Hillock Formation in Platinum Films

Abstract

Hillocks, surface protrusions from thin metal films, have been observed in Al, Al/Cu, Pb, and other materials. Platinum films are widely used as substrates for the deposition of ferroelectric thin films because of their superior oxidation resistance. However, hillock formation in platinum films has not been reported in the literature. In this work, we report the appearance of hillocks in platinum in Pt/Ti bilayers on oxidized silicon wafers. Platinum films 250 to 300 nra were deposited by ion beam sputter deposition at 25°C and 300°C onto a 70 nm Ti film on oxidized Si wafers. The wafers were then heated in flowing argon to 600°C, held 1 hr at 600°C, and cooled to room temperature while the wafer curvature (and hence the film stress) was measured with a laser beam deflection technique. At 600°C, compressive stresses of 0.1 to 0.4 GPa, due to thermal expansion mismatch, developed in the metal films. The platinum surface, initially flat, showed strong hillocking after the anneal. Cross-sectional TEM revealed that severe Ti/Pt interdiffusion occurred, in one case leading to a Ti layer on the top surface.

Analysis of the stress and interfacial reactions in Pt/Ti/SiO2/Si for use with ferroelectric thin films

“…29 The magnitude of the stresses calculated is neither unusual nor high for thin films. 26,28,35 Here, some interaction, namely new phase formation, was noted at a lowest temperature studied, 490°. The porous microstructure found in the evaporated films, due to atomic self-shadowing and low adatom mobility 31 typically has tensile stresses which have been attributed to the attractive forces between columns at grain boundaries or less dense regions.…”

“…Problems which have been experienced with Pt multi-layered substrates include poor adhesion, 25 hillocking 26,27 and oxidation of the Pt surface through the formation of surface TiO x . Problems which have been experienced with Pt multi-layered substrates include poor adhesion, 25 hillocking 26,27 and oxidation of the Pt surface through the formation of surface TiO x .…”

“…26 Oxygen is available from the underlying SiO 2 layer, overlying ferroelectric layer and ambient atmosphere. 26 Oxygen is available from the underlying SiO 2 layer, overlying ferroelectric layer and ambient atmosphere.…”

“…26 Pt films deposited by ion beam sputtering and subsequently furnace annealed at 500-600° formed hillocks readily. In general, hillocking can relieve compressive stress in thin metal films at high temperatures.…”

Structural investigations of Pt∕TiOx electrode stacks for ferroelectric thin film devices

“…Previously, a thin intermediate layer of Ti was often introduced to improve the poor adhesion ability of Pt on SiO 2 . However, the Pt/ Ti metallic electrode shows poor thermal stability and reliability including the formation of Pt hillocks [5][6][7][8] or bumps under thermal conditions, 9 surface morphology evolution during aging, 7 short circuit of ferroelectric capacitors caused by the Pt hillocks, 10 severe diffusion of Ti into Pt, and even encapsulation of Pt hillocks with TiO x during annealing. 6,[9][10][11] In contrast, Sreenivas et al reported that the Pt/ TiO x electrode structure shows an improved thermal stability.…”