Focused ion beam milling is an important tool for LSI modification and failure analysis. For both of these applications, the depth control in milling is a major requirement. The FIB machine we used was equipped with an ion impact photoemission detector for use with both Si and Al. We studied the photoemission change mechanism during LSI milling by altering the acquisition area of the photoemission, and by chemical analysis on the side walls of the milled holes. Si emission varies with the surface area of SiO2. Al emission originates in the edge lines of Al and SiO2at the bottom of the hole, and from the Al redeposited on SiO2of the side walls. On the edge lines and the side walls, ion mixing of SiO2and Al enhances the Al photoemission. We demonstrated that an almost 100% modifying yield in both Al wire cutting and contact hole milling is possible by observing the variance of both emissions.