Novel Silicon Stripixel Detectors on High Resistivity
p-type Magnetic Czochralski Silicon Wafers for USATLAS Upgrade
Zheng Li, D. Lissauer, D. Lynn, P. O’Connor, and V. Radeka
Abstract—a new Si detector system based on a number
of novel aspects of detector structure and detector material is
proposed here for the US ATLAS Upgrade Project. The novel
aspects in this Si detector system include: 1) a novel Si stripixel
detector structure developed at BNL will be used, which provides
2d position sensitivity with single-sided process; 2) high
resistivity magnetic Czochralski (MCZ) Si wafer will be
used, which provides naturally high concentration of
oxygen for improved radiation hardness to charged
particles; and 3) p-type MCZ substrates are used, for
which single-sided process is retained and fast electron
collection is presented, and which experiences no space
charge sign inversion to ensure partial depletion mode
operation at extremely high radiation fluences. In the first
detector design layout, strip pitches in both y and u
directions have been fixed at 80 µm, with a stereo angle of
4.6° between them. The strip length is 3 cm and the
detector chip size is 2.56 cm×6.0 cm, with 2 halves of 2.56
cm×3.0 cm sensitive areas without dead space between
them. Both y and u strips will be read out by the sides
perpendicular to the strips. Simulations on detector
processing as well as the electric properties before and
after radiation by charged particles up to 1.0×1016 neq (1
MeV neutron equivalent)/cm2 have been done. Simulation
results show that up to 2.0×1015 neq/cm2 of charged particle
irradiation, the highest fluence for the inter-medium
region for strip detector in the SLHC, the new detector
with a thickness of 200 µm can still be fully depleted at a
bias of 400 volts.
I. INTRODUCTION
Recently, a novel detector structure based on interleaved
pixel electrodes arranged in a projective x-y readout has been
developed at BNL [1-2]. With this novel detector structure,
named as “stripixel”, a 2d-position sensitive detector can be
Manuscript received on October 18, 2004.
This research was supported by the U.S. Department of Energy: contract No:
DE-AC02-98ch10886
Zheng Li is with Brookhaven National laboratory, Upton, NY 11973-5000,
USA (telephone: 631-344-7604, e-mail: zheng@bnl.gov).
D. Lissauer is with Brookhaven National laboratory, Upton, NY 11973-5000,
USA (telephone: 631-344-4864, e-mail: lissauer@bnl.gov).
D. Lynn is with Brookhaven National laboratory, Upton, NY 11973-5000,
USA (telephone: 631-344-4560, e-mail: lynn@rcf.rhic.bnl.gov).
P O’Connor is with Brookhaven National laboratory, Upton, NY 11973-5000,
USA (telephone: 631-344-7577, e-mail: Oconnor2@bnl.gov).
V. Radeka is with Brookhaven National laboratory, Upton, NY 11973-5000,
USA (telephone: 631-344-4266, e-mail: Radeka1@bnl.gov).
made with single-sided process using double-metal
technology. Two prototypes of stripixel detectors had been
fabricated at BNL for the PHENIX Upgrade at RHIC [3-5].
Laser, electron source and beam tests on these prototype
detectors have shown 2d-position sensitivity with resolutions
of 25 µm in both x and u directions (both are 80 µm in pitch,
and a stereo angle of 4.6°) [4].
For applications in SLHC, where the luminosity will be
increased by a factor of 10, the stripixel detector may be a
good candidate in the inter-medium strip detector region.
Since in this region in the SLHC, the detector strip length may
be limited to a few cm’s in length due to occupancy
requirement. This is an excellent fit for the stripixel detectors
since short strip length will compensated the increase in strip
capacitance due to the interleaving scheme in stripixel
structure [2]. The simplicities in processing, assembly, and
readout in the stripixel detector can result in significant
reductions in costs in material, processing and readout
scheme. Another key issue for the SLHC with increased
luminosity is the radiation hardness of the detectors. The
highest charged particle fluence in the inter-medium region for
strip detectors is about 2.0×1015 neq (1 MeV neutron
equivalent)/cm2. One of the main research approaches to
improve Si sensor radiation hardness is to oxygenate Si by
introducing oxygen into Si during the detector processing [610]. With its natural high oxygen concentration (over 1018
/cm3), high resistivity (≥ 1 kΩ⋅cm) Magnetic Czochralski
(MCZ) Si detectors are a candidate for more improved
radiation hardness [11-12]. In this work, a new detector
system using the novel stripixel structure with p-type MCZ Si
substrate is proposed for the US ATLAS Upgrade for SLHC.
With p-type substrate, the single-sided process advantage in
stripixel processing is preserved (as compared to n on n,
which is two-sided process). In addition, electrons will be
collected with fast rate, no space charge sign inversion will
take place at any radiation level, and detectors can work in
partial depletion mode after extremely high fluence radiation.
Moreover, recent date on oxygenated p-type strip detectors
have shown improved charge collection efficiency, by a factor
of more than 2, over its n-type counterpart [13].
Detector design, detector chip and bonding layout, as well
as simulations in detector processing and electric properties
before and after irradiation will be given.
II.
DETECTOR DESIGN AND CHIP LAYOUT
0-7803-8701-5/04/$20.00 (C) 2004 IEEE
As shown in Fig. 1, the basic stripixel concept is to divide
an ordinary pixel into two parts: the x (or u)-pixel and y-pixel
and read x and y pixels by x and y strips in a projective way.
In order to ensure a signal in both x and y pixel when a
particle passing through the detector, the two parts within a
pixel should be interleaved in such a way that the maximum
distance between them any where should be smaller than the
full width at half maximum (FWHF) for diffusion cloud
during the drifting of the induced charge [2]. For the single
cell design for US ATLAS Upgrade, the pixel pitches are 620
µm in x and 50 µm in y. The y-strip pitch and u-strip pitch are
both 50 µm. The stereo angle between u and y strips is 4.6°.
The line widths for both x and y-pixels are 5 µm, with a
gap of 3.33 µm between them. The maximum distance
between x and y pixels is thus 8.33 µm, less than FWHM = 9
µm for the diffusion cloud [2]. The detector is to be made on
p-type MCZ Si with resistivity of about 3 kΩ-cm.
Fig. 2 shows the schematics of detector processing and
processing parameters. A uniform field (or spray) p+ implant
will be made on the front side to provide channel separation
between the n+ pixels to be implanted later. With this field
implant, only five mask steps are needed for the detector
processing of this new detector system.
u (or Y)-strips
X-strips
u (or Y)-strip
Stereo
angle
X-strip
X-pixel pitch
Y-pixel pitch
Fig. 1 Schematics of the stripixel Si detector system for US ATLAS Upgrade
2 µm
3000 Å
4500 Å
1000 Å
2000 Å
half are separated, there is no guard ring between them;
therefore there is no dead space. There are 512 u strips and
512 X strips in each half. Both u and X strips are read out
from the side using the same double metal process technology
already existed in the stripixel processing (no 3rd metal
needed). The readout lines are grouped in 128 strip bunches,
with 48 µm bonding pitch with stacked bonding pads of 80
µm× 300 µm in size.
III.
n+ implant:
1014/cm2
p+ implant:
1012/cm2
Fig. 2 Schematics a parameters for the processing for the stripixel Si
detector system for US ATLAS Upgrade
Fig. 3 shows the schematic of the wafer layout. On a 4”diameter wafer, there are two detector chips with the size of
2.56cm x 6.0cm each. Each detector chip consists of two
halves, each with a size of 2.56cm x 3.0cm. Strips (both X and
u) in two halves are not connected, which results in a strip
length of about 3.0 cm for both X and u strips in each half.
However, the gap between the two halves is the same as that
between X strips (and that between u strips) at 3.33 µm.
Shown in Fig. 4 is the left half of a detector chip with
chip size of 2.56 cm × 3.0 cm. The right half of the chip is a
mirror image of the left along the mirror line indicated in the
figure. Although the strips in the left half and those in the right
DETECTOR SIMULATIONS
Simulations before irradiation and after irradiation by
charged particles up to 1.0×1016 neq/cm2 have been performed
on novel stripixel Si detectors made on p-type high resistivity
MCZ wafers. The introduction rate of radiation-induced
negative space charges is obtained from ref. [12]. Fig. 4 shows
the detector electrical properties after being irradiated charged
particles to 2.0×1015 neq/cm2, the highest fluence for the strip
detectors in the inter-medium region in SLHC. It is clear that,
even after such high fluence of irradiation by charged
particles, the detector can still be fully depleted at a modest
bias voltage of 400 volts. Further simulations have shown that,
for higher fluences than 2.0×1015 neq/cm2, the detector may be
deleted at higher bias voltages (<1000 volts). However, micro
breakdown along the surface may occur before such biases
being reached. In this case, the detector can be operated at
partial depletion mode with lower biases (400 volts<V<1000
volts) with significant fraction of sensitive volume. However,
0-7803-8701-5/04/$20.00 (C) 2004 IEEE
at fluences close to 1.0×1016 neq/cm2, the trapping effect will
dominate, and is the limiting factor for detector charge
collection efficiency (CCE), and other approaches may be
needed in addition to the approach proposed here.
3cm (48 × 620µm)
u (Y)-strips
2.56 cm
(512 × 50µm)
Chip #1
Chip #2
X-strips
3.33 µm
No dead space
100 mm dia.
Fig. 3 Detector wafer layout for the stripixel Si detector system for US ATLAS Upgrade. The gap between left and right halves on a detector chip is only 3.33 m
to eliminate dead space (shown bigger in the figure for better viewing)
IV.
DETECTOR DESIGN
There are five mask steps in the double metal processing
for the prototype stripixel detectors for US ATLAS Upgrade.
Shown in Fig. 6 is the mask design layout of one detector chip
(half wafer). Both u and Y strips are routed to the top and
bottom sides for wire bonding. 16 readout chips with 128
channels each will be used to read out all u and Y strips. As
shown in Fig. 7, the bonding pads are 48 µm in pitch and
stacked in two rows to get the maximum pad width of 70 µm.
The length of the bonding pads is 300 µm.
Fig. 8 shows the contact vias for both u and X strips
between the 1st Al layer and the 2nd Al layer. The insulation
layer used between the 2 Al layers will be polyimide with a
thickness of about 2 µm. Note here that the width of the via is
only about 5 m, extremely small taking into account the
polyimide thickness. Therefore, extreme caution and special
care should be taken during the processing: if the etching time
of polyimide is too short, then the via may not be open all the
way to the 1st Al, and there will be no contact made to the
strips; if the etching time is too long, then the size of the via
may be too large, resulting in the shorting of u and X strips. In
our experience, the difference between the short time and the
long time is not much, and we do not have too much margin
of error to play with.
Shown in Fig. 9 is the detailed connection scheme
between the Y pixels in the formation of u-strips. While X
pixels in the same row are connected by the 1st Al layer to
form the X strip, the Y pixel in one row is connected to the Y
pixel in the row above it and the next column on the right. In
this way, a stereo angle of 4.6º is formed between X and u
strips. The connection between the two adjacent Y pixels is
made by a connector made by the 2nd Al through the via on
each Y pixel. The connection line is made with minimum
length (about 10µm in current design) to minimized unwanted additional capacitance.
V.
DETECTOR PROCESSING PLAN
By the end of July 2004, the design of the full mask set
has been completed. The mask set was ordered, and it was
delivered in the beginning of September, 2004. The
0-7803-8701-5/04/$20.00 (C) 2004 IEEE
preparation for the fabrication of the first batch of the
prototype stripixel detectors for US ATLAS Upgrade is now
underway, which includes the oxidation of MCZ Si wafers
and the generation of a detector processing receipt. The actual
processing will begin in the beginning of November 2004,
with a delivery date of detectors set for the end of January
2005. Electrical tests, including current-voltage and
capacitance-voltage measurements on test structures and
stripixel detectors will be carried on soon after. Charge
collection tests on the first batch prototype Si stripixel
detectors for US ATLAS Upgrade will be made using a laser
(transient current technique) and by an electron source, before
and after proton radiation. Test results will be published
elsewhere. Improvement will be made on the design of the
second prototype batch base on the test results, and the
processing of the second prototype will probably start in the
Summer of 2005.
Quarter wafer
Side bonding pads ---- zero dead space between two chips
X-bonding
pads
u-strips
X-strips
u-bonding pads
Fig. 4 Detector chip and readout layout for the stripixel Si detector system for
US ATLAS Upgrade
US ATLAS Upgrade
Electric simulation on the
Novel n on p and n on n
2d-sensitive Si Stripixel
detectors on MCZ wafers
For US-ATLAS Upgrade
After 2x1015 neq/cm2
radiation
X-strip
1st Al
Pixel #2
400 volts
Hole concentration
Pixel #3
620 µm
Potential profile
400 volts
Fully depleted at
400 volts
50 µm
Electric field
Pixel #1
Pixel #4
Y-strip
2n Al
Fig. 5 Detector electrical simulations after irradiation for the stripixel Si detector system for US ATLAS Upgrade
0-7803-8701-5/04/$20.00 (C) 2004 IEEE
Detector Chip (half wafer)
Connections between the Y pixels for the formation of u-strips
US ATLAS Stripixel Detector: 2 halves of 48×512 array of 620 µm×50 µm pixels
Interleaving pitch: 8.33 µm, 5 µm line width
512 u and 512 X strips/half with 50 µm pitch and 4.6 stereo angle
Detector size: 6.37 cm×2.98 cm
Sensitive area: 5.97 cm×2.58cm
X Pixel
Y Pixels
u connectors
X strip contacts
Left half
Right half
Fig. 6 ACAD mask design layout of one detector chip for the stripixel Si
detector system for US ATLAS Upgrade
Fig. 9 Connection scheme between adjacent Y pixels for the formation of u
strips.
V. REFERENCES
70 µm
[1]
[2]
300 µm
7 µm
130 µm
Z. Li, BNL Internal Report, BNL #67527, June 10, 2000
Z. Li, “Novel Silicon Stripixel Detector: Concept, Simulation, Design,
and fabrication”, BNL 71393-2003JA, Nucl. Instr. & Meth. A 518, Vol.
3, (2004) pp738-753.
[3] Z. Li et al., “Novel Silicon Stripixel Detector for PHENIX Upgrade”,
presented on 9th Pisa Meeting on Advance Detectors, May 25-31, 2003,
La Biodola, Isola d’Elba, Italy, Nucl. Instr. & Meth. A 518 (2004) 300304.
[4]. J. Tojo et al., “Development of a Novel Silicon Stripixel Detector for
RHIC-PHENIX Detector Upgrade”, presented on IEEE Nucl. Sci.
Symp., October 19-25, 2003, Portland, Oregon, to be published in IEEE
Trans. Nucl. Sci.
[5] Z. Li et al., “Development of 2nd prototype of Novel Silicon Stripixel
Detector for PHENIX Upgrade”, presented on 10th Vienna Conference
on instrumentation, Feb. 16-21, 2004, Vienna, Austria, accepted by
Nucl. Instr. & Meth. A.
270 µm
[6]
48 µm
Fig. 7 Design layout of the bonding side bonding pads for the stripixel Si
detector system for US ATLAS Upgrade
[7]
Contact vias for strips between 1st and 2nd Al layers
u strip contacts
[8]
[9]
Z. Li, H. W. Kraner, E. Verbitskaya, V. Eremin, A. Ivanov, M.
Rattaggi, P. G. Rancoita, F. A. Rubinelli, S. J. Fonash, C. Dale, and P.
Marshall, “Investigation of the Oxygen-Vacancy (A-Center) Defect
Complex Profile in Neutron Irradiated High Resistivity Silicon Junction
Particle Detectors”, IEEE Trans. Nucl. Sci., Vol. 39, No. 6, (1992) pp
1730-1738.
A. Ruzin and CERN Rd48 Collaboration, “Recent results from the RD48 (ROSE) Collaboration” Nucl. Instrum. & Meth. A447, (2000)
pp116-125
RD48 Status Report, CERN/LHCC 2000-009, December 1999.
G. Lindstroem, and CERN RD48 Collaboration, “Radiation hard silicon
detectors––developments by the RD48 (ROSE) collaboration”, Nucl.
Instrum. & Meth. A466, (2001) pp308-326.
[10] G. Lindström, “Radiation Damage in Siliocon Detectors”, Nucl.
Instrum. & Meth. A512 (2003) pp30-43.
[11] J. Harkonen et al., on MCZ.
[12] Z. Li et al, “Radiation Hardness of High Resistivity Magnetic
Czochralski Silicon Detectors after Gamma, Neutron and Proton
Radiations”, IEEE Trans. Nucl. Sci. Vol. 51, No. 4, August (2004)
pp1901-1908.
X strip contacts
Fig. 8 Contact vias and routing of X and u strips
[13] G. Casse et al., “Performances of miniature microstrip detectors
made on oxygen enriched p-type substrates after very high
proton irradiation”, presented at the Vienna Conference of
Instrumentation,
2004,
submitted
to
NIM
A.
0-7803-8701-5/04/$20.00 (C) 2004 IEEE