The position stability of the interferometer is an important performance benchmark. The PicoScale interferometer achieves stabilities in the sub-nm region and can detect vibrations with single-pm amplitudes, which is proven with the measurements described here.
A PicoScale interferometer head and a target mirror were mounted into a box to reduce air fluctuations. The setup was placed in an SEM chamber at ambient pressure. The working distance between interferometer head and mirror was 20 mm. With this setup, position data was recorded from the interferometer for 20 seconds at several different constant frequencies. The Fast-Fourier-Transform of the signal was calculated within the PicoScale software and the RMS position signal over the frequency was plotted. For very high stream frequencies, the data was limited to 500000 data points. In addition, the time signal was recorded and plotted in histograms to show the distribution of the position data over time.
The FFT spectra calculated from the position data at various streaming frequencies are shown in the first gallery below. Each time, the average over 10 FFT spectra was taken. A slight increase in noise level can be seen with increasing frequency, which is due to the increasing bandwidth of the signal. On the other hand, for each streaming frequency, a decline of the noise over the frequency is observed, as is expected for noise of electronic signals (1/f -noise). Despite some disturbance peaks, which may be due to the environment, the noise level is generally below 10 pm.
The histograms of the data at various streaming frequencies are shown in the second gallery. Except of subtracting the average value to center the distribution at zero, no post-processing of the data was carried out. From the histograms it can be seen that the distributions are approximately Gaussian-shaped in all cases, and the standard deviation of the position data can be evaluated. The standard deviation increases slightly with streaming frequency because of the bandwidth, but is below 100 pm for all but the highest streaming frequencies in the MHz range.
Measurement of Small Displacements
A very small sinusoidal excitation was applied to the target mirror with the help of a function generator and a SmarAct amplifier. The voltage was set such as to achieve an excitation amplitude of around 10 pm. The working distance between interferometer head and mirror remained at 20 mm.
The Piezo actuator that was fixed in the previous measurement, is now actuated with amplitudes of around 10 pm. The Fourier spectra calculated with the PicoScale for various excitation frequencies of the Piezo actuator are shown in the gallery below. In order to lower the noise level, the FFT spectra were averaged. The red line in the graphs above shows the 1 pm level and thereby underlines the single-pm resolution achieved in the measurements. The very small external vibration of the mirror caused by the Piezo element clearly stands out from the noise level of the FFT spectra. This proves the capability of the PicoScale to measure vibration down to extremely low amplitudes.
It is clear from the results that the very low noise level of the PicoScale allows to measure and identify vibrational peaks in the single-pm range. Sub-nanometer displacements can be detected due to the position stability below 100 pm. This makes the SmarAct PicoScale interferometer an extremely powerful tool to measure displacements in many different kinds of environments over a very broad frequency and magnitude range.