The most complex Silicon Detectors: Silicon Drift!

1. The most complex Silicon Detectors: Silicon Drift!

2. Silicon Drift Detectors (SDD) E. Gatti, P. Rehak, Semiconductor Drift Chamber - An Application of a Novel Charge Transport Scheme, Nucl. Instr. and Meth. A 225, 1984, pp. 608-614. I rivelatori a deriva sono usati • nell'ambito degli esperimenti con ioni pesanti ultrarelativistici (altissima desità di particelle, tracking con TPC, rivelatori lenti read-out time = ms) • per applicazioni di tipo medico / industriale (rivelazione raggi X) P. Lechner et al., Silicon drift detectors for high resolution room temperature X-ray spectroscopy, Nucl. Instr. and Meth. 1996; A 377, pp. 346-351. Svantaggi / sfide - tempo di raccolta lungo - elettronica complessa - alta tensione (per i silici) - richiede calibrazione accurata della temperatura - richiede calibrazione accurata dell'uniformità del silicio Vantaggi + punti 2D senza ambiguità + alta risoluzione per entrambe le coordinate + costo minore rispetto ai pixel + lettura analogica Particle ID

3. d d 2 2 d - - V V D D 4 4 + + p n n p - V D 4 + = 0 Rivelatori al silicio a deriva Il concetto 1. due giunzioni p+n contrapposte ("back-to-back"), entrambe in polarizzazione inversa, svuotano il silicio dalle cariche libere e creano un campo elettrico con un minimo al centro

4. -V -100 y -90 -100 -90 -80 -70 -60 -50 -40 z -80 -70 -100 -90 -80 -70 -60 -50 -40 -50 -60 = 50 V - n d = 300 µm -50 -50 -40 z y V D + + p n 2. creando una catena di coppie di giunzioni "back-to-back", ciascuna coppia con una tensione decrescente rispetto a quella precedente, si genera il campo di deriva per portare la carica fino all'elettrodo n+

5. Silicon Drift Detectors (1) Particle n+ n+ n+ P+ P+ P+ P+ P+ - + n + - - + P+ P+ P+ P+ P+ P+ d d 2 2 d - - V V D D 4 4 + + p n n p - V D 4 + = 0 Silicon drift detectors are charged partcle detectors capable of providing both two-dimensional position information and ionization measurements. The operating principle is based on the measurement of the time necessary for the electrons produced by the ionization of the crossing particle to drift from the generation point to the collection anodes, by applying an adequate electrostatic field. 7 cm The transport of electrons, in a direction parallel to the surface of the detector and along distances of several centimetres, is achieved by creating a drift channel in the middle of the depleted bulk of a silicon wafer. At the edge of the detector, the electrons are collected by an array of small size anodes. The measured drift time gives information on the particle impact point coordinate y. The charge sharing beween anodes allows the determination of the coordinate along the anode direction x. x y

6. Silicon Drift Detectors (2) In practice, both the drift field and the depletion bias are produced by p+ parallel strips implanted on both faces of the detector. Each strip is polarized with a negative voltage proportional to its distance from the anodes, in order to produce the drift field. In this way, the p+-n junctions are reverse polarized and can assure the depletion of the detector through a n+ ring placed at the periphery of the detector. Potential energy in the Silicon Drift Detectorobtained with a numerical simulation Coordinate x (anode axis) The diffusion and Coulomb repulsion between electrons play a significant role in the drift detectors since the drift time is of the order of a few m. In the thickness direction, they are compensated by the parabolic potential, but generate an increase of the electron cloud size in both other directions. The electron cloud reaches the collection zone with a size increasing as a function of the total drift time. Thus a charge may be collected by more than one anode and the coordinate x is determined as the centroid of the charge deposited on the touched anodes. Typically, a 200 m pitch allows a precision of 30 m. ANODE Coordinate y (drift axis) The signal measured on each anode is amplified and sampled with a typical frequency of a few tens of MHz, depending on the drift velocity and the peaking time of the electronics. The coordinate y is measured by calculating the elapsed time between an external trigger and the arrival of the charge

7. Applicazioni mediche diagnostica con raggi X It consists of a volume of fully depleted high-resistivity silicon, in which an electric field with a strong component parallel to the surface drives electrons generated by the absorption of ionising radiation towards a small sized collecting anode. The electric field is generated by a number of increasingly reverse biased field strips covering one surface of the device The radiation entrance window on the opposite side is made up by a non-structured shallow implanted junction giving a homogeneous sensitivity over the whole detector area. animation • The unique property of this type of detector is the extremely small value of the anode capacitance, which is independent of the active area. This feature allows to gain higher energy resolution at shorter shaping times compared to conventional photo diodes and Si(Li) detectors, recommending the SDD for high count rate applications • Due to the elaborated process technology used in the SDD fabrication the leakage current level is so low that the SDD can be operated with moderate cooling by means of a single stage Peltier element. • The SDD's energy resolution (FWHM < 145 eV @ MnKa, -20oC) can be compared to that of a Si(Li) detector requiring no expensive and inconvenient liquid nitrogen cooling. It surpasses the quality of pin-diodes.

8. Esperimenti di fisica nucleare delle alte energie: CERES - STAR • CERES-NA45 @ CERN SpS • Sistema di Drift circolari usate come rivelatore di vertice (1996-2000) • Problemi legati al partitore esterno • STAR @ RHIC: Silicon Vertex Telescope • 3 piani cilindrici di SDD rettangolari con partitore esterno • smontate dopo 4 anni di presa dati • Problemi legati al partitore esterno

9. SDD per ALICE R&D • inizio progetto 1992 • INFN DSI project in collaboration with CANBERRA Semiconductors. The aim of the project was the production of a large area SDD (5 inch wafers) with integrated high voltage divider • studio del materiale • Neutron Transmutation Doped 5-inch silicon wafers with a resistivity of 3kΩ*cm and a thickness of 300μm. • simulazione del campo di deriva e raccolta • definizione della geometria di catodi, anodi e metallizzazione • definizione dei parametri di funzionamento V/gap = tensione di polarizzazione • simulazione delle zone di guardia • raccolta corrente superficiale • minimizzazione rischi di break-down tra catodi centrali e anello di guardia • test prototipi e studi di radiation damage • Primi prototipi 1993-94  rivelatori unidirezionali • Primo prototipo con geometria quasi definitiva bidirezionale 1998

10. Simulazioni (I) • We need an extensive numerical simulation of SDD electrical behaviour. • The simulation of a complete large area detector cannot be performed because of the huge amount of memory required. Hence the calculation has to be done on limited portions of the device, introducing artificial boundary conditions. • We use a 2D approximation ( 3D differential equations would in principle be required) • This approximation can be done if we consider cross sections orthogonal to the cathodes in such a way that the partial derivative along the third dimension is negligible. • We use ATLAS device simulation software produced by SILVACO. The simulations regard the following regions: • collection zone • drift region • guard region • injectors

11. Simulazioni (II-drift region) • The electric field needed to drift the electron cloud is imposed by a suitable bias of the drift cathodes • In order to obtain a realistic result, the lateral boundaries must be kept far from the region of interest (the central region). • For this reason we used a high number of cathodes (9), leading to a total length of about 1.5mm against a device depth of 0.3mm. • Moreover, the first couple of cathodes on the left are extended in order to further remove the left edge. • An anode is located at the right side in order to guarantee a contact for the bulk.

12. Simulazioni (III-drift region) • The bottom of the potential gutter is perfectly linear; The green line is the potential profile 0.1μm under the Silicon oxide. • It is visible the effect of the field-plate that lowers the potential variations (electric field) near the junctions • It is worthwhile noting that the distance between the red and the green line, passing from one cathode to another, is constant, meaning that there is no influence of the boundary solution.

13. Simulazioni (IV-Collection zone) • The collection zone is one of the critical regions in a SDD. Here the electron cloud is forced to move from the middle plane of the detector toward the anode array in the n-side. The forcing electric field is applied by properly biasing the last few cathodes in the proximity of the anodes. • First we must avoid a trapping of the signal electrons under the oxide when approaching the surface. • Second we have to minimize the non-linearity of the drift speed associated with the transversal movement towards the n-side of the detector. • Third we have to guarantee a good potential separation between anodes and perimeter to avoid inefficiencies of the electron collection.

14. Simulazioni (IV-Collection region) .The picture shows an overlay of the potential map and the trajectory of eight electrons placed at various positions. It is worthwhile noting that the "pull-up" region is very short (200μm), minimising the systematic error on the drift time. Such a kind of collection minimizes also the risk of electron trapping under the oxide because the trajectories are kept far from the surface up to the anode The potential barrier between anodes and n+bulk contact is about 6V.

15. ALICE DB-2 Deriva Deriva SDD Alice: 7.02  7.53 cm  300mm Diviso in due volumi attivi dal catodo centrale polarizzato a -2370V Ciascun volume ha 256 anodi di raccolta (passo di 294 mm). 292 catodi (passo di 120mm) permettono di impostare il campo di deriva tramite un partitore di resistenze impiantate.

16. Caratteristiche principali partitore integrato sul rivelatore doppia catena di resistenze in polisilicio (R=160kΩ) • una per catodi di campo • una per catodi di guardia anodi griglia isolante

17. Guard Zone • At each side of the drift cathode array, p+ implants (guard strips) grade the potential from the highest negative voltages to grounded outer n+ implant ring. Since this region should be as small as possible the electric field needs a careful evaluation. • Furthermore, as the voltage difference between two consecutive guard-zone strips is 16V (see detector description), the punch-through phenomenon should be carefully evaluated

18. Caratterizzazione dei rivelatori Produzione iniziata settembre 2004 (Canberra Semiconductors) Test sui rivelatori nudi svolti presso INFN Trieste Calculating on a yield of 50%, about 500 detectors had been characterized at best • First a visual inspection has to be done to check for interruptions or shorts in the metal. • The second step is to check both the current at the anodes the probe card connects together the anodes in groups of eight • and the linearity of the potentail on the divider checking the voltage drop every ten drift cathodes Connection made by use of two probe cards able to contact the detector safely from both faces (probe pad coordinates) to choose only the well-performing ones with minimal risk of damage to the detector during this operation.

19. Risultati tipici uniformità del partitore corrente agli anodi

20. Detector selection criteria • Linearity of the potential distribution on the integrated divider. • Due to local defect generating high current or to a punch-through current among the cathodes. • Non-linearity of the potential distribution generates a systematic error on the position resolution along the drift direction. Furthermore when the distortion on one side • If the difference on the voltage drop on the resistor connecting two consecutive cathodes is of the order of 0.1V  the electron cloud is shifted dangerously to one of the surfaces • anode leakage current • this determines the noise - and therefore efficiency - of the detector readout (the anode capacitance can be ignored since it will always be small compared to the fixed contribution introduced by the readout microcables). • LIMIT for Anode current = 100 nA

21. Lettura di 64 canali (impulsi di test) Canale Tempo  25ns Lettura di un canale (impulsi di test) Segnale (ADC) Tempo  25ns Esempio di acquisizione di segnali

22. Esempio di un evento dall'esperimento STAR Ampiezza Tempo deriva Anodo

23. Problemi delle SDD • Sensibilità alla temperatura • l'integrazione di strutture di calibrazione • Sensibilità all'uniformita del silicio di base • la mappatura dei sensori

24. 667 V/cm 120 ns 3% T=3.6° Temperature stability dependence La velocità di deriva dipende dal campo, ma anche dalla temperatura (per non dover correggere per l'influenza della temperatura sulla misura, ci vorrebbe una stabilità di 0.1 gradi Celsius) => per calibrare, strutture chiamate "iniettori" sono integrate sul rivelatore permettendo di generare elettricamente segnali da posizioni conosciute

25. iniettori Iniettori • 3 injector lines are inserted between consecutive drift cathodes at distances of 3 mm, 17.6 mm and 34 mm from the anodes. • Each line consists of a metal strip deposited over the oxide. Beneath the strip, separated by a 100 nm thick oxide, it runs a p+ implant interrupted in 33 points that constitute the injection locations. • In these points, 100 mm wide, there is an accumulation of electrons due to the positive oxide charge. Applying a negative pulse to the metal line we push a certain number of these electrons in the silicon bulk. • At the anodes we obtain three sets the drifted images of the 33 injectors.

26. simulazionee risultati dei test di laboratorio Injector event in SDD module mounted on the ladder

27. Uniformita’ del drogaggio (resistivita’) • Se la resistività del silicio del rivelatore non è abbastanza uniforme si creano dei campi elettrici parassiti che spostano la carica dalla traiettoria ideale, • quindi si trova un errore sistematico fra la posizione misurata e la posizione dove la particella è realmente passata

28. Mappe degli errori sistematici per metà di un rivelatore(beam test data) x>0 x = XSDD-XREF x<0 y>0 y = vdrift*tSDD-YREF y<0

29. Scelta del materiale • studi accurati su silicio “Floating zone” e “Neutron Transmutation Doped NTD” 1992-1994 • Floating zone: fluttuazioni di resistività fino a 30% • NTD: <10% (Silicio Wacker) • Produzione ALICE: • iniziata su Wacker -> Fluttuazioni viste in slide precedente  necessità di “mappare” ogni singolo rivelatore per correggere off-line i dati dagli effetti sistematici (stabili nel tempo) • continuata su TOPSIL  fluttuazioni pressoche’ inesistenti • stazione di mappatura presso lab tecnologico INFN Torino usata come stazione di test su “moduli” completi (DETECTOR + FEE + CAVI ALIMENTAZIONE E DATI + schede ausiliarie)

30. FEE amplify-memory-ADC (PASCAL) event buffer chip (AMBRA) Analogue memory Preamplifiers ADCs SDD: front-end

31. SDD module p-side with ladder cables and end-ladder boards LV Board Transition Cable HV Board Heat exchanger at the back of Hybrid LV Board

32. L’Inner Tracking System di ALICE

33. The ALICE Inner Tracking System SSD • 6 Layers, three technologies (keep occupancy ~constant ~2%) • Silicon Pixels (0.2 m2, 9.8 Mchannels) • Silicon Drift (1.3 m2, 133 kchannels) • Double-sided Strip Strip (4.9 m2, 2.6 Mchannels) SDD SPD Lout=97.6 cm Rout=43.6 cm

34. ITS Mechanical assembly Positioning rings CF support cones

35. Reverse biased p-n junction (I) p n -V p+ n E + - - - - - - - - - - - - - + + + + + + + + + + + + - - + + + - - + V + - - -xp xn +V d NA>>ND -xp xn x x

36. Reverse biased p-n junction (II) Depletion voltage: voltage necessary to deplete all the junction thickness How to know the depletion voltage of a diode? Measurement of the capacitance

37. Leakage current The main sources of leakage current in a silicon sensor are: 1) Diffusion of charge carriers from undepleted regions of the detector to the depleted region. Generally well controlled, small contribution ~few nA/cm2 2) Thermal generation of electron-hole pairs in the depleted regions. Temperature dependent, contribution ~ A/cm2 3) Surface currents depending on contamination, surface defects from processing.. It may be the dominant contribution, but it can be reduced processing guard rings

38. p-n junction as detector Metal contact photon Charged particle -V n-type bulk electron hole +V n+-type implant P+ Energy necessary for a m.i.p. to produce a pair of electron-hole in Si: 3.6 eV A m.i.p. produces ~25000e-≈ 4fC Energy lost by a m.i.p. in 1 mm of silicon is ~ 300 KeV. The typical thickness of detectors is ~300m.

39. Fabrication N-type silicon B B n-type wafers are oxidized at 1030oC to have the whole surface passivated. SiO2 Using photolithographic and etching techniques, windows are created in the oxide to enable ion implantation. Different geometries of pads and strips can be achieved using appropriate masks. The next step is the doping of silicon by ion implantation. Dopant ions are produced from a gaseous source by ionisation using high voltage.The ions are accelerated in an alectric field to energy in the range of 10 keV-100 keV and then the ion beam is directed to the windows in the oxide. P+ strips are implanted with boron, while phosphorous or arsenic are used for the n+ contacts. As P+ An annealing process at 600oC allows partial recovery of the lattice from the damage caused by irradiation. n+ Al The next step is the metallisation with aluminium, required to make electrical contact to the silicon. The desired pattern can be achieved using appropriate masks. The last step before cutting is the passivation, which helps to maintain low leakage currents and protects the junction region from mechanical and ambient degradation.

40. Charged particle E Detecting charged particles • The impinging charged particles generate electron-hole pairs • ionization • Electron and holes drift to the electrodes under the effect of the electric field present in the detector volume. • The electron-hole current in the detector induces a signal at the electrodes on the detector faces. Metal contact -V P+-type implant Reverse bias n-type bulk electron hole +V n+-type implant

41. L K Minimum Ionizing Particle Charged particle detection • Energy loss mainly due to ionization • Incident particle interacts with external electrons of Si atoms • All charged particles ionize • Amount of ionization depends on: • particle velocity • particle charge • medium density