Download - 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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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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Lunchtime talk 19/01/06Liam Cunningham
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'
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History of GEMs
GEM foil Electric field
#red lines'
Electrons
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History of GEMs
L. Shekhtman NIM A 494 (2002) 128141
.lose up of GEM field line
distribution
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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
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History of GEMs
3heory of avalanche gain in gas detectors
3he 3ownsend constantis related to the low
current, corona discharge
region of an ionising gas
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History of GEMs
3heory of avalanche gain in gas detectors
Assuming a 5inetic model were is the minimum ionisationenergy we get
=
E
We/p
$
ereis the mean free path andEis the electric field
#$'
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History of GEMs
3heory of avalanche gain in gas detectors
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= #)'
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History of GEMs
3heory of avalanche gain in gas detectors
.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
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History of GEMs
,$ -auli .'(M !" # $%%&' ( l-()'
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Development of GEM foils
(J. Benlloch et al. NIM A 419 (1998) 410-417)
Gain of single GEM foil in
Ar-.
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Development of GEM foils
(J. Benlloch et al. NIM A 419 (1998) 410-417)
=ariation in time response of
gain for different hole profiles
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Development of GEM foils
. !an"en#o$% et al. NIM A &'& (2004) 9'97
Use of GEM foils for neutrondetection using a converter
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Development of GEM foils
. !an"en#o$% et al. NIM A &'& (2004) 9'97
1mages ta5en using GEM
based neutron imagingsystem using a position
sensitive readout system
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Development of GEM foils
!. M.o$mann et al. NIM A &04 (200') 9'98
*chematic of multi-GEM system utilising different photocathodes,
readout is by microstrip detector
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Development of GEM foils
!. M.o$mann et al. NIM A &04 (200') 9'98
3ime response from semi-
transparent cathode multi-GEMsystem detecting U= photons
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Development of GEM foils
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Development of GEM foils
4ther areas for e5perimentation and
de&elopment include! Lo# pressure GEM operation. hech*k et al. NIM A 419 (1998) 42'-428
!
Cryogenic GEM operationA. Bon#a$ 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