Overview of MCP requirements: resistive layer and SEE A. S. Tremsin, O. H. W. Siegmund Space Sciences Laboratory University of California at Berkeley Berkeley,

Overview of MCP requirements: resistive layer and SEE

A. S. Tremsin, O. H. W. Siegmund

Space Sciences Laboratory University of California at Berkeley

Berkeley, CA

LAPD collaboration meeting, October 15-16, 2009, Argonne National Laboratory

Fixed MCP properties

• MCP manufacturing– Specified geometry is selected– Certain MCP resistance is targeted– Good SEE emission layer– Metallization– Preliminary (simple) testing– Storage/transportation

Resistive and emission layers: preconditioning

• MCP manufactured and shipped

• First inspection and operation

• Gain, uniformity, hotspots

• Conformality to each other

• Preconditioning: scrubbing

• Real use

Resistive and emission layers: preconditioning

Would be nice to have MCPs being ready for use as

shipped

MCP preconditioning

• As manufactured MCPs require substantial preconditioning – Geometrical and resistive conformality (MCP stacks)– Outgasing (sealed tubes)– Gain stabilization (high counting rate applications)– Hot spots (can be reduced by self-scrubbing)

• Most of these are defined by the resistive and emissive layer properties

• Present technology: MCP substrate defines both geometry and functional properties (through resistive/emissive layers)

Novel MCP technology

• Separate substrates characteristics from the MCP operational properties

– Nano-engineered films• Synkera with AAO• Arradiance with glass and plastic substrates• LAPD collaboration

• Tune resistive/thermal/outgasing/lifetime properties separately

• Large selection of materials

Two distinct modes of MCP operation

• Current amplification (e.g. image intensifiers)– Low gain (<104)– Moderately to high input fluxes– Usually frame-based readouts (CCD, CMOS)– Limited dynamic range – Timing resolution is limited to readout frame rate

• Event counting– Moderately to high gain for single particle detection (105-106)– Low input fluxes– Typical count rates 0.1 – 106 cps

(can be as high as 108 with low noise readouts, e.g. Medipix)– Both spatial and temporal information on each detected event– More sensitive to gain reduction from ageing, ion feedback

Charge cloudCharge distribution on strips

MCP Pair

Ideal electron amplifier (MCP)• Substrate

– No geometrical distortions– Mechanically robust in large formats– Compatible with large processing temperatures– Low outgasing/contaminating films deposited above– Small pores (ultimate limit of spatial resolution)– Cheap– Easy to manufacture

• Conductive film– Accurately controlled resistance in a wide range (small format MCP/ large

format / large/small pores)– Thermal coefficient of resistance is positive (self regulating/avoiding thermal

runaway)– Does not require high deposition temperatures– Vacuum compatible– Can be baked without changing its properties (required for tube production)– Repeatable

• Emissive film– High secondary electron emission coefficient (high gain, low operational

voltage, smaller L/D/ number of plates– Stable under electron bombardment– Can be baked without changing its properties (required for tube production)– Low outgasing– Efficient charge replenishment– Good photoelectron sensitivity (no need for a separate photocathode)

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Istrip

Ideal electron amplifier (MCP)

• Substrate– No geometrical distortions– Mechanically robust in large formats– Compatible with large processing

temperatures– Low outgasing/contaminating films deposited

above– Small pores (ultimate limit of spatial resolution)– Cheap– Easy to manufacture

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Istrip


• Conductive film– Accurately controlled resistance in a wide

range (small format MCP/ large format / large/small pores)

– Thermal coefficient of resistance is positive (self regulating/avoiding thermal runaway)

– Does not require high deposition temperatures– Vacuum compatible– Can be baked without changing its properties

(required for tube production)– Repeatable

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Istrip


• Emissive film– High secondary electron emission coefficient

(high gain, low operational voltage, smaller L/D/ number of plates

– Stable under electron bombardment– Can be baked without changing its properties

(required for tube production)– Low outgasing– Efficient charge replenishment– Good photoelectron sensitivity (no need for a

separate photocathode)

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V2

Istrip

Existing technology• Substrate

– No geometrical distortions– Small pores (ultimate limit of spatial resolution)– Mechanically robust in large formats– Compatible with high processing temperatures– Low outgassing, not contaminating films deposited above– Cheap– Easy to manufacture

• Conductive film– Accurately controlled resistance in a wide range (small format MCP/ large format /

large/small pores)– Thermal coefficient of resistance is positive (self regulating/avoiding thermal

runaway)– Does not require high deposition temperatures– Vacuum compatible– Can be baked without changing its properties (required for tube production)?– Repeatable

• Emissive film– High secondary electron emission coefficient (high gain, low operational voltage,

smaller L/D/ number of plates)– Stable under electron bombardment– Can be baked without changing its properties (required for tube production)?– Low outgassing– Efficient charge replenishment– Good photoelectron sensitivity (no need for a separate photocathode)

Definitely needs improvement

Relativelygood

Existing technology

• Substrate– No geometrical distortions– Small pores (ultimate limit of spatial resolution)– Mechanically robust in large formats– Compatible with high processing temperatures– Low outgassing, not contaminating films

deposited above– Cheap– Easy to manufacture


Relativelygood

Existing technology

• Conductive film– Accurately controlled resistance in a wide range

(small format MCP/ large format / large/small pores)

– Thermal coefficient of resistance is positive (self regulating/avoiding thermal runaway)


(required for tube production)?– Repeatable


Relativelygood

Existing technology

• Emissive film– High secondary electron emission coefficient

(high gain, low operational voltage, smaller L/D/ number of plates)


(required for tube production)?– Low outgassing– Efficient charge replenishment– Good photoelectron sensitivity (no need for a

separate photocathode)


Relativelygood

– Resistance of the pore• Limited number of counts per pore per

secondnext event with the same gain can only occur after the wall charge is replenished

Typical event transit time ~100 psTypical pore resistance ~1015

Pore current Istrip ~1pA

Positive wall charge builds up on the pore walls, mostly at the bottom where the amplification is the highest.

Typical pore capacitance 10-18 FRecharge time ~ RC = 1 ms

Only portion of that charge replenishes the wall positive charge through tunneling

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Istrip

Pulsed operation: event counting

– Assuming 8” MCP can sustain 60oC operation

– QRad ~ 3.5 Watt for 20 cm MCP– VMCP~1 kV => IStrip ~ 3.5 mA => RMCP~286 M (radiative

heat dissipation only)– Assume we can sustain 10x lower resistance through

heat conduction on the spacers - RMCP~30 M– 20 cm diameter MCP, with 20 m pores on 24 m

centers has ~63E6 pores– RPore ~ 1.9E15 , IPore ~ 0.5 pA– 10% of strip current can be extracted as charge =>

Iout ~ 0.05 pA/pore– Assume output charge value of 106e/pulse, 10 pores

involved in each pulse => 33 events/pore/s

With these assumptions: typical local count rate will be limited to ~100 events/pore/s

However, we observed 10x better performance locally: charge is shared by the neighboring pores (?)

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Rough estimate of MCP stable resistance and local count rate

A.S. Tremsin et al., Proc. SPIE 2808 (1996) pp.86-97.

The ageing effect is not localized to only illuminated area

• Ageing of microchannel plates Gain reduction is due to changes in the

conduction/emission films and/or their interfaces

MCP gain reduction effect: ageing under irradiation

Flat fieldimage

Long integration imageGain~105

Rate >10 MHz/cm2

Accumulated dose~0.01 C/cm2

Uniform flat field illumination

Normalized by initialflat field

No preconditioning of the detector was performed


14 mm

Uniform flat fieldimage (neutrons)

Resolution mask imageGain~105

Rate ~ 3 MHz/cm2


Almost uniform flat field illuminaiton

UV photons


Preconditioning is required for stable gain operation!

It is always done during standard tube manufacturing process.


14 mm

Uniform flat fieldimage (neutrons)

Resolution mask imageGain~105

Rate ~ 3 MHz/cm2


Almost uniform flat field illuminaiton

UV photons


Different applications may require completely different preconditioning procedure:

Rate of scrubbingInput current

Gain/voltage at the scrubbing

High gain detectors are usually scrubbed at low gain to allow more uniform scrub along the pore

What has changed in conduction/emission layers?

• Lower gain - SEE is reduced– Is it due to change in the bulk properties of

the emission layer (impurities/electron traps migration or redistribution)?

– Is it surface contamination?• scrubbing at different pressures should lead to

different ageing curves– Changes in the interface with the conduction

layer?

SEE surface of lead-glass MCPs

A.M.Then, C.G. Pantano, J. Non-crystalline Solids 120(1990) 178

SEE surface of lead-glass MCPs: ageing

B. Pracek, M. Kern, Appl. Surf. Sci. 70/71 (1993) 169

Concentration of K atoms (likely due to ion diffusion process)is greatly increased on the surface after ageing.

Also small increase of carbon contamination was observed.





• Resistance coefficient of MCP: thermal runaway– negative coefficient of resistance– poor heat dissipation in MCP detectors

– certain gain required to detect individual events

Limited local count rate: fixed amount of charge extracted from local area

Tradeoff between gain (resolution) and local count rate

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Istrip

R. Colyer et al., Proc. SPIE 7185-27 (2009)

MCP thermal runaway

A.S. Tremsin et al., Proc. SPIE 2808 (1996) pp.86-97.A.S. Tremsin et al., Nucl. Instr.Meth. 379 (1996) pp.139-151.

Conduction layer and thermal stability of MCPs

A.S. Tremsin et al., Rev. Sci. Instr. 75 (2004) pp.1068-1072

• Need very good control of the resistance value of the conduction layer.

• Not only as manufactured but also through the entire tube production process.

Si MCP thermal coefficient

1.E+08

1.E+09

1.E+10

1.E+11

-20 0 20 40

T(C)

Res

ista

nce

(Ohm

s)

R@100VR@200VR@300V

))(1( 00 TTRR T 1036.0 CT

Different manufacturing process, no lead glass, alkali metal doping;still similar value of TCR

A.S. Tremsin et al., Rev. Sci. Instr. 75 (2004) pp.1068-1072

Can bulk conductive substrate be an alternative to conduction layer?

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Reduced ion feedback

Eions

T. W. Sinor et al., Proc. SPIE 4128 (2000) 5.

Making bulk-conductive glass microchannel platesJay J.L. Yi, Lihong Niu, Proc. SPIE 68900E-1 (2008)

Much more heat will be generated as very small fraction of strip current will be used for charge replenishment

Stable conduction and emission films

•Both thermal coefficient T and voltage-dependent coefficient V of the conduction film should be very small

•Do not change properties under electron bombardment

•A stable SEE layer with low emission is better than high SEE film which changes as device operates

•Both increase of gain and gain reduction are equally bad. The low/high gain can be compensated by accelerating voltage

0

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0 0.005 0.01 0.015 0.02 0.025

Charge extracted (C/cm2)

Rela

tive

MCP

gai

n

Scrubbing interruption points

Si MCP Scrub(corrected for the

input flux variation)

Improved interface between conduction and emission films

•Currently only ~10% of strip current can be extracted as output current, the rest of it is only generating extra heat

•Will be very good if that fraction of useful current can be increased. Pore saturation mechanism is very important.

Conduction and emission film requirements• Conduction film

– Accurately controlled resistance in a wide range (small format MCP/ large format / large/small pores)

– Thermal coefficient of resistance is positive (self regulating/avoiding thermal runaway) or close to zero


(required for tube production)?– Repeatable

• Emissive layer– High secondary electron emission coefficient (high

gain, low operational voltage, smaller L/D/ number of plates)


(required for tube production)– Low outgassing– Efficient charge replenishment

•Compatible with

•large format MCP plates

•visible photocathodes

•tube sealing

•Stable

•Cheap

•Repeatable

Documents

Overview of MCP requirements: resistive layer and SEE A. S. Tremsin, O. H. W. Siegmund Space Sciences Laboratory University of California at Berkeley Berkeley,