Invited Paper TF-TuA1
A Comparison between Conventional- and Spatial ALD; The Effect of Pressure and Exposure Time on Film Properties

Tuesday, October 29, 2013, 2:00 pm, Room 104 A

The last few years have seen important developments in spatial Atomic Layer Deposition (ALD), enabling ALD with high deposition rates. Whereas in conventional ALD, precursors are dosed in a time-separated mode using a purge or pump step, in spatial ALD, precursors are dosed simultaneously and continuously at different physical locations. As a result, deposition rates exceeding 1 nm/s have been reported for spatial atmospheric ALD of Al₂O₃. This has led to the development of high-throughput, industrial scale spatial ALD tools for surface passivation of crystalline silicon wafers for solar cells as well as roll-to-roll spatial ALD concepts for flexible electronics.

From the point of view of chemistry, there is no difference between spatial and conventional ALD, as similar precursors and substrates are used to make similar materials. Important differences between spatial and conventional ALD, though, are the time-scales and pressure regime at which spatial ALD is performed. Whereas in conventional ALD, precursor exposure times typically range from a few tenths of seconds to a few seconds, in spatial ALD precursor exposure times can be as short as several milliseconds, during which the physical and chemical processes (e.g. diffusion, adsorption, reaction, crystallization, etc.) responsible for the film growth have to take place. Furthermore, spatial ALD is often performed at atmospheric pressure, where conventional ALD is usually done at low pressure. This especially impacts plasma-enhanced ALD, as the physics and chemistry of atmospheric pressure plasma’s can be quite different from the low pressure plasma’s typically used in conventional plasma enhanced ALD.

With this in mind, spatial ALD of several different material classes will be discussed, and a comparison with similar, conventional ALD processes, will be made. Similarities, as well as differences, in deposition processes, film properties and their performance in devices will be discussed for

1. Alumina (amorphous dielectric)

2. Zinc oxide (crystalline transparent conductor)

3. Alucone (hybrid material, deposited by spatial Molecular Layer Deposition (MLD))

4. Silver (metal, deposited by plasma enhanced spatial ALD).

Furthermore, possible consequences and opportunities for up-scaling these processes to industrial scales will be discussed.