Gettering Treatments

Unwanted crystalline defects and impurities can be introduced during silicon growth or subsequent wafer fabrication process. To remove metallic impurities from devices, a variety of processing techniques termed "gettering" treatments are available. Gettering is a general term taken to mean a process that removes harmful impurities or defects from the regions in a wafer where devices are fabricated. Pregettering means to pre-treat silicon wafers prior to IC processing. It provides a wafer with sinks that can absorb impurities as they are introduced during device processing.

Gettering processes are divided into two categories:

· Extrinsic gettering involves the use of external means to create the damage or stress in a silicon lattice that leads to the creation of the extended defects or chemically reactive sites at which the mobile impurities are captured. Normally such capturing sites are generated on the backside of the wafer.[3]

· Intrinsic gettering involves the localization of impurities at extended defects which exist within the bulk material of the silicon wafer, and whose origin is due to an "intrinsic" property of the starting material, such as its oxygen content acquired during CZ crystal growth.[2]

Extrinsic Gettering Procedures

· Mechanical damage by abrasion, grooving, or sand blasting have all been used to create stress fields at the backside of wafers. During subsequent annealing steps, dislocations, which relieve these stresses, are generated. The dislocations in turn serve as gettering sites.

· A layer of polysilicon deposited on the wafer backside (1.2-1.5 microns thick) has grain boundaries, and high degree of lattice disorder, which acts as sinks for mobile impurities.

Limitation

The primary limitation of extrinsic gettering procedures is their inherent lack of stability, esp. at high temperatures up to 1250ºC. Such instability manifests itself as the dissolution of gettered metal back into the wafer, and the annealing out of dislocations as the wafer undergoes a number of high temperature steps. Upon being annealed out, dislocations can no longer trap newly introduced impurities, nor recapture the impurities released back into the wafer from gettering sites.

Intrinsic Gettering Procedure

· Intrinsic gettering is based on the principle that under proper conditions, supersaturated oxygen in silicon wafers will precipitate out of solution, and form clusters within the wafer during thermal processing.

· Punching out dislocation loops can relieve the stresses that result as these clusters grow into larger precipitates.

· These dislocations become sites at which unwanted impurities can be trapped and localized. In an effectively designed intrinsic gettering process, these precipitates are only allowed to form in the bulk regions of the wafer.

· They are prevented from forming in the active device regions by reducing the oxygen concentration to levels below the threshold required for precipitation (denuded zone formation). In this manner, unwanted impurities are localized (gettered) only in regions not containing active devices.