Chatterji, S.

Heuser, J. M.

Mueller, W. F. J.

Singla, M.

GSI Repository

Design options for radiation tolerant microstrip sensors for the CBM SiliconTracking System (STS)∗S. Chatterji1, M. Singla2, W.F.J. Mu¨ller1, and J.M. Heuser11GSI, Darmstadt, Germany; 2University of Frankfurt, GermanyThe expected neutron fluence for the CBM STS is1×1014 neqcm−2, after which an upgrade of sensors isforeseen. The main impact of radiation damage is the lossin the charge collection efficiency (CCE) limited by thebreakdown voltage Vbd. Also we aim to minimize the ca-pacitive noise. The dominant contributor to the capacitivenoise comes from the interstrip capacitance Cint. To sum-marize, we aim to develop microstrip detectors having lowCint, high Vbd and maximum CCE.Fluence ProfileTable 1 shows the expected neutron fluence for five yearsof required CBM runtime. In this table, the initial resistiv-ity of silicon has been taken to be 5.33 KΩ-cm,the lifetimesof electrons τe and holes τh have been calculated usingKramberger’s model [1] assuming an operating tempera-ture of -100C. One can observe a deterioration of carrierlife time with fluence which will have an impact on theCCE, especially on the p-side since this side collects lessmobile holes.Table 1: Fluence profile of neutrons for the CBM STS.Year Fluence Neff τe τh Vfd(neq cm−2) (cm−3) (ns) (ns) (V)1 2×1013 2.8×1011 1140 1050 282 4×1013 -1.54×1011 570 527 203 6×1013 -5.35×1011 380 351 444 8×1013 -8.84×1011 285 263 755 1×1014 -12.1×1011 228 211 100Strip IsolationIn order to investigate the life time of sensors, it is imper-ative to extract the CCE as a function of fluence for whichone has to understand the strip isolation in particular on theohmic side. Hence various isolation techniques have beenexplored both through prototyping as also through simu-lations, for example P-stop, P-Spray, Modulated P-spray(conventional isolation techniques) and also a new isolationtechnique, the Schottky barrier. Schottky barrier can be de-fined either through metal work function value or throughbarrier height which in turn depends on the substrate typeand the metal used for Schottky contact. For Aluminum,the barrier height is 0.72 eV for n-type silicon while forp-type silicon, the barrier height is 0.58 eV [2]. A compar-ison of the conventional isolation techniques with Schottkybarrier in terms of Vbd, Cint and CCE is shown in Table 2.One can infer that the Schottky barrier is the best choice∗Supported by EU-FP7 HadronPhysics3 and HICforFAIRin terms of Vbd and Cint. However in terms of CCE, theSchottky barrier gives the worst performance especially af-ter type-inversion. Therefore, Schottky barrier has not beenopted as a suitable isolation technique. Besides P-stop andP-spray, another isolation technique namely modulated P-spray has also been explored. An optimization of modu-lated P-spray has been performed. It has been found thatusing a moderate P-stop width of around 15 μm and verylow P-spray concentration of around 1×1015 neqcm−3gives the best performance in terms of Vbd and Cint, re-ferred to as Optimized Modulated P-spray in Table 3. Fi-nally a comparison of P-stop, P-spray and Optimized Mod-ulated P-spray after one year of operation and the maxi-mum fluence expected at the end of five years of CBM runis shown in Table 3. One can notice from this table thatusing Optimized Modulated P-spray, the Vbd has increasedby around 60% and Cint has reduced by 25% while main-taining the same CCE as with conventional isolation tech-niques. In Tables 2 and 3, Vbd, Cint and CCE are simu-lated values confirmed with measurements. Hence Opti-mized Modulated P-spray is the best choice for isolationtechnique in terms of Vbd, Cint and CCE.Table 2: Comparison of conventional isolation techniqueswith Schottky barrier.Isolation Fluence Vbd Cint CCETechnique (neq cm−2) (V) (pF cm−1) %P-stop 3.93×1012 1010 2.1 91.2520.60×1012 890 2.29 86.25P-spray 3.93×1012 524 2.6 9320.60×1012 450 2.7 86.25Schottky 3.93×1012 1450 2.05 79Barrier 20.60×1012 1350 1.80 77.5Table 3: Comparison between p-stop, p-spray and opti-mized modulated p-spray at low and high fluence.Isolation Fluence Vbd Cint CCETechnique (neq cm−2) (V) (pF cm−1) %P-stop 2×1013 980 2.02 93.151×1014 720 2.03 88.87P-spray 2×1013 513 2.56 93.171×1014 495 2.44 89Opt. Mod. 2×1013 1600 1.58 93.22P-spray 1×1014 1150 1.60 89References[1] V.Cindro et. al., IEEE Trans. Nucl. Sci. N09-2, pp.139-142,2006.[2] http://www.pfk.ff.vu.It/lectures/funkc dariniai/diod/schottky.htm.PHN-NQM-EXP-17 GSI SCIENTIFIC REPORT 201246

Design options for radiation tolerant microstrip sensors for the CBM Silicon Tracking System (STS)

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Design options for radiation tolerant microstrip sensors for the CBM Silicon Tracking System (STS)

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