In chemical vapor deposition of silicon from silane (SiH4), the film growth rate can be limited by either gas-phase mass transport or surface reaction kinetics. At low temperatures, which regime dominates and why?
Think about your answer, then reveal below.
Model answer: At low temperatures, the surface reaction is slow because the decomposition of SiH4 on the surface has an activation energy barrier (~1.6 eV). Mass transport of SiH4 to the surface is relatively fast and temperature-insensitive. Therefore, the overall rate is limited by the surface reaction — this is the reaction-rate-limited (or kinetically controlled) regime. Growth rate increases exponentially with temperature (Arrhenius behavior). At high temperatures, the surface reaction becomes fast enough that the rate is limited by how quickly fresh SiH4 can diffuse through the boundary layer to the surface — the mass-transport-limited regime, where growth rate depends on gas flow and geometry rather than temperature.
The transition between regimes has practical importance: the reaction-limited regime gives better film uniformity because the growth rate depends only on local temperature (which is uniform on a well-designed substrate heater), not on local gas flow patterns. The mass-transport-limited regime gives higher deposition rates. Industrial CVD processes are usually designed to operate in the reaction-limited regime for uniform films or the mass-transport-limited regime when throughput matters more than uniformity.
Question 2 True / False
Sputtering (a PVD technique) can deposit alloy films with compositions matching the target material, which is difficult to achieve by thermal evaporation.
TTrue
FFalse
Answer: True
In thermal evaporation, each element evaporates at a rate proportional to its vapor pressure at the source temperature. Since different elements have different vapor pressures, the film composition differs from the source composition — the more volatile element is enriched. In sputtering, energetic ions (typically Ar+) physically knock atoms out of the target surface. The sputter yield depends on atomic mass and binding energy, not vapor pressure, and the differences between elements are much smaller. As a result, a multi-component target produces a film with nearly the same composition. This makes sputtering the preferred PVD method for depositing alloys and complex oxides.
Question 3 Multiple Choice
Atomic layer deposition (ALD) achieves atomic-level thickness control by using sequential, self-limiting surface reactions. Why is ALD preferred over conventional CVD for depositing conformal films in high-aspect-ratio features?
AALD uses lower temperatures, preventing thermal damage to the substrate
BEach ALD cycle deposits exactly one monolayer regardless of local precursor flux, so growth is uniform even in deep trenches where gas flow is restricted
CALD precursors are less toxic than CVD precursors
DALD films are always crystalline, while CVD films are always amorphous
The key advantage of ALD is self-limiting growth. In each half-cycle, precursor A reacts with the surface until every available site is occupied — then the reaction stops, regardless of how much more precursor arrives. The excess is purged, and precursor B reacts with the A-covered surface, again self-limiting. Each full cycle adds a fixed thickness (typically 0.5-1.5 Angstroms). Because the reaction is self-limiting, it does not matter whether the surface is at the top of a trench or at the bottom — every surface atom gets the same exposure if given enough time. This produces perfectly conformal films even in features with aspect ratios above 100:1.