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Top 20 Most Read Articles
April 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Surface chemistry and surface electronic properties of ZnO single crystals and nanorods J. Vac. Sci. Technol. A 27, 328 (2009); http://dx.doi.org/10.1116/1.3085723 (8 pages) Online Publication Date: 25 February 2009
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The surface chemistry of ZnO single crystals of (0001) and (10
 0) orientations and ZnO nanorods was studied using x-ray and ultraviolet photoelectron spectroscopies. Air drying and UV-ozone preparations were studied in particular as chemical treatments that could be applied to poly(3-hexylthiophene) (P3HT)-ZnO solar cells to enhance performance. The UV-ozone treatment showed negligible effect by photoelectron spectroscopy on the ZnO single crystal surfaces, but brought about electronic shifts consistent with increased upward band bending by ∼ 0.25 eV on the ZnO nanorod surface. Modest interface dipoles of ∼ 0.15 and ∼ 0.25 eV were measured between P3HT and the (100) and (0001) single crystal orientations, respectively, with the dipole moment pointing from ZnO to the P3HT layer. The sol-gel films showed evidence of forming a small interface dipole in the opposite direction, which illustrates the difference in surface chemistry between the solution-grown ZnO and the ZnO single crystals. |
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Dry etching of polydimethylsiloxane for microfluidic systems J. Vac. Sci. Technol. A 20, 975 (2002); http://dx.doi.org/10.1116/1.1460896 (8 pages) Online Publication Date: 7 May 2002
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A fluorine-based reactive ion etch (RIE) process has been developed to anisotropically dry etch the silicone elastomer polydimethylsiloxane (PDMS). This technique complements the standard molding procedure that makes use of forms made of thick SU-8 photoresist to produce features in the PDMS. Total gas pressure and the ratio of O2 to CF4 were varied to optimize etch rate. The RIE recipe developed in this study uses a 1:3 mixture of O2 to CF4 gas resulting in a highly directional and stable etch rate of approximately 20 μm per hour. Selective dry etching can be performed through a photolithographically patterned metal etch mask providing greater precision and alignment with preexisting molded features. The dry etch process is presented in this article along with a brief comparison to recently reported wet etch approaches. © 2002 American Vacuum Society. |
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Work function determination of zinc oxide films J. Vac. Sci. Technol. A 15, 428 (1997); http://dx.doi.org/10.1116/1.580502 (3 pages)
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Zinc oxide-silicon heterojunctions were fabricated using both n- and p-type silicon. The zinc oxide films were deposited by the magnetron sputtering process at various substrate temperatures to form these devices. The electrical properties of these devices were measured and the work function of the zinc oxide was evaluated from these properties. © 1997 American Vacuum Society. |
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J. Vac. Sci. Technol. A 25, 1317 (2007); http://dx.doi.org/10.1116/1.2764082 (19 pages) Online Publication Date: 30 July 2007
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Physical vapor deposition under conditions of obliquely incident flux and limited adatom diffusion results in a film with a columnar microstructure. These columns will be oriented toward the vapor source and substrate rotation can be used to sculpt the columns into various morphologies. This is the basis for glancing angle deposition (GLAD), a technique for fabricating porous thin films with engineered structures. The origin of the columnar structure characteristic of GLAD films is discussed in terms of nucleation processes and structure zone models. As deposition continues, the columnar structures are influenced by atomic-scale ballistic shadowing and surface diffusion. Competitive growth is observed where the tallest columns grow at the expense of smaller features. The column shape evolves during growth, and power-law scaling behavior is observed as shown in both experimental results and theoretical simulations. Due to the porous nature of the films and the increased surface area, a variety of chemical applications and sensor device architectures are possible. Because the GLAD process provides precise nanoscale control over the film structure, characteristics such as the mechanical, magnetic, and optical properties of the deposited film may be engineered for various applications. Depositing onto prepatterned substrates forces the columns to adopt a planar ordering, an important requirement for photonic crystal applications.
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Growth of high-quality SrTiO3 films using a hybrid molecular beam epitaxy approach J. Vac. Sci. Technol. A 27, 461 (2009); http://dx.doi.org/10.1116/1.3106610 (4 pages) Online Publication Date: 30 March 2009
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A hybrid molecular beam epitaxy approach for atomic-layer controlled growth of high-quality SrTiO3 films with scalable growth rates was developed. The approach uses an effusion cell for Sr, a plasma source for oxygen, and a metal-organic source (titanium tetra isopropoxide) for Ti. SrTiO3 films were investigated as a function of cation flux ratio on (001) SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates. Growth conditions for stoichiometric insulating films were identified. Persistent (>180 oscillations) reflection high-energy electron diffraction oscillation characteristic of layer-by-layer growth were observed. The full widths at half maximum of x-ray diffraction rocking curves were similar to those of the substrates, i.e., 34 arc sec on LSAT. The film surfaces were nearly ideal with root mean square surface roughness values of less than 0.1 nm. The relationship between surface reconstructions, growth modes, and stoichiometry is discussed.
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Metal–organic interface and charge injection in organic electronic devices J. Vac. Sci. Technol. A 21, 521 (2003); http://dx.doi.org/10.1116/1.1559919 (11 pages) Online Publication Date: 18 March 2003
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Charge injection at the interface between metallic electrodes and organic semiconductors plays a crucial role in the performance of organic (opto-)electronic devices. This article discusses the current understanding of the formation of the metal–organic contact and the parameters which control the injection current. Organic semiconductors differ significantly from their inorganic counterparts, primarily because they are amorphous van der Waals solids. As a result the electronic states are highly localized, and charge transport is by site-to-site hopping. Organics can also form clean interfaces with many metals, free of interface states in the gap. Nevertheless, there is generally found to be a significant vacuum level offset, the origins of which are not yet fully understood. Organic semiconductors are frequently free of donor and acceptor dopants, and as a result the depletion depth is larger than the organic layer thickness. Thus the Fermi level in the organic and the charge injection barriers depend most directly on the interface offset. The charge injection process is described as thermally assisted tunneling from the delocalized states of the metal into the localized states of the semiconductor, whose energy includes contributions from the mean barrier height, the image potential, the energetic disorder, and the applied electric field. There is no completely satisfactory analytic theory for the field and temperature dependence of the injection current, which, for well characterized interfaces, exhibits behavior relating to both thermionic emission and field-induced tunneling. © 2003 American Vacuum Society. |
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J. Vac. Sci. Technol. A 27, 209 (2009); http://dx.doi.org/10.1116/1.3065679 (8 pages) Online Publication Date: 6 February 2009
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The effect of ion and radical compositions in BCl3/Cl2 plasmas was assessed in this work with a focus on the formation of etch products in patterning hafnium aluminate, a potential high-k gate oxide material. The plasma composition became increasingly more complex as the percentage of boron trichloride was increased, which led to the formation of a significant amount of boron-containing species including B+, BCl+, BCl2+, BCl3+, B2Cl3+, and B2OCl3+ in the plasma. The BCl2+ ions were found to be the dominant species in BCl3 containing plasmas at most conditions; however, increasing the pressure or decreasing the power led to an increase in the formation of higher mass ions. Several compositions of Hf1−xAlxOy thin films ranging from pure HfO2 to pure Al2O3 were etched in BCl3/Cl2 plasmas as functions of ion energy and plasma composition. The etch product distributions were measured and the dominant metal-containing etch products were HfClx and AlClx in a Cl2 plasma and HfClx, HfBOCl4, and AlxCly in a BCl3 plasma, and their concentrations increased with increasing ion energy. Oxygen was detected removed in the form of ClO in Cl2 and as trichloroboroxin ((BOCl)3) in BCl3. Both the etch rate and the etch product formation are enhanced in BCl3/Cl2 plasmas, as compared to those in Cl2 plasmas, due to the change in the composition and reactivity of the dominant ions and radicals.
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Plasma atomic layer etching using conventional plasma equipment J. Vac. Sci. Technol. A 27, 37 (2009); http://dx.doi.org/10.1116/1.3021361 (14 pages) Online Publication Date: 8 December 2008
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The decrease in feature sizes in microelectronics fabrication will soon require plasma etching processes having atomic layer resolution. The basis of plasma atomic layer etching (PALE) is forming a layer of passivation that allows the underlying substrate material to be etched with lower activation energy than in the absence of the passivation. The subsequent removal of the passivation with carefully tailored activation energy then removes a single layer of the underlying material. If these goals are met, the process is self-limiting. A challenge of PALE is the high cost of specialized equipment and slow processing speed. In this work, results from a computational investigation of PALE will be discussed with the goal of demonstrating the potential of using conventional plasma etching equipment having acceptable processing speeds. Results will be discussed using inductively coupled and magnetically enhanced capacitively coupled plasmas in which nonsinusoidal waveforms are used to regulate ion energies to optimize the passivation and etch steps. This strategy may also enable the use of a single gas mixture, as opposed to changing gas mixtures between steps.
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J. Vac. Sci. Technol. A 27, 417 (2009); http://dx.doi.org/10.1116/1.3089242 (6 pages) Online Publication Date: 24 March 2009
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Novel technical and technological solutions enabling effective stress control in highly textured polycrystalline aluminum nitride (AlN) thin films deposited with ac (40 kHz) reactive sputtering processes are discussed. Residual stress in the AlN films deposited by a dual cathode S-Gun magnetron is well controlled by varying Ar gas pressure, however, since deposition rate and film thickness uniformity depend on gas pressure too, an independent stress control technique has been developed. The technique is based on regulation of the flux of the charged particles from ac plasma discharge to the substrate. In the ac powered S-Gun, a special stress adjustment unit (SAU) is employed for reducing compressive stress in the film by means of redistribution of discharge current between electrodes of the S-Gun leading to controllable suppression of bombardment of the growing film. This technique is complementary to AlN deposition with rf substrate bias which increases ion bombardment and shifts stress in the compressive direction, if required. Using SAU and rf bias functions ensures tailoring intrinsic stress in piezoelectric AlN films for a particular application from high compressive −700 MPa to high tensile +300 MPa and allows the gas pressure to be adjusted independently to fine control the film uniformity. The AlN films deposited on Si substrates and Mo electrodes have strong (002) texture with full width at half maximum ranging from 2° for 200 nm to 1° for 2000 nm thick films.
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Nonsinusoidal buckling of thin gold films on elastomeric substrates J. Vac. Sci. Technol. A 27, L9 (2009); http://dx.doi.org/10.1116/1.3089244 (5 pages) Online Publication Date: 24 March 2009
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Buckling of stiff thin films on compliant substrates represents a variety of applications, ranging from stretchable electronics to micro-nanometrology. Different but complementary to previously reported sinusoidal buckling waves, this letter presents a nonsinusoidal surface profile of buckled thin Au films on compliant substrates, specifically, a secondary dip on top of buckling wave or rather broadened wave top with very sharp trough. This nonsinusoidal profile is likely due to tension/compression asymmetry, i.e., different strengths in tension and compression resulted from the polycrystalline, grained microstructure of metal film. Finite element analysis with asymmetric tension/compression material model has reproduced the experiments well qualitatively.
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Plasma deposition of optical films and coatings: A review J. Vac. Sci. Technol. A 18, 2619 (2000); http://dx.doi.org/10.1116/1.1314395 (27 pages)
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Plasma enhanced chemical vapor deposition (PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin film systems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasma systems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others. © 2000 American Vacuum Society. |
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Correlation of chemical composition and electrical properties of rf sputtered alumina films J. Vac. Sci. Technol. A 27, 234 (2009); http://dx.doi.org/10.1116/1.3065978 (11 pages) Online Publication Date: 6 February 2009
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Alumina films were prepared by radio frequency magnetron sputtering from an aluminum oxide target on an indium tin oxide covered glass. The purpose of the study was to test the influence of the sputter parameters on the elemental composition, surface morphology, and electrical insulation properties (breakdown fields and leakage currents). Tested parameters were the sputter gas (Ar) pressure, the sputter rate, the sputter power, the sputter gas composition (Ar:O2), the conditioning of the target, the residual base pressure, and the substrate temperature. The surface morphology of the films was investigated by atomic force microscopy. Depth profiles of the elemental compositions were measured by elastic recoil detection using energetic heavy ions, and the insulation properties were investigated by current voltage measurements and impedance spectroscopy. The main finding is that the leakage currents increase by about five orders of magnitude, if the atomic ratio of O:Al increases from 1.35 to 2.0. In parallel the breakdown fields decrease by a factor of 100, and the character of the breakdowns changes from soft to hard. The highest breakdown fields (2.4 MV/cm) and smallest leakage currents (6.5×10−8 A/cm2 at 2.0 MV/cm) are obtained for slightly Al rich films with small atomic concentrations of H (<0.5 at. %). These films were sputtered in pure Ar and under the lowest possible pressure of the residual gas.
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Nanosphere lithography: A materials general fabrication process for periodic particle array surfaces J. Vac. Sci. Technol. A 13, 1553 (1995); http://dx.doi.org/10.1116/1.579726 (6 pages)
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In this article nanosphere lithography (NSL) is demonstrated to be a materials general fabrication process for the production of periodic particle array (PPA) surfaces having nanometer scale features. A variety of PPA surfaces have been prepared using identical single‐layer (SL) and double‐layer (DL) NSL masks made by self‐assembly of polymer nanospheres with diameter, D=264 nm, and varying both the substrate material S and the particle material M. In the examples shown here, S was an insulator, semiconductor, or metal and M was a metal, inorganic ionic insulator, or an organic π‐electron semiconductor. PPA structural characterization and determination of nanoparticle metrics was accomplished with atomic force microscopy. This is the first demonstration of nanometer scale PPA surfaces formed from molecular materials. © 1995 American Vacuum Society |
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J. Vac. Sci. Technol. A 27, 352 (2009); http://dx.doi.org/10.1116/1.3081966 (4 pages) Online Publication Date: 27 February 2009
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Three types of low-damage radio-frequency (rf) magnetron sputtering processes—an interruptive process, a rotating cylindrical holder method, and an off-axis sputtering method—were designed and studied to reduce the film surface temperature during deposition. Low-damage sputtering processes were investigated to improve the resistivity and optical transmittance in the visible range of Al doped ZnO (AZO) thin films deposited on polymer substrates. In the case of the polyethersulfone substrate, AZO films with a resistivity of 1.0×10−3 Ω cm and an optical transmittance of 75% were obtained by the rotating repeat holder method during rf sputtering.
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J. Vac. Sci. Technol. A 27, 174 (2009); http://dx.doi.org/10.1116/1.3065675 (9 pages) Online Publication Date: 6 February 2009
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The effects of bias voltage on the microstructure and the related tribological properties of CrAlYN/CrN nanoscale multilayer superlattice coatings were investigated. The coatings were deposited at 450 °C substrate temperature by combined high power impulse magnetron sputtering (HIPIMS) and unbalanced magnetron sputtering techniques. The substrates were 304 stainless steel, M2 high speed steel for structural analysis and mechanical testing, as well as cemented carbide substrates end mills for dry high speed milling applications. Substrates were pretreated by HIPIMS etching. The bias voltage Ub was varied between −75 and −150 V. The chemical composition was determined by neutral mass spectroscopy. The microstructure was characterized by x-ray diffraction and cross sectional transmission electron microscopy. All coatings had a single phase B1 fcc structure. The chemical composition was not affected by the bias voltage. Local epitaxial or axiotaxial growth attributed to the HIPIMS etching pretreatment was observed on the large surface areas of the substrate crystals. This turned to columnar growth with {110} texture at low bias voltages Ub between −75 and −120 V, while at Ub = −150 V an equiaxed structure of large crystal sizes developed with {111} texture. At the same time the waviness of the superlattice significantly decreased. An increase in bias voltage resulted in a significant rise in both residual stress levels (from −3.3 to −9.5 GPa) and plastic hardness (from Hp = 34–51 GPa), while the coating/substrate adhesion decreased from 61 to 45 N. The friction coefficient increased from 0.43 (at Ub = −75 V) to 0.55 (at UB = −120 V), while the initial sliding wear rates decreased remarkably (2.6×10−16 m3 N−1 m−1 at UB = −75 V to 3.7×10−17 m3 N−1 m−1 at Ub = −150 V). The life time of 8 mm ball-nosed cemented carbide end mills decreased from 39 min at Ub = −75 V to 19 min when Ub was raised to −150 V. These results highlight that the combination of HIPIMS substrate treatment and designed deposition parameters provides good opportunity to tailor coating structures with optimized properties.
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J. Vac. Sci. Technol. A 19, 963 (2001); http://dx.doi.org/10.1116/1.1368836 (8 pages)
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ZnO:Al/(ZaO) films were deposited on quartz substrates by dc magnetron reactive sputtering from a Zn target mixed with Al. The effect of oxygen flow rate, target to substrate distance, substrate temperature, and Al doping content on the structural, electrical and optical properties of ZAO were investigated. It was observed that the (002) peak position of all films shifts to lower angle comparable to that of bulk ZnO due to the residual stress change with deposition parameters. X-ray photoemission spectroscopy was introduced to analyze the chemical state of Al on the film surface and the results show Al enrichment. The dependences of electrical properties such as resistivity, carrier concentration and Hall mobility on substrate temperature, and Al doping content were measured. The visible transmittance of above 80% and infrared reflectance of above 80% were obtained. The minimum resistivity is 4.23×10−4 Ω cm with the carrier concentration of 9.21×1020 cm−3 and Hall mobility of 16.0 cm2 v1 s−1. The optical band gap was observed to increase with increasing carrier concentration. The probable mechanisms are discussed. © 2001 American Vacuum Society. |
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J. Vac. Sci. Technol. A 27, 436 (2009); http://dx.doi.org/10.1116/1.3097848 (7 pages) Online Publication Date: 24 March 2009
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Undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films have been deposited by plasma enhanced chemical vapor deposition (PECVD) at low temperature with different hydrogen dilutions and rf powers. Large complexity of microstructure in hydrogenated microcrystalline silicon and the existence of different sizes of crystallites are demonstrated by different characterizations. The authors correlate the transport properties with the structural properties of the rf PECVD grown μc-Si:H in the amorphous-to-crystalline transition region. For chamber pressure of 2.0 Torr and rf power density of 310 mW/cm2, the onset of crystallinity is observed for the film deposited at a hydrogen dilution of 94%. At a hydrogen dilution of 95%, amorphous-to-microcrystalline transition have been observed. This film exhibits a dark conductivity of 2.7×10−7 S cm−1 and a crystalline volume fraction of 21%. The mobility-lifetime product for these films are 3.8×10−6 cm2/V and hole diffusion length is 70 nm. Fourier transform infrared study shows mainly monohydride bonding in this film. This film becomes stable after 30 h of light soaking.
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Effects of interfacial layer structures on crystal structural properties of ZnO films J. Vac. Sci. Technol. A 26, 90 (2008); http://dx.doi.org/10.1116/1.2821741 (7 pages) Online Publication Date: 14 December 2007
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Single crystalline ZnO films were grown on Cr compound buffer layers on (0001) Al2O3 substrates by plasma assisted molecular beam epitaxy. In terms of lattice misfit reduction between ZnO and substrate, the CrN and Cr2O3/CrN buffers are investigated. The structural and optical qualities of ZnO films suggest the feasibility of Cr compound buffers for high-quality ZnO films growth on (0001) Al2O3 substrates. Moreover, the effects of interfacial structures on selective growth of different polar ZnO films are investigated. Zn-polar ZnO films are grown on the rocksalt CrN buffer and the formation of rhombohedral Cr2O3 results in the growth of O-polar films. The possible mechanism of polarity conversion is proposed. By employing the simple patterning and regrowth procedures, a periodical polarity converted structure in lateral is fabricated. The periodical change of the polarity is clearly confirmed by the polarity sensitive piezo response microscope images and the opposite hysteretic characteristic of the piezo response curves, which are strict evidences for the validity of the polarity controlling method as well as the successful fabrication of the periodical polarity controlled ZnO structure.
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The microstructure of sputter‐deposited coatings J. Vac. Sci. Technol. A 4, 3059 (1986); http://dx.doi.org/10.1116/1.573628 (7 pages)
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Microstructure is a critical consideration when polycrystalline or amorphous thin films are used for applications such as microcircuit metallization layers and diffusion barriers. The trend in device fabrication toward lower processing temperatures means that such coatings must often be deposited at substrate temperatures T that are low relative to the coating material melting point Tm. The structure of vapor deposited coatings grown under these conditions consists typically of a columnar growth structure, defined by voided open boundaries, which is superimposed on a microstructure which may be polycrystalline (defined by metallurgical grain boundaries) or amorphous. The voided growth structure is clearly undesirable for most applications. Its occurrence is a fundamental consequence of atomic shadowing acting in concert with the low adatom mobilities that characterize low T/Tm deposition, and its formation can be enhanced by the surface irregularities which are common to microcircuit fabrication. This paper reviews some of the recent developments in understanding the fundamental aspects of the relationship between the deposition conditions and the microstructure of sputter‐deposited thin films, with particular emphasis on the origin of the growth structure and its suppression through energetic particle bombardment. |
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Chemistry in long residence time fluorocarbon plasmas J. Vac. Sci. Technol. A 27, 193 (2009); http://dx.doi.org/10.1116/1.3065678 (16 pages) Online Publication Date: 6 February 2009
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The densities of radicals and neutrals in fluorocarbon (FC) plasmas have been investigated in an inductively coupled plasma system to understand the predominant gain and loss mechanisms of dissociative products and their interaction with chamber surfaces. The input parameters varied in this experiment are the source to chuck gap and the F: C ratio of the feed gas. The densities of F, CF2, CF3, CF4, C2F4, SiF4, COF2, CO, and CO2 are measured and analyzed. In addition, two different forms of C4F8, the standard cyclic c and a radical linear l structures are observed in C4F8 containing plasmas. l-C4F8 is shown to be the primary dissociation product of c-C4F8 and, thus, cannot be neglected from calculations of the loss rate of c-C4F8 to electron collisions. This implies that the typically cited dissociative products of c-C4F8 (primarily C2F4) can have dual production channels: one from l-C4F8 and the other directly from c-C4F8. Furthermore, the measured density of CF4 shows strong correlation to the loss of F to the surfaces, indicating its primary production mechanism is from surface recombination. Similarly, CF3 is mainly produced from deposited FC film on chamber surfaces and lost to electron impact reactions. The etch products resulting from etch of the quartz window are shown, and an overall neutral density model is developed.
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