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Development and performance of double sided silicon strip detectors
Nuclear Instruments and Methods in Physics Research A310 (1991) 160-164 North-Holland
NUCLEAR INSTRUMENTS & METHOOS IN PtlY$1CS RESEARCH Secl)on A i
Development and performance of double sided silicon strip detectors G. Batignani ", L. Bosisio b, E. Focardi t,, F. Forti %M.A. Giorgi a, L. Moneta ~, G. Parrini ¢, G. ToneUi ~ and G. Triggiani " a INFN sezione di Pisa, Italy t, INFN sezione di Ptsa, and Scuola Nonnale Superiore di Pisa, Italy r INFN sezione di Pisa, and Unit'ersitd di ih'sa, Italy a INFN .sezione di Pisa, and Unit~ersit~ di Napoli, Italy "INFN sezione di Firenze, and Unicersitd di Firenze, Italy
Microstrip silicon detectors with orthogonal readout on oppc,site sides have been designed and fabricated. The active area of each device is 25 cm2 and the strip pitch is 25 p.m on the junction side and 50 p.m on the opposite ohmic side. A space resolution of 15 p,m on the junction side (100 Ixm readout pitch) and 24 p.m on the ohmic side (200 p.m readout pitch) has been measured. We also report on AC-coupling chips, designed and fabricated in order to allow AC connection of the strips to the amplifiers. These chips are 6.4 × 5.0 mm~- and have 100 p.m pitch. Both AC-couplers and detectors have been installed as part of the ALEPH minivertex.
1. Introduction
2. Latest version of double side detectors
Since several years our group has been developing silicon detectors with microstrip readout on the two opposite sides of the crystal, in order to measure with high precision in two orthogonal coordinates the impact position of ionizing particles. We fabricated detectors on high rcsistivity (greater than 6 k l ! cm), n-type, 300 ~ m thick silicon wafers. The basic idea is to implant on side (namely the "junction side") p+ type strips to form rectifying junctions, and to subdivide the opposite side (namely the "ohmic side") into n ÷ strips. To increase the interstrip resistance of the ohmic side we inserted p "blocking" strips between e v e n two n ÷ strips. This procedure was first studied on small (! × 1 cm:) prototypes and was reported in ref. [1]. Successively, we fabricated 5 × 5 cm 2 detectors with the same strip configuration of the prototypes. We tested them with radioactive sources and with high energy particles [2] and we proved that they indeed measured simultaneously two coordinates of minimum ionizing particles crossing them. In this paper we report (section 2) on the latest version of the 5 × 5 cm 2 detectors, and we discuss electrical propertlc~ ~tnd readout scheme. In section 3 we present the design and fabrication of chips to AC-couple the strips of silicon detectors to the frontend electronics and in section 4 we quote the detector position resolution.
2.1. Design and bias scheme o f the junction sMe
The junction side of our detectors is shown in fig. la (cross scction) and fig. lb (a detail in top view). As in the previous versions, the strips consist of a p + implantation, to make a rectifying junction with the n substrate, plus an aluminum contact metallization. The strips are 5 cm long, 12 ~.m wide and 13 t~m apart. There are 1985 p+ strips and, since the Aluminum is in contact with the implantation, it is possible to perform direct measurements of the electrical properties of e a c h individual diode (e.g. leakage currents). We recently changed the design of the detector in proximity of the edges of the strips. All p+ strips end at 5 g m distance from a p+ guard ring which surrounds the whole active area. The guard ring, which in a previous version was used only to stop the edge currents, can thus be used to bias the diode strips on the junction side. In fact, holding the guard ring at ground and applying a bias voltage Vn to the opposite (ohmic) side, all p ÷ strips bring themselves to a potential V s, because of the punch-through mechanism [3]. A plot of I/s versus V8 is shown in fig. 2a, while in fig. 2b we show the leakage current of one strip, again as a function of V8. We note that the strip leakage current is very low (120 pA for a bias voltage well above the complete depletion voltage, thus corresponding to 4
0168-9002/91/$03.50 ',~, 1991 - Elsevier Science Publishers B.V. All rights reserved
G, Batignani et al. / Double sided silicon strip detecwr~
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Fig. 3. Details of the ohmic side: (a) cross section, (b) top
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Fig. 2. Relevant parameters of p" strips on junction side in function of the bias voltage: (a) potential respect to the guard ring. (b) leakage current. i11. SOLID STATE DETECTORS
G. l~atignatlictal. / D(n~blc sidt'd silk'on strip detectors
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Bias Voltage (volts) Fig. 4. Interstrip resistance (RI) and strip-guard ring resistance (RG) on the ohmic side in function of the bias voltage. n A / c m 2) and that the strip potential is only a few volts higher than that of the guard ring. Another relevant parameter is the capacitance between two strips at 25 p.m pitch, that was measured to be about 4 pF. To implement the biasing scheme described above,
the p+ strips must be AC-connected to the charge amplifiers and for this purpose, we designed capacitor chips, fabricated on quartz wafers, as described later (section 3). The readout pitch can by any multiple of 50 I~m,
Fig. 5. Photograph of 4 in. silicon wafers with a 5 ×5 cm 2 detector fabricated on it, the junction side is on the left and the ohmic side on the right. Around the detector are shown several test structures for process control or other studies.
G. Batignani et al. / Dtmble sided .silicon strip detectors since we provided a bonding pad on every other strips (see again fig. Ib). it is po~,fiblc, however, to build detectors with a bonding pad on each p ~ strip, as we did in a protot.vpe. In fact, exploiting the capacitive charge division mechanism [4], it is possible to read out only a fraction (typically one out of two or four) of the
strips, with small degradation of the spatial resolution. 2.2. Design and bias scheme of the ohmic side In fig. 3a we show a cross section of the ohmic side. The metallized n + strips are 12 ~m wide and have a pitch of 50 p.m. In the middle of two adjacent n ÷ strips, we implanted a 12 p.m wide p+ strip, which is left floating. When the detector is completely depleted,
1~3
the p implantation interrupts the surhtcc electron accumulation layer, which is caused by the fixed oxide charge at the SiO,-Si interface. The sheet resistivity of the accumulation layer dcpcnds hcavily on the fabrication process, and in different batches we mca.~urcd values from 20 k l l / s q u a r e up to 40 k l l / s q u a r e . The interstrip resistance is thus incroa~d from few k f t, a value too low for good position resolution, up to a value which is defined by the detector design at the strip ends. This design is shown it~ fig. 3b, which also shows the surrounding n + guar0 ring, held at the detector bias voltage Va. The p Llocking strips are shaped in such a way that the resistance between an n + strip and the guard ring (due to the conducting channel shaped in the electron accumulation layer) is a
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Fig. 6. AC-coupler chip: (a) pholographic detai|, (b) cross section sketch of a capacitor. Ill. SOLID STATE DETECTORS
164
G. Batignani et al. / Doubh' sided .~ilicon strip dettx'torx
few M I I at a voltage above depletion (fig 4), This accumulation layer resistor can be used to bias the ohmic side, with negligible voltage difference between strips and guard ring. Each n ' strip has a bonding pad (also shown in fig. 3b) for connection to preamplifiers, to be performed via a capacitor in a similar way to the junction side. The readout pitch can thus be any multiple of 50 o,m.
2.3. Detector production yield In two production batches, 40 detectors, designed as described in section 2a and 2b, have been fabricated on high resistivity ( > 6 kf~cm) silicon wafers, see fig. 5. The processing has been carried out by CSEM at Neuchatel (Switzerland). We measured the basic electrical characteristics and defined as good detectors those which, at 10 V higher than the depletion voltage, satisfied the following criteria: (it the total leakage current was less then 2 I~A, (ii) there are at most two diodes with a leakage current above 100 nA, and no one above 500 nA, (iii) no shorts-circuits among p ÷ strips, (iv) interstrip resistance on the ohmic side between 1 and 6 MfL 29 detectors were accepted (production yield 70%). The selected ones have been installed in the A L E P H minivcrtcx [5] at CERN, and an identical, but smaller, one is now used by the R A D I N group [6] for a feasibility study of digital radiography.
3. AC-coupler chips We have fabricated capacitor chips on quartz wafers for the connection of the detectors to electronics. Each chip, 6.35 mm long and 5.0 mm wide, (fig. 6a) supports 64 capacitors, to be connected on one end to one strip of the detector and on the other to the input of the charge amplifiers. The capacitors are made with a double polysilicon layer, interleaved with a dielectric, as shown in fig. 6b. The processing has been made at the CSEM. The measured capacitance is 200 pF (much larger than the capacitance of one strip), the stray capacitance of one channel (mostly with respect to the neighboring ones) is 0.3 pF, the parallel insulation resistance is larger than 100 G ~ and the breakdown voltage is 20 V. Each chip is tested and accepted if all capacitors have the correct values for the previous quantities. The production yield is around 50%. As a general comment, we stress that separation of the decoupling capacitor from the detector is a great help in the yield and costs of the fabrication processes, since it is possible to test separately the two devices.
The major disadvantage is to double the number of connections, but that has not been a problem in our case, since these microbonds arc very. short and parallel to each other and can easily be done with an automatic bonding machine.
4. Spatial resolution measurement Several detectors were assembled with AC-chips and the C A M E X charge amplifiers [7], to form some modules, later installed in the A L E P H minivertex. A space resolution of 15 ~tm (junction side) and of 24 o,m (ohmic side) was measured in the A L E P H test beam
[51. These values are comprehensive of all effects: electronic noise, pickups from the environment, digitization precision, alignement of modules, and accuracy of reconstruction algorithm. Moreover, the quoted resolution is the average value over the whole detector area (25 cm2).
5. Conclusions We fabricated microstrip detectors on silicon, with orthogonal readout on the two opposite sides. Leakage currents are on average 4 n A / c m 2 and the production yield has reached 71)%. The strip pitch is 25 g m on the junction side and 50 0.m on the ohmic side, while the readout pitch can be any multiple of 50 lxm on both sides. AC-coupler chips, consisting of capacitors on a quartz substratc, have bccn fabricated. The capacitor value is 200 pF and the pitch is 100 ~m. A space resolution of 15 ~tm on the junction side (100 Ism readout pitch) and of 24 0,m on the ohmic side (200 ~tm readout pitch) has been measured. These values are the average on the whole detector area and include all effects from the electronic noise to the accuracy of reconstruction algorithm.
References [I] G. Batignani et al.. Nucl. Instr. and Meth. A273 11988) 677. [2] G. Batignani ct al., Nucl. Instr. and Moth. A277 (1989) 147. G. Batignani ct al.. IEEE Trans. Nucl. Sci. NS-36 (1989) 40. [3] J, Ellison et al.. IEEE Trans. Nucl. Sci. NS-36 (1989) 267. [4] J.B. England et al., Nucl. Instr. and Meth. 185 (19811 43. [5] I I.-G. Moser, these Proceedings (2nd London Conf. on Position-Sensitive Detectors, London, UK, 1991l) Nucl. Instr. and Meth. A310 11991) 490. [6] The RADIN collaboration, Phys. Med. 6 11990) 39. [7] W. Burlier et al., Nucl. Instr. and Meth. A273 11988) 778.
NUCLEAR INSTRUMENTS & METHOOS IN PtlY$1CS RESEARCH Secl)on A i
Development and performance of double sided silicon strip detectors G. Batignani ", L. Bosisio b, E. Focardi t,, F. Forti %M.A. Giorgi a, L. Moneta ~, G. Parrini ¢, G. ToneUi ~ and G. Triggiani " a INFN sezione di Pisa, Italy t, INFN sezione di Ptsa, and Scuola Nonnale Superiore di Pisa, Italy r INFN sezione di Pisa, and Unit'ersitd di ih'sa, Italy a INFN .sezione di Pisa, and Unit~ersit~ di Napoli, Italy "INFN sezione di Firenze, and Unicersitd di Firenze, Italy
Microstrip silicon detectors with orthogonal readout on oppc,site sides have been designed and fabricated. The active area of each device is 25 cm2 and the strip pitch is 25 p.m on the junction side and 50 p.m on the opposite ohmic side. A space resolution of 15 p,m on the junction side (100 Ixm readout pitch) and 24 p.m on the ohmic side (200 p.m readout pitch) has been measured. We also report on AC-coupling chips, designed and fabricated in order to allow AC connection of the strips to the amplifiers. These chips are 6.4 × 5.0 mm~- and have 100 p.m pitch. Both AC-couplers and detectors have been installed as part of the ALEPH minivertex.
1. Introduction
2. Latest version of double side detectors
Since several years our group has been developing silicon detectors with microstrip readout on the two opposite sides of the crystal, in order to measure with high precision in two orthogonal coordinates the impact position of ionizing particles. We fabricated detectors on high rcsistivity (greater than 6 k l ! cm), n-type, 300 ~ m thick silicon wafers. The basic idea is to implant on side (namely the "junction side") p+ type strips to form rectifying junctions, and to subdivide the opposite side (namely the "ohmic side") into n ÷ strips. To increase the interstrip resistance of the ohmic side we inserted p "blocking" strips between e v e n two n ÷ strips. This procedure was first studied on small (! × 1 cm:) prototypes and was reported in ref. [1]. Successively, we fabricated 5 × 5 cm 2 detectors with the same strip configuration of the prototypes. We tested them with radioactive sources and with high energy particles [2] and we proved that they indeed measured simultaneously two coordinates of minimum ionizing particles crossing them. In this paper we report (section 2) on the latest version of the 5 × 5 cm 2 detectors, and we discuss electrical propertlc~ ~tnd readout scheme. In section 3 we present the design and fabrication of chips to AC-couple the strips of silicon detectors to the frontend electronics and in section 4 we quote the detector position resolution.
2.1. Design and bias scheme o f the junction sMe
The junction side of our detectors is shown in fig. la (cross scction) and fig. lb (a detail in top view). As in the previous versions, the strips consist of a p + implantation, to make a rectifying junction with the n substrate, plus an aluminum contact metallization. The strips are 5 cm long, 12 ~.m wide and 13 t~m apart. There are 1985 p+ strips and, since the Aluminum is in contact with the implantation, it is possible to perform direct measurements of the electrical properties of e a c h individual diode (e.g. leakage currents). We recently changed the design of the detector in proximity of the edges of the strips. All p+ strips end at 5 g m distance from a p+ guard ring which surrounds the whole active area. The guard ring, which in a previous version was used only to stop the edge currents, can thus be used to bias the diode strips on the junction side. In fact, holding the guard ring at ground and applying a bias voltage Vn to the opposite (ohmic) side, all p ÷ strips bring themselves to a potential V s, because of the punch-through mechanism [3]. A plot of I/s versus V8 is shown in fig. 2a, while in fig. 2b we show the leakage current of one strip, again as a function of V8. We note that the strip leakage current is very low (120 pA for a bias voltage well above the complete depletion voltage, thus corresponding to 4
0168-9002/91/$03.50 ',~, 1991 - Elsevier Science Publishers B.V. All rights reserved
G, Batignani et al. / Double sided silicon strip detecwr~
Ihl
Silicon Dioxide
Metal (AI)
1~" implantation
Silicon Dioxide
p implantation n+ implantation
n - t y p e Silicon
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p+ implantation (B)
m'~~y~'~~501,tm ~N"N~x~x~Metalizalion (AI)
Metallization (AI)
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Fig. 1. Details of the junction side: (a) cross section, (b) top
Fig. 3. Details of the ohmic side: (a) cross section, (b) top
view.
view.
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Fig. 2. Relevant parameters of p" strips on junction side in function of the bias voltage: (a) potential respect to the guard ring. (b) leakage current. i11. SOLID STATE DETECTORS
G. l~atignatlictal. / D(n~blc sidt'd silk'on strip detectors
162
108
107
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-10 4
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,
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20
30
40
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3
/O-
70
Bias Voltage (volts) Fig. 4. Interstrip resistance (RI) and strip-guard ring resistance (RG) on the ohmic side in function of the bias voltage. n A / c m 2) and that the strip potential is only a few volts higher than that of the guard ring. Another relevant parameter is the capacitance between two strips at 25 p.m pitch, that was measured to be about 4 pF. To implement the biasing scheme described above,
the p+ strips must be AC-connected to the charge amplifiers and for this purpose, we designed capacitor chips, fabricated on quartz wafers, as described later (section 3). The readout pitch can by any multiple of 50 I~m,
Fig. 5. Photograph of 4 in. silicon wafers with a 5 ×5 cm 2 detector fabricated on it, the junction side is on the left and the ohmic side on the right. Around the detector are shown several test structures for process control or other studies.
G. Batignani et al. / Dtmble sided .silicon strip detectors since we provided a bonding pad on every other strips (see again fig. Ib). it is po~,fiblc, however, to build detectors with a bonding pad on each p ~ strip, as we did in a protot.vpe. In fact, exploiting the capacitive charge division mechanism [4], it is possible to read out only a fraction (typically one out of two or four) of the
strips, with small degradation of the spatial resolution. 2.2. Design and bias scheme of the ohmic side In fig. 3a we show a cross section of the ohmic side. The metallized n + strips are 12 ~m wide and have a pitch of 50 p.m. In the middle of two adjacent n ÷ strips, we implanted a 12 p.m wide p+ strip, which is left floating. When the detector is completely depleted,
1~3
the p implantation interrupts the surhtcc electron accumulation layer, which is caused by the fixed oxide charge at the SiO,-Si interface. The sheet resistivity of the accumulation layer dcpcnds hcavily on the fabrication process, and in different batches we mca.~urcd values from 20 k l l / s q u a r e up to 40 k l l / s q u a r e . The interstrip resistance is thus incroa~d from few k f t, a value too low for good position resolution, up to a value which is defined by the detector design at the strip ends. This design is shown it~ fig. 3b, which also shows the surrounding n + guar0 ring, held at the detector bias voltage Va. The p Llocking strips are shaped in such a way that the resistance between an n + strip and the guard ring (due to the conducting channel shaped in the electron accumulation layer) is a
Metal (AI)
[-'-]
SiliconDioxide
r"o!isilicon
,[-'7,
LTO
[¢r//////////////.,r~ Todetector ~__~\\x\x\\\xxxx\\\\\x\\\K\.~
~
To elect~ntcs
Fig. 6. AC-coupler chip: (a) pholographic detai|, (b) cross section sketch of a capacitor. Ill. SOLID STATE DETECTORS
164
G. Batignani et al. / Doubh' sided .~ilicon strip dettx'torx
few M I I at a voltage above depletion (fig 4), This accumulation layer resistor can be used to bias the ohmic side, with negligible voltage difference between strips and guard ring. Each n ' strip has a bonding pad (also shown in fig. 3b) for connection to preamplifiers, to be performed via a capacitor in a similar way to the junction side. The readout pitch can thus be any multiple of 50 o,m.
2.3. Detector production yield In two production batches, 40 detectors, designed as described in section 2a and 2b, have been fabricated on high resistivity ( > 6 kf~cm) silicon wafers, see fig. 5. The processing has been carried out by CSEM at Neuchatel (Switzerland). We measured the basic electrical characteristics and defined as good detectors those which, at 10 V higher than the depletion voltage, satisfied the following criteria: (it the total leakage current was less then 2 I~A, (ii) there are at most two diodes with a leakage current above 100 nA, and no one above 500 nA, (iii) no shorts-circuits among p ÷ strips, (iv) interstrip resistance on the ohmic side between 1 and 6 MfL 29 detectors were accepted (production yield 70%). The selected ones have been installed in the A L E P H minivcrtcx [5] at CERN, and an identical, but smaller, one is now used by the R A D I N group [6] for a feasibility study of digital radiography.
3. AC-coupler chips We have fabricated capacitor chips on quartz wafers for the connection of the detectors to electronics. Each chip, 6.35 mm long and 5.0 mm wide, (fig. 6a) supports 64 capacitors, to be connected on one end to one strip of the detector and on the other to the input of the charge amplifiers. The capacitors are made with a double polysilicon layer, interleaved with a dielectric, as shown in fig. 6b. The processing has been made at the CSEM. The measured capacitance is 200 pF (much larger than the capacitance of one strip), the stray capacitance of one channel (mostly with respect to the neighboring ones) is 0.3 pF, the parallel insulation resistance is larger than 100 G ~ and the breakdown voltage is 20 V. Each chip is tested and accepted if all capacitors have the correct values for the previous quantities. The production yield is around 50%. As a general comment, we stress that separation of the decoupling capacitor from the detector is a great help in the yield and costs of the fabrication processes, since it is possible to test separately the two devices.
The major disadvantage is to double the number of connections, but that has not been a problem in our case, since these microbonds arc very. short and parallel to each other and can easily be done with an automatic bonding machine.
4. Spatial resolution measurement Several detectors were assembled with AC-chips and the C A M E X charge amplifiers [7], to form some modules, later installed in the A L E P H minivertex. A space resolution of 15 ~tm (junction side) and of 24 o,m (ohmic side) was measured in the A L E P H test beam
[51. These values are comprehensive of all effects: electronic noise, pickups from the environment, digitization precision, alignement of modules, and accuracy of reconstruction algorithm. Moreover, the quoted resolution is the average value over the whole detector area (25 cm2).
5. Conclusions We fabricated microstrip detectors on silicon, with orthogonal readout on the two opposite sides. Leakage currents are on average 4 n A / c m 2 and the production yield has reached 71)%. The strip pitch is 25 g m on the junction side and 50 0.m on the ohmic side, while the readout pitch can be any multiple of 50 lxm on both sides. AC-coupler chips, consisting of capacitors on a quartz substratc, have bccn fabricated. The capacitor value is 200 pF and the pitch is 100 ~m. A space resolution of 15 ~tm on the junction side (100 Ism readout pitch) and of 24 0,m on the ohmic side (200 ~tm readout pitch) has been measured. These values are the average on the whole detector area and include all effects from the electronic noise to the accuracy of reconstruction algorithm.
References [I] G. Batignani et al.. Nucl. Instr. and Meth. A273 11988) 677. [2] G. Batignani ct al., Nucl. Instr. and Moth. A277 (1989) 147. G. Batignani ct al.. IEEE Trans. Nucl. Sci. NS-36 (1989) 40. [3] J, Ellison et al.. IEEE Trans. Nucl. Sci. NS-36 (1989) 267. [4] J.B. England et al., Nucl. Instr. and Meth. 185 (19811 43. [5] I I.-G. Moser, these Proceedings (2nd London Conf. on Position-Sensitive Detectors, London, UK, 1991l) Nucl. Instr. and Meth. A310 11991) 490. [6] The RADIN collaboration, Phys. Med. 6 11990) 39. [7] W. Burlier et al., Nucl. Instr. and Meth. A273 11988) 778.