The mechanism
Deposition adds material, but different methods deliver it differently. Chemical vapor deposition supplies molecules that react or decompose to form a film. Physical vapor deposition transports material from a source, for example through sputtering or evaporation, to a receiving surface. Geometry matters: a surface that receives little incoming flux can coat differently from an exposed top face. Chemistry, surface reactions and transport can also produce gradients or voids in difficult features. The lesson compares the same trench after representative CVD and directional PVD steps, keeping film thickness, conformity, adhesion and composition as separate outputs rather than labeling one method universally better.
- Choose the intended film function and identify the surfaces that must receive material.
- For CVD, deliver reactants and enable the required surface chemistry; for PVD, generate a material flux.
- Account for transport and reaction along the feature rather than assuming uniform exposure.
- Grow to the required geometry while tracking stress, impurities and possible closure of an opening.
- Measure local coverage and interface performance, not only a blanket-wafer thickness.
Science and assumptions
Coverage ratio
Bottom coverage is bottom-film thickness divided by top-reference thickness for the same feature.
Symbols and units
- C_bottom
- Bottom-to-top thickness coverage ratio
dimensionless
- t_bottom
- Film thickness at the selected feature bottom
nm
- t_top
- Film thickness at its defined top reference
nm
Both thicknesses use the same unit and refer to the same defined feature; the ratio is dimensionless.
Assumptions A local geometric descriptor. It does not capture adhesion, composition, stress or complete three-dimensional void topology.
Engineering example
Synthetic cross-section: 10 nm on a trench top and 3 nm at its bottom gives 0.30 bottom coverage. A blanket thickness measurement reporting 10 nm would miss the weak bottom coverage.
Distinctions and tradeoffs
Equal blanket thickness does not establish equal sidewall coverage or void-free filling.
A method with high deposition throughput may struggle with extreme geometry or a thermal budget; another may require slower cycles or additional preparation.
Contacts, barriers, dielectrics and conductors must cover three-dimensional structures without introducing weak electrical or mechanical interfaces.
Specifications and interfaces
- Thickness
- Length, measured at named locations
Top, sidewall and bottom coverage may differ.
- Conformality
- Local thickness / reference thickness
A defined geometric ratio, not a guarantee of electrical continuity.
Interfaces
- Surface and material supply Input
- Precursors or physically transported atoms
- Deposited film Output
- Geometry, composition, stress and interface adhesion
Check your understanding
A trench opening closes before its interior fills. What does this indicate?
- The local growth profile matters beyond average deposition rate
- The film is necessarily electrically perfect
- All deposition methods are self-limiting
Answer and reasoning
The local growth profile matters beyond average deposition rate
Material can accumulate near the opening and isolate a cavity; changing average thickness alone does not describe this geometry.
Evidence and scope
Cross-sections are representative mechanisms. Actual coverage depends on process, chemistry, pressure, temperature, geometry and surface preparation.
Use the manufacturing investigation to trace compatible inputs, assembly and operating constraints; no current standalone calculator models this lesson’s full mechanism.