Transcript Semiconductor GEMs - Glasgow Experimental Particle Physics

Fabrication of semiconductor
GEMs
or
Why GEMs are still
made from kapton
Liam Cunningham
Lunchtime talk 19/01/06
Overview

Historical info on GEMs
–
What, how etc.

Development of current devices

New developments in GEM technology
–
i.e. what I’ve been doing for 2 years
Liam Cunningham
Lunchtime talk 19/01/06
What is a GEM?
Unfortunately,
not one of these
Liam Cunningham
Lunchtime talk 19/01/06
GEM’s are
Gaseous
•
Electron
•
Multipliers
•
Liam Cunningham
A type of micro-pattern
gas detector which has
been developed for use in
applications requiring high
gain, high speed and low
noise measurement
Lunchtime talk 19/01/06
History of GEMs

First demonstrated by F. Sauli (NIM A 386 ( 1997) 53 l534)

The GEM foil consists of two metal electrodes
separated by an insulating film (kapton,
polyimide, PCB)
Liam Cunningham
Lunchtime talk 19/01/06
History of GEMs
Schematics of first test
GEM structure. GEM
placed inside an
MWPC to replace one
of the cathodes
F.
Sauli (NIM A 386 ( 1997) 53 l-534)
Liam Cunningham
Lunchtime talk 19/01/06
History of GEMs
GEM’s are used to amplify charge created
by incident radiation utilising the
avalanche effect.
Electron
(good)
GEM
Charge detector
(microstrip?)
photon or
particle
Ion (bad)
Liam Cunningham
Pressurised gas
mixture
Lunchtime talk 19/01/06
History of GEMs
GEM foil
Electric field
(red lines)
Electrons
Liam Cunningham
Lunchtime talk 19/01/06
History of GEMs
Close up of GEM field line
distribution
L. Shekhtman NIM A 494 (2002) 128–141
Liam Cunningham
Lunchtime talk 19/01/06
History of GEMs
Theory of avalanche gain in gas detectors
The total multiplication or gas gain from an electron travelling
from cathode to anode is given by :
c
M    dx
a
Where  is the Townsend constant, integrated over the transit
distance from cathode to anode
Liam Cunningham
Lunchtime talk 19/01/06
History of GEMs
Theory of avalanche gain in gas detectors
The Townsend constant
is related to the low
current, corona discharge
region of an ionising gas
Liam Cunningham
Lunchtime talk 19/01/06
History of GEMs
Theory of avalanche gain in gas detectors
Assuming a kinetic model were W is the minimum ionisation
energy we get
1
  W  (1)
  exp 


 E 
Were  is the mean free path and E is the electric field
Liam Cunningham
Lunchtime talk 19/01/06
History of GEMs
Theory of avalanche gain in gas detectors
Taking s as the cross section for ionisation between electrons and gas atoms
gives
were NL is Loschmidts number given by
1

N Ls
(2)
N AP
NL 
RT
(3)
NA Avogadros number, R the gas constant, P/T ambient pressure/ temp
P/T can be expressed as the ratio
Liam Cunningham
qP
T
(4)
Lunchtime talk 19/01/06
History of GEMs
Theory of avalanche gain in gas detectors
Combining these we get
Defining q  R N As


 N As
WN As

exp  
E

q
R
R

q

we can re-write (6) as


 (6)





 W 
1

exp 

q q
 E q q  (6a)



Were W and q are physical parameters of the gas it is easy to see that the gain
depends on E and q
Liam Cunningham
Lunchtime talk 19/01/06
History of GEMs
F.
Sauli (NIM A 386 ( 1997) 53 l-534)
Liam Cunningham
Lunchtime talk 19/01/06
Development of GEM foils
Gain of single GEM foil in
Ar-CO2 atmosphere at
atmospheric pressure
(J. Benlloch et al. NIM A 419 (1998) 410-417)
Liam Cunningham
Lunchtime talk 19/01/06
Development of GEM foils
Variation in time response of
gain for different hole profiles
(J. Benlloch et al. NIM A 419 (1998) 410-417)
Liam Cunningham
Lunchtime talk 19/01/06
Development of GEM foils
Use of GEM foils for neutron
detection using a PP converter
V. Dangendorf et al. NIM A 535 (2004) 93–97
Liam Cunningham
Lunchtime talk 19/01/06
Development of GEM foils
Images taken using GEM
based neutron imaging
system using a position
sensitive readout system
V. Dangendorf et al. NIM A 535 (2004) 93–97
Liam Cunningham
Lunchtime talk 19/01/06
Development of GEM foils
Schematic of multi-GEM system utilising different photocathodes,
readout is by microstrip detector
D. M.ormann et al. NIM A 504 (2003) 93–98
Liam Cunningham
Lunchtime talk 19/01/06
Development of GEM foils
Time response from semitransparent cathode multi-GEM
system detecting UV photons
D. M.ormann et al. NIM A 504 (2003) 93–98
Liam Cunningham
Lunchtime talk 19/01/06
Development of GEM foils
Liam Cunningham
Lunchtime talk 19/01/06
Development of GEM foils

Other areas for experimentation and
development include:
–
Low pressure GEM operation

–
R. Chechik et al. NIM A 419 (1998) 423-428
Cryogenic GEM operation

A. Bondar et al. NIM A 524 (2004) 130–141
Liam Cunningham
Lunchtime talk 19/01/06
GEM applications

Atmospheric pressure and above, GEMs can be used
as an amplifier stage for detection of lightly interacting
particles i.e. MIPS.
–


No further amplification is required in this case
Neutron detector with converter.
Low pressure detectors with CsI photocathode for ultra
soft x-rays and UV photons in single electron counting
operation
–
RICH detectors
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs
Fabrication of GEM foils from rigid semiconductor or
insulating substrates is desirable for a number of reasons
1. Removes effect of sagging as device is powered up
2. Use of reactive gas mixtures could be explored
3. Higher possible baking temperature (improved sealing of
vacuum chambers)
4. Greater density of holes possible due to existing advanced
lithography and processing technology
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs
Very small features and pitches produced in Si using dry
etch technology
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Design for test device
Test structure with 4
different hole diameters
80 – 200 mm
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Design for test device
Single test pattern
Close up on single
hexagonal cell
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Metallisation
The device structure as
shown here is a metallic
layer with an insulating
material separating them.
This implies we need to
passivate the Si surface
and then apply a metallic
film.
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Metallisation
Preliminary attempts used a PECVD (plasma enhanced chemical
vapour deposition) layer of SiO2 with 200 nm of Au as the
metallisation.
The problem is gold
doesn’t stick very
well.
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Metallisation
Metallisation recipe
changed to include
Ti adhesion layer,
this successfully
survives several
future processing
steps.
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching passivation layer
Schematic of
reactive ion etching
(RIE) plasma reactor
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching passivation layer
The theory of the RF plasma operating in glow discharge regime starting from
the force exerted on a single electron
nc= neutral
collision frequency
E= electric field
then taking the x component of the motion and substituting the sinusoidal
electric field it is possible to define
the power
absorbed by the gas
ne= number density of electrons w= E field frequency
m=electron mass Eo= max field strength
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching passivation layer, problems
None.
This is the only step that never had any problems
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si
Schematic of
inductively coupled
plasma (ICP) reactor
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si
The ICP upper chamber this is what
creates the denser plasma responsible
for the faster etching rate. The
frequency is fixed at 13.65 MHz the
power can be varied depending on the
attached power supply.
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si

Parameters for ICP etching
–
–
–
Coil power: Determines the density of the plasma in
the upper chamber
Platen power: determines the potential difference
accelerating ions towards the surface
Pressure: has an effect on the transfer of ionic
species into and out of the etched features
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si
SF6/ O2 mixture used
for etching the initial
features. Preferentially
etching vertically
Liam Cunningham
Plasma chemistry switched to
C4F8
This causes a build up of
polymer over all surfaces
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si
Switching the plasma
gasses back to SF6/O2
starts etching again
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1



ICP (inductively coupled plasma) etching of Si
is very sensitive parameter sensitive.
Incorrect choice of any of the parameters can
lead to non-successful etch.
Recipe design is a fairly time and material
intensive process
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1
Wrong pressure
causes feature to
close up towards the
bottom. This stops
etching after a given
depth.
low pressure
high pressure
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1
Varying the
platen power
modifies the
profile of the
hole.
low platen power
Liam Cunningham
high platen power
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1
Excess passivation
build up caused by
poor cycle time
selection
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1


But, why are these all serious fatal flaws
With poor parameter choice, and subsequent
poor etch profile the depth, diameter and actual
shape of the etched features is pretty vague
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1
These images are of the two ends of the same hole. Obviously there
is a problem, they aren’t circular and they’re different sizes
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1
Improved shape, high
mask erosion is
causing damage to
metal surface
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1
Circular holes
,reduced mask
erosion but still
causing damage to
metal surface
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1
Round holes.
No surface damage
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1
The parameters for the successful etch are as follows
Coil: 900W (etch) / 800W (dep)
Platen: 13W (etch) / 0W (dep)
Etch: SF6/O2 = 130 / 13 sccm
Deposition: C4F8 = 110 sccm
Switch: 11s (etch) / 7s (dep)
Pressure: ~30 mtorr
This process produces an etch rate of 3- 3.5 mm/min
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 1
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 2

ICP switched process etch does not etch SiO2

Need to align from the other side to be able to
etch both SiO2 layers
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 2
First attempt at aligning front to back a
complete and utter mismatch
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 2
Kaleidoscope effect
from partial
rotational mismatch
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 2
Fully etched device
holes circular and
properly aligned.
Looks suitable for
testing
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Etching Si, problems 2
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Testing devices
Constant current ~ 1mA
over large voltage range
need to get lower current,
implying better field
characteristics
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Testing devices

What to do? Change oxide layer, PECVD oxide
has lower resistivity and break down field than
thermal oxide.

Other problems relating to integrity of the layer
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Testing devices
Very low current
<5 pA over large
range looks very
promising
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Testing devices
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Testing devices
Measurements of
changing current as a b
source is applied and
removed from the
sample.
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Testing devices
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Testing devices
Until this was
discovered
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
What next?

Tests showed short between the metal layers and the
Si.
–
Ti diffusion causing conductive TixOy at hole edge

Solution. Change metal again. Use Pd, very low
diffusion in SiO2, sticky unlikely to come off.

Other angle looking at only using one SiO2 layer to cut
down the possibilities of shorts developing
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
What next?

Use Quartz substrate, this has one really big
advantage,
–

This also removes the likelihood of shorts
–

No need for separate passivation
Sounds perfect
Problem, cannot get dry etching facilities for
deep etching in quartz and wet etching is too
isotropic for very deep etching
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Recent developments


Unfortunately not many.
The STS ICP has been down since June.
–


Came back on line last week, making 12 months of
down time in the last 26.
Samples are being etched now with Pd
metallisation.
Masks designed for etching of quartz substrate
Liam Cunningham
Lunchtime talk 19/01/06
Semiconductor GEMs:
Future developments

Adding additional Si3N4 to SiO2 surface to
reduce possibility of interface effects

The next few weeks will produce more
completed devices for testing
Liam Cunningham
Lunchtime talk 19/01/06
ICP theory
Liam Cunningham
Lunchtime talk 19/01/06