Modeling of Boron Diffusion in Polysilicon-on-Silicon Layers
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MODELING OF BORON DIFFUSION IN POLYSILICON-ON-SILICON LAYERS Akif Sultan, Shubneesh Batra, Melvyn Lobo, Keunhyung Park, and Sanjay Banerjee Microelectronics Research Center, University of Texas, Austin, TX 78712 ABSTRACT In the present study we have modeled the diffusion of boron in single crystal silicon from an ion-implanted polysilicon film deposited on a single crystal silicon substrate. Modeling has been done for both BF 2 and boron implants in the polysilicon layer. A new phenomenological model for B diffusivity has been implemented in the PEPPER simulation program using an effective concentration-dependent diffusivity approach. The effective diffusivities of boron in single crystal silicon have been extracted using Boltzmann-Matano analysis. The modeling has been implemented for a wide range of furnace anneal conditions (800 0 C to 950 0 C, from 30 min. to 6 hours), and implant conditions (BF 2 doses varied from 5X101 5 to 2X1016 cm- 2 at 70 keV, boron dose of 5X10 15 cm- 2 at 20 keV). INTRODUCTION The study and modeling of diffusion of dopants in silicon from an ion-implanted polysilicon layer deposited on a silicon substrate is important for obtaining ultra-shallow junctions in polyemitter bipolar transistors and other applications [1]. The dopants are implanted into the polysilicon layer and subsequently diffused into the silicon substrate to achieve ultra-shallow junctions. The diffusion mechanisms in polysilicon-on-single-crystalsilicon structures are not well understood and process simulators such as SUPREM-II [2,3] and PEPPER [4] do not model the impurity profiles accurately for such structures. We have investigated the diffusion of boron in single crystal silicon following a BF 2 or boron implant in a polysilicon layer deposited on a single crystal silicon substrate. Boltzmann-Matano analysis [5] has been used to extract the effective concentrationdependent diffusivities of boron in the single crystal silicon substrate. The PEPPER simulation program provides a convenient platform for testing and implementation of new diffusion models. A new analytical model for concentration-dependent B diffusivities has been incorporated into the PEPPER process simulation program to accurately model the boron diffusion profiles in the single crystal silicon substrate for a polysilicon-on-singlecrystal-silicon structure. The simulations using the new modeling approach have been compared with the default model in PEPPER, as well as with SUPREM-Ill. EXPERIMENTAL PROCEDURE Polysilicon films (300nm) were deposited on RCA-cleaned, lightly-doped (10 ohmcm) silicon wafers (100, n-type) by Low Pressure Chemical Vapor Deposition (LPCVD) at 620 0 C. The polysilicon-on-silicon structure was then ion-implanted with either BF 2 (70 keV, with doses of 5X10 15 , 1X10 16 , and 2X101 6 cm- 2 ) or B (20 keV, with a dose of 5X10 15 cm- 2 ). A 40 nm LPCVD cap oxide was then deposited to prevent B outdiffusion during annealing. The samples were then annealed at 950 0 C (I hr), 900 0 C (6hr,3hr,1 hr), 850 0 C (6hr,3hr,l hr,30min.) and 800
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