(Original blog - https://www.lesker.com/blog/ald-high-purity-alscn-thin-films-achieved-through-ultrahigh-purity-conditions)
As semiconductor and microelectronic devices evolve, requiring atomic-scale manufacturing precision, the purity constraints on materials become extreme. At the 2nm node in semiconductor development, line widths are on the order of 40 atoms! Among the more interesting materials is the ferroelectric material aluminum scandium nitride (AlScN), a wurtzite-structured, solid solution material that combines the superior piezoelectric properties of AlN with the ferroelectric behavior introduced through scandium doping. AlScN has demonstrated breakthrough potential for nonvolatile memory, energy harvesting, microelectromechanical systems (MEMS), Rf filters, and optical devices. All of the applications require conformal, ultra-pure thin films with atomic-level precision.
A recent pre-publication authored by scientists at the Kurt J. Lesker Company, Penn State University, MIT, and the Army Research Laboratory, reports on the development of a unique atomic layer deposition system and process to fabricate AlScN thin films using plasma-enhanced atomic layer deposition (PEALD) under ultra-high purity (UHP-C) conditions (https://doi.org/10.21203/rs.3.rs-6960782/v1). The paper, titled Ferroelectric Aluminum-Scandium Nitride by Plasma-Enhanced Atomic Layer Deposition under Ultrahigh Purity Conditions, details the growth of AlScN thin film under extremely pure conditions which enabled outstanding performance due to the elimination of contaminants, including oxygen and carbon, which suppress ferroelectric performance.
Figure 1: FESEM images of Al(1–x)ScxN deposited on planar Si (100): (a) Top-view showing grain sizes in the range of 10–15nm. (b) Cross-sectional view revealing a columnar microstructure. (c)(d) Show cross-sectional views of Al(1–x)ScxN PEALD on Si (c) vias and (d) trenches with 1:1 aspect ratio, respectively, demonstrating excellent conformal coverage.
The Importance of Ultrahigh Purity
Traditional approaches to the fabrication of high purity thin films of AlScN have been hampered by the impurities inherent in precursors such as the metallic scandium used for sputtering targets, which can exceed 1,000 ppm, or the elevated levels of unwanted gases which are typical of most ALD systems. Oxygen contamination has been shown to degrade ferroelectric properties while broadening switching characteristics and increasing leakage currents. The innovative UHP-C approach uses an alternative ALD precursor for scandium which contains nearly no oxygen – and couples that pristine precursor with a plasma activation step which further enables nitride formation, with monolayer precision.
This research confirms that the reduction of oxygen and carbon contamination in AlScN thin films fabricated by ALD produces films with sharper hysteresis loops, higher remnant polarization, which are substantially more robust when compared with films made by traditional PVD processes.
Figure 2: Goniometer 2θ scan of a PEALD AlScN thin film (ScN:AlN, 1.5:1, 40.2 nm-thick with a 5 nm-thick AlN nucleation layer) deposited on (0002)-oriented GaN on Sapphire substrate at a substrate temperature of 300°C. Inset: Phi scan of AlScN aligned to the (103) reflection.
The AlScN Supercycyle
The innovative process described in the publication features a supercycle which combines sub-cycles to achieve a highly stoichiometric, high purity, film. Alternating subcycles of AlN and ScN are stacked, with an intermittent plasma processing step, which helps establish the AlScN compound. The cycle, repeated thousands of times, builds a highly precise, defect-free, thin film with angstrom-level precision.
A High Purity Deposition Platform for Future Materials
The extraordinary results reported for AlScN by the UHP-C process opens the door to the fabrication of other materials and compounds which can be optimized through the elimination of contaminants. The Lesker ALD 150LX UHP-C system has been validated in this work as an essential tool for the development of next-generation multicomponent nitrides. Designed to break barriers in thin film deposition by ALD, the UHP-C configuration integrates high and ultra-high vacuum system design principles with extreme gas purification technologies to minimize background impurities during film growth. The 150LX provides a scalable, production-relevant tool for the fabrication of conformal thin films for materials whose properties are adversely affected by sub-ppm impurity levels.
As Moore's Law continues to be extended, through 3D architectures, extreme aspect ratios, and shrinking defect budgets, the demand for the highly conformal, ultra-pure thin films fabricated by ALD will become even more essential. AlScN is only one example of multicomponent, conformal, atomically engineered, thin film materials that will be critical to next generation Rf, logic, memory and optoelectronic devices.
Figure 3: PUND test performed at ±40 V with clear current peaks during switching pulses compared to non-switching pulses.
Next Steps
This publication is another example of the power of collaboration between academia and industry which can drive the advancement of an entire industry. The solution put forth in this publication, which combines advanced chemistry with extreme vacuum system design, opens the door for new applications of AlScN as well as providing a platform for the development of other multicomponent thin film materials by ALD.
The full paper, Ferroelectric Aluminum-Scandium Nitride by Plasma-Enhanced Atomic Layer Deposition under Ultrahigh Purity Conditions, is available online at https://doi.org/10.21203/rs.3.rs-6960782/v1.
For more information on the ALD150LX platform, click here.