Vacuum-based long-term thermal stability evaluation of metal-organic precursors for atomic layer deposition

Understanding the long-term thermal stability of metal-organic precursors is critical for maintaining consistent and reliable atomic layer deposition (ALD) performance. Conventional evaluations, such as thermogravimetric analysis (TGA) and bulk long-term thermal stability tests in sealed containers, do not reproduce the vacuum exposure and pressure fluctuations encountered under practical ALD process environments. In this work, we developed a vacuum-based long-term stability evaluation platform that reproduces process-relevant pressure cycling while enabling extended thermal treatment and periodic, atmosphere-free sampling of liquid precursors. Using tetrakis(ethylmethylamino)hafnium (TEMAHf) as a model precursor, we compare non-pumping, 6-h cyclic pumping, and 1-h cyclic pumping environments and track degradation using colorimetry, TGA residual mass, viscosity, and vapor pressure. More frequent pressure cycling accelerates degradation, evidenced by stronger color changes, increased non-volatile residue accumulation, higher viscosity, and reduced vapor pressure. These results indicate that non-vacuum, non-cycling tests can underestimate process-relevant long-term degradation and highlight the need for vacuum- and pressure cycling-based protocols to screen precursor stability and guide precursor and process optimization in ALD.