Galit Levitin

Post Plasma Etch Residue Removal Using CO 2 ­ TMAHCO 3 Mixtures: Comparison of Single-Phase and Two-Phase Mixtures

Photoresist and post etch residue removal were studied as functions of the cleaning-mixture-phase state. Solutions of tetramethylammonium bicarbonate (TMAHCO 3 ) in methanol were used as cosolvents with CO 2 . The phase behavior of this mixture was studied at temperatures between 25 and 70°C and TMAHCO 3 /CH 3 OH mole ratios of 0.127 and 0.02. At a cosolvent flow rate of 0.0262 mol/h, the single-phase mixture was more efficient for photoresist and etch residue removal than the two-phase mixture. Blanket plasma deposited fluorocarbon films were used to investigate the removal mechanism. The removal process was reaction rate limited and depended on the temperature and active species concentrations. This novel cleaning mixture displayed minimal reaction with SiO 2 and Coral films.

Photoelectron spectroscopy studies of plasma-fluorinated epitaxial graphene

Fluorination of graphene has emerged as an attractive approach toward manipulating its physical, chemical, and electronic properties. To this end, we have demonstrated the viability of sulfur hexafluoride plasmas to fluorinate graphene as a safer alternative to the commonly reported techniques of fluorination that include exposures to fluorine and xenon difluoride gas. Incorporation of fluorine moieties on graphene after SF6 plasma-treatment was confirmed by x-ray photoelectron spectroscopy. Modifications in the valence band states of graphene after plasma-treatment were characterized by ultraviolet photoelectron spectroscopy. Increase in work function of plasma-treated graphene demonstrates the ability of plasma-assisted fluorination to modify the electron emission characteristics of graphene. Raman spectroscopy reveals that the majority of carbon atoms in graphene retain their sp2 hybridization after the plasma-treatment.

Solution Activators of Aluminum Electrochemistry in Organic Media

The use of solution activators (dissolved metal salts) to enhance aluminum electrochemical activity in organic media is reported. Aluminum is electrochemically passive in most organic electrolytes and unlike lithium. has not been developed as a battery anode in organic media. We demonstrate significant Al activity in two electrolytes. AlCl 3 in γ-butyrolactone or (C 2 H 5 ) 4 NCl in acetonitrile. These improve the anodic potential by up to I V in comparison to other electrolytes and increase from microampere to milliampere per square centimeter, the sustainable Al oxidation current. Electrolytic metal acetate, chloride, or other salts can further enhance these favorable anodic characteristics. Unlike Sn(II), Sn(IV), or Ga(III), both Hg(II) and In(III) significantly activate Al redox behavior. In I M AlCl 3 /γ-butyrolactone, the Al oxidative polarization is 100, 65. 36, or only 28 Ω cm 2 in electrolytes containing, respectively, 0, 1, 10, 100 mM Hg(II). Hg(II) also shifts negatively the observed E ° Al by 150 to 250 mV, an enhancement sustained during extended anodic galvanostatic discharge. Similarly, as little as I mM In(III) diminishes polarization to 40 Ω cm 2 . Addition of 10 mM In(II) to 0.3 M (C 2 H 5 ) 4 NCl in acetonitrile, improves E ° Al by 0.4 to -1.57 V vs. I 3 -/I.

Fluorinated Graphites as Energetic Cathodes for Nonaqueous Al Batteries

Conventional cathodes utilized in nonaqueous lithium anode batteries, including manganese and vanadium oxides, and molybdenum or titanium sulfides, are incompatible with nonaqueous aluminum anode electrochemical storage processes. Alternative cathodes systems were explored to develop high charge capacity nonaqueous aluminum cells. A series of high storage capacity Al cells, utilizing cathodes composed of CF x fluorinated polymer graphite compounds are demonstrated. Significant CF x cathodic capacities are obtained in aluminum anode cells utilizing a 0.3 M tetraethylammonium chloride, 10 mM Hg(CH 3 COO) 2 acetonitrile electrolyte. The addition of hydroxide to the CF x cathode mixture increases discharge potential, while the addition of nonfluorinated 1 μm graphite increases measured cathodic capacity. The CF x cathode capacity increases with the degree of fluorination from 27 to 35% fluorine, is approximately constant from 35 to 58% fluorine, increases in capacity for 61% fluorine, and increases again with 63% fluorine (the highest available level of 63% fluorine available in the CF x ). Measured cathode capacities exceed 250 mAh g - 1 . For example, in excess of 300 mAh g - 1 , total cathode mass is measured for a 50:50 wt % cathode composite containing 55% fluorinated CF x and half 1 μm nonfluorinated graphite.

Reactions between CO2 and tetramethylammonium hydroxide in cleaning solutions

Supercritical and subcritical CO 2 -based mixtures are being considered as environmentally benign media for photoresist and plasma etch residue removal in electronic device manufacture. Despite many attractive features, supercritical CO 2 has little solvating power for photoresist or inorganic materials. Addition of a basic modifier, such as tetramethylammonium hydroxide (TMAH), to CO 2 can yield a mixture suitable for residue removal. This work investigates chemical reactions between TMAH and CO 2 to gain insight into the cleaning efficiency of subcritical CO 2 mixtures containing TMAH. Results suggest that although tetramethylammonium carbonate/bicarbonate mixtures form, the cleaning ability of TMAH based mixtures in the liquid phase is not significantly affected.

Effect of the polarity of carbon-fluorine bonds on the work function of plasma-fluorinated epitaxial graphene

Work function engineering of graphene facilitates its application as a transparent electrode material in organic electronic devices. Toward this end, we demonstrate the dependence of the work function of plasma-fluorinated epitaxial graphene on the polarity of carbon-fluorine bonds which is controlled by the nature of chemical bonding (ionic, semi-ionic, or covalent) between fluorine and carbon atoms. The work function of fluorinated graphene was measured using ultraviolet photoelectron spectroscopy and the polarity of carbon-fluorine bonds was established using x-ray photoelectron spectroscopy.

Patterning of Cu Films by a Two-Step Plasma Etching Process at Low Temperature

Primarily due to the inability to form volatile etch products for copper (Cu) plasma etching at temperatures <200°C, no effective subtractive etch process for Cu has been reported. We have developed a low temperature (<20°C), reactive plasma etch process for Cu films in an attempt to mitigate the size effect in electrical resistivity, a critical limitation to future integrated circuit device generations. As previously proposed by a thermochemical analysis of the Cu-Cl-H system, the plasma etch process is executed in two steps. In the first step, Cu films are exposed to a Cl 2 plasma to preferentially form CuCl 2 , which is believed to be volatilized as Cu 3 Cl 3 by exposure to a H 2 plasma in the second step. Cu films (100 nm thick) with SiO 2 as a mask have been patterned by this two-step etching process in an inductively coupled plasma system. Experimental parameters such as power applied to the substrate electrode, gas flow rate, and plasma treatment time have been investigated to assess their effect on pattern quality. The addition of argon to the H 2 plasma improved Cu pattern fidelity and gave reasonable etch rates. Possible etch mechanisms for this low temperature process are discussed.

Mechanistic considerations of low temperature hydrogen-based plasma etching of Cu

A simple plasma-based, low temperature etch process is described, which allows subtractive etching of copper (Cu) films and thereby offers an alternative to damascene technology for microelectronic and integrated circuit device fabrication. Hydrogen (H2)-based plasma etching of blanket and SiO2 masked Cu thin films is performed in an inductively coupled plasma reactor at temperatures below room temperature. This process achieves anisotropic Cu features and an etch rate of ∼13 nm/min. Although Ar and He are more efficient sputter gases, Cu etching in these plasma atmospheres displays lower etch rates than those observed with H2 plasmas. Moreover, anisotropy degraded with enhanced ion bombardment due to mask ablation. Cu etch rate and patterning results are consistent with an etch process that involves both chemical and physical characteristics. Specifically, the combination of ultraviolet photon impingement, ion bombardment, and hydrogen interaction with Cu surfaces appears to be responsible for the effici...

Photoresist and Residue Removal Using Gas-Expanded Liquids

Rapid technology development and demands for state-of-the-art generations of integrated circuits bring new challenges to microelectronic device manufacture. Specifically, decreasing dimension sizes may limit the effectiveness of liquid-based removal of photoresist and plasma etch residues. In addition, the hazardous solvents currently in use pose environmental concerns. Gas-expanded liquids (GXLs) and supercritical fluids may offer cleaning or residue removal approaches that overcome some of the drawbacks of current surface preparation methods. Removal of PHOST (polyhydroxystyrene) photoresist films has been demonstrated with CO 2 -expanded ethanol (up to 75 mol % CO 2 ), indicating that the inclusion of CO 2 does not inhibit photoresist removal. In situ interferometry measurements of the PHOST layers allowed insight into the removal mechanism. The GXL mixtures were also invoked to remove post-plasma etch residues using CO 2 -expanded TMAHCO 3 /CH 3 OH. At a temperature of 90°C and pressures above 1000 psig the GXL mixture removed the photoresist and etch residue. Cleaning efficiency depended on CO 2 pressure; at lower pressures the CO 2 reduced the mole fraction of the TMAHCO 3 /CH 3 OH while at higher pressures the CO 2 assisted removal.

Local work function measurements of plasma-fluorinated epitaxial graphene

Plasma-fluorination is an attractive route toward the work function engineering of graphene. The effect of surface topography of epitaxial graphene grown on silicon carbide on the increase in work function after plasma-fluorination was investigated using scanning Kelvin probe microscopy. Results of these studies demonstrate the ability of plasma-treatments to functionalize epitaxial graphene without significant surface roughening. For few-layer epitaxial graphene on the Si-face, work function distribution corresponds to its surface topography. A bimodal distribution is observed before and after fluorination and the separation between the two modes widens after the fluorination. For multi-layer epitaxial graphene on the C-face, no correlation is observed between the work function distribution and the surface topography. After fluorination, the work function is fairly uniform except in few peeled off areas that show a stronger work function contrast.