Process-Induced Charging에 의한 Silicon Pit 발생에 관한 연구

Abstract

It has been well known that silicon pits are frequently observed on the Si wafer surface during the wet etching of oxide layer damaged by ion implantation process for the formation of Si channel. Since the silicon pit results in the yield loss and gate leakage current, it of great importance to prevent the silicon pit formation in the wafer process. However, the mechanism for the silicon pit formation is not clearly understood. In this work, various experiments were carried out for a deeper understanding of the mechanism and kinetics of the silicon pit, and the key processes and parameters which play an important role in the silicon pit formation are suggested. The present work is performed in order to establish the new mechanism of silicon pit formation by introducing the process-induced charging model and to propose the silicon pit-free process flow. For this purpose, the formation of silicon pit is analyzed as functions of the sequence of process flow, the oxide thickness, the doping concentration and energy of ion implantation and the wet etching process. From the variation of the silicon pit formation on each process parameter, it is found that the number and shape of silicon pit are significantly affected by the oxide thickness, the energy and dose amount of the p-type implantation, the chemical concentration and methods of oxide etching, and the implantation or ashing conducted just before the lithography. Based on the test for the parameters above, the suitable surface oxide thickness, the P-MOS silicon substrate implanted by B+ or BF2+, the lithography process, and the wet HF treatment are suggested as the essential factors in the silicon pit formation. PDM (Plasma Damage Monitor) was also employed to analyze the oxide surface charging generated by each process. From the good agreement between PDM map obtained after the lithography and the silicon pit map, it was suggested that the silicon pit was formed only on the locally charged area. This indicates that the process-induced local charging by implantation, ashing and lithography produces a weak point on the oxide surface, which was transformed into the silicon pit during the wet HF treatment. The present experimental results permit us to draw the following mechanism of the silicon pit formation: 1. The electrical field is generated within the oxide layer by the process-induced charging, which brings about the oxide weak point. 2. The excess holes are stored in the p-type silicon surface beneath the oxide weak point by the generated electrical field. 3. During the wet HF treatment, the chemical is penetrated through the path formed in the oxide weak point. The silicon pit is formed by the anodic dissolution reaction between the p-type silicon with the excess holes and the HF chemical. 4. The silicon pit formation is terminated when the oxide charging are eliminated by oxide etching. On the basis of the suggested mechanism, the following methods to prevent the silicon pit can be proposed: the minimization of the surface oxide charging, the elimination of the charging before the wet HF treatment, and the removal of the damaged oxide layer by using the dry etching without the electrolyte.