Sywert Brongersma

Influence of surface and grain-boundary scattering on the resistivity of copper in reduced dimensions

We examine the influence of surface and grain-boundary scattering on the total electrical resistivity of copper as dimensions are reduced close to the bulk electron mean free path (39 nm). Through resistivity and grain size characterization on copper wires with sizes down to 95×130 nm2 in a temperature range of 4.2 to 293 K, it was found that the influence of surface scattering is less than previously speculated, while grain-boundary scattering is dominant. A reduction of the background scattering length due to small grains accounts for the observed behavior. The reflection coefficient varies as expected from impurity enrichment in the grain boundaries.

Self-annealing characterization of electroplated copper films

Self-annealing of electro-chemically deposited copper films is described and studied, with a focus on the effect of process parameters like concentration of the organic additives, current density or thickness of plated copper. Sheet resistance and stress have been monitored and a non-correlated behavior has been observed for these two film characteristics, indicating that other phenomena than the grain growth typically associated with the room temperature recrystallization are likely to be involved. Diffusion/desorption of carbon containing molecules incorporated in the copper film from the bath additives could be a mechanism for stress release.

Grain Growth, Stress, and Impurities in Electroplated Copper

The widely observed secondary grain growth in electroplated Copper layers is shown to be incomplete after the sheet resistance and stress of the layer appear to have stabilized. Instead the layer is in an intermediate state with a grain size distribution that depends on the plating conditions. Further extensive annealing at high temperatures results in an additional considerable enlargement of the grain structure, accompanied by an additional decrease of the sheet resistance and desorption of impurities that were incorporated during plating.

Two-step room temperature grain growth in electroplated copper

Electroplated copper exhibits some surprising changes at room temperature in sheet resistance, stress, and microstructure. This behavior, now known as self-annealing, is shown here to be intimately linked to the composition of the plating bath and the resulting incorporation of organic additives in the Cu layer. Their addition is a necessary condition for self-annealing to occur, but slows down the process for higher concentrations. The phenomenon also depends critically on film thickness, showing an accelerated transformation when film thickness increases. This dependence is explained in terms of a very rapid primary crystallization from the top surface down just after deposition, followed by a slower lateral recrystallization producing large secondary grains. The stress and sheet resistance during recrystallization are identified as two noncorrelated variables.

Gas Detection with Vertical InAs Nanowire Arrays

Nanowire-based devices show great promise for next generation (bio)chemical sensors as evidenced by the large volume of high-quality publications. Here, a nanoscale gas sensing device is presented, based on gold-free grown vertical InAs nanowire arrays. The nanowires are contacted Ohmically in their as-grown locations using an air bridge construction, leaving the nanowire surface free for gas adsorption. Noise measurements were performed to determine the measurement resolution for gas detection. These devices are sensitive to NO(2) concentrations well below 100 ppb at room temperature. NO(2) exposure leads to both a reduction in carrier density and electron mobility.

Growth mechanism and continuity of atomic layer deposited TiN films on thermal SiO2

In atomic layer deposition (ALD), film thickness control by counting the number of deposition sequences is poor in the initial, nonlinear growth region. We studied the growth of TiN films formed by sequentially controlled reaction of TiCl4 and NH3 on thermal SiO2 during the transient, nonlinear period. Using low-energy ion scattering and Rutherford backscattering spectroscopy analysis, we have found that a three-dimensional growth of islands characterizes the ALD TiN growth on SiO2. Growth at different temperatures (350 °C and 400 °C) affects the extent of the transient region and the rapid closure of the film. At 400 °C, a reduced growth inhibition and an earlier start of three-dimensional growth of islands results in film closure at about 100 cycles, corresponding to a TiN thickness of 24±3 A. At 350 °C the minimum thickness at which the TiN layer becomes continuous is 34±3 A, deposited with 150 cycles.

Electrochemical sensing of ethylene employing a thin ionic-liquid layer.

We introduce an electrochemical ethylene sensor that employs a thin layer of ionic liquid as electrolyte. Ethylene is oxidized in a potential window starting ∼600 mV before the onset of the gold working electrode oxidation, which inhibits the ethylene oxidation at high applied potential. The current amplitude and sensor response time depend on the ionic-liquid film thickness, relative humidity, and applied potential, in agreement with a theoretical model based on diffusion. A detection limit of 760 ppb and a linear response up to 10 ppm were achieved. As illustrated by the detection of ethylene, ionic liquids could serve as an alternative electrolyte for many electrochemical gas sensors that heretofore relied on a strongly acidic electrolyte.

Analysis of the size effect in electroplated fine copper wires and a realistic assessment to model copper resistivity

The size effect in electroplated copper wires has been widely studied recently. However, there is no consensus on the role of various scattering mechanisms. Therefore, an in-depth analysis to reveal the origin of the size effect is needed. In this article, we study the resistivity of fine copper wires whose feature sizes shrink in two dimensions. It is shown that the residual resistivity (at 5 K) increases with decreasing wire width or height and the temperature-dependent resistivity slightly deviates from that of bulk copper. This is mainly attributed to surface scattering rather than grain boundary scattering. In fact, the influence of grain boundary scattering in these well annealed copper wires is relatively small. In addition, for copper wires with a constant height, a linear dependence of the copper resistivity on 1/width (w) or 1/cross-sectional area (A), namely ρ=ρic+c*∕w (or ρ=ρic+c**∕A), is derived from the classic surface and grain boundary scattering models and validated experimentally. In this simple description, the contributions of different scattering mechanisms, such as surface reflectivity, p, and grain boundary reflection coefficient, R, defect and impurity density, combine together in parameters of ρic and c* (or c**). Especially, c* is a good indicator of scattering strength, from which one can quantitatively analyze the impact of nonsurface scattering contribution with a reference slope of c*=32.14.The size effect in electroplated copper wires has been widely studied recently. However, there is no consensus on the role of various scattering mechanisms. Therefore, an in-depth analysis to reveal the origin of the size effect is needed. In this article, we study the resistivity of fine copper wires whose feature sizes shrink in two dimensions. It is shown that the residual resistivity (at 5 K) increases with decreasing wire width or height and the temperature-dependent resistivity slightly deviates from that of bulk copper. This is mainly attributed to surface scattering rather than grain boundary scattering. In fact, the influence of grain boundary scattering in these well annealed copper wires is relatively small. In addition, for copper wires with a constant height, a linear dependence of the copper resistivity on 1/width (w) or 1/cross-sectional area (A), namely ρ=ρic+c*∕w (or ρ=ρic+c**∕A), is derived from the classic surface and grain boundary scattering models and validated experimentally. In thi...

Surface and grain boundary scattering studied in beveled polycrystalline thin copper films

The inverse relationship between film thickness and electrical resistivity of metallic films is usually studied by depositing a series of films with different thickness and measuring their sheet resistance with a four-point probe. However, the structure and uniformity of polycrystalline thin films typically depend on thickness, rendering it difficult to establish the dominant electron scattering mechanism. In order to circumvent the uniformity issue we now use beveled films to establish the thickness dependent resistivity of thin copper films. Here the resistivity of films down to a few nm’s can be studied without, e.g., a percolation effect. Additionally, the comparison of data obtained on samples where beveling took place either before or after anneal is used to study the impact of grain size on resistivity. It is shown that the normally observed strong increase in resistivity is dominated by grain boundary scattering. However, the influence of surface scattering can be clearly observed when thick films...

Active Control of the Strong Coupling Regime between Porphyrin Excitons and Surface Plasmon Polaritons

We experimentally demonstrate the active control of the coupling strength between porphyrin dyes and surface plasmon polaritons supported by a thin gold layer. This control is externally exerted by a gas flow and is reversible. The hybridized exciton-polariton branches resulting from the exciton-plasmon coupling display a splitting proportional to the coupling strength of the light-matter interaction. The coupled system changes from the weak (no splitting) to the strong coupling regime (splitting of 130 meV) by controlling the effective oscillator strength in the dye layer, via exposure to nitrogen dioxide. The modification of the coupling strength of the system allows tailoring of the dispersion of the hybridized modes as well as of their group velocity.