Etch rate is the measured amount of a specified material removed per unit time; its meaning depends on the film, surface, geometry, and location. A blanket-film measurement often compares thickness before and after exposure; a patterned feature needs a depth or profile measurement at the location of interest. A fast blanket-film rate alone does not prove that a narrow opening clears, that a mask survives, or that the profile remains vertical.
One rate cannot describe the whole stack
For a patterned etch, ask which layer is being removed, which layer should remain, and where the measurement was made. Selectivity compares the removal rates of two materials under the same studied conditions; it is a ratio, not the target material's rate by itself. Uniformity asks whether removal is similar across locations. Profile describes the shape left behind. These are separate observations: an apparently useful average rate can coexist with mask loss or incomplete clearing in a feature.
Process map
40nm BSI CMOS Image Sensor
Open the STI module in the public 40nm Flow map to locate adjacent Oxide Etch and Nitride Etch, then distinguish target-film and protected-film questions; the overview is not an etch-rate dataset.
Why material choice changes the comparison
In plasma etching, chemical reactions at the surface and energetic-particle assistance can both affect removal. The measured rate therefore belongs to a specified material, surface and process condition; it cannot be transferred to every other stack .
Reactive species and surface films can affect the target and protected layer differently. Selectivity compares their rates under the same specified conditions; it is not a property of the target material in isolation .
An etch-rate statement should therefore say whether it refers to a blanket oxide, a patterned nitride, the mask, or the layer beneath. The selectivity explainer develops the rate-ratio idea; this page uses it only to interpret the etch-rate question.
Why a narrow feature can give a different rate
Transport of reacting species and removal products can change inside a narrow opening. A blanket-film rate may therefore fail to describe the feature bottom, especially as geometry changes during etching .
This is why reporting only a single average rate hides a practical question: did every intended opening clear while the neighboring material and mask retained their needed roles? The answer comes from measurements in the relevant geometry, alongside selectivity and profile evidence, rather than an assumed universal trend.
Locate the question in a current Flow
In the public 40nm BSI CMOS Image Sensor Flow, open the STI module to find Oxide Etch and Nitride Etch next to one another. Its overview is an orientation map: identify the two different target films and ask separately what is removed and what should be protected at each stop. The overview does not contain measured rate data or certify a particular chemistry. The detailed Step prose needs separate content review, so this article links to the Flow overview for the structural relationship.
References
Silicon VLSI Technology - Full
James D. Plummer, Michael D. Deal, Peter B. Griffin
Silicon VLSI Technology · ISBN 978-0130850379