Chapter 5 - Lithography

Chapter 5 - Lithography

• Lithography is the most important process in modern IC manufacturing
• It prints patterns with features 10-20 nm in precision on a substrate.
• Almost all integrated circuits (ICs) are manufactured today using deep-UV (DUV) optical lithography with 193 nm photons.
• Extreme ultraviolet (EUV) lithography (13.5 nm) is used in recent technology generations; DUV systems have reached their resolution limits.

Lecture 1 Outline

• Introduction to lithography
• Historical development and basic concepts
• Optical lithography process
• Light sources
• Wafer exposure systems
• Optics basics (ray tracing and diffraction)
• Projection systems (Fraunhofer diffraction)
• Contact and proximity systems (Fresnel diffraction)

Introduction

• Implementation is expensive and complex
• Demands on resolution, exposure field, placement accuracy, throughput, and defect density.
• Resolution requirements stem from the increasing demand for smaller device structures.
• Exposure field requirements are due to the need for large chips.
• Placement accuracy is crucial because each mask layer needs precise alignment.
• Throughput and defect density are problematic due to industry competition.
• Throughput directly affects the manufacturing cost.
• Defects lead to yield loss and lower profitability.

Photolithography

• A light-sensitive photoresist is spun onto the wafer to form a thin layer.
• The resist is selectively exposed by photons passing through a mask.
• The mask pattern information is transferred to the wafer;
• Develop the resist, complete the pattern transfer, and etch underlying films.
• Used as a mask for ion implantation doping step

Historical Development and Basic Concepts

• The patterning process involves mask design, mask fabrication, and wafer printing
• CAD systems for mask design
  • Layout, simulation, and design rule checking.
• Libraries of previous designs are utilized for efficiency.
• Software tools for routing and connections between functional blocks.
• More sophisticated methods such as electronic beam or laser pattern generators are used for generating and transferring patterns

Mask Fabrication

• Mask level information is transferred to a mask making machine, either electron beam or laser pattern generator.
• Patterns are written on a blank mask using a scanning electron or laser beam The mask is usually a fused silica plate with a chromium layer.
• Resist is developed, then used for the etch mask that transfers the mask pattern to the chromium layer.
• Chromium is dry or wet etched (for large dimensions); photoresist is removed.
• Demagnification for easier fabrication and imperfections checking.
• Defects are detected and repaired by laser ablation or ion beams/chrome deposition.

Mask Fabrication (continued)

• Mask writing using a laser beam or electron beam creating an X-Y pattern across the mask.
• Electron beam systems can be used directly to write images on the wafer.
• This is possible, but the throughput is too slow
• Typical e-beam pattern generators expose a full wafer in tens of minutes.
• Optical steppers have a throughput of over 100 wafers per hour.

Wafer Printing

• Pattern information is transferred to the wafer using a projection exposure system.
• Light collimated and passed through the mask.
• The mask's clear areas transmit light; collected/focused by a lens system(reducing image size typically by 4X)
• The exposure field size is small (a few cm on a side).
• Multiple exposures may need to transfer the entire wafer pattern; the wafer is moved (stepped) to each exposure field.

Lithography Process

• Light source generates photons; ultimately exposing the resist.
• Exposure system images the mask onto the wafer surface.
• Resist contains all issues related to exposure/developing/baking.

Wafer Exposure

• Produces the aerial image at the top surface of the resist, creates mask pattern.
• Positive resist dissolves when exposed (in most applications).
• The light/pattern changes the resist properties producing a 3D latent image of the mask.

Wafer Exposure (continued)

• Aerial image is the dividing line between the lithography system's exposure tool and resist.
• Exposure tool aims for optimal resolution, field, depth, uniformity, photon intensity.
• Photoresist transforms the aerial image into the best 3D replica (geometric accuracy, exposure speed, resistance to subsequent processing).

Light Sources

• Higher resolution when using shorter wavelengths; feature size proportional to the wavelength of the light used
• Lithography systems typically use monochromatic (single wavelength) light.
• From mid-1990s, smaller feature sizes than the light wavelength (subwavelength lithography)
• EUV systems are used today (13.5nm) ; formerly 193nm immersion optical systems.
• Mercury (Hg) lamps used before 1995; arc lamps with Hg vapor.
• Excimer lasers (KrF 248nm and ArF 193nm) for deep UV spectrum.
• EUV (extreme ultraviolet) lithography(13.5 nm) are used for modern systems.
• High-energy laser-pulsed Sn plasma is used to generate photons for EUV systems.

Wafer Exposure Systems

• Three types of optical tools
• Contact
• Proximity
• Projection
• Projection systems are most used today.
• Contact printing: direct mask/resist contact with minimum diffraction effects for high resolution.
• Proximity printing: mask/resist separated by a few microns; minimizes some defects but limited resolution
• Projection printing: uses lens to project mask pattern onto the wafer; most common high volume manufacturing method today.

Optics Basics – Ray Tracing and Diffraction

• Light travels as electromagnetic waves; approximations for large dimension systems using ray tracing.
• Diffraction becomes relevant for smaller features similar to aperture size diffraction limitations.
• Huygens-Fresnel Principle for calculating wavefronts based on secondary wavelets

Optics Basics (continued)

• Diffraction: light bending when passing through an aperture.
• Image becomes more spread out through smaller aperture size.
• Size of the aperture determines the focus and the image spread out
• Optical tools calculate diffraction patterns and image formation.

Projection Systems (Fraunhofer Diffraction)

• Resolution of projection printers in terms of resolution, depth of field, and view.
• Modulation transfer function (MTF) relating to optical system properties and mechanical design.
• Two closely spaced point sources—Airy disk and the Rayleigh limit—represent the maximum resolving power.
• Lens resolution (R) relates to the wavelength (λ), refractive index (n), and the maximum half angle of diffracted light (α).
• NA (numerical aperture) is a measure of the lens' ability to collect diffracted light.

Projection Systems (continued)

• A reduced mask image is transferred onto the wafer; reduced to 4-5 times smaller size.
• MTF (modulation transfer function) is a crucial measure of aerial image quality.
• The value of MTF is between 0 and 1.
• The value of MTF= 1 means high contrast; used in high volume manufacturing.
• MTF depends on the feature size in the image.
  • For larger features, MTF is high.
• For smaller features, MTF falls causing issues for finer details; resolution will impact ability to produce.

Projection Systems (continued)

• Spatial coherence: Ideal point source produces light waves in phase at all points; condenser lenses are used to convert waves into plane waves/same angle.
• If the size of the light source is not sufficiently small, spatial coherence will be reduced.
• S is light source diameter/condenser lens diameter, a useful measure for lithography coherence.

Contact and Proximity Systems (Fresnel Diffraction)

• Diffraction pattern directly influences the aerial image.
• The minimum resolvable feature size is approximately √(gap).
• Contact and proximity printers use multiple wavelengths.
• These printers don’t require perfectly coherent light sources.