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The people now believe that the scaling limit to Moore’s law is lithography because of the need for shorter wavelengths of light to pattern the smaller feature sizes. Therefore, materials are now a key constraint in silicon technology. For example, the current density of conductors in ICs can be increased by using copper instead of aluminium. The RC time delays can be minimized using materials with lower dielectric constants instead of SiO2. Leakage in MOS structures can be minimized using materials, with high dielectric constants, between the gate and the silicon channel in FET in stead of SiO2.
In integrated circuits (ICs), conductive lines provide electrical interconnection among different parts of the ICs, devices, and the outside. The main applications of metallization are classified by gate, contact, and interconnection. Polysilicon and silicides are commonly applied as gates and interconnects in MOS devices. Aluminum, copper, tungsten, silver, titanium, platinum, gold, and palladium are used as these types of contacts, connections, and/or interconnection to external components. Copper has many good properties such as low resistance and high electromigration endurance.
At metal pitches of about 32 and 21 nm, the effective resistivity of damascene Cu wires increases drastically [2] because of:
i) both surface and grain boundary scattering, [3]
ii) Cu diffusion barriers, that cannot be scaled down and the barrier depositions 4 contribute to the low reliability performance of damascene Cu interconnects,
iii) low-k damages induced by both plasma processing. [4]
Table 2034a. Properties of some metals commonly used in ICs.
Property\metal |
Cu |
Al |
W |
Au |
Ag |
Resistivity (10-6 Ω•cm) |
1.7 - 2.0 |
2.66 |
5.65 |
2.35 |
1.59 |
Melting point (°C) |
1085 |
660 |
3387 |
1064 |
962 |
Thermal conductivity (Wcm-1) |
3.98 |
2.358 |
1.74 |
3.15 |
4.25 |
Thermal stress per degree for
films on silicon (107 dyn cm-2 °C-1) |
2.5 |
2.1 |
0.8 |
1.2 |
1.9 |
Coefficient of thermal expansion (CTE)
(10-6 °C-1) |
17 |
23.5 |
4.5 |
14.2 |
19.1 |
Youngs modulus (x10-11 dyn cm-2) |
12.98 |
7.06 |
41.1 |
7.85 |
8.27 |
Corrosion in air |
Poor |
Good |
Good |
Excellent |
Poor |
Specific heat capacity(Jkg-1K-1) |
38 |
917 |
138 |
132 |
234 |
Adhesion to SiO2 |
Poor |
Good |
Poor |
Poor |
Poor |
Table 2034b. Metallization selections in ICs.
Application |
Selection |
Gates, interconnection, and
contacts |
Polysilicon, refractory metal silicides (e.g. MoSix, TaSix, WSix, and TiSix), nitrides,aluminum, copper, and/or refractory metals. |
Diffusion barrier layer |
Ti, TiN, Ta, TaN, Ti-W alloy, and/or silicides |
Top level |
Aluminum, and/or copper |
Metallization
on silicon |
Silicides, tungsten, aluminum, and/or copper |
Figure 2034a. Dependence of recombination lifetime on-metal (Ru, Pt, Cu) concentation in ICs. [1]
| Figure 2034b. Metallic contamination levels on a wafer backside before/after cleaning using the CAN-nitric acid solution then HF-based acid cleaning. [1] |
[1] H. Aoki, K. Watanabe, T. Iizuka,N. Ishikawa and K. Mori, Ruthenium Film Etching and Cleaning Process Using Cerium Ammonium Nitrate (CAN)-Nitric Acid, Extended Abstracts of the 2001 International Conference on Solid State Devices and Materials, Tokyo, 2001, pp. 20-21.
[2] Y. K. Siew et al., Proceedings of the IEEE International Interconnect
Technology Conference (2014), p. 311. 2.
[3] P. Kapur, J. P. McVittie, and K. C. Saraswat, IEEE Trans. Electron Devices 49, 590 (2002).
[4] S. Uchida, S. Takashima, M. Hori, M. Fukasawa, K. Ohshima, K. Nagahata, and T. Tatsumi, J. Appl. Phys. 103, 073303 (2008).
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