Nano fabrication by e-beam lithographie

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

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

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

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

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

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

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

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

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

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

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

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

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

Fundamentals hange of resist solubility a liquid developer e.g. PMMA: MIBK : Isopropanol 1:1, t=30...60 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

Resist Technology Sample Preparation: oating & Tempering 1. Spin oating 2. Tempering (Pre-Exposure-Bake) otplate (or oven) Typical tempering figures: T= 90...210 t = 5...30 min

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

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 0 100 % 0 % Resist hight after development

Resist Technology 1.0 Resist properties: Sensitivity & ontrast Standardized Resist ight 0.8 0.6 0.4 0.2 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) 0.0 10 Dose [µ/cm 2 ] D 1 D 0