Specific Process Knowledge/Thin film deposition/ALD Picosun R200/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 and uniformity

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. Results have been obtained for <100> 100 mm Si wafers with native oxide or BHF treated wafers.

• Titanium dioxide thickness as function of number of cycles
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  Temperature 150 ^oC Silicon wafer with native oxide.
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  Temperature 250 ^oC Silicon wafer with native oxide.
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  Temperature 300 ^oC Silicon wafer with BHF treated surface.
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  Temperature 350 ^oC Silicon wafer with BHF treated surface.
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  Temperature 150 ^oC Silicon wafer with native oxide. Amorphous TiO₂ layer.
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  Temperature 350 ^oC Silicon wafer with BHF treated surface. Anatase polycrystalline layer.

TiO₂ SEM results

Some some SEM images reveals different surface morphology 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 Si surface.
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  Temperature 150 ^oC, 1200 cycles, Si surface with native oxide.
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  Temperature 250 ^oC, 750 cycles, Si surface with native oxide.

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  Temperature 300 ^oC, 1000 cycles, HF treated Si surface.
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  Temperature 300 ^oC, 1000 cycles, Si surface with native oxide.
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  Temperature 350 ^oC, 1250 cycles, HF treated Si surface.

TiO₂ standard recipe characterization across 100 mm Si <100> wafer

Deposition of amorphous and polycrystalline (anatase TiO2)

Deposition conditions at 150 oC (Amorphous TiO2) Number of cycles Thickness (nm) Uniformity across 100mm Si substrate (%) Standard deviation error Refractive index @ 632.8 nm 250 11.95 2.35 0.97 2.38 500 22.64 2.11 1.68 2.42 750 32.66 1.88 2.21 2.40 1000 43.09 2.01 2.80 2.40 Deposition conditions at 350 oC (Polycrystalline anatase TiO2) Number of cycles Thickness (nm) Uniformity across 100mm Si substrate (%) Standard deviation error Refractive index @ 632.8 nm 500 22.41 3.61 0.46 2.31 750 33.57 2.25 0.43 2.37 1000 44.87 3.32 0.87 2.40 1225 56.90 3.45 0.26 2.41

TiO₂ XPS investigation for elemental trace analysis

Typical XPS profile for TiO₂ 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.

• Titanium dioxide deposited at different temperatures on a Si surface
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  Survey scan. Temperatures 120 ^oC and 300 ^oC, 500 cycles.
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  Measurement of the major contamination (Cl). Temperatures 120 ^oC and 300 ^oC, 500 cycles.Raising temperature to 300 ^oC reduces Cl contamination below the detectable level.

TiO₂ deposition on trenches

For ALD deposition of TiO2 on high aspect ratio structures like deep and narrow trenches or holes, a longer nitrogen purge time is needed in order to ensure that the first precursor material is removed everywhere, before the next one is introduced into the ALD chamber. The purge time can be changed, depending on how narrow the trenches or holes are, but you have to be aware of that the ALD deposition then might be very time consuming.

A special recipe called "TiO2T" has been developed for the high aspect ratio structures. Is this recipe each precursors is introduced into the chamber in two steps, first followed by a very short purge time and then followed by a longer purge time.

Recipe: TiO2T

Temperature: 120 ^oC - 350 ^oC

	TiCl4	TiCl4	H2O	H2O
Nitrogen flow	150 sccm	150 sccm	200 sccm	200 sccm
Pulse time	0.1 s	0.1 s	0.1 s	0.1 s
Purge time	0.5 s	20.0 s	0.5 s	20.0 s

Below some SEM images of amorphous TiO₂ deposited at 150 ^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 2000, and this results in a TiO₂ layer of about 90 nm. The purge times corresponds to the ones shown in the table above. From the SEM images it is seen that the TiO₂ layer covers the trenches very well. Trenches prepered using (DRIE-Pegasus). Research results based on this recipe can be found here: LINK

• Amorphous titanium dioxide deposited on Si trenches
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• Anastase titanium dioxide deposited on Si trenches
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Evgeniy Shkondin, DTU Danchip, 2014-2016.