Adsorption of the gaseous precursor onto the surface of the substrate

The first step, the exposure of the vapor-phase precursor to the surface of the substrate, requires that the precursor molecules adsorb onto the substrate. Although the adsorption step preferably involves loss of a ligand and formation of a chemical bond, physically adsorbed species may be acceptable for some ALD systems on flat surfaces so long as the single-layer physisorbed species interacts with the surface more strongly than the precursor molecules interact with a condensed film [33]. The advantage to using physisorbed species is that the coverage of adsorbed precursors will be higher, leading to a higher growth rate [33]. This is acceptable in the semiconductor industry to keep up with the cost of production rate. Uniformity based on physisorbed precursors can be achieved using a flow ALD reactor system on flat samples with short times between pulses, typically 0.1 s; however, for ALD on porous materials, these conditions leading to physisorbed

11 monolayers and multilayers will likely lead to uneven growth of materials in different parts of the surface.

When ALD is carried out on flat substrates, such as silicon wafers, the use of a carrier gas allows the transport of precursors and reactants to be delivered rapidly from the gas stream onto the substrate surface. It has shown success in coating high-aspect ratio structures with straight pores and uniform films have been successfully demonstrated on the surface of trenches with aspect ratios of ~100 etched into silicon [34]. However, the use of a carrier gas poses a serious problem when performing ALD on high-surface-area catalyst support. The rapid convective transport of precursors in the carrier gas flow strongly impede the diffusion of species into the porous samples. This leads to several problems. First, the outermost surface of the sample bed will be exposed to precursor fluxes several orders of magnitude larger than interior surfaces [35]. Without considering the convective transport in this scenario, the precursor molecules have to diffuse past the already-saturated part near the top of the pores before they can reach the unsaturated, deeper parts of the sample. In a realistic ALD process, diffusion of precursor into the pores could also be hindered by counter diffusion of the carrier gas and the presence of products formed by chemisorption. In order to achieve precursor penetration throughout the high surface area material, longer exposure times or multiple exposures are necessary. In one study, precursor exposure times of almost 2 hours were required to saturate just 0.2 grams of SiO2 powder with a specific surface area of 506 m2/g in a fixed-bed reactor. The entire

ALD cycle (consisting of the precursor exposure step, the first purge step, the oxidant exposure step, and the second purge step) lasted 8 hours [36].

12 ALD can be made practical for depositing materials on large amounts of porous samples with the right approach. Using concepts from Kinetic Theory of Gases, Longrie et al. described the adsorption of the precursors onto a bed of particles for a static ALD system [29]. They considered the flux of precursor molecules through the top surface of a bed of particles and calculated the time required to saturate the bed. Their analysis showed that the time required to saturate the surface was given by Equation 1:

𝑡 𝑘𝑇= 3 4 𝑆 𝑟_𝑐2 _𝛥𝑃 √ 𝑀 2𝑅𝑇 𝑙𝑐 2 _{(𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝟏. 𝟏)}

In this equation, rc is the radius of the particles or pores, S is the number of sites per surface

area, lc is the length (or depth) of the bed, M is the molecular weight of the precursor, and

𝛥𝑃 is the dosing pressure of the precursor. This analysis takes into account that, during precursor exposure, the gas must adsorb from the top of the bed, saturate those particles first, and can only then progress deeper into the pores. In their analysis, Longrie, et al. point out that the adsorption time could be hundreds of hours if the experimental parameters are not chosen properly. Fortunately, the equation provides criteria for minimizing the required exposure times. Especially important, the diffusion length needs to be kept short. This can be accomplished by forming the powders into thin wafers rather than having a free-standing bed of powder. Alternatively, Longrie et al. has shown in a different publication that ALD can be performed on high-surface area materials using a home-built static ALD system [35]. Increasing the pressure of the precursor also can significantly decrease the required

13 exposure time. In some cases, development of new, highly volatile precursors may be necessary [37].

In addition to affecting the exposure time, high-surface-area samples also require considerably larger amounts of the precursor. Required precursor amounts for a single ALD exposure can be estimated by assuming a 100 m2/g powder with an adsorption site density of ~1018 _sites/m2 _{[38]. To coat 1 gram of such powder, the amount of precursor}

required for a uniform monolayer coverage is ~10-4 moles (corresponding to 14 mg TMA or 87 mg Ce(TMHD)4). This is a nontrivial amount especially when we consider the

wastage of materials from a flow ALD system.