Nanolithography – strategies for a small world Applications, Raith GmbH

Outline Nanolithography … at the moment situation EBL sytems History – Capabilities – Components EBL writing strategies Process technology Electron–resist–interactions Process simulation Proximity effect and Charging Lift off / etching EBL application examples

Nanolithography … at the moment situation 32nm 103 102 EUV DUV 10 Minimum Requirement For Production 1 X- RAYS ? Electrons by Projection? 10-1 Writing speed cm2/s 10-2 Electrons: Focused Beam (& Shaped beam) 10-3 Mass Production  Need Mask 10-4 STM - AFM 10-5 Direct Writing  Maskless 10-6 0,001 0,01 0,1 1 10 Resolution / m

Optical Lithography Resist Substrate Resist Substrate Substrate Mask to select the exposed area: Contact/ Proximity lithography Projection/ Reduction lithography UV – λ : 435 -157nm Substrate Resist positive resist negative resist Substrate Resist Substrate

Why Electrons? Pros Minimum feature size ~ 5nm Simple to produce / simple to shape Maskless Cons Vacuum required Serial exposure strategy (so far)

Which tasks for EBL? Fabrication of photo masks and imprint templates Direct write applications rapid prototyping low volume production (confined to ultrahigh resolution layer) Nanodevices in R&D

Outline Nanolithography … at the moment situation EBL sytems History – Capabilities – Components EBL writing strategies Process technology Electron–resist–interactions Process simulation Proximity effect and Charging Lift off / etching EBL application examples

History of EBL systems and electron optics Some Milestones in the History of Electron Optics 1897 Discovery of the electron by J.J. Thompson 1924 P. De Broglie: particle/wave dualism 1927 Hans Busch: Electron beams can be focused in an inhomogeneous magnetic field. 1931 Max Knoll and Ernst Ruska built the first TEM 1938 Scanning transmission electron microscope (M. von Ardenne) 1939 First commercial TEM by Siemens (Ruska, von Borries) … 1964 First commercial SEM by Cambridge Instruments 1960s First EBL Systems and EBL Resists

Serial Exposure: EBL Systems R&D R&D systems attachments mask writer throughput, complexity, costs flexibility, versatility

Capabilities of EBL Systems Pattern size? Sample size? Lithography resolution? New technologies? University environment? Process control? Automation? Wafer exposure? R&D mask writer R&D systems attachments fuzzy classification - “systems in between” stitching stitching single field mask making chip exposure small layouts wafers & masks wafers & masks direct write (mix & match) small samples small samples mix & match mix & match full automation automatable jobs high resolution high resolution high resolution imaging imaging metrology multi user multi user nano engineering nano engineering

Most simple EBL system: Upgraded SEM EBL related components Courtesy:SPIE Handbook of Microlithography

EBL systems Courtesy:SPIE Handbook of Microlithography

Electron guns thermal emission, tungsten or LaB6 b) field emission, Source: (Gersley, J. Appl. Phys. 65 (3), 914 (1989)) thermal emission, tungsten or LaB6 b) field emission, cold or thermal advantages: resolution, beam current stability, filament lifetime

Electron beam resolution  DeBroglie wavelength at 1 kV: 0.0387 nm current resolution limit of electron optics is in the range 1 – 2 nm Courtesy: SPIE Handbook of Microlithography

Resolution limit in EBL 4.69 nm Hier an der Flipchart einen prinzipiellen Aufbau STEM und SEM/EBL zeichnen (Pole Piece) ~ 5nm lines exposed in HSQ resist, J. Yang, MIT, and J. Klingfus, Raith, unpublished

EBL systems Courtesy:SPIE Handbook of Microlithography

Beam deflection electro-static electro-magnetic F = q · (E + v × B) fast deflection large deflection

EBL systems beam blanker deflection Courtesy:SPIE Handbook of Microlithography

How is the exposure executed? Serial exposure strategy Need for accurate stage movement: Stitching

How is the exposure executed? Serial exposure strategy Need for accurate stage movement: Stitching

Laserinterferometer controlled stage Compact design No chamber modification required Variable working distance Integral closed loop position control Position control independant of all SEM settings Excellent stability stitching capability

Outline Nanolithography … at the moment situation EBL sytems History – Capabilities – Components EBL writing strategies Process technology Electron–resist–interactions Process simulation Proximity effect and Charging Lift off / etching EBL application examples

EBL techniques Vector scan / Raster scan Vector: only necessary parts are scanned Raster: complete exposure area is scanned Beam is blanked when necessary Gaussian beam / shaped beam / cell projection Gaussian: focused beam (point-by-point) Shaped: projection of variable shapes (2 apertures combined – area exposure) Cell: Projection of a dedicated shape (area exposure) Exposure Stage stationary during exposure (stitching) Write-on-the-fly: moving stage during exposure

Writing strategies: Gaussian beam Vector Scan High resolution electron optics optimised for smallest spot diameters Gaussian Beam Raster Scan Complex electron optics with high speed deflector for 200 MHz – exposure while moving the stage

Writing strategies: Write-on-Fly stage motion beam motion

Writing strategies: Shaped beam Vector Scan Complex electron optics with shaping apertures require high current densities Different writing strategies applied for different tasks „There‘s no good or bad“

Outline Nanolithography … at the moment situation EBL sytems History – Capabilities – Components EBL writing strategies Process technology Electron–resist–interactions Process simulation Proximity effect and Charging Lift off / etching EBL application examples

Process technology pattern transfer spin coating substrate resist spin coating exposure developing wafer coating or stripping step after x process steps remover lift-off etching metal pattern transfer

Process technology pattern transfer spin coating exposure gas EBID remover Lift-Off etching metal pattern transfer substrate resist spin coating exposure developing substrate EBID gas injection metal etching

Electron scattering Forward scattering events resist very often scattering under small angles small-angle hence inelastic generation of Secondary Electrons with a few eV kinetic Energy incident beam BSE SEII SEI resist Backward scattering events occasionally scattering under large angles large angle hence mainly elastic high kinetic energy, range of the primary electrons substrate Organic resist: exposure by secondary electrons (SEI and SEII)

Resist – what happens? Chemical bonds creation or break Organic resist: 6 – 10eV (secondary electrons) Inorganic resist: > 50eV high energy electrons: beam itself  high resolution Positive resist Chemical bond break (e.g.: PMMA, PMMA/CoMAA, PMGI, ZEP520) Negative resist Chemical bond creation (e.g.: ma-N 2400, PMMA, calixarene)

Fragmentation of PMMA m o n C O C C C C C C C C O C O O C O C C O H H

Resist – what happens? chemically amplified negative resist chemical bond break due to acid radical creation thermally activated cross-linking (PEB – Post Exposure Bake) due to molecular weight (e.g.: SU8, KRS-XE, UVNxx, SAL601, SNR200, AZ-PN114) inorganic materials phase or cristallinity change, chemical bond break or auto-developing e.g.: HSQ, WO3, AlF3, SiO2)

Chemically amplified resists SU8 resist substrate spin coating exposure developing prebake chemically amplified resist: postexposure bake chemically amplified resist most applications: used with postexposure bake for high contrast postbake

Some resists Positive resists Negative resists PMMA ZEP UV5 AR-N 7520 or ma-N2403 HSQ SU8 UVN30 more info at: www.microresist.de | www.allresist.de | www.microchem.com http://snf.stanford.edu/Process/Lithography/ebeamres.html

Getting started As an EBL beginner … avoid chemically amplified resist because they need a carefully controlled post exposure bake avoid very sensitive resists make first tests with positive resist, because several tests can be made on one sample  use for example PMMA 950K from Raith’s EBL starter kit

Monte Carlo simulation 1.4 µm PMMA, E = 30 keV, exposed line: 300 nm , Dose 520 µC/cm2

Developing 45 s 40 s 55 s 60 s 35 s 50 s 25 s 5 s 10 s 15 s 20 s 30 s 1.4 µm PMMA, E = 30 keV, exposed line: 300 nm , Dose 520 µC/cm2 with MIBK:IPA = 1:3 45 s 40 s 55 s 60 s 35 s 50 s 25 s 5 s 10 s 15 s 20 s 30 s

Clearing dose Cross-Section after developing for 30 s 460 µC/cm2

Process Window Dose Process window (decreases with resist thickness) Developing (Time, Temperature)

Clearing dose: Rule of thumb incident beam SEII BSE SEI 1. Minor influence of resist thickness on dose 2. Dose depends on beam energy

Proximity effect incident beam BSE SEII SEI Proximity effect corrected uncorrected high dose low dose 1µm Proximity effect depends on beam energy, substrate, pattern various strategies for proximity correction, e.g. dose variation

Displacement and distortion of exposed structures! Charging e- e- insulator resist Displacement and distortion of exposed structures!

Use of conducting layers 1. resist/metal/substrate 2. metal/ resist/substrate insulator resist metal e- insulator metal resist 3. conductive polymer /resist/substrate: disadvantage: sputtering or evaporation system required for metal deposition insulator resist conductive polymer additional spin coating step, but no metal etching required

Exposure Results Optical Images after exposure

Process technology pattern transfer spin coating substrate resist spin coating exposure developing wafer coating or stripping step after x process steps remover lift-off etching metal pattern transfer

Lift-Off Tips & Tricks obtain an undercut resist profile by use a double layer resist use low beam energy over-developing over-exposure use an aspect ratio of resist:metal as large as possible if possible use evaporation, not sputtering

Etching Tips & Tricks Resist Wafer Wafer obtain cross-sections without undercut or overcut by use high beam energy avoide over-exposure and over-developing apply postbake to improve resist stability during etching for organic resists avoid etching processes with oxygen Wafer Resist Wafer

Process technology pattern transfer spin coating Raith support substrate resist spin coating exposure developing Raith support special knowledge, e.g. EBL resist database remover lift-off etching metal pattern transfer general knowledge & support

Raith application lab: Typical instrumentation substrate spin coating exposure developing Cleaning Instrumentation Film thickness measurement Cleaning Spin coating Exposure Developing Inspection wet bench (eye-) shower for accidents with acids storage for chemicals stove / hotplate refrigerator spin coater Film Thickness Probe Raith EBL system optical microscope sputtering machine process step Spin coating Film thickness measurement Exposure Developing Inspection

Outline Nanolithography … at the moment situation EBL sytems History – Capabilities – Components EBL writing strategies Process technology Electron–resist–interactions Process simulation Proximity effect and Charging Lift off / etching EBL application examples

Customer Applications: Nanodevices Nanoelectronics Basic Research Optical Properties CNT Single electron device, Université de Louvain SiO2 Nanowires, Uni Alberta Meta material, Uni Karlsruhe Photonics Nanomechanics EBL Photonic Crystal, Uni Eindhoven Single Electron Transport, LMU Munich

Mix & Match exposure (Overlay) t-gate HEMT, Courtesy F. Robin, ETH Zürich

Mix & Match with/without accurate stage A): no registration after global alignment: highly accurate stage is a must! B): Best overlay required or inaccurate stage: Apply local registration after global alignment Global Wafer Marker Local Writefield Marker

Mix & Match exposure: Example automatic mark scan HEMT Structures Step: large electrodes with 200 nm gap Step: Overlay of 80 nm gate line

Applications – 3D lithography Christchurch New Zealand, 3D pattern in negative resist (OM and AFM image) M. Konijn, University of Canterbury, Christchurch, New Zealand

Application – high resolution map Colombo St 50 microns = 1/20 mm 50 nm = 1/20,000 mm nm-map, R. Blaikie, University of Canterbury, New Zealand

Outline Thanx for listening! Nanolithography … at the moment situation EBL sytems History – Capabilities – Components EBL writing strategies Process technology Electron–resist–interactions Process simulation Proximity effect and Charging Lift off / etching EBL application examples Thanx for listening!