Chemistry Reference
In-Depth Information
2.1. Irradiation
Fig. 1 shows the schematic of the irradiation unit. Multiple ionised heavy ions, for
instance
40
Ar
n+
, impinge on the polyimide foil which is continuously moved
through the ion beam using a reel-to-reel equipment. The ion beam is defocussed
to a size of several square centimetres and is rapidly moved over the whole width
of the polymer foil to obtain a uniform irradiation of the whole foil surface. The
angle of incidence can be varied from normal incidence to about 60°. Especially,
symmetric incidences are possible, as shown in the figure. Ions of noble gases like
argon, krypton and xenon are preferably used. Typical doses are between 10
7
and
10
8
ions per square centimetre.
This “ion bombardment technique” has been used for several decades for the
manufacture of so-called “ion-track membranes” [5]. Such membranes are poly-
mer foils with micro- and nano-sized channels which penetrate the whole thickness
of the foil. They have found a wide range of applications in the field of filtration.
The ion-track technology, which is presented here, relies on a careful control of
the penetration depth of the ions into the polymer foil. The ions transfer their ki-
netic energy to the polymer material in the close environment of their paths. Thus
latent ion tracks with a well-defined length and a diameter of 10 to 30 nm are cre-
ated. The amount of energy deposited along the track and the track length are de-
termined by the species and the kinetic energy of the incoming ions.
It is possible to create latent tracks which penetrate the whole thickness of the
foil (as in the case of membranes) as well as latent tracks with very short lengths.
A foil with such “dead-end” tracks is called an “ion-track foil”. The term “latent
track” is used, as these ion tracks are difficult to be observed using scanning elec-
tron microscopy.
2.2. Etching
After irradiation the polymer film is submitted to a chemical etching process,
which is shown schematically in Fig. 2. Because of the structural changes result-
ing from irradiation, the etching rate in the ion tracks is much higher than in the
unaffected bulk. This effect is used to produce differently shaped microholes that
superpose onto different “surface-depth reliefs”. It is possible to produce cylindri-
cal through-holes, blind holes or cones of different, but well-defined, size and
shape. Examples are given in Fig. 3. The hole sizes and shapes are precisely con-
trolled by a careful choice of parameters related to the irradiation, i.e. ion species,
ion initial energy and incidence angle and parameters related to the etching step,
i.e. etching medium, pH-value, temperature and time. The etching process is
completed by neutralization and rinsing in pure water as shown in Fig. 2.
Examples of modified polyimide surfaces are shown in Fig. 4. On the left, mi-
croholes are visible which were obtained by irradiation under normal incidence
and subsequent etching. On the right, microholes are shown which were obtained
by inclined incidence. From the shadows in the enlarged view d) it is clear that
two symmetric angles of incidence have been used.
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