T. L. Alford

Improved conductivity and mechanism of carrier transport in zinc oxide with embedded silver layer

The effects of an embedded silver layer on the electrical and optical properties of zinc oxide (ZnO)/silver (Ag)/zinc oxide (ZnO) layered composite structures on polymer substrates have been investigated. We have engineered transparent conducting oxide structures with greatly improved conductivity. Optical and electrical properties are correlated with Ag thickness. Film thicknesses were determined using Rutherford backscattering spectrometry. Hall effect, four-point probe, and UV-Vis spectrophotometer analyses were used to characterize electrical and optical properties. The results show that carrier concentration, mobility, and conductivity increase with Ag thickness. Increasing Ag thickness from 8to14nm enhances sheet resistance and resistivity by six orders of magnitude. The optical transmittance of the composite structure decreases when compared to a single ZnO layer of comparable thickness. However, a composite with 12nm of Ag provides conductivity and transmittance values that are acceptable for opto...

Enhanced adhesion of copper to dielectrics via titanium and chromium additions and sacrificial reactions

Abstract Ti and Cr as both interposed layers and alloying components were found to enhance copper adhesion to dielectrics. Films deposited on SiO2, phosphosilicate glass (PSG) and boronphosphosilicate glass (BPSG) were annealed in 95%Ar-5%H2 over the temperature range 400–600 °C. The force required to separate films from substrates was measured by scratch testing. Optical and scanning electron microscopies provided detection of substrate exposure. In the Cu Ti and Cu Cr bilayer systems the force decreases with temperature on all substrates, generally exhibiting better adhesion on SiO2 than on PSG or BPSG. In the Cu(Ti) and Cu(Cr) alloy systems the force increases with temperature with less systematic difference among the three substrates. These results correlate well with tape testing. Ti and Cr segregate out of the Cu layer and react both with the dielectrics and with the ambient gases, as observed by Rutherford backscattering and secondary ion mass spectroscopy. These reactions appear to improve adhesion; however, only a small amount of this reaction is required for the enhancement to occur. We surmise that stress in the copper and/or voiding at the Cu-dielectric interface may play a role as well. We observe a correlation between adhesion and the degree of Cu texturing.

Band gap shift in the indium-tin-oxide films on polyethylene napthalate after thermal annealing in air

Indium-tin-oxide (ITO) thin films on polyethylene napthalate (PEN) with high carrier concentration (∼1021∕cm3) have been grown by electron-beam deposition without the introduction of oxygen into the chamber. The electrical properties of the ITO films (such as, carrier concentration, electrical mobility, and resistivity) abruptly changed after annealing in the air atmospheres. In addition, optical transmittance and optical band gap values significantly changed after heat treatment. The optical band gap narrowing behavior is observed in the as-deposited sample because of impurity band and heavy carrier concentration. The influence of annealing in air on the electrical and optical properties of ITO∕PEN samples can be explained by the change in the free electron concentration, which is evaluated in terms of the oxygen content. Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy analyses are used to determine the oxygen content in the film. Hall effect measurements are used to determine...

Hydroxyapatite powders and thin films prepared by a sol-gel technique

The formation of hydroxyapatite (HA) from a sol-gel precursor was studied using X-ray diffraction (XRD). The sol-gel was processed into both powders and thin films. The effect of drying and firing temperatures on the HA phase formation was studied. Thin HA films up to 1 μm thick were deposited on silicon wafers coated either with borophosphate silicate glass (BPSG) or titanium films, dried and then fired. Increasing the drying temperature raised the firing temperature for initial formation of crystalline HA. It was found that at firing temperatures ranging from 300 to 1000°C, the dominant phase in the powders was HA with small amounts of calcium oxide and β-tricalcium phosphate (β-TCP). As the firing temperature increased the amount of CaO and β-TCP increased. At firing temperatures ranging from 300 to 500°C the only observable crystalline phase in the films was HA.

Characterization of the physical and electrical properties of Indium tin oxide on polyethylene napthalate

Indium tin oxide (ITO) thin films, on polyethylene napthalate (PEN) of both good electrical and optical properties were obtained by radio-frequency sputtering. The optoelectronic properties of the ITO films on PEN substrate were evaluated in terms of the oxygen content and the surface morphology. Rutherford backscattering spectrometry analysis was used to determine the oxygen content in the film. Hall-effect measurements were used to evaluate the dependence of electrical properties on oxygen content. The results showed that the resistivity of the ITO film increases with increasing oxygen content. For an oxygen content of 1.6×1018–2.48×1018atoms∕cm2, the resistivity varied from 0.38×10−2to1.86×10−2Ωcm. Typical resistivities were about ∼10−3Ωcm. UV-Vis spectroscopy and atomic force microscopy measurements were used to determine the optical transmittance and surface roughness of ITO films, respectively. Optical transmittances of ∼85% were obtained for the ITO thin films. Our results revealed that substrate r...

Encapsulation of Ag films on SiO2 by Ti reactions using Ag–Ti alloy/bilayer structures and an NH3 ambient

Thin encapsulated silver films have been prepared on oxidized silicon by nitridation of ∼200‐nm‐thick Ag–19 at. % Ti alloy films and Ag(120 nm)/Ti(22 nm) at 300–700 °C in an ammonia ambient. The encapsulation process has been studied in detail by Rutherford backscattering, and scanning Auger and secondary‐ion‐mass spectrometry, which showed that Ti‐nitride and Ti‐oxide‐silicide formation take place at the surface and the Ag–Ti/SiO2 interface, respectively. Four‐point‐probe analysis of the alloy films suggests that the resistivity is controlled by the residual Ti concentration. Resistivity values of ∼4 μΩ cm were measured in encapsulated Ag alloy films with initial low Ti concentrations. The annealed bilayer structure had minimal Ti accumulations in Ag and the resistivity values were comparable to that of the as‐deposited Ag (∼3 μΩ cm).

Advanced silver-based metallization patterning for ULSI applications

Abstract Silver metallization is being investigated for potential use in future integrated circuits. Unlike the proposed copper metallization, Ag thin films can be reactive ion etched at reasonable rates using a CF4 plasma. This etch technology is an atypical ‘dry-etch’ process since the formation of volatile products is not the main removal mechanism. The primary film removal mechanism, however, is the subsequent resist strip process. The effects of process conditions on the etch rate and post-etch surface roughness is also characterized. Our study shows that the silver etch process in the CF4 plasma depends strongly on the reactive neutrals and the removal rate is enhanced significantly by the presence of energetic ions as well.

Reactive ion etch of patterned and blanket silver thin films in Cl2/O2 and O2 glow discharges

Silver (Ag) is being investigated as a potential metal for interconnect technology because it has the lowest bulk resistivity of any metal and higher electromigration resistance than aluminum. Silver dry etch is an important process step in the implementation of this metal as an interconnect. This article demonstrates both blanket and patterned etch of Ag films in a reactive ion-etch reactor using Cl2/O2 and O2 glow discharges. X-ray diffraction and scanning electron microscopy were used to analyze the postetch materials. Stress caused by volume expansion upon the formation of Ag2O, AgO, AgCl, and AgClOx, and possibly the sputtering and formation of volatile products, are also believed to induce the etch. The etch mechanism initiated by stress led to roughness and residues on the postetch surface for a blanket etch, and jagged edges for a line etch. It is not suitable to use the above chemistries for a blanket films etch; but for pattern etch, it showed suitable line formation after resist removal and cle...

Optimization of Nb2O5/Ag/Nb2O5 multilayers as transparent composite electrode on flexible substrate with high figure of merit

Different multilayer structures of Nb2O5/Ag/Nb2O5 have been deposited onto flexible substrates by sputtering at room temperature to develop an indium free transparent composite electrode. The effect of Ag thickness on the electrical and optical properties of the multilayer stack has been studied in accordance with the Ag morphology. The critical thickness of Ag to form a continuous conducting layer is found to be 9.5 nm. A new conduction mechanism has been proposed to describe the conduction before and after the critical thickness. The effective Hall resistivity of the optimized films is as low as 6.44 × 10−5 Ω-cm with a carrier concentration and mobility of 7.4 × 1021 cm−3 and 13.1 cm2 /V-s, respectively, at the critical Ag layer thickness. The multilayer stack has been optimized to obtain a sheet resistance of 7.2 Ω/sq and an average optical transmittance of 86% at 550 nm without any substrate heating or post-annealing process. The Haacke figure of merit (FOM) has been calculated for the films, and the multilayer with a 9.5 nm thick Ag layer has the highest FOM at 31.5 × 10-3 Ω−1, which is one of the highest FOM values reported for TCE deposited at room temperature on a flexible substrate.

High quality transparent TiO2/Ag/TiO2 composite electrode films deposited on flexible substrate at room temperature by sputtering

Multilayer structures of TiO2/Ag/TiO2 have been deposited onto flexible substrates by room temperature sputtering to develop indium-free transparent composite electrodes. The effect of Ag thicknesses on optical and electrical properties and the mechanism of conduction have been discussed. The critical thickness (tc) of Ag mid-layer to form a continuous conducting layer is 9.5 nm and the multilayer has been optimized to obtain a sheet resistance of 5.7 Ω/sq and an average optical transmittance of 90% at 590 nm. The Haacke figure of merit (FOM) for tc has one of the highest FOMs with 61.4 × 10−3 Ω−1/sq.