1. Edgeless semiconductor sensors for large-area pixel detectors Marten Bosma Annual meeting Nikhef December 12, 2011, Amsterdam

2. Outline Introduction to Medipix detectors From single assembly to large-area detectors Edgeless sensors: –Active-edge –Slim-edge December 12, 20111 Marten Bosma, Nikhef annual meeting, Amsterdam

3. Hybrid pixel detector December 12, 20112 Marten Bosma, Nikhef annual meeting, Amsterdam Semiconductor sensor: Silicon, gallium arsenide, cadmium telluride Read-out chip: Medipix, Timepix, Medipix3 Bump bonds: Copper/tin, Indium/tin 300 μm – 1mm 20 – 70 μm 200 – 700 μm

4. Why Medipix ? Photon counter –High signal-to-noise ratio –Noise rejection High functionality per pixel –Timepix: counting, ToT, ToA –Threshold equalisation –Dark current compensation December 12, 20113 Marten Bosma, Nikhef annual meeting, Amsterdam

5. Medipix3 – Tomorrow’s digital radiography ? December 12, 2011 Marten Bosma, Nikhef annual meeting, Amsterdam 4 Medipix3 features ~1600 transistors per pixel: Charge summing circuitry Simultaneous counting and read-out → no dead time Energy dispersive mode: 7 energy bands Medipix3 pixel layout

6. Mono-crystalline sensors vs. amorphous selenium December 12, 20115 Marten Bosma, Nikhef annual meeting, Amsterdam a-SeGaAsCdTeSi Z 3431;3348;5214 e - mobility (cm 2 /Vs) 3*10 -3 850011001500 h + mobility (cm 2 /Vs) 1*10 -1 400100450 Eh-pair creation energy (eV) 20 – 504.264.433.6

7. 2.8 cm Active detector area December 12, 20116 Marten Bosma, Nikhef annual meeting, Amsterdam 35 x 40 cm 2 Medipix quad module with Relaxd read-out x 200 3 inch X-ray imaging: –Digital radiography detectors High-energy physics: –Close-to-beam/forward physics experiments (TOTEM, LHCb Velo)

8. Tile-ability December 12, 2011 Marten Bosma, Nikhef annual meeting, Amsterdam 7

9. Wire bond → Through-silicon-via December 12, 20118 Marten Bosma, Nikhef annual meeting, Amsterdam Bond padDi-electric LandConductive pathway Through-silicon-via

10. Tile-ability: MPX3 pixel-to-edge distance December 12, 20119 Marten Bosma, Nikhef annual meeting, Amsterdam Courtesy of PANalytical

11. Tile-ability: Sensor edges December 12, 2011 Marten Bosma, Nikhef annual meeting, Amsterdam 10 Rossi, “Pixel Detectors” Slim edgeActive edge Edge effects: Charge generation Surface currents High-field regions Conventionally solved by guard rings

12. Active-edge silicon December 12, 201111 Marten Bosma, Nikhef annual meeting, Amsterdam

13. Active-edge silicon: DRIE/ICP + edge doping December 12, 2011 Marten Bosma, Nikhef annual meeting, Amsterdam 12

14. Leakage current December 12, 201113 Marten Bosma, Nikhef annual meeting, Amsterdam Blade dicing Etching

15. December 12, 201114 Marten Bosma, Nikhef annual meeting, Amsterdam Relaxd read-out interface: 4 detector assemblies in parallel up to 100 frames per second

16. Active-edge silicon: Flood-field image December 12, 201115 Marten Bosma, Nikhef annual meeting, Amsterdam

17. Active-edge silicon: Laser setup December 12, 201116 Marten Bosma, Nikhef annual meeting, Amsterdam λ 660 nm 980 nm 1060 nm Back face Pixel plane

18. Laser data: Edge sensitive volume – 660 nm December 12, 201117 Marten Bosma, Nikhef annual meeting, Amsterdam 1 μm steps -22 μm Indicates VERY small insensitive edge (Spot radius = 20 μm) Effective size of Pixel 1 to 4: 4.62 pixels → {physical width – effective width} = 256.5 μm – 4.62/4 * 220 μm = 2.4 μm insensitive edge 256.5 μm

19. Beam test setup - telescope December 12, 201118 Marten Bosma, Nikhef annual meeting, Amsterdam DUT Courtesy of R. Plackett 120 GeV/c muons and pions Longitudinal

20. Beam data: Long trails Beam December 12, 2011 Marten Bosma, Nikhef annual meeting, Amsterdam 19 ~ 3 cm RIGHTLEFT

21. Beam data: Depth dependence of ToT December 12, 201120 Marten Bosma, Nikhef annual meeting, Amsterdam Beam

22. Beam data: Depth dependence of ToT December 12, 201121 Marten Bosma, Nikhef annual meeting, Amsterdam

23. Slim-edge CdTe & GaAs December 12, 201122 Marten Bosma, Nikhef annual meeting, Amsterdam

24. December 12, 201123 Marten Bosma, Nikhef annual meeting, Amsterdam Detector: 9 slim-edge CdTe pieces Detector readout: Medipix-MXR Sensor specs (Acrorad): Quasi-Ohmic (Pt electrodes) 4.05 x 4.05 x 1 mm 3 36 x 36 pixels of (110 μm) 2 65 μm pixel-to-edge distance

25. Laser data: charge collection efficiency December 12, 201124 Marten Bosma, Nikhef annual meeting, Amsterdam Centre pixel: (μτ) e = 1.25 E-4 Edge pixel: (μτ) e = 1.15 E-4 Fit function described by Hecht relation: Q(U) = Q 0 μτ/L 2 (U – U 0 ) [1 – exp(dL/μτ(U – U 0 )]

26. December 12, 201125 Marten Bosma, Nikhef annual meeting, Amsterdam The leakage current as a function of time after voltage stepping (-200V → 200V → -200V). The current was monitored for one minute per voltage step of 40V Possibly caused by deep–level defects GaAsCdTe

27. High-Z sensor issues December 12, 201126 Marten Bosma, Nikhef annual meeting, Amsterdam General : Material homogeneity and area Grain boundaries – want single crystal Fluorescence Degrades spatial and energy resolution above k-edge CdTe: Only 3” wafers, even smaller dies tellurium inclusions hole trapping/polarisation GaAs: Better single-crystal production (6”) Problem is defects! Shallow defects prevent depletion Carrier lifetimes Te inclusions Edge effect CdTe GaAs

28. Polarisation December 12, 2011 Marten Bosma, Nikhef annual meeting, Amsterdam 27 Non-polarising detector Polarising detector Mobility μ (cm 2 /Vs) Lifetime τ (s) μτ-product (cm 2 /V) Electrons1100~3×10 -6 3×10 -3 Holes100~2×10 -6 2×10 -4 Schubweg: λ e/h = μ e/h × τ e/h × E Φ critical ≈ 10 9 ph/mm 2 s Silicon: μτ e/h ≥ 1

29. Summary Medipix + mono-crystalline high-Z sensors… … have great potential! However, limited active area… edgeless needed! Active-edge silicon: Factor 10 decrease in pixel-to-edge distance. Sensitive edge very close to physical edge. Slim edge high-Z: (Deep-level) defects and poor hole transport. First results indicate edges may not affect charge collection efficiency and noise performance December 12, 201128 Marten Bosma, Nikhef annual meeting, Amsterdam

30. December 12, 201129 Marten Bosma, Nikhef annual meeting, Amsterdam High-energy nuclear interactions with Schottky CdTe. Multipixel-spread splashes, possibly due to polarisation or a cascade of fluorescence effects (?) Bias-voltage and interaction-depth dependence under study. Thank you