Gettering

Gettering is a set of semiconductor processing techniques aimed at removing impurities, particularly metallic contaminants, from the active regions of a device by creating regions or sites that attract and immobilize these impurities. Metal contaminants degrade minority-carrier lifetimes and device performance, so gettering helps improve yield and reliability in silicon devices such as diodes, transistors, and solar cells.

Intrinsic gettering refers to creating gettering sites within the silicon bulk without introducing foreign elements. This is commonly achieved by inducing a high density of defects or oxygen precipitates during high-temperature heat treatments, especially in CZ-grown wafers that contain interstitial oxygen. During subsequent processing, metals diffuse to these internal defects and are trapped, leaving the device region relatively clean.

Extrinsic gettering involves introducing foreign elements or layers that act as sinks for impurities. Phosphorus diffusion gettering (PDG) is widely used: a phosphorus-doped layer or PSG is formed on a back surface and diffused into the bulk to create a gettering layer rich in phosphorus and oxide precipitates. Aluminum or other getters can also be used. After drive-in, impurities migrate toward these gettering centers and away from the active region.

Gettering can also utilize surface gettering, where a high-temperature oxide layer or metallurgical treatment on the wafer surface serves as a sink for contaminants, protecting the device region during later steps.

While effective, gettering adds processing steps and must be controlled to avoid degrading the crystalline quality or leaving residues. It remains a standard tool in silicon fabrication for improving carrier lifetime and device performance in both integrated circuits and photovoltaic cells.