Intrinsic Reliability of BEOL interlayer dielectric
J. Palmer1*, G. W. Zhang1, J. R. Weber2, Che-yun Lin1, C, Perini1, R. Kasim1
1
Quality and Reliability, 2. TCAD, Intel Corporation, Hillsboro, OR, U.S.A
*Email: james.m.palmer@intel.com
2021 IEEE International Reliability Physics Symposium (IRPS) | 978-1-7281-6893-7/21/$31.00 ©2021 IEEE | DOI: 10.1109/IRPS46558.2021.9405089
Abstract — As interconnect dimensions shrink, interlayer
breakdown is increasingly relevant to reliability. This paper
presents TDDB, voltage acceleration, and leakage data for Cu
and Co interconnects. The leakage mechanism is found to be
Poole-Frenkel saturation transitioning to Fowler-Nordheim
tunneling at high field. We conclude the interlayer lifetime is
limited by the low-k dielectric and propose an E-model
acceleration. The reliability of interlayer TDDB must consider
the asymmetric voltage division of the etch stop/low-k stack.
Keywords- Interlayer, TDDB reliability, Low-k, Conduction
mechanism (PF), Fowler-Norheim (FN), Triangular Voltage
Sweep(TVS), TCAD, Poole-Frenkle saturation, Velocity
saturation
I.
INTRODUCTION
Interconnect dimension scaling continues to be an avenue
for technology enhancement, with development of sub-40nm
interconnects [1,2,3]. Unlike intralayer breakdown [2],
interlayer breakdown mechanisms involve both the etch stop
(ES) and low-k dielectric layers, resulting in more complicated
behavior. Fundamental characterization of leakage and
breakdown mechanisms are essential to qualifying future
interconnect process development in order to meet leakage
performance and reliability goals. Additionally, for highvoltage devices it is not possible to design small increments of
additional space like in lateral TDDB, but requires process
engineering to enable reliability. Literature published in recent
years has explored the TTF/breakdown behavior [4,5,6]. This
paper attempts to develop a more detailed model for the
observed electrical behavior of the composite ES/low-k
dielectric stack. We have characterized the dielectric leakage
mechanism based on curve fitting to P-F and F-N conduction
mechanisms and determined that the breakdown is limited by
the intrinsic property of the ILD.
II.
PROCESS AND TEST
measured RVS in both polarities, bottom metal high (BMH)
and top metal high (TMH), as described in Fig. 1.
Fig 1. (a) Breakdown path for interlayer breakdown, dashed line, vs.
intralayer, solid lines. Stress polarity in (b) is top-metal-high (TMH) and (c)
bottom-metal-high (BMH).
III.
RESULTS AND DISCUSSION
The understanding of the interlayer breakdown mechanism
begins with an analysis of the leakage current, which can
indicate potential breakdown mechanisms. Leakage current
density J vs. ramped voltage stress data is plotted in Fig. 2.
These interconnect systems exhibit a saturation in leakage,
which converge to a single field, regardless of the temperature.
The breakdown voltage VBD is typically up to 2.5V lower in
BMH polarity. The analysis in this paper starts with the
saturated leakage behavior at medium electric fields (<
7MV/cm), which exhibits the strongest delta in polarity
followed by the high field (>7MV/cm) leakage.
DETAILS
Samples from wafers processed on Intel’s 10nm process are
used for analysis here [3], with Co-filled and Cu-filled
interconnects. The Co layers utilize Self-aligned quadruple
patterning (SAQP) and have a chemical vapor deposition
(CVD) TiN/WN liner layers, followed by CVD Co seed layers
and electroplating Co metal fill. The Cu layers, utilizing selfaligned double patterning (SADP), have a Ta barrier and Cu
seed deposition followed by Cu electroplating. SiCN is used as
the etch stop material, and all layers use the same low-k
dielectric material.
Electrical reliability tests are performed on interlayer comb
structures stacked between M0-M1 (Co-Co), M1-M2 (Co-Cu)
layers and M4-M5 (Cu-Cu) layers. I-V is measured using
standard ramped voltage stress (RVS) at a variety of
temperatures. TDDB is measured using standard constant
voltage stress (CVS) with log time readouts at 90°C. We
Fig 2: Raw J vs E curves vs. temperature at -30, 0, 30, 60, 90, and 200 oC.
The inset plots are of slope dJ/dE vs. E, illustrating the leakage saturation
region.
Based on a survey of leakage mechanisms it is apparent
that a saturated leakage vs. E-field cannot be due to a contact
effect but must instead be trapping sites in the bulk dielectric.
Poole-Frenkel (P-F) saturation is expected when all the trap
locations are ionized [7,8]. Once all traps are ionized, no
further carriers are available to contribute to the conduction.
See Fig. 3 for the expected band alignment of interlayer
dielectric stack.
𝐽
PF plots, with the standard linearization of ln⁡(𝐸 ) vs √𝐸 are
plotted for both polarities in all three interconnect systems
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(shown in full paper). All three systems were found to have a
P-F slope β of 0.2-0.25. For uncompensated carriers, β is
expected to have a factor two [8]. These values are consistent
with the dielectric constant of the low-k dielectric common to
all three interconnect systems studied in this paper.
Fig 5. Leakage current vs. time for (a) Co-Co, (b), Co-Cu, and (c) Cu-Cu.
Comparison of intra- and interlayer TTF vs E-field for (d) Co and (e) Cu
interconnects.
Fig 3 (a) Band alignment for interlayer system. (b) Energy bands in TMH
polarity and (c) BMH polarity.
The PF trap energy can be extracted from the saturation
field 𝐸𝑠 or the thermal activation energy (shown in the full
paper). 𝐸𝑠 is read from the slope of ln⁡(𝐽/𝐸) vs √𝐸 vanishing. 𝜑𝐵
is calculated from 𝐸𝑠 = 𝜋⁡𝜖⁡𝜑𝐵 2 /𝑞 [8,9]. We see that Es is
between 5-6.5MV/cm in Cu-Cu and 7-8MV/cm for Co-Co,
which corresponds to 𝜑𝐵 = 1-1.1eV and 1.1-1.25eV
respectively. The Arrhenius thermal activation energy follows
𝜑𝑒𝑓𝑓 = ⁡ 𝜑0 - β√𝐸 , where 𝜑0 is the trap energy depth and β is the
P-F slope. In general, both methods arrive at similar trap
energy depths. The trap density is estimated from the
saturation current density (100A/µm2). With reference to the
electron velocity in SiO2 [9] and 𝐽⁡ = ⁡𝜌𝑣, we derived a density
of roughly 5x1020 cm-3.
Based on the data, we conclude that the thermochemical Emodel is the best explanation for breakdown.[10] At electric
fields >8 MV/cm it is possible that there is some additional
degradation due to the tunneling current, but this is not expected
to play any role at use condition where P-F saturation occurs.
Given the estimated trap density and lack of SILC in contrast to
other observations of interlayer TDDB [4], it is questionable
whether traps play any role in the breakdown. We believe this
rules out a 1/E or √𝐸 based acceleration models and instead
propose to follow the thermochemical E-model.
IV. CONCLUSIONS
Based on the leakage data, the interlayer leakage
mechanism is found to be P-F showing carrier density
saturation at medium electric fields and F-N tunneling at high
electric fields. A P-F trap depth of ~1eV is estimated and the FN barrier height is similarly ~1eV. We conclude that the
interlayer lifetime is limited by the low-k dielectric and a
thermochemical E-model is proposed for the lifetime
acceleration. The reliability assessment of interlayer TDDB
must consider the asymmetrical voltage division due to the
unequal dielectric constants in the ES/low-k stack.
REFERENCES
Fig 4. Fit to F-N tunneling form at high field ( >7MV/cm) for (a) Co-Co, (b),
Co-Cu, and (c) Cu-Cu.
Above 7 MV/cm, by plotting ln⁡(𝐽/𝐸) vs √𝐸, excellent fits to
the F-N tunneling form are observed, shown in Fig. 4. The
electron effective mass m* is assumed to be 1me [10], although
dangling bonds may increase m*. The barrier height does not
show consistent polarity dependence, consistent with
overlapping I-V curves in Fig. 2 and is 1+/- 0.2 eV high. We
believe this strongly suggests a form of tunneling at high
electric field.
Leakage vs. time for a single field are plotted in Fig. 5. The
leakage does not show any stress-induced artifacts (SILC). The
voltage acceleration to use condition is reasonable and meets
lifetime goals for the expected use conditions. A comparison of
intra- to interlayer TTF vs E-field is included in Fig. 5.
Interlayer TTF has an acceleration factor comparable to
intralayer in both cases, where γ ~ 5cm/MV, and did not show
a statistically significant departure from the E-model.
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