the surface, but greatly enhances the resistance peak temperature, which corresponds to the transition temperature from insulator to metal. The value of the peak temperature is found to be as high as 264K. Our results suggest an efficient and useful deposition process for high quality colossal magnetoresistivity thin films using for device applications.

Spray pyrolysis deposition of zinc ferrite films from metal nitrates solutions

Well-crystallized spinel ZnFe2O4 thin films with good adhesion have been successfully prepared on glass substrates heated above 350°C by a spray pyrolysis deposition from mixed zinc and iron nitrates aqueous solutions. The solution pH has an influence on the surface morphology of ZnFe2O4 films. A smooth and dense surface was obtained from the mixed solution with a pH of 2.0, while a rough and inhomogeneous surface could be observed from the solution with a pH of 8.0. The crystallite size of ZnFe2O4 films was found to be small, ranging from 10 to 21 nm as calculated from X-ray diffraction patterns, which is of interest for magnetic material applications. The indirect and direct band gap energies of the sprayed films were estimated to be 1.78 and 2.70 eV, respectively. A schematic illustration is proposed to show the possible deposition processes of ZnFe2O4 thin films by spray pyrolysis.

Thin-film CIS alloy PV materials fabricated using non-vacuum, particles-based techniques

CuInSe 2 (CIS) and its alloys with gallium and sulfur are excellent thin-film photovoltaic materials, but the vacuum deposition processes typically used to fabricate thin-film CIS alloy photovoltaic (PV) devices are complex and expensive. A new class of thin-film deposition processes based on non-vacuum processing of particulate materials offers a simple, inexpensive alternative to fabricating high-efficiency CIS alloy PV. Suitable particulate materials can be prepared by a variety of techniques, including precipitation, pulverization, aerosol pyrolysis, vapor condensation, etc. Particulate precursors can be deposited in thin layers by simple techniques such as printing, spraying or dip coating. Porous precursor layers can be densified into dense, polycrystalline films by atmospheric-pressure sintering. By taking advantage of the unique materials science of CIS alloy films, one can mitigate the inherent disadvantages of particle-based processing, e.g. layer porosity. Using non-vacuum particles-based processes, efficiencies of 11.7% and 5% have been confirmed by NREL for CIS alloy cells and modules, respectively.

Performance of hydrothermal PZT film on high intensity operation

The deposition processes of PZT thin films have been described in many reports, yet there have been no descriptions of the performance of PZT films at high intensity operation. Hydrothermally-deposited PZT film transducers were examined at large vibration amplitude levels at resonance frequencies. It was demonstrated that the hydrothermal PZT film expresses superior performance with regard to linearity and maximum vibration velocity. The linearity of frequency responses at around the resonance frequency was excellent and superior to bulk material up to high intensity operation. The characteristics of longitudinal transducers using films formed under two deposition processes were tested. The maximum vibration velocity was 2.24 m/s when the thickness of the film was 38 μm and the driving voltage was 145 V p–p. This velocity value was much larger than bulk PZT materials for high power applications.

Characterization of ZrO2 and SiC ceramic thin films prepared by electrostatic atomization

ZrO2 and SiC ceramic thin films and their bilayer have been successfully prepared by a newly developed electrostatic atomization technique. This technique can generate fine spray of ceramic suspensions in a micrometer sized range with a narrow size distribution which is crucial for preparation of uniform thin films of these ceramic materials. Compared to some other thin film deposition techniques, such as Chemical vapour deposition (CVD), physical vapour deposition (PVD) and plasma spray (PS) etc. the thin film deposition process using electrostatic atomization is not only cheap but also controllable. The prepared ZrO2 and SiC thin films were investigated using scanning electron microscopy (SEM) and energy dispersion analysis (EDA) techniques. These thin films were observed to be homogenous with a particle size less than 10 μm. The ZrO2-SiC bilayer was found to have an abrupt interface, implying that the deposition process is controllable and also that functionally graded ceramic/ceramic materials can be prepared in this way if the thickness of each layer is accurately controlled.

Surface chemistry: from vibrational spectroscopy to photoemission spectromicroscopy

Vibrational spectroscopy and core-level photoelectron spectroscopy are two of the most powerful tools for surface chemical analysis. We have used both techniques in conjunction with others to study hydrocarbon radical interactions with metal and semiconductor surfaces as well as thin film deposition processes. These include the investigations of CH 2 and CH 3 reactions on Cu by high resolution electron energy loss spectroscopy (HREELS), and GaN formation from Ga(CH 3) 3 and NH 3 on SiC by SR-PES. The vibrational analysis has revealed rich surface chemistry of the radicals while the photoemission study has led us to develop new approach for probing ultrafine structures. The effort involves the construction of a scanning photoemission spectromicroscope (SPEM) coupled to a synchrotron radiation light source. Recent results are presented in this brief report.

Surface roughness control of the Al and Al2O3 thin films deposited by using pulsed DC magnetron sputtering

ABSTRACTThe thickness of the Al2O3 layer used in the magnetic tunneling junctions FM1/Al2O3/FM2 is less than 2 nm, here FM1 is for the ferromagnetic layer 1 and FM2 is for ferromagnetic layer 2. In order to obtain ultra-thin Al2O3 layer with higher breakdown voltage and pinhole free, extremely smooth surface roughness of this layer is required. The influence of the sputtering gas pressure, DC pulsed frequency, DC pulsed power, substrate bias and buffer layer on surface roughness and properties of Al thin films were studied. The single layer Al films are usually amorphous, texture (111) Al films can be obtained while using thin Ta 5 nm or Ta5/NiFe2 as underlayer. Very smooth Al thin film can be sputtered on Si/SiO2 (100) wafer with Ta/NiFe buffer layer at f=15 kHz (DC pulsed frequency) and with RF substrate biasing (Vpp is about 21 V). High quality MTJs with high MR ratio up to 44.6% and high field sensitivity up to 19.3%/Oe were finally demonstrated after optimization of thin film deposition process.

多結晶Ｓｉ薄膜トランジスタ製作へのプラズマ技術の応用 高密度マイクロ波プラズマによるシリコンネットワーク構造の制御

The high-density and low-temperature microwave plasma (2.45 GHz) utilizing a spoke antenna was applied for fast deposition of hydrogenated microcrystalline silicon (μc-Si : H) film. Highly crystallized and photoconductive, μc-Si : H film was fabricated under the plasma condition that the electron density ne show the maximum together with the minimum of the “hot electron population”. The precise control of the ion beam energy impinging to the growing surface was very effective for the promotion of the film crystallinity. High rate deposition of μc-Si : Hfilm was also achieved at 47 A/s from the 30 sccm SiH4 plasma. This high-density microwave plasma has a high potential not only for etching but also for the large-area thin film deposition processes such as solar cell and thin film transistors (TFTs).

Diamagnetic Response of Normal-Superconducting Double Layers

The diamagnetism of a normal metal (N: Cu or Au), which is induced by the proximity effect of a superconductor (S: Nb), has been investigated for N-S double layers, which are formed by a thin-film deposition process. Detailed studies of samples, which have different electronic mean-free path ℓ N in N, suggest that ℓ N should be controlled by the impurity concentration rather than the mechanical imperfections in the lattice in order to clarify the ℓ N dependence of the proximity effect. Both the screening distance ρ in N and the parameter ν in ρ∝ T -ν increase with an increase in ℓ N . This result can be understood on the assumption that the normal metal changes its behavior from the “dirty” limit (ξ N >ℓ N ) to the “clean” limit (ξ N <ℓ N ), where ξ N is the coherence length in N.

Etching, smoothing, and deposition with gas-cluster ion beam technology

Gas cluster ion beam (GCIB) processing has recently been introduced as a commercial tool for processing ‘rough’ surfaces, such as polished substrates or thin films. The physical interaction of a gas cluster ion beam with a surface is strikingly different from that of better-known ‘monomer’ ion beams. Clusters are formed by the adiabatic expansion of gas through a nozzle, ionization by electron impact, acceleration, and then impingement upon the surface to be processed. The physics of the surface interaction of the cluster beam strongly depends upon gas composition, cluster size, cluster size distribution, and beam energy. Typical argon GCIBs are composed of clusters ranging from several hundred to several thousand atoms in size. It has been previously shown that Ar clusters can be used to smooth surfaces at a sub-nanometer level. Argon cluster beam smoothing typically occurs in the energy range between 15 and ∼30 keV. As such, the average energy per atom is of the order of 10 eV/atom upon cluster impact with the surface and subsequent dissociation. Ion cluster beams formed with reactive gases such as oxygen and nitrogen can also be formed, but at somewhat lower current densities than those obtainable with argon. Upon impact, reactive gas clusters undergo strong chemical reactions at the substrate surface. An extension of this chemical interaction is to utilize reactive clusters in an ion beam-assisted, thin-film physical vapor deposition process. This has been demonstrated with relatively low energy (E<∼10 keV) oxygen clusters in an electron-beam evaporator to form extremely low resistivity indium–tin oxide films on room-temperature substrates. This paper will describe the basics of GCIB formation and application to atomic scale smoothing of technologically interesting substrates and thin films, as well as reactive GCIB assisted deposition technology. The results presented demonstrate some of the unique physics and materials science that can be achieved with an emerging GCIB technology.

Electrochemical oxidation of o-aminophenol in aqueous acidic medium: formation of film and soluble products

The electrochemical oxidation of o-aminophenol (OAP) has been studied by cyclic voltammetry on platinum (Pt) and glassy carbon (GC) electrodes. Films formed on Pt exhibited a voltammetric response that is dependent on the final potential applied during the electropolymerization. Although a less electroactive film has been obtained on Pt electrodes cycled at higher potentials, modified GC electrodes showed a well-defined redox response independently of the potential range used. This result is attributed to an activation process of the GC electrode rather than to an electroactive film deposited on the electrode. The deposition process of thin films was also investigated with quartz crystal microbalance experiments. Poly-o-aminophenol (POAP) films were insoluble with an electrochromic response from colorless to brown, and a ladder structure confirmed by infrared spectra. Exhaustive electrolysis of OAP produced a reasonable amount of soluble product, which after solvent extraction, was characterized as 2-aminophenoxazin-3-one (APZ), a product also prepared by chemical oxidation of OAP. A route of electrooxidation of OAP has been proposed based on electrochemical and spectroscopic results.

Preparation of SrRuO3 thin film by chemical reactive pulsed laser deposition

The emission characteristics and transport behavior of atoms and ions in laser ablation plume in vacuum and in varied O2 pressure were investigated. A collision-dominated expansion was found in O2 ambient, which enhanced excited atoms and ions, and different atoms and ions tend to get the almost same velocities due to collision. These were thought to give an advantage for in situ reaction during the process of thin film deposition.

Process control and pulsed laser deposition of materials

Pulsed-laser deposition (PLD) is a very powerful and rapid deposition technique, which can produce exceptional-quality thin films. Although PLD has tremendous versatility and potential, PLD capabilities are still constrained by a lack of process control. At the Air Force Research Laboratory on-site at Wright-Patterson AFB, we are developing in-situ/real-time control methodologies for PLD and other thin-film deposition processes. We have identified appropriate sensors for closed-loop feedback control and utilized them with YBa2Cu3O7-x (YBCO) to identify critical process control parameters. Control instrumentation has recently been improved by the addition of moveable fixed-position plume-emission sensors. The reproducibility of YBCO film quality increased significantly when process control was applied to PLD processing. PLD process control techniques were applied during simulated and actual BSTO depositions. A comparison of deposition control with the controlling sensor at different distances from the substrate heater position is discussed. Further process refinement using time-resolved spectral components of the PLD plume on subsequent film quality is also discussed.

Electron-paramagnetic resonance (EPR) and light-induced EPR investigations of CuGaSe2

The chalcopyrite compound CuGaSe2 is considered as potential absorber material for thin film solar cells due to its suitable optoelectronic properties. We investigated intentionally undoped bulk material that is used as source material in a thin film deposition process by electron-paramagnetic resonance (EPR) and light-induced EPR (LEPR). Both methods are well suited to determine the concentration and electronic activity of transition metal ions in the chalcopyrite host lattice. The recently identified Ni+ centre was found by LEPR to act as recombination centre for photogenerated charge carriers. Moreover, the typical signature of Cu2+ could be detected in EPR spectra of aged CuGaSe2 powders indicating an oxidation process to which the material is subjected at ambient conditions. The ageing process is significantly accelerated in water-saturated air of 100% humidity compared to dry air of 50% humidity. The connection of this microscopic process with the electronic degradation of the material during solar cell processing will be discussed.