Chapter 3: Low Temperature Growth of Boron Nitride
3.3 Experimental section
3.3.4 Ellipsometry
An automated angle M-2000F rotating-compensator ellipsometer equipped with an X–Y mapping stage, focusing probes and accompanying software (Complete-EASE 6.39 from J. A. Woollam Co.) was used in this study. Ellipsometric data were acquired in the wavelength range 250–1,000 nm with a resolution of 1.6 nm at incidence angles of 65°, 70° and 75°. The optical properties of both films were determined using the Kramers–Kronig consistent dispersion model using three Lorentz oscillators.
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3.3.5 High-resolution Rutherford backscattering and elastic recoil detection analysis
To investigate the elemental composition of the thin films, high-resolution Rutherford backscattering spectrometry (HR-RBS)18 was performed by irradiating samples with a 450-keV He+ beam generated by an RBS system (HRBS-V500; Kobe Steel). A magnetic-sector analyser with a high resolution of 1.2 keV was used for the measurements of the thin films. By employing the same system, high-resolution elastic recoil detection analysis (HR-ERDA) was simultaneously performed for hydrogen using 500- keV N+ ions. Typical beam currents used in the HR-RBS and HR-ERDA analyses were 40 nA and 6 nA, respectively.
48 3.3.6 Density measurements
Peaks corresponding to the relevant elements (B, N, O and Si) were observed in the HR-RBS spectra.
The areas covered by the peaks reflect both the thickness and the density of these elements. The areal density (atoms per centimetre square) was measured19, enabling the calculation of the a-BN film density by considering the element thickness. Oxygen from surface contamination was observed.
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3.3.7 Breakdown voltage and dielectric constant measurements
The current density–voltage (J–V) and capacitance–frequency (C–f) characteristics of the films in metal/a-BN/n-Si stacks were measured using a Tektronics K4200A-SCS parameter analyser system and a Karl Suss PA-200DS semi-automatic probe station. a-BN-based capacitors were fabricated on BN films directly deposited or transferred onto n-Si substrates. To prevent polymer contamination during device fabrication, a shadow mask with a 200-μm-diameter pattern was used, and a 100-nm-thick Cu electrode was deposited over the a-BN/Si stack. After the device fabrication, capacitance–voltage units in the parameter analyzer system were used to perform the C–f measurements. We carried out the C–f measurements in the frequency range 1 kHz–10 MHz with a hold bias of 0.5 V and an ac. drive of ±30 mV. The measured capacitance values did not change substantially as a function of the applied voltage of 0.5 V. Therefore, the relative dielectric constant was evaluated using the relation κ = Ct/Aε0, where t denotes the a-BN film thickness, A represents the area and ε0 denotes the dielectric constant of vacuum. At high frequencies exceeding 5 MHz, considerable noise levels were observed in the capacitance, probably owing to the low impedance of the a-BN capacitor. Subsequently, the J–V characteristics of both film samples were determined using source measurement units of the parameter analyzer system. The applied voltage was swept from 0 to 10 V with a resolution of 1 pA and a compliance current of 10 mA. Additionally, measurements were carried out at 50-mV voltage steps over 10 power line cycles to prevent degradation due to bias stresses.
50 3.3.8 Diffusion barrier performance
To evaluate the performance of the films as diffusion barriers, ~3-nm-thick samples of a-BN and TiN (deposited by radiofrequency sputtering) were deposited on Si substrates. Subsequently, the samples were coated with 80-nm-thick Co layers using dc. sputtering. After deposition, the samples were placed inside a furnace for annealing. The furnace temperature was ramped up at a rate of 40 °C min−1 in a vacuum of less than 10−4 torr. During annealing, thermally activated diffusion is expected to occur at the interface between Co and the dielectric barrier materials.
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3.3.9 Molecular dynamics simulations and computations
We modelled the Si substrate using a six-layer diamond rectangular slab having its free surface perpendicular to the z axis for BN nucleation and growth. The slab was periodic in the x–y plane with 18 × 18 repetitions of the unit cell, containing a total of 15,552 Si atoms. The top five layers were completely unrestrained during the simulations, whereas the bottom layer was fixed. The system contained 38,000 atoms of boron and nitrogen at a 1:1 ratio, with an additional 1,900 H atoms (~5%) for consistency with the experimental observations. All the simulations were performed using LAMMPS20. Throughout the simulation, the temperature of the substrate was held constant using a Nose–Hoover thermostat in a canonical NVT ensemble at temperature T = 673 K. The film was grown using the following method: all the atoms (boron, nitrogen and hydrogen) were initialized with random velocities in a region of height 40 A above the substrate. They were constantly thermalized at the growth temperature, and allowed to settle and cool on the Si substrate. To prevent premature B–N bond formation, the minimal distance between the initial B and N sources was set at 1.90 A, larger than the B–N bond length of 1.44 A in the hBN lattice. The equation of motion was numerically solved using the velocity Verlet integration scheme. Each simulation was run for more than 15 ns at a time step of 0.25 fs. After the growth process, the systems were further relaxed in an NPT ensemble at T = 300 K.
The extended Tersoff potential for BN was employed to describe the chemical processes (such as bond formation and dissociation) among the atomic species involved.21 This model potential has been specifically designed to correctly describe the dependence of the bonding in B, N and B–N systems on coordination and chemical environment. Thanks to its versatility, it allows the realization of large-scale atomistic simulations with more than a few thousand atoms. To describe the interaction within the silicon substrate, we used the Tersoff model potential, which has been proved to faithfully reproduce both the mechanical and the morphological properties of silicon-based systems.22 We treated the Si–N and Si–B interactions using the parameterized23 Tersoff potential, which has been previously employed to study the compositional and structural features of Si–B–N networks24. Finally, we modelled all the interactions involving hydrogen using a Lennard–Jones potential.
52 3.4 Results and discussion
3.4.1 Introduction of BN thin film deposition
ICP-CVD is configured by adding a device that generates plasma through electromagnetic induction to LPCVD. The addition of a plasma generator eliminates the need to maintain the substrate and the gas atmosphere at a high temperature for chemical reactions.Therefore, ICP-CVD is advantageous for low- temperature deposition.The ICP-CVD system used in this experiment can be seen in figure 1.
53 3.4.2 Optimization of BN thin film deposition
To optimize the borazine flow rate, a flow rate change experiment was conducted. As a result of the flow control experiment conducted at 700 degrees Celsius, it was confirmed that as the flow rate of borazine increased, the deposition rate increased and the surface became rough. (Figure 2) These results prove that it is important to very finely control the borazine flow rate. Then, looking at the result of confirming the trend according to the deposition temperature, if you look at the result of depositing at a temperature near 1000 degrees Celsius, you can see that the surface of the substrate is etched by receiving damage to the plasma. These results indicate that the deposition experiment at 700 degrees Celsius or higher in the current experimental apparatus is meaningless. (Figure 3) Therefore, the experiment was conducted below 700 degrees Celsius.
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Figure 1. ICP-CVD system with borazine MFC for precise control of borazine flow. The nc-BN a-BN were grown on Si substrates at 700 °C and 400 °C, respectively.
55
Figure 2. Surface observation results of BN films deposited at 700 degrees Celsius.
56 Figure 3. Variation of BN deposition with temperature.
57 3.4.2 Microscopic observation
When observing BN films deposited at 700 and 400 degrees Celsius with an atomic force microscope, films with very flat surfaces can be observed. (Figure 4) Transmission electron microscopy imaging and diffraction results shown in Figure 5 reveal that the films are polycrystalline hBN film. Also, it was confirmed that a very small nanocrystalline hBN film with a grain size of about 10 nm was deposited.
The nanocrystalline hBN film shown in Figure 5 results from deposition at 700 degrees Celsius.
Looking at the results in figure 6, it is possible to confirm the fully amorphous BN film. In the case of the BN film deposited at 400 °C, a completely amorphous film was deposited, confirming the new BN allotrope. Figure 7 shows the interpretation of TEM results for crystallinity and elemental analysis through EDS analysis.
58 3.4.3 Spectroscopic measurement and calculation
X-ray photoelectron spectroscopy (XPS) was used to obtain chemical information. The B/N atomic ratio was found to be ~ 1:1.08 (Figure. 8a and b) with B 1s and N 1s peaks at 190.4 eV and 397.9 eV, respectively, – indicating that the films are sp2-bonded B and N.25, 26 Molecular dynamics simulations shown in Figure. 9 confirm the amorphous structure of BN films and the calculated diffraction pattern which is consistent with the result in Fig. 2.
Raman spectra of a-BN and crystalline tri-layer hexagonal-BN (for comparison) reveal that the h-BN E2g mode at 1373 cm-1 is absent in a-BN (Fig. 8c).25, 26 Fourier transform infrared spectroscopy (FTIR) spectrum in Fig. 8d shows that there is an absorption peak near 1370 cm-1 that is attributed to the transverse optical mode of BN in a-BN. Another IR mode located near 1570 cm-1 confirms the amorphous nature of sp2-bonded BN27. We do not observe any N–H or B–H bonds with FTIR (Figure.
6). Detailed chemical and density analysis was conducted with Rutherford Backscattering Spectroscopy (RBS) and Elastic Recoil Detection Analysis (ERDA) – the results of which are shown in Figure. 10.
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Figure 4. BN films observed with an atomic force microscope.
60 Figure 5. Results of TEM observation of nc-BN.
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Figure 6. Atomic structure of amorphous boron nitride. (a) Low-magnification TEM image; (b) Selective area electron diffraction showing diffuse pattern with no discernible crystalline rings;
(c) High-resolution TEM image; (d) Magnified image of red box in (c) demonstrating disordered atomic arrangement; (e) Fast Fourier Transform results for area depicted in (d) demonstrating diffuse diffraction pattern that is typical of an amorphous film;
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Figure 7. Analysis of the reduced radial distribution function obtained from the electron diffraction data and cross-sectional chemical mapping of the a-BN film. a, Azimuthally averaged experimental electron diffraction intensity of a-BN. b, Reduced radial distribution function, G(r), of a-BN obtained from the electron diffraction data. The peak position r = 1.44 Å corresponds to the nearest-neighbour distance of B–N. The G(r) curve was calculated using eRDF Analyser (an open-source interactive GUI for electron reduced density function analysis).
c, High-angle annular dark-field (HAADF) scanning TEM image (left) overlaid with EDS maps of carbon (red), nitrogen (green) and silicon (blue). An image with overlaid EDS maps for all elements is shown on the right. Scale bars, 20 nm.
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Figure 8. Chemical structure of a-BN. XPS profiles for (a) B 1s and (b) N 1s peaks; (c) Raman spectra of a-BN and epitaxially grown tri-layer h-BN (used as reference) on SiO2/Si. The Raman spectrum of bare SiO2/Si substrate is identical to that of a-BN – suggesting that no distinct crystalline h-BN modes are present in a-BN.; (d) FT–IR spectrum measured using s-polarised radiation at an incident angle of 60°; (e) PEY-NEXAFS spectra for the B K-edge of a-BN, measured at incident angles of 30°, 55°, and 70° - showing no dependence on orientation.
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Figure 9. Molecular dynamics simulation. a, b, Side view (a) and top view (b) of a-BN grown on Si substrates at 673 K, calculated using molecular dynamics simulations. Different atomic species are shown in different colours: yellow (Si), blue (N) and pink (B). c, Mass density profile along the transversedirection (z), obtained from the results shown in a and b. Coloured solid lines denote the densities of different chemical species. The simulated density of a-BN is consistent with the experimental result. The black dashed line corresponds to the measured BN mass density.
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Figure 10. FTIR, HR-RBS, HR-ERDA and NEXAFS analyses of a-BN films. a, FTIR spectra of a- BN, showing the absence of B–H and N–H bonds. Abs, absorption. b, c, HR-RBS (b) and HR- ERDA (c) spectra of an a-BN film in the energy range 240–400 keV and 52–68 keV, respectively.
d, Elemental composition calculated using the HR-RBS and HR-ERDA spectra. e, PEY-NEXAFS spectra for the N K edge of a-BN, measured at incident angles of 30°, 55° and 70°, demonstrating a small angular dependence of the N K edge.
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3.4.4 Angle-dependent near-edge X-ray absorption fine structure (NEXAFS) measurement Angle-dependent near-edge X-ray absorption fine structure (NEXAFS) measured in partial electron- yield (PEY) mode at Pohang Light Source-II 4D beam line was used to investigate the chemical and electronic structures of a-BN. In NEXAFS, X-ray absorption excites core electrons of B and N to unoccupied states— that is, 1s electrons are excited to empty π* and/or to σ* states. In the 1s → π*
transition, the spatial orientation of π orbitals strongly impacts the transition probability. Thus, information pertaining to the relative orientation of orbitals in h-BN layers can be obtained by varying the incidence angle of X-rays.28 NEXAFS spectra obtained for a-BN sample at incident angles of 30°, 55°, and 70° are shown in Figure. 8(e). The observed resonance at 192 eV corresponds to the 1s → π*
transition in boron.20 The resonance intensity of the 1s → π* transition in a-BN demonstrates negligible variation with X-ray incidence angle [Figure. 8(e)] – strongly indicating that BN planes are randomly oriented throughout the material. Similar conclusions can be drawn from NEXAFS spectra of N K-edge (Figure. 10). Additionally, NEXAFS confirms that a-BN is completely sp2-hybridised.28, 29
67 3.4.5 Electronic properties
We now discuss the dielectric properties of a-BN. The dielectric constant is a physical measure of how easily electric dipoles can be induced in materials by application of an electrical field. The k 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 frequencies ranging from 10-kHz–30-MHz. The relative dielectric constants (k) for a-BN and h-BN, for comparison, at different frequencies are shown in Figure.
7a. It can be seen that k-values at 100 kHz are 3.28 and 1.78 for h-BN and a-BN, respectively. The values are average of measurements on > 50 devices. The distribution of measured values and the corresponding error bars at 100kHz are provided in Figure. 11b and Table 1. Remarkably, at 1 MHz frequency, the observed k value for a-BN further reduces to 1.16, which is close to the value of air or vacuum. The low k values of a-BN are attributed to nonpolar bonds between BN and also absence of order that prevents dipole alignment even at high-frequencies. The k values for a-BN compare extremely favourably to other reports in the literature, as shown in Table 2. We have confirmed the electrical measurements of k values with those obtained by measuring the refractive index of a-BN with spectroscopic ellipsometry and using the relationship: n2 = k.21 The refractive indices of h-BN and a- BN at 633 nm wavelength were found to be 2.16 and 1.37, respectively, as indicated by the green stars in Figure. 11(b). Thus, k-values for h-BN and a-BN from ellipsometry are 4.67 and 1.88, respectively – closely matching the values obtained with electrical measurements at 100kHz. Low-k dielectric materials are sometimes made porous to exploit the low k value of air but this decreases the density of the material, which in turn results in poor mechanical strength. It can be seen from Figure. 7c that a-BN possesses the lowest dielectric constant at the highest density in comparison with well-known low k materials reported in the literature [Table 2]. In addition, the nano scratch test results showed that the BN films were well attached to the Si substrate, and the strength of the BN films was similar to that of the Si substrate. (Figure 15)
The electrical breakdown strength of a-BN was extracted by measuring the current density with applied bias (Figure. 11d) on vertical sandwich type devices. The data in Figure. 11d reveal that there is a slight increase in current density due to Poole–Frenkel (P–F) tunnelling at low voltages and above 2.2 V, the leakage current sharply increases leading to electrical breakdown. As the thickness of a-BN is 3 nm, the breakdown field is extracted to be 7.3 MV-cm-1 – this is nearly twice that of h-BN (see Table 1) and the highest reported for materials with dielectric constants of less than 2 as shown in Figure. 11e. The a-BN film also exhibits exceptionally low leakage current density of 6.27 µA/cm2 at 0.3 V – thus,
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demonstrating its potential for 3 nm node devices. The key dielectric properties of a-BN and h-BN are summarised in Table 1.
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Electrical properties Film properties
Dielectric constant
@100 kHz / @1 MHz
Breakdown Field (MV-cm-1)
Reflective index (n) @ 633 nm
Density (g-cm-3)
h-BN 3.28 / 2.87 4.0 2.16 2.1
a-BN 1.78 / 1.16 7.3 1.37 2.1~2.3
Table 1. Electrical characteristics of a-BN and h-BN.
70 3.4.6 Diffusion barrier test
A key step in back end of line (BEOL) CMOS fabrication of logic and memory devices is the deposition of a diffusion barrier between the low-k dielectric material and the metal wire interconnects to prevent metal atom migration into the insulator. Ideally, this step can be eliminated if the low-k dielectric material can also serve as the diffusion barrier. We have therefore tested the diffusion barrier properties of a-BN by depositing 80 nm of cobalt film on a-BN and annealing the Co/a-BN/Si devices in vacuum for 1 h at 600 °C. This annealing condition is extremely harsh and under similar conditions severe diffusion of cobalt in Si occurs when industry standard TiN is used as the barrier layer (Figure. 12). In contrast, no diffusion of Co or silicide formation was observed with a-BN in cross-sectional TEM results shown in Figure. 11f (EDS composition line map of the interface is shown in Figure. 13) – suggesting that a-BN can serve as both the low-k dielectric and the diffusion barrier. Our results suggest that a-BN is an excellent low-k material for high performance CMOS electronics. And a-BN has great dielectric strength compare with TiN. (Figure. 14)
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Figure 11. Dielectric properties of a-BN. (a) Dielectric constant determined using capacitance–
frequency measurements on metal–insulator–metal (MIM) structures (thick blue and red lines denote averages; inset illustrates optical image of MIM structure); (b) Distribution of dielectric constant values at 100 kHz and refractive indices (green stars) calculated via ellipsometry measurements; (c) Density versus dielectric constant of low-k materials reported in literature (blue circles) with red circle denoting a-BN reported in this study; (d) Typical current–voltage (J−V) curves for h-BN (approximately 1.2 nm thick; blue curve) and a-BN (3 nm thick; red curve) films; (e) Breakdown field versus dielectric constant for low-k materials reported in literature (blue circles) with red circle denoting a-BN; (f) Cross-sectional TEM images of a-BN after thermal-diffusion test performed for 1 h at 600 °C. The bottom image shows magnified view of red box marked in upper image.
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Figure 12. Cross-sectional TEM images of Co(80 nm)/TiN(3 nm)/ Si films after thermal diffusion tests at different temperatures. a–c, Images obtained after thermal diffusion at 600 °C for 60 min (a), 600 °C for 30 min (b) and 400 °C for 30 min (c). d, Magnified cross-sectional TEM image (right) and EDS line profile (left) of the film shown in a.
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Figure 13. Large-area cross-sectional TEM images, EDS line profiles and maps of a Co/a-BN(3 nm)/Si film after thermal diffusion at 600 °C for 60 min. a, Large-area cross-sectional TEM image and EDS line profiles. b, EDS maps of Co and Si showing that Co is isolated above the a-BN film and does not diffuse into the Si. c, EDS maps of a magnified area in b.
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Figure 14. Breakdown bias at different temperatures for a-BN and TiN barriers.
75 Figure 15. nano scratch test results of BN films.