MICROCHIP MANUFACTURING by S. Wolf Chapter 19 LITHOGRAPHY II: IMAGE-FORMATION and OPTICAL HARDWARE 2004 by LATTICE PRESS
CHAPTER 19 - CONTENTS Preliminaries: Wave- Motion & The Behavior of Light Resolution & Depthof-Focus in Micro- Lithography Applications Lithographic Light-Sources Lithographic Exposure-Tools Projection-Printers Overlay and Wafer-Stages MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 19-2
IMAGE-FORMATION IN PHOTORESIST 3-Stages of Forming a Resist-Image Pattern Selectively expose PR with Actinic-Light & Mask Photochemical-Reaction forms Latent-Image Development forms Resist-Image Pattern 4-Components that Contribute to Forming a Resist-Image Pattern Actinic-Light Mask (or Reticle) Lens of the Exposure System PhotoResist-Film MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 19-3
ELECTROMAGNETIC-RADIATION (LIGHT) Light exhibits wave-behavior - wavelength λ Also exhibits particle-behavior - photon energy = hν Light propagates at speed c of: c = λ ν Radiant-Energy Continuum - Electromagnetic-Spectrum Light used in Microlithography - UV to X-Rays Yellow-Light used in Litho Work-Areas - wont affect PR Regions of the ElectroMagnetic Spectrum. Lithography uses UV & X-Ray Regions MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 19-4
DIFFRACTION OF LIGHT & ITS IMPACT ON LATENT-IMAGE FORMATION IN RESISTS DIFFRACTION: Light-waves bend around edges of objects INTERFERENCE arises when wave passes thru closely-spaced slits in Mask. Diffraction-Pattern projected onto screen LENS behind Mask collects Light & Focuses it onto Screen But, Diffraction-Effects broaden Image of Mask-Pattern (e.g., Slit) projected onto screen. Broadening impacted by: λ, NA, & Width of Slit (a)-(b) Diffraction Pattern caused by light passing thru two closely-spaced slits (c) Definition of Numerical-Aperture (NA) Basic Imaging Principles: (a) Ideal Shadow Imaging; (b) Diffraction- Broadened Projection-Printing MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 19-5
RESOLUTION: Rayleigh-Criteria Rayleigh Resolution-Criterion (for Images of Point-Sources being observed by an Optical-System): Two Point-Sources are just-resolved when the distance between them δ is: δ = 0.61 λ / NA λ = Wavelength, NA = Numerical-Aperture Rayleigh Depth-of-Focus-Criterion (DOF): DOF = ± 0.5 λ / NA 2 (a)-(b) Light-intensity distribution from a point-source projected through a circular aperture. (c)-(d) Images of Point-Sources (Stars). Rayleigh Resolution Criterion is satisfied in c2 & d2. In c1 & d1 Images are not Resolved. MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 19-6
RESOLUTION: Definition for Microlithography Resolution is defined (for Lithographic Applications) as the ability to produce a Line (or Line & Space) or Opening, that meets an acceptable set of Criteria, including: Linewidth Sidewall-Angle Resist-Thickness after Develop All Four Components of the Lithography-Process have an impact on Resolution: Actinic Light-Source Mask (or Reticle) Lens of Exposure-Tool Photoresist-Film Resolution of Patterns Printed on Wafer MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 19-7
LITHOGRAPHY LIGHT-SOURCES: Mercury-Arc Lamps Actinic-Energy used in Lithography is UV-Light Original UV-Light-Source was Mercury-Arc-Lamp (200-2000W) Glow-Discharge (Arc) of Mercury-Vapor Emits UV-Light at High-Intensities at specific λ: g-line (λ = 436-nm) i-line (λ = 365-nm) Illumination System collects UV-light from Arc-Lamp & Projects it onto Mask Mercury-Arc-Lamp-Based Illumination System Emission-Spectrum of Mercury-Arc-Lamp MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 19-8
LITHOGRAPHY LIGHT-SOURCES: Excimer-Laser DUV-Sources For IC-Features Sizes smaller than 0.5-µm, Arc-Lamp i-line-uv-light can no longer Print (Resolve) them Shorter-λ (DUV) & New Light-Source needed - Excimer-Laser (Pulsed Laser-Light Source - 1998) KrF-Excimer-Laser emits 248-nm DUV-light - Used for 0.35, 0.25, & 0.18-µm-CMOS technologies ArF-Excimer-Laser emits 193-nm DUV-light - Used for 0.13, 0.1, &?-µm CMOS technologies F 2 -Excimer-Laser emits 157-nm DUV-light - R&D applications Excimer-Laser System used in Stepper/Scanner Applications MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 19-9
EVOLUTION OF MICROLITHOGRAPHY LITHOGRAPHY was introduced in 1958 to Print Device- Features in the Isoplanar-Process of Hoerni (Fairchild) In 1958 Feature-Sizes were Hundreds of Microns Now < 0.1-micron! Lithography Methods Evolved: Contact-Printing Proximity-Printing Projection-Printing Projection-Printing Now Used Almost-Exclusively: No Contact between Mask & Wafer Better Resolution than Proximity 3-Methods of Wafer-Exposure: (1) Contact; (2) Proximity; (3) Projection Printing MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 19-10
SCANNING-PROJECTION PRINTING (SCANNERS) In Scanning-Projection-Lithography - Mask & Wafer are Simultaneously Scanned through an Arc-Shaped Lens-Field: 1X-Printer (Perkin-Elmer Corp.) Reflective-Optics Uses Broadband-Illumination Can Print Features down to 1.5-micron Scanning Projection Printing: Wafer & Mask are Simultaneously Scanned across Field-Aperture MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 19-11
STEP & REPEAT PRINTING (STEPPERS) For IC-feature-sizes 1.0-micron-to-0.25-micron Step-&-Repeat Reduction-Printing is used 5X & 4X-Reduction; Refractive-Lens Wafer-Exposure Sequence Wafer moved to correct Alignment-Position & Focused Exposure-Field is Illuminated Wafer Stepped to next Exposure-Site High-Precision-Stage Steps Wafer Step & Repeat Projection Systems (Steppers) Expose only One-Field at a time MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 19-12
STEP & SCAN PRINTING For IC-Feature-Sizes smaller than 0.25-micron Step-& Scan Reduction Printing is used Reduction-Lens is used while Wafer is Scanned over one Exposure-Site Lens-Field is a Narrow-Slit Wafer & Reticle Scanned Simultaneously across Slit After Exposure,Wafer Stepped to Next Exposure-Site Catadioptric or Refractive Optics MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Step & Scan Principle combines operations of stepper & scanner. Within each exposure-field, reticle-pattern is scanned across field. 19-13
WAFER-STAGES & OVERLAY Wafers in Stepper & Scanner Systems must be positioned (& held during exposure) with Extreme Accuracy (±100-nm) Wafer-Stage-Subsystems perform this task Laser-Heterodyne-Interferometry Identifies Stage-Position & Linear-Electric-Motor Drives Stage Pattern-Alignment is achieved with Alignment-Marks (or Targets). Marks on Mask Aligned to Marks on Wafer Wafer-Stage of Steppers & Scanners Examples of Overlay-Target Designs (a) Cross-in-Box (b) Frame-in-Frame MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 19-14
SUMMARY OF KEY CONCEPTS The Technology Roadmap for Semiconductors is driven by the desire to continue scaling device-sizes: 0.7X reduction in Linear-Dimension every 3-Years Placement-Accuracy ~1/3 of Feature-Size These goals only achievable by getting Higher-Resolution: Projection-Printing - Using Step-&-Scan Shorter-λ Light-Sources Higher NA-Lenses Advances in Resist-Materials Improve Stage-Positioning Accuracy Resolution-Enhancement Techniques (Chap. 20) Non-Optical Lithography -? (Chap. 20) Whether these challenges can be met represents the biggest uncertainty about the Future of the Roadmap MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 19-15