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Electronic Design

Modeling Holds Key For Converter Advances

ELECTRONIC DESIGN (ED): What is your focus at Linear?

Gross: Function is key for our customers. We provide a set of very high-quality intellectual property functions for our customers. By focusing on the performance, they can create the specific circuits that fit the design requirements of the most critical systems. Manufacturing and the ability to perform specific functions are important to us and our customers.

ED: How do you go about improving performance?

Gross: Transistor count is not the key motive for changes in designs. The technologies and architectures continue to advance in all directions. Converters are moving toward more bits and higher speeds. In these areas, the drivers for change are parallel to the digital world and continue to move toward the smaller feature sizes that enable higher levels of integration.

ED: How do you manage the changes in technologies?

Gross: One key is the ability to get accurate device modeling for the designs. The smaller devices don't model well with the simple models. They used to employ a level 2 MOS model with about 10 parameters. Now they use a BSIM 3 or 4 with over 80 parameters. One problem is that the short channel effects don't correspond as well in the older models. Out of necessity, the foundries have good modeling people on staff. They understand the tools and methodologies needed to extract models that have high correlation to the physical devices in the ICs.

In addition to the manufacturing models, we also need specialized models for our special devices, and the special functions in the chips. These are unique capabilities that enable breakthrough architectures.

One challenge is to get some correlation between the smaller line sizes and the disturbing trend toward lower levels of robustness. For example, electrostatic discharge (ESD) protection is down in the smaller processes. Analog structures need to have ESD protection and low load and parasitic characteristics. We cannot use a digital ESD structure with leakage current of 10 nA in a 25-pA input device. New processes require an empirical process to get the appropriate level of ESD protection, because the protection circuits need to be bulk and not surface conduction devices.

How do your designs affect the manufacturing?

Gross: Testing is another challenge. We have to test all parts to guarantee all parameters. This challenges the testing capabilities and test equipment, because the tester has to be much better than the state-of-the art ICs. In addition, the test engineers have to resolve diametrically opposed characteristics: shortest test time possible and as accurate and complete as possible. A low-noise amplifier is best checked for noise by averaging over a fairly long time, milliseconds or seconds. But the tester needs to be finished with its testing in under 10 ms.

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