Mikael Lindeberg

Interconnected nanowire clusters in polyimide for flexible circuits and magnetic sensing applications

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.

Surface energy as a function of self-bias voltage in oxygen plasma wafer bonding

Abstract A limitation in the use of wafer bonding has been the necessity for high-temperature annealing after contacting the wafers at room temperature. In this paper, we try to find the highest surface energy as a function of self-bias voltage in oxygen plasma-activated wafer bonding, in order to achieve a low-temperature bonding process. The bonding was performed in situ the vacuum chamber. It was found that oxygen plasma has a smoothing effect on the surface roughness, rather independent of the plasma self-bias. However, a moderate self-bias voltage proved to give the highest surface energy for the bonded wafers, both at room-temperature and after annealing at 200°C. We believe that this is due to the fact that a moderate self-bias is the most efficient in removing surface contaminants, like water and hydrocarbons. It was also found that even after annealing at higher temperatures, 480°C and 720°C, the plasma-bonded wafers showed higher surface energy values than wafers bonded in ambient air. This investigation was focused on low-effect plasmas,

Sodium hypochlorite as a developer for heavy ion tracks in polyimide

The developing and etching of heavy ion tracks in polyimide with sodium hypochlorite have been studied to gain control over the parameters that affect the etch result. The shape of the resulting pores is a function of both alkalinity and hypochlorite content of the solution. Sodium hypochlorite decomposes during etching, and the rate constant has been determined as a function of the alkalinity at 62 °C. Polished cross-sections have been examined to determine the pore shape, and this method has shown to be a straightforward way to characterise the pores. Decreasing the alkalinity gives more cylindrical pores, but increases the decomposition rate of the hypochlorite solution and decreases the etch rate.

Vertical Thermopiles Embedded in a Polyimide-Based Flexible Printed Circuit Board

A fabrication process for vertical thermopiles embedded in a 75-mum-thick polyimide foil has been developed for flexible printed circuit boards (flex PCBs). The vertical connections consist of electrodeposited antimony-and nickel-plated through-hole vias. The plated through-hole vias consist of multiple wires, with a total metal content that is 1% of the total via volume. The via fabrication technique is similar to standard flex PCB wet etch and metallization processes. The main difference is that the foils are pretreated with ion irradiation to induce highly selective vertical etch rates. The thermopiles were characterized by measuring their voltage response to an applied temperature difference across the foil thickness.

Reliable small via interconnects made of multiple sub-micron wires in flexible PCB

A fabrication process for small through-hole vias consisting of multiple sub-micron electrodeposited wires has been developed for flexible printed circuit boards (PCB). The resistance of the vias is controlled by adjusting the number of wires per via, as well as the dimensions of the wires. The process steps include modification of the foils by irradiation with energetic ions, wet etching and metallization of the through-hole vias, double-sided surface metallization of the dielectric layer and interconnection lithography. Series of up to 360 interconnected vias of electrodeposited nickel are demonstrated in a flexible PCB foil (75 µm Kapton HN polyimide). The vias have a lateral size of 26 µm. The metal content of the demonstrated vias is 0.12% and 10% of their total volume, corresponding to a metal cross-section of 3.2 µm2 and 270 µm2, respectively. The electrical resistance per interconnected via is 2.6 Ω and 0.07 Ω, respectively. The vias can carry a current density of at least 4 × 106 A cm−2.

A PCB-like process for vertically configured thermopiles

Thermopiles are important components in infrared thermal detectors, thermoelectric generators and thermoelectric coolers. We present a thermopile structure with up to 224 vertically arranged thermocouple legs in a polyimide flex material. The thermopile is optimized for infrared thermal radiation detection and is fabricated using printed circuit board-like (PCB-like) processing. Each thermoelectric leg consists of a bundle of a few hundred sub-micrometre-sized strands of either antimony or nickel. These metal wire bundles were achieved by employing ion track technology on the polyimide foil, resulting in a porous dielectric material. Electrochemical methods were used to grow the thermoelectric materials in the pores. The plating mask was produced in a laminated dry photoresist. A small metal cross section, 20 µm2 (1 vol%), ensured a low heat exchange between the two surfaces of the flex. The typical resistance per thermocouple was 34 Ω. A responsivity to irradiance of 4.3 V mm2 W−1 was measured when heating with a white light source (irradiance 1 mW mm−2).

Plated Through-Hole Vias in a Porous Polyimide Foil for Flexible Printed Circuit Boards

A fabrication process for high aspect ratio plated through-hole vias is presented for flexible printed circuit boards. A 75 ?m thick porous Kapton foil that allows direct definition of high aspect ratio through-hole vias by dry photoresist film lithography and electrodeposition is presented. Pretreatment with swift heavy ion irradiation and wet etching define the pore density and porosity of the foil, similar to ion-track-etched filter membranes. Thin film metallization of a seed layer and lithography of a laminated dry photoresist film define the via sizes and positions. Subsequent through-hole electrodeposition produces vias consisting of multiple wires, where each open pore defines one wire. The via geometries are characterized by scanning electron microscopy. The electrical properties of the vias are characterized by resistance measurements. Vias with an aspect ratio over 2 and a side length of 33 ?m show high yield with low resistance and low variation in resistance.

High aspect ratio “multiple wire” microvias in flexible PCBs

Purpose – The purpose of this paper is to present an update and the latest results from work on high aspect ratio “multiple wire” microvias in porous flexible Kapton foils for printed circuit boards (PCBs).Design/methodology/approach – Kapton foils are made porous by ion track technology and dry resist patterning. In combination with thin film deposition and electroplating the technology is used to define circuits and sensors with microvias made of many individual high aspect ratio wires. The processes are within the reach of many production environments and are suitable for flexible PCB fabrication.Findings – The use of these novel processes enables new types of microvias and multiple wire structures in the foils for millimeter wave circuitry of substrate integrated waveguides and shielding, as well as for sensors with high thermal resistance.Research limitations/implications – Today, through foil electroplating is fairly slow and more work should be made with copper electroplating. Ion track technology ...

30 GHz Litz wires defined by ion track lithography

We have demonstrated the ability to combine microsystem technologies with sub-micron wire fabrication in polyimide based flexible printed circuitry board (PCB) laminates. Clusters of sub-micron wires in each via enables that no increase of resistance is shown for copper wires up to 30 GHz. A toroidal inductor is fabricated as an example of the application of ultra-high density via interconnects.

Ion Track Enabled High Aspect Ratio Flexible PCB Via Technology

By combining ion track technology with ordinary low-resolution printed circuitboard lithography it is possible at low cost to create high aspect ratios via connectors, as solid plugs or consisting of bundles of sub-micron connector wires at a small total cross-section. Ion track enabled microwave circuits in flexible printed circuit boards are suggested to be used in applications like inductors, ferromagnetic resonance microwave filters, circulators and magnetoresistive sensors. In this paper we demonstrate this technology with integrated printed circuitboard devices in two different flexible polyimide-based foils (Espandex and Kapton HN), using the ultra-high-density vias and the sub-micron wires.