Batignani, G

Bosisio, L

Focardi, E

Forti, F

Giorgi, M A

Marangoni, D

Rizzo, G

Tonelli, G

Triggiani, G

CERN Document Server

`”‘**> ' °’a I `@¤§Aé2~ wv A N° " Al°’€`‘OCR OutputAAA ' -* 3;; aAa¢aj“. ,_,;- `‘.r¥ d'°.=—V`.»··a-—‘ "‘»`%*`¥ A»a· g<§gfgj=_y “ '»9·‘ A { a‘·d (9·` ‘‘»a~>2—d **-9‘" y .AA»a‘9"" Tw .: * `¤ Vzi ‘a‘‘k v?¤¤;. A fi;»~ A `~A9 9¥‘ T A A _ a ··9.¤99 · `daa A d` ° A ;}~¢*Yf ~¥¥.’>£¢*‘" ' ’ *g§s;;¥»;"; \‘.i»9a* A A d_d_ ·*‘ 9 {` A \a.¢»z;%a;¢·¤€7¢_AA A `'#l`· 9* .; 9%* a~;·, ~. >—» *ia??;>¢?iY;*—av9 `’d“ =‘· ` =·`MaydA.‘··9Pape! p2228,resented 1994Laar thi! 61h Biodola, Is0PisxcctotBath pP<'€OrYY\·zd\ a méétiflg on aa. 'a d'Elba, ItalyW MGT ’C©$ vauced1Z€Ct0!’ S,0Q6{$O{1‘€\€d in otdct \Y€:cALBYYK O0 A Sk\g\€. vdcusuta thebeen and U5 , CXGXBCXOY , \\l am,Xa dcsigucd vemccsand had W6 xscuss QUNCGSS-dcQCnc Xasx yand xcsxcd in cdxcv d$01*06 cnd€ntears sckn dm €xc\d dYBSUXKS qua<\*dd€»vcm\ madxcds Yksa-XNFN . i apphcadons Cm tha XSSK S which €:i€oi `m<g,<—; \Xabomxowas wc\\. SUUGKUYGS dx&T\K{\G \X\€»~L5cm) ¤sc We KQSGYYBGP»bs\m.u» '2d¤¤b\c»side,d s"f"dcxccmss me prcscm me xcsxs K0\‘{xc0<x strip cunamw usdcxezctou ed m( 5 A da havcw axQQLMx as{2) oxvccsxmd C5} Naw ax mvccsxm 0‘G) awavsm Udx 'Yrxcsu: Umvcrswd ,,y ass , XGA] ‘*mmamd xucnmdx Yxs  aKSF$ dY‘·\m\nd \Nr»~· sermons»¤ ? Z /> A 65‘ CE g Q@scmcd by scmcmc d,dx"`<xasxe,,Xm\g1?vsa\vA\$a*dsg,<x2¤GBadgmvGB ;¤§ x é "’,~ iggvj*9*, LB0s%.sk0\ ¢"¤Hx1,1,0 V9,‘9E, GfY0<xc\\focaidxkmK ,fs Fforix 9, V*WGYriggxdv{x m““0ub\e·Xoxg&*N\.G\ded SK\\c K9,em StripDMzmgcmkUctectdvs n V9,‘iY\s2£YU AEmw X7., 9A I O9\99AMay 22-28, 1994, La Biodola, Isola d'Elba, Italy OCR OutputPaper presented at the 6th Pisa meeting on advanced detectors,detector performances.each wafer in order to measure the relevant, process—dependent, quantities which determine theperformed on a single detector, and we discuss some results on the test structures inserted inALEPH and L3 vertices, and in other field of applications as well. We present the testsbeen designed and tested in our Pisa—lNFN laboratory. These detectors are currently used inIn the last years several hundreds of large (~25cm2) double—sided silicon strip detectors haveAbstract(4) Now at Universita` di Sassari, Italy(3) Now at Universita' di Firenze, Italy(2) Universita’ di Trieste and INFN sezione di Trieste, Italy(1) Universita' di Pisa and INFN sezione di Pisa, Italypresented by G.BatignaniG.Rizzo (1), G.Tonelli (4), G.Triggiani (1)G.Batignani (1), L.Bosisio (2), E.Focardi (3), F.Forti (1), M.Giorgi (1), D.Marangoni (1),'Double-sided Silicon Strip Detectors in Pisa'INFN Pl / AE 94/ O9July 12, 1994a metallized nt guard ring, around the ohmic side, with the same purposes of the one on the junction OCR Outputwhen the detector has been completely depleted, as already discussed in reference [2]. There is alsoA pt strip (without metallization) is inserted between two consecutive nt strips, to isolate themdirection which is ortogonal to the strip direction on the junction side. Each nt strip is metallized.On the opposite side, named "ohmic side", we have implanted nf strips with a 50ttm pitch, in athe detector and to stop the edge currents as well.of the detector is surrounded by a metallized p+ implantation, the guard ring, which is used to biasof 25um, and each strip is directly in contact with an Aluminum metallization. The whole surfacesegmented in strips, thus forming rectifying contacts. These strips are 12tlm wide and have a pitchAll detectors are fabricated doping one side of the wafer, named "junction side", with a p+ implantationl. WORKING PRINCIPLES AND BIASING SCHEME"HODO 4" detectors, fabricated for the present version of the ALEPH vertex detector.present our conclusions (section 4). When it will not be specified, we shall be concerned with themeasurements done on test structures (section 3). Last we briefly discuss the production yield and1), then we report the results on the leakage currents (section2) and on the the most importantIn this paper we first describe the working principles and the biasing scheme of the detectors (sectiondesign of the biasing resistors.The differences among the various versions, apart from their dimensions, mainly consist in thecoordinates of high energy particles crossing them, with a spatial resolution of about 10].Lm or better.All detectors are segmented into strips on both sides, to measure at the same time two orthogonaltracking applications (e.g. beam telescopes)[6].which is used in digital radiography studies by the RADIN group [5], and for high precisionpurposes, like transistors and pixels, and a smaller strip detector, 1.4x1.4 cm2, shown in fig 1a,which determine the detector performances. We have also inserted other test structures for R&Dinserted several test structures, in order to measure the relevant process—dependent quantities,Silicon 4" wafers. They were 300ttm thick, <1ll> oriented. Around the main detector, we haveFigures 1a—1b show pictures of the detectors, fabricated on high resistivity (>6kQ-cm), n-type,version of the ALEPI-l vertex detector [4].22 ~5x6 cm2 detectors in 1994 (fi glb) ("HODO 6") which are the first prototypes for the improvedvertex detector [3],24 ~4x7 cm2 detectors in 1991 ( design named "HODO 5") which formed the first batch for the L3for the ALEPH vertex detector [2],306 ~5x5 cm2 detectors in 1989-93 (figla). This design is named "HODO 4", and is currently used48 ~5x5 cm2 detectors in the years 1987-8, used as prototypes for the ALEPH vertex detector [1],PISA-INFN laboratory. ln particular, we have dealt with:according to a process outline specified by INFN. All testing operations have been done in thedone by lNFN—PISA, while the wafer processing has been carried out by CSEM, Neuchatel,PISA—ALEPH vertex group. The design of the detectors and of the complete mask layout have beenIn the past years, about 400 double—sided Silicon strip detectors have been designed and tested by theINTRODUCTIONdefine a "good" strip if its leakage current is smaller than 10nA (for the ALEPH vertex detector, this OCR Outputof 25;.tm. Few strips have a leakage current greater than 1nA, and only 4 more than 500nA. Weis around 10()pA and on the ohmic side is about twice as large, since the strip pitch is 50p.m insteadcurrent scan is shown in figure 5a (juntion side) and 5b (ohmic side): the current on the junction sidewith respect to the neighbouring ones and the bias resistance to the guard ring. The result of athe current of each individual strip, and on the ohmic side we also measure the isolation of each stripwell, we always make automatic scans of the strips on both sides. On the junction side we measureIn order to measure the number of bad strips and to understand the origin of the leakage current aslower than 1uA.figure 4: the most probable value is around 200nA and 73% of the detectors have a leakage currentmain detector. The complete statistics on the current of the 306 "HODO 4" detectors is reported inring collects also a diffusion current coming from the udepleted region of the wafer surrounding thecurrents after cutting the detectors are usually lower than before, because in the latter case the guardcomplete depletion of the detector, followed by a moderate increase. We have obseved that theboth sides. The total current as a function of the bias voltage is usually almost constant up to theWe have measured the total leakage current of the detectors by simply connecting the guard rings on2. LEAKAGE CURRENT MEASUREMENTSranges from few MQ to SOMQ, depending on the design and on the process spread.since it is dominated by the bulk conductivity, but at depletion it reaches the working value, whichresistance of this channel is shown in figure 3 versus the bias voltage: below the depletion it is lowtype surface channel between each individual n+ strip and the guard ring. The value of the500ttm apart from the guard ring, and the pf blocking strips are shaped in such a way to leave a npositive fixed oxide charge at the Silicon-Silicon dioxide interface. In fact the n+ strips terminateThe bias of the nt strips on the ohmic side is done using the n accumulation layer induced by thecompletely satisfactory, although it is not exactly predictable, since it depends on the leakage cturent.approximately IGQ at l()()pA (the typical value of the strip leakage curent ). This value isbetween the strip and the guard ring is inversely proportional to the leakage current itself, and it isof the bias voltage: the potential in the working point is just a few volts. The dynamic resistancethe punch—through current. This voltage can be measured, and it is reported (figure 2) as a functionopposite side, the pr strips charge up to a voltage such that the leakage current of the strip is equal to4ttm apart from the guard ring, which is kept at ground voltage: when a positive bias is given on theThe strips on the junction side are biased using the punch-through mechanism [7]. All pf strips endtest structures.Nb: 1.4x10cm‘3 up to Nb: 8.4xlOHcm·3, as confirmed by C—V measurements on diodes in theubatches) up to a maximum of about 6OV, corresponding to an effective doping of the bulk fromhigh production yield for the detectors. The depletion voltages range from IOV (for 2 particularsystem, is, according to us, negligible with respect to the advantages of a simpler process and of aRespect to intrinsic AC-coupled detectors , the increased complexity in bonding and assembly of thecapacitor to be AC coupled to electronics: in ALEPH this has been done using an "ad hoc" chip.The detector turns out to be intrinsically DC coupled, and therefore each strip needs an externalside: biasing the detectors and stopping the edge currents.ln the gate—controlled diode (GCD) we measure the surface generation velocity, plotting the diode OCR Outputsuch as 0.4-0.5 s.eq.l. t usually ranges from l0 to l00ms, but on some wafers we have also got larger values,where id is the asymptotic value of the current, A the diode area and the other values are defined afterr = qni A d / 2 id, (eq.4)due to bulk generation, we can evaluate the carrier generation lifetime 1:diode the current reaches a plateau at the depletion: making the hypotesis that this current is totallyThe other measurement we usually perform on diodes is the current- voltage dependence. For a goodshowing that the bulk doping is constant, followed by a plateau.plotting C2 (curve (b) on the left scale): there is a linear dependence up to the depletion voltage,the diode area and a the dielectric constant of Silicon. The depletion voltage is better evaluated bywhere V is the reversed applied voltage, Vld the total depletion voltage, d the detector thickness, Afor V<V,d (eq.3)C(V) :fig.6): it shows a dependence given by:ln the pinnf diode we first measure the capacity versus the bias voltage (curve (a) on the right scale ofstructures placed around the main detector.controlled diodes and the last using MOS capacitors: all of these devices are inserted in the testoxide charge at the Si-SiO2 interface. The first two are measured using diodes, the third using gatethe bulk dopingdensity, the charge carrier generation lifetime, the surface generation velocity and theAs seen in sections l and 2, the most important quantities that determine the detector performance are3. TEST STRUCTURE MEASUREMENTScurrent.ranges from 60 to l4() pA, and we conclude that this is the dominant contribution to the leakagesurface generation velocity. Since SO : l0-25 cm/s , as measured on dedicated test structures, iswhere q, ni, L are the same of eq. l, w is the gap between pf implantated strips and S0 is the(eq. 2)is: qniwLSd/2On the contrary, the surface generation current can be expressed by:for tc the values of 10-l00ms measured in the test structures (section 3), ib ranges from 3 to 30 pA.the wafer thickness (300ttm), L the strip lenght (5cm) and tt is the carrier lifetime generation. Usingwhere q is the electric charge, ni is the intrinsec carrier concentration, p is the strip pitch (25 tim), d(eq. 1)ih: qnipdL/21,can be expressed by the formula:the current of a strip due to bulk generation and to surface generation. At total depletion the formernegligible contribution from edge effects. Moreover, we have tried to evaluate the contributions tothe junction side, 1000 on the ohmic): we conclude that the currents on our detectors have acurrent (100pA on the junction side, 200pA on the ohmic side) times the number of strips (2000 onWe note that the average total current of a detector (~200nA) is just the product of the average stripdetector efficiency, and we use it in the definition of "good" detector.The percentage of the good strips is an important quality factor, since it is proportional to thethe neighbouring ones, and (for the ohmic side strips) if its bias resistance is in the range 5-50MQ.is where the shot noise starts to be a significant fraction of the electronic noise), if it is isolated fromand of the hybrid electronics as well. OCR Outputlaboratory in perfect efficiency and for his professionality and skill in the assembly of the detectorsWe are greatly indebited with Mr. Alessandro Profeti, for his continuous help in keeping ourACKNOWLEDGEMENTcontained.wafer processing, and therefore the costs of the masks and of the wafer processing have beenAt the end, we point out that the DC coupling choice has allowed us to limit the number of steps in thegroup of the detectors rejected because of the current cut.handling. We must specify that, if a detector fails both conditions (i) and (ii), it is computed in thecurrent cut (17.6%) and the number of bad strips (6.9%), three wafers (1%) were damaged in thedefine as the "production yield". The remaining 25.5% detectors are rejected because of theWith these constraints, the percentage of "good" detectors is 74.5%, and this is the quantity that we(ii) percentage of bad strips (defined in section 2) < than 1%.(i) leakage current < 3uA and stable with time,conditions:detector, that is quite safe for the installation in a vertex detector, if it verifies the two followingMoreover, on the basis of the test performed on the 306 "HODO 4" detectors, we can define a "good"comprehension of the device behaviour and in the diagnosis of the bad detectors.centers. We have also presented several measurements done on test structures, to help in the25cmz detectors, and we have shown that this current is mostly (70-90%) due to surface generationdetectors designed by us. The average leakage current at the operation voltage is about 200nA forWe have described the main, large scale, tests performed on the double-sided DC coupled Silicon strip4. CONCLUSIONSvalue for <111> oriented Silicon.Si—SiO2 interface: we obtained values from 2x10u to 9xl0l lcharges/cm2, which is a reasonableat which the fiat-band condition is reached, and we can compute the number of oxide charges at theinversion: the capacitance drops to a lower value. From the curve we can evaluate the voltage (Vtb)accumulation layer is removed and then the bulk is depleted until the surface is brought intooxide there is an electron accumulation layer, due to the positive oxide charges. Increasing V, thefunction of V, the bias voltage. For V=0 the bulk is not depleded, and in the surface under theIn the MOS capacitors we estimate the oxide charge density (QOX) measuring the capacitance (C) as aStandard values for S0 are I0-25 cm/s, a bit high for <111> Silicon.SO = 2Ai / qniS (eq.5)is then given by:surface are collected: an increase Ai in the diode current is measured. The surface generation velocityonly the bulk current is observed, while in depletion also the electron-hole pairs generated at theaccumulation, to depletion and then into the inversion condition. In accumulation and inversionbias voltage. Changing the gate potential, the GCD region under the oxide is brought, starting fromcurrent as a function of the gate voltage: this curve is shown in figure 7 for different values of the[7] P.Holl et al., IEEE Trans. Nucl. Sci., _N _, 251 (1989) OCR Output[6] J.Straver et al., CERN-PPE/94-26[5] M.Conti's talk at this meeting[4] P.Coyle's talk at this meeting[3] B.Alpat et al., Nucl. Instr. and Meth., Ai 197 (1992)[2] G.Batignani et al., Nucl. Phys. (Proc. Suppl.) 235, 297 (1991)[ll G.Batignani etal., Nucl. Instr. and Meth., @7], 147 (1989)REFERENCES7) Gate-controlled diode current in function of the gate voltage for different values of the bias voltage.b) Cd on the left scale.a) the diode capacity (C) on the right scale,6) Reversed bias voltage (V) dependence of:b) ohmic sidea) junction side5) Leakage current in function of the strip number:4) Current distribution at the working voltage for the 306 "HODO 4" detectors.3) Ohmic side resistance in function of the bias voltage between a n+ strip andthe guard ring2) Electric potential of a strip on the junction side as a funcion of the bias voltageb) "HODO 6" (1994)a) "HODO 4" (1989)1) The dctcctors inserted in 4" Silicon wafers:FIGURE CAPTIONS

Double-sided silicon strip detectors in Pisa

Double-sided silicon strip detectors in Pisa

Abstract

Similar works

Full text

Available Versions

CERN Document Server