5.5 Summary 97 3-D circuits, such as visualizing circuits on the individual planes, highlighting features for interplane interconnects, and full 3-D design rule checking. The design rules for 3-D circuits have been largely extended to include aligning the interplane interconnects with the backside vias, which are an additional interconnect structure in the MITLL fabrication technology. Although these tools consider important issues in developing a lay- out environment for 3-D circuits, there is significant room for improvement, as existing capabilities are rather rudimentary and limited to a specific 3-D technology and number of planes. In addi- tion, a complete front- and back-end design flow for 3-D circuits does not yet exist. This situation is further complicated by the lack of a standardized fabrication technology for 3-D circuits. The com- plexity of 3-D integration poses significant obstacles in developing an efficient design flow. Finally, managing thermal effects, which are discussed in the next chapter, requires the thermal analysis pro- cess to be an inseparable element of the physical design process of