High-Q inductive elements

Details High-Q inductive elements High-Q inductive elements

1999-12-14

2001-05-29

Mai, Anh (Department: 2832)

Inductor devices

Coil or coil turn supports or spacers

Printed circuit-type coil

C336S223000, C336S232000

Reexamination Certificate

active

06239684

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to integrated circuits, and more specifically to electrical components of integrated circuits.
BACKGROUND OF THE INVENTION
Analog integrated circuits (ICs) are now being extensively used, for example, in wireless radio frequency (RF) applications such as cellular telephones where high frequencies are encountered. Many analog ICs include inductive elements, such as inductors, formed by a conductor. Inductive elements with a relatively high quality (Q) factor, or low loss, are preferably used in analog ICs. As a result, the analog integrated circuits have superior performance, including selectivity, noise figure, and efficiency. Relatively high Q inductors have been formed on insulating bulk semiconductors, such as gallium arsenide.
Most integrated circuits, however, are formed on silicon. In comparison to gallium arsenide ICs, silicon ICs can be fabricated relatively inexpensively. Also, analog and digital circuits may be readily combined on silicon ICs. However, unlike gallium arsenide, silicon is a conductive bulk semiconductor. As a result, conventional inductive elements formed on silicon are relatively lossy, and thus have relatively low Q factors. For example, Q factors of 3 to 8 are reported for inductors fabricated on silicon in Nguyen et al., “Si IC-compatible inductors and LC Passive Filters,” IEEE Journal of Solid-State Circuits, vol. 25, no. 4, p. 1028-1031, 1990, herein incorporated by reference.
An inductor formed on an IC
101
may be a conventional rectangular spiral inductor
103
, as illustrated in FIG.
1
A. The conventional rectangular spiral inductor
103
includes substantially parallel conductive branches
121
that are mutually coupled to increase the rectangular spiral inductor's
103
effective inductance.
The conventional rectangular spiral inductor
103
is formed in the following manner. A first conductor
109
is patterned on the IC
101
. Then, an insulator, such as resist, defining the location of air bridges
105
, is patterned on the IC
101
. Next, a second conductor
107
is patterned on the IC
101
. However, where an air bridge
105
is to be formed, the insulator separates the first and second conductors
107
,
109
. Finally, conventional air bridges
105
are formed by removing the insulator.
Conventional air bridges
105
, in this example, permit the two conductors
107
,
109
to cross one another, without making electrical contact, as illustrated in FIG.
1
B. Conventional air bridges
105
are formed by substantially perpendicular conductors
107
,
109
to diminish undesired magnetic coupling between the conductors
107
,
109
. Further, relatively low-dielectric-constant air typically separates the conductors
107
,
109
to diminish undesired capacitive coupling between the conductors
107
,
109
.
FIG. 1C
illustrates a prior art first order lumped element electrical model of the rectangular spiral inductor
103
that describes the electrical characteristics of the rectangular spiral inductor
103
below its self-resonant frequency. The self resonant frequency is the maximum frequency at which the rectangular spiral inductor
103
acts as an inductor. Above the self resonant frequency, for example, the rectangular spiral inductor may exhibit capacitive characteristics.
L is the effective inductance of the rectangular spiral inductor
103
. The effective inductance represents the sum of both self and mutual inductances of the branches
121
. The inductance, L, of the rectangular spiral inductor
103
is determined by (1) the length of the branches
121
, (2) the spacing between the branches
121
, and (3) the number of branches
121
, or turns.
The other model elements are parasitics that result from the physical implementation of the rectangular spiral inductor
103
. R
DC
and R
SKIN EFFEECT
are respectively the lumped element equivalent DC and skin effect resistances of the conductors
107
,
109
. R
DC
is determined by the cross-sectional area, length and resistivity of the conductors
107
,
109
. R
SKIN EFFECT
is determined by the thickness of the conductors
107
,
109
. C
S
is a lumped element equivalent capacitance representing the interwinding capacitances between the parallel conductive branches
121
. C
S
is determined by both the distance between adjacent branches
121
, and the dielectric constant of the material proximate to those adjacent branches
121
. The C
P
s are lumped element equivalent capacitances representing capacitances between the conductors
107
,
109
and a ground plane under the IC
101
on which the rectangular spiral inductor
103
is formed. The C
P
s correspond to the width of the conductors
107
,
109
, and the thickness and dielectric constant of the material between the conductors
107
,
109
and the ground plane. R
SUBSTRATE
is a lumped element equivalent resistance corresponding to substrate losses. The Q factor and self-resonant frequency of the rectangular spiral inductor
103
are a function of the reactances and resistances described by the electrical model of FIG.
1
C.
To increase its Q factor, resistances and/or capacitances of the rectangular spiral inductor
103
should be reduced. One technique for reducing the Q factor of the inductor is disclosed in J. N. Burghartz et al., “Integrated RF and Microwave Components in BiCMOS Technology,” IEEE Trans. Electron Devices, vol. 43, no. 9, pp. 1559-1570, 1996 (herein after the “Burghartz Article”), herein incorporated by reference. The Burghartz Article discloses inductors, on silicon ICs, whose conductors are displaced above the silicon, and are encased in oxide. These inductors have Q factors exceeding 10. The higher Q factors arise, in part, because the inductors, disclosed in the Burghartz Article, have relatively lower values of C
P
because the conductors are farther displaced from the IC ground plane by the oxide.
Further, the inductors disclosed in the Burghartz Article require a complex five-level metal silicon technology that is more complicated than conventional two-to four-level metal silicon technologies. Therefore, there is a need for inductors having relatively high Q factors that can be formed with conventional silicon technologies.
SUMMARY OF THE INVENTION
The present invention provides a method of forming air bridges, on a substrate or an integrated circuit, which may be used to form inductors and other devices. A first insulator is formed on a base layer. A first conductor is formed on the first insulator. The first conductor is patterned. A second insulator is formed over the first insulator. A via hole is formed in the second insulator. A second conductor is formed on the second insulator, and is electrically coupled to the first conductor by the via hole. The second conductor is patterned. A cavity is formed under the second conductor, and in the first and second insulators. In one embodiment, the first and second conductors form air bridges.
In another embodiment, a support structure is formed during the step of forming the cavity. In yet another embodiment, a conductive pad is formed over the support structure during the step of patterning the second conductor.
In a further embodiment, the present invention provides an air bridge or inductive element on a substrate or integrated circuit. A first insulator is formed on a base layer. A first conductor is formed and patterned on the first insulator. A second insulator is formed on the first insulator. A via hole is formed in the second insulator. A masking layer is developed on the integrated circuit. A cavity, defined by the developed masking layer, is formed in the first and second insulators. The cavity is filled with a polymer. The integrated circuit is cleaned. A second conductor is formed on the polymer, and coupled to the first conductor by the via hole. The second conductor is patterned. In yet a further embodiment, the cavity is filled with a polymer that is foam.
In yet a further embodiment, the inductive element includes a second via hole in the support structure that couples the

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