6.3 Precursors for chemical vapor deposition of copper

Introduction

Chemical vapor deposition (CVD) is a process for depositing solid elements and compounds by reactions of gas-phase molecular precursors. Deposition of a majority of the solid elements and a large and ever-growing number of compounds is possible by CVD.

Most metallization for microelectronics today is performed by the physical vapor deposition (PVD) processes of evaporation and sputtering, which are often conceptually and experimentally more straightforward than CVD. However, the increasing importance of CVD is due to a large degree to the advantages that it holds over physical vapor deposition. Foremost among these are the advantages of conformal coverage and selectivity. Sputtering and evaporation are by their nature line-of-sight deposition processes in which the substrate to be coated must be placed directly in front of the PVD source. In contrast, CVD allows any substrate to be coated that is in a region of sufficient precursor partial pressure. This allows the uniform coating of several substrate wafers at once, of both sides of a substrate wafer, or of a substrate of large size and/or complex shape. The PVD techniques clearly will also deposit metal on any surface that is in line of sight. On the other hand, it is possible to deposit selectively on some substrate materials in the presence of others using CVD, because the deposition is controlled by the surface chemistry of the precursor/substrate pair. Thus, it may be possible, for example, to synthesize a CVD precursor that under certain conditions will deposit on metals but not on an insulating material such as SiO ₂ , and to exploit this selectivity, for example, in the fabrication of a very large-scale integrated (VLSI) circuit. It should also be pointed out that, unlike some PVD applications, CVD does not cause radiation damage of the substrate.

Since the 1960s, there has been considerable interest in the application of metal CVD for thin-film deposition for metallization of integrated circuits. Research on the thermal CVD of copper is motivated by the fact that copper has physical properties that may make it superior to either tungsten or aluminum in certain microelectronics applications. The resistivity of copper (1.67 mW.cm) is much lower than that of tungsten (5.6 mW.cm) and significantly lower than that of aluminum (2.7 mW.cm). This immediately suggests that copper could be a superior material for making metal interconnects, especially in devices where relatively long interconnects are required. The electromigration resistance of copper is higher than that of aluminum by four orders of magnitude. Copper has increased resistance to stress-induced voidage due to its higher melting point versus aluminum. There are also reported advantages for copper related device performance such as greater speed and reduced cross talk and smaller RC time constants. On the whole, the combination of superior resistivity and intermediate reliability properties makes copper a promising material for many applications, provide that suitable CVD processes can be devised.