Nucl. Tracks Radiat. Meas., Vol. 15, Nos. 1--4, pp. 771-774, 1988 Int. J. Radiat. AppL lnstrum., Part D Printed in Great Britain

0191-278)(/89 $3.00 + .00 Pergamon Press plc

KAPTON NUCLEAR TRACK MICROFILTER

Zhu Tian-cheng (1,2), R. Brandt (2), P. Vater (2), J. Vetter (3)

l) Institute of Atomic Energy, Academia Sinica, Beijing, China 2) Kernchemie, FB 14, Phil ipps-Universit~t, D 3550 Marburg, FR of Germany

3) GSI, D 6100 Darmstadt, FR of Germany

Abstract - Experiments were performed to produce Kapton nuclear track microfi lters (Kapton is a Polyimide, produced by Du Pont). The Kapton fo i l s were irradiated with uranium ions at three di f ferent energies at the UNILAC (GSl, Darmstadt) and then etched in NaClO solution. In order to study the track formation and to obtain the most suitable shape of the etched tracks, the etching conditions were varied systematically. The optimum etching condition is as follows: NaCIO solution (I0% Cl) at 70°C. The response function V=VT/V~ for these ions and the bulk etch rate V~ have been measured. The etched holes consist of two cones having cone angles of ~ (2 to 7) °. The inner ( f i l ter-re levant) diameter and the outer diameter can be calculated using given formulas. The surface of Kapton fo i l s is extremely polished, the etched holes have quite smooth walls. Kapton is rather heat resistent and exhibits good mechanical properties, also at higher temperatures and after exposure to high doses of radiation. Manifold applications are imaginable, accordingly.

I. INTRODUCTION

Since the method concerning the production of microholes in insulating solids was f i r s t invented in 19634, one has t r ied to make microf i l ters with t iny cyl indr ical holes by track etching. In principle, i t is possible by a suitable choice of a thin f o i l of track detector. But this is often d i f f i c u l t from practical point of view. Therefore, i t has been tr ied to obtain small cone angles in a f i l t e r by several projecti les and etching conditions. When a ion has penetrated the track detector, at the beginning of the etching of the latent damage t r a i l , two conical etch pits at the entrance and exi t side of the ion are formed. As the etching time goes on, the microhole consists of two identical funnels standing opposite to each other (Fig. l ) . The outer diameter D of the microhole is larger than the inner diameter d. The value of the cone angle depends on the rat io V=V T /V~ , as ~ = sin-~ ( l /V); here VT - the track etch rate along the damage t r a i l , V~ - the bulk etch rate. The larger the rat io the smaller is the cone angle, the more "cyl indr ical" are holes. In the case of mica, e.g., the angle is very small, therefore the cross section of the holes is everywhere the same. For plastics, the cone angle is always a few degrees resulting in the above discribed funnel holes. One way to produce f i l t e r s with such holes that the inner diameter is not so much dif ferent from the outer diameter is to use very thin fo i l s which are of limited value from the practical point of view. Instead of choosing this method, one should develop such etching conditions that the cone angle be- comes as small as possible. We have irradiated Kapton fo i l s of di f ferent thickness with U-238 ions at three energies

Fig.l.Schematic diagram of the parameters used for the evaluation of the inner and outer diameter of the microhole.

771

772 Z H U T I A N - C H E N G et al.

and found that NaClO solution is the optimum etchant to get small cone angles. The VT for these ions and the V~ have been measured. Knowing these parameters, the inner diameter of the holes in a microfilter of thickness L (before etching) after etching time t can be evaluated from the following equation

d = D [ I - L - ~ V ~ ] (I)

where D=2V$ (VT-V ~ ) t / Y v T2-v~2, which can be measured with a light or scanning electron microscope (SEM). The two deductions can be obtained from equation (1): (A) t o =L/2V T, t e is the etching time when the holes start to be formed in a fo i l ; (B) t~ =L/ZV~, t ~ i s the etching time when the fo i l wi l l be completely dissolved.

2. EXPERIMENT

Irradiation Kapton foi ls of different thickness (12.5, 25 and 50 Nm) were selected for the production of the microfilters. The irradiations were carried out at the UNILAC (GSI, Darmstadt) with U-238 ions having specific energies of 5.9, 13.3 and 16.5 MeV/N, resp. The ranges of these ions in Kapton are about 86, 170 and 210 pm, resp., the ions penetrate the fo i ls at normal incidence.

Track etching The exposed foi ls were etched in NaClO-solution at different temperatures and concen- trations. Fig. 2 shows the V lr and V3 as a function of etching temperature. Fig. 3 gives the relation of V and V~ , resp., with the concentration of NaClO at given temperature. NaClO-solution (I0% Cl) at 70% was found to be the optimum etching condition to obtain the most suitable shape of the etched tracks (Fig. 4) and the ratio V as high as possible.

,° ' / Kapton foil "C

=~ 3O E

~ 20 P

,V, ~0

~0 60 80 100

Etching temperature (°C}

Fig. 2.V-r (16.5 MeV/N U-238) and V~ vs. e t - ching temperature (NaClO solution, I0% Cl ).

> 20 C O . m

u 15 t -

10 t -

O e~

-C r -

E ~ L

1.0 °

5 _~

0.40 m I I I I

2 4 6 8 10

Concentrotion of NoClO-solution (%CI)

Fig. 3.V (16.5 MeV/N U-238) and V~vs. con- centrat ion of NaCI0 so lu t ion (70°C).

Fig. 4.SEM-photos of etched tracks in Kapton due to 16.5 MeV/N U-238 ions (NaClO (IO% Cl), at 70%, 2 h).

KAPTON NUCLEAR TRACK MICROFILTER 773

Determination of VT Sffme o f the fo i ls were simultaneously irradiated at normal and 45 ° incidence in order to measure the VT . The VT was determined by measuring the track lengths according to different etching times by means of an optical microscope. Fig. 5 gives the results. The experimental values have been f i t t ed using a l inear regression. VT for ]6.5 MeV/N U-238 ions was found to be VT =(14.3±O.3)~m/h, and the respective value for 5.9 MeV/N was found to be (67.5±0.5)Nm/h. The VT can also be determined by measuring d and D with a SEM and using the equation VT = [L - 2 (d/D) V~ t ] / 2 ( l -d /D) t . The VT for 16.5 MeV/N U-238 ions found in this way was to be (14.9±0.2)Nm/h (see Fig. 4B).

Determination of V~ The V~ was determined by measuring the weight and the thickness of the fo i l s before and after etching using an analytical balance (0.1 mg accuracy) and a l inear displacement transducer, resp. I t was found to be (0.88±0.02) and (0.9~0.])Nm/h, resp., for the optimum etching condition.

I | I 3O

i ~ 20

I !

30 90 150

Etching time (min)

Fig. 5.Track lengths of ]6.5 MeV/N U-238 ions in Kapton as function of etching time (NaCl0 (I0% C]) at 70%).

Outer diameter of the microf i l ter

60

20

i i 2:15U, 16.5 MeV/N Koptoh toil |L-50~m) 6e" (3.3L't 0.12)lJm"

Dx" (3,33 z 0.16)lJm

FWHM i ,j I

2.2 2.8 3.4 4,0

Outer diorneter O 1pro)

Fig. 6.Distribution of outer diameters at the entrance and exit of ]6.5 MeV/N U ions (NaCl0 (I0% Cl), 60°C, 5 h).

The distr ibut ion of outer diameters in a Kapton microf i l ter is shown in Fig. 6. The averages of outer diameters at the entrance and exi t of 16.5 MeV/N U-238 ions (Etching time: 5 h) were found to be (3.34±0.12) and (3.33~0.16)~m, resp. They are the same within the accuracy of measurements.

Inner diameter of the microf i l ter By knowing V~ and V~ , the inner diameter of the microf i l ter can be evaluated from the measured D using the equation (1). For 16.5 MeV/N U-238 ions, L=50 Nm and t=2h, e.g., d was determined to be d=(0.44±0.02) Nm. This value agrees with the result of a direct measurement of the inner diameter (d=0.45 Nm) obtained by using SEM (see Fig. 7).

Fig. 7.SEM-photo of the inner opening of a hole in a Kapton microf i l ter (L:50 Nm) due to 16.5 MeV/N U-238 ions (NaCl0 (]0% Cl) at 70°C, 2 h).

774 Z H U T I A N - C H E N G et al.

Fig. 8.SEM-photo of the inner opening (NlO0 nm) of a hole in a Kapton microf i l ter (L=12.5 Nm) due to 16.5 MeV/N U-238 ions (NaClO (I0% Cl) at 70%, 27 min.)

In the theory, i t is possible to produce f i l t e r s with every small inner diameter d provided that V T and therewith to are exactly known. The determination of to has an important effect on the real lower l imit of d, because d increases with about 30 nm/min after the perforation of the f o i l . The experimental error in determining to is mainly caused by the measurement of the track lengths ( l imit of the measuring device). From the two values of VT we have measured (14.3 and 14.9 Nm/h, resp.) i t can be concluded that : t o =±2 min. Therefore, for a 50 ~m thick f o i l ~ d can be calculated to be Ad=±60 nm. Similar values of : d hold for 12.5 and 25 Nm thick fo i l s . Fig. 8 shows the SEM-photo of the inner opening ( ~ lO0 nm) of a hole in a 12.5 Nm thick Kapton f i l t e r which was obtained by i rradiat ion with 16.5 MeV/N U-238 ions and etched with NaClO (I0% Cl) at 70 ° for 27 min.

3. CONCLUSIONS

The optimum etching condition of Kapton track detector for the production of microfi l ters is NaClO (I0% Cl) at 70°C. The semi-cone angles due to 5.9, 13.3 and 16.5 MeV/N U-238 ions were found to be #=0.8 ° , 2.5 ° and 3.6 ° , resp.. The holes of the microfi l ters consist of two cones standing opposite to each other. The inner opening is located at the middle of the etched holes, i ts size can be evaluated from the given equation. I t is always smaller than the outer opening. The low l im i t of the inner diameter in a Kapton microf i l ter of 12.5 , 25 and 50 Nm were found to be about 0.06 Nm under similar circumstances. This means the microf i l ter can be produced by using whether thin or thick fo i l s , provided the energy of the projecti le is monoenergetic and high enough for the fo i l used. The determination of V~ has an important effect on the low l im i t of the inner diameter. The V T can also be determined by measuring the rat io (d/D) with help of a SEM, which is highly required to reduce the errors. Provided that a high accuracy-measurement of V T can be made, i t should, in principle, be possible to reduce the low l im i t of the d down to lO nm.

4. ACKNOWLEDGEMENT

This work was supported by the Bundesministerium fur Umwelt, Naturschutz und Reaktorsicherheit (Bonn).

5. REFERENCES

I. R.L. Fleischer et a l . , (1963), Rev.Sci.lnstr. 34, 510-512