Peter M. Hoffmann

Nucleation and Growth of Copper on TiN from Pyrophosphate Solution

In this paper we report on the electrochemical deposition of copper on TiN from pyrophosphate solution. We show that deposition occurs through the instantaneous nucleation of hemispherical clusters followed by diffusion-limited growth over a wide potential range. In this potential regime, the nucleus density increases exponentially with applied potential. The nucleus densities obtained from atomic force microscopy images are in the range of 10 8 -10 11 cm -2 , about two orders of magnitude larger than the highest nucleus density observed in fluoroborate solutions. Such high nucleus densities are essential in depositing continuous thin films because coalescence occurs at relatively low coverages. The near-neighhor distances follow Poisson distributions indicating that the nuclei are randomly distributed over the surface.

Growth Kinetics for Copper Deposition on Si(100) from Pyrophosphate Solution

Pyrophosphate solutions are commonly used in the electronics industry for copper deposition on printed circuit boards. This paper reports on the growth kinetics for copper deposition on silicon from pyrophosphate solution. It is shown that complexation shifts the energetic position of the copper ions in solution with respect to the bandedges for silicon such that deposition occurs via the conduction band. It is shown that the growth kinetics for low Cu(II) concentrations are consistent with progressive nucleation of hemispherical clusters followed by diffusion-limited growth.

Quantitative atom-resolved force gradient imaging using noncontact atomic force microscopy

~Received 4 April 2001; accepted for publication 11 June 2001! Quantitative force gradient images are obtained using a sub-angstrom amplitude, off-resonance lever oscillation method during scanning tunneling microscopy imaging. We report the direct observation of short-range bonds, and the measured short-range force interaction agrees well in magnitude and length scale with theoretical predictions for single bonds. Atomic resolution is shown to be associated with the presence of a prominent short-range contribution to the total force interaction. It is shown that the background longer-range interaction, whose relative magnitude depends on the tip structure, has a significant effect on the contrast observed at the atomic scale.

Direct measurement of interatomic force gradients using an ultra-low-amplitude atomic force microscope

Interatomic force gradients between a W tip and a 7×7 reconstructed Si(111) surface were measured using an off–resonance, ultra–low–amplitude atomic force microscope (AFM) technique. The amplitudes used were less than 1 Å (peak–to–peak), which allowed direct measurement of the interaction force gradients as a function of separation. The force gradient curves are shown to consist of an attractive van der Waals part and short–range attractive and repulsive interactions. The van der Waals background can be subtracted, leaving a short–range interaction with an energy parameter of 1.9–3.4 eV and an interaction length–scale of 0.54–1.26 Å, characteristic of a single atomic bond. This correlates well with our observation of single–atom resolved force gradient images. In general, the interaction is reversible up to the zero intercept of the force gradient (inflection point of the energy). Beyond this point hysteresis tends to be observed and the onset of inelastic deformation can be clearly discerned. An analysis of the atomic scale contact gives reasonable values for the interfacial energy, yield strength, and the energy per atom needed to initiate plastic deformation.

Electrical properties of n-type (111) Si in aqueous K4Fe(CN)6 solution : I. Interface states and recombination impedance

The properties of n-type (111) Si surfaces in aqueous 0.1 M K 4 Fe(CN) 6 + 0.5 M KCl at pH 9 were studied both in the dark and under illumination. In this solution, oxide passivation of the surface gives rise to a variety of electrically active interface states. Both in the dark and under illumination, interface states interacting with the conduction band are present at an energy of about 0.36 eV below the conduction bandedge, giving rise to a characteristic impedance at potentials close to the flatband potential. Furthermore, interface states which act as recombination centers are observed when the surface is illuminated. The density of recombination centers was found to be a function of the light intensity, ranging from 1 x 10 12 to 3 x 10 12 cm -2 , indicating that these states are related to oxidation intermediates. The rate constant for recombination was determined to be about 3 x 10 -9 cm 3 s -1 .

Electrochemical etching of n-type silicon in fluoride solutions

The chemistry of silicon surfaces in aqueous fluoride solutions is complex and exhibits many unique features. The etch rate of the hydrogen-passivated surface passes through a maximum at about pH 7 and is slow at low and high pH. The density of etching intermediates exhibits a sigmoidal behavior of low density in acidic and high density in basic solutions. We show that the pH dependence of both the etch rate and the density of intermediates can be explained by taking into account the fluoride ion concentration, and the concentrations of molecular HF, the dimer (HF) 2 , HF - 2 , as well as OH - and H 2 O.

Analysis of the impedance response due to surface states at the semiconductor/solution interface

Electronic surface states at semiconductor/solution interfaces can mediate processes such as trapping and detrapping of majority and minority charge carriers, recombination, or charge transfer to or from the solution. We have calculated the complete impedance response due to these processes using a kinetic approach. Specific cases are discussed and diagnostic parameters for the capacitance and conductance are presented. Experimental results on n-Si(111) in fluoride solutions are used to illustrate the obtained expressions.

cw carbon monoxide laser with rf excitation in the supersonic flow

A cw CO laser with rf discharge transverse to the supersonic flow is described. Using segmented electrodes an electric power density of 33 W/cm3 or 0.15 eV/CO molecule is deposited in a homogeneous arc‐free plasma at 7 Torr. At a laser power of 60 W the emission occurs predominantly on the V=5→4 and 4→3 transitions.

Performance of N2/CO2 gasdynamic mixing lasers with various injection techniques

The supersonic mixing is the only technique that can exploit the full potential of a thermally excited gasdynamic N2/CO2 laser. A screen‐nozzle mixing technique is compared with two other near‐throat mixing techniques and is found to be the superior method. Averaged values of the small‐signal gain up to 3% cm−1 and specific energies of 10 kJ/kg total mass flow have been measured in a laser channel of 3.5×1.2‐cm2 cross section. Characteristics of both gain and extracted power as a function of the nitrogen stagnation temperature and the CO2 mole fraction are discussed for all three types of CO2 injection.

Laser‐induced fluorescence and emission spectroscopic study of magnetic field effects in a low‐pressure etch plasma

The influence of a magnetic volume field B on the chemical and physical properties of a 13.56‐MHz CF4 plasma was studied by laser‐induced fluorescence (LIF) and by optical emission spectroscopy (OES) at typical low‐pressure etching conditions. The measurements were carried out in a commercial magnetron with a modified magnetic field configuration allowing a continuous variation of B up to a maximum field strength of 80 G. As indicated by LIF the densities of the CF2 radicals in the electronic and vibrational ground state increased by about a factor of 3 when the maximum B field was applied. No concentration gradients were detected by spatially resolved LIF. A similar increase as for CF2 was observed for actinometrically normalized F emission intensities. The ratios of CF2 LIF and CF2 OES signals are compared with the emission intensity behavior of argon, admixed to a small percentage, which suggest an increase of the electron induced production rates (electron densities) significantly stronger than that o...