Structure versus Thermal Stability: The Periodic Structure of Atomic Layer Deposition-Grown Al-Incorporated HfO₂ Films and Its Effects on Amorphous Stabilization

Abstract

Atomic layer deposition (ALD) is used to grow Al-incorporated HfO 2 to study the relationship between the film composition variations in the growth direction and thermal stability of the amorphous phase. Ten nm films with Hf:Al ALD cycle ratios equal to 3:1 and 8:1 remain amorphous up to 900 and 800 °C, respectively, and crystallize after a 30 s annealing at 950 and 850 °C, respectively. Angle-resolved X-ray photoelectron spectroscopy is used to reveal the periodicity of ALDgrown Al-incorporated HfO 2 . Generally, multiple cycles are required to realize a complete layer in ALD. Films with a Hf:Al ALD cycle ratio of g5 have a periodic HfO 2 -HfAl x O y structure, while films with a Hf:Al ALD cycle ratio of <4 appear more like homogeneous films. Increasing the thickness of a homogeneous-like film (Hf:Al = 3:1 ALD cycle ratio) from 10 to 40 nm leads to crystallization when annealed at 900 °C. Amorphous films with a periodic structure (Hf:Al = 8:1, ALD cycle ratio) do not display a dependence of crystallization on film thickness at the same 800 °C annealing condition. The factors controlling the film thermal stability are not only the overall amorphzier incorporation concentration but also how the amorphizer is distributed in the film. Changing the periodic structure of an ALD-grown Al-incorporated HfO 2 film is a potential method to tune the film thermal stability.

Atomic Layer Deposition of Tantalum-Incorporated Hafnium Dioxide: Strategies to Enhance Thermal Stability

“…10 nm [3Hf þ 1Al] and [3Hf þ 1La] films can remain amorphous after 900 C 30 s annealing. Upon increasing film thickness to 40 nm, both [3Hf þ 1Al] and [3Hf þ 1La] crystallize during 900 C annealing for 30 s. 26 The thermal stability of [2Hf þ 1Ta] films, which are more like homogeneous mixtures, show the same thickness dependent characteristic; a 10 nm-thick film is slightly crystallized (Fig. 4b) and a 40 nm-thick film is fully crystallized (not shown) after 800 C RTA processing.…”

“…The overall film thickness for the five samples is 10.1 6 0.4 nm (1 nm interfacial layer excluded). [2Hf þ 1Al] has been found to be more like a homogeneous film by angleresolved XPS analysis, 26 and the local Al concentration in…”

“…The overall film thickness is less important because it is found that the thermal stability of [8Hf þ 1Al] and [6Hf þ 1La] films are independent of film thickness up to 40 nm. 26 With an appropriately designed periodic structure, ALD-grown films may have a stronger thermal stability compared with the homogeneous film with the same incorporated element concentration. This extra thermal stability may disappear when the repeated HfO 2 ultrathin layers are too thick, because thicker HfO 2 ultrathin layer may initiate the crystallization process more easily.…”

“…Detailed explanations about using AR-XPS to verify the existence of periodic structures and the reason why direct methods such transmission electron microscopy (TEM) and XPS depth profile are less likely to work are reported elsewhere. 25,26 To investigate the crystallinity of a sample after annealing, grazing incidence XRD (GIXRD) is performed at a fixed 0.5 incident angle, using a Bruker-AXS D8 Advance Powder Diffractometer with sealed tube Cu Ka radiation. Prior to XRD, samples are annealed under a rapid thermal annealing (RTA) process at different temperatures for 30 s under a N 2 environment.…”

“…We have shown that ALD-grown La and Al-incorporated HfO 2 with low overall La and Al concentrations are composed of repeated HfO 2 -HfLa x O y and HfO 2 -HfAl x O y structures, where ultrathin HfO 2 layers are sandwiched between HfLa x O y or HfAl x O y layers. 25, 26 We propose the film structure, namely the existence of extra HfO 2 -HfM x O y (M ¼ incorporated element) interfaces and ultrathin HfO 2 layers, may add extra influences on the film thermal stability, in addition to the overall amorphizer incorporation concentration.…”