Stylus Stability Test

The accuracy of a profilometer scan is only as good as the mechanical stability of the stylus. To check the stylus stability, lateral force must be applied while checking for scanning aberrations. The top image is an SEM image of a stylus stability standard. It is imaged in secondary electrons at 1.2 kV without coating. It is a lithographed "V" structure in Si. If scanned horizontally from left to right the stylus will experience lateral force towards the bottom of the image as it starts crossing one leg of the "V" and then lateral force towards the top of the image as it starts crossing the the other leg of the "V". The dark region is an SEM imaging artifact. The dark region is where the structure was previously imaged at higher magnification. The interaction of the e-beam with the surface in the presence of diffusion pump vapor results in a reduction of the secondary emission of the surface. When the magnification is then suddenly lowered a "ghost" of the previous scan area is visible.

The bottom image is an actual profilometer scan across both legs of the "V" structure. If the scan is performed horizontally across the "V" structure-- meaning the axis of symmetry of the "V" is perpendicular to the scan direction-- then one should find two structures of the same width. A radical difference in the width of the two legs of the "V" is the result of mechanical instability of the stylus as it experiences a range of lateral forces first in one direction, then the other. It should be noted that if the "V" structure is not scanned exactly horizontally, it will appear as if there is stylus instability.

Current Calibration

Using a 9429 A standard, it is +0.06% off on the kA scale and -0.31% off on the um scale.

Using a 1.74 um standard, it is +0.40% off on the kA scale and +1.15% off on the um scale.

Using a 23.13 um standard, it is -0.95% on the um scale.

Stylus Damage

The profilometer stylus is subject to damage by shear forces-- not normal forces. The stylus normal force is regulated by the instrument, but very large shear forces can be applied to the stylus if the sample stage is inappropriately adjusted with the stylus in contact with the surface.

This SEM image shows a damaged profilometer stylus. Running almost vertically across the image is a deep groove in the stylus shank. Note the crack in the cement used to hold the actual diamond stylus into it's shank at about 11 o'clock. This is very clearly due to the stylus being in contact with the sample surface and then the sample stage being translated to scrape something across the stylus. While this did not visibly damage the stylus itself-- the dark circular region being the cone of the stylus-- it did damage the cement attachment to the stylus shank. This stylus showed profile artifacts consistent with stylus mechanical instability.

Alpha Step 200

The CMMP group at the Florida State University Department of Physics has a Tencor AlphaStep 200 scanning profilometer that is available to the local research community. There are no fees for use of this instrument.

The scanning profilometer is a complement to SEM (scanning electron microscopy) and SPM (scanning probe microscopy). A stylus, typically with a radius of 5.0 um or 12.5 um, is scanned across the surface. The mechanical advantage of a long cantilever attached to the stylus allows precise measurement of surface topography. Common applications include: thin film thickness; profiling photo-lithographed structures; surface roughness (Ra). Profiles can be printed out on thermal paper.

Advantages: fast; very large scan lengths; very large vertical depths; good vertical resolution when lateral resolution is not needed; cost.

Disadvantages: 1D scans, not 2D images; non-trivial stylus normal force; poor lateral resolution.

Specifications:

Vertical Range: +/- 160 kA in kilo-Angstrom mode; +/- 160 um in micrometer mode.
Maximum Scan Length: 10,000 um.
Vertical Resolution: 5 A in kilo-Angstrom mode; 5 nm in micrometer mode.
Horizontal Resolution: 400 A.
Accuracy: ~ 1% typical.
Stylus Force: 1-25 mg.
Maximum Sample Dimensions: 16.5 mm thick x 162 mm diameter.