Lunchtime_talk GEM Detector

7/26/2019 Lunchtime_talk GEM Detector

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Liam Cunningham Lunchtime talk 19/01/06

Fabrication of semiconductorGEMs

or

Why GEMs are still

made from kapton


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Lunchtime talk 19/01/06Liam Cunningham

Overview

istorical info on GEMs

! What" ho# etc$

%e&elopment of current de&ices

'e# de&elopments in GEM technology! i$e$ #hat ()&e *een doing for + years


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What is a GEM?

Unfortunately,not one of these


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GEMs are

A type of micro-pattern

gas detector which hasbeen developed for use in

applications requiring high

gain, high speed and low

noise measurement

Gaseous

Electron

Multipliers


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History of GEMs

,irst demonstrated *y ,$ -auli .'(M !" # $%%&' ( l-()'

he GEM foil consists of t#o metal electrodes

separated *y an insulating film .kapton"

polyimide" C23


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History of GEMs

*chematics of first test

GEM structure+ GEMplaced inside an

M. to replace one

of the cathodes

,$ -auli .'(M !" # $%%&' ( l-()'


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ressurised gas

mi/ture

History of GEMs

GEM0s are used to amplify charge created

by incident radiation utilising theavalanche effect+

Electron

#good'

1on #bad'

photon orparticle

GEM .harge detector

#microstrip2'



History of GEMs

GEM foil Electric field

#red lines'

Electrons



History of GEMs

L. Shekhtman NIM A 494 (2002) 128141

.lose up of GEM field line

distribution



History of GEMs

3heory of avalanche gain in gas detectors

3he total multiplication or gas gain from an electron travellingfrom cathode to anode is given by 4

=c

a

dxM

here is the 3ownsend constant, integrated over the transitdistance from cathode to anode



History of GEMs


3he 3ownsend constantis related to the low

current, corona discharge

region of an ionising gas



History of GEMs


Assuming a 5inetic model were is the minimum ionisationenergy we get

=

E

We/p

$

ereis the mean free path andEis the electric field

#$'



History of GEMs


3a5ing as the cross section for ionisation between electrons and gas atomsgives were 67is 7oschmidts number given by

6AAvogadros number, 8 the gas constant, 93 ambient pressure9 temp

LN

$=

RT

PNN

AL =

#:'

#'

93 can be e/pressed as the ratioT

Pq= #)'



History of GEMs


.ombining these we get

=

q

ER

WN

R

N

q

AA

e/p #"'

;efining we can re-write #"' as ANRq =

=qqE

W

qq

e/p

$

#"a'

ere Wand q are physical parameters of the gas it is easy to see that the

gain depends onEand q



History of GEMs

,$ -auli .'(M !" # $%%&' ( l-()'



Development of GEM foils

(J. Benlloch et al. NIM A 419 (1998) 410-417)

Gain of single GEM foil in

Ar-.




(J. Benlloch et al. NIM A 419 (1998) 410-417)

=ariation in time response of

gain for different hole profiles




. !an"en#o$% et al. NIM A &'& (2004) 9'97

Use of GEM foils for neutrondetection using a converter




. !an"en#o$% et al. NIM A &'& (2004) 9'97

1mages ta5en using GEM

based neutron imagingsystem using a position

sensitive readout system




!. M.o$mann et al. NIM A &04 (200') 9'98

*chematic of multi-GEM system utilising different photocathodes,

readout is by microstrip detector




!. M.o$mann et al. NIM A &04 (200') 9'98

3ime response from semi-

transparent cathode multi-GEMsystem detecting U= photons


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4ther areas for e5perimentation and

de&elopment include! Lo# pressure GEM operation. hech*k et al. NIM A 419 (1998) 42'-428

!

Cryogenic GEM operationA. Bon#a$ et al. NIM A &24 (2004) 1'0141


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GEM applications

tmospheric pressure and a*o&e" GEMs can *e used

as an amplifier stage for detection of lightly interacting

particles i$e$ M(-$! 'o further amplification is re7uired in this case

'eutron detector #ith con&erter$

Lo# pressure detectors #ith Cs( photocathode for ultra

soft 58rays and : photons in single electron countingoperation! ;(C detectors


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emiconductor GEMs

>abrication of GEM foils from rigid semiconductor or

insulating substrates is desirable for a number of reasons

$+ 8emoves effect of sagging as device is powered up

:+ Use of reactive gas mi/tures could be e/plored

+ ?igher possible ba5ing temperature #improved sealing of

vacuum chambers'

)+ Greater density of holes possible due to e/isting advanced

lithography and processing technology


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emiconductor GEMs

=ery small features and pitches produced in *i using dry

etch technology


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emiconductor GEMs


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emiconductor GEMs!

Desi"n for test device

3est structure with )

different hole diameters

!@ :@@ m


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emiconductor GEMs!

Desi"n for test device

*ingle test pattern

.lose up on single

he/agonal cell


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emiconductor GEMs!

Metallisation

3he device structure as

shown here is a metallic

layer with an insulatingmaterial separating them+

3his implies we need to

passivate the *i surface

and then apply a metallicfilm+


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emiconductor GEMs!

Metallisation

reliminary attempts used a E.=; #plasma enhanced chemical

vapour deposition' layer of *i su*strate" this has one really *ig

ad&antage"! 'o need for separate passi&ation

his also remo&es the likelihood of shorts! -ounds perfect

ro*lem" cannot get dry etching facilities fordeep etching in 7uart> and #et etching is too

isotropic for &ery deep etching


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emiconductor GEMs!

(ecent developments

nfortunately not many$

he -- (C has *een do#n since ?une$! Came *ack on line last #eek" making 1+ months ofdo#n time in the last +6$

-amples are *eing etched no# #ith d

metallisation$Masks designed for etching of 7uart> su*strate


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emiconductor GEMs!

Future developments

dding additional -i@'Ato -i4+ surface to

reduce possi*ility of interface effects

he ne5t fe# #eeks #ill produce more

completed de&ices for testing


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)*+ theory

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