Since the beginning of AS-ALD research using inhibitors, the question about the blocking mechanism was explored. The main question here is thus how the inhibitor prevents the precursor from reacting with the surface. Already in the landmark work by Bent and co-workers, the two major mechanisms were proposed that are still discussed today: steric blocking and chemical passivation (Fig. .). Steric blocking is the physical blocking of the surface by a bulky tail group of an inhibitor (SMI or SAM, Fig. .). Chemical passivation means the chemical reaction of the inhibitor with the reactive groups at the surface that would otherwise engage in the ALD reaction with the precursor and/or co-reactant (Fig. .a). For SAMs, as discussed in section 3.4.1.1, the blocking mechanism is clearly steric blocking. The difference in packing of similar SMIs was found, e.g., by Karasulu et al. to lead to more effective steric shielding. Although diffusion through the SAM layer with a possible reaction with nonpassivated surface groups has been hypothesized, there is no clear evidence yet. Thus, the discussion is focused in the following on SMIs. Experimental work indicated already that both mechanisms are at hand in most SMIs, and some trends were discussed. For example, a near linear relationship between SMI size and nucleation delay was found, but the limited number of three data points should be noted. Computational studies can shed more light on this question. For oxide surfaces, the chemical passivation of the reactive OH groups has been found crucial to achieve selectivity. But it was also found that some SMIs only cover approx. 50% of these surface hydroxyls. As discussed in section 3.4.1.2, there are several ways to increase SMI coverage. This was investigated systematically in a computational study for the SMI TMPS on SiO2 and blocking mechanisms dependent on SMI coverage were found. While at low SMI coverages, the precursor is not blocked at all, steric blocking is the major effect at high coverages. However, at medium coverages, there is a variant of the steric blocking effect, termed "reactivity reduction" (Fig. .d). In this case, the precursor can reach the surface and attach to the reactive surface groups, but it is not able to make ligand exchange reactions due to the limited space confined by the surrounding SMI layer. Thus, an unproductive desorption will take place eventually. For Hacac, the question of blocking mechanisms has been investigated in detail from experimental as well as computational perspective by the Mackus and Sandoval groups. They also compared Hacac to acetic acid (HAc) and 2,2,6,6-tetramethyl-3,5-heptanedione (Hthd) to better reveal the factors governing the contributions of the two mechanisms. Their ALD experiments and spectroscopic investigations show that HAc and Hthd are the better SMIs for ALD of SiO2 on