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Atomic Layer Deposition (ALD)

Atomic Layer Deposition (or ALD) is an advanced deposition technique that allows for ultra-thin films of a few nanometres to be deposited in a precisely controlled way. Not only does ALD provide excellent thickness control and uniformity but 3D structures can be covered with a conformal coating for high-aspect-ratio structures.

ALD relies on self-limiting surface reactions and therefore generally provides very low pin-hole and particle levels, which can benefit a wide range of applications. The level of film and interface control and high film quality provided are sought after for many applications. The usage of plasma allows for improved film properties, control thereof and a wide range of possible materials. The flexibility of unique surface pre-treatments allows for low damage processing.

Highlights

  • High quality films grown with ultimate thickness accuracy, one atomic layer at a time
  • Up to 200 mm wafer with typical uniformity <±2%
  • Excellent step coverage even inside high aspect ratio structures
  • Highly-conformal coating
  • Low pin-hole and particle levels
  • Low damage & low-temperature process
  • Reduced nucleation delay
  • Wide range of materials and processes
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ATOMIC LAYER DEPOSITION PROCESS

Atomic Layer Deposition typically involves a cycle of 4 steps that is repeated as many times as necessary to achieve the required deposited thickness. The example shows ALD of Al2O3 using Al(CH3), (TMA) and O2 plasma.

Step 1) Dosing of the substrate with a precursor vapour of TMA, which adsorbs on and reacts with the surface. With the correct choice of precursor and parameters, this reaction is self-limiting.

Step 2) Purging of all residual precursor and reaction products.

Step 3) Low damage remote plasma exposure to the surface with reactive oxygen radicals which oxidize the surface and remove surface ligands, this reaction is self-limiting due to the limited number of surface ligands.

Step 4) Reaction products are purged from the chamber.

Only step 3 varies between H2O for the thermal process or O2 plasma. As the ALD process deposits a (sub)angstrom thickness per cycle, control over the deposition process is obtained at the atomic scale.

 

1st stage of ALD Process showing the first half cycle

1st Half-Cycle

2nd stage of ALD Process showing purge of TMA

Purge

3rd stage of ALD process showing 2nd half-cycle

2nd Half-Cycle

4th stage of ALD process showing purge

Purge

THERMAL ALD

  • Conformal coating can be achieved even in high aspect ratio and complex structures
  • A wide variety of materials is possible with Atomic Layer Deposition, such as:
    • Oxides:
      Al2O3, HfO2, SiO2, TiO2, SrTiO3, Ta2O5, Gd2O3, ZrO2, Ga2O3, V2O5, Co3O4, ZnO, ZnO:Al, ZnO:B, In2O3:H, WO3, MoO3, Nb2O5, NiO, MgO, RuO2
    • Fluorides: MgF2, AlF3
    • Organic-hybrid materials: Alucone
    • Nitrides: TiN, TaN, Si3N4, AlN, GaN, WN, HfN, NbN, GdN, VN, ZrN
    • Metals: Pt, Ru, Pd, Ni, W
    • Sulfides: ZnS, MoS2

PLASMA ENHANCED ALD (PE-ALD)

In addition to the benefits of thermal ALD, PEALD allows for a wider choice of precursor chemistry with enhanced film quality:

  • Plasma enables low-temperature ALD processes and the remote source maintains low plasma damage
  • Eliminates the need for water as a precursor, reducing purge times between ALD cycles - especially for low temperatures
  • Higher quality films through improved removal of impurities, leading to lower resistivity, higher density, etc
  • Effective metal chemistry through use of hydrogen plasma
  • Ability to control stoichiometry/phase
  • Reduced nucleation delay
  • Plasma surface treatment
  • Plasma cleaning of chamber is possible for some materials

Conformal coating of high aspect ratio (15:1) structure with high-rate plasma ALD SiO2

Conformal coating of high aspect ratio (15:1) structure with high-rate plasma ALD SiO2

Al2O3 deposited by FlexAL ALD Courtesy TUE

Al2O3 deposited by FlexAL ALD - Courtesy by Eindhoven University of Technology

Conformal deposition of SiO2, TiO2 and Al2O3 by Plasma ALD

Conformal deposition of SiO2, TiO2 and Al2O3 by Plasma ALD, (CC BY 4.0 license), image library at www.AtomicLimits.com, 2021

KEY FEATURES OF ALD

  • Guaranteed processes set up by our engineers
  • Plasma surface pre-treatments
  • Oxides
    • Low-temperature processing with high material quality
    • Doping and mixing
  • Nitrides
    • Low resistivity
    • Low oxygen content, high refractive index
  • Metals
    • Low nucleation delay with plasma
    • Low-temperature deposition
  • Substrate biasing:
    • During plasma, ALD to control material properties
      • Stress, density, crystallinity (and others)
    • Before plasma ALD to pre-clean substrate surfaces
      • Etches Al2O3, HfO2, SiO2, Si3N4
    • After plasma, ALD to modify material and surface properties
  • Option to have substrate biasing for further process control and improved material properties.
ALD Plasma Process Diagram

ALD Systems & Advantages

PlasmaPro ASP FlexAL
Atomfab
Loading Load lock or Cassette handler Load lock or Cassette
Cassette handler. Brooks MMX with aligner. Optional cool-down station.
Substrates Up to 200mm wafers handling and pieces on a carrier plate Up to 200mm wafers handling and pieces on a carrier plate Can be configured for 200 mm, 150 mm or 100 mm.
Bubbled liquid & solid precursors Up to 6 precursors, bubbled or vapour drawn Up to 8 plus water, ozone and gases Vapour drawn precursors
Max precursor source temperature Up to 200 °C 200ºC High vapour pressure precursors cooled below room temperature for controlled, repeatable dosing 
MFC controlled gas lines with rapid delivery system;  1) thermal gas precursors (e.g. NH<sub>3</sub>, O<sub>2</sub>) 2) plasma gases (e.g. O<sub>2</sub>, N<sub>2</sub>, H<sub>2</sub>) On board 4 configurable (toxic or non-toxic lines) line gas pod and 1 fixed Ar Up to 10 in externally mounted gas pod On board 4 configurable (toxic or non-toxic lines) line gas pod and 1 fixed Ar
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Our Atomic Layer Deposition equipment is built on well over a decade of experience. Key features include of Oxford Instruments systems include:

  • Dose gas pulses down to 10msecs, giving excellent control of dose quantity.
  • Fast recipe control, down to 10msecs.
  • Software control between plasma and thermal ALD.
  • Load lock, Automatic Pressure Control (APC) valve (150 ms open and close during ALD cycle), turbopump for fast cycle times, moisture-sensitive nitrides and metals.
  • Excellent control of ion energy, both through pressure and power control. Option of RF substrate biasing to enhance ion energies for further process control.

WIDE RANGE OF MATERIALS

A wide variety of materials is possible with Atomic Layer Deposition and a wide range of processes can be guaranteed and set up by our process engineers. For novel processes, our extensive process knowledge and vast network allow us to provide starting point recipes that should be good starting blocks to go quickly towards a robust process.

Often plasma-based processes are available utilizing our plasma knowledge and handling of MFC controlled gas mixtures including toxic gases.

2D Materials

FlexAL2D for ALD 2D Materials

2D materials growth can also be grown by ALD which is a new development with the aim to go toward high-quality MoS2 films. ALD chemistry control has the promise to be able to utilize 2D sulphides with their unique properties at CMOS compatible temperatures with precise digital thickness control over a large area (200mm wafers).

Metals

Fluorides

Sulphides

Pt

AlF<sub>3

MoS<sub>2

Ru

MgF<sub>2

Oxides

Nitrides

Al<sub>2</sub>O<sub>3

AlN

Co<sub>3</sub>O<sub>4

Ga<sub>2</sub>O<sub>3

GaN

HfO<sub>2

HfN

In<sub>2</sub>O<sub>3

Li<sub>2</sub>CO<sub>3

MoO<sub>3

Nb<sub>2</sub>O<sub>5

NbN

NiO

SiO<sub>2

Si<sub>3</sub>N<sub>4

SnO<sub>2

Ta<sub>2</sub>O<sub>5

TaN

TiO<sub>2

TiN

WO<sub>3

WN

ZnO

ZrO2

We are very pleased to present the research projects of two PhD students from Eindhoven University of Technology (TU/e). As a lead university in Engineering Science and Technology, TU/e has been working on innovative process technologies dedicated to advancing the industrial application of Atomic Layer Deposition (ALD), an advanced deposition technique that allows for ultra-thin films to be deposited with atomic-level thickness control.

After 15 years of partnership between Oxford Instruments Plasma Technology and the Eindhoven University of Technology, we continue to push the boundaries of ALD research and development, one of the most rapidly evolving techniques used in many applications of nanofabrication. Both research students, Karsten Arts and Marc Merkx, have used Oxford Instruments’ FlexAL ALD system featuring a remote inductively coupled plasma source, enabling high-quality deposition.

OXFORD INSTRUMENTS ALD EQUIPMENT ADVANTAGES

Our Atomic Layer Deposition equipment is built on well over a decade of experience. Key features include of Oxford Instruments systems include:

Atomfab – ALD System for HVM

PACE | PERFORMANCE | PLASMA

Atomfab is the fastest remote plasma ALD system for HVM on the market, specifically designed for manfacturing GaN HEMT and RF Devices.

  • Competitive CoO
  • Quick, easy maintenance
  • Excellent film uniformity
  • High material quality
  • Low substrate damage
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Atomfab ALD system for HVM

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