Pulsed Nd:YAG laser deposition of ruthenium thin films
Ruthenium (Ru) is one of the noble air-stable transition metals, which has excellent thermal chemical stability, low electrical resistivity, and relatively high work function near the valence band edge of Si. Recently, Ru has been introduced into the semiconductor industries as a result of the interesting chemical, physical, and electrical properties it possessed. So far, investigations of ruthenium films have been centered on material properties of Ru layers, growth using direct current/radiofrequency (DC/RF) magnetron sputtering, and chemical vapor deposition. However, comparatively little work has been carried out using the pulsed laser deposition (PLD) technique.
In this research work, the growth of Ru film using PLD was investigated. The Ru films were deposited on silicon (Si) substrates employing 355 nm pulsed Nd:YAG laser source. Laser fluence ranged from 2 to 8 J/cm2 was employed, with deposition duration from 5 to 180 minutes under high vacuum condition. Optical emission spectroscopy (OES) was employed to study the species and purity of the plasma during the deposition. It was observed that intensity of the Ru species spectra increased with increasing laser fluence and more prominent after laser fluence of 4 J/cm2. No impurities were observed. Film thicknesses ranging from 15 to 280 nm were obtained. As the deposition duration and the laser fluence increased, the thickness of the deposited Ru films increased. It is observed that there was a critical deposition duration value, and this value increases as the laser fluence increased. X-ray diffraction (XRD) spectra showed Ru with crystalline orientation of (101), (100), and (002) peaks. The XRD results revealed an enhanced diffraction peak when film thickness increased, under all laser fluences. Grain sizes were deduced from the XRD data by using the Scherrer's formula and the values fall in the range of 20 to 35 nm for the film thickness covering from 50 nm to 250 nm. Besides, the electrical properties of the Ru films were examined. Decreasing trend of resistivity was observed on the deposited Ru film with film thickness below 100 nm, while the increasing trend was observed above 100 nm Ru film. The lowest resistivity of the Ru thin film was found to be around 33 μΩ-cm. Droplets formation was detected on the surface of the Ru film through scanning electron microscopy (SEM) images. It was attributed to the effect of laser fluence modification on the Ru target surface during the deposition. The amount of droplets increased significantly above 4 J/cm 2. In this research work, the experimental threshold laser fluence is found to be around 4 J/cm2 where prominent observations such as increasing Ru I species and increasing droplets were obtained above this threshold. It is correlated well to the calculated theoretical threshold laser fluence which states that significant material removal and luminous plasma plume are expected.
0794: Materials science