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Interconnected nanowire clusters in polyimide for flexible circuits and magnetic sensing applications
Sensors and Actuators A 105 (2003) 150–161
Interconnected nanowire clusters in polyimide for flexible circuits and magnetic sensing applications M. Lindeberg∗ , K. Hjort Ångström Laboratory, Division for Materials Science, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Received 7 March 2002; received in revised form 3 March 2003; accepted 26 March 2003
Abstract By combining nano- and microtechnology we have fabricated three-dimensional (3D) flexible circuits, where clusters of nanowires form the vertical via connections. The nanowires, embedded in foils of polyimide plastic, are interconnected with two lithographically structured metallic surface layers. As the wires are defined by ion track technology they are stochastically distributed with a uniform density in macro-scale. In addition they are highly parallel in well-defined directions in the foil. The key structural element is a junction where overlapping lateral interconnection lines on the surface intersect with clusters of perpendicular or tilted wires. The demonstrated circuit structure is in essence a magnetic field sensor since the wires are made of nickel, a magnetoresistive material. The essential fabrication process comprises: ion track generation by means of heavy ion irradiation, selective ion track etching, electrodeposition of nanowires, and double-sided photolithography. The polyimide, employed commercially in, e.g. flexible printed circuit boards, is for the first time evaluated as a carrier for nanowires. The chemical properties and temperature stability makes the polyimide an appropriate material for implementation of electronic circuitry by ion- and photolithography. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Ion track; Polyimide membranes; Nickel nanowires; Template electrodeposition; Flexible circuitry
1. Introduction Combining conventional microstructure technology (MST) with ion track techniques unfolds vast possibilities of integrating micro- and nano-scale concepts; each range with its inherent advantages and disadvantages. Ion tracks are created upon irradiation of dielectric materials, e.g. polymers, with swift heavy ions passing through the material [1,2]. The continuous tracks, induced along the trajectory of every ion, are permanent and only a few nanometer in diameter consisting of damaged material in a higher energy state, i.e. electronic defects, vacancies, etc. Already in the 1960s ion tracks were studied thoroughly because of their importance in geology and cosmology (cosmic particles that reach through the earth atmosphere can create permanent tracks in many insulating materials). Today, for fundamental studies and for technology, it is more convenient to study and generate ion tracks in an accelerator facility. By operating an ion-beam we can readily produce billions of ion tracks per square centimetre in, e.g. various plastics and metallic oxides. The range of the ion in a material roughly corresponds to its energy per nucleon ∗ Corresponding author. Tel.: +46-18-471-7267; fax: +46-18-471-3572. E-mail address: [email protected] (M. Lindeberg).
divided with the density of the material. For our accelerator conditions the range is around 100 m in most plastic materials (i.e. with densities around 1.3 g/cm2 ). The highly damaged ion tracks generated can then be etched using a chemical solution, an etchant, which attacks the tracks preferentially [3]. The tracks are transformed into fine channels, hereafter referred to as pores or nanopores. The diameter of the pores can be controlled in the range starting from about 20 nm reaching up to 10 m by varying the conditions or the duration of the etching. The aspect ratio, i.e. length to width ratio, for track etched pores in plastic materials is normally high and can reach as high as 104 in polycarbonate [4]. The produced porous membranes can be used as templates or moulds for electrodeposition of very thin wires. Nanoporous membranes exist in the form of commercially available filters (e.g. MilliporeTM , NucleporeTM ). The first wires produced in porous ion track membranes are attributed Possin [5], who studied the fluctuations in superconducting transitions in tin wires in 1970. In the 1990s, the magnetic and magnetoresistive (MR) properties of nanowires became very interesting for fundamental magnetism studies. Hence, several research groups have been studying nanowires for almost a decade. Apart from ion track membranes, where the density of pores can be controlled without restraint, porous anodic alumina structures
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have been used for producing high aspect ration nanowires [6]. Early magnetic studies based on nickel and permalloy wires focused on the anisotropic magnetoresistive (AMR) effect, i.e. changes in the direction of the magnetic field produces a change in resistance [7]. In the late nineties more advanced materials were studied and deposited in such pores: various multilayer structures (also referred to as superlattice structures) were built [8]. For example, cobalt and copper multilayered structures exhibiting the giant magnetoresistive (GMR) effect were successfully grown in nanopores. These structures allow the study and use of current-perpendicular-to-plane (CPP) GMR, since the vertically stacked multilayer structures readily allowed the current to be directed normal to the plane. The small cross-section of the wires also provided a high measurable resistance. Exciting is the large resistance response (up to 65%) to a change in the magnetic field [9]. The motivation for magnetic wires has been their qualification as MR-sensors and non-volatile magnetic bit memories. Today, MR-sensors are found, e.g. in miniaturised pick-up sensors for magnetic hard-disc memories. The prevalent and probably most consistent technique for deposition of nanowires (in the designation nanowires we subjectively include wires up to several hundred of nanometer), is by potentiostatic electrodeposition, i.e. at a constant potential with respect to a reference electrode. The deposition conditions and morphology of the wires have been thoroughly studied by, e.g. Schönenberger et al. [10]. Commercially available filters of 6 m thick polycarbonate has been the material prevailing for template growth of nanowires. These filters have pores of high aspect ratio with diameters in a broad range: from 10 up to 5 m. A potential problem of these membranes is that the porosity is very high and that the pores have an angular distribution of up to 34◦ (according to Structure Probe Inc.). Another drawback of using polycarbonate membranes for technological applications is that it is not readily compatible with photolithography due to its poor resistance to chemicals and elevated temperatures. The polyimide plastic we use (KaptonTM HN polymer foils from Du Pont), hereafter referred to as polyimide, is directly compatible with standard photolithographic processes due to its good chemical resistance, mechanical strength and high temperature stability (up to 250–320 ◦ C). Moreover, we have shown that it is possible to produce pores in polyimide approaching the high aspect ratio of the polycarbonate [11,12]. The untreated polyimide surface normally exhibits hydrophobic properties and low wettability that might hamper electrolyte introduction into the pores for the wire deposition [13]. Nanowire circuitry requires the wires to be interconnected and coupled to a lateral pattern. Thin film metallic layer technology and conventional photolithography have been used to produce metal interconnection patterns on both surfaces of the substrate. However, the process is restrained by several problems: metallic layer adhesion, cracking and delamina-
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tion of metallic layers, UV-transparency of the polyimide, and possibly oxidation of wire ends. The interconnection layers require not only good coupling to the wires, but also a good adhesion of the two metallic layers to the plastic surface is important. Several studies of polyimide surfaces treated in plasmas and etched in wet chemistry to promote adhesion (normally correlated to the wettability) have been described by [13,14]. In fact, it has been shown that treatment of polyimide films in oxygen RF-plasmas can produce a boost in adhesion of some metallic layers with more than a magnitude. On the other hand, storage of the polyimide prior to metallic layer deposition is believed to degrade metal/polyimide adhesion properties since wettability is reduced.
2. Objectives and aim Ion track technology originates from several basic science areas such as particle physics, irradiation science, cosmology, and geology. Recently, a number of applications for the ion track technique have been formulated without formerly having a feasible, conventional microstructuring process to rely on [3]. Examples described are porous biological filtration membranes, membranes for chromatography of chemical substances, ion track modification of the magneto-optics properties of iron garnets and deep, vertical ionlithography. Our objective is to show that a combination of traditional microstructure technology and ion track technology have the advantage of enabling implementation of nano- and microtechnology unified in the same polymer-based system. A few conceivable applications based on nanowire components have been articulated: magnetic field- and magnetic field directional-sensors based on the magnetoresistive effect, e.g. GMR and AMR [9,15]. The prospect of implementing magnetic memories based on GMR spin-valves is also very promising. The process described in this work is capable of producing, in effect, three-dimensional circuit structures, composed of two parts. Firstly, the upright standing nanowire via connections; randomly scattered and embedded in the carrier substrate. Secondly, the two patterned metallic interconnection layers covering both surfaces of the nanowire substrate. The latter structure couples to the nanowire clusters in areas where interconnection lines in both of the two metallic layers intersect and overlap. The structural element where a cluster of nanowires is coupled with an overlapping surface patterns, displaying similarities with conventional via connection (Fig. 1) is referred to as a nanowire cluster link. The number of wires connected in such an area will fluctuate around a mean value identical with µ = irradiation density × area, and the standard deviation, σ, will depend on the stochastical (Poisson) distribution of the irradiation density. For a large expected number of wires (confined inside the contact area) the relative variation is small. Thus, clusters of wires lead to a statistical behaviour with less
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Fig. 1. The nanowire cluster is the basic structural element for all components described in this article. A junction where two overlapping metallic surface layers intersects and couples with a group of parallel nanowires essentially forming a vertical via connection.
variation in response properties compared to a single wire. They also offer a tuneable electrical resistance (or conductor) by allowing the size of two junction pads, hence the projected overlap, to be adjusted, and thus changing the typical number of wires confined in the nanowire cluster. Multiple irradiations at arbitrary angles and with different track densities are also possible. This allows us to use tilted wires in multiple and distinct directions and connect these directions separately by using aligned and shifted double-sided metal interconnection patterns on the surface (Fig. 2). If the sample is tilted 54.7◦ versus plane and rotated in the lateral plane 3 × 120◦ , the ion tracks span a three-dimensional (3D) orthogonal system, which can be used for fabrication of 3D magnetoresistive sensors. Considering that flexible printed circuit boards, an established microelectronic material, consists of a laminate structure of copper and polyimide foils, allows also this substrate to be processed to produce three-dimensional flexible circuitry based on nanowires. In flexible printed circuitry, a constant improvement in the packaging density by util-
Fig. 3. Magnetic field sensor outline, the resistance change, Ω, measured over the two nanowire junctions can be related to the applied magnetic field.
ising smaller line-widths and thinner microvias is desired [16]. Possible structures are interlevel nanowire cluster connections and when combined in arrays: 3D electronic microcomponents, e.g. capacitors and inductor coils are conceivable. Thus, the objective is to be able to start with an uncomplicated raw material: uniformly irradiated foils of polyimide or flexible printed circuit boards; assembling flexible circuitry from this. Hence, the fabrication method based on lithography requires no special irradiation procedures like, e.g. irradiation through stencil/shadow masks or ultra low densities (single ion irradiation). The described process allows us to fabricate, e.g. high aspect ratio through hole microvias based on clusters of nanowires or a solid structure (a full hole produced by merging pores) in the commercially attractive flexible printed circuit board laminates. In this work a magnetoresistive (MR) magnetic field sensor have been fabricated (Fig. 3), consisting of a chain with two nanowire clusters links. Each of these two clusters is built from an average of 10 nickel wires. Nickel was chosen because it is a material with MR-properties and has sufficiently high resistivity to facilitate characterisation of the sensor. The MR-effect of nickel, R/R, is up to 2% for
Fig. 2. The illustration points out the potential of tilted irradiation in combination with microlithography. Double-sided shifted and aligned metal patterns allows coupling to wires of separate directions with different densities if required.
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which reveals that for the above example (100 wires in 100 links), the coefficient of variation (CV) is as low as 1.0% (see also Table 1). For 10 wires per link, CV is 4.5%. In sensor applications, an offset variation of 10% is not uncommon and should not be a hindrance since most sensors are calibrated.
3. Fabrication: process, evaluation and discussion The process below is presented and divided in process steps and placed in sequential order (Fig. 5). 3.1. Irradiation
Fig. 4. This illustrates how the electrical resistance of a component can be regulated by changing the area of the nanowire cluster element or by interconnecting several clusters into a chain.
large applied magnetic fields (≥10 kOe). Using a thick substrate allows us to connect multiple wires in parallel, maintaining the same electrical resistance compared to a single wire link in a thin substrate. In addition, by interconnecting multiple clusters into a long chain we can multiply the total resistance with several magnitudes (Fig. 4). Suppose, for example, that 100 cluster links having an average of 100 wires in each were connected in series. The electrical resistance of this chain would closely correspond to the resistance of one single wire. However, it is reasonable to believe the statistical fluctuations would be very small compared to the one wire case. This is readily verified with a calculation
The substrate used as carrier for the wires, is obtained by irradiating a 25–75 m thick polyimide foils with heavy ions. A cyclotron prepared with 129 Xe27+ ions with an energy of 8.3 MeV/u is used for this purpose. Every ion produces a single full-length track of damage (which later is transformed into one single pore). The penetration depth (ion range) is about 95 m in the polyimide. The range is roughly inversely proportional to the density of the plastic used (1.4 g/cm2 for polyimide)and proportional to the energy per nucleon (MeV/u) used. The ion-beam, having a spot size of about 5 mm (correspond to width of beam), is scanned in a x–y grid across a larger area (60 mm × 60 mm) than the sample area (45 mm × 45 mm). Highly parallel tracks with a divergence of less than 0.1◦ is then obtained. The irradiation density, is one of the most important parameters; between 105 /cm2 and 1010 /cm2 are readily achieved (109 ions/s is generated in the accelerator operating at a typical current of 4 nA). The required irradiation density is accomplished by calibration. A few calibration samples are used to interrelate the acquired density to the beam current (measured before every sample irradiation). Statistically, the number of ion tracks, x, confined inside the area A, is Poisson distributed, x ∈ P(λ · A), or P(x) = eλ·A (λ · A)x /x!. Here the Poisson intensity, λ, corresponds to the ion track density (tracks/cm2 ) and x to the number of wires within this area. Where the expectation, µ, equals λ×A and the standard √ deviation is σ = λ · A. The width (standard deviation) of the theoretical and practical ion track distribution is found in Table 1.
Table 1 Calculated examples show how the coefficient of variation, CV = σ/µ, expected number of wires, E(X), and the expected resistance, E(R), depends on the stochastical variable X, i.e. the number of wires in the nanowire cluster Mean number of wires
Statistically calculated variables from the Poisson distribution
Found ion track distribution
Relative limits for the 95% confidence interval (normal approx.)
E(X)
E(R) ()
CV(X) (%)
CV(R) (%)
CV(R) (%)
Relative resistance limits (%)
100 20 10
1.01 5.28 11.3
10 22.4 31.6
10.2 25.3 44.7
11.6 28.9 51
±22.9 ±56.7 ±100
In this example it is assumed the each wire have a resistance of 100 .
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measurement accuracy is about 1 ◦ C). The batch solution we purchase contains about 0.50 M of NaOH and 6–12% active hypochlorite (NaOCl− ). The pH of this solution is lowered by adding boric acid, H3 BO3 , or increased by raising the NaOH content of the solution. It is worth noticing that one type of flexible printed circuit boards (EspandexTM by Nippon Steel with a dielectric layer of polyimide) can be etched in this solution. The copper layers the printed circuit board can resist this aggressive etch solution for at least 3 h without showing signs of adhesional failure or corrosion. Measurement of the pH is dubious due to the high concentration of hydroxide and hypochlorite ions and the high temperature. It is measured after 60 s of electrode soaking using a MetrohmTM (type 6.0228.00) pH electrode (temperature of the pH meter is then 50–54 ◦ C). Note that the measured (and actual) pH of the solution at elevated temperature is about 0.7 units less than at room temperature (RT). This is because pKw (Kw is the ion product of water and pKw = pH + pOH) of water solutions is 13.3 at 50 ◦ C compared to 14.0 at RT. The hypochlorite content can be decreased by diluting the solution with a NaOH solution with the same alkaline concentration. The geometrical shape of the pores depends strongly on the pH [11] of the solution but also on the hypochlorite content of the solution [12]. It is therefore important to have an understanding of the deterioration of the sodium hypochlorite. The decomposition rate, k2 , of active hypochlorite is governed by the formula dCl% = k2 × (Cl%)2 dt
Fig. 5. Summary of the sensor fabrication process.
3.2. Ion track etching The ion tracks can be transformed into pores simply because the tracks can be etched selectively with respect to the undamaged bulk material. The track can be seen as a continuos trail of damaged material in a higher energy state, consisting of defects, vacancies and electronic damage. Consequently, it is often possible to find an etchant (developer) that attacks preferentially along this track of damage. It is practice to define two etch rates: (i) a track etch rate corresponding to the etch rate into the ion track and (ii) a bulk etch rate counterpart corresponding to the etch rate of the undamaged material (interrelated to the etch propagation rate of the pore radius at surface). To ensure that no particles or grease on the surface is blocking the etchant initially, the irradiated polyimide is pre-cleaned in 100% ethanol followed by a 50% ethanol bath and water rinsing. The development of the polyimide to obtain the nanoporous substrate is performed in a solution of sodium hypochlorite—an alkaline chemical known under the trivial name liquid or laundry bleach, kept at 60 ◦ C (the
where Cl% is the weight percentage active hypochloric, and k2 is an empirical coefficient. The hypochloric content was measured by means of titration with respect to duration of heating at 60 ± 1 ◦ C. Measurements revealed a strong correlation of k2 with the pH of the etching solution for low pH values (pH < 11). Our findings can be described with the following relation: k2 ≈ 2.3 × 10−3 · 1 + 109.8−pH /(hour × Cl%) Satisfactory agreement is confirmed in the pH range of 8.5–13.5. Previously [12], we have documented that the lower the pH, the lower the etchrate and the more cylindrical the pores but the faster is the decomposition rate of the etching solution. We also found that the minimum hypochlorite content required for optimal etching conditions and isotropic etching of the bulk material is dependent on the pH of the etching solution. We found that for higher pH (above 10.6), a minimum content of 5% hypochlorite is recommended. The lower the alkalinity the lower the required hypochloric content. The interior part of the pore, a central section with a length corresponding to 85% of the foil thickness, have actually a more vertical shape than is expressed by the aspect
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ratio. The pore ends are funnel shaped, but the change in cross-section (area) of this central section is less than 0.2%. The optimised solution to obtain cylindrical pores with extremely high aspect ratios contains 40.0 g of H3 BO3 per litre of sodium hypochlorite solution. This corresponds to a measured pH of 10.0 ± 0.15. The maximum pore aspect ratios (i.e. pore length/width) produced with these conditions by our group is ≥4 × 102 when etching simultaneously from both sides of the foil, corresponding to half of this when etching from only one side. The bulk etch rate (i.e. increase rate of the radius) at a pH of 10 have been measured to 81 nm/h (average). The pores for this sensor application were opened up to a diameter of between 300 and 500 nm. 3.3. Conductive seed layer To enable wire growth inside the pores in the substrate, a conductive seed layer is evaporated onto the arbitrary side of the substrate (referred to as the back side). A thickness of 150–200 nm proved satisfactory. The thickness of the metallic layer and the angle between the plane of the foil and the evaporation source determines the penetration depth of the gold in the pores as well as profile of the gold inside the pores (Fig. 6). The initial shape of the grown wire, i.e. solid or tubular geometry, will possibly be determined by the shape of this seed layer. All gold layers (seed layers and interconnection layers) are applied using resistive evaporation at close to perpendicular incidence angle from a molybdenum boat in an Edwards FL400 (Auto 306) Vacuum coater at a base pressure of <3×10−6 mbar. An evaporation rate of about 2 nm/s was used and the thickness was 200 ± 30 nm. Sputtering instead
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of evaporation of gold was tried but did not admit better adhesion. Note that the evaporated metallic layers does not fully cover the pore openings with the used pore sizes (Fig. 6). This would indeed be preferred, but thick evaporated metallic layers exhibit intrinsic stress and the adhesion to the polyimide substrate is impaired. It is however possible to increase the thickness of the seed layer by electrodepositing an additional metal layer, closing the pores and improving the electrical conductance of the layer. 3.4. Wettability of polyimide and metal adhesion The adhesion and fracture properties of both metallic seed layers and interconnection layers are critical issues [17,18], as the foil is subjected to heat treatments and mechanical stress in some of the fabrication steps. Adhesion primarily depends on a number of parameters (some which are interdependent); surface cleanliness, chemical modification, polar groups, chain scission, degree of crosslinking, and surface roughness/microstructure, most of them affecting the surface energy and wettability [13]. The wettability of the polyimide substrate is actually enhanced by the chemical etching of the pores; storage, on the other hand, might impair the modified and chemically active surface. We have measured the diameter of water droplets of a specific volume (2.5 l) on various treated and untreated polyimide surfaces. In Table 2 the contact angles have been derived from simple geometrical considerations (gravitational collapse of the drop was neglected). It has been argued that a gelatinous-like, modified surface layer, is produced after base hydrolysis (such as our chemical etching process), containing low molecular weight polymer
Fig. 6. Evaporated gold seen from underneath (after being removed from the polyimide using an adhesive tape). Tilted evaporation about 40◦ off perpendicular (left); approximately perpendicular evaporation (right).
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Table 2 Contact angles of de-ionised water droplets on the surface of the polyimide for various treatments used in the fabrication process Treatment of the polyimide
Storage time after treatment
Contact angle (◦ )
Comments
Untreated, as-received Cleaned in 100% ethanol Etched in concentrated sodium hypochlorite, 50 min Etched in concentrated sodium hypochlorite, 30 min Treated in oxygen plasma ∼40 min, 300 W
>1 month <1 h <1 h >1 month 1 day < t < 1 week
59 53 29 48 21
Possibly some hydrophobic residues covers the surface Removing hydrophobic stains increase the wettability slightly Contact angle decreases considerable after etching Storage reduces wettability, still less than initially Oxygen plasma gives the most hydrophilic surfaces even after a period of storage
chains, which could have a detrimental effect on the adhesion of metallic layers [14]. We found that heat treatments of the polyimide substrate prior to metal deposition do not substantially improve the adhesive properties of the gold. In fact, to our knowledge it can add to the risk of failure. The gold have a tendency to crack or delaminate when subjected to thermal stress, e.g. when using higher curing temperatures than about 110 ◦ C or thicker metallic layers (exceeding around 500 nm). The dimensional stability during heat treatment of polyimide foils is complicated. The difference in thermal expansion coefficients is expected to be small (in the temperature region 25–100 ◦ C; αKapton–Gold is only 3 ppm/◦ C). But the thermal behaviour of the polyimide is complex. Intrinsic stress in the KaptonTM HN from manufacturing process is released during the first curing step which makes the polyimide shrink approximately 0.2% (specified by Du Pont). 3.5. Electrodeposition of wires Before the electrodeposition step, the carrier substrate is cut into 20 mm × 20 mm pieces. The back, metallic side of the substrate, was first sealed with a special tape (SWT 20 by Nitto Europe NV) to ensure that no electrolyte enters behind the gold layer (the pores are not closed by the gold film). The gold film is very fragile and it is important that neither stresses nor mechanical damage is imposed on the sample in the electrodeposition step, which could deform the sample or generate cracks in the gold film. Sample was therefore sealed by lamination of a flexible laminate around it. The electrolyte enters through a window fabricated in the laminate and the electrodeposition current is supplied to the seed layer through a conductive adhesive copper tape. The electrodeposition of nickel is easy to control since the current versus applied potential follows a rather flat and consistent curve. Electrodeposition was performed under potentiostatic mode in an electrolyte containing 0.40 M (±5%) of nickel sulphate (NiSO4 ) and 0.66 M (±5%) boric acid (H3 BO3 ), having a pH of 3.3 M (±0.1%). Note that the pH is measured at an electrode temperature of approximately 30 ◦ C after 1 min soaking. Prior to electrodeposition, the sample should be cleaned in 50–60% solution of ethanol to enhance the wettability of the polyimide (a higher content of ethanol may remove the adhesive tape used on the back side). The sample is soaked in the electrolyte for between
15 and 45 min before deposition (typically 20 min). No degassing by pumping or ultrasonic treatment was necessary in order to introduce the electrolyte into the pores and initiate the deposition. Potentials between −0.88 and −1.00 V measured with respect to a Ag/AgCl reference electrode was used. For a voltage of −1.00 V the growth rate is 1.0 (±0.2) m/min and for −0.90 V about 0.3 m/min. We have found that the lower the applied voltage the more homogeneous the deposition. For a deposition potential of −0.90 V it was found that 40–50% of all the pores were filled to the top surface. This can be related to a fluctuation of 6–9 min in arrival time, corresponding to <2–3 m in length. However, it is actually possible that a larger fraction than this can later be contacted with the metallic layer since the gold film will penetrate somewhat into the pores. The time for wires to reach substrate surface will be affected by a number of parameters: the fluctuations in thickness of the sample, edge effects, and variation in pore geometry. When to stop the electrodeposition is important to discuss. When the wires reach the front surface the current increases rapidly as the wires grows hemispherical caps, i.e. as the growth is not longer confined inside the geometry of the pores (Fig. 7). When the effective deposition area increases so does the current (Fig. 8). However, if the wires reach the surface at slightly different time or if the electrolyte has a low concentration of metallic ions the increase will not be as sharp. Termination was enforced when the current reached 3–10 times relative the typical deposition current measured in the centre of the pore. 3.6. Metallic interconnection layers The gold, used for both of the two interconnection layers, was chosen because it is non-magnetic material (does not interact with a magnetic field) and has high corrosion resistance. The previously deposited metallic seed layer can be used as-deposited for the back side interconnection layer. The front side layer is applied after the nanowire growth step using the same evaporation conditions. However, a dry-etch step (using oxygen RF-plasma) of the substrate front side is believed to be beneficial for the adhesion. Notably, we found a substantially increased adhesion of the gold for high-porosity substrates possibly because of mechanical interlocking by the unfilled pores.
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Fig. 7. Dry-etched polyimide substrate. The nanowires are laid open after treatment in an oxygen plasma. The plastic body is removed at a higher rate near the wires compared to other parts, producing funnel shaped holes.
3.7. Dry etching of the polyimide In a high pressure oxygen plasma the surface layer (supposedly gelatinous-like) of the plastic is etched and removed without affecting the nickel wires. The plasma etches the plastic around the wires at a faster rate possibly because the sample is on a floating potential causing a potential build-up around the wires. This will expose the wires that are not fully ascended to the surface for the subsequent metallic layer (Fig. 7), essential if wire growth would not be uniform and wires reach the surface at slightly differ-
ent times. A high pressure Tegal Plasmaline 415TM Barrel Asher working at about 1.5 Torr and powered by 300 W is used for this purpose. The etch rate was found to be non-linear; increases with time and temperature, beginning at about 0.1 rising to 1 m/min after 30 min. Normal treatment: 10–15 min (2–6 m etching). The plasma removes the presumed gel-layer, modifies the surface, and is believed to aid the adhesion of the metallic coating [13,14]. However, over-treatment in a plasma is believed to have a disadvantageous effect on the adhesion since it could generate a low molecular surface layer.
Fig. 8. Typical current response during electrodeposition in our 75 m thick porous polyimide membranes.
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3.8. Photolithographic patterning of interconnects The surface interconnection pattern is produced by double-sided photolithography of the metallic layers of the completed nanowire substrate. Note that the metallic layers on the substrate surface cannot readily be structured before wires are deposited, since the resist used during lithography will be forced into pores and is not easily removed in the subsequent photolithographic development process. Below the used process steps follows. 1. The resist is applied to one arbitrary side of the substrate (S1813 positive resist from Shipley Europe Ltd.) is spun at 1000 rpm for 5 s, and then at 3000 rpm for 45 s (using slow acceleration). Instead of using a vacuum chuck where the resist is drained into the unfilled pores creating a non-uniform resist metallic layer and smudging the back side, the sample is mechanically clamped during spinning. 2. Curing (baking) of the resist is performed by heat treatment on a hotplate at 90 ◦ C for 3 min. 3. Resist is spun on the opposite side of the sample using the same parameters as above. 4. Curing by heat treatment in an oven at 90 ◦ C for 20 (up to 30) min. Note that the low curing temperatures of the resist means shorter durations of the wet chemical steps during development. Preferable, because the polyimide quickly absorbs water and acetone, inducing a dimensional expansion of the sample. 5. Two-sided aligned pattern transfer (by UV exposure) from chrome/quartz masks to the sample. A Karl Süss MJB21 (3 in.) double-sided mask aligner with 350 W lamp effect required an exposure time of 12–25 s. Note
that if a large curvature is induced in the sample due to polyimide/gold interface stresses this could lead to cracking of the resist or gold when the sample is clamped between mask plates. 6. The resist is developed in 1% filtered NaOH solution (Microposit 351 from Shipley diluted 1:4) for 25 s at RT. The sample is rinsed in water (without flushing gas) for 45 s. 7. The gold is etched in a gold etchant (100 g KI and 25 g I2 per litre H2 O) for approximately 20 s. 8. The resist is stripped in acetone for a maximum of 20 s since the solvent has a tendency to be absorbed by the polyimide. The sample is rinsed in ethanol and dried with a nitrogen spray gun. 4. Results and discussion The produced sensor structure where the vertical nanowires (extending in the direction into the picture) couples to gold lines is shown in Fig. 9. Hemispherical “caps” are built on the ends of the wires as they reach the top surface and start growing in the radial direction. The ion track density distribution has been measured after opening of the tracks (Fig. 10). The standard deviation, σ, of the ion track density was found to be in average 14% larger than the theoretical value given by the Poisson distribution. Hence, a somewhat larger variation in density than predicted from statistics that can be explained by fluctuations in irradiation current and spot shape. Selected entities are listed in Table 1, exposing the expected statistical behaviour of three types of clusters having different number of wires in average. In Fig. 11, the expected resistance spread is computed
Fig. 9. A light optical microscope image (transmission mode) shows the sensor outline where 40 m wide intersecting gold lines couples (from both sides of the semitransparent polyimide foil) to a cluster of nanowires. The hemispherical wire “caps” of the nanowires are clearly visible.
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Fig. 10. Corresponding irradiation density from measurements in 67 different areas on three samples. The density required was 1.0 × 106 /cm2 . The average number of measured pores on each sample (areas have equal size in same sample) is specified in the graph.
for chains of clusters of various lengths. The calculation is performed by computing the iterative convolutions of the Poisson distribution using a MatlabTM program. Pn =
k
P1 (i) · Pn−1 (k − i)
i=0
where the iteration variable, n = 2, 3, . . . , N is enumerated to the number of links in the chain, P1 is the Poisson probability distribution, index i, is the number of wires in one
cluster. The variable k (the maximum number of wires in a cluster) is assigned a large value where the probability is can be neglected. As the number of elements in the Pn vector used by the computer program increase as 2n , an interpolation method is used by the program to condense the information in this vector. The electrical resistance for one cluster is calculated from the simple relation: R(i) =
Rone wire i
Fig. 11. Computed curves of the expected variation in electrical resistance for chains containing selected numbers of nanowire cluster links. The curves correspond to 1, 2, 5, 10, 20, 50, and 100 nanowire clusters in series each having 10 wires. The typical resistance for every chain is normalised to 1.00.
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Table 3 Measured magnetoresistive responses for five sensor structures with two nanowire clusters in series, each cluster with an average of 10 wires Sample
Maximum MR-ratio, i.e. R/R (%)
1 2 3 4 5
0.76 0.98 0.67 0.78 0.29
The coefficient of variation of the electrical resistance (in zero magnetic field) for our prototype sensor structure was found to be 39% (nine sensor structures on three different electrodeposited samples). This value can be compared with the predicted CV from Poisson statistics, 32%, or with the CV found in our ion track membranes, 36% (i.e. 1.14 × 31.6%). Thus, it seems as the variation in resistance can be predicted well by the statistical theory. Large clusters (with more than 100 wires) can therefore be expected to perform (in dc mode) as a single connection with a very precise and reliable cross-section corresponding to the sum of the individual wire cross-sections. The use of clusters is believed to be beneficial for the technology (single wires are difficult to produce), but still requires certain process reliability and uniformity in, e.g. the electrodeposition step, to obtain a good coupling to all the wires in the cluster. Our belief is that nanowire electrodeposition in membranes of polyimide is not more complicated than in other polymer templates. The potential of scaling down the cluster vias is exciting, the limitations are not in the ion track technique (the ion track density can be multiplied several orders of magnitude), but in the lithographic resolution of the interconnections. We have been able to produce 10 m wide gold lines and with a resolution similar to what our process can achieve on silicon, i.e. about 1–2 m. We have also shown that the cluster are reliable. The samples can withstand rather rough handling without breaking the wires, possibly clusters of wires are more flexible than solid vias. The maximum MR response ratios for five different samples containing around 10 wires per cluster have been measured (Table 3). The gold line intersection area is about 1.6 × 103 m2 . A strong magnetic field (7 kOe) perpendicular to the extension of the wires was applied and the change in electrical resistance with respect to zero field was measured. The change in electrical resistance is deduced from the voltage drop across the wire for an applied current of about 1 A.
5. Conclusions For the first time large area electrodeposition of long (75 m) nickel nanowires (300–500 nm in diameter) have been demonstrated in foils of polyimide plastic, a material suitable also for integration with electronics, e.g. in flexi-
ble circuit board laminates. Although the polyimide plastic exhibits somewhat hydrophobic properties, we have shown that it is possible to wet the surface and allow the electrolyte into the pores. Pore aspect ratios of more than 100 has been obtained. We have shown that it is feasible to combine nano- and microfabrication techniques to integrate high aspect ratio nanostructures (wires) with metallic interconnection patterns produced by conventional microstructure processes. A new fabrication scheme for interconnecting so-called nanowire clusters (arrays of nanowires) in series with surface interconnects is presented (Fig. 5). We conclude that the statistical behaviour of the electrical resistance of these nanowire cluster components/sensors can be theoretically predicted. However, to reduce the variation further, a larger number of wires per clusters or longer cluster-chains can be used.
Acknowledgements The work was carried out within the Centre for Advanced Micro Engineering (AME), financed by the Swedish Foundation of Strategic Research (SSF). Part of the work has also been supported by the Commission of the European Communities under the Research and Training Network EuNITT, Contract No. HPRN-CT-2000-00047. We would like to express our gratitude to Dr. Sven Erick Alm (Division of Mathematics, Uppsala University) for valuable help with statistical theory and to Dr. Laurent Gravier and Dr. Yvan Jaccard (IPE, École Polytechnique Fédéral de Lausanne, Switzerland) for help with MR-measurements and electrodeposition issues.
References [1] R.L. Fleischer, P.B. Price, R.M. Walker, Track registration in various solid-state nuclear track detectors, Phys. Rev. A 133 (1964) 1443– 1450. [2] R.L. Fleischer, P.B. Price, R.M. Walker, Tracks of charged particles in solids, Science 149 (1965) 383–393. [3] R. Spohr, Ion Tracks and Microtechnology: Principle and Applications, Vieweg, Braunschweig, 1990. [4] G. Guillot, F. Rondolez, Characteristics of submicron pores obtained by chemical etching of nuclear tracks in polycarbonate films, J. Appl. Phys. 52 (1981) 7155–7164. [5] E.G. Possin, A method for forming very small diameter wires, Rev. Sci. Instrum. 41 (1970) 772–774. [6] L.F. Thompson, Porous anodic alumina: fabrication characterisation and applications, Thin Solid Films 297 (1997) 192–201. [7] J.E. Wegrowe, S.E. Gilbert, et al., Anisotropic magnetoresistance as a probe of magnetization reversal in individual nano-sized nickel wires, IEEE Trans. Magn. 34 (1998) 903–905. [8] I.K. Schuller, S. Kim, et al., Magnetic superlattices and multilayers, J. Magn. Magn. Mater. 200 (1999) 571–582. [9] A. Fert, L. Piraux, Magnetic nanowires, J. Magn. Magn. Mater. 200 (1999) 338–358. [10] C. Schönenberger, B.M.I. Zande, et al., Template synthesis of nanowires in porous polycarbonate membranes: electrochemistry and morphology, J. Phys. Chem. B 101 (1997) 5497–5505.
M. Lindeberg, K. Hjort / Sensors and Actuators A 105 (2003) 150–161 [11] C. Trautmann, W. Brüchle, et al., Pore geometry of etched ion tracks in polyimide, Nucl. Instrum. Methods B 111 (1996) 70–74. [12] L. Klintberg, M. Lindeberg, G. Thornell, Sodium hypochlorite as a developer for heavy ion tracks in polyimide, Nucl. Instrum. Methods B 184 (2001) 536–543. [13] F.D. Egitto, L.J. Matienzo, in: M.K. Gosh, K.L. Mittal (Eds.), Polyimides, Fundamentals and Applications, vol. 1, Marcel Dekker, New York, 1996, Chapter 15, pp. 389–452. [14] L.J. Matienzo, W.N. Unertl, in: M.K. Gosh, K.L. Mittal (Eds.), Polyimides, Fundamentals and Applications, vol. 1, Marcel Dekker, New York, 1996, Chapter 21, pp. 629–696. [15] J.M. Daughton, GMR applications, J. Magn. Magn. Mater. 192 (1999) 334–342. [16] S. Krongelb, L.T. Romankiw, J.A. Thornello, Electrochemical process for advanced package fabrication—feature article, IBM J. Res. Dev. 42 (1998) 575. [17] T.W. Poon, J. Leu, et al., Adhesion and fracture analysis of metal/polyimide line structures, J. Appl. Phys. 76 (1994) 5515–5523. [18] P.S. Ho, Adhesion and fracture of metal/polyimide fine line structures, in: Proceedings of the ASME International Mechanical Engineering
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Congress and Exposition, San Francisco, USA, 12–17 November 1995.
Biographies M. Lindeberg done MSc in materials science engineering in 1997. He was a research assistant in the thin film solar cell group at Solid State Electronics Division at Uppsala University in 1997. Presently employed as PhD student in the field of ion track technology and microstructure engineering at the Materials Science Division at Uppsala University. K. Hjort done MSc in engineering physics at Uppsala University in 1988 and PhD (Eng) in materials science at Uppsala University in 1993. Since 2001 he is associate professor at the Department of Materials Science, Uppsala University. He is the director of the VINNOVA Competence Centre on Surface and Microstructure Technology, SUMMIT. He is the coordinator of the EC funded European Network on Ion Track Technology, and has contributed to more than 100 international scientific publications in the fields of micro- and nanotechnologies.
Interconnected nanowire clusters in polyimide for flexible circuits and magnetic sensing applications M. Lindeberg∗ , K. Hjort Ångström Laboratory, Division for Materials Science, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Received 7 March 2002; received in revised form 3 March 2003; accepted 26 March 2003
Abstract By combining nano- and microtechnology we have fabricated three-dimensional (3D) flexible circuits, where clusters of nanowires form the vertical via connections. The nanowires, embedded in foils of polyimide plastic, are interconnected with two lithographically structured metallic surface layers. As the wires are defined by ion track technology they are stochastically distributed with a uniform density in macro-scale. In addition they are highly parallel in well-defined directions in the foil. The key structural element is a junction where overlapping lateral interconnection lines on the surface intersect with clusters of perpendicular or tilted wires. The demonstrated circuit structure is in essence a magnetic field sensor since the wires are made of nickel, a magnetoresistive material. The essential fabrication process comprises: ion track generation by means of heavy ion irradiation, selective ion track etching, electrodeposition of nanowires, and double-sided photolithography. The polyimide, employed commercially in, e.g. flexible printed circuit boards, is for the first time evaluated as a carrier for nanowires. The chemical properties and temperature stability makes the polyimide an appropriate material for implementation of electronic circuitry by ion- and photolithography. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Ion track; Polyimide membranes; Nickel nanowires; Template electrodeposition; Flexible circuitry
1. Introduction Combining conventional microstructure technology (MST) with ion track techniques unfolds vast possibilities of integrating micro- and nano-scale concepts; each range with its inherent advantages and disadvantages. Ion tracks are created upon irradiation of dielectric materials, e.g. polymers, with swift heavy ions passing through the material [1,2]. The continuous tracks, induced along the trajectory of every ion, are permanent and only a few nanometer in diameter consisting of damaged material in a higher energy state, i.e. electronic defects, vacancies, etc. Already in the 1960s ion tracks were studied thoroughly because of their importance in geology and cosmology (cosmic particles that reach through the earth atmosphere can create permanent tracks in many insulating materials). Today, for fundamental studies and for technology, it is more convenient to study and generate ion tracks in an accelerator facility. By operating an ion-beam we can readily produce billions of ion tracks per square centimetre in, e.g. various plastics and metallic oxides. The range of the ion in a material roughly corresponds to its energy per nucleon ∗ Corresponding author. Tel.: +46-18-471-7267; fax: +46-18-471-3572. E-mail address: [email protected] (M. Lindeberg).
divided with the density of the material. For our accelerator conditions the range is around 100 m in most plastic materials (i.e. with densities around 1.3 g/cm2 ). The highly damaged ion tracks generated can then be etched using a chemical solution, an etchant, which attacks the tracks preferentially [3]. The tracks are transformed into fine channels, hereafter referred to as pores or nanopores. The diameter of the pores can be controlled in the range starting from about 20 nm reaching up to 10 m by varying the conditions or the duration of the etching. The aspect ratio, i.e. length to width ratio, for track etched pores in plastic materials is normally high and can reach as high as 104 in polycarbonate [4]. The produced porous membranes can be used as templates or moulds for electrodeposition of very thin wires. Nanoporous membranes exist in the form of commercially available filters (e.g. MilliporeTM , NucleporeTM ). The first wires produced in porous ion track membranes are attributed Possin [5], who studied the fluctuations in superconducting transitions in tin wires in 1970. In the 1990s, the magnetic and magnetoresistive (MR) properties of nanowires became very interesting for fundamental magnetism studies. Hence, several research groups have been studying nanowires for almost a decade. Apart from ion track membranes, where the density of pores can be controlled without restraint, porous anodic alumina structures
0924-4247/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-4247(03)00088-8
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have been used for producing high aspect ration nanowires [6]. Early magnetic studies based on nickel and permalloy wires focused on the anisotropic magnetoresistive (AMR) effect, i.e. changes in the direction of the magnetic field produces a change in resistance [7]. In the late nineties more advanced materials were studied and deposited in such pores: various multilayer structures (also referred to as superlattice structures) were built [8]. For example, cobalt and copper multilayered structures exhibiting the giant magnetoresistive (GMR) effect were successfully grown in nanopores. These structures allow the study and use of current-perpendicular-to-plane (CPP) GMR, since the vertically stacked multilayer structures readily allowed the current to be directed normal to the plane. The small cross-section of the wires also provided a high measurable resistance. Exciting is the large resistance response (up to 65%) to a change in the magnetic field [9]. The motivation for magnetic wires has been their qualification as MR-sensors and non-volatile magnetic bit memories. Today, MR-sensors are found, e.g. in miniaturised pick-up sensors for magnetic hard-disc memories. The prevalent and probably most consistent technique for deposition of nanowires (in the designation nanowires we subjectively include wires up to several hundred of nanometer), is by potentiostatic electrodeposition, i.e. at a constant potential with respect to a reference electrode. The deposition conditions and morphology of the wires have been thoroughly studied by, e.g. Schönenberger et al. [10]. Commercially available filters of 6 m thick polycarbonate has been the material prevailing for template growth of nanowires. These filters have pores of high aspect ratio with diameters in a broad range: from 10 up to 5 m. A potential problem of these membranes is that the porosity is very high and that the pores have an angular distribution of up to 34◦ (according to Structure Probe Inc.). Another drawback of using polycarbonate membranes for technological applications is that it is not readily compatible with photolithography due to its poor resistance to chemicals and elevated temperatures. The polyimide plastic we use (KaptonTM HN polymer foils from Du Pont), hereafter referred to as polyimide, is directly compatible with standard photolithographic processes due to its good chemical resistance, mechanical strength and high temperature stability (up to 250–320 ◦ C). Moreover, we have shown that it is possible to produce pores in polyimide approaching the high aspect ratio of the polycarbonate [11,12]. The untreated polyimide surface normally exhibits hydrophobic properties and low wettability that might hamper electrolyte introduction into the pores for the wire deposition [13]. Nanowire circuitry requires the wires to be interconnected and coupled to a lateral pattern. Thin film metallic layer technology and conventional photolithography have been used to produce metal interconnection patterns on both surfaces of the substrate. However, the process is restrained by several problems: metallic layer adhesion, cracking and delamina-
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tion of metallic layers, UV-transparency of the polyimide, and possibly oxidation of wire ends. The interconnection layers require not only good coupling to the wires, but also a good adhesion of the two metallic layers to the plastic surface is important. Several studies of polyimide surfaces treated in plasmas and etched in wet chemistry to promote adhesion (normally correlated to the wettability) have been described by [13,14]. In fact, it has been shown that treatment of polyimide films in oxygen RF-plasmas can produce a boost in adhesion of some metallic layers with more than a magnitude. On the other hand, storage of the polyimide prior to metallic layer deposition is believed to degrade metal/polyimide adhesion properties since wettability is reduced.
2. Objectives and aim Ion track technology originates from several basic science areas such as particle physics, irradiation science, cosmology, and geology. Recently, a number of applications for the ion track technique have been formulated without formerly having a feasible, conventional microstructuring process to rely on [3]. Examples described are porous biological filtration membranes, membranes for chromatography of chemical substances, ion track modification of the magneto-optics properties of iron garnets and deep, vertical ionlithography. Our objective is to show that a combination of traditional microstructure technology and ion track technology have the advantage of enabling implementation of nano- and microtechnology unified in the same polymer-based system. A few conceivable applications based on nanowire components have been articulated: magnetic field- and magnetic field directional-sensors based on the magnetoresistive effect, e.g. GMR and AMR [9,15]. The prospect of implementing magnetic memories based on GMR spin-valves is also very promising. The process described in this work is capable of producing, in effect, three-dimensional circuit structures, composed of two parts. Firstly, the upright standing nanowire via connections; randomly scattered and embedded in the carrier substrate. Secondly, the two patterned metallic interconnection layers covering both surfaces of the nanowire substrate. The latter structure couples to the nanowire clusters in areas where interconnection lines in both of the two metallic layers intersect and overlap. The structural element where a cluster of nanowires is coupled with an overlapping surface patterns, displaying similarities with conventional via connection (Fig. 1) is referred to as a nanowire cluster link. The number of wires connected in such an area will fluctuate around a mean value identical with µ = irradiation density × area, and the standard deviation, σ, will depend on the stochastical (Poisson) distribution of the irradiation density. For a large expected number of wires (confined inside the contact area) the relative variation is small. Thus, clusters of wires lead to a statistical behaviour with less
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Fig. 1. The nanowire cluster is the basic structural element for all components described in this article. A junction where two overlapping metallic surface layers intersects and couples with a group of parallel nanowires essentially forming a vertical via connection.
variation in response properties compared to a single wire. They also offer a tuneable electrical resistance (or conductor) by allowing the size of two junction pads, hence the projected overlap, to be adjusted, and thus changing the typical number of wires confined in the nanowire cluster. Multiple irradiations at arbitrary angles and with different track densities are also possible. This allows us to use tilted wires in multiple and distinct directions and connect these directions separately by using aligned and shifted double-sided metal interconnection patterns on the surface (Fig. 2). If the sample is tilted 54.7◦ versus plane and rotated in the lateral plane 3 × 120◦ , the ion tracks span a three-dimensional (3D) orthogonal system, which can be used for fabrication of 3D magnetoresistive sensors. Considering that flexible printed circuit boards, an established microelectronic material, consists of a laminate structure of copper and polyimide foils, allows also this substrate to be processed to produce three-dimensional flexible circuitry based on nanowires. In flexible printed circuitry, a constant improvement in the packaging density by util-
Fig. 3. Magnetic field sensor outline, the resistance change, Ω, measured over the two nanowire junctions can be related to the applied magnetic field.
ising smaller line-widths and thinner microvias is desired [16]. Possible structures are interlevel nanowire cluster connections and when combined in arrays: 3D electronic microcomponents, e.g. capacitors and inductor coils are conceivable. Thus, the objective is to be able to start with an uncomplicated raw material: uniformly irradiated foils of polyimide or flexible printed circuit boards; assembling flexible circuitry from this. Hence, the fabrication method based on lithography requires no special irradiation procedures like, e.g. irradiation through stencil/shadow masks or ultra low densities (single ion irradiation). The described process allows us to fabricate, e.g. high aspect ratio through hole microvias based on clusters of nanowires or a solid structure (a full hole produced by merging pores) in the commercially attractive flexible printed circuit board laminates. In this work a magnetoresistive (MR) magnetic field sensor have been fabricated (Fig. 3), consisting of a chain with two nanowire clusters links. Each of these two clusters is built from an average of 10 nickel wires. Nickel was chosen because it is a material with MR-properties and has sufficiently high resistivity to facilitate characterisation of the sensor. The MR-effect of nickel, R/R, is up to 2% for
Fig. 2. The illustration points out the potential of tilted irradiation in combination with microlithography. Double-sided shifted and aligned metal patterns allows coupling to wires of separate directions with different densities if required.
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which reveals that for the above example (100 wires in 100 links), the coefficient of variation (CV) is as low as 1.0% (see also Table 1). For 10 wires per link, CV is 4.5%. In sensor applications, an offset variation of 10% is not uncommon and should not be a hindrance since most sensors are calibrated.
3. Fabrication: process, evaluation and discussion The process below is presented and divided in process steps and placed in sequential order (Fig. 5). 3.1. Irradiation
Fig. 4. This illustrates how the electrical resistance of a component can be regulated by changing the area of the nanowire cluster element or by interconnecting several clusters into a chain.
large applied magnetic fields (≥10 kOe). Using a thick substrate allows us to connect multiple wires in parallel, maintaining the same electrical resistance compared to a single wire link in a thin substrate. In addition, by interconnecting multiple clusters into a long chain we can multiply the total resistance with several magnitudes (Fig. 4). Suppose, for example, that 100 cluster links having an average of 100 wires in each were connected in series. The electrical resistance of this chain would closely correspond to the resistance of one single wire. However, it is reasonable to believe the statistical fluctuations would be very small compared to the one wire case. This is readily verified with a calculation
The substrate used as carrier for the wires, is obtained by irradiating a 25–75 m thick polyimide foils with heavy ions. A cyclotron prepared with 129 Xe27+ ions with an energy of 8.3 MeV/u is used for this purpose. Every ion produces a single full-length track of damage (which later is transformed into one single pore). The penetration depth (ion range) is about 95 m in the polyimide. The range is roughly inversely proportional to the density of the plastic used (1.4 g/cm2 for polyimide)and proportional to the energy per nucleon (MeV/u) used. The ion-beam, having a spot size of about 5 mm (correspond to width of beam), is scanned in a x–y grid across a larger area (60 mm × 60 mm) than the sample area (45 mm × 45 mm). Highly parallel tracks with a divergence of less than 0.1◦ is then obtained. The irradiation density, is one of the most important parameters; between 105 /cm2 and 1010 /cm2 are readily achieved (109 ions/s is generated in the accelerator operating at a typical current of 4 nA). The required irradiation density is accomplished by calibration. A few calibration samples are used to interrelate the acquired density to the beam current (measured before every sample irradiation). Statistically, the number of ion tracks, x, confined inside the area A, is Poisson distributed, x ∈ P(λ · A), or P(x) = eλ·A (λ · A)x /x!. Here the Poisson intensity, λ, corresponds to the ion track density (tracks/cm2 ) and x to the number of wires within this area. Where the expectation, µ, equals λ×A and the standard √ deviation is σ = λ · A. The width (standard deviation) of the theoretical and practical ion track distribution is found in Table 1.
Table 1 Calculated examples show how the coefficient of variation, CV = σ/µ, expected number of wires, E(X), and the expected resistance, E(R), depends on the stochastical variable X, i.e. the number of wires in the nanowire cluster Mean number of wires
Statistically calculated variables from the Poisson distribution
Found ion track distribution
Relative limits for the 95% confidence interval (normal approx.)
E(X)
E(R) ()
CV(X) (%)
CV(R) (%)
CV(R) (%)
Relative resistance limits (%)
100 20 10
1.01 5.28 11.3
10 22.4 31.6
10.2 25.3 44.7
11.6 28.9 51
±22.9 ±56.7 ±100
In this example it is assumed the each wire have a resistance of 100 .
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measurement accuracy is about 1 ◦ C). The batch solution we purchase contains about 0.50 M of NaOH and 6–12% active hypochlorite (NaOCl− ). The pH of this solution is lowered by adding boric acid, H3 BO3 , or increased by raising the NaOH content of the solution. It is worth noticing that one type of flexible printed circuit boards (EspandexTM by Nippon Steel with a dielectric layer of polyimide) can be etched in this solution. The copper layers the printed circuit board can resist this aggressive etch solution for at least 3 h without showing signs of adhesional failure or corrosion. Measurement of the pH is dubious due to the high concentration of hydroxide and hypochlorite ions and the high temperature. It is measured after 60 s of electrode soaking using a MetrohmTM (type 6.0228.00) pH electrode (temperature of the pH meter is then 50–54 ◦ C). Note that the measured (and actual) pH of the solution at elevated temperature is about 0.7 units less than at room temperature (RT). This is because pKw (Kw is the ion product of water and pKw = pH + pOH) of water solutions is 13.3 at 50 ◦ C compared to 14.0 at RT. The hypochlorite content can be decreased by diluting the solution with a NaOH solution with the same alkaline concentration. The geometrical shape of the pores depends strongly on the pH [11] of the solution but also on the hypochlorite content of the solution [12]. It is therefore important to have an understanding of the deterioration of the sodium hypochlorite. The decomposition rate, k2 , of active hypochlorite is governed by the formula dCl% = k2 × (Cl%)2 dt
Fig. 5. Summary of the sensor fabrication process.
3.2. Ion track etching The ion tracks can be transformed into pores simply because the tracks can be etched selectively with respect to the undamaged bulk material. The track can be seen as a continuos trail of damaged material in a higher energy state, consisting of defects, vacancies and electronic damage. Consequently, it is often possible to find an etchant (developer) that attacks preferentially along this track of damage. It is practice to define two etch rates: (i) a track etch rate corresponding to the etch rate into the ion track and (ii) a bulk etch rate counterpart corresponding to the etch rate of the undamaged material (interrelated to the etch propagation rate of the pore radius at surface). To ensure that no particles or grease on the surface is blocking the etchant initially, the irradiated polyimide is pre-cleaned in 100% ethanol followed by a 50% ethanol bath and water rinsing. The development of the polyimide to obtain the nanoporous substrate is performed in a solution of sodium hypochlorite—an alkaline chemical known under the trivial name liquid or laundry bleach, kept at 60 ◦ C (the
where Cl% is the weight percentage active hypochloric, and k2 is an empirical coefficient. The hypochloric content was measured by means of titration with respect to duration of heating at 60 ± 1 ◦ C. Measurements revealed a strong correlation of k2 with the pH of the etching solution for low pH values (pH < 11). Our findings can be described with the following relation: k2 ≈ 2.3 × 10−3 · 1 + 109.8−pH /(hour × Cl%) Satisfactory agreement is confirmed in the pH range of 8.5–13.5. Previously [12], we have documented that the lower the pH, the lower the etchrate and the more cylindrical the pores but the faster is the decomposition rate of the etching solution. We also found that the minimum hypochlorite content required for optimal etching conditions and isotropic etching of the bulk material is dependent on the pH of the etching solution. We found that for higher pH (above 10.6), a minimum content of 5% hypochlorite is recommended. The lower the alkalinity the lower the required hypochloric content. The interior part of the pore, a central section with a length corresponding to 85% of the foil thickness, have actually a more vertical shape than is expressed by the aspect
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ratio. The pore ends are funnel shaped, but the change in cross-section (area) of this central section is less than 0.2%. The optimised solution to obtain cylindrical pores with extremely high aspect ratios contains 40.0 g of H3 BO3 per litre of sodium hypochlorite solution. This corresponds to a measured pH of 10.0 ± 0.15. The maximum pore aspect ratios (i.e. pore length/width) produced with these conditions by our group is ≥4 × 102 when etching simultaneously from both sides of the foil, corresponding to half of this when etching from only one side. The bulk etch rate (i.e. increase rate of the radius) at a pH of 10 have been measured to 81 nm/h (average). The pores for this sensor application were opened up to a diameter of between 300 and 500 nm. 3.3. Conductive seed layer To enable wire growth inside the pores in the substrate, a conductive seed layer is evaporated onto the arbitrary side of the substrate (referred to as the back side). A thickness of 150–200 nm proved satisfactory. The thickness of the metallic layer and the angle between the plane of the foil and the evaporation source determines the penetration depth of the gold in the pores as well as profile of the gold inside the pores (Fig. 6). The initial shape of the grown wire, i.e. solid or tubular geometry, will possibly be determined by the shape of this seed layer. All gold layers (seed layers and interconnection layers) are applied using resistive evaporation at close to perpendicular incidence angle from a molybdenum boat in an Edwards FL400 (Auto 306) Vacuum coater at a base pressure of <3×10−6 mbar. An evaporation rate of about 2 nm/s was used and the thickness was 200 ± 30 nm. Sputtering instead
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of evaporation of gold was tried but did not admit better adhesion. Note that the evaporated metallic layers does not fully cover the pore openings with the used pore sizes (Fig. 6). This would indeed be preferred, but thick evaporated metallic layers exhibit intrinsic stress and the adhesion to the polyimide substrate is impaired. It is however possible to increase the thickness of the seed layer by electrodepositing an additional metal layer, closing the pores and improving the electrical conductance of the layer. 3.4. Wettability of polyimide and metal adhesion The adhesion and fracture properties of both metallic seed layers and interconnection layers are critical issues [17,18], as the foil is subjected to heat treatments and mechanical stress in some of the fabrication steps. Adhesion primarily depends on a number of parameters (some which are interdependent); surface cleanliness, chemical modification, polar groups, chain scission, degree of crosslinking, and surface roughness/microstructure, most of them affecting the surface energy and wettability [13]. The wettability of the polyimide substrate is actually enhanced by the chemical etching of the pores; storage, on the other hand, might impair the modified and chemically active surface. We have measured the diameter of water droplets of a specific volume (2.5 l) on various treated and untreated polyimide surfaces. In Table 2 the contact angles have been derived from simple geometrical considerations (gravitational collapse of the drop was neglected). It has been argued that a gelatinous-like, modified surface layer, is produced after base hydrolysis (such as our chemical etching process), containing low molecular weight polymer
Fig. 6. Evaporated gold seen from underneath (after being removed from the polyimide using an adhesive tape). Tilted evaporation about 40◦ off perpendicular (left); approximately perpendicular evaporation (right).
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Table 2 Contact angles of de-ionised water droplets on the surface of the polyimide for various treatments used in the fabrication process Treatment of the polyimide
Storage time after treatment
Contact angle (◦ )
Comments
Untreated, as-received Cleaned in 100% ethanol Etched in concentrated sodium hypochlorite, 50 min Etched in concentrated sodium hypochlorite, 30 min Treated in oxygen plasma ∼40 min, 300 W
>1 month <1 h <1 h >1 month 1 day < t < 1 week
59 53 29 48 21
Possibly some hydrophobic residues covers the surface Removing hydrophobic stains increase the wettability slightly Contact angle decreases considerable after etching Storage reduces wettability, still less than initially Oxygen plasma gives the most hydrophilic surfaces even after a period of storage
chains, which could have a detrimental effect on the adhesion of metallic layers [14]. We found that heat treatments of the polyimide substrate prior to metal deposition do not substantially improve the adhesive properties of the gold. In fact, to our knowledge it can add to the risk of failure. The gold have a tendency to crack or delaminate when subjected to thermal stress, e.g. when using higher curing temperatures than about 110 ◦ C or thicker metallic layers (exceeding around 500 nm). The dimensional stability during heat treatment of polyimide foils is complicated. The difference in thermal expansion coefficients is expected to be small (in the temperature region 25–100 ◦ C; αKapton–Gold is only 3 ppm/◦ C). But the thermal behaviour of the polyimide is complex. Intrinsic stress in the KaptonTM HN from manufacturing process is released during the first curing step which makes the polyimide shrink approximately 0.2% (specified by Du Pont). 3.5. Electrodeposition of wires Before the electrodeposition step, the carrier substrate is cut into 20 mm × 20 mm pieces. The back, metallic side of the substrate, was first sealed with a special tape (SWT 20 by Nitto Europe NV) to ensure that no electrolyte enters behind the gold layer (the pores are not closed by the gold film). The gold film is very fragile and it is important that neither stresses nor mechanical damage is imposed on the sample in the electrodeposition step, which could deform the sample or generate cracks in the gold film. Sample was therefore sealed by lamination of a flexible laminate around it. The electrolyte enters through a window fabricated in the laminate and the electrodeposition current is supplied to the seed layer through a conductive adhesive copper tape. The electrodeposition of nickel is easy to control since the current versus applied potential follows a rather flat and consistent curve. Electrodeposition was performed under potentiostatic mode in an electrolyte containing 0.40 M (±5%) of nickel sulphate (NiSO4 ) and 0.66 M (±5%) boric acid (H3 BO3 ), having a pH of 3.3 M (±0.1%). Note that the pH is measured at an electrode temperature of approximately 30 ◦ C after 1 min soaking. Prior to electrodeposition, the sample should be cleaned in 50–60% solution of ethanol to enhance the wettability of the polyimide (a higher content of ethanol may remove the adhesive tape used on the back side). The sample is soaked in the electrolyte for between
15 and 45 min before deposition (typically 20 min). No degassing by pumping or ultrasonic treatment was necessary in order to introduce the electrolyte into the pores and initiate the deposition. Potentials between −0.88 and −1.00 V measured with respect to a Ag/AgCl reference electrode was used. For a voltage of −1.00 V the growth rate is 1.0 (±0.2) m/min and for −0.90 V about 0.3 m/min. We have found that the lower the applied voltage the more homogeneous the deposition. For a deposition potential of −0.90 V it was found that 40–50% of all the pores were filled to the top surface. This can be related to a fluctuation of 6–9 min in arrival time, corresponding to <2–3 m in length. However, it is actually possible that a larger fraction than this can later be contacted with the metallic layer since the gold film will penetrate somewhat into the pores. The time for wires to reach substrate surface will be affected by a number of parameters: the fluctuations in thickness of the sample, edge effects, and variation in pore geometry. When to stop the electrodeposition is important to discuss. When the wires reach the front surface the current increases rapidly as the wires grows hemispherical caps, i.e. as the growth is not longer confined inside the geometry of the pores (Fig. 7). When the effective deposition area increases so does the current (Fig. 8). However, if the wires reach the surface at slightly different time or if the electrolyte has a low concentration of metallic ions the increase will not be as sharp. Termination was enforced when the current reached 3–10 times relative the typical deposition current measured in the centre of the pore. 3.6. Metallic interconnection layers The gold, used for both of the two interconnection layers, was chosen because it is non-magnetic material (does not interact with a magnetic field) and has high corrosion resistance. The previously deposited metallic seed layer can be used as-deposited for the back side interconnection layer. The front side layer is applied after the nanowire growth step using the same evaporation conditions. However, a dry-etch step (using oxygen RF-plasma) of the substrate front side is believed to be beneficial for the adhesion. Notably, we found a substantially increased adhesion of the gold for high-porosity substrates possibly because of mechanical interlocking by the unfilled pores.
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Fig. 7. Dry-etched polyimide substrate. The nanowires are laid open after treatment in an oxygen plasma. The plastic body is removed at a higher rate near the wires compared to other parts, producing funnel shaped holes.
3.7. Dry etching of the polyimide In a high pressure oxygen plasma the surface layer (supposedly gelatinous-like) of the plastic is etched and removed without affecting the nickel wires. The plasma etches the plastic around the wires at a faster rate possibly because the sample is on a floating potential causing a potential build-up around the wires. This will expose the wires that are not fully ascended to the surface for the subsequent metallic layer (Fig. 7), essential if wire growth would not be uniform and wires reach the surface at slightly differ-
ent times. A high pressure Tegal Plasmaline 415TM Barrel Asher working at about 1.5 Torr and powered by 300 W is used for this purpose. The etch rate was found to be non-linear; increases with time and temperature, beginning at about 0.1 rising to 1 m/min after 30 min. Normal treatment: 10–15 min (2–6 m etching). The plasma removes the presumed gel-layer, modifies the surface, and is believed to aid the adhesion of the metallic coating [13,14]. However, over-treatment in a plasma is believed to have a disadvantageous effect on the adhesion since it could generate a low molecular surface layer.
Fig. 8. Typical current response during electrodeposition in our 75 m thick porous polyimide membranes.
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3.8. Photolithographic patterning of interconnects The surface interconnection pattern is produced by double-sided photolithography of the metallic layers of the completed nanowire substrate. Note that the metallic layers on the substrate surface cannot readily be structured before wires are deposited, since the resist used during lithography will be forced into pores and is not easily removed in the subsequent photolithographic development process. Below the used process steps follows. 1. The resist is applied to one arbitrary side of the substrate (S1813 positive resist from Shipley Europe Ltd.) is spun at 1000 rpm for 5 s, and then at 3000 rpm for 45 s (using slow acceleration). Instead of using a vacuum chuck where the resist is drained into the unfilled pores creating a non-uniform resist metallic layer and smudging the back side, the sample is mechanically clamped during spinning. 2. Curing (baking) of the resist is performed by heat treatment on a hotplate at 90 ◦ C for 3 min. 3. Resist is spun on the opposite side of the sample using the same parameters as above. 4. Curing by heat treatment in an oven at 90 ◦ C for 20 (up to 30) min. Note that the low curing temperatures of the resist means shorter durations of the wet chemical steps during development. Preferable, because the polyimide quickly absorbs water and acetone, inducing a dimensional expansion of the sample. 5. Two-sided aligned pattern transfer (by UV exposure) from chrome/quartz masks to the sample. A Karl Süss MJB21 (3 in.) double-sided mask aligner with 350 W lamp effect required an exposure time of 12–25 s. Note
that if a large curvature is induced in the sample due to polyimide/gold interface stresses this could lead to cracking of the resist or gold when the sample is clamped between mask plates. 6. The resist is developed in 1% filtered NaOH solution (Microposit 351 from Shipley diluted 1:4) for 25 s at RT. The sample is rinsed in water (without flushing gas) for 45 s. 7. The gold is etched in a gold etchant (100 g KI and 25 g I2 per litre H2 O) for approximately 20 s. 8. The resist is stripped in acetone for a maximum of 20 s since the solvent has a tendency to be absorbed by the polyimide. The sample is rinsed in ethanol and dried with a nitrogen spray gun. 4. Results and discussion The produced sensor structure where the vertical nanowires (extending in the direction into the picture) couples to gold lines is shown in Fig. 9. Hemispherical “caps” are built on the ends of the wires as they reach the top surface and start growing in the radial direction. The ion track density distribution has been measured after opening of the tracks (Fig. 10). The standard deviation, σ, of the ion track density was found to be in average 14% larger than the theoretical value given by the Poisson distribution. Hence, a somewhat larger variation in density than predicted from statistics that can be explained by fluctuations in irradiation current and spot shape. Selected entities are listed in Table 1, exposing the expected statistical behaviour of three types of clusters having different number of wires in average. In Fig. 11, the expected resistance spread is computed
Fig. 9. A light optical microscope image (transmission mode) shows the sensor outline where 40 m wide intersecting gold lines couples (from both sides of the semitransparent polyimide foil) to a cluster of nanowires. The hemispherical wire “caps” of the nanowires are clearly visible.
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Fig. 10. Corresponding irradiation density from measurements in 67 different areas on three samples. The density required was 1.0 × 106 /cm2 . The average number of measured pores on each sample (areas have equal size in same sample) is specified in the graph.
for chains of clusters of various lengths. The calculation is performed by computing the iterative convolutions of the Poisson distribution using a MatlabTM program. Pn =
k
P1 (i) · Pn−1 (k − i)
i=0
where the iteration variable, n = 2, 3, . . . , N is enumerated to the number of links in the chain, P1 is the Poisson probability distribution, index i, is the number of wires in one
cluster. The variable k (the maximum number of wires in a cluster) is assigned a large value where the probability is can be neglected. As the number of elements in the Pn vector used by the computer program increase as 2n , an interpolation method is used by the program to condense the information in this vector. The electrical resistance for one cluster is calculated from the simple relation: R(i) =
Rone wire i
Fig. 11. Computed curves of the expected variation in electrical resistance for chains containing selected numbers of nanowire cluster links. The curves correspond to 1, 2, 5, 10, 20, 50, and 100 nanowire clusters in series each having 10 wires. The typical resistance for every chain is normalised to 1.00.
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Table 3 Measured magnetoresistive responses for five sensor structures with two nanowire clusters in series, each cluster with an average of 10 wires Sample
Maximum MR-ratio, i.e. R/R (%)
1 2 3 4 5
0.76 0.98 0.67 0.78 0.29
The coefficient of variation of the electrical resistance (in zero magnetic field) for our prototype sensor structure was found to be 39% (nine sensor structures on three different electrodeposited samples). This value can be compared with the predicted CV from Poisson statistics, 32%, or with the CV found in our ion track membranes, 36% (i.e. 1.14 × 31.6%). Thus, it seems as the variation in resistance can be predicted well by the statistical theory. Large clusters (with more than 100 wires) can therefore be expected to perform (in dc mode) as a single connection with a very precise and reliable cross-section corresponding to the sum of the individual wire cross-sections. The use of clusters is believed to be beneficial for the technology (single wires are difficult to produce), but still requires certain process reliability and uniformity in, e.g. the electrodeposition step, to obtain a good coupling to all the wires in the cluster. Our belief is that nanowire electrodeposition in membranes of polyimide is not more complicated than in other polymer templates. The potential of scaling down the cluster vias is exciting, the limitations are not in the ion track technique (the ion track density can be multiplied several orders of magnitude), but in the lithographic resolution of the interconnections. We have been able to produce 10 m wide gold lines and with a resolution similar to what our process can achieve on silicon, i.e. about 1–2 m. We have also shown that the cluster are reliable. The samples can withstand rather rough handling without breaking the wires, possibly clusters of wires are more flexible than solid vias. The maximum MR response ratios for five different samples containing around 10 wires per cluster have been measured (Table 3). The gold line intersection area is about 1.6 × 103 m2 . A strong magnetic field (7 kOe) perpendicular to the extension of the wires was applied and the change in electrical resistance with respect to zero field was measured. The change in electrical resistance is deduced from the voltage drop across the wire for an applied current of about 1 A.
5. Conclusions For the first time large area electrodeposition of long (75 m) nickel nanowires (300–500 nm in diameter) have been demonstrated in foils of polyimide plastic, a material suitable also for integration with electronics, e.g. in flexi-
ble circuit board laminates. Although the polyimide plastic exhibits somewhat hydrophobic properties, we have shown that it is possible to wet the surface and allow the electrolyte into the pores. Pore aspect ratios of more than 100 has been obtained. We have shown that it is feasible to combine nano- and microfabrication techniques to integrate high aspect ratio nanostructures (wires) with metallic interconnection patterns produced by conventional microstructure processes. A new fabrication scheme for interconnecting so-called nanowire clusters (arrays of nanowires) in series with surface interconnects is presented (Fig. 5). We conclude that the statistical behaviour of the electrical resistance of these nanowire cluster components/sensors can be theoretically predicted. However, to reduce the variation further, a larger number of wires per clusters or longer cluster-chains can be used.
Acknowledgements The work was carried out within the Centre for Advanced Micro Engineering (AME), financed by the Swedish Foundation of Strategic Research (SSF). Part of the work has also been supported by the Commission of the European Communities under the Research and Training Network EuNITT, Contract No. HPRN-CT-2000-00047. We would like to express our gratitude to Dr. Sven Erick Alm (Division of Mathematics, Uppsala University) for valuable help with statistical theory and to Dr. Laurent Gravier and Dr. Yvan Jaccard (IPE, École Polytechnique Fédéral de Lausanne, Switzerland) for help with MR-measurements and electrodeposition issues.
References [1] R.L. Fleischer, P.B. Price, R.M. Walker, Track registration in various solid-state nuclear track detectors, Phys. Rev. A 133 (1964) 1443– 1450. [2] R.L. Fleischer, P.B. Price, R.M. Walker, Tracks of charged particles in solids, Science 149 (1965) 383–393. [3] R. Spohr, Ion Tracks and Microtechnology: Principle and Applications, Vieweg, Braunschweig, 1990. [4] G. Guillot, F. Rondolez, Characteristics of submicron pores obtained by chemical etching of nuclear tracks in polycarbonate films, J. Appl. Phys. 52 (1981) 7155–7164. [5] E.G. Possin, A method for forming very small diameter wires, Rev. Sci. Instrum. 41 (1970) 772–774. [6] L.F. Thompson, Porous anodic alumina: fabrication characterisation and applications, Thin Solid Films 297 (1997) 192–201. [7] J.E. Wegrowe, S.E. Gilbert, et al., Anisotropic magnetoresistance as a probe of magnetization reversal in individual nano-sized nickel wires, IEEE Trans. Magn. 34 (1998) 903–905. [8] I.K. Schuller, S. Kim, et al., Magnetic superlattices and multilayers, J. Magn. Magn. Mater. 200 (1999) 571–582. [9] A. Fert, L. Piraux, Magnetic nanowires, J. Magn. Magn. Mater. 200 (1999) 338–358. [10] C. Schönenberger, B.M.I. Zande, et al., Template synthesis of nanowires in porous polycarbonate membranes: electrochemistry and morphology, J. Phys. Chem. B 101 (1997) 5497–5505.
M. Lindeberg, K. Hjort / Sensors and Actuators A 105 (2003) 150–161 [11] C. Trautmann, W. Brüchle, et al., Pore geometry of etched ion tracks in polyimide, Nucl. Instrum. Methods B 111 (1996) 70–74. [12] L. Klintberg, M. Lindeberg, G. Thornell, Sodium hypochlorite as a developer for heavy ion tracks in polyimide, Nucl. Instrum. Methods B 184 (2001) 536–543. [13] F.D. Egitto, L.J. Matienzo, in: M.K. Gosh, K.L. Mittal (Eds.), Polyimides, Fundamentals and Applications, vol. 1, Marcel Dekker, New York, 1996, Chapter 15, pp. 389–452. [14] L.J. Matienzo, W.N. Unertl, in: M.K. Gosh, K.L. Mittal (Eds.), Polyimides, Fundamentals and Applications, vol. 1, Marcel Dekker, New York, 1996, Chapter 21, pp. 629–696. [15] J.M. Daughton, GMR applications, J. Magn. Magn. Mater. 192 (1999) 334–342. [16] S. Krongelb, L.T. Romankiw, J.A. Thornello, Electrochemical process for advanced package fabrication—feature article, IBM J. Res. Dev. 42 (1998) 575. [17] T.W. Poon, J. Leu, et al., Adhesion and fracture analysis of metal/polyimide line structures, J. Appl. Phys. 76 (1994) 5515–5523. [18] P.S. Ho, Adhesion and fracture of metal/polyimide fine line structures, in: Proceedings of the ASME International Mechanical Engineering
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Biographies M. Lindeberg done MSc in materials science engineering in 1997. He was a research assistant in the thin film solar cell group at Solid State Electronics Division at Uppsala University in 1997. Presently employed as PhD student in the field of ion track technology and microstructure engineering at the Materials Science Division at Uppsala University. K. Hjort done MSc in engineering physics at Uppsala University in 1988 and PhD (Eng) in materials science at Uppsala University in 1993. Since 2001 he is associate professor at the Department of Materials Science, Uppsala University. He is the director of the VINNOVA Competence Centre on Surface and Microstructure Technology, SUMMIT. He is the coordinator of the EC funded European Network on Ion Track Technology, and has contributed to more than 100 international scientific publications in the fields of micro- and nanotechnologies.