Plasma-assisted atomic layer deposition (ALD) is an energy-enhanced method for the synthesis of ultra-thin films with Å-level resolution in which a plasma is employed during one step of the cyclic deposition process. The use of plasma species as reactants allows for more freedom in processing conditions and for a wider range of material properties compared with the conventional thermally-driven ALD method. Due to the continuous miniaturization in the microelectronics industry and the increasing relevance of ultra-thin films in many other applications, the deposition method has rapidly gained popularity in recent years, as is apparent from the increased number of articles published on the topic and plasma-assisted ALD reactors installed. To address the main differences between plasma-assisted ALD and thermal ALD, some basic aspects related to processing plasmas are presented in this review article. The plasma species and their role in the surface chemistry are addressed and different equipment configurations, including radical-enhanced ALD, direct plasma ALD, and remote plasma ALD, are described. The benefits and challenges provided by the use of a plasma step are presented and it is shown that the use of a plasma leads to a wider choice in material properties, substrate temperature, choice of precursors, and processing conditions, but that the processing can also be compromised by reduced film conformality and plasma damage. Finally, several reported emerging applications of plasma-assisted ALD are reviewed. It is expected that the merits offered by plasma-assisted ALD will further increase the interest of equipment manufacturers for developing industrial-scale deposition configurations such that the method will find its use in several manufacturing applications.

Differences in Si surface passivation by aluminum oxide ͑Al 2 O 3 ͒ films synthesized using H 2 O and O 3-based thermal atomic layer deposition ͑ALD͒ and plasma ALD have been revealed. A low interface defect density of D it = ϳ 10 11 eV −1 cm −2 was obtained after annealing, independent of the oxidant. This low D it was found to be vital for the passivation performance. Field-effect passivation was less prominent for H 2 O-based ALD Al 2 O 3 before and after annealing, whereas for as-deposited ALD films with an O 2 plasma or O 3 as the oxidants, the field-effect passivation was impaired by a very high D it .

Two substrate-biasing techniques, i.e., substrate-tuned biasing and RF biasing, have been implemented in a remote plasma configuration, enabling control of the ion energy during plasma-assisted atomic layer deposition (ALD). With both techniques, substrate bias voltages up to −200 V have been reached, which allowed for ion energies up to 272 eV. Besides the bias voltage, the ion energy and the ion flux, also the electron temperature, the electron density, and the optical emission of the plasma have been measured. The effects of substrate biasing during plasma-assisted ALD have been investigated for Al2O3, Co3O4, and TiO2 thin films. The growth per cycle, the mass density, and the crystallinity have been investigated, and it was found that these process and material properties can be tailored using substrate biasing. Additionally, the residual stress in substrates coated with Al2O3 films varied with the substrate bias voltage. The results reported in this article demonstrate that substrate biasing is a promising technique to tailor the material properties of thin films synthesized by plasma-assisted ALD.

Room-temperature atomic layer deposition (RT-ALD) processes are of interest for applications using temperature-sensitive substrates. Challenges with RT-ALD arise when the precursors are not sufficiently volatile, purge times become impractically long, and precursors or co-reactants are unreactive with the surface species. In several cases, the latter two challenges can be overcome using energy-enhanced ALD. Here, we demonstrate RT-ALD (258C) processes for Al 2 O 3 , TiO 2 , and SiO 2 from trimethylaluminum (Al(CH 3 ) 3 , TMA), titanium(IV) tetraisopropoxide (Ti(O i Pr) 4 , TTIP), and bis(diethylamino)silane (SiH 2 (-NEt 2 ) 2 , BDEAS) precursors with an O 2 plasma or O 3 gas as co-reactants. Saturated RT-ALD growth was obtained for all O 2 plasma processes and TMA/O 3 , whereas the TTIP/O 3 and BDEAS/O 3 processes gave no growth. Using these and literature results, the criteria for viable RT-ALD processes are discussed.

The influence of ions and photons during remote plasma atomic layer deposition (ALD) of metal oxide thin films was investigated for different O 2 gas pressures and plasma powers. The ions have kinetic energies of 35 eV and fluxes of $10 12 -10 14 cm À2 s À1 toward the substrate surface: low enough to prevent substantial ion-induced film damage, but sufficiently large to potentially stimulate the ALD surface reactions. It is further demonstrated that 9.5 eV vacuum ultraviolet photons, present in the plasma, can degrade the electrical performance of electronic structures with ALD synthesized metal oxide films.