bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissection of the Calcium Channel Domains Responsible for Modulation of Neuronal Voltage‐Dependent Calcium Channels by G Proteins
Authors: DOLPHIN ANNETTE C. , PAGE KAREN M. , BERROW NICHOLAS S. , STEPHENS GARY J. , CANTÍ CARLES
Published: February 2006

ABSTRACT: The molecular determinants for G‐protein regulation of neuronal calcium channels remain controversial. We have generated a series of α1B/α1E chimeric channels, since rat brain α1E (rbEII), unlike human α1E, showed no G‐protein modulation. The study, carried out in parallel using D2 receptor modulation of calcium currents in Xenopus oocytes of Gβγ modulation of calcium currents in COS‐7 cells, consistently showed an essential role for domain I (from the N terminus to the end of the I‐II loop) of the α1B Ca2+ channel in G‐protein regulation, with no additional effect of the C terminal of α1B. The I‐II loop alone of α1B, or the I‐II loop together with the C‐terminal tail, was insufficient to confer G‐protein modulation of α1E (rbEII). We have further observed that the α1E clone rbEII is truncated at the N‐terminus compared to other a1 subunits, and we isolated a PCR product from rat brain equivalent to a longer N‐terminal isoform. The long N‐terminal α1E, unlike the short form, showed G‐protein modulation. Furthermore, the equivalent truncation of α1B (ΔN1–55) abolished G‐protein modulation of α1B. Thus, we propose that the N terminus of α1B and α1E calcium channels contains essential molecular determinants for membrane‐delimited G‐protein inhibition, and that other regions, including the I‐II loop and the C terminus, do not play a conclusive role alone.
Voltage‐Dependent Calcium Channel Mutations in Neurological Disease
Authors: BURGESS DANIEL L. , NOEBELS JEFFREY L.
Published: February 2006

ABSTRACT: Calcium ion channel mutations disrupt channel function and create recognizable disease phenotypes in the nervous system. The broad array of underlying cellular alterations is commensurate with the expanding genetic diversity of the voltage‐gated calcium ion channel complex and its critical role in regulating cell function. Currently, 16 calcium channel genes are known, and mutations in 7 of these are associated with distinct inherited neurological disorders. These mutations provide new insight into the structure and function of the channels, and link specific subunits to cellular disease processes, including altered excitability, synaptic signaling, and cell death. Studies of mutant channel behavior, subunit interactions, and the differentiation of neural networks demonstrate unique patterns of downstream rearrangement. Developmental analysis of molecular plasticity in these mutants is a critical step to define the intervening mechanisms that translate aberrant ion channel behavior into the diverse clinical phenotypes observed.
Molecular Diversity of K+ Channels
Authors: COETZEE WILLIAM A. , AMARILLO YIMY , CHIU JOANNA , CHOW ALAN , LAU DAVID , McCORMACK TOM , MORENA HERMAN , NADAL MARCELA S. , OZAITA ANDER , POUNTNEY DAVID , SAGANICH MICHAEL , DE MIERA ELEAZAR VEGA‐SAENZ , RUDY BERNARDO
Published: February 2006

ABSTRACT: K+ channel principal subunits are by far the largest and most diverse of the ion channels. This diversity originates partly from the large number of genes coding for K+ channel principal subunits, but also from other processes such as alternative splicing, generating multiple mRNA transcripts from a single gene, heteromeric assembly of different principal subunits, as well as possible RNA editing and posttranslational modifications. In this chapter, we attempt to give an overview (mostly in tabular format) of the different genes coding for K+ channel principal and accessory subunits and their genealogical relationships. We discuss the possible correlation of different principal subunits with native K+ channels, the biophysical and pharmacological properties of channels formed when principal subunits are expressed in heterologous expression systems, and their patterns of tissue expression. In addition, we devote a section to describing how diversity of K+ channels can be conferred by heteromultimer formation, accessory subunits, alternative splicing, RNA editing and posttranslational modifications. We trust that this collection of facts will be of use to those attempting to compare the properties of new subunits to the properties of others already known or to those interested in a comparison between native channels and cloned candidates.
Genomic Organization of Nematode 4TM K+ Channelsa
Authors: WANG ZHAO‐WEN , KUNKEL MAYA T. , WEI AGUAN , BUTLER ALICE , SALKOFF LAWRENCE
Published: February 2006

ABSTRACT: As many as 50 genes in the C. elegans genome may encode K+ channels belonging to the novel structural class of two‐pore (4TM) channels. Many 4TM channels can be grouped into channel subfamilies. We analyzed 4TM channels in C. elegans using methods made possible by having complete genomic sequence. Two genes were chosen for comprehensive analysis, n2P16 and n2P17. By comparing the pattern of conservation in genomic DNA sequences between C. elegans and a closely related species, C. briggsae, we were able to identify all coding regions and predict the gene structure for these two genes. Given the extent of the 4TM channel family, we were surprised to discover that n2P17 produced at least six alternative transcripts encoding a constant central region and variable amino‐ and carboxyl‐termini. Blocks of highly conserved DNA sequences in noncoding regions were also apparent and most likely confer important regulatory functions. The interspecies comparison of the deduced channel proteins revealed that the extracellular loop between M1 and P1 is an apparent hot spot for evolutionary change in both channels. This contrasts with the membrane‐spanning domains that are highly conserved. Analysis of intron positions for 36 channels revealed that introns are frequently present at an identical position within the pore region, but very few are located in membrane‐spanning domains.
The Functional Role of Alternative Splicing of Ca2+‐Activated K+ Channels in Auditory Hair Cells
Authors: JONES E.M. C. , GRAY‐KELLER M. , ART J. J. , FETTIPLACE R.
Published: February 2006

ABSTRACT: Turtle auditory hair cells are frequency tuned by the activity of large‐conductance calcium‐activated potassium (KCa) channels, the frequency range being dictated primarily by the channel kinetics. Seven alternatively spliced isoforms of the KCa channel alpha‐subunit, resulting from exon insertion at two splice sites, were isolated from turtle hair cells. These, when expressed in Xenopus oocytes, produced KCa channels with a range of apparent calcium sensitivities and channel kinetics. However, most expressed channels were less calcium sensitive than the hair cells' native KCa channels. Coexpression of alpha‐subunit with a bovine beta‐subunit substantially increased the channel's calcium sensitivity while markedly slowing its kinetics, but kinetic differences between isoforms were preserved. These data suggest a molecular mechanism for hair cell frequency tuning involving differential expression of different KCa channel alpha‐subunits in conjunction with an expression gradient of a regulatory beta‐subunit.
Structure, G Protein Activation, and Functional Relevance of the Cardiac G Protein‐Gated K+ Channel, IKACh
Authors: WICKMAN KEVIN , KRAPIVINSKY GRIGORY , COREY SHAWN , KENNEDY MATT , NEMEC JAN , MEDINA IGOR , CLAPHAM DAVID E.
Published: February 2006

ABSTRACT: The muscarinic‐gated atrial potassium channel IKACh has been well characterized functionally, and has been an excellent model system for studying G protein/effector interactions. Complementary DNAs encoding the composite subunits of IKACh have been identified, allowing direct probing of structural and functional features of the channel. Here, we highlight recent approaches taken in our laboratory to determine the oligomeric structure of native cardiac IKACh, the mechanism of activation of IKACh by G proteins, and the relevance of IKACh to cardiac physiology.
Potassium Currents in Developing Neurons
Authors: RIBERA ANGELES B.
Published: February 2006

ABSTRACT: In Xenopus spinal neurons, delayed rectifier type voltage‐dependent potassium currents (IKv) are developmentally regulated. These currents play a pivotal role in maturation of the action potential from a long‐duration calcium‐dependent impulse to a brief sodium‐dependent one. Although spinal neurons are heterogeneous, IKv undergoes a synchronized and homogenous developmental functional up‐regulation across this diverse population of motor, sensory, and interneurons. This finding suggested that the diverse population of neurons expressed a common potassium channel. Thus, recent efforts have been directed towards cloning the relevant potassium channel gene. However, these molecular studies reveal an unsuspected heterogeneity in the molecular components of voltage‐dependent potassium channels. Further, synchronous differentiation of IKv is achieved via heterogeneous Kv channel gene expression.
Glutamate Receptor Anchoring Proteins and the Molecular Organization of Excitatory Synapses
Authors: SHENG MORGAN , PAK DANIEL T.
Published: February 2006

ABSTRACT: Ionotropic glutamate receptors are concentrated at postsynaptic sites in excitatory synapses. The cytoplasmic C‐terminal tail of certain glutamate receptor subunits interact with specific PDZ domain‐containing proteins. NMDA receptor NR2 subunits bind to the PSD‐95 family of proteins, whereas AMPA receptor subunits GluR2/3 bind to GRIP. These interactions may underlie the clustering, targeting, and immobilization of the glutamate receptors at postsynaptic sites. By virtue of their multiple protein‐binding domains (e.g., three PDZs in PSD‐95 and seven PDZs in GRIP), PSD‐95 and GRIP can function as multivalent proteins that organize a specific cytoskeletal and signaling complex associated with each class of glutamate receptor. The network of protein‐protein interactions mediated by these abundant PDZ proteins is likely to contribute significantly to the molecular scaffold of the postsynaptic density.
Mice with Genetically Modified NMDA and AMPA Receptors
Authors: SPRENGEL ROLF , SINGLE FRANK N.
Published: February 2006

ABSTRACT: This manuscript summarizes mouse mutants for ionotropic glutamate receptors that were generated by different laboratories to analyze the function of the NMDA and AMPA receptors in the mouse. Thus, NMDA receptor mutant mice that were generated by the “knock‐in” technology demonstrate that the NR1 and the NR2B subunits participate in the formation of NMDA receptors that are involved in vital functions like breathing and suckling of a newborn mouse. Mice that lack NR2A,‐2C, and ‐2D subunits were described to be viable and have been used to study the role of NMDA receptors in adult mice. The depletion of the GluR‐B subunit revealed an NMDA receptor‐independent form of long‐term potentiation (LTP). This AMPA receptor‐mediated LTP at CA3/CA1 synapses was also observed in mice that carry an editing‐deficient GluR‐B allele even though these mice die prematurely after heavy epileptic seizures. In other mutants, the intracellular COOH‐terminal domain of the NMDA receptor was truncated; and when compared to NMDA receptor “knock‐out” mice, a functional knock‐out of the NMDA receptor was observed. However, in the synapses of NR2ΔC/ΔC mutants, gatable NMDA receptors were synaptically activated, indicating that the knock‐out phenotypes mediated by the COOH‐terminally truncated NMDA receptors appear to reflect defective intracellular signaling.
Expression Mechanisms Underlying NMDA Receptor‐Dependent Long‐Term Potentiation
Authors: NICOLL R. A. , MALENKA R. C.
Published: February 2006

ABSTRACT: Long‐term potentiation (LTP) is currently the best available cellular model for learning and memory in the mammalian brain. In the CA1 region of the hippocampus, as well as in many other areas of the CNS, its induction requires a rise in postsynaptic Ca2+ via activation of NMDA receptors. What happens after the rise in postsynaptic Ca2+ is less clear. This paper summarizes experiments performed over the last decade in slice preparations that address the site of expression of LTP. While a large number of laboratories have contributed importantly to this issue, this review will rely primarily on experiments performed in the authors' laboratory. The experiments to be discussed can be broadly divided into two groups: those designed to determine if an increase in glutamate release occurs during LTP and those designed to determine if a change in postsynaptic sensitivity to glutamate occurs during LTP. Experiments in the first category include the analysis of dual‐component excitatory postsynaptic currents (EPSCs), paired‐pulse facilitation, saturating release probability, the use of MK‐801 to measure release probability, and glial glutamate transporter currents to measure directly the synaptic release of glutamate. Experiments in the second category include analysis of miniature EPSC amplitudes, measurements of synaptic potency, the consequences of loading cells with the constitutively activated form of CaM kinase II, and the evidence that during LTP postsynaptically silent synapses become functional. We will argue that, while numerous experiments fail to support a presynaptic expression mechanism, many experiments do point to a postsynaptic expression mechanism. The decrease in synaptic failures during LTP, the only generally accepted experimental result that supports a presynaptic expression mechanism, can be explained by postsynaptically silent synapses. Future directions for research in this field include activity‐dependent targeting of glutamate receptors and the functional consequences of phosphorylation of AMPA receptors.
Molecular Diversity of Neuronal Nicotinic Acetylcholine Receptors
Authors: McGEHEE DANIEL S.
Published: February 2006

ABSTRACT:The potent behavioral and cognitive effects of nicotine highlight the physiological importance of nicotinic acetylcholine receptors (nAChRs). These receptors are part of the superfamily of neurotransmitter‐gated ion channels that are responsible for rapid intercellular communication. Molecular cloning of the protein subunits that make up these receptors has led to greater understanding of the pharmacology and physiology of nAChRs. This review outlines our current understanding of the molecular constituents of these receptors and some of the recent studies of the structural determinants of receptor function.
Heteromeric Complexes of α5 and/or α7 Subunits: Effects of Calcium and Potential Role in Nicotine‐Induced Presynaptic Facilitation
Authors: GIROD R. , CRABTREE G. , ERNSTROM G. , RAMIREZ‐LATORRE J. , McGEHEE D. , TURNER J. , ROLE L.
Published: February 2006

ABSTRACT:Nicotine alters a broad spectrum of behaviors, including attention, arousal, anxiety, and memory. The cellular physiology of nicotine is comparably diverse: nicotine interacts with an array of ionotropic receptors whose gating can lead to direct depolarization of neurons or to an indirect modulation of neuronal excitability by presynaptic facilitation. Furthermore, as many laboratories have shown, the α‐ and β‐type subunits that comprise neuronal nicotinic acetylcholine receptors (nAChRs) are encoded by multiple, homologous genes, yielding at least seven α and three β subunits, distinct in primary sequence. nAChRs that differ in subunit composition differ in pharmacology, conductance, and kinetics as well as in their permeability to and modulation by calcium. We will first discuss recent studies on the biophysics of a special (peculiar?) subset of nAChRs, focusing on heteromeric nAChRs comprised of α4β2 ±α5 or α7 ±β2 and α5. These nAChR channel subtypes are potently and differentially modulated by changes in intracellular calcium ([Ca]). Thus, the Po, τo, and desensitization kinetics of α4β2 channels are altered by changes in [Ca]intfrom 0 to 50 μM; nAChRs that include the α5 subunit are oppositely regulated. Mutagenesis of specific residues within the M1 to M2 domain of α4, β2, and α5 suggest a possible Ca binding “pocket.” The assembly of functional nAChRs that include α5 and/or α7 and the potential role of these novel heteromeric complexes in presynaptic facilitation will also be presented.
The Role of β2‐Subunit‐Containing Nicotinic Acetylcholine Receptors in the Brain Explored with a Mutant Mouse
Authors: LÉNA CLÉMENT , CHANGEUX JEAN‐PIERRE
Published: February 2006

ABSTRACT: Neuronal nicotinic receptors comprise a family of pentameric oligomers made up of a combination of 10 different subunits. The β2 subunit has the widest pattern of expression in the brain and is thus likely to form a significant fraction of neuronal nicotinic receptors. Using mice lacking the β2 subunit, we have shown that nAChRs containing this subunit are responsible for most of the high‐affinity binding sites for nicotine, cytisine, and epibatidine in the brain. Functional receptors containing the β2‐subunit are found in the somatodendritic compartment as well as the axonal compartment of neurons. We have examined the contribution of these receptors to the effects of nicotine on the mesolimbic DA system, which mediates the reinforcing properties of many addictive drugs (including nicotine). Submicromolar doses of nicotine, corresponding to the concentrations of nicotine in vivo in self‐administration paradigms, increased the firing rate of dopaminergic neurons in vitro in normal mice but not in mice lacking the β2 subunit. Consistently, systemic injection of nicotine induced an increase in extracellular dopamine in normal mice but not in mutant mice, and nicotine self‐administration was reduced or suppressed in mutant mice. These data support the view that the β2‐containing receptors are involved in mediating the reinforcing properties of nicotine.
Molecular and Functional Diversity of the Expanding GABA‐A Receptor Gene Family
Authors: WHITING PAUL J. , BONNERT TIMOTHY P. , MCKERNAN RUTH M. , FARRAR SOPHIE , BOURDELLES BEATRICE LE , HEAVENS ROBERT P. , SMITH DAVID W. , HEWSON LOUISE , RIGBY MICHAEL R. , SIRINATHSINGHJI DALIP J. S. , THOMPSON SALLY A. , WAFFORD KEITH A.
Published: February 2006

ABSTRACT:Fast inhibitory neurotransmission in the mammalian CNS is mediated primarily by the neurotransmitter γ‐aminobutyric acid (GABA), which, upon binding to its receptor, leads to opening of the intrinsic ion channel, allowing chloride to enter the cell. Over the past 10 years it has become clear that a family of GABA‐A receptor subtypes exists, generated through the coassembly of polypeptides selected from α1‐α6, β1‐β3, γ1‐γ3, δ, ɛ, and π to form what is most likely a pentomeric macromolecule. The gene transcripts, and indeed the polypeptides, show distinct patterns of temporal and spatial expression, such that the GABA‐A receptor subtypes have a defined localization that presumably reflects their physiological role. A picture is beginning to emerge of the properties conferred to receptor subtypes by the different subunits; these include different functional properties, differential modulation by protein kinases, and the targeting to different membrane compartments. These properties presumably underlie the different physiological roles of the various receptor subtypes. Recently we have identified a further member of the GABA‐A receptor gene family, which we have termed θ, which appears to be most closely related to the β subunits. The structure, function, and distribution of θ‐containing receptors, and receptors containing the recently reported ɛ subunit, are described.
Activity‐Dependent Regulation of GABAA Receptors
Authors: PENSCHUCK SILKE , PAYSAN JACQUES , GIORGETTA OLIVIA , FRITSCHY JEAN‐MARC
Published: February 2006

ABSTRACT: GABAA receptor heterogeneity is based on the combinatorial assembly of a large family of subunits into distinct receptor subtypes. A neuron‐specific expression pattern of receptor subtypes has been demonstrated in adult rat brain, which can be reproduced in vitro in primary neuron cultures. This suggests that genetic programs established during ontogeny govern the expression of γ‐aminobutyric acid (GABAA) receptor subtypes. Activity‐dependent mechanisms nevertheless modulate on a short‐term basis the cell surface expression of GABAA receptors, as demonstrated in cultured hippocampal neurons upon blockade of synaptic transmission or application of brain‐derived neurotrophic factor. Preliminary evidence points to changes in protein phosphorylation as a mechanism underlying short‐term activity‐dependent regulation of GABAA receptors. In vivo, chronic pharmacological modulation of neuronal activity during development, while having marked effects on the rate of cortical growth, failed to influence the expression of GABAA receptor subtypes, suggesting that additional factors are involved.
Structure and Functions of Inhibitory and Excitatory Glycine Receptors
Authors: BETZ HEINRICH , KUHSE JOCHEN , SCHMIEDEN VOLKER , LAUBE BODO , KIRSCH JOACHIM , HARVEY ROBERT J.
Published: February 2006

ABSTRACT: The strychnine‐sensitive glycine receptor (GlyR) is a pentameric chloride channel protein that exists in several developmentally and regionally regulated isoforms in the CNS. These result from the differential expression of four genes encoding different variants (α1‐α4) of the ligand‐binding subunit of the GlyR. Their assembly with the structural b subunit is governed by “assembly cassettes” within the extracellular domains of these proteins and creates chloride channels of distinct conductance properties. GlyR gating is potentiated by Zn2+, a metal ion co‐released with different neurotransmitters. Site‐directed mutagenesis has unraveled major determinants of agonist binding and Zn2+ potentiation. During development, glycine receptors mediate excitation that results in Ca2+ influx and neurotransmitter release. Ca2+ influx triggered by the activation of embryonic GlyRs is required for the synaptic localization of the GlyR and its anchoring protein gepyhrin. In the adult, mutations in GlyR subunit genes result in motor disorders. The spastic and spasmodic phenotypes in mouse as well as human hereditary startle disease will be discussed.
Cyclic Nucleotide‐Gated Channels: Molecular Mechanisms of Activation
Authors: BROILLET MARIE‐CHRISTINE , FIRESTEIN STUART
Published: February 2006

ABSTRACT: Activation of cyclic nucleotide‐gated (CNG) channels represents the final step in the transduction pathways in both vision and olfaction. Over the past several years, CNG channels have been found in a variety of other cell types where they might fulfill various physiological functions. The olfactory and photoreceptor CNG channels rely on the binding of at least two molecules of cAMP or cGMP at intracellular sites on the channel protein to open a nonspecific cation conductance with a significant permeability to Ca ions. A series of elegant experiments with cloned channels and chimeric constructs has revealed significant information regarding the binding and gating reactions that lead to CNG channel activation. These recent studies have identified several regions as well as specific amino acid residues distributed on the retinal or the olfactory CNG channel subunits that play a key role in channel regulation. In this review, we will focus on these specific molecular sites of activation and modulation of CNG channels.
Delayed Reference Control for Hotwire Cutting of Expandable Polystyrene Foam
Authors: Gallina Paolo
Published: February 2006

This paper presents the ideation and implementation of a 2-axes robotic system for hotwire cutting of polystyrene plates. In particular, since the quality of the cutting process is strongly affected by, among others, the interaction force between the hotwire and the workpiece, an accurate force control is required. The force control module, which is referred to as delayed reference control (DRC) belongs to the category of nontime based controllers. According to the DRC theory, the desired input reference xd is a function of time and a variable, which plays the role of a time delay: xd(t−T). The time delay T is properly calculated on-line according to the measured force signal in such a way to improve the cutting process quality during the interaction phase. DRC theory and its practical implementation on a 2-axes robot are presented as well as an accurate description of the cutting process. In fact, experimental results validate theoretical predictions.
Size effects on the electrical activation of low-energy implanted B in Si
Authors: Giannazzo F. , Raineri V. , Bruno E. , Mirabella S. , Impellizzeri G. , Priolo F. , Napolitani E.
Published: February 2006

The phenomena related to the electrical activation of low energy implanted B (3keV,2×1014B∕cm2) in laterally submicron confined Si regions were studied by high resolution quantitative scanning capacitance microscopy (SCM). The B diffusion and its precipitation into electrically inactive B–Si interstitial clusters (BICs) were studied by varying the implant window size from 3.2to0.38μm and annealing at 800°C from 12to200min in N2 ambient. In particular, the electrically active B fraction is followed by calculating the carrier concentration profile from SCM data with increasing the annealing time. Both the B reactivation and diffusion exhibit a strong dependence on the window width. The higher electrically active B fraction is always found in the narrowest window, which also first recovers the almost complete electrical activation. The B diffusivity enhancement for the 3.2μm window size is more than one order of magnitude higher than for the 0.38μm window.
Development of plasma-based implant for silicon devices
Authors: Walther Steve , Liebert Reuel
Published: February 2006

Commercial usage of plasma-based implantation for silicon devices has commenced. This represents the culmination of decades of activity at universities, research labs, and finally semiconductor equipment manufacturers. The technique offers great potential for both economic benefits, as a much higher throughput process than traditional implant, as well as enabling new fabrication options for advanced CMOS, nonplanar implant, thin film formation and modification. This review of plasma-based implant for commercial semiconductor fabrication will focus on results thus far, current status, and the prospects for both implant and nonimplant applications. Due to the unique capabilities of plasma-based implantation for shallow depth and high dose, some discussion of metrology, diagnostics, and the process control needed for a manufacturing technology will be included.
Effect of fluorine on the activation and diffusion behavior of boron implanted preamorphized silicon
Authors: Paul S. , Lerch W. , Colombeau B. , Cowern N. E. B. , Cristiano F. , Boninelli S. , Bolze D.
Published: February 2006

In this study we investigated the effect of position and dose of a separate fluorine coimplant on the activation and diffusion behavior of germanium preamorphized boron implants. Germanium preamorphized silicon was implanted with boron, and fluorine was subsequently implanted with different energies and doses to place it either at the projected range of the boron implant, or between the boron profile and the amorphous-crystalline interface, or at this interface. The wafers were spike annealed at temperatures ranging from 950to1050°C. In terms of sheet resistance it was found that the superposition of B and F profiles leads to decreased activation compared to the wafers without any F implant. Increased boron activation is seen for all the other cases with the biggest effects for the highest fluorine dose. The positioning of F either between the boron projected range and the end of range (EOR) or at the EOR leads to more box-shaped boron profiles with shallower junction depth than the reference wafer.
Ultrashallow profiling using secondary ion mass spectrometry: Estimating junction depth error using mathematical deconvolution
Authors: Yang Ming Hong , Mount Gary , Mowat Ian
Published: February 2006

As implant energies get lower and lower, significant errors can be present in junction depth measurements in secondary ion mass spectrometry (SIMS) ultrashallow depth profiling. Primary beam ion mixing is one of the main sources of errors leading to overestimation of junction depths in SIMS measurements. In this article, we systematically study the correlations between the implant profile trailing edge, junction depth and primary ion beam energy for low energy boron and arsenic implants. Using a mathematical deconvolution model proposed by Yang and Odom [Mater. Res. Soc. Symp. Proc. 669, J4–16 (2001)], we are able to estimate the error of the junction depth and consistently improve the accuracy of junction depth measurements using SIMS.
Quantitative analysis of ultrashallow junction of sub-50nm gate-length transistors: Junction depth, sheet resistance, short channel effects, and transistor performance
Authors: Buh G. H. , Park T. , Yon G. H. , Hong S. J. , Ryoo C. W. , Yoo J. R. , Lee J. W. , Jee Y. J. , Lee J. S. , Jun C. S. , Shin Y. G. , Chung U-ln , Moon J. T.
Published: February 2006

We analyzed causes of dopant loss in ultrashallow junction during complementary metal-oxide-semiconductor (CMOS) fabrication: (1) sputtering-out during ion implantation, (2) wet-etching removal in cleaning process, (3) outdiffusion, and (4) deactivation in post-thermal process. By using low energy electron induced x-ray emission spectrometry and other conventional analytic techniques such as four-point probe (FPP) and SIMS, the dopant losses are quantified. We developed a simple source/drain extension (SDE) sheet-resistance test structure to measure the actual sheet resistance of SDE to avoid sizable error due to FPP probe penetration. By comparing with CMOS electrical data, practical aspect of junction depth and sheet resistance is discussed in terms of short channel effects and on-state current. It is found that deactivation and wet-etching removal of arsenic dopant during cleaning process are the major factors in scaling of n-type transistor, while junction depth is the major one in scaling of p-type transistor.
Dopant loss of ultrashallow junction by wet chemical cleaning
Authors: Buh G. H. , Park T. , Yon G. H. , Kim S. B. , Jee Y. J. , Hong S. J. , Ryoo C. , Yoo J. R. , Lee J. W. , Jun C. S. , Shin Y. G. , Chung U-In , Moon J. T.
Published: February 2006

The loss of the dopant in ultrashallow junction (USJ) by RCA standard clean (SC1) prior to the formation of side-wall spacer is quantified by using transmission electron microscopy (TEM), secondary ion mass spectroscopy, four-point probe, and source/drain extension (SDE) sheet-resistance test structure (SSTS). From the cross-sectional TEM images, the etched depth by one SC1 for n (p)-type SDE was measured to be 1.5nm (0.2nm). From the secondary ion mass spectroscopy profiles, most of the n-type dopant implanted with arsenic at 2keV is expected to be etched-out by four times of SC1 cleaning, while the p-type dopants are immune to SC1 cleaning. We quantified the dopant loss from sheet resistance measurements with the four-point probe and the SSTS. The effect of SC1 cleaning on transistor performance is discussed in terms of on-state current. The dopant loss by SC1 is found to be the most significant factor in process optimization for n-type field effect transistor with USJ.
Noncontact sheet resistance and leakage current mapping for ultra-shallow junctions
Authors: Faifer V. N. , Current M. I. , Wong T. M. H. , Souchkov V. V.
Published: February 2006

Noncontact measurements of p-n junction photo-voltage give direct measures of sheet resistance and leakage current over the entire range of CMOS doping process. Efficient measurement of leakage current on un-patterned monitor wafers enables the practical use of leakage current as a critical process control parameter for ultra-shallow junction (USJ) fabrication. The impact of sub-junction doping and damage is illustrated by theoretical and experimental investigations for the case of USJ formed in CZ test wafers and halo/well profiles. Correlation to other leakage sensitive measurements, surface band-bending under intense light illumination, and diode I–V characteristics is demonstrated. Comparisons to four point probe measurements for leaky and not-leaky USJ are made. The value of combined monitoring and fast mapping of sheet resistance and leakage current for USJ process is emphasized.
Room temperature migration of boron in crystalline silicon during secondary ion mass spectrometry profiling
Authors: Napolitani E. , De Salvador D. , Pesce M. , Carnera A. , Mirabella S. , Priolo F.
Published: February 2006

Recently we have demonstrated that substitutional boron in crystalline silicon can migrate for long distances even at room temperature (RT) and below during secondary ion mass spectrometry (SIMS) profiling. The phenomenon is suppressed after amorphization or by cooling the sample. The above data force to reconsider the observations obtained in the last decades by SIMS in light of possible long-range migration artifacts. Here we show that the use of oxygen flooding (OF) during the analysis enhances the injection of self-interstitials (I) responsible for the migration of B, producing profiles that are more broadened and less accurate than in ultrahigh vacuum. On the other hand, by properly controlling OF, we have obtained new insights on the mechanisms of B diffusion and interaction with intrinsic traps at RT. Moreover, by cooling the sample the migration of B is suppressed even while using OF, allowing measurements of boron deltas in c-Si of unprecedented level of accuracy and sensitivity. Finally, we have clarified the role of the migration phenomena on the profiling of ultra low energy B implants for ultra shallow junctions.
Plasma implanted ultra shallow junction boron depth profiles: Effect of plasma chemistry and sheath conditions
Authors: Walther Steve , Godet Ludovic , Büyüklimanli Temel , Weeman John
Published: February 2006

Plasma based implant systems produce multiple ion species as compared to a beamline implant using a single mass selected ion, such as BF2+. Similarly, plasma implant produces a distribution of ion energies relative to the nearly monoenergetic ions of a beamline implant. To evaluate the significance of these differences, the ultra shallow junction (USJ) dopant depth profiles of these two implant techniques are compared with an advanced secondary ion mass spectrometry (SIMS) profiling technique and a novel diagnostic to measure the “as-implanted” ion mass and energy distributions during the pulse of a plasma implant. The plasma and beamline implants match depth and dose extremely well, consistent with the measured plasma ion characteristics. Additionally, the depth profile effects of plasma pulse timing, relative to the application of the bias voltage pulse, a collisional sheath, and USJ dose loss during device mask removal are explored.
Synthesis of single crystalline silicon nanowires and investigation of their electron field emission
Authors: McClain Devon , Solanki Raj , Dong Lifeng , Jiao Jun
Published: February 2006

A systematic study of the effects of growth parameters on the morphology and field emission performance of silicon nanowires has been undertaken. Single-crystalline silicon nanowires were synthesized by chemical vapor deposition using indium tin oxide-coated glass as a substrate. Morphologies, internal structures and chemical compositions of the resulting nanowires were analyzed using scanning electron microscopy, transmission electron microscopy and energy-dispersive x-ray spectroscopy. The resulting silicon nanowires possessed a highly ordered single crystalline Si core without an external oxide layer and no obvious indium or tin impurities. For silicon nanowires with growth-times of 5, 10, and 20 mins, the turn-on fields were determined to be 7.4, 7.9, and 11.5V∕μm, while the threshold fields were 9.9, 11.8, and 16.9V∕μm, respectively. Field enhancement factors of 540, 270, and 265 were also calculated while peak emission currents in excess of 25μA were observed for nanowires with 5 min growth-times. These results strongly suggest the viability of silicon nanowires as emitters in devices where high emission currents are required.
Fabrication of short-wavelength photonic crystals in wide-band-gap nanocrystalline diamond films
Authors: Baldwin Jeffrey W. , Zalalutdinov Maxim , Feygelson Tatyana , Butler James E. , Houston Brian H.
Published: February 2006

Nanocrystalline diamond films and e-beam patterning techniques have been used to fabricate visible to near-infrared photonic slab crystals (PhCs) with deep submicron feature sizes. Two methods of fabrication, both based on electron-beam lithography, have been explored and are detailed in this Communication. The first method uses direct patterning of flowable oxide as a hard mask for a subsequent highly anisotropic oxygen plasma reactive ion etching of the nanocrystalline diamond film. The second method involves image inversion and employs an organic-inorganic bilayer resist structure that planarizes the surface and provides for a well-controlled undercut. The subsequent metal evaporation and lift-off creates a metal mask with 100nm features demonstrating fine control over edge roughness that is not compromised by the nanocrystalline roughness of the diamond film. Chromium etch mask and oxygen plasma were used to fabricate the diamond PhC. With the proper choice of metal mask and reactive ion etch, this technique can be applied to a wide range of nanocrystalline and polycrystalline films and will enable further scaling into the sub-100nm regime.
Application of flash-assist rapid thermal processing subsequent to low-temperature furnace anneals
Authors: Camillo-Castillo R. A. , Law M. E. , Jones K. S. , Lindsay R. , Maex K. , Pawlak B. J. , McCoy S.
Published: February 2006

The substantial reductions in anneal times, such as in flash-assist rapid thermal processing (fRTP), place considerably more emphasis on the initial condition of the wafer, which may assume a greater role in the dictating diffusion product (Dt). Investigations have been conducted on the effect of low-temperature preanneals prior to fRTP on the extended defect nucleation and evolution and on boron activation. Czochralski grown n-type silicon wafers are preamorphized with 8 Ge+ ions at a constant dose of 1×1015cm−2 and then implanted with 1keV, 1×1015cm−2 B. Low-temperature furnace anneals are performed at 500°C for 30min and the wafers subsequently subjected to flash-assist RTP anneals in the range 1000–1300°C. Four-point probe measurements indicate that the low-temperature anneal results in higher sheet resistance values. Plan-view transmission electron microscopy, secondary-ion-mass spectrometry, and Hall-effect measurements revealed no substantial differences in defect structure, junction depth, or mobility. However, the carrier density was found to be higher for those wafers which were preannealed.
Modeling radiation-induced carbon contamination of extreme ultraviolet optics
Authors: Hollenshead Jeromy , Klebanoff Leonard
Published: February 2006

A comprehensive model of radiation-induced carbon contamination of extreme ultraviolet (EUV) optics is presented. The mathematical model describes the key processes that contribute to the deposition of a carbon film on a multilayer optic when the optic is exposed to EUV radiation in the presence of residual hydrocarbons. These processes include the transport of residual hydrocarbons to the irradiated area, molecular diffusion across the optic surface, and the subsequent dissociation or “cracking” of the hydrocarbon by both direct EUV ionization and secondary electron excitation. Model predictions of carbon growth are compared to measurements taken on optics exposed to EUV in the presence of residual hydrocarbons. Model estimates of hydrocarbon film growth under various conditions of hydrocarbon partial pressures and EUV power demonstrate the sensitivity of film growth to varying operating conditions. Both the model and experimental data indicate that the predominant cause of hydrocarbon dissociation is bond breaking by direct photon absorption, rather than by dissociation processes caused by exposure to secondary electrons. Detailed predictions for carbon deposition for a variety of conditions of EUV power and hydrocarbon pressure are reported. The model successfully predicts that light hydrocarbons (<∼100amu) pose a negligible risk to EUV optics, in general agreement with the experiment. Calculations also predict that modest increases in substrate temperature, on the order of 30°C, will substantially reduce optic contamination by increasing hydrocarbon desorption from the surface. Model investigation of surface diffusion indicates that, while surface diffusion is an important surface phenomenon for light gases, for the heavier hydrocarbons that contribute substantially to contamination, surface diffusion is not an important transport phenomenon.
Dopant profiling in NixSi1−x gates with secondary-ion-mass spectroscopy
Authors: Janssens T. , Pawlak M. A. , Kittl J. A. , Fouchier M. , Lauwers A. , Kottantharayil A. , Vandervorst W.
Published: February 2006

To study the dynamics of dopant redistribution during the silicidation process, accurate secondary-ion-mass spectroscopy (SIMS) profiles of the dopant and matrix elements are needed in partially Ni silicided gates. A point-to-point calibration method was developed to quantify the SIMS profiles, taking into account the strong dependence of the ionization probabilities of As and B on the Ni content. The Ni and Si intensities, which are needed to monitor the variation in matrix composition, are influenced strongly by the used SIMS conditions. The Ni is pushed forward very effectively by an O2+ beam, when low impact angles are used. But also the use of Cs+ beam induces a serious broadening of the Ni profile. To identify the potential artifacts the sample is analyzed from the front and the back side. The use of backside SIMS turns out to be necessary to obtain accurate SIMS profiles of the Ni.
Self-aligned cathodes in recessed geometry for reduced gate currents in nanostructured carbon triodes
Authors: Bae Sungil , Park Kyung Ho , Lee Soonil , Koh Ken Ha
Published: February 2006

We designed and fabricated triode emitters with self-aligned cathodes in recessed geometry to reduce gate currents. Both the self-aligned carbon-nanoparticle and carbon-nanotube cathodes in recessed-cathode triode structure showed substantial gate-current reduction. However, tiny gate currents, a few percent of anode currents, persisted as long as there was gate overhang in triode emitters with recessed-cathode structure, and we were able to reduce gate currents to negligible level only after removing gate overhang. We elucidated the near-complete gate-current reduction of recessed-cathode triodes without gate overhang via two-dimensional electron-trajectory simulation based on numerical calculation of electrostatic potential using the commercial finite-element-method code ANSYS.
Co
Si
x
thermal stability on narrow-width polysilicon resistors
Authors: Chen Yen-Ming , Tu George C. , Wang Ying-Lang , Hwang Gwo-Jen , Lo Cheng-Yao
Published: February 2006

In this study, the thermal stability of polysilicon lines with various widths and different dopant types in 90-nm processes is investigated. The thermal behavior of silicides formed on N+ polyresistors and P+ polyresistors is very different. The worst thermal stability is found on the narrower N+ polyresistors, while an abnormal thermal behavior is observed on P+ polyresistors. This anomalous thermal-stability change with different drawn linewidths of P+ polyresistors is related to the grain-size distribution and the actual polyresistor linewidth. Also, it is interesting to find that the voids formed in P+ polyresistors lead to a stepped increase in sheet resistance.
Superior electrochemical performance of CNx nanotubes using TiSi2 buffer layer on Si substrates
Authors: Fang Wei-Chuan , Huang Jin-Hua , Sun Chia-Liang , Chen Li-Chyong , Papakonstantinou P. , Chen Kuei-Hsien
Published: February 2006

On-chip growth of vertically aligned nitrogen-containing carbon nanotube (CNx NT) arrays was demonstrated. The nanotubes were grown by microwave plasma-enhanced chemical-vapor deposition on different types of silicon substrates (n,p,n+,p+) using a few nanometer thick Fe layer as a catalyst and a Ti buffer layer. The effects of the Ti thickness on the electrochemical (EC) characteristics of the CNx NT arrays were studied. It was found that for a Ti thickness of 20nm, while vertically aligned CNx NTs were produced on all Si substrates, an almost ideal Nerstian behavior was observed only on highly conductive n+ and p+ substrates. As the Ti buffer thickness increased to 200nm, good electrical contacts were established at the bottom end of the CNx NTs and fast electron kinetics were then attainable on all kinds of Si substrates. Nevertheless, the use of thick buffer layers inhibited directional growth. Oxidation treatment of the catalyst Fe layer prior to nanotube growth proved efficient for achieving directional CNx NT formation. Pretreatment of the Ti buffer layer at a temperature of 800°C, leading to the formation of TiSi2, was appropriate for achieving simultaneously enhanced current density and fast electron kinetics comparable to those of CNx NTs on bulk Ti electrodes. The Si-based micro-EC platform established in this work has superior current collection efficiency and is amenable for fundamental EC studies and energy applications.
Experimental characterization of an inductively coupled acetylene/hydrogen plasma for carbon nanofiber synthesis
Authors: Lin Y. Y. , Wei H. W. , Leou K. C. , Lin H. , Tung C. H. , Wei M. T. , Lin C. , Tsai C. H.
Published: February 2006

A plasma-enhanced chemical-vapor deposition process was employed to synthesize carbon nanofibers (CNFs) on glass substrates patterned with Ni catalytic films. At the gas pressure of 20mTorr and the substrate temperature (surface) of ∼500°C, the isolated and vertically aligned carbon nanofibers have been successfully synthesized. This paper reports experimental investigation of plasma properties characterized by the optical emission spectroscopy of the spectral line intensities of the various species such as hydrogen, C2, and CH, as well as the rf characteristics at the biased substrate stage measured by an impedance meter. The measurement results reveal that the C2 density increases with the acetylene/hydrogen flow ratio and the inductively coupled plasma (ICP) source power, as expected. The atomic hydrogen density, however, decreases with the flow ratio but increases with the ICP power. The resulting growth rate of CNFs increases with the C2 density if atomic hydrogen density also increases accordingly, e.g., as the ICP power increases. The trend is reversed if the atomic hydrogen density decreases, due to too much amorphous carbon (a-C) layer formed as a result of oversupply of carbon but not enough atomic hydrogen to remove a-C. The experimental results also show that the etch effect upon the effective removing of the a-C on the surface of catalytic nanoparticles is further enhanced by ion bombardment, e.g., when either the flux (or current) or energy of the ions incident on the substrate surface increases, to give rise to an increase in growth rate. In our ICP reactor, the ion current increases with the ICP power, but it changes little when the bias power is varied. The ion energy increases with the bias power, but it decreases as the ICP power increases while the bias power is fixed. The latter one is because the plasma density increases with the inductively coupled plasma power.