etch profile

# Etch Profile

Etch profile is the cross-sectional shape of an etched feature. It includes depth, width along the opening, sidewall shape and bottom condition. Two features can reach the same depth and still behave differently during the next fabrication step.

## Recognize the basic shapes

A vertical opening has approximately constant width with depth. The illustrated tapered opening narrows toward its bottom; a reverse taper would widen toward the bottom. A bowed opening is wider around its middle than at its top and bottom. These are useful descriptors, not an exhaustive list of possible shapes.

A neck is a local constriction. Undercut describes lateral removal beneath a mask or another overlying feature. Footing, notching, roughness and scalloping need their own definitions and measurement locations. A single average sidewall angle cannot represent every local feature of the wall.

The openings in the diagram start at the material surface. They are intentionally exaggerated for readability and do not represent a qualified recipe or dimensional specification.

## Measure the complete feature

Record top, middle and bottom critical dimensions at defined locations, total depth, and any local maximum or minimum width that matters. Specify the angle reference if reporting sidewall angle; an angle measured from the wafer surface uses 90 degrees for an ideally vertical wall.

Check left and right walls separately when tilt or asymmetry matters. Three widths cannot rule out a neck or bow between the sampled locations. A continuous shape requirement needs a method capable of assessing that shape over the relevant region.

## Keep feature width and density separate

Opening width, depth, pattern density, mask condition and the material stack can all affect the resulting feature. A comparison between a narrow dense array and a wide isolated opening changes more than one factor, so it cannot establish an effect caused by width alone.

Aspect-ratio-dependent etching can produce different etch rates or profiles across feature dimensions. The direction and magnitude depend on the process; a narrower opening is not guaranteed to stop at a particular depth. Investigate width and density through controlled comparisons and representative structures rather than assigning one cause from a schematic.

## Connect profile to the next module

A constriction can restrict a later liner or fill; a widened section can reduce the material separating neighboring features. Fill behavior also depends on the deposition mechanism, film thickness, geometry and integration sequence. Vertical walls and positive taper therefore cannot be ranked as universally better or worse for filling.

For a contact, bottom opening and interface condition can matter alongside shape. For a trench or cavity, the relevant dimensions, wall texture and mechanical or electrical requirements may differ. Derive acceptance limits from the intended function and the evidence for that integration.

## Understand the measurement limits

Cross-section electron microscopy can reveal buried geometry, but the section location, orientation, preparation artifacts and image calibration affect the result. A single section through a three-dimensional opening may miss its narrowest location.

Conventional top-down imaging characterizes visible geometry; it does not by itself establish the full buried profile. Other metrology methods may infer additional dimensions using models, whose assumptions and sensitivity must be validated. Electrical or downstream results provide complementary functional evidence rather than a direct replacement for geometric measurement.

## Retain process context

State whether the image is as-etched, after resist strip, or after the specified clean. Retain the feature layout, stack, recipe revision and chamber condition. Ion transport, surface reactions, passivation, mask erosion and charging can interact; the observed shape alone does not uniquely identify the mechanism.

A useful profile claim states which feature was measured, where and how it was measured, and which requirement it satisfies. For acceptance-band examples, see Etch Profile Budget; the base keyword explains the geometry itself.

## Sources and scope

• [Lam Research — DSiE product family](https://www.lamresearch.com/product/dsie-product-family/): application-specific requirements for profile angle, tilt, asymmetry and uniformity.
• [Gerlt et al. — Reduced etch lag and high aspect ratios by DRIE](https://arxiv.org/abs/2104.02763): process-specific investigation of etch lag and deep silicon structures.

Original schematic illustrations and explanatory synthesis. No universal angle, fill preference or feature-width effect is asserted.