High resolution 3D characterization of silicon detectors using a Two Photon Absorption Transient Current Technique

Journal Pre-proof High resolution 3D characterization of silicon detectors using a Two Photon Absorption Transient Current Technique Marcos Fernández García, Richard Jaramillo Echeverría, Michael Moll, Raúl Montero Santos, Rogelio Palomo Pinto, Iván Vila, Moritz Wiehe

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S0168-9002(19)31291-4 https://doi.org/10.1016/j.nima.2019.162865 NIMA 162865

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Nuclear Inst. and Methods in Physics Research, A

Received date : 30 March 2019 Revised date : 24 September 2019 Accepted date : 25 September 2019 Please cite this article as: M.F. García, R.J. Echeverría, M. Moll et al., High resolution 3D characterization of silicon detectors using a Two Photon Absorption Transient Current Technique, Nuclear Inst. and Methods in Physics Research, A (2019), doi: https://doi.org/10.1016/j.nima.2019.162865. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

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High resolution 3D characterization of silicon detectors using a Two Photon Absorption Transient Current Technique

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Marcos Fern´andez Garc´ıaa,b,∗, Richard Jaramillo Echeverr´ıaa , Michael Mollb , Ra´ul Montero Santosc , Rogelio Palomo Pintod , Iv´an Vilaa , Moritz Wieheb a Instituto

de F´ısica de Cantabria (CSIC-UC), Avda. los Castros s/n, E-39005 Santander, Spain Organisation europ´enne pour la recherche nucl´eaire, CH-1211 Gen´eve 23, Switzerland c SGIker Laser Facility, UPV/EHU, Sarriena, s/n - 48940 Leioa -Bizkaia, Spain d Departamento de Ingenier´ıa Electr´ onica, Escuela Superior de Ingenieros Universidad de Sevilla, 41092 Spain

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b CERN,

Abstract

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The Two Photon Absorption Transient Current Technique (TPA-TCT) is a tool to characterize semiconductor detectors using a spatially confined laser probe. Excess charge carriers are produced by the simultaneous absorption of two sub-bandgap photons in the material. The current induced by the motion of carriers is studied using well known TCT systems. Differently to standard TCT where the energy deposition (pair creation) is continuous along the beam, TPA-TCT reduces this region to an ellipsoidal volume, achieving thus, true 3D spatial resolution. This paper gives an overview of the technique and shows its performance in irradiated detectors, in particular diodes and High Voltage CMOS detectors. Keywords: Two Photon Absorption, Transient Current Technique, RD50, radiation hardness 2010 MSC: 00-01, 99-00 1. Introduction

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As feature size and thickness of silicon radiation detectors are increasingly reducing [1], testing probes of gradually higher spatial resolution are required. Thin detectors are needed for 30 radiation tolerance (higher electric field for the same voltage, lower leakage current) and improved timing (higher slew rate, lower arrival time fluctuations). Small pixels are essential to maintain efficiency (pileup reduction), enhance radiation tolerance (lower leakage current per pixel) and decrease readout 35 noise. Examples of thin detectors with small pixel size are HVHR/CMOS, 3D pixels for HL-LHC or thin LGADs for timing. Visible or near-infrared (NIR) lasers can be micro-focused down to the wavelength limit without diffraction effects [2], therefore providing very good transverse spatial resolution. 40 When a detector is characterized with a beam impinging perpendicular to its surface, the longitudinal resolution (along the propagation direction) is given by the absorption length (penetration depth), ranging from few microns (red laser), to hundreds of microns (NIR laser) in silicon [3]. Therefore longitudinal resolution is achieved only for short wavelengths and it is limited to the surface of the sample. Test beams, using particle 45 accelerators, provide the best transverse 2D spatial resolution yielding, on top, absolute charge calibration. However the energy deposition of a particle is continuous, therefore no spatial resolution along the beam direction is obtained.

Resolution in the bulk of the detector is achieved by grazing angle [4] incidence (particle tracks) or laser edge illumination [5]. In these configurations, either the laser or the particle beam are injected from the edge of the device and depth profiling information is obtained. Yet, due to continuous charge deposition and reach along the beam, carriers can not be confined along the propagation direction of the probe. In the best configuration, pseudo-3D information is obtained combining normal and edge incidence. In this contribution we present, in section 2, a new technique that yields true 3D spatial resolution at any depth in the detector and in one single measurement: the Two Photon AbsorptionTransient Current Technique (TPA-TCT). Section 3 shows its performance on selected irradiated detectors. A summary concludes section 4.

∗ Corresponding

author Email address: [email protected] (Marcos Fern´andez Garc´ıa) Preprint submitted to Journal of LATEX Templates

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2. TPA-TCT introduction Transient Current Techniques (TCT) refer to the observation of the current induced by the transport of excess carriers generated in a semiconductor by an external ionization source [6]. The current induced by the movement of carriers is amplified by a current amplifier and then time resolved using an oscilloscope. From the time development of the signal one can learn about the local conditions the carriers encountered, namely electric field, weighting field and trapping. Normal incidence illumination of the top or bottom sides of a detector using a red laser will decouple the movement of electrons from holes, while IR top or bottom illumination will induce a signal very similar to that of a charged particle. Finally, if the light is inSeptember 24, 2019

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where α and β are the single and two-photon absorption coefficients, N the density of free carriers, σex their absorptivity and (r, z) are cylindrical coordinates. The first term of equation 1 is the well known Beer’s law linear absorption. It is important115 for wavelengths above the bandgap of the material and for low laser irradiance. It leads to an exponential and continuous attenuation of the beam as the material is traversed. The integration of the quadratic term, however, yields a Lorentzian distribution of charges which is much more localized in space (at the fo-120 cus) and leads to a negligible signal away from the focus of the beam, giving TPA-TCT a true 3D spatial resolution. The third

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term is discarded in depleted detectors due to the absence of free carriers in the sensing volume. TPA-TCT exploits charge confinement by using sub-bandgap pulsed lasers (α ∼ 0, no linear term) and high enough power to excite non-linear effects in silicon (second term), while still preventing laser induced micro-plasmas from developing and increasing the free carrier absorption term. Fig. 1 shows a knife-edge scan [10] of the spot from which the Gaussian waist of 1 µm in our experiment is obtained. The inset of the figure depicts the laser probe as an ellipsoid of 13 µm length (calculated from the waist) and 1 µm width. The longer dimension coincides with the propagation direction of the beam. Due to the asymmetric shape of the laser probe, different spatial resolutions are obtained. The elongated dimension dominates spatial resolution in normal (top/bottom) illumination of the sample, while the small waist is used in edgeinjection measurements.

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jected from the side, using a collimated beam, the performance 90 of the detector can be also studied as a function of depth. The power of the laser can be adjusted to simulate a wide range of signal levels, down to a Minimum Ionizing Particle. TCT has been a workhorse for the study of radiation detectors in both ROSE and RD50 collaborations [7, 8]. 95 In standard TCT, photons ranging from visible to IR have enough energy to produce an e-h pair: this is known as Single Photon Absorption (SPA) TCT. No signal is produced if the energy of the photons is smaller than the difference between conduction and valence band. But if two sub-bandgap photons100 arrive in a very short time, a Two Step Process (TSP) transition is possible via a virtual state [9]. The existence of this virtual state is allowed during a period of time not exceeding the uncertainty principle. In order to have two almost coincident photons, pulsed femtosecond pulses are used. In such lasers105 photons are mode-locked into pulses typically 100 fs wide, although the TSP occurs typically within much less than 1 fs. The probability for such process to occur is directly proportional to the square irradiance (power per unit area) of the laser. Because of this square dependence, TPA is a non-linear effect in the material. The attenuation of the beam with the distance (eq. 1) can be expressed as a function of the laser irradiance I(r, z) [9]:

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Figure 2: Collected charge as a function of laser intensity. Full (open) symbols correspond to collected charge with the focus inside (outside) the detector. T=20◦ C.

Figure 1: Knife-edge scan of the TPA laser probe, with fit overimposed and inset representing the laser probe dimension at gaussian 1σ level.

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3. TPA-TCT on irradiated detectors TPA-TCT performance on non-irradiated samples was demonstrated in previous works [11, 12]. In this contribution TPA-TCT of both, neutron irradiated CNM-UMB [13] pin diodes and HV-CMOS sensors [14] is presented. Increase of linear absorption in irradiated silicon Non-ionizing radiation in silicon creates Deep Energy Levels (DLs) in the bandgap. These DLs can trap electrons (or holes) for much longer than the characteristic time of the TPA process. As a consequence, the linear absorption coefficient for single photons will increase, as measured in [15]. Linear absorption leads to spatially continuous generation of e-h pairs (see eq. 1), spoiling the confinement of the TPA process along the beam direction. In the presence of radiation induced linear absorption, a TPA laser will leave a signal even for out-of-focus photons. The carrier generation rate per unit of volume and photon

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Figure 3: Left: Collected charge as a function of focus position for an irradiated 1015 neq /cm2 diode, measured at different bias voltages . Outside from the detector volume the SPA contribution is flat. Right: Invariance of SPA signal with the position of the focus is used to correct the charge distribution and improve β contrast. All measurements at T=-20◦ C.

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where the factor of two in the denominator of the second term accounts for the generation of one electron-hole pair for every two absorbed photons. Fig. 2 shows the number of generated carriers, calculated as collected charge in 14 ns, as a function165 of laser pulse energy for irradiated pin diodes. For the lowest irradiated diode (1013 neq /cm2 ) a clear quadratic behavior is obtained (full dots, focus inside the sample, second term of eq. 2) which implies a low linear absorption contamination (open dots, measured with the focus well outside the detector,170 first term in eq. 2). For a different sample irradiated up to 1015 neq /cm2 the linear contribution is clearly seen (open triangles) and becomes an important contribution to the total charge (full triangles).

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TPA-TCT on irradiated diodes

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Fig. 3 (left) shows collected charge profiles of a CNM-UMB diode irradiated to 1015 neq /cm2 , using normal TPA incidence, as a function of focus position (z coordinate), for different bias180 voltages. Continuous linear absorption is independent of the focus position (flat pedestal measured in fig. 3) since it only depends on the thickness of material traversed (eq. 1). As fig. 3 (right) shows, linear absorption can be effectively removed in normal incidence TPA-TCT. Practically, this is done at raw185 data level (scope waveforms) by subtracting the induced current, when the focus is out of the sensor, to all other waveforms. For this method to work it is important that the full cross section (coordinate r) of the beam is contained within the depletion region. This is not a strong requirement since the cross section of the beam is σ ∼ 1 µm. Therefore one can conclude that linear absorption can be subtracted in any normal incidence190 (top/bottom) TPA-TCT measurement.

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TPA-TCT on HV-CMOS Low resistivity HV-CMOS are challenging detectors for TCT based characterization techniques, since the depletion region can be as small as 10 µm for 100V bias. An HV-CMOS fabricated in the ams H18 high voltage process [16] was successfully measured already in edge-TPA-TCT, see [11]. Here, a device irradiated with neutrons up to 7×1015 neq /cm2 is shown in Fig. 4 under edge-SPA-TCT illumination (left) and edge-TPA-TCT (right). The calculated charge in 6 ns, normalized to unity to ease comparison, of a test diode (DNW without NMOS or PMOS logic) is mapped as a function of the laser injection coordinates in the bulk. In both cases the depletion region grows with increasing Y coordinate. Contrary to SPA-TCT, substructures inside the TPA charge map are visible. In particular the Deep N-Well (DNW) of the detector shows up as a rectangular region of smaller charge collection, due to partial depletion. Once the implant was located, the origin of the TPA charge map was shifted to coincide with the corner of the DNW to p-bulk junction. In the SPA measurement, however, it is not possible to locate the position of the implant. Therefore the evaluation of the depleted thickness width from edge-SPATCT will overestimate the actual value since the position of the junction is not resolved. Fig. 5 shows the depletion depth as a function of voltage, for the irradiated HV-CMOS sample, and the calculated resistivity from its fit [17]. The depth was calculated by Y-coordinate scans (centered in X, at different voltages) as the distance over which the waveforms exhibit only drift characteristics (fast collection times). The junction position and the end of the depleted bulk was identified for each scan. An in-depth analysis of irradiated HV-CMOS will be the subject of a following paper.

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energy ~ω, up to second order approximation, is stated in equation 2

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4. Summary TPA-TCT exploits a non-linear effect shown by any material when illuminated with a high intensity source (for instance, a laser): for certain wavelengths, light absorption (signal) happens only at the focus of the beam. No photons are absorbed

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out of focus. This allows to generate a laser probe confined in space, particularly along the beam direction. Our mea-230 surements employed an elliptical laser probe with measured 1 µm (13 µm) beam waist across (along) beam direction. In irradiated detectors, the linear light absorption coefficient increases with fluence. This leads to a degradation of longitu-235 dinal resolution. For top incidence TPA-TCT, the spatial resolution can be recovered by measuring a signal with the laser focus outside of the sample. TPA-TCT measurements of irradiated HVCMOS have240 shown, even without any linear absorption subtraction algorithm, superior resolving power compared to edge-SPA-TCT. 245

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[1] M. Garcia-Sciveres, N. Wermes, A review of advances in pixel detectors for experiments with high rate and radiation, Reports on Progress in Physics 81 (2018) 066101. [2] M. J. Riedl, Optical Design Fundamentals for Infrared Systems, Second Edition (SPIE Tutorial Texts in Optical Engineering Vol. TT48), SPIE Publications, 2001. [3] M. A. Green, Self-consistent optical parameters of intrinsic silicon at 300K including temperature coefficients, Solar Energy Materials and Solar Cells 92 (2008) 1305 – 1310. [4] B. Henrich, R. Kaufmann, Lorentz-angle in irradiated silicon, Nucl. Instr. and Meth. in Phys. Res. Section A 477 (2002) 304 – 307. 5th Int. Conf. on Position-Sensitive Detectors. [5] G. Kramberger, et al., Investigation of irradiated silicon detectors by Edge-TCT, IEEE Transactions on Nuclear Science 57 (2010) 2294–2302. [6] V. Eremin, N. Strokan, E. Verbitskaya, Z. Li, Nucl. Instr. and Meth. in Phys. Res. Section A 372 (1996) 388 – 398. [7] G. Lindstr¨om, et al., Radiation hard silicon detectors developments by the RD48 (ROSE) collaboration, Nucl. Instr. and Meth. in Phys. Res. Section A 466 (2001) 308 – 326. 4th Int. Symp. on Development and Application of Semiconductor Tracking Detectors. [8] M. Moll, Displacement damage in silicon detectors for high energy physics, IEEE Transactions on Nuclear Science 65 (2018) 1561–1582. [9] M. Rumi, J. W. Perry, Two-photon absorption: an overview of measurements and principles, Adv. Opt. Photon. 2 (2010) 451–518. [10] S. Nemoto, Determination of waist parameters of a gaussian beam, Appl. Opt. 25 (1986) 3859–3863. [11] M. Fern´andez, et al., Nucl. Instr. and Meth. in Phys. Res. Section A 845 (2017) 69 – 71. Proceedings of the Vienna Conference on Instrumentation 2016. [12] M. Fern´andez, et al., Journal of Instrumentation 12 (2017) C01038– C01038. [13] CNM-IMB, Instituto de Microelectr´onica de Barcelona, http://www. imb-cnm.csic.es/index.php/en/, 2019. [14] I. Peric, Active pixel sensors in high-voltage CMOS technologies for ATLAS, J. Inst. 7 (2012) C08002–C08002. [15] C. Scharf, Radiation damage of highly irradiated silicon sensors, Ph.D. thesis, Universit¨at Hamburg, 2018. [16] ams AG, 0.18 µm high-voltage cmos process, https://ams.com/ process-technology, 2019. [17] H. Spieler, Semiconductor Detector Systems, OUP Oxford, 2005.

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This work was performed in the framework of the CERN250 RD50 collaboration under the projects 2016-04, 2017-02. Activity partially supported by the Spanish Ministry of Science grants FPA2015-71292-C2-2-P and FPA2017-85155-C4-1-R; and the European Union’s Horizon 2020 Research and Inno4