New ALD Processes at Cornell on the Oxford FlexAl
Vincent Genova, CNF Process Engineer
Posted October 2012
Since CNF’s purchase of an Oxford Instruments FlexAL ALD system in 2008, we have continued to develop new thin film processes, both on our own and cooperatively with Oxford. Our ALD system has both thermal and plasma enhanced (PEALD) deposition capability for materials derived from hafnium (Hf), aluminum (Al), tantalum (Ta), and silicon (Si) based organometallic and organosilane precursors. In addition to our present list of processes including HfO2, HfN, Al2O3, AlN, and SiO2, we are now pleased to offer TaN, Ta2O5, Si3N4, HfSiO2, HfSiON, and HfAlOx films.
Tantalum based thin films have been widely used for many important applications in nanoscale semiconductor device fabrication. TaN has been used as a diffusion barrier for Cu interconnect technology and is a leading candidate for a metal gate material in CMOS devices. Metal gates along with high-k dielectrics allow for optimum device performance at ever decreasing transistor dimensions. In addition, tantalum oxide thin films have been used as a capacitor dielectric material in dynamic random access memory (DRAM) devices, as well as a gate dielectric for nanoscale CMOS devices. These characteristics along with ALD’s inherent qualities such as high conformality, high film quality at low growth temperatures, uniformity over large areas, and controllability at nanoscale thicknesses, make these and other materials quite attractive.
Our TaN film is derived from a thermal ALD process using an alkylamide precursor pentakis-dimethylamino-Ta (PDMAT) in the first half reaction that is then reduced with ammonia (NH3) in the second half reaction at a deposition temperatures between 225-300C. The saturation curves illustrate excellent self-limiting behavior with deposition rates between 0.41-0.48A/cycle. We have recently developed a plasma based TaN film using PDMAT and H2 based plasma for the second half reaction. This process produces a less resistive film than the thermal TaN ALD process. PEALD has been developed to obtain films with higher quality at lower deposition temperature, largely due to the higher reactivity of the ions and radicals.
|Figure 1: Linear growth of TaN PEALD process using PDMAT and H2 at 110C, 225C, and 300C.|
The tantalum oxide film recently developed is a PEALD process at temperatures from 110-300C. This utilizes the PDMAT precursor along with an oxygen plasma for the second half reaction. PDMAT dosages and oxygen plasma exposure times were optimized to generate saturation curves with self-limiting behavior with growth per cycle rates between 1.04-1.53A/cycle. The index of refraction was measured using spectroscopic ellipsometry and is between 2.1-2.2 at 630nm. X-ray photoelectron spectroscopy (XPS) analysis indicates a film stoichiometry close to Ta2O5, which is targeted. Carbon was detected at levels <4 at.% which is excellent considering an organic precursor, while nitrogen was not detected indicating proper reaction and ligand exchange from the amino precursor. In addition, we have recently developed a tantalum oxide thermal ALD film (figure2).
|Figure 2: Linear growth rates of thermal TaO2 process at 110-300C.|
A silicon nitride PEALD process was recently developed using 3DMAS as the silicon precursor along with an Ar/N2 plasma. There are critical pre-processing steps for successful deposition, including a chamber conditioning and a surface pre-treatment. This film displays the best characteristics at high deposition temperatures around 350C with improving stoichiometry and refractive index (figure 3).
|Figure 3: Refractive index of PEALD silicon nitride vs. temperature.|
The need for high-k dielectrics such as HfO2 is vital for advanced transistor structures. Along with this need is the need for tunable dielectric constant films depending on the device requirements. We recently developed processes for Hf based tertiary and quaternary oxide films such as hafnium aluminate (HfAlOx) (figure4), hafnium silicate (HfSiOx) (figure 5), and hafnium silicon oxynitrides (HfSiON) (figure 6). Many of these films offer higher thermal stability and higher channel electron mobilities than the standard HfO2 film.
|Figure 4: XPS depth profile of HfAlOx PEALD film 200C|
|Figure 5: XPS depth profile of HfSiOx PEALD film 200C.|
|Figure 6: XPS depth profile of HfSiOxNy PEALD film at 200C|
Please contact Vince Genova at the Cornell NanoScale Facility for further information on these ALD processes.