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Liam Cunningham Lunchtime talk 19/01/06
Fabrication of semiconductor GEMs
or
Why GEMs are still made from kapton
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
What is a GEM?
Unfortunately, not one of these
Lunchtime talk 19/01/06Liam Cunningham
GEM’s are
A type of micro-pattern gas detector which has been developed for use in applications requiring high gain, high speed and low noise measurement
Gaseous • Electron• Multipliers•
Lunchtime talk 19/01/06Liam Cunningham
History of GEMs
First demonstrated by F. Sauli (NIM A 386 ( 1997) 53 l-534)
The GEM foil consists of two metal electrodes separated by an insulating film (kapton, polyimide, PCB)
Lunchtime talk 19/01/06Liam Cunningham
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)
Lunchtime talk 19/01/06Liam Cunningham
Pressurised gas mixture
History of GEMs
GEM’s are used to amplify charge created by incident radiation utilising the avalanche effect.
Electron (good)
Ion (bad)
photon orparticle
GEM Charge detector(microstrip?)
Lunchtime talk 19/01/06Liam Cunningham
History of GEMs
GEM foil Electric field(red lines)
Electrons
Lunchtime talk 19/01/06Liam Cunningham
History of GEMs
L. Shekhtman NIM A 494 (2002) 128–141
Close up of GEM field line distribution
Lunchtime talk 19/01/06Liam Cunningham
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
a
dxM
Where is the Townsend constant, integrated over the transit distance from cathode to anode
Lunchtime talk 19/01/06Liam Cunningham
History of GEMs
Theory of avalanche gain in gas detectorsThe Townsend constant is related to the low current, corona discharge region of an ionising gas
Lunchtime talk 19/01/06Liam Cunningham
History of GEMs
Theory of avalanche gain in gas detectorsAssuming a kinetic model were W is the minimum ionisation energy we get
E
Wexp
1
Were is the mean free path and E is the electric field
(1)
Lunchtime talk 19/01/06Liam Cunningham
History of GEMs
Theory of avalanche gain in gas detectorsTaking as the cross section for ionisation between electrons and gas atoms gives were NL is Loschmidts number given by
NA Avogadros number, R the gas constant, P/T ambient pressure/ temp
LN
1
RT
PNN A
L (2)
(3)
P/T can be expressed as the ratioT
Pq (4)
Lunchtime talk 19/01/06Liam Cunningham
History of GEMs
Theory of avalanche gain in gas detectors
Combining these we get
qE
R
WN
R
N
qAA
exp (6)
Defining we can re-write (6) as ANRq
EW
exp1
(6a)
Were W and q are physical parameters of the gas it is easy to see that the gain depends on E and q
Lunchtime talk 19/01/06Liam Cunningham
History of GEMs
F. Sauli (NIM A 386 ( 1997) 53 l-534)
Lunchtime talk 19/01/06Liam Cunningham
Development of GEM foils
(J. Benlloch et al. NIM A 419 (1998) 410-417)
Gain of single GEM foil in Ar-CO2 atmosphere at atmospheric pressure
Lunchtime talk 19/01/06Liam Cunningham
Development of GEM foils
(J. Benlloch et al. NIM A 419 (1998) 410-417)
Variation in time response of gain for different hole profiles
Lunchtime talk 19/01/06Liam Cunningham
Development of GEM foils
V. Dangendorf et al. NIM A 535 (2004) 93–97
Use of GEM foils for neutron detection using a PP converter
Lunchtime talk 19/01/06Liam Cunningham
Development of GEM foils
V. Dangendorf et al. NIM A 535 (2004) 93–97
Images taken using GEM based neutron imaging system using a position sensitive readout system
Lunchtime talk 19/01/06Liam Cunningham
Development of GEM foils
D. M.ormann et al. NIM A 504 (2003) 93–98
Schematic of multi-GEM system utilising different photocathodes, readout is by microstrip detector
Lunchtime talk 19/01/06Liam Cunningham
Development of GEM foils
D. M.ormann et al. NIM A 504 (2003) 93–98
Time response from semi-transparent cathode multi-GEM system detecting UV photons
Lunchtime talk 19/01/06Liam Cunningham
Development of GEM foils
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
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 up2. Use of reactive gas mixtures could be explored3. Higher possible baking temperature (improved sealing of
vacuum chambers) 4. Greater density of holes possible due to existing advanced
lithography and processing technology
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs
Very small features and pitches produced in Si using dry etch technology
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Design for test device
Test structure with 4 different hole diameters80 – 200 m
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Design for test device
Single test pattern
Close up on single hexagonal cell
Lunchtime talk 19/01/06Liam Cunningham
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.
Lunchtime talk 19/01/06Liam Cunningham
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.
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Metallisation
Metallisation recipe changed to include Ti adhesion layer, this successfully survives several future processing steps.
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching passivation layer
Schematic of reactive ion etching (RIE) plasma reactor
Lunchtime talk 19/01/06Liam Cunningham
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
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
c= neutral collision frequency E= electric field
ne= number density of electrons = E field frequencym=electron mass Eo= max field strength
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching passivation layer, problems
None. This is the only step that never had any problems
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si
Schematic of inductively coupled plasma (ICP) reactor
Lunchtime talk 19/01/06Liam Cunningham
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.
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si
SF6/ O2 mixture used for etching the initial features. Preferentially etching vertically
Plasma chemistry switched to C4F8
This causes a build up of polymer over all surfaces
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si
Switching the plasma gasses back to SF6/O2 starts etching again
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 1
low platen power high platen power
Varying the platen power modifies the profile of the hole.
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 1
Excess passivation build up caused by poor cycle time selection
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 1
Improved shape, high mask erosion is causing damage to metal surface
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 1
Circular holes ,reduced mask erosion but still causing damage to metal surface
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 1
Round holes.No surface damage
Lunchtime talk 19/01/06Liam Cunningham
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 m/min
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 1
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 2
First attempt at aligning front to back a complete and utter mismatch
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 2
Kaleidoscope effect from partial rotational mismatch
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 2
Fully etched device holes circular and properly aligned. Looks suitable for testing
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Etching Si, problems 2
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Testing devices
Constant current ~ 1A over large voltage range need to get lower current, implying better field characteristics
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Testing devices
Very low current <5 pA over large range looks very promising
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Testing devices
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Testing devices
Measurements of changing current as a source is applied and removed from the sample.
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Testing devices
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:Testing devices
Until this was discovered
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
Semiconductor GEMs:What next?
Use Quartz substrate, this has one really big advantage,– No need for separate passivation
This also removes the likelihood of shorts– Sounds perfect
Problem, cannot get dry etching facilities for deep etching in quartz and wet etching is too isotropic for very deep etching
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
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
Lunchtime talk 19/01/06Liam Cunningham
ICP theory