EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 1 References: 1. The science and engineering of microelectronic fabrication, Stephen A. Campbell, Oxford, 2001 2. Semiconductor Lithography Principles, practice and materials,Wayne M. Moreau, Plenum Press 3. Silicon Processing for the VLSI Era Vol 4 Deep-submicron Process Technology, S Wolf, Lattice Press 4. Semiconductor Materials and Device Characterization, Dieter K. Schroder, Wiley Interscience 5. Semiconductor Manufacturing Technology, Michael Quirk and Julian Serda Prentice Hall 6. Handbook of Microlithography, Micromachining, and Microfabrication, Volume 1: Microlithography, Editor P. Rai-Choudhury, SPIE Optical Engineering Press, 1997EE6601MicroFabrication Technology A/P Tse man Siu Tel : 6790-4843 Email : emstse@ntu.edu.sg Photolithography Processing Photolithography Technology Photoresist Technology Advanced Lithography Metrology Defect Inspection and Analytical Techniques EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 2 Photolithography and Photoresist Technology Photolithography Process Basic concepts for photolithography, process overview, negative and positive lithography, critical dimension generations, light spectrum, resolution and process latitude, Eight basic steps of photolithography process. Photolithography Technology Purpose of Lithography, Purpose of alignment and exposure, Properties of light for optical lithography, Critical aspects of optics, Resolution and its critical parameters, Photolithography equipment, Reticles and Photomasks, Alignment and Overlay Photoresist Technology Purpose of Photoresist, Components of Photoresist, Conventional g-line and i-line photoresists, Deep UV resists, Metrics of Photoresist, Characterization of Working Windows Advanced Lithography Optical Resolution Enhancement Techniques(RET) for sub-wavelength lithography, Top Surface Imaging, Immersion UV Lithography, Double Patterning Lithography, Extreme UV Lithography, E-beam Lithography, X-Ray Lithography, Nano-Imprint Lithography EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 3 Photoresist Technology 1. Purpose of Photoresist 2. Components of Photoresist 3. Conventional g-line and i-line photoresists 4. Deep UV resists 5. Metrics of Photoresist 6. Characterization of Working Windows EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 4 Implant Diffusion Test/Sort Etch Polish Photo Completed wafer Unpatterned wafer Wafer start Thin Films Wafer fabrication (front-end)Wafer Fabrication Process Flow Note: wafers flow from photo step into only two other areas: etch and ion implant EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 5 Wavelength (nm) EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 6 Material Wavelength (nm) Max. Output (mJ/pulse) Frequency (pulses/sec) Pulse Length (ns) CD Resolution* (m) KrF248300 150050025s 0.25 ArF193175 300 40015s 0.18 F215761020s 0.15 Excimer Laser Sources for Semiconductor Photolithography Move to 193 nm: optical materials have undesirable absorbance and are more sensitive to laser damage, move from quartz to CaF2 lenses Note : * CD Resolution without Resolution Enhancement Techniques (RETs) EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 7 YEARLinewidth (nm)Wavelength (nm)Remarks 19861,200436Hg : g-line 1988800436/365Hg: g-line/i-line 1991500365Hg: i-line 1994350365/248Hg: i-line/DUV 1997250248KrF DUV 1999180248KrF DUV +RETs 2001130248KrF DUV + RETs 200390248/193KrF/ArF DUV +RETs 200565193ArF DUV+RETs 200745193ArF DUV IML 201032193ArF DUV IML, DPL 201422193ArF IML + DPL Minimum linewidth and exposure wavelength EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 8 ) NA (k R1=Reduce Three Ways to Improve Resolution Increase NA Reduce k1 Immersion with NA>1 Shorter wavelength(436 nm365 nm 248nm 193 nm13.5 nm) Improved masks(CD control, Phase Shift masks) Improved lenses (aberrations) Better photoresists Better process controlsResolution Enhancement Techniques (RET) k1 = 0.25 achievable EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 9 22) NA (k DOF=Improvement in Resolution means Depth of Focus becomes an issue Decrease as + Decrease as NA | Decrease as k1 + However, DOF is no longer an issue. Wafers are made PLANAR with chemical mechanical polishing (CMP), allowing the lithography systems to work with smaller DOF.EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 10 To transfer the mask pattern to the photoresist on the top layer of the wafer surface To protect the underlying material during subsequent processing e.g. etch or ion implantation. 1. Purpose of Photoresist in Wafer Fab What is resist? Viscous liquid which has a solid form when solvents are driven out Spin coated on wafer surface to be patterned Exposure of resist to energy/radiation leads to (photo) chemical reactions and changes the resist dissolution rate in the developer Remaining resist is rugged enough to protect (mask) underlying substrate during subsequent processing (etch and implant) EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 11 Better image definition (resolution). Better adhesion to semiconductor wafer surfaces. Better uniformity characteristics. Increased process latitude (less sensitivity to process variations). Progressive Improvements in Photoresist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 12 Two Types of Photoresist Positive Resist Negative Resist CD Capability Conventional g-line, i-line Resist (resolution > 0.5 m) Deep UV Resist (resolution < 0.5 m) Process Applications Non-critical Layers (g-line, i-line resists) Critical Layers (DUV resists) Types of Photoresists EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 13 Photolithography Process Creating the Patterns in Semiconductor Chip Manufacture Negative Resist -Wafer imageis opposite of mask image -Exposed resist hardens and is insoluble -Developer removes unexposed resist Positive Resist -Mask imageis same as wafer image -Exposed resist softens and is soluble -Developer removes exposed resist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 14 Negative Resist Wafer imageis opposite of mask image Exposure causes the resist to cross-link and harden, becoming insoluble in Xylene developer Developer removes unexposed resist Negative resist is uncommon today because of limited resolution (negative DUV resist has poorer performance than positive DUV resist) Positive Resist Mask imageis same as wafer image Exposed resist softens and is soluble in hydroxide developer Developer removes exposed resist Negative Versus Positive Resists EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 15 Additives:chemicals that control specific aspects of resist material Solvent:gives resist its flow characteristics Sensitizers:photosensitive component of the resist material Resin: mix of polymers used as binder; gives resist mechanical and chemical properties 2. Components of Photoresist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 16 Photoresist components Main components for lithographic capability: Polymer (resin) - not opaque at - Chemical reactivity - Etch resistance Sensitizer - Photo Active Compound/Group (PAC/PAG) at Solvent- Keeps photoresist in liquid state Allows spin coating of the photoresist Solvent content determines viscosity and hence thickness! Additives Capability for further process: Etch sensitivity/Implant blocking capability. EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 17 3. Conventional g-line and i-line photoresists Types of Photoresist Positive Optical Resist Matrix (Novolac resin) Sensitizer / dissolution inhibitor (PAC=diazoquinones) Solvent (Propylene Glycol Methyl Ether Acetate (PGMEA),N-Methyl Pyrrolidene(NMP), n-butyl acetate, xylene, etc) Developer: Hydroxides (TMAH, KOH, NaOH etc) Negative Optical Resist Cyclized synthetic rubber resin Sensitizer(PAC=bisarylzide) Solvent (aromatic solvent) Developer: organic solvents EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 18 Areas exposed to light become crosslinked and resist the developer chemical. Unexposed areas remain soluble to developer chemical. Pre-exposure - photoresist Post-exposure - photoresist Post-develop - photoresist UV Oxide Photoresist Substrate Crosslinks Unexposed Exposed Soluble Negative Resist Cross-Linking EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 19 Components of g-line & i-line positive Photoresist ComponentTypical ChemicalsPurpose Remarks Solvent Propylene Glycol Methyl Ether Acetate (PGMEA) gives resist its flow characteristics Compatible to other components Resin Novolac resinmix of polymers used as binder; gives resist mechanical and chemical properties - Chemical Reactivity - Transparent to g-line and i-line UV - Reactive Ion Etch Resistance - Implant blocking capability Sensitizers DiazonapthoQuinone (DNQ), Photo-Active Compound, PAC, photosensitive component of the resist material PAC acts as dissolution inhibition of resin, on absorption of UV light, PACs undergo chemical changes to increase the dissolution in alkaline developer Additives Surfactants,adhesion promoter, dyes chemicals that control specific aspects of resist material dyes can also be added into the photoresist composition to reduce scattered light from the reflection in the resist/substrate interface. EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 20 TheNovolacpolymersolid(aphenolformaldehyde polymers)isanalkali-solubleresin.Thedeveloper solution,typicallywithaconcentrationof0.2Nto 0.26Nor2.36%to2.38%ofTMAH,usedfor photolithographyprocessingisamildbase,all Novolac-based positive photoresist would be dissolved inTMAHdeveloperunlessaninhibitorDNQisadded in the photoresist formulation.The inhibitor is also called the photoactive compound, the PAC. The DNQ (diazonapthoquinone) inhibitor is sensitive to UV light and can be decomposed on absorption of UV light during exposure. A latent photoresist pattern can thus be formed on exposure to UV light through the photomask.The latent pattern in exposed areas of the photoresist film can be developed by dissolving the in TMAH developer solution. In the unexposed areas, the PAC molecules are intact, the resin dissolution in TMAH developer solution is inhibited.DNQ based positive g-line and i-line photoresist Novolac ~ 20% The base resin is novolac a long chain polymer consisting of hydrocarbon rings with 2 methyl groups and 1 OH group attached. DNQ 1 10% EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 21 TheSolventpropyleneglycolmethylether acetate(PGMEA)givestheflowcharacteristics oftheresist.Itmustbecompatiblewiththe NovolacpolymerandthePAC.Itspercentage contentdeterminestheviscosityoftheresist andhencetheresistfilmthicknessoncoating.The solvent is removed during the spin coating, soft bake and hardbake steps.The additives are chemicals that control specific aspects of resist material : Surfactants,adhesion promoter, dyes, DNQ based positive g-line and i-line photoresist PGMEA ~ 70% EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 22 Light exposure breaks N2 bond in DNQ complex molecules, eventually transforming to carboxylic acid Chemical reaction of DNQ based g-line and i-line resists Dissolution inhibition chemistry. One-step chemistry during exposure. ThecarboxylicacidishydrophilicandtheresultingN2bubbles cause the exposed areas of the photoresist film to become porous. These two factors further enhance the dissolving capability of the TMAH developer solution on the UV exposed areas. inden-3-carboxylicAcid (dissolution enhancer) WolfRe-arrangement Ketene PAC The water in this reaction is obtained from humidity in the air and also the small amount of water present in the photoresistfilm. If the air is not humid enough, the remaining carbon bond will bond with the resin, creating an insoluble material. EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 23 Under UV exposure, the weakly bonded N2 free from the carbon ring, leaving a highly reactive carbon site. One of the carbons moves outside the ring to stabilize the structure. The oxygen atom is then covalently bonded to this external carbon atom. This process is known as a Wolff rearrangement. The resultant molecule is called ketene The ketene immediately reacts with water molecule in the resist matrix to form carboxylic acid. Carboxylic acid is soluble in base solution.Dissolution rate in developer (hydroxide) changes Key idea is the differential solubility of about 100:1 Chemical reaction of DNQ based g-line and i-line resists Resin compositionremarkDissolution rate NovolaconlyWithout sensitizer150 /sec Novolacwith DNQWith sensitizer10-20 /sec Novolacwith Indene Carboxylic acidAfter exposure1000-2000 /sec EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 24 Resist exposed to light dissolves in the developer chemical. Unexposed resist, containing PACs, remain crosslinked and insoluble to developer chemical. Pre-exposure + photoresist Post-exposure + photoresist Post-develop + photoresist UV Oxide Photoresist Substrate Soluble resist Exposed Unexposed PAC PAC as Dissolution Inhibitorin Positive i-line Resist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 25 4. DUV photoresists - Chemically Amplified (CA) Resists DNQ resist has large absorption problem below 365 nm wavelength and not suitable for DUV technology DUV resists rely on a new principle, so-called chemically amplified (CA) resists.A catalyst, the photo-acid generator (PAG), replaces the PAC. Absorbance of DNQ resist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 26 Components of Chemically Amplified(CA) DUV Photoresists ComponentTypical ChemicalsPurposeRemarks SolventPropylene Glycol Methyl Ether Acetate (PGMEA) gives resist its flow characteristics Compatible to other components Resin- tertiary-butoxycarbonyl parahydroxystyrene (tBOC-PHS) (248nm) - phenolic copolymer- cyclic olefin / maleic anhydride (COMA) polymers (193nm) tertiary-butoxycarbonyl (tBOC) dissolution inhibitor protection group for 248CA resist;gives resist mechanical and chemical properties Protection group makes it insoluble in developer.

Transparent to 193nm DUV Etch resistance to plasma Sensitizerstriphenylsulfonium saltPhoto-Acid-Generator, PAGphotosensitive component of the resist material PAG acts as H+ catalyst for the de-protection of dissolution inhibitor of resin Additives Low MW additives to increase contrast, Surfactants,adhesion promoter, dyes chemicals that control specific aspects of resist material dyes can also be added into the photoresist composition to reduce scattered light from the reflection in the resist/substrate interface. EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 27 100 80 60 40 20 0 248 nm Relative Intensity (%) KrF laser emission spectrum Emission spectrum of high-intensity mercury lamp 120 100 80 60 40 20 0 200300 400500 600 Wavelength (nm) Relative Intensity (%) g-line 436 nm i-line 365 nm h-line 405 nm DUV* 248 nm DUV Emission Spectrum * I ntensity of mercury lamp is too low at 248 nm to be usable in DUV photolithography applications. Excimer lasers, such as shown on the left provide more energy for a given DUV wavelength.EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 28 Chemically Amplified (CA) Resist for 248nm DUV Two-steps chemistry; dissolution inhibition. PAG triphenylsulfonium salt Polymer/Resin -- PHS - polyhydroxystyrene Chemical reaction of DUV CA resists ExposurePEBEE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 29 Exposure: PAG irradiation forms acid at exposed areas. PEB - Post Expose Bake: Acid react with tBOC-PHS to form soluble PHS + acid; chain reaction and amplification. Acid diffuses into the unexposed area. Chain reaction stops at end of PEB process. Resist base will neutralize the acid Chemically Amplified Resist Resulting line width smaller than optically printed. EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 30 Chemically Amplified Solubility Switch for 248 nm Photoresists during PEB Before exposure, the phenol OH group of PHS is protected by the tBOC group. The base resin is insoluble in the developer solution such as tetramethylammonium hydroxide (TMAH) solution. An triphenylsulfonium salt salt acts as a photo-acid generator and generates the acid during the exposure. The acid-catalyzed reaction induces chain reactions and deprotects tBOC during the PEB. As a result, the phenol OH groups are generated in the exposed area and can be dissolved in the alkaline developer. Chemical amplification: deprotection catalyzed by protons High aromatic content => High etch resistance EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 31 Resist exposed to light dissolves in the developer chemical. Unexposed resist remains crosslinked and PAGs are inactive. Pre-exposure + CA photoresist Post-exposure + CA photoresist Post-develop + CA photoresist UV Oxide Photoresist Substrate Unchanged Exposed Unexposed Acid-catalyzed reaction (during PEB) PAG PAG PAG PAG H+ PAG PAG PAG H+ H+ PAG PAG Chemically Amplified (CA) DUV Resist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 32 1.Resin is phenolic copolymer with protecting group that makes it insoluble in developer. 2.Photoacid generator (PAG) generates acid during exposure. 3.Acid generated in exposed resist areas serves as catalyst to remove resin-protecting group (t-BOC) during post exposure bake (PEB) step. 4.Exposed areas of resist without protecting group are soluble in aqueous developer. Exposure Steps for Chemically-Amplified DUV Resist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 33 5. Metrics of Photoresist Photo activity Resolution Contrast Sensitivity Spectral Response Bleaching Effect Viscosity Etch resistance Thermal Stability Polymer Requirement Adhesion Surface tension Storage and handling Standing waves Effect Anti reflective coating Proximity effect Environmental Effects -Contamination and Particles -T-Top Formation for DUV CA resist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 34 Photospeeds of recording process ProcessASA speedDoseResolution (ergs/cm2)-1 (ergs/cm2) (lines/mm)TV tube10410-4 5 B&W film102 10-2 50 Xerox 11100 Photoresists10-5 105 5000 Source: Semiconductor Lithography Principles, practice and materials, Wayne M. Moreau, 1988 Plenum press, New York. Photo Activity Photoresist is one of the most photo-sensitive chemical processes EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 35 Resolution = how fine a line the resist canreproduce from an aerial image Resolution of resist is determined by Contrast, Thickness, Proximity effects Swelling and contraction after development Contrast = ability of resist to distinguish between light and dark regions Measured by exposing the resist of given thickness to varying radiation dose and measuring dissolution rate EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 36 Poor Resist Contrast Sloped walls Swelling Poor contrast Resist Film Good Resist Contrast Sharp walls No swelling Good contrast Resist Film Resist Contrast EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 37 Positive Resist Negative Resist Resist Contrast D100 = exposure energy dose for complete resist removal D0= threshold exposure energy dose for resist removal Typical values under fixed developer conditions Resists with higher contrast result in better resolution because of more vertical resist profile Contrast Curves EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 38 ( )0 100D / D log1= Large sensitivity for CA DUV resists compared with conventional DQN resist: 20-40 mJcm-2 compared to 100 mJcm-2 typical for DNQs Post-exposure bake very critical in DUV resist technology (chemical reaction occurs) Sensitivity and contrast for resists Remaining photoresist against exposure dose Sensitivity : D100 (Df) Chemical Contrast --- slope of curve defined as : High high resolution/contrast e.g.DNQ g-line/i-line resist : = 2 - 3 DUV CA resist : = 5 - 10 EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 39 Critical Resist Modulation Transfer Function(CMTF) In order to print lines: MTFexposure system CMTFresist Incident energy necessary to produce the photochemical reactions required for defining patterns Related to quantum yield Higher sensitivity required at shorter wavelength because of limited brightness of UV sources and optics efficiency Trade-off between exposure time and brightness Sensitivity absorbed photon of #events induced photon of # = |EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 40 Effect of Sensitivity on Contrast If I is intensity, is the absorption coefficient TR is the resist thickness The resist contrast can be shown to be where is a dimensional constant,Spectral Response Absorbance should not be too high to prevent excessive absorption of UV light Upon exposure, resist must be more transparent EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 41 Bleaching : the absorbance of most resists typically decreases on exposure. The actinic absorbance is defined as the difference between the unexposed and exposed absorbance.Bleaching can provide a more uniform exposure: the top layers of the resist become partially transparent when they are exposed, allowing a fuller exposure of the lower layers, thus achieving a vertical sidewall. DNQ resist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 42 80000 70000 60000 50000 40000 30000 20000 10000 0 1000 2000 300040005000 6000 7000 Spin Speed (RPM) Spin Speed Curve of IX300 (thick DNQ resist) for different viscosity Resist Thickness () 21 cP 110 cP 70 cP Resist Viscocity-- Effect of solvent and -- Spin Speed Curve Solvent keeps photoresist in liquid state Allows spin coating of the photoresist Solvent content determines viscosity and hence thickness! EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 43 Etch Resistance = Once patterned, resist must be able to stand up to etch chemistry (i.e. protect underlying material) and must be thermally stable Etch Resistance Other important resist parameters Thermal stability Adhesion (improved using HMDS-hexamethyldisilizane) Viscosity (% of solid content) Particulates and Metal content Flash point and TLV rating Process latitude, consistency, shelf-life EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 44 Thermal Stability of Photoresist and Enhancement of Thermal Stability with DUV Cure after Development High temperature bake are used to harden resist against further energetic processes such as ion implantation and plasma etching At sufficiently high enough temperature, the resist reflows changing the profile which can be troublesome for patterning some unique structures Post Development DUV Cure improves Thermal Stability EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 45 Chemical Reactivity UV Transparency not opaque Reactive Ion Etch Resistance Technological Requirement for Photoresist Polymer backbonesNovolac based resist Light should be absorbed by PAG, not binder polymer Avoid binder decomposition and outgassing Use light efficiently Allow light to penetrate to bottom of film for straight sidewalls Absorbance < 2.0 m-1,preferably < 1.0 m-1 EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 46 The Need for Technology Shift -Novalac is not transparent at < 300nm for DUV -Parahydroxystyrene (PHS) based polymer backbones for 248nm DUV become opaque at 193nm DUV -2 New material classes suitable for 193nm DUV - Acrylics - cyclic olefin / maleic anhydride (COMA) polymers -Acrylics suffer from poor reactive ion etch resistance etch too fast -Cyclic Olefins are the technological choice for 193nm -perfluoropolymer for future 157nm Technological Requirement for Photoresist Polymer backbonesEE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 47 Absorption Spectra of Photoresist Polymer Backbones EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 48 Design of a 193 nm Photoresist Polymer Etch resistance from polycyclic groups in backbone or side chains Development by de-protection of tertiary alkyl ester EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 49 Design of a 193 nm CA Resist for Immersion Lithography Chemically amplified resists (CARs) utilized at 193nm Immersion Lithography are formulated with the following primary components;- the polymer resin,- casting solvents, - Photo-Acid Generator (PAG), and- base quencher.- Additional components offer additional functionalities as needed in a given formulation.Photo-Acid GeneratorsThe photoacid generators (or PAGs) react with photons, creating an acid which reacts with the polymer resin to deprotect the resin. The acid deprotection then allows the developer to dissolve the polymer chain. Iodonium SaltsTBI-PFOS tert-butylphenyliodonium perfluorooctanesulfonate Sulfonium SaltsTPS-PFBS triphenylsulfonium perfluorobutanesulfonate (or TPS-Nf triphenyl sulfonium nanoflate)TPS-Tf triphenylsulfonium trifluoro sulfonate (or triphenyl sulfonium triflate)EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 50 Polymer ResinsThe components of the polymer resin, generally a ter-polymer (3 components) or tetra-polymer (4 components), are the backbone of the resist matrix.MMA methyl methacrylateTBMA t-butyl methacrylateMAA methacrylic acidMAdMA 2-methyl-2-adamantanol methacrylate (for dry etch resistance)GBLMA gamma butyrolactoneHadA MLMA mevalonic lactone methacrylate (for wetability and adhesion)COMA cycloolefine-maleic anhydride BNC - t-butyl-5-norbornene-2-carboxylate MA - maleic anhydride HNC Hydroxyethyl-5-norbomene-2-carboxylateNC 5-norbornene-2-carboxylic acidVEMA vinyl ether-maleic anhydrideDesign of a 193 nm CA Resist for Immersion Lithography EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 51 Structure of hybrid COMA/methacrylate copolymer(Clariant T2030) Design of a 193 nm CA Resist for Immersion Lithography EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 52 Casting SolventsThe casting solvent functions to dissolve the polymer resins and act as a carrier to uniformly distributethematerialduringtypicalspin-coatingprocesses,thenpredominantly evaporating away during the post-application bake (or PAB).PGMEA propylene glycol methyl ether acetateEL ethyl lactateMAK methyl amyl ketone PGME propylene glycol monomethyl ether Base QuenchersThe base quenchers limit the diffusion of the photogenerated acid and minimize blur.TBAH tetra butyl ammonium hydroxideTEA triethanolamineTPA tripentylamine Dissolution I nhibitorLithocholate DI dissolution inhibitor DeveloperThe developer dissolves the deprotected polymer chains in the resist matrix.TMAH tetramethyl ammonium hydroxide (0.26N industry standard)Design of a 193 nm CA Resist for Immersion Lithography EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 53 Adhesion and Surface tension Si SiSi SiSi Si Si Silicon wafer Si(CH3)3 O Si(CH3)3 O Si(CH3)3 O Si(CH3)3 O Si(CH3)3 O Si(CH3)3 O Si(CH3)3 O OH Si SiSi SiSi Si Si Silicon wafer OHOHOHOHOHOH + NH3 Storage and handling Wafer pre-clean HMDS priming hexamethyldisilazane (HMDS) turns wafer surface from hydrophilic to hydrophobic for better photoresist adhesion. Si-dioxide + H2O + HMDS hexamethyldisiloxane + ammonia Proper storage and handling of chemicals for repeatable photolithography results on wafers EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 54 Polysilicon Substrate STI STI UV exposure light Mask Exposed photoresist Unexposed photoresist Notched photoresist Edge diffraction Surface reflection Photoresist Reflective Notching Due to Light Reflections from non-planarized surface Problems with Reflective wafer Surface EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 55 Standing Wave Effect in photoresist Standing wave effect causes modulation of the developed photoresist edges EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 56 Reflected waves from reflective wafer surface cause standing waves and hence non-uniform exposure along the thickness of the photoresist film and over surface topology. Incident wave Reflected wave Photoresist Film Substrate Incident and Reflected Light Wave Interference in Photoresist Standing Waves Effect EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 57 Cause of Standing Wave Effect Light reflectance inside the resist cause standing waves: Standing Wave intensity depends on:Resist thickness Resist absorbanceLight incident angleThe substrate film: reflectivity, refractive index Periodn 2Period=n : Resist refractive index EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 58 Variation of Exposure Dose due to Standing Wave Effect The interference standing waves may cause modulation in the exposure dose (total UV energy per unit area) for complete exposure of the photoresist to vary with the photoresist film thickness. The standing wave interference effect will also result in the variation in linewidth with changing photoresist thickness and such phenomenon is sometimes called the Swing Curve. Swing Curve ---Variation of Exposure Dose modulation by g-line with resist film thickness Thickness fine tuning done by swing curve chart. Preferred resist thickness: an extreme point of the swing curve to reduce line-width variation. EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 59 Reduction of Standing Wave Effect Reflected waves from wafer surface in particular those reflective layers such as metals causes reflective notching and standing wave effect Standing wave effect is a serious problem for fine line lithography when exposing on reflective surfaces Suppression of reflected waves : - Dyeing the photoresist to increase absorption on exposure and reduce reflected waves intensity - Post Exposure Bake (PEB) - Antireflective coating (ARC) to reduce reflection wave intensity -- - Bottom ARC (BARC) prior to resist spinning - Top ARC (TARC) after resist spinning EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 60 Reduction of Swing Curve Amplitude Increase absorption o by adding dye to the photoresistARC Anti Reflective Coating reduces reflection reduces reflection wave amplitudes (R1 top, R2 Bottom) EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 61 Reduction of Reflection by Dyed Resist The exposure modulation can be partially compensatedusingdyedversionofthe photoresist by increasing the absorbance oftheexposedphotoresistandhence reducingthereflectedUVlightintensity from the reflective substrate.IncreasedabsorbanceofS1813J2 dyedversion photoresist on exposure Absorpbance of S1813 photoresist Reduction of Swing Curve amplitude with dyed photoresist EE6601 MicroFabrication Technology Photoresist Technology NTU-EEE-Tse MS 62 Problem with Dyed Photoresist ---- Excessive Resist Absorption Photoresist(after develop) Substrate Sloping profile 1.The light intensity at the bottom of the resist is considerably less than that received at the top 2. To achieve straight-wall images, the resist absorption