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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.

Some some SEM images of TiO₂ deposited on a silicon surface at different temperatures between 150 ^oC and 350 ^oC are shown below. Some of samples have been treated with HF (hydrofluoric acid) to remove the native oxide layer silicon samples just before the ALD deposition.

Titanium dioxide thickness as function of number of cycles
Temperature 150 ^oC, 1200 cycles, HF treated.
Temperature 150 ^oC, 1200 cycles.
Temperature 250 ^oC, 750 cycles.

Temperature 300 ^oC, 1000 cycles, HF treated.
Temperature 300 ^oC, 1000 cycles.
Temperature 350 ^oC, 1250 cycles, HF treated.

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.