Photoresist Technology Quiz

Chapter 5 - Lithography

• Lecture 2, by Dr. Panagiotis Dimitrakellis & Prof. Dr. Evangelos Gogolides

Outline - Lecture 2

• Historical Development and Basic Concepts
  • Photoresists
    • g-line and i-line resists
    • DUV resists
  • Basic properties and characterization of resists
  • Mask engineering - optical proximity correction and phase shifting
• Manufacturing methods and equipment
  • Wafer exposure systems
  • Photoresists
• Models and simulation
  • Wafer exposure systems

Photolithography

• Basic lithography process: light-sensitive photoresist is spun onto the wafer, forming a thin layer.
• Resist is selectively exposed by photons passing through a mask containing the pattern information for the layer being printed.
• Resist is developed to complete the pattern transfer from the mask to the wafer.
• Resist can be used as a mask to etch underlying films or for ion implantation doping steps.

Photoresists

• Photoresist materials are designed to respond to incident photons by changing their properties.
• Many absorb light; typically this absorption results in electronic processes rather than chemical changes.
• Not useful for lithography since the material needs to hold onto the latent image of the impinging photons until the resist is developed
• A long-lived response generally requires chemical change in the material
• Almost all resists today are fabricated from hydrocarbon-based materials.
• When light is absorbed, chemical bonds break, restructuring the resist material into a new stable form.
• Positive resists become more soluble in the developer solution upon light exposure.
• Negative resists become less soluble upon light exposure
• Today's semiconductor industry favors positive resists for their better resolution.
• Photoresists in use today are liquids at room temperature.
• Applied to wafers by placing liquid on the wafer and spinning it at high speed (spin-coating).
• Spin speed and viscosity of the resist determine the final resist thickness (usually 0.6-1 μm).
• Viscosity is controlled using a solvent, which is removed in a baking step (prebake).

Next steps

• Resist exposure → developing
• Developing is done using liquid developers (immersion, spraying, or a puddle).
• Upon development, the resist is baked again (post-baked) to harden and improve its ability to act as an etch or ion implantation mask.
• After the etching or implantation process, the resist is removed using oxygen plasma or chemical stripping.

A. Sensitivity

• A measure of how much light is needed to expose the resist (measured in mJ cm⁻²).
• For g-line and i-line resists, typical sensitivity is 100 mJ cm⁻².
• Deep UV resists have sensitivities of 20-40 mJ cm⁻² (due to chemical amplification).
• Generally, high sensitivity is important as it reduces exposure time and improves throughput.
• Extremely high sensitivity can lead to instability or temperature sensitivity problems with statistical variations (noise) during exposure.

B. Resolution

• Quality of resist patterns is limited by the exposure system (aerial image).
• Factors such as exposure dose, baking steps, and developing cycles must be precisely controlled to achieve diffraction-limited resolution in resist images.

C. The 'resist' function of photoresists

• "Resist" describes the need for the photoresist to withstand etching or ion implantation after the mask pattern's transfer.
• Practical resists need robustness to resolve small features even when they are reasonably thin.

g-line and i-line photoresists

• Consist of three components: inactive resin (hydrocarbon base), photoactive compound (PAC), and a solvent.
• DUV resists replace the PAC component with a photo-acid generator (PAG).
• This PAG acts as a chemical amplifier or catalyst.
• Most solvent evaporates during spin-coating and prebake before resist exposure.

Most common g-line and I-line resists

• Diazonaphthoquinone or DNQ materials
• Novolac is a common base resin, a polymer with basic hydrocarbon rings.
• It's composed of 2 methyl groups and 1 attached OH group.
• It readily dissolves in developer solutions at a rate of about 15 nm sec⁻¹.
• PACs in these resists are often diazoquinones
• The photoactive part is the component above the SO₂, with remaining abbreviated.
• The role of the PAC is to inhibit the dissolution of the resist material in the developer.
• Diazoquinones are generally insoluble in typical developers, lowering the overall dissolution rate of the resist to about 1-2 nm sec⁻¹
• DNQ materials are essentially insoluble in the developer before exposure to light

When resist exposed to light:

• The diazoquinone molecules chemically changes
• The N2 molecule is weakly bonded, and light breaking this bond leaves behind a reactive carbon site.
• The PAC structure may stabilize itself by rearranging an atom outside the ring and bonding it to the oxygen atom.
• This resultant ketene molecule transforms into a carboxylic acid in the presence of water
• This resultant carboxylic acid is readily soluble in a basic developer

Deep Ultraviolet (DUV) resists

• Based on a completely new chemistry and chemical amplification (CA resists).
• Standard DNQ resists achieve quantum efficiencies (QE) of about 0.3-30% of incoming photons interacting with PAC molecules, leading to better, more efficient resist exposure.
• CA resists use a different exposure process: incoming photons interact with photo-acid generators (PAG) to create acid molecules that catalyze changes in the resist properties during subsequent resist bake.

Key point (CA resists):

• The reactions are catalytic
• Acid molecules regenerate after reactions
• The overall quantum efficiency in a CA resist depends on the initial light/PAG reaction efficiency and the number of catalyzed reactions.

.### Positive vs. Negative resist versions possible

• In positive resist case, PAG initiates a chemical reaction that makes the resist soluble in the developer (Opposite for negative resist).

B. Resolution

• Resist pattern quality is limited by the exposure system, not the resist itself.
• Careful control of exposure dose, baking, and developing cycles is crucial to achieve diffraction-limited resolution in the images.

C. The 'resist' function of photoresists

• "Resist" describes a photoresist's need to withstand etching and ion implantation after transfer.
• In practice, resists must be robust enough to resolve small features.

Mask engineering - optical proximity correction and phase shifting

• Aims to get the desired patterns on the wafer.
• Finite aperture of projection systems leads to some light loss due to diffraction from mask features
• Apertures are circular in projection systems, and patterns may be rounded
• Lost information leads to rounding, changes in width between isolated vs grouped lines, shortening in narrow line ends
• OPC (optical proximity correction): software adjusts mask features to compensate for proximity effects and diffraction
• PSM (phase shift masks): introduce phases to light passing through the mask to improve resolution.
  • Changing the transmission characteristics of the mask in selected areas helps improve resolution
  • A periodic mask with equal lines and spaces, a diffraction grating, as the mask is used.
    • the electric field just after light passes through the mask and also at the wafer.
  • Materials with thickness and index of refraction added to the mask for 180° phase shifting are used.
    • The thickness of layer d = λ/2(n-1), where n is the index of refraction
    • Light intensity is the square of the ɛ field intensity for improvement of aerial image quality.

Manufacturing methods and equipment

• Lithography process dominates manufacturing cost and throughput of modern ICs.
• Contact and proximity printers, though used earlier, are outdated due to low resolution/high defect levels.
• Projection aligners are the current dominant tool, featuring improved optics (adjusting optics for aberrations in small area instead of large area) for high volume manufacturing
• Scanning systems are cost-effective and high-throughput but require 1-x masks
• Steppers eliminate wafer size as a major constraint, reducing image size as it is exposed in limited portions of wafer.

Wafer exposure systems

• Projection aligners feature primary mirrors, secondary mirrors, trapezoid mirrors, illumination, and scanning mechanisms to project the mask's pattern onto a wafer.
• Scanning systems are cost-effective and high-throughput.
• Steppers increase resolution.
• Hybrid systems combine stepping and scanning methods for even higher resolution and field of view
  • Advantages of scanners → precise optical systems
  • Advantages of steppers → reduced mask fabrication, size, lens design
• More complex and more expensive.

Photoresists - process steps and procedures

• Preparation for photoresist application
• Applying the photoresist
• Pre-bake resist
• Align wafer, expose resist
• Post exposure bake
• Develop resist
• Post-bake resist
• These steps are related to preparing the wafer, applying the resist, baking and developing steps, and further baking (post-exposure bake and post-bake).

Models and simulation

• Lithography simulation relies on optics (mathematical light behavior) and chemistry (exposure, baking, developing resist for 3D replica of mask)
• Simulation tools include PROLITH, DEPICT, ATHENA

Wafer exposure system models

• Consider only projection exposure systems, modelling far-field and Fraunhofer diffraction.
• Light acts as electromagnetic waves.
• Electric field patterns are Fourier transforms of the mask and light intensities are the square of electric field magnitudes.

Measurement methods

• Measurement issues include mask dimensions, defects, resist profile, and pattern alignment to underlying wafer features.
• Mask inspection, using microscopes, is unworkable due to the complexity of modern chips
• Scanning over the mask allows for defects if the mask contains more than one chip.
• Defects can be corrected using lasers or ion beams by evaporating or removing unwanted chrome.
• Resist and design databases may differ in actual feature sizes.
• Spot size of e-beam mask making (0.125 - 0.5 µm) can lead to proximity effects on closely spaced features.

Measurement of resist patterns

• Resist development creates a 3D structure, and sometimes involves sloped edges and standing waves.
• Defining and accurately measuring linewidth (least width of a feature that can be printed onto the wafer) is not straightforward in the resist.
• Linewidths greater than 1 μm are typically measured using optical methods (microscopy).
• Smaller features use SEM measurements more prevalent now.

Measurement of etched features

• Photoresist patterns can be transferred to underlying thin films through etching, affecting the quality of the pattern transfer.
• Electrical test structures can be used in combination with SEM images to measure data on linewidths and alignment accuracy (very useful in process development)
• Specific test structures in the scribe lines between chips can also provide information about manufacturing tolerances.

Electrical test structures

• Overall structure assumed as a conducting material
  • (polysilicon, silicide, aluminum, etc.)
  • Right hand part (pads 3-6) is a structure designed to extract sheet resistance of the material
• Geometry chosen to define one square of material
• If a current I_5,6 is forced between terminals 5 and 6, and a voltage V_3-4 is measured between terminals 3 and 4, then the sheet resistance of one square of the material is
  • PS = π V_3-4 / ln(2)*l_5-6

Test your knowledge of photoresist technology with this quiz. Dive into the primary functions, components, and characteristics of g-line, i-line, and DUV photoresists. Understand chemical amplification and the role of photo-acid generators in modern lithography.