TU/e case study:
Using advanced plasma
ALD technology to achieve
conformal deposition on highaspect ratio features, control
over film properties andselective area growth.
Image Source: Bart van Overbeeke Photography

August 2021

Using advanced plasma ALD technology
to achieve conformal deposition on highaspect ratio features, control over film
properties and selective area growth.
We are very pleased to present the research
projects of two PhD students from Eindhoven
University of Technology (TU/e). As a leading
university in Engineering Science and Technology,
TU/e has been working on innovative process
technologies dedicated to advancing theindustrial application of Atomic Layer Deposition(ALD), an advanced deposition technique thatallows for ultra-thin films to be deposited withatomic-level thickness control.
After 15 years of partnership between Oxford
Instruments Plasma Technology and Eindhoven
University of Technology, we continue topush the boundaries of ALD research anddevelopment which is one of the most rapidly
evolving techniques used in many applicationsof nanofabrication. Both research students,
Karsten Arts and Marc Merkx, have used Oxford
Instruments’ FlexAL ALD system featuring aremote inductively coupled plasma source,enabling high quality deposition.

The Vital Role of Film Conformality
Karsten Arts has focused on the influence of ionson film conformality during the plasma‑enhanced
ALD process. Film conformality can bechallenging to achieve, especially in 3D
nanostructures with deep trenches and highaspect ratios, with the thickness uniformity ofthe deposited film being very important for many
applications.

© Oxford Instruments plc, 2021. TU/e case study 2

These deep trenches are challenging to coverby uniform films using conventional methods ofdeposition. In contrast, ALD is renowned for its
ability to cover 3D nanostructured surfaces withextremely high aspect ratio and simultaneously
achieve precise control in material properties.
The ability to obtain highly conformal coatings
with controlled thickness have made ALD
popular for many nanotechnology applications.
Previously, Karsten and the team at TU/ehave found out how to cover extremely deep
aspect ratio features by optimising the process
conditions (e.g. the temperature of the substrates
and the plasma pressures). The reached aspectratio with plasma ALD of silicon oxide (SiO2) andtitanium oxide (TiO2) can approach 1000 which isfar beyond what was previously expected to beachievable by plasma ALD (up to 50 or so).

TiO2 on vertical surfaces (the sidewalls). “Thisinfluence is verified to significantly affect the film
conformality obtained on 3D nanostructures”1,
as the film does not grow with uniformity in thenanostructure.
However, under the right plasma conditions andenergy of ions, Karsten and the research team haveaccomplished deposition of TiO2 with high film
conformality and process reproducibility. The ion
energy and flux can be tuned to influence the film
properties on horizontal surfaces when desired.
Oxford Instruments’ FlexAL ALD system provides
an RF-biased electrode option available forcontrol of film properties and allows low process

Researching Area-Selective ALD
Marc Merkx has focused on understanding
the capabilities of area-selective processes
during atomic layer deposition (ALD). The fieldof area-selective ALD has gained interest in
the semiconductor industry, as it gives more
process control at the atomic scale and can
reduce fabrication errors by replacing top‑down
approaches by self-aligned bottom-up approaches.
Area-selective deposition during ALD enables
production to reduce lithography steps, meaninga more simplified process, and elimination ofalignment issues that might be caused afterlithography.

In his current research paper, Karsten
demonstrates how to achieve TiO2 film
conformality by using plasma-assisted atomic
layer deposition with Oxford Instruments’ FlexAL
system and low-energy ions on vertical trenchnanostructures and microscopic cavity structures.
As Karsten has observed, “The influenceof ions can be significant even under mild
plasma conditions and when using a groundedsubstrate”1. With plasma present, the ions areaccelerated vertically, bombarding the horizontal
surfaces,but they are not interacting with thetemperatures to enable high-quality deposition
on more temperature-sensitive materials suchas polymers. FlexAL enables researchers toprecisely control the ion energy and flux of ionswhich is a crucial factor in influencing the film
growth of silicon oxide and titanium dioxide.

Figure 1: Image of different trenches,with layers of TiO2 (white layers), SiO2(dark grey layers), and a single layerof Al2O3, all grown by plasma ALD
using an Oxford Instruments’ FlexAL
ALD system1.

Marc and the research team achieved highly
selective growth of titanium nitride (TiN), an
important conductive material for application as
diffusion barrier and contact material for instance.
TiN was deposited with Oxford Instruments’FlexAL ALD system, achieving high selectivity on
dielectric surfaces (SiO2, Al2O3) without using any
patterning.

© Oxford Instruments plc, 2021. TU/e case study 3

Figure 2: Schematic illustration of theABCD-type ALD cycle2.

provides remote plasma for low damage
plasma ALD combined with thermal ALD in one
deposition chamber.

The ALD cycle of TiN started by exposing thesubstrate to “small inhibitor molecules that
selectively adsorb on the non-growth area”2 which
prevent the precursor adsorption and ultimately
the precursor molecules are absorbed selectively
only on the growth area. After applying plasmaon the substrate, the adsorbed precursors areconverted into a nitride. The final step of thisprocess uses a remote plasma with substrate bias
to remove any impurities that have been caused
during the process, ensuring that only high-quality
nitride is deposited on the substrate.
Area-selective ALD can been achieved evenon very small surface features (e.g. 50 nm)
and yields “high-quality TiN films in terms ofconductivity and oxygen impurity levels”2. TiN
growth around 6 nanometres has been achieved
with the FlexAL ALD system on SiO2 and Al2O3
surfaces with high selectivity.

As Marc states, “One of the greatest
advantages of FlexAL system is the plasma
capabilities”. Oxford Instruments’ FlexAL offers
maximum flexibility in the choice of precursors,process gases and hardware features andThe revolution of Atomic Layer
Deposition
Both research papers have made great
contributions to the understanding of fundamental
research of ALD. Advancements and new
research projects about film conformality andarea-selective ALD can give great insights that
could improve the control and capability ofnanofabrication in many applications.
With more than 6000 process recipes in ourdatabase, Oxford Instruments Plasma Technology
will continue to support R&D and production
customers and apply our expertise to further
develop their fabrication processes to achieve
novel research and maximise device performance.

Figure 3: Area-selective ALD of TiN2.

© Oxford Instruments plc, 2021. TU/e case study 4

Marc Merkx

Karsten Arts

Worldwide ServiceEmailplasma@oxinst.com

Oxford Instruments is committed to supporting ourcustomers’ success.
We recognise that this requires world class products
complemented by world class support. Our global
service force is backed by regional offices, offering
rapid support wherever you are in the world.

Phone

We can provide:• Flexible service agreements to meet your needs

References
1. Impact of Ions on Film Conformality andCrystallinity during Plasma-Assisted AtomicLayer Deposition of TiO2
Karsten Arts, Harvey Thepass, Marcel A.
Verheijen, Riikka L. Puurunen, Wilhelmus M.
M. Kessels, and Harm C. M. Knoops
Chemistry of Materials Article ASAP
DOI: 10.1021/acs.chemmater.1c00781
2. A
 rea-Selective Atomic Layer Deposition ofTiN Using Aromatic Inhibitor Molecules for
Metal/Dielectric Selectivity
Marc J. M. Merkx, Sander Vlaanderen, Tahsin
Faraz, Marcel A. Verheijen, Wilhelmus M. M.
Kessels, and Adriaan J. M. Mackus
Chemistry of Materials 2020 32 (18),7788‑7795
DOI: 10.1021/acs.chemmater.0c02370• Tailored system training courses• System upgrades and refurbishments• Immediate access to genuine spare parts andaccessories

UK, Yatton+44 (0) 1934 837000
Germany, Wiesbaden+49 (0) 6122 937 161
India, Mumbai+91 22 4253 5100
Japan, Tokyo+81 3 5245 3261
PR China:• Beijing• Guangzhou• Shanghai+86 400 678 0609
Singapore+65 6337 6848
Taiwan+886 3 5788696
US, Canada & Latin America
Concord, MA+1 800 447 4717

plasma.oxinst.com© Oxford Instruments plc, 2021. TU/e case study 5