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Spie Press Book

Advanced Processes for 193-nm Immersion Lithography
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Book Description

This book is a comprehensive guide to advanced processes and materials used in 193-nm immersion lithography (193i). It is an important text for those new to the field as well as for current practitioners who want to broaden their understanding of this latest technology. The book can be used as course material for graduate students of electrical engineering, material sciences, physics, chemistry, and microelectronics engineering and can also be used to train engineers involved in the manufacture of integrated circuits. It provides techniques for selecting critical materials (topcoats, photoresists, and antireflective coatings) and optimizing immersion processes to ensure higher performance and lower defectivity at lower cost. This book also includes sections on shrinking, trimming, and smoothing of the resist pattern to reduce feature sizes and line-edge roughness. Finally, it describes the recent development of 193i in combination with double exposure and double patterning.

Book Details

Date Published: 19 February 2009
Pages: 336
ISBN: 9780819475572
Volume: PM189

Table of Contents
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List of Abbreviations and Acronyms
1 Immersion Lithography and its Challenges
1.1 Basics of Photolithography
    1.1.1 Resolution of the exposure system
    1.1.2 Step and scan
1.2 Immersion Lithography (193i) and Its Advantages
    1.2.1 Depth-of-focus improvement
    1.2.2 Water as the immersion fluid
    1.2.3 Hyper-NA and high refractive index immersion (193i+)
1.3 Challenges for the 193i Process
    1.3.1 Topcoat versus non-topcoat
    1.3.2 Immersion defectivity
    1.3.3 Imaging at hyper-NA
2 Process Steps in the Track
2.1 Coating Module
    2.1.1 Material dispense
    2.1.2 Viscosity of materials
    2.1.3 Film thickness
    2.1.4 Reduction of material consumption
    2.1.5 Coating imperfection and defects Comets Striations Edge bead and backside contamination
    2.1.6 Material drying at nozzle
    2.1.7 Alternative coating techniques
2.2 Baking Module
    2.2.1 Temperature uniformity
    2.2.2 Temperature variation across hotplate during thermal ramp
    2.2.3 Hotplates with temperature gradients
    2.2.4 Sublimation at high-temperature post-apply bakes
    2.2.5 Chemical flare during post-exposure bake
2.3 Development
    2.3.1 Developer dispense
    2.3.2 Optimization of development time
    2.3.3 Deionized water rinse process
2.4 Resist Line-Collapse and Corrective Measures
    2.4.1 Mechanism of line-collapse
    2.4.2 Surfactant rinse to reduce line-collapse
    2.4.3 Evaluation of line-collapse process margin
2.5 Blob Defects
    2.5.1 Reduction of blob defects by process optimization
    2.5.2 Surfactant rinse to reduce the blob defects
2.6 193-nm Immersion-Specific Track Process
    2.6.1 Coating uniformity and stability
    2.6.2 Particle count after coating
    2.6.3 Across-wafer CD uniformity (CDU)
    2.6.4 PEB plate matching
    2.6.5 Developer cup matching
    2.6.6 Batch trend evaluation
3 Resist Leaching and Water Uptake
3.1 Leaching Test Methods
    3.1.1 Water extraction
    3.1.2 Water sample analysis
3.2 Leaching Dynamics
        3.2.1 Leaching dynamics described by a single-exponential model
    3.2.2 Leaching dynamics described by a double-exponential model
    3.2.3 Leaching specifications recommended by scanner suppliers
    3.2.4 Comparing the saturation leaching results
3.3 Leaching with 193-nm Exposure
3.4 Pre-Rinse to Partially Remove Leached Contaminants
3.5 Lens Contamination Caused by Resist Leaching
    3.5.1 Simulation results
    3.5.2 Controlled immersion contamination
    3.5.3 In situ cleaning of the immersion system
3.6 Water Uptake in Resist Film
    3.6.1 Diffusion theory
    3.6.2 Quartz crystal microbalance to measure water uptake
4 Contact Angle of Water on Resist Stacks
4.1 Definition of Static and Dynamic Contact Angles
4.2 Dynamics of the Water Meniscus
4.3 Experimental Results from the Model Immersion Head
4.4 Leakage Mechanism of the Water Meniscus
4.5 Methods to Measure the Contact Angles
    4.5.1 Tilting wafer method
    4.5.2 Captive drop method
    4.5.3 Wilhelmy plate method
    4.5.4 Correlation between static and dynamic contact angles
4.6 Process-Induced Contact Angle Variation
    4.6.1 Surface modification by exposure
    4.6.2 Surface modification by rinse liquid
5 Topcoat and Resist Processes for Immersion Lithography
5.1 Selection of Developer-Soluble Topcoat
    5.1.1 Refractive index and thickness of topcoat
        5.1.2 Chemical compatibility of topcoat and resist
    5.1.3 Dissolution rate of developer-soluble topcoat in developer
    5.1.4 Advanced developer-soluble topcoats
5.2 Lithographic Assessment of Developer-Soluble Topcoats with Resists
    5.2.1 Lithographic assessment
    5.2.2 Exposed resist loss by developer-soluble topcoat
    5.2.3 PEB delay of 193i process with developer-soluble topcoats
5.3 Optimization of Developer-Soluble Topcoat Processes
5.4 193i Resists without Topcoats
    5.4.1 Intrinsic topcoats: balancing the immersion needs with high-resolution performance
    5.4.2 Resolution limits of resists
6 Immersion Defects and Defect-Reduction Strategies
6.1 The Basics of Defect Detection
    6.1.1 ITRS defectivity requirements
    6.1.2 Systematic approach for identifying the sources of defects in the immersion process
    6.1.3 Particle per wafer pass test
6.2 Quality of the Immersion Water
6.3 Appearance of Bubble Defects
    6.3.1 Simulation results Floating bubbles Bubbles attached to the resist surfaces Shape effect of the attached bubble
    6.3.2 Bubble defects observed in resist patterns
6.4 Origins of Bubbles
    6.4.1 Bubbles from the water supply
    6.4.2 Outgassing of resist during exposure
    6.4.3 Entrapment of bubbles on the wafer surface
    6.4.4 Exposure head design
6.5 Defects Caused by Transparent Particles and Blisters
    6.5.1 Formation of antibubble defects
    6.5.2 Sources of resist or topcoat particles and bumps
    6.5.3 Blisters
6.6 Water-Mark Defects
        6.6.1 Mechanism of water-mark defects: water droplets cause local resist photosensitivity losses
    6.6.2 SEM images of water mark defects
    6.6.3 Other evidence of water-mark defects
    6.6.4 How resist components and process parameters affect the formation of water-mark defects
6.7 Strategies for Reducing Water-Mark Defects
    6.7.1 Hydrophobic surfaces help reduce water-marks
    6.7.2 Optimization of routing paths and scan speeds of immersion heads
    6.7.3 DI water rinse process
    6.7.4 Other rinse processes
6.8 Particles
    6.8.1 Particles from the immersion water
    6.8.2 Particles from the wafer stage
    6.8.3 Wafer edge
6.9 Pinholes in Ultrathin Films of Topcoat or Resist
6.10 Microbridging Defects
    6.10.1 Microbridges caused by microbubbles
    6.10.2 Opaque particles
    6.10.3 Intermixing layer between resist and topcoat
6.11 Summary
7 Antireflection Coatings and Underlayer Technology
7.1 General Requirements for Conventional Bottom Antireflection Coatings (BARCs)
    7.1.1 Optical requirements
    7.1.2 Thermal cross-linking
    7.1.3 Sublimation test
    7.1.4 Resist compatibility Resist footing Control of Blob defects
    7.1.5 Etch rate of organic BARCs
7.2 Challenges to Antireflection Control for Hyper-NA Exposure
7.3 Spin-on Dual-Layer BARC and Graded Spin-on BARC
    7.3.1 Spin-on dual-layer BARC
    7.3.2 Graded spin-on BARCs
7.4 Si-Containing BARC and Spin-on Carbon
    7.4.1 Process flow of Resist/Si-BARC/SOC trilayer
    7.4.2 Consideration of antireflection control
    7.4.3 Etch selectivity
    7.4.4 Resist compatibility and tetralayer approach
    7.4.5 Storage stability and solvent rework capability
    7.4.6 Thick Si-BARC ("etch screw") process
7.5 Gap-Fill Materials
    7.5.1 Process flow of gap-fill materials
    7.5.2 Evaluation of filling capability
    7.5.3 Chemical compatibility and etch rate
7.6 Top Antireflection Coatings (TARCs)
    7.6.1 Optical performance of TARC films
    7.6.2 Chemical compatibility and coating issues
    7.6.3 Absorbing TARCs
7.7 Developer-Soluble BARCs (DBARCs)
    7.7.1 Nonphotosensitive DBARCs
    7.7.2 Photosensitive DBARCs Photospeed match with resist Chemical compatibility
8 Resist Shrink and Trim Processes
8.1 Resist Thermal Reflow
    8.1.1 Behavior of thermal reflow
    8.1.2 Reflow bake temperature
    8.1.3 Optical proximity correction (OPC) for thermal reflow
8.2 Chemical Shrink
    8.2.1 Shrinkage behavior
    8.2.2 Defectivity issues
8.3 SAFIER (Shrink Assist Film for Enhanced Resolution)
8.4 Shrinking Via Fluorination Process
8.5 Shrinking Via Silylation Process
8.6 Plasma-Assisted Shrink
8.7 Evaluation of Shrink Processes
8.8 Trim Processes
9 Double Exposure and Double Patterning
9.1 Introduction
    9.1.1 Double exposure (DE)
    9.1.2 Double patterning (DP)
    9.1.3 Resolution capability of DE/DP
    9.1.4 Challenges
9.2 Double Exposure with One Resist Layer
        9.2.1 Combination of interference and projection (regular) lithography
    9.2.2 Exposures with X-dipole and Y-dipole illuminations
    9.2.3 Image-assisted double exposure
    9.2.4 Other approaches
9.3 Double Exposure with Two Full Lithographic Processes
    9.3.1 Double exposure with positive and negative resists
    9.3.2 Freezing of the 1st resist pattern Freezing technique with a surface protective layer Freezing technique with thermal cross-link resist Other "freezing" approaches
    9.3.3 Pack and unpack (PAU) for printing contacts
9.4 Double Patterning
    9.4.1 Double trenching patterning
    9.4.2 Double line patterning
        9.4.3 Silicon-containing resists used as the 2nd resist in double line patterning
    9.4.4 Si-BARC film as hard mask for double patterning
9.5 Self-Aligned Double Patterning
9.6 Novel Approaches
9.7 Additional Comments
10 Line-Edge Roughness of Resist Patterns
10.1 Metrology of Line-Edge Roughness (LER) and Line-Width Roughness (LWR)
    10.1.1 LER
    10.1.2 LWR
    10.1.3 Relationship between LER and LWR
    10.1.4 Correlation length of the roughness
    10.1.5 Spatial frequency spectrum
10.2 Formation of LER
    10.2.1 LER of the mask pattern
    10.2.2 Aerial image contrast at the pattern edge
10.2.3 LER generation in positive chemically amplified (CA) resists
    10.2.4 Effect of BARC and topcoat on resist LER
    10.2.5 Resist dissolution behavior
    10.2.6 Investigation of acid diffusion
10.3 Strategies for Reducing Resist LER
    10.3.1 Developing resists with low intrinsic roughness
    10.3.2 Optimization of resist process parameters
    10.3.3 Smoothing resist patterns through surfactant rinse and hard bake
    10.3.4 Smoothing resist patterns with solvent vapor
    10.3.5 HBr plasma treatment
10.4 Transferring LER from Resist to Substrate: the Effect of Etch
    10.4.1 Isotropic versus anisotropic roughness
    10.4.2 LER reduction after etch
11 Extendibility of 193-nm Immersion Lithography
11.1 Fluids with High Refractive Indices
    11.1.1 Requirements for high-RI fluids
    11.1.2 Measuring the RI of immersion fluids
    11.1.3 Development of high-RI fluids
    11.1.4 Leaching and contact angle
11.2 Materials with High Refractive Indices
11.3 Resists with High Refractive Indices
    11.3.1 Development of high-RI resists
    11.3.2 Aerial image improvements with high-RI material
11.4 Solid Immersion
11.5 Other 193i+ Topics
    11.5.1 Polarization control of exposure light
    11.5.2 Reticle-induced polarization


The benefits of using liquids in optical microscopes were first demonstrated in the 1880s. A century later, in the 1980s, experiments with immersion technology demonstrated its potential for use in modern lithography. In 2002, when 157-nm lithography was delayed by a host of technical problems, the development of 193-nm immersion lithography for use in fabricating integrated circuits gained momentum. The development of 193-nm immersion lithography (193i) occurred much faster than did any previous lithographic technology. Currently, 193i is widely used to manufacture advanced microelectronic devices at the 45-nm node. The entire transition from proof of concept to delivery of a mass production tool took only about four years.

This rapid growth was possible because of the combined efforts of all sectors of the lithography community, including the manufacturers of scanners, materials, and integrated circuits. Much of the research critical to the rapid advancement of 193i has been published in the last few years in various journals and proceedings. One of the goals of this book is to summarize this information so that those new to the field as well as current practitioners may increase their understanding of this important technology.

Thus, while actively involved in evaluating new materials, equipment, and processes for 193i imaging, Yayi Wei began writing the manuscript for this book. During the summer of 2008, Robert Brainard, a researcher developing new resist materials, joined Yayi as his coauthor to help prepare the manuscript. Their collaboration resulted in this timely monograph that presents the knowledge critical for establishing high-yield cost-effective 193i processes and materials. The text can be used as course material for graduate students of electrical engineering, material sciences, physics, chemistry, and microelectronics engineering. It can also be used to train engineers involved in the manufacture of integrated circuits.

A large portion of this book is concerned with the challenges and opportunities of water-based 193-nm immersion lithography. The first chapter provides a broad overview of 193i lithography. The second chapter describes the track where most of the processes occur. The book continues with descriptions of the interactions between the immersion fluid (water) and the resist in terms of contact angle, leaching of resist components, and topcoats. It also provides a comprehensive summary of various immersion-related defects and defect-reduction strategies. It covers topics that were originally developed in "dry" lithography and are extendable to immersion 193-nm lithography, discussing strategies for antireflection control, shrink processes, trim processes, double exposure, double patterning, and line-edge roughness. The book concludes with a chapter describing research efforts aimed at further extensions of immersion lithography to higher numerical aperture (NA) and resolution through the development of high-index lithography. Discussion of some topics (e.g., optical theory of hyper-NA) was kept brief when well described in other monographs.

The knowledge of 193i is still growing and will continue to mature as it is used more frequently in mass production. We appreciate any suggestions from our readers on how to update this material. Your input will help us improve subsequent editions of this book.

Yayi Wei
Altamont, New York

Robert L. Brainard
College of Nanoscale Science and Engineering
University at Albany, State University of New York

January 2009

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