High speed vacuum deposition of organic TFTs in a roll-to-roll facility

Transcription

1 High speed vacuum deposition of organic TFTs in a roll-to-roll facility Dr Hazel Assender University of Oxford 1

2 Prof Martin Taylor Eifion Patchett, Aled Williams Prof Long Lin Prof Steve Yeates Dr John Morrison Dr Hazel Assender Dr Gamal Abbas, Ziqian Ding DALMATIAN TECHNOLOGY

3 Manufacturing capability Source and Drain (Metal) SEMICONDUCTOR EVAPORATION L Org. Semiconductor Gate Substrate (e.g. PET) W Possible interlayer Insulator (e.g. acrylic dielectric) Possible surface modification 3

4 Roll-to-roll processing Camvac Oxford Webspeed up to 5 ms -1 Web width 35 mm vacuum web coating 4

5 Solvent vs. Vacuum deposition Solvent: No pumping Outgassing Most development recently Patterning methods relatively established Vacuum: No solvent/low energy Rapid process (PVD) Multilayers easier High performance metal and ceramic layers No requirement for orthogonal solvents/wettability 5

6 Gate insulator layer: Flash evaporated monomers then cure Roll-to-roll deposition Perhaps surface modification layer: Various options Molecular semiconductor: Evaporation Source and Drain (Metal) Gate: pattern metallization Possible interlayer L W Org. Semiconductor Gate Insulator (e.g. acrylic dielectric) Source and Drain: pattern metallization Polymer smoothing layer: Flash evaporated monomers then cure Substrate (e.g. PET) Build complete device structure on the substrate Possible surface modification Encapsulation layer/gas barrier 6

7 e.g. product tracking tag Circuit design In collaboration with Prof. Martin Taylor, University of Bangor Model circuits based on measured device performances Design circuits around transistor performance and patterning capability Minimise number of transistors Circuit design defines manufacturing priorities 7

8 Polymer deposition Flash evaporation of a monomer Condenses as a liquid on substrate Cure (e.g. e-beam) to solid High speed process Already used for capacitor technology Free of pin-holes over large area Optical Profilometry High quality PEN Acrylic smoothing layer 8

9 Materials: pattern metallisation Anilox Roller and Oil Boiler Winding zone Evaporation zone mbar Unwind Cliché Plate PRINTING RESOLUTION MD: 3-5 micron TD: 3-5 micron Process Drum Rewind Magnification x 6 Magnification x 2 { Source/Drain Electrodes------} 9

10 Getting the manufacture right: pentacene deposition Self-assembling molecules π -orbital overlap gives good carrier mobility. C C p z orbital overlap: π bond plane of sp 2 bonding (rings) Mobility Mobility Mobility ~1-8 cm 2 V -1 s -1 ~1-6 cm 2 V -1 s -1 ~1 cm 2 V -1 s -1 Ordered organic material has higher charge transport mobility IBM J. RES. & DEV. VOL. 45 NO

11 Pentacene Phases Intensity (Counts/s) (1) (1) Intensity 25 nm 8 nm (2) 2θ (degrees) (3) 25 nm (1 ) (3) (3 ) (2) (2 ) 8 nm 25 nm-thick pentacene on SiO 2 - one crystalline phase - lattice spacing of 15.4 Å 8 nm-thick pentacene on SiO 2 - two crystalline phases - lattice spacings 15.4 Å & 14.5 Å θ(degrees) The thin film phase is believed to be responsible for a high mobility in pentacene thin-films FETs 11

12 Microfocal Raman Spectroscopy Intensity (arb. unit) 1154 cm -1 : intermolecular coupling vibrations 1158 cm -1 : end group H vibrations 1178 cm -1 : central H vibrations I 1154 /I nm 8 nm 25 nm 8 nm 115 Raman shift (cm -1 ) 12 The optimal π-orbital overlap was obtained in the 25-nm pentacene film. 12

13 Deposition onto Acrylate or SiOx Intensity Pentacene on Acrylate, N 2 atm Pentacene on SiO θ (deg.) Pentacene film (9nm-thick) grown on acrylate (1.5 µm-thick) is more single phase compared with that grown onto SiO 2 (3 nm-thick). 13

14 Oxygen Plasma Treatment Intensity (arb. unit) Intensity (arb. unit) 1W-2min 2W-1min 2W-2min 1W-1min longer time theta AFM micrographs 1 W 2min 2 W 2min 1 2 2θ (deg.) 2nm 4nm 1.µm 2nm 4nm 1.µm Oxygen-plasma treatment showed a noticeable effect on the diffraction intensities of pentacene films, grown on Si, with longer treatment time. larger grain size associated with higher mobility 14

15 Effect of Background Gases Intensity (counts/s) Intensity θ (deg.) N 2 Vacuum Ar Intensity (arb. unit) 1158 cm cm cm -1 N 2 Vacuum Ar θ (deg.) Raman shift (cm -1 ) Pentacene grown in N 2 ambience best crystalline material. 15

16 Absorbance Absorbance Getting the manufacture right: Smooth, pin-hole free layer High degree of cure Wavenumber (cm -1 ) acrylate_run1 acrylate_run3 acrylate_run4 Absorbance Wavenumber (cm -1 ) insulator layer acrylate_run1 acrylate_run3 acrylate_run Wavenumber (cm -1 ) Well-cured Well-cured Incompletely cured Capacitance (nf) FTIR can determine the degree of cure 3 ma/115nm 2 ma/15nm 4 ma/25 nm ε r =2.1 ε r =3.5 ε r = Frequency (MHz) Impedance spectroscopy: Low loss Can measure dielectric coefficient 16

17 I D (µa) I D (µa) (a) (c) -1V -2V -3V -4V -5V -6V Insulator thickness -1V -2V -3V -4V I D (A) I D (A) (b) V G 96 nm (d) 425 nm V G µ=.3 cm 2 /V.sec V th =~2V I on /I off = 1.18x1 3 µ=~.1 cm 2 /V.sec V th =~1V I on /I off = 1.3x1 3 Comparison of I-V and output characteristics of bottom gate pentacene TFT on (a, b) 96 nm and (c, d) 425 nm thick TRPGDA polymeric dielectric with a 25µm channel length and an aspect ratio of (I D ) 1/2 (µa) 1/2 (I D ) 1/2 (µa) 1/2 17

18 I D (na) I D (na) Curing the acrylic -1V -2V -24V -3V -4V -44V Thermal anneal V -1V -2V -3V -4V Increase cure current I D (µa) I D (A) =-4V I on /I off =1.3x V G -1V -2V -3V -4V (I D ) 1/2 (µa) 1/2 18

19 I D (µa) I D (µa) nm nm -1V -2V -3V -4-4V -4-2 Plasma curing I-V and output characteristics of bottom gate pentacene TFT on plasma cured TRPGDA dielectric of thickness 96nm and 425nm with a 25µm channel length and an aspect ratio of 16. No interfacial modification. -1V -2V -3V -4V I D 1/2 (µa 1/2 ) I D (A) nm =-4V 1 96 nm =-6V V G 425nm =-4V 96nm =-6V V G nm 425 nm I on /I off 2.x x1 3 V th 13 5 µ (cm 2 /Vs)

20 Shelf-life stability of as-deposited FETs I D (A) 1E-5 Week 1 1E-6 1E-7 1E-8 Week 15 1E V G Plasma cured No encapsulation (I D ) 1/2 (µa) 1/ Week Week Week 1 15 I on /I off 2.x x1 2 V th 1-13 µ (cm 2 /Vs).1.7 V G

21 Summary Can make organic electronics in a R2R environment Vacuum technology uses solventless, high-speed processes Build complete devices from multilayers 21