Gettering

Gettering is defined as the process of removing device-degrading impurities from the active circuit regions of the wafer. Gettering, which can be performed during crystal growth or in subsequent wafer fabrication steps, is an important ingredient for enhancing the yield of VLSI manufacturing.

The general mechanism by which gettering removes impurities from device regions may be described by the following steps: 1) the impurities to be gettered are released into solid solution from whatever precipitate they're in; 2) they undergo diffusion through the silicon; 3) they are trapped by defects such as dislocations or precipitates in an area away from device regions.

There are two general classifications of gettering, namely, extrinsic, and intrinsic. Extrinsic gettering refers to gettering that employs external means to create the damage or stress in the silicon lattice in such a way that extended defects needed for trapping impurities are formed. These chemically reactive trapping sites are usually located at the wafer backside.

Several methods have been used to accomplish external gettering. For instance, the introduction of mechanical damage by abrasion, grooving, or sandblasting can produce stresses at the backside of a wafer, which when annealed create dislocations that tend to relieve these stresses. These locations can then serve as gettering sites. The main drawback of this method, of course, is its tendency to initiate and propagate wafer backside microcracks that may compromise the mechanical strength of the wafer.

Diffusing phosphorus into the wafer backside is another technique used for external gettering. P diffusion into silicon result in phosphorus vacancies or dislocations that serve as trapping sites for impurity atoms, such as gold. Another effect of P diffusion is the creation of Si-P precipitates, which have been shown to be capable of removing Ni impurities through interactions between Si self-interstitials and Ni atoms, nucleating NiSi₂ particles in the process.

Introduction of damage by laser is another external gettering method. Scanning a laser beam across the wafer surface induce damage that is very similar to mechanical damage, with the exception that the laser damage is 'cleaner.' Laser subjects the irradiated areas to thermal shock, forming dislocation nests that serve as gettering sites.

Ion bombardment to produce wafer backside damage is another method for external gettering, this time using high-energy ions to induce the necessary stress within the lattices of the wafer backside. Deposition of a polysilicon layer on the wafer backside has also been used for external gettering. Polysilicon layers introduce grain boundaries and lattice disorder that can act as traps for mobile impurities.

Intrinsic gettering refers to gettering that involves impurity trapping sites created by precipitating supersaturated oxygen out of the silicon wafer. The precipitation of supersaturated oxygen creates clusters that continuously grow, introducing stress to the wafer as this happens.

Eventually these stresses reach the point where they need to be relieved. Dislocation loops or stacking faults are thus formed to provide the necessary stress relief. These dislocations and faults subsequently serve as trapping sites for impurities.

A basic requirement of intrinsic gettering is, of course, starting wafers that have sufficient, but not excessive oxygen levels (15-19 ppma).

The advantages of intrinsic gettering over extrinsic gettering are: 1) it does not require subjecting the wafer to any treatment except for heating; 2) its volume of impurity sink is significantly larger than that of external gettering on the wafer backside; 3) its gettering regions are much closer to the device regions.

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