2002 Annual Report Conference on Electrical Insulation and Dielectric Phenomena

Electrical Conduction in Polyimide-FEP Fluoropolymer Films

Riaz Shaikh and G.R. Govinda Raju Department of Electrical and Computer-Engineering

University of Windsor, Windsor, Ontario, N9B 3P4, Canada

Abstract: The aromatic polyimide has excellent mechanical, electrical and chemical properties. Absorption and conduction currents are studied over a temperature range of 50 to 200 'C in Teflon coated film ( 2 5 . 4 ~ Kapton@ base with 12.7pm coating of Teflon@ FEP fluoropolymer resin on one side) and the influence of parameters such as field strengths and electrode materials are investigated. The charging current decays with time according to t-" (decay constant) with the value of index n in the range of 0.66 to 1.25.The discharging current decreases with the value of n in the range of 1.47 to 1.59 for the temperature range investigated. The dielectric loss factors at low frequency were evaluated from the absorption currents. For temperature up to 90 "C one broad peak is observed at frequencies around 3x104 Hz. For temperatures (>I20 "C) the magnitude of the loss factor increases by an order of magnitude and no peaks are observed. Results on conduction currents are also reported for temperatures 90 -200 "C and fields up to 100kV/cm. The activation energy of 0.70 eV and the ionic jump distance in the range 19 -28 nm have been evaluated.

Introduction The highly aromatic nature of the polyimide

polymer contributes to its extremely good thermal and radiation resistance. In addition it has excellent electrical,mechanical properties and is chemically stable. These polyimides have found applications as insulating materials in transformers, motors,generators and in the present days it is increasingly used as packaging material for microelectronics applications[ I]. The current , which flows in the external circuit upon application of the step voltage decays with time until1 steady state is achieved. This current has been termed as charging, resoprtion current[2]. It is thought to be composed of two components. The first is the polarization current which decays with time and follows the Curie-Van Schweindler law:

where I is the current in ampere.A(T) a temperature dependent factor, t is the lapsed time in seconds and n is the decay constant. The second component is the transport current or the conduction current due to the motion of the carriers in the bulk of the material which persists in time. Research on chargingldischarging and

I(t)= A(T)t* (1)

conduction currents have not been reported for polyimide-FEP fluoropolymer film. The results will provide design data for the polyimide user and will provide insight into the conduction process

Experimental Procedure

Measurement of chargingldischarging current and conduction currents were carried on polyimide -FEP fluoropolymer film (commercially available as 150FN019 film manufactured by E.I.DuPont Demours & Co.) having a thickness of 3 1 . 5 ~ . Before using the samples they were heat treated at 200'C to remove any moisture content and to improve the reproducibility of the results. A guard ring was also employed to reduce the effects due to stray field.Before each measurement a blank run was performed in order to free the sample from the extraneous charges. This run was consisted of raising the temperature of the sample at the constant rate at 200 OC with electrodes short circuited.At this temperature the samples were kept short circuited for a period of 12 hours.Infrared spectroscopy analysis is carried out in order to determine the teflon coated side. The currents are measured using a keithley 617 C electrometer. The current and the temperature are recorded simultaneously by data acqusition system. The frequency of the sampling can be adjusted up to 4 KHz but most of the data are taken at 1 Hz.

RESULTS AND DISCUSSION Charging and Discharging currents

a) Time-dependence Fig.1 shows the charging currents for 31.5pm sample respectively with aluminum electrode at constant field and various poling temperatures for duration of 4 hrs. The charging current decays slowly with time and the value of n is in the range of 0.52 to 1.39 for temperatures up to 150 'C. For temperatures greater than 150 'C, the current is seen to decrease with value of n varying between 0.61 to 0.74 for times up to 100 second and for longer times the current increases. The slow decrease of the current in the initial stages can be attributed to the slow filling of the trap. The observed behavior has also been interpreted due to the arrival of the space charge front following transport from the

0-7803-7502-5/02/$17.00 @ 2002 IEEE 578

source electrode [3]. Fig.2 shows the plot of discharging current at constant field of 6 MVm-' and various

-7 0

4 0

~ 120d

X 1 5 0 d

= l8Ooc

3 0 3 0 0

510 -120

-130 0 I 2 S

Log t (s)

Fig.1 Time dependence of charging current at constant field of 6MVm.' and various tempemNres, elechode material-AI

temperatures of 50 -200 'C. For temperatures 50 to 180 OC the current decreases with the value of n in the range of 1.47 to 1.69. For temperature of 200 'C the value of n is found to be 0.57. The decay can be explained in terms of Jonscher universal dielectric theory [4] which states that the time domain response of the current has two value of n. An increase in the value at longer periods is due to dipolar mechanisms while the decrease in the value of n at longer period is due to interfacial polarization.

-12.0 { \

-7.0 , 7 x l r n d

. rnd I

-13.0 J 0 1 4 5

Log t (.i Fig.2 of 6 MVm-I and various trmpa'aNre,electrode material -AI

Time dependence of discharging current at constant field

b) Field dependence

Figure3 shows the charging currents at 200 OC and various field strength of 4 to 12 MVm.'. At 4 MVm-' field the value of n is 0.62 where as for the fields 6 MVm8 and higher the current decreases with a value of n in the range of 0.50 to 0.82 for up to 100 seconds. At times higher than 10' seconds the current increases to reach steady state conduction current. Chouhan [ 5 ] has found similar increase of current for KaptonaH film for temperatures 200 'C and field 8 MVm". Figure 4 shows the discharging current at 200' C and various fields .The current decreases with a value of n in range of 0.56 to

0.60. The current decays fast for times up to 10' seconds where the power law holds good with a value of n in the

-8.0 , I L K 7 -7.0

m8 W l m

A 8 MVlm

010Mv1m

3 a0 3 -9.0

- -10.0

Fig3 Time dependence of charging current at constant temperahue of 200 OC and various field, electrode material- Al

range of 0.74 to 0.81 and for times greater than lo' and the current decay is much less pronounced with n ranging kom 0.02 to 0.05. The charging and discharging currents show mirror images for low temperatures and are dissimilar for all other temperatures and field. The slopes of discharging current shows two values of n one at t < lo'second and other aulO'. This suggests that dipolar processes are operative mechanism at temperatures up to 90 'C and interfacial polarization is the operative mechanism at longer times and temperatures above 180 OC.

-7.0

-8.0

-9.0

40 .0

-11.0

- d 2

0 1 L%gt(ij 4 5

Fig4 Time dependence of discharging current at consfant temperature of 200 OC and various field, electmde material- Al

c) Effect ofElectrode Material

Figure 5 shows the discharging current at 200 OC and field up to 12 MVm-' for silver electrode material. The currents decay with value of n ranging from 0.60 to O.82.The discharging current comparison of figure 4 and 5 does not show very significant dependence on the electrode material except that the current reduces at a faster rate for silver electrodes than that for the aluminum electrodes.

Low frequency dielectric loss (E")

The absorption current, which flows in the dielectric when a step a DC voltage may be used to obtain the

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information on loss factor (E") using Hamons transform [6] and is given by:

-._ -6.0 - -7.0 -

- -8.0 - 2 -9.0

- k

-10.0 -

6.0 7,

W 2 0 &

U200 cc

4.0 s 30.0 J -12.0

-14.0

m6 MVlm

A a M V I ~

x 10 MVlm

0 1 2 3 4 5 Log t (5)

Fig5 material -Silver

Discharging current at 200 OC and various field, eleclrcde

E" = 1 (t)/ 2rrfC0V (2) where I (t) = current at time t, C e geometric capacitance when the assembly replaced with vacuum, V is the step voltage, f is frequency given by 0.M

1.2 1 .o 0.8

w 0.6 0.4 0.2 0.0

-5 -4 -3 log f -2 -1 0

Fig.6 temperaNre. preapplied field of 6 Mmi' , electmde material - aluminum

Fig3 and fig 6 shows the results of the calculated value of (E") at low eequencies for two temperature regimes OS T S 120 OC and 120 S TS 200 OC.

Frequency dependence ofdielechic loss E" at 50- I50 'C

25

-5 -4 -3 lcgf -2 -1 0

Fig.7 180- 200 OC temperature, preapplied field of 6 Mvm-', electrode material -aluminum

Fig 6 shows one broad peak for temperature up to 9OoC. It is seen that the maximum frequencies shift to higher

Frequency dependence of dielectric loss E" at

value with the temperature from 50 to 90 OC indicating the presence of dipolar polarization. Figure7 shows that the magnitude of the loss factor increasing by on order of magnitude and no peaks are observed for higher temperatures which is a manifestation of Interfacial polarization. The FN film has a coating of Teflon and it can be inferred that the carriers are trapped at the interface between the layers of KaptonS and Teflon leading to the process of Interfacial polarization.

Conduction current

The difference between the charging and discharging current is called the transport current or conduction current. Fig 8 shows the conduction current as a function of electric field for various temperatures at IO' seconds. Four mechanism have generally been put forwar& they are ionic conduction, Schottky emission, Pool-Frenkel effect and space charge effect. For mechanisms of ionic hopping the current density at high fields can be explained by the equation

J = 2 e N a v exp (4 KT) sinh eaE/2KT (3)

where e is the charge of the ion, v is the attempt to escape frequency, a is the separation distance between the traps, 4 is the barrier height, E is the electric field, T is the absolute temperature, K is the Boltzman's constant. From the slopes of plot of 1 vs. E the separation distance between the traps can be calculated.

-11.0 - 0 20 40 ,$,,&p 100 120

Fig3 various coostant IemperaNres, electmde material -AI

For higher temperatures the jump distance is found to be increasing. The estimated values of the jump distance are 19.41 nm for 90 OC and 38.64nm for value of 200 oC.Sussi [7] has reported the value of ionic jump distance of 18.8 nm to 37.2 nm in Nomex-Polyster- Nomex and found that the jump distances were increasing with temperature increasing temperatures yielding higher values. Sussi has attributed the large ionic jump distance found in N-P-N due to the presence of different layers. The nonlinear behavior of polymers at high fields may also be due to thermionic emission either from the cathode or from donor like defect states

lswhomal currents BS a h a i o n of applied elechic field at

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in the sample (Poole-Frenkel effect). In the case of field assisted thermionic emissions from the cathode the current density J is expressed for the Schottky equation

J; A, T’ ex& 4% -psELn/ KT) (4)

where A, is the emission constant, T is the temperature, E is the electric field and ps is the Schottky coefficient From the slopes of I vs. Eln plot the value of B, and E

can be calculated. The calculated value of the dielectric constant E. varies between ranges of 0.12 to 0.012. For the Poole Frenkel effect the values of would be four times that obtained for the Schottky dielectric constant. The dielectric constant as obtained for Poole-Frenkel gives a value of 0.46 at 90 ‘C, 0.05 for 200 ‘C. The experimental value of the dielectric constant as obtained from [I] is 2.7. The values are very low as compared with the obtained experimental values. Hence both Schottky emission and Poole Frenkel may be ruled out as the possible mechanisms.

2 5 30 3 5 E “’(MV%) 0 0 05 1 0

Fig.9 Log I vs. E”’ at various temperatures

The zero electric field emission current density is given by equation

J, =A, T’ exp (&KT) ( 5 ) where 4 is the emission constant and +, is the Schottky bamer height. The values of the emission constant can he obtained as 8.17 x to 3.16 x 10’ Acm”R’. These are about 7 orders of ma itudes lower than the expected values of 120 Acm‘ K . Hence this further rules out the Schottky emission as a possible conduction mechanism The activation energy for the steady state conduction can be found out by plotting the extrapolated zero field emission current I , obtained from the figure 8 against reciprocal temperatures. A plot is shown in the figure 10. From the slope the effective work function between the Fermi level of the metal cathode and conduction band of the insulator can be calculated and is obtained as 0.70 eV. Thermally assisted tunneling corresponds to the thermal activation of the electrons over the metal - dielectric interface barrier with the presence of applied field to reduce the barrier height. Although on the basis

2 .$

of the present data alone it is not possible exclusively whether the conduction at high fields is ionic, Poole Frenkel or due to the thermally assisted tunneling yet when these reports are evaluated, it appears that the conduction is due to drift of ions under the influence of applied field.

-17.0

3 -20.0 -21.0

2.2 2.6 1owm 2

Fig.10 PlaIofbasafunct ionofIW~

Conclusions

The charging and discharging currents were investigated for the temperature up to 200 OC and fields up to 12MVm.’ under the influence of various parameters of field, poling temperature and electrode material. The currents shows mirror images for temperature up to 90 ‘C and are dissimilar for all the temperatures investigated. The results show that dipolar processes are operative mechanism at temperatures up to 90 OC and interfacial polarization at longer times and higher temperature.The results of the conduction currents are discussed for fields up to 100KVcm.l and the temperatures up to 200 ‘C. The results did not reveal that either Schottky or the Poole Frenkel mechanism was operative mechanisms. The ionic jump distance were found to he in the range of 19-38 nm following a increasing trend with the temperature.

References [l] Dupont FilmsRihers, “ Technical guide ”

World Wide Web, URL: hnp:// www.duoont.com”. [2] Van Tumhout,Electrets,Topics in Applied Physics, ed by G.M.Sessler ,1980 ,Vol33, Chapter 3. [3] J.LindMayer,J.Appl.Physc., 36 (1),196( 1965). [4] A.K.Jonscher, “The universal dielectric response ” Conference on Electrical Insulation and dielectric phenomena ,PA Oct 28 1990 pp 23-40. [5] M.H.Chohan, H.Mahmood, Farhana Shah, “Absorption-Desorption currents in polyimide film”, J. Material Science Vo1.14, 1995 pp552 [6] B.V.Hamon, “An approximate method to deducing dielectric loss factor from direct current measurement”, Proc IEE Vo199pp 151-155, 1952 [7] High Temperature Insulating Materials”, PhD Thesis, 1992

M.A.Sussi, “Charge Storage and Decay in

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