As the physical size of devices and interconnects on integrated circuits shrinks further and further, the aspect ratio of the metal interconnects increases. The aspect ratio is the via depth divided by the via diameter. While anisotropic etching using high-density plasmas is now commonly practiced to create the via, refilling the deep, narrow interconnect vias with barrier layers and metals remains a substantial technical challenge.

One approach to lining the vias with barrier layers and refilling the vias with metals is Ionized Physical Vapor Deposition (I-PVD), also known as Ionized Metal Plasma (IMP). Metal atoms are ionized in an intense plasma, then can be directed by electric fields perpendicular to the wafer surface. This directed flux of metal is necessary to ensure that the metal species reach the bottom of deep vias.

The figure above is a cross-sectional sketch of an I-PVD plasma reactor. The inside diameter of the reactor is 450 mm. Metal atoms are introduced into the plasma by sputtering from the target at the top of the reactor. A high density plasma is generated in the central volume of the reactor by an Inductively Coupled Plasma (ICP) source. This electron density is sufficient to ionize approximately 80% of the metal atoms incident at the wafer surface. The ions from the plasma are accelerated and collimated at the surface of the wafer by the plasma sheath. The sheath is a region of intense electric field which is directed toward the wafer surface. The field strength is controlled by applying a radio frequency bias to the wafer chuck, as shown. 

The evolution of the ionization process in I-PVD has been studied in the Plasma Engineering Lab. In the plot above, the density of aluminum atoms, aluminum ions, and ion fraction are shown as a function of distance from the source of aluminum atoms. The degree of ionization has been determined by optical emission spectroscopy and by a novel, deposition-based technique which takes advantage of enhanced ion transport through the plasma presheath. The ionization fraction of the flux incident on a wafer's surface is also shown at the bottom of the figure. Here one may observe that over 80% of the arriving aluminum is ionized.

The system has also been modeled by solving the plasma diffusion equations for ions and neutrals. These models are plotted as solid lines on the figure. The agreement between the measured and modeled ionization is seen to be quite good.

The conclusion of this study is that a high degree of ionization is due to constant volume generation of aluminum ions throughout the inductively coupled plasma. The aluminum neutrals, however, are generated only at the target surface. The density of neutral species will decay in the downstream region since their source is confined to the top of the reactor. The result of a more-or-less constant supply of ions and a diminishing density of neutrals is a high ionization fraction in the vicinity of the wafer.

A SEM image of a 1 um wide trench that is lined with titanium using I-PVD

A more detailed description of these experiments is published in the Journal of Vacuum Science and Technology A, 15(4), 2307 (1997).

This research is funded by the National Science Foundation and the Department of Energy under Grant No. DMR-9712988