A Solution in Search of a Problem

Image stitching is an optical technique developed to patch together small fields of view into larger ones. Principally developed for image rendering, not quantitative measurement, it is challenging to use in dimensional metrology applications. However, NIST (Gaithersburg, Md.) and the Center for Nanotechnology at NASA's Ames Research Center (Moffett Field, Calif.) are doing breakthrough work in AFM dimensional metrology, investigating how stitched images that are different manifestations of a systematic error can provide ways to reduce that error's influence.

Although the mathematics — correlation functions, matching derivatives, etc. — is known, this is the first time the technique has been used with AFM dimensional metrology. While there is no obvious application for image stitching in AFM dimensional metrology, the technique found use in researching carbon nanotube tips. Usable nanotube tip yield is low and not well controlled, especially regarding tip characteristics. Problems with mounting different angles and lengths of nanotubes with any significant control have been slowly resolved. However, there is still some angle difference with most nanotube tips with respect to the surface normal. The researchers believe that, under some circumstances, it might be useful to preserve a moderate angle difference.

The investigation considered top-down AFM, ordinarily not used as a linewidth measuring tool. With a nanotube at an angle to the surface, the tip scans over a vertical feature with one sidewall completely visible to it. Because it is at an angle, only one corner images the sidewall as it goes down. With an angled nanotube tip, it should be possible to accurately measure one vertical sidewall. The sample could be rotated to access the other sidewall with the same tip. Although most of the distortion is on the inaccessible sidewall, there is still some residual tip dilation on the sidewall, accessible to the tip.

If these two images could be accurately joined, the composite might be useful for metrology, not just as a rendering or display perspective. Quantitative measurements could be performed from a stitched image.

As detailed in their published work,¹ the researchers took two images, noted the location of several features, and manually combined them. The purpose was to obtain an understanding of, and minimize remaining distortions in, the stitched image. This was accomplished on one linewidth and, although accurate information was obtained, some limitations remained. Although nanotubes are minuscule, they still have a finite width and geometrical shape that must be accounted for in the imaging process.

A montage showing both a schematic illustration of the stitching method and examples of slope images obtained using a carbon nanotube tip with conventional AFM. (Source: NIST)

Regardless of its length, the nanotube has a high aspect ratio and bends while imaging the sidewall. The researchers wanted to determine if it was possible to achieve the kind of sidewall and CD accuracy in a conventional 1-D or top-down AFM that is possible in a CD-AFM. A 1-D AFM has less sophisticated tip position and feedback control, as well as sampling, providing little sidewall information — the sharp tip scans over it quickly, so aside from bending and other geometrical distortions, not much information is obtained to extract accurate edge information.

Joining the images is not a limiting factor; systematic errors in the individual input images are. These arise from nanotube tip interaction with the sample. As tip angles get closer to vertical, the problem should lessen. A sharp and short nanotube tip with an angle approximately equivalent to that of the silicon tip it is mounted on may be the best configuration. Such a tip would only have to image a portion near the top of the sidewall to extract a CD for comparison with a CD-AFM measurement or another reference.

Image stitching seems unlikely to displace other technologies anytime soon. There is no fundamental point where CD-AFM, for example, would be unworkable, making this the best alternative. The procedure is viewed as an independent method to complement (probably offline in the analysis lab) current online methods rather than replacing them.

Presently, image stitching in this application appears to be a solution in search of a problem, offering no obvious advantage outside of the R&D lab. One should, however, take Benjamin Franklin's perspective. In Paris, while witnessing a hot air balloon flight, the founding father was asked, "Of what good is it?" to which he replied, "Of what good is a newborn baby?"