Page 295 - System on Package_ Miniaturization of the Entire System

Radio Fr equency System-on-Package (RF SOP) 269

cofired ceramics (LTCC) and high-temperature cofired ceramics (HTCC), while organics
include a variety of polymers such as liquid crystal polymer (LCP) [21].
LCP has recently received much attention as a high-frequency substrate material
[21–22]. It has impressive electrical characteristics that are environmentally invariant. It
provides a nearly stable dielectric constant (~2.97), has a very low dielectric loss [23], and
temperature stability up to 125°C [24] across a very wide frequency range up to 110 GHz.
Its coefficient of thermal expansion (CTE) can be engineered to match copper, silicon, or
GaAs. Being a polymer, it can be processed considerably cheaper than ceramic materials
[25]. It is flexible, recyclable, and impervious to most chemicals; has low water absorption
[26]; and is physically stable up to 315°C. The availability of two types of LCP substrates
with different melting temperatures makes it possible to realize limited multilayer
architectures. The core LCP layer, commercially available as R/flex 3850 from Rogers,
has a melting point around 315°C, while the bond LCP layer, commercially available as
R/flex 3600, has a melting point around 285°C. The bond and the core LCP layers are
identical in other characteristics except for their melting points. As a result, LCP is one of
the rare organic technologies that allows homogenous multilayer constructions.
While LCP is an emerging RF materials technology, HTCC and LTCC were proven
and widely used for RF and microwave systems [27–28] for several decades. LTCC,
which cofires with appropriate low-temperature conductors such as AgPd, Cu, or Au
around 850°C, allows a multilayer stackup of up to 100 layers. It possesses a combination
of electrical, thermal, chemical, and mechanical properties that cannot be found in most
other material groups. Some of its characteristics that are applicable are
• Stable dielectric constant over a wide range of RF, microwave, and millimeter-
wave frequencies
• Low dielectric loss up to millimeter-wave frequencies
• Engineerable coefficient of thermal expansion (CTE)
• Vertical integration with small vias and lines in a large number of layers
• Very low water and moisture absorption properties

Despite allowing a stackup of a large number of layers, companies like IBM, Kyocera,
TDK, and NTK have developed manufacturing technologies with very high yields.
HTCC, based on aluminum oxide, also called alumina, was the primary workhorse from
the 1960s until LTCC was developed in the 1980s. It is cofired at about 1600°C using
cofirable conductors such as molybdenum or tungsten with higher melting points, which
confer to this technology a superior stability and reliability in harsh environments. The
drawback of these conductors is their higher losses at high frequencies.

5.4.3 Antennas
System-on-package (SOP) can give flexibility to the front-end module by integrating all
functional blocks using multilayer processes and novel interconnection methods. One
of the major issues, however, is to integrate antennas with a high module efficiency and
low cost. Furthermore, the physical sizes of the antennas for low-frequency applications
such as cellular communication and WiFi pose serious challenges because of size.
Fabricating an antenna directly on the package has the advantages of reduced losses
and can result in compact module size. However, integration in the package has other
issues that need to be solved, which include narrow bandwidth characteristics due to
high dielectric constant substrates (which are preferred for integrating capacitors