%20(1).png)
Atomic layer processing methods, including Atomic Layer Deposition (ALD) and Atomic Layer Etching (ALE), have advanced the precision with which metals like gold can be manipulated at the atomic scale. Traditionally, gold has been challenging to process due to its low reactivity, but recent developments have made it possible to deposit and etch gold with atomic-scale control. While Professor Seán Barry’s work has focused on pioneering methods for gold deposition using ALD, Professor Steven M. George and his team have recently demonstrated a successful thermal ALE technique for gold. Together, these breakthroughs represent a new frontier in gold processing, enabling nanoscale applications in electronics, nanotechnology, and catalysis.
Advances in Atomic Layer Deposition (ALD) of Gold: Professor Seán Barry’s Work
Atomic Layer Deposition (ALD) relies on self-limiting surface reactions to grow thin films with atomic precision, and it is ideal for materials where control over layer thickness and uniformity is essential. However, gold presents unique challenges in ALD due to its inertness and lack of reactive sites. Despite this, Professor Seán Barry and his team have developed a plasma-enhanced ALD (PEALD) approach that overcomes these hurdles by using a specialized gold precursor and plasma activation.
Plasma-Enhanced ALD (PEALD) Method
Barry’s team utilized a trimethylphosphine-supported gold(III) precursor, specifically Me₃AuPMe₃, in combination with oxygen plasma to deposit gold layers. The plasma serves to activate the precursor and facilitate the deposition reaction, which would otherwise be hindered by gold’s low reactivity.
Low-Temperature Deposition
The process is achievable at temperatures around 120–130°C, considerably lower than traditional thermal ALD processes. This temperature range minimizes the risk of precursor decomposition, allowing the deposition of smooth and uniform gold films without unwanted by-products.
Deposition Rate and Film Quality
The deposition process achieved a growth rate of approximately 0.5 Å per cycle, providing exceptional control over film thickness. Barry’s PEALD method allows for uniform, conformal gold coatings that are valuable in microelectronics, sensing devices, and other applications where thin films of noble metals are required.
University of Helsinki Unveils Thermal ALD Process for Gold Coating in 3D Applications
The University of Helsinki has developed a groundbreaking thermal Atomic Layer Deposition (ALD) process for gold using the precursor Me₂Au(S₂CNEt₂) with a broad process window (120–250°C), achieving uniform and highly conductive films. This innovation addresses the limitations of plasma-enhanced ALD, which can struggle with coating complex 3D structures. By utilizing ozone as a co-reactant, the researchers achieved continuous gold films with a growth rate of 0.9 Å/cycle at 180°C and low resistivity, ideal for advanced applications requiring precise, conductive coatings. This follows an earlier Helsinki breakthrough in Ruthenium ALD, marking another step forward in atomic-level metal deposition techniques.
Breakthrough in Atomic Layer Etching (ALE) of Gold: Professor Steven M. George’s Method
Building on the advances in ALD for gold, Professor Steven M. George’s recent work on thermal ALE offers a complementary technique to precisely remove gold layers. Published in May 2024, George’s ALE method for gold uses a novel two-step thermal process involving chlorination and ligand addition. This approach bypasses the need for plasma, instead relying on a purely thermal cycle to achieve atomic-level etching of gold.
The study demonstrates a thermal atomic layer etching (ALE) process for gold using sequential reactions: chlorination with sulfuryl chloride (SO₂Cl₂) to form gold chloride, followed by ligand addition with triethylphosphine (PEt₃) to produce a volatile etch product, AuClPEt₃. This method achieved consistent etching at 0.44 ± 0.16 Å per cycle at 150°C on gold films. Mass spectrometry confirmed AuClPEt₃ as the main etch product, while analysis showed that ALE maintained nanoparticle smoothness without surface roughening. The approach was also effective on copper and nickel, offering a versatile ALE pathway for metals through controlled chlorination and ligand-addition reactions. LINK: https://pubs.acs.org/doi/10.1021/acs.chemmater.4c00485
Two-Step Thermal ALE Process
Chlorination: The gold surface is initially chlorinated using sulfuryl chloride (SO₂Cl₂), which forms gold chloride (AuCl) on the surface. This step primes the gold for the ligand addition reaction.
Ligand Addition with Triethylphosphine (PEt₃): After chlorination, triethylphosphine (PEt₃) is introduced to bind with the gold chloride, creating a volatile product, AuClPEt₃, which desorbs from the surface, effectively removing one atomic layer of gold.
Etch Rate and Temperature Control
The ALE process operates in a temperature range of 75 to 175°C, with the optimal and most consistent etch rate of 0.44 ± 0.16 Å per cycle occurring at 150°C. This repeatable, self-limiting reaction cycle ensures precise control over the etching process, which is critical for applications demanding high accuracy.
Experimental Observations and Mass Spectrometry
Quartz crystal microbalance (QCM) measurements tracked mass changes during each ALE cycle, while in situ quadrupole mass spectrometry (QMS) on gold nanopowder confirmed that AuClPEt₃ was the primary volatile product. The intensity of the AuClPEt₃+ ion peaked early in each PEt₃ dose, indicative of a self-limiting reaction where gold is etched in controlled increments.
Structural Integrity of Gold Nanoparticles
Analysis using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) showed that the ALE process did not roughen the surface of gold nanoparticles. This smoothness is crucial for applications in electronics and photonics, where surface quality affects device performance. Additionally, powder X-ray diffraction (XRD) revealed slight broadening of diffraction peaks post-ALE, indicating sintering and suggesting that gold redistribution could contribute to the formation of larger nanoparticles.
Combined Implications of ALD and ALE for Gold
The complementary nature of Barry’s PEALD for gold deposition and George’s thermal ALE for gold etching offers an unprecedented level of control over gold at the atomic level. Together, these methods enable:
High-Precision Patterning: Combined ALD and ALE allow for nanoscale patterning of gold films with atomic precision, benefiting fields such as semiconductor manufacturing and nanotechnology.
Surface Engineering: The smoothness and control over film morphology achieved through these processes make it possible to engineer gold surfaces with specific properties, crucial for sensors, catalysis, and plasmonic devices.
Enhanced Flexibility in Fabrication: The ability to alternate between deposition and etching at the atomic scale provides unparalleled flexibility, especially for creating multilayer structures or complex geometries in microelectronics and MEMS devices.
Sources:
Previous Articles on Gold ALD:
