Area Si Research Articles

Two-dimensional monolithic lead chalcogenide infrared sensor arrays on silicon read-out chips and noise mechanisms

A two-dimensional narrow-gap infrared (IR) focal plane array on an Si substrate where the Si substrate contains the active addressing electronics is described. The array consists of 96 /spl times/ 128 pixels with 75-/spl mu/m pitch and is fabricated in a lead-chalcogenide layer grown epitaxially on the Si read-out chip. The cut-off wavelength is 5.5 /spl mu/m. Each pixel contains a bare Si area, onto which epitaxial growth occurs, and an access transistor. The Si-chips are fabricated in CMOS technology with standard Al-metallization, and the lead chalcogenide layer (PbTe) is grown by molecular beam epitaxy onto the completely processed and tested chips. The photovoltaic IR sensors exhibit mean differential resistances at zero bias Ro of 4 M/spl Omega/ at 95 K, and yield is up to 98%. The sensitivities described by Ro are limited by the density of threading dislocations crossing the active areas of the devices, as each such dislocation causes a shunt resistance on the order of magnitude of 1 G/spl Omega/.

Silicides for 65 nm CMOS and Beyond

AbstractAn overview of silicide development for the 65 nm node and beyond is presented. The scaling behavior of Co based and Ni based silicides to sub-100 nm junctions and sub-40 nm gate lengths was investigated. Co and Co-Ni silicides required a high thermal budget to achieve low diode leakage. Even for lower thermal budgets, the sheet resistance of Co and Co-Ni silicides increased at gate lengths below 40 nm. NiSi had low sheet resistance down to 30 nm gate lengths exhibiting a reverse linewidth effect (sheet resistance decreased with decreasing linewidth), achieved lower contact resistivity than CoSi2 and lower diode leakage for similar sheet resistance values. Bridging issues cannot be ignored for NiSi, in particular for thicker Ni films, higher RTP temperatures and in the presence of Ti. Material issues for the application of NiSi were also investigated. Ni2Si was found to grow with diffusion limited kinetics in the 225-300°C range, with an activation energy of 1.5 eV. Results of the kinetic studies were used to design a two-step RTP process that limited the silicide thickness on small features by a low thermal budget first RTP step, reducing the reverse linewidth effect and avoiding excessive silicidation. In the presence of an interfacial oxide, undesired epitaxial NiSi2 pyramidal grains grew directly at temperatures as low as 310°C on p+ Si. Thermal stability of NiSi was also investigated. We found that the initial mechanism of degradation for thin NiSi films was agglomeration, with activation energies of 2.5-3 eV. The surface after agglomeration remained quite flat with alternating NiSi and exposed Si areas, while the interface roughened significantly. Thick films also degraded initially by agglomeration at low temperatures, but by transformation to NiSi2 at higher temperatures. The addition of Pt improved thermal stability of NiSi films against agglomeration. The Ni/Si-Ge reaction was also studied, finding that the addition of Ge reduced the thermal process window and resulted in a slightly higher resistivity.

Projections from primary somatosensory cortex to the neostriatum: the role of somatotopic continuity in corticostriatal convergence.

We characterized the organization of corticostriatal projections from rodent primary somatosensory cortex (SI), testing the hypothesis that projections from SI areas representing subcomponents of the forelimb exhibit greater neostriatal overlap than projections from areas representing separate body parts. The anterograde tracers Fluoro-Ruby (FR), Alexa Fluor (AF), and biotinylated dextran amine (BDA) were injected into physiologically identified regions of rat SI. Injection locations were confirmed by examining the SI barrel fields and limb representations in tangential sections processed for cytochrome oxidase (CO). Experimental animals were divided into two groups: one group received multiple tracer injections in neighboring SI regions that represent separate body parts (whiskers, forepaw, and hindpaw); the other group received injections in SI areas that represent different components of the forelimb (forepaw, antebrachium, and brachium). The distribution of labeled terminals and their varicosities in the neostriatum and in the thalamus were plotted and quantitatively analyzed. For most animals, tracer overlap in the thalamus was either minimal or completely absent. In the neostriatum, projections from the whisker, forelimb, and hindlimb representations terminated in regions that rarely overlap with each other, while those originating from different parts of the forelimb representation were more likely to terminate in overlapping parts of the neostriatum. To the extent that neostriatal activation depends on corticostriatal convergence, the corticostriatal projections in the sensorimotor channel appeared to be organized so that neostriatal neurons may signal when multiple components of the same body part are activated simultaneously.

Thermal annealing of Si/SiO2 materials: Modification of structural and photoluminescence emission properties

We comparatively study two conventional types of Si/SiO2 materials, Si/SiO2 superlattices (SLs) and Si-rich silica (SiOx) films, prepared with a molecular beam deposition method. Raman scattering, photoluminescence (PL), ultraviolet-visible-infrared absorption, and x-ray photoelectron spectroscopies are employed to characterize the samples. The results show clear parallelism in microstructure and emitting properties of Si/SiO2 SLs and SiOx films. The as-grown material is amorphous, and disordered Si areas are seen in Raman spectra for samples with higher Si contents. Annealing at 1150 °C in nitrogen atmosphere leads to ordering of the Si grains and the typical crystalline size is estimated to be 3–4 nm. For all samples, an annealing-induced increase of PL at ∼1.6 eV is observed, and its resulting position is quite independent of the initial sample architecture. Furthermore, this PL is practically identical for continuous wave and pulsed excitation at 488 nm as well as for pulsed excitation at various wavelengths (266–488 nm), and the order of PL lifetimes is 1–10 μs. No correlation between the crystallite concentration and the PL intensity for the annealed samples is found, and the strongest PL was obtained for two samples with less defined crystallization. The origin of the annealing-induced 1.6 eV PL band is discussed.

Pervasive synchronization of local neural networks in the secondary somatosensory cortex of cats during focal cutaneous stimulation.

Extracellular discharges were recorded from 205 neurons in the secondary somatosensory (SII) cortex of isoflurane-anesthetized cats. Cross-correlation analysis was used to characterize the temporal coordination of SII neurons recorded during cutaneous stimulation with a focal air jet that moved back-and-forth across the distal forelimb. Over two-thirds of the recorded neuron pairs ( n=357) displayed significant levels of synchronized activity during one or both directions of air-jet movement. The probability of detecting correlated activity varied according to the distance separating the neurons. Whereas synchronized responses were observed in 82.3% of the pairs in which the neurons were separated by 200-300 micro m, the incidence of synchronization declined to 52.3% for neurons that were separated by 600-800 micro m. The distance between neurons also had a significant effect on the temporal precision of correlated activity. For neurons that were separated by 200-300 micro m, synchronized responses in the cross-correlograms (CCGs) were characterized by narrow (0.5-1.0 ms) peaks at time zero. For SII neurons that were more widely separated, the peak half-widths were substantially broader and more likely to be displaced from time zero. Analysis of directional sensitivity indicated that only 14.2% of the correlated neurons displayed a directional preference for synchronized activity. By comparison, 63.4% of the neurons displayed a directional preference in their discharge rate. These results indicate that stimulus-induced synchronization is a prominent feature among local populations of SII neurons, but synchronization does not appear to play a critical role in coding the direction of stimulus movement. A comparison of these results with those obtained from similar experiments conducted in primary somatosensory (SI) cortex indicates that neuronal synchronization is more likely in SII cortex. This finding is discussed with respect to the known functional differences between the SI and SII cortical areas.

Activation of somatosensory cortical areas varies with attentional state: an fMRI study

The differing roles of SI and SII areas in the somatosensory system have received relatively little interest in previous research. In the present study fMRI was applied to determine possible changes in activations of these areas as a function of attentional modulation (attending vs. not attending to the stimulation of a finger). The results showed that attention induced larger regional changes, mostly enlargements of activated areas, at SII than at SI. The number of instances where new, emerging activations, not present in the non-attend condition, were observed was larger at SII than at SI. These differential attentional effects indicate that SII areas may have a role in more complex tactile functions such as tactile working memory mechanisms.

Industrially attractive front contact formation methods for mechanically V-textured multicrystalline silicon solar cells

Abstract The formation of the front contact is one major topic for solar cell industry. In addition, for multicrystalline silicon (mc Si) there has been no breakthrough in establishing an efficient and cost effective surface texturisation technology for considerable front surface reflection reduction yet. This paper gives an overview on the integration of the V-texturisation into industrial solar cell processes applying the following economical attractive metallisation methods: screen-printing, roller-printing and buried contact formation. For these metallisation methods cell efficiencies of 16.0–16.3% on BAYSIX mc Si (area 100–156 cm 2 ) have been reached with 4–7% rel. efficiency increase as compared to untextured reference cells. In addition solar cell simulations based on realistic assumptions for the contact geometry and electrical performance are given. Finally, a comparison between solar cell concepts with homogeneous and selective emitter including a mechanical V-texture is presented.

Process requirements for continued scaling of CMOS—the need and prospects for atomic-level manipulation

Since the advent of the Si-based integrated circuit, ever-increasing function has been available at reduced cost and with reduced consumption of power. This revolution has been possible because semiconductor devices have the unique feature that as they become smaller they also become faster, consume less power, become cheaper per circuit, and enable more function per unit area of Si. As the basic device approaches atomic dimensions, it is not clear how far scaling can continue, which current processing technologies lack extendibility, and what innovative process technologies will emerge to take their place. Examination of some of the requirements set forth in the International Technology Roadmap for Semiconductors (ITRS) [1] will expose some of the process modules that are likely to limit scaling. Future work that might be developed to meet the needs of the CMOS roadmap will then be initiated. Among the processes being developed for future needs are a range of selflimited growth and etching reactions as well as other process steps that take advantage of atomic-level control and manipulation to enable new classes of substrate materials. The requirements that drive such a level of control, as well as the progress and prospects for these new techniques, are discussed in this paper.

A Finite Element Analysis of Process-Induced Stress in Scaled-Down MOSFETs. Finite Element Stress Analysis and Evaluation of its Validity by Using Convergent-Beam Electron Diffraction.

The residual stress in thin films composing metal oxide semiconductor field emitter transistors (MOSFETs) was quantified, and a finite element method (FEM) based on the residual stress was proposed to predict stress in a scaled-down MOSFET which consists of a variety of thin films. A numerical analysis of process-induced stress in 130-nm-technology node MOSFETs was also carried out. The strain in the Si area in an actual MOSFET was measured by convergent-beam electron diffraction (CBED) and was compared with the FEM results to verify the validity of the FEM methodology. The FEM analysis results showed that (1) the magnitude of the compressive strain in the channel area increases as the source/drain length is reduced, (2) salicide reduces the magnitude of the compressive strain in the channel area, and (3) depositing tensile-stress SiN film in the sidewall spacer effectively reduces the channel strain. From a comparison of the FEM and CBED results it was found that although differences in the strain values obtained by these two methods are recognized, the results for both methods are in good agreement with regard to the shape of the strain distribution.

Multi-tip cones induced by ion-bombardment

Arrays of multi-tip silicon cones covering large areas of Si(1 0 0) substrates were fabricated using argon ion beam bombardment and nickel as a seeding material. Each cone head split into a number of needle tips with a lateral size of tens of nanometers. The morphology of the cones was studied with scanning electron microscopy (SEM). The transmission electron microscopy (TEM) analysis revealed nickel constituents on the needle tips. In terms of experimental observation the formation mechanism of the multi-tip cones was discussed. Studying the field electron emission properties of fabricated silicon multi-tips showed a turn-on field to be about 25 V/μm.

Influence of Al-doping on electroluminescence of silicon-based films

A series of Al-doped silicon-rich silica (AlSiO) composite films have been prepared by a dual ion beam co-sputtering method with a Al, Si and SiO2 composite target. The content of Al and Si in the films can be adjusted by changing the surface area of Al and Si of the target. Visible electroluminescence (EL) from the samples is found to have only one luminescence band peaked at 510 nm (2.4 eV). Experimental results show that the doping of Al is beneficial to reducing the onset voltage and to increasing the intensity of EL.

Fine-scale organization of SI (area 3b) in the squirrel monkey revealed with intrinsic optical imaging.

Optical imaging of intrinsic cortical activity was used to study the somatotopic map and the representation of pressure, flutter, and vibration in area 3b of the squirrel monkey (Saimiri sciureus) cortex under pentothal or isoflurane anesthesia. The representation of the fingerpads in primary somatosensory cortex was investigated by stimulating the glabrous skin of distal fingerpads (D1-D5) with Teflon probes (3-mm diam) attached through an armature to force feedback-controlled torque motors. Under pentothal anesthesia, intrinsic signal maps in area 3b obtained in response to stimulation (trapezoidal indentation) of individual fingerpads showed focal activations. These activations (ranging from 0.5 to 1.0 mm) were discrete and exhibited minimal overlap between adjacent fingerpad representations. Consistent with previously published maps, a somatotopic representation of the fingerpads was observed with an orderly medial to lateral progression from the D5 to D1 fingerpads. Under isoflurane anesthesia, general topography was still maintained, but the representation of fingerpads on adjacent fingers had higher degrees of overlap than with pentothal anesthesia. Multi- and single-unit recordings in the activation zones confirmed the somatotopic maps. To examine preferential inputs from slowly adapting type I (SA) and rapidly adapting type I (RA) and type II (PC) mechanoreceptors, we applied stimuli consisting of sinusoidal indentations that produce sensations of pressure (1 Hz), flutter (30 Hz), and vibration (200 Hz). Under pentothal anesthesia, activation patterns to these different stimuli were focal and coincided on the cortex. Under isoflurane, activation zones from pressure, flutter, and vibratory stimuli differed in size and shape and often contained multiple foci, although overall topography was maintained. Subtraction and vector maps revealed cortical areas (approximate 250-microm diam) that were preferentially activated by the sensations of pressure, flutter, and vibration. Multi- and single-unit recordings aided in the interpretation of the imaging maps. In conclusion, the cortical signals observed with intrinsic signal optical imaging delineated a somatotopic organization of area 3b and revealed different topographical cortical activation patterns for pressure, flutter, and vibratory stimuli. These patterns were dependent on anesthesia type. Possible relationships of these anesthesia effects to somatosensory cortical plasticity are discussed.

An effect of Si nanoparticles on enhancing Er 3+ electroluminescence in Si-rich SiO 2:Er films

Er-doped Si-rich SiO 2 (SRSO:Er) films have been deposited on n +-Si substrates by the magnetron sputtering technique, and both photoluminescence (PL) and electroluminescence (EL) at 1.54 μm have been observed from the films at room temperature. Dependence of PL and EL intensities on the excess-Si content and annealing temperature has been studied. It is found that proper Si content and annealing temperature can evidently enhance EL intensity. An SRSO:Er film with 20% excess Si (area ratio of the Si target to the whole target) had more intense EL than a SiO 2:Er film without excess Si, both annealed at 800°C, by a factor of 5. This fact clearly demonstrates that energy coupling between Si nanometer particles and Er 3+ ions also exists in the EL process as well as in the PL process. Experimental results also indicate that crystallization is not a prerequisite for NSPs enhancing luminescence in SRSO:Er films. The PL and EL spectra of the SRSO:Er films have much broader full widths at half maximum (FWHM, ∼60 nm) than those of the other Er-doped materials reported. This wide FWHM can perhaps be used in wavelength division multiplexing in optical communication in future.

A New Nucleation-Site-Control Excimer-Laser-Crystallization Method

A new nucleation-site-control method has been proposed for excimer-laser crystallization, where a cross-coupled phase shifter is placed on the sample surface. High thermal conductivity of molten Si flattens its temperature distribution shortly after laser-light irradiation, and, in turn, confines a low-temperature Si region to a very narrow area. Since nucleation can take place only in this narrow area of unmolten Si, not only is the number of nuclei limited to unity, but also the position of a nucleus can be determined. The usefulness of the proposed method is investigated theoretically and experimentally. A single grain as long as 5 µm in diameter was formed at the desired position.

On a dual-polarity on-chip electrostatic discharge protection structure

A novel dual-polarity on-chip electrostatic discharge (ESD) protection structure is designed. The new ESD structure protects IC chips against ESD stressing in the two opposite directions. The ESD structure features symmetric deep-snapback current-voltage (I-V) characteristics, low-impedance active overcurrent discharging path, low holding voltage for overvoltage clamping, fast ESD response of /spl sim/0.18 ns, low leakage (/spl sim/pA), adjustable triggering voltage, and good scalability. It passes 14 KV HBM ESD zapping tests and achieves high ESD-performance-to-Si ratio of /spl sim/80 V//spl mu/m width. The new ESD structure reduces Si areas consumed by ESD protection units and ESD-induced parasitic effects significantly.

Representation of the Ear in Human Primary Somatosensory Cortex

We studied 13 healthy subjects with a multichannel magnetoencephalography (MEG) system to investigate the somatotopic representation of the ear in the primary somatosensory cortex (SI). We stimulated three parts of the left ear: the helix, the lobulus, and the tragus. The somatosensory-evoked magnetic fields (SEFs) were successfully measured in 7 of 13 subjects. Short-latency responses were analyzed using both single dipole and multidipole models (brain electric source analysis, BESA). From the single dipole model, the equivalent current dipole (ECD) following the helix stimulation was estimated to be near the neck area of SI in all the subjects. In the lobulus stimulation, the ECDs were estimated around the neck area of SI in four subjects, in the face area in one subject, and in the deep white matter in two subjects. In the tragus stimulation, the ECDs were estimated around the neck area of SI in three subjects, in the hand area of SI in two subjects, and in the deep white matter in two subjects. When the ECDs were estimated to be located in unlikely sites (hand area and deep white matter), a two-dipole model, (1) the neck area of SI and (2) face area of SI, was found to be the most appropriate. Although this might be a preliminary study due to a relatively small number of subjects, it revealed that receptive fields of some part of the ear, such as the lobulus and tragus, might be present in both the neck and face areas of SI. These findings suggested that the “ear area” of SI has variability between subjects, unlike the other areas of SI, possibly because the ear is located on the border between the neck and face.

Recovery Stage of Polycrystalline-Si Prepared by Excimer Laser Annealing

By irradiating the multi-pulse excimer laser with the energy density smaller than 200 mJ/cm 2 on the amorphous silicon (a-Si), the crystallinity of the Si increases, as increasing the number of the pulse. During the first laser irradiation some part of the melted a-Si becomes the polycrystalline (poly)-Si which corresponds to the nucleus, and after the second irradiation the poly-Si does not melt and the remaining a-Si becomes the poly-Si. The crystal growth of the poly-Si proceeds by the solid phase crystallization (SPC). Crystal growth of poly-Si by excimer laser annealing (ELA) is discussed by considering the recovery stage. This stage is examined from the relationship between the amorphous Si area and the total irradiation time. The fact that the measured data coincides with the theoretical data indicates that the recovery proceeds during the ELA at the low energy density.

Morphological properties of nociceptive and non-nociceptive neurons in primary somatic cerebral cortex (SI) of cat

With the techniques of intracellular recording and labelling, we investigated pain sensation and modulation of the somatic cortical cortex at the neuron’s level. After observing the evoked potentials from stimulating the saphenous nerves (SN) of 654 neurons in SI area of the cats, we labelled 30 of the neurons with Neurobiotin to preserve the distribution and the morphologic characteristics of the neurons in the cortex. Based on the tridimensional reconstruction in addition to the eletrophysiological functions, we found clear morphological distinctions between nociceptive and non-nociceptive neurons (P<0.01). This result provided new experimental material to illustrate the function of nociceptive neurons in somatosensory cortex (SI) and presented further evidence to support the “specificity theory” of pain sensation in terms of morphology.

Magnetic source imaging of tactile input shows task-independent attention effects in SII.

We investigated whether attention to different stimulus attributes (location, intensity) has different effects on the activity of the secondary (SII) somatosensory cortex. Tactile stimuli were applied to the left index finger and somatosensory evoked fields (SEFs) were recorded using a whole-head magnetoencephalography (MEG) system. Two oddball paradigms with stimuli varying in location or intensity were performed in an ignore and an attend condition. Brain sources were estimated by magnetic source imaging. No attention effect was observed for the primary SI area. However, attention enhanced SII activity bilaterally from 55 to 130 ms by 52% in the spatial and 64% in the intensity discrimination task. SII attentional enhancement was very similar in both paradigms and occurred both for deviants and standards.

Composition of trench sidewalls and bottoms for SiO2-masked Si(100) etched in Cl2 plasmas

We have investigated the possible role of redeposition of silicon–chloride etching products on profile evolution by studying the influence of etching product partial pressure on the surface layer formed during chlorine plasma etching of SiO2-masked p-type Si(100). Samples were etched with high and low etching product (SiCly)-to-etchant (Cl, Cl2) concentration ratios by changing the Cl2 flow rate (1.4 or 10.0 sccm, respectively) at a constant pressure of 4 mTorr. Compositional analysis was performed using angle-resolved x-ray photoelectron spectroscopy (XPS). Electron shadowing and differential charging of the insulating SiO2 regions were exploited to spatially resolve the composition of the trench sidewalls and bottoms (2.0, 1.0, 0.5, 0.3, and 0.22 μm wide). Chlorine content and stoichiometry of the etched surfaces were determined by quantifying the XPS intensities of both the Cl(2p) peak and the silicon chloride containing tail of the Si(2p) peak. Comparisons of chlorine content and stoichiometry were also made to unmasked Si areas etched on the same samples. For trenches etched with 10 sccm Cl2, the chlorine coverage (2.6×1015 Cl/cm2, equivalent to ∼3 monolayers) and the silicon chloride stoichiometry (SiCl:SiCl2:SiCl3=1:0.45:0.33) were identical for the unmasked Si areas and the bottoms of the trenches. The trench sidewalls, however, contained roughly 50% less Cl than the unmasked areas, all in the form of SiCl. Virtually identical results were obtained for trenches etched with 1.4 sccm Cl2, indicating that increased SiCly etching product concentrations do not result in the formation of a thick, passivating sidewall layer on trench sidewalls during Cl2 plasma etching of Si masked with SiO2.