Linear Collider vertex detector technology options

The transition from microstrips to pixels, for vertex detectors

Chris Damerell

Detector requirements for the LC Candidate detector architectures

•

CCDs

•

Monolithic APS (including FAPS)

•

DEPFET

•

Hybrid APS

•

SOI-inspired RF pickup suppression – correlated double sampling LCFI Collaboration R&D – selected items The route to convergence on LC vertex detector(s) Synergy with other science

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Since late’70s, successful vertex detectors (for heavy flavour tagging) were mainly based on silicon microstrips

• •

Interesting technology shift is under way. Within 5 years, will mostly be based on silicon pixels Why is this?

•

highest performance b and charm reconstruction in dense track environments has come from two pixel based detectors, NA32 in ’80s, SLD in ’90s

•

extreme radiation environments in the inferno close to IP at future hadron colliders

•

high backgrounds, and high track density in core of jets at future e + e colliders

•

These disparate requirements at hadron and e + e colliders have very different solutions (both of them pixel-based), and are supported by contrasting R&D programmes

•

This transition to pixels implies synergies with other areas of science, where images taken with IR, visible, UV, X-rays benefit from the technologies being developed for HEP vertex detectors, and vice versa

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Of course, it will definitely be silicon pixels at the LC, or will it?

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Detector requirements

• •

5 layers, inner layer at radius 12-15 mm

•

3-hit coverage to cos

q

= 0.96

thin layers (<0.1% X 0 ) for minimal multiple scattering and

g

conversions

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silicon pixels of size ~ 20

m

m square for low cluster-merging in jets

•

support structure with micron precision/stability (specially important for oblique tracks near ladder ends)

•

on-detector signal processing, so almost no external connections

•

power dissipation measured in few tens of watts, so gas cooling is sufficient (this is vital for low material budget)

•

‘adequate’ radiation hardness (tens of krads from pair bgd, plus few times 10 9 neutrons/cm 2 /yr)

•

readout time ~

5

ms for JLC/NLC

(between bunch trains)

~ 50

m

s for TESLA

(20 frames/bunch train)

• 

p

4 sin q •

low mult. scatt term important for efficient charm ID and B vertex charge; quantitative physics examples are now being studied …

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•

The ‘good enough’ vertex detector has yet to be built

• •

Collider vertex detectors have often restricted the capability of their experiments for leading edge physics:

• • •

the possible top signal at 40 GeV in UA1 (early ’80s) the possible Higgs signal in LEP B s signal in SLD [NOT considered when SLD was being designed!] Match to LC physics needs cannot be taken for granted

•

R bp could strike again …

•

Intensive R&D in several technologies will surely be justified (cost effective) in terms of LC physics reach

•

The LC does offer a potential technical advantage (hence enhanced physics reach) wrt the inferno at the heart of LHC

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The SLD experience …

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Candidate detector architectures

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•

MAPS

•

‘Standard CMOS process’: signal charge is collected onto gate of front end transistor from undepleted bulk or epitaxial layer

•

However this isn’t obligatory – early developments by Sherwood Parker et al, with 300 µm fully depleted devices were highly successful

•

First results from Strasbourg group were also based on few mm 2 devices and minimal in-pixel logic

•

Recently, using 0.35 µm CMOS, increasing functionality is being implemented at the periphery of the chip

•

Due to limited g m of in pixel transistors (?) 50 µs readout time requires ‘sideways’ column architecture – MAPS(2)

•

Flexible active pixel idea (Renato Turchetta at RAL) could be a more favourable architecture for TESLA – MAPS(1)

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DEPFET

•

Based on detector-grade high resistivity silicon, fully depleted

•

Requirement of supporting CMOS chips on 2 sides may be a significant limitation

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HAPS (incl new SoI-inspired)

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Read 1 in N pixels, by analogy with capacitive charge division in strip detectors

•

Spatial resolution tends to be somewhat unstable

•

Implications for 2-track resolution?

•

SoI approach could reduce material, but looks pretty complex (?)

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MIMOSA-5 Strasbourg group

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FAPS: RAL group

• •

FAPS could be extended to a full 20 samples per train, stored in pixel If this doesn’t fit with 0.25

m

m CMOS, will surely be OK with 0.13

m

m

• •

Idea is to relax the requirement for fast, precise, signal transmission to chip periphery during train, and so render long columns feasible, with all processing logic outside the detector active volume, as for the CCD architecture Test devices implemented using a 0.25

m

m process – TSMC(imaging)

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DEPFET Bonn/Munich group

Jan 13 2004 MOS transistor instead of JFET A pixel size of ca. 20 x 20 µm² is achievable using 3µm minimum feature size.

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readout chips matrix is read out row-wise 520 x 4000 pixel DEPFET-Matrix (25 x 25 µm pixel) steering chips readout chips Jan 13 2004

• thin detector-area down to 50µm • frame for mechanical stability carries readout and steering-chips

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DEPFET

pixel matrix

Low power consumption Fast random access to specific array regions

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Hybrid Pixel Detector with Interleaved Pixels

HAPS Insubria/Krakow group

Readout pixel Interleaved pixel Polyresistor readout pitch = n x pixel pitch p + n Large enough to house the VLSI front-end cell Small enough for an effective sampling

Charge carriers generated underneath one of the interleaved pixel cells induce a signal on the capacitively coupled read-out pixels, leading to a spatial accuracy improvement by a proper signal interpolation.

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Charge Sharing Studies – Resolution

 

parameterization allows a coordinate reconstruction and resolution measurement

 function Average resolution Resolution vs. spot position • Resolution: – Interleaved pixels (efficient charge sharing): 3 m m Jan 13 2004 C Damerell LC technologies LBL 18

SOI detector

Insubria/Krakow group

Detector  handlable wafer – High resistivity – 300 m m thick Electronics  active layer – Low resistivity – 1.5 m m thick – Readout pixels (min charge sharing): 10 m m Detector: conventional p + -n, DC-coupled Electronics: preliminary solution – conventional bulk MOS technology on the thick SOI substrate Jan 13 2004 19 C Damerell LC technologies LBL

RF pickup suppression

• • • • •

Beam-associated RF radiation penetrating the beam-pipe (even 0.5 mm Be) appears to be negligible However, flanges, BPM cables, etc can permit RF radiation to leak out SLD experience:

•

analogue signals stored securely in CCD buried channel

•

Digital logic (PLL in optical links) was disrupted – fortunately could be restored within some tens of

m

s of collisions) NLC/JLC:

•

could envisage similar settling/restoration before readout TESLA:

•

need to read detector repeatedly during train, to internal storage of sparsified data

• •

each internal frame readout spans ~150 BX, so electronics is hit repeatedly by whatever RF is present For SLD VTX, this would have been fatal

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• • • • • •

The problem: 10 9 signals being read in an electrically hyperactive environment

•

could produce a data deluge

•

contrast between two different collider options and at least 5 detector options Discussion points: Reality check: 300 Mpixels at SLD CCD-based detector at NLC (natural evolution) CCD-based detector at TESLA Other detector technologies at NLC/TESLA

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N M

CCD signal storage and sensing:

N

“Classic CCD”

Readout time  N  M/F out Jan 13 2004

Column Parallel CCD

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• • •

Signal charge from MIP stored safely in buried channel of device During readout, charge is transferred to output node Classical Correlated Double Sampling (CDS): RESET/READ 1/TRANSFER/READ 2 (originally to suppress reset noise)

•

Sparse data scenario permits faster (but equivalent) noise suppression: RESET/READ 1/TRANSFER/READ 2/TRANSFER/READ 3/ …

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In addition, Extended Row Filter (ERF) can suppress pickup:

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SLD experience:

Without ERF, rate of trigger pixels would have deluged the DAQ system

Readout at 5 MHz, during ‘quiet’ inter-bunch periods of 8 ms duration

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•

For NLC, substitute bunch train for bunch

• •

Otherwise, as at SLD, and expect same strategy to work Can again wait many

m

s for beam-related pickup to die away

•

CPCCD lends itself to required functionality in readout chip

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•

For TESLA, one enters uncharted waters

•

Must read during most of 337 ns between bunches (17 samples at 50 MHz in CPCCD)

•

Could cut to say 14 samples giving ~ 50 ns settling time.

Will this suffice?

What will be the noise penalty due to pickup between samples N and N+1?

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DEPFET pixel

• •

DEPFET enjoys same strengths as CCD regarding CDS However, ERF would slow down the readout correspondingly [N samples before and after RESET would imply N-fold increase in readout time]

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Basic MAPS architecture at TESLA

• •

‘Transverse’ readout to satisfy the 50

m

s requirement SAMPLE/RESET at 50

m

s intervals

•

Will be OK for reset noise, but could be catastrophic for pickup from intervening 150 BXs

•

Possible way out: Could do SAMPLE/RESET/SAMPLE within one BX, at 50

m

s intervals

•

This would strongly suppress pickup while sacrificing the suppression of reset noise. Tolerable for C NODE < approx 10 fF

•

Could also (at expense of readout time) implement CCD-like ERF if required

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FAPS or FAPS(CAP) concept: Renato Turchetta

• • •

Flexible APS permits storage of 20 samples during train

•

Readout of above-threshold pixel hits and their neighbours proceeds at leisure in the 200 ms between trains This will permit ‘longitudinal’ readout, with benefit to material budget However, CDS options are no different than for MAPS

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New concept: FAPS(CCD)

•

MIPs which hit the storage register (<10% area) leave a small spurious signal – easily handled by software

•

Lessons being learned about CCDs with reduced clock amplitude (eg without barrier implants) will feed directly into this design concept

•

Increasing availability of mixed CCD/CMOS technology at a few foundries including Sarnoff in USA

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•

Column-pair readout of sparse data (analogue signals to ADCs at ends of ladder)

•

Manufacturability would require not only mixed technology, but also large area precise stitching, etc

•

Could provide the ultimate in pickup immunity, but will this be necessary?

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CONCLUSIONS (on pickup)

•

Pace of development of silicon pixel devices {CCD, MAPS, FAPS(CAP), FAPS(CCD), DEPFET, SOI, HAPS, …} is breathtaking

•

High level of pickup immunity can surely be engineered into some or all of these architectures

•

If at end of this year, we have a warm machine, we can relax and focus mainly on other criteria

•

If TESLA, suggest producing a serious BDS mockup to simulate pickup effects

•

In either case, expect surprises from 10 9 pixels in the LC environment, so it’s probably wise to back contrasting technologies for the (assumed) two detectors, in order to spread the risk

•

At SLD, we were lucky in being able to retro-fit the ERF. Inadvisable to assume this luck will hold, in the unknown territory to be explored next time …

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LCFI R&D programme Novel CCDs and readout electronics

• •

CCD sizes similar to SLD, but readout needs to be 20-2000 times faster

•

Eliminate bulky electronics which would degrade fwd tracking and calorimetry Total of 800 Mpixels, cf 307 Mpixels for SLD

•

TESLA readout requirement stimulated concept of ‘column parallel’ operation

•

This implies an innovative CCD/CMOS hybrid. If successful, this architecture may also be preferred for NLC/JLC. However, for this case, the conventional architecture with a multi-output linear register may suffice

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M N N

“Classic CCD”

Readout time  N  M/F out

Column Parallel CCD

Readout time = N/F out •

CPCCD has max possible readout speed, for given noise performance

•

Readout IC (amp+ADC on 20

m

m pitch) only became available with deep submicron CMOS technology

•

TESLA requires parallel register clocking at 50 MHz: 1 MHz is fine for NLC

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

electronics only at the ends of the ladders



bump-bonded assembly between thinned CPCCD and DSM readout chip



readout chip does all the signal processing, yielding sparsified digital data



CPCCD is driven with high frequency, low voltage clocks (currently 2 V, goal around 1 V peak-peak)



low inductance layout is required for clock delivery

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Standard 2-phase implant Field-enhanced 2-phase implant (high speed) Source followers Direct 2-stage source followers Source followers Direct Metallised gates (high speed) Readout ASIC To pre-amps Readout ASIC Metallised gates (high speed)

Features of our first CPCCD:

•

2 different charge transfer regions

•

3 types of output circuitry

•

Independent CPCCD and readout chip testing possible:

•

without readout chip - use external wire bonded electronics

•

without bump bonding - use wire bonds to readout chip

•

finally, bump-bonded

•

Different readout concepts can be tested (direct charge sensing, and voltage sensing via source follower)

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Jan 13 2004

CPC-1 fully tested standalone, wire-bonded assembly now under test

Direct connections and 2-stage source followers 1-stage source followers and direct connections on 20 μm pitch

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Single pixel events seen in one column of CPC-1 with 2 V peak-peak clocks

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Wire/bump bond pads Charge Amplifiers Voltage Amplifiers 250 5-bit flash ADCs

FIFO

Wire/bump bond pads

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CPR-1 fully tested standalone

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LCFI R&D Thinnest possible detector layers

3 approaches

•

Unsupported silicon

• •

Semi-supported silicon Supported silicon Unsupported approach attractive – ‘like wires in a drift chamber’ Works beautifully along ladder length (sagitta stability around 2 microns) However, processed thin CCD is not like a wire: it’s an inhomogeneous membrane in which transverse stresses may lead to somewhat uncontrollable shape Also, we have concerns about handling issues, for attaching readout chips etc Not abandoned, but semi-supported approach may be more practicable

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Jan 13 2004

Beryllium substrate with adhesive balls Thinned CCD (



20 μm) CCD brought down Shims Assembly after shim removal and curing Adhesive 0.2mm

Beryllium substrate (250 μm) 1 mm

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Thin ladder design concept – technology-independent SLD had glue pads, which implies compression of silicon under cooldown. How to do better in 21 st C?

Micromechanical structure Maybe replace beryllium by some foam material – whatever gives best stiffness for least radiation length, regardless of thermal expansion properties Jan 13 2004 C Damerell LC technologies LBL 47

Route to convergence

Preferred vertex detector technology(ies) to be selected on basis of full-size, fully operational prototype ladders (around 2010?) Choice probably time dependent: what can be ready for startup could well be superseded later [eg at SLC: silicon microstrips were replaced by CCDs in 1990] Convenient access to IR inner detector system) is an essential requirement (for the entire C ommunity should resist pressure from funding agencies to ‘pick the winner’, since a premature choice of technology could seriously degrade the physics potential Good international communication is building a proto-collaboration for the VTX (eg world-wide phone conferences during regional workshops) Probably wise to eventually select two contrasting VTX technologies for the (presumed) two LC detectors – spread the risks

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Construction, commissioning, operation and physics

When choice has been made, some groups (technically oriented) will prefer to develop ‘their technology’ for other applications or possible upgrades Others (particle physics oriented) will wish to contribute to the construction of the first detector(s), followed by commissioning then physics The detector construction should be encouraged as a world-wide endeavour, in spirit of GDN SLD ladders (via UPS) SanJose



SLAC



e2V



Brunel



SLAC



Yale



MIT



SLAC Make mbds Test mbds Fit CCDs Mech QC Functional test Fit blocks Opt survey Intstall

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Synergy with other science

• •

Pixel detectors are uniquely inter-disciplinary Example from ‘fall of the wall’ in structural biology (J Hajdu, TESLA colloquium)

•

120 Hz frame rate needed at LCLS (with 14 bit dynamic range)

•

SNAP, XEUS, biological cell imaging, …

•

Fast Gigapixel-scale imaging systems are widely needed, and the LC vertex detector community is contributing to their development

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5layers 4 layers, double

• •

Clear performance difference between configurations Charm suffers most, B tagging is “easy”

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•

New procedure to attach track to vertices

•

Charged B, up to 89% correct tag, 6-8% worse for 4 layer double thickness configuration

•

Charged D, excellent purity, less difference between configurations

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•

Neutral B: dipole

•

Maintain, develop and improve tools

•

Provide them to the physics community so we can get feed back on detector parameters from various physics channels

•

Make a transition to Java/JAS environment

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