A Comparison of the Physical Properties of Cluster-Based and Vacuum-Evaporated Thin Metal Films
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ABSTRACT
The physical properties of a thin metal film are highly dependent on the process by which the film is formed. Using the Multiple Expansion Cluster Source (MECS), it is possible to form an inert gas aerosol containing neutral metal clusters with uniform diameters in the nanometer size range. Cluster-based (CBD) films can be deposited directly from this aerosol or can be formed from colloidal clusters captured from the aerosol. We present TEM data comparing the initial microstructure of gold films formed by these two CBD processes and also by vacuum-evaporation (PVD). Electrical resistivity versus nominal thickness are also presented for the aerosol deposited CBD and the PVD cases. INTRODUCTION
In microelectronics applications thin metal films are mainly used as interconnect lines to connect components on a chip or to connect the chip to the outside world.[l] As the dimensions of components get smaller, so do the dimensions of these thin metal interconnects. This places greater importance on the physical properties of these thin films. The important characteristics these films should have include: high conductivity, good adhesion to both silicon and dielectric materials, and good stability under thermal and electrical stressing.[2,3] Currently, the material most widely used for metal interconnects is aluminum. However at the high electrical current densities ( > 106 A/cm 2) expected for Ultra Large Scale Integration (ULSI) devices, aluminum interconnects have been found to degrade due to the effects of electromigration.[4] Investigations into averting electromigration damage have focused on producing more reliable aluminum films (grain structure and texture), and on looking at other materials which are more stable at high current densities (gold and copper).[5-8] The use of ionized clusters accelerated toward the substrate by means of an electric field for producing interconnects has been studied by Takagi and Yamada.[9,10] Their ionized cluster beam (ICB) deposition process has been used to produce aluminum lines with increased electromigration resistance when compared to conventional vacuum evaporated interconnects. This reliability enhancement is attributed to the ability of the ICB technique to form good ohmic contacts along with a narrow size distribution of grain sizes. The potential for surface damage due to the relatively high energy of the cluster ions is a major drawback of this technique. In the present work, neutral cluster-beam deposition (CBD) is investigated as an alternative metallization technique for microelectronics applications. The physical properties of gold CBD and vacuum-evaporated (PVD) thin films are compared. EXPERIMENTAL
In the CBD process, a stream of neutral metal clusters is produced using a Multiple Expansion Cluster Source (MECS).[ 11] A schematic of this source is shown in Figure 1. The MECS is a gas-aggregation source where metal is evaporated from a crucible placed in a resistively heated carbon oven held at 100-200 Torr by a flow of helium gas. The metal vapor i
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