Wave transmission lines and networks – Long lines – Strip type
Reexamination Certificate
2001-05-30
2004-02-24
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Long lines
Strip type
C333S246000, C174S1170AS
Reexamination Certificate
active
06696906
ABSTRACT:
TECHNICAL FIELD
The present invention is related to fabrication of circuit boards and microelectronic devices containing signal lines embedded in dielectric substrates and, in particular, to circuit boards and microelectronic devices in which dielectric material surrounding striplines is eliminated, effectively substituting air for the dielectric material, in order to greatly decrease the dielectric loss coefficient for the stripline.
BACKGROUND OF THE INVENTION
In circuit boards and in various microelectronic devices, numerous signals paths electrically interconnect electrical components and subcomponents affixed to, or incorporated within, the circuit boards and microelectronic devices. In many cases, these signal lines, also called “striplines” and “traces,” are composed of copper, or another conductive element or alloy, embedded within a dielectric substrate, such as a fiberglass and epoxy composite or a plastic material. In general, the signals transmitted through signal lines represent information as discrete voltage pulses, with a relatively low voltage state representing a first binary value and a relatively high voltage state representing a second binary value. When the rise/fall time of the signal within the signal line is fast enough that the signal line can change from one logic state to another in less time than it takes for the signal to traverse the length of the signal line, then the signal line is typically mathematically modeled as a transmission line. In a transmission line, the strength, or intensity, of the signal is attenuated as the signal traverses the signal line. The attenuation of the signal a is modeled as the sum of two attenuation coefficients:
&agr;=&agr;
C
+&agr;
D
where &agr;
C
=attenuation coefficient for conduction, and
&agr;
D
=attenuation coefficient for dielectric loss
The attenuation coefficient &agr;is normally expressed in dimensions of decibels/meter (“dB/m”). The attenuation coefficient &agr;
C
arises from resistance of the conductive element or alloy from which the signal line is composed, and the attenuation coefficient &agr;
D
arises from dissipation of energy within the dielectric substrate surrounding the signal line. This dissipation of energy occurs as molecules within the dielectric substrate, having either permanent or induced electric dipole moments, realign themselves within the fluctuating electric field produced by the electric signal transmitted through the signal line. The attenuation coefficient &agr;
D
is proportional to the frequency of the signal, as shown by:
α
D
∝
π
ϵ
r
f
tan
δ
c
where &egr;
R
is the relative permittivity,
tan &dgr;is the loss tangent,
f is signal frequency, and
c is speed of light
The relative permittivity and the loss tangent are characteristic for each different dielectric material. The relative permittivities and loss tangents for a number of materials are provided below, in Table 1:
TABLE 1
Relative Permittivity
Loss tangent
Substrate Material
&egr;
r
tan &dgr;
Alumina 99.5% Pure
9.8
0.0001
FR4 Fiberglass
4.5-4.9
0.01
GaAs
12.9
0.002
PTFE
2.1
0.0003
Quartz
3.78
0.0001
Polyethylene
2.2
0.0002
Dry Air
1.0006
1 × 10
−9
Vacuum
1
0
FIG. 1
shows a graph of the attenuation of signal intensity per unit length versus signal frequency. In
FIG. 1
, the vertical axis
102
represents the relative signal intensity following transmission of the signal through a unit length of signal line, and the horizontal axis
104
represents the logarithm of the signal frequency. Curve
106
represents theoretical signal intensity attenuation due to dielectric loss, or &agr;
D
and curve
108
represents signal attenuation due to resistivity of the conductive element or alloy from which the signal line is composed. Curve
110
is the calculated overall signal attenuation, and curve
112
is an experimentally measured signal attenuation. Note the steep increase in signal attenuation in the gigahertz signal frequency range.
FIG. 2
illustrates the effects of frequency-dependent attenuation of signal strength within a signal line on the signal transmitted within the signal line.
FIG. 2
shows a desirable low-state-to-high-state signal transition
202
and a low-state-to-high-state signal transition
204
when significant signal intensity attenuation occurs due to resistivity of the conducting element or alloy and to dielectric loss. Both low-state-to-high-state transitions are plotted in time against a common time (t) axis
206
and against separate amplitude (“A”) axes
208
and
210
, with the vertically upward direction of increasing amplitude indicated by the vertical arrow in FIG.
2
. For the desirable transition
202
, the signal rises from a low state
212
to a high state
214
within a relatively short period of time &Dgr;t
216
. For the desirable transition, the rise &Dgr;A
218
corresponds to the amplitude or voltage differential generated at the source of the signal. However, when signal attenuation occurs, as described above, the transition time &Dgr;t
220
may increase and the amplitude or voltage differential
222
may decrease. This lower final amplitude is due to conduction losses.
Signal intensity attenuation within a signal line may lead to inoperability of an electronic circuit containing the signal line. Attenuation of the signal intensity may prevent the signal from rising above a voltage or amplitude differential threshold required for signal detection by the destination component or subcomponent connected to the signal line. Increase in the time of transition between low and high voltage or amplitude states may prevent transmission of the signal altogether. As discussed above, with reference to
FIG. 1
, the degradation of transition times and signal intensity is frequency dependent, and greatly increases in the gigahertz range. However, modern microprocessors are currently operated at frequencies in the gigahertz range, and are continuously being enhanced to operate at faster speeds. Transmission of signals between microprocessors and other electronic components within circuit boards and microelectronic devices has become a serious bottleneck constraining overall circuit-board and microelectronic-device processing throughput and speed of operation.
FIG. 3
shows a small section of a circuit board or microelectronic device including two embedded signal lines. In this portion of a circuit board or microelectronic device, the two conductive signal lines
301
and
302
, also called “striplines,” or “traces,” are embedded in a dielectric material
303
parallel to two conductive planes
304
and
305
. The conductive planes
304
and
305
serve as electrical reference planes, or ground planes, for the signal lines
301
and
302
. According to the above discussion, for the traces
301
and
302
to support signal transmission, the signal intensity attenuation must be maintained below a threshold value that depends on the signal response characteristics of subcomponents interconnected by the signal lines and by required times for signal state transitions. As discussed above, signal intensity attenuation is frequency dependent, so as the frequencies of signals carried by the signal lines increases with increasing microprocessor speed, circuit board and microelectronic device designers must more and more carefully control design and material parameters in order to maintain signal intensity attenuation below necessary threshold values.
Currently, circuit board and microelectronic device designers maintain signal intensity attenuation below threshold values by either minimizing the length of signal lines, choosing materials for signal lines having low resistivities, or by choosing dielectric substrate material with low relative permittivities and, most particularly, with low loss tangents. Unfortunately, substrate materials generally increase in cost with decreasing loss tangents. For example, polytetrafluoroethylene (“PTFE”) has a loss tangent several orders of magnitude b
Cherniski Andrew Michael
deBlanc James J.
Dickey David
Hewlett--Packard Development Company, L.P.
Lee Benny
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