Nuclear Instruments and Methods in Physics Research A 439 (2000) 476}482
Laboratory and test beam results from a large-area
silicon drift detector
V. Bonvicini!,*, L. Busso", P. Giubellino", A. Gregorio!, M. Idzik",1, A. Kolojvari",2,
L.M. Montano",3, D. Nouais", C. Petta#, A. Rashevsky!, N. Randazzo#,
S. Reito#, F. Tosello", A. Vacchi!, L. Vinogradov!,2, N. Zampa!
!INFN - Sezione di Trieste, Italy
"INFN - Sezione di Torino, Italy
#INFN - Sezione di Catania, Italy
Abstract
A very large-area (6.75]8 cm2) silicon drift detector with integrated high-voltage divider has been designed, produced
and fully characterised in the laboratory by means of ad hoc designed MOS injection electrodes. The detector is of the
`butter#ya type, the sensitive area being subdivided into two regions with a maximum drift length of 3.3 cm. The device
was also tested in a pion beam (at the CERN PS) tagged by means of a microstrip detector telescope. Bipolar VLSI
front-end cells featuring a noise of 250 e~ rms at 0 pF with a slope of 40 e~/pF have been used to read out the signals. The
detector showed an excellent stability and featured the expected characteristics. Some preliminary results will be
presented. ( 2000 Elsevier Science B.V. All rights reserved.
1. Motivation and detector description
Silicon drift detectors (SDDs [1,2]) have been
adopted to equip the two middle layers of inner
tracking system (ITS) of the ALICE experiment at
LHC [3]. The detector presented in this paper is
the result of an extensive R&D work started in
1992 and carried on by the INFN DSI project. The
* Corresponding author. Tel.: #39-40-3756224; fax: #3940-3756258.
E-mail address: [email protected] (V. Bonvicini)
1 Also at Fac. of Phys. and Nucl. Tech., Acad. of Mining and
Met., Cracow, Poland.
2 Also at Cyclotron Laboratory, St. Petersburg University,
Russia.
3 Also at CINVESTAV, Mexico City, Mexico.
aim of the project was the production of large-area
SDDs with integrated high-voltage divider [4}10].
The use of a large number of SDDs in ALICE
(about 250 detectors) requires the assessment of
large-scale production in industry with good reliability. Following this approach, the program
initially developed by DSI has found a natural
continuity within the ALICE ITS Collaboration.
The detector is the "rst prototype of large-area
SDD for the ALICE ITS and is produced by Canberra Semiconductors N.V. (Belgium) on neutron
transmutation doped (NTD) 5A silicon wafers with
a resistivity of 3 k) cm and a thickness of 300 lm.
Its dimensions are 6.75]8.0 cm2 with a sensitiveto-total area ratio of 86%. Fig. 1 shows a large-area
SDD together with its small (3.5]2.0 cm2) prototype. The detector is a `butter#ya bi-directional
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 8 3 6 - 0
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477
Fig. 2. Details of the collection region: (1) collecting anodes; (2)
guard cathodes; (3) drift cathodes and integrated voltage-divider; (4) independently biased drift cathodes; (5) edge bulk contact.
Fig. 1. Picture showing one large-area SDD near one of the
small prototypes (3.5]2.0 cm2) designed during the DSI R&D
project.
structure, with a drift length of 33 mm and the
drifting charge is collected by two arrays of anodes
(384 anodes for each half) having a pitch of 200 lm.
The cathode strips, having a pitch of 120 lm, are
biased through a high-voltage divider integrated in
the detector and realised with high-resistivity
p` implantations. There are 265 cathodes for each
half of the butter#y and therefore 530 cathodes per
side (the side where the charge is collected and the
opposite one are conventionally referred to as
`anodea or `na side and `pa side, respectively). In
addition, the last cathodes before the anodes (8 on
the anode side and 12 on the p side) are externally
biased and constitute the so-called `collection regiona. Guard electrodes connected to the cathode
strips serve to gradually scale the negative high
voltage of the cathodes down to the ground potential of the edge bulk contact. The potential di!erence between two contiguous guards is twice that of
two contiguous cathodes in the drift region, while
in the collection region the *< between guards is
the same as for the "eld cathodes. The guard pitch
is 40 lm. Fig. 2 shows details of the collection
region of the detector. After careful simulations we
decided to use "eld plates on both cathodes and
guards, to increase the breakdown voltage at the
junctions edges. We used 4.5 lm "eld plates for the
guards and 6.5 lm for the cathodes, symmetrically
on both edges.
Monitoring the uniformity of the drift velocity
across the sensitive area and calibrating the drift
time for temperature variations (due to the dependence of the mobility on ¹, about 1%/K in silicon),
are items of paramount importance when working
with SDDs. The best way to achieve these goals is
to inject charge from a known location into the
detector. The idea of MOS injectors in SDDs was
"rst introduced and demonstrated in Ref. [11]. By
applying a suitable negative pulse to the gate electrode, the electrons accumulated in the potential
`pocketa beneath the oxide can be injected into the
bulk. We have designed a sequence of `point-likea
MOS injectors (placed across the whole sensitive
area of the detector, in the direction parallel to the
anodes) in such a way that they could be controlled
with a single external connection. Fig. 3 shows
details of one of the injectors: under the metal there
is a `screeninga p` implantation that prevents the
formation of an electron accumulation layer. This
is allowed only in certain places, where the implant
V. DRIFT DETECTORS
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Fig. 3. Picture showing details of the MOS injector structure;
one identi"es at the centre one injector, realised by metal on
oxide, while in the remaining region a p` implantation prevents
the formation of an electron accumulation layer.
is interrupted and the surface potential pocket can
be formed and "lled by electrons [9]. The dimensions of a single injector are 100]20 lm2. The
screening implantations are #oating with respect to
the surrounding drift electrodes, which are both at
the most negative potential (<
). The metal gate
BIAS
is not in contact with the screening implantations,
being separated from them by a thin oxide layer.
The DC voltage of the metal gate was measured
and found to be 14 V more positive than <
. We
BIAS
have implemented on the detector a double array of
injectors, 34 for each half of the butter#y, at a pitch
of 2 mm (i.e. one every 10 anodes).
2. Laboratory measurements
The SDD was mounted on a printed circuit
board (the `detector carda) that was in turn
plugged into a motherboard. The motherboard
provided all signals and bias voltages, as well as all
connections and test points for the measurements,
for both the detector and the front-end electronics.
This was a 32-channels, low-noise, bipolar VLSI
circuit (Omni-Purpose Low-Noise Ampli"er, OLA)
speci"cally designed for SDDs [12]. Each channel
features a charge-sensitive preampli"er, a semiGaussian shaping ampli"er and a symmetrical line
driver. We have used 6 OLA chips and thus we
were able to read 192 anodes, corresponding to
38.4 mm, i.e. half of the detector length in the direction perpendicular to the drift.
The leakage current was measured connecting
together 10 anodes at a time. Fig. 4 is a plot of the
resulting average current per anode. Clearly, the
half-detector referred to as `downa presents a much
larger leakage current than the half detector `upa.
We have used for the tests the "rst 192 anodes of
the `upa half chamber. In this region the average
leakage current is about 1 nA/anode at a drift "eld
of 500 V/cm.
The MOS injectors, besides giving the opportunity to calibrate on-line the drift time for temperature variations, are also a powerful instrument to
test the SDD in the laboratory. Fig. 5 shows, as an
example, a scope picture of a signal from the injectors. The upper trace (channel 2) displays the trigger signal from the pulse generator (a positive
TTL), corresponding to the sending of a negative
pulse to the MOS gate. The lower trace (channel 1)
shows the pulse collected at one anode after a time
of #ight of the injected charge of 5.42 ls (the applied drift "eld was 500 V/cm). Fig. 6 shows the
response of all the read out anodes to the charge
simultaneously injected by 17 consecutive injectors.
The measurement conditions were: injection pulse
width of 30 ns and frequency of 50 Hz, pulse amplitude of !7 V and drift "eld of 500 V/cm. As one
can see, all clusters are well separated. The di!erent
cluster amplitudes are most probably due to oxide
inhomogeneities. The interface oxide charge, measured on a test structure placed at the edge of the
wafer from which the detector under test was cut, is
3.9]1011 electrons/cm2. Fig. 7 reports the amplitude of the maximum of one of the clusters as
a function of the injection pulse amplitude measured at three di!erent frequencies. For the actual
V. Bonvicini et al. / Nuclear Instruments and Methods in Physics Research A 439 (2000) 476}482
479
Fig. 4. Plot of the average anode currents for both detector halves at a "eld of 500 V/cm.
Fig. 5. Picture of a signal from the MOS injectors (channel 1)
occurring 5.4 ls after the injection pulse (indicated in channel
2 by the positive TTL trigger output signal from the pulse
generator, synchronous with the negative, 30 ns wide, gate pulse
of 7 V).
Fig. 6. Plot of the amplitude distributions measured at the
anodes from the charge injected by 17 contiguous injectors.
MOS injectors design the main source of electrons
to re"ll the surface potential pocket is bulk and
surface generation in the very vicinity of a MOS
injector as soon as there are only few electrons lying
below the oxide in the direction along the cathodes.
Therefore, the electron replenishment rate is quite
low and the quantity of charge that can be injected
is limited by the frequency of the gate pulses, i.e. by
the time given to the system to re"ll the surface
potential pocket.
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Fig. 7. Plot of the signal amplitude as a function of the amplitude of the injection pulse, for three di!erent frequencies (50, 200 and
500 Hz).
3. Test beam preliminary results
3.1. Set-up and data-taking conditions
The experimental apparatus was installed at the
`T11a secondary beam of the CERN PS East Hall.
The SDD under test was placed in between two
silicon microstrip telescopes, which measured the
particle trajectories. Each telescope is made by six
1 cm]2 cm single-sided silicon microstrip detectors (3 for x and 3 for y), having a strip pitch of
50 lm. A total of four 1 cm]10 cm]0.5 cm trigger
scintillators was used to de"ne the beam spot (one
pair upstream the "rst telescope and one pair
downstream the second telescope). The beam consisted mainly of n with a momentum of 3 GeV/c.
A Struck DL350 FADC system was used to read
the analog data. The FADC channels were driven
by a tuneable sampling clock; during data taking,
we mostly used a sampling frequency of 50 MHz,
but some data were also taken at 25 MHz. Each
FADC channel had a 256 byte ring memory; accordingly, the maximum recorded drift time was
5.1 ls at 50 MHz and 10.2 ls at 25 MHz. Unfortunately, it was impossible during this test beam to
read out the MOS injectors in the DAQ main
stream, and therefore they could not be used for
on-line calibration of the drift time.
3.2. Cluster xnding method and detector ezciency
The charge clusters were identi"ed following
a two-step procedure: "rst, the content of all FADC
channels was analysed. When the signal from the
SDD was higher than a given software threshold
for two consecutive time bins, it was considered as
a `hita. As a second step, the one-dimensional hits
were combined to form two-dimensional clusters:
when hits in adjacent anodes overlapped in time,
they were considered to be part of a two-dimensional cluster. The overall e$ciency is de"ned as
the ratio between the number of tracks for which
a cluster is reconstructed in the SDD and the total
number of tracks as de"ned by the reference telescopes. The results obtained show that at a drift
"eld of 460 V/cm the cluster "nding e$ciency is
V. Bonvicini et al. / Nuclear Instruments and Methods in Physics Research A 439 (2000) 476}482
essentially 100% up to a drift distance of 25 mm,
then progressively decreasing to 93% at the maximum drift distance of 33 mm. This e!ect is due to
the charge di!usion: for a longer drift time the
charge spreads over a larger number of anodes and
therefore the probability that the signal amplitude
goes below the threshold level increases.
481
The linearity along the anode direction is illustrated in Fig. 10, where the cluster centroid position
in the SDD is plotted versus the telescope reference
position. Fig. 11 shows the di!erence between the
3.3. Detector linearity
The detector linearity along the drift direction is
shown in Fig. 8. This is a plot of the electron drift
time in the SDD versus the corresponding coordinate of the projection of the track reconstructed by
the microstrip telescopes. For both "elds the detector presents a good linearity. The drift velocities,
extracted from the slopes of the linear "ts of the two
distributions, are v"4.0$0.02 lm/ns and
v"6.1$0.03 lm/ns. Fig. 9 shows the deviation
between the position evaluated by the SDD and the
reference position given by the microstrip telescopes. The black dots are average values obtained
by considering `slicesa of 500 lm along the drift
direction. The resulting distribution has a maximum spread (peak-to-peak) of +100 lm for the
case of E"460 V/cm.
Fig. 9. Deviation between the position reconstructed by the
SDD along the drift coordinate and the corresponding coordinate of the track reconstructed by the microstrip telescopes.
Fig. 8. Drift time versus the corresponding coordinate of the
track reconstructed by the microstrip telescopes.
Fig. 10. Centroid position along the SDD anode axis versus the
corresponding reference position given by the microstrip telescopes.
V. DRIFT DETECTORS
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4. Conclusions
A very large-area SDD with integrated voltagedivider ("rst prototype for the ALICE ITS) has
been designed and produced on 5A NTD silicon
wafers. The detector has been characterised in the
laboratory, by means of ad hoc designed, integrated
MOS structures, allowing controlled injection of
charge in the detector bulk at known locations. The
SDD has been tested in August 1997 in a 3 GeV/c
pion beam at the CERN PS. The preliminary results show good detector stability and performance
(e$ciency and linearity along both coordinates),
while the spatial resolution was limited by the multiple scattering (due to the relatively low momentum of the particles), enhanced by the long
lever arm between the telescopes and the SDD
under test. In April 1998 a new test beam, dedicated
to precisely evaluate the SDD position resolution,
has been carried out with 380 GeV/c pions at
CERN SPS. The MOS injectors were used for drift
velocity calibration. Data analysis is now under
way.
References
Fig. 11. Deviation between the position reconstructed by the
SDD along the anode direction and the corresponding coordinate of the track reconstructed by the microstrip telescopes.
position along the anodes as reconstructed in the
SDD and the reference position given by the microstrip telescope, for three values of the mean drift
distance. The black dots are average values of the
data contained in `slicesa of 250 lm along the anode direction. In all situations, a non-linearity
probably due to the presence of the high-voltage
divider, occurs at the left side of each distribution.
Excluding the divider region, we see that the maximum #uctuations increase from +50 lm for the
data close to the anodes to +200 lm for the data
far from the anode region.
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