5.1 Photolithography and its limitations

Photolithography is a cornerstone of nanofabrication, using light to pattern tiny features on substrates. It's like painting with light, but instead of a brush, we use masks and special light-sensitive materials called photoresists.

While photolithography can create amazingly small structures, it has limits. The wavelength of light sets a boundary on how small we can go. To push past these limits, scientists have developed clever tricks like immersion lithography and phase-shift masks.

Photolithography Process

Photoresist Application and Properties

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• Photoresist consists of a light-sensitive polymer applied as a thin film to the substrate surface
• Two main types of photoresists include positive and negative resists
• Positive photoresists become more soluble when exposed to light, allowing exposed areas to be removed during development
• Negative photoresists become less soluble when exposed to light, causing exposed areas to remain after development
• Photoresist application involves spin coating the substrate to achieve uniform thickness (typically 0.5-2.5 μm)
• Soft baking removes excess solvent from the photoresist and improves adhesion to the substrate

UV Light Exposure and Mask Alignment

• UV light source emits specific wavelengths (typically 365 nm, 248 nm, or 193 nm) for photoresist exposure
• Mask aligner precisely positions the photomask above the substrate
• Exposure time varies depending on photoresist sensitivity and desired pattern resolution
• Contact, proximity, and projection are three primary exposure methods used in photolithography
• Contact lithography offers high resolution but can damage the mask and substrate
• Proximity lithography reduces damage but sacrifices some resolution
• Projection lithography uses sophisticated optics to project a reduced image of the mask onto the substrate

Mask Design and Fabrication

• Photomask consists of a transparent substrate (quartz) with opaque patterns (chrome)
• Mask patterns define the features to be transferred onto the substrate
• Electron beam lithography often used for high-precision mask fabrication
• Binary masks contain only fully transparent and opaque regions
• Grayscale masks incorporate varying levels of opacity for creating 3D structures
• Mask design software (CAD tools) used to create and optimize mask layouts
• Alignment marks on masks ensure precise positioning relative to previous layers

Resolution and Limitations

Fundamental Resolution Limits

• Resolution limit determines the smallest feature size that can be reliably patterned
• Rayleigh criterion defines the theoretical resolution limit as $R = k_1 \frac{\lambda}{NA}$
• λ represents the wavelength of light used for exposure
• NA denotes the numerical aperture of the projection lens system
• k₁ factor accounts for process-related factors (typically 0.4-0.8)
• Practical resolution limit often 30-50% larger than theoretical limit due to process variations
• Sub-wavelength lithography pushes resolution beyond the wavelength of light used

Diffraction Effects and Pattern Distortion

• Diffraction occurs when light passes through mask openings, causing pattern blurring
• Fraunhofer diffraction dominates in projection systems, while Fresnel diffraction affects contact and proximity lithography
• Airy disk formation limits the minimum resolvable feature size
• Near-field effects become significant for features approaching the wavelength of light
• Line edge roughness results from diffraction and photoresist properties
• Proximity effects cause unintended exposure of nearby features, leading to pattern distortion
• Optical proximity correction techniques compensate for diffraction-induced distortions

Depth of Focus and Process Window

• Depth of focus (DOF) defines the range of acceptable focus positions for pattern transfer
• DOF calculated as $DOF = k_2 \frac{\lambda}{NA^2}$
• k₂ factor typically ranges from 0.5 to 1.0
• Smaller features and higher NA systems result in reduced DOF
• Process window represents the range of exposure and focus conditions that produce acceptable patterns
• Focus-exposure matrix (FEM) used to characterize process window
• Tight process windows require precise control of exposure dose and focus
• Topography variations on the substrate can push features out of the DOF range

Advanced Techniques

Optical Proximity Correction and Mask Enhancement

• Optical proximity correction (OPC) modifies mask patterns to compensate for optical effects
• Rule-based OPC applies predefined corrections based on feature geometry
• Model-based OPC uses sophisticated simulations to predict and correct for optical effects
• Serif features added to corners to reduce rounding
• Assist features improve image contrast for isolated patterns
• Inverse lithography techniques optimize mask patterns using iterative algorithms
• Resolution enhancement techniques (RET) encompass various advanced patterning strategies

Phase-Shift Masks and Interference Lithography

• Phase-shift masks (PSM) manipulate the phase of light to improve image contrast
• Alternating PSM uses 180-degree phase shifts between adjacent features
• Attenuated PSM combines phase shifting with partial transmission
• Chromeless PSM relies solely on phase differences to create patterns
• Interference lithography uses intersecting coherent light beams to create periodic patterns
• Double patterning techniques split dense patterns across multiple exposures
• Self-aligned double patterning (SADP) uses sidewall spacers to define features

Immersion and Extreme Ultraviolet Lithography

• Immersion lithography replaces air with a high-refractive-index fluid between the lens and wafer
• Water immersion increases NA to 1.35, enabling 45 nm and 32 nm node production
• High-index fluids and lens materials push immersion systems to even higher NA values
• Extreme ultraviolet (EUV) lithography uses 13.5 nm wavelength light for next-generation patterning
• EUV light generated by laser-produced plasma or synchrotron sources
• Reflective optics required for EUV due to strong absorption by most materials
• Stochastic effects become significant at EUV wavelengths, requiring new approaches to process control
• Directed self-assembly (DSA) combines top-down lithography with bottom-up material self-organization