CD Measurements with an AFM
CD measurements using an AFM present different challenges than those with a SEM. One advantage is that AFM images cannot be out of focus, so autofocus is unnecessary. However, the following challenges must be overcome:
- AFM is significantly slower than other microscopy techniques, even fast scanning AFMs. Therefore, it is often combined with a light microscope, which can assist with referencing when larger reference structures are present on the sample.
- The AFM image is acquired at the tip position. When the tip is changed, the image shifts, as the tip is commonly at a slightly different position. This can be addressed by using an additional optical microscope to locate the AFM tip. Otherwise, a reference scan is needed to measure the offset each time the tip is replaced.
- The condition of the AFM tip must be evaluated regularly and automatically, as the tip may degrade unexpectedly -- for example, when it collects a particle.
- AFMs are more prone to scan artifacts caused by surface charges or contamination. These artifacts often appear as scan line distortions in the image.
- Low-frequency vibrations and drift alter the line-to-line relationship in the scan. The image must be flattened using a specialized method before being used for CD measurements.
- The AFM image is always a convolution of the tip shape and the sample surface.
These many challenges make the AFM the most hard to use high resolution non-destructive microscope type. The advantages are its height measurement capabilities and the possibility to measurer other physical effects like conductivity.
Image Acquisition
Because of the AFM's slow scanning speed, precise stage positioning is even more critical than in a SEM -- especially when reference structures are not visible in the optical microscope. AFMs cannot generate real overview scans; a typical maximum scan of 100 micrometers in width and height corresponds to a SEM magnification of about 1270 (referenced to a 5-inch polaroid film). Such large AFM scans take a long time and pose a risk to the tip, so they should be avoided. AFMs are most efficient at scan sizes below 10 micrometers. Cross-referencing using FFT, as done in SEMs or light microscopes, is not feasible for AFMs. For CD measurement using AFM, programmed position finding must be used. In many cases, full images are unnecessary -- only a few scan lines may be needed to measure the height or width of individual features.
While SEMs and light microscopes are good at measuring widths, AFMs are best suited for measuring height. The measured structure height in AFM images is minimally affected by tip-sample convolution. To obtain highly accurate, non-destructive results both in height and width, a combined SEM and AFM measurement of the same area is optimal. For such cases, a correlative automation platform like CorrMeas is especially useful, as it enables measurements of the same area by different instruments.