Radiation Damage and Nanofabrication
Increasing signal-collection efficiency
is just as important as reducing damage. Phase-contrast provides the largest signal from light-element samples and has tradi- tionally been carried out using fixed-beam TEM mode. But using recently developed algorithms and perhaps combined with ptychography, STEM might be capable of comparable or greater efficiency [18–22].
Consequences for Small-Probe
Fabrication Ultraviolet lithography is the basis of
microelectronics fabrication and relies on the radiolysis of organic “resists” for pat- terning. Electron-beam lithography can utilize the same resists but is carried out serially using a focused electron beam. A 50 μC/cm2
dose of 20 keV electrons Figure 4: Possible dose-rate effects: direct (due to heating or charging) and inverse (arising from diffusion limits). Te charge density ρe is much higher in STEM mode and might
even increase the amount of bond breaking; time-dependent den- sity-functional calculations [11] can hopefully answer this ques- tion. Such an effect would lead to a direct dose-rate dependence of damage, the amount increasing above some current-density threshold that should increase with increasing primary energy due to the reduced secondary-electron and Auger yield. In an inorganic oxide, the electrostatic field can give rise
to ionic driſt, resulting in phase segregation or phase trans- formation [12] that could be worse in STEM mode. But STEM elemental mapping might benefit from diffusion limits (which reduce radiolytic mass loss) if fast-recording EELS and EDX detectors are available [13]. Charging effects are reduced or eliminated (for example, in
cryo-EM imaging of biomolecules) by coating the sample with a thin layer of amorphous carbon [14,15]. Graphene may work even better, as a conducting layer and diffusion barrier [16]. Aſter irradiation by x-rays, dark progression of damage is
sometimes observed [17]. If it turns out to be significant for elec- tron irradiation of thin specimens, single-frame STEM acquisi- tion would be advantageous [13].
is said to reduce the molecular weight of 70% of PMMA molecules, crosslinking about 2% of them [23]. Αſter chemical development of the latent image, line- widths down to 20 nm are achievable [23], or below 10 nm with 200 kV aberra- tion-corrected optics [24]. For a sensitive
resist, there is always a statistical limit to spatial resolution due to the small number of beam electrons producing the damage [25]. Te resolution of organic polymers may also be limited by their large molecular size [26], which has provided an incentive for using smaller molecules. A focused field-emission probe has been used to create
holes of 1–5 nm diameter in metal halides [27] and oxides [28,29]. If used as a self-developing resist that volatilizes dur- ing exposure, these inorganic materials avoid the need for wet-chemical processing. Each hole could represent one bit of stored information, the small dimensions enabling high infor- mation density, but the sensitivity is well below that of organic resists (Table 2). It may be true that the Encyclopedia Britannica could have been written on a pinhead [30], but that procedure would have taken at least a year. Organics and halides may damage by radiolysis, but hole
drilling in amorphous alumina appears to involve electrostatic charging, as suggested by the existence of a threshold current density. If the charge density ρe
exceeds about 0.1 e/atom, inter-
atomic repulsion causes a Coulomb explosion, and ions are emitted from the specimen surfaces [31]. Any damage process that depends on
Table 2: Approximate values of the dose D, hole diameter d, time to write 106
Material PMMA NaCl Al2O3 MoS2
Graphene Diamond
58
D(C/cm2 2×10−3 100
5000 1600 8000
2×105
) Diameter d 5 nm 4 nm 1 nm
0.3 nm 0.3 nm 0.3 nm
106 holes,
and the damage mechanism for material removal by a beam (1 nm diameter, current 0.4 nA) of 200 keV electrons.
40 ms 30 min 28 hr 9 hr
44 hr 46 day
-time Mechanism Ref. radiolysis radiolysis charging e-sputter e-sputter knock-on
[24] [27] [29] [5]
[35] [35]
inelastic scattering is limited in resolution by the long-range nature of the electrostatic interaction between beam and atomic elec- trons (Coulomb delocalization). Another fundamental limit arises from lateral motion of the secondary electrons, which cause most of the damage in organic materials [32]. As a result of these two effects, it appears that about 50% of the energy is deposited outside a 1 nm diameter probe [33,34]. Tese limits are avoided by basing the
fabrication on knock-on displacement of individual atoms, the only damage process
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