Wet etching removes exposed material through reactions in a liquid. Its selectivity depends on the etchant, the target film, neighboring films and surface conditions; it does not guarantee that an adjacent layer remains untouched. Many wet reactions undercut a mask because the liquid reaches exposed sidewalls. Alkaline etching of single-crystal silicon is an important exception: different crystal planes dissolve at different rates, producing orientation-dependent shapes. The measured orientation dependence of silicon wet etching makes a blanket claim that wet etching has “no directionality” inaccurate. The useful comparison with dry etching is therefore the source of anisotropy and the material-specific selectivity, not a universal wet-versus-dry rule .
Mechanism and boundaries
Reaction products must leave the surface for removal to continue. Oxidation followed by dissolution, direct attack on an existing oxide, and alkaline dissolution of silicon are distinct mechanisms. A chemistry that removes one film preferentially may also attack a mask, an interface or a second exposed material. Temperature, concentration, film density, prior surface state and transport all affect the result; the article does not prescribe a chemical mixture or process setting.
Process checkpoint
Understand Wet Deglaze Etch in context
Identify the residual oxide target after STI planarization; assess neighboring-film compatibility as a condition, not a guarantee.
Process context for “Wet Etching: Selectivity and Anisotropy”: 40nm BSI CMOS Image Sensor · STI · Step 45
Profile and selectivity
For an approximately isotropic reaction, access to a sidewall can cause lateral undercut beneath a mask. For crystalline silicon in some alkaline liquids, the relative rates of exposed crystal planes instead shape the cavity. Neither behavior alone proves that a particular wafer stack has an acceptable selectivity or profile. A dry plasma process can also be designed for material selectivity; its ion-assisted directionality is a separate control. Compare the dry etching overview for that mechanism.
Where Wet Etching Sits in a Modern Flow
The published 40nm flow overview places wet removal around the isolation module. The named Wet Deglaze Etch removes residual oxide after planarization, while SiN Strip removes a nitride pad layer. These names identify different targets; they do not establish a transferable recipe or prove that every other material is unaffected. Inspect the exposed stack and the role of each step before interpreting selectivity. The flow overview is publicly available, and the linked learning route leads to these named steps with their existing access rules.
Real cross-sections from the free 40nm flow — the same steps this article's learning route links to.
Limits and related processes
Undercut, incomplete removal, residues and loss of a neighboring film are possible failure modes. Their relative importance depends on geometry and chemistry. A wet clean may follow pattern transfer, but cleaning, wet etching and hydrofluoric-acid oxide removal are not interchangeable labels. Etch rate and selectivity describe different comparisons: rate concerns one material under stated conditions, while selectivity compares rates between materials under the same stated conditions. A stop layer reduces risk only when the chosen chemistry is suitably selective to that particular stack.
References
Silicon VLSI Technology - Full
James D. Plummer, Michael D. Deal, Peter B. Griffin
Silicon VLSI Technology · ISBN 978-0130850379