## 2 Instrumentation Developments
### AFM-Based 3D Probes  
Although the diameter of the smallest micro-CMM probe may be as small as 25 \(\upmu\)m, it is still too large to measure 3D structures with sizes of a few micrometres or even below. There is a metrology gap between AFMs--the most popular used coordinate measuring techniques for nanostructures--and micro-CMMs. To fill this gap, another idea is a type of 3D probe based on the AFM technique. At PTB, we have developed a 3D-AFM [39] and the so-called assembled cantilever probe [40], which is promising to fill this gap.  
The measurement principle of the 3D-AFM developed by PTB is shown in Figure 4a. It utilizes flared AFM tips. Such tips have an extended geometry near their free end which enables the probing of steep and even undercut sidewalls. The probe element can be regarded as a disc; however, due to its tiny size, it still has a high spatial resolution. Similar to conventional AFMs, the 3D-AFM can measure in contact, intermittent and non-contact mode. Furthermore, a vector approach probing (VAP) method has been applied for enhancing the measurement flexibility and for reducing tip wear [24]. Our preliminary uncertainty estimation indicates that the 3D-AFM is capable of measuring the feature width of nanostructures with an expanded uncertainty down to 1.6 nm (at a confidence level of 95%).  
Currently, such flared probes are commercially available with a diameter up to 850 nm and an effective stylus length of 7.5 \(\upmu\)m [41]. However, the technology can be expa  
Figure 4: **(a)** Schematic diagram showing the measurement principle of a 3D-AFM probe; **(b)** SEM image taken of a flared AFM tip applied in a 3D-AFM; **(c)** schematic diagram of 3D assembled cantilever probe (ACP); and **(d)** a typical probing curve of the 3D-ACP probe. Figures **(c)**, **(d)** are reproduced with permission from [40], Copyright American Institute of Physics, 2007.  
flared tips with diameters of a few micrometres and styli lengths of tens of micrometres using, e.g., Focused Ion Beam (FIB) milling or FIB induced deposition techniques.  
However, due to the limited speed of the FIB microfabrication technique, the size of its manufacturable probing elements is still limited. To further expand the AFM technique for full 3D measurements, a type of assembled cantilever probe (ACP) has also been proposed [40]. Compared to the flared tip where the probing element is directly fabricated on the tip, the ACP technique applies microassembling techniques to create a probing stylus. The ACP also has the advantages of the AFM technique, such as high measurement sensitivity, very low measurement forces (\(\upmu\)N to nN level), a compact structure, low fabrication costs and the ease of probe exchanges. Additionally, the ACP probe is mechanically and electrically compatible with commercial AFMs. It could therefore be applied directly in commercial AFMs to extend their functions from surface topography measurements to 3D measurements, if some minor software modifications are provided.  
Due to its low probing stiffness (typically about 1.5 N/m in lateral directions), the 3D-ACP probe is usually used for point-to-point measurements. It is capable of performing measurements with a probing speed of up to 100 \(\upmu\)m/s. It has a probing repeatability of approximately 25 nm (\(p\)-\(v\)) along the \(z\)-axis, and of approximately 130 nm (\(p\)-\(v\)) in full 3D, as shown in Figure 5. Testing of its probing error according to ISO 10360 has not been performed yet.