LIGA (Part I)

X-ray LIGA ProcessSpeakers: Zhao Wang and Ross AnthonyOutlineLithography Technique OverviewLIGA Process History and originX-ray GenerationX-ray Resist MaterialPrinting MethodElectrochemical Deposition (Electrodepostion)Hot Embossing Mask PreparationApplicationsConclusion

Toward the End of Moores Law

Ultimate limit: single-atom transistor

Quantum mechanics dominates when the size keeps scaling down: Quantum tunneling, Heisenberg uncertainty principle(5nm gate size)

Manufacture technique (e.g lithography system)

Next-generation LithographyCurrent throne holder:Visible-UV lithography, using double patterning. (10 nm)The competitors:EUV lithography (30-40 nm)X-ray lithography (30nm)Focused Ion beam lithography (30 nm)Electron beam lithography (30 nm)Nano-imprint lithography (N/A)(First 22nm fabrication demonstrated by Toshiba in 2007 )

X-ray LIGAWhat is LIGA?LIGA is a German acronym:Lithopraphie(lithography), Galvanik(electroplating),Abformung(molding)First invented in 80s,at IMT, KIT HighlightsHigh freedom to design, high aspect ratios (height vs. width), Nearly vertical side walls, high resolutionDrawbackHigh cost (abandoned by the industry at beginning of 90s)

Origin of LIGA

To make super precision microstructure for nuclear reactor Example: uranium separation nozzle(shown in the above Figure) LIGA ProcessMask Preparation (Intermediate & working masks)X-ray exposure with the working maskElectrodeposition (Female mould, a metallic structure)Mass production by hot embossingMask Preparation Mask fabricated by E-beam lithography & X-ray lithography

Au as absorber iselectrodeposited onthe Ti membrane

X-ray GenerationX-rays wavelength:0.01-10 nm.Electromagnetic radiation emitted from an electron moving at relativistic speedMeans of generation Synchrotron Radiation Undulator or Wiggler RadiationIrradiation is extremely bright, polarized and pulsedSynchrotron radiation

ANKA Synchrotron at IMT

Synchrotron Radiation

Wiggler and Undulator

X-ray Radiation

Radiation comparisonBESSY II Undulator

Compact Superconducting Storage ring X-ray Source at IBM

X-ray Resist MaterialPMMA

C-C bond and CH2-C bond are broken by the photoelectrons and Auger (secondary) electrons generated by high-power X-ray,

Effect after exposure, PMMA with high Mw turns into PMMA with low Mw which dissolves faster in GG developer

Characteristic Doses

Initial distribution of PMMA with high Mw

(After exposure of X-ray)

Distribution at the substrate(Lower dose limit 3.5-4 kJ/cm3)(Development dose)

distribution at the surface(higher dose limit 20 kJ/cm3 )(Damage dose)18

Characteristic DosesLimit of LIGA

Shadow (Proximity) Printing

Projection Printing ?Limitation due to X-ray lens systemOptical lens has no contracting power for X-ray. (n1)Compound refractive lens Each lens contributes about 10-4 refractive index change. It is still not practical as the focal length is on the order of meters

Projection Printing What if we have thousands of strongly curved lens cascaded in a ultra-short distance?Possibly reducing focal length into order of cm or mm by dramatically increasing the refractive index

Future X-ray lens !!Electrodeposition

Electrochemical deposition: coating a metallic layer on to another metal

Hot Embossing (Mirothermoforming)Hot embossing : essential for Nanoimprint Litho.

Pressure given to the polymer by 2 metal plates.Inject into female mould by heating the polymer above the glass transition Temp. temperature (molding)Demolding by cooling down the polymerGlass transition: an intermediate state between solid and liquid, commonly oberved in the polymer or plastic materials Hot EmbossingWide applications of MEMS fabication

Optical waveguideOptical bench for a micro spectrometerMask MaterialAbsorber needs to be able to dissipate heat Absorbers optical and mechanical properties must not change greatly even after long exposure.Material is chosen depending upon: absorption characterstics ease of formation processing conditionsAbsorberSeveral materials can be used for the absorber: Ni, Sn, Ta, Au, PtGold and platinum offer the best absorption

SubstrateSubstrates main purpose is to provide structural supportAlso needs to be able to dissipate heat generated during exposure Common materials are: Ti, Si3N4, Graphite, Be, GlassMask Preparation A mask consists of microstructures made of an x-ray absorbing material placed on top of a substrate which is transparent to x-ray radiation.

Intermediate Mask Depending upon which substrate material is used the process of intermediate mask deposition differs.Titanium is the most common process and has been in use for the last two decadesProcess begins with Si substrate

Intermediate MaskCarbon film is added to substrate2.5 um Titanium is sputtered Titanium undergoes wet oxidation using alkaline peroxide

Intermediate Mask~2.5 um resist is addedAfter baking resist is patterned using e-beam lithography

Intermediate MaskResist is developed 2 um gold is electrodeposited onto material

Intermediate MaskPattern is glued to an Steel Invar frameLifted off the silicon substrate

Working MaskBegins with steel frameFrame is thinnedCoated with 2.8 um Titanium

Working MaskMask is coated with ~30um resistIntermediate Mask is patterned on using soft x-ray lithography

Working MaskResist is developed Followed by electrodeposition of goldGold layer is ~20 um thick

38Final MaskAfter resist is removed mask is ready for deep x-ray lithography

Deep X-ray LithographyExample of final treatment

ElectroformingWhen metal microstructures are needed extra step of electroforming is addedMicrostructure is fabricated on an electrically conducting substrate or the microstructures are covered with a thin layer of gold

Electro formingNickel is deposited onto the resist structures until it is several millimetres thickNickel mould is then stripped from the substrate

42ElectroformingFlood exposure of the resistResist is removedFinal mould is now ready for mass production

ApplicationsOriginally developed for production of Uranium Nozzles in 1980s Applications have since expandedLIGA has been used to construct: Microprism, Microlenses, resonant filters, optical lab benches etcRadiofrequency ApplicationsVariable Capacitor

ConclusionX-ray LIGA: a powerful lithography to achieve extremely high resolution, high aspect ratio and complex 3-D structureVersatile applications: precision mechanics, micro-fluidic structure, photonic device, Biophotonics.Hot Embossing allowing mass productionRelatively high cost for X-ray generation and mask preparationConventional optical system needed46ReferenceGeneral LIGA process description:http://www.imt.kit.edu/english/liga.php2010 summer course Optical Lithography at IMT, KITKorvink, G., Saille, V., Tabata, O., Wallrabe, U. (2009) LIGA and its Applications, 7th edn, WILEY-VCH Verlag GmbH, Weinheim.Thanks for your attention!Question??