The desired properties of metallization for integrated circuits are A. low resistivity. B. low conductivity. C. easy to etch for pattern generation. D. tough to etch for pattern generation. E. no contamination with device or wafers. Choose the correct answer from the options given below :
A, C and E only
Context. Metallization is the step that deposits and patterns the metal layers forming the on-chip interconnects and the contacts to the devices. The requirements follow directly from what those interconnects have to do — carry signals with minimum loss, be shaped into fine lines, and not poison the silicon underneath.
A — low resistivity. DESIRED. Interconnect resistance causes IR drop on power rails and RC delay on signal lines:
\(R=\rho\dfrac{L}{A}, \qquad t_{delay}\propto RC\)
Lower ρ means faster, lower-loss wiring — which is exactly why the industry moved from aluminium (ρ ≈ 2.7 µΩ·cm) to copper (ρ ≈ 1.7 µΩ·cm).
B — low conductivity. NOT DESIRED. Conductivity is the reciprocal of resistivity, \(\sigma = 1/\rho\), so demanding low resistivity and low conductivity is self-contradictory. What is wanted is high conductivity.
C — easy to etch for pattern generation. DESIRED. The metal film must be selectively removed by photolithography and etching to leave fine, accurately defined tracks. A metal that etches cleanly and anisotropically gives sharp line edges and good dimensional control.
D — tough to etch. NOT DESIRED. This is simply the negation of C: a hard-to-etch metal causes residues, ragged edges and shorts between adjacent lines. (Copper is a real example — it does not form volatile etch products, which is why it is patterned by the damascene process, depositing into pre-etched trenches instead of etching the metal itself.)
E — no contamination of device or wafer. DESIRED. Metal atoms that diffuse into silicon act as deep-level generation–recombination centres, raising junction leakage and destroying minority-carrier lifetime; gold and copper are notorious for this. That is why barrier/adhesion layers such as Ti/TiN or Ta/TaN are inserted between copper and silicon.
Other properties worth knowing (not listed here): good adhesion to SiO2, the ability to form a low-resistance ohmic contact to silicon, resistance to electromigration at high current density, and a thermal-expansion coefficient close to that of silicon.
Collecting the desired properties gives A, C and E.
Hence, the correct answer is A, C and E only.
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