Edward V. Barnat

Manipulating Thermal Conductance at Metal−Graphene Contacts via Chemical Functionalization

Graphene-based devices have garnered tremendous attention due to the unique physical properties arising from this purely two-dimensional carbon sheet leading to tremendous efficiency in the transport of thermal carriers (i.e., phonons). However, it is necessary for this two-dimensional material to be able to efficiently transport heat into the surrounding 3D device architecture in order to fully capitalize on its intrinsic transport capabilities. Therefore, the thermal boundary conductance at graphene interfaces is a critical parameter in the realization of graphene electronics and thermal solutions. In this work, we examine the role of chemical functionalization on the thermal boundary conductance across metal/graphene interfaces. Specifically, we metalize graphene that has been plasma functionalized and then measure the thermal boundary conductance at Al/graphene/SiO(2) contacts with time domain thermoreflectance. The addition of adsorbates to the graphene surfaces are shown to influence the cross plane thermal conductance; this behavior is attributed to changes in the bonding between the metal and the graphene, as both the phonon flux and the vibrational mismatch between the materials are each subject to the interfacial bond strength. These results demonstrate plasma-based functionalization of graphene surfaces is a viable approach to manipulate the thermal boundary conductance.

Frequency dependent plasma characteristics in a capacitively coupled 300 mm wafer plasma processing chamber

Argon plasma characteristics in a dual-frequency, capacitively coupled, 300 mm-wafer plasma processing system were investigated for rf drive frequencies between 10 and 190 MHz. We report spatial and frequency dependent changes in plasma parameters such as line-integrated electron density, ion saturation current, optical emission and argon metastable density. For the conditions investigated, the line-integrated electron density was a nonlinear function of drive frequency at constant rf power. In addition, the spatial distribution of the positive ions changed from uniform to peaked in the centre as the frequency was increased. Spatially resolved optical emission increased with frequency and the relative optical emission at several spectral lines depended on frequency. Argon metastable density and spatial distribution were not a strong function of drive frequency. Metastable temperature was approximately 400 K.

Electric fields in a sheath near a metal-dielectric interface

Spatially resolved electric fields in the sheath region near a metal–dielectric junction were measured in a radio-frequency-driven argon plasma. The fields were determined by observing the Stark shifted transitions to the 13d[3∕2]1 Rydberg state by laser-induced fluorescence-dip spectroscopy. Calibration of the Stark shifts for the 13d[3∕2]1 Rydberg state were experimentally obtained in a separate apparatus. Maps of the electric fields illustrate that the structure of the sheath formed around the junction depended on both the surface material and on the configuration of the surface.Spatially resolved electric fields in the sheath region near a metal–dielectric junction were measured in a radio-frequency-driven argon plasma. The fields were determined by observing the Stark shifted transitions to the 13d[3∕2]1 Rydberg state by laser-induced fluorescence-dip spectroscopy. Calibration of the Stark shifts for the 13d[3∕2]1 Rydberg state were experimentally obtained in a separate apparatus. Maps of the electric fields illustrate that the structure of the sheath formed around the junction depended on both the surface material and on the configuration of the surface.

Spatial and frequency dependence of plasma currents in a 300 mm capacitively coupled plasma reactor

There is much interest in scaling rf-excited capacitively coupled plasma reactors to larger sizes and to higher frequencies. As the size approaches operating wavelength, concerns arise about non-uniformity across the work piece, particularly in light of the well-documented slow-surface-wave phenomenon. We present measurements and calculations of spatial and frequency dependence of rf magnetic fields inside argon plasma in an industrially relevant, 300 mm plasma-processing chamber. The results show distinct differences in the spatial distributions and harmonic content of rf fields in the plasma at the three frequencies studied (13.56, 60 and 176 MHz). Evidence of a slow-wave structure was not apparent. The results suggest that interaction between the plasma and the rf excitation circuit may strongly influence the structures of these magnetic fields and that this interaction is frequency dependent. At the higher frequencies, wave propagation becomes extremely complex; it is controlled by the strong electrical nonlinearity of the sheath and is not explained simply by previous models.

Response of the plasma to the size of an anode electrode biased near the plasma potential

As the size of a positively biased electrode increases, the nature of the interface formed between the electrode and the host plasma undergoes a transition from an electron-rich structure (electron sheath) to an intermediate structure containing both ion and electron rich regions (double layer) and ultimately forms an electron-depleted structure (ion sheath). In this study, measurements are performed to further test how the size of an electron-collecting electrode impacts the plasma discharge the electrode is immersed in. This is accomplished using a segmented disk electrode in which individual segments are individually biased to change the effective surface area of the anode. Measurements of bulk plasma parameters such as the collected current density, plasma potential, electron density, electron temperature and optical emission are made as both the size and the bias placed on the electrode are varied. Abrupt transitions in the plasma parameters resulting from changing the electrode surface area are identified in both argon and helium discharges and are compared to the interface transitions predicted by global current balance [S. D. Baalrud, N. Hershkowitz, and B. Longmier, Phys. Plasmas 14, 042109 (2007)]. While the size-dependent transitions in argon agree, the size-dependent transitions observed in helium systematically occur at lower electrode sizes than those nominally derived from prediction. The discrepancy in helium is anticipated to be caused by the finite size of the interface that increases the effective area offered to the plasma for electron loss to the electrode.

Two-dimensional mapping of electron densities and temperatures using laser-collisional induced fluorescence

We discuss the application of the laser-collision induced fluorescence technique to produce two-dimensional maps of both electron densities and electron temperatures in a helium plasma. A collisional-radiative model is used to describe the evolution of electronic states after laser excitation. We discuss generalizations to the time dependant results that are used to simplify data acquisition and analysis. Calibration of the predictions made by the model is achieved using an cw rf discharge that is periodically perturbed via a high voltage pulse. We then demonstrate the capability of the technique by producing images of electron density and temperature of the sheath region formed around a biased electrode.

Radiofrequency sheath fields above a metal-dielectric interface

Two-dimensional maps of the sheath electric fields formed around a metal-dielectric interface were measured in a radio frequency (rf) argon plasma using laser-induced fluorescence-dip spectroscopy. Experimentally determined Stark shifts of the argon Rydberg 13d[3∕2]1 state were used to quantify the electric fields in the sheath as functions of the rf cycle, voltage, and pressure. Both the structure of the sheath fields and the discharge characteristics in the region above the electrode depend on the discharge conditions and the configuration of the surface. Dissimilar materials placed adjacent to each other result in electric fields with a component parallel to the electrode surface.

Unique structure/properties of chemical vapor deposited parylene E

Parylene E, a low κ polymer thin film, with the approximate composition 69% diethylated and 25% monoethylated poly(p-xylylene), has been chemical vapor deposited via a cyclophane precursor at room temperature. It has a dielectric constant of 2.34±0.03 and dielectric loss of <0.005 at 10 kHz from MIMCAP structures. It is particularly unique compared to the other common parylene polymers. Namely, it is nearly optically isotropic and it is soluble in common laboratory solvents such as methylene chloride, chloroform, and toluene. It is shown to have an optical birefringence of −0.0112 at 634.1 nm and a correspondingly low degree of crystallization after a 150 °C postdeposition anneal. The low degree of crystallization results in a smooth film (10 A rms surface roughness for a 2060 A thin film) with no optical loss (in reflection mode), i.e., an extinction coefficient of zero. These properties for parylene E make it appropriate for use as an optical waveguide and for archival conservation or other applications...

Theory of the electron sheath and presheath

Electron sheaths are commonly found near Langmuir probes collecting the electron saturation current. The common assumption is that the probe collects the random flux of electrons incident on the sheath, which tacitly implies that there is no electron presheath and that the flux collected is due to a velocity space truncation of the electron velocity distribution function (EVDF). This work provides a dedicated theory of electron sheaths, which suggests that they are not so simple. Motivated by EVDFs observed in particle-in-cell (PIC) simulations, a 1D model for the electron sheath and presheath is developed. In the model, under low temperature plasma conditions ( Te≫Ti), an electron pressure gradient accelerates electrons in the presheath to a flow velocity that exceeds the electron thermal speed at the sheath edge. This pressure gradient generates large flow velocities compared to what would be generated by ballistic motion in response to the electric field. It is found that in many situations, under comm...

Radio frequency sheath formation and excitation around a stepped electrode

Plasma and sheath structure around a rf excited stepped electrode is investigated. Laser-induced fluorescence dip spectroscopy is used to spatially resolve sheath fields in an argon discharge while optical emission and laser-induced fluorescence are used to measure the spatial structure of the surrounding discharge for various discharge conditions and step-junction configurations. The presence of the step perturbs the spatial structure of the fields around the step as well as the excitation in the region above the step.