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Mar 2012

Volume 30, Issue 2 (partial)

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Novel light sputter ion pump with neodymium iron boron magnets and the low outgassing body

Taekyun Ha, C. D. Park, and Sukmin Chung

J. Vac. Sci. Technol. A 30, 020601 (2012); doi:10.1116/1.3677387 (4 pages)

Online Publication Date: 17 January 2012

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A light sputter ion pump for ultra high vacuum was fabricated using 5-mm-thick neodymium iron boron (NdFeB) magnets. The weight of the pump was reduced by 30%, compared with conventional sputter ion pumps using ferrite magnets. For the sake of uniformity of magnetic field, three pieces of magnet were combined into a 100 mm × 150 mm magnet block for the 30 ℓ/s pumping element. The authors coped with the low working temperature (below 150 °C) of the NdFeB magnets by forming a pure and dense chromium oxide film on the inner surface of the stainless steel pump body. With this vacuum thermal oxidation, the outgassing rate of water and hydrogen was considerably reduced so that a high temperature bakeout was unnecessary. The ultimate pressure achieved by the developed pump was much lower than 10−10 mbar even with a low bakeout temperature at 100 °C.
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07.30.Cy Vacuum pumps
07.55.Db Generation of magnetic fields; magnets
07.30.Bx Degasification, residual gas

SiCl4/Cl2 plasmas: A new chemistry to etch high-k materials selectively to Si-based materials

Paul Bodart, Gilles Cunge, Olivier Joubert, and Thorsten Lill

J. Vac. Sci. Technol. A 30, 020602 (2012); doi:10.1116/1.3679551 (6 pages)

Online Publication Date: 1 February 2012

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Plasma etching of ultrathin layers of high-k materials is one critical step in the fabrication of gate transistors. The main challenge in this process is to achieve an infinite etching selectively between the high-k and the Si (or SiO2 covered silicon) substrate to prevent damaging the source and drain regions of the transistor. State of the art high-k etching plasmas use BCl3 chemistries, sometime at high wafer temperature. However, the process window in which an infinite high-k/Si selectivity can be achieved is very narrow and several issues remain associated with these processes. In this work, we introduced a new high-k plasma etching chemistry: SiCl4/Cl2. It can be used to etch Hf-based and Al-based high-k materials with an infinite selectivity towards Si, SiO2 and SiON, and with a much wider process window than BCl3 plasmas. XPS analyses indicate that the selectivity mechanism relies on the formation of a SiClx deposit selectively on Si-containing materials, which prevent them from being etched. By contrast SiClx radicals and ions are directly involved in the etching of the metallic oxide layer by forming volatile products (most probably SiOClx and HfClx). This new chemistry may replace valuably BCl3-based plasmas for future high-k etching processes and it may also be interesting for other applications.
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81.65.Cf Surface cleaning, etching, patterning
77.55.D- High-permittivity gate dielectric films
78.66.Db Elemental semiconductors and insulators
79.60.-i Photoemission and photoelectron spectra

Porous antimony-doped tin oxide cathodes formed by supercritical CO2 treatment at low temperature for silver electro-deposition

W. C. Tien, A. K. Chu, H. Y. Wen, M. Y. Chang, and W. Y. Huang

J. Vac. Sci. Technol. A 30, 020603 (2012); doi:10.1116/1.3682990 (5 pages)

Online Publication Date: 8 February 2012

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Porous antimony-doped tin oxide (ATO) cathodes for silver electrodeposition devices are proposed. The porous structure of the cathodes is obtained by applying supercritical CO2 (SCCO2) treatment at 60 °C on spin-coated ATO nanoparticles. The morphological, structural, and electrical properties of the ATO cathodes with the SCCO2 treatment are investigated. The 0.5 μm thick ATO cathode grown on ITO glass substrates is transparent. However, black state of the device is observed when silver molecules are anchored onto the surface of the ATO cathode during reduction. The average transmission contrast ratio of 12 is obtained in visible spectrum at a driving voltage of 1.5 V and a saturation current density of 5.8 mA/cm2. In addition, the electrochromic switching time is 4.5 s for a 0.5 × 0.5 cm device with 65 μm cell gap and 0.08 M electrolyte concentration, and its transmission contrast ratio is better than 9.0 at λ = 633 nm.
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81.07.Bc Nanocrystalline materials
81.15.Pq Electrodeposition, electroplating
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.40.Kc Metals, semimetals, and alloys
82.45.Gj Electrolytes
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Microstructure analysis of plasma enhanced atomic layer deposition-grown mixed-phase RuTaN barrier for seedless copper electrodeposition

Tonmoy Chakraborty and Eric T. Eisenbraun

J. Vac. Sci. Technol. A 30, 020604 (2012); doi:10.1116/1.3684597 (5 pages)

Online Publication Date: 10 February 2012

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Plasma enhanced atomic layer deposition (PEALD)-grown mixed phase RuTaN films has been studied as a direct plate material for Cu electroplating within interconnects. It was seen that these liners behaved as excellent Cu diffusion barrier and could be scaled down to sub-5 nm thicknesses. High resolution TEM based structural analysis of these films showed islands of Ru surrounded by amorphous region. The selected area electron diffraction pattern corresponds to the hcp phase of Ru with (101) as the primary crystallographic orientation. Scanning tunneling microscopy and atomic force microscopy suggested the Volmer–Weber growth mechanism of these liners. A series of electroplating experiments with various plating current density and time showed that a uniform bottom-up filling could be achieved in trenches with RuTaN as direct plate liners. Trenches with aspect-ratio as high as 10 could be filled uniformly. The conformality of the PEALD RuTaN process within the trenches was also found to be very promising with step-coverage over 85%.
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68.55.A- Nucleation and growth
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.-y Defects and impurities in crystals; microstructure
81.15.Pq Electrodeposition, electroplating
68.35.Fx Diffusion; interface formation
68.37.Ps Atomic force microscopy (AFM)
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Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

Jason B. Baxter

J. Vac. Sci. Technol. A 30, 021201 (2012); doi:10.1116/1.3676433 (19 pages)

Online Publication Date: 6 February 2012

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Dye sensitized solar cells (DSSCs) have received a tremendous amount of attention since the first report of a 7% efficient cell in 1991. Confirmed record efficiencies are now 11.2% for small cells and 9.9% for submodules, and low-cost production methods are enabling manufacturing of DSSC products for a variety of markets. This review describes the present status of DSSC devices and manufacturing as well as research challenges that must be addressed to continue the rapid commercialization of DSSC technology. These challenges fall into the categories of improving efficiency, stability, and manufacturability. Efficiency improvements will hinge on the development of new combinations of dyes, redox couples, and photoanodes. Best-case lifetimes are determined by the kinetics of various molecular-level processes, and realization of these lifetimes will require improved encapsulation of cells and modules. Low-cost and sustainable manufacturing of DSSC modules depends on use of high-throughput roll-to-roll processing and inexpensive, abundant materials. Prospects for simultaneous improvement of efficiency, stability, and manufacturing are discussed.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells
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Populations of metastable and resonant argon atoms in radio frequency magnetron plasmas used for deposition of indium-zinc-oxide films

L. Maaloul, S. Morel, and L. Stafford

J. Vac. Sci. Technol. A 30, 021301 (2012); doi:10.1116/1.3674162 (8 pages)

Online Publication Date: 6 January 2012

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This work reports optical absorption spectroscopy measurements of the number density of Ar atoms in resonant (3P1, 1P1) and metastable (3P2, 3P0) states in rf magnetron sputtering plasmas used for the deposition of ZnO-based thin films. While the density of Ar 3P2 and 3P0 was fairly independent of pressure in the range of experimental conditions investigated, the density of Ar 3P1 and 1P1 first sharply increased with pressure and then reached a plateau at values close to those of the 3P2 and 3P0 levels at pressures above about 50 mTorr. At such pressures, ultraviolet radiation from resonant states becomes trapped such that these levels behave as metastable states. For a self-bias voltage of −115 V and pressures in the 5–100 mTorr range, similar number densities of Ar resonant and metastable atoms were obtained for Zn, ZnO, and In2O3 targets, suggesting that, over the range of experimental conditions investigated, collisions between these excited species and sputtered Zn, In, and O atoms played only a minor role on the discharge kinetics. The metastable-to-ground state number density ratios were also fitted to the predictions of a global model using the average electron temperature, Te, as the only adjustable parameter. For all targets examined, the values of Te deduced from this method were in excellent agreement with those obtained from Langmuir probe measurements.
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81.15.Cd Deposition by sputtering
81.15.Jj Ion and electron beam-assisted deposition; ion plating
52.77.Dq Plasma-based ion implantation and deposition
78.66.Hf II-VI semiconductors
78.30.Fs III-V and II-VI semiconductors
78.40.Fy Semiconductors

In situ fabrication of blue ceramic coatings on wrought Al Alloy 2024 by plasma electrolytic oxidation

Zhijiang Wang, Xueyuan Nie, Henry Hu, and Riyad O. Hussein

J. Vac. Sci. Technol. A 30, 021302 (2012); doi:10.1116/1.3675610 (7 pages)

Online Publication Date: 12 January 2012

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In situ formation of ceramic coatings on 2024 Al alloy with a blue color was successfully achieved using a plasma electrolytic oxidation process working at atmospheric pressure. This novel blue ceramic coating overcomes the shortcomings of surface treatments resulting from conventional dyeing processes by depositing organic dyes into the porous structure of anodic film, which has poor resistance to abrasion and rapid fading when exposed to sunlight. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy were employed to characterize the microstructure of the blue ceramic coating. The fabricated ceramic coating was composed of CoAl2O4, α-Al2O3, and γ-Al2O3. By controlling the working parameters, the distribution of the CoAl2O4 phase on the surface can be adjusted, and plays a key role in the appearance of the coating. Electrochemical testing, thermal cycling method, and pin-on-disk sliding wear testing were employed to evaluate corrosion, thermal cycling, and wear resistance of the ceramic coatings. The results indicate that the blue ceramic coating has a similar polarization resistance to that of conventional anodic film and can significantly enhance the corrosion resistance of aluminum alloy. There are no destructive horizontal cracks observed within the blue ceramic coating when subjected to 120 times of thermal cycling, which heats the samples up to 573 K and followed by submersion in water at room temperature for 10 min. Compared with the aluminum substrate as well as a conventional anodic film coated aluminum sample, the wear resistance of the blue ceramic coating coated sample was significantly increased while the coefficient of friction was decreased from 0.34 to 0.14.
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81.65.Kn Corrosion protection
81.65.Mq Oxidation
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
82.80.Fk Electrochemical methods
62.20.Qp Friction, tribology, and hardness
81.40.Pq Friction, lubrication, and wear

On the scaling of rf and dc self-bias voltages with pressure in electronegative capacitively coupled plasmas

Ankur Agarwal, Leonid Dorf, Shahid Rauf, and Ken Collins

J. Vac. Sci. Technol. A 30, 021303 (2012); doi:10.1116/1.3676182 (11 pages)

Online Publication Date: 12 January 2012

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Higher gas densities and lower diffusion losses at higher operating pressures typically lead to increased charged species densities (and hence flux) for a constant power deposition in capacitively coupled plasmas (CCP). As a result, one would expect that the bias radio-frequency (rf) voltage required to deposit a given power in a CCP reactor decreases with increasing operating pressure. These observations may not hold true in multiple frequency CCPs, commonly used for dielectric etching in microelectronics fabrication, due to nonlinear interactions between the rf sources. Wafer-based measurements of the rf and self-generated direct current (dc) bias voltages in a dual-frequency capacitively coupled electronegative plasma were made, which indicate that the rf and dc voltages vary nonmonotonically with pressure. These experimental results are presented in this paper and a computational plasma model is used to explain the experimental observations for varying 60 MHz and 13 MHz powers in the Ar/CF4/CHF3 plasma over a pressure range of 25 to 400 mTorr. The authors found that while the ion density increases with pressure, the increase is most dominant near the electrode with the high frequency source (60 MHz). The rf and dc bias voltages are ultimately influenced by both charged species density magnitudes and spatial profiles.
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52.25.Fi Transport properties
52.50.Dg Plasma sources
52.70.Gw Radio-frequency and microwave measurements

Abatement of CF4 and CHF3 byproducts using low-pressure plasmas generated by annular-shaped electrodes

Min Hur, Jae O. K. Lee, Young Hoon Song, and Hoon A. Yoo

J. Vac. Sci. Technol. A 30, 021304 (2012); doi:10.1116/1.3679407 (8 pages)

Online Publication Date: 1 February 2012

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Three different driving schemes are tested for a plasma reactor designed to abate the greenhouse gases emitted by the semiconductor industry. The reactor and electrodes all have a concentric annular shape, which allows them to be easily connected to pre-existing pipelines without any disturbance to the exhaust stream. The destruction and removal efficiencies are measured for CF4 by varying the O2/CF4 ratio and pressure. The influences of adding O2 and H2O to the byproducts of the CHF3 abatement process are investigated by analyzing the spectra resulting from Fourier transform infrared spectroscopy measurements. Based on the experimental results we suggest an appropriate combination of driving scheme and reactant gas species for efficient and economical abatement of a mixture of CHF3 and CF4. Then, the optimal flow rate of the reactant gas is presented. Finally, the reduction rates for global warming emissions are estimated to demonstrate the feasibility of using our device for abatement of greenhouse gases emitted by the semiconductor industry.
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89.60.-k Environmental studies
51.70.+f Optical and dielectric properties
52.77.-j Plasma applications

Temperature dependence of the infrared absorption cross-sections of neutral species commonly found in fluorocarbon plasmas

Caleb T. Nelson, Lawrence J. Overzet, and Matthew J. Goeckner

J. Vac. Sci. Technol. A 30, 021305 (2012); doi:10.1116/1.3679408 (11 pages)

Online Publication Date: 1 February 2012

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This article serves as a reference for the analysis of Fourier transform infrared spectroscopy data from processing plasmas. Until now, there has been a lack of accurate reference data for addressing the problems of species identification and density measurements in cases of increasing gas temperatures. Our results show that, while the integrated absorption cross-sections do not change significantly as temperature increases, the temperature of the absorbing species can be estimated from the rotational band maximum in most cases. Integrated absorption cross-sections for c-C3F6, C4F8, C3F8, C2F6, C2F4, and CF4 are presented for all fundamental bands in the 650 cm−1 to 2000 cm−1 region. In addition, the binary combination bands up to 4000 cm−1 are presented for all species. The temperature of each species has been varied to correspond to neutral temperatures commonly found in processing plasmas.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
82.80.Nj Fourier transform mass spectrometry
33.20.Ea Infrared spectra
52.25.-b Plasma properties

Surprising importance of photo-assisted etching of silicon in chlorine-containing plasmas

Hyungjoo Shin, Weiye Zhu, Vincent M. Donnelly, and Demetre J. Economou

J. Vac. Sci. Technol. A 30, 021306 (2012); doi:10.1116/1.3681285 (10 pages)

Online Publication Date: 6 February 2012

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The authors report a new, important phenomenon: photo-assisted etching of p-type Si in chlorine-containing plasmas. This mechanism was discovered in mostly Ar plasmas with a few percent added Cl2, but was found to be even more important in pure Cl2 plasmas. Nearly monoenergetic ion energy distributions (IEDs) were obtained by applying a synchronous dc bias on a “boundary electrode” during the afterglow of a pulsed, inductively coupled, Faraday-shielded plasma. Such precisely controlled IEDs allowed the study of silicon etching as a function of ion energy, at near-threshold energies. Etching rates increased with the square root of the ion energy above the observed threshold of 16 eV, in agreement with published data. Surprisingly, a substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments ruled out chemical etching by Cl atoms, etching assisted by Ar metastables, and etching mediated by holes and/or low energy electrons generated by Auger neutralization of low-energy ions, leaving photo-assisted etching as the only likely explanation. Experiments were carried out with light and ions from the plasma either reaching the surface or being blocked, showing conclusively that the “sub-threshold” etching was due to photons, predominately at wavelengths < 1700 Å. The photo-assisted etching rate was equal to the ion-assisted etching rate at 36 eV, causing substantial complications for processes that require low ion energies to achieve high selectivity and low damage, such as atomic layer etching. Under these conditions, photo-assisted etching likely plays an important role in profile evolution of features etched in Si with chlorine-containing plasmas, contributing to the commonly observed sloped sidewalls and microtrenches.
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81.65.Cf Surface cleaning, etching, patterning
68.47.Fg Semiconductor surfaces
78.20.Ls Magneto-optical effects
79.20.Fv Electron impact: Auger emission
81.05.Cy Elemental semiconductors
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Blueshift in sulfur treated GaAsP/AlGaAs near surface quantum well

Suparna Pal, S. D. Singh, S. Porwal, S. W. D’Souza, S. R. Barman, and S. M. Oak

J. Vac. Sci. Technol. A 30, 021401 (2012); doi:10.1116/1.3679394 (5 pages)

Online Publication Date: 1 February 2012

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Large blueshift was observed in a near-surface GaAs0.86P0.14/Al0.7Ga0.3As quantum well upon treatment with Na2S·xH2O solution. Very slow etching with simultaneous surface passivation of the quantum well was obtained using this chemical treatment. Photoreflectance (PR) spectra exhibit maximum blueshift of 28 meV after treating the quantum well surface with Na2S·xH2O solution for 30 min (top layer thickness reduced to 10 Å). The blueshift is attributed to an increase in the confinement and/or an image charge effect due to the penetration of the wave function into vacuum. The blueshift is accompanied by a significant reduction in the broadening parameter of the observed e1-lh1 transition in PR spectra indicating effective passivation along with an increase in the confinement.
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78.67.De Quantum wells
81.05.Ea III-V semiconductors
81.07.St Quantum wells
81.65.Cf Surface cleaning, etching, patterning
81.65.Rv Passivation
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Tribological behavior of Ti-Al-Si-C-N hard coatings deposited by hybrid arc-enhanced magnetron sputtering

Guizhi Wu, Shengli Ma, Kewei Xu, and Paul K Chu

J. Vac. Sci. Technol. A 30, 021501 (2012); doi:10.1116/1.3676186 (7 pages)

Online Publication Date: 11 January 2012

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Ti-Al-Si-C-N hard coatings are deposited on high speed steel by hybrid arc-enhanced magnetron sputtering, and the hardness, adhesion, and tribological behavior are studied. On account of the nanocomposite structure, the coatings possess hardness of more than 30 GPa. Failure of the coating during the scratch test is due to the buckling and wedge spallation failure mechanism. Compared to Ti-Al-Si-N, the presence of C in the Ti-Al-Si-C-N coatings leads to reduced friction coefficient and wear rate, indicating effective lubrication rendered by amorphous C. According to the wear tracks examined by scanning electron microscopy, the wear mechanism can be explained by plowing abrasion.
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81.40.Pq Friction, lubrication, and wear
62.50.-p High-pressure effects in solids and liquids
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
81.15.Cd Deposition by sputtering
81.40.Lm Deformation, plasticity, and creep
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure

High rate roll to roll atomic layer deposition, and its application to moisture barriers on polymer films

Eric Dickey and William A. Barrow

J. Vac. Sci. Technol. A 30, 021502 (2012); doi:10.1116/1.3678486 (5 pages)

Online Publication Date: 24 January 2012

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Atomic layer deposition has been shown to provide high quality single layer moisture barrier films on polymer substrates, but conventional pulse-based processes are too slow to be commercially feasible. One way to overcome this speed limitation is to avoid the need to pulse and purge precursors by moving the substrate between zones containing the precursors, passing through intermediate purge zones. Recently, several groups have reported various approaches to accomplishing this, including the approach discussed here in which the flexible web is passed between precursor zones in a serpentine pattern. Al2O3 and TiO2 barrier films 12 to 20 nm thick with water vapor transmission rates in the range of 10−4 g/m2/day have been demonstrated for web speeds in excess of 1 m/s on 100 mm wide polyethylene terephthalate web. Scale-up of this process to 300 mm wide web in a system capable of depositing 10–20 nm of film in a single pass is currently under way. This scale-up effort and the potential for very high volume, low cost moisture barrier production utilizing this technique are discussed.
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68.55.A- Nucleation and growth
68.55.am Polymers and organics
61.41.+e Polymers, elastomers, and plastics
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Twin structures of epitaxial SnO2 films grown on a-cut sapphire by metalorganic chemical vapor deposition

Zhen Zhu, Jin Ma, Caina Luan, Wei Mi, and Yu Lv

J. Vac. Sci. Technol. A 30, 021503 (2012); doi:10.1116/1.3683042 (4 pages)

Online Publication Date: 8 February 2012

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SnO2 films have been grown on a-cut (11math0) sapphire substrates by metalorganic chemical vapor deposition. X-ray diffraction and transmission electron microscopy were employed to characterize the epitaxial relationship and film structure. The films were (101) oriented with pure rutile structure. The in-plane relationship was determined to be SnO2 [010]//Al2O3 [0001] and SnO2 [10math]//Al2O3 [1math00]. There are three kinds of {101} twins in the SnO2 film. These twins caused high density of planar defects in the film and slight misorientation of the growth plane. The film/substrate interface was flat, while the film surface was rough with steps and inclinations.
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68.55.ag Semiconductors
68.35.bg Semiconductors
71.55.Ht Other nonmetals
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
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Fabrication and physical properties of thin TixOy membranes from single crystal TiO2

Maryam Abazari, Jai S. Sim, B. Viswanath, and Shriram Ramanathan

J. Vac. Sci. Technol. A 30, 021601 (2012); doi:10.1116/1.3676197 (6 pages)

Online Publication Date: 11 January 2012

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Utilizing focused ion beam (FIB) milling, we have fabricated thin membranes (also referred to as nanowalls) of TixOy of 100–300 nm thickness starting from rutile titania bulk single crystals. Low probe currents (∼80 pA) in conjunction with XeF2-assisted milling enables minimal contamination during the fabrication process. Transmission electron microscopy studies indicated polycrystallinity and presence of nano-twins in the FIB-milled nanowalls. Formation of such nanoscale twinned structures may be related to high degree of nonstoichiometry, i.e., reduction as a result of milling in TiO2 that is consistent with observations in other oxides in the literature. Compositional analysis in the transmission electron microscope also showed reduced content of oxygen, confirming nonstoichiometry after milling. The authors have studied the temperature dependence of the electrical conductivity behavior in such ultra-thin walls in the temperature range of 300 K < T <520 K as a first effort. Temperature dependence of the electrical resistivity of the nanowall showed semiconducting behavior with an activation energy different from that corresponding to TiO2 single crystal and was attributed to formation of reduced TixOy phases after FIB processing. The results suggest a pathway to fabricate oxide structures for probing mesoscopic conduction phenomena.
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68.55.-a Thin film structure and morphology
68.60.-p Physical properties of thin films, nonelectronic
68.37.Lp Transmission electron microscopy (TEM)
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