In short: Thin-film deposition CVD PVD ALD are the three workhorse methods that grow the nanometre-thick layers every chip is built from. PVD physically sputters or evaporates atoms onto the wafer, CVD reacts gases at its surface, and ALD builds films one self-limiting atomic layer at a time — so conformality and step coverage improve in exactly that order, at the cost of speed.
Every transistor, interconnect and capacitor in a modern integrated circuit is assembled from stacks of thin films — layers of metal, dielectric or semiconductor that can be as thin as a few atoms. Thin-film deposition is the family of processes that grows these layers onto a silicon wafer. The three dominant techniques are CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition) and ALD (Atomic Layer Deposition).
The distinction is fundamental. In PVD, a solid source material is physically converted to vapor — by sputtering with energetic ions or by thermal/e-beam evaporation — and those atoms travel in roughly straight lines to condense on the wafer. In CVD, gaseous precursors are pumped over a heated wafer where they chemically react and decompose, depositing a solid film and releasing volatile by-products. ALD is a special, cyclic form of CVD in which two precursors are pulsed separately so that each step is self-limiting — only one atomic layer can form per cycle.
Conformality describes how faithfully a film follows the underlying topography — flat surfaces, vertical sidewalls and the floor of a deep trench or via. It is quantified by step coverage, usually the ratio of film thickness on the sidewall (or bottom) to the thickness on the top field, expressed as a percentage.
In sputtering, argon is ionised into a plasma and the ions bombard a solid target, knocking atoms loose that then condense on the wafer. In evaporation, the source is heated (resistively or with an electron beam) until it vaporises. PVD runs at relatively low temperatures, deposits very pure metals, and has excellent adhesion — but its directional flux limits it to planar or low-aspect-ratio surfaces.
CVD introduces precursor gases (for example silane for silicon, or tungsten hexafluoride for W) that adsorb and react at the hot wafer. Variants tune the trade-offs: LPCVD (low pressure) improves uniformity, PECVD (plasma-enhanced) lowers the required temperature, and MOCVD uses metal-organic precursors for compound semiconductors. CVD gives good throughput and reasonable conformality.
ALD splits the reaction into two steps separated by inert-gas purges. Precursor A saturates the surface and stops; the chamber is purged; reactant B then reacts only with the chemisorbed A and stops. Repeating the cycle builds the film with atomic-scale thickness control, which is why ALD is the method of choice for ultra-thin high-k gate dielectrics (such as HfO₂) and diffusion barriers.
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Explore CourseTron Courses →CVD deposits a film through a chemical reaction of gaseous precursors at the wafer surface, while PVD physically transfers atoms from a solid source by sputtering or evaporation. CVD generally gives better step coverage; PVD gives very pure, well-adhered metal films but is line-of-sight.
Each ALD half-reaction is self-limiting — it stops once every surface site is occupied, regardless of how much precursor arrived. Film thickness therefore depends only on the number of cycles, not on local gas flux, so deep trenches and flat tops coat to the same thickness.
ALD is a cyclic, surface-saturated form of CVD. It is far slower (growth is a fraction of a nanometre per cycle), but in exchange it delivers atomic-level thickness control and near-perfect conformality that continuous CVD cannot match.
ALD. Gate oxides such as hafnium oxide must be only a couple of nanometres thick, pinhole-free and perfectly uniform — requirements that only the self-limiting ALD process reliably meets at production scale.
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