Liam CunninghamLunchtime talk 19/01/06 Fabrication of semiconductor GEMs or Why GEMs are still made from kapton

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


What is a GEM?

Unfortunately, not one of these


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•


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)


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)


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?)


History of GEMs

GEM foil Electric field(red lines)

Electrons


History of GEMs

L. Shekhtman NIM A 494 (2002) 128–141

Close up of GEM field line distribution


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


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


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)


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)


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

qq

EW

qq

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


History of GEMs

F. Sauli (NIM A 386 ( 1997) 53 l-534)


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



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

Variation in time response of gain for different hole profiles



V. Dangendorf et al. NIM A 535 (2004) 93–97

Use of GEM foils for neutron detection using a PP converter



V. Dangendorf et al. NIM A 535 (2004) 93–97

Images taken using GEM based neutron imaging system using a position sensitive readout system



D. M.ormann et al. NIM A 504 (2003) 93–98

Schematic of multi-GEM system utilising different photocathodes, readout is by microstrip detector



D. M.ormann et al. NIM A 504 (2003) 93–98

Time response from semi-transparent cathode multi-GEM system detecting UV photons





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


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


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


Semiconductor GEMs

Very small features and pitches produced in Si using dry etch technology


Semiconductor GEMs


Semiconductor GEMs:Design for test device

Test structure with 4 different hole diameters80 – 200 m


Semiconductor GEMs:Design for test device

Single test pattern

Close up on single hexagonal cell


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.



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.



Metallisation recipe changed to include Ti adhesion layer, this successfully survives several future processing steps.


Semiconductor GEMs:Etching passivation layer

Schematic of reactive ion etching (RIE) plasma reactor


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


Semiconductor GEMs:Etching passivation layer, problems

None. This is the only step that never had any problems


Semiconductor GEMs:Etching Si

Schematic of inductively coupled plasma (ICP) reactor



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.



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



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



Switching the plasma gasses back to SF6/O2 starts etching again


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



Wrong pressure causes feature to close up towards the bottom. This stops etching after a given depth.

low pressure high pressure



low platen power high platen power

Varying the platen power modifies the profile of the hole.



Excess passivation build up caused by poor cycle time selection



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



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



Improved shape, high mask erosion is causing damage to metal surface



Circular holes ,reduced mask erosion but still causing damage to metal surface



Round holes.No surface damage



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





ICP switched process etch does not etch SiO2

Need to align from the other side to be able to etch both SiO2 layers



First attempt at aligning front to back a complete and utter mismatch



Kaleidoscope effect from partial rotational mismatch



Fully etched device holes circular and properly aligned. Looks suitable for testing




Semiconductor GEMs:Testing devices

Constant current ~ 1A over large voltage range need to get lower current, implying better field characteristics



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



Very low current <5 pA over large range looks very promising





Measurements of changing current as a source is applied and removed from the sample.





Until this was discovered


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


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


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


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


ICP theory

Documents

Liam CunninghamLunchtime talk 19/01/06 Fabrication of semiconductor GEMs or Why GEMs are still made from kapton