Stress and Dopant Activation in Solid Phase Crystalized Si Films
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ABSTRACT Defect creation mechanisms during solid phase crystallization (SPC) of Si thin films were investigated with PECVD amorphous precursor samples produced with various deposition temperatures and thicknesses. These precursor films were implanted with dopant and then crystallized to obtain both SPC and dopant activation. The doping efficiency was found to decrease with the tensile stress level as measured by Raman shift. The stress shows a decrease as the precursor deposition temperature and thickness are lowered. Furthermore, a lower level of stress is induced by rapid thermal annealing when the annealing temperature is high enough to soften the glass substrate on which the films were deposited. We show that by control of stress during the SPC step, intragrain defect density can be lowered and electronic quality of the resulting polycrystalline Si films can be improved. Based on these observations, we propose the following tentative model to explain the defect creation: during SPC, tensile stress evolution is considered to result from the volumetric contraction of Si film when it transforms from the amorphous to crystalline phase. This contraction is retarded by the substrate, which imposes a tensile stress on the film. A high level of stress leads to formation of structural defects inside the grains of the resulting polycrystalline material. These defects trap carriers or complex with the dopant reducing doping efficiency. INTRODUCTION Earlier work in solid phase crystallization (SPC) of amorphous Si thin films focused on grain size enlargement in the resulting polycrystalline silicon (poly-Si) to improve the electronic quality. However, SPC poly-Si films, despite their larger grain sizes of 1-10 jtm cannot compete with the superior mobilities of the eximer laser crystallized films, which have much finer grains of typicaly -0.1 gtm in size. This may possibly be attributed to the dendritic nature of the grain growth in SPC which yields faultier, and also larger surface area grain boundaries. On the other hand inferior mobilities of SPC films may also result from the higher intragrain defect density. In fact, TEM examinations have revealed a high density of threading dislocations, stacking faults and large twins inside the grains of SPC poly-Si films. On the contrary, laser crystalized grains have generally been found to accommodate no defects or only twins in some regions [ 1-3]. In a previous report we addressed the doping inefficiency in SPC films if the film thickness exceeds several thousand A. The inefficiency was found to increase in this thickness regime while grain size remained the same [4]. This observation implies that the intragrain defect creation mechanism may be related to stress evolving during the SPC process. Accordingly, the work here involves the investigation of a correlation between stress and doping efficiency. We also explore ways to minimize the stress by controlling the precursor or anealing parameters on the basis of a model we propose.
225 Mat. Res. Soc. Symp. Proc. Vol. 558 @2000
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