Conformal Sub-monolayer ALD of Al2O3 by TMA -precursor and Water reactant
In recent years, Atomic Layer Deposition (ALD) has evolved as the most applicable thin-film deposition technique. To get highly conformal, ultra-pure and homogeneous thin-film on the substrate, ALD is the best process for micro- and nanofabrication. Experimentally, ALD involves two i.e. self-limited and complementary reactions of different compounds on the substrate to produce sub-monolayer of film growth. One ALD cycle is hence the combination of two half-cycle reactions (Fig. 1 and 2).
In the first half-cycle, precursor molecule is supplied into the deposition chamber. In this reaction, the precursor molecule adsorbs on the substrate forming co-valent bond with the surface molecules (step 1, fig. 1). After some time the substrate is saturated with precursor molecules i.e. no-more precursor molecule can further adsorbs on the surface. Then, the ALD chamber is purged to remove reaction by-product of the first-half cycle along with the unreacted precursor molecules (step 2, fig. 1) from chamber.
In the second half-cycle, the reactant molecule is supplied into the deposition chamber. In this cycle, the reactant molecules react with the precursor molecules depositing very thin-film of desired material. The second half-reaction is such that, in one hand first-layer of desired material is deposited on the substrate and in the other hand surface state evolves same as the initial state (step 3, fig. 1). This phenomenon is termed as the complementary reaction. The deposition chamber is again purged to exhaust non-reacted reactant molecules along with the second-half reaction by-products (step 4, fig. 1) from the chamber.
Fig. 1: Schematic diagram depicting a comple ALD cycle (4 steps-precursor dose, purge, reactant dose and purge), separated into individual half-reactions and purge cycles, on the Substrate .
Fig. 2. Flow-chart of a typical ALD Process
ALD of Al2O3 on Si-Substrate
To deposit Al2O3 on Si-substrate, TMA (Tri-methyl aluminium) (Fig. 3) as a precursor and Water as the reactant molecules are used. In similar fashion as depicted in fig. 1 and 2, a comple ALD cycle of Al2O3 on Si-substrate also involves four major steps or two half-cycle reaction inside a closed ALD-Chamber. ALD half-cycle is explained with reference to ALD process of Al2O3 because lots of research has already been carried out for the ALD of Al2O3 by using TMA as precursor molecule. ALD of Al2O3 by TMA and Water is also called an ideal ALD process. The ALD half-reaction of Al2O3 can be chemically written as follows :
Half -Reaction 1: Self-terminating property
–OH* + Al–(CH3)3 → –O–Al–(CH3)2* + CH4 ….(1)
(Reaction 1: Exposure of TMA on –OH terminated surface.)
Half- Reaction 2: Complimentary behavior
–(CH3)2* + H2O → –OH + CH4 ….(2)
(Reaction 2: Exposure of H2O on –CH3 terminated surface.)
* denotes the surface molecular group.
2 Al(CH3)2 + H2O → Al2O3 + 3 CH4 + ∆H = -376 kcal ....(3)
Fig 3: ALD of Al2O3 on Substrate using TMA as precursor and water as reactant, explained in three steps- (step 1) pre-prepared substrate depicting full-coverage of -OH groups on the substrate, (step 2) TMA adsorption at the -OH sites after exposure of TMA to the substrate, (step 3) reactant reacts with the TMA releasing -CH4 groups out of the TMA in the form of CH4.
Figure 4: Cross-sectional SEM image of an conformal Al2O3 ALD ﬁlm with a thickness of 300 nm on a Si-wafer with a trench structure. (Source: From ref 42. Copyright 1999 John Wiley & Sons).
Result: At the end we get ultra-pure, homogenous, conformal AL2O3 layer on the substrate.
 Thin Solid Films 517 (2009) 2563–2580.
 Chem. Rev. 2010, 110, 111–131