Abstract
Atomic layer deposition (ALD) of alumina using trimethylaluminum (TMA) has technological importance in microelectronics. This process has demonstrated a high potential in applications of protective coatings on Cu surfaces for control of diffusion of Cu in Cu2S films in photovoltaic devices and sintering of Cu-based nanoparticles in liquid phase hydrogenation reactions. With this motivation in mind, the reaction between TMA and oxygen was investigated on Cu(111) and Cu2O/Cu(111) surfaces. TMA did not adsorb on the Cu(111) surface, a result consistent with density functional theory (DFT) calculations predicting that TMA adsorption and decomposition are thermodynamically unfavorable on pure Cu(111). On the other hand, TMA readily adsorbed on the Cu2O/Cu(111) surface at 473 K resulting in the reduction of some surface Cu1+ to metallic copper (Cu0) and the formation of a copper aluminate, most likely CuAlO2. The reaction is limited by the amount of surface oxygen. After the first TMA half-cycle on Cu2O/Cu(111), two-dimensional (2D) islands of the aluminate were observed on the surface by scanning tunneling microscopy (STM). According to DFT calculations, TMA decomposed completely on Cu2O/Cu(111). High-resolution electron energy loss spectroscopy (HREELS) was used to distinguish between tetrahedrally (Altet) and octahedrally (Aloct) coordinated Al3+ in surface adlayers. TMA dosing produced an aluminum oxide film, which contained more octahedrally coordinated Al3+ (Altet/Aloct HREELS peak area ratio ≈ 0.3) than did dosing O2 (Altet/Aloct HREELS peak area ratio ≈ 0.5). After the first ALD cycle, TMA reacted with both Cu2O and aluminum oxide surfaces in the absence of hydroxyl groups until film closure by the fourth ALD cycle. Then, TMA continued to react with surface Al–O, forming stoichiometric Al2O3. O2 half-cycles at 623 K were more effective for carbon removal than O2 half-cycles at 473 K or water half-cycles at 623 K. The growth rate was approximately 3–4 Å/cycle for TMA+O2 ALD (O2 half-cycles at 623 K). No preferential growth of Al2O3 on the steps of Cu(111) was observed. According to STM, Al2O3 grows homogeneously on Cu(111) terraces.
Highlights
Copper is widely used for a variety of applications including water heat exchangers,[1] interconnect and gate electrodes for microelectronics,[2,3] and heterogeneous catalysts for reactions including low temperature water−gas shift (WGS)[4] and methanol steam reforming.[5,6] the use of copper in these applications is limited by corrosion in oxidative environments,[1,7] diffusion into adjacent layers in microelectronics,[2,8] and particle sintering and leaching in Cu-based catalysts.[6]
The Al 2s peak showed the same trend. This BE shift is difficult to explain by the transformation of aluminum hydroxides to aluminum oxide and back: in 0.1 mbar H2O, more hydroxide is expected than following TMA exposure, so a higher Al 2p BE under 0.1 mbar H2O than after TMA exposure was expected, but the opposite trend was observed
To investigate possible alternative mechanisms of TMA interaction with Cu surfaces, we excluded the source of OH groups (H2O) and other possible contaminants in the in situ cell by studying TMA+O2 atomic layer deposition (ALD) under ultrahigh vacuum (UHV) conditions
Summary
Copper is widely used for a variety of applications including water heat exchangers,[1] interconnect and gate electrodes for microelectronics,[2,3] and heterogeneous catalysts for reactions including low temperature water−gas shift (WGS)[4] and methanol steam reforming.[5,6] the use of copper in these applications is limited by corrosion in oxidative environments,[1,7] diffusion into adjacent layers in microelectronics,[2,8] and particle sintering and leaching in Cu-based catalysts.[6] Recently, atomic layer deposition (ALD) of alumina using trimethylaluminum (TMA) has been introduced to form protective coatings on Cu surfaces that prevent corrosion in pohxiodtaotvivoeltaeinc v(irPoVn)mednetvsi,c1e,7s,8diaffnudsiosinnteorfinCguofinCCu-ub2aSsedfilmnasnoinparticles in liquid phase hydrogenation reactions.[6,9,10]. The reaction chamber is purged by inert gas or vacuum. TMA is the most widely used ALD precursor for growth of aluminum oxide films, and water is one of the most common coreactants (see, for instance, reference 12 and references therein)
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