Stylus Profilometry on Thick Film: Check Whether the Tip Can Enter the Feature

Measurement purpose

Determine whether stylus geometry permits access to the surface defining a narrow-feature dimensional result.

Specimens and conditions

Feature geometry

Opening width, walls, depth reference and local obstructions for the actual scan path

Surface state

Known processed layer and evidence that the selected contact does not change the feature

Equipment and records required

• Contact profile measurement: Identified probe shape and condition with appropriate force and scan controls
• Independent validation: Suitable reference geometry or complementary measurement where bottom access remains uncertain

Method sequence

1. Access assessment

   Compare full probe shape with the defined feature

   Record: Geometric limit and assumptions

2. Acquisition

   Retain profiles and check sensitivity to location and contact

   Record: Probe, path and surface-change evidence

3. Interpretation

   Separate actual bottom contact from inaccessible depth

   Record: Supported dimensional result or unresolved feature

Decision and uncertainty

Do not assign true recess depth from a profile when the probe is geometrically prevented from reaching the required surface.

Probe shape, wear, wall geometry, scan direction and contact deformation can alter the apparent profile independently of actual depth.

The profilometry-method owner approves access evidence; the drawing owner defines the physical dimension required for disposition.

Traceable outputs

Define the physical recess the probe must enter

Identify whether the scan concerns a conductor gap, laser-trim kerf, ceramic groove or transition at a glass-overglaze edge. Record its opening width, expected wall shape and the surface that defines depth. A groove beneath a transparent glaze is not accessible merely because it remains visible in a photograph.

State whether the required output is bottom depth, minimum width, edge shape or plateau height. A probe can measure broad plateau separation adequately while misrepresenting the narrow valley between them. Qualification of one output should not be transferred automatically to the others.

Use more than the nominal spherical tip radius

A practical stylus may have a rounded end transitioning into a cone or another shank geometry. The rounded end controls some narrow features, while the cone angle or shank clearance controls deeper access. Retain the relevant probe specification and condition rather than using only a catalogue radius.

Tip wear, adhering material and damage can change access. A nominally smaller tip is not necessarily suitable for every film state because contact stress and surface disturbance also matter. Select force and scan conditions under the instrument method and material requirements, and inspect for changes caused by repeated contact.

Calculate a simple edge-supported descent limit

Consider an ideal rectangular trench with equal-height sharp upper edges, vertical walls and opening width w smaller than twice a spherical tip radius R. When the sphere rests against both upper edges, its lowest point descends only a distance h below the surrounding surface. Circle geometry gives h as R minus the square root of R squared minus one quarter of w squared.

If the actual trench is deeper than this value, the ideal spherical end cannot reach the bottom. If it is shallower, the bottom may be accessible under this restricted geometry, but actual cone, wall and force effects still need checking. The expression is not a general correction for arbitrary groove shapes.

See why a deeper groove can give the same shallow profile

For an illustrative two-micrometre tip radius and two-micrometre opening, the ideal descent limit is approximately 0.268 micrometres. A trench one micrometre deep and another three micrometres deep can both be inaccessible below that same edge-supported level. Increasing vertical data resolution does not reveal their hidden bottoms.

With the same radius and a three-micrometre opening, the ideal descent limit increases to approximately 0.677 micrometres. This sensitivity to opening width means that a changing measured depth can arise from changing access even when actual bottom depth stays constant. These examples are geometric calculations, not expected dimensions for any ChipSimple product.

Check the narrowest accessible opening along the real scan

The stylus follows a path through a three-dimensional feature. A scan that crosses a groove obliquely may encounter a different apparent opening than a perpendicular section, while nearby features can obstruct the probe from another direction. Use the actual path and full local geometry rather than measuring width from an unrelated surface location.

Unequal edge heights, tapered walls and rounded shoulders change the contact condition. Where the ideal model is inadequate, use an appropriate geometric simulation, a suitable reference feature or an independent measurement. Do not force the measured profile into the rectangular-trench equation just because it gives a convenient number.

Distinguish an access limit from an instrument or surface effect

A qualified comparison with another appropriate probe geometry can reveal sensitivity to access, but a changed result does not prove which trace is the true surface. Align the scan location, specimen state and analysis definition. A tip change also affects contact stress and may interact differently with debris or soft material.

Use independent sectioning or another suitable method when the decision requires the hidden bottom. An optical method has its own contrast, steep-slope and transparency limitations. Agreement should be judged against defined surfaces and uncertainty, not by selecting the deepest reported number as automatically correct.

Do not reconstruct hidden depth from a saturated geometric response

Once the probe is supported above the bottom, a range of deeper geometries can produce the same trace. The measured path alone does not identify which hidden depth exists. Repeating or averaging that path improves repeatability but cannot supply the missing geometric information.

A reconstruction method needs independently established probe shape and assumptions about the feature. Report which details are recoverable and which remain nonunique. Do not convert a generic software correction into evidence for a minimum groove depth or conductor clearance when the relevant surface was never reached.

Attach the access decision to the profile result

Retain the probe identity and shape, scan path, force, specimen state, raw profile and the geometric access assessment. Identify whether the reported depth represents actual bottom contact, a bounded accessible descent or an unresolved recess. Keep repeat scans that reveal surface modification rather than discarding them as inconvenient variation.

ChipSimple can review the drawing feature and physical surfaces that an inspection must resolve. A credible result acknowledges the difference between a precise instrument indication and access to the required surface. That distinction prevents a narrow deep feature from being accepted as shallow simply because the probe cannot enter it.