- Email: [email protected]
Kapton nuclear track microfilter
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 N U C L E A R TRACK M I C R O F I L T E R 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, Philipps-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 microfilters (Kapton is a Polyimide, produced by Du Pont). The Kapton f o i l s were irradiated with uranium ions at three d i f f e r e n t 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 t e r - r e l e v a n t ) diameter and the outer diameter can be calculated using given formulas. The surface of Kapton f o 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 i e d to make microfilters with t i n y c y l i n d r i c a l 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 tried to obtain small cone angles in a f i l t e r by several projectiles 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 e x i 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 ratio V=VT / 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 r a t i o the smaller is the cone angle, the more " c y l i n d r i c a l " 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 different from the outer diameter is to use very thin f o 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 becomes as small as possible. Fig.l.Schematic diagram of the parameters We have irradiated Kapton f o i l s of d i f f e r e n t used for the evaluation of the inner thickness with U-238 ions at three energies 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/2VT , t e is the etching time when the holes start to be formed in a f o i l ; (B) t~ =L/ZV~, t ~ i s the etching time when the f o i l w i l l be completely dissolved. 2. EXPERIMENT Irradiation Kapton foils 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 f o i l s at normal incidence. Track etching The exposed f o i l s were etched in NaClO-solution at different temperatures and concentrations. Fig. 2 shows the Vlr 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
r-
C O .m
E
~ P
-C
> 20 u t-
20
E ~L
15 10
t-
,V, ~0
1.0
O e~
5
~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 ).
°
_~ 0.40 m
2
I
I
I
4
6
8
I
10
Concentrotion of NoClO-solution (%CI) Fig. 3.V (16.5 MeV/N U-238) and V ~ v s . conc e n t r a t i o n of NaCI0 s o l u t i o n (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 N U C L E A R TRACK MICROFILTER
773
Determination of VT Sffme o f the f o i l s 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 e d using a linear 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 f o i l s before and after etching using an analytical balance (0.1 mg accuracy) and a linear 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
2:15U,16.5MeV/N
3O
Koptoh toil
|L-50~m)
60
i
i
i
6e" (3.3L't0.12)lJm"
Dx" (3,33z0.16)lJm
~ 20
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%).
2.2
,j FWHM
i
I
2.8 3.4 4,0 Outer diorneter O 1pro)
Fig. 6.Distribution of outer diameters at the entrance and e x i t of ]6.5 MeV/N U ions (NaCl0 (I0% Cl), 60°C, 5 h).
Outer diameter of the m i c r o f i l t e r The distribution of outer diameters in a Kapton m i c r o f i l t e r is shown in Fig. 6. The averages of outer diameters at the entrance and e x i 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 m i c r o f i l t e r By knowing V~ and V~ , the inner diameter of the m i c r o f i l t e r 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 m i c r o f i l t e r (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 m i c r o f i l t e r (L=12.5 Nm) due to 16.5 MeV/N U-238 ions (NaClO (I0% Cl) at 70%, 27 min.) In the t h e o r y , i t is possible to produce f i l t e r s with every small inner diameter d provided that VT and therewith t o are exactly known. The determination of t o has an important effect on the real lower l i m i t of d, because d increases with about 30 nm/min after the perforation of the f o i l . The experimental error in determining t o is mainly caused by the measurement of the track lengths ( l i m i t 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 f o 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 r r a d i a t i o n 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 microfilters 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 microfilters consist of two cones standing opposite to each other. The inner opening is located at the middle of the etched holes, i t s size can be evaluated from the given equation. I t is always smaller than the outer opening. The low l i m i t of the inner diameter in a Kapton m i c r o f i l t e r of 12.5 , 25 and 50 Nm were found to be about 0.06 Nm under similar circumstances. This means the m i c r o f i l t e r can be produced by using whether thin or thick f o i l s , provided the energy of the projectile is monoenergetic and high enough for the f o i l used. The determination of V~ has an important effect on the low l i m i t of the inner diameter. The V T can also be determined by measuring the r a t i o (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 i m i t of the d down to lO nm. 4. ACKNOWLEDGEMENT This work was supported Reaktorsicherheit (Bonn).
by
the
Bundesministerium fur
5. REFERENCES I. R.L. Fleischer et a l . , (1963), Rev.Sci.lnstr. 34, 510-512
Umwelt, Naturschutz
und
0191-278)(/89 $3.00 + .00 Pergamon Press plc
KAPTON N U C L E A R TRACK M I C R O F I L T E R 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, Philipps-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 microfilters (Kapton is a Polyimide, produced by Du Pont). The Kapton f o i l s were irradiated with uranium ions at three d i f f e r e n t 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 t e r - r e l e v a n t ) diameter and the outer diameter can be calculated using given formulas. The surface of Kapton f o 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 i e d to make microfilters with t i n y c y l i n d r i c a l 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 tried to obtain small cone angles in a f i l t e r by several projectiles 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 e x i 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 ratio V=VT / 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 r a t i o the smaller is the cone angle, the more " c y l i n d r i c a l " 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 different from the outer diameter is to use very thin f o 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 becomes as small as possible. Fig.l.Schematic diagram of the parameters We have irradiated Kapton f o i l s of d i f f e r e n t used for the evaluation of the inner thickness with U-238 ions at three energies 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/2VT , t e is the etching time when the holes start to be formed in a f o i l ; (B) t~ =L/ZV~, t ~ i s the etching time when the f o i l w i l l be completely dissolved. 2. EXPERIMENT Irradiation Kapton foils 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 f o i l s at normal incidence. Track etching The exposed f o i l s were etched in NaClO-solution at different temperatures and concentrations. Fig. 2 shows the Vlr 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
r-
C O .m
E
~ P
-C
> 20 u t-
20
E ~L
15 10
t-
,V, ~0
1.0
O e~
5
~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 ).
°
_~ 0.40 m
2
I
I
I
4
6
8
I
10
Concentrotion of NoClO-solution (%CI) Fig. 3.V (16.5 MeV/N U-238) and V ~ v s . conc e n t r a t i o n of NaCI0 s o l u t i o n (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 N U C L E A R TRACK MICROFILTER
773
Determination of VT Sffme o f the f o i l s 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 e d using a linear 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 f o i l s before and after etching using an analytical balance (0.1 mg accuracy) and a linear 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
2:15U,16.5MeV/N
3O
Koptoh toil
|L-50~m)
60
i
i
i
6e" (3.3L't0.12)lJm"
Dx" (3,33z0.16)lJm
~ 20
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%).
2.2
,j FWHM
i
I
2.8 3.4 4,0 Outer diorneter O 1pro)
Fig. 6.Distribution of outer diameters at the entrance and e x i t of ]6.5 MeV/N U ions (NaCl0 (I0% Cl), 60°C, 5 h).
Outer diameter of the m i c r o f i l t e r The distribution of outer diameters in a Kapton m i c r o f i l t e r is shown in Fig. 6. The averages of outer diameters at the entrance and e x i 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 m i c r o f i l t e r By knowing V~ and V~ , the inner diameter of the m i c r o f i l t e r 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 m i c r o f i l t e r (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 m i c r o f i l t e r (L=12.5 Nm) due to 16.5 MeV/N U-238 ions (NaClO (I0% Cl) at 70%, 27 min.) In the t h e o r y , i t is possible to produce f i l t e r s with every small inner diameter d provided that VT and therewith t o are exactly known. The determination of t o has an important effect on the real lower l i m i t of d, because d increases with about 30 nm/min after the perforation of the f o i l . The experimental error in determining t o is mainly caused by the measurement of the track lengths ( l i m i t 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 f o 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 r r a d i a t i o n 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 microfilters 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 microfilters consist of two cones standing opposite to each other. The inner opening is located at the middle of the etched holes, i t s size can be evaluated from the given equation. I t is always smaller than the outer opening. The low l i m i t of the inner diameter in a Kapton m i c r o f i l t e r of 12.5 , 25 and 50 Nm were found to be about 0.06 Nm under similar circumstances. This means the m i c r o f i l t e r can be produced by using whether thin or thick f o i l s , provided the energy of the projectile is monoenergetic and high enough for the f o i l used. The determination of V~ has an important effect on the low l i m i t of the inner diameter. The V T can also be determined by measuring the r a t i o (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 i m i t of the d down to lO nm. 4. ACKNOWLEDGEMENT This work was supported Reaktorsicherheit (Bonn).
by
the
Bundesministerium fur
5. REFERENCES I. R.L. Fleischer et a l . , (1963), Rev.Sci.lnstr. 34, 510-512
Umwelt, Naturschutz
und