Where were we? Simple Si solar Cell! Two Levels of Masks - photoresist, alignment Etch and oxidation to isolate thermal oxide, deposited oxide, wet etching, dry etching, isolation schemes Doping - diffusion/ion implantation Metallization - Materials deposition, PVD, CVD
Conventional Optical Lithography Photolithography is how patterns are transferred into chips, and its the driving force behind Moore s law. Light source and condenser Mask Projection/reduction optics Photoresist Substrate
Photoresist - Basic Process Light: h Light: h Mask Photoresist Wafer Developing Positive Resist Photoresist Wafer Negative Resist
Operating wavelengths, defines Resists Traditionally, the light sources have been high pressure g arc lamps (It s what we use in this course). The aligner (why do we need to align?) filters out specific emission lines from the lamp for exposure of the photoresist g-line - = 436nm i- line - = 365nm (what we have) The drive to smaller feature size also means shorter wavelength. Why? 1) reduced absorption length 2) higher energy per photon 3) less diffraction 4) less refraction Generations after the above include (tend toward excimer laser light sources): =248nm (KrF) =193nm (state-of-the-art, ArF - 30nm features with high index immersion) =157nm (F 2 in development)
Node (International Technology Roadmap for Semiconductors) 10 m 1971 3 m 1975 1.5 m 1982 1 m 1985 800 nm (.80 m) 1989 600 nm (.60 m) 1994 350 nm (.35 m) 1995 250 nm (.25 m) 1998 180 nm (.18 m) 1999 130 nm (.13 m) 2000 90 nm 2002 65 nm 2006 45 nm 2008 32 nm 2010 22 nm 2011 16 nm approx. 2013 11 nm approx. 2015
Photoresists - (the film in the camera) 3 Component Material (relates to i-line and g-line) Matrix or Resin Determines mechanical/chemical properties Partially controls resistance to etching Novolac Industry Standard for + resist C 2 O M O PAC: Photo Active Compound Changes properties due to optical exposure Positive becomes soluble/negative becomes insoluble Controls exposure time/resolution Solvent Dissolve the matrix and PAC Determines Viscosity Controls processing conditions (spin speed, time, bakes, etc.) C 2 M
Photoresist Processing Wafer clean, dehydration, priming Main Goal is Good Adhesion: Critical during developing and etching Clean device integrity Dehydration Organics are hydrophobic Priming Particularly important for oxides MDS: exa Methyl Di Silazane Mist Chamber: Absorb a monolayer Organic Loving R R O Si O O Si O O O Water Loving
Wafer Coating #1: Dispensing Stage Not Critical Few hundred RPM Spread resist over wafer (65-85% solvent) #2: Acceleration Basically not a Problem 2000-6000 RPM in a tenth of a second Key to unifomity (solvents begin evaporating) #3: Spin Speed Controls Thickness Force Balance: Centrifugal vs. Viscous Drag #4: Spin Duration Primary Goal is to Evaporate Solvent (30%) Photoresist Dependenttypically use manufacturers data sheets
Softbake and Exposure Purpose of Softbake Step (80 110 ºC) Drive-off solvent (5%) Improve Adhesion Anneal out stress in polymer film Control film dissolution rate 2 Methods: ot plate: 30 60 seconds Oven: 25-30 minutes Considerations: Throughput, reproducibility Exposure Depends on lamp intensity, spectrum Depends critically on everything else, both before and after Post Exposure Bake? Diffusion of PAC to remove standing waves Chemically Amplified Resist, necessary to activate the photoacid generator
Development and ard Bake Developer (positive DQN) Aqueous NaO/KO Solution Temperature Sensitive: +/- 1ºC Bath or Spray Time again depends on everything else ardbake Improves (hardens) Resistance Detrimental to Profiles otplate or Oven Temperature Sensitive Photolithography in General Complex multidimensional phase space Requires careful optimization and tight process control
Photoresist, the rest of the story -Contrast curves, bleaching, planarization -Positive vs. Negative. Which is better? 1) positive 2) negative -Deep UV photoresists (Novolac absorbs below 250nm, PAC not capable of resolution below 250nm) -Chemically Amplified Resists (CAR) -Polymethyl methacrylate (PMMA) -Resists for Next Generation Lithography - Don t exist -Environmental Issues?