Platinum and platinum-rich alloys are naturally occurring and have been known for a long time since it is often found as native platinum. It occurs naturally in the sands of rivers in South America and it was first used by pre-Columbian natives to produce artefacts. Later in 16th century the Spaniards named the metal “platina,” or little silver, when they first encountered it in Colombia. They regarded platinum as an unwanted impurity in the silver they were mining and it was not until 1748 that platinum was properly reported by Antonio de Ulloa y de la Torre-Giral, a Spanish general of the navy, explorer, scientist, author, astronomer and colonial administrator.
Since the platinum has become known and used because of the outstanding catalytic properties, which it has in common with the other of the six platinum group metals (PGM) – iridium, osmium, palladium, platinum, rhodium, and ruthenium. In addition, platinum’s wear and tarnish resistance characteristics are well suited for making fine jewellery. Other distinctive include:
- high resistance to chemical attack
- excellent high-temperature characteristics
- stable electrical properties.
Because of all these extraordinary properties the PGMs have been exploited for a wide range of industrial applications. Platinum, platinum alloys, and iridium are used as crucible materials for the growth of single crystals, especially oxides. The chemical industry uses a significant amount of either platinum or a platinum-rhodium alloy catalyst to catalyse the partial oxidation of ammonia to yield nitric oxide, which is the raw material for fertilizers, explosives, and nitric acid.
In recent years, a number of PGMs have become important as catalysts in synthetic organic chemistry. Platinum supported catalysts are used in the refining of crude oil, reforming, and other processes used in the production of high-octane gasoline and aromatic compounds for the petrochemical industry. Since 1979, the automotive industry has emerged as the number one consumer of PGMs. Palladium, platinum, and rhodium have been used as oxidation catalyst in catalytic converters to treat automobile exhaust emissions. A wide range of PGM alloy compositions are used in low-voltage and low-energy contacts, thick- and thin-film circuits, thermocouples and furnace components, and electrodes.
It was not until the early 2000 that the platinum and the other PGMs became available as an ALD processes and here below is a summary of the most important fundamental discoveries of platinum ALD.
Thermal ALD of high quality platinum films
It all started with thermal ALD of platinum and ruthenium in Helsinki Finland at the famous Laboratory for Inorganic Chemistry headed by Prof. Markku Leskelä and Prof. Mikko Ritala. Here it was found that high quality platinum films can be grown by thermal ALD from MeCpPtMe3. According to the first publications by Titta Aaltonen (summarized in her PhD Thesis University of Helsinki) the films had strong (111) orientation even down to the lowest growth temperatures. Except for discovering the secrets of thermal ALD of noble metals (Ru, Ir Pt, Pd) Titta Aaltonen made ground breaking studies of their ALD growth mechanism with O2 as the co-reactant. At first it may seem strange that O2, or in her case also laboratory air or pressured air, could be used to grow high quality noble metal films. Titta Aaltonen found that adsorbed oxygen atoms react with the ligands of the noble metal precursor during the metal precursor pulse. Unreacted ligand species that remained on the surface after the metal precursor pulse react with oxygen during the following oxygen pulse. The main reaction by-products detected during the both reaction steps were water and carbon dioxide. For detailed studies of the ruthenium process using RuCp2 it has been concluded that active oxygen that dissolves in the upper most monolayers of the growing noble metal film may be behind the nucleation and growth mechanism of the next “ALD monolayer”.
The growth rates of the platinum films grown at 300 °C from MeCpPtMe3 (78-1350) -CAS 94442-22-5 was reported at about 0.5 Å/cycle both when air and pure oxygen were used as oxygen sources and a 50-nm film grown at 300 °C had a resistivity of 13 μΩcm, which is close to bulk value for platinum. It was also found that the difference between air and O2 co-reactant was in how the films adhered to the substrate. The films grown with air as the oxygen source did not pass the famous scotch tape test, while the films grown with pure oxygen passed the tape test.
Besides having such a beautiful ALD mechanism with such a simple co-reactant as air or O2, one additional very big advantage with the MeCpPtMe3 precursor is that can be vaporized at room temperature, just slightly below its melting point of 30 °C since the vapor pressure of MeCpPtMe3 at room temperature is high enough for delivery into an ALD process chamber. If you need a bit more precursor flow for larger batch type reactors or applications with relying on high surface area you can melt the precursor in a standard stainless-steel ampule or bubbler with carrier gas dip tube to enhance the flow further