Showing papers by "Paul S. Ho published in 1986"

Electronic states at silicide-silicon interfaces

Abstract: A capacitance-spectroscopy technique based on accurate phase detection has been developed to measure the unoccupied states at silicide-silicon contacts. For Pd and Ni silicides, a dispersed group of states was found to exist in the Si band gap with its peak at a level 0.63--0.65 eV above the valence-band edge. Silicide formation alters their density and distribution to reflect the changes in the structural perfection and barrier height. Observations on the epitaxial ${\mathrm{NiSi}}_{2}$-Si(111) interfaces reveal that the characteristics of these states are controlled by the degree of structural perfection of the interface instead of the specific epitaxy. This seems to be the first correlation of the structural and electronic properties of a silicide-silicon interface.

Atomic structure of the epitaxial Al-Si interface

Abstract: The atomic structure of the Al–Si epitaxial interface has been investigated by high-resolution transmission electron microscopy combined with computer image-simulation techniques. A stable epitaxia...

Interaction of metals with model polymer surfaces: Core level photoemission studies

Abstract: Core level photoemission studies have been performed on the in situ interactions of Cr and Cu on two model systems which are related to the pyromellitic dianhydride (PMDA) and oxydianiline (ODA) parts of PMDA–ODA polymide. The model compound which is related to the PMDA constituent is N,N’‐diphenylpyromellitimide and the model polymer which is related to the ODA constituent is poly(2,6‐dimethylphenylene oxide) (PMPO). Cr evaporation on the PMDA model compound changes the PMDA‐related features of the C 1s spectrum, whereas Cu evaporation has comparatively little effect on the PMDA‐related features. The results are consistent with Cr bonding with the PMDA part of the related model compound, or fracturing of the carbonyl bonds and/or other bonds contained in the PMDA structure. Cr evaporation on PMPO results in a new peak at lower binding energy without appreciable changes in the shape of the original bands. Cu predominantly affects the peak in the C 1s spectrum of PMPO which originates from carbons in ether...

Molecular orbital analysis of the XPS spectra of PMDA-ODA polymide and its polyamic acid precursor

Abstract: The calculated carbon 1s (C1s) core energy-level positions of PMDA-ODA polymide and of its polyamic acid precursor are compared with the level positions inferred from XPS measurements. For the polyamic acid, calculation and experiment both yield a difference of approximately 1 eV between the carboxylic acid and the amide carbonyl C1s level positions. The difference in shape between the main C1s XPS peaks of the polyamic acid and polyimide is shown to be related to the difference in C1s core energy-level shifts of the carbon atoms composing the benzene ring adjacent to the amide or imide group. The planar imide or PMDA structure apparently yields larger core level shifts for these atoms. We have previously designated these atoms as “imide carbon atoms” (C-Im) to distinguish them from the aromatic carbon atoms (C-C) of the ODA part of the polymeric repeat unit. Comparison of the carbonyl XPS band intensities with the main peak intensities for the polyamic acid, as well as for the polyimide, suggests that there is a carbonyl deficiency at the surface of both of these materials.

Enhanced adhesion between metals and polymers

Abstract: A technique is described for increasing the adhesion between metals and organic substrates, where the metals are those which normally only very weakly bond to the substrate. These metals include Ni, Cu, Al, Ag, Au, Ta, Pt, Ir, Rh, Pd, Zn, and Cd. The organic substrates include mylar, polyimides, polyesters, polyethylene, polystyrene, etc. Enhanced adhesion occurs when intermixing between the depositing metal atoms and the substrate is optimized to a depth less than about 1000 angstroms into the substrate. This occurs in a critical substrate temperature range of about (0.6-0.8) T c , where T c is the curing temperature of the substrate. The deposition rate of the metal atoms is chosen such that the arrival rate of the metal atoms at the surface of the substrate is comparable to or less than the rate of diffusion of metal atoms into the substrate. This provides optimum intermixing and maximum adhesion.

A method of metallising an organic substrate so as to achieve improved adhesion of the metal

Abstract: Metal-organic substrate adhesion is improved by irradiating the substrate with low energy reactive ions, electrons, or photons to alter the chemical composition of a surface layer of the substrate to a depth of from about 1 nm to a few tens of nanometers. The energy of the incident reactive ions and electrons can be in the range of about 50 to 2000 eV, while the energy of the incident photons can be in the range of about 0.2 -500 eV. Irradiation of the substrate can occur prior to or during metal deposition. For simultaneous metal deposition/particle irradiation, the arrival rates of the metal atoms and the substrate treatment particles are within a few orders of magnitude of one another. The low energy irradiation can be conducted at room temperature or elevated temperatures.

Schottky barrier heights of epitaxial Ni–silicides on Si(111)

Abstract: The correlation of Schottky barrier height and microstructure has been investigated for three different epitaxial Ni–silicides, type‐A, type‐B NiSi2 and NiSi, on n‐Si(111). All these interfaces can be formed to yield a barrier height of 0.78 eV. This high barrier is obtained only for interfaces with almost perfect structures; otherwise less perfect interfaces formed with improper annealing or impurity incorporation yielded low barrier heights of 0.66 eV. The barrier seems to be primarily controlled by the degree of structural perfection, instead of the stoichiometry and the specific type of epitaxy at the interface. A low interface state density can explain the sensitivity of the barrier height of near‐perfect interfaces on minor amounts of impurities, defects, and structural imperfections.

A method of coating an organic substrate with a metal

Abstract: In order to increase the adhesion between an organic substrate and a metal which normally bonds only weakly to the substrate, the substrate is heated to a temperature range , of about (0.6-0.8) T e , where T e is the curing temperature of the substrate, and then held within that temperature range while metal atoms are deposited on the substrate at such a rate that the arrival rate of the metal atoms at the surface of the substrate is comparable with or less than the rate of diffusion of metal atoms into the substrate. This provides optimum intermixing and maximum adhesion. Suitable metals include Ni, Cu, Al, Ag, Au, Ta, Pt, Ir, Rh, Pd, Zn, and Cd. Suitable organic substrates include mylar, polyimides, polyesters, polyethylene, epoxy and polystyrene. Enhanced adhesion occurs when intermixing between the depositing metal atoms and the substrate is optimized to a depth less than about 1000 angstroms (100 nm) into the substrate.

Measurement of interface states in palladium silicon diodes

Abstract: In this paper we present the results of a forward‐biased capacitance measurement on palladium silicon Schottky diodes. The data are interpreted in terms of the interface state density by taking into account the effect of series resistance and using Shockley–Read–Hall statistics. Exchange of charge between the metal and the interface states is included in the model. For the as‐deposited sample, an effective state is postulated to lie opposite the metal Fermi level with a concentration of 1×1012/cm2. Upon annealing and formation of palladium silicide, the density of states decreases by a factor of 2 and changes occur in the capture and emission time constants.

Temperature Dependence of Stress Relaxation at the Copper/Polyimide Interface

Abstract: A Cu/polyimide thin film couple prepared on a thin quartz reed has been used to study interfacial stress relaxation during thermal cycling between room temperature and 400 °C. The polyimide thickness varies from 0 (no polyimide at all) to 10.5μm while the Cu thickness was fixed at 0.53μm. The average copper film stress has been calculated from the curvature of the quartz reed. The information clarifies the relation between the polyimide thickness and the average Cu film stress. The Cu/polyimide interfacial morphology after thermal cycling has also been examined using the cross sectional TEM technique. The results suggest that the interfacial stress is partially released through the deformation of polyimide near the Cu/polyimide interface.

Grain Growth in Vapor Deposited Aluminium Alloys

Abstract: In a number of vapor deposited aluminium alloys grain growth has been investigated systematically by means of quantitative electron microscopy and found to proceed not by grain boundary migration, but by grain coalescence. Parameters influencing the observed mode of grain growth will be discussed with respect to the formation of microstructures with optimal resistance to electromigration, i.e. microstructures with large grain size, high homogeneity in the grain size distribution as well as a strong texture. Analyses of grain size distribution after annealing indicate a strong retardation in grain growth by the solute in all aluminium alloys except Al(Cu). Relative large grain sizes and very small lognormal standard deviations have been observed in Al-l%Cu as well as ternary Al(Cu,Hf) thin films.