Nano fabrication by e-beam lithographie

Transcription

1 Introduction to nanooptics, Summer Term 2012, Abbe School of Photonics, FSU Jena, Prof. Thomas Pertsch Nano fabrication by e-beam lithographie Lecture 14 1

2 Electron Beam Lithography - EBL Introduction Motivation General EBL process Fundamentals Basics and Physics of EBL Resist Technology harging EBL Writing Strategies Interaction between E-beam and Substrate Proximity Effect Proximity Function Periodic Structures Resolution Limits in EBL Some Special Exposure Techniques ell Projection Exposure Variable Dose Exposure Mask Preparation for Lift-ff verlay Exposure

3 Electron Beam Lithography Example: Diffraction grating on 9 fused silica mask blank

4 Motivation Def Lithography: Method for printing on a smooth surfcace Photolith., UV lith., Interference lith., Scanning probe lith., X-Ray lith., EBL, Why EBL? far-field imaging technique diffraction resolution limit (Abbe) sub-100nm-features: l de Broglie ~100nm U electron ~12 ev high technological standard of electron beam control and manipulation capable to structure arbitrary (non-periodic, non-symmetric) patterns Applications of EBL mask fabrication (e.g. chromium on glass) direct writing (rapid prototyping) nano devices in research and development (R&D) Requirements of EBL Extremly complex technological background, clean room Neccessatiy of experienced staff to operate an EBL system

5 General EBL Process Resist Functional layer (optional) Substrate Exposure Development Positive resist Negative resist Subtractive method Dry Etching Additive method Deposition of functional layer... Removal of Resist Lift-ff Final Element Final Element

6 Process Development: Lift-ff 1) Sample preparation 4) Layer deposition Resist Thin T layer Silica substrate Layer stack (Au, Mg, Au) 2) E-Beam Lithography 5) overed resist structure 3) Resist development 6) Wet-chemical lift-off

7 Lift-ff: ritical Steps Achieve an undercut resist profile E-Beam (Thick) Resist & Low beam energy verexposure verdevelopment E-Beam Top-Resist (low sensitivity) Bottom-resist (high sensitivity) Limits of aspect ratio Thick Resist: Thin Resist: J lean Lift-ff, elevated Lift-Layer J igher lateral resolution L Lateral resolution diminished due L Less undercut, Lift-ff unstable to enhanced electron scattering Rule of thumb: Resist thickness ~ 2-3x thickness of layer(s) to be lifted

8 Lift-ff: ritical Steps ighly Directional Deposition Sputtering: Thermal Evaporation: Evaporation is preferable Maximize distance between deposition source and target (>0,5 m) lear / Activate Sample Surface Ar-Plasma, eating,... Flawless Mask Lift-ff Long soaking of sample in solvent (>10 hours) Support Lift-ff by elevated temperature and Mega- / Ultrasonic

9 Etching: ritical Steps Resist oating & Tempering Atomic lattices of different materials do not fit perfectly Resist coating and tempering (180 ) leads to stress induced bubbles and cracks Not suitable for EBL L Dry Etching No reactive dry etching for gold available Physical layer removal by Ion Beam Etching (IBE) Very low selectivity, thick resist mask needed, no etch stop L Resist Removal IBE on metals can lead to redepostion and metal-resist compounds These turn out to be chemically very stable and hardly soluble Pattern often spoiled by garden fences L

10 Fundamentals E-Beam Deflection and Focus electrostatic electromagnetic F = q (E + v x B) Either magnetic or electrostatic fields can be used to focus electrons just as glass lenses are used to focus rays of light. Electron ptics by an electro-magnetical lens system Analogy to Beam-ptics

11 Fundamentals E-Beam Imaging Systems Gaussian beam Variable shaped beam cross over cross over circular aperture electron optics variable angular apertures electron optics Gaussian spot shaped beam resolution: >5nm >50nm writing speed: low fast

12 Fundamentals E-Beam Imaging Systems Gaussian beam Variable shaped beam E-beam writer LIN LV1 masks max. 5 x 5 wafer max. 5 beam 2nm overlay 50nm incr. 2.5 (0.1) nm E-beam writer Vistec SB 350 masks max. 9 x 9 wafer max. 9 resolution 50nm overlay 14nm incr. 1.0nm

13 Fundamentals E-Beam Imaging Systems Gaussian beam Variable shaped beam E-beam writer LIN LV1 masks max. 5 x 5 wafer max. 5 beam 2nm overlay 50nm incr. 2.5 (0.1) nm E-beam writer Vistec SB 350 masks max. 9 x 9 wafer max. 9 resolution 50nm overlay 14nm incr. 1.0nm

14 Fragmentation of PMMA (Polymethylmethacrylate) during exposure n m o Result: PMMA-chains split, enhanced solubility in MIBK (Methylisobutylketone) Fundamentals

15 Fundamentals hange of resist solubility a liquid developer e.g. PMMA: MIBK : Isopropanol 1:1, t= sec, Stop in Isopropanol, N 2 -Drying Positive Resist: Average molecular weight reduced by exposure exposed area is solved much faster in developer and thus removed Negative Resist: Average molecular weight increased by exposure (cross-linking of molecules) unexposed area is removed in developer

16 Electron Beam Lithography - EBL Introduction Motivation General EBL process Fundamentals Basics and Physics of EBL Resist Technology harging EBL Writing Strategies Interaction between E-beam and Substrate Proximity Effect Proximity Function Periodic Structures Resolution Limits in EBL Some Special Exposure Techniques ell Projection Exposure Variable Dose Exposure Mask Preparation for Lift-ff verlay Exposure

17 Resist Technology Sample Preparation: oating & Tempering 1. Spin oating 2. Tempering (Pre-Exposure-Bake) otplate (or oven) Typical tempering figures: T= t = min

18 Resist Technology Resist properties: Sensitivity & ontrast Dose D = Deposited electrical charge per area unit [µ/cm 2 ] I current, t exposure time, F area, j current density ontrast curve measurement: Expose uniform square areas (e.g. 100µm) with increasing dose I t D = = j t F Increase of Dose Value

19 Resist Technology Resist properties: Sensitivity & ontrast ontrast curve measurement Ideal (binary) resist Increase of Dose Value Real (non-binary) resist Increase of Dose Value Sensitivity D 0 = Threshold dose, for which a (large) area of a given resist is completely removed (clearing dose) D 0 D % 0 % Resist hight after development

20 Resist Technology 1.0 Resist properties: Sensitivity & ontrast Standardized Resist ight Ideal resist g = Real resist (Zep520) g =18 ontrast g -1 Ø D ø = log 0 Œ œ Œº Ł D1 łœß Quantitative measure for binary behaviour of a resist (slope in contrast kurve) Dose [µ/cm 2 ] D 1 D 0