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  Research - Atomic Layer Deposition

Also called ALE (Atomic Layer Epitaxy), Atomic Layer Depostion (ALD) is a deposition method that was introduced by Dr. Suntola in 1974 [1], and has the capability of controlling the deposition thickness of thin films to the order of one atomic monolayer.  Chemical gases (precursors) are carefully selected for ALD so that after a single layer of the gas sticks to the surface, the surface is passivated.  Before the next layer is deposited the passivating atoms must be removed in various ways (chemical reactions, thermal spikes, etc).

An example of silicon ALD using thermal spikes as reported by the Nishizawa group[2]

Nishizawa et al. use disilane (Si2H6)as the precursor gas. At 450oC, disilane dissociatively adsorbs into two SiH3 groups that stick to the silicon surface.  At 450oC, unstable hydrogen atoms desorb and form Si-H on the surface.  This hydrogen terminated surface is not reactive so excess disilane molecules are not chemisorbed to the surface.  By spiking the temperature to 550oC, the Si-H bonds are broken and bare silicon atoms stay on the surface.  The bare silicon surface is chemically reactive and another layer of precursor molecules can now be deposited.  In this way, the layer thickness is controlled by the number of repetitions of the ALD cycle.

Plasma-Enhanced Atomic Layer Deposition
The key to Plasma Enhanced Atomic Layer Deposition (PEALD) is to remove passivating hydrogen atoms without the use of a thermal spike.  Our approach is to break the remaining Si-H bonds using energy from Ar plasma, while decreasing the substrate temperature to within the thermal budget of front-end chip processing.  Plasma is a mixture of ions, electrons, excited atoms and neutral atoms.  Argon ions and excited atoms (metastable atoms) have 15.75 eV and >10.55 eV potential energies, respectively.  Therefore, these argon plasma species have sufficient energies to break Si-H bonds (3-4 V), even if the substrate temperature is below 550.  Once the plasma has chemically activated the surface, the next monolayer of precursor can be introduced, completing the ALD cycle.

Experimental PEALD system

An ICP (Inductively Coupled Plasma) source and the matching circuit. The RF-driven coil (center) heats the plasma electrons by induction.  The purple light is the optical emission from argon plasma.

Referenced Material

[1]       Dr. Suntola's homepage, http://www.sci.fi/~suntola/ald.html

[2]       Journal of the Electrochemical Society, 150 (7) G371-G375  2003, Jun-ichi Nishizawa, et. al