IEEE Solid-States Circuits Magazine - Spring 2019 - 48

Single-ended signaling may offer comparable
or even better crosstalk immunity when there is
a limitation on the area available for routing,
which constrains the lane-to-lane pitch.
and differential traces is set to 30 n m
and 25 n m, respectively. The results
without the per-lane pitch constraint
are similar to the silicon interposer.
However, with a constant lane-tolane pitch, the results are different. The simulation results for 2-cm
traces on the organic interposer with
a per-lane pitch constraint are shown
in Figure 9. While there is no clear
winner for FEXT, differential signaling with ground shields offers the
best performance for NEXT, followed
by single-ended with ground shields.
This shows that single-ended signaling may result in performance degradation because of NEXT in organic
interposers, even with ground shields.
This is mainly because of the higher
coupling between C4 bumps, which
is the dominant contributor to crosstalk in organic interposers. NEXT can
be mitigated by adding a ground C4
bump and a ground trace between
the transmit and receive lanes.
Simulation results of this technique are shown in Figure 9(b) with

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NEXT (dB)

FEXT (dB)

and (f) shows that the single-ended
scheme without ground shielding
offers the best immunity to far-end
crosstalk (FEXT). This is because this
configuration provides more spacing
between the wires than other configurations. With respect to near-end
crosstalk (NEXT), the single-ended
scheme with ground shields offers
the best performance, followed by
the single-ended scheme and the
differential scheme with ground
shields. In conclusion, single-ended
signaling may offer comparable or
even better crosstalk immunity when
there is a limitation on the area available for routing, which constrains
the lane-to-lane pitch. Moreover, to
get the best performance and minimize the impact of NEXT, the transmitter and receiver lanes need to be
isolated with ground shields.
The same study is done for a typical organic package substrate. The
constant lane-to-lane pitch for the
organic interposer is set to 120 n m,
while the trace width for the single-ended

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-60
-80

-80

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0

5
10
15
20
Frequency (GHz)
(a)

Single Ended
Differential
Single Ended With Ground Shield
Differential With Ground Shield

5
10
15
20
Frequency (GHz)
(b)

Single Ended
With Ground C4 + Shield

FIGURE 9: A comparison of different signaling schemes on the organic interposer with a
constant lane-to-lane pitch of 120 μm: (a) FEXT and (b) NEXT.

S P R I N G 2 0 19

IEEE SOLID-STATE CIRCUITS MAGAZINE

the dashed blue line. It should be
noted that this technique may not
imply any additional overhead if
the transmitters and receivers are
already physically separated from
each other on the dies (e.g., a fullduplex interface).
In conclusion, when there is
good isolation between neighboring
transmitters and receivers, singleended signaling may not introduce
any additional crosstalk compared to
differential signaling on silicon interposers and organic packaging substrates. Moreover, coupling between
bumps can become the dominant
factor in crosstalk for an organic substrate. Therefore, accurate modeling
of the bumps is required in the study
of crosstalk in USR links.

Termination
In traditional chip-to-chip communication over PCB interconnects, termination at both the transmitter and the
receiver yields better signal integrity
by minimizing reflections. However,
for on-chip interconnects, reflections are not a concern, because
long interconnects tend to be quite
lossy and the reflected signals die off
before they get back to the receiver.
In cases of short on-chip interconnects, where the loss is small, the
time of flight is on the order of the
rise time of the signals. Therefore,
again, reflections do not cause signal integrity issues. The appropriate
termination strategy for USR links
whose properties lie between on-chip
and chip-to-chip interconnects is not
very clear.
In this section, an analysis of the
optimal transmitter/receiver impedance for different interconnects is
covered. To find the optimum termination impedance, we have considered three different metrics: dc gain,
reflection, and group delay. For this
analysis, the lengths of the on-chip,
silicon interposer, organic substrate,
and FR4 interconnect are 1, 3, 14, and
30 cm, respectively. These lengths
provide 15 dB of loss at 15 GHz in
each case, in accordance with the target specified in the HMC specification

IEEE Solid-States Circuits Magazine - Spring 2019

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