Group 11 Precursors for Atomic Layer Deposition: Design and Synthesis of Advanced Precursors Enabled by Investigations in Thermolysis

Abstract

Atomic layer deposition (ALD) of copper seed layers for electrochemical deposition of copper will likely be a step in fabricating future copper interconnects of microelectronic devices. Development of precursors for copper ALD is necessary in order to enable the deposition of suitable seed layers with acceptable properties. Copper(I) amidinate compounds are leading precursor candidates for use in industry. Modifications to their chemical structure was undertaken to improve precursor properties. Copper(I) guanidinates were synthesized and used as single source precursors for copper metal films at 225 °C. Their decomposition mechanism was investigated through solution thermolysis experiments and by unravelling gas phase fragmentation pathways. Evidence for CDI deinsertion occurring in solution was observed by NMR experiments while β-hydrogen elimination was determined as the gas phase pathway by ToF-MS and MI-FTIR experiments. Both pathways were rationalized by DFT calculations. Copper(I) iminopyrrolidinate compounds were specifically designed to block CDI deinsertion and β-hydrogen elimination. As a result, copper(I) tert-butyl-imino-2,2-dimethylpyrrolidinate demonstrated superior thermal stability and adsorbed from the gas phase onto high surface SiO2 at 275 °C with simultaneous loss of its tert-butyl group. Analogous silver(I) and gold(I) compounds were demonstrated to be robust precursors for chemical vapour deposition of metal films at deposition temperatures of 140 °C and 300 °C, respectively. The lack of available monomeric copper(I) amidinate, guanidinate, and iminopyrrolidinate compounds prompted synthetic work to employ N-heterocyclic carbenes and acyclic diamino carbenes as Lewis bases in copper precursors. A large series of monomeric copper(I) hexamethyldisilazide compounds were studied and characterized extensively by thermal gravimetric analysis. Imidazolylidenes were unsuitable for use in copper precursors due to thermal instability. Imidazolinylidenes and formamidinylidenes fashioned several promising precursors demonstrating excellent thermal stability and good volatility. The leading candidate, 1,3-diisopropyl-imidazolin-2-ylidene copper hexamethyldisilazide, had a 1 Torr vapour pressure at 149 °C and could be heated at 130 °C for two weeks without decomposition. Copper metal films were deposited by plasma enhanced ALD at 225°C on SiO2.