Section 2: LithographyThe lithographic processPhotolithographic ProcessPhotomasks - CAD LayoutPhoto MasksLithographic ProcessPrinting TechniquesContact PrintingProximity PrintingProjection PrintingDiffractionAerial Images formed by Contact Printing, Proximity Printing and Projection PrintingPhoton SourcesOptical Projection Printing Modules Optical StepperResolution in Projection PrintingResolution limits in projection printingDepth of Focus (DOF)Slide Number 19Example of DOF problemTradeoffs in projection lithographySub-resolution exposure: Phase Shifting MasksImmersion LithographyImage Quality Metric: ContrastQuestions:Image Quality metric: Slope of imageThe need for high contrastResists for LithographyTwo Resist TypesPositive P.R. MechanismPositive ResistNegative P.R. MechanismPositive vs. Negative PhotoresistsOverlay Errors(1) Thermal Run-in/Run-out errorsRotational / Translational ErrorsOverlay implications: ContactsOverlay implications: Gate edgeTotal Overlay ToleranceStanding WavesStanding waves in photoresistsProximity ScatteringApproaches for Reducing Substrate EffectsElectron-Beam LithographyTypes of Ebeam SystemsResolution limits in e-beam lithographyThe Proximity EffectSlide Number 48Slide Number 49Slide Number 50Slide Number 51Slide Number 52Slide Number 53Dip Pen NanolithographySlide Number 55Slide Number 56Slide Number 57EE143 – Ali Javey Slide 5-1Section 2: LithographyJaeger Chapter 2Litho ReaderEE143 – Ali Javey Slide 5-2The lithographic processEE143 – Ali Javey Slide 5-3Photolithographic Process(a) Substrate covered with silicon dioxide barrier layer(b) Positive photoresist applied to wafer surface(c) Mask in close proximity to surface(d) Substrate following resist exposure and development(e) Substrate after etching of oxide layer(f) Oxide barrier on surface after resist removal(g) View of substrate with silicon dioxide pattern on the surfaceEE143 – Ali Javey Slide 5-4Photomasks - CAD Layout• Composite drawing of the masks for a simple integrated circuit using a four-mask process• Drawn with computer layout system• Complex state-of-the-art CMOS processes may use 25 masks or moreEE143 – Ali Javey Slide 5-5Photo Masks• Example of 10X reticle for the metal mask - this particular mask is ten times final size (10 μm minimum feature size - huge!)• Used in step-and-repeat operation• One mask for each lithography level in processEE143 – Ali Javey Slide 5-6Lithographic ProcessEE143 – Ali Javey Slide 5-7Printing Techniques• Contact printing damages the mask and the wafer and limits the number of times the mask can be used• Proximity printing eliminates damage• Projection printing can operate in reduction mode with direct step-on- waferContact printingProximity printingProjection printingEE143 – Ali Javey Slide 5-8Contact PrintingwaferhvphotoresistResolution R < 0.5μmmask plate is easily damagedor accumulates defectsPhotoMaskPlateEE143 – Ali Javey Slide 5-9Proximity Printingwaferhvg~20μmexposedPhotoresistR is proportional to ( λ g ) 1/2~ 1μm for visible photons,much smaller for X-ray lithographyEE143 – Ali Javey Slide 5-10Projection PrintinghvlenswaferP.R.focal plane~0.2 μm resolution (deep UV photons)tradeoff: optics complicated and expensiveDe-Magnification: nX10X stepper4X stepper1X stepperDiffractionEE143 – Ali Javey Slide 5-11EE143 – Ali Javey Slide 5-12Aerial Images formed by Contact Printing, Proximity Printing and Projection PrintingEE143 – Ali Javey Slide 5-13• Hg Arc lamps 436(G-line), 405(H-line), 365(I-line) nm • Excimer lasers: KrF (248nm) and ArF (193nm) • Laser pulsed plasma (13nm, EUV) Source Monitoring • Filters can be used to limit exposure wavelengths • Intensity uniformity has to be better than several % over the collection area • Needs spectral exposure meter for routine calibration due to aging Photon Sources14Optical Projection Printing ModulesOptical System: illumination and lensMask: transmission and diffractionResist: exposure, post-exposure bake and dissolutionWafer Topography: scatteringAlignment:EE143 – Ali Javey Slide 5-15Optical Stepperwaferscribe line12Imagefieldfield size increaseswith future ICsfield size increaseswith future ICsTranslationalmotion16Resolution in Projection PrintingMinimum separation of a star to be visible.f = focal distanced = lens diameterEE143 – Ali Javey Slide 5-17Resolution limits in projection printingEE143 – Ali Javey Slide 5-18pointDepth of Focus (DOF)EE143 – Ali Javey Slide 5-19EE143 – Ali Javey Slide 5-20ΔFieldOxidePhoto maskDifferent photo imagesExample of DOF problemEE143 – Ali Javey Slide 5-21()() .()106222lNAwant small lDOFNAwant largeDOFmm≅=±λλ(1) and (2) require a compromise between λ and NA !(1) and (2) require a compromise between λ and NA !Tradeoffs in projection lithographyEE143 – Ali Javey Slide 5-22Sub-resolution exposure: Phase Shifting MasksPattern transfer of two closely spaced lines(a) Conventional mask technology - lines not resolved(b) Lines can be resolved with phase-shift technologyEE143 – Ali Javey Slide 5-23•A liquid with index of refraction n>1 is introduced between theimaging optics and the wafer. Advantages1) Resolution is improved proportionately to n. For water, the index of refraction at λ = 193 nm is 1.44, improving the resolution significantly, from 90 to 64 nm.2) Increased depth of focus at larger features, even those that are printable with dry lithography. Immersion LithographyEE143 – Ali Javey Slide 5-24Image Quality Metric: ContrastContrast is also sometimes referred as the Modulation Transfer Function (MTF)Questions:EE143 – Ali Javey Slide 5-25How does contrast change as a function of feature size?How does contrast change for coherent vs. partially coherent light?EE143 – Ali Javey Slide 5-26* simulated aerial image of an isolated lineImage Quality metric: Slope of imageEE143 – Ali Javey Slide 5-27resistresistsubstrateresistsubstrateresistPosition x FinitecontrastInfinitecontrastOptical imageThe need for high contrastEE143 – Ali Javey Slide 5-28Resists for Lithography• Resists– Positive– Negative• Exposure Sources– Light– Electron beams– Xray sensitiveEE143 – Ali Javey Slide 5-29Two Resist Types• Negative Resist – Composition:• Polymer (Molecular Weight (MW) ~65000)• Light Sensitive Additive: Promotes Crosslinking• Volatile Solvents– Light breaks N-N in light sensitive additive => Crosslink
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