Atomic Layer Deposition

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What Is Atomic Layer Deposition

Atomic Layer Deposition (ALD) is a gas phase deposition process based on sequential surface reactions to grow thin films of different materials, wherein the target substrate is exposed, sequentially, to one such reaction at a time.

These reactions are surface-limited, with only a monolayer of a given precursor reacting  with and/or chemisorbing on the surface.  The result is a pinhole free, conformal coating, with superior process control in comparison with other thin film deposition methods, such as Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD).

Key features of ALD that led to its growing applications in the design and fabrication of advanced materials and devices are:

  • thickness control with atomic layer accuracy and precision
  • conformal coverage over high aspect ratio and high surface area materials
  • very low defect density with high uniformity
  • complex and multilayered films (e.g. nanolaminates, nanoalloys and precise doping)
  • a wide variety of insulating, conducting and semiconducting films

Materials Deposited by ALD

ALD of many different materials on flat substrates have been demonstrated.

  • oxides, i.e. Al2O3, ZnOx, InOx, TiO2, ZrO2, SiO2, HfO2, Ta2O5, SnO2
  • nitrides, i.e. SiNx, TaNx, BN, TiN
  • pure metals, i.e. Pt, Ni, Ir, Pd, Ru, Au
  • certain polymers.

New processes continue to emerge for ALD of other materials and composites.   However, not all of these materials could be easily deployed on high surface area materials with high aspect ratio nanopores.  In particular, plasma-based ALD processes  are poorly suited as plasma doesn’t penetrate very deep into the pores.  On the other hand, thermal ALD processes are more amenable for conformal coatings of high aspect ratio nanopores in materials such as AAO.

Atomic Layer Deposition on planar substrates

An example of partial reactions for ALD deposition of zinc oxide:

(A)   ZnOH* + Zn(CH2CH3)2   ->   ZnOZnCH2CH3* + CH3CH3
(B)   ZnCH2CH3* + H2O  ->  ZnOH* + CH3CH3

Reagent 1: diethyl zinc  Zn(CH2CH3)2       Reagent 2: water vapor H2O

AAO as a Platform for High Aspect Ratio ALD

The synergistic combination of ALD with Anodic Aluminum Oxide (AAO) enables control of both the nanoscale geometry and chemistry with precision unavailable with other processes and materials.   The resulting well-characterized nanomaterial platform is ideally suited to support development and validation of high surface area ALD as well as to provide new avenues for expanding the library of AAO-based functional materials and devices.

ALD of alumina was first used for controlled reduction of the pore diameter in anisotropic AAO membranes by George’s group as early as 1997.  Since then, several groups have demonstrated ALD’s potential for precision engineering of nanoscale geometry, surface chemistry and properties of AAO.  Some of the work included contributions by InRedox founders:

ALD Applications

  • Semiconductor manufacturing of advanced microprocessors (gate dielectrics, advanced lithography and barrier / seed)  and memory (capacitor dielectrics)
  • Other microelectronic devices (i.e. hard drives)
  • Biomedical components and devices (i.e. implantable sensors and drug delivery devices)
  • Materials for energy generations and storage (i.e. photocatalytic materials, electrodes for batteries and supercapacitors).

Many applications involve deposition of highly engineered ALD coatings onto high surface area substrates, from integrated circuits with complex topology and high aspect ratio vias to nanoparticles and nanoporous membranes (such as  Anodic Aluminum Oxide (AAO).

How ALD works in High Aspect Ratio Pores

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ALD process inside the pores of AAO
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nanopore surface terminated with -OH

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diethyl zinc, Zn(CH2CH3)2

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purge with inert gas to remove excess diethyl zinc from the gas phase

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surface saturated with -O-Zn-CH2CH3

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water vapor ( H2O)

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purge with inert gas to remove excess water from the gas phase

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monolayer of ZnO has formed, surface terminated with -OH groups

This capability is now offered by InRedox through a unique partnership with Arradiance, a recognized leader in ALD equipment and process development, serving the materials science, nanotechnology, clean energy, nano-electronics and photonics markets. A core ALD capability, for which Arradiance has received more than 12 US patents (with several pending), is the application of functional nanocoatings to high-aspect-ratio, high-surface-area substrates.

InRedox introduces two groups of AAO-ALD products:

  • AAO-ALD Nanocomposites – nanoporous anodic alumina membranes, wafers and films with conformal coatings of different materials (i.e., oxides, metals, nanolaminates, etc)

Recent Examples of AAO-ALD Nanocomposites

Conformal ALD inside high aspect ratio nanopores of InRedox’ AAO films was demonstrated by  Arradiance for platinum (Pt); yittria-stabilized zirconia (YSZ) and platinum- alumina (Pt-Al203) nanolaminates.

Representative SEM of ALD inside the pores of AAO

Nanoporous AAO films on Al substrate with pores closed at the bottom.  Pore diameter 50 nm, pore length or AAO thickness 42 μm, aspect ratio of ~840:1,  total surface area greater than 9 m2.

Other Examples of High Surface Area ALD

Arradiance – InRedox partner for these ALD-AAO nanocomposites – has developed an array of ALD processes and analysis techniques to create conformal nanoengineered coatings in complex high aspect ratio and high surface area environments other than AAO. Examples include:

  • Conductive and Secondary Electron Emissive nanofilms for Microchannel Plates
  • Yittria stabilized zirconia (YSZ) nanofilm for solid oxide fuel cells.
  • Conducting and insulating nanofilms for supercapacitors.