Specific Process Knowledge/Etch/Aluminum Oxide/Al2O3 Etch using HF

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A wet chemical etch of Al₂O₃ can be done with HF. The etch rate depends on the HF concentration.

Experiment and results

Si samples with about 100 nm of ALD deposited Al₂O₃ (1000 cycles at 300^oC) has been etched in different HF concentrations. After the etching, the thickness of the Al₂O₃ layer has been measured using Ellipsometer VASE, and the thickness as function of time has been plotted as shown in the graph below.

• 
  Different etching rates for different HF concentrations.

The etch rates:

• 5% HF → 1.74 nm/s

• 1vol. H₂O : 1vol. 5% HF → 1.05 nm/s

• 2vol. H₂O : 1vol. 5% HF → 0.72 nm/s

Be aware of that the 5% HF etches quite fast. Actually, so fast that it can be tricky to control. Taking the sample out of the solution and placing it in a bigger water container to rinse, resulting a time delay due to the movement and the etch continues. It means the handling things around the fumehood can be a source of errors in this case.

Evgeniy Shkondin, DTU Nanolab, June 2019

Photoresist adhesion

Experiments in April 2025 as shown bad adhesion of AZ 5214E on Al2O3. Total delamination occurred in less than 120 seconds on 50nm and 100nm of Al2O3 deposited using ALD. Thus wetetch is unsuitable to structure an Al2O3 for use as hard-mask for dryetching.
Systematic studies have not yet been done as of April 2025.

Following parameters were tested:

Process parameters

ALD Parameters	Cycles: 500 and 1000 Temperature: 200°C TMA pulse: 0.1s H2O pulse: 0.1s
Lithography parameters	Pretreatment: HMDS Resist type: AZ 5214E Resist thickness: 1.5µm Exposure parameters: Tool: MLA3 Dose: 100 mJ/cm2 Defocus: 1 Mode: Fast Development: TMAH, SP60 Post treatment: Hardbake 120s@120C

Jesper Pan, DTU Nanolab, April 2025