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in materials by application of an electrical field. The κ value of air or
vacuum is 1, but electric polarizability in solid-state matter arises from
dipolar, atomic and electronic components that are most relevant for
high-performance electronics. The contributions from these can be
measured as a function of frequency in the range 10 kHz–30 MHz. The
relative dielectric constants (κ) for a-BN and hBN at different frequen-
cies are shown in Fig. 3a. It can be seen that the κ values for hBN and
a-BN at 100 kHz are 3.28 and 1.78, respectively. The values are aver-
ages of measurements on more than 250 devices. The distribution of
the measured values and the corresponding error bars at 100 kHz are
shown in Fig. 3b and Table 1. Remarkably, at a frequency of 1 MHz, the
observed κ value for a-BN is further reduced to 1.16, which is close to
the value of air or vacuum. The low κ values of a-BN are attributed to
nonpolar bonds between BN and to the absence of order, which pre-
vents dipole alignment. The κ values for a-BN compare very favourably
to other reports in the literature, as shown in Extended Data Table 1.
We validated the κ values from the electrical measurements using
values obtained by measuring the refractive index (n) of a-BN with
spectroscopic ellipsometry and using the relationship n^2 = κ (ref.^22 ).
The refractive indices of hBN and a-BN at a wavelength of 633 nm were
found to be 2.16 and 1.37, respectively, as indicated by the green stars in
Fig. 3b. Therefore, the κ values for hBN and a-BN from ellipsometry are
4.67 and 1.88, respectively—closely matching those obtained with elec-
trical measurements at 100 kHz. Low-κ dielectric materials are some-
times made porous to exploit the low κ value of air, but this decreases
the density of the material, which in turn results in poor mechanical
strength. Figure 3c shows that a-BN possesses the lowest dielectric
constant at the highest density in comparison with well known low-κ
materials (Extended Data Table 1). We also measured the mechanical
properties of the a-BN films to confirm their strength. The results of
nanoindentation measurements shown in Extended Data Fig. 6a, b
indicate that the hardness and stiffness values of the a-BN films are
equal to or greater than those of silicon (>11 GPa). The nanoscratch
test results shown in Extended Data Fig. 6c also suggest that the films
are very well adhered to the substrates.
The electrical breakdown strength of a-BN was extracted by meas-
uring the current density versus the applied bias (Fig. 3d) on vertical
sandwich-type devices. The data in Fig. 3d reveal that there is a slight
increase in current density due to Poole–Frenkel tunnelling at low volt-
ages, whereas above 2.2 V the leakage current increases sharply, leading
to electrical breakdown. Because the thickness of a-BN is 3 nm, the
breakdown field is 7.3 MV cm−1—nearly twice that of hBN (see Table 1 ) and
the highest value reported so far for materials with dielectric constants
of less than 2, as shown in Fig. 3e. The a-BN film also has an exceptionally
low leakage current density of 6.27 μA cm−2 at 0.3 V, thus demonstrating
its potential for use in 3-nm-node devices. The key dielectric properties
of a-BN and hBN are summarized in Table 1.
A key step in the fabrication of back-end-of-line CMOS-compatible
logic and memory devices is the deposition of a diffusion barrier
between the low-κ dielectric material and the metal wire intercon-
nects to prevent metal atom migration into the insulator. Ideally, this
step can be eliminated if the low-κ dielectric material can also serve as
the diffusion barrier. We therefore tested the diffusion barrier prop-
erties of a-BN by depositing an 80-nm-thick cobalt film on a-BN and
annealing the Co/a-BN/Si devices in vacuum for 1 h at 600 °C. These
annealing conditions are extremely harsh, and under similar conditions
severe diffusion of Co into Si occurs when TiN is used as the barrier layer
(Extended Data Fig. 7). By contrast, neither diffusion of Co nor silicide
formation was observed in the cross-sectional transmission electron
microscopy (TEM) results shown in Fig. 3f (additional data in the form
of energy-dispersive spectroscopy composition maps are shown in
Extended Data Fig. 8), suggesting that a-BN can serve as both the low-κ
dielectric and the diffusion barrier. The comparison of the breakdown
bias in the Co/a-BN/Si and Co/TiN/Si devices at various temperatures
suggests that the films are stable at high temperatures (Extended Data
Fig. 9). Our results indicate that a-BN is an excellent low-κ material for
high-performance CMOS electronics.Online content
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published maps and institutional affiliations.© The Author(s), under exclusive licence to Springer Nature Limited 2020Table 1 | Properties of a-BN and hBN thin films
Electrical properties Film properties
Dielectric constant Breakdown
field (MV cm−1)Refractive index,
n, at 633 nmDensity
at 100 kHz at 1 MHz (g cm−3)hBN 3.28 2.87 4.0 2.16 2.1
a-BN 1.78 1.16 7.3 1.37 2.1–2.3