Pinhole Density Research Articles

Very low dislocation density, resistive GaN films obtained using transition metal nitride interlayers

AbstractA threading dislocation density (TDD) reduction method for GaN films is described. Thin amorphous layers of Sc, Hf, Nb, Zr and Cr were deposited on MOVPE‐grown GaN‐on‐sapphire templates with a TDD of 5 × 109 cm–2 and annealed in NH3 to form metal nitrides. The ScN layer remained continuous, with a very low pinhole density, while the HfN layer contained a high pinhole density of approximately 3 × 109 cm–2. The NbN and ZrN layers formed oriented holey network structures. The Cr layer did not nitride. Coalesced GaN epilayers grown on the ScN layers had the lowest dislocation density of 3 × 107 cm–2 (un‐coalesced GaN on ScN had TDDs as low as 5 × 106 cm–2). Unlike GaN films grown using multiple SiNx interlayers, which contain a similar proportion of edge and mixed dislocations, the GaN‐on‐ScN layers contain substantially fewer mixed than edge dislocations, a proportion similar to that of the high‐TDD template. The low‐TDD GaN epilayers grown on ScN are also highly electrically resistive. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantification of pinhole density in ultrathin diamond‐like carbon films

AbstractThe percentage of uncovered area in ultrathin diamond‐like carbon (DLC) films on Si(001) wafers, which assesses the pinhole density of the carbon films, was quantified through angle‐resolved X‐ray photoelectron spectroscopy. The uncovered areas of the silicon substrate are oxidized upon contact with air, so the photoelectron signal from Si+4 could be used for this quantification. The pinhole density exhibited a strong dependence with the growing conditions of the carbon film, even for films of similar thickness. The detection limit of the technique is set by the photoelectron signal noise level, and by the accuracy in the accounting of the background signal. Copyright © 2007 John Wiley & Sons, Ltd.

Ultra-Thin Ruthenium Films by Pulsed Chemical Vapor Deposition

Ruthenium films have been deposited via a pulsed CVD process using tris(2,2,6,6-tetramethyl-3,5-heptanedionato) ruthenium (i.e., Ru(thd)3) , oxygen, and hydrogen at 280 and 300{degree sign}C. Film resistivity near theoretical values (16 μΩ-cm at 3 nm) was achieved with repeatability. Films were smooth and continuous down to 2 nm; a low density of pinholes could be observed on films with 1 nm thickness. Adhesive tape tests indicated good adhesion. Carbon content is low.

Processing and Characterization of Dry-Etch Benzocyclobutene as Substrate and Packaging Material for Neural Sensors

Electrode substrate is one of the most important factors affecting the recording or stimulation efficiency and long-term stability of chronically implanted neural sensors for laboratory research. Various biocompatible polymers have been investigated as potential substrate and packaging material for neural sensors in neuroscience research applications. Dry-etch benzocyclobutene (BCB) is one candidate due to its desirable combination of electrical, mechanical, and thermal properties and its biocompatibility. In this paper, processing techniques were investigated to control the uniformity and pin-pole density of dry-etch BCB film. Dry-etch BCB film as thick as 25 mum with a surface roughness less than 1000 A and a pinhole density less than 1.5 x 10-3 mm-2 has been acquired using an optimized coating and curing recipe. A traditional surface micro-machining technique was used to form the metallization and etch masks during the fabrication of the neural electrode based on dry-etch BCB substrate. Special consideration was given to the study of dry-etch BCB thin film patterning using plasma reactive ion etching dry etching. The optimized plasma etch condition shows that greater than 1 mum etch rate and 65deg via angle are the most suitable for the packaging and patterning of the neural probes. The results show that this fabrication process is optimal for chronically implantable neural sensors based on dry-etch BCB thin film as substrate. The process may find applications in other devices using BCB as substrate.

Structural and chemical investigation of CdSe crystals deposited in a nanoporous sol–gel: Effect of chemistry and defects on photovoltaic properties

Nanoporous cadmium selenide films were prepared by electro crystallization inside a nanoporous SiO 2 network, prepared by a sol–gel process on an electrically conducting indium tin oxide (ITO) substrate. Subsequent removal of the SiO 2 template should, according to literature reports, give a nanoporous network of CdSe in electronic contact with the substrate. The structure/morphology of the film was explored during the preparative steps by grazing incidence small/wide angle X-ray scattering, scanning electron microscopy and atomic force microscopy. The chemistry was studied by time of flight secondary ion mass spectrometry methodologies, such as imaging and depth profiling that maps the in-plane and in-depth distribution of chemistry, i.e. full 3-dimensional chemical imaging of the nanoporous CdSe layer. Finally, the nanoporous CdSe films were tested as electron conductors in hybrid organic/inorganic solar cells employing CdSe as the electron conductor and poly-3-hexylthiophene as the hole conductor. ITO and gold served as the electrodes. It was found that removal of the SiO 2 from the micellar SiO 2–CdSe nanostructure led to a collapse of the resulting CdSe structure with a poor structural conservation. Furthermore, a high pinhole density in the sol–gel resulted in a high degree of shunting paths that led to poor photovoltaic properties.

Suppression of Leakage Current of CdF2/CaF2 Resonant Tunneling Diode Structures Grown on Si(100) Substrates by Nanoarea Local Epitaxy

CdF2/CaF2 resonant tunneling diode (RTD) structures of 100 nm diameter were fabricated in SiO2 hole arrays formed on Si(100) substrates. RTD structures were grown by molecular beam epitaxy (MBE) in SiO2 hole arrays. After the growth of the initial CaF2 layer at a substrate temperature of 120 °C, in situ annealing at 500 °C was carried out to reduce the density of defects or pinholes in the barrier layer. In the measurement of current–voltage (I–V) characteristics, negative differential resistance (NDR) was clearly demonstrated at room temperature. The peak-to-valley current ratio (PVCR) was 106. The valley current was significantly suppressed. Results obtained here strongly indicates that the RTD structures proposed in this study are promising for high-quality NDR elements compatible with Si LSI technology based on Si(100) substrates.

Size-Selective Voltammetry: Modification of the Interface between Two Immiscible Electrolyte Solutions by Zeolite Y

Size- and charge-selective ion transfer across the zeolite-Y-modified interface between two immiscible electrolyte solutions (ZM-ITIES) is described. The zeolite-Y membrane is prepared from pressed disks by healing with tetraethyl orthosilicate (TEOS). Size- and charge-selective transfer of the tetraethylammonium cation, size-selective exclusion of tetrabutylammonium cation, and charge-selective exclusion of the tetrafluoroborate and perchlorate anions are demonstrated at the ZM-ITIES. The exclusion studies suggest that the membrane is coherent and contains a low density of pinholes, after healing with TEOS. Various factors affecting the ion transfer such as analyte concentration, supporting electrolyte concentration, and scan rate are investigated. The diffusion coefficient of tetraethylammonium ions within the zeolite-Y pores is found to be on the order of 10(-8) cm2 s(-1).

Suppression of pinhole defects in fullerene molecular electron beam resists

Molecular resists, such as fullerenes, are of significant interest for next generation lithographies. They utilize small carbon rich molecules, giving the potential for higher resolution and etch durability, together with lower line width roughness than conventional polymeric resists. The main problem with such materials has historically been low sensitivity, but with the successful implementation of chemical amplification schemes for several of the molecular resist families this is becoming less of a concern. Aside from sensitivity the other main obstacle has been the difficulty of preparing good quality thin films of non-polymeric materials. Here we present a study of pinhole defect density in fullerene films as a function of substrate cleanliness, post-application bake, and incorporation of chemical amplification components. Ultrathin (sub 30 nm) films of the previously studied fullerene resist MF03-01, and the polymeric resist PMMA were prepared on hydrogen terminated silicon by spin coating and the density of pinhole defects in the films was studied using atomic force microscopy. It was seen that pinhole density was strongly affected by the quality of the substrates, with the lowest densities found on films spun on freshly cleaned substrates. Aging of the film subsequent to spin coating was seen to have less effect than similar aging of the substrate prior to spin coating. Additionally, the use of a post-application bake significantly degraded the quality of the films. The addition of an epoxy crosslinker for chemical amplification was found to reduce defect density to very low levels.

Low-temperature electron cyclotron resonance plasma-enhanced chemical-vapor deposition silicon dioxide as gate insulator for polycrystalline silicon thin-film transistors

Silicon dioxide films have been deposited at temperatures below 270°C in an electron cyclotron resonance (ECR) plasma reactor from O2, SiH4, and He gas mixture. Pinhole density analysis as a function of substrate temperature for different microwave powers was carried out. Films deposited at higher microwave power and at room temperature show defect densities (<7pinhole∕mm2), ensuring low-temperature process integration on large area. From Fourier transform infrared analysis and thermal desorption spectrometry we also evaluated very low hydrogen content if compared to conventional rf-plasma-enhanced chemical-vapor-deposited (PECVD) SiO2 deposited at 350°C. Electrical properties have been measured in metal-oxide-semiconductor (MOS) capacitors, depositing SiO2 at RT as gate dielectric; breakdown electric fields >10MV∕cm and charge trapping at fields >6MV∕cm have been evaluated. From the study of interface quality in MOS capacitors, we found that even for low annealing temperature (200°C), it is possible to considerably reduce the interface state density down to 5×1011cm−2eV−1. To fully validate the ECR-PECVD silicon dioxide we fabricated polycrystalline silicon thin-film transistors using RT-deposited SiO2 as gate insulator. Different postdeposition thermal treatments have been studied and good device characteristics were obtained even for annealing temperature as low as 200°C.

An Effective Three-Dimensional Micromagnetic Method and ItsApplication to Magnetic Tunnel Junctions

An effective three-dimensional micromagnetic method for predictingequilibrium configuration of magnetization states is presented withconjugate-gradient technique to minimize the total energy, which has amuch faster convergence rate than that of the quasi-Newton method. Asan application, magnetic tunnel junctions with pinholes in theultrathin barrier are simulated. The calculated coupling field Hfincreases with the pinhole density increase, corresponding to thedecrease of barrier thickness. The free layer domain configuration oftunnel junction during the switching process is also demonstrated.

Technique for Preparing Defect-free Spray Coated Resist Film on Three-Dimensional Micro-Electromechanical Systems

Microstructures with high aspect ratio have been fabricated in micro-electromechanical systems (MEMS). The fundamental structures are relatively simple, since the fabrication process relies on planar photolithography. Spray coating of the photoresist is expected to become a new photolithography method for realizing real three-dimensional microstructures. However, as-deposited resist films have pinholes, particularly in the deep regions. This poses limitations to photolithography. Therefore, a new technique is introduced for removing pinholes by means of the migration of resist material on sample surfaces. Thinner vapor assists the migration of resist, generating the driving force of surface tension. The resist material accumulates in the regions with high pinhole density. Consequently, the pinholes are filled efficiently. This effect comes at the cost of thinning the resist coverage in convex corners of a cavity. The removal of pinholes and the decrease of resist film coverage at convex corners may be balanced.

Investigation of Ga oxide films directly grown on n-type GaN by photoelectrochemical oxidation using He-Cd laser

By using a He-Cd laser in a chemical solution of H3PO4 with a pH value of 3.5, Ga oxide films were directly grown on n-type GaN. From the energy-dispersive spectrometer (EDS) measurement and x-ray diffraction (XRD) measurement, the grown Ga oxide film was identified as (104) α-Ga2O3 structure. A small amount of phosphors existed and bonded with oxygen on the grown films. The as-grown films were amorphous. From the XRD analysis, it is evident that annealing of the α-Ga2O3 films led to a change in the microstructure from an amorphous to a polycrystalline phase. In addition, the as-grown low-density films gradually became dense films during the annealing process. Furthermore, the surface roughness of the annealed films also gradually decreased. Hexagonal pinholes on the grown films were observed. The density of the hexagonal pinholes was similar to the defect density of the n-type GaN. From the cross-sectional transmission electron microscopy (TEM) micrographs, it is evident that the hexagonal pinholes originated from defects in the n-type GaN.

Silicon dioxide deposited by ECR-PECVD for low-temperature Si devices processing

Silicon dioxide films have been deposited at temperatures less than 270 °C in an electron cyclotron resonance (ECR) plasma reactor from a gas phase combination of O 2, SiH 4 and He. The physical characterization of the material was carried out through pinhole density analysis as a function of substrate temperature for different μ-wave power ( E w). Higher E w at room deposition temperature (RT) shows low defects densities (<7 pinhole/mm 2) ensuring low-temperatures process integration on large area. From FTIR analysis and Thermal Desorption Spectroscopy we also evaluated very low hydrogen content if compared to conventional rf-PECVD SiO 2 deposited at 350 °C. Electrical properties have been measured in MOS devices, depositing SiO 2 at RT. No significant charge injection up to fields 6–7 MV/cm and average breakdown electric field >10 MV/cm are observed from ramps I– V. Moreover, from high frequency and quasi-static C– V characteristics we studied interface quality as function of annealing time and annealing temperature in N 2. We found that even for low annealing temperature (200 °C) is possible to reduce considerably the interface state density down to 5 × 10 11 cm −2 eV −1. These results show that a complete low-temperatures process can be achieved for the integration of SiO 2 as gate insulator in polysilicon TFTs on plastic substrates.

トンネル接合膜のＴＭＲ特性に及ぼすＡｌ‐Ｎ絶縁層へのイオン照射の効果

Effect of in-situ ion bombardment to Al-N barrier in magnetic tunnel junctions (MTJ) on their tunnel magnetoresistance (TMR) was investigated in correlation with the local conducting properties of Al-N barrier. Slight ion bombardment increased the barrier height of Al-N from 1.2eV to 1.8∼1.9eV, and resulted in the enhancement of TMR ratio from 34% to 37% in as-made MTJ. On the other hand, strong ion bombardment, longer bombardment duration than 10s, produced high conductive channels (pinholes) in the Al-N barrier, and deteriorated the TMR ratio down to 20% and more. Simple calculation for TMR ratio and resistance-area product of MTJs with using the pinhole density and the local conducting properties, determined from conducting-AFM measurement, well explained the experimental features.

Studies of Ga diffusion and the elimination of pinholes in Ga-doped β-FeSi2 films prepared by MBE

Our previous studies on the Ga doping of β-FeSi 2 films proved that Ga atoms could be an effective p-type dopant by using the molecular beam epitaxy (MBE) method. However, a crucial problem was the generation of a great number of pinholes when a high concentration of Ga atoms was introduced into the β-FeSi 2 films. In this report, we perform a qualitative investigation into Ga diffusion in β-FeSi 2 layers with secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES). We found that Ga atoms diffuse much faster in the β-FeSi 2 layers than in Si, and they tend to congregate on the front surface of β-FeSi 2 films during the post thermal annealing at 800 °C. However, the Ga is supposed to evaporate from the surface. Based on these results, we assumed that the formation of pinholes may be closely related to the Ga diffusion and evaporation in β-FeSi 2 layers during the annealing process. In order to suppress the evaporation of Ga, we deposited an SiO 2 capping layer (∼500 nm in thickness) terminating on Ga-doped Fe/Si multilayers prior to the annealing. Preliminary results demonstrated that pinhole density was reduced by the SiO 2 capping layer.

A correlation study between the conformation of the 1,4-dithiane SAM on gold and its performance to assess the heterogeneous electron-transfer reactions

A gold electrode spontaneously modified by 1,4-dithiane (1,4-dt) organosulfur species has been characterized by cyclic voltammetry of [Fe(CN) 6] 3−/4− and cytochrome c (cyt c) probe molecules, SERS (surface-enhanced Raman scattering), and EIS (electrochemical impedance spectroscopy), using [Fe(CN) 6] 3−/4− as the redox-active molecule in solution. The cyclic voltammograms showed a decrease in the faradaic current response with an increase of the immersion time of the gold substrate into the 1,4-dt modifier solution, suggesting that the heterogeneous electron-transfer (hET) process of the [Fe(CN) 6] 3−/4− probe molecule occurs through the surface pinholes. This observation is an indication that the 1,4-dt chemisorption mechanism must involve a slow step that is responsible for the final configuration of the modifier molecules on the surface. The SERS spectra acquired for the surface as a function of immersion time of the gold electrode in the modifier solution presented an intensity enhancement of the bands assigned to the trans and gauche conformation of the 1,4-dt on the surface. The apparent rate constant, k app, estimated by the EIS results, decreases with increase of the immersion time indicating that the pinholes on the surface act as a microarray. This result is consistent with those observed by cyclic voltammetry of the probe molecules, i.e., the pinhole density decreases with increase of the immersion time of the gold electrode in the 1,4-dt modifier solution, and a strong attenuation of the probe molecules interfacial electron-transfer response is observed.

Microstructure and properties of ultrathin amorphous silicon nitride protective coating

The effect of N content on the structure and properties of rf reactively sputtered amorphous silicon nitride (a-SiNx) has been studied by Rutherford backscattering spectrometry, x-ray reflectivity, ellipsometry, and nano-indentation. The N content in the film increased with the N2 concentration in the sputtering gas until the Si3N4 stoichiometry was reached. The hardness of a-SiNx increased with density, which in turn increased with the N content. The maximum hardness of 25 GPa and density of 3.2 g/cm3 were attained at the stoichiometric Si3N4 composition. With the application of a protective overcoat for magnetic disks in mind, thin a-SiNx films were deposited on CoPtCr media to examine their coverage, pinhole density, and wear resistance. According to x-ray photoelectron spectroscopy, the minimum thickness of a-SiNx required to protect the CoPtCr alloy from oxidation was 10 Å, which was 10 Å thinner than that of the reference amorphous nitrogenated carbon (a-CNx). A statistic model showed this lower thickness required for a-SiNx can be attributed to its high density, which corresponds to 93% bulk density of Si3N4. Compared with 45 Å a-CNx coated disks, 15 Å a-SiNx coated disks had lower pinhole defect density and superior wear resistance.

Hot-spot mediated current-induced resistance change in magnetic tunnel junctions

Experimental results on the current-induced resistance change in low-resistance (10-60 /spl Omega//spl middot//spl mu/m/sup 2/) magnetic tunnel junctions with AlO/sub x/ barriers are presented. The observed current-induced resistance change remains unaltered even when the ferromagnetic electrodes are saturated by an external field of 8.5 kOe and, therefore, is not related to the relative magnetization configuration of both electrodes. The critical switching current increases with decreasing barrier thickness (increasing hot-spot density), as well as with increasing junction area. Repeated switching (4000 pulses) leads to junction resistance decrease by 3%-4% and a decrease of the relative current-induced resistance change of about 20%, indicating an increase of hot spots or pinhole density upon current stressing. Critical current increases as current pulsewidth decreases.

Fabrication of device-grade silicon-on-insulator material from appropriate matches of low oxygen implantation dose and acceleration energy

We report the formation of high-quality silicon-on-insulator materials using separation-by-implantation-of-oxygen (SIMOX) technology, with doses ranging from 1.8 to 13.5×1017 cm−2 at acceleration energies of 45–160 keV, and subsequently annealed at a high temperature of over 1300 °C in an oxygen and argon atmosphere for 5 h. The microstructure evolution of SIMOX wafers was characterized by Rutherford backscattering spectroscopy, transmission electron microscopy, modified enhanced Secco, Cu plating, and spectroscopic ellipsometry. The study reveals a series of good matches of dose–energy combination at acceleration energies of 45–160 keV with doses of 1.8–5.5×1017 cm−2, in which the SIMOX wafers have good crystallinity of the top silicon, a sharp Si/SiO2 interface, and a high-integrity buried oxide layer with a low pinhole density and few detectable silicon islands. Furthermore, the higher the oxygen dose, the higher the implanted energy required for the formation of a Si-island-free buried oxide layer. The mechanism of an appropriate dose–energy match is discussed.

Pinhole density measurements of barriers deposited on low- k films

Evaluation of diffusion barrier integrity is an important issue in advanced interconnects. A diffusion barrier separating Cu from low- k must be as thin as possible and must not contain pinholes. We have developed a method for measuring pinhole density in diffusion barriers deposited on low- k materials. The method employs ellipsometric porosimetry for measuring diffusion of toluene in a porous low- k film beneath the barrier in question.