“Underground capacitors”-Very efficient decoupling for high performance UHF signal processing ICs

In a modern IC-process, small-value high-quality capacitors can be designed for placement under metal lines using only existing layout layers and no additional processing. These capacitors may be added to each noise generating block to improve decoupling without using additional silicon area or additional processing. We call this trick "Underground capacitors" but it works under any wide metal line. Two types of circuits were fabricated using a 0.8 μm BiCMOS process with three levels of metallization, characterized and compared in the study: a 32/33 prescaler with internal decoupling capacitors and a 64/65 prescaler with no internal decoupling. The bipolar part of the BiCMOS process was used for the circuit design. The dual-modulus prescaler was chosen as a test vehicle because of its importance for frequency synthesis systems