Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors
The characteristics and radiation tolerance of a double-sided microstrip detector (DSMD) were studied, and the suitability of the detector to the ALICE experiment requirements was analyzed. The sensitive area of the silicon microstrip detector measures 40x75 mm. The DSMD consists of 750 registering...
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| Zitieren: | Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors / A.P. de Haas, P. Kuijer, V.I. Kulibaba, N.I. Maslov, V.L. Perevertailo, V.D. Ovchinnik, S.M. Potin, A.F. Starodubtsev // Вопросы атомной науки и техники. — 2000. — № 2. — С. 26-33. — Бібліогр.: 11 назв. — англ. |
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irk-123456789-822642015-05-28T03:01:54Z Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors de Haas, A.P. Kuijer, P. Kulibaba, V.I. Maslov, N.I. Perevertailo, V.L. Ovchinnik, V.D. Potin, S.M. Starodubtsev, A.F. Experimental methods The characteristics and radiation tolerance of a double-sided microstrip detector (DSMD) were studied, and the suitability of the detector to the ALICE experiment requirements was analyzed. The sensitive area of the silicon microstrip detector measures 40x75 mm. The DSMD consists of 750 registering strips on each side. The strip pitch is 100 mm and the strip length is 40 mm. Strips of the p+-side were oriented parallel to the side edge, the n+-strips were placed at 30 mrad stereo angle with respect to p+-strips and were separated by a common p+-stop structure. Both p+- and n+-strips are biased by integrated polysilicon resistors with a resistance no less than 10 MOhm. The data readout is realized with use of 120 pF coupling capacitors. The radiation tolerance of the microstrip detector was studied using 20 MeV electrons. The leakage current increases from 2 up to 5 nA per one strip and the interstrip resistance decreases from 43 down to 30 GOhm after 10 krad irradiation dose. The other DSMD features remain unchanged under irradiation. To evaluate the detector efficiency, the yield of good coupling capacitors and biasing resistors, as well as strip leakage currents, interstrip resistance and interstrip capacitance were studied. Based on the data obtained, the number of defective strips is found not to exceed 3%; this provides the required detector efficiency of about 97%. 2000 Article Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors / A.P. de Haas, P. Kuijer, V.I. Kulibaba, N.I. Maslov, V.L. Perevertailo, V.D. Ovchinnik, S.M. Potin, A.F. Starodubtsev // Вопросы атомной науки и техники. — 2000. — № 2. — С. 26-33. — Бібліогр.: 11 назв. — англ. 1562-6016 PACS: 29.40.Wk http://dspace.nbuv.gov.ua/handle/123456789/82264 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Experimental methods Experimental methods de Haas, A.P. Kuijer, P. Kulibaba, V.I. Maslov, N.I. Perevertailo, V.L. Ovchinnik, V.D. Potin, S.M. Starodubtsev, A.F. Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors Вопросы атомной науки и техники |
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The characteristics and radiation tolerance of a double-sided microstrip detector (DSMD) were studied, and the suitability of the detector to the ALICE experiment requirements was analyzed. The sensitive area of the silicon microstrip detector measures 40x75 mm. The DSMD consists of 750 registering strips on each side. The strip pitch is 100 mm and the strip length is 40 mm. Strips of the p+-side were oriented parallel to the side edge, the n+-strips were placed at 30 mrad stereo angle with respect to p+-strips and were separated by a common p+-stop structure. Both p+- and n+-strips are biased by integrated polysilicon resistors with a resistance no less than 10 MOhm. The data readout is realized with use of 120 pF coupling capacitors. The radiation tolerance of the microstrip detector was studied using 20 MeV electrons. The leakage current increases from 2 up to 5 nA per one strip and the interstrip resistance decreases from 43 down to 30 GOhm after 10 krad irradiation dose. The other DSMD features remain unchanged under irradiation. To evaluate the detector efficiency, the yield of good coupling capacitors and biasing resistors, as well as strip leakage currents, interstrip resistance and interstrip capacitance were studied. Based on the data obtained, the number of defective strips is found not to exceed 3%; this provides the required detector efficiency of about 97%. |
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Article |
| author |
de Haas, A.P. Kuijer, P. Kulibaba, V.I. Maslov, N.I. Perevertailo, V.L. Ovchinnik, V.D. Potin, S.M. Starodubtsev, A.F. |
| author_facet |
de Haas, A.P. Kuijer, P. Kulibaba, V.I. Maslov, N.I. Perevertailo, V.L. Ovchinnik, V.D. Potin, S.M. Starodubtsev, A.F. |
| author_sort |
de Haas, A.P. |
| title |
Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors |
| title_short |
Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors |
| title_full |
Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors |
| title_fullStr |
Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors |
| title_full_unstemmed |
Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors |
| title_sort |
characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2000 |
| topic_facet |
Experimental methods |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/82264 |
| citation_txt |
Characteristics and radiation tolerance of a double-sided microstrip detector with polysilicon biasing resistors / A.P. de Haas, P. Kuijer, V.I. Kulibaba, N.I. Maslov, V.L. Perevertailo, V.D. Ovchinnik, S.M. Potin, A.F. Starodubtsev // Вопросы атомной науки и техники. — 2000. — № 2. — С. 26-33. — Бібліогр.: 11 назв. — англ. |
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Вопросы атомной науки и техники |
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| fulltext |
E X P E R I M E N T A L M E T H O D S
CHARACTERISTICS AND RADIATION TOLERANCE
OF A DOUBLE-SIDED MICROSTRIP DETECTOR
WITH POLYSILICON BIASING RESISTORS
A.P. de Haas, P. Kuijer
Utrecht University, The Netherlands
V.I. Kulibaba, N.I. Maslov, V.L. Perevertailo, V.D. Ovchinnik, S.M. Potin, A.F. Starodubtsev
National Science Center “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine
The characteristics and radiation tolerance of a double-sided microstrip detector (DSMD) were studied, and the
suitability of the detector to the ALICE experiment requirements was analyzed. The sensitive area of the silicon
microstrip detector measures 40×75 mm. The DSMD consists of 750 registering strips on each side. The strip pitch
is 100 µm and the strip length is 40 mm. Strips of the p+-side were oriented parallel to the side edge, the n+-strips
were placed at 30 mrad stereo angle with respect to p+-strips and were separated by a common p+-stop structure.
Both p+- and n+-strips are biased by integrated polysilicon resistors with a resistance no less than 10 MOhm. The
data readout is realized with use of 120 pF coupling capacitors.
The radiation tolerance of the microstrip detector was studied using 20 MeV electrons. The leakage current
increases from 2 up to 5 nA per one strip and the interstrip resistance decreases from 43 down to 30 GOhm after
10 krad irradiation dose. The other DSMD features remain unchanged under irradiation.
To evaluate the detector efficiency, the yield of good coupling capacitors and biasing resistors, as well as strip
leakage currents, interstrip resistance and interstrip capacitance were studied. Based on the data obtained, the
number of defective strips is found not to exceed 3%; this provides the required detector efficiency of about 97%.
PACS: 29.40.Wk
1. INTRODUCTION
The full-scale prototype of microstrip detector for
ALICE experiments has been developed and
manufactured since 1996. By now, three batches of
microstrip detectors have been manufactured. The
single-sided detectors of the first two batches were used
to investigate the influence of an additional insulating
Si3N4 layer on leakage currents, the breakdown voltage
of coupling capacitors, the interstrip resistance, etc. [1].
The single-sided microstrip detector was developed
to be the p+-side of the double-sided microstrip detector.
In 1998, the full-scale prototype of a DSMD was
manufactured; studies were made of the static
characteristics of the detector and their behavior under a
10-year dose of the ALICE experiment. The results of
the development and studies on static characteristics of
the two-coordinate detector prototype are presented in
this note.
2. DETECTOR DESIGN
To ensure a comprehensive study of the detector's
static characteristics and to provide the control of the
technological process, three groups of test structures for
microstrip, diode and technological test structures on a
4" silicon wafer were designed and manufactured. The
test structures surround the full-scale DSMD located at
the center of the wafer (Fig. 1).
Fig. 1. p+-side (a) and n+-side (b) of 4-inch wafer with microstrip detector (1) and test structures: 2- diodes, 3-5-
microstrip test structures, 6- technological test structures.
26 ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2000, № 2.
Серия: Ядерно-физические исследования (36), с. 26-33.
.
2.1. DOUBLE-SIDED DETECTOR
The sensitive area of the double-sided (DSMD)
silicon microstrip detector measures 40×75 mm2. The
DSMD consists of 750 registering strips on each side.
The strip pitch is 100 µm and the strip length is 40 mm.
Strips of the p+-side were oriented parallel to the side
edge, the n+-strips were placed at a 30 mrad stereo angle
with respect to p+-strips (Fig. 2). The n+-strips are
separated with the common p+- stop structure to exclude
the connection of n+-strips by the accumulation layer.
Both p+- and n+-strips are biased by integrated
polysilicon resistors with a resistance no less than
10 MOhm. The data readout is realized using 120 pF
coupling capacitors. The insulating layer for coupling
capacitors is 0.250 µm of silicon oxide. Two contact
pads on each side of strips are situated in two rows to
ensure the module repairability. Detector protection
against mechanical and atmospheric effects is ensured
by the 1 µm passivating layer of silicon oxide. One
opens up areas in the passivating layer only to Al
contact pads of p-strips, coupling capacitors, guard rings
and biasing line of integrated resistors.
Fig. 2. Microstrip detector corners of the double-sided detector (a- p+side, b- n+side):
1- coupling capacitor pads, 2- polysilicon resistors, 3- biasing pads, 4- p+-guard ring, 5- p+-stop structure.
2.2. TEST MICROSTRIP STRUCTURES
Simultaneously with the main double-sided
microstrip detector, three microstrip test structures
located at the edges of the plate have been manufactured
(Fig. 1). The first test microstrip structure is a double-
sided microstrip detector with 64 registering strips on
each side loaded by integrated resistors. With the
exception of the decreased number of strips the test
structure corresponds to a full-scale detector. The
second test microstrip structure (4) is a single-sided p+-
microstrip detector with 64 strips loaded by integrated
resistors. The third test structure (5) is a 64-strips single-
sided n+-detector with integrated polysilicon resistors
and integrated coupling capacitors.
All these three test structures, like the main detector,
are surrounded by guard rings and coated with a
passivating layer.
The microstrip test structures are provided for
studying signal spectrum, space resolution and their
variation under irradiation.
2.3. DIODE TEST STRUCTURES
Microstrip detector and microstrip test structures are
surrounded by diode test structures (Fig. 1).
The diode test structures serve for studying the
quality of silicon, for measuring the depleting voltage
by a capacitive technique, for preliminary studies of the
microstrip detector characteristics and evaluation of
their behavior under irradiation. Their application
permits one to reduce the necessary number of
microstrip detectors at the stage of research and
development.
The diode test structures are manufactured with
sensitive zones of two sizes: 2×2 mm2 and 5×5 mm2.
The size of the sensitive zone of the smaller diode
structure corresponds to that of a single strip.
2.4. TECHNOLOGICAL TEST STRUCTURES
Test structures are implemented on the wafer to
monitor the technological process, for measuring the
resistance of the layers of polysilicon, p+- and n+-
implantations, for measuring contact resistance and
other characteristics of the detector.
3. PROCESSING OF THE PROTOTYPES
3.1. SINGLE-SIDED DETECTOR
PROCESSING
To manufacture the prototype of the p+-side of a
double-sided strip detector, one used four-inch wafers of
n-silicon with resistivity of 3000-5000 Ohm·cm, <111>
orientation and 350 µm thick, polished from both sides.
In the process of the first oxidation a pyrogeneous
oxide 0.3 µm thick layer was grown at T=900°C with
addition of HCl. This oxide was used as a mask during
27
ion implantation of phosphorus to form n+-guard rings.
Phosphorus ions with the energy E=60 keV and surface
concentration 5×1014 at/cm2 have been used. The back-
side has also been implanted with phosphorus to make
the Ohmic contact but with a higher surface
concentration of ions 2.5×1015 at/cm2.
After the formation of n+-rings, the protective oxide
was removed and a new protective oxide 0.6 µm thick
was grown for masking during boron implantation.
After formation of the pattern of p+- regions, boron with
the energy E=60 keV and surface concentration 5×
1014 at/cm2 was implanted. Phosphorus and boron ions
were implanted through the layer of the dechanneling
oxide 500 Å thick. After that, a SiO2 layer 0.15 µm
thick was grown as the first layer of a two-layer
capacitor dielectric. The layer of a high temperature
Si3N4 0.12 µm thick was the second one. To perform a
comparative analysis, the Si3N4 layer was not deposited
on some of the plates.
To form the resistors, a film of polycrystalline
silicon 0.55 µm thick was deposited and doped by boron
ion implantation up to the level required to obtain
1.5 MOhm resistors.
Contact holes to different layers (p+, n+ and
polysilicon) were obtained by simultaneous wet etching
the SiO2 layers after lithographic formation of contact
areas and plasma-chemical etching the Si3N4 layer in the
region of contact areas.
Strip-detector metallisation was made by depositing
Al with a small silicon admixture (1%). After
lithographing aluminum, the passivating layer of the
low temperature phosphorus-silicate glass 0.9 µm thick
was deposited that covered the total surface of the
detector. The photolithography over the passivating
layer opened only Al areas of contact pads for
connections to the detector and test structures. The final
annealing of the detector was made at T=400°C in
hydrogen during 30 min.
Up to now two sets of single-sided detectors were
produced. In the first set, only double-layer insulation
was used to create the coupling capacitors. In the second
set, the silicon nitride was not deposited on some of the
wafers. In all other respects, all the second set wafers
are identical, processed according to the same
technological operations.
3.2. DOUBLE-SIDED DETECTOR
PROCESSING
The p+-side processing of the double-sided
microstrip detector is the same as the one used for the
processing of the single-sided detector except for the
absence of the Si3N4 additional insulation. Only single
layer SiO2 insulation with 0.25 µm thickness was used
to create the coupling capacitors.
The n+-strips have been implanted with phosphorus
using a surface concentration of ions of 2.5×1015 at/cm2.
The common p+-stop structure was formed by boron ion
implantation at the energy of 60 keV and a dose of 5×
1013 cm-2. As the passivating layer, low temperature
phosphorus-silicate glass 0.9 µm thick was deposited on
the p+- and n+-sides of the detector.
4. TEST CONDITIONS
For measuring static characteristics the probe
stations developed and constructed at KhIPT were used.
A miniature mechanical adapter was also designed and
produced to measure the characteristics of double-sided
detectors before the readout electronics is mounted on
them. The yield of good coupling capacitors was
obtained using a semi-automatic probe station, because
a lot of measurements are required by such tests.
4.1. MECHANICAL ADAPTER
A mechanical adapter was designed and produced to
measure the static characteristics of double-sided
microstrip detectors. The adapter is a mechanical device
comprising two microposition probes (Fig. 3). The
design provides for the arrangement of a silicon wafer,
4 inches in diameter, in the adapter. The wafer is
embedded in a receptacle electrically insulated from the
whole adapter structure. The plate is fixed by two
fasteners with a uniform spring-load around the
perimeter of the plate.
Fig. 3. The mechanical adapter for static charac-
teristics measurements of double-sided microstrip
detectors. 1- two microposition probes, 2, 3- the control
handles of the probes.
The needles of both probes are insulated from the
case and have the leads brought out to the instruments.
The needles are moving independently of one another
and can be put at any point within one-half the detector
plate area. The needle is set in preliminary motion
through a rough and rapid circular displacement
(rotation) around the axis places at the center of the
carriage. The final positioning of the needle is achieved
by a precise movement of the carriage in two directions
using screw pairs. The design of the microprobes
permits the needles to be smoothly lowered and raised
to a distance of 2 to 3 mm with respect to the plate
surface.
The procedure of positioning the probes at the
required site can be performed with any microscope
having a focal distance of no less than 25 mm. After
arranging the intrinsic probes in the required position
and providing a contact, the adapter is flipped over,
mounted on the platform of the measuring station and is
fixed by clips (Fig. 4). The detector plate surface is fully
28
open on this side and the external probes of the
measuring station can be placed at any point.
The microposition probes of the adapter are spring-
loaded and are sufficiently rigid to provide a reliable
contact between the needle and the plate. At the same
time, the weight of the movable probe components is
sufficiently small not to have the contact disrupted as
the adapter is flipped over. The reliability of contacts
was verified in the course of both, tests and
measurements. A properly done contact has never
failed.
Fig. 4. The adapter flipped over and the separated
microposition probe (4).
The flaw in the design of the adapter is that on
lowering the probe needles into the plate the motion of
the needle tip is not exactly perpendicular to the plate,
and the needle position has to be corrected.
4.2. MEASUREMENT OF THE RADIATION
TOLERANCE OF THE MICROSTRIP
DETECTOR
As is known, the action of radiation on
semiconductor detectors is implemented, mainly, via
two mechanisms. The first mechanism of bulk
damaging consists in breaking the crystal symmetry
through displacing atoms from their lattice sites. The
second mechanism of surface damage consists in
changing the charge state of the Si/SiO2 interface
through the oxide ionization [2,3]. In view of this, to
simulate the radiation conditions for the inner tracking
system (ITS) of the ALICE detector, it is necessary to
irradiate the detector with ~600 Gy of ionizing radiation
and a neutron fluence of ~1011 n/cm2 (corresponding to
10 years of ALICE operation). Namely, the neutrons
simulate the action of high energy particles on the bulk
of the detector material [4,5].
The radiation tolerance of microstrip detectors in
this paper was studies using 20 MeV electrons in the
Kharkov Institute of Physics and Technology.
The absorbed dose (Gy) in the specimen irradiated
with electrons was determined using the well-known
values of ionization energy losses and the measured
integral densities of electron fluxes. The absorbed dose
and its distribution over the detector were also measured
with color film dose meters. The color film-ref dose
meters permit one to perform measurements with an
accuracy ≤20% under steady outer conditions and the
temperature not exceeding 60oC. The measurements of
the optical density of dose meters were made with
micro-photometers.
4.3. TEST OF 20 MeV ELECTRONS BULK
DAMAGING EFFICIENCY
In the case of irradiation with high energy electrons,
along with ionization, the action of electrons is
determined by the generation of structure defects in the
bulk of the crystal detector due to the displacement of
substance atoms by accelerated electrons [2, 6, 7].
The efficiency of 20 MeV electron action on bulk
material of the microstrip detector was determined from
the change in the lifetime of charge carriers in silicon
specimens-witnesses [6]. It is known that neglecting the
influence of the surface one can determine the leakage
current of a semiconductor detector from the
relationship [8]
j=qnjWA/2τ, (1)
where q is the electron charge, W is the depth of the
detector depletion, A is the active region of the detector,
τ is the effective minority carrier lifetime and nj is the
intrinsic carrier concentration. The expression for
change of the leakage current of the detector normalized
by one acting particle or one dose unit will have the
form
∆j= 0.5qnjWA·∆τ-1/D, (2)
similar to the well-known expression for the radiation
constant of a semiconductor material [2]
Kτ=∆τ-1/D, (3)
Since the leakage current and the radiation constant
of the detector material have similar dependencies on
the carrier lifetime, then measuring the quantity Kτ for
the detector material, one can judge on the efficiency of
radiation action on a semiconductor detectors.
Measuring Kτ was used in this paper for the
determination of the relative efficiency of 20 MeV
electrons required for the simulation of the action of
14 MeV neutrons with a given fluence on the bulk of
the detector material. The measured data are given in
Table 1.
Table 1. Measurement of relative bulk damaging efficiency of 2 MeV and 3 MeV electrons, 14 MeV neutrons
Particle Energy,
MeV
Measured Kτ,
cm2/s
Kτ [2],
cm2/s
Efficiency of electrons
(relative to 14 MeV neutrons)
n
e
e
14
3
20
1.5·10-6
1.5·10-8
4.1·10-8
1.5–2·10-6
~2·10-8
1
0.01
0.027
29
The results obtained for Kτ for 3 MeV electrons and
neutrons practically coincide with the results of the
paper [2]. Using the efficiency value obtained we have
assumed that the integral flux of 20 MeV electrons 40
times exceeding the fluence of 14 MeV neutrons exerts
on the detector characteristics the close action. We
believe our procedure of irradiation to be valid taking
into account the rather low radiation environment in
ALICE. The predicted changes in the detector
characteristics as a result of irradiation are not large.
Correspondingly, the value of the inverse annealing of
detector characteristics as a result of restructuring the
bulk clusters of defects after irradiation [2, 9, 10] is
negligible.
5. RESULTS
The detector was irradiated in a non-biased state.
The static characteristics of the detector were measured
just before and after irradiation. Table 2 shows the
initial characteristics of the produced single-sided and
double-sided microstrip detectors. Fig. 5 shows the
distribution of leakage currents and biasing resistor
values for 750 strips detector with Si3N4 additional
insulation.
5.1. DEPLETION VOLTAGE
Fig. 6 shows the bulk capacity against the depletion
voltage for different irradiation doses. The
depletionvoltage was studied with the capacitive
technique [11] at 1 kHz and 1 MHz measuring
frequency.
The voltage of total depletion was determined from
the point where the strong and weak variations of
capacity against voltage in log-log coordinates intersect.
Table 2. Microstrip detectors characteristics
Set 1996 set 1997 set 1998 set
Type I I II Double-sided
Design
Ion impl. single-sid.
AC coupled structure
with polysil. resistors
and with oxide passivation
Identical Identical
Ion impl. double-sided
AC coupled structure
with polysil. resistors
and with oxide passivation
Technology 4 inch silicon Identical Identical Identical
Si3N4 layer + + - -
Active area 40×75 mm2 Identical Identical Identical
No of channel 750 750 750 750+750
Pitch 100 µm 100 µm 100 µm 100 µm
Interstrip
Capacitance 4 pF 4 pF 4 pF 5 pF
Interstrip
resistance 3 GOhm 3 Gohm 100 Gohm 43 GOhm
(30 GOhm,10 rad)
Strip leakage
current (FD) 20 nA 20 nA 1 nA <2 nA
(<5 nA, 10 krad)
Biasing
resistor 1.8 MOhm 1.5 MOhm 1.5 MOhm 30 MOhm
Coupling
capacitors 200 pF 200 pF 300 pF 120 pF
Capacitor
breakdown >100 V >100 V >25 V >60 V
Number of
dead strips
(leakage
current)
<1% <1% <1% <1%
Number of
broken
capacitors
<1% <1% <3% <2%
Number of
work resistors 100% 100% 100% 100%
30
Fig. 5. Leakage current (voltage on the integrated resistors) and integrated resistor value distributions for 750
strips detector.
Fig. 6. Bulk capacity against the bias voltage for
different integral doses.
Fig. 7 shows the full depletion voltage against the
irradiation dose. The changes of the full depletion
voltage are slight for the doses expected in the ALICE
experiment. One observes no change of the conductivity
type in the range of doses under study.
5.2. LEAKAGE CURRENTS
5.2.1. INCREASE OF LEAKAGE CURRENTS
OF THE DOUBLE-SIDED MICROSTRIP
DETECTOR AFTER 10 krad DOSE
The radiation tolerance of the double-sided
microstrip detector was studied for a 10 krad dose using
20 MeV electrons. The leakage current increases from 2
up to 5 nA per strip (Fig. 8).
Together with the detector a silicon specimen-
witness, having characteristics similar to those of the
detector, was irradiated. Using the specimen-witness
one has the possibility to control the conditions of
irradiation by comparing the measured radiation damage
constant Кτ with the results obtained previously. It is
well-known, that the 1/τ=f(D) dependence in a wide
dose range has a linear character that is very convenient
for determining the damage constant Кτ. The irradiation
Fig. 7. Full depletion voltage for different
irradiation dose. O, ð , ∆, +– are here the data for four
different detectors.
of the specimen-witness was performed also with the
aim to estimate the influence of surface and other
effects on the change of leakage current during
irradiation. The 8 mm thickness of the silicon specimen
was selected sufficiently high so that the lifetime of
minor charge carriers τNCC can be measured without
taking into account the surface influence of the
commonly used contactless RF-method. At the same
time, the silicon thickness was sufficiently small for
neglecting the electron energy losses over the specimen
thickness under irradiation.
The lifetime of minor charge carriers in the detector
material bulk was obtained from the relationship (1) in
the prediction that the total leakage current is
determined by the generation in the bulk material.
It is seen from the Fig. 8 that the initial lifetime of
minor charge carriers in the detector material,
determined by the leakage current, is less than the initial
one in the silicon specimen-witness. As is well-known
the decrease of the τ value in a detector is due to the
leakage current which appears because of surface
31
imperfection and because of additional recombination
centers in Si in the process of detector construction
Fig. 8. Leakage current (1), inverse lifetimes (2) in
DSMD and inverse lifetime (3) in the silicon specimen
as a function of the dose.
In Fig. 8 are plotted the inverse lifetimes 1/τ in the
silicon specimen and the detector as a function of the
irradiation dose. The damage constant for the silicon
specimen determined from the dependence of the
inverse lifetime on dose is in good agreement with the
previous results (Table 1 and [2]).
The comparison of the values of the slope of 1/τ
=f(D)dependences show that the radiation resistance of
the detector is higher than that of the initial silicon.
Probably, it can be explained by the heterogeneity of the
detector surface as well as by the annihilation centers in
the material bulk created in the process of technological
treatment.
5.2.2. LEAKAGE CURRENTS OF SINGLE-
SIDED DETECTORS AT HIGH IRRADIATON
LEVEL
The leakage currents for single-sided microstrip
detectors (p+-side of the double-sided detector) were
studied in wider dose intervals. Single-sided detectors
with and without additional layer of Si3N4 insulator
were made in one set, using the same thickness for the
SiO2 insulator. The additional Si3N4 layer allows a
coupling capacitor breakdown voltage larger than 100 V
and capacitor yield larger than 99 percent. However, the
leakage current for detectors with double layer insulator
is about 20 nA per strip while the leakage current for the
single layer SiO2 insulated detectors is only 2 nA (Table
2). The 20 nA leakage current leads to additional noise,
450 electrons when the ALICE 128C electronics with a
peaking time of 1.4 microseconds is used. At a 1 nA
leakage current the noise is 100 electrons. The ENC for
an input capacitance of 5 pF is 300 electrons.
Perhaps, the increase of the leakage current for the
detectors with the silicon nitride is associated with the
increase of the surface generation-recombination centers
on the open surface, and, especially, over the strip
perimeter at the contact of the open surface and p+-strip.
The leakage current dependence on diode dimensions
may support this viewpoint. Increasing the sensitive
zone of the diode, the leakage current normalized by the
unit of sensitive area decreases for diodes with Si3N4.
Increasing the sensitive zone area, the ratio of the
open surface of the detector (surface between the p+-
guard ring and p-implanted zone of the diode) to the
sensitive area of the diode decreases. The ratio of the
perimeter to the area of the sensitive zone also
decreases. For the diodes without additional Si3N4
insulation layer the leakage currents normalized by the
unit area are practically constant. Moreover, the value of
the normalized current for the diode coincides with the
one for the microstrip detector.
Fig. 9. Leakage currents variation for microstrip
detectors with (∇) and without Si3N4 (� ).
Fig. 9 shows the variation of leakage currents for
microstrip detectors with and without Si3N4 under
irradiation with 20 MeV electrons. One sees that these
dependences are almost linear. Furthermore, the initial
difference in current of ~20 nA observed for the
detector with Si3N4 persists over the entire range of
doses. It has to be pointed out that the large initial
difference gets relatively much smaller at large
irradiation doses. Fig. 10 shows the distribution of
leakage currents for microstrip detectors with Si3N4 and
without it before irradiation and after irradiation with
the maximum dose. One sees that a good uniformity of
the distribution of leakage currents persists after the
maximum irradiation dose.
32
Fig. 10. Leakage currents distributions before (1, 2)
and after (3, 4) irradiation. 1, 3 - single-layer and 2, 4 -
double-layer insulation.
It was shown that for single-sided silicon microstrip
detectors on 4000 Ohm·cm n-type silicon the use of the
Si3N4 insulation layer increased the leakage current from
1 nA/strip to 20 nA/strip. However, this increase in
leakage current is small compared to the increase of
leakage current induced by a less that 1 Mrad irradiation
doses. After a 2 Mrad dose the leakage currents of both
types of detectors were approximately 250 nA/strip
5.3. INTERSTRIP RESISTANCE
Fig. 11 shows the interstrip resistance for detectors
with single-layer and double- layer insulation against
irradiation dose. The same figure shows the variation of
the ratio of interstrip resistances for the detectors with
single-layer and double-layer insulation.
Fig. 11. Interstrip resistance for the detectors
without (1) and with (2) Si3N4. Interstrip resistances
ratio for the detectors with double-layer and single-
layer insulation (3).
Naturally, the surface conductivity, created by
surface generation-recombination centers in the
detectors with additional insulation, decreases the initial
value of the interstrip resistance. However, one sees that
the surface effect on the interstrip resistance also
becomes insignificant already at irradiation
doses < 1 Mrad.
5.4. INTERSTRIP AND TOTAL STRIP
CAPACITANCE
The large strip pitch (the strip pitch 100 µm and
interstrip gap 60 µm) of the detector provides for small
value of the interstrip and total strip capacitances.
Perhaps, owing to the same reason, the interstrip
capacity and total capacity of the strip show weak
dependence on the irradiation dose.
6. CONCLUSIONS
The technology of production of DSMD at the
KhIPT has been mastered. Test of the static
characteristics on the first batch of ALICE prototype
detectors has been made, as well as a study of the
radiation damage under a 10-years ALICE equivalent
dose.
The main results show that:
1. The number of defective strips does not exceed
3%.
2. The leakage current increases from 2 up to 5 nA
per strip and the interstrip resistance decreases from 43
down to 30 GOhm after a 10 krad irradiation dose. The
other DSMD features remain unchanged under
irradiation.
Having in mind that the first batch was small and
experimental, we have been starting a second batch to
get more representative estimates both on the
production yield and on the detector efficiency.
ACKNOWLEGEMENTS
The authors are very thankful to many colleagues for
the valuable discussions and constructive remarks,
especially P. Giubellino and O. Runolfsson. This work
was supported by INTAS under the Grant 96-0678.
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9. I.D. Konozenko, A.K. Semenyuk and V.I. Khivrich.
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34
National Science Center “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine
PACS: 29.40.Wk
1. INTRODUCTION
2. DETECTOR DESIGN
2.1. DOUBLE-SIDED DETECTOR
2.2. TEST MICROSTRIP STRUCTURES
3. PROCESSING OF THE PROTOTYPES
3.1. SINGLE-SIDED DETECTOR PROCESSING
3.2. DOUBLE-SIDED DETECTOR PROCESSING
ACKNOWLEGEMENTS
REFERENCES
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