5.1. lithography 1,2.final 2013
DESCRIPTION
VLSITRANSCRIPT
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Microelectronics Technology:
Photolithography & Pattern Definition
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PHOTOLITHOGRAPHY
The transfer of an image from the mask to a wafer through the use of Photosensitive material
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Photolithography Requirements For Future Generations of VLSI Technology
Year of 1st DRAM Shipment 1997 1999 2003 2006 2009 1012
DRAM Bits/Chip 256M 1G 4G 16G 64G 256G
Minimum Feature Size nmIsolated Lines (MPU)Dense Lines (DRAM)
Contacts
200250280
140180200
100130140
70100110
507080
355060
Gate CD Control 3s (nm) 20 14 10 7 5 4
Alignment (mean + 3s) (nm) 85 65 45 35 25 20
Depth of Focus 0.8 0.7 0.6 0.5 0.5 0.5
Defect Density (per layer/m2) @ Defect Size (nm)
100 @ 800
80 @ 60
60 @ 40
50 @ 30 40 @ 20 30 @
15
DRAM Chip Size (mm2) 280 400 560 790 1120 1580
MPU Chip Size (mm2) 300 360 430 520 620 750
Field Size (mm) 22x22 25x22 25x36 25x40 25x44 25x52
Exposure Technology 248nm DUV
248nm DUV
248nm or
193nm DUV
193nm DUV or
???
193nm DUV
or ??????
Minimum Mask Count 22 22/24 24 24/26 26/28 28
• 0.7X in linear dimension or decrease by 50% area/transistor every 3 years. • Placement accuracy » 1/3 of feature size.• ≈ 35% of wafer manufacturing costs for lithography.• Note the ???. These represents the single biggest uncertainty about the future of the roadmap.
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Historical Developments and Basic Concepts
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The Pattern Information Transfer and Aerial Imaging
The wafer printing process A: Light source, B. Wafer exposure system, C.Resist• Aerial image is the pattern of optical radiation striking the top of the
resist.• Latent image is the 3D replica produced by chemical processes in the
resist.
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Photolithography:Process Flow
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Dr. G. Eranna Integrated Circuit Fabrication Technology © CEERI Pilani
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Dr. G. Eranna Integrated Circuit Fabrication Technology © CEERI Pilani
Same Mask but Opposite Features
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PHOTOLITHOGRAPHY TECHNIQUES
LIGHT SOURCES
• Optical• Electron Beam• X-Ray• Laser
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A. Light Sources
• Decreasing feature sizes require the use of shorter λ.• Traditionally Hg vapor lamps have been used which generate many
spectral lines from a high intensity plasma inside a glass lamp.• (Electrons are excited to higher energy levels by collisions in the plasma.
Photons are emitted when the energy is released.)
g line - λ = 436 nm
i line - λ = 365 nm (used for 0.5 µm, 0.35 µm)• Brightest sources in deep UV are excimer lasers
Kr+ NF3 KrF photon emission (1)
energy
KrF - λ = 248 nm (used for 0.25 µm)
ArF - λ = 193 nm• Issues include finding suitable resists and transparent optical
components at these wavelengths.
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Wafer Exposure Systems
Contact printing is capable of high resolution but has unacceptable defect densities.Proximity printing cannot easily print features below a few µ m (except for x-ray systems).Projection printing provides high resolution and low defect densities and \ dominates today.Typical projection systems use reduction optics (2X - 5X), step and repeat or step and scan mechanical systems, print ≈ 50 wafers/hour and cost $5 - 10M.
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B1. Optics - Basics and Diffraction
• Ray tracing (assuming light travels in straight lines) works well as long as the dimensions are large compared to λ .At smaller dimensions, diffraction effects dominate.
• If the aperture is on the order of λ, the light spreads out after passing through the aperture. (The smaller the aperture, the more it spreads out.)
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Dr. G. Eranna Integrated Circuit Fabrication Technology © CEERI Pilani
Optics: Basics and Diffraction
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B1: Optics – Basics and Diffraction
• Image formed by a small circular aperture (Airy disk).
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Projection Systems
• These are the dominant systems in use today. Performance is usually described in terms of
• resolution
• depth of focus
• field of view
• modulation transfer function
• alignment accuracy
• throughput
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B2: Projection Systems (Fraunhofer Diffraction)
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B2: Projection Systems (Fraunhofer Diffraction)
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B2: Projection Systems (Fraunhofer Diffraction)
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B2: Projection Systems (Fraunhofer Diffraction)
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B2: Projection Systems (Fraunhofer Diffraction)
• MTF is measure of the contrast (Dark vs Light intensities) in aerial image produced by exposure system.
• Cocept strictly applies to coherent illumination:
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B2: Projection Systems (Fraunhofer Diffraction)
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B2: Projection Systems (Fraunhofer Diffraction)
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Contact and Proximity Systems (Fresnel Diffraction)
• Contact printing systems operate in the near field or Fresnel diffraction regime.• There is always some gap g between the mask and resist.
• The aerial image can be constructed by imagining point sources within the aperture, each radiating spherical waves (Huygens wavelets).
• Interference effects and diffraction result in “ringing” and spreading outside the aperture.
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B3: Contact and Proximity Systems (Fresnel Diffraction)
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B: Wafer Exposure Systems
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C: Photoresists
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Basic Properties of Photoresists
• Adhesion• Resolution• Etch Resistance• Photosensitivity
Contrast: A measure of the resist’s ability to distinguish light from dark areas in the aerial image the exposure system produces
CMTF: A measure of the dark vs light intensities in the aerial image produced by the exposure system
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g-Line and i-Line Resists
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g-Line and i-Line Resists
Photo Active Part of the structure
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g-Line and i-Line Resists
After ExposureChemical change in diazoquinone
Leaves behind highly reactive C site
Carboxylic acid which is soluble in developer
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DUV Resists• g-line and i-line resists have maximum quantum efficiencies < 1 and are typically ≈ 0.3.
• Chemical amplification can improve this substantially.
• DUV resists all use this principle. A catalyst is used.
• Photo-acid generator (PAG) is converted to an acid by photon exposure. Later, in a post exposure bake, the acid molecule reacts with a “blocking”
molecule on a polymer chain, making it soluble in developer AND REGENERATING THE ACID MOLECULE.
catalytic action \ sensitivity is enhanced.
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Basic Properties of Resists• Two basic parameters are used to describe resist properties, contrast
and the critical modulation transfer function or CMTF.
• Contrast is defined as
010log
1
D
Df
• Typical g-line and i-line resists achieve Y values of 2 -3 and Df values of about 100 mJ cm-2. DUV resists have much higher Y values (5 - 10) and Df values of about 20 - 40 mJ cm-2.
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Basic Properties of Photoresist
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Basic Properties of Resists
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Characteristics Positive Resist Negative Resist
Adhesion to Silicon Fair Excellent
Contrast Higher Lower
Developer Aqueous based Organic solvent
Develop Process Window
Small Very wide (insensitive to over development)
Lift-off Yes No
Minimum Feature 0.5 μm and below 2.0 μm
Plasma Etch Resistance Very good Not very good
Proximity Effect Prints isolated holes or trenches better
Prints isolated lines better
Step Coverage Better Lower
Swelling in Developer No Yes
Thermal Stability Good Fair
Comparison of Negative & Positive Photoresist
Dr. G. Eranna Integrated Circuit Fabrication Technology © CEERI Pilani
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Photoresist Exposure: Issues
Ideal Aerial Imag
Exposing Light
• There are often a number of additional issues that arise in exposing resist.
• Resist is applied as a liquid and hence "flows" to fill in the topography.
• Resist thickness may vary across the wafer. This can lead to under or over exposure in some regions and hence linewidth variations.
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Photoresist Exposure: IssuesAerial Image Exposing Light
• Reflective surfaces below the resist can set up reflections and standing waves and degrade resolution.
• In some cases an antireflective coating (ARC) can help to minimize these effects. Baking the resist after exposure, but before development can also help.
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Photoresist Exposure:IssuesPlanarization Layer Etch Mask layer Imaging Layer
• Multi-layer resists are used to produce a thin, uniform imaging layer in which the aerial image is transformed into a latent image.
• Etching is then used to transfer the latent image down through the planarization layer.
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