Axel König, HEPHY Vienna Developing silicon strip sensors produced on 8-inch wafers for upcoming upgrades of LHC experiments Photo: © Infineon Technologies AG IPRD 2016, Siena Axel König, HEPHY Vienna

Content Introduction Strip sensors on 8-inch wafers Radiation hardness Field effect transistors for p-stop studies Summary and conclusion 04.10.2016 Axel König

Content Introduction Strip sensors on 8-inch wafers Radiation hardness Field effect transistors for p-stop studies Summary and conclusion 04.10.2016 Axel König

Demand for tracking detector upgrades The LHC at CERN is continuously increasing its peak luminosity A major upgrade is scheduled around 2023 during the third long shutdown (LS3) Increase in peak luminosity by a factor of ~ 5 LHC long-term schedule Necessity for upgrades of the silicon tracking detectors Reach the end of their designed lifetime until LS3 Accumulated radiation damage Have to cope with the high luminosity conditions present after LS3 Higher granularity needed More radiation hard sensors etc. 04.10.2016 Axel König

Content Introduction Strip sensors on 8-inch wafers Radiation hardness Field effect transistors for p-stop studies Summary and conclusion 04.10.2016 Axel König

Wafer specifications and split groups Manufactured by Infineon Technologies Austria AG High resistive float-zone p-type base material Resistivity of 7 kΩ cm 200 μm physical and active thickness Split groups 22 wafers were delivered to HEPHY Wafers are subdivided into split groups Particular process parameter varied for every group: P-stop or p-spray strip separation Two different p-spray implantation doses Three different p-stop implantation doses Two different thermal budgets concerning p-stop i.e. “early” and “late” Five different polysilicon implantation doses 04.10.2016 Axel König

Layout details Main Sensor Dimension: 15 cm ⨉ 10 cm AC-coupled and biased via polysilicon resistors 2032 strips segmented into two parts The design is very similar to the current strip sensor upgrade design of CMS Several mini Sensors used for P-stop geometry studies Biasing scheme studies Irradiations purposes Several conventional test structures Diodes in different sizes Metal-Oxide-Semiconductor (MOS) structures in different sizes Gate controlled diode …many more New set of test structures 04.10.2016 Axel König

New set of test structures Aims: Process control on a specially confined structure Suitable for measurements in the laboratory and at the manufacturer Transfer challenging measurements to simple ones Elementary cell is called flute Implemented Test structures Diodes, MOS, GCD Meander for sheet resistance measurements Van der Pauw structures Field effect transistors in different designs for p-stop studies etc. 2.2 mm ⨉ 0.6 mm 1 cm ⨉ 1.2 cm 04.10.2016 Axel König

Electrical characterization – global parameters 15 main sensors were characterized Global parameters IV and CV Most sensors show breakthroughs just above full depletion voltage Full depletion voltage of ~ 75 V 04.10.2016 Axel König

Electrical characterization – single strip parameters More than 300.000 measured strips Measured parameters per strip Single strip current Istrip Coupling capacitance Cac Polysilicon resistance Rpoly Current through the dielectric Idiel Results Sufficiently small Istrip and Idiel No pinholes Polysilicon resistance split groups clearly distinguishable Comparable broad distribution in Cac Parameter Median Std. dev. Istrip 184 pA/cm 100 pA/cm Cac 8.9 pF/cm 1.4 pF/cm Rpoly (largest) 391 kΩ 33 kΩ 04.10.2016 Axel König

Inter-strip resistance Measured on sample basis only One sensor of every p-stop and p-spray split group Inter-strip resistance measured on 10 strips for each sensor Larger inter-strip resistance for p-stop “early” split groups P-spray and p-stop “late” samples show similar inter-strip resistances In general, larger inter-strip resistances are desired 04.10.2016 Axel König

Investigations concerning coupling capacitance Frequency and amplitude dependence of Cac Caused by Schottky contacts at the strip implantation Observed broad distribution in Cac Caused by non uniform strip metallization Varying areas of the electrodes result in varying coupling capacitances 04.10.2016 Axel König

Content Introduction Strip sensors on 8-inch wafers Radiation hardness Field effect transistors for p-stop studies Summary and conclusion 04.10.2016 Axel König

Proton irradiations Two mini sensors were irradiated at KIT in Karlsruhe with 23 MeV protons Fluences Sample #1: 6.22⋅1014 neq/cm2 Sample #2: 5.02⋅1015 neq/cm2 Fluences match the expected maximal particle fluences the CMS Tracker will experience Initial investigations: Annealing studies Change of full depletion voltage with respect to annealing time Current decrease with respect to annealing time Estimation of current related damage rate First look at the inter-strip resistance Alexander Dierlamm (KIT) 04.10.2016 Axel König

Annealing studies Both sensors were annealed at a constant temperature of 60°C for increasing time durations IV and CV characteristics were measured after every annealing step Change in full depletion voltage Vdepl corresponds to the predictions: Strong increase in Vdepl directly after irradiation Vdepl decreases up to a total annealing time of ~ 80 min at 60°C Vdepl starts to increase again beyond an annealing time of ~ 80 min at 60°C Leakage current continuously decreases Sample #1 Sample #2 04.10.2016 Axel König

Current related damage rate 𝛂 General relation: ΔI = 𝛼 Φeg V [1] Instead of 𝛼 the geometrical current related damage rate 𝛼* is calculated Geometrical volume instead of actual depleted volume of the sensor is used to calculate 𝛼* At full depletion, 𝛼* should be equal to 𝛼 𝛼* is calculated for every depletion voltage step ΔI was measured after an annealing of 80 min at 60°C and scaled to 20°C measurement temperature Observations: Calculated 𝛼* is always below the predicted 𝛼- value Theory might not be valid anymore for fluences that large (similar observations made in [2]) Plotting ΔI/V vs. Φeg gives an 𝛼-value of 1.71e-17 A/cm [1] M. Moll, Radiation damage in silicon particle detectors, (Ph.D.thesis), Universität Hamburg, 1999 [2] S. Wonsak et al., Measurements of the reverse current of highly irradiated silicon sensors, NIMA, 796 (2015), p. 126-130 04.10.2016 Axel König

Inter-strip resistance of irradiated samples Sample #1: 6.22e14 neq/cm2 Inter-strip resistance before irradiation 12 GΩ - 17 GΩ (for p-stop early samples) Inter-strip resistance measured on sample basis for three strips for each irradiated sensor Both samples show inter-strip resistances in the MΩ region Inter-strip resistances after irradiation are too small in general Related to small inter-strip resistances already present before irradiation Sample #2: 2.05e15 neq/cm2 04.10.2016 Axel König

Content Introduction Strip sensors on 8-inch wafers Radiation hardness Field effect transistors for p-stop studies Summary and conclusion 04.10.2016 Axel König

Field effect transistor (FET) test structures Bias ring Gate Aim: Investigate p-stop properties using the threshold voltage Vth obtained from transfer characteristics of field effect transistors Main difference to standard field effect transistors: Two layers of p-stop between source and drain Simple structure: n+ implanted source and drain Varying distances of 56 μm, 46 μm, 36 μm and 26 μm Each implanted electrode is surrounded by p-stop Fixed width of 6 μm Fixed distance to each electrode of 5 μm One common gate electrode Gate dielectric consists of SiO2 p-stop Source Drain 04.10.2016 Axel König

FET measurement results Transfer characteristics of FET test structures of every p-stop split group were measured Expectation: threshold voltage should be different for every p-stop split group Different implantation depths “early” and “late” Different implantation doses Results Very similar threshold voltages for all p-stop late split groups (A, B and C) Increasing threshold voltages for p-stop early split groups A to C No dependency on pad distance Influence of Schottky contacts on Vth could be observed (more later) 04.10.2016 Axel König

Simulating FET structures using Synopsys TCAD Aim: Investigate the usability of FET test structures for p-stop studies by accessing a larger parameter space using TCAD simulations 2D simulations with 260.000 vertices in total Complete FET geometry including biasing Layer thicknesses implemented as observed in SEM images Interface charge implemented according to MOS measurements Gaussian doping profiles and / or 1D-doping profiles gained from SRP measurement Gate voltage = 6 V Gate voltage = 12 V Gate voltage = 30 V Gate Source Drain Total current density of simulated FETs for varying gate voltages and fixed drain-source voltages. Zoomed into the region of the gate 04.10.2016 Axel König

Simulations Measurements Measurements show typical behavior of Schottky barrier MOSFETs 1D-doping profiles of SRP measurements used for simulation together with TCAD standard Schottky contact model Transfer characteristics Slightly higher drain currents in simulations Smaller Vth in simulations No existing Schottky barrier Increasing currents for decreasing pad distances Output characteristics Much higher drain currents in simulations 04.10.2016 Axel König

Variation of parameters Logarithmic like increase of Vth for increasing p-stop depths Nearly no change in inter-pad resistance Extracted values Threshold voltage from transfer characteristics Inter-pad resistance of fully depleted FET structure Parameters varied P-stop implantation depth P-stop concentration Pad distance Exponential like increase of Vth for increasing p-stop concentrations Exponential like increase of inter-pad resistance for p-stop concentrations up to 1e1016 cm-3 No significant change in Vth Exponential decrease of inter-pad resistance up to 60 μm pad distance 04.10.2016 Axel König

Content Introduction Strip sensors on 8-inch wafers Radiation hardness Field effect transistors for p-stop studies Summary and conclusion 04.10.2016 Axel König

Summary The world’s first AC-coupled silicon strip sensors produced on 8-inch wafers were characterized Issues to be resolved: Non uniformity of strip metallization  spread in coupling capacitance Adjustment of implantation profiles and doses Process optimizations resulting in higher breakthrough voltages Irradiated samples show reasonable results and indicate sufficient radiation hardness Measurements on field effect transistor test structures indicate the suitability for p-stop characterization purposes 04.10.2016 Axel König

Outlook HEPHY received a second batch silicon strip sensors processed on 8-inch wafers very recently Large effort at the manufacturer has been made to resolve the discussed issues using the input of measurements presented here Irradiation studies will continue Tuning of TCAD simulations for implementing Schottky contacts Tuning of inter-pad resistance simulations for a larger parameter space We are close to maturity and looking forward characterizing the second batch of silicon strip sensors processed on 8-inch wafers 04.10.2016 Axel König

Thank you for your attention! 04.10.2016 Axel König

Backup 04.10.2016 Axel König

MOS and Diodes MOS curves very reasonable Flat-band voltage of 1.2 V Oxide charge density of 2e10 cm2 P-spray MOS show irregularities in CV-MOS measurements as expected IV curves of diodes show much better breakthrough behavior than main large sensors Could be related to defect density and the smaller covered area p-spray 04.10.2016 Axel König

Van der Pauw measurements Used to determine the sheet resistance of the p-stop All p-stop “late” variants show similar sheet resistances Corresponds to measurements made on FETs 04.10.2016 Axel König

Strip measurement indicating Schottky contact An IV-curve was measured from DC to DC pad to determine the resistance of the strip implant The shape of the curve indicates an existing Schottky contact 04.10.2016 Axel König

Preliminary results of the 2nd batch Homogenous strip aluminum width No frequency dependence in Cac 04.10.2016 Axel König