Microelectronics Reliability 51 (2011) 904–908
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Microelectronics Reliability
journal homepage: www.elsevier.com/locate/microrel
Thermal stability of SiC Schottky diode anode and cathode metalisations
after 1000 h at 350 °C
Donagh O’Mahony a,⇑, Russell Duane a, Tony Campagno a, Liam Lewis b, Nicolas Cordero a, Pleun Maaskant a,
Finbarr Waldron a, Brian Corbett a
a
b
Tyndall-UCC, Lee Maltings, Prospect Row, Cork, Ireland
Centre for Advanced Photonics & Process Analysis (CAPPA), Cork Institute of Technology, Rossa Avenue, Cork, Ireland
a r t i c l e
i n f o
Article history:
Received 8 June 2010
Received in revised form 4 December 2010
Accepted 8 December 2010
Available online 20 January 2011
a b s t r a c t
The thermal stability of two commercially available silicon carbide Schottky diode types has been evaluated following a 1000 h non-biased storage test under vacuum at 350 °C. The Ti-based Schottky (Anode)
contact shows excellent stability over the duration of the test with less than 5% change in either extracted
Schottky barrier height or ideality values. The Al die attach metalisation on the anode also shows no evidence of degradation after the test. However, a considerable change in series resistance was observed for
both diode types, with up to a factor of 100 measured for one of the diodes. The primary early failure
mode is related to degradation of the NiAg Ohmic (cathode) die attach metalisation. Demixing of the NiAg
alloy, leading to Ag agglomeration is proposed to be the underlying degradation mechanism involved
resulting in delamination of the die attach metalisation and the corresponding series resistance increase.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Silicon carbide (SiC) has emerged as an important electronic
material for high power and high temperature applications in
many sectors such as the aerospace, automotive and more recently
the renewable energy industries [1–3]. It has facilitated the development of a new generation of power devices capable of handling
extremely high currents and voltages and, not surprisingly, reliability is a key issue when targeting these demanding applications.
Despite the considerable progress made in SiC materials and technology development over the last decade there remains some concerns for the reliability SiC devices under long term operation at
high current and/or temperature. Recent reviews [4,5] have highlighted issues such as the generation and propagation of stacking
faults during long term operation of p–n devices under dc forward
bias [6,7] and the stability of the metal–SiC Ohmic contact interface under high temperature [8]. Other studies have focussed on
device reliability under repetitive [9] or dc [8] current stress testing, the latter showing clearly that the Ohmic contact degradation
is significant in the case of SiC–Ni interfaces due to the C residue
remaining after NiSi formation. Another key issue with regard to
SiC device reliability relates to packaging and most commercially
available SiC Schottky diodes are currently only rated for 175 °C
[10] maximum operating (junction) temperature. High temperature packaging solutions have been developed involving gold [11]
⇑ Corresponding author. Address: Tyndall National Institute (UCC), Lee Maltings,
Prospect Row, Cork, Ireland. Tel.: +353 21 490 4177.
E-mail address: donagh.omahony@tyndall.ie (D. O’Mahony).
0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.microrel.2010.12.008
or sintered silver-based die attach [12] materials though these
may not be suitable for high volume production. Here, we have
evaluated the reliability of two commercially available SiC Schottky diodes employing commonly used die attach materials: an
Al-finished anode metalisation for wire-ing and NiAg metalisation
on the cathode bondpad for die attach. The bare dies were stored
unpackaged in a high temperature long duration test under nonbiased electrical conditions in vacuum for 1000 h at 350 °C. It
was found that the Schottky (Anode) contact metalisation is relatively stable throughout the storage test, whereas the Ohmic (cathode) bondpad degraded considerably, leading to a 10–100 fold
increase in diode series resistance.
2. Experimental approach
SiC Schottky diodes obtained from two commercial die vendors,
herein referred to as type 1 and type 2, were both rated for 600 V
maximum reverse breakdown operation. The approximate chip
area and thickness were 2.5 mm2 and 350 lm, respectively. The
Schottky contact metalisation in both cases comprised of a Tibased contact metal with an Al (3 lm thickness) bondpad whereas
the Ohmic back metalisation was a Ni-based contact with a Ni–Ag
bondpad (1.5 lm thickness approx).
Electrical (IV) measurements were carried out on a temperature-controlled Cascade vacuum probe-station in 4-wire configuration under nitrogen purge at various times during the test. In each
case, the oven temperature was reduced to room temperature before removing the die in order to minimise the risk of exposure of
905
D. O’Mahony et al. / Microelectronics Reliability 51 (2011) 904–908
the chips to atmosphere at high temperature. The IV characteristics
of the diodes were measured by scanning voltage (0–1 V, 10 mV
step) in pulsed mode (pulse width = 300 us, period = 500 ms) and
reading current up to 1 A using a Keithley 2430 1 kW source
meter. Reverse biased measurements were carried out with a
Keithley 237 high voltage source meter. 10 bare die each of type
1 and 2 were stored unpackaged at 350 °C in a Heraeus oven
with roughing pump vacuum system (ultimate pressure 1 102 mbar). The die were visually inspected and characterised after
24, 50, 100, 200, 500 and 1000 h. The die attach metalisations were
also analysed using scanning electron microscope imaging (SEM)
and energy dispersive analysis by X-rays (EDAX) for surface
measurements.
3. Results
Table 1
Mean values for diode parameters for both die types extracted using the thermionic
emission expression (Eq. (1)) and experimental data at 25 °C. I0 is the thermionic
emission saturation current density measured (i.e. extracted using Eq. (1)) at 0 V. The
quoted error is the standard deviation from the mean across a sample set of 10 diodes.
Diode
I0 (A/cm2)
uB (eV)
Ideality
Rs (X)
Type l
Type 2
4.0 l015
1.4 1014
1.265+/0.005
1.22+/0.01
1.027+/0.001
1.03+/0.01
0.068+/0.02
0.15+/0.05
fact the diode ideality in both diode types was excellent at less
than 1.05 (Table 1).
The temperature-dependent experimental data was fitted with
the general form of the ideal diode equation for thermionic emission [13,14] in order to determine diode parameters such as Schottky barrier height (uB), diode ideality (n) and series resistance
(Rs):
3.1. Electrical characteristics before storage
Current–voltage characteristics of diodes measured before the
storage test are shown in Fig. 1. The diodes show low turn-on voltage of about 0.8 V and low saturation leakage current at 300 V
(<1 nA). Even at 250 °C, we noted that the leakage current at
300 V was less than 0.1 lA. However, there is a noticeable difference in the on-state characteristic for both diode types with the
type 2 diode showing a greater voltage dependence on current
density. This is particularly evident at the higher measurement
temperature of 250 °C (see linear scale IV plots in Fig. 1). Nevertheless, both diode IV characteristics show excellent linearity over
many orders of magnitude of current density when plotted on a
semi-log scale, which is consistent with a single ideality value. In
J ¼ J 0 exp
qðV IRS Þ
qðV IRS Þ
1 exp
nkT
kT
J 0 ¼ AR T 2 exp
qu B
kT
ð1Þ
ð2Þ
Here, k, q and T are the Boltzmann constant, electron charge and
diode (Schottky) junction temperature during measurement. AR is
the Richardson’s constant assumed to be the value of 256 A/
cm2.K2 previously measured for a Ti Schottky contact to 4H–SiC
[15].
Calibration of the junction voltage vs temperature, using a 3 A
300 ls pulsed current source, showed that the junction temperature increase was negligible for the voltage pulse duration employed here. From the extracted parameters (Table 1) it is
noticeable that while both diodes show similar barrier height
and ideality values there is a significant difference in the extracted
series resistance, with the type 2 diode value twice that of the type
1 die. It is also noticeable that there is a greater spread in extracted
values for the type 2 sample set as evidenced by the greater standard deviation values for these die.
3.2. Non-biased storage test
Fig. 1. IV characteristics of 10 diodes before storage (t = 0 h) for type 1 (left) and
type 2 (right) diodes at 25 °C (solid lines) and 250 °C (lines & symbols). The data is
plotted on log10 and linear scales on the left and right axes, respectively, for each
plot.
The current–voltage characteristics of the diodes measured at
25 °C and 250 °C after the 1000 h storage test are shown in
Fig. 2. Most significant is the fact that at a forward voltage of about
1 V, the current density has reduced by a factor of about 100 for the
type 2 diodes. The reduction in the type 1 diodes is considerably
less but nevertheless still noticeable (approximately a factor of
2–3). The extracted diode parameters, using Eq. (1), are plotted
in Figs. 3 and 4. Clearly, the most significant change is in the series
resistance of the type 2 diodes with up to a 100 fold increase
apparent. There is a small but noticeable decrease in the extracted
barrier heights, especially in the case of the type 1 diodes. There
also appears to be an increase in diode ideality for both diode
types, though the change is relatively small and it is difficult to
say with confidence how significant this change is considering
the large standard deviation values measured across a sample set
of 10 die after 1000 h. Note that the standard deviation values generally increase consistently with storage time illustrating the increased variability in measured and extracted diode parameters.
This is presumably linked with the degradation in diode performance and drift in parameters with increased storage time.
Despite the change in diode parameters, the reverse leakage
current, IR, is relatively unchanged by the storage test indicating
that any degradation of the Schottky junction is negligible
(Fig. 5). There maybe a slight trade-off between the series resistance increase and barrier height reduction in determining the
ID
549396
Title
ThermalstabilityofSiCSchottkydiodeanodeandcathodemetalisationsafter1000hat350°C
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