MICROCHIP MANUFACTURING by S. Wolf

Chapter 19LITHOGRAPHY II:IMAGE-FORMATION andOPTICAL HARDWARE© 2004 by LATTICE PRESS

MICROCHIP MANUFACTURING © 2004 by LATTICE PRESS Sunset Beach CA 19-2

CHAPTER 19 - CONTENTS

• Preliminaries: Wave- Motion & The Behavior of Light• Resolution & Depth- of-Focus in Micro- Lithography Applications

• Lithographic Light-Sources

• Lithographic Exposure-Tools

• Projection-Printers

• Overlay and Wafer-Stages

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IMAGE-FORMATION IN PHOTORESIST3-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 toForming a Resist-Image Pattern

• Actinic-Light• Mask (or Reticle)• Lens of the Exposure System• PhotoResist-Film

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ELECTROMAGNETIC-RADIATION (LIGHT)

Regions of the ElectroMagnetic Spectrum. Lithography uses UV & X-Ray Regions

• Light exhibits wave-behavior - wavelength λλλλ• Also exhibits particle-behavior - photon energy = hνννν• Light propagates at speed c of: c = λλλλ νννν• Radiant-Energy Continuum - Electromagnetic-Spectrum• Light used in Microlithography - UV to X-Rays• Yellow-Light used in Litho Work-Areas - wont affect PR

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DIFFRACTION OF LIGHT & ITS IMPACT ON LATENT-IMAGE FORMATION IN RESISTS

Basic Imaging Principles: (a) IdealShadow Imaging; (b) Diffraction-Broadened Projection-Printing

(a)-(b) Diffraction Pattern caused by lightpassing thru two closely-spaced slits(c) Definition of Numerical-Aperture (NA)

• DIFFRACTION: Light-waves bend around edges of objects• INTERFERENCE arises when wave passes thru closely-spaced slits in Mask. Diffrac- tion-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: λλλλ, NA, & Width of Slit

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RESOLUTION: Rayleigh-Criteria

(a)-(b) Light-intensity distribution from a point-source projected through acircular aperture. (c)-(d) Images of Point-Sources (Stars). Rayleigh ResolutionCriterion is satisfied in c2 & d2. In c1 & d1 Images are not Resolved.

Rayleigh Resolution-Criterion (for Images of Point-Sources being observed by an Optical-System):

λλλλ = Wavelength, NA = Numerical-Aperture

Rayleigh Depth-of-Focus-Criterion (DOF):

DOF = ± 0.5 λλλλ / NA2

δδδδ = 0.61 λλλλ / NA

Two Point-Sources are just-resolved when the distance between them δδδδ is:

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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:

Resolution of Patterns Printed on Wafer

• Linewidth• Sidewall-Angle• Resist-Thickness after Develop

All Four Components of the Lithography-Process havean impact on Resolution:

• Actinic Light-Source• Mask (or Reticle)• Lens of Exposure-Tool• Photoresist-Film

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LITHOGRAPHY LIGHT-SOURCES:Mercury-Arc Lamps

Emission-Spectrum of Mercury-Arc-Lamp

Mercury-Arc-Lamp-BasedIllumination System

• 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 λλλλ: g-line (λλλλ = 436-nm) i-line (λλλλ = 365-nm)• Illumination System collects UV-light from Arc-Lamp & Projects it onto Mask

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LITHOGRAPHY LIGHT-SOURCES: Excimer-Laser DUV-Sources

Excimer-Laser System used inStepper/Scanner Applications

• For IC-Features Sizes smaller than 0.5-µm, Arc-Lamp i-line-UV-light can no longer Print (Resolve) them • Shorter-λλλλ (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

• F2-Excimer-Laser emits 157-nm DUV-light - R&D applications

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EVOLUTION OF MICROLITHOGRAPHY

3-Methods of Wafer-Exposure: (1) Contact;(2) Proximity; (3) Projection Printing

• 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

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SCANNING-PROJECTION PRINTING (SCANNERS)

Scanning Projection Printing: Wafer & Mask areSimultaneously Scanned across Field-Aperture

In Scanning-Projection-Lithography - Mask & Wafer areSimultaneously 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

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STEP & REPEAT PRINTING (STEPPERS)

Step & Repeat Projection Systems (Steppers)Expose only One-Field at a time

• 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

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19-13

STEP & SCAN PRINTING

Step & Scan Principle combines operations of stepper & scanner.Within each exposure-field, reticle-pattern is scanned across field.

• 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 Scan- ned Simultaneously across Slit

• After Exposure,Wafer Stepped to Next Exposure-Site

• Catadioptric or Refractive Optics

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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

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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-λλλλ 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