Characterization of noise isolation structures

SNA's physics based substrate model enables circuit designers to characterize noise isolation structures.  SNA simulates simultaneous propagation of noise through the substrate and metal interconnect. This is a unique capability in the industry.  Using EditEM, Magwel's 3D editor/ viewer, designers are able to analyze the impact of sizing, positioning and grounding of noise isolation structures.  As an example, the characterization of a deep n-well isolation structure is discussed here, more details can be found in an application note. Please contact us to receive this application note.

The noise isolation structure

The deep n-well layer that is available in the latest triple-well CMOS processes is used to shield devices and circuits from noisy substrates. Simulation shows that this structure turns out to be effective for lower frequencies up to 2-3 Ghz. The figure below shows the location of the deep nwell in a noise isolation set up. Noise is injected in the pwell contact. The noise magnitude picked up at the nwell-ring is used as a measure to determine the isolation characteristics of the structure.

Actual doping profiles at different locations in the structure are used during simulations to obtain accurate results. Experiments have demonstrated that doping profiles have a big impact on the results. When approximate block dopings are used, simulation errors of several dB's are observed.

Results with SNA using an accurate model of the substrate

SNA models drift and diffusion currents in the substrate as well as ohmic resistance in the metal. Note that because the simulator is first-principle based, calibration of the simulator is more straightforward than with empirical modelers.

Various experiments have demonstrated that accurate characterization of isolation structures in only possible using a physics based substrate model.