PLANAR scanning probe microscopy FOCUS pulled glass nanofibers. In many cases, however, nanofabrication severely degrades the sensor quality and creates additional constraints on sensor layouts that can be fabricated.
We might, however, ask a radical question: do we really need a tip to do scanning probe microscopy? Surprisingly, the answer is negative. It is remarkably practical to bring an extended,( 10 µ m to millimeter sized), planar sensor surface nanometers close to a planar sample, if the sample is planar itself and free from any protruding contamination. Ensuring these constraints over a micron-to-millimeter large contact surface sounds like a daunting challenge, but is surprisingly well feasible. Already in the 19th century, optical machine shops mastered the technique of optical contact bonding, where two glass surfaces are ground to sub-nanometer scale smoothness and planarity, so that they can then be joined into optical contact and will hold together by intermolecular forces. Every hard drive in a computer employs a similar trick: the read-head slider is a millimeter-sized flat device which flies in nanoscale proximity to a planar recording medium, levitated by an air cushion. While the fly height can vary over the footprint of the slider, e. g. due to tilt, it reaches a minimum of only few nanometers at its trailing edge, where the read head is located. Preparing sufficiently flat and smooth surfaces thus is a century-old well-established technique. While even millimeter to centimeter scale surfaces can be machined with the necessary precision, in many applications it is preferable to restrict the planar surface to a smaller region, for example by fabricating the sensor or the sample into a 10-100 µ m sized pedestal to relax the demands on alignment and contamination.
Mechanically aligning two flat surfaces with nanometer-scale precision parallel to each other is easily accomplished by commercial tip / tilt-positioners( Fig. 2). The key challenge is thus to measure the sensor-sample tilt and distance with sufficient accuracy to perform feedback control when aligning and approaching the sensor and the sample. It turns out that this is readily feasible for several optical schemes, even if this gap is only few nanometers small.
One straightforward choice for this task is interferometric microscopy. Conveniently, it can be implemented without an external reference arm by using reflection interference microscopy, a differential scheme where light reflected from the sensor-air interface interferes with light reflecting at the sample surface creating“ Newton rings” [ 1,2 ]( Fig. 3). This differential scheme largely suppresses common-mode vibrations of the sample and the sensor. An alternative approach uses nanofabricated chirped optical gratings fabricated on the sensor and
Figure 2. The positioning setup for planar scanning is a small modular add-on that can be installed on an inverted optical microscope.
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