“Process development of novel deposition technology of functional thin films”

To bring out more functionality and higher performance of a system within a given size, its structure necessarily becomes more complex. In terms of materials, for instance, electronic and photonic devices increasingly adopt three-dimensional structured Si substrates to enhance device performance., Structural applications rely on composites with coatings or fillings applied to 3D networks of the reinforcing phase with carefully tailored interfaces to enhance the overall strength-to-weight ratio. Thus, it is evident that processing technology for synthesizing complex three-dimensional structures is crucial to realize the new functionality and high performance designed by the material scientist.

We address such technological need through developing film deposition processes enabling coating and filling of functional materials onto complex three-dimensional surfaces. Our research activity is divided into two main works; (i) Process design, in which we analyze the deposition mechanism underlying the process and thereby bring out the optimum properties of the materials, (ii) Process intensification, in which we incorporate new reaction schemes into conventional deposition processes to enhance the process performance (e.g. growth rate, film thickness uniformity, throughput, and crystallinity).

1. Copper (Cu) interconnects in ultra large integration (ULSI)
Development of ultra-narrow Cu interconnects in ULSI with high reliability and multi-functionality against continued miniaturization
Process : CVD; Chemical Vapor Deposition,ALD; Atomic Layer Deposition,PVD; Physical Vapor Deposition
2. Ceramics matrix composite (CMC) for next generation aerospace materials
Development of CMC made of silicon carbide (SiC) with thermal stability and mechanical strength to replace current metal alloy materials (e.g. engines)
Process : CVD; Chemical Vapor Deposition
3. Ceramic hard coating on cemented carbide for machining tools
Development of hard coating materials for machining tools to enable high-speed machining with long lifetimes
Process : CVD; Chemical Vapor Deposition

Theme1 : Copper (Cu) interconnect in ultra large integration (ULSI)

Dramatic improvement of computer performance is attained by the continued miniaturization and high-density integration of the ultra large scale integration (ULSI) technology. We work on the development of multi-layer Cu interconnects to move forward the further miniaturization and densification.

The current Cu interconnects formed in the three-dimensional architecture includes a 2-nm-thick ultra-thin barrier layer to prevent the diffusion of constituent Cu atoms into the surrounding insulation layers. Cu line width and spacing of next generation interconnects is expected to be less than 20nm, which brings out serious problems including increased resistance and electro-migration. Moreover, the thinner barrier layers will reduce its barrier property, increasing the copper diffusion.

We have studied these issues, and proposed a new barrier layer using Co(W) alloy. In addition, we have found that addition of a small fraction of Mn to the Cu interconnects can strengthen the barrier property.

We are therefore developing the deposition process of the Cu(Mn)/Co(W) system focusing on the diffusion behavior of Cu atoms in the barrier, the adhesion strength of the barrier, and the impact of film structure on the resistance. For this research, an emerging characterization technology called atom probe tomography (APT) is actively used to enable visualization of the constituent atoms in the material, thereby providing invaluable data for diffusion studies.

Fabricated Layered Structure
3D mapping observed by APT. Mn is segregated at the grain boundary of Cu.
ULSI section
Cross-section of ULSI Cu interconnect. Replacement of current Ta/TaN stacked layer to Co(W) single layer can lower the effective resistivity of the interconnect while ensuring barrier and adhesion properties.
Trench section
Cross-section of trenches (Aspect ratio of 10) after Co(W) deposition by hot-wire ALD. Co(W) film is evenly formed from top to bottom.
Layered Structure section diagram and picture
Cross-sectional schematic and TEM image of Co(W) film deposited by hot-wire ALD. W is segregated at the grain boundary of Cu to prevent the Cu diffusion.

Theme2 : Ceramics matrix composite (CMC) for next-generation aerospace materials

Recent aviation industry has succeeded in significant weight reductions thereby elongating flight distance, enlarging load capacity, and improving fuel efficiency. These are attributed to the replacement of materials for the wings and cabins from metal alloys to carbon fiber reinforced plastic (CFRP).

Change of engine materials from metal alloys to non-metallic materials with greater temperature tolerance will further contribute to improved fuel efficiency via weight reduction and higher operating temperature. For this, ceramic matrix composite, in particular, SiC/SiC composite of SiC fibers and SiC matrix, is attracting attention.

Chemical vapor infiltration (CVI) is one of the leading candidates for synthesis, in which gaseous precursors containing carbon and silicon atoms are supplied to the SiC bundles to allow chemical deposition reaction on the surfaces of individual SiC fibers. Ultimately, the spaces between fibers are expected to be filled perfectly. Though this process has great potential for this purpose, careful process development is mandatory, especially in the design of the optimal reactor and reaction to control the mass transfer and chemical reaction both in the gas phase and on the growth surface.

We tackle this issue by combining a theoretical approach using quantum chemical calculations and an experimental approach exploiting our unique analysis methods that we have developed. We aim to obtain a solid, quantitative understanding of the chemical reactions and mass transport involved to allow nano-level control of the structure and thereby ultimately tailor the thermal and mechanical properties at the bulk level.

Film forming apparatus
Schematic of reaction chamber. We can see the interior of the chamber because the chamber is heated by the gold image furnace.
SiC impregnation diagram
Schematic of penetration of SiC into fibrous bundles by CVI. The precursors are transported into the spacing between the fibers and react on their surfaces to effectively thicken the fibers.
SiC Fiber section
Perspective of SiC fibers and its cross-section. The fibers are linked by the deposited SiC films.
SiC-CVI Reaction mechanism
Reaction mechanism occurring during SiC-CVI. The precursors are converted to the various intermediate species in the gas phase followed by their reactions on the surface.

Theme3 : Ceramic hard coating on cemented carbide for machining tools

In machining metallic materials such as with a lathe, cemented ceramic insert made of tungsten carbide (WC) with a hard coating is used to increase its hardness and durability, while minimizing its brittleness.

Starting in the late 1960s, the coated material wasinitially made of titanium carbide (TiC). Although the film quality was not necessarily good compared with the current films due to the -immature deposition technology at the time, nevertheless the coating improved the processing speed by 50% and the lifetime by twofold. Currently, alumina (Al2O3) is commonly used, and it is known that nanometer-scale structural control including the crystallinity and crystal size impacts its characteristics.

CVD is one of the leading deposition technology in this field due to uniform film formation being necessary onto the topographic structure with high throughput.

We work on high quality film formation based on the systematic understanding of deposition mechanism underlying the CVD process.

metal surface cutting work
Machining of metallic cylinder surface. Ceramic insert equipped at the tip of the cutting tool scrapes the metal.
Multi-scale analysis conceptual diagram
Scheme of multi-scale analysis of CVD. This is a method to analyze the film thickness distribution in the tubular reactor from the multiple substrates installed in the longitudinal direction, which allows us to know the reaction mechanism.