Supersampled scanning transmission X-ray microscopy for high-resolution vibration-free imaging

Abstract

Scanning transmission X-ray microscopy (STXM) is a nanoscale imaging technique that can utilize several powerful contrast mechanisms for the quantitative mapping of chemical and physical materials properties. Spatial resolutions down to 7~nm at the soft X-ray energy range have been demonstrated. A limiting factor for high-resolution STXM imaging is given by the positioning precision of the sample with respect to the focusing optic, with the current state-of-the-art leading to significant overheads, especially at low pixel dwell times, and being vulnerable to unavoidable external vibrations sources. In this work, we present a method, called supersampled scanning microscopy, that allows for a significant reduction of overhead times while simultaneously removing the effects of vibrational noise by sampling the position of the sample at a rate significantly higher than the vibration spectrum and reconstructing the sample transmission image from the recorded list of positions and detector counts. We demonstrate the performance of the technique with a set of proof-of-concept high-resolution imaging experiments.

Figures (6)

• Figure 1: (a) Heatmap showing the sample positioning uncertainty in idling conditions (control loop attempting to keep position); (b) Same conditions as (a), but with a closed cycle cryostat, normally employed for cooling down samples, activated; (c) Spectrum of the vibrations in both configurations.
• Figure 2: (a) Total scan time determined from STXM scans for a $200 \times 200$ px$^2$, $2 \times 2$ µ m scan in the PP, CV, and with the supersampled configurations as a function of the desired pixel dwell time. (b) Corresponding scan overhead fraction (calculated as the ratio between the overhead time and the nominal scan time).
• Figure 3: (a) Sketch of the setup employed for the supersampled scanning microscopy measurements. The position of the sample, together with the detected photon counts, is recorded at a fast (4 kHz) rate by a dedicated FPGA setup. The piezoelectric stage is scanned with minimal positioning precision requirements using a trajectory generator integrated within the Orocos positioning system of the endstation. (b) Sketch of the operation principle of the CV mode and of the principle of supersampled imaging for the same scan trajectory. In supersampled imaging, the data is assigned to a pixel based on the recorded position, allowing one to overcome possible artifacts caused by vibrations or glitches in the positioning system.
• Figure 4: Proof-of-concept for vibration control. (a) STXM image acquired of a 10 nm Ir Siemens star acquired with the standard STXM setup. In the top half of the image, above the red line, a closed cycle cryostat cooler was turned on to demonstrate the effect of strong mechanical vibrations. (b) Same dataset as in (a), but reconstructed with the supersampling method. The reconstructed image shows no vibrations. Both images have a pixel size of 10 nm. Imaging performed with a 35 nm outermost zone FZP under diffraction-limited conditions. The effective dwell time is of 8 ms/px (Poisson noise of 1.3 %, compared to a contrast of 2.6 %).
• Figure 5: Proof-of-concept for high-resolution imaging. (a) High-resolution standard STXM image of the central area of a 10 nm Ir Siemens star. Several artifacts, marked by the red arrows, are visible in the image. A blur of the spokes in the vertical direction (caused by vibrations) is visible, and the center of the star cannot be fully resolved. (b) Same dataset as in (a), but evaluated from a supersampling stream. Using this method, the artifacts identified in (a) have been corrected, and the center of the star is well resolved. Both images have an effective pixel size of 2.5 nm. Imaging was performed with a 8.8 nm outermost zone FZP under diffraction-limited conditions. The effective dwell time is 38 ms/px (Poisson noise of 0.44 %, compared to a contrast of 2.6 %). (c) Scanning electron micrograph of the Siemens star imaged in (a-b). (d) Profile across the area marked in (b) derived from the standard STXM image, the supersampled reconstruction, and the SEM image. A significant reduction in the linewidth can be observed when using the supersampled reconstruction.