Last Measurement on GEM and Literature review of Conduction in

PolymersGabriele Croci (CERN)

GDD MeetingFebruary, the 21st 2008

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Leakage Current in 10x10 GEM

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ZOOM

Leakage Current in Cu covered Kapton Foil (GEM 10X10 without holes)

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Comparison Holes/No Holes

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The major effect seems to come from surface conduction

REFERENCES[1] P. Keith Watson, “The transport and Trapping of Electrons in Polymers”, IEEE Transaction on

Dielectrics and Electrical Insulation, Vol. 2 No 5. October 1995[2] John J. Simmons, “Poole-Frankel Effect and Schottky Effect in Metal-Insulator-Metal Systems”,

Physical Review, Volume 155,3, 15 March 1967[3] E. Motyl, “Electrode Effects and Electrical Conduction in Polyimide Kapton HN Films”, IEEE

Internation Conference on Conduction and Breakdown in Solid Dielectrics, June 22-25 1998[4] J-P Salveat et al “Onset and growth of conduction in polyimide Kapton induced by swift heavy-

ion irradiation”, Physical Review B, Volume 55, Number 10, 1 March 1997-II[5] A. Rose, “Space-Charge Limited Currents in Solids”, Physical Review Volume 97, Number 6,

March 15, 1955[6] Edward J. Yadlowsky, Robert C. Hazelton, “Radiation Induced conduction in Kapton H Film”,

IEEE Transactions on Nuclear Science, Volume 35, No 4, August 1988[7] R.G. Filho et al, “Induced conductivity Of Mylar and Kapton Irradiated by X-Rays”, IEEE

Transactions on Electrical Insulation Volume EI-21 No. 3, June 1986

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General Polymer Description

• A polymer is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds

• Poliymide (Kapton, dielectric used in GEM) belongs to the polymer family

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General statements about Conduction in Polymers

• Polymers conductivity can be due to the small number of low-mobility charge carriers and to the high trap density [1]

• The traps can play a very important role in the carrier recombination process; they can trap carriers and release them in a successive time [1]

• Mobility changes of several order of magnitude with respect the free (without traps) mobility

• Dependence on temperature, applied electric field and particle (e-, p+, X-rays, Ions..) irradiation

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Energy Band Diagram in a Polymer [1]

• Slight difference from organized structure like metals or semiconductors

• The conduction band edge is substituted by the mobility edge and we can keep the concept of valence band

• The trap levels are usually between this two states

• Tentative to discover the energy distribution of these trapping states injecting electrons inside the polymer

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Some possible origins of trapping centers

• Impurities in the material [2]• Presence of Radicals in the polymer• Chemical structure of polymer chain• Open covalent (0,C) bounds• Regions of free volumes• ….

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Charge Trapping and Decay (1)[1]• The model described in [1] does not take into

account retrapping after a charge is released by a trap: this holds for thin polymers

• The current flowing in the polymer is a function of the energy density of the traps

• Definition of a trapping parameter α=1/μτE (μ:mobility, τ:characteristic time, E: electric field in the polymer)

• Electron is shallow states are rapidly detrapped and are driven more deeply in the material by the field

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Charge Trapping and Decay (2)

• The charge detrapped can contribute to the conduction and can accumulate on the surface of the polymer

• Measurement of Surface Potentials (Vs) with time

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Other Possible Conduction Mechanisms [2],[3],[4]

• Poole-Frenkel effect• Schottky Effect• Hopping• Tunneling• Space charge limited currents

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Poole-Freknel Effect• P-F: field-assisted thermal

ionization; lowering of a Coulombic potential barrier with an electric field; it is associated with the lowering of a trap barrier in the bulk

• Change of work function: W W- eβPFE½

• Change of conductivity: σ=σ0exp(βPFE½/kT)

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Schottky Effect

• Very similar to Poole-Frenkel Effect. It is the attenuation of a metal-insulator barrier arising from electrode image force interaction. It is a surface effect

• Change of conductivity similar to P-F σ=σ0exp(βsE½/kT)

• βPF=2βs

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Hopping Models[4]• Presence of π-conjugated bonds; phonon assisted tunneling

between localized states • Two basic processes: local jumping between adjacent sites

and “percolation” • A hop between two localized electronic states occurs when

the atomic vibratory motion changes the relaive energy of the localized states

• Two kinds of hops– Adiabatic: large electron energy transfer between states; jump

rate not limited by electron energy transfer or distance between sites

– Non Adiabatic: low electron energy transfer; jump rate limited by transfer energy and distances

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Tunneling and Space Charge Limited Current

• Tunneling is the quantum effect of passing through a barrier also if the energy is not enough to overcame the barrier itself

• Space charge limited current[5]: maximum current that can flow in a built-up capacitor charged with static charges. The current can be enhanced by PF effect. Current density has a voltage square dependence

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Radiation Induced Conduction in Kapton H Film [6]

• 8 μm thick kapton irradiated by 45 KeV penetrating electrons

• I-V characteristic depends on the voltage applied to the irradiated sample:– Low Voltage (<50V): Ohmic regime, Linear I-V

Characteristic– Intermediate Voltage (50V<V<700V): Space Charge Limited

Current (SCL) regime, I proportional to V2

– High Voltage (>700V): Trap Filled limit regime (TFL), I exponentially proportional to V

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Conduction Model [6]

• This is the math form of previous statements

It is possible to see three different regimes at different voltage values for current density

The constant A,B,C take into account all the parameters of the material and of the irradiation; h take into account also the energy gap over which the traps are distributed

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kThVTFLSLCohmic eL

VC

L

VB

L

VAJ 2

3

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Induced Conductivity of Mylar and Kapton Irradiated by X-Rays [7]

• Kapton Samples of 80 mm diameter with thickness varying from 6 to 75 μm were irradiated with W X-Rays for several hours

• Electric field (of different intensity) were applied to the samples

• They saw a variation of the Kapton conductivity

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Induced Conductivity of Mylar and Kapton Irradiated by X-Rays [7] (2)

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From my calculation and considering the rate we are using in our lab to test GEM, we are very close to the black curveNext week I will perform this kind of measurement irradiating a 10x10 GEM for several hours to see if there is a variation of the conductivity