Photolithography References: Introduction to Microlithography Thompson, Willson & Bowder, 1994 Microlithography, Science and Technology Sheats & Smith, 1998 Any other Microlithography or Photolithography book Contributors: Dr Nuri Amir, Dr Nava Ariel, Inbar Lifshitz and Oded Cohen 1
Contents Chapter 1: Introduction to Photolithography Chapter 2: Basic Photolithography Optics Chapter 3: Resist Bulk Effects Chapter 4: Characterization of Process Window 2
Integrated Circuits: Isolation Metal P P N N N Well P Well 4
Isolation Metal P P N N N Well P Well 5
Light Mask Isolation Photo - Resist Metal P P N N N Well P Well 6
Isolation Developer Photo - Resist Metal P P N N N Well P Well 7
After rinse: Isolation Photo - Resist Metal P P N N N Well P Well 8
Etch: Isolation Photo - Resist Metal P P N N N Well P Well 9
Strip: Remove of the Photo resist Isolation Photo - Resist Metal P P N N N Well P Well 10
Isolation Metal P P N N N Well P Well 11
Introduction Photolithography Introduction Definition Photolithography: The process of duplicating twodimensional master pattern with the use of light Basic requirements: Mask with the desired pattern (Reticle) Illumination system Flat surface, covered with Photosensitive material (Photo-resist) Carefully controlled environment: vibrations, pressure, humidity, temperature, and light. 12
Introduction Current requirements 2005 requirements in Semiconductors industry (ITRS road map): Print 2D layout with lines as narrow as ~60nm with variation less than 6nm Align plates with maximum error of 20-30nm depending on layer * Source: International Technology Roadmap for Semiconductors website: http://www.itrs.net/links/2005itrs/litho2005.pdf Photolithography: ~1/3 of one chip manufacture total cost 13
Introduction Current requirements Table LITH2 Lithography Technology Requirements Year of Production 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 DRAM DRAM ½ pitch (nm) 36 32 28 25 23 20 18 16 14 13 11 10.0 8.9 8.0 7.1 6.3 CD control (3 sigma) (nm) [B] 3.7 3.3 2.9 2.6 2.3 2.1 1.9 1.7 1.5 1.3 1.2 1.0 0.9 0.8 0.7 0.7 Contact after etch (nm) 36 32 28 25 23 20 18 16 14 13 11 10 8.9 8.0 7.1 6.3 Overlay [A] (3 sigma) (nm) 7.1 6.4 5.7 5.1 4.5 4.0 3.6 3.2 2.8 2.5 2.3 2.0 1.8 1.6 1.4 1.3 k1 (13.5nm) EUVL 0.66 0.59 0.52 0.62 0.55 0.49 0.44 0.51 0.45 0.40 0.47 0.42 0.37 0.33 0.29 0.26 Flash 2014 requirements in Semiconductors industry (ITRS road map): Flash ½ pitch (nm) (un-contacted poly) 22 20 18 17 15 14.2 13.0 11.9 10.9 10.0 8.9 8.0 8.0 8.0 8.0 8.0 CD control (3 sigma) (nm) [B] 2.3 2.1 1.9 1.8 1.6 1.5 1.4 1.2 1.1 1.0 0.9 0.8 0.8 0.8 0.8 0.8 Bit line Contact Pitch (nm) [D] 131 120 110 101 93 113 104 95 87 80 71 64 64 64 64 64 Contact after etch (nm) 36 32 28 25 23 20 18 16 14 13 11 10 8.9 8.0 7.1 6.3 Overlay [A] (3 sigma) (nm) 7.2 6.6 6.1 5.6 5.1 4.7 4.3 3.9 3.6 3.3 2.9 2.6 2.6 2.6 2.6 2.6 k1 (13.5nm) EUVL 0.40 0.37 0.34 0.41 0.38 0.35 0.32 0.38 0.35 0.32 0.37 0.33 0.33 0.33 0.33 0.33 MPU / Logic MPU/ASIC Metal 1 (M1) ½ pitch (nm) 38 32 27 24 21 19 17 15 13 12 11 9.5 8.4 7.5 6.7 6.0 MPU gate in resist (nm) 35 31 28 25 22 20 18 16 14 12 11 9.9 8.8 7.9 6.8 5.9 MPU physical gate length (nm) * 24 22 20 18 17 15 14 13 12 11 9.7 8.9 8.1 7.4 6.6 5.9 Gate CD control (3 sigma) (nm) [B] ** 2.5 2.3 2.1 1.9 1.7 1.6 1.5 1.3 1.2 1.1 1.0 0.9 0.8 0.8 0.7 0.6 Contact after etch (nm) 43 36 30 27 24 21 19 17 15 13 12 11 9.5 8.4 7.5 6.7 Overlay [A] (3 sigma) (nm) 7.6 6.4 5.4 4.8 4.2 3.8 3.4 3.0 2.7 2.4 2.1 1.9 1.7 1.5 1.3 1.2 k1 (13.5nm) EUVL 0.70 0.59 0.50 0.58 0.52 0.46 0.41 0.48 0.43 0.38 0.44 0.39 0.35 0.31 0.28 0.25 Chip size (mm 2 ) Maximum exposure field height (mm) 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 Maximum exposure field length (mm) 33 33 33 33 33 33 33 33 33 33 33 33 33 33 33 33 Maximum field area printed by exposure tool (mm 2 ) 858 858 858 858 858 858 858 858 858 858 858 858 858 858 858 858 Wafer site flatness at exposure step (nm) [C] 38 34 30 27 24 21 19 17 15 13 12 11 10.0 9.0 8.0 7.0 Number of mask Counts MPU [E] 50 54 44 50 Number of mask Counts DRAM [E] 41 33 38 Number of mask Counts Flash [E] 43 31 Wafer size (diameter, mm) 300 300 300 300 300 450 450 450 450 450 450 450 450 450 450 450 NA required for logic (single exposure) 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 NA required for double exposure (Flash) 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 NA required for double exposure (logic) 1.12 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 EUV (13.5nm) NA 0.25 0.25 0.25 0.33 0.33 0.33 0.33 0.43 0.43 0.43 0.56 0.56 0.56 0.56 0.56 0.56 P n le A o Manufacturable solutions exist, and are being optimized Manufacturable solutions are known Interim solutions are known Manufacturable solutions are NOT known Photolithography: ~1/2of one chip manufacture total cost 14 * Source: International Technology Roadmap for Semiconductors website: http://www.itrs.net/links/2005itrs/litho2013.pdf
Introduction The process The Photolithography Process: Printing process consists of 3 steps: Coat, Expose, & Develop Performed by two machines linked together: Stepper & Track 1. Track: Coats the Si wafer with Photosensitive resist material 2. Stepper: Exposes the resist (1) Coat (Spin) by the Mask pattern (2) Expose 3. Track: Develops the exposed resist (3) Develop the Mask pattern is left on the wafer 15
Introduction The process Stepper & Track link: Chill plate Track (Coater/Developer) 16
Introduction The process The Mask The Mask (Reticle) One mask per wafer layer Made of Quartz (transparent at UV) & Chrome Must be perfect (+/-2 nm divided by 4) Cost: $1K - $500K, depend on it s complexity Side view Top view 17
Introduction photolithography printing machine Performance Trend Motivation for scaling (reduction of transistor size): Functionality, Speed, Power Economics - more chips per wafer higher yield Necessary progress - photolithography printing machine From: Intel technology Journal Q3 98 http://www.intel.com/technology/mooreslaw/index.htm 18
Introduction photolithography printing machine Technology Evolution Year Light Projection Method Size Ratio Light Source (l) Wave-length 1970 Contact 1:1 Hg Lamp 463nm (G-Line) 1980 Proximity 1:1 Hg Lamp 463nm (G-Line) 1985 Step & Repeat 1:5 Hg Lamp 463nm (G-Line) 1991 Step & Repeat 1:5 Hg Lamp 365nm (I-Line) 1994 Step & Scan 1:4 Hg Lamp 256nm (DUV) 1998 Step & Scan 1:4 Excimer Laser 248nm 2001 Step & Scan 1:4 Excimer Laser 193nm 2009 Step & Scan 1:4 Wet+Excimer Laser 193nm 201? Step & Scan 1:4 EUV 13.6nm 19
Introduction Printing Methods Contact Printing Printing methods: Contact Printing Properties Mask is in physical contact with the wafer Mask covers the entire wafer Limitations Mask gets dirty and damaged Wafer non-flat surface affects printing quality 20
Introduction Printing Methods Proximity Printing Printing methods: Proximity Properties Mask covers the entire wafer Small gap d between mask and wafer Limitations Resolution limit: minimum feature size ~ 3 ld (for l=365nm minimum feature size d~24 m) 3 m for 21
Introduction Printing Methods Stepping Printing methods: Step & repeat Projection printing Expose one or more dies at a time (one field) Use reduction lens (1:5) Focus correction at each step Limited ~25X25mm field size Today steppers can print 350nm with l = 248nm Mask wafer 22
Introduction Printing Methods Stepping & Scanning Printing methods: Step & Scan Step (between fields) and Scan (within field) Scan: Both reticle and wafer move during exposure Requires stage and reticle excellent sync Reduced lens active area 25X8mm higher quality (uniformity) Mask Focus while scanning Resolution: 65nm wafer 23
Introduction Light Sources Light Sources: Hg Lamp Hg lamp (365nm: I-Line, 256nm: DUV): Simple for use / easy to replace Low power at small wavelength a resolution limiter Wide band width: needs filtering at illuminator sensitive to chromatic (wavelength driven) aberrations 24
Introduction Light Sources Light Sources: Excimer Laser Advantages High Power at UV Bandwidth narrowing (highly coherent source) Problems Pulsed (~30nsec, ~100Hz) can damage the optics Cost, space, facilities 25
Chapter 2 Basic Photolithography Optics Light Geometrical Optics Wave optics Projection optics Resolution limit Coherence of light 26
Optics Basics The Electro-Magnetic Wave The duality of light Is it a ray of particles? Is it a wave? De Broglie s equation 27
Optics Basics The Electro-Magnetic Wave The Electro-Magnetic Wave Light is composed of perpendicular components of electrical and magnetic fields Both components are perpendicular to the wave propagation direction C=lsn=const. E=hsn h - plank s constant 28
Optics Basics The Electro-Magnetic Wave The Electro-Magnetic Spectrum 29
Optics Basics Geometrical Optics Geometrical Optics The wave nature of light is neglected D (feature size) >> l (wavelength) Basic Principles: i r Propagates in straight line within the same medium Incident angle i = reflectance angle r Snell law - light changes direction by the difference in refractive indices n 1 *sin(α) = n 2 *sin(β) n1 n2 30
Optics Basics Interaction of Light an Matter Interaction of Light and Matter When an EM wave strikes an atom, it make its electrons cloud oscillate. That oscillation produce a time delayed EM wave. n1 n2 [light in matter will travel at a different speed v. v C ALWAYS The index of refraction: n=c/v 31
Optics Basics Geometrical Optics Lens Lens From practical considerations spherical lens are used: Spherical lens approximation for near main axis illumination Focal length f: 1 f ( n i 1 1 1)( ) R 1 R 2 Image Formation: R1,2 - lens rad D1,2 - object, image F - focal distance 1 d 1 1 d 2 1 f object f image 32
Optics Basics Geometrical Optics Lens aberrations Lens Aberration Lens imperfections (aberrations) are inevitable As long as the total aberration induced error << l (1/10) the effect on patterning/resolution will be acceptable diffraction limited optics Aberration types: Chromatic Spherical Coma Astigmatism Hubble Space Telescope 33
Optics Basics Interaction of Light an Matter Dispersion: n=n(l) Typical lens material have High n change at UV n l 34
Optics Basics Geometrical Optics Lens aberrations Spherical Spherical h The focal plane depends on the ray s distance from the focal axis (h) Optimal focal plane Optimal focal plane changes with h(max) Can be minimized with a proper choice of lens 35
Optics Basics Geometrical Optics Lens aberrations Chromatic Chromatic n=n(l) Optimal focal plain depends on the band width Two wavelengths can be achromatized by a proper lenses combination Photolithography: Using a very narrow bandwidth or monochromatic light, for example laser: l=248nm, 0.08pm bandwidth 36
Optics Basics Geometrical Optics Lens Stepper optics Stepper optics Stepper simplified optical scheme: Light Source Condenser lens Mask Objective lens Wafer Designed objective lens involves complex set of ~30 individual lenses: Disadvantage power loss! 37
Optics Basics Wave Optics Diffraction Wave Optics: Diffraction Geometrical optics: An opaque object makes a sharp shadow. Reality: The light bends around the edge. 38
Optics Basics Wave Optics Diffraction Wave Optics: Diffraction Geometrical optics: An opaque object makes a sharp shadow. Reality: The light bends around the edge. 39
Optics Basics Wave Optics Diffraction Fresnel and Fraunhofer diffraction Fresnel (near-field) diffraction: The screen is close to the source The image of the aperture is projected onto the screen. Fraunhofer (far-field) diffraction: The screen in very far: R the distance between aperture and screen. d the aperture s greatest width. l - the wavelength of the light R d The diffraction pattern will be the Fourier Transform of the aperture. 2 l 40
Diffracted Light Light passing trough a grid is diffracted Diffracted rays are on the order of m = 0, (+/-)1, (+/-)2... 0-2 -1 1 2 Single slit (w wide) diffraction: Sin m l w The diffracted pattern originates from interference of different waves and the phase relations between them 41
Optics Basics Projection Optics Collecting the diffracted rays The order of the diffracted rays follows the Fourier series terms. For perfect image transfer all orders should pass through the lens In order to form an image at least two orders must pass through the lens 42
Optics Basics Projection Optics Projection Optics Mask to lens: FT of image -carries information about image s spatial frequencies Lens to Wafer: Lens transforms the FT back into image at focal plane Mask FT Inv(FT) Objective lens Wafer 43
Optical Limits of resolution d * sin f = m*l, m= ( ) 1,2, m diffraction order l wavelength of light d The narrowest line-period at the reticle (Line + space) Critical Dimension: CD= ~d/2 (AKA pitch) d -2 f 2-1 CD 0 1 f Angle of diffraction order NA=sin NA(min)= l/d=sin f 44
Optics Basics Resolution Limit Optical Limits of resolution d * sin f = m*l, m= ( ) 1,2, a NA(min)= l/d CD effect: As CDs (d) get smaller angle between the orders of diffraction increases: min CD (d/2) ~ 0.5 l / NA l effect: The smaller the wavelength: more orders of interference, hence better image quality l1 l2 l1>l2 45
Optics Basics Resolution Limit Optical Limits of resolution Depth of focus: Wafer printed out of optimal focus signal is smeared Loss of CD control High NA High NA reduces depth of focus but improves resolution: Low NA Depth of Focus = k 2 l / NA 2 Resolution = k 1 l / NA (Rayleigh s formula) 46
Optics Basics Resolution Limit Optical Limits of resolution Min CDs : ~ l / NA The smaller the better Need to find the balance Depth of Focus: ~ l / NA 2 The larger the better 47
Chapter 3 Photo-resist Properties Resist Properties Resist Reaction Resist Adhesion Resist thickness control Standing waves effect Proximity effect 48
Photo- Resist Properties Resist properties Basic resist requirements: Sensitivity to Radiation at desired wavelength solubility in developer Good adhesion Flat and homogeneous coating of the wafer Controlled resist thickness Long shelf life 49
Photo- Resist Properties Resist properties Resist main components: Polymers: A long chain of molecules (phenolic resins) combined with a radiation sensitive compound (carbon rings with changing cross linking induced by light) Solvent Some resists need to be heated pre exposure in order to alter solubility 50
Introduction The process The Resist The Photo-Resist Negative and Positive Resist: Expose: Negative Positive Resist Base Light Develop: Etch: Strip: 51
Photo- Resist Properties Resist reaction Chemically Amplified Resist positive resist reaction: Phenolic resins are hydrophilic and soluble in solvents DQ is hydrophobic and causes the phenolic resins to be hydrophobic as well Light transforms DQ into an acid - ICA (Indene Carboxylic Acid) which is hydrophilic, developer can dissolve the exposed areas 52
Photo- Resist Properties Chemically Amplified Resist: After Exposure: Acid formation at exposed areas Resist reaction PEB: Post Expose Bake: Acid makes Polymer soluble and hydrophilic More acid is evolved (Chain reaction) Acid diffuses into unexposed area not desired Unexposed resist film Exposure H+ H+ H+ H+ H+ H+H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+H+ H+ End of PEB will stop the chain reaction. H+ H+ Develop: Developer spreads on wafer. Developer and developed resist are rinsed H+ H+ PEB Develop H+ H+ H+ H+ H+ H+ H+ Line width at wafer: Smaller than optical printed line 53
Resist Properties Features width (CD) Proximity Effects 1D Effects 1D Proximity Effects Features on reticle will come out with different size on the wafer depending on their proximity (interference) Wafer CD's as function of reticle pitch (Pitch =Chrome line + space ) Isolated features Dense features Resolution limit 0 Space between features 54
Resist Properties Proximity Effects 2D Effects 2D Proximity Effect Pattern on Mask: Pattern on wafer: Destructive intereference in corners resist was not exposed, corner is rounded Lines shorter Rounded corners 55
Resist Properties Proximity Effects 2D Effects 2D Proximity Effect Can be fixed by OPC: Optical Proximity Correction OPC require smaller CDs 56
Chapter 4 Characterization of Working Window Focus Exposure Matrix DOF 57
Working window Resist line profile: DOF Wafer Negative focus Focus below the resist Wafer Optimal focus Positive focus Focus above the resist Wafer Changing focus is done by controlling the distance between the wafer and the lens 58
Working window Negative focus Focus above the resist DOF Positive focus Focus below the resist 59
Characterization Of Working Window Focus position Working window Working window Resist profile: Exposure dose Positive focus Focus below the resist Negative focus Focus above the resist 60
Characterization Of Working Window Focus Exposure Matrix Focus Exposure Matrix Selecting optimal focus point: Wafer stage in optimal location when CDs don t vary with focus Selecting optimal energy dose: print bias: mask size to wafer size delta depending on desired CDs Proximity effect focus by isolated Exposure energy Focus 61
Resist profile Anti Reflective Coating ARC is used to reduce standing waves which damage resist profile 62
Back up files 63
Smaller chips Higher yield 64
Smaller chips Higher yield Edge Defects 65
EBR Edge Bid removal The resist at the edge of the wafer needs to be removed since it creates particles and makes the wafer edge sticky. There are two ways to remove it: Optical exposure (if the resist is positive) Chemical removal. 66