- Email: [email protected]
Flexible and stretchable electronics for wearable health devices
Solid-State Electronics xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Flexible and stretchable electronics for wearable health devices Jeroen van den Brand a,⇑, Margreet de Kok a, Marc Koetse a, Maarten Cauwe b, Rik Verplancke b, Frederick Bossuyt b, Michael Jablonski b, Jan Vanfleteren b a b
Holst Centre/TNO, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands Imec CMST Technology Park 914, B-9052 Ghent, Belgium
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Flexible electronics Stretchable electronics
a b s t r a c t Measuring the quality of human health and well-being is one of the key growth areas in our society. Preferably, these measurements are done as unobtrusive as possible. These sensoric devices are then to be integrated directly on the human body as a patch or integrated into garments. This requires the devices to be very thin, flexible and sometimes even stretchable. An overview of recent technology developments in this domain and concrete application examples will be discussed. Ó 2015 Published by Elsevier Ltd.
1. Introduction
2. Technology advancements
Today’s weaable healthcare tools are complex systems, based on advanced electronics (mixed-signal front-ends, DSP, transceiver, power management, memory). Nevertheless, they predominantly rely on rather traditional system design and technology for integration and packaging. These devices are typically built using conventional board-level and/or package-level integration. The result is mostly a system that is uncomfortable to wear because it is rigid, large and heavy. From a user point of view, the device should preferably be comfortable and unnoticeable. Standard integration technologies typically not fulfill these requirements. An interesting approach to achieve better wearability is by transforming the flat rigid device into a flexible or stretchable electronics device that can better follow the shape of the human body. The current paper addresses recent technology developments at our institutes that enable this. Two generic complementary platforms will be discussed. The first one is based on organic and large area electronics (OLAE) technologies. It is specifically targeting low cost, large area devices like health patches. The second one is based on thin film technologies. This platform targets more advanced wearable and medical devices. The paper also addresses progress in two complementary technologies that are used in both platforms: thin chip integration and a technology for realizing stretchable devices from flexible foils. Finally, concrete application prototypes will be shown in which the platforms and technologies are being exploited.
2.1. Organic and large area electronics platform
⇑ Corresponding author. E-mail address: [email protected] (J. van den Brand).
Organic and large area electronics (OLAE) are built on thin and flexible plastic foils. Mostly polyesters like PET and PEN are chosen because of their low cost and transparent nature. A large part of the electronic functionality is printed as thin layers using functional inks. The end result is a very thin and flexible but also cost effective electronics device. OLAE technologies are currently already being industrially employed for wearable health and medical devices – for example for ECG patches but also for glucose diagnostic devices. Up to now this primarily concerns devices having a low complexity. Only coarse pitch circuitry traces are being printed while all more complex functionality (e.g. microcontrollers, radio chips) is integrated into the system on a separate, often rigid, printed circuit board (PCB). Recent advances in materials, technologies and processes enable the realization of more complex devices using OLAE technologies. Screen printing and inkjet printing [1] are the most commonly used printing techniques in OLAE. Of these two, screen printing is the most appropriate for the wearable health and medical devices as considered in this paper. As compared to inkjet printing it more easily enables the realization of complex multilayer circuits and the printed structures typically have a better conductivity owing to a thicker print deposit. Circuitry patterns with resolutions down to 50 lm line/spacing can routinely be achieved. These resolutions are well in-line with what is needed for wearable health and medical devices. An example of such a fine line printed pattern is given in Fig. 1. It shows the fanout circuitry for a bare die silicon chip. The circuitry was printed
http://dx.doi.org/10.1016/j.sse.2015.05.024 0038-1101/Ó 2015 Published by Elsevier Ltd.
Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024
2
J. van den Brand et al. / Solid-State Electronics xxx (2015) xxx–xxx
Fig. 1. Example of a printed circuitry pattern. Shown is a fanout circuitry for a bare die silicon chip. At the bonding position of the chip, the circuitry was printed at a minimum pitch of 100 lm (50 lm line and spacing).
with a nanoparticle Ag-based screen printing paste (DuPont) and using special high-resolution printing screens (SPG Prints). At the bonding position of the chip, the circuitry was printed at a minimum pitch of 100 lm (50 lm line and spacing). Realizing more complex circuits also implies the need for multilayered circuitry patterns. Also here advances have been made in recent years. Fig. 2 shows an example of a three layer printed circuit by printing alternating layers of conducting (gray) and dielectric ink (white) on top of each other. Fig. 2 also emphasizes one of the clear advantages of OLAE technologies: the base foil has a thickness of only 50 lm while the 5 sequentially printed layers (conductors, dielectrics) only add a maximum of 35 lm to the thickness. This results in a very flexible 3 circuit layer PCB having a thickness of less than 150 lm.
2.2. Thin film electronics platform More complex, high-end wearable and medical devices often need device (e.g. thinness, flexibility) and circuit (e.g. pitch, IO count, conductivities) performance that cannot be achieved with OLAE technologies. Therefore, a technology platform is being developed which allows realizing complex systems or subsystems. So, not only standalone patches can be realized, but also subparts of patches, which then can be combined with OLAE, taking advantage of the complementary nature of both approaches. The thin film electronics (TFE) platform is enabled by combining spin-on polyimide films with thin-film metallization. Polyimide films have been used as dielectric and passivation layers
Fig. 2. Overview image of a 3 layer printed circuitry patterns on a 50 lm PET foil. Circuitry is printed with screen printing technology using Ag-filled conductive ink (gray) and dielectric ink (white).
in the microelectronics industry for many years, as they exhibit exceptional electrical, thermal and mechanical properties. Thin-film metallization offers the obvious advantage that fine-pitch metallization schemes can be realized, ensuring compatibility with interconnection pitches of conventional, commercially available dies. Vacuum deposition processes, such as sputtering or evaporation, are thus preferred techniques to metallize the polyimide films. In addition, the deposition processes allow for use of a variety of metals: copper is generally used because of its electrical conductivity, but also biocompatible alternatives such as gold or even platinum can be considered. The process starts from a glass carrier substrate onto which a polyimide layer is spin coated and cured, resulting in a stress-free polyimide film with a typical thickness in the range of 5–10 lm. Next, thin-film copper metallization (1 lm thickness) is sputter deposited, preceded by an adhesion enhancement layer composed of titanium tungsten, deposited in the same vacuum cycle. The metallization is then patterned via a sequence of photolithography and wet etching to realize the interconnects, and covered by depositing another polyimide film. Electrical contacts are provided by selectively removing the covering polyimide film on top of specific metal pads. This is done by laser ablation, using a nanosecond KrF excimer laser (248 nm). By repeating this procedure, i.e. the sequence of metal deposition, polyimide coating and via ablation, multilayer circuitry can be enabled. To release the devices from its carrier, a laser release methodology is used to ablate a sub-micron layer of polyimide at the glass/polyimide interface. This ablation procedure can be adapted according to the final device geometry. 2.3. Thin chip integration Most medical and wearable devices need some form of intelligence. For example microcontrollers (to process sensor data) or radio chips (to communicate this data with the outside world). Integrated circuits are normally supplied in packaged format. Using these bulky and rigid packages has a big impact on the thickness and flexibility advantages of the base foils. At thicknesses below 25 lm, it is known that silicon becomes flexible [2]. It would thus be preferred to integrate the needed silicon functionality as bare die and in a thinned down form. Several approaches exist to achieve thin silicon chips. Grinding and polishing is one of the more common routes but for example the ChipFilm technology of IMS Chips [2] results in inherently thin silicon chips. The integration of these unpackaged, thinned dies is done using either of two distinct approaches. The first approach, used in combination with the TFE platform, the ultra-thin chips are embedded with the active side facing upwards (face-up) in between two sequentially deposited spin-on polyimide layers. Although height irregularities in the order of the thickness of the die are introduced, electrical connectivity can be provided using the described processing sequence. Fig. 3 shows a typical example of this approach, in which an interconnection test die is provided with fan-out metallization. In the second approach, the active components are assembled face-down on the plastic foil. Both conductive adhesives and solders can be used to establish the interconnects. In particular the integration of silicon chips with OLAE foils is a challenge because of the low thermal stability of the polyesters. PEN has a glass transition temperature (Tg) of 130 °C and PET of 85 °C while poly(imide) has a Tg of 350 °C [3]. This limitation in thermal stability excludes many well- established processes for making electronic products, as for example soldering. The integration of silicon chips with OLAE foils is most commonly done using a flip chip process based on conductive
Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024
3
J. van den Brand et al. / Solid-State Electronics xxx (2015) xxx–xxx
10 # opens circuitry-bump resistance (Ohm)
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Fig. 4. Frequency of the output waveforms of thinned microcontroller dies with three different thicknesses as a function of inverse bending radius.
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adhesives. Specifically work was performed on achieving a reliable and robust chip-foil interconnect with low temperature cure anisotropic conductive adhesive (ACA) [4,5]. Fig. 5 shows typical reliability results of a 25 lm thick silicon chip with a pitch of 250 lm attached to a PET foil using an anisotropic conductive adhesive. It can be seen that the low chip thickness results in excellent thermal shock behavior, while the accelerated humidity shows some failure due to hygroscopic swelling of the adhesive. Current activities involve achieving even finer pitches for even thinner chips. Instead of ACA’s, high resolution printed isotropic conductive adhesives are being used for OLAE-based applications. In parallel, alternative technologies are being pursued to integrate thin chips [6,7]. Considerable work has been performed to better understand the mechanical and functional stability of such thin chips [8] both for the OLAE and the ultra-thin electronics platforms. A dedicated four-point mechanical bending test setup has been built in which the bending and electrical behavior of a thin chip can be monitored in-situ. Fig. 4 shows some recent results obtained with this setup. Microcontroller chips of different thicknesses were tested for the stability of their internal clock as a function of bending radius. This result shows that not only the mechanical strength of the silicon needs to be taken into account when choosing a chip thickness to achieve a given bending radius, but also the effect of the generated stress on the active layers inside the chip.
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2.4. Stretchable and conformable electronics Flexible electronics can meet to a large extent the irregularities of the human body. For increased user comfort, it would however be preferred that the electronics device can be deformed in more than one direction simultaneously. The device would then need to be conformable or stretchable. In the past years, a technology has been developed which enables the realization of stretchable systems from flexible foils [9]. The basic principle is shown in Fig. 5. The electronic functionality is distributed onto islands that are interconnected to each other by meander-shaped interconnects. The whole system is embedded into a stretchable rubber to keep everything together and to protect the device. An advantage of this technology over competing stretchable electronics technologies [10] is that it uses standard flex foil manufacturing technologies. The process starts from a normal full area flex foil (polyester, polyimide or other plastic foil materials). Circuitry is made with standard printed circuit board technologies (e.g. printing or lithography). Components are also assembled using traditional interconnect technologies (adhesives, soldering). Once finished, the meanders are structured into the foil using for example a laser or die cutting. Finally, the structured foil is embedded inside a rubber material by lamination of a polyurethane or overmoulding with silicone. Overall good stretchability and reliability can be achieved with the technology. Fig. 6 shows an overview of typical stabilities for different foil types embedded in silicone (Dow Corning, Sylgard 186). Unsupported meanders can be stretched almost a thousand
Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024
4
J. van den Brand et al. / Solid-State Electronics xxx (2015) xxx–xxx
Fig. 6. Schematic principle of meander-based stretchable electronics technology.
times at 10% elongation. A supporting layer of PET or PEN increases that tenfold, while a 50 lm PI cladding can survive over 100.000 cycles at 10% elongation or more than 1000 cycles at 50% elongation. Much effort was spent on the proper design of the meanders [11]. An aim of the design and modeling activities was to minimize stress and strain concentrations in the meander under deformation. Therefore, the stress has to be distributed as much as possible along the meander. The current meander shape of choice is the horseshoe shape. For minimal stress, the width (W) of the track should be as small as possible. This minimum W is determined mainly by technological constraints. In case of conventional PCB manufacturing, where the conductors are patterned by lithography and wet etching of a 17 or 35 lm thick Cu sheet, conductor tracks with W = 100 lm provide circuits with sufficient process yield. The meander radius (R) and angle (h) are design parameters. The values chosen for these parameters will depend on the application requirements (e.g., the maximum desired stretchability of the circuit). A further improvement of the mechanical reliability of the meanders is achieved by supplying the meanders with a flexible support (polyimide, PEN, PET, etc.). Numerical modeling showed that the width is the main parameter, influencing (in fact reducing, compared to non-supported meanders) the maximum plastic strain in the Cu meander [12,13]. This can be explained by the redistribution of the plastic strain in the metal. Fig. 7 gives an overview of the typical stretch reliability that can be achieved.
Fig. 8. Phototherapy device realized using meander-based stretchable electronics technology. Work performed in the framework of the FP7 EU Project PlaceIT.
Fig. 9. Flexible sensor label based on OLAE technologies. Circuitry has been printed and on top of this bare die ultrathin (25 lm thick) silicon chips were assembled together with peripheral 01005 sized components.
3. Application examples As an illustration of the advancements in the technology, some application example prototypes are shown in Figs. 8–12. Fig. 8 until 10 concern devices built on the OLAE platform while Figs. 11 and 12 concern devices built on the TFE platform. Fig. 8 shows a wrist-based phototherapy device. Heavy use was made of the stretchable electronics technology to ensure the electronics follows the shape of the hand. The device is essentially a Fig. 10. Wearable low cost energy expenditure patch. Based on OLAE technologies. Circuitry has been printed with screen printing. On top of this, multiple chips were assembled using conductive adhesives.
Fig. 7. Graph showing the reliability of the meander technology for different substrate types.
‘light-engine’ consisting of 54, blue LEDs spread over an active of around 150 cm2 and fully encapsulated in a silicone (PDMS, Dow Corning MS1003 on top, Xiameter RTV- 4230-E, bottom). LEDs were connected in parallel by a stretchable meander. The complete engine fits into a prototyped textile wrap to enable easy fixation on the hand. Reliable functionality under significant stretching deformation has been confirmed for the device. Around 1% of relative efficiency loss was obtained after 120,000 cycles of tensile elongation where interconnects were strained by 9.3% and 8300 cycles at 18.6% interconnect strain. This proves that the meanders allow low
Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024
J. van den Brand et al. / Solid-State Electronics xxx (2015) xxx–xxx
Fig. 11. Lightweight, conformable system consisting of a 4 4 microelectrode array and ASIC for multi-channel brain recording and stimulation.
5
The device consists of a 4 4 microelectrode array (shown in the inset), which is routed toward a 30 lm thick custom designed ASIC. The ASIC enables multi-channel brain recording and stimulation. As a final example, Fig. 12 shows a first proof-of-concept of a stretchable LED-based microdisplay. The stretchability of the display is meant to give it a better mechanical robustness than would be possible with a flexible display. This is for example relevant in wearable applications like garments. The TFE-based base foil consisted of 0.5 mm sized ‘LED islands’ which are interconnected to each other by 0.5 mm sized meanders for making the display stretchable. A multitude of bare die LEDs (CREE, size 200 200 50 lm) were flip chip assembled onto the base foil using conductive adhesives. Thin sheets of thermoplastic polyurethane (50 lm) were laminated on both sides to encapsulate and protect the device. Initial stretchability tests show that the device can be stretched up to 10% without damage. 4. Summary and conclusion An overview has been given of the current technology developments and their status at our institutes, for the application of wearable health and medical devices. In the past years, several new technologies have been developed at our institutes that can enable a next generation of devices with an improved form factor as compared to the devices currently on the market. Moreover, these technologies have reached a maturity state at which their advantages can be investigated by building actual devices with them. Some examples of this have been shown in the current paper. Future work will continue to focus on building devices and further optimization of the underlying technologies.
Fig. 12. Stretchable LED microdisplay. Consisting of bare die LEDs assembled on a TFE electronics foil.
and stable resistance throughout device fatigue life. Possible improvements could be made by cleaner definition of the PDMS boundaries in order to avoid tearing at higher extensions. Fig. 9 shows an example of a thin (<150 lm) multi-purpose RFID-based flexible sensor patch. The patch is for be attachment to the body and can monitor environmental parameters like humidity and gas levels. Two circuit layers, the RFID antenna and the humidity sensor were successfully and reproducibly realized with screen-printing on a polyester foil. Ultrathin chips (microcontroller, radio chip, 25 lm thickness) and peripheral 01005 sized components were successfully integrated using conductive adhesives. Another sensor patch is shown in Fig. 10. This device is more advanced in functionality but less advanced in form factor. The patch has integrated Bluetooth low energy functionality, accelerometer, temperature sensor and humidity sensor. It consists of an OLAE base foil having three circuitry layer screen printed pattern. Directly on top of this, the various chips were assembled as packaged components using conductive adhesives. The final device was encapsulated using a silicone to protect it against moisture. Fig. 11 shows an example of a more complex healthcare related device, realized using the TFE platform. The developments are part of the Flemish government supported project BrainstaR. The project focuses on the development of a fully implantable microsystem for brain stimulation and recording in small rodents. Its compact, lightweight and conformable nature makes the system ideally suited for use with small rodents, such as rats.
References [1] van Osch THJ, Perelaer J, de Laat AWM, Schubert US. Inkjet printing of narrow conductive tracks on untreated polymeric substrates. Adv Mater 2008;20:343–5. [2] Burghartz JN, Appel W, Harendt C, Rempp H, Richter H, Zimmermann M. Ultrathin chip technology and applications, a new paradigm in silicon technology. Solid-State Electron 2010;54:818–29. [3] Fjelstad J. Flexible circuit technology. 3rd ed. BR Publishing Inc.; 2007. [4] van den Brand J, Veninga E, Kusters R, Podprocky T, Dietzel A. Flipchip bonding of Si chip on flexible PEN foil using novel electronic 100 lm pitch fan-out circuitry. In: Materials research society symposium proceedings, vol. 1030; 2008. p. G05-10-17 [5] van den Brand J, Kusters R, Heeren M, Remoortere BV, Dietzel A. Flipchip bonding of ultrahin Si dies onto PEN/PET substrates with low cost circuitry. Presented at the ESTC 2010, Berlin; 2010. [6] Cauwe M, Vandecasteele B, De Baets J, van den Brand J, Kusters R, Sridhar A, et al. A chip embedding solution based on low-cost plastic materials as enabling technology for smart labels. In: 2012 4th Electronic systemintegration technology conference (ESTC); 2012. p. 6. [7] Sridhar A, Cauwe M, Fledderus H, Kusters RHL, van den Brand J. Novel interconnect methodologies for ultra-thin chips on foils. In: 2012 IEEE 62nd electronic components and technology conference (ECTC); 2012. p. 238–44. [8] van den Ende D, Verhoeven F, van der Eijnden P, Kusters R, Sridhar A, Cauwe M, et al. High curvature bending characterization of ultra-thin chips and chip-onfoil assemblies. In: 2013 European microelectronics packaging conference, New York: IEEE; 2013. [9] Bossuyt F, Vervust T, Vanfleteren J. Stretchable electronics technology for large area applications: fabrication and mechanical characterization. IEEE Trans Compon Pack Manuf Technol 2013;3:229–35. [10] Rogers JA, Someya T, Huang Y. Materials and mechanics for stretchable electronics. Science 2010;327:1603–7. [11] Gonzalez M, Axisa F, BuIcke MV, Brosteaux D, Vandevelde B, Vanfleteren J. Design of metal interconnects for stretchable electronic circuits. Microelectron Reliab 2008:825–32. [12] Yung-Yu H, Gonzalez M, Bossuyt F, Vanfleteren J, De Wolf I. Polyimideenhanced stretchable interconnects: design, fabrication, and characterization. IEEE Trans Electron Dev 2011;58:2680–8. [13] Hsu YY, Gonzalez M, Bossuyt F, Axisa F, Vanfleteren J, De Wolf I. The effects of encapsulation on deformation behavior and failure mechanisms of stretchable interconnects. Thin Solid Films Jan 2011;519:2225–34.
Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Flexible and stretchable electronics for wearable health devices Jeroen van den Brand a,⇑, Margreet de Kok a, Marc Koetse a, Maarten Cauwe b, Rik Verplancke b, Frederick Bossuyt b, Michael Jablonski b, Jan Vanfleteren b a b
Holst Centre/TNO, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands Imec CMST Technology Park 914, B-9052 Ghent, Belgium
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Flexible electronics Stretchable electronics
a b s t r a c t Measuring the quality of human health and well-being is one of the key growth areas in our society. Preferably, these measurements are done as unobtrusive as possible. These sensoric devices are then to be integrated directly on the human body as a patch or integrated into garments. This requires the devices to be very thin, flexible and sometimes even stretchable. An overview of recent technology developments in this domain and concrete application examples will be discussed. Ó 2015 Published by Elsevier Ltd.
1. Introduction
2. Technology advancements
Today’s weaable healthcare tools are complex systems, based on advanced electronics (mixed-signal front-ends, DSP, transceiver, power management, memory). Nevertheless, they predominantly rely on rather traditional system design and technology for integration and packaging. These devices are typically built using conventional board-level and/or package-level integration. The result is mostly a system that is uncomfortable to wear because it is rigid, large and heavy. From a user point of view, the device should preferably be comfortable and unnoticeable. Standard integration technologies typically not fulfill these requirements. An interesting approach to achieve better wearability is by transforming the flat rigid device into a flexible or stretchable electronics device that can better follow the shape of the human body. The current paper addresses recent technology developments at our institutes that enable this. Two generic complementary platforms will be discussed. The first one is based on organic and large area electronics (OLAE) technologies. It is specifically targeting low cost, large area devices like health patches. The second one is based on thin film technologies. This platform targets more advanced wearable and medical devices. The paper also addresses progress in two complementary technologies that are used in both platforms: thin chip integration and a technology for realizing stretchable devices from flexible foils. Finally, concrete application prototypes will be shown in which the platforms and technologies are being exploited.
2.1. Organic and large area electronics platform
⇑ Corresponding author. E-mail address: [email protected] (J. van den Brand).
Organic and large area electronics (OLAE) are built on thin and flexible plastic foils. Mostly polyesters like PET and PEN are chosen because of their low cost and transparent nature. A large part of the electronic functionality is printed as thin layers using functional inks. The end result is a very thin and flexible but also cost effective electronics device. OLAE technologies are currently already being industrially employed for wearable health and medical devices – for example for ECG patches but also for glucose diagnostic devices. Up to now this primarily concerns devices having a low complexity. Only coarse pitch circuitry traces are being printed while all more complex functionality (e.g. microcontrollers, radio chips) is integrated into the system on a separate, often rigid, printed circuit board (PCB). Recent advances in materials, technologies and processes enable the realization of more complex devices using OLAE technologies. Screen printing and inkjet printing [1] are the most commonly used printing techniques in OLAE. Of these two, screen printing is the most appropriate for the wearable health and medical devices as considered in this paper. As compared to inkjet printing it more easily enables the realization of complex multilayer circuits and the printed structures typically have a better conductivity owing to a thicker print deposit. Circuitry patterns with resolutions down to 50 lm line/spacing can routinely be achieved. These resolutions are well in-line with what is needed for wearable health and medical devices. An example of such a fine line printed pattern is given in Fig. 1. It shows the fanout circuitry for a bare die silicon chip. The circuitry was printed
http://dx.doi.org/10.1016/j.sse.2015.05.024 0038-1101/Ó 2015 Published by Elsevier Ltd.
Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024
2
J. van den Brand et al. / Solid-State Electronics xxx (2015) xxx–xxx
Fig. 1. Example of a printed circuitry pattern. Shown is a fanout circuitry for a bare die silicon chip. At the bonding position of the chip, the circuitry was printed at a minimum pitch of 100 lm (50 lm line and spacing).
with a nanoparticle Ag-based screen printing paste (DuPont) and using special high-resolution printing screens (SPG Prints). At the bonding position of the chip, the circuitry was printed at a minimum pitch of 100 lm (50 lm line and spacing). Realizing more complex circuits also implies the need for multilayered circuitry patterns. Also here advances have been made in recent years. Fig. 2 shows an example of a three layer printed circuit by printing alternating layers of conducting (gray) and dielectric ink (white) on top of each other. Fig. 2 also emphasizes one of the clear advantages of OLAE technologies: the base foil has a thickness of only 50 lm while the 5 sequentially printed layers (conductors, dielectrics) only add a maximum of 35 lm to the thickness. This results in a very flexible 3 circuit layer PCB having a thickness of less than 150 lm.
2.2. Thin film electronics platform More complex, high-end wearable and medical devices often need device (e.g. thinness, flexibility) and circuit (e.g. pitch, IO count, conductivities) performance that cannot be achieved with OLAE technologies. Therefore, a technology platform is being developed which allows realizing complex systems or subsystems. So, not only standalone patches can be realized, but also subparts of patches, which then can be combined with OLAE, taking advantage of the complementary nature of both approaches. The thin film electronics (TFE) platform is enabled by combining spin-on polyimide films with thin-film metallization. Polyimide films have been used as dielectric and passivation layers
Fig. 2. Overview image of a 3 layer printed circuitry patterns on a 50 lm PET foil. Circuitry is printed with screen printing technology using Ag-filled conductive ink (gray) and dielectric ink (white).
in the microelectronics industry for many years, as they exhibit exceptional electrical, thermal and mechanical properties. Thin-film metallization offers the obvious advantage that fine-pitch metallization schemes can be realized, ensuring compatibility with interconnection pitches of conventional, commercially available dies. Vacuum deposition processes, such as sputtering or evaporation, are thus preferred techniques to metallize the polyimide films. In addition, the deposition processes allow for use of a variety of metals: copper is generally used because of its electrical conductivity, but also biocompatible alternatives such as gold or even platinum can be considered. The process starts from a glass carrier substrate onto which a polyimide layer is spin coated and cured, resulting in a stress-free polyimide film with a typical thickness in the range of 5–10 lm. Next, thin-film copper metallization (1 lm thickness) is sputter deposited, preceded by an adhesion enhancement layer composed of titanium tungsten, deposited in the same vacuum cycle. The metallization is then patterned via a sequence of photolithography and wet etching to realize the interconnects, and covered by depositing another polyimide film. Electrical contacts are provided by selectively removing the covering polyimide film on top of specific metal pads. This is done by laser ablation, using a nanosecond KrF excimer laser (248 nm). By repeating this procedure, i.e. the sequence of metal deposition, polyimide coating and via ablation, multilayer circuitry can be enabled. To release the devices from its carrier, a laser release methodology is used to ablate a sub-micron layer of polyimide at the glass/polyimide interface. This ablation procedure can be adapted according to the final device geometry. 2.3. Thin chip integration Most medical and wearable devices need some form of intelligence. For example microcontrollers (to process sensor data) or radio chips (to communicate this data with the outside world). Integrated circuits are normally supplied in packaged format. Using these bulky and rigid packages has a big impact on the thickness and flexibility advantages of the base foils. At thicknesses below 25 lm, it is known that silicon becomes flexible [2]. It would thus be preferred to integrate the needed silicon functionality as bare die and in a thinned down form. Several approaches exist to achieve thin silicon chips. Grinding and polishing is one of the more common routes but for example the ChipFilm technology of IMS Chips [2] results in inherently thin silicon chips. The integration of these unpackaged, thinned dies is done using either of two distinct approaches. The first approach, used in combination with the TFE platform, the ultra-thin chips are embedded with the active side facing upwards (face-up) in between two sequentially deposited spin-on polyimide layers. Although height irregularities in the order of the thickness of the die are introduced, electrical connectivity can be provided using the described processing sequence. Fig. 3 shows a typical example of this approach, in which an interconnection test die is provided with fan-out metallization. In the second approach, the active components are assembled face-down on the plastic foil. Both conductive adhesives and solders can be used to establish the interconnects. In particular the integration of silicon chips with OLAE foils is a challenge because of the low thermal stability of the polyesters. PEN has a glass transition temperature (Tg) of 130 °C and PET of 85 °C while poly(imide) has a Tg of 350 °C [3]. This limitation in thermal stability excludes many well- established processes for making electronic products, as for example soldering. The integration of silicon chips with OLAE foils is most commonly done using a flip chip process based on conductive
Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024
3
J. van den Brand et al. / Solid-State Electronics xxx (2015) xxx–xxx
10 # opens circuitry-bump resistance (Ohm)
0,10
8
0,08 6 0,06 4 0,04
# opens
bump-circuitry resistance (4p, Ohm)
accelerated humidity testing (60 C/90%RH) 0,12
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adhesives. Specifically work was performed on achieving a reliable and robust chip-foil interconnect with low temperature cure anisotropic conductive adhesive (ACA) [4,5]. Fig. 5 shows typical reliability results of a 25 lm thick silicon chip with a pitch of 250 lm attached to a PET foil using an anisotropic conductive adhesive. It can be seen that the low chip thickness results in excellent thermal shock behavior, while the accelerated humidity shows some failure due to hygroscopic swelling of the adhesive. Current activities involve achieving even finer pitches for even thinner chips. Instead of ACA’s, high resolution printed isotropic conductive adhesives are being used for OLAE-based applications. In parallel, alternative technologies are being pursued to integrate thin chips [6,7]. Considerable work has been performed to better understand the mechanical and functional stability of such thin chips [8] both for the OLAE and the ultra-thin electronics platforms. A dedicated four-point mechanical bending test setup has been built in which the bending and electrical behavior of a thin chip can be monitored in-situ. Fig. 4 shows some recent results obtained with this setup. Microcontroller chips of different thicknesses were tested for the stability of their internal clock as a function of bending radius. This result shows that not only the mechanical strength of the silicon needs to be taken into account when choosing a chip thickness to achieve a given bending radius, but also the effect of the generated stress on the active layers inside the chip.
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2.4. Stretchable and conformable electronics Flexible electronics can meet to a large extent the irregularities of the human body. For increased user comfort, it would however be preferred that the electronics device can be deformed in more than one direction simultaneously. The device would then need to be conformable or stretchable. In the past years, a technology has been developed which enables the realization of stretchable systems from flexible foils [9]. The basic principle is shown in Fig. 5. The electronic functionality is distributed onto islands that are interconnected to each other by meander-shaped interconnects. The whole system is embedded into a stretchable rubber to keep everything together and to protect the device. An advantage of this technology over competing stretchable electronics technologies [10] is that it uses standard flex foil manufacturing technologies. The process starts from a normal full area flex foil (polyester, polyimide or other plastic foil materials). Circuitry is made with standard printed circuit board technologies (e.g. printing or lithography). Components are also assembled using traditional interconnect technologies (adhesives, soldering). Once finished, the meanders are structured into the foil using for example a laser or die cutting. Finally, the structured foil is embedded inside a rubber material by lamination of a polyurethane or overmoulding with silicone. Overall good stretchability and reliability can be achieved with the technology. Fig. 6 shows an overview of typical stabilities for different foil types embedded in silicone (Dow Corning, Sylgard 186). Unsupported meanders can be stretched almost a thousand
Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024
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Fig. 6. Schematic principle of meander-based stretchable electronics technology.
times at 10% elongation. A supporting layer of PET or PEN increases that tenfold, while a 50 lm PI cladding can survive over 100.000 cycles at 10% elongation or more than 1000 cycles at 50% elongation. Much effort was spent on the proper design of the meanders [11]. An aim of the design and modeling activities was to minimize stress and strain concentrations in the meander under deformation. Therefore, the stress has to be distributed as much as possible along the meander. The current meander shape of choice is the horseshoe shape. For minimal stress, the width (W) of the track should be as small as possible. This minimum W is determined mainly by technological constraints. In case of conventional PCB manufacturing, where the conductors are patterned by lithography and wet etching of a 17 or 35 lm thick Cu sheet, conductor tracks with W = 100 lm provide circuits with sufficient process yield. The meander radius (R) and angle (h) are design parameters. The values chosen for these parameters will depend on the application requirements (e.g., the maximum desired stretchability of the circuit). A further improvement of the mechanical reliability of the meanders is achieved by supplying the meanders with a flexible support (polyimide, PEN, PET, etc.). Numerical modeling showed that the width is the main parameter, influencing (in fact reducing, compared to non-supported meanders) the maximum plastic strain in the Cu meander [12,13]. This can be explained by the redistribution of the plastic strain in the metal. Fig. 7 gives an overview of the typical stretch reliability that can be achieved.
Fig. 8. Phototherapy device realized using meander-based stretchable electronics technology. Work performed in the framework of the FP7 EU Project PlaceIT.
Fig. 9. Flexible sensor label based on OLAE technologies. Circuitry has been printed and on top of this bare die ultrathin (25 lm thick) silicon chips were assembled together with peripheral 01005 sized components.
3. Application examples As an illustration of the advancements in the technology, some application example prototypes are shown in Figs. 8–12. Fig. 8 until 10 concern devices built on the OLAE platform while Figs. 11 and 12 concern devices built on the TFE platform. Fig. 8 shows a wrist-based phototherapy device. Heavy use was made of the stretchable electronics technology to ensure the electronics follows the shape of the hand. The device is essentially a Fig. 10. Wearable low cost energy expenditure patch. Based on OLAE technologies. Circuitry has been printed with screen printing. On top of this, multiple chips were assembled using conductive adhesives.
Fig. 7. Graph showing the reliability of the meander technology for different substrate types.
‘light-engine’ consisting of 54, blue LEDs spread over an active of around 150 cm2 and fully encapsulated in a silicone (PDMS, Dow Corning MS1003 on top, Xiameter RTV- 4230-E, bottom). LEDs were connected in parallel by a stretchable meander. The complete engine fits into a prototyped textile wrap to enable easy fixation on the hand. Reliable functionality under significant stretching deformation has been confirmed for the device. Around 1% of relative efficiency loss was obtained after 120,000 cycles of tensile elongation where interconnects were strained by 9.3% and 8300 cycles at 18.6% interconnect strain. This proves that the meanders allow low
Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024
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Fig. 11. Lightweight, conformable system consisting of a 4 4 microelectrode array and ASIC for multi-channel brain recording and stimulation.
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The device consists of a 4 4 microelectrode array (shown in the inset), which is routed toward a 30 lm thick custom designed ASIC. The ASIC enables multi-channel brain recording and stimulation. As a final example, Fig. 12 shows a first proof-of-concept of a stretchable LED-based microdisplay. The stretchability of the display is meant to give it a better mechanical robustness than would be possible with a flexible display. This is for example relevant in wearable applications like garments. The TFE-based base foil consisted of 0.5 mm sized ‘LED islands’ which are interconnected to each other by 0.5 mm sized meanders for making the display stretchable. A multitude of bare die LEDs (CREE, size 200 200 50 lm) were flip chip assembled onto the base foil using conductive adhesives. Thin sheets of thermoplastic polyurethane (50 lm) were laminated on both sides to encapsulate and protect the device. Initial stretchability tests show that the device can be stretched up to 10% without damage. 4. Summary and conclusion An overview has been given of the current technology developments and their status at our institutes, for the application of wearable health and medical devices. In the past years, several new technologies have been developed at our institutes that can enable a next generation of devices with an improved form factor as compared to the devices currently on the market. Moreover, these technologies have reached a maturity state at which their advantages can be investigated by building actual devices with them. Some examples of this have been shown in the current paper. Future work will continue to focus on building devices and further optimization of the underlying technologies.
Fig. 12. Stretchable LED microdisplay. Consisting of bare die LEDs assembled on a TFE electronics foil.
and stable resistance throughout device fatigue life. Possible improvements could be made by cleaner definition of the PDMS boundaries in order to avoid tearing at higher extensions. Fig. 9 shows an example of a thin (<150 lm) multi-purpose RFID-based flexible sensor patch. The patch is for be attachment to the body and can monitor environmental parameters like humidity and gas levels. Two circuit layers, the RFID antenna and the humidity sensor were successfully and reproducibly realized with screen-printing on a polyester foil. Ultrathin chips (microcontroller, radio chip, 25 lm thickness) and peripheral 01005 sized components were successfully integrated using conductive adhesives. Another sensor patch is shown in Fig. 10. This device is more advanced in functionality but less advanced in form factor. The patch has integrated Bluetooth low energy functionality, accelerometer, temperature sensor and humidity sensor. It consists of an OLAE base foil having three circuitry layer screen printed pattern. Directly on top of this, the various chips were assembled as packaged components using conductive adhesives. The final device was encapsulated using a silicone to protect it against moisture. Fig. 11 shows an example of a more complex healthcare related device, realized using the TFE platform. The developments are part of the Flemish government supported project BrainstaR. The project focuses on the development of a fully implantable microsystem for brain stimulation and recording in small rodents. Its compact, lightweight and conformable nature makes the system ideally suited for use with small rodents, such as rats.
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Please cite this article in press as: van den Brand J et al. Flexible and stretchable electronics for wearable health devices. Solid State Electron (2015), http:// dx.doi.org/10.1016/j.sse.2015.05.024