Search

A very thin non-contact IR temperature sensor has been fabricated in a polymer foil. A thermoresistor was placed in the isothermal back-layer of a vertical configured IR-sensor. The IR-sensor is a ...

PurposeThe 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/approachKapton 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.FindingsThe 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/implicationsToday, through foil electroplating is fairly slow and more work should be made with copper electroplating. Ion track technology works well on polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyimide (PI) but should also be studied for novel polymer foils such as liquid crystal polymers (LCPs).Originality/valueThe paper details how ion track and PCB technology can be combined to enable a new type of through the foil via interconnect that consists of a multitude of wires. With these porous substrates, double‐sided circuits with high aspect ratio microvias and other multiple wire structures can be created using only lithography, thin film deposition, and electroplating. A new type of electrothermal sensorfoil is presented with several advantages over its competing micro electro mechanical systems (MEMS) based Si sensors.

As the call for higher wiring density in packaging and vertical microvia interconnections (microvias) rapidly evolves, the need for smaller lateral dimensions in printed circuit boards (PCB) microvias must be met. The ion track lithography described in this paper allows for high throughput micromachining of small, deep, vertical microvias in flexible PCB and all-polymer laminates. Ion track lithography makes use of swift heavy ion irradiation to enhance the selectivity and directionality of chemical etching. Within the areas exposed to the ion irradiation, small sub-micron pores (capillaries) are created, one for every ion. If etching is prolonged, the pores become merged. Electrodeposition from a metallic seed layer is used to fill these structures with metal. The lithography masks define either the areas where the ion tracks are developed or where the tracks are metallized. The smallest achievable size of the microvias is only limited by the resolution of the mask; microvias below 10 μm in diameter can also be achieved also in thick polyimide foils. Since each impinging ion forms one track, the foil’s porosity can be controlled by adjusting the irradiation dose, as well as by etching the pores to a suitable size. Depending on the porosity and material, the resultant metallized microvia consists of either individual or interlaced wires (like strands in a bundle wire), or is a solid. As an individual sub-micron wire may have an aspect ratio of several hundreds, this allows for the fabrication of truly vertical microvia structures, allowing ultra-high density microvia batch production. Demonstrator microstructures with highly vertical microvias have been fabricated in foils up to 125 μm thickness. Several components integrated in flexible PCB have been presented by us, e.g. magnetoresistive sensors, thermopile IR-sensors and microwave components like inductor elements.

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.

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.

Ion track technology makes it possible to produce low-cost templates for nanowires. In this study we have studied ion tracks in a polyimide resist on silicon. Spin coated, 1.5 μm thick polyimide resist on 4 in. Si wafers was irradiated by 129Xe27+ ions at low fluence. The irradiated resist was etched in sodium hypochlorite (NaClO) solution, and Ni nanowires were electroplated in the pores. Using scanning electron microscopy observations the ion track etch properties were obtained. The linear track etch rate of 0.6–1.0 μm/min was calculated from the conical aspect ratio of the nanowire, and from the pore diameter at the surface at different etch times the bulk etch rate in the halo at a distance longer than 5 nm from the ion axis was found to be 0.3 nm/min, while at a long distance from the ion axis the track etching velocity of the virgin material was 0.01 nm/min. Taking into account the big difference in the track etch velocity between the latent track and the material outside, a latent track diameter of 11 nm could be estimated. Such a diameter value is in good agreement with a prediction obtained from an inelastic thermal spike model. Furthermore, fabrication of 1.5 μm long magnetoresistive nanowires electroplated in ion tracks in the IC compatible polyimide resist may be a way to develop low-cost CMOS integrated magnetoresistive sensors.

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.

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.

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

We present the design, fabrication and characterization of vertical thermopile infra-red (IR) sensors. The sensors are produced in batches in a standard flexible printed circuit board (flex PCB) material. The vertical design opens up for large area detection and longer thermopile chains. The sensors consist of series of 88 vertically interconnected thermocouples in a 75 mum thick polyimide foil (Kapton HNtrade). The sensors showed a voltage response to an IR pulse of 1.2 V/(W/mm2).

High aspect ratio microstructures are today of utterly importance in MEMS. The process described in this paper can be used to produce deep, vertical microstructures in polyimide based materials, used in e.g. flexible printed circuit boards (FPCBs). Stacked polyimide and metal layers have the potential of integrating microelectronic circuits with above applications. The process is capable of producing regions with perpendicular sub-micron metal wire connections at numerous, arbitrarily specified locations. The flexible laminate is irradiated with heavy ions creating a vertical damage anisotropy (ion tracks) in the polymer layer. Lithographically defined apertures in the metallic layer define the geometry. The tracks exposed are selectively developed forming nanometer-wide pores. Metallic structures have been replicated in these pores by electrodeposition of metals. Demonstrator microstructures and highly vertical, through hole microvias have been fabricated. Ion track technology is promising for ultra-high density via batch production and has a potential of further miniaturizing via dimensions.

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.

We have demonstrated the ability to combine microsystem technologies with nanowire fabrication for magnetic field sensor applications. The substrate material used is a thick polyimide foil, the very same material flexible printed circuit boards are composed of. Thus, compatible and appropriate for microelectronic integration.

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).

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.

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.