Radiation Damage and Nanofabrication in TEM and STEM
Ray Egerton Physics Department, University of Alberta, Edmonton, Canada T6G 2E1
regerton@ualberta.ca
Abstract: Different aspects of electron-beam damage are summa- rized, together with some quantitative evaluation. TEM and STEM are compared in terms of
Keywords: transmission electron microscope, electron-beam
damage, knock-on damage, electron charge damage, electron heat damage
Introduction Radiation damage in the transmission electron micro-
scope (TEM) has been a problem ever since researchers in the late 1940s put organic specimens into the electron beam, hop- ing to see details beyond the limit of a light microscope, or even processes occurring within living cells. But as in the initial studies of radioactive materials or the first atom-bomb tests, there was little recognition of the damaging effect of ionizing radiation. Tis process starts with the breaking of chemical bonds, causing structural damage on an atomic scale (observ- able from TEM diffraction patterns), and leads to longer-range disruption of structure (seen in TEM images) and the removal of some chemical elements (known as mass loss, measurable by energy-loss or x-ray spectroscopy). With the development of brighter electron sources,
improved aberration-corrected optics, and better electron detectors, damage has emerged as an important limiting factor in many areas of electron microscopy. Electron-beam damage has many aspects, as described
in a recent review paper [1], and what follows is, in part, an update to that article, collecting together various mathematical formulas and references to recent publications. From time to time, people find new ways of using electron beams to fabricate small structures, so that creative aspect of radiation damage will also be discussed briefly.
Radiation Effects in an Electron Microscope An electron beam can cause permanent change to a TEM
specimen (thickness t) through several physical mechanisms as discussed below. Heating. Within the irradiated volume (radius R), inelas-
(2κ)], typically 1 ps for a small probe of diameter 2R=1 nm. Te final temperature rise is ΔT ∼ Ib
=R2
, the temperature ln(R0/R) [ρCp
and κ is the thermal conductivity of the specimen. For most materials, κ > 0.1 W/m/K at room temperature and ΔT does
where Em (∼ 7Z for atomic number Z<30) is the mean energy loss per inelastic collision, λi
Em(eV) ln(R0/R)/(2πκλi is the inelastic mean free path, 56 doi:10.1017/S1551929521000663
tic scattering generates heat that is removed by conduction (for example, to an annular heat sink at distance R0 specimen of density ρ and specific heat Cp rise is exponential with a time constant τh
away). For a /
),
Figure 1: Beam current (in nA) giving ΔT=10 K for specimens of thermal con- ductivity κ, calculated assuming Em
=40 eV, λi=100 nm, R0 =30 µm.
www.microscopy-today.com • 2021 May information-to-damage ratio. Electron-beam
fabrication is briefly considered in terms of resolution and writing speed.
not exceed 10 K for beam currents typical of a modern TEM (Figure 1) so thermal decomposition cannot account for most damage observations. However, polymers such as polystyrene (with κ ∼ 0.01 W/m/K at T=100 K) can soſten and fall apart in the beam, assisted by forces due to electrostatic charging. With the condenser aperture removed, thermionic-source TEMs have allegedly generated beam currents high enough to melt metallic specimens. Charging. Inelastic scattering causes emission of second-
ary and Auger electrons, creating a local charge Q, a radial conduction current Ic
and permittivity εr =Q/(πR2
that reduces the total yield Y(Vs εr
(πσt) and a charge density within the irradiated volume given by ρe
= Ib Y(Vs), with Vs=Ib Y(Vs ) ln(R0 highest at the edge of the probe: dV/dr=Vs t) = 2ε0 εr Ib Y(Vs)/(πσtR2 /R)].
Tis charge accumulation has an RC time constant τq (Q/Vs)=ε0
/σ that depends on the electrical conductivity σ of the sample, but not on the beam cur-
=(Vs /Ic
rent or radius. For a poor-quality insulator, the result may be a steady-state condition: Ic
/R)/
). Te voltage gradient is /[R ln(R0
Table 1 shows estimates based on these macroscopic for- mulas, taking t=100 nm, R0=30 µm, and Y(Vs) = 10−2 STEM probe (2R=1 nm, Ib tion (2R=5 µm, Ib
=0.4 nA) and for TEM illumina- =100 nA, values given in parentheses). Te
charge density ρ is given in terms of electron units per atom. Radiolysis. Inelastic scattering causes ionization damage
(radiolysis): the breakage of chemical bonds and loss of atomic structure followed by ejection of atoms from the sample (mass loss). For an incident beam with uniform current density, the
, for a
, and an increasing surface potential Vs ) from each surface (Figure 2).
)
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