## 2 Instrumentation Developments  
### Upgrade of a Nanomeasuring and Nanopositioning Machine (NMM)  
Several micro/nano-CMMs are operated at PTB, including a SIOS NMM, a Zeiss F25, and a Werth VideoCheck UA CMM. Here, we detail the recent upgrade of the NMM in collaboration with the SIOS Company, Ilmenau, Germany.  
The measurement principle of the NMM is briefly shown in Figure 1a. Its motion platform consists of a mirror corner which comprises three high-precision planar mirrors attached orthogonally to each other. With three high-precision interferometers (\(x\)-, \(y\)- and \(z\)-interferometer), the displacement of the motion platform can be measured with respect to the metrology frame (Zerodur frame) with a resolution of 0.08 nm. In addition, there are two angle sensors available for measuring all three angular DOFs of the motion platform with a resolution of better than 0.01\". Thus, all six DOFs of the motion platform are accurately measured. The motion platform is moved by three stacked mechanical stages driven by voice coil actuators (not shown). By utilizing a digital signal processor (DSP) servo controller based on the measured six DOF values, the NMM is capable of positioning and measuring with nm accuracy. For micro/nano-CMM measurements, the sample is fixed on the mirror corner and the CMM probe is typically located at the intersection point of three measurement beams of laser interferometers. Thus, the measurement is performed fully in compliance with the Abbe principle along all three axes.  
The recent upgrade of the NMM was undertaken with regard to a number of components, as detailed below:  
* The geometry of the corner mirror has been improved to fix samples better so that the stress introduced into the optical component due to the sample fixing is greatly minimized. In addition, the height of the mirrors has been increased so as to allow higher objects (up to 22 mm).
* The interferometers and the angle sensors have been improved for easier adjustment, better thermal behaviour and better stability.
* A new motorized spring mechanism has been installed for the weight compensation of the motion stage. As a result, the heat generation of the z-driving motors is greatly reduced, allowing much better temperature stabilization.
* All the control electronics have been upgraded. They now have an increased servo frequency response up to 1 kHz for better stage control performance.
* An improved instrument chamber for better thermal and acoustic insulation and an improved vibration damping stage are applied.  
To demonstrate how much the performance of the NMM has improved, the positioning noise along the \(z\)-axis before and after the upgrade, measured at the same sampling frequency of 6.25 kHz, is compared in Figure 1b. It can be seen that the noise level has been significantly reduced from 1\(\sigma\) = 0.52 nm to 1\(\sigma\) = 0.13 nm. In Figure 1c, a positioning example is shown where the NMM is commanded to move along the \(x\)-, \(y\)- and \(z\)-axes simultaneously by several steps of 10 \(\upmu\)m with a speed of 5 \(\upmu\)m/s. The position noise along the \(x\)-, \(y\)- and \(z\)-directions after arriving at the target positions is shown in Figure 1d, indicating an excellent positioning and measurement performance.