Ion Track Technology


Reimar Spohr

Ion Tracks

A Single Particle Approach to Nano Technology

A single particle approach to nanotechnology

Ion tracks may have been observed unwittingly during the late 19th century by H. Baumhauer who studied etch pits in minerals. Half a century later, D.A. Young found an explanation for naturally occurring ion tracks in minerals: fission fragments from traces of uranium. Each track corresponds to exactly one fission fragment. In minerals the ion tracks can remain unaltered for millions of years. At known uranium concentration, their number density tells about the time when the mineral has solidified from its melt and is used as a geological clock.

The discovery of ion tracks spread rapidly. Among its first applications was filtration. But its applications range much wider.


Ion accelerator - key to ion track generation

Heavy ion accelerators provide parallel beams with high flux density. The total number of ions available each second ranges from one individual ion up to billions of ions per square meter. The beam spot diameter ranges between micrometers and meters. The distance traveled by the ion in the solid, the ion range, varies between micrometers and millimeters. Scribing with individual ions is possible with an aiming precision of one micrometer at present. A refinement of the aiming precision down to the nanometer scale is possible. Ion track channels down to less than 10 nanometer diameter (about 100 atomic diameters) can be fabricated. The unique combination of materials, range, and precision provides a basis for fascinating innovations based on ion track technology. Applications range from single bio sensors to water repellent surfaces.

Ion accelerators provide an immense technological potential for applications. They are the driving force behind ion track technology.

Scribing with single particles

The ion track technique uses heavy particles for scribing. Each track corresponds to the passage of exactly one ion through the solid.

In fractions of a second, thousands of artifacts can be created per square micron. Heavy ions are decelerated smoothly in the material and stop at a well defined range. The interaction corresponds to that of a heavy body traveling in a gas of light particles. Heavy ions therefore provide a precise stop. The technique can be applied as deep-cutting method for micromechanical systems. The unique advantage of ion tracks is that ion tracks can be tilted. Scribing is not restricted to vertical scribing. The large scale fabrication of tilted textures is possible. Oblique walls are a unique starting point for novel applications.

In contrast to the ion track technique, conventional lithography requires radiation sensitive films. The films are transformed into a developable state using visible light, ultraviolet radiation, electrons and x-rays. The transformation changes the whole volume of the film. A mask permits the irradiation of selected areas. Photons and electrons act only in swarms. Only collective effects count. The scattering of photons and electrons prevents defining a straight etch path over long distances. Due to their intrinsic properties, conventional techniques don't provide tilted textures.

Amplifying ion induced effects

When a fast ion penetrates through a polymer an ion track is obtained. The technique uses etching as a chemical amplifier. In this way the effect is magnified by orders of magnitude. The etching process accentuates the invisible defect raising it to a useful level. Due to the high energy density of ions many homogeneous dielectric materials are susceptible to selective etching.

Access to radiation resistant materials

In comparison with photons and electrons: accelerated ions provide a much higher energy density. The acceleration energy of heavy ions exceeds classical techniques by orders of magnitude. The ion energy is released in a narrow channel along the ion path. Classical lithography requires radiation sensitive resists for scribing. Ion tracks are not restricted to resists. They cover a much wider range of materials. Even radiation resistant materials can be treated. They are usually non-conducting and homogeneous down to the molecular scale. Quartz is one example of such a radiation resistant material that can be treated. Many technical polymers are susceptible to selective ion track etching. Sodium hydroxide solutions are most commonly used for polymers. After short etching, small etch troughs are formed. After long etching, penetrating channels are formed. At high energy deposition density cylindrical perforations are possible.

Access to cylindrical channels

For applications in micro and nano-technology, one has to master the fabrication of nearly identical nano-channels. In many cases cylindrical geometry is preferred.

Soap bubbles consist of self organized mono-layers of tenside molecules forming a flexible membrane. Similar to biological membranes the membrane is impermeable for hydrated ions. Tenside molecules have a water-loving (hydrophilic) and a water repellent (hydrophobic) end. They fill a larger volume than the hydrated ions of the etching medium (Na+ and OH-). A flexible, but impenetrable mono layer of tenside molecules forms between the etching medium and the channel wall. Its thickness corresponds to the length of the tenside molecule. As soon as the channel diameter suffices for the tenside molecules to penetrate into it, a protective mono-layer forms on the channel wall. However, the tenside molecules are too large to penetrate into the tip, where selective etching occurs as usual. The result is a cylindrical channel.

Fabricating bio-sensors

Chemical modification of the channel wall changes its interaction with passing particles. Different wall claddings bind to specific molecules and delay their passage. In this sense, the wall recognizes the bound molecule. As an example, DNA fragments are selectively bound by their complementary fragments. The attached molecules reduce the channel volume. The induced resistance change reflects the molecule's concentration.

Classifying micro- and nanoparticles

The resistance of a channel filled by an electrolyte depends on the volume of the particle passing through it. The technique is applied to counting and sizing of red blood cells, bacteria, and virus particles.

Electrically conducting channels

Charged pores have a high electrical conductivity. A cloud of counter ions is formed close to the wall. The cloud is highly mobile and responsible for most of the charge transport. Surface conductivity exceeds volume conductivity. Hydrogen ions have a much higher mobility than heavier ions. Their contribution to the conductivity is high.

Current rectifying pores

Asymmetric pores are obtained by one-sided etching. The geometric asymmetry translates into a conduction asymmetry. The phenomenon is similar to electric valves. The pore has two conduction states, open and closed. For applied voltages above a certain value the valve is open. Below a certain voltage the valve is closed. For small pores a periodic flipping between these two conduction states occurs. The oscillation is analogous to bio pores in cell membranes. The valve depends on the acidity of the medium filling it. High concentration of hydrogen ions (high acidity) corresponds to a high current. The diode senses the acidity (pH) of the solution.

Bio-specific channel sensors

A tiny modification renders a pH sensitive to specific bio molecules. One example is the measurement of the glucose level in blood. For this purpose an oxidation enzyme (glucose oxidase) is attached to the pore wall transforming glucose into lactic acid. The acid increases the conductivity of the pore. In this way bio-specific diodes are formed.

Fabricating nanowires

Etched tracks can be filled by liquids. The liquids can be transformed into solids. In this way a replica of the original artifact is fabricated. One method is electro-deposition. For this, a cathode film is deposited on one side of the track etched membrane. It is negatively charged with respect to the anode. The membrane is immersed in a metal salt solution. As an example, copper sulfate is obtained by dissolving copper in sulfuric acid. The salt solution contains positive copper ions and negative sulfate ions. The positive copper ions are pulled toward the cathode. They are neutralized at the cathode. The deposited metal forms a compact layer. Fast deposition leads to polycrystalline deposits. Slow deposition leads to single crystals. Gradually the channels fill with metallic copper. The length of the nano wires is controlled by the deposition time. Embedded wire arrays act as oriented antenna arrays. They render the embedding matrix highly anisotropic. Free standing wire arrays are obtained by dissolving the polymer. A platform covered with many free standing wires acts as large area field emitter.

Magnetic multilayers

Each metal has a characteristic resistance to corrosion given by its electronegativity. A defined electric potential is needed to keep a metal atom bound to the cathode. The tendency to leave the cathode in the form of a charged ion increases with decreasing electronegativity. Mixed electrolytes enable to adjust the overall wire composition by the applied voltage. Multilayer wires are fabricated by varying the deposition voltage as function of time.

Nano-wires consisting of alternating magnetic/nonmagnetic layers act as magnetic sensors. As an example, cobalt/copper nanowires are obtained from an electrolyte containing both metals. At low voltage, pure copper is deposited while cobalt resists electro-deposition. At high voltage both metals are deposited as an alloy. If the electrolyte contains predominantly cobalt, a magnetic cobalt-copper alloy is deposited with a high fraction of cobalt.

The electrical conductivity of the multilayer wire depends on the applied external magnetic field. The magnetic order of the cobalt layers increases with the applied field. Without magnetic field, neighboring magnetic layers prefer the anti-parallel order. With magnetic field, the magnetic layers prefer the orientation parallel with the magnetic field. The parallel orientation corresponds to a reduced electrical resistance. The effect is used in reading heads of magnetic storage media (GMR effect).

Mimicking bio-textures

Bio engineering is an inspiring source of innovation.

Moles, living underground, are in steady contact with soil but never get dirty. Their fur consists of tiny hairs. Each hair consists of tiny scales pointing away from the body of the animal. The scales are hydrophobic. The rubbing of the hairs against each other moves dust particles away from the body of the mole.

Ion track technology simulates such oriented scales more closely than any other available technique. Tilted textures with a hydrophobic coating are superhydrophobic and anisotropic - similar to animal hair. Tilted textures show a preferred direction of transport. Water drops run-off preferentially along the stroke direction of the tilted texture. The effect has been demonstrated to transform vibration into motion.