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Vertical Resistance Test Structure

IP.com Disclosure Number: IPCOM000125911D
Original Publication Date: 2005-Jun-22
Included in the Prior Art Database: 2005-Jun-22
Document File: 3 page(s) / 163K

Publishing Venue

IBM

Abstract

Disclosed are test structures and a methodology designed to isolate and extract the vertical resistance from the metal silicide down to either N-well or P-well. Using these structures and methodology, vertical resistance can now be extracted independently from the other resistances found in any device that includes underlying wells. Further, these test structures and methodology can be used to characterize in more detail the well sheet resistivities in various different layouts.

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Vertical Resistance Test Structure

Vertical resistance is a key parameter for passive semiconductor devices such as varactors, and diodes. The vertical resistance contributes to the total series resistance found in a particular device. In the case of varactors, the series resistance effects the overall Q (quality factor) of the device, Q being an important parameter for the quality and usefulness of the device. In the case of band-gap reference circuit diodes, the series resistance effects the roll off behavior of the IV curves, thereby limiting the useful operating range of the diode. So, the characterization of any vertical resistances found in a device is a key component of the complete modelling of the device's behavior. In fact, in many cases the vertical resistance is the dominate portion of the overall series resistance.

In order to extract the vertical resistance, test structures are created by making two sets of contacts of length (L) and width (W) to the particular well (N or P) being characterized. The contacts are placed with a separation distance of D. These three parameters (L,W, and D) are varied in different layouts to provide a set of data from which the vertical resistance, as well as the sheet resistance of the well, is extracted.

The figures below show the layouts for two test structures. Figure 1(2) shows a typical layout for the structure used to characterize the vertical resistance to an N-well(P-well). Two well contacts with width (W) and length (L) are used to contact to the wells. These two contacts are separated by a distance (D). Note that the N-well is surrounded by a substrate contact to bias the substrate to help ensure that there is no leakage out of the N-well. The N-well has a total width of W and a total length of D+2*L (as seen visually in the picture). The P-well has an N-well ring around it to provide a boundary for the P-well. This keeps the current paths in the P-well narrowly defined. D in this case is held constant, at a minimum distance, to help minimize the current leaking down under the N-well boundary and away from this device. Using these two methods, the leakage on these structures is minimized.

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Figure 1: Vertical Resi...