Sputter Synthesis of Multifunctional Hafnium Oxide Based Thin Films for Gate Dielectric and Memory Applications

Abstract

Over the past decade, tremendous research has drawn considerable attention for incorporation of high-k dielectric materials to replace the conventional low-k dielectric material such as SiO2. Among them, the hafnium oxide (HfO2) has received a great deal of consideration and appears to be the most promising material because of high dielectric constant ( ~18), a relatively large energy bandgap ( ~5.7 eV) with sufficient band offsets with silicon and a good thermal stability in contact with silicon and metal gates. Due to the excellent material properties, HfO2 has attracted a great deal of interest for its use as gate dielectric in Metal oxide semiconductor field effect transistor (MOSFET). In recent years, HfO2 thin films have shown an excellent resistive switching memory effect by using metal-oxide-metal structures and found suitable for memristor device applications. If these devices can be monolithically integrated to fabricate one transistor one resistor (1T1R) structure, a high speed, high storage density, low consumption of power will be achieved. This can be possible by using multifunctional high-k dielectric material such as HfO2.

Thin films of HfO2 have been synthesized by RF magnetron sputtering technique. The effect of various sputtering parameter such as RF power, substrate temperature, working pressure and Ar:O2 gas flow ratio are studied to evaluate the structural, morphological and electrical properties. The effect of rapid thermal annealing (RTA) conditions like RTA temperature and RTA duration on HfO2 films properties are also investigated. Doped (Ti, Zr, Gd and Ta) HfO2 films are also synthesized on silicon substrate by RF co-sputtering technique. The dopant concentration are varied by changing the RF power of the dopant target, while keeping the RF power of Hf target at optimized value.

A preliminary investigation has been carried out to study RS behaviour of the doped HfO2 films using metal/oxide/metal structure for memristor device application. This study is aimed to investigate the improved switching behaviour of co-sputtered HfO2 film with different dopant concentration. In particular, the influences of dopants/native defects on the switching behavior of dielectric films are explored. The structural properties of as-deposited and doped HfO2 thin films are investigated by X-Ray diffraction system (Rigaku ultima IV). The morphological and compositional properties of the sputtered films are carried out by field emission scanning electron microscope (Nova NanoSEM/ FEI) and X-ray Photoelectron spectroscope (XPS). Raman spectroscopy studies are carried out in order to study the vibrational mode HfO2 in doped samples. In order to fabricate MOS capacitors, thermally evaporated aluminum electrodes are used as top and bottom contacts. The capacitance-voltage(C-V) and leakage current–voltage (I-V) measurements for the Al/HfO2/SiMOS structures are performed using an Agilent E4980A precision LCR meter and Keithley 6487 Picoammeter/Voltage source, respectively. The resistive switching current-voltage measurements of metal-insluator-metal (MIM) structure are carried out by using Keithley 2410 source meter.

The thesis concludes by suggesting scope of further research on the topics highlighted in the present work.