CCD-based Pixel Detectors for Linear Collider Flavour Identification

1. CCD-based Pixel Detectors by LCFI Andrei Nomerotski (U.Oxford) on behalf of LCFI collaboration May 7 2006, UK HEP Forum Outline • LCFI Collaboration • Pixel Sensors and their Readout • Column-Parallel CCDs • Storage Pixels : ISIS • Mechanical Studies • Summary

2. LCFI :Linear Collider Flavour Identification Valencia Goals : Development of technologies and algorithms for the ILC vertex detector

3. Pixel Sensors • Traditionally LFCI develops CCD-based sensors • Builds on successes of the SLD vertex detector The VXD3 upgrade vertex detector: 96 large CCDs, 307 Million pixels (1996)

4. Vertex Detector for ILC Main requirements: • Excellent point resolution (3-4 μm), ~1 Gigapixel 20x20 μm • Low material budget ( 0.1% X0 per layer) • Low power dissipation • Moderate radiation hardness ( 20 krad/year) • Tolerance to Electro-Magnetic Interference (EMI) • Operation in 5T magnetic field • Fast Readout – The Challenge

5. 337 ns 0.2 s 2820x 0.95 ms The Challenge LC BeamTime Structure: What readout speed is needed? • If read once per train : occupancy ~200 hits/mm2 : too slow • Need to read once accumulated occupancy ~10 hits/mm2 • => 20 times per train = 50µs/MPixel • Fastest commercial CCDs ~ 1 ms/MPixel • Two approaches • Parallel Readout of traditional CCD: CPCCD – information leaves the sensor as fast as it can • Storage Sensors : each pixel has a ‘memory’ filled up during collisions and read out between trains at slow rate: ISIS technology – information is stored in the sensor = one train

6. M M N N “Classic CCD” Readout time  NM/fout Column Parallel CCD Readout time = N/fout Column Parallel CCD • Simple idea : read out a vector instead of a matrix  Readout time shortened by orders of magnitude BUT • Despite ‘parallel processing’ readout rate is still challenging : 50 MHz clock moves charge 2500 times in 50 µs. 2500x20 µm = 50 mm • Every column needs own amplifier and ADC  requires readout chip

7. Column Parallel CCD • Readout Chip is a difficult but clearly feasible problem • Geometrically concept of columns is similar to the silicon strips - strip detectors have complex readout chips integrated in ladder • However density of channels and 0.1% Xo constraint requires bump-bonding – non-trivial anyway • Difference wrt Strips: to move the charge need to clock all columns of the CCD simultaneously • Simple exercise : • typical capacitance of CCD sensor : 100nF • 50MHz  10 ns rise time • Clock current = 100 nF x 2 V / 10 nsec = 20A ! • Voltage drops 20 A x 0.1 Ohm = 2 V • Inductance of 1mm long bond wire = 1 nH : corresponds to 0.3 Ohm at 50 MHz • Driving a full area CPCCD is a major challenge! • Need a special high current clock driver • Need to be extra careful with the design of clock distribution

8. CPCCD : LCFI R&D Milestones • Established proof of principle for small area sensors : CPC1 • Established proof of principle for readout chip : CPR1, developed and produced more sophisticated CPR2 • Moved on to large area sensors : CPC2 • Need to handle the problem of clock driver • Design dedicated clock driver : CPD1 • Find ways to reduce the CCD capacitance • Find ways to reduce the required clock voltage Next slides: Results from prototypes

9. CPC1 Bump-bonded to CPR1 • CPC1 : Two phase CCD, 400 (V)  750 (H) pixels, 20 μm square; • CMOS readout chip (CPR1) designed by the Microelectronics Group at RAL: • 0.25 μm process • Charge and voltage amplifiers matching the outputs of CPC1 • Correlated double sampling • 5-bit flash ADCs and 132-deep FIFO per column • Everything on 20 μm pitch • Size : 6 mm 6.5 mm • Manufactured by IBM • Bump-bonded by VTT (Finland) using solder bumps Bump-bonded CPC1/CPR1 in a test PCB

10. CPC1/CPR1 Performance 5.9 keV X-ray hits, 1 MHz column-parallel readout Voltage outputs, non-inverting (negative signals) Noise 60 e- Charge outputs, inverting (positive signals) Noise 100 e- K.Stefanov RAL • First time e2V CCDs have been bump-bonded • High quality bumps, but assembly yield only 30% : mechanical damage during compression suspected • Differential non-linearity in ADCs (100 mV full scale) : addressed in CPR2 Bump bonds on CPC1 under microscope

11. Next Generation CPCCD Readout Chip – CPR2 Voltage and charge amplifiers 125 channels each Analogue test I/O Digital test I/O 5-bit flash ADCs on 20 μm pitch Cluster finding logic (22 kernel) Sparse readout circuitry FIFO Bump bond pads CPR1 CPR2 • CPR2 designed for CPC2 • Results from CPR1 taken into account • Numerous test features • Size : 6 mm 9.5 mm • 0.25 μm CMOS process (IBM) • Manufactured and delivered February 2005 Wire/Bump bond pads Steve Thomas, RAL

12. CPR2 Test Results Sparsified output Test clusters in • Parallel cluster finder with 22 kernel • Global threshold • Upon exceeding the threshold, 49 pixels around the cluster are flagged for readout • Tests on the cluster finder: works! • Several minor problems, but chip is usable • Design occupancy is 1% • Cluster separation studies: • Errors as the distance between the clusters decreases • Reveal dead time • Many of the findings have already been input into the CPR2A design Tim Woolliscroft, Liverpool U Tim Woolliscroft, Liverpool U

13. Next Generation CPCCD : CPC2 No connections this side Clock bus Charge injection Extra pads for clock monitoring and drive every 6.5 mm Image area Four 2-stage SF in adjacent columns Standard Field-enhanced Standard Temperature diode on CCD Four 1-stage and 2-stage SF in adjacent columns Main clock wire bonds Main clock wire bonds CPR1 CPR2 • Three different chip sizes with common design: • CPC2-70 : 92 mm  15 mm image area • CPC2-40 : 53 mm long • CPC2-10 : 13 mm long • Compatible with CPR1 and CPR2 • Two charge transport sections • Choice of epitaxial layers for different depletion depth: 100 .cm (25 μm thick) and 1.5 k.cm (50 μm thick)

14. CPC2 + ISIS1 Wafer ISIS1 • 5” wafers • One CPC2-70 : 105 mm  17 mm total chip size • Two CPC2-40 per wafer • 6 CPC2-10 per wafer • 14 In-situ Storage Image Sensors (ISIS1) • 3 wafers delivered CPC2-70 CPC2-40 CPC2-10

15. CPC2-40 in MB4.0 Transformer CPR1/CPR2 pads Clock monitor pads Johan Fopma, Oxford U • Transformer drive for CPC2 • “Busline-free” CCD: the whole image area serves as a distributed busline • 50 MHz achievable with suitable driver in CPC2-10 and CPC2-40 • First clocking tests have been done

16. CPC2: First Results 55Fe spectrum from CPC2-10 at 1 MHz K.Stefanov RAL • First 55Fe spectrum at 1 MHz, -40 C, reset every pixel

17. Brian Hawes, Oxford U Clock Drivers for CPC2 Transformer Driver • Requirements: 2 Vpk-pk at 50 MHz over 40 nF (half CPC2-40); • Planar air core transformers on 10-layer PCB, 1 cm square • Parasitic inductance of bond wires is a major effect – fully simulated; IC driver: CPD1 • Transformer is bulky: IC driver could be a better solution; • Design of the first CPCCD driver chip (CPD1) has started, manufacture in June • CPD1: 2-phase CMOS driver chip for 20 Amp current load at 25 MHz (L2-L5 CCDs) • 0.35 μm process, size  3 x 8 mm2 • 32 W peak power but 0.5% duty cycle • Thermal and electromigration issues seems to be under control

18. Next Steps for CPCCD • Evaluate performance of CPC2 bump-bonded to CPC2/CPR2 • Designing with e2V test devices to study how to reduce CCD capacitance and how to reduce clock voltage • Theoretically can achieve factor of 4 reduction in C • Design of CPC3 will depend on results of these tests

19. Radiation Damage Effects in CCDs: Simulations Simulation at 50 MHz Operating window Signal density of trapped electrons in 2D • Full 2D simulation based on ISE-TCAD developed • Trapped signal electrons can be counted • CPU-intensive and time consuming • Simpler analytical model also used, compares well with the full simulation • Window of low Charge Transfer Inefficiency (CTI) between -40 C and 0 C • Will be verified by measurements on CPC2 L. Dehimi, K. Bekhouche (Biskra U); G. Davies, C. Bowdery, A.Sopczak (Lancaster U)

20. Storage Pixels • Industry analogy is “Burst mode” : capturing a limited number of images at short intervals • Burst mode imagers are available commercially (ex. DALSA) with rates up to 100MHz : the rate is limited by the charge transfer between neighboring cells • Memory is implemented as a CCD register associated with an imaging pixel : whatever one can fit in an area of one pixel 2003: Dart bursting a ballon : 100 consecutive frames at 1M frame/sec

21. Storage Pixels as Particle Detectors • ILC requirements : capture charge every 50 us 20kHz – no problem Challenges : • Used as particle (not visible light) detector – need efficient charge collection from the whole area • Need to fit 20 cell CCD register into 20x20 square micron pixel (together with photogate and some logic) • Need a more complicated than pure CCD process

22. Reset transistor Source follower Row select transistor reset gate output gate storage pixel #1 transfer gate storage pixel #20 VDD row select sense node (n+) photogate To column load n+ buried channel (n) p+ well p+ shielding implant reflected charge Charge collection reflected charge High resistivity epitaxial layer (p) In-situ Storage Image Sensor : ISIS • Charge is collected into a photogate • Each pixel has its own 20-cell CCD register : store raw charge during collisions • Increased resistance to RF • Column-parallel readout during quiet time at ~1 MHz: much reduced clocking requirements

23. In-situ Storage Image Sensor (ISIS) 5 μm Global Photogate and Transfer gate • Additional ISIS advantages: • ~100 times more radiation hard than CCDs – less charge transfers • Easier to drive because of the low clock frequency: 20 kHz during capture, 1 MHz during readout • ISIS combines CCDs, active pixel transistors and edge electronics in one device: specialised process • Development and design of ISIS is more ambitious goal than CPCCD • “Proof of principle” device (ISIS1) designed and manufactured by e2V Technologies ROW 1: CCD clocks ROW 2: CCD clocks On-chip switches On-chip logic ROW 3: CCD clocks ROW 1: RSEL Global RG, RD, OD RG RD OD RSEL Column transistor

24. The ISIS1 Cell • 1616 array of ISIS cells with 5-pixel buried channel CCD storage register each; • Cell pitch 40 μm  160 μm, no edge logic (pure CCD process) • Chip size  6.5 mm  6.5 mm Output and reset transistors OG RG OD RSEL Column transistor OUT Photogate aperture (8 μm square) CCD (56.75 μm pixels)

25. Tests of ISIS1 K.Stefanov RAL Tests with Fe-55 source • The top row and 2 side columns are not protected and collect diffusing charge • The bottom row is protected by the output circuitry • ISIS1 without p-well tested first and works OK • ISIS1 with p-well has very large transistor thresholds, permanently off

26. Mechanical Options Target of 0.1% X0 per layer (100μm silicon equivalent) • Unsupported Silicon • Longitudinal tensioning provides stiffness • No lateral stability • Not believed to be promising • Thin Substrates • Detector can be thinned to epitaxial layer (~20 μm) • Silicon glued to low mass substrate for lateral stability • Longitudinal stiffness still from moderate tension • Beryllium has best specific stiffness • Rigid Structures • Foams look very promising • Will start to investigate shell structure supports

27. Laser displacement meter Z precision ~ 1 µm 2D motorised stage X-Y precision < 1 µm Ladder in cryostat: ∆T ~ 100C Fast: 1D scan < 1 minute Scan during cooling Laser Survey System E.Johnson RAL

28. Beryllium substrate Minimum thickness 0.15% X0 Good qualitative agreement from FEA models and measurement Carbon Fibre substrate Better CTE match than Be ~0.09% X0, no rippling to <200K lateral stability insufficient Silicon Tension Beryllium Glue Ladder testing with Be and Carbon Fibre J.Goldstein RAL

29. Rigid Structures: Foams • Properties: • Open-cell foam • Macroscopically uniform • No tensioning needed • 3% RVC prototype • Sandwich with foam core • 0.09% X0 • Mechanically unsatisfactory • Working on glue application • 8% Silicon Carbide prototype • Single-sided: substrate + foam • 0.14% X0 • 3-4% believed possible 20 µm silicon 1.5 mm silicon carbide

30. Silicon Carbide Foam Glue “pillars” (right plot) are better than thin glue layer (left plot)

31. Gas cooling test stand established Cold nitrogen flow Model of 1/4 detector Measurements from test stand: CFD simulation of the same setup In agreement with mesurements Next step:more detailed comparison Cooling Studies G.Leithall RAL S.Yang, Oxford

32. Summary • LCFI is a viable and growing collaboration to develop technologies and algorithms for VD • First generation sensors extensively studied • Column parallel CCD principle proven • First results from the second generation of sensors and readout chips • Detector-scale CCDs • Sparsified readout • Developing advanced clock drivers • First prototypes of storage devices • Mechanics : 0.1% X0ladders seems achievable