Effects of substrate resistances on LNA performance and a bondpadstructure for reducing the effects in a silicon bipolar technology

摘要

The effects of substrate resistances on the performance of 5.8-GHznlow-noise amplifiers (LNAs) have been evaluated through a combination ofnexperimental and simulation studies. The substrate resistive network fornthe LNA has been constructed by fabricating and measuring a testnstructure. The substrate resistances can be significantly affected bynthe die area and thickness, which raises a serious concern for on-waferntesting and optimization of circuits using the test results. Thensubstrate resistances reduce the simulated gain by more than 10 dB andnincrease the noise figure by 2.7 dB. The simulation study has shown thatnthe dominant substrate resistances are those associated with thenbondpads. To reduce the effects of the substrate resistances, anground-shielded bondpad structure, which consists of a Metal 2 pad andnan n+ plug grounded shield separated by a composite oxidenlayer, has been developed. It reduces the resistance to ground to almostnzero by conducting the signal away from the substrate to ground throughnthe low-resistivity n+ plug layer. The pad structure innaddition improves the interpad isolation by as much as 35 dB. However,nto harness this isolation improvement, the inductance between the IC andnPC board ground should be made small by using a low ground inductancenpackage. Using this ground-shielded bondpad, the measured gain and noisenfigure of a 4.5-GHz tuned amplifier were improved by 10 and 2 dB,nrespectively, over the same circuit implemented using the conventionalnbondpad