| AVS 61st International Symposium & Exhibition | |
| Selective Deposition as an Enabler of Self-Alignment Focus Topic | Wednesday Sessions |
| Session SD-WeM |
| Session: | Fundamentals of Selective Deposition |
| Presenter: | John R. Abelson, University of Illinois at Urbana-Champaign |
| Correspondent: | Click to Email |
When performed at low substrate temperature, the growth of thin films by chemical vapor deposition can be strongly inhibited by the reversible adsorption of precursor, byproduct, or neutral molecule species on the active surface. The microscopic mechanism is typically that of site blocking: as the surface coverage of inhibitor species increases, the reaction probability of arriving precursor molecules drops, and can reach values as low as 10-6 under realistic growth conditions. In specific cases, the mechanisms of associative precursor desorption or coverage-dependent film growth rate also occur.
We will show that site blocking by the precursor itself can afford extremely conformal film growth in structures with aspect ratio > 100:1, and that the addition of a neutral molecule inhibitor to a ‘non-conformal’ precursor can provide good step coverage in features with aspect ratio ~ 10:1. We also solve the diffusion-reaction equation to predict the regimes of precursor pressure and substrate temperature that afford conformal growth, and map the boundaries onto a conformal zone diagram. Under conformal conditions the surface roughness is exceptionally low due to the smoothing effect of precursor re-emission, which mitigates the ‘shadowing’ of the incident flux by peaks in the surface morphology.
We then show that an inhibitor molecule can be used to control the film nucleation step. The inhibitor molecule must have a greater adsorption energy on the deposited material than on the bare substrate surface, or vice versa, such that the equilibrium coverage of inhibitor is large only on the strongly binding surface. For strong binding to the film, the deposit consists of a high density of nm-scale nuclei that coalesce into an ultra-smooth film; we give the example of HfB2 growth on SiO2 using NH3 as the inhibitor. Conversely, when the inhibitor slows the nucleation rate, the deposit consists the sparse distribution of islands in a narrow size distribution that may be useful in photonic or catalytic applications; we give the example of Cu growth using VTMS as the inhibitor.
The use of inhibitors may provide a pathway towards selective deposition if film growth can be completely shut off on the surface that is intended to remain bare. Systems in which the inhibitor can drive associative desorption of the precursor are predicted to be especially useful, in that they remove unwanted precursor molecules. Another possibility is the use of activated species such as atomic H, generated by a remote plasma, that fully passivate covalently bonded surfaces but recombine rapidly (and therefore have no effect) on metallic surfaces.