Copper in ICs
- Practical Electron Microscopy and Database -
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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.

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

 

 

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