WO2014202678A1 - Method for fabricating a flexible electronic membrane and flexible electronic membrane fabricated by such method - Google Patents
Method for fabricating a flexible electronic membrane and flexible electronic membrane fabricated by such method Download PDFInfo
- Publication number
- WO2014202678A1 WO2014202678A1 PCT/EP2014/062845 EP2014062845W WO2014202678A1 WO 2014202678 A1 WO2014202678 A1 WO 2014202678A1 EP 2014062845 W EP2014062845 W EP 2014062845W WO 2014202678 A1 WO2014202678 A1 WO 2014202678A1
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- WO
- WIPO (PCT)
- Prior art keywords
- flexible
- membrane
- thin
- electronic
- layer
- Prior art date
Links
- 230000009975 flexible effect Effects 0.000 title claims abstract description 135
- 239000012528 membrane Substances 0.000 title claims abstract description 101
- 238000000034 method Methods 0.000 title claims abstract description 37
- 239000010409 thin film Substances 0.000 claims abstract description 59
- 229920000642 polymer Polymers 0.000 claims abstract description 46
- 239000002904 solvent Substances 0.000 claims abstract description 18
- 238000000151 deposition Methods 0.000 claims abstract description 7
- 239000000758 substrate Substances 0.000 claims description 30
- PNEYBMLMFCGWSK-UHFFFAOYSA-N Alumina Chemical compound [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims description 27
- 239000011888 foil Substances 0.000 claims description 27
- 229920003023 plastic Polymers 0.000 claims description 16
- 239000004033 plastic Substances 0.000 claims description 16
- 239000004065 semiconductor Substances 0.000 claims description 14
- 239000004372 Polyvinyl alcohol Substances 0.000 claims description 12
- 229920002451 polyvinyl alcohol Polymers 0.000 claims description 12
- 229920000052 poly(p-xylylene) Polymers 0.000 claims description 11
- XLOMVQKBTHCTTD-UHFFFAOYSA-N zinc oxide Inorganic materials [Zn]=O XLOMVQKBTHCTTD-UHFFFAOYSA-N 0.000 claims description 8
- 239000011787 zinc oxide Substances 0.000 claims description 7
- 230000004410 intraocular pressure Effects 0.000 claims description 6
- 229960001296 zinc oxide Drugs 0.000 claims description 6
- 238000012544 monitoring process Methods 0.000 claims description 4
- 238000004528 spin coating Methods 0.000 claims description 4
- 238000001704 evaporation Methods 0.000 claims description 3
- 230000008020 evaporation Effects 0.000 claims description 3
- OFIYHXOOOISSDN-UHFFFAOYSA-N tellanylidenegallium Chemical compound [Te]=[Ga] OFIYHXOOOISSDN-UHFFFAOYSA-N 0.000 claims description 3
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims 4
- 229910052804 chromium Inorganic materials 0.000 claims 4
- 239000011651 chromium Substances 0.000 claims 4
- 239000004020 conductor Substances 0.000 claims 1
- 238000005452 bending Methods 0.000 description 14
- 238000004519 manufacturing process Methods 0.000 description 10
- 239000000463 material Substances 0.000 description 10
- 239000004642 Polyimide Substances 0.000 description 8
- 230000005669 field effect Effects 0.000 description 8
- 229910052710 silicon Inorganic materials 0.000 description 8
- 239000010703 silicon Substances 0.000 description 8
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 7
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 6
- 229920001721 polyimide Polymers 0.000 description 6
- 239000010936 titanium Substances 0.000 description 6
- 229910052719 titanium Inorganic materials 0.000 description 6
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 5
- 239000010931 gold Substances 0.000 description 5
- 229910052737 gold Inorganic materials 0.000 description 5
- 238000004901 spalling Methods 0.000 description 5
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 5
- CSCPPACGZOOCGX-UHFFFAOYSA-N Acetone Chemical compound CC(C)=O CSCPPACGZOOCGX-UHFFFAOYSA-N 0.000 description 4
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 4
- 206010040844 Skin exfoliation Diseases 0.000 description 4
- 239000002041 carbon nanotube Substances 0.000 description 4
- 229910021393 carbon nanotube Inorganic materials 0.000 description 4
- 238000004544 sputter deposition Methods 0.000 description 4
- 238000013459 approach Methods 0.000 description 3
- 239000012634 fragment Substances 0.000 description 3
- 239000000976 ink Substances 0.000 description 3
- 238000007639 printing Methods 0.000 description 3
- 208000010412 Glaucoma Diseases 0.000 description 2
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 description 2
- 239000008186 active pharmaceutical agent Substances 0.000 description 2
- 229910021417 amorphous silicon Inorganic materials 0.000 description 2
- 229910021419 crystalline silicon Inorganic materials 0.000 description 2
- 230000001419 dependent effect Effects 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 229920002457 flexible plastic Polymers 0.000 description 2
- 230000002209 hydrophobic effect Effects 0.000 description 2
- 150000002500 ions Chemical class 0.000 description 2
- 239000011159 matrix material Substances 0.000 description 2
- 229920001155 polypropylene Polymers 0.000 description 2
- 239000004753 textile Substances 0.000 description 2
- XOLBLPGZBRYERU-UHFFFAOYSA-N tin dioxide Chemical compound O=[Sn]=O XOLBLPGZBRYERU-UHFFFAOYSA-N 0.000 description 2
- 229910001887 tin oxide Inorganic materials 0.000 description 2
- 238000012546 transfer Methods 0.000 description 2
- 230000001052 transient effect Effects 0.000 description 2
- 238000001039 wet etching Methods 0.000 description 2
- 206010059837 Adhesion Diseases 0.000 description 1
- 239000004743 Polypropylene Substances 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 238000009530 blood pressure measurement Methods 0.000 description 1
- 238000004891 communication Methods 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 208000037265 diseases, disorders, signs and symptoms Diseases 0.000 description 1
- 238000009510 drug design Methods 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 230000003203 everyday effect Effects 0.000 description 1
- 230000001747 exhibiting effect Effects 0.000 description 1
- 239000010408 film Substances 0.000 description 1
- 229920005570 flexible polymer Polymers 0.000 description 1
- 238000007667 floating Methods 0.000 description 1
- 239000011521 glass Substances 0.000 description 1
- AMGQUBHHOARCQH-UHFFFAOYSA-N indium;oxotin Chemical compound [In].[Sn]=O AMGQUBHHOARCQH-UHFFFAOYSA-N 0.000 description 1
- 238000001459 lithography Methods 0.000 description 1
- 230000007774 longterm Effects 0.000 description 1
- 238000004377 microelectronic Methods 0.000 description 1
- 239000002070 nanowire Substances 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- SLIUAWYAILUBJU-UHFFFAOYSA-N pentacene Chemical compound C1=CC=CC2=CC3=CC4=CC5=CC=CC=C5C=C4C=C3C=C21 SLIUAWYAILUBJU-UHFFFAOYSA-N 0.000 description 1
- 239000002985 plastic film Substances 0.000 description 1
- 229920003223 poly(pyromellitimide-1,4-diphenyl ether) Polymers 0.000 description 1
- -1 polypropylene Polymers 0.000 description 1
- 230000002035 prolonged effect Effects 0.000 description 1
- 230000002000 scavenging effect Effects 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 238000002207 thermal evaporation Methods 0.000 description 1
- 239000012780 transparent material Substances 0.000 description 1
- 238000001429 visible spectrum Methods 0.000 description 1
- YVTHLONGBIQYBO-UHFFFAOYSA-N zinc indium(3+) oxygen(2-) Chemical compound [O--].[Zn++].[In+3] YVTHLONGBIQYBO-UHFFFAOYSA-N 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
- H01L29/76—Unipolar devices, e.g. field effect transistors
- H01L29/772—Field effect transistors
- H01L29/78—Field effect transistors with field effect produced by an insulated gate
- H01L29/786—Thin film transistors, i.e. transistors with a channel being at least partly a thin film
- H01L29/78603—Thin film transistors, i.e. transistors with a channel being at least partly a thin film characterised by the insulating substrate or support
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/02—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers
- H01L27/12—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being other than a semiconductor body, e.g. an insulating body
- H01L27/1214—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being other than a semiconductor body, e.g. an insulating body comprising a plurality of TFTs formed on a non-semiconducting substrate, e.g. driving circuits for AMLCDs
- H01L27/1259—Multistep manufacturing methods
- H01L27/1262—Multistep manufacturing methods with a particular formation, treatment or coating of the substrate
- H01L27/1266—Multistep manufacturing methods with a particular formation, treatment or coating of the substrate the substrate on which the devices are formed not being the final device substrate, e.g. using a temporary substrate
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/66007—Multistep manufacturing processes
- H01L29/66075—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials
- H01L29/66227—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
- H01L29/66409—Unipolar field-effect transistors
- H01L29/66477—Unipolar field-effect transistors with an insulated gate, i.e. MISFET
- H01L29/66742—Thin film unipolar transistors
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
- H01L29/76—Unipolar devices, e.g. field effect transistors
- H01L29/772—Field effect transistors
- H01L29/78—Field effect transistors with field effect produced by an insulated gate
- H01L29/786—Thin film transistors, i.e. transistors with a channel being at least partly a thin film
- H01L29/7869—Thin film transistors, i.e. transistors with a channel being at least partly a thin film having a semiconductor body comprising an oxide semiconductor material, e.g. zinc oxide, copper aluminium oxide, cadmium stannate
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29D—PRODUCING PARTICULAR ARTICLES FROM PLASTICS OR FROM SUBSTANCES IN A PLASTIC STATE
- B29D11/00—Producing optical elements, e.g. lenses or prisms
- B29D11/0074—Production of other optical elements not provided for in B29D11/00009- B29D11/0073
- B29D11/00807—Producing lenses combined with electronics, e.g. chips
-
- G—PHYSICS
- G02—OPTICS
- G02C—SPECTACLES; SUNGLASSES OR GOGGLES INSOFAR AS THEY HAVE THE SAME FEATURES AS SPECTACLES; CONTACT LENSES
- G02C7/00—Optical parts
- G02C7/02—Lenses; Lens systems ; Methods of designing lenses
- G02C7/04—Contact lenses for the eyes
Definitions
- 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.
- flexible is to be understood as mechanically flexible, in particular bendable and/or stretchable .
- 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 .
- 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 .
- the soluble polymer is water-soluble and the non-soluble polymer is hydrophobic.
- 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.
- 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.
- 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”.
- the thin-film electronic cir ⁇ cuit is a thin-film transistor (TFT; see e.g., TFT; see e.g., TFT
- 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.
- FIG. 2 shows schematic drawings (Figs. 2a)
- Fig. 4 shows a schematic drawing of a flexi ⁇ ble electronic device according to the invention in form of an electronic contact lens.
- FIG. 1 illustrates 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 .
- the cross-sectional area may be of different size.
- the thickness of the layer of the soluble polymer 3 is in particular 400 nm.
- 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 ⁇ (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.
- 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.
- the thin-film transistor 5 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.
- 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.
- 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) .
- conducting layers in particular consisting of titanium (TI) and/or gold (AU) .
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 ⁇ (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.
- 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.
- the electrical properties of flexible electronic device 20 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.
- 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
- the thin- film transistor 5 stays fully functional (see Cherenack,
- 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 ⁇ (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 ) .
- 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 .
- a polypro ⁇ pylene foil for example with a thickness of 100 ⁇ (mi ⁇ crometres) may be used.
- 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.
- 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.
- 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.
- 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.
- 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 ⁇ (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 ⁇ (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 .
- 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).
- 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.
- 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.
- 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.
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Physics & Mathematics (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Computer Hardware Design (AREA)
- Ceramic Engineering (AREA)
- Manufacturing & Machinery (AREA)
- Thin Film Transistor (AREA)
Abstract
The invention relates to a method for fabricating a flexible electronic membrane (12) comprising forming of an electronic chip (1) by providing a carrier layer (2), depositing a layer of a soluble polymer (3) on top of the carrier layer (2), depositing a layer of a non-soluble polymer (4) on top of the layer of the soluble polymer (3), the layer of the non-soluble polymer (4) forming a flexible membrane, and forming a thin-film electronic circuit (5) on top of the flexible membrane (4), wherein the electronic chip (1) is placed in a solvent (11), the solvent (11) dissolving the layer of the soluble polymer (3), such that the flexible membrane (4) with the thin-film electronic circuit (5) is released, thereby forming the flexible electronic membrane (12). The invention further relates to a flexible electronic membrane (12) formed by such method.
Description
Method for fabricating a flexible electronic membrane and flexible electronic membrane fabricated by such method
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 cm2/Vs and an Ion/I0ff ratio of 104 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 cm2/Vs and an Ion/I0ff ratio of 106 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 (AI2O3) . 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 cm2. 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 (AI2O3) 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: I0f f as low as 50 fA/μιη with the ratio I0ff/I0n ratio being larger than 107 for VDS (drain to source voltage) = VGs (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 cm2/Vs, and an output resistance of ap¬ proximately 625 kQ for VDS = VGs = 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 cm2/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 ID - gate to source voltage VGs 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 cm2/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.
Claims
1. A method for fabricating a flexible elec¬ tronic membrane (12) comprising forming of an electronic chip (1) by
- providing a carrier layer (2),
- depositing a layer of a soluble polymer (3) on top of the carrier layer (2),
- depositing a layer of a non-soluble polymer (4) on top of the layer of the soluble polymer (3), the layer of the non-soluble polymer (4) forming a flexible membrane, and
- forming a thin-film electronic circuit (5) on top of the flexible membrane (4),
wherein the electronic chip (1) is placed in a solvent (11), the solvent (11) dissolving the layer of the soluble polymer (3), such that the flexible membrane (4) with the thin-film electronic circuit (5) is re¬ leased, thereby forming the flexible electronic membrane (12) .
2. The method according to claim 1, wherein the flexible membrane (4) and/or the thin-film electronic circuit (5) are transparent.
3. The method according to claim 1 or 2, wherein polyvinyl alcohol (PVA) is used as soluble poly¬ mer that is deposited on top of the carrier layer (2) in particular by means of spin coating.
4. The method according to one of the preced¬ ing claims, wherein parylene is used as non-soluble poly¬ mer that is deposited on top of the layer of the soluble polymer (3) in particular by means of evaporation.
5. The method according to one of the preced- ing claims, wherein the thin-film electronic circuit (5) is a thin-film transistor, in particular a bottom gate thin-film transistor.
6. The method according to claim 5, wherein for forming of the thin-film transistor (5) a gate contact (6), in particular a chromium (CR) gate contact, is formed on the flexible membrane (4) .
7. The method according to claim 6, wherein for forming of the thin-film transistor (5) further a gate isolator (7) is deposited on top of the gate contact
(6) and the flexible membrane (4), the gate isolator (7) in particular being formed by a layer of aluminium oxide (A1203) .
8. The method according to claim 7, wherein for forming of the thin-film transistor (5) further a semiconductor layer (8) is deposited on the gate isolator
(7) , the semiconductor layer (8) in particular being formed by an amorphous indium-gallium-zinc-oxide (IGZO) semiconductor layer.
9. The method according to claim 8, wherein for forming of the thin-film transistor (5) further a source contact (9) and a drain contact (10) are formed by depositing conducting layers.
10. A method for fabricating an electronic device (20; 21), comprising fabricating a flexible electronic membrane (12) by the method of one of the preced¬ ing claims, wherein the flexible electronic membrane (12) is transferred onto a destination substrate (13) to form the electronic device (20; 21) .
11. The method according to claim 10, wherein the destination substrate (13) is a flexible destination substrate, in particular a flexible foil.
12. A flexible electronic membrane fabricated according to a method of one of the claims 1 to 9, com¬ prising a thin-film electronic circuit (5) positioned on top of a layer of a non-soluble polymer (4), the layer of the non-soluble polymer (4) forming a flexible membrane.
13. The flexible electronic membrane accord¬ ing to claim 12, wherein the flexible membrane (4) and/or the thin-film electronic circuit (5) are transparent.
14. The flexible electronic membrane accord¬ ing to claim 12 or 13, wherein the non-soluble polymer (4) is parylene.
15. The flexible electronic membrane accord- ing to claim 12 or 14, wherein the thin-film electronic circuit (5) is a thin-film transistor, in particular a bottom gate thin-film transistor.
16. The flexible electronic membrane accord¬ ing to claim 15, wherein the thin-film transistor (5) comprises a gate contact (6), in particular a chromium (CR) gate contact, formed on the flexible membrane (4) .
17. The flexible electronic membrane accord¬ ing to claim 16, wherein the thin-film transistor (5) further comprises a gate isolator (7) deposited on top of the gate contact (6) and the flexible membrane (4), the gate isolator (7) in particular given by a layer of aluminium oxide (AI2O3) .
18. The flexible electronic membrane accord¬ ing to claim 17, wherein the thin-film transistor (5) further comprises a semiconductor layer (8), in particular an amorphous indium-gallium-zinc-oxide (IGZO) semi¬ conductor layer, deposited on the gate isolator (7).
19. The flexible electronic membrane accord¬ ing to claim 18, wherein the thin-film transistor (5) comprises a source contact (9) and a drain contact (10) in form of conducting layers.
20. An electronic device comprising a flexi¬ ble electronic membrane (12) according to one of the claims 12 to 19 placed on top of a destination substrate (13) .
21. The electronic device according to claim 20, wherein the destination substrate (13) is a flexible destination substrate, in particular a flexible foil.
22. The electronic device according to claim 20 or 21, wherein the destination substrate (13) compris¬ es a plastic contact lens, and wherein a strain gauge sensor (14) is arranged on the flexible membrane (4) in
electrical connection with the thin-film electronic circuit ( 5 ) .
23. Electronic contact lens for intraocular pressure monitoring, comprising a flexible electronic de- vice (20) according to claim 22.
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