development of radiation hard sensors for very high luminosity colliders—cern-rd50 project

Nuclear Instruments and Methods in Physics Research A 511 (2003) 97–105

Development of radiation hard sensors for very highluminosity colliders—CERN-RD50 project

Michael Moll*

CERN, EP Division, Geneva 23 1211, Switzerland

On behalf of the RD50 Collaboration1

Abstract

The requirements at the Large Hadron Collider (LHC) at CERN have pushed today’s silicon tracking detectors to

the very edge of the current technology. Future very high luminosity colliders or a possible upgrade of the LHC to a 10

times increased luminosity of 1035 cm2 s1 will require semiconductor detectors with substantially improved properties.

Considering the expected total fluences of fast hadrons above 1016 cm2 and a possible reduced bunch-crossing interval

of E10 ns, the detector must be ultra radiation hard, provide a fast and efficient charge collection and be as thin aspossible. The newly formed CERN-RD50 project is aiming to provide detector technologies, which are able to operate

safely and efficiently in such an environment. This article describes the approaches and first results of RD50 to develop

ultra radiation hard sensors by optimizing existing methods and evaluating new ways to engineer the silicon bulk

material, the detector structure and the detector operational conditions as well as investigating the possibility to use

semiconductor materials other than silicon.

r 2003 Elsevier B.V. All rights reserved.

PACS: 29.40.Gx; 29.40.Wk; 61.82.Fk

Keywords: Radiation damage; Semiconductor detectors; Defect engineering

1. Motivation

During the last decade advances in the field ofsensor design and improved base materials havepushed the radiation hardness of the currentsilicon detector technology to impressive perfor-mance. It should allow operation of the trackingsystems of the Large Hadron Collider (LHC)

experiments at nominal luminosity (1034 cm2 s1)for about 10 years [1–3]. However, the predicted1MeV neutron equivalent fluences of fast hadrons,ranging from 3 1015 cm2 at a radius ofR ¼ 4 cm to 3 1013 cm2 at R ¼ 75 cm for anintegrated luminosity of 500 fb1, will lead tosubstantial radiation damage of the sensors anddegradation of their performance. For the inner-most silicon pixel layers a replacement of thedetectors may become necessary before 500 fb1

has been reached. One option that has recentlybeen discussed to extend the physics reach of theLHC, is a luminosity upgrade to 1035 cm2 s1,

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*Tel.: +41-22-76-72495; fax: +41-22-76-79330.

E-mail address: [email protected] (M. Moll).1A full list of RD50 members (52 Institutes) can be found on

http://cern.ch/rd50/.

0168-9002/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0168-9002(03)01772-8

http://cern.ch/rd50/

envisaged after the year 2012 [4]. An increase ofthe number of proton bunches, leading to a bunch-crossing interval of the order of 10–15 ns isassumed to be one of the required changes. Presentpixel detector technology, applied at larger radius(e.g. R > 20 cm), may be a viable but very costextensive solution. However, the full physicspotential can only be exploited if the current b-tagging performance is maintained. This requires atracking layer down to RE4 cm where one wouldface fast hadron fluences above 1016 cm2

(2500 fb1). The current silicon detectors areunable to cope with such an environment. Thenecessity to separate individual interactions at acollision rate of the order of 100MHz may alsoexceed the capability of available technology.In order to meet these challenges and to have a

reliable detector technology, both for particlesensors and electronics, available for an LHCupgrade or a future high luminosity hadroncollider, focused and coordinated R&D projectshave to be launched now. As a side effect, anyincrease of the radiation hardness and improve-ment in the understanding of the radiation damagemechanisms achieved before the luminosity up-grade will be highly beneficial for the interpreta-tion of LHC detector parameters and a possiblereplacement of pixel layers. Following thesemotivations the CERN-RD50 project ‘‘Develop-ment of Radiation Hard Semiconductor Devicesfor Very High Luminosity Colliders’’ [5,6] wasinitiated and approved in June 2002 by the CERNResearch Board.

2. Radiation damage

The three main macroscopic effects on high-resistivity silicon detectors following energetichadron irradiation are:

* Change of the effective doping concentrationNeff with severe consequences for the operatingvoltage needed for total depletion.

* Fluence proportional increase in the leakagecurrent, caused by the creation of generation-recombination centers.

* Deterioration of charge collection efficiency(CCE) due to charge carrier trapping leadingto a reduction of the effective drift length bothfor electrons and holes.

A detailed description of the radiation effects isbeyond the scope of this article and can be foundelsewhere (e.g. Refs. [7–10]). Recent research onradiation hard silicon detectors was focused on theunderstanding of the detector behavior afterexposure to neutron or charged hadrons fluencesof up to 1015 cm2. Beyond this fluence (up to themaximum expected fluence of 1.6 1016 cm2 foran upgraded LHC [4]) only little or no data exist.However, already after an 1MeV neutron equiva-lent fluence of 1015 cm2 severe changes to thedetector macroscopic parameters are observedwhich are summarized in the following (datacalculated for an annealing of B14 days at roomtemperature).The sign of the space charge changes from

positive to negative due to the radiation-inducedgeneration of deep acceptors. Accordingly, thedepletion of the type inverted detector starts fromthe n-contact and the overall negative space chargeof non-oxygenated silicon requires more than1000V for a full depletion of a 300 mm thickdetector [7]. Storing at room temperature leads toreverse annealing, a further increase of thenegative space charge and depletion voltage,respectively (see e.g. Ref. [7]). The leakage currentincreases to I=VE40mAcm3 (20C) [11] makingcooling of the detectors inevitable to reduce noiseand power consumption. Due to the high con-centration of trapping centers the effective carrierdrift length at an electric field strength of 1V mm1

is reduced to about 140–190 mm for electrons andto about 50–80 mm for holes [12,13]. The combina-tion of trapping and incomplete depletion leads toa strong deterioration of charge collection effi-ciency (CCE) and consequently signal-to-noiseratio, both for pixel and strip detectors.

3. Approaches to obtain radiation hard sensors

Based on achievements of past and presentR&D projects (e.g. Refs. [14–17]) and recent

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M. Moll / Nuclear Instruments and Methods in Physics Research A 511 (2003) 97–10598

discoveries in radiation hard semiconductor devicetechnologies, three fundamental strategies havebeen identified by RD50 to develop radiationharder tracking detectors:

* Material engineering stands for the deliberatemodification of the detector bulk material. Oneapproach is defect engineering of silicon, whiche.g. includes the enrichment of the silicon basematerial with oxygen, oxygen dimers or otherimpurities. Another approach is the use of othersemi-conductor materials than Si such as e.g.SiC.

* Device engineering stands for the improvementof present planar detector structures by e.g.modification of the electrode configuration orthinning of the bulk material and the develop-ment of new detector geometries such as ‘‘3Ddetectors’’.

* Variation of detector operational conditions.Investigations of the optimum detector opera-tional conditions include for example theoperation of silicon detectors at low tempera-tures or under forward bias.

Although these three approaches might befollowed within RD50 along separate researchlines it is expected that the ultimate radiation harddetector will be achieved by a judicious combina-tion of two or more of the above-mentionedstrategies. Of course this solution will dependstrongly on the specific application, the radiationenvironment and financial restrictions.Following the above strategies and taking into

account the big size of the collaboration as well asthe vast scientific program, a scientific organiza-tion as presented in Table 1 was chosen (details inRef. [6]). It consists of two major research linesand six sub-projects and implies that the opera-tional conditions are evaluated within everyproject.Obviously the different research projects cannot

be strictly separated and a strong interaction isexpected and encouraged. Furthermore, there arecertain studies that are common to severalresearch projects which are organized in workinggroups dealing, e.g. with common mask designs,inter-calibration of microscopic and macroscopic

measurement equipment and setups, device simu-lations or organization of irradiation experiments.In the following a brief overview of the six

RD50 research projects will be given.

4. Material engineering

4.1. Defect engineering

The presently most successful example for defectengineered silicon is oxygen-enriched silicon. Itwas introduced to the HEP community as Diffu-sion Oxygenated Float Zone (DOFZ) silicon bythe RD48 (ROSE) collaboration in 1998 [16] andconsists of diffusion of oxygen (e.g. for 24 h at1150C) into the silicon bulk from an oxide layergrown via a standard oxidation step. RD48 foundthat the built-up of negative space charge (corre-sponding to an increase of depletion voltage aftertype inversion) after irradiation with high ener-getic charged hadrons is strongly suppressed. Areduction by a factor 3 in the damage parameter b(slope of Neff vs. particle fluence) was reported[18]. This beneficial effect was recently found to beeven more pronounced after gamma irradiation[19,20] as shown in Fig. 1. While the standardsilicon type inverts and shows a strong built-up ofnegative space charge the effective space charge ofthe oxygenated silicon remains almost constantand even a small generation of donors is observed.However, after neutron irradiation no effect [9,10]or only small influences [21] of the oxygenation onthe radiation hardness have been observed. The

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Table 1

Scientific organization of RD50

Material engineering Device engineering

Defect/material

characterization

Pad detector

characterization

Defect engineering New structures

New materials Full detector systems

The two research lines are coordinated by the spokesperson and

deputy and are each subdivided into three projects managed by

project conveners.

M. Moll / Nuclear Instruments and Methods in Physics Research A 511 (2003) 97–105 99

leakage current after hadron irradiation is notinfluenced by the oxygen content [22] while aftergamma-irradiation it is reduced in oxygen enrichedsilicon [20].Recent investigations also show that radiation

hardness is not as clearly depending on the oxygenconcentration as thought in the beginning. This isdemonstrated in Fig. 2. Two identical sets ofsilicon wafers originating from different ingotswere processed with and without an oxygenationstep. As a result a wide variation is observed forthe standard silicon set while for the oxygenatedset only little difference is seen. Furthermore, somestandard samples show exactly the same behavioras their oxygenated counterparts. A confusingresult, which several groups also have observed foroxygenated and standard detectors produced bySintef (Norway) (e.g. Ref. [23]). These experimentsclearly demonstrate that besides the oxygen con-tent also other up to now not identified parametersplay an important role. This and other openquestions regarding oxygen-rich silicon are dis-cussed in more detail in Refs. [9,10].Czochralski silicon (CZ) with a resistivity that is

suitable for detector applications has only recentlybecome available. Since it is grown in a quartzcrucible it has high oxygen content in the order ofsome 1017–1018 cm3. First irradiation experimentswith charged hadrons show remarkable and verypromising results [21]. As shown in Fig. 3 theoverall change of depletion voltage is smaller thanin oxygenated or standard FZ silicon. Even moresurprising are Transient Current Technique (TCT)

measurements showing that the material is nottype inverted after a fluence as high as 1015 cm2

(190MeV pions) [24]. Therefore the increase indepletion voltage is believed to be due to theformation of donors (most probably thermal

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0 200 400 600 800 1000dose [Mrad]

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Fig. 1. Co60-gamma irradiation of standard and oxygenated

(DOFZ) silicon detectors [20].

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DOFZ silicon

Fig. 2. Proton irradiation of standard (upper figure) and

oxygenated (lower figure) detectors produced by ST Microelec-

tronics (Italy) from Wacker (Germany) silicon.

0 2.1014 4.1014 6.1014 8.1014 1015

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Cz-TD generatedCz-TD killed

Fig. 3. Pion irradiation of standard FZ, oxygenated FZ (open

symbols) and two types of high resistivity Czochralski silicon

(filled symbols) [21].


donors), which overcompensate the radiation-induced acceptors. Since also the price of thismaterial is expected to be lower than for FZmaterial [25], CZ is a good material for p+–ndetectors since even after high fluences the highelectric field will remain on the p+-electrode (notype inversion). Meanwhile first full size stripdetectors have been produced and successfullytested [25,26] and irradiation tests are under way.The only known drawback of a material with veryhigh oxygen content is that it requires specialattention to avoid thermal donor creation (andthus a change of effective doping concentration)during processing.Other approaches under investigation by RD50

are the use of epitaxial silicon (showing similarradiation hardness properties as CZ silicon [21]) orsilicon enriched with oxygen dimers [27], hydrogenor other impurities like e.g. germanium.

4.2. Defect and material characterization

Defect and material characterization as well ascomputer simulations of defect formation anddefect properties are indispensable tools to achievea profound understanding of the radiation damageprocess. Only on such basis can the link betweenmicroscopic defects and macroscopic detectorproperties be fully understood in order to defectengineer the sensor material in a coordinated way.Only recently has a clear correlation between

radiation-induced defects in silicon and changes ofmacroscopic detector properties been revealed[28]. It was shown that the values of the detectordepletion voltage after gamma irradiation can becoherently predicted using defect parametersdescribed in Refs. [28,29]. One of the characterizeddefects was tentatively attributed to be the longsearched Di-Vacancy-Oxygen (V2O). A defectaccredited to play a key role in the radiationdamage process not only after gamma, but alsoafter hadron irradiation [7,30]. Computer simula-tions (AIMPRO) have predicted that this defecthas an acceptor level at 0.57 eV below the bandgap [31], which coincides well with the value of0.55 eV given in Ref. [28]. However, another groupwithin RD50 also claims to have possibly identi-fied the V2O defect [32] having an acceptor level at

0.46 eV and a second acceptor level at 0.20 eVabove the conduction band. The later observationcoincides with the prediction in Ref. [31] that V2Oshould have a second acceptor level, whichhowever is expected to be at 0.11 eV above theconduction band. Both groups are now workingtogether within RD50 to solve this riddle. In-dependent of the disclosure of the physical andchemical structure of the observed defects thefinding of a defect with electrical propertiesexplaining macroscopic detector properties in itselfis a huge step forward in the understanding ofradiation damage.The material and defect characterization group

within RD50 is furthermore working on issues likethermal donors (formation, stability, influence ondefect generation during irradiation, etc.), pre-sence of hydrogen and its influence on oxygendiffusion and defect formation and the impact ofthe nitrogen and carbon content on defect kinetics.Theoretical solid state physicists are working onthe simulation of the physical properties of defectsin silicon as well as on defects in other newmaterials, currently under investigation in RD50.Last but not least, microscopic characterizationtechniques are improved and developed (e.g. Ref.[33]).

4.3. New materials

It is well possible that a fluence of 1016 cm2

corresponds to the operating limit of siliconsensors for temperatures close to room tempera-ture. Other sensor materials are therefore underinvestigation. Diamond detectors are investigatedby the RD42 collaboration [15] and silicon carbide(SiC) and semi-insulating GaN, which haverecently been recognized as potentially radiationhard, are now studied by RD50.In Table 2 we list some characteristics of 4H–

SiC compared with diamond, silicon and GaN.The relatively large band gap leads, as fordiamond and GaN, to a very low leakage current.Since the ionization energy lies between those ofdiamond and silicon also the resulting average e–hpair generation of 51 mm1 for a minimum ionizingparticle (mip) lies between those of silicon(110 mm1) and diamond (36 mm1). Finally, the

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displacement threshold of 25 eV indicates a poten-tially higher radiation hardness than silicon.However, this has to be tested since SiC is notmonoatomic but a compound material.Semi-insulating 300 mm-thick SiC substrates

equipped with ohmic contacts were tested in thepast with a 90Sr b-source [34]. A response ofB2000 e(500V) has been measured correspond-ing to a charge collection distance ofB39 mm. Dueto polarization effects, most probably caused by ahigh defect concentration in the bulk, this signalwas found to decay very quickly and irreversiblydown to 800e. This undesirable effect led to theconclusion that SiC is no promising material forparticle detection.However, epitaxial 4H–SiC with very high

crystal quality is now available and Schottkybarrier detectors have been studied with a-particlesfrom an 241Am source [35–37]. Charge collectionefficiency (CCE) of 100% has been measuredobserving no polarization effects. Radiation da-mage of these devices has been tested with a 60Cog-source and 8.2MeV electrons up to a dose of40MRad and with 24GeV c1 protons up to afluence of E9 1013 cm2. Although the effectivespace charge density (Neff ) was observed todecrease from the initial value of 7.7 1015 to1.4 1015 cm3 after the highest electron dose of40MRad, no influence on the CCE was observed[36]. Also after the highest gamma dose andproton fluence a CCE of 100% was observed atroom temperature. Furthermore, the leakagecurrent decreased after irradiation [37].First studies on the response of epitaxial 4H–

SiC Schottky barriers to b-particles from a 90Sr-

source have been performed in the frameworkof RD50 [38,39]. Schottky diodes of 1–2mmdiameter have been manufactured on epitaxialwafers produced by CREE (Durham, USA).The thickness of the active epilayer withNeffB6 1014 cm

3 was 21 mm. The charge collec-tion signal is stable and reproducible, showing anabsence of priming and polarization effects. Thecollected charge increases as a function of V1/2 andsaturates at approximately 240V, with a maximumvalue of B1100e, corresponding to B100%charge collection efficiency over the total epilayerthickness. RD50 is now planning to measure thecharge collection efficiency of 4H–SiC detectorsproduced with thicker epilayers in the range 30–70 mm and to produce microstrip and pixelsdevices with epitaxial 4H–SiC to investigate thefeasibility of position sensitive detectors with thismaterial.RD50 is also developing GaN based detectors.

First tests of SI–GaN Schottky detectors with a-particles showed a CCE of 100% (for details seeRef. [40]).

5. Device engineering

5.1. Pad detector characterization

The research activity within this project isfocused on systematic studies of the macroscopicpad detector properties by means of IV/CV andCCE (laser, mips and a-particles) measurementsduring isothermal and isochronal annealing ex-periments performed after irradiation with differ-ent particle types and fluences. In a first step theseactivities will be performed with devices remainingfrom previous research activities (e.g. RD48) andlater on with newly developed or defect engineeredmaterial. The main goal is to measure andparameterize the change of detector properties upto the maximal expected fluence for an upgradedLHC (1.5 1016 cm2). Such parameterization isnot only useful for calculations of LHC-likedamage scenarios but is also an input for under-standing basic radiation damage processes as wellas testing the Non Ionizing Energy Loss (NIEL)-hypothesis. Lately several violations of this

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Table 2

Properties of diamond, 4H–SiC, Si and GaN crystals

Property Diamond 4H–SiC Si GaN

Eg (eV) 5.5 3.27 1.12 3.39

me (cm2V s1) 1800 800 1500 1000

mh (cm2V s1) 1200 115 450 30

e–h energy (eV) 13 8.4 3.6 B8–10Displacem. (eV) 43 25 13-20 B10–20Density (g cm3) 3.52 3.21 2.33 6.15

Radiation length, X0 (cm) 12.2 8.7 9.4 2.7

e–h pairs/X0 (106 cm1) 4.4 4.5 10.1 B2–3


hypothesis have been observed and still a widerange of particle types and energies has not beeninvestigated with respect to the NIEL. Of specialinterest are e.g. irradiations with low energyprotons for which a NIEL violation is predicted[41] and with high energetic neutrons in order tounravel the charged hadron–neutron puzzle ofoxygenated silicon [7]. Also irradiations with veryhigh-energy electrons have so far only rarely beenperformed [42].

5.2. New structures

There are several approaches to optimize thedetector structure to achieve, even after the bulkmaterial has been severely radiation damaged, asufficiently high CCE in a required collection time.An optimized multi-guard ring systems allows

to push the operation voltage up to B1000V andthus allows for full depletion up to higherirradiation fluences. However, the hazard of abreakdown is still present and the power con-sumption is raised. Detector structures requiringonly a reasonable low operation voltage aretherefore under development.So-called ‘‘3D-detectors’’ [43] and ‘‘Semi-3D

detectors’’ (for details see Ref. [44]) offer thisfeature. The electrodes of a 3D-detector are anarray of vertical columns that are penetrating thedetector bulk. The first processing step is thereforethe formation of holes by dry etching, electro-chemical etching or laser drilling [45] (see Fig. 4.)with a diameter ofB5–10 mm and a pitch ofB30–100 mm. These holes are then filled with p+ and n+

electrode material (or are metallized) to formp+–n junctions (or Schottky contacts). Such astructure has still the same ionization layer asstandard planar detectors (e.g. 300 mm), but due tothe smaller distance between the electrodes thedepletion voltage is smaller and the chargecollection is much faster.Thinning down planar detectors to about 50 mm

thickness is another approach and different thin-ning techniques are investigated (e.g. Ref. [46]).Thin detectors offer a low material budget andhave a reduced depletion voltage. However, thecollectable charge is strongly reduced (e.g. Ref.

[47]), which sets higher requirements on thereadout electronics.

5.3. Full detector systems

The Full Detector Systems subgroup of RD50 isthe subgroup closest to the application of theimproved radiation hard detectors into futureHEP experiments. Its goals are the investigationof miniature and full size strip and pixel detectorsmade with different silicon substrates and detectorgeometries, using LHC speed electronics. Already,there have been studies performed with prototypesof the future LHC-b vertex locator detector usingboth p-strip and n-strip read-out and irradiated atthe CERN PS. CCE studies confirmed the super-iority of n-side read-out after irradiation, andshowed limited signal loss (20%) even after afluence of 7 1014 p cm2 [48,49]. Other CCEstudies performed on segmented detectors havealso observed an improvement for oxygenatedsilicon material compared with standard silicon.Since the charge collected at a given voltage isreduced both by the trapping, for which n-sideread-out helps, and by the changes to the effectivedoping concentration, for which oxygenated sili-con material helps, the combination of thesetechniques yields a good performance of trackingdetectors up to the fluences expected for the LHC.As mentioned previously, similar studies have to

be performed on such detectors using the findingsof the other subgroups of RD50 to study their

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Fig. 4. Holes in silicon obtained by inductively coupled plasma,

diameter 10 mm, pitch 85 mm. Top view (left) and cross-sectionof a 130mm deep hole (right) [45].


performance at fluences characteristic of the post-LHC era.

6. Conclusion

A need for R&D on ultra radiation hardtracking detectors for very high luminosity colli-ders has been identified and consequently theR&D collaboration ‘‘CERN-RD50’’ was formed.RD50 follows various scientific approaches totackle this challenge including Defect Engineering,Device Engineering and the Optimization ofOperational Conditions. It is expected that inorder to achieve ultra radiation hard sensors acombination of the above mentioned approachesdepending on the radiation environment, applica-tion and available electronics will be the bestsolution for the next generation high luminositytracking detectors. First scientific results and anoutline of future work have been given.

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