Transformer comprising stacked inductors

Details Transformer comprising stacked inductors Transformer comprising stacked inductors

2001-03-01

2003-08-19

Trinh, Michael (Department: 2822)

Active solid-state devices (e.g., transistors, solid-state diode

Integrated circuit structure with electrically isolated...

Passive components in ics

C257S528000, C257S728000, C438S381000

Reexamination Certificate

active

06608363

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally in the field of fabrication of electronic circuit components. In particular, the present invention is in the field of fabrication of inductors used to build transformers used in electronic circuits.
2. Background Art
It is known in the art that there is an ever-present demand for decreasing electronic circuit component sizes and geometries. The demand is fueled, in large part, by the consumer' desire for ever-smaller portable communication and information processing devices, such as cellular telephones, hand-held wireless information assistants, laptop computers, and networking devices. The requirement to decrease the size of these consumer communication and information processing devices has resulted, among other things, in a general trend in the market to integrate everything on a chip—to have systems on a chip. In addition to the size reduction obtained with a system on a chip, there is also a manufacturing cost reduction. These systems on a chip have resulted in Ultra Large Scale Integration (ULSI) chips containing over a million components per chip. However, the transformer, an off-chip electronic component, has not benefited from this dramatic decrease in size. Attempts in the art to realize an on-chip transformer encounter various problems, such as size, unwanted capacitance, low quality factor, low self-resonance frequency, and low coefficient of coupling of the transformer's cross-coupled inductors. These problems become more severe as the operating frequency of the transformer increases. For example, these problems greatly hinder the design of transformers for use in RF applications in the commercially important wireless communication range of 800 to 2400 MHz.
The problems mentioned above that are encountered in the art in an attempt to realize an on-chip transformer will be illustrated by reference to an exemplary transformer shown in top view
100
in FIG.
1
A and in cross-sectional view
122
in FIG.
1
B.
FIG. 1A
shows a top view
100
of an exemplary transformer on an area of a semiconductor die. A first inductor is also referred to as primary winding
104
in
FIGS. 1A and 1B
. A second inductor is also referred to as secondary winding
106
in
FIGS. 1A and 1B
. For illustration purposes, primary winding
104
and secondary winding
106
of the exemplary transformer shown in
FIGS. 1A and 1B
are patterned in a manner known in the art in metal level one. However primary winding
104
and secondary winding
106
can be implemented at any metal level in the semiconductor die. The distance between adjacent segments of primary winding
104
and secondary winding
106
is referred to by numeral
108
in FIG.
1
A. The diameter of primary winding
104
is referred to by numeral
118
in FIG.
1
A.
FIG. 1B
shows cross-sectional view
122
of the exemplary transformer along the line B—B in FIG.
1
A. The width of a segment of primary winding
104
is referred to by numeral
114
while the width of a segment of secondary winding
106
is referred to by numeral
124
. Thickness
110
refers to the thickness of primary winding
104
and secondary winding
106
. Dielectric
120
having thickness
112
is situated between the lower surface of primary winding
104
(or the lower surface of second winding
106
) and silicon substrate
116
.
By way of background, a transformer is comprised essentially of two cross-coupled inductors. The magnetic coupling between the two inductors is called mutual inductance. The quality factor (“Q”) of an inductor is determined by the formula Q=2&pgr;fL/R, where L is the inductance of the inductor, f is the operating frequency of the inductor, and R is the resistance of the inductor. A relatively low quality factor signifies a relatively high energy loss. Therefore, by increasing the respective quality factors of a transformer's cross-coupled inductors, the energy loss in the transformer can be decreased. Consequently, increasing the inductance of the transformer's inductors will decrease the energy loss in a transformer. Also, decreasing the resistance of the transformer's inductors can also decrease the energy loss in a transformer.
Each of the primary or secondary windings in the exemplary transformer shown in
FIGS. 1A and 1B
is analogous to a “square spiral inductor” known in the art. The problems encountered when attempting to increase the inductance of the primary winding or secondary winding of the exemplary transformer shown in
FIGS. 1A and 1B
are thus analogous to the is problems encountered in increasing the inductance of a square spiral inductor. For example, typical inductor values for a square spiral inductor used in mixed signal circuits and in RF applications range from 1 to 100 nano-henrys. To achieve a square spiral inductor having a value of 30 nano-henrys using a fabrication process with a metal pitch of 5.0 microns, the inductor would require approximately 17 “metal turns” and would have a “diameter” of approximately 217 microns. As such, even a 30 nano-henry conventional on-chip inductor would require a considerable amount of die space.
Moreover, for a given diameter
118
, the inductance is proportional to n
2
, where n is the number of “metal turns” of primary winding
104
. Therefore, increasing the number of “metal turns” of primary winding
104
can increase the inductance of primary winding
104
. However, as the number of “metal turns” increases, the overall resistance of primary winding
104
will also increase. As stated above, the quality factor of an inductor is inversely proportional to the inductor's resistance. Thus, the increase in the overall resistance of the inductor, for example primary winding
104
, will decrease the quality factor of the inductor. As also explained above, a decrease in the quality factor of an inductor results in a greater energy loss in the inductor. Therefore, if primary winding
104
were cross-coupled to another similar on-chip inductor, for example to secondary winding
106
, the resulting transformer would suffer a corresponding energy loss.
It is known in the art that the self-resonance frequency of an inductor is inversely related to the capacitance of the inductor. It is also known in the art that the self-resonance frequency of an inductor should be greater than twice the operating frequency of the inductor for optimal operation. As such, optimal functioning of the inductor requires that the inductor have a low inherent capacitance value. Moreover, the need to reduce the inductor's capacitance increases as the operating frequency of the inductor increases. As such, the inductor's capacitance becomes a limiting factor in the use of on-chip inductors and consequently on-chip transformers, at relatively high frequencies, such as 800 to 2400 MHz.
It is known in the art that the capacitance of an inductor that is patterned into a metal level of a semiconductor die is proportional to (l*w)/d, where “l” is the total length of all the segments of the inductor, “w” is the width of a segment of the inductor, and “d” is the distance between the metal level the inductor is patterned in and the silicon substrate or the next lower metal level of the chip.
In our exemplary transformer shown in
FIGS. 1A and 1B
, the capacitance of primary winding
104
can be decreased by either decreasing width
114
of primary winding
104
or increasing the distance between the metal level the inductor is patterned in and the silicon substrate. However, by decreasing width
114
of primary winding
104
in an attempt to decrease the capacitance of primary winding
104
, the resistance of primary winding
104
is increased. Since the quality factor of an inductor, such as primary winding
104
, is inversely proportional to its resistance, increasing the resistance of primary winding
104
decreases its quality factor. Therefore, decreasing width
114
of primary winding
104
is not an effective method of reducing the capacitance of primary winding
104
.
Anothe

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