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282 Cha pte r F i v e

Quality
Factor Inductance (nH) Frequency (GHz)
Silicon
Low resistivity 52.8 [94] 1.38 13.6
High resistivity 30 [95] 4 1–2
Micromachined 150 [79] 1 8–23
Wafer-level packaging 38 [80] 1 4.7
LTCC 93 [81] 9.6 1.15
Organic laminate 180 [42] 4.8 2.2

TABLE 5.1 Comparison of Technologies for Inductor Integration

As mentioned earlier, the parameters of interest for the design of inductors are its
inductance, Q factor, and self-resonant frequency (SRF). The design of inductors with
any substrate can be optimized using a combination of full-wave electromagnetic
solvers such as method-of-moment based tools and quasi-TEM approaches. Quasi-TEM
approaches are faster compared to full-wave solvers, and at lower frequencies (< 8 GHz)
they provide a better approximation for the associated loss in devices with thicker
metallization (>10 μm). For passives that use a circular topology, full-wave solvers such
as from Sonnet are typically used for purposes of optimization.
In summary, Table 5.1 compares the various technologies for fabricating inductors
with the highest quality factor. The corresponding inductance and frequency of
operation are also listed in the table. This table does not represent the ultimate limit for
that technology but provides an indication of what has been achieved. Silicon-based
processing includes inductor fabrication on low- and high-resistivity silicon and
micromachining. Wafer-level packaging refers to the realization of inductors above the
passivation layer using thin-film postprocessing techniques on the silicon wafer. LTCC
and organic laminates are the substrate technologies discussed earlier.

5.4.5 RF Capacitors
RF applications such as filters and resonators need stringent tolerance, a low temperature
coefficient of capacitance (TCC), and a high Q (quality factor). This is in sharp contrast
to decoupling capacitors that do not have such stringent requirements but require
capacitance densities that are much higher. Current RF capacitors are either polymer-
based with low capacitance density or high-temperature vacuum-deposited (metal-
organic chemical vapor deposition (MOCVD) thin-film, thick-film LTCC-based
composites), which causes limitations for RF integration. Among embedded RF passives,
emerging applications with embedded RF capacitors require the development of
organic compatible dielectric materials with thermally stable high dielectric constant,
low loss, and improved electrical performance. Thin-film and thick-film processes
should therefore be engineered to be compatible with the low-cost substrate wiring and
other embedded RF component technologies.

Electrical and Material Parameters
Capacitors are one of the most basic elements in RF systems. RF capacitors are needed
for filtering (<10 pF) and capacitive coupling (<500 pF). These applications need a
2
capacitance density of about 1 nF/cm . RF capacitors require a Q of ≥ 200 to meet the