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The ALD window for titanium dioxide (TiO₂) ranges from 120 ^oC to 350 ^oC.

A low temperatures between 120 ^oC and 150 ^oC an anatase TiO₂ layer is grown in the ALD, and at higher temperatures between 300 ^oC and 350 ^oC an amorphous TiO₂ layer is grown. At temperatures between 150 ^oC and 300 ^oC the TiO₂ layer will be a mixture of both anatase and amorphous TiO₂.

For Si wafers, anatase TiO₂ is best grown on wafers without native oxide (removed using BHF), and amorphous TiO₂ is best grown on wafers with native oxide.

XPS measurements shows that at temperaturs below 120 ^oC the TiO₂ layer will be contaminated with about 1-3 % chlorine molecules from the TiCl₄ precursor. This can be seen as small white dots in SEM images of the anatase TiO₂ layers.

Recipe: TiO2

Temperature: 120 ^oC - 350 ^oC

	TiCl₄	H₂O
Nitrogen flow	150 sccm	200 sccm
Pulse time	0.1 s	0.1 s
Purge time	3.0 s	4.0 s

In the graphs below the TiO₂ thickness as function of the number of cycles for deposition temperatures between 150 ^oC and 350 ^oC can be seen. From the equations the number of cycles required for a certain thickess can be calculated.

Titanium dioxide thickness as function of number of cycles
Temperature 150 ^oC.
Temperature 250 ^oC.
Temperature 350 ^oC.

Below some SEM images of anatase TiO₂ deposited at 120 ^oC on Si trenches are shown. The width of the trenches is 200 nm, and the depth is 4 µm, i.e. the aspect ratio is 1:20. The number of cycles is 500, and this results in a TiO₂ layer of about 25 nm. From the SEM images it is seen that the TiO₂ layer covers the trenches very well.

Below some SEM images of amorphous TiO₂ deposited at 300 ^oC on Si trenches are shown. The width of the trenches is 200 nm, and the depth is 4 µm, i.e. the aspect ratio is 1:20. The number of cycles is 500, and this results in a TiO₂ layer of about 26 nm. From the SEM images it is seen that the TiO₂ layer covers the trenches very well.

Evgeniy Shkondin, DTU Danchip, 2014.