Ion Implantation Damage Annealing


Ion Implantation is the process of depositing chemical dopants into a substrate by directly bombarding the substrate with high-energy ions of the chemical being deposited.  This process involves the collision of the highly-energetic bombarding ions with the atoms of the substrate, and is therefore destructive to the material structure of the substrate being bombarded.



Silicon damage caused by ion implantation include: 1)  the formation of crystal defects such as Frenkel defects, vacancies, di-vacancies, higher-order vacancies, and interstitials; 2) the creation of local zones of amorphous material within its supposedly crystalline structure; and 3) formation of continuous amorphous layers as the localized amorphous regions grow and overlap. Damage types 1 and 2 are categorized together as 'primary crystalline damage'. Restoring the ion-implanted substrate to its pre-implant condition requires the substrate to be subjected to a reparative thermal process known as annealing.


Ion Implantation Damage Annealing has five (5) major components: 1)  electrical activation of the implanted impurities; 2) primary crystalline damage annealing; 3)  annealing of continuous amorphous layers; 4) dynamic annealing; and 5) diffusion of implanted impurities. It is conducted in a neutral environment, such as in Ar or N2 atmosphere.


Electrical activation of the implanted impurities refers to the process of increasing the electrical activity of newly implanted impurity atoms during annealing, which usually don't occupy substitutional sites after being implanted. The temperature range up to 500 deg C remove trapping defects, releasing carriers to the valence or conduction bands in the process. Electrical activity decreases again at 500-600 deg C, because of the formation of dislocations.  Beyond 600 deg C, however, electrical activation increases until it peaks at around 800-1000 deg C.


Primary crystalline damage annealing basically consists of: 1) recombination of vacancies and self-interstitials in the low temperature range (up to 500 deg C); 2) formation of dislocations at 500-600 deg C which can capture impurity atoms; and 3)  dissolution of these dislocations at 900-1000 deg C.  


Annealing of the continuous amorphous layers that extend to the surface has been shown to occur by solid-phase epitaxy between 500-600 deg C. Under this phenomenon, the crystalline substrate beneath the amorphous layers initiates the recrystallization of the amorphous layers, with the regrowth proceeding towards the substrate surface. Factors affecting the recrystallization rate include crystal orientation and the implanted impurities.  Amorphous layers that don't extend to the surface anneals differently, with the solid-phase epitaxy occurring at both amorphous-single crystal interfaces and the regrowth interfaces meeting below the surface.



Dynamic annealing effects simply refers to the healing of implant damage even as the implantation process is still occurring.  This takes place because the heat applied to the wafer during implantation makes the point defects more mobile.


Diffusion of implanted impurities pertains to the mass transport of implanted species across a concentration gradient within an implanted layer during the annealing process. The presence of implant damage makes this diffusion process more complex than what would occur in an undamaged single-crystal substrate. Diffusion of implanted impurities during annealing can degrade devices that have shallow junctions or narrow base and emitter regions if the thermal processing is not done rapidly enough. 


See Also:  Ion ImplantationDiffusion Gettering;  Crystal Defects





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