Matthew H. Ervin

Atom Counting at Surfaces

Multiphoton resonance ionization has been combined with energetic ion bombardment to examine dopant concentrations ofindium on the surface of silicon. The results yield a linear relation between the indium concentration and the known bulk values and a detection limit of 9 parts per trillion, at a mass resolution exceeding 160. This measurement, which surpasses the limits of any previous surface analysis by a factor of 100, has been made possible with an experimental configuration that optimizes sampling and detection efficiency while reducing background noise to virtually zero. During the analysis, submonolayer quantities of the surface are removed, so that as few as 180 surface atoms may be counted.

Annealing of electron beam induced deposits of platinum from Pt(PF3)4

Electron beam induced deposition (EBID) is of interest as a damage-free and resist-less means of incorporating nonconventional materials such as polymer fibers, nanowires, and carbon nanotubes into integrated circuits. A novel inorganic platinum precursor—Pt(PF3)4—is investigated in this work, as it has previously been demonstrated to deposit Pt with resistivities close to bulk Pt when deposited using high dose rates. In this work, the effects of 100 and 200 °C anneals on these Pt deposits are examined. Annealing these deposits is observed to shrink the deposits and decrease their P content. As a result, the measured Pt resistivities decreased with annealing by factors of 1.6–9.9.

Electronic and nuclear effects in ion-induced desorption from NaCl {100}

Multiphoton resonance ionization (MPRI) spectroscopy has been employed to investigate the ejection mechanisms of neutral and ionic particles from an ion‐bombarded NaCl{100} single crystal. The results are used to reveal the similarities and the differences between ion bombardment and electron irradiation of alkali halides. The mass spectra of neutral species and positive and negative ions have been measured. The yield of Na+ ions is found to be two orders of magnitude higher than in measurements with electron bombardment. It is suggested that the secondary ions are created by direct emission from the collision cascade. The ejection of neutral Na atoms is observed to be very sensitive to the temperature of the target, the angle of incidence, and the state of the surface as determined by the time‐of‐flight (TOF) measurements. In particular, it is found that most of the neutral Na atoms are emitted with thermal energies, which indicates that desorption via electronic transitions dominates over ejections from...

Approach to optimizing n-SiC Ohmic contacts by replacing the original contacts with a second metal

Nickel (Ni) contacts to n-type silicon carbide (n-SiC) have good electrical properties, but the physical contact, and therefore the reliability, is poor. An approach is described for using the good electrical properties of Ni Ohmic contacts, while using another metal for its superior topological, mechanical, thermal, or chemical properties for the chosen application. In the present work, we show that once the Ni contact has been annealed, forming nickel silicides, it can be etched off and replaced by a second metal chosen for its desired properties. This second metal displays an as-deposited contact resistance as low as the original annealed Ni contact, indicating that the critical feature responsible for Ohmic contact formation has not been removed when the silicide phases were etched off. Not only does this approach provide more flexibility for optimizing the contact for a given application but it also provides some insight into the Ohmic contact formation mechanism.

Multiphoton resonance ionization of molecules desorbed from surfaces by ion beams

It is known that high molecular weight, thermally labile molecules can be desorbed intact using keV ion beams. This knowledge has led to numerous applications of fast atom bombardment (FAB) and secondary ion mass spectrometry (SIMS) by mass spectrometric detection of the desorbed ions. Here we show that these measurements can be significantly enhanced by using resonance enhanced laser ionization to softly ionize the neutral component of the desorbed flux. This experimental configuration can produce sensitivity improvements of several orders of magnitude over SIMS while adding a certain degree of selectivity to the ionization process itself. Examples of this performance will be presented using a wide variety of molecules including aromatic hydrocarbons, a number of biologically relevant compounds and organic polymer substrates. In some cases, detection limits in the attomole range can be achieved.

Energy and angular distributions of particles desorbed from surfaces using multiphoton resonance ionization detection

Multiphoton resonance excitation has been employed to ionize neutral atoms and molecules desorbed from surfaces bombarded by 5 keV Ar+ ion beams. By positioning a laser beam above the target it is possible to image the ions onto a microchannelplate detector and to obtain energy and angle-resolved distributions. These distributions may be compared directly to classical dynamics computer simulations of the ion/impact event. Results are presented using the (001), (111) and (331) crystal faces of Rh to illustrate how the distributions contain specific structural information. Moreover, we compare the distributions from (111) to secondary ion angular distributions. This comparison suggests that there are special impact points which lead to ion formation from clean metal surfaces. Finally, we present preliminary measurements for the MPRI of pyrene molecules desorbed from polycrystalline gold surfaces. The results suggest this technique may be valuable for monitoring reaction intermediates present at very low concentration on catalyst surfaces.