GOSSIP: Gas On Slimmed Silicon Pixels - A CMOS Readout for a Thin Gas Vertex Detector

1. GOSSIP: a vertex detector combining a thin gas layer as signal generator with a CMOS readout pixel array GOSSIP Gas On SlimmedSIlicon Pixels

2. Time Projection Chamber (TPC): 2D/3D Drift Chamber The Ultimate Wire (drift) Chamber track of charged particle E-field (and B-field) Wire plane Wire Plane + Readout Pads Pad plane

3. Let us eliminate wires: wireless wire chambers 1996: F. Sauli: Gas Electron Multiplier (GEM)

4. 1995 Giomataris & Charpak: MicroMegas Ideally: a preamp/shaper/discriminator channel below each hole….

5. The MediPix2 pixel CMOS chip 256 x 256 pixels pixel: 55 x 55 μm2 per pixel: - preamp - shaper - 2 discr. - Thresh. DAQ - 14 bit counter - enable counting - stop counting - readout image frame - reset We apply the ‘naked’ MediPix2 chip without X-ray convertor!

6. Cubic drift volume: 14 x 14 x 14 mm3 Cathode (drift) plane: - 700 V Drift space: 15 mm (gas filled) Micromegas: - 350 V Baseplate MediPix2 pixel sensor Brass spacer block Printed circuit board Aluminum base plate cosmic muon Very strong E-field above (CMOS) MediPix!

9. 55Fe, 1s No source, 1s 55Fe, 10s 14 mm Friday 13 (!) Feb 2004: signals from a 55Fe source (220 e- per photon); 300 m x 500 m clouds as expected The Medipix CMOS chip faces an electric field of 350 V/50 μm = 7 kV/mm !! We always knew, but never saw: the conversion of 55Fe quanta in Ar gas

10. Single electron efficiency • no attachment • homogeneous field in • avalanche gap • low gas gain • simple exponential grown • of avalanche •  • No Curran or Polya • distributions but simply: Prob(n) = 1/G . e-n/G Eff = e-Thr/G Thr: threshold setting (#e-) G: Gas amplification

11. New trial: NIKHEF, March 30 – April 2, 2004 Essential: try to see single electrons from cosmic muons (MIPs) Pixel preamp threshold: 3000 e- (due to analog-digital X-talk) Required gain: 5000 – 10.000 New Medipix New Micromegas Gas: He/Isobutane 80/20 !Gain up to 30 k! He/CF4 80/20 …… It Works!

12. He/Isobutane 80/20 Modified MediPix Sensitive area: 14 x 14 x 15 mm3 Drift direction: Vertical max = 15 mm

13. He/Isobutane 80/20 Modified MediPix Sensitive area: 14 x 14 x 15 mm3 Drift direction: Vertical max = 15 mm

14. He/Isobutane 80/20 Modified MediPix δ-ray? Sensitive area: 14 x 14 x 15 mm3 Drift direction: Vertical max = 15 mm

15. MediPix modified by MESA+, Univ. of Twente, The Netherlands Non Modified Modified Pixel Pitch: 55 x 55 μm2 Bump Bond pad: 25 μm octagonal 75 % surface: passivation Si3N4 New Pixel Pad: 45 x 45 μm2 Insulating surface was 75 % Reduced to 20 %

19. Vernier, Moire, Nonius effect Pitch MediPix: 55 μm Pitch Micromegas: 60 μm Periodic variation in gain per 12 pixels Non-modified MediPix Modified MediPix has much less Moire effect Focussing on (small) anode pad Continues anode plane is NOT required Reduction of source capacity! No charge spread over 2 or 4 pixels

20. De-focussing Modified focusing De-focussing focusing Non Modified InGrid: perfect alignment of pixels and grid holes! Small pad: small capacitance!

21. ‘Micromegas’ INtegrate Micromegas GRID and pixel sensor InGrid By ‘wafer post processing’ at MESA+, Univ. of Twente

22. Integrate GEM/Micromegas and pixel sensor: InGrid ‘GEM’ ‘Micromegas’ By ‘wafer post processing’

26. For KABES II, there are two options. • The TPC with transverse drift option would need strips rather than pixels. • But it could be interesting to have an InGrid-like integrated mesh. • The thin Si or CMOS+gas option would need a very high rate capability. • CAST (CERN Axion Solar Telescope) seems to be a more straightforward application. • It simply requires a possibility of triggering a common stop. • This is why Esther Ferrer-Ribas, from CAST, will join us. • - The polarimetry application (challenging Belazzini) is very interesting • for people from the Astrophysics division. The requirement is very similar to CAST's. • - The MicroTPC might have applications in nuclear physics or in Babar, for instance. • - There are other applications (X-ray beam monitor for SOLEIL) which I can talk about tomorrow. • - The protection issue is essential in all Micromegas applications.

27. ! With 1 mm layer of (Ar/Isobutane) gas we have a fast TPC! • thick enough for 99 % MIP detection efficiency • thin enough for max. drift time < 25 ns (LHC bunchX) • Replace {Si sensor + amplifier} by gas layer: •  tracker for intense radiation environment After all: until 1990 most vertex detectors were gas detectors! Si solved granularity problems associated with wires.

28. GOSSIP: Gas On Slimmed SIlicon Pixels MIP MIP Micromegas (InGrid) Cathode foil CMOS pixel array CMOS pixel chip Drift gap: 1 mm Max drift time: 16 ns

29. Essentials of GOSSIP: • Generate charge signal in gas instead of Si (e-/ions versus e-/holes) • Amplify # electrons in gas (electron avalanche versus FET preamps) • Then: • No radiation damage in depletion layer or pixel preamp FETs • No power dissipation of preamps, required for Si charge signals • No detector bias current • 1 mm gas layer + 20 μm gain gap + CMOS (almost digital!) chip • After all: it is a TPC with 1 mm drift length (parallax error!) Max. drift length: 1 mm Max. drift time: 16 ns Resolution: 0.1 mm  1.6 ns

30. Ageing Efficiency Position resolution Rate effects Radiation hardness HV breakdowns Power dissipation Material budget

31. Ageing Remember the MSGCs…… • Little ageing: • the ratio (anode surface)/(gas volume) is very high w.r.t. wire chambers • little gas gain: 5 k for GOSSIP, 20 – 200 k for wire chambers • homogeneous drift field + homogeneous multiplication field • versus 1/R field of wire. Absence of high E-field close to a wire: • no high electron energy; little production of chemical radicals • Confirmed by measurements(Alfonsi, Colas) • But: critical issue: ageing studies can not be much accelerated!

32. Efficiency • Determined by gas layer thickness and gas mixture: • Number of clusters per mm: 3 (Ar) – 10 (Isobutane) • Number of electrons per cluster: 3 (Ar) - 15 (Isobutane) • Probability to have min. 1 cluster in 1 mm Ar: 0.95 • With nice gas: eff ~ 0.99 in 1 mm thick layer should be possible • But……. • Parallax error due to 1 mm thick layer, with 3rd coordinate 0.1 mm: • TPC/ max drift time 16 ns; σ = 0.1 mm; σ = 1.6 ns: feasible! • Lorentz angle • We want fast drifting ions (rate effect) • little UV photon induced avalanches: good quenching gas

33. Position resolution 0 Q 20 ns • Transversal coordinates limited by: • Diffusion: single electron diffusion 0 – 40/70 µm • weighed fit: ava 20/30 µm • 10 e- per track: σ = 8/10 µm • pixel dimensions: 20 x 20 – 50 x 50 μm2 • Note: we MUST have sq. pixels: no strips (pad capacity/noise) • Good resolution in non-bending plane! • Pixel number has NO cost consequence (m2 Si counts) • Pixel number has some effect on CMOS power dissipation • δ-rays: can be recognised & eliminated • 3rd (drift) coordinate • limited by: • Pulse height fluctuation • gas gain (5 k), pad capacity, # e- per cluster • With Time Over Threshold: σ = 1 ns ~~ 0.1 mm

34. Rate effects 0 Q 20 ns SLHC @ 2 cm from beam pipe: 10 tracks cm-2 25 ns-1 400 MHz cm-2! • ~10 e- per track (average) • gas gain 5 k • most ions are discharged at grid • after traveling time of 20 ns • a few percent enter the drift space: time • Some ions crossing drift space: takes 20 – 200 μs! • ion space charge has NO effect on gas gain • ion charge may influence drift field, but this does little harm • ion charge may influence drift direction: change in lorentz angle ~0.1 rad • B-field should help

35. Data rate Hit Pixel (single electron) data: 8 bit column ID 8 bit row ID 4 bit timing leading edge 4 bit timing trailing edge total 24 bits/hit pixel 100 e-/ 25 ns cm2 380 Gb/s per chip (2 x 2 cm2) Cluster finding: reduction factor 10: 40 Gb/s Horisberger: Data rate, DAQ, data transmission is a limiting factor for SLHC Required: rad hard optical links with 1 mm3 light emittors per chip!

36. Radiation hardness • Gas is refreshed: no damage • CMOS 130 nm technology: ? TID • ? NIEL • ? SEU: design/test • need only modest pixel input stage • How is 40 Gb/s hit pixel data transferred? • need rad hard optical link per chip!

37. HV breakdowns: InGrid issue 1) High-resistive layer 3) ‘massive’ pads 2) High-resistive layer 4) Protection Network

38. Power dissipation • For GOSSIP CMOS Pixel chip: • Per pixel: • - input stage (1.8 μA/pixel) • monostable disc/gate • Futher: data transfer logic • guess: 0.1 W/cm2 •  Gas Cooling feasible!

39. Detector Material budget ‘Slimmed’ Si CMOS chip: 20 μm Si Pixel resistive layer 1 μm SU8 eq. Anode pads 1 μm Al Grid 1 μm Al Grid resistive layer 5 μm SU8 eq. Cathode 1 μm Al

40. Gas instead of Si • Pro: • no radiation damage in sensor • modest pixel input circuitry • no bias current, no dark current (in absence of HV breakdowns..!) • requires (almost) only digital CMOS readout chip • low detector material budget • Typical: Si foil. New mechanical concepts: • self-supporting pressurised co-centric balloons • low power dissipation • (12”) CMOS wafer  Wafer Post Processing  dicing 12” pcs • no bump bonding • ‘simple’ assembly • operates at room temperature • less sensitive for X-ray background • 3D track info per layer • Con: • Gas chamber ageing: not known at this stage • Needs gas flow (but can be used for cooling….)

41. How to proceed? • InGrid 1 available for tests in October: • rate effects (all except change in drift direction) • ageing (start of test) •  Proof-of-principle of signal generator: Xmas 2004! • InGrid 2: HV breakdowns, beamtests with MediPix (TimePix1 in 2005) • TimePix2: CMOS chip for Multi Project Wafer test chip • GOSSIPO ! Dummy wafer

42. Essential Ingredients of GOSSIP CMOS chip • RATE • Assume application in Super LHC: • Bunch crossing 25 ns • 10 tracks per (25 ns cm2) • 10 e- per track (average: Landau fluct.) • So: 4 MHz/mm2 tracks!, 40 MHz/mm2 single electrons!

43. Q 10 - 20 ns Chargesignal on pixel input pad • Signal shape is well defined and uniform • No bias current, no dark current • Signal is subject to exponential distribution • may be large, but limited by • chamber ageing • space charge (rate) effects

44. Input Pad capacity • preamp stage, noise, power • Input pads may be small: focusing • Too small pads: chamber ageing • capacity to neighbors & metal layers • capacity due to gas gain grid • Pixel size: 50 x 50 - 20 x 20 μm2 • 4 fF seems feasible

45. Time resolution • preamp-disc speed, noise, power • Measurement 3rd coordinate: σdrift time: 25/16 = 1.5 ns • Time over threshold: slewing correction • drift time related to BX • Record: leading edge - BX • trailing edge - BX • BX ID

46. Data Readout ALL data: 80 MB s-1 mm-2 ( 15 GB/s per chip) Maybe possible in 10 years from now: - optical fibre per chip - Vertex can be used as trigger For SLHC: Use BX ID info (typical Vertex policy) - tell BX ID to all (Rows/Columns/Pixels) - get data from (Row/Column/Pixel)

47. Gossipo • MultiProjectWafer submit in 130 nm CMOS technology • Test of essential GOSSIP ingredients: • Low power, low input capacity preamp/shaper/discriminator • 1.5 ns TDC (per discriminator output) • Data transfer • Maybe not all of this in a first submit • Maybe with less ambitious specifications

48. Amplifier-shaper-discriminator • How to apply a test pulse? • using gas gain grid (all channels fire) • capacitive coupling test pulse strip • reality: with a gas gain grid(!) • What to do with the output? • (bonded) contact: digital feedback?! • TDC + DAQ? • TDC • - 1.5 ns clock: derived on-board from 40 MHz BX clock? • 640 MHz clock distribution (per pixel?!) • DLL?

49. (My) goal of this meeting: • Are there any showstoppers in this stage? • can we define a Gossipo concept (block diagram)? • Can we estimate the amount of work?