Photolithography

Photolithography transfers a pattern defined by light into a photosensitive photoresist coating. After exposure and development, the remaining photoresist forms a relief pattern that can protect selected regions during etching or another material-transfer step.^[ref]

A photomask can place bright and dark regions in the projected optical image. A positive photoresist develops more rapidly where exposed, so sufficiently exposed regions are removed; a negative photoresist retains the exposed regions instead. The final geometry depends jointly on the optical image and the material response. In particular, diffraction limits the image delivered by the optics, while photoresist contrast determines how a gradual variation of exposure is converted into a material boundary. Photolithography therefore connects wave optics, photochemistry and semiconductor device fabrication.

Aerial-image modulation measures the variation of optical intensity in the image delivered to a photoresist before its chemical and development response. A common contrast measure is $M=(I_{\max }-I_{\min })/(I_{\max }+I_{\min })$, for nonnegative intensities with nonzero denominator. Equal bright and dark intensities give $M=0$; a dark minimum with a positive maximum gives $M=1$.

For example, intensities $9$ and $1$ give $M=0.8$. Multiplying both by the same exposure factor leaves $M$ unchanged, although it changes the deposited dose and can change the printed pattern. Image modulation and photoresist contrast therefore describe different stages of photolithography: the optical signal and the material's conversion of that signal into a developed profile.^[ref]

A photomask is a patterned optical component used to define the illumination reaching a photosensitive layer in photolithography. Its pattern controls transmission, reflection or optical phase, depending on the imaging system. In projection lithography, optics form a reduced image of the mask on the substrate.^[ref]

The printed boundary depends on diffraction, illumination and photoresist response as well as the drawn mask edge. Optical proximity correction deliberately changes mask geometry to compensate for those effects. A mask feature is therefore part of an imaging-and-material process, rather than a guarantee that an identical outline will appear on the wafer.

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.

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.

Photoresist contrast measures how sharply a photoresist changes its developed thickness as the exposure dose changes. For a positive photoresist, let $h(D)$ be the remaining thickness after a specified development process and $h_0$ the initial thickness. A contrast curve plots $h(D)/h_0$ against ${\log }_{10}D$. The measured photoresist contrast is the magnitude of the slope near clearing; it need not describe the entire curve.^[ref]

In an idealized model, the thickness decreases linearly with ${\log }_{10}D$ between a retention exposure dose $D_0$ and a clearing exposure dose $D_c$, where $0<D_0<D_c$. ThenOutside that interval the idealized thickness is clipped to $1$ or $0$. A large $\gamma$ means a smaller exposure dose ratio suffices to change from retention to clearing. This is why photoresist contrast influences the critical modulation for a photoresist; the inverse-logarithm formula assumes this particular contrast-curve model.

The critical modulation for a photoresist is the smallest aerial-image modulation that permits a common exposure time to clear bright regions while retaining full thickness in dark regions. Consider a positive photoresist with retention exposure dose $D_0>0$ and clearing exposure dose $D_c>D_0$. Write the extreme image intensities asFor exposure time $t$, the requirements are $t{I}_{\max }\ge D_c$ and $t{I}_{\min }\le D_0$. A feasible $t$ exists exactly whenwith $M=1$ handled directly since the dark-region exposure dose is zero. Rearrangement givesThis derivation assumes exposure dose is intensity times exposure time and uses fixed retention and clearing thresholds. In the idealized linear contrast-curve model of photoresist contrast, $D_c/D_0=10^{1/\gamma }$, soFor example, $D_c=4D_0$ gives $M_c=3/5$. Greater photoresist contrast lowers the required aerial-image modulation in this model.^[ref]