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Lithography Process: IC & MEMS Fabrication
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Lecture 08: Lithography Process Instructor: Dongjin Lee Text reading: Jaeger Ch 2 Prof. Dongjin Lee History • Joseph Nicephore Niepce in 1826 - first photolithography, • • engraving of Cardinal d’Amboise • Resolution: ~1 mm More than 100 years later – another try: Louis Minsk developed the synthetic photosensitive polymer, polyvinylcinnamate, the first negative photoresist In 1960’s – large amounts of transistors Engraving, 1650 Prof. Dongjin Lee Copy of the Engraving Heliograph, 1826 Photolithography • Photo-litho-graphy: Latin word meaning light-stone-writing • 사진석판인쇄술 or 노광공정 in Korean • The manufacturing process where micro- & nanopatterns on • transparent glass (mask) are transferred onto silicon wafer via photoresist by optical means (appropriate wavelength) The photoresist patterns play a role of masking layer for postprocess • Up to 30% of overall semiconductor processes, 35% of manufacturing cost Photolithography Photo: light Litho: stone Graphy: writing Prof. Dongjin Lee Photolithography • Photolithography is an optical means of transferring patterns from the mask onto a substrate • Patterns are firstly transferred to an imageable photoresist layer • Photoresist is a liquid film that can be spread out onto a substrate, exposed to UV light with a desired pattern, and developed for subsequent processing (etching or deposition) • Photolithography is a binary pattern transfer: there is no grayscale, color, nor depth to the image Prof. Dongjin Lee Photolithography • Lithography is the most important process in IC and MEMS fabrication • The minimum feature size is determined by lithography process • The National Road Map of Semiconductor is driven by the desire to continue scaling down minimum feature size in devices Year of 1st DRAM Shipment DRAM Bits/Chip 1999 1G 2003 4G 2006 16G 2009 64G 2012 256G Minimum Feature Size nm Isolated Lines (MPU) Dense Lines (DRAM) 140 180 100 130 70 100 50 70 35 50 Gate CD Control 3σ (nm) 14 10 7 5 4 Depth of Focus (µm) 0.7 0.6 0.5 0.5 0.5 248 nm DUV 248 nm or 193 nm DUV 193 nm DUV or ??? 193 nm DUV or ??? Exposure Technology ??? Prof. Dongjin Lee IC Processing Flow Materials IC Fab Metallization CMP Dielectric deposition Test Wafers Thermal Processes Implant PR strip Etch PR strip Packaging Masks Photolithography IC Design Prof. Dongjin Lee Final Test Kinds of Lithography • Photolithography: photon (UV, EUV) & photoresist • X-Ray lithography: X-ray & X-ray resist • Electron-beam (e-beam) lithography: e-beam & e- beam resist • Ion beam lithography : ion beam & ion beam resist resist = photoresist Prof. Dongjin Lee Electromagnetic Wave λ ν Prof. Dongjin Lee Electromagnetic Wave • Speed of electromagnetic wave or light (c): 3.00ⅹ108 m/s • Max Planck: An atom or molecule emits discontinuous magnitudes of energy and the energy emitted from a quantum is proportional to frequency of the quantum • Relationship of wavelength (λ), frequency (ν), period (T): • E = hυ = hc/λ (Plank constant: h=6.63ⅹ10-34 J·s) • The energy has an unit of J or eV Prof. Dongjin Lee Overview of Photolithography Process • Surface preparation: wafer cleaning • Prebake (followed by priming) • (Spin) Coating photoresist onto wafer • Soft bake • Alignment • Exposure (followed by post-exposure bake) • Develop • Hard bake • Other processing using the patterned photoresist as a masking • • layer (etching, liftoff, etc.) Strip photoresist Post process cleaning Experiment movie: https://www.youtube.com/watch?v=YVjZzjqsAA0&list=PL7FguTECNv8qO_7zHalC DUyVubgMRXbn1 Comprehensive movie for IC (4.5 min): https://www.youtube.com/watch?v=_8HwIHyVD1E Prof. Dongjin Lee Exemplary Photolithographic Process Mask a. Deposition of silicon dioxide (SiO2) b. Spin coating of (positive) photoresist (PR) c. UV exposure d. Develop e. Etching of silicon dioxide (SiO2) f. PR stripping 3D image of fabricated device Process flow (공정도): usually cross-sectional view is used Prof. Dongjin Lee History • Joseph Nicephore Niepce in 1826 - first photolithography, • • engraving of Cardinal d’Amboise • Resolution: ~1 mm More than 100 years later – another try: Louis Minsk developed the synthetic photosensitive polymer, polyvinylcinnamate, the first negative photoresist In 1960’s – large amounts of transistors Engraving, 1650 Prof. Dongjin Lee Copy of the Engraving Heliograph, 1826 Wafer cleaning • Typical “dirt” that must be removed before applying • photoresist Standard degrease: • 2-5 min soak in acetone with ultrasonic agitation • 2-5 min soak in methanol with ultrasonic agitation • 2-5 min soak in DI (deionized: 18 MΩ·cm) water with ultrasonic agitation • 30 sec rinse with DI water • N2 blow off dry or spin rinse dry • For particularly troublesome grease, oil, or wax stains • Start with 2-5 min soak in 1,1,1-trichloroethane (TCA) or trichloroethylene (TCE) with ultrasonic agitation prior to acetone • Organic residues: O2 plasma (2-3 min @ 200-300 W) • Hazards: • TCE is carcinogenic; 1,1,1-TCA is less hazardous • Acetone is flammable • Methanol is toxic by skin adsorption Prof. Dongjin Lee Wafer Cleaning: RCA cleaning • Most wafer manufacturers use a final cleaning method developed by RCA in 1970 • It has been being used for cleaning of new silicon wafers out of box • The 3-step process starts with an SC1 solution to remove organic impurities and particles from the wafer surface • Next, native oxides and metal impurities are removed with hydrofluoric acid (HF) • Finally, the SC2 solution causes super clean new natural oxides to grow up on the surface Prof. Dongjin Lee Wafer Priming • Adhesion promoters are used to assist resist coating • Resist adhesion factors: • Moisture content on the surface • Wetting characteristics of resist • Delay in exposure after the pre-bake • Type of primer, resist chemistry, surface smoothness, stress from coating process, surface contamination, etc. • Ideally want no H2O on wafer surface: wafer should be baked • right before priming To improve adhesion photoresist primers are used • Widely used: Hexamethyldisilazane (HMDS) • Wafer priming should not be always performed! Prof. Dongjin Lee Primer Vapor Coating Prep Chamber Primer Layer HMDS Vapor Wafer Hot Plate Pre-Bake: dehydration Wafer Primer Vapor Coating • Remove moisture from wafer surface • Usually around 110°C for 1-2 min • Promote adhesion between photoresist and wafer surface • HMDS vapor coating prior to PR spin coating • HMDS can also be spun Prof. Dongjin Lee Spin Coating of Photoresist • Wafer is kept on a spinner chuck by vacuum and photoresist is applied uniformly by spin coating • Typically 3000-6000 rpm for 15-60 seconds • Resist thickness depends on: • Primarily photoresist viscosity • Secondarily spinning velocity • Most resists are 0.5-2 μm thick • Special type of resist (e.g. SU-8) can make up to 200 μm Prof. Dongjin Lee Spinning Process Spin speed profile RPM Time Prof. Dongjin Lee Spinning Curve kvS t= rpm t : PR thickness k : Spinner constant v : PR viscosity S : Percent solid content 3.5 100 cSt 3.0 Thickness (mm) 15 Film thickness (µm) 12 9 6 2.5 2.0 50 cSt 27 cSt 1.5 20 cSt 1.0 10 cSt 0.5 5 cSt 3 0 0 200 400 600 800 1000 Spin speed (rpm) 0 2k 3k 4k 5k 6k 7k Spin speed (rpm) Dynamic viscosity: 1 cSt = 0.01 St = 1x10-6 m2/s = 1 mm2/s Prof. Dongjin Lee Photoresist coating: Automated Production Systems Rite Track 88e wafer processing system (Courtesy of Rite Track Services, Inc.) Animation in separate file Also edge bead removal Prof. Dongjin Lee Photoresist • Photoresist is a photosensitive polymeric material which has a very important role in patterning features on a Si substrate • There are two types of photoresist: positive resist and negative resist • The exposure changes solubility due to photochemical reaction when exposed to UV light • Positive PR: from insoluble to soluble • Negative PR: from soluble to insoluble • Needs high resolution, high etch resistance, good adhesion • Thinner PR film has a higher resolution • Thinner PR film has a lower the etching resistance Prof. Dongjin Lee Types of Photoresist Negative Photoresist Positive Photoresist • Becomes insoluble after exposure • Becomes soluble after exposure • When developed, the exposed part is NOT dissolved • Exposed part becomes cross-linked polymer • Cheaper • When developed, the exposed parts dissolved • Exposed part loses cross-link • Better resolution • Novolac resin polymer in acetate solvent • Polyisoprene type in xylene solvent Prof. Dongjin Lee − PR + PR Film Film Substrate Substrate Negative vs. positive photoresists Photoresist Spin coating of PR Substrate UV light Mask Photoresist Substrate UV exposure Negative Photoresist Substrate Develop Positive Photoresist Substrate Prof. Dongjin Lee Soft Bake • Used to evaporate the solvent from the spun photoresist • Typically: • Convective type: 80-90°C for 20-30 min in a oven • Conductive type: 90-100°C for 1-2 min on a hot plate • Hot plate is usually faster, more controllable and does not trap • • solvent like in oven Commercially, microwave heating or IR lamps are also used in production lines As a result of soft bake: • Improves adhesion • Improves etch resistance • Improves uniformity • Improves line-width control • Optimizes light absorbance characteristics of photoresist Prof. Dongjin Lee Soft Bake • Convection oven: • Solvent at surface of resist is evaporated first, which can cause resistance to develop impermeable skin, trapping the remaining solvent inside Heating must go slow to avoid solvent burst effects • • Conduction hot plate • Need an extremely smooth surface for good thermal contact • • and heating uniformity Temperature rise starts at bottom of wafer and works upward, more thoroughly evaporating the coating solvent Generally much faster and more suitable for automation • Widely use in industry Prof. Dongjin Lee Baking Systems Wafer Heater Microwave Source Heated N2 Photoresist Chuck Wafers Vacuum Wafer Heater Hot plate Prof. Dongjin Lee Convection oven Vacuum Microwave oven Alignment & Exposure • Need to cool down to ambient temperature before exposure • Silicon thermal expansion rate: 2.5×10-6/°C • For 8 inch (200 mm) wafer, 1°C change causes 0.5 μm difference in diameter Wavelength of UV: 436 nm: g-line 405 nm: h-line 365 nm: i-line Prof. Dongjin Lee Exposure Straightness of light • One of most critical process for IC & MEMS fabrication • Determines the minimum feature (pattern) size Prof. Dongjin Lee Alignment and Exposure Hardware Contact Proximity 1 μm 2-5 μm Space: ~ 10 μm Projection • Separate for alignment • Contact for exposure CD ≈ λ g • Poorer image than contact align: mask lifetime may be longer 0.1-1 μm • Reproduction ratio may be changed (max 1:1) Prof. Dongjin Lee Scanning Projection System Light Source Slit Lens Mask Synchronized mask and wafer movement Lens Photoresist Wafer Prof. Dongjin Lee Step & Repeat Alignment System Light Source Projection Lens Reticle Projection Lens Wafer Wafer Stage Prof. Dongjin Lee Step & Repeat Alignment System Light Source Reference Mark Alignment Laser Reticle Stage Reticle Interferometer Laser Projection Lens Y X Wafer Interferometer Mirror Set Wafer Stage Prof. Dongjin Lee Photomask • Photomasks have the master patterns which are transferred to • wafers through PR Types: • Photographic emulsion on soda lime glass (cheapest) • Fe2O3 on soda lime glass • Cr on soda lime glass • Cr on quartz glass (most expensive, needed for deep UV lithography) • Dimensions: • 4 in by 4 in by 0.06 in for 3 inch wafers • 5 in by 5 in by 0.06 in for 4 inch wafers • Polarity • Light-filed: mostly clear, drawn feature opaque • Dark-field: mostly opaque, drawn feature clear Prof. Dongjin Lee Examples of Photomask Bright field mask Prof. Dongjin Lee Dark field mask Example Layout • pMOS Metal-Gate Process: Basic 4-mask process 1. p-diffusion 2. Thin oxide 3. Contacts 4. Metal Mask design is performed using CAD software Prof. Dongjin Lee Example Layout p-diffusion Contacts Prof. Dongjin Lee Thin oxide Metal Mask Alignment • Each mask must be carefully • • aligned to the previous layer Some forms of alignment marks are used Automated alignment and exposure in production lines Prof. Dongjin Lee Post Exposure Bake (PEB) • PEB is needed usually for negative PR • PEB normally uses hot plate at 110 to 130°C for about 1 min • For the same kind of PR, PEB usually requires a higher • • • temperature than soft bake Over-baking will cause polymerization and affect photoresist development Baking temperature is higher than the photoresist glass transition temperature (Tg) Thermal movement of photoresist molecules is induced Prof. Dongjin Lee Develop • Each commercial photoresist has its recommended developer, a • • chemical that dissolves exposed (unexposed in case of negative resist) areas of the photoresist Developer solvent dissolves the softened part of photoresist Some photoresists can be “inversed” by special treatment Prof. Dongjin Lee Development: Immersion Develop Prof. Dongjin Lee Rinse Spin Dry Development: Spin Develop Animation in separate file Prof. Dongjin Lee Development Profiles PR PR Substrate Substrate Normal development Incomplete development PR PR Substrate Substrate Under development Over development • Positive PR normally uses weak base solution • The most commonly used one is the tetramethyl ammonium hydride (TMAH: (CH3)4NOH) Prof. Dongjin Lee Develop Positive resist Negative resist Prof. Dongjin Lee Pattern Inspection • See if • There is surface irregularities such as scratches, pin holes, stains, contamination, etc • Critical dimension (CD) is observed • Overlay or alignment • Run-out/ run-in, reticle rotation, wafer rotation, misplacement in xand y- direction • If PR pattern is not realized correctly, patterned PR can be strip ped and rework is performed • Photoresist pattern is temporary • Rework cannot be done after etching or ion implantation • Scanning electron microscope (SEM) or optical microscope is u sed for pattern inspection Prof. Dongjin Lee Critical Dimension PR PR PR Substrate Substrate Substrate Good CD CD Loss Sloped Edge Prof. Dongjin Lee Misalignment Cases Run-out Run-in θ Reticle or wafer rotation Misplacement in x -direction Misplacement in y-direction • If the wafers pass the inspection, they will move out of photo bay and go to the next process steps • Either etch or ion implantation Prof. Dongjin Lee Hard bake • Used to improve adhesion to the substrate and harden the • • • developed photoresist prior to processing steps Hard bake is especially needed for acid- and Ar-ion etching It is not needed for processes in which a soft resist is desired, e.g. metal lift-off process Longer and hotter bake makes subsequent resist removal more difficult • Typically, 110-130°C for 1-2 min (all depend on photoresist) Prof. Dongjin Lee Post Process: Etch vs. Lift-off • The final thin-film pattern is the same as photoresist pattern • Photoresist is not in contact with substrate Prof. Dongjin Lee • The final thin-film pattern is negative with respect to photoresist pattern • Photoresist is in contact with substrate (chemical reaction can occur) Photoresist Removal (stripping) • Simple solvents are generally sufficient for non- baked photoresists: • Positive photoresists: acetone, trichloroethylene (TCE), phenol• based strippers Negative photoresist: methyl ethyl ketone (MEK, CH3COC2H5), methyl isobutyl ketone (MIBK, CH3COC4H9) • Oxygen-plasma stripping is very effective in general for removing organic polymer residues • Some photoresists (SU-8) are very hard to remove: can be used as parts of a final device Prof. Dongjin Lee Process Recipes • A collective wisdom Prof. Dongjin Lee Photolithography Resolution • Resolution is achievable and repeatable minimum feature size • Resolution is determined by wavelength of the light • (diffraction) and the numerical aperture To reduce diffraction and achieve the higher resolution, the exposure system can use: • Shorter wavelengths of light: DUV, EUV to X-ray (ArF excimer laser at 193 nm, Hg-vapors lamp, Xe-lamp) • High numerical aperture (NA) lenses to project the light Prof. Dongjin Lee Photolithography Resolution Light source for higher resolution KrF excimer laser: 248 nm (0.18 um) ArF excimer laser: 193 nm (0.10 um) F2 excimer laser: 157 nm (0.07 um) Prof. Dongjin Lee
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