Etch Profile Budget

# Etch Profile Budget

An etch profile budget defines the allowed cross-sectional shape of a feature and the variation that must be checked before the feature is accepted. It connects width, depth, sidewall shape and local defects to the needs of the next fabrication step.

A width at the top and another at the bottom are useful measurements. Together, they still cannot establish the complete shape between those locations.

## Look beyond the two end widths

In this schematic, all three openings have the same top and bottom width. The straight opening stays constant between them. The necked opening becomes narrower near the top; the bowed opening becomes wider around the middle. A measurement limited to the two end widths would miss both differences.

A narrow section can constrain a later liner or fill. A widened section can reduce the material separating neighboring features. The relevant consequence depends on the device and integration, so neither a vertical wall nor a particular taper is automatically the correct target for every process.

## Define the shape that matters

Record the material stack, feature type, nominal depth and the surfaces used as measurement references. Define the width at each specified depth and how depth is normalized when comparing features of different total depth. State whether the acceptance envelope applies continuously or only at designated measurement locations.

A useful specification can include top, middle and bottom widths; maximum bow or necking; left and right sidewall angles; tilt; undercut; and local footing or notching near an interface. For processes that produce scallops or other texture, define the relevant roughness or scallop metric and the measurement scale. These metrics are not interchangeable.

Avoid replacing the profile with one average angle. Different wall shapes can have the same average angle, and a symmetric width measurement can hide a feature that is tilted or asymmetric. Define the angle reference explicitly if an angle is reported.

## Apply the limits at the specified locations

The second illustration uses an invented acceptance band: the opening must be 38-46 nm wide at each of three sampled depths. The top is 42 nm and the bottom 40 nm, both inside the band. The middle is 50 nm, outside the band. This example fails the stated sampled-width criterion even though its end widths pass.

The band is a teaching value, not a recommended specification. The three sample points also do not prove that every location between them lies inside the band. A continuous profile requirement needs a measurement and analysis method capable of assessing that requirement.

## Separate limits, variation and uncertainty

An engineering limit comes from an integration requirement. Observed process variation describes measured results. Measurement uncertainty describes limits in knowledge of those results. Label each separately instead of treating an observed range as a specification or adding every contribution without a defined model.

If a profile margin is reported, identify the metric and boundary. For an upper width limit, a simple signed difference is limit minus measured width; for a lower limit it is measured width minus limit. In this example the middle width has a signed margin of 46 - 50 = -4 nm to the upper boundary before any uncertainty treatment. It is not a complete yield or capability calculation.

Define the decision rule for uncertainty near a boundary. Do not use the apparent precision of a plotted point as evidence that measurement error is negligible.

## Sample representative structures

Include relevant widths, pitches, pattern densities and wafer locations. Retain the recipe revision, chamber condition and process sequence associated with each result. Changes to the incoming mask or film stack can change the final shape and should remain traceable.

Document cross-section location, orientation, image calibration and the edge-detection method. A single section through a three-dimensional feature need not capture its worst constriction or asymmetry. Section preparation and imaging can introduce artifacts; resolve suspected artifacts before attributing them to the etch.

## Use the budget through the process flow

Check whether the reported profile is as-etched, after resist removal, or after the specified post-etch clean. Those are different process states. If the downstream requirement applies after cleaning, an as-etched image alone does not establish that requirement.

When a feature misses its envelope, compare the actual shape with mask condition, depth and protected-layer condition. Use controlled process evidence to investigate the cause. A bow, neck or notch is a geometric observation, not by itself proof of one particular chemistry or equipment fault.

Define the full shape, measure the relevant locations, and retain the acceptance rule. A profile budget makes the intended geometry visible and keeps a passing end width from hiding an unacceptable feature.

## 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 in deep silicon etching.
• [Oxford Instruments — Deep reactive ion etching](https://plasma.oxinst.com/technologies/drie-new-template): sidewall smoothness and mask-undercut control among deep silicon process objectives.

Original teaching diagrams and acceptance-band example. The profile-budget framework and numeric example are explanatory synthesis; they are not a vendor specification, a qualified process window or a universal acceptance standard.