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Investigation of an abnormal pattern of
leakage currents in silicon microstrip detectors
I. Rashevskaya on behalf of the Slim5 Collaboration, Trieste Group
Abstract Some batches of microstrip detectors fabricated by FBK-irst showed an odd and peculiar pattern of the strip leakage currents: the first and last few strips of the detector
had low leakage, whereas all the strips in between showed very high current (3-5 orders of magnitude higher). Due to the peculiar shape of the strip current plot, the phenomenon has
been called “panettone effect”. This characteristic was common to all detectors within the affected batches. A detailed study of detectors and test structures made with a few
variations of the fabrication process led us to conclude that the high strip currents were due to stress induced in the silicon by the combination of LPCVD-deposited layers of TEOS
oxide and nitride. The strips close to detector ends had low current because there the stress was locally relaxed by the presence along the bias ring of a continuous contact opening
through the dielectric layers. In accordance with this interpretation, some modifications have been introduced in the fabrication process, resulting in detectors with low leakage
current for all strips, showing no particular distribution pattern.
2.0E+06
-1.8E-08
1.9E+06
-1.6E-08
1.8E+06
-1.4E-08
1.7E+06
-1.2E-08
1.6E+06
Strip current
-1.0E-08
1.5E+06
Strip resistance
-8.0E-09
1.4E+06
-6.0E-09
1.3E+06
-4.0E-09
1.2E+06
-2.0E-09
1.1E+06
0.0E+00
1.0E+06
128
0
16
32
48
64
80
96
112
Example of the current distribution on a 128-strip detector.
The current of the first and the last few strips is low, whereas
all the strips in between have very high current (3-5 orders of
magnitude higher). This peculiar phenomenon, common to
ALL six different detectors in EACH wafer of the batch, has
been called “Panettone effect”.
T1
T2
T3
Strip number
First 20 strips
SLIM-01(EPI) S1
-1.2E-07
-1.0E-07
-1.0E-07
The possible causes of the effect
-8.0E-08
-6.0E-08
-6.0E-08
-4.0E-08
-4.0E-08
-2.0E-08
Strip current
0.0E+00
top
3
S1baby
S1
S1
S2
top
5
S2baby
T6
T5
0.0E+00
2.0E-08
5
10
15
20
364
369
Strip number
374
379
2.0E-08
384
Strip number
Current distribution on a “CAP” test structure
SLIM-01 CAP TS2T Vback=60V
-4.0E-08
-3.5E-08
-3.0E-08
I (A)
-2.5E-08
-2.0E-08
-1.5E-08
Although Gated-Diodes included in test structures showed low surface
currents, the characteristics of the currents measured on strip detectors
led us to suspect that surface generation played a major role there. In
order to verify this hypothesis, the condition of the Si-SiO2 interface in
the interstrip gaps of a sensor has been controlled with a gate, obtained
-3.5E-07
by extending the metal strips with a layer of conductive paint.
-3.0E-07
The total detector current (I_back) shows the V_gate
-2.5E-07
dependence typical of a Gated Diode (plot (a)).
-2.0E-07
In accumulation and depletion, the current measured on a strip
-1.5E-07
(I_strip) follows exactly the same shape of I_total, with a scale
-1.0E-07
factor ≈ 119 ≈ number of high-current strips (see plot (b)).
-5.0E-08
Once surface inversion channels start to form, the strip current
0.0E+00
grows much higher, because it collects the charge from many
30
other strips. Eventually, the strip and backside currents
-4.2E-08
decrease to low values when all the surface is inverted,
-3.7E-08
suppressing the surface generation.
-3.2E-08
The high current measured on strip detectors is
-2.7E-08
-2.2E-08
quantitatively compatible with this surface-generated
-1.7E-08
current in the interstrip gaps. It must be concluded that in
-1.2E-08
the gaps between the strips of the detectors the interface
-6.8E-09
-1.8E-09
has much worse characteristics than in the gated diodes.
Silver paint
5.E-06
IDC
0.0E+00
0
14
28
Strip number
42
(a)
2.E-06
1.E-06
0.E+00
20
10
0
-10
-20
-30
-40
-50
V gate (V)
5.E-06
4.E-06
IDC
Iback
3.E-06
(b)
2.E-06
I back (A)
The fact that the first and last strips show normal current suggests that the
proximity to some feature of the detector edge could be a factor influencing
the current. Two candidates are the Bias Ring or the n-type implant running
all along the cut lane. In order to distinguish between these possibilities, we
measured the currents of a test structure, designed for interstrip capacitance
measurements, which has four groups of 13 strips, separated by a common
Bias Ring. The peculiar pattern of strip currents is reproduced within each
group of strips. The number of low-current strips at the ends is the same as in
large sensors having the same strip pitch. Confronting detectors with
different pitches, we see that low-current strips extend up to about the same
distance from the bias ring. This suggest that proximity to the bias ring,
rather than to the n-type edge implant, is what makes low-current possible.
3.E-06
I back (A)
-5.0E-09
4.E-06
Iback
I DC (A)
-1.0E-08
Role of surface generation current
I DC (A)
0
Additional measurement
S2
bot
6
bot
4
-2.0E-08
Strip current
The “panettone effect” has been observed on wafers with and
without gettering. Although the current of test diodes and of the first
strips is two orders of magnitude lower on the gettered wafers, the
central strips have about the same current. Therefore, bulk generation
is not likely to be the source of the high current of central strips.
The strip current grows moderately with bias voltage, but the
“panettone effect” is already present at low bias voltages, and there is
no change when depletion voltage is exceeded. Then one can
conclude that the effect cannot be connected to defects on the back
side.
In order to verify a possible connection of the problem to the
design of the SLIM detectors, single side detectors from another
batch (denoted as “RadHard”), having the layout of Babar Model 2,
have been tested. All show the “panettone effect”. The same has been
observed on other detectors fabricated by FBK-irst on various types
of substrates (epitaxial, float zone, p-typee) with various designs.
Four wafers without passivation (batch SLIMbis) have shown the
same “panettone effect” of lot SLIM1. Therefore the passivation
cannot be responsible for the problem.
T4
Vback=60V
-1.2E-07
-8.0E-08
I (A)
Last 20 strips
Vback=60V
I (A)
SLIM-01(EPI) S1
Possible causes of the effect
Possible causes
-2.0E-08
Rpoly (Ohm)
I (A)
Panettone effect
Strip current
scan
ofVback=60V
an “S2” detector
SLIM-142(FZ)
S2BB
1.E-06
0.E+00
30
20
10
0
-10
-20
-30
-40
-50
V gate (V)
Proposed explanation for the origin of the effect
Strip
Sequence of layers above the silicon substrate
Current
OXIDE NITRIDE POLY TEOS Al PASSIV Panettone
OXIDE NITRIDE POLY TEOS Al
Panettone
OXIDE POLY NITRIDE
Al
OK
OXIDE
NITRIDE
Al
OK
OXIDE NITRIDE POLY TEOS Al
Panettone
OXIDE OXIDE
POLY TEOS Al PASSIV
OK
OXIDE NITRIDE
Al PASSIV
OK
-1.0E-08
By comparing the technologies of different
detector lots fabricated by FBK-irst, we
observe that the peculiar 'Panettone Effect'
is correlated with the combined presence
of two LPCVD-deposited dielectric layers:
silicon nitride and TEOS oxide.
We can hypothesize that this combination produces a high level of stress, which induces defects at the silicon/oxide
interface, leading to a high rate of surface generation. These dielectric layers are interrupted in the contact areas between
metal and (implanted) silicon. This locally releases the stress in a region around the contact. Since the Bias Rings of the
detectors (as well as the Guard Rings of the test structures) have a continuous contact opening along their length, the local
release of the stress can explain the fact that the strips within a certain distance from the Rings have low leakage current.
The fact that gated diode test structures show low surface generation currents is due to their small size: all regions of the
structure are within a short distance from the surrounding Guard Ring. It should be noted that this picture assumes the
interface defect generation due to stress to be a reversible process: defects are removed once the stress is released by
opening the contact.
Making use of a modified technology excluding the TEOS oxide, a batch of striplet detectors has been fabricated. They
showed no “panettone effect”, and have been successfully employed in the SLIM5 beam test at CERN in September 2008.
-8.0E-09
-6.0E-09
I [A]
Fabbrication
Lot
SLIM1
SLIMbis
SLIMter
SLIMter NO Poly
RadHard BaBar2
SD3 Babar1 Babar2
ALICE
Further support of the interpretation
-4.0E-09
Strip current
(NO POLY)
AC broken (manca
poly)
-2.0E-09
0.0E+00
0
50
100
150
Strip number
200
250
In a few cases, a number of strips
in the high-current central region
of the sensor showed a reduced
current. Visual inspection revealed
that the polysilicon layer was
missing along a fraction of the
length of a few strips. This
caused the contact etch between
metal and poly to penetrate
down to the implanted strip, thus
cutting through all dielectric
layers.
According to our interpretation,
this locally releases the stress,
yielding lower current for the
strips with interrupted
polysilicon and a few of their
neighbors.
Related Presentation on the 2009 Pisa Meeting
• Lorenzo Vitale on behalf of SLIM5 collaboration,
SLIM5 Beam Test Results for Thin Striplet Detector and Fast Readout Beam
Telescope
•Mauro Villa on behalf of SLIM5 collaboration,
Beam-Test Results of 4k pixel CMOS MAPS and High Resistivity Striplet Detectors
Equipped with Digital Sparsified Readout