Platinum Hillocks in Pt/Ti Film Stacks Deposited on Thermally Oxidized Si Substrate

Abstract

The stress dependence of platinum hillock formation during post thermal cycling was investigated in Pt/Ti electrode stacks. Annealing temperatures were varied from room temperature (RT) to 650 • C. High compressive stress was generated during electrode annealing by the Ti diffusion into the platinum layer followed by oxidation in the platinum grain boundaries. The compressive stress was the major driving force for the hillock formation on the platinum surface. Thus, the Ti glue layer was oxidized before platinum deposition to reduce the Ti diffusion. The Pt/TiO x electrode stack retained its smooth platinum surface after the electrode annealing of 650 • C for 30 min in O 2. The Pt/TiO x interface remained flat even after the ferroelectric annealing at 800 • C, which was performed after SrBi 2 Ta 2 O 9 (SBT) deposition. Moreover, the remanent polarization (2P r) of the SBT capacitor was increased to 17 µC/cm 2 on the Pt/TiO x electrode stack, up from 13 µC/cm 2 , which was the value on the Pt/Ti electrode stack.

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

“…Previously, a similar but much stronger density decrease of the Pt layer in Pt/ Ti bilayer electrode upon the annealing was reported and attributed to the strong diffusion of Ti and its following oxidation. 21,22 Though the diffusion at the Pt/ TiO x interface was not strong 6,11 or even undetectable as reported, 18 the limited diffusion and incorporation of Ti and O into Pt have been observed previously [23][24][25][26] and confirmed by scanning transmission electron microscopy. 16 Ti and O can migrate along the grain boundaries of Pt when the sample is annealed.…”

“…[8][9][10] For Pt films deposited using dc sputtering at low temperatures, a compressive stress is generally present. 10,[18][19][20] The stress is relaxed under high-temperature treatments through the formation of Pt hillocks and becomes tensile upon cooling down to room temperature because of the thermal strain. 8,19,20 That is the case for the unannealed Pt/ TiO x electrode in the present study.…”

Bottom electrode crystallization of PZT thin films for ferroelectric capacitors

“…As previously reported, Pt films made at low temperatures are under compressive stress, 12,15 whereas the ones deposited at high temperature are under tensile stress after cooling due to the larger thermal expansion coefficient for Pt compared to the one for the substrate (8.8 × 10 −6 and 2.6 × 10 −6 K −1 at room temperature, respectively). 16 For the deposition conditions used in this study, Pt films made at 200 • C should be under compressive or low tensile stress whereas the ones deposited at 500 • C are under tensile stress. This behavior is independent of the type of adhesion layer due to the big difference in thickness between the Pt and Ti/Zr films (200 and 30 nm, respectively).…”

Annealing effect of platinum-based electrodes on physical properties of PZT thin films

“…However, this method tends to have major instability problems, such as Pt hillock formation [4,5]. Pt hillocks are a major concern because they can lead to capacitor failure [6]. In addition, there is also a possibility that inter-diffusion between each layer creates a Pt-Ti compound, which can alter the crystal structure of the bottom electrode, affecting the orientation and crystal structure of the ferroelectric materials [7,8].…”