Recently we have presented stretchable circuits, based on printed circuit board technology [1]. These circuits are not very well suited for applications where a high degree of biocompatibility is required (e.g. for implantation purposes), because they use standard Cu as the electrical interconnection material. Moreover fine interconnection pitches (smaller than 100 micrometer) cannot be achieved because patterning resolution is limited by the Cu thickness which usually is 17µm or 35µm in PCB technology. Therefore thin-film versions of the PCB technology based technology for elastic circuits have been developed and will be presented in this contribution. They use sputter deposited TiW/Au thin-film metal layers as metal interconnects. TiW (typically 50nm thick) is used as an adhesion layer, while Au (typical thickness 250nm) is a biocompatible metal with low resistivity. This biocompatible thin-film metal stack is supported by polyimide which is a flexible polymer material. The metal layer, as well as its polyimide support are patterned as meanders, which after embedding in biocompatible elastic PDMS (poly-dimethyl siloxane, silicone rubber) material, allows in plane deformation (stretching) of the meander without loss of its electrical interconnection functionality. The polyimide starting material can either be a spin-on material (e.g. HD Microsystems 2611), or a sheet, as used in standard flexible circuits (e.g. Dupont Kapton®). The metal meanders are formed by lithography and wet etching, for the polyimide meanders dry etching with a hard metal mask is used for patterning. Metal meandering lines with widths as low as 20 micrometer were successfully patterned in this way. Supporting the thin-film metal meanders with a flexible polyimide drastically increases the mechanical reliability, compared to non-supported meanders. Measurements for HD2611 supported TiW/Au meanders, embedded in Dow Corning Silastic® MDX4-4210 PDMS show a minimum lifetime of 500’000 cycles at a strain of 10%, without any measurable change in meander resistance. Moreover, in the case of use of the spin-on polyimide we have demonstrated for the first time the possibility to integrate an ultra-thin chip package (UTCP, [2]) together with thin-film stretchable interconnections in the same soft PDMS substrate. The chip is thinned down to a thickness of 20 to 30 micron. It is embedded in HD2611 spin-on polymide, which simultaneously serves as the support for the stretchable interconnects. The TiW/Au metal layer is used, not only as stretchable interconnection metallization, but also as contact material to the chip pads, and fan-out to the stretchable interconnects. In this contribution we will describe the different fabrication processes in further detail and show examples of fabricated devices, using these technologies. [1] J. Vanfleteren et al., MRS-B, Vol.37, pp.254-260 , 2012. [2] W. Christiaens et al., IEEE Trans. Comp. Pack. Techn., 33 (4): pp. 754-760, 2010.
Stretchable circuits with polyimide supported thin-film metal meanders
SALVO, PIETRO;
2012-01-01
Abstract
Recently we have presented stretchable circuits, based on printed circuit board technology [1]. These circuits are not very well suited for applications where a high degree of biocompatibility is required (e.g. for implantation purposes), because they use standard Cu as the electrical interconnection material. Moreover fine interconnection pitches (smaller than 100 micrometer) cannot be achieved because patterning resolution is limited by the Cu thickness which usually is 17µm or 35µm in PCB technology. Therefore thin-film versions of the PCB technology based technology for elastic circuits have been developed and will be presented in this contribution. They use sputter deposited TiW/Au thin-film metal layers as metal interconnects. TiW (typically 50nm thick) is used as an adhesion layer, while Au (typical thickness 250nm) is a biocompatible metal with low resistivity. This biocompatible thin-film metal stack is supported by polyimide which is a flexible polymer material. The metal layer, as well as its polyimide support are patterned as meanders, which after embedding in biocompatible elastic PDMS (poly-dimethyl siloxane, silicone rubber) material, allows in plane deformation (stretching) of the meander without loss of its electrical interconnection functionality. The polyimide starting material can either be a spin-on material (e.g. HD Microsystems 2611), or a sheet, as used in standard flexible circuits (e.g. Dupont Kapton®). The metal meanders are formed by lithography and wet etching, for the polyimide meanders dry etching with a hard metal mask is used for patterning. Metal meandering lines with widths as low as 20 micrometer were successfully patterned in this way. Supporting the thin-film metal meanders with a flexible polyimide drastically increases the mechanical reliability, compared to non-supported meanders. Measurements for HD2611 supported TiW/Au meanders, embedded in Dow Corning Silastic® MDX4-4210 PDMS show a minimum lifetime of 500’000 cycles at a strain of 10%, without any measurable change in meander resistance. Moreover, in the case of use of the spin-on polyimide we have demonstrated for the first time the possibility to integrate an ultra-thin chip package (UTCP, [2]) together with thin-film stretchable interconnections in the same soft PDMS substrate. The chip is thinned down to a thickness of 20 to 30 micron. It is embedded in HD2611 spin-on polymide, which simultaneously serves as the support for the stretchable interconnects. The TiW/Au metal layer is used, not only as stretchable interconnection metallization, but also as contact material to the chip pads, and fan-out to the stretchable interconnects. In this contribution we will describe the different fabrication processes in further detail and show examples of fabricated devices, using these technologies. [1] J. Vanfleteren et al., MRS-B, Vol.37, pp.254-260 , 2012. [2] W. Christiaens et al., IEEE Trans. Comp. Pack. Techn., 33 (4): pp. 754-760, 2010.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
