Isolation Techniques for Wafer Fabrication

Wafer Fab Isolation Techniques

The individual components that make up the circuit on a monolithic die need to have electrical isolation from each other in order to function. The most common techniques used for achieving component isolation during wafer fabrication include the following: 1) by employing reverse-biased p-n junctions; 2) through what is known as mesa isolation; 3) by wafer bonding to an insulating substrate; 4) by oxide isolation; 5) by trenching; and 6) through a combination of any of these processes.

A reverse-biased p-n junction has an extremely low leakage current, which is why its use as an isolation technique during wafer fabrication is very common. By doping two adjacent regions with opposite types of conductivity and providing them with adequate reverse biasing, they become effectively isolated from each other. Under such a situation, the coupling between the regions is only capacitive in nature, which becomes an issue only at high frequencies.

Another somewhat obvious technique for achieving component isolation is known as mesa isolation. This involves the building of the components on an active film which was grown on an insulating (or semi-insulating) film, and then etching moats around the components. This results in the components becoming individual 'islands', or 'mesas', hence the name 'mesa isolation' given to this isolation technique. Circuits fabricated on silicon on insulators, as well as those made on epitaxial GaAs over semi-insulating (SI) GaAs substrate, are examples of applications of mesa isolation.

Wafer bonding to an insulative substrate may be considered as a variant of mesa isolation. This isolation technique takes advantage of the fact that any two flat, smooth, clean, and hydrophilic surfaces can be bonded at ambient temperature without the use of external forces. Wafer bonding can be applied to widely dissimilar materials. Once the moats are etched around the 'mesas', isolation is provided by the insulating substrate.

As its name implies, oxide isolation techniques consist of a series of material deposition and removal steps that leads to the formation of active single-crystal tubs that are completely surrounded by an oxide layer. Such oxide layers, once formed, provide near-perfect isolation between the active tubs.

Trenching is a process wherein anisotropic wet etching or reactive ion etching is employed to dig a trench around the active region. The dug trench is then filled up with isolating material. Planarization is done after filling up the trenches.

Primary Reference: Sorab K. Ghandhi, VLSI Fabrication Principles, Wiley-Interscience

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