Specific Process Knowledge/Thin Film deposition/ALD/TiO2 deposition using ALD

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

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

TiO₂ standard recipe

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	4.0 s	5.0 s

TiO₂ deposition rates

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 to be deposited can be calculated. All results have been obtained for Si wafers with native oxide.

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

TiO₂ results

Some some SEM images of TiO₂ deposited on a Si 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 just before the ALD deposition.

• Titanium dioxide deposited at different temperatures on a Si surface
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  Temperature 150 ^oC, 1200 cycles, HF treated.
• 
  Temperature 150 ^oC, 1200 cycles.
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  Temperature 250 ^oC, 750 cycles.

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

XPS measurements of TiO₂ deposited at 120 ^oC and 300 ^oC are shown below. From the XPS measurements it can be calculated that at temperatures below 120 ^oC the TiO₂ layer will be contaminated with about 1-3 % chlorine molecules from the TiCl₄ precursor. This can be also seen as small white dots in the SEM image of the amorphous TiO₂ layers above.

XPS measurements of titanium dioxide.

• TiO₂ deposition on trenches using ALD

Below some SEM images of amorphous 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.

• Amorphous titanium dioxide deposited on Si trenches
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Below some SEM images of anatase 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.

• Anastase titanium dioxide deposited on Si trenches
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Evgeniy Shkondin, DTU Danchip, 2014.