Isolating analog circuits from digital interferences

Recent advances in analog integrated circuit techniques and continued improvement in VLSI technology are enabling more and more mixed-signal analog digital integrated circuits to be implemented on the same chip. One of the important topics in mixed signal circuit design is the coupling of digital switching noise onto sensitive nodes in the analog circuits. In this paper we consider isolating analog circuits using a post processing method. Using an inexpensive mask, a trench can be etched using potassium hydroxide, KOH, from the backside of the wafer all the way to the under-surface of the field oxide. To evaluate the practical aspects of the basic idea, we have implemented some test structures consisting of analog sensing transistors surrounded by conventional guard ring, and large digital inverters a few hundred microns away.