In STEM measurements, the electronic noise or interference by any stray fields in the environment can superimpose on the ramping current in the scanning coils when STEM images are generated by rastering the electron beam on specimens. The high frequency fluctuation leads ‘tearing’ noise in the images, while the medium- and low-frequency noises induce distortion of the images. All those noise and interference especially affect the precision of spatial coordinates of the recorded atomic features.
The electronic noise in EDS detectors is mathematically given by,
Ctot -- The total capacitance,
af -- The 1/f noise coefficient,
IL -- The leakage current,
gm -- The transconductance,
A1 -- The filter constants,
T -- The shaping time constant,
q -- The electron charge.
The first term on the right side of Equation 4654d represents the thermal noise,
the second term the 1/f noise, and the third term the leakage current. The characteristics of various EDS detectors is listed on a table on page4655.
The natural line width of the emitted X-rays (e.g. Mn Kα line) in EMs (electron microscopes) is only a few eV but the line widths obtained with EDS systems are normally larger than 100 eV, for instance, the EDS-detected FWHM (full width at half maximum) at the energy of Mn Kα is 135-165 eV. The natural energy distribution of the characteristic X-rays of a single line is well described by the Lorentzian probability distribution instead of Gaussian shape obtained by the energy-dispersive detectors. The electronic noise in the EDS system introduces the peak broadening that is a major source causing the difference between the EDS-detected and natural energy resolutions. The width of the electronic noise is also described as the ‘point-spread function’ of the detector.