large area cylindrical silicon drift detector 1992

8/7/2019 Large area cylindrical silicon drift detector 1992

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619IE EE T R A " r m N S ON NU CL EA R S CIEN CE , V O L.39, NO. 4,1992Large Area Cylindrical Silicon Drift Detector

W. Chen, H. Kraner, Z. Li, P. RehakBrookhaven National Laboratory, Upton NY 11973

E. Gatti, A . Longoni, M. SampietroPolitecnico di Milano, 32 Piaeza Leonard0 da Vinci, 20133 Milano, Italy*

P. Holl, J. KemmerTU Miinchen, 8046 Garching andKETEK GmbH, Hauptstrafle 41d, 8048 Haimhausen, Germany

U. Faschingbauer, B.Schmitt, A. Worner, J. P. WurmMax-Planck-Institut for Nuclear Physics, D-6900 Heidelberg 1, Germany

AbstractAn advanced silicon drift detector, a large ar ea cylin-drical drift detector, was designed, produced, tested andinstalled in the NA45 experiment. The active area of thedetector is practically the total area of a 3 inch diame-ter wafer. Signal electrons created in the silicon detector

by fast charged particles drift radially outside toward anarray of 36 0 anodes located on the periphery of the de-tector. Th e drift time measures the radial coordinateof the particle's intersection; the charge sharing betweenanodes measures the azimuthal coordinate.

The detector provides unambiguous pairs of r,d co-ordinates for events with multiplicities up to severalhundred. Its use in the experiment aims at a positionresolution of 20 pm (rms) in each direction giving about2 loe two-dimensional elements.

There is a small hole in the center of the detectorto allow the passage of the noninteracting particle beam .The longest drift distance is about 3 cm . The nomi-nal value of the drift field is 50 0 V/cm resulting in amaximum drift t ime of 4 ps.

This manuscript has been authorized under contractnumber DE-AC02-76CH00016 with the U.S. Departmentof Energy . According ly, the U. S. Government retainsa non-exclusive, royalty-free licence to publish or repro-duce the published form of this contribution, or allowothers to d o so, for U.S. Government purposes.(* ) This research is also supported by the Italian INFN,MURST and CNR.

I. INTRODUCTIONThe semiconductor drift detector [I ] uses the trans-port of electrons in a direction parallel to the large de-tector surface. Th e direction of the transpo rt is imposedby means of rectifying jun ctio ns at different poten tials onboth sides of the wafer. Th e shape of the junctions aswell as the applied potential on them can vary to achievedifferent drift geometries. Up to now only linear geome-tries and cylindrical geometries with the drift of electronstoward the center were realized [2-31.An ideal detector for the experiment NA45 a t theC ER N SPS [4] should measure x and y coordinatesof several hundred charged particles in a n unambiguousway. Th e required resolution is abo ut 20 pm in both co-ordinates within a circular area with a radius of 3 cm.

A cylindrical silicon drift detector can fulfill those re-quirements of the experiment. In reality the cylindricalgeometry of the detector is very close to t he ideal geome-try for most of fixed target experiments with unpolarieedbeams.Fig. 1 shows the detector and it s supporting assem-bly. Th e active part is practically the en tire 3 inch siliconwafer with a small hole in the center. Signal electronsproduced by fast particles drift inside the silicon radi-ally towards the outside edge of the detector. Close tothe outside edge there is an array of 360 anodes a t thesame radius each collecting electrons from 1 deg seg-ments. Anodes are bonded to traces on a ceramic boardto which the silicon wafer is glued. The traces on theceramic board contact traces on an FR-4 (G10) mother

board leading to the inputs of charge sensitive pream-plifiers. Th e contacts ar e accomplished by a layer ofelastomer interconnectors [5]. The mechanical forces forall connectors are provided by 12 screws. The ceramic0018-9499/92$03.00 0 1992IEEE


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Figure 1: Pho tograp h of the n-side of the cylindricaldetector within its assembly. Th e detector is the siliconwafer of 3 inch diameter with a small hole in the centerto allow the passage of the noninteracting beam. Anodesare connected to the outside preamplifiers . A motherboard for the preamplifiers fills the outside radius. (160")board holding the detector can be thus quickly discon-nected from the FR-4 mother board a nd heated to ashigh a tempera ture as 3 O OOC fo r a possible annealing ofthe radiation damage.

The preamplifiers are of a hybrid type [6] with 3channels in each unit. Th e photograph shows the socketsinto which the preamplifier units are plugged. Th eoutside diameter of the assembly is 1 6 0 m m .

Each preamplifier drives about 5 m long 50S2 cablewith a gaussian shaper [6] a t the rece iving end. Outpu tsof the shapers are sampled at 50 MHz by fast 6 bi tnonlinear flash ADCs [7].

The present paper treats mainly the design aspectsof the detector and is organized as follows. The secondsection studies the transp ort of electrons in a cylindricalgeometry. T he consequences of the analysis of thetransport for the pitch of anodes and for the numberof read out channels is shown. Th e third section trea tsthe flow of the leakage current generated at th e Si -S i 0 2 interface into a sink anode. T he diversion of theleakage current away from the signal anodes improvesthe performance of' the detector. Moreover, th e increaseof the current generation on the S i - Si02 interface dueto anticipated radiation dam age effects less the detector.Th e de tec tor is thus expected to be radia t ion harder thanone would first estimate.

Th e four th section shows some results obtained fromlabora tory tes ts and from the run of the de tec tor a t theNA45 experiment at CER N. Results from the experimentare very preliminary and more beam results will bepublished later. A list of conclusions closes the paper.11. TRANSPORT O F ELECTRONS

A. Theoretical considerationsDuring the drift time the electrons diffiise in the driftdirection ( r) and in a direction perpendicular t o the drift(4) . Th e diffusion in th e drift direction is not influencedby the drift field. However, in th e 4 direction the effectof the diffusion is combined with the defocusing causedby the radial divergence of the drift field.The general problem of the electron transport in aradial field can be simplified in our case of a constantradial drift velocity v. We treat the motion of electronsin a coordinate system having the origin at the centerof gravity of the entire electron cloud. (Moving awayfrom the center with a constant radial velocity v . ) Let

us define direction X = ~4 perpendicular to the radiusdirection at the center of the electron cloud. Th e meanradial velocity of an electron at X ha s a component ofvelocity vx = v $ i n the X direction. Th e net flux ofelectrons in th e X direction th us can be w ritten as:

X anj x = nu- - D-T ax

where n(t,X ) is the electron density and D is the diffusionconstan t of electrons in silicon. Comb ined with thecontinuity equation we obtain the diffusion equation forelectrons in X direction

where TO is the initial radial coordinate of the electroncloud.The solution of the above differential equation isa Gaussian distribution of n(t,X) with respect to thecoordinate x

n( t ,X)= -e 2~1 -q&U

where the tim e dependence enters through U

The second term is the square of the initial spread ofthe electron cloud multiplied by the square of theradii ratio. This is the simple geometrical projection ofthe initial s igma to the final radius. Th e first term isdue to the diffusion of electrons during the drift . Thewell known expression for the diffusion in a linear field


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Figure 4: Closer look on the n-side of the cylindricaldetector and it s assembly. Th e magnification is about 2.6times. is 500 pm .Figure 5: Photography of the part of the n-side of thedetector under a magnification of 8 times. Anode pitch

curves between these two extremes have the center of theelectrons shifted at 10% of the anode pitch. The r.m.s ofthe electron cloud is 400 p m in the Fig . 2 and we s ta r tto see the dependence of the resolution on the coordinaterelative to the anode position for the anode pitch above50 0 p m .The other interesting detail shown in Fig. 2 is theworsening of resolution w ith a smaller anode pitch (regionbetween an anode pitch of 200 pm t o 50 0 p m ).Theeffect is due to the contribution of the series noiseof the preamplifiers to the final resolution. In thisdetector the preamplifiers are ex ternal as opposed to onesintegrated on the silicon wafer. Th e tota l preamplifierinput capacitance is dominated by the capacitance ofthe firs t transistor a nd the capacitance of the connectionand is independent of the anode size and pitch. Th eamplifier noise of an individua l anode channel is thus alsoindependent of the anode pitch. For smaller anode pitchthe signal is read through more channels adding noisefrom more preamplifiers in to the p osition determination.The worsening of the resolution can be avoided byintegrating the firs t transistor onto the wafer of thedetector [8]. When the capacitance of the firs t transistoris made equal to the anode capacitance the noise of

the read out channel is proportional to the anode size.Th e total noise considered for th e position determinationdepends only on the spread of electrons and there is nodeterioration of the resolution with a finer anode pitch.C. Implementation

Fig. 4 shows the coexistence of the linear and thecylindrical geometry on the detector. Due to the limi-tations in the production of masks for the detector, thecylindrical detector is in reality a polygon with 120 sides.The detector is linear within 3" sectors and then thereis a 177' angle between two neighbor sectors. Th e elec-tric field is applied on 24 1 concentric rings (or polygonswith 12 0 sides) just resolved in Fig. 4 . The signal elec-trons are transported in the middle of the wafer, that is ,a bou t 12 5 pm from the surface. Th e electric field i n th emiddle of the wafer does not have a sharp angle and thefield under a 177" angle between two sectors is rounded.At small radii where the linear dimension of a 3 O sec-tor is less than 20 0 pm the drift field inside the wafer ispractically cylindrical.

T heblack spots almost randomly scattered across the siliconThere are other features visible on Fig. 4.


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Figure 8: Th e same part of the detector n-side under amagnification about 25 times.

detector are integrated resistors of the voltage divider.There are 240 resistors implanted on each side of thewafer. An effort was made to place the resistors on thewafer in such a way that the power dissipated in theresistors is distributed uniformly across the whole waferarea. The drift field is constant and therefore the voltagedifference between the neigh bour rings is constant . Th earea covered by individual rings depends on the radius ofthe ring. To dissipate the same power per unit area t hepower dissipated per resistor should be proportional tothe radius of the ring. As a compromise, the dissipationis constant for 12 neighbouring rings and then changesto some other value for the next 12 rings. In to tal, thereare 20 different groups of resistors on each side of th edetector. Th e right values of the drift voltages must beapplied to the rings at the boundary of each group. Thebond wires visible at the left bottom of Fig. 4 bring thedrift voltages from a n outside divider t o every 12-th ring.

The bonds at the left han d side of Fig. 4 are connec-tions between the anodes and the gold plated traces ona ceramic board. Holes in a FR-4 board above the ce-ramics are exit holes for the forced radial air flow whichcools the detector.

Figure 7: Negative potential in a radial cross section(called Y) of the detector. Equip otentials imposed atboth surfaces of the detector are the rectifying junctions.Surface between rectifying junctions is covered by SOz.Potential a t this surface depends on the global design ofthe detector.

111. SURFACE PHENOMENATh e second im port ant feature of the design of thecylindrical drift detector is the collection of the currentgenerated on the Si - Si02 interface on the guard (sink)anode. Th e design of drift detector was made in such away tha t t he electrons generated on the interface are notcollected at the detector anode. The designed detectorcollects all electrons generated at the Si - Si02 interfaceon a guard anode. Th e total leakage current collectedon the d etector anodes is the sum of the bulk generationcurr ent an d junct ion diffusion curren ts only.Fig. 5 and Fig . 6 shows the n-side of the detecto runder larger magnifications 8 and 25 times respectively.Individual rings are well visible. The white rings arethe rectifying p+n junctions. The white appearance ofthe junction is due t o the aluminum which covers thejunctions. Th e remain ing silicon areas are covered by

thermally grown SiOz.Fig. 7 shows the negative potential in a radial crosssection of th e detector. Equipo tentials impo sed at bothsurfaces of the detector are the rectifying junctions. Thepicture shows very clearly the realization of the main


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Figure 8: Detail of negative potential in a cross sectiof the de tector c lose to the surface. Th e potent id .o nis

valley in the potential energy for electrons (negative po-tential). In this valley the signal electrons are transportedfrom the point of creation by the ionization of fast parti-cle to the anode. Now let us concentrate our attent ion,however, to secondary valleys located right below sur-face covered by thermally grown SiOz. The existenceof the secondary valleys is a consequence of fixed posi-

, t ive charges in the Si02 close to th e Si- Si02 interface.The presence of these charges bends the energy band insuch a way that electrons are held close to the interface(simple electrostatic attr acti on between mobile electronswithin the silicon and fixed positive charges in the oxideclose to the interface).

Potential shown in Fig. 7 was calculated assumingtha t there a re no e lec trons a t the Si - Si02 interfaceto compensate positive charge of the SiOz. This isone extreme condition which is hard to realize and isnot very desirable. Th e surface of silicon under are ascovered by Si02 is depleted, and a relatively large currentis generated on the Si - Si02 interface [9]. Holesgenerated at the interface (one can visualize holes as

bubbles on figures where the negative potential is plotted)are immediately absorbed by surrounding p + n junctions.Electrons, however, (one can visualize the motion ofelectrons as heavy balls) fall to the minimum of thepotential energy right at the interface. To follow theseelectrons we have to know the shape of the interface inthe coordinate not shown in Fig. 7.

In a case of cylindrical symme try or in a fully sym-metrical geometry of polygon of 120 sides, the potentialis identical at each cross section independent of the az-imuthal angle 4. Electrons falling ont o the interface haveno way to escape. They s ta r t t o accumula te at the re-gions of secondary valleys forming an accumulation layerat the interface. Due to th e negative electric charge ofelectrons th e positive oxide charges are partially compen-sa ted and the band bending at the interface decreases.After a while an equilibrium situation is reached wherethere is the maximum density of electron in the accumu-lation layer at the interface. Th e boundary between thesecondary valley and t he m ain valley has disappeared a tone point along the secondary valley. Mobile electrons in


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Figure 10: Microphotography of the part of the n-sideof th e detcctor under a magnification of 170 times. Twodifferent aluminu m layers are visible. Th e non sinteredaluminum covers Si02 and bridges above the river toconnect the sintered aluminum (rougher surface) coveringrectifying junctions. At the botto m left there is astructure which allows rivers current to reach the sinkanode at the outside radiu s while avoiding signal anodes.

the accumulation layer form almost an equipotential sur-face and an arrival of one electron anywhere along theinterface leads to the emission of one electron from thesecondary valley into the main valley which is collectedlater a t the signal anode of the detector.To prevent the inclusion of electrons generated atSi - Si02 interface from the secondary valley in thesignal anode of the detector a part of the interface

charge must be drained away from the interface beforethe maximum electron density is reached. One wayof draining is by breaking the cylindrical symmetry byincorporating of small openings into the rectifying rings.Small openings let the electrons accumulated in a regionbetween two rectifying rings fall int o the next inter-ringregion. The negative potential in the next region issimilar to the previous one but the potential difference ofone ring (6 V) . Thu s electrons generated a t the interfacemove from one inter-ring region into the next region. Th e

Figure 11: Microphotography of the river regionunder a rnagniflcatisn of 600 times. Flow of electrons iatowards left bottom of the picture. Entrance opening ofthe river is always smaller to prevent excess drainage ofelectrons from the Si - SiOa interface.

following ring is interrupted as well. T hese interruptionsform rivers where the surface current flows.Several rivers are visible as radial lines in Fig. 6and Fig. 6 . These rivers have to carry away all currentgenera ted a t th e Si-Si02 interface. At a small radius ofthe detector, the surface is small and there is only littlecurrent which was generated at a smaller radius to becarried away. A t larger radii there is larger quanti ty of thegenerated current and also current generated upstreammust flow in rivers. The number of rivers increasesfrom 2 at the center to 40 at the largest radius of thedetector.The draining of surface generated current should bedone at a correct rate. Not enough drainage would letthe surface leakage current reach the detector anode.

Too much drainage, however, would deplete larger partof the interface surface and increase the amount of thesurface leakage curre nt. Fig. 8 shows the detail of thenegative potential at the interface where there are stillsome electrons left at the interface. Th e potential barrierto prevent electrons generated at Si - Si02 interface is


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Figure 12: Microphotography of one resistor of the volt-age divider under a magnification of 60 0 times. Resistorsare made during t he sam e production step a s the rectify-ing junctions. Resistance of the implant is about 8 k n / U .Changing the width of the connection between two ringsdifferent values of resistance can be obtained.still present. Conductivity d ue to t he electrons left on th einterface is just r ight t o carry generated electrons fromlocation of origin to rivers. This direction correspondsto a vertical f low between rectifying junc tions as shownin Fig. 9. Electrons generated a t the upper (botto m)half of the picture are transported downwards (upwards)towards the river. Th e electric f ield along the interfaceis small enough that it does not introduce any azimuthalcomponent of the drift field for signal electrons movingiu th e middle of the m ain valley in th e radial direction.Fig. 1 0 shows a r iver terminating at the s inkanode outside signal anodes. Th e rectifying electrodecircumvallating signal anodes is interru pted between twoanodes to let the surface current to pass signal anodesand flow into the sink anode. Th e second layer of nonsintered aluminum covering partially Si02 is visible asa smooth metallization layer. This layer defines thepotentia l a t the top of Si02 surface and also makesbridges across rivers. T he bridges connect sinteredaluminum a t the top of rect ifying junctions of the samering after being interru pted by rivers. Connections

aieEza000OX04

IALL ANODES1 ~ x y ~ x ~ ~ x ~ ~ ~ x - x - x . - x - x - - ~

Slope=7.2hA/cmZt I I I I I I T I I I 13 25 37 49 61 73 85 97 1 09 121 133145157

I I RING # I I5 10 15 20 25s (cm2)

Figure 13: Dependence of the leakage current flowinginto all signal anodes an d the leakage current f lowing tothe sink anode on the active sise of th e B1 1 detector.

are needed to pu t t he entire r ing at the same potentia l.The second aluminum layer was non sintered, therebyeliminating the possibility of spiking at the oxide cutcorners.Microphotography of Fig. 11 shows details of ringinter ruption to form a river. Surface electrons flowtoward the left botto m of the picture. Th e entranceopening of the river is the pa rt of the river to avoid theexcessive drainag e of the surface electrons. Qua lity of thealignment and of the photolithography can be seen fromthis picture and from the microphotography shown inFig. 12 . Registration of alignment to 5 2 pm is requiredin this design.

IV.RESULTSA.Tests in Laboratory

Several cylindrical drift detectors were produced atBNL and TU Munich. F ig. 13 shows the functioning ofthe designed collection mechanism for the surface currentqualitatively. The leakage current f lowing to th e all signalanodes a s well as the leakage current f lowing to the sink


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r-812. Ring ,169 OL - 7OOV rg 229 oL -5OOV 04-04-1991

Sun anode Is O.L282E+04 nR. Rvrg. g rd 0.3154E+04 nR

Figrire 14: Leakage current for each of the 36 0 anodesof a cylindrical detector B12. The r ing # 16 9 was biaseda t -700 V. The upper curve (left l inear scale) shows thecurrent flowing into th e sink anod e.

anode are plotted as function of the active size of thedetector . Th e measurement was done with the de tec torB11. This detector has no defect between rings number13 and number 16 9 a t the inner radius . Th e slope ofthe leakage current to all s ignal anodes is 7 .2 nA/cm2.The slope of the leakage current to the sink anode is18 0 nA/cm2.Th e improvem ent (decrease) of the leakagecurrent is a factor of 25. We have other indications thatthe surface leakage current was drained to o hard by abo uta factor of 3 which gives a more realistic improvementfactor of 8. We would like to stress tha t the diversion ofthe surface generated leakage current into th e sink anodemade the detector less sensitive to potential radiationdamage of the Si - Si02 interface.

Fig. 14 shows the leakage current of individual an-odes in B1 2 detector. 90 % of the detector surface wasactive and most of the anodes have the leakage currentbelow 1 nA under operating field and bias.B. Beam P e r f o r m a n c e

In summer of 1991 th e B12 cylindrical detector wasinstal led in to the NA45 Experiment a t the CERN SPS.

3M) 5320 1280 F240

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Figure 15: Frequency of hits as a function of the anodenumber or the az imuthal coordina te . 3O sectors arewell visible. The entries into histogram were selectedaccording to the information from a silicon pad detectorright in front of the cylindrical silicon drift detector.Only ,even ts wi th no hi t in an ae imuthal sec tor in thepad detector were entered. A hole in the frequency ofhits at arround the anode number 23 0 corresponds t o thelocation of the selected p ad.

Three figures presented here (Fig. 16, Fig. 16 an d Fig. 17 )are preliminary.

V. CONCLUSIONSAn advanced silicon drift detector, a large areacylindrical drift detector, was designed, produced, testedand ins ta l led in the NA45 experiment . The de tec torprovides unam biguous pairs of r,+ coordinates for events

with multiplicities u p to several hundred. Th e positionresolution of the detector of 20 p m (rms) in each directionprovides about 2 10 two-dimensional elements.

Although somewhat complicated, it has been pos-sible to produce nearly fully operational devices on thewhole area of 3 inch diameter wafers. The surface cur-rent is efficiently collected by a sink anode as designed.Th e observed anod e current is primarily bulk generatedcurrent at an average level of 7 nA / cm 2 .


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DRl.r[TIUE DISTR A L LFigure 16: Frequency of hits a s a function of drift t ime.No selection of entries into the histogram. Th e increaseof number of events per bin with the increase of the drifttime is due to the physics of the particle production inthe experiment.

REFERENCES1. E. Gat t i and P. Rehak, Nucl. Instr. and Meth. 225,

608 (1984).P. Rehak et al., Nucl. Instr. and Meth. 248, 36 7(1986).P. Rehak et al., IEEE Transactions on Nucl. Science-36, 203 (1989).U. Faschingbauer e t al., Proposal to t he SPSC C ER N,GD type low resistance connectors from Shin-EtsuPolymer America, Inc., 34135 7-th Street, UnionCity, CA 94587.

2 .3.

4. SYSC 88-25, S P S C / P 237.5.

70 -w -

50 -

40 -

DR l l T lYE D l 5 lR ALL

Figure 17: Frequency of hi ts as a function of drift time.The entries into the histogram were selected accordingto the pad detector . This t ime only events with no hitsbetween two radii w ere plotted.

6. Preamplifiers and shapers were developed at BNLand produced by AST ER TE CHNOL OGY Inc . , P .O.Box 729, Ramsenburg, NY 11960.

7. ADCs were produced by Dr. B. Struck, 2000 Tang-s tedt /Hamburg, Germany.8. P. Rehak et al., Nucl. Instr. and Meth. m,168A.S. Grove, Physics and Technology of Semicondmc-tor Devices, J , Wiley and Sons, Inc., New York, 1967,Page 298.

(1990).9.

large area cylindrical silicon drift detector 1992

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