- Email: [email protected]
Advances in the development of pixel detector for the SuperB Silicon Vertex Tracker
Nuclear Instruments and Methods in Physics Research A 731 (2013) 25–30
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Advances in the development of pixel detector for the SuperB Silicon Vertex Tracker E. Paoloni p,r,n, D. Comotti a, M. Manghisoni a,b, V. Re a,b, G. Traversi a,b, L. Fabbri c,d, A. Gabrielli c,d, F. Giorgi d, G. Pellegrini d, C. Sbarra d, N. Semprini-Cesari c,d, S. Valentinetti c,d, M. Villa c,d, A. Zoccoli c,d, A. Berra e,f, D. Lietti e,f, M. Prest e,f, A. Bevan g, F. Wilson h, G. Beck g, J. Morris g, F. Gannaway g, R. Cenci i, L. Bombelli k,l, M. Citterio l, S. Coelli l, C. Fiorini k,l, V. Liberali j,l, M. Monti l, B. Nasri k,l, N. Neri l, F. Palombo j,l, A. Stabile j,l, G. Balestri r, G. Batignani p,r, A. Bernardelli r, S. Bettarini p,r, F. Bosi r, G. Casarosa p,r, M. Ceccanti r, F. Forti p,r, M.A. Giorgi p,r, A. Lusiani q,r, P. Mammini r, F. Morsani r, B. Oberhof p,r, A. Perez r, G. Petragnani r, A. Profeti r, G. Rizzo p,r, A. Soldani r, J. Walsh r, L. Gaioni b, A. Manazza o,b, E. Quartieri o,b, L. Ratti o,b, S. Zucca o,b, G.-F. Dalla Betta s,n, G. Fontana s,n, L. Pancheri s,n, M. Povoli s,n, G. Verzellesi n,m, L. Bosisio t,u, L. Lanceri t,u, I. Rashevskaya u, C. Stella v,u, L. Vitale t,u a
Università degli Studi di Bergamo, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Italy c Università degli Studi di Bologna, Italy d Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy e Università dell'Insubria, Como, Italy f Istituto Nazionale di Fisica Nucleare, Sezione di Milano Bicocca, Italy g School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK h STFC Rutherford Appleton Laboratory, Harwell, Oxford Didcot OX11 0QX, UK i University of Maryland, USA j Università degli Studi di Milano, Italy k Politecnico di Milano, Italy l Istituto Nazionale di Fisica Nucleare, Sezione di Milano, Italy m Università degli Studi di Modena e Reggio Emilia, Italy n Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Italy o Università degli Studi di Pavia, Italy p Università degli Studi di Pisa, Italy q Scuola Normale Superiore, Pisa, Italy r Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Italy s Università degli Studi di Trento, Italy t Università degli Studi di Trieste, Italy u Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy v Università degli Studi di Udine, Italy b
art ic l e i nf o
a b s t r a c t
Available online 25 June 2013
The latest advances in the design and characterization of several pixel sensors developed to satisfy the very demanding requirements of the innermost layer of the SuperB Silicon Vertex Tracker will be presented in this paper. The SuperB machine is an electron positron collider operating at the ϒð4SÞ peak to be built in the very near future by the Cabibbo Lab consortium. A pixel detector based on extremely thin, radiation hard devices able to cope with rate in the tens of MHz/cm2 range will be the optimal solution for the upgrade of the inner layer of the SuperB tracking system. At present several options with different levels of maturity are being investigated to understand advantages and potential issues of the different technologies: thin hybrid pixels, Deep N-Well CMOS MAPS, INMAPS CMOS MAPS featuring a quadruple well and high resistivity substrates and CMOS MAPS realized with Vertical Integration
Keywords: Vertex detectors Monolithic active pixels Charged particle tracking
n Corresponding author at: Università degli Studi di Pisa, Italy. Tel.: +39 0502214246. E-mail address: [email protected] (E. Paoloni).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.06.070
26
E. Paoloni et al. / Nuclear Instruments and Methods in Physics Research A 731 (2013) 25–30
technology. The newest results from beam test, the outcomes of the radiation damage studies and the laboratory characterization of the latest prototypes will be reported. & 2013 Elsevier B.V. All rights reserved.
1. Introduction The high energy physics community is facing the seemingly paradoxical condition in which the Standard Model (SM) minimally extended to include the neutrino mass terms is confirmed by all the particle accelerator based experiments while some astronomical [1] and cosmological [2] observations apparently cannot be accommodated within it. Many theoretical models of New Physics (NP) are available to reconcile this discrepancy waiting for confirmations from the collider based experiments. In this respect, the B-factories of the second generation (namely SuperB [3] and Super-KEKB [4]) will be very powerful tools with a complementary role with respect to the LHC experiments. Each one of these two future colliders will produce, in a 5 years time span, a data set consisting of 50–75 109 B mesons pairs and as many τ leptons pairs and D mesons. These data samples will be larger by two orders of magnitude with respect to the present available ones allowing for a significantly improved determination of several SM parameters [5]. The more precise measurements will permit to put more stringent limits on the NP models or, hopefully, will exhibit statistically significant deviations with respect to the SM predictions enabling the determination of the NP parameters. The SuperB collider was approved by the Italian Government as part of the National Research Plan in December 2010 and canceled in December 2012 as a consequence of the per-during economical crisis. SuperB was conceived as an accelerator complex consisting of two intersecting storage rings with low emittance beams colliding with a luminosity of 1036 cm−2 s−1 .
2. The SuperB silicon vertex tracker The SuperB Silicon Vertex Tracker (SVT) consists of six layers of silicon sensors. The five external layers made by double-sided silicon micro-strip sensors, with the strips on the opposite sides of each sensor orthogonally oriented, are very similar to the BaBar ones [6]. The SVT must provide the information needed to reconstruct the proper time difference Δt between the decays of the two B mesons produced in collisions, namely the position in space of the B decay vertices. Neglecting the motion of the B mesons in the center of mass frame, Δt∼Δz=βγ where Δz is the distance among the B decay vertices, β the velocity of the center of mass frame in the laboratory frame and γ the usual Lorentz factor. This task is more challenging in SuperB than in BaBar because the new machine can provide a smaller boost with a βγ ¼ 0:24 to be compared with the BaBar one, βγ ¼ 0:56. The average value of Δz was 250 μm in BaBar and 107 μm for the new machine hence to achieve the same resolution on Δt a proportionally better resolution on Δz is needed. The BaBar SVT spatial resolution is already at the state of the art and was limited by the multiple Coulomb scattering term. The only viable way to improve it is to reduce the lever arm of the extrapolation from the innermost tracking device to the Interaction Point (IP). Fast Monte Carlo studies indicate that it is possible to obtain in SuperB a resolution on Δt comparable to the BaBar one with a BaBar like SVT providing a point resolution of 15 μm at normal incidence integrated with a very light X=X 0 o 1% inner layer (called L0) at a distance smaller than 1.5 cm from the IP providing a point resolution of 10 μm (see Fig. 1 for the overall configuration and Fig. 2 for the Δt resolution performances).
The luminosity scaling background rates for a detector so close to the IP as the L0 are a real concern. Detailed evaluations of the effects of pairs production (i.e. eþ e− -eþ e− eþ e− ) and radiative Bhabha scattering (i.e. eþ e− -eþ e− γ) give an estimate of the hit rate in the 20 MHz/cm2 range. Single beam backgrounds (i.e. beam gas, Touschek and synchrotron radiation from the focusing system) seem by far less worrisome. The L0 readout architecture requirements have been set to sustain a hit rate of 100 MHz/cm2 without significant inefficiency, to accommodate for at present unforeseen or underestimated background sources.
3. The SuperB L0 The SuperB collaboration proposes a striplet based L0 detector for the beginning of data taking at low luminosity. The background resilience of this technology is however marginal at nominal luminosity, hence a pixel based tracking device is under active development. A thin hybrid pixel prototype called SuperPix0 had been successfully tested on the SPS beam at CERN. On the other hand, the tight requirement on the material budget (X=X 0 o 1%) makes the CMOS Monolithic Active Pixel Sensor (MAPS) technology [7] very appealing [8]. Several prototype chips (the “APSEL” series) have been built using the ST Microelectronics, 130 nm triple well technology and demonstrated that the proposed approach is very promising for a thin and fast pixel detector. The ST Microelectronics, 130 nm triple well process [9] is suitable for high performances mixed signals circuits. It allows a P-well to be placed inside a deep N-well, resulting in three types of structures, as shown in Fig. 3. This third type of well is useful for isolating the analog circuitry within it from other digital sections on the chip by the reverse bias between the deep N-well and the P-substrate. A classical optimum signal processing chain for capacitive detectors together with the CMOS latch logic was implemented at the pixel level in these prototypes featuring a pitch of 50 μm. The deep N-well provided by the ST process is used to collect the electrons released in the P-doped substrate by the ionising particle. A charged particle detection efficiency of 92% was measured at the SPS for the APSEL4D prototype [8]. The main limitation of the triple-well “APSEL” series stems from its limited charge collection efficiency, which is related to the effects of the shallow N-well in which the digital p-mos devices are realized. As a matter of fact, the charge is collected by diffusion both by the deep N-well and parasitically by the shallow N-well. To understand quantitatively this phenomenon, the ionization and diffusion had been simulated with a fast Monte Carlo program based on the Bichsel parametrization of the straggling in thin silicon detectors [10] together with a discrete random walk [11]. The simulations show an island of inefficiency for particles impinging the detector near the shallow N-well and far away from the collecting deep N-well (see Fig. 4). The inefficient area covers a surface roughly equal to 5% of the pixel surface. Two approaches are under study to overcome this effect, which is detrimental in view of both the increased shallow N-well area needed to host the more advanced pixel logic necessary to cope with the 100 MHz/cm2 hit rate and of the reduced charge collection efficiency induced by radiation damage. The first one (3D MAPS) consists of the vertical integration of two silicon tiers, one for the sensor and the analog front end and
760,52 SVT Layer 2 SVT Layer 1
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490,00 SVT Layer 3 SVT Layer 4
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SVT Layer 5
30,00
292,75
390,00
430,00 390,00
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Fig. 1. The SuperB interaction region horizontal cross-section. The dimensions are quoted in mm. The 6 layers of the SVT with respect to the Interaction point are represented. On the left and right side of the drawing are also visible the cryostats hosting the magnetic quadrupoles of the final focus.
5 μm thick standard resistivity ð∼10 Ω cmÞ, 12 μm thick standard resistivity ð∼10 Ω cmÞ, 12 μm high resistivity ð∼1 kΩ cmÞ.
1.5 L0 radius = 1.4 cm
1.4
L0 radius = 1.6 cm
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BaBar
1.2 1.1 1 0.9 0.8 0.7 0.6 0.5
0
0.2
0.4
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Fig. 2. SuperB Δt resolution for the B-ΦKs decay mode as a function of the L0 material budget X=X 0 and radius. The dashed line represents the BaBar performance.
the other for digital readout [12]. The competitive effect of the shallow N-well hosting the p-mos devices can be kept at a minimum while providing all the needed space for a flexible and fast in-pixel logic. The second one (APSEL4WELL) is based on the INMAPS process [13] which is an evolution of the triple-well process providing an additional deep P-implant. A deep P-well can be formed in this way around the shallow N-well (see Fig. 3). The small potential barrier at the boundary between this deep P-well and the epitaxial layer is sufficient to keep the electrons generated by the ionizing radiation away from the shallow N-well junction inhibiting its charge collection detrimental effects. The additional advantage of the INMAPS process is the possibility to build the device on a high resistivity epitaxial layer providing a better charge collection efficiency and a higher level of radiation tolerance. 3.1. The APSEL4WELL The APSEL4WELL is a 50 μm pitch pixel matrix built with the INMAPS 180 nm process. The foundry options for the epitaxial layer chosen for the prototypes are:
The version built on the high resistivity epitaxial layer exhibits the best charge collection efficiency thanks to the larger depleted volume at the inversely biased junction between the p-epitaxial layer and the deep N-well electrode. The collecting electrode is made by 4 square diodes with a side length of 1:5 μm (see Fig. 5) connected to a charge amplifier. In such a configuration, the equivalent noise charge (ENC) is kept at a minimum. The analog processing chain realized at the pixel level is shown in Fig. 6. The 4 collecting diodes are connected to the integrated charge to voltage converter formed by the first amplifier and by the capacitance C FB in its feedback loop. The baseline is restored by the P-mos transistor MFB biased to operate in the deep subthreshold region. The second stage is AC coupled to the first one. It is a shaper block featuring a current mirror in the feedback network to discharge the capacitance C 2 at a constant current set by I mirr: . The shaped pulse is then compared to a matrix-wide threshold voltage by means of a threshold discriminator. The noise, the gain and the cluster collected charge were measured on 13 small matrices formed by 9 pixels arranged in a square grid with 50 μm pitch. Each pixel provides an analog readout of the shaped charge pulse. I mirr: was set to provide discharge time of 200 ns for the expected typical MIP pulse-height. The gain (920 mV/fC with a 10% dispersion) and ENC (30e− with a 20% dispersion) measured by injecting a known charge on the collecting electrodes while measuring the analog signal at the shaper output are in good agreement with the post-layout simulations. Tests with the 55Fe source are harder to interpret for the low resistivity devices. As a side effect of the attempt to reduce at an extreme minimum the ENC of the device by reducing the capacitance of the collecting diode junction, the volume in which the charge released by the 55Fe γ is entirely collected (i.e., the depleted volume of the above mentioned junction) is so small that an exceedingly small fraction of γ photons is absorbed within it. The overwhelming majority is absorbed away from the depleted volume so that just a fraction of the charge is collected making the 55Fe γ peak hardly visible. It is still possible to estimate the gain from the end point of the pulse height distribution taking into account the noise contribution.
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Fourth well of the INMAPS process
Standard triple well process analog NMOS
N+ Pwell N+ Deep-Nwell
analog PMOS digital NMOS digital PMOS digital PMOS in a collecting Nwell in a collecting Nwell in a non collecting Nwell
P+
P+ Nwell
N+
Pwell
N+
P substrate
P+ P+ Nwell
P+ P+ Nwell Deep-Pwell
Fig. 3. Schematic cross-section of a CMOS wafer showing the additional deep P-well implant available in the INMAPS process. Not drawn to scale.
Fig. 4. Charged particle detection inefficiency map. The geometry of the collecting electrode and of the shallow N-well are sketched together with the impinging point of particles for which the collected charge is less than 266 e− .
This evaluation is in agreement within 10% with the gain measured with the charge injection method. The spectrum observed with the pixel realized on the high resistivity layer shows instead a clear peak (see Fig. 7) thanks to the sizable depleted volume for which the charge collection efficiency is approximately 100%. Another nice demonstration of the better charge collection properties of the high resistivity p-epitaxial layer had been obtained comparing the results obtained by exposing the matrices to a 90Sr β source (cf. Fig. 8). The matrices built on the high resistivity p-epitaxial layer exhibit a most probable value (MPV) of the charge collected by the matrices near 780e− while the low resistivity matrices with the same epitaxial layer thickness exhibit a smaller 550e− . The MPV for devices with the 5 μm thick epi-layer is even smaller, being roughly equal to 308e− . Besides the small 3 3 matrix featuring analog output for all the 9 pixels, a larger 32 32 matrix with a novel digital readout had been realized. The bad/dead pixel rate is ∼0:3% on a sample of 3072 pixels from 3 different chips. The novel digital readout architecture main feature is the possibility to operate the matrix both in data push and triggered mode. A time stamp bus signal broadcasts to the whole matrix in order to store at pixel level the Time Stamp at which the pixel Fired (TSF). The matrix provides an asynchronous FAST OR signal
Fig. 5. Simplified layout of the APSEL4WELL pixel: N-wells are represented in darker grey and the deep P-well layer in lighter gray. The collecting N-wells are the four small dark grey squares called “sensors” in this figure. The other N-wells hosting the analog front-end PMOS and the digital front-end PMOS are also represented.
Fig. 6. The APSEL4WELL in pixel analog signal processing chain.
for each column to notify the peripheral logic that at least a pixel in the column had fired. In the triggered mode of operation, the peripheral logic receives from an external device the Time Stamp TS0 of the hits to be retrieved from the matrix. The peripheral logic interrogates only the columns for which a FAST OR signal was received during TS0, asking for the hits having TSF ¼ TS0 . In the data push operation the pixels are read out by the peripheral logic right after each rising edge of the time stamp signal. The behaviour of the digital readout architecture has been simulated in both operation modes assuming a hit rate of 100 MHz/cm2. In the
E. Paoloni et al. / Nuclear Instruments and Methods in Physics Research A 731 (2013) 25–30
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200 150 100 50 0 100
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Fig. 7. The pulse height distribution in mV of the 55Fe X-rays source observed by a typical APSEL4WELL pixel on the high resistivity p-epitaxial layer.
Fig. 9. 3D MAPS detection efficiency as a function of the comparator threshold expressed in units of electron charge. QclusterTrgseed_all 1 Mean
120 High resistivity p-epitaxial layer Preliminary.
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0.0002312
Constant 595.2 ± 17.1
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measure the charged particle detection efficiency and the spatial resolution will be done by November 2012 at the SPS.
The first APSEL-like Deep N-Well (DNW) MAPS devices were realized within the 3DIC Consortium to explore the 130 nm Chartered/Tezzaron process. While good 3D chips have still to be tested on the beam line (the first submission was doomed by bad 3D interconnections) some tests on the layers hosting the sensor and the analog front-end confirmed the expected improvement in the charge collection efficiency. A 3 3 matrix realized with this process providing analog readout for all the 9 pixels had been characterized with radioactive sources in the laboratory and on the beam at the SPS. The gain measured with the 55Fe γ is 320 mV/ fC, the ENC is 45e− and the most probable charge collected by the matrix is 1044e− for a MIP. The detection efficiency as a function of the comparator threshold expressed in units of electron charge is shown in Fig. 9. In the next 3D run (Tezzaron/Global Foundries) two pixel matrices will be realized as prototypes for the SuperB L0. Both chips will share a new readout architecture working both in the data push and the triggered configuration. The “3D APSEL” chip will be a 128 100 matrix made by a low resistivity CMOS MAPS sensor while “SuperPIX1” will be a smaller 128 32 matrix connected to a high resistivity pixel sensor.
4. Conclusions 0
100
200
300 400 Pulse height (mV)
500
600
Fig. 8. Comparison of the pulse height spectrum obtained with a 90Sr β source (high resistivity p-epitaxial layer on top and the low resistivity one on bottom). The shaded histogram (visible only on the low resistivity spectrum) is the noise hits pulse height distribution.
triggered mode the efficiency is higher than 98% for a trigger latency shorter than 6 μs with a time stamp granularity of 100 ns. In the data push mode the efficiency remains higher than 99.6% for time stamp granularity smaller than 2 μs. The correct behaviour of these two operating modes has been verified by comparing the threshold scans obtained in the two readout modes. A beam test to
The SuperB detector and machine communities are finalizing the design of their apparatus [14] facing the challenging requirements set by the ambitious goal of building a collider with the unprecedented luminosity of 1036 Hz/cm2. The striplet based design for the L0 can barely meet the requirements at nominal luminosity while the investigated CMOS MAPS approach, successfully realized and tested so far, does not show the desired resilience against radiation damage [14]. Two new approaches are now under active investigation: 3D MAPS and 2D INMAPS. 3D MAPS is a very attractive option, but as of now (October 2011) a fully functional system is still lacking. The aggressive construction schedule SuperB (aiming for the first collisions in 2016) leaves a tight time slot for the development of this technology. On the
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other hand the 2D INMAPS option (even if lacking some of the very intriguing features of 3D MAPS) appears as a more mature technology that seems to better fit the SuperB schedule. References [1] M. Persic, P. Salucci, F. Stel, Monthly Notices of the Royal Astronomical Society 281 (1996) 27. [2] E. Komatsu, et al., Astrophysical Journal Supplement Series 192 (2011) 18. [3] M. Bona, et al., SuperB: a high-luminosity asymmetric e+ e- super flavor factory, Conceptual Design Report, 2007. [4] S. Hashimoto, M. Hazumi, J. Haba, J.W. Flanagan, Y. Ohnishi, et al., Letter of Intent for KEK Super B Factory 2004. [5] B. O'Leary, et al., SuperB Progress Reports—Physics, 2010.
[6] C. Bozzi, et al., Nuclear Instruments and Methods in Physics Research Section A 447 (2000) 15. [7] R. Turchetta, J. Berst, B. Casadei, G. Claus, C. Colledani, et al., Nuclear Instruments and Methods in Physics Research Section A 458 (2001) 677. [8] G. Rizzo, C. Avanzini, G. Batignani, S. Bettarini, F. Bosi, et al., Nuclear Instruments and Methods in Physics Research Section A 650 (2011) 169. [9] STMicroelectronics, Deep sub-micron processes, in: CMP Annual User Meeting, January 2012, Paris, 2012. [10] H. Bichsel, Reviews of Modern Physics 60 (1988) 663. [11] C. Itzykson, J. Zuber, Quantum Field Theory, 1980. [12] W. Dulinski, G. Bertolone, R. de Masi, Y. Degerli, A. Dorokhov, et al., Thin, Fully Depleted Monolithic Active Pixel Sensor Based on 3D Integration of Heterogeneous CMOS Layers, 2009. [13] J. Ballin, J. Crooks, P. D. Dauncey, A.-M. Magnan, Y. Mikami, et al., Monolithic Active Pixel Sensors (MAPS) in a Quadruple Well Technology for Nearly 100% Fill Factor and Full CMOS Pixels (eprint arXiv:0807.2920), 2008. [14] SuperB Technical Design Report, in preparation.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Advances in the development of pixel detector for the SuperB Silicon Vertex Tracker E. Paoloni p,r,n, D. Comotti a, M. Manghisoni a,b, V. Re a,b, G. Traversi a,b, L. Fabbri c,d, A. Gabrielli c,d, F. Giorgi d, G. Pellegrini d, C. Sbarra d, N. Semprini-Cesari c,d, S. Valentinetti c,d, M. Villa c,d, A. Zoccoli c,d, A. Berra e,f, D. Lietti e,f, M. Prest e,f, A. Bevan g, F. Wilson h, G. Beck g, J. Morris g, F. Gannaway g, R. Cenci i, L. Bombelli k,l, M. Citterio l, S. Coelli l, C. Fiorini k,l, V. Liberali j,l, M. Monti l, B. Nasri k,l, N. Neri l, F. Palombo j,l, A. Stabile j,l, G. Balestri r, G. Batignani p,r, A. Bernardelli r, S. Bettarini p,r, F. Bosi r, G. Casarosa p,r, M. Ceccanti r, F. Forti p,r, M.A. Giorgi p,r, A. Lusiani q,r, P. Mammini r, F. Morsani r, B. Oberhof p,r, A. Perez r, G. Petragnani r, A. Profeti r, G. Rizzo p,r, A. Soldani r, J. Walsh r, L. Gaioni b, A. Manazza o,b, E. Quartieri o,b, L. Ratti o,b, S. Zucca o,b, G.-F. Dalla Betta s,n, G. Fontana s,n, L. Pancheri s,n, M. Povoli s,n, G. Verzellesi n,m, L. Bosisio t,u, L. Lanceri t,u, I. Rashevskaya u, C. Stella v,u, L. Vitale t,u a
Università degli Studi di Bergamo, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Italy c Università degli Studi di Bologna, Italy d Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy e Università dell'Insubria, Como, Italy f Istituto Nazionale di Fisica Nucleare, Sezione di Milano Bicocca, Italy g School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK h STFC Rutherford Appleton Laboratory, Harwell, Oxford Didcot OX11 0QX, UK i University of Maryland, USA j Università degli Studi di Milano, Italy k Politecnico di Milano, Italy l Istituto Nazionale di Fisica Nucleare, Sezione di Milano, Italy m Università degli Studi di Modena e Reggio Emilia, Italy n Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Italy o Università degli Studi di Pavia, Italy p Università degli Studi di Pisa, Italy q Scuola Normale Superiore, Pisa, Italy r Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Italy s Università degli Studi di Trento, Italy t Università degli Studi di Trieste, Italy u Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy v Università degli Studi di Udine, Italy b
art ic l e i nf o
a b s t r a c t
Available online 25 June 2013
The latest advances in the design and characterization of several pixel sensors developed to satisfy the very demanding requirements of the innermost layer of the SuperB Silicon Vertex Tracker will be presented in this paper. The SuperB machine is an electron positron collider operating at the ϒð4SÞ peak to be built in the very near future by the Cabibbo Lab consortium. A pixel detector based on extremely thin, radiation hard devices able to cope with rate in the tens of MHz/cm2 range will be the optimal solution for the upgrade of the inner layer of the SuperB tracking system. At present several options with different levels of maturity are being investigated to understand advantages and potential issues of the different technologies: thin hybrid pixels, Deep N-Well CMOS MAPS, INMAPS CMOS MAPS featuring a quadruple well and high resistivity substrates and CMOS MAPS realized with Vertical Integration
Keywords: Vertex detectors Monolithic active pixels Charged particle tracking
n Corresponding author at: Università degli Studi di Pisa, Italy. Tel.: +39 0502214246. E-mail address: [email protected] (E. Paoloni).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.06.070
26
E. Paoloni et al. / Nuclear Instruments and Methods in Physics Research A 731 (2013) 25–30
technology. The newest results from beam test, the outcomes of the radiation damage studies and the laboratory characterization of the latest prototypes will be reported. & 2013 Elsevier B.V. All rights reserved.
1. Introduction The high energy physics community is facing the seemingly paradoxical condition in which the Standard Model (SM) minimally extended to include the neutrino mass terms is confirmed by all the particle accelerator based experiments while some astronomical [1] and cosmological [2] observations apparently cannot be accommodated within it. Many theoretical models of New Physics (NP) are available to reconcile this discrepancy waiting for confirmations from the collider based experiments. In this respect, the B-factories of the second generation (namely SuperB [3] and Super-KEKB [4]) will be very powerful tools with a complementary role with respect to the LHC experiments. Each one of these two future colliders will produce, in a 5 years time span, a data set consisting of 50–75 109 B mesons pairs and as many τ leptons pairs and D mesons. These data samples will be larger by two orders of magnitude with respect to the present available ones allowing for a significantly improved determination of several SM parameters [5]. The more precise measurements will permit to put more stringent limits on the NP models or, hopefully, will exhibit statistically significant deviations with respect to the SM predictions enabling the determination of the NP parameters. The SuperB collider was approved by the Italian Government as part of the National Research Plan in December 2010 and canceled in December 2012 as a consequence of the per-during economical crisis. SuperB was conceived as an accelerator complex consisting of two intersecting storage rings with low emittance beams colliding with a luminosity of 1036 cm−2 s−1 .
2. The SuperB silicon vertex tracker The SuperB Silicon Vertex Tracker (SVT) consists of six layers of silicon sensors. The five external layers made by double-sided silicon micro-strip sensors, with the strips on the opposite sides of each sensor orthogonally oriented, are very similar to the BaBar ones [6]. The SVT must provide the information needed to reconstruct the proper time difference Δt between the decays of the two B mesons produced in collisions, namely the position in space of the B decay vertices. Neglecting the motion of the B mesons in the center of mass frame, Δt∼Δz=βγ where Δz is the distance among the B decay vertices, β the velocity of the center of mass frame in the laboratory frame and γ the usual Lorentz factor. This task is more challenging in SuperB than in BaBar because the new machine can provide a smaller boost with a βγ ¼ 0:24 to be compared with the BaBar one, βγ ¼ 0:56. The average value of Δz was 250 μm in BaBar and 107 μm for the new machine hence to achieve the same resolution on Δt a proportionally better resolution on Δz is needed. The BaBar SVT spatial resolution is already at the state of the art and was limited by the multiple Coulomb scattering term. The only viable way to improve it is to reduce the lever arm of the extrapolation from the innermost tracking device to the Interaction Point (IP). Fast Monte Carlo studies indicate that it is possible to obtain in SuperB a resolution on Δt comparable to the BaBar one with a BaBar like SVT providing a point resolution of 15 μm at normal incidence integrated with a very light X=X 0 o 1% inner layer (called L0) at a distance smaller than 1.5 cm from the IP providing a point resolution of 10 μm (see Fig. 1 for the overall configuration and Fig. 2 for the Δt resolution performances).
The luminosity scaling background rates for a detector so close to the IP as the L0 are a real concern. Detailed evaluations of the effects of pairs production (i.e. eþ e− -eþ e− eþ e− ) and radiative Bhabha scattering (i.e. eþ e− -eþ e− γ) give an estimate of the hit rate in the 20 MHz/cm2 range. Single beam backgrounds (i.e. beam gas, Touschek and synchrotron radiation from the focusing system) seem by far less worrisome. The L0 readout architecture requirements have been set to sustain a hit rate of 100 MHz/cm2 without significant inefficiency, to accommodate for at present unforeseen or underestimated background sources.
3. The SuperB L0 The SuperB collaboration proposes a striplet based L0 detector for the beginning of data taking at low luminosity. The background resilience of this technology is however marginal at nominal luminosity, hence a pixel based tracking device is under active development. A thin hybrid pixel prototype called SuperPix0 had been successfully tested on the SPS beam at CERN. On the other hand, the tight requirement on the material budget (X=X 0 o 1%) makes the CMOS Monolithic Active Pixel Sensor (MAPS) technology [7] very appealing [8]. Several prototype chips (the “APSEL” series) have been built using the ST Microelectronics, 130 nm triple well technology and demonstrated that the proposed approach is very promising for a thin and fast pixel detector. The ST Microelectronics, 130 nm triple well process [9] is suitable for high performances mixed signals circuits. It allows a P-well to be placed inside a deep N-well, resulting in three types of structures, as shown in Fig. 3. This third type of well is useful for isolating the analog circuitry within it from other digital sections on the chip by the reverse bias between the deep N-well and the P-substrate. A classical optimum signal processing chain for capacitive detectors together with the CMOS latch logic was implemented at the pixel level in these prototypes featuring a pitch of 50 μm. The deep N-well provided by the ST process is used to collect the electrons released in the P-doped substrate by the ionising particle. A charged particle detection efficiency of 92% was measured at the SPS for the APSEL4D prototype [8]. The main limitation of the triple-well “APSEL” series stems from its limited charge collection efficiency, which is related to the effects of the shallow N-well in which the digital p-mos devices are realized. As a matter of fact, the charge is collected by diffusion both by the deep N-well and parasitically by the shallow N-well. To understand quantitatively this phenomenon, the ionization and diffusion had been simulated with a fast Monte Carlo program based on the Bichsel parametrization of the straggling in thin silicon detectors [10] together with a discrete random walk [11]. The simulations show an island of inefficiency for particles impinging the detector near the shallow N-well and far away from the collecting deep N-well (see Fig. 4). The inefficient area covers a surface roughly equal to 5% of the pixel surface. Two approaches are under study to overcome this effect, which is detrimental in view of both the increased shallow N-well area needed to host the more advanced pixel logic necessary to cope with the 100 MHz/cm2 hit rate and of the reduced charge collection efficiency induced by radiation damage. The first one (3D MAPS) consists of the vertical integration of two silicon tiers, one for the sensor and the analog front end and
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490,00 SVT Layer 3 SVT Layer 4
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Fig. 1. The SuperB interaction region horizontal cross-section. The dimensions are quoted in mm. The 6 layers of the SVT with respect to the Interaction point are represented. On the left and right side of the drawing are also visible the cryostats hosting the magnetic quadrupoles of the final focus.
5 μm thick standard resistivity ð∼10 Ω cmÞ, 12 μm thick standard resistivity ð∼10 Ω cmÞ, 12 μm high resistivity ð∼1 kΩ cmÞ.
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Fig. 2. SuperB Δt resolution for the B-ΦKs decay mode as a function of the L0 material budget X=X 0 and radius. The dashed line represents the BaBar performance.
the other for digital readout [12]. The competitive effect of the shallow N-well hosting the p-mos devices can be kept at a minimum while providing all the needed space for a flexible and fast in-pixel logic. The second one (APSEL4WELL) is based on the INMAPS process [13] which is an evolution of the triple-well process providing an additional deep P-implant. A deep P-well can be formed in this way around the shallow N-well (see Fig. 3). The small potential barrier at the boundary between this deep P-well and the epitaxial layer is sufficient to keep the electrons generated by the ionizing radiation away from the shallow N-well junction inhibiting its charge collection detrimental effects. The additional advantage of the INMAPS process is the possibility to build the device on a high resistivity epitaxial layer providing a better charge collection efficiency and a higher level of radiation tolerance. 3.1. The APSEL4WELL The APSEL4WELL is a 50 μm pitch pixel matrix built with the INMAPS 180 nm process. The foundry options for the epitaxial layer chosen for the prototypes are:
The version built on the high resistivity epitaxial layer exhibits the best charge collection efficiency thanks to the larger depleted volume at the inversely biased junction between the p-epitaxial layer and the deep N-well electrode. The collecting electrode is made by 4 square diodes with a side length of 1:5 μm (see Fig. 5) connected to a charge amplifier. In such a configuration, the equivalent noise charge (ENC) is kept at a minimum. The analog processing chain realized at the pixel level is shown in Fig. 6. The 4 collecting diodes are connected to the integrated charge to voltage converter formed by the first amplifier and by the capacitance C FB in its feedback loop. The baseline is restored by the P-mos transistor MFB biased to operate in the deep subthreshold region. The second stage is AC coupled to the first one. It is a shaper block featuring a current mirror in the feedback network to discharge the capacitance C 2 at a constant current set by I mirr: . The shaped pulse is then compared to a matrix-wide threshold voltage by means of a threshold discriminator. The noise, the gain and the cluster collected charge were measured on 13 small matrices formed by 9 pixels arranged in a square grid with 50 μm pitch. Each pixel provides an analog readout of the shaped charge pulse. I mirr: was set to provide discharge time of 200 ns for the expected typical MIP pulse-height. The gain (920 mV/fC with a 10% dispersion) and ENC (30e− with a 20% dispersion) measured by injecting a known charge on the collecting electrodes while measuring the analog signal at the shaper output are in good agreement with the post-layout simulations. Tests with the 55Fe source are harder to interpret for the low resistivity devices. As a side effect of the attempt to reduce at an extreme minimum the ENC of the device by reducing the capacitance of the collecting diode junction, the volume in which the charge released by the 55Fe γ is entirely collected (i.e., the depleted volume of the above mentioned junction) is so small that an exceedingly small fraction of γ photons is absorbed within it. The overwhelming majority is absorbed away from the depleted volume so that just a fraction of the charge is collected making the 55Fe γ peak hardly visible. It is still possible to estimate the gain from the end point of the pulse height distribution taking into account the noise contribution.
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Fourth well of the INMAPS process
Standard triple well process analog NMOS
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Fig. 3. Schematic cross-section of a CMOS wafer showing the additional deep P-well implant available in the INMAPS process. Not drawn to scale.
Fig. 4. Charged particle detection inefficiency map. The geometry of the collecting electrode and of the shallow N-well are sketched together with the impinging point of particles for which the collected charge is less than 266 e− .
This evaluation is in agreement within 10% with the gain measured with the charge injection method. The spectrum observed with the pixel realized on the high resistivity layer shows instead a clear peak (see Fig. 7) thanks to the sizable depleted volume for which the charge collection efficiency is approximately 100%. Another nice demonstration of the better charge collection properties of the high resistivity p-epitaxial layer had been obtained comparing the results obtained by exposing the matrices to a 90Sr β source (cf. Fig. 8). The matrices built on the high resistivity p-epitaxial layer exhibit a most probable value (MPV) of the charge collected by the matrices near 780e− while the low resistivity matrices with the same epitaxial layer thickness exhibit a smaller 550e− . The MPV for devices with the 5 μm thick epi-layer is even smaller, being roughly equal to 308e− . Besides the small 3 3 matrix featuring analog output for all the 9 pixels, a larger 32 32 matrix with a novel digital readout had been realized. The bad/dead pixel rate is ∼0:3% on a sample of 3072 pixels from 3 different chips. The novel digital readout architecture main feature is the possibility to operate the matrix both in data push and triggered mode. A time stamp bus signal broadcasts to the whole matrix in order to store at pixel level the Time Stamp at which the pixel Fired (TSF). The matrix provides an asynchronous FAST OR signal
Fig. 5. Simplified layout of the APSEL4WELL pixel: N-wells are represented in darker grey and the deep P-well layer in lighter gray. The collecting N-wells are the four small dark grey squares called “sensors” in this figure. The other N-wells hosting the analog front-end PMOS and the digital front-end PMOS are also represented.
Fig. 6. The APSEL4WELL in pixel analog signal processing chain.
for each column to notify the peripheral logic that at least a pixel in the column had fired. In the triggered mode of operation, the peripheral logic receives from an external device the Time Stamp TS0 of the hits to be retrieved from the matrix. The peripheral logic interrogates only the columns for which a FAST OR signal was received during TS0, asking for the hits having TSF ¼ TS0 . In the data push operation the pixels are read out by the peripheral logic right after each rising edge of the time stamp signal. The behaviour of the digital readout architecture has been simulated in both operation modes assuming a hit rate of 100 MHz/cm2. In the
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Fig. 7. The pulse height distribution in mV of the 55Fe X-rays source observed by a typical APSEL4WELL pixel on the high resistivity p-epitaxial layer.
Fig. 9. 3D MAPS detection efficiency as a function of the comparator threshold expressed in units of electron charge. QclusterTrgseed_all 1 Mean
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measure the charged particle detection efficiency and the spatial resolution will be done by November 2012 at the SPS.
The first APSEL-like Deep N-Well (DNW) MAPS devices were realized within the 3DIC Consortium to explore the 130 nm Chartered/Tezzaron process. While good 3D chips have still to be tested on the beam line (the first submission was doomed by bad 3D interconnections) some tests on the layers hosting the sensor and the analog front-end confirmed the expected improvement in the charge collection efficiency. A 3 3 matrix realized with this process providing analog readout for all the 9 pixels had been characterized with radioactive sources in the laboratory and on the beam at the SPS. The gain measured with the 55Fe γ is 320 mV/ fC, the ENC is 45e− and the most probable charge collected by the matrix is 1044e− for a MIP. The detection efficiency as a function of the comparator threshold expressed in units of electron charge is shown in Fig. 9. In the next 3D run (Tezzaron/Global Foundries) two pixel matrices will be realized as prototypes for the SuperB L0. Both chips will share a new readout architecture working both in the data push and the triggered configuration. The “3D APSEL” chip will be a 128 100 matrix made by a low resistivity CMOS MAPS sensor while “SuperPIX1” will be a smaller 128 32 matrix connected to a high resistivity pixel sensor.
4. Conclusions 0
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Fig. 8. Comparison of the pulse height spectrum obtained with a 90Sr β source (high resistivity p-epitaxial layer on top and the low resistivity one on bottom). The shaded histogram (visible only on the low resistivity spectrum) is the noise hits pulse height distribution.
triggered mode the efficiency is higher than 98% for a trigger latency shorter than 6 μs with a time stamp granularity of 100 ns. In the data push mode the efficiency remains higher than 99.6% for time stamp granularity smaller than 2 μs. The correct behaviour of these two operating modes has been verified by comparing the threshold scans obtained in the two readout modes. A beam test to
The SuperB detector and machine communities are finalizing the design of their apparatus [14] facing the challenging requirements set by the ambitious goal of building a collider with the unprecedented luminosity of 1036 Hz/cm2. The striplet based design for the L0 can barely meet the requirements at nominal luminosity while the investigated CMOS MAPS approach, successfully realized and tested so far, does not show the desired resilience against radiation damage [14]. Two new approaches are now under active investigation: 3D MAPS and 2D INMAPS. 3D MAPS is a very attractive option, but as of now (October 2011) a fully functional system is still lacking. The aggressive construction schedule SuperB (aiming for the first collisions in 2016) leaves a tight time slot for the development of this technology. On the
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other hand the 2D INMAPS option (even if lacking some of the very intriguing features of 3D MAPS) appears as a more mature technology that seems to better fit the SuperB schedule. References [1] M. Persic, P. Salucci, F. Stel, Monthly Notices of the Royal Astronomical Society 281 (1996) 27. [2] E. Komatsu, et al., Astrophysical Journal Supplement Series 192 (2011) 18. [3] M. Bona, et al., SuperB: a high-luminosity asymmetric e+ e- super flavor factory, Conceptual Design Report, 2007. [4] S. Hashimoto, M. Hazumi, J. Haba, J.W. Flanagan, Y. Ohnishi, et al., Letter of Intent for KEK Super B Factory 2004. [5] B. O'Leary, et al., SuperB Progress Reports—Physics, 2010.
[6] C. Bozzi, et al., Nuclear Instruments and Methods in Physics Research Section A 447 (2000) 15. [7] R. Turchetta, J. Berst, B. Casadei, G. Claus, C. Colledani, et al., Nuclear Instruments and Methods in Physics Research Section A 458 (2001) 677. [8] G. Rizzo, C. Avanzini, G. Batignani, S. Bettarini, F. Bosi, et al., Nuclear Instruments and Methods in Physics Research Section A 650 (2011) 169. [9] STMicroelectronics, Deep sub-micron processes, in: CMP Annual User Meeting, January 2012, Paris, 2012. [10] H. Bichsel, Reviews of Modern Physics 60 (1988) 663. [11] C. Itzykson, J. Zuber, Quantum Field Theory, 1980. [12] W. Dulinski, G. Bertolone, R. de Masi, Y. Degerli, A. Dorokhov, et al., Thin, Fully Depleted Monolithic Active Pixel Sensor Based on 3D Integration of Heterogeneous CMOS Layers, 2009. [13] J. Ballin, J. Crooks, P. D. Dauncey, A.-M. Magnan, Y. Mikami, et al., Monolithic Active Pixel Sensors (MAPS) in a Quadruple Well Technology for Nearly 100% Fill Factor and Full CMOS Pixels (eprint arXiv:0807.2920), 2008. [14] SuperB Technical Design Report, in preparation.