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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.
Atomic Layer Etching typically involves a cycle of 4 steps that is repeated as many times as necessary to achieve the required etch depth. This example shows ALE of AlGaN etching 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.
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.
25nm wide Si trenches etched to 110nm depth by ALE, HSQ mask still in place.
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, left = before etching (Ra = 600pm), right = after etching (Ra = 300pm). The surface has been smoothed by ALE.
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 |
MoS2 |
Cl2 |
Ar |
Si or a-Si |
Cl2 |
Ar |
SiO2 |
CHF3 or C4F8 |
Ar or O2 |
AlGaN or GaN |
Cl2, BCl3 |
Ar |
AlGaN or GaN |
N2O |
BCl3 |
GaAs or AlGaAs |
Cl2, BCl3 |
Ar |
InP or InGaAsP etc. |
CH4, Cl2 |
Ar |
SiN |
H2 |
Ar |
Al2O3 |
BCl3 |
Ar |
Graphene |
O2 |
Ar |
HfO2, ZrO2 |
Cl2, BCl3 |
Ar |
AlGaN ALE Process Cycle
AlGaN Etch per Cycle with and without Chlorine dose
The PlasmaPro 100 ALE delivers precise process control of etching for next-generation semiconductor devices. Specially designed for processes such as recess etching for GaN HEMT applications and nanoscale layer etching, the system's digital/cyclical etch process offers low damage, smooth surfaces.
As layers become thinner to enable the next-generation semiconductor devices, there is a need for ever more precise process control to create and manipulate these layers. The PlasmaPro 100 ALE delivers this by enhancing our Cobra ICP platform with specialised hardware for atomic layer etching.
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Figure caption (LHS): Higher AlGaN surface roughness (0.8 nm Ra) for ICP-RIE process.
Figure caption (RHS): Reduced AlGaN surface roughness (0.4 nm Ra) for ICP-RIE & ALE process.
Figure caption: ALE for surface smoothing, low roughness processing of p-GaN devices to enable improved device performance.
Figure caption (LHS): GaN surface roughness (0.2 nm Ra) before ALE to demonstrate fully-recessed device with through-AlGaN etch.
Figure caption (RHS): GaN surface roughness (0.1 nm Ra) after ALE to demonstrate fully-recessed device with through-AlGaN etch.
Figure caption: ALE for accurate etch thickness control of remaining AlGaN to ±0.5 nm for partially-etch recess to enable normally-off devices and improved device reliability.
Figure caption: TEM verification across 3 samples of Etchpoint accuracy for AlGaN layer. Targeted AlGaN remaining thickness after ALE of 5 nm ±0.5 nm achieved, which was correlated to Etchpoint etch traces.
Etchpoint is a patent-pending UV reflectance-based endpoint technique with the optimised wavelength selected to allow for unrivalled accuracy of etch layer depth for GaN and AlGaN. Other endpoint solutions can typically achieve ±2 nm resolution which limits the capability to reliably fabricate some GaN HEMT device structures. This new etch-depth monitoring solution has been exclusively developed and optimised by Oxford Instruments in collaboration with LayTec. Etchpoint is fully integrated with both the hardware and software of the PlasmaPro 100 ALE system.
Platforms may be clustered to combine technologies and processes with either cassette or single wafer loading options. Hexagonal or square transfer chamber configurations are available.
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