Abstract:
Spiral inductors are a key component of mixed-signal and analog integrated
circuits (IC's). Such circuits are often fabricated using silicon-based technology,
owing to the inherent low-cost and high volume production aspects. However,
semiconducting substrate materials such as silicon can have adverse effects on
spiral inductor performance due to the lossy nature of the material. Since the
operating requirements of many high performance IC's demand reactive components
that have high Quality Factor's (Q's), and are thus low loss devices, the
need for accurate modeling of such structures over lossy substrate media is key to
successful circuit design.
The Q's of commonly available off-chip inductors are in the range of 50-
100 for frequencies ranging up to a few gigahertz. Since off-chip inductors must
be connected through package pins, solder bumps, etc., which all contribute additional
loss and thus lower the apparent Q of an external device, the typical on-chip
Q requirement for a given RFIC design is generally lower than that for an off-chip
spiral solution. However, a spiral inductor that was designed and fabricated originally
in a low loss technology such as thin-film alumina may have substantially
worse performance in regard to Q if it is used in a silicon-based technology, owing
to the conductive substrate. For this reason, it is imperative that semiconducting
substrate effects be accurately accounted for by any modeling effort for monolithic
spirals in RFICs.
This thesis presents a complete modeling solution for both single and multi-level
spiral inductors over lossy silicon substrates, along with design considerations
and methods for mitigation of the undesirable performance effects of semiconducting
substrates. The modeling solution is based on Spectral Domain Approach
(SDA) solutions for frequency dependent complex capacitive (i.e. both capacitance
and conductance) parasitic elements combined with a quasi-magnetostatic
field solution for calculation of the frequency dependent complex inductive (i.e.
both inductance and resistance) terms. The effects of geometry and process variations
are considered as well as the incorporation of Patterned Ground Shields
(PGS) for the purpose of Q enhancement. Proposals for future extensions of this
work are discussed in the concluding chapter.
