**5.2 Novel conductivity on interconnected 100** f g **vertical side-surfaces**

Three-dimensional integrated circuits, which contain multiple layers of active devices, have the potential to dramatically enhance chip performance, functionality, and device packing density. Recent 3D structured field-effect transistors (FETs) have surfaces with different orientations; for instance, a fin-type tri-gate structure has one top-surface and two side-surfaces [35, 36].

> was uniformly grown on the Si facet-surface with a smooth interface without any depressions or protrusions. No breaks or discontinuity of the Au wires was observed in SEM images, even at the top facet-edges. **Figure 15(b)** shows current-voltage properties at 100 K for 3D angularly interconnected Au wires of 2 *μ*m width (circles) and 5 *μ*m width (squares) in the parallel (red) and perpendicular (blue) configurations (insets). The length of the Au wires was 100 *μ*m. Both perpendicular wires have an approximately one-order larger resistance (smaller current) than the parallel wires of the same width. For the other wire widths, the resistance in the perpendicular configuration was also larger than that in the parallel configuration. The simplified resistance ratio, defined as the inverse of the current ratio at a fixed voltage, was 3–10 for the measured wire widths. Note that the dimensions (cross section and length) and crystallinity of the Au film were almost the same in the parallel and perpendicular configurations. Thus, the significant anisotropic resistance in these configurations is ascribed to the properties of the geometric shapes of the Au wires, that is, 3D angular interconnects. Indeed, the perpendicular wires have more facet-edges than the parallel wires; the perpendicular channel crossed over 11 top and 23 bottom facet-edges, while the parallel channel crossed

> *(a) Typical SEM image for a facet sample on which a 10-nm-thick Au layer was deposited. (b) Current vs. voltage curves of Au wires with the channel area (W*�*L) of 2*�*100 and 5*�*<sup>100</sup>* <sup>μ</sup>*m<sup>2</sup> at 100 K in the parallel (red) and perpendicular (blue) configurations (insets). The upper left inset shows a schematic of the Au film on the Si*f g 111 *facet sample. The Au film is assumed to consist of bottom-surface (B), facet-surface (F), top facetedge (TE), and bottom facet-edge (BE) regions with resistivity of RB, RF, RTE, and RBE, respectively.*

*Creation and Evaluation of Atomically Ordered Side- and Facet-Surface Structures of Three…*

*DOI: http://dx.doi.org/10.5772/intechopen.92860*

**Figure 15.**

**105**

over one top and two bottom facet-edges for a wire width of 5 *μ*m.

The electric connection between metal wires on these surfaces—that is, the wiring interconnects at sharp edges of top- and side-surfaces—is one of the issues in the development of 3D devices. Although the conductivity in metal wires on isolated 2D planar or side-surfaces has been well discussed, there are no reports on the metal conductivity of interconnections between 3D surfaces with different orientations, probably owing to the difficulty in measuring the intrinsic conductivity in 3D angular interconnects, which is mainly caused by diffuse scattering on rough surfaces [37–48]. One of the most outstanding factors contributing to the conductivity in the 3D angular interconnects is the facet-edge, which is a boundary of two surfaces with different crystalline orientations. To extract an intrinsic conductive property for metal wires interconnected at facet-edges, atomically flat surfaces in 3D structured substrates are required for the evaluation of the 3D interconnect resistivity by eliminating extra factors, such as roughness. To elucidate the impact of the 3D geometric effect on the conductive property, angularly interconnected Au wires with two configurations, crossing over and parallel to the facet-edges, were produced on atomically flat facet 111 f g surfaces (**Figure 13**).

**Figure 15(a)** shows a typical cross-sectional SEM image of a 111 f g facet sample on which a 10-nm-thick Au layer was deposited. The SEM image indicates that Au

*Creation and Evaluation of Atomically Ordered Side- and Facet-Surface Structures of Three… DOI: http://dx.doi.org/10.5772/intechopen.92860*

#### **Figure 15.**

left-side surface (**Figure 14(c)**) showed that fcc-Ag epitaxially grew with an atomi-

*reciprocal lattice rods in (a), and spots indicated by orange arrows correspond to the 2* � *2 superstructure in*

*RHEED patterns obtained from (a) Si*ð Þ 111 *-Ag left-side surface and (b) Si*ð Þ 111 *-Fe right-side surface annealed at 773 K in UHV. Spots in the L0 Laue zone indicated by orange arrows correspond to the* ffiffiffi

Our results clearly show that a coherently grown (ultra) thin film was realized on the vertical side-surface with the growth alternating between the out-of-plane and inplane directions. The siliciding reaction can be controlled on the side-surfaces. Therefore, highly developed thin-film formation techniques are applicable for the vertical side-surface of 3D patterned substrates, and the material stacking direction can be perfectly switched between the out-of-plane and in-plane directions.

Three-dimensional integrated circuits, which contain multiple layers of active devices, have the potential to dramatically enhance chip performance, functionality, and device packing density. Recent 3D structured field-effect transistors (FETs) have surfaces with different orientations; for instance, a fin-type tri-gate structure

The electric connection between metal wires on these surfaces—that is, the wiring interconnects at sharp edges of top- and side-surfaces—is one of the issues in the development of 3D devices. Although the conductivity in metal wires on isolated 2D planar or side-surfaces has been well discussed, there are no reports on the metal conductivity of interconnections between 3D surfaces with different orientations, probably owing to the difficulty in measuring the intrinsic conductivity in 3D angular interconnects, which is mainly caused by diffuse scattering on rough surfaces [37–48]. One of the most outstanding factors contributing to the conductivity in the 3D angular interconnects is the facet-edge, which is a boundary of two surfaces with different crystalline orientations. To extract an intrinsic conductive property for metal wires interconnected at facet-edges, atomically flat surfaces in 3D structured substrates are required for the evaluation of the 3D interconnect resistivity by eliminating extra factors, such as roughness. To elucidate the impact of the 3D geometric effect on the conductive property, angularly interconnected Au wires with two configurations, crossing over and parallel to the facet-edges, were

**Figure 15(a)** shows a typical cross-sectional SEM image of a 111 f g facet sample on which a 10-nm-thick Au layer was deposited. The SEM image indicates that Au

� �, without any visible defects or dislocations.

<sup>3</sup> <sup>p</sup> � ffiffiffi 3 p

� �∥ Si 111

*(b). Cross-sectional TEM images of (c) Ag-deposited and (d) Fe-deposited side-surfaces at RT.*

**5.2 Novel conductivity on interconnected 100** f g **vertical side-surfaces**

has one top-surface and two side-surfaces [35, 36].

produced on atomically flat facet 111 f g surfaces (**Figure 13**).

cally matched interface, Ag 111

*21st Century Surface Science - a Handbook*

**Figure 14.**

**104**

*(a) Typical SEM image for a facet sample on which a 10-nm-thick Au layer was deposited. (b) Current vs. voltage curves of Au wires with the channel area (W*�*L) of 2*�*100 and 5*�*<sup>100</sup>* <sup>μ</sup>*m<sup>2</sup> at 100 K in the parallel (red) and perpendicular (blue) configurations (insets). The upper left inset shows a schematic of the Au film on the Si*f g 111 *facet sample. The Au film is assumed to consist of bottom-surface (B), facet-surface (F), top facetedge (TE), and bottom facet-edge (BE) regions with resistivity of RB, RF, RTE, and RBE, respectively.*

was uniformly grown on the Si facet-surface with a smooth interface without any depressions or protrusions. No breaks or discontinuity of the Au wires was observed in SEM images, even at the top facet-edges. **Figure 15(b)** shows current-voltage properties at 100 K for 3D angularly interconnected Au wires of 2 *μ*m width (circles) and 5 *μ*m width (squares) in the parallel (red) and perpendicular (blue) configurations (insets). The length of the Au wires was 100 *μ*m. Both perpendicular wires have an approximately one-order larger resistance (smaller current) than the parallel wires of the same width. For the other wire widths, the resistance in the perpendicular configuration was also larger than that in the parallel configuration.

The simplified resistance ratio, defined as the inverse of the current ratio at a fixed voltage, was 3–10 for the measured wire widths. Note that the dimensions (cross section and length) and crystallinity of the Au film were almost the same in the parallel and perpendicular configurations. Thus, the significant anisotropic resistance in these configurations is ascribed to the properties of the geometric shapes of the Au wires, that is, 3D angular interconnects. Indeed, the perpendicular wires have more facet-edges than the parallel wires; the perpendicular channel crossed over 11 top and 23 bottom facet-edges, while the parallel channel crossed over one top and two bottom facet-edges for a wire width of 5 *μ*m.

Au wires on the 3D facet structure can be separated into four regions: flat bottom-surface, flat facet-surface, convex top facet-edge, and concave bottom facet-edge regions with resistances of *RB*, *RF*, *RTE*, and *RBE*, respectively, as shown in the inset of **Figure 15(b)**. The geometries of crystal grain boundaries in the polycrystalline Au wire may increase the electrical resistance [49]. The aggregation of crystal grain boundaries in the interconnect region, leading to the reduction in effective cross-sectional area, was attributed to the large *α* (≈30–180), i.e., *RB* = *RF*�*αRTE*,�*αRBE*.

Seamless Technology Transfer Program through target-driven R&D (A-STEP) from the Japan Science and Technology Agency (JST) (No. JPMJTM19CM), Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research B (Nos. 18H01871 and 20H02483), the Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University, Nos. F-16-OS-0012 and F-16-OS-0016), and the Research Program of "Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials" in "Network Joint Research Center for

*Creation and Evaluation of Atomically Ordered Side- and Facet-Surface Structures of Three…*

Materials and Devices" (No. 20203017).

*DOI: http://dx.doi.org/10.5772/intechopen.92860*

**Author details**

Japan

**107**

Azusa N. Hattori<sup>1</sup> and Ken Hattori<sup>2</sup>

Technology, Ikoma, Nara, Japan

provided the original work is properly cited.

\*Address all correspondence to: khattori@ms.naist.jp

\*

1 Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

2 Graduate School of Science and Technology, Nara Institute of Science and

We found that the conductivity passing through the interconnects is sensitive to the alignment of the facet-edges in the electric path, and the series configuration showed a 3–10-fold larger resistance than the parallel configuration, which originated from the increased resistivity across the facet-edges by a factor of �30–180, as calculated from the circuit model. This work provides a fundamental understanding of the impact of the 3D angular interconnects of a metal wire on electric transport and guidelines for the comprehensive investigation of the intrinsic interconnect transport properties on 3D structures, which is expected to produce critical benefits in the semiconductor industry.

## **6. Conclusion**

This chapter demonstrated the creation and evaluation of atomically ordered side-surfaces and inclined facet-surfaces in nanofabricated 3D Si architectures toward the realization of sequentially 3D integrated and stacked devices for More Moore [3]. Atomically flat and well-ordered 3D structured surfaces play an important role in creating high-performance films on arbitrarily oriented 3D surfaces, similar to films grown on ordinary planar substrates. We emphasized that electron diffraction techniques such as RHEED and LEED are convenient for evaluating atomically ordered 3D surfaces, while conventional SEM has no atomic resolution. The points for recognizing 3D surfaces in diffraction patterns were explained along the basic concept of diffraction in reciprocal space; the intersection of reciprocal lattice rods with an Ewald sphere led to diffraction spots on arcs, and the spots elongate in the surface normal direction.

By diffraction, we evaluated several systems with side- or facet-surfaces of Si 111 f g or 100 f g on Si 110 ð Þ or Si 001 ð Þ substrates, which were fabricated by lithography dry and wet etching processes, followed by annealing in vacuum. Metal deposition on the well-ordered 3D surfaces (followed by annealing) also produced well-ordered films of nanometer thickness fabricating with arbitrary orientations. This is one of the fundamental techniques for defect-free material construction on 3D architectures. The electrical conductivity of metal wires on atomically flat 3D facet-surfaces crossing facet-edges was also measured toward the design of nanoscale steric wiring. We consider that these techniques for creating and evaluating 3D surfaces are promising for the realization of future 3D architecture devices.

#### **Acknowledgements**

The authors appreciate Dr. Shohei Takemoto, Dr. Haoyu Yang, Prof. Hiroshi Daimon, and Prof. Hidekazu Tanaka for their contribution to these studies. We also thank Ms. Saeko Tonda, Ms. Michiko Sakuma, Mr. Shoichi Sakakihara, and Mr. Takeshi Ishibashi for their helpful support in the fabrication of 3D Si samples and Ms. Liliany N. Pamasi and Mr. Ken Maetani for their support in the diffraction measurement of Si samples. This work was partially supported by Adaptable and

*Creation and Evaluation of Atomically Ordered Side- and Facet-Surface Structures of Three… DOI: http://dx.doi.org/10.5772/intechopen.92860*

Seamless Technology Transfer Program through target-driven R&D (A-STEP) from the Japan Science and Technology Agency (JST) (No. JPMJTM19CM), Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research B (Nos. 18H01871 and 20H02483), the Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University, Nos. F-16-OS-0012 and F-16-OS-0016), and the Research Program of "Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials" in "Network Joint Research Center for Materials and Devices" (No. 20203017).
