Study of unique pseudo buried layer in 0.18um SiGe BiCMOS process
SiGe is an important semiconductor material after Si and GaAs. SiGe HBT has obvious advantages over Si HBT in terms of frequency characteristics, DC characteristics and noise characteristics, especially compatible with mature Si process. In our low cost and high performance 0.18um SiGe BiCMOS with collector epitaxial layer free, a unique pseudo buried layer is developed to replace the conventional n-type buried layer under collector epitaxial layer. This pseudo buried layer is important to keep the high performance of SiGe HBT due to the skipping of collector epitaxial layer and conventional n-type buried layer. The lithography is the most crucial step in pseudo buried layer process. In this paper, we report the study result of lithography process of the pseudo buried layer. The lithography conditions suitable for fabrication are achieved. A negative tone photoresist is selected with 7770A thickness and 80 ∼ 100nm resolution. The mask bias is 100nm per side and STI side wall angle 82 degrees. The process window (CD + OVL) control is within 50nm and the recommended minimum window CD of the device is 300nm.