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Atomic Layer Etching (ALE)

Atomic Layer Etching (or ALE) is an advanced etch technique that allows for excellent depth control on shallow features. As device feature size reduces further and further ALE is required to achieve the accuracy required for peak performance. 

High fidelity pattern transfer (etching) is essential for the fabrication of today’s advanced microelectronic devices. As features shrink to sub-10nm levels, and novel devices make use of ultra-thin 2D materials, there is an increasing need for atomic-scale fidelity.

This has led to a growing interest in a technique known as Atomic Layer Etching (ALE), which overcomes the limitations of conventional (continuous) etching at the atomic scale. Plasma-based atomic layer etching is a cyclical etching process of gas dosing and ion bombardment that removes material layer by layer and has the potential to remove single atomic layers with very low damage.

Highlights

  • Ultra low damage
  • High selectivity
  • Ultra accurate depth control
  • Ultra low power operation
  • Ability to etch in ALE or normal etch mode
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Atomic Layer Etching Process

Atomic Layer Etching typically involves a cycle of 4 steps that is repeated as many times as necessary to achieve the required etch depth. The example shows ALE of Si with Cl2/Ar:

Step 1) Dosing of the substrate with an etching gas, which adsorbs on and reacts with the etch material. The etch gas is often plasma dissociated to enhance the rate of adsorption. With the correct choice of dosing gas and parameters, this can be self-limiting, if the chemical dose stops after adsorbing one monolayer.

Step 2) Purging of all residual dose gas.

Step 3) Bombardment of the surface with low energy inert ions which removes the reacted surface layer. This can be self-limiting if the energy of the ions is sufficient to remove the chemically modified layer, but insufficient to (sputter) etch the underlying bulk material.

Step 4) Etching products are purged from the chamber.

ALE Process of Si With Cl2 Ar

 Benefits of Atomic Layer Etching

  • Low damage etching, due to the use of low ion energies
  • Precise control of etching depth
  • Ultra-thin layer removal
  • Self-limiting behaviour
  • High selectivity, since dose gas and ion energy can be tailored to minimise etching of mask layers or underlying materials
  • Etch rate is less affected by the aspect ratio of etched features (i.e. reduced ARDE), since the supply of radicals and surface ion bombardment have been separated into independent steps
  • Improved uniformity, due to its self-limiting nature
  • Smooth etch surfaces
  • Anisotropic in nature, due to the reliance on ion bombardment

Read the article

Atomic Layer Etching: What for?

Read our article in Compound Semiconductor

Atomic layer etching promises to improve the quality of GaN-based HEMTs and eradicate the damage associated with high etching rates.

Written by Dr Mike Cooke and Dr Andy Goodyear for Compound Semiconductor magazine.



ALE Features

  • Etch rates 2 to 7Å/cycle
  • Demonstrated results in a-Si, Si, SiO2, MoS2, GaN, AlGaN layer etching
  • Fast recipe control down to 10ms
  • Atomic Layer Deposition-style gas dose delivery with 10ms open-close response 
25nm wide Si trenches etched to 110nm depth by ALE, HSQ mask still in place

25nm wide Si trenches etched to 110nm depth by ALE, HSQ mask still in place.

ALE of MoS2 shows no Raman defect

ALE of MoS2 shows no Raman defect peak after etching, highlighting the low damage etching capabilities of ALE.

 

AlGaN surface roughness after 200 ALE cycles

AlGaN surface roughness after 200 ALE cycles, left = before etching (Ra = 600pm), right = after etching (Ra = 300pm). The surface has been smoothed by ALE.

Wide Range of Materials

ALE is suitable for a wide range of materials, including Si, a-Si, MoS2, SiO2, GaN, AlGaN, III-V’s, Si3N4, graphene, HfO2, ZrO2, Al2O3, metals etc.

Material Etched Dose Gas Etch Gas
MoS<sub>2 Cl<sub>2 Ar
Si or a-Si Cl<sub>2 Ar
SiO<sub>2 CHF<sub>3</sub> or C<sub>4</sub>F<sub>8 Ar or O<sub>2
AlGaN or GaN Cl<sub>2</sub>, BCl<sub>3 Ar
AlGaN or GaN N<sub>2</sub>O BCl<sub>3
GaAs or AlGaAs Cl<sub>2</sub>, BCl<sub>3 Ar
InP or InGaAsP etc. CH<sub>4</sub>, Cl<sub>2 Ar
SiN H<sub>2 Ar
Al<sub>2</sub>O<sub>3 BCl<sub>3 Ar
Graphene O<sub>2 Ar
HfO<sub>2</sub>, ZrO<sub>2 Cl<sub>2</sub>, BCl<sub>3 Ar
ALE AlGaN cycle

AlGaN ALE Process Cycle

AlGaN EPC

AlGaN Etch per Cycle with and without Chlorine dose

Oxford Instruments Plasma Technology: ALE Equipment Advantages

Our Atomic Layer Etching equipment is built on 13+ years of ALD experience. Key features include:

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Atomic Layer Etching Systems