22 September 1995 Prevention of auto-doping-induced threshold voltage shifts
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The maintenance of consistent transistor threshold voltages is critical to the optimal performance of modern microelectronic devices. Such devices frequently have two levels of polysilicon. The doping processes associated with each polysilicon layer results in a series of highly doped structures on the surface of the wafer. Because they are exposed during subsequent thermal processes, these doped structures can also result in unintended auto-doping of associated regions on the wafer's surface. Such auto-doping can cause threshold voltage variation and is highly undesirable. This paper describes a problem encountered with a 0.8 micron double-level metal, double-level polysilicon device which exhibited erratic lot-to-lot threshold voltage variation. The device featured transistors on both levels of polysilicon, but only the second- level devices displayed the threshold voltage shift. It was postulated the voltage shift resulted from an auto-doping phenomenon caused by Phosphorous out-gassing from the doped first level of polysilicon as well as from the doped backside of the wafer. A series of experiments were conducted to confirm this hypothesis. In addition, the development of a process modification involving a barrier oxide which eliminated the auto-doping is presented. This process modification is robust, reliable, and has resulted in uniform lot-to-lot threshold voltages and improved yield.
© (1995) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Tony T. Phan, Tony T. Phan, Jerry T. Healey, Jerry T. Healey, William R. Kent, William R. Kent, } "Prevention of auto-doping-induced threshold voltage shifts", Proc. SPIE 2635, Microelectronic Manufacturing Yield, Reliability, and Failure Analysis, (22 September 1995); doi: 10.1117/12.221444; https://doi.org/10.1117/12.221444

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