Specific Process Knowledge/Characterization/XRD/XRD SmartLab/ALD deposited alumina and titania XRR

Purpose of the work

The work described below was conducted in 2019 by Sarah Elisabeth Hussein El Dib (s135170) and Evgeniy Shkondin, DTU Nanolab as a part of the master project in collaboration with the DTU Photonics Department. The XRR fitting results has been corrected in november 2021.

The prepared samples were investigated by X-ray Reflectivity (XRR) method. The idea is to investigate the initial growth of Al₂O₃ and TiO₂ on conventional Si samples (with native oxide) and measure the thickness of the layers in the range of 1 to 10 nm, where Spectroscopic Ellipsometry can be limited.

The deposition of ultrathin layers (with thickness below 10 nm) is difficult to characterize with ellipsometry alone, because of the model complexity. The prepared samples consist of a thin ALD film, but in order to properly distinguish it, the native oxide and surface moisture (with adventitious carbon contamination) should be taken into account since whey both have thicknesses comparable to ALD ultra-thin layer. For this purpose, the XRR method was employed.

Page has been updated in January 2022 with minor densities correction.

Deposition characteristics

A sequence of samples has been made by deposition Al₂O₃ and TiO₂ on flat natively oxidized Si (100) samples. The thickness setpoints were chosen to be 2, 4, 6, and 8 nm, and a number of cycles calculated from the published growth rates which were verified for thicker films (20-100nm).

Deposition of Al₂O₃

The standard recipe for deposition of Al₂O₃ on flat and high aspect ratio structures has been used.

Equipment: ALD-1

Recipe: AL2O3

Assumed deposition rate: 0.097 nm/cycle

Temperature: 200 ^oC

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

Number of cycles: 20, 40, 60, 80 for 2nm, 4nm, 6 nm and 8 nm respectively.

Deposition of TiO₂

The standard recipe for deposition of amorphous TiO₂ on flat and high aspect ratio structures has been used.

Recipe: amorphous TiO2,

Equipment: ALD-2

Assumed deposition rate: 0.0385 nm/cycle

Temperature: 150 ^oC

	TiCl₄	H₂O
Nitrogen flow	150 sccm	150 sccm
Pulse time	0.1 s	0.1 s
Purge time	4.0 s	6.0 s

Number of cycles: 52, 104, 156, 208 for 2nm, 4nm, 6 nm and 8 nm respectively.

X-ray reflectivity results

X-ray reflectivity (XRR) profiles for Al₂O₃ and TiO₂ films at different number of cycles has been obtained using Rigaku XRD SmartLab equipment. The voltage and current settings for the Cu X-ray tube were standard 40kV and 30mA. The incident optics contained a Ge220x2 monochromator and 5 ° Soller slit. Other slits: IS=0.03mm RS1=0.03mm and RS2=0.032mm. Step size: 0.01 and measurement time - 10s for each point. Fitting procesure was performed using commercial GlobalFit software assuming the model based on Si substrate with native oxide followed by the deposited ALD layer with thin moisture surface. The results are summarized in a tables below.

Sample model used in XRR analysis.
Sample considers to have ALD layer with top moisture deposited on natively oxidized silicon.

Si reference samples

Before the depositions of thin films, the Si samples has been analyzed with and without native oxide. It wil be used as a reference. Removing of native oxide has been done using 5% HF solution and rinsing with water. Results are shown below:

All XRR scans.
Measuring Si reference sample with and without native oxide.

Physical properties of Si reference sample with and without native oxide

	Si native oxide			Si substrate			Fitting parameters
Sample	SiO₂ Thickness (nm)	SiO₂ Density (g/cm³)	SiO₂ Roughness (nm)	Si Thickness (nm)	Si Density (g/cm³)	Si Roughness (nm)	Intensity	Background	Fitting area ( $2\Theta$ )	R	$\mathrm {X} ^{2}$
Si without native oxide	-	-	-	$\infty$	2.328	0.1	1.23109	2.95508E-7	0.4045-2.0	0.00864	0.00056
Si with native oxide	1.6	1.92	0.412	$\infty$	2.328	1.12	1.18812	3.22806E-7	0.4045-6.0	0.02357	0.01376

Fitting of Delta and Beta values
Sample	SiO₂		Si
Sample	Delta	Beta	Delta	Beta
Si without native oxide	-	-	7.5796E-6	1.7601E-7
Si with native oxide	6.2243E-6	8.1116E-8	7.5796E-6	1.7601E-7

Thin films samples

Below is an overview of all samples in total. XRR spectra are shifted in respect to each other in y-direction for better comparison. The reader can click on XRR Spectra for individual samples to see them in highere resolution.

All XRR scans.
All Al₂O₃ samples scan.
All TiO₂ samples scan.

Fitting resilts for Al₂O₃

XRR results for Al₂O₃ layers

	Top layer (moisture)			Main layer			Si native oxide			Si substrate
Number of cycles	Moisture thickness (nm)	Moisture density (g/cm³)	Moisture roughness (nm)	Al₂O₃ thickness (nm)	Al₂O₃ density (g/cm³)	Al₂O₃ roughness (nm)	SiO₂ thickness (nm)	SiO₂ density (g/cm³)	SiO₂ roughness (nm)	Si thickness (nm)	Si density (g/cm³)	Si roughness (nm)
20	1.22	0.99	0.46	2.11	3.30	1.04	1.33	1.60	1.01	$\infty$	2.328	0.02
40	1.01	1.82	0.36	4.00	2.88	0.41	2.10	2.11	0.50	$\infty$	2.328	0.31
60	1.00	1.80	0.37	6.29	2.86	0.40	2.07	2.10	0.48	$\infty$	2.328	0.27
80	0.99	1.85	0.38	8.46	2.97	0.46	2.02	2.10	0.56	$\infty$	2.328	0.38

Fitting parameters
Sample	Moisture		Al₂O₃		Si native oxide		Si substrate		Fitting parameters
Sample	Delta	Beta	Delta	Beta	Delta	Beta	Delta	Beta	Intensity	Background	Fitting area ( $2\Theta$ )	R	$\mathrm {X} ^{2}$
20 cycles	3.2291E-6	7.4986E-8	1.0491E-5	1.2221E-7	5.1604E-6	6.7251E-8	7.5796E-6	1.7601E-7	1.15165	4.1694E-7	0.3846 - 5.000	0.02218	0.01138
40 cycles	5.9060E-6	1.3693E-7	9.1612E-6	1.0654E-7	6.8125E-6	8.8635E-8	7.5669E-6	1.7544E-7	1.11031	1.24726E-8	0.3714 - 5.000	0.02677	0.01663
60 cycles	5.8344E-6	1.3527E-7	9.0960E-6	1.0578E-7	6.7815E-6	8.8233E-8	7.5669E-6	1.7544E-7	0.92337	2.27778E-7	0.3714 - 5.000	0.02671	0.01677
80 cycles	6.0443E-6	1.4036E-7	9.4679E-6	1.1029E-7	6.8062E-6	8.8700E-8	7.5796E-6	1.7601E-7	0.98482	6.13396E-8	0.3714 - 5.000	0.02601	0.01598

Fitting resilts for TiO₂

XRR results for TiO₂ layers

	Top layer (moisture)			Main layer			Si native oxide			Si substrate
Number of cycles	Moisture thickness (nm)	Moisture density (g/cm³)	Moisture roughness (nm)	TiO₂ thickness (nm)	TiO₂ density (g/cm³)	TiO₂ roughness (nm)	SiO₂ thickness (nm)	SiO₂ density (g/cm³)	SiO₂ roughness (nm)	Si thickness (nm)	Si density (g/cm³)	Si roughness (nm)
52	0.41	1.00	1.01	2.54	3.80	1.02	1.31	1.55	1.01	$\infty$	2.328	0.01
104	0.57	0.80	0.60	5.09	3.45	0.02	2.01	2.13	0.43	$\infty$	2.328	0.20
156	0.54	0.78	0.61	7.54	3.42	0.45	2.04	2.10	0.41	$\infty$	2.328	0.21
208	0.52	1.10	0.41	9.68	3.38	0.38	2.10	2.07	0.42	$\infty$	2.328	0.41

Fitting parameters
Sample	Moisture		Al₂O₃		Si native oxide		Si substrate		Fitting parameters
Sample	Delta	Beta	Delta	Beta	Delta	Beta	Delta	Beta	Intensity	Background	Fitting area ( $2\Theta$ )	R	$\mathrm {X} ^{2}$
52 cycles	3.2558E-6	7.5605E-8	1.1697E-5	5.7265E-7	5.0270E-6	6.5513E-8	7.5796E-6	1.7601E-7	1.20038	8.88999E-9	0.3846 - 5.000	0.02639	0.01450
104 cycles	2.5465E-6	5.9040E-8	1.0612E-5	5.1877E-7	6.9037E-6	8.9823E-8	7.5669E-6	1.7544E-7	1.12213	1.89786E-7	0.3846 - 5.000	0.01616	0.00516
156 cycles	2.5465E-6	5.9040E-8	1.0527E-5	5.1462E-7	6.8032E-6	8.8515E-8	7.5669E-6	1.7544E-7	1.18524	6.07729E-8	0.3846 - 5.000	0.02059	0.00854
208 cycles	3.5635E-6	8.2621E-8	1.0393E-5	5.0808E-7	6.6905E-6	8.7048E-8	7.5669E-6	1.7533E-7	1.21905	6.06500E-8	0.3846 - 5.000	0.01954	0.00725

Graphical illustration of thickness dependence for Al2O3 and TiO2 ultrathin layers

Deposition rate in the initial stage.
Al₂O₃ samples.
TiO₂ samples.