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Sep 2012

Volume 30, Issue 5, Articles (05xxxx)

Issue Cover Spotlight Figure

J. Vac. Sci. Technol. B 30, 051202 (2012); http://dx.doi.org/10.1116/1.4739425 (5 pages)

Gvidas Astromskas, Mattias Borg, and Lars-Erik Wernersson
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Creating nanoscale Ag patterns on the Si(111)–(√3 × √3)R30°-Ag surface via guided self-assembly

Alex Belianinov, Barış Ünal, Michael C. Tringides, and Patricia A. Thiel

J. Vac. Sci. Technol. B 30, 050601 (2012); http://dx.doi.org/10.1116/1.4738745 (4 pages)

Online Publication Date: 20 July 2012

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Patterns of Ag nanostructures can be created on the Si(111)–(√3 × √3)R30°-Ag surface, using a simple two-step process in ultrahigh vacuum. First, patterns are created using the tip of a scanning tunneling microscope. Second, Ag is deposited at room temperature. The Ag diffuses over long distances on the surface and selectively aggregates at the patterned regions. The size of the Ag features is ∼3–4 nm.
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81.16.Rf Micro- and nanoscale pattern formation
81.15.Dj E-beam and hot filament evaporation deposition
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation
81.16.Dn Self-assembly

Proximity error correction method for continuous moving stage electron beam writing

Sachin Kasture, Nikesh V. V., Gajendra Mulay, and Achanta Venu Gopal

J. Vac. Sci. Technol. B 30, 050602 (2012); http://dx.doi.org/10.1116/1.4746259 (5 pages)

Online Publication Date: 22 August 2012

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Fabrication of high density waveguide-like structures using electron-beam lithography is challenging due to concerns such as stitching errors and proximity issues, which lead to irregularities in the fabricated structure. Continuous moving stage writing method is used to avoid the stitching errors for the long waveguide-like structures, but conventional proximity error correction methods cannot be applied in such cases. The authors propose a simple theoretical method to proximity correct such structures and experimentally demonstrate it in the case of high density millimeter long waveguide-like or grating structures. This method is ideal for high aspect ratio writing, which involves structures that have elements that are much longer than the separation between them. Also, in this method, every element can be assigned a single dose and thus does not need fracturing of individual elements. Experimental results agree well with the theoretically obtained corrections.
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85.40.Hp Lithography, masks and pattern transfer
42.79.Gn Optical waveguides and couplers
81.16.Nd Micro- and nanolithography

Effect of niobium doping on the optical and electrical properties in titanium dioxide grown by pulsed laser deposition

Hyojung Bae, Jun-Seok Ha, Seunghwan Park, Toyohiro Chikyow, Jiho Chang, and Dongcheol Oh

J. Vac. Sci. Technol. B 30, 050603 (2012); http://dx.doi.org/10.1116/1.4750373 (4 pages)

Online Publication Date: 7 September 2012

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The influence of niobium (Nb) on both the electrical and optical properties in titanium dioxide (TiO2) grown by pulsed laser deposition was investigated. Nb atoms with a critical doping ratio of 8 at. % were carefully controlled in a 100 nm-thick TiO2 layer (Nb:TiO2). The Hall effect results revealed that the electrical resistivity and electron concentration of the resulting Nb:TiO2 film were 30 times lower and 2 orders of magnitude higher than those of un-doped TiO2 (u-TiO2), respectively, leading to a slight degradation of optical transmittance in the visible region. The room temperature photoluminescence results showed that the emission intensity at the near band of Nb:TiO2 was greatly enhanced, while, that at the impurity band created by oxygen vacancy (Vo) was almost constant. Detailed investigation of the T-dependent conductivity indicated that the Nb:TiO2 film possessed an activation energy as shallow as 20.8 meV, which suggested that the electrical properties were dominantly determined by Nb shallow impurity rather than point defects such as Vo.
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61.72.up Other materials
73.61.Le Other inorganic semiconductors
78.40.Ha Other nonmetallic inorganics
78.55.Hx Other solid inorganic materials
78.66.Li Other semiconductors
81.15.Fg Pulsed laser ablation deposition
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Interplay of native point defects with ZnO Schottky barriers and doping

Leonard J. Brillson, Yufeng Dong, Filip Tuomisto, Bengt G. Svensson, Andrei Yu. Kuznetsov, Daniel Doutt, H. Lee Mosbacker, Gene Cantwell, Jizhi Zhang, Jin Joo Song, Z.-Q. Fang, and David C. Look

J. Vac. Sci. Technol. B 30, 050801 (2012); http://dx.doi.org/10.1116/1.4732531 (11 pages)

Online Publication Date: 29 June 2012

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A combination of depth-resolved electronic and structural techniques reveals that native point defects can play a major role in ZnO Schottky barrier formation and charged carrier doping. Previous work ignored these lattice defects at metal–ZnO interfaces due to relatively low point defect densities in the bulk. At higher densities, however, they may account for the wide range of Schottky barrier results in the literature. Similarly, efforts to control doping type and density usually treat native defects as passive, compensating donors or acceptors. Recent advances provide a deeper understanding of the interplay between native point defects and electronic properties at ZnO surfaces, interfaces, and epitaxial films. Key to ZnO Schottky barrier formation is a massive redistribution of native point defects near its surfaces and interfaces. It is now possible to measure the energies, densities, and in many cases the type of point defects below the semiconductor-free surface and its metal interface with nanoscale precision. Depth-resolved cathodoluminescence spectroscopy of deep level emissions calibrated with electrical techniques show that native point defects can (1) increase by orders of magnitude in densities within tens of nanometers of the semiconductor surface, (2) alter free carrier concentrations and band profiles within the surface space charge region, (3) dominate Schottky barrier formation for metal contacts to ZnO, and (4) play an active role in semiconductor doping. The authors address these issues by clearly identifying transition energies of leading native point defects and defect complexes in ZnO and the effects of different annealing methods on their spatial distributions on a nanoscale. These results reveal the interplay between ZnO electronic defects, dopants, polarity, and surface nanostructure, highlighting new ways to control ZnO Schottky barriers and doping.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts
78.60.Hk Cathodoluminescence, ionoluminescence
61.72.jd Vacancies
61.72.uj III-V and II-VI semiconductors
61.72.Cc Kinetics of defect formation and annealing
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Formation of high quality nano-crystallized Ge films on quartz substrates at moderate temperature

Cong Li, Jun Xu, Wei Li, Shenghua Sun, Xiaofan Jiang, and Kunji Chen

J. Vac. Sci. Technol. B 30, 051201 (2012); http://dx.doi.org/10.1116/1.4739432 (6 pages)

Online Publication Date: 25 July 2012

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The hydrogenated amorphous Ge films were prepared by plasma enhanced chemical vapor deposition technique. Post-thermal annealing was applied to obtain nano-crystalline Ge (nc-Ge) films on quartz substrates. The evolution of microstructure, optical, and electrical properties was studied during the transition process from amorphous to nano-crystalline phase. It was found that the nano-crystalline Ge with size of 10–30 nm can be formed at the moderate annealing temperature (450–600 °C). Moreover, systematic investigation on carrier transport was carried out for samples with and without annealing. It was found that the room temperature conductivity can be increased by 6 orders of magnitude after the formation of nc-Ge. Hall measurements suggested that the nano-crystalline Ge films showed the p-type behaviors and the hole mobility can reach as high as 211 cm2 V−1 s−1 for samples annealed at 450 °C, which can be used in large area Ge-channel thin film transistors.
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81.05.Cy Elemental semiconductors
81.07.Bc Nanocrystalline materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.ag Semiconductors
73.61.Cw Elemental semiconductors
78.66.Db Elemental semiconductors and insulators

Thin InAs membranes and GaSb buffer layers on GaAs(001) substrates

Gvidas Astromskas, Mattias Borg, and Lars-Erik Wernersson

J. Vac. Sci. Technol. B 30, 051202 (2012); http://dx.doi.org/10.1116/1.4739425 (5 pages)

Online Publication Date: 26 July 2012

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Thin InAs layers and membranes are fabricated on GaAs substrates using GaSb buffer layers grown by MOVPE. The quality of the GaSb buffer layers is optimized and epitaxial InAs layers are grown on GaSb layers of various thickness. The best GaSb buffer layers are obtained for a nucleation temperature of 450 °C and a subsequent growth temperature of 570 °C with a V/III ratio of 3, as confirmed by both the structural (high-resolution XRD, AFM) and electrical (Hall) measurements. Furthermore, a clear relationship between the structural quality of the GaSb and InAs layers is established. Finally, free-standing InAs structures are fabricated where Hall measurements reveal a mobility that depends on the film thickness.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.ag Semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects
73.61.Ey III-V semiconductors

Studies on InAs/GaSb superlattice structural properties by high resolution x-ray diffraction

Yi Zhou, Jianxin Chen, Qingqing Xu, and Li He

J. Vac. Sci. Technol. B 30, 051203 (2012); http://dx.doi.org/10.1116/1.4739428 (6 pages) | Cited 2 times

Online Publication Date: 2 August 2012

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This paper presents work on InAs/GaSb superlattice structural property studies. The superlattice materials were grown by molecular beam epitaxy and measured by high resolution x-ray diffraction, and measured x-ray rocking curves were fitted to the simulated ones in order to fully analyze the superlattice structures. A four-layer model including an InAs layer, a GaSb layer and two interface layers was used for simulation. The results show that the two interface layers are ternary compounds of InSbAs, which have, respectively, an Sb composition of 0.99 at the InAs-on-GaSb interfaces and an Sb composition of 0.01 at the GaSb-on-InAs interfaces. This is the first article, to our knowledge, on the detailed analysis of the InAs/GaSb superlattice interface structures. The experiments also demonstrate that the As flux during the epitaxy growth affects the interface layer InSbAs compositions and hence the lattice mismatch between the superlattices and the substrates. With an As beam equivalent pressure change from 1 × 10−5 to 3 × 10−6 Torr, the lattice mismatch decreases from 3.2 × 10−3 to 5 × 10−4. Measurements on and analysis of Bragg peak broadening under different diffraction geometries show that the broadening depends on both the superlattice period and the lattice mismatch.
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68.65.Cd Superlattices
81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.Ct Interface structure and roughness

Boron filling of high aspect ratio holes by chemical vapor deposition for solid-state neutron detector applications

Kuan-Chih Huang, Rajendra Dahal, Nicolas LiCausi, James J.-Q. Lu, Yaron Danon, and Ishwara B. Bhat

J. Vac. Sci. Technol. B 30, 051204 (2012); http://dx.doi.org/10.1116/1.4742856 (6 pages) | Cited 2 times

Online Publication Date: 9 August 2012

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A multiple deposition and etching process has been developed to enable high fill factor boron deposition in high aspect ratio holes fabricated in a (100) silicon substrate. The boron deposition was carried out using low-pressure chemical vapor deposition and the etching was done by inductively coupled plasma reactive ion etching technique. The boron deposition processes were carried out under different conditions in order to find a baseline process condition. The boron etching processes done under different conditions with the photoresist as the mask are also discussed. Finally, the fabricated neutron detector with the highest fill factor was characterized for the thermal neutron detection efficiency.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.65.Cf Surface cleaning, etching, patterning
68.55.ag Semiconductors
29.40.Wk Solid-state detectors
52.77.Bn Etching and cleaning

In situ grown Ge in an arsenic-free environment for GaAs/Ge/GaAs heterostructures on off-oriented (100) GaAs substrates using molecular beam epitaxy

Mantu K. Hudait, Yan Zhu, Nikhil Jain, Siddharth Vijayaraghavan, Avijit Saha, Travis Merritt, and Giti A. Khodaparast

J. Vac. Sci. Technol. B 30, 051205 (2012); http://dx.doi.org/10.1116/1.4742904 (11 pages) | Cited 5 times

Online Publication Date: 10 August 2012

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High-quality epitaxial Ge layers for GaAs/Ge/GaAs heterostructures were grown in situ in an arsenic-free environment on (100) off-oriented GaAs substrates using two separate molecular beam epitaxy (MBE) chambers, connected via vacuum transfer chamber. The structural, morphological, and band offset properties of these heterostructures are investigated. Reflection high energy electron diffraction studies exhibited (2 × 2) Ge surface reconstruction after the growth at 450  °C and also revealed a smooth surface for the growth of GaAs on Ge. High-resolution triple crystal x-ray rocking curve demonstrated high-quality Ge epilayer as well as GaAs/Ge/(001)GaAs heterostructures by observing Pendellösung oscillations and that the Ge epilayer is pseudomorphic. Atomic force microscopy reveals smooth and uniform morphology with surface roughness of ∼0.45 nm and room temperature photoluminescence spectroscopy exhibited direct bandgap emission at 1583 nm. Dynamic secondary ion mass spectrometry depth profiles of Ga, As, and Ge display a low value of Ga, As, and Ge intermixing at the Ge/GaAs interface and a transition between Ge/GaAs of less than 15 nm. The valence band offset at the upper GaAs/Ge-(2 × 2) and bottom Ge/(001)GaAs-(2 × 4) heterointerface of GaAs/Ge/GaAs double heterostructure is about 0.20 eV and 0.40 eV, respectively. Thus, the high-quality heterointerface and band offset for carrier confinement in MBE grown GaAs/Ge/GaAs heterostructures offer a promising candidate for Ge-based p-channel high-hole mobility quantum well field effect transistors.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
71.20.Nr Semiconductor compounds
78.55.Cr III-V semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.bg Semiconductors

Interface studies on high-k/GaAs MOS capacitors by deep level transient spectroscopy

Souvik Kundu, Yelagam Anitha, Supratic Chakraborty, and Pallab Banerji

J. Vac. Sci. Technol. B 30, 051206 (2012); http://dx.doi.org/10.1116/1.4745882 (8 pages)

Online Publication Date: 10 August 2012

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An experimental analysis has been performed in high-k/GaAs MOS devices to investigate the slow and fast interface traps (Dit) using high frequency capacitance-voltage and deep level transient spectroscopic (DLTS) measurements. Prior to deposition of high-k gate dielectric, an ultrathin layer of ZnO was deposited on GaAs by metalorganic chemical vapor deposition. The number of slow interface traps was found to be 2.80 × 1011 cm−2, whereas the fast interface trap density was measured to be 1.80 × 1011 eV−1 cm−2. The activation energy, capture cross section, and concentration of majority carrier traps were measured to be 0.30 eV, 5.70 × 10−19 cm2, and 4.93 × 1015 cm−3, respectively. Combining conventional DLTS with insufficient-filling, the trap location was found to be at 0.14 eV. Therefore, the traps are not exactly at the interface of GaAs and high-k but in the GaAs surfaces very close to the interfaces. According to the trap energy level position, Dit was found to be 5.3 × 1011 eV−1 cm−2. The leakage current is found to reduce in ZnO passivated devices due to an increase in valance band offset by 0.49 eV. Such an improvement is due to a higher surface potential resulting from the wide bandgap of ZnO.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Formation and morphological evolution of InAs quantum dots grown by chemical beam epitaxy

Jihene Zribi, Denis Morris, and Richard Arès

J. Vac. Sci. Technol. B 30, 051207 (2012); http://dx.doi.org/10.1116/1.4746072 (5 pages)

Online Publication Date: 20 August 2012

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In this work, we study the formation and the morphological evolution of InAs quantum dots (QDs) grown by chemical beam epitaxy on GaAs (001) substrate. A series of samples having different nominal InAs thicknesses has been investigated using atomic force microscopy (AFM) and low-temperature photoluminescence (PL) experiments. AFM results show that large two-dimensional (2D) clusters evolve into three-dimensional (3D) islands that change in size and density as the quantity of deposited InAs material increases. The 2D–3D growth mode transition occurs at an InAs thickness of 1.6 monolayer (ML). The QD density reaches a maximum value of about 8 × 1010 cm−2 at 2.4 ML and dot coalescence is observed for larger InAs thicknesses. These results are consistent with PL measurements performed on samples having an additional GaAs cap layer. A broad QD PL band appears when the InAs thickness reaches 1.6 ML and this emission band is redshifted for thicknesses above 2.4 ML.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.07.Ta Quantum dots
78.67.Hc Quantum dots

Inductively coupled plasma deep etching of InP/InGaAsP in Cl2/CH4/H2 based chemistries with the electrode at 20 °C

Andreas Wieczorek, Vladimir Djara, Frank H. Peters, James O’Callaghan, Kevin Thomas, and Brian Corbett

J. Vac. Sci. Technol. B 30, 051208 (2012); http://dx.doi.org/10.1116/1.4748807 (7 pages)

Online Publication Date: 29 August 2012

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A Cl2/CH4/H2 inductively coupled plasma process without additional heating or wafer bonding is developed for the InP/InGaAsP material system. Vertical and smooth sidewalls can be observed in the scanning electron microscope images. The main factors of etch rate, selectivity, and sidewall roughness are analyzed relative to the gas concentration in a full factorial design of the experimental procedure. Under optimized conditions, an etch depth of more than 3 μm with smooth and vertical sidewalls can be obtained. A strong indication of a passivation effect of CH4 is obtained.
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52.77.Bn Etching and cleaning
81.65.Rv Passivation
81.05.Ea III-V semiconductors

193 nm excimer laser lift-off for AlGaN/GaN high electron mobility transistors

Xiaotie Wang, Chien-Fong Lo, Lu Liu, Camilo V. Cuervo, Ren Fan, Stephen J. Pearton, Brent Gila, Michael R. Johnson, Lin Zhou, David J. Smith, Jihyun Kim, Oleg Laboutin, Yu Cao, and Jerry W. Johnson

J. Vac. Sci. Technol. B 30, 051209 (2012); http://dx.doi.org/10.1116/1.4751278 (5 pages)

Online Publication Date: 7 September 2012

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AlGaN/GaN HEMTs grown on both-side-polished sapphire substrates were successfully lifted-off with a 193-nm UV excimer laser system. The photon energy of the 193 nm laser is larger than the band gap of AlN and thus it can be used to lift-off AlGaN HEMT structures with AlN or AlGaN interfacial layers grown on sapphire substrates prior to growth of the GaN buffer layers. The lifted-off HEMT chip was warped and showed 25–42% reduction of the saturation drain current. There was no degradation observed either in the forward or reverse gate current-voltage (I-V) characteristics or on the drain punch-through voltage. Based on comparisons of cross-sectional electron micrographs, no additional dislocations were created in the HEMT structures after the laser lift-off process. Reduction in saturation drain current was attributed to relaxation of the lifted-off HEMT structures. Newton's rings and Raman spectrum E2 peak shifts were used to estimate the strain relaxation of the laser lifted-off samples.
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85.30.Tv Field effect devices

Effects of polarity and surface treatment on Ga- and N-polar bulk GaN

Michael Foussekis, Josephus D. Ferguson, Joy D. McNamara, Alison A. Baski, and Michael A. Reshchikov

J. Vac. Sci. Technol. B 30, 051210 (2012); http://dx.doi.org/10.1116/1.4751276 (10 pages)

Online Publication Date: 12 September 2012

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The effects of polarity and surface treatment on the morphological, electrical, and optical behaviors in bulk GaN have been investigated. Kelvin probe, atomic force microscopy (AFM), and photoluminescence (PL) techniques were utilized to examine a set of freestanding, bulk GaN samples, which were grown by halide vapor phase epitaxy. The Ga- and N-polar surfaces were treated with either a mechanical polish (MP) or chemical mechanical polish (CMP), which influences the morphology, surface photovoltage (SPV), and PL behaviors. Topography studies indicate that the CMP-treated, Ga-polar surface is the smoothest of the sample set, whereas the MP-treated, N-polar surface has the highest root mean square roughness. Local current–voltage spectra obtained with conducting AFM reveal a higher forward-bias, turn-on voltage for the N-polar versus Ga-polar surfaces. Using a Kelvin probe, intensity-dependent SPV measurements are performed on samples with CMP-treated, Ga- and N-polar surfaces, and provide band bending values of 0.83 and 0.70 eV, respectively. The restoration of the SPV from CMP-treated surfaces behaves as predicted by a thermionic model, whereas restoration from MP-treated surfaces has a faster rate than expected. This result is possibly due to enhanced electron conduction via hopping between defect states to the surface. The quantum efficiency of the PL from the CMP- and MP-treated surfaces at room temperature is ∼1% and 1 × 10−5%, respectively, suggesting high quenching of the PL for MP-treated surfaces by near-surface defects. Therefore, AFM, PL, and SPV data indicate that the MP-treated surfaces have a significantly higher density of surface defects.
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81.65.Ps Polishing, grinding, surface finishing
68.37.Ps Atomic force microscopy (AFM)
73.20.Hb Impurity and defect levels; energy states of adsorbed species
68.35.B- Structure of clean surfaces (and surface reconstruction)
72.40.+w Photoconduction and photovoltaic effects
78.55.Cr III-V semiconductors
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Large contrast enhancement by sonication assisted cold development process for low dose and ultrahigh resolution patterning on ZEP520A positive tone resist

Landobasa Y. M. Tobing, Liliana Tjahjana, and Dao Hua Zhang

J. Vac. Sci. Technol. B 30, 051601 (2012); http://dx.doi.org/10.1116/1.4739053 (7 pages) | Cited 1 time

Online Publication Date: 20 July 2012

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The authors demonstrate a robust, low dose, high contrast, and ultrahigh resolution patterning process based on sonication assisted development of ZEP520A positive tone resist in both room and cold temperature. The contrast as high as γ ∼ 25 and γ ∼ 9.14 can readily be achieved in 6 °C and room temperature development, respectively, in diluted n-amyl acetate solution. The high contrast is demonstrated on 90 nm thick ZEP resist at 20 kV acceleration voltage, from which 20 nm thick titanium lift-off of 60 nm pitch lines and 50 nm pitch dots can be successfully achieved.
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81.16.Rf Micro- and nanoscale pattern formation

Outgassing and photochemical studies of photosensitive films upon irradiation at 13.5 nm

Grace H. Ho, Chih-H. Shao, Jia-J. Sung, Fu-H. Kang, Chih-B. Kao, Wei-L. Hung, Yu-L. Chou, and Yen-H. Huang

J. Vac. Sci. Technol. B 30, 051602 (2012); http://dx.doi.org/10.1116/1.4739000 (10 pages)

Online Publication Date: 27 July 2012

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Neutral outgassed species from polymethylmethacrylate (PMMA), a model resist (denoted RRR), and three underlayer thin films have been characterized and their absolute outgassing rates determined using a quadrupole mass spectrometer following irradiation at 13.5 nm. The radiation, which was in the extreme ultraviolet (EUV) energy range at 13.5 nm, was delivered from the 08A1-beamline at the National Synchrotron Radiation Research Center in Taiwan. The side chains of acryl-type polymers that are vulnerable to fragmentation and outgassing upon EUV irradiation were characterized and the important outgassed species (H2O and H3O+) were identified from the Si-containing photosensitive materials. The authors propose that the Si-OH formed on the Si-containing surfaces upon EUV irradiation likely undergoes subsequent incremental H2O and H3O+ outgassing upon cumulative EUV irradiation. A benchmark comparison revealed that the absolute outgassing rates of PMMA, RRR, and a Si-containing underlayer sample determined in this work were consistent with values previously reported in the literature. Thus, we have effectively demonstrated that the Taiwanese facility is capable of evaluating absolute resist outgassing rates, making it one of a limited number of EUV resist evaluation sites worldwide. The authors have provided direct evidence that the extent of ionic and neutral outgassing from 80, 100, and 125 nm PMMA and RRR films is not thickness-dependent. The authors have further demonstrated that the absolute outgassing rates of PMMA and RRR films of various thickness and two oxy-hydrocarbon underlayer samples correlated strongly with the 13.5 nm photoabsorption (σabs) and the structural toughness [double-bond-equivalence-per-backbone (DBEPB)] of the polymers. The authors revealed that the formula σabs/DBEPB could be used as a generic metric to predict the resistance of photosensitive films to EUV irradiation for the oxy-hydrocarbons in this work and four previously studied photoresists.
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82.50.Hp Processes caused by visible and UV light
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
61.41.+e Polymers, elastomers, and plastics
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Thermally reflowed ZEP 520A for gate length reduction and profile rounding in T-gate fabrication

Brian P. Downey, David J. Meyer, Robert Bass, D. Scott Katzer, and Steven C. Binari

J. Vac. Sci. Technol. B 30, 051603 (2012); http://dx.doi.org/10.1116/1.4740502 (5 pages)

Online Publication Date: 1 August 2012

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The characteristics of thermally reflowed ZEP 520A-7 (ZEP), a resist commonly used in electron beam lithography, are presented for use as a gate stem resist layer in T-gate process development. As-developed ZEP lines possess a resist sidewall profile that displays varying amounts of undercut, which are determined by the conditions used to expose the line. The authors find that after thermal reflow, the top of the ZEP profile becomes substantially rounded in shape, mitigating “metal cathedraling” problems, a yield-affecting issue that becomes more pronounced as the gate length is reduced. In addition to profile rounding, a linewidth reduction of over 100 nm is observed, and this process has been used to produce gate lengths in the 30–40 nm range. The changes in feature size and the final profile shape depend on the as-developed sidewall angle. Additionally, the ZEP reflow process saturates after a certain amount of time, so reproducibility is not hindered by a lack of precise control in timing. As larger lines can be used to produce smaller features via reflow, the potential for faster throughput exists due to the use of a higher beam current, which would normally limit the attainable minimum feature size.
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85.40.Hp Lithography, masks and pattern transfer
81.16.Nd Micro- and nanolithography

Simulation study of cleaning induced extreme ultraviolet reflectivity loss mechanisms on mask blanks

Mihir Upadhyaya, Gregory Denbeaux, Arun John Kadaksham, Vibhu Jindal, Jenah Harris Jones, Byunghoon Lee, and Frank Goodwin

J. Vac. Sci. Technol. B 30, 051604 (2012); http://dx.doi.org/10.1116/1.4746245 (8 pages)

Online Publication Date: 14 August 2012

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It is widely recognized in the semiconductor industry that getting to defect-free extreme ultraviolet (EUV) mask blanks is critical in achieving high volume chip manufacturing yield beyond the 22 nm half-pitch node. Finished mask blanks are normally subjected to a cleaning process to get rid of the loosely adhered particles on the top. It is important that this cleaning process does not degrade the properties of the multilayer blank or introduce additional particles or pits during the process. However, standard cleaning processes used to clean multilayer blanks can result in EUV reflectivity loss, loss of uniformity in reflectivity, increased roughness, and add pits and particles on mask blanks. The standard cleaning process consists of multiple steps, each of which may cause the oxidation of the ruthenium capping layer, as well as the underlying bilayers, etching of the multilayer stack, and increased roughness of the bilayers, thus leading to a loss in EUV reflectivity. It is a challenging task to experimentally correlate the processing steps to the resulting damage and to quantify the reflectivity loss. Further, due to the high cost of materials we have not been able to perform extensive experiments to determine the root cause of problems. In this work, we have combined mask blank cleaning using standard processes, x-ray photoelectron spectroscopy, transmission electron microscope cross section, and atomic force microscope studies with simulations to quantify the impact of the multilayer oxidation, etching, and roughness on the EUV reflectivity loss and mask blank degradation.
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81.65.Cf Surface cleaning, etching, patterning
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.20.Bh Theory, models, and numerical simulation

Mitigation of extreme ultraviolet mask defects by pattern shifting: Method and statistics

Alfred Wagner, Martin Burkhardt, Alexander B. Clay, and James P. Levin

J. Vac. Sci. Technol. B 30, 051605 (2012); http://dx.doi.org/10.1116/1.4751280 (10 pages)

Online Publication Date: 7 September 2012

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Currently the mask blanks used in extreme ultraviolet lithography cannot be fabricated free of defects. A rapid method of determining the optimum placement of mask patterns on the blank to avoid these defects is described. Using this method, the probability of fabricating defect-free masks, when the pattern is (1) randomly placed on the mask blank or (2) positioned optimally to avoid defects, is determined for a variety of integrated circuit designs, defect densities, and defect sizes. In addition to circular defects, oval and clusters of defects are also considered. Finally, simple analytical expressions for the probability of obtaining a defect-free mask in the case of random placement of the mask pattern is presented and compared to Monte Carlo simulations.
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85.30.De Semiconductor-device characterization, design, and modeling

Scanned-spot-array extreme ultraviolet imaging for high-volume maskless lithography

Kenneth C. Johnson

J. Vac. Sci. Technol. B 30, 051606 (2012); http://dx.doi.org/10.1116/1.4752112 (12 pages)

Online Publication Date: 11 September 2012

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High-volume maskless lithography systems typically operate by raster-scanning a large array of focused radiation spots across an exposure surface while the spot intensities are modulated. This paper outlines a design concept and optical simulation results for a spot-scanning extreme ultraviolet (EUV) system using a 10×-reduction, flat-image Schwarzschild projection system consisting of only two mirrors. The spots are generated in the system's object space by means of blazed, multilevel zone-plate microlenses configured as Schupmann achromatic doublets, which are highly efficient and are designed to nullify geometric aberration in the projection system. Coherent proximity effects are eliminated by partitioning the exposure radiation into discrete, diffraction-limited image spots, which have convergence cones of numerical aperture 0.3 at the 13.5-nm operating wavelength. The image spot separation is 2.5 μm, and the spot array covers a 10-mm square image field, sufficient to achieve printing throughput of order 30 (300-mm) wafers per hour. For simplicity, the spot intensities are all controlled by a single source modulator, allowing printing of 2.5 -μm periodic patterns without a spatial light modulator. The microlens manufacturing technology would be similar to that used for EUV mirrors and phase-shift masks, but with dramatically less stringent tolerance requirements.
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85.40.Hp Lithography, masks and pattern transfer
81.16.Nd Micro- and nanolithography
42.79.Bh Lenses, prisms and mirrors
42.15.Fr Aberrations
42.79.Hp Optical processors, correlators, and modulators
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Influence of film thickness and deposition rate on surface quality of polyparylene coatings

Florian Schamberger, Astrid Ziegler, and Gerhard Franz

J. Vac. Sci. Technol. B 30, 051801 (2012); http://dx.doi.org/10.1116/1.4740049 (6 pages)

Online Publication Date: 8 August 2012

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Polyparylene is a polymer with outstanding properties when compared to other synthetics. Therefore, chemical vapor deposition of polyparylene is making inroads in several technical fields. Even applications demanding tight requirements on coating quality, like gate dielectrics for the semiconductor industry and semipermeable layers for drug eluting implants in medical science, are coming within its purview. In this paper, the impact of film thickness and deposition rate of the polyparylene CVD process on surface parameters like roughness and pinhole density is investigated. Surface roughness and pinhole density were monitored by atomic force microscopy (AFM) measurement. Furthermore, the influence of the pinhole density on tunneling current and breakdown voltage of the deposited layers is also investigated. The results obtained are used to establish an easy method to estimate the pinhole density by electrical measurement rather than the AFM method, which is not only time consuming but also limited to small flat areas. To deposit very thin layers below a thickness of 1 μm, conventional coating techniques for polyparylene have been replaced by a completely new method that features a steep slew rate for the deposition rate. The new coating method is superior to the well-known Gorham method with respect to the temperature dependent deposition rate, which makes the coating of very thin layers impossible.
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64.60.Q- Nucleation
64.70.Hz Solid-vapor transitions
68.37.Ps Atomic force microscopy (AFM)

First-principles calculations and XPS measurements of gold segregation at the Cu3Au(111) surface

Marcos Dionízio Moreira, Giselle N. Fontes, Horst Niehus, Carlos A. Achete, and Rodrigo B. Capaz

J. Vac. Sci. Technol. B 30, 051802 (2012); http://dx.doi.org/10.1116/1.4745891 (5 pages) | Cited 1 time

Online Publication Date: 10 August 2012

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A combination of first-principles calculations based on density-functional theory, pseudopotentials, and x-ray photoelectron spectroscopy (XPS) measurements is used in order to study Au segregation in Cu3Au(111) surfaces. Our theoretical results suggest Au compositions from 50% to 75% in the topmost layer, depending on the chemical potentials of the atomic species. This strong Au segregation is restricted to the topmost surface plane and it is supported by the XPS measurements in a semiquantitative manner.
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68.35.Md Surface thermodynamics, surface energies

Void detection in copper interconnects using energy dispersive x-ray spectroscopy

Menelaos Tsigkourakos, Wilfried Vandervorst, Thomas Hantschel, Alexis Franquet, Thierry Conard, and Laureen Carbonell

J. Vac. Sci. Technol. B 30, 051803 (2012); http://dx.doi.org/10.1116/1.4742855 (5 pages)

Online Publication Date: 24 August 2012

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The formation of small voids in narrow sub-50 nm Cu interconnect lines during their fabrication is a major challenge for the development of integrated circuits. Focused ion beam sectioning and scanning electron microscopy imaging are being used for void detection during process development, but as they are destructive, they are less suited for in-line metrology. Therefore, the authors developed a nondestructive method based on energy dispersive x-ray spectroscopy where voids can be detected as changes in x-ray intensity due to the direct proportionality of the generated characteristic x-ray intensity with the amount of copper atoms contained in the interaction volume. The procedure allows the detection of small voids down to 15 nm in a fast and nondestructive way. This is possible for thin and chemically mechanically polished Cu interconnects where intensity variations due to thickness variations can be ignored.
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61.72.Qq Microscopic defects (voids, inclusions, etc.)
81.16.-c Methods of micro- and nanofabrication and processing
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
85.40.Ls Metallization, contacts, interconnects; device isolation

ac heating–dc detecting method for Seebeck coefficient measurement of the thermoelectric micro/nano devices

Tingting Miao, Weigang Ma, and Xing Zhang

J. Vac. Sci. Technol. B 30, 051804 (2012); http://dx.doi.org/10.1116/1.4750497 (5 pages)

Online Publication Date: 5 September 2012

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A novel ac heating–dc detecting method is developed to measure the Seebeck coefficient of thermoelectric micro/nano devices. The suspended thermoelectric device in vacuum is heated by an ac current to generate a temperature difference composed of static and harmonic components and corresponding dc and harmonic thermoelectric voltage. The Seebeck coefficient can be extracted from the ratio of the dc thermoelectric voltage and the static temperature difference. Furthermore, it has been deduced that the dc thermoelectric voltage is proportional to the square of the heating current and the Seebeck coefficient can be directly extracted from the corresponding slope. This approach has been verified by numerical simulation on a 22.0 nm thick Au-Pt heterojunction and experiment applied on a 25.4 μm thick Chromega–Alomega thermocouple, and the measured Seebeck coefficient corresponds well with the nominal value.
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07.10.Cm Micromechanical devices and systems

Substrate temperature and electron fluence effects on metallic films created by electron beam induced deposition

Samantha G. Rosenberg, Kees Landheer, Cornelis W. Hagen, and D. Howard Fairbrother

J. Vac. Sci. Technol. B 30, 051805 (2012); http://dx.doi.org/10.1116/1.4751281 (10 pages) | Cited 1 time

Online Publication Date: 12 September 2012

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Using three different precursors [MeCpPtMe3, Pt(PF3)4, and W(CO)6], an ultra-high vacuum surface science approach has been used to identify and rationalize the effects of substrate temperature and electron fluence on the chemical composition and bonding in films created by electron beam induced deposition (EBID). X-ray photoelectron spectroscopy data indicate that the influence of these two processing variables on film properties is determined by the decomposition mechanism of the precursor. For precursors such as MeCpPtMe3 that decompose during EBID without forming a stable intermediate, the film's chemical composition is independent of substrate temperature or electron fluence. In contrast, for Pt(PF3)4 and W(CO)6, the initial electron stimulated deposition event in EBID creates surface bound intermediates Pt(PF3)3 and partially decarbonylated Wx(CO)y species, respectively. These intermediates can react subsequently by either thermal or electron stimulated processes. Consequently, the chemical composition of EBID films created from either Pt(PF3)4 or W(CO)6 is influenced by both the substrate temperature and the electron fluence. Higher substrate temperatures promote the ejection of intact PF3 and CO ligands from Pt(PF3)3 and Wx(CO)y intermediates, respectively, improving the film's metal content. However, reactions of Pt(PF3)3 and Wx(CO)y intermediates with electrons involve ligand decomposition, increasing the irreversibly bound phosphorous content in films created from Pt(PF3)4 and the degree of tungsten oxidation in films created from W(CO)6. Independent of temperature effects on chemical composition, elevated substrate temperatures (>25 °C) increased the degree of metallic character within EBID deposits created from MeCpPtMe3 and Pt(PF3)4.
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68.55.-a Thin film structure and morphology
82.80.Dx Analytical methods involving electronic spectroscopy
68.43.Rs Electron stimulated desorption
79.60.Bm Clean metal, semiconductor, and insulator surfaces
81.65.-b Surface treatments

Laser polishing of niobium for application to superconducting radio frequency cavities

Senthilraja Singaravelu, J. Michael Klopf, Chen Xu, Geoffrey Krafft, and Michael J. Kelley

J. Vac. Sci. Technol. B 30, 051806 (2012); http://dx.doi.org/10.1116/1.4752216 (7 pages)

Online Publication Date: 14 September 2012

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Superconducting radio frequency niobium cavities are at the heart of an increasing number of particle accelerators. Their performance is dominated by a several nanometer thick layer at the interior surface. Maximizing the smoothness of this surface is critical, and aggressive chemical treatments are now employed to this end. The authors describe laser-induced surface melting as an alternative “greener” approach. Selection of laser parameters guided by modeling achieved melting that reduced the surface roughness from the fabrication process. The resulting topography was examined by scanning electron microscope and atomic force microscope (AFM). Plots of power spectral density computed from the AFM data give further insight into the effect of laser melting on the topography of the mechanically polished (only) niobium.
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81.65.Ps Polishing, grinding, surface finishing
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Ps Atomic force microscopy (AFM)
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
68.35.bt Other materials
64.70.dj Melting of specific substances

Band alignment of zinc oxide as a channel layer in a gate stack structure grown by plasma enhanced atomic layer deposition

Chiyu Zhu, David J. Smith, and Robert J. Nemanich

J. Vac. Sci. Technol. B 30, 051807 (2012); http://dx.doi.org/10.1116/1.4752089 (8 pages)

Online Publication Date: 18 September 2012

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A gate stack structure with a thin ZnO layer between an oxidized Si(100) surface and an alloyed hafnium and lanthanum oxide (HfO2-La2O3) layer was prepared by plasma enhanced atomic layer deposition at ∼175 °C. High resolution electron microscopy indicated an amorphous structure of the deposited layers. The electronic properties were characterized with x-ray and ultraviolet photoemission spectroscopy. A significant amount of excess oxygen was observed in the as-deposited ZnO and (HfO2-La2O3) layers. A helium plasma postdeposition treatment can partially remove the excess oxygen in both layers. The band alignment of this structure was established for an n-type Si substrate. A valence band offset of 1.5 ± 0.1 eV was measured between a thin ZnO layer and a SiO2 layer. The valence band offset between HfO2-La2O3 (11% HfO2 and 89% La2O3) and ZnO was almost negligible. The band relationship developed from these results demonstrates confinement of electrons in the ZnO film as a channel layer for thin film transistors.
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68.55.ag Semiconductors
81.05.Dz II-VI semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
85.40.Sz Deposition technology
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
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Characterization of enhancement-mode n-channel sulfur-treated InP MOSFET with liquid phase deposition-TiO2 gate oxide

Chih-Feng Yen and Ming-Kwei Lee

J. Vac. Sci. Technol. B 30, 052201 (2012); http://dx.doi.org/10.1116/1.4739057 (3 pages)

Online Publication Date: 25 July 2012

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TiO2 films were prepared with aqueous solutions of hexafluorotitanic acid and boric acid on an InP substrate with an ammonium sulfide treatment. The films showed greatly enhanced electrical characteristics compared to those prepared without the ammonium sulfide treatment. The leakage currents of the resulting TiO2/InP capacitor reached 2.1 × 10−7 and 7.4 × 10−7 A/cm2 at ±0.5 MV/cm. The dielectric constant and the effective oxide charges were 43 and −2 × 1011 C/cm2, respectively. The interface state density was 3.6 × 1011 cm−2 eV−1. The fabricated enhancement-mode n-channel InP MOSFET exhibited good electrical characteristics with a maximum gm of 43 mS/mm and electron mobility of 348 cm2/Vs.
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85.30.Tv Field effect devices
84.32.Tt Capacitors

Formation of large-area GaN nanostructures with controlled geometry and morphology using top-down fabrication scheme

Dipak Paramanik, Abhishek Motayed, Geetha S. Aluri, Jong-Yoon Ha, Sergiy Krylyuk, Albert V. Davydov, Matthew King, Sean McLaughlin, Shalini Gupta, and Harlan Cramer

J. Vac. Sci. Technol. B 30, 052202 (2012); http://dx.doi.org/10.1116/1.4739424 (13 pages)

Online Publication Date: 26 July 2012

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This paper details the fabrication of GaN nanoscale structures using deep ultraviolet lithography and inductively coupled plasma (ICP) etching techniques. The authors controlled the geometry (dimensions and shape) and surface morphology of such nanoscale structures through selection of etching parameters. The authors compared seven different chlorine-based etch chemistries: Cl2, Ar, Cl2/N2, Cl2/Ar, Cl2/N2/Ar, Cl2/H2/Ar, and Cl2/He/Ar. The authors found that nitrogen plays a significant role in fabricating high quality etched GaN nanostructures. This paper presents the effects of varying the etch parameters, including gas chemistry, gas flow rate, ICP power, rf power, chamber pressure, and substrate temperature, on the etch characteristics, including etch rate, sidewall angle, anisotropy, mask erosion, and surface roughness. Dominant etch mechanisms in relation to the observed characteristics of the etched features are discussed. Utilizing such methods, the authors demonstrated the fabrication of nanoscale structures with designed shapes and dimensions over large area. Nanocolumns with diameter of 120 nm and height of 1.6 μm with sidewall angle of 86° (90° represent a vertical sidewall) were fabricated. Nanocones with tip diameter of 30 nm and height of 1.6 μm with sidewall angle of 70° were demonstrated. Such structures could potentially be used in light-emitting diodes, laser diodes, photodetectors, vertical transistors, field emitters, and photovoltaic devices. This study indicates the feasibility of top-down methods in the fabrication of next-generation nitride-based nanoscale devices, with large-area uniformity and scalability.
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81.16.Nd Micro- and nanolithography
81.65.Cf Surface cleaning, etching, patterning
68.35.bg Semiconductors
52.77.Bn Etching and cleaning
61.46.-w Structure of nanoscale materials

Cu film thermal stability on plasma cleaned polycrystalline Ru

Xin Liu, Chiyu Zhu, Brianna S. Eller, Tianyin Sun, Christopher J. Jezewski, Sean W. King, and Robert J. Nemanich

J. Vac. Sci. Technol. B 30, 052203 (2012); http://dx.doi.org/10.1116/1.4742906 (7 pages) | Cited 2 times

Online Publication Date: 14 August 2012

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The first part of this study examined oxide stability and cleaning of Ru surfaces. The surface reactions during H2 plasma exposure of Ru polycrystalline films were studied using x-ray photoelectron spectroscopy (XPS). The ∼2 monolayer native Ru oxide was reduced after H-plasma processing. However, absorbed oxygen, presumably in the grain boundaries, remains after processing. A vacuum thermal anneal at 150 °C substantially removes both surface oxide and absorbed oxygen which is attributed to a reduction by carbon contamination. The second part of the study examined the thermal stability of Cu on a Ru layer. The thermal stability or islanding of the Cu film on the Ru substrate was characterized by in situ XPS. After plasma cleaning of the Ru adhesion layer, the deposited Cu exhibited full coverage. In contrast, for Cu deposition on the Ru native oxide substrate, Cu islanding was detected and was described in terms of grain boundary grooving and surface and interface energies. The oxygen in the grain boundary has a negligible contribution to the surface energy and does not contribute to Cu islanding.
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81.65.Cf Surface cleaning, etching, patterning
65.40.gp Surface energy
61.72.Mm Grain and twin boundaries
68.35.Fx Diffusion; interface formation
68.35.Np Adhesion
68.60.Dv Thermal stability; thermal effects
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