Ruthenium may replace copper interconnects in next-generation very-large-scale integration (VLSI) circuits. However, interfacial bonding between Ru interconnect wires and surrounding dielectrics must be optimized to reduce thermal boundary resistance (TBR) for thermal management. In this study, various adhesion layers are employed to modify bonding at the Ru/SiO2 interface. The TBRs of film stacks are measured using the frequency-domain thermoreflectance technique. TiN and TaN with high nitrogen contents significantly reduce the TBR of the Ru/SiO2 interface compared to common Ti and Ta adhesion layers. The adhesion layer thickness, on the other hand, has only minor effect on TBR when the thickness is within 2–10 nm. Hard X-ray photoelectron spectroscopy of deeply buried layers and interfaces quantitatively reveals that the decrease in TBR is attributed to the enhanced bonding of interfaces adjacent to the TaN adhesion layer, probably due to the electron transfer between the atoms at two sides of the interface. Simulations by a three-dimensional electrothermal finite element method demonstrate that decreasing the TBR leads to a significantly smaller temperature increase in the Ru interconnects. Our findings highlight the importance of TBR in the thermal management of VLSI circuits and pave the way for Ru interconnects to replace the current Cu-based ones.

1. Background and purpose Silicon tin (SiSn) alloys are attractive candidate for the next-generation group-IV semiconductors. It is well known that Si and Ge change from indirect to direct transition types with the addition of Sn. Among them, SiSn alloys are expected to be applied for the near-infrared optical devices because it is predicted to become a direct band-gap appropriate for the optical communication with sufficient Sn content [1]. Although the Sn composition that changes from indirect to direct band-gap has been reported using several calculations, there are few reports of experimental results. In addition, the optical properties of single crystalline Si1-x Sn x have not been sufficiently clarified owing to the difficulty of crystal growth. In this study, we evaluated the optical properties to clarify the band structure of single crystalline Si1-x Sn x . 2. Experimental method Strained 30 nm-thick Si1-x Sn x films were grown on Si substrate by molecular beam epitaxy (MBE), and epitaxial growth was confirmed by X-ray diffraction (XRD) two-dimensional reciprocal space mapping (2DRSM) [2]. The Sn composition was determined by 2DRSM. The Sn fraction x of Si1-x Sn x samples used in this study were 0.005, 0.009, 0.018, 0.022, and 0.06. The spectroscopic ellipsometry measurements were performed with an incident angle of 70°, using a 70 W xenon lamp with a wavelength range of 200 to 1600 nm for the light source. The functions used in the analysis are the Tauc-Lorentz and Lorentz function. 3. Results and Discussion Figure 1(a) and (b) show real part and imaginary part of the complex dielectric functions for Si1-x Sn x (x=0.005 - 0.06) and pure-Si, respectively. From Fig. 1, it can be confirmed that the spectra shift with increasing Sn composition and the peaks appear around 1.8 eV and 2.8 eV for the samples with high Sn composition (2.2% and 6.0%). The peak shift characteristic with the addition of Sn is similar to the results of germanium tin (GeSn) [3]. The complex dielectric functions of the semiconductor include much information about the electronic band structure. In Fig. 1, the sharp peaks that are due to direct band transitions show what is known as critical points (CPs). In the case of Si, E'0 CP (Γ) and E 1 CP (L) energy are around 3.2 eV [4]. Therefore, it can be considered that the peak at 2.8 eV indicates the separation of the E'0 and E 1 CP energies due to the change in the band gap at the Γ point with the addition of Sn. The peak at 1.8 eV is unique to Si1-x Sn x , which is not found in Si. We consider that this peak is due to the split-off valence band caused by the Sn addition. The behavior of the valence band is strongly affected by strain. These results suggest a reduction of the band gap at the Γ point and the formation of an optical transition region due to the Sn addition, and reveal important findings for the application of SiSn for the near-infrared devices. Acknowledgment This work was partly supported by the Japan Society for the Promotion of Science (JSPS) (17J08240, 19K21971, and 21H01366). References [1] M. Kurosawa et al., Appl. Phys. Lett. 106, 171908 (2015). [2] R. Yokogawa et al., ECS Trans. 98. 291 (2020). [3] M. Medikonda et al., J. Vac. Sci. Technol. B 32, 061805 (2014). [4] P. Lautenschlager et al., Phys. Rev. B 36, 9 (1987). Figure 1

1. Background and purpose Currently, it is expected that Ru is going to be introduced in place of Cu, which is the mainstream interconnect material in the very large scale integrated (VLSI) circuit. As high integration has been proceeding, the interconnect is expected not only having excellent electrical characteristics and reliability, but also providing an effective pathway for the removal of the heat generated by device operation [1]. The increasing current density in the narrowing interconnect produces Joule heat. The generated heat is removed by a heat sink attached to the substrate through the interconnect. Therefore, if the thermal resistance between the substrate and the interconnect is high, the heat cannot be removed efficiently and the interconnect temperature rises, lead to a degradation of VLSI reliability [2].In this study, we evaluated chemical bonding states of Ru/TaN/(SiOx/)Si stacking structure to clarify the mechanism contributing thermal resistance for the superior thermal management in VLSI. 2. Experimental method A 5 nm TaN was deposited by reactive sputtering (Deposition conditions: 25°C, 300 W, 1.0×10-5 Pa, Ta target) and subsequently 10 nm Ru was deposited by magnetron sputtering on a Si substrate with approximately 2 nm-thick native oxide. We prepared four samples with Ar:N2 gas ratios of 19:1, 17:3, 16:4, and 15:5, respectively, during the TaN deposition. The total thermal resistance was measured by the Frequency Domain Thermoreflectance (FDTR) method under vacuum condition.We also evaluated the chemical bonding states at the interface by Laboratory Hard X-ray Photoelectron Spectroscopy (Lab. HAXPES). The measurement conditions were kinetic energy of 9.25 keV, energy resolution of 0.5 eV, energy step of 50 meV, beam diameter of 50 μm2, and detectable depth of 50 nm, respectively. Au 4f was used as a reference of the energy calibration during for the analysis. 3. Results and Discussion The thermal resistance measured from the entire Ru/TaN/(SiOx/)Si stacking structure was reported by previous studies [1]. It can be seen that the thermal resistance decreases with the increase of N2 flow rate. In order to discuss the mechanism for determination of the thermal conductance, we performed nondestructive measurements by Lab. HAXPES.Figures 1 and 2 show the photoelectron spectra obtained from Ru 3d (Ar:N2=19:1, 15:5) and Si 1s (Ar:N2=19:1, 15:5), respectively. From Fig. 1, no significant change is observed in Ru 3d spectra even within the N2 gas flow rate changes. Therefore, the cause of the change in thermal resistance with the N2 gas flow rate change cannot be found at the Ru/TaN interface. On the other hand, from the Si 1s spectrum shown in Fig. 2, it can be confirmed that the peak intensity due to Si-N or Si-O-N bonds increases with increasing N2 gas flow rate. Therefore, it can be considered that the Si-N or Si-O-N bonds formed at the TaN/(SiOx/)Si interface as the N2 gas flow rate increased promoted heat transport and contributed to the reduction of thermal resistance.In summary, the impor...

1. Background and purpose Since carbon (C) has a smaller lattice constant than silicon (Si), Si doped with C approximately 1% or less (Si:C: carbon-doped silicon) also has a smaller lattice constant than Si. By using Si:C as the source / drain material of the n-type metal-oxide-semiconductor field-effect transistor, the tensile strain can be applied to the channel region to improve electron mobility [1]. However, the nano-fabrication of Si:C causes strain relaxation, which hinders the improvement of electron mobility. We have reported the in-plane biaxial strain relaxation for Si:C nanowires evaluated with Raman spectroscopy, assuming that there is no out-of-plane strain relaxation [2]. In this study, we evaluated anisotropic strain relaxation, including not only in-plane but also out-of-plane direction, for Si:C nanowire by reciprocal lattice space mapping (RSM) measurement using synchrotron radiation X-rays. 2. Experimental method Si:C thin films were grown on (001) Si substrate by molecular beam epitaxy, and then nano-fabricated into nanowire by electron beam lithography and dry etching. The film thicknesses were 43, 50, and 37 nm, with C concentration of 0.60%, 0.83%, and 1.1%, respectively, confirmed by cross-sectional transmission electron microscope and secondary ion mass spectrometry measurements. In the Si:C nanowire, the length direction was parallel to the [110] direction and the width direction was parallel to the [-110] direction, respectively. The nanowire width (W) was varied as 1000, 500, 200, and 100 nm, while the nanowire length (L) was fixed at 10 μm. In order to obtain sufficient signals for RSM, 30,000 identical nanowires in the case of W = 1,000 and 500 nm, and 50,000 identical nanowires in the case of W = 200 and 100 nm were fabricated in approximately 1.5 mm × 1.5 mm areas, respectively. The X-ray energy was set to 10 keV. The RSMs of Si:C nanowires were obtained around 337 diffractions for C concentrations of 0.60% and 0.83% samples (Si:C0.83%), and around 115 diffraction for C concentration of 1.1% sample, respectively. 3. Results and Discussion Figures 1 (a) and (b) show the RSMs of the unprocessed and the nanowires with W of 500 nm of Si:C0.83% films, respectively. In Fig. 1 (a) and (b), the profiles near qx = 4.91 Å- 1, qz = 8.11 Å- 1 correspond to the hem of the 337 diffraction profiles for the Si substrate, and the peaks around qz = 8.165 Å- 1 are the diffraction profiles for the Si:C0.83% film or the Si:C0.83% nanowires, respectively. Here, qx and qz are components of scattering vector in the in-plane and out-of-plane directions, respectively. Figure 1 (a) shows that the profiles for Si substrate and Si:C0.83% film were obtained on the same qx, which indicates that the in-plane lattice constant of Si:C0.83% film before nano-fabrication was equal to that of Si substrate, and the tensile strain has been applied to Si:C0.83% film. Figure 1 (b) shows that qx increases and qz decreases for the Si:C0.83% nanowires with W of 500 nm as compared with the Si:C0.83% film, which indicates that...

We evaluated the optical properties and the band structure of strained single crystalline Si1-x Sn x using spectroscopic ellipsometry. The results suggest a reduction of the band gap at the Γ point and the formation of an optical transition by Van-Hove singularity with higher Sn fraction. In addition, since the reduction of the band gap with increasing Sn fraction at the Γ point is larger than at other points, it is expected the indirect transition type Si1-x Sn x used in this study may eventually change to the direct transition type. Although higher Sn fraction are required to achieve direct transition type Si1-x Sn x , the band structure of strained single crystal Si1-x Sn x was experimentally clarified.