Simulation Techniques for Improving Fabrication Yield of RF-CMOS ICs

Simulation Techniques for Improving Fabrication Yield of RF-CMOS ICs

Amparo Herrera (University of Cantabria, Spain)
DOI: 10.4018/978-1-4666-0083-6.ch004
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One of the industry sectors with the largest revenue in the telecommunication field is the wireless communications field. Wireless operators compete for being the first to place their products in the market to obtain the highest revenues. Moreover, they try to offer products that fulfill the user demands in terms of price, battery life, and product quality. All these requirements must be also fulfilled by the designer of the MMIC (Microwave Monolithic Integrated Circuits) circuits that will be used in those wireless terminals, achieving a reliable design, with high performance, low cost, and if possible, in one or two foundry iterations so as to bring the product out to the market as soon as possible. Silicon based technologies are the lowest cost. The demand to use them is simply based on that fact, but their usage in these applications is limited by the ease of use for the designer, in particular, by the lack of adequate simulation models. These technologies don’t include some essential components for the design of RF circuits, which leads to measurement results quite different from those simulated. On the other hand, GaAs based technologies, more mature in the RF and microwave field, provide very accurate models, as well as additional tools to verify the design reliability (yield and sensitivity analysis), allowing good results often with only one foundry iteration. The deep study of the problems presented when designing Si-based RF circuits will convince the reader of the need to use special tools as electromagnetic simulation or coo simulation to prevent it. The chapter provides different simulation techniques that help the designer to obtain better designs with a lower cost, as foundry iterations are reduced.
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Transmission Line Theory

In general the books dedicated to electromagnetic signal propagation explain the complete theory of signal transmission in circuits and equipment (Wadell, 1991) . All of this theory is applicable to circuits and/or distributed circuits with propagation in mind. On the other hands, circuit theory with localized components (L, C, R) are based on network analysis. For deciding when to use one or the other theory, the operation frequency and the substrate used for transmission must be taken into account.

Next, we will briefly outline one of the theories used to explain electromagnetic signal propagation.

Signal transmission in RF & Microwave circuits or systems takes place through structures that are capable of guiding waves. These structures are basically of two types: transmission lines that guide waves TEM (transverse electromagnetic) between two (or more) conductors, which can be defined and measured in each plane perpendicular to the direction of propagation by the voltage difference between the two conductors. In each transverse plane, the total current flowing through the conductors can also be defined and measured. Otherwise, and more generally, the waveguides are structures that guide electromagnetic signals which are not TEM waves. These structures may have one or more dielectric, in the first case, homogeneous structures and the second one inhomogeneous.

For the study of propagation on transmission lines there are two possible approaches: a rigorous one calculating the electromagnetic fields in the structure and thereafter the voltage and current in each transverse plane. The other one is based on the study of the equivalent circuit of a differential segment of transmission line (Figure 1) and then solving the differential equations that appear to apply circuital concepts. Here, the latter method will be used to obtain the main magnitudes of a transmission line.

Figure 1.

Transmission line

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