GRAIN BOUNDARY DIFFUSION IN POLYCRYSTALLINE SILICON FILMS ON SiO2

The enhanced grain boundary diffusion in cw laser processed LPCVD polycrystalline silicon films on insulating SiC>2 has been investigated and the grain boundary diffusion coefficient D' for Phosphorus has been determined. Mesa diodes were fabricated in the large grain poly-Si films by standard photolithographic and etching methods and subsequent ion implantation. The electron beam induced current mode (EBIC) of the 3EI4 allowed direct measurement of the dopant diffusion length along the grain boundaries ^intersecting the p-n junction. We have found a t' dependence of the penetration depth and Arrhenius behavior for the temperature dependence of the diffusion coefficient. The numerical evaluation of the experimental data is based on 'Whipple's exact solution of the grain boundary diffusion problem. 1.Introduction.Grain boundaries play an essential role in polycrystalline materials and because of the technological impact grain boundary diffusion has been an active area of research [1,2 ]. Current interest in polycrystalline silicon films arises from its potential use as electronic material for the fabrication of thin film transistors. Recent experiments have demonstrated large improvements in MOS device performance in polycrystalline Si whose grain size has been increased by laser processing [3,4,5,6 ]. In general, beam crystalArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982148 C1-364 JOURNAL DE PHYSIQUE Fig-la): SEM micrograph showing Fig.lb): corresponding EBIC image typical surface morphology of of the same sample laser processed large grain area polycrystalline Si on insulator lization enlarges the grain size to dimensions comparable to those of the thin film devices and hereby minimizes the detrimental effects of grain boundaries such as reduced lifetime and mobilities[ 7 1 . Very small devices, however, exhibit large leakage currents as grain boundaries form channels for rapid diffusion of source and drain dopants which act as shunt paths [ 6 ] . This paper describes the determination of the grain boundary diffusion coefficient of phosphorus in poly-Si across a p-n junction by the electron beam induced current mode (EBIC) of the SEM. 2. Experimental Procedure.Thin polycrystalline Si films were deposited by low-pressure-chemical-vapor-deposition (LPCVD) to 0.5um thickness on wafers covered with lum of steam oxide. These wafers were sybsequently recrystallized by irradiation with a 10 W beam of a cw Ar laser. In the irradiated area the film is completely melted so that heterogeneous nucleation in the wake of the molten zone results in the formation of large 5lOum Si grains. The EBIC investigation required a p-n junction in the sample and for this purpos~~the wafers were uniformly implanted with a Boron ion dose of 2.5*10 /cm2 at 163 keV. The wafers were oxidized to form a lOOnm thick SiO layer on the Si film, which was then patterned by standard lithograpzic and etching methods to create an array of 120um diamete hol5s. Through these holes P ions were implanted with a dose of 7 *10f5/cm at 35 heV. In this way many lateral p-n junction diodes were fabricated on a single 3" wafer.Figure la k x shows the qeneral surface morpholoqy of the laser processed large grain poi$-~i film and lb) the corresponding EBIC image. Prior to microscopical observation the oxide film was chemically stripped from the surface and the qrain boundaries I Y have been delineated by a 5 sec Schimmel etch [ 8 ] . The diffusion experiments were carried out in a furnace under flowing Nitrogen atmosphere. The individual samples were subjected to different tem1 Fig.2: Schematic of isoconcentration contours caused by rapid diffusion along grain boundary slab of thickness