Diamond Pixel Modules for the High Luminosity
ATLAS Inner Detector Upgrade
ATLAS Upgrade Document No:
Institute Document No.
Created: 15/05/2007
Page: 1 of 14
Modified: 21/12/2007
Rev. No.: 1.8
Abstract
The goal of this proposal is to construct diamond pixel modules as an option for the ATLAS
pixel detector upgrade. This proposal is made possible by progress in three areas: the
recent reproducible production of high quality polycrystalline Chemical Vapour Deposition
diamond material in wafers, the successful completion and test of the first diamond ATLAS
pixel module, and the operation of a diamond after irradiation to 1.8x1016 p/cm2. In this
proposal we outline the results in these three areas and propose a plan to build 5 to 10
ATLAS diamond pixel modules, characterize their properties, test their radiation hardness,
explore the cooling advantages made available by the high thermal conductivity of diamond
and demonstrate industrial viability of bump-bonding of diamond pixel modules. Based on
availability and size polycrystalline Chemical Vapour Deposition diamond has been chosen
as the baseline solution. The use of single crystal Chemical Vapour Deposition diamond is
reserved as a future option if the manufacturers can attain sizes in the range 16mm x 16mm.
Contact Person: Marko Mikuž (marko.mikuz@cern.ch)
Prepared by:
Checked by:
H. Kagan (Ohio State University)
M. Mikuž (Jožef Stefan Institute, Ljubljana)
W. Trischuk (University of Toronto)
Distribution List
ATLAS High Luminosity Steering Group
Approved by:
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Introduction
The super Large Hadron Collider (sLHC) is designed to increase the luminosity of the LHC by a
factor of ten to 1035 cm-2s-1. In sLHC scenarios the total expected fluence at a radius of about 5cm will
exceed 1016 particles/cm2[1]. Several studies are now being performed to find solutions for detectors
which have to operate in these radiation environments. So far it is found that at such high fluences the
operational conditions are extreme. The availability of a very radiation hard detector material and
electronics will be a necessity in view of the future luminosity upgrades planned for the sLHC. Chemical
Vapour Deposition (CVD) diamond is one material to consider for such detectors.
During the last two years polycrystalline CVD (pCVD) diamond material became readily
available, in the form of wafers. The collection distance measure of the quality of material from these
wafers routinely exceeds 300m. Figure 1 shows three such diamond wafers with as-grown collection
distances of 315m, 310m and 305m respectively. A few years ago difficulties were encountered in
the processing of the samples at Element Six [2]. In some cases, a loss of charge collection was observed
after the final processing which depended on the details of the processing technique. Element Six has
fixed this problem. After characterizing four full wafers, all indications are that Element Six can
reproducible grow and process high-quality detector-grade CVD diamond material.
Figure 1: Photograph of the growth side of three full 12cm diameter wafers metallized with gold contacts
1cm apart for testing. The collected charge was measured at each contact on each wafer using a 90Sr
source in the laboratory. The largest collection distance on these wafers is 315m, 310m and 305m
respectively. The rightmost figure shows where sensors have been cut out near the edge of the wafer.
These sensors have comparable quality to those taken from nearer the centre of previous wafers.
The production of high-quality polycrystalline CVD material has allowed RD42 collaborators
(developing and characterizing detectors) in collaboration with the Bonn University ATLAS group
(developing front end electronics) and the Fraunhofer Institute for Reliability and Microintegration (IZM)
[3] (performing bump-bonding) to construct new pixel detectors. This collaboration constructed the first
full diamond ATLAS pixel module (46,080 channels) using diamond from one of the wafers described
above, with the final ATLAS pixel rad-hard electronics, and tested it in the ATLAS test beam at CERN.
During this test we found that all 16 chips worked, the noise in the module was <140e, the module could
be operated at thresholds <1500e, observed hits in over 90% of the pixels and mapped out the beam
profile. Unfortunately the test beam was cut short before the complete module was tested. The module
was then transported to DESY and tested in its 6 GeV electron beam. Again the module performed quite
well with a noise of 136e, threshold of 1450e and an in-time efficiency of >97%. In the fall 2006 we
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moved the diamond pixel module to CERN and tested it in the 120 GeV pion beam. Once again we
observed that the module performed quite well with a noise of 136e, threshold of 1450e. The data is
presently being analyzed by Markus Mathes in Bonn.
As part of the radiation hardness program we irradiated the highest quality polycrystalline CVD
diamond at that time (ccd=215m) to a fluence of 1.8x1016 p/cm2 [4]. After this irradiation the diamonds
were characterized in the SPS test beams. We observed that after 1.8x1016 p/cm2 operating at an electric
field of 1V/m the diamonds still function well with 25% of the un-irradiated charge and operating at an
electric field of 2V/m they yielded 33% of the un-irradiated charge. As a result of these signal
measurements we expect a S/N for a full ATLAS Pixel module after 1.8x1016 p/cm2 of >15:1 (details are
in section 4). In 2007 we irradiated the diamond pixel module to 1.5x1015 p/cm2 and characterized it in
tests beams at CERN. This data is presently being analysed.
In addition to having the necessary radiation tolerance diamond sensors have proven easy to
handle, metalise, have high thermal conductivity and low leakage current. The latter properties
significantly reduce the shot noise and there is no possibility of thermal runaway and hence allow much
simpler cooling schemes to be implemented. This will be one of the focii of the R&D proposed here.
2
Participating Institutions
The proposal is submitted by ATLAS institutions with expertise in the development of radiation
hard CVD diamond sensors and detector prototypes. Many of these institutions are long-time members of
the RD42 diamond detector collaboration. We expect that in the future more institutions will join this
project. The following is a list of institutions who have agreed to participate at this time.
CanadaCarleton
D. Asner;
Toronto
M. Cadabeschi, D. Tardif and W. Trischuk;
Germany
Bonn
M. Cristinziani, M. Mathes, J. Velthuis and N. Wermes;
Slovenia
Jožef Stefan Institute A. Gorišek and M. Mikuž ;
Switzerland CERN
B. DiGirolamo, H. Pernegger and S. Roe;
U.S.A.
Ohio State
S. Cline, K.K. Gan, H. Kagan, R. Kass and S. Smith.
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3 The Status of pCVD Diamond Detector Prototype R&D
Over the last few years, we have worked closely with Element Six (now Diamond Detector Ltd.)
to achieve major improvements in the quality and uniformity of CVD diamond. The pCVD diamond
growth recipes have been migrated to production reactors. The measured collection distance and pulse
height distribution, using a 90Sr source, of one point on an as grown wafer is shown in Figures 2 and 3. To
obtain these distributions metal contacts were placed on each side of the diamond wafer 1 cm apart. The
mean charge observed is 11,340e, the most probable charge is ~8000e and 99% of the distribution is
above 4000e. The mean charge observed, <Q>, is related to the collection distance, d, the average
distance the electron and hole move apart by:
d = <Q>[e] / 36e/m
where 36e/m is the mean number of electron-hole pairs generated by a minimum ionizing particle
along a 1m path in diamond. The mean charge of 11,340e corresponds to a collection distance of
315m.
Figure 2: Measured charge collection distance as a function of electric field for a point on an as-grown wafer
1.3mm thick.
.
Figure 3: The pulse height distribution of the diamond measured with a 90Sr source in the laboratory. The data is
collected using a scintillator trigger. The upper histogram is the observed pulse height with 0V applied to the
diamond. The lower histogram is the observed pulse height at an electric field of 0.7 V/m.
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The diamonds used in our tests are cut from wafers and then thinned to approximately 500m
thickness. One measure of the reliability of this processing is the range of collection distances observed in
final production parts. In Figure 4 we show the collection distance of the last 26 diamonds purchased
from Element Six by RD42 measured in the lab with a 90Sr source. The collection distance ranges from
205m to 255m.
Figure 4: Collection distance from production diamonds received by RD42
In Figure 5 the leakage current versus electric field is shown for a sample, which had the standard
mechanical surface processing before contacts were applied. The contacts on both sides were dots with
guard rings around them. The leakage current is of the order of tenths of a picoamp up to an electric field
of 1.5V/m and nearly symmetric for positive and negative voltages. Polycrystalline CVD diamond
sensors have been observed with erratic dark currents [5] but this is not a problem for the detection of
single particles or when they are operated in a magnetic field.
Figure 5: I-V curve for a surface processed pCVD sample.
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3.1 Results from the First Diamond ATLAS Pixel Module
As stated earlier, in collaboration with the Bonn University group and IZM, using high quality
diamond, we constructed the first full diamond ATLAS pixel module, bump-bonded it to the final
ATLAS IBM 0.25 m rad-hard electronics, and tested the assembly at CERN and DESY. In the normal
production of an ATLAS module, a silicon wafer is laser cut after bump-bonding. However, laser cutting
a diamond wafer after bump-bonding graphitises the surface [6]. Thus we decided to first remove the 2cm
x 6cm diamond part from the wafer, perform the photo-lithography and then install the diamond in a
carrier for bump-bonding. In Figure 6 we show a close-up view of the metal pixel pattern on the diamond
after the under-bump metal has been applied.
Figure 6: Photograph of the ATLAS diamond pixel pattern after the under-bump metal is deposited.
In Figure 7 we show the final diamond pixel module with 16 pixel integrated circuit readout chips
ready for external cables and testing. This module was dressed in Bonn (Figure 8) and tested at CERN in
the ATLAS stand-alone beam using the Bonn ATLAS telescope for external tracking. We were able to
obtain an image of the beam in the ATLAS diamond pixel module.
Figure 7: Photograph of (a) the detector side and (b) the electronics side of the first ATLAS pixel module.
Figure 8: Photograph of the fully dressed diamond ATLAS Pixel Module ready for test.
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In Figure 9 (left) we show the hit map in the diamond pixel module in the CERN test-beam for
events which triggered the pixel module. A clear beam image is evident indicating that all 16 chips work.
The missing stripe in the CERN data is in the ganged pixel region where tracking information is required
to unravel the pixel ambiguity introduced by the ganging of multiple pixels to a single electronics
channel. During this test no tracking information was available. In this plot ambiguous hits are not
displayed. To continue these tests the diamond module and ATLAS telescope were moved to DESY.
Figure 9 (right) shows the corresponding hitmap of the same module at DESY.
Figure 9: ATLAS pixel module hit maps in the (a) 180 GeV pion beam at CERN and (b) the 4-6 GeV electron
beam at DESY. At DESY, with telescope tracking, one observes the edge of the scintillator trigger and that the
ganged pixel region has been resolved.
Figure 10 shows the bare module threshold and noise obtained in laboratory tests before the
diamond detector was attached. We observe an average noise of <140e, a threshold of 1500e and a
threshold spread of 26e (using TurboDAQ).
Figure 10: The electronics threshold and noise of the bare pixel module measured in the laboratory before the
diamond pixel sensor was attached.
Figure 11 shows the full module threshold and noise obtained in laboratory tests before the DESY
test-beam. The noise, threshold and threshold spread are unchanged from the bare module tests.
Compared to an ATLAS silicon pixel module the diamond pixel module exhibits smaller noise (137e vs
180e), operates at low threshold (1450e), has a lower threshold spread (25e vs. 60e) and requires no
cooling. Figure 12 shows the tracking results obtained at DESY. The spatial resolution observed is
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23m in the x-view where the pixels have 50m pitch and a characteristic “top-hat” distribution in yview where the pixels are 400 and 600m long. In the low energy 4-6 GeV beam at DESY the spatial
resolution is dominated by multiple scattering. For example the silicon detectors in the telescope have an
observed resolution of roughly 11m per plane at CERN and 30m per plane at DESY. Multiple
scattering also affects the measured efficiency since some events with poor tracking are included. At
present the analysis of the low energy DESY test indicates the in-time efficiency has a lower limit >97%
(see Figure 13). To make a better determination of the efficiency requires more analysis and/or a high
energy test-beam run. This analysis is under way. Even with the incomplete analysis at hand it seems
evident that this generation of pixel detectors are suitable for applications at the sLHC.
Figure 11: The full diamond ATLAS pixel module results for threshold and noise at DESY.
Figure 12: (a) The diamond ATLAS pixel module correlation with the tracking telescope. (b) The diamond pixel
module spatial resolution in the test-beam at DESY. The contribution from multiple scattering dominates the
resolution and has not been unfolded.
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Figure 13: The diamond ATLAS pixel module preliminary efficiency distribution in a tested region of the module
during the 6 GeV DESY test.
This device was tested in Fall 2006 in the 120 GeV pion beam at CERN. During this test the
module noise was again 140e and the module operated at a threshold of 1450e. Figure 14 shows the raw
online hit correlation with the telescope planes for all scintillator triggers. Good correlation between the
module and the telescope planes is already observed at the raw trigger level.
Figure 14: Correlation of the ATLAS diamond pixel module with the tracking telescope in a) y and b) x during the
CERN 2006 test-beam.
The analysis of this data is under way with final results expected in the next month. Preliminary
results from this analysis are shown in Figure 15. The analysis to date has shown that during this run the
tracking telescope was not working optimally and had an extrapolation error at the diamond module of
11m. We observe a raw position resolution of 18m in the diamond module. This translates into a
module resolution of 14m, a bit under the digital resolution. The spatial resolution is determined by the
amount of charge sharing and will be remeasured in further test beams.
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Figure 15: Preliminary results for the module spatial resolution in the x and y directions from 120GeV pions.
3.2 Radiation Hardness
For the sLHC the RD42 group has shown from previous irradiations that up to 2.2 x 10 15 p/cm2
pCVD diamonds lose at most 15% of their most probable charge [7]. The diamonds in those studies had a
characteristic initial collection distance of about 150 microns. More recently the RD42 group has
increased the irradiation level by nearly a factor of ten and irradiated pCVD diamond samples, with
collection distances of 190 and 215 microns, to fluences of 1.8 x1016 p/cm2 corresponding to a dose of
roughly 500 MRad. In Figure 16 we show the summary of irradiation results when the diamond is
operated at an electric field of 1 V/m and 2 V/m. The damage curve (1/ccd = 1/ccd0+k) is shown
for ccd0=215m diamond indicating a damage constant k=0.7x10-18 m-1cm2. At an electric field of 1
V/m and after irradiation to 1.8 x 1016 p/cm2 the pCVD diamond retains 27% of its original pulse
height. Since the diamond signal is not saturated, one can collect more charge by raising the operating
voltage. Also shown in Figure 16 is the preliminary result when the diamond is operated at an electric
field of 2V/m where ~33% of the original pulse height is retained.
Figure 16: Summary of proton irradiation results for pCVD diamond at an electric field of 1 V/m (blue) and the
measurement at an electric field of 2 V/m (green). The damage curve (1/ccd=1/ccd0+k) is shown.
As indicated above the diamond pixel module, due to its low capacitance and low leakage current
operates at a lower noise level than a comparable silicon detector. We can compare the noise a diamond
pixel module with that of a standard silicon pixel module or a 3D silicon single-chip pixel module all
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operating with the same electronics. During the fall 2006 test-beam at CERN the noise levels for unirradiated devices were 140e for diamond, 180e for silicon and 310e for 3D silicon single-chip module.
Diamonds and diamond modules are operated at ambient temperature without any cooling even after
irradiation. After 2x1016 p/cm2on the sensor, we observe the same capacitance, leakage current and
electronic noise as before irradiation. In Figure 17 we show the signal to noise as a function of fluence
for diamond pixels. The signal data for the diamond is taken from Figure 16 and we have used the noise
figure (140e) above since we see no change in noise with fluence. We expect to observe an S/N ~17 after
1.8 x 1016 p/cm2, a remarkable result after this fluence.
Figure 17: The expected Signal-to-Noise for irradiated diamond pixel modules. The diamond module noise is 140e.
3.3 Room Temperature Operation
The above results were obtained with the diamonds and diamond modules at ambient temperature
without any cooling. After 2x1016 p/cm2on the sensor, we observe the same capacitance, leakage current
and electronic noise as before irradiation. Thus the sensors in a system constructed from diamond
material will not need to be cooled. Of course, the front end electronics will still need cooling. Still this
feature should allow for a mechanical support structure of less material than one which necessarily
includes cooling. This is one major advantage of diamond material.
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R&D Plan and Resources
The results described above were presented at the LHCC meeting at CERN (Jan. 2007) and before
that at the US ATLAS Pixel meeting at LBNL (Mar. 2006). At these meetings we received some
encouragement to propose an R&D project to build and test more modules to ascertain if diamond pixel
modules are a viable solution for the sLHC. We propose to construct and test at least five full diamond
pixel modules. The number of modules we construct is limited primarily by funding and will be adjusted
to available funding. If the new modules perform as expected we would be in a position to construct
more modules. Part of our first year goal is to transfer the metallization technology to IZM so that they
may produce entire modules in house. The proposed path is completely compatible with any changes to
the pixel chip or dimensions since this will only require a new production mask using the same diamond.
This allows us to take advantage of developments in the FE, MCC, and optical chips as they develop. To
achieve an upgrade solution, R&D is needed to verify that the results from the first pixel module are
reproducible, to test the radiation hardness of the entire system, to transfer the technology to industry and
to evaluate the costs to construct an upgrade or b-layer replacement. The following is the proposed R&D
plan. The plan complements other R&D efforts without duplication.
4.1 Securing Diamond
The diamond material will be obtained by the University of Toronto and Jožef Stefan Institute
from Element Six Ltd. Diamond material will be ordered in the roughly 2cm x 6cm size necessary to
construct pixel modules, in 1cm x 1cm test pieces suitable for single chip assemblies and in 5mm x 5mm
pieces for irradiation studies.
4.2 Detector Construction
The cleaning and pattering of the diamonds will initially be performed at Ohio State University
and then shipped to IZM in Berlin for bump-bonding. The procedure used will be the same as that which
we developed for the first diamond ATLAS pixel module [4, 6]. The cost for bump-bonding is expected
to dominate this phase of development and has a large initial charge. In parallel we will transfer the
cleaning and metallization procedures to IZM so that they can produce the remaining modules in house
from raw material.
4.3 Front-end electronics
We already have secured approval from the pixel group for electronics for the second module.
We will require electronics for the remaining modules we produce. We are participating in the purchase
of wafers of FE-I3 pixel chips for these purposes. Our Bonn, Carleton and CERN colleagues will
characterize the front-end electronics to be used in the assembly of the diamond pixel prototype modules.
4.4 Module Preparation
Our Bonn University colleagues will work with industrial partners (such as IZM) to prepare the
bump-bonded sensor/readout chip assemblies. They will also wire-bond and dress the next modules.
4.5 Module Tests
Laboratory tests of the completed module will be performed by the Bonn University, Carleton and
CERN groups. The will be used to understand and correct weaknesses in the assembly process and to
characterise modules before/after irradiations (see below).
4.6 Test-beam Studies
For beam tests we hope to use the Bonn telescope (BAT) at CERN. Ohio State University,
University of Toronto, CERN and Jožef Stefan Institute personnel will join the Bonn University group in
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this endeavour. The CERN group will provide the necessary infrastructure to carry out the test-beam datataking while students in the other groups will participate in the analysis of the test-beam data.
4.7 Irradiations
All components need to be irradiated to the sLHC dosage. This work will be carried out by Jožef
Stefan Institute along with the Ohio State University. We will concentrate on the irradiation using the 24
GeV protons at the PS (CERN) at which our group has extensive experience using the facility for the
development silicon detectors, diamond detectors [7] and the development of the opto-link for the present
pixel system. During the irradiation of the most recent pCVD sensor material, we will measure the
leakage current and monitor the charge collection distance. The irradiations will be staged since the
present electronics is not expected to survive 2x1016 p/cm2.
4.8 Mechanical Support Structures and Intrinsic Cooling
Our University of Toronto colleagues will work in conjunction with the ATLAS pixel group to
design a prototype mechanical system. While we have already assembled two full diamond pixel
modules, it remains to quantify the heat transfer possible through the diamond sensor material to
understand how much additional cooling a stave or sector built of such modules would require. The goal
of this project is to quantify how much the support mechanics could be reduced as a result of the relaxed
cooling requirement of diamond. This system will be compared in terms of material and cost with the
present pixel support system and any other proposed systems.
4.9 Cost Evaluation
Part of our basic research program is to evaluate the cost of constructing a diamond b-layer
replacement or diamond upgrade solution for ATLAS. Thus this evaluation will logically occur after we
have constructed at least 5 modules since the cost evaluation requires the information we will attain in the
construction of the modules. Our experience has been that to minimize the material cost we will need to
have multiple suppliers. Our Ohio State University, University of Toronto and Jožef Stefan Institute
colleagues will lead this effort. They have already begun working with additional diamond suppliers and
are presently testing their material. The second piece of information necessary for a cost evaluation is the
cost of constructing a module. More information will become available as we proceed with 4.2. Finally,
a large part of the cost evaluation involves evaluating the advantage, if any, from the minimal cooling and
support structures required for a diamond solution. This should follow logically from the results of 4.8.
We observed this past year that we are manpower limited in items 4.4 (Module Preparation), 4.5
(Module Tests), 4.6 (Test-beam Studies), 4.8 (Mechanical Support Structures) and 4.9 (Cost evaluation).
We welcome additional groups joining our overall effort and in particular these areas.
5.
Schedule
The schedule of the initial R&D program is summarized in Table I. The program is directed at
answering the questions necessary for a complete proposal. The schedule is determined by funding. The
irradiation dates are best estimates.
Goal
Test of the module #1 at CERN
Irradiation of module #1 at CERN
Test of irradiated module#1 at CERN
Timeframe
Oct 2006 – done
Spring/Summer 2007 – done
Summer/Fall 2007 – in progress
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Construction of module #2, 3
Test of module #2, 3
Irradiation of small samples
Irradiation of modules #2, 3
Test-beam of irradiated modules
Construction of modules #4, 5
Initial cost estimate
Test of modules #4, 5
Irradiation of modules #4, 5
Test Beam of modules #1-5
Construction of modules #6-10
Test of modules #6-10
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Fall 2007/Winter/Spring 2008 – in progress
Spring/Summer 2008
Spring/Summer 2008
Summer/Fall 2008
Fall 2008
Winter 2009
Winter 2009
Spring/Summer 2009
Summer/Fall 2009
Fall 2009
Optional - 2010
Optional - 2010
Table I: Schedule of the initial R&D program.
References
[1] First Workshop on Radiation Hard Semiconductor Devices for Very High Luminosity Colliders,
CERN, 28-30 Nov. 2001; see also the ATLAS Radiation Task Force web page:
http://bosman.home.cern.ch/bosman/Radiation_maps.html.
[2] Element Six Ltd., King's Ride Park, Ascot, Berkshire, SL5 8BP, United Kingdom.
[3] Fraunhofer Institut Zuverlassigkeit Mikrointegration, Gustav-Meyer-Allee 25, D-13355, Berlin,
Germany.
[4] W. Adam et al. (RD42 Collaboration), “Development of Diamond Tracking Detectors for High
Luminosity Experiments at the LHC”, Status Report/RD42, CERN/LHCC 2006-010.
[5] A.J. Edwards et al., “Radiation Monitoring with Diamonds Sensors in BaBar”, IEEE Trans. Nucl.
Sci.. 51 (2004) 1808..
[6] W. Adam et al. (RD42 Collaboration), “The First Bump-bonded Pixel Detectors on CVD Diamond”,
Nucl. Instr. and Meth. A436 (1999) 326.
[7] W. Adam et al. (RD42 Collaboration) “Pulse Height Distribution and Radiation Tolerance of CVD
Diamond Detectors”, Nucl. Instr. and Meth. A447 (2000) 244.