Spatial Atomic Layer Deposition

Despite excellent advantages in thin film fabrication, conventional ALD is a rather slow deposition technique and usually involves processing in vacuum, which makes it complicated and expensive to scale up. In recent years, an alternative approach to conventional ALD, namely atmospheric pressure spatial atomic layer deposition (AP-SALD), has received much attention.^[3–5] Curiosity enough, despite being mentioned in Suntola and Antson’s patent in 1977, it was not until 2008 that the first scientific article on AP-SALD was published.^[3] In this novel concept, schematically shown in Figure 1, the precursors are continuously injected to the substrate from different locations of a fixed gas injector. In addition, the reactive precursors are spatially separated thanks to inert gas barriers located alternatively between precursor flows. The inert gas outlets act not only as effective barriers to keep precursors chemically isolated but also as a purge area to ensure a single monolayer of precursors chemisorbed onto sample surface after each exposure. The substrate oscillates at very short distances (20 µm – 200 µm) from the fixed gas injector, making gas separation effective and replicating conventional ALD cycles. Because of the short gap between the substrate and the injector, this design for open-air ALD is also called close-proximity SALD reactor, published for the first time by Levy et al. from Kodak.^[3,6]

Figure 2 shows a zoom in the cross-section view of the gas injector for two gap distances of 70 µm and 230 µm between the gas injector and the substrate (gas velocity represented in color scale, computed using COMSOL Multiphysics Simulation). As can be seen, the reactive precursor (trimethylaluminum - TMA in this illustration) is kept away from the oxidant by exhaust channels and inert gas flows (nitrogen in this illustration). This example shows the possibility to perform ALD at the atmospheric pressure and even in the open-air, i.e. without using any deposition chamber or the need of unrealistically high flows. The open-air version of SALD is thus called Atmospheric Pressure SALD (AP-SALD), which shares the same unique assets of ALD, but it has a numerous additional advantages including i) several orders of magnitudes faster than ALD since no purge steps, no vacuum chamber is required, ii) possibility to scale up since it is not limited by the size of the chamber as in the case of conventional ALD, iii) Integration of in-situ characterization techniques can be easily performed.

Due to the suitability of the AP-SALD approach for industrial applications, many patents have been filled since its birth date in 1977. The first one by Levy (Kodak) dated from 2008, and in a rather quick time, the technique has indeed reached industrial commercialization. According to our best knowledge, there are currently 5 companies (Applied Materials, Jusung, Solaytech, Levitech, Beneq) and 14 laboratories in the whole world, developing, fabrication AP-SALD equipment. Some "close-proximity" approaches developed by several research groups are shown in Figure 3. In the Kodak initial design, the gas injector lays at the bottom (the outlets facing upward), with the substrate oscillating on top, as shown in Figure 3.a. Another type of AP-SALD approach was also presented by Poodt et al. in TNO group in 2010 (in the 10^th International Conference on Atomic Layer Deposition, Seoul, R. South Korea), and soon after they have published an article in Advanced Materials journal, in which high-quality Al₂O₃ layer deposited at deposition speeds of at least 1.2 nm/s for application in solar cells was demonstrated. In their AP-SALD reactor design, the gas injector is fixed while the substrate holder is rotating, and both are placed in an oven. The precursor and by-product gas lines are connected to the reactor head through an opening in the top of the oven, as shown in Figure 3.b. Other groups such as Driscoll’s group in University of Cambridge, Dasgupta’s group in University of Michigan or Chen and co-workers in Huazhong University of Science and Technology, China have also developed their own SALD reactors based on close-proximity approach, as shown in Figure 3.c, Figure 3.d and Figure 3.e.

Several materials including intrinsic, doped and mixed oxides have been produced using AP-SALD reactors, for instance Al₂O₃, ZnO, SnO₂, HfO₂, TiO₂, Cu₂O, Nb₂O₅, ZnO:N, Al:ZnO, Zn_1-xMg_xO, In:ZnO, ZnO:S, TiO_2-xCl_2x... The applications range from active and passive components for thin film transistors (TFTs), barrier passivation layers to active components for new-generation solar cells.

In 2013, Muñoz-Rojas and co-workers in Prof. Driscoll’s group in University of Cambridge have investigated their close-proximity AP-SALD for depositing amorphous TiO₂ blocking layers at 100 °C for application in inverted bulk heterojunction solar cells. Some years later, Dr. Muñoz-Rojas moved to LMGP, CNRS, Grenoble, France to build his own research group, in which Viet Huong Nguyen did his Ph.D. and Postdoc. The AP-SALD system shown in Figure 3.f. has been built in February 2016, which allows currently depositing several high-quality metal oxide materials including ZnO, Al₂O_3, TiO₂, Cu₂O, SiO₂ and their compounds.

In the ALD Research Group at Phenikaa University, a home-build 'close-proximity' SALD system is currently under construction.

References