Micro- and Nano-Technology... for Optics

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1 Micro- and Nano-Technology for Optics 3.2 Lithography U.D. Zeitner Fraunhofer Institut für Angewandte Optik und Feinmechanik Jena

2 Electron Beam Column electron gun beam on/of control magnetic deflection system and objective aperture detector stage positioning system x/y-stage Laser interferometer (position feedback)

3 Beam Diameter (Example) here: about 6nm beam size with proper systems 0.5nm beam size is achievable

4 Material Interaction Photons Electrons electron beam 20keV Dose 5-8µm (material dependent) scattering of electrons in the material distribution of deposited dose exponential absorption (Lambert-Beer) complex distribution

5 Electron Deceleration deceleration: numerous material dependent secondary effects: secondary electrons Auger-electrons characteristic x-ray radiation Bremsstrahlung radiation primary electrons resist substrate direction changes in statistical order

6 Interaction Volume primary electrons increasing beam energy resist substrate scattering volume

7 Monte-Carlo Simulation of Electron Scattering electron beam resist substrate Proximity Function

8 relative energy density Proximity Function log region 2: back scattered electrons L... total path length of an electron 0,5µm r L region 3: x-ray radiation and extensions of the beam region 1: primary electrons 0 r 0, 5µm radius r

9 Direct Exposure of a NaCl-Crystal exposure with high dose atoms are ionized and can be released from the crystal direct image of the beam pattern, realized by a fine electron beam on a NaCl crystal

10 Statistics of the Exposure Process PMMA 250µC/cm² 10nm desired structure without diffusion with diffusion of molecules

11 Statistics of the Exposure Process FEP µC/cm² 10nm desired structure without diffusion with diffusion of molecules

12 Statistics of the Exposure Process comparison of structures in the resist 10nm desired structure PMMA 250µC/cm² FEP µC/cm²

13 High resist sensitivity in EBL no more statistical independency Resist exposure dose (µc/cm²) e - /(10nm x 10nm) LER (nm) PMMA nm ZEP nm FEP (6)nm Photoresists photons/(10nm x 10nm) DUV 5,000 20,000 2nm EUV ?? DUV Photoresist PMMA ZEP 520 FEP

14 Roughness caused by statistic electron impact experiment (resist pattern FEP 171) schematic modeling (polymer deprotection) 400nm modeling parameters dose: 0.65 e - /nm² (10 µc/cm²) Gauss: 30 nm diffusion: 10 nm no quenching, no proximity effect

15 The Vistec SB350 OS e-beam writer basis system: SB350 OS (Optics Special), Vistec Electron Beam electron energy: 50keV max. writing field: 300mm x 300mm max. substrate thickness: 15mm resolution (direct write): <50nm number of dose levels: 128 address grid: 1nm overlay accuracy: 12nm (mask to mean) writing strategy: variable shaped beam / cell projection vector scan write-on-the-fly mode 43nm wafer resist grating 100nm period 500 nm

16 The Vistec SB350 OS e-beam writer 50keV electron column substrate loading station

17 E-beam writing strategies Gaussian beam Variable shaped beam Cell-Projection incident beam cross-section aperture angular apertures lattice aperture electron optics Gaussian spot shaped beam resolution: writing speed: >1nm low >30nm fast >30nm extreme fast

18 E-Beam Lithography: Example Structures binary grating 400nm period photonic crystal 2µm effective medium grating

19 resist depth [nm] E-Beam Lithography: Variable Dose Exposure fit model: h = a Exp(b D) + c a = ( ) nm b = ( E-7) cm 2 /µc c = (53 3.1) nm measured fit blazed grating µm ARP 610 exposure: 0.5A/cm 2, dose layer 1.0, 1.2, 1.5µC/cm 2 development:60s ARP-developer + 15s Isopropanol 20s ARP-developer + 15s Isopropanol electron dose [µc/cm 2 ] diffractive element

20 Multilevel Profile Fabrication Principle: multiple executions of a binary structuring step mask 1 mask 2 mask 3 8 level profile N masks/exposures and etching steps 2 N levels

21 diffraction efficiency [%] Expected Diffraction Efficiency (for a grating) scalar theory: h sinc 2 1 N N h % % % % % number of phase levels N

22 Efficiency normalized to ideal element [%] Diffraction Efficiency reduced by overlay error level element simulation 4-level measurement due to random alignment error misalignment normalized to pixel size [%] Alignment error in x and y normalized to pixel size [%] 90% of the design efficiency 6% misalignment allowed pixel size misalignment allowed 500nm 30nm 250nm 15nm

23 Diffraction Efficiency in Reality Diffraction efficiency expected (scalar theory) diffraction efficiency h The real diffraction efficiency depends on: - Overlay error - line width error - depth error - edge angle - design - wavelength - deflection angle - number of diffraction orders number of phase levels You will not get the best efficiency with the highest number of phase levels!!!! N

24 Resist melting technique for micro-lens fabrication resist substrate resist coating Courtesy of A. Schilling, IMT UV - light photo mask photolithography modeling of the melting development - thermal resist melting - or reflow in solvent atmosphere

25 Simplified lens design d L R h L r L focal length: f refraction index: n curvature radius of the lens: r L f ( n n ) air L d C h C h L r Ideal: diameter resist cylinder = diameter lens volume resist cylinder = volume lens L r 2 L 1 4 d 2 L resist cylinder substrate h C 1 2 h L 2 3 h d 3 L 2 L

26 NA limitation by wetting angle The rim angle R of the lens must be larger than the wetting angle W W R Typical wetting angle resist substrate ca. 25 deg dent If not: W 35 and n = 1.46 NA min 0.35 How to overcome this problem?

27 Reflow process 1) exposure resist substrate light 3) reflow solvent atmosphere pedestal 2) development 4) baking reflow technique reduces the wetting angle edge of pedestal or passivation limits the spreading Wetting angle < 1deg possible