Materials and Nanoscience Seminar with Steve George. 

Thermal Atomic Layer Etching (ALE):  ALD in Reverse

Thermal atomic layer etching (ALE) can be performed using sequential self-limiting surface reactions that yield controlled etching at the atomic level.  Thermal ALE is the reverse of atomic layer deposition (ALD).  The sequential reactions during thermal ALE are based on surface modification followed by volatile release of the modified surface layer.  Surface modification typically results from fluorination.  Volatile release can be accomplished using metal precursors that undergo ligand-exchange reactions with the fluorinated surface layer. 

Thermal Al₂O₃ ALE using HF for fluorination and Al(CH₃)₃ (trimethylaluminum (TMA)) for ligand-exchange is a model system for thermal ALE.  The temperature-dependent etch rate for thermal Al₂O₃ ALE varies from 0.14 Å/cycle at 250°C to 0.75 Å/cycle at 325°C.  The etch rate is also dependent on HF pressure because thicker fluoride layers are formed at higher HF pressure.  Thermal ALE can be very selective because the etching is dependent on the stability and volatility of the etch products.  ZrO₂ ALD monolayer etch stops can also be deposited that block the etching of Al₂O₃ and then can be removed later using ZrO₂ ALE.  

The deposit and etchback method can achieve ultrathin films with low pinhole density using ALD together with thermal ALE.  Nucleation difficulties often lead to pinholes in ultrathin ALD films.  By growing thicker initial Al₂O₃ ALD films and then etching them back using thermal Al₂O₃ ALE, ultrathin Al₂O₃ films with lower pinhole density can be achieved compared with Al₂O₃ ALD films of the same thickness.  The deposit and etchback method can be employed to form ultrathin pinhole-free films of various materials on flat substrates or particles.