An Antiferroelectric Zirconia MOS Capacitor on Degenerately Doped Silicon for Memory Applications
Author(s)
Gaskell, Anthony Arthur
Advisor(s)
Editor(s)
Collections
Supplementary to:
Permanent Link
Abstract
Antiferroelectric oxides are functional materials that undergo a first-order phase transition between a non-polar parent phase and two polar ferroelectric phases under applied
electric fields of the respective polarity. While antiferroelectricity was first observed in lead-zirconate (PbZrO3), it was recently been discovered in the complementary metaloxide-
semiconductor (CMOS) compatible binary oxide zirconia (ZrO2). ZrO2 has already been investigated extensively as a high- gate dielectric in CMOS logic devices and dynamic random access memory (DRAM), but foundries are careful to maintain the amorphous
phase to prevent the high variations in film properties associated with the random
orientation of grains in polycrystalline films, especially in ultra-scaled technology nodes.
The first experiment in this work showed that when a 10 nm ZrO2 thin film was deposited
using low temperature (below 250 C) atomic layer deposition (ALD), and encapsulated
in titanium nitride (TiN) electrodes in a metal-insulator-metal (MIM) configuration,
a minimum annealing temperature of 350 C in a nitrogen atmosphere was required to obtain the antiferroelectric polarization-electric field (P-E) response. A higher annealing temperature led to a larger peak switching current in the response, which glancing incidence
x-ray diffraction (GI-XRD) and scanning transmission electron microscopy (STEM) revealed
was correlated to larger mean grain sizes in the ZrO2 film. It was therefore inferred that while the majority of the material in the thin film had been transformed to the nonpolar
tetragonal phase during the anneal, grains smaller than a specific threshold were not able to undergo the first order phase transformation between the parent phase and the polar
orthorhombic phases. Deposition experiments also showed that if the 10 nm ZrO2 thin film was deposited
at 400 C with only a bottom TiN metallic layer, it nonetheless crystallized in the parent
tetragonal phase. This negated the need for a top metallic capping electrode and postmetallization
anneal. The research in this work also considered the effects of alloying a 10 nm ZrO2 MIM film film with lanthanum oxide (La2O3) by alternating deposition cycles. P-E measurements
showed that the typical antiferroelectric was still discernible, albeit with an increase in the critical electric field magnitudes required to induce the phase transformation, until a La cationic concentration of 3.5 % was reached. Beyond this concentration, the characteristic double hysteresis loops of the antiferroelectric ZrO2 thin film were no longer observable, and the x-ray diffraction (XRD) Bragg’s peaks of the tetragonal phase, which is responsible for antiferroelectric behavior, decreased in intensity and broadened. The resulting film was found to show a decrease in overall crystallinity, leading to an amorphous linear dielectric.
These results were in agreement with the decreased nano-crystallite size modeled from the results. The La2O3 acting upon the tetragonal ZrO2 thin film like a bi-axial tensile strain, thereby decreasing its unit cell tetragonality, was concluded to be the underlying mechanism for the suppression of the antiferroelectric phase.
Finally, a heavily doped MOS capacitor with a crystallized 10 nm thick ZrO2 oxide film and 20 °A SiO2 interfacial layer was fabricated and measured to determine whether its the polarization-voltage (P-V) response displayed non-volatile binary memory. The response showed all 4 typical antiferroelectric switching current peaks, corresponding to first order phase transitions between the non-polar and polar phases of both polarities. However, when compared to a conventional antiferroelectric MIM capacitor, the critical electric field values at which the thin film transitioned from either of the polar orthorhombic phases to the nonpolar tetragonal phase lagged beyond zero, indicating that the thin film rested in a polar domain once the electric field was removed. This revealed a novel non-volatile hysteresis suitable for a solid-state memory applications.
Sponsor
Date
2020-05-17
Extent
Resource Type
Text
Resource Subtype
Thesis