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STXM end station

The second branch of the DEMETER beamline is ended with Scanning Transmission X-ray Microscope (STXM).

Scanning transmission X-ray microscopy (STXM) is a method to obtain a microscopic image of the raster-scanned sample by detecting the transmission intensity of the focused X-rays.

fig. The principle of sof x-ray STXM microscope.

One of the main elements of the STXM microscope is the zone plate (known as Fresnel zone plate FZP). The FZP is focusing a monochromatic beam of photons delivered by the synchrotron. After passing through the Fresnel lens, the photon beam passes through the order-sorting aperture (OSA), used to select the diffraction fringes of the specific order. An unfocused zero-order beam is stopped by the OSA aperture and the central stop (CS) on the FZP lens. The sample is located at the focal point of the Fresnel lens. The focal distance from the FZP to the sample is typically several mms. The sample position is raster scanned with piezoelectric scanner, and then monitored with laser interferometic sensors.

Fast, low-noise avalanche photodiode (APD) is often used to detect the transmitted X-rays. In the case of weak signals, a photomultiplier tube (PMT) is better solution. PMT counts the pulses created by excitation of a specific scintillator with X-rays transmitted through the sample. Fully digital control electronics are used to realize a fast scan by selecting the dwell time at each pixel.

The standard working mode at STXM microscope is transmission. However, but thick samples, or samples with small concentration of a given element, x-ray fluorescence yield (XRF) mode is available. Note, that in fluorescence mode the resolution is slightly lower in comparison to transmission and sample is scanned longer.

In principle, all the optics should be in vacuum, however, the ultra-high vacuum is not necessary. For samples that cannot be in vacuum, helium (He) atmosphere can be provided in the microscope chamber. Other way is to sealed the sample between two membranes (made a so-called "sandwich"). The STXM chamber containing the elements from FZP to the detector is separated from the synchrotron beam by a thin Si3N4 silicon nitride membrane window. This allows a quick replacement of the sample and feasibility for in situ environments.

The samples measured by the STXM should not be too thick (this results in no transmitted signals) or too thin (no significant absorption). Ideally, the sample should have an optical density OD = ln (Io / I) ≈ 1 at the target photon energy range. For this purpose, the specimens for STXM are often prepared by dispersing the particles onto Si3N4 membranes or grid meshes with support films. Larger samples are usually sectioned by using microtomes or focused ion beam (FIB).

The spatial resolution, of STXM microscope, is typically 20-100 nm and it is mainly determined by the diffraction limit of the lithographically produced Fresnel lenses FZPs.

The most important measurement mode in STXM is the so-called "image stack" - a series of images are collected as a function of photon energy to obtain a dataset with space (XY) and energy (E) dimensions. A local absorption spectrum can be obtained from the arbitrary region of interest at the image. It allows a detail chemical composition analysis of a measured sample.

Polarization-dependent experiments such as XMCD and LD are possible as with the bulk spectroscopy. In comparison with PEEM microscope were dichroic measurements are performed using bulk samples, in STXM only thin layers can be studied. Magnetic domains are obtained by calculating the asymmetry A = (IL - IR) / (IL + IR) from the images taken at two opposite circular polarizations (left and right).

Application of soft X-ray absorption on the nanoscale

X-ray transmission scanning microscope (STXM) was designed and built in the Solaris synchrotron as close cooperation with PhD. T. Tyliszczak (Advanced Light Source, Berkeley, USA).

Another working mode at STXM microscope is x-ray absorption spectroscopy XAS often termed NEXAFS (Near Edge X-Ray Absorption Fine Structure). This spectroscopy provides elemental and chemical specificity, as well as sensitivity to polarization effects related to the magnetic and crystal structure of the materials utilizing XMCD and XMLD effects. By using a scanning X-ray microscope, one can obtain chemical information on a scale from few nanometers to a millimeters.

Sample requirements:

  • The sample should be sufficiently thin to allow for minimum 5% transmission for X-rays in the required energy range.
  • Samples can be placed on Si3N4 membranes or TEM grids. Recommended membrane thickness is from 50 nm to 200 nm, depending on the X-ray energy range.
  • The vapour pressure of the samples have to be smaller than the minimum measurements pressure at the STXM chamber
  • The standard frame size is 5.0x5.0mm (square) or Ø=5.0mm (circular). Minimum frame size is 2.5x2.5mm or Ø=3.0mm, respectively. Maximum sample size is limited up to 19x10mm (HxV). In this case, it is possible to scan only 8 fields with a diameter 2.5mm. Sample thickness: max. 1.0mm. See: Al plate for sample holders.

Experimental conditions and environment

  • Available gases: He, Ar, O2, N2, CO2
  • Measurements pressure range: 1e-7 mbar do 1100mbar (the standard operating conditions at STXM is He atmosphere with pressure from 10 to 1000 mbar)
  • Temperature range: at this moment room temperature measurements are only available. Otherwise – only to special holders (equipped with heater)
  • Maximum field of view: +/-50µm2 (piezo stage), +/-1,25mm2 (linear stage)

Equipment

  • Available detectors:
    1D: photodiode Si (PD), avalanche photodiode (APD), photo multiplier tube (PMT),
    2D: fast soft X-ray CMOS camera 4Mpx
  • Solid drift detector (SDD) - Amptek X123 Fast SSD detector with sensitivity for photons with energy down to about 200 eV.

Measurement technique

  • X-ray transmission (single point or arbitrary line)
  • Fluorescence yield

Measurement geometry

Samples are scanned in a plane perpendicular to the beam in a focal point of a focused beam. In special cases, samples can be rotated with respect to the photon beam (max. +/- 45º).