Atomic Layer Deposition of Boron-Containing Layers Using a Combined Trimethylborate- and Water-Based Plasma

Trimethylborate (TMB), a commonly used boron precursor for depositing boron-containing thin films via atomic layer deposition (ALD), poses several limitations, including low boron incorporation, growth inhibition, and process temperature constraints. Despite TMB’s high vapor pressure and thermal stability, its limited reactivity leads to slow and incomplete surface reactions in thermal processes. Plasma-enhanced processes employing oxygen plasma as reactant, although effective in avoiding carbon impurities, tend to produce films with a low boron content. To overcome these limitations, we initially explored the use of TMB plasma as a reactant, inspired by the success of trimethyl phosphate (TMP) plasma in the synthesis of various ALD metal phosphates. However, TMB alone in plasma form proved to be ineffective as a reactant due to the absence of a temperature window for self-limiting growth on the substrate surface. In response, we developed an approach that combines TMB with H₂O as a coreactant in the plasma phase. This method demonstrated self-limiting growth at and above 250 °C, a significantly higher growth per cycle (∼3.5 Å), and a marked increase in boron concentration (>25% increase vs O₂ plasma process and >85% increase vs thermal process) in the aluminum borate thin films. The underlying growth mechanisms were analyzed using in situ ellipsometry, in vacuo X-ray photoelectron spectroscopy (XPS), and time-resolved quadrupole mass spectrometry (QMS). These findings not only provide valuable insights into the deposition of aluminum borate films but also provide a promising pathway for the development of other metal borates or boron-containing layers by the ALD technique.