Platinum(100) hillock growth in a Pt/Ti electrode stack for ferroelectric random access memory

Abstract

The Pt hillock in a Pt/Ti electrode stack has been the main concern in ferroelectric random access memory due to the reliability problem. The origin of the hillock formation is the compressive stress, and the main mass transport mechanism for hillock formation is the grain boundary diffusion for thin films with a columnar structure. However, the hillock growth orientation and mechanism have not been reported. In this study, we found that an orientation relationship of Pt(100) hillock //Pt(111) thin film existed between the Pt hillock and the thin film. The Pt hillock was a single crystal having facets with polyatomic steps. From these results, we suggest that the Pt hillock growth mechanism is the layer growth of flat faces, which shapes the hillock into a tetrahedron single crystal. O 9 ͑SBT͒ thin films have many advantages when compared to Pb (Zr x Ti 1Ϫx )O 3 ͑PZT͒ thin films, such as being fatigue free, having a low leakage current, and having stable imprint characteristics for the application of ferroelectric random access memory ͑FeRAM͒. 1,2 In addition, Pt is chemically stable in a high processing temperature, which is necessary for crystallizing the ferroelectric oxide film. However, the adhesion of Pt to substrates such as silicon oxide (SiO x ) is poor. 3 Therefore, a Ti glue layer has, typically, been employed.The Pt/Ti electrode stack deposited by a sputtering method is likely to have a major instability problem in Pt hillock formation, which is reportedly the main cause of capacitor shorts. 4,5 The compressive stress generated in thin film is widely accepted as being responsible for the occurrence of hillocks in metallic thin film. 6,7 In the case of the Pt/Ti electrode stack, the Pt hillock formation is, reportedly, strongly dependent on the magnitude of the extrinsic compressive stress, which is generated by Ti diffusion into the Pt layer followed by oxidation in the Pt grain boundary during electrode annealing. 5 Accordingly, the Pt/TiO x electrode stack, which was substituted for a Ti glue layer, kept the Pt film surface flat, even when annealed at 650°C for 30 min in air atmosphere. 5 The main mass transport mechanism for the growth of the hillock on thin film with a columnar structure is the grain boundary diffusion. 8,9 Although the orientation relationship between the Pt hillock and Pt thin film has not been reported up to now because of the small hillock and the general lack of concern, it may be a critical clue for understanding the hillock growth mechanism. In this study, we found the orientation relationship between the Pt hillock and the thin film to be Pt(100) hillock //Pt(111) thin film , and we made a suggestion on the hillock growth mechanism.Both Pt and Ti films were sputter deposited on a ͗100͘oriented 8 in. silicon substrate, passivated with a thermally grown SiO x layer of 1000 Å thickness. The respective thicknesses of Pt and Ti were 2000 Å ͑sputtering rate 40 Å/s͒ and 200 Å ͑sputtering rate 10 Å/s͒. The details for the fabrication of the Pt/Ti electrode stack can be found in th...

Stress relaxation induced faceted Cu and W particles on the surfaces of Cu–Zr and W thin films

“…2e-o shows the high magnification of several typical Cu particles. Unlike the hillocks observed in other studies [1][2][3][4][5][6][7][8], these Cu particles are very regular, and most of them are polyhedron with clear edges and corners. Among them, the hexagonal prism, the deformed decahedron, the deformed icosahedron and the hexagonal disk can be observed in Fig.…”

“…As well known, residual stress is very common in thin films, and even reaches several GPa, which can usually lead to hillocks on thin films during annealing [1][2][3][4][5][6][7][8]. Copper (Cu) rich hillocks [2] and pure Cu hillocks [9] were observed during the annealing of Cu-Mo composite thin films as well as in the electromigration test on Cu-Sn systems.…”

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

“…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. However, the population and size of the Pt hillocks increase upon the further thermal annealing, which strongly suggests the generation of a compressive stress during the annealing.…”

“…The Pt hillocks with radii typically ranging from 20 to 70 nm for the case at hand are much smaller than those on Pt/ Ti electrode stacks as reported earlier. [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.…”

“…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.…”