Radiation Damage and Nanofabrication
TEM signal (for example, diffracted- peak intensity) is oſten proportional to exp(-D/Dc
(product of current density and exposure time) and Dc
), where D is the electron dose is a critical or characteris-
tic dose, dependent on the accelerating voltage and specimen temperature. For a non-uniform beam, the decay is no lon- ger exponential (Figure 3). Radiolysis in an organic specimen is
reduced (by a factor of typically 2 to 20) by cooling to around 100o
K, supposedly
by suppressing the diffusion of radia- tion-induced reaction products (single atoms, radicals, ionized species). Chem- ical scavengers such as antioxidants may also curtail structural damage at room temperature [2], but not at 100o
K where
diffusion is already largely eliminated. Encapsulation in ice or between layers of carbon is found to reduce damage and is employed in cryo-EM (together with cooling) to permit structural measure- ments on proteins and other important biological molecules [3]. Any substantial heating effect of a
beam will increase the radiation sensitiv- ity, giving a direct dose-rate effect (Dc
fall-
Figure 2: Left, radial heat flow from the irradiated volume of specimen (radius R) to a heat sink at distance R0 and accumulated charge Q.
Table 1: Charging calculated assuming classical electrostatics and ohmic conduction. Specimen σ(S/m) Al2O3
εr
ing with increasing dose rate). Conversely, mass transport and associated structural damage may be diffusion-limited, giving an inverse response (Figure 4). Knock-on damage. High-angle elastic scattering causes
Ice (-5°C) Pure Si am-C
10−8 10−7 10−3 103
90 τ
2.5 2 ms 8 ms
12 0.1 μs Vs (volt)
70 (6000) 7 (600)
10 0.01 ps 10−9
ρe (e/atom) dV/dr (MV/m) 104
10−3 (0.1) 10−5 (10−7
) 10−11
0.5 (10−5 (10−10 (10−16
)
0.14 (10−6 )
) 103
(103 (102
) 10−7
) )
0.3 (0.02) (10−13
)
knock-on displacement of individual atoms of the specimen. Tis process is inefficient, so it is most commonly observed in metals and semiconductors where radiolysis is absent. Te interaction is highly localized and has been used to produce changes on an atomic scale [4]. Tere is evidence that inelastic scattering can supply some of the energy needed for displace- ment of a particular atomic species, allowing modification at primary-beam energies below the threshold value calculated from relativistic kinematics [5].
STEM versus TEM for Radiation-Sensitive Specimens A question of longstanding concern is how much damage
scanning transmission electron microscopy (STEM) produces, relative to TEM and for the same amount of retrieved information. Beam heating is less, but only because the current achievable in a small electron probe is much below that used for TEM imaging. Charging effects are time-dependent and sensitive to dose rate, which is typically a factor of 103
–105 higher in STEM.
As seen in Table 1, the charging time for resistive specimens may exceed the pixel dwell time (0.16 μs for a 250×250 raster recorded in 10 s). Te specimen would then discharge between adjacent frames if there were no charge trapping. Random or sparse sampling (as opposed to conventional raster scanning) might allow less charge accumulation and damage [6]. A simi- lar situation may apply to short-pulse beams [7,8], except that
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Coulomb repulsion between beam electrons precludes sub-μm probes with very high current density [9,10]. For good insulators, including most organic specimens, the
electric field just outside a STEM probe well exceeds the breakdown strength (100–1000 MV/m), as seen in the last column of Table 1. Calculated assuming ohmic conduction, Vs
and ρ will then be over- estimates, but local dielectric breakdown might itself be damaging. .
Right, electron current arising from emission of secondary and Auger electrons, creating a surface potential Vs
Figure 3: Damage response for a Gaussian or a diffraction-limited probe (blue curve) compared with the exponential decay calculated for a current density equal to half the peak value.
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