Edgeless sensors for full-field X-ray imaging 12 th iworid in Cambridge July 14 th, 2010 Marten Bosma 12 th iworid, Cambridge - July 14 th, 2010
Human X-ray imaging High spatial resolution Low-contrast resolution High dynamic range Medipix Good noise performance Human X-ray imaging (30 120 kev) High detective quantum efficiency Good charge collection efficiency Good quality high-z sensors High absorption (CdTe-based) Full field imaging Large area (edgeless detectors) Good spectrometric properties Medipix 3 12 th iworid, Cambridge - July 14 th, 2010
Current status Four Medipix chips underneath one large sensor (Quad module). ~ 7.5 cm Four Quads in one cooling rig. ~ 0.5 cm Inactive seam of ~ 0.5 cm along one dimension. 12 th iworid, Cambridge - July 14 th, 2010
Edgeless modules Quad Sensor with Guard Ring Medipix ASIC Medipix ASIC PCB Ball-Grid-Array Through-Silicon-Via Medipix ASIC Edgeless Quad Sensor Medipix ASIC PCB
Edge termination Edge issues: Minimization of charge injection; Confinement of the depletion region; Prevention of high-field regions. Edgeless reduction of inactive periphery by: Less-deleterious dicing: Etching Laser dicing (stealth dicing) Design: Active edge Current termination (stop ring) Pixel Detectors Rossi 12 th iworid, Cambridge - July 14 th, 2010
Separation steps 1 st etch (SF 6 C 4 F 8 SF 6 etc.) 1. p + n + n-type Si bulk 2. phosphorus diffusion for: ii. i. Edge activation Surface state passivation Mask material 3. Phosphorus (n + ) 2 nd etch
Conventional vs. Edgeless 500 µm 50 µm
Stop ring functionality Effect of grounding the stop ring n + stop ring V bias p + stop ring 12 th iworid, Cambridge - July 14 th, 2010 V bias
Fabrication by Canberra Samples Medipix pixel sensors (10 30 µm edge distance) TOTEM strip sensors (30 µm and 50 µm edge distance) Rectangular test diodes (13 300 µm edge distance) Circular test diodes (13 150 µm edge distance) DRIE dicing and phosphorus diffusion by IMEC Characterization by Nikhef Medipix sensor TOTEM strip sensor Rectangular diode Circular diode 12 th iworid, Cambridge - July 14 th, 2010
Humidity No significant dependence on humidity level (except for severely damaged structures)
Temperature Normal temperature dependence of thermal generation current
J V curves T = 294 296 K R.H. = 35 38 % V dep 25 V (from J-V fits) N d (substrate) 5 10 11 cm -3 (from resistivity) V fd (V) J gen @ 50V (na/cm 2 ) J surf @ 50V (na/cm 2 ) Conv. 26 4.1 1.5 Edgeless 21 46 70 At 50 V reverse bias, current density levels are: 20 45 na/cm 2 for p + stop ring structures; 115 140 na/cm 2 for n + stop ring structures; having a 50 µm edge distance (one pixel column). Breakdown at (160 190) V
C V curves T = 295 K; R.H. = 30 % f = 100 khz; V AC = 30 mv RMS V dep 20 30 V (from C-V curves) N d (substrate) (5 7) 10 11 cm -3 (1/C 2 -V curves) At V dep the capacitance is approx. 10 pf
Proof-of-principle Edgeless vs. Conventional
J V measurements Inverse correlation: Edge distance and stop ring width current density Structures with a 50 µm edge distance and a 15µm wide stop ring show acceptable current densities at 50 V reverse bias.
300 µm edge dist. and 25 µm SR C V curve effects 13 µm edge dist. and 5 µm SR
Charges at Si/SiO 2 interface 10 11 charges/cm 3 at the Si/SiO 2 interface TCAD simulation 12 th iworid, Cambridge - July 14 th, 2010
Summary Integrity checks: No significant influence of humidity and normal temperature dependence. Depletion voltage is approximately 25 V; Breakdown at (160 190) V reverse bias; The edgeless and conventionally processed devices show comparable current densities. The p+ stop ring structures show lower current densities than the n+ stop ring ones: 20 45 na/cm 2 for p + stop ring structures; 115 140 na/cm 2 for n + stop ring structures; with 50 um edge distance, at 50 V reverse bias. Inverse correlation between leakage-current density and both edge distance and stop ring width: Structures with a 50 µm edge distance and a 15 µm wide stop ring show acceptable current-density values. 12 th iworid, Cambridge - July 14 th, 2010
What s next? Edge effects in CdTe-based (high-z) sensors. Examination of other dicing techniques (e.g. stealth dicing) Blade diced Stealth diced 12 th iworid, Cambridge - July 14 th, 2010
Acknowledgements Canberra: P. Burger, O. Evrard Imec: P. De Moor, K. De Munck, D. S. Tezcan Nikhef: J.L. Visschers, J. Visser 12 th iworid, Cambridge - July 14 th, 2010
Back-up slides 12 th iworid, Cambridge - July 14 th, 2010
12 th iworid, Cambridge - July 14 th, 2010
Dicing Blade dicing Deep Reactive Ion Etching
Humidity Severely damaged edge No significant dependence on humidity level (except for severely damaged structures)
Integrity checks Temperature
J V curve fits V V V V FD FD : : I A V BV C leak I A V BV C leak FD A [na/( Vcm 2 )] B [na/(vcm 2 )] C [na/cm 2 ] V fd (V) J gen @ 50V (na/cm 2 ) J surf @ 50V (na/cm 2 ) Conv. 0.80 0.03 22 26 4.1 1.5 Edgeless 10 1.4-1.0 21 46 70
Electrostatic potential p + stop ring 5 µm SR; 10 µm edge dist. 50 µm SR; 100 µm edge dist SR floating SR biased
Stop ring functionality No stop ring 50 µm wide stop ring Hole-current density (changing parameter: electron and hole recombination velocity at the edge)