Dale E. Ewbank dale.ewbank@rit.edu unl081413_microe.ppt 2013 Dale E. Ewbank page 1
OUTLINE Masks Optical Lithography Photoresist Sensitivity Processing Exposure Tools Advanced Processes page 2
MICROLITHOGRAPHY The production of ultra-small three dimensional relief images based on exposure and subsequent development of a photon sensitive polymer called photoresist. DRAM LITHO FEATURE EXPOSURE TOOL YEAR SIZE WAVELENGTH COST 1985 1000 nm 436 nm $0.8 MILLION 1988 750 436 $1.0 1991 500 365 $3.0 1994 350 365 $5.5 1997 250 248 $7.0 2003 110 248 $10 2006 65 193 $20-30 2009 45 193 or 193i $50-80 2012 22 193i double patterning $70-90 2012 Dale E. Ewbank page 3
MASKS Reduction reticle with pellicle: ASML, Canon, Nikon, others 1X projection with pellicle: Perkin Elmer Micralign page 4
TRANSMISSION PROPERTIES OF OPTICAL GLASS page 5
Ultra Violet Radiation Ultra Violet Radiation 193 nm ArF laser 248 nm KrF laser 365 nm i-line 436 nm g-line Modified from http://www.osram.co.at/_global/pdf/professional/display_optic/semiconductor_medical/hbo-ic/hbo_ic.pdf page 6
Optical Lithography Illumination System Source Illumination System Source Condenser Lens Condenser Lens Mask Gap (z) Mask Substrate Objective Lens Proximity/Contact Projection NA = n sinq q Substrate page 7
DIFFRACTION page 8
FRESNEL DIFFRACTION Proximity/Contact System Resolution Wmin ~ 0.7 ( Z) 1/2 W = space opening width on the mask = exposing wavelength Z = gap (distance) from the mask to the wafer Note that Z cannot go be zero because of resist thickness page 9
RAYLEIGH CRITERIA Projection System Resolution P/2 = k 1 / NA NA = n sinq DOF = +/- k 2 / (n sin 2 q) k 2 = 0.5 g-line i-line KrF KrF ArF ArF ArF k 1 0.61 0.61 0.50 0.35 0.43 0.40 0.26 [nm] 436 365 248 248 193 193i 193i NA 0.28 0.50 0.50 0.70 0.93 1.20 1.35 P/2 [nm] 950 450 250 124 90 65 38 DOF[+/- nm] 2780 730 500 253 111 95 54 2011 Dale E. Ewbank page 10
NEGATIVE AND POSITIVE PHOTORESISTS Light Photomask Photoresist Film to be Patterned Substrate Negative Resist Positive Resist Rendered Insoluble Rendered Soluble After Development After Etch and Strip page 11
ONE AND TWO COMPONENT RESIST HIERARCHY Positive Resist Chemically Amplified Resist Components Components ONE TWO TWO Acrylates (PMMA) Diazo naphtho quinone Novolac (Shipley 1400) Acid Generator -- Onium salt Polymer backbone -- Polystyrene page 12
NOVALAC RESIN OH OH CH 2 CH 2 n CH 3 CH 3 Novalac resins are soluble in organic solvents, exhibit good film forming characteristics, are capable of combining with orthoquinone diazide sensitizers page 13
PHOTOACTIVE COMPOUND (PAC) O N 2 hu R Naphthoquinone diazide (PAC) R Keto-carbene and Nitrogen O + N 2 R R C O Stable ketene molecule add H 2 O H O C OH Base soluble carboxylic acid page 14
POSITIVE NOVOLAC PHOTORESIST Novolac Base Matrix Resin - 5-15% Sensitizer or Photoactive Compound (PAC) - 5-15% Casting Solvent - 60-80% Additives - 10% PAC - such as naphthoquinone diazide Solvents - such as EL (Ethyl Lactate) or PGMEA (Propylene Glycol Monomethyl Ether Acetate) Additives - such as adhesion promoters, surfactants, dyes, antioxidants, polymerization inhibitors Unexposed resists dissolves in an alkali developer at a rate of 2 nm/sec Partially exposed resists dissolves in an alkali developer at a rate of 200 nm/sec page 15
Absorbance SENSITIVITY unexposed resist 0.6 Resist absorbs light in the base resin and in the PAC. The difference is the more important 0.4 parameter difference 0.2 exposed resist 300 340 380 420 460 Wavelength (nm) 500 page 16
Deep ULTRA VIOLET RADIATION page 17
CHEMICALLY AMPLIFIED RESIST Poly(t-butoxycarbonyloxystyrene) with onium salt cationic photoinitiator for acid generation page 18
THICKNESS LOG DOSE g Dose to clear The higher the slope, gamma, then the smaller the difference needs to be between exposure in areas to be cleared and areas to leave resist. That is the required aerial image modulation is smaller. page 19
MODULATION Mask m(x) Modulation = Imax - Imin Imax + Imin Aerial image I(x) 1 Imax Imin 0 Ideal Actual Wafer page 20
PROCESSING Substrate Cleaning Coat track Priming if needed BARC coat and bake Spin Coating Photoresist Soft-Bake (PAB) Top Coat and bake if needed Stepper Exposure Develop track Post Exposure Bake (PEB) Develop Rinse Hard-Bake (PDB) Etching Stripping page 21
SUBSTRATE CLEAN AND PRIME Cleaning is done with a high pressure (2000 psi) water scrub A dehydration bake is typically done on a hot plate at 250 C for 1 min. (Wafers are clean and dry just after removing from oxide growth furnace) HMDS (hexa-methyl-di-silizane), - Adhesion promoter or primer: Are commonly applied as a liquid or vapor. HMDS attaches to remaining OH molecules releasing ammonia gas and creating an organic-like surface improving adhesion Too much HMDS is detrimental to sensitivity and adhesion. page 22
HMDS PRIME page 23
SPIN COATING Most spin coating is performed at spin speeds from 3000 to 6000 RPM for 20 to 60 seconds, producing coating uniformities to +/- 100 Å page 24
SOFT-BAKE The main purpose is to reduce the solvents from a level of 20-30% down to 4-7%. Baking in a convection oven about 20 minutes is equivalent to hot plate baking for about 1 minute. Forced Air Oven Exhaust Photoresist wafer Hot Plate Fan 90 TO 100 C page 25
EXPOSURE E = I t where E is exposure dose in mj/cm 2 I is irradiance in mw/cm 2 t is exposure time in seconds Humidity should be 40-44% because exposed PAC requires water to convert to carboxylic acid page 26
POST EXPOSURE BAKE Post exposure bake increases speed of resist Post exposure bake reduces standing wave effects Post exposure bake is require for chemically amplified and image reversal resists (100 to 115 C for 1 min.) page 27
DEVELOP AND RINSE Develop is done in an alkali solution such as NaOH or KOH (Metal Containing Developers) Trace quantities of these metals can cause transistor threshold voltage shifts. Metal Ion Free Developers are available (TMAH) Developer Concentration and Temperature of Developer are the most important parameters to control. page 28
HARD BAKE Hard Bake is done at or slightly above the glass transition temperature. The resist is crosslinked (and is toughened prior to plasma etch). The resist flows some as shown below. Pinholes are filled. Improves adhesion also. No flow should occur at the substrate. Photo stabilization involves applying UV radiation and heat at 110 C for dose of 1000 mj/cm 2 then ramping up the temperature to 150-200 C to complete the photo stabilization process. After Develop After Hard Bake 125 to 140 C for 1 min. page 29
ETCHING Isotropic Etching - etches at equal rate in all directions Oxide wet etched Photoresist Wet Chemical Etching - is isotropic Anisotropic Etching - etches faster vertically than horizontally Plasma Etching (Dry Etch or Reactive Ion Etching, RIE) is either isotropic or anisotropic depending on ion energy and chemistry of etch. page 30
STRIPPING Oxygen Plasma Ashing Plasma Damage is Possible to sensitive gate oxide layers Hot Sulfuric Acid and Hydrogen Peroxide If underlying layers are not etched by these chemicals Organic Solvents are available page 31
GCA 6700 G-LINE Stepper page 32
ASML 5500/200 NA = 0.48 to 0.60 variable = 0.35 to 0.85 variable With Variable Kohler, or Variable Annular illumination Resolution = K1 /NA = ~ 0.35µm for NA=0.6, =0.85 Depth of Focus = k 2 /(NA) 2 = > 1.0 µm for NA = 0.6 i-line Stepper = 365 nm 22 x 27 mm Field Size Microlithography 2011 Dale E. Ewbank page 33
STEPPER UV-Light Source Illumination Diffuser Filter Condenser Lens, NA C Reticle Alignment Motors Fiducial Marks Reticle Thru Lens Alignment Detector Aperture Blades Alignment Microscope and TV Zeeman Two Frequency Laser Interferometer 5X Reduction Lens, NA O Auto Focus Stage Motors Reference Mirror X Y Baseline Auto Leveling X-Y Stage page 34
STEP AND SCAN page 35
ADVANCED PROCESSES Tri-layer Process Bi-layer Process Lift-off Process Reversal Process Dyed Resists Anti-reflective Coatings Contrast Enhancement Materials Chemically Amplified Resists Silylation Process page 36
TRI-LAYER Film to be Etched Substrate with Topology Coat with Planarizing Layer Coat with Barrier Layer Coat with Photoresist Image Photoresist Etch Barrier Layer Reactive Ion Etch Planarizing Layer Etch Film page 37
SILYLATION Film to be Etched Polymerized Areas Coat with Planarizing Layer of Photoresist SiO2, 10% in Photoresist Expose Desired Pattern causing exposed areas to become polymerized Soak in HMDS Vapor Note: HMDS is incorporated into the non polymerized areas only Reactive Ion Etch in Oxygen coverts silicon into SiO2, Only polymerized areas are etched. Silicon containing areas form and in-situ mask After Reactive Ion Etch Etch Film page 38
LIFT-OFF 1. Create a reverse slope or undercut resist edge profile 2. Deposit film by evaporation 3. Chemically strip photoresist and lift off film, leaving film in desired pattern Substrate Photoresist Film page 39
REVERSAL PROCESS No Extra Processing Normal Process After Development Photoresist Substrate Coat Expose Reversal Process Post Exposure Reversal Bake Flood Expose After Development page 40
REFERENCES Microlithography Science and Technology, Second Edition, edited by Kazuaki Suzuki and Bruce W. Smith, CRC Press, 2007. Introduction to, Second Edition, Edited by Larry F. Thompson, C.Grant Willson and Murrae J. Bowden, ACS Professional Reference Book, American Chemical Society, Washington, DC 1994., David Elliott, McGraw Hill Book Company, 1986. www.osram.co.at/_global/pdf/professional/display_optic/semiconductor_medical/hbo- IC/hbo_ic.pdf, accessed 03/13/2009. www.mellesgriot.com, accessed 06/10/2008. http://www.itrs.net/reports.html, accessed 03/14/2011. 2012 Dale E. Ewbank page 41