Multilayer Organic Technology for RF/Wireless Components in Multiband Applications
Today, low-temperature co-fired ceramic (LTCC), multilayer ceramic (MLC) and ceramic mono-block technologies are the dominant choices for the implementation of surface mount components for RF passive filters, diplexers, baluns and front end modules. LTCC is the most popular ceramic technology since it uses miniature lumped components, which can be optimized for operation over a wide band of frequencies, whereas mono-block and MLC components use different materials for different frequencies and limit the integration of devices for multiband applications. LTCC, with its ability to integrate in excess of 20 layers, has become a platform for the integration of modules in multiband applications that combine several lumped element filters, baluns and diplexers for cellular and WLAN applications.
It is typical for such modules, as well as the components, to consist of 15 or more metal layers with microvias connecting layers. The need for many layers typically translates into additional design time, higher tooling costs and inadequate model to hardware correlations. Moreover, LTCC has lower performance due to process tolerances (15 percent component tolerance) and higher dielectric losses (Dissipation Factor, Df =0.005-0.007 at 1 GHz) compared to MLC and their mono-block counterparts (2 to 5 percent component tolerance and Df = 0.0005).
With the advent of a new class of thin, low loss, organic dielectrics, large area fabrication techniques and unique lumped element design topologies, high Q, low loss RF components have been realized. These new materials can be made as thin as 5um, with a dielectric constant (Dk) of 7.6 and a Df of 0.002 at 10 GHz. These materials are stable over frequency and have very little moisture uptake, typically < 0.04 percent. In the past, organics were typically shunned for such applications due to their variability with changes in temperature and humidity (typical FR4/5 materials). However, this is no longer the case, as these materials can attain Moisture Sensitivity Levels to MSL 1 at 260oC.
An MLO component consists of one or more RF dielectric layers embedded between layers of other laminates to provide routing, shielding and bonding pads for SMT placement. The MLO technology can also be used as an RF or mixed signal substrate, in which the layers support placement of both RF and digital ICs. Figure 1 illustrates a typical MLO cross-section. Variations of this stack-up may be used if, for example, thinner components are required. The dielectric layer must have low loss at the common wireless frequency ranges and, at the same time, have a high Dk to provide high capacitance density. Unfortunately, these are counteracting properties for most materials. High Dk is usually obtained by filling polymers with dielectric materials, which can increase loss.