bluuebunny/crossref_metadata_2025 huggingface dataset:
Issues and Challenges of Cleaning for Nano-Scale NAND Flash Memory Manufacturing
Authors: Ryu Choon-Kun , Song Young-Tak , Jin Gyu-An , Park Sung-Ki
Published: September 2007

In comparison to other IC devices, the unique device structure of a flash memory is a floating gate as a memory cell unit. One of critical steps of the flash memory process integration is the wet etch process of effective field oxide which affects the coupling ratio and the threshold voltage of a cell transistor. Since the gate of a flash memory cell is composed of multi-layers, the gate sidewall damage should be minimized with non-selective and minimal loss during the clean after the gate etch. As contacts and trenches shrink to nano-scale, highly selective and damage-free cleaning should be developed to minimize sidewall loss and prevent pattern collapse. Since complete global and local planarization is required to satisfy stringent requirements of nano-lithography, the number of CMP processes increase gradually. During the cleaning step after the CMP, slurry particles and residues must be removed completely.
Maintaining a Stable Etch Selectivity between Silicon Nitride and Silicon Dioxide in a Hot Phosphoric Acid Bath
Authors: Liu Lewis , Kashkoush Ismail , Chen Gim , Murphy Casey
Published: September 2007

Phosphoric acid (H3PO4) - water (H2O) mixtures at high temperature have been used for many years to etch silicon nitride (Si3N4) selective to silicon dioxide (SiO2) layers. The production requires removing the Si3N4 completely while keeping SiO2 loss to a minimum. The challenge for batch wafer cleaning is how to maintain a high etching selectivity of Si3N4 to SiO2 for a longer bath life. Experiments showed that increasing water concentration in the bath results in higher selectivity: more Si3N4 etched and less SiO2 etched. With the installations of a water concentration monitor and a water spiking apparatus, the bath is able to control the water concentration. The partial replacement of the chemical bath after each product lot reduces dissolved silicon concentration. The addition of H2SO4 saves initial conditioning time. With all of the efforts, the bath starts at a high selectivity and maintain this selectivity for an extended bath life.
Removal of Backside Particles by a Single Wafer Megasonic System
Authors: Sohn Hong-Seong , Hong Uk Sun , Park Chan-Gun , Lee Eui-Kang , Lee Hyuk-Jun , Brause Eric , Park Jin-Goo
Published: September 2007

Single wafer megasonics were used to remove backside particle contamination resulting from vacuum chucks and chambers. Process evaluation was done using blanket wafer experiments with Si3N4 particles. To increase the difficulty of the particle removal test vehicle, aged contaminated wafers were used. The impact of film type, megasonic power, process chemicals and etch amount were investigated. The impact of performing a backside megasonic clean prior to the lithography process was shown
Particle Removal from Si Substrates in Organic Solvents using Megasonic Energy
Authors: Barbagini Francesca , Janssens Tom , Bearda Twan , Armini Silvia , Van Hoeymissen Jan , Mertens Paul , Fransaer Jan
Published: September 2007

In this paper, particle removal experiments from silicon wafers in organic solvent using megasonic energy are presented. Nanoparticles are first deposited on Si wafers via controlled-contamination process. The particle removal efficiency as a function of megasonic power is studied both for freshly-deposited and aged particles. Several cleaning recipes are evaluated, by changing the processing conditions and rinsing time/chemistry. When low megasonic power is used, particles are found to cluster and re-deposit on the wafer surface after being detached. This phenomenon could be explained by charging phenomena of particles and silicon surfaces in the specific solvent. A surfactant is added to prevent clustering and re-deposition, leading to a noticeably improvement in particle removal efficiency.
Quantitative Measurement of Pattern Collapse and Particle Removal Force
Authors: Kim Tae-Gon , Wostyn Kurt L. , Mertens P. , Busnaina Ahmed A. , Park Jin-Goo
Published: September 2007

The pattern collapse and the particle removal force should be quantitatively understood and measured to develop the effective cleaning process without pattern damages. The method of measuring these forces by atomic force microscope (AFM) was introduced in this paper. Pattern collapse forces of a silicon oxynitride (SiON), silicon oxide (SiO2) and photoresist (PR) line pattern were measured. The particle removal force was measured on a 100 nm PSL particle on silicon. The force for pattern collapse increased linearly as a function of pattern width. PR pattern requires a smaller force than the SiON and SiO2 pattern at the same line width. Particle removal force of aged PSL on hydrophilic Si surface was two orders smaller than the collapse force of same size of patterns.
The Metallic Contamination Related Whisker Defect During Deposition and Its Removal
Authors: Choi Geunmin
Published: September 2007

We have investigated the whisker defect generation during SiH4 -based silicon film deposition, which is mostly related to metallic impurities on the surface. The metallic impurities from various dry etching systems are heavily detected during device fabrication. The contamination level on silicon surface is around three orders of magnitude higher than that of allowance level. Furthermore, the copper impurities contaminated on the a-Si surfaces are hardly removed even by the acidic solutions with high oxidation-reduction potential values. The contaminants generated the abnormal film growth which was detected after a-Si film deposition. The defect is more than 500nm in size (denote Whisker defect). They can cause serious CMP scratching on the surface or pattern lifting at post process as well as device yield drop. When the a-Si film was deposited on a-Si film, the amount of defect observed was greater than those of other surfaces.
All-Wet Strip Approaches for Post-Etch Photoresist Layers After Low-K Patterning
Authors: Claes Martine , Le QuocToan , Kesters Els , Lux Marcel , Urionabarrenetxea Ariana , Vereecke Guy , Mertens P. , Carleer R. , Adriaensens P.
Published: September 2007

Wet-only removal of post-etch (meth-)acrylate based photoresist on porous low-k dielectrics applied in either resist or metal hard mask based patterning is investigated. Characterization of photoresist degradation by etch is applied to support the selection of wet cleaning chemistries and/or cleaning processes. FTIR and 1H-NMR analysis results indicate a common degradation mechanism for both patterning schemes with formation of double carbon bonds in a cross-linked crust that is insoluble in organic solvents. Even though a common degradation method is observed, resist cleanability in metal hard mask applications is by far more difficult due to a higher degree of resist cross-linking. Cleaning approaches by dissolution in solvents in combination with physical forces, for both patterning schemes, are discussed in detail.
Effect of Ashing, Strip and Annealing Process on the Dopant Concentration of Silicon
Authors: Lee Mong Sup , Park ImSoo , Kang DaeHyuk , Han DongGyun , Son YoonHo , Lee KunTack , Hong ChangKi , Kang ChangJin , Moon JooTae
Published: September 2007

In this study, we investigated the change of dopant concentration during ashing, strip and annealing process. Combined HF-based strip and annealing process causes the loss of dopants, phosphorous and arsenic, while the ashing process does not affect the dopant loss. This is because HF cleaning removes the silicon oxide acting as the capping layer of dopant evaporation and out-diffusion in annealing process.
Post W CMP Cleaning Without HF Cleans
Authors: Kang Young-Jae , Yang Chan-Ki , Kwon Tae-Young , Park Jin-Goo , Jo Jung-Hun , Lim Geun-Sik
Published: September 2007

In general, HF cleaning is well known to remove the slurry residue particles in W plugs. HF chemistry lifts off the particles by etching the plug during scrubbing and effectively removes particles. It is sometimes impossible to apply HF chemistry on W plug due to the degradation of electrical characteristics of a device. In this paper, a post W CMP cleaning process is proposed to remove residue particles without applying HF chemistry. After W CMP, recessed plugs are created, therefore they easily trap slurry particles during CMP process. These particles in recessed plug are not easy to remove by brush scrubbing when NH4OH chemistry is used for the cleaning because the brush surface can not reach the recessed area of plugs. Higher profiles make the brush contact much more effectively and result in similar particle removal efficiency even in NH4OH cleaning to that in HF brush scrubbing.
Achievement of Higher-k and High-Φ in Phase Controlled HfO2 Film Using Post Gate Electrode Deposition Annealing
Authors: Watanabe Yukimune , Ota Hiroyuki , Migita Shinji , Kamimuta Yuuichi , Iwamoto Kunihiko , Takahashi Masashi , Ogawa Arito , Ito Hiromi , Nabatame Toshihide , Toriumi Akira
Published: September 2007

Cubic-HfO2 film was formed by the post-gate-electrode-deposition annealing (PGA) process and the highest permittivity 39 was obtained. The optical bandgap (Ф) is also confirmed to be as wide as that of the low-permittivity monoclinic phase. Thus higher-k and high-Ф is achieved in cubic-HfO2 films. We found, however, that the permittivity degrades sensitively with both gate-electrode structure and PGA temperature. Analyses using x-ray diffraction, far-infrared spectroscopy, and TEM observation revealed that the cubic phase is not stable and orthorhombic and tetragonal phases appear in the degraded films. We calculated the stress in stacked structures during annealing and obtained the relationship between the stress in HfO2 films and the crystalline phases. It is shown that the stress induced in HfO2 through the PGA process has a strong impact on controlling the crystalline phases and thus the permittivity.
First-Principles Study of Dynamical and Dielectric Properties of Orthorhombic Phases of Group IVb Transition Metal Oxides.
Authors: Rignanese Gian-Marco
Published: September 2007

Using density-functional theory, we investigate the structural, vibrational and dielectric properties of the orthorhombic phases of the group IVb transition metal dioxides (HfO2, ZrO2, TiO2). We discuss the Born effective charge tensors, the phonon frequencies at the Γ point of the Brillouin zone, and the dielectric permittivity tensors.
Impact of Hf-Precursor Choice on Scaling and Performance of High-k Gate Dielectrics
Authors: Maes Jan Willem , Fedorenko Yanina , Delabie Annelies , Ragnarsson Lars-Aåke , Swerts Johan , Nyns Laura , Van Elshocht Sven , Wang Changgong , Wilk Glen
Published: September 2007

Downscaling of CMOS gate stacks requires introduction of ultra-thin high-k dielectrics such as HfO2. Atomic Layer Deposition has excellent characteristics for depositing such films. We have compared HfCl4/H2O and TEMAH/O3 ALD between 245ºC and 370ºC. Growth behavior and electrical performance are discussed as function of process parameters. Precursor choice has a clear impact on HfO2 deposition process and device performance. Both precursors demonstrate good growth nucleation on -OH rich starting surfaces. TEMAH shows better nucleation on low -OH density surfaces, suggesting a potential advantage for interface optimization towards low EOT. However, with HfCl4 lowest leakage currents were obtained as well as excellent interface scaling results by using well chosen interface layers. The leakage current of TEMAH/O3 layers strongly depends on the ALD process temperature and oxidizer dose, suggesting the impact of C impurities on the dielectric properties. Overall the HfCl4 approach seems to be the most promising solution.
Control of Flat Band Voltage by Partial Incorporation of La2O3 or Sc2O3 into HfO2 in Metal/HfO2/SiO2/Si MOS Capacitors
Authors: Adachi Manabu , Okamoto Kouichi , Kakushima Kuniyuki , Ahmet Parhat , Sugii Nobuyuki , Tsutsui Kazuo , Hattori Takeo , Iwai Hiroshi
Published: September 2007

High-k/SiO2 interfacial properties are most critical factors determining the high-k gate MOSFET characteristics. We fabricated MOS capacitors of metal/HfO2/SiO2/Si structures in which were contained in the HfO2 layer. Flat-band voltage (VFB) shifts were measured by changing composition in metal/HfO2/(HfO2)1-x(La2O3)x/SiO2/Si and metal/HfO2/(HfO2)1-x(Sc2O3)x/SiO2/Si structures. It was found that VFB shift arises mainly from high-k/SiO2 interface rather than metal/high-k interface. VFB could be effectively controlled by incorporating La2O3 or Sc2O3 near the high-k/SiO2 interface.
Improvement of Interface Properties of W/La2O3/Si MOS Structure Using Al Capping Layer
Authors: Tachi Kiichi , Kakushima Kuniyuki , Ahmet Parhat , Tsutsui Kazuo , Sugii Nobuyuki , Hattori Takeo , Iwai Hiroshi
Published: September 2007

Electrical characteristics of W/La2O3/Si MOS capacitors have been investigated under various annealing condition. Relatively high interface state densities (Dit) of 1012 cm-2eV-1 observed after annealing at 200 to 500 oC was effectively reduced down to 1011 cm-2eV-1 by using an Al capping layer in the annealing process. However, a trade off between high capacitance density and low Dit has been observed in this method.
Role of the Ionicity in Defect Formation in Hf-Based Dielectrics
Authors: Umezawa Naoto , Shiraishi Kenji , Miyazaki Seiichi , Uedono Akira , Akasaka Yasushi , Inumiya Seiji , Oshiyama Atsushi , Hasunuma Ryu , Yamabe Kikuo , Momida Hiroyoshi , Ohno Takahisa , Ohmori Kenji , Chikyow Toyohiro , Nara Yasuo , Yamada Keisaku
Published: September 2007

Ionicity has been raised as an important factor in discussing defect formation in Hf-based oxides. It has been elucidated from our first-principles calculations, that the stability of defects is dominated by Coulomb interactions between charged defects and surrounding ions. For instance, the formation energy of positively charged oxygen vacancies (VO+2) is markedly decreased when they are coupled with substitutional N atoms at O sites, as the nominal charges of N3- are negatively greater than those of the oxygen ions O2-. Our computational results have further revealed that the ionic character of Hf causes generation of a low-lying Si dangling bond level in Si doped HfSiOx. These results suggest that one must take into account the ionicity of the Hf-based compounds, which possess totally different properties from the conventional gate insulator SiO2 in terms of defect formation.
Hydrogen Trapping in Oxygen-Deficient Hafnium Silicates
Authors: Driemeier Carlos , Fonseca Leonardo , Ribeiro Mauro , Xavier Ademir , Baumvol Israel
Published: September 2007

Isotopic substitution, nuclear reaction analysis, and x-ray photoelectron spectroscopy were employed to show that oxygen-deficient hafnium silicates trap hydrogen atoms. Based on this experimental observation, we used first-principles calculations to investigate the structure, energetics, and electronic properties of H interacting with O vacancies in a hafnium silicate model. We found that O vacancies close to a Si atom are energetically favored when compared to vacancies in HfO2-like regions, implying that close-to-Si O vacancies are more likely to occur. Trapping of two H atoms at a close-to-Si O vacancy passivates the vacancy-induced gap states. The first H interacts with neighbor Hf atoms, while the second H binds to the Si atom.
Efficient Gate Capacitance and Current Modeling of High-k Gate Stacks
Authors: Register Leonard F. , Hasan Mohammad M. , Li Fei , Banerjee Sanjay
Published: September 2007

Computationally efficient and accurate physically-based self-consistent compact models of CMOS gate-capacitance (Cg-Vg) and gate-current (Ig-Vg) for both ultra-thin SiO2 and high-k gate-stacks down to ~ 0.5 nm EOT are reviewed. The compact model produces results comparable to those obtained via computationally intense simulators. A modified Levenberg-Marquardt algorithm has been used in combination with these models to provide an efficient material and device parameter extraction capability. In a few seconds, parameters such as effective oxide thickness, surface substrate doping concentrations, flat-band voltages and poly-Si doping concentrations can be obtained from measured Cg-Vg, and physical thicknesses, band offsets, dielectric constants and tunneling masses for the gate-dielectrics can be extracted from measured Ig-Vg.
In-Line Non-Contact Micro-Kelvin Measurements Applied to ZrO2/Al2O3/ZrO2 Dielectric Stacks in the Active Capacitor Cell Areas of Advanced DRAM
Authors: Kim Jae-Hyun , Kim Chul-Hong , Yoo Hyung-Won , Kim Hong-Lyeol , Son Sang-Hoon , Lee Chang-Hwan , Kim Tae-Kyoung , D'Amico John , Wilson Marshall , Wilson Matthew , Kim Se-Hwi , Park Sung-Hwan
Published: September 2007

We present micro corona - Kelvin data acquired on ZrO2/Al2O3/ZrO2 (ZAZ) dielectric stacks in the active capacitor cell areas of advanced DRAM production wafers. Specifically, we compare micro - SASS (Self Adjusting Steady State) voltage data measured on patterned wafers to similarly acquired data on blanket ZAZ dielectric stacks. From this comparison, we formulate a model that interprets active area micro - SASS voltage as being proportional to the ZAZ dielectric stack capacitance. Validation of the method and accompanying model is made through measurement of a production baseline that demonstrates sensitivity to various process deviations. Further validation is achieved through comparison of in-line active cell area micro - SASS data to End - of - the - Line Electrical Parametric Measurement Capacitance and Breakdown Voltage data. We also demonstrate that the method, which includes corona charging and measurement on the active device areas, does not adversely impact final device yield.
Electrical Passivation of the (100)Ge Surface by Its Thermal Oxide
Authors: Bellenger Florence , Houssa Michel , Delabie Annelies , Conard Thierry , Caymax Matty , Meuris Marc , De Meyer Kristin , Heyns Marc
Published: September 2007

The physical and electrical properties of Ge/GeO2/high-κ gate stacks, where the GeO2 interlayer is thermally grown in molecular oxygen, are investigated. The high-κ layer (ZrO2, HfO2 or Al2O3) is deposited in-situ on the GeO2 interlayer by atomic layer deposition. Detailed analysis of the capacitance-voltage and conductance-frequency characteristics of these devices provides evidence for the efficient passivation of the (100)Ge surface by its thermal oxide layer. A larger flat-band voltage hysteresis is observed in HfO2-based gate stacks, as compared to Al2O3 gate stacks, which is possibly related to the more pronounced intermixing observed between the HfO2 and GeO2.
Germanium: The Past and Possibly a Future Material for Microelectronics
Authors: Brunco David P. , De Jaeger Brice , Eneman Geert , Satta Alessandra , Terzieva Valentina , Souriau Laurent , Leys Frederik E. , Pourtois Geoffrey , Houssa Michel , Opsomer Karl , Nicholas Gareth , Meuris Marc , Heyns Marc
Published: September 2007

Germanium possesses higher bulk mobilities than silicon and was used for the first transistors. However, by the 1960s its use was largely supplanted with Si due largely to Si's high quality thermal oxide. Today, with the 45 nm technology node in production, high k dielectrics are beginning to replace SiO2 in the gate, and as such, one of the key reasons for using Si is no longer as relevant. This, combined with performance concerns for Si based devices for and beyond the 22 nm node has made Ge a worthy area for research for high performance devices. In this paper, we give an overview of some of the major issues for Ge MOSFETs, illustrating recent progress using data from IMEC. Key results include a factor of 10 reduction in threading dislocations for epi Ge on Si by the use of an ~850{degree sign}C anneal, the first successful use of As halo implants, progress on optimization of activation anneals, the use of a thin epitaxial Si passivation and its impact on threshold voltage, a 2-step anneal NiGe process flow to drastically reduce defects, and the importance of a hydrogen anneal following metallization.
Investigation on Molybdenum and Its Conductive Oxides as p-type Metal Gate Candidates
Authors: Li Zilan , Schram Tom , Witters Thomas , Cho Hag-Ju , O'Sullivan Barry , Hooker Jacob , De Gendt Stefan , De Meyer Kristin
Published: September 2007

In this work, we studied molybdenum (Mo) and its conductive oxides (MoOx) for p-type metal gate application. Three compositions of MoOx have been investigated in this work. The resistivity of Mo/MoOx was found to significantly increase with increasing oxygen incorporation. Clear phase separation of all MoOx was observed after high temperature thermal treatment. Incorporation of oxygen was found to be effective to increase the work function (WF) of Mo. However, after full integration process, significant WF decrease of MoOx was also observed, which may be induced during the high temperature junction activation process. The interfacial silicon oxide layer growth and the EOT dependent Vfb roll-off behavior will also be discussed.
Recent Advances in Functional Fluoropolymers for Fuel Cell Membranes
Authors: Souzy Renaud , Boyer Cyrille , David Ghislain , Kostov Georges , Ameduri Bruno M.
Published: September 2007

Novel fluorinated copolymers based on vinylidene fluoride (VDF) were synthesized by iodine transfer copolymerization with hexafluoropropene (HFP) and a-trifluoromethacrylic acid (MAF), either in solution or in emulsion without any surfactant. a,w-Diiodoperfluoroalkanes were used as the chain transfer agents (CTAs) and acted as interesting labels in 19F NMR to assess the molecular weights of the resulting terpolymers. According to the CTA concentrations in feed, molecular weights were ranging from 2,000 to 220,000 g/mol. Chemical modification of these fluorinated terpolymers led to original functionalized copolymers containing either sulfonic acid or benzimidazole functions which were processed into membranes for fuel cells, the properties of which were investigated.
Effect of IEC and Loading of Cathode SPEEK Binder on Kinetic and Transport Properties of All-SPEEK MEAs
Authors: Sambandam Satheesh Kumar , Ramani V.
Published: September 2007

The choice of polyelectrolyte in the catalyst layer is an important consideration in the formation and efficacy of the triple phase boundary. It also influences the stability of the electrolyte/electrode interface for long term fuel cell performance. The polyelectrolyte composition in the electrode has to be optimum for higher Pt utilization and hence better MEA performance. MEAs with SPEEK (of different IECs) as binder in the electrode layer and SPEEK (IEC 1.75 meq/g) membrane were prepared by decal transfer. MEAs were tested at 800 C and 75% RH. Steady state polarization curves were obtained with air and oxygen. Polarization data for MEAs with different binders in the electrode were analyzed to determine the relative contributions of the different sources of polarization, namely activation losses, membrane resistance losses, ohmic losses in the electrode, and mass transport losses in the GDL and electrode.
Preparation and Application of IrO2/Pt Electrocatalyst for Regenerative Fuel Cells
Authors: D'Urso Claudia , Lizbeth Morales , Di Blasi Alessandra , Baglio Enzino , Ornelas Ruben , Orozco Germán , Arriaga Luis G. , Antonucci Vincenzo , Arico Antonino
Published: September 2007

Electrocatalysts of the type Pt-IrO2 (at different concentrations) have been synthesized and characterized in Regenerative Fuel Cell (RFC) configuration. Commonly IrO2 has been mainly used as oxygen evolution electrocatalyst. These oxide-metal mixtures were prepared by incipient wetness technique and characterized by XRD and SEM, obtaining in all cases nanoparticles. The powder materials were impregnated by hot-spray technique on Nafion 117 in order to evaluate water electrolysis/fuel cell performance. The catalyst loading in oxygen electrodes was about 6 mg•cm-2. Water electrolysis/fuel cell performance has been evaluated using a single cell test fixture with a geometric area of 5 cm2. It is derived that for fuel cell operation an excess of Pt favours the oxygen reduction process whereas IrO2 promotes oxygen evolution.
A Kinetics Study of Oxygen Reduction Reaction on Palladium-Nickel Alloy
Authors: Li Bing , Amiruddin Shabab , Prakash Jai
Published: September 2007

Carbon supported palladium-nickel and nickel were synthesized by chemical reduction of the metal salts. Polycrystalline Pd, Ni and Pd-Ni were electrodeposited on gold substrates. The structure of these materials was characterized using X-ray diffraction (XRD). Catalytic activity study of Pd-Ni/C and Ni/C for oxygen reduction reaction (ORR) in acidic and alkaline media respectively was carried on a glass carbon rotating disk electrode (RDE) using Nafion as binder. Electrodeposited Pd-Ni on gold RDE was also investigated for ORR in acidic media. The performance of membrane electrode assembly (MEA) with Pd-Ni/C cathode, Nafion 117 membrane electrolyte and Pt/C anode was studied in a 5cm2 polymer electrolyte membrane (PEM) fuel cell.
Polymer-Stabilized PdRuSe Nanoparticles for Oxygen Electrocatalysis
Authors: Kalu Egwu E. , Ramos Kristine , Waryoba Daudi , Kalu Peter
Published: September 2007

Carbon-supported tri-metallic Pd-based (PdRuSe/C) electrocatalysts were synthesized and characterized. The electrocatalysts were synthesized by a polymer-modified impregnation method. The SEM and XRD results demonstrate that the synthetic approach leads to the formation of metal nanoclusters at very low activation temperature. Cyclic voltammetry results indicated that the PdRuSe catalyst is comparable to Pt in its ORR catalytic activity. The synthesized catalyst has methanol tolerance.
Synthesis and Characterization of Pt-Carbon Nano Capsule Electrocatalyst for Hydrogen Oxidation Reaction
Authors: Chang Yun-Min , Wu Pu-Wei , Hsieh Yu-Chi , Tsai Chia-Fan
Published: September 2007

Synthesis of nanoparticulate Pt and PtRu were conducted in typical wet chemical approach on carbon nanocapsules (CNCs) and VXC72R for electrochemical characterization in acidic electrolyte. XRD of the as-synthesized powders indicated Pt and PtRu in FCC phase with particle sizes around 2~15 nm. Cyclic voltammetry of Pt/CNCs and PtRu/CNCs was analyzed for hydrogen oxidation reaction, revealing impressive catalytic power. In contrast, Pt/VXC72R and PtRu/VXC72R demonstrated moderately reduced performance. Our work suggested CNCs to be superb material as electrocatalyst support.
Performance of PEMFCs with Sputter Deposited Pt and Pt Alloy Cathodes
Authors: Kreidler Eric , He Ting
Published: September 2007

A fabrication and testing method was developed to assess the performance of thin film Pt and Pt alloy cathode catalysts in an operating fuel cell. Pt and Pt alloy thin films were sputter deposited onto carbon/Nafion decals and subsequently hot pressed with the catalyst towards the membrane. The results show that improved mass performance and catalyst utilization is observed with Pt thin films and that increased mass activities are achieved with PtCo (76:24) and PtCr (80:20) alloys as compared to pure Pt. Mass activity improvements of 14 mV and 8 mV were observed for the PtCo and PtCr alloys with respect to a pure Pt film with a similar mass loading under 300/350 kPa hydrogen/oxygen operation. Details of fabrication and testing are provided including testing nuances of cathode thin films due to poor water management.
Spatially Resolved Electrode Diagnostic Technique for Fuel Cell Applications
Authors: Carter Robert N. , Brady Brian K. , Subramanian Kannan , Tighe Thomas , Gasteiger Hubert A.
Published: September 2007

A non-destructive, diagnostic technique for evaluating the extent of degradation in electrodes on membrane-electrode assemblies (MEAs) has been developed that can be applied to parts from application-scale proton exchange membrane fuel cells. The modes of electrode degradation that are examined include: catalyst activity loss, catalyst area loss, and electrode support structure damage through carbon corrosion. The technique incorporates a current distribution tool to enable spatially resolved analysis at a length scale of approximately 1 cm. The MEA parts are evaluated through a combination of cyclic voltammetry and polarization curves performed at conditions designed to expose degradation in one of the modes described above.
Stochastic Analysis of Flooding in PEM Fuel Cells by Electrochemical Impedance Spectroscopy
Authors: Roy S. , Orazem M. E.
Published: September 2007

Steady-state and impedance measurements were correlated to gain insight into flooding of a single PEM fuel cell with interdigitated flow channels. While the values of the impedance can be related to the onset of flooding, the stochastic character of the formation of water droplets and subsequent removal by gas flow gives rise to a significant increase in the noise level of the impedance measurement. The standard deviation of the impedance measurements was compared to a base-noise level evaluated using a measurement model analysis for cells operated at minimal levels of flooding. The increase in stochastic noise associated with flooding is more apparent at low frequencies, due to the closer match to the characteristic frequency associated with the growth and removal of water droplets. The present work demonstrates how the stochastic character of flooding may be exploited to detect onset of flooding without the need to regress impedance spectra.
Novel Technique of MEA Sample Preparation Using a Focused Ion Beam for Scanning Electron Microscope Investigation
Authors: Kuroda Carlos S. , Yamazaki Yohtaro
Published: September 2007

A novel technique using a focused ion beam (FIB) set up with a cryo-stage was developed to prepare a flat surface on a membrane electrolyte assembly (MEA) for scanning electron microscope (SEM) observations. The microsample was first trimmed by an ultramicrotome for use in the novel technique. The standard ultramicrotome epoxy block was prepared by "partially-embedding" techniques to suppress the MEA electrode fill up by epoxy resin. An MEA specimen was sampled from the epoxy block and stick to a silver block. The silver block was placed in to an FIB chamber on a cryo-stage and milled at -150{degree sign}C. The SEM image showed flat surface without any thermal damage composed of an interpenetrate structure of carbon particles and ionomer. From the MEA surface acquired by this novel method, we can understand the flooding phenomenon in an electrode porous structure. In addition, the proton conduction through the catalyst layer can be predicted.
In Situ AC Impedance Diagnosis of Hydrogen Oxidation in a PEMFC
Authors: Xu Fan , Cheng Xuan , Zhang Ying , Fan Qinbai
Published: September 2007

The effects of gas flow rate, temperature and applied voltage on hydrogen oxidation at Pt-Ru/C electrodes were systematically investigated by electrochemical impedance spectroscopy (EIS) and liner sweep voltammetry (LSV). The kinetic parameters of polarization resistance (Rp/R2) and double layer capacitance (C2) were obtained by fitting the impedance data with the equivalent circuit. The charge transfer resistance (Rct) which is identical to Rp in this study was also determined by analyzing the LSV data. The contribution of hydrogen oxidation to the cell lifetime was studied by in situ impedance measurement at different periods of time during durability test using a simulated PEMFC. It was found that the R2 or Rct values increased with decreases in gas flow rate and temperature and increase of applied voltage. Anode impedance increased rapidly after 100h of continuous test, and was nearly 1/3 to the cell impedance at 25oC. The changes in high-frequency parameters were related to the microstructure of the catalyst layer.
Freeze-Induced Damage and Purge Based Mitigation in Polymer Electrolyte Fuel Cells
Authors: Kim Soowhan , Chacko Charlie , Ramasamy Ramaraja P. , Mench Matthew M.
Published: September 2007

The objective of this work is to investigate damage of polymer electrolyte fuel cell (PEFC) materials subjected to freeze-thaw cycling. Effects of microstructures of membrane electrolyte assembly (MEA) and diffusion media (DM) were investigated by ex-situ freeze-thaw cycling tests and revealed a strong direction for the material choices in the PEFC and confirmed the previous computational model in the literature (1, 2). Specifically, the membrane electrode assemblies were found to be a source of water that can damage the catalyst layers under freeze/thaw conditions. Damage was found to occur almost exclusively under the channel, and not under the land. Conceptually, the best material to mitigate freeze damage is a crack free virgin catalyst layer on a reinforced membrane that is as thin as possible, protected by a stiff diffusion media. In addition to ex-situ cycling test, freeze-thaw cycling of fuel cells was also carried out. Without purge of liquid water during the shut down, the fuel cell experienced irreversible damage. Removal of free liquid water from the catalyst layer pores is integral to the reduction of freeze-induced damage.
Study on Water Content of Perfluorosulfonic Acid Ionomer Membranes Below Water Freezing Point
Authors: Koiwai Akihiko , Kamiya Atsushi , Kawasumi Masaya
Published: September 2007

Non-freezing water contents in perfluorosulfonic acid ionomer membranes at temperatures below water freezing point were measured by 1H-NMR. At temperatures below water freezing point, the non-freezing water contents decreased because relative humidity decreased with decreasing temperature. As the result, proton conductivity of the membranes decreased rapidly below water freezing temperature.
An Empirical Model for the Flooding Behavior of Ultra-thin PEM Fuel Cell Electrodes
Authors: Debe Mark K. , Steinbach Andrew J.
Published: September 2007

A quantitative empirical model is applied to explain the onset of cathode flooding behavior of PEM fuel cell membrane electrode assemblies having the ultra-thin 3M NSTF (nanostructured thin film) electrocatalysts and first generation Toray carbon paper gas diffusion layers (GDL). An empirical method is used to define the current density at the onset of cathode flooding, and calculated vapor phase water removal rates at specific water balance conditions are found to give excellent quantitative agreement with the measured rates of water generation for three different first generation GDL's. Effective water vapor diffusion coefficients porosites and saturation levels for the GDL's can be extracted from the water balance data at the in-situ operating conditions. Application of the method to later generation GDL's for which the flooding occus at much lower temperatures is also discussed, with the conclusion that liquid phase transport plays a greater role for these GDL's.
An Artificial Neural Network for Capillary Transport Characterization of Fuel Cell Diffusion Media
Authors: Kumbur Emin C. , Sharp Kendra V. , Mench Matthew M.
Published: September 2007

This study addresses the development of a design algorithm based on artificial neural network (ANN) that can precisely predict the capillary transport characteristics of fuel cell diffusion media (DM). A three-layered ANN architecture processing the feed-forward error back propagation methodology has been constructed. The designed neural network was systematically trained with the novel benchmark data generated from direct measurements of capillary pressure-saturation of differently engineered DMs under a wide range of conditions [8-10]. Once the trained network learned the complex non-linear relationship between the transport properties and measured parameters of the tested DM samples, it was utilized to predict the capillary pressure of the DM as a function of the hydrophobic additive content and assembly compression pressure at the intermediate conditions, in which the experimental data are not available.
Cyclic Voltammograms from First Principles
Authors: Karlberg Gustav S. , Jaramillo Thomas , Skulason Egill , Rossmeisl Jan , Bligaard Thomas , Norskov Jens
Published: September 2007

Cyclic voltammetry is a fundamental experimental tool for characterizing electrochemical surfaces. Whereas cyclic voltammetry is widely used within the field of electrochemistry, a way to quantitatively and directly relate the cyclic voltammogram to ab initio calculations has been lacking, even for the simple case of electroadsorption and desorption of H. In the following we derive the cyclic voltammogram for H adsorption and desorption on Pt(111) and Pt(100) based solely on density functional theory calculations and standard molecular tables. The method will also be extended to include the potential dependence of the OH coverage during electrochemical water splitting on Pt(111) and Pt3Ni(111).
Effect of Calcium Ion on the Performance of PEMFCs
Authors: Ohashi Masato , Smith Carl S. , Van Zee J. W.
Published: September 2007

The effect of Calcium ion on the performance of polymer electrolyte membrane fuel cell (PEMFC) was studied. The calcium hydroxide solutions were applied directly to the anode of the cell to soak GORETM PRIMEA® membrane electrode assembly (MEA) in situ when there was no load applied to PEMFC. During operation, the performance decreased when the calcium solution was applied repeatedly, or when the concentration or soaking time of calcium solution was increased. With EPMA (Electro probe micro analysis) technique, no calcium was observed in the gas diffusion layers. Calcium mainly accumulated in the membrane and less amount of calcium accumulated in both anode and cathode catalyst layers. Those results indicate that calcium was exchanged with proton in the membrane and catalyst layers by an ion exchange reaction of calcium and sulfonic group in MEA, and that the performance decreased by this ion exchange reaction.
In Situ XAS Investigation of Electrocatalysts Surface Poisoning by Halides
Authors: Arruda Thomas , Shyam Badri , Ziegelbauer Joesph , Ramaker David , Mukerjee Sanjeev
Published: September 2007

X-Ray Absorption Spectroscopy was utilized to study the poisoning of actual fuel cell catalysts under in situ conditions. The delta mu analysis technique of X-Ray Absorption Near Edge Structure on in situ Pt/C electrocatalysts is utilized at various potentials and concentrations of KCl. Comparison with FEFF8.0 calculated delta mu curves suggest a line shape consistent with 3-fold bonded Cl-. Inspection of the delta mu peak intensities implies dramatic suppression of Pt-O[H] adsorption in 10mM Cl-, but with both Cl- and O[H] adsorption competitively occurring in 1mM Cl-. This is the first time that XAS has been employed to probe the poisoning effects of Cl- on Pt electrocatalysts.
Temperature-Dependence of Hydrogen Oxidation Rates in the Presence of CO at Pt, PtRu, and Pt3Co Catalysts Dispersed on Carbon Black
Authors: Uchida Hiroyuki , Izumi Kenji , Watanabe Masahiro
Published: September 2007

The kinetically-controlled current density jK for hydrogen oxidation reaction (HOR) at Pt, PtRu and Pt3Co nanocatalysts (d = ca. 3 nm) supported on carbon black with and without CO poisoning in 0.1 M HClO4 solution were examined with a channel flow electrode at 30 to 90 oC. The apparent rate constant kapp for the HOR was calculated based on jK/2F = kapp[H2]. It was found that the values of kapp at three kinds of supported catalysts agreed beautifully with those of corresponding bulk electrodes in the whole temperature range. The dependence of the jK on CO coverage at each supported catalyst was also identical to that of bulk one. Hence, no particle-size effect was observed on the HOR with and without CO-poisoning, at least, down to d = 3 nm.
Hybrid Catalysts for Degradation Mitigation in PEFCs
Authors: Trogadas Panagiotis , Ramani V.
Published: September 2007

Carbon-tungsten oxide (C/WO3), Carbon-phosphotungstic acid (C/PTA) and Platinum/Carbon-PTA (Pt/C-PTA) hybrid catalysts were prepared by wet chemical methods. Rotating ring-disk electrode (RRDE) experiments were performed using these materials to estimate the amount of hydrogen peroxide (H2O2) formed during the oxygen reduction reaction (ORR). C-WO3 15 wt% and C-PTA hybrid catalysts produced 30-40% less hydrogen peroxide than C and Pt/C-PTA had 20-30% less peroxide production than the baseline Pt/C electrocatalyst. The hybrid catalysts were used to prepare membrane electrode assemblies that were tested at 90{degree sign}C and 30% RH at open circuit with pure hydrogen as fuel and oxygen as the oxidant. The concentration of F- in the anode condensate over 24 hours was found to be reduced by a factor of 50-60% when C-WO3 15wt% and C-PTA replaced C as the catalyst.
Modelling of Carbon Corrosion in a PEMFC caused by Local Fuel Starvation
Authors: Hu Jingwei , Sui Pang Chieh , Kumar Sanjiv , Djilali Ned
Published: September 2007

A two-dimensional, along-the-channel model to predict the carbon corrosion for a cathodic catalyst layer of a PEM fuel cell with a starvation zone is reported. The model is able to capture the abrupt decrease of electrolyte potential along the interface of normal and starvation zone, as reported by Reiser et al. (1). Our simulation results show that at higher operational potentials, lower protonic conductivity of the electrolyte, faster carbon corrosion kinetics and higher oxygen permeability will result in higher carbon corrosion rate.
Effect of Scan Range on Pt Surface Area Loss in Potential Cycling Experiments
Authors: Waje Mahesh , Li Wenzhen , Chen Zhongwei , Larsen Paul , Yan Yushan
Published: September 2007

Effect of upper and lower potential limits on Pt stability in potential cycling experiments is studied for 20 wt. % Pt/C E-TEK catalyst in acidic medium. The Pt electrochemical surface area (ECSA) loss is observed as a parameter to study different potential ranges. It is observed that at upper potential limits the ECSA loss is influenced by electrochemical Pt dissolution while at lower potential limits reduction of β Pt oxide phase plays important role in ECSA loss.
Methanol Oxidation on Pt(111)/Ru in Alkaline Media
Authors: Spendelow Jacob S. , Goodpaster Jason , Johnston Christina , Kenis Paul , Wieckowski A.
Published: September 2007

Dehydrogenation and oxidation of methanol has been studied on well-ordered Pt(111), vicinal Pt(111), and Pt(111)/Ru in alkaline media. Multiple mechanisms of promotion have been identified. Steps with (110) orientation promote methanol dehydrogenation through a geometric effect, while they promote CO oxidation through a change in electronic structure. Adsorbed Ru promotes methanol dehydrogenation through two distinct electronic (ligand) effects: it increases the intrinsic activity of nearby Pt sites for methanol dehydrogenation, and it decreases the local CO coverage at the active sites even when the overall CO coverage is high. Ru also promotes methanol oxidation through a ligand effect on CO oxidation. The bifunctional effect of Ru is concluded to be much weaker in alkaline media than in acidic media. STM imaging confirms the stability of adsorbed Ru even during extended potential cycling and oxidizing treatments, demonstrating the promising characteristics of Pt/Ru surfaces for fuel cells.
A Novel PtSnIr/C Anodic Catalyst for Direct Ethanol Fuel Cell
Authors: Cao Lei , Sun Gongquan , Li Huanqiao , Yan Shiyou , Xin Qin
Published: September 2007

PtSnIr/C catalyst with a nominal atomic Pt/Sn/Ir ratio of 3: 1: 0.5 and Pt loading of 20 wt. % was synthesized with NaBH4 as a reducing agent. In comparison with Pt3Sn/C catalyst with the same nominal Pt loading, catalysts were characterized using X-ray diffraction (XRD), cyclic voltammetry (CV) and chrono-amperometry methods and single-cell tests. Pt3SnIr0.5/C catalyst showed enhanced electro-catalytic activity towards ethanol oxidation and better cell performance.
Evaluation of Solid Ammonium Formate Oxidation for PEM Fuel Cells
Authors: Ohyama Kazutoshi , Kimura Chiharu , Aoki Hidemitsu , Kuwabata Susumu , Sugino Takashi
Published: September 2007

We have first attempted to develop high performance and safe fuel cells using ammonium formate as solid (powder) fuel. This solid fuel has potential to realize safe portability compared with liquid fuel such as methanol from the viewpoint of toxicity and flammability. In order to make use of some advantages of ammonium formate, we investigate oxidation characteristics of ammonium formate toward Pt electrode. From cyclic voltammograms, ammonium formate has high activity to be oxidized with a Pt electrode. We have found that oxidation of ammonium formate can be improved by additional Ir catalysts with Pt catalysts. There is a high potentiality that ammonium formate is useful as solid (powder) fuel for polymer electrolyte membrane (PEM) fuel cells. This new fuel will be driving force of developing safe fuel cells for PEM.
Simple Synthesis of a Variety of Nano-structures Using Silicide Alloys with Ga Droplets
Authors: Ogino Kazuki , Honda Satoshi , Yasuda Takahito , Tatsuoka Hirokazu , Inaba Takashi , Kominami Hiroko , Nakanishi Yoichiro , Murakami Kenji
Published: September 2007

A variety of nano-structures, such as nanofibers, nanotubes, nanocapsules, nanoribbons and nanorods, were synthesized using silicide alloys with Ga droplets. It was found that the growth morphology and the structural property of the nanostructures significantly depended on the silicides seed materials. In addition, the amorphous SiOx nanofibers show strong ultraviolet and/or visible light emissions, and the PL spectra of the nanofibers depended on the seed materials. For some of the nanofiber syntheses, the formation of nanoflakes or nanoribbons of β-Ga2O3 occurs, and a variety of growth morphologies for the β-Ga2O3 nanostructures was obtained. The series of morphological and structural images are shown for a variety of nanostructures. The obtained SiOx nanofibers are expected to be materials for use in cheap, abundant and safe luminescent devices for visible light applications.
Formation of Ultra-Thin Quantum Dot Films by Mist Deposition
Authors: Price Steven C. , Shanmugasundaram Karthik , Zhu Ting , Zhang Fan , Xu Jian , Mohney Suzanne , Zhang Q. , Ruzyllo J.
Published: September 2007

Films of CdSe colloidal semiconductor quantum dots were deposited on several substrates including bare silicon, silicon dioxide, aluminum, and ITO coated glass using mist deposition. The process parameters, including deposition time, solution concentration, and electric field, were varied to change the thickness of the deposited film. Blanket films and films deposited through a shadow mask were created to investigate the method's ability to pattern films during the deposition process. Smooth films were formed with thicknesses from about 39 nm, or 7 monolayers, to a single layer. The results show that mist deposition of quantum dots is a viable method for creating thin quantum dot films.
End Point Detection of GaAs Backside Via Etching in Capacitor on Via
Authors: Nam Peter , Luo Wen-Ben , Zeng Xianglin , Wang J. , Uyeda Jansen , Biedenbender Mike , Farkas David , Barsky Mike , Olson Scott , Yajima M. , Dai Fred
Published: September 2007

We present an optimized GaAs backside via etching process using optical emission spectrometry (OES) based end point detection for capacitor on via circuit topology. A strong and repeatable end point was observed using the 416.8 nm Gallium by-product emission line, where 6-7 % normalized signal drop indicated the end point. To establish the production process, the signal threshold was set at 3 %, and we conducted an over-etching study varying the percentage of over-etching from 5%~30%. 20 % over-etching setting showed consistent and good uniformity of the ramped breakdown voltage for capacitor on via test structures.