Micro- and Nano-Technology... for Optics

Transcription

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

2 Printing on Stones Map of Munich Stone Print

3 Shadow Printing Photomask Wikipedia Mask Wafer Curtesy: R. Völkel, Suss Microoptics

4 Contact Printing light mask resist substrate

5 Mask Aligner

6 Mask Aligner

7 Mercury Emission Spectrum high pressure Hg-vapor lamp i h g e - line

8 Proximity Printing light mask proximity gap resist substrate

9 Pattern Generation by Photolithography Standard contact photolithography with a Mask Aligner: mask illumination photomask z diffraction pattern geometric shadow printing reduction of resolution with increasing z

10 The inverse microscope microscope lithography light source image object microscope lens object projection lens image light source

11 Projection Lithography light mask Resolution: R = k 1 λ NA projection optics resist R minimum feature size k 1 optics dependent factor wavelength NA numerical aperture of imaging system substrate

12 High-End Lithography Tool microelectronic chips on Si-wafers DUV lithography stepper, =193nm (ASML) EUV lithography stepper, =13.5nm (ASML) very low flexibility

13 resist thickness after development Photo Resist UV-exposure: Photoinitiator creates reactive species Chemical solubility in alkaline media changes Example: DNQ-based positive resist: Positive Resist: D 1 hard resist Dose D th x D log 10 D resist D 2 exposure dose D

14 Printing Result: Hard Resist Dose D th x Resist pattern: resist (almost) binary profile

15 resist thickness after development Gradation Curve D 1 hard resist suitable for binary pattern soft resist D log 10 D D 2 exposure dose D dose range for variable dose writing of continuous surface reliefs

16 Printing Result: Soft Resist Dose x Resist pattern: resist continuous surface profile (typically nonlinear wrt. exposure dose)

17 Technology for continuous profiles variable dose exposure: intensity modulated exposure beam proportional transfer (RIE): Ions (e.g. CF 4 ) resist substrate development: t 1 t 2 dose dependent profile depth in resist after development process element profile transferred into substrate material

18 Photolithography Examples

19 ASML-Stepper

20 Stepper Objective for DUV-Lithography aspheric lenses Zeiss SMT, WO 2003/075049

21 Double Patterning

22

23 Pre-Compensation of Diffraction Effects Optical Proximity Correction (OPC) without OPC with OPC mask layout serifs Example: image on wafer edge rounding line shortening

24 Physics of Half-Tone- and Gray-Tone-Masks filling factor: small medium high grating period or pitch > Principle of half tone masks blocking of higher orders by a lens grating period ore pitch < brightness in the wafer plane Principle of gray tone masks higher orders do not exist - Sub wavelength masks - HEBS glass masks - LDW glass masks

25 Half-Tone Lithography type of masks pulse width pulse density Also possible: - combinations - Error diffusion half tone mask Courtesy of K. Reimer, ISIT/FhG objective gray tone image

26 Holography Example: resist structure laser beam 1 laser beam 2 resist substrate lithographic exposure with an interference pattern source: Horiba Jobin Yvon

27 Holography Setup Amplitude split by beam-splitter (Ar + laser) (pinhole for spatial coherence)

28 Holography Examples single exposure two crossed exposures

29 Mask Aligner With Collimated Illumination normal incidence Suss MA6-NFH oblique incidence mercury lamp collimator polarizer interference filter cold-light mirror mask substrate special features: adjustable angle of incidence: 0deg- 55deg ( 1deg ) low divergence: 0.1deg interference filter: 313nm, 365nm, 435nm 6 7

30 Principle of Pattern Transfer Mask Resist Substrate L 0 d b -1 0 th -1 st h Parameters: Wavelength / Pitch d Angle of incidence Groove depth h Duty cycle f = b / d Two beam interference only 0 th and -1 st order wavelength 2d 3 2d Symmetric diffraction angles Littrow - mounting angle of incidence sin L 2d Equal intensities rigorous calculations duty cycle and groove depth of the mask grating

31 Experimental Results Phase mask Amplitude mask Mask 1 µm Copy 1 µm 1 µm

32 Incidence Angle p p m p p=p m /2 /2 < p < 3 /2 /2 < p < also usable for gratings with different orientations (e.g. circular gratings)

33 Laser Lithography

34 Laser Lithography Scanning Beam AOM AOD U~ profile U~ deflection angle mirror focusing lens scan width substrate motion

35 DWL 400-FF Laser Writer HIMT

36 DWL 400-FF Laser Writer basis system: Laser: max. writing field: min. spot size: autofocus system: writing mode: writing speed: DWL 400, Heidelberg Instruments =405nm (laser diode) 200mm x 200mm 1µm optical variable dose (max. 128 level) spot positioning by stage movement and beam deflection lateral scan (width up to 200µm at max. resolution) mm²/min on planar substrates (depending on structure) writing on curved substrates: substrate table: cardanic mount, tilt in two orthogonal axes min. radius of curvature: 10mm max. surface tilt angle: <10 max. sag: 30mm

37 Lithography with variable dose exposure e-beam, laser beam variable dose exposure: intensity modulated exposure beam resist substrate y writing path x substrate movement development: t 1 t 2 dose dependent profile depth after development process high flexibility for arbitrary surface profiles

38 Laserlithography Example Structures refractive lens array profile depth: 35µm diffractive lens array profile depth: 1.5µm diffractive beam shaper profile depth: 1.2µm refractive beam shaper depth: 1.7µm refractive beam shaper profile depth: 6µm

39 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)

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

41 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

42 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

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

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

45 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

46 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

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

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

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

50 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

51 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

52 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

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

54 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

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

56 E-Beam Lithography: Variable Dose Exposure resist depth [nm] fit model: h = a Exp(b D) + c a = ( ) nm b = ( E-7) cm 2 /µc c = (53 3.1) nm 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 measured fit blazed grating electron dose [µc/cm 2 ] diffractive element

57 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

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

59 Diffraction Efficiency reduced by overlay error Efficiency normalized to ideal element [%] simulation 4-level measurement 20 due to random alignment error misalignment normalized to pixel size [%] Alignment error in x and y normalized to pixel size [%] 4-level element 90% of the design efficiency 6% misalignment allowed pixel size misalignment allowed 500nm 30nm 250nm 15nm

60 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

61 Micro-Lenses in Nature Water droplets Surface tension generates small droplets with ideally spherical surface shape lens

62 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

63 Simplified lens design d L curvature radius of the lens: r L R h L focal length: refraction index: n f 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

64 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?

65 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