Micro- and Nano-Technology...... for Optics 3.2 Lithography U.D. Zeitner Fraunhofer Institut für Angewandte Optik und Feinmechanik Jena
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)
Beam Diameter (Example) here: about 6nm beam size with proper systems 0.5nm beam size is achievable
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
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
Interaction Volume primary electrons increasing beam energy resist substrate scattering volume
Monte-Carlo Simulation of Electron Scattering electron beam resist substrate Proximity Function
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
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
Statistics of the Exposure Process PMMA 250µC/cm² 10nm desired structure without diffusion with diffusion of molecules
Statistics of the Exposure Process FEP 171 10µC/cm² 10nm desired structure without diffusion with diffusion of molecules
Statistics of the Exposure Process comparison of structures in the resist 10nm desired structure PMMA 250µC/cm² FEP 171 10µC/cm²
High resist sensitivity in EBL no more statistical independency Resist exposure dose (µc/cm²) e - /(10nm x 10nm) LER (nm) PMMA 250 1560 1-3nm ZEP 520 30 187 3nm FEP 171 9.5 59 10(6)nm Photoresists photons/(10nm x 10nm) DUV 5,000 20,000 2nm EUV 200-500?? DUV Photoresist PMMA ZEP 520 FEP
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
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
The Vistec SB350 OS e-beam writer 50keV electron column substrate loading station
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
E-Beam Lithography: Example Structures binary grating 400nm period photonic crystal 2µm effective medium grating
resist depth [nm] E-Beam Lithography: Variable Dose Exposure 0-200 -400-600 -800-1000 fit model: h = a Exp(b D) + c a = (-54.4 0.74) nm b = (0.00139 7.9E-7) cm 2 /µc c = (53 3.1) nm measured fit blazed grating -1200-1400 -1600 3µm 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 0 5 10 15 20 25 electron dose [µc/cm 2 ] diffractive element
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
diffraction efficiency [%] Expected Diffraction Efficiency (for a grating) 100 8 16 32 90 80 70 60 50 40 30 2 4 scalar theory: h sinc 2 1 N N h 2 40.5% 4 81.1% 8 95.0% 16 98.7% 32 99.7% 20 10 0 0 5 10 15 20 25 30 35 number of phase levels N
Efficiency normalized to ideal element [%] Diffraction Efficiency reduced by overlay error 100 4-level element 80 60 40 20 simulation 4-level measurement due to random alignment error 0-15 -10-5 0 5 10 15 20 25 30 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
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 -... 0 2 4 8 16 32 number of phase levels You will not get the best efficiency with the highest number of phase levels!!!! N
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
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
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?
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