III. Gm-C Filtering - Epublications - Université de Limoges

II.6 Technological Opportunities and Limitations
A fully active structure relies on the technological choice of the transistors, but it also
relies on the available passive components in the chosen technology.
II.6.a Integrated Passive Components
II.6.a.i Inductors
On-chip inductors are mainly realized as spirals to enhance the inductive effect.
However, the planar spiral geometry is far from being solenoidal and the inductance does not
scale linearly with the number of turns. Hence, on-chip inductors often require large silicon
space.
The quality factor of such an inductor is limited by series loss due to the resistivity of
the metal layer used for the spiral. Thick copper layers can be used to reduce the resistivity
and to increase the inductor quality factor. Nevertheless this may require additional masks, so
the cost of the final chip is increased. Furthermore, despite copper metal line, on-chip
inductors quality factors stay relatively low. This parameter is also strongly dependent on the
silicon area. Indeed, the larger the inductor area, the higher the quality factor, but the larger
the capacitive parasitics which lower the self resonant frequency.
II.6.a.ii Capacitors
Having a look to the capacitance formula, given by
ε 0ε
r S
C = , (III.70)
t
ox
it is worth noticing that it strongly depends on the insulator thickness tox and on the surface S.
To decrease the capacitance area, the insulator thickness has to be decreased. According to the
various materials constituting the capacitances, different types can be used. A MOS
capacitance (Metal-Oxide-Semiconductor) allows the use of high density capacitances due to
a high permittivity coefficient oxide layer, made of TaO5 for instance. However, they are
strongly non-linear and they require a bias. Using additional lithography masks, MIM (Metal-
Insulator-Metal) capacitors may be used. Because they use a very thin insulator, MIM
capacitances are able to achieve high capacitance values and also show a high density
(between 2 and 6fF/µm 2 ).
II.6.a.iii Resistors
Resistors can be made with various materials according to the wanted resistance value.
Very reliable models are required, especially in terms of parasitic and also of process and
temperature variations, in order to achieve very stable and robust designs.
II.6.b Transistors
One major opportunity of fully active, therefore transistor based, filtering is the ability
to take advantage of technology scaling. Higher density of integration and smaller
consumption thanks to a lower supply voltage are well known advantages. A more advanced
technology node allows using more digital gates in a same area in order to improve the analog
RF performances by means of calibration.
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