As the dimensions of semiconductor devices continue to shrink, conventional fabrication processes face major challenges, including quantum tunneling through thin gate oxides, which leads to increased gate leakage current. To address this, atomic layer deposition (ALD) offers precise, conformal thin film growth on complex surfaces. ALD has been adopted in industry to deposit high-k metal oxides, replacing conventional SiO2 gate dielectrics. As a subcategory of ALD, area-selective ALD (AS-ALD) is a promising technique that enables deposition only on targeted regions by leveraging the differences in surface functional groups. This study first investigated the initial film growth of two different aluminum (Al) precursors in a custom-built ultra-high vacuum (UHV) chamber. Trimethylaluminum (TMA), a standard and widely used Al2O3 precursor, was examined first, followed by bis-dimethylamino-diamino-aluminum (BDMADA–Al), a nonpyrophoric alternative to TMA. In-situ characterization was conducted using quadrupole mass spectrometry (QMS) and X-ray photoelectron spectroscopy (XPS). AS-ALD of Al2O3 was then performed using BDMADA–Al as the precursor and dimethylamino trimethylsilane (DMATMS) as the blocking molecule in a custom-built viscous flow reactor. Three different deposition processes were tested to investigate blocking performance and film composition. Ex-situ water contact angle (WCA) and XPS measurements confirmed effective blocking when DMATMS was reapplied between each cycle of ALD.