Photolithography

Transfer of a light-defined pattern into a resist and then into a material.

Controlled removal of a material by a chemical, plasma or physical process. etch selectivity compares removal rates for different materials; directionality compares removal rates along different geometric directions. High selectivity does not imply a vertical profile, and high directionality does not guarantee an undamaged surface.

A durable patterned layer used to protect underlying material during an etch. Its thickness and etch selectivity must survive the intended depth; erosion and lateral bias can change final feature dimensions even when the starting pattern is correct.

Chemical stripping of organic material, commonly photoresist, by reactive species produced in a plasma. oxygen radicals convert suitable organic material into volatile oxidation products. A remote source reduces direct energetic ion bombardment, but exposed materials can still oxidize and inorganic residue can remain.

An etch that removes a material in different directions without a strongly preferred geometrical direction. It can undercut an overhang and release a sacrificial layer, but can also widen a patterned opening. Access and transport can make the realized undercut nonuniform even with locally isotropic chemistry.

Creating a dense final pattern through several less-dense exposures or through additional pattern-transfer and pitch-division steps. Overlay and etch control replace part of the burden that would otherwise fall on single-exposure optical resolution.

Mask-based optical pattern transfer across a small noncontact gap. Fresnel diffraction across the gap blurs edges; a common resolution estimate is $w=k\sqrt{\lambda g}$ for wavelength $\lambda$, gap $g$ and process factor $k$.

Imaging a patterned mask onto a resist-coated substrate using projection optics.

Splitting a final pattern between separate lithographic steps to reduce the feature density that each exposure must resolve. The steps need a suitable intervening fixation, transfer or spacer process. Minimum single-feature width, pattern decomposition and overlay remain limitations.

A sequence that exposes and transfers one sparse pattern, then applies fresh resist and transfers a second aligned pattern. Their union defines a denser final mask or etched layer. Pattern transfer between exposures prevents simple addition of optical dose from replacing two independently resolved exposures.

Pattern-dependent modification of mask edges or assist features to compensate the finite imaging and pattern-transfer process. It is assessed by the resulting printed dimensions, not by similarity of the mask to the desired layout.

Illumination using noncentral pupil directions to place useful mask diffraction orders inside a projection lens. Grating orientation, coherence and process window affect the benefit.

A lithographic mask that controls optical phase as well as transmission to improve useful interference contrast. Benefits and phase conflicts depend on the pattern.

Pattern projection using extreme-ultraviolet radiation, multilayer reflective optics and a reflective mask.

Direct pattern writing by a scanned electron beam in an electron-sensitive photoresist.

Unintended exposure near a written feature caused by electron scattering in the resist and substrate. The deposited-energy distribution is broader than the incident probe, requiring dose or geometry correction for dense and isolated patterns.

Nonlocal dose deposition that changes developed features through scattering and neighboring exposures.

Small-angle scattering that broadens an incident electron probe as it travels through resist.

Adjustment of exposure dose or shape to compensate for energy deposited outside the intended write location.

Projection lithography using a liquid between the final lens and wafer to increase numerical aperture through its refractive index, with stringent optical and chemical compatibility requirements.

The ratio of etch rates of a target and a protected or underlying material. Endpoint control and surface damage matter as well as this ratio.

Patterning a deposited film by dissolving an underlying patterned sacrificial resist. Film in exposed openings remains while film on the resist is removed.

The axial image-side range over which a specified image criterion remains satisfied. Lithographic scaling is often written $\mathrm{DOF}=k_2\lambda /{\mathrm{NA}}^2$; specify whether a quoted range is full width or one-sided.

A radiation-sensitive patterning film. Positive resist becomes more soluble after suitable exposure; processing sets its dose response.

Selective dissolution of resist after exposure to reveal the written pattern.

A photoresist whose exposed regions dissolve faster in the developer than unexposed regions.

A photoresist whose exposed regions become less soluble in developer over the relevant process window, so adequately exposed regions remain after development.

Radiant energy per exposed area. For steady intensity $I$ and exposure duration $\tau$, $D=I\tau$; the developed photoresist profile also depends on chemical processing.

The inverse logarithmic dose range between retention and clearing, $\gamma =[{\log }_{10}(D_c/D_r){]}^{-1}$. Larger contrast permits a smaller aerial-image intensity ratio.

The smallest aerial-image modulation that permits full clearing in bright regions while preserving the initial thickness in dark regions.