0.35μm UMOSFET Backside Process Analysis And Improvement

Backside metal process is the most important step of UMOSFET process. In the traditional backside metal process, IC early failure always happens because of the unstabele adhesive ability between backside metal and wafer backside, which is caused by some reasons such as over backside grinding, surface roughness mismatch, monolayer metal evaporation, etc. In order to improve the yield and the reliability, the new wafer process baseline of backside roughness and metal evaporation is set up and all kinds of defect solving methods is offered in the paper.Firstly, for the backside wafer roughness, the two different etching solutions are researched: the acid solution and the KOH one. In the acid solution, the most appropriate volume ratio of 33% HF and 70% HNO3 is 7:1 and add water by 10%, which offers the best roughness wafer surface and easy control etch rate for the mass production. In the KOH one, after 15 minutes reaction in the 30% KOH solution, the wafer gets the reasonable surface condition, which is suitable for the next metal evaporation step. it’s proved that the acid solution is better than the KOH one in the mass production based on lots of experiment raw data.Secondly, the backside metal evaporation process parameters are optimized. The multilayer metal (Ti, Ni, and Ag) system is selected. The wafer has better backside metal adhesive ability after preheated to 200℃than the one without that. The metal surface particle problem is basically solved by adding the metal pre-molten time (1Min as Ti, 7Min as Ni, and 3Min as Ag).At last, the causes of some defects such as wafer bending, frontside discolor and backside metal peelings are analyzed, and the corresponding solving methods are summarized. A complete series flow of backside metal is finalized in this paper. As the flow studied above has been implemented in mass production, 0.35μm UMOS product gets stable reliability, and the yield is greatly improved as well.