J O U R N A L OF MATERIALS SCIENCE LETTERS 14 (1995) 552-553

Absorption-desorption currents in polyimide film

M. H. C H O H A N , H. M A H M O O D , F A R H A N A S H A H *

Departments of: Electronics and *Computer Science, Quaid-i-Azam University, Islamabad, Pakistan

The highly aromatic character of polyimides con- tribute to their extremely good thermal and radi- ation resistance. This group of polymers has excel- lent mechanical and electrical properties. The low dielectric constant, high breakdown strength and high resistivity allow their use in demanding micro- electronic devices and in aircraft and space applica- tions [1].

A lot of work on polyimide films has been carried out [2-6]. Some studies reveal a current-time dependence, whereas in other cases no such time dependence has been found. The time-independent values of current have been found to vary with temperature and applied electric field. The time- dependent currents at initial and later stages, how- erer, suggest different conduction mechanisms.

An experimental study on a 12.5#m thick polyimide film (poly-4,4'oxidiphenylene-pyromel- litimide), supplied by Dupont under the trade name Kapton H-Film, has been made. Aluminium elec- trodes 1 cm in diameter were deposited on two sides of the sample in a vacuum of 1.33 x 10 -s Pa using an evaporation technique. Absorption and desorption currents in the sample were measured using a temperature controlled sample holder.

Fig. 1 shows the absorption current measure- ments. The current decay is such that there is broad plateau region up to 104 S, after which the current

-5 i

-6

-7 ~'-----.-.«_~

g E -8

I I i I

2 0 4 ] C + + . _ _ _ _ _ _ ~ . . +

O

= 170°C

B o -~ -10

° \ 5 - - ~ °°°c _e ~1 - ,<-<<-~~ -12 ~ . . . . . -

-13 - i + i i h -1 0 1 2 3 4 5

log t ime (s)

Figure I Absorption currents for 12.5/~m polyimide at 100 V.

decays to the steady d.c. level. At times less than 100 ms, the decay is proportional to t - 1 o r possibly t «/2. The decay is unusual in that at the higher temperatures, i.e. greater than 200 °C, the current tends to increase with time. At 237 °C, the maximum measurement temperature, the current increases by a factor of two and then remains steady.

Fig. 2 shows the desorption current. Once again, only in the initial stages does the desorption current mirror the absorption current. Of particular interest are the deviations from linearity, which move to earlier times with increasing temperature. This suggests the use of the Hamon transform [7] modified to allow for zero d.c. contribution (since these are desorption currents rather than the absorp- tion currents, for which the transform was de- signed). Fig. 3 shows the results of applying the transform. A complete set of loss peaks are shown which move to higher frequencies with increasing temperature. Although the peaks do not follow an

-8

-9

P -10

Œ

O -11

"{3 Q }

o -12

-13

i i i i i

I I i I +

0 1 2 3 4 log t ime (s)

Figure2 Desorption currents for 12.5/~m polyimide at 100V. + 204 °C; • 170 °C; O 137 °C; x 66 °C; A 39 °C.

i i i i i

10-6Hz 10-5Hz I ---, ~ 7-8 x 10-SHz -^-2- ~~.~ ù . / f ' ° ~ ° ~ . j I U I - IZ

õ ~ ~~-+~~-° "ZL~ ~ .

_ 3 i f i i i

-6 -5 -4 -3 -2 - 1 0 log frequency (Hz)

Figure3 Hamon transform results (from Fig. 2): /k 204°C; • 170 °C; O 137 °C; x 66 °C; + 39 °C.

5 5 2 0261-8028 © 1995 Chapman & Hall

Arrhenius relation, they do appear to have the same dependence on temperature as the d.c. conductivity. The peak height also decreases with increasing temperature.

At present it is difficult to give any satisfactory explanation for the ultra-low frequency loss peak found for polyimide. From the position of the peaks on the frequency scale, a Maxwell-Wagner type of polarization could be inferred, where mobile carriers are blocked either at some internal boundary separ- ating different phases (e.g. amorphous/crystalline) or at the electrodes. In the latter case the carriers will then oscillate between the electrodes. From the similarity of the temperature dependence of the d.c. conductivity (log I oc V1/2) and the position of the loss peaks, it can further be inferred that the same carriers are responsible for both phenomena, i.e. when the field is first applied, carriers which can

move unobstructed to the electrodes, are discharged and cause d.c. conductivity: other carriers may be trapped or blocked at phase boundaries creating a space charge and consequently polarization.

References 1. R . A . L A R S E N , IBMJ. Res. Dev. 24 (1980) 268. 2. J . R . H A N S O M B and J. H. C A L D E R W O O D , J. Phys. D6

(1973) 1093. 3. V. A D A M E C and J. H. C A L D E R W O O D , ibid. D8 (I975)

551. 4. E. S A C H E R , IEEE Trans. Electr. Insul. El-14 (1979) 85. 5. L . B . R O T H A M , J. Electrochem. Soc. 127 (1980) 2216. 6. B . L . S H A R M A and P. K. C. P ILLAI , Polymer 23 (1982)

17. 7. B . V . H A M O N , Proc. IEE 99 (1952) 151.

Received 25 August and accepted 2 November 1994

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