The application of SiO2 in VLSI technology is mainly A. to use it as a 'Mask'. B. to provide electrical isolation. C. to provide surface activation. D. to provide electrical conduction. E. as a etching material. Choose the most appropriate answer from the options given below :
A and B only
Why SiO2 matters. Silicon owes much of its dominance to the fact that its native oxide, SiO2, is easy to grow (simply heat the wafer in oxygen or steam), is chemically stable, adheres perfectly to the silicon beneath, and is an outstanding insulator with a breakdown field of about 107 V/cm. Germanium has no comparable oxide, which is one reason it lost out to silicon.
A — "to use it as a mask." TRUE. The common dopants — boron, phosphorus, arsenic — diffuse orders of magnitude more slowly in SiO2 than in silicon, and a thick oxide also stops implanted ions. So an oxide layer patterned by photolithography exposes windows through which dopants enter only where wanted. This planar process, in which the oxide defines the geometry of every junction, is the foundation of IC manufacture.
B — "to provide electrical isolation." TRUE. SiO2 serves as field oxide (LOCOS/STI) isolating neighbouring devices, as the gate dielectric separating the gate from the channel in a MOSFET, and as the inter-metal dielectric separating stacked interconnect layers.
C — "to provide surface activation." FALSE. Oxide does the opposite of activating the surface: it passivates it, tying up dangling bonds, reducing surface states and protecting the junctions from moisture and contamination. "Activation" refers to the annealing step that moves implanted dopants onto substitutional lattice sites — nothing to do with SiO2.
D — "to provide electrical conduction." FALSE. SiO2 is a wide-bandgap insulator (Eg ≈ 9 eV); conduction is exactly what it is used to prevent. Conduction paths are made by metals and by doped polysilicon.
E — "as an etching material." FALSE. Oxide is a material that gets etched (by HF wet etch or fluorine-based plasma), not an etchant. Its ability to be etched selectively supports the masking role but is not itself an application.
Hence, the correct answer is A and B only.
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