In-Situ Stress Measurement during Heat Treatment of Amorphous Tantalum Oxide

Abstract

Heat treatment is known to improve the electrical and mechanical properties of amorphous tantalum oxide (ATO) in tantalum capacitor anodes. In-situ, multi-beam optical stress sensor (MOSS) measurements were performed on the ATO / tantalum system during heat-treatment in air and vacuum. Thermal desorption mass spectroscopy was also used to monitor species desorbing from the oxide during heating in vacuum. In addition, scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the heat-treated samples. The results suggest that the ATO/Ta system undergoes changes as a function of annealing temperature and atmosphere. At temperatures below ∼400 • C, the main effects are oxide dehydration and densification, indicated by a net compressive stress. After heat-treatment at temperatures above 400 • C in air, the net stress becomes increasingly compressive with increasing temperature. Oxide growth due to oxygen uptake is the likely cause for the compressive stress. In vacuum, the net stress remains tensile even after heat-treating at high temperatures above 400 • C. Dehydration and the formation of a high density of oxygen vacancies at the metal-oxide interface during heat-treatment are the likely causes for the net tensile stress-thickness. Tantalum capacitors are widely used in various electrical systems.1-3 Amorphous tantalum oxide (ATO) serves as the dielectric. The use of tantalum capacitors in high reliability applications generated intensive research on the electrical properties of the ATO, specifically on phenomena related to dielectric breakdown in high electric fields.3-7 Oxide defects such as interstitial oxygen ions and oxygen vacancies can significantly influence electrical and mechanical properties of ATO. [8][9][10] Heat treatment below the crystallization temperature of ATO is known to improve its electrical properties. 11-15However, some of the details of the oxide transformation and structural change of ATO during heat treating below its crystallization temperature remain unclear.14, [16][17][18][19][20] Temperature and atmosphere are the main parameters controlling the oxide transformation. Specifically, it is known that heating the as-grown, amorphous ATO to high temperatures around will result in the formation of crystallization oxide. 13,21,22 Phase transformations and reactions in solids are typically accompanied by volume changes that can induce stresses when the material is subjected to a mechanical constraint. In Ta capacitors, this occurs because the ATO dielectric is constrained by the underlying Ta metal. In-situ stress measurements have been previously employed to explore the details of phase transformations and reactions in thin solid films. 3,18,[23][24][25] In our recent work, this approach has been used to study the amorphous-to-crystalline phase transition in ATO under high electrical fields.3,26 For example, compressive stress of 0.06 GPa was generated in the ATO along with morphological changes that lead to cracking in the dielectric. 26 In the present stud...