Experimental study of the boron redistribution in bilayer films silicon-based by SIMS technique
The present work focuses on the study two sets of films bilayers obtained by Low Pressure Chemical Vapor Deposition (LPCVD), for use as material to MOS gate structures (transistors, chemical sensor ISFET, etc.). The first series of films are composed by two layers, silicon amorphous un-doped layer (poly1) and polysilicon boron doped in situ (poly2). The second series are constituted by boron doped polysilicon (polySi) and nitrogen doped polysilicon (NIDOS). These films (poly1/poly2 and polySi/NIDOS) are annealed in tem the same conditions of deposit and annealing. The boron concentration is monitored by secondary ion mass spectrometry (SIMS). The superposition between the SIMS profiles of poly1/poly2 and polySi/NIDOS films have shown that low thermal annealing budget at 600°C/2h, ensures long boron redistribution to the interface poly2/SiO2. At the contrary, a high thermal budget the second layer (poly2) was recristallyzed and reached to the doped oxide. For polySi/NIDOS films, SIMS profiles confirmed the presence of low nitrogen (X = 1%) which can effectively suppress the boron penetration at the interface NIDOS/SiO2.
Boron diffusion and activation in polysilicon multilayer films for P+ MOS structure: Characterization and modeling
This work deals with in situ boron diffusion and activation in multilayer films: polysilicon (Poly1)/amorphous silicon (Poly2). These films are deposited by LPCVD technique. However, several heat treatments were carried in order to determine the optimal annealing conditions to suppress boron penetration from the gate to the substrate through the gate oxide in MOS structure. The boron concentration is monitored by secondary ion mass spectrometry (SIMS). To investigate SIMS profiles we proposed a model of boron diffusion into these multilayer structures. It is important to note that the parameter values of the studied films such as the diffusion coefficient, the activation percentage of boron as well as the acceleration rate of boron diffusion are deduced from adjustment of simulated profiles with experimental profiles. From these results, we inferred that the boron is electrically active and its distribution does not reach the oxide layer and consequently, the Poly2 may reduce the boron diffusion in optimal annealing conditions.
Physical Models of Boron Diffusion in Ultrathin Gate Oxides
Based on a network defect model for the diffusion of B in SiO 2, we propose that B diffuses via a peroxy linkage defect whose concentration in the oxide changes under different processing conditions. We show that as the gate oxide is scaled below 80 A in thickness, additional chemical processes act to increase B diffusivity and decrease its activation energy, both as a function of the distance from the Si/SiO 2 interface. For a 15 A oxide, the B diffusivity at 900°C would increase by a factor of 24 relative to diffusion in a 100 A oxide. The role of nitridation of SiO 2 to create a barrier to B diffusion is modeled by assuming the N atoms compete with B for occupation of diffusion-defect sites. The model predicts that nitridation is ineffective in stopping B penetration when BF 2 implants are used to dope the polysilicon gate, and similarly for B implants when the gate oxide thickness decreases below approximately 30 to 40 A.