With technology scaling down to sub 0.16um and metalization exceeding 7 levels, the development of reproducible backside silicon sample preparation techniques becomes increasingly important to accurately localize defects. Bulk silicon thinning is a critical step in the backside sample preparation process. This paper will discuss two different ways for silicon thinning: reactive ion etching (RIE) alone, and RIE in conjunction with mechanical milling. In addition, the characterization and optimization of the RIE process for backside silicon thinning will be discussed in this paper. We have found mechanical milling works well for many package types; however, we have had difficulty reproducibly thinning certain package types such as very small die or packages where the wire bonds are in the plane of the silicon die and are in very close proximity to the edge of the die. In these cases, we have found that reactive ion etching (RIE) can be used successfully. We have also found that for package types where mechanical milling works, the combination of mechanical milling and reactive ion etching process is a useful technique for accurately controlling the final thickness of the silicon. This technique combines the speed of mechanically milling and the advantage of RIE process to accurately control the etch rate and etch process in the final stages of thinning the silicon die.