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.

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How Atomic Layer Deposition Works

Highlights

Self-limiting atomic layer-by-layer growth
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

Purge

2nd Half-Cycle

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

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 (FlexAL)
- 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 SYSTEMS

Our products offer the following capabilities:

	FLEXAL	ATOMFAB
Loading	Load lock or Cassette	Clusterable cassette
Substrates	Up to 200mm wafers handling and pieces on a carrier plate
Bubbled liquid & solid precursors	Up to 8 plus water, ozone and gases
Max precursor source temperature	200ºC
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>)	Up to 10 in externally mounted gas pod

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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

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
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

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:

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.

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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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