Technical Field

The invention relates to a method for fabri- eating a flexible electronic membrane and to a flexible electronic membrane according to the preambles of the in ^¬ dependent claims. The term "flexible" is to be understood as mechanically flexible, in particular bendable and/or stretchable .

Background Art

Thin-film transistors and other thin-film electronic devices have attracted considerable attention since they enable cost effective and large scale elec ^¬ tronic device manufacturing. Fabricating thin-film electronic circuits on flexible plastic foils results in me ^¬ chanically flexible electronic devices. Such flexible electronic devices can, for example, be used in rollable displays, in rollable keyboards, as conformable sensors, flexible plastic solar cells, and flexible batteries. They promise to have a strong impact on electronics in everyday life, presumably establishing new application areas such as biotechnology, energy scavenging and nowa- days unconventional fields of electronics use (see e.g., Wong, W.S. & Salleo, A., "Flexible electronics: materials and applications", publisher: Springer, 2009; Thompson, M.J., "Thin film transistors for large area electronics", Journal of Vacuum Science & Technology B: Microelectron- ics and Nanometer Structures, vol. 2, 827-834, 1984; Rog ^¬ ers, J. A., Someya, T. & Huang, Y., "Materials and mechan ^¬ ics for stretchable electronics", Science, vol. 327, 1603-1607, 2010; Hwang, S.-W. et al . , "A physically transient form of silicon electronics", Science, vol. 337, 1640-1644, 2012; Crawford, G., "Flexible flat panel dis ^¬ plays", Wiley, 2005; Krebs, F.C., Gevorgyan, S.A. & Al- strup, J., "A roll-to-roll process to flexible polymer solar cells: model studies, manufacture and operational stability studies", Journal of Material Chemistry, vol. 19, 5442-5451, 2009; Rogers, J. A. et al . , "Paper-like electronic displays: Large-area rubber-stamped plastic sheets of electronics and microencapsulated electropho- retic inks", Proceedings of the National Academy of Sci ^¬ ences, vol. 98, 4835-4840, 2001).

While effective commercialization of such flexible electronic devices has been prevented mainly by cost and performance constraints, flexible electronic de ^¬ vices have been implemented in some specific application areas where their advantages of flexibility, biocompati- bility, conformability and light weight counterbalance the aforementioned drawbacks (Someya, T. et al . , "A large-area, flexible pressure sensor matrix with organic field-effect transistors for artificial skin applica ^¬ tions", Proceedings of the National Academy of Sciences of the United States of America, vol. 101, 9966-9970, 2004; Hwang, S.-W. et al . , "A physically transient form of silicon electronics", Science, vol. 337, 1640-1644, 2012). Methods for fabricating such flexible electronic devices have thus received some attention ( Sirringhaus , H. et al . , "High-resolution ink et printing of all- polymer transistor circuits", Science, vol. 290, 2123- 2126, 2000; Forrest, S.R., "The path to ubiquitous and low-cost organic electronic appliances on plastic", Na ^¬ ture, vol. 428, 911-918, 2004; Munzenrieder , N., Petti, L., Zysset, C, Salvatore, G.A., Kinkeldei, T., Perumal, C, Carta, C, Ellinger, F., & Troster, G., "Flexible a- IGZO TFT amplifier fabricated on a free standing polyi- mide foil operating at 1.2 MHz while bent to a radius of 5mm", 2012 IEEE International Electron Devices Meeting ( IEDM) , Proceedings, pp. 5.2.1-5.2.4, 2012).

A flexible electronic device including an electronic circuit can, for example, be obtained by di- rect fabrication on a plastic foil (Munzenrieder , N., Zysset, C, Kinkeldei, T., & Troster, G., "Design Rules for IGZO Logic Gates on Plastic Foil Enabling Operation at Bending Radii of 3.5 mm", IEEE Transactions on Elec ^¬ tron Devices, vol. 59, 2153-2159, 2012), by peeling off a polymer layer spin-coated on a rigid substrate, the poly ^¬ mer layer comprising the electronic circuit (Takei, K. et al . , "Nanowire active-matrix circuitry for low-voltage macroscale artificial skin", Nature materials, vol. 9, 821-826, 2010; Tsamados, D. et al . , "Double-gate penta- cene thin-film transistor with improved control in sub ^¬ threshold region", Solid-State Electronics, vol. 54, 1003-1009, 2010), or by spalling the thin top layer from a crystalline silicon wafer after fabrication, the top layer comprising the electronic circuit (Shahrjerdi, D. & Bedell, S.W., "Extremely Flexible Nanoscale Ultrathin

Body Silicon Integrated Circuits on Plastic", Nano let ^¬ ters, vol. 13, 315-320, doi : 10.1021/nl304310x, 2012).

For direct fabrication on a plastic foil printing technologies can be employed and large-area man- ufacturing achieved, but the approach suffers from me ^¬ chanical instabilities of the plastic substrates/foils and hence from limited feature resolution ( Sirringhaus , H. et al . , "High-resolution ink et printing of all- polymer transistor circuits", Science, vol. 290, 2123- 2126, 2000) .

Peeling off a polymer layer from a rigid substrate and spalling the thin top layer from a crystalline silicon wafer both suffer from the relatively high strains that need to be applied during the separation process/step (i.e., the peeling off or the spalling, re ^¬ spectively) and/or the limited mechanical flexibility of the resulting electronic devices. While the spalling ap- proach may be promising in terms of costs, performance and achievable system complexity, limited bendability of the silicon wafer/foil, difficulties in controlling final thickness and process reliability are still unresolved issues.

Direct fabrication on plastic foils and peel- ing-off a spin coated polymer layer from a rigid sub ^¬ strate, and also the spalling approach typically result in minimum bending radii in the order of millimetres, bending radii being limited by the strain-induced damage to the electrically active layers. Induced strain during bending is the main cause of failure in the resulting electronic circuits (Gleskova, H., Wagner, S., & Suo, Z., "Failure resistance of amorphous silicon transistors un- der extreme in-plane strain", Applied Physics Letters, vol . 75, 3011-3013, 1999) .

Smaller bending radii can be achieved either by using materials with appropriate intrinsic mechanical capabilities (Yi, H.T., Payne, M.M., Anthony, J.E., & Podzorov, V., "Ultra-flexible solution-processed organic field-effect transistors", Nature Communications, vol. 3, article no. 1259, 2012; Liu, X. et al . , "Rational Design of Amorphous Indium Zinc Oxide/Carbon Nanotube Hybrid Film for Unique Performance Transistors", Nano letters, vol. 12, 3596-3601, 2012; Wang, C. et al . , "Extremely bendable, high-performance integrated circuits using sem ^¬ iconducting carbon nanotube networks for digital, analog, and radio-frequency applications", Nano letters, vol. 12, 1527-1533, 2012) or by encapsulating the electronic cir- cuit(s) placed on the zero-strain plane of the device.

Organic transistors and organic electronic circuits still operating when bent/folded into a radius of 100 μιη (micrometres) have been realized by sandwiching the active electronic layer comprising the transis- tor/electronic circuit between the substrate and an en ^¬ capsulation layer (Sekitani, T., Zschieschang, U., Klauk, H., & Someya, T., "Flexible organic transistors and cir- cuits with extreme bending stability", Nature materials, vol. 9, 1015-1022, 2010). However, combining small bending radii with high performance is still rather challenging. E.g., for organic electronic circuits with minimum bending radius of 100 μιη operating voltages of 2 V, field effect mobility of 0.5 cm ² /Vs and an I _on /I _0ff ratio of 10 ⁴ can be achieved, while for example with a flexible elec ^¬ tronic circuit based on semiconducting carbon nanotube networks (as described in Wang, C. et al . , "Extremely bendable, high-performance integrated circuits using sem ^¬ iconducting carbon nanotube networks for digital, analog, and radio-frequency applications", Nano letters, vol. 12, 1527-1533, 2012) with albeit higher minimum bending radius of 2.5 mm operating voltages of 5 V, field effect mo- bility of 50 cm ² /Vs and an I _on /I _0ff ratio of 10 ⁶ can be achieved .

Disclosure of the Invention It is an object of the invention to provide an alternative method for fabricating a flexible elec ^¬ tronic membrane that is easy to implement. It is a fur ^¬ ther object of the invention to provide a method for fab ^¬ ricating a highly flexible electronic membrane with rela- tively high performance. It is a still further object of the invention to provide a flexible high-performance electronic membrane that is highly bendable. A flexible electronic membrane is defined as a flexible membrane combined with electronic components, in particular an electronic circuit.

In order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, a method for fabri ^¬ cating a flexible electronic membrane is provided. Ac- cording to the method of the invention first an electronic chip is formed. For forming of the electronic chip a carrier layer is provided on top of which a layer of a soluble polymer is deposited. The carrier layer is pref ^¬ erably formed from a silicon wafer. Then, a layer of a non-soluble polymer is deposited on top of the layer of the soluble polymer, the layer of the non-soluble polymer forming/constituting a flexible membrane. The flexible membrane is in particular in that case stretchable when the layer of the soluble polymer is stretchable.

The soluble layer may, of course, consist of several soluble (sub-) layers that are preferably soluble by different solvents. The term "non-soluble" in particu ^¬ lar means not being soluble by a solvent that dissolves the layer of the soluble polymer or one its ( sub- ) layers . Preferably, the soluble polymer is water-soluble and the non-soluble polymer is hydrophobic.

After that a thin-film electronic circuit is formed on top of the flexible membrane, i.e. on top of the layer of the non-soluble polymer. Of course, several thin-film electronic circuits and other electronic compo ^¬ nents such as sensors may be formed on top of the flexi- ble membrane. See http://en.wikipedia.org/wiki/Thin-film for a definition of "thin film".

Thereafter, the such formed electronic chip is placed in solvent to dissolve the layer of the soluble polymer (or at least one of its ( sub- ) layers )) , thereby releasing the flexible membrane with the thin-film elec ^¬ tronic circuit from the carrier layer. Preferably, water is used as solvent. The released flexible membrane with the thin-film electronic circuit forms or represents, re ^¬ spectively, the flexible electronic membrane according to the invention.

The flexible electronic membrane may then be transferred onto a destination substrate, thereby forming an electronic device according to the invention. The des ^¬ tination substrate is preferably flexible, for example a flexible foil, such that the resulting electronic device is flexible. The flexible electronic membrane fabricated with the method of the invention comprises (preferably: consists of) the flexible membrane (i.e., the layer of the non-soluble polymer) and the thin-film electronic circuit placed on top of the flexible membrane.

Polyvinyl alcohol (PVA) is preferably used as soluble polymer that is preferentially deposited on top of the carrier layer by means of spin coating. PVA is water-soluble .

Parylene is preferably used as non-soluble polymer that is deposited on top of the layer of the sol ^¬ uble polymer preferentially by means of thermal evapora ^¬ tion. Parylene is hydrophobic. The layer of the non- soluble polymer has preferably a thickness of not more than 1 μιη (micrometer) .

Preferentially, the thin-film electronic cir ^¬ cuit is a thin-film transistor (TFT; see e.g.,

http : / /en . wikipedia .org/wiki/Thin-film_transistor ) , in particular a bottom gate thin-film transistor. The thin- film transistor preferably comprises a gate contact, a gate isolator (layer), a semiconductor layer, a drain contact and a source contact. The gate isolator is pref ^¬ erably realized by a layer of aluminium oxide (AI ₂ O3) . The semiconductor layer is preferably realized by a layer of amorphous indium-gallium-zinc-oxide (IGZO).

Transparent materials may be chosen for the flexible membrane and preferentially also for the thin- film electronic circuit, for example for ophthalmic ap ^¬ plications .

With the method of the invention a flexible electronic membrane can be fabricated that is ultra- flexible, light in weight, highly conformable, and fully transparent if desired, while demonstrating proper func ^¬ tionality even when bent to extremely small radii such as 50 μιη (micrometers), and in particular also when folded, curled and/or crumpled by hands or other means. With the flexible electronic membrane and the electronic device according to the invention strain induced into its thin- film electronic circuit can be minimized. The flexible electronic membrane of the invention can be applied to almost any arbitrarily shaped surface and meets require- ments of applications hardly addressable by previously known approaches.

The methods of the invention for fabricating a flexible electronic membrane and an electronic device with such a flexible electronic membrane are optimized regarding performance of the flexible electronic mem ^¬ brane/the electronic device, low temperature fabrication, thicknesses of possibly employed brittle material, and adhesion between different (material) layers to achieve long term reliability and high bendability.

Brief Description of the Drawings

Further advantageous features and applica ^¬ tions of the invention can be found in the dependent claims, as well as in the following description of the drawings illustrating the invention. In the drawings like reference signs designate the same or similar

parts/components throughout the several figures of which: Fig. 1 shows an electronic chip as produced during the method of the invention,

Fig. 2 shows schematic drawings (Figs. 2a),

2b) , 2c) ) illustrating the method of the invention for fabricating a flexible electronic membrane,

Fig. 3 shows a schematic drawing of a bent flexible electronic membrane according to the invention, and

Fig. 4 shows a schematic drawing of a flexi ^¬ ble electronic device according to the invention in form of an electronic contact lens.

Mode(s) for Carrying out the Invention Figures 1 and 2 illustrate the method of the invention. First an electronic chip 1 as depicted in Fig ^¬ ure 1 is formed as follows: A carrier layer 2 is formed from a - preferably p-doped - silicon wafer (not shown) with a preferred width of 4 inches. The carrier layer 2 may be a dye cut with a cross-sectional area of for exam ^¬ ple 2 x 2 cm ² . Of course, the cross-sectional area may be of different size.

A layer of a soluble polymer 3, in particular a layer of polyvinyl alcohol (PVA) 3, is deposited on the carrier layer 2 by means of spin coating, wherein the spin coating is preferably performed at 4000 rpm for about 60 seconds. Subsequently the layer of PVA 3 and the carrier layer 2 are preferably baked, preferentially at 100°C for about 120 seconds. The thickness of the layer of the soluble polymer 3 is in particular 400 nm.

After that a layer of a non-soluble polymer 4, in particular a layer of parylene, is deposited on top of the layer of the soluble polymer 3 through thermal evaporation, the layer of the non-soluble polymer 4 preferably having a thickness of 1 μιη (micrometer) , preferentially exhibiting an average roughness of about 6 nm in case of parylene. The layer of the non-soluble polymer 4 forms a flexible membrane. Thus, a thin and ultra- flexible membrane 4 is formed that is very light. The flexible membrane 4 is preferably transparent which can be achieved by using parylene.

On top of the flexible membrane 4 a thin-film electronic circuit 5, in particular a thin-film transis ^¬ tor is formed. The thin-film transistor 5 preferably comprises a gate contact 6, a gate isolator 7, a semiconduc ^¬ tor layer 8, a source contact 9 and a drain contact 10. The thin-film transistor 5 or at least some of its layers 6-10 are preferably transparent in particular for oph ^¬ thalmic applications, the same applying to any other electronic circuit or electronic component formed on top of the flexible membrane 4.

The gate contact 6 is preferably formed as bottom gate contact, preferentially by depositing chromi- urn (CR) onto the flexible membrane 4, in particular by e- beam evaporation. The thickness of the gate contact 6 is in particular 35 nm. In case the gate contact 6 shall be transparent as for the contact lens application described below, the gate contact 6 is preferably formed from indi- urn tin oxide (ITO), in particular by sputtering ITO onto the flexible membrane 4. ITO may be sputtered by room temperature. The thickness of the ITO gate contact can for example be 100 nm.

The gate isolator 7 is preferably given by a layer of aluminium oxide (AI ₂ O3) 7 being deposited on top of the gate contact 6 and the flexible membrane 4 (inso ^¬ far as the flexible membrane 4 is not covered by the gate contact 6) . The thickness of the gate isolator 7 is in particular 25 nm. The dielectric constant of the layer of aluminium oxide 7 is 9.5. The layer of aluminium oxide 7 is preferably deposited by means of atomic layer deposi ^¬ tion (ALD) at a temperature of 150°C which preferentially is the highest temperature encountered during performing of the method of the invention. Using only temperatures not exceeding 150 °C has the advantage that damaging of the layer of polyvinyl alcohol 3 can be avoided.

The semiconductor layer 8 is formed on top of the gate isolator 7, in particular by sputtering and afterwards wet etching of the used semiconductor material. Preferably amorphous indium-gallium-zinc-oxide (IGZO) is used as semiconductor material. I.e., the semiconductor layer 8 is preferably given by an amorphous indium- gallium-zinc-oxide (IGZO) layer 8. The thickness of the semiconductor layer 8 is in particular 15 nm. Etching may be performed with a HC1 (hydrochloric acid) solution.

A source contact 9 and a drain contact 10 are formed on top of the semiconductor layer 8 by depositing conducting layers, in particular consisting of titanium (TI) and/or gold (AU) . In case contacts 9, 10 consisting of titanium and gold are used, the/each titanium layer has in particular a thickness of 10 nm, whereas the/each gold layer in particular has a thickness of 60 nm. Ace ^¬ tone lift off may be used for structuring the source con ^¬ tact 9 and the drain contact 10. In case transparent source 9 and drain contacts 10 are required, as for the below-mentioned contact lens application, the source con- tact 9 and the drain contact 10 may be formed from indium tin oxide (ITO), in particular by sputtering ITO onto the semiconductor layer 8. ITO may be sputtered by room temperature. The thickness of the ITO source and drain con ^¬ tacts can for example be 100 nm.

For structuring the layers 6-10 of the thin- film transistor 5 standard UV lithography may be used. An overall thickness of the thin-film transistor 5 of only 145 nm can be achieved. Employing a carrier layer 2 made of silicon, which is rather rigid, and layers of PVA 3 and parylene 4, which are chemically stable, ensures that the method of the invention advantageously allows use of techniques such as acetone lift-off and wet etching.

Moreover, using the rigid carrier layer 2 on which the flexible membrane 4 is formed (separated by the layer of the soluble polymer 3) instead of forming the flexible membrane 4 directed on a plastic foil circumvents inher ^¬ ent mechanical instabilities of the plastic foil, which may hamper the fabrication process. Such mechanical in ^¬ stabilities may, for example, be expansion during fabri- cation due to temperature gradients, which would limit achievable resolution.

The electronic chip 1 is then placed into/put in the solvent 11 as shown in Figure 2a) (for example in a beaker) , the solvent 11 surrounding at least the layer of the soluble polymer 3. Preferably water is used as solvent 11. Consequently, the soluble polymer, in partic ^¬ ular given by PVA, dissolves and the flexible membrane 4 with the thin-film electronic circuit 5 on top of it is released (Figure 2b) ) . The layer of the soluble polymer 3 is therefore also called sacrificial layer. Dissolving of the layer of PVA 3 may take about 10 minutes.

After the layer of the soluble polymer 3 has dissolved, the flexible membrane 4 with the thin-film electronic circuit 5 remains floating in/on the solvent 11 (Figure 2c)), while the carrier layer 2 sinks. The flexible membrane 4 with the thin-film electronic circuit 5 on top represents the flexible electronic membrane 12 of the invention. This flexible electronic membrane 12 may then be transferred onto a destination substrate 13 to form a flexible electronic device 20, 21 according to the invention (see Figures 5 for a contact lens example) .

The layer of parylene 4 that is preferably used as flexible membrane is transparent, exhibits ex ^¬ treme flexibility and conformability and good adhesion properties which enable the transfer to almost any arbi ^¬ trarily shaped surface acting as destination substrate 13. It is noted that human skin may also act as destina ^¬ tion substrate.

For transferring the flexible membrane 4 with the thin-film electronic circuit 5 can be fished out of the solvent 11 either directly by the flexible destina- tion substrate 13 or by means of a fishing tool that transfers it to the flexible destination substrate 13.

The destination substrate 13 may be a flexi ^¬ ble foil, in particular a polyimide foil (e.g. a Kapton foil) with a thickness of, e.g., 50 μιη (micrometres), that is dipped into the solvent 11, moved beneath the flexible electronic membrane 12 and is then used to lift the flexible electronic membrane 12 out of the solvent 11. In such manner curling of the flexible electronic membrane 12 can be avoided. After transferring of the flexible electronic membrane 12 onto the destination sub ^¬ strate 13, in particular the flexible foil, the flexible electronic membrane 12 and the destination substrate/foil 13 are preferably baked for, e.g., approximately 10 minutes at about 70 °C temperature to evaporate any re ^¬ maining solvent /water . Baking improves adhesion of the flexible electronic membrane 12 to the destination sub- strate 13, which is preferably a polyimide foil, making subsequent release of the flexible electronic membrane 12 from the destination substrate 13 practically impossible.

Preferably before placement into the solvent 11 the electronic chip 1 is tested regarding its electri- cal properties. Preferentially, the electronic chip 1 has the following electrical properties: I _{0f f} as low as 50 fA/μιη with the ratio I ₀ ff/I ₀ n ratio being larger than 107 for V _DS (drain to source voltage) = V _G s (gate to source voltage) = 5 V, a threshold value of approximately 2.7 V, a subthreshold swing of 245 mV/dec, a transconductance of approximately 0.7 μΞ/μιη, a field effect mobility of ap ^¬ proximately 21 cm ² /Vs, and an output resistance of ap ^¬ proximately 625 kQ for V _DS = V _G s = 5 V.

Furthermore, the electrical properties of flexible electronic device 20 (and/or the flexible elec ^¬ tronic membrane 12) are preferably tested after fabrica ^¬ tion. Using a polyimide foil as destination substrate 13 may result in a decrease of the gate leakage current due to the insulating properties of the polyimide foil. Fur- thermore, an average decrease of the output resistance by a factor of approximately 6, a shift of the threshold voltage by approximately -0.3 V, and/or an increase of the field effect mobility by approximately 1 cm ² /Vs, which is mirrored by increased transconductance and in- creased I _on / I off ratio may be observed. Shift of threshold voltage, the change in field effect mobility and a modu ^¬ lation of the hysteresis in the drain current I _D - gate to source voltage V _G s curve is presumably due to absorp ^¬ tion of the solvent /water 11 (Park, J.-S., Jeong, J.K., Chung, H.-J., Mo., Y.-G., & Kim, H.D., "Electronic transport properties of amorphous indium-gallium-zinc- oxide semiconductor upon exposure to water", Applied Physics Letters, vol. 92, 072104, 2008).

Bending the electronic device 20 of the in ^¬ vention for example around a rod with radius of 5 mm can result in a tensile strain of about 0.5 % parallel to a channel formed between the source contact 9 and the drain contact 10 of the thin-film transistor 5. The tensile strain is presumably caused by the destination substrate

13 (in particular the polyimide foil) . However, the thin- film transistor 5 stays fully functional (see Cherenack,

K.H., Munzenrieder , N.S, & Troster, G., "Impact of mechanical bending on ZnO and IGZO thin-film transistors", Electron Device Letters, IEEE, vol. 31, 1254-1256, 2010, for typical behaviour of IGZO thin-film transistors under tensile strain; see Munzenrieder, N., Cherenack, K.H., & Troster, G. "The effects of mechanical bending and illu ^¬ mination on the performance of flexible IGZO TFTs", IEEE Transactions on Electron Devices, vol. 58, 2041-2048, 2011, on the much inferior performance of thin-film tran- sistors directly formed on a 50 μιη thick polyimide foil) .

For the flexible electronic membrane 12 of the invention with parylene used as non-soluble polymer a tensile strain of only 0.01 % may be achievable when bent to a radius of 5 mm. The assumed limit for full function- ing of the thin-film transistor 5 may be reached first at the much smaller bending radius of 50 μιη (micrometres; Gleskova, H., Wagner, S., & Suo, Z., "Failure resistance of amorphous silicon transistors under extreme in-plane strain", Applied Physics Letters, vol. 75, 3011-3013, 1999 ) .

Transferring the flexible electronic membrane 12 of the invention onto a fragment of human hair 14 lying on a glass substrate (not shown) , the hair fragment

14 having a radius of approximately 50 μιη (as shown in Figure 3) the flexible electronic membrane 12 exhibits good conformability and wraps around the hair fragment 14. The bent thin-film transistor 5 is fully operational, showing good DC performance with a field effect mobility of about 26 cm ² /Vs, a subthreshold swing of about 90 mV/dec and a threshold voltage of about 3.4 V. The gate dielectric is still properly working and the gate leakage current remains below 10 pA for the entire operating range .

As destination substrate 13 also a polypro ^¬ pylene foil, for example with a thickness of 100 μιη (mi ^¬ crometres) may be used. For the layer of parylene 4 adhe- sion is sufficient to keep it attached to the polypropyl ^¬ ene foil, albeit adhesion being less than with a polyi- mide foil as destination substrate. However, the lower, but sufficient adhesion facilitates handling and manipu ^¬ lation of the flexible electronic membrane 12 and leads to less possible tensile strain being induced from the polypropylene foil into the flexible electronic membrane 12 and thus the thin-film electronic circuit 5.

The flexible electronic membrane 12 is pref ^¬ erably transferable onto any arbitrarily shaped sur- face/destination substrate 13. For example, the flexible electronic membrane 12 may be transferred onto a plastic contact lens 13, in particular to measure intraocular pressure for glaucoma disease monitoring. High intraocu ^¬ lar pressure is considered one of the major risk factors for glaucoma. In this case the plastic contact lens acts as flexible destination substrate 13 or forms at least part of it (see Figure 4) . A strain gauge sensor 15 for measuring the intraocular pressure is formed/provided on the flexible membrane 4 in electrical connection with the thin-film electronic circuit 5. In Figure 4 the strain gauge sensor 15 is only shown schematically. Its position may deviate from the one depicted.

Thus, an electronic device 20 in form of an electronic contact lens 21 for intraocular pressure moni- toring is provided, wherein the flexible electronic mem ^¬ brane 12 is attached to a plastic contact lens 13 acting as destination substrate and a strain gauge sensor 15 is provided (directly or indirectly) on top of the flexible membrane 4 in electrical connection with the thin-film transistor 5.

The strain gauge sensor 15 preferably com- prises a titanium-gold-stack with one or more titanium layers, each of preferentially 10 nm thickness, and one or more gold layers, each of preferentially 60 nm thick ^¬ ness, that may be formed by a combination of acetone lift-off and e-beam evaporation. The strain gauge sensor 15 preferably has a flat resistance of 300 Ω (Ohm) . Indi ^¬ um tin oxide (ITO) may be used instead of titanium and gold to create a transparent strain gauge sensor 15. A typical soft plastic contact lens 13 has a thickness of about 150 μιη (micrometres) and a bending radius of about 8 mm, while for the flexible electronic membrane 12 of the invention a total thickness of only 1145 nm can be achieved .

Nowadays typically a Goldmann tonometry is used to determine intraocular pressure (Ehlers, N., Bram- sen, T., & Sperling, S., "Applanation tonometry and central corneal thickness", Acta ophthalmologica, vol. 53, 1975; Martinez-de-la-Casa, J.M. et al . , "Effect of corne ^¬ al thickness on dynamic contour, rebound, and Goldmann tonometry", Ophthalmology, vol. 113, 2156-2162, 2006). Although Goldmann tonometry is precise and reliable, it does not allow continuous and prolonged monitoring of in ^¬ traocular pressure, which may, however, facilitate detec ^¬ tion of pressure anomalies. While it has been previously tried to integrate bulky silicon-based electronics on top of a plastic contact lens (Rosengren, L., Backlund, Y., Sjostrom, T., Hok, B., & Svedbergh, B., "A system for wireless intra-ocular pressure measurements using a sili ^¬ con micromachined sensor", Journal of Micromechanics and Microengineering, vol. 2, 202, 1992), with the present invention a fully transparent thin-film electronic cir ^¬ cuit based on one or more thin-film transistors 5 in electrical connection with a strain gauge sensor 15 can by arranged via a transparent flexible membrane 4 on top of a plastic contact lens 13. With gate contact 6, source contact 9 and drain contact 10 formed by ITO sputtering as described above, a transparent flexible electronic membrane 12 may be realized, having a total thickness of only about 1145 nm, that exhibits optical transmission losses of only 20 % in the visible spectrum at maximum.

Further applications of the flexible elec ^¬ tronic device 20 and/or the flexible electronic membrane 13 according to the invention lie for example in the are ^¬ as of solar cells, in particular ultra-light solar cells, implantable devices, electronic textiles, in particular smart-skin electronic textiles due to superior conforma- bility and adhesion properties of the flexible electronic membrane 12.