Low-temperature-cofired-ceramic (LTCC) circuit technology supports compact, multilayer designs and is now widely used in wireless applications, especially in RF modules and system-in-package (SiP) designs. It has a number of advantages over laminate technologies, although the process is similar to those used for laminated printed-circuit-board (PCB) materials. Typical advantages are lower dielectric loss, higher packaging density, and integrated/embedded passive components (resistors, inductors, and capacitors). There are a wide range of tape materials and processes available for LTCC designs.

A multilayer LTCC structure generally shrinks during the low-temperature cofiring process. However, there are some manufacturers who offer "zero-shrink" materials, where shrinkage is restricted only to the z-dimension. These materials may cost more than standard LTCC tape materials and processes. Shrinkage has added a challenge to achieving high performance with LTCC designs and limited the yield of LTCC components or subsystems. As a result, it may prevent the use of LTCC for applications requiring high performance and high yield. Still, the use of a Design for Manufacturing (DFM) methodology can help to achieve first-pass design success with LTCC, even accepting the shrinkage.

The DFM approach for LTCC involves the development of a design flow to generate broadband models for embedded passive components in LTCC. Those models, which are used for first-pass design success, will be presented along with several passive LTCC circuits developed from the DFM technique. The passive circuits were developed with the help of the Advanced Design System (ADS) and Momentum software tools from Agilent Technologies (www.agilent.com/find/eesof). ADS is a popular electronic-design-automation (EDA) software tool that includes circuit/system simulators and layout tools for RF integrated circuits (RF ICs), monolithic microwave integrated circuits (MMICs), SiPs, modules, and circuits. ADS can also perform statistical design studies, such as Monte Carlo analysis. (Momentum is a three-dimensional (3D) planar electromagnetic (EM) simulation tool that can study current flow and planar field behavior for a wide range of 3D planar high-frequency structures.) Momentum accepts arbitrary design geometries such as multi-layer structures, and then it accurately simulates complex EM effects such as coupling and parasitics. Multilayer LTCC is well suited to simulation with a 3D planar tool like Momentum.

The typical front end found in a wireless handset includes a transmitter section with a directional coupler for power-control measurements . The purpose of the power control is to assure that transmit power is within the regulatory limits for a given handset. Maintaining transmitter power within these limits is essential for spectral mask compliance since the operating range of the unit's RF power amplifier must be maintained in its linear range for amplitude-modulated (AM) signals. The power control loop relies on the directional coupler for sensing incident power; any other-than-specified directivity in the directional coupler may lead to erroneous readings in the measured power. Because the handset power amplifier can generate unwanted levels of harmonic energy, an additional lowpass filter is added to the transmitter architecture to maintain spectral emissions within regulatory limits.

To ensure handset compliance within regulatory limits, a robust design technique is needed for both directional coupler and lowpass filter. The two components will be used as examples of how to apply a DFM methodology in studying the process variations and layout parameters of LTCC and its impact on certain output parameters such as insertion loss. Some variations are to be expected in designing passive circuits with LTCC; typical variations include changes in dielectric constant, substrate thickness, transmission line width, and layer alignment. Such variations are expected to be monitored and controlled during the manufacturing process but, nevertheless, must be accounted for in order to achieve first-pass design success with LTCC.

The flow chart in Fig. 1 illustrates the cross-interaction of these parameters to some of directional coupler's output parameters, insertion loss, directivity, and coupling ratio. In this chart, ε, T, W, and AL represent dielectric constant, substrate thickness, trace width, and alignment, respectively. Also, the "plus" and "minus" signs denote the extreme case of the upper and lower specifications, respectively. Based on the LTCC material supplier's data, variations in dielectric constant are minimal, hence the other three parameters, the substrate thickness, trace width, and alignment should be taken into consideration.

The directional coupler used as an example here has broadside embedded coupled lines. The coupler has four ports: RF input, coupled port, isolated port, and RF output. Figure 2 shows the layout (with port definitions). The performance of the directional coupler was simulated with Momentum; Fig. 3 shows the measured versus simulated results for the coupler's insertion loss and coupling ratio. The simulation data agrees closely with the measured results. To demonstrate this approach, it will also be applied to the design of an example lowpass filter (Fig. 4).

Such variations in process and layout parameters may be unavoidable in the course of the design cycle. Circuit components can even suffer variations in value, typically expressed as component tolerance. Such changes in component values, manufacturing process variations, and related layout parameter variations are usually difficult to fix later in the design cycle. Consequently, taking them into consideration at the early stages of a design will help ensure high-yield first-pass design success.

Among all possible variations in process and layout parameters, some are more critical than others to affecting output parameters. Understanding the sensitivity of output parameters to those critical parameter variations is a simple but effective first step for a DFM methodology. For example, insertion loss may be affected differently by variations in trace width or substrate thickness. In order to achieve less performance variations in a design, it is critical to understand and control the most sensitive parameters first. Sensitivity analysis in simulation software involves taking partial derivatives of the performance response with respect to a design variable of interest. This can help pinpoint variables that contribute disproportionately to performance variances. The ADS software provides sensitivity analysis as a part of its basic statistical package.

The directional coupler's insertion loss, directivity, and coupling ratio vary as a function of three different cases of substrate thickness, trace width, and alignment. The three cases represent the nominal, lower, and upper extreme. For instance, W0 stands for the nominal value of trace width and W0+ for the upper extreme case. Momentum EM simulations were used extensively to collect the variation data for this case study.

Although designers can make some analogical conclusions about the sensitivity from these charts, it is much easier and more useful to use a graphical display of the results, such as in a Pareto chart showing the percentage ratio of certain parameter variations to performance. Figure 5 shows a Pareto chart for the parameters or factors that contribute to the directional coupler's performance variations. The chart shows that variations in substrate thickness are more influential in the insertion loss performance than any other parameters or their combinations. For instance, 60 percent of the variations in performance stem from the effects of substrate thickness variations.